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MARINE  BIOLOGICAL  LABORATORY. 


Received      June    27*    1939 
Accession  No.       50458 
Given  by       Amer  loan    Book    Co* 
Place,  New   York    City 


*,t*flo  book  OP  pamphlet  is  to  be  removed   ttom   the  Iiab- 
oratopy  tuithout  the  pepmission  of  the  Trustees. 


BIOLOGY 

The  Story  of  Living  Things 


GEORGE  WILLIAM  HUNTER 

Lecturer  in  Methods  of  Science  Teacliimj 

Department  of  Education 

Claremont  Colleges 

HERRERT   EUGENE  WALTER 

Professor  of  Biology,  Brown  Unirersity 

GEORGE  WILLIAiVI   HUNTER,  III 

Assistant  Professor  of  Biology,  Wesleyan  Unirersity 


"^1 

c 


AMERICAN   BOOK   COMPANY 


NEW    YORK  CINCINNATI 

BOSTON  ATLANTA  DALLAS 


CHICAGO 

SAN   FRANCISCO 


Copyright,  1937,  by 
AMERICAN  BOOK  COMPANY 


All  rights  reserved 


COLLEGE  BIOLOGY,  H.  W.  &  H. 
W.  P.  2 


MADE  IN  U.  S.  A. 


This  hook  is  gratefully  dedicated  to  our  wives,  to  whom 
much  of  the  credit  and  none  of  the  blame  is  due. 


1>11EFACE 


Here  are  a  few  chips  left  over  from  the  authors'  workshop. 

First  of  all  we  do  not  pretend  to  ha\'e  presented  herein  the  last 
word  in  a  field  already  overcrowded  by  worthy  ri\'als.  The  "last 
word"  has  an  undesirable  mortuary  connotation  quite  out  of  keeping 
in  a  book  about  living  things. 

The  authors  have  been  teaching  biology  for  a  total  of  ninety-four 
academic  years,  in  addition  to  over  sixty  seasons  of  strenuous  service 
in  summer  field  work  with  classes  at  marine  and  fresh-water  labora- 
tories, and  they  can  truthfully  and  enthusiastically  say  that  they 
have  enjoyed  this  experience. 

If  what  they  would  pass  on  to  other  students  of  biology  appears 
from  the  table  of  contents  to  bear  the  familiar  marks  of  old  stuff,  the 
reason  is  that  it  represents,  in  their  minds  at  least,  what  remains 
after  many  years  of  trial  and  elimination  at  the  hands  of  an  army  of 
different  teachers  and  scholars.  The  fact  that  much  material  that  has 
been  worked  over  before  it  was  retained  does  not  necessarily  prevent, 
it  is  hoped,  some  degree  of  freshness  in  its  presentation.  Any  text- 
book, the  authors  hold,  should  be  somewhat  like  a  dish  of  uncracked 
nuts,  accompanied  by  a  good  substantial  nutcracker.  It  is  desirable 
that  the  reader  should  have  some  of  the  fun  of  wielding  the  nutcracker, 
for  no  pedagogical  cellophane  can  preserve  nuts  already  shelled  in  an 
entirely  fresh  and  satisfactory  condition  for  a  very  long  time. 

An  inevitable  handicap  that  the  textbook  method  of  presentation 
of  any  subject  is  bound  to  suffer,  is  the  fact  that  between  the  covers 
of  a  book  the  whole  banquet  is  set  upon  the  table  at  once  in  a  more  or 
less  complete  array.  It  is  the  part  of  the  instructor  to  break  up  the 
feast  into  courses  and  to  serve  them  in  digestible  portions.  Perhaps 
the  method  of  suspense  employed  in  magazine  serials  woidd  furnish  a 
better  way  of  arriving  at  the  desired  end  than  presenting  the  matter 
all  at  once  in  l^ook  form,  since  sufficient  time  shoukl  always  be  pro- 
vided between  the  planting  and  harvesting  of  intellectual  ideas  to 
allow  for  unforced  sprouting  and  growth. 

In  the  use  of  any  textbook  it  is  well  to  remember  that  the  pages 
may  be  turned  backward  as  well  as  forward,  and  that  it  is  no  crime 
either  to  skip  or  to  reread. 

Every   studious   and   effective   reader,   moreover,   is  wary  about 


vi  PREFACE 

accepting  witliout  question  whatever  he  may  come  across  in  print, 
for  even  textbooks  are  often  known  to  l)o  incomplete  and  liable  to 
error. 

Again,  if  the  art  of  reading  between  the  hnes  has  not  been  culti- 
vated, it  does  not  greatly  avail  simply  to  scan  the  printed  hnes 
themselves.  Every  opening  that  induces  the  reader  to  seek  further 
should  be  gratefully  prized. 

Goethe  once  said:  "Wer  nicht  mit  der  Bewunderung  anfangt, 
werdet  nie  in  das  innere  Heiligthum  eindringen."  Wonder  is  truly 
the  mother  of  wisdom,  for  once  the  capacity  for  wonder  slips  away, 
one  is  prone  to  become  blase,  imcomfortably  sophisticated,  and 
intellectually  slothful. 

With  this  explanation  of  the  way  it  is  hoped  that  this  book  will 
be  used,  the  authors  unite  in  cordially  inviting  the  reader  to  join 
them  in  exploring  the  following  pages. 


ACKNOWLEDGMENTS 


The  authors  wish  to  make  grateful  acknowledgment  to  all  who 
have  aided  them  in  the  preparation  of  a  college  textbook  in  biology. 

In  particular,  mention  should  be  made  of  the  members  of  the  Biology 
Department  of  Wesleyan  University  who  so  willingly  collaborated  in 
trying  out  the  ecological  approach  to  a  study  of  general  biology  for 
several  years  prior  to  publication  of  this  book.  Grateful  acknowledg- 
ment is  also  made  to  them  for  innumerable  suggestions  as  well  as  for 
their  willingness  to  include  certain  successful  features  of  the  course  in 
the  text.  Their  help  and  advice  has  frequently  been  sought  and 
willingly  given. 

Special  mention  should  also  be  made  of  the  tireless  effort  and 
willing  help  of  Wanda  S.  Hunter  and  Alice  Hall  Walter,  both  of  whom 
read  the  manuscript  and  proof  and  contributed  much  to  whatever 
success  this  book  may  attain. 

It  is  impossible  here  to  enumerate  all  who  have  aided  in  the 
production  of  this  book,  but  the  following  names  must  be  men- 
tioned :  Dr.  E.  C.  Schneider,  Shanklin  Biological  Laboratory, 
Wesleyan  University,  for  reading  the  entire  manuscript ;  Dr.  Francis 
R.  Hunter,  Rhode  Island  State  College,  for  reading  the  entire  proof ; 
Dr.  Aurel  O.  Foster,  Gorgas  Memorial  Laboratory,  Panama,  for 
reading  section  XII ;  Dr.  Hurbert  B.  Goodrich,  Shanklin  Biological 
Laboratory,  Wesleyan  University,  for  reading  sections  XIX-XXIII ; 
Dr.  Frederick  L.  Hisaw,  Biological  Laboratories,  Harvard  University, 
for  reading  section  XVIII ;  Dr.  John  A.  AIcGeoch,  Psychological 
Laboratory,  Wesleyan  University,  and  Dr.  Bernard  C.  Ewer,  Depart- 
ment of  Psychology,  Pomona  College,  for  reading  section  XVH ; 
Dr.  Philip  A.  Munz,  Department  of  Biology,  Pomona  College,  for 
reading  the  botanical  portions  of  the  book ;  Messrs.  Emil  Kotcher 
and  Wilson  C.  Grant  for  aid  in  preparing  the  index. 

Acknowledgment  is  also  made  to  organizations  and  individuals 
without  whose  co-operation  it  would  have  been  impossible  to  secure 
many  of  the  instructive  and  attractive  illustrations. 


vn 


CONTENTS 


PAGE 
1 


NATURAL   HISTORY 

CHAPTER 

I.   The  Stage  Settin"g  (Ecology) 

II.   The  Biological  Conquest  of  the  Would  .         .         .      20 

III.  The  Interdependence  of  Living  Things  —  The  Web  of 

Life 44 

IV.  Roll  Call (33 


FUNDAMENTALS   OF  STRUCTURE  AND   FUNCTION 


V.    Life  and  Protoplasm 
VL   Cells  and  Tissues 


125 

138 


ORGANISMS   ILLUSTRATING   BIOLOGICAL   PRINCIPLES 


VII.  Beginnings:  The  Large  Group  of  the  Smallest  Organlsms 

VHI.  The  Development  of  Sexuality  in  Plants 

IX.  Division  of  Labor  in  Coelenterates 

X.  Being  a  Worm 

XI.  The  Popular  Insect  Plan  . 

NIL  The  Art  of  Parasitism 

XIII.  Advantages  of  Being  a  Vertebrate 


151 
lf)8 
17!i 
187 
199 


.) 


233 


THE   MAINTENANCE   OF  THE   INDIVIDU.VL 

XIV.  The  Role  of  Green  Plants 

XV.  The  Metabolic  Machinery  of  Animals 

XVI.  Support,  Motion,  and  Sensation 

XVII.  The  Display  of  Energy 

XVIII.  Chemical  Regulators 


237 
274 
32() 
3C.4 
390 


THE  MAINTENANCE  OF  SPECIES 


XIX.   Reproduction  and  Life  Cycles 
XX.   The  Great  Relay  Race 


IX 


405 
434 


IS 


X  CONTENTS 

THE  CHANGING  WORLD 

CHAPTER  PAGL 

XXI.   Time  Spent  (Palaeontology) 473 

XXII.   The  Epic  of  Evolution 483 

XXIII.  That  Animal,  Man  (Anthropology)          ....  530 

MAN  AS  A  CONQUEROR 

XXIV.  Man's  Conquest  of  Nature       ......  567 

XXV.   Conservation  and  Its  Meaning 589 

XXVI.   Man's  Fight  for  Survival 608 

XXVII.   The  Next  Million  Years 637 

Index 645 


NATURM.    IIISToin 


T 


THE   STAGE   SETTI\(;    (ECOLOGY) 

Preview.  Ecology  of  a  typical  region  •  How  to  study  ocolog>'  •  IMaiit 
and  animal  associations  •  Basic  environments  :  water  as  a  factor  ;  tempera- 
ture ;  light  as  a  factor ;  chemical  factors ;  gravity  as  a  factor ;  substratum  ; 
molar  agencies ;  biotic  factors  •  Life  in  the  water  •  Life  in  the  air  •  Life  on 
land  •  Suggested  readings. 

PREVIEW 

"My  heart  is  fixed  firm  and  stable  in  the  belief  that  ultimately  the  sun- 
shine and  the  summer,  the  flowers  and  the  azure  sky,  shall  Ijecome,  as  it 
were,  interwoven  into  man's  existence.  He  shall  take  from  all  their  beauty 
and  enjoy  their  glory."  —  Richard  Jefferies  :   The  Life  of  the  Fields. 

There  is  a  lure  in  knowing  something  intimate  about  i)lant  and 
animal  neighbors,  their  habits  and  the  places  where  thoy  live.  A 
trout  fisherman  finds  almost  as  keen  enjoyment  in  watching  a  king- 
fisher make  its  catch  as  in  having  a  trout  take  his  own  fly.  Flic 
banks,  meadow\s,  and  woods  along  a  trout  stream  are  aliv'e  with 
interesting  plants  and  animals.  Even  a  slight  acquaintance  with 
what  may  be  expected  along  the  path  makes  a  hike  througii  the 
forest  and  field  immensely  more  worth  while.  An  early  morning 
walk,  if  one  knows  a  few  permanent  bird  residents  and  can  recognize 
a  migrant  here  and  there,  takes  on  an  absorbing  interest  for  tlie 
observer.  Such  trips  in  the  open  are  eventful  experiences,  the  joy 
of  which  is  not  easily  forgotten.  One  may  see  the  beauty  of  living 
things,  and  enjoy  the  songs  of  birds  and  the  gay  colors  of  insects,  or 
get  a  thrill  out  of  the  sight  of  the  first  violet  or  bluebird,  as  he  drinks 
in  the  sweet  odors  of  the  flowery  meadow.  From  the  standpoint  of 
the  more  observant,  another  side  than  passive  enjoyment  of  nature 
is  to  be  found.  It  is  discovered  in  asking  and  trying  to  answer  the 
how  and  why  of  life  aroimd  us. 

Charles  Elton  has  called  the  science  of  ecology  "scientific  natural 
history."  This  deals  with  the  occurrences  and  behavior  of  organ- 
isms in  a  given  habitat  or  home.  Anyone  who  feels  a  genuine  response 
to  the  call  from  the  natural  environment  surrounding  him  cammf 

1 


2  NATURAL   HISTORY 

fail  to  find  an  interest  in  this  approach  to  natural  history.  Why,  for 
example,  do  eertaiii  kinds  of  animals  live  in  the  swift  water  of  trout 
streams,  while  different  ones  are  associated  with  plants  in  a  quiet 
pond?  Why  are  the  types  of  life  found  along  the  seashore  so  unlike 
those  around  the  edge  of  an  inland  lake?  Why  do  forest  trees  grow 
tall  in  the  dense  woodland,  more  spreading  in  the  open,  and  stunted 
near  the  tops  of  mountains?  These  and  hundreds  of  like  questions 
can  be  answered  truthfully  with  the  background  afforded  by  the 
science  of  ecology. 

Ecology  of  a  Typical  Region  ^ 

New  England  scenery  is  characterized  by  rounded  granite  hills, 
often  heavily  wooded  with  second  or  even  third  growth.  In  the 
hollows  surrounded  by  these  hills  nestle  little  lakes,  bodies  of  water 
varying  in  area  from  a  few  hundred  square  feet  of  surface  to  many 
scores  or  even  square  miles  in  extent. 

A  survey  of  the  inhabitants  of  one  of  these  smaller  lakes,  chosen  as  a 
typical  example,  reveals  relatively  few  fish  and  fewer  plants  in  the 
open  water.  Nearer  shore  are  found  unmistakable  zoning  of  plants 
and  animals,  depending  on  whether  the  shore  is  rocky,  sandy,  or 
muddy.  In  sheltered  bays  having  a  bottom  of  soft  mud  are  found 
numbers  of  pond  lilies  and  other  aquatic  plants,  which  give  shelter 
to  pickerel,  bass,  and  smaller  fish,  as  well  as  a  vast  array  of  small 
crustaceans,  insect  larvae,  and  microscopic  plants  and  animals. 

Part  of  the  lake  shore  is  a  sandy  beach,  at  one  end  of  which  a  slug- 
gish stream,  after  meandering  through  a  meadow,  empties  into  the 
lake.  This  constitutes  quite  a  typical  environment  and  will  yield 
abundant  material  if  searched  carefully. 

The  edge  of  the  lake  bordering  on  the  beach  contains  relatively 
few  plants  and  animals.  It  is  exposed  to  the  wind  and  consequently 
to  wavelets  which  cause  more  or  less  movement  of  the  loose  sand,  thus 
giving  slight  protection  to  living  things.     We  find  here  almost  no 

'  BOOKS    USEFUL    FOR    FIELD    WORK 

Downing,  Our  Living  World,  Longmans,  Green,  1924. 

Johnson  and  Snook,  Seashore  Animals  of  the  Pacific  Coast,  Maemillan,  1927. 

Lntz,  Field  Book  of  Insects,  Putnam,  1921. 

Mann  and  Hastings,  Out  of  Doors,  Holt,  19.32. 

Morgan,  Field  Book  of  Ponds  and  Streams,  Putnam,  19.30. 

Needhani  and  Needham,  Guide  to  the  Study  of  Fresh  Water  Biology,  3rd  ed.,  Comstock  Publ.  Co., 

193.-). 
Weaver  and  Clement,  Plant  Ecology,  McGraw-Hill,  1929. 


THE  STAGE   SETTING 


./.  N   lOinkin.  Jr. 

A  slow-ilowing  stream  presents  a  habitat  for  characteristic  plants  ami  animals 
adapted  to  this  type  of  environment.     Head  i)afjes  ."5-1. 

plants  and  only  occasional  bass,  pickerel,  or  minnows.  A  few 
dragonfly  nymphs  live  under  the  small  stones  in  shallow  water,  while 
ninnerous  snails  (Campeloma)  are  foimd  buried  in  the  sand  or  crawl- 
ing along  the  bottom.^  It  is  possible  to  collect  a  few  specimens  of 
plankton,  which  consists  of  minute  free-swimming  or  floating  organ- 
isms, but,  on  the  whole,  it  is  a  relatively  inhospitable  environment 
inhabited  by  comparatively  few  organisms. 

Within  a  few  yards  of  this  beach  the  stream  flows  gently  over  a 
shallow  sandbar,  flanked  by  cattails  and  rushes.  Here  are  nimierous 
representatives  of  several  groups  of  plants :  in  the  water  a  \'ariety 
of  algae,  Spirogyra  (pond  scum),  streaming  filaments  of  Ocdogonium, 
Oscillatoria,  and  Cladophora,  and  iimiunerable  unicellular  organisms, 
such  as  desmids  and  diatoms.  Water  cress,  water  plantain,  water 
smart-weed,  and  burr-weed  grow  along  the  banks,  while  in  sheltered 
bays  the  surface  of  the  water  may  be  covered  with  duckwec'd  or  per- 
haps yellow  and  white  water  lilies.  H(>re  and  there  in  boggy  i)I;ic(>s 
are  dense  masses  of  cattails,  yellow  flowering  rushes,  and  numerons 
sedges,  while   on  the  banks   are   fotmd  grasses  of  se\'eral  species. 

'It  is  expected  that  the  student  will  make  free  use  of  IV,  '  Roll  Call."  for  cencral  idcntiBcation 
and  of  the  books  of  reference  noted  for  more  intimate  and  exact  classification. 


4  NATURAL   IIISTOHY 

buttercups,  Jack-in-the-pulpit,  bog  arrow-grass,  and  a  few  shrubs 
such  as  button-bush  and  willow.  The  vegetation  shows  a  zonal 
arrangement  of,  first,  submerged  or  floating  water  plants,  then  emer- 
gent forms,  growing  in  the  water  and  along  the  banks,  while  other 
plants  such  as  grasses  and  shrubs  are  found  at  a  little  distance  from 
the  water.  This  zonal  distribution  is  characteristic  of  shore  associa- 
tions of  plants  and  animals. 

In  the  slow-flowing  stream  live  two  species  of  sunfish,  two  or  three 
species  of  pickerel,  bass,  three  species  of  frogs,  bullfrogs,  green  frogs, 
and  pickerel  frogs  with  their  tadpoles,  also  an  occasional  painted 
turtle  and  water  snake.  Of  birds,  the  redwing  blackbirds  are  numer- 
ous, with  occasional  kingfishers,  and  more  rarely  a  great  blue  heron. 
Although  no  mammals  are  in  sight,  a  telltale  mound  of  sticks  shows 
that  muskrats  live  there.  Of  the  smaller  organisms,  the  nymphs  and 
larvae  of  the  dragonfly  and  Mayfly  are  the  most  abundant.  The 
water  swarms  with  two  species  of  water  bugs  and  diving  beetles,  while 
beetle  larvae  and  the  larvae  of  mosquitoes  are  numerous.  Many 
crustaceans,  tiny  amphipods  and  isopods,  may  be  seen  swimming  or 
feeding  on  the  aquatic  plants.  The  snails,  Physa  and  Lymnaea,  are 
very  abundant,  while  a  few  aquatic  worms,  Tuhifex,  may  be  found  in 
the  mud.  Colonies  of  bryozoans  may  also  be  found,  incrusting  the 
stems  of  water  plants,  as  well  as  an  occasional  mass  of  fresh-water 
sponge. 

These  two  regions,  the  lake  shore  and  the  stream,  although  only  a 
few  yards  apart,  present  tremendous  differences  in  populations. 
Why  these  differences?  At  first  sight,  one  might  say  it  was  due 
entirely  to  abundance  of  food,  but  this  is  only  begging  the  question. 
Evidently  many  factors  are  at  work.  The  fauna  and  flora  of  other 
localities  visited  would  show  even  greater  changes.  Across  the 
meadow  and  up  into  the  nearby  woods  each  locality  would  be  found 
to  be  inhabited  by  groups  of  living  plants  and  animals  differing  in 
many  respects  from  those  in  neighboring  localities.  In  each  of  these 
localities  there  would  be  certain  dominant  organisms  better  fitted 
than  any  others  to  live  there.  These  become  permanent  species  in 
that  locality. 

How  to  Study  Ecology 

To  understand  much  al)out  ecology,  one  must  be  able  to  do  much 
more  than  simply  study  a  book.  The  place  to  study  the  stage  setting 
is  the  stage.     The  place  to  learn  about  the  relation  of  living  things 


THE  STM^E   SKTTINd 


to  their  cnviroiuncnt  is  the  luihitat.  Kltoii '  in  his  iiiteivstiiijr  intro- 
duction to  ecology  cU^scribes  the  attack  on  a  ceilain  ccolofrical  prob- 
lem in  these  words : 

"Suppose  one  is  studying  the  factors  limiting  the  distribution  of  animals 
living  in  an  estuary.  One  would  need  to  know  amongst  other  things  what 
the  tides  were  (but  not  the  theories  as  to  how  and  why  they  occur  in  a  par- 
ticular way) ;  the  chemical  composition  of  the  water  and  how  to  estimate 
the  chloride  content  (but  not  tlie  reasons  why  silver  nitrate  precipitates 
sodium  chloride) ;  how  the  rainfall  at  different  times  of  the  year  affected 
the  muddiness  of  the  water;  something  about  the  physiology  of  sulphur 
bacteria  which  prevent  animals  from  living  in  certain  parts  of  the  estuary ; 
the  names  of  common  plants  growing  in  salt-marshes ;  sometliing  about  the 
periodicity  of  droughts  (but  not  the  reasons  for  their  occurrence).  One 
would  also  have  to  learn  how  to  talk  politely  to  a  fisherman  or  to  the  man 
who  catches  prawns,  how  to  stalk  a  bird  witli  field-glasses,  and  possibly  how 
to  drive  a  car  or  sail  a  boat.  Knowing  all  these  things,  and  a  great  deal 
more,  the  main  part  of  one's  work  would  still  be  the  observation  and  coUeo 
tion  of  animals  with  a  view  to  finding  out  their  distribution  and  habits." 

This  gives  us  our  approach.  Our  own  interests,  our  reading,  and 
the  time  involved  must  largely  determine  the  extent  to  whicii  we 
solve  the  ecological  problems 
of  our  own  environment. 


Plant  and  Animal 
Associations 

In  making  an  ecological 
study  of  living  communities 
we  notice  that  one  kind  of 
plant  or  one  kind  of  animal 
is  never  found  li^-ing  entirely 
alone.  Plants,  for  example, 
are  associated  together  by 
lack  or  abundance  of  water ; 
those  living  under  abundant 
water  conditions  being  called 
hydrophytes ;  those  associated 
in  a   condition   of   moderate 


Water  lilies,  catta 
(•haracteristi( 


hulriislu's 
pliyle."*. 


1  From  Elton,  Charles.  Animal  Ecology,  p.  35.     By  permission  of  The  Macmillan  Company. 

publishers. 


NATllRAr.   HISTORY 


Typical  xerophytic  plants  of  the  desert  areas. 


Hau'oTtIt 


water  supply,  mesophytes ;  and  those  which  associate  in  desert  condi- 
tions, xewphytes.  Animals  which  live  in  the  water  are  said  to  be  aquatic, 
those  on  land  terrestrial,  while  those  that  live  both  on  land  and  in 
water  are  called  amphibious.  Animals  and  plants  associated  in  still 
water  are  quite  different  from  those  in  running  water,  while  different 
types  of  plants  and  animals  are  found  close  to  shore,  in  deep  water,  in 
rapid  water,  on  rocky  shores  or  on  sandy  shores,  in  salt  or  in  fresh 
water,  and  in  tidal  pools  or  on  the  sand.  Everywhere  we  find  dif- 
ferent associations  of  plants  and  animals.  Many  explanations  are 
given,  but  no  one  explains  everything.  One  investigator,  Merriman, 
emphasizes  temperature  as  an  all-important  factor ;  Walker  gives 
atmospheric  pressure  ;  Heilprin,  food  ;  and  Shelford,  in  recent  experi- 
ments, indicates  that  the  conditions  under  which  an  animal  breeds 
may  greatly  influence  its  distribution.  He  experimented  with  tiger 
beetles,  using  different  soils  such  as  clay,  clay  and  humus,  humus, 
humus  and  sand,  and  pure  sand.  The  beetles  lay  their  eggs  only  in 
moist  soil,  therefore  this  factor  was  constant  with  all  the  soils.  In 
this  experiment  the  soils  were  also  placed  at  a  level  and  on  slants. 


THE  STAGE  SETTING 


Eighty  per  cent  of  all  the  eggs  were  laid  in  steep  elay,  and  <)S  per 
cent  in  sloping  soil.  Thus  ho  concludes  that  the  egg-laying  hahits 
of  these  beetles  determine  tiieir  habi- 
tat, for  if  they  could  not  get  the  kind 
of  soil  and  the  slope  needed,  they 
would  not  breed.  In  this  case  the 
fluctuation  and  distribution  of  a  spe- 
cies would  be  dependent  upon  a  single 
factor.  This  may  be  true  in  the  dis- 
tribution of  a  great  many  plants  and 
animals. 

Basic  Environments 

There  are  three  states  of  matter, 
gas,  liquid,  and  solid.  These  are  evi- 
dent in  the  land,  the  water,  and  the 
air  in  which  living  things  are  found. 
Life  is  only  found  in  conditions  where 
it  is  at  least  partially  fitted  or  adapted 
to  live.  These  conditions,  called  factors  of  the  en\iroinnent.  are  air 
or  its  contained  gases;  water  or  moisture;  temperature;  light; 
chemical  constituents  in  soil,  water,  or  foods;  gravity;  the  presence 
of  a  substratum  on  which  the  organism  rests,  such  as  soil,  moving 
objects  in  the  water,  or  the  sea  bottom ;  molar  agencies,  such  as 
wind,  water  currents,  or  any  moving  force  in  the  environment ;  and 
finally,  biotic  factors  which  come  through  the  interaction  of  other 
organisms  in  the  same  environment. 


A  birch  forest  is  composed  of 
typical  me.sophyte.s. 


Water  as  a  Factor 

Water  is  absolutely  essential  to  life,  from  40  to  95  per  cent  of  all 
living  things  being  formed  of  this  substance.  It  is  generally  true  that 
no  growth  or  life  process  of  either  plants  or  animals  can  take  place 
without  water.  An  example  of  this  relationship  of  moisture  to  life  is 
shown  in  the  story  of  the  British  Mu.seum  snail  related  by  Mr.  Baird.' 

"  On  the  25th  of  March  1846  two  specimens  of  Helix  desertorum,  colloc-ted 
by  Charles  Lamb,  Esq.,  in  Egypt  some  time  previously,  were  fixed  ui>on 
tablets  and  placed  in  the  collection  among  the  other  ^h)llusca  of  the  .Musmnn. 
There  they  remained  fast  gummed  to  the  tal)let.  About  the  loth  of  .Marcli 
1850,  having  occasion  to  examine  some  shells  in  the  same  ca.se,  Mr.  Il'iird 

1  Ann.  Mag.  Nat.  Hisl.  (2)  vi.  (1850).  p.  68. 
H.  W.  H,  —  2 


NATURAL   HISTORY 


w|i»4«jiK. 


'-t**-». 


WiiylU  J'itrcc 

These  photographs  were  taken  from  the  same  spot  on  the  Mohave  desert  floor. 
The  upper  was  made  at  the  end  of  the  rainy  season,  the  lower  about  two  months 
later.     What  one  factor  causes  this  difference.!^ 


noticed  a  recently  formed  epiphragm  over  the  mouth  of  one  of  these  snails. 
On  removing  the  snails  from  the  tablet  and  placing  them  in  tepid  water,  one 
of  them  came  out  of  its  shell,  and  the  next  day  ate  some  cabbage  leaf.  A 
month  or  two  afterwards  it  began  repairing  the  lip  of  its  shell,  which  was 
broken  when  it  was  first  affixed  to  the  tablet." 


THE  STAGE  SETTING  ., 

The  uses  to  which  water  is  put  by  an  organism  are  nianit'old.  It 
is  necessary  as  a  solvent  for  foods  within  the  body.  In  HvIiik  tissues 
it  becomes  a  medium  of  exchange  between  different  parts  of  tlio  body, 
while  in  higher  animals  it  carries  off  body  heat,  thus  helping  in  tiic 
regulation  of  their  temperature.  In  air  it  causes  humidity.  In  soil 
it  carries  the  raw  food  materials  of  green  plants.  In  many  alkali 
lakes,  such  as  Great  Salt  Lake,  fish  life  is  practically  absent  and  the 
numbers  of  insects  and  crustaceans  inhabiting  such  water  are  greatly 
reduced  because  of  the  high  mineral  content  of  the  water.  On  the 
other  hand  certain  crustaceans,  such  as  the  brine  shrimps,  are  only 
found  in  water  containing  a  high  concentration  of  salts.  Acid  lakes 
and  streams  contain  only  certain  types  of  fish,  and  according  to  in- 
vestigation by  Jewell  ^  are  lacking  in  snails,  possibly  because  of  the 
absence  of  lime  from  which  snails  build  their  shells. 

Temperature 

Differences  in  climate  (which  after  all  are  largely  differences  in 
temperature  and  water  supply)  are  accompanied  by  changes  in  the 
appearance  and  kinds,  of  plants  and  animals.  The  life  processes  of 
organisms  proceed  between  certain  maximum  and  minimum  limits 
of  temperature.  Somewhere  between  these  is  an  optimum  temi^era- 
ture  at  which  the  life  processes  function  most  normally.  In  i)lants 
optimum  temperatures  vary  greatly  for  different  species,  and  are 
largely  instrumental  in  determining  what  plants  will  grow  in  a  gi\-cn 
locality.  For  example,  apple-raising  regions  must  have  a  mean 
summer  temperature  of  not  more  than  70°  F.  The  optimum  of 
most  tropical  plants  ranges  over  90°  F.,  while  alpine  species  require 
a  temperature  slightly  above  freezing.  The  temperature  of  plants 
changes  rapidly,  depending  on  the  amount  of  external  heat  they  re- 
ceive. This  has  an  important  bearing  on  horticulture.  Lemons  on 
the  trees,  for  example,  freeze  at  a  temperature  of  28°  F.,  and  oranges 
at  26°  F.  They  are  often  kept  from  freezing  by  means  of  heaters. 
Plant  injuries  caused  by  freezing  are  due  to  the  rapid  withdrawal  of 
water  from  the  soft  parts,  therefore  plants  with  a  high  water  con- 
tent are  more  easily  injured.  This  accounts  for  the  freezing  of  the 
young  tips  of  trees.  Seeds  which  have  a  small  water  conteut  are 
capable  of  withstanding  very  low  temi)eratures. 

In  animals,  as  in  plants,  the  lif(>  processes  proceed  best  at  oi)timuiii 
temperatures  which  differ  with  the  species.     Mast  protozoa  divide 

1  Jewell,  ••  The  Fishes  of  an  Acid  Lake."     Tran..  Amer.  M  \ol.  XLIII,  1924.  pp.  77-84. 


10 


NATURAL  HISTORY 


^g» 

4 

*"■  ■  ■>^---''^'>j*S3B||)iiN^:'- 

Hk 

nil                   wl^^^^^^^H 

y^^: 

(,?)  ir.  L.  Macchtlin 
During   the   freezing  weather  in  January,    1937,  in   California,   citrus  groves 
which  were  adequately  protected  by  heaters  lost  relatively  little  fruit,  while  many 
unprotected  groves  suffered  a  complete  loss  of  fruit  as  well  as  some  trees. 

much  more  rapidly  at  warm  than  at  cold  temperatures,  and  this  is  true 
of  the  reproduction  of  many  animals.  Many  tropical  animals  may 
withstand  cold  temperatures,  but  will  not  propagate  at  those  tem- 
peratures. H.  B.  Ward  ^  has  made  observations  on  the  sockeye 
salmon  which  indicate  that  these  fish  in  swimming  up  rivers  to  spawn 
always  take  the  river  of  slightly  cooler  temperature,  a  difference  of 
1°  F.  being  sufficient  to  divert  the  fish.  Seasonal  cycles  of  activity 
are  largely  influenced  by  temperature,  this  being  particularly  true  of 
reproductive  activity,  which  plays  a  part  in  the  migrations  of  birds, 
the  rapid  multiplication  of  plankton  and  other  forms.  Some  animals 
respond  to  a  cold  temperature  by  going  into  a  resting  state  or  hiberna- 
tion, while  others  go  into  a  dormant  condition  because  of  unfavorable 
conditions  of  heat  and  dryness.  This  latter  state,  aestivation,  is  often 
seen  in  regions  having  marked  periods  of  alternating  rain  and  drought. 

I  Ward,  H.  B.  "  Some  Responses  of  Sockeye  Salmon  to  Environmental  Influences  during  Fresh- 
water Migration."     Ann.  and  Mag.  of  Nat.  Hist.,  Vol.  VI,  pp.  18-36. 


THE  STAGE  SETTING 


II 


Animals  are  said  to  be  warm-blooded  or  cold-blooded.  The  foriiK  r 
term  means  that  they  have  a  constant  body  temi)erature  {honwio- 
thermal),  while  the  latter  means  that  the  body  temperature  varies 
with  the  external  temperature  {poikilothcnnal) .  Frogs  can  often 
be  frozen  stiff  and,  when  thawed  out  gradually,  will  live.  This  is 
true  of  many  animals  and  is  an  undoubted  adaptation  which  enables 
them  to  withstand  great  cold.  Homoiothermal  animals,  however, 
are  more  or  less  independent  of  the  external  temperature  because 
their  internal  body  heat  remains  at  a  constant  temperature  regard- 
less of  outside  fluctuations. 

Animals  are  divided  into  two  groups  depending  on  whether  they 
can  easily  stand  changes  in  external  temperature,  some  being 
restricted  to  a  relatively  narrow  range  of  temperature  changes  {steno- 
thermaV),  while  others  have  not  only  the  ability  to  withstand  a  large 
range  of  temperature,  but  also  may  become  acclimated  to  new  tem- 
perature ranges  if  they  are  changed  gradually  from  one  environment 
to  another  {eur y thermal) .  A  classic  series  of  experiments  by  Dallinger 
with  protozoans  showed  that  he  could  change  their  li\'ing  conditions 
from  15.6°  to  70°  C.  without  having  the  animals  die.  It  is  this  ability 
that  gives  us  the  plant  and  animal  populations  in  some  hot  springs. 

Light  as  a  Factor 

Light  is  a  form  of  radiant  energy.  Passed  througli  a  prism  it  is 
broken  up  into  the  primary  colors  of  the  spectrum,  each  of  which  has 


Left :   A  nasturtium  plant  exposed  to  ordinary  Kreeniu...se  lif^hl  sin.e  ^.vnuuMum. 
Right:  Same  plant  exposed  to  onr-sideddlunnnal.on  for  .SIX  hours. 


I^ettuce.  a  long-day  plant. 


Salvia,  a  short-day  plant.     These  series  of  plants  were  grown  experimentally 
at  the  Boyce  Thompson  Institute  for  Plant  Research,  Yonkers,  N.  Y. 


12 


THE  STAGE  SETTING 


i:i 


a  different  wave  length.  In  addition  there  is  the  non-visible  radiant 
energy  of  the  ultra-spectrum.  These  different  wave  lengths  ha\c 
various  effects  on  plants  and  animals.  Chlorophyll,  the  gnnMi  color- 
ing matter  of  plants,  which  depends  on  the  presence  of  light,  absorbs 
light  waves  only  from  the  red  and  blue  bands  of  the  spectrum. 
Whereas  most  of  the  radiant  energy  absorbed  by  a  plant  changes 
to  heat,  a  very  small  part  of  it,  estimated  at  not  more  than  0.5  per 
cent  to  3  per  cent,  is  used  by  the  chlorophyll  in  the  process  of  starch 
making.  As  in  the  case  of  temperature,  optimum  light  is  necessary 
for  the  best  work  of  plants,  some  preferring  shade  and  others  living 
at  their  best  in  bright  sunlight. 

Light  causes  movements  in  leaves  and  stems  as  well  as  changes 
in  the  size  and  shape  of  these  organs.  Plants  respond  to  light,  tlie 
leaves  being  placed  so  as  to  get  the  most  light  possible.  The  amount 
of  light  largely  determines  the  shape  of  the  entire  plant,  trees  in  a  thick 
forest  having  a  very  different  shape  from  similar  trees  in  the  open. 

The  length  of  daylight  has  an  effect  on  plants.  Some  plants,  like 
the  radish,  spinach,  and  clover,  require  a  long  day  to  produce  flowers 
and  fruit,  while  fall  flowers,  such  as  cosmos,  dahlia,  and  ragweed, 
require  a  short  day  in  order  to  form  flowers  and  fruit.  It  has  been 
shown  experimentally  that  for  each  species  there  appears  to  be  a 
most  favorable  length  of  day  for  flowering,  fruiting,  tuber  formation, 
and  other  food-storing  activities.  This  discovery  is  of  great  value  to 
agriculturists. 


60 


50 


40 


30  V 


20 


10 


Legend 


Diaptomus  Lake  Eaton 

Holopfdium  8<Q  Sirnon  Pond 

Cladocera  LaVe  Madeleine 

Diaptomus 


13  Noon  2 


10 


12 


6  8  10  13  Noon 

.V.  Y.  State  Conscrralion  Drpt. 


Curves  showing  the  variation  in  nmnbers  over  a  period  of  21  hours  of  scvcriil 
species  of  plankton  organisms  from  three  Adirondack  lakes.  W  hiil  factors  might 
be  expected  to  influence  their  dislribiilion? 


14 


NATURAL  HISTORY 


Animals  definitely  respond  by  movement  to  the  stimulus  of  light, 
but  unlike  green  plants,  some  respond  positively  and  others  nega- 
tively. The  unicellular  Ameba  is  killed  by  too  much  light.  Earth- 
worms and  some  other  animals  are  definitely  repelled  by  light.  The 
moth,  on  the  other  hand,  is  attracted  to  light.  Although  of  great 
importance,  light  may  be  injurious  to  some  forms,  for  bacteria  and 
some  animals  are  killed  by  long  exposure  to  it.  The  dangers  from 
certain  wave  lengths  of  light  are  seen  in  a  bad  case  of  sunburn. 

Light  influences  animals  in  other  ways.  Light  stimulus  coming 
through  the  eyes  of  flounder  is  said  to  give  rise  to  changes  in  the  pig- 
ment of  the  skin.  Thus  the  surface  of  the  skin  takes  on  the  general 
color  and  markings  of  its  background.  Some  animals  in  caves  lack 
pigment,  and  there  seems  to  be  a  general  relationship  between  light 
and  pigment  in  the  skin.  There  is  a  day  and  night  rhythm  in  the  lives 
of  many  animals.  Land  snails  feed  at  night,  while  activities  of  most 
birds  are  confined  to  the  daytime.  Bees  go  to  flowers  during  day- 
light. Migrations  of  plankton  are  influenced  by  light,  many  crusta- 
ceans coming  to  the  surface  only  at  night  and  going  deep  down  into 
the  water  during  the  daytime. 


A  dry  alkali  lake. 


Life  is  practically  absent  in  such  areas. 
why  this  is  so.'> 


II  riiilil  I'll  rcc 

Can  you  explain 


THE  STAGE  SETTING 


Fishing  boats  at  the  mouth  of  the  Klamath  River  in  northern  Cahfornia. 
Salmon  run  in  on  the  outgoing  tide  apparently  in  response  to  the  fresh  water 
coming  out  through  the  narrow  mouth  of  the  river. 

Chemical  Factors 

Under  this  heading  are  inckided  all  of  the  chemical  factors  in  the 
environment  of  living  things.  Such  are  soil,  rocks,  and  the  various 
salts  and  chemical  substances  found  in  food  and  water.  Experi- 
mental evidence  shows  that  certain  mineral  substances  are  needed 
for  plant  growth,  and  that  these  minerals  are  found  in  the  composi- 
tion of  living  matter. 

Alkali  soils  form  a  great  problem  of  agriculture.  In  sixteen  west- 
ern states  this  is  the  greatest  problem  outside  of  the  water  supply. 
In  thirteen  irrigated  states  there  is  enough  alkali  present  to  be  harm- 
ful to  crops.  Alkalies  are  chiefly  harmful  because  their  presence 
causes  the  soil  water  to  become  permeated  with  these  salts,  thus 
hindering  absorption  of  water  by  the  plant. 

Acidity  of  the  soil  is  another  problem  for  the  agriculturist.  It  is 
produced  by  a  number  of  factors,  such  as  the  removal  of  calciinn  from 
the  soil,  or  the  production  of  acids  by  certain  bacteria  or  from  decom- 
position. Acid  affects  the  plant  growth  by  checking  the  multii)lica- 
tion  of  useful  bacteria  and  keeps  earthworms  and  other  useful  animalN 
out  of  the  soil.  However,  some  species  of  plants  demand  aciti  soils. 
Mountain  laurel,  rhododendron,  blueberries,  and  cranberries  are 
examples,  as  are  sphagnum  mosses  found  in  certain  bogs. 


16 


NATURAL   HISTORY 


The  distribution  of  fishes  and  other  organisms  in  water  depends 
largely  on  whether  these  waters  are  neutral,  acid,  or  alkaline.  Brook 
trout,  for  example,  are  usually  found  in  acid  and  neutral  waters, 
while  sunfish,  bass,  perch,  and  certain  other  fish  are  typically  asso- 
ciated with  alkaline  waters. 

Carbon  dioxide  in  the  atmosphere  is  another  factor  which  deter- 
mines plant  distribution,  three  parts  to  10,000  being  necessary  if 
plants  are  to  make  starch.  Oxygen  is  essential  for  living  things. 
Certain  so-called  anaerobic  bacteria  and  a  few  animals  appear  to  be 
able  to  live  without  oxygen.  Some  insect  larvae,  worms,  and  molluscs 
live  a  part  of  the  year  in  deep  lakes  where  little  or  no  free  oxygen  is 
present,  due  to  decomposition  of  the  algae.  Certainly  one  factor  in 
the  distribution  of  aquatic  animals  appears  to  be  the  oxygen  content 
of  the  water. 

Gravity  as  a  Factor 

The  pull  w^e  call  gravity  brings  about  differences  in  pressure  both  of 
air  and  of  water.  Plants  and  animals  must  adjust  themselves  to  this 
factor.  In  a  general  way  gravity  determines  the  size  of  organisms. 
Insects  and  birds  which  move  about  swiftly  in  the  air  must  be  small, 
otherwise  gravity  would  bring  them  down.     Gravity  is  important  in 

the  growth  and  orienta- 
tion of  plants.  It  is  a 
stimulus  for  the  direc- 
tion taken  by  the  plant 
body,  apparently  caus- 
ing the  root  to  grow 
downward  and  the  stem 
to  grow  upward,  while 
horizontal  branches  are 
neutral  to  the  pull  of 
gravity.  This  same 
force  acts  upon  sessile  or 
rooted  animals,  such  as 
hydroids  and  sponges. 
Adaptations  to  offset 
the  force  of  gravity  are  seen  in  the  air  spaces  of  floating  plants,  oil 
drops  in  eggs,  spines  and  long  hairs  on  the  surfaces  of  aquatic  plants 
and  animals,  and  the  air  spaces  in  bones  and  other  tissues  of  birds, 
and  in  the  construction  of  feathers. 


Successive  positions,  from  photographs,  showing 
effect  of  gravity  on  a  green  plant  {Impatiens  glandii- 
ligera).  —  After  Pfeffer. 


THE  STAGE   SETT1\(; 


'  2^ 

'•  r^' 

ij^ 

f^ 

1 

.l/M/irs 


Cypress  trees  have  become  adapted  to  live  in  swampy  lands  by  developing 
buttressed  bases  of  the  trunks  and  erect  growths  (knees)  from  the  roots.  Tliese 
enable  the  tree  to  get  sufficient  air. 

Substratum 

Anything  in  which  a  plant  grows  or  on  which  an  animal  comes  to 
rest  is  known  as  substratum.  Types  of  soil  differ  from  cold,  dense, 
clayey  soils,  which  though  they  hold  water  do  not  readily  give  it  up 
to  humus  that  is  well  aerated,  has  a  high  nitrogen  content,  hokls 
water,  and  gives  it  up  readily.  The  distribution  of  plants  depends 
to  a  considerable  extent  on  the  kind  of  soil  found  in  a  given  locality. 
For  example,  mosses  and  ferns  grow  in  moist  soil,  while  cacti  are 
found  in  sandy  desert  soils.  Varying  soil  temperatures  are  brought 
about  by  the  kind  of  soil,  whether  coarse  or  fine ;  by  the  pre.'^ence 
of  a  blanket  of  living  things  over  it  ;  by  its  color  (dark  soils  absorb 
heat  more  readily  than  light-colored  soils) ;  and  by  the  water  it  will 
hold  (wet  soils  are  cooler  than  dry).  Great  variations  occur  in  the 
air  content  of  soils  and  this  again  determines  the  plants  and  animals 
found  in  a  given  area.  Water-soaked  soil,  for  examj^le,  contains 
practically  no  air  and  does  not  ordinarily  have  a  large  jjiant  or 
animal  population.  In  some  cases  a  plant  adapts  itself  to  water- 
soaked  soil,  as  seen  in  th(>  bald  cypress. 


18 


NATURAL  HISTORY 


Animals  also  differ  with  different  types  of  soil.  This  is  particularly 
true  of  the  bottoms  of  lakes  or  streams.  A  different  fauna  is  found 
on  the  rocky  stream  bed  from  the  soft  mud  of  the  pool  below.  Mud 
contains  more  food,  but  it  is  also  more  difficult  for  organisms  living 
in  it  to  carry  on  respiration.  Soil  is  also  the  home  of  such  burrowing 
animals  as  nematode  worms,  earthworms,  ants,  beetles,  digger  wasps, 
and  the  larvae  of  various  insects. 

Molar  Agencies 

Such  are  any  moving  agencies.  Running  water  and  winds  erode ; 
ice  moves  soil  and  rocks.  Tides  cause  great  differences  in  aggrega- 
tions of  plant  and  animal  life,  animals  living  between  tides  having 
different  problems  to  face  from  those  below  the  tidal  flow.  Moving 
air  has  a  definite  effect  on  vegetation,  as  is  often  seen  in  the  wind- 
blown trees  on  mountainsides  or  plains.  Moving  air  acts  upon  seeds, 
tumbleweeds,   spores,  and  fruits,   thus  spreading  plants  over  vast 


American  Museum  uf  Xatural  History 

Tidal  shores,  along  the  New  England  coast,  show  wide  variations  in  habitat. 
The  flora  and  fauna  of  the  intertidal  zone  differs  greatly  from  that  of  the  regions 
above  and  below  the  tidal  flow. 


TUE  STAGE  SETTING 


1«» 


\y ritjlil  /'it  re, 

The  ell'ect  of  differences  in  environiiient  upon  the  same  s[)e(ies  of  tree  (Piiins 
ponderosa).  Here  molar  agencies  are  largely  responsible  for  Ihi;  changed  ai)pear- 
ance  of  the  tree. 

areas,  but  it  also  fells  much  timber,  breaks  off  branches,  and  destroys 
crops.  Winds  may  either  help  or  hinder  in  the  migration  of  insects. 
The  cotton  boll  weevil  travels  north  more  rapidly  in  the  years  when 
more  wind  is  recorded.  Winds  blow  birds  and  insects  out  to  sea, 
thus  destroying  them,  or  they  may  land  them  in  a  new  location  where 
they  may  multiply  rapidly.  Currents  of  air  as  well  as  water  currents 
distribute  plants  and  animals.  Many  animal  forms  react  to  wind 
and  water  currents.  Fish  head  upstream,  an  adaptation  favorable  to 
food-getting.  The  swiftness  of  the  current  not  only  determines  tiic 
distribution  of  fishes,  but  also  of  other  forms,  such  as  caddis  fly 
larvae  and  "water  pennies." 

Biotic  Factors 

These  are  factors  arising  from  the  presence  of  other  li^•ing  organ- 
isms. One  is  concerned  when  studying  ecology  not  only  with  the 
environment  of  living  things  but  also  with  how  li\ing  things  react 
on  others  in  their  immediate  environment.  There  is  competition 
not  only  between  plants  and  animals,  but  al.so  between  plants  of  the 


20  NATURAL   HISTORY 

same  and  of  different  species  for  a  place  under  tlie  sun  ;  literallj'-  under 
the  sun,  for  competition  is  caused  by  the  limited  amount  of  light  that 
will  fall  in  a  given  area.  Young  plants  often  die  because  of  the  shad- 
ing by  the  parent  plant,  and  larger  plants  preempt  areas  of  soil  which 
give  little  or  no  space  for  young  growth.     Feeding  by  animals,  such 


Effect  of  sheep  grazing  upon  trees.     Thousands  of  young  trees  are  destroyed 

every  year  in  this  way. 

as  rabbits  or  sheep,  may  change  the  entire  flora  of  a  region,  while 
parasitic  organisms  injure  a  vast  number  of  the  hosts  on  which  they 
live.  There  are  also  marked  cases  of  partnership  between  organisms, 
bacteria  in  the  soil  giving  and  taking  from  both  plants  and  animals, 
and  helping  to  create  a  cycle  of  food  substances  which  pass  through 
the  bodies  of  both  animals  and  plants.  The  feeding  of  animals  is  their 
biggest  business  in  life,  and  the  presence  of  a  food  supply  determines 
very  largely  the  presence  of  animals  in  a  given  locality.  It  is  said 
that  the  oak  tree  serves  as  food  for  over  500  species  of  insects,  the 
apple  for  400,  clover  and  corn,  over  200  each.  Thus  man  with  his 
tilling  of  the  soil,  destruction  of  the  forest,  and  domestication  of 
plants  and  animals  has  changed  the  fauna  and  flora  of  the  land. 

Having  discussed  the  effects  of  the  factors  of  the  environment  on 
living  (jrganisms,  let  us  now  see  how  the  interaction  of  these  factors 
affects  life  in  the  situations  that  living  things  are  forced  to  meet. 
Animals  and  plants  must  be  adapted  to  live  either  in  the  water,  in 
air,  or  on  land.     The  pages  that  follow  show  some  of  these  adaptations. 


THE  STAGE  SETTING 


21 


Life  in  the  Water 

Plants  are  adapted  for  lite  in  water  hy  a  mucli  reduced  root  system, 
by  leaves  which  either  float,  are  ribbonlike,  or  are  finely  divided  with 
air  passages  and  air  spaces.  The  latter  spaces  help  buoy  up  the  plant 
and  also  allow  for  an  accumulation  of  oxyu-en  and  carbon  (hoxidc 
Green  coloring  matter  is  abundant,  such  plants  being  better  fitted 
for  vegetative  propagation  than  reproduction  by  flowers  and  fruits, 
as  is  shown  by  their  numerous  horizontal  and  thickened  stems.  In 
general,  aquatic  plants  are  restricted  to  relatively  shallow  water, 
many  species  being  found  floating  near  the  surface. 

Animals,  usually  locomotor  and  having  definite  adaptations  for 
movement  in  the  water,  have  a  much  wider  ^•ertical  range.  The 
bodies  of  most  fishes  are  more  or  less  streamlined,  and  protected  by 
mucus  which  covers  the  backward-pointing  scales,  their  fins  being 
placed  where  they  offer  the  least  possible  resistance  to  the  medium. 
In  some  animals,  the  limbs  are  transformed  into  flii)pers,  while  in 
lower  types,  such  as  protozoa,  threads  of  living  matter,  cilia,  are 
used  as  whiplike  organs  of  locomotion.  Since  the  oxygen  content 
of  water  is  only  about  1  per  cent  as  against  over  20  per  cent  in  air, 
we    find    special    adaptations 


for  taking  in  oxygen.  These 
structures  are  usually  in  the 
form  of  gills,  delicate  struc- 
tures which  will  be  discussed 
more  fully  later. 

The  water  forms  an  ideal 
medium  for  vast  numbers  of 
small,  free-swimming,  or  float- 
ing organisms,  the  plankton. 
Oceans  and  lakes  swarm  with 
them.  Every  small  pool  has 
its  plankton,  and  even  rapidly 
flowing  waters  will  disclose 
some  of  these  tiny  organisms. 
In  certain  tested  regions  in 
the  Atlantic,  plants  form  about  56  per  cent  and  animals  44  jx-r  cent 
of  the  total  plankton.  The  flora  consists  mostly  of  diatoms,  bac- 
teria, and  many  forms  of  algae,  while  the  fauna  includes  numerous 
dinoflagellates  and  other  one-celled  animals,  eggs  of  fish,  molluscs, 


Diatoms  have  various  forms  aFid  may  !><• 
colonial  as  well  as  unicellular.  riicrc  arc 
probably  I. "),()()(»  species  known. 


22 


NATURAL   HISTORY 


numerous  crustaceans  mostly  copepods,  jellyfish,  and  the  larvae  of 
many  crustaceans,  molluscs,  and  fish.  Some  of  the  plankton,  such 
as  small  crustaceans,  tunicates,  medusae,  small  fishes,  and  larger 
algae,  may  be  visible  to  the  naked  eye,  but  most  of  it  is  microscopic. 


LAKE 

PLACID 

UPPER 
SARANAC 

MIDDLE 
SARANAC 

LOWER 
SARANAC 

deptm\ 

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LJ 

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CO 
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a 
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Ui 

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CO 
Q- 

BLUE  GREEN 
ALGAE 

1  M 

i_ 

_ 

■ 

■ 

BOTTOM 

^_ 

GREEN  ALGAE  : 

1    M 

„ 

■ 

l_ 

BOT  1  OM 

^^ 

^ 

DIATOMS 

1   M 

■ 

■ 

^ 

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■ 

^ 

BOTTOM 

J_ 

Mi 

iV.  y.  Siaie  Conseruation  Dept. 

Comparison  of  the  distribution  of  nannoplankton  (minute  forms  that  will 
pass  through  the  meshes  of  a  plankton  net)  at  the  surface  and  bottom  of  four 
Adirondack  lakes. 


The  larger  pelagic  organisms  mostly  found  in  the  ocean,  such  as  fish, 
squid,  whales,  turtles,  and  seals,  are  called  collectively  nekton. 

Currents,  wind  action,  the  shapes  of  bays  and  coasts,  migrations 
of  various  animals,  all  cause  differences  in  the  horizontal  distribution 
of  plankton.  Sometimes  given  forms,  as  Cladocera,  will  multiply  very 
rapidly,  even  coloring  the  water  in  a  large  area.  The  vertical  dis- 
tribution is  much  more  regular  with  reference  to  plants,  since  algae 
and  other  green  plants  depend  upon  sunlight.  Plants  get  very  little 
light  at  a  depth  of  100  meters.  At  75  meters'  depth,  only  half  as  many 
plants  are  found  as  at  50  meters,  and  careful  investigation  in  various 
areas  shows  that  most  of  the  plant  plankton  lives  within  a  few  feet 
of  the  surface.  On  the  other  hand,  animals  exist  at  great  depths. 
Beebe  reports  jellyfish,  shrimps,  and  other  plankton  at  a  depth  of 
over  1000  feet  and  the  tunicate,  Salpa,  as  well  as  fishes,  at  his  greatest 
depth  of  3028  feet.  Dredgings  from  the  "Challenger"  and  other 
expeditions  reveal  many  living  organisms,  particularly  protozoans, 
in  the  abysmal  depths. 


Till':  sr\(;|.:  si:rnN(;  o., 

Towins  with  a  phnikton  i.cl  (a  llun-mcshcd  uv\  cf  l,oltii,K  ,-l„tl,) 
near  the  surlacc  of  tlu>  ocean  on  an  early  summer  day  would  yield 
a  very  different  distribution  of  organisms  from  those  collected  "on  ;, 
fall  or  winter  day.  There  is  a  seasonal  variation  in  distrihutioii. 
The  eggs  and  larvae  of  animals  ar(>  abundant  in  the  spring  and  early 
summer,  while  great  numbers  of  algae  appear  then  which  are  Hot 
found  later.  This  rhythm  of  plant  life  is  believed  to  be  correlated 
with  a  turn-over  of  the  available  phosphates  and  nitrates  in  the  water. 
In  the  winter,  the  (^ooler  top  layer  of  water  sinks  and  pushes  up  the 
water  rich  in  the  salts  necessary  for  plant  growth  from  underneath, 
so  that  with  the  coming  of  warmer  weather  tlu^  life  cycle  goes  on  and 
a  seasonal  rhythm  of  algae  appears.  This  turnover  of  plant  and 
animal  life  is  very  great.  The  fishing  industry  on  the  Grand  Banks 
and  in  the  North  Sea  is  largely  due  to  the  occurrence  of  this  great 
seasonal  rhythm  of  plankton. 

There  is  also  a  considerable  variation  in  the  numbers  of  plankton 
near  the  surface  of  the  water  during  the  day  and  night.  Many  crus- 
taceans, for  example,  come  to  the  surface  at  night  and  go  down  in 
the  daytime,  while  green  algae  are  usually  nearer  the  surface  during 
the  day. 

In  oceans  and  lakes,  there  is  a  more  or  less  distinct  zoning  of  living 
forms,  depending  on  the  depth  of  water,  the  type  of  shore,  or  the  kind 
of  bottom.  A  very  different  fauna  and  flora  exist  on  a  rocky  coast 
from  that  along  a  sandy  beach.  The  forms  of  botii  plants  and  ani- 
mals are  different  in  salt  and  fresh  water  areas. 

Life  in  the  Air 

Here  life  is  more  circumscribed.  There  are  no  true  air  plants  unless 
they  be  the  so-called  epiphytes  of  the  tropical  rain  forest,  some  algae, 
such  as  the  Pleurococcus  found  on  the  bark  of  trees,  or  the  lichens, 
which  encrust  rocks  and  tree  trunks.  The  reproducti\'e  bodies  of 
plants,  such  as  spores,  seeds,  and  fruits,  are  furnished  with  adai)ta- 
tions  which  enable  them  to  pass  long  distances  through  the  air,  thus 
allowing  new  areas  to  be  populated.  In  animals  where  locomotion 
is  possible  various  special  adaptations  exist.  Flying  animals  hiivv 
their  wings  placed  wdiere  they  will  not  onlv  cause  the  liody  to  mo\e 
forward,  but  also  assist  in  balancing  it.  Instead  of  one  ])ropeller 
placed  astern,  as  in  fish,  flying  animals  have  two  paired  i)roi)ellers 
placed  forward  at  a  greater  breadth  of  beam.  The  body  is  not  onl.\- 
streamlined,  but  in  higher  forms  special  adaptations  exist  for  protec- 
H.  w.  H.  —  3 


21. 


NATURAL   HISTORY 


Epiphytes  in  a  semitropical  forest.     Note  the  aerial  roots  for  securiiif?  moisture 

from  the  air. 


tion  against  low  temperatures  and  moisture.  Oiled  skin  and  feathers 
of  birds  are  examples.  Bones  are  hollow  and  large  air  spaces  are 
found  between  muscles.  In  insects  a  special  aerating  system  exists, 
since  in  these  heavier-than-air  machines  a  very  rapid  oxidation  of 
fuel  material  must  take  place  if  the  organism  is  to  be  efficient  in 
the  medium. 

Life  on  the  Land 

Adaptations  in  plants  for  life  on  the  land  are  seen  in  the  widely 
branching  root  systems,  the  woody  stiffened  stems,  the  leaves  placed 
in  positions  where  light  may  reach  them,  and  in  the  various  adaptive 
movements  which  enable  green  plants  to  get  a  share  of  the  much 
needed  light.  In  tropical  rain  forests,  this  relation  to  light  is  seen  in 
a  vertical  zoning  where  sun  plants  form  long  twining  stems,  making 
their  way  up  the  tall  trimks  of  trees  to  an  upper  zone  where  light  is 
available,  while  in  the  lower  areas  are  found  shade-loving  plants  which 
prefer  less  sunlight.  In  animals,  where  movement  is  much  more 
evident,  there  are  special  adaptations  in  the  form  of  legs,  which 
support  the  body  off  the  ground  and  allow  of  various  types  of  loco- 
motion such  as  climbing,  crawling,  walking,  running,  and  leaping. 


THE  STAGE  SETTLNG 

Various  other  types  of  movement  are  found  as,  for  example,  tlir 
waves  of  muscular  contraction  in  the  foot  of  the  slug ;  the  crawling 
of  earthworms  where  tiny  setae  are  used  as  levers;  the  erawlinfr 
of  the  snake  with  its  definite  use  of  scales  as  "ground  grippers"  ;  the 
adaptations  for  leaping  in  the  grasshopper  and  the  frog;  adaptations 
for  climbing,  such  as  the  sucking  disks  on  tiie  toes  of  tree  frogs  {Ilyln) 
and  of  some  lizards,  or  the  arrangement  of  the  toes  in  climbing  birds. 
These  and  scores  of  other  adaptations  for  obtaining  food,  for  brcatii- 
ing,  and  for  protection  may  be  recalled. 

SUGGESTIONS   FOR   FURTHER   READLXG 

Borradaile,  L.  A.,  The  Animal  and  Its  Environment,  Oxford  University  Press, 

London,  1923. 

A  general  book  on  the  natural  history  of  animals. 
Elton,  C.,  Animal  Ecology,  The  IVIacmillan  Co.,  1927.     Chs.  I,  II,  III,  I\',  \'. 

A  fascinating  book,  written  in  a  charming  style.    Accurate  and  authentic. 
Jordan,  D.  S.,  and  Kellogg,  V.  L.,  Animal  Life,  D.  Appleton  tV:  Co.,  1900. 

Contains  some  valuable  chapters  fundamental  to  an  understanding  of 

ecology. 
Needham,  J.  C,  and  Lloyd,  J.  T.,  The  Life  of  Inland  Waters,  Charles  C. 

Thomas,  1930.     Chs.  Ill  and  V. 

Interesting  aquatic  natural  history. 
Pearse,  A.  S.,  Animal  Ecology,  McGraw-Hill  Book  Co.,  192G.     Chs.  II  and  III. 

Rather  technical. 
Shelford,  V.  E.,  Animcil  Communities  in  Temperate  America,  University  of 

Chicago  Press,  1913. 

A  pioneer  work,  but  still  reliable  and  usable. 
Shelford,   V.  E.,   Laboratory  and  Field  Ecology,  The  Williams  &   Wilkins 

Co.,  1929. 

Very  usable  for  field  work. 
Weaver,  J.  E.,  and  Clements,  F.  E.,  Plant  Ecology,  McGraw-Hill  Book  Co., 

1929.     Chs.  IX,  X,  XI,  XII,  XIII,  XV. 

Authentic  and  well  written.     It  should  be  of  great  value  in  the  field. 


II 


THE   BIOLOGICAL   CONQUEST   OF  THE   WORLD 

Preview.  A  comparison  of  two  forests  •  The  why  of  distribution ; 
barriers ;  successions  and  their  causes ;  overpopulation  and  its  results  • 
The  shifting  world  of  organisms  •  Ways  of  locomotion  •  Adaptability  to 
new  conditions  •  Human  interference  •  Life  zones  •  Life  Realms  •  Sug- 
gested readings. 


PREVIEW 

The  science  of  Ecology,  or  the  distribution  of  animals  and  plants  in 
a  given  habitat,  was  considered  in  the  preceding  section.  Chorology 
attempts  to  determine  the  laws  governing  the  distribution  of  animals 
and  plants  over  the  surface  of  the  earth. 

So  long  as  man  accepted  the  naive  assum]3tion  that  the  earth  was 
originally  populated  by  means  of  isolated  creative  acts,  there  was 
no  point  in  attempting  to  explain  the  distribution  of  living  things. 
They  had  all  been  put  arbitrarily  in  the  places  where  they  occurred, 
and  that  was  all  there  was  to  it.  With  the  rise  of  the  belief  which 
culminated  in  Darwin's  famous  theory,  that  dissimilar  species  have 
arisen  by  modification  from  other  species,  and  that  all  organisms  are 
related,  more  or  less  distantly,  to  one  another,  the  interpretation  of 
plant  and  animal  distribution  became  a  very  interesting  and  challeng- 
ing field  for  study. 

How  about  the  varied  populations  of  living  things  in  arctic,  tem- 
perate, and  torrid  climates ;  the  absence  of  animals  and  plants  from 
areas  quite  suited  to  their  existence?  Why  is  it  that  tapirs  are 
found  only  in  South  America  and  the  East  Indies,  while  certain 
fishes,  such  as  the  pickerel,  occur  only  in  North  America  and  north- 
em  Europe  ?  Equally  difficult  aspects  of  distribution  cropped  out, 
notably  in  the  Australian  fauna  and  flora,  which  differ  so  greatly 
from  that  of  the  rest  of  the  world,  while  most  perplexing  of  all, 
probably,  the  habit  of  migration  that  makes  certain  animals,  such 
as  birds,  seals,  salmon,  and  eels,  change  residence  regularly  from  one 
region  to  another.  A  gradual  suspici(jn  that  two  environments  quite 
similar  in  general  appearance  might  nevertheless  be  populated  by 
species  of  plants  and  animals  different  from  each  other  gave  the  clue 

26 


TfTE   BIOLOGICAL   CONQUEST   OF   Till:    Would  27 

to  a  scientific  differoiitiution  of  specific  distribution,  AVWor///,  Irom 
.aionoral  distribution,  Choroloc/!/. 

A  Comparison  of  Two  Forests 

Two  writers,  Victor  E.  Shclford,  the  well-known  ecologist.  and 
William  Beebe,  ornithologist  and  naturalist,  have  given  two  widely 
different  pictures,  one,  an  accurate  description  of  a  hard-wood  forest 
in  Illinois,  and  the  other,  a  survey  of  life  in  a  British  Guiana  jungle 
forest. 

A  typical  beech-maple  forest,  such  as  Dr.  Shelford  describes,  can 
be  found  anywhere  in  the  vicinity  of  Chicago.  A.ssociated  with  the 
two  dominant  trees  are  ash,  elm,  walnut,  linden,  and  a  wealth  of 
smaller  trees  and  shrubs  forming  a  lower  layer  under  the  higher  trees. 
Wild  cherry,  sassafras,  and  dogwood  are  abundant,  and  in  some  of 
the  more  northern  forests,  azalea  and  rhododendron  form  an  inter- 
mediate growth.  The  floor  of  the  forest  is  covered  with  herbs  and 
flowering  plants,  large  and  small,  which  change  with  the  season.  In 
spring,  trilliums,  violets,  wild  geraniums,  anemones,  phlox,  and  scores 
of  other  plants  are  in  bloom,  succeeded  in  the  fall  by  asters  and  other 
composites,  in  areas  having  ample  light.  A  relatively  large  number 
of  plants  having  spiny  or  hooked  fruits  occur,  which  aid  in  their 
accidental  distribution  by  wandering  animals.  A  few  large  mammals, 
deer,  fox,  and  hares,  are  found  occa.sionally,  though  are  rarely  seen. 
The  woodchuck  is  perhaps  the  mo.st  numerous  of  the  mammals,  and 
the  red,  gray,  and  fox  squirrels  are  not  uncommon.  Of  birds  the 
crested  flycatcher,  wood  pewee,  blue  jay,  scarlet  tanager,  wood 
thrush,  and  red-eyed  vireo  nest  in  the  lower  trees,  while  the  oven-bird 
conceals  its  curious  architecture  on  the  ground.  The  wood  frog, 
red-backed  salamander,  and  Pickering's  tree  frog  are  found,  although 
not  always  in  evidence,  and  insects  abound,  especially  those  that  live 
on  trees,  such  as  borers  of  various  sorts,  beetles,  millipeds,  spiders, 
and  in.sect  larvae.  Inhabiting  the  lower  layer  of  the  forest  are  snails, 
centipedes,  sowbugs,  and  earthworms.  This  represents,  \\\\]\  \aria- 
tions,  a  typical  association  of  life  in  a  northern  deciduous  forest. 

At  first  sight  the  jungle  forest  does  not  appear  to  be  very  difTerent 
from  the  northern  forest.  Both  contain  large  and  small  trees,  the 
larger  ones  in  the  jungle,  such  as  mora  and  greatheart.  towering  to  a 
height  of  two  hundred  feet  or  more,  but  here  the  likeness  stops. 
There  is  an  almost  complete  absence  of  large  horizontal  branches  in 
the  tropical  forest,  the  trunks  of  trees  shooting  straight  up  for  si.xty 


■m 


NATURAL  HISTORY 


}\'iUiam  Beebe  American  Museum  of  Xatural  History 

CouTtestj  U.  S.  Forest  Serrice 

A  comparison  of  two  widely  separated  forests.  The  right-hand  photograph  is  a 
typical  northern  mesophyte  beech-maple  association,  the  left-hand  photograph  a 
tropical  rain  forest  of  British  Guiana.  Note  the  superficial  likenesses  and  dif- 
ferences. 

or  seventy  feet  without  a  branch,  festooned  with  long  cHmbing  hanas, 
which  in  this  way  work  from  the  forest  floor  into  the  upper  zones. 
Four  general  horizontal  regions,  or  zones  of  life,  are  distinguishable, 
namely,  the  forest  floor,  the  lower  jungle  up  to  about  twenty  feet,  the 
mid-jungle  up  to  seventy  feet,  and  the  tree-tops,  towering  a  hundred 
and  fifty  or  two  himdred  feet  high.  Life  at  first  seems  almost  absent 
in  the  jungle  to  the  casual  observer,  but  if  one  stops,  and  simply 
looks,  the  jungle  wakes  up  and  life  appears  everyu^here.  The  forest 
floor  is  covered  with  the  accumulated  debris  of  ages,  fallen  trees  in 
different  stages  of  decay,  fungi,  mosses,  and  lichens,  with  a  generous 
covering  of  brown  leaves,  for  here  the  leaves  fall  all  the  year  around, 
instead  of  only  in  the  autumn  season  as  in  northern  regions.  The 
ground  area  is  occupied  by  occasional  deer,  paca,  and  tapirs,  with 
agoutis  and  armadillos  found  more  frequently.  Partridge  and  the 
strange  tropical  tinamou  are  seen  here  and  there,  as  well  as  jungle 
mice  and  rats,  salamanders,  frogs,  a  few  snakes,  innumerable  scorpions, 
beetles,  grubs,  worms,  and  rarely,  the  unique  and  interesting  Peripatus. 
In  the  low  jungle  are  found  manikins  of  several  species,  ant-birds, 
with  trumpeters  and  jungle-wrens,  while  at  night  opossums  climb 


THE   BiULUCilCAL   CONQUEST   OF   Till-:    Would  u) 

about  through  the  underbrush.  During  the  daytime  tiie  wonderful 
morphous  butterflies,  brilhant  spots  of  blue,  add  a  touch  of  col.,!-  to 
the  picture. 

The  mid-jungle  contains  the  most  life.  Here  iimumeraljlc  birds, 
curassows,  guans,  pigeons,  barbets,  jacamars,  trogons,  and  smaller 
feathered  species  abound,  in  company  witli  ant-eaters,  sloths,  squir- 
rels, bats,  coatis,  and  small  monkeys  such  as  marmosets. 

The  upper  jungle  of  the  tree-tops  is  the  mo.st  difficult  region  to 
know.  Red  howlers  and  be.som  monkeys  move  about  in  the  tree- 
tops,  and  occasional  glimpses  may  be  had  of  toucans,  macaws,  and 
great  flocks  of  parakeets  and  parrots  that  live  ther(\  Fierce  ants 
prevent  tree-climbing,  and  the  relatively  great  height  and  mass  of 
foliage  make  living  things  not  easily  acce.ssible  to  observers  in  this 
upper  layer  of  the  tropical  rain  forest. 

These  two  forests,  the  northern  maple-birch  and  the  jungle,  by  their 
entirely  dissimilar  populations  illustrate  contrasts  that  might  be 
found  in  many  parts  of  the  world.  Sometimes  conditions  in  widely 
separated  areas  may  be  almost  similar,  with  diverse  populations 
inhabiting  them,  and  again,  localities  close  at  hand  may  show  remark- 
able diversities  in  their  living  inhabitants.  When  regions  far  apart 
have  similar  populations,  which  does  not  commonly  happen,  the 
biologist  is  faced  by  a  puzzling  problem. 

The  Why  of  Distribution 

Jordan  and  Kellogg  ^  give  three  laws  to  account  for  the  distril)u- 
tion  of  organisms  which  they  state  as  follows  :  E\ery  species  is  found 
everywhere  that  conditions  are  suitable  for  it  unless  (I)  it  was  unable 
to  reach  there  in  the  first  place,  or  (2)  having  reached  there  it  was 
unable  to  stay  because  it  could  not  adapt  itself  to  the  new  condi- 
tions, or  (3)  having  entered  the  new^  environment  it  became  modified 
into  another  species.  It  is  not  only  the  normal  habitat  that  deter- 
mines the  presence  of  a  given  plant  or  animal,  but  its  accessibility 
from  the  place  of  origin. 

Although  every  species  originated  historicaUy  from  some  i)receding 
species  at  some  definite  place,  its  present  distribution  results  from 
the  working  of  two  opposing  factors,  expansion  and  repression.  The 
factors  of  expansion  will  be  mentioned  later,  lliose  of  n^pression 
are,  first,  inadequate  means  of  dispersal  because  slow-moving  animals 


1. Jordan,  D.  S..  and  KellofiK.  V.  L.,  Animal  Life.     Appletou,  190(). 


30 


NATURAL   HISTORY 


are  necessarily  limited  in  their  distribution.  A  second  means  of 
repression  lies  in  the  poor  adaptability  of  organisms  to  new  localities 
which  they  have  invaded.  A  round  peg  will  not  fit  in  a  square  hole, 
nor  a  square  peg  in  a  round  hole,  but  if  the  peg  consists  of  a  plastic 
material  it  will  adapt  itself.  The  normal  habitat  for  a  species  is  the 
place  where  the  organism  is  most  nearly  in  physiological  equilibrium, 
the  geographic  range  being  determined  by  the  fluctuation  of  a  factor, 
or  factors,  which  are  necessary  for  the  life  of  a  species. 

Barriers 

Each  species  widens  its  range  of  distribution  as  far  as  possible  and 
tries  to  overcome  obstacles  which  nature  has  put  in  its  way.  These 
obstacles  may  be  chemico-physical,  geographical,  or  biological  bar- 
riers. 

In  general  chemico-physical  barriers  are  climatic  in  nature,  such 
as  unfavorable  conditions  of  moisture,  soil,  or  temperature.  Soil 
deficiencies,  salinity,  the  presence  or  absence  of  light,  or  character 
of  the  surrounding  medium  might  also  be  mentioned.     These  climatic 


Friislur 


Why  might  such  a  mountain  barrier  restrict  the  distribution  of  certain  plants 

and  animals  .3 


THE  BIOLOGICAL  CONQUEST   OK   TIIL    WolU.O 


:U 


barriers  may  be  in  vertical  zones,  extending  from  tlie  ocean  level  to 
mountain  tops,  as  well  as  horizontal,  spreading  out  north  and  south 
from  the  equator  in  zones  of  latitude. 


Map  showing  ancient  and  modern  ranges  of  the  elephants  and  their  ancestors. 
The  shaded  area  shows  the  former  habitat  of  the  maniniolh  and  mastodon, 
ant^estor  of  the  modern  elephant.  A  land  connection  probal)Iy  existed  I)etween 
Asia  and  North  America.  Note  the  restricted  range  of  the  present-day  elephants 
indicated  by  heavy  shading.     How  can  this  be  accounted  for.^ 

Sometimes  natural  barriers  occur,  such  as  high  mountain  ranges 
with  eternal  snow,  deserts  with  unfavorable  conditions  of  moisture, 
or  in  the  case  of  water-distributed  animals  such  as  fishes,  high  water- 
falls may  prevent  them  from  moving  up  a  stream  beyond  a  cciiain 
point.  The  barrier  for  one  organism,  however,  might  l)e  a  highway 
for  another.  A  desert  would  be  an  impassable  barrier-  to  a  squirn^l 
but  not  to  a  camel. 

Geographical  barriers  have  not  always  been  fixed.  Geological 
history  reveals  the  fact  that  some  land  surfaces  were  once  occujiied  by 
water  and  what  is  now  water  may  have  been  land.  The  presence  of 
fo.ssil  sea.shells  in  the  Panama  Canal  area  indicates  that  the  Isthmus 
was  formerly  submerged,  and  there  is  evidence  that  as  late  as  Eocene 
times  there  was  a  land  connection  acro.ss  Bering  Straits.  As  bar- 
riers have  changed  so  has  the  resulting  distribution  of  organisms. 
Distribution  often  indicates  the  geography  of  the  i)ast.  .Mnnbers 
of  the  same  genus  may  differ  widely  in  certain  isolated  localities,  as, 
for  example,  the  tapirs  found  in  tropical  America  and  the  Malay 


32  NATURAL   HISTORY 

Peninsula  with  its  adjacent  islands.  In  early  geological  times  mem- 
bers of  this  genus  were  widespread  and  abundant,  whereas  now,  due 
to  the  disappearance  of  former  land  connections,  there  are  but  two 
widely  isolated  species  in  existence. 

The  distribution  of  animals  is  bound  up  in  their  food  supply. 
Hence  carnivorous  animals  are  restricted  to  areas  wiiere  the  animals 
on  which  they  prey  live.  Often  a  biological  barrier  is  created  by  the 
presence  of  animals  which  are  parasitic  on  a  given  form.  The  tsetse 
fly,  Glossina,  which  frequents  the  river  bottoms  and  shores  of  lakes 
in  certain  parts  of  Africa,  prevents  the  ranging  of  other  than  native 
cattle  in  these  areas  because  of  the  fact  that  they  transmit  a  blood 
parasite  fatal  to  such  animals.  Man  himself  is  most  active  in  both 
creating  and  breaking  down  barriers.  He  introduces  new  animals 
and  plants  either  purposely  or  by  chance  into  areas  where  they  thrive 
and  replace  other  species,  or  by  building  dams,  irrigating,  deforesta- 
tion, or  accidentally  burning  over  areas,  he  destroys  one  kind  of  life 
perhaps  never  to  replace  it  with  another. 

Successions  and  Their  Causes 

Succession  means  that  in  a  given  area  organisms  succeed  one  an- 
other because  of  changes  in  the  environment,  migration  taking  place 
so  that  they  may  reach  conditions  favorable  to  their  development. 
An  example  of  plant  succession  may  be  seen  in  almost  any  pond  that 
is  gradually  drying  up.  In  deep  water  there  are  a  few  submerged 
aquatic  plants ;  in  water  from  6  to  8  feet  deep  floating  plants  such  as 
pond  lilies  are  found ;  in  shallow  water  from  1  to  4  feet  deep,  cat- 
tails and  reeds  are  abundant ;  while  at  the  edge  we  find  a  meadow  of 
sedges  and  some  bushy  plants.  As  the  pond  becomes  drier,  these 
plants  slowly  push  outward  until  eventually  it  may  be  completely 
filled  with  plants  which  build  up  soil,  making  first  a  swamp  and 
eventually  a  meadow,  while  around  the  edge  of  the  former  pond  will 
now  be  a  forest  of  trees  and  bushes.  In  the  tropical  oceans  different 
corals  succeed  each  other,  growing  on  the  skeletons  of  other  species, 
thus  building  their  way  into  shallow  and  warmer  water,  or  along  the 
ocean  shore  colonial  diatoms  may  occur,  to  be  followed  by  hydroids  and 
seaweeds,  the  latter  becoming  a  dominant  climax  formation,  a  group  of 
species  that  are  better  fitted  to  survive  in  that  habitat  than  any  others. 

Erosion,  which  carries  away  the  original  inhabitants,  or  a  deposit 
of  new  soil  by  running  water,  wind,  or  other  agencies,  gives  oppor- 
tunity for  the  establishment  of  new  life  in  a  region  thus  devastated. 


THE   BI()LO(;iC,\L   CONQUEST   ()\-     nil]    \\(,|u,|) 


M\ 


The  question  of  how  long  seeds  will  survive,  uiidci-  whal  condilions 
they  will  germinate,  and  how  fast  they  will  grow  is  of  g,vat  inipor- 
tance  in  the  repopulation  of  areas  after  soil  erosion  oi-  fire.  Beale 
reports  an  experiment  where 
ten  out  of  twenty-two  species 
of  seeds  sprouted  after  hav- 
ing been  buried  in  open  bot- 
tles in  moist  sand  at  a  depth 
of  three  feet  for  over  forty 
years.  After  a  coniferous  for- 
est has  been  devastated  by 
fire,  an  entirely  new  series  of 
plants  spring  up  in  the  area  ; 
first  herbs,  such  as  fireweed 
or  wild  mustard ;  then  trees 
or  bushes,  the  seeds  of  which 
may  be  brought  by  birds,  as 
raspberry,  blackberry,  or  wild 
cherry ;  later  a  stage  of  trees 
having  wind-blown  or  bird- 
carried  seeds,  such  as  aspen, 
cottonwoods,  or  birches.  Still 
later  the  forest  may  become 
repeopled  by  its  original  in- 
habitants, which  becomes  the 
climax. 

Conditions  of  wind,  mois- 
ture, sunlight,  and  weather,  the  sum  total  of  which  constitutes  climate, 
play  a  most  important  part  in  succession.  If  drought  destroys  life  in 
a  given  region,  an  entirely  new  group  of  plants  may  come  to  occujiy 
that  area,  bringing  with  them  a  new  group  of  animals.  Migrations 
of  animals  may  be  brought  about  by  changing  seasons. 

The  biotic  conditions  governing  successions  are  many.  Man, 
through  clearing  forests,  throwing  wastes  into  ri\ers,  or  introducing 
new  plants  or  animals  which  may  compete  with  existing  species,  often 
completely  upsets  the  balance  of  life  and  causes  succe.'^sioiis.  Indus- 
trial pollution  may  completely  depopulate  streams  of  fish  life,  bac- 
terial growth  replacing  the  original  plants  and  animals.  Sometimes 
new  organisms  add  so  many  competing  mouths  to  feed  in  a  gix'cn  terri- 
tory that  it  becomes  necessary  for  some  to  break  away  if  any  are  to  li\('. 


Wriijlu  I'itrct 

A  lypiciil  undt'CKruwtti  succession  after  a 
I'orcsl  (ire. 


34  NATURAL   HISTORY 

Overpopulation  and  Its  Results 

One  of  the  I'aclors  in  dcteniiiiiiug  tlie  .spread  and  distribution  of 
organisms  is  overpopulation.  An  annual  plant,  for  example,  pro- 
ducing only  two  seeds  a  year,  which  is  far  below  the  actual  number, 
and  always  developing  these  into  mature  plants,  in  only  twenty-one 
years  would  have  1,048,576  descendants.  A  pair  of  common  house- 
flies  which  usually  produces  eggs  six  times  a  year,  each  batch  con- 
taining 150  to  200  eggs,  with  the  young  flies  beginning  in  turn  to 
lay  eggs  in  about  fourteen  days  after  hatching  and  repeating  the  life 
cycle,  might,  it  is  calculated,  beginning  to  breed  in  April,  if  all  the 
eggs  were  hatched  and  no  individuals  died,  give  rise  to  191,010,000,- 
000,000,000,000  descendants  by  the  end  of  August.  However,  each 
species,  year  in  and  year  out,  tends  to  remain  about  stationary  in 
number.  Indeed,  many  species  are  actually  disappearing.  The 
reasons  for  this  check  of  potential  populations  are  found  in  lack  of 
adequate  food  supply,  lack  of  favorable  breeding  conditions,  and  in 
the  fact  that  many  animals  and  plants  become  food  for  others. 

The  Shifting  World  of  Organisms 

There  is  no  doubt  that  desire  for  food  furnishes  the  greatest  urge 
to  locomotion  and  exploration  in  animals.  Dr.  Crothers  once  said 
in  one  of  his  essays  that  the  "haps  and  mis-haps  of  the  hungry  make 
up  natural  history."  Indirectly  there  is  the  same  necessity  for  food 
on  the  part  of  plants,  but  here  the  urge  is  expressed  not  so  much  in 
locomotion  as  in  a  struggle  for  position  with  reference  to  light,  which 
is  essential  to  every  green  plant  in  the  manufacture  of  its  own  food. 

Changing  environmental  conditions  may  force  the  movements  of 
organisms  and  produce  faunal  and  floral  repopulations.  For  example, 
it  is  known  that  drifting  coconuts  frequently  float  long  distances  and 
grow  into  trees  upon  some  distant  shore.  A  recent  cataclysm  of 
nature  has  given  us  an  opportunity  to  see  the  repopulation  of  a 
devastated  area  taking  place.  In  1883,  the  volcanic  island  of  Kra- 
katao  was  literally  blown  to  pieces  by  a  series  of  terrific  explosions 
that  destroyed  every  living  thing  on  the  island.  Less  than  three 
years  after  the  volcano  became  quiescent,  a  Dutch  botanist  visiting 
the  island  found  the  ash  which  covered  its  surface  completely  car- 
peted with  a  layer  of  bacteria,  diatoms,  and  primitive  blue-green  algae. 
Here  and  there  ferns  were  found,  along  with  several  kinds  of  mosses. 
There  were  even  a  few  flowering  plants,  but  no  trees  or  shrubs.     In 


THE  BIOLOGICAL  CONQUEST   OF  THE   Would  r, 

that  short  time,  the  naked  land  had  been  partially  ropoijulatcd  with 
these  several  low  forms  of  life,  by  spores  or  seeds  blown  througli  tho 
air,  or  floated  in  water  from  the  nearest  islands,  wliich  wore  aboiil 
fifteen  miles  away.  Twenty-three  years  after  the  explosion,  I'rofessor 
Ernst  visited  Krakatao  and  reijorted  a  forest  of  cocoinit  pahns  and 
figs  growing  near  the  shore  line,  a  luxuriant  jungle  in  the  interior,  and 
considerable  animal  life,  represented  by  species  that  could  either  fly 
or  drift  to  the  island  on  floating  wood.  Ernst  estimated  that  within 
another  fifty  years  this  island  would  differ  in  no  respect  from  its 
neighbors,  a  prediction,  however,  wliich  seems  doomed  to  failure  of 
confirmation  because  the  volcano  has  again  gone  on  a  rampage. 

Variations  in  temperature,  brought  about  by  the  changing  seasons, 
are  a  factor  in  the  movements  of  animals.  This  is  particularly  true 
in  the  case  of  the  annual  migrations  of  such  animals  as  crabs,  lobsters, 
and  squid,  which  go  into  deep  water  in  winter,  returning  to  shallow 
shore-water  in  spring.  Movements  apparently  dependent  to  some 
extent  upon  temperature  occur  in  the  case  of  many  marine  fishes,  and 
birds,  certain  butterflies,  and  bats,  that  go  north  and  south  according 
to  the  season.  Many  animals  mo\T  up  and  down  mountain  slojjes 
probably  for  the  same  reason. 

Sometimes  other  factors  than  scarcity  of  food,  em'ironmental 
changes,  or  seasonal  differences  cause  migration.  I^emmings.  for 
instance,  small  rodents  living  in  the  mountainous  districts  of  Scan- 
dinavia, at  intervals  of  from  five  to  twenty  years  suddenly  mo\'e 
forth  in  vast  numbers,  with  no  apparent  Pied  Pi])er  of  Hamelin  to 
lead  them,  but  always  in  the  same  general  direction,  swimming  rivers 
and  lakes,  overcoming  all  sorts  of  obstacles,  and  eventually  ending 
the  mysterious  trek  in  the  ocean.  Although  they  feed  on  the  way 
and  consume  enormous  amounts  of  food  material,  the  search  for  food 
is  not  sufficient  to  explain  their  fatal  pilgrimages. 

The  relation  of  different  degrees  of  salinity  to  th(>  breeding  habits 
of  food-fishes  jjrobably  influences  their  distribution  also,  by  det(>r- 
mining  the  character  of  organisms  in  their  feeding  grounds.  Tetters- 
son  found  that  herring  only  enter  the  Baltic  when  the  .salinity  g<'ts 
to  a  certain  degree,  whereas  Galtsoff  found  that  in  America  tli(>  mi- 
gration of  mackerel  is  due  not  so  much  to  salinity  as  to  temperature. 
Thus,  different  factors  appear  to  influence  different  species  in  deter- 
mining their  movements. 

Birds,  becau.se  of  their  ability  to  fly,  are  better  aide  to  seek  out  :i 
favorable  place  for  abode  than  most  animals.     Many  tlifferent  reasons 


36 


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38 


NATURAL   HISTORY 


have  been  given  to  account  for  the  long-distance  migrations  of  ducks, 
geese,  the  Arctic  tern,  golden  plover,  and  other  remarkable  feathered 
travelers.  Food  cannot  be  the  deciding  factor,  jjecause  many  birds 
leave  for  the  south  while  food  is  still  abundant.  Neither  can  tempera- 
ture be  the  only  cause,  because  a 
majority  of  migrating  birds  go 
south  when  the  weather  is  still 
warm,  while  robins  and  other 
lairds  often  stay  behind  and  win- 
ter successfully  in  cold  climates. 
Humidity,  atmospheric  pressure, 
winds,  have  all  been  considered 
as  playing  a  part  in  migration, 
but  it  is  more  likely  that  some- 
thing within  the  bird  rather  than 
any  external  environmental  factor 
is  the  impelling  cause  for  this 
impressive  phenomenon.  For  in- 
stance, among  the  hormones  pro- 
duced by  the  ductless  glands,  are 
sex  hormones  which  may  stimu- 
late the  bird  to  the  extraordinary 
activity  that  results  in  long  mi- 
gratory flights.  How  to  account 
for  the  direction  and  exactness  of 
these  migratory  flights  is  another 
matter,  even  more  difficult  to 
explain. 

Changing  climatic  conditions 
probably  influence  plants  more 
directly  than  animals,  because  the 
latter  are  more  capable  of  move- 
ment, and,  consequently,  better  able  to  escape  from  unfavorable 
surroundings.  Nevertheless,  living  things  make  up  a  world  of 
shifting  organisms,  always  on  the  move. 

Ways  of  Locomotion 

Much  of  the  delight  that  the  naturalist  experiences  comes  from 
observing  and  interpreting  the  ways  and  devices  by  which  the  move- 
ments of  organisms  are  brought  about. 


WINTER    HOME 


The  annual  migration  routes  of  the 
Arctic  tern.  It  covers  about  22,000 
miles  in  its  yearly  round  trip  from  its 
winter  range  in  the  Antarctic  to  the 
summer  breeding  range  in  the  Arctic. 
Note  the  different  routes  taken  going 
and  coming. 


THE   BTOLOGICAI.   CONQUEST   OF  'll||.;   Woiuh 


.'.'» 


II  riijhl  I'll  rci 

The  Russian  thistle   {Salsolu)  introduced   into  this  country  in   1!571.     Today  it 
covers  the  entire  country.     What  adaptations  have  enabled  this  pesi  to  do  tliis? 

In  the  world  of  attached  animals,  like  sea-anemones  and  corals,  that 
apparently  are  doomed  to  remain  in  one  place,  the  free-swimming 
larvae  seize  the  opportunity  to  break  away  from  the  maternal  apron- 
strings  before  settling  down  for  life,  just  as  stationary  plants  by  means 
of  spores,  seeds,  and  chmbing  or  trailing  vegetative  parts  are  enabled 
to  shift  about  and  occupy  new  territory.  Seeds  of  orchids  and  certain 
spores  of  fungi,  mosses,  and  ferns,  for  example,  are  light  as  dust  and 
may  be  wafted  hundreds  of  miles  in  the  air  before  settling  down  to 
germinate  on  some  distant  soil.  Seeds  of  dandelions  and  other  plants, 
such  as  milkweed,  willow,  and  cottonwood,  have  feathery  paracluite- 
like  structures,  which  support  them  in  the  air  for  some  time,  e\-en  in  a 
wind  blowing  only  two  miles  an  hour.  Insects,  ballooning  spiders,  and 
birds  make  use  of  air  currents,  sometimes  being  carried  long  distances, 
particularly  by  heavy  winds.  Whole  plants,  like  the  Russian  lliistl(\ 
and  the  "resurrection  plants"  of  desert  regions,  may  dry  uj)  and 
break  loose  from  their  anchoring  roots,  and  roll  along  the  ground  or 
ride  the  breeze  scattering  their  seeds,  thus  taking  root  in  newly  invatknl 
regions. 

H.  w.  H.  —  4 


40 


NATURAL  HISTORY 


Estimate  ok  Seeds  Produced  by  a  Single  LARtiB  WE>;n 


Dandelion     . 
Cockle-bur    . 
Oxeye  daisy 
Prickly  lettuce 
Beggar's  ticks 
Ragweed 


1,700 

9,700 

9,750 

10.000 

10,500 

23,000 


Crabgrass    . 
Russian  thistle 
Pigweed 
Purslane  (large) 
Tumble  mustard 
Lamb's-quarters 


89,600 

150,000 

305,000 

1,250,000 

1,500,000 

1,600,000 


Some  fruits,  like  those  of  violets  and  the  witch-hazel,  explode,  send- 
ing their  seeds  to  a  distance.  Even  gravity  may  sometimes  be  re- 
sponsible for  spreading  plants  by  means  of  soil-slides,  while  animals 
in  such  accidentally  disturbed  soil  may  be  carried  considerable  dis- 
tances to  a  new  situation. 

Birds  inadvertently  scatter  fruits  and  seeds  by  first  swallowing 
and  then  depositing  them  elsewhere  with  their  droppings.  As  a 
result  cherry  bushes  and  poison-ivy  vines  may  often  be  seen  growing 
along  fences  where  birds  have  roosted. 

Adaptability  to  New  Conditions 

The  fact  that  some  organisms  do  not  invariably  adapt  themselves 
to  new  localities  which  they  have  invaded  is  a  great  deterrent  to 
their  permanent  spread.  Successful  invaders  that  gain  a  new  foothold 
as  pioneers,  and  retain  it  as  settlers,  are  conspicuous  enough  to  be 
discovered  and  remembered,  but  unsuccessful  ones,  reaching  the 
Promised  Land  but  unable  to  establish  themselves  there,  escape  atten- 
tion. Indian  corn,  for  example,  seems  unable  to  reproduce  and  main- 
tain itself  if  allowed  to  run  wild.  The  yellow-fever  mosquito  has  a 
certain  dead-line,  north  of  which  it  cannot  successfully  continue  to  live. 

Just  as  in  economic  life,  so  in  communities  of  plants  and  animals, 
undesirable  individuals  frequently  appear,  bumming  their  way  into 
places  where  they  are  not  wanted.  Weecis  are  notorious  plant- 
hoboes  that  are  pre-eminently  successful  on  their  own  part,  but  are 
unwanted  by  man,  and  reckoned  as  outlaws  with  a  bad  reputation, 
because  they  rob  other  plants  which  man  favors,  of  food,  moisture,  and 
sunlight.  Having  great  natural  vitality,  they  are  successful  because 
they  usually  grow  even  in  unfavorable  conditions  which  would  kill 
competing  plants,  and  produce  enormous  numbers  of  seed.  Their 
persistence  and  varied  means  of  seed  dispersal  are  easily  realized  by 
anyone  who  has  tried  to  pick  "beggar's  ticks,"  and  "sticktights," 
and  burrs  from  his  clothes  after  a  ramble  in  the  autumn  woods. 


THE  BlULOCilCAL   CONQUEST   OF  THE   WOULD  n 

Human  Interference 

Man  is  often  the  unwitting  cau.se  of  sliifts,  .sometimes  with  serious 
results,  of  animal  and  ])lant  ixjpulations.  Tlie  Russian  thistle, 
already  mentioned,  was  introdueed  into  South  Dakota  in  1S74  with 
flax-seed  from  Europe.  By  1888,  it  was  reported  as  a  troublesome 
weed  in  both  the  Dakotas.  By  1898,  it  had  covered  all  the  area  east 
of  the  Rocky  Mountains  from  the  Gulf  of  Saskatchewan,  and  today 
ranges  over  the  whole  country. 

There  are  many  curious  cases  of  the  accidental  transport  by  human 
agency  of  animals  and  plants  to  regions  far  from  their  point  of  origin. 
Recently  a  tropical  boa  landed  in  Middletown.  Connecticut,  with  a 
bunch  of  bananas.  Tropical  tarantulas,  too,  are  known  to  be  carried 
over  long  distances  in  the  shipment  of  this  fruit.  Such  instances  as 
these,  however,  usually  have  no  lasting  effect  on  the  general  spread 
of  organisms,  yet  they  emphasize  the  fact  that  unanticipated  develop- 
ments in  distribution  are  quite  jiossible  from  very  insignificant  and 
unsuspected  beginnings.  Man's  interferences  with  the  distribution 
of  organisms  have  by  no  means  always  been  unfortunate  or  disastrous. 
In  many  instances  his  rearrangements  of  plant  and  animal  popula- 
tions have  been  eminently  successful.  The  planting  of  various 
species  of  trout  in  new  streams  has  proved  to  be  a  wise  move,  \\hile 
the  introduction  of  reindeer  into  Alaska  and  Labrador  is  of  incal- 
culable benefit  to  both  man  and  beast.  The  list  of  cases  where  man 
has  lifted  the  lid  of  Pandora's  box  and  set  free  plants  and  animals 
for  weal  or  woe  into  new  localities  could  be  extended  indefinitely. 

Life  Zones 

Reference  has  already  been  made  to  a  zonal  distribution  of  i)hints 
and  animals  in  a  pond.  A  similar  condition  is  easily  seen  in  climbing 
any  high  mountain.  Life  zones  are  often  rather  sharply  marked,  but 
usually  show  transitional  areas  between  them.  A  region  which  has 
been  carefully  studied  and  which  shows  this  zonal  distribution  in  a 
marked  way  is  the  San  Francisco  mountain  region  in  north  Arizona. 
Here,  a  mountain  nearly  13,000  feet  in  height  rises  out  of  a  desert 
plain.  This  mountain  shows  successively  two  tyj^es  of  desert  zone, 
a  lower  and  upper,  each  with  its  own  desert  fauna  and  flora,  cacti, 
sagebrush,  a  few  birds,  mice,  lizards,  and  snakes.  Then  a  r(>gion  at 
between  6000  and  7000  feet  of  pinon  pines  and  red  cedars,  inhabited 
by  more  birds  and  a  small  number  of  mammals.     Between  7000  and 


42 


NATURAL  HISTORY 


Zonal  distribution  of  flora  on  a  moun- 
tain peak  rising  from  a  desert  area. 
How  would  you  account  for  these  differ- 
ent life  zones  ? 


8200  feet  there  are  forests  of  Douglas  and  balsam  fir,  with  such  mam- 
mals as  meadow  mice,  chipmunks,  deer,  lynx,  and  puma.     Higher  still 
between  8200  and  9500  feet,  is  a  typical  Canadian  vegetation,  timber 

pine,  Douglas  and  balsam  fir, 
and  aspens,  while  the  wood- 
chuck,  porcupine,  rabbit,  mar- 
ten, fox,  wolf,  and  other  northern 
forms  are  found.  From  9500  to 
11,500  feet  we  find  a  fauna  and 
flora  almost  like  that  of  northern 
Canada  and  called  Hudsonian. 
Stunted  spruce  and  pine  exist 
up  to  the  timber  line  with  a 
few  typical  mountain  mammals 
such  as  the  marmot,  and  pika 
or  mountain  hare.  Above  this 
area  lies  the  rocky  Alpine  zone, 
snow-clad  for  half  the  year  even 
in  this  warm,  sunny  climate. 
Lichens  on  the  rocks  and  a  few 
stunted  herbs  are  the  only  plant  life  visible,  while  a  limited  number 
of  insects  and  an  occasional  mammal  from  the  Hudsonian  zone  are 
the  only  signs  of  animal  life. 

The  facts  that  the  chorologist  has  discovered  concerning  life  zones 
have  been  put  to  practical  use  by  the  Biological  Survey  of  the  United 
States  Department  of  Agriculture.  A  life  zone  map  has  been  pre- 
pared so  that  the  settler  going  into  a  new  region  will  know  at  once 
the  kind  of  plants  and  animals  best  adapted  to  live  there.  In  addi- 
tion, information  is  available  about  the  character  of  the  soil,  the 
rainfall,  temperature  range,  and  the  particular  cereals,  fruits,  and 
vegetables  that  can  be  grown  in  the  region. 

Life  Realms 

Different  parts  of  the  world,  each  with  its  several  life  zones,  have 
been  separated  into  life  regions,  or  realms.  If  we  plot  the  distribu- 
tion of  a  given  family  of  animals  or  plants,  we  often  find  that  species 
within  the  group  have  a  wide  distribution,  in  some  instances  covering 
more  than  a  single  continent.  Australia  has  long  been  set  aside  as 
a  distinct  realm  because  its  peculiar  fauna  and  flora  differ  from  those 
in  other  parts  of  the  earth  and  so  is  called  the  Australian  Realm. 


THE  BIOLOGICAL   CONQUEST   OF  Till:    WOULD 


\:\ 


"L,_   Hblarctic   p,. 


3  / 


^^^^^4i/^• 


Ethiopian 


Auslralia-ri      ^:;» 
"Rsalin 


Map  showing  life  realms. 


Similarly  there  are  the  South  American,  or  Xcotroi)i('al,  Etliioi)iaii, 
Oriental,  and  Holarctic  realms,  the  latter  comprising  most  of  the 
land  surface  of  the  Tropic  of  Cancer.  Each  of  these  regions  has 
animals  and  plants  peculiar  to  itself,  although  resemblances  are  often 
found  between  inhabitants  in  different  realms. 


SUGGESTED   READINGS 

Beebe,  C.  W.,  Hartley,  G.,  Howes,  P.  G.,  Tropical  Wild  Life  in  Britif^h 

Guiana,  New  York  Zool.  See,  1917.     Ch.  VI. 

Contains  an  interesting  description  of  a  tropical  rain-forest. 
Borradaile,  L.  A.,  The  Animal  and  Its  Environment,  O.xford  University  Press. 

London,  1923.     Chs.  VII,  VIII,  X,  XI,  XIII. 

Excellent  for  general  reading. 
Elton,  C,  Animal  Ecology,  The  Macmillan  Co.,  1927.     Chs.  Ill,  V,  X. 

Fascinating  reading. 
Jordan,  D.  S.,  Kellogg,  V.  L.,  and  Heath,  H.,  Animals,  D.  Applcton  ct  Co., 

1909.     Chs.  VII,  XVI. 

Old  but  reliable. 
Pearse,  A.  S.,  A7iimal  Ecology,  McGraw-Hill  Rook  Co.,  192G.     Ch.  IV. 

Rather  a  book  of  reference  than  a  reading  book. 
Roule,  L.,  Fishes,  Their  Journeys  and  Migrations,  W.  ^^■.  Norton  &  Co.,  1933. 

All  of  this  book  makes  interesting  reading. 
Walter,  H.  E.,  Biology  of  the  Vertebrates,  The  Macmillan  Co.,  192S.     Cli.  III. 

Interesting  and  reliable. 
Weaver,  J.  E.,  and  Clements,  F.  E.,  Plant  Ecology,  McGraw-Hill  Book  Co., 

1929.     Chs.  IV,  V,  VII,  XVIII. 

Very'  scientific  and  yet  interesting. 


II J 


THE   INTERDEPENDENCE   OF   LIVING   THINGS  — 
THE  WEB   OF   LIFE 

Preview.  Relations  between  members  of  the  same  species;  care  of 
eggs  by  parents;  care  of  young  •  Relations  of  mutual  aid  •  Animal  can- 
nibalism •  Relations  of  competition  •  Relation  of  members  of  different 
species  ■  Adaptations  for  food-getting  in  animals  •  Scavengers  •  Food- 
getting  in  plants ;  carnivorous  plants  •  Symbiosis  •  Commensalism  •  Par- 
asitism •  The  chemical  relationship  of  plants  and  animals  •  Life  habits  of 
bacteria  •  Relation  of  bacteria  to  free  nitrogen  •  Rotation  of  crops  •  The 
relations  between  insects  and  flowers  •  Suggested  readings. 

PREVIEW 

Those  who  have  been  fortunate  enough  to  be  in  California  or  Flor- 
ida when  the  oranges  are  in  bloom  will  never  forget  their  odor ;  nor 
will  they,  when  examining  the  grove,  fail  to  notice  the  large  number 
of  bees  vi-siting  the  flowers.  The  bees  are  after  nectar  and  pollen, 
yet  without  these  winged  agents,  the  crop  of  oranges  for  the  follow- 
ing year  would  probably  be  small.  This  interrelationship  between 
insects  and  flowers  was  noticed  by  Charles  Darwin,  who  pointed  out 
that  the  size  of  the  clover  crop  in  England  depended  upon  the  num- 
ber of  cats  in  a  given  region.  His  friend  Huxley,  who  knew  better 
than  Darwin  how  to  popularize  science,  immediately  went  him  one 
better  and  added  that  the  size  of  the  clover  crop  depended  upon  the 
number  of  old  maids.  When  asked  to  explain,  he  gave  this  logical  se- 
quence of  events.  Old  maids  keep  cats  ;  cats  prey  upon  field  mice  ; 
mice  provide  nesting  places  for  bumblebees ;  bumblebees  pollinate 
clover,  upon  which  pollination  the  next  year's  crop  depends.  So  he 
had  a  perfectly  logical  chain  of  events.  Throughout  nature  there  is 
this  give  and  take  between  different  organisms  which  we  call  the  web  of 
life.  When  man  interrupts  or  displaces  a  link  in  the  chain  of  interre- 
lationships, the  web  is  broken  and  the  whole  fauna  or  flora  of  a  region 
may  be  changed,  as  in  the  case  of  the  Englishman  who  took  a  bit  of 
water  cress  to  Australia,  planting  some  in  a  nearby  stream  to  remind 
him  of  home.  This  foreign  plant,  having  no  enemies  and  finding 
conditions  favorable  for  its  growth,  literally  overran  the  waterways 
until  today  the  rivers  of  Australia  are  choked  with  water  cress.    Look- 

11 


THE   INTERDEPENDENCE  OF  LIVING   THINGS  i:, 

ing  over  the  world  of  plants  and  animals  an  unescajDabie  dcixMidenro 
of  one  form  of  life  upon  another  is  found  in  the  food  relationship 
by  which  green  plants  supply  animals  with  food  and  in  the  shelter 
relationship,  by  which  animals  find  safety  in  the  protection  given 
by  plants.  Reducing  this  search  for  food  and  shelter  to  its  ultimate, 
we  find  that  all  animals  are  dependent  upon  green  plants. 

But  does  the  green  plant  get  anything  from  the  animal  ?  At  first 
sight  it  would  seem  as  though  it  were  all  give  and  no  take.  As  we 
study  the  situation  more  closely,  however,  we  find  that  food-making 
is  dependent  upon  certain  raw  materials,  some  of  which,  such  as 
nitrogenous  wastes,  can  only  be  supplied  from  the  dead  bodies  of 
organisms  or  their  excreta.  Moreover,  another  important  raw 
material,  carbon  dioxide,  used  by  green  plants  in  starch-making,  is 
given  off  as  a  respiratory  by-product  by  animals,  and  in  this  same 
process  oxygen  is  released. 

All  of  these  facts  suggest  certain  problems.  Why,  for  example, 
when  some  animals  produce  enormous  numbers  of  eggs  and  others 
only  a  few,  do  not  the  former  outnumber  the  latter?  Of  what 
significance  is  the  mutual  aid  so  frequently  observed  in  nature? 
What  is  symbiosis  and  why  is  it  significant?  What  is  the  \'alue  of 
pollination  by  insects  as  compared  with  pollination  by  other  means? 
What  part  do  bacteria  play  in  the  fives  of  plants  and  animals? 
What  is  the  reason  for  parasitism  ?  Can  the  oft-repeated  statement 
that  green  plants  make  food  for  the  world  be  proved  ?  A  start  on  the 
answers  to  some  of  these  questions  will  be  made  in  the  pages  that  follow. 

Relations  between  Members  of  the  Same  Species 

Many  examples  of  helpful  relationships  can  be  .seen  between  ani- 
mals of  the  same  species,  especially  in  the  care  of  young.  Although 
in  low  forms,  such  as  sponges,  coelenterat(>s.  echinoderms,  and  a  good 
many  fishes,  large  numbers  of  eggs  are  laid  and  given  little  or  no 
parental  care,  the  production  by  the  male  of  immense  numbers  of 
sperm  cells  in  the  vicinity  of  the  eggs  insures  chance  fertilization  and 
continuity  of  the  species.  For  example,  Norman  '  reports  that  a  cod 
w^hich  weighed  21^  pounds  produced  over  6,650,000  eggs.  At  tiie 
time  of  egg  laying  each  male  of  the  above  .species  throws  billions  of 
sperm  cells  into  the  water  near  the  eggs.  Higher  in  the  animal  scale 
we  find  greater  provision  for  care  of  the  young  correlatcnl  with  a  re- 
duction in  the  number  of  eggs  laid.     Many  insects  lay  their  eggs  on 

1  Norman,  J.  R.,  A  History  of  Fishes.     Stokes,  1931. 


46 


NATURAL  HISTORY 


Bruinu  II 

Ichneumon  fly  {Ophion  macnirum) 
laying  eggs  in  the  cocoon  of  a  Cecropia 
moth. 


plants  which  will  become  food  for  the  larvae  or  caterpillars.     Others 
lay  their  eggs  either  in  the  ground  where  they  are  protected,  or  in 

dead  bodies  of  animals  on  which 
the  larvae  may  feed,  as  in  the  case 
of  certain  beetles,  or  in  a  ball  of 
dung,  as  in  the  case  of  the  dung 
beetle.  Certain  ichneumon  flies 
bore  deep  into  tree  trunks  in  order 
to  lay  their  eggs  in  the  larvae  of 
wood-boring  insects.  Some  w^asps 
paralyze  caterpillars  or  spiders, 
laying  eggs  in  the  still  living  victim 
so  that  when  the  eggs  hatch  the 
young  larvae  will  have  food.  In 
many  animals,  food  is  provided  in 
the  yolk  of  the  egg,  the  eggs  of 
fish  and  birds  being  examples. 
Spiders  and  earthworms  form 
cocoons,  which  in  the  case  of  the 
earthworm  are  usually  filled  with  a  nutritive  fluid  on  which  the  young 
feed  after  they  are  hatched,  while  in  the  cocoon  of  the  spider  the 
young  feed  upon  each  other,  the  strongest  of  the  group  surviving. 

Care  of  Eggs  by  Parents 

Some  of  us  as  youngsters  have  angled  for  sunfish  and  will  always 
remember  the  thrill  that  came  when  a  brightly  colored  male  dashed 
at  the  bait  dangled  over  the  hollowed  nest  containing  eggs  which  he 
was  guarding.  From  the  simple  nest  of  sunfish  and  salmon  through 
the  more  complicated  nests  of  the  stickleback  or  lake  catfish  we  come 
to  the  more  elaborate  nesting  habits  of  birds.  Some  birds,  as  terns, 
sandpipers,  or  gulls,  simply  make  shallow  holes  in  the  sand,  as  does 
the  sand  ostrich.  Grebes  and  rails  make  nests  of  floating  decaying 
vegetation.  Nuthatches  and  woodpeckers  make  nests  in  holes  in 
trees  where  the  young  are  protected.  At  the  top  of  the  ladder  are 
more  elaborate  nests  such  as  those  of  the  oriole  and  oven-bird  of  our 
latitude  or  the  tailor  bird  and  weaver  bird  of  the  tropics. 

Care  of  Young 

Sir  Arthur  Newsholme  has  said  that  the  most  dangerous  work  in 
the  world  is  that  of  being  a  baby.     If  the  young  of  plants  and  ani- 


THE   INTKHDEI'llNDENCE  OF   MV|\(;    ril|\(;s 


M 


A.  1'.  .sV((/i  CtinserrniUin  iJcpl 

Stickleback  and  nest.     Of  what  advantage  would  this  be  to  the  species? 

mals  survive  this  dangerous  stage,  their  chances  of  growing  to  adults 
are  very  considerable.  Although  parental  care  is  not  associated 
with  plants,  nevertheless  in  low  forms  of  plant  lif(>  locomotor  stages 
occur,  called  zoospores  or  swarm  spores,  by  means  of  which  the  plants 
gain  footholds  in  new  areas.  Many  devices  have  already  been  men- 
tioned by  means  of  which  seeds  are  scattered  far  from  the  parent 
plant.  In  higher  plants,  hard  shells,  spiny  coverings,  or  inedible  pulp 
protect  seeds  within  the  mature  fruit,  thus  giving  greater  ojjpor- 
tunity  for  the  scattering  and  germination  of  seeds. 

Adaptations  for  the  protection  of  young  are  more  evident  among 
animals.  In  crustaceans,  the  larvae  of  which  form  the  chief  food 
for  great  numbers  of  fish,  there  are  not  a  few  protective  adaptations. 
In  some  instances  crustaceans  have  brood  pouches  in  which  the  young 
are  kept,  or,  as  in  the  case  of  crayfish  and  lobster,  the  developing 
eggs  are  cemented  to  the  abdominal  appendages  of  the  mother  and 
carried  around  by  her.  The  male  bullhead  .swims  arountl  with  and 
broods  over  his  young,  while  the  male  sea  horse  has  a  brood  pouch  in 
which  the  young  are  held.  In  some  worms  and  crustaceans,  the  eggs 
may  be  retained  in  the  burrow  of  the  parent,  or  they  may  be  held 
in  the  mantle  cavity  or  a  space  similar  to  it,  as  in  the  fresh-water 
mussels,  barnacles,  and  tunicates.  Some  spiders,  notably  the  wolf 
spiders,  carry  the  egg  cocoon  about  with  them  and  when  the  yoimg 
are  hatched,  they  are  carried  on  the  backs  and  legs  of  the  female 


48 


NATURAL  HISTORY 


Huijh  Spiricer 

A  spider  with  its  egg  cocoon. 


until  large  enough  to  care  for  themselves.     The  male  of  the  so-called 

midwife  toad  (Alytes)  carries  the  eggs  entangled  around  the  legs. 

The  male  Surinam  toad  places  the  eggs  on  the  back  of  the  female, 

where  each  sinks  into  a  tiny  pouch  as  it  develops. 

Animals  that  lay  eggs  which  hatch  outside  of  the  mother's  body 

are  said  to  be  oviparous.     A  modified  form  of  this  procedure  is  seen 

in  some  nematodes,  arthropods, 
fish,  amphibia,  and  reptiles.  Here 
the  eggs  remain  in  the  oviduct  or 
uterus  of  the  mother  until  they 
are  almost  ready  to  hatch,  the 
body  of  the  mother  acting  as  an 
incubator.  Such  forms  are  said 
to  be  ovoviparous.  Most  of  the 
mammals  which  retain  the  eggs  in 
the  body  until  the  young  are  born 
are  said  to  be  viviparous.  Here 
the  young  are  held  as  embryos 
within  the   body   of   the   mother 

and  nourished  by  means  of  an  organ  called  the  placenta.     The  young 

of  mammals  are  suckled  at  the  breasts  of  the  mother  until  they 

are  able  to  eat  solid  food. 

Relations  of  Mutual  Aid 

A  certain  amount  of  protection  is  afforded  plants  from  their  habit 
of  living  in  communities.  Examples  are  the  aggregations  of  cacti  in 
our  western  deserts  or  the  acacia  and  "thorn  bush"  communities  of 
Australia.  The  animal  world,  too,  shows  many  examples  of  protec- 
tion among  gregarious  forms.  The  schooling  of  fishes  not  only  is  a 
defense  for  the  group  from  larger  fish,  but  it  also  enables  small  fish, 
working  concertedly,  to  prey  on  organisms  much  larger  than  them- 
selves. The  driver  ants  in  Africa,  traveling  in  great  swarms,  often 
overcome  and  devour  animals  hundreds  of  times  larger  than  them- 
selves. Wolves  hunt  in  packs,  several  of  them  rushing  together  to 
bring  down  their  larger  prey.  Deer  and  other  herbivorous  animals 
move  in  herds  for  mutual  protection. 

Another  relation  of  mutual  aid  results  from  the  development  of 
division  of  labor  among  certain  animals.  Although  social  division 
of  labor  is  well  seen  in  the  human  species,  there  are  many  examples 
in  the  insect  world,  particularly  among  the  social  bees  and  wasps, 


THE  INTERDEPENDENCE  OF   LIV1\(,   Tl||\f;s 


49 


such  as  tho  division  of  the  colony  into  castes  thai  include  nuih-s 
(drones),  fertile  females  ((lueens),  and  infertile  females  (workers). 
Castes  are  even  more  mmierous  among  ants,  there  being  winged  and 
wingless  females,  intermediates  between  females  and  workers,  soldiers, 
several  groups  of  workers,  and  winged  and  wingless  males.  Not  all  of 
these  forms,  however,  are  found  in  any  one  species.  By  means  of  such 
division  of  labor,  life  in  the  colony  goes  on  at  a  very  efficient  level. 

Animal  Cannibalism 

Most  of  us  have  had  the  experience  of  having  some  pet  destroy 
her  young  when  they  were  in  danger,  or  of  having  laboratory-bred 
rats  or  mice  eat  their  newborn  young.  This  is  probably  a  perverted 
instinct,  but  nevertheless  animal  cannibalism  is  .seen  rather  fre- 
quently. The  destruction  of  a  wounded  member  of  a  pack  of  wolves 
when  hunting  is  usual.  The  female  spider  usually  kills  the  male 
after  fertilization  of  the  eggs,  this  habit  being  common  to  some 
other  forms.  Similarly  the  eggs  may  be  destroyed  by  the  male, 
as  in  the  case  of  the  mole  cricket  and  centipede,  w^hich  eat  the  eggs 
shortly  after  they  are  laid,  the  mothers  resorting  to  numerous  pro- 
tective devices  in  order  to  thwart  the  cannibalistic  fathers.  Many 
fish  eat  the  eggs  of  their  own  species.  Even  the  domestic  hen  at 
times  will  eat  her  own  eggs. 

Relations  of  Competition 

Evidences  of  competition  in  the  plant  world  are  numerous.  Be- 
cause of  their  sessile  habit,  older  plants  may  overshadow  and  crowd 
out  the  young  ones,  or  one  group  of  plants  may  prevent  the  growth 
of  other  plants  in  the  vicinity.  Weeds  and  plants  in  general  pro- 
duce enormous  quantities  of  seed,  which  are  kept  from  germinat- 
ing by  the  rapid  growth  of  the  older  plants.  Many  grasses  and  some 
shrubs  grow  rapidly  by  means  of  underground  shoots,  in  this  way 
securing  territory  which  might  be  used  by  other  plants.  Thus  plants 
with  favorable  adaptations  may  completely  pre-empt  new  territory  for 
themselves  at  the  expense  of  others  le.ss  able  to  use  the  environment. 

In  animals,  competition  between  individuals  of  a  species  is  almost 
universal.  Males  fight  each  other  for  the  possession  of  females,  or 
sometimes  just  for  the  sake  of  fighting.  There  is  a  contituial  struggle 
for  food,  for  water,  and  for  a  place  to  live.  I>arger  animals,  as  we 
have  seen,  prey  on  smaller  ones  and  in  general  those  best  fitted  to 
compete  in  the  battle  of  \Uo,  survive. 


50 


NATURAL   HISTORY 


Wright  Pierce 

This  desert  weed,  rabbit  brush  (Chrysothamnus  nauseosus)  has  pre-empted  newly 
cleared  areas  along  the  border  of  the  Mohave  Desert.  How  would  you  account 
for  its  rapid  spread  ? 


Relation  of  Members  of  Dififerent  Species 

No  one  who  has  carefully  watched  the  life  that  goes  on  in  a  grove 
or  forest  can  escape  seeing  there  the  enactment  of  a  drama  that  repre- 
sents the  larger  picture  of  relationships  between  living  things  the 
world  over.  Insects  are  flying  through  the  air,  crawling  along  the 
ground,  or  burrowing  into  decaying  logs  and  the  ground.  Spiders 
and  ground  beetles  may  occasionally  be  observed  making  off  with  a 
victim,  while  here  and  there  birds  such  as  woodpeckers,  flycatchers, 
and  warblers  may  be  seen  feeding  on  adult  insects  or  their  larvae, 
while  a  hawk  may  be  watching  to  pounce  upon  some  one  of  the 
insect-eating  birds.  If  we  were  able  to  make  a  prolonged  study  of 
the  area  we  would  find  that  squirrels,  rabbits,  and  wood  mice  are  food 
for  larger  flesh-eating  animals  or  carnivores,  such  as  foxes.  In  such 
an  area  we  might  also  find  a  series  of  herbivorous  animals  ranging 
from  plant  lice  (aphids)  living  on  the  leaves  of  trees  to  occasional  deer 
which  browse  on  the  leaves  of  the  same  plants. 


zSn 


plants  rrxike 
the  fooct  fbr- 
tha-  worloC 


THE   INTERDEPENDENCE  OF  L1VT.N(;   TIIIN(JS  :,i 

It  will  be  noted  in  the  illustrations  given  that  animals  almost 
mvanably  feed   upon   others  smaller  than   themselves.   The  same 
relationship  is  seen  in  lakes  or  oceans  where  microscopic  plants  and 
animals  (plankton)  form  the  food  of  other  larger  organisms,  especially 
fish.     These  living  things  form 
definite    "food    chains"   in 
which  larger  animals  feed  on 
smaller  and  smaller  ones  until 
ultimately  the    lowest   forms 
subsist  on  tiny  green  plants 
or  bacteria.     For  example,  in 
a   small    pond   we   may  find 
billions  of  diatoms,  unicellular 
algae,  and  protozoa  and  feed- 
ing on  them  millions  of  small 
crustaceans.     With  them  are 
thousands    of    insect    larvae, 
hundreds  of  small  fish,  and  a 
few  large  fish,  such  as  bass, 
pickerel,  or  perch,  which  are 
dependent  upon  all  the  other 
forms  of  life.     In  this  case  a 
few  large  animals  are  depend- 
ent for  food  upon  the  development  of  myriads  of  smaller  organisms, 
the  basis  of  this  food  being  very  simple  plants.     Take  away  any  link 
in  the  food  chain  and  life  in  the  pond  becomes  disorganized,  with  the 
ensuing  death  of  many  of  the  inhabitants. 

Since  smaller  animals  reproduce  more  rapidly  than  larger  ones, 
the  food  supply  for  those  "on  the  top  of  the  heap"  remains  fairly 
constant.  It  should  be  borne  in  mind,  however,  that  the  larger 
animals  require  a  range  of  sufficient  size  to  support  them. 

Adaptations  for  Food-getting  in  Animals 

Protozoans,  if  ameboid,  engulf  their  food,  but  in  other  members  of 
this  group,  food  passes  into  the  cell  through  a  definite  opening  or 
through  the  plasma  membrane.  Sponges  and  many  molluscs  pick 
up  microscopic  food  as  it  comes  to  them  in  water  currents.  Some 
molluscs  bore  holes  through  the  hard  shells  of  bivalves,  in  that  way 
securing  the  soft  parts  of  the  animal  for  food.  Insects  have  biting, 
chewing,  or  sucking  mouthparts,  each  type  being  fitted  to  utilize  a 


drassViopptrs- 
°  Gat  gi-ass 


\\  hy  will  a  break  in  the  food  chain  often 
cause  disorganization  of  life  in  that  locality  ? 


NATURAL   HISTORY 


Wright  Pierce 

Adaptations  of  beaks  of  birds  for 
food -getting. 


different  kind  of  food.  Carnivorous 
mammals  have  sharp  teeth  fitted  for 
tearing  and  holding  prey  ;  herbivorous 
mammals  have  flat,  corrugated  teeth  ; 
rodents,  gnawing  or  chisel-like  teeth  ; 
while  snakes,  which  swallow  their  prey 
whole,  have  pointed,  needlelike  teeth 
to  hold  their  food  securely.  More 
striking  adaptations  for  food-getting 
are  found  in  birds  whose  beaks  and  feet 
both  give  clues  to  their  food  habits. 
The  flesh-eating  birds  have  hooked 
beaks  and  curved  claws ;  aquatic 
birds  have  feet  shaped  like  paddles 
and  scooplike  bills  for  straining  out 
small  organisms  from  the  water ;  wad- 
ing birds  display  a  remarkable  variety 
of  highly  specialized  beaks  and  feet ; 
and  the  smaller  land  birds  show 
equally  interesting  adaptations  for  se- 
curing food.  Bizarre  adaptations  for 
procuring  food  characterize  the  giraffe, 
with  its  long  neck  that  enables  it  to 
reach  up  to  feed  on  branches  of  trees 
fifteen  feet  from  the  ground,  the  ant- 
eater,  with  its  sticky  tongue,  and  the 
walrus,  which  digs  bivalves  with  its 
tusks. 

Scavengers 

Some  forms  of  life  are  not  only  om- 
nivorous in  their  diet,  but  are  actually 
scavengers,  living  on  dead  organic  ma- 
terials. The  bacteria,^  smallest  of  all 
plants,  feed  upon  or  destroy  millions 
of  tons  of  organic  wastes  which  other- 
wise would  make  life  on  earth  impossi- 
ble. Think  of  a  world  without  decay. 
Land  and  water  would  soon  become 


'  See  pages  165-166. 


THi:   INTEHDKPKNDKNCE   OF   I.IVINC    T|||\,;s  ;,.•, 

roverod  witli  the  dead  bodi(>s  of  plants  and  animals.  TUr  i)acteriu  of 
decay  are  very  numerous  in  rich,  damp  soils  containing  large  amounts 
of  organic  material.  They  decompose  organic  materials,  changing 
them  to  compounds  that  can  be  absorbed  by  plants  to  be  used  ii, 
building  protoplasm.  Without  decay  life  would  be  impossible.  f„r 
green  plants  would  otherwise  be  unable  to  get  the  raw  food  materials 
to  make  food  and  living  matter. 

In  general  all  plants,  both  colorless  and  green,  may  be  said  to  play 
a  part  in  ridding  the  earth  of  organic  wastes.  The  fungi,  or  colorless 
plants,  get  their  nourishment  from  the  dead  bodies  of  plants  and 
animals,  while  the  green  plants  take  organic  wastes  from  the  soil 
to  be  used  in  the  manufacture  of  foods. 

Many  animals  also  take  part  in  scavenging.  Some  of  the  food  of  the 
protozoa  is  made  up  of  decaying  unicellular  material  and  the  bacteria 
which  cause  its  decay.  Certain  forms,  especially  insects,  feed  upon  and 
lay  their  eggs  in  decaying  flesh,  while  myriads  of  insects  and  their 
larvae  help  to  break  down  decaying  wood  in  a  forest.  These  are 
only  a  few  instances  of  this  important  function. 

Food-getting  in  Plants 

Although  green  plants  make  foods  and  use  raw  food  materials  '  from 
their  environment  to  do  this,  there  are  some  that  destroy  foods. 
Fungi,  such  as  bacteria,  molds,  smuts,  and  rusts,  ruin  billions  of  dollars' 
worth  of  food  plants  and  plant  jiroducts  each  year.  This  is  seen  in 
damage  to  crops,  fruits,  stored  foods,  and  animals  used  as  food  by 
man. 

Carnivorous  Plants 

A  curious  exception  to  ordinary  green  plant  nutrition  exists  in 
carnivorous  plants,  which  also  illustrates  a  different  interrelationshij) 
between  plants  and  animals.  Carnivorous  plants  add  to  their  nitro- 
gen requirement  in  several  ways.  The  fresh-water  aquatic  plants 
known  as  bladderworts  {Utricidaria)  catch  water  fleius  and  other 
small  crustaceans  in  hving  bladderlike  traps.  Just  what  lure  urges 
the  crustaceans  to  destruction  is  hard  to  say,  but  the  fact  that  they 
are  caught  in  numbers  is  verified  by  their  decomposed  remains  found 
in  the  bladders.  Other  animal-eating  forms  are  the  various  pitch(>r 
plants  (Sarracema  sp.),  some  of  which  are  found  in  our  northern 
swamps.     Insects   are   apparently   lured   to   the   urn-shaped    leave^' 

>  See  pages  253-262. 


54 


NATURAL  HISTORY 


by  a  trail  of  sweet  nectar  secreted  just  outside  the  mouth  of  the 
pitcher.  Once  inside,  a  shppery  surface  and  incurving  hairs  prevent 
egress,  and  the  insect  is  soon  digested  by  the  enzymes  in  the  fluid 
contained  inside  the  pitcher.  Still  another  leaf  modification  with  a 
similar  function  is  seen  in  the  sundew  (Drosera  sp.).  Here  the  leaves 
are  covered  on  one  surface  by  sticky  glandular  hairs,  which  close 


The  leaf  of  a  bladder  wort  {Uiricu- 
laria  vulgaris).  Many  of  its  numerous 
divisions  bear  bladders  (6),  especially 
near  the  place  of  attachment  to  the 
main  leaf  axis  (a).  Note  the  aper- 
tures of  the  bladders  (p)  into  which 
small  aquatic  animals  may  crawl  or 
swim. 


The  modified  leaf  of  a  sundew  {Drosera 
rotund  if olia)  showing  the  conspicuous 
glandular  hairs  (g)  covering  the  upper 
surface,  the  hairs  at  the  right  having 
caught  an  insect.  Note  that  the  hairs 
are  tipped  by  a  drop  of  secreted  liquid 
{d),  which  attracts  insects  to  the  leaf  and 
also  entangles  them.  —  After  Kerner. 


over  the  insect,  hold  it  fast,  and  ultimately  digest  it  and  absorb  its 
juices.  In  the  Venus's-flytrap  (Dionaea  sp.),  another  carnivorous 
plant  found  in  some  parts  of  this  country,  the  leaves  have  two  sensi- 
tive lobes  provided  with  marginal  hairs.  If  an  insect  lights  on  a 
leaf,  the  two  lobes  close  over  it  and  the  insect  is  trapped.  After  its 
prey  is  digested,  the  lobes  of  the  leaf  open  up  and  the  plant  is  ready 
for  action  again. 

Symbiosis 

The  process  of  living  together  for  mutual  advantage  is  called 
symbiosis.  Plants  may  join  forces  as  may  animals,  or  in  some 
instances,  plants  with  animals.     Lichens,  for  example,  illustrate  this 


THE   INTERDEPENDENCE   OF   LIVI\(.   TIIIN(;S 


W  I  III  III  I'll  rci 

An  encnistiiif^  licht-n.      Why  docs  lliis  pl;inl  suc- 
ceed in  such  an  unfavorable  cm  ironiucul  i' 


mutual    partnership    in 

an  interesting  way.     A 

lichen    is    composed    of 

two   kinds  of  plants,   a 

green  alga  and  a  fungus, 

one    of    which    at    least 

may    live   alone.     The 

two  plants  form  a  part- 
nership for  life,  the  alga 

making    the    food    and 

nourishing    the    fungus, 

while  the  latter  gives  the 

alga  raw  food  materials, 

protects    it,    and    keeps 

it  from  dying  when  the 

humidity  of  the  air  is  low. 

Other  examples  are  bac- 
teria  and    the   mycelial 

filaments  of  fungi   {my- 

corhiza)  which  live  sym- 

biotically  on  the  roots  of  certain  plants,  taking  food  from  the  plants, 

but  giving  them  nitrogen  in  a  usable  form  in  return. 

A  common  example  of  symbiosis  between  plants 
and  animals  is  the  green  Hydra  {Chlorohydra  viri- 
dissima),  which  holds  in  its  body  wall  a  unicellular 
alga  known  as  Zoochlorella.  These  plants  contain 
chlorophyll,  using  the  sun  to  make  food.  In  this 
partnership,  the  algae  get  carbon  dioxide  and  ni- 
trogenous wastes  from  the  animal,  to  which,  in 
turn,  they  give  food  and  the  oxygen  set  free  in  the 
process  of  starch-making.  There  are  numerous 
examples  of  this  kind  of  symbiosis  in  tlio  animal 
world,  as  is  seen  in  many  of  the  protozoa,  sjionges, 
A  root' tip  of  the   coelenterates,  flatworms,  molluscs,  and  sea  urchins. 

European    beech       xhe  symbiotic  relationship  of  animals  to  each 

{Fagus   sylvalica),       ^^^  -^  ^j^^^^.^^  y      ^j^^  ^j,       protozoans  iixing  in 

showing  ectotrophic  '^  „  .  i  •.  * 

mycorhiza,  the  fun-   the  digestive   tracts  of  termites  or   white   ants. 

gal  hyphae  forming  These  Httlc  animals  act  as  digestive  cells  for  the 
:nS:the?tLtLt  termites,  making  it  possible  for  them  to  use 
-After  Frank.  wood  fibers  on  which  they  live.     In  return   th(> 

H.   W.   H.  —  5 


56 


NATURAL  HISTORY 


a  shark- 


s-cccker- 


protozoans  receive  food  and  are  protected  by  their  hosts.  A  some- 
what similar  situation  prevails  in  the  large  intestine  of  man,  where 
certain  types  of  useful  bacteria  are  found.  These  forms  help  keep 
down  putrefying  bacteria,  receiving  in  return  a  home,  food,  and 
a  favorable  temperature  in  which  to  live.  Certain  species  of  ants 
protect  and  feed  aphids,  in  turn  feeding  upon  the  sweet  fluid  secreted 
by  the  aphid. 

Commensalism 

Some  associations  are  not  obviously  to  the  advantage  of  either 
organism,  the  two  feeding  together   as   messmates.     Animals   like 

the  small  crabs  that  live 
in  the  water  canals  of 
certain  sponges,  or  the 
tiny  fishes  that  live  in 
the   lower   part  of   the 
body  of  a  "trepang,"  a 
sea    cucumber,    are   ex- 
amples.    The  young  of 
some  species  of  rudder- 
fish    (Stromateidae)    ac- 
The  shark  sucker  (Remora  brachypiera,  Lowe)     company  large  jellyfish, 
showing  sucking  disk  and  its  method  of  attach-    geekina;      shelter      under 
ment  to  the  shark.     The  Remora  gets  free  trans-     ,,     ■        +•      •  j.      4.      i 

portation  and  makes  sudden  forays  after  food  as  their  stmgmg  tentacles 
well  as  sharing  the  "left-overs"  of  the  shark's  food,  when  chased  by  larger 
But  it  seems  doubtful  if  the  shark  gains  anything  ggj^  while  another  fish 
from  the  association.  .   ^  . 

{Nomeus)  lives  in  con- 
stant association  with  the  beautiful  coelenterate  known  as  the 
Portuguese  man-of-war. 

Parasitism 

Not  all  life  is  give  and  take.  Some  plants  and  animals  live  at  the 
expense  of  others,  giving  nothing  and  taking  all.  These  are  known 
as  parasites,  the  organism  which  entertains  them  being  called  the 
host.  From  the  lowest  to  the  highest  forms  in  the  plant  and  animal 
kingdom  there  are  few  which  are  not  attacked  by  parasites  at  some 
stage  of  their  existence. 

Parasitism  implies  plenty  of  food,  shelter,  and  a  relatively  protected 
life  for  the  parasite,  but  it  also  usually  spells  degradation  in  structure 
and  loss  of  activity.  It  may  mean  only  inconvenience,  but  more 
Ukely  a  shorter  and  disturbed  life  for  the  host,  especially  if  the  parasite 


THE   INTERDEPENDENCE  OF  LIVING   THINGS 


57 


causes  disease.     In  some  instances,  the  complicated  life  history  is  so 
bound  up  with  more  than  one  host  that  if  one  of  the  hosts  is  absent 
a  hnk  in  the  chain  of  life  is  broken,  the  life  cycle  cannot  be  completed' 
and  the  parasite  dies.     The  black-stem  grain  rust,  which  ref,uires 


^^ 


recC  spore 
blov/n   to 
anotber^  stem 

recC  or 
Sxtmreierc 
rixst  on 
•wheat  stem 


barlserrv  rtcst 
spore  in?ectintf 
ths  cells  of -^heat 
stem  ira 
spring- 


"bocrbei^ry 
leaves 


mfscts  stem 

through 

breo-^hing" 

?«""«.  red  rust  syjrecuil^  , 
from  stem  to  stem/ 
cCixriijg'  Sixmme'P 

blaclc  or 
^vinter  rust 
lives    on 


straw  thrcuflh , 
winter-     *  " 


infection  form', 
barberry  rust 
onborberrjlea.^ 
a  cluster  cup' 


The  life  history  of  black  stem  grain  rust. 

controlled. 


r,  ■!,  black  spora 

inject  ing^ 
bocfics,  sporicCa 

a  spoT~id.ium 

infects    the 

Cells    of  a 

barberry  leaf 

Explain  how  this  rust  may  he 


both  the  barberry  plant  and  the  wheat  to  complete  its  life  history ; 
the  pine  tree  blister,  which  lives  on  the  currant  or  gooseberry  at  one 
stage  of  its  life  history,  and  on  the  pine  at  another ;  and  the  parasite 
causing  malaria,  which  requires  both  the  anopheline  mosquito  and  the 
blood  of  man  to  complete  its  cycle,  are  examples. 

The  Chemical  Relationship  of  Plants  and  Animals 

The  study  of  plant  and  animal  ecology  may  be  said  to  be  analogous 
to  the  study  of  human  economics.  Social  conditions  among  men, 
animals,  and  plants  are  all  determined  by  the  environmental  factors 
present,  but  chiefly  by  the  availability  and  abundance  of  food.  The 
world's  food  supply  in  the  long  run  depends  upon  the  chemical  ele- 
ments making  up  the  environment  and  energy  derived  from  the  sun. 


58 


NATURAL  HISTORY 


Plants  and  animals  are  made  out  of  the  same  chemical  elements. 
Burn  some  beans  or  a  piece  of  beefsteak,  a  piece  of  wood  or  a  bit  of 
living  bone,  an  entire  green  plant  or  a  dead  mouse,  and  the  chemist 
would  tell  us  that  the  same  chemical  elements  are  present  in  animals 
and  plants ;  that  certain  of  these  elements  passed  off  in  the  smoke, 
others  into  the  air  as  colorless  gases,  leaving  still  others  as  a 
whitish  ash.  All  living  things  are  composed  mainly  of  carbon, 
oxygen,  hydrogen,  nitrogen,  with  about  twelve  other  chemical  ele- 
ments found  in  very  minute  quantities.  These  elements  are  all 
present  in  the  immediate  environment  of  plants  and  animals,  air, 
water,  and  soil. 

How  they  get  from  the  basic  environment  into  living  things  can 
be  briefly  stated.  Carbon,  which  is  contained  in  all  organic  foods 
and  in  this  condition  is  taken  into  the  animal  body,  can  only  be 
absorbed  in  the  form  of  carbon  dioxide  by  food-making  green 
plants.  This  gas,  which  is  present  in  the  atmosphere  to  the  average 
amount  of  about  0.03  per  cent,  gets  there  as  a  result  of  oxidative 
processes  taking  place  in  plants  and  animals,  as  well  as  by  the  com- 
bustion of  organic  substances.  Factories  and  volcanoes  alike  form 
their  quota  of  carbon  dioxide  to  diffuse  out  into  the  atmosphere. 
The  cycle  of  the  passage  of  carbon  from  plants  to  animals  and  from 
animals  back  to  plants  is  shown  in  the  accompanying  figure. 

Hydrogen,  another  component  part 
of  living  things,  cannot  be  used  in 
its  pure  state  by  either  plants  or 
animals.  In  water  (H2O) ,  it  becomes 
an  important  part  of  the  food  of 
animals,  and  as  water  vapor  it  is 
used  in  starch-making  by  green 
plants. 

Oxygen  is  freely  available  to  both 

plants  and  animals.    As  a  gas,  making 

up  over  20  per  cent  of  the  air,  capable 

of  being  dissolved  in  water  for  aquatic 

plants  and  animals,  it  is  used  by  all 

living  things  in  respiration.     Green 

plants  add  this  gas  to  the  air  during  the  process  of  starch-making. 

Nitrogen  is  one  of  the  most  important  elements  found  in  living 

things.     Making  up  79  per  cent  of  the  air,  it  is  not  usable  in  the 

form  of  a  gas  except  by  the  nitrogen-fixing  bacteria. 


The  carbon  and  oxygen  cycles  in  a 
balanced  aquarium.  Trace  the  pas- 
sage of  an  atom  of  carbon  from  a 
green  plant  back  to  the  plant. 


THE   INTERDEPENDENCE   OK   LIVING   THINGS 


:><> 


The  other  mmeral  components  of  living  matter,  of  wliich  sulpliur 
phosphorus,  calcium,  potassium,  and  iron  are  among  tlie  most  impor- 
tant, are  all  found  either  in  water,  soil,  or  both.  How  the  plant  makes 
use  of  them  and  turns  them  over  for  the  use  of  animals  is  an  interesting 
story  to  be  told  later.  But  enough  has  been  said  to  show  that  foods 
made  by  the  green  plants  form  the  supply  on  which  all  animals  live. 

carbohydrates^,, .^^^^  carbon  iiox.ae^^ /'0>^yg«?'v 

proteins^   X  -^-ureot  /^  >\    n'til«ts 

slltsZI^   AnimaU  (Gr^enPlantC r^f"' 

>.^ter- \^  y-sctlts        V  y^-^^'■-1^<.^ 

The  food  relationships  between  green  plants  and  animals. 

Life  Habits  of  Bacteria 

In  this  web  we  call  life,  bacteria  play  a  most  important  part.  Since 
bacteria  contain  no  chlorophyll,  they  are  unable  to  make  carbo- 
hydrate food,  and  must  obtain  their  foods  from  decaying  organic 
matter.  In  order  to  absorb  such  food  it  must  be  made  soluble  so 
that  it  will  pass  into  their  bodies.  This  they  do  by  digesting  food 
substances  by  means  of  enzymes  ^  which  they  secrete.  Bacteria 
that  grow  or  thrive  in  the  presence  of  oxygen  are  called  aerobic,  while 
those  which  live  without  free  oxygen  are  called  anaerobic.  The  latter 
need  oxygen,  like  other  living  things,  obtaining  it  by  breaking  down 
the  foods  on  which  they  live,  and  utilizing  oxygen  freed  in  this  process. 

Relation  of  Bacteria  to  Free  Nitrogen 

It  has  been  known  since  the  time  of  the  Romans  that  the  growth  of 
clover,  peas,  beans,  and  other  legumes  causes  soil  to  become  more 
favorable  for  the  growth  of  other  plants,  but  the  reason  for  this  was 
not  discovered  until  modern  times.  On  the  roots  of  the  plants 
mentioned  are  found  little  nodules,  or  tubercles,  in  each  of  which 
are  millions  of  nitrogen-fixing  bacteria  {Rhizobium  leguminosarum) , 
that  take  nitrogen  gas  from  the  air  between  the  soil  particles  and 
build  it  into  nitrites  which  arc  tlu>n  converted  by  otluM-  bacteria 
(Nitrobacter)  into  nitrates.  In  this  form  it  can  be  used  by  plants. 
Nitrogen-fixing  bacteria  live  in  a  symbiotic  relationship  with  the 
plants  on  which  they  form  tubercles,  their  hosts  pro\iding  them  with 
organic  food. 

J  See  pages  127-128. 


60 


NATURAL  HISTORY 


fjring' 
JoactcHee 


Bacteria  also  act  upon  ammonia  formed  from  plant  and  animal 
wastes,  one  kind  (Nitrosomonas)  producing  nitrites,  or  nitrate  salts, 
and  others  (Nitrobader)  converting  the  nitrites  into  the  more  stable 
nitrates.  Thus  all  of  the  compounds  of  nitrogen  are  used  over  and 
over,  first  by  plants,  then  as  food  by  animals,  eventually  returning  to 
the  soil,  or  in  part  being  released  as  free  nitrogen.     This  process  is 

called  the  nitrogen  cycle. 
Although  free  nitrogen  is 
fixed  for  use  by  means  of 
electrical  discharges  dur- 
ing thunderstorms,  by 
man-made  machines,  by 
ultraviolet  light  (which 
is  estimated  to  return 
100,000,000  tons  a  year 
to  the  earth's  surface), 
and  from  other  sources, 
yet  these  means  give  an 
almost  negligible  amount 
of  usable  nitrogen  to  the 
soil,  compared  with  what 
is  used  in  crop  produc- 
tion, especially  since  so  much  nitrogen  is  lost  from  the  soil  in  various 
ways.  The  nitrogen-fixing  bacteria  supply  the  deficiency,  thus  form- 
ing one  of  the  most  important  inter-relationships  between  plants  and 
animals  because  of  their  direct  relationship  to  the  production  of  the 
food  of  the  world. 

Rotation  of  Crops 

Plants  that  are  hosts  for  the  nitrogen-fixing  bacteria  are  raised 
early  in  the  season,  then  plowed  under  and  a  second  crop  of  a  differ- 
ent kind  is  planted.  The  latter  grows  quickly  and  luxuriantly  be- 
cause of  the  nitrates  left  in  the  soil  by  the  bacteria  which  lived  with 
the  first  crop.  For  this  reason,  clover  is  often  grown  on  land  used 
later  for  corn,  or  cowpeas  will  be  followed  by  a  crop  of  potatoes. 
On  well-managed  farms,  different  crops  are  planted  in  succession 
in  a  given  field  in  different  years  so  that  one  crop  may  replace  some 
of  the  elements  taken  from  the  soil  by  the  previous  crop.  This  is 
known  as  rotation  of  crops. ^ 

'  Crop  rotation  is  not  only  a  process  to  conserve  the  fertility  of  the  soil,  but  also  a  sanitary  meas- 
ure to  prevent  infection  of  the  soil. 


The  nitrogen  cycle.     What  additions  could 
made  to  this  diagram? 


be 


THE   INTERDEPENDENCE  OF  LIVINt;   Tll|\(;s 


<)l 


Wriilhl  I'Urcc 

Tiger  Swallow-tail  (Papilio  turnns)  on  rose. 


The  Relations  between  Insects  and  Flowers 

One  of  the  most  interesting  symbiotic  relationships  is  that  which 

exists  between  msects  and  flowers.     Flowering  plants  produce  .seed.s 

and  fruits,  and  from 

these  come  new  gen- 
erations   of    plants, 

but  if  it  were  not  for 

the  visits  of  insects, 

many   plants   would 

not    produce    seeds. 

Insects  visit  flowers 

in    order    to    obtain 

nectar,  a  sugary  sub- 
stance formed  by  the 

nectar    glands,    and 

pollen.     The    glands 

which    produce    the 

nectar  are  usually  so 

placed  that  an  insect 

has  to  push  its  way  past  the  stamens  and  pistil  of  the  flower  in  order 

to    reach    the   desired    food.     In 
doing    this,    pollen    grains    may 
adhere  to  the  hairy  covering  of 
the  insect  and  be  transferred  to 
the  sticky  surface  of  the  upper 
end  of  the  pistil  (stigma).     Inside 
the  pollen  grains  are  the  male  re- 
productive  cells    (sperms),   while 
in    the   ovary    of    the   pistil   are 
held  the  female  reproductive  cells 
(eggs).     In  order  to  ha\e  develop- 
ment of  a  new  plant,  it  is  essen- 
tial for  a  sperm  cell  to  unite  with 
an  egg  cell.     Pollen  grains  on  the 
stigma  are  stimulated  to  send  out 
hairlike    tubes,    wiiich    j)enetrate 
the  stalk  (style)  of  the  j)istil  and 
eventually  reach  the  ovary.     The 
pollen   tube  carries  one  or  more 


•derminatino: 


anther* 


•filameriti 


ovulell'' 


.  .-finicropyle-' 


A  longitudinal  section  of  the  repro- 
ductive organs  of  a  flower  showing  the 
penetration  of  a  pollen  tube  through 
the  opening  in  the  pistil  called  the 
micropyle,  and  the  growth  of  the  pollen 
tube  to  the  ovule. 


62  NATURAL  HISTORY 

sperm  cells,  which  are  thus  enabled  to  unite,  each  with  a  single  egg 
cell,  in  the  ovule  of  the  pistil.  The  union  of  the  sperm  nucleus  with 
its  egg  nucleus  is  called  fertilization.  As  a  result  of  this  process  the 
fertilized  egg  develops  into  an  embryo  or  young  plant  which  is  held 
in  the  seed.  When  favorable  conditions  arise,  this  embryo  m.ay 
develop  into  a  plant. 

Bees  are  the  chief  pollinizing  agents,  although  butterflies,  moths, 
flies,  and  a  few  other  insects  perform  this  service  as  well.  Hum- 
mingbirds often  pollinate  tubular  flowers,  while  other  small  birds, 
snails,  and  even  bats  are  agents  in  the  pollination  of  certain  forms. 
Man  and  animals  may  accidentally  pollinate  flowers  in  brushing  past 
them  through  the  fields.  The  value  of  cross-pollination  is  obvious 
and  is  an  example  of  the  close  weaving  of  life  in  which  man,  animals, 
and  plants  are  all  inescapably  entangled. 

SUGGESTED   READINGS 

Borradaile,  L.  A.,  The  Animal  and  Its  Environment,  Oxford  University  Press, 

London,  1923.     Chs.  IV,  V,  XIV. 

Excellent  for  reference. 
Elton,  C,  Animal  Ecology,  The  Macmillan  Co.,  1935.     Chs.  V,  VI,  VIII. 

Particularly  valuable  on  the  animal  community  and  the  relationship  of 

animals  to  a  food  supply. 
Needham,  J.  C,  and  Lloyd,  J.  T.,  Life  of  Inland  Waters,  2nd  ed.,  Charles  C. 

Thomas,  1930.     Ch.  V. 

Interrelationships  among  fresh-water  organisms. 
Pearse,  A.  S.,  Animal  Ecologij,  McGraw-Hill  Book  Co.,  1926.     Chs.  VIII,  X. 

A  wealth  of  material  on  interrelationships. 
Rau,  Phil.,  Jungle  Bees  and  Was-ps  of  Barro  Colorado  Island,  privately  printed, 

Kirkwood,  St.  Louis,  1933. 

An  ecological  study  of  a  tropical  environment. 
Wallace,  A.  R.,  The  Geographical  Distribution  of  Animals,  1876.     Books  I 

and  II.     Ch.  IV,  especially. 

This  book  forms  the  basis  for  most  of  the  modern  work  in  distribution. 

All  of  Part  III,  Books  I  and  II,  is  extremely  interesting. 
Weaver,  J.  E.,  and  Clements,  F.  E.,  Plant  Ecology,  McGraw-Hill  Book  Co., 

1935.     Ch.  XVI. 

An  interesting  chapter  on  relations  between  plants  and  animals,  with 

especial  emphasis  on  insect  pollination. 
Wells,  H.  G.,  Huxley,  J.  S.,  and  Wells,  C.  P.,  The  Science  of  Life,  Doubleday, 

Doran  &  Co.,  1931.     Book  6,  Chs.  IV  and  V. 

A  fascinating  book  for  general  reading. 


IV 


ROLL  CALL 

Preview.  Earh^  contributions  to  classification  •  Binomial  nomencla- 
ture ■  Law  of  priority  •  What  is  a  species?  •  A  classification  of  plants  and 
animals  •  Classification  of  the  plant  kingdom  •  Classification  of  the  animal 
kingdom  ■  Glossary  of  terms  occurring  in  the  Roll  Call. 

PREVIEW 

It  is  hoped  that  this  section  will  be  freely  used  by  the  student. 
It  is  not  expected  that  the  classification  of  plants  and  animals  will  be 
learned  by  rote,  but  rather  used  for  reference  from  time  to  time  as 
new  forms  are  seen.  By  this  means  the  diagnostic  characteristics 
of  different  phyla  and  classes  will  gradually  be  learned  as  needed,  and 
the  relationship  of  one  group  to  another  become  more  apparent. 

In  order  to  enjoy  hikes  or  longer  trips,  the  student  should  be  able 
to  recognize  the  larger  groups  of  the  plant  and  animal  kingdoms. 
Fortunately  there  are  museums,  botanical  gardens,  and  zoological 
parks  to  which  one  may  refer,  all  the  more  intelligently  of  course  if  he 
has  himself  first  discovered  living  animals  and  plants. 

Identifying  plants  and  animals  correctly  becomes  more  of  a  plea.s- 
ure  than  a  task,  if  the  principles  of  scientific,  as  well  as  common, 
nomenclature  are  understood.  Both  scientific  and  common  names 
will  be  encountered.  The  former  are  written  in  the  dead,  unchanging 
Latin  language,  and  are  of  more  universal  usefulness,  since  the  latter 
are  frequently  misleading  and  confu.sing,  as  more  than  one  common 
name  may  be  applied  in  different  countries,  or  in  different  parts  of  the 
same  country,  to  a  single  plant  or  animal.  For  example,  the  common 
"chain  pickerel"  is  listed  under  the  scientific  name  of  Esox,  indicating 
the  larger  or  generic  group  to  which  the  fish  belongs,  and  niger,  which 
is  its  specific  name,  but  it  has  at  least  twenty-two  connnon  names  in 
different  parts  of  this  country.  Here  are  a  few  of  them:  black 
pickerel,  pike,  common  eastern  pickerel,  duck-bill  pickerel,  green  p'lkv, 
little  pickerel,  and  lake  pickerel.  The  terms  pike,  pickerel,  aii.l  lake 
pickerel  are  also  quite  commonly  used  in  some  parts  of  the  country 
to  designate  another  fish,  the  great  northern  pike,  Esox  lucius.  In 
still  other  localities  "pike"  refers  to  an  entirely  different  group,  the 
pike-perches,  belonging  to  the  genus  Stizostedion.     This  examph^  will 

63 


64  NATURAL  HISTORY 

serve  to  indicate  the  necessity  for  the  use  of  Latin  scientific  names  in 
classification.  There  may  be  other  members  of  the  genus  Esox,  but 
there  is  only  one  niger,  although  varieties  of  the  same  are  possible  in 
different  environments.  The  terms  of  genus  and  species  were  intro- 
duced to  the  scientific  world  in  the  middle  of  the  18th  century  by 
Carl  von  Linn^  (1707-1777),  of  Sweden. 

The  study  of  classification  is  called  Taxonomy  and  is  subdivided 
into  zoological  taxonomy,  or  Systematic  Zoology,  and  botanical  taxon- 
omy, or  Systematic  Botany. 

Early  Contributions  to  Classification 

In  order  to  secure  an  idea  of  the  development  of  taxonomy  it  is 
necessary  to  go  back  several  hundred  years  to  some  of  the  earlier 
biologists  and  glance  at  a  few  of  the  contributions  of  these  students. 
Obviously  such  an  excursion  can  hope  to  touch  upon  only  a  few  of 
the  more  important  workers.  Logically,  one  should  go  all  the  way 
back  to  Aristotle's  time,  but  lack  of  space  forbids  such  an  interesting 
excursion.  Consequently  we  must  confine  ourselves  to  the  immediate 
forerunners  of  Linne,  or  Linnaeus  as  he  came  to  be  called,  who  intro- 
duced the  concept  of  binomial  nomenclature  and  with  it  a  more  ade- 
quate idea  of  genus  and  species. 

In  1576,  Matthias  de  TObel  published  an  important  work  on  plants. 
This  was  an  attempt  to  arrange  plants  according  to  their  structure. 
He  took  the  shape  of  the  leaf  as  the  basis  for  this  classification,  and  it 
led  him  to  put  such  things  as  ferns  in  the  same  group  with  trees  because 
the  fronds  of  the  fern  bore  a  superficial  resemblance  to  the  needles 
of  the  hemlock.  Another  botanist  was  the  Swiss,  Kasper  Bauhin 
(1560-1624),  who  described  in  order  6000  species  of  plants,  beginning 
with  the  ones  he  considered  most  primitive.  He  approached  the 
concept  of  genus  and  species,  because  he  grouped  together  plants 
which  resembled  one  another  externally. 

John  Ray  (1627-1705)  deserves  recognition  along  with  Linnaeus 
as  the  founder  of  the  science  of  systematic  biology.  This  enthusiast 
published  a  catalogue  of  British  plants  in  1670  and  later  works  (1703) 
in  which  he  introduced  and  explained  the  groups  of  Monocotyledons 
and  Dicotyledons.  He  also  made  less  extensive  contributions  to  the 
classification  of  animals.  Some  of  these  he  published  with  his  good 
friend  Willughby  (1635-1672).  Ray  gave  evidence  in  his  work  that 
he  realized  the  fundamental  differences  between  genus  and  species ; 
furthermore,  he  had  the  keenness  to  group  together  both  related  plants 


ROLL  CALL  63 

and  animals.     Ray  also  advanced  the  idea  that  fossils  are  extinct 
species. 

Linnaeus  was  born  in  1707,  the  son  of  a  Swedish  clergyman.  Ho 
would  have  been  destined  to  become  a  cobbler  had  it  not  been  for  the 
influence  of  a  physician  who  recognizcnl  the  lad's  abilities.  To  make  a 
long  story  short,  he  finally  secured  his  medical  degree,  aided  in  no 
small  amount  by  the  contributions  of  his  fiancee,  and  eventually 
became  a  professor  of  natural  history  at  Upsala.  It  seems  that  Lin- 
naeus had  a  passion  for  natural  history  and  for  classifying  everything 
which  came  to  hand.  He  initiated  several  changes  in  the  study  of 
systematic  biology,  many  of  which  are  still  in  use  today. 

Binomial  Nomenclature 

The  most  important  contributions  of  Linnaeus  center  about  (1)  brief, 
clear,  and  concise  diagnoses ;  (2)  sharper  divisions  between  groups ; 
and  (3)  a  definite,  clear-cut  system  of  scientific  terminology,  known 
as  hinomial  nomenclature.  These  innovations  appeared  in  the  1753 
edition  of  Species  Plantarum  and  the  1758,  or  tenth,  edition  of  his  great 
work,  the  Systema  Naturae.  The  tenth  edition  of  this  latter  work  is 
taken  as  the  starting  point  of  zoological  nomenclature.  Linnaeus 
divided  the  plant  and  animal  kingdoms  into  Classes,  Orders,  Genera, 
and  Species.  This  was  a  great  step  over  the  use  of  popular  common 
descriptive  terms,  as  you  can  now  appreciate  if  you  refer  back  to  the 
example  of  the  pickerel.  However,  a  big  mistake  made  by  Linnaeus 
was  his  concept  of  fixity  of  species. 

In  1898  the  International  Congress  of  Zoology  appointed  an  inter- 
national commission  which  drew  up  a  set  of  rules  ajjplying  to  the 
divisions  of  the  animal  kingdom.  Thus  classification  today  is  really 
an  expansion  of  the  Linnaean  system  which  now  includes  in  the  case 
of  the  animal  kingdom,  for  example,  the  following : 

Animal  Kingdom  —  is  made  up  of 

Phyla  —  each  of  which  is  composed  of 
Classes  —  in  turn  made  up  of 
Orders  —  then 

Families  —  and  finally 
Genera  —  and 
Species. 

In  the  plant  kingdom  a  comparable  arrangement  is  utilized,  beginning 
with  Divisions  (=  phyla). 


66  NATURAL  HISTORY 

Law  of  Priority 

In  describing  species  it  sometimes  happened  that  more  than  one 
person  described  the  same  form,  giving  it  different  names.  In  such 
cases  the  name  assigned  by  the  one  who  first  described  it  is  used,  the 
second  being  considered  a  synonym.  This  is  the  reason  for  writing 
the  describer's  name  and  date  of  pubHcation  after  the  specific  name. 
Ordinarily  the  date  and  frequently  the  describer's  name  is  omitted. 
Thus  the  true  daisy  is  properly  Bellis  perennis,  Linn.  1758,  or  the 
English  sparrow,  Passer  domesticus,  Linn. 

What  Is  a  Species? 

We  have  taken  a  glimpse  at  the  contributions  of  some  of  the  con- 
temporaries and  near  contemporaries  of  Linnaeus  and  have  gained  a 
sHght  concept  of  the  problems  these  early  workers  faced  in  defining 
and  describing  a  species.  Biological  scientists  of  today  are  still 
working  on  this  problem.  The  principle  involved  is  readily  under- 
stood when  we  look  at  a  sheep,  a  cat,  and  a  dog.  One  can  easily  sepa- 
rate them  from  each  other,  various  cats  being  put  in  one  group  and 
diverse  dogs  in  another.  All  domestic  cats,  whether  they  be  the  alley 
variety  or  pet  Persians,  and  all  dogs,  whether  they  be  a  "dog  in  the 
manger"  or  "man's  best  friend,"  fall  into  well-marked  and  easily 
separable  groups,  known  as  species.  To  continue  further,  one  finds 
in  looking  over  representative  mammals  that  many  other  species  such 
as  the  jaguars,  ocelots,  jaguarundis,  and  cougars,  all  have  certain  char- 
acteristics in  common  with  our  domestic  cats.  These  characteristics 
are  size,  build,  shape  of  head,  nature  of  claws,  teeth,  and  fur.  The 
zoological  systematist,  therefore,  places  them  in  one  larger  group  or 
genus  which  is  called  Felis,  a  relationship  expressed  below. 


Kingdom.  Animal .  Plarzb 

'Phylum  -  Onovdcdxx .  Arthropod  a.  'Koiltx£r<i.a..  eh=>. 
Gla55  -  "rlocmiTacxIioD ,  Pisces  .Pept/ilia,  Aves.cstc-. 
Order"-  Carpi vonx .  'R'ocCe-aticc.Chiroptera,ete- 
Kimil/-  PeUcLcce^ .  <Zo.n idoe  ."LCr^icCoca.etc. 

Genzxs .  "feli^ .  Lumhricus,  eto. 

r 

-SpecieS-  domesticcc.leo.tigTjs.eu^. 

Tndi vidical .  lorn,  ,Dick  .Harry,  etc. 


ROLL  CALL  57 

However,  species  have  otlicr  characteristics  besides  oxtornal  or 
morphological  similarities.  They  breed  true,  that  is,  cats  produce 
cats,  and  dogs  produce  dogs.  Usually  diflferent  species  cannot  be 
crossed.  There  are  exceptions,  for  sometimes  one  species  crossed  with 
another  may  yield  a  sterile  hybrid.  Thus  a  horse  crossed  with  an  ass 
produces  a  mule.  But  on  the  whole  the  preceding  statement  holds 
true. 

Two  criteria  have  been  used  in  classifying  organisms,  first,  struc- 
tural differences,  or  appearance,  which  really  means  comparative  mor- 
phology,  checked  physiologically  and  genetically  by  the  cross-breeding 
of  species,  and  second,  the  approach  through  a  study  of  the  early 
development  and  the  life  cycle,  emhnjology,  and  the  distribution 
of  the  organism,  ecology.  The  latter  leads  to  a  consideration  of 
varieties,  subspecies,  and  races,  which  through  mtergradations  often 
complicate  the  problem  of  determining  species. 

Such  a  study  may  be  made  either  more  complicated  or  facilitated 
according  to  whether  a  so-called  natural  classification  or  artificial 
classification  is  utilized.  Thus  bats  and  birds  might  be  artificially 
classified  together,  simply  because  they  both  fly,  just  as  whales  and 
fishes  are  placed  together  by  the  ignorant,  because  both  inhabit 
the  water.  A  careful  study  of  the  anatomy  and  development  of  these 
animals  would  indicate  that  if  one  is  trying  to  show  relationships, 
which  is  what  a  classification  should  do,  bats  and  whales  would  both 
have  to  be  put  in  the  mammalian  group. 

Determination  of  the  type  of  symmetry  present  is  useful  in  clas- 
sification. Some  organisms  possess  a  universal  symmetry,  as  the 
protozoan  Volvox.  In  such  cases  the  organism  is  divided  into  equal 
halves  by  any  plane  that  passes  through  the  center.  Starfish  and 
hydra,  on  the  other  hand,  are  well-known  examples  of  radial  sytn- 
metry.  In  such  forms  there  is  a  single  axis,  as  may  be  seen  in  a 
cylinder,  and  a  number  of  planes  through  such  an  axis  would  di\-id(^ 
the  organism  into  symmetrical  halves.  Most  of  the  more  highly 
developed  forms  possess  bilateral  symmetry,  which  is  characterized 
by  similar  halves  on  either  side  of  a  main  axis.  Other  secondarv' 
planes  occur  in  bilaterally  symmetrical  animals,  resulting  in  anterior- 
posterior,  and  in  dorso-ventral  differentiation.  Sometimes  segmenta- 
tion, or  metamerism,  is  apparent,  as  in  the  case  of  the  earthworm  and 
many  of  the  Arthropods. 

If  one  attempts  a  classification  that  is  based  primarily  upon  struc- 
ture, it  is  necessary  to  differentiate  between  homology  and  analogy. 


68  NATURAL  HISTORY 

The  former  refers  to  similarity  of  structure  and  the  latter  to  similarity 
of  function.  Thus  the  f orelimbs  of  a  bat,  bird,  cat,  and  turtle  are  ail 
homologous,  while  the  wings  of  a  bat  or  a  bird  are  analogous  to  the 
wings  of  a  butterfly,  but  they  are  not  homologous  since  they  differ  in 
structure. 

A  Classification  of  Plants  and  Animals 

As  stated  earlier,  the  appended  scheme  of  classification  is  simply  a 
tool  to  be  used  by  the  student.  Remember  that  a  scheme  of  classi- 
fication is  not  only  the  "who's  who"  of  the  plant  and  animal  world 
but  it  shows  relationships  as  well,  indicating  what  we  know  at  present 
in  this  field.  Classification  involves  a  knowledge  of  the  occurrence, 
distribution,  development,  and  structure  of  the  form  studied,  and 
so  is  much  more  than  simply  applying  a  scientific  name  to  an  ani- 
mal or  plant.  The  use  of  scientific  names  cannot  readily  be  avoided, 
as  will  be  realized  from  a  study  of  these  pages.  If  one  really  desires 
to  excel  in  biological  work,  he  must  set  out  cheerfully  and  with 
determination  to  acquire  an  understanding  of  the  use  of  these  tools  as 
an  indispensable  aid  to  a  comprehension  of  the  interrelationship  of 
organisms  to  one  another. 

In  the  first  place  it  is  hoped  that  diagrams  which  accompany  this 
classification  are  detailed  enough  to  give  the  student  some  concept  of 
the  more  common  or  important  kinds  of  representative  organisms 
occurring  in  each  group.  It  is  unfortunately  impossible  in  these 
drawings  to  represent  the  different  animals  according  to  scale.  The 
student  hardly  needs,  however,  to  be  reminded  that  whales  and 
protozoans  should  be  interpreted  as  decidedly  different  in  size.  In 
most  cases,  the  classification  will  be  carried  only  as  far  as  the  class, 
although  in  a  few  groups,  as  with  the  Arthropoda  and  Tetrapoda, 
it  is  necessary  to  go  to  the  orders.  In  some  instances  attempts  have 
been  made  to  simplify  the  classification  in  order  to  avoid  unnecessary 
scientific  terminology.  It  should  be  added  that  the  classification 
here  presented  is  only  one  of  many  that  may  be  encountered  in  various 
books,  differing  in  details  but  agreeing  in  essential  particulars. 

It  is  impossible  to  designate  readily  all  of  the  characteristics  which 
are  utilized  in  the  separation  of  the  larger  plant  and  animal  groups. 
However,  it  is  of  importance  to  know  (1)  whether  we  are  dealing  with 
a  one-  or  many-celled  form  (uni-  or  multi-cellular) ;  (2)  the  number 
of  germ  layers  present  in  the  organism,  diplohlastic  — •  two  (ecto- 
derm and  endoderm) ;  triplohlastic  —  three  (ectoderm,  endoderm,  and 


ROLL  CALL 


m 


mesoderm) ;  (3)  the  nature  of  the  body  —  usually  divisible  into  tubes 
within  tubes  or  sacs ;  (4)  the  symmetry  —  radial  or  bilateral ;  (5)  the 
nature  of  the  appendages  —  if  present,  whether  jointed  or  non-jointed, 
paired  or  unpaired ;  (6)  whether  the  organism  is  segmented  or  non- 
segmented  ;  (7)  which  organ  systems  or  organs  are  present,  in  what 
form  they  occur  and  how  they  function ;  (8)  type  of  skeleton,  ab- 
sent, exo-  or  endoskeleton ;  (9)  the  presence  or  absence  of  a  noto- 
chord  ;  (10)  the  presence  or  absence  of  special  organs  ;  (11)  the  type 
of  tissues  present,  as  bark,  phloem,  muscular,  or  circulatory. 

Inasmuch  as  some  of  these  and  other  terms  appearing  in  the  scheme 
of  classification  are  new,  a  short  glossary  is  included.  This  is  designed 
to  elucidate  terms  used  in  the  appended  classification.  Other  words 
are  defined  as  they  are  first  used  in  the  text  and  may  be  found  by 
reference  to  the  index. 

CLASSIFICATION  OF  THE   PLANT   KINGDOM 

(mainly  after  Sinnott) 

All  members  of  the  plant  kingdom  are  characteristically  ses.sile ;  typically 
possess  chlorophyll;  usually  take  food  in  inorganic  form;  cell  walls  of  cellulose 
or  hydrocarbon. 

DIVISION  I  —  THALLOPHYTA  —  Thallus  Plants  (algae,'  fungi,  bacteria). 
Chabacteristics  :    Small,  often  minute,  little  differentiated  plants  some- 
times possessing  chlorophyll;    se.x  organs,  when  present,   typically  one 
celled ;  spore-bearing  organs  are  single  celled ;  80,000  species. 

Subdivision  A  —  Algae  —  Composed  mostly  of  blue-green,  green,  brown,  or  red 

algae. 
Characteristics  :    Chlorophyll  frequently  associated  with  other  pigments ; 
manufactures  own  food. 

Class  I  —  Cyanophyceae  —  Blue-green  algae  {Gloeocapsa,  Nostoc,  Oscilla- 
tor ia). 
Characteristics  :  Simplest  and  lowliest  of  green  plants ;  body  consists  of  a 
single  cell  with  nucleus ;  sap  cavity  and  chloroplasts  absent ;  often  tend- 
ing to  adhere  in  colonies;  usually  in  threadlike  rows  ( filaments);  cyto- 
plasm homogeneous,  pigment  evenly  dispersed,  or  a  colored  outer  and  a 
colorless  inner  zone  may  be  distinguishable ;  blue-green  color  probably  due 
to  chlorophyll  mixed  with  blue  pigment  (phycocyanin) . 

Class  II  -  Chlorophyceae  -  Green  algae  (Cartena,  Ulva,  Ulothrix,  Oedogo- 
nium,  Vaucheria,  Spirogyra). 


1  Genera  in  boldface  type  indicates  that  the  form  is  illustrated  in  this  unit. 


70 


NATURAL  HISTORY 


Class  I 

Cyanophyacaofi/ 

blua-ghsen  olgcus 


5UBOIV(5ION  A 

ALGAE 


l.OsciUatoi-ioc 


1.     d  ^-^^  —  ^v. 

LaTY^inarig  Txjccus     wiopfccryx 


Class  12" 

Vhaeaphyceoiz 

brov/n  algae 


Class  3Zr 

Kicxtomoceae/ 

cCicctoms 


S.FragillariCL 

2'Pe.nticula 


Cl?ciropby<2<2a.<s 

sborje/Nx/'ocr-t/S 


CYiar-a 


2- 


Staphylo  Coitus 


Olcxss  "SC 

"RbocCop^^yeeoa 


THALLOPMYTA 


ovass  I. 
bojcte-r-ict 


Class  is: 
'Phycomycetes 

ol^-like  fitnga. 


astreptococcTxs  4.E»cu:ilIuS  5.Spirillurai.$cipi'o1egmgt^^i:ggi3^ 


Class  IT 
SacAhca-omyceLejs 

yeast/S  


*spor<3 


z^;' 

//^y 


2.  Cbmatr  ic"ha 
l.Kemitrichia  s.Trichompboro 


Class  HE. 
slime    "Yucn^i 


,  2, 


-1.  ., 
£>coa5Cir^ 


Class  "C. 
Ascomycetes 

gac  ■{^UT7^i 


wheat 


2 
smuts  ^"puff  balls 


SUBDIVISION  B 
FU-NGI 


3ctsicCiomycetes 
smutts  ondrtxstls 


ROLL  CALL  71 

Chakacteristics  :  Chlorophyll  associated  with  carotin  and  xantlKjphyll; 
marine  or  fresh  water  organisms,  or  inhabitants  of  moist  hind;  nucleus 
and  one  or  more  chloroplasts  present;  starch  synthesizeil  in  pi/rentmls; 
plant  composed  of  single  cells,  colony,  filament,  or  plate  of  cells;  most 
species  produce  motile  i-eproductive  cells  (zoospores) ;  botli  equal  {iso-)  and 
different  sized  (hetero-)  gametes  present. 

Class  III  —  Charophyceae  —  Stonewarts  {Chara  and  Nitella). 

Characteristics:  Vegetative  body  consisting  of  long,  jointed  stems  with 
whorls  of  short  branches  arising  at  joints  {nodes) ;  asexual  spores  absent  ; 
more  complicated  antheridia  and  oogonia  than  found  in  Thallophytes  borne 
along  branches. 

Class  IV  —  Phaeophyceae  —  Brown  algae,  kelps,  rockweeds,  sargassum  {Lami- 
naria,  Fucus,  Ulopteryx). 
Characteristics:   Multicellulate;  exclusively  marine ;  brown  color  (due  to 
one  or  more  brown  pigments  associated  with  chlorophyll) ;  normally  found 
in  intertidal  zone. 

Class  V  —  Diatomaceae  —  Diatoms  (Meridion,  Diatoma,  Denticula  Fragillaria). 
Characteristics  :    Large  group  of  unicellular  algae ;    related  in  color  to 
brown  algae ;  common  as  plankton  organisms  in  both  fresh  and  salt  water ; 
siliceous  walls. 

Class  VI  —  Rhodophyceae  —  Red  algae  {Nemalion,  Polysiphonia,  Phyllophora, 
Corallopsis) . 
Characteristics  :  Mostly  marine ;  characteristically  reddish  in  color ; 
branched,  vegetative  body  filamentous  and  delicate;  grow  entirely  sub- 
mersed; cell  wall  often  thick,  gelatinous;  color  due  to  pigment,  phyco- 
erythrin ;  no  motile  cells ;  sexual  reproduction  highly  specialized. 

Subdivision  B  —  Fungi  —  Fungi,  bacteria,  and  molds. 

Characteristics  :   Chlorophyll  lacking;  exist  as  parasites  or  saprophj-tes. 

Class  I  —  Schizomycetes  —  Bacteria  (Diplococcus,  Staphylococcus,  Streptococ- 
cus, Bacillus,  Bacterium,  Spirillum). 
Characteristics  :   Unicellular  plants,  usually  without  pigment,  dividing  in 
one,  two,  or  three  planes;    apparently  structureless,  but  probably  con- 
taining a  diffuse  nucleus. 

Class  II  —  Saccharomycetes  —  Yeasts  (Saccharoimjces). 

Characteristics:    Sometimes   regarded  as   reduced  Ascomycetes;    single 
cells  with  definite  nucleus ;    cytoplasm  and  sap  cavity ;    buds  a.sexua!ly ; 
under  unfavorable  conditions  forms  four  spores,  in  a  modified  ascus. 
Class  III  —  Myxomycetes  —  Slime  fungi,  slime  molds   (Hemitrichia,   Coma- 
tricha,  Trichamphora) . 
Characteristics:    Border-Hne  plants;    spores  borne  by   fruiting  bodies, 
germinating  into  small,  naked  mass  of  protoplasm  without  a  wall ;    indi- 
vidual cells  fuse,  forming  a  Plasmodium. 
Class  IV  —  Phycomycetes  —  Algalike  fungi,  molds,  and  bliglits  {Saprolegnia, 
Mucor). 

H.  w.  h.  —  6 


72 


NATURAL  HISTORY 


Hcpa'ticoce 

1 L  vei^-NVortS 


BRYOPHyfA 

,  iverwor ts ,  mosses 


r^^^-Sporxs  capsule 


CD  6'phcc^nixxn 
peoct  moss 


•e^C 


Ccctbcarinia 

acomrrion  moss 


arche^nium   Qnt"hei4diuTn 
of    corartiorL    moss 


Olo-SS  IC 


ROLL  CALL 


Characteristics  :  Resemble  algae ;  plant  body  consists  of  filaments  {hyphae) 
which  are  not  divided  into  cells  by  cross  walls ;  multinucleate. 

Class  V  —  Ascomycetes  —  Sac  fungi  {Morchella,  Exoascus,  Microsphaera). 
Characteristics:  Includes  over  20,000  species,  mostly  saprophytes  or 
parasites;  body  consists  of  branching  mycdium  throughout  substratum 
and  a  definite  fruiting  body  at  surface ;  produce  spore  sacs  {asci)  contain- 
ing eight  spores  (ascospores) ;  group  of  asci  embedded  in  sterile  hyphae 
may  or  may  not  be  surrounded  by  protective  envelope. 

Class  VI  —  Basidiomycetes  —  Basidia  fungi,  smuts  and  rusts,  wheat  rust 
(Puccinia),  puff  balls. 
Characteristics:  Large  and  varied  group;  specialized  reproductive  struc- 
ture (basidium)  is  swollen  terminal  cell  of  hypha,  in  mushrooms  the  ija- 
sidium  usually  bears  four  basidios pores,  each  carried  on  a  delicate  stalk 
{sterigma) ;  sexual  reproduction  rare;  lichens  —  composite  plants  in  which 
algal  cells  are  entangled  in  mycelium.  Usually  regarded  as  a  parasitism 
of  algal  member  rather  than  an  example  of  symbiosis. 

DIVISION  II  —  BRYOPHYTA  —  Liverworts  and  Mosses. 

Characteristics  :  Alternation  of  generations  in  which  sexual  (gametophyfic) 
stage  dominates;  asexual  (sporophyiic)  stage  typically  parasitic  upon 
the  gametophyte ;  archegonium  and  nmlticcUulate  antheridium  pre.sent ; 
gametophyte  contains  x  number  of  chromosomes  while  the  2  x  number 
occurs  in  the  sporophyte;  careful  study  of  archegonium  reveals  typical 
flask  shape,  with  sterile  cells  (neck  and  venter)  surrounding  the  egg  and 
associated  cells ;  antheridium  more  or  less  stalked  and  consisting  of  layer 
of  jacket  cells  surrounding  cuboidal  sperm  mother  cells. 

Class  I  —  Hepaticae  —  Liverworts  (Marchantia,  Riccia). 

Characteristics:  Intermediate  between  green  algae  and  higher  plants; 
thaUus  flattened  and  attached  to  soil  by  rhizoids;  growth  by  repeated 
division  of  single  large  apical  cell. 

Class  II  —  Musci  —  Mosses  (Sphagnum,  Polytrichum,  Catherinia). 

Characteristics:  In  every  habitat  except  salt  water;  very  common  in 
alpine  and  arctic  regions;  gametophyte  erect,  consisting  of  stalk  with 
spirally  arranged  leaves ;  attachment  by  rhizoids. 


74 


NATURAL  HISTORY 


Subdivision  A 
pi"!  mi  live, 
vasculctr  plants 


WTJVjynia 
Devonian  plant  ^ 

RsilophyCalcs  ^-"^ 
-.i   <'..^  .(2)p5notum 


(3)Tmesipteris 


$UBDIVI5I0M  B 
Lycopsicta 

club  mosses 


sporopVr/te    y,- 


Lycopodium. 


:5UBD  I  VISION  C 
ophejiopsido. 


/ 


TI?ACH£OPHYTA 

vascular'  plants 


Subdivision!) 
Pberopsida 

ferns .  seed  plants, 


polkw 


<j'gametopViyt<2 


tipof 

pollen 

tube 


^''\  cells  ' 


txtbe 


@,^/C<3ll 

^mtoikr 

>>^    cell      i 


eicxssi 

Filicineae 


arc 


Gymr205perma<2 


dicotyledon  ii20i20Ccit/kfGn 


AndioSpermae 

oclVc,  iTioLple .  elm ,  ccc . 


ROLL  CALL 

<■> 

DIVISION  III  -  TRACHEOPHYTA  -  Vascular  Plants. 

Chakacteristics  :  Fibro- vascular  system  for  transportati..,i  „f  raw  mate- 
rials up  and  food  down;  separation  of  specialized  cl.l..njphyll-lK.ari..K 
tissue ;  adaptation  to  absorption  of  water  from  soil. 

Subdivision  A  —  Primitive  Vascular  Plants  —  {Psilotum  and  Tmesipteris,  Rhynia). 
Characteristics  :   Fossil  primitive  vascular  plants  giving  rise  in  tliree  lines 
to  Lycopsida,  Sphenopsida,  and  Pteropsida. 

Subdivision  B  —  Lycopsida  —  Club  mosses,  ground  pines  (Lycopodium,  Selagi- 
nella). 

Characteristics:  Stem  clothed  with  small,  numerous,  spirally  arranged 
leaves;  sporangia  borne  on  upper  surface  of  spowphyll;  latter  usually 
grouped  into  terminal  cones. 

Subdivision  C  —  Sphenopsida  —  Horsetails  (Equisetum). 

Characteristics  :  Hollow,  typically  jointed  stems,  bearing  small  leaves  at 
joints  (nodes);  stems  ribbed;  diaphragms  often  across  stem  at  nodes; 
sporangia  borne  in  groups  on  stalked  shield-shaped  structures  forming 
terminal  cones ;  ribs  opposite  fibro-vascular  bundles  which  are  associated 
with  small  air-filled  canal;  abundant  in  Paleozoic  age;  now  only  about 
35  species. 

Subdivision  D  —  Pteropsida  —  Ferns  and  seed-bearing  plants. 

Characteristics:  Typically  large  leaves;  sporophytic  generation  domi- 
nates ;  sporangia  relatively  large. 

Class  I  —  Filicineae  —  Ferns. 

Characteristics  :  Small,  herbaceous  plants  with  typical  pinnately  com- 
pound leaves  (fronds) ;  stem  relatively  weak  and  inconspicuous ;  roots 
numerous  but  do  not  form  an  extensive  system;  small  sporangia  borne 
on  lower  surface  of  leaf  in  groups  usually  protected  by  membrane  (iiulu- 
simu) ;  spore  germinates,  forming  small,  thin  gametophyte  (prothallus), 
which  in  turn  bears  antheridial  and  archegonial  structures.  About  15,000 
species,  some  of  which  reach  a  height  of  30  feet.  From  forms  like  the 
ferns  evolved  the  higher  vascular  plants  whic-h  dominate  the  earth's 
surface  today. 

Class  II  —  Gymnosperil\e  —  Evergreens,  pines,  hemlocks,  spruces,  junipers. 
Characteristics:  Seeds  freely  exposed  to  air;  usually  nondeciduous 
types;  megaspore  retained  within  megasporangium  where  it  germinates 
producing  female  gametophyte;  integument,  a  new  structure,  enclo.ses  a 
sporangium  and  embryo  sac;  reduced  male  gametophyte  transferred 
directly  to  vicinity  of  female ;  male  obtains  access  to  female  gametophyte 
by  new  structure  (pollen  tube);  young  sporophyte  develops  in  contact 
with  and  at  expense  of  parental  sporophyte;  gametophyte  with  haploid 
(x)  number  of  chromosomes  entirely  parasitic  upon  sporophyte.  Mem- 
bers of  this  group  are  phylogenetically  ancient;  only  about  450  living 
species. 

Class  III  —  Angiospermae  —  Deciduous  trees  and  plants.  Dicotyledons,  oak, 
maple,  beech ;  Monocotyledons,  corn. 


76 


NATURAL  HISTORY 


Class  I 

SarcocCina 


(2) 
ATCella 


C3)  "'-J 
"Radiolarla 


Clctss  IE 
Kastigbphorec 


PROTOZOA 

one    cellecC  animals 


c'C®)  \^ 


m^ 


•«oY^ 


^^-^  ctsexuctl     \^^9© 
Cycle  in.      / 

1 

mosquito 


Bisali- 


PlasmocCium. 


Class  JSL 

5porq3oa 


Vorticella 


(2)  ^^ 

Stentor^ 

(3) 


5tyionych  i  cc 


Class  IST 

ly^ftcsoricc 


ROLL   CALL 


77 


Characteristics:  Seeds  enclosed  by  a  case  (ovary),  so  that  pollen  Rrain 
does  not  reach  the  ovule  but  rests  on  surface  of  carpel ;  closure  to  form 
case  probably  arose  by  folding  together  of  edges  of  megasporophyll  {carpd) ; 
pollen  received  on  special  organ  (stigma)  at  tip  of  ovary.  Members  of 
this  group  probably  were  derived  from  gymnosperm  stock ;  now  number 
135,000  species  and  are  subdivided  into  dicotyledons  and  monocotyledons 
which  may  be  separated  by  the  following  ciiaracteristics : 


Dicotyledons 

Monocotyledons 

Number  of  cotyledons 
of  embryo       .     .     . 

two 

one 

Vascular  bundles    .     . 

arrange  to  form  vas-cylinder 
enclosing  pith 

scattered 

Leaves      

open  venation,   veinlets  end- 
ing  freely   in   margin,    which 
is  often  toothed  or  lobed 

closed  venation   (i.e.  parallel) 
margin  therefore  entire 

Flowers 

in  sets  of  four  or  five 

in  sets  of  three 

CLASSIFICATION  OF  THE   ANIMAL  KINGDOM 

(mainly  after  Hegner) 

All  members  of  the  animal  kingdom  are  characteristically  free-moving  organ- 
isms; generally  capable  of  assimilating  organic  foods;  rarely  possessing  chloro- 
phyll ;   cell  membranes  composed  of  protoplasm  or  proteins. 

PHYLUM  I  —  PROTOZOA  —  One-celled  animals. 

Characteristics  :    Single  cells  or  colonies  of  loosely  aggregated  unspecial- 
ized  cells ;  rarely  differentiated  into  germ  cells ;  8500  species. 

Class  I  —  Sarcodixa  —  Naked  protozoa  (Ameba,^  Arcella,  Radiolaria). 
Characteristics  :  Locomotion  by  means  of  pseudopodia. 

Class  II  —  Mastigophora  —  Flagellate  protozoa  (Euglena,  Trypanosoma). 
Characteristics  :  Locomotion  by  means  of  flagella. 

Class  III  —  Sporozoa  —  Parasitic  protozoa  (Lankesieria,  Myxosporidia,  Plas- 
modium). 
Characteristics  :  Xo  organs  of  locomotion  in  adults ;  endo-parasites  repnn 
ducing  by  schizogony  and  spore  formation. 

Class    IV  —  Infusoria  —  Ciliate    protozoa    (Vorticella,    Stentor,   Stylorjychia, 
Paramecium). 
Characteristics  :  Locomotion  by  means  of  cilia. 


1  See  footnote  at  beginning  of  classification  of  Plant  Kingiioni. 


78 


NATURAL  HISTORY 


Class  T 

Calcarea 


KexactiY^ell  ioCa 


Grantia 


(1^ 
Euplectella 


PORIFERA 


sponges 


(D 


@ 


Spongilla 

fresh- wcxter-Spon^e 


ELc5pong"ia 


ROLL  GALL  79 


PHYLUM  II  —  PORIFERA  —  Sponges. 

Characteristics  :  Usually  considered  as  diploblastic  animals ;  body  con- 
sists of  a  perforated  (inhalent  pores)  cylinder,  leading  to  central  canal 
opening  to  outside  through  exhalent  pore;  [peculiar  flagellate,  collared 
cells  (choanocyfes)  typically  present;  body  structure  frequently  compli- 
cated by  budding ;  2500  species. 

Class  I  —  Calcarea  —  (Grantia). 

Characteristics  :  Small  marine  sponges  possessing  one-,  two-,  or  four-rayed 
calcareous  spicules. 

Class  II  —  Hexactinellida  —  Deep-sea  sponges  (Euplectella). 
Characteristics  :  Sponges  with  six-rayed  siliceous  spicules. 

Class  III  —  Desmospongia  —  Finger  sponge,  bath  sponge  (Chalina,  Spongilla, 
Euspongia) . 
Characteristics:    Diverse  groups  of  sponges  possessing  spicules  of  silicon, 
not  six-rayed,  with  spongin,  or  a  combination  of  spicules  and  spongin. 


80 


NATURAL  HISTORY 


Olass  I 

H/cCro3oa; 


(1^ 
Otoe-lia 


C3) 


PhyBcclicc 
Portuguese    mar?- of -^/ar 


COELENTERATA 


(IV 

Aurelia 


Class     IE 

5c/pbo3)Oa 


Secc  anerrzor^e 


Astra  n^ioc 


Class  HE 
Antbo^oa 


ROLL  CALL 


ni 


PHYLUM  III  —  COELENTERATA  —  Jellyfishes  and  corals. 

Characteristics:  Mostly  marine;  radially  syinniotrioal ;  diploblastic- ani- 
mals with  a  noncellular  layer  of  niesoglea  lying  between;  po.ssL's.sing 
tentacles,  armed  with  nematocysts;  body  composed  of  a  single  gastro- 
vascular  cavity ;  4500  species. 

Class  I  —  Hydrozoa  —  Fresh-water  polyps,  jellyfishes,  and  a  few  stony  corals 
{Hydra,  Obelia,  Physalia). 
Characteristics  :  Mostly  marine ;  usually  hydroid  and  jellyfish  forms 
occur  in  the  same  life  cycle;  the  jellyfish  (medusae)  po.ssess  a  shelflike 
velum  extending  inward  from  the  margin  toward  the  mouth  (manubrium) ; 
a  few  species  like  Hydra  possess  no  medusoid  stage;  the  stony  coral, 
Millepora,  represents  a  colony  with  a  coral-like  skeleton  of  calcium  car- 
bonate. 

Cl.\ss  II  —  Scyphozoa  —  (Amelia). 

Characteristics:  Entirely  marine,  with  the  medusoid  stage  dominating; 
produced  from  subordinate  polyp  by  terminal  budding  (strobilalion) ; 
velum  usually  absent;  lobate,  typically  eight-notched. 

Class  III  —  Anthozoa  —  Sea-anemones,  sea-pens,  and  stony  corals  (Metridium, 
Pennatula,  Astrangia,  Sagartia). 
Characteristics  :  Entirely  marine  with  medusoid  stage  suppres.sed ;  organ- 
isms characterized  by  an  introverted  ectodermal  mouth  (sto7nodaeum)  anti 
vertical  radiating  mesenteries  extending  inward  from  the  body  wall;  one, 
two,  or  more  rarely  three  cihated  gullet  grooves  (siphonoglijphs)  carry 
a  stream  of  oxygenated  water  to  interior.  Corals  produce  islands  and 
reefs;  in  addition  they  sometimes  protect  a  shore  from  wave  action. 


82 


NATUIIAL   HISTORY 


a) 


CTENOPHORA 


ytonna  iphorroc 

Comb   i<2-^Vy  "'^i^^'S^^w^'i^pM 


C2) 

Venas*  gxindLie 


ROLL  C^LL  8:{ 


PHYLUM  IV  —  CTEXOPHORA  —  Sea-walnuts  {Cestus,  Hormiphora,  Mnemi- 
opsis) . 
Characteristics  :    Eight  radially  arranged  rows  of  comhjlike  plates  typi- 
cally present;    fundamentally  bilaterally  syninictrical;    with  a  distinct 
mesodermal  layer  (therefore  triploblastic);   no  nematocysts :    100  species. 


84 


NATURAL  HISTORY 


Class  I 
Tarbellaria 


#'G>' 


■■^ 


m 


1. 

'Planoria  ^'  Microsbmum 


Clccss  IT 
TrematooCa 


twoflUKZf 

Yrom, 
Turtles 
mouth 


3 

'PnGUtTiono®<ie5 
frog  lun^  fluke/' 


PLATYnaKINTHES 


(a^Taa.nioe 


Cb  J^s  bicerC'Lcs 


uterus 


J?^^ 


:•••    Cv55 


ovary-  ^telloricx. 
2(aO  proglottv 


Class  HI 

CsstodUx 


Diphyllobothrium 

brocccC  tapeworm 

of  mctn_ 


3. 

cionorc^is 
liver  fluke 
of  -man. 


•prohoscis.. 

naphridia 

lo«^.  naPVB J 

ovary - 


hrodn 
Jnoufh. 


Olsons 

ofcx 

inemertine 


Class  ISL 


ROLL  CALL  ^^. 


PHYLUM  V  —  PLATYHELMLXTHES  —  Flatworms. 

Characteristics  :  Dorso-ventrally  flattened,  soft  bodies,  bilaterally  sym- 
metrical, animals  lacking  true  segmentation  and  blood  vascular  system; 
no  anus;  excretory  system  of  flame-cell  type;  only  Class  I  free-living,  all 
others  parasitic ;  4600  species. 

Class    I  —  Turbellaria  —  Free-living    flatworms    {Planaria,    Microstomum, 
Bdelloura). 

Characteristics:  Typically  free-living,  possessing  a  ciliated  ectoderm; 
some  ectodermal  cells  secrete  mucus,  or  produce  rodlike  bodies  (rhnMite.s) ; 
classification  into  orders  depends  upon  nature  of  intestine. 

Cl.\ss  II  — Trematoda  —  Flukes  (Polystoma,  Pneumonoeces,  Clonorchis). 

Characteristics:  Parasitic  flatworms  with  non-ciliated  ectoderm  in  the 
adult,  possessing  one  or  more  suckers;  highly  specialized  for  parasitic 
existence;  many  are  internal  parasites  having  complicated  life  cycle, 
occupying  as  many  as  four  hosts  during  development ;  digestive  system 
present. 

Class  III  —  Cestoda  —  Tapeworms  {Taenia,  Diphyllobothrium). 

Characteristics  :  Members  of  this  group  are  completely  parasitic,  living 
as  adults  in  the  alimentary  canal  of  vertebrates;  digestive  tract  absent; 
body  typically  divided  into  a  chain  of  segments  (proglottids),  except  for 
Cestodaria,  budded  from  neck,  gradually  increasing  in  diameter  towards 
posterior  end;  the  head  (scolex)  typically  bearing  organs  of  adhesion  in 
the  form  of  hooks  and  suckers. 

Class  IV  —  Nemertinea  —  Nemertines  (Micrura,  Cephalothrix,  Cerebralidus). 
Characteristics  :  Members  of  this  gnnij)  because  of  uncertain  systematic 
position  not  always  placed  with  the  flatworms ;  characteristically  found  in 
moist  earth  or  fresh  water,  most  forms  being  marine;  characterized  by 
possessing  alimentary  canal  with  mouth  and  anus,  definite  blood-va.scular 
system,  and  a  long  proboscis  enclosed  in  a  proboscis  sheath. 


86 


NATURAL  HISTORY 


CAccss  I 

KematooCa 


^^^     ,  ■    '    ^"       (2)  (5)    ^ 

Trichi^Gllo:  spiralis  Trichuris  ovis  'NecatDr  onnericaTiiK 

■pork    TDundvorra      NK-'J^ip  '•v^orm        yiooy<:\i/or-m 


NmAtnELMINTHES 


•rotxncC-wox^ms 


;^'<^'t,C:iife^^i^^^^'- 


ClotSS  IE 

Gordiacsa 


Leptorty-nchoicLes  £hecatus 


Clots  s  HE 
Aj:iar2thoc<2.pbala 


ROLL  CALL  37 


PHYLUM  VI  —  NEMATHELMINTHES  —  Roundworms. 

Characteristics:  Bilaterally  symmetrical ;  cylindrical,  unsegmented,  long 
and  slender  worms ;  usually  a  distinct  alimentary  canal  with  mouth  and 
anus ;  primitive  body  cavity  present ;  papillae  or  spines  at  anterior  tip  of 
body. 

Class  I  —  Nematoda  —  Threadworms  (TrichineUa,  Trichuris,  Necator,  Oxyuris). 
Characteristics  :  Members  of  this  group  art;  l)oth  free-living  and  parasitic 
on  plants  and  animals;  mouth  usually  terminal  and  alimentary  canal 
composes  a  relatively  straight  tube  with  anal  opening  near  posterior  end 
of  body ;  body  cavity  not  lined  by  epithelium  but  bounded  directly  by 
muscles  of  the  body ;  four  thickenings  of  the  ectoderm,  one  dorsal,  one 
ventral,  and  two  lateral,  produce  ridges  containing  excretory  canals  and 
nervous  system ;  sexes  separate. 

Class  II  —  Gordiacea  —  Hairworms  {Gordius,  Paragordius). 

Characteristics  :  Long,  slender,  and  hairlike ;  free-living  adults  in  water ; 
larvae  usually  parasitize  aquatic  insect  larvae  (often  Mayflies) ;  asually 
reach  a  second  host,  as  beetle  or  grasshopper,  in  which  development  con- 
tinues ;  escape  to  water  made  by  breaking  through  body  wall ;  no  lateral 
lines  present;  body  cavity  hned  by  distinct  peritoneal  epithelium  derived 
from  mesoderm ;   eggs  discharged  into  body  cavity  instead  of  to  outside. 

Class  III  —  Acanthocephala  —  Spiny-headed  worms  (Leptorhynchoides,  Neo- 
echinorhynchus,  Macracanthorhynchus) . 
Characteristics:    Protrusible  proboscis  armed  with  hooks;    alimentary 
canal  absent;    reproductive  sy.stem  complex;    entirely  parasitic,   larval 
stage  in  Arthropods. 


h.  w.  h.  —  7 


88 


NATURAL  HISTUllY 


TROCHEUMINTHES 


(1^ 

'Philodina 
^iKSisa.  animalcule 


Clccss   I 

li^otifera 


(1) 
ChoQXandtus 


OlccSS  31 

Gastrotricboc 


ROLL  CALL 


W 


PHYLUM  VII  —  TROCHELMINTHES  —  Rotifers,  Gastrotricha. 

Characteristics  :  Small,  frequently  microscopic,  identifiable  by  cilia  around 
the  mouth  region  ;  about  1300  species. 

Class    I  —  Rotifera  —  Wheel    animalcules    (Philodina,    Notommata,    Trocho- 
sphaera). 
Characteristics  :  Mostly  free-living,  inhabiting  fresh  water ;  distinct  nerv- 
ous system ;   universally  characterized  by  presence  of  jaws  inside  pharynx 
(mastax) ;  usually  a  foot. 

Class  II  —  Gastrotricha  —  (Chaetonotus). 

Char.acteristics  :  Microscopic  organisms  reaching  maximum  length  of 
about  0.5  mm. ;  animal  divided  into  indistinct  head,  neck,  and  body  ;  oral 
bristles  on  side  of  head ;  often  a  forked  tail  containing  cement  glands ; 
locomotion  by  ciliary  bands  or  by  long  bristles. 


90 


NATURAL  HISTORY 


Class    I 
Br/ojoa 


'Pec:*tir?atella 
fresh -water  hr/oysan 


M0LLU5CpiDEA 

moss  aniYnals  ana.  lamp  svjetis 


ejdsrnol  viev 


(1)  lyTageWania 


Class  IE 

BrachiopocCa 


m 

Phoronie 


Class  IL 

Phor-onidea 


ROLL  CALL 


91 


PHYLUM  VIII  —  MOLLUSCOIDEA  —  Moss  animals  and  lamp-shells. 

Characteristics  :  Unsegmented,  sessile,  typicull}'  marine,  bilaterally  S3'm- 
metrical  animals  possessing  a  ridge  (lophophore)  bearing  ciliated  tentacles 
which  surrounds  the  mouth ;  5700  species,  including  fossils. 

Class  I  —  Bryozoa  —  Moss  animals  (Electro,  Pectinatella,  Iletniseptella,  Bugula, 
Plumatella). 
Characteristics:  Colonial,  sessile,  free-living  animals;  mostly  marine; 
lophophore  usually  horseshoe-shaped ;  alimentary  canal  L'-shaped ;  divi- 
sion into  subclasses  depends  upon  whether  anus  opens  within  or  without 
lophophore. 

Class  II  —  Brachiopoda  —  Lamp-shells  (Magellania). 

Characteristics  :  Marine  organisms  possessing  characteristic  lophophore ; 
body  covered  by  calcareous,  dorso-ventrally  arranged  bivalve  shell,  usu- 
ally attached  by  a  stalk  (peduncle). 

Class  III  —  Phoronidea  —  (Phoronis). 

Characteristics  :  Small,  marine,  sedentary  animals  living  in  tubes ;  unseg- 
mented adults  are  hermaphroditic,  possessing  a  body  cavity  as  well  as 
characteristic  horseshoe-shaped  lophophore;  two  excretory  organs  and  a 
vascular  system. 


92 


NATURAL  HISTORY 


Class    T 

Arc>2ia]f7r?elicCa 


Polygordlius 


cMass  "K 
Cl2aetopocCa 


Nsreis  Chaetoptert£5 

c\ccro..^»/'or-m.      tube  vorm.  (^3) 


ANNELIDA 

segmentecC    "^orms 


KirucCo 
•■medicinal    leech 


Class  HL 

Hirudinaa 


Lambricud 

Garthvorm  1 


fhaecolosoTQa 


arrow  vorm 


Class  sr 


Csrephyrea 


Cla&S"y 

Chaetognatha 


ROLL  CALL  y , 


PHYLUM  I X  —  ANNELIDA  —  Segmented  worms. 

Characteristics  :  Segmented  animals  bearing  distinct  head,  digestive  tube, 
coelom,  and  sometimes  nonjointed  appendages;  frequently  supplied  with 
chitinous  bristles  (setae) ;   6500  species. 

Class  I  —  Archiannelida  —  (Polygordius). 

Characteristics  :  Marine  worms  lacking  setae  or  parapodia ;  trochophore 
larvae  present. 

Class  II  —  Chaetopoda  —  Clam  worms,  tube  worms,  earthworms  (Nereis, 
Glycera,  Chaetopierus,  Lumbricus). 
Characteristics  :  Members  of  this  class  marine,  terrestrial,  or  fresh  water ; 
paired  setae  characteristically  arranged  in  integumentary  pits  or  upon 
parapodia ;  further  subdivision  based  upon  number  of  setae  present : 
Oligochaeta,  a  few ;   Polychaeta,  many. 

Class  III  —  Hirudinea  —  Leeches  (Hirudo,  Glossiphonia). 

Characteristics  :  Hermaphroditic,  dorso-ventrally  flattened  annelids  with 
32  body  segments,  two  suckers,  one  surrounding  mouth,  the  other  the 
posterior  end ;  setae  and  parapodia  absent ;  growth  of  mesenchyniatous 
cells  reduces  coelom. 

Class  IV  —  Gephyrea  —  Sipunculid  worms  (Phascolosoma). 

Characteristics  :  Non-segmented  when  adult,  without  setae  or  parapodia ; 
characterized  by  a  large  coelom  and  trochophore  larvae. 

Class  V  —  Chaetognatha  —  Arrow  worms  (Sagitta). 

Characteristics:  Small,  transparent,  marine  invertebrates  with  well- 
developed  body  cavity,  alimentary  canal,  nervous  system,  two  eyes; 
lobes  on  sides  of  mouth  armed  with  bristles  which  aid  in  capturing  food. 


94 


NATURAL  HISTORY 


Class  I 

Asteroidea 


Clccss  X 
Ophijiroidea 


OphioglypVja 
brittle -star- 


ECHINODERMATA 

starfishes,  etc 


m'w 


■'-i'lyfi"'-:.- 


K'#^-  Her) 

Arbcxoicx 

sect  urcHin 


i\» 


Thj/one. 
sea:  -  cucuiTzber 


EcVjinarachniuS 
sccncC  dCollocr 


Class  is: 

Holothuroidea 


Class  IE 

Echirzoidea 


l^er^tacrmus 


Class  ^ 

Crinoidea 


ROLL  CALL  95 


PHYLUM  X  —  ECHINODERMATA  —  Starfishes,  sea-urchins,  sea-curumbors. 

Characteristics  :    Adults  radially  symmetrical    (pentamerous) ;    marine ; 

tube-feet,  water  vascular  system,  distinct  alimentary  canal,   large  body 

cavity  usually  present ;   frequently  a  spiny  skeleton  of  calcareous  plates ; 

larvae  bilaterally  symmetrical ;  4800  species. 

Class  I  —  Asteroidea  —  Starfishes  (Asterias,  Mediaster). 

Characteristics  :  Typically  five  rays  or  arms  not  marked  off  from  central 
disk ;  each  ray  possessing  ventral  ambulacral  groove  through  which  numer- 
ous tube-feet  extend ;  gastric  pouches  and  hepatic  caeca  extend  into  rays ; 
blunt  spines  and  pedicellariae  present;   respiration  by  dermal  branchiae. 

Class  II  —  Ophiuroidea  —  Brittle-stars  (Ophiopholis,  Ophiothrix,  Ophioglypha, 
Ophioderma). 
Characteristics:    Typically  pentamerous  with  arms  sharply  marked  off 
from  disk ;  no  ambulacral  groove ;  hepatic  caeca  and  anal  opening  lacking. 

Class  III  —  Echinoidea  —  Sea-urchins,  sand-dollars,  spatangoids  (Arbacia, 
Strong ylocentrotus,  Echinarachnius,  Spatangus,  Moira). 
Characteristics  :  Typically  pentamerous  without  arms  or  free  rays ;  test 
of  calcareous  plates  bears  movable  spines;  i)ediceilariae  usually  three- 
jawed  ;  mouth  with  five  conspicuous  teeth  constituting  part  of  Aristotle's 
lantern. 

Class  IV  —  Holothuroidea  —  Sea-cucumbers   {Holothuria.    Thyone,    Leptosy- 
napta). 
Characteristics  :    Long,  ovoid,  soft-bodied  cchinoderms ;    tentacles  about 
mouth;   body  wall  muscular ;   skeleton  greatly  reduced. 

Class  V  —  Crinoidea  —  Sea-lilies  or  feather-stars  {Antcdon,  Halhromelra,  Co- 
rnadinia,  Pentacrinus). 
Characteristics  :  Usually  five  branched  arms,  possessing  featherlike  divi- 
sions (pinnules) ;  aboral  pole  sometimes  possessing  cirri  but  more  gener- 
ally a  stalk  for  temporary  or  permanent  attachment ;  a  few  modern  types, 
most  forms  known  as  fossils. 


96 


NATURAL  HISTORY 


Class  I 

AiTiphineura 


Class    I 
GastropocCa 


Class  HE 

ScaphopocCa 


1 5cb  r2och  itoio. 
chiton 


Helix 
Iccnd.  snccil 


marine 

Snail 


C5). 

Limccx 


tootlri  snocil 


MOLLUSC  A 

clams ,  Snccils,  etc 


soctllop 


Class  3Sr 

PslecypocCa 


ra^or-shell  Cicom 


(2) 

OCtopLCS 


Class  ^ 

Cephalopoda 


ROLL  CXLL  97 


PHYLUM   XI  —  MOLLUSCA  —  Snails,  clams,  and  oysters. 

Characteristics  :  Unsegmented,  bilatorally  synunotiical,  triijloblastic  ani- 
mals bearing  a  shell,  muscular  foot,  and  mantle;  four  main  pairs  of  nerv- 
ous ganglia ;  70,000  species. 

Class  I  —  Amphineura  —  Chitons  (Chaetopleura,  Ischnochiton) . 

Characteristics:  Bilaterally  symmetrical;  shell  typically  composed  of 
eight  transverse  calcareous  plates  with  many  pairs  of  gill  filaments. 

Class  II  —  Gastropoda  —  Snails,  slugs,  whelks  {Umax,  Physa,  Helix,  Lymnaea, 
Campelotna,  Busy  con). 
Characteristics  :    Asymmetrical  animals  with  well-developed  head ;    spi- 
rally-coiled shell. 

Class   III  —  Scaphopoda  —  Elephant's-tusk   shells    (Dentalium,  Siphonodenta- 
lium). 
Characteristics:   Both  shell  and  mantle  tubular;   protrusible  foot ;    rudi- 
mentary head. 

Class  IV  —  Pelecypoda  —  Clams,  mussels,  oysters,  and  scallops  {Ensis,  Ano- 
donta,   Venus,  Teredo,  Ostrea,  Pecten). 
Characteristics:     Usually   bivalved   shells   with    two-lobed   mantle;     no 
head ;   body  laterally  compressed ;   bilaterally  symmetrical. 

Class    V  —  Cephalopoda  —  Squids,    cuttlefishes,    octopus,    nautilus    (Loligo, 
Polypus,  Dosidieus). 
Characteristics:    Bilaterally  symmetrical;    with  foot  divided  into  siphon 
and  arms  provided  with  suckers;    well-developed  nervous  system  con- 
centrated in  head;  mouth  possesses  strong  jaws. 


/.<^ 


V 


98 


NATURAL  HISTORY 


ClctSS    I 

Cmstacea 


Olci-ss   -jn 
Oiiychopbortt 


4 


PecLiculus   /^TXi-jh 


Class  IS" 

liasecta 


"Po-pilio         . 


ciccss"sr 
Aractiiaoidea 


ROLL  CALL  ^,j 

PHYLUM     XII  -  ARTHROPODA  -  Lobsters,    crabs,    spider.,     millir>odes 
insects.  '       * ' 

Characteristics:  External  evidence  of  segmentation,  body  at  least  beine 
divisible  into  a  well-defined  head,  thorax,  and  abdomen;  jointed  append- 
ages ;  chitinous  exo-skeleton ;  nervous  system  of  ladder  f  vpo  witl,  tondcnry 
toward  concentration  in  head  region;  main  longitudinal  blood  vessel 
with  heart  dorsal  to  alimentary  canal;  coelom  reduced;  body  cavity 
filled  with  blood  (hemocele) ;  640,000  species. 

Class  I  —  Crustacea  —  Crayfish,  crabs,  water  fleas,  barnacles,  sowbugs  (Cam- 
barus,  Callinectes,  Gammarus,  Asellus,  Trior thrus). 
Characteristics:  Mostly  aquatic;  usually  bearing  gills;  with  two  pairs 
of  antennae  (feelers) ;  chitinous  exo-skeleton ;  body  divided  into  head, 
thorax,  and  abdomen ;  head  and  thorax  sometimes  fused  {cephalolhorax) ; 
further  subdivision  depending  largely  upon  characteristics  of  carapace. 

Class  II  —  Onychophora  —  Annelidlike  arthropods  (Peripatus). 

Characteristics:  Tropical,  primitive,  wormlike  tyi)os  j)resumably  inter- 
mediate between  the  segmented  worms  and  the  arthropods;  excretory 
system  of  annelid  type  (nephridial) ;  respiratory  organ  resembles  tracheae 
of  insect  group ;  external  appendages  ringed,  suggesting  segmentation  of 
arthropods. 

Class  III  —  Myriapoda  —  Centipedes  and  millipedes  {Scolopendra,  Spirobolus). 
Characteristics  :    Body   relatively  long  and   definitely   nietamcric ;    one 
pair  of  antennae ;    appendages  segmented  ;    legs  similar ;    respiratory  sys- 
tem of  tracheal  type ;    in  millipedes  there  are  two  pairs  of  legs  per  somit«, 
in  centipedes  one. 

Class  IV  —  Insecta  —  Insects,  as  butterflies,  grasshoppers,  beetles,  bees. 

Characteristics:    L^sually  possess  wings;   one  pair  of  antennae;   tracheal 
respiratory  system ;  segmented  legs. 
Order  1  —  Thysanura  —  Bristletails,  Silverfish  (Lepistna,  Campodea,  Thermobia). 
Characteristics  :  Wingless  arthropods ;  primitive ;  probably  derived  from 
wingless  ancestors ;    11  abdominal  segments;   chewing  mouth  parts;   usu- 
ally two  or  three  long,  threadlike,  segmented  caudal  appendages;    less 
than  20  species  in  the  United  States ;  no  metamorphosis. 
Order  2  —  Collembola  —  Springtails  (Archorules). 

Characteristics  :  Primitive  wingless  insects  with  chewing  or  sucking  mouth 
parts;  four  segmented  antennae;  usually  no  tracheae;  six  abdominal 
segments;  a  springing  organ  (furcida)  present  on  ventral  side  of  fourth 
abdominal  segment  in  most  species ;  no  metamorphosis. 
Order  3  —  Orthoptera  —  Grasshoppers,  cockroaches,  walking  sticks  {Melanoplus, 
Periplaneta,  Diapheromera). 
Characteristics:  Members  of  this  order  are  characterized  by  two  pairs 
of  wings  (sometimes  greatly  reduced) ;  the  fore  wings  usually  thickened. 
sometimes  leathery ;  hind  wings  folded  fanlike  beneath  fore  wings ;  biting 
mouth  parts ;  gradual  or  simple  metamorphosis. 


100  NATURAL  HISTORY 

Order  4  —  Isoptera  —  Termites  or  white  ants  {Reticulitermes) . 

Characteristics  :  Four  similar  wings  lying  flat  on  back  when  at  rest ; 
workers  are  wingless;  chewing  mouth  parts;  abdomen  joined  directly  to 
thorax ;  gradual  or  simple  metamorphosis. 

Orders  —  Neuroptera  —  Dobson  flies,  alder  flies,  lacewings,  ant-lions  {Corydalis, 
Chrysopa,  Myrmeleon). 
Characteristics  :  Four  membranous  wings  with  many  veins ;  chewing 
mouth  parts ;  larvae  carnivorous ;  tracheal  gills  usually  present  on  aquatic 
larvae;  the  larvae  of  the  horned  Corydalis  known  as  hellgrammites  are 
used  by  fishermen  as  bait ;   complete  metamorphosis. 

Order  6  —  Ephemerida  —  Mayflies  (Ephemera). 

Characteristics  :  Mouth  parts  of  adult  vestigial ;  two  pairs  of  membra- 
nous, more  or  less  triangular,  wings ;  fore  wings  larger  than  hind  wings ; 
caudal  filaments  and  cerci  very  long;  aquatic  larvae  breathe  by  tracheal 
gills,  usually  located  on  either  side  of  abdomen ;  adult's  span  of  life  short ; 
mouth  parts  poorly  developed,  probably  making  organism  incapable  of 
taking  food;  nymph  remains  one  to  three  years  in  water;  adults  moult 
within  24  hours  after  acquiring  wings,  therefore  called  sub-imagos ;  gradual 
or  simple  metamorphosis. 

Order  7  —  Odonata  —  Dragonflies  and  damsel  flies  (Macromia,  Agrion). 

Characteristics  :  Chewing  mouth  parts ;  two  pairs  of  membranous  veined 
wings;  characteristic  joint  (nodus)  on  anterior  margin  of  each  wing;  eyes 
large,  compound ;  nymphs  are  aquatic ;  gradual  or  simple  metamorphosis. 
When  at  rest  dragonflies  hold  their  wings  horizontally  and  at  right  angles 
to  body,  while  damsel  flies  maintain  theirs  vei-tically. 

Order  8  —  Plecoptera  —  Stone  flies  (Allocapnia,  Taeniopteryx). 

Characteristics  :  Chewing  mouth  parts  often  poorly  developed  in  adults ; 
two  pairs  of  wings;  hind  wings  usually  larger  and  folded  beneath  fore 
wings ;  nymphs  aquatic,  bearing  filamentous  tracheal  gills ;  usually  be- 
neath stones  in  flowing  water;  gradual  or  simple  metamorphosis.  The 
salmon  fly,  Taeniopteryx  pacifica,  is  a  dangerous  pest  in  the  State  of 
Washington  because  it  destroys  buds. 

Order  9  —  Corrodentia  —  Book-  and  bark-lice  (Trodes). 

Characteristics  :  Either  wingless,  or  two  pairs  of  membranous  wings  char- 
acterized by  a  few  prominent  veins;  fore  wings  larger  than  hind  wings; 
when  at  rest  held  over  body  like  sides  of  a  roof;  chewing  mouth  parts; 
gradual  metamorphosis.  Book-lice  often  eat  paper  and  bindings  of  old 
books. 

Order  10  —  Mallophaga  —  Chewing  lice  or  bird-lice  (Menopon,  Trichodectes). 

Characteristics  :  Chewing  mouth  parts ;  wings  absent ;  eyes  degenerate ; 
metamorphosis  gradual  or  wanting.  Members  of  this  group  are  ecto- 
parasitic  upon  hair  and  scales  of  birds  and  mammals. 

Order  11  —  Embiidina  —  Emhiids  {Emhia). 

Characteristics  :  Chewing  mouth  parts ;  wingless  or  possessing  two  pairs 
of  delicate  membranous  wings  with  few  veins ;  cerci  present  on  two  seg- 
ments ;  males  usually  winged,  females  wingless ;  gradual  metamorphosis. 
These  organisms  live  under  stones,  etc.,  in  tunnels  formed  of  silk  produced 
in  tarsal  glands. 


ROLL  CALL  101 

Order  12—  Thysanoptera  —  Thrips  (Thnps,  Franklinella,  Crypiolhnps). 

Characteristics:  Piercing  mouth  parts;  either  wingless  or  with  two  pairn 
of  long,  narrow  membranous  wings,  practically  veinless;  large,  free  pro- 
thorax;  feet  clawless  but  possessing  small  protrusible  membranous  sacs 
for  clinging;   manj^  parthenogenotic  ;   gradual  metamorphosis. 

Order  13  —  Anoplura  —  Sucking  lice  {Pediculus,  I'hthirins). 

Characteristics:  Wingless  ectoparasitic  lice  with  piercing  and  sucking 
mouth  parts;  eyes  poorly  developed  or  absent;  parasitic  on  bodies  of 
mammals ;  gradual  metamorphosis.  At  least  two  species,  the  head  louse 
and  crab  louse,  occur  on  man. 

Order  14  — Hemiptera  —  True  bugs  {Artocorixa,  Lethocercus). 

Characteristics  :  Either  wingless,  or  with  two  })airs  of  wings ;  in  such  cases 
fore  wings  are  thickened  at  base ;  mouth  parts  adapted  for  piercing  and 
sucking;  gradual  or  simple  metamorphosis.  Members  of  this  group  con- 
tain many  interesting  and  sometimes  economically  important  forms.  The 
water-boatmen  (Corixidae)  have  long,  flat,  fringed  metathoracic  legs  which 
are  adapted  for  swimming.  These  peculiar  forms  carry  a  film  of  air  about 
body  when  under  w^ater.  The  leaf  bugs  (Xeridae)  are  frequently  numer- 
ous and  injurious  to  plants.  Bedbugs  (Cimicidae)  have  been  accused  of 
transmitting  various  diseases.  The  cabbage  bug  does  damage  to  garden 
vegetables. 

Order  15  —  Homoptera  —  Cicadas,  aphids,  leaf-hoppers,  and  scales  {Euscclis, 
Empoasca,  Rhopalosiphum) . 
Characteristics  :  Mouth  parts  adapted  for  piercing  and  sucking ;  two  pairs 
of  wings  of  uniform  thickness  held  over  back  like  sides  of  a  roof.  The  cica- 
das (Cicadidae)  are  better  known  as  the  "seventeen-year  locust."  Plant- 
lice  (Aphididae)  are  mostly  small  green  insects  that  suck  juices  from 
plants  and  have  a  gradual  metamorphosis. 

Order  16  —  Dermaptera  —  Earwigs  (Anisolabis,  Labia). 

Characteristics  :  Either  wingless,  or  possessing  one  or  two  pairs  of  wings ; 
in  such  cases  fore  wings  are  small  and  leathery,  meeting  in  straight  line 
along  back;  chewing  mouth  parts ;  gradual  metamorphosis.  Earwigs  are 
nocturnal  and  feed  principally  upon  vegetation. 

Order  17 — Coleoptera  —  Beetles  and  weevils  {Hydrous,  Dytiscits,  Photinus, 
Anthonomus). 
Characteristics  :  Either  wingless  or  with  two  pairs  of  wings,  fore  wings 
being  hard  and  sheathlike  {elytra);  hind  wings  membranous  and  are 
folded  two  ways  under  elytra;  large  movable  prothorax;  chewing  mouth 
parts;  complete  metamorphosis.  Many  forms  are  found  in  this  group. 
as  the  tiger  beetles,  fireflies,  click  beetles,  whirligig,  ladybird,  and  leaf 
beetles. 

Order  18  —  Strepsiptera  —  Stylopeds  {Xenos). 

Characteristics:  Mouth  parts  reduced  or  wanting;  nutrition  by  absorp- 
tion; males  possessing  club-shaped  fore  wings  and  large  membranous 
hind  wings ;  females  wingless  and  legless ;  life  cycle  complex ;  para.sitic  on 
bees,  wasps,  and  homopterous  bugs. 


102  NATURAL  HISTORY 

Order  19  —  Mecoptera  —  Scorpion-flies  {Panorpa,  Bittacus). 

Characteristics  :  Members  of  this  group  are  wingless  or  characterized  by 
two  pairs  of  long  membranous  wings  containing  many  veins;  head  pro- 
longed into  beak;  antennae  long  and  slender;  mouth  parts  adapted  for 
chewing ;  males  with  olasping-organ  on  caudal  extremity  resembling  sting 
of  a  scorpion  ;   metamorphosis  complete. 

Order  20  —  Trichoptera  —  Caddis  flies  {Phryganea,  Molanna). 

Characteristics  :  Adults  with  vestigial  mouth  parts ;  two  pairs  of  mem- 
branous wings  obscurely  colored  by  long  silky  hairs  and  narrow  scales; 
antennae  long  and  slender;  metamorphosis  complete;  larvae  and  pupae 
aquatic,  constructing  portable  cases  of  sand  grains  or  vegetable  debris 
fastened  together  with  silk  from  modified  salivary  glands. 

Order  21  —  Lepidoptera  —  Butterflies  and  moths  {Tinea,  Alsophila,  Papilio). 

Characteristics  :  Wingless,  or  with  two  pairs  of  membranous  wings  cov- 
ered with  overlapping  scales;  sucking  mouth  parts  coiled  beneath  head 
consist  of  two  maxillae  fastened  to  form  a  tube;  metamorphosis  com- 
plete ;  larvae  known  as  caterpillars ;   many  species  known. 

Order  22  —  Diptera  —  Flies  and  mosquitoes  (Tipula,  Culex,  Prosimulium,  Musca, 
Drosophila). 
Characteristics  :  One  pair  of  membranous  fore  wings  on  mesothorax,  or 
wingless ;  knobbed  threads  (halteres)  on  metathorax  represents  hind  wings ; 
mouth  adapted  for  piercing  and  sucking,  forming  proboscis ;  larvae  known 
as  maggots ;  complete  metamorphosis. 

Order  23  —  Siphonaptera  —  Fleas  (Ctenocephalus,  Pulex). 

Characteristics  :  Wingless  insects  with  laterally  compressed  body ;  head 
small ;  no  compound  eyes ;  mouth  adapted  for  piercing  and  sucking,  legs 
for  leaping;  metamorphosis  complete;  ectoparasites  of  mammals  and 
more  rarely  birds. 

Order  24  —  Hymenoptera  —  Saw  flies,  ichneumon  flies,  ants,  wasps,  and  bees 
(Cladius,  Ophion,  Formica,  Vespa,  Apis). 
Characteristics  :  Wingless  or  with  two  pairs  of  membranous  wings ;  fore 
wings  usually  larger ;  venation  reduced ;  wings  held  together  on  each  side 
by  hooks  (hamuli);  mouth  parts  adapted  for  chewing  or  sucking;  first 
abdominal  segment  fused  with  thorax ;   complete  metamorphosis. 

Class  V  —  Arachnoidea  —  Spiders,  scorpions,  ticks,  mites,  and  king  crabs 
{Caddo,  Lycosa,  Phalangium,  Buthus,  Argas,  Sarcoptes,  Limulus). 
Characteristics  :  No  antennae  nor  true  jaws ;  two  of  six  pairs  of  jointed 
appendages  modified  for  mouth  parts;  respiration  by  lung-books  or 
tracheae;  first  pair  of  appendages  usually  contain  poison  glands,  second 
pair  used  as  jaws ;  terminal  portions  as  sensory  organs ;  body  usually 
divided  into  anterior  cephalothorax  and  posterior  abdomen ;  former  bears 
four  pairs  of  legs  for  locomotion. 


THE   ANIMAL   KINGDOM  > 


KM 


Phvllm 


Chordata 


Arthropoda 


Mollusca 
Echinodermata 

Annelida  (Annulata) 
Molluscoidea 
Platyhelminthes 
Nemathelminthes 

Troehelmi  n  thes 
Coelenterata 

Porifera 
Protozoa 


Claj; 


Mammalia 

Aves 

Reptilia 

Amphibia 
Pisces 
Miiior 
Cl  asses 


Onychophora 
Crustacea 

Myriapoda 

Insecta 

Arachnoidea 


Examples 


Kmtimatek 

iNl-MMEU    (IK    I.IVIM. 
.Si'EciEM    DkhciiiiiEU 


VERlEliUATES 

Man,  cat,  horse,  bat,  whale 
liirds,  fowls 

Turtles,  snakes,  lizards,   alli- 
gators 
I'rofis,  toads,  salamanders 
I'ishos 

Tunicates,  Balanoglossus,  etc. 
Total  Chordata 

INVERTEBRATES 


Crayfish,    crabs,   water   fleas, 

barnacles,  sowliugs 
Centipedes,  millipedes,  etc. 
All  true  insects 
Spiders,  scorpions,  ticks, 

mites,  and  king  crabs 

Total  .\rthropoda 

Snails,  slugs,  clams,  oysters 

Starfish,     sand     dollar,     sea- 
urchin 

Earthworm,  leeches 

Bryozoa,  Ijrachiopods 

P'latworms,  flukes,  tapeworms 

Roundworms,  Trichinclla, 
Filaria 

Rotifers,  wheel  animalcules 

Jelly-fishes,      coral     animals. 
Hydra 

Sponges 

Ameba,  Paramecium, 

Euglena,     malarial    organ- 
isms, trypanosomes 

Grand  total 


:{.7.')() 
l.l.."j(t() 

•4.000 

1.7.">0 

lIl.jOO 

1,500 


70 

20,000 

2,430 

625,000 

27.500 


38.000 


fi75,000 

S0.(JO0 

5,000 
5,000 
2,5(X) 
G.500 

.3.500 
1.500 

5.000 
3.000 


15,000 


S40,000 


'  Modified  from  Metf-alf  and  Flint,  Destmrtirp  and  Useful  Insects.  By  pprmis.xjon  r)f  the  McOrnw- 
Hill  Book  Company,  publishers.  The  discrepancies  between  this  table  and  the  tt'Xt  illusirato  the 
pragmatic  nature  of  taxonomy. 


H.    W.    H.  —  8 


104 


NATURAL   HISTORY 


Sub -phylum     I 

HEKIC«ORT>ATA 
half  -  CL '  cViorcC 


Sub  •  pliylum    H 

Urockorbata 

chor*<jC -  in-  t<xil 


CI) 
Tunicate. 


CI.) 


sute-pViylvcm,    uc 

CEPHALOCKORDOTA 

cViorcC-in- Vj«acC 


^ 


(1^  ri'^^i 

Arophioxus ' 
Iccnc-elet 


ammals  '*\/it*h  a  -notocVjorcC 


(A)  SUPERCLASS    AGNATHA 


(c)  Superclass   tetrapoda 


(ii  fossil  Ostracoderm 

'Pbsrichthys 


(2)  Cyclostoma.to. 


^etronwjjon. .  lamprey 

CB)  SUPERCLASS      PI5CE6 


(1)     ^ 

A-mphibia 
frog- 


(3) 

Aves 
bird 


"ReptiLia 
turtle 


sub-phylura  IS" 

VERTEBRATA 

'->vit/Vi  "toccc-Vctoones 


"Mammalia 


ROLL   CALL  jqs 


PHYLUM   XIII  —  CHORD  ATA  —  Animals  with  notochord. 

Characteristics:  All  possess  a  dorsal  supporting  rod  or  notochord  and 
pharyngeal  gill  clefts  at  some  stage  in  life  cycle;  tubular  nerve  cord 
dorsal  to  digestive  tract ;  36,000  species. 

Sub-Phylum  I  —  Hemichordata  —  (Balanoglossus). 

Characteristics  :  Wormlike  marine  oiganisms  of  doubtful  relationship  that 
burrow  in  sand  and  resemble  the  larval  echinoderms  in  development ;  head- 
end with  proboscis  and  collar;  with  or  without  a  notochord. 

Sub-Phylum  II  —  Urochordata  —  Tunicates  and  ascidians. 

Characteristics  :  Marine  organisms  with  saclike  covering  {tunic) ;  larvae 
resemble  tadpoles,  possessing  notochord  in  tail;  gill  slits  and  endostyle 
present  in  pharynx. 

Sub- Phylum  III  —  Cephalochordata  —  Lancelots  (Amphioxus). 

Characteristics  :  Segmented  primitive  chordates,  burrowing  in  sand ;  lat- 
erally compressed ;  notochord  extending  from  anterior  tip  to  tail. 

Sub-Phylum  IV  —  Vertebrata  (or  Craniata)  —  Vertebrates. 

Characteristics  :  Animals  with  definite  head,  sense  organs,  closed  circula- 
tory system,  and  axial  notochord  at  some  period  in  life  cycle;  skull  and 
vertebral  column  present  either  in  cartilaginous  or  bony  stage. 

Super-Class  A  —  Agnatha  —  Fossil,  armored  Ostracoderms,  lampreys  and  hag- 
fishes    (Cyclostomata).     Primitive  fishlike  forms   (Pterichthys,  Petromy- 
zon). 
Characteristics:    Animals  w'ithout  jaws;    sucking  mouth  and  primitive 
brain  present. 

Super-Class  B  —  Pisces  —  True  fishes. 

Characteristics  :  Organisms  with  true  jaws ;  typically  scaled ;  charac- 
teristically aquatic;  appendages  developed  into  fins;  two-chambered 
heart. 


106 


NATURAL  HISTORY 


Clocss    I 

Ela5mobranchn 


Class    IE 
Holocepholi 


(1^ 


Chimaera 
spook  fislT. 


(B)  Superclass  PISCES 

FISHES        PROPER. 


1£> 

Protopterus 

lungfish 


Class  12: 


9^ 

'Pe^rccc 


class  "JT 

Tsleostei 


l\OLL  CALL  jy^ 


Class  I  —  Elasmobranchii  —  Gristle-fishes  {Sgualus,  Raia). 

Characteristics:  Cold-blooded  fishlike  vertebrates  witli  jaws;  charac- 
terized by  a  cartilaginous  skeleton,  i)(>rsistent  notochord  and  placoid  scales; 
upper  jaw  suspended  to  ci-aniuin  indirectly  by  means  of  ligaments  and 
cartilages  (hyostylic). 

Class  II  —  Holocephali  -^  Elephant-fishes  (Chimaera). 

Characteristics  :  Immovable  upper  jaw  fused  with  cranium  (autostyiic) 
resembling  higher  forms;  gill  slits  covered  by  flap  (operculum);  tail 
heterocercal. 

Class  III  —  Ganoidei  —  Enamel-scaled  fishes  (Acipenser,  Lepisosteus,  Polyp- 
terus). 
Characteristics  :  More  or  less  armored  fish ;  remnants  of  group  dominant 
in  Devonian  seas;  degenerating  spiral  valve  in  intestine  associated  with 
presence  of  pyloric  caeca;  scales  usually  rhomboidal,  fitting  together 
rather  than  overlapping;   dorsal  fin  usually  close  to  caudal  fin. 

Class  IV  —  Dipnoi  —  Lung-fishes  (Neoceratodus,  Lepidosiren,  Protopterus). 

Characteristics:  Semitropical  fishes,  passing  dry  season  by  aestivating  in 
slimy  cocoon ;  during  period  of  active  life  use  gills,  and  while  aestivating 
breathe  air,  the  modified  swim  bladder  acting  as  a  lung;  cycloid  scales; 
auricle  of  heart  partially  divided. 

Class  V  —  Teleostei  —  Bony  fishes  (Ctenolabriis,  Perca,  Gadus,  Microptcrtis). 
Characteristics  :  Bony  fishes,  breathing  primarily  by  gills ;  well-develoiM-d 
operculate  bones,  cycloid  or  ctenoid  scales ;  tail  homocercal.     These  fishes 
constitute  about  90  per  cent  of  all  known  varieties. 


108 


NATURAL  HISTORY 


E 


OrcCe^r-  (i) 


OrdiQr  (2") 

jg^css  amphibia 


Rftstorations 
Steg'ooepViali 


(1"^ 

Cccecilicc 
"blincC"  ^vo^mUke 
amphibian. 


(C)5icperclass  TE 
CLASS  I    AMPH 


'RAPODA 

IBIA 


(1^ 

Triturus 
spottccC  incvt 


Necturzxs 


Order  (3) 

UrodLela  . .  .  ., 
gtnpbibitt  wttn  taits 


OncCer-  (4-) 

Anurcc 

taillC96    amphibia 


ROLL  CALL  ,„y 


Super-Class  C  —  Tetrapoda  —  Four-footed  vertebrates. 

Characteristics  :  Well-defined  limbs  witli  hands  and  feet  typically  con- 
structed on  plan  of  five  digits;  stapes  or  coluniolla  present  in  ear;  Rirdles 
adapted  to  bear  weight  on  land;  body  divisible  into  neck  and  trunk,  tail 
present. 

Class  I  —  Amphibia  —  Frogs  and  salamander.'?. 

Characteristics:    Cold-blooded,   naked  vertebrates   undergoing  a  meta- 
morphosis ;     usually   with   five-fingered   limbs    (pentadactylous) ;    young 
u.sually  aquatic,  breathing  by  gills;  adults  using  lungs  and  skin,  u-suallj' 
air  breathers. 
Order  1  — Stegoccphalia  —  Extinct  fossil  amphibians  (Erynps,  Loxomma). 

Ch.^racteristics  :    Fossil  forms   resembling  amphibia,   flourishing  in  car- 
boniferous age ;  probably  earliest  four-footed  air  breathers. 
Order  2  —  Apoda  —  Legless  amphibia  (Herpeles,  Siphonops,  Caecilia). 

Characteristics  :  Small,  tropical,  wormlike,  often  blind  amphibia,  burrow- 
ing in  ground. 
Order  3 — Urodela  —  Salamanders    {Desmognathus,    Necturus,     Cryptobranchus, 
Triturus). 
Characteristics:    Tadpole-like  tail  retained  throughout  life;    some  never 
emerge  from  water;   a  few  retain  external  gills  in  adult  stage. 
Order  4  —  Anura  —  Frogs  and  toads  {Rana,  Bufo,  Ilyla). 

Characteristics  :  Tailless  upon  completing  their  metamorphosis ;  capable 
of  singing ;   characterized  by  the  possession  of  movable  eyelids. 


no 


NATURAL  HISTORY 


OrcCeJr,  1  ,      ^. 

RViy  n  cho  ctepnou  loc 

'tJhe.   old,  t-imer-^'' 


Spherzodon 


OrcL©3~    2    • 

Cro<iociilicc 

Crocodilea .  aUigators 


Alligator 


Soft  iheileat  turtle 


(C)5LqDercla55  TETRAPODA 
CLA5S  I     PEPTIU  A 


"i5o>:  turtle^ 


OrcCer  -3 

Chelonia 

turtles,  tortoises 


^  'osoxers 
fish -like  reptile^ 


sub  ordter  Soci^ria 
lixccr-cCs 


Plesiosaurs 


Pberodoc'tyls 


./(2)  -^^.^^.ffla^       (4)         

''  sub  order  Serper^teS  Hiriosaurs 
SriccKe.S  ^lant  reptiles 


Ordei^   4r 

5c|uamata 
snokss ,  li3ards 


Orders  5-8  fossil  rejjtiles 
Ichthyosouria  ,Plesio5auria 
Ptcrc?ctactylia,T)inoscturia 


UOLL  CALL  ijj 


Class  II  —  Reptilia  —  Turtles,  snakes,  alligators,  and  lizards. 

Characteristics:  Cold-blooded;  usually  covered  with  scales  and  fre- 
quently bony  plates ;  air  breathers. 

Order  1  —  Rhynchocephalia  —  "The  old-timers,"  Sphenodon. 

Characteristics  :  Biconcave  vertebrae  often  containing  remnants  of  noto- 
chord;  quadrate  bone  immovable;  parietal  eye  present.  This  group  i.« 
represented  by  one  genus  of  lizards,  Sphenodon,  found  only  in  New  Zea- 
land. 

Order  2  —  Crocodilia  —  Crocodiles  and  alligators  {Crocodiliis,  Alligator). 

Characteristics  :  Anterior  appendages  bearing  five  digits,  jiosterior  four 
with  trace  of  fifth ;  longitudinal  slit  constitutes  cloacal  opening;  vertebrae 
procoelous. 

Order  3  —  Chelonia  —  Turtles  and  tortoises  (Amyda,  Eretmochelye,  Terrapene, 
Testudo,  Chelonia). 
Characteristics  :   Body  surrounded  by  bony  case  forming  a  carapace  and 
plastron;    toothless  jaws ;    immovable  quadrate  bone;    appendages  typi- 
cally with  five  digits. 

Order  4  —  Squamata  —  Snakes  and  lizards  (Phrynosoma,  Heloderma,  Tham- 
nophis). 
Characteristics:  Usually  with  horny  epidermal  scales  or  plates;  movable 
quadrate  bone;  vertebrae  usually  procoelous;  ril)s  with  single  heads. 
This  order  is  usually  subdivided  into  two  sub-orders:  lizards  (Sauria) ; 
and  snakes  (Serpentes). 

Orders  5-8  —  /)z/;o.s<7;/m  —  Fossil  reptiles  (Ichthyosaurs,  Plesiosaurs,  Pterodac- 
tyls, Dinosaurs). 

In  these  groups  belong  such  forms  as  the  fishlike  reptiles  (Ichthyosaurs) ; 
the  long-necked  reptiles  (Plesiosaurs) ;  the  flying  reptiles  (Pterodactyls) ; 
and  the  giant  reptiles  (Dinosaurs). 


112 


NATURAL  HISTORY 


Subcbss  A  -  Arcbaeomithes 
r|     fossil  reptile-like  "bircCs 

I 


SUPERCLASS   TETRAPO"DA 

ClassI  AVE5   BIRDS 


Kestserornithifbrmes  ^, 
^5sil  toothe^blrasl 

Aptery^iforme? 
« Ca5i:arii|brTOes  Kivi^^ 

^Caseovarie^  ^        Ciconiifomes 

C)Icbtbxor^^i7orm<25     ^         /«        stork -like  bircfs 


Grui  formes 
rails  at2ct coots 


(10) 

cViarBucCrji^nTMs 
glover,  5nipi«  ,^115 


11^       ^      .  ^ 

m.^s??^  Cuculi  formes 

stratHorjiformes     |f^^        M-^WxV      Talcomfl^nes 
Afncan  oitrich     jV    ]j         col^'mbilbrmcs  falcon-like-binis 
(-r)  <?4rV;;^V--      l-oons  and  Grcbas  /^ 

i)moTnitbi(t>rmes        ^"^^W  ^^^'^S^l     ^^^^        -"^ssi^*^ 
Moots  ^-r-'^'^/      j'^^'^,.//     Coraciiformas 

extinct        :^^/  <^^f<  —    ^'^ 

(8)  (^^^  ^  Gcclhformss 


=^    elephant  birds      aibatrossa.s  .petrels 


RheifoT^m©©     . 
American  ostrioh 


Subclass  D  -Neorrzitbes 


(2/) 

fixsseriformes 
percViing-  birds 


HOI.L   CALL  ,lj 

Class  III  —  Aves  —  Birds. 

Characteristics:  Typically  featliered  and  toothless;  \varm-l)Ioo(lo(l. 

Subclass  A  —  Archaeornithes  —  Fossil  birds  (Archaeopteryx). 

Characteristics:    Ancient  re[)tilelike  fossil  birds;    only  three  specimens  of 
a  single  genus  (Archaeopteryx)  are  known. 

Subclass  B  —  Neornithes  —  Recent  birds. 

Characteristics:    Mostly  composed  of  birds  which  are  represented   by 
living  forms;   21  orders. 
Order  1  —  Hesperornithiformes  —  (Hesperornis). 

Characteristics  :  Fossil,  toothed  birds  from  America ;  teetli  set  in  a  groove. 
Order  2  —  Ichthyornithiformes  —  {I chthyornis) . 

Characteristics  :   Fossil,  toothed  birds  from  America,  whose  teeth  are  set 
in  sockets. 
Order  3  —  Struthioniformes  —  Ostriches  (Stridhio). 

Characteristics  :    Naked  head,  neck,  and  legs ;   flightless,  terrestrial  forms ; 
feet  with  two  toes;  no  keel  on  breastbone  {sternum). 
Order  4  —  Rheiformes  —  Rheas  (Rhea). 

Characteristics  :    Distinguished  from  preceding  order  by  a  partially  feath- 
ered  head   and   neck;     flightless   terrestrial   birds,   with   three-toed   feet; 
feathers  without  aftershaft. 
Order  5  —  Casuariiformes  —  Cassowaries  and  emus  (Dromalus). 

Characteristics:     Terrestrial,    flightless    birds,    possessing   small   wings; 
feathers  with  large  aftershaft. 
Order  6  —  Crypturiformes  —  Tinamous  (Rhynchotus) . 

Characteristics  :  Flying,  terrestrial  birds,  with  a  short  tail ;  no  pygostyle. 
Order  7  —  Dinornithiformes  —  Moas  (Palapteryx). 

Characteristics  :    Recently  extinct,  flightless,  terrestrial  birds,  with  large 
hind  limbs ;  wing  bones  absent. 
Order  8  —  Aepyornithiformes  —  Elephant  birds  (Aepyornis). 

Characteristics:   Extinct  terrestrial  flightless  birds  with  large  hind  limbs; 
small  sternum  and  wings ;   large  eggs. 
Order  9  —  Apterygiformes  —  Kiwis  (Apteryx). 

Characteristics:   Small  flightless  terrestrial  birds;   hairlike  feathers  with- 
out aftershaft. 
Order  10  —  Sphenisciform.es  —  Penguins  (Eudyptes). 

Characteristics:    Marine  antarctic  birds,  incapable  of  flight,  with  small 
scalelike  feathers;   wings  modified  as  paddles  for  swimming. 
Order  11  —  Colymbiformes  —  Loons  and  grebes  (Gavia,  Podiceps). 

Characteristics  :  Aquatic  birds  with  feet  far  back  with  webbed  or  lobed  toes. 
Order  12  —  Procellariiformes  —  Albatrosses  and  petrels  (Diomedea,  Hydrobates). 
Characteristics  :   Marine  birds  with  great  powers  of  flight ;    webbed  toes ; 
bill  sheath  of  several  pieces. 
Order    13  —  Ciconiiformes  —  Storks,    birds,    pelicans,    cormorants,    snake-birds, 
herons,  ibises,  and  flamingos  (Phalacrocorax,  Ardea,  Phoetncoptcru.f). 
Characteristics:    Long-legged  aquatic  marsh  birds  with  feet  adapted   for 
wading. 


114  NATUllAL  lilSTORY 


Order  14  —  Anseriformes  —  Swans,  geese,  and  ducks  (Mergus,  Anas,  Cygnus). 
Characteristics  :    Aquatic  birds  whose  beak  is  covered   by  soft  sensitive 
membrane  edged  with  horny  lamellae. 
Order  15  —  Falconiformes  —  Falcons,  vultures,  eagles,  hawks,  and  secretary-birds 
{Cathartes,  Gymnogyps,  Sagittarius,  Falco). 
Characteristics  :     Carnivorous   birds   with   curved,    hooked   beak ;     feet 
adapted  for  perching  and  provided  with  sharp,  strong  claws. 
Order    16  —  GalUformes  —  Tui'keys,    fowls,    quails,    and    pheasants ;    also    the 
hoactzin  (Meleagris,  Colinus,  Bonasa). 
Characteristics  :   Arboreal  or  terrestrial  birds ;  feet  adapted  for  perching. 
Order  17  —  Gruiform.es  —  Rails  and  cranes  {Rallus,  Gallinula,  Fulica). 

Characteristics  :   Mostly  marsh  birds. 
Order  18  —  Charadriiform.es  —  Plovers,  snipes,  gulls,  terns,  auks,  and  pigeons 
{Jacana,  Larus,  Rhynchops). 
Characteristics  :   Marine,  arboreal,  or  terrestrial  forms. 
Order  19  —  Cuculiformes  —  Cuckoos  and  parrots  {Conuropsis,  Coccyzus). 

Characteristics  :    Arboreal  birds,  first  and  fourth  toes  directed  backwards ; 
the  latter  may  be  reversible. 
Order  20  —  Coraciiformes  —  Kingfishers,  owls,  hummingbirds,  swifts,  and  wood- 
peckers (Streptoceryle,  Antrostomus) . 
Characteristics  :  Tree-inhabiting  forms  with  short  legs. 
Order  21  —  Passeriformes  —  Perching  birds  (Passer,  Sayornis,  Tyrannus). 

Characteristics  :  More  than  half  of  all  known  birds  belong  in  this  order. 
In  America  representatives  of  25  families  are  found.  A  few  of  these  are 
the  flycatchers,  larks,  thrushes,  thrashers,  wrens,  warblers,  swallows, 
shrikes,  nuthatches,  crows,  orioles,  finches,  and  creepers. 


ROLL  CALL  ,,- 

Class  IV  —  Mammalia  —  Mammals. 

Characteristics:  Members  of  this  class  are  readily  distiiiKuisl)0(l  by  a 
covering  of  hair  at  some  time  in  their  existence;  the  females  pos.seKs 
mammary  glands  which  secret(>  milk  for  nourishment  of  young. 

Subclass  A  —  Prototheria  —  Monotremes  {Echidna  and  Ornithorhynchus). 
Characteristics:    Egg-laying  mammals;    in   case  of  Ecliidim   the  egg  is 
placed  in  a  temporary  pouch  and  incubated  until  hatched. 

Subclass  B  —  Metatheria  —  Marsupials  {Didelphys,  Petrogale,  Macropus). 
Characteristics  :    Carry  young  in  marsupium  or  pouch  ;  allantoic  placenta 
typically  absent. 

Subclass  C  —  Eutheria  —  Viviparous  mammals. 

Characteristics  :    Bring  forth  their  young  alive ;   young  never  carried  in 
pouch  ;  nourished  before  birth  by  placenta. 
section  a  —  unguiculata  —  Clawed  mammals. 
Order  1  —  Insectivora  —  Insect-eaters,  moles,  and  European  hedgehogs. 

Characteristics  :   Small  terrestrial  clawed  mammals  with  typically  planti- 
grade feet ;   molar  teeth  enameled,  rooted,  and  tuberculate. 
Order  2  —  Dermaptera  —  Flying  lemurs. 

Characteristics:    Members  of  this  group  resemble  the  insectivores  in  the 
structure  of  the  skull  and  the  canine  teeth ;   only  two  genera  are  known, 
which  inhabit  the  forests  of  Malaysia  and  the  Philippines. 
Order  S  —  Chiroptera  —  Insectivorous  bats,  fruit-bats,  and  blood-sucking  vam- 
pires. 
Characteristics  :   Mammals  with  claws  whose  fore  limbs  are  modified  for 
flight. 
Order   4  —  Carnivora  —  Flesh-eating   mammals,   hyenas,    raccoons,    dogs,    cats, 
weasels,  bears,  sea-lions,  seals,  and  walruses.  « 

Characteristics  :     Carnivorous  mammals  with  claws  and  large  projecting 
canine  teeth;   incisors  small;   premolars  adapted  for  flesh-cutting. 
Order  5  —  Rodentia  —  Gnawing  animals,  hares,  rats,  mice,  squirrels,  beavers, 
porcupines,  guinea  pigs. 
Characteristics  :    Members  of  this  group  are  usually  separated  into  two 
suborders  depending  upon  the  possession  of  one  or  two  pairs  of  incisors 
in  upper  jaw. 
Order  6  —  Edentata  —  So-called  toothless  mammals,  three-toed  sloth,  armadillo, 
and  pangolin. 
Characteristics:     Clawed   mammals;    teeth   entirely   absent   or   mi.-^sing 
from  anterior  part  of  jaw ;    teeth  usually  without  enamel ;    tongue  often 
long  and  protractile. 
section  b  —  primates. 

Order  7  —  Primates —  M&mmaAs  with  nails;    tarsiers,  lemurs,  monkeys,  apes, 
man. 
Characteristics:     Toe   or  thumb   usually   is  opposable  to  other  digits; 
dentition  rather  primitive ;   eye  orbits  directed  forward ;    posture  usujilly 
semierect. 


116 


NATURAL  HISTORY 


Subclass  A  PROTOTHERIA 
MONOTREMES 

®gig-layin^  mamnrzals 


OrnitlQorbyr2c"bu4 
cUxckbill 


SubcktssB  NETATHEQIA 

HAR5(JP)AL5 

mammals  >vitl3 

torOQgt  pOLCCVx 


Macropas 
l<ocng"a.-roo 


(C)  5uperda55TETRAP0DA 

Cla$$  32^  MAMMALIA. 


Section  a 

UNGUICULATA 

clawea  mammals 


SubclassC  EUTHERIA 

viviparous  mamrcKxis 


Cccmivora 


Order  1 .  Tnolss.e.tc. 

Insectivora 


S|>^ 


OrcCai-  v5 

RocCent-icc 

gnaviog  mammals 


Dermaptera 

flying  ]emtxi~5 


OroCer  6 
Edentata 


OrcCejT  8 

Artiodoctyla 

even- toed.       \^ 


Section   C/ 

UNGULATA 

hoof«gcC  mammali 


M^ 


OrcLai-  9  ' 

PerissododMct 
octd-toedL    "^ 


OroCer  lO 
Probosrcixfea 

trttnk  ondC  tusl<S 


ordterll  secccovs 
Siren  ice 


ROLL  CALL  jj- 


SECTION  c  —  UNGULATA  —  Hoofed  mammals. 

Order    8  —  Artiodactyla  —  Even-toed    ungulates;     hippopotamus,    camel,    deer, 
moose,  domestic  cattle,  giraflfe. 
Characteristics;    An  even  number  of  digits,  axis  of  symmetry  passing 
between  digits  three  and  four. 
Order  9  —  Perissodactyla  —  Odd-toed  ungulates,  horse,  zebra,  tapir,  rhinoceros. 
Characteristics  :    An  uneven  number  of  digits,  axis  of  symmetry  passing 
through  digit  three 
Order  10  —  Proboscidea  —  Elephants. 

Characteristics  :    Ungulates  characterized  by  long,  prehensile  proboscis ; 
incisors  developed  to  form  tusks ;  broad  molars. 
Order  11  —  Sirenia  —  Sea-cows,  dugong,  manatee. 

Characteristics:  Aquatic  ungulate-type  Eutheria;  fore  limbs  finlike ;  hind 
limbs  absent ;   tail  with  horizontal  fin. 
Order  12  —  Hyracoidea  —  Hyrax  and  coneys. 

Characteristics  :    Small  rodent-like  mammals  with  reduced  tail  and  short 
ears ;  four  digits  on  fore  limbs ;  three  digits  on  hind  limbs. 
section  d  —  cetacea  —  Whales  and  dolphins. 

Characteristics  :   Aquatic  mammals ;   probably  derived  from  the  Ungui- 
culata  or  Ungulata. 
Order  13  —  Odontoceti  —  Toothed  whales,  dolphin,  porpoise,  grampus. 

Characteristics  :    Cetacea  with  teeth  (at  least  on  lower  jaw) ;    no  whale- 
bone. 
Order  14  —  Mystacoceii  —  Whalebone  whales,  fin  whale,  right  whale. 

Characteristics:    Cetacea  without  teeth  in  adult;    mouth  provided  witli 
plates  of  whalebone. 


118  NATURAL  HISTORY 

GLOSSARY  OF  TERMS  OCCURRING  IN   ROLL  CALL 

Abdomen  —  the  posterior  region  of  the  body,  behind  the  thorax  of  an  insect ;  the 
region  of  the  body  below  the  chest  in  man. 

Aestivating  —  passing  the  summer  in  a  torpor. 

Aftershaft  —  an  accessory  plume  arising  from  the  posterior  side  of  the  shaft  of 
the  feathers  of  many  birds. 

Alimentary  canal  —  food  tube  of  animal,  beginning  with  mouth  and  ending  with 

anus. 
Allantoic  —  pertaining  to  a  respiratory  sac  which  in  early  fetal  life  grows  out  from 

the  hind-gut  of  an  embryo. 

Ambulacral  groove  —  groove  in  which  tube-feet  are  located. 

Antennae  —  paired  appendages,  which  are  sensory  in  function,  on  the  head  of  an 

insect  or  crustacean. 
Antheridium  —  organ  or  receptacle  in  which  male  sex  cells  of  ferns  are  produced. 
Anus  —  posterior  opening  of  alimentary  canal. 

Appendage  —  an  organ  or  part  attached  to  a  body,  as  a  leg,  arm,  fin,  or  tail. 
Arboreal  —  pertaining  to  forms  frequenting  trees. 

Archegonium  —  a  female  organ  in  which  the  young  plant  begins  development. 
Aristotle's  lantern  —  masticating  apparatus  of  sea-urchin. 
Ascomycetes  —  sac  fungi. 

Ascospore  —  one  of  a  set  of  spores  contained  in  a  special  sac  or  ascus. 
Ascus  —  a  membranous  spore  sac  of  fungi. 
Asexual  —  having  no  sex. 

Axial  —  pertaining  to  the  fundamental  central  line  of  a  structure. 
Basidiospore  —  a  spore  formed  on  a  basidium. 

Basidium  —  the  spore-producing  organ  of  certain  of  the  higher  fungi. 
Bilaterally  symmetrical  —  having  two  symmetrical  sides  about  an  axis. 
Bill  sheath  —  protective  covering  for  bill. 
Bivalve  —  consisting  of  two  shells  or  valves. 
Body  cavity  —  space  in  which  the  viscera  lie. 
Branchiae  —  gills. 
Calcareous  —  containing  lime  or  calcium,  chalky. 

Canine  tooth  —  a  pointed  tooth  situated  between  an  incisor  and  a  bicuspid  or 
premolar  tooth. 

Carapace  —  a  bony  or  chitinous  case  covering  an  animal's  back,  as  in  the  crayfish. 

Carotin  —  yellow  pigment  of  plants ;    associated  with  chlorophyll  and  xantho- 
phyll. 

Carpel  —  a  pistil,  or  one  of  the  members  composing  a  compound  pistil  or  seed- 
vessel. 

Cartilaginous  —  gristly  substance  forming  part  of  the  skeleton. 

Caudal  —  of,  or  pertaining  to,  the  tail. 

Cerci  —  bristlelike  structures. 

Chitin  —  a  carbohydrate  derivative  forming  the  skeletal  substance  in  arthropods. 

Chlorophyll  —  green  coloring  matter  found  in  plants  and  some  animals. 

Chloroplasts  —  small  bodies  of  protoplasm  containing  chlorophyll. 

Chromosome  —  a  deeply  staining  body  in  the  nucleus  of  a  cell,  supposed  to  carry 

the  determiners  of  hereditary  characters. 
Cirri  —  slender  extensions  found  on   bodies  and  appendages  of  many  forms, 

which  are  used  for  various  functions. 


HULL   CALL  Hy 

Clasping  organ  —  specialized  holdfast  structure  of  certain  males  used  in  conula- 

tion.  ' 

Coelenteron  —  internal  cavity  of  a  coelenterate,    which    servos  as  a  diKcstivc 
tract  as  well  as  body  cavity. 

Coelom  —  body  cavity. 

Columella  —  rodhke  bone  of  middle  ear  of  anura  formed  from  hyomandihular 

bone. 

Compound  eye  —  made  up  of  several  simple  eyes. 

Cotyledon  —  embryonic  leaf,  in  a  seed. 

Ctenoid  scales  —  scales  with  a  comblike  or  serrate  margin. 

Cuticle  —  an  outer  layer  of  the  skin. 

Cycloid  scales  —  scales  with  evenly  curved  free  border. 

Cytoplasm  —  the  living  substance  of  the  cell  outside  of  the  nucleus  and  inside 
the  cell  membrane. 

Deciduous  —  falling  off  at  maturity. 

Dentition  —  number,  arrangement,  and  kind  of  teeth. 

Diaphragm  —  (Bot.)  a  septum  or  membranous  layer. 

Dicotyledon  —  a  plant  that  bears  seeds  having  two  cotyledons. 

Digits  —  terminal  divisions  of  limb  in  any  vertebrate  above  fishes. 

Diploblastic  —  having  two  distinct  germ  layers. 

Direct  development  —  no  metamorphosis,  i.e.,  the  young  when  hatched  closely 
resemble  adult  except  for  size. 

Dorsal  —  pertaining  to  the  back  or  top  side  of  (as  of  a  leaf). 

Dorsoventral  —  pertaining  to   structures  which  extend   from  dorsal  to  ventral 
side. 

Ectoderm  —  the  outer  embryonic  layer  in  a  multicellular  animal. 

Ectoparasite  —  a  parasite  that  lives  on  the  exterior  of  an  organism. 

Elytra  —  the  anterior  wings  of  beetles,  hard  and  caselikc. 

Embryo  sac  —  the  megaspore  in  plants. 

Endoparasite  —  a  parasite  which  lives  within  the  body  of  its  host. 

Endostyle  —  ciliated  groove  whose  cells  secrete  mucus.     Foimd  in  urochordate,s 

and  cephalochordates. 
Epithelium  —  cellular  tissue  covering  a  free  surface  or  lining  a  tube  or  cavity. 
Excretory  —  pertaining  to  organs  of  elimination. 
Exo-skeleton  —  an  outside  skeleton  such  as  the  shell  of  a  lobster. 
Fibrovascular  bundles  —  collections  of  tubular  cells,  supported  by  woody  cells, 

which  conduct  fluids  in  plants. 
Filamentous  —  composed  of  long,  threadlike  structures. 
Flagella  —  threadlike  projections  of  cells,  which  are  used  for  locomotion. 
Flame  cell  —  the  terminal  cells  of  l)ranches  of  excretory  system  in  flatwornis, 

with  cavity  continuous  with  lumen  of  duct,  and  containing  a  ciliuni  or  l)uncli 

of  cilia,  the  motions  of  which  give  a  flickering  appearance  similar  to  that  of 

a  flame. 
Foot  —  thick  muscular  locomotor  organ  of  molluscs. 
Furcula  —  a  forked  process  or  structure. 
Gamete  —  a  mature  sex  cell. 
Gametophyte  —  a  stage  in  the  life  history  of  a  moss  or  fern  in  which  sex  cells 

are  produced. 
Ganglion  —  a  group  of  nerve  cells  situated  outside  of  the  brain  or  spinal  .•oluinn. 
Gastric  —  pertaining  to  or  in  region  of  stomach. 
H.  w.  H.  —  9 


120  NATURAL  HISTORY 

Gastrovascular  —  serving  both  digestive  and  circulatory  purposes. 

Germ  cell  —  sex  cell. 

Gill  cleft  —  a  branchial  opening  formed  on  the  side  of  the  pharynx. 

Gill  filaments  —  the  soft  filamentous  structures  on  the  respiratory  organs  (gills) 
of  aquatic  animals. 

Gland  —  an  organ  which  secretes  material  to  be  used  in,  or  excreted  from,  the 

body. 
Halteres  —  a  pair  of  small  capitate  bodies  representing  rudimentary  wings  in 

flies,  used  as  balancers. 

Haploid  —  having  the  number  of  chromosomes  characteristic  of  mature  germ- 
cells  for  the  organism  in  question. 

Hemocoele  —  an  expanded  portion  of  the  blood  system  which  takes  the  place  of 
a  true  coelom. 

Hepatic  caecum  —  blind  pouch  or  diverticulum  of  or  in  region  of  liver. 

Hermaphroditic  —  pertaining  to  an  organism  with  both  male  and  female  repro- 
ductive organs. 

Heterocercal  —  having  vertebral  column  terminating  in  upper  lobe  of  fin,  which 
is  usually  larger  than  the  lower. 

Homocercal  —  with  equal  or  nearly  equal  lobes,  and  axis  ending  near  middle  of 
base. 

Hypha  —  one  of  the  filaments  composing  the  mycelium  of  a  fungus. 

Incisors  —  front  chisel-like  teeth  of  either  jaw. 

Incubate  — ■  to  keep  warm  and  under  other  favorable  conditions  for  hatching. 

Indirect  development  —  undergoing  metamorphosis,  i.e.,  showing  a  decided  change 
in  form  and  appearance  from  time  of  hatching  until  maturity. 

Integument  —  a  covering  or  protective  layer;  skin. 

Keel  —  ridgelike  process. 

Lamellae  —  thin  platelike  structures. 

Larvae  —  young  stages  in  the  development  of  some  forms  of  animals,  which  be- 
come self-sustaining  but  which  do  not  have  the  characteristics  marking  adults. 

Lateral  line  —  longitudinal  line  at  each  side  of  body  of  certain  aquatic  animals 
marking  position  of  sensory  cells. 

Laterally  compressed  —  narrow  from  side  to  side. 

Ligament  —  a  band  of  connective  tissue  binding  one  bone  to  another. 

Lobate  —  divided  into  lobes. 

Lophophore  —  ridge  bearing  tentacles. 

Lung-books  —  respiratory  organs  formed  like  a  purse  with  numerous  compart- 
ments or  a  book  with  edges  of  leaves  exposed. 

Mammary  glands  —  milk-secreting  glands. 

Mantle  —  the  soft  outer  fold  of  skin  in  molluscs  which  secretes  the  outer  shell. 

Medusoid  —  like  a  jellyfish  or  medusa. 

Megasporangium  —  a  macrospore-producing  sporangium  in  plants. 

Megaspore  —  larger  spore  of  heterosporous  plants,  regarded  as  female;  embryo- 
sac  cell  of  seed  plant. 

Membranous  —  resembling  or  consisting  of  a  membrane ;  pliable  and  semi- 
transparent. 

Mesenchymatous  —  pertaining  to  mass  of  tissue  intermediate  between  ectoderm 

and  endoderm,  derived  from  mesoderm. 
Mesenteries  —  peritoneal  folds  serving  as  a  bridge  for  blood  vessels  and  for 

holding  organs  to  body  wall. 
Mesoderm  —  the  middle  layer  of  tissue  in  a  young  animal  embryo. 


ROLL   CALL  ,o, 

Mesoglea  —  an  intermediate  non-cellular  layer  in  sponges  and  coeleiiterate.s. 

Metamorphosis  —  change  in  form  or  structure  of  an  animal  in  its  devclotimerif 

from  embryo  to  adult. 
Gradual  or  simple  metamorphosis  —  young  resemble  adults  at  hatcliing  cxccDt 

for  absence  of  wmgs  or  color,  shape,  and  structure  of  some  appendages  ' 
Complete  metamorphosis  —  young  differ  from  adults  in  appearance    habitat 

etc.,  and  undergo  several  changes  in  form  such  as  larvae,  pupae,  and  adult.  ' 
Metathorax  —  posterior  segment  of  insect's  thorax. 
Molars  —  grinding  teeth. 

Monocotyledon  —  a  plant  that  bears  seeds  having  but  one  cotyledon. 
Mother  cell  —  primary  cell  before  division  occurs. 

Mycelium  —  the  threadlike  body  of  a  mold,  or  other  fungus ;  made  up  of  indi- 
vidual threads  called  hyphae. 

Nematocyst  —  a  stinging  cell. 

Nervure  —  one  of  riblike  structures  which  support  membranous  wing  of  insect. 

Notochord  —  a  rod  of  cells  forming  the  supporting  axis  of  lower  chorda tes ;  found 
in  early  stages  of  development  in  all  vertebrates. 

Nucleus  —  the  center  of  activity  in  the  living  cell. 

Nymph  —  larva  of  aquatic  forms  which  undergo  gradual  or  simple  metamor- 
phosis. 

Oogonia  —  female  reproductive  organs  in  certain  Thallophytes ;  the  mother  egg 
cells. 

Operculum  —  a  lid  or  cover. 

Oral  —  pertaining  to  the  mouth ;   side  on  which  mouth  lies. 
Ovary  —  (Bot.)  the  base  of  a  pistil,  containing  the  ovules. 
(Zool.)  —  the  egg-containing  organ. 

Ovule  —  egglike  cell  of  a  plant. 

Papillae  —  any  small  nipplelike  projections  or  parts. 

Parapodia  —  paired  appendages  used  in  locomotion,  attached  to  body  segments 

of  some  marine  worms. 
Parasites  —  animals  or  plants  which  live  at  the  expense  of  other  organisms. 
Parietal  eye  —  rudimentary  eye  ari-sing  as  an  evagination  on  the  median  dorsiil 

surface  of  the  brain. 
Parthenogenesis  —  reproduction  without  fertilization  by  a  male  element. 
Pedicellariae  —  minute  pincerlike  structures  studding  the  surface  of  some  of  the 

echinoderms. 
Pentamerous  —  made  up  of  five  parts. 

Peritoneum  —  membrane  which  lines  the  abdominal  walls  and  invests  the  con- 
tained viscera. 
Phylogenesis  —  history  of  evolution  of  species. 
Placenta  —  organ  through  which  the  mammalian  embryo  is  nourished  by  the 

mother. 
Placoid  scales  —  embedded  scales  and  dermal  teeth  of  elasmobranchs. 
Plantigrade  —  walking  with  sole  of  foot  touching  the  ground. 
Plasmodium  —  a  single  mass  of  living  material  which  contains  many  nuclei. 
Plastid  —  small  bodies  of  .specialized  protoplasm  lying  in  cytoplasm  of  some 

cells  —  especially  plants  and  certain  protozoans. 
Plastron  —  ventral  bony  shield  of  tortoises  and  turtles. 

Poison  gland  —  gland  which  secretes  poison,  used  for  protection  or  food-getting. 
Pollen  tube  —  a  tubular  process  developed  from  pollen  grains  after  attachnu-nt 

to  stigma. 


122  NATURAL  HISTORY 

Polyp  —  a  separate  zooid  of  a  colonial  animal. 
Prehensile  —  adapted  for  holding. 

Premolars  —  bicuspid  teeth  between  canine  and  molar  teeth. 
Proboscis  —  any  of  various  tubular  prolongations  of  the  head  of  animals ;   mus- 
cular protrusible  part  of  the  alimentary  canal  in  certain  worms. 
Procoelus  —  with  concave  anterior  face. 

Prothallus  —  a  small,  thin,  gametophytic  mass  of  tissue  developed  from  spores 
of  ferns. 

Prothorax  —  anterior  thoracic  segment  of  arthropods. 

Protractile  —  capable  of  being  thrust  out. 

Pseudopodia  —  protrusions  of  protoplasm  (false  feet)  serving  for  locomotion  and 
prehension  in  protozoa. 

Pygostyle  —  an  upturned  compressed  bone  at  end  of  vertebral  column  of  birds, 
formed  by  fusion  of  caudal  vertebrae. 

Pyloric  caecum  —  a  blind  diverticulum  or  pouch  in  the  pyloric  region. 

Pylorus  —  the  aperture  between  the  stomach  and  the  small  intestine. 

Pyrenoid  —  a  colorless  plastid  of  lower  plants,  a  center  of  starch  formation. 

Quadrate  bone  —  the  bone  with  which  the  lower  jaw  articulates  with  the  cranium 
in  some  forms. 

Radially  symmetrical  —  having  similar  parts  arranged  on  either  side  of  a  central 

axis. 

Reproduction  —  the  process  by  which  organisms  produce  off  .spring.  In  asexual 
reproduction  a  new  organism  is  formed  by  the  separation  of  a  cell  or  cells 
from  a  single  parent;  in  sexual  reproduction  two  cells  from  two  plants  or 
two  animals  of  different  sexes  join  together  to  form  a  new  individual. 

Rhizoids  —  rootlike  organs. 

Rhomboidal  —  shaped  more  or  less  like  an  equilateral  parallelogram,  having  its 
angles  oblique. 

Rodent  —  animal  with  a  habit  of  gnawing  or  nibbling. 

Sap  cavity  —  a  vacuole,  filled  with  water  and  dissolved  substances  in  mature, 
live  plant  cells. 

Saprophytes  —  organisms  which  live  on  dead  and  decaying  organic  matter. 

Scale  —  a  flat,  small,  platelike  external  structure,  dermal  or  epidermal. 

Schizogony  —  repeated  division  of  the  nucleus  without  immediate  cell  division. 

Sedentary  —  not  free-living;  animals  attached  by  a  base  to  some  substratum. 

Segmentation  —  the  division  or  splitting  into  segments  or  portions ;  cleavage  of 
an  ovum. 

Sessile  —  stationary  or  attached,  opposite  of  free-living  or  motile. 

Setae  —  bristlelike  structures. 

Siphon  —  a  tube  through  which  water  may  pass  into  and  out  from  the  mantle 

cavity  of  a  mollusc. 
Sperm  —  male  sex-cell. 

Spicules  —  siliceous  or  calcareous  secreted  skeletal  structures  of  sponges. 
Spiral  valve  —  a  spiral  infolding  of  intestinal  wall. 
Spongin  —  material  of  skeletal  fibers  of  certain  sponges. 
Sporangium  —  a  sac  containing  spores. 
Spore  —  a  type  of  reproductive  cell,  usually  asexual,  with  a  protective  covering 

enabling  it  to  survive  unfavorable  environmental  conditions. 
Sporophyll  —  a  sporangium-bearing  leaf  of  ferns. 
Sporophjrte  —  spore-bearing  stage  in  the  life  cycle  of  a  plant. 
Stapes  —  stirrup-shaped  innermost  bone  of  middle  ear  of  mammals. 


ROLL  CALL  12:j 

Sterigma  —  a  slender  filament  arising  from  basidium,  giving  rise  to  spores  by 
abstriction. 

Stigma  —  the  part  of  a  pistil  which  receives  the  pollen  grains. 

Tarsal  —  pertaining  to  the  tarsus  or  last  region  of  the  leg  of  an  insect ;  the  ankle 
bones  of  vertebrates. 

Tentacles  —  flexible  organs  at  the  oral  region  of  an  animal,  used  for  feeling, 
grasping,  etc. 

Thallus  —  a  simple  plant  body  not  differentiated  into  root,  stem,  and  leaf. 

Thorax  —  the  part  of  the  body  between  the  head  and  abdomen. 

Tracheae  —  respiratory  tubes  of  insects. 

Tracheal  gills  —  small  winglike  respiratory  outgrowths  from  the  abdomen  of 
aquatic  larvae  of  insects. 

Triploblastic  —  with  three  primary  germinal  layers. 

Trochophore  —  free-living  pelagic,  ciliated  larval  stage. 

Tube-feet  —  organs  of  locomotion  of  echinoderms. 

Tuberculate  —  resembling  or  having  root-swellings  or  nodules. 

Ungulates  —  hoofed  animals. 

Vascular  —  consisting  of  or  containing  vessels  adapted  for  transmission  or  cir- 
culation of  fluid.  •*; 

Vein  —  branched  vessel  which  carries  blood  to  heart;  rib  or  nervure  of  insect 
wing. 

Velum  —  a  membranous  partition  likened  to  a  veil  or  curtain. 

Ventral  —  pertaining  to  the  belly  surface  or  under  side. 

Vertebrae  —  bones  of  the  vertebral  column  (backbone). 

Vestigial  —  small  and  imperfectly  developed ;  rudimentary. 

Whorl  —  (Bot.)  circle  of  flowers  or  parts  of  a  flower  arising  from  one  point. 
{Zool.)  spiral  turn  of  univalve  shell. 

Xanthophyll  —  a  yellow  pigment  invariably  associated  with  chlorophyll  in  higher 
plants. 

Zoospore  —  a  motile  spore  of  either  plants  or  animals. 


FUNDAMENTALS  OF  STIU  CTURE  AND   Fl  NC'lioN 


V 


LIFE  AND   PROTOPLASM 

Preview.  What  is  being  alive?  •  Metabolism  •  Some  signs  of  mani- 
festation of  life  •  The  production  and  use  of  enzymes  associated  with  living 
things  ■  Structure  of  protoplasm  •  Protoplasm  and  the  cell  •  Chemical  or- 
ganization of  living  matter  •  Protoplasm  a  complex  mixture  •  Protoplasm 
a  colloidal  mixture  •  Diffusion  ■  Osmosis  and  its  significance  to  living  cells  • 
Suggested  readings. 

PREVIEW 

Being  alive  is  something  that  we  all  know  a  little  about.  Liveliness 
is  associated  with  those  of  one  group  who  are  "up  and  coming," 
those  who  are  active,  both  mentall}^  and  physically.  If  living  things 
are  thought  about  a  little  more  closely,  certain  things  are  attributed 
to  them  :  they  move,  feed,  grow,  are  sensitive,  and  they  reproduce  their 
kind.  The  scientist  goes  a  step  further  and  compares  the  living  thing.s 
with  those  which  do  not  possess  this  mysterious  something  we  call 
life.  He  says  life  is  a  manifestation  of  forces,  like  a  flame,  or  elec- 
tricity. He  goes  beyond  superficial  observation  and  asks  himself 
a  good  many  questions  about  the  make-up  and  action  of  the  living 
things  which  fill  his  environment.  Some  of  the  problems  with  which 
one  is  confronted  are  relatively  simple  and  may  be  solved  with  a  little 
close  observation,  even  without  the  aid  of  a  microscope,  but  other 
problems  are  speculative  and  may  never  be  answered  in  full. 

If  the  problems  were  to  be  assembled  with  a  view  to  attempting 
their  solution,  some  of  the  more  important  might  be  the  following : 
What  is  being  alive?  What  differentiates  living  stuff  from  non- 
living? What  is  known  about  the  ultimate  composition  of  the  living 
stuff  ?  Is  it  different  for  animals  and  for  plants  ?  And  what  common 
characteristics  can  be  found  for  i)lants  and  animals? 

It  is  obvious  that  our  problems  resolve  themselves  into  two  gr()Ui)s. 
those  which  are  more  or  less  speculative  and  those  which  depend  on 
the  knowledge  provided  by  the  physicist,  chemi.st,  and  biologist. 
The  newer  knowledge  of  chemistry  and  physics  and  the  u.se  of  the 
refinements  of  the  compound  microscope  have  made  possible  mudi 
that  was  undreamed  of  a  i'o^^■  decades  ago.     It  is  only  260  years  since 


12.') 


126  FUNDAMENTALS   OF  STRUCTURE   AND   FUNCTION 

the  first  simple  microscopes  of  the  Dutchman,  Antony  van  Leeuwen- 
hoek,  but  enormous  advances  have  been  made  in  this  period.  The 
most  rapid  advances  in  chemistry,  physics,  and  biology  have  come 
in  the  last  two  or  three  decades,  but  we  are  still  far  from  the  solution 
of  the  great  riddle  of  the  universe  —  What  is  life,  and  from  whence 
did  it  come  ? 

What  Is  Being  Alive? 

The  chemist  or  the  biologist  weighs  the  food  an  organism  eats  and 
thus  finds  out  that  much  of  the  energy  locked  up  in  food  is  trans- 
formed within  the  body  of  the  organism,  ultimately  to  be  released 
in  another  form,  either  in  heat  production  or  in  work  of  some  kind. 
Not  only  do  living  things  release  energy  but  they  also  grow  and  are 
able  to  repair  parts  that  are  wasted  or  lost.  Think  of  the  athlete, 
hale  and  hearty,  winning  points  for  his  team ;  losing  weight  in  a 
football  game,  making  it  up  after  the  game  at  the  training  table,  or 
imagine  the  same  athlete  recovering  from  a  severe  illness,  or  with 
his  leg  in  a  cast  after  an  accident.  One  may  feel  fairly  sure  that  he 
will  soon  be  well  again.  The  living  stuff  of  which  he  is  made  will 
not  only  use  the  food  to  release  energy  for  his  normal  processes,  but 
will  also  rebuild  the  expended  body  material  and  rid  itself  of  such 
wastes  as  result  from  the  process.  Put  in  another  way,  this  living 
stuff  of  which  an  athlete  is  composed  has  the  ability  to  take  in  food, 
to  use  this  food  for  the  release  of  the  energy  stored  up  in  it,  or,  under 
certain  conditions,  to  make  some  of  the  food  over  into  living  ma- 
terial. Living  things  thus  have  the  capacity  for  growth,  for  waste 
and  repair,  and,  like  man-made  machines,  have  the  abihty  to  use  food 
fuel,  and  to  release  energy  from  it. 

Metabolism 

The  sum  total  of  all  the  processes  involved  in  the  business  of  being 
alive  is  called  metabolism.  This  series  of  processes  is  twofold  :  first, 
constructive  metabolism,  or  anabolism,  in  which  the  food  material 
becomes  a  part  of  the  living  organism,  the  energy  being  held  there  in 
a  potential  form  ;  and  second,  by  destructive  metabolism  or  katabo- 
lism,  in  which  the  body  material  is  broken  down  to  release  energy, 
and  in  which,  as  a  result,  there  is  the  production  of  work  and  a 
passing  off  of  waste  products.  This  text  is  concerned,  by  and  large, 
with  the  various  phases  of  metabolism  which  will  be  considered  in 
greater  detail  later. 


LIFE   AND    PROTOl'LASM  joy 

Some  Signs  of  Manifestation  of  Life 
One  sign  of  life  is  the  release  of  cnersy,  which  is  a  rosult  of  respir-i- 
tion.     It  occurs  in  all  living  things,  ho  it  a  tree,  a  frog,  or  a  man 
when  oxygen  is  taken  into  the  body,  whore  it  combines  with  oxidiz- 
able  materials  to  release  energy.     The  by-products  are  carbon  dioxide 
and  water,  which  are  given  off  by  the  organism. 

Living  things  are  sensitive  to  and  respond  to  various  stimuli  in 
their  environment.  The  plant  in  the  window,  the  earthworm  in 
the  ground,  the  fish  in  the  water,  and  the  bird  in  the  tree  are  all 
sensitive  to  and  respond  in  different  ways  to  the  stimulus  of  light. 
Temperature,  chemical  substances,  gravity,  electricity,  radiations, 
and  mechanical  factors,  all  are  stimuli  which  affect  living  things  in 
different  ways.  It  is  this  characteristic  of  li\ing  things  that  we  call 
irritability  or  sensitivity. 

One  direct  outcome  of  the  ability  of  living  things  to  respond  to 
stimuli   in   their   environment    is   their   adaptiveness.      Thus   li\ing 
organisms  ha\e  the  capacity  to  adjust  themseh-es  to  changes  in  con- 
ditions.    Some  plants  throw  out  new  roots,  or  suckers,  or  trailing 
stems,  by  means  of  which  they  can  get  a  foothold  in  slightly  different 
environments  from   those  in  which   they  are  accustomed   to  live. 
Certain  low  forms  of  plants  have  even  become  adapted  to  li\e  in  the 
hot  springs  as  those  in  the  Yellowstone  National  Park,  in  a  habitat 
many  degrees  warmer  than  that  of  their  near  relatives  found  in 
adjoining  pools.     Fishes,  and  certain  small  crustaceans,  may  similarly 
adapt  themselves  to  life  in  water  containing  a  high  concentration  of 
salts.     This  power  of  adaptation  is  a  quality  of  the  organism  as  a 
whole,  and  results  in  adjustment  between  the  external  environment 
and  the  internal  body  material. 

The  Production  and  Use  of  Enzymes  by  Living  Things 

In  recent  years  a  good  deal  of  work  has  been  done  by  physiologists 
to  see  how  the  cell  is  able  to  perform  the  cycl(\  in  which  material 
is  taken  into  the  organism  as  food  or  is  made  into  food  as  in  the  case 
of  green  plants.  Food  is  changed  into  a  j^oluble  form  so  that  it  may 
pass  through  the  delicate  living  membrane  of  every  coll.  Meantime 
each  cell  is  using  oxygen,  which  also  has  to  be  taken  in  through  the 
cell  membrane,  while  wastes  are  given  off  by  the  same  road.  Physi- 
ologists seem  agreed  that  those  living  processes,  called  digestion, 
absorption,   respiration,   and  excretion,  are  made   possible   by   the 


128 


FUNDAMENTALS   OF  STRUCTURE   AND   FUNCTION 


presence  of  substances  called  enzymes,  which  act  as  catalyzing  agents, 
thus  hastening  by  their  presence  the  performance  of  such  functions. 
(See  pages  279-280.)  Enzymes  are  manufactured  in  certain  cells 
and  it  is  believed  that  every  cell,  even  an  egg  cell,  contains  enzymes 
which  are  capable  of  digesting  food  substances,  as  well  as  those  which 
aid  in  oxidation  within  the  cell. 

In  plants,  enzymes  seem  to  be  made  in  almost  any  cell  that  is  active 
and  these  enzymes  usually  have  a  reversible  action.  For  example, 
certain  insoluble  foods  may  be  broken  down  or  hydrolyzed  in  the 
cells  of  the  leaf,  so  that  they  are  soluble,  then  they  will  pass  in  that 
condition  to  the  stem,  the  roots,  or  the  fruit,  where  a  reverse  action 
takes  place  and  the  food  is  stored  in  an  insoluble  condition.  In 
animals,  the  hydrolyzing  enzymes  which  make  digestion  possible  are 
usually  formed  by  groups  of  cells  forming  glands. 

Structure  of  Protoplasm 

This  living  material,  known  as  protoplasm,  has  been  called  by  the 
biologist  Huxley  "the  physical  basis  of  life."     It  is  this  stuff  that  is 

always  present  in  things 
that  are  ahve.  In  our 
present  state  of  knowl- 
edge we  may  liken  it  to 
the  albumen  or  white 
of  egg,  a  nearly  colorless 
and  translucent  sub- 
stance, like  soft  jelly. 
This  substance  seen  un- 
der the  compound  microscope  has  many  granules  floating  in  it. 
It  is  more  or  less  elastic,  although  in  some  cases  it  will  flow  like 
a  dense  liquid.  Seen  under  a  high  magnification  it  may  be  almost 
homogeneous  in  structure  or  may  appear  foamy  or  spongelike,  or 
even  fibrillar  in  appearance.  A  study  of  living  cells  shows  that  it 
is  obviously  quite  different  in  structure  at  different  times  and  in 
different  animals  and  plants. 


^^C^. 


gretntjtlav 
"structure 


Stritctvtre 


ctlveolecr 
structure. 


Three  states  of  protoplasm.     2  and  3  have  much 
higher  magnification  than  1. 


Protoplasm  and  the  Cell 

Although  cells  were  first  described  in  1665  by  Robert  Hooke,  it 
was  not  until  the  nineteenth  century  that  the  cell  theory  came  into 
the  spotlight.  The  knowledge  that  all  organisms,  plant  and  animal, 
are  composed  of  fimdamentally  identical  protoplasmic  units,  or  cells, 


LIFE   AND   PROTOPLASM 


120 


forms  ono  of  the  most  important  corner  stones  in  th..  roun.JMli.,,,  of 
biology. 

Wiiile  plant  and  animal  colls  possess  some  rather  striking  cJifT.T- 
ences  m  organization,  they  ar(>  fnndamentally  similar.     Prac-tic-iUy 
every  cell  that  is  microscopically  visible  possesses  several  difTerent 
kmds  of  structures  located  within  its  borders.     Some  of  these  struc- 
tures are  alive,  some  lifeless.     In  the  first  group 
may  be  placed  the  -plastiih  of  plant  cells,  the 
mitochondria  or  chrondrio somes,  some  of  which 
probably  give  rise  to  plastids,  fibers  of  various 
kinds,  the  Golgi  bodies  and  the  centrosomes,  the 
latter  of  importance  in  animal  cell  division.     In 
the  second  group  may  be  placed  such  inclusions 
as  yolk,  or  other  food  substances,  fatty  droplets, 
granules  of  pigment  or  of  secretions  (as  in  gland 
cells),  and  crystals  of  various   kinds,   such  as 
calcium  oxalate  in  plant  cells.     To  this  list  may 
also  be  added  vacuoles,  which  in  plant  cells  often 
occupy  the  major  space  within  the  cell  membrane. 
All  of  these  structures  are  confined  to  the  cyto- 
plastn  or  part  of  the  protoplasm   outside  the 
nucleus.     In  Elodea,  the  cells  present  a  green       t^. 
appearance,  due  to  the  presence  of  many  tmy    pi.yii  cell  of  a  K-af; 
ovoid  bodies,  the  chloroplasts,  which  are  plastids    <"•  ihl()ro[)last ;  n,  nu- 
containing  chlorophyll.     Careful  obser^•ation  of 
a  single  cell  shows  that  the  chloroplasts  move 
slowly  down  one  side  of  the  cell,  across  one  end,  and  uj)  the  other  side, 
keeping  rather  close  to  the  outer  edge  of  the  cell  during  the  process. 
This  is  due  to  the  movement  of  the  cytoplasm.     In  the  cells  of  the 
hairlike  stamen  of  Tradescantia,  the  movement  of  the  cytopla.'^m  is 
also  evident.     Here  it  can  be  seen  actively  streaming  in  currents 
within  the  cell,  carrying  along  within  it  tiny  crystals  of  inorganic 
origin,  as  well  as  colorless  plastids  and  granules.     The  latter  term  is 
usually  applied  to  inert  materials,  such  as  granules  of  stored  food  in  the 
form  of  starch  grains  (in  plants),  fat  or  yolk  granules,  or  pigment 
granules  which  frequently  occur  scattered  througiiout  |)r()toi)lasm. 
Between    the    strands    of    (ytoplasm    are    spaces    or    vaeuolrs   filled 
with   a   watery   fluid,    called   cell   sap.     In   young   jilant    cells,   the 
vacuoles  are  small  and  the  cytoplasm    occupies    the   greater    part 
of  the  cell,  but  in  mature  plant  cells  the  cytoplasm  is  found  clo.se 


{'k'us ;       r 
u\  cell  wal 


\  a<n(il(' 


130 


FUNDAMENTALS   OF   STRUCTURE  AND   FUNCTION 


Xell  membrane 

Cytoplasm 

..Cenfrosomes 
...Nucleolus 

Nucleus 
Plasiid 

Vacuole 


Diagram  of  a  typical  animal  cell. 


to  the  outer  part  of  the  cell,  while  the  vacuoles  form  large  sap  cavities 
within  the  cell.  Although  Golgi  bodies  appear  much  less  stable  and 
more  changeable  in  form  than  plastids,  they  are  found  in  many  kinds 
of  plant  and  animal  cells.     Fibrils  of  various  kinds,  such  as  those 

seen  in  a  muscle  cell,  are  frequently 
found.  In  plants  the  cell  wall,  a 
delicate  but  rigid,  secreted  cellulose 
covering,  is  lined  with  a  delicate 
living  membrane  which  separates 
the  living  stuff  from  the  cell  wall. 
At  one  point  can  be  found  a  slightly 
denser  jellylike  part  of  the  proto- 
plasm called  the  nucleus.  Both 
the  vacuoles  and  the  nucleus  are 
separated  from  the  cytoplasm  by 
delicate  membranes.  In  many  but 
not  all  nuclei,  dense,  dark-staining 
nucleoli  appear.  While  their  func- 
tion is  not  clearly  understood,  they 
generally  break  up  and  disappear 
during  cell  division.  The  nucleus  proper  is  a  vital,  definite  part  of 
every  living  cell  and  is  of  great  importance  in  cell  division  which 
must  take  place  if  a  many-celled  organism  is  to  grow  in  size,  for 
growth  takes  place  by  an  increase  in  the  number  of  cells,  not  in  the 
size  of  the  cells.  The  nucleus  is  filled  with  nuclear  sap,  in  which 
is  found  a  network  of  linin  fibers.  On  these  fibers  are  scattered 
numerous  granules  of  chromatin.  This  material,  which  as  we  will 
see  later  forms  chromosomes,  is  of  the  greatest  importance,  as  through 
it  plants  and  animals  are  able  to  pass  on  to  successive  generations 
their  inheritable  qualities. 

Chemical  Organization  of  Living  Matter 

A  dozen  or  more  of  the  ninety-odd  elements  recognized  by  the 
chemist  are  found  in  living  protoplasm,  —  carbon,  hydrogen,  oxy- 
gen, and  nitrogen  comprising  the  greatest  bulk.  These  elements  also 
form  the  basis  of  our  so-called  organic  foodstuffs,  which  are  called 
proteins,  carbohydrates,  and  fats.  The  two  latter  groups  of  sub- 
stances are  made  up  of  carbon,  hydrogen,  and  oxygen,  while  the  pro- 
teins have  the  element  nitrogen  added  to  their  constitution,  along 
with  sulphur,  phosphorus  and  sometimes  iron.     In  a  simple  carbo- 


LIFE  AND   PROTOPLASM  ,  ,, 

hydrate,  such  as  ghicose,  for  example,  the  chemist  writes  a  fornuihi 
representing  a  molecule  of  the  substance.  In  such  a  simple  nujlecule 
the  atoms  of  hydrogen  and  oxygen  are  usually  united  in  the  same 
proportion  as  in  water  and  the  empirical  formula  is  written  CcHi-Oa. 
This  water  proportion  (H,.0)  is  maintained  in  other  more  complex 
carbohydrates  such  as  starch,  but  here  the  chemist  writes  an  x  after 
the  empirical  formula  (CeHioOa)^.  This  means  that  the  molecular 
formula  is  not  exactly  known  but  in  the  case  of  starch  the  x  should 
probably  be  about  200,  which  makes  the  molecule  very  much  larger 
than  that  of  the  simple  sugar.  The  simple  sugars  with  their  small 
molecules  are  ea.sily  soluble  in  water,  while  the  complex  molecule  of 
the  starch  is  not  .so  soluble.  In  fatty  substances,  oxygen  is  present  in 
a  much  smaller  proportion  than  in  the  carbohydrates.  An  example 
might  be  oleic  acid,  one  of  the  components  of  butter  fat,  (Ci8H,3402). 
Proteins  have  still  more  complex  molecules.  In  the  first  place  tliey 
are  built  up  of  simpler  substances,  called  amino  acids,  and  in  some 
cases  other  radicals  are  added  to  them.  For  example,  in  the  cell 
nuclei  the  protein  is  combined  with  nucleic  acid,  which  has  the  aston- 
ishingly complex  formula  C38H49029Xi5P4,  which  really  means  very 
little  except  to  the  student  of  chemistry. 

Protoplasm  a  Complex  Mixture 

Living  stuff,  having  the  same  elements  as  the  complex  foodstuffs 
for  a  ba.sis,  is  even  more  intricate.  No  chemical  compounds  in  nature 
are  quite  as  complicated  in  composition,  for  protoplasm  not  only 
is  made  up  of  the  foodstuffs  but  it  also  consists  largely  of  water. 
One  estimate  by  weight  gives  80  per  cent  water,  15  per  cent  proteins, 
3  per  cent  fats,  1  per  cent  carbohydrates  and  other  organic  substances, 
and  1  per  cent  inorganic  salts.  It  has  been  determined  that  carbon, 
nitrogen,  hydrogen,  oxygen,  and  phosphorus  are  alwaj's  present  in 
protoplasm  and  are  called  the  primary  elements.  Magnesium,  pota.^- 
sium,  iron,  and  sulphur  appear  equally  nece.s.sary  for  life.  Sodium 
and  chlorine  are  always  found  in  animal  but  only  infrequently  in  jilant 
tissues,  and  calcium  appears  necessary  for  life  in  the  higher  forms. 
Other  elements,  bromine,  fluorine,  iodine,  silicon,  boron,  manganese, 
and  even  copper,  zinc,  and  aluminum,  are  found  in  some  organisms. 

While  some  of  these  elements  are  solids  and  others  gases,  none  of 
them,  except  oxygen,  typically  occurs  to  any  marked  extent  free 
in  the  organism.  Nor  are  th(>y  found  free  in  the  foods  or  waste 
products,  but  rather  as  various  kinds  of  chemical  compounds  which 


132 


FUNDAMENTALS   OF   STRUCTURE   AND    FUNCTION 


may  be  further  subdivided  into  inorganic  and  organic  compounds. 
The  former  comprise  most  of  the  non-Hving  compounds  such  as  soil 
and  rocks  and  their  decomposition  products.  However,  in  proto- 
plasm, inorganic  compounds  are  usually  present  as  water,  salts,  or 
gases.  Water  is  important  not  only  because  it  comprises  70-98  per 
cent  of  protoplasm  by  weight,  but  also  because  it  dissolves  so  many 
different  substances.  Furthermore,  water  is  an  important  factor  in 
promoting  the  dissociation  of  many  salts  into  their  constituent  ions. 
The  inorganic  salts  which  occur  in  marine  organi.sms,  for  example, 
are  usually  those  commonly  found  in  sea  water.  Some,  such  as  ni- 
trates and  nitrites,  occur  chiefly  in  plants,  while  compounds  con- 
taining sodium  and  chlorides  are  characteristic  of  animal  tissues. 
Only  three  gases  are  found  in  varying  amounts  in  the  living  cell,  — 
free  oxygen,  carbon  dioxide,  and  ammonia. 

Protoplasm  a  Colloidal  Mixture 

Matter  exists  in  three  states,  gaseous,  liquid,  and  solid.  Frequently 
it  passes  from  one  state  to  another,  as  when  ice  melts  under  the  in- 
fluence of  heat,  turn- 
ing to  steam  as  the 
water  boils  away. 
That  protoplasm  at 
different  times  and 
under  different  con- 
ditions varies  in  ap- 
pearance is  probably 
due  to  the  fact  that 
A  B  c  it  is  a  colloid  and  as 

Colloidal  constitutions.     The  continuous  phase  in  a    such  can  change  from 
being  fluid;  in  a  jell  (C)  solid;  while  in  the         "sol  "  or  liouid    tO 


sol  (A 

intermediate  phase  (B)  the  solid  forms  a  net  through 

which  the  fluid  is  continuous. 


a  "gel,"  or  solid  state 
and  then,  under  cer- 
tain conditions,  back  again.  The  scientist  examines  protoplasm 
under  the  ultramicroscope  and  finds  tiny  dancing  particles  which 
are  invisible  under  the  ordinary  illumination  of  the  microscopic  field 
(Brownian  movement).  This  condition  is  known  as  a  dispersion, 
the  dispersed  particles  being  carried  in  the  dispersion  medium, 
in  this  case  water.  A  fog  composed  of  tiny  droplets  of  water  is 
an  example  of  dispersion  in  nature.  If  the  particles  in  a  disper- 
sion are  small,  the  substance  is  called  a  crystalloid,  when  large  it 


LIFE   AND    PROTOPLASM 


i:{3 


is  called  a  colloid.  Xow  these  terms  are  not  api)lie(l  t(j  fixed  sub- 
stances but  to  states  of  matt(M-.  Gelatin  passes  from  a  li{|uid  to  ii 
solid  state  on  being  heated  or  cooled.  A  study  of  the  diap;ram  shows 
how  this  might  be  possible.  In  the  left-hand  diagram  the  solid  i)ar- 
ticles  are  floating  freely  in  the  fluid  of  the  medium  ;  in  the  middle 
diagram  the  solid  portion  is  becoming  a  loose  mesh;  while  in  the 
right-hand  diagram  the  mesh  has  become  a  solid  mass,  including  the 
liquid  within  it .  The  protoplasm  within  the  cells  of  plants  and  animals 
probably  behaves  in  a  similar  manner,  under  some  conditions  assum- 
ing the  "sol,"  and  at  others  the  "gel"  state.  Remembering  that 
protoplasm  is  not  a  single  protein  substance,  but  rather  a  mi.xture 
of  proteins,  fats,  carbohydrates,  and  sometimes  even  other  sub- 
stances, it  is  clear  why  there  are  many  slightly  different  protoplasms 
depending  on  the  part  of  the  animal  or  plant  examined.  This  fact 
may  help  us  to  see  why  the  living  matter  of  a  muscle,  the  blood,  or 
the  brain  differs  visibly  in  structure.  For  one  thing,  the  water  con- 
tent differs  greatly.  Living  bone  is  said  to  be  25  per  cent  water, 
muscles  about  75  per  cent,  the  jellyfish  almost  99  per  cent,  and  some 
fruits  as  high  as  98  per  cent  water. 

Diffusion 

We  have  spoken  of  the  work  of  the  enzymes  in  making  food  sub- 
stances soluble.  Let  us  now  see  why  .'^olul)ility  is  necessary  for  the 
life  processes  of  cells.  The  physical  phe- 
nomenon of  diffusion  is  easily  demonstrated 
by  the  slow  spread  of  red  ink  when  a  droj) 
is  put  into  a  glass  of  water.  Brownian 
movement  of  dancing  particles  visible  under 
the  high  power  of  the  microscope  is  a  mani- 
festation of  molecular  kinetic  energy  caused 
by  the  water  molecules  bombarding  these 
particles.  It  is  a  similar  movement  of 
molecules  that  occurs  when  diffusion  takes 
place.  Molecules  of  any  substance  are 
always  in  motion.  If  this  substance  is 
soluble  (the  solute)  in  another  substance 
(the  solvent),  there  is  always  a  tendency 
for  these  molecules  to  move  from  the 
place  of  their  greatest  concentration  to  i^laces  where  they  are  not 
so  highly  concentrated,  until   an  equilibrium  is  reached  and   there 


Lonfjit  udiiial  sect  ion 
through  a  tumbler  of  watt-r 
containing  soluble  crystal. 
sliowiiif;  by  arrows  tlic direc- 
tion of  (lillusion.  ami  by 
(lotted  circles  the  lines  of 
equal  concentration. 


134 


FUNDAMENTALS   OF   STRUCTURE   AND   FUNCTION 


are  just  as  many  molecules  of  the  solute  in  one  part  of  the  solvent 
as  in  another.  In  the  case  of  the  diffusion  of  red  ink  in  water, 
the  eosin  (which  is  the  coloring  material  used)  was  more  concen- 
trated in  the  drop  than  in  the  water,  so  the  molecules  of  eosin  began 
moving  away  from  this  place  of  high  concentration  until  they  were 
equally  dispersed  throughout  the  water.  As  a  general  rule  we  may 
say  that,  if  other  conditions  are  equal,  the  diffusion  rate  between 
two  points  is  proportional  to  the  differences  in  concentration  of  the 
substances  at  these  two  points.  One  thing  which  affects  the  diffusion 
rate  is  the  nature  of  the  medium,  w^hether  it  be  a  gel,  emulsion,  or 
some  sort  of  semisolid  (porous).  Gelatin,  for  example,  which  is  a 
gel,  offers  no  effective  resistance  to  the  diffusion  of  molecules  of  a 
crystalloid  nature  through  its  meshes,  but,  upon  the  other  hand, 
this  network  may  serve  to  block  effectively  the  passage  of  colloidal 
substances. 

Suppose  a  membrane  were  stretched  crosswise  in  a  jar  where 
diffusion  was  taking  place.  Could  the  molecules  of  the  diffusing 
substance  pass  through  the  membrane?  This  depends  on  whether 
the  membrane  is  permeable  to  the  diffusing  substance.  In  some 
membranes  the  ultramicroscopic  "pores"  are  believed  to  be  quite 
large,  thus  letting  through  molecules  of  larger  sizes,  while  in  other 
membranes  the  "pores"  through  which  substances  can  diffuse  are 

very  small.  Other  substances 
penetrate  in  proportion  to  their 
lipoid  solubility.  Thus  some 
membranes  allow  certain  sub- 
stances to  pass  through,  while 
they  keep  out  others.  Such  mem- 
branes are  said  to  be  selectively 
'permeable.  An  ordinary  parch- 
ment  membrane   will   allow   the 


'^l 


ff^ 


".•-•'•.^«.--  ■',■  ■' i'ly. l,v-i  .'•:~-r 


'■f.^:.:^-. 


■■"iPV^i^- 


a 


W 


e         p  "'"-"-^f 


V 


Diagram    of    an    imaginary    section  .  i    •.       t->    -  -u 

through  the  cell  wall  and  protoplast  to  eosm  to  pass  through  it.     But  the 

show  a,  outer  water ;  iv,  cell  wall ;  c,  ecto-  cell  membrane  does  not  act  in  the 

plast   or    cytoplasmic   membrane   next  ^^^^  manner,   as   it   is   a   vldsma 
to  the  cell  wall;   p,  general  cytoplasm;  i         i     j.-      i 

/.  tonoplast  or  inner  cytoplasmic  mem-  membrane,    and    selectively    per- 

brane  next  to  the  water,  thus  forming  meable. 

a  continuous   pathway  which  carries       ^j^^    plasma   membranes   sur- 
solutes  irom  (a)  to  (?)) ;  i\  vacuole.  ,.        ,.    .  ,    ,.         , 

rounding  living  cells  are  believed 

to  be  colloidal  in  nature,  made  of  a  combination  of  fatty  and  protein 

substances.     Careful   experiments   have   demonstrated   them  to  be 


LIFE   AND   PROTOPLASM 


i.r, 


selectively  permeable.  Most  living  cells  allow  oxygen  and  carhon 
dioxide  to  pass  freely  through  their  mcmijranes,  while  diss(jlved 
sugars  and  digested  proteins  in  the  form  of  amino  acids  dilTu.sc 
through  more  slowly.  Water  of  course  passes  through,  acting  as  a 
vehicle  for  other  substances.  Such  membranes  are  impernicablc  to 
certain  salts  and  not  to  others.  The  permeability  of  living  cells 
to  dissolved  substances  differs  with  the  cell,  and  naturally  with  tlu; 
organism.  Salt-  and  fresh-water  fishes  are  examples  of  types,  the 
cells  of  whose  gills  exhibit  different  permeabilities.  Dead  cell  mem- 
branes are  usually  permeable  to  crystalloid  solutes,  while  living 
cell  membranes  permit  but  few  salts  to  enter.  In  general,  cells  are 
not  permeable  to  colloids,  because  of  the  large  size  of  the  particles 
constituting  the  colloid. 

Osmosis  and  Its  Significance  to  Living  Cells 

We  have  already  seen  that  if  a  membrane  is  sc^lectiveiy  permeable, 
then  some  substances,  such  as  water  or  certain  solutes,  will  pass 
through  readily,  but  other 
solutes  may  not.  because 
their  molecules  are  too  large 
to  pound  their  way  through 
the ultramicroscopic  "pores" 
of  the  membrane.  The 
process  by  which  substances 
diffuse  through  membranes 
is  known  as  osmosis.  It  is 
of  the  greatest  importance 
to  living  cells,  as  it  is  by  this 
means  that  dissolved  gases, 
such  as  oxygen,  and  dis- 
solved food  substances  get 
into  the  cell,  as  well  as  the 
process  by  which  waste  ma- 


- sugar 


^littion... 


lJ»  selectively 
lJ.pcrroeab\^ 


t 


-'-'4* 


molcciclc 


^       •  •wat«r'mo\eculti   =^       °'TS^^ 


-penTKoBe 


=1.  "WCCt&V 


vater- 


.XLi» 


Diagram  to  explain  osmotic  pressure.  Sii>:ar 
solution  is  of  equal  density  in  ea<h  tube. 
Explain  rise  of  solution  in  left  hand  tube. 


terials  pass  out.     Perhaps  a  further  word  of  oxplana  .on  ,s  u,  orcler. 
Other  things  being  equal,  if  two  soh.tions  «' '''«""": ™"''™'7,;" 
are  separated  by  a  permeable  membrane,  the  chffus.on  ^v,ll  ^fll    , 
in  the  direetion  of  the  greater  to  the  lesser  ™';'-"-  "'  ;    V^ 
it  a  sugar  solution  be  .separated  by  a  pern,eabIo  men,b,a,.o 
another  more  dilute  sugar  solution,  diffusion  -l'/^'  ;;■'," 
the  more  coneentrated.     If,   however,  we  separate   «at,.   from  .. 

H.   W.    H.  —  10 


136 


FUNDAMENTALS   OF   STRUCTURE   AND   FUNCTION 


sugar  solution  by  a  selectively  permeable  membrane,  the  water  mole- 
cules tend  to  pass  through  the  membrane  (since  it  is  permeable  to 
water)  from  the  water,  to  the  sugar  solution  where  the  water  is  in 
less  concentration.  Actually  it  is  a  question  of  the  water  molecules 
of  the  solvent  reaching  an  equilibrium. 

Osmotic  pressure,  in  living  cells,  is  one  of  the  factors  that  accounts 
for  the  rise  of  water  in  roots  and  up  the  stems  of  plants.  Its  effects 
can  easily  be  demonstrated  experimentally  in  the  laboratory  by  plac- 
ing, for  example,  living  cells  of  Spirogyra  in  a  10  per  cent  solution  of 
salt  and  water.  The  water  from  within  the  cell  (where  it  is  in  greater 
concentration)  passes  out  through  the  cell  membrane  to  enter  the  salt 
solution  (where  water  is  in  less  concentration  than  in  the  cell).  The 
result  is  that  the  cell  body  shrinks  away  from  the  cell  w^all  and  the 
shrunken  cell  is  said  to  be  plasmolyzed.     A  solution  which  contains 

a  greater  number  of  mole- 
cules of  the  substance  in 
solution  (solute)  per  vol- 
ume than  the  interior  of 
the  cell  is  said  to  be  hy- 
perosmotic; if  it  has  less 
concentration  than  the  in- 
terior of  the  cell  it  is 
hyposmotic;  and  if  it  has 
the  same  number  of  solute 
molecules  per  unit  volume 
as  the  interior  of  the  cell 
the  solution  is  isosmotic  to 
the  cell. 

When  a  cell  is  placed  in 
a  hyposmotic  solution  it 
will  tend  to  swell  up,  be- 
cause water  is  diffusing 
more  rapidly  inward,  and  so,  unless  the  cell  is  surrounded  by 
heavy  walls  as  in  the  case  of  plants,  the  cell  will  tend  to  burst. 
When  this  happens  it  is  called  cytolysis.  This  may  be  demon- 
strated when  human  red  blood  corpuscles  are  placed  in  distilled 
water.  It  is  evident,  therefore,  that  osmotic  pressure  differs  greatly 
in  the  cells  of  different  organisms,  possibly  depending  on  whether 
they  live  in  fresh  or  salt  water  and  the  consequent  concentration  of 
salts  present.     As  a  matter  of  fact,  fresh- water  organisms  live  in  a 


pelliole  — 


.cytoplasm. . 


YlllClsZJiS. 


..ceU\/cdl 


Plasmolysis  in  a  plant  and  an  animal  cell. 
Note  how  the  cytoplasm  has  shrunk  away  from 
the  wall  in  the  case  of  the  plant  cell  and  the 
pellicle  in  the  case  of  Paramecium.  Why  has 
this  occurred  ? 


LIFE   AND   PROTOPJASM  I37 

hyposmotic  solution.  In  plants,  the  cell  walls  prevent  the  cells  from 
swelling  up,  while  in  animals  there  are  special  ways  of  ridding  the 
body  cells  of  excess  water. 

SUGGESTED   READINGS 

Calkins,  G.  N.,  Biology  of  the  Protozoa,  Lea  and  Febiger,  192().     Cli.  L 

A  full  and  scientific  approach  to  the  cell. 
Plunkett,  C.  R.,  Outlines  of  Modern  Biology,  Henry  Holt  &  Co.,  1930.     Chs. 

I  and  IV. 

A  chemical  and  physical  approach  to  the  study  of  protoplasm. 
Singer,  C.  J.,  The  Story  of  Limng  Things,  Harper  &  Bros.,  193 L     Chs.  IV 

and  IX. 

An  interesting  history  of  biology.     Chapters  IV  and  IX  deal  with  the 

historical  approach  to  the  cell  theory. 
Wilson,  E.  B.,  The  Cell  in  Development  and  Heredity,  The  Macmillan  Co., 

1925. 

A  classic  authority  on  the  ceU. 


VI 


CELLS  AND   TISSUES 

Preview.  Living  things  composed  of  cells  •  Plant  and  animal  cells 
differ  in  size,  shape,  and  structure  •  Why  cells  divide  •  How  plant  cells 
divide  •  How  animal  cells  divide  •  Tissues  •  The  tissues  in  plants;  the 
meristematic  tissues ;  the  protective  tissues ;  the  fundamental  tissues ;  the 
conducting  tissues  ;  the  tissues  in  animals ;  the  epithelial  tissues  ;  the  sup- 
porting tissues ;  the  muscular  tissues ;  circulatory  tissue ;  the  nervous  tissues  ; 
reproductive  tissues  •  Why  are  living  organisms  so  called?  •  Suggested 
readings. 

PREVIEW 

One  characteristic  of  living  things  is  that  they  are  organized  into 
tiny  units  of  Uving  matter  which  have  been  called,  rather  inaptly, 
"cells,"  because  an  Englishman,  Robert  Hooke,  as  early  as  1665, 
described  the  construction  of  cork  which  he  saw  under  a  lens  as 
"little  boxes  or  cells  distinguished  from  one  another."  He  cut 
cross  sections  with  a  penknife  and  saw  that  they  were  "all  cellular  or 
porous  in  the  manner  of  a  honeycomb,  but  not  so  regular."  What 
Hooke  saw  was  the  woody  walls  enclosing  spaces  which  in  younger 
plants  would  be  filled  with  living  matter. 

From  a  comparison  with  the  simplest  organisms,  it  is  evident  that 
the  more  complex  forms  are  built  up  of  cells,  and  that,  although  each 
cell  can  function  as  an  organic  whole,  far  more  efficient  results  are 
obtained  when  groups  of  cells  organized  into  tissues  do  the  work.  The 
consideration  of  groups  of  cells,  according  to  their  structure  and  func- 
tion, constitutes  in  itself  a  major  chapter  in  biological  study,  called 
Histology.  The  study  of  individual  cells,  which  make  up  the  sub- 
ject matter  of  Cytology,  is  absolutely  indispensable  to  a  proper 
understanding  of  the  organism  as  a  whole. 

The  problems  for  reading  and  further  study  are  so  numerous  that 
we  might  spend  the  major  part  of  our  available  time  in  discussing 
them.  Why  and  how  do  cells  divide?  What  are  the  differences 
between  plant  and  animal  cells?  What  are  the  reasons  for  having 
tissues  and  organs?  How  did  many-celled  organisms  come  into 
existence,  and  why?  The  pages  which  follow  will  enable  the  student 
to  make  at  least  a  start  on  some  of  these  interesting  questions. 

138 


CELLS   AND   TISSUES  ,.,,, 

Living  Things  Composed  of  Cells 

A  very  small  proportion  of  living  plants  on  the  earth  :uc  uniccliui-ir 
but  accordnig  to  Hegncr,  tiio  number  of  species  of  protozoa  or  single- 
celled  ammals  must  be  nearly,  if  not  quite,  as  great  as  all  the  other 
species  of  animals  put  together.  He  bases  his  estimate  upon  the  fa<-t 
that  practically  every  kind  of  animal  has  its  own  species  of  parasitic 
protozoa  living  upon  or  within  it.  Nevertheless  the  mctazoa,  as  the 
many-celled  animals  are  called,  make  up  most  of  the  living  animals 
that  we  know  about  on  earth  today,  just  as  the  many-celled  plants 
make  up  the  visible  and  familiar  plant  life. 

Just  how  the  many-celled  forms  of  life  evolved  from  the  unicellular 
forms  is  a  matter  of  conjecture.  Two  theories  of  origin  in  animals 
have  arisen,  one  of  which,  the  colonial  theory,  postulates  many-celled 
organisms  evolving  as  colonies  of  cells,  which  hold  together  after  fi.ssion 
to  form  plants  or  animals,  instead  of  separating  into  individual  isf>- 
lated  cells.  As  these  cell  masses  evohcd,  they  became  more  and 
more  complex,  different  systems  of  organs  appearing  in  more  highly 
organized  forms.  In  the  animal  series  shown  on  ])age  146,  this 
theory  seems  to  be  pretty  well  substantiated.  But  another  theory, 
the  organismal  theory,  considers  the  living  thing  as  a  whole,  being 
divided  into  units  of  structure  in  the  many-celled  organism.  Accord- 
ing to  such  a  theory  unicellular  organisms  would  become  first  much 
chfferentiated  within  their  own  bodies,  as  is  .seen  in  many  of  the 
protozoa.  These  theories  need  not  concern  us  further  at  present. 
Both  have  many  facts  to  support  them,  substantiatetl  by  the  devel- 
opment and  structure  of  various  types  of  organisms. 

Plant  and  Animal  Cells  Differ  in  Size,  Shape,  and  Structure 

An  examination  of  the  figure  on  page  140,  will  sliow  that  cells 
are  far  from  uniform  in  size  and  shape.  They  differ  in  size  from  the 
smallest  bacteria  which  can  just  be  distinguished  with  an  ultra-micro- 
scope that  magnifies  3000  diameters,  to  cells  that  can  be  seen  with  the 
naked  eye.  The  egg-cell  of  the  chick,  for  example,  includes  the  con- 
spicuous yolk,  while  certain  cells  in  the  human  spinal  cord,  altluiu^h 
microscopic  in  size,  may  have  prolongations  reaching  down  irito  the 
muscles  of  the  fingers  or  toes.  Cells  are  not  of  n(>cessity  lar-jcr  in 
large  animals  or  plants,  some  of  our  largest  cells  being  found  li\ing 
isolated  and  alone.  But  under  normal  conditions  a  cell  of  a  given 
size  and  shape  always  reproduces  the  same  kind  of  cell  as  itself. 


140 


FUNDAMENTALS   OF  STRUCTURE   AND   FUNCTION 


idL  "bouiillus/ 


Anthro^  bacillus^ 


As  to  shapes,  their  name  is  legion.  A  typical  cell  might  be  thought 
of  as  a  spherical  or  ovoid  body,  but  we  find  them  cubical,  flat,  thread- 
like, spindle-shaped,  columnar,  or  irregular  in  outline.     They  are  often 

modified  by  being  com- 
pressed by  other  cells, 
but  frequently  if  given 
opportunity  will  resume 
their  original  form  when 
released  from  pressure. 

Structural  differences 
exist  between  plant  and 
animal  cells,  the  chief  of 
which  is  the  cellulose  wall, 
characteristic  of  plants, 
which  gives  such  cells 
the  rigidity  and  yet  the 
flexibility  found  in  woody 
stems.  Other  physiolog- 
ical differences  will  be 
discussed  in  the  following 
chapters. 


red.  corynxsc^s^ 
.of    f  nog. 


a.no5om.<a 


r<Ed  corp 
of  )inoa7 


Cugle: 


no. 


Spex-m  ofTnarj. 


human 
livei-  cell 


Comparative  size  of  cells.  The  anthrax  bacillus 
shown  is  among  the  largest  of  the  bacteria,  while 
the  human  liver  cell  is  not  large  as  cells  go. 
(After  Wells,  Huxley,  &  Wells.) 


Why  Cells  Divide 

Every  cell  has  its  limits 
of  size  and  when  that 
size  is  reached,  if  food  is 
sufficient  and  conditions 
favorable,  it  will  divide.  In  both  plant  and  animal  cells,  the  mech- 
anism and  the.  end  results  reached  by  cell  division  are  similar,  in 
that  the  chromatin  from  within  the  nucleus  is  redistributed  so  that 
the  daughter  cells  have  approximately  the  same  amount  of  chromatin 
and  eventually  the  same  size  as  the  parent  cell  from  which  they 
came.  Cell  division  is  a  universal  phenomenon  and  seems  to  be  a 
part  of  the  normal  life  of  cells.  Theories  advanced  to  account  for 
cell  division  are  (1)  colloidal  changes  in  the  protoplasm  of  which 
they  are  composed,  (2)  electrical  changes  within  the  cell,  (3)  oxidative 
changes  within  the  cell,  and  (4)  changes  in  surface  tension.  The 
latter  can  be  experimentally  proven  by  treating  unfertilized  eggs  with 
certain  chemicals  which  cause  a  change  in  surface  tension  and  initiate 
subsequent  cell  division. 


CELLS  AND   TISSUES 
How  Plant  Cells  Divide 


III 


Both  plant  and  animal  cells  are  said  to  divide  by  a  proc-ss  c,f  n-ll 
division  called  mitosis.  In  plants,  the  resting  cell  has  a  nuclcM..  wl,i,-l. 
contains  a  network  of  linin  fibers,  on  the  strands  of  which  are  f,.,n„l 
irregular  chromatin  granules.  When  the  cell  is  activated  to  divide 
these  granules  assume  the  form  of  a  thickened,  irregularly  coiled  thread' 
called  a  spireme.  This  thread  splits  lengthwise  into  two  thr(.-„is 
which  remam  so  close  together  that  for  some  time  they  appear  as 
one,  finally  splitting  crosswise  into  a  number  of  chromo.sonus  that 


resLind 
cell   ^ 


prophets  e.       metapWe 


anaphase         telophase 

Mitosis  in  plant  cells.     Read  the  text  and  ('\|)laiii  the  (li;i;:r;iiii 


celll 


sr 


are  constant  in  number  in  all  cells  of  a  given  species.  While  this  proc- 
ess is  going  on  there  has  appeared  in  the  cytoplasm  on  oj)j")osite  sides 
of  the  nucleus  two  caplike  masses  of  delicate  fibers,  which  later  will 
give  rise  to  the  so-called  spindle  fibers.  Now  the  iiuchar  membrane 
disappears  and  the  fibers  grow  into  the  center  of  the  nucleus,  where 
some  become  attached  to  the  chromosomes  while  others  join  with 
fibers  from  the  opposite  side  or  pole.  This  series  of  changes  is 
known  as  the  prophase.  These  two  cone-shapetl  ma.s.ses  of  fibers 
form  the  spindle,  while  the  split  chromo.somes  arrange  themselves 


142  FUNDAMENTALS   OF   STRUCTURE  AND   FUNCTION 

in  a  plane  in  the  middle,  or  equator,  of  the  spindle,  this  being 
known  as  the  meta phase.  Next  the  half  or  split  chromosomes 
appear  to  be  pulled  apart  by  the  spindle  fibers  so  that  an  equal 
number  move  toward  each  pole,  where  they  come  to  rest.  These 
changes  are  called  the  anaphase. 

Here  the  spindle  fibers  which  extended  from  one  pole  to  the 
other  begin  to  thicken  at  the  equator.  The  swellings  grow  larger, 
fuse,  and  spread  out  to  form  a  delicate  plate,  which  eventually  extends 
clear  across  the  mother  cell.  This  cell  plate  is  in  the  nature  of  a 
plasma  membrane  which  splits  into  two,  forming  the  new  cell  wall 
between  the  two  new  cells.  The  fibers  of  the  spindle  now  disappear 
and  cell  division  is  completed.  Meantime  the  recently  split  chromo- 
somes lose  their  identity  and  again  take  on  the  netlike  appearance  as 
in  the  original  resting  cell.  The  last  series  of  changes  comprises 
the  telophase. 

How  Animal  Cells  Divide 

The  resting  animal  cell  undergoes  a  similar  process  in  division. 
However,  in  the  animal  cell  a  new  structure  is  found,  called  the 
centrosphere,  which  is  a  small  body  lying  in  the  cytoplasm  near  the 
nucleus.  A  central  granule,  called  the  centrosome  or  centriole,  is 
found  within  this  centrosphere.  The  centriole  usually  divides  to 
form  two  of  these  granules  at  the  beginning  of  mitosis.  The  initial 
stages  of  cell  division,  collectively  called  the  prophase,  occur  when  the 
particles  of  chromatin  scattered  throughout  the  nucleus  take  the  form 
of  the  spireme  or  tangled  thread.  This  thread  thickens  and  shortens 
and  then  breaks  up  into  the  individual  chromosomes.  The  number 
of  chromosomes  for  the  body  cells  of  the  individual  of  a  species  is  always 
constant.  Among  plants,  for  example,  in  the  pea  there  are  always 
14,  in  the  onion  16,  and  in  the  lily  24 ;  while  examples  taken  at 
random  among  animals  show  4  for  certain  roundworms,  8  for  the  fruit 
fly,  Drosophila,  of  which  you  will  hear  more  later,  32  in  one  of  the 
common  earthworms,  200  in  one  of  the  crayfishes,  24  Mn  a  common 
locust,  24  in  one  of  the  frogs,  and  48  in  man. 

During  the  formation  of  the  spireme  the  threads  of  the  future  spindle 
are  growing  out  from  radiations,  called  asters,  which  appear  around  the 
centrioles.     (See  figure  on  page  143.)    As  the  process  continues  the  two 

'  This  is  not  quite  exact,  for  it  has  been  found  that  in  some  animals  at  the  time  when  the  chromo- 
somes are  reduced  in  number  in  the  process  of  maturation  (see  page  429) ,  there  is  an  even  number 
in  the  female  sex  cells  but  an  odd  number  in  the  male  sex  cells  or  vice  versa. 


CELLS  AND   TISSUES  ,,, 

centrioles  move  farther  apart,  the  spindle  fibers  elongate,  the  n.icl.-ir 
membrane  disappears,  some  spindle  fibers  api)ear  to  attach  to  tlic 
chromosomes,  and  gradually  the  longitudinally-split  chromosomes 
collect  at  the  equator  of  the  spindle.  The  next  step  in  mitosis,  known 
as  the  metaphase,  is  the  arrangement  of  the  chromosc^mes.  with  each 
split  body  on  opposite  sides  of  the  equator  of  the  spindle.  Then  the 
two  sets  of  chromosomes  begin  to  move  toward  the  opposite  poles  of 
the  spindle,  the  fibers  which  are  attached  to  them  getting  shorter  tunl 
shorter.     At  this  time  comes  the  first  external  appearance  of  cell 

6enLrosphere 
-  (tentrosome 
^    chromatin 

spireme 

\nwi\ear- 
fnamhrexna. 
resting  cell 

oCiso.ppeo.'ns 
-spindle  thread 

.CentroSome 


linVm     -  pi'ophase 


spireme  sViortens 
anoL  thickens 


and  of  prophase 
nuclecLr  membi*ane 
<A-isccppe<ir-s 


metxx-phaSe 


arjapbase 


end  of  anaphase 


te\ophasa 
nuclear  membranes 


ctoir^ter    Cells 

restivxT  stage. 


Mitosis  in  animal  cells.     Compare  this  diagram  with  that  on  page  i  11. 


division,  a  slight  constriction  appearing  in  the  cell  body.  The 
constriction  in  the  cell  becomes  more  evident  and,  as  the  i)rocess 
continues,  the  chromosomes  become  grouped  so  as  to  form  the  new- 
nuclei  of  the  two  daughter  cells.  These  progressive  changes  are 
collectively  known  as  the  anaphase.  In  the  final  stage,  or  Idophasc, 
the  two  sets  of  chromosomes  gradually  lo.-;e  their  individuality  and 
become  Httle  masses  of  chromatin  grouped  on  linin  fibers  in  the  new 
nuclei  around  which  a  nuclear  membrane  is  formed.  .Meantime  the 
constriction  in  the  cell  has  gone  far  enougii  to  form  two  daughter  cells, 
the  new  separating  partition  appearing  along  the  line  nf  the  (-(luator 


144  FUNDAMENTALS   OF   STRUCTURE   AND   FUNCTION 

of  the  spindle.  The  centriole  in  many  daughter  cells  divides  imme- 
diately into  two,  although  in  some  cells  it  remains  as  a  single  body 
until  a  new  mitosis  begins. 

Tissues 

Cells  form  aggregates  called  tissues,  examples  of  which  may  be  seen 
in  the  woody  cells  making  up  the  greater  part  of  the  stem  of  a  plant ; 
the  elongated  cells  in  this  same  stem  which  form  the  conducting 
tissue;  the  flat  protective  cells  covering  the  outside  of  the  leaf,  called 
collectively  the  epidermis;  and  the  large  columnar  cells  filled  with 
green  chloroplasts  that  form  the  parenchyma  layer  directly  under  the 
epidermal  cells.  In  our  own  body,  we  find  numerous  examples  of  famil- 
iar tissues  set  apart  for  doing  some  particular  work,  such  as  the  epithe- 
lial, or  protective,  tissues ;  the  connective  tissues,  which  serve  to  bind 
the  various  groups  of  cells  together ;  the  muscular  tissues,  of  several 
kinds ;  the  supporting  tissue  cells,  which  help  to  build  the  bones ; 
glandular  tissues  ;  the  nervous  tissues  of  several  kinds  ;  and  the  blood, 
which,  though  fluid,  yet  contains  cells,  and  is  classed  as  a  circulating 
tissue. 

The  Tissues  in  Plants 

It  is  a  difficult  matter  to  make  a  classification  of  tissues  that  will 
fit  all  plants  and  yet  be  simple  enough  to  use  at  this  stage  of  our 
biological  knowledge.  But  the  following  will  give  us  a  general  survey 
which  can  later  be  expanded  by  the  student  of  botany. 

The  Meristematic  Tissues.  These  cells  in  general  are  small, 
thin  walled,  and  rich  in  protoplasm.  They  are  found  in  the  rapidly 
growing  parts  of  plants,  the  buds,  the  tips  of  the  roots,  and  in  growing 
layers.     They  represent  the  primitive  and  embryonic  tissues. 

The  Protective  Tissues.  Such  are  the  epidermal  cells  covering 
leaves.  These  are  often  waterproofed  with  a  waxy  material  called 
cutin.  Such  layers  are  found  on  the  outside  of  the  stem,  root,  and 
even  the  fruit,  forming  a  protective  covering.  In  the  stem  and  the 
root,  the  epidermis  is  often  replaced  by  a  layer  of  corky  cells,  while  on 
leaves,  stems,  and  flowers  the  epidermal  cells  frequently  develop 
hairs  or  scales,  which  sometimes  secrete  sticky  substances. 

The  Fundamental  Tissues.  These  groups  of  cells  form  the  great 
mass  of  plant  tissue,  such  as  the  soft  green  parts  of  the  leaf,  the  pith 
or  cortex  of  plant  stems,  the  soft  parts  of  flowers  and  fruits.  These 
cells  differ  greatly  in  size  and  shape  in  different  parts  of  the  plant, 


CELLS  AND   TISSlJi:s 


II.-) 


but  in  general  they  are  alive  and  act  as  storage  cells.  Some  of  the 
parenchyma  cells,  called  collectively  collnichynw,  become  f  liickcncd 
at  the  corners,  as  seen  in  a  cross  .section,  and  serve  as  strengthcnii.n 


'menStQWatiC     P'^'^^'^.X"'-^  colknch^roa    sderenchv^xt 
tissues  "tancCameatial      tissues 

oooo 


■  Sedition 

epjdermis 


"plant-  -,  I",     y,  ... 

...  i:icxir^         :jcylem  M    '.phloem 

prouective  t-issues         <:tondxxctin<^  tissues 


Types  of  plant  tissue  cells. 

units  in  the  outer  part  of  the  stem.  The  walls  of  the  other  funda- 
mental tissue  cells  become  much  thickened  and  are  called  sclcrcrichyma 
cells,  which  may  become  fibrous,  helping  to  .support  the  stem,  while 
others  form  stone  cells  making  up  the  covering  of  nuts  antl  other 
hardened  parts. 

The  Conducting  Tissues.  In  the  liigher  plants,  woody  bundles 
of  elongated  cells  act  as  tubes  for  the  conduction  of  water  and  food 
substances.  The  water-conducting  tissues  are  collectivelj'  known 
as  the  xylem  and  consist  largely  of  supporting  dead  cells  (trachcids) 
impregnated  with  a  strengthening  substance  called  lignin,  and  long 
tubular  cells  (vessels)  which  have  lost  their  cross  walls.  Scattered 
amongst  them  are  various  other  types  of  cells,  including  jiarenchynia. 
The  tissues  which  conduct  food  materials  down  the  stem  from  the 
leaves,  where  food  is  made,  are  known  collectively  as  the  phlonn. 
The  characteristic  conducting  cells  of  the  j^hloem  are  known  as  sifir 
tubes,  which  have  perforations  in  the  end  or  .side  walls  known  as  the 
sieve  plates.     Long  threads  of  cytoplasm  pa.ss  through  these  holes. 


146 


FUNDAMENTALS   OP^   STRUCTURE  AND   FUNCTION 


connecting  cell  with  cell  and  making  a  pathway  for  the  food  sub- 
stances. Small  companion  cells  are  attached  to  the  sieve  tubes. 
The  phloem  is  also  provided  with  parenchyma  and  fibrous  cells, 
which  give  strength  to  the  tubular  bundle. 

The  Tissues  in  Animals 

Although  the  histologist  makes  a  much  more  detailed  classification 
of  tissues,  a  convenient  grouping  for  animals  is  the  following : 

The  Epithelial  Tissues.  Not  only  do  these  cells  form  the  outer 
layer  of  the  animal  body,  but  they  also  are  responsible  for  the  forma- 
tion of  such  protective  body  structures  as  the  calcareous  shells  of 


reticular  odiposs 


emooth. 


grlonduP 


^   ,    fibrous      Z,^    ..^      --^  °  ^'w^-  -'.^^^l? 

stratifM  ^.,^.:.:-:,^,:mm\       Striated: 

^m^mmSiim 


mm 


Columnar 


epithelial 
ti55ue6 


vv  ;.:>: -v: :.  ;;,U^         loom. 

Yiyoline  Cartilage 

supporting 


■tissues 


red. 
©  <!orpuscles 

circulcctorv 


r<2/procCuclive 


tissu:<2^ 

Types  of  animal  tissue  cells.     Into  how  many  groups  may  they  be  classified  ? 

clams  and  oysters,  the  chitinous  covering  of  the  insect,  or  the  outer 
covering  of  the  crayfish.  These  tissues  line  all  body  surfaces  as 
well  as  the  digestive  tract  and  other  inpocketings  of  the  outer  body 
covering.  They  are  of  the  utmost  importance  because  they  also 
form  the  glands  of  the  body,  structures  which  secrete,  for  example. 


CELLS  AND   TISSUES  ,  1^ 

digestive  enzymes  or  the  waste  products  of  metabolism,  such  as 
perspiration.  They  also  form  a  largo  portion  of  many  of  the  sense 
organs  of  the  body.  In  shape,  the  cells  of  epithelial  tissues  as  they 
lie  side  by  side  may  be  fiat,  cuboidal,  columnar,  or  even  ovoid. 

The  Supporting  Tissues.  These  tissues  .serve  to  bind  together  or 
support  the  various  parts  of  the  body.  They  include  bone,  cartilage, 
and  connective  tissue,  and  they  differ  from  other  ti.ssues  in  that  it  is 
the  material  formed  by  the  cells,  rather  than  the  cells  themselves,  that 
is  of  functional  importance.  In  bone  or  cartilage,  for  example,  the 
supporting  portion  or  matrix  is  produced  by  the  cytoplasm  of  cells 
and  surrounds  it.  Fat  cells  are  connective  ti.ssue  cells  in  which  the 
body  of  the  cell  becomes  a  storehouse  for  a  drop  of  fat,  the  living  part 
of  the  cell  being  much  reduced.  Pigment  cells  are  branched  irregular 
structures  of  a  somewhat  similar  nature.  Most  characteristic  of 
true  connective  tissues  are  the  white  non-elastic  fibers  that  make  a 
network  in  certain  parts  of  the  body,  or  form  the  glistening  cords  or 
tendons  \\\nQh.  connect  bones  with  muscle,  or  ligaments,  which  connect 
bones  with  bones.  Other  forms  of  connecti\e  ti.ssue  that  might  be 
mentioned  are  the  areolar,  which  forms  an  elastic  padding  underneath 
the  skin;  and  the  yellow  elastic  fibers  found  in  the  air  tubes  of  the 
lungs  and  the  walls  of  arteries. 

The  Muscular  Tissues.  Motion  of  certain  cells  is  produced  by 
ameboid  movement,  or  by  the  lashing  of  tiny  threads  of  protoplasm, 
that  is,  flagella  or  cilia.  But  in  higher  animals  movement  is  brought 
about  by  the  muscle  cells  in  which  the  propert}'  of  contractility  is 
greatly  developed.  In  higher  animals,  muscles  are  groups  of  highly 
specialized  cells  bound  together  by  connective  tissues.  There  are 
three  kinds  of  muscle  cells,  namely,  smooth,  striated,  and  cardiac. 
Smooth  muscle  cells  are  long  with  an  outer  contractile  fibrillar  layer 
surrounding  a  central  area  of  semifluid  protoplasm  containing  a 
nucleus.  In  vertebrate  animals,  smooth  muscle  is  found  i^articularly 
in  the  walls  of  the  blood  vessels  and  the  walls  of  the  digesti\-e  tract. 
Striated  muscle  fibers  in  higher  animals  are  groups  of  cells  slu)wing  n(» 
cell  boundaries  and  held  together  by  connective  ti.ssue.  They  s1k)w 
curious  cross  striations  and  on  the  whole  in  man  are  under  control  (»f 
"  the  will,"  hence  are  called  voluntary  muscles.  A  third  type  o( 
muscle,  the  cardiac,  is  striated,  but  involuntary  in  action,  making 
up  the  tireless  muscles  of  the  heart. 

Circulatory  Tissue.  Although  the  blood,  lymph,  and  other 
fluids  that  serve  to  transport  foods  and  wastes  in  the  body  are  <-on- 


148  FUNDAMENTALS   OF   STRUCTURE   AND   FUNCTION 

stantly  in  motion,  we  must  classify  them  as  tissues,  for  they  contain 
living  cells  or  corpuscles  of  various  kinds,  carried  about  in  a  fluid 
matrix  or  plasma.  These  tissues  are  of  the  utmost  importance  to 
animals,  as  it  is  only  by  means  of  them  that  the  living  cells  of  the  body 
receive  nourishment  and  oxygen,  and  get  rid  of  their  wastes. 

The  Nervous  Tissues.  Even  in  its  simplest  form  we  have  seen 
that  protoplasm  is  sensitive  and  responds  to  stimuli.  In  higher 
animals  this  sensitivity  and  conductivity  of  sensations  is  taken  over 
by  the  nervous  tissues.  The  unit  of  structure  is  the  neuron,  or  nerve 
cell.  The  elongated  fibers  from  these  cells  are  bound  together  into 
nerves  or  conducting  ])athways  for  nerve  impulses.  All  parts  of  the 
vertebrate  body,  with  the  exception  of  the  cartilages  and  epidermal 
derivatives,  are  supplied  with  nervous  tissue,  which  may  be  said  to 
be  the  master  tissue  of  the  body. 

Reproductive  Tissues.  These  cells  which,  as  one  author  puts 
it,  are  "within  the  body  though  perhaps  not  of  the  body,"  form  tis- 
sues, eggs  and  sperms,  that  have  to  do  with  the  futures  of  all  animals. 

Why  Are  Living  Organisms  So  Called? 

In  the  preceding  pages,  we  have  referred  to  living  things  as  organ- 
isms. The  anatomist  calls  collections  of  tissues,  which  do  specific 
kinds  of  work,  organs.  The  hand  is  an  example  of  an  organ  which  is  a 
collection  of  tissues.  Muscles  are  attached  to  the  hones  by  means  of 
tendons  and  bones  are  joined  together  by  ligaments.  The  skm, 
composed  of  several  different  kinds  of  tissue  cells,  is  supplied  with 
blood  and  nervous  tissues,  while  the  whole  organ  is  interlaced  through 
and  through  with  other  connective  tissues.  Living  things  are  made  up 
of  organs,  and  we  call  them  organisms.  The  living  world  about  us, 
plant  and  animal,  is  a  collection  of  organisms,  some  very  simple, 
others  aggregates  of  simple  cells,  still  others  formed  of  untold  billions 
of  differentiated  cells,  grouped  into  tissues  forming  an  organism, 
such  as  an  insect,  a  fish,  a  tree,  or  a  man.  Yet  all  these  different  and 
complex  entities  basically  are  made  of  the  living  stuff  called  proto- 
plasm. In  animals,  this  grouping  of  organs  which  are  united  in  the 
performance  of  some  general  function  gives  us  a  number  of  organ- 
systems.  There  is,  for  example,  the  integumentary  system,  or  outer 
body  covering ;  the  supporting  system,  which  forms  the  body  frame ; 
the  systems  which  have  to  do  with  the  nutrition  of  the  body,  the 
digestive,  respiratory,  circulatory,  and  excretory  systems ;  the  nervous 
system,  which  controls  the  activity  of  the  body ;    and  the  reproduc- 


CELLS  AND    J  ISSUES  1  j., 

live  system,  which  has  to  do  witli  the  contiiiuaiice  of  Hfc  It  is  on 
the  structural  development  of  these  systems,  ({('velojx'd  to  ;i  greater 
or  lesser  extent  in  all  of  the  many-celled  animals,  that  the  various 
groups  of  the  metazoa  are  classified. 

SUGGESTED   READINGS 

Dahlgren,  U.,  and  Kepner,  W.  A.,   Textbook  of  the  Principles  of  Animal 
Histology,  The  Macmillan  Co.,  1908.     Chs.  I,  II.  and  \'. 

Holmau,  K.  M.,  and   Robbins,  W.  W.,  Elements  of  Jiotany,  2iul  cd.,  John 
Wiley  &  Sons,  Inc.,  1928.     Ch.  III. 

Maximow,  A.  A.,  Textbook  of  Histology,  edited  by  W.  Bloom,  W.  B.  Saunders 
Co.,  1930. 
Rather  technical.     Chapters  I  and  II  useful. 

Stohr,  Philip,  A  Textbook  of  Histology,  .')th  ctl.  (arranged  by  J.  L.  Bremer), 
P.  Blakiston's  Son  &  Co.,  193(3. 
Chapters  I  and  II  make  excellent  reading. 

Wilson,  E.  B.,    The  Cell  in  Development  and  Heredity,  3rd  cd.,  The  Mac- 
millan Co.,  1925. 

The  most  authoritative  text  on  the  cell.     Rather  advanced,  but  with 
excellent  figures.     Chapters  I  and  II  especially  us(>ful. 


ORGANISMS   ILLUSTRATING  BlOL()(;i(;\| 

PRLNCIPLES 


MI 


BEGINNINGS:    THE   LARGE   CROIP  OF  THE 
SMALLEST   OHGA.MSMS 

Preview.  Some  forms  found  in  a  drop  of  fresh  water:  Ainoba.  an 
animal  cell ;  Euglena ;  Paramecium  ;  Diatoms  :  Desmids  ;  Bacteria  ■  Func- 
tional differences  between  plant  and  animal  cell  •  Suggested  readings. 

PREVIEW 

Over  two  hundred  and  sixty  years  ago,  when  the  Dutchman,  Antony 
van  Leeuwenhoek,  examined  what  he  called  "little  animals"  under 
his  homemade  microscopes,  he  made  the  first  real  exploration  of  a 
drop  of  water  ever  attempted.  His  microscopes  were  simple  affairs, 
consisting  of  a  single  lens.  They  had  no  tube  or  mirror  such  as  our 
microscopes  of  today  have.  When  objects  were  examined  they  had 
to  be  brought  into  position  and  focus  through  the  use  of  rather  coarse 
screws. 

Besides  being  the  first  person  actually  to  see  the  capillary  circulation 
of  the  blood  (a  thing  that  Harvey  knew  must  be  so,  but  which  he  wa.s 
unable  to  prove),  van  Leeuwenhoek  made  numerous  other  llhJ^sio- 
logical  and  anatomical  observations  which  gave  him  the  title  of 
"founder  of  histology."  One  thinks  of  him  most  often  as  the  first 
man  who  saw  protozoa,  unicellular  plants,  and  own  bacteria  in 
standing  water. 

Let  us  read  his  own  description  and  judge  for  ourseh-es  a.s  to  what 
he  saw.  The  following  extract  is  taken  from  a  letter  written  on 
October  9,  1676,  to  Henry  Oldenburg,  First  Secretary  of  the  Koyal 
Society  of  London.  It  describes  the  finding  of  "little  animals"  in  a 
drop  of  rain  water. 

"Of  the  first  sort  that  I  discovered  in  the  said  water,  I  saw,  after  divers 
observations,  that  the  bodies  consisted  of  5,  6,  7,  or  8  ver>'  clear  globules, 
but  without  being  able  to  discern  any  membrane  or  skin  that  held  these 
globules  together,  or  in  which  they  were  inclosed.  When  these  aninuilcules 
bestirred  'emselves,  they  sometimes  stuck  out  two  little  hnrns,  which  were 
continually  moved,  after  tlie  fashion  of  a  horse's  ears.     The  i)art  l)etween 

H.   W.    H. —  11  131 


152       ORGANISMS   ILLUSTRATING  BIOLOGICAL   PRINCIPLES 

these  little  horns  was  flat,  their  body  else  being  roundish,  save  only  that  it 
ran  somewhat  to  a  point  at  the  hind  end ;  at  which  pointed  end  it  had  a 
tail,  near  four  times  as  long  as  the  whole  body,  and  looking  as  thick,  when 
viewed  through  my  microscope,  as  a  spider's  web.  At  the  end  of  this  tail 
there  was  a  pellet,  of  the  bigness  of  one  of  the  globules  of  the  body ;  and 
this  tail  I  could  not  perceive  to  be  used  by  them  for  their  movements  in 
very  clear  water.  .  .  . 

"I  also  discovered  a  second  sort  of  animalcules,  whose  figure  was  an  oval ; 
and  I  imagined  that  their  head  was  placed  at  the  pointed  end.  These  were 
a  little  bit  bigger  than  the  animalcules  first  mentioned.  Their  belly  is  flat, 
provided  with  divers  incredibly  thin  little  feet,  or  little  legs,  which  were 
moved  very  nimbly,  and  which  I  was  able  to  discover  only  after  sundry 
great  efforts,  and  wherewith  they  brought  off  incredibly  quick  motions. 
The  upper  part  of  their  body  was  round,  and  furnished  inside  with  8,  10,  or 
12  globules  :  otherwise  these  animalcules  were  very  clear.  These  little  ani- 
mals would  change  their  body  into  a  perfect  round,  but  mostly  when  they 
came  to  lie  high  and  dry.  Their  body  was  also  very  yielding  :  for  if  they  so 
much  as  brushed  against  a  tiny  filament,  their  body  bent  in,  which  bend  also 
presently  sprang  out  again ;  just  as  if  you  stuck  your  finger  into  a  bladder 
full  of  water,  and  then,  on  removing  the  finger,  the  inpitting  went  away." 

His  description  of  the  cause  of  movement  in  his  little  creatures  is 
amusing,  yet  it  shows  that  he  saw  cilia  plainly  and  estimated  their 
size  quite  clearly. 

"But  many  of  the  things  we  imagine,  and  the  natural  objects  that  we 
inquire  into,  are  very  insignificant;  and  especially  so,  when  we  see  those 
little  living  animals  whose  paws  we  can  distinguish,  and  estimate  that  they 
are  more  than  ten  thousand  times  thinner  than  a  hair  of  our  beard ;  but  I 
see,  besides  these,  other  living  animalcules  which  are  yet  more  than  ten 
thousand  times  than  a  hair  of  our  beard ;  but  I  see,  besides,  these  other 
living  animalcules  which  are  yet  more  than  a  hundred  times  less,  and  on 
which  I  can  make  out  no  paws,  though  from  their  structure  and  the  motion 
of  their  body  I  am  persuaded  that  they  too  are  furnished  with  paws  withal : 
and  if  their  paws  be  proportioned  to  their  body,  like  those  of  the  bigger 
creatures,  upon  which  I  can  see  the  paws,  then,  taking  their  measure  at  but 
a  hundred  times  less,  it  follows  that  a  million  of  their  paws  together  make 
up  but  the  thickness  of  a  hair  of  my  beard ;  while  these  paws,  besides  their 
organs  for  motion,  must  also  be  furnished  with  vessels  whereby  nourishment 
must  pass  through  them."  ' 

Van  Leeuwenhoek  was  made  a  member  of  the  Royal  Society  for  his 
clear  reports  of  what  he  saw  and  at  his  death  he  had  sent  the  Society  a 

1  Dobell,  C,  Antony  van  Leeuwenhoek  and  his  "Little  Animah,"  pp.  118  and  180,  Harcourt, 
Brace  and  Co.     By  permission  of  the  publishers. 


THE  LARGE  GROUP  OF  THE  SMALLEST  ()IU;\NISMS  m 

case  containing  26  of  his  microscopes,  a  gift  which  was  later  lost  ( )„,. 
of  the  few  remaniing  of  the  419  lenses  put  up  at  auction  after  van 
Leeuwenhoek's  death  was  recently  examined  by  an  expert  who 
reported  that  the  biconcave  lens  that  he  inspected  "was  very  good 
indeed"  and  proved  that  its  maker  had  attained  "a  very  high  degree 
of  proficiency  in  grinding  extremely  small  glasses." 

With  the  modern  microscope  of  the  college  laboratory,  infinitely 
better  work  can  be  done  than  with  this  old  pioneer.  The  best  of  \an 
Leeuwenhoek's  lenses  are  said  to  have  magnified  not  more  than  270 
diameters,  while  the  "  high  dry  "  power  of  the  average  modern  micro- 
scope gives  a  magnification  of  about  440  diameters,  so  that  the  college 
freshman  today  has  a  far  better  physical  equipment  than  did  this 
famous  Dutchman.  He  also  has  much  more.  In  the  years  that  have 
intervened  between  the  time  of  van  Leeuwenhoek  and  the  present, 
patient  observations  of  minute  forms  of  life  ha\-c  been  made  by 
hundreds  of  scientists  whose  results  may  be  found  in  these  pages  and 
in  other  books  suggested  for  collateral  reading.  With  this  intro- 
duction the  student  might  begin  the  study  of  simple  organisms  in 
some  such  way  as  Antony  van  Leeuwenhoek  did,  by  examining  a 
drop  of  pond  water. 

Some  Forms  Found  in  a  Drop  of  Fresh  Water 

The  pages  that  follow^  will  serve  to  give  us  a  slight  acquaintance 
with  some  of  the  simplest  plant  and  animal  forms  that  are  likely  to  be 
met  in  the  examination  of  a  drop  of  pond  water  or  water  from  a 
laboratory  aquarium.  Li  addition  to  the  unicellular  organisms, 
scores  of  other  higher  forms  are  likely  to  be  seen.  Countless  protozoa, 
including  the  many  tiny  species  of  monads,  dart  across  the  field  of  the 
microscope ;  others  many  times  larger,  with  their  highly  specialized 
cell  parts,  as  Euplotcs  or  Stylonychia,  may  be  found  browsing  on  tiny 
plants.  Frequently  one  also  encounters  threads  of  the  filamentous 
algae,  Zygneyna  or  Sjpirogyra,  while  debris,  consisting  of  tiny  bits  of 
wood,  sand  grains,  and  the  glasslikc  cases  of  diatoms  and  desmids. 
may  abound. 

Many  tiny  crustaceans,  water  fleas,  and  cojx'pods  are  usually 
present,  and  in  addition  one  finds  the  easily  recognizable  rotifers, 
with  their  whirling  wheels  of  cilia,  their  prominent  grinding  organ 
or  mastax,  and  their  slender  toelike  posterior  foot  by  means  of  which 
they  often  become  attached  to  .solid  objects.  Sometimes  a  small 
roundworm  may  be  found  working  its  way  through  the  dt^bris.  while 


154      ORGANISMS   ILLUSTRATING   BIOLOGICAL   PRINCIPLES 


hyaline  cap.. 


pseuctopodiam 


\ 
ptemagsl.m  o  _, 


many  types  of  insect  larvae  and  pupae  may  also  be  seen.  This  brief 
list  includes  only  a  few  of  the  many  new  acquaintances  to  be  found 
in  a  drop  of  water. 

AmebOj  an  Animal  Cell 

Ameba  is  the  classic  representative  of  a  single-celled  animal  which 
illustrates  the  action  of  living  protoplasm.  Found  in  ooze  taken 
from  the  bottom  of  small  ponds  or  sluggish  streams,  it  is  seen  to  be 

an  irregular  and  almost 
transparent  cell.  When 
in  motion  the  protoplasm 
of  its  body  apparently 
flows  out  into  newly 
formed  bulging  projec- 
tions of  the  body  called 
pseudopodia  (Gk.  pseu- 
dos,  false;  pous,  foot). 
The  cell  body  consists  of 
two  substances,  an  inner, 
more  fluid,  granular  por- 
tion, the  endoplasm  and 
a  more  viscous  area,  the 
ectoplasm,  on  the  outside. 
The  whole  Ameba  is 
surrounded  by  a  deli- 
-foodvacuole  cate  plasma  membrane. 
When  the  animal  moves, 
the  protoplasm  appears 
to  flow  into  the  pseudo- 
podia. According  to  S. 
O.  Mast  of  the  Johns 
Hopkins  University, 
when  an  Ameba  is  mov- 
ing in  a  given  direction  the  endoplasm  sol  pushes  out  in  a  pseudopo- 
dium  and  becomes  changed  to  a  gel,  the  "gel"  at  the  other  end  of  the 
cell  becoming  a  "sol"  that  moves  into  the  cefl  body.  This  illustrates 
a  characteristic  of  protoplasm  mentioned  earlier. 

This  cell,  like  others  of  its  kind,  has  a  nucleus  containing  chromatin. 
Certain  vacuoles  are  present,  some  of  which  are  filled  with  a  watery 
fluid,  others  hold  food  in  different  states  of  digestion,  while  a  single 


---■nuclexcs 


li.L.-fooct  vacuole. 


Contractile, 
vacuole— 


Ameba  proteus.  The  direction  of  progress  of 
the  cell  is  shown  by  arrows.  What  happens  to 
the  protoplasm  in  the  extreme  anterior  end  during 
movement.     (After  Mast.) 


THE  LARGE  GROUP  OF  THE  SMALLEST  OI\(;\Ms\is       i:..-, 


vacuole,  called  the  contractile  vacuole,  rhythmically  collects  and  expels 
fluid.  The  function  of  the  contractile  vacuole  may  he  to  eliminate 
wastes  from  the  cell,  or  it  may  have  a  hydrostatic  function,  that  is, 
it  may  control  the  amount  of  water  contained  in  the  ccjl.  Food 
particles  are  actually  ingested  or  taken  into  the  cell  by  the  proto- 
plasm which  flows  around  the  food,  engulfs  it,  and  then  surrounds  it 
with  digestive  fluids  in  a  food  vacuole. 

A  recent  series  of  observations  by  Mast  and  Hanliart '  indicate 
that  the  Ameba  selects  certain  kinds  of  food,  ])referring,  for  instance, 
Chilomonas  to  Monas,  although  both  are  flagellates  of  about  the 
same  size,  form,  and  activity.  It  was  further  sliown  that  Monas 
was  not  digested  in  the  food  vacuoles,  while  Chilajnonas  was,  and 
also,  some  organisms,  such  as  mold  spores,  certain  algae,  and  other 
flagellates,  might  be  eaten  but  were  not  digested. 

The  process  of  constructive  and  destructive  metabolism  may  take 
place  in  a  single  cell.  Indigestible  waste  materials  are  pa.s.sed  out 
any^vhere  from  the  surface 
of  the  cell  body,  while 
respiration  takes  place  by 
means  of  an  osmotic  ex- 
change of  the  gases,  oxy- 
gen and  carbon  dioxide, 
through  the  cell  mem- 
brane. 

As  a  result  of  the  taking 
of  food,  the  cell  gradually 
increases  in  size  and  then 
divides  by  a  process  known 
as  binary  fission.  Accord- 
ing to  a  recent  study  by 
Chalkley  and  Daniel  -  the 
division  of  the  nucleus 
shows  the  typical  stages 
of    mitotic    division,    the 

entire  process  lasting,  under  normal  temperature  conditions,  about 
half  an  hour.     During  the  process  the  Ameba  is  quiescent  and  the 


late  prophorse. 


mid- anaphase 


Gccriy  anaphase 


•metaphose 


Mitotic  division  in  the  nucleus  of  Vruflia.      i,  After 
GliiilklcN  ;uni  Daniel. ' 


iMast,   S.  0.,  and  Hanhart,   W.   L.,   "  Feedins,  Digestion, 
(Leidy)."     Phusiol.  -Zool..  Vol.  8,  lO.'?.").     Pp.  2,5,5-272. 

-C 
and   the 
Pp.  592-619. 


and  StarviiliiiM  in  .■inwrbii   pmlrw 


Iv)."     Ph„.no}. 'Zool..  Vol  8.  \9^r,.     Pp.  2,5,5-272.  ,i„„,vll 

Chalkley,  H.  W..  and  Daniel.  G.  E..  "The  Relation   between  the   lorn,   of     he    I.  v   .«      r  1 
the    Nuclear   Pha.ses  of  Division  in   Amoeba   protcu..  (Leidy).        Phmol.  looU  \o\.  rt.   !..,«•«. 


156      ORGANISMS   ILLUSTRATING   BIOLOGICAL   PRINCIPLES 


pspiidopodia  are  relatively  small.  After  the  nucleus  divides,  the  cell 
body  separates  into  two  equal  parts,  each  of  which  grows  into  a 
full-sized  individual. 

Euglena 

Although  Ameba  is  usually  looked  upon  as  the  simplest  of  all  animal 
cells,  there  is  another  group  of  organisms  containing  equally  simple 

forms,    making    up    a    large 

flagsllum 
cytostome/. 


stigma  .^^<a. 

I^lagsllcu-  granule 

rs-Ser-vDirr 

basal  granule 

Contractile, 
vacuoles  -•'- 

nuole-us — 

Central  bcx^ 

cbromatopbore 
pyrenoid 

striation$ — 


lent 


Euglena  viridis.  Read  your  text  and  give 
the  functions  of  each  of  the  structures  shown. 
Note  that  the  drawing  makes  the  cell  appear 
flat  whereas  in  cross  section  it  is  oval.  What 
evidence  of  holophytic  nutrition  is  seen  in 
this  diagram  ?     (After  Hegner.) 


proportion  of  the  microscopic 
plankton  of  the  ocean  and 
bodies  of  fresh  water.  This 
grotip,  w^hich  comprises  one 
of  the  classes  of  the  Phylum 
Protozoa,  includes  the  Masti- . 
gophora,  or  flagellates,  cells 
that  move  by  means  of  one  or 
more  long,  whiplash  threads 
of  protoplasm.  Certain  of 
the  Mastigophora  bear  a  close 
relationship  to  plants,  and  the 
organism  Euglena,  selected  as 
a  representative  of  the  group, 
is  often  claimed  as  a  plant  cell 
by  botanists.  Euglena  may 
be  found  in  shallow  and  some- 
times temporary  freshwater 
ponds,  where  it  often  grows 
with  such  rapidity  as  to  give 
a  dull  greenish  color  to  the 
water.  When  unfavorable 
conditions  set  in,  the  organism 
settles  to  the  bottom,  becomes 
covered  with  a  resistant  coat 


or  cyst,  and  is  only  recalled 
to  active  life  by  a  recurrence  of  favorable  environmental  conditions. 
Some  species  of  Euglena  have  conspicuous  spiral  markings  on  the 
surface  of  the  body,  which  is  roughly  ovoid,  with  a  depression  at  the 
anterior  end,  called  the  gullet.  A  single  flagellum  has  its  origin  near 
the  base  of  the  gullet,  in  the  form  of  a  long  axial  filament  anchored 
in  the   protoplasm,  that   gives   the   filament   free   movement.     By 


THE   LARGE   GROUP  OF    rilE   SMALLEST   OUCXMSMS       ir,7 

means  of  a  rotary  movement  of  the  Hagelluni,  the  cell  is  pulled  forward 
on  a  spiral  course,  which  is  caused  partly  by  the  way  the  flaRellum 
moves  and  partly  by  the  irregular  shape  of  the  cell.  At  the  same 
time  a  current  of  water  is  swept  into  the  gullet,  bearing  with  it  par- 
ticles of  potential  food.  The  niembranous  covering  of  the  bod\- 
allows  the  shape  of  the  cell  to  change,  often  moving  by  what  is  known 
as  euglenoid  motion,  that  is,  by  a  wave  of  contraction  over  tlie  whole 
body,  thus  causing  a  slow  movement  like  that  characteristic  of  Ameha. 

Although  some  species  of  Euglena  appear  to  ingest  the  food  i)ar- 
ticles  that  are  swept  into  the  gullet,  the  ordinary  nutrition  is  tiie  same 
as  that  of  a  green  plant.  The  imuM-  protoplasm  of  the  cell  is  filletl 
with  chloroplasts  (chromatophores)  by  means  of  which  the  raw  mate- 
rials, water,  carbon  dioxide,  and  mineral  salts,  are  synthesized  into 
food,  thus  storing  the  energy  of  sunlight.  Different  species  of  Euglena 
are  sensitive  to  different  degrees  of  sunlight  and  are  found  to  turn 
towards  a  source  of  light,  the  anterior  part  of  the  cell,  which  contains 
a  red  "eyespot,"  being  most  sensitive  to  the  light  stimulus.  When 
they  are  exposed  to  strong  sunlight,  they  change  their  direction, 
coming  to  rest  in  an  area  of  moderate  or  "optimum  light."  Respira- 
tion is  carried  on  as  in  any  other  unicellular  form  by  exchange  of 
gases  through  the  membrane  covering  the  body.  During  the  period 
when  starch  is  being  made  in  the  sunlight  enough  oxygen  is  released 
within  the  cell  body  to  supply  its  needs.  Excretion  of  waste  products 
appears  to  be  taken  care  of  by  a  number  of  very  small  contractile 
vacuoles,  that  collect  fluids  from  the  cell,  eliminating  them  period- 
ically into  a  small  reservoir  which  empties  into  the  gullet.  The 
individual  cell  in  some  respects  acts  hke  a  plant,  and  in  others  like  an 
animal.  It  is  a  borderline  representative,  and  as  such  must  be 
regarded  as  a  very  primitive  organism. 

Reproduction  takes  place  as  in  other  sim])le  forms  by  fission,  the 
free-swimming  cell  splitting  lengthwise*.  The  si)Iit  begins  at  the 
anterior  end,  the  two  new  cells  finally  having  the  same  structures  as 
are  found  in  the  parent  cell.  In  some  species  of  Euglena  that  encyst, 
the  cell  divides  by  fission  during  the  quiescent  period,  so  that  two 
or  more  cells  are  eventually  released  from  the  cyst.  In  some  instances 
as  many  as  32  have  been  released  from  a  single  cyst. 

Paramecium 

Although  protozoa  are  single  cells,  some  representatives  ..f  the 
phylum  are  much  more  highly  specialized  than  tiie  snnple  Ameha.  o. 


158      ORGANISMS   ILLUSTRATING  BIOLOGICAL   PRINCIPLES 


Euglena.  These  living  cells  may  often  be  seen  with  the  naked  eye  as 
whitish  specks,  moving  slowly  near  the  surface  of  a  laboratory  hay 
infusion  that  has  been  standing  for  some  time.  There  are  several 
different  species  commonly  found,  some  larger  than  others,  although 

in  a  drop  of  infusion 
much  variation  in  size 
within  the  same  species 
may  be  found.  The 
class,  Infusoria,  con- 
tains a  large  number  of 
forms,  one  of  which, 
longitudinal  fiber  Paramecium,  or  the 
— -trichocvst         "slipper  animalcule,"  is 


.ei\ 


IOC 


^basal  i^TamAs. 


1-^-  -  thread  of  attachment 


Diagram  showing  structure  of  the  pelHcle  in  Para- 
nicciurri  nnillimicronucleata.  Under  high  power 
of  the  microscope  the  peUicle  is  seen  to  form  minute 
hexagonal  areas,  from  the  center  of  each  of  which  a 
cilium  protrudes.  The  cilia  arise  from  basal  gran- 
ules (microsomes)  which  are  located  on  strands  of 
protoplasm  (longitudinal  fibers).  Where  do  the 
t  richocysts  lie  with  reference  to  the  cilia  .•*  Where 
are  the  openings  through  which  the  trichocysts  are 
discharged?     (After  Lund.) 


very  common.  It  has  a 
somewhat  flat,  elliptical 
body  with  the  anterior 
thinner  end  more  blunt 
and  the  broader  poste- 
rior end  more  pointed. 
The  cell  body  of  Para- 
mecium is  almost  trans- 
parent and  is  made  up 
of  an  outer,  non-granu- 
lar layer,  the  ecto'plasm, 
and  an  inner  semifluid,  granular  layer,  the  endoplasm.  The  ectoplasm 
is  covered  with  a  delicate,  elastic,  but  lifeless  covering  called  the 
pellicle.  Under  it  is  the  living  cell  membrane  and  through  the  pellicle 
project  numerous  threads  of  protoplasm,  the  cilia,  which  are  distrib- 
uted over  the  surface  of  the  body  in  regular  rows.  The  cilia  are 
quite  uniform  in  size  except  at  the  posterior  end  of  the  cell,  where 
they  are  a  little  longer.  It  is  by  means  of  a  lashing  movement  of 
these  cilia  that  locomotion  takes  place.  Embedded  in  the  clear  ecto- 
plasm are  also  fotmd  nimierous  defensive  structures,  called  trichocysts. 
Under  certain  conditions,  delicate  filaments  or  threads  are  discharged 
from  them  which  serve  as  organs  of  offense  and  defense.  It  is  be- 
lieved that  they  may  contain  minute  quantities  of  poison  which 
paralyzes  other  protozoa. 

On  one  side  a  depression,  the  oral  groove,  runs  diagonally  from  the 
anterior  end  of  the  body  to  about  the  middle.  This  oral  groove  ends 
in  a  gullet,  which  in  turn  leads  to  the  interior  of  the  cell.     The 


THE   LARGE   GROUP  OF  THE  SiMVLLEST   ()IU;\MsM< 


!■)<» 


diagonally  beating  cilia  which  cover  the  body  cause  the  rotation  of 
the  Paramecium  on  the  longitudinal  axis.     Since  the  cilia  in  the  oral 
groove  are  longer  and  capable  of  more  vigorous  motion,  th(>  b<,<lv 
tends  to  swerve  toward  the  left.     As  the  water  passes  down  the  <,ral 
groove  towards  the  gullet,  the  waving  undulatinq  mnnhranv   f<,r,ne,l 
of  ciha  fused  together,  guides  particles  of  potential  food  down  the 
gullet  by  means  of  its  wavelike  motion.     At  the  inner  e.id  f..od 
vacuoles  are  formed  within  the  body.     The  food  vacuoles  an<l  other 
granular  inclu.sions  shift  about  in  a  definite  course  within  the  cndo- 
plasm  of  the  cell.     Gradually  the  food  particles  within  a  given  vacuole 
are   digested   by  means   of 
enzymes  formed  in  the  endo- 
plasm  and  released  into  the 
food  vacuole.     The  digested 
food    material   is   absorbed 
into  the  protoplasm,  there 
to  build  up  living  matter  or 
to  be  used  later  in  the  release 
of  energy.     Food  wastes  are 
passed  out  of  the  cell  through 
the  anal  spot.     Excretion  of 
wastes  may  also  take  place 
through  the  cell-membrane 
by  diffusion,  or  through  two 
contractile  vacuoles,  one  at 
each  end  of  the  cell,  which 
consist  of  a  central  cavity 
with    canals    radiating   out 
from  it  into  the  endoplasm. 
Many   experiments   have 
been  made  to  test  the  sensi- 
tiveness of  Paramecium  to 
various  stimuli.     As  in  other 
living    cells    responsive    to 
stimuli,  factors  of  the  envi- 
ronment have  a  distinct  in- 
fluence upon  its  movements. 
Paramecium  swims  in  a  spi- 
ral course  partly  as  a  result  of  its  shape  and  the  arrangement  and 
diagonal  beating  of  the  cilia,  and  partly  on  account  of  the  anteriorly 


anLericfT  end. 

i pellicle- 


ccxTial 

■^^CXOLCole 

—  ectoplccSm. 
•--endoplasm 


--oral  droavQ, 
- -moLcth 

;- gullet.. 

....■Cood,\roc\jio\Q. 

arxxl  ^pot 

Contractile  ,, 

^_^    -.tricHocy^t 

.;\^,,<o^..„ cilicx. 

'"'^<:'.. posterior  ond. 

Internal  structure  of  Pdrdnirritirn  attula- 
lurn.  The  cilia  cover  the  cut  ire  surface  of 
the  cell  and  are  somewhat  Ioiiltit  .iI  I  In"  jmis- 
terior  em]. 


160      ORGANISMS   ILLUSTRATING   BIOLOGICAL   PRINCIPLES 

pointed  groove  which  turns  the  cell  to  the  left  as  it  progresses  through 
the  water.  When  moving  into  an  unfavorable  environment  or  hitting 
against  a  solid  object,  Paramecium  reverses  the  direction  of  its  ciliary 
lashings,  backs  away,  and  goes  forward  again  in  a  slightly  different 
course,  repeating  the  performance  until  the  obstacle  is  eventually 
avoided.  Other  reactions  take  place  with  reference  to  light,  gravity, 
heat,  dissolved  chemicals,  electricity,  and  water  currents,  all  of  which, 
whether  positive  or  negative,  are  co-ordinated  by  means  of  a  so-called 
neuromotor  mechanism  within  the  cell  that  enables  it  to  adjust  itself 
to  its  environment.  Under  careful  methods  of  staining  a  number  of 
very  minute  fibrils  may  be  found  in  the  cell  which  arise  in  a  central 


I  I 

Binary  division  of  Paramecium  caiidaiiim.  Note  the  position  and  structure  of 
micro-  and  niacronucleus  in  I.  Follow  these  structures  to  the  formation  of  the 
daughter  cells  (IV).  Do  both  micro-  and  macronuclei  divide  by  mitosis?  What 
other  changes  take  place  in  the  cells  .3     (After  Hegner.) 

body  near  the  nucleus  and  radiate  out  to  the  bases  of  the  cilia.  This 
apparatus  apparently  aids  in  co-ordinating  the  action  of  different 
parts  of  the  cell. 

Occupying  a  central  area  in  the  cell  are  two  denser  bodies,  the 
larger,  knowni  as  the  macronucleus,  has  to  do  with  the  metabolic 
activities  of  the  cell,  while  the  smaller,  or  micronucleus,  contains  the 
chromatic  material  which  is  associated  with  heredity. 

In  a  hay  infusion  Paramecia  may  be  found  dividing  by  simple 
fission.  In  this  process  both  macro-  and  micronucleus  elongate,  and 
then  divide.  '  A  new  gullet  buds  off  from  the  original  one,  two  new 
contractile  vacuoles  appear,  and  the  cell,  which  has  been  constricting 
in  the  middle,  pulls  apart  to  form  two  new  cells.  This  process  may 
continue  for  a  good  many  generations  where  food  is  plentiful  and 
conditions  of  life  favorable.     Woodruff  has  kept  one  culture  of  Par- 


THE   LARGE   GROl  P  OF  THE  SMALLEST  ()U(;.\M.sM> 


IM 


amecia  in  his  laboratory  at  Yalo  University  lor  thirty  yoars  and  (hir- 
ing that  period  over  twelve  thousand  generations  vv.t,.  I,,-,,!  I.y  fissi,,,, 
It  has  been  observed  in  these  cultures,  however,  (haf  after  4()  or  inor.- 
divisions  have  occurred,  a  i)rocess  called  cndomixis  takes  plac.-.  in 
which  the  old  active  niacronucleus  is  replaced  by  a  new  one  made 


12- 


Endomixis  in  Paramecium  aurelia.  The  normal  condition  of  Parainociiiin  is 
shown  in  I  showing  niacronucleus  and  two  niicronuclei.  Follow  throufrli  the 
series  pictured.  What  happens  to  the  niacronucleus?  How  many  iniiTonuclci 
are  formed?  What,  happens  next?  Note  in  IV  that  only  one  daughler  cell  is 
shown.  How  does  this  cell  obtain  the  normal  number  of  niicronuclei?  Where 
does  the  new  niacronucleus  come  from?  This  rhythm  of  cell  actixity  seems  to 
occur  with  considerable  regularity  every  10  to  .^0  generations  and  it  gi\  es  the  new 
macronucleus  chromatin  from  the  reserve  sujiply  held  in  the  micronucleiis.  This 
process  does  not  appear  in  all  ciliates  and  is  not  beliexed  to  be  necessary  for 
normal  growth.     (After  Hegner.) 

from  chromatin  of  the  reserve  micronucleus.  This  process  is  similar 
in  many  respects  to  conjugation,  except  that  no  foreign  chromatin 
is  added. 

Under  normal  conditions,  another  process  known  as  amphimixis  or 
conjugation  takes  place  somewhat  resembling  the  sexual  procc^^ses  of 
higher  animals.  Two  cells  come  to  lie  with  their  gullet  surfaces  next 
to  each  other  and  a  bridge  of  protoplasm  forms  between  them,  \\hile 
this  is  going  on  the  micronucleus  in  each  cell  moves  away  from  the 


162      ORGANISMS   ILLUSTRATING   BIOLOGICAL   PRINCIPLES 


macronucleiis,  elongates  and  divides  twice  in  rapid  succession.  Three 
of  the  micronuclei  thus  formed  in  each  cell  disappear,  but  the  fourth 
one  divides  again.  In  this  last  division  two  irregular  masses  of  chro- 
matin are  formed.     This  process  has  been  likened  to  a  similar  division 


inr 


"Xcr 


Conjugation  in  Paramecium  caudafiim.  Shortly  after  the  conjugating  pair 
come  together  with  their  ventral  surfaces  opposed  (I)  a  protoplasmic  bridge  is 
formed,  the  macronucleus  breaks  down  (II)  and  each  micronudeus  divides  a 
second  time  (III).  What  happens  to  three  of  the  four  micronuclei?  Compare 
this  stage  with  the  figure  on  page  429  (maturation).  Next  the  micronuclei 
remaining  in  the  cell  divide  into  two,  the  smaller  (migratory)  micronucleus  passing 
over  by  the  protoplasmic  bridge  into  the  opposite  cell,  there  to  unite  with  the 
larger  (stationary)  nucleus  (VI).  Trace  the  subsequent  divisions  of  the  fused 
micronucleus  (VII,  IX).  How  do  we  get  back  to  the  original  cell  condition? 
(X-XIV).     (After  Hegner.) 

that  takes  place  in  the  eggs  of  animals,  at  the  period  known  as  matu- 
ration, when  the  sex  cells  are  losing  part  of  their  chromatic  material 
in  preparation  for  fertilization  of  the  egg  by  the  sperm  cell.  The 
smaller  mass  is  thought  to  correspond  to  a  sperm  cell  of  the  many- 
celled  animals,  while  the  larger  one  corresponds  to  the  egg  cell.  In 
any  event,  each  of  the  smaller  micronuclei  migrates  reciprocally  over 


THE  LARGE  GROUP  OF  THE  SMALLEST  ()H(;.\MSMs 


i«.:{ 


the  protoplasmic  bridge,  and  unites  with  the  larger  niicroiiuclcus  of 
the  cell  left  behind.  The  two  conjugating  cells  now  separate,  and  the 
newly  fused  nucleus,  composed  of  a  male  and  female  microinu'lcus, 
is  left  in  each  cell.  Then  a  series  of  divisions  of  this  nucleus  takes 
place  until  eight  nuclei  are  formed,  four  of  which  become  macro-  and 
four  micronuclei.  Three  of  the  micronuclei  next  disintegrate,  leaving 
the  cell  with  four  macro-  and  one  niicionucleus.  The  latter  divides 
again  and  with  it  the  cell,  so  that  two  cells  result,  each  witli  a  inicro- 
and  two  macronuclei.  A  second  division  leaves  the  daughter  cells 
each  with  a  single  macro-  and  micronucleus,  which,  thus  rejuvenateil. 
start  off  on  a  series  of  several  hundred  cell  divisions  until  another 
period  of  old  age  comes  on,  when  conjugation  or  endomixis  is  repetited. 

Diatoms 

These  beautiful  microscopic  plants,  sometimes  called  "jewels  of  the 
plant  world,"  are  among  the  most  numerous  of  the  one-celled  plants. 
Over  2000  species  have  been  identified  and  named.  They  form  one 
of  the  most  abundant  components  of  plankton  in 
both  fresh  and  salt  water,  and  are  also  found  in 
damp  earth  and  on  moist  rocks,  where  they  may 
occur  singly  or  massed  together  in  groujis.  Certain 
species  stick  together  because  of  a  gelatinous  ma- 
terial which  they  secrete.  Some  diatoms  move 
with  a  slow  gliding  motion  when  they  are  in  con- 
tact with  solid  objects,  although  lacking  visil)le 
organs  of  locomotion.  They  secrete  a  glasslike 
shell  exquisitely  marked  by  tiny  ridges  and  rows 
of  extremely  minute  holes. 

Diatoms  have  been,  and  still  are,  among  the 
most  abundant  of  li\ing  organisms.  So  abundant 
were  they  in  past  ages  that  large  deposits  of  their 
shells  exist  in  the  form  of  diatomaceous  earth. 
In  California,  there  are  deposits  of  diatomaceous 
earth  lying  hundreds  of  feet  thick  over  an  area  of 
many  square  miles,  while  the  floor  of  the  ocean  is 
covered  with  ooze  made  up  of  skeletons  of  diatoms, 
which  after  death  sink  to  the  bottom  of  the  water. 
This  diatomaceous  material  is  used  as  a  basis  for  i^ohshnig  lu.wders 
in  the  manufacture  of  bacteriological  filters,  and  of  certain  kinds  o\ 
porcelains  and  glass. 


The  (lialoiii  \(i- 
vinild  («)  Niilxt-sidf, 
{h^  ;:inilc  side.  sIkiw- 
inf.'  IIk'  rcliilion  of 
tlic\;iK('s.  ThiMiii- 
clfiis  iiiid  the  two 
rililmiilikc  chlorn- 
pliislsitrc  iiol  sliowii. 
(After  Plil/«T.i 


164      ORGANISMS   ILLUSTRATING  RIOLOGICAL  PRINCIPLES 


One  of  the  most  common  diatoms  found  in  pond  water  is  Navicula. 
In  this  form  the  cell  wall  consists  of  two  valves,  one  of  which  fits  into 
the  other.  The  part  that  fits  over  the  inner  valve  is  called  the  girdle. 
The  cell  appears  quite  different  in  structure  when  seen  from  the  valve 
side  or  the  girdle  edge.  In  the  latter  view,  a  bridgelike  mass  of  pro- 
toplasm containing  a  nucleus  appears,  while  in  a  valve  view  a  line 
running  down  the  center,  called  the  raphe,  is  seen,  that  shows  three 
tiny  spots,  one  in  the  middle  and  one  at  each  end.  A  mucilaginous 
material  exudes  through  a  series  of  pores  which  form  the  base  of  the 
raphe.  Navicula  has  two  chloroplasts,  colored  yellowish-brown  by  a 
pigment  called  carotin.  These  can  be  seen  best  when  the  cell  is 
viewed  from  the  flat  side.  At  the  time  of  cell  division,  the  chloro- 
plasts first  increase  in  size,  pushing  the  two  valves  apart  so  that  they 
barely  touch.  Then  the  nucleus,  chloroplasts,  and  cytoplasm  of  the 
cell  divide,  an  inner  valve  forming  for  each  cell.  Each  of  the  new 
cells  thus  formed  is  much  smaller  than  the  parent  cell. 

Desmids 

Another  one-celled  form  common  in  fresh  water  is  the  bright 
green  desmid,  Closterium.  Like  diatoms,  desmids  are  of  various 
shapes  and  sizes.     They  are  beautiful  symmetrical  structures  with 

large,  bright  green  chloro- 
plasts, which  may  be  lobed, 
starshaped,  or  platelike.  The 
cell  wall  is  thin  and  transpar- 
ent, the  granular  protoplasm 
within  being  obscured  by 
chlorophyll,  but  the  nucleus, 
in  the  center  of  the  cell,  may 
be  easily  recognized. 

Desmids  divide  by  a  simple 
transverse  splitting,  forming 
two  cells,  each  new  desmid 
consisting  of  half  of  an  old 
cell  from  which  an  entire  cell 
is  formed.  In  addition,  a  process  of  conjugation  takes  place,  in 
which  two  cells  come  together,  each  sending  out  a  protoplasmic 
protuberance  that  forms  a  connecting  canal.  The  contents  of  the 
two  cells  meet  in  this  tube,  fuse,  and  form  a  single  cell  which  grows  a 
thick  wall,  whereupon  it  remains  as  a  dormant  spore  or  zygote  until 


Closterium. 


Two  cells  undergoing  conjuga- 
.  tion. 


THE   LARGE   GROUP  OF  THE   SMALLEST  OlUiVMsMs       |r,.-. 

conditions  are  favorable  for  germination.     When   the   zyjrotr  .l,„..s 
germinate,  two  new  individuals  come  direetly  from  it. 

Many  other  forms  of  algae  may  be  found  in  fresh  and  .s.h  w.-.i.t. 
Some,  like  Scenedcsmus,  occur  in  colonies,  their  end  cells  being  ..ftcn 
provided  with  characteristic  spines.  Another  colony  of  gr,.,.,,  cells 
Pediastrium,  made  up  of  a  flat  plate  of  sixteen  cells,  is  also  frecpiently 
seen.  These  are  only  a  few  of  the  many  forms  of  green  algae  that 
may  be  found  in  a  drop  of  water  debris  tak(Mi  from  a  (iniet  poml 
bottom. 

Bacteria 

Various  kinds  of  bacteria  are  common  in  a  drop  of  i)ond  water  or 
hay  infusion.  They  are  sometimes  seen  moving  through  the  water, 
but  more  often  are  massed  together  in  a  scum  covering  the  surface 

""a^o?.  O^tfJ  /f#^^' 


cocci 


op    oo  oo 


«■&  S? 


QO  QO   ^ 

ig^Hfl      GO ..  ao 

rmcrococcA    diplococci    staphylococci    streptococci 
Forms  of  l);icleria. 

of  the  water.  Three  large  groups  of  bacteria  ha\-e  been  established 
according  to  their  shape,  coccus,  baccillus,  and  spirillum.  The  coccus 
or  spherical-shaped  bacteria  may  live  singly,  as  micrococci.  Anotlier 
form,  the  diplococci,  divides  and  remains  attaclied  s(j  as  to  form 
pairs  ;  a  third,  streptococci,  reproduces  to  form  chains  ;  while  a  fourth. 
staphylococci,  forms  irregular  groups  of  eight  cells  or  more,  resem- 
bling a  bunch  of  grapes;  Sarcina  divides  in  three  directions  to  pro- 
duce cubical  packets.  The  rod-shaped  bacteria,  or  bacilli,  \i\ry  a 
good  deal  in  size  and  shape,  as  well  as  in  tiieir  ability  to  form  spores, 
some  being  very  short,  others  many  times  longer  than  wide.  The 
third  type,  comprising  the  spirilla,  are  cur\e(l  or  twisted  in  shape, 
and  move  through  the  water  rapidly  by  spiral  movement.  This 
form  can  often  be  seen  hi  a  droj)  of  pond  water  or  hay  infusion. 
BacilH  and  spirilla  move  by  means  o(  Jlagdla,  protoplasmic  threads 


166      ORGANISMS   ILLUSTRATING   BIOLOGICAL   PRINCIPLES 

which  are  difficult  to  see  except  under  the  highest  power  of  the 
microscope. 

The  cell  wall  of  a  bacterium  is  usually  considered  as  a  selectively 
permeable  membrane,  very  delicate,  and  secreted  by  the  cytoplasm. 
A  gelatinous  capsule  may  be  formed  by  some  bacteria,  so  that  groujis 
of  them  clump  together  in  masses.  Although  pigments  are  often 
present,  bacteria  contain  no  chlorophyll,  and  consequently  most  of 
them  are  dependent  on  other  organisms  for  their  food.  They  feed 
both  on  living  and  dead  organisms,  using  not  only  organic  foodstuffs, 
such  as  starches,  sugars,  and  proteins,  but  even  leather  or  wood. 
Since  their  food  must  be  liquid  in  order  to  be  absorbed,  they  form 
digestive  enzymes  within  the  cell  which  exude  to  digest  the  food  out- 
side of  the  cell  body. 

In  addition  to  these  foods,  bacteria  need  certain  mineral  salts  that 
are  found  in  protoplasm,  water,  and  nitrogen  in  a  usable  form.  Not 
all  bacteria  are  capable  of  nitrogen  fixing,  but  many  obtain  their 
supply  of  nitrogen  for  tissue  building  as  green  plants  do,  in  the  form  of 
compounds  of  ammonia  or  nitric  acid. 

The  chromatin  material  is  scattered  through  the  cell,  there  being 
no  distinct  nucleus  in  most  bacteria.  Bacteria  need  moisture,  a  favor- 
able temperature,  and  food,  in  order  to  grow.  Under  favorable  con- 
ditions they  multiply  with  great  rapidity  by  simple  fission.  Under 
unfavorable  conditions,  many  bacterial  cells  can  contract,  lose  con- 
siderable water,  and  form  resistant  coats,  thus  making  spores,  which 
can  stand  extreme  conditions  of  dryness  and  temperature.  While 
bacteria  are  usually  killed  by  heating  to  100°  C,  some  spores  can 
withstand  this  temperature  for  long  periods. 

Functional  Differences  between  Plant  and  Animal  Cells 

A  comparison  of  the  several  types  of  unicellular  organisms  described 
might  seem  at  first  to  show  hard  and  fast  distinctions  between  plant 
and  animal  cells.  Although  chlorophyll  is  associated  with  plants,  it  is 
sometimes  found  in  borderline  animals,  while  many  plants,  such  as 
the  fungi  and  bacteria,  lack  chlorophyll.  Locomotion  is  not  exclu- 
sively an  animal  characteristic.  Some  animal  cells,  as  Vorticella,  are 
fixed  during  a  part  of  their  life  history,  while  many  unicellular  plants 
move  freely  through  the  water.  Other  plants,  although  fixed  for  part 
of  their  lives,  produce  sex  cells  that  are  motile  in  water.  The  greatest 
difference  exists  in  methods  of  nutrition.  In  the  green  plant  cell,  for 
instance,  food  substances  are  made  inside  the  cell  in  the  presence  of 


THE   LARGE   GROUP  OF  THE   SMALLEST   ORGANISMS       167 

sunlight  while  in  animal  cells,  food  is  made  outside  and  has  to  be 
absorbed  before  it  can  be  used.  The  method  of  nutrition  used  by  the 
green  plant  is  called  holophytic,  and  that  of  the  animal  cells,  holozoic. 
The  differences  between  these  two  types  of  nutrition  are  summed  up 
in  the  table  below. 


Animal  Cell 

Plant  Cell 

No  chlorophyll 

Chlorophyll  present 

Cannot  make  organic  foods 

Can  synthesize  organic  foods  out  of  law 

food  materials 

Only  source  of  energy  is  organic  food 

Source  of  energy  is  the  sun 

Ingests  solid  food 

Cannot  ingest  solid  food 

Usually  moves  about  after  food,  therefore 

Does  not  ordinarily  move  about,  and  uses 

greater    destructive  metabolism 

sun's  energy,  therefore  greater  construc- 

tive metabolism 

Depends  on  other  organisms  for  food 

Supplies  other  organisms  with  food 

SUGGESTED  READINGS 

Calkins,  G.  N.,  Biology  of  the  Protozoa,  Lea  &  Febiger,  1926.     Chs.  I,  III, 

and  IV,  especially. 
Dobell,  C.,  Antony  van  Leeuwenhoek  and  his  "Little  Animals,"  Harcourt,  Brace 

and  Co.,  1932. 

The  entire  book,  which  contains  excellent  translations  of  most  of  the 

original  letters  of  van  Leeuwenhoek,  is  well  worth  reading.     It  is  a 

most  authentic  picture  of  this  interesting  Dutchman  and  his  times. 
Giltner,  W.,  Textbook  of  General  Microbiology,  P.  Blakiston's  Son  &  Co.,  1928. 

Ch.  III. 
Holman,  R.  M.,  and  Robbins,  W.  W.,  Elements  of  Botany,  3rd  cd.,  John 

Wiley  &  Sons,  Inc.,  193(3.     Ch.  X. 
Locy,  W.  A.,  Biology  and  Its  Makers,  Henry  Holt  &  Co.,  1908.     Ch.  V. 

An  excellent  historical  survey. 
Needham,  J.  G.,  and  Lloyd,  J.  T.,  Life  of  Inland  Waters,  2nd  ed.  Charles  C. 

Thomas,  1930.     Ch.  IV. 

Excellent  descriptions  and  illustrations  of  the  life  found  in  pond  water. 
Singer,  C.  J.,  The  Story  of  Living  Things,  Harper  &  Bros.,  1931.     Ch.  IV. 

An  interesting  and  authentic  history  of  biology. 
Ward,  H.  B.,  and  Whipple,  G.  C,  Fresh-Water  Biology,  John  Wiley  &  Sons, 

Inc.,  1918. 

This  book  is  invaluable  for  reference.     Chapters  VI,  IX,  and  XVII  arc 

especially  useful. 


H.  w.  H. 


12 


VIII 


THE   DEVELOPMENT   OF  SEXUALITY   IN   PLANTS 

Preview.  The  beginnings  of  sex  in  the  algae  •  Oedogonium  •  A  repre- 
sentative fungus  •  Alternation  of  generations  in  the  plant  kingdom  •  Sug- 
gested readings. 

PREVIEW 

The  one  unescapable  fact  that  stands  out  in  the  observation  of 
plants  and  animals  in  the  world  about  us  is  the  remarkable  variety 
among  living  things.  They  range  from  tiny  forms  too  small  to  be  seen 
with  the  unaided  eye  to  huge  organisms  such  as  elephants  or  trees. 

The  biologist  is  not  satisfied  with  random  looking.  He  looks  for 
certain  things,  tries  to  interpret  what  he  sees,  but  as  Thoreau  once 
said,  "We  must  look  a  long  time  before  we  can  see."  One  of  the 
striking  facts  already  noted  in  the  Roll  Call  of  forms  of  life  is  that 
both  plants  and  animals  may  be  placed  in  groups  having  similar 
characters,  and  that  these  groups  arrange  themselves  in  a  series  of 
gradually  increasing  intricacy  of  structure,  which  goes  hand  in  hand 
with  an  ever  increasing  complexity  in  functions.  Simple  plants  or 
animals  do  things  simply.  Almost  any  part  of  the  one-celled  Ameba 
can  do  any  part  of  the  work  of  the  cell  although  lacking  organs  found 
in  higher  forms.  More  refined  ways  of  doing  things,  and  a  more' 
efficient  division  of  work,  come  with  increasing  complexity  of  organic 
structure.  The  true  investigator  is  ever  alert  to  find  forms  that 
illustrate  this  increasing  division  of  labor,  and  is  always  asking  why 
and  how  such  things  come  about.  Biologists  have  picked  out  certain 
representative  forms  that  clearly  suggest  certain  facts  and  principles 
that  are  worth  knowing.  It  is  possible,  for  example,  through  the 
study  of  some  simple  forms  of  organisms,  such  as  the  Thallophytes, 
to  discover  the  beginnings  of  sexuality  in  plants. 

The  Thallophytes  include  most  of  the  simplest  plants  and  are 
divided  into  two  great  groups,  algae  and  fungi,  the  latter  containing 
no  chlorophyll.  Wliile  there  are  six  classes  of  algae,  four,  namely, 
the  blue-green,  the  green,  the  brown,  and  the  red,  are  classified 
largely  on  color.  All  of  the  four  groups  are  essentially  water- 
loving  plants,  showing  in  many  ways  that  they  are  simple  and 
rather   primitive   organisms.     In   size   they   range  from   tiny   uni- 

168 


THE   DEVELOPMENT   OF   SEXUALITY    IN    PLANTS 


169 


cellular  forms  to  some  of  the  great  brown  seaweeds,  or  kelps  of 
the  California  coast  which  may  be  several  hundred  feet  in  length. 
Ascending  the  scale  of  increasing  complexity  in  structure,  we  find  the 
appearance  first  of  sex  cells  and  later  of  sex  organs  evolved  to  form 
and  protect  these  sex  cells. 

By  selecting  other  representatives  from  the  higher  plant  groups, 
such  as  mosses,  ferns,  and  flowering  plants,  we  can  follow  this  evolu- 
tion of  sex  through  the  entire  plant  kingdom.  The  pages  that  follow 
will  at  least  give  us  a  start  on  the  answer  to  the  question  :  How  and 
where  does  sex  originate  in  plants  and  what  is  its  meaning  ? 


The  Beginnings  of  Sex  in  the  Algae 

Pleurococcus,  or  Protococcus  as  it  is  sometimes 
called,  is  one  of  the  simplest  of  all  living  plants, 
familiar  to  most  of  us  as  the  green  "moss" 
usually  seen  on  the  north  side  of  trees.  Indians 
used  it  to  find  their  direction  through  the  forest, 
as  persons  lost  in  the  woods  do  today.  Its 
habitat  suggests  that  the  life  of  the  plant  has 
direct  relation  to  moisture,  temperature,  and 
light.  It  would  be  injured  by  the  direct  rays 
of  the  sun,  because  some  rays  such  as  those  of 
ultraviolet  light  are  injurious  to  unprotected 
protoplasm. 

The  cell  of  Pleurococcus  is  very  simple  as  seen 
under  a  microscope.  It  is  found  single,  in  twos, 
threes,  fours,  or  flat  colonies  of  several  cells 
hanging  together.  Examination  of  a  single 
cell  discloses  the  presence  of  a  thin  wall  sur- 
rounding a  mass  of  green  protoplasm,  the  protoplast,  which  almost 
completely  fills  the  cell.  If  a  drop  of  iodine  solution  is  placed  under 
the  coverslip,  the  detailed  structure  of  the  cell  becomes  more  evident. 
The  nucleus  is  completely  surrounded  by  one  large,  spherical  chloro- 
plast.  The  cell  is  a  complete  entity,  in  spite  of  the  fact  that  it  is 
often  attached  to  other  cells.  Physiologically  it  is  able  to  carry  on 
all  the  functions  of  a  living  green  plant,  making  food,  and  digesting 
it  as  well  as  absorbing  food  and  water.  It  grows  to  a  certain  size 
and  then  reproduces  by  simple  fission,  part  of  the  mother  cell  going 
into  one  daughter  cell  and  part  into  the  other.     Theoretically  the 


Reproduction  in  Pleu- 
rococcus. Each  cell  is 
considered  as  an  indi- 
vidual, although  colonies 
(seen  above)  may  be 
formed.  The  protoplasm 
of  the  cell  body  is  not 
shown,  the  single  chloro- 
plast  being  surrounded 
by  protoplasm  in  active 
cells. 


170      ORGANISMS   ILLUSTRATING  BIOLOGICAL   PRINCIPLES 


protoplasm  of  the  pleurococcus  is  immortal,  since  it 
passes  from  cell  to  cell  by  means  of  cell  division. 

Spirogyra  is  one  of  the  multicellular  green  algae. 
It  is  a  slimy  thread,  called  "pond  scum,"  found  near 
the  surface  of  a  pond,  often  buoyed  up  by  bubbles  of 
gas  which  it  forms.  The  filamentous  plant  body 
consists  of  several  cells  joined  end  to  end,  each  with  a 
characteristic  spirally-banded  chloroplast. 

Examination  of  a  single  cell  shows  a  colorless  cell 
wall,  the  cytoplasm  of  the  cell  mostly  adhering  to  its 
inner  surface.  Strands  of  cytoplasm  radiate  from  a 
central  colorless  nucleus,  which  is  suspended  in  a  large 
vacuole  or  sap  cavity.  The  most  characteristic  fea- 
ture is  the  large  twisted  chloroplast,  on  which  are 
scattered  many  pyrenoids,  bodies  which  contain  some 
of  the  starch  manufactured  by  the  chloroplast.  In- 
dividuals grow  in  size  by  forming,  through  transverse 
division  of  the  cells,  longer  or  shorter  filaments,  de- 


A  Spirogyra   pending  upon  the  environmental  conditions. 


cell  showing  the 
spiral  chloro- 
plast containing 
pyrenoids,  and 
the  nucleus. 


At  certain  times  in  the  year,  the  plants  form  resting 
spores  called  zygospores.  Two  adjoining  filaments 
come  to  lie  parallel,  the  cells  opposite  to  each  other 
sending  out  bulging  outgrowths  which  meet  to  form  a 
connecting  tube.  Meantime,  owing  to  the  dissolving  of  the  cell  wall 
at  the  end  of  the  outgrowths,  water  gets  inside  of  the  cells,  so  that 
they  show  signs  of  plasmolysis,  rounding  up  into  ovoid  masses. 
Curiously,  however,  the  cells  of  one  filament  remain  stationary, 
while  the  cell  contents  from  the  other  filament  move  over  through 


Conjugation  of  Spirogyra.     Explain  what  happens.     (After  Coulter.) 


THE   DEVELOPMENT   OF   SEXUALITY    IN    PLANTS 


171 


the  tube  and  fuse  with  th(^  quiescent  cells.  When  this  fusion  takes 
place,  the  nuclei  unite  so  that  a  single  resting  cell  is  formed,  called  the 
zygote,  which  develops  a  thick  wall,  very  resistant  to  drought  and  cold. 
The  zygote  is  heavy  enough  to  sink  to  the  bottom  of  the  pond  when 
the  rest  of  the  filament  dies,  and  under  favorable  conditions  will 
germinate,  giving  rise  to  a  new  filament. 

Since  these  cells  from  different  filaments  join  or  fuse,  somewhat 
after  the  manner  of  conjugation  in  Paramecium,  we  think  of  them  as 
sex  cells,  or  gametes.  Although  the  two  cells  are  of  the  same  size, 
yet  one  is  active  and  the  other  passive.  In  higher  plants  and  animals, 
the  active  cell  is  referred  to  as  the  male  gamete,  or  spei^m,  and  the 
non-active  cell  as 
the  female  gamete, 
or  egg.  A  compari- 
son of  Spirogyra 
with  higher  forms 
suggests  a  very  sim- 
ple type  of  sexual 
reproduction,  known 
as  conjugation. 

In  another  fila- 
mentous form,  Ulo~ 
thrix,  certain  cells 
are  modified  to  be- 
come free-swimming 
zoospores,  provided 
with  four  cilia  which 
may  swim  about  for 
as  long  as  an  hour 
before  settling 
down.  It  is  obvious 
that  such  a  free- 
swimming  cell  may  plant  a  new  individual  at  some  distance  from 
the  original  filament.  Gametes  of  Vlothrix  are  also  formed  as  free- 
swimming  cells,  all  alike,  having  two  cilia  instead  of  four.  These 
gametes  fuse  by  conjugation  and  produce  a  zygote,  which,  like  that 
of  Spirogyra,  has  a  thick  resistant  wall,  and  is  capable  of  developing 
even  after  exposure  to  very  unfavorable  conditions. 

In  the  formation  of  the  conjugating  gametes  of  both  Vlothrix  and 
Spirogyra  a  significant  thing  happens  to  the  nuclei  of  the  cells  before 


Ulothrix:  a.  base  of  filament  with  holdfast;  b,  fila- 
ment producing;  zoospores  or  gametes;  c.  young  filament 
developed  from  zoospore;  d.  filament  discharging  zoo- 
spores and  gametes ;  e.  an  escaped  zoospore  ;  /,  escaped 
and  pairing  gametes ;  g,  zygospores ;  h,  zygospore  pro- 
ducing zoospores  by  reduction  division,  (a-f/.  After 
Coulter;  //,  after  Dodel-Port.) 


172      ORGANISMS  ILLUSTRATING  BIOLOGICAL  PRINCIPLES 


fusion 


they  conjugate.  By  a 
series  of  divisions,  such 
as  is  shown  in  the  dia- 
gram, the  number  of 
chromosomes  in  the  nu- 
clei of  the  zygote,  result- 
ing from  the  union  of  the 
two  gametes,  is  reduced 
. .  to  half  this  number.  If 
\  spelling         disappear        g^j^^g  g^^j^  dcvice  as  this 

'  ^  /      \  were  not  used,  every  time 

sex  cells  united,  the  num- 
ber  of  chromosomes 
would  be  doubled.  How- 
ever, by  this  so-called 
reduction  division  during 
the  formation  of  the  ga- 
Diagram  to  show  how  reduction  division  takes  metes,  which  OCCUrs  in 
place  in  the  zygote  of  Spirogyra.  both  plants  and  animals, 

the  number  of  chromosomes  is  halved.  We  speak  of  the  single  number 
of  chromosomes  as  haploid  and  the  double  number,  which  comes  with 
the  union  of  the  two  gametes,  as  diploid. 


■2.y^U 


first  division, 
reduction 
of  ci-ji-TomoSomcS 
from    ?T-;  to  ri  , 
mat.u.ra.t,ion 


/second 
cLivision, 
TTJitjDSiS 


Oedogoniiim 

In  another  of  the  filamentous  algae, 
Oedogonium,  there  is  the  first  appearance 
of  two  kinds  of  sex  cells.  This  alga  repro- 
duces by  zoospores  and  in  addition  forms 
two  sex  organs,  structures  called  anther- 
idia,  which  produce  a  number  of  ciliated 
sper7n  cells  and  oogonia,  the  latter  holding 
a  single  egg  cell.  The  sperm  cells  swim 
through  the  water  from  the  antheridia, 
one  uniting  with  the  egg  cell,  and  almost 
immediately  a  thick  wall  is  formed  about 
the  fertilized  egg.  This  oospore  does  not 
produce  a  new  plant  directly,  but  gives 
rise  to  zoospores,  which  in  turn  eventu- 
ally become  new  plants. 


Life  history  of  Oedogonium. 


THE   DEVELOPMENT   OF   SEXUALITY    IN    PLANTS  173 

Another  form  of  Oedogonium  forms  antheridia  and  oogonia  on 
separate  filaments,  the  male  filament  being  much  smaller  than  the 
female  filament.  Thus  the  filamentous  algae  illustrate  three  big 
ideas,  namely,  division  of  labor,  development  of  sex,  and  reduction  of 
chromosomes. 

In  the  simplest  plants  all  cells  tend  to  do  the  same  work,  but  in  the 
more  specialized  algae  there  is  a  differentiation  of  work  and  an 
accompanying  differentiation  of  cells  to  accomplish  it.  In  the 
development  of  sex  and  of  structures  to  take  care  of  the  sex  cells,  as 
found  in  the  forms  described,  the  contribution  of  the  sex  cells  seems 
to  be  to  provide  a  greater  vigor  to  the  offspring,  especially  when  the  sex 
cells  come  from  different  individuals.  Most  important  of  all  is  the 
fact  that  cells  which  fuse,  as  in  the  case  of  the  sex  cells,  must  have 
some  way  of  reducing  the  number  of  their  chromosomes,  else  they 
would  be  doubled  each  time  two  sex  cells  united.  This  is  accom- 
plished by  the  reduction  division  referred  to  above,  by  which  process 
the  number  of  chromosomes,  doubled  at  the  time  of  fertilization,  is 
halved.  This  reduction  process  occurs  in  both  plants  and  animals, 
and  although  in  plants  it  occupies  a  different  place  in  the  life  cycle, 
its  ultimate  effect  is  the  same  in  both  cases. 

A  Representative  Fungus 

Bread  mold,  Rhizopus  nigricans,  one  of  the  most  common  of  the 
fungi,  may  easily  be  grown  in  the  laboratory  by  exposing  a  moist  piece 
of  bread  to  the  air  for  a  few  moments.  Mold  spores  are  so  numerous 
everywhere  that  under  ordinary  conditions  a  growth  of  mold  will  be 
evident  within  one  or  two  days,  first  appearing  as  a  white,  fluffy 
growth  that  rapidly  covers  the  surface  of  the  bread.  This  is  the 
mycelium,  which  consists  of  branching  tubelike  filaments,  or  hyphae, 
containing  many  nuclei,  but  without  cross  walls.  The  absence  of 
chlorophyll  shows  the  inability  of  the  mold  to  make  its  own  foods 
and  explains  why  the  mycelium  sends  down  into  the  bread,  root- 
like branches  called  rhizoids,  that  secrete  enzymes,  by  means  of 
which  the  food  substances  in  the  bread  are  digested.  Some  of  the 
hyphae  form  long  branches  called  stolons,  which  run  along  the  sur- 
face of  the  bread,  forming  new  plants.  At  points  where  rhizoids 
are  developed,  there  arise  later  numbers  of  erect  branches,  or  spo- 
rangiophores,  on  the  tips  of  which  are  developed  sporangia,  or  spore- 
bearing  organs. 


174      ORGANISMS   ILLUSTRATING  RIOLOGICAL   PRINCIPLES 


Great  numbers  of  tiny  spores  are  produced  by  division  of  the  dense 
terminal  portions  of  the  sporangiophores.  As  a  sporangium  becomes 
mature  an  outer  wall  is  formed  and  the  spores  turn  black  in  color. 

When  this  outer  wall 
breaks,  the  minute  spores 
are  scattered  far  and  wide 
by  air  currents. 

Molds  also  reproduce 
sexually,  by  means  of  con- 
jugation. Rhizopus  has 
two  different  strains  of 
mycelia,  one  of  which  is 
called  a  plus  (  +  )  and  the 
other  a  minus  (  — )  strain. 
If  hyphae  of  two  such 
strains  come  in  contact 
with  each  other,  zygo- 
spores are  formed.  Short, 
club-shaped  branches  are 
developed  from  the  hy- 
phae, the  dense  proto- 
plasmic tips  are  cut  off  from  the  end  of  each  by  cell  walls,  and  these 
"cells,"  each  of  which  contains  several  nuclei,  unite  to  form  a 
zygote.  The  zygote  with  the  hyphae  which  develop  from  it  proba- 
bly represents  the  diploid  stage  of  chromosome  in  the  life  cycle, 
the  haploid  stage  being  reached  when  the  spores  on  the  sporangium 
germinate. 

The  fungi  are  of  even  more  interest  by  reason  of  their  method  of 
nutrition.  They  are  typically  neither  holozoic  nor  holophytic,  since 
they  live  as  saprophytes  on  dead  organic  materials.  This  means  that 
they  must  absorb  food  materials  which  are  supplied  to  them  from 
outside  sources  after  digesting  them  by  means  of  enzymes,  when 
absorption  takes  place  through  the  plasma  membrane  of  the  cell. 

Alternation  of  Generations  in  the  Plant  Kingdom 

The  most  important  difference  in  the  life  cycle  between  the  Bryo- 
phytes  or  Mosses  and  lower  forms,  aside  from  a  greater  differentiation 
of  the  plant  body,  is  the  alternation  of  an  asexual  with  that  of  a  sexual 
generation  in  the  hfe  cycle.  The  asexual  generation,  which  produces 
spores,  is  called  the  sporophyte,  while  the  sexual  generation,  which 


Reproduction  in  bread  mold  (Rhizopus  nigri- 
cans). Read  the  text  and  then  explain  the 
diagrEun. 


THE   DEVELOPMENT   OF   SEXUALITY   IN   PLANTS  175 


gives  rise  to  gametes  of  two  different  sexes,  is  known  as  the  gameto- 
phyte.  The  latter  generation  is  the  conspicuous  green  plant  that 
manufactures  food  and  serves  as  host  for  the  sporophyte  generation 
which  is  permanently  attached  to  it. 

The  gametophyte  of  the  simple  moss,  Funaria  hygrometrica,  is  a 
short  upright  stalk  bearing  usually  three  spiral  rows  of  simple  leaves, 


sperm 


embryo 


Sfertili^ect  egg 


rontheridiunz. 


mum. 


dbcmetophone , 


\ 


bud 


protonemol 
threocC 


ycrunS  gb-metophyt' 


The  life  cycle  of  Funaria,  a  moss.     Which  stage  is  more  prominent, 
gametophyte  or  sporophyte? 

each  containing  numerous  chloroplasts.  At  the  lower  end,  a  group  of 
small  brown  rkizoids  furnish  the  means  of  attachment  to  the  sub- 
stratum. The  moss  plant  is  dioecious,  having  separate  sex  organs 
on  different  plants.  The  male  gametophytes  are  shorter  than  the 
female  gametophytes  and  bear  at  the  upper  tip  a  cluster  of  structures 
known  as  antheridia.  Each  mature  antheridium  looks  like  a  tiny 
club  with  a  wall  formed  of  rather  large,  thin  cells,  which  forms  a  recep- 
tacle for  numerous  motile  sperm  cells.  The  female  gametophyte 
bears  at  the  apex  of  the  short  stem,  although  in  the  mature  plant 


176      ORGANISMS   ILLUSTRATING   BIOLOGICAL   PRINCIPLES 

hidden  by  leaves,  a  cluster  of  flask-shaped  structures  called  archegonia, 
at  the  bottom  of  each  of  which  is  a  single  rather  large  egg  cell. 

Fertilization  of  the  egg  can  take  place  only  when  the  antheridia  and 
archegonia  are  wet  from  rain  or  dew.  In  such  an  event  the  sperm 
cells  ooze  out  in  a  mucilaginous  substance  secreted  from  the  walls  of 
the  antheridium  and  pass  in  drops  of  water  to  the  necks  of  the 
flask-shaped  archegonia.  Here  they  are  chemically  attracted  by  a 
substance  exuded  from  the  inside  of  the  archegonium  and  swim  down 
the  tubular  neck  until  one  meets  the  egg  cell,  when  fertilization  takes 
place.  The  gametophytic  phase  of  the  moss  is  the  haploid  stage  of  the 
chromosomes,  fertilization  of  the  egg  restoring  the  diploid  number 
characteristic  of  the  sporophyte.  This  generation  begins  with  the 
cell  division  which  follows  the  fertilization  of  the  egg  in  the  archego- 
nium and  results  in  the  growth  of  a  tiny  stalk,  bearing  at  its  upper  end 
a  capsule,  that  in  the  adult  sporophyte  is  filled  with  asexual  spores. 
During  the  formation  of  the  spores  within  the  capsule,  the  formative 
tissues  produce  a  number  of  large,  rounded  spore  mother  cells,  from 
each  of  which  by  nuclear  divisions  tetrads,  or  groups  of  four  spores, 
are  formed.  During  this  tetrad  formation,  a  reduction  division 
takes  place  so  that  the  spores  contain  only  the  haploid  number  of 
chromosomes. 

The  moss  capsule  is  quite  a  complex  structure  with  a  cap,  or  oper- 
culum, that  covers  an  urn-shaped  affair  bearing  at  its  upper  end  a 
circle  of  teethlike  structures  collectively  called  the  peristome.  As  the 
sporophyte  ripens  it  dries  up  and  the  numerous  ripe  spores  are  scat- 
tered by  the  action  of  the  peristome  teeth,  the  latter  being  very 
hygroscopic,  or  sensitive  to  moisture.  When  the  weather  is  humid  or 
wet,  the  teeth  of  the  peristome  curl  up  and  when  dry  they  straighten 
out,  thus  expelling  the  spores,  which  may  then  be  scattered  by  the 
wind.  The  germinating  spore  does  not  grow  directly  into  a  leafy 
plant,  but  first  forms  a  protonema  or  algalike  filament  from  which 
upright  stalks  later  arise,  while  rhizoids  grow  downwards  from  it, 
thus  forming  again  the  moss  plant.  This  life  cycle  with  its  alterna- 
tion of  gametophytic  and  sporophytic  stages  is  characteristic  of  the 
life  cycle  of  mosses  and  liverworts,  as  well  as  the  higher  group  of  the 
ferns  (Filicinae). 

In  the  flowering  plants  (Angiospermae),  one  finds  an  almost  com- 
plete suppression  of  the  gametophytic  generation,  the  sex  cells  or 
gametes  being  produced  in  modified  leaflike  parts  of  the  flower.  The 
floral  parts  —  sepals,  petals,  stamens,  and  carpels  —  are  thought  of 


THE   DEVELOPMENT   OF   SEXUALITY    IN    PLANTS 


177 


as  leaves  which  have  become  metamorphosed  from  their  vegetative 
form  and  function  to  hold  the  sex  structures.  The  stamens  and 
pistil  (carpel)  contain  spore-forming  tissues  which,  by  means  of 
reduction  division,  produce  pollen  grains  containing  microspores 
(sperms),  while  ovules  produce  a  female  gametophyte  and  its  egg. 
The  sperm  cells  are  formed  in  the  pollen  grains,  while  the  egg  cells 


germinating  poller 


tube 


osUs  of 
anLher 


form  pollen  grains      with— ^    sperm  2,-^  ^; 

tuba  nuclau^ 


TRe  embr/o  sac  contains  a  dividimg  nuclaus  ^  v '/   bolUn 
eight  cxne  jlnally  formed,  tuJo  forn-i  fusion  rnAcleu^  /^tL-ubo 

Development  of  male  and  female  gametophyte  in  the  flowering  plants.  Only 
the  cells  which  actually  form  these  structure  are  shown.  The  parts  of  the  sporo- 
phyte  upon  which  the  gametophyte  is  parasitic  are  omitted  for  the  sake  of  clarity. 
Read  the  text  carefully  and  then  use  the  diagrams. 

are  held  within  the  ovary  of  the  pistil  as  has  been  previously  stated. 
In  the  angiosperms  or  flowering  plants  the  male  gametophyte  is  so 
much  reduced  that  it  consists  of  only  three  cells,  a  tube  nucleus  and 
two  generative  cells  (see  figure).  Just  previous  to  the  formation  of 
the  pollen  grains  (male  gametophyte)  reduction  di\ision  takes  place 
so  that  its  cells  contain  the  haploid  number  of  chromosomes.  The 
female  gametophyte  is  also  greatly  reduced.  After  reduction  divi- 
sion, the  megaspore  divides  (see  figure)  one  nucleus  migrating  to  each 
end  of  the  etnbryo  sac  (female  gametophyte).  The  nuclei  continue 
to  divide  until  eight  are  formed  in  two  groups  at  opposite  ends  of 
the  embryo  sac.  From  each  group  a  single  nucleus  then  unites  with 
the  other  to  form  a  fusion  nucleus  (see  figure).  At  this  stage  the  egg 
nucleus  is  ready  for  fertilization  by  the  sperm  nucleus.  A  double  ferti- 
lization now  takes  place,  the  sperm  nucleus  fuses  with  the  egg  nucleus 
and  the  second  sperm  nucleus  unites  with  the  fusion  nucleus.     The 


178      ORGANISMS   ILLUSTRATING   BIOLOGICAL   PRINCIPLES 

former  gives  rise  to  the  young  plant,  the  latter  to  its  food  supply,  the 
endosperm.  The  transfer  of  pollen  in  flowers  of  the  same  species 
may  result  in  the  fertilization  of  the  egg  and  subsequent  growth  of 


Division   I 

Thallopbyta 

algcxe.      "^ 


Division  I 

Bryophyta 


Division  BE  Tr-ccchsopViyta.-  vasculcti-  T=lcL"ts 

subdivision  A*B;vc  ^abdVvi/ion  D     PteropsicCcc 

ferns       ^^rnnospe4*ms      angiospcrrrjs 


primiuve  plants 

Lvcopsida  ,Sl*enopJic£a 


^ener-cction 


Diagram  showing  relation  of  sporophyte  and  gametophyte  generations  in  the 

plant  kingdom, 

the  plant  body  (sporophyte  generation).  The  evolution  of  sporo- 
phytic  and  gametophytic  generations  in  the  plant  kingdom  is  shown 
in  the  above  chart. 

SUGGESTED  READINGS 

Coulter,  J.  M.,  Barnes,  C.  R.,  and  Cowles,  H.  C.,  A  Textbook  of  Botany, 

Vol.  I,  American  Book  Co.,  1930. 

This  text  gives  an  excellent  foundation  for  the  understanding  of  sexu- 
ality in  plants. 
Gager,  C.  S.,  General  Botany,  P.  Blakiston's  Son  &  Co.,  1926. 

A  general  botany  which  gives  much  information  on  economic  questions, 

as  well  as  sex  development  in  simple  plants. 
Robbins,  W.  J.,  and  Rickett,  H.  W.,  Botamj,  D.  Van  Nostrand  Co.,  1929. 

Chs.  XV-XXIV. 

Excellent  diagrams  help  in  the  understanding  of  the  development  of  sex. 
Sinnott,  E.  W.,  Botany,  Principles  and  Problems,  3rd  ed.,  McGraw-Hill  Book 

Company,  1935.     Chs.  XI  and  XIV-XXIII. 

A  thoroughly  up-to-date  treatment  of  the  subject. 
Wilson,  C.  L.,  and  Haber,  J.  N.,  Plant  Life,  Henry  Holt  &  Co.,  1935. 

An  interesting  and  well-written  elementary  text. 


I 


IX 


DIVISION   OF  LABOR   IN   THE   COELENTERATES 

Preview.  The  Hydra,  a  representative  of  the  phylum  Coelcnterata ;  the 
ectoderm  and  its  functions ;  the  endoderm  and  its  functions ;  reactions  to 
stimuli ;  reproduction ;  regeneration  ■  Hydroids  •  Suggested  readings. 

PREVIEW 

It  has  already  been  shown  that  unicellular  animals  may  exhibit 
considerable  complexity  of  structure,  and  that  associated  with  this 
complexity,  there  is  a  separation  of  functions  in  different  parts  of  the 
cell,  but  we  have  not  traced  this  division  of  labor  into  the  many- 
celled  animals  or  metazoa.  The  colonial  forms,  such  as  Pandorina, 
Eudorina,  and  Volvox,  claimed  by  both  botanists  and  zoologists,  are 
interesting  exam]iles  of  aggregations  of  many  cells  showing  little 
evidence  of  organization  or  division  of  labor.  Even  in  the  colony  of 
Volvox,  most  of  the  cells  have  common  functions,  only  the  reproduc- 
tive cells  being  set  off  from  the  others. 

The  Hydra,  a  tiny  animal  little  higher  in  the  scale  of  life,  gives  every 
evidence  in  its  structure  of  being  a  simple  organism  and  not  just  a 
collection,  or  colony,  of  cells.  It  shows,  in  a  convincing  manner,  how 
a  simple,  many-celled  organism  lives.  It  answers  the  question  of 
how  division  of  labor  might  arise  among  the  cells  of  a  simple  organism, 
For  this  reason  it  is  chosen  as  a  type  in  most  courses  in  biology  and 
so  has  a  place  in  this  text. 

The  Hydra,  a  Representative  of  the  Phylum  Coelenterata 

Hydras  are  quite  abundant  in  many  ponds  or  slow-moving  streams, 
where  they  may  be  collected  on  the  stems  and  leaves  of  aquatic  plants. 
In  an  aquarium,  they  often  leave  these  plants  and  become  attached 
to  the  glass  walls  of  the  aquarium,  where  they  appear  as  tiny  brown 
or  green  cylinders  one-half  of  an  inch  or  more  in  length.  At  the  free 
or  so-called  oral  end,  a  circle  of  tentacles  surrounds  a  conelike  area, 
the  hypostome,  in  which  the  mouth  is  found.  The  opposite,  or  aboral, 
end  forms  a  disklike  structure  which  is  provided  with  mucous  cells 
that  aid  it  in  sticking  to  a  surface.  Hydras  are  able  to  move  slowly 
by  a  looping  motion  of  the  body.    The  green  ones,  which  are  much  more 

179 


180      ORGANISMS   ILLUSTRATING  BIOLOGICAL   PRINCIPLES 

active  than  the  brown  ones,  frequently  change  their  position  if  food 
is  not  abundant.  They  respond  to  chemical  stimuli  of  food,  to  light, 
and  to  unfavorable  temperatures,  food  being  the  chief  factor  in  their 
environment.     The  color  of  green  hydras  is  due  to  the  presence  of 


Hydra  is  able  to  change  its  position  both  by  turning  "handsprings"  as  shown 
in  the  diagram  and  also  by  contraction  and  expansion  of  the  basal  portion  of  the 
body. 

minute  green  algae,  called  Zoochlorellae,  that  live  in  a  symbiotic 
relationship  within  the  endodermal  cells. 

The  term,  Coelenterata,  which  is  the  name  of  the  phylum  to  which 
the  common  Hydra  vulgaris  belongs,  comes  from  the  Greek  words 
koilos,  hollow,  and  enteron,  intestine,  which  may  be  translated  "hav- 
ing an  internal  digestive  cavity,"  an  apt  title,  since  a  Hydra  is  really 
a  hollow,  double-walled  bag. 

The  Ectoderm  and  Its  Functions 

The  bulk  of  the  outer  layer  of  cells  (ectoderm)  is  made  up  of  large 
epitheUo-muscular  cells,  having  a  layer  of  muscle  fibers  placed  lon- 
gitudinally at  their  bases,  that  enable  the  animal  to  lengthen  or 
shorten  its  body.  A  similar  layer  of  fibers  on  the  inner  layer  of  cells 
which  run  circularly  around  the  body  allows  it  to  expand  or  contract 
in  diameter.  Between  the  epithelio-muscular  cells  and  near  the  inner 
margin  of  the  ectoderm  are  found  numerous  smaller  interstitial  cells 
from  which  are  derived  numerous  other  cells,  including  the  cnido- 
blasts.  Nerve  cells  are  likewise  scattered  throughout  the  ectoderm, 
forming  a  nerve  Jiet  at  the  base  of  the  epithelial  cells. 

Cnidoblasts  are  most  abundant  on  the  tentacles,  although  they  are 
found  on  all  parts  of  the  body  exclusive  of  the  basal  disk.  They  hold 
four  kinds  of  stinging  capsules,  nematocysts,  by  means  of  which  the 
animal  paralyzes  living  prey  that  comes  in  contact  with  its  tentacles. 
The  nematocysts  are  capsules  containing  a  hollow  inverted  thread 
which  under  certain  conditions  can  be  thrown  out,  together  with  a 
poisonous  substance,  hypnotoxin,  that  has  the  power  to  paralyze  any 
other  small  animal  which  it  touches.     The  nematocyst  reacts  to  cer- 


I 


DIVISION   OF  LABOR   IN   THE   COELENTERATES  IJil 

tain  chemical  stimuli  that  apparently  cause  a  change  of  osmotic  pres- 
sure within  the  cell,  thus  forcing  out  its  threadlike  portion.  After  a 
nematocyst  is  protruded,  the  cnidoblast  dies  and  is  soon  replaced  by 
another. 


stinSina 


"nerve 
cell 


"muscular 


absorbing 
cell       ^ 

.flagellum 

-sensory 
cell      -^ 


cell       ® 

cxxnthmd 
cell 


The  Endoderm  and  Its  Functions 

By  cutting  a  section  through  the  body  of  a  Hydra  its  similarity  to  a 
two-walled  sac  is  evident.  Between  the  ectoderm  and  the  inner  layer 
of  cells  (endoderni)  a  thin,  structureless  layer  called  the  mesoglea 
forms  as  a  secretion 
from    the    cells    of    the  «^toclerm      j  e«dod^m 

inner  and  outer  layers. 
Mesoglea  forms  much 
of  the  bulk  of  other 
coelenterates  like  the 
jellyfishes.  The  endo- 
derm consists  principally 
of  large  vacuolated  cells 
that  have  flagella  at  the 
free  or  inner  end,  al- 
though they  are  also 
capable  of  developing 
pseudopodia  at  this 
end.  Circular  contrac- 
tile fibers  are  developed 
at  their  basal  end.    Thus 

they  are  endothelial-muscular  cells.  In  the  third  of  the  body  nearest 
the  basal  end,  gland  cells  develop,  which  secrete  digestive  enzymes. 
Nerve  and  sensory  cells  are  also  found  in  the  endoderm. 

For  a  simple  animal,  the  Hydra  seems  to  have  many  kinds  of  cells. 
What  is  the  use  of  so  many  ?  The  answer  is  found  in  the  way  it  gets 
food,  ingests  it,  and  finally  absorbs  it  into  the  body  cells.  By  watch- 
ing a  hydra  in  the  aquarium  it  will  be  seen  that  its  tentacles  are  con- 
stantly moving  as  if  seeking  food.  If  a  tiny  bit  of  raw  beef  is  placed 
within  reach,  the  animal  will  bend  over  and  carry  the  meat  to  the 
mouth,  the  edges  of  which  soon  close  around  it,  forcing  it  inside.  If 
the  piece  is  too  large  to  be  taken  in,  the  Hydra  actually  turns  inside 
out  in  an  attempt,  usually  successful,  to  put  the  meat  inside  the 
gastrovascidar  cavity.  Once  inside  the  cavity,  digestive  enzymes  from 
the  glandular  cells  act  upon  the  food,  gradually  breaking  it  down  into 


Sections  through  the  body  wall  of  hydra  showing 
the  two  layers  of  cells  separated  by  the  striated 
lamella  secreted  by  the  basal  parts  of  the  ecto- 
dermal and  endodermal  cells. 


182      ORGANISMS  ILLUSTRATING  BIOLOGICAL  PRINCIPLES 

smaller  and  smaller  fragments.  Digestion  appears  to  be  aided  by 
the  churning  movements  caused  by  expansion  and  contraction  of 
the  body  wall.  Ultimately  some  of  the  food  is  reduced  to  a  soluble 
state,  and  absorbed  into  the  endodermal  cells.  Meanwhile  some  of 
the  large  vacuolated  cells  put  out  pseudopodia  and  engulf  some  of 
the  undigested  food  particles,  finishing  the  digestive  process  inside 
their  own  cell-bodies.  Thus  Hydra  has  two  types  of  digestion,  one 
intracellular,  like  that  found  in  all  unicellular  animals  and,  there- 
fore, more  primitive ;  the  other,  extracellular,  that  is,  taking  place  in 
the  digestive  cavity.  Most  of  the  food  of  the  Hydra  is  digested  in 
the  latter  way,  the  cells  lining  the  cavity  absorbing  the  digested  food 
before  passing  it  along  to  the  cells  of  the  ectoderm.  According  to 
Hegner,  part  of  the  absorbed  food  is  in  the  form  of  oil  globules  which 
are  passed  over  to  the  cells  of  the  ectoderm  and  stored  there  for  future 
use.  Unusable  or  undigested  material  is  thrown  out  of  the  digestive 
cavity  by  a  sudden  contraction,  there  being  no  other  way  of  eliminat- 
ing such  wastes  except  through  the  surface  of  the  body,  as  in  lower 
forms.  Hydra  like  other  animals  uses  oxygen  to  release  energy. 
Respiration  probably  takes  place  through  the  surface  of  the  entire 
body,  the  cells  receiving  oxygen  and  giving  off  carbon  dioxide  by 
diffusion  through  the  cell  membranes. 

Reactions  to  Stimuli 

Hydra  show  very  definite  reactions  to  certain  stimuli,  most  of 
which  have  to  do  with  obtaining  food.  Hungry  Hydra  are  much 
more  active  than  well-fed  ones,  and  respond  to  various  chemical 
stimuli  besides  reacting  to  mechanical  stimuli,  to  heat,  to  light,  and 
to  electricity,  all  of  which  indicates  the  possession  of  some  sort  of 
simple  nervous  system,  since  the  movements  made  are  more  or  less 
co-ordinated.  If  touched  lightly  on  a  tentacle  with  a  needle,  only 
the  tentacle  contracts,  but  with  increased  stimulation,  the  other 
tentacles  contract,  until  finally,  the  whole  animal  draws  down  into 
a  little  ball.  Its  physiological  condition,  according  to  Jennings,^ 
determines  whether  it  "  shall  creep  upward  to  the  surface  and  toward 
the  light,  or  sink  to  the  bottom  ;  how  it  shall  react  to  chemicals  and 
to  solid  objects ;  whether  it  shall  remain  quiet  in  a  certain  position, 
or  reverse  this  position  and  undertake  a  laborious  tour  of  exploration." 

The  nervous  system  of  Hydra  forms  a  nerve  net.  It  consists  of  a 
concentration  of  primitive  nerve  cells  about  the  base  of  the  hypostome 

1  Jennings,  Behavior  of  the  Lower  Organisms.     Columbia  Univ.  Press,  1915,  p.  231. 


DIVISION   OF   LABOR    IN   THE   COELENTERATES 


183 


and  the  foot.  This  network  of  cells  lies  in  the  ectodermal  layer  of 
the  animal,  and  receives  impulses  from  sensory  cells  as  well  as  trans- 
mitting them  to  the  muscle  fibrils.  The  sensory  cells  of  the  ectoderm 
.vary  in  their  location ;  one  type  occurs  on  the  tentacles,  one  on  the 
hypostome,  and  a  third  on  the  foot 
(base).  Neuro-sensory  cells  which 
are  located  in  the  mid-body  area 
.also  resemble  nerve  cells,  except  that 
they  send  processes  to  muscle  fibrils 
and  so  become  intermediate  between 
those  receiving  stimulation  and  those 
making  the  response.  Some  nerve 
cells  appear  in  the  endodermal  layer 
but  are  not,  so  far  as  can  be  deter- 
mined, connected  with  the  ecto- 
dermal nerve  net. 


Reproduction 

Probably  the  most  important 
function  of  the  interstitial  cells  is 
their  growth  into  sex  cells.  Most 
Hydras  are  hermaphroditic,  that  is, 
have  both  kinds  of  sex  cells  present 
in  the  same  individual,  but  since  the 
sperm  cells  and  ova  ripen  at  different 
times,  fertilization  is  accomplished 
by  sex  cells  from  different  indi- 
viduals. Sperm  cells  are  produced 
by  the  mitotic  division  of  interstitial 
cells,  each  of  which  first  produces  a 
number  of  parent  male  cells,  contain- 
ing the  somatic  number  of  chromosomes. 


The  nerve  net  in  a  young  hydra 
as  seen  with  an  intravitani  methylen- 
blue  stain.  Note  the  ringlike  ar- 
rangement in  hypostome  and  foot. 
What  effect  might  such  an  arrange- 
ment have  on  movement?  (After 
J.  Ilodzi.) 


These  cells  divide  four 
times  and  in  the  process  a  reduction  division  takes  place,  leaving  the 
sperm  cells  with  just  half  as  many  chromosomes  as  the  body  cells. 
A  somewhat  similar  process  takes  place  in  the  formation  of  the  ova. 
One  interstitial  cell  becomes  larger  than  the  others,  rounds  into  a 
sphere,  and  is  surrounded  by  other  interstitial  cells,  which  serve  as  an 
ovary  for  the  growing  egg.  The  latter  continues  to  grow  in  size,  form- 
ing yolk  from  the  surrounding  cells.  Just  before  the  egg  becomes 
mature,  the  process  of  maturation  takes  place  (see  page  429),  dur- 
H.  w.  H.  — 13 


184      ORGANISMS   ILLUSTRATING  BIOLOGICAL  PRINCIPLES 


ing  which  the  number  of  chromosomes  is  reduced  to  half  the  body- 
number.  Spermaries  and  ovaries  can  be  seen  in  the  Hving  Hydra  as 
little  lumps  on  the  ectoderm.     The  spermaries  are  always  found  near 

the  free  end  of  the  body,  the 
ovaries,  when  present,  being 
nearer  the  base.  The  egg 
is  fertilized  while  still  at- 
tached to  the  parent  and 
develops  into  an  embryo 
surrounded  by  a  protective 
chitinous  case,  in  which 
stage  it  sinks  to  the  bottom 
of  the  pond  for  a  resting 
period  before  emerging  as 
an  adult. 

Asexual  development 
also  takes  place.  A  small 
bulging  area,  formed  by 
the  interstitial  cells,  ap- 
pears on  the  side  of  the 
body,  which  more  or  less 
rapidly  grows  into  a  short 
column  surrounded  by 
tentacles,  depending  on  the 
food  supply  available  for 
the  parent  Hydra.     When 

* 

fully  developed  the  bud 
may  separate  from  the 
parent  and  lead  a  separate 
existence.  A  Hydra  fre- 
quently produces  more  than 
one  bud  on  a  single  animal. 


young 
hixd 


sperrr? 
—  cells 
forming 


ec.todJ2.rm 
endocferm- 


jonriing" 


Longitudinal  section  through  the  body  of  a 
Hydra,  showing  both  sexual  and  asexual  repro- 
ductive structures. 


Regeneration 

Although  regeneration  takes  place  in  other  groups  of  animals  it  is 
best  seen  in  the  phylum,  Coelenterata.  The  primitiveness  of  Hydra 
is  shown  by  the  fact  that  it  can  regenerate  or  replace  lost  parts  by 
growth  of  the  body  cells.  It  may  be  cut  lengthwise  or  crosswise,  or 
even  into  small  pieces,  and  the  fragments  will,  under  favorable  con- 
ditions, give  rise  to  complete  individuals. 


DIVISION   OF   LABOR   IN   THE   C0ELENTE1\ATES 


185 


Hydroids 

Hydra  vulgaris  is  a  fresh  water  form,  but  many  more  representatives 
of  the  Coelenterate  group  are  found  in  salt  water,  the  most  famiUar 
being  the  hydroids  found  attached  to  the  piles  of  wharfs  and  other 
submerged  objects.     Among  the  most  common  hydroids  are  members 


hydranth. 


-gbnobVzsca 


^9  ^onacC 


medusa     /;<^^^»v^^",'^^ 
^^^  (^..fertiTe 

asexual  *~-^-Viyctrorhi3a.  -  y 

Stage  / 

^..blastula 


^T-planula 


Life  cycle  of  Obelia  —  showinff  alternation  of  generations.'     Compare  with  text 
pages  18.5-186  for  explanation  of  diagram. 

of  the  genus  Obelia.  These  animals  form  colonies,  in  which  the  indi- 
viduals, called  polyps,  or  zooids,  are  attached  to  each  other  by  means 
of  hollow  stalks,  covered  with  a  chitinous,  cellophanelike  perisarc. 
At  the  tip  of  each  branch,  the  covering  expands  into  a  cuplike  hydro- 
theca,  which  surrounds  the  living  polyp.  As  in  Hydra,  each  individual 
polyp  of  Obelia  is  hollow  and  two  layered,  with  a  circle  of  tentacles 
about  the  raised  hypostome,  in. which  the  mouth  is  located.  The 
tentacles  are  provided  with  nematocysts  that  act  in  the  same  manner 
as  in  the  Hydra.  The  food  cavity,  however,  extends  down  each  stalk- 
like branch  or  individual  and  is  continuous  with  that  of  the  other 
polyps,  thus  forming  a  common  gastrovascular  cavity  in  which  food 


186      ORGANISMS  ILLUSTRATING  BIOLOGICAL  PRINCIPLES 

is  digested.  There  are  also  cells,  as  in  Hydra,  which  perform  intra- 
cellular digestion. 

Obelia  gives  rise  to  another  type  of  polyp  than  the  nutritive  individ- 
ual just  described.  This  is  the  reproductive  polyp,  or  gonangium  that 
grows  out  as  a  bud,  expands  into  a  knoblike  central  axis  known  as 
the  hlastostylc  within  a  chitinous,  closed  vase,  called  the  gonotheca. 
On  the  sides  of  the  blastostyle  budlike  structures,  called  medusa  buds, 
develop.  These  break  off  and  swim  away  as  tiny  bisexual  jellyfish, 
or  medusae,  representing  the  sexual  stage  in  the  life  history.  A  sperm 
cell  from  one  of  these  medusae  fertilizes  an  egg  from  another,  which, 
after  a  developmental  period,  becomes  a  free-swimming  ciliated  larva, 
called  a  planula.  After  a  short  time  the  planula  settles  down  and 
produces  a  new  asexual  colony  of  Obelia.  Other  related  forms  as  the 
jellyfish,  Aurelia,  possess  a  predominating  free-swimming  stage,  while 
the  sessile,  non-sexual  generation  is  reduced. 

This  life  cycle  is  reminiscent  of  a  similar  condition  in  plants,  which 
also  have  an  alternation  of  generations.  During  the  maturation  of  the 
sperm  and  egg  cells,  reduction  division  takes  place  in  which  the  chro- 
mosomes of  the  sex  cells  are  reduced  to  half  the  body  number.  In 
alternation  of  generations  of  plants,  all  the  cells  of  the  gametophytic 
generation  are  haploid,  but  as  in  animals  only  the  mature  sex  cells  are 
haploid,  the  body  cells  having  the  same  number  of  chromosomes  as 
the  body  cells  of  the  sexual  generation.  The  end  result  accomplished 
in  both  plants  and  animals  is  the  same. 

SUGGESTED  READINGS 

Curtis,  W.  C.,  and  Guthrie,  M.  J.,  Textbook  of  General  Zoology,  2nd  ed., 

John  Wiley  &  Sons,  Inc.,  1933,  pp.  278-301. 
Guyer,  M.  F.,  Animal  Biology,  Harper  &  Bros.,  1931,  pp.  197-206. 
Hegner,  R.  W.,  College  Zoology,  The  Macmillan  Co.,  1936.     Ch.  X. 

An  authentic  description  of  Hydra  and  its  activities. 


X 


BEING  A  WORM 

Preview.  A  typical  worm  ;  external  structure  of  the  earthworm  {Lum- 
bricus  terrestris) ;  the  digestive  tract  and  its  functions ;  how  blood  circulates, 
the  blood  and  its  functions  ;  organs  of  excretion  ;  the  muscles  and  their  work  ; 
reactions  to  stimuli ;  the  nervous  system  and  its  functions  ;  the  reproductive 
system  and  reproduction  •  Regeneration  •  Suggested  readings. 

PREVIEW 

Passing  from  the  simple  two-layered  development  of  the  Hydra,  in 
which  division  of  labor  among  the  cells  is  slight,  we  come  to  the  earth- 
worm, another  lowly  animal,  but  one  which  represents  the  big  idea 
of  a  typical  three-layered,  segmented  form. 

In  Hydra,  the  egg  develops  into  an  adult  form  having  two  layers, 
namely,  edodertn  and  endoderm,  but  in  the  earthworm,  a  third 
layer,  the  mesoderm,  appears,  w^iich  is  characteristic  of  all  the  higher 
animals.  These  three  germ  layers  are  of  great  significance  in  the 
study  of  animals,  for  all  of  the  complex  tissues  of  the  body  are  derived 
from  them. 

Another  reason  why  the  earthworm  is  chosen  for  study  is  because  it 
represents  a  very  simple  type  of  segmented  or  metameric  animal  of 
which  a  great  variety  is  found  not  only  among  worms  but  also  among 
insects  and  crustaceans.  Judging  by  the  insects,  segmented  animals 
are  the  most  abundant  and  successful  of  all  animals,  since  they  out- 
number all  other  species.  The  pages  that  follow  will  concern  them- 
selves chiefly  with  the  "hows  and  whys"  of  the  activity  of  the 
common  ''night  crawler,"  some  of  which  are:  How  far  has  division 
of  labor  progressed  ?  What  organ  systems  are  well  developed  ?  How 
does  co-ordinated  movement  take  place,  and  how  do  worms  become 
aware  of  their  surroundings  ? 

A  Typical  Worm 
External  Structure  of  the  Earthworm  (Lumbricus  terrestris) 

The  body  of  the  earthworm  is  divided  into  segmented  parts,  or 
metameres,  which  in  adult  worms  may  number  over  one  hundred. 
The  body  tapers  bluntly  at  each  end,  the  anterior  end  being  easily 

187 


188      ORGANISMS   ILLUSTRATING  BIOLOGICAL   PRINCIPLES 

distinguished  by  the  rounded  mouth  which  is  just  ventral  to  or  under 
a  small  protuberance,  the  'prostomium,  while  the  anus,  or  posterior  end 
of  the  digestive  tract,  is  a  tiny  slit  in  the  last  segment.  The  posterior 
end  is  also  flattened,  and  between  segments  32  to  37,  not  counting 
the  prostomium  enclosing  the  mouth  as  the  first,  there  is  found  a 
swollen  region,  called  the  clitellum,  important  in  reproduction. 

The  upper  or  dorsal  side  may  be  distinguished  by  its  darker  color, 
while  the  ventral  side  is  slightly  flattened  and  contains  four  double 
rows  of  tiny  projections  called  setae,  which  give  the  worm  a  grip  on 
the  ground  when  in  locomotion.     The  dorsal  side  is  devoid  of  any 


The  common  earthworm,  Lunibricus  ierresiris. 


Wright  Pierce 

Note  the  swollen  area,  or  clitellum. 


openings  except  some  very  minute  dorsal  pores  that  communicate 
with  the  body  cavity,  or  coelom,  but  the  ventral  side  has  several 
paired  openings,  difficult  to  find,  which  lead  to  the  reproductive  and 
excretory  organs.  The  surface  of  the  body  is  covered  with  a  delicate 
iridescent  cuticle,  secreted  by  the  living  epithelial  cells  of  the  skin,  but 
which  is  itself  dead.  Its  iridescence  is  caused  by  the  presence  of 
numerous  grooves  (striae),  and  its  surface  is  pierced  with  small  holes, 
which  are  openings  for  the  mucous  gland  cells  of  the  skin.  The  coelom 
or  body  cavity  is  cut  up  into  small  compartments  by  partition  walls, 
or  septa,  that  are  absent  or  incomplete  in  the  extreme  anterior  region, 
between  the  18th  and  19th  segments,  and  in  the  region  posterior  to 
the  reproductive  organs.     The  coelom  in  the  living  worm  is  filled 


BEING  A  WORM 


189 


Septum 
Vnuscle,--- 


hear-ts 

also  3,4-.S 


seroinal  — 
receptacle 


with  fluid  which  passes  from  one  segment  to  another  through  single 
perforations  in  each  of  the  septa.  The  fluid  contains  ameboid  cells, 
that  probably  serve  as  scavengers,  and  it  acts  as  blood,  bathing  and 
nourishing  the  tissues  and  carrying  away  wastes. 

The  Digestive  Tract  and  Its  Functions 

The  food  of  earthworms,  bits  of  animal  or  vegetable  matter  mixed 
with  soil,  is  taken  into  the  mouth  by  means  of  suction.     A  muscular 
pharynx,  previously  moistened  by  the  fluid  poured  out  from  small 
glands  in  its  wall,  is  able  to 
pull    the    material    into    the 
esophagus,  a  thin-walled  part 
of  the  tube  which  extends  from 
the  6th  to  the  15th  segment, 
beside  whose  walls,  between 
segments  10  to  12,  there  are 
embedded  three  pairs  of  whit- 
ish structures,  the  calciferous 
glands.     These  glands  produce 
a  limy  secretion  supposed  to 
neutralize  the  food  materials. 
The   esophagus   leads  into  a 
thin-walled    crop,    occupying 
the  15th  and  16th  segments, 
which    opens    into    a    thick- 
walled,  muscular  gizzard   ex- 
tending over  segments  17  and 
18.     The  latter  organ  has  an 
internal  chitinous  wall,  and  is 
probably  used  to  macerate  bits 
of  undigested  food  by  means 
of  muscular  contraction.     The 
remainder  of  the  food  tube,  ex- 
tending from  the  19th  segment 
to  the  anus,  is  called  the  intestine.     Its  inner  surface  is  increased  by  a 
fold  on  the  dorsal  side  (typhlosoJc) ,  while  surrounding  it  there  is  a  layer 
of  yellow-brown  tissue  cMorogogen  cells,  which  are  thought  to  aid  in 
excretion  and  possibly  digestion  of  food.     The  wall  of  the  intestine 
contains  gland  cells  that  secrete  at  least  three  kinds  of  enzymes,  which 
digest  starches,  fats,  and  proteins.     The  digested  food  is  absorbed 


Seminal 
vesicle.—. 


cConsal 
vessel 


.pViarxnx 
.<?5c>p'hag"tc5 


.Caldfe.r<3US 
glancfs 


.crop 
intestine 


t/pbloSole 
rzerve  CorcC 


ventral 
vessel 

three  ofhen-.-r^-l 
vessels  *■ 

The  earthworm  {Lnmhricns  ierrestris) 
opened  from  dorsal  side  to  show  internal 
structure.     (After  Sedgwick  and  \\  ilson.) 


190      ORGANISMS   ILLUSTRATING  BIOLOGICAL   PRINCIPLES 


through  the  walls  of  the  intestine,  most  of  it  passing  into  the  blood 
and  directly  into  the  coelomic  fluid,  where  it  may  continue  to  the 
muscular  wall  outside  the  coelom.  Unusable  material,  mostly  earth, 
is  passed  off  by  muscular  contraction  through  the  anus,  and  may  often 
be  seen  on  lawns  as  little  piles  of  "castings." 

How  Blood  Circulates 

Since  in  the  earthworms  there  is  a  very  different  arrangement  than 
in  Hydra,  where  food  is  directly  available  to  all  the  cells,  we  would 
expect  to  find  some  means  of  distributing  it  to  the  tissues  where  it 

may  be  used.  This  is  accomplished  by 
means  of  a  closed  system  of  blood  vessels. 
Some  idea  of  the  circulation  may  be  derived 
by  a  study  of  the  accompanying  diagrams. 
Five  large  blood  vessels  run  lengthwise 
through  the  body,  one  dorsal  vessel,  close 
to  the  food  tube,  into  the  walls  of  which 
it  sends  two  pairs  of  lateral  vessels  in  each 
segment ;  another,  the  ventral  vessel,  runs 
just  ventral  to  the  digestive  tract  and  also 
sends  lateral  branches  into  its  wall.  There 
are  also  three  others,  the  paired  lateral 
neural  vessels  and  the  suhneural  vessel, 
which  run  longitudinally,  the  latter  directly 
under  the  nerve  cord,  and  two  other  smaller 
ones  lying  parallel  one  on  each  side  and 
above  the  nerve  cord.  Five  "hearts,"  so 
called  because  of  their  frequent  contrac- 
tions, encircle  the  esophagus  in  the  region 
of  the  7th  to  the  11th  segments,  connecting 
the  dorsal  with  the  ventral  vessel.  Blood 
passes  into  the  dorsal  vessel  especially 
from  a  long  typhlosolar  vessel  which  helps 
drain  absorbed  foods  from  the  intestinal 
walls,  flowing  forward  until  it  reaches  the 
"  hearts."  Its  forward  movement  is  caused 
by  slow,  regular  contractions  of  the  dorsal 
blood  vessel.  The  blood  passes  posteriorly  through  the  "hearts" 
and  then  flows  into  the  ventral  blood  vessel.  Here  it  passes  poste- 
riorly,   although   some   of   it   moves    from   the   hearts   toward    the 


nerves 

buccal  cavity 
Esuprcicsophatfeol , 
tiixurnssoiohigeal 
Sub  esopho^ol 


.„psai, 
vessel 

lateral 
vessel 


■esof>hogus 


..ventral 

VGSjel 


-  -  (trop 


.nsrve  ccnet 
■with.  IntM-al 
neurai  vesssis 

intastina. 


The  circulatory  system  of  the 
earthworm. 


BEING  A  WORM 


191 


anterior  end  of  the  body.  Blood  also  passes  tliroush  two  intestino- 
integumentary  vessels  which  pass  off  at  the  10th  segment  to  supply 
the  walls  of  the  esophagus  and  the  skin,  and  to  nephridia  of  that 
region.     Parietal  vessels  connect  the  dorsal  and  subneural  vessels, 


cross    Section  of  typyosolar  vessel 

/ 


'>_Jat^rccl-y2eu:ral  vessel 


V  nerve 
CorcL 


The  '"hearts"  of  the  earthworm.     How  do  they  function  in  circulation.^ 

that  branch  from  the  ventral  vessel  to  supply  the  body  muscle  walls 
and  nephridia.  Blood  also  passes  from  the  ventral  vessel  to  the  body 
walls,  and  to  nephridia,  and  returns  to  flow,  after  passing  through 
capillaries,  into  the  lateral  neural  trunks.  In  the  subneural  vessel, 
the  blood  flows  posteriorly  and  thence  up  by  way  of  the  parietal 
vessels  into  the  dorsal  vessel.  Both  dorsal  and  ventral  vessels  supply 
the  anterior  part  of  the  worm. 


The  Blood  and  Its  Functions 

The  blood  of  the  earthworm  consists  of  a  liquid  plasma,  carrying 
colorless  corpuscles  which  are  flattened  spindle-shaped  bodies.  The 
red  color  is  due  to  hemoglobin,  the  same  oxygen-carrying  substance 
found  in  the  blood  of  man.  But  in  the  earthworm  the  plasma  is 
colored  rather  than  the  corpuscles.  The  exchange  of  food  and 
oxygen,  which  the  blood  picks  up  in  the  intestine  and  body  walls, 
respectively,  occurs  in  the  tiny  lymph  spaces  around  the  individual 
cells.     Respiration  takes  place  through  the  moist  outer  membrane 


192       ORGANISMS   ILLUSTRATING  BIOLOGICAL   PRINCIPLES 

of  the  skin,  where  the  oxygen  is  picked  up  and  combined  with  the 
hemoglobin,  to  be  later  released  in  the  cells  of  the  body  where  work 
is  done.  Carbon  dioxide  and  wastes  are  here  taken  up  by  the  blood 
and  carried  back  to  the  skin  and  to  the  nephridia  or  excretory  organs. 
One  can  easily  demonstrate  the  network  of  tiny  capillaries  in  the  skin 
where  this  exchange  takes  place. 


Organs  of  Excretion 

The  paired  nephridia  are  essentially  coiled  tubular  organs,  made 
up  of  a  ciliated  funnel  or  nephrostome  that  opens  into  the  coelom, 

a  thin  ciliated  glandular 


^-^epttcin 


like  region 


tube,  that  loops  on  itself 
about  three  times,  and  a 
pore,  the  ncphridiopore, 
through  which  the  excre- 
tory products  pass  to  the 
exterior.  Some  excretory 
materials  are  probably 
taken  directly  from  the 
coelomic  fluid  by  means 
of  the  currents  caused 
by  the  cilia,  while  other 
wastes  may  be  taken 
directly  from  the  blood- 
capillaries  which  cover 
the  surface  of  the  glan- 
dular tubules.  One  characteristic  feature  of  the  nephridium  is  that  it 
always  passes  through  the  septum  separating  two  segments. 


A  nephridium  of  an  earthworm.  Trace  the 
passage  of  fluid  from  the  coelom  to  the  exterior  of 
the  worm.  Note  the  ciliated  surface  of  the  neph- 
rostome.   What  is  its  function  .!^     (After  Wolcott.) 


The  Muscles  and  Their  Work 

Movement  is  brought  about  by  muscular  contraction.  As  an 
earthworm  crawls,  a  wave  of  contraction  from  the  posterior  toward 
the  anterior  appears  to  move  up  the  body  of  the  worm.  A  careful 
examination  shows  that  movement  is  brought  about  by  the  contrac- 
tion and  relaxation  of  two  opposing  groups  of  muscle  fibers  and  by 
the  movement  of  the  rows  of  setae  on  the  ventral  surface.  The 
muscles  are  arranged  in  two  layers  just  under  the  skin,  an  outer 
circular  layer  running  around  the  body  and  an  inner  longitudinal 
layer.     When  the  worm  lengthens,  the  longitudinal  muscles  relax 


BEING  A  WORM 


193 


and  the  circular  muscles  contract,  while  a  shortening  of  the  worm 
results  from  a  contraction  of  the  longitudinal  muscles  and  a  relaxing 
of  the  circular  muscles.  Each  stiff  seta  is  placed  in  a  little  sac, 
from  which  it  extends  out  beyond  the  surface  of  the  body.  Inside 
the  sac,  attached  to  the  seta  and  to  the  outer  body  wall,  are  two  pairs 


endocterra  ^ 
•muscle.-, 

peritoneum  ^^^ 
TOphridium 


^Cuticle    ectoderm       Circtxlar 


^."peritoneum. 


muscle 

^nephricCiTopore 
<-Seta 


/         verztro-l  vessel 
lataml  vessel 


'wentral  rjerve-  cord. 
subnsLcral   vess-©! 


Cross  section  through  earthworm.  Compare  this  with  cross  section  of  Hydra. 
What  advances  in  complexity  of  structure  flo  you  find:'  In  the  earthworm  the 
most  noticeable  difl'erence  is  seen  in  the  coelom.  which  is  formed  by  a  sphtting  of 
the  mesodermal  bands  in  the  embryo  (seen  on  page  197).  Note  that  the  coeiom 
is  completely  lined  by  a  delicate  membrane,  the  peritoneum.  Notice  also  the 
longitudinal  fold  or  typhlosole  which  gives  more  surface  to  the  inner  wall  of  the 
intestine.  What  is  its  function  .^  In  the  diagram,  the  funnels  of  the  nephridia 
are  not  shown.     Explain  why  this  is  so. 

of  muscles  by  means  of  which  the  seta  can  be  directed  forwards  or 
backwards,  depending  on  the  direction  the  worm  is  traveling.  When 
the  worm  is  moving  forward,  the  anterior  end  is  extended,  the  setae, 
that  are  pointed  backward,  are  set  into  the  ground,  serving  as  an- 
chors, while  the  posterior  end  of  the  worm  is  pulled  forward  by  means 
of  the  contraction  of  the  longitudinal  muscles. 


194      ORGANISMS   ILLUSTRATING   BIOLOGICAL  PRINCIPLES 

Reactions  to  Stimuli 

Earthworms  live  in  soil  and  make  burrows  which  extend  from  a  few 
inches  to  several  feet  under  ground.  They  are  nocturnal  and  lie  in 
their  burrows  not  far  from  the  surface  during  the  day  time,  coming 
out  at  night  to  forage  for  food.  In  winter,  they  go  below  the  frost 
line,  remaining  there  inactive.  In  hot  and  dry  weather,  they  go  as 
far  down  as  possible  into  the  earth,  while  a  heavy  rain  will  bring  them 
out  of  their  burrows  in  great  numbers.  Earthworms  react  positively 
to  mechanical  stimuli.  A  vibration  on  the  earth  will  send  them  down 
into  their  burrows.  They  are  positively  attracted  to  surfaces  of  solid 
objects,  as  can  be  seen  if  worms  are  placed  on  moist  blotting  paper  in 
a  covered  pan.  They  will  soon  be  found  lying  along  the  edges  of  the 
pan,  where  two  surfaces  are  in  contact  with  the  body.  This  response 
to  contact  apparently  keeps  them  quite  constantly  in  their  burrows. 
They  react  positively  to  certain  chemical  substances,  like  foods,  and 
move  away  from  others.  A  match  that  has  been  dipped  in  ammonia 
and  placed  near  the  anterior  end  of  an  earthworm  will  demonstrate 
this  reaction.  They  respond  positively  to  moderate  moisture,  which 
is  needed  for  respiration  through  the  body  covering,  and  to  different 
intensities  of  light,  by  withdrawing  from  bright  areas  and  moving 
toward  weak  illuminations.  Like  Hydra,  however,  reactions  to 
stimuli  depend  largely  on  the  "physiological  condition"  of  the  worm, 
that  is,  upon  internal  rather  than  upon  external  factors. 

The  Nervous  System  and  Its  Functions 

The  earthworm  has  a  simple  type  of  central  nervous  system  con- 
sisting of  a  ventral  nerve  cord,  with  thickenings,  called  ganglia,  in 
each  segment,  a  dorsal  "brain"  or  supraesophageal  ganglion,  made  up 
of  two  ganglia,  and  a  "ring"  of  nervous  tissue,  called  the  circum- 
esophageal  connectives,  which  extends  around  the  esophagus,  connect- 
ing the  "brain"  with  the  ventral  nerve  cord.  Lateral  nerves,  which 
leave  the  "brain"  and  cord  to  end  in  muscles,  skin,  and  other  organs, 
form  a  peripheral  nervous  system.  The  worm  does  not  have  visible 
organs  of  sensation,  but  the  skin,  especially  at  the  anterior  and 
posterior  ends,  is  dotted  with  groups  of  tiny  sensory  cells.  Some  of 
these  are  sensitive  to  light,  and  still  others  probably  to  odor.  Stimuli 
received  by  these  cells  are  transmitted  to  the  central  nervous  system 
by  means  of  nerve  fibers.  Those  which  lead  from  the  sensory  cells 
to  the  central  nervous  system  are  known  as  afferent  fibers,  while  out- 


BEING  A   WOIIM 


195 


going  fibers  which  originate  in  nerve  cells  within  the  cord  are  known 
as  efferent  or  motor  fibers,  since  they  end  in  muscle  cells  and  stimulate 
them  to  contract,  thus  causing  motion.  The  unit  over  which  these 
impulses  travel  is  called  a  neuron,  which  is  the  term  given  to  the  nerve 
cell  and  its  prolongations.     (See  page  340.)    In  the  earthworm  sensory 


anterior- 


SerjSory  c<=ll5{r<2cepto«) 
epidermis.': 


■muscle  cells 
;e|^fecton$) 


,^—Septu.rrL     \j 


•postsrior 

The  nerve  cord  of  the  earthworm  showing  neurons  concerned  in  the  reflex 
arc.  Explain  how  adjustment  to  an  unfavorable  condition  might  be  affected. 
How  might  movement  in  another  segment  of  the  worm  be  co-ordinated  with  the 
one  shown  in  the  diagram.''      (After  Curtis  and  Guthrie.) 

impulses  are  passed  longitudinally,  both  anteriorly  and  posteriorly, 
by  means  of  the  peripheral  nervous  system,  and  these  impulses  are 
modified  by  means  of  adjustor  neurons  in  the  central  nervous  system. 
This  accounts  for  the  co-ordination  between  segments  as  the  worm 
crawls  toward  a  desirable  object  or  suddenly  withdraws  from  a  harm- 
ful situation. 

The  Reproductive  System  and  Reproduction 

Earthworms  have  both  testes  and  ovaries  in  the  same  animal, 
and  are  therefore  hermaphroditic,  but  they  are  not  capable  of  self- 
fertilization.     Two  pairs  of  testes  lie  attached  to  the  anterior  walls  of 


196      ORGANISMS  ILLUSTRATING  RIOLOGICAL  PRINCIPLES 


segments  10  and  11,  and  are  enclosed  by  the  ventral  unpouched 
portion  of  two  of  the  three  seminal  vesicles.  Dorsally  the  three  pairs 
of  large  pouches  of  the  seminal  vesicles  in  segments  9,  11,  and  12  are 
light-colored  structures  easily  seen  in  a  dissection.  Immature  sperm 
cells  are  passed  from  the  testes  to  complete  their  development  in  the 
seminal  vesicles.  Two  pairs  of  vasa  efferentia  in  somites  10  and  11 
fuse  to  form  the  paired  vas  deferens  that  carry  the  sperm  to  the 
exterior  through  the  male  openings  on  segment  15.  A  pair  of  tiny 
ovaries  are  attached  to  the  anterior  septum  of  segment  13,  the  eggs 


i^..-^ Semirzal  rsceptocle 

--.'tes'tis 
.--fur\T\©l 


^^..)... seminal  vesicle 

ovctr^ 

ovicCuct 
spsrm.  duct^ 


Reproductive  organs  of  the  earthworm.  The  seminal  vesicles  are  cut  away  on 
one  side  to  show  the  funnels  of  the  sperm  ducts.  Read  your  text  carefully  and 
explain  how  reproduction  takes  place. 

passing  from  this  into  the  oviducts  which  open  to  the  surface  on  seg- 
ment 14.  Fertilization  of  the  eggs  is  accomplished  by  the  process  of 
copulation  in  which  two  worms,  placing  themselves  in  opposite 
directions,  become  "glued"  together  on  their  ventral  surfaces  by 
means  of  mucus  secreted  from  the  glands  of  the  clitellum  region. 
While  they  are  thus  placed  a  mutual  transfer  of  sperm  cells  from  the 
seminal  vesicles  of  one  worm  to  the  seminal  receptacles  of  the  other 
takes  place,  rhythmic  muscular  contractions  of  the  body  helping  to 
force  the  sperms  along.  Then  the  worms  separate.  Later,  when  the 
eggs  are  to  be  laid,  a  cocoonlike  band  of  mucus  is  formed  by  the  clitel-. 
lum,  which  is  forced  forward  by  movements  of  the  worm,  and  as  it 
passes  by  the  oviducal  pores,  receives  the  ripe  eggs.  When  it  passes 
over  the  opening  of  the  seminal  receptacles  on  the  ventral  surface  of 


BEING  A   WORM 


191 


sperm 


...mesooCarm. 


mesoderm 


onus 
V. 


segments  9  and  10,  it  receives  sperm  cells  from  the  other  worm  that 
have  been  stored  there.  The  girdle  is  passed  down  over  the  anterior 
end  of  the  worm,  slipped  off,  forming  a  closed  case  which  contains  the 
eggs,  sperms,  and  a  nutritive  fluid.  These  capsules  may  be  found  in 
late  spring  under  stones, 
boards,  logs,  or  in  manure 
heaps.  After  fertilization, 
the  egg  of  the  earthworm 
divides  first  into  two,  then 
four,  then  eight  cells,  and 
so  on,  continuing  until  a 
hollow  ball  of  cells,  called  a 
hlastula,  is  formed.  These 
cells  are  not  all  the  same 
size,  larger  cells  appearing 
on  the  lower  pole  of  the 
sphere,  which  begins  to 
flatten  and  show  a  depres- 
sion, forming  eventually 
a  hollow  cuplike  affair, 
called  the  gastrula.  This 
process  known  as  gastrula- 
tion  places  the  larger  cells 
of  the  lower  pole  on  the  in- 
side of  the  cup  where  they 
become  the  endoderm, 
leaving  the  outer  cells  of 
the  sphere  to  form  the 
ectoderm.  Meantime  a 
third  layer  of  cells  which 
lies  between  the  other  two 
layers  buds  off  and  be- 
comes the  mesoderm.  This  latter  layer  gives  rise  to  the  musculature, 
blood  vessels,  and  most  of  the  excretory  and  reproductive  tissues  ;  the 
endoderm  forms  the  food  tube  and  much  of  the  glandular  material  con- 
nected with  it ;  the  ectoderm  gives  rise  to  the  epiderms,  the  nervous 
system  and  sense  organs,  and  the  outer  portions  of  the  nephridia,  repro- 
ductive ducts,  and  digestive  tracts.  The  young  worms  remain  in  th(^ 
egg  case  until  they  are  about  an  inch  in  length.  When  first  hatched 
they  have  no  clitellum,  since  this  organ  appears  only  in  mature  worms. 


gostrola 


Stages  in  development  of  earthworm.  Fig- 
ures II-V.  Segmentation  of  egg  and  formation 
of  blastula.  Figures  VI-VIII.  Sections,  show- 
ing formation  of  mesoderm  as  a  band  of  cells. 
IX.  Late  stage  of  gastrula,  showing  coelomic 
spaces  in  mesoderm  bands.  X.  Longitudinal 
section  of  young  worm  showing  food  tube,  mouth 
and  anus.     (After  Sedgwick  and  Wilson.) 


198      ORGANISMS  ILLUSTRATING  BIOLOGICAL  PRINCIPLES 

Regeneration 

Earthworms,  like  other  members  of  the  lower  phyla  of  the  animal 
kingdom,  have  the  ability,  under  certain  conditions,  to  grow  new 
parts.  Experiments  have  been  made  by  Hazen,  Morgan,  and  others 
that  show  if  a  sufficient  number  of  segments  are  present  a  worm  may 
regenerate  a  new  posterior  end,  or  even  a  new  anterior  end.  Earth- 
worms have  even  been  successfully  grafted  end  to  end. 

SUGGESTED  READINGS 

Curtis,  W.  C,  and  Guthrie,  M.  T.,  Textbook  of  General  Zoology,  2nd  ed.,  John 

Wiley  &  Sons,  Inc.,  1933. 

Excellent  chapter  on  the  Annulata,  pp.  350  to  375. 
Darwin,  Ch.,  Formation  of  Vegetable  Mould,  D.  Appleton  &  Co. 

An  easily  read  classic  which  ought  to  be  known  to  every  student  of 

biology. 
Hegner,  R.  W.,  College  Zoology,  4th  ed.,  The  Macmillan  Co.,  1936. 

Chapter  XV  is  a  well-written  and  authentic  chapter  on  the  Annulata. 


XI 


THE  POPULAR   INSECT  PLAN 

Preview.  The  insect  body  plan;  the  head  and  its  appendages;  the 
thorax  and  its  appendages ;  honey  manufacture ;  digestion ;  circulation, 
respiration,  and  excretion ;  the  nervous  system  •  Reproduction  and  life  his- 
tory •  The  life  in  the  hive  •  Suggested  readings. 

PREVIEW 

It  would  seem  right  in  a  text  on  biology  that  a  representative  of  the 
largest  and  most  successful  group  of  animals  should  be  described  and 
that  more  than  a  passing  glance  be  given  to  this  enormous  group, 
which  contains  far  more  than  half  of  all  living  animals.  We  are 
always  meeting  insects,  because  they  are  so  plentiful  rather  than 
from  choice.  They  annoy  us  when  we  are  in  the  woods,  they  bite 
us  when  we  are  lolling  on  the  beach  at  the  seashore,  they  get  into  our 
foods  and  render  them  unfit  for  use,  or  they  eat  our  stored  clothes. 
Worse  than  this,  they  defoliate  trees,  and  sometimes  destroy  forests, 
and  take  their  tithe  of  the  nation's  food  crops.  A  good  many  have 
been  implicated  in  the  transfer  of  disease  and  some  have  actually 
rendered  regions  uninhabitable  by  man. 

Biologists  have  a  good  reason  for  a  study  of  representatives  of  the 
great  phylum,  Arthropoda,  because  the  arthropod  plan  of  structure  is 
the  one  employed  by  the  majority  of  the  species  of  the  animal  king- 
dom. In  its  simplest  form,  it  represents  an  organism  made  up  of 
segments,  each  body  segment  bearing  a  pair  of  jointed  appendages. 
The  head  always  bears  at  least  one  pair  of  jointed  antennae  or  feelers, 
jointed  mouth  parts,  and  usually  compound  eyes.  The  body  is  pro- 
tected by  an  exoskeleton  composed  of  chitin  secreted  by  the  cells 
beneath.  A  digestive  tract  passes  straight  through  the  body  and 
there  is  a  nervous  system  such  as  we  saw  in  the  Annelids,  consisting 
of  a  ventral  nerve  cord,  a  dorsal  "brain,"  and  a  nerve  ring  about  the 
esophagus.  Dorsal  to  the  food  tube  is  an  elongated  heart,  there 
being  no  closed  system  of  blood  vessels.  Such  a  simple  arthropod 
would  be  difficult  to  find  for  laboratory  purposes,  so  we  have  to  use 
other  more  specialized  forms. 

From  the  strictly  biological  point  of  view  there  is  another  reason 
for  the  study  of  an  insect.  It  offers  an  example  of  a  segmented  ani- 
H.  w.  H.  — 14  199 


200       ORGANISMS    ILLUSTRATING   BIOLOGICAL   PRINCIPLES 

mal  that  has  gone  in  for  specialization  in  a  big  way.  The  insects 
are  a  subdivision  of  the  Arthropods,  animals  that  have  jointed  legs 
and  jointed  bodies,  and  as  such  show  definite  repetition  of  similar 
parts,  or  metamerism,  a  phenomenon  previously  noted  in  the  Annelids. 
As  a  group  they  have  become  differentiated  to  such  an  extent  from 
their  not  so  distant  relatives  that,  like  the  man  on  the  flying  tra- 
peze, they  ''fly  through  the  air  with  the  greatest  of  ease."  In  no 
other  group  except  the  birds  has  this  ability  been  so  exploited.  In 
addition  some  forms,  such  as  the  bees,  ants,  and  wasps,  show  an 
astonishingly  complex  social  life. 

As  a  successful  group  insects  show  numerous  adaptations,  not  only 
in  structure  but  in  life  habits.  They  are  not  only  active  but  often 
so  inconspicuous  as  to  pass  unnoticed  by  their  enemies.  Insects  are 
characterized  by  a  rapidly  growing  larval  period  associated  with  an 
abundance  of  food.  The  protected  pupa  is  characterized  by  internal 
changes  fitting  the  organism  for  the  active  reproductive  life  of  an 
adult.     They  deserve  our  careful  consideration  as  a  type  for  study. 


The  Insect  Body  Plan 

Adult  insects  are  readily  identified  because  the  body  is  made  up  of 
three  parts,  an  anterior  head,  a  mid  region  or  thorax,  and  a  posterior 
region,  the  abdomen.     The  body  may  be  further  subdivided  into 


Wright  Pierce 

The  large  vagrant  grasshopper  {Schistocerca  vaga  Scudder)  normal  size.     A  typical 
insect.    Give  all  the  distinguishing  marks  of  an  insect  as  shown  in  this  photograph. 


THE  POPULAR  INSECT  PLAN 


201 


segments  and  has  three  pairs  of  jointed  thoracic  legs.  These  charac- 
ters distinguish  any  insect.  If  you  will  refer  to  the  "Roll  Call" 
you  will  see  that  the  various  orders  of  insects  are  distinguished  by  still 
other  characters,  such  as  the  presence  or  absence  of  different  kinds  of 
wings,  or  differences  in  the  structure  of  the  mouth  parts,  which  may  be 
modified  for  various  purposes.  All  insects  breathe  through  tracheal 
tubes  and   have   a  body 

.^./Vclypeus 


upper  Up 


mandibla 

rTncuciUotry   pcdptxs 


hypoph<xrynyc 


palpife 


COL-rdc). 


maxiilcc 


^     TTjcxxilla 

-Subment-um. 


lotbium 


Mouth  parts  of  the  locust. 


armor  of  chitin,  a  protein 
substance  something  like 
cow's  horn. 

Many  zoologists  like  to 
use  a  locust  or  "grass- 
hopper" as  a  laboratory 
type  for  study.  This  is 
because  the  body  parts 
are  easy  to  see  and  be- 
cause it  is  a  form  ha^'ing 
relatively  simple  mouth 
parts.  It  is  provided 
with  two  pairs  of  jaws,  a 
forklike  pair,  the  7?iax- 
illae,  and  a  pair  of  hard 
toothed  jaws,  the  mandi- 
bles.  These  parts  when 
not  in  use  are  covered  by  two  flaps,  the  upper  and  lower  lips  (labrum 
and  labiujn).  Such  mouth  parts  are  found  in  the  bee,  although  some- 
what modified  from  the  more  primitive  type  seen  here.  Moreover, 
the  locust  is  a  more  typical  insect  because  it  has  three  distinct  thoracic 
segments,  known  as  the  pro-,  meso-,  and  metathorax,  and  it  also  has 
a  more  nearly  typical  number  of  abdominal  segments,  which  in  most 
insects  is  ten  or  eleven.  The  bee,  although  not  such  a  typical  insect, 
shows  so  many  adaptations,  and  in  addition  has  so  complex  a  social 
life,  that  it  is  selected  as  a  representative  of  the  class  Insecta. 

The  honey  bee  {Apis  mellifica)  forms  colonies  which  include  three 
kinds  of  individuals ;  first,  workers,  bees  with  undeveloped  female  sex 
organs,  which  form  by  far  the  largest  number  in  the  colony ;  second, 
drones,  or  males ;  and  third,  a  queen,  or  fertile  female.  An  average- 
sized  colony  of  bees  may  contain  from  35,000  to  50,000  workers, 
several  hundred  drones,  and  one  adult  queen.  In  the  following 
description  the  worker  bee  is  used,  unless  otherwise  specified. 


202       ORGANISMS  ILLUSTRATING  BIOLOGICAL  PRINCIPLES 


A  study  of  the  accompanying  illustration  indicates  that  the  bee, 
like  other  insects,  has  three  body  divisions  —  head,  thorax,  and  abdo- 
men, but  instead  of  the  usual  three  thoracic  parts,  there  are  four, 
since  one  segment  from  the  abdomen  becomes  fused  with  the  thorax, 


ocellus  or- 

Simple^  eye 
Compound  eye. 

ctntcnna. 


•momcCible 

Tnaxilla 
ccndi  othei^ 
mouth  ports-- 


Sting 


Worker  bee,  lateral  view,  hairs  removed,  showing  parts  of  body  and  appendages 

on  left  side.     (After  Snodgrass.) 

leaving  only  six  visible  segments  in  the  abdomen.  The  head  bears 
a  pair  of  jointed  antennae,  or  "feelers,"  large  compound  eyes,  and 
mouth  parts  much  modified  from  the  plan  shown  by  the  locust. 
Three  pairs  of  jointed  legs  and  two  pairs  of  membranous  wings  are 
attached  to  the  thorax,  the  wings  growing  out  of  the  meso-  and  meta- 
thorax.  At  the  posterior  end  of  the  abdomen  an  ovipositor  in  the 
female  is  modified  in  the  worker  into  a  sting,  which  is  withdrawn 
inside  its  sheath  within  the  body  when  not  in  use.  The  body  is 
covered  with  a  horny  three-layered  coat  made  up  of  an  outer  chitiii- 
ous  cuticula  that  covers  the  entire  body  except  at  the  joints,  where 
it  becomes  membranous,  thus  allowing  movement ;  a  middle  layer 
of  cells  called  the  hypodermis ;  and  an  inner  delicate  basement  mem- 
brane. 

Protruding  from  the  chitinous  covering  are  many  hairs  and  bristles, 
outgrowths  formed  by  the  hypodermis,  in  which  there  are  several 
kinds  of  cells,  some  forming  the  chitinous  coat,  others  the  hairs,  and 
still  others  gland  or  sensory  cells.  In  some  cases  the  hairs  are  hollow 
and  contain  sensory  nerve  endings.  We  must  picture  these  animals 
covered  with  heavy  armor,  through  which  sensation  is  impossible 


THE   POPULAR    INSECT   PLAN 


2o:{ 


except  where  sensory  nerve  endings  penetrate  the  armored  surface, 
ending  in  various  sense  organs  such  as  compound  eyes,  antennae, 
and  sensory  hairs. 


.epicCar  misl  cuticulo. 


5P 
cte 


ermis 


structure-  of    bocCj^  woJl 
(..  hair 

chitin.. 


.Cell  cf  h/podermis 
basement  membrane 


msmbrone  •/*^^^ 


•chit  11 


cuticula 

g^^hypodermis      «o  Chitin  in  fSlcts  or  at  joints 

some,  celts  form  bains 

The  body  wall  and  its  modifications.  The  epidermal  portion  of  the  body  wall 
is  composed  of  a  horny  substance  called  chitin,  the  dermal  portion  having  a  some- 
what different  chemical  nature,  like  cellulose.  In  places  where  movement  is 
necessary  the  chitin  is  replaced  by  a  flexible  membrane.  Several  types  of  hairs 
are  found,  some  solid,  others  hollow,  all  outgrowths  of  the  exoskeleton.  (After 
Snodgrass. ) 

The  Head  and  Its  Appendages 

According  to  the  observations  of  embryologists  the  head  of  the 
bee  is  made  up  of  six  segments  that  are  fused  together  in  the  adult. 
This  statement  is  based  on  the  well-estabhshed  fact  that  every  seg- 
ment in  its  embryonic  condition  bears  a  pair  of  appendages.  Two 
compound  eyes,  which  are  very  large  in  the  drones,  are  placed  on  each 
side  of  the  head,  while  between  them  in  a  triangle  on  the  top  and  front 
of  the  head  are  three  simple  eyes,  or  ocelli.  Below  and  between  the 
compound  eyes  are  the  jointed  antennae.  The  mouth  parts  consist 
of  lahrimi  and  labium,  the  latter  a  complicated  structure  which  con- 
tains the  long,  flexible  Ugula  or  tongue  with  a  spoonlike  labellum  used 
by  the  bee  in  withdrawing  nectar  from  flowers.  Attached  to  each 
side  of  the  ligula  are  two  jointed  labial  palps.  The  base  of  the  labium 
consists  of  two  pieces,  the  submcntum  and  mentum.  The  upper  jaws 
or  mandibles  are  on  each  side  of  the  labrum,  while  the  lower  jaws  or 
maxillae,  with  their  tiny  palps,  fit  closely  and  laterally  over  the  men- 
tum. The  liquid  food,  nectar,  is  first  collected  by  means  of  the  hairs 
on  the  ligula,  the  maxillae  and  labial  palps  being  formed  into  a  tube 
through  which  the  ligula  works  up  and  down  with  a  kind  of  pumping 


204      ORGANISMS   ILLUSTRATING  BIOLOGICAL   PRINCIPLES 


motion  although  the  entire  labium  aids  in  the  process.  While  feed- 
ing, the  flap  of  the  labrum  or  epipharynx  is  lowered,  making  a  pas- 
sageway for  the  nectar  to  pass  into  the  mouth.  Thus  the  mouth 
parts,  which  are  all  present  in  the  locust  as  separate  structures, 

here  form  a  sort  of  pro- 
boscis, that  when  not  in 
use  is  folded  back  under- 
neath the  head. 

Bees  also  feed  on  solids 
such  as  pollen  and  "bee 
sugar,"  which  they  mois- 
ten with  regurgitated 
honey  and  saliva  before 
swallowing.  The  mandi- 
bles and  maxillae  are  both 
used  in  feeding  on  solids, 
but  the  chief  uses  of  the 
mandibles  are  in  building 
honeycomb. 

Bees  are  well  provided 
with  sensory  structures. 
Experiments  by  Mclndoo 
and  Von  Frisch  indicate 
that  bees  can  distinguish 
between  different-tasting 
substances,  for  some  of 
which  they  show  strong 
preferences.  But  whether 
they  can  actually  taste  or 
whether  they  distinguish  substances  by  means  of  a  sense  of  smell 
is  difficult  to  prove.  Several  experiments  have  been  made  that 
prove  the  presence  of  a  well-developed  perception  of  odor.  Among 
the  most  convincing  experiments  were  those  in  which  Von  Frisch 
trained  bees  to  select  certain  odors,  such  as  oil  of  orange  peel,  out 
of  43  other  odors.  He  concludes  that  not  only  can  bees  discover 
feeding  places  through  a  sense  of  smell  but  they  tell  other  bees  of  the 
existence  of  food  supplies  by  means  of  a  "round  dance"  in  which  the 
successful  bee  probably  holds  the  odor  of  the  particular  flowers  on 
which  she  has  been  feeding  and  disseminates  it  to  the  bees  that  crowd 
around  her  in  the  hive. 


Wright  Pierce 

Head  of  worker  bee.  Anterior  view.  Com- 
pare this  with  the  accompanying  hne  drawing 
and  identify  as  many  structures  as  you  can. 


THE   POPULAR   INSECT   PLAN 


205 


Experiments  in  which  the  antennae  were  removed,  together  with 
evidence  from  microscopic  examinations  of  the  antennae,  indicate 
that  they  hold  many  of  the  sense  organs  which  perceive  odors.  Small 
pits,  in  which  these  sensory  cells  are  located,  are  found  on  the  surface 


A.'-- 


-;-simple.  eyes 


compcurjct  eye 


labrum. 


n^andible. 


maxilla 


maxillary 
palp 


Simpla  eyes 
Compound.-  eye^ 


clypeu? 
■labram 

^a-ndiWe 

palp 

ma^cilla 


labium. 
..labial  palp 


■■prob^ 


oseis- 


labium. 

labial  palp 

IT 


Ij.... tongue^  (glossal 
i*»-labellu.ra       X 

L  Head  of  worker,  lateral  view,  mouth  part  labeled.  H.  Head  of  worker, 
lower  view,  lower  part  of  proboscis  cut  away.  Compare  these  mouth  parts  with 
those  of  the  locust.      Which  shows  the  more  primitive  condition!' 

of  the  antennae.  The  queen  has  about  1600  of  these  pits  on  each 
antenna,  the  workers  about  2400,  and  the  drones  about  37,800.  This 
large  number  probably  makes  it  possible  for  the  drones  to  find  the 
queen  during  her  nuptial  flight,  at  which  time  sperm  cells  are  placed 
within  her  body  so  as  to  insure  fertilization  of  the  eggs  as  they  are  laid. 
The  eyes  of  the  bee,  as  well  as  those  of  other  insects  and  crustaceans, 
are  compound.  This  means  that  they  are  composed  of  individual 
units  called  ommatidia.  Each  onmiatidium  consists  of  the  retinula, 
a  group  of  elongated  sensory  cells,  which  encloses  a  rodlike  rhahdom, 
the  latter  made  up  of  the  sensory  edges  of  the  retina]  cells.  At  the 
outer  edge  is  a  corneal  lens,  under  which  is  formed  a  crystalline  cone. 
The  retinal  cells  are  connected  with  the  optic  nerve  fibers,  the  entire 
apparatus  being  covered  with  a  layer  of  i)igment  cells,  so  that  each 
ommatidium  is  a  unit,  and  according  to  experimental  evidence,  is 


206      ORGANISMS  ILLUSTRATING  BIOLOGICAL  PRINCIPLES 


-crystalline  lens 


used  as  a  single  eye,  in  conjunction  with  the  several  hundred  others  in 
the  compound  eye.     Such  eyes  are  not  very  efficient.     It  is  probable 

that  they  do  not  have  any  sharp  vision 
for  distant  objects  and  not  very  clear 
vision  for  near  objects.  Bees  have 
been  conditioned  to  visit  boxes  of 
different-colored  flowers  in  order  to  get 
honey,  but  recent  experiments  by  Lutz 
and  others  indicate  that  they  are 
guided  to  flowers  by  odor  rather  than 
by  color. 

Bees  also  have  a  tactile  sense  which 
comes  through  tactile  hairs  on  various 

oCistol  retir?alar  nicclexcs     P^rts  of  the  body,  these  hairs  being 

most  numerous  on  the  antennae. 


-Cr/stalline  cone 

.outer  piginent.  Cell 

jcorneal  pignQenL  cell 
-T-hctbcCom 

.i_retinalcti--  cell 


-Outer  pigment/  oall 


The  Thorax  and  Its  Appendages 

The  entire  body  of  the  bee  is  covered 
with  hairs,  which  indirectly  play  an 
important  part  in  pollen  collection  and 
cross  pollination,  for  the  bee  in  rubbing 
against  the  stamens  of  a  flower  gets  a 
good  deal  of  pollen  on  the  head  and 
back.  The  thorax  is  armored  and  thus 
serves  well  its  purpose  as  a  base  for 
the  attachment  of  legs  and  wings.  The 
delicate  membranous  wings,  with  their 
ramifying  veins  and  veinlets  serving  as 
supporting  structures,  are  outgrowths 

appendages.  A  wing  in  flight  describes 
a  figure  eight  course,  its  rapid  move- 
ments being  caused  by  four  pairs  of 
muscles. 

The  legs  have  most  interesting  special  adaptations  for  the  several 
trades  which  the  worker  bee  carries  on.  It  is  a  typical  insect  leg,  of 
five  divisions  consisting  of  a  heavy  basal  coxa,  a  short  piece  called 
the  trochanter,  a  long  femur  which  with  the  adjoining  tihia  is  pro- 
vided with  long  hairs,  and  a  five-jointed  tarsus.  The  tarsus  is 
provided  at  the  tip  with  a  pair  of  strong  claws,  between  which 


♦..ne-rve^ 


Detail  of  an  ommatidium. 


THE   POPULAR   INSECT   PLAN 


207 


is  found  an  adliesive  organ  that  enables  the  animal  to  hold  fast  to 
slippery  surfaces. 

Each  pair  of  legs  bears  different  structures  which  are  of  use  in  pollen 
gathering  and  the  making  of  wax.  The  anterior  pair  of  legs  has  along 
the  anterior  margin  of  the  tibia  a  fringe  of  short,  stiff  hairs,  eye  brushes, 
used  for  cleaning  pollen  or  other  materials  from  the  compound  eyes. 


femsxr:.,/^ 


coxa 


Spina  of  the 
cl«aner 


front  ^ 
of  vorker 
'hDneyb<?e- 


g....  tibia 

eyebmsbes 


>  tarsus 


^-articularis 


■poiten 
cojtib 


middle  leg' 
of  worker 
honey  beer- 


„f)dten  Ijasket 
vnelatorsxTS 


hind  leg" 
o^-workei" 
Money  bee 


-^lanta. 


inner  .surf a<ie 
ofmatatarsus 
of  hincC  le^ 


These  appendages  are  used  for  more  purposes  than  locomotion.     Find  all  the 
adaptations  shown  and  give  the  use  of  each  adaptation  to  the  bee. 

The  first  joint  of  the  tarsus  is  provided  with  long  hairs  which  form  a 
pollen  brush.  This  is  used  to  collect  pollen  grains  scattered  over  the 
hairs  of  the  body.  At  the  base  of  the  first  joint  of  the  tarsus  is  a 
semicircular  notch  lined  with  short,  stiff  bristles,  while  a  flat  spur 
projects  from  the  distal  end  of  the  tibia.  This  apparatus  is  the 
antennae  cleaner.  To  accomplish  this  function  the  front  leg  is  ex- 
tended with  the  notch  placed  at  the  base  of  the  antenna,  which 
when  drawn  backward  through  the  notch  is  effectively  cleaned  of 
pollen. 

The  middle  pair  of  legs  is  not  so  highly  specialized  as  the  anterior 
pair.  There  is  a  large  spine  near  the  outer  end  of  the  tibia  which  is 
used  as  a  pick  for  removing  flakes  of  wax  secreted  from  the  wax  pockets 
located  under  the  abdomen.  The  flattened  basal  segment  of  the 
tarsus  is  called  the  planta.  Its  hairy  surface  is  used  for  brushing 
pollen  from  the  body  hairs. 

The  hind  legs  are  larger  and  broader  tlian  the  two  anterior  pairs. 
They  carry  most  of  the  pollen  gathered  from  flowers  to  the  hive. 
The  slightly  concave  outer  surface  of  the  tibia,  called  the  pollen 
basket,  is  lined  by  long  outward-curving  hairs,  and  may  often  be  seen 


208      ORGANISMS   ILLUSTRATING  BIOLOGICAL  PRINCIPLES 

filled  with  pollen  in  bees  returning  to  the  hive.  The  inner  surface 
of  the  first  tarsal  segment,  or  metatarsus,  is  covered  with  rows  of  stiff 
bristles  forming  the  pollen  comb,  while  the  lower  edge  of  the  tibia  ends 
in  a  row  of  spines,  called  the  pecten.  The  pecten  of  one  leg  is  scraped 
over  the  pollen  comb  of  the  opposite  leg,  the  pollen  thus  obtained 
being  pushed  up  into  the  pollen  basket  by  means  of  a  Uttle  projection 
on  the  upper  edge  of  the  metatarsus. 

Honey  Manufacture 

Although  bees  make  honey,  which  is  a  good  energy-releasing  food, 
they  do  not  live  entirely  upon  it  because  of  its  lack  of  proteins  needed 
for  building  up  the  body.  Both  adults  and  larvae  use  pollen  mois- 
tened with  saliva  and  honey,  which  forms  "bee  bread."  Bees  suck 
up  nectar  from  flowers,  pass  it  through  the  esophagus  into  a  thin- 
walled  crop,  or  honey  stomach.  This  organ  is  an  extensile  sac  which 
when  filled  holds  only  a  drop  or  two  of  fluid  so  that  numerous  trips 
to  and  from  the  hive  are  necessary  to  fill  a  single  cell  of  honeycomb. 
The  gathered  nectar  remains  in  the  honey  stomach  until  the  bee 
returns  to  the  hive,  when  it  is  regurgitated  and  placed  in  the  cells  of 
the  honeycomb.  As  honey,  it  is  still  too  watery,  so  some  of  the 
workers,  by  a  rapid  vibration  of  their  wings,  cause  enough  water  to 
evaporate  to  bring  it  to  the  right  consistency.  Just  before  the  honey 
is  capped  in  the  comb,  the  worker  places  a  minute  amount  of  formic 
acid  from  its  poison  glands  in  the  cell.  This  aids  in  the  preservation 
of  the  honey.  Bees  store  somewhat  over  two  pounds  of  honey  a  day 
for  the  average  hive.  This  is  in  addition  to  what  the  adults  eat  and 
what  is  fed  the  young.  Honey  storage,  of  course,  varies  with  the 
weather.  Bees,  like  human  outdoor  laborers,  do  not  work  on  rainy 
days. 

Dr.  L.  Armbruster  of  Berlin  made  some  interesting  computations 
on  the  number  of  visits  of  bees  to  flowers  necessary  to  store  up  about 
two  and  one  half  pounds  of  honey.  He  found  that  bees  have  to  visit 
at  least  6,000,000  clover  heads,  as  clover  honey  seems  to  require  the 
most  work.  Peas,  at  the  bottom  of  the  scale,  called  for  as  low  as 
80,000  visits  from  the  bees,  and  other  honey-producing  plants  fell 
within  these  two  limits.  Among  the  most  important  honey-produc- 
ing plants  are  white  clover,  buckwheat,  and  fruit  trees  in  the  East 
and  North  ;  alfalfa,  sweet  clover,  and  a  few  trees,  as  the  tulip  tree,  in 
the  Central  West ;  the  citrus  fruits,  palmettos,  and  mangrove  in  the 
South  ;  and  alfalfa,  sages,  citrus  and  other  fruit  trees  in  the  far  West. 


THE   POPULAR   INSECT   PLAN 


209 


-Salivary  glarjcCS 
esophogors 


-  -  honay  Stomach 
..prov©ntricultC5 


Digestion,  Circulation,  Respiration,  and  Excretion 

The  digestive  tract  posterior  to  the  crop  has  to  do  with  the  digestion 
of  food.  The  stomach,  a  large  cyhndrical  structure,  has  a  valvelike 
arrangement  between  it  and  the  crop  to  prevent  nectar  not  used  as 
food  from  going  further.  It  leads  into  a  small  intestine,  which  in 
turn  expands  to  form  the 
rectinn  at  the  posterior  • 

end  of  the  body.  -       (^'W'^'^  ■  -  pl^^'^yn&al  glands 

Attached   to    the   an-  x%  '^-^A<a#-P°stc«nlbml%iands 

terior  end  of  the  intestine 
is  a  circle  of  Malpighian 
tubules,  about  one  hun- 
dred in  number,  named 
after    their     discoverer, 

Marcello  Malpighi,  who  '^V^^^^^^'^^^-'J^f 
first  pictured  them  in 
his  Anatomy  of  the  Silk- 
worm published  in  1669. 
The  tubules  are  excre- 
tory in  nature,  as  is 
proven  by  the  fact  that 
small  crystals  of  nitrog- 
enous wastes  are  formed 
in  them. 

In  the  insects  and 
crustaceans,  there  is  no 
closed  system  of  blood  vessels  as  was  found  in  the  earthworm,  but 
in  the  former  there  is  a  well-developed,  dorsally  placed,  tubular 
heart,  located  in  the  abdomen  and  perforated  by  paired  openings, 
or  ostia,  through  which  blood  enters.  Blood  is  forced  out  of  the 
anterior  end  into  spaces,  or  sinuses,  which  in  the  insects  are  found 
throughout  the  body  cavity  and  take  the  place  of  blood  vessels. 
The  heart  acts  somewhat  like  a  rubber  bulb  syringe  in  a  pail  of 
water,  serving,  along  with  the  muscular  movements  of  the  insect, 
to  keep  the  blood  in  motion  through  the  blood  sinuses.  Snod- 
grass  ^  shows  that  there  is  a  rapid  and  complete  circulation  of 
blood  through  the  main  sinuses,  the  blood  being  forced  backward 
into  the  abdomen  on  the  ventral  side  of  the  body  by  the  pulsat- 


smdl  intestine 
ventriculus. 

rectal  gkncC 


The  food  tube  of  worker  bee  and  glands  con- 
nected with  it.  The  pharyngeal  glands  form  the 
royal  jelly  or  brood  food  given  to  the  larvae  by  the 
workers.  The  postcerebral  glands  secrete  a  fatty 
substance,  which  is  thought  to  be  mixed  with  wax 
in  making  honeycomb.  The  salivary  glands  are 
true  digestive  glands.     (After  Snodgrass.) 


1  Snodgrass,  Anatomy  and  Physiology  of  the  Honey  Bee,  McGraw-Hill,  1925,  pp.  189-190. 


210      ORGANISMS  ILLUSTRATING  BIOLOGICAL   PRINCIPLES 


-airsccC/ 


.tracheal  tube 


spiracle^  1— 


ing  vibrations  of  the  so-called  ventral  diaphragm,  a  sheet  of  thin 
tissue  which  is  stretched  across  the  ventral  part  of  the  abdominal 
cavity,  while  on  the  dorsal  side  it  is  pumped  by  the  heart  toward  the 
anterior  end  of  the  body.  The  blood,  which  bathes  all  the  tissues, 
consists  of  a  plasma,  and  colorless  blood  corpuscles  or  leucocytes. 
The  plasma  is  rich  in  food  substances,  but  there  are  no  oxygen- 
carrying  substances  in  it,  so  that  insect  blood  only  carries  foods  and 
wastes. 

Oxygen  is  brought  directly  to  the  tissues  by  a  very  efficient  type 
of  respiratory  organ,  the  so-called  tracheae  and  their  branches.  Along 
the  sides  of  the  thorax  and  abdomen  of  insects  are  found  paired 
openings  called  spiracles.     In  the  worker  bee  there  are  ten  pairs  of 

these  openings,  three  pairs 
in  the  thorax  and  seven 
in  the  abdomen.  The 
spiracle  is  an  oval  open- 
ing which  can  be  opened 
and  closed  by  means  of 
a  flat  plate  attached  to 
its  rim.  Each  spiracle 
leads  into  a  tracheal  tube, 
the  wall  of  which  is 
strengthened  by  a  spiral 
thread  of  chitin,  thus 
keeping  the  tube  filled 
with  air.  These  tubes 
branch  again  and  again 
until  they  finally  end  in 
tiny  tubules  between  the 
body  cells.  Expansion 
and  contraction  of  the 
muscles  of  the  body  wall 
force  air  in  and  out 
through  the  tracheae,  thus  securing  circulation  of  oxygen  to  all  body 
cells.  In  addition  to  the  tracheae,  large  air  sacs  are  developed  in 
the  thorax  and  abdomen,  as  are  seen  in  the  above  diagram.  Since 
insects  that  fly  rapidly  usually  have  better  developed  air  sacs  than 
those  that  are  sedentary,  it  is  evident  that  the  air  sacs  must  serve 
to  "lighten  the  load"  of  the  body  in  its  flight  as  a  heavier-than-air 
machine. 


Spiracle  3.. 

spiracle  4....^ 
Spiracle  5... 
Spiracle  6- 
spiracle  7- 


-i|K — anrsac 


-,-tube5  join 
■  dorsal  sclcs 


If.  .commissure 


A  portion  of  the  tracheal  system  of  the  worker 
bee.  The  dorsal  trachea  and  air  sacs  have  been 
removed.  Three  spiracles  are  not  shown.  What 
advantages  are  there  in  having  this  type  of  re- 
spiratory system?     (After  Snodgrass.) 


THE   POPULAR   INSECT   PLAN 


211 


The  Nervous  System 

The  nervous  system  of  the  bee  is  well  developed,  consisting  of  a 

series  of  ganglia,  forming  a  double  ventral  nerve  cord  with  a  dorsal 

cerebral  ganglion  (brain), 

antennal . 


rain. 


optic  loloe 


to  -vin^? 


to  leg 


connected  by  a  circum- 
esophageal  nerve  ring  with 
a  subesophageal  ganglio7i 
directly  underneath  it. 
Although  typically  in  a 
segmented  animal  there 
should  be  one  ganglion 
for  each  segment,  we  find 
fewer  ganglia  than  seg- 
ments in  the  adult  bee. 
This  is  because  certain  of 
the  ganglia  have  fused, 
there  being  seven  in  the 
ventral  ner^'e  cord  of  the 
bee.  From  each  of  these 
nerves  efferent  fibers  ex- 
tend to  the  muscles  while 
afferent  fibers  from  sense 
cells  end  in  the  ganglia  to 
make  up  the  reflex  arc 
previously  described  (page 
195).  Not  all  co-ordi- 
nation of  muscles  is  controlled  by  the  brain,  for  a  headless  bee  will 
still  walk  and  experiments  have  shown  that  the  body  ganglia  are 
independent  centers  of  control  over  the  appendages.  Insects  of  the 
order  Hymenoptera,  to  which  the  bee  belongs,  have  the  best  brain 
development  of  any  of  the  insects,  a  fact  that  seems  to  be  correlated 
with  their  complex  social  habits  and  their  keen  senses. 


Nervous  system  of  worker  bee.  Why  are  the 
ganglia  in  the  thorax  so  much  larger  than  those 
of  the  abdomen?  Note  that  the  brain  is  on  the 
dorsal  side,  the  esophagus  (not  shown)  passing 
between  the  two  nerves  that  connect  it  with  the 
first  thoracic  ganglion.      (After  Snodgrass.) 


Reproduction  and  Life  History 

Although  the  workers  possess  undeveloped  ovaries,  all  the  eggs  are 
laid  by  the  fertile  female  or  queen.  While  a  worker  may  live  about 
six  weeks  in  summer  and  never  more  than  a  few  months,  the  queen 
lives  three  or  fotir  years,  or  even  longer.  The  ovaries  of  the  queen  are 
made  up  of  a  number  of  tubules,  in  which  are  eggs  in  all  stages  of 


212      ORGANISMS  ILLUSTRATING  BIOLOGICAL  PRINCIPLES 

development.  The  fully  developed  eggs  pass  out  through  the  oviduct 
into  the  vagina,  where  they  are  fertilized  by  sperm  cells  that  were 
placed  in  a  sac  called  the  seminal  receptacle  by  a  drone  during  the  nup- 
tial flight  of  the  queen.  The  drones  form  the  sperm  cells  in  two  testes, 
but  the  sperms  are  stored  in  seminal  vesicles  from  which,  during  mat- 
ing, they  are  transferred  to  the  seminal  receptacles  of  the  queen. 

The  queen  lays  fertilized  eggs  in  honeycomb  cells  of  the  worker 
and  unfertilized  eggs  in  the  larger  drone  cells.     Just  how  she  controls 

the   actual   fertilization 
cocoon.  .W-^ 


of  the  egg  is  not  known. 
According  to  Nolan,^  the 
queen  produces  an 
average  of  about  900 
eggs  a  day  during  the 
season,  but  may  lay  as 
many  as  2000  a  day 
during  the  period  of 
greatest  honey  making. 
The  queen  places  the 
eggs  in  the  cells  by 
means  of  an  ovipositor, 
which  in  the  workers  is 
modified  into  a  sting. 
The  latter  structure  is 
made  up  of  two  darts, 
closely  applied  to  each  other  so  as  to  form  a  tube  through  which 
poison  from  a  poison  sac  flows  when  the  darts  are  forced  out  of  their 
sheath  as  the  bee  stings.  Two  different  poisons  are  produced,  one  of 
which  is  formic  acid,  the  other  an  alkahne  substance.  Worker  bees 
usually  die  after  stinging,  as  the  sting  with  its  attached  parts,  along 
with  some  of  the  intestine,  is  left  in  the  wound.  The  queen,  which 
also  has  a  sting,  uses  it  only  in  combat  with  other  queens  and  does 
not  lose  her  life  in  its  use. 

The  life  history  of  the  bee  is  rather  brief.  Three  days  after  fertiliza- 
tion the  egg  hatches  into  a  larva  which  lies  in  the  cell  surrounded  by 
a  plentiful  supply  of  "bee  milk,"  a  mixture  of  digested  honey,  pollen, 
and  saliva.  After  three  days  of  feeding  by  the  young  "nurse"  bees, 
the  larvae  are  given  more  and  more  undigested  food.     Drones  are 


<^©er2  cell 


Cells  of  hive  of  honey  bee.  Note  the  stages  in 
development  of  worker.  How  many  kinds  of  cells 
are  shown?     (Read  page  213.) 


1  Nolan,  "  Egg-laying  Rate  of  the  Queen  Bee."     Gleanings  in  Bee  Culture,  Vol.  52,  1924,  pp.  428- 


431. 


THE   POPULAR    INSECT   PLAN  2i;{ 

fed  undigested  honey  and  pollen  after  the  fourth  day,  while  young 
queens  are  fed  upon  an  especially  nutritious  albuminous  "royal 
jelly"  until  they  pupate.  During  the  larval  period,  the  young 
insects  grow  rapidly,  changing  their  skins  or  molting  several  times 
during  the  process.  About  the  end  of  the  fifth  day  the  larvae  are 
given  their  last  food  by  the  attendant  bees  and  the  cell  is  capped  with 
wax.  Then  the  larva  spins  a  cocoon,  molts  for  the  last  time,  and 
becomes  a  pujM.  In  this  stage  it  begins  to  assume  adult  characters 
and,  after  the  next  molt,  emerges  from  the  cell  as  an  adult.  This 
process,  in  which  the  insect  undergoes  certain  changes  not  in  line 
with  its  direct  development,  is  called  a  metamorphosis.  The  last 
molt  in  which  the  young  adult  is  ready  to  emerge  from  the  cell  takes 
place  about  20  days  from  the  time  the  egg  was  laid.  The  young 
adult  bee,  or  imago,  chews  its  way  out  of  the  cell,  usually  emerging 
on  the  21st  day.  The  metamorphosis  of  the  drone  takes  24  days  and 
the  queen  16,  the  greater  rapidity  of  the  latter  probably  being  due  to 
the  more  nutritious  food  received. 

The  Life  in  the  Hive 

The  activities  in  a  bee  hive  are  numerous  and  interesting.  Besides 
collecting  nectar  and  pollen  and  making  honey,  the  most  important 
work  is  that  of  building  the  wax  cells  of  the  comb.  Wax  is  secreted 
by  the  wax  glands  on  the  abdomen  and  transferred  to  the  mouths  of 
the  workers,  where  it  is  mixed  with  saliva,  kneaded  by  the  mandibles, 
and  shaped  into  the  familiar  hexagonal  cells  of  the  honeycomb. 
Six  kinds  of  cells  are  made  :  (a)  drone  cells,  (6)  worker  cells,  (c)  queen 
cells,  {d)  transition  cells  between  worker  and  drone  cells,  (<?)  attach- 
ment cells  which  fasten  the  comb  in  place,  and  (/)  honey  cells. 
Worker  bees  also  bring  back  propolis  or  "bee  glue,"  resinous  materials 
collected  largely  from  the  buds  of  trees.  The  propolis  is  used  to  fill 
up  cracks  in  the  hive  and  to  strengthen  the  comb.  Water  is  also 
carried  to  the  hive  in  dry,  hot  weather.  Besides  the  above  activities, 
others  must  be  performed  if  life  in  the  hive  is  to  go  on.  The  workers 
must  have  plenty  of  fresh  air,  for  they  do  hard  work.  To  this  end 
certain  of  the  bees  are  delegated  to  the  task  of  vibrating  their  wings 
rapidly,  thus  creating  currents  of  air  through  the  hive.  Some  workers 
rid  the  hive  of  excreta,  dead  bees,  or  any  other  substances  that 
interfere  with  its  cleanliness.  Still  other  bees  guard  the  entrance  of 
the  hive  against  such  enemies  as  bee  raoths  or  yellow-jacket  hornets, 
which  come  to  steal  honey. 


214      ORGANISMS  ILLUSTRATING  BIOLOGICAL  PRINCIPLES 

One  other  activity  is  that  of  swarming.  In  early  summer,  the  hive 
frequently  becomes  overcrowded,  and  when  such  conditions  arise 
several  queen  cells  are  built  and  young  queens  are  raised.  When  a 
young  queen  hatches,  the  old  queen  gathers  together  several  thousand 
of  the  workers,  who  fill  their  honey  stomachs  with  honey  and  then  set 
out  to  form  a  new  colony.  Sometimes  scouts  are  sent  out  in  advance 
to  seek  a  place  for  the  new  hive,  which  may  be  in  a  hollow  tree. 
Often  the  swarm,  forming  a  large  ball  about  the  queen,  will  come  to 
rest  on  the  branch  of  a  tree  and  the  beekeeper  may  then  hive  it  in  an 
artificial  hive.  It  is  interesting  to  note  that  our  honey  bee  {Apis 
meUifica)  is  an  emigrant  from  Europe  and  that  there  are  no  native 
honey  bees  in  this  country. 

This  social  life  with  its  accompanying  division  of  labor  is  seen  in 
varying  degrees  all  through  the  order  Hymenoptera.  Beginning  with 
the  solitary  bees,  we  find  increasing  social  complexity  of  life  until,  in 
the  ants,  a  highly  organized  group  is  developed  having  several  different 
kinds  of  workers,  soldiers,  and  males.  If  you  want  fascinating  reading 
along  this  line,  look  into  William  Beebe's  Jungle  Life,  or  better,  into 
Wheeler's  masterly  volume  on  Ants. 

SUGGESTED  READINGS 

Carpenter,  G.  H.,  The  Biology  of  Insects,  The  Macmillan  Co.,  1928. 

Chapters  II,  III,  IV,  V,  VII,  and  IX  make  interesting  reading. 
Fernald,  H.  T.,  Applied  Entomologij,  McGraw-Hill  Book  Co.,  1935.     Chs.  IV, 

V,  and  XXXIII. 

A  useful  book  of  reference. 
Kellogg,  V.  L.,  American  Insects,  Henry  Holt  &  Co.,  1908. 

Still  an  authentic  book  of  reference. 
Metcalf,  C.  L.,  and  Flint,  W.  P.,  Fundamentals  of  Insect  Life,  McGraw-Hill 

Book  Co.,  1932.     Chs.  II,  IV,  and  V. 
Plath,  0.  E.,  Bumblebees  and  Their  Ways,  The  Macmillan  Co.,  1934. 

A  fascinating  study  of  one  type  of  social  insect. 
Snodgrass,  R.  E.,  Anatomy  and  Physiology  of  the  Honeybee,  McGraw-Hill 

Book  Co.,  1925. 

Parts  of  Chapters  II,  III,  and  IV  are  particularly  useful,  but  a  student 

can  cull  much  from  the  entire  book. 
WeUs,  H.  G.,  Huxley,  J.  S.,  Wells,  G.  P.,  The  Science  of  Life,  Doubleday, 

Doran  &  Co.,  1931. 

Pp.  1147-1182  give  one  phase  of  insect  life  worth  reading  about. 


XII 


THE  ART   OF   PARASITISM 

Preview.  Who  qualifies?  •  Some  host-parasite  relationships  :  The  host- 
parasite  conflict ;  effects  of  a  parasitic  life ;  keeping  the  cycle  going  ■  The 
complexity  of  parasitic  relationships :  External  parasites ;  temporary  para- 
sites, periodic  parasites,  pennanent  parasites ;  internal  parasites ;  parasites 
requiring  one  host,  parasites  requiring  two  hosts,  malaria,  parasites  requiring 
more  than  two  hosts  •  Suggested  readings. 

PREVIEW 

According  to  the  definition,  a  parasite  is  one  that  "lives  on  or 
within,  and  at  the  expense  of  some  other  organism,"  and  thus  might 
include  forms  from  the  smallest,  such  as  filtrable  viruses  and  bacteria, 
to  some  of  the  largest  species.  As  a  matter  of  fact,  parasitism  is 
well-nigh  universal,  for  examples  are  found  among  nearly  all  groups 
of  plants  and  animals. 

In  many  instances  there  appears  to  be  a  remarkable  balance 
between  the  parasite  and  its  host.  A  dead  host  is  of  little  use  to  a 
parasite  since  it  implies  a  loss  of  free  transportation  as  well  as  board 
and  lodging.  Consequently  the  existence  of  a  parasite  must  be  a 
compromise,  for  it  must  be  able  to  secure  enough  nourishment  to 
maintain  and  reproduce  itself  and  yet  do  this  either  without  injuring 
too  much  the  vitality  of  its  host,  or  actually  reducing  its  own  numbers. 
As  a  result  of  this  rather  elaborate  compromise  parasites  have  become 
so  adapted  that  they  usually  destroy  only  small  portions  of  the  host 
tissue  which  usually  can  be  replaced  by  regeneration. 

Whenever  a  parasite  reaches  a  final  host,  the  problem  of  propaga- 
tion arises.  Most  parasites  produce  large  numbers  of  eggs,  cysts,  or 
spores  that  are  discharged  with  the  waste  products  of  the  host. 
Through  the  medium  of  food  or  drink,  these  reach  the  next  host,  which 
is  sometimes  intermediate  or  secondary,  the  parasites  thus  becoming 
dependent  upon  the  food  habits  of  more  than  one  organism  to  main- 
tain their  cycles. 

Most  animal  parasites  are  essentially  carnivorous  in  their  feeding 

habits.     True    carnivores,    however,    destroy    their    prey,    whereas 

parasites  as  a  rule  do  not,  and  while  carnivores  are  much  larger  than 

their  prey,  parasites  are  smaller.     Elton  says,  to  summarize,  "The 

H.  w.  H.— 15  21. "S 


216      ORGANISMS   ILLUSTRATING   BIOLOGICAL   PRINCIPLES 


difference  between  the  methods  of  a  carnivore  and  a  parasite  is  simply 
the  difference  between  Hving  upon  capital  and  upon  income ;  between 
the  habits  of  the  beaver,  which  cuts  down  a  whole  tree  a  hundred 
years  old,  and  the  bark  beetle,  which  levies  a  daily  toll  from  the  tissues 
of  the  tree  ;  between  the  burglar  and  the  blackmailer.  The  general 
result  is  the  same,  although  the  methods  employed  are  different."  ^ 

Who  Qualifies? 

Parasites  vary  greatly  in  their  types  of  relationships.  Some  may 
be  classified  as  either  internal  or  external,  according  to  their  location  in 
or  upon  the  host.  They  may  be  otherwise  classified  as  temporary,  or 
free-living  during  a  part  of  their  life  cycles ;  yermanent,  or  parasitic 

throughout  their  life  span  ; 
and  'periodic,  only  visit- 
ing their  hosts  to  obtain 
nourishment.  Actually 
there  are  almost  as  many 
gradations  and  variations 
in  the  degree  of  parasitism 
among  animals  and  plants 
as  there  are  kinds  of  par- 
asites. Mosquitoes  and 
some  fleas  visit  their  hosts 
just  long  enough  to  sat- 
isfy their  appetites.  The 
cattle  tick,  Boophilus  an- 
nulatus,  never  leaves  its 
host  except  when  ready  to 
lay  eggs.  Scab  mites  and 
some  lice  are  permanent, 
living  upon  the  same  host 
from  one  generation  to  the  next,  only  leaving  or  being  transferred 
by  direct  contact.  In  between  these  extremes  occur  such  well-known 
forms  as  the  hookworm,  which  has  a  free-living  larval  stage,  and 
the  botflies  that  pass  their  larval  existence  as  parasites. 

Among  plants,  the  large  and  heterogeneous  group  of  bacteria  exhibit 
many  varieties  of  parasitism,  while  higher  in  the  plant  scale  such  forms 
as  dodder  and  broomrape  exemplify  true  parasitism.  Other  groups 
are  partially  parasitic  during  their  life  cycle. 

■  From  Elton,  C,  Animal  Ecology.     By  permission  of  The  Macmillan  Company,  1935. 


Wright  Pierce 

Dodder,  an  example  of  a  plant  parasite  which 
starts  life  as  a  self-respecting  plant  growing  in 
soil. 


THE  ART  OF  PARASITISM 


217 


Some  Host-Parasite  Relationships 

In  the  event  of  parasitism,  the  association  is  definitely  in  favor  of 
the  parasite,  since  it  usually  "lives  on"  the  second  party  concerned, 
the  host.  Such  a  relation- 
ship constitutes  a  fourth 
type  of  habitat,  namely 
parasitic,  that  is  available 
to  both  plants  and  ani- 
mals along  with  the  well- 
recognized  terrestrial, 
fresh-water,  and  marine 
habitats.  That  many  or- 
ganisms take  advantage 
of  this  type  of  existence 
may  be  clearly  proved  by 
observing  the  plants  and 
animals  of  any  locality. 

The  Host-Parasite 
Conflict 

Theoretically  a  conflict 
exists  between  the  para- 
site and  the  host.  The 
latter  has  as  its  chief 
weapon  a  lytic  or  dissolv- 
ing power  which  is  a  nor- 
mal physiological  reaction.  Likewise  the  weapons  that  probably 
were  first  brought  by  the  parasite  from  its  hypothetical  free  living 
ancestral  state  must  also  have  been  of  a  lytic,  toxic,  or  otherwise 
destructive  nature.  In  many  cases  the  host  seems  to  have  adapted 
itself  to  bear  the  burden  of  parasitism  with  the  least  possible 
outlay  of  energy  on  its  own  part,  so  that  eventually  there  has  devel- 
oped a  balance  between  the  two  organisms,  which  might  be  called 
a  Jwst-parasitc  equilibrium.  In  order  to  reach  this  equilibrium  the 
parasite  has  likewise  gradually  e^'olved  some  sort  of  protectixe  device, 
often  a  capsule  which  becomes  interpolated  in  the  cycle,  or  an  anti- 
enzyme  or  anticoaguUn  to  counteract  the  destructive  action  of  the 
host's  secretions,  thus  necessitating  a  counter  attack  upon  the  part 
of  the  host.     This  apparently  was  made,  first,   through   the  o\or- 


W right  I'icTCe 

The  large  masses  in  this  tree  represent  a  true 
plant  parasite,  the  mistletoe. 


218      ORGANISMS   ILLUSTRATING  BIOLOGICAL   PRINCIPLES 

development  of  the  host  mechanism  which  elaborates  specific  pro- 
tective substances  called  antibodies,  and  secondly,  through  "the 
adaptation  of  certain  normally  phagocytic  cell  groups  to  the  intern- 
ment and  gradual  destruction  of  the  parasite."  ^ 

Sometimes  the  introduction  of  a  parasite  has  a  visible  effect  upon 
the  host.  In  the  case  of  certain  gall  insects,  such  as  the  Cynipoidea, 
the  deposition  of  an  egg  by  the  female  in  a  plant  tissue,  or  the  subse- 
quent movement  of  the  larva,  furnishes  the  stimulus  which  causes 


H^HI^^^^^^^HBf^#i  ^ 

^^H 

I^^HRP^' 

^Hf^l 

^^^^^^H^^.' 

iPn  ■'■ 

^^^^^^^^^.^TlK^^^^I 

Wright  Pierce 

Unopened  oak  gall  beside  one  which  has  been  opened  to  show  the  enclosed 

larvae. 

abnormal  proliferation  of  tissue,  resulting  in  the  enclosing  of  the  insect 
larva  and  the  production  of  a  so-called  gall.  The  type  of  gall  pro- 
duced on  a  given  plant  appears  to  be  specific,  whether  it  occurs  in 
root,  leaf,  twig,  or  stem.  Usually  a  gall  ceases  to  grow  about  the 
time  when  the  enclosed  larva  finishes  feeding.  In  such  instances  it 
dries  and  forms  a  protective  covering  inside  of  which  the  insect 
pupates,  ultimately  gnawing  its  way  out. 

Effects  of  a  Parasitic  Life 

Parasitism  as  a  biological  phenomenon  probably  has  a  more  far- 
reaching  effect  upon  the  structure  of  the  parasite  than  upon  the  host. 
In  the  first  place  the  former  no  longer  has  to  worry  about  locomotion 
or  the  securing  of  food  because  these  two  important  functions  are 
taken  care  of  by  the  host.  Consequently  a  gradual  simplification 
of  the  organs  of  a  parasite  takes  place,  until  in  forms  like  the  tape- 

>  Smith,  T.,  Parasitism  and  Disease,  Princeton  Univ.  Press,  1934,  p.  111. 


THE   ART   OF    PAIUSITISM  219 

worm,  and  the  spiny-headed  worm,  for  example,  there  is  no  trace 
whatever  of  a  digestive  tract  in  the  adult.  Such  worms,  however, 
have  access  to  various  digested  foods  which  are  ready  for  absorption 
by  the  host  and  it  appears  certain  that  these  gutless  forms  must  be 
able  to  absorb  and  utilize  materials  from  the  alimentary  canal  of 
their  benefactor.  Other  worms,  such  as  the  flukes  or  trematodes,  and 
roundworms,  possess  a  well-developed  alimentary  canal,  the  secretions 
of  which,  in  some  instances  at  least,  cause  a  liquefaction  of  the  tissue 
in  the  immediate  vicinity  of  the  parasite,  thus  making  it  available  as 
food  for  the  organism. 

Another  problem  which  parasites  have  had  to  solve  is  that  of  respira- 
tion. In  the  case  of  cellular  or  blood-inhabiting  forms  the  parasite 
obviously  has  access  to  plenty  of  oxygen,  whereas  intestinal  parasites 
face  a  difficulty,  since  the  alimentary  canal  is  known  to  contain  little 
oxygen.  Many  investigators  now  believe  that  these  worms  secure 
their  energy  from  the  breakdown  of  dextrose.  This  substance  results 
from  the  hydrolysis  of  more  complex  carbohydrates  and  is  the  form 
in  which  it  is  absorbed  from  the  intestine  into  the  blood  stream. 
Presumably  oxygen  is  secured  during  the  process  of  anaerobic  fermen- 
tation that  results  in  the  splitting  of  dextrose  or  glycogen  (if  the 
carbohydrate  has  been  converted  into  glycogen  during  the  metabolism 
of  the  parasite)  into  fatty  acids  and  carbon  dioxide.  This  type  of 
metabolism  is  characteristic  of  some  bacteria  and  yeasts. 

One  of  the  most  striking  effects  of  the  parasitic  habit  lies  in  the 
tremendous  development  of  the  reproductive  capacity  of  the  parasite, 
a  process  undoubtedly  correlated  with  the  numerous  hazards  which 
must  be  met  if  its  life  cycle  is  to  be  completed.  The  development 
occurs  in  two  ways,  —  first  by  the  production  of  enormous  numbers 
of  eggs,  and  secondly  by  the  interpolation  of  asexual  stages  in  the  cycle. 
Thus  it  has  been  estimated  that  a  single  free-swimming,  ciliated  stage 
{miracidium)  of  a  fluke  may  be  the  indirect  parent  of  as  many  as 
10,000  free-swimming,  tailed  larvae  {cercariae). 

External  or  ectoparasites  also  show  marked  evidence  of  adaptation  to 
their  type  of  existence,  as  shown  by  the  piercing  and  sucking  mouth 
parts  of  the  parasitically  inclined  arthropods  or  the  degeneration 
of  the  mouth  parts  in  the  case  of  the  adult  botflies,  as  well  as  by 
the  laterally  compressed  body  of  the  flea,  and  the  loss  of  wings  in 
lice  and  bedbugs.  Limitation  as  to  the  host  and  as  to  the  location 
on  the  host  shows  specialization  among  this  group.  These  factors 
tend  to  illustrate  stages  in  the  development  of  ectoparasitism. 


220      ORGANISMS   ILLUSTRATING  BIOLOGICAL   PRINCIPLES 

Keeping  the  Cycle  Going 

The  chief  problem  of  any  species  centers  about  maintaining  itself, 
a  statement  to  which  there  is  no  exception  in  the  world  of  parasites. 
Obviously  those  organisms  which  have  become  adapted  as  ectopara- 
sites are  not  faced  with  complicated  problems  relating  to  the  transfer 
from  host  to  host.  By  means  of  simple  contact  a  new  host  may  be 
reached,  or  if  a  portion  of  the  life  cycle  of  the  parasite  is  free  living, 
it  may  leave  the  host  to  deposit  its  eggs.  Even  in  cases  where  the 
eggs  are  laid  among  hairs  of  the  host  they  usually  fall  to  the  ground 
to  develop. 

A  more  difficult  problem  of  maintaining  the  species  must  be  faced 
when  internal  parasites  are  involved.  Bacteria  which  are  capable  of 
producing  protective  capsules  or  spores  of  one  sort  or  another  are 
tided  over  unfavorable  periods  and  so  aided  in  reaching  new  hosts. 
They  are  adapted  also  for  rapid  reproduction.  One  worker  has  esti- 
mated that  if  the  multiplication  of  bacteria  were  unchecked  one  cell 
would  be  the  parent  of  281,500,000,000  bacteria  in  two  days.  Such  a 
mass  at  the  end  of  the  third  day  would  weigh  about  148,356,000 
pounds. 

Many  parasitic  protozoa  as  well  as  metazoa  are  adapted  to  be 
transferred  from  one  host  to  the  next  by  means  of  resistant  cysts 
secreted  by  the  organism.  Others,  like  the  blood-inhabiting  try- 
panosomes  or  malarial  organisms,  secure  transference  by  adapting 
themselves  to  insects  which  act  as  wholesale  distributors  for  the 
parasites.  Some  produce  harmful  toxins  which  occasionally  kill  the 
host.  In  such  instances,  however,  one  may  be  sure  that  the  host  is 
abnormal  and  the  parasites  have  not  become  adapted  to  it.  In  the 
case  of  the  trypanosomes  of  man  in  Africa,  antelopes  are  their  natural 
hosts  and  are  quite  tolerant  to  these  blood  parasites.  Since  man  and 
domestic  animals  are  unnatural  hosts,  they  are  consequently  much 
more  severely  affected  by  them. 

The  Complexity  of  Parasitic  Relationships 

The  most  satisfactory  way  to  secure  a  general  idea  of  the  surpris- 
ingly varied  adaptations  to  a  parasitic  existence  is  by  a  study  of 
a  few  examples.  Such  a  study  emphasizes  clearly  the  almost  uncanny 
adaptations  which  have  been  made  by  parasites  to  insure  the  com- 
pletion of  their  life  cycles.  While  various  types  of  parasitism  clearly 
exist,  nevertheless  the  line  that  demarks  one  kind  of  parasite  from 


THE   ART  OF   PARASITISM 


221 


another  may  not  always  be  sharply  drawn.  However,  for  the  sake  of 
convenience  an  attempt  will  be  made  to  outline  briefly  a  few  examples 
of  such  relationships. 

External  Parasites 

External  parasites  are  found  throughout  the  plant  and  animal  king- 
doms. Even  among  the  minute  protozoa,  ectoparasitic  organisms 
occur,  such  as  Cyclochaeta,  a  parasite  on  fishes,  which  may  cause  an 
appreciable  economic 
loss  under  epidemic 
conditions.  The  lam- 
prey eel  among  the 
chordates  is  a  large 
external  parasite  on 
certain  fishes. 

For  the  sake  of  con- 
venience, external  par- 
asites may  be  classified 
as  to  whether  they  are 
temporary,  periodic,  or 
permanent.  Some 
forms,  like  the  house 
fly,  do  not  really  belong 
in  any  of  these  cate- 
gories. Yet  the  house 
fly  certainly  deserves 
mention,  since  it  serves  as  a  mechanical  carrier  from  one  host  to 
another  for  the  transfer  of  numerous  bacteria  and  their  spores,  as 
well  as  the  cysts  and  eggs  of  various  other  parasites. 

Temporary  Parasites.  As  an  example  of  temporary  parasitism 
may  be  mentioned  the  parasitic  Hymenoptera  that  lay  their  eggs 
on  the  eggs,  larvae,  or  even  the  adults  of  other  insects.  During  the 
developmental  interlude  they  remain  as  true  parasites  within  the 
body  of  the  host  until  they  eventually  destroy  it,  at  which  time  they 
cease  their  parasitic  existence  and  become  free  living.  The  ichneu- 
mon flies,  that  belong  in  this  group  of  parasitic  Hymenoptera,  each 
year  attack  and  destroy  great  numbers  of  injurious  as  well  as  some 
beneficial  insects.  Another  example  of  a  temporary  parasite  is  the 
ox  botfly,  the  free  living  adult  of  which  attaches  its  eggs  to  hairs  on 
the  legs  of  cattle.     Upon  hatching,  the  larvae  penetrate  the  hide  and 


1^^ 

"-^l^'^^^LjEPmS^^^^^Mnr 

ll^Sj^Slmr' 

1^'      '^HP 

^^^^^S.1 

fW^'^^J 

American  Museum  of  Natural  History 

These  brook  lampreys  are  close  relatives  of  a 
larger  form  which  frequently  attacks  fish  and  remains 
as  a  temporary  external  parasite  until  the  host  is 
destroyed.  What  type  of  mouth  is  characteristic 
of  this  group  ? 


222      ORGANISMS   ILLUSTI\ATING  BIOLOGICAL   PRINCIPLES 

wander  through  the  underlying  tissues  of  the  host  until  in  the  spring 
of  the  year  they  come  to  lie  beneath  the  skin,  which  is  soon  punctured 
to  serve  as  an  air  vent.  Finally,  when  the  larvae  are  full  grown  they 
burrow  out,  fall  to  the  ground,  and  there  pupate,  finally  emerging 
as  adult  free  living  flies,  destined  to  ruin  many  million  dollars'  worth 
of  hides  annually. 


i>e;Comes 


larva  fall?  to 
drouncC ,  pupatss 


rnatas, 
Iccys 


becomes  j 

lodgecC 

xxndzr 

biole. 

tovaroC 

spring 


penetratss 
hide  of 
cattle   and  ,^^ 

tissue  during 
the.  vinter 


Life  cycle  of  the  ox  botfly. 


Periodic  Parasites.  Other  arthropods  definitely  fall  into  the 
group  of  those  that  are  periodically  parasitic.  Such  forms  are 
predators,  and  most  of  them  are  blood  suckers,  in  which  manner  they 
may  serve  as  a  link  in  a  chain  of  parasitism.  Thus  the  female  mos- 
quito serves  as  the  carrier  for  organisms  that  cause  malaria,  yellow 
fever,  and  filariasis.  Others  like  the  tick  or  rat  flea  may  not  only 
secure  a  meal  of  blood  from  one  host  but  at  the  same  time  be  the  means 
of  transmitting  Rocky  Mountain  spotted  fever  or  bubonic  plague  to 
some  other  host.  Certain  species  fall  into  the  realm  of  parasites  by 
their  own  right,  the  tick  and  botfly  clearly  belonging  in  this  latter 
group. 


thp:  art  of  parasitism 


223 


\\  ri(jhi  I'iirct 

Longitudinal  section  showing  mistletoe  invading 
the  tissues  of  its  host. 


Permanent  Parasites.  Comparatively  few  organisms  belong  in 
this  category.  Some  of  the  flukes  with  a  continuous  life  cycle  like 
the  marine  Epidella  melleni,  or  the  gill  fluke,  Ancyrocephalus,  pass 
their  entire  cycle  upon  the 
same  host,  adding  their 
progeny  to  the  same  ani- 
mal and  so  on  ad  infinitum. 
Similarly,  the  female  head 
louse  that  cements  her 
eggs,  or  "nits,"  to  human 
hair  from  which  newly 
hatched  lice  appear  within 
six  to  ten  days  is  another 
example  of  a  permanent 
parasite.  The  new  addi- 
tions to  the  community  of 
head  lice  must  soon  feed 
upon  the  roots  of  the  host's 
hair  or  else  they  will  die. 

The  parasitical  mistletoe  is  practically  permanent  in  habit,  since 
it  not  only  taps  the  life-giving  fluids  of  its  host  but  also  lives  for 
many  years  upon  the  same  tree. 

Internal  Parasites 

The  food  cycle  plays  a  vital  role  in  the  dispersal  of  all  internal 
animal  parasites.  It  frequently  happens  that  animals  which  suck 
the  juices  of  plants  or  the  blood  of  other  animals  play  an  important 
part  as  an  intermediate  host.  It  should  be  borne  in  mind  that  when 
carnivores  are  included  in  the  chain  of  parasitism,  the  cycle  tends 
toward  greater  complexity.  A  few  examples  will  serve  to  illustrate 
this  point. 

Parasites  Requiring  One  Host.  The  adult  hookworm,  Necator 
americanus,  lives  in  the  small  intestine  of  man,  where  the  adult  female 
is  attached  to  the  walls  of  the  intestine  and  produces  great  numbers 
of  eggs  which  are  eliminated  from  the  digestive  tract  in  the  early 
developmental  stages.  Under  proper  conditions  of  soil,  temperature, 
and  moisture,  development  of  the  larvae  proceeds  rapidly,  so  that 
hatching  may  take  place  within  24  hours.  The  small  larval  form  is 
only  about  0.25  mm.  in  length,  but  by  the  end  of  the  third  day  it  has 
nearly  doubled  in  length  and  soon  molts  twice,  then  being  in  the 


224       ORGANISMS   ILLUSTRATING  BIOLOGICAL   PRINCIPLES 

infective  stage.  Hookworm  larvae  may  enter  the  body  in  food  or 
drink,  but  the  normal  method  of  entry  is  by  actively  boring  through 
the  skin  of  the  human  hand  or  foot.  For  this  reason  the  disease  is 
called  "ground  itch"  because  of  the  inflamed  areas  caused  by  the 


to  stomach 


egg  passed, 
in    fexies 


larva  ready'' 
to  infect  -^ 
human  iTost 


Life  cycle  of  the  liookworm. 


r-yo 


entrance  of  the  larvae.  Once  liaving  effected  entrance  into  the  host, 
the  minute  worms  are  passively  carried  through  the  blood  stream  to 
the  heart  and  thence  to  the  lungs,  where  they  migrate  out  from  the 
capillaries  of  the  lungs  and  work  their  way  through  the  delicate 
walls  of  the  air  sacs  into  the  lung  cavities.     They  next  migrate  up 


THE  ART   OF   PARASITISM  22:, 

the  lung  passages  over  the  "saddle"  to  the  esophagus,  and  there 
are  swallowed,  reaching  the  stomach  and  eventually  the  intestine. 
Within  the  next  fortnight  two  more  molts  occur,  after  which  the 
parasites  reach  maturity,  copulate,  produce  eggs,  and  continue  the 
cycle. 

The  large  roundworm,  Ascaris  lumhricoides,  lays  eggs  which  de- 
velop into  infective  embryos  within  three  weeks  under  proper  con- 
ditions of  temperature  and  moisture.  After  reaching  the  digestive 
tract  of  the  host  together  with  food  or  drink,  the  newly  hatched 
larva  burrows  through  the  mucous  layer  and  starts  on  a  "10-day 
tour"  following  essentially  the  same  itinerary  as  that  of  the  hook- 
worm. 

Among  the  protozoa  the  Ameba,  Endamcha  histohjtica,  the  cause 
of  amebic  dysentery,  is  transmitted  from  one  human  host  to  another 
and  thence  to  the  outside  world,  and  back  again  to  the  human  large 
intestine  by  means  of  resistant  cysts  carried  in  contaminated  food 
and  drink. 

Parasites  Requiring  Two  Hosts.  The  dread  pork  roundworm, 
Trichinclla  spiralis,  while  a  permanent  parasite  having  a  relatively 
simple  life  cycle,  nevertheless  requires  two  hosts  to  complete  its  cycle. 
The  encysted  larvae  occur  in  a  variety  of  hosts,  but  are  normally 
secured  by  man  through  eating  insufficiently  cooked  pork.  The 
parasites  mature  rapidly  in  the  small  intestine  and  reproduce  within 
twenty-four  to  forty-eight  hours  of  their  arrival.  Each  viviparous 
female  produces  between  10,000  and  15,000  larvae,  which  are  depos- 
ited directly  in  the  lymph  or  capillaries  lining  the  intestine,  and  are 
thus  circulated  by  the  blood  until  they  reach  the  voluntary  muscles  of 
the  body.  There,  these  minute  roundworms  leave  the  blood  stream, 
enter  the  muscle  fibers,  where  within  a  month  a  lemon-shaped  cyst  is 
deposited  about  them.  Since  man  is  not  cannibalistic,  the  introduc- 
tion of  these  parasites  into  his  body  becomes  a  blind  alley  so  far  as 
completing  the  life  cycle  is  concerned.  Unfortunately,  when  these 
parasites  are  once  established  in  the  body,  there  is  no  way  of  getting 
rid  of  them.  In  due  course  of  time,  calcium  carbonate  is  deposited 
about  the  cyst  and  eventually  the  parasite  dies,  but  the  obnoxious 
cyst  remains  to  remind  the  infected  person  of  his  injudicious  meal 
by  frequent  muscular  pains  which  may  accompany  this  infection 
for  years.  The  normal  hosts  of  TrichineUa  seem  to  be  the  rat, 
mouse,  and  pig.  The  former  are  commonly  found  in  numbers  about 
slaughter  houses  and  the  percentage  of   their  infection  is  usually 


226      ORGANISMS   ILLUSTRATING  BIOLOGICAL  PRINCIPLES 


liigh.     A  great  number  of  other  animals  have  been  experimentally 
infected. 

Nearly  all  of  the  taenioid  tapeworms  have  a  rather  simple  life 
cycle.  In  the  case  of  the  beef  and  pork  tapeworms,  for  example,  the 
infective  stage  occurs  in  the  flesh  of  the  host  in  a  milky  white  cyst. 
When  this  larval  tapeworm,  or  cysticerus,  consisting  of  an  inverted 
head  or  scolex  and  its  outer  cyst  wall,  is  ingested  by  man,  the  head 


if  imppcfparly 
cooked. ,  pork  with 
"^  cysLs  In  musda. 
f  ibens  may  develop 
vhan  eaten  'by 


encysts   in   pork 
if  hog'  is  "host- 


vulva 


oviduct 


tacome  adults 

in  smoll  intestine, 

vithin  afev  dajs- 

■females  burro*^  into 

U5C  mucosa ,  depos-it 

over  10,000  larvae^ 


intestine 
of  hog- 


encyst  in  human 
Yntcscl©  if  man 
is  host 


larvae  enter  blooeC 
stream ,  are  carried 
to  vol  untoj'y  muscles 


•muscles  ^  ,  .  ,    , 

of  xnan  which  larvae  penetrate  and  then 

The  life  cycle  of  Trichinella. 

becomes  everted,  and  then  attached  to  the  intestinal  wall,  where  the 
worm  starts  budding  segments  or  proglottids  and  soon  reaches  sexual 
maturity.  Proglottids  of  Taenia  saginata,  or  proglottids  together 
with  free  eggs  in  the  case  of  T.  solium,  are  passed  with  the  feces  and, 
when  eaten  by  the  proper  intermediate  host,  develop  into  cysticerci. 
Cattle,  buffalo,  giraffes,  and  llamas  may  harbor  the  larval  form  of  the 
beef  tapeworm,  while  the  hog,  camel,  monkey,  dog,  and  man  are  the 
only  known  hosts  for  the  pork  tapeworm.  The  chief  difference 
between  the  cycle  of  these  two  parasites  centers  around  the  possibility 
of  auto-infection  in  the  case  of  the  latter.  This  occurs  by  ingesting 
the  eggs  destined  for  the  outside,  which  hatch  in  the  intestines, 


THE   ART   OF   PARASITISM 


227 


lorozoitsi 


ifeetecL 
sctUvory 
glcmcd 


gametoejtc 


migrate  to  the  blood  stream  and  so  reach  various  parts  of  the  body, 
there  producing  cysticerci.  As  in  the  case  of  Trichinella,  human 
infection  really  becomes  a  blind  alley  for  the  parasite. 

Malaria.  One  of  the  most  economically  important  parasites  is 
the  causative  organism  of  malaria,  a  minute  spore-forming  protozoan 
of  the  genus  Plasmodium. 
The  infective  stage,  or  spo- 
rozoite,  reaches  the  blood 
stream  of  man  in  the  saliva 
of  the  mosquito,  which  is 
poured  into  the  wound  im- 
mediately after  the  victim 
is  punctured.  This  minute 
parasite  promptly  pen- 
etrates a  red  corpuscle  and 
starts  to  de\'elop  asexually, 
growing  until  it  fills  about 
one  half  of  the  corpuscle. 
It  is  now  ready  to  undergo 
the  asexual  reproductive 
cycle.  The  chromatin  mate- 
rial is  gradually  separated 
into  a  number  of  tiny 
masses,  each  one  of  which 
finally  becomes  surrounded 
by  a  bit  of  cytoplasm. 
Growth  continues  until  the 
red  corpuscle  is  filled  with  e  eye  e  o 

a  number  of  new  indi\-iduals  called  merozoites.  Soon  the  corpuscle 
bursts,  liberating  these  merozoites,  each  one  of  which  seeks  out  a 
new  corpuscle  and  begins  the  asexual  cycle  all  over  again. 

This  asexual  cycle  recurs  regularly,  the  intervals  depending  upon 
the  species  of  parasite  infecting  the  blood  stream.  Thus  in  the  case 
of  tertian  malaria,  schizogony  is  completed  every  twenty-four  hours, 
while  in  the  quartan  type  it  takes  forty-eight  hours  to  complete  it. 
The  periodic  chills  and  fever  so  characteristic  of  malaria  occur  at 
the  time  of  the  bursting  of  the  red  corpuscles  with  the  subsequent 
release  of  the  asexually  formed  merozoites  and  the  accompanying 
waste  matter.  Quinine  is  the  most  widely  used  drug  to  combat  the 
infection  as  it  destroys  the  newly  "hatched"  merozoites. 


jneTO*oiteS^&S® 


228      ORGANISMS   ILLUSTRATING  BIOLOGICAL   PRINCIPLES 


After  a  number  of  asexual  generations  have  been  produced,  special 
larger,  sausage-shaped  crescents  appear  within  the  red  corpuscles. 
These  are  the  gametocytes,  or  sexual  forms.  If  a  female  mosquito 
sucks  blood  from  a  person  having  mature  male  and  female  stages  of 

the  parasite  in  the  blood,  such 
i^a,  parasites  are  taken  mto  the  diges- 

tive tract  of  the  mosquito,  where 
union  of  the  male  and  female 
gametocytes  takes  place.  After 
conjugation  the  resulting  zygote 
forms  an  ookinete  or  cyst  that 
enters  the  lining  of  the  stomach 
of  the  mosquito,  in  the  outer 
walls  of  which  a  complicated  de- 
velopment then  ensues  for  about 
twelve  days,  ending  with  the 
formation  of  a  large  number  of 
spindle-shaped  structures  called 
sporozoites.  The  cyst  then  bursts 
and  the  sporozoites  migrate  to  the 
salivary  gland  of  the  mosquito. 
After  that  time,  if  the  female  mos- 
quito bites  an  uninfected  human 
host  she  infects  him  with  the  sporo- 
zoites, which  enter  red  blood  cells. 
Animals  are  not  the  only  group 
having  complicated  parasitic 
cycles.  The  various  smuts,  mildews,  and  rusts  are  plant  parasites 
that  annually  take  their  toll  throughout  the  country.  Wheat  rust  is 
probably  one  of  the  most  destructive  of  the  parasitic  fungi.  This 
rust  has  been  the  most  dreaded  of  plant  diseases  because  it  destroys 
the  harvest  upon  which  the  civilized  world  is  most  dependent.  Wheat 
rust  has  long  been  associated  with  barberry  bushes.  As  early  as 
1760,  laws  were  enacted  in  New  England  providing  for  the  destruction 
of  barberry  bushes  near  wheat  fields,  although  nothing  was  actually 
known  of  the  relationship  between  the  barberry  and  rust  until  com- 
paratively recent  years.  It  is  now  known  that  wheat  rust  may  pass 
part  of  its  life  as  a  parasite  on  the  barberry,  whence  it  migrates  to  the 
wheat  plant  and  there  undergoes  a  complicated  life  history.  Since 
the  nourishment  and  living  matter  of  the  wheat  are  used  as  food  by 


Diagram  of  eggs,  larva,  pupa,  and 
adult  of  Culex  (left)  and  the  malarial 
carrying  Anopheles  (right).  How  could 
you  tell  the  eggs,  larvae,  and  adults  of 
these  two  genera  apart  ? 


THb:   AllT   OF   PAllASITISM 


229 


the  parasite,  the  plant  is  weakened  and  Httle  or  no  grain  is  produced. 
A  few  of  the  wheat  rusts  do  not  require  two  hosts  but  complete  their 
life  cycle  on  wheat  alone.  Such  rusts  pass  the  winter  by  means  of 
thick-walled  spores  which  may  remain  in  the  stubble  or  in  the  ground 
until  the  young  wheat 
plant  appears  the  follow- 
ing year,  or  the  spores  are 
carried  by  the  wind  from 
other  regions. 


Parasites  Requiring  More 
Than  Two  Hosts 

Tapeworms  show  a  va- 
riety of  adaptations  and 
exhibit  a  unique  and  deli- 
cate balance  that  permits 
the  completion  of  their 
various  cycles.  Roughly 
they  may  be  divided  into 
two  groups,  one  in  which 
the  eggs  reach  water,  sub- 
sequently passing  through 
some  aquatic  organism, 
and  a  second  in  which  ova 
are  scattered  in  the  soil 
and  reach  the  intermedi- 
ate host  by  means  of  food 
or  drink.  In  the  first 
group  are  the  broad  tape- 
worm of  man,  the  bass 
tapeworm,  and  many 
others,  while  the  second 
includes  the  various  tae- 
nioid  worms  and  their 
relatives.  All  of  these  par- 
asites show  a  remarkable 
degree  of  specialization. 


N.  Y.  State  Conservaiion  Depi. 
The  life  cycle  of  the  bass  tapeworm  {P.  arnblo- 
plilis).  (1)  The  mature  tapeworm  occurs  in  the 
intestines  of  the  large-  and  small-mouthed  hass. 
(2)  Contact  with  water  causes  the  proglottids 
to  liberate  the  eggs  which  are  eaten,  (.3)  by 
various  copepods.  \\  hen  infected  copepods  are 
eaten  by  many  species  of  plankton-feeding  fish 
(1)  a  larval  tapeworm  (plerocercoid)  develops  in 
the  mesenteries,  liver,  spleen,  or  gonads  of  these 
fish.  Heavy  infections  in  the  small-mouthed 
bass  affect  reproduction.  The  tapeworm  reaches 
maturity  when  fish  infected  with  the  larval  stage 
are  eaten  by  larger  ones.  How  could  this  cycle 
be  controlled  in  fish  hatcheries!' 


The  broad  tapeworm  of  man,  Diphyllobothrium  latum,  was  brought 
to  this  country  sometime  during  the  last  century  by  immigrants  from 
the  shores  of  the  Baltic  Sea.     The  worm  matures  in 


the  digesti\e 


230      ORGANISMS   ILLUSTRATING   BIOLOGICAL   PRINCIPLES 

tract  of  the  host,  producing  a  string  of  as  many  as  3000  to  4000  seg- 
ments or  proglottids,  often  reaching  a  length  of  ten  meters.  Mature 
proglottids,  passed  from  the  host  with  the  feces,  must  reach  water, 
where  the  eggs  are  shed.  After  a  developmental  period  in  the  water, 
the  eggs  hatch  into  free-swimming  larvae  (coracidia),  which  to  continue 
development  must  be  eaten  by  a  copepod.  The  parasites  penetrate 
the  intestinal  wall  and  so  reach  the  body  cavity  of  this  host,  where 
they  develop  until  the  copepod  is  in  turn  eaten  by  a  fish,  when  they 
usually  penetrate  to  the  flesh  of  the  host  and  grow  to  approximately 
six  millimeters  in  length.  Various  fishes,  such  as  the  northern  pike, 
Esox  lucius,  wall-eyed  pike,  Stizostedion  vitreum,  sand  pike,  S.  cana- 
dense  griseum,  as  well  as  the  burbot,  Lota  maculosa,  may  all  serve 
as  second  intermediate  hosts  for  this  important  parasite.  Man  and 
other  carnivores  acquire  the  infection  by  eating  improperly  cooked 
fish. 

The  bass  tapeworm  which  matures  in  large-  and  small-mouthed 
black  bass  also  requires  three  hosts  —  copepods,  small  fishes  which 
carry  the  larval  stage  encysted  in  the  viscera,  and  the  final  host,  or 
adult  bass.  The  life  cycle  of  this  parasite  illustrates  very  clearly 
the  interdependence  of  organisms  necessary  for  the  completion  of  the 


Adult  yellow  grub,  enlarged 
from  mouth  cavity  of  hejxn« 


•i>;^@  e-Maturtegg    ^o^'  — "— "^^--tS 


N.  Y.  State  Conservation  Dept. 

Diagram  of  the  life  cycle  of  the  yellow  grub  of  bass  (C.  marginatum).  (1)  The 
adult  fluke  in  buccal  cavity.  (2-4)  Embryo  within  egg  hatches  as  free  living 
miracidium  which,  upon  entering  snail,  produces  a  mother  sporocyst  and  two 
generations  of  rediae  (5-8),  cercariae  (8-9),  liberated  by  the  daughter  redia, 
penetrate  many  species  of  fish  (10-11)  and  mature  when  eaten  by  various 
herons  (12). 


THE   ART  OF   PARASITISM 


2:{i 


cycle.  The  adult  tapeworm  matures  sexually  in  the  spring  of  the 
year,  the  mature  eggs  being  shed  into  shallow  water  where  the  fishes 
come  inshore  to  spawn.  The  eggs  of  the  parasites  are  soon  eaten  by 
copepods  and  the  developmental  period  necessary  for  the  larval 
parasite  to  reach  its  second  infective  stage  is  closely  correlated  with 
the  time  interval  between  the  laying  of  the  bass  eggs  and  the  absorp- 
tion of  the  yolk  sac  of  the  bass  fry.  At  the  time  the  young  fishes 
begin  feeding  upon  plankton,  the  copepods  in  the  vicinity  of  bass 
nests  are  found  to  be  much  more  heavily  parasitized  than  at  other 
seasons  of  the  year.  It  is  adaptations  such  as  these  which  enable 
parasites  to  complete  complex  life  cycles. 

Flukes,  or  trematodes,  probably  undergo  more  complicated  cycles 
than  any  other  group  of  parasites.  In  considering  the  complex 
cycle  of  a  trematode  one  should  keep  in  mind  that  there  are  usually 


Diagram  explaining  the  life  cycle  of  endoparasitic  trematodes. 

two  free-living  stages,  —  the  miracidium  and  the  cercaria.  The 
variations  that  may  be  expected  in  such  a  cycle  are  apparent  upon 
inspecting  the  above  diagram. 

The  frequent  presence  of  a  second  intermediate  host  suggests  a 
characteristic   of  most  trematodes.     For  example,  the  great  blue 

H.    W.    H.  —  16 


232      ORGANISMS  ILLUSTRATING  BIOLOGICAL  PRINCIPLES 

heron  harbors  an  adult  fluke,  Clinostomum  marginatum,  in  its  mouth 
cavity.  Eggs  discharged  by  the  parasite  reach  the  water  and  soon 
hatch,  the  miracidia  penetrating  snails.  After  several  generations 
in  the  snail,  fork-tailed  cercariae  emerge  to  penetrate  under  the 
scales  into  the  flesh  and  sometimes  on  the  fins  of  many  species 
of  fresh-water  fish.  Here  they  grow  into  the  typical  yellow  grubs 
commonly  found  surrounded  by  a  cyst  formed  by  the  connective 
tissue  of  the  host. 

As  a  result  of  the  above  discussion  of  parasitism  it  is  hoped  that 
some  concept  of  the  elaborate  food  chains  and  interrelationships  and 
interdependence  characteristic  of  the  various  groups  of  parasites  and 
their  hosts  may  be  gained.  Because  these  relationships  are  so  com- 
plicated and  form  so  intricately  woven  a  pattern,  it  becomes  prac- 
tically impossible  "to  predict  the  precise  effects  of  twitching  one 
thread  in  the  fabric." 

SUGGESTED  READINGS 

Cowdry,  E.  V.,  et  al.,  Human  Biology  and  Racial  Welfare,  P.  B.  Hoeber,  1930. 

Ch.  XVII. 

Popular  discussion,  resistance,  etc.,  from  the  bacteriological  point  of 

view. 
Elton,  C.,  Animal  Ecology,  The  Macmillan  Co.,  1935.     Chs.  V,  VI. 

Excellent  readable  discussion  of  parasitism  from  an  ecological  view- 
point. 
Massee,  George,  Diseases  of  Cultivated  Plants  a>}d  Trees,  The  Macmillan  Co., 

1910,  pp.  1-23,  59-77. 

A  good  discussion  of  parasitic  plants. 
Needham,  J.  G.,  Frost,  S.  W.,  Tothill,  B.,  Leaf-Mining  Insects,  The  Williams 

&  WUkins  Co.,  1928.     Ch.  I. 

Deals  with  natural  history  of  group. 
Smith,  T.,  Parasitism  and  Disease,  Princeton  University  Press,  1934. 

Excellent  general,  but  somewhat  technical,  discussion  of  the  parasitic 

habit. 


XIII 


ADVANTAGES  OF  BEING  A  VERTEBRATE 

Preview.  Vertebrate  cliaracteristics  •  Skeletons  •  Invertebrate  attempts  • 
The  vertebrate  endoskeleton  •  Suggested  readings. 

PREVIEW 

How  fortunate  it  is  that  we  are  vertebrates,  not  only  vertebrates 
in  general  but  mammalian  vertebrates  of  the  royal  primate  line  which 
has  blossomed  finally  into  human  beings  ! 

When  one  thinks  over  the  myriads  of  lowly,  less  endowed  animals 
scattered  along  the  devious  highways  of  evolution,  who  might  have 
been  our  near  relatives,  it  is  a  real  privilege  to  claim  relationship 
with  such  highly  endowed  primates  as  monkeys  and  apes.  With 
the  inclusive  vertebrate  type,  to  say  nothing  of  the  specialized  Pri- 
mates, there  are  certain  outstanding  structures  and  qualities  which 
we  as  mankind  are  thankful  to  possess.  They  are  so  famihar  to  us, 
however,  that  we  are  apt  to  forget  how  far  our  fortunate  biological 
heritage  is  dependent  upon  them. 

Only  a  few  of  these  distinctive  vertebrate  characteristics  that  give 
us  occasion  for  self-congratulation  may  be  pointed  out  here.  A 
consideration  of  the  Vertebrates  as  such  forms  a  biological  science 
in  itself,  set  forth  in  a  voluminous  Hbrary  of  descriptive  and  inter- 
pretative books. 

Vertebrate  Characteristics 

Even  a  partial  list  of  the  distinctive  vertebrate  endowments 
would  include  the  following:  1,  a  highly  developed  nervous  system, 
based  upon  a  hollow  dorsally-located  nerve  cord  ;  2,  a  unique  embry- 
onic skeletal  axis,  called  the  notochord,  which  is  the  foundation  for  a 
living  internal  skeleton,  adaptable  to  the  changing  demands  of  growth  ; 
3,  a  peculiar  kind  of  blood,  that  in  the  higher  forms  makes  the  mainte- 
nance of  a  constant  body  temperature  possible  regardless  of  the  sur- 
rounding temperature  ;  4,  various  devices  for  effectually  transporting 
the  blood  to  every  part  of  the  body,  devices  that  are  as  superior  to  the 
methods  employed  by  non-vertebrates  as  modern  highways  and 
means  of  transportation  are  better  than  the  conditions  encountered 
in  the  days  of  the  trackless  wilderness ;    and  5,  locomotor  organs  for 

233 


234      ORGANISMS  ILLUSTRATING  BIOLOGICAL  PRINCIPLES 

getting  about  on  land,  in  water,  and  in  air,  far  surpassing  those 
employed  by  lower  animals. 

Skeletons 

Let  us  consider  briefly  just  one  vertebrate  feature,  namely,  the 
living  inside  skeleton,  which  gives  the  name  "vertebrate"  to  the  group. 
It  is  the  culmination  of  an  endless  array  of  experiments  and  adapta- 
tions that  have  been  going  on  since  the  beginnings  of  life  on  this 
planet,  and  there  is  every  indication  that  the  end  is  not  yet.  The 
skeleton  of  man,  for  example,  is  by  no  means  the  final  mechanism  of 
its  kind.  There  are  to  be  expected  in  the  future  other  models  nearer 
to  perfection,  though  based  upon  all  that  has  gone  before. 

Invertebrate  Attempts 

The  microscopic  protozoa  made  brave  experiments  with  the  idea 
of  a  skeleton,  in  their  case  an  armor  mostly  for  protective  pur- 
poses and  consequently  found  located  on  the  outside  of  the  animal 
itself.  In  fact,  protection  seems  to  be  the  primary  service  of  skeletal 
structures  in  general,  although  secondarily  supplanted  largely  by  the 
function  of  support  and  muscle  attachment.  It  still  plays  an  important 
role  even  in  the  vertebrates,  since  the  brain  and  cord,  being  ex- 
tremely liable  to  injury,  are  enclosed  within  a  protective  skull  and 
enveloping  vertebral  arches,  while  the  viscera  are  in  part  stowed  away 
within  a  bony  thoracic  basket. 

In  the  great  group  of  arthropods,  that  includes  both  crustaceans 
and  insects,  the  skeleton  is  plainly  a  protective  external  covering 
which,  being  a  lifeless  excretion  of  the  skin,  does  not  change  in  size 
after  it  has  been  laid  down.  As  the  anima'l  grows,  the  dead  inelastic 
skeletal  armor  thus  formed  fits  more  and  more  tightly  over  the 
enlarging  body  until  finally  it  has  to  crack  open  in  order  that  the 
animal  may  emerge  and  become  refitted,  after  an  interval  of  rapid 
bodily  expansion,  with  a  new  and  larger  skeletal  garment.  This 
complicated  process  is  called  molting.  To  elaborate  and  then  periodi- 
cally to  reject  all  this  material  is  not  only  a  physiologically  expensive 
performance  but  it  is  also  a  hazardous  one,  since  while  shifting  into 
a  more  commodious  suit  of  armor,  the  animal  may  lose  a  leg  or  two, 
and  is  always  exposed  for  some  time  to  enemies  while  in  its  defenseless 
shell-less  condition. 

Insects,  caught  in  the  same  evolutionary  blind  alley  with  their 
crustacean  cousins,  have  taken  an  upward  step  by  secreting  a  much 


ADVANTAGES   OF  BEING  A   VERTEBRATE  235 

thinner  chitinous  envelope  than  the  more  cumbersome  "crust"  of  the 
crustaceans.  Instead  of  molting  at  repeated  intervals  throughout 
life,  they  have  hit  upon  the  idea  of  metamorphosis,  whereby  they  do 
all  their  molting  early  during  the  growing  larval  stages.  Then,  as 
adults  of  established  and  unchanging  size,  they  live  happily  ever 
after  without  being  troubled  by  the  inconveniences  and  perils  of 
growth  within  an  unada])tive  external  encasement. 

Another  and  paramount  objection  to  a  protective  exoskcleton  is 
the  increasing  burden  of  a  heavy  armor  which  soon  becomes  insup- 
portable, necessitating  a  limit  to  the  size  of  the  body  encased  within 
it.  The  largest  known  representative  of  the  enormous  group  of  the 
insects  is  probably  smaller  even  than  the  smallest  adult  vertebrate. 

The  mxoUuscs  have  gone  at  the  problem  of  evolving  a  skeleton  in 
another  way.  Although  the  skeleton  is  still  on  the  outside,  excreted 
and  consequently  lifeless,  it  is  never  wastefully  molted  after  the 
crustacean  fashion.  The  parsimonious  molluscs  keep  every  particle 
of  their  old  dead  shells  and  simply  add  new  layers  on  the  inside,  as 
growth  demands.  The  layers,  being  a  little  more  extensive  with  each 
addition,  form  by  their  edges  the  familiar  "lines  of  growth"  showing 
as  parallel  ridges  on  the  outside  of  the  shell.  This  particular  experi- 
ment in  skeletons,  however,  has  cost  the  group  of  molluscs  dear,  for 
the  heavy  shell,  together  with  the  accompanying  policy  of  passive 
defense,  has  either  impeded  the  power  of  locomotion  with  all  attendant 
advantages  that  would  accrue  for  the  evolution  of  the  nervous  system, 
or  has  brought  about  its  complete  abandonment.  The  clams  and 
their  allies,  therefore,  have  stuck  conservatively  in  the  mud  and 
lagged  behind  in  the  race  for  life,  while  other  animals  without  the 
incubus  of  a  molluscan  shell  have  toiled  successfully  on  to  higher 
levels  of  attainment  in  working  out  their  organic  salvation. 

The  Vertebrate  Endoskeleton 

The  vertebrates  alone  exploit  a  fundamentally  different  model  in 
skeletal  structure. 

An  increase  in  size  being  necessary  for  dominance  in  the  struggle  for 
existence,  an  adequate  supporting  scaffolding  for  the  body  is  de- 
manded, and  as  a  result  the  skeletal  function  of  protection  now  be- 
comes secondary.  Levers  and  muscles  to  work  them  to  attain 
locomotion,  with  ample  skeletal  surface  for  their  attachment,  are  also  in- 
dispensable for  animals  that  are  to  develop  a  successful  nervous  sys- 
tem.    The  vertebrate  skeleton  provides  for  these  adaptive  advances. 


236      ORGANISMS  ILLUSTRATING  BIOLOGICAL  PRINCIPLES 

The  fact  that  the  vertebrate  skeleton  is  inside  the  body  makes  it 
a  changeable  living  structure  which,  by  reason  of  its  capacity  for 
continuous  growth,  keeps  pace  with  the  increasing  demands  of  the 
enlarging  organism.  With  the  introduction  of  such  a  scheme  of 
mechanical  support,  the  ban  upon  size  imposed  by  a  lifeless  exoskele- 
ton  is  lifted,  so  that  during  the  Age  of  Mesozoic  monsters  there  were 
dinosaurs  and  similar  beasts,  for  example,  that  were  able  to  lift  tons 
of  flesh  into  the  air  upon  majestic  bony  scaffoldings.  These  prehis- 
toric giants  proved  impracticably  large,  however,  and  vanished 
forever  from  the  face  of  the  earth  after  recording  by  means  of  their 
fossil  remains  the  results  of  these  colossal  experiments  in  the  mech- 
anism of  living  inside  skeletons.  There  still  remain  today,  elephants 
on  land  and  whales  in  water  as  living  illustrations  of  how  far  it  is 
possible  to  go  in  the  matter  of  size  when  an  adequate  living  internal 
support  is  provided. 

The  remarkable  superiority  of  the  vertebrate  endoskeleton  over 
all  other  skeletal  devices  is  evident.  It  would  be  possible  to  go  much 
further  and  to  unfold  some  of  the  marvelous  details  of  adaptation 
which  every  separate  part  of  the  vertebrate  skeleton  presents.  That 
would  call  for  many  pages.  It  is  the  task  of  the  comparative  anatomist 
to  assemble  and  elucidate  the  innumerable  facts  about  the  vertebrate 
plan  of  structure,  of  which  those  involved  in  the  skeleton  are  a 
sample,  and  to  point  out  wherein  we  are  fortunate  to  be  constructed 
as  we  are.  This  is  a  fertile  field,  inviting  intellectual  adventure  for 
those  who  have  the  curiosity  to  explore  it. 

SUGGESTED  READINGS 

Adams,  L.  A.,  An  Introduction  to  the  Vertebrates,  John  Wiley  &  Sons,  Inc., 

1933. 

A  fine  text. 
Keith,  Sir  A.,  The  Engines  of  the  Human  Body,  J.  B.  Lippincott  Co.,  1919. 

Parallels  intriguingly  worked  out  for  the  mechanically  minded. 
Neal,  H.  V.,  and  Rand,  H.  W.     Comparative  Anatomy,  P.  Blakiston's  Son 

and  Co.,  1936. 

Written  by  two  masters  of  the  subject. 
Newman,  H.  H.,  Vertebrate  Zoology,  The  Macmillan  Co.,  1920. 

Just  what  the  title  indicates. 
Walter,  H.  E.,  Biology  of  the  Vertebrates,  The  Macmillan  Co.,  1928. 

Many  illustrations.     Bibliography. 
Wilder,  H.  H.,  History  of  the  Human  Body,  Henry  Holt  &  Co.,  1923. 

Told  with  literary  grace  without  sacrifice  to  accuracy. 


THE   MAINTENANCE  OF  THE   INDIVIDUAL 


XIV 


THE  ROLE  OF  GREEN  PLANTS 

Preview.  Structure  of  green  plants  •  The  raw  food  materials  used  by 
plants  •  The  root  and  its  work  •  The  stem,  structure  and  function  •  The 
structure  of  the  leaf  •  How  green  plants  make  food ;  carbon  dioxide  as  raw 
material ;  the  role  of  water ;  chlorophyll  and  light ;  relation  of  artificial 
light  to  food  making ;  what  goes  on  in  the  green  leaf  in  sunlight ;  chemistry 
of  food  making  •  Enzymes  and  their  work  •  How  food  is  used  by  the  plant 
body  •  Respiration  •  Transpiration  •  The  rise  of  water  in  plants  •  Produc- 
tion of  oxygen  by  plants  •  Suggested  readings. 

PREVIEW 

It  is  a  trite  statement  to  say  that  the  destiny  of  man  on  the  earth 
depends  upon  green  plants.  All  living  stuff  is  made  up  of  the  ele- 
ments found  in  air,  in  water,  and  under  the  earth's  surface,  yet 
no  laboratory  technician  has  ever  been  able  to  put  this  material 
together  and  make  protoplasm.  That  energy  is  displayed  by  plants 
and  animals  is  obvious,  but  no  man  has  ever  been  able  to  energize 
matter  and  create  a  living  organism.  We  know  that  the  units  of 
structure,  the  cells,  do  release  energy  and  that  this  energy  comes,  as 
does  all  other  energy,  from  the  oxidation  of  fuel  substances.  Such 
fuels  used  by  living  things  we  call  foods.  Moreover,  these  foods, 
be  they  from  plant  or  animal,  in  the  long  run  depend  upon  energy 
derived  from  the  sun.  The  Biblical  declaration  that  ''all  flesh  is 
grass"  is  literally  true,  for  without  green  plants  animals  would  have 
no  food. 

We  do  not  think  of  plants  as  very  dynamic  objects  compared  with 
animals.  Nevertheless,  if  we  look  at  the  soil  pushed  up  by  growing 
seeds,  the  pavement  broken  by  the  growth  of  trees,  and  even  the 
hardest  rocks  split  apart  by  the  wedge  action  of  growing  stems  and 
roots,  we  realize  that  plants  are  very  much  alive.  They  respond  to 
the  various  stimuli  in  the  environment,  reacting  like  animals  to 
temperature  changes,  to  gravity,  to  various  chemical  substances,  or 
to  the  directive  force  of  currents  of  water. 

Unlike  animals,  whose  metabolism  is  catabolic,  the  green  plant's 
metabolism  is  more  completely  anabolic.     It  builds  up  materials  to 

237 


238  THE  MAINTENANCE  OF  THE  INDIVIDUAL 

a  greater  extent  than  it  tears  them  down  in  the  metabolic  process. 
Compare  the  growth  of  an  animal  with  that  of  a  plant  such  as  a  big 
tree.  While  the  animal  is  more  fixed  in  size  and  limited  in  age,  the 
tree  grows  for  a  longer  period  of  time  and  grows  to  a  greater  size. 
These  differences  are  due  to  a  continuous  growth  of  the  meristemic 
tissue  already  mentioned,  and  also  to  the  fact  that  new  tissues  and 
organs  grow  continuously  from  this  area  of  meristem  that  is  found  in 
growing  buds,  stems,  and  roots.  The  most  important  difference 
between  green  plants  and  animals  lies  in  the  fact  that  the  green  plant 
can  make  use  of  the  sun's  energy  to  manufacture  foodstuffs  on  which 
not  only  it,  but  also  the  animals  which  eat  it,  depend. 

In  an  investigation  of  a  living  green  plant,  two  methods  of  study 
present  themselves.  We  can  rather  carefully  dissect  and  study  each 
system  of  structures  which  makes  up  the  living  organism  and  direct 
our  attention  to  the  microscopic  make-up  of  each  part.  In  this  way 
a  fairly  complete  picture  will  be  had  of  the  organism  in  its  entirety. 
But  such  a  picture  will  lack  vitality.  If  the  plant  is  a  living  thing, 
then  why  not  study  it  from  the  point  of  view  of  function,  of  what  it 
does  and  how  it  lives,  using  only  so  much  reference  to  the  structures 
as  will  make  intelligible  the  work  of  the  parts  of  the  plant?  This  is 
the  viewpoint  adopted  for  this  unit.  The  plant  is  to  be  thought  of  as 
a  living,  working  organism,  performing  the  same  metabolic  functions 
as  any  other  organism,  but  in  addition  doing  a  different  kind  of  work 
from  that  of  animals,  that  of  synthesizing  organic  foodstuffs  out  of 
chemical  raw  materials  from  the  air,  the  soil,  and  the  absorbed  water. 
This  unit,  then,  will  bring  up  a  number  of  important  points.  Among 
them  will  be  such  questions  as  these :  What  are  the  adaptations 
which  enable  the  green  plant  to  do  its  work?  Where  does  the  raw 
material  from  which  food  is  manufactured  come  from  and  how  does 
the  plant  get  it  ?  Under  what  conditions  is  the  work  of  food  manu- 
facture performed?  Where  is  food  made  and  how  does  it  get  to  the 
cells  where  work  is  done?  Why  is  light  necessary  for  green  plants 
and  why  do  they  bleach  in  the  darkness?  Are  green  plants  really 
as  important  as  is  here  indicated  ?  These  and  similar  questions  will 
be  answered  in  the  pages  that  follow. 

Structure  of  Green  Plants 

It  is  not  easy  to  give  a  general  description  of  a  green  plant.  In  the 
higher  plants  it  is  obvious  that  there  are  several  well-defined  regions 
which  are  called  root,  stem,  leaves,  flowers,  and  fruits.     In  these 


Wright  Pierce 


(1)  Eucalyptus  trees,  natives  of  Australia ,  which  have  found  California  so  well 
suited  to  their  needs  that  they  are  a  dominant  form  there.  (2)  Rose,  shrub  showing 
bushy  habit.  (3)  Snapdragon,  a  common  annual.  (4)  Carrot,  a  biennial;  note 
the  food  storage  in  the  root.     Of  what  value  is  this  to  the  plant  ? 

239 


240 


THE   MAINTENANCE  OF  THE   INDIVIDUAL 


e  e  i 


regions  several  different  methods  of  growth  occur  which  will  be 
described  later.  Some  plants  that  grow  more  or  less  continuously, 
forming  a  woody  body  which  resists  cold  and  storm,  are  called  trees 
or  shrubs.  Others  die  down  at  the  end  of  the  year,  although  they 
have  some  wood  fiber  in  the  body.  These  are  the  herbaceous  plants, 
examples  of  which  are  peas,  beans,  and  a  variety  of  garden  plants  and 

roadside  weeds.    Herbaceous  plants 
DiCotyledoiv     MoaocotyledoR      that  produce  seeds  and  die  before 

the    following    winter    are    called 
annuals. 

A  second  group  of  herbaceous 
plants,  called  biennials,  store  food 
in  the  roots  or  underground  portion 
of  the  stem.  After  the  upper  part 
of  such  a  plant  is  cut  down  by  un- 
favorable weather  conditions,  the 
following  spring  the  underground 
portions  send  up  a  new  shoot  from 
the  subterranean  food  supply.  This 
gives  rise  to  flowers  and  seeds  at 
the  close  of  the  second  year.  Ex- 
amples of  biennials  are  carrots, 
parsnips,  and  beets. 

A  third  type  of  herbaceous  plants 
is  the  perennial,  which  grows  each 
spring  from  the  underground  parts 
that  remain  alive  during  the  winter. 
Many  of  our  common  weeds  have 
this  habit,  which  makes  them 
difficult  to  eradicate. 

Woody  plants,  such  as  trees  and 
shrubs,  as  we  have  seen  in  the 
unit  on  classification,  are  grouped  either  as  conifers  (the  softwoods, 
pines,  firs,  hemlocks,  and  their  relatives)  or  as  deciduous  hardwoods. 
The  latter  are  placed  with  the  flowering  plants,  and  may  be  either 
monocotyledons  or  dicotyledons.  These  two  groups  have  differences 
in  the  structure  of  leaf,  stem,  and  seed.  The  monocotyledons  usu- 
ally have  parallel-veined  leaves,  like  those  of  grass  or  lily.  Their 
stems  have  scattered  "closed"  woody  vascular  bundles  and  a  single 
cotyledon  in  the  seed.     The  dicotyledons  have  netted-veined  leaves, 


iS  tern 

Differences  between  monocoty- 
ledons and  dicotyledons;  c,  cotyle- 
don ;  e,  endosperm ;  fb,  fibrovascular 
bundles;    h,  hypocotyl;    p,  plumule. 


THE   ROLE  OF  GREEN   PLANTS  241 

sudh  as  are  seen  in  the  elm,  oak,  or  sassafras;  stems  with  "open" 
vascular  bundles  which  usually  appear  as  a  ring  of  growing  tissue; 
and  seeds  with  two  cotyledons  or  seed  leaves.  These  structures  will 
be  referred  to  in  more  detail  later. 

The  Raw  Food  Materials  Used  by  Plants 

For  a  good  many  centuries  after  the  time  of  the  Greek  philosophers 
who  first  hold  this  theory,  it  was  thought  that  green  plants  absorbed 
food  from  the  soil,  but  it  was  not  until  the  time  of  the  Belgian  philoso- 
pher van  Helmont,  who  lived  in  the  sixteenth  century  (1577-1644), 
that  it  was  clear  that  water  played  a  very  important  part  in  the 
growth  of  a  plant.  One  of  van  Helmont's  experiments  consisted  of 
placing  a  willow  slip  weighing  five  pounds  in  a  vessel  containing  two 
hundred  pounds  of  dried  soil.  For  five  years  he  watered  the  tree  with 
distilled  water,  making  careful  observations  on  it  until  it  had  grown 
to  be  a  sapling  weighing  one  hundred  and  sixty-nine  pounds  and  three 
ounces.  But  when  he  weighed  the  soil  in  which  the  tree  had  grown, 
he  found  it  had  lost  only  two  ounces.  Clearly  then,  the  gain  came 
largely  from  sources  other  than  soil,  and  he  rightly  concluded  that 
water  was  largely  responsible  for  the  increase  in  weight.  In  the  first 
half  of  the  eighteenth  century,  an  English  clergyman,  Stephen  Hales, 
worked  out  the  daily  water  consumption  of  a  plant  by  ascertaining 
the  relation  between  leaf  and  stem  surface  and  the  quantity  of  water 
absorbed.  He  went  a  step  further  than  van  Helmont  in  suggesting 
that  plants  take  something  from  the  air  as  well  as  the  soil  with  which 
to  build  up  their  body  material.  In  1779,  Ingen-Housz,  a  Dutch  phy- 
sician, who  was  a  co-worker  with  the  famous  surgeon,  John  Hunter, 
showed  that  the  green  part  of  a  plant,  when  exposed  to  light,  uses  the 
free  carbon  dioxide  of  the  atmosphere,  but  that  it  does  not  have  this 
power  when  kept  in  darkness.  A  little  later,  in  1804,  de  Saussure, 
by  a  series  of  experiments,  proved  that  carbon  dioxide  and  water 
were  both  used  by  plants  in  the  sunlight  and  that  as  carbon  dioxide 
was  taken  from  the  atmosphere,  about  the  same  amount  of  oxygen 
was  returned  to  it.  He,  however,  missed  the  use  of  the  green  coloring 
matter  of  the  leaf  in  its  connection  with  the  sun's  energy  in  building 
living  matter  and  food.  The  real  explanation  of  the  function  of  this 
green  substance  (chlorophyll)  was  left  for  Julius  von  Sachs,  a  famous 
botanist  of  the  nineteenth  century.  He  was  the  first  investigator  to 
demonstrate  the  fact  that  green  plants  make  food  for  the  world.  Just 
how  they  do  this  is  still  not  fully  known,  although  plant  physiologists 


242  THE   MAINTENANCE   OF  THE   INDIVIDUAL 

have  been  experimenting  and  are  still  experimenting  in  the  attempt 
to  solve  the  problem. 

With  this  background,  our  point  of  view  is  to  consider  the  living 
green  plant  as  an  organism,  faced  by  the  same  kinds  of  problems  as  a 
living  animal,  taking  a  living  from  its  environment,  storing  up  food 
for  the  inevitable  time  of  food  shortage,  and  eventually  forming 
fruits  to  hold  the  seeds  which  are  necessary  to  pass  the  stream  of  life 
on  to  the  next  generation.  Unlike  an  animal,  the  green  plant  takes 
raw  food  materials  from  its  environment  and,  under  certain  favorable 
conditions,  synthesizes  them  into  organic  foods,  a  process  effected 
by  means  of  a  number  of  adaptive  structures,  in  certain,  favorable 
environmental  conditions,  the  chief  of  which  is  sunlight. 

By  burning  the  body  of  a  hving  plant  until  nothing  but  ash  remains, 
and  then  making  a  careful  analysis  of  this  residue,  frequently  as  many 
as  thirty  chemical  elements  are  found.  Twelve  are  nearly  always 
present,  eight  of  which  are  essential  to  plant  growth.  The  latter  are 
boron,  calcium,  iron,  magnesium,  manganese,  phosphorus,  potassium, 
and  sulphur.  It  will  be  noticed  that  this  list  does  not  agree  exactly 
with  the  previous  list  of  elements  usually  found  in  the  protoplasm  of 
Hving  things  (page  131),  but  the  implication  is  clear.  The  chemical 
elements  found  in  living  matter,  as  previously  noted,  are  also  found 
in  rocks  or  soil,  air,  and  water.  The  stage  is  set  and  it  remains  for 
the  scientist  to  discover  just  how  these  elements,  found  in  the  environ- 
ment, can  be  made  into  food  and  living  stuff  by  the  green  plant. 

A  good  many  experiments  have  been  made  with  plants  to  determine 
more  exactly  the  function  of  these  elements.  It  has  been  shown 
that  if  green  plants  are  placed  in  a  nutrient  solution  containing  the 
necessary  elements,^  growth  will  take  place.  If,  however,  certain 
elements  are  subtracted  from  the  solution,  the  plants  will  not  develop, 
or  their  growth  will  be  considerably  slowed  down.  Such  experiments 
give  us  our  first  clue  to  one  important  use  of  the  root.  It  is  evidently 
an  absorbing  organ  through  which  the  plant  takes  in  not  only  water, 
but  some  of  the  essential  mineral  materials  necessary  for  its  growth. 

1  A  list  of  the  most  commonly  used  nutrient  solutions  for  plant  growth  are  given  below. 

Crone's  solution  :   Water,  2.0  1. ;  KNO3,  1.0  g. ;  FeP04,  0.5  g. ;  CaS04,  0.25  g. ;  MgSOj,  0.25  g. 

Detmer's  solution:  Water,  1000  g. ;  Ca(N03)2,  1.0  g.;  KCl,  0.25  g. ;  MgS04,  0.25  g. ;  KH2PO4, 
0.25  g. ;  FeClj,  trace. 

Knop's  solution:  Water,  1000  g. ;  Ca(N03)2,  1.0  g. ;  KNO3,  0.25  g. ;  KH2PO4,  0.25  g. ;  MgS04, 
0.25  g. ;  FeP04,  trace. 

Pfeffer's  solution :  Water,  3-7  1. ;  Ca(N03)2,  4  g. ;  KNO3,  1  g. ;  MgS04,  1  g. ;  KH2PO4,  1  g. ;  KCl, 
0.5  g. ;  FeCls,  trace. 

Sach's  solution  :  Water,  1000  g. ;  KNO3,  1.00  g. ;  NaCl,  0.50  g. ;  CaS04,  0.50  g. ;  MgS04,  0.50  g. ; 
Ca3(P04)2,  0.50  g. ;  FeCls,  0.005  g. 


THE  ROLE   OF   GREEN   PLANTS 


243 


The  Root  and  Its  Work 

Recent  experiments  made  by  Weaver  ^  and  others  show  that  plants 
have  extremely  comphcated  root  systems.  The  roots  of  an  old  oat 
plant,  for  example,  although  extending  through  only  about  two  cubic 
yards  of  soil,  were  found  to  have  a  total  length  of  over  450  feet. 
Weaver  found  that  hardy  wheat  plants  sent  their  rootlets  into  the 
soil  six  feet  below  the  surface 


^,CeJ7tml  Cylinder- 
-_>v&ocf^  bundle 


-root  "hciiT~ 


ictermis 


of  the  ground.  In  the  bush 
morning-glory,  a  common 
plant  of  the  mid-western 
plains,  the  roots  may  extend 
ten  feet  into  the  ground  and  a 
distance  of  twenty-five  feet 
away  from  the  parent  plant. 
The  roots  of  corn  extend 
laterally  three  to  four  feet 
from  the  stem  and  sometimes 
over  seven  feet  into  the  soil. 
All  this  is  evidence  for  the 
great  importance  of  the  root 
as  an  absorbing  organ. 

Examination  of  longitudinal 
sections  cut  from  growing 
roots  shows  that  the  body  of 
a  root  is  made  up  of  a  central 
woody  cylinder  surrounded 
by  layers  of  softer  cells,  collec- 
tively called  the  cortex.  Over 
the  lower  end  of  the  root  is 
found  a  collection  of  cells, 
most  of  which  are  dead,  ar- 
ranged in  the  form  of  a  cap 

covering  the  growing  tip.  As  the  root  pushes  through  the  soil,  the 
outer  cells  of  this  root  cap  are  sloughed  off,  and  are  rapidly  replaced 
by  growing  cells  of  meristem  that  are  just  under  the  root  cap.  The 
root  cap  proper  is  evidently  a  protective  adaptation.  In  the  woody 
region  of  the  root  are  vascular  tissues  consisting  of  xijlc77i  and  phloem. 
These  tissues  form  a  series  of  tubelike  structures  which  together  with 


.  >■-' J~OOt/ 


cctp 


Root  of  a  dicotyledon,  greatly  magnified. 
Find  the  functions  of  each  part  labeled. 
How  might  soil  water  get  from  the  outside 
of  the  plant  into  the  woody  bundles.'* 


'  Weaver,  Root  Development  of  Field  Crops,  McGraw-Hill  Book  Co. 


244 


THE  MAINTENANCE  OF  THE  INDIVIDUAL 


strong  supporting  woody  cells  constitute  the  vascular  bundles  that 

put  the  root  in  connection  with  the  stem  and  leaves  above  it. 

If  mustard  seeds,  for  example,  are  germinated  in  a  moist  chamber, 

a  few  days  after  germination  the  lower  part  of  the  root  will  be  found 
to  be  covered  with  a  delicate,  fuzzy  growth.  Ex- 
amination of  the  root  at  this  stage  shows  an  actively 
growing  area  of  meristem,  an  elongating  zone  of  tissue 
directly  back  of  it,  with  a  zone  of  maturing  tissue 
above,  which  together  make  a  zone  of  growth  coincid- 
ing more  or  less  directly  with  an  area  covered  with 
fuzzy  structures  known  as  the  root  hairs. 

Root  hairs  vary  in  length  according  to  their  posi- 
tion on  the  root,  the  longer  ones  being  found  at  some 
distance  from  the  tip.  They  are  outgrowths  of  the 
outer  layer  of  epidermis.  A  single  root  hair  examined 
under  the  compound  microscope  is  found  to  be  a 
threadlike,  almost  colorless  structure.  The  delicate 
cellulose  wall  is  lined  by  the 
protoplasm  of  the  cell,  the 
outer  layer  of  which  forms  a 
selectively  permeable  mem- 
brane.    Inside  the  root  hair 

are  found  numerous  vacuoles  filled  with  cell 

sap.     A  nucleus  is  always  present  and  may  be 

found  in  the  basal  part  of  the  cell,  or  in  the 

hairlike   portion   itself.     The  root  hairs  are 

evidently  living  epidermal  cells. 

An  examination  of  a  young  plant  growing 

in  moist  soil  shows  that  the  root  hairs  reach 

out  between  the  particles  of  soil,  apparently 

being  closely  cemented  or  attached  in  places 

to  them.     Each  particle  of  soil  is  surrounded 

by  a  delicate  film  of  water,  which,  with  the 

dissolved  minerals  found  in  it,  is  absorbed 

into  the  root  hair  by  the  process  of  osmosis. 

The  wall  of  the  root  hair  is  covered  with  a 

delicate  layer  of  mucilagelike  pecten  formed 

by  the  outer  layer  of  the  cell  wall  and  is  also 

in  contact  with  the  moist  protoplasm  within 

the  cell,  which  forms  a  delicate  membrane 


Root  hairs  of 
corn,  showing 
their  relation  to 
the  root  tip. 


Root  hair,  showing  its 
relation  to  an  epidermal 
cell.  How  do  you  account 
for  the  attachment  of  the 
soil  particles  to  the  surface 
of  the  root  hair  ? 


THE   ROLE  OF   GREEN   PLANTS  245 

just  under  the  wall.  Diffusion  takes  place  following  the  laws  of 
osmosis,  according  to  which  water  passes  through  a  selectively  per- 
meable membrane  from  a  point  of  its  greater  to  a  point  of  its  lesser 
concentration.  This  means  that  water  passes  from  the  soil  into  the 
cell  sap,  which  has  a  higher  concentration  of  solutes  than  does  the 
water.  Since  the  cell  sap  within  the  root  hair  has  received  a  greater 
quantity  of  water,  it  in  turn  becomes  a  point  of  higher  concentration 
of  water  than  the  cells  lying  next  to  it  interiorly,  and  consequently, 
the  flow  continues  from  these  outer  cells  to  the  adjoining  cells  which 
have  a  higher  concentration  of  solutes.  In  this  manner  water  is 
passed  through  the  cells  of  the  root  i)ito  the  woody  cylinder  inside 
the  cortex.  Once  having  reached  this  region  it  passes  up  the  tubes 
into  the  stem  and  on  into  the  leaves  as  will  be  shown  later. 

The  Stem,  Structure  and  Functions 

In  thinking  of  the  tree  as  a  li^'ing  organism,  we  are  not  so  much 
concerned  with  the  internal  structure  of  the  stem  as  with  the  way  it 
functions.  For  many  centuries  it  has  been  known  that  water  passes 
up  through  the  wood.  If  a  tree  is  girdled  —  that  is,  if  a  narrow  strip 
of  bark  extending  inward  as  far  as  the  wood  is  removed  —  the  tree  will 
keep  its  leaves  for  some  time,  indicating  the  upward  passage  of  water 
which  keeps  them  from  wilting.  If,  however,  a  strip  of  wood  directly 
under  the  bark  is  removed,  enough  of  the  bark  being  left  intact  to 
allow  for  passage  of  fluids,  the  leaves  will  wilt  within  a  very  few  mo- 
ments. A  cut  branch  of  apple  or  willow  placed  in  red  ink  after  a  few 
hours  shows  by  a  red  circle,  visible  in  sections  cut  across  the  stem, 
that  the  colored  water  has  passed  up  through  the  outer  layers  of  the 
new  wood. 

In  order  to  understand  better  the  pathways  for  the  rise  of  sap  in 
the  dicotyledon  stem,  one  must  study  its  growth.  When  seen  in 
cross  section,  the  vascular  tissues  of  such  stems  are  arranged  in  a  circle. 
In  some  herbaceous  stems,  the  woody  bundles  are  separated  by  a 
parenchyma,  but  in  trees,  shrubs,  and  a  good  many  herbs,  the  bundles 
are  united  to  form  a  complete  ring  around  the  stem.  These  vascular 
bundles  are  open  at  each  side  and  grow  more  or  less  continuously 
from  a  single  row  of  meristem  or  embryonic  cells  which  form  a  layer 
around  the  stem.  This  layer  is  called  the  cambium,  and  the  growth 
of  the  wood  and  bark  of  our  large  trees  is  due  to  the  activity  of  this 
always  youthful  layer  of  cells  which,  like  the  cells  of  embryonic  tissue, 
continually  divide  and  multiply  to  form  internally  the  xylem  or  wood 


246 


THE  MAINTENANCE  OF  THE  INDIVIDUAL 


and  externally  the  phloem  tissue.  In  the  spring  when  this  tissue  is 
very  active,  it  forms  a  soft  layer  of  cells  that  allows  of  the  easy  sepa- 
ration of  bark  from  wood,  a  fact  well  known  to  any  small  boy  who 
has  made  a  willow  whistle. 

It  is  not  necessary  to  go  into  the  details  of  stem  structure,  except 
to  note  that  the  cambium  layer  gives  rise  each  year  to  new  layers  of 


Cross  section  of  stem  of  Ricinus  communis,  a  dicotyledon,  showing  cambium 
ring.     In  what  area  of  the  diagram  does  growth  take  place  ? 

tissues,  both  internally  and  externally.  The  inner  layers  made  up 
of  secondary  xylem  are  from  the  annual  rings  of  a  tree.  In  spring 
the  growth  of  the  tissue  is  rapid,  while  in  winter  it  is  very  slow  indeed 
or  stops  entirely,  thus  making  the  differences  in  the  cross  section 
shown  in  the  figure.  As  the  tree  ages,  changes  may  be  noticed  in  the 
appearance  of  the  older  woody  area  forming  the  interior  of  the  trunk. 
This  wood  becomes  darker  in  color,  its  chemical  composition  changes, 
and  it  loses  its  ability  to  conduct  water.  It  is  known  as  the  heart- 
wood  as  distinguished  from  the  outer  rings  of  wood  called  sap-wood. 


THE  ROLE  OF   GREEN   PLANTS 


247 


The  latter  conducts  water,  while  the  heart-wood  functions  merely  as 
a  supporting  tissue.     As  the  tree  increases  in  diameter,  the  area  of 

,_bark 

.-Cambium 
layer- 
annual 


pith 
rays 


Section  through  a  dicotyledonous  stem.     Explain  its  method  of  growth. 

heart-wood  increases  while  the  sap-wood,  although  greater  in  cir- 
cumference, gets  proportionately  smaller  in  extent. 

The  bark,  or  area  outside  the  cambium,  is  made  up  of  several 
different  tissues,  which  have  a  somewhat  different 
arrangement  in  conifers  than  in  deciduous  trees.  The 
area  known  as  phloem  is  formed  immediately  outside 
the  cambium.  This  area  contains  many  living  sieve 
tubes  through  which  elaborated  food  is  carried  down 
from  the  upper  part  of  the  plant.  The  sieve  tubes 
in  the  conifers  are  more  or  less  regular  in  arrangement 
while  in  deciduous  trees  they  are  scattered.  In  both 
stems  they  are  all  surrounded  by  parenchyma. 
Scattered  through  the  bark  of  deciduous  trees  are 
masses  of  tough,  stringy  schlercnchyma  cells  of  two 
types,  phloem  fibers  —  fibrous,  elongated  cells  that 
give  strength  and  elasticity  to  the  trunk  —  and  thick- 
walled,  hard  stone  cells.  Outside  the  latter  area  is 
formed  the  corky  layer,  produced  by  a  layer  of  growing 
cells  known  as  the  cork  cambium.  Cork  cells,  which 
have  their  walls  impregnated  with  an  insulating  sub- 
stance called  suherin,  are  of  great  value  to  the  tree 
because  they  prevent  a  rapid  loss  of  water  from  the 
tissues.  It  is  this  layer  in  the  Spanish  cork  oak  which 
is  of  commercial  value.  In  some  trees,  such  as  the 
redwoods,  the  bark  forms  a  coating  highly  resistant 
to  fire. 

H.  w.  H.  — 17 


Above,  sieve 
\  ('  s  s  e  1  (of 
phloem)  with 
c()mf>anion  cell; 
below,  sieve 
plate,  with 
section  of  com- 
panion cell. 
(After  Stras- 
burger.) 


248 


THE  MAINTENANCE  OF  THE   INDIVIDUAL 


Wright  Pierce 

The  characteristic  lenticels  of  the  white  birch 
{Betiila  populi folia).  Note  the  placement  of  the 
lenticles. 


Scattered  over  the  surface  of  twigs  and  young  tree  trunks  are 

found  many  lenticels,  openings  in  the  corky  layer  which  become  filled 

with  loose  masses  of  cells. 
They  are  found  both  on 
roots  and  stems  and  act 
as  pores  which  allow  for 
the  exchange  of  gases  be- 
tween the  living  cells  of 
the  cortex  and  the  me- 
dium outside.  Lenticels 
are  often  spoken  of  as 
"breathing  jDores"  and 
experimental  evidence 
seems  to  make  this  title 
valid. 

As  the  stem  or  trunk 
of  a  tree  grows  larger  in 
diameter,  there  is  an  in- 
creasing  area   that   uses 

water  and  foods.     Cells  cannot  grow  without  food,   and  food  in  a 

growing  plant  cannot  be  made  without  water.     The  structures  which 

put  the  water-conducting  tissue  of 

the  inside  of  the  stem  in  connection 

with  the  phloem  of  its  outer  part 

are  known  as  vascular  rays.     They 

may  be  seen  in  almost  any  cross 

section    of    a     tree     which     has 

produced    secondary    xylem    and 

phloem.     Here  the  cambium  has 

rows    of    irregularly    placed    cells 

that  instead  of  forming  xylem  and 

phloem  produce  ingrowing  masses 

of  more  uniform  parenchymatous 

cells    making     vertically     placed 

strings    of    tissue.     These    bands 

act      as      conducting      pathways 

for    water    from    the    xylem    to 

the    phloem    and    also    as   chan- 
nels for  elaborated  food  from  the  phloem  to  the  xylem,  thus  dis- 
tributing these   materials  to  the  growing  trunk.     Experiments  by 


phi 


.oem 


(i:am\:)i.imri 


\  — •^yiein 


pith 


Note  the  bands  of  living  parenchym- 
atous tissue  that  grow  inward  toward 
the  pith. 


THE  ROLE  OF   GREEN   PLANTS 


219 


Aiichtor  ^  have  shown  that  food  and  water  are  not  transferred  from 
one  side  of  a  tree  to  the  other,  but  instead  that  ahnost  all  of  the 
water  taken  in  is  used  directly  above  where  it  is  absorbed,  while 
food  passes  down  from  the  leaves  on  the  same  side  of  the  tree.  There 
is  seemingly  little  cross  transfer  of  food  or  water  in  a  plant  stem. 

Vascular  rays  must  not  be  confused  with  the  so-called  pith  rays 
which  are  formed  in  herbaceous  stems  such  as  Ranunculus  or  in  the 
stem  of  Clematis  where,  as  the  primary  wood  bundles  grow  in  the  pith, 
the  pith  forms  narrow  plates  between  the  bundles.  These  appear  as 
the  pith  rays  in  a  cross  section. 

Conditions  of  growth  upon  which  the  passage  of  food  and  water 
depend  differ  in  monocotyledons  from  those  in  dicotyledons.  If  a 
stalk  of  celery  or  asparagus  is  placed  in  red  ink  over  night,  the  color 
is  seen  to  be  located  in  little  fibrous  bundles  of  tissue  which  are  scat- 
tered throughout  the  stem.  If  such  a  stained  stem  is  examined  in 
cross  section  under  the  microscope,  it  is  found  to  be  made  up  of  pa- 
renchyma or  pith  which  is  dotted  with  little  groups  of  woody  cells 
of  irregular  size  and  shape.     These  are  the  vascular  bundles  which, 


Transverse  section  of  stem  of  corn,  a  monocotyledon,  showhiK  the  "  scattered  " 
vascular  bundles  which  are  cut  in  cross  section. 


■  Auchter,  E.  C. 
in  Woody  Plants?' 


"  Is  There  Normally  a  Cross  Transfer  of  Foods,  Water,  and  Mineral  Nutrients 
Univ.  Maryland  .\gric.  Exp.  Station,  Bull.  251,  Sept.  1923. 


250 


THE   MAINTENANCE   OF  THE   INDIVIDUAL 


instead  of  being  located  in  a  ring  as  in  the  dicotyledons,  are  scattered 
through  the  pith  although  more  concentrated  toward  the  outer  edge  of 
the  stem.  Examination  of  this  outer  edge  or  rind  shows  that  there 
is  no  true  bark,  but  that  this  outer  area  is  made  up  of  these  same 
woody  bundles  closely  massed  together.  Under  high  power,  the 
bundles  are  seen  to  have  outer  strengthened  walls  of  wood  cells 

enclosing  tubelike  cells  of 
different  diameters  of 
which  the  larger  have 
pitted  surfaces.  The  area 
containing  these  tubes  is 
the  xylem.  Other  elon- 
gated tubular  cells  having 
their  ends  perforated  with 
small  holes  like  a  sieve, 
form  the  sieve  tubes, 
w^hich  are  the  conducting 
tissues  of  the  phloem.  In 
the  phloem,  the  tubes  pass 
foods  down  from  the 
leaves,   while   the    xylem 

A  cross  section  through  a  closed  monocotyle-  carries  water  up  from  the 
donous  bundle.  Note  that  the  thick-walled  roots  to  the  leaves.  The 
xylem  cells  completely  enclose  the  cells  of  the    entire  WOody  bundle  is  en- 

^   °^™'  closed  w^th  a  tough  wall  of 

sclerenchyma  which  gives  strength  and  resiliency  to  the  stem.  Since 
this  hard  tissue  binds  the  entire  bundle,  it  is  called  a  closed  bundle. 
Monocotyledonous  stems  grow,  then,  through  an  increase  and 
lengthening  of  closed  bundles  in  the  parenchyma  of  the  stem. 

The  end  result  in  both  monocotyledon  and  dicotyledon  stems  is  the 
same.  The  vascular  bundles  put  the  root,  stem,  and  leaves  in  direct 
communication.  The  root  hairs  at  one  end  and  the  cells  of  the  leaf 
at  the  other  end  are  the  opposite  terminals  of  long  communicating 
woody  tubes.  These  tubes  carry  water  and  solutes  up  from  the  soil 
to  the  cells  of  the  leaf,  and,  as  will  be  shown  presently,  carry  elaborated 
food  materials  down  from  the  leaves  to  various  parts  of  the  plant, 
where  they  may  be  stored  for  future  consumption  or  used  immediately 
to  liberate  the  energy  needed  in  growth  and  in  destructive  metabolic 
changes.  The  vascular  bundles  which  leave  the  stem  to  enter  the 
leaves  do  so  by  way  of  the  petiole  or  leaf  stalk.     As  they  enter  the  blade 


THE   ROLE   OF   CxREEN   PLANTS 


251 


of  the  leaf,  they  branch  into  bundles  of  ever  smaller  and  smaller 
diameter  to  form  the  veins  of  the  leaf.  In  the  monocotyledonous 
leaves,  these  veins  run  in  a  more  or  less  parallel  direction  as  seen  in 
grass  blades  or  palm  leaves.  In  the  case  of  the  dicotyledonous  plants 
characteristic  irregular  and  netted  veins 
are  found,  reminding  one  of  the  branch- 
ing of  the  capillaries  in  the  human  body. 
These  veins  are  made  up  structurally 
of  tracheids  and  tracheal  vessels,  ser\'ing 
as  water-conducting  tissues ;  sieve  tubes, 
which  carry  out  food  materials  from  the 
leaf;  and  supporting  tissue,  which 
makes  up  the  mechanical  framework  of 
the  veins.  Thus  the  veins  act  as  a  sup- 
porting skeleton  for  the  leaf  as  well  as 
conduits. 

The  Structure  of  the  Leaf 


The  outer  covering  of  the  leaf  (epi- 
dermis) is  composed  of  a  layer  of 
irregularly  shaped  cells,  usually  rather 
flattened.  In  some  plants,  like  the 
mullein,  these  cells  are  prolonged  into 
hairs,  or  again  the  layer,  as  a  whole,  is 
frequently  covered  with  a  waxy  cuticle 
which  is  impermeable  to  gases  and 
water.  The  under  surface  of  the  leaf, 
as  seen  through  the  compound  micro- 
scope, shows  many  tiny  oval  openings, 
which  are  called  stomata.  The  position 
of  the  stomata  varies  in  different  leaves. 
Some  plants,  as,  for  example,  water 
lilies,  whose  leaves  float  on  the  surface 
of  the  water,  have  them  in  the  upper 
epidermis.  Others  have  them  on  the 
under  .side,  and  .still  others  have  them 
on  both  surfaces.  The  estimated  num- 
ber of  these  openings  varies.  Mac- 
Dougal  estimates  that  as  many  as  two 
million  are  on  the  under  surface  of  an 


Stomata  from  the  loaf  of  an 
Easter  lily  (Lilium  lonyiflorum) : 
Above,  a  stoma,  as  seen  in  sur- 
face view,  showing  the  two 
kidney-shaped  guard  cells  {g), 
which  enclose  the  stomatal  aper- 
ture (s),  the  more  deeply  shaded 
portion  representing  the  central 
slit ;  note  the  chloroplasts  in  the 
guard  cells;  (b)  subsidiary  cells. 
Below,  a  stoma,  as  seen  in  cross 
section  ;  note  the  guard  cell  (g) 
next  to  the  subsidiary  cell  (6) ;  the 
outer  slit  (o)  is  enclosed  between 
the  cutinized  outer  guard-cell 
ridges  (r),  the  enlarged  area  just 
below  being  the  outer  vestibule 
(o') ;  below  the  central  slit  (s)  is 
the  inner  vestibule  (('),  which 
here  opens  directly  into  the 
cavity  (c)  underneath  the  stoma. 


252 


THE   MAINTENANCE   OF  THE   INDIVIDUAL 


oak  leaf  of  ordinary  size,  while  four  or  five  hundred  thousand  to  a 
leaf  is  a  common  estimate.  Surrounding  the  opening  of  each  stoma 
are  found  two  kidney-shaped  cells,  the  guard  cells,  which  can  easily 
change  their  shape  under  certain  conditions.  They  are  of  great 
importance  in  the  life  of  the  plant,  since  they  control  to  a  great 
extent  the  amount  of  moisture  that  may  be  lost  from  the  leaf's  sur- 
face. The  guard  cells  are  noticeably  greener  than  the  epidermal  cells, 
the  color  being  due  to  many  tiny  green  chloroplasts. 

If  the  leaf  is  cut  in  cross  section  and  examined  under  the  microscope, 
it  will  be  found  to  be  made  up  largely  of  a  tissue  known  as  mesophyll. 

Lying  close  to  the  epi- 
dermis are  one  or  two 
layers  of  elongated  cells 
with  the  long  axis  placed 
at  right  angles  to  the  sur- 
face of  the  leaf.  These 
layers  of  cells  are  collec- 
tively called  the  palisade 
layer.  Each  cell  of  this 
layer  contains  numerous 
chloroplasts  which  are 
found  in  the  protoplasm 
close  to  the  cell  wall.  It 
has  been  estimated  that  a 
square  inch  of  a  sunflower 
leaf  contains  as  many  as 
thirty  million  of  these 
chloroplasts,  which  are 
most  important  structures 
in  the  plant  so  far  as  food 
making  is  concerned. 
Below  the  palisade  layer 
is  a  layer  of  numerous  irregular  cells  containing  fewer  chloroplasts. 
These  cells  are  known  collectively  as  the  spongy  parenchyma.  Be- 
tween them  are  found  air  spaces  connected  with  the  exterior  of  the 
leaf  through  the  stomata.  We  have  already  noted  that  the  veins 
form  the  framework  of  the  leaf  and  in  a  cross  section  are  often  found 
occupying  part  of  the  area  of  spongy  parenchyma.  These  veins 
connect  the  vascular  tissue  of  the  root  and  stem  with  the  leaf.  The 
petiole,  or  leaf  stalk,  is  made  up  largely  of  vascular  and  supporting 


Cross  section  through  a  leaf;  e,  upper  epider- 
mis, e',  lower  epidermis,  showing  stomata  (s) ; 
I,  intercellular  spaces  in  the  spongy  parenchyma. 
Note  the  cross  section  of  the  vein  (v).  Why  is 
the  palisade  layer  (p)  so  placed  ? 


THE   ROLE   OF   GREEN   PLANTS 


253 


woody  tissue.  At  one  point  on  the  petiole,  usually  close  to  the  main 
stalk,  a  little  time  before  the  leaves  drop  from  deciduous  trees  in  the 
fall,  a  layer  of  delicate,  thin-walled  cells  is  formed  which  extends 
completely  across  the  petiole.  This  is  called  the  separation  or  ab- 
scission layer,  and  it  is  at  this  point  that  the  leaf  is  cast  off. 

How  Green  Plants  Make  Food 

The  general  biologist  is  concerned  not  so  much  with  the  structure 
of  the  organism  or  with  detailed  minutiae  as  with  the  general 
metabolism  of  an  organism  as  a  whole.  He  wants  to  know  how  plants 
and  animals  act  as  living  things,  both  alone  and  in  relation  to  each 
other.  We  have  examined  the  green  plant  from  the  standpoint  of 
structure  and  are  ready  to  consider  it  as  an  organic  whole,  as  a  living 
organism  that  releases   en- 


leof  on  live  plant 
+  light- 


boilecL 

■— r^  in  -^oodi 
■\>^  alcoViol 


positive   reaction 
"where  sta.r-cVi  was 
locctte<:C 


ergy,  respires,  feeds,  repro- 
duces, and  in  time  dies. 
But  we  must  remember  that 
in  addition,  the  green  plant 
makes  food,  and  it  is  this 
process  upon  which  we  will 
now  focus  our  attention. 

It  is  a  relatively  simple 
matter  to  prove  that  sun- 
light is  necessary  for  starch 
making  in  a  leaf.  Place  a 
healthy  green  plant  in  dark- 
ness for  a  couple  of  clays. 
Then  pin  strips  of  black 
cloth  over  parts  of  some  of  the  leaves  and  expose  the  plant  to  bright 
sunlight  for  a  few  hours.  Later,  remove  the  leaves  and  boil  them  to 
soften  the  tissues,  adding  alcohol  to  extract  the  chlorophyll,  and 
finally,  place  them  in  a  solution  of  iodine.  A  blue  color  will  appear  in 
those  parts  of  the  leaves  exposed  to  sunlight,  while  the  covered  areas 
will  be  colorless.  The  appearance  of  the  blue  color  in  the  presence  of 
iodine  is  the  regular  test  for  starch,  thus  showing  clearly  that  sunlight 
is  necessary  for  starch  making. 

Another  simple  experiment  may  be  performed  to  show  that  air  is 
also  a  necessary  factor.  Place  a  healthy  green  plant  in  darkness  for 
two  or  three  days,  then  carefully  smear  vaselin(>  on  th(>  ui)i)(>r  and 
lower  surface  of  two  or  three  l(?aves,  leaving  the  others  uiitoiiclicd. 


Proof  that  light  is  necessary  for  starch  forma- 
tion in  green  leaves. 


254  THE   MAINTENANCE   OF  THE   INDIVIDUAL 

Place  the  plant  in  full  sunlight  for  a  few  hours,  then  remove  the 
vaselined  and  untouched  leaves,  and  treat  both  in  the  manner  de- 
scribed in  the  last  experiment.  The  leaves  to  which  no  air  penetrated 
will  be  shown  to  have  no  starch. 

The  need  of  carbon  dioxide  in  the  process  of  starch  making  may  also 
be  demonstrated  by  a  relatively  simple  experiment.  If  plants  are 
grown  under  similar  conditions  in  two  bell  jars,  but  in  one  case  the 
carbon  dioxide  in  the  atmosphere  is  removed  by  means  of  soda  lime, 
while  the  other  plant  is  left  in  the  bell  jar  containing  normal  air,  the 
latter  continues  to  grow  while  the  one  lacking  carbon  dioxide  does 
not  increase  in  size. 

By  burning  a  plant  in  a  hot  flame,  it  can  be  ultimately  reduced  to 
mineral  ash  equaling  about  4  to  5  per  cent  of  the  entire  weight.  Ac- 
cording to  Raber,  from  1  to  55  per  cent  of  the  plant  is  consumed, 
while  from  40  to  95  per  cent,  roughly  speaking,  consists  of  water. 
Since  a  green  plant  is  immobile  and  since  it  has  no  way  of  obtaining 
material  except  from  the  air,  water,  and  the  soil  that  surrounds  it, 
it  may  be  safely  assumed  that  if  food  is  found  in  the  plant  body, 
it  must  be  made  there.  That  foods  are  found  in  plants  is  common 
knowledge.  We  eat  roots,  stems,  fruits,  and  leaves  of  plants.  Grains 
form  our  staples  of  food.  Roots  and  various  types  of  fruits  form 
part  of  our  dietary,  while  herbivorous  animals  live  upon  grasses  and 
fodder  crops.  This  brings  us  then  to  the  sources  of  the  raw 
materials  out  of  which  these  elaborated  foods  must  be  formed. 

Carbon  Dioxide  as  Raw  Material 

Carbon  dioxide  is  not  only  a  product  of  respiration  of  animals  but  of 
plants  as  well.  A  man  gives  off  about  nine  hundred  grams  of  carbon 
dioxide  daily  into  the  air.  Carbon  dioxide  also  gets  into  the  air  from 
the  combustion  of  inflammable  materials.  Volcanic  eruptions  and 
other  sources  of  combustion  increase  the  amount,  while  decaying 
organisms  and  the  oxidation  of  rocks  and  soils  add  a  very  appreciable 
amount  daily  to  the  store.  While  it  is  estimated  that  there  are  only 
two  grams  of  carbon  in  each  ten  liters  of  air,  nevertheless  the  fact 
that  carbon  dioxide  is  universally  available  in  the  air  and  oceans  close 
to  the  surface  of  the  earth  shows  that  it  may  readily  be  made  use  of 
by  growing  plants.  Its  need  in  food  manufacture  is  well  illustrated 
by  the  statement  that  the  world  crop  of  wheat  requires  annually  one 
hundred  and  fourteen  million  tons  of  carbon  dioxide  in  order  to  pro- 
duce the  seventy  million  tons  of  carbohydrates  which  form  this  crop. 


Wrijjla  Pierce 

The  role  of  water.  Upper  photograph :  The  Mohave  River  near  Victorville. 
This  river  rises  in  the  San  Bernardino  Mountains  and  loses  itself  in  a  desert  sink. 
What  effect  does  it  have  upon  the  desert  .►> 

Lower  photograph:  An  irrigated  orange  ranch  in  the  desert  near  Clareniont, 
California.  Thousands  of  acres  of  trees  now  grow  where  desert  conditions 
existed  before  irrigation. 

255 


256  THE   MAINTENANCE   OF  THE   INDIVIDUAL 

The  Role  of  Water 

Water  as  a  raw  material  needs  little  mention.  The  soil  always  con- 
tains more  or  less  water,  and  the  original  source  of  water  in  its  cycle 
through  the  oceans,  the  air,  the  clouds,  and  rain  gives  the  earth  a 
never  ending  water  supply.  When  mm  aids  Nature  in  carrying 
water  to  dry  areas  by  irrigation  the  desert  literally  is  made  "to  blos- 
som as  the  rose."  Certain  chemical  elements  find  their  way  into  the 
plant  body  with  this  water.  If  the  green  plant  is  to  manufacture 
organic  food  substances,  it  is  evident  that  the  elements  carbon,  oxy- 
gen, and  hydrogen  must  come  from  the  water  and  air.  Various 
mineral  salts,  taken  in  by  the  root,  furnish  the  necessary  amounts  of 
calcium,  iron,  potassium,  sodium,  and  other  elements,  which  leaves 
only  nitrogen  to  be  accounted  for.  Although  nitrogen  makes  up 
approximately  four  fifths  of  the  atmosphere,  it  is  nevertheless  unusable 
in  that  free  form.  It  is  an  extremely  inert  gas  and  does  not  unite 
readily  in  combination  with  other  substances.  By  means  of  the  proc- 
ess of  decay,  however,  and  particularly  through  the  nitrogen-fixing 
bacteria  found  on  the  roots  of  certain  types  of  plants,  this  highly  im- 
portant element  is  made  available  to  plants.  So  much  for  the  raw 
materials.     Now  let  us  turn  to  the  machinery  of  food  manufacture. 

Chlorophyll  and  Light 

Common  observation  shows  that  there  is  a  relation  between  light 
and  the  green  color  of  plants.  We  are  familiar  with  the  bleaching  of 
celery  stalks,  with  the  curious  blanched  elongated  shoots  of  a  potato 
which  sprouts  in  darkness,  and  with  the  fact  that  young  seedlings  are 
devoid  of  chlorophyll  until  after  they  have  sprouted.  Seedlings 
grown  with  light  coming  from  one  side  turn  to  the  source  of  light,  while 
plants  grown  in  a  dark  box  having  a  hole  on  one  side  work  their  way 
toward  the  light.  Obviously  light  has  a  very  potent  effect  on  the 
plant. 

Sunlight  passed  through  a  prism  is  broken  up  into  seven  primary 
colors  ranging  from  violet  to  red,  but  passed  through  a  spectroscope 
shows  numerous  dark  lines  traversing  different  areas  in  the  spectro- 
prism.  The  most  conspicuous  are  used  as  landmarks  by  physicists 
and  for  convenience  have  been  designated  by  the  letters  A  to  H  by 
Fraunhofer,  their  discoverer.  These  several  wave  lengths  of  light  can 
be  measured  and  it  has  been  fovmd  that  they  vary  from  0.00076  mm. 
at  the  red  end  of  the  spectrum  to  0.00039  mm.  at  the  violet  end. 


THE    HOIJ-:   OF   r.REF^.N    PLVNTS  257 

Rays  of  greater  and  shorter  length  are  also  found  at  eaeh  end  of  the 
spectrum  forming  the  ultraviolet  and  infrared  portions.  The  heat 
of  light  rays  varies,  Ijcing  greater  at  the  r(>d  end  of  the  spectrum. 
Since  all  life  depends  upon  this  I'adiant  energy  whose  source  is  the 


1        z      ^  4  I  n  m  EA 

When  a  green  leaf  is  placed  in  the  path  of  light  passing  through  a  {)risni.  dark 
strips  appear,  due  to  the  partial  or  conipleh^  blocking  of  the  light  energy.  These 
are  shown  in  the  absorption  spectra  above.  .4,  chlorophyll  of  Alliumiirsi- 
mim  in  alcohol;  B,  chlorophyll  of  English  ivy  {Iledera  helix)  in  alcohol; 
C,  chlorophyll  of  OscUlatoria  in  alcohol;  D,  carotin.  1,  2,  3,  4.  absorption  bands 
of  chlorophyll;  /,  //,  III.  absorption  bands  of  carotin;  EA,  end  absorption. 
The  lettered  broken  lines  mark  the  position  of  the  principal  absorption  hnes  of  the 
solar  spectrum  (Fraunhofer  lines);  the  numbered  solid  lines  form  a  scale  from 
which  wave  lengths  (X)  in  nullionths  of  a  millimeter  may  be  found  by  adding 
a  cipher;  note  the  increasing  dispersion  from  left  (red)  1o  right  (violet). 
(After  Kohl.) 

sun,  the  green  plant  is  no  exception  to  this  rule.  Certain  parts  of 
the  plant,  however,  are  more  susceptii)le  than  other  portions  to  ra- 
diant energy.  While  the  green  leaf  as  a  whole  needs  sunlight,  it  is 
only  chlorophyll  in  the  chloroplasts  that  is  al:)le  to  utilize  it  for  food 
making. 

If  a  chloroplast  is  examined  under  a  very  high  magnification  of  the 
microscope,  it  is  found  to  be  a  mass  of  living  matter  somewhat 
denser  than  the  protoplasm  surrounding  it.  In  its  disk-shaped  struc- 
ture the  green  coloring  matter  is  arranged  around  the  outer  part  of 
the  chloroplast,  while  the  central  portion  usually  contains  a  clear  area 


258 


THE   MAINTENANCE  OF  THE   INDIVIDUAL 


filled  with  fluid.  Chlorophyll  is  a  very  complex  protein,  apparently 
made  up  of  two  substances  known  as  Chlorophyll  A,  having  the  chemi- 
cal formula  C55H7205N4Mg,  and  Chlorophyll  B,  C55H7o06N4Mg. 
It  is  found  to  be  somewhat  like  the  hemoglobin  of  the  human  blood 
except  that  it  has  an  atom  of  magnesium  instead  of  iron  and  the 
property  of  fluorescence,  its  color  being  different  in  transmitted  or 
reflected  light.  Chlorophyll  in  solution,  when  extracted  from  the 
leaf  by  means  of  alcohol,  appears  green  as  light  passes  through  it,  but 
red  when  light  is  reflected  from  it  against  a  black  background.  Other 
pigments  are  closely  associated  with  chlorophyll,  a  group  of  yellow 

pigments  called  carotins, 
which  give  the  yellow 
color  to  carrots  and  other 
fruits  or  vegetables,  and 
xmithophylls,  pigments 
that  help  give  color  to 
leaves  in  the  fall. 

Numerous  experiments 
have  been  made  to  dis- 
cover how  chlorophyll 
does  its  work.  It  has 
been  found  that  if  light  is 
passed  through  this  sub- 
stance and  then  broken 
up  by  a  prism,  that  part 
of  the  light  which  is 
absorbed  by  the  chlorophyll  may  be  detected  by  the  presence  of 
absorption  bands  in  the  spectrum  where  the  chlorophyll  has  taken 
out  the  light.  By  this  means  we  learn  that  the  red  band  of  the  spec- 
trum is  most  active  while  parts  of  the  blue,  violet,  and  indigo  regions 
of  the  spectrum  are  also  absorbed.  A  classic  experiment  by  Engel- 
mann  illustrates  this  in  another  w^ay.  A  filament  of  an  alga  was 
placed  in  a  culture  of  bacteria  which  were  active  only  in  the  pres- 
ence of  oxygen.  The  filament  was  then  put  in  darkness  until  the 
bacteria  had  used  up  all  the  oxygen  present.  Then  the  slide  con- 
taining filament  and  bacteria  was  placed  on  a  microscope  under  a 
solar  spectrum.  In  a  short  time  the  bacteria  were  found  to  mass 
themselves  in  abundance  at  the  red  end  of  the  spectrum  and  to  a  lesser 
extent  at  the  blue  end,  because  at  these  points  more  oxygen  was 
given  off  by  the  alga,  thus  indicating  activity  in  starch  formation. 


al 

3   C 

5 

E 

l\ 

D         F 

'.■.'/• 

ii;| 

y.*''"^'--"'.v-/ 

■'■*. 

«%.,. 

'-■■■.' 

■'«■-  •-•'.■'..•?-'- 

^/A_      _   _ 

:  •.•.-'•*  .■-*■.■.'.■?...  . 

m 

1                    ^ 

:::;•:.;; 

■■^.■'■'•'.•.■■--  X-' 

;p^^'-" 

."*■'■■ 

-,•': 

;■;v■■iv^,v:;:: 

^^^'^^^S:0' 

/\ 

•'  ''Z^'J^' 

I"." 

Engelmann's  experiment  to  show  the  areas  in 
the  spectrum  most  favorable  for  oxygen  release 
in  a  green  alga.     The  dots  represent  bacteria. 


THE  ROLE  OF   GREEN   PLANTS 


259 


Relation  of  Artificial  Light  to  Food  Making 

We  have  already  noted  that  there  are  great  differences  in  the 
amount  of  sunhght  required  by  plants.  As  a  matter  of  fact,  very 
strong  sunlight  may  cause  harm  since  it  overheats  the  protoplasm, 
thus  endangering  the  life  of  the  plant.  Moreover,  it  increases  the 
rate  of  transpiration  so  that  water  is  evaporated  too  rapidly.  Experi- 
mental evidence  with  growing  plants  shows  also  that  too  much  sun- 
light may  retard  growth.     Some  plants  are  shade  loving,  as  may  be 


Shade  loving  plants  on  a  forest  floor.     Note  the  leaf  arrangement  with  reference 

to  light. 

seen  in  any  field  trip  to  a  forest.  The  differences  in  illumination  are 
correlated  with  differences  in  the  structure  of  the  leaf,  the  ])lants 
which  are  exposed  to  bright  sunlight  having  a  well  developed  palisade 
layer,  while  the  spongy  parenchyma  is  not  so  well  developed.  The 
reverse  is  true  in  shade-loving  plants.  In  addition,  plants  that  live 
in  the  shade  are  apt  to  have  a  very  thin  epidermis  and  usually  ha\-e 
dull  leaf  surfaces  which  do  not  reflect  the  light  as  reatlily. 

Contrary  to  common  belief,  it  is  possible  to  grow  i:)lants  without 
sunlight  as  pro^'ed  by  recent  experiments  (Harvey)  with  a  large 
number  of  different  crop  plants  such  as  grains,  tomatoes,  squash, 
peas,  potatoes,  and  others.     Plants  exposed  continuously  to  the  light 


260 


THE   MAINTENANCE  OF  THE   INDIVIDUAL 


of  nitrogen-filled  tungsten  lamps  of  from  200  to  1000  watts  produced 
both  viable  fruits  and  seeds.  The  bearing  of  this  experiment  upon 
growing  crops  in  areas  where  the  days  are  short  and  the  intensity  of 
sunlight  not  great  is  readily  seen.  Lamps  have  been  put  on  the 
market  for  use  in  the  home  which  provide  space  directly  underneath 
the  bulb  for  stimulating  plant  growth  during  the  winter  season. 

What  Goes  On  in  the  Green  Leaf  in  Sunlight 

When  we  examine  the  green  leaf  to  see  how  it  is  adapted  to  use  the 
energy  of  sunlight,  several  interesting  facts  are  discovered.  One  is 
that  a  plant  places  its  leaves  so  that  they  get  the  largest  possible 
amount  of  sunlight,  in  a  given  period.  Petioles  and  even  stems  of 
plants  turn  with  the  sun  so  that  a  maximum  amount  of  green  surface 
is  exposed  to  its  rays.     Looking  at  a  tree  from  above  as  the  bird  sees 


Diagram  to  show  the  cells  of  the  palisade  layer  of  a  leaf  at  two  different  times 
during  the  day.     Which  of  the  two  receives  full  sunlight  ? 

it,  leaves  are  found  to  be  so  arranged  that  there  is  a  minimum  amount 
of  overshading,  the  leaves  forming  a  sort  of  mosaic  or  pavement  on 
which  the  sunlight  falls.  Examination  of  the  internal  structure  of  the 
leaf  also  shows  that  the  palisade  layers  which  contain  the  greatest 
number  of  chloroplasts  per  cell  are  massed  close  under  the  upper  part 
of  the  epidermis.  It  is  this  layer  of  palisade  cells  wdiere  most  of  the 
work  of  starch  or  sugar  making  takes  place.  In  the  cells  themselves, 
the  green  chloroplasts  are  so  placed  that  a  maximum  amount  of  light 
falls  upon  them.  When  the  sun's  rays  are  slanting  during  the  morning 
and  afternoon,  light  can  reach  all  of  them  readily,  while  at  the  period 
of  greatest  illumination,  when  the  sun's  rays  are  direct,  less  light 
reaches  them  as  they  lie  one  above  the  other.  Their  position  may  be 
changed  in  the  protoplasm,  their  movement  being  controlled  by  the 


THE   ROLE   OF   GREEN   PLANTS  o^.i 

liviii<i;  substance  in  which  they  rest.  The  ciiloroi)hists  are  the 
structures  in  the  cells  which  utilize  the  sun's  rays,  and  it  is  within 
them  that  the  raw  materials,  carbon  cUoxide  and  water,  are  manu- 
factured into  sugar. 

Chemistry  of  Food  Making 

The  actual  processes  of  sugar  and  starch  formation  in  (he  ](-if  are 
not  fully  known.  The  end  process  can  easily  l)e  shown  by  the 
equation : 

6  COo  +  6  HoO  =  CeHioOe  +  6  O2 

(carbon  dioxide  plus  water  =  gluco.se  plus  oxygen) 

but  how  this  glucose  actually  comes  into  existence  is  still  problem- 
atical. Many  theories  have  been  advanced  to  account  for  the  con- 
version of  raw  materials  into  foods.  The  one  proposed  by  von 
Baeyer  in  1870  is  still  accepted  with  modifications.  He  assumed  that 
formaldehyde  is  formed  by  the  breaking  down  of  carbon  dioxide  into 
carbon  monoxide  and  oxygen  at  the  same  time  the  water  in  the  leaf 
is  broken  up  into  hydrogen  and  oxygen.  The  carbon  monoxide  and 
hydrogen  unite  to  form  formaldehyde,  which  is  then  built  into 
glucose  as  shown  by  the  following  formula  : 

CO2  — ^  CO  +  O 
H2O  — >■  \h  +  O 
CO  +  H2  — ^  CH2O  (formaldehyde) 
6  CH2O  — ^  C6H12O5  (sugar) 

One  objection  to  this  theory  is  that  carbon  monoxide  is  extremely 
poisonous  and  is  almost  never  found  free  in  plants,  while  the  product 
formaldehyde  is  also  a  poison.  Later  theories  postulate  that  by 
first  reducing  carbon  dioxide  and  water  to  carbonic  acid,  then  to 
formic  acid  and  hydrogen  peroxide  by  the  addition  of  a  molecule  of 
water,  formaldehyde  and  hydrogen  peroxide  result,  the  peroxide  being 
finally  reduced  to  water  and  oxygen : 

C02+H20  =  H2C03  (carbonic  acid) 
H2C03+H20  =  HCOOH  (formic  acid)+H202  (hydrogen  peroxide) 
HCOOH+H2O  =  CH2O  (formaldehyde)  +H2O2 
2Ho02  =  2H20+02 

The  last  step  in  this  process  is  brought  about  by  an  enzyme,  known 
as  catalase.     Plant  physiologists  believe  that  although  formaldehyde 


262  THE   MAINTENANCE   OF  THE   INDIVIDUAL 

is  a  poison,  it  is  probably  changed  into  sugar  so  rapidly  that  at  no 
time  is  there  much  present  in  the  cells  of  the  leaf.  The  last  part  of 
this  process,  that  of  changing  the  formaldehyde  to  sugar,  seems  to  be 
brought  about  by  the  action  of  the  two  chlorophylls,  A  and  B.  One 
recent  writer,  Gordon, >  has  given  the  following  suggestive  formula: 

6  C55H70O6N4Mg    +   6  H2O    =    6  C55H7205N4Mg   +   602 
(Chlorophyll  B)  (Chlorophyll  A) 

6  C55H7205N4Mg  +  6  CO2  =  6  C,r,H7o06N,Mg  +  CeHisOs 

(Chlorophyll  A)  (Chlorophyll  B)  (sugar) 

To  the  amateur  chemist  this  means  very  little,  but  it  suggests  the 
double  action  of  the  two  chlorophylls  in  the  formation  of  sugar. 
All  we  really  know  is  that  sugar  is  first  formed  in  the  green  leaf  and 
that  later  this  is  changed  to  starch  and  stored  in  that  form  in  various 
parts  of  the  plant. 

Of  the  manufacture  of  foods  other  than  sugar  very  little  is  known. 
There  are  tiny  droplets  of  fat  in  the  vacuoles  inside  the  chloroplasts. 
We  know  that  fats  can  be  synthesized  out  of  carbohydrates  by 
animals.  Therefore,  a  similar  process  may  take  place  in  plants. 
Fatty  tissue  is  undoubtedly  manufactured  out  of  the  carbon,  oxygen, 
and  hydrogen  contained  in  the  sugar  molecule.  Probably  a  like 
situation  exists  in  the  chloroplasts  of  the  leaves,  although  we  do  not 
know  just  how  this  process  takes  place. 

Proteins  are  even  more  complex  than  carbohydrates  and  fats. 
Their  molecule  contains  nitrogen  and  a  number  of  mineral  salts, 
in  addition  to  carbon,  oxygen,  and  hydrogen.  Protein  foods  are 
found  not  only  in  leaves,  but  in  most  of  the  storage  organs  of  the  plant. 
Apparently  proteins  can  be  synthesized  out  of  the  sugar  plus  the 
elements  nitrogen,  sulphur,  and  phosphorus,  wiiich  combine  with 
the  carbon,  oxygen,  and  hydrogen  of  the  glucose.  Proteins  are 
probably  manufactured  in  other  cells  than  those  containing  chloro- 
phyll, wherever  .starches,  sugar,  and  the  essential  salts  are  found, 
although  light  does  not  seem  to  be  a  necessary  factor  in  the  process. 
Proteins  are  undoubtedly  used  in  any  of  the  cells  of  the  plant,  just  as 
they  are  in  animal  cells,  for  the  making  of  protoplasm,  since  the  plant 
is  a  living  organism  composed  of  cell  units  each  of  which  is  doing  a 
common  work  for  the  plant  as  a  whole. 

1  Gordon,  R.  B.  :  "  Suggested  Equation  for  the  Photo-synthesis,  Action."  Ohio  Journal  of 
Science,  29:  131,  1929. 


THE   ROLE   OF   GREEN    PLANTS  26:5 

Enzymes  and  Their  Work 

The  changes  just  described  which  take  place  in  food  making  as  well 
as  in  food  storage,  all  belong  to  a  series  of  oxidative  and  reducing 
changes  that  are  presided  over  and  brought  about  by  enzyme  action, 
another  indication  of  the  importance  of  these  omnipresent  substances. 

We  have  already  spoken  of  enzymes  and  their  work,  but  reference 
to  them  again  may  not  be  amiss  at  this  point.  They  are  found 
practically  everyw^here  in  the  living  cells  of  plants  and  animals,  being 
much  more  numerous  than  was  at  first  believed.  Although  their 
nature  is  not  fully  known,  we  do  know  that  they  are  colloidal  sub- 
stances, because  they  will  pass  through  porcelain  filter,  but  not 
through  membranes.  We  also  know  that  some  of  them  are  doubtless 
proteins,  and  that  they  are  sensitive  to  light  and  ultraviolet  rays  as 
well  as  to  heat,  acid,  alkali,  and  substances  which  are  toxic  to  proto- 
plasm. They  are  powerful  catalyzers,  as  is  shown  by  the  fact  that  a 
single  gram  of  the  enzyme  invertase,  for  example,  will  quickly  hy- 
drolyze  one  million  times  its  weight  of  sugar.  Enzymes  are  found  in 
all  living  cells  and  are  specific  in  action,  that  is,  one  enzyme  will  only 
do  a  certain  type  of  w^ork.  In  general,  they  may  be  divided  into  a 
number  of  groups,  depending  upon  their  function,  such  as  the  hy- 
drolases, that  act  in  the  digestive  processes  of  plants  and  animals  by 
hydrolyzing  materials  ;  the  oxidases,  which  enable  cell  respiration  to 
take  place ;  the  fermentases,  as,  for  example,  remiin,  that  is  used  in 
cheese  making,  and  the  coagulascs,  to  which  pedasc  belongs  that  is 
used  commercially  in  substances  sold  for  use  in  jelly  making;  and 
finally,  the  carboxylases,  which  cause  organic  acids  to  split  into  carbon 
dioxide  and  other  simpler  substances. 

Specific  examples  of  these  various  plant  enzymes  include  the  en- 
zyme, diastase,  that  causes  the  digestion  of  starch.  Another  enzyme, 
maltase,  aids  in  the  digestion  of  maltose  to  glucose,  a  still  simpler 
sugar.  A  similar  action  takes  place  by  means  of  the  enzyme,  ptyalin, 
in  our  own  salivary  digestion.  Bacteria  carry  on  a  slightly  different 
type  of  digestion  in  which  cellulose  or  wood  fiber  is  broken  down  and 
used  as  food.  Here  another  enzyme,  cellulase,  causes  this  digestive 
change.  Still  another  enzyme,  called  lipase,  is  instrumental  in  the 
digestion  of  fats.  In  fruits  and  seeds  rich  in  fat,  such  as  the  avocado, 
Brazil  nut,  walnut,  almond,  or  pecan,  the  fats  are  broken  down  into 
fatty  acids  and  glycerine  just  as  in  animals  where  lipase  is  formed  by 
the  pancreas. 

H.  w.  H.  — 18 


264  THE   MAINTENANCE   OF   THE   INDIVIDUAL 

Protein  digestion  is  l^rought  about  by  a  different  group  of  enzymes, 
called  proteases.  These  enzymes  are  found  in  abundance  in  leaves  and 
germinating  seeds  of  plants  and  to  a  lesser  extent  in  practically  all 
plant  tissues.  In  the  living  plant,  the  digestive  enzymes  carry  on  a 
necessary  and  important  work.  If  plants  make  foods  in  the  green 
leaf,  and  they  do,  and  if  they  store  foods  in  the  root,  stem,  fruit,  and 
seed,  then  there  must  be  some  way  to  transfer  the  foods  made  in  the 
leaf  in  a  soluble  form  to  those  parts  of  the  organisms  where  the  food 
is  finally  used.  This  work  of  changing  insoluble  foods  to  soluble  foods 
is  obviously  performed  by  enzymes.  A  still  more  interesting  phenom- 
enon sometimes  takes  place.  Many  of  these  enzymes  under  certain 
conditions  are  capable  of  reversing  their  actions,  that  is,  of  converting 
a  soluble  substance  like  sugar  into  an  insoluble  one  such  as  starch, 
or  of  changing  proteins  to  soluble  forms  so  that  they  can  be  transported 
through  the  vascular  system  of  the  plant  and  stored  in  insoluble  form 
in  seeds,  nuts,  and  roots. 

The  changes  from  sugar  to  starch  may  take  place  in  leaves  wherever 
certain  plastids  known  as  amyloplasts  exist.  These  bodies  have 
the  power  to  form  starch  in  the  presence  of  a  series  of  enzymes  which 
first  bring  about  the  transformation  of  simple  sugars  to  more  complex 
sugars,  and  then  to  an  intermediate  substance  between  sugars  and 
starches,  called  dextrin.  Dextrin  is  changed  into  soluble  starch  by  the 
enzyme,  amijlase,  and  finally  the  soluble  starch  is  converted  into 
insoluble  starch  by  the  enzyme,  coagulase.  Thus  we  see  that  the 
work  of  enzymes  is  absolutely  essential  to  the  life  of  the  plant.  Al- 
though plants  and  animals  obtain  their  foods  in  different  ways,  they 
probably  assimilate  it  in  much  the  same  manner,  for  foods  serve 
exactly  the  same  purposes  in  plants  and  in  animals,  namely,  they  are 
oxidized  to  release  energy  and  they  build  up  living  matter. 

How  Food  Is  Used  in  the  Plant  Body 

Although,  basically,  the  uses  of  food  are  production  of  energy  and 
making  of  protoplasm,  certain  substances  are  produced  by  plants 
which  are  not  found  in  animals.  For  example,  the  plant  cell  is  charac- 
terized by  its  cellulose  wall  which  in  old  cells  is  strengthened  by  the 
addition  of  a  complex  substance,  known  as  lignin.  This  forms  the 
useful  substance  we  call  wood.  In  addition,  other  products  charac- 
teristic of  plant  activity  should  be  mentioned  :  the  fatty  substances, 
known  as  cutin  and  suherin,  as  well  as  waxes  which  give  the  "bloom" 
to  certain  fruits ;   the  essential  oils  in  resins,  such  as  lemon,  pepper- 


THE   ROLE   OF   GREEN   PLANTS 


265 


mint,  wintergreen,  menthol,  eucalyptus,  camphor,  and  the  like ;  va- 
rious alkaloids ;  poisonous  substances  such  as  nicotine  and  strych- 
nine ;  acids  such  as  mahc,  citric,  and  tartaric.  Plant  protoplasm,  in 
addition,  as  we  have  seen,  manufactures  many  characteristic  enzymes 
and  produces  pigments  like  the 
chlorophylls  and  carotins  al- 
ready mentioned.  The  carotin 
present  in  green  grass  fed  to 
dairy  cows  gives  the  deeper 
color  so  much  desired  in  cream 
and  butter.  Another  interest- 
ing substance  found  in  carotin 
is  a  precursor  of  Vitamin  A 
which  exists  in  plant  bodies  as 
a  form  of  carotin  and  is  prob- 
ably transformed  by  the  liver  of 
animals  into  Vitamin  A.  This 
is  another  example  showing  how 
closely  the  lives  of  plants  and 
animals  are  interwoven.  (See 
pages  277-279.) 


00 

A  diagram  of  the  outer  portion  of  a 
cross  section  of  a  wheat  grain  showing  the 
various  layers  of  tissues  :  h,  the  different 
integuments  of  the  ovary  and  seed  which 
make  up  the  husk ;  o,  the  cells  of  "  tileu- 
rone  layer"  of  the  endosperm,  which  are 
loaded  with  protein  grains  :  and  b,  the  layer 
of  starch-bearing  cells.     (After  Cobb.) 


Respiration 

Respiration  is  essentially  the 
same  process  in  plants  as  in 
animals.  In  its  simplest  terms  it  is  the  release  of  potential  energy 
from  foods  by  means  of  the  process  of  oxidation,  whereby  oxygen 
is  used  and  carbon  dioxide  is  given  ofT.  Glucose  is  perhaps  the 
chief  fuel  of  the  plant  body,  although  fats  also  serve  this  purpose. 
The  latter  are  probal^ly  changed  to  sugar  before  actually  being 
utilized  in  the  respiratory  process. 

In  order  to  have  respiration  take  place,  there  must  be  an  exchange 
of  gases  through  a  selectively  permeable  membrane.  This  means  that 
there  will  be  an  exchange  of  oxygen  and  carbon  dioxide  in  the  cells 
where  the  oxidative  process  is  taking  place.  Sin.ce  respiration  occurs  in 
all  living  cells  and  since  there  is  a  greater  volume  of  carbon  dioxide  and 
oxygen  in  parts  of  plants  that  are  growing  rapidly,  it  is  obvious  that 
growing  roots  must  have  a  supi)ly  of  oxygen.  This  is  a  reason  for  the 
loosening  of  soil  particles  around  plants  in  cultivation  to  allow  air  to 
have  access  to  the  root  hairs.     The  actual  oxidative  |)rocess  is  con- 


266 


THE   MAINTENANCE   OF  THE   INDIVIDUAL 


siderably  influenced  by  external  conditions.  Low  temperatures  slow 
up  the  process  as  do  very  high  temperatures,  there  being  an  optimum 
temperature  for  each  organism  at  which  the  rate  of  respiration  goes  on 
best.  Seeds  have  survived  a  temperature  of —250°  C.  Experiments 
with  leaves  show  that  the  respiratory  rate  increases  rapidly  from 
0°-40°  C,  from  which  point  it  falls  slowly  until  the  death  of  the 
organism.  The  amount  of  food  present  in  the  plant  is  a  second 
factor  influencing  the  rate  of  respiration,  while  the  rate  also  varies 
with  the  amount  of  protoplasm  in  the  cells.  Light  usually  increases 
the  respiratory  rate,  probably  because  of  a  parallel  increase  in  food 
and  temperature.  It  is  also  found  that  wave  lengths  which  increase 
photosynthesis  also  increase  the  respiratory  rate.  Finally,  the  rate 
of  respiration  is  greatly  affected  by  poisons  or  anesthetics,  at  first 
being  increased,  but  later  slowing  down  rapidly.  In  brief,  respiration 
in  plants,  as  in  animals,  is  induced  by  the  action  of  enzymes,  and 
results  in  the  release  of  energy. 

Transpiration 

If  a  healthy  potted  plant  is  placed  in  a  dry  bell  jar  and  left  in  the 
sun  for  a  few  minutes,  drops  of  water  are  seen  to  gather  on  the  inside 
of  the  jar.     By  covering  the  pot  with  a  rubber  tissue  to  exclude  the 


large. 
battery 
jar-. 


(Jovcrect 
vith  5heet 
rubbe'T.... 


moifture 


star-t- 


24  Viours  later 


Experiment  to  show  transpiration.     Read  your  text  and  explain  what  has 

happened. 


possibility  of  the  evaporation  of  water  through  its  surface  and  return- , 
ing  it  to  the  jar  under  similar  conditions,  drops  of  moisture  are  again 
found  after  a  time  on  the  inner  surface  of  the  jar.     Obviously,  water 
must  come  out  through  the  leaves  or  stem  of  the  plant,  a  fact  which 
can  be  demonstrated  by  weighing  it  before  placing  it  in  the  jar,  and 


THE   ROLE  OF   GREEN   PLANTS 


267 


again  after  a  brief  period  of  exposure  to  sunliglit,  when  it  will  be  found 
to  have  lost  weight.  This  loss  ol'  water  takes  place  through  the  sto- 
mata  and  to  some  extent  through  the  lenticels  of  the  stem,  a  loluniom- 
enon  closely  associated  with  the  process  of  i)hotosynthesis,  for  which 
a  relatively  enormous  amount  of  water  is  required.  The  reasons  for 
this  are  that  living  matter  is  largely  composed  of  water  ;  that  the  pro- 
cess of  food  making  cannot  take  place  in  plants  unless  the  interior  of 
their  leaves  are  moist ;  and,  in  the  third  place,  because  water  is  one  of 
the  raw  materials  used  in  making  sugar.  The  amount  of  water  given 
off  by  plants  through  transpiration  is  very  great.  Early  in  the  eight- 
eenth century  Stephen  Hales  (see  p.  241)  estimated  that  an  average 
crop  of  cabbages  loses  from  three  to  four  tons  of  water  per  day  per  acre 
in  warm  weather.  An  acre  of  pasture  grass  is  said  to  give  off  over 
100  tons  of  water  on  a  hot,  dry  day.  A  medium-sized  tree  will  evapo- 
rate about  five  to  six  tons  of  water  on  a  hot  day.  One  writer,  von 
Hohnel,  estimated  that  an  acre  of  large  beech  trees  would  transpire 
30,000  barrels  of  w^ater  in  one  summer.  Such  figures  show  that  a 
green  plant  loses  water  very  rapidly  during  hot,  dry  days. 

The  amount  of  water  lost  differs  greatly  under  different  conditions. 
If  the  air  is  humid,  or  if  the  temperature  is  lowered,  or  if  the  tempera- 
ture of  the  plant  becomes  low,  the  rate  of  transpiration  is  greatly 


Diagrammatic  cross  section  through  a  stoma  to  show  movement  of  guard  cells. 
The  dotted  lines  show  the  closed  position.  Closure  is  brought  about  by  the 
guard  cells  becoming  more  elongated  and  flattened,  while  the  outer  wall  (w) 
remains  in  place,  the  ventral  wall  (/)  and  dorsal  wall  (V/)  assume  the  positions  (/') 
and  id')  moving  toward  the  central  slit  (s)  of  the  opening  of  the  stoma.  This 
movement  is  largely  brought  about  through  the  change  in  position  of  the  base 
or  hinge  {h)  {h')  of  the  guard  cell.     (After  Schwendener.) 

reduced.  The  stomata  tend  to  close  under  certain  conditions,  thus 
helping  to  prevent  evaporation.  The  opening  and  closing  of  the 
stomata  depend  on  changes  in  turgor  of  the  guard  cells.     The  stomata 


268 


THE    MAINTENANCE   OF  THE   INDIVIDUAL 


open  when  the  guard  cells  become  more  than  normally  turgid,  but  if 
the  turgor  of  all  of  the  living  cells  of  the  leaf  is  reduced  by  water  loss, 
then  the  stomata  seem  to  close  automatically. 

Light  increases  the  amount  of  sugar  formed  in  the  guard  cells 
because  of  the  chloroplasts  present,  which  results  in  a  concentration 
of  sugar,  thereby  causing  a  change  in  turgor.  When  the  leaf  is  not 
illuminated  by  direct  sunlight,  or  at  night,  the  amount  of  sugar  con- 
centration in  the  guard  cells  becomes  less,  and  consequently  the 
stomata  close.  They  usually  are  closed  at  night  but  remain  open 
from  shortly  after  sunrise  until  late  in  the  afternoon.  Toward  the 
middle  of  the  afternoon  they  begin  to  close,  thus  decreasing  the 
amount  of  water  lost  in  the  latter  part  of  the  day.  Plants  wilt  on 
hot,  dry  days  because  they  cannot  obtain  water  rapidly  enough  from 
the  soil  to  make  up  for  the  loss  through  the  leaves.  Many  adapta- 
tions are  found  in  the  leaves  which  help  prevent  this  water  loss,  such 
as  waterproofing  of  the  outer  cells,  hairs  on  the  leaf  surface,  the 
absence  of  leaves,  as  in  the  cactus  where  the  minute  leaves  are  early 
replaced  by  spines,  or  the  actual  turning  of  the  leaves  in  order  to 
place  a  small  surface  to  the  sun,  as  in  the  compass  plant,  thus  causing 
the  rate  of  evaporation  to  decrease. 


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Capillary  tubes  of  various  sizes.     Is  there  any  relation  between  the  size  of 
the  bore  of  the  tube  and  the  water  level  in  the  tube  ?     Explain. 


THE  ROLE  OF  GREEN   PLANTS 


269 


The  Rise  of  Water  in  Plants 

We  have  spoken  of  the  passage  of  water  from  the  root  up  the  stem 
into  the  leaf.  Osmotic  pressure  has  been  shown  to  be  sufficient  to 
start  this  column  of  water  on  its  way  up  the  stem,  but  it  is  not  enough 
to  account  for  the  rise  of  water  sometimes  hundreds  of  feet  into  the 
air  in  the  stems  of  trees.  Several  theories  have  been  advanced  to 
account  for  this  phenomenon.  The  most  satisfactory  of  these  is  the 
theory  that  such  a  column  of  water  is  held  together  by  the  force  of 
cohesion.  Experimental  evidence  shows  that  the  cohesive  quality  of 
water  in  capillary  tubes  is  very  great.  The  core  of  water  acts  as  a 
fine,  extremely  ductile  wire.  When  we  realize  that  a  core  of  water  in 
a  tube  2^  of  an  inch  in  diameter  will  withstand  a  pressure  of  over 
4600  pounds  to  the  square  inch,  it  will  be  seen 
that  such  resistance  is  a  factor  in  the  rise  of 
water  through  the  very  tiny  tubes  found  in 
the  vascular  bundles  of  a  tree.  Another 
factor  in  the  rise  of  water  in  a  plant  or  tree  is 
the  evaporation  that  takes  place  through  the 
leaves,  causing  a  pull  on  the  cores  of  water  in 
the  tubes  of  the  vascular  bundles.  During 
the  daytime  this  is  undoubtedly  the  chief 
factor  in  causing  the  rise  of  fluids  in  the 
stem. 

Production  of  Oxygen  by  Plants 

A  good  many  years  ago  the  botanist  Sachs 
proved  that  a  green  plant  placed  in  the  sun- 
light will  give  off  oxygen,  an  experiment  easily 
shown  in  the  laboratory.  If  an  aquatic  plant 
such  as  Elodca  is  placed  under  an  inverted 
funnel  in  a  bell  jar  of  water,  and  an  inverted 
test  tube  of  water  is  placed  over  the  mouth 
of  the  funnel,  bubbles  of  a  gas  are  seen  to 
leave  the  plant  and  gradually  displace  the 
water  in  the  test  tube.  If  a  sufficient  amount  of  this  gas  is  collected, 
it  can  be  tested  with  a  glowing  splint  of  wood  and  proved  to  be 
oxygen.  The  amount  of  the  gas  can  be  shown  to  depend  approxi- 
mately on  the  amount  of  sunlight  and  consequently  the  rate  of 
photosynthesis.     Going  back  to  the  formula  which  shows  the  making 


How  would  you  pro\e 
that  the  gas  on  the  test 
tube  w  as  oxygen  ? 


270  THE   MAINTENANCE   OF  THE   INDIVIDUAL 

of  sugar  in  the  leaf,  we  find  oxygen  is  given  off  as  a  by-product. 
The  reaction  may  be  expressed  by  the  following  formula : 

6  CO2  +  6  H2O  +  energy  from  sunlight  =  CeHi^Oe  +  6  O2 

(glucose) 

The  value  of  this  reaction  to  mankind  is  obvious.  The  by-product 
oxygen,  which  is  poured  into  the  air  by  green  plants,  is  used  by 
animals  as  well  as  plants  in  their  respiratory  processes.  This  exchange 
of  oxygen  and  carbon  dioxide  by  plants  and  animals  gives  us  one  of 
the  most  significant  and  far-reaching  interrelationships  seen  in  the 
organic  world. 

Briefly  summing  up  the  process  of  food  making  in  plants  we  find 
that  raw  materials  pass  in  the  form  of  water  and  soil  solutes  from 
the  soil  through  the  root  hairs  and  up  the  vascular  bundles  of  xylem 
into  the  leaf,  where  water  is  taken  into  the  individual  green  cells. 
Carbon  dioxide  reaches  the  cells  from  the  air  through  the  stomata 
and  to  a  lesser  extent  probably *in  the  water  stream  through  the  roots. 
In  sunlight,  the  process  of  photo.synthesis  takes  place.  Elaborated 
foods  made  in  the  form  of  sugars  may  be  changed  by  enzymes  to 
starches  and  immediately  stored  in  the  leaf,  or  may  be  passed  down 
through  the  sieve  tubes  of  the  phloem  to  various  parts  of  the  plant 
where  they  may  be  used  or  stored.  Fats  are  probably  synthesized 
from  carbohydrates  in  the  green  parts  of  plants,  while  proteins  seem 
to  be  formed  in  the  cells  irrespective  of  the  presence  of  chlorophyll. 
Enzymes  play  a  very  important  role  both  in  the  manufacture  and 
in  the  use  of  food  and  are  essential  to  respiration  and  oxidation.  The 
digestive  processes  which  go  on  in  the  leaf  and  other  cells  of  the  plant 
are  also  due  to  enzymes. 

All  that  has  been  said  in  the  preceding  pages  leads  to  the  most 
important  plant  function,  the  reproduction  of  the  species.  With 
vegetative  propagation  by  means  of  budding,  runners,  underground 
stems,  tubers,  or  some  of  the  other  asexual  means  of  continuing  life, 
plants  would  not  go  far.  To  establish  outposts  in  far-flung  dominions 
they  must  have  means  of  travel.  These  can  only  be  obtained  through 
free  moving  parts.  Such  plants  are  seeds  and  fruits,  which  may  be 
dispersed  by  outside  agencies  far  from  the  parent  plant. 

The  life  of  the  flowering  plant  culminates  in  the  production  of  seeds 
and  fruits.  As  growth  progresses  and  food  is  accumulated,  a  time 
comes,  sooner  or  later,  when  the  energies  of  the  plant  are  directed  to 
the  rapid  production  of  the  reproductive  organs.     Often  this  growth 


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272 


THE   MAINTENANCE   OF  THE   INDIVIDUAL 


is  much  more  rapid  than  vegetative  growth,  and  almost  overnight, 
flowers  appear. 

The  flower,  as  has  been  previously  shown,  holds  the  gametophyte 
generation  of  the  plant  and  produces  from  fertilized  eggs  the  seeds 
which  hold  the  embryos  or  future  plants.  The  fruit  arises  from  the 
ovary  of  the  flower,  together  with  the  parts  that  may  be  attached  to  it. 


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Adaptations  in  certain  fruits  for  seed  distribution.     Can  you  describe  the 
specific  adaptation  in  each  case? 

Sometimes  the  parts  are  fleshy,  forming  edible  fruits  such  as  apples, 
pears,  or  plums ;  occasionally  they  form  hard  coverings  such  as  the 
shefls  of  nuts,  and  often  they  are  prolonged  into  feathery  outgrowths 
which  aid  in  the  distribution  of  the  fruit  and  seeds.  Enough  has  been 
said  of  distribution  for  us  to  grasp  the  significance  of  such  adaptations, 
the  ultimate  purpose  of  which  is  to  place  the  embryo  in  new  areas  so 
that  when  the  seed  germinates  it  may  develop  into  a  new  plant  and 
thus  complete  the  life  cycle. 

SUGGESTED  READINGS 

Biisgen,  M.,  and  Miinch,  G.  (translated  by  Thompson),  The  Structure  and 
Life  of  Forest  Trees,  John  Wiley  &  Sons.,  Inc.,  1929, 
Interesting  and  authentic, 


THE   ROLE   OF   GREEN    PLANTS  273 

Ganong,  W.  F.,  The  Living  Plant,  Henry  Holt  &  Co.,  1913. 

A  not  too  technical  account  of  how  plants  Hve. 
Holman,  R.  M.,  and  Robbins,  W.  W.,  Textbook  of  General  Botany,  3rd  ed.,  John 

Wiley  &  Sons,  Inc.,  1933. 

Excellent  chapter  on  photosynthesis. 
Macdougal,  D.  T.,  The  Green  Leaf,  D.  Appleton  &  Co.,  1930. 

A  fascinating  account  of  the  work  of  the  green  leaf.     Readable  and 

authentic. 
Raber,  0.  L.,  Principles  of  Plant  Physiology,  The  Macmillan  Co.,  1928. 

A  readable,  but  thoroughly  scientific,  book  of  reference.     Especially 

valuable  are  chapters  IV,  VI,  XVI,  XIX,  XX,  XXI,  XXII,  and  XXIV. 
Sinnott,  E.  W.,  Botany,  Principles  and  Problems,  3rd  ed.,  McGraw-Hill  Book 

Co.,  1935. 

Chapters  IV,  V,  VI,  VII,  and  VIII  are  useful  for  reference.     Note  the 

many  suggestive  questions  at  ends  of  chapters. 
Wilson,  C.  L.,  and  Haber,  J.  M.,  Introduction  to  Plant  Life,  Henry  Holt  & 

Co.,  1935. 

A  general  botany  with  a  new  point  of  view.     Readable  and  usable. 


XV 


THE   METABOLIC   MACHINERY   OF   ANIMALS 

Preview.  Section  A .  Intake  devices  and  how  they  function  •  Foods  and 
their  uses ;  energy  producers ;  non-energy  producers ;  vitamins  ■  The  acti- 
vators —  enzymes  •  Digestion  in  lower  animals  •  Digestion  in  higher  ani- 
mals ;  methods  of  increasing  digestive  surfaces ;  parts  of  the  digestive 
system :  The  oral  cavitj^,  the  pharynx  and  esophagus,  the  stomach,  the 
small  intestine,  the  large  intestine ;  the  digestive  glands  and  their  enzymes : 
The  salivary  glands,  the  gastric  glands,  the  intestinal  glands,  the  pancreas, 
the  liver,  the  secretions  of  the  small  intestine ;  absorption  and  the  fate  of 
absorbed  foods  •  Section  B.  The  how  and  why  of  circulation  •  Why  a 
transportation  system  •  Unspecialized  transportation  systems  ■  Open  cir- 
culatory systems  •  Closed  circulatory  systems  :  Among  invertebrates ;  among 
vertebrates  •  The  blood  •  The  lymph  •  The  conduits  —  arteries,  veins,  and 
capillaries  •  The  heart  •  The  aortic  arches  •  The  course  of  blood  in  the 
body ;  functions  of  the  blood  •  Section  C.  Respiratory  devices  •  Respira- 
tion ;  the  protein,  hemoglobin ;  external  respiration  :  Respiratory  papillae, 
respiratory  pouches  or  trees,  lung-books,  the  body  surface,  gills,  tracheae, 
lungs,  internal  respiration ;  respiratory  system  in  man  •  Section  D.  Ex- 
cretory mechanisms  •  Excretion ;  types  of  excretory  devices :  Contractile 
vacuoles,  intracellular  excretion,  other  excretory  devices ;  excretory  devices 
of  vertebrates  —  kidneys  ;  the  mammalian  excretory  system  :  The  liver, 
other  devices  for  waste  elimination,  the  kidneys  •  Suggested  readings. 

PREVIEW 

The  body  has  often  been  compared  to  a  machine.  This  analogy 
probably  holds  best  when  speaking  of  the  preparation  of  food  for 
combustion,  the  actual  release  of  energy,  and  the  resulting  work 
done,  as  well  as  the  disposal  of  the  end  products.  It  is  this  group  of 
processes  with  which  we  will  here  be  concerned.  All  animals  are  in 
constant  competition  with  one  another  for  food.  If  herbivorous  they 
may  be  competing  amongst  themselves  directly  for  plant  food ;  if 
carnivorous,  the  competition  is  more  indirect.  Food,  whether  it  is 
animal  or  plant  by  nature,  is  being  continuously  sought  to  maintain 
that  complex  series  of  processes  called  by  some  authors  the  "flame 
of  life."  An  earlier  unit  describes  how  plants  take  raw  materials, 
such  as  water,  carbon  dioxide,  and  nitrogenous  compounds,  and  build 
them  up  into  foods  which  may  then  be  used  or  stored.     The  plant 

271 


THE   METABOLIC   MACHINERY   OF   ANIMALS  275 

in  order  to  transport  or  to  utilize  this  stored  material  must  first 
break  it  down  into  simpler  soluble  compounds  so  that  it  may  pass  to 
the  cells  of  the  organism  where  it  is  utilized.  A  somewhat  similar 
situation  occurs  in  animals  since  complex  protoplasmic  material  of 
animal  or  vegetable  nature  is  taken  in  by  the  organism,  broken  down 
into  simpler  units,  and  then  utilized  or  stored  in  the  cells  of  the 
body  during  the  normal  processes  of  metabolism.  This  breakdown 
of  foods  is  known  as  digestion,  the  intricacies  of  which  make  a  fas- 
cinating study. 

There  are  a  number  of  important  and  interesting  problems  which 
present  themselves  at  this  point.  The  most  important  problems 
involved  are  :  What  is  food  and  how  is  it  digested  ?  How  is  it 
disposed  of  after  absorption?  How  is  energy  released?  How  are 
waste  products  removed?  Briefly,  they  center  around  questions 
which  we  should  answer,  for  it  is  both  interesting  and  profitable 
to  understand  something  of  the  human  machine.  Consequently, 
although  other  animals  are  mentioned,  the  fact  should  not  be  lost 
sight  of  that  we  have  a  selfish  interest  and  are  anxious  to  know  about 
ourselves.  The  answers  to  these  stimulating  questions  will  be  found 
in  the  discussions  that  follow. 


SECTION   A.     INTAKE   DEVICES  AND   HOW  THEY 

FUNCTION 

Foods  and  Their  Uses 

Any  substance  taken  into  the  body  that  can  be  utilized  for  the 
release  of  energy,  for  the  regulation  of  body  processes,  or  for  the 
building  and  repair  of  tissues  falls  into  the  category  of  food.  If  this 
broad  definition  of  food  is  adopted,  then  water,  inorganic  salts,  vita- 
mins, carbohydrates,  proteins,  and  fats  should  be  included.  Food 
substances  may  be  further  subdivided  into  those  capable  of  releasing 
the  latent  or  potential  energy  bound  up  within  the  molecule  and  those 
which,  though  non-energy  producers,  are  still  essential  to  life.  Energy 
which  is  so  essential  to  the  metabolism  of  an  organism  is  largely 
secured  through  the  breakdown  of  a  complex  series  of  molecules  into 
simpler  ones.  Non-energy  producers  are  equally  as  essential  to  the 
well-being  of  the  organism  since  water  and  inorganic  salts,  for  ex- 
ample, are  necessary  for  the  maintenance  of  the  normal  composition 
of  tissue. 


276 


THE  MAINTENANCE  OF  THE  INDIVIDUAL 


tVjermometer 


Energy  Producers 

Carbohydrates,  proteins,  and  fats  are  the  sources  from  which  energy 
within  the  animal  body  is  derived.     Of  these,  carbohydrates  and  fats 

are  more  readily  oxidizable  than  proteins, 
a  fact  which  is  taken  advantage  of  by 
the  Eskimo,  who  secures  much  of  his 
energy  from  oils  and  fats.  The  white 
man  in  the  tropics  uses  carbohydrates 
chiefly  for  the  same  reason. 

No  two  foods  contain  the  same  per- 
centages of  carbohydrates,  proteins,  or 
fats.  At  water  analyzed  many  foods  in 
the  calorimeter  which  bears  his  name. 
Such  a  bomb  calorimeter  consists  essen- 
tially of  an  outer  insulated  chamber  sur- 
rounding one  containing  a  known  amount 
of  water.  The  inner  compartment  in 
turn  encloses  the  metallic  chamber  in 
which  a  certain  amount  of  oxygen  and 
food  are  placed  and  burned  by  means  of. 
an  electric  current.  The  amount  of  heat 
generated  is  transmitted  to  the  water  in 
the  chamber  surrounding  the  bomb  and 
the  value  of  this  in  terms  of  calories 
is  then  determined.  It  will  be  recalled 
that  a  calorie  is  the  amount  of  heat  necessary  to  raise  one  gram  of 
water  one  degree  centigrade. 


l— ^ater  around,  bomb 
■varmect  by  bixcmug' 
of  fooct 

Diagram  of  a   bomb  calorim 
eleri     How  does  it,  work.^ 


Non-Energy  Producers 

Three  widely  diversified  groups  are  represented  by  water,  inorganic 
salts,  and  vitamins.  All  serve  the  common  end  of  keeping  the  animal 
in  a  state  of  well-being,  yet  each  group  does  so  in  a  very  different  way. 

Water  constitutes  a  large  portion  of  the  animal  body  which  may 
compose  even  as  much  as  five  sixths  of  the  daily  intake.  Estimates 
vary  from  62  to  nearly  75  per  cent  of  water  by  weight  in  the  case  of 
the  human  body.  The  quantity  in  the  different  tissues  varies  accord- 
ing to  the  metabolic  state  of  the  tissues  of  the  organism.  It  is  well 
established  that  bone  contains  only  about  22  per  cent  of  water,  while 
other  organs,  as  the  liver,  muscles,  kidney,  and  brain,  contain  much 


THE   METABOLIC   MACHINERY    OF    \NIMALS  277 

larger  amounts.  In  the  case  of  man  the  amount  of  water  in  tlie  adult 
body  remains  approximately  the  same  under  normal  conditions,  but 
if  decreased  beyond  a  certain  point  intolerable  thirst  results.  On  the 
other  hand,  if  the  amount  of  water  is  increased,  the  blood  pressure  is 
raised  in  the  renal  capillaries  and  the  excretion  of  urine  is  stimulated. 
The  consumption  of  a  hberal  supply  of  water  is  a  characteristic 
biological  process  as  it  favors  the  removal  or  dilution  of  waste  and 
poisonous  materials  from  the  body. 

Along  with  water,  the  presence  of  certain  chemical  elements  such  as 
sodium,  potassium,  calcium,  magnesium,  iodine,  iron,  chlorine,  phos- 
phorus, sulphur,  silicon,  and  fluorine  is  necessary  to  maintain  the 
various  kinds  of  tissues.  Much  experimental  work  has  been  per- 
formed upon  various  animals,  indicating  the  importance  of  a  proper 
balance  of  these  elements  in  the  diet.  The  absolute  withdrawal  of 
any  of  these  may  end  in  the  death  of  the  organism. 

Since  these  salts  form,  a  part  of  all  tissues  and  serve  a  variety  of 
functions  it  is  impossible  to  mention  all  of  them.  The  important 
part  w^hich  calcium  salts,  for  example,  play  in  the  formation  of 
bone  is  well  realized.  In  this  connection  it  has  been  said  that 
there  is  enough  lime  in  a  human  body  "  to  whitewash  a  small  hen- 
coop." 

Certain  parts  of  foods  rich  in  carbohydrates  contain  indigestible 
material  that  serve  as  roughage  and  are  useful  in  stimulating  the 
muscles  of  the  large  intestine.  Bran,  whole  wheat,  fresh  vegetables, 
and  fruit  provide  some  of  the  best  sources  of  these  materials.  Other 
examples  may  be  found  in  the  cellulose  of  plant  cells  which  can  be 
used  as  food  by  only  a  few  animals. 

Flavorings,  stimulants,  and  condiments,  such  as  pepper,  mustard, 
tea,  coffee,  and  cocoa,  are  not  true  foods.  However,  they  have  a 
real  value  in  making  food  more  appetizing. 

Vitamins 

It  might  seem  that  an  organism  could  be  kept  alive,  well,  and 
healthy  upon  a  balanced  diet  of  the  necessary  inorganic  salts  and 
water,  together  with  energy  producers  and  tissue  builders,  such  as 
amino  acids,  carbohydrates,  and  proteins.  Modern  scientific  work 
has  dispelled  this  illusion  by  a  series  of  laboratory  experiments  and 
by  observations  of  experiments  performed  in  nature.  We  now  know 
that  regulating  substances,  called  vitamins,  are  some  of  the  most 
essential  ingredients  of  all  foods. 


278  THE   MAINTENANCE  OF  THE  INDIVIDUAL 

These  health  regulators  have  been  lettered  and  are  known  as 
Vitamins  A,  B,  C,  D,  E,  and  G.  More  recent  experiments  show  that 
what  was  previously  believed  to  be  a  single  vitamin  may  prove  to 
be  a  mixture  of  two  discrete  fractions.  These  may  be  referred  to  as 
A\  A^,  and  so  on,  or  they  may  be  given  a  new  letter,  as,  for  example, 
H.  Thus  Vitamin  B  has  become  subdivided  into  B  or  B^  the  anti- 
neuritic  vitamin,  the  absence  of  which  results  in  a  disease  known  as 
heriheri,  and  B-  or  G,  the  antipellagric  vitamin,  the  lack  of  w^hich 
produces  pellagra. 

The  initiation  of  scientific  work  in  this  field  is  usually  credited  to 
Eijkman,  who  in  1897  produced  beriberi  in  fowls  by  feeding  them  on 
certain  restricted  diets.  This  was  really  "putting  the  cart  before  the 
horse,"  for  through  the  pioneering  contributions  of  Grijns  (1901)  it 
was  shown  that  the  disease  is  produced  by  the  absence  of  some  essential 
constituent  of  the  diet.  This  important  conclusion  has  been  corrob- 
orated and  extended  materially  through  the  efforts  of  Hopkins  in 
England  and  McCollum,  Eddy,  Osborne,  and  Mendel  in  the  United 
States.  Research  in  this  field  has  taken  great  strides  since  1910  and 
is  still  going  on. 

Vitamin  A  is  found  in  the  fatty  and  oily  constituents  of  such 
foods  as  butter  and  cream,  egg  yolk,  liver,  carrots,  cod-liver  oil, 
yellow  corn,  and  leafy  vegetables.  Experiments  have  demonstrated 
that  this  vitamin  is  a  necessary  adjunct  to  growth.  Without  it  rats 
die,  but  if  even  such  minute  amounts  as  0.005  mg.  of  the  purified 
vitamin  are  added  to  the  normal  diet,  the  sick  animals  are  restored 
to  general  health. 

Scurvy  has  long  been  the  curse  of  those  embarking  upon  long  sea 
voyages  or  expeditions  where  it  has  been  necessary  to  provide  diets 
deficient  in  fresh  meats  and  vegetables.  It  has  also  been  known  that 
such  a  disease  can  be  cured  by  the  use  of  fresh  vegetables  and  fruits. 
As  early  as  1804,  lemon  juice  was  issued  regularly  to  British  sailors, 
who  became  known  thereafter  as  "limeys."  It  is  only  within  com- 
paratively recent  years,  however,  that  this  remedy  has  been  known  to 
be  due  to  the  presence  of  Vitamin  C,  the  antiscorbutic  vitamin.  It 
may  be  secured  most  conveniently  in  oranges,  lemons,  or  tomatoes. 
Apparently  food  can  be  dried  or  canned  without  marked  injury  to 
the  vitamin.  Almost  as  soon  as  this  vitamin  is  eliminated  from  the 
diet  degenerative  changes  begin,  although  some  time  is  necessary 
before  the  first  symptoms  appear.  This  has  been  designated  as  the 
depletion  period. 


THE   METABOLIC   MACHINERY   OF   ANIMALS  279 

Vitamin  D,  better  known  as  the  antirachitic  vitamin,  is  chiefly  con- 
cerned with  maintaining  an  adequate  supply  of  phospliorus  and 
calcium  in  the  blood,  bones,  and  teeth.  The  discovery  of  this  vitamin 
is  associated  with  a  study  of  rickets.  Early  workers  noted  that  cod- 
Uver  oil  had  a  beneficial  effect.  The  cure  was  attributed  to  Vitamin 
A  until,  in  1923,  McCollum  of  the  Johns  Hopkins  University  and  his 
co-workers  showed  that  the  efficacy  of  cod-liver  oil  remained  even 
after  treatment  which  destroyed  Vitamin  A,  an  observation  which 
led  to  the  identification  of  Vitamin  D.  The  best  sources  of  this 
vitamin  are  cod-liver  oil,  butter,  and  egg  yolk. 

More  recently  it  has  been  shown  that  the  precursor  or  "pro- 
vitamin" of  Vitamin  D,  a  substance  known  as  ergosterol,  will  yield  the 
vitamin  after  irradiation  with  ultraviolet  light.  Ultraviolet  rays  of 
the  sun,  or  X-rays,  are  likewise  a  great  help  in  overcoming  rickets. 
At  the  present  time  four  methods  are  used  to  increase  the  amount  of 
Vitamin  D  in  the  bodj' :  (1)  irradiation  of  the  skin  by  exposure  to 
sunlight  or  other  sources  of  ultraviolet  light ;  (2)  the  addition  of 
cod-liver  oil  to  the  diet ;  (3)  the  introduction  of  irradiated  ergosterol 
(viosterol) ;  and  (4)  the  use  of  Vitamin  D  concentrates  in  foods.  This 
latter  method  has  been  most  successfully  introduced  by  Zucker,  by 
the  addition  of  this  concentrate  to  milk,  thus  facilitating  its  adminis- 
tration to  young  children. 

A  survey  of  the  prevalence  of  rickets  shows  that  this  disease  is 
much  more  common  than  has  been  supposed,  especially  in  young 
children,  a  fact  strikingly  brought  out  when  83  per  cent  of  a  group 
of  over  200  children  from  New  Haven,  Connecticut,  who  were  exam- 
ined by  X-ray,  showed  mild  evidence  of  rickets. 

Vitamin  E  is  commonly  known  as  the  antisterility  vitamin.  This 
important  substance  has  been  shown  to  be  present  in  greatest  quantity 
in  lettuce,  whole  wheat,  and,  to  a  somewhat  lesser  degree,  in  egg  yolk 
and  milk.  It  is  fat-soluble  and  quite  resistant  to  heat.  There  is 
evidence  suggesting  that  the  animal  body  has  the  ability  to  store  this 
vitamin. 

The  Activators  —  Enzymes 

It  will  be  recalled  that  the  metabolic  processes  of  plants  and  animals 
include  about  every  type  of  reaction  known  to  the  chemist.  It  has 
been  demonstrated  that  enzymes  not  only  are  essential  for  diges- 
tion, but  also  that  all  chemical  changes  in  the  body  are  mediated  by 
enzymes.     Glucose  may  be  taken  as  an  example.     The  decomposition 

H.    W.    H.  —  19 


280  THE   MAINTENANCE   OF  THE   INDIVIDUAL 

and  oxidation  of  this  simple  sugar  produces  over  100  different  sub- 
stances. The  living  cell  yields  only  a  few  of  these,  and  then  in  a 
regular  succession.  Such  remarkable  specificity  and  speed  of  reaction 
in  the  living  cell  is  largely  due  to  the  action  of  enzymes  which  have 
the  property  of  accelerating  some  particular  reaction.  As  was 
pointed  out  previously  (p.  128),  enzymes  may  be  regarded  as  catalysts 
because  they  are  not  expended  and  primarily  serve  to  speed  up  a 
reaction. 

While  the  properties  of  particular  enzymes  will  be  discussed  in  some 
detail  as  they  are  encountered  later,  certain  of  their  general  char- 
acteristics as  determined  by  the  biochemist  will  be  briefly  mentioned. 
In  this  connection  it  is  interesting  to  note  that  six  enzymes  have  been 
prepared  in  crystalline  form,  and  all  are  proteins.  While  this  evidence 
is  not  conclusive  it  suggests  the  probable  chemical  nature  of  a  con- 
siderable number  of  these  activators.  Most  enzymes  have  what  the 
chemist  calls  a  reversible  reaction  and  so  may  be  capable  of  serving 
as  a  catalyst  for  both  hydrolyses  and  syntheses.  However,  it  should 
not  be  forgotten  that  under  some  conditions  an  action  may  be  practi- 
cally irreversible.  Such  is  the  case  with  glucose  which,  although 
theoretically  capable  of  reacting  in  several  different  ways,  continues 
to  react  in  one  direction  because  of  the  presence  and  concentration 
of  a  particular  enzyme.  Nearly  all  enzymes  appear  to  have  an  opti- 
mum working  temperature  of  about  40°  C.  (104°  F.).  Furthermore, 
enzymes  appear  to  be  specialized,  at  least  to  the  extent  of  requiring 
a  definite  acidity  or  alkalinity  of  the  surrounding  medium.  One 
classic  example  is  the  pepsin  of  the  stomach,  which  reacts  only  in  an 
acid  environment. 

Many  enzymes  seem  to  have  the  common  function  of  splitting  com- 
plex molecules  into  simpler  ones,  a  process  usually  accomplished 
through  the  addition  of  water,  or  hydrol3^sis.  Enzymes  acting  in 
this  manner  may  be  described  as  hydrolytic,  the  term  being  formed 
by  adding  the  suffix  lytic  to  the  Greek  stem  for  water.  The  enzymes 
themselves  are  designated  by  adding  the  ending  ase  to  the  name  of 
the  substance  upon  which  each  acts,  as,  for  example,  maltase  or  lipase, 
signifying,  respectively,  action  upon  maltose  or  the  lipins  (fats). 
Such  activators  may  be  spoken  of  collectively  as  hydrolases  since 
they  act  through  the  addition  of  water.  Similarly  the  catalyzing 
enzymes  for  oxidations  and  reductions  are  spoken  of  collectively  as 
oxidases.  A  few  other  enzymes  do  not  fall  into  either  of  the  above 
categories. 


THE   METABOLIC   MACHINERY   OF  AMMALS  281 

Digestion  in  Lower  Animals 

Digestion  within  the  animal  kingdom  is  primarily  of  two  sorts, 
intracellular  taking  place  within  the  cell  and  extracellular  which  is 
carried  on  outside  the  boundaries  of  the  cell.  Sometimes  both  types 
of  digestion  occur  in  the  same  organism.  The  complexity  of  the 
picture  among  one-celled  forms  may  be  appreciated  when  it  is  real- 
ized that  within  the  confines  of  a  single  cell  are  carried  on  all  the 
essential  processes  characteristic  of  a  many-celled  organism. 

Euglcna,  for  example,  shows  evidence  of  being  a  rather  generalized 
physiological  type  (see  page  157).  Within  the  group  to  which  it 
belongs  three  types  of  nutrition  are  found:  (1)  holophijtic  nutrition 
carried  on  by  the  aid  of  chlorophyll ;  (2)  saprophytic  nutrition  cor- 
responding to  that  carried  on  by  the  chlorophyll-less  molds  and 
fungi ;  and  (3)  holozoic  nutrition,  involving  the  ingestion  of  solid 
food  particles,  a  type  characteristic  of  animals.  Both  Ameba  and 
Paramecium  are  characterized  by  relatively  simple  intracellular 
digestion,  the  potential  food  reaching  the  interior  of  the  cell  by 
means  of  a  food  vacuole,  the  indigestible  particles  being  egested 
from  the  cell  later. 

In  sponges  ingestion  and  digestion  principally  occur  in  the  collared, 
or  choanoflagellate,  cells  where  food  vacuoles  are  formed  and  wastes 
egested.  The  nutritive  material  is  then  passed  from  one  cell  to  the 
other  and,  according  to  Hegner,  may  be  circulated  to  a  certain  extent 
by  wandering  ameboid  cells  found  in  the  middle  region  by  a  similar 
intracellular  action. 

In  the  coelenterates  one  first  finds  evidence  of  extracellular  digestion. 
Here  a  special  layer  of  cells  called  the  endoderm,  which  lines  the  prim- 
itive gastrovascular  cavity,  is  set  aside.  This  cavity  appears  in  Hydra 
as  a  simple  sac  lined  by  cells  possessing  the  ability  to  send  out  either 
flagella  or  pseudopodia.  Some  of  these  cells  are  glandular  and 
secrete  digestive  enzymes  which  are  passed  into  the  gastrovascular 
cavity,  making  digestion  an  extracellular  process.  A  certain  amount 
of  intracellular  digestion  does  take  place,  however,  since  some  of  the 
food  particles  are  surrounded  by  pseudopodia  and  so  brought  within 
the  walls  of  the  endodermal  cells. 

Most  of  the  parts  of  the  digestive  system  found  in  v(^rtebrates  are 
represented  in  the  earthworm  (see  page  189).  The  digestive  system 
of  a  crayfish  will  be  discussed  here  as  representative  of  the  Arthro- 
poda.     Its  food  consists  of  such  organisms  as  frogs,  tadpoles,  small 


282 


THE  MAINTENANCE  OF  THE  INDIVIDUAL 


fish,  insect  larvae,  snails,  and  decaying  organic  matter.  The  max- 
illipeds  and  maxillae  around  the  mouth  are  used  to  hold  the  food 
while  the  mandibles  crush  it  into  small  pieces  that  are  then  passed 
into  the  esophagus.  The  large  stomach  contains  a  series  of  chitinous 
ossicles,  forming  the  gastric  mill,  which  grinds  the  food.     When  the 

food  has  been  broken  up 


bloctde-r: 

mancLiblsS 
and.  mou-th- 

Gsophagixs —■ r 

sixbesophodeal-  " 
ganglior^ 


-«"J 


Supruesopba6ea.l 

g-a  rig  lion.       * 

circumagophcxSeol 
conn<activ<e.s  ^ 

grinding  stomach 


16 


sbemal 
artsry" "" 

opeTiing" 

vas  d&^&rsns^ 


r-i-estis 
..'haor-t 
--ostium 

...dorsal  artery 


:I1 


12 


iJ.-inLesLine. 


sufficiently,  it  passes 
through  the  strainer  into 
the  pyloric  chamber,  where 
the  digestive  glands  or 
"liver"  empty  their  secre- 
tions through  hepatic  ducts. 
These  glands  secrete  en- 
zymes which  digest  both 
proteins  and  fats.  From 
this  chamber  the  dissolved 
food  passes  into  the  in- 
testine where  nutritive  ma- 
terial is  absorbed  through 
the  intestinal  wall. 

Digestion  in  Higher 
Animals 


^3 


anus— w 

Sagittal  section  of  crayfish  showing  diges- 
tive system. 


In  the  vertebrate  series 
the  parts  of  the  digestive 
systems  are  analogous  and 
even  homologous  with  some 
invertebrate  structures. 
All  except  the  lowest  and 
parasitic  types  of  inverte- 
brates are  characterized  by 
an  alimentary  canal.  Dif- 
erences  which  occur  in  the  digestive  tracts  of  vertebrates  are  largely 
attributable  to  the  different  kinds  of  food  handled  by  different  types 
of  systems.  Carnivores  digest  their  foods  more  rapidly  than  herbi- 
vores and  so  can  get  along  with  a  shorter  alimentary  tract. 

Methods  of  Increasing  Digestive  Surfaces 

One  of  the  first  problems  in  digestion  is  the  production  of  an  ade- 
quate absorptive  surface.     Greater  digestive  surfaces  may  be  prO' 


THE   METABOLIC   MACHINERY    OF   ANIMALS 


283 


cured  by  increasing  the  length  or  diameter  of  the  aUmentary  tract,  or 
by  the  formation  of  pockets,  or  caeca,  of  different  sizes  and  shapes.  A 
carnivore  such  as  a  cat  has  an  ahmentary  tract  which  is  only  three  to 
five  times  the  body  length,  whereas  a  cow,  being  herbivorous,  supports 
one  over  twenty  times  the  length  of  its  body.  Man,  who  is  interme- 
diate as  well  as  an  "omnivorous  beast,"  has  one  about  ten  times  longer 


small  intestine. 


esopVia^us 


man     °PP^-^^^    rabbit 


appendix    dixode«UTn 


Cctt 


rabbit- 


^3) 


Comparison  of  digestive  tracts  of  a 
carnivore  and  herbivore.  How  can  the 
differences  in  the  size  of  the  caeca  and 
length  of  the  gut  be  explained  P  (After 
WeUs,  Huxley,  and  WeUs). 


£..r<3ctcx\  glandi 


Alimentary  canal  of  the  dogfish. 
State  the  function  of  the  vahular 
intestine.  What  are  the  principal  dif- 
ferences between  this  digestive  tract 
and  that  of  the  rabbit  P 


than  the  body.  A  modification  quite  characteristic  of  some  groups  is 
the  caecum,  which  is  noticeably  large  in  rodents.  Other  types  of  caeca, 
like  the  pyloric  caeca,  sometimes  occur  near  the  juncture  of  the 
stomach  and  the  intestine  among  fishes,  and  there  should  be  men- 
tioned here  the  longitudinal  fold,  or  typhlosole,  of  the  earthworm. 
When  such  a  longitudinal  fold  is  twisted  spirally,  there  results  a 
structure  known  as  a  spiral  valve,  v/hich  is  characteristic  of  sharks. 


284 


THE   MAINTENANCE   OF  THE   INDIVIDUAL 


Circular  folds  or 
plicae  circulares  in 
the  intestine  of 
man.  These  occur 
from  the  duodenum 
to  the  anus. 


Other  devices  such  as  throwing  the  surface  into  transverse  ridges 
are  quite  common,  for  example,  in  man  they  occur  in  the  intestine 
and  colon  as  plicae  circulares. 

Parts  of  the  Digestive  System 

The  Oral  Cavity.  The  various  mouth  cavities 
of  vertebrates  are  all  developed  for  one  fundamental 
purpose,  namely,  the  ingestion  of  food.  The 
mouth  cavity  is  specialized  in  many  different  ways 
and  is  further  complicated  in  air-breathing  forms 
by  the  necessity  for  completely  separating  the  air- 
intake  apparatus  from  the  digestive  tract.  This 
is  accomplished  quite  readily  in  water-inhabiting 
species  through  the  use  of  gill-slits.  In  land  forms, 
however,  the  external  nares  (nostrils)  and  associated 
nasal  passages  are  dorsal  and  the  lungs  ventral  to 
the  opening  of  the  digestive  tract.  It  is  neces- 
sary therefore  to  arrange  in  the  pharyngeal  region 
for  the  crossing  of  the  air  passageways  over  the 
food  tube.  This  separation  is  facilitated  in  most 
forms  by  the  presence  of  a  hard  bony  plate  known  as  the  hard  palate 
that  lines  the  roof  of  the  mouth.  At  the  posterior  end  of  the  hard 
palate  is  attached  a  flap  of  soft  tissue,  the  soft  palate,  which  further 
expedites  the  separation  of  respiratory  and  digestive  tracts. 

The  oral  cavity  is  lined  throughout  by  a  mucous  membrarie  the  cells 
of  which  secrete  mucus  that  serves  as  a  lubricant  facilitating  the 
passage  of  food.  This  same  tissue  is  found  throughout  the  entire 
surface  of  the  food  tube.  In  various  parts  of  the  alimentary  canal  are 
found  openings  of  various  glands  which  add  their  digestive  ferments 
to  the  mucus.  These  glands  will  be  considered  in  detail  under  the 
digestive  processes  of  man. 

Usually  the  surface  of  the  palate,  especially  the  posterior  part, 
known  as  the  soft  palate  and  uvula,  contains  numerous  mucus-secreting 
glands  called  the  palatine  glands.  The  secretions  of  these  glands  help 
to  keep  the  cavity  of  the  mouth  moist.  In  many  animals,  especially 
carnivores,  there  appear  a  number  of  washboardlike  palatine  ridges 
that  appear  to  be  an  adaptation  to  enable  its  owner  to  secure  a  surer 
grip  upon  the  unfortimate  victim  that  has  been  seized  in  its  jaws. 
The  large,  bulky  tongue,  which  occupies  practically  the  entire  floor  of 
the  buccal  cavity,  likewise  plays  an  important  role  in  eating. 


THE   METABOLIC    IVIACHINERY   OF   AMMAJ.S 


285 


Teeth  are  found  in  the  vertebrate  group  from  fishes  up  to  man. 
While  derived  from  a  common  embryological  source,  they  have 
developed  in  many  different  ways  during  the  course  of  e\olution  to 
serve  such  various  uses  as  grasping,  grinding,  or  cutting  food.     In 


skull 


frontal 

Sinus 


turbirTok/.V 

inteTTjol 
nostrij. 

external 
nostril 


SCCtlp 

ctura  mater 


pineal  glanct 
pitccito-ry 
"pons 

cersbellum 
--mectulicc 


cpiglotti: 
larynx 

trcuibea.... 
esophagus 
thyroid. 


Sagittal  section  of  human  head. 

many  of  the  lower  fishes  they  are  unspecialized  and  are  continu- 
ously being  replaced  as  worn  out.  Thus  the  shark  always  has  a 
new  set  developing  behind  the  old,  a  device  suggestive  of  an  end- 
less chain. 

The  garpike  has  a  series  of  long,  pointed,  unspeciahzed  teeth  which 
are  used  merely  as  holdfast  organs.  In  such  types,  teeth  are  not 
crushing  or  tearing  devices.  The  amphibia  and  reptiles  show  little 
tendency  toward  specialization,  except  among  the  poisonous  reptiles 


I^H 

IB 

m 

^^s* 

^^^^^H 

^< 

,   -tir^ 

i 

^- 

I^H 

^B 

fM 

X .  V.  Sinir  CnnxcnaHo)!  />ipl. 

Unspecialized  teeth  of  the  garpike,  Lc/x'sosleus  ossciis. 


286 


THE   MAINTENANCE   OF  THE   INDIVIDUAL 


with  their  fangs  and  the  toothless  jawed  turtles  that  make  up  for  the 
lack  of  teeth  by  sharp  cutting  horny  beaks  suggestive  of  the  bird's 
beak. 

The  greatest  development  and  specialization  of  teeth  occurs  among 
the  mammals.     According  to  their  shape  and  function  they  are  divided 

into  incisors,  or  cutting 
chisels,  canines,  or  graspers 
and  tearers,  premolars,  or 
grinders,  and  molars,  or 
crushers.  Here  we  find 
a  real  relationship  between 
the  type  of  teeth  and  the 
diet  of  the  organism.  In 
the  carnivores,  for  exam- 
ple, the  anterior  grinders 
are  so  constructed  that 
they  slide  like  shears  while  the  canines  are  specialized  for  grasp- 
ing animal  food,  the  back  molars  tending  towards  degeneracy.  In 
herbivorous  animals  except  the  rodents  the  front  teeth,  especially 
the  canines,  are  reduced  while  the  molars  become  greatly  developed. 
The  teeth  of  man  play  a  definite  role  in  the  mechanical  preparation 
of  food  for  digestion.     Instead  of 


Skull  of  a  squirrel,  a  rodent  (left),  and  a  cat, 
carnivore  (right).  Compare  carefully  for  differ- 
ences in  dentition. 


holding  the  prey,  they  crush, 
grind,  and  tear  the  food  so  that  a 
greater  surface  may  be  exposed  to 
the  action  of  digestive  juices. 
Man  like  some  other  organisms 
develops  more  than  one  set  of 
teeth.  The  first,  or  milk  teeth, 
are  only  twenty  in  number  while 
there  are  thirty-two  secondary, 
or  permanent  teeth. 

Each  tooth  is  divisible  into  an 
upper  gum-protruding  crown,  a 
lower  embedded  root,  and  an  in- 
termediate neck.  The  outer  part 
of  the  crown  is  protected  by  the 
hardest  substance  of  the  body, 
enamel,  that  surrounds  the  bony 
dentine.     This    in    turn    protects 


0xrTu.. 


JCTW 


enamel 
dentiYie 

.pulp 

.ner-v© 

-bloocL 
vessel 

cement. 


Sagittal  section  through  a  tooth, 
are  cavities  painful.^ 


Why 


THE   METABOLIC   MACHINERY   OF  ANIMALS 


287 


the  pulp  cavity  where  during  the  Ufe  of  the  tooth  nerves  and  blood 
vessels  are  housed.  Each  tooth  is  held  in  a  socket  of  the  jaw  by 
means  of  another  hard  tissue,  the  cement. 

Nearly  every  vertebrate  organism  possesses  some  sort  of  tongue 
which  serves  a  variety  of  functions.  The  lassoing  tongue  so  char- 
acteristic of  certain  amphibia,  for  example,  is  provided  with  special 
glands  secreting  glutinous  mucus  that  helps  to  ensnare  insects.  In 
lizards  the  tongue  may  become  extremely  long  and  extensile,  it 
also  servii^ig  to  aid  jn  capturing  food,  while  among  some  of  the 
birds  it  may  even  be  adapted  for  impaling  insects,  as  in  the  case 
of  the  "horny,  spearlike  tongue"  of  the  woodpecker.  The  mam- 
malian tongue  is  likewise  specialized,  for  in  many  of  the  herbivores  it 
is  definitely  muscular  and  prehensile,  being  used  to  grasp  tufts  of  grass 
which  are  then  cut  off  against  the  lower  incisors,  while  in  dogs  and 
cats  it  is  used  as  a  spoon  to  take 
up  liquids.  The  tongue  helps  me- 
chanically in  swallowing  and  in 
man  it  also  plays  a  vital  part  in 
speech.  The  tongue  of  higher 
forms  is  covered  with  a  variety 
of  sensory  structures  which  test 
the  various  foods  before  they  are 
swallowed. 

The  Phaeynx  and  Esophagus. 
This  region  is  both  membranous 
and  muscular.  We  may  think  of 
the  pharynx  in  all  air  or  land  verte- 
brates as  being  an  irregular  cavity 
supplied  with  openings.  Dorsally 
and  anteriorly  are  two  posterior 
nares,  or  internal  nostrils,  laterally 
the  openings  of  the  Eustachian  tubes 
connecting  with  the  middle  ear, 
while  medianly  and  ventrally  lies 
the    opening    to    the    oral    cavity. 

Posteriorly  there  are  two  openings,  one  down  the  esophagus  and  the 
other,  the  glottis,  leading  into  the  trachea  (see  fig.,  page  285).  Above 
the  soft  palate  is  a  mass  of  lymphoid  tissue,  known  as  the  adenoids, 
or  pharyngeal  tonsils,  while  anteriorly  and  laterally  lie  the  true,  or 
palatine  tonsils. 


cr'op 


$l:omocc"h 


.gi3)5ard; 


intestine 


Stomach"  of  bird.     What  are  the 
functions  of  the  different  parts  .^ 


288 


THE   MAINTENANCE   OF  THE   INDIVIDUAL 


The  Stomach.  The  stomach  of  vertebrates  is  likewise  subject  to 
considerable  variation.  In  the  case  of  grain-eating  birds  a  distended 
esophageal  region,  the  crop,  is  developed  for  the  storage  of  food. 
Below  this  region  is  the  stomach  proper,  divisible  into  a  glandular 
stomach,  which  secretes  digestive  enzymes,  and  a  muscular  gizzard, 

or     grinding     stomach, 
that    compensates    for 


esophagjxs 


rtcmen, 


psalterium 


ahomasutn 


Stomach  of  a  ruminant.  What  is  the  function 
of  the  valve  and  what  is  the  significance  of  "chew- 
ing the  cud"  ?     (After  Walter.) 


the  absence  of  teeth. 

A  second  example  of 
an  outstandingly  differ- 
ent type  of  stomach  ap- 
pears in  the  compound 
stomach  of  ruminants 
as,  for  example,  a  cow. 
Here  there  are  four 
parts,  namely,  the  ru- 
men, recticulum,  psalte- 
rium,  and  abomasum, 
the  first  two  being 
derivatives  of  the  esoph- 


agus. The  more  solid  food  is  temporarily  stored  in  the  rumen,  or 
paunch,  as  fast  as  it  is  ingested,  gradually  being  passed  on  into  the 
reticulum  where  it  is  mixed  further  with  digestive  juicfes  and  softened. 
From  time  to  time,  ball-like  masses  of  this  food  are  regurgitated  from 
the  reticulum  and  thoroughly  mixed  with  saliva  by  chewing.  This 
process  is  commonly  known  as  "chewing  the  cud."  After  a  time  the 
food  is  swallowed  a  second  time  and  if  the  chewing  has  sufficiently 
reduced  the  mass  to  a  small  slippery  wad,  it  passes  directly  into  the 
psalterium  and  thence  to  the  abomasum,  where  it  undergoes  gastric 
digestion. 

The  human  stomach  as  compared  with  the  compound  stomach  of 
the  ruminants  is  of  a  more  simple  type,  although  divisible  both 
histologically  and  physiologically  into  several  parts.  The  esophagus 
enters  an  expanded  cardiac  region  the  entrance  of  which  is  guarded 
by  a  ringlike  sphincter  muscle.  The  stomach  is  always  curved  to 
some  extent,  the  inner  or  concave  surface  being  known  as  the  lesser 
curvature  and  the  outer  or  convex  as  the  greater  curvature.  The  blind, 
rounded  part  of  the  stomach  lying  to  the  left  and  usually  opposite  the 
entrance  of  the  esophagus  is  called  the  Jundus,  while  the  region  closest 
to  the  point  of  entrance  of  the  esophagus  is  called  the  cardiac  portion ; 


THE   METABOIJC    MACHINERY   OF  ANIMALS 


289 


the  lower  end  is  known  as  the  pyloric  part,  the  extreme  Hmit  of  which 
is  indicated  by  a  groove  called  the  pylorus.  The  pyloric  and  fundic 
parts  of  the  stomach  differ  in  the  nature  of  their  musculature  as  well 
as  in  their  physiological  activity  during  digestion.  The  pyloric  part 
is  separated  from  the  small  intestine  by  a  sphincter  muscle,  called  the 
sphincter  pylorus.  The  shape  and  position  of  the  stomach  may  \'ary 
according  to  the  posture  and  amount  of  food  ingested.  Thus,  while 
the  stomach  is  supposed  to  lie  in  an  "obliquely  transverse  position," 


Cocrdiac   region. 
funcLus 


-T^egion. 


£oT2_ 


ga-s  bubble 

.CarcCiac 
region 

intermecCiate 
portioi-x 


ctuocCsr2Ltri>. 

pyloras-j^ 

pyloric 


The  human  stomach  (1)  as  usually  depicted,  (2)  the  shape  and  position  of  the 
stomach  as  shown  by  X-ray,  (3;  stomach  and  large  intestine  showing  position  of 
food  at  varying  hours  after  ingestion.      (After  Howell.) 

it  really  assumes  a  J-shape  as  detected  by  X-rays.  The  folded  wall 
of  the  fundus  is  dotted  with  thousands  of  tiny  pits,  the  mouths  of 
gastric  glands,  or  little  tubes  the  epithelial  lining  of  wliich  secretes 
the  gastric  juice.     (See  page  294.) 

As  in  the  case  of  the  remainder  of  the  digestive  tract,  the  stomach 
wall  is  made  up  of  several  layers  of  tissue.  Beginning  with  the  inside 
is  the  soft,  thick,  glandular  mucosa,  usually  thrown  into  folds,  or 
rugae,  which  tend  to  disappear  when  the  stomach  is  distended. 
A  second  layer,  the  submucosa,  composed  of  loose  connective  tissue 


290 


THE   MAINTENANCE   OF  THE   INDIVIDUAL 


lies  between  the  mucous  and  muscular  layers.  The  latter  is  made  up 
of  three  layers  of  involuntary  muscles,  an  inner,  poorly  developed 
obUque  layer  over  which  lies  a  circular  layer  that  in  turn  is  enclosed 
by  an  outer  layer  of  longitudinal  muscles.     The  fourth  or  outermost 

coat  is  known  as  the  serosa, 


ynoufh 


capillar 
lumen 


parietal' 
cells 


tissoe  ^ 


:^'J> 


A  typical  gastric  gland.     Explain  the 
functioning  of  each  part. 


which  is  continuous  with  the 
peritoneum  and  as  such  covers 
both  organs  and  their  associ- 
ated glands.  This  covering  is 
moist  and  serves  not  only  as 
a  protection  but  also  facilitates 
the  movement  of  one  portion 
over  the  other. 

Food  in  order  to  reach  the 
stomach  must  be  rolled  into 
boluses  and  then  swallowed. 
This  is  a  complicated  reflex 
movement  which  apparently 
may  be  more  or  less  volun- 
tarily initiated  as  the  bolus 
passes  into  the  pharyngeal 
region,  past  the  trap  door  (epi- 
glottis) which  covers  the  open- 
ing into  the  larynx  and  trachea. 


Failure  of  this  flap  to  close  properly  results  in  food  "going  down  the 
wrong  way,"  when  the  mass  is  expelled  after  a  paroxysm  of  choking 
and  coughing. 

Liquids  and  soft  foods  reach  the  stomach  in  about  0.1  second  while 
more  solid  boluses  are  passed  along  by  a  series  of  slow-moving  wavelike 
contractions,  called  peristalsis.  Boluses  require  about  six  seconds  to 
reach  the  stomach.  The  entrance  of  food  into  the  stomach  is  prob- 
ably controlled  by  the  cardiac  sphincter.  Solid  food  may  remain 
in  the  stomach  for  several  hours.  One  of  the  first  noteworthy  obser- 
vations of  this  process  was  made  upon  Alexis  St.  Martin,  a  Canadian 
voyageur  who  was  studied  by  Beaumont  in  1847.  The  adventurer 
had  a  permanent  opening  into  his  stomach  as  a  result  of  a  gunshot 
wound,  which  permitted  direct  observation  of  processes  going  on  within 
the  stomach.  These  and  other  studies  indicate  that  the  fundus  largely 
fimctions  as  a  reservoir  which  retains  the  bulk  of  the  food  while  the 
more  muscular  pyloric  portion  churns  it,  forcing  it  periodically  into 


THE   METABOLIC   MACHINERY   OF   ANIMALS  291 

the  first  part  of  the  small  intestine  (duodenum).  It  is  interesting  to 
remember  that  carbohydrates  pass  out  of  the  stomach  soon  after 
ingestion,  remaining  only  about  one  half  as  long  as  proteins.  Fats 
hkewise  remain  a  long  time  within 
the  stomach  even  when  combined 
with  other  foodstuffs. 

The  Small  Intestine.  The 
intestine  is  subdivided  into  a 
region  principally  devoted  to 
absorption     of    digested    foods, 

namely  the  small  intestine  and  the  (Y 

large  intestine  which  to  a  lesser  M 

extent  is  devoted  to  a  continua-  '^i^^cting'    nioUon    shoNvn  in. 

tion  of  absorption,   and   to    the  ^■'^•^  ■  '^^/'^hmic.Segnrjenting- 

collection     of     waste     products.  ''T^overrjenLs. 

The  entire  small  intestine  of  man,  r\ f^^"--^ 

some  twenty  feet  in  length  and  e^ "^^^-^ 

about    an   inch    in    diameter,    is      cLiastalsis  is  Cannon's  nanxe. 
concerned  with  the  digestion  and      ^°^  '^^'^  perisLxlLic  wave  xvl^ich 
„.  ,.  c    c      1  1.,-         Tnoves   olonS  U^e  intestine, 

absorption    of    foods    and    their  prccecCad    Joy -inhibition 

transfer  to  the  blood  stream.     It  t^.  ,    .„  •     ,  • 

,        ,     ,.  ,     ,  Diagram  to  illustrate  peristalsis. 

IS  also  believed  that  some  waste 

materials  are  actually  excreted  into  the  lumen  of  the  gut.  These 
functions  are  accomphshed  by  a  series  of  adaptations,  one  of  which 
is  the  extraordinary  length  of  the  small  intestine,  together  with 
numerous  small  circular  ridges,  -plicae  circulares,  which  serve  the 
double  function  of  giving  an  increased  absorptive  surface  and  of 
retarding  the  rate  of  passage  of  foodstuffs.  The  other  but  by  no 
means  the  least  important  adaptation,  is  the  presence  of  millions  of 
small  knoblike  projections,  or  villi.  These  tiny  structures  according 
to  Howell  move  actively  either  by  lateral  lashings  or  by  extension 
and  retraction.  It  is  believed  that  these  movements  are  associated 
with  the  act  of  absorption  and  probably  play  an  important  part  in 
emptying  the  lymph  sac,  or  lacteal,  lying  in  the  center  of  each  villus. 
By  means  of  the  plicae  circulares  and  the  villi,  the  small  intestine  is 
estimated  to  have  an  absorbing  surface  equal  to  twice  that  of  the 
surface  of  man's  body. 

The  internal  structure  of  the  villus  is  best  seen  in  a  longitudinal 
section.  The  outer  wall  is  composed  of  a  thin  layer  of  epithelial  cells 
in  which  the   more   complex   fats   are   resynthesized  before   being 


292 


THE   MAINTENANCE   OF   THE   INDIVIDUAL 


passed  to  the  ladeals.  Beneath  this  is  a  mass  of  connective  tissue 
permeated  by  a  network  of  capillaries  that  in  turn  surround  the 
central  lymph  channel  (lacteal)  into  which  fat  is  absorbed.  Between 
the  villi  are  found  the  openings  of  the  intestinal  glands  which 
are  associated  with  the  compound  duodenal  glands  in  the  production 
of  intestinal  juice.  Aggregations  of  two  types  of  lymph  nodules 
appear,  solitary  lymph  nodes  about  the  size  of  a  pin  head  and  groups 
spoken  of  as  Peyer^s  patches.  The  latter  are  sometimes  the  seat  of 
local  inflammation  and  ulceration  as  in  typhoid  fever. 

The  same  four  coats  which  were  found  about  the  stomach  occur 
in  the  small  intestine  except  that  the  oblique  layer  of  muscles  is 
missing,  while  the  mucous  layer  is  very  thick  and  vascular. 

The  Large  Intestine.  The  large  intestine  of  man  has  somewhat 
the  same  anatomical  structure  as  the  small  intestine  except  that  it 
lacks  villi  and  has  a  greater  diameter.  It  is  separable  into  a  shallow 
blind  pouch  at  the  juncture  of  the  small  and  large  intestines,  and  an 
enlarged  colon  and  rectum,  terminating  with  the  anus.  The  entrance 
of  material  into  the  large  intestine  is  regulated  by  the  ileo-caecal 
valve,  formed  by  two  flaps  of  mucous  membrane,  which  permits  entry 
into  it  but  effectively  prevents  back  flow.     At  the  end  of  the  caecum 

is  a  A'estigial  continuation  of 
it,  the  vermiform  appendix,  a 
blind  pouch  usually  about  three 
inches  long.  Inflammation  of 
this  structure  usually  results 
in  a  condition  recognized  as 
appendicitis. 

The  colon  of  man  is  divisible 
into  four  parts  known  respec- 
tively as  the  ascending,  trans- 
igrr?oJd    verse,  descending,  and  sigmoid 
-Oiorv     colons.     In    other    mammals, 
the  colon  may  not  always  be 
rectum  separated   into   these   parts 

The  caecum,  appendix,  and  colons  of  although  the  juncture  of  the 
man.  Why  is  the  appendix  so  frequently  .n^alland  large  intestines  is 
the  seat  ol  bacterial  inlections.J  ^ 

clearly  set  off  by  an  ileo-caecal 

valve  and  a  caecum.  The  anus  is  guarded  by  both  an  external  and 
an  internal  sphincter  which  keep  the  orifice  closed  except  during 
defecation.     The  external  sphincter  is  composed  of  striated  muscle 


caecum 

appendi 


THE    METABOLIC    MACHINERY   OF   ANIMALS  293 

and  is  under  the  direct  control  of  the  will,  while  the  internal  sphincter 
is  derived  from  one  of  the  coats  of  the  rectum  and  consists  of  un- 
striated  or  involuntary  muscle. 

The  process  of  absorption  is  thought  to  be  continued  to  a  limited 
extent  in  the  large  intestine  as  its  contents  are  retained  for  a  consider- 
able time.  The  secretions  of  this  region  are  alkaline,  containing  much 
mucus  l)ut  apparently  no  enzymes.  By  the  time  the  contents  reach 
the  large  intestine  the  water  content  is  considerably  reduced  through 
absorption.  Bacteria,  which  compose  nearly  50  per  cent  of  the  human 
feces,  carry  on  putrefactive  protein  fermentation  in  the  large  intestine. 

The  Digestive  Glands  and  Their  Enzymes 

The  chemical  processes  of  dige.stion  occur  largely  through  the  activ- 
ity of  enzymes  which  are  produced  in  a  variety  of  different  glands. 
Practically  all  vertebrates  possess  salivary  and  gastric  glands,  a  liver, 
pancreas,  and  various  intestinal  glands. 


v/^^*>,*.^i^^^  SLcblirj^t^cd  duct 

submaxillary  ^^itblii' 
glancC  gl^^c 

Salivary  glands  in  man.     What  enzyme  do  these  glands  secrete  ?     (After  Walter.) 

The  Salivary  Glands.  Saliva,  which  acts  as  a  lubricant  in  the 
mouth,  is  manufactured  in  the  cells  of  three  pairs  of  glands  that 
empty  into  the  mouth  by  ducts,  and  which  are  called,  according  to 
their  position,  the  parotid  (beside  the  ear),  the  submaxillary  (imder 
the  jawbone),  and  the  sublingual  (under  the  tongue).  In  addition, 
the  salivary  glands,  which  are  absent  in  most  aquatic  forms,  secrete 
a  digestive  enzyme,  ptyalin,  that  acts  upon  starch  in  an  alkaline 
medium,  splitting  it  partially  or  entirely  into  a  disaccharide  sugar 
known  as  maltose.  Ptyalin  is  present  in  all  mammals  except  those 
which  are  entirely  carnivorous. 


294  THE   MAINTENANCE   OF  THE   INDIVIDUAL 

The  chewing  process  theoretically  inixes  food  with  saliva  thoroughly 
but  in  man  the  bolus  is  invariably  swallowed  before  the  ptyalin  has 
completed  its  action.  Recent  studies  indicate  that  salivary  digestion 
continues  in  the  stomach  for  some  time  until  stopped  by  the  hydro- 
chloric acid  of  the  stomach. 

The  Gastric  Glands.  The  inner  surface  of  the  stomach  is 
covered  with  cells  producing  mucus,  the  entire  region  being  dotted 
with  thousands  of  tiny  gastric  glands  secreting  gastric  juice.  Most 
of  the  lumen  of  each  gland  is  lined  by  columnar  epithelial  cells  called 
chief  cells,  while  between  the  basement  membrane  and  the  chief  cells  of 
the  glands  lie  scattered  parietal  cells.  The  chief  cells  of  the  neck 
of  the  gland  secrete  mucus  while  those  lower  down  secrete  an  in- 
activated enzyme  or  zymogen,  called  pepsinogen.  Oval  parietal  cells 
secrete  hydrochloric  acid,  which  activates  the  pepsinogen,  converting 
it  into  an  active  enzyme  (pepsin),  that,  in  the  presence  of  this  acid, 
breaks  down  proteins  to  the  intermediate  products,  peptones  and 
proteoses.  Gastric  juice  is  slightly  acid  in  its  chemical  reaction, 
containing  about  0.2-0.4  per  cent  of  free  hydrochloric  acid  together 
with  another  enzyme  called  rennin.  The  latter  curdles  or  coagulates 
casein,  a  protein  found  in  milk,  which  is  the  basis  of  cheese.  After 
milk  is  curdled  pepsin  is  able  to  act  upon  it.  "Junket"  tablets, 
which  contain  rennin,  are  used  for  this  purpose  in  the  preparation  of 
a  dessert  which  has  milk  as  a  basis. 

The  stomach  is  the  place  where  the  digestion  of  proteins  is  initiated 
and  where  digestion  of  carbohydrates  may  be  continued.  Some 
investigators  believe  that  emulsified  fats  such  as  cream  are  digested 
by  a  gastric  lipase.  However,  since  saponification  and  emulsification 
must  take  place  before  absorption,  and  after  the  fats  reach  the  intes- 
tine, it  appears  probable  that  fats  undergo  no  digestive  changes  in 
the  stomach. 

Although  little  or  no  absorption  takes  place  in  the  stomach,  under 
certain  conditions  water,  salts,  alcohol,  and  drugs  may  be  absorbed. 
There  appears  little  evidence  at  present  to  support  the  contention  that 
sugars  and  peptones  are  appreciably  absorbed  in  this  organ. 

Food,  after  being  mixed  with  gastric  juice,  becomes  increasingly 
liquid  and  is  known  as  chyme,  in  which  state  it  passes  through  the 
pylorus.  The  next  step  is  facilitated  by  the  muscular  movements  of 
the  small  intestine,  which  are  primarily  of  two  kinds.  The  first, 
peristalsis,  helps  pass  the  food  slowly  along  the  intestine.  The  second, 
rhythmical  contractions  or  segmentation,  may  be  described  as  a  series  of 


THE   METABOLIC   MACHINERY   OF   ANIMALS  295 

local  constrictions  occurring  at  points  where  the  food  masses  lie. 
Such  contractions  break  up  the  food  into  a  number  of  segments 
enabling  the  enzymes  to  reach  all  parts. 

The  Intestinal  Glands.  The  partly  digested  food  in  the  small 
intestine  comes  in  contact  almost  simultaneously  with  secretions  from 
the  liver,  pancreas,  and  intestinal  glands. 

The  Pancreas.  As  the  acid  chyme  enters  the  duodenum  it 
activates  some  "prohormone,"  probably  -prosecretin,  which  is  first 
absorbed  into  the  capillaries  of  the  blood  vessels  and  then  carried 
throughout  the  body.  Some  secretin  ultimately  reaches  the  pancreas, 
which  is  then  stimulated  to  further  activity  causing  the  chemical 
secretion  of  the  pancreatic  juice.  The  pancreas  is  one  of  the  most 
important  digestive  glands  in  the  human  body.  It  is  anatomically 
a  rather  diffuse  structure  resembling  the  salivary  glands  in  form. 
Its  duct,  joined  with  the  bile  duct  from  the  liver,  empties  into  the 
small  intestine  a  short  distance  below  the  pylorus  near  the  juncture 
of  the  duodenum  and  the  ileum. 

The  secretions  of  the  pancreas  or  "stomach  sweet  bread"  contain 
three  groups  of  enzymes,  (1)  amylopsin,  (2)  trypsin  and  some  erepsin, 
and  (3)  lipase,  which  act  respectively  upon  carbohydrates,  proteins, 
and  fats.  The  first,  amylopsin,  breaks  down  starches  by  hydrolysis 
to  double  sugars,  finally  yielding  the  disaccharide  maltose,  and  dextrin. 
Maltose  is  further  broken  down  by  maltase  into  a  monosaccharide, 
glucose  (dextrose),  which  may  then  be  absorbed. 

Second,  in  order  for  absorption  to  take  place  in  proteins  they  must 
be  broken  down  into  their  constituent  arnino  acids  by  the  action  of 
at  least  trypsin  and  erepsin.  Protein  material  reaches  the  first  por- 
tion of  the  small  intestine,  or  duodenum,  in  the  acid  chyme  which  is 
generally  neutralized  somewhat  before  the  proteolytic  enzymes  do 
their  work. 

Third,  fats,  thus  far  unchanged  in  the  process  of  digestion  except  to 
be  melted  by  the  heat  of  the  body,  are  then  emulsified  by  the  bile  and 
finally  are  hydrolyzed  in  the  intestine  by  the  action  of  lipase  into 
glycerol  (glycerin),  and  also  one  or  more  fatty  acids.  These  are 
absorbed  by  the  epithelial  cells  of  the  villi,  resynthesized  into  more 
complex  fats,  and  passed  into  the  lymph  channels,  or  lacteals. 

Aside  from  the  noteworthy  office  of  "secretor  of  the  pancreatic 
juice,"  the  pancreas  has  another  important  function.  One  might 
say  that  it  is  one  of  the  "board  of  directors"  governing  the  li(>altli 
of  the  body.     When  the  sugar  content  of  the  blood  becomes  too  high 

H.  V.  H.  —  20 


296  THE   MAINTENANCE  OF  THE   INDIVIDUAL 

and  sugar  appears  in  the  urine,  diabetes,  a  disease  caused  by  a 
dearth  of  insulin  in  the  blood,  occurs.  Insuhn  is  a  hormone*  formed  by 
groups  of  cells  collectively  called  the  islands  of  Langcrhans,  which 
function  as  ductless  glands.  Since  1921,  when  Banting,  Best,  and 
Macleod  found  that  insulin  injected  into  animals  showing  symptoms 
of  diabetes  caused  a  decrease  of  sugar  in  blood  and  urine,  this  pan- 
creatic hormone  has  become  a  veritable  lifesaver  to  man. 

The  Liver.  The  liver  is  the  largest  gland  in  the  body,  and  in  man 
is  found  just  below  the  diaphragm,  a  little  to  the  right  of  the  mid 
line  of  the  body.  It  is  not  primarily  a  digestive  gland,  although  it 
secretes  daily  about  a  quart  of  bile,  which  while  containing  no  en- 
zymes may  have  the  power  of  rendering  the  lipase  of  the  pancreatic 
fluid  more  active.  Bile  when  mixed  with  the  pancreatic  juice  helps 
emulsify  liquid  fats  into  minute  separate  droplets,  in  this  way  pre- 
paring them  for  digestion.  Certain  substances  in  the  bile  aid  espe- 
cially in  the  absorption  of  fats.  Another  important  function  of  bile 
is  the  neutralization  (wholly  or  in  part)  of  the  acid  chyme  when 
it  enters  the  duodenum,  thus  preparing  it  for  the  action  of  the 
pancreatic  juice.  Bile  also  stimulates  the  peristaltic  movements  of 
the  intestine,  thus  preventing  extreme  constipation.  It  is  also  thought 
by  some  to  have  a  slight  antiseptic  effect  in  the  intestine.  Bile  seems 
to  he  mostly  a  waste  product  from  the  blood.  Its  color  is  due  to 
certain  substances  wiiich  result  from  the  destruction  of  worn-out  red 
corpuscles  of  the  blood. 

Besides  these  digestive  and  excretory  functions  the  liver  is  also 
concerned  with  the  formation  of  a  nitrogenous  waste,  urea,  CO(NH2)2. 
This  product  is  largely  ]:)roduced  in  the  liver,  whence  it  is  transferred 
to  the  blood  and  carried  to  the  kidneys  where  it  is  excreted. 

Perhaps  the  most  important  function  of  the  liver  is  the  formation 
and  storing  of  an  animal  starch,  or  glycogen.  The  liver  is  supplied 
with  blood  from  two  sources,  some  from  the  heart,  but  a  greater 
amount  directly  from  the  walls  of  the  stomach  and  intestine.  This 
latter  blood  supply  is  very  rich  in  food  materials  and  from  it  the  cells 
of  the  liver  take  out  sugars  in  the  form  of  glucose  (dextrose),  which  is 
synthesized  into  animal  starch  in  the  liver.  Glycogen  is  stored  in 
the  liver  until  such  time  as  energy  is  needed.  It  is  then  reconverted 
to  the  monosaccharide  form,  glucose,  and  carried  by  the  blood  stream 
to  the  tissues  where  it  is  oxidized  with  an  accompanying  release  of 
energy.  A  limited  amount  of  glycogen  may  be  found  and  stored 
in  the  muscles  and  it  is  also  thought  to  be  produced  from  proteins  and 


THE   METABOLIC    MACIIINEHY   OK   ANIMALS  297 

possibly  fats  as  well  as  carbohydrates.  Storage  of  glycogen  in  the 
liver  has  been  demonstrated  by  taking  two  rabbits,  which  were  fed 
heavily  on  clover  after  a  period  of  starvation.  After  allowing  suit- 
able time  for  digestion  and  assimilation,  one  rabbit  was  killed  and 
glycogen  was  demonstrated  in  the  liver  cells,  while  the  other  was 
given  strenuous  exercise  before  being  sacrificed  to  science.  Upon 
examination  the  second  rabbit  showed  a  greatly  reduced  quantity  of 
glycogen  in  the  liver  cells. 

The  Secretions  of  the  Small  Intestine.  There  can  be  no 
doubt  of  the  importance  of  the  part  played  by  the  pancreas  and  liver 
in  digestion  which  is  supplemented  by  secretions  of  the  intestinal 
wall,  called  collectively  intestinal  juice,  or  succus  entericus,  a  substance 
containing  five  important  enzymes  secreted  by  small  intestinal 
glands  of  the  mucosa  (see  figure  of  villus).  The  first,  enterokinase, 
acts  as  a  co-ferment  on  proteins  and  was  formerly  thought  to  be  an 
activator  for  trypsinogen.  Erepsin,  while  appearing  to  be  the  same 
as  that  appearing  in  the  pancreas,  hydrolyzes  peptides  to  amino  acids  ; 
maltasc,  as  previously  noted,  converts  maltose  into  dextrose,  while 
lactase  hydrolyzes  milk  sugar  into  the  simple  compounds  of  galactose 
and  dextrose,  and  invertase  converts  ordinary  table  sugar  into  levu- 
lose  and  dextrose.  The  la.st  three  are  frequently  spoken  of  col- 
lectively as  inverting  enzymes. 

It  should  be  remembered  that  the  large  intestine  produces  no 
enzymes,  wherefore  it  is  as.sumed  that  little  or  no  digestion  takes 
place  there.  The  bacteria  of  the  large  intestine  attack  any  protein 
material  which  has  escaped  digestion  and  break  it  down  by  putre- 
factive fermentation. 

Absorption  and  the  Fate  of  Absorbed  Foods 

In  animals  that  possess  circulatory  systems  the  diffusible  end-prod- 
ucts of  foods  are  passed  through  the  epithelium  of  the  gut  into  the 
blood  stream,  or,  in  the  case  of  fats,  through  the  lymphatics  to  the 
blood.  In  higher  vertebrates  most  of  the  absor})tion  takes  place  in 
the  walls  of  the  small  intestine.  While  diffusion  and  osmosis  are  im- 
portant factors  in  the  passage  of  food  and  water  through  the  walls 
of  the  intestine,  many  physiologists  agree  that  the  living  matter  in 
the  cells  lining  the  intestine  exerts  energy  which  affects  the  absorption 
of  the  substances  that  pass  into  the  blood  and  lacteals.  This  is  proved 
by  the  fact  that  if  these  cells  are  injured  or  poisoned,  absorption 
follows  the  laws  of  osmosis  and  diffusion.     Ordinarily  the  cells  lining 


298 


THE  MAINTENANCE  OF  THE  INDIVIDUAL 


the  intestine  are  like  tiny  chemical  laboratories.  Carbohydrates  in 
the  form  of  monosaccharides,  or  glucose  (dextrose),  are  absorbed 
through  the  epithelial  cells  lining  the  villi  and  reach  the  capillaries  of 
the  circulatory  system.  Proteins  in  the  form  of  amino  acids  likewise 
reach  the  blood  stream  in  this  way.  Glycerin  and  fatty  acids  are 
absorbed  by  the  epithelial  cells,  resynthesized  in  these  minute  chemical 
laboratories  into  more  complex  fats,  and  are  then  passed  on  to  the 
lymph  channels,  ladeals,  of  the  lymphatic  system  in  the  villi.     This 


gobWt  cell  ,cap\lk 


Cells    of 
epithaliUnj 


intestm-al 
glcxrjcC 


niuscler 
vein-*/; 


Icccteal- 
arter^i 

Diagram  of  intestinal  villi  and  glands.     Can  you  explain  the  part  played  by 
the  villi  in  absorbing  digested  "  foods  "  ? 


fluid  or  lymph  then  passes  into  the  other  lymphatics,  eventually  reach- 
ing the  blood  through  the  thoracic  duct  which  enters  the  jugular  vein 
in  the  neck.  On  the  other  hand,  simple  sugars  and  amino  acids  pass 
directly  into  the  blood  and  reach  the  blood  vessels  which  carry  them 
to  the  liver,  where,  as  we  have  seen,  sugar  is  taken  from  the  blood 
and  stored  as  glycogen.  From  the  liver  the  food  within  the  blood  is 
carried  to  the  heart  and  is  then  pumped  to  the  tissues  of  the  body. 
A  large  amount  of  water  and  some  salts  are  also  absorbed  through 
the  walls  of  the  stomach  and  intestines.  The  greatest  loss  of  water, 
however,  occurs  in  the  large  intestine. 


THE   METABOLIC   MACHINERY   OF  ANIMALS 


299 


We  have  already  traced  the  changes  taking  place  in  the  absorbed 
sugars,  chiefly  dextrose,  and  have  shown  how  they  may  be  taken  from 
the  blood  stream,  converted  into  glycogen,  and  temporarily  stored. 
Some  of  this  sugar  is  usually  available  in  the  circulating  blood  which 
contains  0.1  to  0.15  per  cent  of  it.  The  muscles  likewise  store  glyco- 
gen that  is  used  as  work  is  done.  Carbon  dioxide  and  water  are 
the  final  products  of  carbohydrate  oxidation.  Experimental  evidence 
indicates  that  glycogen  may  be  produced  from  some  of  the  metabolic 


PTOcass:  builds  protoplasm; 
.  ^lastss.  mostly  crccttinin 
and.  pifTin.  booCies 


by 

deamination 


oarboViydrcLtes 
formecL  by 
daatninotion 


process :  excretion  of  ^fastes, 

as  arao.  anct  uric  acict, 

r'eptilas  a.not  bircts . 


etirect 
Combustion 


cLirect 
combltsCion. 


^    JT 


oCir-ect 
Combustioa 


process:  oxidation, 

Gnergsy  relsasscC  for~ 
msto-Dolisra  ■  wastes, 
■woter  ancCearpon  dioxide 


/" 


Summary  of  metabolic  processes. 


products  of  proteins.^  The  production  of  glycogen  from  fats  still 
lacks  conclusive  evidence,  although  there  is  some  indication  of  indirect 
conversion. 

The  proteins  which  have  been  absorbed  may  be  utilized  in  two 
ways :  (1)  in  the  rebuilding  of  broken-down  protoplasm ;  (2)  in  the 
supply  of  energy  for  work.  Consequently,  protein  substances  are 
often  differentiated  into  tissue  builders  and  energy  producers. 

Fats  ultimately  reach  the  circulating  blood  from  which  they  are 
taken  up  and  used  by  the  various  tissues.  Fats  may  be  oxidized 
within  the  cell  to  supply  energy.  In  such  cases  the  final  products 
are  carbon  dioxide  and  water.     When  excess  fat  is  eaten  it  is  held  in 


1  Howell,  Textbook  on  Physiology,  12th  ed.     Saunders,  p.  869. 


300  THE   MAINTENANCE   OF  THE   INDIVIDUAL 

reserve  in  adipose  tissues.  Sonic  animals  must  build  up  a  large  sui)ply 
of  fat  so  that  they  may  draw  upon  it  when  their  food  supply  is  low. 
This  is  particularly  true  of  such  hibernating  animals  as  the  bear  that 
emerges  in  the  spring  from  a  period  of  sleep  at  a  time  when  its  fat 
supply  is  depleted.  Fat  storage  in  man,  upon  the  other  hand,  is 
entirely  unnecessary  from  a  physiological  point  of  view  and,  due  to 
the  frequency  of  meals,  is  usually  quite  involuntary. 

SECTION  B.     THE  HOW    AND  WHY  OF    CIRCULATION 

Why  a  Transportation  System? 

Within  the  body  of  nearly  all  of  the  metazoa  evidence  of  a  highly 
specialized  system  of  internal  transportation  is  found.  The  degree  of 
development  of  such  a  system  depends  mostly  upon  the  size  of  the 
organism,  the  amount  of  activity  it  displays,  the  complexity  of  its 
internal  organization,  and  whether  or  not  it  is  a  warm  blooded  animal. 
The  size  of  the  body,  the  speed  and  frequency  with  which  the  animal 
moves  are  some  of  the  factors  that  determine  how  "specialized  and 
well  trained"  the  "handy  man"  about  the  body,  i.e.,  the  circulatory 
system,  must  be.  With  specialization  comes  greater  division  of 
labor,  yet  specialized  parts  such  as  nerve  cells  and  muscle  fibers  require 
not  only  nourishment  but  also  the  elimination  of  waste  products 
from  their  immediate  vicinity  as  well  as  favorable  conditions  of 
temperature.  The  solution  of  the  problem  is  met  in  part  by  more  or 
less  bathing  all  cells  in  lymph  which  serves  for  bringing  food  to  the 
cells  and  for  the  removal  of  wastes.  In  order  to  secure  a  continuous 
food  supply  and  to  insure  the  adequate  removal  of  wastes  such  a 
transportation  system  is  necessary. 

In  all  but  the  simplest  organisms  such  a  system  is  composed  of 
vessels  containing  lymph  which  brings  its  contents  to  locations  where 
it  can  eliminate  the  wastes,  take  up  the  energy-releasing  oxygen,  and 
pick  up  food  for  the  tissues.  Without  such  a  system  the  organism 
cannot  exist. 

Unspecialized  Transportation  Systems 

Unicellular  animals  obviously  have  no  need  for  a  circulatory 
system  as  each  individual  cell  is  in  a  position  to  excrete  its  own 
wastes  and  secure  oxygen  and  food  for  itself  through  its  own  cell 
membrane.     Even  in  slightly  more  specialized  forms,  such  as  the 


THE   METABOLIC    MAClllNEin    OF   ANIMALS  :i()l 

coelenterates,  tliere  is  no  need  for  a  specialized  transportation  system 
for  circulating  digested  foodstuffs  other  than  that  furnished  by  the 
ramifications  of  the  gastrovascular  system.  Since  the  organism  is 
composed  of  only  two  layers  of  cells,  each  is  capable  of  securing  the 
necessary  materials  forits  metabolism  either  from  outside  of  the  body 
or  from  a  neighboring  cell  lining  the  cavity. 

However,  in  the  flatworm  Planaria,  a  more  highly  developed  gastro- 
vascidar  system  appears.  In  animals  of  this  type  the  gut  ramifies 
between  nonspecialized  cells  composing  the  parenchymatous  tissue 
in  which  the  various  organ  systems  of  the  body  are  embedded.  As 
the  food  is  digested  it  is  circulated  directly  throughout  the  gastro- 
vascular cavity  by  means  of  contractions  of  the  body,  the  food  readily 
passing  from  the  branched  gut  to  surrounding  tissues  of  the  body 
by  diffusion.  The  waste  products  reach  the  gastrovascular  cavity 
and  by  similar  muscular  contractions  are  passed  to  the  outside,  or 
they  may  be  excreted  through  the  flame  cell  excretory  system  (see 
page  320). 

Still  further  advances  in  the  development  of  specialized  circulatory 
devices  occur  in  types  having  a  body  cavity,  or  coelom.  In  a  number 
of  invertebrates  the  coelom  is  filled  with  a  lymphlike  fluid  which  may 
contain  corpuscles  resembling  white  corpuscles,  or  leucocytes.  This 
may  be  looked  upon  as  an  advance  over  the  gastrovascular  type  of 
distributing  system.  And,  as  we  ascend  the  animal  scale  and  the  cir- 
culatory devices  tend  to  become  more  complex,  we  note  the  tendency 
to  develop  definite  tubes  in  which  the  circulatory  fluids  may  l)e  con- 
fined. These  types  are  usually  muscular  and  contractions  of  the 
body  facilitate  the  movement  of  the  fluid.  In  segmented  forms  like 
the  earthworm  the  coelomic  fluid  supplements  the  work  of  the  regular 
circulatory  system. 

Open  Circulatory  Systems 

This  type  of  transportation  reaches  its  peak  of  development  in  the 
Crustacea.  The  lobster  or  crayfish,  both  aquatic  forms,  furnish 
familiar  examples,  in  which  the  blood  serves  the  three  purposes 
of  respiration,  transportation  of  foodstuffs,  and  the  elimination  of 
wastes.  As  in  all  well-developed  circulatory  systems,  there  is  a 
muscular  pumping  mechanism,  or  heart,  which,  by  its  contractions 
forces  the  blood  along  a  group  of  so-called  arteries.  The.se  in  turn 
usually  break  down  into  smaller  vessels  terminating  in  the  tissues. 
The  blood  bathes  the  tissues  and  then  finds  its  way  back,  usually  along 


302  THE   MAINTENANCE   OF  THE   INDIVIDUAL 

a  system  of  sinuses,  through  the  gills  to  the  pericardial  sinus  surround- 
ing the  heart.  It  passes  into  the  heart  by  means  of  a  series  of  openings 
called  ostia,  guarded  by  one-way  valves. 

Insects,  a  still  more  highly  specialized  group,  have  a  very  direct 
respiratory  system  called  a  tracheal  system,  which  takes  over  the  job 
usually  handled  by  the  blood  stream,  bringing  the  oxygen  directly  to 
the  tissues  through  a  network  of  tubules,  or  tracheae.  This  has  been 
discussed  previously  in  detail  (pages  209-210). 

Closed  Circulatory  Systems 

Among  Invertebrates 

Systems  of  this  general  type  are  found  in  a  large  and  diversified 
group  of  organisms  beginning  with  the  invertebrates  and  extending 
throughout  the  vertebrate  group.  The  motive  power  of  such  cir- 
culatory devices  consists  essentially  of  a  central  pumping  plant  or 
heart,  from  which  extends  a  series  of  arteries  that  break  down  into 
minute  capillaries  in  the  tissues  and  then  pass  into  gradually  larger 
vessels  known  as  veins  which  return  the  blood  to  the  heart.  Some- 
where in  the  capillary  circuit  the  blood  is  aerated,  giving  off  carbon 
dioxide  and  taking  in  oxygen.  The  earthworm  furnishes  an  example 
of  such  a  system  in  the  invertebrates. 

Among  Vertebrates 

In  all  of  the  vertebrates  there  is  a  well-developed  closed  type  of 
circulatory  system,  although  the  supplementary  lymphatic  system 
might  be  construed  as  a  sort  of  open  system.  In  order  to  understand 
the  work  performed  by  these  systems  we  must  turn  our  attention  to 
the  various  component  parts  involved  and  consider  their  functions. 

The  Blood 

Blood  is  a  red  fluid  which,  examined  microscopically,  is  seen  to  be 
composed  of  three  types  of  corpuscles,  red  and  white,  circulating  in  a 
liquid  plasma,  and  the  much  smaller  blood  platelets.  The  first  con- 
tains hemoglobin,  which  combines  with  oxygen  in  a  loose  combination 
forming  oxyhemoglobin ,  useful  in  respiration.  The  white  corpuscles, 
on  the  other  hand,  are  the  scavengers  of  the  body.  They  are  ame- 
boid in  shape  and  are  concerned,  in  part  at  least,  with  the  defense 
of  the  body  against  bacterial  invasion.     Under  certain  stimuli  great 


THE   METABOLIC   MACHINERY    OK   ANIMALS  303 

numbers  of  one  sort  or  the  other  of  these  blood  eells  are  produced. 
The  blood  platelets  are  now  generally  beheved  to  play  an  important 
role  in  the  clotting  of  blood. 

In  the  web  of  a  frog's  foot  the  blood  may  be  seen  rushing  along 
through  relatively  large  vessels  which  break  down  into  smaller  ones 
until  reaching  the  capillaries,  through  which  the  corpuscles  slide  in 
single  file  at  a  much  slower  gait.  It  is  here  that  oxygen  and  food 
diffuse  by  osmosis  to  the  surrounding  lymph  and  so  reach  the  tis- 
sues. Under  the  microscope  the  blood  appears  to  be  traveling  at  a 
headlong  pace,  due  to  the  fact  that  this  instrument  magnifies  only 
space  without  reference  to  time.  The  pace  of  the  corpuscles  quick- 
ens again  as  they  reach  the  larger  venules  which,  after  anastomos- 
ing, ultimately  lead  to  the  heart  as  veiris.  Two  interesting  facts 
might  be  mentioned  here,  one  dealing  with  the  capillaries  and  the 
other  with  blood.  Dr.  Krogh,  a  Nobel  prize  winner  from  Denmark, 
says  that  if  an  average  human  being  was  selected  and  all  of  his  capil- 
laries were  opened  up  and  spread  out  flat,  their  total  area  would  nearly 
cover  that  encompassed  by  an  average  city  block.  The  other  fact 
centers  about  the  numbers  of  corpuscles  present,  of  which  various 
estimates  have  been  made.  In  normal  women  and  men  there  should 
be  4,500,000  to  5,000,000  red  corpuscles  (erythrocytes)  per  cubic  mil- 
limeter of  blood,  while  somewhere  between  5000  and  10,000,  nor- 
mally about  7500  white  corpuscles  (leucocytes),  is  considered  an 
average  count.  Red  corpuscles  vary  in  number  with  altitude,  a 
greater  number  being  necessary  in  high  altitudes  where  less  oxygen  is 
present  in  the  atmosphere  and,  consequently,  greater  numbers  are 
needed  to  transport  the  amount  of  air  necessary  for  life. 

The  plasma  of  the  blood  also  contains  a  great  variety  of  protective 
substances  which  are  known  under  the  general  heading  of  antibodies. 
They  are  induced  by  bacteria  and  other  parasites  which,  acting  as 
foreign  proteins,  stimulate  some  living  body  cells  to  manufacture  them 
(see  page  626)  and  turn  their  protective  substances  loose  into  the 
blood  stream. 

The  Lymph 

Even  though  capillaries  are  distributed  widely,  each  is  surrounded 
by  narrow  lymph  spaces,  that  are  filled  with  plasma  and  white 
corpuscles,  the  latter  being  mostly  lymphocytes.  Lymph  is  concerned 
with  the  transportation  of  food,  oxygen,  and  other  substances  neces- 
sary for  the  successful  metabolism  of  the  organism.     It  is  lymph  which 


304 


THE  MAINTENANCE  OF  THE  INDIVIDUAL 


comes  into  contact  with  the  tissues  and  serves  as  the  go-between  for 
the  blood  and  cells.  Lymph  gradually  flows  from  the  lymph  spaces 
into  lymph  capillaries,  which  in  turn  unite  to  form  larger  and  larger 
lymph  vessels,  interspersed  with  numerous  lymph  glands  and  lymph 

nodes.  Finally  the  lymph  vessels  unite  into  a 
large  thoracic  duct  emptying  into  the  jugular 
vein  in  the  neck  region. 


The  Conduits  —  Arteries,  Veins,  and 
Capillaries 

Having  considered  the  "stuff"  that  blood  is 
made  of,  we  can  now  turn  to  a  consideration 
of  the  vessels  through  which  it  passes.  The 
chief  function  of  the  capillaries  centers  about 
the  exchange  of  the  products  of  metabolism 
with  the  lymph.  Some  of  the  plasma  of  the 
blood  actually  transudes  through  the  walls  of 
the  capillaries,  while  certain  types  of  leuco- 
cytes also  pass  through  the  walls,  which  are 
composed  of  nothing  more  or  less  than  a 
single-celled  layer  of  epithehal  cells,  called 
endothelium. 

Distinct  structural  differences  exist  between 
the  capillaries  and  the  arteries  and  veins  of  all 
vertebrates.  Both  arteries  and  veins  are  cov- 
ered externally  by  a  rough  protective  coat  of 
connective  tissue.  Between  this  and  the  inner 
endothelial  lining  lies  a  layer  of  elastic  mus- 
cular fibers.  -In  veins,  this  layer  is  relatively 
thin,  while  in  the  arteries,  it  is  quite  well  de- 


Principal  lymph  chan- 
nels of  man.  Note  the 
abundance  of  lymph  ves- 
sels in  the  region  of  the 
intestine.  What  function 
do  they  serve  ?  Find  the 
thoracic  duct  emptying 
into  the  jugular  vein. 


veloped,  probably  being  correlated  with  the 
greater  pressure  to  which  arteries  are  subjected  as  evidenced  by  the 
periodic  spurting  of  blood  whenever  an  artery  is  cut. 

In  the  veins  blood  is  prevented  from  flowing  back  away  from  the 
heart  by  a  series  of  cuplike  valves  that  open  in  the  direction  of  the 
blood-flow  toward  the  heart  but  which  close  when  the  reversed  move- 
ment is  attempted.  They  are  quite  similar  to  the  semilunar  valves 
of  the  heart  (page  308). 

Veins  collapse  when  cut  while  arteries  do  not.  This  fact  proved 
a  stumbling  block  to  the  proper  interpretation  of  the  anatomy  and 


THE   METABOLIC   MACHINEMY   OF    WIMAr.S 


SOS 


Connective 
tissue... 


Epitlididl. 
layer 


Muscle, 
cells 


physiology  of  arteries  and  veins  by  the  early  scientists.  William 
Harvey  (1578-1657)  was  the  first  to  understand  thoroughly  the  cir- 
culatory system,  but  other  earlier  and 
contemporary  workers  were  not  far 
behind  him.  The  great  artist,  Leo- 
nardo da  Vinci  (1452-1519),  left  in 
manuscript  numerous  drawings  and 
notes  on  the  heart  and  other  vessels, 
stating  that  the  aorta  "subdivides 
into  as  many  principal  branches  as 
there  are  principal  parts  to  be  nour- 
ished, branches  which  continue  to 
ramify  ad  infinitum.''  Vesalius 
(1514-1564)  in  his  famous  anatomical 
treatise,  Fahrica,  first  published  in 
1543,  expressed  doubt  as  to  the  exist- 
ence of  the  connecting  "pores"  be- 
tween the  two  sides  of  the  heart. 
This  was  an  attack  upon  one  of  the 
main  features  of  the  teachings  of 
Galen,  who  believed  there  was  "an  ebb 
and  flow  of  blood  within  both  veins 
and  arteries  throughout  the  system." 
blood  and  the  arteries  vitalized  blood. 

(130- 


Epithelial 
Isyer. 


An  artery 

A  capillary 

Comparison  of  the  walls  of  an 
artery,  vein,  and  t-apillary. 


Diagram  showing  how  valves  of  a  vein 
prevent  the  back  flow  of  blood. 


The  former  contained  crude 
Yet  neither  Vesalius  nor  Galen 
-200  A.D.)  apparently  under- 
stood the  circulatory  system. 

William  Harvey  is  rightfully 
known  as  the  father  of  physiology 
for  in  1616  he  began  presenting  his 
views  on  the  circulation  of  the 
blood.  His  book,  however,  did  not 
appear  until  1628.  In  it  we  find 
evidence  for  the  thesis  that  the 
heart  is  the  pump,^  that  the  arteries 
dilate  passively  as  the  heart  forces 
the  blood  into  them,  that  the  blood 
goes  from  the  right  ventricle 
through  the  lungs  to  the  left  auricle, 

'  All  stages  of  this  phase  of  the  argument  are 
not  outlined  fully. 


306 


THE   MAINTENANCE   OF  THE   INDIVIDUAL 


and  that  the  amount  and  rate  of  flow  of  the  blood  from  the  heart  makes 
it  necessary  to  assume  that  most  of  it  must  return  to  the  heart.  This 
latter  fact  was  shown  by  assuming  that  the  ventricle  held  only  two 
ounces ;  then,  if  the  pulse  beats  72  times  per  minute,  in  an  hour  it 
would  force  72  X  60  X  2,  or  8640  ounces,  or  540  pounds,  into  the  aorta, 
which  is  considerably  more  than  the  weight  of  man.  The  return  of 
the  blood  to  the  heart  is  accomplished  by  veins,  thus  completing  the 
circuit.  This  summarizes  briefly  the  gist  of  Harvey's  contributions 
on  circulation.  Small  wonder  that  after  so  many  misleading  beliefs 
this  master  should  be  acclaimed  for  his  careful  thinking  and  his 
accurate  observations  upon  the  action  of  the  heart.  His  study  in- 
volved examinations  of  about  forty  species  of  animals,  and  ulti- 
mately led  to  the  fundamental  concept  of  the  circulation  of  blood. 

The  Heart 

The  vertebrate  heart  is  really  a  pumping  station  which  in  its 
simplest  form,  as  found  in  the  fishes,  consists  of  a  receiving  auricle 
and  a  pumping  ventricle.  Back  flow  is  prevented  by  a  series  of  valves 
placed  at  strategic  points.  Ascending  the  vertebrate  scale  and  leav- 
ing behind  water-inhabiting  forms,  we  find  the  circulatory  system 


ampl-^ibian        Tept.il<2. 


hlrd.  and, 
raccynmod 


Evolution  of  four-chambered  heart.     Contrast  situation  in  fish  and  amphibia 

with  reptiles,  birds,  and  mammals. 


becoming  more  complicated  and  the  heart  evolving  from  a  two- 
chambered  form,  typical  of  fish,  to  a  four-chambered  type  found  in 
birds  and  mammals.  Intermediate  stages  in  this  progression  appear 
in  the  amphibia  and  reptiles. 

The  heart  of  man  is  a  cone-shaped,  muscular  organ  about  the  size 
of  the  fist.     It  is  surrounded  by  a  loose  membranous  bag  called  the 


THE   METABOLIC   MACHINERY   OF   ANIMALS 


307 


pericardium,  the  inner  lining  of  which  covers  the  heart  and  secretes 
the  pericardial  fluid  in  which  the  organ  lies.  The  heart  of  an  adult 
mammal  may  be  divided  into  a  right  and  left  side,  each  having  no 
direct  internal  connection  with  the  other.     Each  half  may  likewise 

innomlnatfi-    left, 
ar-tery....  subclavian 

;  arterx.^ 


pLdmonory 
artery. 


Superior" 


.bicuspid. 


inferior- 
vsna  casid} 

tricuspicC- 
vctlve 

dnorduojs. 
tancunaxa. 

right 
ventricle 


papillar/ 
muscle^ 

.      left,    .    1 
— -vetttncle 


A  section  through  the  mammalian  heart.     Read  the  text  carefully  and  trace 
the  course  of  blood  through  the  heart. 

be  divided  transversely  into  an  upper  relatively  thin-walled  auricle 
and  a  more  muscular  lower  ventricle.  The  right  side  contains 
unoxygenated  or  venous  blood,  while  the  left  auricle  and  ventricle 
contain  arterial  blood  saturated  with  oxygen. 

The  right  auricle  receives  the  venous  blood  by  two  vessels  known 
as  the  superior  vena  cava,  or  precava,  entering  on  the  anterior  surface 
and  bringing  the  blood  from  the  head  and  neck,  and  the  inferior  vena 
cava,  or  postcava,  which  empties  into  the  lower  portion  of  the  right 
auricle,  returning  the  blood  from  parts  of  the  body  below  the  dia- 
phragm. The  blood  passes  into  the  right  ventricle  through  the; 
tricuspid  valve  which,  as  the  name  suggests,  is  composed  of  three 
irregularly  shaped  flaps.     The  tips  of  these  flaps  project  into  the 


308  THE   MAINTENANCE   OF  THE   INDIVIDUAL 

ventricle,  where  they  are  attached  by  tendinous  chords,  the  chordae 
tendineae,  to  small  muscular  projections  called  the  papillary  -muscles, 
extending  from  the  wall  of  the  ventricle.  Back  flow  is  prevented 
upon  contraction  of  the  ventricle  by  the  closing  of  the  flaps  due  to 
pressure,  while  a  reversal  of  their  position  is  prevented  by  the  chordae 
tendineae  and  the  contraction  of  the  papillary  muscles.  Thus  the 
blood  passes  from  the  right  ventricle  into  the  pulmonary  artery,  the 
lower  portion  of  which  is  guarded  against  back  flow  by  three  lialf- 
moon-shaped  cups,  called  the  semilunar  valves.  The  blood  has  now 
started  toward  the  lungs  through  the  pulmonary  artery,  which  is  the 
only  artery  carrying  unoxygenated  blood,  to  the  lungs,  where  car- 
bon dioxide  is  given  off  and  oxygen  taken  in  by  the  hemoglobin  in 
the  red  blood  corpuscles.  It  then  passes  into  one  of  the  larger 
pulmonary  veins  and  so  reaches  the  left  auricle  of  the  heart.  Here 
the  process  described  for  the  right  half  of  the  heart  is  repeated  ex- 
cept that  the  left  auricular- ventricular  orifice  is  guarded  by  the 
bicuspid  valve,  w^hile  the  semilunar  valves  on  this  side  of  the  heart  lie 
in  the  aorta  which  is  the  outgoing  artery  carrying  the  blood  about  the 
body. 

The  "beating"  of  the  heart  is  a  more  complicated  story  than  can 
be  elaborated  here.  First,  as  the  ventricles  relax,  blood  flows  from  the 
veins  into  the  auricles  and  ventricles,  then  the  two  auricles  contract 
simultaneously,  further  dilating  the  two  ventricles.  This  is  followed 
by  the  immediate  contraction  of  the  two  ventricles.  Then  follows 
a  brief  period  of  relaxation  or  rest  during  which  the  auricles  and 
ventricles  are  being  filled  again,  after  which  the  cycle  is  repeated. 
This  forces  the  blood  from  the  heart  in  a  series  of  spurts,  accounting 
for  the  type  of  bleeding  noted  when  an  artery  is  severed,  and  for  the 
expansion  of  the  elastic  arteries  as  the  blood  is  forced  out  of  the  heart 
into  them. 

The  Aortic  Arches 

As  the  blood  goes  out  through  the  pulmonary  artery  it  is  passing 
through  the  embryological  remains  of  the  aortic  arches.  Originally 
six  in  number,  these  paired  aortic  arches  are  of  great  interest  to 
students  of  evolution  since  embryological  and  comparative  anatomical 
studies  have  yielded  a  very  striking  picture  of  the  changes  in  this 
region  involved  in  the  shift  of  vertebrates  from  water  to  land.  From 
fishes  on  up  to  mammals  only  these  functional  aortic  arches  have 
persisted,  although  six  pairs  of  aortic  arches  are  usually  reckoned  as 


THE   METABOLIC    MACllINEUY    OF   ANIMALS  ,509 

the  fundamental  numbcM-.      An  idea  of  the  changes  involved  from  life 
in  the  water  to  life  on  land  may  be  secured  from  tli(>  figure. 

11  w  E        "2:        "St 

^.^.^ ^ 

duarsoX  aorta 

primitive  condition 

. .  r  ^ventro.!     aorta. 

carotid  j-  _   i  ► 

dorsal  aorta 

^      -^= ^ 

yauriclez^^^^)  - .  . 

-ventral  aorta~ri^^ri;;^:^i 

*^T    -  t; — '^^\iM?lliTl  Ir^l  f  --^^  Oorsol  OLorto. 

'\~\^  \'\\'\  '\'\\sA^^Na\x4\\^  \V  ^^^^^^  amphibian 

V  ventrol    oorxa 
carotjdC  left  systemic  arch. 

,^;^'_-'^-^  -i-"^  -  -  CZ;X!r  ''''r^~^-"r;m^ — -^  dorsql  Oprto. 

uAV^^^^^  ^igber amphibia 
^_____^      ^    ot^  reptiles 

'pvuTOonary  trunk 

Carotid.)  ri^t  systemic  arcVj      , 

*^ "'''rr"'-'-  '\^^'-^' '^.' ■  /  -   -  -^■■;-..-^Wx,   pv.\trtort(xrir  a.-rtti^y 

■       Vi   /^^'  ^7 — \  V,  lAjTl  ^^— --•^    cAorsol  oiorto. 

-.;..>  ,^ , ,.,  ^  wi^-- -'-^^i-^^^^  bird: 

^^^j;!?'^-^..      ^ S"^e^^ig!l. great  arch  of  oorta 

7T— '•TY'; — '■  r\\\Vl"\\/9 '"•';' ""V'"'7yf(  aors-o-i  aorcct 

'--■"■^"-'-:!"....i^^.^  '''^;;::_-----"'^xJpuiTnonarx  tnmk 
Fate  of  the  aortic  arches  of  the  vertebrates  as  seen  from  the  side.     (After  (iiiyer.) 


The  Course  of  the  Blood  in  the  Body 

There  are  two  distinct  systems  of  circidation  in  the  body.  The 
pulmonary  circulation,  noted  in  connection  with  the  study  of  the 
heart,  takes  blood  from  the  right  auricle  and  ventricle  to  the  lungs, 
passing  it  back  to  the  left  auricle.  The  longer  circulation  is  known 
as  the  systemic  circulation  in  which  the  blood  leaves  the  left  ventricle 
through  the  dorsal  aorta  and  through  ever-branching  arteries  pa.sses 
to  the  muscles,  nervous  system,  kidneys,  skin,  and  other  organs  of 
the  body.  It  gives  food  and  oxygen  to  these  tissues,  receives  the 
waste  products  of  oxidation  while  passing  through  the  microscopic 
capillaries,  and  returns  to  the  right  auricle  through  veins. 


310  THE   MAINTENANCE  OF  THE   INDIVIDUAL 

Some  of  the  blood  on  its  way  from  the  heart  passes  to  the  walls 
of  the  food  tube  and  so  on  to  its  glands.  From  these  parts  it  is  sent 
with  its  load  of  absorbed  food  to  the  liver.  Here  the  portal  vein 
that  carries  the  blood  breaks  up  into  capillaries  around  cells  of  the 
liver,  which  take  out  the  excess  sugar  from  the  blood  and  store  it  as 
glycogen.    From  the  liver  the  blood  passes  directly  to  the  right  auricle. 

Functions  of  the  Blood 

The  blood  being  the  circulatory  tissue  plays  a  very  important  part 
in  the  maintenance  of  the  organism.  Most  waste  products  of  the 
tissues  are  carried  by  the  blood  from  their  point  of  origin  to  some 
other  region  of  the  body  which  is  adapted  for  their  elimination. 
Thus  the  nitrogenous  waste,  urea,  is  carried  to  the  kidneys.  Other 
wastes  are  eliminated  through  the  sweat  glands  of  the  skin  or  the 
lungs.  The  blood  stream  is  also  concerned  with  the  transportation  of 
oxygen  from  the  lungs,  and  nutrient  material  from  the  intestines  to  the 
tissues.  In  addition,  it  carries  the  products  of  one  tissue  to  another ; 
for  example,  internal  secretions  which  are  produced  in  glands  must 
be  transported  elsewhere  to  do  their  work.  Secretin,  already  referred 
to,  will  serve  as  an  example  of  this  type  of  action. 

In  addition  to  the  three  transportation  jobs  already  mentioned 
the  blood  also  serves  to  remove  various  waste  products  of  metabolism 
from  the  point  of  their  formation  to  the  organs  which  excrete  them, 
i.e.,  the  lungs,  skin,  intestines,  and  kidneys.  Through  its  accessi- 
bility to  the  various  organs  and  glands  of  the  body,  the  blood  may  aid 
in  maintaining  the  normal  acid-base  balance  of  the  tissues  as  well  as 
the  water  content  of  the  body. 

We  know  that  oxidation  generates  heat,  which  means  that  in 
the  human  body  heat  is  being  constantly  released  by  the  working 
cells.  It  is  carried  by  the  blood  stream  to  the  outside  layers  of  the 
body  and  there  dissipated  in  the  surrounding  environment  unless 
special  heat-regulating  devices  are  present.  Man  regulates  his  body 
temperature  very  largely  by  controlling  the  heat  loss  through  nerve 
impulses  causing  contraction  of  the  minute  blood  vessels  in  the  skin. 
The  expansion  of  these  blood  vessels,  resulting  from  the  stimulus  of 
the  vasomotor  center  of  the  medulla  oblongata,  allows  greater  radiation 
and  consequent  loss  of  heat  (see  page  351).  What  is  of  perhaps  still 
greater  importance  to  man  in  cooling  his  body  is  the  ability  of  sweat 
glands  to  increase  their  action  under  proper  nervous  stimulation 
and  to  pass  out  more  sweat  to  be  evaporated.     Heat  is  required 


THE   MET.VBULIC    MACHINERY   OF  ANIMALS  311 

to  vaporize  the  sweat  on  the  body  surface,  and  body  heat  is  lost. 
Conversely,  by  performing  muscular  work,  heat  may  be  produced  in 
greater  quantity  through  the  increase  of  oxidations  in  the  body. 

Clotting  is  another  very  important  function  of  the  blood.  We  are 
all  familiar  with  the  fact  that  while  blood  is  fluid  when  drawn  from 
the  body  it  soon  becomes  viscous  and  later  gelatinous.  Finally  a 
clot  is  formed,  which  may  be  seen  floating  in  the  blood  serum.  It 
was  initiated  in  part  by  the  dissolution  products  of  the  blood 
platelets.  In  the  gelatinous  stage,  both  red  and  white  corpuscles 
are  caught  in  the  fibrin  network,  and  as  the  clot  shrinks  the  red 
cells  are  held  more  tightly  by  needlelike  fibers  of  fibrin.  There  are 
too  many  theories  of  clotting  to  present  here,  but  when  blood  is 
exposed  to  air  chemical  changes  finally  transform  the  soluble  fibrino- 
gen, which  occurs  normally  in  the  blood  stream,  into  insoluble  fibrin. 
The  blood  of  a  normal  person  ordinarily  clots  in  about  five  minutes. 
The  blood  of  a  few  persons,  however,  forms  clots  very  slow^ly  or 
refuses  to  clot  at  all.  Such  a  condition  is  known  as  hertiophilia,  and 
the  person  affected  as  a  hemophiliac. 

Finally,  the  blood  plays  an  important  part  in  health  and  disease 
both  through  the  distribution  of  antibodies  and  the  defense  mechan- 
ism of  the  white  corpuscles  against  bacterial  invasion. 

SECTION   C.     RESPIRATORY  DEVICES 

Respiration 

Every  living  organism  requires  oxygen  for  its  metabolic  processes, 
which  demands  that  every  cell  shall  take  in  oxygen  and  give  off 
wastes,  largely  carbon  dioxide  and  water.  This  exchange  of  free 
oxygen  and  carbon  dioxide  is  necessary  for  combustion.  In  all  ver- 
tebrates respiration  may  be  divided  into  two  types,  external  and  inter- 
nal respiration.  The  former  involves  the  exchange  of  gases  between 
the  atmosphere  and  the  blood  through  some  specialized  device  such 
as  gills  or  lungs,  while  internal  respiration  is  an  interchange  between 
the  blood  and  the  cells  of  the  body. 

In  looking  into  the  story  of  respiration,  one  finds  the  first  relevant 
suggestions  coming  from  John  Mayo  who  in  1668  suggested  that  res- 
piration and  combustion  were  analogous  processes.  His  work  was 
antedated  by  another  early  worker,  Robert  Hooke,  the  same  man 
who  described  the  dead  cells  in  cork,  and  who  demonstrated  by  the 
use  of  experiments  that  air  is  necessary  for  the  maintenance  of  life 
H.  w.  H.  —  21 


312  THE   MAINTENANCE   OF  THE   INDIVIDUAL 

in  animals.  It  was  Priestley  (1733-1804),  however,  who  discovered 
oxygen  and  recognized  its  great  importance  to  all  living  matter.  The 
name  of  one  more  important  early  worker,  Lavoisier  (1743-1794), 
should  remain  in  our  memory  as  he  was  the  first  man  to  attempt  a 
quantitative  scientific  study  of  the  phenomenon  of  respiration.  It 
was  he  who  first  stated  ''life  is  a  chemical  action"  and  who  realized 
that  animal  heat  was  the  result  of  an  oxidation  process  involving 
substances  of  the  body.  Both  he  and  LaPlace  (1749-1827)  carried 
on  numerous  experiments  on  respiration  and  its  relation  to  the  pro- 
duction of  animal  heat.  Out  of  this  humble  beginning  has  come  all 
the  later  fascinating  studies  upon  respiration  by  such  workers  as 
Liebig,  Voit,  Rubner,  Pettenkofer,  Atwater,  Rosa,  Benedict,  and 
others. 

The  Protein,  Hemoglobin 

Before  turning  attention  to  the  various  devices  developed  to  meet 
the  problem  of  respiration -one  mechanism  that  is  universally  present 
in  the  vertebrates  should  be  mentioned,  namely,  the  respiratory 
pigment  hemoglobin.  This  is  a  protein  compound  found  in  the  red 
corpuscles  of  vertebrates.  It  has  the  ability  of  combining  readily 
with  oxygen  to  form  oxyhemoglobin,  thus  enabling  the  blood  stream 
to  carry  much  more  oxygen  than  it  could  possibly  do  by  saturating 
the  plasma. 

The  interchange  of  oxygen  and  carbon  dioxide  may  be  explained  by 
physical  laws.  It  is  known  that  a  gas  tends  to  pass  in  the  direction  of 
the  least  pressure.  Even  when  a  moist,  permeable  membrane,  or  a  se- 
lectively permeable  membrane,  such  as  the  epithelium  of  the  lungs  and 
capillaries,  is  placed  between  different  gases  the  molecules  pass  freely 
back  and  forth.  In  the  event  of  a  difference  in  pressure  between  the 
two  sides  of  the  membrane,  the  gases  pass  through  from  the  region  of 
greater  pressure  to  that  of  the  lower  pressure  until  it  is  equalized. 
Oxygen  constitutes  nearly  21  per  cent  of  the  atmosphere  and  is  pres- 
ent in  sufficient  amounts  to  furnish  enough  pressure  to  transfer  it 
to  regions  of  lower  pressure.  If  we  keep  in  mind  the  fact  that  the 
pressure  of  oxygen  outside  the  body  must  always  be  greater  than  that 
in  the  blood  stream  in  the  lungs,  we  can  readily  understand  why 
oxygen  must  pass  through  the  moist  permeable  membranes  and  into 
the  blood  stream,  thus  giving  us  the  explanation  of  external  respiration. 
On  the  same  basis  internal  respiration  may  be  explained.  The  first 
step  involves  the  liberation  of  oxygen  from  the  blood  to  the  lymph. 


THE   METABOLIC   MACHINERY   OF   ANIMALS  3L3 

while  the  next  centers  around  its  transfer  to  the  cells  of  the  body. 
An  examination  of  the  first  stage  shows  the  blood  passing  through  the 
capillaries  which  are  bathed  in  lymph  where  the  oxygen  pressure  is 
very  low.  This  condition  brings  about  dissociation  of  the  oxyhemo- 
globin to  such  a  degree  that  it  loses  over  a  third  of  its  oxygen  during 
its  brief  passage  through  the  capillaries.  The  lymph  in  turn  loses  oxy- 
gen to  the  cells  in  the  same  way.  While  oxygen  is  being  liberated 
carbon  dioxide  is  being  returned  to  the  blood  stream  in  exactly  the 
same  manner,  for  carbon  dioxide  is  present  in  greater  concentration 
in  the  cells  than  in  the  lymph  and  in  the  blood  stream  respectively. 

External  Respiration 

While  the  phenomenon  of  respiration  is  a  common  one  yet  it  is 
accomplished  in  manj'  different  ways.  Small,  single-celled,  or  rela- 
tively simple  organisms  have  no  need  of  a  complicated  respiratory 
system.  However,  it  is  well  to  remember  that  while  the  surface  of  a 
body  varies  as  the  square,  its  volume  varies  as  the  cube  of  its  diam- 
eter. This  means  that  as  an  object  increases  in  size  the  ratio  of  its 
surface  to  its  volume  becomes  smaller.  By  transferring  this  thought 
to  biological  fields  we  can  readily  appreciate  that  as  animals  increase 
in  size  respiratory  systems  become  a  real  necessity. 

A  survey  of  the  animal  kingdom  shows  that  organisms  have  met 
this  need  in  a  great  variety  of  complex  and  sometimes  rather 
peculiar  ways.  Four  types  of  respiration  are  commonly  found, 
namely,  respiration  through  the  surface  of  the  body,  by  means  of 
gills,  tracheae,  and  lungs.  Three  other  methods  are  less  commonly 
found,  namely,  by  means  of  respiratory  papillae,  respiratory  trees,  and 
lung-hooks. 

Respiratory  Papillae.  These  occur  as  evaginations  from  the 
dorsal  surface  of  such  forms  as  the  starfishes,  where  they  are  known 
as  dermal  branchiae.  They  are  really  outpocketings  of  the  body 
wall. 

Respiratory  Pouches  or  Trees.  These  tubular  and  more  or 
less  branched  pouches  occur  in  such  groups  as  the  sea  urchins,  holo- 
thuroideans,  and  some  starfish.  In  the  first  group  the  pouches  are 
outgrowths  from  the  mouth,  while  in  the  holothuroidea  they  are 
outpocketings  from  the  rectal  region  (see  figure,  page  314). 

Lung-books.  Such  structures  consist  of  a  series  of  folds  suggest- 
ing the  pages  of  a  book.  Each  "leaf"  is  filled  with  blood  spaces  and 
is  exposed  on  two  sides  to  the  air.     Respiratory  devices  of  this  type 


314 


THE   MAINTENANCE   OF  THE   INDIVIDUAL 


inte-stlne/ 


respiratory 


body  vail 


are  found  in  many  spiders  while  a  similar  structure  called  a  gill- 
book  occurs  in  the  horseshoe  crab,  Limulus.     Gill-books  may  more 

properly  be  considered  as 
"^'^'^  modified  gills. 

The  Body  Surface. 
This  type  of  respiratory 
system  is  probably  the 
most  simple.  It  consists 
of  an  exchange  of  gases 
through  the  surface  of 
the  body.  It  is  found, 
however,  not  only  in  such 
simple  one-celled  animals 
as  the  protozoa,  which 
have  no  specialized  sys- 
tem for  respiration,  but 
also  in  sponges  and  coe- 
lenterates.  Even  in  the 
parasitic  and  free-living, 
flat  worms  and  some 
roundworms,  respiration 
is  of  the  same  type. 
Some  of  the  smaller  forms 
of  the  higher  groups  may 
also  resort  to  this  method 
of  gaseous  exchange. 

In  some  of  the  more 
highly  specialized  forms 
such  as  the  earthworms,  a  circulatory  system  is  present  although 
respiration  still  takes  place  through  the  cuticle.  The  blood  of  the 
earthworm  is  red  and  contains  hemoglobin  which  is  dissolved  in  the 
plasma,  just  the  opposite  of  the  situation  in  the  vertebrates  where 
hemoglobin  occurs  in  the  red  blood  cells. 

Complete  dependence  upon  integumentary  respiration  does  not 
occur  among  vertebrates.  Probably  the  closest  approach  to  such  a 
situation  is  in  the  lungless  salamanders  (Plethodontidae)  and  in 
certain  other  urodeles,  such  as  the  hellbender,  Cryptohranchus.  In 
the  former,  integumentary  respiration  is  usually  supplemented  by  a 
capillary  network  in  the  pharyngeal  region  and  is  therefore  designated 
as  buccopharyngeal  respiration.     A  highly  developed  system  of  capil- 


Cl. 


The 


OCXCCL 


'lungs,"  or  respiratory  tree  of  the  sea 
cucumber,  a  holothurian. 


THE  METABOLIC   MACHINERY  OF  ANIMALS 


315 


giU. 


laries  which  almost  penetrate  to  the  outer  surface  of  the  epidermis 
is  found  in  the  integument  of  many  amphibians.  In  some  amphibia 
as  much  as  74  per  cent  of  the  carbon  dioxide  is  given  off  through  the 
skin.  Such  adaptations  are  possible  only  where  a  cool  environment 
keeps  down  the  metabolic  rate  of  these  forms. 

Gills.  Gills  are  either  flattened  or  feathery,  and  are  external  or 
internal  in  their  location.  Invariably  the  blood  circulate.s  in  them  and 
is  separated  from  the  surrounding  water  by  a  thin  membranous  wall 
through  which  the  dissolved  gases  are  exchanged.  Among  the 
invertebrates  the  position  of  the 
gills  varies  in  accordance  with 
the  habitat  of  the  animal.  In 
such  forms  as  the  crayfish  for    to  pericardial 

example,  they  are  in  a  protected    Sirjus 

outer  chamber  covered  by  chitin. 
Circulation  is  accomplished  by 
the  creation  of  a  water  current 
through  the  action  of  the  swim- 
merets  and  certain  appendages 
about  the  mouth.  In  fishes 
and  amphibians,  water  typically 
enters  the  mouth  where  it  is 
passed  to  and  o^^er  the  pharyn- 
geal gills  and  from  there  through 
slits  to  the  outside. 

Tracheae.  These  are  com- 
posed fundamentally  of  air- 
carrying  tubules,  which,  by  a 
series  of  anastomoses  and  rami- 
fications, penetrate  to  nearly  all 
parts  of  the  body.  They  are 
characteristically  found  in  most 
insects,  myriapods,  protracheates,  and  some  arachnids.  Such  a  sys- 
tem starts  with  a  series  of  openings  known  as  spiracles,  occurring 
along  the  outer  surface  of  the  thoracic  and  abdominal  segments. 
Leading  from  the  spiracles  are  air  tubes,  or  tracheae,  which  show 
great  numbers  of  anastomoses,  frequently  forming  abdominal  reser- 
voirs, or  air  sacs.  The  tracheae  are  nothing  more  or  less  than  a 
series  of  pipes,  for  they  are  lined  with  chitin  and  stiffened  by  a  spiral, 
fiberlike  thickening:.     The  finer  subdivisions  of  the  tracheae  extend 


efferent  branchial 

V(3.5S<2^1. .- 

afferent  branchial 
arter/. 


branchio^tcgite 

from,  lateral 
bloocC  $inix.$ 

l^jrtion  of  gills  of  crayfish. 


protecting  branchiostegite. 
blood  aerated  .3 


Note  the 
How  is  the 


316  THE   MAINTENANCE   OF  THE   INDIVIDUAL 

to  all  inner  jjarts  of  the  body  where  they  end  blindly  making  possible 
the  delivery  of  oxygen  directly  to  the  cells.  Here  again  external  res- 
piration takes  place  in  the  spiracular  region,  while  internal  respiration 
centers  about  the  diffusion  of  gases  to  and  from  the  tracheae  and  the 
cells.  The  efficacy  of  this  system  is  suggested  by  the  rapid  and  sus- 
tained metabolism  common  to  many  of  the  insects. 

Lungs.  This  type  of  respiratory  system  is  found  best  developed 
among  the  birds  and  mammals.  The  lungs  of  birds  are  specialized 
for  a  high  metabolic  rate  and  for  making  lighter  the  load  which  must 
be  lifted  in  flight.  Air  sacs  connected  with  the  lungs  are  found 
throughout  the  viscera,  and  even  the  bones  are  filled  with  air  and  so 
are  very  light.  The  connection  between  these  and  the  rest  of  the 
respiratory  system  has  been  demonstrated  by  closing  the  trachea  and 
opening  the  air  sac  in  an  upper  wdng  bone.  The  fact  that  the  bird 
continues  to  breathe  demonstrates  this  connection. 

The  mammalian  respiratory  system  is  essentially  the  same  regard- 
less of  the  form  studied.  The  most  important  part  of  the  lungs  are 
the  terminal  air  sacs  called  alveoli,  in  which  the  inspired  air  contacts 
the  many  capillaries  of  the  circulatory  system  found  throughout 
the  moist  mucous  membranes.  Oxygen  and  carbon  dioxide  diffuse 
through  the  capillary  walls  surrounding  the  alveoli  and  so  the 
exchange  of  gases  is  effected. 

Internal  Respiration 

It  has  been  shown  in  the  case  of  very  simple  animals,  such  as 
Paramecium,  that  when  oxidation  of  food  takes  place  in  the  cell 
energy  results.  In  forms  which  possess  complicated  circulatory  sys- 
tems, external  respiration  must  first  take  place,  after  which  oxygen 
is  transported  by  the  hemoglobin  of  the  blood  to  the  various  parts 
of  the  body  where  the  actual  work  is  to  be  done.  Here  real  or  in- 
ternal respiration  takes  place,  since  cell  activity  depends  upon  food 
and  oxygen. 

As  aerated  blood  passes  through  the  capillaries  these  are  bathed 
in  plasma  in  which  the  oxygen  pressure  is  low.  The  oxyhemoglobin, 
a  compound  of  oxygen  and  hemoglobin,  is  stable  only  in  an  environ- 
ment where  oxygen  pressure  is  comparatively  high.  Therefore  the 
hemoglobin  delivers  itself  of  the  oxygen  to  the  lymph,  which  in  turn 
transfers  it  to  the  cells.  The  pressure  of  carbon  dioxide  on  the  other 
hand  is  higher  in  the  cells  thus  facilitating  its  transfer  to  the  lymph 
and  so  to  the  blood  stream  proper. 


THE   METABOLIC   MACHINERY   OF  ANIMALS 


;n7 


Respiratory  System  in  Man 

Air  passes  from  tjie  nostrils  through  the  shthke  glottis  into  the 
windpipe.  This  tube,  called  the  trachea,  the  top  of  which  may  easily 
be  felt  as  the  "  Adam's  apple  "  of  the  throat,  is  supported  by  a  series  of 
cartilaginous  rings  complete  in  front  but  incomplete  behind  and  divid- 
ing into  two  hronchi.  Within  the  lungs,  the  bronchi  break  up  into  a 
great  number  of  smaller 
tubes,  the  hronckiolcs,  which 
divide  somewhat  like  the 
small  branches  of  a  tree 
and  arc  lined  with  ciliated 
epithelial  cells.  The  re- 
mainder of  the  tubes  are 
also  lined  with  ciliated  cells, 
the  cilia  of  which  are  con- 
stantly in  motion  lashing 
with  a  quick  stroke  toward 
the  outer  end  of  the  tube, 
that  is,  toward  the  mouth. 
Hence  any  foreign  material 
in  the  tubes  will  be  ex- 
pelled first  by  the  action 
of   the  cilia  and   then   by 


pharynjc 

xtvulcc. 
-epiglottis 

esophagits 


.bronchial  tubes 
orbr^chioles 


coughing  or 


"clearing  the 


The  respiratory  system  of  man.      Note  the 
cartilagenoiis  rinjjs  supporting  the  ducts. 


throat." 

The  bronchial  tubes  end, 
as  already  noted,  in  very 
minute  air  sacs  called  al- 
veoli. Great  numbers  of  these  are  present,  thereby  increasing  the 
respiratory  surface  tremendously.  These  tiny  pouches  have  elastic 
walls  into  which  air  is  taken  when  we  inspire  or  take  a  deep  breath. 
Around  the  walls  of  the  pouches  and  separated  by  a  xcry  thin 
membrane,  are  numerous  capillaries  from  the  pulmonary  artery 
which  brings  the  blood  from  the  right  ventricle  of  the  heart  to  the 
lungs.  Through  the  very  thin  walls  of  the  air  sacs  a  diffusion  of  gases 
takes  place,  which  results  in  the  blood  giving  up  carbon  dioxide  and 
taking  in  oxygen.  Consequently  the  blood  becomes  a  brighter  red, 
due  to  formation  of  oxyhemoglobin  by  the  combination  of  oxygen  with 
the  hemoglobin  in  the  red  corpuscles. 


318  THE   MAINTENANCE   OF  THE   INDIVIDUAL 

COMPOSITION  OF  FRESH  AIR  AND  THAT  EXPIRED  FROM  THE  LUNGS 


Constituents 


Oxygen        

Carbon  dioxide    .     .     .     . 
Nitrogen  and  other  gases 
Water  vapor 


In  Outdoor  Aik 


20.96 
.04 
79.0 
variable 


In  Air  Expired 
FROM  THE  Lungs 


16.4 

4.1 

79.0 

.5 


As  shown  in  the  above  table,  there  is  a  loss  of  nearly  5  per  cent  of 
oxygen  and  a  corresponding  gain  in  carbon  dioxide  and  water  vapor 
in  expired  air. 

The  lungs  are  located  in  a  triangular,  air-tight  sac  or  thoracic  cavity, 
with  the  sternum  or  breastbone  in  front,  the  ribs  on  the  side,  the 
immobile  vertebral  column  at  the  back,  and  the  convex  diaphragm 
below.  The  ribs,  connected  to  the  breastbone  in  front  and  the  back- 
bone behind,  are  united  to  each  neighboring  rib  by  a  sheet  of  intercostal 
muscles.  Furthermore  the  articulation  of  the  rib  with  the  vertebral 
column  is  higher  than  its  connection  with  the  sternum,  and  the  shape 
is  such  that  when  the  lungs  are  empty  the  "convexity  of  the  curve 
points  slightly  downwards."  Inspiration  results  from  the  contraction 
of  the  intercostal  and  associated  muscles  which  not  only  pull  the 
ribs  toward  a  horizontal  position  but  also  force  the  sternum  ventrally. 
The  diaphragm.,  which  also  assists,  is  a  combination  of  a  membrane 
and  muscle  and  forms  a  partition  between  the  thoracic  and  abdominal 
cavities.  The  concave  surface  of  the  diaphragm  is  towards  the  pos- 
terior, that  is,  down.  Contraction  reduces  the  concavity  so  that  the 
result  is  an  increase  in  the  capacity  of  the  thoracic  cavity.  Keeping 
in  mind  that  the  chest  cavity  is  air  tight,  the  lungs  elastic,  and  that 
the  sole  entrance  of  air  is  from  the  trachea,  it  is  not  difficult  to  see 
that  when  the  capacity  of  the  chest  cavity  is  increased  by  the  move- 
ments described  above,  the  lungs  naturally  expand  and  inspiration 
takes  place.  Expiration  is  produced  in  part  by  special  muscles,  the 
relaxation  of  the  diaphragm  and  walls  of  the  chest  cavity,  and  the 
elasticity  of  the  lungs  themselves. 

The  nervous  mechanism  that  controls  this  process  is  found  in  the 
respiratory  center  of  the  medulla  oblongata  (see  page  351.)  Under 
normal  conditions  respiration  results  from  the  alternate  stimulation  of 
two  sets  of  fibers  in  the  vagus  nerve  leading  from  the  lungs  to  the 
respiratory  center.  The  inspiratory  fibers  are  stimulated  at  each  ex- 
piration by  the  collapse  of  the  lungs,  which  results  in  an  increase  in 


THE   METABOLIC   MACHINERY   OF  ANIMALS  319 

the  rate  of  inspiratory  discharge  from  the  center  down  the  cord  to  the 
various  levels  where  the  relay  apparatus  or  sympathetic  system  causes 
inspiration.  As  the  inspiration  occurs  the  expiratory  fibers  of  the 
vagus  are  stimulated  by  the  expansion  of  the  lungs  and  the  inspiration 
is  partially  inhibited.  Experiments  clearly  indicate  that  the  gases  in 
the  blood  have  a  direct  effect  upon  the  activity  of  the  center  since, 
for  example,  an  increase  of  carbon  dioxide  in  the  blood  results  in  an 
increase  in  the  force  or  rate  of  the  respirations.  This  however  does 
not  tell  the  whole  story.  Recently  accumulated  data  furnish  evidence 
for  the  belief  that  the  activity  of  the  respiratory  center  is  controlled 
by  the  hydrogen-ion  concentration  of  the  blood  passing  through  it, 
which  in  turn  is  affected  by  the  pressure  of  carbon  dioxide  in  the 
blood. 

SECTION   D.     EXCRETORY  MECHANISMS 

Excretion 

This  term  is  used  to  cover  the  separation,  collection,  and  elimi- 
nation of  the  waste  products  of  metabolism  from  the  body.  These 
waste  products  naturally  vary  within  the  organism  itself  from  time  to 
time,  and  show  even  greater  variation  between  different  species  of 
animals.  Fundamentally  such  devices  center  about  mechanisms 
which  are  adapted  in  different  ways  for  the  elimination  of  one  funda- 
mental by-product  —  nitrogenous  wastes.  In  addition  liquids  in  the 
form  of  water,  dissolved  inorganic  salts,  and  gases,  as,  for  example, 
carbon  dioxide,  are  likewise  eliminated  by  excretory  devices.  Like- 
wise the  digestive  tract  furnishes  the  avenue  through  which  solid 
wastes  may  be  eliminated,  although  this  latter  method  should  not 
be  regarded  as  true  excretion.  Furthermore  it  should  be  realized 
that,  in  the  vertebrates  at  least,  there  is  a  constant  elimination  or 
sloughing  off  of  the  exposed  cells  on  various  epithelial  surfaces,  as 
well  as  from  the  linings  of  various  tubes  and  ducts  which  connect 
more  or  less  directly  with  the  outside.  This  section,  however,  is 
primarily  concerned  with  the  various  urinary  devices  for  the  disposal 
of  liquid  wastes. 

In  highly  specialized  forms  such  as  mammals  a  number  of  devices  are 
adapted  in  one  way  or  another  for  the  elimination  of  waste  products. 
Before  studying  these  mechanisms  in  any  detail,  we  shall  consider 
briefly  the  various  types  of  excretory  systems  found  throughout  the 
animal  kingdom. 


320  THE   MAINTENANCE   OF  THE   INDIVIDUAL 

Types  of  Excretory  Devices 

Contractile  Vacuoles.  Protozoa  are  usually  characterized  by 
some  sort  of  contractile  vacuole  which  serves  to  eliminate  such  sub- 
stances as  carbon  dioxide,  surplus  water,  and  perhaps  some  non- 
volatile nitrogenous  substances.  In  addition  to  contractile  vacuoles, 
protozoa  may  store  and  later  eliminate  more  solid  wastes  by  the 
formation  of  granules  or  crystals  within  vacuoles  in  the  body. 

Intracellular  Excretion.  In  some  of  the  simplest  metazoa  a 
so-called  intracellular  excretion  takes  place.  This  involves  the  inges- 
tion of  particles  of  waste  products  by  certain  ameboid  cells  which 
leave  the  body  and  disintegrate,  freeing  the  excretory  matter  within 
their  protoplasm.  Associated  with  this  process  is  the  excretion  of 
other  wastes  from  the  surface  of  the  body,  as  is  characteristic  of  some 
of  the  sponges.  In  addition,  certain  cells  may  store  waste  products 
or  there  may  be  localized  areas  for  excretion. 

Other  Excretory  Devices.  In  some  of  the  coelenterates  the 
first  evidence  of  true  excretory  organs  appears  in  the  form  of  pores 
connected  with  the  alimentary  tract  through  the  canal  system  {e.g., 
Hydra  and  Discomedusae).  Although  other  types  exist  they  are 
unimportant  for  our  purposes  and  may  be  omitted. 

Among  slightly  higher  forms  than  sponges  and  coelenterates  the 
waste  products  are  carried  to  the  outside  through  a  complicated 
system  of  connecting  tubules  in  which  are  located  occasional  ciliated 
cells,  whose  function  appears  to  be  to  keep  the  fluids  in  motion.  The 
blind  ends  of  these  tubules  are  capped  by  minute  ciliated  cells  of  the 
protonephridial  excretory  system  called  flame  cells.  These  lie  in  the 
parenchyma  and  by  their  movement  initiate  the  flow  of  liquid  and 
soluble  waste  products  which  they  have  secreted  through  the  wall. 
The  waving  of  the  tuft  of  cilia  in  each  cell  is  responsible  for  the  intro- 
duction of  the  term  flame  cell.  In  some  cases  it  is  believed  that  the 
cells  of  these  convoluted  tubules  may  also  reabsorb  food  material 
from  the  passing  "wastes"  as  well  as  contribute  excreta  to  the  stream. 

Reaching  the  higher  segmented  worms  like  the  earthworm,  the 
excretory  apparatus  is  composed  of  a  system  of  paired  nephridia  for 
each  somite.  Such  nephridial  systems  are  really  a  series  of  separate 
units,  each  of  which  is  composed  of  a  ciliated  funnel,  or  nephrostome, 
and  a  duct  that  passes  through  the  posteriad  septum  to  empty  to  the 
outside.  A  portion  of  the  canal  is  usually  glandular  or  secretory  in 
function  and  serves  to  discharge  waste  products  into  the  tubule  and 


THE   METABOLIC   .MACHINERY   OF   ANIMALS  321 

possibly  to  reabsorb  any  nutrient  materials  which  escaped  in  wastes 
from  the  fluid  in  the  body  cavity  (see  figure,  page  192). 

In  the  insects  still  another  type  of  excretory  system  is  composed  of 
special  tubules  called  Malpighian  tubules.  The  cavity  of  each  tubule 
is  surrounded  by  large  cells  covered  by  a  peritoneal  lining,  emptying 
into  the  intestinal  canal.  The  free  ends  of  the  tubules  lie  in  the  body 
cavity,  where  they  are  bathed  in  blood.  The  waste  products  pass 
into  the  Alalpighian  tubules  from  the  blood.  This  interpretation  is 
supported  by  the  detection  of  considerable  quantities  of  nitrogenous 
material  in  the  tubules  (see  figure,  page  210). 

Excretory  Devices  of  Vertebrates  —  Kidney 

The  excretory  organs  of  vertebrates  are  known  as  kidneys.  While 
several  different  forms  of  kidneys  are  known  to  exist,  they  are  all 
derived  embryologically  from  paired  segmented  structures,  which  in 
many  of  the  lower  types  may  be  connected  with  the  body  cavity 
by  a  series  of  ciliated  funnels  reminiscent  of  the  earthworm.  Along 
with  the  complex  changes  of  the  various  systems  of  organs  found 
in  the  higher  forms,  especially  of  the  circulatory  system,  there  is  a 
much  more  intimate  association  of  the  circulatory  and  excretory 
systems  and  a  decrease  in  the  importance  of  the  part  played  by  the 
body  cavity  in  the  removal  of  wastes. 

The  Mammalian  Excretory  System 

A  typical  mammalian  excretory  system  is  a  complex  affair,  for  it 
involves  not  only  the  kidneys  and  their  associated  duets,  but  also 
the  bladder  and  portions  of  the  circulatory  system  as  well.  This 
does  not  tell  the  entire  story,  for  the  lix'er,  lungs,  skin,  and  alimentary 
tract  also  play  an  important  part  in  the  excretion  of  wastes. 

The  Liver.  The  liver,  which  was  considered  in  connection  with 
the  digestive  system,  also  plays  a  vital  role  in  the  elimination  of  cer- 
tain wastes  from  the  body.  Proteins  are  absorbed  from  the  digestive 
tract  in  the  form  of  amino  acids.  Too  heavy  a  protein  diet  results  in 
the  absorption  of  more  nitrogen-containing  material  than  can  be 
utiUzed  by  the  cells  of  the  body  for  tissue  building.  The  cells  of  the 
liver  have  the  ability  to  split  off  the  nitrogen-containing  radical  and 
in  some  instances  resynthesize  the  remaining  materials  to  carbohy- 
drates and  even  fat.  The  nitrogen  which  is  thus  left  behind  may 
have  been  removed  as  ammonia  (NH3)  which  is  quite  toxic  to  the 


322 


THE   MAINTENANCE   OF  THE   INDIVIDUAL 


body,  especially  the  nerve  centers,  but  the  liver  also  splits  off 
hydrogen  and  unites  it  with  carbon  dioxide  to  produce  a  relatively 
harmless  substance  called  urea  (CO(NH2)2),  and  water,  thus 


2  NH3  +  COo 


^0  =  C 


NH2 
NH5 


+  H0O 


which  in  turn  is  removed  from  the  blood  stream  by  the  kidneys. 
Other  products  which  are  eliminated  by  the  hver  include  bile,  its 
pigments,  as  well  as  various  salts,  neutral  fats,  cholesterin,  and 
lecithin. 

Other  Devices  for  Waste  Elimination.  There  are  parts  of 
other  systems  that  should  be  mentioned  in  a  consideration  of  the 
phenomenon  of  excretion.  These  are  the  lungs,  skin,  and  alimentary 
canal.  The  former,  as  previously  noted,  excretes  through  the  alveoli 
most  of  the  carbon  dioxide  produced  in  the  body  of  man.  This  may 
be  indicated  in  tabular  form  ^  for  man  as  follows : 


Organs 

Essential 

Incidental 

Lungs 

Kidneys     

Alimentary  canal    .     . 
Skin 

Carbon  dioxide 

Water  and  soluble  salts,  re- 
sulting from  metabolism  of 
proteins,  neutralization  of 
acids,  etc. 

Solids,  secretions,  etc. 

Heat  regulator 

Water,  heat 
Carbon  dioxide,  heat 

Water,  carbon   dioxide,  salts, 

heat 
Water,  carbon  dioxide,  salts, 

hair,  nails,  and  dead  skin 

The  skin  serves  a  variety  of  purposes,  one  of  the  most  important 
being  regulation  of  the  elimination  of  small  amounts  of  carbon  dioxide. 
When  the  kidneys  are  not  functioning  properly  the  skin  may  be 
stimulated  to  excrete  more  waste  substances.  The  alimentary  canal 
serves  to  rid  the  body  of  nondigested  and  nondigestible  substances 
which,  through  the  processes  of  digestion,  have  yielded  up  their 
content  of  foods.  Furthermore,  the  alimentary  canal  actually  excretes 
waste  products  through  its  walls  into  the  lumen  of  the  canal. 

The  Kidneys.  We  think  of  these  structures  as  the  principal  organs 
of  excretion,  and  perhaps  rightfully  so.  Nevertheless  ehmination  of 
wastes  is  not  the  only  important  function  of  the  kidneys.     They  help 


'From  Kimber,  Gray,  and  Stackpole,  A  Textbook  of  Anatomy  and  Physiology.     By  permission  of 
The  Macmillan  Company,  publishers. 


THE   METABOLIC   MACHINERY   OF  ANIMALS 


323 


to  keep  the  ingredients  of  the  plasma  of  the  blood  standardized,  thus 
regulating  the  salt  content  of  the  blood  by  altering  the  ratio  of  salt  to 
water  produced  in  the  urine,  depending  upon  the  amount  taken  into 
the  body.  The  normal  healthy  person  eliminates  the  following 
amounts  of  waste  per  day,  through  the  kidneys :  30  grams  of  urea 
(converted  ammonia) ;  15  grams  of  urea  salt ;  10  grams  of  other 
soluble  urea  substances.  The  remainder,  96  per  cent  by  weight,  is 
water,  making  a  total  of  one  to  one  and  a  half  liters  that  is  eliminated. 
A  sagittal  section  through  the  kidney  reveals  the  expanded  upper 
end  of  the  ureter  on  the  median  side  draining  the  basinlike  pelvis 
of  the  kidney.  The  outer  portion  is  a  compact  region  called  the 
cortex,  while  the  inner  striated  portion  ending  in  the  irregular  margin 
of  the  pelvis  is  known  as  the  medullary  substance  of  the  kidneys. 


-Cortex 


proximal 
diistal  tubule. 

.glomerulus']  sift 


descending  limt       A  i 
ascending  limb    ,^  f 

Hanles  loop 


papillary  diccts 
:r!:^papUla 

Diagram  of  the  human  excretory  system. 


blocCdCar- 


.  wr-athra 
How  do  urea,  water,  and  inorganic 


salts  reach  the  pelvis  i* 


The  inner  margin  of  the  medullary  substance  forms  renal  pyramids 
the  tips  of  which  are  projections,  or  papillae,  that  lie  in  closely  invest- 
ing cuplike  depressions  of  the  pelvis,  called  calyces.  The  tip  of  each 
papilla  is  dotted  with  the  openings  of  the  collecting  ducts,  which  in 
turn  are  formed  from  the  union  of  several  renal  or  uriniferous 
tubules. 


324  THE   MAINTENANCE   OF  THE   INDIVIDUAL 

These  uriniferous  tubules  begin  in  an  expansion  (Bowman's  capsule) 
about  a  little  arterial  knot  of  capillaries,  called  a  glomerulus,  which 
together  make  up  the  functional  unit  of  the  excretory  system,  known 
as  a  renal  or  Malpighian  corpuscle.  In  order  to  understand  the  work- 
ings of  these  million  odd  excretory  units,  it  is  necessary  to  understand 
the  anatomy  of  the  kidney. 

The  main  trunk  line  of  the  arterial  system  gives  off  a  pair  of  renal 
arteries  that  are  broken  down  into  many  very  small  afferent  vessels 
each  of  which  enters  the  glomerulus,  leaving  as  a  smaller  efferent 
vessel  that  breaks  down  into  a  typical  capillary  network  over  the 
convoluted  surface  of  the  tubule.  As  the  wall  of  Bowman's  capsule 
surrounding  the  glomerulus  is  thin,  it  is  believed  that  water  and 
inorganic  salts  are  mechanically  filtered  out  into  the  cavity  by  means 
of  differences  in  pressure  between  the  blood  vessels  and  the  lumen  of 
the  tubule.  In  the  second  set  of  capillaries  the  urea  and  other  specific 
urinary  constituents  are  first  transferred  by  the  cells  and  so  secreted 
in  the  uriniferous  tubule.  Water  and  certain  salts  are  reabsorbed 
into  the  blood  stream  at  this  point. 

In  any  event,  the  kidneys  remove  the  waste  products  from  the  blood 
stream,  transferring  them  to  the  pelvis  of  the  kidney,  and  thence 
down  the  ureters  to  the  bladder.  Here  the  urine  is  stored  until 
finally  released  to  pass  to  the  outside  through  the  urethra. 


SUGGESTED  READINGS 

Clendenning,  L.,  The  Human  Body,  Alfred  A.  Knopf,  Inc.,  1930.    Chs.  III-VII. 

INIore  popularized  anatom}^  and  phj^siology. 
Haggard,  H.  H.,  Devils,  Drugs,  and  Doctors,  Harper  &  Bros.,  1929.     Ch.  VI. 

A  popular  account  of  early  anatomy  and  physiology. 
Howell,  W.  H.,  Textbook  of  Physiology,  17th  ed.,  W.  B.  Saunders  Co.,  1933. 

Chs.  XXIII,  XLI,  XLII,  XLIII,  and  XLV. 

A  detailed,  technical  account  of  physiology. 
Kimber,  D.  C,  Gray,  C.  E.,  and  Stackpole,  C.  E.,  Textbook  of  Anatomy 

and  Physiology,  9th  ed..  The  Macmillan  Co.,  1934.     Chs.  XVII  and 

XXI. 

An  anatomy  and  physiology  of  the  human  respiratory  system.     Technical 

but  condensed. 
Locy,  W.  A.,  The  Growth  of  Biology,  Henry  Holt  &  Co.,  1925.     Ch.  X. 

An  account  of  Harvey's  contribution  to  our  knowledge  of  the  circula- 
tory system.    See  also  other  books  by  this  author,  or  others  on  the 

history  of  biology, 


THE   METABOIJC    MACHINERY   OF   ANIMALS  325 

Metcalf,  C.  L.,  and  Flint,  W.  P.,  Fundamentals  of  Insect  Life,  McGraw-Hill 

Book  Co.,  1932.     Chs.  Ill  and  IV. 

A  brief  account  of  insect  anatomy  and  physiology. 
Pearse,  A.  S.,  and  Hall,  E.  G.,  Homoiothermism,  John  Wiley  &  Sons,  Inc., 

1928. 

An  interesting  discussion  of  the  origin  of  warm-blooded  vertebrates. 
Plunkett,  C.  R.,  Outlines  of  Modern  Biology,  Henry  Holt  &  Co.,  1930.     Ch.  V. 

A  good  physico-chemical  account. 
Rogers,  C.  G.,  Textbook  of  Comparative  Physiology,  McGraw-Hill  Book  Co., 

1927.     Chs.  XVI,  XVII,  XXII.  XXIII,  and  XXVII 

An  advanced  account  of  physiological  digestive  processes  from  a  com- 
parative viewpoint. 
Wells,  H.  G.,  Huxley,  J.  S.,  and  Wells,  C.  P.,  The  Science  of  Life,  Doubleday, 

Doran  &  Co.,  1934.     Ch.  II,  Sec.  7,  Book  1 ;  Ch.  II,  Sees.  4,  5,  and  6. 

A  readable,  popular  account. 


XVI 


SUPPORT,   MOTION,  AND   SENSATION 

Preview.  Section  A.  Skeletal  devices  •  The  interdependence  of 
parts  •  The  kinds  of  skeletons :  Exoskeletons ;  endoskeletons ;  the  axial 
skeleton ;  the  appendicular  skeleton  •  Functions  of  skeletons :  Support ; 
protection;  movement  •  Section  B.  Devices  for  movement  •  The  "why" 
of  motion  and  locomotion ;  protoplasmic  extensions ;  demio-muscular 
sacs ;  water  vascular  systems  •  Muscles  and  muscular  systems :  Smooth 
or  involuntary  muscles,  skeletal  or  striated  muscles,  heart  muscle,  muscular 
contractions  •  Section  C.  Mechanisms  of  sensation  and  co-ordination  • 
The  morphological  unit  —  The  neuron  •  The  physiological  unit  —  The 
reflex  arc  •  Types  of  nervous  systems  :  Neuromotor  mechanisms  ;  co-ordina- 
tion by  a  network ;  co-ordination  by  a  nerve  ring ;  co-ordination  by  a  linear 
nervous  system ;  co-ordination  by  a  dorsal  tubular  nervous  system  •  Pro- 
tective devices  for  the  central  nervous  system  •  Anatomy  and  development 
of  the  brain  :  The  early  development  of  the  central  nervous  system ;  the 
parts  of  the  vertebrate  brain :  The  cerebrum  or  telencephalon,  the  'Twixt- 
brain  or  diencephalon,  the  mid-brain  or  mesencephalon,  the  cerebellum  or 
metencephalon,  the  medulla  oblongata  or  myelencephalon  ■  The  cranial 
nerves  •  The  spinal  cord  ■  The  spinal  nerves  •  The  autonomic  nervous 
system  •  The  sense  organs  —  Receptor  devices  :  taste  ;  smell ;  simple  light 
receptors ;  compound  eyes ;  camera  eyes ;  ears ;  cutaneous  sense  organs  • 
Suggested  readings. 

PREVIEW 

It  will  be  seen  from  the  preceding  unit  that  one  of  the  most  impor- 
tant essentials  for  an  animal  is  to  carry  on  successfully  its  metabolic 
processes.  This  is  equally  necessary  for  plants  although  they  have 
the  advantage  of  being  able  to  secure  most  of  the  raw  food  materials 
they  need  from  their  immediate  environment.  Animals  have  to 
move  to  get  their  food.  The  necessity  for  motion  involves  three 
factors,  a  mechanism  to  support  the  body  when  seeking  food, 
machinery  to  do  the  moving,  and  an  apparatus  to  detect  the  location 
of  food.  In  order  to  locate  food,  a  co-ordination  of  eye  and  Hmb 
under  control  of  the  nervous  system  is  required.  The  eye  receives  a 
stimulus  the  instant  that  the  color  or  shape  of  food  is  noted  by  the 
receptor  devices  in  the  retina.  The  motions  of  the  arms  and  legs  then 
supplement  the  desire  for  food,  followed  by  the  act  of  taking  it.     In 

326 


SUPPORT,    MOTION,   AND   SENSATION  327 

this  triple  process  some  of  the  thousands  of  pressure  endings  that 
are  scattered  over  the  body  come  into  phiy.  Many  of  these,  in  t\m 
case  of  man,  are  conveniently  concentrated  in  the  finger  tips  which 
relay  messages  to  the  brain.  It  is  readily  seen  that  the  process  of 
getting  food  requires  co-operative  action  of  the  skeletal,  muscular, 
and  nervous  systems. 

The  limb  action  involving  stooping,  standing,  and  reaching  calls 
into  play  different  sets  of  voluntary  or  skeletal  muscles.  This  empha- 
sizes one  of  the  fundamental  principles  of  the  study  of  muscles 
(myology),  namely,  that  for  every  muscle  group  there  is  an  opposing 
set  which  performs  the  opposite  type  of  movement.  Muscles  are 
effective  during  contraction  and  not  during  relaxation.  We  speak 
of  the  muscles  that  extend  the  arm  or  leg  as  extensors  and  those 
which  bend  them  as  flexors.  Such  muscles  are  very  different  from 
the  smooth,  involuntary  muscles  in  the  walls  of  the  intestines.  Here 
the  food  undergoes  rhythmic  segmentation  and  is  broken  up  into 
boluses  by  the  intermittent  contractions  of  smooth  muscle  cells. 
Fortunately,  the  control  of  these  involuntary  muscles  is  taken  off 
the  hands  of  the  voluntary  or  central  nervous  system.  Such  routine 
functions  are  put  under  the  control  of  the  autonomic  nervous  system, 
which  frees  the  brain  of  the  necessity  of  "willing"  all  these  things  to 
happen  and  leaves  the  central  nervous  system  free  for  "higher  evolu- 
tionary adventures"  by  taking  over  the  "drudgery  of  living."  In 
order  to  understand  these  processes,  commonly  taken  as  a  matter  of 
course,  we  must  investigate  carefully  the  "why  and  how"  of  loco- 
motion and  then  try  to  see  how  this  complicated  performance  is 
controlled. 

SECTION  A.     SKELETAL   DEVICES 

The  Interdependence  of  the  Parts 

The  material  covered  in  this  unit  consists  of  representatives  of  three 
well-defined  and  anatomically  separable  systems,  namely,  the  skeletal, 
muscular,  and  nervous  systems.  Although  they  are  frequently  con- 
sidered separately  for  the  sake  of  clearness  it  should  be  kept  in  mind 
that,  physiologically,  the  muscles,  skeleton,  nerves,  and  blood  supply 
are  all  intimately  interwoven.  In  the  human  body,  there  are  numer- 
ous muscles  most  of  which  are  under  voluntary  control  and  as  such 
are  concerned  with  posture,  with  maintaining  the  relationship  of  the 
various  skeletal  parts  to  one  another,  or  with  some  sort  of  movement. 
H.  w.  H.  —  22 


328  THE  MAINTENANCE  OF  TFIE   INDIVIDUAL 

All  of  these  muscles  are  under  the  control  of  the  nervous  system,  while 
energy  for  their  continued  movement  must  be  furnished  by  means  of 
absorbed  food  transported  through  the  circulatory  system  to  every 
part  of  the  body.  To  visualize  this  inter-relationship  think  of  the 
sustained  movement  of  an  arm  or  leg  which  is  dependent  upon  the 
activity  of  numerous  muscles.  The  action  of  the  muscles  is  in  turn 
controlled  by  the  nerves  which  conduct  messages  to  the  tissues  from 
the  brain  and  spinal  cord.  The  entire  network  of  nerves  and  their 
branches  has  often  been  likened  to  a  telephone  system  with  its  compli- 
cated series  of  connections  and  relay  wires.  Closely  associated  with 
the  nerves  are  the  arteries  and  veins,  forming  the  triumvirate  so 
often  pictured  in  histological  or  medical  texts. 


The  Kinds  of  Skeletons 

Skeletal  support  is  of  common  occurrence  in  the  animal  kingdom. 
Skeletons  may  be  divided  typically  into  outer  coverings,  or  exo- 
skeletons,  and  inner  supporting  devices,  or  endo skeletons. 

Exoskeletons 

Generally  speaking,  any  creature  or  organism  possessing  07ily  an 
exoskeleton  belongs  to  the  large  group  of  invertebrate,  or  non-chordate, 
animals.  Such  forms  may  be  present  in  some  members  of  a  given 
phylum  and  not  in  others.  Even  in  the  protozoa,  for  example,  the 
shelled  arcellidae  occur  in  the  same  class  with  the  naked  Ameba. 
Other  examples  within  this  same  group  are  the  foraminifera  and 
radiolaria  which  possess  limy  or  glassy  skeletons.  This  suggests  that 
on  the  whole  these  types  of  exoskeleton  are  not  essential  for  loco- 
motion but  are  primarily  protective  devices.  That  is  certainly  true  of 
the  sessile  sponges,  corals,  sea-lilies,  and  lamp-shells  (brachiopoda), 
and  would  also  probably  hold  for  most  of  the  clams,  snails,  star- 
fishes, and  brittle-stars.  In  the  great  phylum  of  the  arthropods,  the 
exoskeleton  is  specialized  and  definitely  associated  with  an  equally 
highly  adapted  muscular  system,  the  two  being  definitely  designed  for 
effecting  locomotion.  Even  among  the  vertebrate  chordates  an 
exoskeleton  as  well  as  an  endoskeleton  sometimes  occurs,  as,  for 
example,  in  the  turtles.  In  such  forms  the  vertebral  column  becomes 
fused  to  the  dorsal  shell  which  is  formed  by  the  flattened  ribs  plus 
dermal  costal  plates. 


SUPPOllT,    MOTION,   AND   SENSATION  329 

Endoskeletons 

Endoskeletons  are  characteristic  of  chordatc  animals.  An  internal 
supporting  rod  {notochord)  is  clearlj^  present  in  the  larvae  of  the 
tunicatcs  and  in  the  adult  amphioxus,  while  a  well-developed  endo- 
skeleton  is  found  in  all  of  the  so-called  higher  forms  from  fishes  to  man. 

The  skeleton  of  vertebrates  is  divided  typically  into  three  parts : 
the  axial  skeleton,  which  includes  the  skull,  thoracic  basket,  main 
spinal  column,  and  tail ;  the  appendicular  skeleton,  which  pertains  to 
the  appendages ;  and  the  visceral  skeleton,  which  is  developed  in  con- 
nection with  the  various  modifications  of  the  gill  region.  In  adult 
fish,  the  visceral  skeleton  forms  the  cartilaginous  or  bony  bars  (gill 
arches).  In  other  vertebrates,  the  visceral  skeleton  becomes  con- 
verted into  various  highly  modified  structures  involved  in  the  forma- 
tion of  the  jaws,  the  hyoid  support  of  the  tongue,  the  larynx,  accessory 
parts  of  the  skull,  and  even  the  bones  in  the  middle  ear. 

The  Axial  Skeleton 

Anteriorly,  the  axial  skeleton  of  vertebrates  is  specialized  into  a 
skull,  a  bony  case  covering  the  expanded  anterior  end  of  the  spinal 
cord,  or  brain.  Incorporated  into  this  skull  are  specialized  protective 
capsules  for  several  of  the  major  sense-organs,  namely,  the  eyes,  ears, 
and  nose. 

Many  bones  are  fused  to  form  the  skull.  These  are  of  two  sorts, 
either  memhranous  or  cartilaginous.  The  former  are  developed 
directly  from  a  connective  tissue  membrane,  while  the  latter  type 
pass  through  a  preliminary  cartilaginous  stage  before  becoming 
bone.  In  primitive  vertebrates,  the  brain  is  protected  by  cartilage 
which  later  in  the  evolutionary  picture  becomes  ossified.  Still 
later,  this  original  cartilaginous  cranium  is  further  protected  by 
the  addition  of  a  group  of  thin,  flat  membrane  bones,  shingled  over  the 
skull.  In  higher  forms  the  number  of  embryonic  bones  in  the  skull 
has  been  reduced.  The  skull  of  a  dog,  for  example,  contains  fewer 
bones  than  that  of  a  codfish.  A  study  of  the  earlier  stages  of  develop- 
ment in  mammals  shows,  however,  that  representatives  (or  homo- 
logues)  of  many  of  the  bones  present  in  the  cod  skull  may  be  found. 
These  embryonic  elements  fuse  in  later  development,  making  the 
smaller  number  of  skull  bones  found  in  the  adult.  In  the  skull  of  a 
reptile,  for  example,  there  are  four  occipital  bones  surrounding  the 
point  of  exit  of  the  spinal  cord  from  the  skull,  which  in  most  adult 


330 


THE   MAINTENANCE   OF   THE   INDIVIDUAL 


mammals  are  fused  into  a  single  occipital  hone.  Further  study  of  a 
series  of  forms  from  fish  to  man  would  furnish  remarkable  evidences 
of  homology  besides  emphasizing  the  interpretative  importance  of 
the  study  of  comparative  anatomy. 


parietal 


temporal... 
YTCCSal 


--frontal 

V  L  1  u:m'vm  r — ^^<2-noicC 

■^  1  "^^^is^y/  \  loccriTTial 

Wi\     *^  -^^ incclccr' 

■<-/^ ■mccicjllcc 

mancCible 


parietal 


temponxl.. 
occipital.. 


Tnasto\di    process../    / 
styloid   process.../ 


frontal 
" '      ..-.ethynoid 

nccsocl 

— lacrimal 
mcclcc-T' 

<- .  fe> nxxjcilloL 

ynarzdible- 


Bones  of  a  human  skull.     (After  Walter.) 

The  skull  bones  of  man  are  frequently  divided  into  cranial  bones, 
which  surround  the  brain  itself,  and  those  which  are  designated  as 
facial  hones. 

The  remainder  of  the  axial  skeleton  is  composed  of  the  vertebral 
column  and  its  associated  bones.  In  aquatic  forms  hke  the  fishes, 
this  part  of  the  axial  skeleton  is  comparatively  unspecialized,  being 
divisible  into  the  rib-bearing  vertebrae  of  the  trunk,  and  those  without 
ribs,  called  caudal  vertebrae,  which  go  to  make  up  the  tail.  With 
the  evolution  of  land  animals,  protection  of  the  under  side  of  the 
body  became  essential  and  therefore  a  ''thoracic  basket"  was  de- 


I 


SUPPORT,    MOTION,   AND   SENSATION 


331 


veloped,  composed  of  ribs  attached  to  a  ventral  breastbone  (sternum) 
and  to  the  dorsal  backbone.  Ascending  the  evolutionary  tree  farther 
the  organism  became  better  adapted  to  turn  the  head.  A  fish  or 
frog  must  not  only  roll  the  eyes  but  also  change  the  entire  position 
of  the  body  in  order  to  look  behind.     Not  so  with  a  cat,  which  may 


cervical 

vertebrae" 


rooCitcs 
iclncc 


Cranxum. 


clavicl© 


L /..scapula. 

-thoracic     sternuTn 
vertebrae 


lumbar 
v©rt<atorcc© 


-5ctortxm 


Carpus 


patella 

tibia, 
fxbula,. , 

1 1  fl  /  f  i// 

In/  1/  i//j 

.tarsus 
mata-torsLrs 

Human  skeleton.     Can  you  recognize  the  bones  of  a  disarticulated  skeleton.^ 

roll  its  eyes  and  is  also  able  to  turn  its  head.  This  ability  to  rotate 
the  head  is  due  to  varying  numbers  of  cervical,  or  neck,  vertebrae. 
Four-footed  animals  are  further  characterized  by  four  other  sets  of 
vertebrae,  thoracic  (with  ribs),  lumbar  (without  ribs),  sacral  (for  the 
attachment  of  the  pelvic  girdle),  and  caudal. 


332 


THE   MAINTENANCE  OF  THE   INDIVIDUAL 


procoroioidr 
CoracoicC 


radius 


ScapLcla 
glenoid  -j^ssa 

xtlnct 


.carpccls 
f  ooi\. .■Tneta<;arpal  5 


The  Appendicular  Skeleton 

A  study  of  any  group  of  land  animals  shows  a  fundamental  simi- 
larity of  limb  construction.  Even  such  apparently  diverse  structures 
as  the  flippers  of  a  whale  or  a  seal  and  the  wings  of  a  bird  are  found  to 
be  identical  in  fundamental  plan.  All  sorts  of  land  animals  typically 
possess  shoulder  and  hip  girdles,  respectively  known  as  pectoral  and 
pelvic  girdles.  These  girdles  are  attached  directly  or  indirectly  to  the 
axial  skeleton,  thus  providing  rigidity  and  facilitating  movement  of 
the  appendages.     It  is  significant  that  the  pentadactyl  limb  of  the 

land  vertebrates  is  built 
upon  a  generalized  plan, 
in  which  each  girdle  is 
formed  of  three  bones. 
Each  front  and  hind  leg 
is  likewise  composed  of 
three  major  bones.  In 
the  anterior  limb,  a  single 
humerus  articulates  with 
two  bones,  the  ulna,  a 
process  of  which  forms 
the  "funny  bone "  of  the 
elbow,  and  the  radius. 
In  the  posterior  limb  the 
corresponding  bones  are 
the  femur,  which  is  typi- 
cally characterized  by  a 
prominent  "ball"  at  one 
side  of  the  main  axis 
fitting  into  a  socket  in 
the  pelvic  girdle ;  the 
tibia,  or  shin-bone  ;  and 
the  smaller  fibula.  In 
addition  to  these  larger 
bones  is  the  group  of 
wrist  {carpal)  and  ankle 
(tarsal)  bones,  followed  by  the  metacarpal  and  metatarsal  bones, 
depending  upon  whether  they  belong  to  the  anterior  or  posterior 
limb.  The  bones  of  the  fingers  or  toes  are  technically  known  as 
phalanges. 


•pre  lirnb 


^  c  c  V 


...phoclanges 


femu"r 


pubis- 
ischium 


tibia 


ODD tarsals 

f  h  '^  \ 
I  ;Br   ■ 

Diagram  of  the  bones  of  the  fore  and  hind  hmbs 
arranged  to  show  their  homology. 


'\imd.  HtoId 


--'pbalan^e-S 


SUPPORT,   MOTION,   AND   SENSATION 


333 


The  feet  of  animals  show  many  remarkable  adaptations.  Foot 
posture  involves  more  than  fallen  arches ;  it  determines  the  speed 
at  which  an  animal  can  travel. 
If  the  wrist  and  ankle  are  raised 
from  the  ground  the  result  is  a 
longer  leg  capable  of  a  longer 
stride,  which  means  covering 
more  ground  in  the  same  inter- 
val of  time.  Anatomists  dis- 
tinguish three  types  of  feet : 
plantigrade,  the  primitive  fiat- 
footed  type  found  in  man  and  the 
bear ;  digitigradc,  characteristic 
of  cats  or  dogs  that  are  literally 
"on  their  toes  "  all  the  time  ;  and 
the  unguligradc,  restricted  to 
forms  which  walk  on  their  nails, 
like  horses,  cows,  and  camels. 

Functions  of  Skeletons 


digitigixxcCe         -unguligracCe^ 


Types  of  mammalian  feet.  State  the 
advantages  and  disadvantages  of  each 
type  of  foot.  (After  Pander  and  D'Al- 
ton.) 


Skeletal  devices  usually  serve 
one  of  three  functions,  namely, 
support,  protection,  or  movement.     Examples  of  each  type  will  be  given, 
although  it  is  sometimes  difficult  to  separate  these  functions. 

Support 

It  is  quite  apparent  that  organisms  living  in  water  have  much  less 
necessity  for  a  supporting  framework  than  land-inhabiting  animals. 
This  is  due  to  the  fact  that  the  body  approximates  more  closely  the 
density  of  the  surrounding  medium  and  is  consequently  buoyed  up  by 
it.  Cuttlefishes  and  jellyfishes  maintain  their  shape  in  their  natural 
environment  but  out  of  water  collapse  more  or  less  completely. 

In  like  manner,  the  bivalve  shells  of  clams  and  mussels  form  a 
supporting  skeleton,  to  which  is  attached  the  mantle  that  in  turn 
encloses  the  viscera.  Crayfish  and  lobsters  offer  still  another  ex- 
ample of  skeletal  support,  for  their  movement  is  largely  brought 
about  through  the  interaction  of  a  well-developed  exoskeleton  and 
inside  muscles. 

In  land-inhabiting  forms,  the  function  of  the  skeleton  as  a  sup- 
porting device  becomes  most  apparent.     It  is  hard  to  envisage  any 


334  THE   MAINTENANCE   OF  THE   INDIVIDUAL 

other  form  of  mechanical  supporting  mechanism  which  would  permit 
the  general  physiological  setup  as  we  know  it  in  land  animals  today. 

Protection 

It  is  difficult  to  speak  of  the  skeleton  without  associating  it  with  the 
idea  of  protection.  Special  devices  suggestive  of  protection  are 
scattered  throughout  representatives  of  most  of  the  phyla.  Certain 
types  of  spicules  in  the  sponges,  the  calcareous  exoskeleton  of  stony 
corals,  and  the  thickened  horny  layer  of  other  branching  colonial 
coelenterates  (hydroids)  probably  serve  for  the  protection  of  these 
animals.  Skeletal  protective  devices  are  also  quite  obvious  in  snails, 
starfishes,  sea-urchins,  arthropods,  armored  fishes,  fossil  armored 
reptiles,  and  turtles. 

Movement 

Movement  is  one  of  the  almost  universal  characteristics  of  animals. 
Even  in  the  protozoa  special  locomotor  organs  such  as  pseudopodia, 
flagella,  and  cilia  are  found.     The  earthworm  uses  its  setae  in  crawling. 

The  greatest  use  of  the  skeleton  for  movement,  however,  occurs  in 
the  arthropods  and  vertebrates,  two  highly  specialized  groups.  The 
former  have  well-developed  exoskeletons  while  the  latter  are  charac- 
terized by  an  endoskeleton.  This  means  that  in  the  case  of  insects, 
for  example,  the  muscles  are  inside  the  skeleton  while  in  vertebrates 
they  are  outside.  In  both  groups,  however,  the  skeletal  elements 
articulate  with  one  another,  usually  by  means  of  curved  and  rounded 
surfaces  permitting  free  movement  of  one  part  upon  the  other. 


SECTION   B.     DEVICES   FOR   MOVEMENT 

The  "  Why  "  of  Motion  and  Locomotion 

In  the  first  place,  animals  must  actively  seek  food  and  must  be 
constantly  on  the  move  if  they  are  to  keep  from  starving.  In  addi- 
tion, many  animals,  especially  the  higher  vertebrates,  give  evidence 
of  enjoying  play,  another  type  of  muscular  activity.  This  is  more 
apt  to  be  true  of  the  young,  but  is  also  characteristic  of  many 
adults.  If  an  organism  is  to  survive  in  the  struggle  for  existence,  the 
ability  to  become  adapted  to  different  environments  by  moving  from 
one  place  to  another  is  a  third  essential.     For  example,  grazing  ani- 


SUPPORT,   MOTION,   AND   SENSATION  335 

mals  must  be  able  to  go  from  one  feeding  area  to  another.  This 
holds  good  not  only  from  the  standpoint  of  competition  for  food  but 
also  from  that  of  avoiding  unfavorable  climatic  conditions,  such  as 
drought,  which  destroys  those  animals  that  are  unable  to  keep  on 
moving  to  a  better  feeding  ground.  Other  animals  use  this  same 
ability  of  movement  in  flight  and  so  survive  by  being  able  to  escape 
capture.  Lastly,  the  part  played  by  motion  in  perpetuating  species 
should  be  mentioned.  The  strutting  and  bowing  of  a  male  pigeon, 
or  the  battle  between  two  male  deer  in  the  silence  of  the  forest  are 
common  examples  of  movement  employed  in  the  perpetuation  of  the 
species. 

Protoplasmic  Extensions 

The  concept  of  movement  is  usually  associated  with  the  contraction 
of  muscles,  but  muscles  do  not  tell  the  whole  story.  Three  distinct 
types  of  locomotor  devices  —  namely,  pseudopodia,  flagella,  and  cilia, 
which  are  so  characteristic  of  the  protozoa,  have  already  been 
described. 

The  cirri  of  protozoa  are  probably  the  most  highly  specialized  of  all 
unicellular  motile  structures  as  they  may  be  moved  in  any  direction. 
Certain  organisms  like  Stylonychia  or  Euplotes  actually  walk  or  run  on 
the  tips  of  their  cirri.  The  action  of  the  cirri  is  thought  by  some  to 
be  controlled  by  a  so-called  "neuromotor  apparatus"  present  in  these 
"simple"  one-celled  organisms. 

Der mo-Muscular  Sacs 

Many  of  the  soft  bodied  invertebrates  possess  locomotor  muscles 
concentrated  in  the  outer  layers  of  the  body.  The  earthworm  is  an 
example  of  such  a  type.  The  body  is  shortened  by  the  contraction 
of  the  inner  longitudinal  muscles  and  elongated  by  the  action  of  the 
outer  circular  set  lying  immediately  beneath  the  cuticula  and  hypo- 
dermis. 

Water  Vascular  Systems 

The  echinoderms  have  exclusi^'e  patents  on  this  method  of  loco- 
motion that  functions  by  means  of  water  pressure  in  their  numerous 
tube-feet.  The  apparatus  opens  on  the  dorsal  surface  of  a  starfish, 
for  instance,  through  a  sievelike  structure,  called  the  m.adreporite.  Sea 
water  may  be  added  to  the  so-called  amhulacral  fluid  through  the 


336 


THE   MAINTENANCE   OF  THE   INDIVIDUAL 


madreporite  by  means  of  cilia  which  send  it  along  the  stone  canal. 
The  latter  structure  leads  straight  down  to  the  circumoral  ring  canal. 
Five  radial  canals  branch  from  this  and  extend  down  the  five  arms 
sending  off  smaller  branches  which  end  in  the  tube-feet  lying  along  the 
ambulacral  grooves.     The  proximal  end  of  each  foot  has  a  muscular 


ws/  s  :Q 


Vertical  section  through  an  arm  of  a  starfish :  b,  ampulla ;  d.  water  canal 
opening  at  madreporite  plate,  sl\  i,  radial  water  tube;  m,  mouth;  //,  tube  feet; 
py,  digestive  gland  ;   sic,  stomach  ;    a,  anus ;    v,  ring  canal ;    n,  nerve  ring. 

bulb,  the  ampulla,  which  is  capable  of  contracting,  thus  forcing  the 
ambulacral  fluid  into  the  tube-foot.  When  the  sucking  disks  at  the 
free  end  of  the  distended  tube-feet  become  attached  to  an  object,  the 
muscles  of  these  tubular  organs  contract,  forcing  the  water  back  into 
the  ampullae,  and  the  animal  through  its  grip  is  enabled  to  move 
forward. 


Muscles  and  Muscular  Systems 

Great  differentiation  of  muscles  is  invariably  related  to  a  well- 
developed  skeletal  system.  In  two  large  diverse  groups  of  animals, 
the  arthropods,  with  a  chitinous  exoskeleton,  and  the  vertebrates, 
with  a  calcareous  endoskeleton,  individual  muscles  rather  than 
muscle  layers  have  been  developed.  Examples  of  exoskeletal  muscles 
are  the  colorless,  transparent,  or  yellowish-white  muscles  typical  of 
the  insects.  Although  soft  and  almost  gelatinous  in  appearance,  these 
muscles  which  are  usually  striated  are  very  efficient,  as  may  be  seen 
in  the  common  house  fly  whose  wings  beat  over  300  strokes  per 
second.  Among  vertebrates  there  are  found  smooth  or  involuntary 
muscles,  skeletal  or  striated  muscles,  and  heart  muscles. 

While  the  muscles  of  a  frog  and  those  of  a  man  may  be  homologous, 
that  is,  comparable  embryologically  and  morphologically,  it  does 
not   necessarily  follow  that  they  are    analogous,  that  is,   alike  in 


SUPPORT,   MOTION,   AND   SENSATION 


337 


the  particular  function   which    they   perform.     The   frog's   leg,   for 

example,  is  relatively  incapable  of  more  than  a  flexing  motion  or  a 

straight  swing  of  the  limb,  whereas  the  human  arm  responds  to  flexing, 

rotating,    or    swinging, 

origin  of 


according  to  the  way 
in  which  it  is  moved. 
Human  musculature  is 
much  more  complex 
than  that  of  a  frog  be- 
cause it  has  many  more 
diverse  functions  to  per- 
form. 

Evidently  there  is  a 
definite  relationship 
between  the  types  of 
motion  which  are  possi- 
ble from  the  standpoint 
of  skeletal  structure 
and  the  development 
of  muscles  that  make 
such  movements  effec- 
tive. Actual  movement 
results  from  the  con- 
traction of  muscles  and 
is  stimulated  into  activ- 


raixsole. 


musctle 


terjoCon  cf 
Achilles  ' 


lir— .insertion 
f\   of  muscle 


man 


Comparison  of  the  arrangement  of  the  muscles 
and  supporting  skeleton  oi  an  insect's  and  verte- 
brate's leg.     (Former  after  Berlese.) 


ity  by  nerves.  Since  the  muscles,  nerves,  and  skeleton  are  closely 
correlated  parts,  their  degree  of  usefulness  depends  to  a  marked 
degree  upon  the  proper  development  and  functioning  of  all  the 
parts. 

Smooth  or  Involuntary  Muscles.  This  tissue  is  characterized 
by  the  absence  of  striations  and  the  presence  of  a  single  nucleus  in 
each  cell  or  fiber.  It  is  the  type  of  muscle  which  carries  on  most  of 
the  internal  movements  of  the  body.  The  walls  of  the  intestines 
are  lined  by  layers  of  circular  and  longitudinal  involuntary  muscles. 
The  muscles  in  the  walls  of  blood  and  lymph  vessels,  the  tracheal 
tube,  reproductive  ducts,  the  ureters,  and  tlie  skin  are  also  of  this 
type.  Typically  sluggish  in  contraction,  they  are  the  principal  kind 
of  muscles  found  in  the  lower  animals. 

Skeletal  or  Striated  Muscles.  In  this  category  fall  all  of  the 
muscles  which  are  under  the  control  of  the  central  nervous  system  and 


338  THE   MAINTENANCE   OF  THE   INDIVIDUAL 

which  move  the  boiios  of  tlie  skeleton.  There  are  approximately  over 
five  hiindnnl  sucli  mviseles  distinguishable  in  man.  They  form  the 
body  wall,  thus  constituting,  through  a  three-ply  arrangement,  the 
chief  means  of  keeping  the  viscera  in  position.  They  regulate  the 
position  of  the  head  and  the  degree  of  curvature  of  the  backbone,  as 
well  as  the  shape  of  the  thigh  and  the  calf  of  the  leg,  and  the  contour 
of  the  arm.  Since  these  muscles  are  responsible  for  all  quick,  con- 
sidered movements,  as  well  as  simple  reflex  actions,  they  must  be 
built  upon  a  plan  whereby  one  set  of  muscles  through  contraction 
may  perform  an  opposite  type  of  movement  from  the  other,  that  is, 
work  in  opposition  to  each  other. 

Individual  skeletal  muscle  fibers  may  reach  something  over  an  inch 
in  length,  but  average  only  about  5^^  of  an  inch  in  diameter.  If  a 
single  fiber  of  skeletal  muscle  is  examined  under  the  microscope,  the 
regular  rows  of  striations  become  visible.  Careful  study  reveals  a 
series  of  dense  strands  of  protoplasm  running  the  entire  length  of  the 
muscle  fiber,  between  which  are  spaces  filled  with  a  watery  proto- 
plasmic material.  It  is  believed  that  these  delicate  protoplasmic 
strands  are  capable  of  forcible  contraction  which,  by  mass  action, 
results  in  the  shortening  of  the  entire  muscle  fiber.  Each  muscle 
fiber  is  enclosed  in  a  modified  elastic  connective  tissue  membrane 
called  sarcolemma,  that  bears  scattered  nuclei  on  its  inner  surface. 
Practically  every  muscle  fiber  cell  is  stimulated  by  a  nerve  ending. 
Groups  of  these  muscle  fibers  are  bound  together  with  connective 
tissue,  numbers  of  these  bundles  forming  the  muscle  proper,  which 
is  then  spoken  of  as  a  biceps,  triceps,  and  so  on. 

The  ends  of  a  muscle  are  usually  tapered.  One  end  is  anchored 
to  an  immovable  portion  of  the  skeleton,  and  is  termed  the  origin, 
while  the  opposite  end,  which  is  attached  to  the  portion  of  the  skeleton 
to  be  moved,  is  termed  the  insertion.  The  helly  of  a  fusiform  muscle 
is  the  mid-portion  between  origin  and  insertion  which  swells  during 
contraction.  The  tough  sheath  of  connective  tissue  surrounding  the 
muscle  becomes  continued  as  a  tendon  merging  into  the  periosteum  of 
the  bone,  thus  giving  a  firm  attachment.  Striated  muscles  are  also 
arranged  in  flat,  fan-shaped  masses,  or  in  thin  sheets. 

Heart  Muscle.  This  variety  of  muscle  occurs  in  all  of  the  higher 
animals.  Although  it  has  characteristics  similar  to  the  muscles 
previously  described,  cytological  and  physiological  differences  place 
it  in  a  category  by  itself.  Notwithstanding  the  fact  that  the  action 
of  the  heart  is  involuntary,  the  cells  composing  heart  muscle  are 


SUPPORT,   MOTION,   AND   SENSATION  339 

striated  and  nucleate,  resembling  skeletal  muscles  in  being  capable 
of  rapid,  powerful  contractions,  but  unlike  other  muscles  by  reason 
of  their  regular  automatic  contraction  and  relaxation. 

Muscular  Contractions.  That  muscle  contraction  is  stimu- 
lated by  a  nerve  impulse  in  the  living  animal  has  long  been  proved, 
but  this  is  as  far  as  we  can  at  present  safely  go,  for  in  seeking  a  physico- 
chemical  explanation  of  what  actually  happens  within  the  cell  itself 
we  are  treading  upon  dangerous  ground.  At  the  present  time  there 
does  not  appear  to  be  an  accepted  theory  that  accounts  completely 
and  satisfactorily  for  muscle  contraction. 

Certain  things,  however,  are  definitely  known.  In  the  first  place, 
muscles  shorten  when  they  contract.  Under  the  microscope,  the  light 
and  dark  bands  so  readily  seen  in  striated  muscle  appear  to  exchange 
places.  In  reality,  the  light  bands  have  become  dark  and  the  dark 
ones  light  so  that  there  has  been  no  actual  exchange  of  position  but 
only  a  change  in  physical  make-up.  Chemically,  muscular  action  is 
due  to  a  series  of  complex  chemical  reactions  which  imdergo  a  number 
of  complicated  changes,  yielding  in  the  end  specific  amounts  of  lactic 
acid.  It  is  known  that  the  shortening  of  the  muscle  fibers  occurs 
before  and  independeyiily  of  the  formation  of  the  acid  and  therefore  it  is 
difficult  to  believe  that  the  two  are  unrelated.  When  muscular  activ- 
ity is  prolonged,  or  when  it  is  carried  out  under  conditions  implying  a 
lessened  supply  of  oxygen,  there  is  an  accumulation  of  so-called  waste 
products,  especially  of  lactic  acid.  According  to  Hill  (1923)  experi- 
ments on  man  caused  an  increase  of  from  29  to  104  mg.  of  lactic  acid 
per  100  cc.  of  blood  in  the  case  of  violent  exercise  carried  on  for  one 
and  a  half  minutes.  This  large  increase  in  acid  has  been  interpreted 
as  meaning  that  the  supply  of  oxygen  to  the  contracting  muscles  was 
inadequate.  Even  with  increased  respiration  and  circulation,  lactic 
acid  accumulated  in  the  muscles  and  was  given  off  to  the  blood,  thus 
creating  an  "oxygen  debt"  to  the  muscles.  This  phenomenon  is 
associated  with  the  condition  of  fatigue  and  has  been  studied  in  ath- 
letes, especially  track  men,  where  it  was  found  that  an  accumulation 
of  lactic  acid  hinders  muscular  relaxation.  In  races  the  intake  of 
oxygen  is  of  course  determined .  by  the  efficiency  of  the  lungs  and 
heart.  In  long  distance  running  the  athlete  reaches  an  equilibrium 
between  his  oxygen  intake  and  lactic  acid  production.  In  short  races 
he  may  breathe  but  once  or  not  at  all  and  so  builds  up  a  large  oxygen 
debt.  In  such  cases  a  state  of  exhaustion  may  be  reached  in  a  few 
seconds. 


310 


THE  MAINTENANCE  OF  THE  INDIVIDUAL 


SECTION   C. 


MECHANISMS  OF  SENSATION  AND 
CO-ORDINATION 


The  Morphological  Unit  —  The  Neuron 

In  order  to  get  at  the  secret  of  control  of  skeletal,  muscular,  and 
nervous  systems,  it  is  necessary  to  examine  the  various  nervous  devices 
foimd  throughout  the  animal  kingdom  which  have  been  developed  as 
co-ordinators.  All  animals,  except  perhaps  the  protozoa,  are  built 
up  of  a  number  of  essentially  similar  cell  units.  The  complexity  of 
the  adjustment  device  is  directly  related  to  the  way  these  units  are 

put  together,  as  well  as  to  the 
actual  number  of  the  units 
comprising  the  nervous  system. 
Since  the  fundamental  unit  of 
structure  of  the  nervous  system 
is  the  nerve  cell  (neuron),  we  will 
do  well  to  examine  it  further. 
A  typical  neuron  consists  of  a 
cell  body  and  two  kinds  of  out- 


r 


direction 
oP  impulse'. 


5— deridrite^ 

nucleus 

Icellbody 

.■naksd.  a^on 
;<.jCoUateral 


nucleus  of— . 
yieurilemma 


..medullary  5beatb    growths,    the    many    branched 

dendrites  which  receive  impulses. 


indicate^    _ .   . 

6reat  lendtVi  fi^         ,     ro 

^  ^      Vi..r2odsofl?arJvier 

1 


and  the  elongated  axon,  that 
conducts  messages  away  from 
the  body  of  the  cell,  and  ter- 
minates in  the  end  organs.  The 
''naked"  axon  is  characteristic 
of  the  gray  matter  of  the  cen- 
tral nervous  system.  Around 
many  of  the  axons  is  a  thin, 
membranous  protective  cover- 
ing, called  the  neurilemma,  or 
Schwann^s  sheath.  This  is  liv- 
ing tissue  as  shown  by  the 
nuclei  scattered  through  it,  and 
by  the  fact  that  it  may  be 
regenerated  after  injury.  Neu- 
rons of  this  latter  type  are  found  in  most  invertebrate  nervous 
systems,  in  some  of  the  prochordates,  and  in  some  of  the  peripheral 
nerves.  In  parts  of  the  central  nervous  system  of  vertebrates  the 
neurilemma  is  replaced  by  segments  of  white  fatty  substance,  called 


^-neurilemma 


\j2rmir2a\  branched 

A  typical  multipolar  nerve  cell. 


SUPPORT,   MOTION,   AND   SENSATION 


341 


the  medullary  sheath,  while  other  periplieral  nerves  possess  both  a 
medullary  sheath  and  an  outer  neurilemma. 

The  manner  in  which  neurons  operate  depends  upon  their 
"hook  up."  Contact  without  fusion  (synapse)  is  made  between 
the  end  organ  of  one  neuron  and  the  dendrite  of  another,  resulting 
in  continuity  from  the  physiological  point  of  view. 


The  Physiological  Unit  —  Reflex  Arc 

The  physiological  unit  of  the  nervous  system  is  a  reflex  arc.  Such 
arcs  are  made  up  of  two  or  more  neurons  and  a  muscle  or  gland  ele- 
ment.    A  simple  arc  consists  of  a  receptor  neuron,  the  dendrites  of 


recsptoT- 


stimttlus- 


"Sensory    neuron. 


synocpss 


epi 


fbeli 


lum. 


I 


2. 


csffectox^ 


Simpk$t  form  reflex ai 

of  a  reflex:  arc       J  vith 

]^~  ne,u.ro-ns 


re$ponss-« 


association 


syn<xp^e 


associat  ior> 
neixrorL    ' 


■muscle  fibers 

Diagram  of  reflex  arcs.     Explain  why  this  is  often  called  the  "physiological  unit 

of  the  nervous  system." 

which  receive  the  stimulus  and  transmit  it  via  the  axon  to  the  spinal 
cord  where  a  synapse  with  the  dendrites  of  an  effector  cell  occurs. 
The  impulse  is  then  transmitted  by  means  of  the  hitter's  axon  to 
the  muscle  or  gland  cell.  Reflex  arcs  generally  require  one  or  more 
adjuster  neurons  in  the  circuit  between  the  receptor  and  effector 
cells.  Such  adjuster  cells  are  usually  located  in  the  spinal  cord  or 
in  the  brain. 

Even  so  brief  a  discussion  of  reflexes  cannot  be  concluded  without 
mention  of  the  compound  reflex  arcs  which  are  formed  by  a  single 
receptor  neuron  and  two  or  more  effectors  that  may  be  widely  sepa- 
rated in  the  body,  or  l)y  two  or  more  receptors  and  a  single  effector. 
Varying  complexities  of  these  latter  types  are  made  by  the  inter- 


342  THE   MAINTENANCE  OF  THE   INDIVIDUAL 

polation  of  adjustor  neurons.  It  is  a  moot  question  whether  or  not 
the  simplest  type  of  refiex  arc,  involving  only  two  neurons,  ever  occurs 
in  vertebrates.  Most  of  the  so-called  "refiex  actions"  of  man  are 
usually  not  isolated  from  the  rest  of  the  nervous  system. 

In  lower  vertebrates,  such  reflexes  as  are  concerned  with  locomotion, 
breathing,  swallowing,  and  escape  from  danger  are  automatic  spinal 
cord  reflexes.  When  it  comes  to  forms  with  complicated  and  highly 
developed  nervous  systems,  such  as  man,  many  actions  become  auto- 
matic, relieving  the  brain  ordinarily  of  any  responsibility  concerning 
them.  In  this  category  fall  such  phenomena  as  breathing,  sneezing, 
and  shivering.  Certain  actions,  namely  jerking  of  the  knee,  dodging 
a  blow,  closing  the  eyes  to  keep  out  foreign  particles,  are  reflexes 
which  may  be  controlled  or  inhibited  by  a  conscious  effort.  Still  more 
complex  reflexes  are  called  into  action  when  playing  a  musical  instru- 
ment, or  in  walking,  talking,  swimming,  or  driving  a  car. 

Types  of  Nervous  Systems 

One  of  the  fundamental  characteristics  of  protoplasm  is  irritability. 
In  simple  types  of  animals,  like  Ameba  or  the  sponges,  where  co-ordi- 
nation between  parts  is  not  essential,  no  specialized  nervous  system  ia 
developed.  With  the  aggregation  of  cells  in  higher  forms  thera 
arises  the  necessity  of  correlating  the  interaction  of  component  parts 
and  consequently  some  sort  of  definite  nervous  system  has  been 
evolved.  To  be  sure,  such  devices  are  quite  unspecialized  compared 
with  the  complicated  nervous  apparatus  of  a  vertebrate,  but  never- 
theless they  appear  to  be  reasonably  effective. 

Neuromotor  Mechanisms 

Ameba,  although  a  very  simple  type  of  organism,  gives  evidence  of 
being  definitely  affected  by  stimuli.  This  is  shown  by  the  passage  of 
stimuli  from  one  point  on  the  surface  to  the  general  mass  of  the  body, 
causing  the  animal  to  move  away  from  the  source  of  stimulation 
and  resulting  in  the  formation  of  pseudopodia  on  the  opposite  side. 
Experiments  upon  Ameba  suggest  that  stimuli  are  transmitted  in  the 
clear  outer  layer  of  ectoplasm. 

Probably  the  highest  development  of  a  co-ordinating  system  among 
the  protozoa  appears  in  some  of  the  ciliates.  We  have  already 
discussed  movement  in  Stylonychia  and  in  Euplotes.     Considerable 


SUPPORT,    MOTION,   AND   SENSATION  343 

experimental  work  has  been  performed,  largely  by  K(jfoid  and  his 
students  at  the  University  of  California,  upon  co-ordination  in  the 
latter  form.  Euplotcs  is  characterized  by  a  group  of  anal  cirri,  while 
the  anterior  surface  possesses  an  undulating  membrane,  near  one  end 
of  which  lies  a  co-ordinating  center,  or  motorium.  From  this,  fine 
protoplasmic  threads  emanate  leading  to  various  parts  of  the  ciliate. 
Five  of  these  strands  lead  to  five  anal  cirri.  Cutting  these  proto- 
plasmic threads  causes  disruption  of  the  rhythm  of  their  movement, 
thus  furnishing  experimental  evidence  of  the  existence  of  a  neuromotor 
apparatus  in  certain  ciliates. 


Co-ordination  by  a  Network 

In  some  of  the  most  primitive  metazoan  forms,  such  as  the  sponges 
and  the  lower  coelenterates,  there  is  evidence  of  a  very  elementary 
and  simple  type  of  co-ordinating  mechanism.  A  form  like  Hydra, 
which  makes  a  variety  of  different  movements,  reacts  to  various 
stimuli  since  it  feeds,  contracts,  expands,  creeps,  and  occasionally 
turns  cart-wheels.  The  mechanism  which  makes  such  acrobatics 
possible  in  Hydra  has  been  described  as  a  nerve  net,  and  as  such  forms 
a  part  of  the  sensory-neuro-muscular  mechanism,  or  as  it  is  sometimes 
called,  the  receptor-effector  system. 

Co-ordination  by  a  Nerve  Ring 

Only  two  of  all  the  great  phyla  of  animals,  the  coelenterates  and 
the  echinoderms,  are  apparently  radially  symmetrical.  The  nervous 
system  of  the  first  of  these  radially  symmetrical  groups  has  just  been 
described  and  it  can  be  seen  how  unspecialized  are  its  co-ordinating 
devices.  Turning  to  the  echinoderms,  as  examples  of  the  second 
radially  symmetrical  group,  we  find  that  in  spite  of  the  fact  that 
embryos  of  these  invertebrates  are  bilaterally  symmetrical,  the  nerv- 
ous system  of  the  adults  has  developed  along  the  lines  of  radial  sym- 
metry. This  type  of  nervous  system  is  composed  of  several  parts,  the 
relative  development  of  which  varies  in  the  different  classes,  the  star- 
fish having  numerous  nerve  cells  lying  among  the  ectodermal  cells. 
Some  of  these  nerve  cells  may  connect  with  nerves  from  the  fairly 
definite  ridges  of  nerve  tissue  known  as  the  radial  nerve  cords  nnming 
the  length  of  each  arm  and  uniting  to  join  a  nerve  ring  that  encircles 
the  mouth.  In  addition  there  may  be  an  apical  nervous  systern  that 
H.  \y.  H.  —  23 


344  THE  MAINTENANCE  OF  THE   INDIVIDUAL 

innervates  the  dorsal  muscles  of  the  arms.  As  might  be  expected, 
the  tube-feet  in  the  starfishes  (Asteroidea)  are  supplied  with  sensory 
organs.  It  is  also  interesting  to  note  that  at  the  tip  of  each  arm  of  a 
starfish  there  occurs  a  light-perceiving  organ. 

Co-ordination  by  a  Linear  Nervous  System 

Once  the  flatworms  are  reached  in  the  evolutionary  series  one  finds 
the  beginning  of  a  linear  type  of  nervous  system.  In  the  segmented 
worms,  or  annelids,  the  nervous  system  is  composed  of  two  main 
longitudinal,  closely  associated  nerve  trunks  from  which  the  several 
branches  in  each  somite  pass  laterally.  Each  segment  of  the  worm 
usually  contains  one,  or,  if  the  longitudinal  cords  are  widely  separated, 
two  ganglia  arranged  in  parallel  lines.  In  such  cases  the  ganglia  are 
connected  by  a  transverse  commissure.  This  ladderlike  type  of  nerve 
co-ordination  reaches  its  peak  in  the  arthropods,  where  well-developed 
ganglia  occur  in  most  somites.  In  nearly  all  types  of  the  higher 
invertebrates  there  is  in  the  head  end  a  ganglionic  mass  of  nervous 
tissue  which  has  been  dignified  by  the  appellation  of  "a  brain," 
whereas  it  should  have  been  more  properly  known,  because  of  its 
position,  as  a  supraesophageal  ganglion.  All  nerve  cords  of  similar 
type  are  ventral  in  position  and  lie  beneath  the  gut.  In  order  to  reach 
the  supraesophageal  ganglion,  the  nerve  cord  splits  at  the  large 
infra-  or  subesophagcal  ganglion,  and  passes  around  the  esophagus  by 
means  of  the  circumesophageal  connectives  or  loop. 

Reaching  the  arthropods,  the  primary  change  in  the  central  nervous 
system  is  found  to  be  a  greater  concentration  of  ganglia.  In  the 
larval  forms  of  insects,  there  is  little  change  from  the  linear  nervous 
system  of  annelids.  In  adult  insects,  however,  ganglia  are  concen- 
trated, and  even  fused,  in  the  regions  of  special  organs.  For  instance, 
the  "brain"  and  subesophagcal  ganglia  are  connected  with  the  ocelli, 
antennae,  and  mouth  parts,  while  thoracic  ganglia  are  associated  with 
the  wings  and  other  appendages.  An  autonomic  (sympathetic)  nerv- 
ous system,  which  is  believed  to  control  the  action  of  the  heart, 
digestive  system,  and  spiracle  muscles,  makes  its  debut  in  the 
arthropods. 

Co-ordination  by  a  Dorsal  Tubular  Nervous  System 

Among  the  vertebrates  there  is  a  highly  developed  dorsal,  tubular, 
central  nervous  system  with  evidence,  even  in  the  lower  forms,  of 


SUPPORT,    MOTION,   AND   SENSATION  345 

distinct  cephalization.  The  nervous  system  serves  to  correlate  move- 
ments and  to  give  information  of  changes  in  the  environment.  In- 
numerable fibers  extend  from  an  elaborate  central  controlling  device 
to  all  parts  of  the  body.  Such  a  nerve  mechanism  may  be  subdivided 
into  several  parts.  For  example,  in  man  there  is  a  central  nervous 
system,  a  pcj-iphcral  nervous  system,  and  an  autonomic  or  sympathetic 
nervous  system. 

Protective  Devices  for  the  Central  Nervous  System 

As  this  centralized  ner^'ous  system  is  the  master  which  controls  all 
voluntary  acts  and  indirectly  all  parts  of  the  body,  it  is  of  primary 
importance  to  protect  so  delicate  a  mechanism  from  injury.  Since 
the  situation  is  essentially  the  same  among  the  different  members  of 
the  large  group  of  vertebrates,  attention  will  be  primarily  directed  to 
the  system  as  it  is  found  in  mammals,  and  more  particularly  in  man. 

The  skull  and  the  A^ertebral  column  serve  as  the  ''first  line  of 
defense"  for  the  all-important  brain  and  spinal  cord  against  possible 
attack  or  injury.  However,  "secondary  defenses"  must  also  be 
present.  The  inner  surface  of  the  skull  and  the  vertebral  column, 
therefore,  is  lined  with  a  tough  membrane  of  fibrous  connective  tissue, 
called  the  dura  mater.  Inside  the  dura  mater  the  central  nervous 
system  itself  is  also  covered  with  a  thin,  closely  investing  membrane, 
the  pia  mater,  while  between  it  and  the  dura  mater  lies  the  delicate 
serous  membrane  known  as  the  arachnoid.  These  three  membranes 
furnish  additional  protection  to  the  central  nervous  system,  but  they 
would  be  relatively  ineffectual  without  the  buffering  effect  of  the  cere- 
brospinal fluid  which  fills  the  spaces  between  the  arachnoid  and  pia 
mater.  Thus  the  vertebrate  nervous  system  is  insulated,  cushioned, 
or,  to  put  it  more  graphically,  furnished  with  "shock  absorbers,"  that 
enable  man  and  other  vertebrates  to  withstand  severe  shocks  without 
injury. 

Anatomy  and  Development  of  the  Brain 
The  Early  Development  of  the  Central  Nervous  System 

Amphioxus  gives  Init  slight  evidence  of  an  enlargement  of  the 
cephalic  end  of  the  dorsal  tubular  nerve  cord,  but  in  the  bony  fishes 
there  are  already  five  main  divisions  in  the  adult  brain.  These  same 
divisions  are  to  be  found  in  every  one  of  the  different  vertebrate 
classes,  and  all  representative  vertebrate  brains  have  a  similar  embry- 


346 


THE   MAINTENANCE   OF  THE   INDIVIDUAL 


ological  history.     In  other  words,  these  structures  are  both  homolo- 
gous and  in  a  general  way  analogous. 

Some  of  the  more  important  changes  in  the  growth  and  expansion 
of  the  nerve  cord  are  as  follows.  Early  in  its  embryonic  develop- 
ment, before  the  five  regions  of  the  brain  are  developed,  the  anterior 
portion  of  the  growing  nerve  cord  becomes  differentiated  into  three 
enlargements,  designated,  beginning  anteriorly,  as  the  iore-  (prosen- 
cephalon), mid-  (mesencephalon),  and  hind-  (rhombencephalon)  brains. 


encepWlon. 


A 


prosencephalon 


mess 


/'clienogphcvloii 
telencephalon  / 
1/         ''  ^ 


■metencepholon 
mescncepholoa      v  tnyelencephcJon. 


rrbombencephobn. 
jncephalon 


Development  of  the  vertebrate  brain  from  a  simple  encephalon. 

Most  of  the  subsequent  development  takes  place  in  the  fore-  and 
hind-brains  (page  347).  As  growth  continues,  the  anterior  part  of 
the  fore-brain  divides,  grows  out  into  two  pouchlike  lateral  lobes, 
called  the  cerebral  hemispheres  (telencephalon),  or  collectively  the 
cerebrum.  The  jiosterior  portion  of  the  primitive  fore-brain  is  now 
designated  as  the  'twixt-brain  (diencephalon) .  The  mid-brain  (mesen- 
cephalon) meanwhile  remains  imdivided,  while  the  hind-brain  becomes 
separated  into  an  anterior  dorsal  outgrowth,  called  the  cerebellum 
(metencephalon) ,  and  a  posterior  medulla  oblongata  (myelencephalon) , 
which  is  continuous  with  the  cord. 


SUPPOMT,    MOTION,    AND   SENSATION 


347 


Before  going  further  with  a  consideration  and  (hseussiou  of  the 
human  nervous  system,  a  comparison  of  brains  of  (Hfferent  verte- 


amphibian 


rsptile. 


Cerebrtc 


'bineo.l 
bocL' 


opLic 

lobe,'  ,, 
c^.r-ebe-llum' 


optic 
lobe 


mccCitlla. 


snake^ 


brates  should  be  made 
for  the  sake  of  clearness. 
Remembering  that  the 
brain  of  a  fish  is  not 
folded  upon  itself  as  is 
the  brain  of  a  mammal, 
it  is  easy  to  see  that  the 
introduction  of  flexures 
tends  towards  greater 
compactness.  Another 
outstanding  develop- 
ment is  the  increase  in 
size  of  some  of  the  re- 
gions of  the  brain.  In 
lower  forms,  the  domi- 
nating portions  of  the 
brain  from  the  stand- 
point of  mass  are  the 
optic  lobes  of  the  mid- 
brain and  the  medulla 
oblongata,  and,  as 
might  be  inferred,  both 
the  cerebrum  and  cere- 
bellum are  quite  small. 
In  the  higher  mammals, 
however,  these  organs  become  two  of  the  most  important  centers 
in  the  brain,  increase  in  the  size  of  the  cerebrum  being  in  direct  pro- 
portion to  the  intelligence  of  the  animal. 

An  examination  of  a  few  of  the  more  important  landmarks  of  the 
divisions  of  the  brain  in  order  to  secure  a  general  idea  of  the  function- 
ing of  each  of  these  parts  will  furnish  a  background  for  the  discussion 
of  the  ''  Display  of  energy." 

The  Parts  of  the  Vertebrate  Brain 

The  Cerebrum  or  Telencephalon.  As  the  adult  condition  is 
approached,  certain  other  characteristic  structures  appear.  From 
the  anterior  portion  of  the  cerebrum  grow  the  paired  olfactory  lobes. 
In  lower  xertebrates  these  may  extend  into  expanded  olfactory  hulhs. 


d-'CX.t-' 


Representative  vertebrate  brains.  o.L,  olfactory 
lobes.  W  hat  reffions  increase  noticeably  in  mass  from 
fish  to  manmials  ?  How  are  these  changes  correlated 
with  the  shift  from  water  to  land:*     (After  Guyer.) 


348  THE  MAINTENANCE  OF  THE   INDIVIDUAL 

from  which  the  olfactory  nerves  pass  directly  to  the  nostrils,  thus 
receiving  stimuli  which  are  interpreted  in  the  brain  as  odors. 

The  cerebral  hemispheres  contain  cavities  known  as  the  first  and  sec- 
ond ventricles,  which  are  continuous  posteriorly  with  the  third  ventricle, 
found  in  the  'twixt-brain  or  diencephalon.  Dorsally  and  laterally  the 
cerebral  walls  are  known  as  the  pallium,  that  furnishes  a  foundation 
on  which  the  outer  layer,  or  coriex  is  developed.  In  the  higher  verte- 
brates a  connecting  bridge  of  white  fibers,  called  the  corpus  callosum, 
unites  the  two  cerebral  hemispheres.  The  higher  the  mammal  is  in 
the  scale  of  life  the  more  convoluted  the  cortical  surfaces  of  the  hemi- 
spheres become,  and  the  more  the  cerebrum  weighs  in  proportion  to 
the  rest  of  the  organ.  A  rabbit's  cerebnun  composes  slightly  more 
than  half  of  the  mass  of  the  entire  brain,  while  in  man  it  exceeds  four 
fifths  of  the  total  weight. 

The  sui)erficial  cortical  layer  of  the  cerebrum  forms  a  mass  made 
up  of  numberless  nerve  cells  interwoven  into  an  intercommunicating 
network.  The  axons  of  some  of  these  neurons  pass  over  the  bridge 
of  the  corpus  callosum  from  one  side  to  the  other  while  other  axons 
extend  downward  in  great  bands  as  far  as  the  cerebellum  and  to  other 
more  posterior  centers.  It  has  been  demonstrated  that  this  portion 
of  the  brain  is  the  seat  of  consciousness  and  the  controlling  center  of  all 
our  higher  mental  life.  As  the  cerebral  functions  increase,  the  instinc- 
tive reflexes  retire  further  into  the  background.  Herein  lies  the  chief 
difference  between  the  so-called  lower  animals  and  the  higher  ones. 
The  former  are  chiefly  at  the  mercy  of  their  hereditary  limitations  and 
their  environment,  while  the  latter  have  risen  sufficiently  above  their 
environmental  conditions  to  begin  at  least  to  become  "the  captains  of 
their  souls."  A  series  of  experimental  operations  on  a  dog,  in  which 
the  entire  cerebrum  was  finally  removed,  illustrates  the  importance  of 
this  part  of  the  brain  to  the  higher  animals.  The  dog  in  question 
apparently  became  an  idiot,  unable  to  associate  experiences  or  to 
learn.  It  had  no  ability  to  differentiate  between  solid  objects  in  its 
path  and  patches  of  sunlight  on  the  floor,  which  could  in  no  way 
hinder  its  progress. 

The  'Twixt-Brain  or  Diencephalon.  This  region  is  compara- 
tively inconspicuous,  but  very  essential  to  the  biologist,  since  the 
ventral  floor  of  the  'twixt-brain  gives  rise  to  an  outgrowth  called  the 
infundibulum,  which  fuses  with  a  dorsal  outgrowth  from  the  roof  of 
the  mouth  to  form  the  pituitary  gland  (hypophysis),  the  "generalissimo" 
of  the  ductless  glands.     Possibly  because  of  its  importance  it  has 


SUPPOlir,    MO'I'ION,   AND   SENSATION 


319 


,<— . 


frcntal 
ctrscr 


occipital' 
areo: 


temporal   arecc 

Two  I'lKurt's  illustralinK  tiie  intercoiniiiunicating  pathways  of  nerve  libers  in 
the  human  brain. 

a.  Various  association  fibers  of  the  human  brain.  ,1,  between  adjacent  areas; 
B.  between  frontal  and  occipital  areas;  (;,  D,  between  frontal  and  temporal 
areas;  E,  between  occipital  and  temporal  areas.  Note  the  corpus  callosum 
which  contains  larf):e  groups  of  association  fibers  and  connects  the  cortex  of  the 
right  cerebral  hemisphere  with  that  of  the  left.  The  caudate  nucleus,  CN,  and 
the  thalamus,  OT,  both  contain  gray  matter. 


■m^-fr^^t 


b.  Scheme  of  projection  fibers  connecting  the  cerebrum  and  other  parts  of 
the  brain.  .1,  tracts  fnmi  frontal  lobe  to  the  pons  varolii  and  thence  to  the 
cerebellum  via  (!;  B,  motor  (pyramidal)  tracts;  C,  sensory  tracts;  D  and  E, 
the  visual  and  auditory  tracts,  respectively;  F,  fibers  connecting  the  cerebrum 
and  cerebellum ;  6',  fibers  connecting  the  cerebellum  and  the  brain  stem ;  11, 
fibers  between  the  cerebellum  and  the  cord  ;  ./,  fibers  connecting  the  auditory 
nucleus  and  the  brain  stem ;  A',  crossing  over  of  motor  (pyramidal)  tracts  in  the 
brain  stem ;  V7,  fourth  ventricle.  The  numbers  refer  to  the  cranial  nerves. 
(Both  modified  from  Starr.) 


350  THE   MAINTENANCE   OF  THE   INDIVIDUAL 

become  exceptionally  well  protected.  In  all  mammals  this  little  gland 
is  lodged  in  a  protective  median  depression  in  the  sphenoid  bone  of  the 
skull  called  the  "Turk's  saddle,"  or  sella  turcica.  The  'twixt-brain 
also  gives  rise  laterally  to  outgrowths  of  the  lateral  wall  which  form 
the  optic  stalks  that  are  essential  to  the  development  of  the  eyes.  In 
this  region  of  the  brain  several  problematical  structures,  of  particular 
interest  to  the  comparative  anatomist,  such  as  the  pineal  eye,  have 
their  origin.     The  cavity  of  the  'twixt-brain  is  called  the  third  ventricle. 

The  Mid-Brain  or  Mesencephalon.  This  portion  of  the  brain 
has  kept  many  of  its  primitive  embryonic  characters,  its  gray  matter 
being  still  found  largely  in  ganglionic  masses.  Anatomically,  it  is  a 
small  region,  the  lumen  of  which,  communicating  anteriorly  with  the 
third  ventricle,  is  called  the  aqueduct.  In  lower  forms  like  the  fishes 
and  amphibia,  the  roof  of  this  cavity  is  expanded  dorsally  into  two 
rounded  protuberances,  the  optic  lobes,  or  corpora  bigemina.  The 
optic  lobes  of  reptiles,  birds,  and  mammals  become  further  divided  into 
two  pairs  of  centers  known  as  the  corpora  quadrigemina,  from  which 
are  sent  out  bands  of  fibers,  the  anterior  pair  being  connected  with  the 
eyes,  and  the  posterior  pair  with  the  ears.  In  forms  below  the  mam- 
mals the  mid-brain  functions  as  a  co-ordinating  center  for  impulses 
entering  through  the  eye,  ear,  and  certain  nerves  of  the  body.  In 
the  mammals  much  of  this  co-ordinating  function  has  been  taken  over 
by  the  cerebrum.  Upon  the  latero-ventral  surface  of  the  mid-brain 
may  be  seen  a  band  of  fibers,  the  crura  cerebri,  forming  a  highway  of 
communication  between  the  cerebrum  and  the  posterior  parts  of  tlie 
central  nervous  system.  Two  motor  cranial  nerves,  the  oculomotor 
(III)  and  the  trochlear  (IV),  which  supply  muscles  of  the  eye,  arise 
here. 

The  Cerebellum  or  Metencephalon.  While  the  surface  of  the 
cerebellum  is  not  convoluted  in  the  same  manner  as  the  cerebrum, 
nevertheless  its  surface  of  gray  matter  is  increased  by  being  thrown 
into  numerous  furrows.  It  is  composed  of  two  hemispheres  connected 
by  a  bridge,  the  vermis,  and  has  consequently  been  likened  to  a  butter- 
fly with  the  bridge  forming  the  body.  The  cerebellum  lies  just  pos- 
terior to  the  cerebrum  and  dorsal  to  the  mid-brain.  When  cut  in 
sagittal  section  it  is  seen  to  be  composed  of  radiating  folds,  arranged 
in  an  outer  layer  of  gray  matter  and  an  inner  core  of  white  matter. 
Taken  as  a  whole  the  white  matter  somewhat  resembles  a  tree  and  so 
has  been  called  the  arbor  vitae,  or  "tree  of  hfe." 

Ventrally,   a  swollen   band  of   fibers,   called   the   pons  varolii,  is 


SUPPORT,    MOTION,   AND   SENSATION  351 

plainly  evident  because  of  the  transverse  direction  of  its  fiber,  crossing 
from  one  side  of  the  cerebellum  to  the  other.  Nerve  fibers  arising  in 
the  frontal,  parietal,  and  occipital  lobes  of  the  cerebrum  reach  the 
cerebellar  hemispheres  by  way  of  the  anterior  -peduncles  in  front  of  the 
pons,  the  latter  bearing  some  resemblance  to  a  pair  of  legs  supporting 
the  body  of  the  cerebellum.  There  is  a  second  pair  of  lateral  "legs" 
behind  the  pons,  the  posterior  peduncles,  which  contain  communi- 
cating fibers  between  the  cerebellar  lobes  and  the  posterior  regions 
of  the  central  nervous  system.  Thus,  a  highway  of  communication 
with  the  cerebrum  is  at  hand  and  herein  lies  a  partial  explanation  of 
man's  ability  to  perform  purposive  acts  as  the  result  of  the  various 
visual,  auditory,  and  other  impressions  of  the  senses.  Experimental 
evidence  indicates  that  this  portion  of  the  brain  is  primarily  a  seat  of 
muscular  co-ordination. 

If  the  cerebellum  of  a  dog  is  removed,  the  animal  is  unable  to 
co-ordinate  its  movements  at  first.  Later  it  learns  to  walk,  but  the 
gait  is  always  slow  and  staggering.  In  a  similar  condition,  a  pigeon 
is  unable  to  fly,  but  like  the  dog,  may  eventually  learn  to  walk  again, 
although  resembling  the  proverljial  inebriate  in  its  gait.  It  has  often 
been  claimed  that  man  would  make  a  better  recovery  after  removal  of 
the  cerebellum  than  either  the  dog  or  bird  since  his  more  highly 
developed  cerebrum  would  compensate  the  loss.  In  any  case,  from 
these  experiments  the  importance  of  the  cerebellar  region  of  the  brain 
in  our  everyday  activities  is  better  understood. 

The  Medulla  Oblongata  or  Myelencephalon.  The  brain  at 
this  point  is  anatomically  little  more  than  an  expanded  region  of  the 
si)inal  column,  but  it  is  the  sole  means  of  communication  between  the 
cerebrum  and  the  body.  Its  dorsal  surface  is  partially  covered  by 
the  posterior  peduncles,  and  there  is  also  a  very  thin  non-nervous 
roof,  the  mctatela,  which  covers  the  fourth  ventricle,  or  the  large  cavity 
of  the  brain  of  this  region.  Ventrally,  two  raised  convex  columns  of 
fibers  may  be  seen,  known  as  the  pyramids. 

In  the  gray  matter  of  the  medulla,  the  controlling  centers  for  many 
of  the  essential  functions  of  life  are  found,  for  example,  the  reflexes 
concerned  with  the  vasomotor  and  respiratory  functions.  Numerous 
other  centers  that  control  swallowing,  coughing,  sucking,  sneezing, 
salivary  secretions,  gastric  secretions,  heart  inhibition,  and  other 
activities  connected  with  the  living  body  are  located  in  the  medulla. 

All  of  the  cranial  nerves,  except  the  first  four  [)airs,  arise  from  this 
region.     It  is  here,  too,  that  the  pyramidal  tracts  of  transmitting  fibers 


352 


THE   MAINTENANCE   OF  THE   INDIVIDUAL 


cross  from  one  side  of  the  brain  to  the  other,  like  the  letter  "X," 
so  that  the  control  of  the  left  side  of  the  body  is  located  in  the  centers 
of  the  right  side  of  the  brain  and  vice  versa. 

The  Cranial  Nerves 

There  are  typically  ten  pairs  of  cranial  nerves  in  the  lower  verte- 
brates and  twelve  in  mammals,  arising  from  different  parts  of  the 
brain.  Of  these,  four  pairs  are  of  particular  interest.  Three  pairs 
(I,  II,  and  VIII)  are  concerned  with  the  innervation  of  the  organs  of 
special  sense,  while  the  fourth  (X)  is  that  great  wanderer,  the  vagus. 

The  first,  or  olfactory  nerve  (I)  receives 
stimuli  from  the  nose  and  conveys 
them  to  the  brain.  The  second,  or 
optic  nerve  (II)  emerges  from  the 
lateral  floor  of  the  diencephalon,  its 
fibers  more  or  less  completely  crossing 
in  the  optic  chiasma,  that  lies  just 
anterior  to  the  infundibular  outgrowth 
of  the  pituitary  body  already  men- 
tioned. This  nerve  transfers  the  im- 
pulses which  are  interpreted  in  the 
brain  as  sight.  The  third  pair  of  cranial 
nerves  associated  with  a  special  sense 
is  known  as  the  auditory  nerve  (VIII), 
and  has  the  dual  function  of  hearing 
and  equilibration. 

The  remainder  of  the  cranial  nerves 
will  be  omitted  from  further  discussion 
except  the  vagus  (X),  the  ramifications  of  which  are  more  extensive 
than  those  of  any  of  the  other  cranial  nerves.  The  vagus  is  a 
mixture  of  motor  and  sensory  elements,  the  former  supplying  muscles 
of  the  pharyngeal  and  laryngeal  region,  most  of  the  digestive  tract 
and  the  liver,  pancreas,  and  spleen,  the  kidneys  as  well  as  the  heart, 
and  certain  blood  vessels.  The  sensory  fibers  are  distributed  to  the 
mucous  membranes  of  the  larynx,  trachea,  lungs,  esophagus,  stom- 
ach, intestines,  and  gall  bladder.  Inhibitory  fibers  also  reach  the 
heart  and,  in  addition,  this  versatile  nerve  supplies  the  gastric  and 
pancreatic  glands  with  secretory  fibers.  Much  of  phylogenetic  in- 
terest may  be  gleaned  from  a  careful  comparative  study  of  the 
distribution  of  this  and  other  cranial  nerves  from  fish  to  man. 


Diagram  showing  the  oplic 
chiasma  in  man.  Note  that  the 
crossing  is  not  complete,  a  con- 
dition probably  related  to  the 
binocular  method  of  vision. 


SUPPORT,   MOTION,   AND   SENSATION  353 

The  Spinal  Cord 

The  medulla  ohlonsata  is  {'outiiniecl  almost  im])erceptibly  over  into 
the  spinal  cord,  which  extends  in  adult  man  from  the  foramen  magnum 
of  the  skull  posteriorly  through  the  vertebral  column  for  seventeen  to 
eighteen  inches.  The  spinal  cord  is,  roughly  speaking,  the  size  of  the 
Uttle  finger,  or  about  0.4  of  an  inch  in  diameter.  Two  enlargements 
occur  in  it,  one  in  the  region  of  the  shoulder-blades,  and  the  other 
below  the  small  of  the  back,  respectively  knowm  as  the  cervical  and 
lumbar  enlargements. 

The  internal  structure  of  the  cord  is  quite  characteristic.  In  cross 
section,  the  central  gray  matter  somewhat  re.sembles  the  letter  "H," 
the  position  of  the  gray  and  white  matter  being  apparently  reversed 
from  their  ])osition  in  the  brain.  As  a  matter  of  fact,  in  both  cord 
and  brain  the  gray  matter  is  disposed  inside  close  around  the  cavity 
that  extends  throughout  the  whole  central  nervous  system.  Outside 
this  central  gray  matter  are  the  transmission  fibers  which  app(>:ir 
white.  In  the  cerebellum  and  cerebrum  of  the  brain  there  is  super- 
imposed an  outer  layer  of  gray  matter  that  constitutes  the  centers  of 
adjustment.  This  secondary  gray  layer  is  .so  pronounced  in  the  brain 
that  it  gives  rise  to  the  popular  impression  of  a  reversal  in  the  arrange- 
ment of  white  and  gray  matter  between  the  cord  and  brain.  The  gray 
matter  is  composed  of  a  ventral,  or  anterior,  and  a  dorsal,  or  posterior, 
column,  divided  into  these  two  parts  by  the  tran.sverse  bar  of  the  "H." 
The  white  matter  may  also  be  subdivided  into  three  parts  on  either 
side,  a  ventral,  lateral,  and  dorsal  funiculus. 

The  Spinal  Nerves 

The  nerves  in  this  group,  like  the  cranials,  are  paired,  there  being 
31  pairs  in  man.  Each  nerve,  moreover,  is  "mixed,"  that  is,  it  is 
composed  of  a  dorsal  or  sensory  root  containing  receptor  neurons, 
carrying  messages  toward  the  brain,  and  a  ventral  or  jnotor  root  bearing 
effector  neurons,  which  carry  messages  away  from  the  brain  to  muscles 
and  glands.  It  will  be  noted  that  some  of  the  cranial  nerves,  unlike 
the  spinal  nerves,  have  lost  this  original  ability  to  transmit  messages 
both  ways  and  have  been  reduced  to  one-way  traffic,  for  example,  the 
three  pairs  of  eye-muscle  nerves  (III,  IV,  and  VI)  handle  only  outgoing 
impulses,  wdiile  the  auditory  nerve  (VIII)  can  only  transmit  .stimuli 
inward  toward  the  brain.  From  the  point  in  a  mixed  nerve  where  the 
incoming  and  the  outgoing  roots  fuse  are  typically  given  off  the 


354 


THE   MAINTENANCE   OF  THE   INDIVIDUAL 


following  braticlios  :  (] )  a  dorsal  branch;  (2)  a  nioro  prominent  ventral 
branch,  whicli  supplies  the  skin  and  body  nuisculaturc  ;  (,S)  a  com- 
municating branch,  going  to 
the  ganglia  of  the  autonomic 
system  and  thence  to  the 
viscera ;  and  (4)  a  small 
meningeal  branch,  going  back 
to  the  protective  layers  of 
the  cord.  Thus  the  nerves 
emanating  from  this  point  of 
fusion  are  mixed  in  character 
while  their  roots  are  not. 


mesonterii 
ganglion 


cfcrsol  raot 


ganglion^ 


dorsal  root 
Ventral  root 


rocmrcs 
Communi- 


\ir.. 

;  visntroli? 


Components  of  a  spinal  nerve.  Somatic 
motor  fibers  are  indicated  by  solid  lines; 
\  isceral  motor  by  lonp  dashes;  somatic  sen- 
sory in  short  dashes ;  \  isceral  sensory  by 
dotted  lines. 


The  Autonomic  Nervous 
System 

The  term  autonomic  nervous 
system  embraces  all  nerves 
and  ganglia  located  outside  of 
the  spinal  cord,  which  reg- 
ulate the  activities  of  smooth, 
or  involuntary,  muscle  and 
various  glands.  It  should 
also  be  thought  of  as  an  auxiliary,  or  perhaps  more  properly  a  relay 
apparatus  to  supplement  the  work  of  the  central  nervous  system. 

Anatomically  the  system  consists  of  two  ''longitudinally  con- 
nected" chains  of  ganglia  lying  on  either  side  and  just  ventral  to  the 
cerebrospinal  cord  together  with  various  ganglia  scattered  throughout 
the  viscera  and  groups  of  connecting  nerves  extending  to  the  central 
nervous  system.  This  system  is  divisible  into  two  parts.  The  first 
is  called  the  thoracicolumbar  jiart,  and  consists  of  the  double  chain  of 
ganglia  mentioned  above  together  with  the  connections  through  the 
spinal  nerves.  It  reaches  the  blood  vessels,  heart,  digestive  tract,  and 
many  other  parts  of  the  body.  The  second,  or  parasympathetic  part, 
is  characterized  by  having  three  centers,  two  cranial  centers,  one  in 
the  mid-brain,  one  in  the  medulla  region,  and  a  posterior  center  in  the 
sacral  region. 

Masses  of  nervous  tissue  are  scattered  as  ganglia  which  are  located 
in  various  organs,  such  as  the  walls  of  the  digestive  tract,  where  they 
are  known  as  the  solar,  cardiac,  or  aortic  plexvses.  These  serve  as  relay 
centers  for  impulses  coming  from  the  main  trimk  line  of  the  autonomic 


SUPPORT,    MOTION,   AND   SENSATION 


355 


system,  and  since  each  of  these  centers  usually  presents  a  fanlikc 
arrangement  of  efferent  fibers,  they  serve  to  increase  the  number  of 
available  pathways. 

The  autonomic  system  is  full  of  contradictions,  for  there  appears  to 
be  an  antagonistic  action  on  the  part  of  the  thoracicolumbar  to 


ciiiar 


'  sphsnogcJotirgg>T^lacrimal  gland 


1  \xxmhar 


i  Sacral 


Submaxillary^^^  "osa .  palctta. 

■i^-2fe^^i^  subling,ual  ^. 

mucous  rtiem. 
■mouth. 

panoticC  glancC 
hacwrC 

louynx 
trcLchao. 
^hronchtcs 

^vsssds  of  abcL 

liver  and. 
dLucts 


^^  pancrsQ-S 

fjcxdcmna-l 

Smail    . 
intestine 


Colo 


in. 


rectum 


Serf,  or^n 


e)t±ernal 
genitalia. 


Diagram  of  the  autonomic  nervous  system.     The  parasympathetic  part  appears 
in  solid  lines  and  the  thoracicolumbar  part  in  dotted  hues. 

impulses  from  the  cranial  or  sacral  parts  of  the  parasympathetic 
system.  Thus  the  cranial  part  slows  the  heart  while  the  thoracicolum- 
bar accelerates  it.  This  has  often  been  spoken  of  as  "reciprocal 
innervation, "  a  principle  which  plays  a  very  important  role  in  the 
proper  functioning  of  various  organs. 


356 


THE   MAINTENANCE  OF  THE   INDIVIDUAL 


The  origin  of  the  autonomic  system  has  been  the  subject  of  con- 
siderable speculation.  Some  investigators  believe  that  it  has  been 
secondarily  derived  from  the  central  nervous  system  probably  by  the 
migration  of  cells.  Others  support  the  idea  that  it  is  in  reality  a 
primitive  ancestral  apparatus  which  is  more  or  less  homologous  with 
the  nervous  system  of  invertebrates.  According  to  this  theory  the 
autonomic  system  has  become  secondarily  subservient  to  the  volun- 
tary nervous  system  of  the  vertebrates. 

The  Sense  Organs  —  Receptor  Devices 

The  mechanism  and  functioning  of  many  of  the  different  parts  of 
the  vertebrate  nervous  system  have  been  considered  in  some  detail 
for  the  purpose  of  showing  how  the  voluntary  system  controls  actions, 
and  also  how  the  involuntary  system  has  taken  over  the  burden  of 
running  the  body.  It  now  remains  to  trace  the  various  devices  that 
have  been  developed  to  help  an  animal  keep  in  touch  with  its  environ- 
ment, in  other  words  the  sensory  receptors,  which  range  from  special- 
ized to  rather  generalized  structures  and  are  usually  classified  as 
organs  of  taste,  smell,  sight,  hearing,  and  the  tactile  sense. 


Taste 

In  the  lower  vertebrates  the  sense  of  taste  is  quite  widely  dis- 
tributed.    For  example,  in  some  of  the  fishes  sensory  cells  of  chemical 

reception  are  scattered 
^,fV^,'P^2-^-,— ^-^-^      somewhat  widely  over  the 

body  surface.  In  higher 
vertebrates  such  organs  are 
mostly  restricted  to  the  sur- 
face of  the  tongue  and  are 
known  as  taste  buds.  Most 
people  labor  under  the  de- 
lusion that  they  can  dis- 
tinguish between  a  great 
variety  of  flavors.  Actually, 
however,  buds  are  sensitive 
to  only  four  kinds  of  stim- 
uli, sweet,  sour,  bitter,  and 
salty.  The  confusion  results  from  the  inclusion  of  interpretations 
of  sensations  received  by  the  olfactory  senses. 


epithelial 
cell 


tosLe- 
Cell... 


■nerve/ 


!trrh\".^. 


A  taste  bud.     Explain  how  it  functions. 


SUPPORT,    MOTION,   AND   SENSATION 


357 


epithelial 


jj^torx 


Smell 

This  is  one  of  tlio  more  important  organs  of  special  sense.  Even 
aquatic  forms  have  been  shown  to  possess  a  fairly  keen  sense  of  smell. 
In  land  forms,  the  nasal  chamber  becomes  supplied  with  sensory 
olfactory  cells  that  are  quite  primi- 
tive, or  undifferentiated.  The  in- 
sects, which  in  some  cases  have  a  keen 
sense  of  smell,  have  the  olfactory 
organs  located  on  the  antennae. 
Loeb  performed  an  experiment  that 
clearly  demonstrated  the  acuteness  of 
this  sense  in  a  butterfly,  by  suspend- 
ing a  female  butterfly  in  a  box  and 
then  opening  the  window.  In  less 
than  half  an  hour  a  male  butterfly  of 
the  same  species  was  nearby.  It 
soon  reached  the  window,  flew  into 
the  room,  and  perched  on  the  box. 
Two  other  males  also  came  during  the  afternoon.  Their  sense 
of  smell  no  doubt  was  responsible  for  their  discovery  of  the  female. 
Man,  whose  sense  of  smell  is  by  no  means  as  keen  as  that  of  some 
other  animals,  can,  nevertheless,  detect,  for  example,  one  part  in  a 
million  of  iodoform. 


.sa^^rtir^g 


.-'srve 
^iber 


Olfactory  cells. 


Simple  Light  Receptors 

The  reaction  of  animals  to  light  is  one  of  the  most  characteristic 
responses  found  in  the  animal  kingdom.  In  the  simplest  organisms  it 
has  been  demonstrated  that  this  reaction  may  be  classified  as  a  positive 
or  negative  attraction  to  light.  The  ability  to  react  to  light  indicates 
the  presence  of  cells  or  tissues  in  the  animal  which  are  photosensitive. 
Since,  in  lowTr  forms,  the  response  to  light  may  be  detected  by  the 
manner  in  which  the  animal  reacts  in  the  presence  or  absence  of  light, 
or  in  avoiding  illuminated  areas,  it  appears  probable  that  there  is  a 
more  or  less  direct  connection  between  the  photoreceptor  cells  and 
the  muscles.  The  responses  to  light  of  such  animals  as  the  protozoa, 
hydroids,  and  earthworms  apparently  fall  into  this  category,  and 
has  led  to  their  being  designated  as  positively  or  negatively 
phototropic.  Much  interesting  experimental  work  has  been  done 
along  these  lines. 


358  THE   MAINTENANCE   OF  THE   INDIVIDUAL 

Compound  Eyes 

The  intergradation  from  the  type  of  photosensitive  cells  mentioned 
above,  to  a  primitive  eye,  or  eye  spot,  is  a  gradual  one.  One  of  the 
first  steps  in  the  production  of  a  simple  eye  spot  appears  to  be  the 
concentration  at  a  given  point  of  a  number  of  light-sensitive  cells  con- 
nected with  nerves.  From  such  simple  beginnings  two  types  of  eyes 
have  been  evolved  in  the  animal  kingdom,  the  compound  eye  of  the 
insects  and  Crustacea,  and  the  camera  eye  of  certain  molluscs  and  the 
vertebrates.  The  compound  eye  is  composed  of  a  varying  number  of 
complete  individual  eyes  called  ommatidia.  Each  ommatidium  is  di- 
rectly connected  with  the  brain  and  produces  a  separate  image  that, 
joined  to  others,  gives  a  unified  picture.  It  has  been  ascertained  by 
counting  the  exposed  surfaces,  or  facets,  of  the  ommatidia  that  there 
may  be  present  any  number  from  a  few  dozen  up  to  several  thousand. 
Some  ants  have  about  fifty,  while  the  swallowtail  butterfly  has  seven- 
teen thousand,  and  dragonflies  still  more  in  each  eye.  The  walls  of 
each  ommatidium  are  surrounded  with  pigment  cells  that  absorb  all 
tangential  rays,  consequently  only  those  rays  which  penetrate  straight 
in  through  the  facet  reach  the  sensory  areas  located  in  the  retinular, 
or  photoreceptive,  cells.  On  account  of  this  restricted  intake,  each 
ommatidium  receives  for  interpretation  only  a  small  portion  of  the 
rays  entering  through  the  cornea.  It  is  believed  that  there  is  no 
marked  overlapping  of  images  since  each  image  is  recorded  in  a  differ- 
ent spot,  the  end  result  being  a  series  of  small  images  one  next  the 
other,  which  act  to  produce  the  completed  picture,  called  an  erect 
mosaic  (see  figure,  page  206). 

Camera  Eyes 

The  camera  type  of  eye  in  invertebrates  reaches  its  peak  in  the 
molluscan  squid  and,  among  the  vertebrates,  in  the  human  eye. 
These  two  types  offer  a  good  illustration  of  analogous  structures.  A 
study  of  the  development  of  these  two  types  of  eyes  shows  that  the 
position  of  certain  elongated  cells  of  the  retina,  called  the  rods  and 
cones,  are  reversed  in  the  two  forms,  and  consequently  while  their 
function  is  in  general  the  same,  or  analogous,  their  type  of  structure, 
or  homology,  is  different.  The  vertebrate  eye  is  almost  spherical, 
and  fits  into  a  funnel-shaped  socket  of  bone,  called  the  orbit,  while 
the  stalklike,  optic  nerve  connects  the  eye  directly  with  the  brain. 
Free  movement  is  made  possible  by  means  of  six  small  muscles  which 


SUPPORT,    MOTION,   AND   SENSATION 


;}5g 


are  attached  to  the  outer  coat  of  the  eyeball  and  to  the  bony  wall 
around  the  eye. 

The  wall  of  the  eyeball  is  made  up  of  three  coats.  The  outer  tough 
white  coat  of  connective  tissue  is  called  the  sclerutic  coat.  In  front, 
where  the  eye  bulges  out  a  little,  the  outer  coat  becomes  transparent, 
forming  the  cornea.     A  second  coat,  the  choroid,  is  supplied  with  blood 


pupil 
conjuTictV;^ 
Cornea 


vitreous 
humor 


Sagittal  section  of  a  inaiiimalian  eye. 

vessels  and  cells  containing  considerable  quantities  of  black  pigment. 
The  iris,  which  shows  through  the  cornea  as  the  colored  part  of  the 
eye,  is  a  part  of  this  coat.  In  the  center  of  the  iris  there  is  a  small 
circular  hole,  the  -pupil.  The  iris  is  under  the  control  of  involuntary 
muscles,  and  may  be  adjusted  to  varying  amounts  of  light,  the  hole 
becoming  larger  in  dim  light  and  smaller  in  bright  light.  The  inmost 
layer  or  coat  of  the  eye,  called  the  retina,  is  double,  consisting  of  an 
outer  pigmented  and  an  inner  sensory  part.  This  is  perhaps  the 
most  delicate  layer  in  the  entire  body.  Despite  the  fact  that  the 
retina  is  less  than  ^^  of  an  inch  in  thickness,  it  is  composed  of  several 
layers  of  cells.  The  optic  nerve,  made  up  of  a  chain  of  relaying 
neurons,  enters  the  eye  from  behind  and  spreads  out  over  the  surface 
of  the  retina.  At  its  point  of  entry  a  cross  section  of  the  optic  nerve 
shows  that  the  nerve  consists  only  of  axons  of  neurons,  and  conse- 
quently this  "blind  spot"  is  not  sensitive  to  light.  The  ultimate 
photoreceptors  are  numerous  elongated  cells,  called  rods  and  cones. 
The  function  of  the  rods  is  a  highly  specialized  sensitivity  to  light, 
and  of  the  cones  the  perception  of  color.  In  the  optical  center  of  the 
H.  w.  H.  —  24 


360 


THE  MAINTENANCE  OF  THE   INDIVIDUAL 


l)osterior  part  of  the  retina  lies  a  region  known  as  tlie  yellow  spot,  or 
macula  lutea.  The  central  pitlike  portion  of  the  macula  lutea,  where 
cones  predominate,  almost  to  the  exclusion  of  rods,  is  designated  as 
the  fovea  centralis,  since  it  is  here  that  the  keenest  vision  occurs. 
The  retina  is  thinner  at  this  point  and  the  black  pigment  of  the  outer 
layer  shows  through  from  behind,  making  it  dark  purple  in  color,  due 
to  a  layer  of  cells  next  to  the  choroid  coat.     The  retina  acts  as  the 

sensitized  plate  in  a  camera,  for 


outar  Surface  of  retivict 


*-  pigment 
layer 

....l-oct 

...Cone- 


outar 

nuclear 

layer 

(outer  , 
I  crarjular 
*^layar 


inner- 
groiT7ixlaTr 
layer 


1  ganglionic. 

jlell^layer 
fibens-  of 
optic  nerve 


on  it  are  received  the  impressions 
of  light  and  shade  and  color  which 
are  transformed  and  sent  to  the 
brain  resulting  in  sensations  of 
sight.  Like  the  camera,  the  eye 
has  a  lens  formed  of  transparent 
elastic  material,  a  circumstance 
permitting  a  change  of  its  form 
and,  in  consequence,  a  change  of 
focus  upon  the  retina.  By  means 
of  this  change  in  form,  or  accom- 
modation, both  near  and  distant 
objects  may  be  seen.  In  fishes, 
unlike  mammals,  accommodation 
is  accomplished  by  shifting  the 
position  of  the  lens,  as  in  a  camera, 
rather  than  by  changing  its  shape. 
In  front  of  the  lens  is  a  small 
cavity,  divided  by  the   iris   into 


inner  surface  of"  retina. 
Detail  of  retina  showing  rods  and  cones. 

two  chambers  that  communicate  through  the  pupil,  filled  with  a 
watery  fluid,  the  aqueous  humor,  while  behind  it  is  the  main  cavity 
of  the  eye,  filled  with  a  transparent,  almost  jellylike,  vitreous  humor. 
The  lens  lies  directly  behind  the  iris  and  is  attached  to  the  choroid 
coat  by  means  of  delicate  ligaments  and  by  pressure  of  the  two 
liquid  media. 

In  order  to  function  properly,  the  surface  of  the  eye  must  be  kept 
moist,  and  various  glands  are  located  in  the  cavernous  orbit  of  the  eye 
and  along  the  edges  of  the  eyelids  which  serve  this  purpose.  The  best 
known  are  the  tear  or  lachrimal  glands  with  their  associated  ducts  that 
open  into  the  nasal  chamber.  These  glands  increase  their  normal 
production  of  moisture  to  form  visible  tears  when  the  surface  of  the  eye 
is  irritated  by  foreign  particles  or  when  the  emotions  gain  control. 


SUPPORT,    MOTION,   AND   SENSATION 


361 


Ears 

The  structures  making  up  the  compUcated  mechanism  of  iiearing 
primarily  serve  two  purposes,  namely  equilibration  and  hearing.  Of 
these  functions  the  first  is  luidoubtedly  the  more  primitive. 

Most  invertebrates,  whether  jellyfish,  molluscs,  or  crayfish,  main- 
tain their  equilibrium  by  some  sort  of  otocyst.  Roughly  described, 
this  consists  of  a  sac  lined  throughout  or  in  part  by  cell-receptors  and 
containing  concretions  called  otoliths.  As  the  animal  changes  its 
position  the  otolith  shifts  due  to  the  forces  of  gravity  and  thus  stimu- 
lates by  contact  the  different  receptor  nerve  cells,  which  transmit  the 
impulse   of   pressure   to  the  ^      ^  

endolymphatic 
Sac 

endolymphcctic  duct 


anter-ior 

Semicircular-/ 

CxxnaL 


posterior- 
Semicircular- 
Corjal 

utriculLcs 


brain,  where  it  is  interpreted 
so  as  to  enable  the  animal  to 
right  itself. 

The  ecjuilibratory  mecha- 
nism of  vertebrates  functions 
principally  through  stimuli 
received  from  nerve  cells 
located  in  the  arnqmllae  or 
swollen  ends  of  three  semi- 
circular canals,  occupying 
roughly  the  three  planes  of 
space.  The  animal  is  enabled 
to  adjust  its  position  wdth 
reference  to  the  stimuli  re- 
ceived through  the  influence 
of  gravity.  In  such  cases  the 
fluid  within  the  semicircular 
system  stimulates  differen- 
tially the  nerve  endings  in 
the  ampullae.  Stimuli  reach 
the  nerve-receptors  in  the  same  manner  as  they  do  in  the  lower 
forms,  being  carried  by  branches  of  the  auditory  nerve  (VIII)  in  the 
brain.  The  entire  structure  is  protected  by  a  surrounding  mass  of 
cartilage  which  in  higher  forms  becomes  ossified. 

As  to  the  function  of  hearing,  it  is  possible  that  in  the  case  of 
fishes  vibrations  are  transmitted  by  the  water  through  the  skull  to 
the  sensory  inner  ear.  However,  when  air  is  substituted  for  water 
as  the  chief  environment  some  other  more  sensitive  device  must  be 


'hor-L3onta.l 

Semicirculctr- 

Cocnccl 

The  inner  ear  of  a  fish  showing  the  essential 
features  of  this  balancing?  organ.  Where  are 
the  ampullae!'  These,  together  with  areas  in 
the  alriculus  and  sacculus,  contain  patches  of 
sensory  cells  connected  with  branches  of  the 
auditory  nerve.  How  are  such  areas  stimu- 
lated:*    The  lagena  produces  the  cochlea. 


362 


THE   MAINTENANCE   OF  THE   INDIVIDUAL 


developed.  In  the  land  vertebrates  amplifying  devices  are  developed 
in  the  form  of  a  vibrating  ear  drum  or  tympanic  membrane  beneath  the 
skin,  and  a  chain  of  middle  ear  hones  that  transmit  the  vibrations  to 
the  inner  ear  where  the  sensitive  receptor-cells  are  located.     Thus 


A  cross  section  of  the  coiled  cochlea  which  contains  the  or^an  of  Corti  in  which 
the  sensitive  hair  cells  are  located.  The  scala  media  is  filled  with  fluid  endolymph 
which  is  separated  from  the  fluid  perilymph  of  the  scala  vestibuli  by  Reissner's 
membrane.  Vibrations  of  the  ear  drum  are  transmitted  throuKh  the  middle 
ear  bones  which  cause  the  vibration  of  a  membrane  at  one  end  of  the  scala 
vestibuli,  thus  disturbing  the  perilymph  in  the  scala  vestibuli.  How  are  the  hair 
cells  stimulated  ? 

there  is  gradually  developed  an  elaborate  mechanism  by  which  vibra- 
tions are  transmitted  and  amplified  through  the  ear  drum  and  the 
three  bones  of  the  middle  ear  to  the  spirally  coiled  portion  of  the  inner 
ear,  or  cochlea,  where  the  receptor-cells  are  located.  These  essential 
cells  receive  stimuli  which  are  carried  by  branches  of  the  auditory 
nerve  to  the  brain  for  interpretation. 

Cutaneous  Sense  Organs 

There  remains  for  consideration  that  diverse  group  of  sense  organs 
located  in  the  integument.  In  fishes,  the  tactile  sense  consists  princi- 
pally of  pressure  receptors,  which  are  usually  concentrated  along  the 


SUPPORT,    MOTION,    AND   SENSATION  363 

lateral  line.  The  entire  surface  of  the  body  of  \ertehrates  in  general 
is  practically  covered  with  receptors  capable  of  iiitcrpretinp;  touch  or 
pressure,  temperature,  and  pain. 

These  integumentary  receptors,  of  which  there  are  many  modifica- 
tions, are  not  located  with  imiform  density  over  the  body  surface. 
It  has  been  estimated  that  there  are  between  3,000,000  and  4,000,000 
pain,  500,000  pressure,  150,000  cold,  and  16,000  warm  receptors 
located  in  the  human  skin. 

An  understanding  of  the  sensations  and  impulses  which  are  received 
from  the  organs  of  special  sense  is  the  primary  means  of  keeping  our- 
selves informed  about  changes  taking  place  in  our  immediate  sur- 
roundings. From  these  sensations  and  impulses  are  built  up  definite 
reactions  as  well  as  certain  convictions  or  attitudes  which  enable  us  to 
secure  the  maximum  (^r  minimvmi  out  of  life. 

SUGGESTED   READINGS 

Clendenning,  L.,  The  Human  Body,  Alfred  A.  Knopf,  Inc.,  1930.     Pp.  53-70, 

223-250. 

More  popular  reading. 
Howell,  W.  H.,  A  Textbook  of  Physiology,  12th  ed.,  W.  B.  Saunders  Co. 

1933.     Chs.  I-V. 

A  thorough  technical  account  of  the  j)hysiolog3'  of  muscle  and  nerve. 
Rogers,  C.  G.,  Textbook  of  Com-parative  Physiology,  McGraw-Hill  Book  Co., 

1927.     Chs.  XXVI  and  XXVIII. 

Advanced  account  from  the  comparative  viewpoint. 
Wells,  H.  G.,  Huxley,  J.  S.,  Wells,  C.  P.,  Science  of  Life,  Doubleday,  Doran 

&  Co.,  1931.     Pp.  32-38,  523-524,  697-698,  1200-1226. 

Popular  account  with  emphasis  on  man. 


XVII 


THE   DISPLAY   OF   ENERGY 

Preview.  Why  living  things  are  responsive  •  Various  kinds  of  stimuH  • 
Tropisms  •  Nature  of  responses  •  Mechanism  of  response  in  plants  •  Mech- 
anisms of  response  in  animals  •  Tropisms,  reflexes,  and  native  behaviors  • 
Native  behaviors  may  be  modified  •  Habit  formation  •  Conditioned  behav- 
iors •  Are  behaviors  adaptive  responses?  •  When  are  animals  conscious?  • 
Emotional  responses  •  What  is  intelligence?  •  Intelligence  of  apes  •  Intelli- 
gence of  man  •  The  measurement  of  intelligence  •  Suggested  readings. 

PREVIEW 

The  display  of  energy  is  characteristic  of  all  living  things.  We 
may  predict  quite  accurately  what  forms  energy  will  take  in  very 
simple  plants  and  animals,  since  they  react  variously  but  consistently 
to  factors  of  the  environment,  such  as  light,  temperature,  and  mois- 
ture, by  making  definite  turning  movements,  growth  movements,  or 
by  other  behavior.     These  expressions  of  behavior  are  called  tropisms. 

When  it  comes  to  answering  the  question,  "Why  do  we  behave 
like  human  beings?"  we  are  faced  with  a  much  more  difficult  problem, 
for  the  more  complex  the  organism,  the  more  complicated  are  its 
behavior  patterns. 

Comparing  the  behavior  of  plants  with  that  of  animals  and  using 
the  same  stimuli  in  each  case,  we  find  in  general  that,  correlated  with 
the  lack  of  muscles  and  a  nervous  system,  in  plants  responses  to 
stimuli  are  slow  and  usually  expressed  as  growth  movements.  In 
animals  which,  except  in  the  lowest  forms,  have  both  muscles  and  a 
nervous  apparatus,  the  reaction  to  a  given  stimulus  is  a  response  in 
the  form  of  some  sort  of  motion  such  as  swimming,  flying,  crawling, 
walking,  or  running. 

Two  very  definite  theories  of  animal  behavior  are  held.  One 
theory  recognizes  animals  as  living  machines,  giving  definite  and 
unchangeable  responses  to  certain  stimuli.  In  such  a  mechanistic 
view  of  life  the  organism  is  considered  in  terms  of  groups  of  cells  and 
tissues,  or  of  the  elements  of  which  it  is  composed.  When  the  ma- 
chine is  very  complex  its  actions  are  less  predictable  because  the 
same  stimulus  may  cause  a  different  reaction  to  a  different  part  of 
the  machine.     Light,  for  example,  would  evoke  a  response  only  from 

361 


THE   DISPLAY   OF  ENERGY  :56r> 

photoreceptive  organs,  while  differences  in  temperature  might  affect 
many  different  groups  of  tissues  or  organs  in  different  ways 

Another  view,  quite  opposite  to  this,  is  the  organismal  theory. 
Here  the  unity  of  the  organism  as  an  interacting  whole  is  stressed.  It 
is  considered  as  an  individual  and  not  as  a  collection  of  cells  and  tis- 
sues. The  study  of  embryology  bears  out  this  idea,  for  in  the  develop- 
ment of  the  egg  certain  regions  of  protoplasm,  instead  of  certain  cells, 
develop  into  the  future  embryo.  The  egg  at  an  early  stage  shows 
polarity,  a  right  and  left  side  as  well  as  an  anterior  and  posterior  end 
of  the  future  organism,  some  time  before  it  divides  into  cells.  Profes- 
sor Child  of  the  University  of  Chicago  has  developed  and  tested  a 
theory  of  the  unity  of  the  (organism  which  he  calls  the  axial  gradient 
theory  that  helps  in  understanding  the  complex  response  patterns  ob- 
served in  the  higher  forms  of  life.  He  considers  an  animal  as  having 
definite  axes  of  polarity,  or  symmetry,  the  anterior  end  containing 
the  most  sensitive  recei^•ing  structures.  Since  the  brain  is  the  most 
active  protoplasmic  substance  its  metabolic  rate  is  higher  than  that 
of  the  rest  of  the  organism,  while  its  activity  controls  other  parts  of 
the  organism. 

This  concept  of  the  organism  is  an  aid  to  a  better  understanding  of 
the  complicated  reactions  and  responses  that  are  found  in  higher 
animals.  It  is  difficult  to  explain  the  complex  response  patterns  of 
vertebrates  unless  they  are  considered  to  be  organized  masses  of  proto- 
plasm which  respond  as  units  to  the  total  pattern  of  stimuli  rather 
than  to  individual  stimuli.  Living  animals,  at  least  those  high  in  the 
scale  of  life,  respond  to  total  situations  rather  than  to  isolated  stimuli. 
Such  a  point  of  view  is  taken  by  the  "Gestalt "  group  of  psychologists, 
who  use  the  term  insight  to  describe  an  organized  response  at  the  con- 
scious behavior  level.  Such  a  response  can  be  shown  to  be  directed 
toward  a  goal,  the  complex  movements  being  organized  in  relation  to 
that  goal,  the  result  of  which  is  that  the  animal  is  able  to  solve  its  life 
problems.  According  to  this  theory,  a  child  who  is  learning  to  walk 
does  not  make  random  "trial  and  error"  movements.  The  uncer- 
tainty of  its  first  steps  is  due  to  a  lack  of  maturity  of  the  muscles  and 
of  the  nervous  system,  and  not  to  the  lack  of  a  goal.  This  can  be  seen 
in  a  comparison  of  two  children  of  the  same  age,  one  of  whom  is 
allowed  to  walk  early,  the  other  who  has  been  kept  off  its  feet  for  fear 
of  having  the  legs  bowed.  The  latter  will  walk  almost  at  once  when 
allowed  to  try  the  new  "stunt."  When  maturity  of  muscles  and 
nerves  is  attained  it  becomes  possible  for  a  total  behavior  pattern  to 


366  THE   MAINTENANCE  OF  THE   INDIVIDUAL 

appear  and  walking  takes  place.     The  second  child  has  both  condi- 
tions present. 

This  explanation  of  the  display  of  energy  helps  us  to  understand  the 
mental  life  of  higher  animals,  especially  with  reference  to  a  directed 
urge  toward  definite  goals  of  behavior.  In  the  pages  that  follow  an 
attempt  will  be  made  to  show  how  conscious  life  has  developed.  No 
set  theories  or  beliefs  will  be  imposed  on  the  reader,  but  a  brief  presen- 
tation of  the  facts  will  be  given  as  we  see  them.  The  student  can 
then  do  his  own  thinking. 

Why  Living  Things  Are  Responsive 

Life  has  been  likened  by  many  writers  to  a  flowing  river  which 
continually  moves  in  one  direction.  Meeting  obstacles,  it  is  diverted 
from  its  course,  moving  rapidly  over  steep  declivities  and  meandering 
slowly  in  level  valleys.  We  do  not  think  of  a  river  in  terms  of  water 
alone,  but  also  in  terms  of  the  rocks  in  its  bed,  of  its  banks  of  gravel 
or  soil,  even  of  the  forests  in  which  it  takes  its  source,  and  of  the 
wharves  and  bridge  abutments  of  the  cities  through  which  it  passes 
in  its  course.  We  know  that  eddies  in  the  river  mark  submerged 
rocks,  that  sharp  curves  may  be  caused  by  areas  too  hard  for  the  river 
to  erode,  that  ledges  may  cause  waterfalls.  It  is  not  possible  to  think 
of  the  river  without  the  environment  which  surrounds  it. 

Guided  by  this  comparison,  we  note  the  cause  of  sensitiveness  of 
living  matter  of  which  an  organism  is  made  up  in  the  fact  that  wher- 
ever factors  of  the  environment  impinge  upon  the  organism,  changes 
in  the  latter  are  sure  to  take  place.  These  factors,  forces,  or  things 
that  cause  changes  in  the  life  activities  of  plants  or  animals  are 
called  stimuli,  and  changes  in  relation  between  the  organism  and  its 
surroundings,  reactions  to  stimuli.  Such  responses  may  be  sudden, 
as  the  involuntary  start  which  comes  as  a  result  of  some  unexpected 
noise  or  the  quick  withdrawal  of  one's  hand  from  a  hot  object,  or 
they  may  be  extremely  slow  and  continuous,  as  is  seen  in  the  gradual 
turning  movements  of  a  plant  placed  in  an  area  of  unequal  illumina- 
tion. The  sum  total  of  all  the  reactions  of  an  organism  to  the  stimuli 
which  impinge  upon  it  constitutes  its  behavior. 

Various  Kinds  of  Stimuli 

In  order  to  understand  what  causes  behavior,  we  must  analyze  the 
various  kinds  of  stimuli  which  act  upon  plants  and  animals,  as  follows  : 


THE   DISI'IAY    OF   ENERGY  367 

1.  Thermal,  that  is,  changes  of  temperature,  as  extremes  of  heat  or 
cold. 

2.  Photic,  Hght  changes  both  in  direction,  intensity,  and  color. 

3.  Chemical,  changes  that  occur  in  the  concentration  of  certain 
substances  which  may  come  in  contact  with  the  organism. 
Such  changes  might  be  the  presence  of  salts,  acids,  alkalies,  or 
other  substances  in  the  soil,  or  various  types  of  chemical  sub- 
stances such  as  are  found  in  the  food  of  animals. 

4.  Electric,  changes  in  the  direction  and  strength  of  electric  cur- 
rents. Since  the  modern  concept  of  matter  is  interpreted  in 
terms  of  electricity,  it  must  be  realized  that  these  changes  may 
have  a  profound  effect  on  living  organisms. 

5.  Mechanical  stimuli,  such  as  changes  in  osmotic  pressure  within 
cells,  the  pull  of  gravity,  changes  in  pressure  of  the  medium. 
Contact  with  various  objects,  and  sound  waves,  are  also  impor- 
tant. Many  animals  and  plants  respond  definitely  also  to  cur- 
rents of  air  or  water. 

In  unicellular  organisms  responses  are  usually  more  predictable  than 
in  higher  organisms  because  the  latter  are  complex  structures  in  which 
different  parts  may  be  differently  affected  by  the  same  stimulus.  For 
example,  gravity  may  act  negatively  on  the  stems  of  green  plants  and 
positively  on  the  roots  of  the  same  plant.  While  the  stem  of  a  plant 
may  be  influenced  to  grow  toward  light  the  roots  grow  away  from  it. 
These  examples  might  be  multiplied  many  times. 

Tropisms 

In  1918  Jacques  Loeb,  one  of  the  foremost  investigators  in  this 
country,  brought  out  a  book  entitled,  Forced  Movements,  Tropisms, 
and  Animal  Conduct.  The  author  took  for  his  thesis  the  mechanistic 
point  of  view  of  life.  To  him,  and  to  other  members  of  his  school,  living 
organisms  are  mechanisms  whose  activities  are  directly  influenced  by 
the  stimuli  in  their  environment,  the  sum  total  of  behavior  being  the 
direct  result  of  their  reactions  to  various  stimuli.  In  a  series  of  con- 
vincing experiments,  Loeb  showed  that  animals  are  forced  to  do  certain 
things  because  of  a  purely  mechanical  effect  brought  about  by  the 
stimuli  impinging  upon  them.  If,  for  example,  the  common  shrimp 
(Palaemonetes)  is  placed  in  a  trough  through  which  an  electric  current 
flows,  with  its  head  toward  the  anode  pole,  the  tail  at  once  becomes 
stretched  out .  If  it  is  placed  with  its  head  toward  the  cathode  pole,  the 
tail  is  bent  under  the  body.     In  the  latter  case  the  animal  can  only 


368 


THE   MAINTENANCE  OF  THE   INDIVIDUAL 


swim  backwards,  while  in  the  former  case  it  can  only  crawl  forward.  In 
both  cases  the  change  in  position  is  caused  by  the  action  of  the  current 
on  the  flexor  and  extensor  muscles,  which  in  one  case  are  contracted 
and  the  other  case  extended,  thus  causing  the  animal  to  assume  the  po- 
sitions mentioned.  Experiments 
such  as  these  give  rise  to  the  theory 
of  tropisms,  which  is  simply  another 
term  for  a  series  of  responses  of  an 
organism  to  the  various  factors  of 
its  environment.  Tropisms  may 
be  briefly  classified  as  phototro- 
pisms,  or  responses  to  light ;  geo- 
tropisms,  or  responses  to  gravity  ; 
hydrotropisms,  or  responses  to 
water ;  chcmotropisms,  or  responses 
to  chemical  substances  ;  thermotro- 
pisms, or  responses  to  temperature 
changes ;  galvanotropisms,  or  re- 
sponses to  electricity;  thigmotro- 
pisms,  or  responses  to  contact ; 
rheotropisms,  or  responses  to  water 
currents ;  and  aneinotropisms,  or 
responses  to  air  currents. 

A  tropism  is  a  kind  of  directional 
urge.  It  represents  a  condition 
within  an  organism,  resulting  from 
the  interaction  between  its  struc- 
ture (nervous)  and  the  stimuli  of  the  environment.  Loeb  explained 
tropisms  as  specific  irritabilities  or  sensitivities  to  stimuli  at  the 
surface  of  the  body,  and  in  terms  of  body  symmetry,  since  corre- 
sponding parts  on  two  sides  of  the  body  would  show  the  same  sen- 
sitivities. Noncorresponding  parts,  according  to  this  theory,  would 
show  unequal  sensitivities,  resulting  in  directive  movements. 

Loeb  explained  his  famous  example  of  the  reversal  of  tropisms  in  a 
caterpillar  by  showing  that  the  caterpillar  moves  toward  light  when 
hungry  and  is  irresponsive  to  light  when  satisfied.  The  result  is 
most  useful  to  a  caterpillar,  because  as  it  leaves  its  nest  when  hun- 
gry, it  is  near  the  surface  of  the  ground  and  is  drawn  by  light  to  the 
tips  of  the  branches  where  young  edible  leaves  are  sprouting,  returning 
to  the  lower  branches  when  nonresponsive  to  light. 


Position  taken  by  lejjs  of  shrimp 
when  current  goes  laterally  through 
animal,  from  left  to  right.  (After  Loeb 
and  Maxwell.) 

Which  direction  would  the  animal 
be  forced  to  take  in  movement  .3 


THE  DISPLAY   OF   KNERGY  369 

The  typical  moth  is  positively  phototropic.  'IMiis  is  an  advantage 
in  its  natural  environment  because  it  flies  at  night  and  gets  its  food 
largely  from  white  flowers  which  are  mon^  conspicuous  at  night.  If, 
however,  the  factor  of  artificial  light  is  introduced,  the  moth  flies  to 
its  death.  This  is  not  because  it  "thinks"  it  sees  a  white  flower,  but 
because  its  eyes,  its  central  nervous  system,  and  its  wings  are  all 
connected  as  a  unit,  so  that  the  animal  has  to  turn  in  flying  to  the 
flame  not  once,  but  again  and  again. 

Jennings  found  Paramecium  equally  responsive  to  paper,  silk,  or 
particles  of  carmine  placed  in  its  immediate  environment,  thus 
showing  a  purely  mechanical  response.  It  took  these  foreign  sub- 
stances into  its  gullet  and  the  material  was  passed  into  the  body. 
Such  responses  are  not  advantageous.  On  the  other  hand  a  purely 
thigmotropic  response  may  be  advantageous  to  these  animals.  Para- 
mecia  feed  on  bacteria,  which  may  form  raftlike  masses.  As  soon  as 
a  Paramecium  comes  in  contact  with  such  a  mass,  its  response  to  this 
stimulus  causes  it  to  remain  quiet,  while  it  feeds  upon  the  bacteria. 
Its  sensitivity  to  other  stimuli  at  this  time  is  decreased,  making  it 
seem  as  if  its  attention  were  "fixed  upon  its  meal." 

Nature  of  Responses  * 

The  nature  of  a  response  to  a  stimulus  depends  upon  the  intensity 
and  nature  of  the  stimulus  as  well  as  upon  the  structure  of  the  part 
stimulated.  The  nature  of  this  response  may  differ  greatly.  In 
unicellular  organisms  the  entire  cell  may  move  in  response  to  a 
stimulus,  though  sometimes  there  is  only  a  turning  or  the  movement 
of  cilia  on  one  side.  If  a  simple  animal  such  as  Hydra  is  touched, 
withdrawal  of  the  tentacles  touched  may  occur,  or,  if  the  stimulus  is 
more  intense,  the  entire  body  may  contract.  In  plants,  responses  to 
stimuli  may  result  in  movements  caused  by  diff'erences  in  osmotic 
pressure  of  the  cells,  or  in  turning  movements  brought  about  by  the 
growth  or  turgor  of  certain  cells.  There  may  be  glandular  responses, 
too,  such  as  the  production  of  nectar  in  flowers,  or  the  flow  of  saliva, 
or  the  dry  mouth  of  "stage  fright"  in  man.  The  newt  gives  off  slime 
when  touched,  and  the  gland  cells  in  the  skin  of  a  toad  exude  poison 
when  it  is  roughly  handled. 

As  a  result  of  response  to  pressure,  gas  is  secreted  into  the  swim- 
bladder  of  some  fish.  Certain  areas  in  jellyfish  or  in  fireflies  become 
luminous  when  touched,  while  some  fishes  and  other  animals,  such  as 
squid,  octopuses,  tree  frogs,  and  chameleons,  respond  to  change?  of  the 


370 


THE   MAINTENANCE   OF  THE   INDIVIDUAL 


•>i*'':-:  ■•■.-■-  ■    *■  ■  ■."■■   ,■ 

..."  ••■^{;.-'.  ■    •          »        >•        », 

••   V;..  ,. v.- ■■•,'■.■■■     ■■■•... '-J,'      •>■■■•    ■-'•;■' 

Francis  B.  Sumner 

Dr.  Sumner's  experiments  with  flounders  show 
a  response  of  the  animal  to  different  backgrounds. 
How  would  you  attempt  to  account  for  this  .•* 


environment  by  chang- 
ing their  color  pattern. 
There  may  even  be  elec- 
tric responses  to  stimuli 
as  seen  in  the  discharge 
of  as  much  as  300  volts 
from  the  electric  organ  of 
the  electric  eel,  a  shock 
sufficient  to  kill  a  horse. 
In  the  higher  animals 
where  well-developed  or- 
gans have  been  evolved, 
an  organ  is  usually  at- 
tuned to  one  kind  of 
stimulus  and  responds 
only  to  that  particular 
stimulus.  The  eye,  for 
example,  responds  to  light 
waves,  but  to  no  other 
ether  waves,  while  the 
organ  of  Corti  in  the  mam- 
malian ear  distinguishes 
with  accuracy  betw^een 
different  wave  lengths 
which  cause  sounds.  Thus 
the  nature  of  responses 
depends  not  upon  the 
stimulus,  but  upon  the 
kind  of  cells  stimulated. 


Mechanisms  of  Response  in  Plants 

It  is  much  easier  to  show  that  plants  respond  to  stimuli  than  to 
explain  how  they  do.  Most  of  the  responsive  activities  of  plants  do 
not,  as  one  author  puts  it,  result  in  "discriminating  movement"  so 
much  as  in  ''discriminating  growth."  If  a  growing  root  is  photo- 
graphed every  ten  or  fifteen  minutes  and  these  pictures  greatly 
magnified  are  projected  as  a  slow  motion  motion  picture,  the  root 
seems  to  act  like  an  intelligent  "white  worm,"  pushing  aside  soil 
particles,  avoiding  obstacles,  and  ultimately  finding  its  way  to  an  area 
where  water  exists. 


THE   DISPLAY   OF   ENERGY  371 

In  spite  of  the  work  of  Sir  J.  C.  Bose,  the  distinguished  Indian 
botanist,  who  used  very  dehcate  instruments  to  measure  tlie  irrita- 
biUty  of  plants,  scientists  as  a  group  have  not  accepted  his  behef  that 
t\\o  transmission  of  stimuh  in  plants  is  by  means  of  a  mechanism 
similar  to  the  nerves  of  animals.  There  is  no  doubt  that  certain 
parts  of  the  plant  stem  do  conduct  stimuli  more  rapidly  than  others, 
but  it  is  doubtful  whether  the  conducting  strands  of  protoplasm  in  the 
sieve  tubes  of  the  phloem  are  actually  the  areas  of  special  transmission. 
Experiments  have  been  made  in  which  the  stimulus  of  an  electric 
current  can  be  cut  out  by  the  use  of  anesthesia,  just  as  in  the  case  of 
the  nerves  of  animals,  but  since  the  cells  in  the  area  where  the  stimulus 
is  transmitted  are  much  shorter  than  the  neurons  in  the  animal, 
transmission  is  naturally  slower  and  anesthetics  have  the  same 
effect  on  living  protoplasm  in  each  case.  One  investigator,  Ricca, 
has  shown  that  a  stimulated  region  of  a  plant  secretes  a  hormone  that 
travels  to  the  region  of  response,  causing  a  reaction  to  the  stimulus. 
Other  workers  have  even  shown  that  if  the  tip  of  one  plant  is  grafted 
to  another  plant  from  which  the  tip  has  been  removed,  the  stimulus 
will  be  transmitted  to  the  responsive  region  of  the  latter  plant.  A 
number  of  experiments  upon  plants  indicate  that  stimuli  are  trans- 
mitted by  means  of  hormones  which  are  carried  in  the  transpiration 
stream  through  the  vascular  bundles.  Too  little  is  knawn  at  the 
present  time  to  say  with  certainty  exactly  what  effect  hormones 
have,  but  it  is  quite  evident  that  they  do  play  a  part  in  the  trans- 
mission of  stimuli. 

One  of  the  most  studied  responses  is  geotropism.  Roots  are 
assumed  to  respond  positively  to  the  pull  of  gravity  while  stems  are 
considered  to  be  negatively  geotropic.  Branches  and  leaves  usually 
grow  at  right  angles  to  the  force  of  gravity  while  some  roots  place 
themselves  at  a  definite  angle  to  this  force.  Gravity  has  been  shown 
to  be  a  stimulus  by  experiments  which  either  replaced  it  by  some  other 
force,  or  neutralized  its  effect.  For  example,  plants  are  placed  on  a 
slowly  revolving  disk  called  a  clinostat.  If  the  })lant  is  revolved 
horizontally  on  the  disk,  which  rotates  parallel  to  the  long  axis  of  the 
plant,  the  roots  and  stems  will  continue  to  grow  in  the  same  direction 
as  they  did  at  the  beginning  of  the  experiment.  Gravity  in  this  case 
acts  on  all  sides  of  the  plant  eciually,  with  the  result  that  there  is  no 
change  in  the  position  of  the  plant's  organs.  In  the  famous  experi- 
ment of  Thomas  Andrew  Knight,  who  worked  in  the  early  part  of  the 
nineteenth  century,  plants  were  placed  on  a  rapidly  rotating  disk  in 


372 


THE   MAINTENANCE  OF  THE   INDIVIDUAL 


which  centrifugal  force  was  substituted  for  gravity.  In  this  experi- 
ment the  roots  grew  outward  while  the  stems  grew  toward  the  center 
of  the  revolving  disk,  instead  of  assuming  the  normal  geotropic 
positions. 


Roots  of  Vicia  faba  with  tips  in  glass  slippers:  at  left,  a,  b,  c,  three  stages 
in  the  curvature  of  the  same  root,  0  to  20  hours;  at  right,  a,  b,  two  stages  of  the 
same  root;   h,  18  hours  after  being  placed  in  position  a.     (After  Czapek.) 

Experiments  by  Czapek,  in  which  the  tips  of  growing  roots  were 
placed  in  glass  slippers  smaller  than  those  used  by  Cinderella,  show 
that  the  region  sensitive  to  the  pull  of  gravity,  "at  least  in  certain 
plants,"  is  located  in  the  last  two  millimeters  of  the  root-tip.  Recent 
investigators  have  tried  to  account  for  this  location  of  the  response. 
In  animals,  definite  organs  which  "perceive "  gravity  are  found.  Such 
are  the  otocysts  of  the  crustaceans  and  the  balancing  organs  (semi- 
circular canals)  of  higher  animals.  In  the  crustaceans  small  but 
relatively  heavy  particles,  known  as  otoliths,  give  the  animal  its  sense 


.y.doc;>3C 

sensory 
iMY'    "hairxs 


part  of  an 
antsnnule 


sand 


otolit'bs— .„ ^,      >, 

enlarged  view 
of  otocysts 

Balancing  organs  of  a  crustacean.     How  do  they  function.^ 


THE   DISPLAY   OF   ENERGY 


.17:} 


of  position  in  space  when  they  come  to  rest  on  the  sensory  hairs  which 
Hue  tiie  httle  pits,  or  otocysts.  A  somewhat  similar  explanation  has 
been  advanced  to  account  for  the 
response  to  gravity  in  plants. 
Cells  of  plants  are  filled  with  fluid, 
but  they  also  have  in  them  various 
solid  bodies,  some  of  which  are 
starch  grains,  and  others  tiny  crys- 
tals of  calcium  oxalate,  or  other 
minerals.  It  is  thought  that  the 
movement  of  these  bodies  within 
the  cell  may  give  the  stimulus  for 
the  turning  movements  attributed 
to  gravity.  The  twining  move- 
ment and  spiral  growth  of  stems 
also  seems  to  be  related  to  the 
stimulus  of  gravity,  for  if  such 
plants  are  placed  on  a  rotating  clinostat,  the  twining  movement  ceases. 
There  are  many  other  kinds  of  responses,  but  the  mechanism  of  the 
response  is  not  always  clear.  Roots  travel  for  long  distances  toward  a 
source  of  water.  A  case  is  cited  in  California  of  a  eucalyptus  tree 
which  sent  out  its  roots  over  100  feet  underneath  a  boulevard,  the 
fine  roots  ultimately  clogging  a  cement  water  pipe  on  the  other  side 


Perceptive   region 
of  Roripa  amphibia; 
of  the  granules   in 
Nemec.) 


in    the   root   cap 

with  the  position 

the  cells.      (After 


^^v^^^^^^ 

Q 

" 

1^ 

H 

■mkM 
1 

Wrhjhl  I'iirct' 

The  Sensitive  Plant  (Mimosa  pudica)  before  and  after  stitmilation.     Time 
required  for  reaction  can  be  measured  in  seconds. 


374 


THE   MAINTENANCE   OF  THE   INDIVIDUAL 


of  the  boulevard.  The  Carolina  ])oplar  has  lost  its  vogue  as  a  tree  for 
city  planting  largely  because  of  this  habit  of  clogging  drain  })ipes  by 
the  response  of  the  roots  to  water.  The  movements  seen  in  the  wilting 
of  leaves,  or  the  changes  in  the  position  of  leaves  in  bright  sunlight 

and  in  slight  illumination,  are 
familiar  to  all.  There  may 
even  be  a  quite  rapid  opening 
and  closing  of  flower  petals, 
and  there  are  also  definite 
noticeable  changes  in  the  posi- 
tion of  the  leaflets  of  clover, 
alfalfa,  oxalis,  and  other 
plants  in  the  morning  and  at 
night.  The  relatively  rapid 
responses  of  the  leaves  of 
the  sensitive  plant,  Mimosa 
pudica,  are  all  brought  about 
by  the  functioning  of  struc- 
tures called  pulvini,  cushion- 
like enlargements  of  the 
petiole  of  the  leaf  at  the  point 
of  its  insertion  in  the  stem. 
When  the  leaflets  of  the  large 
compound  leaves  of  the  mi- 
mosa are  stimulated  by  heat, 
pressure,  or  anesthetics,  they 
tend  to  droop,  the  stimulus 
from  the  leaflets  being  trans- 
mitted at  the  extraordinarily  rapid  rate  (for  plants)  of  from  one  to  three 
centimeters  per  second.  When  the  stimulus  reaches  the  pulvinus 
where  the  cells  are  large  and  are  rich  in  water,  a  change  in  turgor  takes 
place  in  these  cells,  with  the  result  that  the  leaf  stalk  droops.  In  some 
plants  there  is  a  rapid  and  temporary  fluctuation  in  growth  on  opposite 
sides  of  the  leaves.  This  causes  a  comparatively  rapid  turning  move- 
ment, but  it  is  evident  that  these  forces  are  not  in  themselves  sufficient 
to  explain  all  the  changes  that  take  place  in  such  plants  as  the  Mimosa. 


Leaf  motility  in  the  sensitive  plant 
(Mimosa  pudica):  above,  an  open  leaf; 
l)elow,  a  leaf  whose  leaflets  (/)  have  been 
closed  by  niechanieal  impact ;  note  also  that 
the  petiole  (p)  has  dropped;  s,  stipule; 
m,  pulvinus. 


Mechanisms  of  Response  in  Animals 

The  mechanism  of  the  reflex  arc  has  already  been  described  in  some 
detail  in  the  discussion  of  the  various  types  of  nervous  systems  found 


THE   DISPLAY   OF   ENERGY 


375 


..oral  cUia 


...orccl  ^roov(2^ 

...Contractile 

vcxcixol<2^ 


in  animals.  It  will  not  be  amiss,  however,  even  at  the  risk  of  repeti- 
tion, to  take  up,  from  the  standpoint  of  function,  the  effects  of  some 
of  the  forms  of  animal  behavior. 

In  simple  animal  cells,  such  as  Ameba,  the  outer  portion  of  the 
cell  is  in  contact  with  the  stimulus  which  is  transmitted  through  the 
protoplasm  to  the  in- 
terior of  the  cell.  In 
cells  with  cilia,  continua- 
tions of  these  structures 
that  reach  down  into  the 
protoplasm  apparently 
act  as  organs  for  recep- 
tion of  stimuli.  Euglena 
has  a  pigmented  "eye- 
spot"  which  is  definitely 
sensitive  to  light.  In 
some  specialized  proto- 
zoan cells  a  motoriuni  or 
co-ordinating  center  is 
found. 

In  higher  forms  of 
animals  there  are  defi- 
nite receptors  in  the  form 

of  sense  cells,  organs  which  act  as  stimulating  centers  with  nerves 
serving  as  conductors  to  the  parts  that  are  fitted  for  resi)onse,  the 
so-called  effectors.  Examples  of  such  effectors  are  the  muscle  cells, 
gland  cells,  and  the  cells  of  such  organs  as  the  luminous  areas  of  the 
fire-fly,  and  the  electric  organ  of  the  electric  eel.  In  the  nerve  net  of 
such  animals  as  Hydra,  or  the  jellyfish,  apparently  no  synapses 
exist  between  the  cells,  the  nervous  system  being  a  tangled  net  through 
which  the  nerve  impulse  flows.  In  such  a  nervous  apparatus  the 
nerve  activity  is  slower  than  in  a  type  of  nervous  system  found  in 
animals  like  the  earthworm.  The  so-called  "ladder  nervous  system" 
exists  in  worms  and  in  arthropods  generally,  and  is  seen  at  its  highest 
development  in  the  insects,  where  there  is  a  series  of  units  in  which 
the  neurons  are  connected  by  synapses.  Such  types  of  nervous 
systems  are  more  effective  because  the  nerve  impulses  travel  only  in 
one  direction  through  a  neuron,  while  in  the  nerve  net  they  may  travel 
in  any  direction.  Receiving  neurons  in  the  sense  organs  are  found 
at  the  surface  or  in  a  situation  where  they  may  be  exposed  to  stimuli. 

H.  w.  H.  — 25 


.caudal  Cirri 


Euplotes,  a  hypotrichous  ciliate.  Note  the 
thickened  cilia  or  cirri  by  means  of  which  the 
animal  is  able  to  make  siuUlen  jumping  move- 
ments. 


376 


THE  MAINTENANCE  OF  THE  INDIVIDUAL 


Connecting  neurons  tie  up  these  with  the  effector  neurons  which  stimu- 
late the  muscles  to  contract,  or  the  glands  to  secrete.     The  dorsally 

placed  vertebrate  nervous  sys- 


a  few  cells  of  -tfic^ 
nerve.  neLof 

N 


\adde-r system      systsra  of  odultrnid^ 
of  a  myriapod  ■   *^'"     ' 


tern  is  considered  the  most 
highly  developed  type  of  all. 
Here  centralized  function  is 
found  at  the  anterior  end  of 
the  body  in  the  so-called  brain. 
In  the  animal  series  all  animals 
except  those  built  on  the  radial 
plan  show  a  very  distinct 
centralization  of  sense  organs 
(receptors)  at  the  anterior  end. 
The  organs  of  sight,  hearing, 
taste,  and  smell  are  found  in 
a  relatively  small  area  on  the 
head  close  to  the  brain.  It  is 
easy  to  see  how  evolutionary 
development  has  brought  this 
about,  since  it  is  the  anterior 
end  which  is  constantly  ex- 
ploring for  the  rest  of  the 
Upon  the  success  of  this  exploratory  ability  rests  the  suc- 


Three  types  of  nervous  systems.  What 
are  the  general  Hkenesses  and  diflerences  ? 
Which  would  be  called  the  highest  type  and 

why  i' 


animal 

cess  of  the  animal  in  its  struggle  for  existence. 

Tropisms,  Reflexes,  and  Native  Behaviors 

The  term  reflex  action  has  been  given  to  the  response  which  comes 
from  the  stimulation  of  a  single  reflex  arc,  a  receptor  with  its  neuron 
leading  inward  to  an  effector  neuron  which  in  turn  causes  movement 
through  the  effector  muscles  or  glands.  In  most  if  not  all  cases,  how- 
ever, there  is  more  than  a  single  series  of  neurons  engaged  in  the 
action  of  the  reflex  arc.  There  is  always  a  direct  response  in  the 
reflex.  The  response  is  quite  predictable  and  results  in  movement 
of  a  relatively  small  part  of  the  animal's  body.  A  tropism,  on  the 
other  hand,  may  be  considered  as  a  steady  response  to  a  continued 
stimulus.  As  one  writer  well  puts  it,  the  tropism  is  "a  steady  under- 
lying bias  in  behavior  brought  about  by  a  constant  stimulus."  The 
tropism  affects  the  organism  as  a  whole,  the  reflex  directly  affects 
only  a  small  portion  of  it.  The  activities  of  all  animals,  but  espe- 
cially the  lower  forms,  are  a  continual  series  of  reflexes  and  tropisms. 


THE   DISPLAY   OF   ENERGY  377 

When  reflexes  follow  one  another  in  an  orderly  succession  involving 
a  chain  of  reflexes,  one  step  of  which  determines  the  next,  they  are 
called  native  behavior  'patterns.  That  these  are  inherited  patterns  is 
seen  in  such  acts  as  cocoon-making,  egg-laying,  or  mating  behavior, 
which  only  take  place  once  in  the  life  of  the  individual. 

There  have  been  two  lines  of  e\olution  in  behavior  patterns,  one 
culminating  in  the  insects,  the  other  in  nian.  These  two  groups  are 
the  most  successful  in  the  animal  kingdom.  The  insect  group 
embraces  probably  over  625,000  species,  while  man  is  but  a  single 
species.  It  is  estimated  that  many  insects,  particularly  the  ants, 
have  undergone  no  significant  structural  changes  since  the  Oligocene 
period  some  thirty  million  years  ago.  They  are  at  the  summit  of 
their  development  while  man  is  just  beginning.  Insects  mark  the 
top  notch  of  these  native  behavior  patterns.  Their  innate  stereo- 
typed functions  make  them,  in  the  words  of  one  writer,  "a  bag  of 
tricks."  Their  actions  depend  upon  a  series  of  associations  which 
form  a  sequence  or  chain  of  events.  These  chain-reflexes  in  many 
cases  have  formed  so  complicated  a  pattern  that  the  ensuing  actions 
appear  to  be  intelligent.  However,  when  these  actions  are  carefully 
analyzed,  by  means  of  experiments,  they  exhibit  a  far  different  type 
of  response.  The  well-known  example  given  by  Fabre  will  suffice  to 
illustrate  how  such  a  chain  of  reflexes  works.  One  of  the  Sphex  wasps 
habitually  paralyzes  a  cricket  by  stinging  it,  and  then  drags  it  to  its  nest 
as  food  for  its  larvae.  After  the  female  w^asp  has  dragged  the  paralyzed 
victim  to  the  entrance  of  the  burrow,  she  leaves  it  there  and  goes  inside, 
apparently  to  inspect  conditions.  In  his  experiment,  Fabre  moved  the 
cricket  a  short  distance  from  where  it  was  left  and  when  the  wasp  came 
out,  finding  the  cricket  out  of  its  original  position,  she  seized  it  again  and 
dragged  it  back  to  the  mouth  of  the  nest,  and  again  went  in.  Fabre  re- 
moved the  cricket  forty  times,  and  for  forty  times  the  wasp  repeated  its 
actions.  As  Huxley  has  so  aptly  said,  all  she  knew  was,  "drag  cricket 
to  the  threshold — pop  in  —  pop  out  —pull  cricket  in."  In  this  case  the 
initial  stimulus  that  started  this  whole  chain  of  events  was  the  maturing 
of  the  egg  in  the  body  of  the  wasp,  and  the  breaking  of  a  single  link  in  the 
chain  of  associations  was  sufficient  to  break  the  sequence  of  events.  Ex- 
amples of  these  chain  reflexes,  which  have  been  called  instincts  for  v/ant 
of  a  better  term,  are  so  numerous  that  volumes  have  bcnni  written  about 
them.  The  many  fascinating  books  of  Fabre,  the  intriguing  volume 
on  wasps  by  the  Peckhams,  the  still  interesting  classic  entitled,  Ants, 
Bees  and  Wasps,  by  Sir  John  Lubbock,  are  all  worth  reading. 


378  THE   MAINTENANCE  OF  THE   INDIVIDUAL 

Native  Behaviors  May  Be  Modified 

Although  native  behavior  is  usually  predictable,  there  is  some 
evidence  that  it  may  be  modified  under  certain  conditions.  Howes  ^ 
gives  such  a  case.  The  sphecid  wasp  places  a  single  paralyzed  cicada 
in  its  burrow  after  laying  an  egg  in  the  body  of  the  unfortunate  vic- 
tim. The  burrow  is  then  sealed  with  earth,  the  young  wasp  feeding 
on  the  paralyzed  insect  until  the  larva  pupates.  The  adult  wasp 
carries  the  cicada,  which  is  larger  than  itself,  by  means  of  two  power- 
ful up-turned  hooks  on  each  side  of  its  hind  legs.  Howes  removed 
these  hooks  from  the  legs  of  a  sphecid  wasp  and  after  several  hours 
replaced  the  wasp  near  the  burrow,  but  close  to  a  cicada  which  it 
had  previously  captured  and  paralyzed.  The  wasp  paid  no  attention 
to  the  cicada  but  flew  off,  shortly  returning  with  another  victim 
which  it  carried  between  the  first  and  second  pairs  of  legs.  This 
shows  a  marked  modification  of  its  original  instinctive  behavior. 

The  following  examples  show  how  in  two  nearly  related  species  there 
may  be  differences  in  behavior.  The  mud-dauber  wasp  builds  a 
small  nest  of  from  eight  to  ten  cells,  filling  each  cell  with  paralyzed 
insects  or  spiders  which  are  used  as  food  for  the  developing  young. 
In  filling  the  cell,  Howes  found  that  the  wasp  averaged  one  spider  for 
every  seven  minutes  of  time  until  its  tenth  visit,  when  it  brought  a 
small  pellet  of  mud  which  it  flattened  and  placed  across  the  opening  of 
the  cell.  This  was  not  enough  to  close  the  cell,  so  the  insect  flew  away 
to  get  more  mud.  While  it  was  gone  Howes  removed  the  spiders  and 
the  cell  cap.  The  wasp,  upon  returning,  resealed  the  cell  without 
examination  and  without  depositing  spiders  or  another  egg.  In  the 
case  of  the  paper  wasp,  a  near  relative,  when  Howes  replaced  an 
unfinished  cell  with  one  of  papier-mache  the  wasp  immediately  tore 
the  papier-mache  cell  down  and  proceeded  to  build  a  proper  one. 
This  indicates  that  the  chain  of  native  behaviors  in  some  cases  may 
never  be  broken  without  a  complete  re-acting  of  the  whole  scene, 
while  in  others  modification  of  behavior  which  looks  like  a  low-grade 
intelligence  is  found. 

In  considering  the  insect  with  its  "bag  of  tricks,"  all  of  which  can 
be  played  expertly  but  which  cannot  be  changed,  we  must  think  in 
terms  of  structure  as  well  as  in  terms  of  function.  Contrast,  for 
example,  the  strongly  built  claws,  legs,  or  mouth  parts  of  an  insect, 
or  a  crustacean,  with  the  hands  of  a  man.     The  former,  each  of  which 

I  Howes,  P.  G.,  Insect  Behavior,  Badger,  1919. 


THE   DISPLAY   OF   ENERCIY 


379 


is  fitted  to  perform  a  very  limited  number  of  unchangeable  acts,  are 
rigid.  The  latter,  on  the  other  hand,  are  plastic,  extremely  flexible 
and  adaptable,  capable  in  some  instances  of  playing  a  Chopin  noc- 
turne, or  in  others  of  fashioning  the  cunning  work  of  a  Cellini. 

Habit  Formation 

The  patterns  of  behavior  that  we  call  habits  are  closely  allied  to 
native  behaviors.  If  animals  can  make  associations,  any  act  which 
comes  as  a  result  of  a  contiguity  of  stimulation  and  useful  associa- 
tion tends  to  be  repeated.  If  there  are  many  repetitions  the  per- 
formance of  such  an  act  becomes  more  and  more  certain.  In  other 
words,  it  becomes  a  habit.  It  has  been  said  that  our  lives  are 
bundles  of  habits.  This  is  particularly  true  of  man,  since  many  of  the 
activities  learned  in  early  life,  such  as  walking,  learning  to  drive  a  car, 
riding  a  bicycle,  skating,  swimming,  writing,  typewriting,  and  hun- 
dreds of  other  activities  common  to  this  machine  age,  are  habitual. 

One  object  of  education  is  the  training  of  different  cerebral  areas 
so  that  they  will  do  their  work  efficiently.  In  learning  to  write  one 
exerts  a  conscious  effort  in  order  to  make  the  letters  at  first.  Later, 
the  actual  forming  of  letters  is  done  without  conscious  effort,  for  by 
training  the  act  has  become  habitual. 

Conditioned  Behaviors 

More  than  thirty  years  ago  the  famous  Russian  physiologist 
Pavlov  began  a  series  of  experiments  that  have  changed  much  of 
our  thinking  regarding  the 
fixity  of  animal  behavior.  His 
best  known  work  was  done 
with  dogs.  It  is  proven  that 
when  food  is  offered  to  a 
dog  saliva  is  secreted.  This 
effect  is  partly  psychic  and 
partly  mechanical,  as  can  be 
seen  when  one  thinks  of  a 
particularly  sour  pickle  or 
lemon,  or  chews  dry  food. 
Pavlov  found  that  the  dog's 
saliva,   which   was   normally 


f-^  -f^paroticC 
inUmoa  duct   t^JJ  'dlcincC 

"^a"0^  lintsrncU) 


external 
t-ube 


<4r-r 


-  submayrillar)^ 
gland- 


drop  of.^alivtt^ 


Diagram  to  show  Pavlov's  experiment. 
Under  what  conditions  would  saliva  be 
caught  in  the  tube?  Explain  why  he  ob- 
tained a  conditioned  reflex. 


secreted  when  the  dog  saw  food,  could  be  caused  to  flow  by  the 
ringing  of  a  bell,  or  by  the  presentation  of  a  plate  of  a  given  color. 


380  THE   MA.INTENANCE  OF  THE   INDIVIDUAL 

But  this  behavior  was  only  hroufilit  about  through  tlie  presentation 
of  food  many  times  in  succession  at  the  same  time  or  just  after  the 
ringing  of  the  bell,  or  the  use  of  the  colored  plate,  thus  forming 
an  association  between  food  and  bell,  or  food  and  plate.  Eventually 
when  food  was  not  presented  but  the  bell  rang,  or  the  plate  was 
shown,  saliva  would  flow  from  the  parotid  gland  just  as  if  food  was 
present.  The  reflex  established  originally  with  food  was  changed 
through  association  of  food  with  bell  or  plate.  Thus  Pavlov  estab- 
lished a  law  of  the  conditioned  reflex,  which  may  be  stated  thus : 
"If  a  new  indifferent  external  stimulus  is  many  times  present  along 
with  one  which  has  also  a  definite  response,  the  subsequent  presenta- 
tion of  the  new  stimulus  causes  the  reflex  to  be  given." 

Conditioned  reflexes  have  been  demonstrated  in  forms  as  simple 
as  the  Ameba,  earthworm,  crab,  snail,  octopus,  as  well  as  in  higher 
animals.  It  is,  however,  unlikely  that  conditioning  plays  a  very 
important  part  in  the  lives  of  lower  animals.  In  the  case  of  fishes, 
reptiles,  amphibians,  and  vertebrates,  the  "training"  which  comes 
through  the  conditioning  of  behavior  may  play  a  minor  part.  In  the 
highest  vertebrates,  apes  and  men,  conditioning  undoubtedly  plays  a 
very  definite  part  in  the  learning  process.  Experiments  made  in 
Pavlov's  laboratory  have  shown  that  while  a  dog  may  take  from 
thirty  to  one  hundred  trials  before  it  is  conditioned  to  food,  a  young 
child  may  show  the  same  conditioned  effect  in  from  ten  to  twenty-five 
trials. 

Are  Behaviors  Adaptive  Responses? 

It  is  easy  to  show  that  all  responses  to  stimuli  are  useful  to  a  plant 
and,  therefore,  enable  it  to  adapt  itself  more  easily  to  the  environment 
in  which  it  lives.  The  turning  of  stems  and  leaves  toward  light,  the 
"seeking"  of  roots  for  water,  the  twining  movements  of  plants  are 
all  well-known  examples. 

When  it  comes  to  animals,  there  are  two  views  of  their  response  to 
stimuli,  one  mechanistic,  the  other  adaptive.  The  first  considers  the 
organism  to  be  a  machine  that  responds  blindly  to  the  various  physical 
and  chemical  stimuli  which  impinge  upon  it,  regardless  of  the  conse- 
quence to  the  organism.  This  is  much  easier  to  see  in  simple  animals 
than  in  more  complex  ones,  because  in  the  latter  the  behavior  of  the 
organism  is  influenced  not  only  by  different  combinations  of  stimuli 
but  also  by  the  reinforcement  or  weakening  of  stimuli.  The  behavior 
of  the  organism  at  a  given  time  will  be  determined,  not  by  a  single 


Tin:  DISPLAY  OF  i:nkr(;y  :m 

stimulus  but  by  the  aggregate  of  all  the  stimuli  which  impinge  upon 
it.  The  stimulus  pattern  causes  the  behavior  pattern.  The  fact 
that  organisms  behave  in  a  purposeful  way  and  that  frequently  their 
behaviors  are  modified  or  "conditioned,"  has  given  rise  to  the  point 
of  view  that  behaviors  are  adaptive. 

To  understand  this  philosophy  it  is  necessary  to  go  back  to  the  Avork 
of  Child.  In  recent  years  he  has  shown  that  all  organisms  exhibit  a 
definite  polarity.  Even  in  a  single-celled  organism,  polarity  is  shown 
not  only  in  an  anterior  and  posterior  end,  but  also  in  a  physiological 
gradient  which  extends  from  the  surface  to  tlie  interior.  The  proto- 
plasm at  the  surface  exhibits  the  highest  rate  of  metabolism,  the 
protoplasm  near  the  center  the  lowest  rate.  If  the  organism  is  cut 
in  two,  a  new  center  forms  as  far  away  from  the  surface  as  possible 
and  a  new  field  of  metabolism  comes  into  existence. 

The  following  test  of  this  metabolic  gradient  was  made  with  flat- 
worms,  animals  so  simple  in  structure  that  they  lend  themselves 
readily  to  experimentation.  After  removing  the  head  and  tail  end  of 
a  number  of  worms,  the  remaining  part  of  the  worm  was  cut  into  four 
pieces,  as  many  as  a  hundred  worms  at  a  time  being  used  in  the 
experiment.  After  sorting  the  cut  pieces  into  groups  of  anterior 
sections,  second,  third,  and  fourth  sections,  it  was  found  that  the 
metabolic  rate  in  these  groups  was  constant,  the  most  anterior  group 
using  the  most  oxygen  and  giving  off  the  most  carbon  dioxide.  The 
most  posterior  group  used  the  least  oxygen  and  gave  off  the  least 
carbon  dioxide.  There  was  thus  a  chemical  gradient  of  physiological 
activity  correlated  with  the  nervous  differentiation  of  the  organism, 
the  latter  acting  as  a  physiological  unit  and  not  as  a  cell  aggregate. 

Physiological  gradients  are  seen  every^vhere.  Eggs  exhibit  polarity, 
the  potential  energy  at  one  end  being  much  greater  than  at  the  other. 
Gland  cells  are  j^olarized  so  that  they  always  secrete  in  a  certain 
direction,  while  nerve  cells  in  higher  animals  invariably  conduct 
impulses  in  only  one  direction.  In  embryos,  an  early  polarization 
takes  place  and,  as  we  have  seen,  all  animals  except  radially  sym- 
metrical ones  exhibit  polarity. 

The  beginnings  of  behavior  in  embryonic  animals  start  as  mass 
movements  of  the  organism  as  a  whole.  This  has  been  found  to  be 
due  to  the  fact  that  the  central  nervous  system  has  not  grown  out 
into  the  surface  of  the  body.  A  new  group  of  behaviorists  start  with 
the  general  thesis  that  behaviors,  such  as  tropisms,  are  organized 
responses  to  a  total  pattern  of  stimuli,  the  organism  modifying  its 


382 


THE   MAINTENANCE   OF  THE   INDIVIDUAL 


behavior  according  to  the  stimulus  pattern  it  receives.  This  modi- 
fication results  in  a  change  or  tension  on  the  part  of  the  organism, 
leading  it  toward  a  goal.  This  goal  may  be  food,  or  some  other 
"desirable"  situation.  Many  experiments  have  been  made  which 
indicate  that  modifications  of  behavior  which  result  in  learning  take 
place  very  early  in  the  animal  scale.  Schaeffer  ^  made  an  interesting 
experiment  with  Ameba.  He  put  a  particle  of  glass  close  to  an 
Ameba  that  had  been  starved  for  some  time,  making  the  glass  vibrate 
by  means  of  a  rod.  The  Ameba  immediately  surrounded  the  glass, 
forming  a  food  vacuole,  but  after  six  minutes  expelled  the  glass. 
Five  minutes  later  the  glass  was  again  presented,  and  again  the 
Ameba  ingested  it,  this  time  expeUing  the  glass  after  three  and  a  half 
minutes.  In  a  third  presentation  the  glass  was  only  partially  ingested 
and  two  more  trials  gave  slight  food  response.  All  further  trials 
showed  the  Ameba  completely  indifferent  to  the  glass.  This  con- 
tinued placing  of  nonedible  material  in  front  of  the  animal  set  up  a 
tension  in  the  protoplasm  which  resulted  in  a  modified  behavior, 
causing  indifference  to  the  vibrating  glass. 

In  animals  which  have  only  a  nerve  net,  modification  of  behavior  is 
also  possible.     In  the  famous  experiments  of  Loeb  and  of  Parker  bits 

of  meat  were  presented 
to  the  tentacles  of  a  sea 
anemone,  the  meat  being 
passed  by  the  tentacles 
into  the  mouth.  Then 
the  same  tentacles  were 
fed  with  bits  of  filter 
paper  which  had  been 
soaked  in  beef  juice.  At 
first  the  animal  made  no 
distinction  between  food 
and  filter  paper  but  after 
ten  trials  learned  that 
filter  paper  was  not  food  and  constantly  rejected  it.  Hundreds  of 
other  examples  might  be  given  to  show  that  behaviors  are  modifiable. 
But  if  lower  animals  live  in  a  world  of  present  actions  and  "have  no 
thought  for  the  morrow,"  then  it  is  doubtful  if  this  conditioning  of 
behavior  means  much  in  the  ultimate  solution  of  their  life  problems. 


meat 


filter  papei~  N/ilb  meat  jui<ie, 
is  token  , , 


taken  to 
mouth 


popct-  is  rejecteoC 


A  sea  anemone  will  learn  to  distinguish  between 
meat  and  filter  paper  flavored  by  meat  juice. 


1  Schaeffer,  A.  A.,  "Choice  of  Food  in  Amoeba."     Journal  of  Animal  Behavior,  1917,   Vol.  VII, 
220-252. 


THE   DISPLAY   OF   ENKIIGY  383 

When  Are  Animals  Conscious? 

If  we  accept  Loeb's  mechanistic  point  of  view,  no  animals  lower  than 
man  would  be  considered  conscious.  As  Professor  Hodge  once  said, 
"A  house  fly  is  about  as  intelligent  as  a  shot  rolling  down  the  board." 
Once  a  chain  of  behavior  is  set  in  motion,  it  continues  until  the  life 
cycle  is  completed  by  egg-laying. 

The  theory  most  commonly  accepted  among  psychologists  today 
is  that  when  an  animal  improves  its  responses  through  the  use  of 
experience,  then  it  has  some  degree  of  consciousness.  Just  because 
an  earthworm  may  "learn"  to  turn  to  the  right  instead  of  the  left 
in  a  T-tube  to  avoid  an  electric  shock  does  not  mean  that  it  has  either 
consciousness  or  memory  in  the  true  sense  of  those  terms.  Nor  does 
the  dog  which  can  be  conditioned  necessarily  have  a  consciousness  of 
the  sound  of  a  bell  or  of  color  in  the  sense  that  man  does.  In  insects, 
for  example,  where  there  is  a  highly  developed  nervous  system  of  a 
specialized  type,  the  animals  live  largely  in  a  world  of  odor.  Their 
perception  of  food,  nest,  or  surroundings  is  largely  dependent  upon 
odor.  Ants  recognize  each  other  and  their  tribal  enemies  by  odor. 
The  male  moth  recognizes  its  mate  by  odor. 

We  must  be  careful  not  to  read  our  own  sensations  into  the  re- 
sponses of  simple  animals.  As  Wells,  Huxley,  and  Wells  aptly  say, 
"The  jelly-fish  only  pulsates.  A  sea  urchin  with  its  nerve-net  has  no 
sense  of  wholeness."  It  is  a  mistake  to  assume  that  lower  animals 
live  in  a  world  where  space  and  time  play  a  conscious  part.  The  eyes 
of  worms,  insects,  or  most  molluscs  do  not  "see"  in  our  sense  of  the 
word.  The  insect  may  perceive  colors  and  moving  objects,  but  to 
many  animals  the  world  is  a  world  of  light  and  shadow.  Three  states 
of  existence  are  probably  found,  —  that  of  mere  reception  of  stimuli ; 
another,  in  which  objects  become  stimuli ;  and  in  higher  animals  a 
perception  of  space  and  time.  The  world  of  recognized  cause  and 
effect  is  probably  open  only  to  the  highest  animals,  such  as  apes  and 
man.  Therefore,  consciousness  is  a  very  variable  term  and  at  most 
does  not  mean  much  to  the  psychologist. 

Emotional  Responses 

The  emotional  responses  of  higher  animals  are  a  type  of  nervous 
and  glandular  activity  that  plays  a  tremendous  part  in  their  lives. 
Feelings,  joys  and  sorrows,  fear,  anger,  worry,  or  optimism,  how 
much  they  govern  the  life  patterns  of  the  average  man  !    Biologically 


381  THE  MAINTENANCE  OF  THE  INDIVIDUAL 

such  activities  are  closely  tied  up  with  hormone  activity.  Definite 
changes  in  the  body  are  recognized  as  associated  with  certain  emotions. 
Sudden  fright  causes  the  heart  to  beat  faster,  the  hair  "stands  on  end," 
the  face  blanches,  and  the  digestive  glands  cease  their  accustomed 
activity.  The  biologist  sees  in  these  physical  accompaniments  of 
the  "feelings,"  changes  that  hark  back  to  native  behaviors,  actions 
that  make  for  self-preservation.  Under  the  stress  of  unpleasant 
emotions  the  glandular  activities  of  the  digestive  tract  are  reduced, 
so  that  more  blood  flows  to  the  muscles,  thus  allowing  greater  muscular 
activity.  The  automatic  sympathetic  nervous  system  invokes  secre- 
tion from  the  adrenal  glands  which  in  turn  tune  up  the  sense  organs 
to  greater  sensitivity  and  the  circulatory  and  respiratory  systems  to 
greater  activity,  with  a  resulting  increase  in  oxygen  and  in  food  to  the 
muscles.  Emotions  are  evidently  self-preserving  activities,  but  they 
also  add  and  subtract  much  from  the  lives  of  men.  The  highly 
emotionalized  person  who  has  his  "ups  and  downs"  may  get  more  out 
of  life  than  his  lethargic  neighbor,  but  he  also  suffers  more  deeply  and 
may  make  more  mistakes  in  judgment  when  under  emotional  stress. 

What  Is  Intelligence? 

The  term  intelligence  has  been  much  misused,  for  we  are  apt  to  read 
our  own  point  of  view  into  the  actions  of  lower  animals.  Psychologists 
say  that  an  animal  is  intelligent  when  behavior  is  flexible  enough 
to  make  it  profit  by  experience.  Stereotyped  functions  having  a 
pattern  handed  down  by  heredity  have  been  shown  to  be  native 
behaviors.  Patterns  of  conduct  not  inherited,  but  acquired  by 
many  repetitions,  are  habits.  The  intelligent  act  shows  choice.  It 
involves  analysis  of  a  situation  and  the  comparison  of  past  experi- 
ences in  relation  to  the  present,  that  is,  there  must  be  memory  or  a 
record  of  past  events.  In  addition,  the  intelligent  act  also  involves  a 
synthesis  with  past  experiences  built  up  with  the  aid  of  memory  and 
imagination.  Intelligent  animals  show  a  certain  amount  of  insight. 
Intelligence  involves  the  solving  of  problems,  in  other  words,  the 
directional  mind  set  toward  a  goal. 

Intelligence  in  animals  appears  to  be  correlated  with  a  definite 
development  of  the  cortical  layer  of  the  cerebrum.  Although  the  size 
and  weight  of  the  brain  have  little  to  do  with  intelligent  action,  the 
size  of  the  cerebrum  in  relation  to  the  rest  of  the  brain  is  definitely 
correlated  with  intelligence.  More  than  this,  the  number  of  convo- 
lutions in  the  surface  of  the  cerebrum,  with  a  consequent  increase 


THE   DISPLAY   OF   ENERGY  .•^8.j 

in  the  number  of  cortical  brain  cells,  has  a  decided  correlation  with 
the  degree  and  kind  of  intelligence  that  an  animal  shows.  A  comi)ar- 
ison  of  the  brains  of  normal  with  those  of  feeble-minded  individuals 
shows  that  in  the  latter  the  number  or  depth  of  the  cerebral  convo- 
lutions is  much  less  than  in  the  former,  thus  giving  an  anatomical 
evidence  for  differences  in  intelligence  in  man. 
As  one  of  the  authors  ^  has  said. 

"With  an  increase  of  cerebral  function  the  instinctive  reflexes  take  more 
and  more  to  the  background,  and  therein  is  a  great  distinction  between 
'  lower '  animals  which  are  largely  at  the  mercy  of  their  environment  and 
heredity,  and  the  '  higher  '  animals,  which  to  an  increasing  degree  have  risen 
above  environmental  conditions,  and  become  more  and  more  '  the  captains 
of  their  souls.'  The  most  prized  possession  of  mankind  is  the  '  capacity  for 
individuality,'  yet  even  what  passes  for  '  free  will '  has  its  basis  in  the  neurons 
and  reflexes  built  up  in  the  brain,  that  after  all  must  be  regarded  as  the 
mechanism  through  which  consciousness,  memory,  imagination,  and  will  are 
affected,  rather  than  as  the  seat  of  these  manifestations  of  intellectual  life." 

Types  of  intelligence  differ  widely  in  the  animal  scale.  The  so- 
called  "Gestalt"  psychologist  would  consider  modified  or  conditioned 
behavior  as  evidence  of  some  degree  of  intelhgence.  Perkins  and 
Wheeler  have  shown  that  goldfish  could  be  trained  to  make  correct 
responses  to  light  of  various  intensities  even  when  the  absolute  in- 
tensities of  the  lights  were  changed  as  well  as  their  positions.  Scores 
of  similar  experiments  performed  wuth  higher  animals  could  be  cited 
to  show  adaptative  configurational  behaviors.  If,  however,  w'e  take 
the  criteria  given  in  the  above  paragraphs  it  would  seem  that  memory 
and  a  synthesis  of  previous  action  are  necessary  to  the  possession  of 
true  intelligence.  The  "hold-up"  bear  of  Yellowstone  Park  appears 
to  be  intelligent  when  it  lumbers  out  of  the  forest  and  holds  up  the 
passing  autoist  for  candy.  It  simply  associates  the  moving  cars  and 
their  contents  with  sweets.  Probably  chance  started  it  on  its  nefar- 
ious career.  A  dog  taught  to  do  certain  tricks  seems  intelligent  but 
has  simply  formed  associations  between  the  food  given  as  a  reward 
and  the  act  learned.  A  dog  which  welcomes  its  master  after  a  long 
absence  probably  does  not  remember  or  have  a  deep  attachment 
for  its  master,  but  simply  responds  to  a  blind  though  increasing  urge 
brought  about  by  a  stimidus  pattern  in  which  associations  exist 
between  master  and  food,  or  some  other  goal. 

1  From  Walter,  H.  E.,  Biology  of  the  Vertebrates,  p.  631.  By  permission  of  Tiie  Macmillan 
Company,  publishers. 


386 


THE   MAINTENANCE   OF  THE   INDIVIDUAL 


Intelligence  of  Apes 

Because  of  their  relationship  to  man,  the  higher  apes  have  been  the 
source  of  much  fruitful  experimentation  of  late  years.  Kohler  ^  has 
demonstrated  that  the  chimpanzee  shows  evidence  of  emotionalized 
response  as  well  as  a  comparatively  high  degree  of  intelligence. 
A  chimpanzee  shows  emotion  not  only  by  actions,  but  also  in  facial 


Yale  LaboTotOTies  of  Primate  Biology 

Chimpanzees  are  the  most  emotional  as  well  as  intelligent  of  the  apes. 

expression.  The  ape  jumps  up  and  down  to  show  excitement,  knocks 
its  head  on  the  floor  of  the  cage  when  unable  to  solve  a  difficult 
problem,  or  looks  vacuously  into  space  and  smiles  when  lost  in  con- 
templation of  some  object  that  interests  it.  Yerkes  ^  shows  that 
chimpanzees  have  wide  differences  in  emotional  or  intelligent  conduct. 
One  may  be  gloomy,  another  happy,  one  lethargic,  another  active, 
one  dull  mentally,  and  another  bright.  They  may  be  as  tempera- 
mental as  some  human  beings  or  just  as  stoical.  They  also  show 
great  differences  in  mentality  and  like  man  have  their  "off"  days. 

Kohler  describes  one  series  of  experiments  which  show  that  apes 
have  intelligence  to  solve  problems  difficult  enough  to  test  the  inge- 
nuity of  a  young  human  child.  The  ape  Koko  was  the  subject. 
In  his  cage  was  placed  a  box  and  from  the  top  of  the  cage  a  banana 
was  suspended  well  out  of  reach.     The  ape  first  tried  jumping  for  the 


1  Kohler,  W.,  The  Mentality  of  Apes,  Kegan,  London,  1924. 

2  Yerkes,  R.  M.,  and  Learned,  B.  W. :    Chimpanzee  Intelligence  and  Its  Vocal  Expression. 
liams  and  Wilkins  Co. 


Wil- 


THE   DISPLAY   OF   ENERGY  387 

fruit,  but  finding  this  did  not  work,  approached  the  l)ox  and  gave  it 
a  push  toward  the  food,  looking  meanwhile  at  the  banana.  Dr.  Kohler 
then  made  the  goal  more  interesting  by  adding  a  piece  of  an  orange. 
After  a  brief  pause,  Koko  went  back  to  the  box,  pushed  it  vigorously 
until  it  was  directly  under  the  fruit,  then  climbed  up  on  the  box  and 
got  his  reward.  The  same  problem  was  given  Chica,  another  ape. 
This  was  solved  successfully  several  times  until  one  day  her  com- 
panion Teserca  was  resting  on  the  box.  While  this  was  happening 
Chica  jumped  in  vain  for  the  fruit,  finally  giving  up  in  despair  though 
not  attempting  to  use  the  box.  Presently  Teserca  got  down  from  the 
box.  At  once  Chica  dragged  the  box  under  the  fruit,  climbed  up,  and 
got  her  reward.  Evidently  the  box  on  which  Teserca  was  resting 
meant  to  Chica  something  to  "rest  on"  and  not  until  the  box  alone 
was  seen  with  the  fruit  did  it  mean  "something  with  which  to  get  the 
fruit."  This  simple  problem  was  made  more  difficult  by  raising  the 
fruit  to  a  greater  height  and  adding  three  boxes  which  had  to  be  piled 
one  on  the  other  before  the  fruit  could  be  reached.  Such  a  problem 
was  solved  by  Sultan,  an  ape  of  unusual  intelligence.  Yerkes  ^  made 
a  similar  experiment  with  the  gorilla  Kongo  in  which  three  boxes  had 
to  be  stacked  in  order  to  reach  food.  K  year  after  the  successful 
solution  of  this  problem,  the  gorilla  was  furnished  with  a  similar 
problem,  the  three  boxes  being  of  slightly  different  size.  The  problem 
was  solved  immediately,  thus  showing  evidences  of  memory. 

The  most  difficult  problem  of  all  was  solved  by  Dr.  Kohler's  ape 
Sultan.  Food  was  placed  just  out  of  reach  outside  the  bars  of  the 
cage  and  Sultan  was  given  two  sticks,  one  of  which  would  fit  into  the 
other.  Sultan  made  a  good  many  useless  and  rather  stupid  move- 
ments before  he  finally  "got  the  idea"  with  the  aid  of  the  experi- 
menter, who  had  put  one  finger  into  the  hole  of  one  stick  while  holding 
the  stick  close  to  the  animal.  Sultan,  after  playing  with  the  sticks, 
got  the  two  sticks  in  a  straight  line  and  at  once  pushed  the  thinner  one 
into  the  opening  of  the  thicker  one.  Once  having  made  a  long  pole 
with  the  two  sticks,  he  immediately  drew  the  banana  into  the  cage 
and  was  so  well  pleased  with  his  performance  that,  without  waiting  to 
eat  the  fruit,  he  proceeded  by  means  of  the  double  stick  to  pull  in 
other  pieces  of  fruit.  The  second  time  the  experiment  was  tried 
Sultan  almost  immediately  stuck  one  stick  into  the  other  and  got 
the  fruit.  In  a  later  experiment  he  was  given  two  similar  sticks  the 
smaller  of  which  was  a  little  too  large  to  go  into  the  hole  of  the  latter. 

1  Yerkes,  R.  M.,  "  The  Mind  of  the  Gorilla."     Comp.  Psy.  Mon.,  1928,  Vol.  V,  No.  2. 


;588  THE   MAINTENANCE   OF  THE   INDIVIDUAL 

Sultan  chewed  the  smaller  stick  down  into  a  wedge  and  then,  inserting 
it  into  the  larger  hollow  stick,  proceeded  to  get  the  fruit.  This  is  a 
degree  of  intelligence  such  as  might  be  seen  in  primitive  cave  men  who 
chipped  stones  to  make  weapons,  or  hollowed  out  trees  to  make  canoes. 

Intelligence  in  Man 

Man,  however,  is  a  long  step  above  the  ape  because  he  not  only 
can  do  things  that  the  ape  can,  but  in  addition,  he  has  memory 
which  enables  him  to  make  complex  abstractions  and  to  think  of 
objects  and  things  which  are  not  present.  This  ability  to  form  com- 
plex abstractions  and  to  use  them  in  thinking  are  things  that  an  ape 
never  could  do.  As  Herrick^  has  well  said,  "The  chimpanzee  does 
not  know  the  meaning  of  F-  =  2  PX,  and  he  never  can  find  out  J' 
In  addition,  man  has  a  tool  which  the  apes  cannot  use,  and  that  is 
language.  One  ape  has  been  taught  a  very  few  words,  but  it  is 
doubtful  whether  these  words  have  any  meaning  to  him.  The  reader 
of  these  lines  not  only  can  see  the  printed  word,  but  can  understand 
the  meaning  of  the  symbols  employed  and  can  express  it  in  terms  of 
speech.  He  has  traveled  a  long  way  further  than  the  apes  because 
he  can  read,  write,  and  speak. 

The  Measurement  of  Intelligence 
Most  young  people  today  hear  a  good  deal  about  "I.  Q's."  Nu- 
merous tests  have  been  devised  which  are  supposed  to  measure  the 
intelligence  of  the  human  being.  The  experts  who  prepared  the  tests 
have  established  norms,  or  average  scores,  for  different  ages.  The 
I.  Q.,  or  intelligence  quotient,  is  found  by  establishing  a  ratio  between 
the  mental  age  (M.  A.)  and  the  chronological  age  (C.  A.)  of  the  subject. 
If,  for  example,  a  child's  chronological  age  is  9  and  he  makes  a  score 
which  is  that  of  a  child  of  10,  his  I.  Q.  is  found  by  dividing  his  mental 
age  (M.  A.)  by  his  chronological  age  (C.  A.)  and  multiplying  by  100. 
In  this  case  he  would  have  ^  X  100,  or  an  I.  Q.  of  111.  An  I.  Q. 
of  from  90  to  1 10  is  about  normal.  If  a  person  has  over  140  I.  Q. 
he  is  considered  to  be  a  genius,  only  about  1  per  cent  of  all  persons 
falling  in  this  group.  A  glance  at  the  chart  shows  the  normal  dis- 
tribution of  intelligence  as  foimd  by  testing  large  numbers  of  people. 
While  the  tests  now  used  are  far  from  perfect,  testing  factual  knowledge 
rather  than  ability  to  think,  they  do  indicate  in  most  cases  intelligence 
with  reference  to  the  subject  tested,  and  so  fulfill  a  practical  purpose. 

'  Herrick,  C.  J.,  Brains  of  Rats  and  Man.     Univ.  of  Chicago  Press,  1925. 


THE   DTSPI.AY   OF   ENKRCY 


:\m 


257, 

- 

- 

''^^9/ 

20% 



^B 

<n    15?. 

~- 

^^B 

tfl 

/////////////////// 

'o 

- 

'fM/, 

^^^mw 

'Wf/A 

V  lor. 

^P 

^^X§MM 

?    .■St 

- 

w 

^^^B 

'W§/ 

^^^^B 

-_      01 

'$m 

W///A 

"Mm, 

W/W/a 

W////// 

mmm, 

25'. 


14% 


5% 
2% 


I.Q         69and below  70-79       80-89         90-99      I00-IO9     ilO-H9      120-129     laoondtip 

Distribution  of  intelligence  in  school  children.      I.  Q.'s  are  shown  below  graph. 

SUGGESTED  READINGS 

Caldwell,  0.  W.,  Skinner,  0.  E.,  and  Tietz,  J.  W.,  Biological  Foundations  of 

Education,  Ginn  &  Co.,  1931.     Chs.  XIV,  XV. 

An  elementary  luit  valuable  text. 
Kohler,  Wolfgang,  The  Mentality  of  Apes,  Harcourt,  Brace  and  Co.,  1926. 

Fascinating  reading. 
Pavlov,  I.  P.,  Lectures  on  Conditioned  Reflexes,  International  Publishing  Co., 

N.  Y.,  1928, 

Authoritative  and  based  on  experimental  evidence. 
Walter,  H.  E.,  Biology  of  the  Vertebrates,  The  Macmillan  Co.,  1929.     Chs. 

XIX,  XX. 

A  well-established  authority  easily  read. 
Wells,  H.  G.,  Huxley,  J.  S.,  Wells,  C.  P..  The  Science  of  Life,  Doubleday, 

Doran  &  Co.,  1934.     Book  VIII. 

Interesting  discussion  of  consciousness  in  animals. 
Wheeler,  R.  H.,  and  Perkins,  T.  H.,  Principles  of  Mental  Development,  The 

Thomas  Y.  Crowell  Co.,  1932.     Chs.  Ill,  V. 

An  excellent  exposition  of  gestalt  psycholog>\ 
Yerkes,  R.  M.,  and  Learned,  B.  W.,  Chimpanzee  Intelligence  and  Its  Vocal 

Expression,  Williams  and  Wilkins  Co.,  1925. 
Yerkes,  R.  M.,  and  Yerkes,  A.,  The  Great  Apes,  Yale  University  Press,  1929. 

Both  of  Dr.  Yerkes'  books  give  the  latest  experimental  work  on  the  emo- 
tional and  mental  life  of  the  apes. 


XVIII 


CHEMICAL  REGULATORS 

Preview.  Chemical  co-ordination  •  Regulators  of  digestive  processes  • 
Regulators  of  general  metabolism  :  Adrenals,  thyroid,  parathyroids,  pan- 
creas ■  Growth  regulators  :  Thyroid  ;  gonads  and  pituitary ;  pineal  •  Repro- 
ductive organs  as  regulators  •  The  master  gland  or  "generalissimo,"  the 
pituitary :  the  anterior  lobe,  growth  stimulation,  gonad  stimulation,  lacta- 
tion hormone,  thyreotropic  hormone,  adrenotropic  hormone,  blood  sugar 
raising  principle,  fat  metabolism-regulating  principle,  parathyreotropic 
principle  ;   the  intermediate  lobe ;   the  posterior  lobe  •  Suggested  readings. 

PREVIEW 

Co-ordinating  devices  are  necessary  as  soon  as  cells  become  grouped 
together  in  large  enough  masses  to  isolate  the  inner  ones  from  external 
stimuli.  As  the  cell  mass  increases  in  size,  there  is  a  tendency  for 
greater  division  of  labor  to  be  developed,  and  we  find  organisms  evolv- 
ing with  special  tissues  to  perform  specific  functions.  These  tissues  in 
turn  are  woven  into  more  complex  systems  that  call  for  a  still  greater 
division  of  labor. 

Probably  the  chief  co-ordinating  mechanism  which  keeps  the 
organism  in  touch  with  its  external  environment  is  the  nervous 
system.  Even  the  primitive  nerve  net  of  the  coelenterates  serves 
quite  adequately  in  this  capacity,  while  the  linear  type  of  nervous 
system  with  its  more  highly  specialized  co-ordinating  centers  fur- 
nishes a  more  complex  and  efficient  mechanism  in  the  higher  forms. 
There  is  another  equally  important,  though  far  less  thoroughly  under- 
stood mechanism  that  acts  as  an  "internal  co-ordinator,"  since  both 
nervous  and  chemical  correlation  is  necessary  to  secure  a  symmetrical 
development  and  orderly  functioning  of  the  related  parts. 

The  study  of  chemical  co-ordination  is  a  field  literally  bristling 
with  thousands  of  unanswered  questions  and  holding  promise  of 
becoming  one  of  the  most  productive  phases  of  modern  physiological 
and  medical  research.  Within  its  pages  are  already  told  some  of 
the  most  thrilling  tales  of  intellectual  adventure  one  could  hope  to 
encounter.  Only  a  few  of  these  will  be  enumerated,  but  we  might 
well  seek  an  answer  to  such  questions  as :  What  are  the  controlling 
devices  of  the  body  for  producing  and  regulating  normal  growth? 

390 


CHEMICAL   REGULATORS  :591 

What  starts  and  governs  the  changes  in  voice  and  body  that  accom- 
pany the  maturing  of  reproductive  systems?  Wluit  is  the  explana- 
tion of  that  last  little  ounce  of  strength  which  enables  a  sprinter 
to  put  on  the  final  burst  of  speed?  How  can  such  correlation  be 
possible  without  the  existence  of  a  ''master  mind"  for  the  l)ody? 
Answers  to  these  questions  will  be  found  in  the  pages  that  follow. 

Chemical  Co-ordination 

Our  knowledge  of  chemical  regulation  of  the  body  is  far  from 
complete.  Workers  in  the  field  of  endocrinology,  as  in  other  fields, 
are  continuously  pushing  back  the  frontiers  of  ignorance,  at  best  a 
slow  process.  Nevertheless,  each  year  new  excitants,  or  hormones, 
are  discovered,  their  effect  noted,  and  their  refinement  or  synthesis 
accomplished.  Rarely  there  occurs  the  discovery  of  the  existence  of 
a  new  and  hitherto  unsuspected  gland  that  produces  one  of  these 
hormones.  Thus  far  we  know  quite  definitely  that  the  thyroids, 
parathyroids,  pituitary,  gonads,  liver,  placenta,  adrenals,  pancreas, 
the  mucosa  of  the  stomach  and  intestine,  and  possibly  the  pineal 
and  thymus  glands,  function  as  ductless  or  endocrine  glands.  In 
some  instances  more  evidence  is  needed,  but  on  the  whole  a  ma- 
jority of  scientists  are  in  agreement  regarding  this  list. 

Early  zoologists,  including  such  leaders  as  Johannes  Miiller  and 
Jakob  Henle,  failed  to  attach  enough  significance  to  the  ductless 
glands.  According  to  Rogers,^  the  former  stated  that  "the  ductless 
glands  are  alike  in  one  particular  —  they  either  produce  a  change  in 
the  blood  which  circulates  through  them,  or  the  lymph  which  they 
elaborate  plays  a  special  role  in  the  formation  of  blood  or  chyle." 

Probably  the  first  experimental  study  in  endocrinology  was  made 
by  A.  A.  Berthold  of  Gottingen  in  1849  when  he  began  a  study  of 
the  results  following  the  removal  of  the  testes  of  fowls.  Shortly  after 
this  Claude  Bernard,  Addison,  and  Brown-Sequard  made  significant 
contributions.  The  first  of  these  investigators  worked  on  the  liver, 
while  the  other  two  studied  the  adrenals.  Brown-Sequard  actually 
extirpated  the  adrenal  glands  and  noted  that  the  accomjjanying 
weakness  and  death  could  be  prevented  by  transferring  blood  from  a 
normal  animal  to  the  one  from  which  the  adrenals  had  been  removed. 

From  the  physiological  point  of  view,  endocrine  glands  may  be  di- 
vided into  five  large  groups  as  regulators  of  :  (1)  digestion  ;  (2)  general 

'  Quoted  by  permission  of  the  publishers  from  Rogers,  C.  G.,  Textbook  of  Comparative  Phyni- 
oiogy,  p.  361.     McGraw-Hill  Book  Company,  1927. 
H.   W.    H.  —  26 


392  THE   MAINTENANCE  OF  THE   INDIVIDUAL 

metabolism  ;  (3)  growth  ;  (4)  reproduction  ;  and  (5)  the  master  center 
which  serves  as  the  "generahssimo"  of  all  the  endocrines.  Such  an 
arrangement  means  that  a  gland  which  produces  more  than  one 
hormone  may  have  to  be  considered  in  more  than  one  category. 

Regulators  of  Digestive  Processes 

Little  can  be  added  to  the  description  of  the  hormone  secretin 
except  to  point  out  that  physiologists  are  somewhat  uncertain  as  to 
whether  the  hormone  is  first  produced  as  an  inactive  substance  called 
prosecretin,  or  as  the  active  agent  known  as  secretin.  The  action  of 
this  internal  co-ordinating  mechanism  may  be  seen,  for  example, 
in  the  secretion  of  the  pancreatic  juice  which  is  always  poured  at 
apparently  just  the  proper  time  into  the  small  intestine.  At  first, 
this  co-ordination  was  believed  to  be  due  to  some  sort  of  undiscovered 
nervous  reflex  mechanism  that  was  stimulated  as  the  food  passed  a 
given  point.  This  interpretation  was  discounted  by  two  promi- 
nent English  physiologists,  Bayliss  and  Starling,  who  in  1902  showed 
that  it  is  the  passage  of  the  acidulated  food  past  the  pylorus  into  the 
upper  part  of  the  small  intestine  {duodenum)  which  stimulates  the 
production  of  a  hormone,  secretin.  This  substance  is  absorbed  by 
the  blood  and  carried  throughout  the  body,  the  portion  reaching  the 
pancreas  furnishing  the  necessary  stimulus  to  effect  the  release  of  its 
digestive  enzymes. 

Regulators  of  General  Metabolism 

Adrenals 

The  paired  adrenal  gland  is  composed  of  an  outer  cortex  and  an 
inner  medulla,  each  part  having  a  different  embryonic  origin  and  pro- 
ducing a  different  hormone.  In  the  lower  vertebrates  the  cortex  is 
represented  by  an  elongated  mass  of  glandular  tissue  called  the 
interrenal,  lying  between  the  kidneys  and  derived  embryologically 
from  the  lining  of  the  body  cavity.  The  medulla  on  the  other  hand 
is  at  first  a  separate  structure,  composed  of  so-called  chromaffin 
cells,  which  have  their  origin  in  the  nervous  tissue  of  the  autonomic 
nervous  system.  In  the  higher  vertebrates  the  interrenal  and  the 
chromaffin  cells  become  incorporated  to  form  the  adrenal  gland. 

The  outer  portion,  or  the  cortex  of  the  adrenals,  secretes  a  hormone 
known  as  cortin  which  has  been  proved  to  be  essential  to  life.     If 


CHEMICAL   REGULATORS 


303 


pancreatio 
IslocncCs 


there  is  a  deficiency  of  this  hormone  in  the  human  body  heart  action 
slows  down,  the  skin  becomes  discolored,  and  the  vital  energy  is 
overcome  by  a  growing,  and  usually  fatal  lassitude,  symptoms 
characteristic  of  Addison's  disease.  Biologists  and  the  medical  pro- 
fession were  led  to  this  conclusion  as  to  the  effects  of  adrenal  hormones 
through  numerous  observations  and  experiments.  Swingle,  of 
Princeton,  recounts  how  cats 
with  extirpated  adrenals 
barely  survived  eight  to  ten 
days.  During  this  time  their 
temperature  fell  six  to  seven 
degrees.  Yet  such  animals, 
at  the  brink  of  death,  were 
saved  and  restored  to  ap- 
parent health  within  seventy 
hours  by  the  subcutaneous 
injection  of  beef  cortin. 

Cortin  appears  to  have  an- 
other property,  namely,  to 
stimulate  the  development  of 
the  sex  organs.  This  has  been 
shown  by  a  series  of  experi- 
ments on  young  male  rats, 
in  which  the  injected  animals 
showed  a  much  more  rapid 
growth  of  the  sex  organs  than 
the  controls.  These  studies 
suggested  that  the  occasional 
precocious  sexual  develop- 
ment of  young  children  may  be  due  to  an  over-enlargement  of  the 
adrenals  through  the  presence  of  a  tumor  either  in  the  cortex  of  the 
gland  or  in  the  pituitary  gland  which  largely  regulates  general 
endocrine  balances.  Young  girls  under  similar  conditions  develop 
masculine  characters.  More  or  less  complete  cases  of  the  reversal 
of  secondary  sexual  characteristics  in  women  are  on  record,  in  a  few 
of  which  the  removal  of  tumors  involving  the  adrenals  has  restored 
a  normally  characteristic  feminine  condition. 

The  secretion  of  the  medulla  or  inner  portion  of  the  adrenal  gland  has 
been  known  to  science  for  some  time  as  adrenin  or  epinephrine. 
This  hormone  was  first  isolated  by  Takamine  in  1901  and  has  since  been 


The  location  of  the  ductless  glands. 


391 


IHE   MAINTENANCE   OF  THE   INDIVIDUAL 


synthesized.  Its  effect  is  very  interesting.  It  is  known  that  small 
amounts  of  adrenin  are  being  continuously  secreted  and  passed  into 
the  blood  stream  to  have  an  effect  upon  the  involuntary  muscles 
of  the  body.  In  cases  of  emotional  excitement  there  is  an  increased 
secretion  of  adrenin,  in  consequence  of  which  there  results  a  more 
rapid  heartbeat,  together  with  an  increase  of  blood  flow  and  of  the 
glucose  output  from  the  liver.  This  in  turn  brings  about  greater 
efficiency  of  the  muscles  and  so  increases  the  capacity  for  work.  If 
this  portion  of  the  gland  is  not  operating  normally  such  symptoms 

as  muscular  fatigue,  cold  hands 


and  feet,  sensitiveness  to  cold, 
mental  indecision,  and  sometimes 
collapse  and  heart  failure  ensue. 
Adrenin  is  efficacious  in  reliev- 
ing severe  bronchial  spasms  dur- 
ing attacks  of  asthma  and  it  has 
also  been  successfully  used  to 
mitigate  the  distress  caused  by 
hives  and  by  hay  fever. 


.thyroid  cartilage 

pry  amid    lobe 
of-^VnyroicL 

-P  (xra-thyro  i  cC 

-isLhrotcS 
-left  lobe-^of 
thyroid.  ^lancC 
-pa.rat  by  ro  i  cC 


-trcLCh 


ecc 


viewed,   from  |i'«nt/ 

Diagram  showing  the  location  and 
relationship  of  the  thyroid  and  para- 
thyroid glands. 


Thyroid 

Some  sort  of  thyroid  gland  is 
present  in  all  of  the  vertebrates. 
In  every  instance  it  arises  as  an 
outgrowth  from  the  pharyngeal 
region  and  is,  therefore,  a  deriva- 
tive of  the  digestive  tract.  In  man  the  thyroid  is  definitely  bilobed 
and  in  cases  of  goiter  may  be  considerably  enlarged. 

The  secretion  of  the  thyroid  gland,  thyroxin  (C11II10O3NI3),  was  first 
isolated  by  Kendall  in  1914  and  later  improved  isolation  methods  gave 
Ci5Hii04NI,i  (Harrington,  1926).  Under  normal  conditions  but  little 
of  this  substance  is  secreted  at  a  time,  in  evidence  of  which  is  the  fact 
that  three  and  one-half  tons  of  fresh  thyroid  glands  are  necessary  to 
produce  36  grams  of  crystalline  thyroxin.  This  substance  regulates 
the  rate  of  the  transformation  of  energy  in  the  body,  thus  controlling 
the  metabolic  rate.  Its  potency  is  almost  uncanny,  as  is  evidenced 
by  the  fact  that  one  milligram  of  thyroxin  produces  a  two  per  cent 
increase  in  the  total  oxidation  of  a  resting  adult  body. 

One  concept  of  the  rate  of  metabolism  in  the  human  body  may  be 
secured  through  the  basal  metabolism  test,  a  device  to  measure  the 


CHEMICAL   REGULATORS 


39- 


oxygen-carbon  dioxide  Ixilance,  which  determines  the  amount  of 
energy  required  to  keep  th(>  body  aHve,  maintain  its  temjierature, 
muscle  tone,  rate  of  breathing,  and  heartbeat.  It  has  been  con- 
ckisively  shown  by  a  sufficient  number  of  studies  that  a  comparison 
of  people  who  have  been  placed  under  similar  conditions  may  be 
made,  and  it  is  now  pos- 
sible, as  a  result  of  these 
tests,  to  gain  a  highly 
accurate  idea  of  the  meta- 
bolic rate  of  different 
people,  and  so  to  detect 
an  over-  or  under-activity 
of  the  thyroid  gland. 
Both  conditions  are  ab- 
normal, indicating  rather 
serious  metabolic  malad- 
justments. 

If  the  thyroid  is  over- 
active, a  person  so  affected 
usually  has  a  high  basal 
metabolic  rate.  Such  a 
person  finds  his  combus- 
tion rate  speeded  up  and 
is  a  heavy  eater,  but  at 
the  same  time  that  he 
burns  his  food  products 
rapidly,  he  gradually  be- 
comes weaker  and  weaker.  Evidence  of  a  high  metabolic  rate  shows 
further  in  nervousness  and  irritability.  The  individual  is  also  char- 
acterized by  protruding  eyeballs,  an  increased  and  more  irregular 
heartbeat,  as  well  as  a  higher  temperature,  insomnia,  and  general 
nervousness,  which  in  advanced  cases  may  seriously  undermine  both 
the  mind  and  health.  This  general  picture  of  overactivity  is  typically 
associated  with  the  variety  of  goit(T  known  as  exophthalmic  goiter. 

Another  type  of  goiter,  "common  goiter,"  is  frequently  encoun- 
tered in  regions  wdiere  there  is  a  material  lack  of  iodine  in  the  water  and 
soil.  In  such  cases  there  is  an  insufficient  supply  of  thyroxin  secreted, 
which  is  sometimes  due  to  a  decrease  of  iodine  in  the  chemical  com- 
position of  the  thyroxin  molecule.  Nature  apparently  endeavors  to 
compensate  for  this  by  increasing  the  size  of  the  gland  with  rather 


Xew  York  AcfuUmy  of  Medicine 

An  example  of  a  goiter.     What  type  is  it .3 
What  caused  it  !> 


:}96  THE  MAINTENANCE  OF  THE   INDIVIDUAL 

grotesque  results.  Fortunately  this  condition  may  l)e  alleviated  in 
the  early  stages  by  the  addition  of  iodine  to  the  diet,  or  in  advanced 
cases,  by  extirpation  of  a  portion  of  the  gland. 

Occasionally  an  individual  gives  evidence  of  an  underactivity  of 
the  thyroid  gland.  In  such  a  case,  the  amount  of  thyroxin  produced 
by  the  gland  is  actually  decreased.  Well-defined  and  characteristic 
symptoms  result.  Ingested  food  is  not  utilized,  with  the  result  that 
the  excess  is  soon  deposited  as  fat,  and  definite  obesity  becomes  visible. 
Certain  other  symptoms  are  quite  characteristic,  as  a  slowing  down  of 
the  mental  and  nervous  activities,  which  may  result  finally  in  feeble- 
mindedness or  imbecility.  If  the  thyroid  gland  be  hereditarily  de- 
fective or  non-functional  a  lamentable  condition  known  as  cretinism 
develops.  In  such  cases  skeletal  development  is  arrested  and  a  stunted 
misshapen  individual  results  since  normal  growth  becomes  impossible. 
This  condition  is  alleviated  by  administering  thyroxin. 

Parathyroids 

The  parathyroid  glands,  likewise  outgrowths  of  the  pharyngeal 
region  of  the  body  of  nearly  all  vertebrates,  vary  slightly  in  number 
and  position  with  the  form  under  consideration.  In  man,  there  are 
normally  four  parathyroid  glands  having  a  total  weight  of  not  over 
0.4  gram.  Nevertheless,  they  persist  during  life  and  are  now  known  to 
play  a  very  important  part  in  maintaining  the  calcium  balance  of  the 
body,  and  by  means  of  it,  the  irritabiUty  of  the  cells.  The  active 
hormone  of  the  parathyroid  glands  was  demonstrated  by  Hanson  in 
1925  and  later  isolated  by  Collip. 

While  the  exact  nature  and  method  of  the  functioning  of  these 
glands  is  not  thoroughly  understood,  it  is  known  that  their  removal 
is  usually  fatal.  The  first  effect  of  the  removal  of  these  glands  is  that 
the  calcium  salts  are  reduced  and  the  threshold  of  stimulation  thereby 
lowered ;  the  peripheral  nerves  and  muscles  of  the  organism  become 
more  irritable  and  the  various  reflexes  become  extreme.  Severe 
muscular  contraction,  or  tetanus,  finally  results  and  the  death  of  the 
organism  usually  ensues  unless  calcium  is  added  to  the  blood.  This 
may  be  done  by  the  injection  of  the  hormone  or  solutions  of  calcium 
salts. 

Pancreas 

While  the  pancreas  has  long  been  recognized  as  a  gland  secreting 
various  important  digestive  enzymes,  it  was  not  until  1889  that  it 


CHEMICAL   REGULATORS  397 

was  shown  to  have  an  equally  important  role  as  a  ductless  gland, 
producing  hormones.  Von  Mering  and  Minkowski  showed  that  its 
extirpation  was  followed  in  all  cases  by  the  appearance  of  sugar  in 
the  urine.  Evidence  has  accumulated  indicating  that  the  oval  or 
spherical  islands  of  Langerhans,  that  are  embedded  in  the  pancreatic 
tissue,  are  the  source  of  the  hormone  now  called  insulin,  which 
regulates  the  sugar  metabolism  of  the  body. 

The  story  of  the  long  struggle  of  scientists  to  demonstrate  the 
existence  of  this  hormone  is  a  fascinating  one.  Banting,  Best,  and 
Alacleod  ^  in  1921  gave  the  first  successful  demonstration  of  the 
isolation  of  insulin.  We  now  know  that  the  general  physical  and 
mental  condition  of  people  suffering  from  diabetes  can  be  markedly 
improved  through  the  administration  of  this  hormone.  While  the 
exact  nature  of  the  reaction  is  not  fully  understood,  it  is  certain  that 
the  amount  of  sugar  in  the  blood  stream  is  reduced  sharply  after  the 
injection  of  insulin. 

Growth  Regulators 

Thyroid 

One  must  think  of  the  thyroid  as  a  gland  with  a  dual  function.  We 
have  already  noted  the  effect  which  the  secretions  of  this  gland  have 
upon  general  metabolism.  The  second  effect  is  its  influence  upon 
growth.  When  the  thyroid  is  removed  in  young  dogs,  for  example,  a 
retardation  of  growth  occurs  in  a  few  weeks.  These  experiments 
substantiate  observations  made  upon  children  with  congenital  lack 
of  thyroid. 

Gonads  and  Pituitary 

As  will  be  noted  in  more  detail  later,  these  glands  are  both  associated 
with  growth,  and  play  an  imj^ortant  role  in  the  normal  development  of 
the  individual. 

Pineal 

Although  the  function  of  the  pineal  gland  is  not  clear,  it  should  be 
mentioned  at  this  point.  It  is  a  small  body  which  appears  in  nearly 
all  vertebrates  as  an  outgrowth  from  the  roof  of  the  'twixt-brain 
(diencephalon).  The  pineal  body  reaches  its  greatest  development  in 
man  at  about  the  seventh  year.     After  that  age,  and  particularly 

'  Banting,  Best,  Macleod.     Am.  Jour.  Physiol.,  59  :   479.     1922. 


:59«  THE   MAINTENANCE  OF  THE   INDIVIDUAL 

after  puberty,  it  undergoes  involution  and  finally  disappears,  its 
place  being  taken  by  fibrous  tissue.  While  there  is  some  evidence 
that  extirpation  of  the  pineal  gland  accelerates  the  development  of 
the  sexual  organs  of  the  male,  as  found  in  experiments  on  the  guinea 
pig,  its  functioning  is  still  a  moot  question. 

While  the  thymus  has  long  been  a  subject  of  controversy,  it  now 
appears  likely  to  many  students  that  this  gland  will  not  prove  to 
belong  to  the  endocrine  group. 

Reproductive  Organs  as  Regulators 

It  has  been  known  for  many  years  that  the  gonads  are  structures 
in  which  are  produced  the  eggs  and  spermatozoa  that  are  essential 
to  reproduction  in  most  forms  of  life.  However,  scientists  have 
learned  within  comparatively  recent  years  that  the  reproductive  or- 
gans function  also  as  ductless  glands,  producing  hormones  associated 
with  the  development  of  those  features  known  as  secondary  sexual 
characters.  One  group  of  these  hormones,  though  partially  under  the 
control  of  that  "generalissimo"  of  the  endocrines,  the  pituitary,  is 
really  responsible  for  the  normal  cyclical  functioning  of  the  sex 
glands.  Besides  producing  eggs  and  sperm,  the  ovaries  and  testes 
play  a  vital  part  in  the  development  of  those  mental  and  physical 
characteristics  which  constitute  maleness  or  femaleness.  The  exist- 
ence of  some  regulatory  mechanism  has  been  clearly  demonstrated 
in  various  animals  by  the  removal  of  the  sex  glands  and  the  subsequent 
failure  of  certain  secondary  sexual  characteristics  to  develop.  Nu- 
merous examples  might  be  cited.  Male  deer  (Cervidae),  for  instance, 
are  typically  adorned  with  antlers  that  are  annually  renewed.  A 
young  castrated  buck  fails  to  grow  antlers,  thus  suggesting  that  the 
key  to  this  phenomenon  lies  in  the  production  of  some  secretion  of 
the  testes. 

Many  other  experiments,  performed  in  recent  years  upon  the  lower 
vertebrates,  tend  to  support  the  idea  that  such  secretions  are  indis- 
pensable to  the  proper  development  of  many  male  and  female  char- 
acteristics. When  emasculated  male  rats  or  guinea  pigs  are  given 
ovarian  transplants,  the  skeleton  and  hair  soon  begin  to  resemble 
those  of  a  female  and  before  long  the  mammary  glands  enlarge  to 
functional  size.  These  results  suggest  that  the  effect  is  due  to  some- 
thing secreted  in,  or  by,  certain  cells  of  the  transplanted  gonad. 

Other  experiments  indicate  that  the  hormones  of  one  sex  dominate 
expression  of  the  other  sex.     Such  a  case  is  that  of  the  "free-martin," 


CHEMICAL   UEGULATOllS  399 

which  is  a  sterile  female  calf,  born  with  a  normal  male  twin.  Lillie 
discovered  in  these  instances  that  there  was  a  fusion  of  the  embryonic 
circulations  between  the  twins  and  that,  since  the  male  gonads  develop 
before  those  of  the  female,  the  male  hormone  appeared  first  in  the 
united  fetal  circulation  and  not  only  interfered  with  the  growth  of  the 
ovary  to  such  an  extent  as  to  cause  sterility,  but  even  caused  a  tend- 
ency toward  the  assumption  of  secondary  male  characters. 

Evidence  relating  to  a  second  type  of  secretion  associated  with  the 
rhythmical  recurrence  of  ovulation  in  the  female  of  all  vertebrates 
leads  to  the  belief  that  in  mammals  at  least  two  ovarian  hormones 
occur,  —  one  derived  from  the  follicular  cells  surrounding  the  egg 
before  it  escapes  from  the  ovary,  and  the  other  from  the  mass  of  cells, 
or  corpus  lutcum,  that  fills  the  follicle  after  rupture. 

The  cells  of  the  follicle  secrete  a  hormone  known  as  ocstrin  into  the 
follicular  fluid.  This  substance  has  the  dual  function  of  initiating 
some  changes  in  the  female  and  completing  other  reactions.  Ocstrin 
is  secreted  by  the  ovaries  of  all  vertebrates  which  have  been  studied  so 
far.  It  is  a  growth-promoting  hormone  which  governs  the  develop- 
ment of  the  secondary  sexual  characters,  including  the  reproductive 
tract  of  the  female,  while  the  corpus  luteum,  as  known  at  present,  is 
really  a  mammalian  gland  w^hich  has  appeared  in  association  with 
lactation  and  viviparity.  The  corpus  luteum  hormone,  progestin, 
prepares  the  uterus  for  the  reception  of  a  fertilized  egg,  and  if  one 
does  not  appear  the  corpus  luteum  involutes,  the  uterus  returns  to  a 
resting  condition,  and  a  new  cycle  is  started.  Progestin  quiets  the 
uterus  by  inhibiting  its  rhythmic,  spontaneous  contractions.  In  the 
strict  sense  the  corpus  luteum  may  be  regarded  as  a  gland  of  preg- 
nancy. Several  interesting  experiments  have  been  performed  on 
various  mammals.  It  is  well  known  that  the  mating  instinct  is  lost 
when  a  normal  female  is  spayed  (removal  of  ovaries).  Allen  and 
Doisy  were  able  to  produce  characteristic  cychcal  changes  in  the  genital 
tract  of  spayed  rats  and  mice  by  the  injection  of  the  hormone  from 
the  follicular  fluid. 

The  interstitial  cells  of  the  testes  evidently  yield  hormones  which 
produce  secondary  sexual  characters  in  a  castrated  male.  Much 
work  still  remains  to  be  done  on  this  point. 

The  Master  Gland  or  "Generalissimo,"  the  Pituitary 

The  pituitary  gland,  or  hypophysis,  might  well  be  regarded  as  the 
commander-in-chief  of  all  the  endocrine  glands.     Embryologically  the 


400  THE  MAINTENANCE  OF  THE   INDIVIDUAL 

anterior  part  of  the  gland  arises  as  a  dorsal  evagination  (Rathke^s 
pocket)  from  the  buccal  ectoderm,  while  the  posterior  part  develops 
as  a  downward  outgrowth  {infundihulum)  from  the  portion  of  the 
brain  (diencephalon)  lying  directly  over  the  mouth.  The  anterior 
outgrowth  in  man  finally  produces  the  anterior  lobe,  a  small  inter- 
mediate lobe,  and  a  thin  layer  extending  to  the  brain  as  the  pars 
tuberalis,  while  the  posterior  portion  forms  the  so-called  posterior 
lobe,  or  pars  nervosa. 

The  Anterior  Lobe 

There  appears  to  be  fairly  good  evidence  of  the  existence  of  at 
least  five  and  possibly  eight  hormones  produced  by  this  portion  of  the 
pituitary  gland.  It  is  probable,  since  the  histology  of  the  gland 
indicates  remarkably  little  diversity  of  tissue,  that  the  substances 
produced  are  very  closely  related  chemically. 

A.  Growth  Stimulation.  If  overactivity  of  tliis  portion  of  the 
gland  occurs  when  young,  giants  will  result.  On  the  other  hand  a 
similar  overactivity  arising  when  adult,  results  in  excessive  growth 
of  the  bones  of  hands,  feet,  and  face  —  a  condition  known  as  acro- 
megaly. The  intraperitoneal  injection  of  fresh  anterior  pituitary 
extracts  resulted  in  the  production  of  giant  rats.  Additional  evidence 
has  been  secured  through  the  autopsies  of  various  giants,  who  showed 
a  greatly  enlarged  pituitary. 

B.  Gonad  Stimulation.  During  comparatively  recent  years  it 
has  been  shown  that  anterior  pituitary  implants  will  produce  sexual 
precocity  in  sexually  immature  mammals.  This  operation  has  been 
performed  on  all  of  the  more  common  laboratory  mammals,  includ- 
ing cats,  dogs,  and  monkeys,  and  thus  far  holds  for  all  vertebrates 
studied.  In  the  female  such  implants  stimulate  the  development  of 
both  follicles  and  corpora  lutea,  which  are  associated  with  the  growth 
of  the  female  secondary  sexual  characters.  Implants  in  the  male 
stimulate  the  development  of  the  semeniferous  tubules  and  inter- 
stitial tissue  correlated  with  the  growth  of  male  secondary  characters. 
These  effects,  as  determined  by  hormone  isolation,  are  due  to  two 
hormones  secreted  by  the  anterior  pituitary,  —  one  which  stimulates 
the  growth  of  follicles  in  the  ovaries  and  tubular  growth  in  the  case 
of  the  male,  and  the  second  which  produces  the  formation  of  the  corpus 
luteum  and  the  secretions  of  the  interstitial  cells  of  the  testis. 

C.  Lactation  Hormone.  Knowledge  of  the  existence  of  this 
hormone  is  comparatively  recent.     Various  workers  reported  that 


CHEMICAL   REGULATORS  401 

they  were  able  to  induce  lactation  in  spayed,  virgin  rabbits,  which 
had  developed  mammary  glands  prior  to  the  operation,  througii 
the  injection  of  a  substance  secured  from  the  anterior  pituitary. 
Some  years  later  'prolactin  was  extracted  in  an  im|)uro  form  which, 
while  not  causing  development  of  the  mammary  gland,  nevertheless 
brought  about  the  onset  and  continuation  of  the  secretory  phase. 
Prolactin  \^^ls  effective  after  castration. 

D.  Thyreotropic  Hormone.  While  many  investigators  have 
demonstrated  a  close  relationship  between  the  pituitary  and  the 
thyroid  gland,  it  was  not  until  1927  that  the  pituitary  gland  of  the  rat 
was  removed  to  show  that  the  thyroid  is  dependent  upon  this  structure 
for  stimulation.  In  1933,  a  purified  extract  under  the  name  of  the 
thyreotropic  hormone  was  prepared. 

E.  Adrenotropic  Hormone.  It  was  shown  in  1930  that  if  the 
anterior  lobe  of  the  pituitary  is  removed  in  rats  atrophy  of  the  cortex 
of  the  adrenals  follows,  although  normality  may  be  restored  by  in- 
jecting pituitary  extracts.  Later  Houssay  and  his  co-workers  showed 
that  the  active  agent  in  such  experiments  is  a  product  of  the  an- 
terior lobe,  also  proving  the  existence  of  this  adrenotropic  principle. 

Most  biologists  now  concede  the  existence  of  these  five  hormones 
from  the  anterior  lobe  of  the  pituitary  gland.  Evidence  is  rapidly 
accumulating  which  supports  the  idea  of  the  existence  of  three  more, 
F  to  H. 

F.  Blood  Sugar-raising  Principle.  It  has  been  previously 
shown  that  the  removal  of  the  pancreas  results  in  the  appearance 
of  sugar  in  the  urine,  that  is,  experimental  diabetes  is  produced. 
Overactivity  of  the  pituitary,  as  acromegaly,  for  example,  is  usually 
associated  with  hyperglycemia  (over  the  normal  amount  of  sugar  in 
the  blood)  and  glycosuria  (sugar  in  the  urine).  Furthermore,  a  nor- 
mal animal  develops  the  same  condition  when  injected  with  anterior 
pituitary  extracts.  Now,  when  the  hypophysis  is  removed  hypo- 
glycemia results  and  the  animal  is  very  sensitive  to  insulin.  Also  it 
has  been  shown  that  if  both  the  anterior  pituitary  and  the  pancreas 
are  removed  the  experimental  diabetes  resulting  from  the  loss  of  the 
pancreas  is  greatly  decreased.  It  is  apparent,  then,  that  in  the 
absence  of  the  pancreas  the  anterior  pituitary  tends  not  only  to 
increase  the  blood  sugar  but  also  to  make  the  animal  sensitive  to 
insulin.  This  clearly  indicates  that  there  is  a  balance  between  these 
two  glands.  It  might  be  added  that  extracts  of  the  anterior  pitui- 
tary increase  the  blood  sugar  in  the  absence  of  the  pancreas,  thy- 


402  THE   MAINTENANCE   OF  THE   INDIVIDUAL 

roids,  adrenal  medulla,  and  sympathetic  system.  It  appears  quite 
conclusive,  therefore,  that  the  action  of  the  anterior  pituitary  hor- 
mone is  at  least  partially  direct. 

G.  Fat  Metabolism-regulating  Principle.  Several  groups  of 
experimenters  have  produced  evidence,  since  1930,  that  the  anterior 
lobe  of  the  pituitary  gland  also  produces  a  hormone  that  regulates 
the  metabolism  of  fats  in  the  body. 

H.  Parathyreotropic  Principle.  While  the  evidence  is  not 
irrefutable  there  are  some  grounds  for  believing  that  the  control  of  the 
parathyroids  is  made  possible  by  a  secretion  from  the  anterior  lobe  of 
the  pituitary  gland. 

The  Intermediate  Lobe 

This  portion  of  the  pituitary  gland  produces  a  hormone  known  as 
intermedin,  which  has  been  found  in  all  vertebrates  so  far  studied. 
The  effects  of  this  hormone  may  be  readily  demonstrated  in  frogs  and 
other  amphibians.  At  the  present  time  its  function  in  mammals  is 
not  known. 

The  Posterior  Lobe 

The  posterior  lobe  of  the  pituitary  gland  consists  of  contributions  by 
the  pars  nervosa  and  the  pars  intermedia.  It  is  possible,  therefore, 
that  its  products  may  contain  secretions  from  both  sources.  Two 
fractions  have  been  isolated  from  the  posterior  lobe,  called  respectively 
pitressin  and  pitocin.  However,  much  work  on  these  hormones  still 
remains  to  be  done  before  the  various  effects  noted  on  the  cardio- 
vascular, respiratory,  uterine,  renal  organs,  and  the  smooth  muscle- 
tissue  of  the  intestine  and  mammary  glands  are  proved  to  be  due  to 
one  or  to  several  discrete  fractions.  Several  characteristic  reactions, 
however,  might  be  noted.  First  there  is  the  pressor  effect  which  is  char- 
acterized by  an  increased  blood  pressure  and  a  decreased  heart-rate. 
Injections  of  the  posterior  lobe  cause  an  increased  secretion  of  urine 
and  also  bring  about  a  contraction  of  the  plain  muscle  of  the  uterus. 
This  latter  action  has  been  made  use  of  by  the  medical  profession  to 
stimulate  the  contractions  of  the  uterus  at  childbirth.  If  an  animal 
is  lactating,  injections  of  the  posterior  lobe  will  bring  on  an  increased 
flow  of  milk. 

From  this  brief  account  may  be  gathered  some  idea  of  the  way  in 
which  this  small  endocrine  gland  functions  as  the  commander-in-chief 
of  the  metabolism  of  the  body.     Although  much  work  remains  to  be 


CHEMICAL   RIJGULATOHS  4():{ 

done  in  this  connection,  it  is  nevertheless  apparent  that  the  pituitary 
gland  exercises  an  interlocking  directorate  over  the  remaining  lesser 
lights  of  the  endocrine  constellation. 

SUGGESTED   READINGS 

Clenaenning,  L.,  The  Human  Body,  Alfred  A.  Knopf,  Inc.,  1930.     Ch.  IX. 
Interestingly  written  account  of  the  problem  of  co-ordination. 

Cobb,  I.  G.,  The  Organs  of  Internal  Secretion,  4th  ed.,  Wm.  Wood  and  Co., 
1933. 

Fairly  technical  discussion  of  functioning  and  non-functioning  of  endo- 
crine glands. 

Rogers,  C.  G.,  Textbook  of  Comparative  Physiology,  McGraw-Hill  Book  Co., 
1927.     Ch.  XXV. 

Discussion  of  the  endocrines  and  their  work  from  a  comparative  view- 
point. 


THE    MAINTENANCE  OF  SPECIES 


XIX 


REPRODUCTION  AND   LIFE  CYCLES 

Preview.  Where  did  life  come  from  ?  :  Refutation  of  spontaneous  genera- 
tion ;  other  theories  of  the  origin  of  hfe ;  Hfe  produces  Ufe  •  Regeneration  • 
Asexual  types  of  reproduction  :  Budding ;  fission  •  Sexual  reproduction  in 
the  invertebrates :  jjrotozoa ;  (^tlier  invertebrates ;  hermaphroditism  •  Par- 
thenogenesis •  Paedogenesis  •  Alternation  of  generations  •  Sexual  reproduc- 
tion and  development  in  the  vertebrates :  Germ  cells  versus  soma  cells ; 
fertilization,  results  of  fertilization ;  early  cleavage  and  variations  caused 
by  yolk ;  blastulation  ;  gastrulation  ;  mesoderm  formation  ;  early  differ- 
entiation of  the  embr\'o  •  Tissue  fonnation  •  Protective  devices  for  the  em- 
bryo :  Egg  shells  ;  the  yolk  sac  ;  amnion  and  chorion  ;  allantois  ;  placenta  • 
Elaboration  of  germ  cells  or  gametogenesis  :  Formation  of  sperm  —  spermato- 
genesis ;  formation  of  ova  —  oogenesis  •  The  new  embryology :  Genes ; 
environment ;   natural  potencies ;  organizers  •  Suggested  readings. 

PREVIEW 

It  was  obvious  to  the  early  philosophers  that  the  earth  preceded  the 
living  things  upon  it  and  they  advanced  the  interesting  idea  that 
living  things  arose  spontaneously  from  their  surroundings.  The 
Bible  alludes  to  this  belief  when  Samson  propounded  his  riddle, 
"Out  of  the  eater  came  forth  meat  and  out  of  the  strong  came  forth 
sweetne.ss."  Samson  saw  flies  coming  out  of  the  decaying  body  of  a 
lion,  took  the  flies  for  bees,  which  he  believed  were  arising  spontane- 
ously from  the  lion's  body,  hence  the  riddle.  The  story  of  the  long 
struggle  to  disprove  spontaneous  generation,  ending  with  the  conclu- 
sive demonstrations  of  Louis  Pasteur,  makes  one  of  the  fascinating 
bits  of  reading  in  the  field  of  biology. 

With  the  disproval  of  the  existence  of  spontaneous  generation  and 
the  perfection  of  the  microscope,  great  interest  was  evidenced  in  the 
many  different  ways  in  which  plants  and  animals  reproduced.  Today 
the  student  of  embryology  sees  the  apparently  many  diverse  ways  of 
reproducing  the  species  reduced  to  a  few  essentially  similar  funda- 
mental patterns. 

Likewise  the  exactness  with  which  the  chromatin  is  segregated  and 
divided  within  the  developing  germ  cell  is  a  never-ending  source  of 

405 


406  THE  MAINTENANCE  OF  SPECIES 

wonder  to  the  biologist.  Another  interesting  study  centers  about 
the  development  of  the  various  protective  devices  that  surround  the 
embryo  and  keep  it  from  injury  until  it  is  hatched  or  born.  The 
infinite  care  with  which  these  devices  have  been  developed  is  a  credit 
to  the  ingenuity  of  Mother  Nature. 

In  this  unit  the  student  will  find  the  answer  to  questions  arising 
in  his  mind  concerning  the  nature  of  these  reproductive  devices. 

Where  Did  Life  Come  From? 

Greek  and  Roman  literature  is  full  of  references  to  the  possible 
origin  of  life  and  to  the  probability  that  it  arose  spontaneously.  A  few 
brave  souls  dared  to  doubt  this  almost  universally  accepted  concept. 
However,  even  as  late  as  the  17th  century  Alexander  Ross  writes, 

"So  may  we  doubt  whether  in  cheese  and  timber  worms  are  generated, 
or  if  beetles  and  wasps  in  cow-dung,  or  if  butterflies,  locusts,  shellfish, 
snails,  eels,  and  such  life  be  procreated  of  putrefied  matter,  which  is  to 
receive  the  form  of  that  creature  to  which  it  is  by  formative  power  disposed. 
To  question  this  is  to  question  reason,  sense,  and  experience.  If  he  doubts 
this,  let  him  go  to  Egypt,  and  there  he  will  find  the  fields  swarming  with 
mice  begot  of  the  mud  of  Nylus,  to  the  great  calamity  of  the  inhabitants." 

Refutation  of  Spontaneous  Generation 

Belief  in  spontaneous  generation  was  first  shaken  by  the  Italian 
physician  Redi,  who  noticed  that  flies  were  attracted  to  decaying 
meat.  In  an  experiment  he  put  sterilized  meat  into  several  jars, 
covered  one  lot  with  parchment,  another  lot  with  a  fine  netting,  and 
the  third  he  left  open.  Fly  maggots  were  found  later  in  the  meat 
in  the  open  jars,  fly  eggs  on  the  netting,  and  no  maggots  in  the  parch- 
ment-covered jars.  This  experiment  should  have  exploded  the  belief 
that  maggots  arose  spontaneously  from  rotting  meat.  However, 
the  belief  kept  constantly  recurring  because  it  was  very  difficult  to 
prevent  the  invasion  of  food  materials  by  bacteria,  even  after  the 
substances  and  vessels  containing  them  were  apparently  sterilized. 
The  Abbe  Needham,  seventy  years  after  the  Redi  demonstration, 
experimented  with  living  germs,  and  because  of  the  errors  arising  from 
improper  sterilization  found  living  germs  in  flasks  of  nutritive  fluid 
that  had  first  been  heated  and  then  were  sealed  with  a  resinous 
cement.  A  little  later  the  Itahan,  Spallanzani  (1729-1799),  placed 
nutrient  fluids,  such  as  meat  and  vegetable  juices,  in  glass  flasks,  the 
necks  of  which  were  sealed  in  a  flame ;   then  he  placed  the  flasks  in 


REPRODUCTION   AND   LIFE   CYCLES  407 

boiling  water  for  three  quarters  of  an  hour.  The  contents  of  the 
flasks  remained  unchanged.  Spallanzani  then  op(!ned  the  flasks  and 
after  a  short  period  they  were  found  to  be  full  of  living  organisms. 
Needham  objected  to  the  experiments  on  the  ground  that  the  boiling 
had  killed  the  "vegetative  force"  of  the  infusion.  However,  the 
idea  of  spontaneous  generation  was  not  finally  disproved  until  the 
time  of  Pasteur  and  T3mdall,  who  proved  that  living  germs  may 
be  carried  about  by  dust  in  the  air  and  that  only  when  air  con- 
taining dust  particles  can  be  excluded  from  substances  it  is  certain 
that  bacteria  will  not  grow  in  them. 

Other  Theories  of  the  Origin  of  Life 

The  theory  of  the  simultaneous  creation  of  life  and  this  planet  does 
not  agree  with  such  theories  as  the  scientists  offer  to  account  for  the 
origin  of  the  eartli.  Whether  we  accept  the  nebular  hypothesis  of 
LaPlace,  the  later  planetesimal  hypothesis  of  Chamberlin,  or  the  still 
later  theories  of  Green  or  Shapley,  we  are  confronted  in  all  of  them  by 
the  formation  of  our  jjlanet  from  material  far  too  hot  to  sustain  life. 
As  Jeans  says,  ''The  physical  condition  under  which  Hfe  is  feasible  is 
only  a  tiny  fraction  of  tlie  range  of  physical  conditions  which  pre\^ail 
in  the  universe  as  a  whole."  The  theory  sometimes  advanced  that 
life  may  have  been  transferred  from  another  planet  does  not  help  us 
much,  for  we  still  have  to  account  for  life's  origin.  As  has  been  so 
well  said  of  life  on  Mars,  which  of  any  of  our  planetary  neighbors 
has  concUtions  the  most  possible  for  supporting  life,  "Man  recon- 
structed to  walk  on  Mars  would  be  crushed  to  death  by  his  own 
weight  on  the  eartli."  Special  creation  as  advocated  by  the  early 
Church  does  not  help  the  scientist  very  much,  for  it  still  leaves  life 
to  be  accounted  for.  It  allows  of  no  scientific  investigation  and  so 
it  cannot  be  used  by  the  biologist. 

Probably  the  theory  which  has  the  most  hope  of  ultimate  solution 
is  the  belief  that  at  some  time  life  arose  by  a  chance  combination  of 
chemical  elements  of  which  the  earth  is  made.  Evidence  found  in 
the  rocks  indicates  that  the  earth  is  much  older  than  its  inhabitants. 
Professor  Henry  F.  Osborn  pointed  out  the  striking  similarity  of  the 
salts  found  in  the  blood  and  those  found  in  sea  water.  He  made  the 
suggestion  that  life  might  have  originated  in  some  pool  in  which  the 
saUne  contents  contained  the  life  elements  found  in  protoplasm. 
Would  it  be  too  much  to  speculate  on  the  origin  of  some  simple  form 
of  life  by  allowing  a  flash  of  lightning  to  release  the  pure  nitrogen 
H.  w.  H.  — 27 


408  THE   MAINTENANCE  OF   SPECIES 

of  the  air  in  some  form  of  nitrate  which  would  combine  with  the  life 
elements  found  in  sea  water  and  the  carbon  dioxide  of  the  air  ?  This 
theory  is  in  reality  a  refurbished  concept  of  spontaneous  generation. 
In  discussing  it  two  points  should  be  kept  in  mind.  First,  if  sponta- 
neous generation  of  this  sort  did  occur  at  one  time,  the  contrast  between 
the  physical  environments  of  the  past  and  present  would  be  great. 
Second,  even  if  conditions  were  right  for  the  similar  production  of 
life  today,  it  appears  likely  that  such  simple  beginnings  would  be 
almost  immediately  destroyed  by  better  established  forms  of  life. 
Both  serve  as  explanations  of  why  we  do  not  have  life  produced 
spontaneously  today. 

Life  Produces  Life 

Since  the  time  of  William  Harvey,  court  physician  of  Charles  I  of 
England,  the  statement  "Omne  vivum  ex  ovo"  has  been  used.  Living 
things  come  from  other  living  things,  not  always  from  eggs,  as  Harvey 
said,  but  in  the  case  of  unicellular  animals  and  plants  by  the  cell 
dividing  to  form  two. 

Each  organism,  plant  or  animal,  has  a  definite  life  cycle,  a  series  of 
changes  which  it  goes  through  from  its  simplest  form  as  an  egg  to  its 
ultimate  adult  structure.  More  than  this,  sooner  or  later  it  will  die. 
In  some  unicellular  forms  the  life  cycle  takes  a  very  brief  period 
indeed,  but  in  the  elephant  it  is  over  a  hundred  years,  and  some 
trees,  like  the  giant  sequoias,  live  thousands  of  years.  Sooner  or  later 
life  activities  cease  and  the  Biblical  statement  of  "dust  to  dust"  is 
justified.  Death  comes  as  a  final  close  of  all  activity  and  normally 
after  the  animal  or  plant  has  produced  offspring. 

New  individuals,  whether  complicated  mammals  or  simple  protozo- 
ans, arise  from  the  same  kind  of  pre-existing  organisms.  The  exact 
method  of  reproduction,  however,  varies  markedly  in  different 
groups.  Protozoa,  at  one  end  of  the  scale,  produce  new  individuals 
by  the  simple  process  of  cell  division,  while  the  mammals,  at  the  other 
extreme,  show  evidence  of  considerable  division  of  labor  with  special 
organs  involved  in  the  production  and  functioning  of  the  highly 
specialized  sex  cells.  In  order  to  understand  these  various  processes 
it  is  desirable  to  summarize  the  different  reproductive  devices  which 
appear  in  the  animal  kingdom. 

Regeneration 

The  replacement  by  an  organism  of  lost  or  injured  tissue  is  included 
in  this  discussion  of  reproduction  on  the  ground  that  such  a  phe- 


REPRODUCTION   AND   LIFE   CYCLES 


409 


iiomenoii,  involving  the  creation  of  new  cells  by  cell  division,  is  a 
fundamental  type  of  growth.  The  ability  to  regenerate  lost  parts 
seems  to  be  correlated  inversely  with  the  degree  of  specialization 
and  the  extent  to  which  division  of  labor  appears.  For  example,  an 
unspecialized  sponge  when  pressed  through  silk  bolting-cloth  into 
small  fragments  will  reproduce  new  individuals.     Other  more  highly 


tyo  rzev  rays 


two  newm^s 


WTone  old 
ray 

two  new  Tays 


tliree  old  rays  '  ^^^^ 


Vlanavw 

Examples  of  regeneration  in  representatives  of  four  different  phyla.     How  may 

such  phenomona  be  explained  ? 

specialized  forms  show  less  ability  to  regenerate  so  completely,  but 
many  of  the  coelenterates  as  well  as  certain  worms  and  echinoderms 
possess  this  facility  of  regeneration  to  a  high  degree.  Starfish,  long 
the  enemy  of  oysters,  have  increased  rapidly  in  part  due  to  the  care- 
less practice  of  oystermen  who  tore  them  apart  and  left  the  frag- 
ments in  the  water.  It  is  now  known  that  such  disjointed  parts,  if 
containing  portions  of  the  central  disk,  are  capable  of  regenerating 
into  new  individuals. 


410  THE   MAINTENANCE  OF   SPECIES 

Lobsters,  crabs,  spiders,  and  some  insects  have  tiie  uncanny  ability 
of  breaking  off  an  injured  appendage  near  its  base,  a  phenomenon 
known  as  autotomy.  In  such  instances  new  appendages  are  usually 
regenerated  and  the  animal  emerges  as  a  successful  contestant  in 
another  skirmish  in  the  struggle  for  existence.  Vertebrates,  how- 
ever, show  but  slight  ability  to  replace  lost  parts.  Of  course  a 
break  in  the  skin  is  soon  healed  by  regeneration,  although  more 
extensive  damage  to  the  part  results  merely  in  the  elaboration  of 
some  connective  tissue  and  skin  and  not  in  complete  restoration. 
A  crushed  toe,  for  example,  usually  necessitates  an  amputation,  for 
in  such  cases  one  never  finds  a  new  toe  replacing  the  old. 

It  is  a  rather  striking  fact  that  the  more  limited  type  of  regeneration 
common  among  the  higher  vertebrates  is  almost  indistinguishable 
from  the  normal  metabolic  processes  so  characteristic  of  growth  and 
repair.  It  is  only  a  step  from  such  methods  of  growth  to  the  highly 
specialized  type  known  as  reproduction. 

Asexual  Types  of  Reproduction 

Budding  and  fission,  or  simple  cell  division,  comprise  the  usual 
asexual  methods  of  reproduction.  A  brief  consideration  of  these 
methods  at  this  point  will  serve  to  link  regenerative  processes  with 
those  of  higher  types  of  reproduction.  The  former  may  be  thought  of 
as  reproduction  by  an  unequal  cell  division,  a  mode  of  division  not 
infrequently  found  among  one-celled  organisms.  In  more  complex 
organisms,  as  Hydra,  repeated  divisions  of  totipotent  cells  may  occur 
to  produce  a  bud.  Fission  merely  involves  the  division  of  an  organ- 
ism into  two  or  more,  usually  approximately  equal  parts. 

Budding 

Organisms  which  undergo  budding  might  easily  be  confused  with 
those  exhibiting  regeneration.  These  phenomena  closely  resemble 
each  other,  the  chief  difference  being  that  budding,  unlike  regener- 
ation, does  not  typically  result  from  injury.  It  is,  moreover,  an 
important  type  of  reproduction  occurring  quite  generally  in  plants 
as  well  as  widely  throughout  the  lower  animal  kingdom. 

The  fresh-water  sponge  reproduces  by  means  of  two  kinds  of  buds, 
the  first  type  being  liberated  to  take  up  a  separate  existence  while 
the  second  remains  as  a  kind  of  internal  bud,  called  a  gemmule.     It 


UEiMlODUCTION   AND    LIFE   CYCLES  Ml 

has  been  previously  shown  that  in  Hydra  the  new  bud  extends  out 
from  the  body,  developing  tentacles,  mouth,  and  hy[)ostome  at  the 
distal  end  of  the  organism.  After  growing  sufficiently  the  base 
constricts  and  the  two  animals,  parent  and  offspring,  become  sepa- 
rated, each  taking  up  an  independent  existence  (page  184). 

In  the  higher  worms  such  as  the  palolo  worm  and  the  Naididae,  a 
type  of  budding  occurs  which  might  be  described  as  fragmentation. 
The  number  of  fragments  apparently  depends  upon  tlu;  size  of  the 
worm,  each  piece  usually  producing  all  of  the  missing  parts. 

Fission 

This  variety  of  asexual  reproduction  is  the  most  common.  The 
one-celled  protozoa  rely  almost  exclutijvely  upon  this  type  of  develop- 
ment, seldom  resorting  to  the  more  complicated  "sexual"  methods. 
In  binary  fission  the  nucleus  appears  to  take  the  initiative,  since  it 
divides  first  and  is  followed  by  the  division  of  the  cytoplasm  of  the 
cell. 

Fission  is  rather  closely  allied  to  budding.  Many  of  the  turbel- 
larian  and  nemertin(\an  flatworms  utilize  this  method,  as,  for  example, 
the  turbellarian,  Microsiomum,  which  often  divides  into  two,  four, 
or  even  sixteen  pieces.  These  parts  produce  all  of  the  necessary 
structures  except  eye-spots  and  often  remain  attached  in  chains  for 
long  periods  of  time. 

Sexual  Reproduction  in  the  Invertebrates 

Protozoa 

Sexual  reproduction  involves  the  union  of  two  cells  produced  usually 
by  two  animals  of  different  sexes.  This  phenomenon  appears  in 
practically  every  group  of  the  animal  kingdom.  Even  in  the  protozoa 
there  are  two  types  of  reproduction  which  may  be  thought  of  as 
initiating  the  sexual  method.  In  the  first  type  there  is  either  a 
complete  union  of  two  individual  cells  of  equal  or  of  unequal  size, 
or  there  may  be  specialized  cells  called  gametes.  Many  variations 
of  this  type  are  to  be  found  among  different  species. 

The  second  type  of  sexual  reproduction  occurring  in  the  protozoa 
is  called  conjugation,  which  has  already  been  described  (page  161). 
Briefly,  conjugation  means  that  two  single-celled  organisms  come 
together  temporarily,  form  some  sort  of  protoplasmic  bridge,  exchange 


412  THE   MAINTENANCE   OF  SPECIES 

nuclear  material,  and  finally  separate.  If  the  conjugating  forms  are 
of  equal  size,  as  in  the  case  of  Paramecium,  both  usually  survive  and 
continue  to  reproduce,  by  asexual  means.  On  the  other  hand,  when 
the  conjugants  are  of  unequal  size  it  frequently  happens  that  the 
smaller,  or  micro-conjugant,  degenerates  soon  after  conjugation. 

Other  Invertebrates 

As  division  of  labor  among  the  cells  of  an  organism  progresses  there 
is  increasing  evidence  of  a  gradual  but  none  the  less  clear  demarcation 
into  two  sorts  of  cells,  the  soma  or  body  cells,  and  the  germ  or  sex  cells. 
These  groups  are  separated  early  in  the  development  of  the  individual, 
the  former  being  burdened  with  the  responsibilities  of  movement, 
protection,  securing  food,  and  in  some  cases  caring  for  the  young. 
The  second,  comprising  the  germ  cells,  is  solely  concerned  with  the 
elaboration  of  highly  specialized  cells  adapted  for  the  production  of 
new  individuals,  and  so  serving  for  the  maintenance  of  the  race. 

Since  sexual  reproduction  undergoes  many  modifications  in  the 
invertebrates,  it  appears  logical  to  consider  some  of  these  phenomena 
before  undertaking  a  detailed  study  of  sexual  reproduction  in  the 
higher  vertebrates. 

Hermaphroditism 

Many  of  the  lower  invertebrates  exhibit  a  kind  of  sexual  reproduc- 
tion in  which  both  the  male  and  female  organs  are  found  in  the  same 
individual.  A  complete  set  of  male  and  female  reproductive  organs 
occurs,  for  example,  in  a  single  Hydra.  In  this  genus  the  syermary 
producing  the  spermatozoa  is  situated  closer  to  the  tentacular  region 
than  the  ovary  which  is  located  near  the  foot.  These  gonads  rupture 
when  mature,  and  one  of  the  liberated  spermatozoa  finally  fertilizes 
the  ovum  contained  in  a  disrupted  ovary.  When  both  gonads  are 
functional  on  the  same  individual  self-fertilization  may  occur. 

The  earthworm  likewise  contains  a  complete  set  of  male  and  female 
reproductive  organs  in  the  same  individual,  but  here,  as  in  many  of 
the  trematode  flatworms,  copulation  takes  place  between  two  separate 
individuals.  In  such  cases  the  exchange  of  spermatozoa  results  in 
cross-fertilization. 

While  hermaphroditism  is  unusual  in  the  vertebrates,  it  is  believed 
to  occur  normally  in  a  few  instances  such  as  certain  hagfishes  (cyclo- 
stomes)  which  are  known  to  be  hermaphroditic.     In  these  forms, 


REPRODUCTION   AND   LIFE   CYCLES  113 

however,  cross-fertilization  occurs,  since  the  ova  and  spermatozoa 
mature  at  different  times.  Reported  cases  of  functional  hermaphro- 
ditism among  mammals  appear  to  be  highly  doubtful. 

Parthenogenesis 

The  development  of  an  egg  without  fertilization  by  a  sperm  occurs 
quite  commonly  under  natural  conditions  in  some  invertebrate  forms. 
Usually  there  is  a  cessation  of  activity  on  the  part  of  the  males  for  a 
period  of  time  when  ova,  produced  by  the  females,  develop  into 
apparently  normal  individuals.  In  some  few  instances  males  are 
permanently  absent.  The  rotifers,  water  fleas  (Cladocera),  and  plant 
hce  (aphids)  all  exhibit  this  type  of  development  at  times.  In  cladoc- 
era, of  which  Daphnia  is  a  well-known  example,  the  females  produce 
parthenogenetic  eggs  during  the  warm  weather.  From  two  to  twenty 
eggs,  depending  upon  the  species,  are  deposited  and  nourished  in  the 
brood-sac.  Usually  several  generations  of  females  will  be  produced 
in  this  fashion.  Eventually  male  as  well  as  female  daphnids  are 
produced,  and  the  eggs  from  this  generation  of  females  must  be 
fertilized  by  the  males.  When  fertilization  occurs,  the  eggs  are 
covered  by  the  highly  resistant  protective  portion  of  the  brood-sac 
(ephippium)  which  enables  them  to  withstand  desiccation  and  the 
rigors  of  winter. 

Numerous  experimenters  have  been  interested  in  attempts  to  induce 
artificial  parthenogenesis  in  various  invertebrate  eggs  by  means  of 
chemical  or  physical  stimuli  ranging  all  the  way  from  simple  salts 
and  complex  fatty  acids  to  mechanical  means,  such  as  pricking  with  a 
needle,  shaking,  or  raising  the  temperature  of  the  water  surrounding 
the  experimental  organisms.  Mead  first  successfully  induced  arti- 
ficial parthenogenesis  with  the  ova  of  annelids  and  Loeb  extended 
the  experiments  to  include  starfishes,  sea  urchins,  molluscs,  and  even 
frogs,  which  underwent  at  least  partial  development  by  means  of 
various  chemical  or  physical  stimuli  aptly  described  as  parthenoge- 
netic agents. 

Most  of  the  experimental  efforts  to  induce  parthenogenesis  in 
vertebrates  have  been  rewarded  by  failure.  In  a  few  instances  tad- 
poles have  been  produced  through  mechanically  initiating  cleavage 
of  the  egg  by  pricking  with  a  needle  and  introducing  a  small  amount  of 
blood  serum  at  the  same  time.  Pincus  has  also  been  able  to  carry  a 
mammal  embryo  through  early  developmental  stages  after  partheno- 
genetic stimulation. 


414  THE   MAINTENANCE   OF   SPECIES 

Paedogenesis 

Reproduction  by  immature  individuals  is  called  paedogenesis. 
As  it  is  rarely  encountered  in  the  animal  kingdom,  only  two  examples 
need  be  mentioned.  The  first  occurs  in  the  trematodes,  where  imma- 
ture larval  forms,  such  as  sporocytes  and  rediae,  appear  to  produce 
the  next  generation  parthenogenetically.  These  in  turn  often  give 
rise  to  another  generation  through  paedogenetic  reproduction.  In 
the  vertebrates  the  best  known  example  is  the  Mexican  axolotl,  a 
urodelous  amphibian.  This  interesting  animal,  while  still  remaining 
in  its  larval  form,  reproduces  its  kind  sexually  without  undergoing 
metamorphosis  or  losing  its  external  gills. 

Alternation  of  Generations 

Alternation  of  a  sexual  with  an  asexual  generation  is  called  metagen- 
esis, or  simply  alternation  of  generations.  Several  of  the  invertebrates, 
especially  the  coelenterates,  normally  exhibit  metagenesis.  In  the 
hydroid  Obelia,  for  example,  the  asexual  generation  is  represented 
by  a  sessile,  colonial  hydroid  and  the  sexual  generation  by  the  mature, 
bisexual  medusa  buds  (see  page  185). 

Sexual  Reproduction  and  Development  in  the  Vertebrates 

Germ  Cells  versus  Soma  Cells 

The  early  growth  and  later  development  of  the  embryo  and  its 
systems,  organogeny,  are  to  be  considered  in  some  detail.  To  com- 
plete the  picture  it  is  necessary  to  envision  the  continued  growth  of 
the  organism  until  it  matures,  reproduces  its  kind,  and  dies.  The 
life  of  every  organism,  whether  plant  or  animal,  is  involved  with  the 
mathematical  concepts  of  division,  multiplication,  addition,  and 
subtraction.  In  the  formation  of  a  new  individual  by  two  parents, 
two  germ  cells  are  added  together  {fertilization).  In  order  that  the 
hereditary  genes  thus  united  may  not  be  disastrously  doubled  in 
each  generation,  one  half  of  those  present  from  each  contributing 
parent  are  subtracted  by  the  elimination  of  either  the  maternal  or  the 
paternal  member  of  each  chromosome  pair  just  prior  to  maturation. 
Thus,  a  constant  number  of  chromosomes  with  their  respective  genes 
is  maintained  in  each  body  cell  of  any  species.  After  this  preliminary 
process  of  subtraction  and  addition  has  been  accomplished,  the  newly 
combined  germinal  cell,  that  is,  the  fertilized  egg,  or  ovule,  initiates 


REPRODUCTION   AND   LIFE   CYCLES 


415 


an  exhaustive  series  of  divisions,  whereby  each  cell  repeatedly  becomes 
two  (growth).  The  result  of  these  successive  divisions  is  an  enormous 
multiplication  of  differentiating  cells  to  form  the  entire  body  of  the 
individual  (development) . 

In  the  present  connection  it  is  only  desirable  to  emphasize  that 
this  complicated  process  of  cell-division  (mitosis)  has  been  exhaus- 
tively studied,  so  that  its  essentials  are  now  well  known.  In  a  word, 
the  end  result  is  the  final  distribution,  to  every  one  of  the  innumerable 
cells  that  form  the  individual,  of  equal  germinal  contributions  from 
the  two  parents  in  the  form  of  gene-bearing  chromosomes. 


azrosons. 

heocLJJ 

centrosonze 
middle 
piece- 


tail. 


sheccth- 

axial  — 
filament 


>|^^  animal   pole 

nacleus 


encL  — J 
pieces  I 


vacuole 

vitelline, 

rnembroLne- 


^i-cxnixle 


veg'eLal  polt 


Generalized  diagram  of  spernialozoan  (left)  and  ovum  (right)  ready  for 
fertilization.  Note  the  two  views  of  the  spernialozoan.  The  head  contains 
much  nuclear  material  plus  the  acrosome.  The  middle  piece  contains  two  disk- 
like centrosomes,  twisted  milochondria  and  cytoplasm,  while  the  tail  has  an  outer 
sheath  and  axial  filament.  Eggs  are  always  larger  than  spermatozoa  and  con- 
tain varying  amounts  of  reserve  food.  Yolk  settles  toward  the  vegetal  pole. 
(After  McEwen.) 

Sexual  reproduction  in  the  vertebrates  is  essentially  identical 
regardless  of  the  group  considered.  In  every  case  there  is  a  special 
organ  in  the  male  called  a  testis,  or  spermary,  for  the  production  of 
sperm,  and  an  ovary  in  the  female  in  which  eggs  are  elaborated.  Each 
sperm  or  ovum  is  a  single  cell.  Both  kinds  of  germ  cells  differ  in 
shape  and  size  throughout  the  vertebrate  series. 

The  tadpole-shaped  spermatozoa  are  always  much  smaller,  quite 
active,  and  lack  nutrient  material  within  their  bodies,  as  contrasted 
with  the  sedentary  ova  in  which  food  is  stored  for  the  prospective 
embryo.     Sperm  may  be  divided  morphologically  into  three  parts, 


416 


THE   MAINTENANCE   OF   SPECIES 


^  first  and. 
^ ,®  SeconoC 


polar 
toocCy 


the  head,  middle,  and  tail  pieces. 
The  head  is  composed  chiefly  of 
chromatin  and  is  usually  more  or 
less  pointed.  The  middle  piece  con- 
stitutes the  general  region  imme- 
diately posterior  to  the  head  and 
contains  cytoplasm,  m,itochondria, 
centrioles,  and  the  axial  filament, 
while  the  tail  piece  appears  to  be 
primarily  a  locomotor  device. 

Ova,  on  the  other  hand,  are  always 
non-motile  and  much  larger  than 
the  sperm,  due  primarily  to  the  fact 
that  ova  contain  nutritive  material, 
or  yolk,  which  is  utilized  after  fertil- 
ization. The  amount  of  yolk  present 
in  eggs  of  the  various  classes  of  ver- 
tebrates differs  widely.  In  all  forms 
in  which  the  eggs  develop  outside 
of  the  body,  as,  for  example,  the 
fish,  amphibians,  reptiles,  and  birds, 
there  must  be  enough  nutritive  mate- 
rial present  in  the  form  of  yolk  to 
supply  the  embryo  until  it  hatches 
and  can  feed  itself. 

Fertilization 

Fertilization  consists  of  the  union 
of  a  sperm  and  an  ovum.  This  fusion 
may  occur  either  outside  of  the  body 
of  the  female,  as  in  the  case  of  most 
of  the  teleost  fishes  and  other  water- 
inhabiting  animals,  or  within  the 
oviduct  of  the  female.  Literally 
millions  of  sperm  are  liberated,  but 
usually  only  a  single  sperm  enters  an 


Generalized  diagram  of  fertiliza- 
tion. (I)  shows  the  formation  of  the 
first  polar  body,  the  maturation 
spindle  of  the  second  maturation 
division  (see  p.  429),  and  the  pene- 
tration of  the  spermatozoan.  The 
second  polar  body  is  formed  by  the 
second  maturation  division  and  the 
egg  nucleus  starts  towards  the  cen- 
ter of  the  egg.  The  sperm  nucleus, 
or  male  pronucleus,  starts  towards 

the  center  (II)  via  the  entrance  path,  but  turns  (III)  toward  the  center  on  its 
copulation  path  to  meet  the  egg  nucleus  and  be  arranged  on  the  equatorial  plate 
(IV)  for  the  first  cleavage  division.  Note  that  the  centrosomes  for  this  division 
are  supplied  by  the  male  pronucleus.     (After  McEwen.) 


REPRODUCTION   AND   LIFE  CYCLES  417 

egg  and  in  any  event  only  one  normally  effects  fertilization.  The 
head  and  middle  pieces  usually  become  separated  from  the  tail  piece 
as  penetration  is  effected,  leaving  the  tail  at  the  p(>rii)hcry  of  the 
ovum  in  much  the  same  way  that  sandals  are  left  at  the  portal  of  a 
Japanese  house.  The  continued  penetration  of  the  remainder  of  the 
sperm  is  made  possible  through  movements  of  the  cytoplasm  within 
the  egg.  The  male  element,  which  is  now  known  as  the  male  pro- 
nucleus, absorbs  water,  enlarges,  and  finally  becomes  arranged  on 
the  equatorial  plate  with  the  female  -pronucleus  of  the  ovum,  and  the 
initial  cell  division  follows. 

Results  of  Fertilization.  The  more  important  effects  of  ferti- 
lization may  be  briefly  summarized  as  follows :  (1)  Reproduction. 
This  is  accomplished  by  restoring  the  normal  (diploid)  number  of 
chromosomes  and  by  so  doing  producing  a  new  center  of  cell  division. 
(2)  Variation.  As  will  be  seen  later,  the  whole  phenomenon  of 
maturation  of  the  germ  cells  and  the  consequent  reduction  of  chromo- 
somes to  the  haploid  number  makes  possible  new  combinations  and 
variations  between  fertilized  ova,  or  zygotes,  upon  which  natural 
selection  may  act.  (3)  Rejuvenescence.  For  years  fertilization  and 
the  concomitant  stimulation  of  protoplasm  have  been  thought  neces- 
sary to  revivify  an  organism.  Data  have  been  collected  both  in 
support  of  and  in  contradiction  to  this  theory.  Endomixis,  as  shown 
by  Woodruff  (page  161),  apparently  acts  as  the  rejuvenating  agent  in 
nonconjugating  strains  of  protozoa. 

Early  Cleavage  and  Variations  Caused  by  Yolk 

Once  fertilization  has  occurred,  cell  division  proceeds  rapidly  and 
the  zygote  gives  way  to  the  early  cleavage  stages.  In  tlie  simplest 
types  each  plane  of  cleavage  typically  passes  at  right  angles  to  the 
preceding  plane,  the  cells  multiplying  from  the  two-celled  to  the  four- 
celled  stage,  and  so  on  up  imtil  the  number  in  a  given  cleavage  stage 
cannot  easily  be  determined. 

The  amount  of  yolk  present  in  the  egg  affects  the  cleavage  rate  and 
even  the  pattern  of  development,  since  yolk  is  denser  than  typical 
cytoplasm  and,  therefore,  settles  toward  the  lower  side  of  the  egg. 
Its  presence  affects  the  rate  of  cell  division  by  slowing  it  down.  If 
yolk  is  present  in  large  amounts  as  in  bird  and  reptile  eggs,  it  tends 
to  occupy  most  of  the  available  space  in  the  ovum.  In  such  ova 
the  embryo  develops  in  the  upper  polar  area,  or  in  a  restricted  disk 
called  the  blastoderm  lying  on  top  of  the  yolk  mass.     The  ova  of 


4ia 


THE   MAINTENANCE  OF  SPECIES 


amphioxus  and  of  mammals  contain  but  a  small  amount  of  equally 
distributed  nutritive  material,  while  a  third  type  of  distribution 
occurs  in  some  insect  eggs  where  the  yolk  is  concentrated  in  the  center 
of  the  ovum. 

Blastulation 

In  isolecithal  eggs,  in  which  the  yolk  is  distributed  throughout  the 
egg,  the  cells  produced  by  successive  divisions  are  all  of  approxi- 
mately the  same  size,  and  cleavage  progresses  with  regularity  until 

,      ,  /Polair    body, 

polar  Dod^  ^  -^ 

"      :rivi  tell  ins 
space 


fertili3atibn: 


arcbenteron  -^ 
ectoderm^      blcrstocoel 


ventrcU 
lip 


blastopore  ^' 


ET  ..     .    ^ 

Cleavage  in  Amphioxus.  Note  fertilization  membrane  (I)  and  decrease  in  cell 
size  as  blastulation  occurs  (II-IV).  Gastrulation  (V,  \T)  follows  with  a  reduction 
of  blastocoel  and  formation  of  gut  {archenleron).     (After  Conklin.) 

the  embryo  is  a  mass  of  increasingly  smaller  undifferentiated  cells. 
A  central  cavity  is  produced  as  soon  as  the  scanty  yolk  is  used 
up  to  furnish  fuel  for  cell  division.  As  a  result  the  entire  mass  re- 
sembles a  rubber  ball  with  the  surface  representing  the  layer  of  out- 
side cells  and  the  cavity  inside  of  the  liall  forming  the  hlastocoel. 
This  stage  is  called  a  hlastula,  and  the  process  whereby  it  is  formed 
is  known  as  blastulation. 

Gastrulation 

As  mitosis  continues  after  blastulation,  the  cells  on  the  side  con- 
taining the  yolk  gradually  become  larger  and  eventually  are  pushed 
inward  much  as  one  would  push  in  the  side  of  a  hollow  rubber  ball 
with  the  finger.  The  new  cavity  thus  formed  represents  the  primitive 
gut,  or  archenleron,  and  the  embryo  is  now  spoken  of  as  a  gastrula. 


REPRODUCTION   AND   LIFE   CYCLES 


419 


Thus  far  two  germ  layers 
can  be  differentiated,  an  outer 
layer  of  ectoderm  and  an  inner 
one  of  endoderm  which  lines  the 
archenteron,  while  the  dimin- 
ishing remains  of  the  blasto- 
coel  lie  between.  This  stage  is 
suggestive  of  those  organisms, 
like  the  coelenterates,  which 
characteristically  possess  only 
two  germ  layers  even  in  the 
adult  condition,  and  are  there- 
fore designated  as  diplohlastic. 

Mesoderm  Formation 

The  details  of  the  further 
development  of  the  embryo 
vary  considerably,  depending 
upon  the  form  studied,  but 
all  of  the  higher  forms  above 
the  coelenterates  produce  a 
third  germ  layer  called  the 
mesodcrjyi.  The  elaboration  of 
mesodermal  tissue  may  come 
from  either,  or  possibly  both, 
of  the  primary  germ  layers. 
In  all  of  the  vertebrates,  two 
sheets  of  mesodermal  cells  are 
formed,  an  inner  splanchnic 
layer  associated  with  the  inner 
tube,  or  developing  gut,  and  an 
outer  so7natic  layer,  which  is 
contiguous  with  the  ectoderm. 
Loosely  scattered  mesodermal 
cells  {mesenchyme  cells),  de- 
rived from  these  more  compact 
layers,  fill  in  the  narrow  spaces 
between  the  gut  and  splanch- 
nic layer  and  between  the 
somatic  layer  and  ectoderm. 


mccUxllarx  plate 

.Tn€.ciunary  fold 

iwtochortt 
mesoderm 

endoderm 
enterpcoeli<t 

sctoderm 


YnedujcWaxy  fokL 

iTiyotome/ 
tnyoco€/l 
denrjCLtome- 
nephrotome 

notoebord; 
e.ndoderni 

Sorrxxt'id. 
mesoderm 

$plctnch.n\c 
mesodema 

ectode-rm. 

>ieura.l  cocnal 
ocnd,  tijcbe. 
myotorne 
rnyocoel 
dermatome. 

fn<as«ntery 

Somatic  and. 

splanchnic 

mesooLerm 

©ncCocC<sr-rn 
.<SCtodernri 

Diagram  of  a  generalized  vertebrate  to 
show  the  origin  and  early  differentiation  of 
the  ectoderm,  endoderm.  and  mesoderm. 
(I)  shows  the  mesoderm  arising  by  means 
of  the  enterocoelic  pouches  budfling  off 
from  the  archenteron.  Above  and  between 
these  pouches  lie  the  beginnings  of  the 
notochord.  In  (II)  the  medullary  plate  has 
formed  the  neural  tube  and  the  mesoderm 
has  become  differentiated  into  regions 
which  will  form  somites  (myotomes),  kid- 
neys, and  linings  of  the  body  cavity.  This 
differentiation  goes  still  further  in  (III). 
(After  McEwen.) 


420 


THE   MAINTENANCE   OF  SPECIES 


blastopore 


Diagram  to  show 
the  closure  of  the 
blastopore  in  a  frog. 
Figures  I  to  III  are 
views  from  the  vege- 
tal pole.  The  rota- 
tion so  typical  of  de- 
veloping amphibian 
eggs  has  been  started 
in  III  and  completed 
in  IV.  The  view  in 
IV  is  from  the  poten- 
tial ventral  side  of 
the  embryo.  (After 
Jenkinson.) 


Early  Differentiation  of  the  Embryo 

It  must  be  borne  in  mind  that  the  changes  out- 
lined follow  a  definite  pattern  and  that  some  of 
them  are  going  on  simultaneously.  One  of  the 
first  changes  after  gastrulation  is  a  gradual  in- 
crease in  the  length  of  the  embryo  due  largely 
to  the  rapid  cell  divisions  about  the  lips  of  the 
blastopore,  which  forms  the  exterior  opening  of 
the  archenteric  cavity.  The  result  is  a  gradual 
fusion  by  a  backward  growth  of  the  lips  of  the 
blastopore,  which  thus  produces  an  elongated 
line,  the  primitive  streak.  This  is  one  of  the  best 
known  embryological  landmarks.  Anterior  to 
the  primitive  streak  there  soon  develops,  partially 
produced  by  a  sinking  of  the  ectoderm,  two 
closely  associated  parallel  folds  of  ectoderm, 
which  extend  anteriorly  forming  the  walls  of  the 
neural  groove.  Gradually  an  anterior-posterior 
fusion  of  the  walls  of  the  groove  produces  the 
central  nervous  system,  a  dorsal  tubular  structure 
characteristic  of  the  vertebrates.  Sheets  of  meso- 
derm likewise  grow  anteriorly  and  laterally  from 
the  region  of  the  primitive  streak,  soon  splitting 
distally  to  form  the  splanchnic  and  the  somatic 
layers.  Meantime  beneath  this  the  gut  is  form- 
ing and  being  pinched  off  from  the  yolk  beneath. 
In  its  anterior  part,  the  pharyngeal  gill-pouches 
and  later  the  gill-slits  appear,  together  with  out- 
growths which  form  the  lining  of  the  thyroid  and 
thymus  glands.  Posterior  to  this  region  there 
soon  develops  a  ventral  out-pocketing  of  the  gut, 
which  later  forms  the  lungs  in  land  animals,  while 
still  further  posteriad  lie  the  forerunners  of  the 
liver  and  pancreas. 

The  degree  of  closure  of  the  gut  along  the 
ventral  surface  of  the  embryo  is  largely  depend- 
ent upon  the  quantity  of  yolk  present  in  the  egg. 
An  egg  containing  little  or  a  moderate  amount  of 
yolk,  as  in  Amphioxus  or  the  frog,  respectively, 


REPRODUCTION   AND   LIFE   CYCLES 


421 


has  the  ventral  body  wall  completed  early  in  development.  In  such 
forms  the  yolk  that  remains  is  carried  within  the  body  of  the  embryo 
and  is  accessible  as  fuel  for  further  metabolism. 


ectoderm-*^ 


.hldi 
*ilastocoele^ 


IbU.  isoWcifhal 

ed'tf  .    amphioxa? 


f  "blastopore 


fiCtocCarm 
endodarm 


"blC. 


telol<2cLthal 
egig.  amphibicin 

ectoderm  ^Wc. 

-blp. 
endode.] 


giastrocoelQ 


teldleciti2al 
eg"g  -  toiT'd 

Diagram  showing  effect  of  yolk  on  the  formation  of  the  gastrula.     Read  text  p.  420 
and  attempt  to  describe  the  etfect  of  yolk  on  gaslrulation.      (After  Patten.) 

In  many  of  the  fishes  that  are  relatively  large-yolked  forms,  develop- 
ment is  similar.  Young  fry  of  the  small-mouthed  bass  carry  around 
enough  yolk  to  maintain  their  "  flame  of  life  "  for  about  two  weeks, 
after  which  they  begin  feedhig  on  the  usually  plentiful  plankton 
organisms.  Whereas  in  a  macrolecithal  type  with  an  abundant 
supply  of  yolk,  such  as  a  bird's  egg,  the  gut  fails  to  close  until  a 
much  later  date,  the  embryo  literally  floating  on  top  of  the  mass  of 
potential  food.  Even  as  development  continues  there  is  such  a  vast 
quantity  of  yolk  present  that  it  appears  impossible  for  the  embryo 
to  complete  the  ventral  body  wall  until  much  of  the  potential  food 
material  has  been  absorbed.  As  this  process  takes  some  time  the 
embryo  remains  independent  of  other  sources  of  food  material  until  it 
gradually  depletes  the  supply,  and  surrounds  the  remainder  of  the 
yolk  with  the  continued  outgrowth  of  the  gradually  extending  germ 
layers. 


422  THE   MAINTENANCE   OF   SPECIES 

Tissue  Formation 

Each  of  the  three  primary  germ  hiyers  produces  a  number  of 
different  tissues  that  in  turn  form  the  various  organ  systems.  Briefly 
summarized,  the  ectoderm  forms  all  of  the  nervous  tissue,  which  in 
turn  makes  up  the  nervous  system,  as  well  as  the  organs  of  special 
sense  that  are  developed  in  connection  with  it.  The  ectoderm  also 
gives  rise  to  the  epidermis  of  the  integument  and  its  various  derivatives 
such  as  scales,  hair,  horn,  nails,  feathers,  and  the  enamel  of  the  teeth. 
In  addition  the  linings  of  the  mouth,  anus,  and  nasal  passages  also 
come  from  the  ectodermal  epithelial  tissue. 

The  endoderm  forms  the  epithelial  tissue  lining  the  digestive  tract 
with  the  exception  of  its  extremities  which  come  from  the  ectoderm. 
Many  zoologists  believe  that  all  the  various  outgrowths  from  the 
digestive  tract,  for  example,  the  lungs,  air  tubes,  and  liver,  as  well 
as  various  out-pocketings  from  the  pharynx  such  as  the  thymus  and 
thyroid,  contain  a  significant  endodermal  contribution.  In  some 
chordates,  the  notochord  buds  oE  from  the  endoderm.  It  should  be 
noted,  however,  that  in  the  case  of  the  lungs  and  liver  considerable 
amounts  of  mesodermal  tissue  also  enter  into  the  formation  of  these 
organs. 

The  mesoderm  is  the  largest  contributor  to  the  tissues  and  different 
systems  of  the  body.  The  circulatory  tissue  is  derived  from  the 
mesenchyme  of  the  mesoderm,  while  both  skeletal  and  muscular 
tissues  and  frequently  the  notochord  come  from  this  germ  layer. 
Likewise,  both  the  excretory  and  reproductive  systems  are  derived 
from  the  mesoderm,  which  also  makes  some  contribution  to  the 
respiratory  system.  Finally  the  derma  of  the  skin,  cartilage,  con- 
nective tissues,  such  as  ligaments  and  tendons,  and  the  peritoneal 
lining  of  the  coelomic  cavity,  may  be  classified  as  mesodermal 
derivatives. 

Protective  Devices  for  the  Embryo 

Egg  Shells 

Various  and  sundry  varieties  of  protective  envelopes  for  ova  are 
found  throughout  the  animal  kingdom.  Although  protozoa  do  not 
have  eggs,  encysted  forms  are  protected  from  unfavorable  environ- 
mental conditions  by  hard  coats  analogous  to  shells.  For  example, 
the  cyst  of  Endameba  histolytica,  the  causative  organism  of  amebic 


REPRODUCTION   AND    LIFE   CYCLES 


42.3 


dysentery,  passes  from  the  alimentary  canal  of  man  safely  protected 
by  a  thick,  hyaline  coat,  until  such  time  as  ingestion  by  a  suitable 
host  brings  about  its  dissolution  in  the  host's  stomach.  The  eggs  of 
some  of  the  tapeworms  and  roundworms  are  surrounded  by  dense 
impervious  shells,  rendering  them  viable,  in  the  ca.se  of  Ascaris,  for 
five  or  six  years.  Some  of  the  parasitic  roundworms  are  ovovivipa- 
rous,  retaining  the  eggs 
within  the  body  of  the 
parent  until  thoy  are 
nearly  ready  to  hatch. 

A  few  fishes,  like  some 
of  the  skates,  produce 
an  egg  surrounded  by  a 
hard,  leatherlike  ca.se, 
which  is  drawn  out  into 
entangling  tendrils 
that  readily  become 
enmeshed  in  seaweeds, 
thus  affording  protec- 
tion to  the  egg.  Most 
of  the  fresh-water  fishes 
and  amphibians,  how- 
ever, lay  eggs  which  are 
protected  by  nothing 
more  than  a  gelatinous 
mass  which  .swells  after 
the  eggs  are  laid  in  the 
water  and  are  fertilized 

by  the  sperm.  Among  the  reptiles  and  birds  a  hard  shell  is  usually 
produced  which  gives  protection  to  the  enclosed  ovum  with  its 
stored  food.  Only  one  small  group  of  mammals,  the  monotremes, 
lay  eggs,  all  others  being  viviparous. 

The  Yolk  Sac 

Among  the  fishes  which  lay  telolecithal  eggs  containing  enough 
yolk  to  render  the  cleavage  pattern  irregular,  a  mass  of  undivided 
yolk  accumulates  beneath  the  developing  embryo.  Soon,  however, 
the  blastoderm  upon  which  the  embryo  lies  grows  down  over  the 
yolk,  eventually  enclosing  it.  This  mass  of  tissue  is  composed  of  an 
inner  layer  of  endoderm  and  an  outer  lining  of  mesoderm  and  is  called 
H.  w.  H.  —  28 


Embryo  and  egg  case  of  skate.  Such  cases  afford 
protection  against  wave  action.  What  other  types 
of  adaptations  are  there  for  the  protection  of  eggs 
and  embryos.^      (After  Walker.) 


4.24 


THE  MAINTENANCE  OF  SPECIES 


brain 


^pirzccL  CarcC 


digestive, 
•trcxct/ 


the  yolk  sac.  Gradually 
blood  vessels  develop 
in  the  mesenchyme  of 
the  yolk  sac,  facilitat- 
ing the  transportation 
of  food  to  the  develop- 
ing embryo. 

Amnion  and  Chorion 

In  addition  to  the 
protection  afforded  by 
egg  membranes  or  shells 
and  the  yolk  sac,  the 
higher  vertebrates, 
namely,  the  reptiles, 
birds,  and  mammals, 
elaborate  additional 
embryonic  membranes 
that  serve  not  only  as 
supplementary  protec- 
tive devices  to  keep  the 
embryos  from  mechan- 
ical injury  but  also  tem- 
porarily handle  the  problems  of  respiration,  excretion,  and  nutrition. 
In  order  to  understand  their  functions,  and  the  fact  that  their 
evolution  is  intimately  tied  up  with  that  of  the  land-inhabiting 
reptiles,  birds,  and  mammals,  one  must  trace  their  embryological 
development. 

As  long  as  organisms  returned  to  the  water  during  the  breeding 
season,  as  the  amphibians  still  do,  the  exchange  of  gases  and  elimination 
of  wastes  takes  place  directly,  since  the  surrounding  water  not  only 
contains  sufficient  dissolved  oxygen  but  also  it  soon  dissipates  waste 
products  which  are  passed  through  the  egg  membranes  and  elimi- 
nated. With  the  acquisition  of  a  land  habitat,  the  inability  to  return 
to  the  water  to  spawn  presented  new  problems,  centering  about  the 
control  of  metabolism  in  the  embryo.  These  needs  were  met  through 
the  elaboration  of  a  series  of  embryonic  membranes,  which  were 
apparently  developed  to  facilitate  the  carrying  on  of  normal  metabolic 
processes  through  a  permeable  egg  shell.  They  occur  in  modified 
forms  in  all  land  vertebrates. 


SomotopW 

5plar2diT7op'l<aure 

Diagram  of  a  developing  fish  embryo.     Note  the 
"  contained  "  yolk  sac.     What  is  its  ultimate  fate  ? 


REPRODUCTION   AND   LIFE   CYCLES 


125 


The  first  of  these  new  membranes  to  be  considered  are  the  amnion 
and  chorion.  They  may  be  best  understood  by  studying  their  origin. 
It  will  be  recalled  that  in  telolecithal  eggs  the  endoderm  does  not 
succeed  at  once  in  growing  ventrally  to  meet,  and  so  to  close,  the 
digestive  tube.  Instead  the  unclosed  tube  lies  flat  upon  the  surface 
of  the  yolk.  Both  the  ectoderm  and  mesoderm  grow  laterally  over 
the  endoderm  dii'ectly  over  the  yolk  on  the  inner  layer  of  the  blasto- 
derm.    The  mesoderm  as  a  whole  divides  into  three  portions,  the 


tmbTYo.. 


ectoderm 

.Tnesoderni 

...endbcterm. 


coelom. 


allantoic  cavity 
amnion   \      aWantois 


amniotic- 
cavitv 

znhryoy 


shell 

niembn 

olbixmen...'? 


embryo. 


■c.^voll-c" 


chorion! - 


extra--' 

embr/onid 

coelom 


">.,viLe!Une 
mambrana 


,ai-nniotic  Cavity 


allantois 

ommotic 
CCLVity.. 


choriotv 


allantpis 
chorion., 
aranion 


yolk 
vitelline 
menabrans 

.^  allantoic  cavity^^^^.^  sWlc 
'   ,">tolk  scut. 


--yolk 

^/itilli-ne 
iTjsmbran 


._yolk' 

vitelline 
iriembrane 


Development  of  the  extra-embryonic  membranes  in  the  chick.  State  the 
contribution  of  each  germ  layer  to  the  amnion,  chorion,  and  yolk  sac.  (After 
Patten.) 


first  of  which  is  the  upper  epimere  part  immediately  flanking  the 
developing  neural  tube  and  producing  the  somites.  Beneath  the  epi- 
mere lies  a  small  mesomeral  portion  that  later  develops  the  excre- 
tory and  reproductive  systems  from  a  ridge  lying  in  the  dorsal  wall  of 
the  coelom.  The  mesoderm  below  the  mesomere  is  the  hypornere, 
which  soon  divides  into  an  outer  somatic  and  inner  splanchnic  layer 
of  mesoderm.  In  large-yolked  eggs  this  hypomeral  portion  extends 
laterally  over  the  endoderm  which  is  covering  the  surface  of  the 
yolk.     In  all  of  the  higher  groups,  beginning  with  the  reptiles,  the 


426  THE   MAINTENANCE   OF  SPECIES 

superficial  ectoderm  and  the  outer  or  somatic  mesoderm  are  con- 
tiguous, and  together  are  called  the  somatopleure.  They  grow  up 
from  the  surface  to  produce  folds  known  respectively  as  head,  tail,  and 
lateral  folds,  and  these  folds  in  turn  grow  up  and  over  the  embryo  from 
the  head  posteriorly  until  they  meet  and  fuse.  Upon  dissolution 
of  the  wall  at  the  point  where  these  folds  meet,  two  new  complete 
layers  covering  the  embryo  are  produced,  the  inner  layer  of  somato- 
pleure being  known  as  the  amnion,  and  the  outer  as  the  chorion.  The 
amniotic  cavity  between  the  amnion  and  the  embryo  is  lined  with 
ectoderm  and  becomes  filled  with  a  shock-absorbing  amniotic  fluid 
which  serves  the  additional  function  of  keeping  the  embryo  moist. 
Outside  the  amnion  is  the  chorionic  cavity  which  is  lined  with  somatic 
mesoderm. 

All  of  the  time  that  the  head,  tail,  and  lateral  folds  of  the  amnion 
are  developing,  the  yolk  is  being  reduced  and  the  splanchnopleure, 
composed  of  the  endoderm  and  splanchnic  mesoderm,  is  growing  down 
and  around  it  to  complete  the  yolk  sac.  The  outer  margins  of  the 
somatopleure  at  the  base  of  the  developing  amniotic  folds  likewise 
continue  to  grow  down  and  around  the  yolk  sac  until  they  finally  meet 
ventrally.  This  new  layer  may  really  be  called  a  continuation  of 
the  chorion,  while  the  cavity  lying  between  the  outer  surface  of  the 
yolk  sac  and  the  inner  side  of  the  chorion  is  in  reality  but  a  continu- 
ation of  the  body,  or  coelomic  cavity.  Because  of  its  position  this 
portion  of  the  coelomic  cavity  becomes  known  as  the  extra-embryonic 
coelom.  It  will  be  seen  from  the  figure  (page  425)  that  the  chorionic 
cavity  is  nothing  but  an  outgrowth  from  this. 

Allaniois 

A  yolk  sac  is  developed  in  all  of  the  egg-laying  types  of  reptiles  and 
birds.  Even  in  the  mammals,  it  is  present  in  a  reduced  form.  Rep- 
tiles, birds,  and  mammals,  however,  develop  a  fourth  embryonic  struc- 
ture called  the  allantois,  which  serves  as  an  excretory  and  respiratory 
organ.  While  the  yolk  sac  is  attached  by  a  yolk-stalk  to  the  mid- 
gut region,  the  allantois  develops  as  a  diverticulum  from  the  ventral 
surface  of  the  hind-gut.  Its  growth  does  not  start  until  after  the 
amnion  and  chorion  are  in  the  process  of  formation.  Almost  at  once, 
however,  this  out-pocketing  encounters  the  inner  layer  of  mesoderm 
so  that  the  allantois  comes  to  be  lined  by  endoderm  on  the  inside  and 
covered  by  splanchnic  mesoderm  on  the  outside.  The  outgrowth 
continues,  extending  out  into  the  extra-embryonic  coelom  and  up 


REPRODUCTION  AND  LIFE  CYCLES 


427 


into  the  chorionic  cavity.  Thus  the  allantois  in  reptiles  and  birds 
comes  to  He  close  to  the  porous  shell,  where  it  is  well  supplied  with 
blood  vessels  and  so  readily  becomes  a  membrane  through  which 
oxygen  may  be  secured  and  the  various  waste  products  of  metabolism 
eliminated. 

Placenta 

In  all  mammals  except  the  egg-laying  types  and  the  marsupials, 
who  bring  forth  their  young  in  an  immature  stage  of  development,  a 
new  mechanism,  the  placenta,  is  evolved  to  supply  the  metabolic  needs 


placenta 
yolk  $ojt 

allantois  .r> 
chorion... 

<xmn\on.. 


amniotic 

cavity 

fillecCVitlrj 

ocrnniotiC' 

fluid. 


Fallopian  tube 


Cavitvof 
tfie  uterus 


mucus  plug 


muscular  volls  oj^lctefifS- 


Diagrammatic  sagittal  section  of  human  uterus.     What  devices  do  you  find  for 

protection  and  nourishment  .^ 

of  the  embryo.  Other  important  changes  are  associated  with  the 
formation  of  this  structure.  In  the  first  place  the  developing  embryo 
reaches  the  uterus  and  becomes  implanted  in  the  uterine  mucosa  at 
about  the  time  of  gastrulation.  The  amnion  is  formed  and  serves  the 
same  protective  function  as  in  the  lower  types,  while  the  chorion  is 
intimately  associated  with  the  maternal  tissue  lining  the  uterus  and 
so  becomes  concerned  with  respiration,  excretion,  and  nutrition. 
Blood  vessels  invade  this  modified  chorion,  extending  from  it  down  the 


428  THE   MAINTENANCE   OF   SPECIES 

umbilical  cord  to  the  embryo.  From  the  surface  of  the  chorion 
fingerhke  projections,  or  villi,  push  out  which  interdigitate  with 
similar  fingerhke  processes  of  the  uterine  wall,  thus  facilitating  the 
maintenance  of  metabolism.  This  portion  of  the  chorion  together 
with  the  wall  of  the  uterus  in  which  the  embryo  is  embedded  is  usually 
designated  as  the  placenta.  While  there  is  no  exchange  of  blood 
between  the  parent  and  embryo,  their  two  blood  streams  in  the  case 
of  the  primates  are  separated  only  by  the  lining  of  the  fetal  capilla- 
ries, the  connective  tissue  surrounding  them,  and  the  epithelial  layer 
on  the  surface  of  the  chorionic  villi.  While  the  allantois  does  develop 
in  the  mammalian  embryo,  it  is  incorporated  into  the  growing  placenta 
and  in  primates  is  really  functionless,  except  for  the  proximal  portion 
which  is  transformed  into  the  urinary  bladder  of  mammals.  As  the 
embryonic  membranes  are  not  permanent  structures  they  are  dis- 
carded at  birth. 

Elaboration  of  Germ  Cells,  or  Gametogenesis 

It  should  be  borne  in  mind  that  the  germ  cells  themselves  can  be 
traced  back  in  the  developing  embryo  only  to  a  certain  point  which 
varies  in  different  groups.  In  the  chick,  for  example,  the  germ  cells 
may  be  traced  to  the  anterior  margin  of  the  blastoderm.  In  some 
invertebrates,  such  as  Ascaris  megaloccphala  hivalvens,  it  has  been 
shown  that  the  germ  cells  may  be  detected  at  the  thirty-two  cell  stage. 
In  the  latter  instance  the  primordial  germ  cell  may  be  readily  detected 
by  its  size. 

While  the  primordial  germ  cells  are  present  early  in  the  life  of  the 
individual,  it  frequently  happens  that  the  organism  does  not  mature 
for  some  time  and  consequently  the  development,  or  maturation,  of 
functional  germ  cells  is  delayed.  Usually  the  maturation  process 
covers  a  considerable  period  of  time  which,  in  the  case  of  a  male, 
terminates  in  the  elaboration  of  sperm.  Hence  the  entire  process  is 
called  spermatogenesis,  while  in  the  female  the  production  of  ova  is 
known  as  oogenesis.  Both  phenomena  may  be  spoken  of  collectively 
as  gametogenesis. 

Formation  of  Sperm  —  Spermatogenesis 

The  primordial  germ  cells  of  the  male  undergo  an  extended  period 
of  division,  the  resulting  cells  of  which  are  designated  as  sperma- 
togonia.    These   reproduce   other   spermatogonial   cells   by   normal 


REPRODUCTION  AND   LIFE  CYCLES 


429 


mitotic  cell  division,  and  when  ready  for  the  final  maturing  stages 
they  first  undergo  a  period  of  growth  in  which  the  cells  increase  some- 
what in  size.  At  this  point  one  must  look  inside  the  cell  to  see  what  is 
happening  within  the  nucleus.  Here  the  chromosomes  are  paired. 
Each  member  coming  from  the  male  or  from  the  female  parent,  re- 
spectively, is  identical  as  to  shape  and  size  with  the  exception  in 


first 
polar  hsdy 


OOi 


SeconcC 
bocCy 


primordiaX    "penod.  of 
germ  cells     rnitoU'c 
divisiorz 


drowth  period 
synapsis  and. 
tatrods  formed. 


first  msiotjo  ^eixnd        moXicnz^ 
ciivision.     meioticdiViSiai  germ  cells 


spermatogonia 


Secondary  


Diagram  illustrating  meiosis  and  the  maturation  of  the  germ  cells.  Explain 
how  a  constant  number  of  chromosomes  is  maintained  for  a  given  species.  (After 
Curtis  and  Guthrie.) 


certain  cases  of  the  pair  of  so-called  sex  chromosomes.  The  sperma- 
togonium has  now  been  transformed  by  this  process  into  a  primary 
spermatocyte.  When  mitosis  takes  place  each  chromosome  instead  of 
being  split  longitudinally  as  in  the  case  of  normal  mitosis  becomes 
separated  so  that  one  entire  member  of  each  pair  of  homologous 
chromosomes  is  passed  to  each  daughter  cell.  This  brings  about  an 
actual  reduction  of  the  numbers  of  chromosomes  present  in  each 
daughter  cell  by  one  half.  This  division  (meiosis)  is  spoken  of  as  the 
reduction  division  and  the  number  of  chromosomes  as  the  haploid 
number  in  contrast  with  the  normal  or  diploid  number  found  in  nil 


430  THE   MAINTENANCE   OF   SPECIES 

other  cells.  Each  of  the  daughter  cells  is  now  a  secondary  spermatocyte 
producing  two  spermatids  by  the  next  division  in  which  each  of  the 
remaining  chromosomes,  as  in  usual  mitosis,  splits  longitudinally  in 
half,  thus  maintaining  the  haploid  number  in  each  cell.  Each  sperma- 
tid eventually  undergoes  a  metamorphosis  into  an  active  sperm  with- 
out further  cell  divisions.  Thus,  each  primary  spermatocyte  pro- 
duces four  functional  sperm. 

Formation  of  Ova  —  Oogenesis 

Oogenesis  differs  from  spermatogenesis  only  in  certain  essential 
respects,  although  the  corresponding  stages  must  necessarily  be 
designated  differently.  Thus  the  primary  germ  cells  produce  oogonia 
which  in  turn  produce  primary  and  secondary  oocytes,  polar  bodies,  and 
finally  ova.  In  the  period  of  growth  intervening  between  the  oogonium 
and  its  transformation  into  a  primary  oocyte  there  is  a  large  accumu- 
lation of  stored  food  and  an  accompanying  increase  in  size.  In  the 
next  stage,  when  the  primary  oocyte  undergoes  its  reduction  division 
the  resulting  cells  are  of  unequal  size,  one  becoming  much  larger 
than  the  other,  having  monopolized  all  of  the  yolk.  The  smaller 
one  is  in  reality  an  aborted  secondary  oocyte  and  is  called  the  first 
polar  body.  The  second  maturation  division  again  results  in  the 
formation  of  a  relatively  large  egg  and  a  tiny  second  polar  body. 
Sometimes  the  first  polar  body  likewise  undergoes  division,  formmg 
a  total  of  three  small  polar  bodies  and  one  large  ovum. 

The  process  of  fertilization  brings  together  the  male  and  the  female 
pronuclei,  each  of  which  contains  the  haploid  number  of  chromosomes. 
By  this  means  the  diploid  number,  or  full  complement  of  chromosomes, 
is  restored.  Each  chromosome,  moreover,  is  composed  of  a  number 
of  genes  arranged  on  it  like  a  string  of  beads.  The  manner  in  which 
this  mechanism  functions  in  bringing  about  variations  in  the  offspring 
will  be  considered  in  the  unit  on  genetics  (page  457) . 

The  New  E)mbryology 

The  question  as  to  just  how  far  back  one  can  trace  the  develop- 
mental pattern  of  an  embryo  is  one  which  has  long  fascinated  the 
zoologist.  Great  strides  along  this  line  have  been  made  in  recent 
years  by  the  students  of  experimental  embryology.  We  know  that 
fertilized  ova  develop  with  great  rapidity  into  well-formed  embryos, 
characterized  first  by  germ  layers,  later  by  tissues,  and  finally  by 


REPRODUCTION    AND   LIFE   CYCLES  431 

systems  of  organs.  The  modern  experimental  cmbryologist  raises 
the  specter  of  the  old  controversy  of  rpigenesis  or  preformation,  by 
inquiring  into  the  question  of  how  much  of  the  development  is  depend- 
ent upon  the  contents  of  the  fertilized  egg  and  how  much  is  due 
to  environmental  factors. 

Genes 

All  of  the  evidence  which  has  been  gathered  to  date  indicates  that 
the  development  of  an  embryo  is  a  highly  complicated  process.  As 
a  starting  point  one  might  mention  the  character-controlling  genes 
of  the  chromosomes  that  are  brought  together  in  the  formation  of  a 
zygote.  The  vital  part  w^iich  these  play  in  altering  developmental 
patterns  has  been  clearly  demonstrated  many  times. 

Environment 

The  second  important  factor  is  the  environment.  Changes  in  the 
normal  environment  frequently  result  in  abnormalities.  It  is  well 
known  that  temperature  is  a  vital  factor,  since  in  all  except  viviparous, 
warm-blooded  forms,  a  change  in  temperature  will  affect  the  rate  of 
development.  Under  some  conditions,  for  example  when  gastrula- 
tion  is  occurring,  atypical  forms  may  result.  Likewise  variations  in 
temperature  may  produce  apparent  changes  in  the  genes  themselves. 
When  certain  kinds  of  fruit  flies  are  kept  at  a  higher  temperature, 
there  is  a  decrease  in  the  number  of  ommatidia  produced  in  each  eye. 
Subsequent  breeding  experiments  and  a  lowering  of  the  temperature, 
however,  result  in  a  return  to  the  original  type.  Another  example  of 
the  environmental  influence  which  upsets  the  normal  metabolism  of 
the  embryo  so  that  abnormalities  result  may  be  seen  in  the  alter- 
ation of  the  oxygen,  or  food  supply.  The  introduction  of  poisons 
also  has  similar  effects. 

Changes  in  the  metabolic  rate  of  an  organism  are  definitely  cor- 
related with  environmental  factors  as  shown  by  the  work  of  Child 
and  his  associates,  who  demonstrated  the  presence  of  definite  "meta- 
bolic gradients."  The  axial  gradient  theory  accounts  for  differences  in 
dominance  of  certain  areas  in  the  developing  organism,  beginning 
with  the  axis  occurring  between  the  two  poles  of  an  egg.  The  dorsal 
lip  of  the  blastopore  soon  becomes  established  as  the  region  of  greatest 
metabolic  activity  and  so  determines  the  rate  of  development  of  the 
other  parts.     It  is  at  this  region  of  highest  metabolic  activity  that  the 


432  THE   MAINTENANCE  OF  SPECIES 

head  develops.  Such  differences  in  metaboHc  rates  between  differ- 
ent parts  of  an  organism  have  been  demonstrated  experimentally 
and  it  is  probable  that  they  are  related  to  differences  in  the  oxygen 
supply. 

Natural  Potencies 

Great  differences  normally  occur  between  the  so-called  "potencies" 
of  various  species  of  eggs.  Some  species  of  animals  produce  toti- 
potent eggs.  These  are  eggs  in  which  the  formative  material  is  equally 
distributed  throughout  the  component  cells,  or  hlastomeres  during 
early  development.  The  resulting  cleavage  is  called  indeterminate 
because  all  cells  up  to  a  certain  stage  are  totipotent,  a  condition 
that  may  be  demonstrated  by  separating  the  various  blastomeres, 
for  example,  from  the  two-celled  to  the  sixteen-celled  stage  in  some 
of  the  jellyfish,  and  securing  normal,  though  perhaps  dwarfed,  indi- 
viduals from  each.  Cleavage  in  man  is  apparently  of  this  type,  and 
is  the  logical  explanation  of  the  production  of  identical  twins. 

In  the  case  of  non-totipotent  species  the  cleavage  pattern  is  said  to 
be  determinate.  There  is  little  doubt  that  many  of  the  determinative 
factors  are  already  present  in  the  cytoplasm  of  an  egg  before  it  is 
fertilized.  In  such  forms  as  the  mollusc,  Dentalium,  or  the  tunicate, 
Styela,  the  cytoplasm  of  the  egg  itself  appears  to  be  arranged  in  a 
definite  pattern  with  respect  to  its  future  development.  In  such 
cases  the  early  separation  of  blastomeres  results  in  the  formation  of 
partial  embryos. 

Organizers 

Certain  parts  of  embryos  are  called  organizers  because  they  appear 
to  be  more  or  less  directly  responsible  for  the  development  of  other 
closely  associated  regions.  Much  experimental  work  has  been  done 
abroad  by  Spemann  and  his  co-workers,  and  in  this  country  by 
Harrison  and  his  students,  all  of  which  demonstrates  the  presence  of 
such  organizers.  Perhaps  one  of  the  most  important  organizers  is  the 
dorsal  lip  of  the  blastopore.  That  this  region  is  normally  associated 
with  the  development  of  a  neural  plate  may  be  demonstrated  by 
transplanting  it  to  a  region  beneath  the  ventral  ectoderm  of  a  frog's 
gastrula,  where  one  would  normally  expect  the  formation  of  epidermis, 
but  instead  an  aberrant  neural  plate  appears.  Such  experimental 
evidence  has  been  most  carefully  checked  and  rechecked  by  all  manner 


REPRODUCTION   AND   LIFE   CYCLES  433 

of  transplantation  experiments.  Naturally  the  stage  of  development 
reached  at  the  time  of  transplantation  affects  the  results  obtained. 
Much  work,  however,  remains  to  be  done  in  this  fascinating  field. 

SUGGESTED   READINGS 

Huxley,  J.  S.,  and  DeBeer,  R.  G.,  Elements  of  Experimental  Embryology,  The 

Macmillan  Co.,  1934. 

Scientific  but  readable  account  of  modern  embryology. 
McEwen,  R.  S.,  Vertebrate  Embryology,  rev.  ed.,  Henry  Holt  &  Co.,  1931. 

A  standard  elementary  text  for  reference. 
Morgan,  T.  H.,  Embryology  and  Genetics,  Columbia  University  Press,  1934. 

Popularly  written  attempt  to  tie  up  modern  embryology  and  genetics. 
Patten,  B.,  Early  Embryology  of  the  Chick,  3rd  ed.,  P.  Blakiston's  Son  & 

Co.,  1929. 

Excellent  account  of  avian  development. 
Richards,  A.,  Outline  of  Comparative  Embryology,  John  Wiley  &  Sons,  Inc., 

1931.     Pp.  20-90. 
Wells,  H.  G.,  Huxley,  J.  S.,  Wells,  C.  P.,  The  Science  of  Life,  Doubleday, 

Doran  &  Co.,  1934.     Pp.  150-159. 

Popular  account  of  human  development. 


XX 


THE  GREAT  RELAY  RACE 

Preview.  Seed  and  soil  •  Independence  of  the  germplasm  •  Lines  of 
approach  •  The  experimental  method  :  The  usefulness  of  hybrids  ;  Mendelism  ; 
what  Mendel  did  ;  monohybrids,  dihybrids,  trihybrids,  and  other  crosses : 
Unit  characters  and  factors,  modified  ratios,  different  kinds  of  factors  • 
Practical  breeding :  Selection,  mass  selection,  pedigree  breeding,  progeny 
selection;  inbreeding  and  cousin  marriage;  outbreeding  and  hybrid  vigor; 
asexual  propagation  ■  The  germplasmal  method  :  Chromosomes ;  genes ; 
linkage  and  crossing-over;  chromosome  maps  •  The  role  of  cytoplasm  • 
Sex  in  heredity  •  Suggested  readings. 

PREVIEW 

"Now  these  are  the  generations  of  Pharez :  Pharez  begat  Hezron,  and 
Hezron  begat  Ram,  and  Ram  begat  Amminadab,  and  Amminadab  begat 
Nahshon,  and  Nahshon  begat  Salmon,  and  Salmon  begat  Boaz,  and  Boaz 
begat  Obed,  and  Obed  begat  Jesse,  and  Jesse  begat  David." 

As  will  be  remembered,  along  came  Ruth  at  the  Boaz  stage  and 
injected  a  welcome  bit  of  romance  into  these  dry  statistics.  It  is  not, 
however,  the  vivid  story  of  this  Moabite  woman,  who  was  in  her  day 
so  young  and  charming,  that  is  the  reason  for  introducing  this  quo- 
tation from  the  Book  of  Ruth,  but  rather  the  bare  record  of  names 
in  itself,  together  with  the  indispensable  "begats,"  that  claims  our 
immediate  attention  now.  The  generations  of  mankind  have  always 
been  hooked  up  in  this  chainlike  fashion.  The  spark  of  life  has 
always  been  borne  forward  for  certain  intervals  of  time  by  indi- 
viduals, and  then  transmitted  to  individuals  of  another  generation  to 
carry  on.  This  is  the  Great  Relay  Race,  participated  in  alike  by  all 
human  beings,  lower  animals,  and  plants.  It  depends  upon  the 
co-operation  of  long  lines  of  separate  mortal  individuals  who  play 
their  temporary  part  and  then  inevitably  die,  while  the  immortal 
enduring  line  of  life  itself  persists.  The  science  of  genetics  attempts 
to  explain  how  such  a  relay  race  is  run. 

A  single  microscopic  streptococcus,  a  solitary  wandering  housefly, 
or  a  chance  weed  pulled  up  from  the  wayside,  each  can  boast  of  a 
longer  pedigree  than  can  the  King  of  England.     This  universal 

434 


THE   GREAT   RELAY   RACE  435 

principle  of  continuous  inheritance,  although  not  always  recognized, 
has  been  used  and  practiced  as  an  art  from  the  begiiuiing,  not  only  in 
the  case  of  man  himself,  but  also  with  domestic  animals  and  cultivated 
plants.  The  real  factors  of  heredity,  however,  together  with  the 
orderly  "laws"  which  indicate  their  manner  of  working,  have  not 
been  analyzed  and  made  into  a  science  until  within  comparatively 
recent  times.  The  very  word  "genetics"  was  first  employed  by 
Bateson  in  1906. 

To  agree  in  advance  to  conduct  any  would-be  excursionist  down  the 
rapidly  flowing  genetic  river  to  a  definite  landing  place  is  both  pre- 
sumptuous and  unwise,  for  there  are  at  present  too  many  long,  un- 
charted stretches  and  too  much  that  is  unknown  to  make  positive 
textbook  promises  of  this  kind  probable  of  fulfillment.  Nevertheless, 
the  general  direction  in  which  the  river  of  genetics  flows,  in  spite  of 
its  shifting  changes,  is  plain  to  all,  and  the  tales  of  returning  travelers 
invite  us  to  intellectual  adventure.  Students  in  this  field  today, 
however,  must  make  up  their  minds  at  the  start  to  be  alert  explorers 
and  ambitious  pioneers,  rather  than  passive,  personally  conducted 
excursionists. 

Seed  and  Soil 

In  the  relay  race  of  heredity  the  continuous  thing  that  is  handed  on 
from  generation  to  generation  is  not  the  lighted  torch,  but  rather 
something  that  corresponds  to  a  box  of  matches  with  which  another 
torch  may  be  lighted.  Biological  inheritance,  unlike  legal  inheritance 
by  which  material  possessions  are  transferred  from  parents  to  children, 
consists  in  the  transmission  of  genes,  or  ultra-microscopic  chemical 
units  possessing  the  uncanny  capacity,  under  suitable  conditions,  of 
expanding  into  visible  structures  or  traits  that  resemble  those  in  the 
parental  make-up. 

Heredity  binds  the  generations  together  and  is  absolutely  essential, 
but  in  itself  it  is  not  enough.  The  potent  genes,  which  are  the 
determiners  of  heredity,  must  have  a  suitable  setting  in  which  to 
unfold  their  potentialities.  This  necessary  setting  is  called  the 
environment.  It  expresses  and  represents  the  spread  that  occurs 
within  the  Hmits  of  the  hereditary  possibilities,  for  the  hereditary 
pattern  may  be  enhanced  or  dwarfed  in  its  expression  by  the  action 
of  the  environment.  Stated  another  way,  the  environment  does  not 
change  the  quality  of  hereditary  characters,  although  it  makes  possible 
either  a  greater  or  a  lesser  development  of  them. 


436  THE   MAINTENANCE  OF   SPECIES 

Long  ago  Semper  demonstrated,  for  example,  that  the  size  to  which 
fresh-water  snails  will  grow  is  somewhat  dependent  upon  the  spacious- 
ness of  the  aquarium  in  which  they  are  kept,  and  Baur  has  shown 
that  red-flowering  primroses  may  be  made  to  produce  white  flowers 
if  subjected  to  continuous  high  temperature  (30°  C.)  for  a  week  or  so 
immediately  before  blooming. 

The  heredity  factor  is  so  important,  nevertheless,  that  organisms 
can  after  all  breed  only  their  own  kind,  regardless  of  the  environment 
in  which  they  are  placed.  It  is  quite  as  futile,  therefore,  to  argue  the 
relative  importance  of  heredity  and  environment  as  it  would  be  to 
debate  which  of  the  two  surfaces  of  a  sheet  of  paper  is  more  essential 
in  making  it  a  sheet  of  paper.  Naturally  the  biologist  is  impressed 
with  the  contribution  which  heredity  makes  in  the  formation  of  a  new 
individual,  while  the  sociologist,  as  would  be  expected,  emphasizes 
the  environmental  factor.  Although  no  seed  is  so  poor  that  it  may 
not  be  improved  by  good  soil  and  nurture,  and  no  seed  is  so  good  that 
it  will  not  imperfectly  develop  in  poor  soil,  yet  it  is  not  within  the 
capacity  of  tares  under  any  circumstances  to  produce  wheat,  nor  can 
we  expect  dogs  to  engender  cats.  Former  President  Lowell  of  Harvard 
once  said,  "There  is  a  better  chance  to  raise  eaglets  from  eagle  eggs  in  a 
hen's  nest,  than  from  hen's  eggs  in  an  eagle's  nest."  Neither  heredity 
nor  environment  is  effective  alone.  In  the  formation  of  any  individual 
organism,  the  environment  is  the  force  that  works  from  without  in, 
while  heredity  works  from  within  out.  Both  are  as  indispensable  in 
producing  a  plant  or  an  animal  as  land  and  water  are  in  the  formation 
of  a  shore  line. 

Moreover,  there  is  extra-biological  or  social  inheritance  to  reckon 
with,  that  makes  us  the  "heirs  of  the  ages."  CiviHzation  in  itself 
may  be  regarded  as  the  collective  achievements  of  mankind,  and 
as  time  goes  on  these  environmental  collections  multiply  and  accu- 
mulate. We  live  today,  for  example,  in  a  world  of  skyscrapers, 
automobiles,  stock  exchanges,  airplanes,  chain-stores,  movies,  ocean 
liners,  and  radios,  the  acquisition  of  which  our  ancestors  of  three 
hundred  years  ago  never  even  dreamed  of.  If  we  may  seem  to  have 
a  larger  horizon  and  to  sec  farther  than  our  ancestors,  it  is  not  so 
much  because  we  are  taller  than  they  were,  as  it  is  because  we  stand 
on  their  shoulders  with  respect  to  these  extra-biological  acquisitions. 

There  is  no  doubt  that  the  environment  of  mankind  has  undergone 
more  modification  than  human  heredity  has.  When  we  consider,  for 
example,  the  intellectual  and  artistic  output  of  ancient  Greece,  a  small 


THE  GREA-'r   1 11:1. W   llACIi; 


4:n 


ENI/IR0NMEN7 


HELRLDITY 


AJ 


country  in  classical  times  with  restricted  environment,  and  contrast 
it  with  the  corresponding  output  of  the  whole  enlarged  modern  world, 
with  its  highly  elaborated  setting,  there  is  occasion  to  wonder  whether 
the  intrinsic  capabilities  of  man  have  increased  as  much  as  his  oppor- 
tunities. It  has  always  been  easier  for  man  to  modify  his  surroundings 
than  to  control  his  own  heredity.  To  quote  Joseph  Jastrow,  the 
psychologist,  "The  fact  that  modern  schoolboys  are  far  better 
equipped  to  withstand,  utilize,  and  control  the  forces  of  nature  than 
was  Aristotle,  is  not  due  to  the  superiority  of  the  schoolboys,  but  to 
the  contributions  of  the  Aristotles  of  past  generations." 

Furthermore,  the  range  of  hereditary  possibilities,  particularly  in 
the  case  of  man,  may  be  considerably  influenced  by  training  or  educa- 
tion, which  is  a  hopeful  factor  that  perhaps  cannot  be  entirely  ac- 
counted for  either  by  heredity  or  environment.  Education  in  itself 
forms  no  part  of  the  hereditary  stream,  since  it  is  only  the  capacity 
to  acquire  education  in  a  yroper  environment  that  can  be  handed  on 
from  parent  to  child.  In  the 
case  of  plants,  and  those  ani- 
mals whose  automatic  in- 
stincts make  it  unnecessary 
for  them  to  learn  how  to  live, 
the  factor  of  training  or  edu- 
cation does  not  play  as  domi- 
nant a  part  as  in  man. 

In  the  accompanying 
diagram  an  attempt  has  been 
made  to  indicate  the  mutual 
dependence  of  heredity  and 
environment,  in  the  forma- 
tion of  three  different  hypo- 
thetical individuals.  A,  B,  and  C,  represented  by  the  rectangles  in  the 
figure.  When  the  parallel  edge  indicating  the  environment  is  shoved 
back  and  forth,  like  a  slide  rule,  different-sized  rectangles  result. 
The  act  of  shoving,  particularly  when  the  slide  rule  is  shortened  and 
the  "rectangular  individual"  is  consequently  enlarged,  is  much  like 
the  process  of  education  or  training.  In  each  case  it  will  be  noted 
that  neither  the  whole  of  the  hereditary  nor  the  whole  of  the  envi- 
ronmental edge  is  involved  in  the  resulting  individual.  This  cor- 
responds with  our  common  observation  and  conviction  that  neither 
our  capacities  nor  our  opportunities  are  all  ever  entirely  utilized. 


£Nl/IRONMENT 


B 


HLRLDITY 


£:n/ironment 


HEREDITY 

A  "slide-rule"  diagram,  showing  how  the 
interplay  between  heredity  and  environment 
may  result  in  different  individuals,  A,  B, 
and  C. 


438 


THE  MAINTENANCE  OF  SPECIES 


Independence  of  the  Germplasm 

The  germplasm,  or  the  sexual  cells  that  carry  the  load  of  hereditary 
possibilities,  and  the  somatoplasm,  which  makes  up  the  body  of  the 
individual,  although  to  a  certain  extent  dependent  upon  each  other 

in  a  nutritional  way,  are 
remarkably  independent. 
Despite  the  popular  idea  to 
the  contrary,  it  is  extremely 
improbable  that  changes 
wrought  by,  or  impressed 
upon,  the  somatoplasm  ex- 
ercise any  modifying  influ- 
ence upon  the  accompanying 
germplasm.  The  somato- 
plasm is  simply  like  a  casket 
in  which  the  jewel  of  germ- 
plasm reposes.  No  decora- 
tion or  elaboration  of  the 
casket  will  have  any  material 
effect  upon  the  jewel  within. 
This  point  has  been  con- 
vincingly brought  out,  along 
with  other  cumulative  evi- 
dences, in  a  critical  experi- 
ment performed  in  1911  by 
Castle  and  PhiUips.  These 
investigators  successfully 
transplanted  the  ovaries  of 
a  black  guinea  pig  into  a  white  guinea  pig  whose  own  ovaries  had 
been  removed.  Later,  after  recovery  from  the  operation,  when 
this  white  female  with  the  borrowed  ovaries  of  the  black  female  was 
mated  with  a  white  male  guinea  pig,  the  offspring  were  all  black, 
although  both  their  parents  were  white,  and  under  ordinary  circum- 
stances would  produce  only  white  offspring.  This  shows  that 
temporary  residence  within  a  white  somatoplasm  did  not  in  any  way 
affect  the  character  of  the  black-producing  germplasm  that  had  been 
grafted  into  the  white  body. 

The  establishment  of  the  fact  of  the  practical  ineffectiveness  of 
somatic  influence  upon  the  germplasm  has  far-reaching  applications 


Diagram  of  ovarian  transplantation  experi- 
ment by  Castle  and  Phillips,  to  show  the  lack 
of  somatic  influence  on  the  f^erniplasm.  The 
ovaries  of  a  black  guinea  pig  were  engrafted 
into  a  female  albino  whose  ovaries  had  been 
removed.  Upon  recovery  this  female  was 
mated  three  times  with  an  albino  male.  All 
the  progeny  were  black.  (From  Walter, 
Genetics,  by  permission  of  The  Macmillan 
Company,  publishers.) 


THE   GREAT   RELAY   RACE  439 

in  any  theory  of  heredity.  It  means  tliat  modifications  acquired 
within  the  Hfetime  of  the  individual  are  not  transferred  to  the  parental 
germplasm,  and  do  not  consequently  reappear  as  hereditary  charac- 
ters in  the  next  generation.  If  this  conclusion  seems  perhaps  dis- 
couraging to  prospective  parents  who  would  gladly  have  whatever 
success  in  the  building  of  character,  the  development  of  intelligence, 
or  the  attainment  of  artistic  or  other  ability  that  they  have  been  able 
to  bring  about  in  their  own  lifetime  perpetuated  in  their  children, 
they  may  well  be  reminded  of  the  other  side  of  the  picture,  namely, 
that  parental  failures  in  accomplishment  during  life  likewise  form  no 
part  in  their  children's  biological  inheritance.  Each  child,  therefore, 
starts  out  with  his  ancestral  biological  inheritance  unimpaired  by 
either  parental  failures  or  successes.  In  any  case,  the  honest  scien- 
tifically-minded person  is  bound  to  accept  the  facts  whatever  they  are, 
if  they  can  be  ascertained,  regardless  of  the  conclusions  to  which  they 
lead,  rather  than  to  place  dependence  upon  unproven  propositions 
that,  with  wishful  thinking,  he  would  like  to  believe  are  true. 

It  should  be  pointed  out  clearly  that  the  only  biological  opportunity 
where  it  is  possible  to  improve  the  germinal  chances  of  the  next  genera- 
tion is  not  after  the  germinal  equipment  has  already  been  assigned 
to  the  prospective  parent  from  his  ancestors,  but  at  the  critical  time 
of  mating  when  two  streams  of  germplasm  are  selected  for  combina- 
tion. Picking  out  the  right  mother  is  the  most  important  contribu- 
tion which  any  man  can  make  for  his  future  children. 

Thus,  the  individual  somatoplasm  is  simply  the  guardian  and 
executor  of  the  germinal  possibilities  committed  to  its  care.  Heredi- 
tary possibilities  do  not  come  directly  from  the  parents,  but  through 
them  down  the  long  ancestral  line.  When  and  how  remote  ancestors 
have  picked  up  the  gifts  of  biological  inheritance  which  they  present  to 
posterity  forms  one  of  the  most  intriguing  riddles  in  the  science  of 
genetics.  It  is  encouraging  to  know  that  the  results  of  modern 
researches  have  hopefully  opened  up  the  way  to  a  possible  answer  to 
this  question,  which  may  be  more  suitably  developed  later  on. 

Lines  of  Approach 

There  are  two  fundamental  lines  of  approach  to  genetics :  first,  by 
way  of  the  more  visible  so7natoplasmof  organisms,  and  second,  thegerm- 
plasmal  approach,  which  involves  recourse  to  microscopic  technique. 
The  former  approach  may  be  subdivided  into  at  least  three  lines  of 
attack,  namely,  observational,  statistical,  and  experimental. 

H.  w.  H.— 29 


440  THE  MAINTENANCE  OF  SPECIES 

The  observational  method  has  been  practiced  from  time  immemorial, 
and  to  it  is  due  most  of  the  accumulations  of  our  general  knowledge 
concerning  heredity  up  to  about  the  turn  of  the  present  century  in 
1900.  The  phrase  "like  produces  like"  expresses  the  general  impres- 
sion that  is  gained  from  observation,  although  there  are  plenty  of 
exceptions  to  the  apparent  rule.  We  say  that  children  in  a  general 
way  "take  after"  their  parents,  although  there  are  conspicuous  in- 
stances when  it  becomes  necessary  for  parents  to  "take  after"  their 
children,  in  order  that  they  may  be  made  to  conform  to  a  family 
tradition,  whatever  it  may  be.  It  is  repeatedly  observed  that  not 
only  individuals  of  one  generation  may  be  in  general  like  their  pred- 
ecessors, but  that  certain  noticeable  characteristics  in  the  make-up 
of  an  individual  may  occur  more  often  in  some  family  lines,  breeds  of 
animals,  or  strains  of  plants  than  in  the  general  population  of  which 
they  are  a  part.  Whenever  this  is  so  we  are  led  to  suspect,  even  when 
we  may  not  be  entirely  convinced,  that  such  characteristics  are 
hereditary.  General  but  more  or  less  vague  observations  of  this  sort, 
while  useful  in  establishing  the  simple  fact  of  inheritance,  do  not  go 
very  far  in  determining  and  analyzing  the  causes  of  heredity  and  the 
laws  of  procedure  that  underlie  the  mechanism  of  inheritance,  which 
it  is  necessary  to  know  in  order  to  establish  a  real  science  of  genetics. 

The  statistical  method  recognizes  the  desirability  of  arranging  quali- 
tative data  in  quantitative  terms,  as  a  necessary  process  in  reducing 
random  observations  and  guesses  to  definite  scientific  form.  Recourse 
must  always  be  made  to  mathematical  treatment  in  formulating  any 
science,  and  genetics  is  no  exception.  Mathematics,  however,  is 
simply  a  useful  tool  to  be  employed  in  arranging  the  facts  and  in 
bringing  them  together  in  convenient  form  for  interpretation.  There 
are  repeated  occasions  when  it  is  not  only  desirable  but  indispensable 
to  focus  isolated  and  scattered  facts  into  a  single  comprehensive  pic- 
ture which  can  only  be  accomplished  by  statistical  treatment.  Statis- 
tics, however,  to  be  of  value  in  solving  problems  of  heredity,  must  be 
based  upon  careful  observations  and  accurate  measurements  pre- 
viously obtained.  Biometry,  the  science  of  measurement  when  ap- 
plied to  biological  data,  is  powerless  to  extract  true  conclusions  out 
of  faulty  observations  or  findings. 

The  biometrical  approach  is  about  the  only  way  available  in  which 
to  investigate  the  problems  of  heredity  as  applied  to  mankind.  It  is 
obviously  not  feasible,  even  if  it  were  desirable,  to  plan  and  execute 
controlled  experiments  in  human  breeding,  of  sufl&cient  magnitude 


THE   GREAT   RELAY    RACE  441 

and  duration,  to  be  of  general  significance  in  establishing  the  laws  of 
inheritance.  Not  only  would  any  such  ambitious  program  take  too 
many  generations  to  reach  any  satisfactory  conclusions,  even  if  it 
were  possible,  but  also  it  would  involve  too  many  insuperable  social 
diflSculties.  In  the  case  of  mankind,  therefore,  we  are  forced  to 
resort  to  experiments  in  marriage  and  other  sexual  relations  that  have 
already  been  made  in  the  past,  for  collecting  data,  and  this  type  of 
investigation  demands  the  technique  of  statistical  treatment. 

The  third  method  of  approach  in  storming  the  citadel  of  genetics  is 
the  ex'perimental  method.  This  has  proven  to  be  very  successful.  By 
controlling  breeding  of  animals  and  plants  and  observing  the  outcome, 
which  is  not  open  to  the  objections  encountered  when  human  material 
is  employed,  it  has  become  possible  to  find  out  much  concerning  the 
modus  operandi  of  inheritance.  The  same  biological  laws  and  pro- 
cedures that  are  found  to  be  true  of  plants  or  animals  may  then,  to  a 
large  extent  at  least,  be  applied  to  man.  This  method  will  be  elabo- 
rated somewhat  in  the  following  sections. 

All  of  these  methods,  namely,  observation,  statistical  treatment, 
and  experimental  breeding,  are  concerned  primarily  wnth  somato- 
plasms.  The  germplasmal  method  of  approach,  on  the  other  hand,  is 
concerned  with  the  concealed  beginnings  of  the  life  story,  rather  than 
with  its  visible  sequel  in  the  bodies  of  organisms.  The  germplasmal 
approach  has  to  do  wdtli  the  astonishing  behavior  of  the  genes,  which 
are  the  determiners  of  subsequent  somatoplasmal  manifestations. 
This  underground  phase  of  the  heredity  problem  is  proving  in  recent 
years  to  be  most  illuminating,  and  some  consideration  of  it,  together 
with  the  experimental  method  just  mentioned,  will  make  up  the 
essential  remaining  part  of  this  section  on  genetics. 

The  Experimental  Method 

The  Usefulness  of  Hybrids 

In  order  to  learn  the  secrets  of  inheritance  by  the  controlled  crossing 
of  plants  and  animals,  it  is  necessary  to  use  parental  stocks  that  differ 
from  each  other  in  some  of  their  characteristics.  When  this  is  done, 
hybrids  are  produced  in  which  the  respective  contributions  to  the 
offspring  from  the  two  parents  may  be  determined,  and  thus  the 
first  steps  made  in  the  analysis  of  the  problems  of  inheritance. 

If  both  parents  and  the  consequent  offspring  are  alike,  then  a  color- 
less monotony  results  that  gives  no  differential  clue  as  to  how  heredity 


442  THE   MAINTENANCE   OF  SPECIES 

works.  Just  as  in  the  evolution  of  species  during  long  periods  of 
geological  time,  variation  must  somewhere  have  entered  in  to  make 
it  possible  that  an  elephant  and  a  mouse  could  have  arisen  from  a 
common,  remote  ancestor,  so  in  the  relay  race  of  heredity  we  cannot 
picture  the  details  of  how  a  succession  of  generations  comes  about, 
when  all  the  individuals  concerned  are  alike.  The  uniform  bulk  of  in- 
heritance passes  unnoticed.  It  is  only  the  "  sore  thumbs  "  of  variation 
that  stand  out,  for  although  hereditary  succession  may  and  does  occur 
in  the  absence  of  variation  it  is  only  when  a  visible  variable  is  intro- 
duced from  one  parent  or  the  other,  that  we  can  see  how  the  inherit- 
ance of  a  characteristic  jumps  from  one  side  of  the  house  to  the  other, 
skips  a  generation,  doubles  up,  or  behaves  in  some  other  manner. 
One  outstanding  way  in  which  hybrid  variation  is  brought  about  in 
nature  is  by  sexual  reproduction,  in  which  two  different  streams  of 
germplasm  unite  to  form  a  new  generation. 

Pure  hereditary  strains,  on  the  other  hand,  are  probably  not  nearly 
as  common  in  nature  as  are  hybrids.  In  self-fertilized  plants,  for 
example,  we  may  not  expect  to  find  much  in  the  way  of  hereditary 
variation,  since  no  different  outside  germplasmal  potentialities  have 
been  introduced  in  the  production  of  offspring.  Likewise,  in  par- 
thenogenetic  organisms,  which  develop  progeny  without  any  contribu- 
tion from  the  male  parent,  as  well  as  in  all  kinds  of  asexual  propaga- 
tion, where  a  fragment  of  the  parental  body  gives  rise  without  germinal 
modification  to  a  new  individual,  one  may  expect  to  encounter 
monotony  so  far  as  hereditary  variations  are  concerned.  Transient 
variations  that  are  induced  by  environmental  causes,  like  the  tanned 
skin  of  a  lifeguard  at  the  seashore,  or  the  luxuriant  growth  of  a 
pigweed  on  a  manure  pile,  do  not  carry  over  in  heredity.  That 
hereditary  variations  frequently  do  appear  in  the  absence  of  hybrid 
combinations  is  to  be  accounted  for  by  the  occurrence  of  mutations, 
or  spontaneous  hereditary  variations,  which  are  mentioned  in  the 
section  on  Evolution. 

In  the  early  days  of  the  nineteenth  century,  certain  scientifically- 
minded  botanists  in  Europe  began  to  explore  the  possibilities  of 
hybridization  by  artificially  crossing  plants.  Koelreuter  (1733-1806) 
and  Gaertner  (1772-1836)  in  Germany,  Naudin  (1815-1889)  in 
France,  and  Knight  (1759-1839)  in  England  were  conspicuous 
pioneers  in  this  field  of  experimentation.  It  remained,  however,  for 
Gregor  Johann  Mendel  (1822-1884)  of  Austria  to  become  the  master 
hybridizer  of  them  all,  and  to  carry  his  experiments  through  to  results 


THE   GREAT   RELAY   R-\CE  443 

and  conclusions  that  mark  him  as  the  patron  saint  of  the  modern 
science  of  Genetics. 

Mendelism 

Gregor  Johann  Mendel,  with  peas  and  arithmetic,  not  only  demon- 
strated the  existence  of  an  orderly  system  of  inheritance  that  bears 
his  name,  but  was  himself  a  living  example  of  the  extent  to  which 
innate  hereditary  ability  can  dominate  an  environment  none  too 
favorable.  He  was  an  Augustinian  monk,  attached  to  a  monastery 
in  Briinn,  Austria  (now  Brno,  Czechoslovakia),  where,  with  ordinary 
garden  peas,  he  carried  through  a  remarkable  series  of  breeding 
experiments  extending  over  several  years.  During  the  first  part  of 
his  career,  when  working  on  these  famous  experiments,  he  was 
handicapped  by  having  only  a  small  patch  of  a  cloister  garden  in 
which  to  operate.  Later  on,  when  he  finally  became  abbot  of  the 
monastery  and  could  control  garden  space  at  will,  he  was  necessarily 
so  occupied  with  the  administrative  duties  of  his  office  that  he  did 
not  have  much  time  to  devote  to  scientific  pursuits.  Yet,  in  spite  of 
these  limitations,  and  regardless  of  the  fact  that  his  associates  were 
not  particularly  sympathetic  with  his  unpriestly  avocations,  he 
carried  to  completion  by  himself  this  remarkable  piece  of  fundamental 
investigation  which  insures  for  him  a  permanent  place  in  the  biological 
Hall  of  Fame. 

His  results  were  finally  published  in  1866  in  the  obscure  "Pro- 
ceedings" of  a  small,  unimportant  local  Natural  History  Society. 
They  did  not  at  the  time  gain  appreciative  attention  and  were 
promptly  forgotten,  due  in  part  perhaps  to  the  preoccupation  of  the 
scientific  world  at  the  time  with  the  newly  launched  Thconj  of  Natural 
Selection  (1859)  of  Charles  Darwin.  Unrecognized  and  unknown, 
Mendel  died  in  1884,  with  the  confident  declaration  on  his  lips, 
"Meine  Zeit  wird  schon  kommen  ! "  Some  years  later  this  prophecy 
came  true  when,  in  1900,  three  scientists,  Correns  in  Germany,  von 
Tschermak  in  Austria,  and  DeVries  in  Holland,  independently 
rediscovered  Mendel's  forgotten  contribution,  and  because  of  it, 
initiated  the  remarkable  era  in  the  study  of  heredity  that  has  resulted 
in  establishing  the  science  of  Genetics  as  we  know  it  today. 

What  Mendel  Did 

Mendel's  genius  is  shown  by  the  fact  that  he  did  not  make  his 
experiments  blindly,  but  set  for  himself  the  clearly  defined  problem 


444  THE   MAINTKNANCE   OF   SPECIES 

of  reducing  the  phenomena  of  inheritance  to  a  measurable  mathemat- 
ical basis.  For  this  purpose  he  wisely  chose  for  experimentation  gar- 
den peas,  which  not  only  are  easily  grown,  but  also  possess  readily 
recognized  constant  characteristics.  Since  peas  are  normally  self- 
fertilized,  they  represent  at  the  start  comparatively  pure  hereditary 
strains.  Moreover,  hybridization  in  peas  can  be  controlled  from 
contamination  by  insects.  Since  fertilization  occurs  before  the  flowers 
open,  the  hooded  structure  of  the  flowers  is  such  that  interference  from 
their  chance  visits  is  prevented.  As  is  well  known,  insects  may  carry 
on  involuntary  hybridization  experiments  of  their  own  in  connection 
with  many  plants,  by  transferring  pollen  grains  from  the  stamen  of 
one  flower  to  the  stigma  of  another. 

Instead  of  considering  the  whole  complex  individual  in  the  light  of 
a  "hybrid"  unit,  as  former  hybridizers  had  done  with  much  result- 
ing confusion,  Mendel  focused  his  attention  upon  single  alternative 
characters,  one  pair  at  a  time,  that  were  unlike  in  the  two  contributing 
parents.  This  simplification  of  the  problem  made  the  collection  of 
data  less  complicated,  and  the  analysis  of  results  possible.  Finally, 
he  not  only  combined  single  pairs  of  characters  into  hybrids,  but  he 
went  further  and  followed  up  the  results  obtained  by  breeding  these 
known  hybrids  together  through  several  generations,  meanwhile  tak- 
ing meticulous  pains  to  account  for  all  the  offspring  of  whatever  sort 
in  each  case,  so  that  ratios  of  relative  occurrence  could  be  computed. 
For  example,  he  dealt  with  seven  pairs  of  alternative  characteristics 
found  in  different  strains  of  peas,  as  follows  :  ^ 

1.  Smooth  seeds  or  wrinkled  seeds  ; 

2.  Yellow  seed-coats  or  green  seed-coats ; 

3.  Tall  vines  or  dwarf  vines  ; 

4.  Colored  flowers  or  white  flowers ; 

5.  Axial  flowers  or  terminal  flowers  ; 

6.  Inflated  pods  or  constricted  pods ; 

7.  Green  pods  or  yellow  pods. 

In  every  case  when  these  pairs  of  characters  were  put  together 
the  hybrids  thus  produced  were  not  intermediate  in  appearance,  but 
were  alike,  and  resembled  one  of  the  parents  and  not  the  other. 
When  these  hybrids  in  turn  were  interbred  with  each  other,  or  allowed 
to  be  normally  self-fertilized,  which  amounts  to  the  same  thing,  the 


'  Dr.  O.  E.  White,  of  the  Brooklyn  Botanical  Garden,  as  early  as  1917  reported  thirty-four  pairs 
of  hereditary  characters  in  peas  on  which  determinative  experimental  studies  have  been  made. 


THE   GREAT   RELAY   RACE 


445 


progeny  always  fell  into  two  groups  in  appearance  like  the  two  grand- 
parents, in  the  ratio  of  3:1.  Thus,  when  smooth  peas  were  arti- 
ficially crossed  with  wrinkled  peas,  the  hybrids  were  all  smooth  peas, 
and  when  these  smooth  hybrids  in  turn  were  allowed  to  cross  inter  se, 

Parents   CP) 

Gametes 

Hqbrid  children  (f,) 

Gametes 
Grandchildre 

Diagram  of  the  ancestry  and  progeny  of  a  typical  monohybrid,  formed  from 
smooth  and  wrinkled  garden  peas.  The  inner  circles  represent  germplasm,  en- 
closed in  the  outer  circles,  or  somatoplasm.  S,  determiners  of  smooth  peas; 
s,  determiners  of  wrinkled  peas. 

the  resulting  grandchildren  could  be  grouped  in  the  ratio  of  three 
smooth  peas  to  one  w^rinkled  pea.  These  results  are  indicated  dia- 
grammatically  in  the  accompanying  figure,  with  the  smooth  charac- 
teristic represented  by  a  single  letter  as  *S,  and  the  wrinkled  kind  by 
s.  The  germinal  make-up  of  each  individual  is  thus  represented  by 
two  letters,  since  it  is  always  derived  from  two  parental  gametes. 

If  smooth  and  wrinkled  gametes  come  together  in  the  same  indi- 
vidual, the  smooth  determiner  covers  up,  or  "dominates,"  the 
wrinkled  one,  and  is  consequently  called  a  dominant,  while  the 
wrinkled  gamete  recedes  from  visible  expression  for  the  time  being, 
and  is  designated  as  a  recessive.  Which  one  of  the  alternate  pair  of 
parental  characters  will  be  dominant  and  which  recessive  in  the 
offspring  in  any  given  case  cannot  be  learned  in  advance  by  inspec- 
tion. It  is,  therefore,  necessary  to  resort  to  the  breeding  test  in  order 
to  make  the  determination. 

Further  crosses  on  IVIendel's  part  showed  that  SS  peas  were  pure 
stock,  like  one  of  the  grandparents  with  which  he  started,  and  when 
interbred  produced  only  SS  peas,  although  coming  from  impure  or 
hybrid  parents.  Similarly  the  ss  peas  were  also  pure  like  the  other 
grandparent,  and  likewise  always  gave  rise  only  to  ss  peas  when 
allowed  to  inbreed  with  their  own  kind.     The  hybrid  Ss  peas,  on  the 


446 


THE   MAINTENANCE  OF  SPECIES 


other  hand,  being  constituted  like  their  hybrid  parents,  when  interbred 
furnished  again  the  typical  3  :  1  ratio.  In  Mendel's  original  experi- 
ments there  were  actually  obtained  from  the  Ss  peas  5474  smooth 
and  1850  wrinkled  peas,  which  is  very  near  the  expected  3 :  1  ratio. 
Such  pure  SS  peas  and  hybrid  Ss  peas  are  said  to  be  phenotypically 
alike  and  genotypically  different.  That  is,  they  look  alike,  but  have 
different  possibilities  when  it  comes  to  producing  gametes.  The 
way  to  distinguish  the  one  from  the  other  is  to  breed  them  hack  with 
the  recessive  ss  peas,  which  can  conceal  nothing,  and  observe  the  kind 
and  proportion  of  the  offspring  produced.  SS  X  ss  gives  100  per  cent 
Ss  (phenotypically  smooth),  while  Ss  X  ss  gives  50  per  cent  Ss 
(phenotypically  smooth)  and  50  per  cent  ss  (phenotypically  wrinkled), 
as  shown  in  the  checkerboard  below,  in  which  the  gametes  of  the  two 
sexes  are  placed  outside  the  double  lines,  and  the  resulting  kinds  of 
individuals  are  represented  by  double  letters  within  the  squares. 


s 
s 

S 

Ss 

Ss 

S 
Ss 
Ss 

S 

s 

s 

Ss 

ss 

s 

Ss 

ss 

Sometimes  dominance  may  be  incomplete,  in  which  case  it  is  not 
necessary  to  back-cross  with  the  corresponding  recessive  in  order  to 
determine  which  are  pure  and  which  are  hybrid  dominants.  The 
four-o'clock  {Mirahilis  jalapa),  as  pointed  out  by  Correns,  furnishes 
a  well-known  demonstration  of  this  point,  for  the  hybrid  produced  by 
red  X  white  flowers  is  not  dominant  red,  as  might  be  expected,  but 
pink.  The  pink  hybrids  give  in  turn  the  proportion  of  three  colored 
flowers  (one  red  and  two  pink)  to  one  white. 

Mendel  carried  through  the  same  hybridization  procedure  and  sub- 
sequent follow-up,  with  each  of  the  seven  pairs  of  contrasting  char- 
acters, and  found  that  the  approximate  3 :  1  result  always  obtained, 
regardless  of  whatever  other  characters  were  present  in  the  individual 
plants.  Each  pair  of  characters,  in  other  words,  behaved  inde- 
pendently of  every  other  pair.  This  is  called  the  principle  of  inde- 
pendent assortment. 

It  is  apparent,  moreover,  that  the  determiner  for  each  character 
retains  its  integrity,  reappearing  in  the  next  generation  true  to 
itself,  regardless  of  the  company  it  has  been  keeping  within  the  germ 
cell.     This  integrity  of  the  hereditary  determiners,  together  with  the 


THE   GREAT   RELAY   RACE 


447 


uncontaminated  reappearance  of  the  character  in  the  next  generation, 
is  termed  the  principle  of  segregation. 

Thus,  out  of  simple  but  perfectly  controlled  experiments  with  garden 
peas,  Mendel  was  able  to  lay  down  three  "laws,"  namely,  dominance, 
independent  assortment,  and  segregation,  which  together  constitute  the 
essential  features  of  what  is  known  as  "Mendelism."  These  funda- 
mental laws  have  been  confirmed  many  times  over,  in  a  great  variety 
of  plants  and  animals  by  a  host  of  critical  investigators,  and  their  use 
now  makes  possible  a  precise  prediction  of  results  in  experimental 
breeding  that  was  quite  impossible  before  their  formulation. 

Monohybrids,  Dihybrids,  Trihybrids,  and  Other  Crosses 

The  fundamental  Mendelian  laws  as  illustrated  by  a  monohyhrid, 
that  is,  a  hybrid  with  respect  to  a  single  pair  of  characters,  are  com- 
paratively simple.  When  two  monohybrids  are  bred  together,  as 
shown  in  the  preceding  paragraphs,  the  resulting  progeny  occur  in  the 
phenotypic  ratio  of  3  :  1,  and  the  genotypic  ratio  of  1  :  2 :  1.  Dihy- 
brids, trihybrids,  tetrahybrids,  etc.,  are  increasingly  comphcated, 
but  are  quite  understandable  when  it  is  remembered  that  they  are 
nothing  more  than  combinations  of  monohybrids,  resulting  from  the 
independent  assortment  of  the  characters  involved.  The  expecta- 
tions for  such  crosses  are  show^l  in  the  following  table  : 


Number  of 

Pairs  of 
Characters 

Possible  Combi- 
nations When 
Crossed 

Number  of 

Phbnotypes  in 

Progeny 

Number  of 

Genotypes  in 

Progeny 

Monohybrid 

1 

4 

2 

3 

Dihybrid       .... 

2 

16 

4 

9 

Trihybrid     .... 

3 

64 

8 

27 

Tetrahybrid      .     .     . 

4 

256 

16 

81 

Etc 

7 

? 

? 

? 

As  an  example  of  the  way  in  which  a  dihybrid  works  out,  black 
color  in  horses  is  dominant  over  chestnut  color,  and  trotting  gait  over 
pacing.  These  two  pairs  of  characters  are  independent  of  each  other, 
so  that  when  a  black  pacer  is  mated  with  a  chestnut  trotter,  all  the 
offspring  of  the  hybrid  generation  will  be  black  trotters,  since  black 
color  and  trotting  gait  are  dominant  characters.  Then  when  such 
hybrid  black  trotters  are  mated  together  there  will  be  sixteen  possible 


448 


THE   MAINTENANCE   OF   SPECIES 


BOARD 


combinations,  falling  into  four  phenotypic  groups,  and  nine  genotypic 
groups,  as  shown  in  the  following  checkerboard,  in  which  the  double 
gametes  of  the  dihybrid  parents  are  represented  outside  the  double 
hnes,  and  their  combinations  in  the  offspring  indicated  within  the 
sixteen  squares.  The  arbitrary  symbols  used  are,  B  (black)  ;  b  (chest- 
nut) ;  T  (trotter) ;  t  (pacer).  It  will  be  seen  from  the  checkerboard  that 
DIHYBRID  CHECKER-      ^^^^  chances  out  of  sixteen  are  possible  that 

a  black  trotter  will  result,  since  both  B  and  T 
are  present  at  least  once  in  their  make-up. 
There  are  three  chances  that  a  black  pacer 
(Bt)  will  occur,  three  chances  for  a  chestnut 
trotter  (bT),  and  one  chance  in  sixteen  that 
the  dihybrid  parents  will  produce  a  chestnut 
pacer  (bt) .  Thus,  the  phenotypic  ratio  in  the 
case  of  a  dihybrid  is  typically  9:3:3:1.  The 
checkerboard  further  shows  that  the  sixteen 
possibilities  fall  into  nine  genotypic,  or  ac- 
tually different,  groups  represented  by  dif- 
ferent combinations  of  the  four  symbolic  letters  within  the  squares. 
The  expectation  when   two  trihybrids  are  crossed  is  shown  by 


BT 

Bt 

bT 

bt 

BT 

BT 
BT 

BT 
Bt 

BT 

bT 

BT 

bt 

Bt 

Bt 
BT 

Bt 
Bt 

Bt 

bT 

Bt 

bt 

bT 

bT 
BT 

bT 
Bt 

bT 
bT 

bT 
bt 

bt 

ht 
BT 

bt 
Bt 

bt 
bT 

bt 
bt 

TRIHYBRID   CHECKERBOARD 


YTA 

YTa 

YtA 

Yta 

yTA 

yTa 

ytA 

yta 

YTA 

YTA 
YTA 

YTA 
YTa 

YTA 
YtA 

YTA 

Yta 

YTA 
yTA 

YTA 

yTa 

YTA 

ytA 

YTA 

yta 

YTa 

YTa 
YTA 

YTa 
YTa 

YTa 

YtA 

YTa 
Yta 

YTa 

yTA 

YTa 
yTa 

YTa 

ytA 

YTa 

yta 

YtA 

YtA 
YTA 

YtA 
YTa 

YtA 
YtA 

YtA 
Yta 

YtA 
yTA 

YtA 

yTa 

YtA 

ytA 

YtA 

yta 

Yta 

Yta 
YTA 

Yta 
YTa 

Yta 
YtA 

Yta 
Yta 

Yta 
yTA 

Yta 
yTa 

Yta 

ytA 

Yta 
yta 

yTA 

yTA 
YTA 

yTA 
YTa 

yTA 
YtA 

yTA 
Yta 

yTA 

yTA 

yTA 
yTa 

yTA 
ytA 

yTA 

yta 

YTa 

yTa 
YTA 

yTa 
YTa 

yTa 
YtA 

yTa 
Yta 

yTa 
yTA 

yTa 
yTa 

yTa 
ytA 

yTa 
yta 

ytA 

ytA 
YTA 

ytA 
YTa 

ytA 
YtA 

ytA 
Yta 

ytA 
yTA 

ytA 
yTa 

ytA 
ytA 

ytA 
yta 

yta 

yta 
YTA 

yta 
YTa 

yta 
YtA 

yta 
Yta 

yta 
yTA 

yta 
yTa 

yta 
ytA 

yta 
yta 

THr:   (iHKAT   WELW   RACE 


l<) 


Mendel's  garden  peas,  in  which  three  alternative  pairs  of  characters  are 
selected,  namely,  yellow  (Y)  and  green  (y)  peas;  tall  (7')  and  dwarf 
(0  vines;  and  axial  (A)  and  terminal  (a)  flowers.  The  trihybrids  in 
this  case  will  have  the  genotypic  formula  YyTtAa,  and  will  be  pheno- 
typically  yellow,  tall,  and  axial.     Such  hybrids,  because  of  the  inde- 


GENOTYPES 


PHENOTYPES 


27  VTA  (yellow,  tall,  axial) 


9  YTa  (yellow,  tall,  terminal) 


9  YtA  (yellow,  dwarf,  axial) 


3  Yta  (yellow,  dwarf,  terminal) 


9  yTA  (green,  tall,  axial) 


3  yTa  (green,  tall,  terminal) 


3  ytA  (green,  dwarf,  axial) 


1  2fta  (green,  dwarf,  terminal) 


64 


Kinds  of  trihybrids 

pendent  assortment  of  their  characters,  can  produce  eight  possible 
kinds  of  gametes,  or  mature  germ  cells,  each  carrying  three  charac- 
ters, as  follows:  YTA,  YTa,  YtA,  Yta,  yTA,  yTa,  ytA,  yta.  When 
these  triple  gametes  unite,  there  are  sixty-four  (8  X  8)  possible  com- 
binations, as  shown  in  the  accompanying  checkerboard,  which  will 
fall  into  eight  different  phenotypic  groups  in  the  ratio  of  27  :  9  :  9  :  3  : 
9:3:3:  1  (total  64),  and  they  may  be  further  classified  into  twenty- 
seven  genotypically  different  groups,  represented  above  as  three 
monohybrid  ratios  combined. 


450 


THE   MAINTENANCE   OF  SPECIES 


In  actual  practice,  if  a  combination  of  three  or  more  characters 
is  desired,  one  character  at  a  time  in  either  pure  dominant  or  recessive 
form  is  obtained.  By  this  method,  since  the  expectation  of  either  a 
pure  dominant  or  a  pure  recessive  in  a  monohybrid  is  one  out  of  four, 
early  reahzation  of  the  desired  combination  is  likely. 

Unit  Characters  and  Factors 

A  great  deal  has  been  learned  about  heredity  through  the  experi- 
mental breeding  of  plants  and  animals  since  Mendel's  laws  became 
available.  Many  of  the  facts  gained,  however,  are  at  first  sight  in 
apparent  contradiction  to  these  laws,  but  the  value  of  the  fundamental 
concepts  of  dominance,  independent  assortment,  and  segregation  in  the 

^  ,.  interpretation  of  inherit- 

nSreQIiary  OOmailU  wcice     remains     unques- 

Determiners  Characters   tioned.    Any  adequate 

\_)    .«...___^  consideration  of  the  ap- 

parent departures  from 
the  clear-cut  conclusions 
of  Mendelism  would  re- 
quire many  more  pages 
than  are  available  in  this 
book. 

For  one  thing,  Men- 
del's experiments  led  him 
to  the  idea  of  Unit 
Characters,  each  spon- 
sored by  a  single  germinal 
determiner.  There  is 
now  abundant  evidence 
that  whatever  it  is  in  the 
germplasm  that,  under  suitable  environmental  conditions,  becomes 
eventually  expressed  as  a  single  character,  it  is  often  made  up  of  more 
than  one  unit.  This  discovery  has  led  to  the  development  of  the  factor 
hypothesis,  which  implies  that  there  is  usually,  if  not  always,  an  in- 
terplay between  different  hereditary  factors  in  determining  the  con- 
tribution which  inheritance  furnishes  to  the  formation  of  a  character 
in  an  individual.  Moreover,  a  constellation  of  interacting  hereditary 
factors  may  be  responsible,  in  certain  instances,  for  the  expression  of 
more  than  one  visible  character. 

Modified  Ratios.     The  existence  of  factors,  or  fractional  rather 


B 


Diagram  of  the  relation  between  hereditary 
determiners  and  resulting  somatic  characters. 
A,  three  or  more  determiners  may  combine  to 
produce  a  single  visible  character,  or  B,  a  single 
hereditary  determiner  may  find  expression  in  a 
number  of  difTerent  somatic  characters. 


THE   GREAT   RELAY   RACE 


451 


than  unit  determiners,  is  particularly  apparent  when,  for  example,  the 
typical  dihybrid  ratio  of  9  :  3  :  3  :  1  becomes  modified  into  other  than 
the  usual  phenotypic  groups.  The  following  ratios  have  been  dem- 
onstrated in  various  dihybrid  crosses:  3:6:3:1:2:1,  9:3:4, 
10 :  3  :  3,  12  :  3  :  1,  9  :  6 :  1,  9  :  7,  10  :  6,  13  :  3,  and  15  :  1.  In  each  of 
these  cases  it  is  still  a  dihybrid,  made  up  of  two  monohybrids  and 
totaling  sixteen  possibilities  involved. 

To  work  out  a  single  illustration  of  how  the  factor  idea  gives  rise 
to  a  modified  phenotypic  ratio,  let  us  take  Bateson's  famous  case  of 
sweet  peas,  that  resulted  in  the  9  :  7  ratio  of  flower  color.  Bateson 
dealt  with  two  different  strains  of  white-flowering  sweet  peas  that 
bred  true  to  the  white  color  as  long  as  they  were  not  out-crossed. 
When  the  two  white  strains  were  artificially  crossed  with  each  other, 
however,  all  the  progeny  in  the  first  generation  produced  purple 
flowers.  This  purple  color  was  found  to  be  due  to  the  combination  of 
two  factors,  which  may  arbitrarily  be  designated  as  A  and  B,  one  of 
which  was  furnished  by  each  parental  strain.  Neither  factor  alone 
could  produce  the  purple  color  since  the  parents  were  both  white. 
When  the  purple  hybrids  in  turn  formed  their  possible  kinds  of 
gametes  and  were  crossed  with  each  other,  there  resulted  the  custom- 
ary sixteen  combinations  of  a  dihybrid,  as  shown  in  the  checker- 
board. AAhh  (white)  X  aaBB  (white)  =  AaBh  (purple).  Gametes 
from  AaBb  =  AB,  Ab,  aB,  ah. 


AB 

Ab 

aB 

ab 

AB 

A  BAB 

ABAb 

ABaB 

ABah 

Ab 

AbAB 

AbAh 

AbaB 

Abah 

aB 

aBAB 

aBAb 

aBaB 

aBah 

ab 

abAB 

abAb 

abaB 

abab 

Of  the  sixteen  possibilities,  the  nine  possessing  at  least  one  A  factor 
and  one  B  factor  produced  purple  flowers,  while  the  remaining  seven, 
which  did  not  possess  both  the  A  and  B  factors,  were  whilse.  It  will 
be  seen  that  the  seven  phenotypically  white-flowering  possibilities 
fall  into  three  genetically  different  groups,  namely,  3  AAhh  or  3  Aahh, 
3  aaBB  or  3  aaBh,  and  1  aahh.  By  breaking  up  the  seven  kinds 
of  white-flowering  sweet  peas  into  the  genetically  different  groups 
3:3:  1,  and  adding  them  to  the  nine  purple-flowering  kinds,  the 
underlying  Mendelian  dihybrid  ratio  of  9  :  3  :  3  :  1  is  restored.     This 


452  THE  MAINTENANCE  OF  SPECIES 

is  a  case  of  complementary  factors,  because  one  factor  is  required  to 
complement  the  other  in  order  to  bring  the  character  into  expression 
while  neither  is  effective  alone. 

Different  Kinds  of  Factors.  There  are  also  supplementary 
factors,  where  one  factor  alone  may  produce  a  visible  effect,  but  a 
second  factor  may  change  its  manifestation ;  or  inhibiting  factors, 
where  the  expression  of  a  factor  is  prevented  by  the  interference  of 
another;  or  duplicate  factors,  where  separate  "doses"  of  the  same 
thing  combine  to  produce  a  cumulative  effect ;  or  lethal  factors,  which 
are  so  disharmonious  that  if  they  arrive  together  from  both  parental 
sources,  the  unfortunate  individual  sooner  or  later  dies,  although  able 
to  survive  when  only  a  single  lethal  factor  comes  from  one  parent ; 
or  sex-linked  factors,  that  are  tied  up  with  either  the  maternal  or  the 
paternal  side  of  the  house.  In  all  these  cases  the  factors  in  their 
behavior  obey  the  fundamental  Mendelian  laws,  although  the  resulting 
ratios  furnish  intriguing  complications  that  Mendel  himself  did  not 
anticipate. 

It  is  hoped  that  the  reader  will  be  stimulated  to  explore  in  books 
devoted  primarily  to  Genetics  (see  bibliography)  further  than  the 
general  survey  presented  in  this  chapter. 

Practical  Breeding 

Selection 

Long  before  Mendel  pointed  the  way  by  which  to  control  the 
operations  of  heredity,  man  was  active  in  fixing  desirable  characters  in 
animals  and  plants  by  means  of  artificial  selection,  and  in  doing  this 
was  only  following  in  the  footsteps  of  Mother  Nature,  who  has  been 
exercising  ''natural  selection"  from  time  immemorial.  Many  of  the 
forms  selected  and  nurtured  by  man  never  could  have  survived  if  left 
to  the  more  exacting  demands  of  nature. 

We  know  today,  thanks  to  Mendel,  that  phenotypes  do  not  always 
reproduce  their  own  kind,  and  that  the  genotype  is  the  all-important 
thing  to  get  at  in  heredity.  It  must  be  admitted,  however,  that  in 
spite  of  difficulties  encountered,  our  pre-Mendelian  forebears,  in  estab- 
lishing lines  of  domesticated  animals  and  cultivated  plants  by  the 
method  of  blind  selection  of  phenotypes,  attained  a  remarkable  degree 
of  success.  Even  the  ancient  lake-dwellers  of  prehistoric  Switzerland, 
it  is  said,  developed  ten  different  kinds  of  cereals  from  wild  plants. 

There  are  three  different  methods  of  phenotypic  selection  which  are 


THE  GREAT  RELAY  RACE  453 

still  practiced  with  gratifying  results  by  practical  breeders,  namely, 
mass  selection,  pedigree  breeding,  and  progeny  breeding. 

Mass  Selection.  In  mass  selection  a  general  population,  exhibit- 
ing desirable  qualities  on  the  average,  is  drawn  upon  to  furnish  pro- 
genitors for  the  following  generation  in  the  faith  that  "like  produces 
like."  There  are  two  ways  in  which  a  desirable  population  to  breed 
from  may  be  obtained.  A  crop,  for  example,  may  be  grow^n  under 
the  most  favorable  conditions  of  cultivation  and  environment  and 
the  improved  individuals  resulting  chosen  as  seed.  This  method  of 
procedure  is  based  upon  the  questionable  belief  that  acquired  charac- 
ters reappear  in  the  next  generation.  Or  the  same  crop  may  be  grown 
under  adverse  conditions  and  those  individuals  which  are  pheno- 
typically  most  promising  chosen,  with  the  idea  that,  since  they  have 
made  good  in  spite  of  unfavorable  surroundings  and  poor  nurture, 
they  must  obviously  possess  desirable  inherent  or  hereditary  qualities. 

The  limitations  of  this  common  practice  of  mass  selection  lie  in  the 
fact  that  selection  must  be  made  over  and  over  again,  since  nothing 
dependable  has  been  established.  Moreover,  the  best  individuals  in 
this  wholesale  procedure  are  often  swamped  by  the  average  ones,  so 
that  all  are  reduced  to  a  mediocre  level. 

Pedigree  Breeding.  Pedigree  breeding,  based  likewise  upon  the 
fallacy  that  like  always  produces  like,  narrows  selection  definitely  to 
single  individuals  or  lines,  rather  than  hopefully  employing  a  confusion 
of  many  unknown  lines.  It  is  a  method  that  has  been  particularly 
successful  in  breeding  race  horses  and  various  kinds  of  domestic 
animals,  and  depending  upon  stud-books  and  zealously  recorded  pedi- 
grees. Even  human  beings  are  known  to  indulge  in  "blue  books" 
and  proud  genealogical  records  that  characterize  pedigree  breeding. 

Progeny  Selection.  Progeny  selection  depends  upon  the  princi- 
ple that  the  only  way  to  determine  the  character  of  the  essential 
germplasm  in  plants  and  animals  is  to  see  what  kind  of  somatoplasms 
it  produces.  In  the  poultry  pens  at  the  Massachusetts  Agricultural 
Station  at  Amherst,  for  example.  Hays  and  Sanborn  established  a 
strain  of  hens  in  which  the  annual  egg  production  was  raised  from  145 
to  235.  This  was  done  by  selecting  cocks  that  bred  pullets  which 
made  good  by  producing  an  increased  yield  of  eggs.  Thus  it  was 
demonstrated  that  the  male  has  a  hand  in  the  heredity  of  egg  pro- 
duction, although  it  is  the  female  that  does  the  real  work. 

In  similar  fashion,  bulls  siring  heifers  that  prove  to  be  high  milk- 
producers  are  selected  for  building  up  a  herd  of  dairy  cows.     Bulls 


454  THE   MAINTENANCE  OF  SPECIES 

cannot  produce  milk  but  they  can  sire  heifers  that  do.  In  these 
cases,  instead  of  predicting  what  the  offspring  will  do  by  observing 
the  parental  performance,  the  offspring  themselves  are  taken  to  show 
what  their  parents  can  do  in  producing  desirable  progeny.  Mendelism 
has  shown  that  selection  of  any  kind,  in  order  to  be  effective,  must 
deal  with  genotypes  rather  than  phenotypes,  and  that  the  material 
from  which  selection  is  made  must  be  hybrid  rather  than  pure  in  its 
composition  if  progress  is  to  result. 

Inbreeding  and  Cousin  Marriage 

Inbreeding  in  various  degrees  of  consanguinity  or  blood  relationship 
tends  to  produce  uniformity,  or  purity,  in  the  hereditary  stream. 
Notwithstanding  popular  opinion  to  the  contrary,  inbreeding  in  itself 
is  not  harmful.  It  simply  tends,  in  the  case  of  hybrids,  to  bring 
recessive  traits  out  into  the  open,  and  these  are  in  many  instances 
less  desirable  than  dominant  characters.  Cousin  marriage  in  highly 
hybridized  human  stocks  is  a  potent  way  of  unearthing  "skeletons  in 
the  closet,"  for  cousins,  being  of  approximately  the  same  hereditary 
make-up,  are  apt  to  carry  concealed  the  same  recessive  characters, 
which  thus  have  a  Mendelian  chance  of  getting  together  and  becom- 
ing somatically  visible.  On  the  other  hand,  when  people  not  closely 
related  are  mated  together,  their  undesirable  recessive  traits,  being 
different  in  each  'parent,  are  likely  to  remain  concealed  or  covered  up 
by  corresponding  dominants  contributed  by  the  other  parent.  For 
example,  \i  Aa  and  Aa  represent  two  similar  related  individuals  of 
the  same  make-up  so  far  as  the  characteristics  A  and  a  are  concerned, 
there  is  one  chance  in  four,  according  to  the  Mendelian  monohybrid 
ratio,  that  the  undesirable  combination  of  aa  will  appear  in  the  off- 
spring. If,  however,  two  unrelated  individuals,  Aa  and  Bh,  carry 
undesirable  gametes  represented  by  the  small  letters  a  and  h,  there 
is  only  one  dihybrid  chance  in  sixteen  that  the  individual  showing  the 
undesirable  recessive  combination  aahh,  with  no  concealing  dominant 
to  interfere,  will  appear,  and  there  are  only  three  additional  chances 
each  out  of  sixteen  that  either  the  aa  or  the  hb  recessive  characteristic 
will  come  to  light.     (See  checkerboard  on  page  451.) 

In  nature  there  are  many  instances  where  inbreeding  is  enforced. 
Wheat,  and  cereals  generally,  as  well  as  the  legumes  to  which  Mendel's 
peas  belong,  are  habitually  self-fertilized,  and  this  is  even  closer 
inbreeding  than  brother  and  sister  mating,  to  say  nothing  of  the 
pairing  of  cousins. 


THE  GREAT  RELAY  RACE  455 

Outbreeding  and  Hybrid  Vigor 

Outbreeding,  on  the  contrary,  introduces  variety  and  tends  to 
cover  up  recessive  defects  by  the  introduction  of  new  dominant  char- 
acters, although  it  does  not  permanently  eliminate  the  former. 

In  nature  probably  most  animals  and  plants  outbreed.  Even 
hermaphroditic  animals  such  as  earthworms  and  snails,  in  which  both 
sexes  are  included  in  one  individual,  usually  mature  their  eggs  and 
sperm  at  different  times,  as  already  noted,  thus  insuring  outbreeding. 
The  same  thing  is  true  to  a  large  extent  of  the  great  array  of  plants  in 
which  both  pollen  grains  and  ovules  are  housed  in  the  same  flower. 

One  of  the  beneficial  results  of  outbreeding  is  hybrid  vigor,  which 
usually  accrues  to  the  first  generation  of  hybrids.  This  result  may  be 
accounted  for  as  the  summation  of  desirable  dominant  characters 
from  the  two  diverse  parents.  The  advantages  gained  by  this  type 
of  cross,  however,  do  not  endure  in  successive  generations,  when 
inbreeding  comes  in  with  its  leveling  effects.  The  former  confusion 
and  uncertainty  about  the  consequences  of  inbreeding,  outbreeding, 
and  hybrid  vigor  is  straightened  out  when  one  goes  behind  the  scenes 
with  the  insight  made  possible  by  Mendel's  laws. 

Asexual  Propagation 

Another  practical  way  of  maintaining  desirable  hereditary  quali- 
ties, particularly  in  plants,  when  once  they  have  been  obtained,  is 
by  asexual  propagation  through  slipping  or  grafting.  This  is  the 
method  employed  in  maintaining  strains  such  as  navel  oranges,  w^hich 
produce  no  seeds,  and  also  in  plants  which  do  produce  seeds  whose 
phenotypes  are  known  desirable  somatoplasms  but  whose  genotypes 
are  hidden  in  unknown  problematical  seeds.  By  this  procedure  of 
asexual  propagation  the  desired  combination  is  continued,  without 
the  introduction  of  any  disturbing  germinal  modification  from  the 
outside.  Many  of  Luther  Burbank's  famous  plant  "creations," 
such  as  the  spineless  cactus  and  the  white  blackberry,  have  been 
established  and  made  available  by  this  method. 

The  Germplasmal  Method 

The  foregoing  somatoplasmal  methods  of  approach  in  studying 
heredity,  although  to  a  remarkable  degree  successful,  are  at  best  only 
indirect.     It  is  more  and  more  apparent  that  the  most  hopeful  line 
H.  w.  H.  —  30 


456  THE  MAINTENANCE  OF  SPECIES 

of  future  advance  is  concerned  with  the  direct  analysis  of  the  germ- 
plasm  itself,  that  is,  of  the  basic  chemical  materials  (genes)  out  of 
which  somatoplasms  are  derived.  This  has  been  made  all  the  more 
possible  within  the  last  half  century  by  the  increased  efficiency  of 
greatly  improved  microscopes  and  microtomes,  and  through  the 
development  of  staining  technique  by  means  of  aniline  dyes  which 
render  visible  and  differentiated  microscopic  details  of  structure  that 
were  formerly  unseen. 

Chromosomes 

Every  germ  cell,  as  well  as  each  of  the  somatic  cells  that  are  the 
building  stones  of  the  body,  contains  a  nucleus,  within  which,  at 
certain  times  in  the  life  cycle  of  the  cell,  chromosomes  may  be  seen. 
These  structures  stain  more  deeply  with  certain  dyes  than  do  other 
parts  of  the  cell,  thus  becoming  visible  under  the  microscope. 

It  is  doubtful  that  Mendel  ever  saw  chromosomes,  for  it  was  not 
until  the  late  seventies,  after  his  scientific  career  was  practically  over, 
that  the  invention  and  development  of  aniline  dyes  made  possible 
their  discovery.  Each  pair  of  chromosomes  has  a  characteristically 
different  shape  and  size,  whereby  it  is  usually  possible  to  distinguish 
them  from  every  other  pair.  Chromosomes,  moreover,  retain  their 
specific  identity,  in  spite  of  the  fact  that  they  may  change  their  form 
temporarily,  or  for  a  time  disappear  entirely  from  view.  When 
germ  cells  undergo  maturation  to  form  their  gametes  as  a  preliminary 
to  fertilization,  the  total  number  of  chromosomes  in  each  cell  is 
reduced  to  one  half.  An  entire  pair  is  never  normally  eliminated, 
although  this  sometimes  occurs  under  abnormal  circumstances 
{non-disjunction).  The  result  is  that  ordinarily  there  is  left  behind 
one  complete  outfit  of  all  the  chromosomes  characteristic  of  the 
species,  with  their  determinative  genes,  both  in  the  mature  egg  and 
the  mature  sperm.  As  pointed  out  previously,  fertilization  restores 
pairs  of  chromosomes  and  then  ever  afterwards,  by  means  of  the 
meticulous  machinery  of  mitosis,  these  pairs  are  handed  on  to  all 
subsequent  cells  of  the  body  that  arise  from  the  fertilized  egg. 

One  of  the  evidences  that  chromosomes  play  an  important  part  in 
heredity  lies  in  the  fact  that  they  are  the  only  parts  of  the  germ  cells 
in  which  the  two  sexes  contribute  equally  to  the  formation  of  the 
fertilized  egg  in  animals,  or  ovule  in  plants,  that  initiates  a  new 
individual.  It  is  common  observation  that  each  parent  in  the  long 
run  is  equally  responsible  for  hereditary  traits  in  the  offspring,  and 


THE   GREAT   RELAY   RACE  457 

this  agrees  in  general  with  the  fact  of  equal  contributions  to  the 
following  generation  of  chromosomes  from  each  parent.  It  has  been 
repeatedly  shown  by  experiment,  as,  for  example,  with  the  eggs  of 
sea-urchins,  that  when  more  than  one  sperm  enters  and  fertilizes  an 
egg,  thus  involving  the  presence  of  an  atypical  number  of  chromo- 
somes, the  resulting  larvae  are  monstrous,  or  at  least  abnormal,  and 
do  not  long  survive.  Evidently  this  is  a  case  of  too  much  father! 
The  conviction  of  the  responsibility  of  chromosomes  in  heredity  is 
further  strengthened  by  a  very  large  number  of  remarkably  ingen- 
ious investigations  made  in  the  last  twenty  years,  centering  about 
the  occasional  abnormal  behavior  of  chromosomes  during  the  matura- 
tion divisions,  particularly  with  the  much-studied  banana-fly  Droso- 
phila,  maize,  and  the  jimson-weed  Datura.  It  is  not  possible  in  this 
limited  summary  to  do  more  than  to  call  attention  to  this  brilliant 
and  complicated  work,  which  goes  far  in  confirming  the  importance 
of  chromosomes  in  heredity.  It  is  earnestly  hoped,  nevertheless, 
that  the  reader  may  eventually  have  the  opportunity  to  explore  this 
fairyland  of  fact.  Although  it  involves  a  somewhat  discouraging 
array  of  strange  technical  terms,  such  as  non-disjunction,  transloca- 
tion, coincidence,  inversion,  duplication,  deficiency,  deletion,  interfer- 
ence, and  ploidy,  it  turns  out  that  the  terms  used  are  not  at  all 
formidable  upon  closer  acquaintance. 

Genes 

Although  the  chromosomes  of  the  male  and  female  germ  cells 
unite  to  build  the  "imponderably  small"  bridge  over  which  the 
hereditary  load  passes  from  one  generation  to  another,  they  are  not 
in  themselves  the  actual  units  of  heredity.  These  ultimate  bearers 
of  inheritance,  which  are  borne  by  the  chromosomes,  are  known  as 
genes,  a  name  given  them  by  the  Danish  botanist  Johannsen  (1859- 
1927).  Dr.  W.  E.  Castle  has  defined  a  gene  as  "the  smallest  part  of 
chromatin  capable  of  varying  by  itself."  In  other  words,  genes  are 
the  ultimate  invisible  hereditary  units  and  as  such  form  the  essential 
subject  matter  of  genetics. 

That  no  one  has  ever  surely  seen  genes  under  the  microscope  does 
not  lessen  the  fact  of  their  reality.  Like  the  atoms  of  the  chemist  and 
the  electrons  of  the  physicist,  of  whose  reality  there  is  no  doubt,  they 
are  too  small  to  be  seen  by  any  means  at  present  at  our  disposal. 
We  know  next  to  nothing  about  the  structure  and  chemical  composi- 
tion of  these  ultimate  hereditary  units  ;  nevertheless,  we  already  know 


458  THE   MAINTENANCE   OF   SPECIES 

a  good  deal  about  their  behavior,  although  the  scholarly  attack  upon 
the  gene  in  the  light  of  what  is  sure  to  follow  can  be  said  to  have 
hardly  begun. 

It  is  plain  that  there  are  many  more  distinguishable  traits  and 
characters  present  in  an  organism  than  there  are  chromosomes.  In 
Drosophila,  for  example,  which  has  only  four  pairs  of  chromosomes  to 
a  cell,  over  five  hundred  hereditary  differences  have  been  accurately 
identified.  Consequently,  many  determining  genes  must  be  located 
in  each  pair  of  chromosomes.  What  has  been  found  to  be  true  of 
Drosophila  in  this  respect,  is  undoubtedly  true  of  other  organisms. 
So  much  of  our  knowledge  of  genes  in  general  has  been  acquired  by 
investigations  upon  the  ubiquitous  banana-fly  that  genetics  stands  in 
some  danger  of  becoming  Drosophiletics.  These  tiny  flies,  that  have 
never  even  heard  of  birth  control,  lend  themselves  very  favorably  to 
the  study  of  genes.  Within  a  month  a  single  pair  can  become  grand- 
parents of  so  many  grandchildren  that  it  is  difficult  to  keep  track  of 
them.  Millions  have  actually  been  experimentally  bred  and  critically 
examined  one  hy  one  by  different  workers  within  the  past  three  decades 
since  their  scientific  usefulness  has  been  discovered.  They  even 
gained  the  Nobel  prize  award  (1934),  with  the  aid  of  Dr.  Thomas 
Hunt  Morgan  and  his  associates. 

It  has  not  only  been  possible  for  the  investigators  of  Drosophila 
to  determine  more  than  five  hundred  determining  genes  in  these  flies, 
but  also  even  to  locate  these  several  genes  definitely  in  particular 
pairs  of  chromosomes,  and  to  arrange  them  with  reference  to  each 
other  at  definite  distances  apart  within  a  single  chromosome.  All 
that  has  been  learned  by  the  followers  of  Mendel  about  the  interaction 
of  what  are  termed  "factors"  appUes  to  the  invisible  genes.  For 
example,  it  is  not  likely  that  single  genes,  any  more  than  single 
factors,  "determine"  single  somatic  traits  or  characteristics.  Rather 
the  genes  must  work  together  to  bring  about  visible  results,  since 
"genie  balance"  is  essential  to  somatic  success. 

Linkage  and  Crossing-over 

Although  there  is,  as  Mendel  demonstrated,  independent  assortment 
between  different  chromosomes  during  the  formation  of  the  gametes, 
the  genes  that  are  located  in  any  single  chromosome  tend  to  hang 
together  in  succeeding  generations  and  not  to  become  separated  from 
each  other.  This  is  called  linkage.  By  means  of  it,  whole  blocks  of 
genes  may  act  together  as  a  unit  in  heredity. 


THE   GREAT   RELAY   RACE  159 

Mendel  did  not  hit  upon  linkage,  because  it  fortunately  so  happened 
that  the  determiners  of  the  seven  characters  (page  444)  with  which  ho 
dealt  were  each  located  in  separate  chromosomes,  of  which  there  are 
known  to  be  seven  pairs  in  garden  peas.  This  was  a  happy  accident, 
for  if  Mendel  had  chanced  upon  genes  linked  together  in  a  single 
chromosome,  he  might  never  have  been  able  to  establish  the  law  of 
independent  assortment,  which  is  so  essential  in  determining  the 
Mendelian  ratios. 

In  mitosis  it  sometimes  happens,  however,  as  shown  by  the  sub- 
sequent breeding  results,  that  chromosomes  emerge  which  contain  a 
different  combination  and  arrangement  of  genes  than  that  in  the 
originals  from  which  they  came.  In  other  words,  linkage  is  broken 
up.  The  way  this  comes  about  is  as  follows.  During  the  process  of 
the  preparation  of  the  germ  cells  for  sexual  union  (mciosis),  as  has 
been  repeatedly  observed,  the  maternal  and  paternal  chromosomes  in 
each  pair  of  egg  or  sperm  come  to  lie  close  together  side  by  side. 
They  may  even  twist  around  each  other.  This  intimate  contact  of 
homologous  chromosomes  from  the  two  parents  is  called  synapsis. 
It  will  be  recalled  how  later  the  still  entire  chromosomes  separate  or 
unwind  from  their  mates  and  migrate  to  opposite  poles  of  the  germ 
cell,  during  the  unique  reduction  division,  thus  forming  two  new  cells 
each  containing  but  half  the  normal  number  of  chromosomes  in  each 
cell.  This  means  that  either  the  maternal  or  the  paternal  chromosome 
of  each  pair  is  missing  in  the  resulting  daughter  cells,  while  the  end 
result  of  ordinary  mito.sis,  or  cell  division,  is  the  production  of  two 
new  cells,  each  with  a  complete  equipment  of  chromosome  pairs 
representing  the  maternal  and  paternal  contributions. 

After  synapsis,  the  two  chromosomes  in  each  pair  may  separate  and 
go  their  different  ways  with  all  their  genes  linked  together  exactly  as 
they  were  before  intimate  contact  with  each  other,  or  during  synapsis 
they  may  stick  temporarily  together  and  then  later  break  into  frag- 
ments and  become  reassembled  in  a  new  relationship,  with  a  part  of 
a  paternal  chromosome  attached  the  supplementary  part  of  a  mater- 
nal chromosome.  When  such  an  interruption  of  linkage  occurs  it  is 
termed  crossing-over.  It  is  as  though,  following  an  ardent  embrace, 
Jack's  head  should  be  found  perched  on  Jill's  shoulders,  and  in 
exchange,  Jill's  head  should  turn  up  on  Jack's  shoulders.  That  this 
extraordinary  kind  of  performance  actually  does  happen  with  the 
chromosomes  has  been  amply  demonstrated  over  and  over  by  observ- 
ing the  ratios  in  which  the  offspring  appear  following  a  dihybrid  cross. 


160  THE   MAINTENANCE   OF   SPECIES 

An  illustrative  case  may  serve  to  make  both  linkage  and  crossing- 
over  plainer.  In  corn,  colored  kernel  (C)  is  dominant  over  colorless 
kernel  (c),  and  plump  starchy  grains  (S)  are  dominant  over  wrinkled 


B 


D 


Diagram  of  the  steps  in  crossing-over.  A,  an  allelomorphic  pair  of  chromo- 
somes, with  genes  represented  as  soHd  or  open  circles ;  B,  synapsis,  or  the  con- 
tact of  homologous  chromosomes ;  C,  breakage  of  chromosomes  at  the  point  of 
contact ;  D,  reassembly  of  chromosome  fragments,  resulting  in  a  cross-over  of 
genes,  making   a  new  combination. 

sugary  grains  (s).  Thus,  when  pure  colored-starchy  corn  (CCSS)  is 
crossed  with  pure  colorless-wrinkled  corn  (cess),  the  resulting  hybrid 
will  be  colored-starchy  like  the  dominant  parent  in  appearance  but 
with  the  genotypic  formula  of  CcSs.  When  in  turn  these  hybrids  are 
back-crossed  with  the  recessive  parent  (cess),  in  order  to  reveal  the 
different  kinds  of  offspring  which  they  are  capable  of  producing,  the 
expected  result,  if  there  is  independent  assortment,  would  be  the  ratio 
of  1  CS  :lCs:  IcSilcs,  as  shown  below. 


Hybrid  Gametes 

CS 

Cs 

cS 

cs 

Recessive  gametes  cs 

CScs 

Cscs 

cScs 

cscs 

In  an  actual  experiment,  however,  when  the  hybrid  was  back-crossed 
to  the  recessive  parent,  the  offspring  were  phenotypically  4032 
CS :  149  Cs  :  152  cS  :  4035  cs.  This  is  approximately  the  ratio  of 
48  :  2  :  2  :  48,  instead  of  the  expected  1:1:1:1.  The  explanation 
of  this  result  is  that  out  of  a  total  of  8368  cases  there  were  8067 
instances  in  which  the  characters  CS  and  cs,  that  entered  into  the 


THE   GREAT   RELAY   RACE  46i 

hybrid  combination  together,  stayed  together  in  linkage,  while  in  the 
remaining  301  cases  out  of  8368,  crossing-over  occurred  between  the 
colored  (C)  and  the  starchy  (*S)  genes  derived  originally  from  one 
grandparent,  with  the  corresponding  colorless  (c)  and  wrinkled 
(s)  genes  furnished  by  the  other  grandparent.  These  cross-overs  were 
a  new  combination  in  corn,  namely,  colored-wrinkled  (Cs),  and 
colorless-starchy  (cS). 

Chromosome  Maps 

By  experiment,  particularly  with  Drosophila,  which  lends  itself 
especially  to  this  kind  of  investigation,  varying  percentages  of  crossing- 
over  between  different  pairs  of  genes  located  in  the  same  pair  of  chro- 
mosomes have  been  determined.  This  method  of  taking  advantage 
of  the  occurrence  of  crossing-over  has  led  to  the  determination  of  the 
distance  hetween  individual  genes  in  particular  chromosomes,  depending 
upon  the  principle  that  the  nearer  together  two  pairs  of  genes  are, 
the  more  likely  they  are  to  remain  linked  when  the  chromosomes  twist 
about  one  another  and  subsequently  break  and  rejoin,  while  the 
farther  apart  they  are,  the  more  likely  they  are  to  shift  from  one 
chromosome  to  the  other  during  synapsis. 

For  example,  if  the  percentage  of  crossing-over,  as  shown  by  the 
results  of  breeding,  between  the  hypothetical  genes  Aa  and  Bh,  is  five, 
and  that  between  Bh  and  Cc  is  twenty,  then  the  cross-overs  between 
Aa  and  Cc  ought  to  be  twenty-five  (5  +  20)  if  the  order  of  the  genes  in 
the  chromosomes  is  A-B-C,  or  fifteen  (20  -  5)  if  the  order  of  arrange- 


15 /^-5^B 


20- 


I             I             I             I             I 
-15 ^^ 25 


The  determination  of  the  order  of  genes  on  a  chromosome. 

ment  is  C-A-B.     This  kind  of  confirmation  has  been  repeatedly 
verified  in  actual  breeding  experiments. 

By  an  extension  of  this  technique  it  has  been  possible  to  construct 
chromosome  maps,  in  which  the  location  of  the  different  invisible  genes 
in  the  various  chromosomes  can  be  determined  with  astonishing 
accuracy.  Such  a  map  of  the  four  different  chromosomes  in  Droso- 
phila, as  far  as  it  had  been  completed  in  1926.  when  Morgan  published 
"The  Theory  of  the  Gene,"  is  shown  on  page  462.  Today  the  chro- 
mosome map  of  Drosophila,  like  a  recent  map  of  the  world  as  com- 


462 


THE   MAINTENANCE   OF   SPECIES 


pared  with  one  of  Marco  Polo's  day,  shows  many  new  additions, 
thanks  to  the  patient  and  tireless  labors  of  the  small  army  of  Droso- 
philologists. 


I 


II 


III 


w 


0.0 

(yellow 

0.0  i,             J  hairy-wing 

0.+J\       /{scute 

0.3 -^i 

=^~  kthal-1 

0.0 1 

- 

0.6  <^- 

1.0  yr 

1-5  '//Z 

'^  broad 
~\  V  prune 
"V\\  whilp 

2.0 

- —  star 

3.±  — 

arigtaless 

3.0W/- 

zmfo^t 

6.*  — 

"-"  extended 

3.±m~ 

^^notch 

i.hf// 

W^  abnormal 

5.5W 
6.97 

Vo  echirttts 
\  bifid 

12  +  — 

-guU 

7.5V- 
13.7  V- 

i6.±  y- 

-\\ruby      , 
-^^  cross-veinless 

13.0 -: 
14.±/- 
16.0^ 

- —  truncate 

-^  dack^ous 

\  streak 

20.0  —Z 

cut 

21.0^ 

^^  singed 

^'\ 

ytan 

27.7  ---:  = 

lozenge 

33.0  ^_ 

Vermillion 

31.0 

dachs 

36.1  \ 

^  miniature 

35.0 

ski-n 

36.2-^= 

-~^^  dusky 

38.  ±  ^- 

~^furrowed 

43.0  \_ 

^^  sable 

41.0 

-^  jammed 

44.4  --- 

— garnet 

46.± 

-^  minute-e 

48.5  \_ 

-black 

48.7^" 

\  jaunty 

54.2  V 

^ small-wing 

54.5  -i: 
56.5  \_ 

rudimentary 

-forked 

54.5 

purple 

57.0  -'- 

~"^^6ar 

57.5 

■  cinnabar 

58.5;^= 

59.0  y 
59.6  y- 
62,±  y- 

65.0  ' 

~\^  small-eye 
_  \^ fused 

60.± 

— —  safranin 

\y  beadez 
\  \  minute-^ 

-\ufi 

64.i  — 

67.0 

68.  ±''- 

pink-wing 

— —  vestigial 
^  telescope 

70.0  /         ^bobbed 

72.0 

.  lobe 

74.± 

gap 

75.5 

--—curved 

83.5  — 


90.0 humpy 


-fringed 


.  arc 
/,  plexus 


99.5  \ 
100.5- 

los.on- 

105.: 

106.  ± . —  purploid 

107.0  J7        \Xspeck 
107.5'  \  balloon 


-'Atethal-Uo 
6.01^- -^jbroum 
S.±J\        /{blistered 


0.0  — -y —  roughoid 


20.0 — — divergent 


X.0 

26.5^ 


35.0 

36.5 

40.1  \ 

40.2  -^: 
40.4 


42!2- 


K- 


*^-°46:±^. 

46.5-" 

49.7  y 

50.±V 
50.- 
54.8' 

58.2  ~. 
58.5  ^y. 
58.7  </ 
59.5 -V" 
62.0  V' 
63.1-^  . 
66.2^ 
69.5  v.^ 

70.7  -;: 

72.0  ■^' 

75.7--; 

76.2''^ 


91.1 

93.0  — 


1.8-7 


0.0  --^—  bent 


y/i- 


^.< 


h.tn ^shaven 

9.0  '  \  eyeless 


;■— -sepia 
^  hairy 


—  rose 

. — crcam-111 

^  minute-h 
:^  tilt 
,^dicka€te 
thread. 


scarlet 


deformed 


warped 

:^^^ki:in_ 
.^  pink 
'  Y^  maroon 

Xjduarf 

^curled 

hairy-wing  sup, 
: —  stubble 
;^  sj)ineles3 

\  bitkorax 
'\^bithorax-b 
'  V>  stripe 
\  glass 

■^  delta 

^^  hairless 
'■ —  ebony 

\  band 
:-—  cardinal 
\  white-ocelli 


• —  rough 
•  —  crumpled 

V^  beaded 
\  pointed 

^  claret 

\  minute 


94. 
100.7  v^ 

101.0  ^' 

106.2  — -*- — minute-g 


(After  Morgan) 


"  When  it  is  remembered  that  Drosophila  is  a  very  tiny  fly ;  that  paired 
reproductive  organs  occupy  only  a  small  part  within  its  abdomen ;  that  each 
of  these  reproductive  organs  in  the  male  is  made  up  of  several  tubules ;  that 
within  these  tubules  may  eventually  be  found  the  sperm  cells  with  plenty  of 
room  in  which  to  move  about ;    that  within  each  sperm  cell  is  a  nucleus ; 


THE   GREAT   RELAY   RACE 


163 


that  after  half  the  contents  of  the  nucleus  has  been  disposed  of  there  remains 
four  chromosomes;  that  within  each  chomosome  there  are,  beyond  the 
range  of  vision,  hundreds  of  genes ;  and  that  it  is  possible  within  a  single 
chromosome  to  determine  not  only  the  relative  arrangement  of  the  many 
genes,  but  also  to  find  out  the  relative  distance  between  any  two  of  these 
genes,  it  wUl  be  realized  that  the  analysis  of  the  germplasm  has  gone  a  long 
way." ' 

The  Role  of  the  Cytoplasm 

In  spite  of  the  demonstrated  importance  of  the  chromosomes  and 
their  genes  in  the  mechanism  of  heredity,  they  are  not  the  whole  story. 
There  is  the  cytoplasm  to  be  reckoned  wdth,  particularly  in  the  egg. 
In  no  cell  can  either  the  nucleus  or  the  cytoplasm  lead  an  independent 
existence.  Each  depends  upon  the  other.  Hence,  while  the  un- 
doubted significance  of  the  chromosomes  is  being  emphasized,  it  is 
well  to  remember  also  the  indispensable  cytoplasm.  Is  there  such 
a  thing  as  cytoplasmic  inheritance,  in  addition  to  that  of  the  genes  ? 

In  answer  to  this  question  it  is  necessary  in  the  first  place  to  dis- 
tinguish the  part  that  cytoplasm  plays  in  development  as  well  as  its 
possible  function  in  hereditary  transmission. 

The  nuclear  membrane  separates  the  chromosomes  from  the  sur- 
rounding cytoplasm  during  the  resting  stage  of  every  cell  cycle, 
resulting  in  some  degree  of  temporary  independence.  However, 
every  time  mitosis  is  repeated  this  protective  membrane  vanishes  for 
the  time  being,  leaving  the  chromosomes  directly  exposed  to  the 
cytoplasm.  Here,  then,  is  furnished  an  opportunity  for  exchange  of 
materials  between  chromosomes  and  cytoplasm,  and  this  exchange 
does  undoubtedly  occur.  During  mitosis,  it  will  be  remembered, 
each  chromosome  splits  lengthwise,  and  the  half  chromosomes  thus 
formed,  mingling  freely  with  the  cytoplasm,  migrate  to  their  respec- 
tive poles.  Meanwhile  they  are  restored  to  their  original  dimensions 
by  the  intake  of  material  from  the  cytoplasm  itself.  Thus  a  part 
of  the  cytoplasm  of  the  cell  becomes  made  over  into  chromosomal 
material. 

In  the  long  series  of  successful  mitoses  by  means  of  which  the 
zygote  eventually  becomes  an  adult  individual,  the  chromo.somes  in 
each  newly  formed  cell  still  maintain  their  original  genetic  make-up  as 
to  form  and  numbers  of  pairs.  The  cytoplasm  of  these  various  cells, 
on  the  other  hand,  undergoes  transformation  to  constitute  the  different 

'  From  W'alter,  H.  E.,  Genetics.     By  permission  of  The  Macmillan  Cominiiiy,  publishers. 


464  THE  MAINTENANCE  OF  SPECIES 

tissues  of  the  body.  In  other  words,  while  there  is  accompUshed  an 
equal  distribution  of  chromosomes,  an  unequal  distribution  and 
elaboration  of  the  cytoplasm  takes  place.  It  is  plain,  therefore,  that 
in  this  process  the  genes  not  only  take  in  material  to  be  elaborated 
from  the  cytoplasm,  but  that  in  turn  something  must  go  out  from 
them  to  bring  about  the  differentiation  of  the  surrounding  cytoplasm. 
That  there  is  a  chemical  difference  between  what  is  in  the  chromo- 
somes and  what  is  outside  of  them  is  proven  by  the  differential  way 
in  which  these  substances  respond  to  certain  stains.  Apparently 
there  is  carried  on  from  generation  to  generation  throughout  life  an 
elaborate  and  extensive  performance  of  "give  and  take"  between  the 
germplasmal  chromosomes  and  the  somatoplasmal  cytoplasm  of  the 
cells.     Dr.  H.  S.  Jennings  states  the  matter  in  the  following  words : 

"  This  process  of  changing  the  cytoplasm  by  the  action  of  the  genes  is  the 
fundamental  thing  in  development.  The  genes  repeat  this  process  over  and 
over  again,  taking  in  cytoplasm,  modifying  it,  giving  it  off  in  changed  condi- 
tion, and  leaving  the  genes  themselves  unaltered."  ^ 

In  the  light  of  the  intimate  relationship  between  genes  and  cytoplasm, 
and  recognizing  the  dominant  part  taken  by  it  in  developmental 
processes,  can  we  assign  any  truly  hereditary  role  to  the  cytoplasm 
itself,  except  as  it  is  first  taken  in  and  made  a  part  of  the  chromosome 
complex  ? 

It  is  common  knowledge  that  apple  blossoms,  when  fertilized  with 
foreign  pollen,  produce  only  apples  like  the  maternal  parent  because 
the  apple  itself  is  merely  an  elaboration  of  the  maternal  tissue  of  the 
ovary,  determined  in  its  character  before  the  ovule  in  the  ovary  is 
fertilized.  Seeds  of  such  apples,  however,  grow  into  trees  that 
produce  fruit  showing  paternal  as  well  as  maternal  characters.  This 
sort  of  "maternal  inheritance"  suggests  the  presence  of  some  heredi- 
tary factor  outside  the  genes  that  keeps  an  apple  a  sweet  apple,  for 
instance,  although  its  blossom  is  fertilized  by  pollen  from  a  sour 
apple  tree.  It  is  only  necessary,  however,  to  remember  that  the 
cytoplasm  of  the  sweet  apple  is  already  determined  by  the  germinal 
contributions  of  the  preceding  generation,  both  maternal  and  paternal, 
rather  than  by  the  fertilizing  pollen  in  the  present  case,  in  order  to 
find  a  satisfactory  explanation  that  does  not  involve  cytoplasmic 
determination. 

'  From  Jennings,  H.  S.,  Genetics,  p.  233.    By  permission  of  W.  W.  Norton  &  Company,  publishers. 


THE  GREAT   RELAY   RACE  465 

In  practically  all  groups  of  plants  there  are  certain  structures 
embedded  in  the  cytoplasm  called  plastids,  which  are  centers  of  meta- 
bolic activity.  They  are  composed  of  packets  of  various  materials 
essential  to  plant  life,  such  as  starch  grains,  chlorophyll,  oil  droplets, 
and  the  like,  having  a  definite  chemical  composition  and  easily  visible 
under  the  microscope.  It  is  generally  agreed  that  ])lastids  are 
derived  from  preceding  plastids,  quite  as  chromosomes  are  from 
preceding  chromosomes,  and  that  they  are  not  formed  anew  in  each 
cell.  Unlike  chromosomes,  however,  they  do  not  undergo  orderly 
mitosis  when  they  divide,  thus  securing  in  daughter  plastids  an 
accurate  halving  of  material  as  in  the  case  of  chromosomes  and 
genes,  nor  do  they  always  follow  the  Mendelian  laws  in  their  redis- 
tribution. A  case  in  w^hich  the  chromosomes  and  genes  do  not 
apparently  play  their  usual  equal  parts,  but  in  which  it  looks  as  if  the 
inheritance  is  through  self-perpetuating  plastids  in  the  female  cyto- 
plasm and  never  through  the  male  gametes,  is  found  in  plants  with 
variegated  or  striped  leaves.  In  these  plants,  the  cells  with  plastids 
carrying  chlorophyll  {chloro-plasts)  determine  the  green  areas  in  the 
leaf,  while  cells  with  plastids  that  lack  the  chlorophyll  (leucoplasts) 
account  for  the  white  areas.  Branches  and  flower  buds  occur  with 
either  chloroplasts  or  leucoplasts.  When  crosses  are  made  between 
flowers  borne  upon  a  green  branch,  and  those  from  a  white  branch  of 
such  variegated  plants,  the  resulting  offspring  are  white  or  green 
according  to  the  kind  of  plastids  present  in  the  maternal  parental 
branch,  irrespective  of  the  kind  of  pollen  employed.  The  grand- 
parental  genes  determine  the  character  of  the  maternal  plastids,  which 
in  turn  cause  the  new  branch  or  plant  to  be  white  or  green. 

Current  opinion  about  the  whole  matter  is  summarized  in  the 
statement  of  Dr.  E.  M.  East  to  the  effect  that  "though  the  nucleus 
and  cytoplasm  co-operate  in  development,  the  only  ascertained  agent 
of  heredity  is  the  nucleus."  What  the  future  may  disclose  still 
remains  a  question  unanswered,  but  at  present  it  appears  that 
"cytoplasmic  inheritance"  is  unproven. 

Sex  in  Heredity 

While  it  is  quite  possible  for  one  generation  to  arise  from  another 
by  various  asexual  methods,  yet  it  is  evident  that  the  whole  mech- 
anism of  heredity  has  been  revolutionized  by  the  rise  of  sex. 

As  previously  pointed  out  in  the  section  on  "The  Usefulness  of  Hy- 
brids" (page  441),  in  the  study  of  heredity  so  long  as  level  uniformity 


466 


THE   MAINTENANCE   OF   SPECIES 


characterizes  the  succession  of  generations,  there  is  no  way  by  which 
the  laws  of  inheritance  may  be  detected.  Distinctive  alternative 
characters  must  be  introduced  from  unlike  parents  and  combined  in 
various  ways  in  order  to  make  the  manner  of  inheritance  in  the 
progeny  recognizable.  Transitory  environmental  variations,  since 
they  play  no  part  in  inheritance,  only  cloud  the  picture.  It  is  germ- 
plasmal  variations  alone  that  can  be  of  service  in  inheritance,  and 
such  variations  are  provided  in  double  measure  by  the  device  of 
sexual  reproduction.  Thus  sex  is  not  only  the  major  means  by  which 
inheritance  is  effected  but  it  also  furnishes  the  key  that  unlocks  the 
mystery  of  how  evolution  is  brought  about. 

The  way  in  which  sexual  recombination  can  change  the  flow  of 
germplasm  from  one  generation  to  another  is  suggested  in  the  figure, 


Two  different  biparental  streams  of  germplasm,  A  and  B,  may  form  four  new 
different  biparental  streams  of  germplasm,  a,  b,  c,  and  d,  in  the  next  generation. 

which  reduces  the  matter  to  terms  so  simple  that  it  is  consequently 
entirely  inadequate  to  represent  the  actual  complexity  and  possible 
rearrangement  accompanying  sexual  reproduction. 

Although  Mother  Nature's  children,  that  is,  plants,  animals,  and 
even  mankind,  have  successfully  utilized  the  mechanism  of  sex  for 
an  incomprehensible  span  of  time,  it  is  only  in  recent  years  that  man 
has  come  to  understand,  with  anything  like  scientific  accuracy,  the 
way  in  which  it  works. 

In  the  eighteenth  century,  the  "ovists"  held  that  the  egg  was  the 
all-important  factor,  and  that  the  sperm  simply  served  to  start  the 


THE  GREAT  RELAY  RACE  467 

egg  on  its  developmental  way.  An  opposing  school  of  "spermists" 
maintained  that  the  egg  was  only  useful  as  a  means  of  food  storage 
for  the  essential  sperm.  Notwithstanding  the  fact  that  the  ancient 
Assyrians  were  well  aware  that  date  palms  would  not  mature  fruit 
unless  pollen  from  male  trees  was  dusted  on  the  blossoms  of  the 
female  trees,  it  was  less  than  a  century  ago  that  it  was  finally  estab- 
lished by  Leuckhart  (1822-1898)  that  both  egg  and  sperm  are  homol- 
ogous partners  in  heredity. 

It  was  not  until  the  beginning  of  the  present  century,  after  Mendel's 
laws  had  been  re-established  and  chromosomes  had  been  discovered, 
that  sex  was  recognized  as  a  hereditary  trait  in  itself,  dependent 
principally  upon  genes.  That  other  factors  besides  genes  may  con- 
tribute to  the  determination  of  sex  is  no  doubt  true.  For  example, 
Dr.  Oscar  Riddle,  of  the  Carnegie  Institution  of  Washington,  has 
advanced  a  well-grounded  theory  of  the  metabolic  determination  of  sex, 
based  upon  exhaustive  experiments  extending  over  many  years,  in 
breeding  doves  at  Cold  Spring  Harbor,  Long  Island.  Other  inves- 
tigators have  emphasized  the  modifying  influence  of  the  external 
environment,  and  of  the  internal  hormones,  but  no  one  denies  the 
action  of  the  genes  as  the  primary  effective  factor  in  sex  determi- 
nation. 

The  theory  most  generally  accepted  today  to  account  for  the 
approximate  equality  of  the  sexes  in  the  offspring  of  any  species  is 
that  of  Correns,  who  postulated  that  the  gametes  of  one  parent  are 
of  two  kinds,  male-producing  and  female-producing,  while  the  gametes 
of  the  other  parent  are  alike  so  far  as  sex  determination  is  concerned. 
This  idea  has  been  amply  substantiated  by  the  discovery  in  many 
forms  of  plants  and  animals  of  what  has  subsequently  been  designated 
as  sex  chromosomes. 

As  has  been  repeatedly  emphasized,  chromosomes  occur  in  homolo- 
gous pairs,  one  member  from  each  parent.  McClung  in  1902,  dis- 
covered that  in  the  germ  cells  of  the  male  locust,  Xiphidium  fasciatum, 
there  occurred  an  odd  chromosome  without  a  mate  while  in  the 
female  immature  germ  cells  every  chromosome  was  supplied  with  a 
corresponding  mate.  Consequently,  this  being  the  case,  when  the 
members  of  the  chromosome  pairs,  following  synapsis,  separate  to 
form  the  gametes,  the  odd  chromosome  joins  one  group  of  daughter 
chromosomes,  leaving  the  other  group  one  chromosome  short.  The 
former  sort  of  gametes,  carrying  the  odd  chromosome,  upon  union 
with  a  normal  female  gamete  having  a  full  quota  of  chromosomes, 


468  THE   MAINTENANCE  OF  SPECIES 

forms  a  zygote  that  will  produce  a  female,  while  the  latter  sort  without 
the  odd  chromosome,  when  uniting  with  the  normal  female  gamete, 
produces  a  zygote  that  is  destined  to  become  a  male.  Thus,  if  XX 
represents  the  sex  chromosomes  of  the  female,  and  XO  those  of  the 
male,  the  result  is  diagrammatically  as  follows : 

Germ  cells  XX  XO 


Gametes 
Zygotes 

In  many  instances  it  has  been  observed  that  the  formula  XY,  instead 
of  XO,  represents  the  male  sex  chromosome  pair,  while  the  female 
remains  XX.  That  is,  instead  of  an  odd  unpaired  sex  chromosome, 
there  is  a  mismated  pair.  The  accompanying  figure,  showing  the 
chromosomes  in  Drosophila,  serves  as  an  example  of,  such  a  case.  It 
will  be  seen  in  this  figure  that  in  the  male  there  are  present  three  pairs 
of  chromosomes  in  which  the  mates  are  alike,  but  that  one  chromo- 
some of  the  fourth  pair  is  rodlike,  while  its  mate,  the  F-chromosome, 
has  a  bent  tip.  By  substituting  Y  for  the  0  in  the  preceding  diagram, 
the  same  explanation  for  the  equality  in  number  of  the  sexes  among 
the  offspring  is  reached,  as  in  the  case  of  McClung's  locusts.  In 
both  of  these  examples  it  is  the  number  of  X-chromosomes  present, 

that  is,  one  or  two,  that  deter- 
V  CT  mines  the  sex  of  the  offspring. 

Other  variations  of  this  funda- 
mental idea  have  been  found  in 
the  copious  investigations  which 

nj  C  have  been  made  on  the  heredity 

f    •  of  sex,  but  all  agree  with  Correns' 

^  ^  original  interpretation  of  unlike 

The  four  pairs  of  chromosomes  in  Dro-    ^^^  gametes  in  one  parent  and 
sopiiila  melanogaster.     (Alter  Morgan.)        .       °  .  ^ 

like  gametes  in  the  other. 

The  great  majority  of  plants  and  animals  that  have  been  examined 
show  that  the  male  ordinarily  is  the  sex  that  produces  two  kinds  of 
sex-determining  gametes.  Birds,  butterflies,  and  moths  form  an 
exception  to  this  general  rule,  for  in  them  all  the  sperm  gametes  are  of 
one  kind,  while  two  kinds  of  mature  eggs,  male-producing  and  female- 
producing,  occur.  The  result  of  approximate  equality  of  the  sexes  in 
the  progeny,  however,  is  the  same  as  in  the  former  instances. 


-^.^  -^..^ 


THE  GREAT   RELAY   RACE 


469 


In  mankind  there  are  twenty-four  pairs  of  chromosomes,  of  which 
twenty-three  pairs,  common  to  both  sexes,  are  called  autosomes,  and 
to  these  is  added  one  pair  of  sex  chromosomes,  designated  XF  in  the 
male  and  XX  in  the  female.  A  curious  fact  about  F-chromosomes  in 
general  is  that,  with  few  exceptions,  breeding  experiments  prove  them 
to  be  devoid  of  genes.  They  play  a  dummy  hand.  Thus  the 
F-chromosome  exerts  the  same  non-contributory  role  in  heredity  as 
the  0  element  does  in  the  XO  combination.  The  X-chromosome,  on 
the  other  hand,  not  only  plays  a  part  in  sex  determination,  but  it  also 
harbors  additional  genes  that  control  the  appearance  of  other  traits 
and  characters.  These  are  called  sex-linked  traits.  Their  existence 
is  demonstrated  in  the  male  because  there  is  nothing  in  the  F-chromo- 
some mate  to  conceal  them. 

This  point  may  be  made  clear  by  citing  Morgan's  now  famous  case 
of  the  white-eyed  Drosophila.  Many  years  ago  in  one  of  his  cultures 
of  normal  red-eyed  flies,  there  appeared  a  single  white-eyed  male 
mutant  individual.  The  conjunction  of  Professor  Morgan's  seeing 
eye  with  the  white  eye  of  this  particular  tiny  fly  marks  an  event  in  the 
history  of  genetics  comparable  to  what  happened  to  the  science  of 
physics  when  the  falhng  apple  and  Sir  Isaac  Newton's  head  came 
together.  In  both  cases  an  exceptional  brain  was  fortunately 
stimulated,  with  far-reaching  benefits  to  science.  When  Morgan's 
unique  white-eyed  male  fly  was  mated  with  a  normal  red-eyed  female, 
all  the  offspring  were  red-eyed,  thus  showing  the  dominance  of  the 
red-eyed  character  over  white-eye.  When  these  red-eyed  hybrids 
were  mated  together,  the  expected  Mendelian  ratio  of  three  reds  to 
one  white  resulted,  but  all  the  males  were  white-eyed.  Omitting  the 
autosomes  and  representing  only  the  sex  chromosomes,  the  matter 
may  be  diagrammed  as  foUow^s.  (The  underscored  A"  indicates  that 
red-eye  color  is  linked  with  the  sex  chromosome.  The  absence  of 
underscoring  means  white-eye.) 


Parents 
Gametes 

XX 

X 

\- 

/ 

X 

i^r 

Fj  offspring 
Gametes 

X 

^■■, 

/ 

~-x_ 

Fg  offspring 

XX- 

— xa: ■ 

"■^^xr 

~~~~ 

^XY 

470 


THE   MAINTENANCE   OF   SPECIES 


In  order  to  obtain  a  white-eyed  female,  it  was  necessary  to  mate  a 
wliite-eyed  male  to  a  hybrid  red-eyed  female,  which  works  out  as 
follows : 


Parents 

,xx 

/    \ 

Gametes 

X- ___      A' 

XHr^:^ 

F,  offspring 

XX 

In  this  type  of  sex-linked  inheritance,  the  paternal  character  may 
be  transferred  directly  in  50  per  cent  of  the  cases  from  father  to 
son  and  from  mother  to  daughter.  There  is  another  type  of  sex- 
linkage,  as  exemplified  by  some  kinds  of  color-blindness  in  man,  in 
which  the  inheritance  is  never  direct  from  father  to  son  and  from 
mother  to  daughter,  but  indirect,  or  zigzag,  as  from  father  through 
daughter  to  grandson.  This  is  called  criss-cross  inheritance.  Thus, 
when  a  female,  normal  for  color-blindness,  is  mated  with  a  color-blind 
male,  the  trait  skips  a  generation  before  it  reappears. 


Parents 


Gametes 


F.  children 


J(Y 

color-blind   (>^ 


Gametes  ■^" 

F2  grandchildren  XX 

normal  ( 

It  will  be  seen  that  in  addition  to  regular  Mendelian  inheritance, 
which  has  to  do  with  the  genes  located  in  the  various  autosomes  and 
which  results  in  the  typical  3  :  1  ratio  when  the  hybrids  are  bred 
together,  there  are  two  other  types  of  inheritance,  involving  the  sex 
chromosomes.  One  of  these  is  the  direct  type  in  which  the  character 
may  be  handed  on  from  father  to  son  or  from  mother  to  daughter,  and 
the  other  is  the  indirect  type  of  criss-cross  inheritance  in  which  the 
father  cannot  give  the  character  to  his  son,  but  may  pass  it  along  to 
his  grandson  by  way  of  his  daughter. 

In  drawing  this  section  to  a  close,  it  is  worth  while  to  quote  the 
opinion  of  the  eminent  English  geneticist,  C.  C.  Hurst,  who  says, 
"Perhaps  there  is  nothing  which  has  helped  the  study  of  genetics 
more  than  the  existence  of  sex."  It  would  take  us  too  far  afield  to 
follow  out  the  enticing  vistas  of  heredity  opened  up  by  the  phe- 
nomenon of  sex.     Some  of  the  many  aspects  of  heredity  which  might 


THE   GREAT   RELAY   RACE  471 

be  considered  in  this  connection  are  suggested  by  such  terms  as  sex 
hormones,  sex  determination,  sex  reversal,  parthenogenesis,  hermaph- 
roditism, gynandromorphs,  gonad  transplantation,  sterility,  free- 
martins,  and  identical  twins.  In  order  to  go  on,  the  interested  student 
must  have  recourse  to  books  and  source  material  devoted  entirely  to 
genetics.  Even  with  such  aids  much  that  is  new  and  illuminating  in 
this  rapidly  developing  science  will  be  found  wanting, 

SUGGESTED   READINGS 

Castle,  W.  E.,  Genetics  and  Eugenics,  3rd  ed.,  Harvard  University  Press,  1924. 

An  authoritative  summary  by  a  pioneer  in  genetics. 
Crew,  F.  A.  E.,  Animal  Genetics,  Edinburgh,  1925. 

The  way  a  brilliant  Scotchman  sees  heredity. 
Dunn,  L.  C,  Heredity  and  Variation,  The  University  Society,  1934. 

Brief  and  very  readable. 
Jennings,  H.  S.,  Genetics,  W.  W.  Norton  &  Co.,  1935. 

Particular  emphasis  upon  the  chromosomal  aspect. 
Morgan,  T.  H.,  The  Theory  of  the  Gene,  Yale  University  Press,  1926. 

The  statement  of  a  Nobel  prize  winner. 
Schwesinger,  G.  C,  Heredity  and  Environment,  The  Macmillan  Co.,  1934. 

Emphasis  upon  the  genesis  of  psychological  characteristics. 
Sinnott,  E.  W.,  and  Dunn,  L.  C,  Principles  of  Genetics,  2nd  ed.,  McGraw-Hill 

Book  Co.,  1932. 

A  widely  used  text. 
Snyder,  L.  H.,  The  Principles  of  Heredity,  D.  C.  Heath  &  Co.,  1935. 

A  very  excellent  up-to-date  book. 
Walter,  H.  E.,  Genetics,  3rd  ed..  The  Macmillan  Co.,  1930. 

An  elementary  presentation. 
Wilson,  E.  B.,  The  Cell  in  Development  and  Heredity,  The  Macmillan  Co., 

1925. 

A  masterly  storehouse  of  reliable  information. 


H.    W.    H.  — 31 


I 


THE   CHANGING  WORLD 


XXI 


TIME  SPENT   (PALEONTOLOGY) 

Preview.  The  stretch  of  time  •  Measures  of  time  •  Kinds  of  fossils  • 
Fossils  as  time  markers  •  The  testimony  of  extinct  types  •  The  role  of  pale- 
ontology •  Suggested  readings. 

PREVIEW 

There  are  two  things  with  which  living  creatures  are  inseparably 
involved  and  from  which  there  is  no  escape,  space  and  time. 

Although  everyone  has  a  working  idea  of  what  is  meant  by  these  two 
common  words  and  uses  them  freely  and  constantly  in  all  sorts  of  con- 
nections, it  is  somewhat  surprising  how  difficult  it  is  to  define  them 
satisfactorily  without  making  use  of  other  words  that  require  definition 
as  well.  Try  it !  Just  what  is  time  ?  Do  not  resort  to  the  dictionary 
until  you  are  willing  to  give  up.  You  will  probably  find  the  dictionary 
disappointing.  Is  time,  perhaps,  that  particular  bit  of  eternity  to 
which  we  can  set  limits?     If  so,  what  is  eternity? 

There  are  two  sciences  in  this  connection  that  are  profitable  to 
explore,  if  only  to  enlarge  our  intellectual  sky  lines.  The  first  and 
older  science  is  Astronomy,  which  serves  to  expand  our  ideas  of  space, 
and  of  which  man  alone  can  have  any  inkling.  The  second  is  Paleon- 
tology. Although  this  has  been  developed  more  recently,  it  is  never- 
theless concerned  with  very  old  things.  One  benefit  to  be  gained  from 
the  study  of  paleontology  is  that  it  stretches,  and  makes  more  spa- 
cious, our  concept  of  time. 

It  is  not  the  purpose  of  this  section  to  present  an  outline  of  paleon- 
tology, but  simply  to  consider  very  briefly  the  relation  between  time 
and  living  things.  The  role  of  living  things  with  reference  to  space 
has  already  been  touched  upon  in  unit  II  under  the  title,  "The 
Biological  Conquest  of  the  World." 

The  Stretch  of  Time 

Whatever  time  is,  the  geologist  has  plenty  of  convincing  evidence 
that  an  enormous  amount  of  it  already  has  been  spent  upon  this  earth 
since  it  became  the  earth,  for  time  was  passing,  ''with  no  vestige  of  a 

473 


474 


THE  CHANGING  WORLD 


beginning  and  no  prospect  of  an  end,"  even  before  the  "  everlasting 
hills  "  were  born.  The  geological  evidences  of  the  passage  of  time  are 
plain  and  unmistakable  to  everyone. 

A  visit  to  the  Grand  Canyon  of  the  Colorado,  for  example,  and  an 
inspection  of  the  gigantic  stone  book  there  revealed,  with  its  leaves 
of  stratified  rock  piled  one  upon  another,  must  impress  even  the  most 

flippant  traveler  with  the 
record  of  time  spent  that 
is  there  displayed.  Strati- 
fied rocks  made  out  of 
sediment  such  as  those 
which  form  the  walls  of 
that  stupendous  gorge 
were  built  up  first  some- 
what slowly  through  the 
erosion  of  land  masses, 
then  the  sediment  was 
collected  and  borne  away 
by  flowing  streams  and 
finally  deposited  bit  by 
bit  in  horizontal  beds 
under  water.  These  sedi- 
ments were  subsequently 
compressed,  cemented, 
and  hardened  into  layers 
of  stone,  varying  in  thick- 
ness. 

Sooner  or  later  there 
might  follow  the  gradual 
shifting  of  the  levels  of 
land  and  water,  possibly 
caused  by  the  aging  and  consequent  wrinkling  on  a  large  scale  of 
the  earth's  crust.  At  any  rate,  whatever  the  cause,  there  has  resulted 
an  eventual  submergence  here  and  there  of  what  was  once  land,  as 
well  as  a  slow  up-thrust  of  the  neighboring  ocean  bed  to  form  newly 
emerged  land. 

Meanwhile  rain  fell,  not  continuously  in  delugelike  floods,  but 
from  time  to  time  just  as  it  does  at  present,  with  considerable  intervals 
between  the  rainy  spells.  In  fact,  there  is  every  reason  to  believe 
that  all  the  processes  leading  to  the  formation  of  sedimentary  rocks 


U.  S.  Geological  Survey 

Erosion  in  the  Grand  Canyon  of  Colorado  has 
laid  bare  stratifications  of  soil  formation  de- 
posited in  centuries  past. 


TIME  SPENT   (PALEONTOLOGY)  475 

in  the  past  were  gradual  and  time  consuming,  exactly  as  they  are  seen 
to  be  before  our  very  eyes  today. 

Such  repeated  rainfalls  drain  down  the  slopes  of  the  newly  emerged 
land,  and,  after  countless  contributions  from  lesser  streams,  combine 
into  rivers  which  cut  slowly  into  the  elevated  accumulations, of  sedi- 
mentary rock  and  wear  it  away.  Thus,  in  the  course  of  long  eons  of 
time,  a  river  with  its  abrasive  sediment  scours  out  and  fashions  a 
gorge. 

In  the  case  of  the  Grand  Canyon  the  rushing  Colorado  River,  now 
down  a  mile  deep  from  the  rim  of  the  gorge,  is  still  grinding  away 
unceasingly  at  its  uncompleted  task  of  recording  spent  time.  What 
a  majestic  open  diary  of  the  passage  of  time ! 

Measures  of  Time 

The  biologist  finds  it  not  only  convenient  but  indispensable  to 
establish  some  sort  of  foot-rule  by  means  of  which  the  continuous 
and  incomprehensible  past  may  be  divided  into  understandable  por- 
tions. To  this  end,  the  stratified  or  sedimentary  rocks  of  the  geologist 
prove  to  be  of  the  greatest  use.  Even  so,  only  through  much  persist- 
ent study  by  experts  has  anything  like  a  satisfactory  time-scale  been 
evolved. 

Sedimentary  rocks,  for  example,  sandstones,  limestones,  and  shales, 
do  not  envelop  the  entire  earth  in  continuous  layers  in  the  way  that 
an  onion  is  made  up.  They  occur  only  in  patches  here  and  there, 
where  once  was  water  in  which  they  could  be  deposited  from  the 
surrounding  land  masses.  However,  when  the  various  patches  of 
sedimentary  rocks  the  world  over  are  examined  and  compared,  it  is 
quite  possible  to  piece  them  together,  like  a  jig-saw  puzzle,  into  a  total 
column  of  layers  one  above  the  other. 

For  purposes  of  identification  and  description,  the  time  consumed  in 
the  formation  of  this  column  may  be  divided  into  eras  and  sub- 
divided into  periods.  While  the  opinion  of  experts  may  differ  with 
respect  to  the  limits  and  details  of  these  arbitrary  divisions  of  past 
time,  there  is  universal  agreement  as  to  their  orderly  sequence. 
Such  a  time-scale  of  eras  and  periods  is  given  on  the  following  page. 

In  this  time-scale  stratified  rocks  can  be  employed  as  a  standard 
of  estimation  for  only  approximately  the  last  half  of  known  time,  i.e., 
45  per  cent.  The  rocks  of  the  Proterozoic  and  Archeozoic  eras  that 
characterize  the  older  approximate  half,  i.e.,  55  per  cent  of  the  time- 
scale,  are  either  of  the  original  fire-fused  sort  which  has  never  been 


476 


THE  CHANGING  WORLD 


GEOLOGICAL   TIME-SCALE    (Lull) 


Eras 

Periods 

Estimated  Percentage  of 
All  Known  Time 

Psychozoic 

Recent  (Post-glacial) 

Cenozoic 

Pleistocene  (Glacial) 

Pliocene 

Miocene 

Oligocene 

Eocene 

Paleocene 

4% 

Mesozoic 

Cretaceous 

Jurassic 

Triassic 

11% 

45% 

Paleozoic 

Permian  (Glacial) 

Carboniferous 

Devonian 

Silurian 

Ordovician 

Cambrian 

30% 

Proterozoic 

25% 

55% 

Archeozoic 

30% 

subjected  to  erosion  and  stratification,  or  those  which,  even  if  they 
may  once  have  been  sedimentary,  have  lost  their  stratified  character, 
due  to  crushing  pressure  or  to  transforming  association  with  vulcanic 
forces.  Marble,  for  example,  laid  down  originally  in  layers  following 
the  disintegration  of  calcareous  skeletons,  or  by  the  deposition  of 
dead  shells  of  myriads  of  microscopic  marine  organisms,  is  metamor- 
phosed sedimentary  limestone,  while  quartzite  and  gneiss  are  rocks 
that,  by  the  action  of  heat  and  pressure,  have  been  made  over  out  of 
sandstone,  which  was  also  once  stratified. 

Sedimentary  biology,  or  the  horizontal  arrangement  of  fossil  remains 
in  sedimentary  rocks,  practically  begins  with  the  Paleozoic  era, 
although  there  are  shadowy  evidences,  such  as  the  graphite  traces  of 
primitive  seaweeds,  showing  that  life  occurred  as  far  back  as  the 
Archeozoic  era.  In  the  rocks  of  the  Proterozoic  era  have  also  been 
found  scanty  traces  of  calcareous  algae,  primitive  sponges,  and  shells 
of  radiolarians,  but  most  of  the  remains  of  life  during  this  enormous 
expanse  of  time  have  been  obhterated.  Only  a  part  of  the  Proterozoic, 
and  some  of  the  Paleozoic,  era  are  represented  in  the  famous  walls  of 
the  Grand  Canyon  of  the  Colorado. 


TIME   SPENT    (PALEONTOLOGY) 


477 


The  Pleistocene  period,  in  which  modern  man  finally  made  his 
appearance,  and  which  probably  does  not  include  more  than  50,000 
or  100,000  years,  is  such  an  insignificant  fragment  of  the  whole  that 
it  is  scarcely  worth  while  to  attempt  to  include  it  in  a  percentage 
column  of  known  time. 

Kinds  of  Fossils 

A  fossil  is  an  indication  of  past  life,  not  of  recent  past  life  but  of 
something  that  lived  so  long  ago  that  ordinarily  it  would  be  forgotten 
and  disregarded  entirely,  except  for  the  interest  of  the  curious  inquir- 
ing paleontologist. 

Fossils  are  of  many  kinds.  They  may  be  the  actual  remains  of 
organisms  preserved  indefinitely  from  decay,  as,  for  example,  mixed-up 
bones  of  struggling  animals  caught  in  the  ancient  asphalt  pits  at 
Rancho  La  Brea  in  California ;    insects  imprisoned  in  transparent 


Courtesy  of  Los  Anueles  Mixscum 

Skulls  and  bones  of  bison,  horse,  and  dire  wolf  are  recognizable  in  this  mass  of 
fossil  bones  ready  for  removal  from  one  of  the  tar  pits  at  Rancho  La  Brea, 
California. 


478 


Tllli:  CHANGING   WORLD 


Mammoths  preserved  in  arctic  ice. 


amber,  which  is  hardened  Oligocene  pitch ;    or  mammoths  frozen 
centuries  ago  in  arctic  mud  and  ice,  with  no  opportunity  since  then 

to  thaw  out,  of  which 
at  least  a  score  of 
authentic  instances 
are  known. 

Petrifactions  of  bone 
or  shell  or  wood  are 
another  kind  of  fossils, 
formed  by  the  filtra- 
tion of  dissolved  min- 
erals into  spaces  left 
after  the  decay  of  the 
original  organic  mat- 
ter. In  such  fossils 
the  inorganic  part  has 
resisted  disintegration 
long  enough  to  serve  as  a  matrix  or  a  mold,  and  thus  to  preserve  the 
original  shape.  Sometimes  the  mineral  replacement  of  minute  parts 
may  be  so  gradual  and  complete  that  the  bone  or  shell  or  tree-trunk 
is  said  to  be  "turned  to  stone,"  often  with  histological  details  faith- 
fully retained.  Limestone  is  often  composed  of  innumerable  shells 
of  minute  organisms,  such  as  foraminifera  and  the  skeletons  of  corals 
that  extract  from  the  water  the  necessary  calcareous  materials. 

Still  other  types  of  fossils  are 
casts  and  molds  in  which  the 
organisms  or  parts  of  them  re- 
main undestroyed  long  enough 
to  permit  the  taking  of  a  perma- 
nent death  mask  of  some  kind, 
which  is  then  all  that  is  finally 
preserved.  Some  beautiful  ex- 
amples, which  may  reproduce  in 
great  detail  the  character  of  the 
original,  are  impressions  of  ferns 
and  leaves,  or  of  insect  wings, 
occasionally  to  be  found  when 
shale  or  slate  rock  is  split  open. 
Under  favorable  circum- 
stances, tracks  and  trails  left  by  A  Paleozoic  fernlike  plant. 


TIME   SPENT    (PALEONTOLOGY) 


479 


Milloii  li.  Wtid 

Dinosaur  footprints  in  Connecticut. 


animals  may  be  preserved,  showing  that  the  animal  in  question  was 
once  a  going  concern.  Just  as  rabbit  tracks  in  the  snow  register 
the  fact  that  a  rabbit  has 
passed  that  way,  so  the 
many  stone  footprints 
which  Professor  Hitch- 
cock of  Amherst  College 
originally  cHscovered  up 
and  down  the  Connecticut 
Valley  are  dinosaur  auto- 
graphs, signed  in  the  great 
stone  book,  that  record 
who  were  once  travelers 
there. 

Particularly  curious  fos- 
sils are  the  so-called  copro- 
lites,  which  are  hardened 
feces  of  animals.  These,  in  some  instances,  by  their  twisted  form, 
give  a  hint  as  to  the  structure  of  the  vanished  soft  parts  of  the 
posterior  part  of  the  intestine,  which  were  able  to  shape  excreta  in 
such   a   fashion.     Some    coprolites,    furthermore,    even   enable   the 

paleontologist  to  deter- 
mine the  bill-of-fare  of 
an  animal  that  lived 
perhaps  a  million  years 
ago. 

Finally,  coal  and  oil 
deposits,  wherever  found 
in  nature,  mark  the  place 
and  time  of  former  vege- 
tative life. 

In  all  these  cases  what 
we  call  a  fossil  is  a 
truthful  and  undeniable 
witness  of  the  former 
existence  of  a  living  thing. 
They  are  not  to  be  con- 
fused with  artifacts  which 
are  structures  fashioned 
by  the  hand  of  man. 


:v<_' 


( '.  .s.  Geological  .Surrey 

Folded  beds  of  limestone  on  the  south  coast  of 
Alaska. 


480  THE  CHANGING  WORLD 

Fossils  as  Time  Markers 

Just  as  the  inclusion  of  contemporary  documents  of  various  sorts 
within  the  corner  stone  of  a  building,  or  the  carving  of  a  date  over 
the  door,  indicates  the  time  when  the  building  was  erected,  so  the 
presence  of  fossils,  found  embedded  within  a  particular  layer  of  sedi- 
mentary rock,  serves  to  fix  the  approximate  time  when  the  sedimenta- 
tion occurred. 

Since  fossils  succeed  each  other  over  long  reaches  of  time  in  a  cumu- 
lative series,  they  aid  in  establishing  the  date  when  a  particular  layer 
of  the  earth's  crust  was  formed,  as  was  first  pointed  out  by  William 
Smith  (1769-1839),  who  succeeded  in  homologizing  certain  scattered 
rock  formations  in  England  by  means  of  typical  key  fossils  found  in 
them.  Moreover,  the  kind  of  stratified  rock  in  which  fossils  are 
found  in  turn  helps  to  determine  when  the  organisms  which  resulted 
in  fossils  lived.  Thus,  the  confirmation  works  both  ways.  This  is 
not  as  much  of  a  vicious  circle  as  it  may  seem  to  be,  for  the  progres- 
siveness,  or  upward  evolution,  of  organic  forms  is  not  taken  advantage 
of  in  estimating  the  relative  ages  of  different  strata  until  after  the 
strata  themselves  have  been  surely  arrayed,  by  painstaking  obser- 
vation and  interpretation,  in  their  unmistakable  natural  order  of 
occurrence. 

The  Testimony  of  Extinct  Types 

A  ruined  castle  on  the  Rhine,  with  broken  battlements  and  tumbling 
towers,  is  a  mute  witness  to  many  years  employed  first  in  building, 
followed  by  a  probably  extended  period  of  occupation,  and  by  a  final 
period  of  gradual  decay  and  abandonment.  It  is  quite  unlike  the 
flimsy  tent  of  the  camper,  which  is  quickly  put  up  at  night  and  taken 
down  in  the  morning.  The  castle  stands  for  the  lapse  of  time.  The 
tent  does  not.  The  same  story  of  the  flight  of  time  is  told  more 
emphatically  by  fossil  animals  and  plants. 

While  there  have  been  innumerable  individual  animals  and  plants 
that  have  lived  and  died  in  the  past,  usually  without  leaving  any  trace 
of  their  former  existence,  there  are  also  whole  groups  of  organisms, 
that  is,  species,  genera,  families,  orders,  and  even  classes,  which  have 
likewise  become  entirely  extinct,  and  are  now  known  to  have  existed 
only  because  of  the  occasionally  fossilized  remains  of  their  representa- 
tives. To  have  developed  these  extensive  groups  by  any  process  of 
evolution,  and  then  to  allow  time  enough  for  the  bringing  about  of 


TIME  SPENT   (PALEONTOLOGY) 


481 


their  gradual  downfall  and  elimination,  naturally  calls  for  more  than 
the  work  of  a  day. 

When  one  visits  a  museum,  like  the  American  Museum  of  Natural 
History  in  New  York  City,  and  there  encounters  the  unbelievable 
genuine  framework  of  some  towering  dinosaur,  he  is  compelled  to 
admit  that  it  must  have  taken  a  great  deal  of  time  to  evolve  such  a 
creature  by  any  possible  process  of  slow  successive  adaptations. 


I8fc- 


Comparative  sizes  of  man  and  dinosaur. 

Moreover,  not  one  kind  of  dinosaur  alone  but  many  diverse  kinds, 
which  have  taken  time  enough  to  branch  o&  from  the  original  stock, 
whatever  that  was,  have,  without  the  least  shadow  of  doubt,  also 
lived  and  died.  Probably  the  slow  processes  that  have  led  up  to  such 
bizarre  manifestations  of  former  life  in  many  cases  ran  concurrently, 
like  jail  sentences,  but  even  so,  enormous  quantities  of  time  must 
have  been  demanded  for  the  accomplishment  of  these  known  results. 
It  does  not  seem  likely  that  a  sane  and  reasonable  Creator  ever  made 
one  of  these  dinosaurs  de  novo,  "out  of  whole  cloth."  They  bear  every 
mark  of  having  been  repeatedly  cut  over  and  reassembled  out  of 
preceding  garments.  There  is  no  evidence,  moreover,  that  dinosaurs 
came  to  a  sudden  catastrophic  end  all  at  once.  It  took  long  periods  of 
additional  time  finally  to  undo  the  gigantic  task,  and  to  bring  about 
the  wreckage  and  gradual  extinction  of  these  elaborate  creatures. 

The  age-long  episode  of  the  rise  and  fall  of  the  dinosaur  dynasty, 
for  example,  which  endured  for  some  millions  of  years,    has  been 


482  THE   CHANGING   WORLD 

repeated  over  and  over  again  in  the  case  of  other  animal  and  plant 
groups.  Thus,  not  only  is  the  fact  of  the  existence  of  all  sorts  of 
fossils,  marking  various  remote  stages  of  past  life,  evidence  of  the 
vast  extent  of  known  time,  but  also  the  slow  rise  and  fall  of  plant  and 
animal  groups  as  a  whole  emphasizes  the  same  point. 

The  Role  of  Paleontology 

To  learn  the  kinds  of  animals  and  plants  that  have  lived  in  former 
times ;  to  determine  just  when  they  lived  and  what  they  did ;  and 
to  find  out  which  lines  failed  to  maintain  survivors  down  to  the 
present  time,  and  why,  are  some  of  the  concerns  of  paleontology. 

There  is  a  seductive  lure  in  fossil  hunting,  like  that  which  stimulates 
the  prospector  for  gold,  only  in  the  case  of  the  paleontologist  it  is 
intellectual  gold  that  he  is  after,  the  acquisition  of  which  is  of  much 
more  inestimable  value  than  the  discovery  of  the  yellow  metal. 

For  those  who  care  to  look  into  this  matter  of  past  life,  and  for  those 
who  would  like  to  share  some  of  the  joys  of  the  exploring  paleon- 
tologist, there  follows  a  short  list  of  books,  which  is  recommended 
to  point  the  way. 

SUGGESTED  READINGS 

Lucas,  F.  A.,  Animals  before  Man  in  North  America,  D.  Appleton  Co.,  1902. 

An  excellent  popular  presentation  of  ancient  animal  life. 
Lull,  R.  S.,  Fossils,  The  University  Society,  1931. 

A  short  stimulating  introduction  to  the  life  of  other  days. 
Merriam,  J.  C.,  The  Living  Past,  Charles  Scribner's  Sons,  1930. 

On  the  subject  of  earth  history,  the  President  of  the  Carnegie  Listitu- 

tion  can  converse  with  the  young  as  well  as  with  the  old. 
Shimer,  H.  W.,  An  Introduction  to  the  Study  of  Fossils,  The  Macmillan  Co., 

1914. 

A  valuable  textbook  of  paleontology  interpreted  through  the  study  of 

existing  forms. 
Sternberg,  C.  H.,  The  Life  of  a  Fossil  Hunter,  Henry  Holt  &  Co.,  1909. 

The  adventures  of  a  typical  American  who  hunted  fossils  when  railroads 

were  new  in  Kansas,  Texas,  and  the  Dakotas,  and  Indians  were  more  in 

evidence  than  automobiles. 


XXII 


THE  EPIC  OF  EVOLUTION 

Preview.  The  universality  of  change  •  Adaptations  •  Making  the  best 
of  it  •  Kinds  of  organic  adaptations :  Structural ;  embryological ;  physio- 
logical ;  psychological ;  genetical ;  ecological ;  physical ;  biological  •  Evo- 
lution •  Evolution  and  miraculous  creation  •  The  nature  of  scientific 
evidence  •  Evidence  from  comparative  anatomy  ;  the  key  to  comparative  an- 
atomy is  organic  evolution  •  From  embryology  •  From  classification  •  From 
distribution  •  From  fossils  •  From  serology  •  From  human  interference  • 
Environmental  theory  of  Lamarck  •  Natural  selection  theory  of  Darwin: 
Variation  ;  overpopulation ;  struggle  for  existence ;  survival  and  elimination ; 
inheritance ;  isolation  •  Mutation  theory  of  DeVries  •  Germplasm  theory  of 
Weismann  •  Other  theories  •  Conclusion  •  Suggested  readings. 

PREVIEW 

Observable  inborn  as  well  as  acquired  CHANGES  in  animals  aiid 
plants,  and  in  their  surroundings,  necessitate  ADAPTATIONS  and  ad- 
justments on  the  part  of  organisms  which,  if  inherited,  residt  in 
EVOLUTION. 

To  challenge,  analyze,  and  expand  the  ideas  contained  in  this 
statement  is  a  large  order.  It  will  require  full  and  willing  co-operation 
on  the  part  of  the  reader,  who  is  expected  to  think  as  he  reads  of 
cases  from  his  own  observations  and  experience  that  bear  upon  the 
general  propositions  advanced. 

It  is  freely  admitted  that,  with  such  an  ambitious  thesis  as  this, 
one  is  tempted  to  take  now  and  then  to  the  aerial  route  of  specula- 
tion, and  to  generalize  with  panoramic  views  of  the  w^liole,  when  to 
particularize  with  illustrative  details  might  be  more  illuminating  and 
to  the  point.  The  contributing  reader  is  consequently  hereby  warned 
in  advance  to  keep  one  eye  at  least  on  the  solid  ground  of  fact  below, 
whenever,  by  flights  of  fancy  and  theory,  he  finds  himself  being 
hurried  to  his  destination  by  the  more  rapid  and  less  substantial  air 
route  of  speculation. 

The  Universality  of  Change 

It  is  a  matter  of  common  experience  that  everything  which  we  can 
observe  about  us  eventually  undergoes  change. 

483 


484  THE  CHANGING  WORLD 

Although  it  may  be  necessary  to  extend  the  duration  of  observa- 
tion in  order  to  detect  the  occurrence  of  something  different  as 
happening,  nevertheless,  it  always  comes  about  in  the  end.  The 
apparently  stationary  hour-hand  of  a  clock,  for  example,  is  known  to 
shift  its  position  during  the  day,  in  spite  of  its  appearance  of  stand- 
ing still. 

That  no  structure  or  action  remains  constant  and  enduring  is 
particularly  evident  in  living  things,  in  which  change  is  inevitable 
from  the  cradle  to  the  grave,  not  only  in  mankmd  but  also  in  the 
daily  life  of  every  animal  and  plant. 

Furthermore,  if  it  were  possible  to  take  a  complete  census  of  all 
the  different  kinds  of  organisms  represented  on  the  earth  today,  for 
comparison  with  similar  censuses  taken  during  the  different  geologi- 
cal periods,  sweeping  changes  in  the  character  of  whole  groups  would 
at  once  be  apparent.  Even  with  the  partial  census  which  biologists 
have  been  able  to  make  of  organisms  laiown  to  have  existed  in  the 
past,  as  contrasted  with  the  catalogue  of  living  forms  thus  far  dis- 
covered, it  is  proven  without  a  doubt  that  the  Law  of  Change  is  now, 
and  always  has  been,  everywhere  in  constant  operation. 

The  causes,  or  sequences  of  events,  leading  up  to  all  sorts  of  changes 
are  naturally  diverse  and  numerous.  With  organisms  they  may  be 
inborn,  that  is,  genetic  in  character,  or  largely  external  and  environ- 
mental, but  in  any  case,  the  fact  of  change  with  its  consequences 
is  observable  and  can  be  analyzed,  even  though  the  underlying 
causes  that  bring  these  changes  about  are  often  uncertain  and  un- 
known. 

Sometimes  changes,  in  themselves  slight,  may  have  far-reaching 
consequences.  For  example,  when  single-celled  organisms,  accus- 
tomed from  time  immemorial  to  divide  periodically  each  one  into 
two  individuals,  discovered  the  great  advantages  of  partnership  and 
remained  attached  to  each  other  after  fission  occurred,  instead  of- 
separating  and  going  their  independent  ways,  right  then  was  born 
the  pregnant  idea  of  tissues  and  organs.  The  device  of  cell  multipli- 
cation opened  up  consequent  possibilities  of  the  working  together  of 
parts  for  greater  effectiveness,  through  the  fertile  principle  of  "divi- 
sion of  labor."  This  change  was  a  great  historical  event  in  the  world 
of  life,  with  extensive  sequels. 

Again,  when  by  gradual  changes  the  shift  from  a  single  parent  to 
the  sexual  method  of  two  parents  came  about,  another  great  bio- 
logical epoch  began  in  which,'  by  utilizing  two  hereditary  streams 


THE   EPIC   OF  EVOLUTION  m^y 

instead  of  one,  the  possibilities  of  the  offspring  of  successive  genera- 
tions were  more  than  doubled. 

Take  one  more  illustration  of  a  series  of  changes  that  has  altered 
the  whole  course  of  subsequent  biological  events.  The  bilateral  sym- 
metry of  locomotor  animals,  that  is,  those  having  a  head  end  and 
right  and  left  sides  to  the  body,  was  preceded  by  the  radial  symmetry 
of  attached  forms  like  Hydra,  an  arrangement  making  it  possible 
from  a  point  of  anchorage  to  explore  the  surroundings  for  food  in 
every  direction  without  the  machinery  of  locomotion.  When  ani- 
mals with  radial  symmetry  become  free-swimming,  like  jellyfish 
and  sea-urchins,  they  go  at  random  in  any  direction.  It  is  only 
after  one  definite  part  in  the  circumference  of  a  radially  symmetrical 
animal  constantly  takes  to  leading  the  way  that  a  head  end  is  initi- 
ated, with  a  bram  center  to  direct  the  increasing  activities  of  the 
changing  animal.  The  connecting  hnks  in  this  chain  of  changes 
between  radial  and  bilateral  animals  are  to  be  seen  in  certain  tur- 
bellarian  flatworms,  whose  fundamental  plan  is  the  same  as  that 
which  would  be  formed  if  a  radial  jellyfish  were  stretched  out  length- 
wise with  one  horizontal  axis  elongated,  thus  forming  a  polar  arrange- 
ment with  head  and  tail  ends.  It  may  be  a  long  call  to  return  thanks 
to  these  lowly  creatures  for  discovering  the  advantages  of  a  head  with 
a  directive  brain  in  it,  but  perhaps  it  is  not  too  late  at  least  to  register 
oiu'  gratitude. 

There  is  much  variation  in  the  degree  of  plasticity  shown  by 
organisms  and  in  the  range  of  changes  which  they  undergo.  Some 
forms,  like  certain  brachiopods  and  shell-bearing  protozoans  of  the 
deep  sea,  are  so  well  fitted  to  the  constant  habitat  in  which  they 
five,  where  there  is  no  variation  of  pressure  or  temperature,  and 
where  no  day  or  night  intrudes  upon  their  tranquillity,  that  those 
living  now  have  not  changed  perceptibly  in  appearance  from  their 
extremely  ancient  forebears. 

On  the  other  hand,  in  the  strenuous  environment  of  the  tidal  zone, 
where  land  and  restless  waters  meet,  the  inhabitants  are  kept  con- 
stantly busy  and  alert  in  matching  structural  and  functional  changes 
with  insistent  and  recurring  changes  in  their  surroundings.  Thus  it 
is  that  changes  in  the  environment  necessitate  continuous  adjust- 
ments on  the  part  of  plants  and  animals.  Whatever  may  happen 
meantime  to  the  individual  actor,  the  show  must  go  on.  This  tradi- 
tion of  the  dramatic  stage  is  quite  as  true  also  for  the  larger  stage 
of  changing  life. 


486  THE   CHANGING    WORLD 

ADAPTATIONS 
Making  the  Best  of  It 

Adaptations  are  biological  charigcs  that  organisms  make  in  adjusting 
themselves  to  physical  changes  which  constantly  occur  in  the  environ- 
ment. Here  are  two  variables  to  analyze  and  consider,  namely, 
changes  made  by  the  organism,  and  those  that  come  to  pass  in  the 
environment  of  the  organism.     How  do  they  interact? 

Organic  adaptations  vary  widely  in  the  degree  of  perfection.  They 
may  be  incipient,  partial,  and  ineffective,  or,  at  the  other  extreme, 
they  may  have  gone  so  far  as  to  result  in  overspecialization.  This 
latter  condition  is  frequently  dangerous  to  its  possessor,  since  the 
specialist,  having  all  his  eggs  in  one  basket,  cannot  help  sacrificing 
some  of  the  saving  adaptability  necessary  to  meet  a  changing  turn 
of  the  environmental  wheel.  This  is  particularly  true  in  human 
affairs.  The  hobo  who  is  limited  to  snow-shoveling  is  unemployable 
in  the  summer  season. 

Many  instances  of  adaptation  are  entirely  obvious.  Others  are 
obscure  and  speculative,  but  in  any  case  the  extensive  gallery  of 
common  adaptations  furnishes  abundant  and  intriguing  material  for 
the  biologist.  "Maeterlinck's  essay  on  the  adaptations  of  the  bee," 
says  Henshaw  Ward,  "makes  the  Arabian  Nights  seem  flat." 

Kinds  of  Organic  Adaptations 

The  classification  which  follows  is  entirely  arbitrary  and  by  no 
means  complete.  It  is  simply  an  attempt  to  arrange  certain  cate- 
gories of  adaptation  temporarily  for  purposes  of  description.  There 
is  such  a  wealth  of  illustrative  material  that  it  is  almost  hopeless 
to  attempt  to  pick  and  choose.  Consequently,  resort  will  be  made 
more  to  suggestion  than  to  detailed  elaboration  of  particular  cases. 
Here  is  an  excellent  opportunity  for  the  reader  to  fill  in  omissions 
with  supplementary  material  of  his  own. 

Structural  Adaptations 

The  elaborate  mouth-parts  of  insects  are  all  designed  apparently 
on  the  same  fundamental  plan,  but  this  plan  is  carried  out  quite 
differently  in  the  "tobacco-chewing  grasshopper,"  which  feeds  on 
\'egetation,  and  in  the  prodding  mosquito,  that  sucks  blood  out  of 
protesting  humans. 


THE   EPIC   OF   EVOLUTION 


/|87 


The  size  of  an  animal  may  be  in  itself  a  structural  contribution  to 
success  in  life.  A  single  horse  that  weighs  a  ton  and  a  ton  of  mice 
both  require  in  general  the  same  sort  and  amount  of  food,  but  the 


.-..secor^d  y77axilla 

maxillary  I 
palp 

-first 
maxilla. 


—  )T7axillar/  palp 


/.mandible. 
\abr-am.j 
Second,  maxilla 

The  cockroach  (left)  and  the  female  mosquito 
(right)  inherit  a  homologous  set  of  mouth  parts, 
which  have  become  considerably  modified  to  meet 
the  conditions  imposed  by  different  functions. 


mice  stand  the  better 
chance  of  getting  about 
and  securing  food 
necessary  for  main- 
taining a  ton  of  proto- 
plasm. 

Some  other  random 
suggestions  of  examples 
of  structural  adapta- 
tions are  radial  sym- 
metry in  sessile  ani- 
mals, the  histological 
structure  of  leg  bones 
adapted  to  bear  body 
weight,  the  handy, 
prehensile  tails  of  South 
American  monkeys,  the 
sharp  claws  of  certain  bloodthirsty  carnivores,  the  sticky  protrusible 
tongue  of  ant-eaters,  the  snowshoelike  feet  of  the  Mexican  jacanas, 
which  get  their  insect  food  while  skipping  lightly  over  floating  lily- 
pads,  the  elongated  snouts  of  chestnut  weevils  that  have  the  problem 
of  spiny  burrs  to  solve,  and  the  shoelike  hoofs  of  heavy  ungulates. 

Embryological  Adaptations 

Reptiles  and  birds,  that  hatch  by  breaking  through  an  enclosing 
eggshell  after  making  a  preliminary  start  in  life,  and  mammals,  which 
go  through  the  early  stages  of  their  development  in  safety  within 
the  mother's  body,  have  to  be  fitted  successively  for  two  quite  dif- 
ferent sets  of  conditions.  Such  embryos,  during  the  period  of  their 
imprisonment,  employ  two  notable  adaptive  devices,  the  amnion  and 
the  allantois,  which  are  discarded  upon  emergence.  The  amnion  is 
an  enveloping  antenatal  robe,  filled  with  fluid,  within  which  the  deli- 
cate, rapidly  growing  embryo  floats,  protected  from  mechanical  shock 
and  from  growth-checking  exposure  to  a  dry  world.  It  is  an  adapta- 
tion to  land  life  quite  unnecessary  in  the  case  of  fishes  and  amphibians, 
whose  usually  shell-less  eggs  are  deposited  in  the  water  during  the 
period  of  their  preliminary  development.  The  allantois  is  a  make- 
u.  w.  H.  — 32 


488  THE   CHANGING   WORLD 

shift  respiratory  device,  effective  within  the  eggshell,  or,  in  the  case 
of  mammals,  in  the  uterus  of  the  mother,  before  it  is  possible  for  the 
lungs  of  the  young  individual  to  take  over  the  task  of  respiration. 

Curiously,  the  embryologist  often  has  to  describe  a  different  organ 
from  that  which  the  anatomist  cites  for  the  accomplishment  of  the 
same  function  in  the  animal  body.  An  adult  anatomical  structure 
over  and  over  again  succeeds  a  transitory  embryonic  forerunner. 
Thus,  temporary  nephroi  are  followed  by  permanent  kidneys  ;  downy 
lanugo  is  replaced  by  hair,  more  or  less  permanent ;  the  gauzy  em- 
bryonic covering  of  epitrichium  gives  way  to  the  adult  skin ;  there  is 
a  succession  of  teeth ;  the  intestine  replaces  the  yolk  sac ;  the  primi- 
tive vitelline  circulation  gives  over  its  temporary  emergency  service  as 
the  systemic  circulation  arises ;  the  two-chambered,  fishlike,  embry- 
onic heart  of  the  mammal  becomes  replaced  by  the  three-chambered 
amphibian  stage  before  the  final  four-chambered  heart  is  established  ; 
while  for  the  embryonic  vertebrate  skeleton,  patterned  largely  in 
cartilage,  there  is  eventually  substituted  a  more  efficient  bony  frame- 
work. 

All  these  illustrations  and  many  more  indicate  adaptations  to 
adult  life,  following  the  different  preliminary  conditions  imposed  by 
embryonic  existence. 

Physiological  Adaptations 

When  for  any  reason  one  kidney  is  removed,  or  put  out  of  com- 
mission, the  remaining  kidney  assumes  the  double  task  and  increases 
correspondingly  in  size.     This  is  a  physiological  adaptation. 

The  apparatus  of  the  sweat  glands  is  a  physiological  device  enabling 
mammals  to  adjust  themselves  to  the  greater  variation  in  tempera- 
ture which  occurs  on  land,  as  contrasted  with  that  to  which  sweatless 
water  animals  are  exposed. 

A  grasshopper,  with  its  large  immovable  compound  eyes  facing 
everywhere  except  below  where  the  mouth  is  located,  is  not  able 
to  see  the  food  that  it  is  eating,  so  tactile  palps,  that  are  sensory 
modifications  of  the  mouth-parts,  become  adapted  to  function  in- 
stead of  eyes  in  the  examination  of  food. 

Darwin  cites  the  strange  case  of  a  certain  species  of  parrot  in  New 
Zealand  which,  after  the  introduction  of  large  herds  of  sheep  into 
its  habitat  and  after  somehow  getting  the  taste  of  blood,  gave  up 
its  former  vegetarian  habit  of  life  and  became  a  murderous,  blood- 
thirsty carnivore,  living  entirely  on  the  flesh  of  sheep. 


THE  EPIC  OF  EVOLUTION 


489 


The  activation  upon  occasion  of  the  mammary  glands,  as  well  as 
the  formation  of  antitoxins,  and  the  acquisition  of  immunity  to  cer- 
tain disea"ses,  are  further  examples  of  physiological  adaptation  to 
bodily  needs. 

Psychological  Adaptations 

Patterns  of  instinctive  behavior  which  adapt  an  untaught  cater- 
pillar to  spin  a  cocoon  of  a  definite  sort,  or  direct  an  insect  to  lay 
its  eggs  upon  a  particular  food-plant  specific  for  its  offspring  which  it 
will  never  see,  as  well  as  the  inner  urge  that  causes  birds,  and  certain 
other  animals,  to  migrate  periodically,  may  possibly  be  cited  as  ex- 
amples of  psychological  adaptation.  At  any  rate,  the  exercise  of  the 
nervous  system  that  enables  actors  to  repeat  their  lines  unconsciously 
in  dozens  of  plays,  and  by  which  musicians  are  able  upon  repetition 
to  perform  complicated  and  extensive  scores  without  conscious  effort, 
comes  close  to  being  an  adaptation  of  a  psychological  nature. 

Genetical  Adaptations 

Adaptations  frequently  work  for  the  benefit  of  the  species  rather 
than  for  the  welfare  of  the  individual. 

The  clever  dandelion  grows  close  to  the  ground  in  a  flat  rosette. 
This  habit  enables  it  to  escape  from  browsing  animals  to  a  consider- 
able degree  and  to  with- 
stand trampling.  Its  yel- 
low blossom  lies  low  and 
bides  its  time  until  all  is 
ready  and  then,  just  at 
the  critical  time,  the  hol- 
low stem  shoots  up  like  a 
fire  ladder  into  the  air 
almost  overnight,  bearing 
a  cluster  of  white-tufted 
aviating  seeds  that  are 
all  prepared  for  distribu- 
tion. They  are  so  deli- 
cately poised,  pincushion- 
fashion,  in  their  elevated  position  that  the  slightest  breeze  is 
sufficient  to  waft  them  on  their  way. 

All  the  many  reproductive  modifications,  both  structural  and  func- 
tional, which  are  involved  in  the  fertilization  of  animal  eggs  as  well 


^  —  "€_. 


The  adaptable  dandelion,   providing  for  itself 
and  for  its  progeny. 


490  THE  CHANGING   WORLD 

as  in  the  formation  of  spores  and  seeds  of  plants,  make  up  a  world 
of  adaptations  in  themselves  that,  since  they  have  to  do  with  the 
maintenance  of  species,  may  be  considered  as  genetic  in  character. 

Another  genetic  adaptation,  that  is  so  universal  as  to  be  properly 
regarded  as  a  law  of  nature,  is  shown  in  those  animals  and  plants 
whose  reproductive  products  are  particularly  exposed  to  great  perils, 
and  which  in  consequence  produce  a  correspondingly  larger  number 
of  eggs  or  seeds  than  do  those  whose  offspring  are  better  safeguarded. 
Nest-building  in  all  its  diverse  forms,  as  well  as  the  multitudinous 
devices  employed  by  plants  to  secure  pollination  and  the  dispersal 
of  seeds  are  further  examples  of  genetic  adaptation. 

Ecological  Adaptations 

Any  group  of  varying  organisms,  adjusted  more  or  less  imperfectly 
to  a  certain  habitat,  tends  in  the  course  of  time  to  branch  out  and 
to  occupy  different  neighboring  habitats.  Adaptation  to  new  habi- 
tats is  ecological  adaptation.  This  type  of  adaptation  has  been 
somewhat  amplified  in  unit  II,  on  the  "Biological  Conquest  of  the 
World,"  and  innumerable  examples  of  conditions  met  in  the  great 
primary  habitats  of  water,  land,  and  air  will  come  to  the  mind  of 
every  observing  reader.  If  animals  could  talk  and  had  intelligence 
enough  to  know  what  to  say,  think  what  tales  they  could  rehearse 
of  the  troubles  they  have  known  and  the  satisfactions  they  have 
experienced  in  becoming  adapted  to  their  particular  niches  in  nature  ! 
Imagine,  for  instance,  a  Thousand  and  One  Nights  spent  in  listening 
to  such  representative  spokesmen  as  the  hermit  crab,  the  nocturnal 
earthworm,  the  carrion  beetle,  the  golden  plover,  the  sperm  whale, 
the  liver  fluke,  the  snake  in  the  grass,  and  the  bullfrog  on  the  bank. 
Even  the  plant  world  could  be  profitably  admitted  to  take  part  in 
such  a  symposium.  For  example,  what  might  the  northern  pine 
and  the  southern  palm,  the  roadside  weed  and  the  head  of  rice,  to 
say  nothing  of  the  bacteria  of  "  Typhoid  Mary,"  have  to  tell  us  of 
ecological  adaptation ! 

Physical  Adaptations 

Certain  factors  in  the  make-up  of  the  physical  environment,  such 
as  temperature,  pressure,  and  light,  set  limits  within  which  organ- 
isms must  adapt  themselves  in  order  to  live.  The  range  of  livable 
possibilities  imposed  by  these  physical  factors  varies  greatly  with  the 
organism.     Professor  Brues  of  Harvard  reports  that  there  are  algae 


THE  EPIC  OF  EVOLUTION 


491 


.  /(///  "'    \nturiil  Histnry 

A  deep-sea  group  of  fish  :  left,  Macrurid, 
and  right,  Brotulid. 


and  the  larvae  of  certain  insects 
that  are  adapted  to  live  in  hot 
springs,  the  temperature  of  which 
is  sufficient  to  coagulate  the  pro- 
toplasm of  most  organisms.  Some 
animals,  frogs  for  example,  can 
survive  a  degree  of  freezing  that 
would  be  fatal  to  others.  Trees 
and  woody  shrubs  can  successfully 
withstand  low  temperatures  that 
cause  most  of  the  less  woody 
plants  to  succumb.  The  varying 
range  of  frost  and  heat  to  which 
plants  of  different  sorts  are  sus- 
ceptible is  common  knowledge  to 
every  farmer. 

Adaptive  devices,  such  as  gem- 
mules  of  fresh-water  sponges,  the 
winter  eggs  of  daphnids,  and  the 
statoblasts  of   certain  bryozoans, 

carry  these  lowly  animals  through  the  freezing  winter  into  another 
summer  quite  as  effectively  as  the  various  coats  and  shells  of  seeds 
and  nuts.     Again,  warm-bloodedness  is  an  adaptation  fitting  birds 

and  mammals  to  cope  successfully 
Avith  great  and  often  sudden  shifts 
in  temperature  on  land,  to  which 
the  cold-blooded  inhabitants  of 
water  are  not  subjected. 

Pressure  is  another  physical 
factor  to  which  every  organism, 
in  order  to  live,  must  be  adjusted. 
Most  animals  and  plants  living  on 
the  surface  of  the  earth,  beneath 
a  uniform  blanket  of  atmosphere, 
are  not  subjected  to  much  differ- 
ence in  pressure,  but  deep-sea 
fishes,  with  an  additional  weight 
of  superimposed  water,  have  quite 

American  Museum  of  y.aturnl  Histunj  ^  ^  ;  -i 

Oceanic  angler  fish,  Linophryne.    The     a  different  problem  to  meet.    This 
beard  is  probably  luminous.  particular    form    of    adaptation 


492  THE  CHANGING  WORLD 

consists  not  so  much  in  protective  envelopes  of  one  kind  and  another, 
that  must  inevitably  be  crushed,  as  in  the  development  of  easily  per- 
meable tissues  through  which  the  pressure  is  equalized. 

Light  is  essential  to  photosynthetic  plants.  These  exhibit  many 
adaptations  by  way  of  the  arrangement  and  form  of  their  leaves  to 
secure  adequate  exposure  to  light.  There  are  many  kinds  of  animals 
on  the  other  hand,  Uke  cave-dwellers  and  deep-sea  forms,  as  well  as 
fungi  among  plants,  that  can  dispense  with  light  entirely.  In  such 
animals,  the  eyes  and  other  adaptations  to  a  world  of  light  and  shadow 
are  either  entirely  wanting  or  have  become  degenerate.  To  com- 
pensate animals  that  live  in  darkness  for  their  loss  of  light,  tactile 
devices  of  various  sorts  develop,  enabling  them  to  find  their  food 
and  to  accomplish  the  business  of  living,  while  in  the  case  of  plants 
the  saprophytic  method  of  living  a  chlorophyll-less  life  is  adopted. 

The  reflex  mechanism  in  the  iris  of  the  eye  by  which  the  size  of 
the  pupil  is  made  to  vary  and  the  amount  of  light  admitted  to  the  ret- 
ina is  regulated  is  a  beautiful  adaptation  of  the  organism  to  amount 
of  light.  In  fact,  the  whole  vertebrate  eye  is  an  exquisite  example  of 
cumulative  organic  adaptation  to  the  environmental  factor  of  light. 

Biological  Adaptations 

The  association  of  organisms  with  each  other  gives  rise  to  a  great 
variety  of  biological  adaptations,  such  as  symbiosis,  commensalism, 
saprophytism,  parasitism,  gregariousness,  and  social  life. 

Flowering  plants  evolve  ways  of  attracting  the  visits  of  insects 
and  of  inveigling  them  to  transfer  pollen  in  the  production  of  seeds. 
Insects  in  turn  are  so  modified  as  to  take  advantage  of  what  the 
flowering  plants  offer  them  by  way  of  nectar  and  other  desirable 
forms  of  food.  It  is  significant  that  flowering  plants  did  not  develop 
in  geological  time  until  after  insects  appeared. 

Carnivorous  hunters  are  fitted  to  pursue  their  prey,  and  the  hunted, 
by  developing  speed  in  flight  or  by  wits  with  which  to  outguess  the 
pursuer,  are  adapted  to  escape.  Mother  Nature  impartially  gives 
both  the  hunter  and  the  hunted  a  sporting  chance. 

Through  protective  coloration  and  camouflage,  by  bluffing  with 
warning  colors,  or  by  intimidating  behavior,  some  animals  escape 
their  enemies,  while  others  are  blackmailed  into  surrendering  to  their 
captors  a  part  of  themselves  and  escaping  with  a  viable  residue, 
having  in  reserve  the  adaptive  resource  of  regeneration  of  lost  parts. 

There  is  a  curious  European  toad,  Bombinator  igneus  by  name. 


THE  EPIC   OF  EVOLUTION  493 

that  has  developed  a  bright  scarlet  belly  and  a  taste  nauseous  to 
birds,  in  the  course  of  its  adventures  in  adaptation  to  an  environ- 
ment unfortunately  shared  with  toad-devouring  storks.  When  a 
stork  by  chance  seizes  a  bombinator,  the  victim  is  usually  ejected 
because  of  the  acrid  taste  produced  by  the  skin-glands  of  the  toad. 
Neither  stork  nor  toad  gains  anything  by  this  performance,  and,  to 
lessen  the  likelihood  of  its  occurrence,  whenever  a  stork  swoops  down 
upon  a  pond  where  bombinators  are  socially  congregated  around 
the  margin,  the  little  animals  quickly  flop  over  and  expose  their 
conspicuous  scarlet  bellies  to  view,  thus  furnishing  red-light  signals 
for  the  stork  to  "stop,"  before  an  accident  happens  that  both  would 
regret. 

A  great  variety  of  defensive  devices,  such  as  armor,  shells,  spines, 
fangs,  horns,  hoofs,  and  stingers,  have  been  developed  in  different 
animals.  The  nonchalant  skunk  is  so  well  assured  by  its  defensive 
fire  extinguisher  mechanism  that  it  does  not  run  away  from  danger. 
This  may  be  why  so  many  of  them,  upon  the  intrusion  of  the  jugger- 
nautlike automobiles  into  their  habitat,  are  run  over  and  killed, 
while  the  more  cautious  rabbits  and  other  wayside  animals  escape. 

Plants  display  a  great  range  of  biological  adaptations  in  the  attempt 
to  defend  themselves  against  browsing  herbivores  and  devouring 
insects.  Some  plants  have  bitter  or  unpalatable  chemical  substances 
lodged  in  their  tissues.  Cacti  and  thistles  bristle  with  discouraging 
spines,  while  shrubs  and  trees  are  provided  with  tough  resistant  bark. 
Desert  plants  develop  fuzzy  hairs  that  hinder  transpiration,  or  are 
coated  over  with  an  impervious  varnish  that  tends  to  prevent  the 
loss  of  water  from  their  tissues. 

There  may  be  still  other  categories  of  adaptations,  and  some  of 
the  foregoing  examples  could  perhaps  be  assigned  to  other  classifica- 
tions, but  the  undeniable  fact  remains  that  adaptations  of  infinite 
variety  characterize  the  living  world  about  us. 

EVOLUTION 

Evolution  and  Miraculous  Creation 

Perhaps  two  of  the  most  famous  scholars  of  the  ancient  English 
universities  of  Oxford  and  Cambridge  were  John  Milton  (1608-1674) 
and  Charles  Darwin  (1809-1882).  Each  wrote  an  immortal  book 
upon  the  same  epic  theme  of  how  living  things  in  this  world  came 
to  be  as  they  are.     Milton's  book  was  entitled  Paradise  Lost,  and 


494  THE  CHANGING   WORLD 

Darwin's,  The  Origin  of  Species.  Milton's  answer,  couched  in  stately 
poetry,  was  that  the  forms  of  hfe  were  suddenly  created  by  divine 
fiat  out  of  the  ''dust  of  the  ground,"  without  any  organic  predeces- 
sors, while  Darwin's  plain  prose  presented  overwhelming  evidence 
of  the  gradual  evolution  of  present  animals  and  plants  from  earlier 
forms  of  life. 

This  latter  conception  did  not  originate  with  Darwin.  The  ancient 
Greeks,  unhampered  by  any  Biblical  tradition  of  "creation,"  fore- 
shadowed the  idea  of  the  rise  of  organisms  by  the  slow  operation  of 
natural  laws.  Centuries  later,  Saint  Augustine  of  Numidia  (354-430), 
with  panoramic  vision,  expounded  the  same  view,  and  still  others 
from  time  to  time  got  glimpses  of  the  majestic  canvas  depicting  the 
progressive  pageant  of  life.  Among  the  more  recent  of  Darwin's 
forerunners  was  his  own  grandfather,  Erasmus  Darwin  (1731-1802), 
who  wrote  a  compendious  work  of  rather  poor  poetry  entitled  "The 
Botanic  Garden,"  in  which  the  theme  of  organic  evolution  was 
developed.  Major  Leonard  Darwin  (1850-  ),  distinguished  son  of 
Charles  Darwin  and  leader  in  England  today  in  developing  the 
related  field  of  eugenics,  has  carried  on  the  Darwin  family  tradition 
of  making  evolution  plain  to  the  world.  He  defined  evolution  in  his 
book  entitled  The  Need  for  Eugenic  Reform  as  "the  gradual  building 
up,  in  accordance  with  the  laws  of  nature,  of  the  world  as  we  now 
find  it,  from  some  unknown  beginning." 

It  is  common  observation  that  one  individual  arises  from  another. 
Organic  evolution  is  simply  an  extension  of  this  principle  to  include 
those  groups  of  organisms  called  species.  The  evolutionary  principle 
is  everywhere  observable,  even  in  other  than  strictly  biological  fields. 
The  earth,  the  solar  system,  and  the  far  distant  heavenly  galaxies 
have  all  been  evolved.  Human  society,  language,  and  customs  have 
come  about  by  the  operation  of  the  same  type  of  universal  sequences. 
Even  our  idea  of  God  has  evolved  from  that  of  the  originally  exclu- 
sive individual  household  god,  up  through  tribal  gods,  and  the  more 
inclusive  national  gods,  until  finally  there  has  been  accepted  the  idea 
of  universal  human  brotherhood  with  one  God  over  all. 

The  idea  of  miraculous  creation,  which  was  quite  acceptable  to 
the  mystical  Eastern  mind  centuries  ago,  has  lost  its  potency  with 
the  logical  Western  mind  of  today.  There  are  everywhere  observ- 
able too  many  partial  and  imperfect  adaptations  and  misfits  to 
represent  the  handiwork  of  an  intelligent  and  skillful  creator,  if 
miraculous  creation,  with  the  possibility  of  immediate  perfection, 


THE   EPIC   OF  EVOLUTION  495 

was  the  method  employed.  It  is  illogical  and  impious  to  postulate 
the  Creator  as  a  bungling  and  slipshod  workman. 

In  spite  of  controversial  echoes  from  the  past,  there  is  nothing 
alarming  or  unsettling  in  the  concept  of  biological  evolution.  There 
is  no  more  conflict  between  it  and  religious  faith  than  there  is  in 
Galileo's  demonstration,  which  so  worried  his  contemporaries,  that 
the  earth  moves  around  the  sun.  The  religious  person  and  the 
evolutionist  both  approach  the  citadel  of  truth  with  equal  reverence, 
but  from  somewhat  different  directions.  There  is  nothing  to  prevent 
their  harmonious  meeting  within  the  portals. 

While  the  facts  of  evolution  are  comparatively  plain,  the  factors 
that  determine  how  it  came  about  are  still  uncertain  and  debatable. 
Darwin,  in  The  Origin  of  Species,  not  only  marshaled  in  thorough- 
going and  masterly  fashion  the  facts  in  support  of  evolution,  but 
he  also  went  further  and  advanced  his  "Theory  of  Natural  Selection," 
to  be  considered  in  a  later  section,  in  the  attempt  to  explain  how 
evolution  has  occurred.  A  consideration  of  the  facts  of  evolution 
calls  for  evidences  which  are  derived  from  many  sources.  Just  as 
"all  roads  lead  to  Rome,"  so  various  lines  of  evidence  about  to  be 
presented  converge  to  establish  the  general  truth  of  organic  evolution. 

The  Nature  of  Scientific  Evidence 

What  is  evidence  to  one  person  may  not  be  to  another.  The 
yokel  at  the  circus  who  exclaimed,  "There  ain't  no  such  beast," 
when  he  saw  a  giraffe  for  the  first  time,  could  not  easily  accept  the 
evidence  of  his  own  eyes.  It  would  be  futile  to  try  to  convince  a 
cat  that  a  picture  of  a  mouse,  however  well  done,  really  represents 
a  live  mouse.  The  cat  lacks  experience  in  judging  pictures  and  is 
unable  to  gain  such  experience,  so  that  the  idea  that  a  picture  has 
anything  whatever  to  do  with  a  real  mouse  is  beyond  the  cat's  com- 
prehension. 

Sufficient  intelligence,  a  background  of  experience,  and  an  open 
mind  are  essentials  in  understanding  what  any  evidence  means. 
The  more  technical  the  matter  presented,  the  greater  the  intelligence 
and  experience  required  to  evaluate  it.  Moreover,  since  it  is  quite 
out  of  the  question  to  acquire  at  first  hand  all  the  knowledge  and 
experience  of  which  we  make  use,  it  is  necessary  many  times  to 
accept  the  judgment  of  others  who  are  experts  in  fields  more  or  less 
unfamiliar  to  us.  Distinguishing  marks  of  a  truly  educated  man 
are  not  the  only  possession  of  a  considerable  store  of  first  hand 


496  THE  CHANGING  WORLD 

information,  but  also  the  ability  and  willingness  to  appreciate  what 
others  have  accomplished,  and  to  judge  with  discrimination  what 
persons  are  properly  qualified  to  serve  as  authorities  on  any  par- 
ticular subject. 

The  attitude  of  the  uninformed  and  unintelligent,  when  confronted 
with  evidence  that  lies  somewhat  beyond  their  horizon,  is  frequently 
bewilderment,  retreat  to  the  strongholds  of  prejudice  and  hearsay,  or 
indulgence  in  the  smoke-cloud  of  contempt  or  derision  for  that  which 
they  do  not  comprehend.  It  is  among  those  that  lack  discrimina- 
tion in  the  acceptance  of  evidence,  and  who  fail  to  pick  properly 
qualified  authorities  on  whom  to  depend  for  whatever  lies  beyond 
their  ken,  that  all  sorts  of  undesirable  propaganda  and  muddy  think- 
ing find  soil  in  which  to  flourish.  Moreover,  it  is  usually  a  waste 
of  time  to  present  evidence  to  a  prejudiced  and  closed  mind,  to  one 
that  confuses  argument  with  evidence,  or  who  is  guided  by  emotional 
likes  and  dislikes  rather  than  by  a  deep-seated  confidence  in  truth, 
wherever  it  may  lead.  Many  lines  of  evidence  in  any  case  have  to 
be  taken  on  faith,  since  they  cannot  fall  under  the  direct  inspection 
of  the  senses. 

"Never  in  its  life  has  the  sun  seen  the  shade, 
Never  in  its  life  seen  a  shadow  where  it  falls ; 

There,  always  there,  in  the  sun-swept  glade, 

It  lurks  below  the  leaf ;  behind  bodies,  under  walls, 

Creeps,  clings,  hides.     Be  it  millions,  be  it  one  — 
The  sun  sees  no  shadow,  and  no  shadow  sees  the  sun."  ^ 

What  the  poet  says  of  sun  and  shadow  clearly  expresses  the  idea 
that  evidence  may  be  both  true  and  acceptable,  although  as  elusive 
to  the  senses  as  shadows  are  to  certain  sunlike  minds.  In  a  consid- 
eration of  the  evidences  of  evolution,  it  is  appropriate  to  call  in  the 
testimony  of  various  biological  sciences,  which  presuppose  more  or 
less  familiarity  with  them,  and  training  therein,  to  fully  appreciate 
the  force  of  the  facts  presented. 

Evidence  from  Comparative  Anatomy 

Comparative  anatomy  is  a  biological  science  rich  in  significant 
problems  and  their  solution.  It  arose  from  the  dead  descriptive 
level  of  the  older  science  of  human  anatomy,  which  in  turn,  unless 
interpreted  in  the  light  of  comparison  with  that  of  other  animals, 

'Laurence  Houseman,  from  Shipley,  in  Life,  page  31.  By  permission  of  The  Macmillan 
Company,  publishers. 


THE   EPIC   OF   EVOLUTION  497 

remains  largely  meaningless  and  puzzling.  The  problems  that  human 
anatomy  presents  are  illuminated  and  largely  solved  by  recourse  to 
comparative  anatomy. 

The  Key  to  Comparative  Anatomy  Is  Organic  Evolution 

A  mere  description  of  the  structure  of  different  kinds  of  animals  and 
plants  would  in  itself  be  monotonous  and  colorless,  an  uninterpreted 
mass  of  isolated  facts,  were  it  not  that  a  thread  of  relationship,  con- 
necting these  forms  of  life  with  each  other,  gives  significance  to  the 
whole.  Identity  of  plan,  based  upon  derivation  from  a  common  stock, 
with  adaptive  variation  in  the  working  out  of  that  plan  to  meet  changing 
environments  or  new  functions,  is  the  creed  of  the  comparative  anato- 
mist, by  which  he  makes  sense  out  of  what  he  observes. 

Goethe  (1749-1832),  in  whom  superlative  excellence  as  a  poet 
overshadowed  his  real  greatness  as  a  pioneer  biologist,  pointed  out 
that  the  sepals,  petals,  stamens,  and  pistils  of  flowers  are  to  be  inter- 
preted as  modified  leaves,  crowded  together  on  a  short  stem,  to  meet 
the  requirements  of  a  different  function.  Again,  an  examination  of 
the  hearts  of  different  vertebrates,  for  example,  shows  an  evolving 
series  of  structures  as  illustrated  on  page  306.  The  fish  heart  is  a 
single  pump,  with  a  thin-walled  atrium  receiving  the  returning  blood, 
and  a  muscular  ventricle  for  pumping  it  over  the  body.  In  the 
amphibian  the  single  pump  begins  to  become  double  by  the  introduc- 
tion of  two  atria,  thus  making  a  sort  of  heart-and-a-half  arrangement, 
while  in  most  reptiles,  by  the  formation  of  a  partial  ventricular  sep- 
tum, the  organ  is  advanced  to  become  a  heart-and-three-quarters. 
Finally,  in  the  crocodiles,  birds,  and  mammals,  the  heart  becomes  a 
double  pump  with  two  auricles  and  two  ventricles.  This  continuous 
series  of  modifications  the  comparative  anatomist  interprets  as  due 
to  progressive  evolution  based  upon  relationship. 

Inspired  by  Darwin's  "Origin  of  Species,"  the  eminent  German 
anatomist,  Robert  Wiedersheim,  has  written  a  remarkable  book^  in 
which  he  describes  in  charming  and  scholarly  manner  a  long  array  of 
human  anatomical  structures  that  in  every  instance  are  matched  by 
corresponding  details  exhibited  by  other  vertebrates.  He  concludes 
that  there  is  nothing  unique  or  original  in  the  "structure  of  man." 
Even  such  differences  as  appear  in  the  distinctive  human  brain  as 
compared  with  the  brain  of  his  "poor  relations"  in  the  animal  king- 
dom, are  quantitative  rather  than  qualitative. 

'  Der  Bau  des  Menschen. 


498  THE   CHANGING   WORLD 

An  outstanding  example  of  common  origin  that  is  frequently  cited 
is  the  case  of  homologous  bones  in  the  wing  of  a  bird,  the  leg  of  a 
quadruped,  the  flipper  of  a  whale,  and  the  arm  of  man,  which  con- 
form to  a  common  plan  but  develop  into  diverse  structures  for  differ- 
ent uses. 

One  of  the  blood  ties  that  suggests  the  cousinship  of  all  verte- 
brates is  the  fact  that  they  are  limited  to  two  pairs  of  lateral  append- 
ages, although  some  of  them  have  lost  one  or  both  of  these  pairs. 
A  learned  doctor's  thesis  in  biology  that  could  satisfactorily  explain 
the  presence  of  wings  in  addition  to  arms  on  the  shoulders  of  the 
angels  which  Raphael  painted  on  the  ceiling  of  the  Sistine  Chapel 
in  Rome  would  be  as  famous  as  the  frescoes  themselves.     The  only 
possible  conclusion  acceptable  to  the  comparative  anatomist  would  be 
that  angels  and  men  are  entirely  unrelated,  which  may  be  true  enough. 
When  representatives  of  animal  groups  are  passed  in  review  in 
the  mind's  eye,  all  sorts  of  different  structures,  such  as  skeleton, 
kidneys,  teeth,  sense  organs,  brains,  and  respiratory  devices,  fall  into 
line  as  being  made  up  of  a  continuous  series,  explainable  upon  the 
supposition  that  they  have  evolved  one  from  another  during  the 
long  course  of  geologic  time,  but  that  otherwise  are  unintelligible. 
It  is  quite  impossible  for  a  comparative  anatomist,  grounded  in  the 
knowledge  of  many  details,  not  to  be  convinced  that  the  leg  of  a 
horse  is  one  end  of  a  series  of  structural  modifications  that  began 
with  the  fin  of  a  fish.     Or  that  the  plan  represented  in  the  life  cycle 
of  the  flowering  plants  is  not  the  outcome  of  the  alternation  of  gamet- 
ophytes  and  sporophytes  so  apparent  in  the  mosses  and  ferns.     All 
the  necessary  connecting  links  are  there,  which  would  be  senseless 
indeed  if  they  did  not  fall  into  line  to  spell  continuity.     Innumerable 
examples  of  this  sort  are  familiar  to  the  biologist. 

As  has  been  pointed  out  by  certain  doubting  Thomases,  the  fact 
that  things  may  be  arranged  in  a  continuous  series  does  not  neces- 
sarily mean  genetic  relationship.  Weapons  of  human  defense,  for 
example,  may  be  traced  from  their  earliest  beginnings  in  the  form 
of  stones  and  clubs,  up  through  spears,  bows  and  arrows,  and  fire- 
arms of  increasing  efficiency,  to  the  deadly  machine  gun,  yet  no  one 
would  say  that  this  is  genetic  derivation  of  one  kind  from  another, 
because  weapons  of  defense  do  not  reproduce  their  kind  as  do  living 
things.  The  U.  S.  Patent  Office  is  well  aware  that  the  same  idea 
often  turns  up  from  widely  different  sources  having  no  possible 
immediate  connection. 


THE   EPIC   OF   EVOLUTIOiN  499 

Likewise  there  is  frequently  a  convergence  of  structure  on  the  part 
of  diverse  organisms,  as  a  result  of  adaptation  to  a  single  kind  of 
environment.  When  this  happens  in  the  organic  world,  it  is  called 
convergent  evolution,  and  it  puts  the  comparative  anatomist  on  his 
guard,  since  resemblance  between  organisms  does  not  always  signify 
relationship.  Pelagic  animals  of  the  open  ocean,  belonging  to  the 
quite  different  groups  of  coelenterates,  molluscs,  crustaceans,  worms, 
tunicates,  and  fishes,  often  become  more  or  less  transparent,  which 
makes  it  difficult  for  them  to  be  seen  by  ravenous  fishes  from  below, 
or  by  preying  birds  diving  down  from  above.  Snakes,  blind  caecil- 
ians,  legless  lizards,  and  eels,  all  have  attained  a  similar  body  form, 
but  innumerable  other  anatomical  features  that  they  severally  pos- 
sess prove  them  to  be  not  closely  related,  in  spite  of  their  external 
resemblance.  Most  cases  of  convergent  evolution  are  functional 
rather  than  structural.  Organisms  show  their  relationship  to  each 
other  by  their  structure,  that  is,  by  what  they  are,  rather  than  by 
their  function,  that  is,  by  what  they  do. 

Vestigial  structures,  such  as  the  well-known  degenerating  vermi- 
form appendix  in  man,  that  is  absent  in  the  cat  but  excessively 
developed  in  the  rabbit,  are  anatomical  parts  gradually  disappear- 
ing below  the  horizon  of  usefulness.  Like  certain  finicky  parlor 
boarders,  they  often  make  trouble  and  there  is  no  accounting  for 
their  presence  except  upon  the  theory  of  evolution.  Again,  as  a 
final  example,  may  be  cited  the  lumbar  plexus,  which  is  a  union  of 
spinal  nerves  to  supply  the  hind  legs  of  vertebrates.  The  fact  that 
some  snakes  have  a  lumbar  plexus,  although  they  have  no  legs  to  be 
supplied  with  nerves,  indicates  that  they  are  still  hanging  on  to  the 
documentary  evidence  that  establishes  their  relationship  to  other 
vertebrates,  although  they  have  diverged  far  from  the  ancestral 
stock. 

Evidence  from  Embryology 

There  is  a  suggestive  parallel  between  the  embryonic  develop- 
ment of  the  individual  and  the  more  extensive  course  of  organic 
evolution.  A  whale  and  a  mouse,  both  mammals,  are  more  alike  in 
their  development  than  a  whale  and  a  fish,  which  are  outwardly 
more  similar.  Again,  that  curious  living  fossil,  the  horseshoe  crab 
Limulus,  which  has  retained  its  conservative  individuality  as  a  species 
since  paleozoic  times,  looks  anatomically  like  a  crustacean.  Its 
embryological  development,  however,  as  Kingsley  has  demonstrated. 


500 


THE  CHANGING  WORLD 


makes  more  probable  its  relationship  to  spiders  and  scorpions,  a 
conclusion  which  is  confirmed  physiologically  by  blood  tests,  as  will 
be  pointed  out  later. 

The  microscopic  water  flea  Daphnia,  the  sedentary  rock  barnacle 
Balanus,  that  according  to  Huxley  "stands  on  its  head  and  kicks 

food  into  its  mouth  with 
its  legs,"  the  amorphous 
parasitic  degenerate 
lump  Sacculina,  some- 
times infesting  the  ab- 
domen of  crabs,  and  the 
familiar  free-swimming 
lobster  Homarus,  are  very 
diverse  in  adult  appear- 
ance. No  one  would 
ordinarily  suspect  them 
of  being  related,  yet  an 
examination  of  their  em- 
bryonic history  reveals 
unmistakably  that  they 
are  crustacean  cousins 
all  of  one  blood.  Thus 
does  embryology,  the  sci- 
ence concerned  with  the 
development  of  the  indi- 
vidual, furnish  evidence  of  relationship  that  otherwise  may  not  at 
once  be  apparent. 

The  everyday  miracle  of  adult  organisms  developing  from  eggs  or 
seeds  loses  much  of  its  force  because  of  its  very  familiarity.  We 
cease  to  wonder  that  a  chick  can  hatch  out  of  a  hen's  egg,  or  that  a 
gorgeous  flower  can  arise  from  a  seed  buried  in  the  ground,  because 
its  repeated  occurrence  comes  within  the  span  of  our  everyday  experi- 
ence. If  we  could  live  and  observe  for  a  million  years,  no  doubt  the 
panorama  of  evolution  would  become  as  obvious,  and  be  as  unques- 
tionably acceptable,  as  that  of  individual  development. 

The  fact  that  our  horizon  is  limited  by  a  span  of  "  three  score  years 
and  ten"  foreshortens  our  vision  so  that  we  lose  the  perspective 
needed  to  make  the  picture  clear  and  in  focus.  The  limitations  of 
human  life  are  in  this  respect  a  decided  handicap  to  a  more  complete 
understanding  of  evolutionary  processes. 


These  four  extremely  unlike  animals,  fitted  for 
quite  different  careers,  all  have  in  common  the 
heritage  of  the  crustacean  plan. 


THE  EPIC   OF  EVOLUTION  301 

The  presence  of  useless  vestigial  structures,  left  behind  during  the 
forward  march  of  individual  development,  furnishes  a  hint  of  former 
differences  in  the  ancestral  make-up,  and  is  evidence  that  evolution 
has  occurred.  For  example,  the  useless  downy  hair  (lanugo)  that 
covers  the  entire  body,  including  even  the  face,  of  the  human  embryo 
during  its  earlier  stages  is  at  least  a  reminder  of  other  mammals 
that  are  clothed  all  over  with  hair.  It  is  hard  to  explain  such  cases 
except  upon  the  supposition  of  relationship  and  the  evolution  of  one 
form  from  another. 

A  garage  with  horse  stalls  and  mangers  in  it  would  obviously  be  a 
horse  barn  made  over  to  meet  the  modern  demands  of  the  automo- 
bile. The  stalls  and  mangers,  like  the  vestigial  organs  that  it  has 
not  been  imperative  to  remove,  indicate  not  a  "specially  created" 
garage,  but  the  evolution  of  a  horse  barn  into  a  garage.  Embry- 
ology is  rich  in  instances  of  vestigial  organs  which  point  to  the  fact 
that  evolution  has  been  going  on. 

But  the  idea  that  ontogeny,  or  the  development  of  the  individual, 
faithfully  repeats  phylogeny,  that  is,  the  ancestral  evolution  of  the 
race,  though  certainly  very  suggestive,  nevertheless  has  its  limita- 
tions. In  the  first  place  it  is  too  much  to  ask  of  a  hen's  egg,  which 
can  develop  into  a  chick  in  three  weeks,  to  rehearse  word  for  word 
a  phylogenetic  story  that  has  required  a  million  years  to  accom- 
plish. Countless  episodes  would  naturally  have  to  be  omitted. 
Certain  embryonic  structures,  moreover,  such  as  the  yolk  sac,  the 
amnion,  and  the  allantois,  have  no  counterpart  in  the  adult  ancestry 
of  the  race.  Another  limitation  is  that  the  larval  stages,  exposed  to 
environmental  adjustments,  may  become  modified  into  temporary 
emergency  devices  having  no  phylogenetic  significance. 

There  have  been  various  attempts  to  make  sense  out  of  the  obvi- 
ous parallel  between  embryonic  development  and  organic  evolution. 
The  Recapitulation  Theory,  or  as  Haeckel  (1834-1919)  named  it,  the 
''Biogenetic  Law,"  assumes  that  higher  forms  of  life  during  their 
embryonic  development  pass  through  stages  attained  by  adult  organ- 
isms of  the  lower  orders.  Fishes,  amphibians,  reptiles,  birds,  and 
mammals,  for  instance,  beginning  alike  at  the  egg  stage,  become 
adult  by  stopping  at  various  levels,  as  shown  in  the  figure  at  the 
bottom  of  page  502,  in  which  the  vertical  lines  represent  embryonic 
development,  and  the  horizontal  lines  the  attainment  of  the  adult 
condition.  The  vertical  line  of  mammals  at  the  right  represents 
ontogeny  repeating  phylogeny. 


502 


THE  CHANGING  WORLD 


amphibiocns 


fis'hes 


Earlier  Agassiz  (1807-1873)  pointed  out  that  the  embryo  of  higher 
forms  does  not  so  much  resemble  the  adults  of  lower  living  forms  as 
mammals  it  does  adults  of  lower  fossil  forms. 

I    birds  Von  Baer  (1792-1876),  "Father  of 

Embryology,"  maintained  that  the 
resiles  j^Qj-g  nearly  the  adults  of  two  groups 
resemble  each  other,  the  longer  their 
embryonic  development  follows  an 
identical  path.  To  the  evolutionist 
resemblance  of  this  kind  means  rela- 
tionship. This  concept  is  diagram- 
matically  shown  in  the  figure  at  the  left 
in  which  it  is  evident  that  mammals, 
for  instance,  are  more  nearly  related 
to  reptiles  than  to  fishes,  because  they 
tread  the  embryological  road  together 
for  a  longer  period. 

Morgan  (1866-  )  proposed  a 
"Repetition  Theory,"  in  which  the 
embryonic  stages  run  along  parallel  lines,  rather  than  in  one  com- 
posite line,  to  diverge  eventually  into  adult  stages.  This  idea  pre- 
cludes the  possibility  that  the  embryonic  stages  in  any  one  group 
could  be  represented  by  adult  ancestral  stages  of  another  group. 
The  adult  mammal,  for  instance,  does  not  look  back  upon  the  adult 


I 


I  ^ 

.  bircCs         I  ^ 

reptilss    \ '" 

.  amphibians  \  ^ 
d 


-["ishes 


s\ 


i 


fishes  amphibians  nsptiks    birsls  mammals 

Diagram  illustrating  ontogeny  and  phylogeny. 

fish  as  one  of  the  embryonic  stages  through  which  it  has  passed,  but 
apparent  resemblances  are  to  be  accounted  for  on  the  ground  that 
the  development  of  the  mammal  runs  parallel  with  that  of  the  fish, 
and  consequently  resemblances  are  to  be  expected. 


THE  EPIC  OF  EVOLUTION 


SOS 


Hurst  (1870-     ),  on  the  other  hand,  thought  of  the  lines  represent- 
ing the  embryonic  stages  as  divergent  rather  than  parallel,  since 

the   farther   back  one  traces  r   s*.     ^l    ^ 

1  .      .,  ccdxjtity    stages 

the  ontogeny,  the  greater  the  —  ^ 

resemblance  between  different     ,      am^y,i>^ian5^ ,      birds 

Imes.     Morgan  m  reply  sug-    |i5>^s   S^  reptlkS/    mammals 

gested  that  the  reason  for  this  "^ 

observation,  indisputably  true, 

may  be  because  there  are  fewer 

available   diagnostic   features 

upon  which  to  base  differences 

the  farther  one  goes  back  in 

development. 

Finally,  O.  Hertwig  (1849- 

1922)  has  emphasized  the  fact 

that  the  different  lines  do  not 

all  start  alike  with  the  same 

egg   stage.     There  are  eggs   and   eggs. 

possibilities  not  attained  by  the  fish  egg.     Thus,   the  eggs  of  the 

various  groups,  or  "species-cells"  as  Hertwig  calls  them,  have  ac- 
complished an  evolution  in  themselves  and  attained  different  levels 

mammals  *-*^  possibility,  with  the  result  that 
the  mammalian  egg  has  a  flying 
start  over  the  fish  egg,  and  be- 
comes   in    consequence   an   adult 


6    6 
Morgan's  "  Repetition  Theory." 

The  mammalian   egg  has 


reptiles 


amphibians 
fisV2es 


Divergence  theory  of  Hurst. 


farther  up  the  scale.  This  rela- 
tion is  shown  in  the  figure.  It  is 
evident  that  the  lower  adult  type 
of  the  fish  can  take  no  more  than 
a  deceptively  apparent  part  in 
the  developmental  steps  through 
which  the  mammal  passes.  The 
egg  of  the  reptile,  for  example,  in 
a  certain  sense  has  reached  a  stage 
of  advancement,  in  possibilities 
at  least,  somewhat  comparable 
with  the  adult  attainment  of  the 


amphibian.  The  dotted  lines  in  the  figure  show  what  would  be 
necessary  to  assume  in  order  to  picture  how  ontogeny  repeats 
phylogeny. 

H.  w.  H.— 33 


504 


THE  CHANGING  WORLD 


However,  all  the  foregoing  unproven  speculations  do  not  invali- 
date the  outstanding  fact  that  similarity  of  development  suggests 
relationship,  particularly  among  forms  that  have  come  to  be  unlike 


Tnamrnals 


amphibian 
fishes^ 


birds 


amphibians 


See  text. 


each  other,  and  implies  that  evolution  has  occurred.  Why,  for  ex- 
ample, should  a  mammal  in  its  development  "go  around  Robin 
Hood's  barn"  in  order  to  pass  through  a  fishlike  stage  with  useless 
gill  pouches,  unless  such  structures  were  once  present,  and  not  yet 
discarded,  in  ancestral  fishes? 

Evidence  from  Classification 

The  most  natural  question  anyone  asks  upon  seeing  a  new  or 
unknown  animal  or  plant  is,  "What  is  it?"  In  the  science  of  classi- 
fication, or  taxonomy,  the  first  essential  is  to  identify  and  to  name 
different  organisms.  This  is  no  mean  task,  as  there  are  many  thou- 
sands of  different  kinds  of  plants  and  animals.  According  to  the 
Biblical  account,  the  first  piece  of  work  that  any  human  being  is  on 
record  as  having  accompUshed  was  in  the  field  of  taxonomy.  "And 
Adam  gave  names  to  all  the  cattle,  and  to  the  fowls  of  the  air,  and 
to  every  beast  of  the  field"  (Genesis  2 :  20). 


THE   EPIC   OF  EVOLUTION  505 

Following  the  identification  and  naming  of  living  things,  in  order 
to  have  any  intellectual  peace  of  mind  it  is  necessary  to  arrange 
them  in  some  sort  of  order.  "Mother  Nature"  does  not  do  that  for 
us,  since  her  household  is  everywhere  always  in  bustling  delightful 
confusion.  We  are  forced,  therefore,  to  regulate  natural  things  for 
ourselves,  if  we  would  approach  the  study  of  all  these  forms  in  any 
satisfactory  scientific  way. 

When  elaborating  a  reasonable  scheme  of  classification,  the  tax- 
onomist  runs  invariably  into  evidences  of  evolution.  If  no  other 
line  of  evidence  had  ever  been  established  to  prove  the  truth  of 
evolution,  that  from  classification  alone  would  be  conclusive.  The 
criteria  which  have  been  found  to  be  most  useful  in  grouping  organ- 
isms together  intelligibly  are  not  superficial  or  functional  character- 
istics, but  the  more  deep-seated  anatomical  structures  that  indicate 
genetic  relationship.  It  would  be  quite  futile  to  depend  upon  a 
superficial  characteristic  like  the  presence  of  spines,  for  example,  as 
a  standard  of  classification,  since  it  would  bring  together  such  strange 
bedfellows  as  porcupines,  thistles,  Murex  shells,  sea-urchins,  and 
cacti,  ending  in  as  much  confusion  as  ever.  On  the  other  hand,  if 
some  more  deep-seated  anatomical  character  is  selected,  like  the 
backbone,  then  there  can  be  gathered  into  one  proper  fraternal  group 
forms  of  such  diverse  appearance  as  bird,  beast,  and  fish.  Or  in 
flowering  plants  if,  for  example,  such  a  superficial  character  as  yellow 
color  is  employed  for  purposes  of  classification,  then  representatives 
of  families  as  diverse  as  dandelions,  roses,  sunflowers,  and  witch- 
hazels  would  be  incongruously  bunched  together,  and  everyone  knows 
that  would  never  do. 

There  are,  instead,  many  available  fundamental  differences,  such 
as  the  number  and  arrangement  of  the  floral  organs,  which  are 
satisfactory  and  dependable  criteria  for  classification,  because  they 
indicate  relationship.  External  features,  that  are  naturally  exposed 
directly  to  molding  environmental  influences,  register  where  an  organ- 
ism has  been.  Internal  characteristics  more  often  signify  true  rela- 
tionship, and  what  an  organism  actually  is.  Although  clothes  may 
distinguish  a  prince  from  a  pauper,  underneath  both  robes  and  rags 
"a  man's  a  man  for  a'  that."  As  Kipling  has  it,  "the  Colonel's 
lady  an'  Judy  O'Grady  are  sisters  under  their  skins." 

Taxonomy  actually  resolves  itself  into  anatomical  and  embryo- 
logical  description,  since  this  sort  of  detail  is  necessary  as  a  basis  for 
discrimination. 


S06 


THE  CHANGING  WORLD 


The  unit  of  the  taxonomist  is  the  species,  just  as  the  unit  of  the 
anatomist  is  the  organ,  and  that  of  the  physiologist  a  functioning 
system  of  organs.  Exactly  what  constitutes  a  species  is  still  a  matter 
of  controversy.  Someone  has  said  that  a  species  is  simply  a  com- 
promise of  opinions  on  the  part  of  experts.  A  species  represents  a 
real  entity,  nevertheless,  for  it  is  something  that  outlasts  the  mor- 
tal individuals  composing  it.  For  our  present  purpose  it  may  be 
described  as  a  group  of  individuals  more  like  each  other  than  they 
are  like  any  other  individuals. 

According  to  a  time-honored  system,  larger  groups  than  species,  of 
increasing  inclusiveness,  are  employed  in  classification,  such  as  gen- 
era, families,  orders,  classes,  and 
phyla.  (See  unit  IV.)  Linnaeus, 
past  master  in  taxonomy,  regarded 
species  as  entirely  separate  groups, 
to  be  arranged  as  if  in  the  pigeon- 
holes of  a  desk.  This  was  before  the 
evidences  of  evolution  were  as  well 
known  as  they  are  today.  Connect- 
ing links,  however,  have  played 
havoc  with  the  pigeonhole  idea  in 
classification.  A  good  example  of 
connecting  links,  dating  back  to  the 
Jurassic  Period,  is  Archacoptcryx,  the 
earliest  known  reptile-bird,  sporting 
feathers,  teeth,  and  a  lizardlike  tail. 
Biological  literature  is  full  of  such 
connecting  links,  plainly  indicating 
relationship  and  the  occurrence  of 
evolution. 

The  attempt  to  sort  out  different 
species  of  such  groups  of  organisms 
as,  for  example,  sedges,  mosses, 
grasshoppers,  violets,  or  fishes,  im- 
mediately brings  difficulty,  because 
the  representatives  of  these  groups 
grade  into  each  other.  It  takes  a  specialist  to  do  it.  The  more 
nearly  two  species  are  related,  the  fewer  and  finer  are  the  diag- 
nostic features  that  can  be  found  and  utilized  to  distinguish  them. 
The  dipterologist,  for  example,  is  obliged  to  resort  to  very  minute 


Archaeopteryx,  the  oldest  known 
bird,  drawn  from  the  Berhn  specimen. 
Note  teeth,  three  fingers,  feathers, 
and  a  Hzardlike  tail.  The  only  other 
known  specimen  is  in  the  British 
Museum.  (After  Parker  and  Has- 
weU.) 


THE   EPIC   OF  EVOLUTION  507 

technical  details  in  pigeonholing  the  thirty  thousand  or  so  kinds  of 
flies  that  occur  in  the  United  States  alone.  It  is  little  wonder  that 
many  half  brothers  turn  up  in  such  an  extensive  fraternity. 

So  it  comes  about  that  the  taxonomist,  while  still  resorting  to  con- 
venient pigeonholes  in  classifying  plants  and  animals,  comes  more 

"mammals 

acraniotes.    ^  Yv^«f;U*  birds       / 

I      c/cbstomes  reptiles  /  / 

I  /fishes  aTnpbibicoi$p-...™<<<Archaeopte90< 

I  I        V    /  I'-y^- Sauropsida. 

\       \         y._^^tf^.....lchthxopsicta 

\      I  ^^x^?r. Gnath  oStomatcc 

L^^rfr. Craniota 


The  theoretical  branching  of  the  Craniote  tree.     X  marks  the  spot  where  the 

reptile-bird  Archaeopleryx  roosts. 

and  more  to  picture,  in  his  mind  at  least,  a  branching  tree  as  the 
proper  symbol  by  which  to  represent  the  obvious  relationships  that 
connecting  links  indicate.  A  tree  with  its  trunk  giving  rise  to 
branches  and  twigs  is,  in  fact,  a  perfect  diagrammatic  picture  of  the 
evolutionary  process.  Such  a  zoological,  phylogenetic  tree,  includ- 
ing only  vertebrates,  however,  is  shown  in  the  figure,  with  X  marking 
the  spot  where  Archaeopleryx  can  comfortably  roost. 

In  a  similar  taxonomic  tree,  enlarged  to  include  all  animal  creation, 
a  watery  floating  jellyfish  on  one  of  the  lower  branches  might  humbly 
look  up  to  an  earthworm,  with  its  wonderfully  prophetic  head  end, 
while  arrogant  human  beings  in  the  very  tree-top  look  down  patron- 
izingly upon  the  scatter-brained  monkey,  and  all  other  biological 
way-stations. 

Similar  taxonomic  trees  may  also  be  constructed  to  show  what 
is  known  about  the  possible  relationship  between  different  groups 
of  plants. 

Finally,  in  summation  it  may  be  repeated  that  the  key  to  classifica- 
tion is  relationship ;  that  is,  the  derivation  of  one  form  from  another, 
which  is  evolution. 


508  THE  CHANGING  WORLD 

Evidence  from  Distribution 

The  peculiar  way  in  which  species  of  animals  and  plants  are  dis- 
tributed in  oceans  and  upon  land  finds  no  sensible  explanation 
unless  it  is  assumed  that  evolution  has  occurred,  when  it  plainly 
becomes  a  matter  of  untangling  historical  events,  such  as  past  geo- 
logical changes  and  the  migrations  of  plants  and  animals,  and  finding 
out  their  proper  sequence.  From  their  original  home  the  members 
of  each  species  scatter,  due  to  overcrowding,  the  search  for  food, 
and  various  other  reasons,  until  they  encounter  barriers  that  limit 
their  advance.  The  species  may  settle  in  a  new  habitat,  or  undergo 
transformations  and  adaptations  that  make  further  exploration  of 
the  world  possible.  A  species  may  also  perish  in  the  attempt  to 
live  in  a  changing  environment  to  which  it  cannot  adapt  itself. 
Fossil  records  are  filled  with  examples  of  this  sort.  Therefore,  the 
key  to  the  present  distribution  of  organisms  lies  in  a  knowledge  of  the 
vicissitudes  experienced  in  the  past. 

It  was  the  unusual  distribution  of  life  on  the  Galapagos  Islands 
that  started  Charles  Darwin  in  his  yeasty  thinking  about  evolution, 
and,  as  everyone  knows,  he  started  others  to  thinking.  These  vol- 
canic islands  lie  500  miles  off  the  coast  of  Ecuador,  and  were  visited  by 
Darwin  during  his  famous  voyage  around  the  world  on  the  Beagle. 
There  he  found  an  assemblage  of  peculiar  animals,  all  unmistakably 
patterned  after  South  American  forms,  but  yet  modified  somewhat 
from  the  continental  types.  It  is  evident  that  originally  there  must 
have  been  land  connection  between  South  America  and  what  is 
now  this  archipelago  of  volcanic  islands,  making  a  bridge  over  which 
continental  animals  could  migrate.  With  the  gradual  subsidence  of 
the  oceanic  floor,  the  tops  of  the  volcanoes  were  left  as  isolated 
islands,  and  the  islanders  found  themselves  cut  olT  from  their  rela- 
tives on  the  mainland.  Survival  on  these  isolated  islands  called  for 
nice  adaptation,  different  in  each  different  habitat. 

South  America  and  Africa  have  in  general  the  same  climate  and 
would  be  suitable  habitats  for  the  same  organisms.  Nevertheless, 
the  faunas  and  floras  of  these  regions  are  quite  different.  In  Africa 
are  found  the  rhinoceros,  lion,  wart  hog,  zebra,  baboon,  giraffe, 
gorilla,  okapi,  and  aardvark.  None  of  these  animals  occur  in  South 
America,  which  in  turn  is  the  home  of  the  armadillo,  sloth,  vampire- 
bat,  llama,  peccary,  tapir,  agouti,  and  marmoset,  not  one  of  which 
is  found  in  Africa.     Such  diverse  distribution  indicates  that  these 


THE   EPIC   OF  EVOLUTION 


509 


EVOLUTION    OF   THE   CAMELS 


Plexstocew 


Recent 


Wiocene 


Miocene 


Ol\goccnc 


Sliull 


Feet 


Teeth 


Procomelus 


Pocbrotherium 


Eocerie 


PrOtylopuS 


two  continents  have  been  separated  long  enough,  even  if  they  were 
ever  in  communication,  to  allow  their  characteristic  faunas  to  evolve 
independently. 

There  is  geological  evidence  of  an  ancient  Pleistocene  land-bridge 
in  the  region  of  Bering  Strait,  between  North  America  and  Eurasia. 
The  presence  of  this  former  bridge  explains  why  similar  native  ani- 
mals, such  as  bears,  sheep,  antelopes,  moose,  bison,  and  caribou, 
occur  in  these  two  great  regions  now  separated  from  each  other,  and 
are  not  represented  on 
other  continents. 

One  of  the  best  exam- 
ples of  the  connection 
between  evolutionary 
processes  and  distribu- 
tion, which  has  been  un- 
earthed quite  completely 
by  paleontologists,  is  that 
of  the  camel-like  mam- 
mals. The  ancestral 
home  of  these  animals, 
as  shown  by  fossils,  was 
North  America,  where 
they  went  through  a  long 
preliminary  evolution, 
but  where  none  of  them 
are  present  today.  Pro- 
tylopus  was  an  Eocene 
''camel";  Poebrotherium 
followed  in  Oligocene 
times ;  and  Procamelus 
in  the  Miocene  period. 
Later,  in  the  Pliocene  period  some  of  these  ancestral  camels  migrated 
in  two  directions  from  their  birth  place.  One  stream  went  across 
the  Bering  Sea  bridge  into  Eurasia  and  evolved  into  Bactrian  camels 
with  two  humps  and  the  Arabian  dromedaries  with  a  single  hump. 
The  other  stream  migrated  southward  over  the  Isthmus  of  Panama 
into  South  America,  and  became  modified  into  the  wild  guanacos  and 
vicunas,  from  w^hich  much  later  the  domestic  llamas  and  alpacas  were 
derived.  Thus,  the  transformed  descendants  of  these  pecuhar  an- 
cient fossil  forms  of  North  America  are  found  today  occupying  habi- 


f1 


^ 


£1 


Meso/oic  or  Age  of  Reptiles 


Hypothetical  fiKe-toed  Ancestor 


American  Miisevm  of  Natural  History 

Evolution  of  the  camel. 


510  THE  CHANGING   WORLD 

tats  far  apart,  and  are  quite  different  in  general  appearance,  although 
unmistakably  relatives. 

In  conclusion  may  be  quoted  the  eminent  paleontologist,  W.  B. 
Scott  (1858-  ),  who  says,  "The  main  outline  of  the  problem  of 
distribution  has  been  satisfactorily  explained  on  the  evolutionary 
theory,  and  no  other  theory  even  pretends  to  account  for  the  facts." 

Evidence  from  Fossils 

It  would  be  quite  as  impossible  to  describe  Niagara  Falls  without 
mentioning  either  water  or  honeymooners,  as  to  write  about  evidences 
of  evolution  from  fossils  without  citing  the  remarkable  known  history 
of  the  horse.  Everyone  is  led  to  refer  to  this  famous  pedigree, 
extending  back  for  something  like  forty  million  years,  because  it 
furnishes  a  perfect  and  well-authenticated  demonstration  of  evolu- 
tion. 

The  earliest  known  "horse"  was  Eohippus,  of  which  thirteen 
species  have  been  identified  from  the  Eocene  period.  A  full  grown 
Eohippus  was  scarcely  more  than  a  foot  high.  It  had  four  toes  and 
a  remnant  of  a  fifth  on  each  front  foot,  with  three  toes  and  parts 
of  a  first  and  fifth  on  each  hind  foot.  These  feet  were  well  adapted 
for  living  on  the  soft  ground  of  forest  areas.  The  teeth  of  Eohippus 
were  piglike  in  character  and  not  at  all  like  the  highly  modified 
teeth  of  modern  horses.  In  fact,  during  Eocene  times  all  the  mam- 
mals were  in  a  decidedly  primitive  stage,  not  yet  having  become 
differentiated  into  carnivores  and  herbivores,  with  corresponding 
modifications  of  their  teeth  and  general  structure. 

If  we  now  leap  the  intervening  years  and  come  down  to  the  mod- 
ern horse,  Equus,  we  find  a  very  different  animal.  Its  adult  size  is 
much  larger.  It  is  the  only  quadruped  which,  like  a  toe-dancer, 
stands  upon  a  shoelike  hoof  at  the  tip  of  a  single  toe  on  each  foot 
with  its  heel  high  off  the  ground.  It  is  adapted  for  rapid  flight  over 
open  plains,  for,  since  the  days  of  Eohippus,  carnivorous  school- 
masters have  appeared  to  teach  it  how  to  run  for  its  life.  Its  teeth 
are  unique.  The  molars  are  all  practically  alike,  with  high  crowns 
constructed  in  such  a  way,  with  hard  enamel  and  softer  dentine  side 
by  side,  that  these  substances  wear  away  unequally,  thus  always 
leaving  sharp  grinding  enamel  surfaces.  Moreover,  its  teeth  con- 
tinue to  grow  for  some  years,  instead  of  attaining  maturity  early  in 
life,  and  so  are  enabled  to  keep  up  the  life  long  grind  to  which  they 
are  subjected. 


THE  EPIC   OF   EVOLUTION 


511 


There  are  forty-five  fossil  species  of  Equus,  and  seven  wild  species 
now  living  in  Asia  and  Africa,  from  which  domesticated  horses  and 
donkeys  have  been  derived.  Between  the  extremes  of  Eocene  Eohip- 
pus  and  modern  horses,  eight  other  genera  have  been  found,  con- 
taining over  one  hundred  species,  and  forming  a  continuous  series 
with  no  gaps  of  importance.  The  arrangement  of  this  series,  and 
the  approximate  duration  in  time  of  each  type,  is  shown  in  the 
accompanying  table.  In  addition  to  the  main  line  that  eventuated 
in  Equus,  there  have  been  various  side  lines  which  became  extinct. 

The  original  home  of  Eohippus,  and  other  genera  of  fossil  horses, 
was  North  America,  and  here  for  millions  of  years  they  w^orked 
out  their  evolutionary  salvation.  In  Pleistocene  times,  they  made 
repeated  migrations  back  and  forth  across  the  Bering  Sea  bridge 
to  Eurasia  and  Africa,  w^here  their  descendants,  the  wild  asses  and 
zebras,  carry  on  today.  Meanwhile,  all  the  horses  of  North  America 
became  extinct,  not  suddenly  but  gradually  over  a  stretch  of  thou- 
sands of  years.  What  caused  their  extinction  is  unknown.  Perhaps 
it  was  the  Pleistocene  glaciers,  or  it  may  be  that  they  were  finally 
wiped  out  by  their  carnivorous  enemies.  The  suggestion  has  even 
been  made  that  the  deadly  tsetse  flies,  fossils  of  wiiich  have  been 
found  in  the  Florissant  shales  of  Colorado,  might  have  caused  their 
downfall.  These  villainous  flies,  with  the  aid  of  parasitic  protozoans 
which  they  transfer  to  mammalian  hosts,  have  made  it  impossible 
for  any  except  native  cattle  and  horses  to  live  in  considerable  river 
bottom  areas  of  Africa  today. 


million   50 
years 


side  lines 


AQ_ 


30 


20 


PLEISTOCtNE^    7 


mnnline 


EOCENE 


OLlGOCEMt 


losp. 
Orobippus 


lohippus 
■ibsp. 


Epibippus 
2sR 


Caenertx 


four-toedt  horses 


16  sp. 
nssohippus 


"Miobippus 
nsp. 


tbree-toed: 
all  toes  used 


MIOCENE 


^ 


PLIOCENE 


16  Sp. 

Pambippus  Pli|ohippus 


Msiychippii 


three -toed: 

only  central 
toe^  used 


17  sp. 


Equus 


Ple5ippu5 
isp 


8 


one  -toed 


The  40,000,000  year  old  pedigree  of  the  horse,  involving  in  a  direct  line  at  least 
ten  genera  and  over  one  hundred  species. 


512  THE   CHANGING   WORLD 

The  final  episode  in  equine  history  occurred  in  very  recent  times, 
even  as  late  as  after  Adam's  ancestors  had  become  human  beings 
and  had  passed  through  a  series  of  many  civilizations  that  arose  and 
fell.  Then,  preceding  the  comparatively  recent  Christian  era,  there 
came  long  dark  ages  until  yesterday,  in  the  seventeenth  century, 
adventurous  Spaniards  brought  to  South  America  domesticated 
descendants  of  the  European  branch  of  this  long,  royal  equine  line. 
Some  of  these  much  traveled  horses,  being  set  free,  "went  native," 
and  became  the  wild  mustangs  and  broncos  which  spread  from 
South  America,  and  finally  came  to  reoccupy  their  ancestral  plains 
in  North  America.  Thousands  of  skeletons  of  fossil  horses  all  along 
the  evolutionary  line  have  been  discovered,  and  may  be  seen  in  vari- 
ous museums.  Their  sequence  is  so  plain  that  even  the  uninitiated 
can  understand  it  and  be  convinced  of  the  truth.  When  once  this 
documentary  evidence  is  realized,  the  fact  of  evolution  is  established 
beyond  any  doubt. 

A  common  difficulty  in  accepting  the  evidences  of  organic  evolu- 
tion is  inability  to  appreciate  the  length  of  time  that  it  has  taken. 
It  never  could  have  come  about  within  a  few  thousand  years.  How 
ridiculous  it  is  to  expect  anything  like  a  laboratory  demonstration 
of  an  accomplishment  which  has  taken  millions  of  years  to  effect ! 
The  geologist,  however,  presents  us  with  all  the  years  we  could 
possibly  need  to  enable  us  to  allow  for  the  slow  processes  of  evolu- 
tion ;  so  many,  in  fact,  that  we  grow  intellectually  footsore  and 
weary  traveling  backward  in  time.  The  story  of  the  horse,  for 
example,  occupied  only  a  part  of  the  Cenozoic  era,  or  about  one 
tenth  of  the  time  since  the  dawn  of  the  Paleozoic  era  in  which  the 
general  record  of  fossils  begins.  The  fact  that  at  this  early  time  all 
the  large  groups  of  animals  except  vertebrates  were  represented  in 
great  diversity  makes  it  reasonable  to  suppose  that  evolution  of 
organic  forms  did  not  start  then,  but  had  already  been  going  on 
long  enough  to  lead  up  to  the  Paleozoic  differentiation  of  types.  It 
should  be  remembered  that  only  in  sedimentary  rocks,  the  earliest 
of  which  belong  to  the  Paleozoic  era,  are  fossil  epitaphs  recorded. 
Many  lines  die  out.  Even  the  horse  is  on  its  last  legs  in  an  evolu- 
tionary sense,  with  only  a  paltry  half  dozen  or  so  living  species  left 
out  of  all  the  past. 

Some  of  the  inescapable  conclusions  of  the  occurrence  of  evolution 
that  are  reached  by  an  examination  of  the  evidence  from  fossils  are : 
(a)  that  there  is  a  general  increase  in  complexity  of  organisms  as  time 


THE  EPIC  OF  EVOLUTION 


513 


goes  on ;  (6)  that  organic  forms  are  derived  from  preceding  forms ; 
(c)  that  connecting  Unks,  beyond  Darwin's  fondest  dreams,  are  now 
known  ;  (d)  that  the  rate  of  evohition  varies  in  different  kinds  of  ani- 
mals and  plants ;  (e)  that  when  a  species  is  once  extinct,  there  is  no 
reappearance  of  it. 

A  table,  indicating  the  rise  and  expansion  of  typical  groups  of 
organisms,  is  shown  in  the  accompanying  record  of  geological  chro- 
nology. 


TABLE   OF   GEOLOGICAL   CHRONOLOGY    (Modified  from  Lull) 


Eras 

Periods 

Advances  in  Life 

Dominant  Life 

Psychozoic 

Recent  (Post- 
glacial) 

Era  of  mental  life 

Man 

Pleistocene 

(Glacial  period) 

Extinction  of  great  mam- 
mals 

Pliocene 

Origin  of  man 

Cenozoic 

Miocene 

Culmination  of  mammals 

Mammals 

Oligocene 

Rise  of  higher  mammals 

Eocene 

Rise  of  horses 

Paleocene 

Dominance  of  archaic 
mammals 

Cretaceous 

Extinction  of  great  reptiles 
Rise  of  flowering  plants 

Mesozoic 

Jurassic 

Rise  of  birds  and   ptero- 
dactyls 

Reptiles 

Triassic 

Rise  of  dinosaurs 

Permian 

(Glacial  period) 

Rise  of  ammonites 
Last  of  trilobites 

Amphibians 

Carboniferous 

Rise  of  reptiles  and  insects 
Abundant  land  plants 

Paleozoic 

Devonian 

Rise  of  amphibians 
First  land  flora 
Ganoid  fishes 
Corals  and  brachiopods 

Fishes 

Silurian 

Rise  of  lung  fishes 
First  air-breathers 

Ordovician 

Land  plants  and  corals 
Armored  fishes 

Higher  shelled  inverte- 
brates 

Cambrian 

Rise  of  molluscs 
Dominance  of  trilobites 
First  known  marine  faunas 

Proterozoic 

Evidences  of  life  scanty 

Shell-less  invertebrates 

Archeozoic 

Unicellular  organisms 

514  THE  CHANGING   WORLD 

Evidence  from  Serology 

When  serum  from  human  blood  is  injected  at  repeated  intervals 
into  a  rabbit,  the  rabbit  eventually  develops  antibodies  and  becomes 
sensitized  to  human  blood.  When  the  blood  thus  prepared  is  added 
to  human  blood,  it  produces  a  precipitate,  a  chemical  change  that 
does  not  occur  if  it  is  added  to  the  blood  of  other  animals.  Unsensi- 
tized  rabbit  blood  does  not  react  to  human  blood.  Rabbit  blood 
can  also  be  sensitized  to  horse  serum,  or  to  that  of  other  animals 
such  as  the  pig  or  fowl. 

Such  blood  tests  constitute  a  chemical  method  which  enables  the 
experimenter  to  determine  whether  or  not  a  specimen  of  unknown 
blood  is  human,  a  technique  that  has  proved  very  useful  to  the 
criminologist.  In  Germany  blood  tests  have  even  been  employed 
to  determine  the  composition  of  suspected  sausages. 

By  this  method  also  the  degrees  of  relationship  between  different 
animals  can  be  determined.  In  the  case  of  man,  for  example,  when 
human-sensitized  rabbit  blood  is  added  to  that  of  anthropoid  apes, 
the  precipitation  is  almost  as  complete  as  with  human  blood.  The  re- 
action occurs  in  diminishing  degree  with  Old  World  catarrhine  mon- 
keys, and  still  less  with  New  World,  long-tailed  platyrrhine  monkeys. 
It  does  not  react  in  any  appreciable  degree  with  the  blood  of  lemurs. 
This  is  in  confirmation  of  anatomical  and  embryological  evidence 
that  the  order  of  relationship  among  primates  is  man,  apes,  Old 
World  monkeys.  New  World  monkeys,  and  lemurs. 

Rabbit  blood  sensitized  to  horse  serum  will  react  against  the  blood 
of  a  zebra,  but  less  positively  than  to  horse  blood.  By  similar  blood 
tests  it  is  shown  that  whales  are  more  akin  to  ungulates  than  to  car- 
nivores, that  birds  are  closer  to  turtles  than  to  lizards,  and  that  the 
horseshoe  crab  Limulus,  as  already  mentioned,  is  more  of  a  scorpion 
or  spider  than  it  is  a  crab,  which  it  externally  resembles. 

Evidence  from  Human  Interference 

The  part  man  has  played  in  directing  the  course  of  evolution  is 
apparent  in  domesticated  animals  and  plants,  which  often  differ  to  a 
remarkable  degree  from  wild  ancestral  forms,  as  a  visit  to  a  flower, 
dog,  or  poultry  show  demonstrates. 

What  man  has  done  has  not  been  creative  but  selective.  He  has 
employed  neither  laws  nor  methods  which  were  not  already  in  opera- 
tion.    Nature  has  furnished  the  plastic  variable  organisms,  and  man 


THE   EPIC   OF  EVOLUTION  515 

has  simply  picked  out  and  fostered  those  forms  that  have  suited  his 
purpose.  If  man,  during  the  comparatively  short  time  he  has  been 
an  actor  on  the  evolutionary  stage,  has  been  able  to  bring  about  such 
considerable  changes  in  the  population  of  the  earth  as  is  shown  by 
domesticated  animals  and  plants,  it  appears  reasonable  to  suppose 
that  "  Mother  Nature,"  in  the  enormous  span  of  time  which  has  been 
available  for  her  experiments,  would  certainly  be  able,  without 
human  help,  to  have  something  to  show  in  the  way  of  evolution. 
The  origin  of  some  domestic  races,  such  as  maize,  is  quite  lost  in 
antiquity,  but  the  wild  forebears  of  most  domestic  forms  are  known. 
It  is  quite  well  established  that  the  numerous  varieties  of  poultry 
came  from  two  original  stocks,  the  jungle-fowl  of  India  and  the 
Malayan  azeel  fowl.  The  many  kinds  of  pigeons  —  fantails,  barbs, 
carriers,  pouters,  tumblers,  and  others  —  all  came  from  an  original 
single  stock,  namely,  the  wild  rock  pigeon.  Pigs,  sheep,  cattle,  horses, 
rabbits,  guinea  pigs,  roses,  forage  plants,  all  trace  their  ancestry  to 
wild  forms.  From  the  plastic  wolflike  ancestor  of  the  dog  has  been 
evolved  by  the  selective  hand  of  man  a  most  remarkable  array  of 
descendants.  Think  of  great  danes  and  pomeranians ;  long-nosed 
collies  and  snuffling  pekinese ;  waddling  bowlegged  dachshunds  and 
dainty  dancing  black-and-tans ;  woolly  poodles  and  Mexican  hairless 
dogs ;  spindle-legged  greyhounds  with  sharp  projecting  features  and 
stocky  bulldogs  with  faces  pushed  in,  and  all  the  other  kinds  of 
dogs ! 

In  many  cases  domesticated  forms  of  living  creatures  could  not 
survive  in  nature,  since  man  has  picked  out  different  qualities  than 
impartial  nature  would  have  selected.  Someone  has  said,  "the  best 
bred  hog  can  only  grunt,  and  snooze,  and  die.  The  prairie  rooter  of 
a  hundred  years  ago  had  more  wit  than  all  the  Chester- Whites  and 
Poland-Chinas  of  today." 

Another  fruitful  line  of  human  interference  with  evolutionary 
processes  is  that  of  experimental  breeding,  which  has  come  to  flower 
in  the  last  forty  years  since  some  knowledge  of  the  hereditary  laws, 
furnished  by  Mendelism,  has  made  it  possible.  This  is  not  the 
place  to  explain  what  is  involved  in  Mendelism  (see  XXII),  except  to 
say  that  it  has  to  do  with  the  controlled  combination  of  hereditary 
lines  which  may  result  in  evolutionary  changes  in  organisms.  If 
what  man  can  accomplish  rather  abruptly  by  controlled  matings  can 
also  take  place  in  nature  where  promiscuous  matings  occur,  then  a 
great  side  light  is  thrown  upon  evolution. 


516  THE  CHANGING  WORLD 

Huxley  once  said  that  he  "beheved  in  justification  not  by  faith 
but  by  experiment."  In  1904  the  Carnegie  Institution  of  Wash- 
ington, in  order  to  make  possible  programs  of  scientific  research 
looking  far  into  the  future,  established  at  Cold  Spring  Harbor,  Long 
Island,  New  York,  a  department  under  the  significant  title  of  Experi- 
mental Evolution.  This  more  recently  has  been  renamed  the 
Department  of  Genetics,  since  it  was  realized  that  controlled 
hybridization  furnishes  the  most  practical  line  of  approach  to  the 
larger  problem  of  evolution.  What  has  already  been  accomplished  by 
the  remarkable  staff  of  scientists  at  this  unique  station  is  a  story  of 
world-wide  interest.  It  is  a  very  good  sign  that  intellectual  curiosity 
does  not  let  us  rest  simply  with  the  evident  conclusion  that  evolution 
of  organic  life  has  occurred  in  the  past,  but  that  it  seeks  to  go  further 
and  tries  to  find  out  how  evolution  actually  may  come  about. 

The  Environmental  Theory  of  Lamarck 

In  the  stormy  days  following  the  French  Revolution,  a  famous 
Frenchman,  with  his  head  in  the  clouds  above  the  turmoil  of  human 
history,  brought  out  a  book  in  which  appeared  the  first  attempt  to 
explain  how  evolution  occurred.  This  was  Jean  Baptiste  Lamarck 
(1744-1829),  whose  book,  La  Philosophie  Zoologique,  appeared  in 
1809,  the  year  Charles  Darwin  was  born. 

Lamarck  was  the  eleventh  child  of  his  parents.  When  a  young 
man,  he  ran  away  from  the  Jesuit  College,  where  he  was  in  training 
for  the  priesthood,  to  become  a  soldier  in  the  French  army.  He 
distinguished  himself  for  bravery  on  the  field  of  battle,  was  disabled 
for  further  military  service,  and  returned  to  scholarly  pursuits. 
Devoting  himself  to  botanical  studies,  he  published  important  books 
in  this  field,  and  was  also  instrumental  in  establishing  the  famous 
Jar  din  des  Plantes  in  Paris.  In  1794,  he  was  appointed  to  a  chair 
of  Invertebrate  Zoology  in  this  Institution  at  the  age  of  fifty  years, 
deserting  botany  to  become  a  zoologist.  His  extensive  observations 
in  zoology  led  him  to  formulate  his  theory  of  evolution,  at  a  time 
when  everyone,  including  his  influential  fellow  countryman,  Georges 
Cuvier  (1769-1832),  "Father  of  Comparative  Anatomy,"  held  to  the 
Linnaean  idea  of  the  constancy  and  independence  of  miraculously 
created  species. 

Lamarck's  conception  of  the  transformation  of  species  may  be 
thought  of  as  standing  on  three  legs :  the  molding  effects  of  en- 
vironment ;    the  results  of  use  and  disuse ;    and  lastly,  an  inner  urge 


THE  EPIC  OF  EVOLUTION  517 

or  desire  on  the  part  of  the  organism  to  meet  new  conditions.  The 
changes  wrought  by  these  means  during  the  lifetime  of  the  individual, 
he  postulated,  were  then  handed  on  through  heredity  to  following 
generations.  The  latter  assumption  turned  out  to  be  a  weak  link 
in  the  chain. 

There  is  plenty  of  evidence  that  environment,  directly  in  the  case 
of  plants  and  indirectly  through  the  nervous  system  in  animals, 
does  cause  modification  in  the  structure  and  behavior  of  animals 
and  plants.  Arctic  animals,  for  example,  develop  a  thick  pelt,  and 
wind-swept  trees  grow  in  a  leaning  attitude  in  accordance  with 
prevailing  winds.  There  is  no  doubt,  either,  of  the  truth  of  his  second 
assumption.  Use  does  increase,  and  disuse  decrease,  the  develop- 
ment of  muscles,  as  every  athlete  knows.  The  lungs  of  opera  singers 
become  enlarged,  while  unused  organs  in  general  tend  to  diminish. 

The  third  postulate  in  his  theory  is  not  so  obvious,  and  Lamarck 
himself  did  not  stress  it.  According  to  this  idea,  which  Lamarck's 
opponents  made  ridiculous,  the  desire  of  the  ancestral  deer,  for 
example,  to  browse  on  leaves  higher  up  off  the  ground  caused  it  to 
stretch  its  neck  until,  after  some  generations  of  stretching,  it  became 
a  giraffe.  It  goes  without  saying  that  plants,  lacking  the  mechanism 
of  a  nervous  system  through  which  to  express  "desires,"  are  excluded 
from  this  method  of  attaining  evolutionary  ends. 

There  is  no  doubt,  however,  that  certain  changes  during  the  lifetime 
of  the  individual  are  everywhere  brought  about  by  environmental 
influences  and  the  effects  of  use  and  disuse.  All  such  evidence  would 
offer  an  obvious  explanation  of  the  method  of  evolution  if  only  there 
was  assurance  that  during  the  lifetime  of  the  individual  acquisitions 
are  passed  on.  Lamarck  did  not  question  that  this  was  so,  nor  did  his 
contemporaries.  It  remained  for  August  Weismann  (1834-1914)  to 
point  out  many  years  later  the  improbability  of  the  "inheritance  of 
acquired  characters."  The  great  service  of  Lamarck  was  to  over- 
throw the  old  idea  of  the  fixity  of  species,  and  to  suggest  a  reason- 
able hypothesis  concerning  the  origin  of  variations,  which  must  be 
the  point  of  departure  for  every  theory  of  evolution. 

Following  Lamarck's  work  there  were  three  avenues  open  to  the 
seeker  after  truth  about  evolution  :  (a)  to  retain  belief  with  Linnaeus 
and  Cuvier  in  the  fixity  of  species,  with  no  evolutionary  transforma- 
tion ;  (6)  to  accept  Lamarck's  theory  of  the  causes  of  variation  and 
the  inheritance  of  acquired  characteristics ;  or  (c)  to  find  some  other 
explanation  of  how  evolution  came  about. 


518  THE  CHANGING  WORLD 

The  Natural  Selection  Theory  of  Darwin 

The  Theory  of  Natural  Selection  was  arrived  at  independently 
and  simultaneously  by  Charles  Darwin  (1809-1873)  and  Alfred 
Russell  Wallace  (1822-1913).  It  is  greatly  to  the  honor  of  these 
two  gentlemen  that  neither  one  jealously  claimed  priority  for  the 
idea.  They  remained  throughout  life  friends  rather  than  rivals. 
Darwin's  elaboration  was  the  more  exhaustive  of  the  two,  and 
consequently  his  name  is  more  often  the  one  associated  with  the 
theory. 

When  a  young  man,  Darwin,  as  naturalist  on  board  the  Beagle, 
which  was  employed  by  the  British  Government  in  making  extensive 
surveys  for  navigation,  spent  five  years  ''seeing  the  world."  For  the 
next  twenty  years  he  mulled  over  what  he  had  seen,  adding  to  it 
by  exhaustive  study  and  experiment,  before  he  was  ready  to  publish 
his  results.  The  Origin  of  Species  appeared  in  November,  1859,  and 
the  entire  first  edition  was  sold  out  on  the  first  day.  There  is  no  doubt 
that  it  is  the  most  famous  scientific  book  of  the  nineteenth  century. 
It  has  gone  through  many  subsequent  editions  and  has  been  trans- 
lated into  many  languages.  It  is  the  parent  of  whole  libraries  of 
intellectual  children.  The  thoroughness  with  which  the  work  was 
done,  and  the  restraint  and  caution  employed,  explains  why  the 
edifice  of  Natural  Selection  there  set  forth  has  withstood  the  battering 
storms  of  controversy  during  subsequent  years.  That  part  of  the 
theory  which  has  been  modified  necessarily  to  make  it  square  today 
with  the  advance  of  biological  knowledge  has  to  do  largely  with  the 
nature  of  variations  and  the  mechanism  of  heredity.  The  central 
thesis  stands. 

Darwin  was  impressed  with  the  effectiveness  of  human  selection 
in  the  formation  of  domestic  species,  and  extended  this  idea  to  include 
nature  as  the  selective  agent  instead  of  man.  The  essentials  of 
Darwin's  theory  are  as  follows  :  (a)  variation  occurs  in  all  organisms  ; 
(6)  universal  prodigality  of  reproduction  tends  to  overpopulation; 
(c)  a  struggle  for  existence  results,  which  tends  to  check  overpopula- 
tion ;  (d)  survival  of  the  best  adapted  to  survive,  and  the  elimination 
of  the  unsuccessful,  follows  the  struggle  for  existence ;  (e)  the  life- 
saving  qualities  so  selected  by  nature  are  transmitted  to  the  offspring 
and  become  the  cumulative  heritage  of  the  race ;  (/)  hereditary 
characteristics  acquired  through  natural  selection  are  prevented  by 
isolation,  either  geographical  or  physiological,  from  cancellation  or 


THE  EPIC  OF  EVOLUTION  519 

swamping  with  parent  stocks ;    and  finally  (g)  result  in  a  newly 
ADAPTED  SPECIES.     Somc  brief  expansion  of  these  six  points  follows. 

Variation 

Darwin  started  out  with  the  universally  observable  fact  of  varia- 
tion among  organisms  as  an  axiom.  Unlike  Lamarck,  he  did  not 
make  any  particular  attempt  to  find  out  the  underlying  causes  of 
variation.  He  pointed  out,  how^ever,  that  even  things  so  apparently 
alike  to  the  casual  observer  as  a  flock  of  sheep  invariably  reveal  in- 
dividual differences  to  the  shepherd  who  knows  his  sheep.  Each 
structural  feature  of  an  organism  may  exhibit  variation,  making 
an  enormous  range  of  variability  possible  among  a  group  of  similar 
individuals.  Variations  are  of  different  kinds  so  far  as  they  affect 
the  survival  of  individuals.  Some  are  useful  in  survival,  some  are 
indifferent,  and  some  are  either  harmful  or  even  lethal,  "There  is 
none  perfect,  no  not  one." 

Organisms  do  not  vary  in  order  to  become  better  adapted  to  their 
environment,  as  Lamarck  assumed  in  the  case  of  his  fantastic  deer 
that  became  a  giraffe,  but  they  may  be  better  adapted  to  the  en- 
vironment as  a  result  of  the  occurrence  of  variation. 

Overpopulation 

Both  Darwin  and  Wallace  were  much  impressed  by  the  writings 
of  an  English  clergyman,  Thomas  Malthus  (1766-1834).  This  math- 
ematically inclined  gentleman,  who  lived  in  the  prolific  days  of  large 
families,  was  much  concerned  by  the  fact  that  mankind  was  ap- 
parently increasing  faster  than  the  food  supply.  Darwin  goes  to 
considerable  length  to  point  out  that  every  organism  produces  many 
more  offspring  than  can  possibly  grow  to  maturity,  even  under  the 
most  favorable  conditions.  A  single  toadstool,  for  example,  may 
easily  produce  a  million  spores,  while  a  termite  queen  can  furnish 
an  average  of  an  egg  a  minute  for  a  year  at  a  time.  Even  the  animals 
breeding  most  slowly  would  require,  if  all  their  young  succeeded  in 
becoming  adult,  only  a  few  centuries  of  unlimited  geological  time  to 
overrun  the  entire  world.  Yet,  in  general,  organisms  do  no  more 
than  hold  their  own  year  after  year,  except  occasionally  when  the 
''balance  of  nature"  gets  upset,  and  plagues  of  grasshoppers,  star- 
lings, weeds,  gypsy  moths,  and  what  not,  flare  up  locally  for  a  limited 
period. 

H.  w.  H.  —  34 


520  THE  CHANGING  WORLD 

Struggle  for  Existence 

As  a  consequence  of  the  prodigality  of  overproduction,  there  follows 
a  struggle  for  existence,  which  is  simply  the  result  of  an  effort  on  the 
part  of  every  creature  to  live  and  leave  descendants.  This  struggle 
may  be  against  environmental  conditions,  between  individuals  of  the 
same  species,  or  between  individuals  of  different  species. 

Bumpus  describes  a  case  of  the  first  sort  in  which  some  sixty 
English  sparrows  out  of  a  colony  wintering  in  a  church  belfry 
perished  in  a  sleet  storm.  When  they  were  statistically  compared 
with  an  equal  number  of  survivors  with  respect  to  ten  measurable 
anatomical  features,  it  was  found  that  those  which  perished  in  the 
struggle  for  existence  under  the  adverse  environment  of  the  sleet 
storm  were  the  most  variable  ones  at  either  extreme,  that  is,  the 
anatomical  geniuses  and  dullards,  while  the  conservative  average 
ones  survived. 

An  example  of  the  second  kind  of  struggle  for  existence  is  found  in 
the  competition  between  a  parent  plant  and  its  offspring  for  moisture, 
standing  room,  air,  light,  and  nutriment.  Most  plants  have  well- 
known  devices  for  lessening  this  competition  by  scattering  their  seeds 
outside  the  immediate  parental  environment.  Among  animals,  as 
also  even  among  humans,  there  are  all  sorts  of  inducements  to  make- 
the  young  shift  for  themselves  and  not  to  continue  to  live  off  their 
parents. 

The  cobweb  house-spider,  Theridium,  hangs  up  a  little  pear-shaped 
woven  bag  with  several  dozen  tiny  eggs  in  it.  When  these  hatch  out 
within  the  bag,  there  is  nothing  for  the  young  spiders  to  eat  except 
brothers  and  sisters,  which  they  proceed  to  devour.  The  first  ones  to 
hatch  have  a  decided  advantage,  and  finally,  only  two  or  three  of  the 
whole  lot  triumphantly  emerge  out  of  the  woven  bag.  The  worse 
a  spider  is  ethically,  according  to  human  standards,  the  better  that 
spider  is  as  a  spider.  It  is  thus  seen  that  not  only  the  movements 
of  the  heavenly  bodies  are  subject  to  Einstein's  law  of  relativity,  but 
that  ethics  are  also. 

There  is  finally  an  age-long  struggle  also  between  carnivores  and 
their  prey,  and  between  different  organisms  of  all  sorts  for  food,  and 
for  whatever  else  is  necessary  for  the  maintenance  of  life.  It  is  not 
all  competition,  however,  since  co-operation  frequently  enters  into  the 
struggle  for  existence,  as  is  instanced  by  the  mutual  protection  secured 
in  flocks  and  herds.     This  gives  an  altruistic  touch  to  the  picture. 


THE   EPIC   OF  EVOLUTION  521 

Moreover,  the  struggle  for  existence  is  not  necessarily  a  cruel,  bloody, 
hand-to-hand  encounter.  On  the  contrary  it  is  unconscious  in  most 
cases,  and  when  death  comes  it  is  usually  during  the  earlier  stages  of 
life,  painless  and  without  worrisome  premonition  or  warning.  Na- 
ture's ways  are  simply  the  way  things  are,  wholesome  and  innocent, 
and  not  tinged  either  with  the  bitterness  of  human  hate  or  with 
sweet  sentimentality.  It  is  wise  to  remember  that  most  of  the  sup- 
posed joys  and  sorrows  of  animals  and  plants  are  quite  beyond 
our  ken. 

Survival  and  Elimination 

Left  to  herself,  Nature  either  "mends  or  ends."  The  result  of  the 
struggle  for  existence  is,  in  most  instances,  the  survival  of  the  fittest, 
that  is,  of  those  best  adapted  to  cope  with  the  circumstances  to  which 
they  are  subjected.  Stated  another  way,  it  is  the  elimination  of  the 
unfit,  namely,  those  that  fail  to  make  good.  Both  are  processes  that 
tend  to  provide  better  ancestors  for  succeeding  generations. 

It  is  not  always  the  ''fittest"  by  any  means  that  survive,  for  the 
best  do  not  invariably  succeed  in  living.  Sometimes  it  is  the  lucky 
ones  rather  than  the  best.  When  a  whalebone  whale,  for  example, 
strains  out  a  million  microscopic  crustaceans  from  the  sea-water 
in  taking  one  gigantic  swallow  of  the  animated  sea-soup  which 
constitutes  its  food,  those  that  escape  are  not  necessarily  the  best 
fitted  to  survive. 

The  more  we  examine  details,  however,  the  fewer  are  the  cases 
in  which  there  does  not  appear  some  factor  of  structure  or  be- 
havior that  plays  a  determining  part  in  survival  or  elimination. 
Sudden  environmental  changes  usually  result  disastrously  in  the 
extinction  of  organisms,  while  gradual  changes  tend  to  allow  latent 
adaptive  possibilities  in  plastic  plants  and  animals  to  come  into 
expression. 

Specialization  is  hazardous,  because,  although  by  means  of  it  an 
organism  may  become  better  fitted  to  one  set  of  conditions,  it  results 
in  a  loss  of  plasticity  and  of  the  organic  resources  necessary  to  meet 
changes  successfully.  Better  adaptation  means  having  more  re- 
sources for  survival.  The  great  group  of  insects,  for  example,  have 
gained  their  dominance  in  the  animal  world,  as  demonstrated  by  their 
great  numbers  and  diversity,  partly,  without  doubt,  because  of  their 
small  size,  short  life-cycle,  and  infinite  variety,  all  factors  that  have 
aided  them  to  survive. 


522  THE  CHANGING  WORLD 

Inheritance 

New  species  are  not  formed  by  survival  alone.  They  are  only  sorted 
out  in  that  way  from  variations  that  have  appeared. 

After  potentially  better  parents  have  been  "selected"  through  the 
processes  of  the  survival  of  the  fittest  and  the  ehmination  of  the 
unfit,  then  some  effective  way  must  be  found  for  the  transmission  of 
these  life-winning  qualities  to  the  next  generation,  or  there  can  be 
no  evolution  of  the  race.  Here  enters  again  the  old  question  of 
whether  the  cumulative  acquisitions  of  a  lifetime  are  transmissible, 
as  Lamarck  held,  or  whether  there  is  some  other  possible  way  to  get 
from  Ameba  to  man. 

Although  no  doubt  Darwin  sensed  something  of  the  uncertain 
nature  of  acquired  characteristics,  he  did  not  deny  their  adequacy 
as  a  means  of  evolutionary  advance,  and  in  that  regard  Darwinism 
offers  no  improvement  over  Lamarckianism.  What  he  did  do  was 
to  emphasize  the  importance  of  inborn  rather  than  environmental 
characteristics,  as  of  greater  value  in  the  selective  process.  It  is  not 
so  much  what  an  animal  becomes  or  accomplishes  in  its  lifetime 
that  is  of  hereditary  importance  to  the  offspring,  as  what  it  has  within 
itself  to  accomplish.  Among  human  kind,  success  in  life  may  be  more 
of  a  family  affair  (heredity)  than  a  matter  of  education  (environment 
and  training).  Inherent  possibilities,  whatever  their  origin,  are 
plainly  transmissible,  and  furnish  the  needful  material  on  which 
selection  may  act  for  cumulative  improvement. 

Darwin  tried  to  imagine  how  acquired  characters  could  become 
inherent,  and  so  transmissible  along  with  other  hereditary  character- 
istics. To  this  end  he  elaborated  his  supplementary  pangenesis  hy- 
pothesis, which  briefly  is  that  specific  determiners,  or  pangenes,  are 
formed  by  every  part  of  the  body.  These  pangenes,  like  instructed 
delegates  representing  various  constituencies,  collect  together  to  make 
up  the  germ  cells  from  which  a  new  individual  arises.  When  such 
germ  cells  unfold  their  possibilities  in  development,  every  part  of  the 
parental  body,  including  acquired  characters,  is  represented,  and 
consequently  may  reappear  in  the  new  individual. 

Pangenesis  was  a  brilliant  attempt  to  strengthen  the  weakest  link 
in  the  chain  of  explanation  of  how  natural  selection  might  give  rise  to 
new  species.  There  are  too  many  ifs,  however,  to  this  delightfully 
simple  hypothesis.  It  must  be  remembered  that  it  was  suggested 
before  the  astonishing  story  of  the  chromosomes  was  known,  and 


THE  EPIC  OF  EVOLUTION  523 

before  Mendel  and  his  followers  had  laid  bare  the  essential  mechanism 
of  heredity.  In  a  later  paragraph,  reference  will  be  made  to  how 
Darwin's  great  German  disciple,  Weismann,  came  to  the  rescue  and 
made  the  hypothesis  of  pangenesis  unnecessary. 

Isolation 

"The  nearest  relative  of  any  species  is  not  to  be  found  in  the  same 
area,  nor  in  a  far  distant  area,  but  in  a  nearby  area,  separated  from 
it  by  barriers."  This  is  the  Law  of  Isolation  as  formulated  by 
David  Starr  Jordan.  Unless  some  sort  of  isolation  prevents  or 
minimizes  the  swamping  effect  of  promiscuous  interbreeding,  a 
newly  "selected"  species  has  difficulty  in  maintaining  its  indepen- 
dence. 

There  are  first  of  all  geographic  barriers  that  lead  to  isolation,  as, 
for  example,  the  water  barrier  when  continental  islands  are  cut  off 
from  the  mainland.  In  such  cases,  since  the  island  types  can  no 
longer  breed  with  the  continental  forms  from  which  they  arose,  there 
is  furnished  an  opportunity,  due  to  isolation,  for  them  to  maintain 
the  modifications  which  make  them  different  species.  Oceanic  islands 
also  illustrate  the  part  isolation  plays  in  establishing  new  species. 
For  instance,  the  volcanic  island  of  Oahu,  on  which  Honolulu  is 
situated,  is  fluted  with  valleys  as  the  result  of  erosion,  and  each 
valley,  as  Gulick  has  shown,  has  its  own  peculiar  species  of  land  snail 
of  the  genus  Achatinella.  These  snails  live  in  trees  and  are  isolated, 
each  species  in  its  own  valley,  because  the  mountainous  ridges  be- 
tween the  valleys  furnish  a  barrier  to  their  intermingling  with  each 
other. 

In  addition  to  geographic  barriers  there  are  biological  harriers  that 
provide  isolation  for  newly  formed  species,  protecting  them  from 
the  leveling  effects  of  mixture  with  the  parental  stocks.  Plants, 
for  example,  may  maintain  their  individuality,  even  while  remaining 
in  the  same  habitat  with  contaminating  relatives,  by  practicing 
self-fertilization,  or  by  establishing  a  different  period  of  sexual 
maturity. 

There  are  a  dozen  different  species  of  albatross  in  the  Southern 
Hemisphere  which  mingle  freely  throughout  the  whole  range  of  their 
wanderings  except  during  the  breeding  season,  when  the  members 
of  each  species  are  segregated  in  their  own  quarters  to  reproduce. 
This  behavior  is  true  of  migrating  birds  in  general  so  that,  so  far  as 
breeding  goes,  there  is  virtual  isolation  among  them. 


524  THE   CHANGING   WORLD 

Dr.  Vernon  L.  Kellogg,  an  authority  on  bird-lice  (Mallophaga),  is 
acquainted  with  several  hundred  species  that  live  parasitically  among 
the  feathers  of  birds.  He  finds  that  nearly  every  kind  of  bird  enter- 
tains its  own  particular  species  of  bird-lice.  Since  birds  of  different 
species  in  their  aerial  activities  do  not  often  come  into  bodily  contact, 
these  wingless  bird-lice  are  isolated,  as  if  on  an  island,  and  each  species 
is  passed  around  among  nest  mates  of  bird  hosts  of  one  kind.  This 
peculiar  type  of  isolation  on  specific  hosts  helps  to  explain  why 
so  many  different  sorts  of  Mallophaga  have  evolved  and  maintained 
their  distinctive  differences. 

The  Mutation  Theory  of  DeVries 

Darwin  devoted  a  large  portion  of  The  Origin  of  Species  to  a  dis- 
cussion of  anticipated  objections  to  the  theory  of  natural  selection. 
An  attempt  to  review  these  controversial  matters  is  aside  from  the 
purpose  of  this  book.  They  form  a  pile  of  straw  that  has  been  thor- 
oughly threshed  over,  not  only  by  Darwin  himself  but  by  biologists 
generally.  Suffice  it  to  say  that,  after  all  the  objections  to  this  theory 
have  been  considered,  Darwin's  contribution  to  the  fundamental 
problem  of  evolution  remains  an  enduring  monument  to  his  genius, 
the  influence  of  which  extends  far  beyond  the  realm  of  biology. 

One  of  the  difficulties  that  has  often  been  emphasized  has  to  do  with 
variations,  which  are  the  indispensable  materials  for  selection  to  act 
upon.  The  kind  of  variation  on  which  Darwin  depended  was  the 
minute  modifications  everywhere  evident.  Natural  selection  does  not 
satisfactorily  explain  how  such  slight  variations  can  become  life- 
determining.  In  order  to  assume  importance  in  the  survival  of  the 
individuals  possessing  them,  that  is,  to  become  of  selective  value, 
these  slight  variations  must  accumulate  and  increase  until  they 
acquire  a  life-and-death  significance  in  the  struggle  for  existence. 
The  greenness  of  a  katydid,  for  example,  is  a  life-saving  feature  which 
renders  its  possessor  largely  invisible  to  its  bird  enemies,  against  a 
background  of  green  leaves.  A  slight  departure  towards  greenness 
from  the  ancestral  conspicuous  brown  color  of  the  species  would  be 
of  no  use  in  concealment.  Natural  selection  cannot  take  hold  until 
there  is  enough  greenness  developed  to  provide  safety  by  concealment. 
One  suggestion  is  that  useless  variations  are  often  correlated  with 
useful  ones  and  so  are  rescued  from  oblivion,  just  as  in  a  "landshde" 
during  a  political  election  many  insignificant  minor  officials  arrive 
in  office  on  the  coat-tails  of  the  real  winner. 


THE  EPIC  OF  EVOLUTION  525 

Another  theoretical  escape  from  the  dilemma  is  furnished  by.  the 
assumption  that  the  variations  employed  in  survival  are  not  all  made 
out  of  accumulations  of  the  useless  sort,  which  are  often  transitory 
effects  of  the  environment  and  not  heritable,  but  are  a  particular 
kind  of  variation  that  is  of  hereditary  significance  from  the  start. 
Evidence  of  the  existence  of  such  a  distinctive  kind  of  germinal 
variation,  whose  transmissibility  is  not  questionable,  has  been 
furnished  by  the  Dutch  botanist  Hugo  DeVries  (1846-1935)  in  his 
book  entitled  Die  Mutationstheorie.  By  chance  DeVries  discovered 
among  some  wild  evening  primroses,  Oenothera,  certain  individuals 
so  decidedly  different  from  the  original  type  that  they  would  be 
regarded  by  a  botanist  as  distinct  species  if  the  history  of  their 
origin  was  not  known.  There  was  evidence  that  these  new  forms 
did  not  evolve  gradually,  but  that  they  appeared  suddenly  with  no 
warning  of  imminent  change.  Moreover,  when  isolated  they  repro- 
duced their  distinctive  characteristics.  Variations  of  this  kind  that 
breed  true  DeVries  called  mutations. 

It  is  now  known  that  the  occurrence  of  mutations  is  widespread 
among  both  plants  and  animals.  Several  hundred  distinct  mutations, 
for  example,  have  been  described  from  the  much-studied  fruit  fly, 
Drosophila,  alone.  Mutations  may  be  useless  or  useful  in  survival, 
but  in  any  case  they  are  heritable  and  thus  furnish  raw  materials 
for  the  selection  mill.  In  other  words,  all  mutations  must  still  run 
the  gauntlet  of  natural  selection. 

DeVries'  theory  made  it  clear  that  it  is  not  necessary  to  wait  for  the 
slow  accretions  of  insignificant  useless  chance  variations  to  provide 
characteristics  of  selective  value,  since  mutations  furnish  the  necessary 
materials  which  evolution  demands,  ready  made  and  transmissible. 
Thus,  existing  organisms  are  to  be  regarded  as  the  sum  of  the  muta- 
tions that  have  survived  since  the  dawn  of  life. 

The  reality  of  mutations  has  been  amply  demonstrated.  The 
causes  of  this  type  of  variation,  however,  are  still  a  matter  for  further 
study  and  investigation,  in  which  considerable  progress  has  already 
been  made.  It  is  likely,  furthermore,  that  the  mutations  of  DeVries 
do  not  represent  the  introduction  of  something  entirely  new.  but 
rather  a  new  combination  of  characters  already  present.  The  great 
service  of  DeVries'  work  lies  in  the  fact  that  the  explanation  of  the 
method  of  evolution  has  been  shifted  by  means  of  it  from  the  un- 
scientific field  of  argument  to  the  more  scientific  and  dependable 
field  of  experimentation. 


526  THE  CHANGING  WORLD 

Germplasm  Theory  of  Weismann 

August  Weismann  (1834-1914),  who  was  an  ardent  supporter  of 
Darwin,  went  straight  at  the  heart  of  another  difficulty,  which  not 
only  Darwin  himself  but  also  Lamarck  had  encountered,  namely, 
the  problem  of  the  manner  in  which  inheritance  takes  place. 

A  critical  examination  of  available  facts  convinced  Weismann  that 
only  inborn  characteristics  are  handed  on  from  generation  to  genera- 
tion, and  that  peculiarities  picked  up  by  parents  during  their  lifetime 
come  to  an  end  with  the  death  of  the  individual.  This  led  to  the 
formulation  of  the  Germplasm  Theory,  namely,  that  the  germ  cells 
from  which  the  individuals  arise,  and  which  are  the  bearers  of  the 
hereditary  possibilities,  are  quite  different  from  the  innumerable 
transitory  cells  that  make  up  the  rest  of  the  body.  According  to 
Weismann's  theory,  germinal  material  forms  a  continuous  chain, 
from  which  in  successive  generations  individual  organisms  tempora- 
rily develop.  The  germinal  material,  although  it  is  subject  to  death 
with  the  mortal  body  of  the  organism,  is  potentially  immortal,  because 
in  the  process  of  reproduction  it  may  continue  from  generation  to 
generation. 

This  conception  led  Weismann  to  question  the  possibility  of  the 
transmission  of  bodily  acquisitions  from  one  generation  to  another, 
since  the  avenue  of  transmission  is  by  way  of  the  germinal  stream  and 
not,  as  popularly  supposed,  from  one  body  to  another.  The  body  is 
simply  the  visible  expression  of  the  germinal  characters  handed  on 
from  its  ancestry,  and  for  which  it  serves  only  as  a  temporary  carrier. 
The  body  of  the  individual  does  not  produce  the  germ  cells,  as 
Darwin's  hypothesis  of  pangenesis  assumed,  but  the  germ  cells  produce 
the  body.  It  must  be  admitted  that  Weismann  did  a  thorough  job 
in  discrediting  the  supposed  inheritance  of  acquired  characters, 
for  today  biologists  are  quite  generally  agreed  that  such  inheritance 
does  not  occur,  or  if  it  does,  only  to  an  insignificant  extent.  The 
court  of  last  resort  for  those  who  are  unconvinced  is  appeal  to  further 
facts,  to  be  obtained  by  decisive  experimentation.  The  value  of 
Weismann's  speculative  thought  was  largely  due  to  the  fact  that  it 
stimulated  further  research  and  discovery. 

The  whole  course  of  evolution  thus  finally  resolves  itself  into  what 
occurs  in  the  unseen  germplasm,  as  opposed  to  what  takes  place  in  the 
visible  parts  of  the  body.  Selection  of  variations  of  any  sort  is  of 
importance  only  when  those  bodily  characters  are  recognized  as 


THE   EPIC   OF   EVOLUTION  527 

carriers  of  hereditary  qualities  that  have  come  down  the  long  ancestral 
line  through  the  continuous  germinal  stream. 

Other  Theories 

It  would  be  going  too  far  afield  to  attempt  here  to  review  all  of  the 
other  theories  that  have  been  advanced  to  account  for  evolution  in 
whole  or  in  part.  Some  of  these  are  subsidiary  to  the  theory  of 
natural  selection,  as,  for  instance,  Darwin's  own  theory  of  Pangenesis, 
already  mentioned,  and  also  his  theory  of  Sexual  Selection. 

Perhaps  the  largest  group  of  alternative  theories  of  descent  are 
those  which  center  around  the  idea  of  Orthogenesis.  These  theories 
hold  that  variation  is  not  qualitative  and  random  in  character  in 
every  direction,  but  quantitative,  that  is,  either  plus  or  minus  modifi- 
cations and  in  one  direction  only.  According  to  this  idea  variations 
form  a  determinative  series  that  goes  forwards  or  backwards  relent- 
lessly, with  little  reference  to  adaptation  and  in  spite  of  environmental 
influences.  Overspecialization,  as  in  the  case  of  the  gigantic  antlers 
of  the  extinct  Irish  elk,  finds  in  orthogenesis  an  easy  explanation, 
for  cumulative  variations  of  this  kind  may  gain  such  headway  in  one 
direction  that  they  overshoot  the  goal  and  lead  to  eventual  destruc- 
tion. 

Weismann  in  his  supplementary  theory  of  Germinal  Selection  and 
Wilhelm  Roux  (1850-1924)  with  his  Kamjpf  der  Teile,  or  struggle 
between  the  parts,  have  transferred  the  struggle  for  existence  from 
individuals  to  the  component  parts  of  individuals,  while  various 
vitalistic  attempts,  like  Bergson's  Elan  Vital,  and  George  Bernard 
Shaw's  Life  Force,  have  been  made,  which  dodge  the  whole  issue  by 
invoking  some  mystical  agency  that  is  beyond  the  reach  of  scientific 
testing  by  experiment. 

Conclusion 

Darwin's  great  service  was  that  he  formulated  a  plausible  explana- 
tion for  the  theory  of  descent  which  did  not  beg  the  whole  question 
by  resorting  to  the  supernatural.  "  ^Mother  Nature  "  is  not  a  directive 
personality  substituted  by  "ungodly  scientists"  for  the  supernatural 
Creator  of  all  things.  There  is  no  more  personality  in  natural  selec- 
tion than  there  is  in  the  wind,  which  "selects"  the  grain  from  the 
chaff.  Nor  is  there  necessarily  any  more  design,  any  more  purpose 
or  moral  bearing  to  natural  selection  than  there  is  in  the  action  of  the 
law  of  gravity,  or  in  the  shaping  of  water-worn  rocks  by  the  surf 


528  THE   CHANGING   WORLD 

at  the  seashore.  Natural  factors  that  can  be  observed  and  measured, 
and  whose  effects  can  be  predicted,  are  all  that  are  involved. 

Truth-seekers  do  well  to  exhaust  first  of  all  what  may  be  proven 
or  disproven  by  observation  as  well  as  by  experiment  with  natural 
things  and  processes  that  are  within  reach,  before  appealing  to  the 
supernatural,  which  lies  beyond  the  realm  of  science.  Natural  law 
is  an  observed  and  verifiable  sequence  of  events  that  is  dependable 
and  makes  the  prediction  of  future  events  possible  or  probable. 
For  example,  under  the  same  atmospheric  pressure  water  always  boils 
at  the  same  temperature,  you  can  depend  on  it.  On  the  other  hand, 
the  supernatural  is  an  interference  with  natural  sequences,  and  is 
neither  predictable  nor  dependable. 

It  is  not  the  scientific  way  of  disposing  of  difficulties  to  shake  the 
head  and  look  wise,  or  to  call  in  unknown  supernatural  aids,  as  long 
as  unexhausted  natural  resources  remain  at  hand.  Dr.  W.  E.  Ritter's 
wise  advice  to  scientists  might  well  be  taken  to  heart  by  everyone, 
"Investigate  things  as  they  are,  not  as  they  might  he,  or  ought  to  be." 
Darwin  did  just  this,  and  consequently  his  concept  of  the  "Origin 
of  Species  hy  Means  of  Natural  Selection"  is  much  more  than  an 
attempted  explanation  of  how  evolution  came  about.  It  is  a  model 
exposition  of  the  scientific  method  of  thinking,  which  finds  universal 
application  in  all  fields  of  human  endeavor.  Darwin  and  Abraham 
Lincoln  were  born  on  the  same  day.  Both  were  great  emancipators 
in  different  fields.  That  the  theory  of  natural  selection  falls  short 
in  certain  particulars  is  not  important.  It  has  served  its  purpose  in 
stimulating  and  giving  direction  to  further  investigation,  which  is 
what  makes  life  worth  living.  Any  theory  is  like  a  temporary 
scaffolding  to  be  discarded  after  the  building  is  erected,  for  if  it  is 
still  retained  intact,  it  may  obscure  the  building  itself. 

Robert  Boyle,  the  physicist  (1627-1691),  once  said  with  reference 
to  theories  in  general, 

"Having  met  with  many  things  for  which  I  can  assign  no  possible  cause, 
and  with  some  for  which  many  different  ones  might  be  alleged,  I  dare  speak 
positively  and  confidently  of  very  few  things  except  of  matters  of  fact." 

William  Harvey  (1578-1657),  who  discovered  the  circulation  of  the 
blood,  also  summed  up  the  scientific  attitude  in  these  words : 

"Some  .  .  .  persons  vainly  seek  by  dialectics  and  far-fetched  arguments, 
either  to  upset  or  to  establish  things  that  are  only  to  be  founded  on  ana- 
tomical demonstration,  and  believed  on  the  evidence  of  the  senses.     He 


THE   EPIC   OF   EVOLUTION  529 

who  truly  desires  to  be  informed  on  the  question  at  hand,  and  whether  the 
facts  alleged  be  sensible,  visible,  or  not,  must  be  bound  either  to  look  for 
himself  or  take  on  trust  the  conclusions  to  which  they  have  come  who  have 
looked,  and  indeed  there  is  no  higher  method  of  attaining  to  assurance  and 
certainty." 

In  conclusion  this  whole  section  of  the  theoretical  aspects  of  biology 
is  well  epitomized  by  Dr.  A.  D.  Mead. 

"The  centuries  of  biological  research  could  not  change  the  order  of  nature. 
The  increased  knowledge  may  not  even  mean  greater  wisdom  in  handling 
knowledge.  It  may  not,  perhaps,  though  it  ought  to,  make  men  more  sensi- 
tive to  the  wonder  of  it  all.  But  it  has  thoroughly  involved  man  in  the 
laws  that  govern  plants  and  animals  in  general,  and  has  deeply  altered  our 
conception  of  what  those  laws  are." 

SUGGESTED   READINGS 

Kellogg,  V.  L.,  Evolution,  D.  Appleton  Co.,  1924. 

A  popular  and  very  readable  account  of  the  history  and  evidence  of 

evolution. 
Kerr,  J.  G.,  Evolution,  The  Macmillan  Co.,  1926. 

The  book  is  written  for  beginners,  who  will  find  the  author's  reasoning 

easy  to  follow  and  understand. 
Lull,  R.  S.,  Ways  of  Life,  Harper  &  Bros.,  1925. 

An  excellent  book  for  the  layman  who  wishes  a  clear,  concise  statement 

of  the  scientific  data  relating  to  the  evidences  of  evolution  and  the  origin 

and  history  of  living  things. 
Parker,  G.  H.,  What  Evolution  Is,  Harvard  University  Press,  1925. 

A  clear,  brief  outline. 
Ward,  H.,  Evolution  for  John  Doe,  Bobbs-Merrill  Co.,  1925. 

Non-technical,  as  readable  and  interesting  as  a  detective  story. 


XXIII 


THAT  ANIMAL,   MAN   (ANTHROPOLOGY) 

Preview.  The  process  of  becoming  human  ■  Our  primate  cousins  •  The 
downward  ascent  of  man  •  The  consequences  of  an  upright  hfe  •  The  great- 
est wonder  in  the  world  •  FHnt  and  metal  history  •  Getting  the  upper  hand 
of  things  •  Gaining  ideas  and  passing  them  on  •  Skeletons  in  the  Pleistocene 
ice  chest:  Java  man;  Heidelberg  jaw;  Charles  Darwin's  neighbor;  the 
first  lady  of  China ;  the  Neanderthalers ;  wild  horse  hunters ;  reindeer 
hunters  •  Races  •  Passing  muster  •  The  biological  Garden  of  Eden  •  Sug- 
gested readings. 

PREVIEW 

"My  favorite,  and  I  might  say  my  only  study,  is  man." 

—  George  Borrow. 

If  a  board  sidewalk  belted  the  earth  at  the  equator,  and  the  entire 
present  human  population,  estimated  at  1,700,000,000,  should  fall 
into  a  lockstep  procession  on  it,  the  line  would  girdle  the  globe  some 
seventy  times.  The  sidewalk  would  need  to  be  at  least  one  hundred 
and  fifty  feet  wide  in  order  to  allow  the  procession  to  move,  and  even 
then  there  would  be  considerable  shoving  and  crowding,  and  countless 
toes  would  be  stepped  on. 

In  spite  of  the  fact  that  mankind  forms  one  of  the  most  recent 
species  to  be  evolved,  no  other  animal  is  so  widespread  over  the  earth, 
and  adapted  to  occupy  successfully  so  many  diverse  habitats  all  the 
way  from  "Greenland's  icy  mountains  to  India's  coral  strand." 

That  man  is  an  animal,  subject  to  the  same  biological  laws  as 
other  animals,  was  recognized  in  1755  by  Linnaeus  (1707-1778)  when 
he  included  Homo  sapiens  in  his  classification  of  animals,  although 
without  the  idea  of  relationship,  which  culminated  later  with  Darwin. 
Although  there  are  countless  detailed  evidences  of  animal  relationship, 
man  is  in  many  ways  quite  unique  and  stands  head  and  shoulders 
above  other  animals.  Man  is  the  only  animal  that  can  make  such 
a  claim  and  put  it  in  writing.  No  other  animal  can  communicate 
abstract  ideas.  No  other  animal  can  measure  the  distance  between 
the  stars,  or  build  a  steamboat,  or  speak  a  sentence,  or  compose  a 
symphony,  or  commit  a  sin  and  be  sorry  for  it,  or  levy  taxes,  or  take 
thought  for  the  morrow,  but  mankind  as  a  whole  can  do  them  all, 
and  much  besides. 

530 


THE  ANIMAL,    MAN    (ANTHROPOLOGY)  531 

The  science  of  man  is  called  Anthropology,  and  it  might  well  claim 
our  attention  for  more  than  the  space  which  it  is  possible  to  allot  to 
it  in  this  book,  for,  as  Professor  Shaler  (1841-1906)  once  wrote,  "The 
cry  of  what  is  man  from  the  Hebrew  singer  has  been  re-echoed  in  all 
ages  and  lands  wherever  men  have  attained  the  dignity  of  contem- 
plation." 

The  Process  of  Becoming  Human 

No  one  knows  the  total  duration  of  life  upon  the  earth,  not  even 
paleontologists,  but  evidences  are  unmistakable  that  millions  of 
years  have  elapsed  since  the  dawn  of  life,  when  animals  and  plants 
first  appeared.  It  does  not  matter  that  the  estimates  of  experts  are 
at  great  variance.  The  fact  remains  that  an  enormous  stretch  of 
years  was  involved  in  the  evolutionary  preface  to  mankind. 

So  far  as  is  known,  man  emerged  from  the  evolutionary  welter  only 
about  500,000  years  ago,  although  a  long  chain  of  events  extending 
over  millions  of  years  led  up  to  his  advent.  Someone  has  estimated 
that  if  a  moving  picture  of  the  successive  geological  ages,  in  which 
there  is  a  known  fossil  record  of  life,  could  be  speeded  up  and  com- 
pressed into  a  continuous  show  of  fourteen  hours  beginning  at  10  a.m., 
man  would  appear  first  on  the  screen  about  five  minutes  before  mid- 
night. Such  a  picture  would  begin  with  simple  unicellular  organisms, 
at  first  neither  plant  nor  animal  but  gradually  evolving  into  one  or 
the  other,  and  followed  eventually  by  a  multitudinous  host  of  proto- 
zoans. These  emerging,  and  later  becoming  diversified,  would  be 
seen  to  foretell  in  miniature  something  of  future  possibilities  by  reason 
of  having  worked  out,  even  with  a  body  made  up  of  only  a  single  cell, 
varieties  resembling  superficially  Lilliputian  Hydras,  worms,  snails, 
sea-urchins,  crabs,  and  other  higher  forms  of  life.  Since  protozoans 
in  reproduction  habitually  produce  twins  by  the  process  of  fission, 
the  story  continues  with  the  rise  of  the  long  dynasty  of  the  metazoans, 
or  multicellular  forms,  that  developed  when  protozoan  twins,  like 
"Siamese  twins,"  got  the  habit  of  hanging  together. 

Next  follows  the  pattern  of  sponges,  each  a  loosely  aggregated  mob 
of  individual  cells,  which,  as  time  went  on,  in  more  complicated  forms 
higher  up  the  scale,  became  organized  and  differentiated  into  orderly 
tissues,  thus  making  possible  the  development  of  organs.  Then  coe- 
lenterates,  lowly  plantlike  animals  generally  of  sessile  habit,  grad- 
ually became  free-swimming  and  adventurous,  while  their  radial  type 
of  symmetry  in  consequence  was  in  due  time  transformed  into  the 


532  THE   CHANGING   WORLD 

bilateral  type,  making  possible  the  revolutionary  head  end.  In  this 
device  a  suitable  home  was  furnished  for  the  brain  when  it  should 
appear,  with  exploratory  sense  organs  near  by,  placed  handily  to  best 
advantage  for  receiving  impressions  from  the  immediate  surroundings. 
Following  this  by  the  device  of  metamerism,  which  is  a  division  of 
the  body  into  segments,  primitive  fiatworms  took  on  annelid  charac- 
teristics, thus  acquiring  flexibility  after  the  manner  of  a  train  of  cars, 
and  also  gaining  survival  insurance  against  accidental  loss  of  parts. 

Locomotor  legs  soon  came  in  to  lift  the  long  crawling  body  off  the 
ground,  thereby  much  lessening  friction  in  traveling  about.  Legs 
developed  joints,  adding  the  mechanical  advantage  of  levers.  Various 
experiments  in  the  number  of  legs  were  tried  out  with  a  result  of 
increasing  efficiency.  Myriapods  and  centipedes  had  too  many. 
Crustaceans  began  a  reduction.  Spiders  and  their  allies  brought 
the  number  down  to  eight,  while  the  great  group  of  insects  finally 
settled  upon  six  legs  as  the  prevailing  fashion.  It  remained  for  verte- 
brates to  get  along  at  first  with  only  four  legs,  one  under  each  corner 
of  a  horizontal  body,  like  the  legs  of  a  table.  Eventually  in  the  case 
of  birds  and  man,  and  some  other  vertebrates,  only  a  single  pair  of 
legs  remains  at  the  end  of  an  upright  vertical  body.  It  is  likely  that 
evolution  has  reached  its  limit  with  reduction  to  one  pair  of  locomotor 
legs,  since  a  one-legged  animal  would  obviously  be  at  a  disadvantage. 
However,  with  these  legs  we  have  run  ahead  of  our  story. 

When  skeletal  parts  for  muscular  attachment  first  developed, 
making  locomotor  legs  workable,  the  skeleton  was  an  exoskeleton  on 
the  outside  of  the  body.  Being  secreted  by  the  underlying  tissues, 
and  consequently  a  dead  structure  unyielding  and  hampering  to  the 
enlarging  body  within,  it  is  soon  outgrown  and  has  to  be  discarded, 
frequently  at  considerable  peril  and  physiological  expense,  to  allow 
for  future  growth.  It  was  a  great  day  for  us  when  our  ancestors 
put  their  skeletons  inside  the  body  and  became  vertebrates.  The 
vertebrate  endoskeleton  can  remain  alive  and  continue  growing,  and 
can  thus  keep  up  with  the  demands  of  increasing  body  size. 

Many  other  evolutionary  experiments  were  tried  out  in  the  course 
of  time,  resulting  in  the  establishment  of  the  great  major  phyla  of  the 
animal  kingdom,  but  it  was  the  vertebrate  idea  which  finally  forged 
ahead  upward  toward  man. 

The  fishes  served  a  long  apprenticeship  as  the  lowest  vertebrates, 
principally  in  the  waters  of  the  great  oceans  that  cover  most  of  the 
globe,  until  eventually  there  developed  those  adventurous  pioneers, 


THE   ANIMAL,   MAN    (ANTHROPOLOGY)  53.3 

the  amphibians,  that  emerged  upon  land.  Following  thi.s  notable  ac- 
complishment, modifications  and  adaptations  came  thick  and  fast, 
or  the  aquatic  amphibians  could  never  have  met  the  demands  of  the 
new  land  habitat  and  become  successful  settlers  there.  They  never 
did  make  a  great  success  of  it,  for  the  transition  from  water  to  land 
was  so  gigantic  an  enterprise  that  the  poor  creatures  barely  succeeded 
in  entering  even  into  the  edge  of  the  Promised  Land.  Consequently 
they  have  always  been,  and  remain  today,  the  smallest  and  most 
helpless  of  all  the  vertebrate  groups,  but  to  their  glory  be  it  said  that 
they  did  mark  the  road  and  pave  the  way  over  which  advancing 
hordes  of  reptilian  successors  were  enabled  to  press  on  to  greater 
achievements.  We  do  not  appreciate  the  part  reptiles  have  played  in 
the  making  of  man,  partly  by  reason  of  the  insignificance  of  the  rep- 
tiles living  today.  Only  a  few  cold  blooded  crawling  snakes,  repulsive 
crocodiles,  furtive  lizards,  and  sluggish  turtles  are  left  to  remind  us 
of  a  reptilian  aristocracy  that  dominated  the  Mesozoic  world  for  at 
least  ten  million  years,  and  laid  the  foundation  for  the  next  great 
stride  upward.  In  passing,  we  may  remind  ourselves  that  a  good  deal 
happened  during  the  gray  Mesozoic  millenniums,  of  which  we  have 
some  few  hints  in  the  fossil  remains  of  extinct  reptiles,  but  we  must 
not  now  be  diverted  from  our  upward  quest  by  the  stirring  saga  of 
those  particular  past  events,  marking  as  they  did  the  rise  and  domi- 
nance of  life  in  the  Reptilian  age. 

There  were  two  ways  of  escape  out  of  this  long-drawn-out  ancient 
reptiUan  "civilization,"  namely,  by  way  of  the  birds,  or  by  way  of  the 
mammals.  Birds  do  not  immediately  concern  us  in  this  connection, 
for  the  reason  that  they  have  sacrificed  every  other  future  prospect 
in  becoming  specialized  for  flight  in  the  air,  and  in  conquering  the  new 
aerial  realm.  Now  at  last  they  find  themselves  trapped  in  a  lane 
that  has  no  turning,  and  apparently  without  any  future  evolutionary 
outlet.     They  certainly  did  not  lead  the  way  to  man. 

Mammals,  on  the  other  hand,  chose  the  better  part  by  retaining  a 
wider  range  of  evolutionary  resources,  and  meanwhile  by  putting  off, 
to  a  considerable  extent,  the  narrowing  effects  of  organic  special- 
ization. The  first  mammals  were  small  insignificant  creatures,  that 
were  no  doubt  looked  down  upon  or  ignored  by  their  reptilian  con- 
temporaries. They  possessed,  however,  certain  secrets  of  warm- 
bloodedness,  prolonged  parental  care,  and  other  physiological  and 
anatomical  innovations,  of  which  the  cold-blooded  reptiles,  with  their 
lesser  brains,  never  dreamed. 


534 


THE  CHANGING  WORLD 


Changes  and  advances  in  many  directions  came  thick  and  fast, 
once  the  mammaUan  idea  was  introduced.  Out  of  all  the  emerging 
orders  of  mammals  it  was  probably  the  insedivores  that  became  the 
forerunners  of  the  primates  to  which  man  belongs.     These  lowly 

creatures,  of  which  the  shrews, 
moles,  and  the  European 
hedgehog  are  living  representa- 
tives, somehow  kept  within 
the  broad  highway  of  struc- 
tural generalization,  not  being 
lured  into  blind  alleys  of  spe- 
cialization as  was  the  fate  of 
the  hoofed  ungulates,  bats,  and 
leviathanlike  whales.  Cer- 
tain of  these  insectivores,  the 
tree  shrews,  quite  unlike  their 
modern  burrowing  cousins,  the 
moles,  took  to  arboreal  life, 
thus  gaining  shelter  and  escap- 
ing in  some  degree  from  their 
terrestrial  enemies.  Accord- 
ing to  certain  biologists,  they 
initiated  the  Grand  Order  of 
the  Primates,  from  which  man 
has  finally  emerged. 

So  it  came  about  that  all 
along  the  long  trail  "from 
Ameba  to  man"  there  were 
innumerable  casualties.  Ex- 
ploring parties  left  the  main 
line.  Many  were  lost,  but  some  have  kept  on  in  diverging  pathways 
until  today,  although  separated  from  the  main  highway  that  has  led 
up  to  man.  The  final  episode  in  the  ascent  of  man  concerns  the 
story  of  the  primates. 

Our  Primate  Cousins 

Since  man  is  the  only  animal  that  can  wTite  a  book,  we  find  the 
mammalian  Order  to  which  man  belongs  naively  designated  as 
Primates,  or  the  first.  If  a  horse  could  make  a  classification  of 
mammals,  no  doubt  the  Ungulates  would  be  placed  first,  for  they 


American  Museum  of  Natural  History 

Tupaia,  the  tree  shrew,  that  discovered 
the  possibilities  of  tree  hfe  and  thus  became 
the  probable  forerunner  of  our  arboreal 
ancestors. 


THE   ANIMAL,    MAN    (ANTHROPOLOGY) 


535 


have  the  most  highly  speciahzed  feet  of  any  vertebrate,  and  if  whales 
could  express  themselves,  they  would  naturally  claim  first  place  for 
the  Cetacea,  on  account  of  their  dominant  size  and  extreme  special- 
ization.    Thus  are  the  advantages  of  literacy  evident  ! 

Primates  include  the  arbo- 
real lemurs,  the  curious  hob- 
goblin Tarsius,  monkeys,  apes, 
and  man.  The  greatest  num- 
ber of  primate  species  are 
lemurs,  which  first  appeared 
some  three  million  years  ago 
in  Eocene  times.  According 
to  Dr.  D.  G.  Elliot,  there  are 
eighty-three  species  living  to- 
day, mostly  to  be  found  in 
the  forests  of  Madagascar,  as 
well  as  many  extinct  species 
known  only  by  their  fossil 
remains.  Living  lemurs  vary 
in  size  from  that  of  a  mouse 
to  that  of  a  cat,  although  the 
largest  known  extinct  repre- 
sentative was  as  big  as  a 
donkey. 

In  habit  the  ghostly  "wailing  lemurs,"  which  are  practically  con- 
fined to  tree-tops,  are  mostly  agile  night-prowlers  that  avoid  trouble 
by  retirement  during  the  daytime.  Although  their  place  is  unmis- 
takably among  the  primates,  they  exhibit  certain  anatomical  features 
of  a  non-primate  character,  such  as  a  low  type  of  brain,  the  absence 
of  a  bony  back  wall  to  the  orbits  of  the  eyes,  and  a  reminiscent  claw 
on  the  second  toe  of  each  hind  foot,  while  on  all  the  other  digits  the 
claws  are  flattened  out  into  primate  nails.  The  remoteness  of  their 
relationship  to  other  primates  is  further  indicated  by  the  fact  that 
unlike  other  primates  they  have  an  ungulatelike  placenta,  possess 
groin-nipples  as  well  as  breast-nipples,  and  habitually  prockice  several 
young  at  a  time.  Of  particular  interest  to  the  anthropologist  is  the 
related  genus  of  Tarsius,  comprised  of  a  half  dozen  species  living  in 
Borneo  and  Java.  Tarsnis  is  not  much  larger  than  a  rat.  It  sits 
up  and  takes  notice  with  its  enormous  eyes  directed  straight  in  front, 
an  arrangement  that  is  made  possible  by  reason  of  the  snout  and  jaws 
H.  w.  H.  — ■  35 


\tu'  York  Zoological  Suciely 

A  tree-dwelling  lemur.     What  different 
uses  for  such  a  tail  ? 


536 


THE   CHANGING   WORLD 


being  very  much  shortened,  thus  allowing  the  large  laterally  placed 
eyes  to  swing  around  in  front  into  a  spectroscopic  position.  When 
this  curious  animal  wishes  to  look  behind  itself,  instead  of  rotating  the 

eyeballs,  its  whole  head  swivels 
around  in  an  alarmingly  weird 
and  dislocated  manner.  Mem- 
bers of  this  genus  are  the  only 
primates  with  a  single  incisor 
tooth  in  each  half  of  the  lower 
jaw,  other  primates  having  two. 
Their  fingers  and  toes  are  much 
elongated  and  terminate  not  only 
with  nails  instead  of  claws,  but 
also  with  adhesive  disks,  which 
are  very  useful  in  arboreal  life. 
On  account  of  their  long  fingers 
they  are  decidedly  hand-feeders, 
and  are  also  able  to  take  hold 
of  objects  and  to  bring  them  up 
close  to  their  staring  eyes  for  in- 
spection, while  with  the  hind  legs, 
adapted  for  hopping  and  spring- 
ing from  limb  to  limb,  they  some- 
what resemble  miniature  kangaroos  in  their  movements.  According 
to  some  authorities,  these  grotesque  little  animals- are  a  direct  link  in 
the  evolutionary  chain  leading  to  man,  while  other  lemurs,  monkeys, 
and  apes  are  held  to  be  side  deflections  from  the  main  line.  The 
possible  relationship  of  Tarsius  to  mankind  is  based  mostly  upon 
anatomical  evidence,  too  technical  to  be  enlarged  upon  here.  The 
interested  reader  is  urged  to  look  beyond  the  pages  of  this  book  for 
further  details. 

Monkeys  form  two  great  groups,  those  in  the  New  World  being 
more  primitive  than  their  Old  World  relatives.  The  New  World 
broad-nosed  (platyrrhine)  monkeys  do  not  have  an  opposable  thumb, 
but  are  partly  compensated  for  this  handicap  by  possessing  a  pre- 
hensile tail  that  serves  them  as  a  fifth  hand  in  their  aerial  adventures 
in  the  tree-tops.  They  have  a  generous  mouthful  of  thirty-six  teeth. 
Marmosets  are  the  smallest  of  the  New  World  monkeys,  and 
"howlers,"  the  prima  donnas  of  American  tropical  forests,  are  the 
largest,  with  spider  monkeys,  capuchins,  and  other  species  inter- 


Tarsiiis,  the  goggle-eyed  lemur  which 
arrests  the  attention  of  anthropological 
ancestor  hunters. 


THE   ANIMAL,   MAN    (ANTHROPOLOGY) 


537 


mediate  in  size.  The  Old  World  narrow-nosed  (catarrhine)  monkeys 
have  a  rather  small,  more  or  less  opposable  thumb  and  big  toe,  a 
stiiblike  tail  less  useful  than  ornamental,  and  thirty-two  teeth,  the 
same  number  as  in  man.  To  this  group  belong  macaques,  mandrills, 
baboons,  and  proboscis  monkeys,  with  some  other  species. 

There  are  four  kinds  of 
living  apes  (Anthropoids) , 
namely,  gibbons,  orang- 
utans, chimpanzees,  and 
gorillas.  The  anatomical 
gap  separating  these  apes 
from  monkeys  may  be  as 
great,  if  not  greater,  than 
that  between  apes  and 
man. 

The  gibbons,  natives  of 
Southeast  Asia,  Borneo, 
Sumatra,  and  Java,  walk 
quite  upright  on  the 
groimd,  often  swinging 
along  by  using  their  arms, 
which  are  of  enormous 
length,  like  a  pair  of 
crutches.  They  are  most 
at  home  in  trees,  however, 
where  they  travel  with 
astonishing  rapidity  and 
acrobatic  skill.  This  method  of  locomotion  is  graphically  described 
by  W.  T.  Hornaday  :  ^ 

Tlie  gibbon  "progresses  by  swinging  himself  end  over  end,  holding  by  his 
hands  while  he  gives  his  body  a  long  swing  toward  another  branch.  His 
body  becomes  horizontal,  he  grasps  the  branch  with  his  feet,  and,  letting 
go  with  his  hands,  swings  head  downward  and  backward,  until  he  comes 
right  side  up  again,  lets  go  with  his  feet  and  goes  flying  through  the  air  to 
the  next  branch.  He  grasps  with  his  hands,  swings  the  other  end  of  himself 
forward  again,  and  so  on.  .  .  .  By  this  revolutionary  method  he  goes  just 
as  well  as  if  he  had  a  head  on  each  end  of  his  body." 


Xcw  Yiirk  Zoological  Societij 

A  representative  of  the  New  World  long-tailed 
monkeys. 


'  From    Hornaday, 
publishers. 


Two    Years    in    the   Juiiyles.     By   permission    of   Charles   Scribner's    Sons, 


538 


THE   CHANGING  WORLD 


The   short-legged   orang-utans   of  Borneo   and   Sumatra,  though 
larger  than  gibbons,  are  likewise  denizens  of  tropical  forests,  being 

more  at  home  in  the  tree-tops 
than  on  the  ground.  They 
frequently  build  for  them- 
selves temporary  nests  or 
shelters  of  sticks  and  twigs, 
and  exhibit  an  increased  men- 
tal capacity  over  that  of  the 
gibbons. 

Probably  the  chimpanzees 
of  tropical  Africa  are  the  best 
known  of  the  apes  because  of 
their  teachability,  and  con- 
sequent exploitation  on  the 
vaudeville  stage,  at  Holly- 
wood, and  elsewhere.  In  the 
last  twenty  years.  Dr.  Robert 
M.  Yerkes  of  Yale  University, 
with  a  stafT  of  assistants,  has 
been  studying  intensively  the 
behavior  of  these  disconcert- 
ingly "almost  human"  apes, 
maintaining  for  the  purpose 
a  considerable  colony  of  them 
under  constant  observation  in 
Florida,  and  another  smaller 
group  at  New  Haven,  Connec- 
ticut. His  painstaking  and 
arduous  investigations  are 
adding  very  much  to  our  accu- 
rate knowledge  of  the  dawn 
of  intelligence  and  of  the  an- 
cestral sources  of  human  be- 
havior. 

The  gorillas  of  Africa  are  the 
largest  apes,  and  perhaps  the 
least  known,  because  of  their  in- 
accessibility and  the  difficulty  of  maintaining  them  in  captivity.  Their 
strength  is  prodigious  and  their  courage  is  said  to  be  unbounded. 


A'rw  York  Zoological  Society 

The  gibbon  and  chimpanzee  are  representa- 

apes. 


tivcs  of  the 


THE   ANIMAL,    MAN    (ANTHROPOLOGY) 


539 


Although  the  average  body  of  a  gorilla  is  perhaps  twice  as  heavy 
as  that  of  a  man,  the  brain  is  only  about  half  as  large. 

Of  the  four  kinds  of  apes  no  one  of  them  stands  nearer  to  man 
than  all  the  others  in  all  particulars.  The  fact  that  both  fossil  and 
embryo  apes  present 
more  human  charac- 
teristics than  either 
living  or  adult  apes 
indicates  their  diver- 
gence from  the  main 
primate  stem,  and  their 
cousinship  to  man. 
rather  than  any  direct 
lineal  relationship.  No 
scientist  assumes  that 
man  has  arisen,  in  the 
course  of  evolution, 
from  any  contempo- 
rary species  of  pri- 
mates. Humankind 
as  compared  with  apes 
presents  among  other 
characteristics  a  less 
protrusible  face, 
smaller  eyebrow  ridges, 
slighter  jaws,  less  hairi- 
ness, larger  and  more 
elaborate  brains,  to- 
gether with  the  ability 
to  speak  and  to  com- 
municate abstract 
ideas.  On  the  other 
hand,  apes  and  man 
are  subject  not  only  to  the  same  diseases,  showing  similarity  in  blood 
tests,  but  they  also  resemble  each  other  in  a  great  array  of  anatomical 
features.  The  distinctive  differences  between  apes  and  man  are 
quantitative  rather  than  qualitative.  For  these  reasons,  if  for  no 
others,  since  man  dwells  in  such  an  anatomical  glass  house,  he  should 
hesitate  before  throwing  contemptuous  stones  at  his  anthroiwid 
cousins. 


A(7/'  Yurk  Z.Kiloiiical  Sucitty 

The  fjorilla  and  the  orans-utan,  members  of 
the  ape  group,  should  be  compared  with  those  on 
page  .538. 


540  THE   CHANGING   WORLD 

The  Downward  Ascent  of  Man 

It  is  related  that  an  anxious  and  somewhat  ilhterate  maiden  lady 
once  inquired  at  a  bookstore  for  a  copy  of  a  book  of  which  she  had 
vaguely  but  hopefully  heard,  entitled  The  Decent  Man.  Her  disap- 
pointment was  great  when  The  Descent  of  Man  by  Charles  Darwin 
was  finally  identified  as  the  probable  book  in  question,  confirming 
her  suspicions  that  her  lifelong  quest  was  hopeless  as  usual.  The 
"descent  of  man,"  however,  remains  another  story. 

Probably  the  ancestral  home  of  the  primates  was  in  tropical  tree- 
tops.  The  majority  of  living  representatives  of  the  Order  still 
retain  the  same  arboreal  headquarters,  only  a  few  kinds,  among  them 
man,  having  subsequently  taken  to  more  insecure  and  adventurous 
life  on  the  ground.  As  previously  suggested,  it  was  doubtless  tree- 
shrews,  in  the  primitive  mammalian  Order  of  the  Insectivora,  that 
first  broke  away  from  the  terrestrial  habitat  of  ancestral  reptiles,  and 
adventured  into  tree-tops.  There  are  anatomical  reasons  that  lead 
us  to  suspect  that  the  tree  shrews,  educated  in  their  aerial  manual 
training  school,  gave  rise  in  the  course  of  time  to  lemurs  and  other 
primates.  Modern  representatives  of  the  tree  shrews  are  still  to  be 
found  in  the  forests  of  Borneo.  They  are  small,  generalized,  planti- 
grade animals  with  five  digits  on  each  foot,  and  a  long  pointed  snout, 
housing  well-developed  organs  of  smell. 

Arboreal  life  wrought  profound  modifications  in  these  primate 
explorers  of  the  new  tree  habitat.  The  poking  insectivorous  snout, 
with  its  keen  sense  of  smell,  as  its  usefulness  off  the  ground  was 
lessened,  gradually  retreated,  while  the  eyes  and  the  tactile  sense, 
indispensable  in  arboreal  life,  came  into  dominance.  The  front  legs 
were  now  lengthened  and  adapted  for  sustaining  the  hanging  weight  of 
the  body,  while  the  hind  legs  became  not  only  organs  of  support, 
but  also  were  fitted  for  springing  and  leaping  from  limb  to  limb. 
When  at  rest  in  trees,  the  sitting  posture  was  naturally  adopted,  so 
that  the  originally  horizontal  quadruped  became  a  more  or  less 
vertical  animal,  a  change  entailing  a  long  list  of  further  anatomical 
adjustments.  Chief  among  the  advantages  accruing  from  sitting  up 
on  end  was  the  release  of  the  front  legs  from  the  function  of  support. 
Frogs,  however,  which  are  also  famous  sitters,  still  use  their  front  legs 
for  bracing  support,  and  so  gain  nothing  new  by  assuming  the  semi- 
vertical  posture  of  contemplation.  Generally  speaking,  the  front  legs 
of  sitting  primates  were  transformed  into  reaching  arms  with  grasping 


THE  ANIMAL,   MAN    (ANTHROPOLOGY)  541 

hands,  whereby  their  surroundings  could  be  explored  and  objects 
of  interest  brought  up  close  to  their  sense-organs  for  examination. 
This  method  was  a  vast  improvement  over  the  necessity  of  moving 
the  sense-organs,  body  and  all,  into  the  immediate  neighborhood  of 
objects  to  be  examined. 

As  long  as  primates  kept  to  the  comparative  security  of  aerial 
apartments  in  trees,  they  necessarily  could  not  attain  large  militant 
size,  for  trees  are  no  suitable  place  for  heavy  or  bulky  animals.  So 
the  time  inevitably  came  when  certain  of  the  primates,  after  a  long 
period  of  arboreal  schooling,  ventured  more  and  more  down  upon 
the  ground  until  finally,  in  the  case  of  man,  the  descent  was  made 
permanent.  The  descent  of  ancestral  man  from  an  arboreal  habitat, 
however,  resolves  itself  after  all  into  an  evolutionary  ascent,  or  step 
upward,  for  life  is  much  more  worth  living  on  the  ground  than  in  trees. 
There  are  more  enemies  to  combat,  and  more  necessity  and  oppor- 
tunity for  sharpening  the  wits.  The  table  also  is  spread  with  more 
available  food,  in  particular  a  greater  variety  of  vegetation,  and, 
in  the  animal  world,  creepers,  crawlers,  burrowers,  runners,  jumpers, 
and  swimmers,  all  good  to  eat,  that  are  out  of  easy  reach  of  tree- 
dwellers. 

It  w^as  comparatively  easy  with  increasing  intelligence  to  make 
the  transition  from  sitting  in  trees  to  walking  vertically  on  the  ground, 
and  every  human  baby  faithfully  repeats  the'  ancestral  story  by 
first  sitting  up  on  end  before  balancing  on  its  hind  legs  in  learning 
to  walk. 

Finally,  it  may  again  be  repeated  that  none  of  the  primates  existing 
today  are  to  be  regarded  as  directly  ancestral  to  man,  as  is  often 
popularly  supposed. 

The  Consequences  of  an  Upright  Life 

As  long  as  the  forerunner  of  man  went  about  on  all  fours,  whatever 
brain  was  present,  being  encased  in  a  heavy  bony  skull,  had  to  be 
carried  out  in  front  of  the  body,  at  considerable  mechanical  dis- 
advantage. In  the  case  of  the  horse,  for  example,  a  large  unwieldy 
neck,  made  up  of  vertebrae  and  abundant  muscles  and  tendons,  is 
necessary  to  guy  the  heavy  head  to  the  long,  bladelike,  bony,  spinous 
processes  which  stand  up  in  a  row  behind.  The  check-rein  of  a 
driving  harness  is  man's  contribution  to  the  horse's  age-old  problem 
of  holding  up  its  head. 


542 


THE   CHANGING   WORLD 


After  the  upright  posture  was  hit  upon  in  the  course  of  evolution, 
however,  this  particular  disadvantage  largely  disappeared,  since  the 
increasing  heaviness  of  the  encased  brain  was  amply  provided  for  by 
poising  the  whole  head  on  the  top  of  a  supporting  pillar-like  vertebral 
column. 

Experiments  in  assuming  an  upright  posture  and  in  going  about 
on  the  hind  legs  did  not  originate  with  the  primates.  There  were 
Mesozoic  dinosaurs,  for  example,  that  habitually  reared  up  their 
gigantic  bodies  on  end,  also  the  whole  class  of  birds,  as  well  as  vari- 
ous kinds  of  jumping  animals  such  as  kangaroos. 

In  these  cases,  however,  while  stand- 
ing up,  the  legs  are  bent  in  a  sitting 
attitude,  with  the  knees  projecting  for- 
ward. It  remained  for  man  to  become 
the  most  straight-legged  upright  biped 
of  them  all,  and  this  fact  has  resulted 
in  the  modification  of  practically  every 
part  of  his  anatomy. 

The  single  archlike  curving  backbone 
of  a  typical  quadruped,  from  which  the 
weight  of  the  body  hangs  suspended, 
became  in  man  a  vertical  supporting 
column,  partially  straightened  by  a  new 
curve  in  the  opposite  direction,  forming 
the  "small  of  the  back."  Young  babies 
are  flat-backed  at  first,  and  only  acquire 
this  compensating  curvature  later  when 
they  develop  into  walking  bipeds. 

The  centra,  or  bodies  of  the  vertebrae, 
become  flat-faced,  thus  stacking  up  into 
a  firmer  column  than  would  have  been 
possible  with  the  original  ball-and-socket  centra,  while  the  spinous 
processes  of  the  vertebrae  all  come  to  slope  backward  and  downward, 
instead  of  in  the  anticlinal  fashion,  as  in  quadrupeds  generally. 

Furthermore,  the  axis  and  atlas  of  the  cervical  vertebrae  become 
modified  to  permit  easy  rotation  of  the  head,  allowing  the  eyes  of 
erect  man  to  sweep  the  horizon,  as  no  quadruped  can  easily  do, 
at  the  same  time  permitting  the  eyes  themselves  to  gaze  straight 
out  from  the  vertical  face  instead  of  looking  downward  along  a 
projecting  snout. 


Adult 


Infant 


Diagrams  showing  the  dif- 
ference in  the  curvature  of  the 
backbone  between  the  infant 
and  the  adult.  (From  Walter, 
The  Human  Skeleton.  By  per- 
mission of  The  Macmillan 
Company,  pubUshers.) 


THE   ANIMAL,   MAN    (ANTHROPOLOGY) 


543 


Skeleton  of  a  hippopotamus,  showiiif;:  the  anti- 
clinal arrangement  of  \  ertebral  spines  character- 
istic of  quadrupeds,  but  not  of  bipeds.  (After 
Hesse.) 


At  the  other  end  of  the  spinal  column  the  tail  bones,  no  longer 
useful  in  any  of  the  former  ways,  telescoped  together  to  form  the 
coccyx.  This  fusion  of 
the  caudal  vertebrae 
formed  a  mass  which  bent 
in  and  became  embedded 
under  the  skin,  forming 
a  part  of  the  floor  of  the 
pelvic  basin,  now  a  neces- 
sary underpinning  for  the 
support  of  the  shifting 
visceral  weight.  So  it 
came  about  that  man  in 
tucking  his  ancestral  tail 

between  his  legs  turned  this  apologetic  performance  to  advantage. 
The  thorax  with  its  encircling  ribs  became  flattened  and  widened 
as  a  consequence  of  upright  posture,  while  the  sternal  bones,  relieved 
from  visceral  weight,  became  firmly  fused  together  and  shortened, 
allowing  more  freedom  and  effectiveness  of  motion  for  the  respiratory 
muscles. 

The  legs  of  man  straightened,  with  a  greater  resultant  efficiency 
in  leverage,  leaving  the  arms  relatively  shorter,  since,  with  the  passing 
of  locomotion  on  all  fours,  legs  and  arms  no  longer  needed  to  be 

of  the  same  approximate 
length. 

The  human  foot  met 
its  new  responsibilities  in 
a  variety  of  adaptable 
ways.  Being  squarely 
plantigrade  on  the  ground, 
the  bones  involved  be- 
came arranged  in  two 
arches,  one  longitudinal 
and  one  transverse,  to 
provide  sprightliness  to 
the  gait,  as  well  as  an  ade- 
quate support  to  body-weight.  One  of  the  ankle  bones,  the  calcaneus, 
projected  out  behind  forming  a  heel,  thus  lessening  the  likelihood 
that  the  balancing  biped  might  tend  to  tip  over  backward.  In  the 
hind  foot  of  a  quadruped  such  a  development  of  a  heel  was  unneces- 


The  two  arches  of  the  human  foot.  (From 
Walter,  The  Unman  Skeleton.  By  permission  of 
The  Macmillan  Company,  publishers.) 


544  THE  CHANGING  WORLD 

sary.  The  big  toe,  which  in  apes  and  babies  diverges  laterally 
from  the  second  toe,  straightened  and  lengthened  in  adult  man  into 
an  efficient  organ  of  support.  In  quadrupeds  this  responsibility  is 
thrown  mainly  on  the  middle  toe,  which  in  such  animals  as  horses 
becomes  the  only  line  of  contact  with  the  ground. 

One  of  the  farthest  reaching  results  of  uprightness  was  the  emanci- 
pation of  the  arms  from  bearing  the  weight  of  the  body  in  locomotion. 
This  freedom  allows  the  hands,  with  opposable  thumbs,  to  be  em- 
ployed in  exploratory  touch,  in  defense,  and  in  taking  hold  of  things. 
The  hinged  wrist  of  man,  with  the  rotating  radius,  increases  the 
availability  of  the  grasping  hand,  so  that  it  can  be  used  in  a  great 
variety  of  positions.  Thus,  instead  of  an  organ  specialized  for  a 
single  purpose,  like  the  hoof  of  a  horse,  or  the  wing  of  a  bat,  the  hand 
remained  fortunately  a  generalized  structure  capable  of  many  uses. 
Bats  are  mammals  that  are  able  to  fly,  but  at  the  price  of  losing  their 
hands.  Dr.  Hooton,  the  anthropologist,  in  contrasting  the  foot  and 
hand  of  man,  happily  describes  the  foot  as  a  "specialist,"  and  the 
hand  as  a  "general  practitioner." 

The  many  adjustments  resulting  from  erect  posture  are  by  no 
means  confined  to  the  skeleton.  The  pathologist,  whose  business 
it  is  to  seek  out  the  weak  spots  in  the  human  frame  and  to  discover 
the  causes  of  human  ills,  has  a  great  light  shed  upon  his  problems 
when  he  remembers  that  man  is  still  in  the  making,  and  that  his 
remote  ancestors  went  about  on  all  fours. 

The  Greatest  Wonder  in  the  World 

The  human  brain  is  the  greatest  wonder  in  the  world,  for  through 
it  alone  are  all  the  other  wonders  made  known.  It  is  the  brain  that 
above  all  else  is  responsible  for  man's  evident  superiority  over  every 
other  creature,  since  intelligence,  rather  than  brute  strength,  is  the 
greatest  winning  factor.  Other  parts  of  the  bodily  mechanism  may 
gain  more  perfect  elaboration  in  various  animals  than  in  man,  but 
the  human  brain  in  its  evolution  has  easily  outstripped  all  other 
anatomical  achievements. 

The  marvelous  details  of  the  rise  of  the  nervous  system  with  the 
brain  are  a  story  for  the  comparative  anatomist  to  tell  elsewhere. 
It  would  fill  a  very  bulky  volume,  for  the  dawn  of  the  mind,  the  most 
wonderful  of  all  dawns,  is  also  the  most  engaging  episode  in  the  whole 
evolutionary  pageant.  Moreover,  only  an  animal  with  a  human 
brain  can  realize  anything  about  it. 


THE   ANIMAL,    MAN    (ANTHROPOLOGY) 


545 


Although  no  other  brain,  oxcopt  that  of  the  great  elephants  and 
the  gigantic  whales,  is  actually  larger  than  the  human  brain,  it  is  not 
the  gross  size  and  weight  of  man's  brain  that  determines  its  pre- 
eminent dominance.  Quality  as  well  as  quantity  of  brain  must 
enter  in  as  an  important  factor,  as  the  relative  size  between  the 
weight  of  brain  and  body  shows.  It  will  be  seen  from  the  accom- 
panying table  that  the  hummingbird  has  three  times  as  much  brain, 
compared  to  its  body  weight,  as  man,  yet  no  one  would  say  that  it  is 
as  intelligent  as  man.  It  is  obvious  that  there  are  brains  and  brains, 
with  reference  to  quality  as  well  as  quantity. 


TABLE  OF  RATIOS  BETWEEN  THE  BRAIN  AND 
BODY  WEIGHT 


Tuna  fish 

.37,000 

Ostrich    . 

12,000 

Horse 

500 

Frog   .      . 

170 

Gorilla    . 

120 

Lemur     . 

40 

Man 

.35 

Rat  .... 

Marmoset 
Hummingbird 


1  :28 
1  :  22 
1  :  12 


The  cerebral  cortex,  an  interwoven  tissue  of  nerve  cells  overlying 
the  anterior  part  of  the  brain  of  the  higher  vertebrates,  is  of  supreme 
importance  as  the  center  of  intellectual  life.     An  expert  estimate 


abdomen 
"tr-utriVf 


;;^fissare  of* 


Diagram  of  the  human  cerebrum,  showing  the  general 
distribution  of  sensory  and  motor  centers. 

has  been  made  that  the  human  cortex,  which,  if  spread  out  flat 
instead  of  being  wrinkled  and  folded,  would  occupy  about  a  foot  and 
a  half  square,  contains  over  9,000,000,000  nerve  cells.  These  cells 
are  so  .small  that  altogether  they  weigh  a  little  more  than  a  dozen 


546  THE   CHANGING   WORLD 

grams,  and  they  could  all  be  packed  into  a  cubic  inch  of  space.  It  is 
this  restricted  sheet  of  cortical  cells  that  constitutes  the  marvelous 
headquarters  of  control  for  human  behavior.  In  it  are  centers  or 
patches  of  specialized  nerve  cells  for  the  reception  of  impressions 
received  through  each  of  the  various  sense  organs,  such  as  eyes,  ears, 
and  touch  endings,  with  neighboring  areas  devoted  to  the  control  of 
bodily  movements.  It  will  be  seen  from  the  map  of  brain  locali- 
zation, shown  in  the  figure,  that  the  receptive  center  for  hearing  is 
located  in  the  temporal  lobes  of  the  cerebrum,  that  of  touch  in  the 
parietal  lobes,  and  that  of  sight  in  the  occipital  lobe,  while  the  out- 
going control  of  muscular  movements  is  spread  along  the  edge  of  the 
deep  groove,  the  fissure  of  Rolando,  marking  the  boundary  between 
the  frontal  and  the  parietal  regions. 

The  cortical  centers  of  reception  and  disbursement  are  so  hooked 
up  and  interrelated  that  together  they  form  an  intricate  but  unify- 
ing and  efficient  switchboard,  reminding  one  of  the  central  telephone 
exchange  in  a  large  city.  This  arrangement  makes  possible  associa- 
tions of  various  sorts,  and  furnishes  a  mechanism  for  the  formation 
of  ideas,  as  well  as  providing  for  the  storage  of  garnered  experiences, 
that  have  been  embalmed  in  the  preservative  of  memory  and  kept 
available  for  future  reference. 

In  the  course  of  vertebrate  evolution  the  sense  of  smell  was  the 
first  to  acquire  significant  representation  in  the  cortex,  since  it  was 
the  most  useful  of  the  senses  in  the  case  of  lowly  animals  sniffing 
around  with  their  noses  close  to  the  ground.  As  time  went  on, 
however,  particularly  in  connection  with  arboreal  life  of  the  primates, 
cortical  centers  of  sight,  hearing,  and  muscular  control  gained  a 
relative  ascendancy,  enabling  natural  selection  to  take  a  fresh  lease 
on  the  task  of  sharpening  the  wits  and  elaborating  the  brain.  The 
process  is  by  no  means  completed  yet,  but  even  now  it  has  gone  so 
far  that,  in  the  case  of  man,  a  brain  has  been  developed  which  en- 
ables him  to  perform  such  wonderful  feats  as  weighing  a  star,  or 
splitting  off  an  electron  from  an  atom,  intellectual  feats  that  are 
quite  unthinkable  in  the  case  of  any  other  animal. 

Flint  and  Metal  History 

Man,  with  his  handy  hands,  is  the  master  mechanic,  and  the  only 
animal  that  can  use  all  sorts  of  tools.  Monkeys  and  raccoons  have 
grasping  hands,  but  they  are  not  very  successful  tool-users,  for 
the  reason  that  their  brains  have  not  caught  up  with  their  hands. 


THE   ANIMAL,    MAN    (ANTHROPOLOGY)  547 

Animals  whose  brains  outrun  their  hands  would  be  equally  handi- 
capped, since  they  would  have  no  adequate  outlet  for  action.  A  fish 
equipped  with  a  human  brain  would  go  crazy,  with  only  fins  to  do 
with  instead  of  hands. 

The  use  of  tools  and  weapons  is  particularly  important  to  man, 
because  he  is  otherwise  comparatively  helpless,  not  having  horns, 
fangs,  hoofs,  claws,  or  any  such  specialized  anatomical  instruments 
built  into  his  body.  He  is  also  particularly  fortunate  in  being  able 
to  shift,  with  his  handy  hand,  from  one  tool  to  another,  as  animals, 
whose  tools  are  a  part  of  their  bodily  structure,  cannot  do.  The 
origin  of  tool-using  is  of  special  interest  to  the  anthropologist,  be- 
cause tool  history  can  be  traced  back  much  farther  than  written 
history,  or  even  the  fossil  record  left  by  human  bones.  It,  there- 
fore, constitutes  the  very  earliest  evidence  of  man's  presence  on 
the  earth. 

The  material  out  of  which  the  earliest  tools  were  fashioned  was 
mostly  flint,  although  other  kinds  of  stone,  as  well  as  volcanic  glass, 
or  obsidian,  were  used.  No  doubt  wood  w^as  used  extensively  too 
in  various  ways,  but,  due  to  its  perishable  nature,  no  witness  of  the 
fact  remains.  The  first  traces  of  human  flint  tools,  according  to 
Professor  Bean,  date  back  about  300,000  years.  Their  evolution 
can  be  traced  through  various  stages  of  improvement  down  to  his- 
toric times,  when  metals  came  to  be  employed  largely  in  their  stead. 
The  successive  cultural  stages  of  tool-making  are  known  as  the  Stone 
age,  the  Copper  age,  the  Bronze  age,  and  the  Iron  age.  Today  the 
subsequent  Steel  age,  w^ith  its  many  instruments  of  precision,  may 
be  regarded  as  the  high  peak  in  this  long  evolution. 

These  successions  did  not  occur  simultaneously  the  world  over, 
since  advancement  was  much  more  rapid  in  certain  parts  of  the  globe 
than  in  others.  For  example,  the  rude  primitive  inhabitants  of  the 
British  Isles  were  still  back  in  the  phases  of  flint  culture  at  the  time 
when  the  Greeks  and  Romans  around  the  Mediterranean  Sea  had 
learned  the  use  of  bronze  and  iron.  Furthermore,  one  kind  of  tool 
always  overlapped  and  replaced  another  gradually,  just  as,  in  the 
matter  of  transportation,  ox-carts,  horses  and  carriages,  bicycles, 
automobiles,  and  airjilanes  have  succeeded  each  other  without 
crowding  out  their  predecessors  all  at  once. 

The  Stone  age  has  been  divided  into  three  divisions,  Eolithic, 
Paleolithic,  and  Neolithic,  according  to  the  degree  of  perfection 
attained  in  fashioning  stone  tools  or  weapons. 


548 


THE   CHANGING   WORLD 


E0LITH6 


Eolithic  implements  are  somewhat  uncertain  in  character,  although 
very  stimulating  to  the  imagination.  They  are  "handy"  stones, 
sometimes  rudely  chipped  without  any  definite  design,  except  that 

they  fit  the  hand  and  some- 
times show  evidence  of  having 
been  used.  Whether  they 
ever  did  fit  into  a  calloused 
prehistoric  human  hand  is 
problematical.  Doubtless  the 
first  tools  and  weapons  were 
not  made  but  were  found  and 
picked  up,  already  sufficiently 
fashioned  by  such  natural 
forces  as  frost  and  erosion. 
It  was  not  until  later  that 
flints  were  made  over  by 
himian  agency  into  shapes  for 
a  definite  purpose. 

PaleoUths  show  unmistaka- 
ble evidences  of  having  been 
fashioned  by  man.  The 
earliest  ones  are  of  very 
crude  workmanship,  perhaps 
roughly  sharpened  at  one 
end,  or  chipped  on  one  side 
only.  Tools  of  more  im- 
proved workmanship  fol- 
lowed —  stone  axes,  cleavers, 
scrapers,  punches,  spear  and 
arrow  heads,  and  flakes  with 
notched  sawlike  margins  or 
sharp  knifelike  edges.  Some 
of  these  show  a  very  remark- 
able degree  of  skill  in  their 
manufacture.  The  joy  that 
a  modern  boy  experiences  in  the  possession  of  his  first  jack-knife  is  a 
possible  echo  of  the  delight  which  our  cave-dwelling  ancestors  felt  when 
they  succeeded  in  splitting  off  a  knife-blade  flake  from  a  core  of  flint. 
The  paleolithic  toolmakers  worked  for  many  thousands  of  years, 
as  evidenced  by  the  associated  remains  of  extinct  animals,  before  they 


PALEOLITHS 


NEOUTH 


Flints,  representinf;  three  periods  of  the 
Stone  age.  Drawn  from  specimens  in  the  col- 
lection at  Brown  University,  (From  Walter, 
Biology  of  the  Vertebrates.  By  permission  of 
The  Macmillan  Company,  publishers.) 


THE   ANIMAL,    MAN    (ANTHROPOLOGY)  549 

learned  to  polish  their  flint  axes,  chisels,  and  other  tools  and  weapons, 
thus  making  the  characteristic  neolithic  instruments.  Meanwhile 
harpoons  and  needles  of  bone  had  been  invented,  and  the  begin- 
nings of  human  vanity  were  recorded  in  the  form  of  beads  and  other 
ornaments  made  of  bone  and  shell. 

"The  change  from  the  Stone  age  to  the  Age  of  Metals,"  says 
Professor  MacCurdy,  "was  the  most  revolutionary  step  ever  taken 
by  man."  Of  the  metals,  copper  was  first  employed  in  Egypt, 
mostly  at  first  for  ornaments,  as  early  as  5000  B.C.  This  was  fol- 
lowed by  malleable  bronze,  and  finally  by  iron,  which  is  not  at  all 
easy  to  smelt  out  of  the  rocks  where  it  occurs  in  nature.  Iron,  and 
particularly  its  modification  in  the  form  of  steel,  has  come  to  be 
devoted  to  so  many  uses  that  if  it  were  all  magically  withdrawn 
today,  our  civilization  would  collapse. 

Getting  the  Upper  Hand  of  Things 

Something  of  man's  later  successes  and  failures  in  the  control  of 
his  environment  is  related  in  the  following  pages  on  "Man,  the 
Conqueror."  In  this  connection,  however,  may  properly  be  men- 
tioned a  few  of  the  very  first  problematical  steps  that  led  to  his 
ultimate  triumph  as  a  human  being. 

Primitive  man  was  without  doubt  overwhelmed  and  molded 
by  a  dominating  environment,  and  was  to  a  very  large  extent  the 
slave  of  his  surroundings.  He  could  not  have  been  aware  of  very 
much  in  the  make-up  of  w^hat  was  about  him,  in  the  sense  in  which 
modern  man  knows  his  external  world,  any  more  than  ants,  running 
about  busily  in  the  grass,  realize  the  clouds  floating  in  the  sky  over- 
head. The  revelations  and  mysteries  of  nature  which  the  man  of 
today  senses  on  all  sides,  as  w^ell  as  the  orderly  sequences  of  cause 
and  effect  that  make  up  events,  probably  made  very  little  impression 
on  our  remote  animal-like  ancestors  in  the  days  when  they  were 
becoming  human.  They  were  probably  unaware  even  of  the  existence 
of  these  surrounding  factors,  just  as  starfish  are  ignorant  of  stars, 
or  ants  are  unaware  of  clouds. 

Flashing  lightning  and  crashing  thunder  primitive  man  did  not 
understand,  and  it  terrified  him  into  superstitious  subjection  to  the 
unknown  forces  about  him.  His  dawning  mind  was  enslaved  because 
he  did  not  yet  know  his  world.  Intellectual  freedom,  based  upon 
a  knowledge  of  the  laws  of  nature,  was  to  come  only  after  long  years 


550  THE   CHANGING   WORLD 

of  endeavor,  and  was  eventually  to  mark  his  most  substantial  triumph 
in  emerging  from  his  humble  origin. 

For  thousands  of  years  the  best  that  he  could  contrive  by  way  of 
a  protection  from  devastating  storms  and  climatic  inclemency  was 
to  retreat  to  natural  caves  and  rock  shelters,  where  he  disputed 
possession  with  cave-bears,  cave-hyenas,  and  other  formidable  beasts. 
Whenever  food  was  abundant  he  gorged  himself.  When  it  was 
scarce  he  starved.  The  artificial  production  of  food,  in  order  to 
secure  a  constant  supply,  he  had  not  yet  dreamed  of,  any  more 
than  did  the  wild  animals  about  him. 

Gradually  "in  man's  ceaseless  struggle  to  achieve  his  destiny," 
inventions  beyond  the  possibilities  of  any  animal  with  a  lesser  brain 
began  to  appear  in  the  form  of  tools,  weapons,  weaving,  pottery,  the 
wheel,  dugout  canoes,  and  devices  for  shelter.  There  was  at  first 
probably  little  spare  time  in  which  to  develop  these  higher  arts  and 
accessories  of  living,  for,  as  in  the  case  of  wild  animals,  the  day's  work 
largely  consisted  in  barely  keeping  alive.  Moreover,  whatever  lei- 
sure was  available  could  have  been  but  imperfectly  applied  to  the 
higher  life,  since  the  intellectual  equipment  necessary  for  this  accom- 
plishment was  still  wanting  to  a  considerable  extent.  Even  today 
modern  man,  already  liberated  more  and  more  by  machinery  from 
continuous  toil,  is  not  always  mentally  equipped  to  dispose  of  his 
spare  time  with  entire  edification  to  himself  and  to  others. 

Another  human  accomplishment  which  no  animal  has  ever  attained 
centers  around  commerce  or  the  acquisition  and  exchange  of  property. 
The  great  gap  between  mankind  and  even  the  most  intelligent  of 
animals  is  evident  when  it  is  realized  how  foreign  to  any  animal 
behavior  are  even  the  most  primitive  forms  of  barter.  Beginnings 
of  hoarding,  or  the  possession  of  property,  are  perhaps  shown  by 
honey-bees  and  nut-storing  squirrels,  but  it  is  a  long  call  from  this 
instinctive  behavior  to  the  intelligent  exercise  of  forethought  that  is 
practiced  by  economic  man. 

Thus,  by  means  of  agriculture,  domestication,  the  use  of  fire,  the 
development  of  fundamental  inventions,  the  beginnings  of  economic 
practices,  and  above  all  by  the  gradual  emancipation  of  the  mind 
from  the  terrors  of  superstition  and  the  misunderstandings  of  igno- 
rance, did  emergent  man  begin  to  get  the  upper  hand  of  things,  and 
to  make  the  grand  transition  from  the  more  or  less  animal-like  soli- 
tary life  of  cave-dwelling  to  the  co-operative  social  and  intellectual 
life  of  modern  man. 


THE  ANIMAL,   MAN    (ANTHROPOLOGY) 


551 


Gaining  Ideas  and  Passing  Them  On 

Once  the  brain  of  man  had  evolved  far  enough  to  incubate  ideas, 
speech  came  to  the  rescue  and  made  possible  the  transfer  of  ideas 
from  one  individual  to  another.  Thus,  the  intellectual  accumula- 
tions of  experience  and  tradition  were  preserved  and  utilized,  and 
the  emancipating  process  of  learning  made  possible.  Language, 
it  goes  without  saying,  has  been  one  of  the  most  important  factors 
in  human  evolution. 

There  are  various  ways  in  which  animals  can  communicate  with 
each  other.  Ants  pass  the  time  of  day  by  touching  antennae  together, 
and  dogs  comply  with  the  social  conventions  of  the  dog  world  largely 
through  the  sense  of  smell,  but  humankind  has  spoken  and  written 
language  as  the  primary  means  of  communication. 

There  are  certain  skeletal  differences  in  the  lower  jaws  of  apes  and 
humans  which  help  to  explain  why  one  speaks  and  the  other  does 
not.  In  man  the  lower  jaw  spreads,  like  a  letter  V,  while  in  the  apes 
it  is  more  U-shaped,  due  in  part  to  the  projecting  canine  teeth  that 
make  a  "corner"  between 
the  incisors  in  front  and  the 
premolars  and  molars  that 
are  arranged  behind  along 
the  side  of  the  jaw.  There 
is  thus  more  room  for  the 
tongue  within  the  arch  of 
the  human  lower  jaw  than  in 
that  of  the  ape,  which  is  of 
prime  importance  in  speech. 
Moreover,  the  ape  does  not 
have  a  projecting  chin  to  pro- 
vide more  room  for  play  of 
the  tongue,  as  in  the  case 
of  man,  although  the  whole 
face  projects  more.  This  is 
an  important  difference,  for 
the  two  halves  of  the  lower 

jaw  are  anchored  together  by  a  bony  formation  on  the  inside, 
the  so-called  "simian  shelf,"  a  horizontal  junction  which  reduces 
decidedly  the  available  space  for  the  tongue  and  its  muscular  attach- 
ment. In  man,  the  simian  shelf  disappears  with  the  outside  de- 
H.  w.  H.  —  36 


Lower  jaw  of  man  (above)  and  ape   ([)elow! 


552  THE  CHANGING  WORLD 

velopment  of  the  projecting  chin,  and  instead  genial  tubercles,  small 
s])ines  of  bone  projecting  backward  for  the  attachment  of  the  genio- 
glossal muscles  of  speech,  are  present  on  the  anterior  inside  angle  of 
the  lower  jaw,  just  in  front  of  the  spot  which  in  tlie  a\)OH  is  the  loca- 
tion of  the  simian  shelf.  As  Professor  Hooton  remarks,  "The  slang 
expression  'chinning,'  meaning  'talking,'  seems  to  have  a  certain 
evolutionary  justification,"  but  it  is  not  enough,  however,  to  possess 
the  anatomical  machinery  for  speech.  A  parrot  has  that.  There 
must  be  cortical  centers  developed  in  the  brain  sufficient  to  make 
possible  the  realization  of  the  significance  of  what  is  said  in  speech. 
Many  animals  are  vocal  and  make  a  variety  of  sounds.  It  is  said 
that  chimpanzees  have  a  vocabulary  of  at  least  a  dozen  words  by 
which  they  express  various  emotions.  Dogs  can  modify  their  bark- 
ing to  indicate  different  things,  and  crows  modulate  their  "caws." 
No  animal  except  man,  however,  puts  together  even  a  short  sentence, 
and  there  can  be  no  such  thing  as  an  animal  grammar. 

Skeletons  in  the  Pleistocene  Ice  Chest 

When  did  man  become  human?  How  long  has  it  been  since  he 
emerged  from  among  his  nonhuman  relatives  to  occupy  a  definite 
place  on  the  evolutionary  stage?  Research  and  discovery  in  recent 
years  have  made  it  possible  to  give  a  tentative  answer  to  these  ques- 
tions, which  would  not  have  been  the  case  a  century  ago.  There  is 
no  doubt  as  to  the  existence  of  contemporary  human  beings  all  about 
us,  for  they  fall  within  personal  observation.  Tradition  and  his- 
torians are  able  to  carry  back  the  story  of  humanity,  with  diminishing 
certainty,  through  the  Dark  Ages  at  least  to  classical  times.  Beyond 
that  period  the  uncertainty  deepens,  even  when  persistent  archae- 
ologists with  their  spades  uncover  buried  cities,  often  built  one  above 
the  other,  and  thus  push  back  still  further  the  outposts  of  human 
antiquity.  The  builders  of  these  ancient  cities  fade  from  view, 
so  far  as  archaeologists  are  able  to  inform  us  at  present,  about 
5000  B.C.,  and  when  the  thread  is  again  picked  up  some  5000  years 
earlier,  that  is,  about  10,000  B.C.,  it  is  the  vanishing  prehistoric  traces 
of  cave-dwellers  which  tell  of  the  existence  of  man.  Such  troglodytic 
evidences  of  man  are  spread  over  a  long  indefinite  interval  of  time, 
during  which  the  ancestors  of  modern  man  probably  endured  a 
precarious  existence,  limited  to  life  in  small,  struggling,  isolated  family 
groups.  How  to  dwell  together  in  anything  like  larger  co-operative 
relationship  had  not  yet  been  learned. 


THE   ANIMAL,   MAN    (ANTHROPOLOGY) 


553 


The  critical  emergence  from  long  centuries  of  cave  life,  up  through 
the  beginnings  of  agriculture  to  community  life,  must  have  come 
during  the  transitional  millenniums  between  10,000  b.c,  after  the 
retreat  of  the  last  ice  cap  at  the  close  of  the  Pleistocene  period,  and 
the  earliest  known  traces  of  community  or  city  life,  around  5000  b.c. 
There  were  in  the  entire  Pleistocene  period,  at  least  in  the  northern 
hemispheres,  four  great  invasions  of  arctic  climates,  periods  of  per- 
petual winter  with  unmelted  snow  and  ice,  when  an  extensive  gla- 
cial blanket  covered  the  land  the  year  around.  Between  these  ice 
ages  intervened  warmer  centuries  without  perpetual  ice,  when  at 
times  even  tropical  conditions  obtained.  It  was  probably  within 
this  span  of  Pleistocene  time,  in  which  there  was  such  a  wide  range  of 
alternating  climates  to  keep  adaptable  organisms  on  the  qui  vive  in 
order  to  maintain  themselves,  that  man  put  in  his  initial  appearance 
and  gradually  established  himself  among  the  existing  forms  of  life. 
The  Pleistocene  period,  therefore,  is  called  the  Age  of  Man,  in  dis- 
tinction to  the  Cenozoic  era,  of  which  it  is  a  part,  and  which  is 
designated  as  the  Age  of  Mammals. 

There  have  been  various  attempts  to  estimate  the  relative  duration 
of  the  three  great  geological  eras.  Paleozoic,  Mesozoic,  and  Cenozoic, 
that  are  represented  by  sedimentary  rocks  from  which  fossil  remains 
of  animals  and  plants  have  been  recovered.  The  following  table, 
derived  from  ^-arious  sources,  shows  the  guesses  made  by  a  dozen 
investigators,  in  which  the  relative  duration  of  the  three  fossiliferous 
eras  is  represented  in  percentages  of  the  entire  time  that  has  elapsed 
from  the  beginning  of  the  Paleozoic  era  down  to  the  present. 

TABLE   OF   PERCENTAGES   OF   TIME 


Paleozoic 

Mesozoic 

Cenozoic 

Sollas 

47.83 
66.00 
63.63 
63.17 
63.18 
74.64 
60.00 
65.57 
60.96 
66.67 
70.83 
77.89 

65.03 

27.27 
22.00 
25.45 
26.00 
26.30 
14.92 
30.00 
24.60 
29.38 
24.44 
22.92 
18.63 
24.33 

24.90 

MacCurdy 

Bean 

12.00 
10.92 

Walcott 

10.83 

Wells 

10.52 

Boule 

10.44 

Osborn       

Bretz 

10.00 
9.83 

Schuchoit 

Lull 

9.65 

8.89 

Buttel-Reepen 

Barrell 

Average 

6.23 
3.48 

10.64 

554  THE   CHANGING   WORLD 

These  estimates  have  been  arrived  at  by  various  methods.  That 
of  Sollas,  for  instance,  is  based  upon  observed  rates  of  erosion  and 
sedimentation,  although  such  rates  are  known  to  vary  considerably 
with  the  conditions  involved.  Barrell's  computations,  on  the  other 
hand,  depend  upon  the  transformation  of  radio-active  substances  in 
the  earth's  crust.  This  latter  method  is  probably  the  most  reliable 
criterion  for  measuring  the  passage  of  time,  for  the  reason  that  it  has 
been  experimentally  demonstrated  that  the  rate  at  which  the  trans- 
formation of  radio-active  substances  occurs  is  constant.  Thus,  it 
serves  as  a  reliable  time-meter  for  determining  the  age  of  the  rocks 
in  which  these  substances  are  found.  Uranium  salts,  for  example, 
by  discharging  three  helium  atoms,  become  transformed  into  radium, 
which,  in  turn,  undergoes  still  further  progressive  change,  accom- 
panied by  the  release  of  energy,  by  shooting  off  five  more  helium 
atoms  when  it  finally  becomes  stable  in  the  form  of  inert  lead.  Conse- 
quently, since  this  accurately  timable  transformation  takes  place 
at  a  definite  rate,  the  time  of  the  laying  down  of  a  stratum  of  rock 
in  the  earth's  crust  containing  uranium-lead,  or  other  radio-active 
elements  in  various  stages  of  transformation,  can  be  dated  with 
considerable  accuracy. 

It  will  be  seen  from  the  table  that  an  average  of  the  opinions  of  the 
twelve  experts  cited  indicates  that  the  lapse  of  time  during  the 
Cenozoic  era  covered  10.64  per  cent  of  the  time  since  the  first  known 
plants  and  animals  lived.  Furthermore,  the  Cenozoic  era  is  sub- 
divided into  periods  of  varying  duration,  of  which  the  last,  or 
Pleistocene  period,  meaning  "most  recent,"  is  estimated  to  be 
approximately  one  sixth  of  the  entire  Cenozoic  era,  or,  according 
to  a  most  conservative  guess,  about  500,000  years.  This  is  the 
spacious  stretch  of  time  in  which  we  are  to  hunt  for  our  earliest 
human  ancestors.  Since  our  primate  cousins  are  known  from  their 
fossil  remains  to  have  existed  as  far  back  as  the  Oligocene  period, 
there  is  no  occasion  to  apologize  for,  or  to  feel  in  any  way  embarrassed 
by,  the  grotesque  character  of  relatives  so  remote. 

Aside  from  the  indirect  testimony  of  comparative  anatomy  and 
embryology,  based  upon  the  probable  time  needed  to  evolve  so  com- 
plex an  organism  as  man,  there  are  two  lines  of  indisputable  evidence 
of  the  great  antiquity  of  mankind.  The  first  deals  with  artifacts, 
or  the  tools  and  weapons  considered  in  a  previous  section,  which  could 
only  have  been  fashioned  by  human  hands,  and  the  second,  with  the 
occurrence  of  human  fossils,  the  "poor  Yoricks"  that  have  frequently 


THE   ANIMAL,    MAN    (ANTHROPOLOGY)  555 

been  found  associated  with  the  remains  of  species  of  animals  known  to 
have  been  long  extinct.  These  fragments  of  human  skek^tons,  pre- 
served in  the  vast  Pleistocene  ice  chest  which,  as  already  pointed  out, 
was  restocked  with  ice  at  least  four  times,  piece  out  for  us  something 
of  the  extensive  pre-history  of  man. 

The  absorbing  interest  in  human  fossils  is  greatly  enhanced  by 
their  scarcity.  Not  only  destructive  processes  of  decay  but  also  the 
inevitable  exposure  of  dead  bodies  to  devouring  animals  were  condi- 
tions to  which  primitive  man  was  particularly  liable. 

The  outstanding  and  much  studied  examples  of  Pleistocene  man 
have  nearly  all  been  discovered  since  Darwin's  day.  They  have, 
in  the  majority  of  cases,  been  recovered  from  the  debris  of  limestone 
caverns,  or  found  embedded  in  sedimentary  deposits,  along  with  the 
bones  of  extinct  animals  that  serve  to  establish  the  time  when  they 
lived.  For  the  most  part  they  have  l^een  found  in  European  coun- 
tries, such  as  France,  Spain,  Belgium,  Germany,  and  Austria,  which 
have  been  more  thorouglily  explored  by  anthropologists  than  other 
countries,  although  a  few  notable  specimens  have  come  from  such 
diverse  regions  as  China,  South  Africa,  Australia,  and  Java.  Human 
fossils  from  North  and  South  America  are  in  no  authentic  instance, 
according  to  Dr.  Hrdlicka  of  Washington,  of  the  great  antic}uity 
characteristic  of  the  famous  representatives  of  early  man  from 
Europe  and  other  parts  of  the  Old  World.  As  a  matter  of  fact,  it  is 
not  at  all  easy  for  any  newly  unearthed  human  fossil  to  run  the 
gantlet  of  critical  anthropologists,  and  to  be  admitted  to  good 
standing  in  the  ancient  and  honorable  society  of  genuine  primitive 
man.  It  may  be  possible  to  fool  some  of  these  cautious  investigators 
some  of  the  time,  but  it  is  cjuite  impossible  to  fool  all  of  them  in  the 
end.  To  these  experts  we  must  turn  for  information  in  this  field 
of  study  which  lies  beyond  the  opportunity  and  capacity  of  ordinary 
laymen  to  explore.  Alluring  as  the  ancient  story  is,  there  is  a  chance 
here  to  do  no  more  than  call  a  roll  of  a  few  of  our  most  famous  known 
fossil  ancestors,  and  to  refer  those  interested  in  the  subject  to  the 
bibliography  at  the  end  of  the  chapter  for  further  exploration  and 
information. 

Java  Man 

The  oldest  authentic  fossil  primate  suspected  of  being  human  is 
Pithecanthropus  erectus  of  Java,  who  lived  either  around  the  beginning 
of  the  Pleistocene  period  or  at  the  end  of  the  preceding  Pliocene 


556  THE   CHANGING   WORLD 

period,  some  500,000  years  ago.  Only  the  skull-cap,  left  femur,  and 
three  teeth  of  the  fossil  were  found,  far  enough  apart  to  suggest 
accidental  burial,  j'-et  these  fragments  were  sufficient  to  indicate  the 
essentially  primitive  character  of  this  famous  individual.  It  has 
been  briefly  described  as  ''more  apelike  than  any  man,  and  more 
human  than  any  ape."  With  it  were  found  the  remains  of  twenty- 
seven  different  kinds  of  mammals,  mostly  of  extinct  types. 

Heidelberg  Jaw 

Homo  heidelhergensis  is  known  only  by  a  lower  jaw,  decidedly  ape- 
like in  conformation,  but  supplied  with  teeth  unmistakably  human. 
This  ancient  being  appeared  on  earth  about  250,000  years  ago,  al- 
together too  soon  to  matriculate  at  the  venerable  university,  founded 
as  recently  as  1386  a.d.,  in  Heidelberg,  Germany,  near  which  it  was 
discovered  in  1907.  The  fact  that  the  jaw  bone  was  buried  under 
eighty-two  feet  of  undisturbed  sedimentary  rocks,  along  with  the 
bones  of  such  extinct  animals  of  early  Pleistocene  times  as  Elephas 
antiquus  and  Rhinoceros  etruscus,  indicates  with  considerable  certainty 
when  it  lived. 

Charles  DarwirCs  Neighbor 

In  1911  fragments  of  a  human  skeleton  were  found  in  England, 
different  enough  from  all  other  humans  to  be  classified  not  only  in 
a  separate  zoological  species,  but  even  in  a  distinct  genus  from  that 
of  modern  man.  This  individual,  now  named  Eoanthropus  dawsoni, 
had  a  human  cranium  but  an  apelike  jaw,  and  was  found  in  sur- 
roundings indicating  a  time  of  around  150,000  years  ago.  Piltdown 
in  Sussex,  where  the  bones  were  found,  is  only  about  thirty  miles 
from  Charles  Darwin's  home  at  Down,  but  Darwin  died  without 
any  knowledge  of  his  famous  neighbor,  in  whom  he  would  no  doubt 
have  been  keenly  interested  had  he  been  aware  of  his  existence. 
Somewhat  later  parts  of  a  second  contemporary  skeleton  were  found 
near  the  same  locality. 

The  First  Lady  of  China 

Quite  recently,  in  1929,  in  the  cave  deposits  of  Chou  Kou  Tien 
thirty-seven  miles  southw^est  of  Peking,  were  discovered  the  fossil 
remains  of  the  "first  lady  of  China,"  Sinanthropus  pekinerisis  by 
name.     The  fact  that  she  was  securely  embedded  in  limestone  under 


THE   ANIMAL,   MAN    (ANTHROPOLOGY)  557 

one  hundred  and  ten  feet  of  cave  deposits,  together  with  representa- 
tives of  the  early  Pleistocene  fauna,  vouches  for  her  very  remote  origin, 
although  anthropologists  are  not  yet  completely  agreed  as  to  the 
probable  time  when  she  lived.  This  fossil  is  the  first  discovered 
evidence,  accompanied  with  definite  geological  data,  of  the  existence 
of  early  Pleistocene  man  north  of  the  Himalayas. 

The  Meander  thaler  s 

Coming  down  to  times  extending  from  approximately  100,000  b.c. 
to  30,000  B.C.,  there  is  ample  and  convincing  fossil  evidence  of  the 
existence  of  a  peculiar  race  of  cave-dwellers,  principally  scattered 
over  what  is  now  Europe,  that  were  enough  unlike  modern  man  to 
be  placed  in  a  separate  species  by  themselves.  This  is  the  species  of 
Homo  neanderthalensis,  of  which  over  a  score  of  authentic  specimens, 
more  or  less  complete,  have  been  found  and  critically  described. 
They  had  brains  and  brawn  enough  to  have  lived  somehow  through 
the  strenuous  grisly  days  of  the  later  ice  ages,  along  with  mammoths, 
woolly  rhinoceroses,  cave-bears,  cave-hyenas,  and  other  such  ancient 
companions.  The  Neanderthalers  made  flint  instruments  and  knew 
the  use  of  fire.  Sometimes  they  even  buried  their  dead,  and  occa- 
sionally they  disposed  of  them  in  cannibalistic  feasts,  as  revealed 
by  broken  and  charred  bones.     Those  were  the  "good  old  days" ! 

Wild  Horse  Hunters 

Following  the  Neanderthalers,  and  perhaps  instrumental  in  their 
final  disappearance,  came  two  other  races  of  mankind,  the  Aurigna- 
cians  and  the  Crdmagnons,  who  likewise  dwelt  in  caves.  No  one 
yet  knows  whence  they  came,  but  there  is  plenty  of  evidence,  fossil 
and  otherwise,  that  they  invaded  Europe,  eventually  replacing  the 
Neanderthalers  then  living  there.  No  anatomical  reason  appears 
for  placing  these  two  races  in  different  zoological  species  from  that 
of  modern  man,  namely,  Homo  sapiens.  The  Aurignacians  were 
hunters  of  mammoths  and  wild  horses,  that  in  their  day  roamed 
over  what  is  now  Europe.  Living  some  30,000  years  ago,  they  made 
enduring  pictures  of  considerable  artistic  merit  upon  the  walls  of 
the  caverns  which  they  frequented,  dei^icting  principally  the  animals 
which  they  hunted.  Many  of  these  drawings,  fortunately  sheltered 
from  the  devastating  tooth  of  time,  are  still  preserved  today. 


Amirican  Musntm  of  Natural  Hlxlory 


Painted  grotto  drawings,  reproduced  from  the  original  in  the  great  rock 
shelter  of  Cuevas  del  Civil,  near  Albocacer,  Castellon.  A  group  of  men,  most 
of  them  armed. 

558 


THE  ANIMAL,   MAN    (ANTHROPOLOGY)  559 

Reindeer  Hunters 

The  Cromagnons  were  likewise  hunters,  cave-dwellers,  and  artists. 
The  animals  they  drew  were  largely  reindeer  rather  than  wild  horses, 
showing  that  the  climate  where  they  lived  had  become  cold  in  their 
day,  because  the  reindeer  inhabits  only  cold  regions.  Fossil  remains 
show  that  the  Cromagnons  were  physically  a  well-developed  race, 
and  it  is  generally  believed  that  they  were  the  immediate  ancestors 
of  modern  man.  They  emerged  from  the  last  ice  age,  and  can  be 
traced  down  to  about  10,000  b.c,  when  gradual  emancipation  from 
cave  life  and  the  beginnings  of  agriculture  had  their  origin.  Both 
Aurignacians  and  Cromagnons  were  expert  flint  workers. 

Human  landmarks  throughout  the  Pleistocene  period  are  roughly 
indicated  on  the  chart  on  page  560.  The  beaded  line,  in  which 
each  interval  between  the  beads  represents  1000  years,  is  drawn 
folded  up  like  an  accordion,  in  order  to  accommodate  the  diagram 
to  a  single  page,  with  the  last  10,000  years  laid  down  horizontally 
at  the  right.  To  obtain  a  proper  appreciation  of  the  lapse  of  time 
in  which  man,  although  one  of  the  most  recent  animals  to  occupy 
the  earth,  is  involved,  the  entire  beaded  line,  in  imagination,  should 
be  pulled  out  straight. 

Races 

Anthropologists  agree  that,  zoologically  speaking,  modern  man 
constitutes  a  single  species,  called  Homo  sapiens,  although  it  has 
been  definitely  established  that,  during  the  Pleistocene  period,  other 
species  of  human  beings,  now  extinct,  existed.  It  is  obvious,  how- 
ever, that  Homo  sapiens  today  is  made  up  not  only  of  varying 
individuals,  no  two  of  which  are  alike,  but  of  fairly  well  defined 
diverse  groups  of  human  beings,  which  correspond  to  what  biologists 
designate  among  animals  as  different  breeds.  These  groups  in  the 
case  of  mankind  are  called  races. 

The  science  of  Ethnology  is  concerned  with  sorting  out  different 
races  by  means  of  an  analysis  of  their  several  characteristics,  besides 
tracing  the  origins  of  races,  and  mapping  the  migrations  and  dis- 
persals from  points  of  origin,  through  which  man  has  come  to  occupy 
practically  the  entire  earth. 

There  is  considerable  unavoidable  confusion  in  defining  just  what 
is  included  in  a  particular  race  of  human  beings,  because  racial  classi- 
fications may  be  based  upon  either  geographical,  linguistic,  political, 
cultural,  or  religious  standards,  as  well  as  upon  biological  criteria. 


560 


THE   CHANGING  WORLD 


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THE  ANIMAL,   MAN    (ANTHROPOLOGY)  561 

Furthermore,  whatever  the  criteria  that  are  employed,  it  is  quite 
certain  that  in  no  case  does  there  anywhere  exist  today  a  i)ure  race 
of  mankind,  vuicontaminated  by  any  other  race.  The  nearest  ap- 
proach to  biological  racial  purity  would  be  expected  among  inbred 
peoples,  which  have  been  isolated  from  diverse  parental  stocks  for 
a  long  time,  as,  for  example,  the  Eskimo  tribes  of  the  north,  the  hairy 
Ainu  aborigines  of  northern  Japan,  the  Igorots  of  the  Philippines, 
the  Veddahs  of  southern  Ceylon,  and  the  Pygmies  of  Africa. 

Dr.  Hrdlicka  divides  Homo  sapiens  into  three  primary  races  ac- 
cording to  the  pigmentation  of  the  skin,  namely,  white,  yellow-brown, 
and  black.  Skin  color  in  itself  is  no  measure  of  either  inferiority 
or  superiority.  Its  only  biological  significance  is  that  it  may  possibly 
be  regarded  as  an  environmental  adaptation  to  prevailing  amounts 
of  ultraviolet  light  in  different  regions  of  the  earth.  Even  if  a 
correlation  between  skin  pigment  and  sunlight  is  demonstrated,  the 
probable  cause  of  it  is  more  reasonably  explained  as  an  hereditary 
adaptation  enabling  the  darker  races  to  live  successfully  in  tropical 
regions  of  greater  intensity  of  sunlight,  rather  than  that  dark  skin 
is  the  result  of  exposure  to  excessive  sunlight,  which  has  become 
hereditary.  Unfortunately  it  is  a  superficial  criterion  that  cannot 
be  applied  to  our  fossil  Pleistocene  ancestors  of  whose  skin  we  have 
no  knowledge.  There  is  evidence,  however,  that  these  three  great 
primary  divisions  of  mankind  were  differentiated  from  each  other 
before  the  time  when  the  records  of  written  history  were  begun. 

The  White  race,  which  may  include  individuals  all  the  way  from 
light  blondes  to  dark  brunettes,  is  frequently  further  divided  into 
at  least  four  sub-races,  namely,  the  Mediterranean,  Armenoid, 
Alpine,  and  Nordic.  Individuals  of  the  Mediterranean  sub-race  are 
typically  short,  slender,  olive-skinned,  narrow-nosed,  and  long- 
headed. They  include  various  peoples  of  the  Mediterranean  coasts, 
Spaniards,  Portuguese,  Greeks,  Cretans,  some  Italians,  Persians, 
Berbers,  Arabs,  Phoenicians,  most  Egyptians,  and  some  English. 
The  Armenoid  sub-race,  characterized  frequently  by  a  prominent 
convex  nose,  have,  in  many  instances,  a  decided  flair  for  commerce, 
and  an  outstanding  capacity  for  survival  and  advancement  even 
under  adversity.  They  include  Armenians,  Turks,  Syrians,  some 
Persians,  and  certain  Jews.  The  Alpine  sub-race  is  made  up  for  the 
most  part  of  stocky,  round-headed  people,  including  some  Russians, 
Greeks,  Swiss,  North  Italians,  South  Germans,  Balkans,  Czechs, 
Poles,  and  French.     The  members  of  the  Nordic  sub-race  are  charac- 


562 


THE  CHANGING  WORLD 


teristically  tall,  fair-haired,  and  blue-eyed,  with  narrow  faces  and 
well-developed  chins.  They  include  Scandinavians,  North  Germans, 
Netherlanders,  Flemings,  many  English,  Scotch,  and  Russians. 

The  Yellow-Brown  race  is  characterized  usually  by  straight  dark  hair, 
high  cheek  bones,  apparently  slanting  eyes,  and  broad  heads.  They  in- 
clude the  various  yellow  Mongolians,  American  Indians,  Eskimos,  and 
Malays,  also  early  inhabitants  of  the  New  World,  such  as  the  Aztecs  of 
Mexico,  the  Mayas  of  Central  America,  and  the  vanished  Incas  of  Peru. 

Finally,  the  people  of  the  Black  race,  with  narrow  heads,  wide 
noses,  thick  nonapelike  lips,  include  Negroes,  Pygmies,  Melanesians, 
and  Polynesians. 

In  the  great  centers  of  population,  such  as  Europe  and  the  Orient, 
as  well  as  in  the  immigrant-filled  land  of  the  United  States,  hybridi- 
zation of  different  strains  of  humanity  has  gone  so  far  that  it  has 
become  very  difficult  to  draw  racial  lines  of  demarcation  upon  any 
satisfactory  biological  basis.  Moreover,  it  makes  considerable  dif- 
ference who  does  the  classifying,  for  it  seems  to  be  almost  impossible 
to  ehminate  the  subjective  factor  of  prejudice,  when  pride,  patriotism, 
and  personal  bias  are  involved,  as  they  are  in  racial  matters. 

Mongolian  America-n  Alnina  "Jbt^clic 

Indian     QlP'"®      /  yvediterranean. 

Semitic 
HamitJC 


fVelyravidian 
AusLra-lian 


Kectnderthal 
Derivation  of  races.     (After  Bean.) 

The  accompanying  diagram  indicates  one  expert's  idea  of  the 
relation  of  the  principal  races  and  sub-races  of  mankind  to  each  other. 


THE  ANIMAL,   MAN    (ANTHROPOLOGY) 


563 


Passing  Muster 

The  analysis  of  physical  differences,  by  means  of  which  individuals 
can  pass  muster  in  order  to  be  assigned  to  a  particular  race  of  man- 
kind, is  based  upon  certain  generally  accepted  measurements.  Lord 
Kelvin,  the  physicist,  once  wrote : 


Landmarks  for  making  anthropological  measurements,  o/,  alare ;  en,  eurion ; 
gh  glabella;  gn,  gnathion ;  na,  nasion ;  op,  opisthocranium ;  sn,  subnasale; 
zy,  zygnion.     (After  Sullivan.) 

"  I  often  say  that  when  you  can  measure  what  you  are  speaking  about, 
and  express  it  in  numbers,  you  know  something  about  it,  but  when  you 
cannot  express  it  in  numbers,  your  knowledge  is  of  a  meager  and  unsatis- 
factory kind ;  it  may  be  the  beginning  of  knowledge,  but  you  have  scarcely 
in  your  thoughts  advanced  to  the  stage  of  science,  whatever  the  matter 
may  be." 

The  science  of  measuring  man  is  called  Anthro'pometry.  Dr. 
Louis  R.  Sullivan,  formerly  anthropologist  at  the  American  Museum 
of  Natural  History  in  New  York  City,  has  prepared  a  compact 
pocket  manual,  entitled  Essentials  of  Anthropometry,  designed 
particularly  to  aid  travelers  and  students  generally,  who  may  be 
interested  in  the  biological  side  of  racial  problems  among  the  various 
peoples  with  whom  they  come  in  contact.  In  this  excellent  little 
manual  Dr.  Sullivan  indicates  six  essential  dimensions  as  a  minimum, 
namely,  head  breadth,  face  breadth,  nasal  width,  head  length,  face 
height,  and  nasal  height,  from  which  four  critical  ratios,  or  indices, 


564 


THE  CHANGING  WORLD 


may  be  derived.  They  are  cephalic,  cephaHc-facial,  facial,  and  nasal 
indices.  To  these  measurements  he  adds  a  list  of  nine  easy  ohserva- 
tions  which  it  is  desirable  to  make  in  order  to  supplement  the  indices, 
and  from  this  small  array  of  fundamental  data  "we  have  a  key  to  the 
relationship  of  racial  groups  in  any  part  of  the  world." 

In  making  the  measurements  for  these  indices  it  is  necessary  to 
locate  eight  landmarks  on  the  head,  which  are  shown  in  the  diagram. 
The  measurements  are  not 
difficult  to  make,  if  one 
is  supplied  with  calipers, 
such  as  are  illustrated. 
The  nine  observations 
recommended  are  to  de- 
termine skin  color,  hair 
color,  hair  form,  eye  color, 
the  presence  or  absence 
of  the  epicanthic  fold  in 
the  inner  angle  of  the 
eye,  thickness  of  the  lips, 
character  of  incisor  teeth, 
amount  of  beard  natu- 
rally present,  and  the 
degree  of  hairiness  of  the 

body.  While  many  other  measurements  and  observations  are  fre- 
quently made  by  scientists  with  particular  ends  in  view,  these  six 
essential  dimensions,  and  the  ratios  derived  from  them,  together  with 
the  nine  supplementary  determinations  by  observation,  are  regarded 
as  a  minimum  sufficient  to  furnish  a  definite  racial  picture. 

The  satisfaction  of  human  curiosity  in  such  matters  as  obtaining 
anthropological  data  was  not  always  regarded  as  a  commendable 
pursuit.  Professor  Hooton  points  out  that  an  Act  of  Queen  Eliza- 
beth (1579-1598)  declared  all  persons  "fayning  to  have  a  knowledge 
of  Phisiognomie  or  like  Fantasticall  Ymaginaccions "  were  liable 
"to  be  stripped  naked  from  the  middle  upward  and  openly  whipped 
untill  his  body  be  bloudye."  Thus  were  the  beginnings  of  scientific 
endeavor  penalized  in  the  days  of  Shakespeare  and  good  Queen  Bess  ! 

The  Biological  Garden  of  Eden 

Among  the  unsolved  riddles  that  engage  the  anthropologist  is  the 
question  of  the  actual  time  and  place  of  human  origin.     One  of  the 


Spreading  calipers  as  they  are  held  while  being 

used. 


THE   ANIMAL,   MAN    (ANTHROPOLOGY)  565 

outstanding  differences  between  man  and  his  animal  relatives  is  his 
insatiable  intellectual  curiosity,  that  leads  him  to  speculate  even  upon 
questions  which  he  cannot  always  answer.  Just  wluni  and  where  did 
mankind  graduate  from  the  long  drawn-out  school  of  animal  life, 
and  become  qualified  to  be  called  human?  Did  man's  emergence 
from  his  animal  ancestry  occur  once  only  in  a  hypothetical  Garden  of 
Eden  from  which  starting  point  he  spread  over  the  earth,  as  is  in- 
ferred to  be  the  manner  of  origin  generally  assumed  in  the  case  of 
different  animal  and  plant  species,  or  was  the  faint  but  important 
original  line  of  demarcation  between  animals  and  mail  crossed 
repeatedly  in  various  localities  by  different  ancestral  lines,  which 
have  contributed  eventually  to  the  compound  make-up  of  what  we 
call  a  human  being?  No  direct  answer  can  be  made  now  to  the 
question  of  human  origin,  nor  in  all  probability  can  ever  be  made. 
What  difference  does  it  make,  when  and  where  man  first  came  upon 
the  stage?  The  important  thing  is  that  he  has  arrived  and  domi- 
nates life  on  the  earth  today. 

Asia  is  regarded  by  many  scientists  as  the  probable  original  home 
of  humanity.  One  of  the  reasons  for  this  opinion  is  the  fact  that  the 
vast  continent  of  Asia  is  geographically  adequate  to  have  been  the 
region  from  which  man  set  out  to  overcome  the  world.  It  has 
a  sufficiently  large  area  and  is  now,  or  has  been  in  the  past,  linked 
by  land  bridges  with  other  important  land  areas  on  the  earth,  which 
would  allow  for  human  dispersal  as  we  see  it  today.  Moreover, 
there  is  geological  evidence  that  it  has  been  continental  land  since 
long  before  the  Pleistocene  Period,  when  the  first  known  traces  of 
man  appeared.  The  earliest  civilizations  marked  by  historical  re- 
mains, and  the  first  known  domestic  animals  also,  are  of  Asiatic  origin. 
That  famous  quartet  of  the  oldest  authentic  human  fossils,  namely 
the  Piltdowner  of  England,  the  Heidelberg  Jaw  of  Germany,  the 
Peking  skeletons  of  China,  and  the  Ape-man  from  Java  are  all  far 
distant  from  the  ancient  central  plateau  of  Asia.  Dr.  W.  D.  Matthews 
has  pointed  out  that  in  evolution  the  most  highly  specialized  and  the 
most  recent  types  of  a  series  will  be  found  distributed  near  their 
point  of  origin,  while  the  more  primitive  and  older  representatives 
of  a  species,  which  have  had  more  time  to  explore  the  world,  will  be 
found  farthest  away  from  the  original  starting  point.  This  is 
"Matthews'  Law,"  and  it  is  borne  out  in  the  case  of  man,  if  Asia  be 
regarded  as  the  "Garden  of  Eden"  in  which  mankind  began  his 
notable  career.     Naturally  the  spread  of  mankind,  from  whatever 


566  THE  CHANGING  WORLD 

point  of  origin,  extended  over  a  long  period  of  time,  and  must  not  be 
pictured  in  terms  of  modern  means  of  travel.  There  is  very  little 
reason  to  suspect  that  the  great  transition  from  nonman  to  man 
was  in  any  way  an  abrupt  event. 

It  may  be  appropriate  to  bring  this  chapter  on  Anthropology  to  a 
close  with  the  following  quotation  from  that  genial  old  Roman  drama- 
tist, Terence :  Homo  sum !  humani  nihil  a  me  alienum  puto.  (I  am 
a  man ;  and  I  think  nothing  appertaining  to  mankind  is  foreign  to 
me.) 

SUGGESTED  READINGS 

Andrews,  R.  C.,  On  the  Trail  of  Ancient  Man,  G.  P.  Putnam's  Sons,  1926. 

The  barren  wastes  of  the  Gobi  Desert,  sand  storms  and  blizzards,  hostile 

tribes,  chasing  antelopes  in  automobiles,  and  toilsome  digging  for  old 

bones  alternate  in  these  vivid  pages. 
Bean,  R.  B.,  The  Races  of  Man,  The  University  Society,  1932. 

Differentiation  and  dispersal  of  mankind,  treated  clearly  in  a  few  read- 
able pages. 
Carrel,  Alexis,  Man,  the  Unknown,  Harper  &  Bros.,  1935. 

A  stimulating  book  by  a  Nobel  prize  man. 
Gregory,  W.  K.,  Our  Face  from  Fish  to  Man,  G.  P.  Putnam's  Sons,  1929. 

An  admirable  illustrated  account  of  human  evolution  as  related  by  one 

of  our  foremost  comparative  anatomists. 
Jones,  F.  W.,  Arboreal  Man,  Longmans,  Green  &  Co.,  1916. 

The  thesis  of  man's  arboreal  origin  convincingly  presented. 
Haddon,  A.  C,  Races  of  Man  and  Their  Distribution,  The  Macmillan  Co., 

1925. 

Standard  presentation  for  the  beginner.     One  of  the  best. 
Hooton,  E.  A.,  Uy  from  the  Ape,  The  Macmillan  Co.,  1931. 

Evolution  of  man  from  the  primates.     Although  scholarly  and  rather 

exhaustive,  it  is  alluringly  readable  because  of  welcome  oases  of  humor. 
MacCurdy,  G.  G.,  The  Coming  of  Man,  The  University  Society,  1932. 

Prehistoric  man,  his  remains  and  phases  of  his  culture,  and  his  relation 

to  other  primates,  treated  authoritatively  with  clearness  and  brevity. 
MacCurdy,  G.  G.,  Human  Origins,  D.  Appleton  &  Co.,  1924. 

A  scientific  study  of  man's  culture  in  the  Old  and  New  Stone  ages,  and 

the  ages  of  Bronze  and  Iron. 
Osborn,  H.  F.,  Men  of  the  Old  Stone  Age,  Charles  Scribner's  Sons,  1919. 

Authoritative,  detailed,  and  fully  illustrated. 
Sullivan,  L.  R.,  Essentials  of  Anthropometry,  Am.  Mus.  Nat.  Hist.,  1923. 
Wilder,  H.  H.,  The  Pedigree  of  the  Human  Race,  Henry  Holt  &  Co.,  1926. 
Yerkes,  R.  M.,  Almost  Human,  The  Century  Co.,  1925. 

A  popular  account  of  experiences  with  apes. 


MAN   AS   A   CONQUEROR 
XXIV 


MAN'S   CONQUEST   OF  NATURE 

Preview.  Has  man  conquered  his  environment?  •  The  historical  set- 
ting •  Methods  employed  •  Economic  value  of  plants  and  animals :  Uses 
of  animals ;  indirect  economic  value  of  plants  and  animals  •  The  other 
side  of  the  picture  •  Harm  done  by  plants  •  Harm  done  by  animals  • 
Methods  of  control  •  Suggested  readings. 

PREVIEW 

If  any  one  of  us  could  have  looked  in  on  a  group  of  our  caveman 
ancestors  with  a  view  to  comparing  their  control  of  the  environment 
with  that  of  the  average  man  of  today,  there  is  no  doubt  of  what  we 
would  say.  Modern  man  has  quite  thoroughly  conquered  his  environ- 
ment and  has  control  of  its  living  as  well  as  its  nonliving  factors.  He 
has  by  means  of  his  superior  mental  make-up  gained  control  over  his 
lower  brute  companions  and  molded  their  lives  to  his  needs.  He  has 
conquered  the  forces  of  nature ;  harnessed  water  serves  him  with 
power;  irrigation  ditches  make  desert  areas  available  for  his  crops 
and  herds.  He  has  analyzed  soil  so  that  he  knows  what  crops  grow 
best  under  given  soil  conditions ;  he  has  harnessed  winds  and  made 
them  pump  water  and  hoist  loads  ;  he  has  learned  how  to  use  the  sun's 
heat  and  how  to  protect  himself  from  the  numbing  cold ;  he  has 
controlled  water  and  lighted  his  cities  and  his  homes,  and  yet,  is  he  a 
real  conqueror?  Are  all  of  his  efforts,  directed  as  they  are  by  science, 
ultimately  successful?  Is  he  truly  the  conqueror  of  his  environment 
and  the  master  of  his  future?  Physically  man  has  done  much  and 
done  it  well,  yet  he  has  made  mistakes  due  to  lack  of  complete 
knowledge,  to  misdirected  enthusiasm,  or  to  bias.  Potentially  man 
is  a  conqueror,  but  he  cannot  always  overcome  selfishness,  egotism, 
and  the  lack  of  complete  knowledge  which  is  essential  to  an  attack  on 
any  scientific  problem.  He  cuts  away  forests  to  clear  land  which  will 
produce  his  crops,  at  the  same  time  bringing  down  floods  and  disaster ; 
he  builds  dams  to  harness  water  power,  while  neglecting  to  provide 
the  right  kinds  of  waterways  for  fish  that  spawn  in  the  upper  reaches 
of  those  rivers  ;  he  overcomes  one  pest  but  introduces  another  in  his 
H.  w.  H.  — 37  567 


568  MAN  AS   A   CONQUEROR 

anxiety  to  obtain  cheaper  building  materials.  He  makes  mistakes 
and  those  mistakes  cost  him  dearly. 

Other  factors  enter  into  the  picture.  The  biologist  knows  that  the 
insects  which  inhabited  this  earth  millions  of  years  before  man  came 
on  it  have  been,  and  still  are,  the  most  successful  group  of  animal?. 
They  are  adapted  in  many  ways  to  escape  enemies.  They  reproduce 
in  great  numbers  and  very  frequently.  They  are  omnivorous  feeders, 
and  numerically  they  outnumber  all  the  other  species  of  animals. 
Dr.  Howard  in  a  recent  work  ^  points  out  the  fact  that  while  man  has 
jumped  to  the  fore  through  his  intelligence,  this  same  intelligence 
may  ultimately  be  his  undoing,  because  he  is  giving  to  his  insect  com- 
petitors through  his  agricultural  presents  to  them  more  and  more 
food  and  thus  opportunity  for  more  rapid  increase.  These  facts 
certainly  should  make  us  question  man's  supremacy,  unless  he  can 
plan  more  wisely  for  the  future. 

There  are  many  agencies  working  toward  the  goal  of  man's  ultimate 
conquest  of  his  natural  environment.  Most  of  these  agencies  are 
well  directed,  sane,  and  based  on  the  best  findings  of  science.  But 
man,  with  his  foibles,  his  illogical  thinking,  his  greed  and  selfishness, 
introduces  other  factors.  Particularly  we  have  in  this  democracy  of 
ours  the  leadership  of  the  politician,  the  grafter,  and  mercenary 
private  interests  to  contend  with.  To  fight  these  obstructive  forces 
we  must  know  the  facts  and  then  go  ahead  as  real  scientists,  prepared 
to  use  the  facts  w^isely.  The  pages  that  follow  should  help  clarify  our 
thinking  concerning  some  of  the  problems  of  economic  biology  and 
biological  conservation. 

Has  Man  Conquered  His  Environment? 

A  little  over  three  hundred  years  ago  our  Pilgrim  forefathers  landed 
on  the  shores  of  Massachusetts  Bay.  They  found  wooded  lands, 
rocky  hills,  with  clear  streams  winding  through  shallow  valleys  filled 
with  heavy  undergrowth.  The  land  was  gradually  cleared,  farms  were 
established,  and  settlements  came  into  being.  Today  the  countryside 
looks  very  different  from  the  days  when  those  colonists  reached  an 
inhospitable  shore.  And  yet  in  the  last  fifty  years,  changes  have  been 
going  on  that  are  beginning  to  show  how  nature  takes  a  part  even 
when  man  has  seemingly  made  a  complete  conquest  of  the  land  which 
he  set  out  to  conquer.  In  the  last  half  century  many  New  York  and 
New  England  farms  have  been  abandoned,  the  countryside  between 

'  Howard,  L.  O.,  The  Insect  Menace,  Century  Company,    1934. 


MAN'S   CONQUEST   OF   NATURE  569 

numerous  towns  and  villages  going  back  to  its  original  state  of  wood- 
land. Everywhere  in  nature  we  see  this  tendency  to  establish  a  bal- 
ance and  whenever  man  steps  in  to  upset  the  balance  that  nature  has 
established,  sooner  or  later  other  living  things  tend  to  re-establish  it. 
In  the  case  where  man  cuts  the  forests,  clears  the  land,  and  does 
not  grow  crops  this  balance  is  lost.  With  trees  and  cover-plants 
destroyed,  the  soil  is  unprotected  against  storms  of  rain  or  wind  and 
consequently  water  digs  gullies  and  wind  carries  off  the  surface  soil, 
to  the  ultimate  wastage  of  the  land.  If  man  covers  the  cleared  area 
with  crops,  a  certain  amount  of  protection  is  insured  the  land,  but  the 
original  fauna  and  flora  will  probably  never  again  be  established. 
Our  prairies  were  once  covered  with  plants  that  have  now  disappeared 
as  a  living  covering.  They  have  been  replaced  by  crops  of  domesti- 
cated grasses  and  grains,  or  by  various  "hitch  hikers"  from  the  ends 
of  the  earth  —  outcasts  from  man's  estate  —  weeds.  Indigenous 
animals  to  a  great  extent  are  gone  also,  often  being  replaced  by  the 
hangers-on  of  man's  migrations,  rats  and  mice,  dogs  and  cats,  and 
foreigners  such  as  English  sparrows  and  starlings.  Man  may  seem  to 
have  conquered  his  environment,  but  when  we  note  dust  storms  in 
the  central  west,  hurricanes  in  the  east,  and  frosts  in  our  semitroi3ical 
southlands,  along  with  countless  hordes  of  insect  pests,  we  may  with 
justice  wonder  if  man  really  is  in  absolute  control  of  the  situation. 

The  Historical  Setting 

The  history  of  man's  domestication  of  plants  and  animals  is  a  story 
which  is  only  partly  known.  Just  when  this  process  began  is  con- 
jecture. We  do  know  that  at  a  very  early  period  primitive  men  living 
in  the  southern  part  of  Europe,  as  well  as  an  area  in  Asia  and  northern 
Africa,  probably  began  the  domestication  of  some  of  our  common 
plants  and  animals.  The  how  and  why  of  man's  control  is  also  largely 
problematical.  As  nomadic  life  changed  to  a  more  settled  form  of 
residence  it  is  easy  to  see  that  a  food  supply  that  did  not  have  to  be 
hunted  was  desirable.  Doubtless  women  first  discovered  the  values 
of  wild  grains  and  fruits,  resulting  in  primitive  methods  of  cultivation 
that  led  to  the  selection  of  seeds  from  better  fruits  for  future  plantings. 
We  know  that  rice  has  been  cultivated  for  over  5000  years  and  many 
of  our  common  grains  for  an  even  longer  period.  The  remains  of 
Swiss  lake  dwellings  which  date  back  to  about  10,000  B.C.  show  that 
oats,  barley,  millet,  flax,  and  such  fruits  as  the  apple,  pear,  and  grape 
were  known  and  probably  cultivated.     In  the  Americas,  corn  was 


570  MAN   AS   A   CONQUEROR 

cultivated  in  great  terraced  fields  at  the  time  of  the  Incas.  When 
Jacques  Cartier  first  viewed  the  site  of  the  present  city  of  Mon- 
treal, he  saw  there  a  village  surrounded  by  cornfields.  From  earli- 
est times  the  growing  of  grains  and  the  progress  of  civilization  have 
gone  hand  in  hand. 

Sheep,  cattle,  swine,  and  dogs  appear  to  have  been  domesticated 
as  far  back  as  the  Bronze  Age.  The  dog  was  probably  one  of  the 
first  animals  used  by  man,  its  domestication  making  possible  that  of 
other  animals,  especially  sheep,  goats,  and  cattle.  The  horse,  which 
must  have  roamed  wild  in  Europe  during  the  Old  Stone  Age,  was  then 
used  for  food  by  the  savage  cavemen.  Later  horses  were  domesti- 
cated, but  there  are  no  authentic  records  of  their  use  until  about 
2000  B.C.,  when  they  were  used  in  Babylon,  and  three  hundred  years 
later,  when  they  were  introduced  into  Egypt.  They  reached  their 
peak  of  usefulness  in  quite  recent  times. 

Looking  back  on  the  history  of  agriculture  we  find  that  it  is  a 
story  of  very  gradual  crop  improvement,  both  in  yield  and  quality 
of  product.  Take,  for  example,  the  staple  wheat.  While  the  exact 
form  of  the  parent  wheats  is  not  known,  we  do  know  that  a  wild  wheat 
(an  emmer)  grows  today  without  cultivation  in  the  highlands  of  Syria 
and  Palestine.  As  far  back  as  300  B.C.  Theophrastus,  the  Greek 
"Father  of  Botany,"  reported  several  varieties  of  wheat.  Different 
types  of  Indian  corn,  flint,  sweet,  soft,  and  popcorn,  were  known  as 
early  as  800  a.d.  in  the  Mayan  cities  of  Yucatan,  while  as  many  as 
1000  varieties  of  rice  are  said  to  exist  in  India  and  China,  where  rice 
was  probably  first  cultivated. 

The  early  use  of  plants  must  have  been  merely  to  piece  out  the 
family  food  supply  as  hunting  became  poorer.  Then  as  domestication 
of  animals  took  place  and  man  ceased  a  nomadic  existence,  grains 
were  used  as  food  for  cattle  and  horses.  At  still  later  stages  of  his 
civilization  man  began  to  work  for  qualities,  which  were  not  thought 
of  in  earlier  civilizations ;  more  abundant  or  better  fruits  and  grains, 
stronger  beasts  of  burden,  swifter  horses,  a  better  milk  supply,  and 
fleece  that  would  supply  better  material  for  yarns. 

Since  man  only,  of  all  the  animals,  is  able  to  make  a  record  of  what 
he  has  learned  and  to  hand  this  knowledge  down  to  the  next  genera- 
tion, the  results  of  this  social  inheritance  are  seen  in  the  plant  and 
animal  production  of  today.  First  man,  or  more  likely  the  woman 
who  did  the  work,  must  have  noticed  that  certain  plants  grew  better 
and  produced  larger  crops  and  more  desirable  fruits  when  given  more 


MAN'S  CONQUEST  OF  NATURE  571 

sunlight,  water,  cultivation,  or  fertilizer.  Along  with  this  eame  the 
seizing  upon  favorable  variations  and  their  continuance  by  cultivation. 
Lack  of  precise  knowledge  prevented  certain  success,  and  progress 
was  slow.  Crop  production,  moreover,  has  always  been,  and  will 
continue  to  be,  to  a  large  degree  deperdent  upon  the  vagaries  of  the 
weather,  as  the  effects  of  the  recent  draughts  in  the  United  States 
prove.  Nevertheless,  as  familiarity  with  different  crop  requirements 
increased,  improvement  in  planting  and  care  of  the  land  has  resulted. 

Methods  Employed 

As  far  back  as  Roman  times,  agriculture  was  well  advanced,  for  the 
Roman  farmer  plowed,  fertilized,  and  irrigated  his  land.  Later,  un- 
der the  feudal  system  of  the  Dark  Ages  agriculture  declined  for  the 
reason  that  the  peasants  were  uneducated  and  their  lords  interested  in 
war  rather  than  in  the  pursuits  of  peace.  It  was  not  until  the  coming 
of  the  eighteenth  century  that  revolutionary  changes  began  to  take 
place  in  agricultural  methods  through  the  practice  of  crop  rotation, 
and  the  growing  of  such  crops  as  would  pro^'idc  food  for  stock  during 
the  winter  season.  Agriculture  at  the  present  time  has  become  a 
science,  and  should  be  looked  upon  as  a  profession.  Knowledge 
necessary  to  increased  crop  or  stock  production  is  disseminated 
through  various  channels,  such  as  farm  bureaus,  the  publications  of 
the  Department  of  Agriculture,  various  state  agencies  such  as  agri- 
cultural schools  and  colleges,  the  public  school,  and  public  press.  The 
application  of  science  to  disease  in  both  animals  and  plants  has  played 
an  important  part  in  promoting  agriculture  and  animal  husbandry, 
as  is  seen  in  the  successful  battle  waged  against  many  plant  and  animal 
parasites.  The  science  of  entomology  aids  the  farmer  by  furnishing 
him  with  the  knowledge  of  life  histories  of  insects,  of  their  methods  of 
feeding,  and  of  their  natural  enemies,  indigenous  or  imported.  Ani- 
mals and  plants  introduced  from  the  far  corners  of  the  earth  have 
been  made  available  with  resulting  benefit  to  the  farmer. 

It  should  be  noted  that  relatively  little  advance  in  plant  and  animal 
improvement  would  have  been  possible  had  it  not  been  for  the  applica- 
tion of  certain  scientific  principles  explained  in  other  pages  of  this 
book.  Although  man  had  bred  plants  and  animals  for  many  thou- 
sands of  years,  it  had  been  a  very  unscientific  procedure,  conducted  by 
a  "  hit  and  miss  "  method.  Long  before  the  rediscovery  of  Mendel's 
laws  in  1900,  man  had  used  selection  to  improve  his  stock  and  nature 
had  helped  by  occasionally  producing  hybrids  which  could  be  propa- 


572  MAN  AS  A  CONQUEROR 

gated  asexually.  Burbank's  well-known  adventure  with  the  potato 
seed-ball  was  doubtless  due  to  the  fact  that  the  flower  which  produced 
this  seed-ball  had  been  pollinated  from  another  plant  with  different 
qualities  from  those  of  the  Early  Rose  potato  plant  that  produced  the 
seed-ball.  All  that  is  known  of  this  story  is  what  has  been  told  by 
Mr.  Burbank,  how  he  discovered  the  seed-ball,  watched  it  develop, 
and  the  following  year  planted  its  seeds.  He  tells  of  the  great 
variation  in  the  offspring  which  grew  from  these  seeds  and  of  his 
selection  for  propagation  of  the  tubers  from  one  of  the  plants  that 
gave  rise  to  the  famous  Burbank  potato,  still  one  of  the  most  popular 
products  of  the  potato  industry. 

The  case  just  cited  illustrates  one  of  the  most  common  methods  used 
by  plant  and  animal  breeders  today.  It  has  been  recognized  that 
two  types  of  variations  exist  in  nature.  The  first  is  that  of  so-called 
fluctuating  variations,  seen  in  all  living  things,  which,  for  example, 
result  in  the  bearing  of  a  number  of  fruits  or  seeds  of  different  sizes 
by  a  single  plant,  or  leaves  of  slightly  differing  shape  by  a  tree.  Such 
variations,  however,  as  the  agriculturist  knows  are  not  handed  down 
from  one  generation  to  the  next.  The  second  type  of  variations  is 
called  mutations  or  discontinuous  variations.  This  knowledge  has 
quite  revolutionized  the  methods  of  plant  and  animal  breeders,  and 
they  now  attempt  to  find  and  propagate  mutants,  instead  of  trying 
to  make  use  of  variations  that  are  not  capable  of  being  handed  down 
to  the  next  generation. 

Methods  used  in  selection  have  also  changed.  We  use  selection 
for  plant  and  animal  betterment,  but  we  do  not  necessarily  always 
select  the  best  appearing  fruits  or  largest  seeds  for  future  planting. 
As  Donald  F.  Jones  has  well  said,  "Science  now  shows  how  a  bumper 
crop  of  all  good  ears  may  be  grown  from  nubbins,  but  they  must  be 
the  right  kind  of  nubbins."  ^ 

Most  important  of  the  investigations  in  the  research  program  of  the 
Department  of  Agriculture  is  the  search  for  a  "superior  germplasm." 
When  such  a  superior  stock  becomes  available,  it  is  perfected  and 
the  results  turned  over  to  the  practical  breeder  for  perpetuation. 
The  isolation  of  strains  having  superior  breeding  possibilities  is  of 
tremendous  value  to  the  farmer  because  it  not  only  enables  him  to 
grow  more  plants  in  a  given  area,  but  also  plants  of  better  quality. 
In  1935,  the  parasitic  organism,  stem  rust,  cost  the  farmers  in  North 
Dakota  alone  $100,000,000.     Since  over  100  strains  of  black  stem  rust 

1  JSast,  E.  M.,  Biology  in  Human  Affairs,  McGraw-Hill,  1931. 


MAN'S   CONQUEST   OF   NATURE  573 

have  already  been  found,  it  is  a  very  serious  enemy  of  the  wheat 
crop.  Fortunately,  in  the  epidemic  of  1935  a  new  spring  wheat, 
the  Thatcher,  developed  by  the  Minnesota  Agricultural  Experiment 
Station  in  co-operation  with  the  United  States  Department  of  Agri- 
culture, proved  resistant  to  all  known  strains  of  rust. 

A  recent  exhibition  in  the  Department  of  Agriculture  displayed 
about  150  new  superior  varieties  of  field  crops.  No  less  than  eight  su- 
perior wheats,  among  them  Turkey,  Thatcher,  Marquis,  and  Kanred, 
with  several  new  varieties  of  oats  and  barley,  are  now  cultivated  on 
more  than  40  million  acres  of  crop  land  each  year.  New  varieties 
of  potatoes,  such  as  the  Katahdin,  resist  some  of  the  serious  diseases 
of  potatoes.  Peas  and  melons  unaffected  by  parasitic  wilt  have 
been  developed,  while  fruits  of  superior  color,  appearance,  and  keep- 
ing quality  have  been  evolved. 

In  livestock,  animals  have  been  produced  that  show  greater  resist- 
ance to  disease,  larger  body  size,  better  growth,  better  performance, 
and  greater  fecundity.  In  breeding  these  animals,  it  has  been  found 
that,  through  a  use  of  Mendel's  laws,  certain  of  these  characteristics 
are  shown  to  breed  true,  since  they  are  alike  in  both  parents.  An 
outstanding  successful  strain  of  cattle,  known  as  the  Santa  Gertrudis, 
has  been  recently  developed  in  Texas.  The  Department  of  Agricul- 
ture is  now  experimenting  with  crosses  of  Brahman  and  Aberdeen- 
Angus  breeds  of  cattle  to  establish  certain  desirable  characteristics. 
A  new  strain  of  sheep  known  as  the  "Columbia  type,"  which  is 
particularly  adapted  to  the  rather  rigorous  regions  of  the  Northwest, 
has  been  developed  from  the  Rambouillet  and  Lincoln  breeds  of 
sheep.  Crosses  of  poultry  have  been  bred  which  produce  as  many  as 
300  eggs  per  hen  per  year,  as  against  less  than  100  eggs  from  the 
average  hen.  At  some  state  experiment  stations  certain  cows  of  su- 
perior breeds  have  been  found  to  produce  as  much  as  1000  pounds 
of  butterfat  per  year,  while  the  average  cow  produces  little  more 
than  200  pounds  per  year.  These  are  only  a  few  of  the  accomplish- 
ments brought  about  by  practical  breeding  experiments  in  this 
country. 

Economic  Values  of  Plants  and  Animals 

The  results  of  this  gradual  domestication  of  plants  and  animals  are 
seen  today  in  the  very  great  value  of  our  agricultural  products  and 
farms.  According  to  census  reports  the  value  of  farm  property  in 
the  United  States,  in  spite  of  the  long  period  of  depression,  is  more  than 


574  MAN  AS  A  CONQUEROR 

that  invested  in  the  manufactures  of  this  great  producing  country 
of  ours.  Diversified  farming  is  becoming  more  and  more  general. 
Market-gardening  forms  the  lucrative  business  of  many  thousands 
of  people  near  our  great  cities,  and  in  many  of  our  southern  states 
where  raising  cotton  has  given  place  to  diversified  farming.  With 
improved  methods  of  canning  and  preserving,  over  $165,000,000  worth 
of  fruits  and  vegetables  are  used  annually  in  addition  to  fresh  garden 
products  sold  in  markets  or  consumed  by  the  grower. 

Orchard  and  other  fruits  play  an  important  part  in  agriculture. 
The  citrus  crop  of  the  world  has  greatly  increased  in  recent  years 
because  of  the  dissemination  of  knowledge  of  its  value  in  producing 
vitamins.  Grapes  are  commercially  valuable  for  wine  and  raisins, 
while  figs,  olives,  and  dates  play  important  parts  as  staple  foods  in 
many  parts  of  the  world.  Nuts  of  various  kinds  are  valued  sources  of 
oils  and  proteins.  Sugar  comes  from  sugar  cane,  beets,  and  the  maple, 
its  manufacture  ranking  as  an  important  industry  in  many  parts  of 
the  world. 

Tea  leaves  with  coffee  and  cocoa  beans  form  the  basis  of  man's 
most  important  beverages.  The  annual  tea  production  of  the  world 
is  estimated  at  over  17,000,000,000  pounds,  while  coffee  has  a  yearly 
production  of  over  3,000,000,000  pounds.  Cocoa,  with  an  annual 
production  of  close  to  1,000,000,000  pounds,  is  used  in  candy-making 
as  well  as  furnishing  the  basis  for  a  variety  of  beverages. 

Spices  of  various  kinds,  vegetable  oils,  and  various  drugs  are  all 
plant  products  of  considerable  economic  importance. 

Fiber  plants  rank  high  in  our  list  of  economically  valuable  crops. 
Cotton,  in  addition  to  its  use  in  the  home,  has  an  important  place 
in  the  manufacture  of  cellophane,  guncotton,  smokeless  powder, 
and  as  the  basis  of  celloidin  lacquers  and  varnishes  so  necessary 
in  the  automobile  industry.  From  its  seeds  a  valuable  oil  is  de- 
rived, while  its  refuse  makes  fodder  for  cattle.  Other  important 
fiber  crops  are  flax,  the  bast  fibers  of  which  are  made  into  linen, 
while  hemp,  abaca,  sisal,  and  henequen  are  used  for  making  twine 
and  rope. 

The  values  of  forest  products  need  only  be  mentioned.  Wood  is 
important  in  the  construction  of  buildings,  shipbuilding,  airplane 
construction,  furniture,  and  trim  as  well  as  in  the  rayon  and  paper 
industries.  Scores  of  important  chemicals  are  derived  from  wood. 
Man  still  uses  a  surprisingly  large  amount  of  wood  for  fuel,  especially 
where  forests  are  still  existent.     The  latex  of  the  BraziHan  rubber  tree 


MAN'S   CONQUEST   OF  NATURE  575 

(Hevea  hrasiliensis)  and  other  rubber-producing  plants,  various  resins 
and  gums,  tannin,  and  cork  are  all  important  forest  products. 

Uses  of  Animals 

It  would  seem  unnecessary  to  list  all  of  the  animal  series  that  man 
uses  as  food,  but  we  cannot  look  at  the  census  statistics  without  seeing 
the  direct  value  in  dollars  and  cents  of  our  meat-producing  mammals. 
Three  bilhon  dollars'  worth  of  such  animals  is  a  pretty  large  investment, 
even  in  so  rich  a  country  as  the  United  States.  In  addition,  there  are 
the  various  products  which  come  from  cattle,  namely,  milk,  butter, 
cheese,  and  leather.  A  few  wild  mammals  such  as  deer,  bears,  and,  in 
the  arctic  regions,  seals  and  walruses  are  also  used  for  food.  Birds 
both  wild  and  domesticated,  and  their  eggs,  form  part  of  our  food 
supply,  although  wild  game  birds  are  disappearing  so  rapidly  that  we 
cannot  consider  them  as  a  source  of  food  except  among  the  Eskimos 
of  the  arctic  region.  Amphibians,  for  example  the  large  bullfrogs,  fur- 
nish food  for  epicures,  while  some  reptiles,  such  as  the  iguana  and 
even  snakes,  are  eaten  in  some  parts  of  the  world.  There  are  edible 
salt-water  turtles,  too,  many  of  large  size,  the  leatherback  and  the 
green  turtle  often  weighing  six  to  seven  hundred  pounds  each.  The 
flesh  of  the  diamond-back  terrapin,  an  animal  found  in  the  salt  marshes 
along  our  southeastern  coast,  is  highly  esteemed  as  food. 

Fish  is  a  food  the  world  over.  Among  fresh-water  species,  white- 
fish,  pike,  and  the  various  members  of  the  trout  family  are  valued 
food  and,  especially  in  the  Great  Lakes  region,  are  so  abundant 
as  to  warrant  the  establishment  of  important  fisheries.  By  far  the 
most  important  food  fishes,  however,  are  those  which  are  taken  in 
salt  water. 

Among  invertebrates  used  for  food  the  much  desired  lobster  should 
not  be  omitted.  Because  of  the  esteem  in  which  it  is  held,  it  has  been 
almost  exterminated  in  many  localities.  The  canning  of  lobsters, 
crabs,  and  shrimp  ranks  as  an  important  industry  in  many  parts  of 
the  world.  Molluscs,  especially  oysters,  clams,  and  scallops,  are 
much  sought  as  delicacies,  and  form  the  basis  for  important  industries, 
particularly  along  our  eastern  coast.  Lower  forms  are  little  used  as 
food  although  the  Chinese  are  very  fond  of  holothurians,  which  are 
preserved  by  drying  and  are  called  "trepang."  In  the  West  Indies 
the  soft  parts  of  sea-urchins  are  considered  a  delicacy.  Finally,  the 
honey-bee  furnishes  us  with  honey,  of  which  over  60,000,000  pounds 
are  used  every  year  in  this  country. 


576  MAN  AS  A  CONQUEROR 

Although  the  advance  of  civilization  has  been  coupled  with  the 
domestication  of  animals,  particularly  as  beasts  of  burden,  many 
other  values  might  be  noted.  The  furs  of  many  wild  animals, 
especially  the  carnivores,  such  as  seals,  otters,  sables,  minks,  and 
others,  are  of  much  economic  importance.  Among  the  domesticated 
animals,  sheep.  Angora  and  Cashmere  goats,  the  camel,  and  alpaca 
are  most  used.  Nor  can  we  omit  the  larva  of  the  moth,  Bombex 
mori,  which  produces  raw  silk,  the  basis  of  an  important  industry 
in  China,  Japan,  Italy,  and  France. 

Many  other  economic  values  depend  upon  animals.  In  past  ages 
protozoans,  as  well  as  diatoms,  had  an  important  part  in  rock- 
building  and  today  their  skeletons  form  the  basis  of  some  of  our 
polishing  powders.  Nor  must  we  forget  their  place  in  the  formation 
of  oil  deposits,  since  the  shells  of  diatoms  and  foraminifera  in  the 
deep  borings  are  almost  always  indicative  of  the  presence  of  oil. 
Corals  have  played  a  considerable  part  in  the  formation  of  islands 
and  the  red  coral  of  the  Mediterranean  is  valued  for  ornamental 
purposes.  Pearls,  the  finest  of  which  come  from  the  north  coast  of 
the  island  of  Ceylon,  are  formed  by  the  secretion  of  mother-of-pearl 
by  the  mantle  of  the  clam  or  oyster  around  some  irritating  substance, 
such  as  a  grain  of  sand  or  a  parasite.  The  pearl  button  industry  in 
this  country  is  largely  dependent  upon  fresh-water  mussels,  shells  of 
which  are  cut  into  buttons. 

Whale  oil,  obtained  from  the  ''blubber"  of  several  species  of  whales, 
and  formerly  used  for  illumination,  has  now  become  a  commercial 
lubricating  oil.  Neat's-foot  oil,  derived  from  the  hoofs  of  cattle,  is 
another  commercial  lubricant.  Tallow,  from  both  cattle  and  sheep, 
and  lard  from  hogs  have  many  well-known  uses.  Cod-liver  oil,  a 
by-product  of  the  codfish,  is  used  for  medical  purposes.  There  is 
obtained,  too,  from  the  menhaden  of  the  Atlantic  coast,  an  oil  used  in 
dressing  leather  and  making  paints.  Great  quantities  of  menhaden 
go  into  the  manufacture  of  fertilizers.  Leather  made  from  the  skins 
of  cattle,  horses,  sheep,  goats,  alligators,  and  snakes  is  put  on  the 
market  in  the  form  of  shoes,  pocketbooks,  coats,  gloves,  and  other 
articles.  Horns  and  bones  are  utilized,  for  making  glue  as  well  as 
combs,  buttons,  and  handles  for  brushes.  Ivory  is  obtained  from 
the  tusks  of  the  elephant,  walrus,  and  other  animals.  The  musk 
deer,  musk  ox,  and  muskrat  furnish  musk  used  in  the  preparation  of 
certain  perfumes.  Ambergris,  a  basis  for  delicate  perfumes,  is  formed 
in  the  intestines  of  the  sperm  whale. 


MAN'S   CONQUEST   OF   NATURE  577 

Indirect  Economic  Value  of  Plants  and  Animals 

The  Biblical  .statement,  "All  flesh  is  grass,"  is  literally  true  of  the 
herbivorous  animals,  which  eat  not  only  grass  but  also  untold  masses 
of  weeds  that  otherwise  would  crowd  out  useful  plants.  Just  as 
plants  furnish  food  for  some  animals,  so  do  some  animals  for  carnivo- 
rous species.  Protozoa  and  many  kinds  of  tiny  plants  form  the 
food  supply  of  forms  higher  in  the  scale,  especially  crustaceans  and 
worms,  which  in  turn  are  eaten  by  fishes.  Many  fishes  live  on 
plankton  or  on  smaller  fishes  that  feed  on  plankton.  Thus  we  see 
the  aquatic  world  is  a  great  balanced  aquarium.  Man  disturbs  this 
ecological  balance  when  he  dumps  untreated  sewage  and  factory 
wastes  into  a  stream  near  its  source,  as  in  the  case  of  the  Illinois  River. 
The  immediate  result  of  this  unsanitary  custom  was  the  destruc- 
tion of  fish  life  for  a  distance  of  about  100  miles.  It  has  been  esti- 
mated by  Professor  Forbes  that  the  Illinois  River,  before  it  was 
polluted  by  the  Chicago  drainage  canal,  produced  annually  over 
150,000,000  pounds  of  fish  food.  On  the  other  hand,  diluted  sewage 
when  emptied  into  a  river  is  utilized  by  bacteria  upon  w^hich  micro- 
scopic animals  feed,  and  these  in  turn  furnish  food  for  crustaceans  and 
snails,  later  eaten  by  fishes. 

We  have  already  seen  the  great  value  of  the  hymenopterous 
and  lepidopterous  insects  to  the  agriculturist.  There  is  yet  to  be 
mentioned  the  indirect  value  of  insects  as  food  for  useful  animals. 
Dr.  Forbes,  for  instance,  has  estimated  that  over  50  per  cent  of  the 
food  of  many  fresh-water  fishes  is  made  up  of  insects,  mostly  aquatic 
larvae.  Nor  should  we  forget  the  service  rendered  by  parasitic  insects, 
native  and  imported,  in  their  war  upon  harmful  insects.  Ichneumon 
flies  and  ladybird  beetles  stand  high  in  this  category.  Insects  also 
eat  enormous  numbers  of  weeds,  often  acting  as  scavengers.  Many 
beetles  and  some  species  like  the  lac  insect,  which  furnishes  the  basis 
of  shellac ;  gall  insects,  from  the  galls  of  which  pyrogallic  acid  is 
made ;  and  the  cochineal  insect,  one  of  the  plant  scales,  produce 
substances  useful  to  man. 

The  toad  is  of  great  economic  importance  to  man  because  of  its  diet. 
It  is  known  to  eat  no  less  than  eighty-three  species  of  insects,  mostly 
injurious.  On  the  whole,  our  common  snakes  are  beneficial  to  man. 
Even  the  rattlesnake  and  copperhead  feed  upon  harmful  rodents. 

The  food  of  birds  makes  them  of  great  importance  to  agriculture. 
Investigations   undertaken   by  the   United   States   Department   of 


578  MAN  AS  A  CONQUEROR 

Agriculture  (Division  of  Biological  Survey)  show  that  a  surprisingly 
large  number  of  birds  once  believed  to  harm  crops  really  perform 
a  service  to  farmers  by  killing  injurious  insects.  Even  the  much 
maligned  crow  eats,  as  well  as  grain  and  fruit,  mice  and  harmful 
insects,  notably  grasshoppers,  and  feeds  its  nestlings  many  more. 
A.  H.  Howell,  in  Bulletin  29  of  the  Biological  Survey,  hsts  85  species 
of  birds  known  to  eat  boll-weevils,  based  on  stomach  examinations 
of  3114  birds.  The  bluebird  includes  grasshoppers,  ants,  spiders, 
weevils,  tent  caterpillars,  army-worms,  cutworms,  and  the  codling 
moth  in  its  diet.  Swifts  and  swallows  eat  flies,  and  cuckoos  and  blue 
jays  eat  hairy  caterpillars,  relished  by  few  other  birds,  while  much  of 
the  winter  food  of  chickadees  consists  of  eggs  of  aphids  or  plant  lice. 
Ants  are  eaten  by  many  species  of  birds.  Larvae  of  beetles,  mostly 
injurious,  are  preferred  by  crows,  blackbirds,  and  robins.  Many 
observations  indicate  that  nesting  birds  eat  a  large  amount  of  food  in 
proportion  to  their  size,  and  consequently  destroy  vast  numbers  of 
injurious  insects.  A  young  robin  three  weeks  old  has  been  observed 
to  eat  70  cutworms  in  one  day ;  a  young  tanager,  150  cutworms  in  a 
day  besides  other  food ;  and  a  young  phoebe  just  out  of  the  nest,  as 
many  as  200  good-sized  grasshoppers  in  a  day. 

In  addition  to  eating  insects,  nearly  300  species  of  birds  eat  the  seeds 
of  weeds  and  other  injurious  plants.  Our  native  sparrows,  the  mourn- 
ing dove,  bobwhite,  rose-breasted  grosbeak,  horned  lark,  crow  black- 
bird, and  other  birds  feed  largely  upon  the  seeds  of  numerous  common 
weeds.  An  examination  of  the  stomachs  of  a  number  of  these  birds 
showed  that  they  had  consumed  over  one  hundred  kinds  of  weed  seeds. 
Tree  sparrows  alone  are  estimated  to  eat  875  tons  of  weed  seeds  every 
winter  in  the  state  of  Iowa. 

Some  birds,  such  as  cormorants,  pelicans,  herons,  ospreys,  bitterns, 
kingfishers,  gulls,  and  terns,  are  active  fishers,  and  thus  may  destroy 
food  fish  and  distribute  parasites.  But  gulls,  as  well  as  the  buzzards 
of  the  West  and  South  and  the  vultures  of  India  and  semitropical 
countries,  are  of  immense  value  as  scavengers.  Birds  of  prey  (hawks 
and  owls)  eat  living  mammals,  including  many  harmful  rodents,  such 
as  gophers,  field  mice,  and  rats. 

In  addition  to  their  commercial  value,  mammals  are  useful  in 
many  ways.  Browsing  cattle  keep  down  weeds,  along  with  their 
consumption  of  grass  and  other  forage.  A  few  mammals  are  insectiv- 
orous, notably  bats  and  moles,  both  of  which  destroy  injurious  insects. 
Some  carnivorous  animals,  such  as  skunks,  weasels,  raccoons,  coyotes. 


MAN'S   CONQUEST   OF   NATURE  579 

and  foxes,  destroy  harmful  rodents  and  may  be  considered  of  more 
use  than  harm  to  the  farmer. 

The  Other  Side  of  the  Picture 

If  we  accept  the  statement  that  man  is  a  rather  doubtful  conqueror 
of  his  environment  and  the  living  things  within  it,  we  should  look  at 
the  other  side  of  the  picture  and  then  attempt  to  strike  a  balance 
between  the  forces  which  aid  and  which  hinder  man  in  his  quest  for 
complete  control  over  nature.  Biological  science  must  do  more  than 
catalogue  lists  of  economic  victories  over  nature,  or  of  battles  won  or 
lost  in  the  field  of  plant  or  animal  husbandry.  The  facts  noted  in  the 
preceding  pages  ought  to  give  the  student  a  basis  on  which  to  build  an 
argument  which  will  place  man  either  on  the  defensive  or  in  control  of 
the  forces  of  nature  that  surround  him.  The  bare  facts  related  here 
should  be  supplemented  by  much  reading  and  investigation  before  a 
conclusion  is  reached.  When  the  facts  are  weighed,  one  sees  that 
man  is  by  no  means  a  complete  conqueror,  and  that  in  some  places 
he  even  seems  to  be  playing  a  losing  game.  By  noting  some  of  the 
damage  wrought  by  plant  and  animal  enemies  of  man  in  the  economic 
world,  and  then  adding  the  plants  and  animals  that  attack  him,  di- 
rectly causing  disease  and  death,  we  will  be  in  a  better  position  to 
decide  man's  position  as  a  potential  conqueror. 

Harm  Done  by  Plants 

In  a  general  survey  of  harmful  agents,  bacteria  and  fungi  stand  out 
as  the  most  destructive.  Leaving  out  death  and  illness  due  to  bac- 
teria which  cause  human  disease,  there  is  still  a  formidable  list  of 
plant  enemies  which  do  much  economic  harm.  Of  the  two  billion 
dollars  of  damage  done  yearly  to  the  crops  of  this  country  probably  a 
third  comes  from  bacteria  and  fungi.  Bacterial  infections  cause  such 
diseases  as  wilts,  which  attack  cucumbers  and  melons;  fire  blight, 
due  to  a  bacillus  attacking  fruit ;  bean  blight ;  the  black  rot  of  cab- 
bage ;  and  the  soft  rots  that  destroy  many  vegetables  in  storage. 
The  brown  galls  of  fruit  trees  have  been  proven  to  be  of  bacterial 
origin,  as  well  as  the  watermark  disease  of  the  English  willows,  a  blow 
to  the  cricket  players  of  England.  Potato  scab  is  caused  by  an 
organism  {Actinoryiijces  scabies)  closely  related  to  bacteria. 

The  algaelike  fungi,  or  Phycomycctes,  include  water  mold  (*Sap- 
rolegnia),  the  downy  mildews,  and  the  true  molds.     The  plant  disease 


580 


MAN  AS  A  CONQUEROR 


called  "damping  off,"  which  attacks  seedlings,  white  "rust,"  the 
brown  rot  of  lemons,  numerous  downy  mildews  that  attack  grapes  or 
garden  vegetables,  and  the  once  dreaded  "rot"  of  potatoes  {Phy- 
tophthora  intestans)  are  among  this  group.  The  Ascomycetes,  one 
of  the  largest  classes  of  fungi,  produce  spores  in  a  spore  case  called 


•  AOOtdWA' 


MOOaiSTOW^O 


OSHAV 

aafifT 


U.  S.  Dept.  of  Agric. 

Map  showing  spread  of  Dutch  elm  disease  from  July,  1933,  to  February,  1934. 
The  black  circles  show  centers  of  infection. 


an  ascus.  They  include  the  powdery  mildews  so  common  on  many 
garden  plants,  the  black  knot  of  plums  and  cherries,  the  brown  rots 
of  stone  fruits,  the  black  rot  of  tobacco,  the  wilts  of  cotton  and 
watermelon,  peach  leaf  curl,  apple  and  pear  scab,  bitter  rot  of  apples, 
the  blue  and  green  molds,  and  the  yeasts,  the  latter  of  which  are  on 
the  whole  useful. 

Shortly  before  1910,  an  importation  of  Japanese  chestnut  trees  to 
an  estate  near  New  York  City  introduced  a  blight  which  attacked 
our  native  chestnuts  and  spread  so  rapidly  that  today,  in  the  eastern 


MAN'S   CONQUEST   OF   NATURI<: 


581 


part  of  the  United  States,  they  have  almost  been  exterminated. 
Even  more  serious  is  the  more  recent  introduction  of  the  Dutch  elm 
tree  disease,  a  wilt  that  was  introduced  in  an  importation  of  European 
elm  logs  shortly  before  1930.  At  that  time,  this  disease  had  been 
found  in  several  localities  extending  as  far  west  as  Indiana  and  Ohio 
and  as  far  south  as  Norfolk,  Virginia,  marking  places  where  the 
infected  logs  had  been  shipped.  It  attacks  the  wood  and  is  spread  by 
the  European  bark  beetle  as  well  as  by  other  means.  A  determined 
campaign  is  now  being  waged  to  stamp  out  this  disease,  which,  unless 
controlled,  will  doom  our  native  elms  to  destruction  as  it  has  those  of 


USUAL  inular  \o    -jJJ 

Dutch  elm  disease.     Brood  galleries  of  Scolylus  muUisfriatus,  an  imported  beetle. 

Europe  in  the  past  fifteen  years.  The  latest  estimate  by  Charles 
Lathrop  Pack  calls  for  the  destruction  of  25,000,000  trees  in  order  to 
save  the  remaining  elms  on  this  continent.  In  view  of  a  program  of 
this  magnitude  it  would  seem  impossible  to  save  our  elms,  because  of 
the  difficulty  in  completely  eliminating  the  fungus. 

The  most  important  class  of  the  fungi  from  the  economic  viewpoint 
are  the  Basidiomycetes,  fungi  that  bear  asexual  spores  on  a  charac- 
teristic structure  called  a  hasidium.  Among  the  worst  pests  of  this 
kind  are  the  corn  smut,  which  causes  the  commonly  seen  smut  balls 
in  ears  of  corn,  many  different  grain  smuts,  grain  rusts,  and  one  white 
pine  blister  rust,  besides  many  fungus  diseases  of  wood.  In  this 
class  are  also  found  the  mushrooms,  both  edible  and  poisonous. 

If  we  add  to  the  above  list  the  poisonous  plants  of  this  country,  such 
as  loco-weed,  jimson  weed,  poison  ivy,  poison  oak,  and  poison  sumac,  we 
have  a  formidable  list  of  plants  contending  with  man  for  supremacy. 


582  MAN  AS  A  CONQUEROR 

One  of  the  most  serious  factors  against  which  man  has  to  fight  has 
recently  been  called  to  the  attention  of  scientists  by  Professor  E.  C. 
Stackman  of  the  University  of  Minnesota,  and  that  is  the  rapid 
appearance  of  new  strains  of  harmful  fungi.  A  single  reproductive 
cell  of  a  grain  smut  was  isolated  and  grown  under  laboratory  condi- 
tions. In  a  relatively  short  time  112  distinct  physiological  strains 
were  produced  from  the  original  plant.  This  means  that  under  nat- 
ural conditions  there  are  new  strains  constantly  arising,  that  will  in 
time  attack  new  crops  as  they  are  planted,  some  living  on  varieties 
of  wheat,  others  on  oats,  barley,  or  rye.  In  other  words,  nature  is 
constantly  at  work  producing  new  varieties,  either  through  muta- 
tion or  through  sexual  crossing  of  existing  varieties,  thus  forming 
hybrids  which  are  different  from  the  original  parents  and  which  have 
the  possibilities  of  attacking  different  grains  from  those  their  parents 
live  upon.  It  looks  as  if  man  was  less  than  one  jump  ahead  of  such 
plant  parasites. 

Harm  Done  by  Animals 

It  is  not  the  purpose  of  these  pages  to  do  more  than  call  attention 
to  some  of  the  animals  harmful  to  man,  but  we  should  note  that 
some  of  the  most  dreaded  diseases,  such  as  rabies,  malaria,  sleeping 
sickness,  and  amebic  dysentery,  are  laid  at  the  door  of  the  protozoa. 
Among  the  echinoderms,  starfish  do  much  damage  to  shellfish  and 
thousands  are  dredged  up  and  destroyed  each  year  by  oystermen. 
Cestodes  are  parasitic  in  food  animals  such  as  cattle,  swine,  and 
fishes,  and  from  these  hosts  may  infect  man.  The  class  Trematoda 
also  includes  many  parasitic  flukes,  some  of  which  may  infect  man. 
The  Nemathelminthes  include  the  hookworm  (Necator)  and  Trichina 
as  well  as  the  Filaria,  which  sometimes  causes  elephantiasis.  Para- 
sitic worms  also  destroy  annually  large  numbers  of  fishe?,  birds,  and 
mammals  used  as  food.  Among  the  mollusca  that  do  harm  are  the 
whelks  which  destroy  other  edible  molluscs,  and  the  shipworm 
{Teredo)  that  destroys  submerged  timber,  such  as  the  piles  of  wharfs 
and  the  hulls  of  vessels.  Of  crustaceans,  crayfish  may  become  a 
serious  pest  to  cotton  raisers  by  destroying  young  cotton  plants. 
A  few  poisonous  spiders  exist,  such  as  the  notorious  "black  widow" 
and  the  tarantula.  The  ticks  are  of  much  importance  because  of 
their  parasitic  habits  and  the  fact  that  they  carry  other  parasites, 
such  as  the  protozoan  that  causes  Texas  cattle-fever. 

It  is  the  insects,  however,  that  must  rank  highest  as  man's  com- 


MAN'S  CONQUEST  OF  NATURE 


583 


petitors.  The  most  successful  and  most  numerous  of  all  animals, 
estimated  to  do  from  $1,000,000,000  to  $2,000,000,000  annual  damage 
to  our  crops  as  well  as  unestimated  harm  to  man's  health  and  comfort, 
they  are  indeed  to  be  reckoned  with.  Insects  are  of  especial  impor- 
tance to  man  because  of  their  relation  to  his  food  supplies.  Plagues 
of  locusts  have  scourged  many  lands  since  earliest  history,  but  with 
increased  cultivation  and  the  introduction  of  new  crops,  most  insect 
pests  have  miore  recently  turned  from  their  original  diet  of  weeds 
or  grasses  to  feed  upon  the  introduced  food-plants.  The  chinch 
bug  originally  inhabiting  the  Great  Plains  regions  and  living  on 
wild  prairie  grasses,  with  the  coming  of  the  settler  and  the  raising 
of  cereal  crops  changed  its  food  supply  and  became  a  pest  to  the 
farmer.  The  potato  "bug,"  a  beetle  that  a  few  generations  ago 
was  an  inconspicuous  and  not  extremely  numerous  insect  living  on 
wild  native  plants  of  the  family  Solanaceae  to  which  the  potato  be- 
longs, upon  the  introduction  of  the  potato  to  Colorado  promptly 
changed  from  its  original 
diet  to  the  new  food  and 
spread  to  new  areas  where 
the  potato  was  cultivated. 
Within  a  few  years  it  had 
reached  all  parts  of  the 
United  States  and  re- 
cently has  appeared  in 
England.  These  are  only 
a  few  examples  of  many 
similar  cases  that  illus- 
trate the  fact  that  man, 
in  spite  of  all  he  can  do, 
is  spreading  and  aiding 
insect  pests  which  are 
getting  a  large  portion  of 
his  basic  food  supplies. 

But  native  forms  of  in- 
sect pests  are  not  enough. 
With  the  expansion  of 
commerce  and  the  intro- 


introcCir  ced 
in  Ke*v  Jersey 


^^ran^e  1916-1927 
range   1927-1930 


The  present  range  of  the  Japanese  beetle.  An 
imported  pest.  What  steps  would  you  advocate 
to  stop  its  rapid  spread :' 


duction  of  airplanes  as  well  as  railways  and  steamships,  man  is  con- 
tinually called  upon  to  battle  new  importations  of  destructive  insects, 
which  in  spite  of  strict  quarantine  laws  are  gaining  a  foothold  on  our 
H.  w.  H.  —  38 


584 


MAN   AS   A   CONQUEROR 


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MAN'S   CONQUEST   OF   NATURE  585 

shores.  During  the  war  of  the  Anicrican  Revolution,  these  stowaways 
began  to  arrive.  Witness  the  Hessian  fly  cocoons  brought  in  with 
straw  imported  from  Germany  to  feed  the  horses  of  the  Hessian 
troops.  The  progeny  of  these  flies,  by  destroying  wheat,  have  done 
more  damage  in  this  country  than  all  the  Hessians  who  fought  during 
the  war.  One  of  the  most  recent  importations  and  serious  pests  is 
the  Japanese  beetle.  Introduced  in  the  soil  around  the  roots  of 
iris  plants  imported  from  Japan,  it  was  first  observed  in  New  Jersey 
in  1916.  At  the  present  writing  it  has  spread  over  300  miles  from 
the  point  of  its  introduction  and  has  become  a  very  serious  menace 
over  several  thousand  square  miles  of  territory.  The  Mediterranean 
fruit-fly,  since  its  discovery  in  Spain  in  1842,  has  spread  to  all  parts 
of  the  world,  gaining  a  foothold  in  Florida  in  April,  1929.  Because 
this  fly  breeds  in  citrus  and  other  fruits,  as  well  as  in  peppers, 
tomatoes,  lima  beans,  and  eggplants,  its  introduction  was  a  seri- 
ous menace  to  the  crops  of  this  region.  The  situation  called  for 
strong  action  in  which  the  state  of  Florida  and  the  national  govern- 
ment took  immediate  part.  A  quarantine  was  declared  and  no 
fruit  shipped  from  the  infected  area.  All  trees,  vines,  or  plants  on 
which  the  flies  fed  were  destroyed,  and  trees  in  nearby  areas 
thoroughly  sprayed  at  frequent  intervals.  This  treatment  was  so 
effective  that  by  November  16,  1930,  no  flies  or  infected  fruits  or 
vegetables  being  found,  the  quarantine  was  lifted.  But  we  are  not 
always  as  fortunate  with  imported  pests.  Take,  for  example,  the 
European  corn  borer.  Because  of  the  nocturnal  habit  of  the  moth, 
which  produces  the  caterpillar,  it  was  not  discovered  in  this  coun- 
try until  it  was  too  late  to  combat  it  effectually.  Now,  as  the  map 
published  by  the  Department  of  Agriculture  shows,  it  has  spread 
widely  over  the  entire  northeastern  part  of  the  United  States  and  is 
rapidly  approaching  the  corn  belt.  The  story  of  the  incredibly  rapid 
increase  of  some  of  these  insect  pests  is  repeated  again  and  again. 
Our  cereal  crops  are  attacked  at  every  stage  of  their  existence. 
Weevils  destroy  the  stored  grain,  cutworms  attack  the  plants  in 
their  early  stages,  biting  insects  such  as  locusts  destroy  the  leaves, 
and  bugs  suck  the  plant  juices,  while  various  boring  insects  such  as 
the  corn  borer  or  the  codling  moth  destroy  the  grain  or  fruit. 

What  is  true  of  food  plants  is  true  also  of  the  fiber  crops  and  for- 
ests of  our  country.  The  cotton  boll-weevil,  imported  from  Mexico  in 
1892,  has  spread  over  the  entire  South,  in  some  places  entirely  chang- 
ing the  economic  life  of  the  farmer,  and  causing  replacement  of  the 


586 


MAN  AS  A  CONQUEROR 


cotton  crops  with  other  types  of  agricultural  products.  The  cotton 
boll- weevil  lays  its  eggs  in  the  young  flower  bud,  while  the  larvae  feed 
on  the  substance  within  the  bud,  causing  it  to  drop  off,  with  the 
consequent  non-production  of  cotton  fiber.  Beetles  also  lay  their 
eggs  in  the  young  bolls  of  cotton,  with  the  result  that  they  become 
discolored,  thus  ruining  the  cotton  produced.     It  is  estimated  that 


SPREAD  OF  THE  COTTON    BOLL  WEEVI  L,  1892-1932 


U.5.0CPARTMErtT  or  ACRICUCT 


tAU  or  AGRICULTUdAi   CCONOMlCS 


over  half  of  the  cotton  crop  is  destroyed  by  the  boll-weevil.  Because 
of  protection  offered  by  the  cotton  boll,  the  weevil  is  difficult  to  ex- 
terminate. Parasitic  insects  have  been  introduced  to  prey  upon  this 
pest ;  infected  bolls  and  stalks  are  burned  ;  crops  are  rotated  and  the 
ground  plowed  under  for  two  or  three  years  at  a  time  in  order  to 
destroy  the  wintering  weevil  pupae.  However,  nothing  has  succeeded 
in  stopping  the  boll-weevil's  advance  over  the  cotton-raising  South. 
Today  it  is  considered  to  be  one  of  the  greatest  crop-destroying 
imported  pests.  The  gypsy  moth,  the  cabbage  butterfly,  the  codling 
moth,  and  scores  of  others  give  rise  to  untold  billions  of  caterpillars 
each  year  which  strip  our  trees  and  shrubs  of  their  leaves.  Locusts 
move  in  swarms  across  the  country,  leaving  a  wake  of  devastation  in 
their  path.  Plant  lice  and  scale  insects  take  their  toll  of  fruit  and 
forest  trees,  and  beetles,  too,  such  as  the  hickory  borer,  threaten  the 
existence  of  all  of  the  hickories  in  the  eastern  part  of  this  country. 
The  Englemann  spruce  beetle  and  the  mountain  pine  bark  beetle, 
which  have  already  done  enormous  damage  to  the  forests  of  the  Far 
West,  are  rapidly  spreading  their  areas  of  destruction.  Insects  from 
almost  every  order  do  harm  to  man,  so  why  multiply  the  list. 


MAN'S   CONQUEST   OF   NATURE  SSI 


Methods  of  Control 


It  might  seem  a  hopeless  fio-ht  that  man  is  waging  with  his  insect 
foes,  especially  as  he  is  constantly  introducing  new  species  and  just 
as  constantly  providing  more  food  for  them.  It  seems,  indeed,  like 
an  endless  chain  of  difficulty.  Nevertheless,  man  has  his  brains  and 
his  social  inheritance  to  aid  in  the  fight.  He  has  organized  liis 
forces  through  such  agencies  as  the  United  States  Department  of 
Agriculture  and  its  various  bureaus,  state  agricultural  agencies, 
public  and  private  research  laboratories,  as  well  as  control  and  quaran- 
tine offices  in  various  parts  of  the  country.  A  very  large  number  of 
highly  trained  scientists  are  at  work,  both  in  this  country  and  abroad, 
studying  life  histories,  looking  u})  plants  fitted  to  withstand  insect 
attacks,  and  running  down  parasitic  enemies  of  harmful  insects. 

Methods  of  control  have  been  worked  out  along  several  lines. 

(1)  Natural  enemies  of  insects  which  do  harm  are  found  and 
encouraged.  Many  of  these  enemies  are  already  "on  the  job." 
Insect-eating  birds,  toads,  frogs,  and  snakes,  as  well  as  insect- 
feeding  mammals,  are  examples.  Many  insects  are  attacked  by 
parasitic  fungi.  To  find  enemies  for  imported  crops  it  is  often  nec- 
essary for  entomologists  to  go  to  the  original  country  from  which  a 
given  plant  has  come  in  order  to  study  its  insect  enemies  there,  and 
to  note  how  these  are  kept  in  check.  The  historic  example  of  the 
discovery  of  a  ladybird  beetle  as  an  enemy  of  the  cottony-cushion 
scale,  which  threatened  the  orange  industry  of  California,  may  be 
cited.  In  this  case  a  natural  enemy  of  the  destructive  scale  was 
found  in  Australia,  which,  when  imported  to  California,  soon  had 
the  situation  under  control.  Our  latest  enemy,  the  Japanese  beetle, 
has  possibility  of  control  through  an  imported  roundworm,  while 
our  native  birds  are  also  beginning  to  include  it  in  their  dietary. 
The  importation  of  such  insects  as  new  species  of  ichneumon  flies, 
that  parasitize  many  harmful  caterpillars,  or  of  damsel  flies  or  man- 
tises,  both  of  which  feed  on  injurious  insects,  are  examples  of  this 
kind,  of  control.  Scouts  from  the  Bureau  of  Entomology  in  the 
Department  of  Agriculture  are  now  at  work  in  foreign  coimtries 
seeking  parasitic  enemies  of  the  European  corn  borer,  the  Mexican 
cotton  boll-weevil,  and  our  worst  forest  pests. 

(2)  A  second  method  used  in  fighting  insect  pests  is  to  study  the 
life  histories  of  both  the  pest  and  the  crop  which  it  attacks,  and  then 
either  to  change  the  crop  in  a  given  area  to  another  on  which  the  pest 


588  MAN   AS   A   CONQUEROR 

will  not  feed,  or  else  to  plant  a  given  crop  earlier  or  later  so  it  will 
mature  a  little  ahead  or  behind  the  ai)pearance  of  the  insect  enemy. 
Such,  for  example,  is  the  early  planting  of  cotton  in  the  southern 
areas  where  the  boll-weevil  is  a  pest,  or  the  late  planting  of  spring 
wheat  in  order  to  escape  damage  from  the  Hessian  fly. 

(3)  A  third  method  of  fighting  insects  comes  through  a  study  of 
their  feeding  habits.  Beetles,  caterpillars,  and  locusts  bite  holes  in 
plants  and  chew  their  food,  whereas  bugs  suck  the  juices  of  plants. 
In  the  case  of  the  former,  poisons  are  sprayed  on  the  leaves  which  are 
eaten  by  the  insect.  Such  poisons  as  lead  arsenate  or  Paris  green 
are  used  against  the  potato  beetle  and  cabbage  moths.  In  the  case  of 
the  sucking  insects,  an  oil  spray  or  emulsion,  that  clogs  up  the  spiracles 
and  eventually  kills  them,  is  used.  Mixtures  containing  nicotine,  oil, 
soap,  or  kerosene  are  called  contact  poisons.  In  addition  to  these 
methods,  picking  off  or  shaking  the  insects  into  pans  and  then  de- 
stroying them,  drenching  planted  areas  with  creosote  or  other  sub- 
stances, or  banding  tree  trunks  with  tar  are  employed. 

The  battle  between  man  and  his  insect  foes  has  only  begun.  Each 
year  sees  new  developments  in  both  agriculture  and  animal  husbandry, 
and  each  year,  with  an  increase  of  food  plants  and  new  strains  of  food 
animals,  new  species  of  plant  and  animal  parasites  as  well  as  of  pests 
are  appearing,  either  introduced  from  other  countries  or  developed 
in  nature's  own  way  from  mutants  or  crosses.  Will  man  ultimately 
win  the  battle  ?     Who  knows  ? 

SUGGESTED  READINGS 

East,  E.  M.,  Biology  in  Human  Affairs,  McGraw-Hill  Book  Co.,  1931. 

An  interesting  and  authentic  survey  of  biological  knowledge  as  it  is  ap- 
plied for  the  benefit  of  mankind.  Written  by  a  dozen  leading  specialists 
in  the  fields  of  genetics,  medicine,  foods,  public  health,  and  psychology. 

Gager,  C.  S.,  General  Botany,  P.  Blakiston's  Son  &  Co.,  1926. 

A  general  botany  with  much  economic  material  included.  Valuable  for 
reference. 

Henderson,  J.,  The  Practical  Value  of  Birds,  The  Macmillan  Co.,  1927. 
Useful  for  reference. 

Howard,  L.  0.,  The  Insect  Menace,  Century  Co.,  1934. 

The  title  suggests  the  contents.  An  interesting  discussion  of  a  timely 
subject  by  an  eminent  authority. 

Metcalf,  C.  L.,  and  Flint,  W.  P.,  Insects,  Man's  Chief  Competitors,  Williams 
and  Wilkins,  1932. 
The  title  explains  the  point  of  view. 


XXV 


CONSERVATION  AND   ITS  MEANING 

Preview.  The  usefulness  of  forests  •  Forest  waste  and  methods  of 
conservation  •  Waste  and  conservation  of  animal  life  :  Fisheries  ;  shellfish  ; 
birds;  Mammals  •  Conservation  of  wild  life  •  Present  methods  of  conser- 
vation •  Organizations  for  conservation :  State  conservation  departments ; 
Biological  surveys  ;  Federal  agencies  ;  Bureau  of  Fisheries ;  National  Parks  • 
Is  there  a  unified  program?  •  Suggested  readings. 

PREVIEW 

This  country  has  been  blessed  beyond  many  areas  of  the  earth  in 
its  abundance  of  Nature's  resources.  The  first  settlers  found  forests, 
inhabited  with  game,  covering  the  land,  streams  and  rivers  alive  with 
fish,  and  great  plains  supporting  herds  of  buffalo.  Yet  today,  with 
our  country  embarking  on  almost  its  one  hundred  and  fiftieth  year 
of  national  life,  its  wild  life  is  almost  exterminated,  its  forests  are  only 
one  eighth  of  their  former  size,  and  its  oil,  coal,  and  mineral  deposits 
are  rapidly  approaching  depletion.  Increasing  population  has  meant 
the  use  of  more  power,  more  fuel,  more  mineral  wealth.  Consequently 
man  has  disturbed  more  and  more  the  balance  of  nature,  sometimes 
with  disastrous  results.  No  one  who  has  traveled  through  a  cut-over 
or  burned-over  forest  area,  or  through  an  exhausted  coal  or  oil  region, 
can  escape  seeing  the  necessity  for  immediate  and  drastic  control  of 
our  waste.  No  fisherman  or  hunter  who  remembers  the  bounty  of 
the  streams  and  forests  of  former  days  can  escape  understanding 
why  there  are  now  restrictions  on  the  size  of  the  bag  of  game  or  limit 
of  fish.  All  thinking  citizens  must  realize  not  only  the  need  for 
conservation  of  what  is  left  of  our  natural  resources,  but  also  the 
necessity  of  intelHgently  adding  to  our  supplies  of  living  things  by 
means  of  reforestation,  fish  culture,  stocking  of  streams  and  lakes, 
as  well  as  providing  more  food  supplies  and  refuges  for  wild  life. 

The  Usefulness  of  Forests 

Forests  have  indirect  values  and  uses  other  than  commercial  which 
mean  more  to  man's  future  welfare  than  a  supply  of  lumber  or  fuel 
or  forest  products,  important  as  these  are.  History  shows  that  as  man 
has  cut  down  forests,  tilled  land,  and  built  cities,  destructive  physical 

589 


390 


MAN  AS  A   CONQUEROR 


changes  have  resulted.  When  the  earth's  surface  is  covered  with 
trees,  their  roots  make  a  soil-mat  which  holds  water  in  the  ground, 
preventing  a  rapid  run-off.  The  blanket  of  foliage  above  intercepts 
the  moisture  and  allows  a  more  gradual  passage  of  water  into  the 
ground,  while  the  soil  under  the  trees,  rich  with  an  accumulation  of 
humus  and  dead  leaves,  holds  the  water,  so  that  a  forest  floor  is 
estimated  to  absorb  and  hold  back  for  some  time  a  rainfall  of  four  to 
five  inches.  But  let  the  forest  cover  be  destroyed  by  fire  or  poor 
lumbering,  and  what  is  the  result?  As  soon  as  the  forest  cover  is 
gone,  the  first  heavy  rain  washes  off  the  soil,  carrying  it  to  places  where 
it  will  be  of  little  value.  The  water  lost  by  surface  run-off  in  level 
areas  like  the  Great  Plains  region  varies  from  15  per  cent  after  light 
showers  to  50  per  cent  during  heavy  storms.  In  sloping  areas  the 
run-off  naturally  is  much  greater.  It  is  estimated  that  the  annual 
wastage  of  soil  in  the  country  from  erosion  amounts  to  1,500,000,000 
tons,  containing  over  126,000,000,000  pounds  of  soluble  mineral 
material  necessary  for  the  life  of  plants.  In  dollars  and  cents  this 
wastage  is  estimated  at  $2,000,000,000  annually.  Most  of  this  loss 
comes  through  man's  carelessness  with  fire  or  downright  rapacity  in 
denuding  forests  through  cutting  without  proper  provisions  for 
replanting. 

The  relation  of  forest  fires  to  erosion  and  floods  is  seen  in  the  fol- 
lowing table,  which  was  made  after  the  devastating  flood  near  Los 
Angeles,  California,  during  the  storm  of  December  30,  1933,  to 
January  1,  1934.  This  area  had  been  burned  over  a  few  months  pre- 
viously, with  the  result  that  the  cover  of  chaparral  and  small  trees 
was  completely  destroyed. 


Watershed 

Rainfall, 
Inches 

Per  Cent  of 
Area  Burned 

Per  Cent  of 

Area 

Unburned 

Run-off 

Maximum, 

Cubic  Feet 

per  Second 

Erosion, 

Cubic  Yards 

PER  Square 

Mile 

Verdugo     . 
San  Dimas 
Haines        .     . 

19.1.3 
16.85 
11.26 

67 

0 

68 

33 

100 

32 

1000 

51 

1000 

50,000 

56 

67,000 

A  study  by  C.  A.  Connaughton,  of  the  United  States  Forest  Serv- 
ice, on  3000  test  plots  of  burned-over  forest  areas,  comprising  both 
cut-over  areas  and  virgin  timber,  showed  in  general  that  the  degree 
of  subsequent  erosion  varied  with  the  severity  of  the  fires.  Plots  of 
virgin  timber  burned  over  showed  only  about  half  as  much  erosion  as 
cut-over  land  having  the  same  degree  of  forest  fire. 


CONSERVATION   AND   ITS   MEANING 


591 


It  has  been  estimated  that  the  transporting  power  of  water  varies 
as  the  sixth  power  of  its  velocity.  This  means  the  carrying  power  of 
water  is  increased  sixty-four  times  if  its  rate  of  flow  is  doubled.  From 
such  figures  it  is  easy  to  account  for  the  enormous  destruction  by 
streams  at  a  time  of  flood,  as  was  witnessed  in  March,  1936,  in  the 
eastern  part  of  the  United  States.  The  annual  damage  by  "spring 
freshets"  in  cut-over  areas  in  the  East,  the  recent  floods  in  the  Ohio 
and  Mississippi  valleys,  where  the  forest  cover  has  given  place  to  farms 
and  cities,  and  the  damage  done  by  cloudbursts  in  denuded  areas  in 
the  Southwest,  all  testify  to  the  power  of  uncontrolled  water  and  show 
the  need  of  forest  cover  to  hold  back  flood  waters. 


Erosion  areas  in  the  United  States. 
1,    Area  of  most  serious  erosion  :   2,    harmful  erosion  widespread  ;   3.    fiat  areas, 
slight  erosion  ;    4,    erosion  generally  not  serious ;    5,    much  serious  wind  erosion  ; 
6.    much  erosion  from  over-grazing. 

But  erosion  does  damage  in  a  more  insidious  way  than  through 
spectacular  floods.  A  plowed  area  on  a  hillside  allows  more  rain  to 
run  off  than  a  similarly  located  area  covered  with  grass.  A  plowed 
field  allows  more  soil  to  be  carried  off  by  wind  than  does  a  similar 
field  covered  with  grass.  A  plowed  field  will  be  left  covered  with 
mud  after  a  heavy  rain  and  the  pores  of  soil  will  be  found  to  be 
clogged  with  soft  mud,  making  plant  growth  practically  impossible. 
A  glance  at  the  map  shows  the  very  large  area  in  this  country  in 
which  more  or  less  serious  erosion  takes  place.  A  check-up  with  the 
map  showing  forest  areas  on  page  605  makes  clear  that  the  areas  of 


592 


MAN   AS   A  CONQUEROR 


least  erosion  are  those  which  are  still  covered  with  forests,  while  those 
that  show  the  greatest  destruction  by  erosion  are  the  areas  where  the 
cover  has  been  destroyed  without  adequate  replacement.  The  farms 
in  the  mountains  of  Tennessee,  Kentucky,  and  on  the  eastern  slope 
of  the  Southern  Appalachians  suffer  most  from  water  erosion,  while 
great  regions  in  the  Middle  West  have  been  made  subject  to  wind 
erosion  through  the  removal  of  large  areas  of  protective  cover,  thus 
giving  the  name  of  "dustbowl"  to  this  region.  This  does  not  mean 
that  farmers  should  not  plow  land  and  plant  crops,  but  it  does  mean 
a  lack  of  intelligent  farm  planning  on  the  part  of  many  farmers. 
Leaving  a  few  trees  here,  or  planting  others  there  to  form  a  windbreak, 
the  use  of  grasses  or  grains  in  wind-exposed  tracts,  cutting  up  large 
fields  into  smaller  ones  in  which  diversified  crops  may  be  grown, 
planting  grass  along  banks,  and  placing  check  dams  in  gullies  already 
eroded  are  some  of  the  ways  in  which  farm  erosion  may  be  prevented. 

Forest  Waste  and  Methods  of  Conservation 

When  the  white  man  first  settled  our  eastern  coast  three  hundred 
years  ago,  there  was  eight  times  as  much  virgin  forest  as  there  is  today. 
The  present  total  forest  of  the  United  States  is  less  than  494,000,000 
acres.     More  than  80,000,000  acres  of  this  area  have  been  burned  or 

cut,  so  that  they  are  now 
waste  land.  It  is  esti- 
mated that  fire,  worms, 
and  insects  destroy  each 
year  in  the  United  States 
about  7,000,000,000 
board-feet  of  standing 
timber.  Add  to  this  a 
timber  production  from 
10,000,000,000  to  over 
44,000,000,000  board-feet 
a  year,  depending  on  the 
building  demand,  and  we 
can  see  the  wastage  that 
is  taking  place  in  our 
American  forests.  It  is 
estimated  that  we  are 
deforesting  at  the  rate  of 
The  tragedy  of  forest  fires.         ""  about   10,000,000  acres  a 


}l  incur  III 


CONSERVATION  AND   ITS  MEANING  593 

year.  Forest  fires  caused  by  man's  carelessness  as  well  as  by  lio;ht- 
ning  have  laid  waste  over  12,000,000  acres  of  forest  land  in  a  single 
year.  A  forest  fire  does  much  more  than  burn  trees,  for  a  severe 
fire  usually  destroys  the  organic  material  of  the  forest  floor  known 
as  duff,  thus  preventing  the  growth  of  new  forests  for  years  to  come, 
and  in  addition,  it  drives  out  or  kills  much  wild  life. 

Other  enemies  to  forests  are  parasitic  fungi  that  destroy  trees,  and 
various  insects  which  eaf  their  leaves  and  tender  shoots  or  bore  into 
the  wood.  The  caterpillars  of  the  gypsy  and  brown-tail  moths  are 
chief  agents  of  destruction  in  the  first  category,  while  various  beetles 
may  be  listed  in  the  second  group.  The  Engelmann  spruce  beetle 
has  destroyed  millions  of  feet  of  timber  in  the  Rocky  Mountain  region, 
while  the  Black  Hills  beetle  has  done  similar  damage  in  South  Dakota. 
Much  damage,  too,  is  done  by  grazing  animals,  especially  sheep.  The 
recent  Taylor  Act,  which  throws  the  entire  165,000,000  acres  of  the 
Public  Domain  open  to  cattle  and  grazing  interests,  is  a  serious  menace 
to  our  forests  and  wild  life.  In  addition  to  all  of  this  kind  of  wastage, 
if  we  add  that  caused  through  waste  in  lumbering,  at  the  mills,  through 
nonutilization  of  by-products,  and  especially  in  pulpwood  cutting 
for  the  paper  industry,  where  millions  of  small  trees  are  sacrificed, 
we  can  see  many  reasons  for  a  general  and  more  scientific  conservation 
of  our  forest  resources. 

Fortunately  this  country  is  beginning  to  awaken  to  the  need  of 
forest  conservation  and  has  numerous  agencies  both  Federal  and 
commercial  at  work  toward  this  end.  Many  lumber  companies  are 
replanting  cut-over  areas  and  selecting  with  greater  care  the  trees  to 
be  lumbered.  Forests  are  being  treated  as  crops  to  be  harvested 
when  ripe.  Waste  products  are  being  utilized  to  a  greater  extent. 
All  sawdust  formerly  had  to  be  burned,  but  now  alcohol,  beaver- 
board,  and  other  by-products  are  obtained  from  this  source.  Although 
much  bark  is  used  for  tanning,  still  there  is  wastage  here.  More 
and  more  lumber  is  being  treated  each  year  with  creosote  or  other 
chemicals  as  a  protection  against  insects,  thus  effecting  another  saving. 
It  is  estimated  that  the  treating  of  railway  ties  with  creosote  has  re- 
sulted in  an  annual  saving  of  around  1 ,500,000,000  board-feet.  Much 
wood  was  formerly  utilized  in  the  making  of  boxes,  for  which  sub- 
stitutes are  used.  The  Forest  Products  Laboratory  of  the  United 
States  Forest  Service  works  upon  the  various  chemical  products 
obtained  from  wood  and  has  shown  a  list  of  uses  given  on  page  594, 
many  of  which  are  still  not  utilized.     The  trim  of  homes  has  been  re- 


594 


MAN   AS   A   CONQUEROR 


Chemical  Products  from 

Wood  ' 

baking  powder 

lignin     (at     present 

one 

printing  ink 

cellophane 

fourth  of  wood  is  w 

aste) 

rayon 

cellulose 

pyroligneous  acid 

resin 

cellulose  acetate 

oleo  resin 

sausage  casings 

cellulose  nitrate 

oxalic  acid 

sealing  wax 

collodion-film 

paint 

smokeless  powder 

ethyl  alcohol 

paper 

soap 

furfural 

paper  size 

sugars 

galactin 

patent  leather 

synthetic  camphor 

hydrolyzed  sawdust 

plastics 

turpentine 

lacquer 

polish 

varnish 

placed  by  metal  or  other  substances,  and  in  general  the  rate  of  forest 
cutting  has  been  somewhat  reduced  by  these  and  other  means.  Rail- 
roads are  planting  areas  for  the  production  of  ties  and  each  year  sees 
more  emphasis  placed  on  the  care  and  protection  of  forested  areas. 
Recent  figures  indicate  that  over  a  five-year  period,  although  the  num- 
ber of  forest  fires  in  national  forests  increased  from  7601  for  a  previous 
five-year  period  to  9512,  yet  the  number  of  acres  burned  over  was  less 
than  half  that  burned  in  the  previous  period.  Forest  Service  officials 
give  credit  to  the  members  of  the  C.C.C.  for  this  saving.     Not  only 


Wright  Pierce 

Flood  control  work  1)>  the  Civilian  Conservation  Corps.  Ry  means  of 
wire  mesh  and  small  stones  this  mountain  torrent  is  kept  within  bounds  at 
a  time  of  flood . 


'  U.  S.  Forest  Service,  Forest  Products  Lab.,  Madison,  Wis. 


CONSERVATION   AND   ITS   MEANING  595 

have  they  built  roads,  cleaned  up  fallen  timber  and  "slash,"  and  cut 
fire  breaks,  but  they  also  were  on  the  ground  early  enough  to  pre- 
vent many  of  the  fires  from  spreading. 

The  government  agency  which  has  to  do  with  the  carrying  out  of  our 
forest  policy  is  the  United  States  Forest  Service,  a  branch  of  the 
United  States  Department  of  Agriculture.  Forest  rangers  keep  in 
the  field,  continually  patrolling  forest  areas.  Fire  towers  are  built 
from  which  observations  are  made,  airplanes  scout  during  the  season 
of  fire  hazard  in  order  to  locate  outbreaks  of  fire,  and  trained 
foresters  are  constantly  at  work  repairing  injured  trees,  cleaning  up 
areas  that  are  fire  hazards,  and  replanting  burned  or  waste  areas  with 
seedling  trees. 

Waste  and  Conservation  of  Animal  Life 

Fisheries 

Fish  have  been  an  important  food  supply  since  earliest  times,  but 
we  find  that  the  drain  caused  by  overfishing,  commercially  as  well  as 
in  sport,  is  making  severe  inroads  on  the  original  fish  population. 
Every  sportsman  well  knows  that  with  the  coming  of  the  automobile 
his  former  haunts  have  been  pre-empted  by  others  and  that  the  supply 
of  game  fish  has  rapidly  decreased.  To  an  even  greater  extent  over- 
fishing has  occurred  in  the  oceans,  due  to  the  demands  of  increased 
population. 

It  is  well  known  that  fishes  change  their  habitat  at  different  times 
in  the  year,  a  fact  which  is  made  use  of  by  sport  and  commercial 
fishermen.  Although  temperature  changes  and  the  quest  for  food 
play  an  important  part  in  the  migration  of  some  fish,  it  should  be 
noted  that  this  habit  in  fishes  as  in  birds  seems  to  be  due  to  the  growth 
of  the  gonads  and  the  ripening  of  eggs  and  sperms.  In  the  ocean, 
migrations  in  a  general  way  follow  the  coast  lines.  The  continental 
shelf  which  exists  along  the  eastern  coast  of  the  United  States,  giving 
rise  to  the  Grand  Banks  off  the  coast  of  Newfoundland,  marks  the 
northern  limit  of  the  range  of  immense  numbers  of  food  fishes,  par- 
ticularly the  cod.  Consequently  this  area  of  the  ocean  has  been 
fished  to  a  very  considerable  degree,  being  the  principal  source  of 
pollack,  haddock,  and  cod. 

The  relation  of  the  spawning  habits  of  fish  to  commercial  fisheries  is 
important.  Many  of  the  most  desirable  food  fish,  such  as  salmon, 
shad,  sturgeon,  and  smelt,  swim  in  from  the  ocean  up  rivers  in  order 


596  MAN   AS  A   CONQUEROR 

to  deposit  their  eggs  in  fresh  water.  The  Chinook  salmon  of  the 
Pacific  coast,  which  is  the  species  most  used  in  the  salmon-packing 
industry,  travels  up  the  Columbia  River  over  a  thousand  miles  in 
order  to  deposit  its  eggs  near  the  headwaters.  Several  runs  of  salmon 
occur  at  different  points  along  the  western  coast,  different  species  of 
salmon  entering  different  rivers  to  deposit  their  eggs.  The  run  of  the 
sockeye  or  blueback  salmon  (Oncorhynchus  nerka)  on  the  Columbia 


The  romance  of  the  Alaska  salmon.  After  the  eggs  are  laid  in  the  headwaters 
of  the  rivers  frequented  by  the  various  species  of  salmon,  the  old  fish  die.  The 
young,  after  several  months  to  a  year,  make  their  way  to  the  sea,  where  all  traces 
of  them  are  lost.  But  when  adult,  these  same  fish  apparently  make  their  way 
back  to  the  streams  where  they  were  hatched,  to  complete  their  life  cycle. 

begins  in  March  or  April  and  ends  in  July  or  August  near  the  head- 
waters of  the  Salmon  River  in  Idaho.  The  same  species  begins  to 
run  in  the  Fraser  a  little  later,  reaching  its  spawning  ground  in  August 
and  September,  wiiile  in  Alaska  the  sockeye  has  a  relatively  short  run. 
The  Chinook  (Oncorhynchus  tschawytscha)  begins  to  run  on  the 
Columbia  in  February  or  March,  and  spawns  as  late  as  November  or 
December  in  the  high  headwaters  of  the  Columbia.  Wherever  the 
spawning  beds  may  be,  it  has  been  found  that  egg-laying  does  not 
take  place  until  the  water  has  fallen  to  about  54°  F.     The  relation  of 


CONSERVATION   AND   ITS   MEANING 


597 


uncontrolled  fishing  to  spawning  is  an  instance  of  the  need  of  wise 
legislation.  Salmon  and  other  food  fish  of  similar  habits,  such  as  the 
sturgeon,  are  in  much  danger  of  extermination  because  of  this  rela- 
tionship. 

A  further  danger  to  fish  is  the  pollution  of  streams.  Thus  the 
salmon  have  not  only  been  depleted  by  overfishing  and  wasteful 
methods  of  fishing,  but 
have  been  "discouraged" 
from  ascending  such 
streams  as  the  Connecti- 
cut River  by  the  great 
quantities  of  polhition 
present.  It  is  reported 
that  the  salmon  formerly 
ran  up  this  river  in  such 
quantities  that  the 
farmers  used  to  back  their 
wagons  to  the  edge  of  the 
stream  and  take  them 
out  by  the  wagon  load 
for  food  and  fertilizer. 
A  somewhat  similar  story 
may  be  told  of  Lake 
Champlain.  At  the  time 
of  the  Revolution,  salmon 
used  to  run  up  from  the 
St.  Lawrence  River  into  the  lake  and  would  then  spawn  in  its 
tributary  streams.  Early  maps  of  this  region  published  in  1776  and 
1779  indicate  the  location  of  "salmon  fisheries"  and  records  from  the 
diary  of  the  settlers  run  as  follows  : 

"Sunday,  Aug.  26,  1789.     The  water  raised  and  Salmon  run  plentifully 
for  the  first. 
Monday,  Oct.  8,  1789.     365  Sahnon  taken." 

The  salmon  industry  in  this  region  declined  so  in  the  next  half  century 
that  we  find  "only  straggling  individuals  are  met  with  in  Lake  Cham- 
plain."  1  Nothing  remains  but  the  memory  —  or  controversy  —  over 
when,  and  by  whom,  the  last  salmon  was  seen  or  caught  in  the  lake. 

I  From  Thompson's  Vermont.  Published  1842.  Quoted  in  Supplement  to  the  19th  Annual 
Report  of  the  N.  Y.  State  Conservation  Department,  1929.  A  Biological  Survey  of  the  Champlain 
Watershed.      1930. 


ICn  .liiKiii  Kodak 

Salmon  leaping  falls.  Proper  lishways  should 
be  constructed  where  dams  interfere  with  fish 
migration. 


598 


MAN   AS   A   CONQUEROR 


Shellfish 

What  has  been  said  with  regard  to  the  great  destruction  of  fish  is 
equally  true  of  shellfish.  The  oysters  in  Chesapeake  Bay  were 
thought  to  be  inexhaustible  until  they  were  almost  exterminated,  when 
the  state  of  Maryland  found  that  in  order  to  preserve  this  great 
natural  asset,  oyster  culture  was  necessary.  Oysters  are  now  con- 
served here  and  in  other  states  by  cultivation.  In  certain  areas  of 
shallow  water  old  oyster  shells,  broken  stone,  or  bunches  of  fagots 
are  placed  on  which  the  young,  free-swimming  larvae  may  attach 
themselves.  After  these  "seed  oysters"  have  grown  to  a  sufficient 
size,  they  are  removed  and  placed  in  beds  in  shallow  water,  where 
they  are  later  harvested. 

There  are  two  general  aspects  of  the  question  dealing  with  the 
shellfish  industry,  namely,  as  it  affects  marine  or  fresh-water  forms. 
Among  the  problems  of  fresh-water  biology  those  relating  to  the 

life  history  of  the  fresh- 
water mussel  should  be 
noted. 

It  is  known  that  the 
propagation  of  fresh-water 
mussels,  the  shells  of  which 
are  used  in  the  pearl  button 
industry,  depends  upon 
whether  the  larval  stage, 
or  glochidium,  finds  the 
proper  temporary  fish  host. 
This  tiny  larval  mussel 
attaches  itself  to  the  fins  or  gills  of  certain  species  of  fish.  The 
host  builds  a  protecting  cyst  wall  about  it,  when  it  undergoes  fur- 
ther development.  After  reaching  a  sufficiently  advanced  stage, 
it  drops  from  its  host,  which  by  this  time  may  have  reached 
quite  a  different  locality,  and  continues  its  own  battle  for  life. 
The  rivers  of  the  mid-west,  especially  the  Mississippi  and  Ohio 
systems,  are  the  great  producers  of  fresh-water  mussels.  The  con- 
struction of  dams  and  the  addition  of  pollution  to  these  streams  have 
killed  off  a  large  percentage  of  these  mussels.  Here  the  United  States 
Bureau  of  Fisheries  came  to  the  rescue  and  used  the  fish  they  were 
salvaging  from  the  back  waters  of  the  Mississippi  system  as  hosts  for 
the  glochidia  of  the  mussels.     While  the  effectiveness  of  this  type  of 


^  "by  ambaddind  in  a  ■FilaTrvsnt 
thess  infect  2^  j-  ^^cr\^irM\3 


tocLoLult 

-for-m  on 

bdtXom. 


Life  history  of  fresh-water  mussel. 


CONSERVATION   AND   ITS   MEANING  599 

artificial  propagation  is  hard  to  determine,  it  is  believed  that  it  has 
been  moderately  successful.  The  Bureau  is  also  attempting  to  raise 
the  glochidia  in  artificial  media  in  quantities  large  enough  to  make  it 
possible  to  keep  the  supply  from  being  depleted  by  the  pearl  button 
industry.  Here  again  hope  lies  in  the  work  of  the  biologist  who  must 
solve  the  problem. 

Birds 

In  the  matter  of  bird  life  the  story  is  the  same.  The  American 
passenger  pigeon,  which  once  was  so  numerous  that  in  1869  one  town 
in  Michigan  marketed  11,880,000  pigeons  in  forty  days,  became 
extinct  by  1914.  In  early  Colonial  times  the  heath  hen  was  abundant 
along  the  eastern  coast  from  Maine  to  the  Carolinas.  The  last  sur- 
viving member  of  this  species  died  on  Martha's  Vineyard  Island  in  1932. 
The  snowy  egret  has  been  practically  exterminated  in  the  South  and 
the  prairie  chicken  has  suffered  the  same  fate  in  the  Central  West. 
Unless  adequate  protection  is  given,  the  red-head,  canvasback,  and 
ruddy  duck  may  become  exterminated  at  least  in  parts  of  their 
range.  The  Labrador  duck  was  exterminated  in  1875,  a  victim  of 
reckless  exploitation.  Measures  have  been  taken  to  protect  the 
dangerously  depleted  wood  duck,  which  is  now,  under  a  closed  season 
extending  over  some  years  in  certain  states,  showing  a  hopeful  though 
slow  increase  in  numbers. 

The  relation  of  bird  migration  to  conservation,  as  in  the  case  of 
fishes,  is  close,  for  the  annual  journeys  that  birds  make  have  been 
made  use  of  by  sportsmen  in  shooting  duck  and  other  wild  fowl. 
While  biologists  have  been  trying  to  explain,  with  a  certain  amount  of 
success,  the  factors  back  of  bird  migration,  an  army  of  hundreds  of 
thousands  of  licensed  hunters,  with  very  definite  success,  have  been 
slaughtering  migrating  birds  by  millions,  until  today  many  of  our 
wild  fowl  are  in  imminent  danger  of  extermination.  Dr.  William  T. 
Hornaday  ten  years  ago  estimated  that  the  stock  of  game  birds  and 
quadrupeds  left  at  that  time  was  only  about  2  per  cent  of  what  had 
existed  fifty  years  before,  and  today  even  this  remnant  has  been 
greatly  reduced.  A  carefully  planned  program  of  restoration  of 
breeding  and  feeding  areas,  destroyed  by  drought,  cultivation  of  the 
land  for  crops,  or  by  other  agencies,  is  now  being  carried  out  by  the 
Federal  government  in  certain  localities  in  the  United  States,  where 
not  many  years  ago  water  and  shore  birds  could  be  found  in  great 
numbers.  Sportsmen  are  combining  with  agencies  seeking  a  biolog- 
ical approach  to  the  problem. 
H.  w.  H.  —  39 


600  MAN  AS  A  CONQUEROR 

Mammals 

The  story  is  repeated  with  the  mammals.  Whales  are  almost  ex- 
terminated, whalebone  whales  for  the  plates  of  baleen  used  in  strain- 
ing out  the  tiny  marine  organisms  on  which  they  feed,  and  the  right, 
sperm,  and  other  species  for  oil.  Among  rodents  the  beaver,  once 
having  a  distribution  reaching  practically  all  over  the  United  States 
north  of  the  Gulf  of  Mexico  and  the  Rio  Grande,  is  now  found  only 
in  a  few  protected  areas.  They  have  been  practically  wiped  out 
because  of  the  value  of  their  fur.  Among  the  carnivores  the  marten, 
the  fisher,  the  mink,  the  fox,  and  many  others  wanted  for  furs  have 
practically  disappeared.  Even  the  lion  and  tiger,  with  their  wide 
African  and  Indian  ranges,  are  becoming  rarities  and  are  only  seen 
in  protected  areas.  Thus  the  onward  march  of  civilization  ruth- 
lessly exacts  its  toll. 

Conservation  of  Wild  Life 

Coming  to  wild  life,  we  find  that  efforts  toward  conservation  are 
still  too  loosely  and  ineffectively  put  forth  to  be  of  much  avail.  There 
is  need  for  a  broad  scheme  of  education  that  shall  reach  every  part  of 
the  country  and  help  to  mold  public  opinion  with  reference  to  the 
conservation  of  our  wild  life  resources.  It  is  true  that  as  far  back  as 
1884  a  start  was  made  by  the  American  Ornithologists'  Union  to  im- 
prove the  legal  status  of  wild  birds.  This  resulted  not  only  in  the  for- 
mation of  the  Audubon  Society,  but  also  laid  the  foundation  in  1885  for 
the  organization  of  the  Biological  Survey  along  scientific  lines  of  inquiry 
into  the  life  histories  and  economic  value  of  birds  and  mammals. 
It  was  not  until  1909,  however,  that  the  Federal  government  framed  a 
law,  known  as  the  Lacey  Amendment,  prohibiting  the  shipping  of 
birds  from  a  state  where  it  was  illegal  to  kill  them.  In  1913,  a  Federal 
law  went  into  effect  stopping  spring  shooting  of  all  migratory  birds, 
and  the  slaughter  of  songbirds,  including  most  insectivorous  birds. 
This  law  gave  a  closed  season  on  fifty-four  out  of  sixty  species  of  shore 
birds  and  shortened  the  open  season  on  northern  waterfowl  to  three 
months,  all  of  which  has  helped  greatly,  but  more  especially  in  the 
protection  of  land  birds.  In  1916,  the  Migratory  Bird  Treaty  Act 
with  Great  Britain  was  devised  and  in  1918  signed,  protecting  over 
five  hundred  species  of  migrating  birds  in  this  country  and  Canada. 
In  1929,  a  Federal  Bird  Refuge  Law  was  passed  providing  money  for 
the  establishment  of  bird  sanctuaries  and  funds  for  their  maintenance. 


CONSERVATION    AND   ITS   MEANING 


601 


^^^^^ 


Biological   Survey 

•  Sfi^  Pefuge  — 

Forest  Service 

©   Bira  Refuse  and  Game  /^eaerfc 
Ottier  Federal  Agencies 

A   Bird  Pefu^e  and  0-ifnf   Preserve 

FederalBird  Refuges  anoGame  Preserves 

APRIL    IS.  I3£a 


U .  a.  Bureau  of  Biul.  Suney 


What  are  the  differences  between  the  bird  refuges  of  the  Biological  Survey 
and  other  F^ederal  agencies?  Why  are  dilVerent  agencies  administering  these 
refuges  ? 

Already  over  one  hundred  wild  life  refuges  have  been  set  aside  by  the 
Federal  Government  and  these  are  augmented  by  many  private  bird 
and  game  refuges  and  preserves,  estimated  at  the  present  time  to 
include  over  800,000  acres  in  this  country  and  over  150,000  acres  in 
Canada. 

Much  also  has  been  done  in  the  way  of  conservation  of  mammals. 
Not  many  years  ago  the  supply  of  American  buffalo,  or  bison,  was 
thought  to  be  inexhaustible,  but  today  after  nearly  complete  extermi- 
nation, a  few  thousand  exist  protected  by  law.  The  Alaskan  fur  seal 
is  another  valuable  mammal  that  was  almost  exterminated  by  over- 
hunting.  Great  herds  were  reduced  from  millions  to  a  little  over 
200,000  in  1910.  At  that  time  the  Federal  government  assumed 
control,  preventing  hunting  during  the  l^reeding  season,  with  the 
result  that  today  the  herd  consists  of  over  600,000  head. 


Present  Methods  of  Conservation 

Considering  in  more  detail  some  of  the  methods  used  by  the  modern 
conservationist,  we  find  that  the  old  hit  or  miss  methods  are  giving 
way  to  new  ideas.  One  method  centers  about  attempts  to  devise 
ways  of  restoring  the  normal  balance  of  nature,  which  has  been  upset 


602  MAN  AS  A  CONQUEROR 

by  man's  interference,  so  that  reproduction  may  occur  normally. 
Stocking  at  random,  for  example,  regions  native  to  grouse  and  quail 
with  the  introduced  pheasant,  or  planting  fish  in  streams  without 
specific  knowledge  of  conditions  essential  to  survival,  such  as  adequate 
food  supply,  the  effect  of  climatic  extremes  of  temperature,  and  pro- 
tected breeding  areas,  are  seen  to  be  makeshift  methods  at  best,  often 
ill  adapted  to  advance  the  welfare  of  either  the  wild  population  or  of 
man. 

Organizations  for  Conservation 

Through  organization,  conservation  is  likely  to  enter  upon  a  new 
and  more  encouraging  era.  The  Wild  Life  Conference  which  met  in 
Washington,  February  3-7,  1936,  had  for  its  purpose  the  building  of  a 
nation-wide  organization  to  undertake  the  task  of  a  co-ordinated 
survey  of  the  status  of  wild  fife  in  each  state  with  united  support 
for  the  enactment  or  revision  of  laws  devised  for  the  betterment  of 
conditions. 

There  are  many  organizations  that  are  interested  in  the  program  of 
conservation.  Each  state  has  many  different  local  fish  and  game 
clubs  that  have  more  or  less  to  do  with  problems  of  one  kind  of 
conservation.  Many  of  these  groups  are  selfishly  interested  because 
as  individuals  they  desire  better  hunting  or  fishing.  Consequently, 
the  emphasis  has  been  to  assist  in  one  way  or  another  in  increasing  the 
local  output  of  pheasants  or  trout.  Upon  the  other  hand,  national 
organizations,  like  the  Izaak  Walton  League,  also  exhibit  an  interest 
in  the  broader  problems  of  conservation,  such  as  the  establishment  of 
game  refuges  and  protection  against  river  pollution  by  factories  and 
cities. 

State  Conservation  Departments 

Local  chapters  and  clubs,  whether  or  not  they  have  national  ties, 
usually  work  through  their  State  conservation  departments,  and  to  a 
somewhat  lesser  extent  through  some  Federal  agency.  The  various 
State  departments  compile  statistics  of  the  vast  quantities  of  fish  and 
game  that  they  have  planted.  Nearly  all  of  these  figures  bring  out 
the  rather  astounding  fact  that  hundreds  of  thousands  of  fish  and  game 
are  planted  annually  without  any  appreciable  increase  in  the  numbers 
available  for  the  sportsman  and  nature  lover,  and  in  not  a  few  cases 
losses  are  recorded.     Nature,  even  with  artificial  help  in  propagation. 


CONSERVATION   AND   ITS   MEANING  603 

does  not  seem  able  to  hold  its  own.     What  happens  to  all  of  these 
animals  which  are  planted  and  which  do  not  appear  to  survive  ? 

Biological  Surveys 

In  order  to  answer  this  question  more  intelligently,  various  scientific 
studies  of  one  sort  or  another  have  been  undertaken.  Most  of  these 
have  been  aimed  at  providing  an  adequate  stocking  policy  for  either 
fish  or  game.  Perhaps  the  most  complete  survey  of  this  sort,  designed 
to  determine  an  adequate  stocking  policy,  deals  with  the  waters  of 
New  York. 

In  1926,  the  New  York  State  Conservation  Department  organized  a 
biological  survey  that  undertook  over  a  period  of  years  a  most  careful 
study  of  the  various  watersheds  of  the  state.  The  cost  of  the  survey 
was  borne  by  receipts  from  fishing  and  hunting  license  fees.  Practi- 
cally every  phase  of  the  life  histories  of  game  fish  was  investigated. 
Such  matters  as  the  existing  fauna  of  the  streams,  ponds,  lakes,  and 
rivers,  together  with  the  food,  weed  areas,  chemistry  of  the  water, 
extent  of  pollution,  bottom  and  plankton  organisms,  as  well  as  the 
great  variety  of  parasites  which  infect  the  fish,  were  given  the  most 
careful  consideration.  On  the  basis  of  the  assembled  data  stocking 
policies  were  then  determined  and  some  estimate  was  made  of  the 
number  and  kind  of  fish  which  should  be  planted. 

A  number  of  other  states  have  adopted  survey  programs  to  help 
determine  stocking  and  planting  policies  for  fish  and  game.  Both 
Michigan  and  California  are  doing  splendid  work  along  these  lines 
and  somewhat  similar  programs  are  contemplated  or  are  actually 
under  way  in  other  states.  The  most  discouraging  feature  of  such 
programs  is  that,  in  most  instances,  it  is  a  case  of  locking  the  barn 
door  after  the  horse  is  stolen,  since  so  much  damage  has  already  been 
done,  some  of  which  is  irreparable. 

Federal  Agencies 

There  are  several  Federal  agencies  acting  either  directly  or  indi- 
rectly along  the  line  of  conservation.  The  Bureau  of  Biological 
Survey  has  various  problems  under  consideration,  dealing  principally 
with  matters  of  the  migration,  distribution,  economic  value,  and  life 
histories  of  various  birds  and  mammals.  In  the  Bureaus  of  Plant  and 
Animal  Industry  centers  the  work  of  parasitologists  who  are  concerned 
with  problems  of  identification  and  control  of  various  types  of  plant 


604 


MAN  AS  A  CONQUEROR 


and  animal  parasites.  In  addition  to  the  field  workers,  hundreds  of 
research  workers  in  government  and  state  laboratories  are  investigat- 
ing problems  connected  with  conservation.  Some  of  these  relate  to 
soil,  insects,  plant  diseases  and  methods  of  overcoming  them,  while 
others  are  problems  requiring  a  genetical  approach.  Bacteriologists 
and  plant  pathologists  are  discovering  bacteria  or  fungi  that  are 
inimical  or  restrictive  to  species  of  their  kind  causing  damage.  An 
example  of  such  work  has  recently  come  from  the  laboratory  of  the 
University  of  Idaho  School  of  Forestry,  where  it  has  been  found  that 
the  white  pine  blister  fungus,  a  serious  enemy  of  the  white  pine,  can  be 
destroyed  by  another  fungus  which  is  parasitic  upon  it.  Investiga- 
tions are  only  beginning  in  this  fertile  field. 

The  Bureau  of  Fisheries 

This  Bureau  is  concerned  with  the  propagation  of  various  types  of 
commercial  and  game  fishes,  as  well  as  shellfish.  During  recent  years 
research  problems  have  covered  three  major  fields,  marine  and  fresh- 
water fisheries  investigations,  agricultural  investigations,  and  investi- 
gations on  shellfish. 

Under  the  first  heading  are  studies  concerned  with  the  conservation 
and  replenishing  of  cod,  haddock,  mackerel,  and  other  salt-water  fishes, 
as  well  as  of  trout  and  salmon,  together  with  various  problems  relat- 
ing to  other  fresh-water  fishes.  In  the  second  group  are  the  problems 
relating  to  the  improvement  of  feeding  and  breeding  trout,  the  treat- 
ment and  cure  of  diseases  of  hatchery  fishes,  studies  on  fish  nutrition 
and  investigation  of  inland  waters  with  respect  to  pollution.  In  the 
third  group  of  investigations  is  the  propagation  of  the  pearl  mussels,  as 
well  as  surveys  of  the  waters  in  our  various  National  Parks.  Fortu- 
nately the  work  of  this  department  appears  to  be  better  correlated 
with  the  programs  of  the  various  State  conservation  departments 
than  some  other  governmental  agencies. 

During  the  year  1933  a  total  of  ninety-one  agencies  were  concerned 
with  the  output  of  fish  for  the  Federal  government.  These  agencies  are 
recorded  as  having  distributed  the  astounding  number  of  7,202,155,600 
fish  and  eggs. 

One  of  the  most  interesting  problems  facing  the  fish-culturist  is 
the  question  of  how  many  of  the  planted  eggs  and  fish  can  survive. 
Although  over  2,000,000,000  artificially  fertilized  eggs  of  the  cod  are 
released  every  year,  it  is  doubtful  if  this  helps  nature  to  any  great 
extent.     When  it  comes  to  the  question  of  stocking  with  fish  eggs, 


CONSERVATION   AND   ITS   MEANING 


fry,  or  fingerlings  secured  from  hatcheries,  it  is  essential  that  there 
should  be  a  high  survival  curve.  Recent  work  tends  to  show  that  in 
the  case  of  trout,  at  least,  the  larger  the  fish,  the  better  the  chance 
it  has  of  meeting  and  overcoming  the  vicissitudes  of  life  when  stocked 
in  an  open  stream. 


k. 


National  Forests 
National  Parks 


^'       s 


National  Parks  and  National  Forests  in  the  United  States.     How  do  you  account 
for  the  geographic  distribution  of  these  areas  .•> 

National  Parks 

Among  the  most  valuable  Federal  agencies  for  the  conservation  of 
wild  life  are  the  National  Parks,  of  which  there  are  twenty-two  in  the 
United  States  and  eighteen  in  Canada.  In  these  areas  all  wild  life  is 
protected,  including  bison,  antelope,  moose,  white-tailed  deer,  mule- 
deer,  elk.  Rocky  Mountain  sheep,  black  and  grizzly  bears,  along 
with  numerous  smaller  predatory  animals.  National  Parks  are  in 
the  truest  sense  the  pleasure  grounds  of  the  conservationist. 

Is  There  a  Unified  Program? 

The  thinking  American  would  undoubtedly  admit  the  necessity  for 
a  definite  long-range  plan  of  conservation  for  all  of  our  natural 
resources,  regardless  of  whether  or  not  he  is  concerned  with  oil, 
forests,  minerals,  or  fish  and  game.  Since  our  various  conservation 
agencies  are  centered  in  the  Federal  government,  it  is  legitimate  that 


606  MAN  AS  A  CONQUEROR 

some  plan  should  emanate  from  this  source.  The  fact  is  that  about 
fourteen  Federal  agencies  have  worked  more  or  less  at  cross  pur- 
poses because  of  the  intricacies  of  red  tape,  and  the  results  are  far 
from  satisfactory. 

One  agency  of  the  Federal  government,  for  example,  reserves  a 
large  breeding  area  such  as  Tule  Lake  in  northern  California  for  the 
use  of  various  species  of  waterfowl.  This  vast  area  is  the  nesting 
place  of  thousands  of  our  wild  migratory  waterfowl,  and  was  set 
aside  for  breeding  purposes  by  the  late  Theodore  Roosevelt.  A  few 
years  ago  $824,000  was  allocated  to  the  Reclamation  Bureau  for  the 
purpose  of  draining  this  vast  lake  and  converting  it  into  farm  land, 
the  maximum  worth  of  which  could  not  possibly  exceed  $300,000. 
As  a  result  it  was  reported  that  in  the  spring  of  1935  agents  of  the 
Reclamation  Service  burned  the  cattails  and  rushes  along  the  borders 
of  the  lake  and  literally  cooked  the  eggs  in  about  800  wild  goose 
nests,  —  and  this  to  improve  the  grazing  conditions  of  the  region. 

Another  example  of  destructive  conservation  appears  in  the  appro- 
priation of  $200,000  for  the  eradication  of  snails  in  four  states  of  the 
Pacific  Northwest.  The  purpose  of  this  was  the  control  of  the  sheep 
liverfluke.  No  one  denies  the  desirability  of  helping  the  sheep  owner 
with  the  problem  of  controlling  parasites  which  do  much  damage  to 
his  flocks.  But  the  method  proposed  for  accomplishing  this  end  is  in 
many  respects  worse  than  the  disease.  The  plan  called  for  the  placing 
of  copper  sulphate,  a  deadly  protoplasmic  poison,  in  the  streams  of 
the  region.  This  is  an  efficacious  method  of  getting  rid  of  snails, 
but  unfortunately  it  also  kills  all  the  other  organisms  which  play  such 
an  important  part  in  the  economics  of  stream  life. 

One  of  the  most  disturbing  actions  with  reference  to  fish  conserva- 
tion is  the  construction  of  huge  power  dams  on  rivers  which  are  high- 
ways for  migrating  salmon  on  their  way  to  the  spawning  beds.  Espe- 
cially is  this  a  serious  menace  in  the  case  of  high  dams,  where  the  fish 
attempt  to  enter  the  current  flowing  from  the  power  plant  instead  of 
ascending  the  fish  ladders  that  are  provided,  and  thus  die  without 
being  able  to  deposit  their  eggs.  There  is  little  doubt  that  the  dams 
now  projected  in  the  Columbia  River  may,  within  a  short  period  of 
time,  sound  the  knell  of  the  salmon-fishing  industry  in  this  region. 

Clearly  the  answer  to  the  questions  raised  in  the  preceding  para- 
graphs can  only  be  furnished  by  the  formation  of  some  Federal 
Bureau  which  has  the  power  to  regulate  all  agencies  for  conservation. 
The  first  annual  meeting  of  the  North  American  Wild  Life  Conference 


CONSERVATION  AND  ITS  MEANING  607 

held  in  St.  Louis  in  March,  1937,  had  this  end  in  view  and  formed  the 
Wild  Life  Federation.  Such  a  Bureau  must  have  the  necessary  fore- 
sight to  enable  it  to  plan  wisely  and  well  a  long-term  conservation 
program  which  will  meet  the  ultimate  needs  of  this  great  country  of 
ours  and  preserve  our  wealth  of  natural  resources.  Otherwise  in 
the  years  to  come  the  American  people  may  be  looked  upon  as  the 
greatest  "desert  makers"  of  all  time. 

SUGGESTED  READINGS 

Hornaday,  W.  T.,  Wild  Life  Conservation  in  Theory  and  Practice,  Yale  Uni- 
versity Press,  1914. 

A  fund  of  information  concerning  the  conservation  of  wild  life  up  to  1915. 
Hornaday,  W.  T.,  Thirty  Years  War  for  Wild  Life,  Charles  Scribner's  Sons, 

1931. 

A  valedictory  by  one  of  this  country's  most  ardent  conservationists. 
Pack,  C.  L.,  Trees  as  Good  Citizens,  American  Tree  Association,  1922. 

An  interesting  and  popular  account  of  the  value  of  shade  trees,  with 

suggestions  for  conservation. 
Rowan,  W.,  The  Riddle  of  Migration,  The  Williams  and  Wilkins  Co.,  1932. 

Applications  to  the  conservation  of  wdld  birds. 


XXVI 


MAN'S  FIGHT  FOR  SURVIVAL 

Preview.  What  is  health?  •  What  is  the  biological  significance  of 
death  ?  •  Causes  of  disease :  Unfavorable  environmental  factors  •  Degenera- 
tive diseases  •  Man  and  his  parasitic  worms :  Parasites  acquired  through 
improperly  prepared  foods;  parasites  acquired  directly  by  man;  parasites 
acquired  indirectly  by  man;  malaria  as  an  economic  problem;  yellow 
fever  and  its  relation  to  insect  vectors ;  typhus ;  other  diseases  carried  by 
insects;  animals  other  than  insects  may  spread  disease;  the  relation  of 
bacteria  to  disease ;  certain  bacteria,  called  pathogens,  cause  disease ;  how 
do  bacteria  enter  the  body?  some  important  bacterial  diseases  •  What  is 
immunity?  The  mechanism  of  immunity;  active  acquired  immunity; 
some  examples  of  diseases  where  active  immunity  is  practiced ;  bacterins 
and  their  use ;  the  menace  of  the  carrier ;  vaccines  and  attenuated  organ- 
isms; hay  fever;  passive  acquired  immunity  •  Are  parasitic  diseases  con- 
querable? •  Suggested  readings. 

PREVIEW 

The  growth  of  knowledge  of  man's  relation  to  parasites  and  the 
prevention  of  disease  has  been  a  matter  of  evolution.  Primitive  man 
used  charms  and  incantations  to  ward  off  disease.  During  Roman 
times  traditional  methods  were  handed  down  from  the-  Greek  and 
Roman  philosophers.  The  first  glimpse  of  real  knowledge  came  in 
the  seventeenth  century  with  such  discoveries  as  that  of  the  circula- 
tion of  blood  by  Harvey,  the  relatively  modern  diagnostic  work  of  the 
physician,  Sydenham,  and  the  surgical  skill  of  John  Hunter.  In 
the  eighteenth  century  progress  was  marked  by  the  work  of  Jenner  in 
relation  to  vaccination  for  smallpox.  In  the  nineteenth  century  a 
rapid  advance  began  with  Pasteur's  discovery  of  bacteria  as  one  of 
the  causes  of  disease,  the  isolation  of  some  of  man's  most  deadly 
enemies  by  Robert  Koch  and  others,  and  the  foundation  of  modern 
antiseptic  surgery  by  Lister.  During  the  latter  part  of  this  period 
many  discoveries  of  ways  and  means  of  disease  prevention  were 
made,  such  as  the  beginnings  of  water  filtration,  the  pasteurization 
of  milk,  and  more  emphasis  on  the  control  and  prevention  of  various 
diseases.  The  twentieth  century  marks  a  notable  departure  into  the 
field  of  public  health  and  a  rapid  development  of  public  health  work. 

608 


MAN'S   FIGHT   FOR  SURVIVAL  609 

The  discovery  of  the  importance  of  disease  carriers,  both  insect  and 
human,  has  played  an  miportant  part  in  the  improvement  of  Uving 
conditions,  while  even  more  important  is  the  advance  of  knowledge 
in  relation  to  immunity  and  the  factors  which  bring  it  about.  The 
life  span  has  steadily  advanced  as  a  result  of  these  and  other  discover- 
ies and  their  applications,  a  gain  brought  about  largely  through  the 
mastery  of  disease  caused  by  bacteria,  especially  those  diseases  which 
are  fatal  to  young  children. 

Since  the  question  of  the  survival  and  progress  of  civilization 
depends  upon  a  knowledge  of  the  means  of  successful  control  of  disease 
and  preservation  of  health,  a  study  of  man's  fight  for  survival  should 
be  of  the  greatest  value  to  college  students,  who  later  must  take  their 
places  as  intelligent  citizens  and  voters.  Upon  their  knowledge  of  the 
facts  concerning  the  successes  in  this  battle  against  parasites  and 
disease,  further  progress  will  depend. 

Although  our  knowledge  should  be  preventive  rather  than  curative, 
since  it  is  the  duty  of  the  physician  to  take  care  of  the  disease  when 
it  comes,  the  average  citizen  ought  to  be  well  informed  enough  to 
answer  all  of  the  following  questions  intelligently.  What  are  the 
causes  of  disease?  Which  parasites  do  man  most  bodily  harm? 
Where  is  he  most  likely  to  meet  them  and  how  may  he  prevent  their 
attacks?  What  are  the  facts  about  human  carriers?  Do  present 
laws  adequately  protect  man  against  them?  What  is  natural 
immunity  and  how  does  the  body  protect  itself?  What  is  the 
present  status  of  protective  immunity  and  what  are  the  ways  in  Avhich 
man  may  bring  it  about  ?  These  questions  are  taken  up  in  the  pages 
that  follow. 

What  Is  Health? 

Health  is  evidently  something  to  be  sought  after  as  a  primary  ob- 
jective of  life.  The  old  Anglo-Saxon  word,  hoelth,  from  which  the 
word  health  is  derived,  meant  ivhole  as  opposed  to  its  opposite,  soec 
which  meant  sick  or  not  whole.  The  implication  is  clear.  Health 
is  a  condition  in  which  body  and  mind  are  free  from  disease.  One 
writer  puts  health  on  a  higher  plane  and  defines  it  as  "the  quality  of 
life  that  renders  the  individual  fit  to  live  most  and  serve  best."  ^ 
Such  a  definition  gives  to  life  a  higher  responsibility,  and  is  one 
that  should  be  adopted  by  every  man  and  woman.     If  education 


1  Williams,  J.  G.,  Personal  Hygiene  Applied,  Saunders,  1926. 


610  MAN  AS  A  CONQUEROR 

means  leadership,  then  it  should  mean  healthy  leadership  in  the 
best  sense. 

The  human  body  has  been  likened  to  an  engine  by  many  writers. 
It  requires  fuel  and  oxygen  in  order  to  release  energy,  forms  wastes 
which  have  to  be  eliminated,  and  must  have  frequent  rest  in  order  to 
do  its  work  most  efficiently.  Both  machine  and  body  eventually 
wear  out,  but  we  do  not  refer  to  a  sick  machine  although  we  do  speak 
of  a  sick  person.  Anyone  may  abuse  his  body  through  lack  of  sleep, 
exercise,  or  improper  food  so  that  it  will  not  function  properly.  He 
may  poison  it  with  alcohol  and  nicotine  and  injure  some  of  the  internal 
organs  so  that  they  never  will  have  their  former  efficiency.  He  may 
meet  with  an  accident  and  be  crippled,  or  he  may  be  attacked  by 
microscopic  foes  such  as  bacteria  or  protozoa  and  thus  suffer  from 
disease.  Not  only  in  these  respects  does  the  body  differ  from  the 
machine,  but  also  for  the  reason  that  it  can  repair  itself,  a  thing  which 
no  machine  can  do.  When  it  is  in  perfect  condition  it  is  called  a 
healthy  body. 

What  Is  the  Biological  Significance  of  Death? 

In  old  age,  the  body  machinery  begins  to  wear  out,  the  normal 
functions  slow  down,  tissues  wear  away,  the  liver,  kidneys,  reproduc- 
tive and  nervous  tissues  shrink  and  cease  from  more  active  function- 
ing. Muscles  lose  their  tone,  the  weight  becomes  less,  special  sense 
organs  lose  their  accuracy  as  the  body  reacts  more  slowly  to  stimuli 
and  the  skin  does  not  shrink  as  fast  as  do  the  tissues  beneath.  Diges- 
tion does  not  function  as  well  as  formerly  and  apparently  there  comes 
a  slow  poisoning  of  the  tissues,  since  the  cells  give  out  wastes  more 
rapidly  than  they  are  eliminated.  The  body  machine  wears  out 
because  it  cannot  eliminate  the  poison  fast  enough.  At  length  some 
part  of  the  system  gives  out.  In  most  cases  the  muscles  of  the 
heart,  that  have  been  constantly  at  work  since  before  birth,  sud- 
denly stop,  or  the  arteries,  which  have  become  brittle  through  faulty 
calcium  metabolism,  break  and  death  ensues. 

We  often  think  of  an  animal  as  dead  if  its  head  is  cut  off.  But 
under  such  circumstances  the  heart  of  a  frog  or  a  snake  continues 
beating.  Obviously  such  an  animal  is  not  all  dead.  The  work  of 
Carrel  with  excised  tissues  gives  evidence  that  the  individual  cells  of 
the  body,  if  in  a  favorable  environment,  will  continue  living  perhaps 
indefinitely.  Physically,  death  means  the  breaking  of  the  plasma 
membranes  of  the  cells  so  that  their  selectively  permeable  properties 


MAN'S  FIGHT  FOR  SURVIVAL  611 

are  lost.  When  a  cell  is  killed,  substances  which  in  life  could  not  pass 
in  or  out  by  osmosis  can  easily  do  so,  with  the  result  that  it  loses  the 
salts  and  sugars  essential  for  life,  as  well  as  its  turgor,  aad  becomes 
limp.  It  is  dead,  for  it  no  longer  has  the  ability  to  regulate  its  outgo 
and  intake. 

Causes  of  Disease 

The  causes  of  disease  are  many.  These  may  be  listed  as  food 
deficiencies,  endocrine  maladjustments,  hereditary  deficiencies,  un- 
favorable environmental  factors,  bad  health  habits  which  result  in 
body  poisons,  diseases  of  middle  and  old  age  (wearing  out  of  the 
machine),  parasitic  diseases,  and  infections.  Health  examinations 
of  some  1500  men  entering  Cornell  University  showed  that  over  50 
per  cent  had  defective  eyes,  over  25  per  cent  bad  posture,  over 
22  per  cent  skin  disease,  22  per  cent  enlarged  thyroid  glands,  and  over 
20  per  cent  flat  feet,  all  of  which  physical  handicaps  are  correctable.* 
Wood's  estimate  made  in  1918  of  16,000,000  school  children  with 
physical  defects  or  ailments  either  preventable  or  remediable  has  not 
changed  greatly  in  recent  years.  These  conditions  in  children  and 
young  adults  are  largely  due  to  lack  of  proper  care  in  running  the 
human  machine.  Improper  diet,  overfatigue,  poor  posture,  over- 
stimulation through  drugs  or  alcohol,  heedlessness  of  warning  symp- 
toms —  these  are  the  most  frequent  causes  of  bodily  illness.  Dr.  Vin- 
cent, former  president  of  the  Rockefeller  Foundation,  recently  stated 
that  more  than  80  per  cent  of  the  illnesses  of  man  could  be  avoided 
if  people  were  willing  to  obey  the  laws  of  health  and  live  as  well  as 
they  knew  how  to  live.  The  running  of  the  human  machine  is  up 
to  the  individual  and  it  is  only  through  his  willingness  to  take  care 
of  himself  that  an  individual  health  program  can  be  established. 

Unfavorable  Environmental  Factors 

In  the  past  this  factor  has  been  overstressed.  There  is  no  doubt 
that  overcrowding,  unsanitary  conditions,  and  lack  of  a  pure  water 
supply  help  to  raise  the  death  rate.  Tuberculosis,  for  example,  is 
closely  correlated  with  social  conditions.  Factors  which  lower  the 
bodily  resistance  also,  such  as  fatigue,  exposure  to  conditions  of  wet 
and  cold,  poor  ventilation  in  working  and  living  quarters,  are  all 
menaces  to  health,  but  the  old  idea  that  the  products  of  decomposi- 
tion of  animal  and  vegetable  material  cause  disease  is  untrue.     A  few 

»  Smiley  and  Gould,  A  College  Textbook  of  Hygiene,  Macmillan,  1926,  p.  3. 


612  MAN  AS  A  CONQUEROR 

decades  ago,  interest  centered  in  civic  clean-ups  because  it  was 
believed  that  clean  streets  meant  a  lower  death  rate,  but  street 
cleaning  or  house  cleaning  will  not  control  epidemics  of  disease.  On 
the  other  hand,  there  are  unfavorable  environmental  factors  which 
directly  contribute  to  outbreaks  of  epidemics,  such  as  impure  milk 
and  polluted  water  supply,  the  control  of  which  is  of  the  utmost 
importance  to  the  health  of  the  individual.  Not  only  food  supplies 
containing  the  proper  vitamins  necessary  to  life  are  essential  to 
health,  but  also  the  assurance  that  all  foods  handled  are  clean  and  that 
the  handlers  of  foods  are  also  clean  and  free  from  disease.  Selfishness 
of  neighbors  is  a  large  factor  in  the  health  of  a  given  community, 
since  communicable  diseases  are  spread  through  carelessness  on  the 
part  of  those  who  have  them.  The  publicity  of  scientific  knowledge  is 
a  large  factor  in  public  health.  The  increase  of  interest  on  the  part  of 
the  public  is  today  correlated  with  clinics  for  the  care  of  babies,  for 
prenatal  care,  for  the  care  of  venereal  disease  and  tuberculosis,  and 
above  all  with  clinics  where  treatment  for  immunity  against  certain 
diseases  may  be  received.  Health  knowledge  disseminated  by  means 
of  bulletins,  lecture  bureaus,  radio  talks,  and  particularly  school 
health  programs  and  public  nursing  services,  are  all  factors  which 
help  to  control  unfavorable  conditions  in  a  given  community. 

Degenerative  Diseases 

After  the  age  of  forty,  the  greatest  numlier  of  deaths  are  caused  by 
heart  disease,  cerebral  hemorrhage,  arteriosclerosis,  cancer,  paresis, 
and  nephritis.  Along  with  these,  pneumonia  and  tuberculosis  claim 
many  victims.  The  statistician,  Louis  Dublin,  states  that  approxi- 
mately 2  per  cent  of  the  total  population  of  the  United  States  suffer 
from  organic  heart  defects  and  that  the  number  of  deaths  from  this 
cause  is  over  200,000  annually.  It  is  the  chief  cause  of  death  after 
the  age  of  forty-five  years.  The  origins  of  this  disease  often  date 
back  to  childhood,  when  heart  lesions  may  have  resulted  from  early 
infections.  A  large  percentage  of  heart  trouble  is  also  due  to  syphilis. 
In  the  case  of  cancer  there  is  a  constantly  mounting  mortality.  We 
know  what  cancer  is,  but  we  do  not  know  what  causes  it.  Apparently 
certain  groups  of  cells  go  wild,  growing  without  restraint  until  they 
destroy  their  victim.  Two  types  of  cancer  are  knowm,  only  one  of 
which  is  malignant.  Education  ought  to  make  people  realize  the 
necessity  of  immediate  diagnosis  and  an  operation,  when  necessary,  if 
cancer  is  to  be  overcome.     Nephritis,  a  disease  of  the  kidneys,  slows 


MAN'S  FIGHT  FOR  SURVIVAL 


613 


down  their  efficiency,  allowing  poisons  to  accumulate  in  the  body 
which  eventually  cause  death.  In  the  case  of  cerebral  hemorrhage, 
as  well  as  apoplexy  and   arteriosclerosis,  the   only  help  comes  in 


-siiicicCe- 17.9 

other  respiratory        > 

/ear/- 


,  "homicide/  .9.0 
4au^  ojtdcCents 


apople^.etc. 


Death  rates  in  cities  of  the  United  States,  based  on  the  latest  available  informa- 
tion.    What  types  of  diseases  exact  the  greatest  toll  of  life  ? 

moderation  both  in  diet  and  in  bodily  activity.  Degenerative  dis- 
eases are  the  natural  result  of  the  gradual  wearing  out  of  the  body  and 
all  that  we  can  expect  to  do  is  to  lessen  the  death  rate  from  these 
causes. 


Man  and  His  Parasitic  Worms 

Various  parasitic  worms  have  been  known  for  countless  centuries  as 
enemies  of  man.  In  the  Ebers  papyrus  of  the  sixteenth  century  B.C. 
there  is  a  record  of  certain  diseases  attributed  to  the  presence  of  the 
"bowel  worm."  The  fiery  serpent  which  the  Israelites  encountered 
in  the  wilderness  of  Sinai  was  undoubtedly  none  other  than  a  round- 
worm, Fullebornius  (Dracunculus)  medinensis.  Evidence  of  the  sa- 
gacity of  Moses  lies  in  his  separation  of  animals  into  "clean"  and 
"unclean"  on  the  basis  of  the  presence  or  absence  of  parasites. 
Thus  all  scavenger  beasts  were  prohibited  as  food.  As  civilization 
progressed  and  man  used  more  cooked  food,  the  number  of  parasites 


614 


MAN  AS  A  CONQUEROR 


which  could  be  acquired  through  the  ingestion  of  raw  meats  was  much 
reduced.  However,  there  are  still  epidemics  of  various  parasitic 
diseases  due  to  worms,  although  they  usually  occur  in  widely  scattered 
localities. 


Parasites  Acquired  through  Improperly  Prepared  Foods 

Fortunately  there  are  only  a  few  tapeworms  which  may  affect  the 
health  of  human  beings.  Meats  that  pass  from  one  state  to  an- 
other are  inspected  by  the  Federal  government  for  the  presence  of 
larval  stages  of  such  parasites.  Not  all  beef  and  pork,  however,  is 
examined  for  the  encysted  larval  stage  of  the  beef  and  pork  tape- 
worms (T.  saginata  and  T.  solium).  This  means  that  meat  obtained 
through  local  abattoirs  would  not  be  inspected  by  a  Federal  repre- 
sentative and  so  might  be  infected.     The  descriptions  of  the  life 


Regions  in  North  America  where  fish  infected  with  the  larval  stages  of  the  broad 
tapeworm  of  man  have  been  taken.     (After  Ward.) 

histories  of  these  worms,  as  well  as  that  of  the  broad  tapeworm  of 
man  {Diphyllohothrium  latum),  are  found  on  pages  226-230. 

A  study  of  the  distribution  of  the  broad  tapeworm  in  the  United 
States  suggests  that  it  was  introduced  from  the  continent  by  various 
immigrants  who  were  infected  when  admitted  to  this  country.  A 
very  high  percentage  of  the  population  living  near  the  shores  of  the 
Baltic  Sea,  like  the  Finns,  are  infected  with  this  tapeworm.     One  of 


MAN'S   FIGHT   FOR  SURVIVAL  615 

the  first  endemic  centers  of  the  broad  tapeworm  of  man  in  this 
country  was  in  the  region  in  and  about  Ely,  Minnesota,  which  is  a 
community  with  a  high  percentage  of  Finns.  More  recently,  the 
parasite  has  been  found  to  be  spreading  to  other  parts  of  the  country 
and  it  is  possible  that  it  may  prove  to  be  one  of  the  more  important 
parasitic  worms  with  which  health  authorities  have  to  deal,  since 
both  of  the  intermediate  hosts  are  found  in  nearly  all  of  our  inland 
waters. 

Another  parasite  that  is  perhaps  the  most  universally  distributed 
form  in  this  country  is  the  pork  roundworm  Trichinella  spiralis, 
an  organism  so  minute  that  the  government  does  not  take  the  respon- 
sibility of  inspecting  for  it.  Great  care  should  be  exercised  in  pre- 
paring pork  to  have  it  thoroughly  cooked.  The  life  history  of  this 
worm  is  described  on  page  225. 

The  presence  of  the  larvae  of  Trichinella  in  the  blood  stream  stimu- 
lates the  production  of  one  group  of  white  blood  corpuscles,  the 
eosinophils,  which  is  a  characteristic  symptom  of  trichinosis,  as  this 
parasitic  disease  is  called.  At  the  time  of  the  penetration  of  the 
larvae  into  the  muscles,  severe  muscular  pain,  especially  in  the 
extensors  and  flexors,  is  usually  experienced,  which  is  followed  by  a 
period  when  the  patient  becomes  emaciated  and  anemic,  and  is  fre- 
quently succeeded  by  a  secondary  complication  in  the  form  of  pneu- 
monia.    Death  may  ensue  due  to  exhaustion  or  pneumonia. 

On  the  basis  of  1895  autopsies  made  between  1881  and  1910,  39 
(2.5  per  cent)  were  infected  with  Trichinella  spiralis.  More  recently 
Queen  (1931)  reported  a  total  of  18.6  per  cent  in  403  autopsies  and  Hall 
(1936),  13.7  per  cent.  This  does  not  necessarily  mean  that  trichinosis 
is  on  the  increase  in  this  country  but  rather  that  the  methods  of  de- 
tection have  improved,  more  representative  samplings  of  the  popula- 
tion have  been  made,  and  that  the  examinations  are  more  careful.  It 
appears  probable  from  the  above  that  a  much  greater  proportion  of 
the  population  harbors  this  parasite  than  was  previously  supposed. 

Parasites  Acquired  Directly  by  Man 

There  are  several  rather  important  parasites  of  man  found  in  this 
country  that  are  not  carried  by  an  intermediate  host,  but  which  reach 
him  directly.  The  two  most  important  forms  are  the  hookworm 
(Necator  americanus)  and  the  roundworm,  Ascaris.  Children  fre- 
quently pick  up  other  parasitic  worms,  but  these  two  are  probably 
the  most  important  from  the  standpoint  of  public  health. 
H.  w.  H.  —  40 


616  MAN  AS  A  CONQUEROR 

The  hookworm  was  first  recognized  as  an  insidious  cause  of  dis- 
ease in  this  country  by  Dr.  C.  W.  Stiles  in  1902.  He  considered 
it  a  major  factor  responsible  for  the  condition  of  indigent  and  shift- 
less people  known  as  "poor  whites"  throughout  the  southeastern 
part  of  the  United  States.  Infection  by  hookworm  has  recently 
been  found  to  be  almost  universal  in  some  tropical  countries  and  is 
widespread  in  all  tropical  countries  at  the  present  time.  The  Negro 
is  apparently  much  more  resistant  to  the  debilitating  effects  of  this 
parasite  than  his  white  brother.  The  survey  work  of  the  Rockefeller 
Sanitary  Commission  has  made  it  possible  to  follow  the  progress  of 
educational  campaigns  throughout  the  world  to  combat  hookworm 
disease  as  well  as  to  study  the  effects  of  the  treatments  administered 
for  its  suppression.  Between  1910  and  1915,  a  survey  was  conducted 
in  the  United  States  under  the  auspices  of  this  commission  and  it 
was  found  that  children  between  six  and  eighteen  years  of  age  carry 
the  heaviest  infection.  Of  approximately  90,000  children  examined, 
55.1  per  cent  were  infected.  Between  1920  and  1923,  an  inspection  of 
more  than  44,000  children  from  some  of  the  same  areas  showed  that 
the  infection  had  dropped  to  27.8  per  cent.  In  this  same  connection 
it  should  be  noted  that  the  infection  was  also  much  lighter,  some 
school  children  harboring  but  few  worms. 

The  question  arises  as  to  the  way  in  which  the  infection  becomes 
established,  and  how  it  happens  that  children  are  more  heavily 
infected  than  their  parents.  The  fact  that  youngsters  are  usually 
barefoot  while  a  much  greater  proportion  of  the  adults  wear  shoes 
has  a  direct  relation  to  the  spread  of  the  infection.  The  control  of 
the  hookworm  is  due  largely  to  education  in  community  sanitation 
in  addition  to  therapeutic  measures.  In  the  poorer  districts  of  the 
South,  sanitary  privies  were  rarely  found,  hence  the  soil  in  many 
localities  was  literally  alive  with  hookworm  larvae.  With  the  build- 
ing of  privies  and  educating  people,  both  young  and  old,  to  wear  shoes 
as  a  means  of  prevention,  the  danger  of  infection  in  these  regions 
was  greatly  decreased.  Several  substances  have  been  used  as  ver- 
mifuges, carbon  tetrachloride  in  a  chemically  pure  form  having  been 
found  most  efficient. 

In  foreign  tropical  countries  aid  given  to  over  seventy  different 
countries  or  states  through  the  International  Board  of  the  Rocke- 
feller Foundation  has  reduced  hookworm  infection  on  an  average  of 
50  per  cent  in  Ceylon,  India,  the  Philippines,  and  Siam,  as  well  as  in 
some  South  American  countries. 


MAN'S   FIGHT   FOR  SURVIVAL 


617 


Another  parasitic  infection  caused  by  the  roundworm  Ascaris 
lumhricoides  involves  a  rather  large  proportion  of  tlie  population, 
especially  in  the  southern  parts  of  this  country  where  the  weather  is 
warmer  and  presumably  the  conditions  necessary  for  its  develop- 
ment are  more  nearly  ideal.  Prior  to  1921  various  surveys  indicated 
that  the  eleven  states  extending  east  of  the  Mississippi  River  and 
south  of  the  Ohio,  with  Texas,  had  an  average  infection  of  13.8  per 
cent  of  the  population.  Further  studies  were  carried  on  in  1934  with 
the  result  that  certain  regions  in  mountainous  sections  where  soil 
conditions  were  just  right  for  its  spread  showed  an  infection  rate  as 
high  as  30  per  cent.  However,  it  was  demonstrated  that  through 
the  use  of  suitable  sanitary  methods  this  worm  can  be  controlled, 
as  seen  by  reference  to  its  life  history,  page  225. 

Most  of  the  parasites  mentioned  which  infect  man  are  intestinal 
forms.  In  the  midwest,  however,  bathers  at  a  few  summer  resorts 
have  encountered  a  different  variety.  Certain  fork-tailed  free- 
swimming  larvae,  cercariae,  of  some  of  the  blood  flukes  which  nor- 
mally penetrate  the  skin  of  some  of  the  lower  mammals  to  invade 
their  blood  streams  apparently  mistake  man  for  their  normal  host. 
Fortunately  they  do  not  continue 
their  development  in  this  unusual 
host,  although  causing  an  intense 
itching  during  and  after  penetra- 
tion of  the  skin,  chiefly  among 
susceptible  people. 


Parasites  Acquired  Indirectly  by 
Man 

In  the  higher  as  well  as  in  many 
of  the  lower  organisms  which 
parasitize  man,  the  life  history  of 
the  invader  is  often  found  in  two 
or  more  different  hosts.  Contact 
with  the  parasite,  obviously,  is 
necessary  in  order  to  have  the 
disease  germs  enter  the  body. 

In  the  case  of  protozoan  para- 
sites which  affect  man  and  in  some  cases  of  bacterial  infection  a  carrier 
usually  becomes  necessary  in  order  that  the  infective  organism  may 
reach  the  interior  of  the  body.     Among  these  carriers  there  are  two 


CLAW  FROM  TIP  OF  FOOT 

0»BACILU 


■:^mmmi,^^ 


Diagram  to  show  how  bacteria  might  be 
carried  on  the  foot  of  the  hou.se  fly. 


618  MAN  AS  A  CONQUEROR 

distinct  types.  In  many  cases  insects,  that  are  called  vectors,  pick 
up  the  destructive  organism  incidentally  and  carry  it.  Such  an 
insect  carrier  is  the  house  fly,  which  has  been  inveighed  against  by 
many  writers  as  being  one  of  our  most  deadly  enemies  as  a  carrier 
of  intestinal  diseases. 

Many  other  insects  have  criminal  records  of  this  sort,  for  example, 
the  malarial  organism  is  carried  by  a  specific  mosquito,  Anopheles. 
Yellow  fever  is  directly  related  to  the  Aedes  mosquito,  while  in  the 
Far  East  another  species  carries  the  filarial  worm,  which  causes  the 
terribly  deforming  disease  known  as  elephantiasis.  In  certain  areas 
in  Africa,  the  tsetse  fly  Glossina  transmits  the  dreaded  sleeping 
sickness,  and  almost  universally  lice,  fleas,  ticks,  and  mites  may  all 
be  added  to  the  list  of  organisms  responsible  for  spreading  disease. 

Insects  that  carry  parasites  dangerous  to  man's  health  and  welfare 
maybe  divided  into  two  groups,  first,  casual  carriers,  such  as  the  house 
fly,  in  which  the  parasite  carried  has  no  relation  whatever  to  the  life  of 
the  insect  carrier,  and  secondly,  predatory  insects,  or  those  which  suck 
blood,  and  in  which  the  parasite  passes  a  part  of  its  life  cycle.  Such 
insects  may  often  be  dangerous  carriers,  as  shown  by  the  blood-sucking 
mosquitoes  that  carry  malaria  and  yellow  fever. 

Malaria  as  an  Economic  Problem 

The  economic  problem  of  malaria  has  been  very  serious  in  almost  all 
temperate  and  tropical  parts  of  the  world.  In  this  country,  the 
problem  has  affected  over  13,000,000  of  the  inhabitants,  principally 
those  living  in  the  South,  where  in  some  states  as  high  as  90  per 
cent  of  the  population  live  in  districts  where  the  malarial  mosquito 
is  normally  found.  Statistics  in  this  country  show  that  millions  of 
dollars  are  lost  each  year  through  workers  who  are  incapacitated  and 
whose  efficiency  is  materially  affected  by  the  disease.  It  is  estimated 
that  for  each  death  attributable  to  malaria  there  is  a  loss  of  from 
2000  to  4000  days  by  illness. 

Among  the  effective  preventive  measures  are  oiling  of  standing 
water  to  prevent  breeding  of  mosquitoes,  draining  of  marshes,  the 
introduction  of  certain  species  of  fish  which  feed  upon  the  larvae,  and 
screening  of  houses  in  districts  where  malaria  is  present.  The  most 
recent  method  of  control  is  by  spraying  standing  water  with  finely 
powdered  Paris  green.  The  anopheline  larvae  eat  this  material  and 
are  poisoned  by  it.  In  some  parts  of  Italy  where  malaria  has  been 
extremely  prevalent  in  the  past,  it  was  found  that  towns  in  areas 


MAN'S   FIGHT   FOR   SURVIVAL  619 

where  the  Paris  green  treatment  had  been  used  have  almost  com- 
pletely eliminated  malaria,  while  in  towns  only  a  few  miles  away 
where  no  such  treatment  was  used,  four  fifths  of  the  inhabitants 
contracted  the  disease  in  a  single  season. 

Yellow  Fever  and  Its  Relation  to  Insect  Vectors 

Although  we  do  not  think  of  yellow  fever  as  being  an  important 
disease  today,  it  was  not  more  than  a  century  and  a  half  ago  that  it 
played  a  very  important  part  in  the  health  of  this  country.  As  late 
as  1878,  the  disease  ravaged  the  Mississippi  Valley,  where  in  34  cities 
there  were  nearly  70,000  cases  and  over  16,000  deaths. 

The  story  of  the  conquest  of  yellow  fever  is  one  of  the  most  thrilling 
in  medical  annals.  After  the  Spanish-American  War,  when  yellow 
fever  was  so  prevalent  in  Havana,  a  military  commission  consisting 
of  Major  Walter  Reed,  James  Carroll,  A.  Agramonte,  and  Jesse 
Lazear  was  established  to  investigate  the  control  of  the  disease. 
After  a  series  of  experiments  which  resulted  in  the  death  of  Dr.  Lazear 
and  the  severe  illness  of  several  army  volunteers,  the  mosquito  Aedes 
was  proven  to  be  the  carrier  of  this  dread  disease.  Methods  of  pre- 
vention adopted  as  a  result  of  these  experiments  were  almost  im- 
mediately successful  in  Cuba  and  in  other  parts  of  the  world  where 
the  disease  had  been  endemic.  Yellow  fever  has  always  been 
limited  to  areas  near  the  seacoast  or  along  the  banks  of  navigable 
rivers.  It  is  prevalent  during  hot  seasons,  but  much  less  of  a 
menace  in  cold  weather.  Now  that  we  know  the  relation  of  the 
disease  to  its  transmission  by  the  mosquito  Aedes  some  of  these  points 
clear  up. 

No  one  has  yet  seen  the  causal  agent  of  the  disease.  In  1918 
the  Japanese  parasitologist,  Noguchi,  working  for  the  Rockefeller 
Institute,  found  a  spiral  organism  which  he  believed  was  the  cause. 
Later  he  lost  his  life  on  the  Gold  Coast  of  Africa  while  still  seeking 
the  causal  agent.  It  is  now  believed  that  the  organism  is  not  a 
spirochete,  but  a  filtrable  virus.  Even  though  the  organism  is  not 
known,  the  fact  that  the  carrier  is  has  made  it  possible  practically  to 
eliminate  the  disease  from  areas  where  as  late  as  1900  it  was  endemic. 

Typhus 

Another  disease  closely  connected  with  an  insect  carrier  is  typhus. 
During  the  seventeenth  and  eighteenth  centuries  epidemics  of  this 
disease  were  frequent  in  crowded  and  unsanitary  areas,  especially 


620  MAN   AS  A  CONQUEROR 

where  conditions  of  famine  and  war  were  found.  In  1909,  the  trans- 
mission of  typhus  was  first  correlated  with  the  bite  of  the  body  louse 
or  "cootie."  During  the  World  War  the  disease  was  kept  under  con- 
trol through  the  disinfection  not  only  of  wearing  apparel  but  also  of 
the  soldiers  themselves  in  "de-lousing"  plants  which  were  established 
back  of  the  front-line  trenches. 

Other  Diseases  Carried  by  Insects 

Numerous  protozoan  diseases  are  carried  by  insects.  In  tropical 
countries,  especially,  several  diseases  of  cattle  as  well  as  of  man  are 
caused  by  trypanosomes,  tiny  protozoans  belonging  to  the  group 
of  the  flagellates.  One  species  (7".  gamhiense)  produces  the  African 
sleeping  sickness  while  another  form  {T.  cruzi)  causes  Chagas'  disease 
in  South  and  Central  America. 

Many  other  diseases  of  man  are  caused  by  parasitic  protozoans. 
Amebic  dysentery  is  caused  by  the  presence  of  Endameba  histolytica, 
which  lives  in  the  colon  of  the  digestive  tract.  These  parasites  are 
much  more  widely  spread  than  was  formerly  thought,  for  even  in  this 
country  from  5  to  10  per  cent  of  the  population  carry  this  parasite. 
Amebic  dysentery  received  considerable  publicity  during  the  recent 
World's  Fair  at  Chicago  when  several  carriers  were  discovered 
handling  food  and  a  number  of  cases  were  traced  to  Chicago.  Among 
other  ijisect-borne  diseases  are  kola  azar,  a  tropical  fever,  which 
is  thought  to  be  carried  by  fleas  and  bedbugs ;  dengue,  a  disease 
caused  by  a  filtrable  virus  carried  by  mosquitoes ;  pappataci,  a 
tropical  disease  believed  to  be  caused  by  a  filtrable  virus  and  carried 
by  a  sand-fly;  and  possibly  poliomyelitis,  which  is  thought  to  be 
carried  by  flies. 

Animals  Other  Than  Insects  May  Spread  Disease 

The  arachnids  or  ticks  are  serious  enemies  of  higher  animals, 
especially  cattle,  because  they  transmit  such  diseases  as  Texas  fever, 
and  in  the  case  of  man,  the  Rocky  Mountain  spotted  fever.  The 
relapsing  fevers  of  the  tropics  are  also  believed  to  be  carried  by  ticks 
as  well  as  by  bedbugs,  fleas,  and  some  biting  flies. 

Bubonic  plague,  the  Black  Death  of  the  Middle  Ages,  is  estimated 
to  have  killed  over  25,000,000  people  in  Europe  during  the  fourteenth 
century.  It  even  reached  this  country  about  1900,  killing  more  than 
100  persons  in  California  during  the  succeeding  four  years.  At  pres- 
ent, there  are  several  endemic  foci  of  the  disease,  one  in  China,  one 


MAN'S  FIGHT  FOR  SURVIVAL  621 

in  India,  a  third  in  Arabia,  and  a  fourth  in  the  interior  of  Africa,  to 
which  must  now  be  added  a  fifth  area  on  our  western  coast.  Plague 
is  really  a  disease  of  rats  and  ground  squirrels,  but  through  the 
activity  of  fleas  it  can  be  transferred  from  a  sick  rat  to  the  body  of 
man,  where  it  thrives.  Over  a  million  rats  were  killed  in  fighting 
the  last  outbreak  of  bubonic  plague  in  California  and  great  care  has 
to  be  used  in  quarantine  to  prevent  rats  from  reaching  our  shores 
through  ships  from  countries  where  the  plague  is  endemic. 

The  Relation  of  Bacteria  to  Disease 

Bacteria  are  present  almost  everywhere  as  parasites.  They  are 
found  inside  as  well  as  outside  of  the  human  body,  existing  in  countless 
milHons  in  the  mouth,  on  the  teeth,  and  particularly  in  the  lower  part 
of  the  food  tube.  There  has  been  a  good  deal  of  discussion  as  to 
whether  the  bacteria  in  the  food  tube  are  harmful  or  useful.  Experi- 
ments indicate  that  in  some  animals,  at  least,  bacteria  live  as  mess- 
mates in  the  digestive  tract,  actually  helping  the  host  by  breaking 
down  waste  from  foods.  Several  recent  experiments  have  shown  that 
intestinal  bacteria  are  not  necessary,  however,  in  the  life  process  of 
the  host. 

Certain  Bacteria  Called  Pathogens  Cause  Disease 

These  organisms,  like  other  living  things,  take  in  food  and  form 
organic  wastes  within  their  own  bodies  which  they  give  off  as  toxins. 
Toxins  that  diffuse  through  the  body  tissues  of  the  host  where  the 
infection  occurs  are  called  exotoxins,  while  those  retained  within  the 
bodies  of  the  bacteria  to  be  released  at  their  death  are  referred  to  as 
endotoxins.  Every  species  of  pathogenic  bacteria  forms  a  particular 
toxin  which  has  a  specific  action  on  the  host,  frequently  causing  symp- 
toms of  a  definite  disease.  When  bacteria  die,  as  they  may  in  great 
numbers  during  the  progress  of  a  disease,  they  break  down,  releasing 
protoplasmic  constituents  that  separate  from  each  other,  splitting  into 
smaller  and  smaller  molecular  groups  as  proteins  do  when  changed 
to  amino  acids  during  digestion.  These  split  proteins,  as  they  are 
called,  may  be  extremely  poisonous  and  act  in  many  cases  as  toxins. 
Bacteria  also  break  down  body  tissues  of  the  host,  in  some  cases 
destroying  the  intestinal  lining,  blood  corpuscles,  or,  as  in  tuber- 
culosis, definite  tissue  cells.  Parasitic  bacteria  that  cause  boils  and 
abscesses  are  believed  to  send  out  enzymes  which  dissolve  the  white 
corpuscles  so  that  they  may  be  used  by  bacteria. 


622  MAN  AS  A  CONQUEROR 

Like  other  parasites  that  have  been  mentioned,  bacteria  show  con- 
siderable variation  as  to  choice  of  host.  Some  few,  such  as  those 
causing  typhoid,  Asiatic  cholera,  or  syphilis,  are  restricted  to  man 
and  apparently  cannot  gain  and  maintain  a  hold  in  the  bodies  of  other 
hosts.  Another  group,  bubonic  plague,  anthrax,  rabies,  and  glanders, 
that  normally  live  in  other  hosts  than  man,  have  become  adapted  to 
his  body  through  his  contact  with  lower  animals.  One  of  the  best 
examples  of  accidental  parasitic  attack  on  man  is  bubonic  plague, 
which  came  through  the  introduction  of  the  rat  as  a  hanger-on  m 
homes.  A  third  group  of  bacteria  which  includes  the  tubercle  bacilli 
as  well  as  the  group  of  the  streptococci  and  pneumococci  appear  to 
live  in  several  different  hosts.  Certain  of  the  cocci  are  parasitic  in 
other  animals  as  well  as  in  man.  The  bovine  tubercle  bacillus  may 
live  in  the  pig  or  in  man  as  well  as  in  its  original  cattle  host.  It  was 
this  habit  among  certain  types  of  bacteria  of  living  in  a  variety  of 
hosts  that  gave  the  clue  to  some  of  the  early  discoveries  with  reference 
to  disease.  Robert  Koch  noticed,  for  example,  tiny  rods  in  the  blood 
of  sheep  that  had  just  died  from  splenic  fever.  He  could  not  afford 
to  purchase  sheep  to  experiment  with  since  he  was  a  poor  country 
doctor,  but  he  could  afford  mice.  He  found  that  inoculations  of  the 
mice  with  infected  sheep's  blood  caused  the  death  of  the  mice  and, 
moreover,  that  the  same  symptoms  appeared  in  both  mice  and  sheep. 
This  fact  led  to  the  discovery,  through  the  making  of  pure  cultures, 
that  one  specific  germ  causes  the  disease  anthrax.  Many  other 
similar  discoveries  have  hinged  on  the  biological  factor. 

How  Do  Bacteria  Enter  the  Body? 

Microorganisms  causing  infectious  diseases  enter  the  body  through 
some  body  opening,  respiratory,  digestive,  genital,  or  urinary,  or 
through  wounds  in  the  skin.  The  most  frequent  means  of  infection  is 
through  direct  contact  or  by  a  spray  of  tiny  droplets  which  is  expelled 
into  the  air  while  talking.  Other  avenues  of  infection  are  dust,  which 
spreads  germ.s  of  tuberculosis ;  impure  water  or  contaminated  milk, 
which  may  contain  typhoid  germs  ;  soil,  from  which  the  tetanus  bacilli 
may  be  picked  up ;  raw  foods,  which  may  spread  such  diseases  as 
septic  sore  throat  and  typhoid ;  and  handling  of  articles  used  by 
persons  suffering  from  an  infectious  disease.  In  addition  to  these 
means  there  is  the  introduction  of  infection  through  carriers,  such  as 
insects  or,  in  some  cases,  man.  The  human  carrier,  as  we  will  see 
later,  is  a  most  serious  menace  to  society. 


MAN'S  FIGHT  FOR  SURVIVAL  623 

It  might  be  thought  that  with  all  of  these  bacterial  foes  and  so 
many  means  of  infection  the  human  body  would  succumb  without 
even  making  a  fight.  However,  man  has  several  definite  ways  of 
resistance.  In  the  first  place  a  good  state  of  health  does  much  to 
give  effective  resistance  to  entering  bacteria.  The  skin,  if  healthy, 
is  an  effective  barrier  and  is  far  more  effective  if  it  has  no  abrasions. 
Many  secretions  given  off  from  the  protective  tissues,  such  as  tears 
which  cover  the  conjunctiva  of  the  eye,  the  various  juices  of  the 
digestive  tract,  and  even  the  lymph  that  surrounds  the  body  cells, 
contain  resistive  substances  that  prevent  the  growth  of  bacteria,  pro- 
vided the  body  is  in  a  healthy  condition. 

Some  Important  Bacterial  Diseases 

Although  modern  medicine  is  rapidly  conquering  many  diseases, 
some  still  remain  unvanquished.  Of  these,  tuberculosis  stands  out 
as  one  of  the  most  serious  enemies  of  man.  While  the  common  cold 
causes  more  days  of  illness  and  is  perhaps  economically  the  most 
important,  it  is  not  as  serious  a  menace  as  tuberculosis,  which  is 
probably  responsible  for  one  tenth  of  all  the  deaths  due  to  diseases 
to  which  man  is  subject.  In  1900,  the  death  rate  from  tuberculosis 
was  195.2  for  each  100,000  inhabitants  in  the  registered  area  of  the 
United  States.  In  1935,  the  death  rate  in  the  same  area  had  dropped 
to  51.2  per  100,000.  While  this  is  encouraging  in  the  extreme,  it  does 
not  mean  that  the  disease  is  conquered. 

Tuberculosis  is  caused  by  the  growth  of  tubercle  bacilli  within  the 
lungs  or  other  tissues  of  the  body.  In  the  lungs  they  form  small 
tubercles  which  close  up  the  delicate  air  passages,  while  they  also 
attack  other  parts  of  the  body,  causing  tuberculosis  of  the  bones, 
scrofula,  and  other  diseases.  Tuberculosis  is  usually  contracted  from 
other  people  who  have  the  disease,  although  in  the  case  of  children 
the  bovine  tuberculosis  germ  may  cause  the  disease.  Dr.  William  H. 
Park,  a  noted  authority  on  bovine  tuberculosis,  states  that  in  a  very 
large  number  of  cases  investigated,  57  per  cent  of  abdominal  tubercu- 
losis in  young  children  and  47  per  cent  of  such  tuberculosis  in  children 
under  five  years  of  age  was  of  the  bovine  type.  It  is  needless  to  say 
that  all  milk  should  come  from  tuberculin-tested  cows  or  at  least  be 
pasteurized,  especially  if  the  milk  is  of  doubtful  origin,  since  this 
method,  if  properly  used,  will  kill  the  tuberculosis  germs.  About 
one  per  cent  of  the  beef  cattle  show  tuberculosis  by  test,  but  the 
meat  from  such  cattle,  if  properly  cooked,  is  not  a  menace. 


624  MAN  AS  A  CONQUEROR 

Tuberculosis  is  unfortunately  tied  up  with  social  conditions  and  for 
this  reason  is  extremely  difficult  to  combat.  Ten  times  as  much 
tuberculosis  has  been  found  in  the  heads  of  families  earning  less 
than   $500  a  year  as  among  those  earning  $700  and   over.     The 

disease  is  not  inherited, 
1940  ?  death  rates       ^ut    where    people    live 

fSOfSn  '^^^siZ^°'^       crowded    together    with 

lyjJ  Zj51[MftMi\  tuV®rculos'is      other  tubercular  people, 

1930  liVBvIt  it  is  extremely  hard  to 

1925  If  SNIltD  prevent  infection,  espe- 

AAAARAAAAAAn  cially    it    they    live    m 

1 920  VVVhUIIIIIHI  homes  with  little  ventila- 

1915  OOQt&IIINIIIM)  fr     I"  New  York  City 

1910  ossfloniiiiiititc    ht\s"Lr:u 

1905  9@SI@lllllllf  lltltO         ^h^^h    were    known    to 

1900  omiitiiiiitiiiiii)  ^rl^tzz 

What  factors  are  responsible  for  the  steady         tuberculosis  existed  there 
dechne  in  deaths  from  tuberculosis  ? 

year  after  year.  Tuber- 
culosis is  also  closely  related  to  certain  trades,  especially  the  so-called 
dusty  trades.  Any  work  that  lowers  the  resistance  through  poor 
ventilation,  long  hours,  insufficient  nutrition,  and  dusty  occupations 
paves  the  way  for  tuberculosis.  The  chief  factor  in  combating 
tuberculosis  is  keeping  up  a  high  resistance  to  all  diseases.  This 
is  obtained  only  through  proper  amounts  of  sleep  and  rest,  plenty  of 
fresh  air,  proper  food  with  a  large  amount  of  milk,  and,  particularly, 
freedom  from  worry.  Since  all  of  these  conditions  are  difficult  to 
obtain  in  the  lower  social  scale,  it  is  obvious  why  the  disease  is  so 
hard  to  combat.  A  form  of  vaccination,  the  Calmette  vaccine,  is 
now  being  used  with  some  degree  of  success,  especially  in  the  case 
of  young  children. 

In  the  year  1920,  influenza  and  pneumonia  were  responsible  for 
twice  as  many  deaths  in  the  United  States  as  were  caused  by  tubercu- 
losis. Those  of  us  who  remember  the  frightful  epidemic  which  lasted 
from  September,  1918,  to  June,  1919,  have  reason  to  dread  influenza. 
There  have  been  over  fourteen  epidemics  of  influenza  and  pneumonia 
since  the  sixteenth  century.  In  the  great  outbreak  during  the  World 
War  there  were  635,000  deaths  from  these  diseases  as  against  a  normal 
mortality  of  135,000  for  the  same  period.     Of  a  total  population  of 


MAN'S  FIGHT  1011  SURVIVAL  625 

104,000,000  in  this  country,  it  is  estimated  that  over  30,000,000  had 
influenza.  While  much  work  has  been  done  to  discover  the  causative 
organism  of  influenza,  the  fact  that  the  organism  works  in  conjunc- 
tion with  several  others,  including  the  pneumococcus  germ,  has  made 
it  difficult  for  the  disease  to  be  fought  by  means  of  vaccines  or  im- 
mune sera.  At  present  these  two  diseases  may  be  named  among  the 
most  serious  enemies  of  mankind. 

Although  it  is  impossible  to  do  more  than  mention  the  many  diseases 
caused  by  bacteria,  emphasis  should  be  placed  on  the  fact  that  among 
the  most  common  infections  are  those  caused  by  the  Streptococceae. 
Pneumonia,  septic  sore  throat,  which  often  appears  in  severe  epi- 
demics, erysipelas,  and  apparently  catarrh  and  some  forms  of  colds 
are  caused  by  them.  The  StapJujlococci  are  responsible  for  boils  and 
abscesses.  A  member  of  the  genus  Neisseria  causes  gonorrhea  and 
probably  cerebro-spinal  meningitis.  Anthrax,  tetanus,  whooping 
cough,  gas  gangrene,  cholera,  bubonic  plague,  Malta  fever,  one  type 
of  dysentery,  and  hundreds  of  other  diseases  are  due  to  specific  forms 
of  bacteria. 

What  Is  Immunity? 

It  is  a  matter  of  common  knowledge  that  certain  members  of  a 
family  will  have  a  very  light  attack  of  a  communicable  disease  while 
the  others  may  suffer  severely  from  it.  Some  may  be  exposed  many 
times  to  a  given  disease  and  not  take  it,  while  others,  who  are  more 
susceptible,  will  come  down  with  the  disease.  This  resistance  on  the 
part  of  the  body  to  disease  is  called  immunity.  Adults  are  practically 
immune  to  certain  children's  diseases,  such  as  measles,  chicken-pox, 
and  scarlet  fever.  On  the  other  hand  infants  appear  to  be  immune, 
especially  early  in  life,  to  both  diphtheria  and  measles.  A  theory 
has  been  advanced  that  this  early  immunity  is  restricted  to  breast- 
fed babies  because  the  material  (colostrum)  secreted  in  the  mother's 
breasts  shortly  after  childbirth  contains  substances  w^iich  protect 
the  child  against  these  and  other  early  infections. 

Eskimos,  Indians,  the  Irish,  Scandinavians,  and  Negroes  are  very 
susceptible  to  tuberculosis,  while  Jews  are  relatively  immune  to  this 
disease,  probably  due  to  the  fact  that  the  American  Jews  have  lived 
an  urban  life  where  they  have  been  constantly  exposed  to  tuberculosis 
and  so  have  built  up  an  immunity  to  it.  The  inhabitants  of  the  Fiji 
Islands  were  almost  wiped  out  by  exposure  to  measles,  a  relatively 
mild  disease  to  the  European.     The  Negro  seems  to  have  a  natural 


626  MAN  AS  A  CONQUEROR 

immunity  to  diphtheria,  while  the  North  American  Indian  is  some- 
what immune  to  scarlet  fever.  The  natives  of  South  America  are 
much  more  resistant  to  malaria  and  yellow  fever  than  are  whites  from 
more  northern  territories.  Evidently,  then,  immunity  may  be  racial 
as  well  as  individual. 

Immunity  is  also  brought  about  through  an  attack  of  infectious 
diseases.  One  Greek  historian  who  visited  Athens  more  than  twenty 
centuries  ago  noted  that  those  who  recovered  from  a  visitation  of 
plague  did  not  take  the  disease  a  second  time.  Immunity  which 
lasts  for  a  greater  or  lesser  period  is  usually  found  after  attacks 
of  smallpox,  chicken-pox,  measles,  mumps,  scarlet  fever,  whooping 
cough,  and  many  other  diseases. 

The  Mechanism  of  Immunity 

All  toxins,  when  entering  the  human  body,  cause  the  body  cells 
and  blood  to  react  to  these  poisons,  through  the  protection  of  various 
substances  known  as  antibodies.  These,  when  produced  in  the  body, 
have  the  effect  of  either  neutralizing  the  toxins  or  actively  fighting 
bacteria.  In  addition  to  antibodies  there  is  also  a  protective  mecha- 
nism (phagocytes)  in  the  white  corpuscles  of  the  blood.  If  bacteria  get 
into  a  wound,  for  example,  the  phagocytes  are  apparently  drawn  to 
the  spot,  possibly  through  some  chemical  stimulus,  and  attack  the 
bacteria  by  engulfing  them.  The  blood  contains  certain  types  of 
antibodies  which  are  known  as  opsonins.  These,  which  are  specific  for 
different  diseases,  enable  the  phagocytes  to  engulf  and  digest  invad- 
ing bacteria. 

Certain  other  antibodies  called  lysins  act  directly  on  the  bacteria 
themselves,  causing  them  to  dissolve.  Still  another  group  of  anti- 
bodies called  agglutinins  cause  the  bacteria  in  the  blood  to  clump 
together  in  tiny  inactive  masses  and  are  doubtless  acted  upon  by 
both  opsonins  and  lysins  so  that  they  become  an  easy  prey  for  the 
phagocytes.  Yet  another  group  of  antibodies,  known  as  precipitins, 
cause  the  bacteria  to  precipitate  out  from  the  blood  in  masses  that 
are  easily  discernible  under  the  microscope.  Agglutinins  and  precipi- 
tins have  become  of  great  value  to  physicians  in  determining  whether 
or  not  certain  diseases  are  present.  For  example,  a  test  known  as  the 
Widal  test  has  been  developed  to  determine  whether  a  person  has 
typhoid  fever.  A  few  drops  of  the  patient's  blood  are  allowed  to 
stand  until  the  serum  has  separated,  and  this  is  then  diluted  with  a 
weak  salt  solution  to  which  are  added  live  typhoid  bacteria.     If  the 


MAN'S  FIGHT   FOR  SURVIVAL 


627 


person  whose  blood  is  tested  has  typhoid,  the  bacteria  will  imme- 
diately become  clumped  together  or  agglutinated,  thus  showing  that 


'i(//M/^/lT////////////////////^, 


■^^, 


■^/////////,y.//^,^M,//y/////yl4^ 


Agglutination  test  for  typhoid.  The  diagram  at  the  left  shows  free-swimming 
bacteria,  at  the  right  the  bacteria  have  become  clumped  together  by  theagglutins 
produced  by  the  body  cells. 

the  antibodies  are  already  formed  and  are  at  work.  Just  as  each 
disease  is  caused  by  a  specific  kind  of  organism  producing  a  specific 
type  of  toxin,  so  the  blood  forms  a  specific  type  of  antibody  for  each 
toxin. 

Another  method  of  receiving  immunity  has  been  recently  discovered 
independently  by  two  investigators,  Twort  and  d'Herelle.  The 
latter  made  a  suspension  of  feces  from  a  convalescent  case  of  dysen- 
tery, filtered  the  material,  and  then  added  the  filtrate  to  a  broth  culture 
of  dysentery  and  found  that  some  substance  in  the  filtrate  killed 
the  bacteria.  This  substance  he  called  bacteriophage.  It  is  ultra- 
microscopic,  specific,  being  produced  by  specific  bacteria,  and  appears, 
under  certain  conditions,  to  produce  immunity  to  specific  diseases. 


Active  Acquired  Immunity 

It  has  long  been  known  that  immunity  can  be  acquired  through  an 
attack  of  a  given  contagious  disease.  The  idea  underlying  this  type 
of  immunity,  later  developed  by  Pasteur,  is  that  the  causal  organism 
may  be  weakened,  then  inoculated  into  a  person's  body,  and  a 
slight  attack  of  the  disease  thus  induced.  Active  immunity  is  now 
brought  about  in  different  ways  through  the  introduction  of  (1)  living 
organisms  causing  the  disease,  (2)  attenuated  or  weakened  organisms, 
(3)  dead  organisms,  or  (4)  extracts  of  products  of  the  organism.  All 
of  these  substances  may  be  called  vaccines.     The  underlying  prin- 


628  MAN  AS  A  CONQUEROR 

ciple  in  this  type  of  immunity  is  the  same  in  all  of  these  cases. 
Certain  cells  of  the  body  are  roused  or  activated  to  form  anti- 
bodies. Thus  the  invading  organisms  are  destroyed  and  their  toxins 
neutralized.  In  other  words,  the  body  is  active  and  does  its  own 
work  by  means  of  lysins,  precipitins,  agglutinins,  and  other  defense 
mechanisms. 

Some  Examples  of  Diseases  Where  Active  Immunity  Is 

Practiced 

Smallpox  is  a  very  ancient  disease,  having  been  known  for  thousands 
of  years.  Always  epidemic,  in  the  eighteenth  century  it  is  said  to 
have  caused  60,000,000  deaths  in  Europe.  The  disease  was  brought 
to  America  by  the  Spanish  early  in  the  sixteenth  century,  and  three 
and  a  half  millions  of  Mexicans  died  as  a  result.  The  American 
Indians  were  almost  wiped  out  by  epidemics  of  smallpox  that  began 
in  early  Colonial  days. 

The  famous  discovery  of  vaccination  for  smallpox  by  Edward 
Jenner  was  a  matter  of  evolution.  The  Chinese  and  Turks  used  a 
form  of  inoculation  against  smallpox.  Lady  Mary  Wortley  Montagu, 
a  famous  beauty  of  her  time,  and  wife  of  the  English  minister  to 
Turkey,  believed  so  much  in  the  inoculation  practiced  by  the  Turks 
that  she  had  her  own  boy  inoculated  and  introduced  the  practice  into 
England  in  1721,  a  date  considerably  earlier  than  that  of  Jenner's 
experiments  with  inoculation.  For  nearly  twenty  years,  Jenner 
made  observations  and  experiments,  until  in  May,  1796,  he  vaccinated 
a  boy  of  eight  with  lymph  taken  from  cowpox  pustules  on  the  hand 
of  a  milkmaid.  Shortly  after  this  the  boy  was  inoculated  with  some 
pustules  of  smallpox  and  failed  to  take  the  disease.  This  discovery 
resulted  in  making  possible  the  conquest  of  smallpox.  The  present 
method  of  preparing  vaccine  virus  is  painstakingly  safeguarded. 
Healthy  calves,  preferably  from  six  months  to  two  years  old,  are  kept 
under  sanitary  conditions  until  it  is  certain  that  they  have  no  disease. 
They  are  then  inoculated  with  smallpox  virus  on  carefully  steri- 
lized areas  on  the  ventral  side  of  the  body.  Later  these  areas  be- 
come covered  with  small  vesicles  which  contain  the  smallpox  virus. 
This  virus  is  then  collected,  placed  in  sterile  containers,  treated  with 
glycerol  and  distilled  water,  and  allowed  to  stand  three  to  four 
weeks.  It  is  then  ground  up  and  put  into  small  containers  for  use 
by  physicians.  Every  step  in  the  process  is  carefully  protected, 
so  that  if  fresh  virus  is  used  there  is  absolutely  no  danger  to  the 


MAN'S  FIGHT  FOR  SURVIVAL 


629 


patient  in  vaccination,  and  almost  certain  immunity  against  small- 
pox is  conferred. 

Nevertheless,  smallpox  is  still  with  us.  Frequent  outbreaks  still 
occur  and  it  is  much  to  our  shame  that  the  United  States  has  one  fifth 
of  all  the  smallpox  in 
the  civilized  world.  Dur- 
ing the  years  1921-1926 
Massachusetts,  with  a 
population  of  4,197,000, 
had  64  cases  of  smallpox, 
though  only  2  deaths, 
while  CaUfornia,  with  a 
population  of  400,000 
less,  had  26,985  cases  and 
392  deaths.  This  differ- 
ence in  smallpox  rate  was 
not  due  to  climate  or 
conditions  of  inhabitants. 


Deaths  from  smallpox  occur  almost  entirely  in 
states  that  do  not  enforce  compulsory  vaccination 
laws. 


but  simply  to  the  fact  that  in  1911,  laws  compelling  vaccination  as 
a  prerequisite  for  school  attendance  in  California  were  repealed  and 
in  1921  all  compulsory  vaccination  laws  were  repealed,  while  in 
Massachusetts,  vaccination  is  compulsory.  In  areas  where  vaccina- 
tion is  required  the  rate  of  smallpox  is  almost  zero. 

In  the  case  of  typhoid  we  have  a  nearly  conquered  enemy.  Pri- 
marily a  disease  of  the  digestive  tract,  the  bacilli  enter  the  body  with 
raw  foods  and  leave  the  body  in  the  feces.  Hence,  any  food  or  drink 
that  is  contaminated  with  sewage  becomes  a  potential  source  of  infec- 
tion. Prior  to  1890,  the  death  rate  from  typhoid  was  frequently  as 
high  as  200  per  100,000  inhabitants,  w^hile  today  in  the  country  at 
large  the  death  rate  from  typhoid  and  paratyphoid  is  only  a  little  over 
3  per  100,000.  This  change  has  been  brought  about  first  through  the 
knowledge  that  epidemics  are  usually  due  to  contaminated  water  or 
milk.  Filtration  plus  chlorination  of  water  supplies  has  cut  out  the 
offending  bacillus  from  water.  Pasteurization  of  milk  has  almost 
eliminated  this  source  of  danger,  although  there  are  still  epidemics 
which  are  due  to  poor  milk  supplies.  As  late  as  1927  Montreal, 
Canada,  had  an  epidemic  of  4755  cases  of  typhoid  which  were  dis- 
tributed through  milk.  A  report  of  the  epidemic  says  that  "surface 
streams  were  commonly  used  as  sources  of  water  for  the  milk  houses 
(houses  where  the  milk  was  prepared  for  shipment)  and  for  the  dis- 


630  MAN  AS  A   CONQUEROR 

posal  of  sewage  from  the  homes  up  stream,"  and  in  one  milk-receiving 
station  "the  water  used  mainly  for  washing  the  cooling  vats  and 
other  equipment  was  pumped  from  the  river." 

Bacterins  and  Their  Use 

Typhoid  fever  has  been  largely  brought  under  control  by  means  of  a 
vaccine  known  as  a  haderin  because  it  is  made  from  dead  causative 
bacteria.  The  principle  underlying  vaccination  is  that  the  body 
works  up  an  active  immunity  by  the  introduction  of  large  numbers  of 
dead  typhoid  germs.  The  presence  of  the  dead  bacilli  stimulates 
certain  living  cells  in  the  body  to  make  antibodies,  thus  causing  the 
body  to  acquire  immunity.  The  immunity  acquired  probably  does 
not  last  more  than  two  or  three  years,  so  that  typhoid  inoculation 
should  be  given  within  this  period  if  continued  immunity  is  to  be 
expected.  Bacterins  are  now  used  as  protective  agencies  against 
cholera  and  plague.  During  the  World  War  a  mixture  of  four  vac- 
cines (typhoid  bacilli,  paratyphoid  bacilli  A  and  B,  and  cholera 
spirilla)  was  used  successfully  by  Castellani  in  Serbia  to  control  these 
diseases.  A  vaccine  made  of  living  bovine  bacilli  cultivated  in  the 
laboratory  long  enough  to  make  them  lose  their  virulence  is  the  basis 
of  the  Calmette  vaccine  which  is  used  as  a  preventive  against  tuber- 
culosis. There  seems  to  be  divided  opinion  as  to  the  value  of  this 
treatment. 

The  Menace  of  the  Carrier 

Although  we  can  protect  our  milk  and  water  supplies  and  to  a  very 
large  degree  control  typhoid  through  the  use  of  cooked  rather  than 
raw  foods,  we  cannot  protect  ourselves  adequately  from  the  one 
menace  that  keeps  typhoid  and  certain  other  intestinal  diseases  con- 
stantly with  us.  People  recovering  from  typhoid  frequently  carry 
bacteria  in  the  body  for  some  time  after  the  disease.  Such  people  are 
called  temporary  carriers.  Frequently  the  germs  are  carried  for 
longer  periods,  the  person  being  apparently  well.  People  have  been 
found  to  be  carriers  when  no  typhoid  history  can  be  traced.  Such  a 
chronic  carrier  was  the  cook  known  as  "Typhoid  Mary."  Presum- 
ably the  typhoid  bacilli  were  transferred  to  food  by  means  of  her 
dirty  hands.  During  a  period  of  fourteen  years  she  was  responsible 
for  forty-nine  cases  of  typhoid.  The  typhoid  carrier  is  more  com- 
mon than  is  usually  realized,  and  since  isolating  carriers  is  a  form 


MAN'S  FIGHT  FOR  SURVIVAL  631 

of  attacking  personal  liberty,  a  serious  legal  problem  is  involved  in 
their  control. 

In  order  to  stamp  out  parasitic  diseases  absolutely,  there  must  be 
effective  control  of  the  activities  of  carriers.  This  is  a  difficult  matter 
to  carry  out  because  of  the  injustice  worked  on  the  carrier  who  fre- 
quently must  make  a  living.  Perhaps  medical  discoveries  will  find 
some  way  to  make  carriers  safe,  but  at  least  they  must  be  educated 
as  to  their  potential  danger  to  others.  Upon  their  co-operation,  the 
health  of  a  community  frequently  depends. 

Vaccines  and  Attenuated  Organisms 

The  story  of  the  use  of  vaccines  in  the  fight  against  germ  disease  is 
tied  up  closely  with  the  work  of  Louis  Pasteur.  In  1880,  while  he  was 
engaged  in  an  investigation  of  chicken  cholera,  several  virulent  cul- 
tures of  cholera  bacteria  were  overlooked  and  left  in  the  laboratory. 
Some  days  later  these  organisms  were  used  to  inoculate  healthy  fowls. 
To  Pasteur's  surprise  the  birds  did  not  die  and  later  were  found  to 
be  immune  to  the  deadly  chicken  cholera  germs.  This  discovery 
gave  Pasteur  the  idea  of  using  weakened  or  attenuated  cultures  of 
bacteria  in  inoculation  as  a  protection  against  disease.  Continued 
study  showed  that  anthrax,  if  grown  in  the  laboratory  at  a  relatively 
high  temperature,  was  also  much  weakened  and  could  be  used  suc- 
cessfully in  inoculation  against  anthrax  in  sheep  and  cattle. 

The  same  idea  was  used  in  Pasteur's  famous  and  successful  attack 
on  rabies.  It  is  a  dramatic  episode  worth  the  telling.  Rabies,  a  dis- 
ease of  dogs  transmissible  to  man,  had  long  been  known  as  a  dread 
and  incurable  enemy  of  mankind.  Pasteur  first  unsuccessfully  tried 
to  make  vaccine  from  the  saliva  of  rabid  dogs,  but  later  found  that, 
since  the  disease  attacks  the  central  nervous  system,  the  dried  nerve- 
cord  of  infected  animals  gave  him  a  source  for  the  inoculating  virus. 
He  dried  nerve  cords  of  infected  rabbits  for  a  period  of  fourteen  days 
and  found  that  by  that  time  the  organism  had  lost  its  virulence  so  that, 
when  inoculated  into  dogs,  it  had  no  effect.  Beginning  with  cords 
dried  for  thirteen  days  and  continuing  inoculations  made  from  crushed 
fragments  of  cords  which  had  only  dried  one  day,  Pasteur  was  able 
to  prove  that  dogs  bitten  by  other  rabid  dogs  w^ere  protected  against 
the  disease.  But  to  carry  this  experiment  over  to  human  beings  was 
another  matter.  Ultimately  a  small  boy  from  the  province  of  Alsace, 
terribly  lacerated  by  a  mad  dog,  was  brought  to  his  laboratory.  It 
was  a  life  or  death  case  and  Pasteur  made  the  inoculations  with  fear  and 

H.    W.    H. 41 


632  MAN  AS  A  CONQUEROR 

misgivings.  The  treatment  proved  successful  and  the  praise  of  Pasteur 
was  sung  all  over  the  world.  One  more  disease  had  been  conquered 
through  the  use  of  vaccines.  In  this  particular  case,  the  causal  agent 
has  never  actually  been  found,  but  it  is  thought  to  be  a  filtrable  virus, 
which  once  within  the  body  attacks  the  central  nervous  system. 

Rabies  has  been  dreaded  most,  not  because  of  its  prevalence,  but 
because  of  its  deadly  nature.  In  well-developed  cases  recovery  is 
very  rare,  the  mortality  being  practically  100  per  cent.  In  1886, 
when  treatments  at  the  Pasteur  Institute  were  first  being  undertaken 
on  a  large  scale,  2671  persons  were  treated  with  a  mortality  of  less 
than  1  per  cent.  By  1912  the  mortality  was  reduced  to  0,  showing 
the  efficacy  of  this  treatment. 

Hay  Fever 

Still  another  type  of  disease  is  fought  by  means  of  the  principle  of 
active  immunity.  Sufferers  from  hay  fever  and  from  hay  fever 
hives  and  certain  forms  of  food  poisoning  are  found  to  be  susceptible 
to  certain  proteins.  These  may  be  in  the  form  of  pollens  in  the  case 
of  hay  fever  sufferers,  or  in  the  form  of  certain  types  of  foods,  or  other 
proteins,  such  as  hair,  feathers,  and  even  dust,  in  the  case  of  asthma 
or  food-poisoning  symptoms.  In  order  to  discover  what  causes  the 
susceptibility,  extracts  of  different  pollens  or  different  food  substances 
are  placed  on  small  abrasions  in  the  skin.  An  almost  immediate 
reddening  welt  is  formed  if  the  patient  is  susceptible  to  the  substances. 
Much  relief  is  afforded  and  sometimes  a  total  cure  of  these  symptoms 
is  found  in  an  antigen  manufactured  from  the  offending  proteins  which 
is  inoculated  in  gradually  increasing  doses  until  the  body  builds  up 
resistance  sufficient  to  give  tolerance  to  the  offending  substance. 

Passive  Acquired  Immunity 

Another  type  of  immunity  depends  not  on  the  use  of  bacteria,  but 
instead,  on  their  products  or  toxins.  Such  antitoxin  treatment 
consists  of  neutralizing  the  toxin  given  off  by  bacteria  in  the  body  with 
immune  bodies  which  have  been  developed  by  other  organisms.  The 
use  of  antitoxin  is  associated  with  diphtheria,  since  it  was  in  connection 
with  this  disease  that  this  method  of  treatment  was  first  worked  out. 

In  1888,  Roux,  working  in  Pasteur's  laboratory,  found  that  the 
diphtheria  germ  produces  a  toxin  which  causes  the  symptoms  of  the 
disease,  and  a  little  later  the  German,  von  Bering,  found  that  a 
serum  made  from  the  blood  of  animals  that  had  been  made  immune 


MAN'S  FIGHT  FOR  SURVIVAL 


6:53 


to  diphtheria  could,  when  inoculated  into  other  animals,  confer  this 
immunity  upon  them.  A  protective  antitoxin  was  first  used  in  1893 
in  Berlin  and  a  perfected  antitoxin  made  from  the  blood  of  the  horse 
was  used  with  startling  success  in  this  country  in  1895.  In  1916,  a 
modified  treatment  in  which  the  toxin  of  the  germ  was  injected  along 
with  the  antitoxin  resulted  in  a  better  protection  because  the  nat- 
ural defenses  of  the  body  were  stimulated  by  the  small  amount  of 
toxin  injected  to  form  antibodies,  wliile  the  antitoxins  protected  the 
body  from  harmful  effects.  This  toxin-antitoxin  treatment  was  in 
turn  improved  upon  in  1923  by  two  workers,  one  in  France  and  the 
other  in  England,  who  found  that  diphtheria  toxin  treated  with 
formalin  lost  its  toxic  power  but  at  the  same  time  continued  to  pro- 
duce immunity.  This  substance,  called  a  toxoid,  bids  fair  to  become 
the  only  method  used.  It  will  be  noted  that  this  is  an  active  immunity 
and  not  passive  such  as  that  produced  by  antitoxin. 

Another  control  measure  against  diphtheria  has  been  found  in  the 
so-called  Schick  test,  named  after  its  discoverer,  Bela  Schick.  This 
test  shows  immediately 
whether  a  person  is  sus- 
ceptible or  immune  to  the 
disease.  A  very  minute 
amount  of  diphtheria  toxin 
is  injected  into  the  outer 
skin  and  if  the  person  is 
susceptible,  an  almost  im- 
mediate reddening  of  the 
skin  takes  place.  In  1926, 
a  five-year  program  to 
eliminate  diphtheria  was 
tried  in  New  York  State 


deaths    per  100,000  cVjildrsn  uncCar  iS 


IcCiphtheria  in. 
Kev>6rkXy| 


Education  of  all  parents  plus  the  findings  of  medical 
science  will  ultimately  stamp  out  diphtheria. 


in  which  several  agencies  co-operated.  In  general,  the  program  con- 
sisted of  Schick  testing  all  young  children,  the  susceptible  children 
being  immediately  treated  with  toxin-antitoxin.  That  this  program 
was  not  completely  successful  was  due  to  the  fact  that  some  people 
avoided  their  responsibility.  It  would  be  possible  to  wipe  out 
diphtheria  by  very  early  treatment  of  all  babies  with  toxoid. 

Another  disease  of  children  which  has  been  responsible  for  a  large 
number  of  deaths  and  much  unnecessary  illness  is  scarlet  fever.  In 
this  disease  a  new  test  devised  by  Dr.  and  Mrs.  Dick  and  known  as  the 
Dick  test  is  used  in  the  same  way  as  the  Schick  test.    A  dilute  toxin 


634  MAN   AS  A   CONQUEROR 

produced  ])y  the  bacteria  which  causes  scarlet  fever  when  injected 
into  the  arm  indicates  susceptibiUty  by  a  sHght  swoUing  and  redness 
of  the  area.  If  the  scarlet  fever  toxin  is  inoculated,  the  body  will 
work  up  an  immunity  against  the  disease.  Another  treatment  con- 
sists in  using  an  antitoxic  serum  which  combats  the  toxins  of  scarlet 
fever  in  the  same  way  as  the  diphtheria  antitoxin  combats  the  similar 
diphtheria  toxm.  Still  another  child's  disease  which  is  now  fought 
by  means  of  passive  immunity  is  measles,  where  a  serum  obtained 
from  convalescent  measles  patients  is  used  as  an  antitoxic  measure. 

Other  antitoxins  are  used  against  tetanus,  a  much  dreaded  infec- 
tion. During  the  World  War  soil-infected  wounds  were  immediately 
treated  with  this  antitoxin  and  with  another  worked  up  against 
gas  gangrene.  In  consequence  the  mortality  from  these  infections 
was  much  reduced.  Antitoxins  are  also  used  against  certain  snake 
venoms,  the  mechanism  of  immunity  being  apparently  the  same  in 
poisoning  from  snake  venom  as  in  toxic  poisoning  from  bacteria  or 
other  organisms. 

Are  Parasitic  Diseases  Conquerable? 

In  answer  to  this  question,  one  has  only  to  look  at  statistics 
showing  the  lengthening  life  span.  Certain  diseases  are  nearly 
conquered.  Smallpox,  diphtheria,  typhoid,  yellow  fever,  and  rabies 
are  all  almost  in  sight  of  the  time  when  they  will  be  under  absolute 
control.  Some  diseases  are  more  difficult  of  conquest  but  are  rapidly 
coming  under  control,  for  example,  children's  diseases  such  as  measles, 
whooping  cough,  and  scarlet  fever,  all  of  which  are  being  attacked 
through  immune  sera  or  vaccines.  The  difficulty  here  is  that  because 
of  the  length  of  the  incubation  period,  children  often  infect  others 
when  their  parents  do  not  actually  know  that  they  have  a  given 
disease.  Malaria,  tuberculosis,  and  hookworm  are  also  rapidly 
coming  under  control,  due  to  the  application  of  recent  discoveries. 
Certain  of  our  parasitic  enemies  still  remain  unconquered.  Pneu- 
monia and  influenza  are  among  the  greatest  causes  of  death  when 
they  go  on  epidemic  rampages.  The  common  cold  still  remains  an 
unconquered  enemy  both  because  of  its  insidiousness  and  because 
people  do  not  consider  it  serious  enough  to  treat  as  a  real  disease. 
Infantile  paralysis,  meningitis,  and  many  tropical  diseases  are  also  as 
yet  uncontrolled.  The  two  venereal  diseases,  gonorrhea  and  syphilis, 
are  much  more  serious  enemies  than  is  realized,  not  only  because 
they  are  difficult  to  control  but  also  because  of  the  intimate  nature 


IMAM'S   Ih.m    FOR   SURVIVAL  (,.33 

of  the  diseases  and  the  social  stigma  connected  witli  Iheni.  Many 
women,  particularly,  suff(n-  for  considerable  periods  of  time  before 
they  understand  the  nature  of  the  affection.  These  social  diseases 
deserve  much  more  serious  consideration  than  is  given  them. 

Undoubtedly  science  will  eventually  be  able  to  con(}uer  all  parasitic 
diseases  theoretically  because  it  is  worth  while  to  do  so,  but  such 
diseases  can  never  be  entirely  eliminated  until  Mr.  Everyman  is 
willing  to  bear  liis  share  of  the  responsibility.  Not  only  must  he  be 
educated  as  to  methods  of  control,  but  he  must  also  be  unselfish 
enough  to  abide  by  ciuarantine  laws  and  regulations,  enforcing  them 
himself,  and  seeing  that  others  also  keep  them.  The  reasons  for 
quarantine  are  obvious  when  one  remembers  that  the  incubation 
period  of  a  disease,  especially  children's  diseases,  is  the  most  effective 
time  for  passing  on  the  disease  to  others.  Children  coming  down  with 
serious  diseases  often  apparently  have  a  slight  cold  in  the  head,  the 
nose  runs,  they  cough,  and  perhaps  have  a  little  fever.  During 
such  a  period  the  germs  can  most  readily  be  passed  to  others,  hence 
the  reason  for  protection  during  this  time  as  well  as  later  on.  With- 
out quarantine  the  control  of  infectious  diseases  is  impossible,  since  a 
leakage  of  disease  germs  through  unwillingness  to  co-operate  with 
authorities  means  disaster  and  epidemic. 

There  must  also  be  a  wider  knowledge  about  diseases  and  control 
measures  on  the  part  of  the  average  voter  and  citizen.  There  is 
need  for  Mr.  Everyman  to  know  how  to  spend  the  money  which  goes 
into  taxes.  Less  than  2^  per  cent  of  the  total  expenditures  of  253 
cities  in  the  United  States  was  used  for  "conservation  of  health"  in 
1921.  The  picture  would  not  be  very  different  today.  In  1923-1924, 
$100,000,000  of  the  Federal  budget  was  appropriated  for  rural  post 
roads  and  $50,000  for  rural  health  work.  A  survey  of  American 
cities  made  in  1923  showed  the  average  distribution  for  health  work 
at  $0.71  per  capita  out  of  a  total  per  capita  expenditure  of  $25.09. 
Figures  today  would  be  slightly  higher,  but  the  proportion  would  not 
differ  greatly.  While  communicable  disease  may  not  be  controlled 
by  departments  of  health  or  even  by  a  well-trained  medical  pro- 
fession, it  can  be  stamped  out  through  the  use  of  these  agencies 
plus  intelligent  action  on  the  part  of  taxpayers  through  individual 
co-operation  and  understanding.  It  should  be  the  place  of  the  col- 
lege trained  men  and  women  who  reacl  these  pages  to  assume  their 
responsibility  in  making  the  world  safer  from  the  attacks  of  com- 
municable disease. 


636  MAN   AS  A   CONQUEROR 

SUGGESTED  READINGS 

Broadhurst,  Jean,  How  We  Resist  Disease,  J.  B.  Lippincott  Co.,  1923. 

The  best  book  of  its  kind,  although  now  not  up  to  date.     Interesting 

and  authentic  as  far  as  it  goes. 
Dublin,  L.  I.,  Health  and  Wealth,  Harper  &  Bros.,  1928. 

A  comprehensive  discussion  of  economic  factors  as  related  to  health. 
De  Kruif,  Paul,  Microbe  Hunters,  Harcourt,  Brace  &  Co.,  1926. 

The  first,  and  still  among  the  best,  of  many  popular  books  on  the  con- 
quering of  parasitic  diseases. 
Downing,  E.  F.,  Science  in  the  Service  of  Health,  Longmans,  Green  &  Co., 

1930. 

An  elementary  but  interesting  account  of  the  conquest  of  parasitic 

disease  by  men  who  gave  their  all  for  science. 
Haggard,  H.  W.,  What  You  Should  Know  about  Health  and  Disease,  Harper 

&  Bros.,  1928. 

General,  but  interesting  and  authentic. 
Moore,  H.  H.,  Public  Health  in  the  United  States,  Harper  &  Bros.,  1923. 

Valuable  for  statistical  information  up  to  date  of  publishing. 
Park,  W.  H.,  and  WiUiams,  A.  W.,  Who's  Who  among  the  Microbes,  Cen- 
tury Co.,  1929. 

Practical  applications  pertaining  to  public  health  and  preventive  medi- 
cine which  have  been  made  from  the  study  of  bacteria. 
Roddis,  L.  H.,  Edward  Jenner  and  the  Discovery  of  Smallpox  Vaccination, 

George  Banta  Publishing  Co.,  1930. 

Interesting  account  of  the  man  and  his  work. 
Smith,  Theobald,  Parasitism  and  Disease,  Princeton  University  Press,  1934. 

One  of  the  latest  and  best  books  on  the  subject  by  one  who  has  done  his 

part  in  conquering  parasites. 
Tobey,  J.  A.,  Riders  of  the  Plagues,  Charles  Scribner's  Sons,  1930. 

An  interesting  history  of  outbreaks  of  parasitic  diseases  from  the  time 

of  the  Crusades  to  the  present. 
Vallery-Radot,  R.,  The  Life  of  Pasteur,  Doubleday,  Doran  &  Co.,  1926. 

(Garden  City  Pub.  Co.,  1926.) 

A  classic  (translated). 


XXVll 


THE  NEXT   MILLION   YEARS 

Preview.  The  period  of  man  •  Human  betterment  ■  Difficulties  in  any 
eugenic  program  •  Biological  background  of  eugenics  •  The  moral  at  the 
end  of  the  tale  •  Suggested  readings. 

PREVIEW 

The  predictions  in  this  chapter  apply  only  to  the  next  million  years. 
Beyond  that  time  we  do  not  venture  to  go,  nor  are  we  here  con- 
cerned with  the  possible  future  events  of  the  next  few  years  which 
may  fall  within  the  span  of  our  own  lifetime,  wherein  we  may  be 
shown  to  be  mistaken  in  our  owlish  prognostications.  Somewhere 
between  the  immediate  unfolding  future  and  a  million  years  hence 
there  lies  an  immense  territory  of  safety  for  the  would-be-wise 
prophet  over  which  the  speculative  imagination  may  freely  roam 
unchallenged. 

In  any  case  much  is  bound  to  happen  in  this  vast  coming  time,  since 
the  laws  of  inevitable  change  are  shown  to  be  continuous  and  un- 
changeable. They  have  been  in  operation  upon  this  planet  for  so 
many  million  years,  and  have  always  resulted  apparently  in  so  con- 
sistent a  swing  of  events,  that  whatever  is  likely  to  occur  in  the  next 
million  years  is  in  a  general  way  reasonably  predictable. 

The  probable  advent  of  mankind  in  the  Pleistocene  period  some 
500,000  years  ago  forms  a  comparatively  recent  episode  biologically 
in  the  grand  drama  of  life,  although  since  Pithecanthro'pus'  day  the 
human  pattern  has  been  repeated  and  modified  by  probably  over 
20,000  successive  generations.  When  we  venture  still  farther  back 
into  the  evolutionary  past  and  remember,  for  instance,  that  our 
remote  amphibian  ancestors  were  able  to  pave  the  way  for  the  develop- 
ment of  an  animal  with  a  human  brain,  what  unthinkable  changes 
may  we  not  expect  to  arise  in  the  next  comparatively  short  million 
years  from  mankind,  with  his  unfathomable  potentialities  as  a  start- 
ing point ! 

The  Period  of  Man 

In  this  changing  world  during  recent  geological  years,  man  has  been 
coming  more  and  more  into  his  own.     Some  of  the  ways  in  which 

637 


638  MAN   AS   A   CONQUEROR 

this  has  taken  ])lac('  aro  sot  forth  in  the  unit  on  "Man's  C^onquost 
of  Nature,"  and  certain  of  the  i)ossil)ilities  of  future  iiuniau  control 
of  the  enviroinncMit  are  j)ointed  out  in  otiier  units. 

There  is  no  doubt  tliat  modern  science  in  the  hands  of  intelligent 
man  has  become  a  magic  key  admitting  him  to  castles  of  mystery  and 
delight,  as  well  as  opening  to  him  storehouses  of  energy  by  means  of 
which  he  will  be  able  still  further  to  control  and  transform  the  world. 

The  invention  of  labor-saving  devices  and  the  dawn  of  the  Machine 
Age  have  liberated  mankind  from  much  of  the  time-consuming 
drudgery  which  forms  an  inevitable  part  of  daily  living,  and  have 
provided  him  with  a  larger  leisure  for  intellectual  adventure  and  a 
more  abundant  life.  It  is  not  enough,  however,  to  secure  leisure. 
The  important  thing  is  what  will  be  done  with  it  when  it  is  gained.  If 
it  simply  turns  out  that  with  increasing  leisure  "Satan  finds  some- 
thing for  idle  hands  to  do,"  then,  in  a  very  literal  sense,  there  will 
be  the  devil  to  pay  in  the  future.  The  most  important  question 
relating  to  the  future  of  mankind  on  the  earth  is  not  what  kind  of 
world  will  our  descendants  find  to  live  in,  but  what  kind  of  individuals 
will  they  he  f 

Human  Betterment 

Biological,  as  contrasted  with  social,  control  of  the  potent  stream 
of  humanity  is  the  field  of  Eugenics.  As  an  organized  science  it  is 
still  in  its  swaddling  clothes,  although  as  an  art  it  has  been  practiced 
more  or  less  blindly  ever  since  there  have  been  animals  that  were 
human.  W.  H.  P.  Faunce  once  said,  "To  neglect  eugenics  today  is 
to  neglect  the  whole  future  of  humanity  and  to  insure  catastrophe." 

One  reason  why  the  fallow  field  of  human  heredity  has  not  attracted 
the  scientific  husbandman  earlier  is  that  its  rewards  are  mostly  pro- 
jected so  far  into  the  future.  Why  labor  to  plant  slow-growing  seed- 
lings of  forest  trees  which  promise  scanty  or  no  returns  imtil  after 
one  is  dead  and  gone,  when  one  can  sow  a  field  of  wheat  with  the 
prospect  of  an  early  harvest  ?  It  is  difficult  to  visualize  and  to  become 
enthusiastic,  or  even  academically  interested,  in  remote  great-great- 
great-great-grandchildren  whom  we  can  never  know,  when  there  is 
so  much  of  immediate  pressing  concern  presented  to  us  by  contempo- 
raries whom  we  can  daily  see  about  us. 

Obviously  there  are  two  outstanding  ways  by  which  to  contribute 
towards  a  better  future  world  for  our  followers  to  live  in  on  this  earth. 
One  way  is  that  of  Euthenics,  that  is,  by  the  modification   and 


THE   NEXT   MILLION    YEARS  639 

amelioration  of  the  environment.  It  involves  the  accumulation  and 
transfer  of  material  things,  such  as  property  and  possessions  of  all 
sorts,  inventions  and  the  triumphs  of  applied  science,  traditions  and 
literatures,  in  short,  everything  that  contributes  to  a  better  stage 
setting.  This  method,  however,  is  uncertain  and  transitory.  The 
frequent  failure  of  legally  drawn  wills,  designed  to  secure  financial 
and  social  security  for  following  generations,  illustrates  how  the  grasp 
of  the  dead  man's  fingers  may  weaken  and  relax.  In  a  larger  way  the 
perspective  of  history  shows  repeatedly  how  different  civilizations  in 
the  past  have  been  replaced  or  dissipated,  and  there  is  reason  to 
believe  that  no  civilization  ])ossesses  the  germs  of  permanence.  The 
flowers  of  the  en\'ironment  fade,  but  meanwhile  the  seeds  of  heredity 
live  on  and  furnish  the  essential  living  source  from  which  renewal  is 
possible. 

The  other  way  of  providing  for  human  betterment  is  by  Eugenics, 
which  has  been  defined  as  "race  betterment  through  good  ancestry." 
It  provides  better  actors  to  utilize  the  stage  setting. 

Whether  we  consciously  direct  the  stream  of  human  germplasm  or 
not,  it  is  bound  in  the  long  run  to  be  the  most  fundamental  and 
important  of  all  the  factors  destined  to  mold  the  world  of  the  future. 
In  proN'iding  for  any  Utopia,  the  program  of  euthenics  is  designed  to 
keep  humanity  out  of  hell,  while  the  purpose  of  eugenics  is  to  keep 
hell  out  of  humanity.     Both  objectives  are  desirable. 

In  cultivating  the  human  garden  it  is  to  be  noted  that  less  advance- 
ment has  been  made  than  in  the  cultivation  of  animals  and  plants,  due 
to  the  peculiar  difficulties  encountered.  William  Penn  is  credited 
with  the  gently  sarcastic  comment,  "Men  are  more  commonly  careful 
of  the  breed  of  their  horses  and  dogs  than  of  their  children." 

Difficulties  in  Any  Eugenic  Program 

The  reason  for  the  obvious  lag  in  the  development  of  eugenics,  or 
human  genetics,  is  to  a  large  extent  due  to  the  peculiar  difficulties 
encountered. 

Owing  to  the  long  lapse  of  time  between  the  generations  of  mankind, 
and  the  comparatively  few  children  produced  in  each  family,  it  is 
not  practical,  even  if  it  were  socially  permissibl{%  to  set  up  experi- 
ments in  human  breeding  in  order  to  establish  or  to  disprove  theories 
of  inheritance.  Life  is  not  long  enough  to  arrive  at  satisfactory 
conclusions  from  controlled  breeding  experimentation  with  man. 
Conseciuently,  the  data  about  the  heredity  of  man  must  come  largely 


640  MAN   AS  A  CONQUEROR 

from  uncontrolled  experiments  in  human  matings  already  performed. 
The  evaluation  of  such  data  can  be  adequately  handled  only  by  means 
of  the  elusive  and  illusive  technique  of  statistical  treatment.  More- 
over, the  collection  of  facts  about  human  beings  is  inevitably  colored 
and  distorted  by  pride  and  prejudice.  Plants  and  animals  do  not 
tell  lies  about  themselves,  but  some  human  beings  do. 

The  fact  that  here  one  more  often  deals  with  complex  traits  rather 
than  more  directly  with  the  elementary  genes,  and  that  the  smoke 
screen  of  training  and  education  plays,  in  man,  a  particularly  con- 
fusing role  by  covering  up  the  contrasting  effects  of  heredity  and 
environment,  makes  the  analysis  of  the  human  hereditary  picture  all 
the  more  difficult. 

While  there  is  no  doubt  that  the  fundamental  laws  of  Mendelism, 
which  go  so  far  to  elucidate  hereditary  procedure  in  plants  and  ani- 
mals generally,  are  equally  applicable  to  mankind,  they  cannot  be 
subjected  to  the  same  demonstrable  proof.  Even  the  most  ardent 
disciple  of  eugenics  would  hesitate  to  propose  the  back-cross  of  a 
man  with  his  recessive  grandmother  in  order  to  determine  his  genetic 
constitution. 

The  fact  that  a  problem  is  difficult,  however,  does  not  mean  that  it 
cannot  be  solved.  The  more  difficult  it  is  the  greater  the  challenge 
presented  and  the  greater  the  final  satisfaction  when  a  successful 
solution  is  eventually  found. 

Biological  Background  of  Eugenics 

In  spite  of  obvious  difficulties,  a  workable  program  of  eugenics  is 
by  no  means  a  hopeless  proposition,  since  biological  science  has 
already  furnished  much  solid  ground  for  eugenics  to  stand  upon.  It 
is  quite  definitely  established,  for  example,  that  biological  inheritance 
in  man,  as  in  other  organisms,  depends  primarily  upon  continuity  of 
the  germplasm  rather  than  upon  somatic  contributions  acquired  during 
the  lifetime  of  the  parents,  and  that  consequently  any  characteristic 
which  an  individual  possesses  arises  not  from,  but  through,  the  bodies  of 
the  parents  from  more  remote  ancestral  sources.  The  parents,  there- 
fore, are  to  be  regarded  not  as  the  source  of  the  child's  heredity,  but 
simply  as  the  trustees  and  guardians  of  the  hereditary  stream  whose 
springs  lie  far  back  in  the  cloud-covered  mountains  of  the  evolutionary 
past. 

Mendel  has  shown  us  how  purity  can  arise  from  impurity,  not  by 
any  miraculous  process  of  the  "forgiveness  of  sins,"  but  by  the 


THE  NEXT    MILLION   YEARS  6li 

segregation  of  genes.  He  also  makes  plain  why  too  close  inbreeding, 
among  those  strains  that  possess  too  many  hidden  "skeletons  in  the 
closet,"  is  hazardous.  Relatives  are  apt  to  carry  the  same  kinds  of 
undesirable  recessive  characteristics,  which  thus  have  a  sporting  Men- 
delian  chance  of  joining  hands  and  becoming  evident  whenever  cousins 
marry.  On  the  other  hand,  recessive  traits,  desirable  or  undesirable, 
may  be  carried  on  for  an  unlimited  number  of  generations  hidden 
under  the  shadow  of  corresponding  dominant  traits  contributed 
through  the  other  parent. 

This  leads  to  the  practical  idea  that  the  way  to  discover  the  genetic 
potentialities  of  any  individual  is  not  simply  to  take  account  of  the 
characteristics  which  the  person  in  question  presents,  but  to  observe 
what  shows  up  among  the  immediate  relatives,  who  are  presumably 
exploiting  the  same  general  mixture  of  germplasm.  The  mother-in- 
law  joke  is  no  joke.  Every  man  in  a  eugenical  sense  marries  all  his 
wife's  relatives. 

Finally,  one  of  the  most  significant  contributions  of  biological 
science  to  eugenics,  which  is  often  not  appreciated  even  by  those 
who  have  heard  about  it,  is  the  well-established  fact  of  the  peculiar 
behavior  of  the  germ  cells  before  their  union  to  form  a  new  individ- 
ual, whereby  half  of  the  hereditary  potentialities  carried  by  each 
parent  is  irretrievably  lost.  The  consequence  is  that  the  amount 
inherited  from  any  'particular  ancestor  is  not  dependent  upon  the 
number  of  generations  that  ancestor  is  removed,  but  upon  the  de- 
vious fortunes  of  chance  assortment  and  elimination  of  the  genes 
during  the  preparation  of  the  eggs  and  the  sperm  for  their  union. 


The  Moral  at  the  End  of  the  Tale 

We  cannot  change  our  individual  biological  inheritance.  That  was 
determined  for  us  once  and  for  all  and  entirely  without  our  connivance 
at  the  time  when  the  egg  from  which  we  developed  was  fertilized. 
The  cards  were  then  dealt  and  all  that  we  can  do  now  is  to  play  out 
the  hand.  Fortunately  this  may  be  done  in  a  variety  of  ways,  a 
fact  that  makes  every  individual  life  worth  living. 

"  I  am  the  legatee  of  fierce  desires, 

A  strange  bequest  of  sundry  hopes  and  fears, 
Loves,  hates,  and  hidden  smouldering  fires. 
Has  come  to  me  unsought  far  down  the  years 


642  MAN   AS  A  CONQUEROR 

From  whose  name  I  bear ;  themselves  the  heirs 
Of  time,  and  race,  through  every  bygone  age 

Of  man.     And  I  am  not  myself,  but  theirs 
Who  so  devised  this  jumbled  heritage. 

"  Yet  I  thank  God,  and  thank  Him  with  a  song, 

That  He  gave  me  a  will  that  is  my  own, 
And  made  me  free  to  choose  the  right  and  wrong, 

And  fight  and  fashion  life  as  I  shall  choose. 
And  with  this  gift  I  sigh  for  no  man's  shoes. 

Nor  envy  any  king  upon  his  throne. 
So  fare  I  forth  intent  at  least  to  be 

Master,  not  slave,  of  my  strange  legacy."  ' 

Finally,  the  possibility  of  eugenic  control,  or  changing  the  hereditary 
stream,  arises  only  when  a  mate  is  chosen  with  whose  germplasm  our 
own  may  be  combined.  This  is  shuffling  the  cards  and  dealing  a  new 
hand.     It  is  the  task  of  eugenics  to  see  that  it  is  intelligently  done. 

"Eugenics  indicates  a  new  method  of  striving  for  human  welfare  which, 
if  combined  with  an  equal  striving  for  improvements  in  human  surroundings, 
more  truly  justifies  a  hopeful  outlook  than  any  other  which  has  yet  been 
tried  in  the  whole  history  of  the  world."  ^ 

The  prospect  for  the  next  million  years  would  be  bright  indeed  if 
everyone  heeded  the  eugenic  golden  rule,  that  is,  Do  unto  your 

DESCENDANTS   AS   YOU    WOULD   HAVE   HAD    YOUR    ANCESTORS   DO    UNTO 
YOU. 

SUGGESTED  READINGS 

Darwin,  L.,  What  Is  Eugenics?  Galton  Pub.  Co.,  1929. 

A  popular  exposition  by  the  Honorary  President  of  the  International 

Federation  of  Eugenic  Organizations. 
Holmes,  S.  J.,  Human  Genetics  and  Its  Social  Import,  McGraw-Hill  Book  Co., 

1936. 

The  newest  of  several  books  this  biologist  has  written  upon  eugenics. 
Huntington,  E.,  Tomorrow's  Children,  John  Wiley  &  Sons,  1935. 

Questions  and  answers  concerning  eugenics. 


'WUliam  Woodford  Rock  in  the  Christian  Century,  May  7,  1925.  By  permission  of  the  pub- 
lishers. 

2  From  Leonard  Darwin,  Eugenic  Reform.  By  permission  of  D.  Appleton  &  Company,  pub- 
lishers. 


THE   NEXT   MILLION  YEARS  613 

PoponuG,   P.,  atid   Joliiison,  1{.   H..  Applied   Eugenics,  The  M.'U'iiiillati  Co., 

i9;«. 

I 'articular  atUiiitidii  to  social  aspects. 
Wliite,  F.  W.,  Poskrity,  Galtoii  l\il).  Co.,  19.'JU. 

A  small  book  of  sane  propaganda  from  England. 
Wiggam,  A.  E.,  The  Next  Age  of  Man,  Blue  Ribbon  Books,  1931. 

Easy  and  worth  while  to  read. 
Eugenics  Record  Office,  Cold  Spring  Har])or,  Long  Island,  X.  Y. 

Headquarters  for  information  of  all  sorts  about  eugenics. 


INDEX 


Italicized  numbers  indicate  illustrations. 


Aardvark,  508 

Abiogenesis,  408 

Abomasum,  288 

Absorption,  by  root  hairs,  244-245 

Absorption  spectra  of  green  plants,  257 

Acanthocephala,  87 

Achatinella,  523 

Acid-base  balance,  310 

Adaptability,  to  new  conditions,  40 

Adaptations,  486-493 

biological,  492-493 

embryological,  487-488 

for  food  getting  in  animals,  51-52 

for  food  getting  in  birds,  52 

for  life  in  air,  23-24 

for  life  in  water,  21-22 

for  life  on  land,  24-25 

for  protection  of  young,  47 

for  seed  dispersal,  272 

genetical,  489-490 

in  desert  plants,  36,  37 

in  worker  bees,  207-208 

organic,  486 

physical,  490-492 

physiological,  488-489 

psychological,  489 

structural,  486-^57 
Adaptiveness,  definition  of,  127 
Addison,  391 
Addison's  disease,  393 
Adenoids,  287 
Adjustor  neuron,  341 
Adrenals,  392-394 
Adrenin,  393,  394 
Adrenotropic  hormone,  401 
Aedes  mosquito,  618,  619 
Aepyornithiformes,  113 
Aerating  devices,  in  plant,  17 
Aestivation,  10 
Afferent  fibers,  in  bee,  211 

in  earthworm,  194,  195 
Agassiz,  502 
Ages,  cultural,  547 
Agglutination  test  for  typhoid,  627 
Agglutinins,  626 
Agnatha,  105 
Agouti,  508 
Agriculture,  550 
Ainu  aborigines,  561 
Air  sacs,  315 

function  of,  210 

in  insects,  210 


Alare,  563 
Albatross,  113,  523 
Algae,  69,  71,  169-172 
Alimentary  canal,  282 
Allantois,  426,  427,  487,  501 
Alligators,  103,  HI 
Alpine  races,  561 
Alternation  of  generations,  414 

angiosperms,  176-178 

hydroids,  186 

plants,  174-176,  175 
Alveolus,  317 
Ambulacral  fluid,  335 
Ambulacral  grooves,  336 
Ameba,  77,  103,  328,  342 

food  of,  155 

mitotic  division  in,  155 

structure  of,  154-156 
Ameba  proteus,  154 
Amebic  dysentery,  620 
American  Ornithologists'  Union,  600 
American  passenger  pigeon,  599 
Amino  acids,  definition  of,  131 
Ammonites,  513 
Amnion,  424,  425,  4^7,  487,  497 
Amniotic  fluid,  426 
Amphibia,    103,    109,   497,    501,   513, 

533 
Amphimixis,  161,  162,  163 
Amphineura,  97 
Amphioxus,  105,  329 
Amphioxus  ova,  418 
Ampulla,  echinoderms,  336 

mammalian  ear,  361 
Amylopsin,  295 
Anabolism,  126 
Analogy,  67 

Anal  spot  of  Paramecium,  159 
Anaphase,  142,  143 
Anatomy,  comparative,  496-497 

human,  496 
Ancyrocephalus,  223 
Angiospermae,  75 
Angiosperms,  alternation  of  generation 

in,  176-178 
Animal  breeding,  methods  of,  571-573 
Animal  cell,  typical,  130 
Animals 

as  food,  575 

economic  value  of,  576-578 

harm  done  by,  582-586 

in  fresh  water,  153-154 


646 


INDEX 


Annelida,  93,  103,  344 
Annual  rings,  246,  2Jilf 
Annuals,  240 
Anodonta,  97 
Anopheles  mosquito,  618 
Anoplura,  101 
Anseriformes,  114 
Ant-eater,  487 
Antelopes,  509 
Antennae  cleaner,  207 
Antennae,  insects,  199,  202,  205 
Anterior  limb,  332 
Anterior  peduncles,  351 
Antheridia,  moss,  175 

Oedogonium,  172 
Anthozoa,  81 
Anthrax,  622 

Anthropological  measurements,  563 
Anthropology,  531 
Anthropometry,  563 
Antibodies,  218,  303,  311,  514 
Anticoagulin,  217 
Antienzyme,  217 
Antigen,  632 

Antitoxin,  diphtheria,  632 
Antitoxins,  489 
Ants,  549 
Anura,  109 
Aorta,  309 

Aortic  arches,  308,  309 
Aortic  plexuses,  354 
Ape-man,  565 

Apes,    115,    233,    514,    535,    5S8,   539, 
544,  551 

experiments  with,  386-387 
Aphids,  413 

Apical  nervous  system,  343 
Apoda,  109 
Appendages,  329 
Appendix,  292 
Apterygiformes,  113 
Aqueduct,  350 
Aqueous  humor,  360 
Arabs,  561 
Arachnoidea,  102 
Arachnoid  membrane,  345 
Arbacia,  95 
Arbor  vitae,  350 
Arcella,  77 
Arcellidae,  328 
Archaeopteryx,  113,  506,  507 
Archaeornithes,  113 
Archegonia,  moss,  175,  176 
Archenteron,  4^8 
Archeozoic  era,  475,  476,  513 
Archiannelida,  93 
Arctic  tern,  migi'atioii  of,  38 
Aristotle,  437 
Armadillo,  115,  508 


Armbruster,  L.,  208 
Armenians,  561 
Armenoid  races,  561 
Armor,  protective,  493 
Arms,  evolution  of,  544 
Arteries,  304,  305 

Ai-thropoda,  67,  68,  99,  103,  328,  334, 
336,  344 

structure  of,  199 
Arthropods,  234 

parasitic,  222 
Artifacts,  479,  554 
Artificial  light,  relation  to  food-making 

in  green  plants,  259-260 
Artiodactyla,  117 
Ascaris,  423 

infection  by,  617 
Ascaris  lumbricoides,  225 
Ascaris  megalocephala  bivalvens,  428 
Ascidians,  105 
Ascomycetes,  71,  73 
Asexual  development,  in  Hydi'a,  184 
Asexual  propagation,  455 
Asexual   reproduction,   types  of,  410- 

411 
Ash,  plant  residue,  242,  254 
Associations 

plant  and  animal,  5-20 

maple-birch,  28 
Asteroidea,  95 
Asthma,  394 
Astrangia,  81 
Astronomy,  473 
Atwater,  276,  312 
Auchter,  E.  C,  249 
Auditory  nerve,  352,  361 
Augustine,  Saint,  494 
Aurelia,  81 
Auricle,  308 
Aurignacians,  557,  560 
Autonomic  nervous  system,  354,  355 
Autosomes,  469 
Autotomy,  410 
Aves,  103,  113 

Axial  filament,  sperm,  415,  416 
Axial  gradient  theory,  365,  381,  431 
Axon,  340,  359 
Azeel  fowl,  515 
Aztecs,  562 

Baboon,  508,  537 
Bacillus,  71,  165 
Backbone,  curvature  of,  54-2 
Bacteria,  69,  71,  216 

aerobic,  59 

anaerobic,  59 

ecological  adaptations  in,  490 

forms  of,  165 

how  they  enter  the  body,  622 


INDEX 


617 


lifo  habits  of,  59 

relation  to  diseaso,  021 

relation  to  free  nitrogen,  oO-CiO 

spores  of,  16(5 

structure  of,  165-166 
Bacterial  infections  of  plants,  579 
Bacterins  and  their  use,  630 
Bacteriophage,  627 
Bacterium,  71 
Baird,  7 

Balancing  organ,  crustacean,  372 
Balanoglossus,  105 
Balanus,  500 
Balkans,  561 
Banting,  296,  397 
Bark,  247,  248 
Barnacles,  99,  500 
Barrell,  estimate  of  geological  time  by, 

553,  554 
Barriers 

biological,  32,  533 

chemico-physical,  30 

geographical,  31,  523 

mountain,  30 
Basic  environments,  7 
Basidiomycetes,  73 
Bateson,    experiments    of    with    sweet 

peas,  451 
Bats,  115,  534 

specialization  of  wing,  544 
Bauhin,  Kasper,  64 
Bayliss,  392 

"Beagle,"  voyage  of,  508,  518 
Bean,  Professor 

estimate  of  age  of  flints  by,  547 

estimate  of  geological  time  bv,  553 
Bears,  385,  509 
Bee 

head  of  worker,  £04 

mcjuth  parts,  203-;^0.5 
Beebe,  William,  22,  27 
Beetles,  101 

Bergson,  theory  of  evolution  of,  527 
Behavior,  definition  of,  366 
Behavior  patterns,  381-382 
Behaviors,  as  adaptive  responses,  380- 

381 
Belly,  of  muscle,  338 
Benedict,  312 
Berbers,  561 
Bering  Sea,  511 
Bering  Strait,  509 
Bernard,  Claude,  391 
Berthold.  A.  A.,  391 
Best,  296,  397 
Biceps,  338 
Biennials,  240 

Bilateral  symmetry,  evolution  of,  532 
Bile,  296 

H.  w,  H.— 42 


Binary  fission,  41 1 

Anieba,  155 

Paramecium,  160 
Binomial  nomenclature,  63,  64,  65 
liiogenetic  Law,  501 
Biological  surveys,  603 
Biometrv,  440 
Birds,  103,  113 

economic  value  of,  577-578 

embryological  adaF)tati(>ns  in,  487 

evolution  in,  532 

hearts  of,  497 

in  mesozoic  era,  452 

in  recapitulation  theory,  501 

insect  eating,  578 

physical  adaptations  in,  491 

relation  of  blood  to  other  animals, 
514 

rise  of  in  evolution,  513 

seed-eating,  578 

sex  inheritance  in,  468 

species  of  lice  on,  524 
Bison 

distribution  of,  509 

La  Brea  fossils  of,  477 
Blackberry,  white,  455 
Black  stem  grain  rust,  57 
Bladderwort,  leaf  of,  54 
Blastocoel,  418 
Blastoderm,  417 
Blastomeres,  432 
Blastopore,  4^0 
Blastostvle,  Obelia,  186 
Blastula,  418 

earthworm,  197 
Blastulation,  418 
Blind  spot,  359 
Blood 

earthworm,  191-192 

insects,  210 

transportation  of  in  vertebrates,  233 
Blood  platelets,  302-303 
Blood  sinuses,  bee,  209 
Blood  sugar  raising  principle,  401 
Blood  vessels,  earthworm,  190-191 
Body  cavity,  426 
Body  parts,  insect,  200 
Body  surface,  respiration,  314 
Body  wall,  insect,  2^2-203 
Bolus,  290 

Bombinator  (toad),  biological  adapta- 
tions in,  492-493 
Bones,  ankle,  332 

cartilaginous,  329 

cranial,  330 

facial,  330 

membrane,  329 

metacarpal,  332 

metatarsal,  3S2 


648 


INDEX 


Bones  (cont.) 

occipital,  329,  330 

skull,  329,  330 

wrist,  332 
Bony  fishes,  107 
Boophilus  annulatus,  216 
Bose,  J.  C,  371 
Botany,  systematic,  64 
Botflies,  216 
Boule,  estimate  of  geological  time  by, 

553 
Bowman's  capsule,  323,  324 
Boyle,  Robert,  528 
Brachiopoda,  91,  328 
Brachiopods,  485 

rise  of,  513 
Brain,  329,  344 

anatomy  and  development  of,  345- 
352 

human,  544,  545,  551 

parts  of  the  vertebrate,  34? 
Branchiostegite,  315 
Breastbone,  331 
Breeding,  practical,  452-454 
Bretz,  estimate  of  geological  time  by, 

553 
Brittle-stars,  328 

Broad  tapeworm,  distribution  of,  614 
Bronchioles,  317 
Bronchus,  317 
Bronze  age,  547 
Broomrape,  216 
Brown-Sequard,  391 
Brues,   work  of,  on   physical  adapta- 
tions, 490-491 
Bryophyta,  73 
Bryozoa,  91 
Bryozoans,  491 
Bubonic  plague,  222,  620-621 
Buccopharyngeal  respiration,  314 
Budding,  410 
Bugula,  91 
Bullfrog,  490 

Bulls,  pedigree  breeding  with,  453 
Bumpus,  report  of  on  survival  in  spar- 
rows, 520 
Burbank,  plant  breeding  of,  455 
Busycon,  97 
Buttel-Reepen,  estimate  of  geological 

time  by,  553 
Butterflies,  sex  heredity  in,  468 

Cacti,  biological  adaptations  in,  493 

Cactus,  spineless,  455 

Caeca,  283 

Caecilia,  109 

Caecilians,  499 

Calcaneus,  543 

Calcarea,  79 


Calciferous  glands,  earthworm,  189 

Calcium,  277 

Calorimeter,  bomb,  276 

Calyces,  323 

Cambium,  245,  246,  248 

Cambrian  period,  476,  513 

Camel,  evolution  of,  509 

Camera  eye,  358 

Camouflage,   a   biological  adaptation, 

492 
Cancer,  612 
Canines,  286 
Cannibalism,  animal,  49 
Capillaries,  304,  305,  309 

lymph,  304 
Capillary  tube,  268 
Capsule,  moss,  176 
Capuchins,  536 
Carbohydrates,  276,  277,  298 
Carbon  dioxide,  318 

use  of  in  starch-making,  254 
Carboniferous,  476,  513 
Cardiac  plexuses,  354 
Cardiac  portion  of  stomach,  288 
Caribou,  distribution  of,  509 
Carnegie  Trust,  516 
Carnivora,  115 

Carnivores,  and  struggle  for  existence, 
520 

blood  relationships  in,  514 

structure  adaptations  in,  487 

teeth  of,  52 
Carnivorous  plants,  53-54 
Carotin,  258 
Carpal  bones,  332 
Carrel,  Alexis,  610 
Carriers 

disease,  630-631 

mechanical,  221 
Carrion  beetle,  490 
Carrot,  239 
Cartier,  Jacques,  570 
Carteria,  69 
Cartilage,  329 
Cassowaries,  113 
Castings,  earthworm,  190 
Castle 

definition  of  gene  by,  457 

experiments  of  with  germplasm,  438 
Casts,  fossil,  478 
Casuariiformes,  113 
Cat,  495,  499 
Catalyst,  280 
Catherinia,  73 
Cattle,  511,  515 

superior  strains  of,  573 
Cave-bears,  550,  557 
Cave-dwellers,  492,  500,  552,  557 
Cave-hyena,  550,  557 


INDEX 


649 


Cavity,  buccal,  284 
Cell  division 

mitotic,  141-144 

theories  concerning,  140 
Cell  of  mesophvll  of  leaf,  129 
Cell  theory,  128,  138,  139 
Cell  wall  and  protoolasm,  134 
Cells 

comparative  size  of,  IJfO 

tissue,  144-148 
Cement,  287 

Cenozoic  era,  476,  512,  513,  553,  554 
Centipedes,  532 
Central  nervous  system,  337,  420 

early  development  of,  345,  346 
Centrioles,  416 
Centrosome,  142 
Centrosphere,  142 
Cephalochordata,  105 
Cephalopoda,  97 
Cercariae,  219,  231 
Cerebellum,  346,  350,  353 
Cerebral  hemispheres,  346 
Cerebrospinal  fluid,  345 
Cerebrum,  346,  347,  353 

human,  545 
Cestoda,  85 
Cestus,  83 
Cetacea,  117 
Chaetognatha,  93 
Chaetonotus,  89 
Chaetopoda,  93 
Chalina,  79 
Chamberlin,  407 
Change,  universality  of,  483-485 
Chara,  71 

Charadriiformes,  114 
Charophyceae,  71 
Chelonia,  111 

Chemical  co-ordination,  391-392 
Chemical  relationships  of  plants  and 

animals,  57-59 
Chemistry,     food     making     in     green 

plants,  261-262 
Chestnut  weevil,  487 
Chick,  500 
Child,  365,  381,  431 
Chimpanzee,  386,  537,  538 
Chin,  development  of,  551-552 
Chiroptera,  115 
Chitons,  97 
Chlorine,  277 

Chlorogogen  cells,  earthworm,  189 
Chlorophyceae,  69 
Chlorophyll 

action  of  in  starch  making,  258,  262 

and  light,  256-258 

chemical  composition  of,  258,  262 

definition  of,  13 


Chloroplasts 

definition  of,  129 

effect  of  light  on,  260 

in  green  leaf,  252,  260 

part  played  bv  in  heredity,  465 

structure  of,  257-258 
Choanoflagellate  (collared)  cells,  281 
Chordae  tendineae,  307,  308 
Chordata,  103,  105 
Chordates,  328 
Chorion,  424,  425 
Chorionic  cavity,  426 
Choroid,  359 

Chorology,  definition  of,  26 
Chromaffin  cells,  392 
Chromosome  map,  461,  462 
Chromosomes,  456-457 

numbers  of,  142 
Chronological  age,  388 
Chyme,  294 
Ciconiiformes,  113 
Cilia,  21,  334 
Circulation,  300-310 

bee,  209-210 

earthworm,  190 
Circulatory  system,  closed,  302 
Circumesophageal  connectives,  344 

bee,  211 

earthworm,  194 
Circumesophageal  loop,  344 
Cirri,  335 

Civilian  Conservation  Corps,  594,  595 
Cladocera,  413 
Clams,  97,  103,  235,  328,  333 
Classification,  64-117 

animal,  77-117 

evolutionary  evidence  from,  504 

plants,  69-77 
Climax  formation,  definition  of,  32 
Clinostomum  marginatum,  life  cycle  of, 

230,  232 
Clitellum,  earthworm,  188 
Closed  fibrovascular  bundle,  250 
Closterium,  structure  of,  164 
Clotting,  311 
Club  mosses,  75 
Cnidoblasts,  Hydra,  180 
Coal,  479 
Coccus,  165 

Coccyx,  evolution  of,  543 
Cochlea,  362 

Cockroach,  mouth  parts  of,  487 
Cocoa  production,  annual,  574 
Codfish,  329 

Coelenterata,   81,   103,   180,  281,   301, 
320,  343,  499,  531 

colonial,  334 
Coelom,  earthworm,  188 
Coelomic  cavity,  426 


650 


INDEX 


Coffee  production,  annual,  574 

Cohesion,  in  rise  of  water,  269 

Coincidence  in  chromosomes,  457 

Cold  receptors,  363 

Coleoptera,  101 

Collembola,  99 

CoUip,  396 

Colloid,  definition  of,  132 

Colon,  m2 

Colonial  theory,  139 

Colostrum,  625 

Colymbiformes,  113 

Comatrichia,  71 

Commensalism,  56,  492 

Communicable  diseases,  spread  of,  612 

Comparative  anatomy,  330 

evolutionary  evidence  from,  496 
Competition,  relations  of,  49-50 
Complex  reflexes,  342 
Compound  eyes,  358 
Compound  reflex  arcs,  341 
Condiments,  277 
Conditioned  behavior,  379-380 
Conditioned  reflex,  law  of,  380 
Coneys,  117 
Conjugation,  411 

Connecting  links  in  evolution,  506 
Connecting  neurons,  376 
Connective  tissue,  338 
Connective  tissue  membrane,  329 
Consciousness,  in  animals,  383 
Conservation 

lack  of  unified  program  in,  605-607 

methods  of,  601-602 

of  wild  life,  600-601 

organizations  for,  602-603 
Contractile  vacuoles,  320 
Co-ordination  by  a  dorsal  tubular  nerv- 
ous system,  344-345 
Co-ordination  by  a  linear  nervous  sys- 
tem, 344 
Co-ordination  by  a  nerve  ring,  343- 

344 
Co-ordination  by  a  network,  343 
Copper  age,  547 
Coprolites,  479 
Copulation,  earthworm,  196 
Coracidia,  230 
Coraciiformes,  114 
Corallopsis,  71 
Corals,  81,  103,  328 

rise  of,  513 

skeletons  of,  478 
Cork  cambium,  247 
Corn,  linkage  and  crossing  over  in,  460 
Corn  stem,  cross-section  of,  ^49 
Cornea,  359 
Corpora  bigemina,  350 
Corpora  quadrigemina,  350 


Corpus  callosum,  348 

Corpus  luteum,  399 

Correns,   experiments  of  in  heredity, 

443,  446,  467 
Corrodentia,  100 
Cortex,  adrenal,  392 
Cortex,  cerebral,  348,  545 
Cortex,  kidney,  323 
Corti,  organ  of,  370 
Cortin,  392,  393 
Costal  plates,  328 
Cotton  boll-weevil,  585-586 
Cousin  marriage,  454 
Cows,  breeding  of,  453 
Coxa,  206,  207 
Crab,  horseshoe,  499 
Crabs,  99,  103,  531 
Cranial  nerves,  351,  352 

oculomotor  (III),  350 

trochlear  (IV),  350 
Craniota,  105,  507 
Cranium,  329,  330 
Crayfish,  333 

Creation,  miraculous,  493-495 
Creator,  527 

Cretaceous  period,  476,  513 
Cretans,  561 
Crinoidea,  95 
Crocodiles,  533 

heart  of,  497 
Crocodilia,  111 
Cromagnons,  557,  559,  560 
Crop,  288 

earthworm,  189 

insect,  208 
Crops,  rotation  of,  60 
Cross-fertilization,  412 
Crossing-over,  4:58-460 
Crow,  552 
Crown,  tooth,  286 
Crura  cerebri,  350 
Crustacea,  99,  103,  302 
Crustaceans,  234,  499,  521,  532 
Cryptobranchus,  314 
Crypturiformes,  113 
Crystalloid,  definition  of,  132 
Ctenophora,  83 
Cucuhformes,  114 
Cutaneous  sense  organs,  362-363 
Cutin,  264 
Cuttlefishes,  333 
Cuvier,  516,  517 
Cyanophyceae,  69 
Cycles,  carbon  and  oxygen,  S8 
Cyclochaeta,  221 
Cyclostomata,  105 
Cynipoidea,  218 
Cypress  (bald),  17 
Cysticercus,  226 


INDEX 


651 


Cysts,  422 

Cytology,  definition  of,  138 
Cytolosis,  136 
Cytoplasm,  416 

definition  of,  129 

role  of  in  heredity,  463-464 
Czechs,  561 

Dandelion,  505 

seed  dispersion  in,  4^9 
Daphnia,  413,  500 

Darwin,  Charles,  443,  488,  493,  495, 
508,  513,  516,  518,  519,  522,  523, 
524,  526,  527,  528,  530,  540,  555, 
556 
Darwin,  Erasmus,  494 
Darwin,  Leonard,  494,  642 
Datura,  457 
da  Vinci,  Leonardo,  305 
Death,  biological  significance  of,  610- 

611 
Deer,  335 

Deficiency,  in  chromosomes,  457 
Deletion,  in  chromosomes,  457 
Desmospongia,  79 
Dendrites,  340 
Dengue,  620 
Dentalium,  97,  432 
Denticula,  71 
Dentine,  286 

Dermo-muscular  sacs,  335 
Dermoptera,  101,  115 
de  Saussure,  241 
Desmids,  164-165 
Determinate  cleavage,  432 
Determiners,  450 
Development,  415 
Devonian  period,  476,  513 
DeVries,  443,  524,  525 
Dextrin,  295 
Diabetes,  296 
Diaphragm,  318 
Diatoma,  71 
Diatomaceae,  71 
Diatoms,  21,  163-164: 

shells  of,  163 

uses  of,  163 
Dicotyledons,  64,  75,  77 

definition  of,  240 
Dick  test,  633-634 
Diencephalon,  346,  348,  397,  400 
Differentiation  of  the  embryo,  420-421 
Diffusion,  133-lSA 
Digestion 

Ameba,  154-155 

extracellular,  281-284 

in  higher  animals,  282 

Hydra,  182 

insects,  209 


intracellular,  281 

in  lower  animals,  281-282 

Paramecium,  159 
Digestive  glands,  282 
Digestive    systems    (intake    devices), 

275 
Digestive  tract 

earthworm,  189 

insect,  209 
Digitigrade  feet,  333 
Dihybrids,  447,  448 
Dinornithiformes,  113 
Dinosaura,  111 

Dinosaurs,  236,  479,  481,  513,  542 
Dioecious  plants,  175 
Diphyllobothrium  latum,  229 
Diploblastic  organisms,  68,  419 
Diplococcus,  71 
Diploid  number,  429 
Dipnoi,  107 
Diptera,  102 
Discomedusae,  320 
Discontinuous  variations,  572 
Diseases 

causes  of,  611 

death  rates  from,  613 

degenerative,  612-613 

relations  of  environment  to,  611-612 
Display  of  energy,  animals,  366 
Distribution,     evolutionary     evidence 

from,  508-510 
Divergence  theory,  503 
Division  of  labor 

Coelenterata,  179 

Hymenoptera,  214 
Divisions  of  plants,  65 
Division  series,  415 
Diverticula,  digestive,  282 
Dodder,  216 
Dogs,  329,  515,  552,  639 
Dolphins,  117 
Domestic  animals,  576 
Domestication,  550 

of  plants  and  animals,  569-571 
Dominance,  447-449 
Dorsal  cerebral  ganglion,  bee,  211 
Dorsal  pores,  earthworm,  188 
Double  fertilization,  177-178 
Dragonflies,  100 
Drone,  201,  212 

Droplet  method  of  infection,  622 
Drosophila,  457,  458,  461,  468,  525 
Dublin,  Louis,  612 
Ductless  glands,  391-402 
Duodenum,  291,  392 
Duplication,  in  chromosomes,  457 
Dura  mater,  345 
Dustbowl,  592 
Dutch  elm  disease,  580, 581 


652 


INDEX 


Ear,  361-S62 
Ear  drum,  362 
Early  cleavage,  417,  418 
Earthworm,  67,  93,  103,  187-197,  188, 
334,  335,  409,  507 

circulatory  system  of,  190,  191 

cross  section  of,  193 

development  of,  197 

digestive  tract  of,  189 

"hearts"  of,  190,  191 

hermaphroditic,  195 

nephridium  of,  192 

nervous  system  of,  194,  195 

reactions  to  stimuli,  194 

regeneration  in,  198 
East,  Dr.  E.  M.,  465 
Echinodermata,  95,  103,  335,  343 
Echinoidea,  95 

Ecological  balance,  man's  effect  on,  577 
Ecology 

definition  of,  1,  26 

how  to  study,  4,  5 

typical  region,  2-4 
Economic  value  of  plants  and  animals, 

573-579 
Ectoderm,  419 

Hydra,  180-/Si 
Ectodermal  cells,  343 
Ectodermal  derivatives,  422 
Ectoparasites,  219 
Ectoplasm 

Ameba,  154 

Paramecium,  158,  159 
Eddy,  278 
Edentata,  115 
Education,  437 
Eels,  499 

Effector,  definition  of,  375 
Effector  cell,  341 
Effector  neurons,  376 
Efferent  fibers 

bee,  211 

earthworm,  195 
Egg  shells,  422-423 
Eggs,  490,  503 

care  of,  by  parents,  46 

theories  regarding,  in  heredity,  466 
Egyptians,  561 
Eijkman,  278 
Einstein,  520 
Elasmobranchii,  107 
Elbow,  332 

Elements,  in  tissues,  277 
Elephant,  117,  545 
Elephant-fishes,  107 
Elephants  and  ancestors,  range  of,  SI 
Elephas,  556 

Elimination  of  the  unfit,  521 
Elk,  527 


Elliot,  Dr.  D.  G.,  535 

Elton,  215 

Embiidina,  100 

Embryonic  membranes,  4^4-4^6,  4^7 

Embryology,  500 

Embryo  sac,  177 

Emotional  responses,  383-384 

Emus,  113 

Enamel,  286 

Endameba  histolytica,  225,  422 

Endoderm,  281,  419 

formation  of,  in  earthworm,  197 

Hydra,  181,  182 
Endodermal  derivatives,  422 
Endomixis,  161,  417 
Endoplasm 

Ameba,  154 

Paramecium,  158,  159 
End  organs,  340 
Endoskeleton,  235,  328,  329,  334,  336, 

532 
Endothelial  muscle  cells  in  Hydra,  181 
Endothelium,  304 
Endotoxins,  621 
Energy 

non-producers  of,  276 

producers  of,  276 

release  in  plants,  237 
Engelmann,  experiment  of,  with  oxygen 

release  in  algae,  258 
English,  561,  562 
Enterokinase,  297 
Environment,  431 

basic,  7 

factors  of,  7-20 

effect  of,  in  heredity,  435,  437,  517, 
522 

effect  on  diseases,  611-612 

man's  effect  on,  568-569 
Enzymes,  279,  280,  396 

inverting,  297 

production  of,  127-128 

reversible,  280 

types  of,  in  plants,  263-264 
Eoanthropus,  556,  560 
Eocene  period,  476,  510,  511,  513 
Eohippus,  510,  511 

Eolithic  division  of  Stone  age,  547,  560 
Ephemerida,  100 
Ephippium,  413 
Epicanthic  fold,  563 
Epidella  melleni,  223 
Epigenesis,  431 
Epiglottis,  290 
Epimere,  425 
Epinephrine,  393 
Epipharynx,  bee,  204 
Epiphytes,  23,  24 
Epithelial  cells,  291 


INDEX 


6.-,:{ 


Epithelio-muscular  cells  in  Hydra,  180 

Epitrichian,  488 

Equilibration,  361 

Equisetum,  75 

Equus,  510,  511 

Eras  of  time,  475 

Erect  mosaic,  358 

Erepsin,  295,  297 

Ergosterol,  279 

Erosion 

areas  of  in  the  United  States,  591 
damage  from,  590-592 

Erythrocytes,  303 

Eskimo,  561,  562 

Esophagus,  287,  288 
earthworm,  189 

Ethnology,  559 

Eucalyptus,  239 

Eugenics,  638,  639,  642 
Euglena,  77,  103,  281 
eyespot  of,  375 
nutrition  in,  157 
reproduction  in,  157 
structure  of,  156 
Euplectella,  79 
Euplotes,  335,  342,  343,  375 
Eurion,  563 

European  corn  borer,  range  of,  584,  585 
Euspongia,  79 
Eustachian  tubes,  287 
Euthenics,  638-639 
Eutheria,  115 
Evaporation,  effect  of,  on  rise  of  water 

in  plants,  269 
Evening  primroses,  mutations  in,  525 
Evidence,  scientific,  of  evolution,  495 
Evolution,  493-529 
Excretion,  319 
Exoascus,  73 
Exophthalmic  goiter,  395 
Exoskeletons,  235,  328,  334,  336,  532 
Exotoxins,  621 
Expiration,  318 
Extensors,  327 
External  nares,  284 
External  respiration,  312,  313,  316 
Extra-embryonic  coelom,  426 
Eye,  physical  adaptations  in,  488,  492 
Eye  brush,  bee,  207 

Facets,  358 

Factor  hypothesis,  450 

Factors,  hereditary,  450,  458 

complimentary,  452 

inhibiting,  452 

sex-linked,  452 

supplementary,  452 
Factors  of  the  environment 

biotic,  19-^0 


chemical,  /5-16 

gravity,  16 

light,  11-\A 

molar  agencies,  18-/9 

substratum,  17-18 

temperature,  9-11 

water,  7-9,  8 
Falconiformes,  114 
Fangs,  493 
Fat   metabolism,    regulating   principle 

of,  402 
Fat  synthesis  in  plants,  262 
Fats,  276,  282 
Fatigue,  muscular,  339 
Faunce,  W.  H.  P.,  638 
Federal  agencies,  603-604 
Federal  bird  and  game  refuges,  601 
Feet,  adaptations  of,  333 
Female  pronucleus,  417 
Femur,  332 

insect,  206,  207 
Ferns,  75,  498 
Fertilization,  414,  416,  456 

definition  of,  62 

earthworm,  196-197 

moss,  176 

results  of,  417 
Fibrin,  311 
Fibrinogen,  311 
Fibula,  332 
Filicineae,  75 
Fins,  498 
Fire,  use  of,  550 
First  polar  bodv,  430 
Fish,  499,  501,  532,  547 

armored,  334 

heart  of,  497 

physical  adaptations  in,  491 

rise  of,  513 
Fisheries,  595-596 

Bureau  of,  604-605 
Fission,  411 
Flagella,  334 

bacteria,  165 
Flame  cells,  320 

Flatworms,  85,  103,  344,  485,  532 
Flemings,  562 
Fleas,  216 
Flexors,  327 
Flexures,  347 
Flies,  102 
Flints,  547,  548 
Florissant  shales,  511 
Flower,  function  of,  272 
Flowering  plants,  rise  of,  513 
Fluctuating  variations,  57^* 
Flukes,  85,  103,  219 
Fluorine,  277 
Follicular  cells,  399 


^4 


INDEX 


Food,  275,  297 

of  fishes,  577 
I'\)od  chains,  51 
Food  gettiiiK  in  plants,  53-54 
Food  making  in  green  plants 

factors  in,  253-254 

summary  of,  270 
Food  tube,  worker  bee,  209 
Foot,  evolution  of,  5^3 
Forage  plants,  515 
Foramen  magnum,  352 
Foraminifera,  328,  478 
Forbes,  Prof.,  577 
Fore-brain,  346 
Forest  fires,  592 

relation  to  floods,  590 
Forest  products,  574,  593-594 
Forest  service,  work  of,  593-595 
Forest  waste,  592-593 
Forests 

enemies  o*",  593 

usefulness  of,  589-590 
Fossils,  477-480,  510,  533,  554 
Four-o'-clock,  446 
Fovea  centralis,  360 
Fowl,  514 
Fragillaria,  71 
Fragmentation,  411 
Fraunhofer's  lines,  256,  257 
Free-martin,  398,  471 
French,  561 
Fresh-water    mussel,    life    history    of, 

598 
Frogs,  103,  109,  336,  491,  540,  545 
Fruit  fly,  525 
Fucus,  71 
Ftnmria 

gametophyte  of,  175 

life  cycle  of,  175 

sporophyte  of.  174 
Fundus,  288 
Fungi,  71,  228 

harm  done  by,  579-580 

method  of  nutrition  in,  174 
Funiculus,  ventral,  lateral,  and  dorsal, 
353 

Gaertner,  442 
Galapagos  Islands,  508 
Galen,  305 
Galileo,  495 
Gall,  218 
Galliformes,  114 
Gall  insects,  218 
Galtsoff,  35 
Gametes,  411,  446,  460 

Ulothrix,  171 
Gametocytes,  228 
Gametogenesis,  428 


Gametophytes,  498 

development  of  in  flowering  plants, 
/  / 
Gametophyte  generation,  evolution  of, 

178 
Ganoidei,  107 

Garden  of  Eden,  biological,  564-565 
Garpike,  teeth  of,  285 
Gastric  gland,  293,  294 
Gastric  Hpase,  294 
Gastric  mill,  282 
Gastropoda,  97 
Gastrotricha,  89 
Gastrovascular  cavity,  281 

Hydra,  181-182 
Gastrovascular  system,  301 
Gastrula,  418 
Gastrulation,  418 

earthworm,  197 
Geese,  114 
Gemmule,  410,  491 
Genes,  430,  431,  435,  456,  457,  461,  640 

subtraction  of,  414 
Genetics,  434,  435,  516 
Genial  tubercles,  552 
Genotype,  446,  449 
Genus,  concept  of,  64-68 
Gephyrea,  93 
Germans,  561,  562 
Germ  cells,  412 

addition  of,  414 
Germinal  selection,  527 
Germ  plasm,  438-439,  466,  526,  640 
Gestalt  psychology,  385 
Gibbons,  537,  538 
Gill  arches,  329 
Gill  books,  314 
Gills,  315 

Giraffe,  117,  495,  508,  517,  519 
Girdle 

hip,  332 

pectoral,  332 

pelvic,  332 

shoulder,  332 
Gizzard,  288 
Glabella,  563 
Glacial  period,  513,  560 
Gland  cell,  341 
Glands 

duodenal,  292 

gastric,  290 

lymph,  304 

intestinal,  292 
Gloecapsa,  69 
Glomerulus,  323,  324 
Glottis,  287,  317 
Glucose,  279,  280,  295,  298 
Glycerin  (glycerol),  295,  298 
Glycogen,  296,  298 


INDEX 


655 


Glycosuria,  401 

Gnathion,  563 

Gnathostomata,  507 

God,  ideas  concerning,  494 

Goethe,  497 

Goiter,  395 

Golden  plover,  490 

Gonad  stimulation,  400 

Gonad  transplantation,  471 

Gonads,  397,  398 

Gonotheca,  Obelia,  185,  186 

Gordiacea,  87 

Gordius,  87 

Gordon,  K.  B.,  262 

Gorilla,  508,  537,  538,  539,  545 

Grand  Canyon,  4^4 

Grantia,  79 

Grasshopper,  99,  506,  519 

mouth  paits  of,  201 

physiological  adaptations  in,  483 

structural  adaptations  in,  486 

vagrant,  200 
Gravity,  effect  of,  on  plants,  16 
Gray  matter,  340,  353 
Grebes,  113 
Greeks,  561 
Gregariousness,  492 
Grijns,  278 
Gristle-fishes,  107 
Ground  pines,  75 
Growth  curve,  inflorescence  in  Yucca, 

271 
Growth  of  cell,  415 
Growth  stimulation,  400 
Grubs,  yellow,  230,  232 
Gruiformes,  114 
Guanaco,  509 
Guard  cells,  251,  252,  267 
Guinea  pig,  438,  515 
Gulick,  523 
Gulls,  114 
Gymnospermae,  75 
Gynandromorphs,  471 
Gypsy  moths,  519 

Habits,  formation  of,  379 

Haeckel,  501 

Hair  color,  563 

Hair  form,  563 

Hairiness,  563 

Hairwoi-ms,  87 

Hales,  Stephen,  241,  267 

Hands,  547 

Hanson,  396 

Haploid  number,  429 

Hard  palate,  284 

Harrington,  394 

Harrison,  432 

Harvey,  William,  305,  408,  528 


Hawks,  114 

Hay  fever,  394,  632 

Head  fold,  426 

Head  piece,  sperm,  416 

Health,  definition  of,  609 

Health  work,  expeiiditurps  for,  in  the 

United  States,  ()35 
Hearing,  361 
Heart,  30(9-308,  307,  488,  497 

beating  of,  308 

insect,  209 
Heart  action,  bee,  209 
Heart  muscl(\  338 
Heart-wood,  246-247 
Heath  hen,  599 
Hedgehog,  534 
Heidelberg  jaw,  556,  565 
Hemichordata,  105 
Hemiptera,  101 
Hemitrichia,  71 
Hemoglobin,  312 
Hemophilia,  31 1 
Henle,  391 

Hens,  breeding  of,  453 
Hepaticae,  73 
Herbaceous  plants,  240 
Herbivorous  animals,  teeth  of,  52 
Heredity,  435,  437,  522,  640 
Hermaphrodites,  471 
Hermaphroditism,  412 
Hermit  crab,  490 
Herrick,  C.  J.,  388 
Hertwig,  O.,  503 
Hesperornithiformes,  113 
Hessian  fly,  585 
Hexactinellida,  79 
Hibernation,  definition  of,  10 
Hind-brain,  346 
Hippidium,  511 
Hippopotamus,  543 
Hirudinea,  93 

Histology,  definition  of,  138 
Hitchcock,  479 
Hodge,  Prof.,  383 
Hog,  515 
Holocephali,  107 
Holophytic  nutrition,  281 
Holothuroidea,  95 
Holozoic  nutrition,  281 
Homarus,  500 
Homologous  bones,  498 
Homology,  67 
Homoptera,  101 
Homo  heidelbergensis,  556,  560 
Homo  neanderthalensis,  557 
Homo  sapiens,  530,  557.  559,  560,  561 
Honey  bee,  201-202,  203,  550 

life  history  of,  212,  213 
Honey  manufacture,  208 


656 


INDEX 


Honey  stomach,  bee,  208,  S09 
Hoofs,  493 

Hooke,  Robert,  138,  311 
Hookworm,  216 

control  of,  616 

infection  by,  615-616 

larvae  of,  223 

life  cycle  of,  22S-224 
Hooton,  544,  552,  563 
Hormiphora,  83 
Hormones,  310,  391,  396,  399,  402,  467 

sex,  471 
Hornaday,  W.  T.,  537,  599 
Horns,  493 
Horses,  447,  477,  487,  515,  541,  544,  545 

blood  relationship  of,  514 

evolution  of,  511,  512,  513 

wild,  557 
Horseshoe  crab,  514 
Host,  217 

Host-parasite  conflict,  217 
Host-parasite  equilibrium,  217 
Host-parasite  relationships,  217 
Hotsprings,  491 
House  fly,  336 

foot  of,  617 
Houseman,  Laurence,  496 
Houssay,  401 
Howard,  Dr.,  568 
Howell,  A.  H.,  578 
Hrdlicka,  555,  561 
Humerus,  332 
Humming  bird,  545 
Hurst,  C.  C,  470,  503 
Huxley,  500,  516 
Hybrid  vigor,  455 
Hybrids,  441 
Hydra 

budding  in,  184 

digestion  in,  182 

locomotion  in,  180 

maturation  in,  183,  184 

nerve  net  of,  183 

neuro-sensory  cells  in,  183 

reaction  to  stimuli  by,  182-183 

regeneration  in,  184 

reproduction  in,  183-184 

reproductive  organs  of,  184 

respiration  in,  182 
Hydroids,  185-186,  334 
Hydrolase,  280 
Hydrolytic,  280 
Hydrophytes,  5 
Hydrotheca,  185 
Hydrozoa,  81 
Hygroscopic,  176 
Hymenoptera,  102 

parasitic,  221 
Hyoid,  329 


Hyperglycemia,  401 
Hyperosmotic,  definition  of,  136 
Hyphae,  mold,  173 
Hypnotoxin,  180 
Hypohippus,  511 
Hypomere,  425 
Hypophysis,  348,  399 
Hyposmotic,  definition  of,  136 
Hypostome,  179 
Hyracoidea,  117 

Ichneumon  fly,  46,  221 
Ichthyopsida,  507 
Ichthyornithiformes,  113 
Identical  twins,  432 
Igorots,  561 
Ileo-caecal  valve,  292 
Imago,  bee,  213 
Immunity,  489 

active,  627-632 

definition,  625 

mechanism  of,  626-627 

passive,  632-634 

racial,  625-626 

types  of,  625-626 
Inbreeding,  454 
Incas,  562 
Incisors,  286,  563 
Inclusions,  cell,  129 
Incubation  period  in  diseases,  635 
Independent  assortment,  446,  447,  450, 

458,  460 
Indeterminate  cleavage,  432 
Indians,  562 

Inferior  vena  cava  (postcava),  307 
Influenza,  624-625 
Infraesophageal  ganglion,  344 
Infundibulum,  348,  400 
Infusoria,  77 
Ingen-Hausz,  241 
Inheritance,  522 

criss-cross,  4^0 

cytoplasmic,  463 

extra-biological,  436 

maternal,  464 

social,  436 
Inner  ear,  361 

Insect  pests,  enemies  of,  587 
Insect  poisons,  588 
Insecta,  99,  103 
Insectivora,  115 
Insectivores,  534,  540 
Insects,   99,    103,   2M,  315,   334,   336, 
344,  513,  521,  532 

body  plan  of,  200-203,  202 

casual  carriers,  618 

characteristics  of,  200-201,  202 

damage  by,  583,  586 

diseases  carried  by,  617-621 


INDEX 


657 


methods  of  controlling,  587-588 

predatory  as  carriers,  618-619 
Insertion,  of  muscle,  337,  338 
Inspiration,  318 
Insulin,  296 
Insight,  365 
Intelligence,  definition  of,  384-385 

in  apes,  386-388 

in  man,  388 

measure  of,  389 
Intelligence  quotient,  definition  of,  388 
Intercostal  muscles,  318 
Interference,  in  heredity,  457 
Intermediate  lobe,  pituitary,  402 
Intermedin,  402 
Internal  nostrils,  287 
Internal  respiration,  312,  316 
Inter-relationships 

between  members  of  same  species, 
45-48 

between  plants  and  animals,  59 
Interrenal,  adrenal,  392 
Interstitial  cells,  399 
Intestinal  glands,  295 
Intestine 

earthworm,  189 

insect,  209 
Intracellular  excretion,  320 
Inversion,  in  heredity,  457 
Invertase,  297 

Invertebrate  nervous  systems,  340 
Invertebrates,  103,  328,  335 
Iodine,  277 
Iris,  359,  492 
Iron,  277 
Iron  age,  547 

Islands  of  Langerhans,  296,  397 
Isolation,  518,  523 
Isolecithal,  418,  4^1 
Isoptera,  100 

Isosmotic,  definition  of,  136 
Italians,  561 

Jacana,  487 

Japanese  beetle,  range  of,  583,  585 

Japanese  chestnut  blight,  580 

Jastrow,  Joseph,  437 

Java  man,  555-556 

Jaw,  Heidelberg,  556,  565 

Jaws,  329 

Jbrhs  407 

Jellyfi'shes,  81,  103,  333,  485,  507 

Jenner,  Edward,  628 

Jennings,  H.  S.,  369,  464 

Jews,  561 

Johannsen,  457 

Jones,  D.  F.,  572 

Jordan,  523 

Jordan  and  Kellogg,  29 


Jugular  vein,  304 
Jungle-fowl,  515 
Jurassic  period,  476,  513 

Kala  azar,  620 

Kangaroo,  542 

Katabolism,  definition  of,  126 

Katydid,  524 

Kellogg,  Vernon  L.,  524 

Kelvin,  Lord,  562 

Kendall,  394 

Kidneys,  321,  322,  323,  488,  498 

Kingsley,  499 

Kipling,  505 

Kiwis,  113 

Klamath  River,  fishing  in,  15 

Knight,  442 

Knight,  experiment  of,  with  plants,  371- 

372 
Koch,  R.,  622 
Koelreuter,  442 
Kohler,  W.,  386-387 
Krakatao,  re  population  of,  34-35 
Krogh,  303 

Labellum,  203,  205 
Labial  palps,  203,  205 
Labium,  201,  203,  205 
Labrum,  201,  203,  205 
Lachrimal  gland,  360 
Lactase,  297 
Lactation  hormone,  400 
Lacteals,  291,  292,  298 
Lactic  acid,  in  muscles,  339 
Lake-dwellers,  452 
Lamarck,  516,  517,  519,  522,  526 
Laminaria,  71 
Lamprey  eel,  221 
Lamp-shells,  91,  328 
Land-inhabiting  forms,  333 
Language,  551 
Lankesteria,  77 
Lanugo,  488,  501 
LaPlace,  312,  407 
Large  intestine,  291,  292 
Larva 

bee,  212 

coelenterates,  186 
Larynx,  329 
Lateral  fold,  426 
Lateral  line,  363 
Lavoisier,  312 
Law  of  Priority,  66 
Leaf 

adaptations  for  movement  in,  374 

cross  section  of,  252 

functions  of,  253-270 

of  sensitive  plant,  374 

structure  of,  251-253 


658 


INDEX 


T;caf  arraneoinent,  250 
Leeches,  93,  103 
Leg 

fore,  332 

hind,  332 
Legs,  evolution  of,  532,  543 

of  worker  bee,  207 
Lemurs,  U5,  514,  535,  536,  545 
Lens,  359,  360 
Lenticels,  2Jf8 
Lepidoptera,  102 
Leptorhynchoides,  87 
Leuckhart,  467 
Leucocytes,  303 
Leucoplasts,  465 
Lice,  216 

head,  223 
Lichen,  55 
Liebig,  312 
Life 

in  air,  23-24 

definition  of,  126 

in  the  hive,  213 

in  water,  21-23 

on  land,  24-25 

signs  of,  127 
Life  realms,  42,  43 
Life  zones,  41-42 
Light,  490,  492 

a  stimulus,  14 
Light-perceiving  organ,  344 
Light  receptors,  357 
Lignin,  264 
Ligula,  203,  205 
Limax,  97 
Limestone,  478 
Limulus,  314,  499,  514 
Linin  fibers,  141 
Linkage,  458-461 
Linnaeus  (Linne),  64,  65,  66,  506,  517, 

530 
Linophryne,  491 
Lion,  508 
Lipase,  280,  295 
Liver,  293,  296,  321 
Liver  fluke,  490 
Liverworts,  73 
Living  matter,   chemical  organization 

of,  130-131 
Lizards,  103,  111,  499,  514,  532 
Llama,  508,  509 
L'Obel,  Matthias,  64 
Lobsters,  99,  333,  500 
Locomotion 

the  "why"  of,  334 

ways  of,  38-40 
Locomotor  organs,  233,  334 
Locust,  sex  inheritance  in,  467,  468 
Loeb,  Jacques,  357,  367-368 


Loons,  113 
Louse,  body,  620 
Lumbar  plexus,  497 
liUmbricus,  93 
Lung  books,  313 
Lung-fishes,  107 

rise  of,  513 
Lungs,  316,  322 
Lycopodium,  75 
Lycopsida,  75 
Lymph,  298,  299,  304 
Lymphatic  system,  304 
Lymph  nodes,  292,  304 
Lymphocytes,  304 
Lysins,  626 
Lytic  power  of  parasites,  217 

Macaques,  537 

MacDougal,  D.  T.,  251 

McClung,  467,  468 

McCollum,  278,  279 

MacCurdy,  549,  553 

Mclndoo,  204 

Macleod,  296,  397 

Macrolecithal,  421 

Macronucleus,  Paramecium,  160,  161, 

162 
Macropus,  115 
Macula  lutea,  360 
Madreporite,  335 
Magellania,  91 
Magnesium,  277 
Maize,  457,  515 
Malaria,  220 

economic  importance  of,  618-619 

life  cycle  of,  227 

preventive  measures  for,  618-619 
Malays,  562 
Male  pronucleus,  417 
Mallophaga,  100,  524 
Malpighi,  209 
Malpighian  corpuscle,  324 
Malpighian  tubules,  209,  321 
Maltase,  280,  295,  297 
Malthus,  519 
Maltose,  293 
Mammalia,  103,  115 
Mammals,  488,  491,  497,  501,  513,  533, 
553 

conservation  of,  600 
Mammary  glands,  489 
Mammoth,  478,  557 
Man,  115,  513,  532,  545,  553,  637,  638 

parasitic  worms  of,  613-614 
Mandibles 

bee,  203,  205 

locust,  201 
Mandrill,  537 
Marchantia,  73 


INDEX 


659 


Market  gardening,  574 
Marmoset,  508,  545 
Marsupials,  115 
Mast,  S.  O.,  154,  155 
Mastax,  rotifers,  153 
Mastigophora,  77 
Matthews,  565 
Matthews'  Law,  565 
Maturation,  428,  456 
Maxilla 

bee,  203,  205 
locust,  201 
Mayas,  562 
Mayflies,  100 
Mavo,  John,  311 
Mead,  A.  D.,  413,  529 
Mechanisms  of  response 
animals,  374-376 
plants,  370-374 
Mechanisms  of  sensation  and  co-ordi- 
nation, 340 
Mecoptera,  102 
Medulla,  adrenal,  392 
Medulla  oblongata,  310,  318,  346,  351, 

353 
Mediterranean  fruit-fly,  585 
Mediterranean  races,  561 
Medullary  sheath,  341 
Medusae,  Obelia,  186 
Meiosis,  459 
Melanesians,  562 
Membrane 
plasma,  134 

selectively  permeal)le,  134 
Mendel,  278,  442,  443-452,  459,  523,  640 
Mendelism,  443,  515,  640 
Mendel's  laws,  455,  467 
Mental  age,  388 
Meridion,  71 
Meristem,  243 
Merozoite,  227 
Mesencephalon,  S46,  350 
Mesenchyme  cells,  419 
Mesoderm  formation,  419 

in  earthworm,  197 
Mesodermal  derivatives,  422 
Mesohippus,  511 
Mesomeral,  425 
Mesophyfl,  252 
Mesophytes,  6,  7 
Mesozoic  era,  476,  513,  533,  553 
Metabolic  gradients,  381,  431 
Metabolism,  definition  of,  126 
Metacarpal  bones,  332 
Metagenesis,  414 
Metameres,  earthworm,  187 
Metamerism 
earthworm,  187 
insect,  200 


Metamorphosis,  235 

honey  bee,  212-213 
Mctaphase,  142,  143 
Metatarsal  bones,  332 
Metatarsus,  bee,  207,  208 
Metatela,  351 
Metathcria,  115 
Metazoa,  139 
Metencephalon,  3^6,  350 
Method 

experimental,  441 
germplasmal,  441,  455 
observational,  440 
statistical,  440 
Metridium,  81 
Mexican  axolotl,  414 
Mice,  487 

Micro-conjugant,  412 
Micronucleus,    Paramecium,   160    161, 

162 
Microsphaera,  73 
Microstomum,  41 1 
Micrura,  85 
Mid-brain,  346.  350 
Middle  ear,  329,  362 
Middle  piece,  sperm,  Jft5,  416 
Mildews,  228 
Milhpedes,  99,  103 
Milton,  John,  493 
Minkowski,  397 
Miocene  period,  476,  511,  513 
Mirabilis  jalapa,  446 
Miracidium,  219 
Mistletoe,  217,  223 
Mites,  102 
Mitochondria,  416 
Mitosis,  141-144,  415,  456,  459 
animal,  US 
plant,  141 
Mixed  nerves,  353 
Moas,  113 

Modified  ratios,  450-452 
Molars,  286 
Molds,  71 

bread,  173-/74 
fossil,  478 

reproduction  in,  174 
Moles,  115,  534 
Molluscoidea,  91,  103 
Mollusca,  97,  103 
Molluscs,  235,  499,  513 
Molting,  234 
Mongolians,  562 
Monkeys,    233,    487,    507,    514,    535, 

536,  .537,  546 
Monocotyledonous  stem,  ^.^5-251,  250 
Monocotyledons,  64,  75,  77,  240 
Monohybrids,  447 
Monotremes,  115,  423 


660 


INDEX 


Moose,  509 

Morchella,  73 

Morgan,  458,  461,  462,  468,  469,  502, 

503 
Mosquitoes,  102,  216,  222 

Anopheles,  £28 

Culex,  238 

mouth  parts  of,  487 
Mosses,  73,  498,  506 
Moths,  468 

Motion,  the  "Why"  of,  334 
Motorium,  343,  375 
Mouse,  495,  499 

Mouth  parts,  homologous,  486,  487,  488 
Movement,  devices  for,  334 
Mucor,  71 
Mucosa,  289 
Mucous  membrane,  284 
Miiller,  Johannes,  391 
MultipHcation  of  cells,  415 
Murex,  505 
Musci,  73 
Muscle  cell,  341 
Muscle  bands,  339 

Muscles  and  muscular  systems,  336-340 
Muscles 

circular,  335,  337 

earthworm,  192-193 

exoskeletal,  336 

fusiform,  338 

heart,  336 

inner  longitudinal,  335 

involuntary,  327,  336,  337 

longitudinal,  337 

origin  of,  337,  338 

skeletal,  336,  337,  338 

smooth,  327,  336,  337 

striated,  336,  337,  339 

voluntary,  327 
Muscular  activity,  339 
Muscular  contractions,  339 
Muscular  relaxation,  339 
Musculature,  human,  337,  338 
Mussels,  333 
Mutations,  442,  524-525 
Mutual  aid,  48 
Mycelium,  mold,  173 
Mycorhiza,  55 
Myelencephalon,  346,  351 
Myology,  327 
Myriapoda,  99,  103,  315 
Myriapods,  532 
Mystacoceti,  117 
Myxomycetes,  71 
Myxosporidia,  77 

Naididae,  411 
Nannoplankton,  22 
Nasion,  663 


National  Parks  and  Forests,  605 

Native  behavior  patterns,  377-378 

Natural  potencies,  432 

Natural  selection,  443,  518,  524 

Naudin,  442 

Navicula,  structure  of,  163 

Neanderthalers,  557,  560 

Necator  americanus,  223 

Neck,  286 

Necturus,  109 

Needham,  406 

Negroes,  562 

Nekton,  22 

Nemalion,  71 

Nemathelminthes,  87,  103 

Nematocysts,  180 

Nematoda,  87 

Nemertinea,  85 

Neolithic  division  of  Stone  age,  547,  560 

Neornithes,  113 

Nephridia,  earthworm,  192 

Nephroi,  488 

Nephrostome,  192,  320 

Nereis,  93 

Nerve  cell,  340,  343 

Nerve  cord,  233 

Nerve  impulse,  339 

Nerve  net,  343 

Hydra,  183 
Nerve  ring,  343 
Nerves,  peripheral,  340 
Nervous  system,  233 

bee,  211 

physiological  unit,  341 

protective  devices  for,  343 

types  of,  342,  376 

unit  of  structure,  340 
Nest-building,  a  genetical  adaptation, 

490 
Netherlanders,  562 
Neural  groove,  420 
Neurilemma,  340 
Neuromotor  apparatus,  335,  342 
Neuron,  340 

definition  of,  195 
Neuroptera,  100 
Nitella,  71 
Nitrogen  cycle,  60 
Nitrogenous  wastes,  319 
Nodes,  lymph,  304 
Noguchi,  619 
Nolan,  212 
Non-chordates,  328 
Non-disjunction,  456,  457 
Nordic  races,  561 
Norman,  J.  R.,  45 
Nostoc,  69 
Nostrils,  284,  317 
Notochord,  69,  233,  329 


INDEX 


66] 


Nutrient  solutions  for  plants,  242 
Nutrition,  plant  and  animal  compared. 

167 
Nucleus,  130,  456,  465 

Obelia,  81,  414 

life  cycle  of,  185 
Ocelli,  bee,  203 
Octopus,  97 
Odonata,  100 
Odontoceti,  117 

Odor,  perception  of,  in  bees,  204-205 
Oedogonium,  69 

structure  of,  172-173 
Oenothera,  525 
Oestrin,  399 
Oil 

in  plants,  264-265 

occurrence  of,  479 
Okapi,  508 
Olfactory  bulbs,  347 
Olfactory  lobes,  347 
Olfactory  nerve,  352 
Olfactory  pits,  bee,  205 
Oligocene  period,  476,  511,  513,  554 
Ommatidia,  205,  206,  358 
Onychophora,  99,  103 
Oogenesis,  430 
Oogonia,  430 

Oedogonium,  172 
Ookinete,  228 
Oospore,  Oedogonium,  172 
Operculum,  moss,  176 
Ophioglypha,  95 
Ophiuroidea,  95 
Opisthocranium,  563 
Opsonins,  626 
Optic  chiasma,  352 
Optic  lobes,  350 
Optic  nerve,  352,  358 
Optic  stalks,  350 
Oral  (buccal)  cavity,  284 
Oral  groove,  Paramecium,  158,  159 
Orang-utan,  537,  539 
Orbit,  358 
Orchard  fruits,  574 
Orchard  heaters,  10 
Ordovician  period,  476,  513 
Organ,  506 

Organ  of  Corti,  362,  370 
Organismal  theory,  139,  365 
Organisms 

definition  of,  148-149 

shifting  of,  34-38 
Organizers,  432 
Organogeny,  414 
Origin  of  life,  406,  407 
Ornithorhynchus,  115 
Orohippus,  511 


Orthogenesis,  527 

Orthoptera,  99 

Osborn,  estimate  of  geological  time  by, 

553 
Osborn,  Henry  F.,  407 
Osborne,  278 
Oscillatoria,  69 
Osmosis,  135 

Osmotic  pressure,  in  plants,  135-136 
Ostia,  insect  heart,  209 
Ostium,  302 
Ostracoderms,  105 
Ostriches,  113,  545 
Otocyst,  372 
Otoliths,  361,  372 
Outbreeding,  455 
Ova,  415,  430 
Ovary,  412,  415 
Overpopulation,  518,  519 

results  of,  34 
Oviducts 

bee,  212 

earthworm,  196 
Ovipositor 

bee,  202 

queen  bee,  212 
Ovists,  466 
Owls,  114 
Ox  botfly,  221,  222 
Oxidase,  280 
Oxygen,  318 

production  of,  by  green  plants,  269 

release  of,  by  green  plants,  269-270 
Oxyhemoglobin,  303,  312,  316,  317 
Oysters,  97,  103 

Pack,  C.  L.,  581 

Paedogenesis,  414 

Pain  receptors,  363 

Palatine  ridges,  284 

Palatine  tonsils,  287 

Paleocene  period,  476,  513 

Paleolithic  division  of  Stone  age,  547, 

560 
Paleontology,  473,  482 
Paleotherium,  511 
Paleozoic  era,  476,  478,  513,  553 
Paleozoic  period,  512,  513 
Pallium,  348 
Palolo  worm,  411 
Pancreas,  293,  295,  396 
Pangenes,  522 
Pangenesis,  522,  526.  527 
Papillary  muscles,  308 
Pappataci,  620 
Parahippus,  511 
Paramecium,  77,  157-163,  316 

locomotion  of,  159-160 

structure  of,  159 


662 


INDEX 


Parasites,  215 

blood-inhabiting,  219 

digestive  tract  of,  219 

eggs  of,  219 

external,  216,  219,  221 

indirectly  acquired,  617-621 

internal,  216,  223 

maintenance  of  cycle,  220 

periodic,  216,  222 

permanent,  216,  223 

relationships  of,  216 

reproductive  capacity  of,  219 

requiring  one  host,  223 

requiring  more  than  two  hosts,  229 

requiring  two  hosts,  225 

respiration  of,  219 

temporary,  216,  221 
Parasitic  diseases,  634-635 
Parasitic  Hymenoptera,  221 
Parasitic  life,  effects  of,  218 
Parasitic  worms,  harm  caused  by,  582 
Parasitism 

as  a  biological  adaptation,  492 

art  of,  215 
Parasympathetic,  354 
Parathyroid,  396 

principle  of,  402 
Parenchyma,  245,  252 
Park,  Wm.  H.,  623 
Parker,  experiment  with  sea  anemone, 

382 
Parotid  gland,  293 
Parrots,  114,  488,  552 
Pars  intermedia,  402 
Pars  nervosa,  400,  402 
Pars  tuberalis,  400 
Parthenogenesis,  413,  471 

in  mammals,  413 
Parthenogenetic  agents,  413 
Passeriformes,  114 
Passive  immunity,  632-634 
Pasteur,  407,  631-632 
Pasteur  Institute,  632 
Pathogens,  621 
Pavlov,  work  of,  575-380 
Peas,  garden,  444,  445,  454,  459 

sweet,  451 
Peccary,  508 
Pectin,  244 

on  bee's  leg,  £07,  208 
Pectinatella,  91 
Pectoral  girdle,  332 
Pedigree  breeding,  453 
Peking  man,  565 
Pelecypoda,  97 
Pellicle,  structure  of,  in  Paramecium, 

158 
Pelvic  girdle,  331,  332 
Pelvis  (of  kidney),  823 


Penguins,  113 
Penn,  William,  639 
Pentacrinus,  95 
Pentadactyl  limb,  332 
Pepsinogen,  294 
Peptones,  294 
Perching  birds,  114 
Perennial,  240 
Period 

post  glacial,  476 

recent,  476 

time,  475 
Periosteum,  338 
Peripatus,  99 
Peripheral  nervous  system,  earthworm, 

194 
Perisarc,  185 
Perissodactyla,  117 
Peristalsis,  290,  291,  294 
Peristome,  moss,  176 
Permian  period,  476,  513 
Perpetuation  of  species,  335 
Persians,  561 
Petals,  497 
Petiole,  250-251 
Petrifactions,  478 
Petromyzon,  105 
Pettenkofer,  312 
Pettersson,  35 
Peyer's  patches,  292 
Phaeophyceae,  71 
Phagocytes,  626 
Phalanges,  332 
Pharyngeal  tonsUs,  287 
Pharynx,  287 

earthworm,  189 
Phascolosoma,  93 
Phenotype,  446,  449 
Phillips,  438 
Philodina,  89 

Phloem,  243,  246,  £47,  248,  250 
Phoenicians,  561 
Phoronidea,  91 
Phoronis,  91 
Phosphorus,  277 
Photoreceptors,  358,  359 
Phycomycetes,  71 
Phyla,  65 
Phyllophora,  71 
Phylogeny,  501 
Physalia,  81 
Pia  mater,  345 
Pig,  514,  515 
Pigeons,  114,  335 
Piltdown  man,  556,  565 
Pincus,  413 
Pineal  eye,  350 
Pineal  gland,  397,  398 
Pines,  75 


INDEX 


663 


Pisces,  103,  105 

Pistils,  497 

Pithecanthropus,  555,  560,  637 

Pitli  rays,  2^7,  249 

Pituitary  gland,  348,  397,  398,  399 

anterior  lobe,  400 

intermediate  lobe,  400 
Placenta,  427,  428 
Planaria,  85 

Plankton,  13,  lA,  21,  22 
Planta,  bee,  207 
Plant    and    animal    cells,     functional 

differences,  166-167 
Plant  breeding,  methods  of,  571-573 
Plantigrade  feet,  333 
Plant  lice,  413 

Plant  parasites,  216,  223,  288 
Plants 

harm  done  by,  579-582 

long  and  short  day,  12,  13 

parts  of,  238-240 ' 

role  of  green,  237-273 

small,  in  fresh  water,  153-154 

types  of,  239 
Plants  and  animals 

cells  of,  139-140 

economic  values  of,  573-579 
Planula,  Obeha,  185,  186 
Plasma,  303 
Plasmodium,  77,  227 
Plasmolysis,  136 

Plastids,  part  played  by,  in  heredity,  465 
Platelets,  311 
Platyhelminthes,  85,  103 
Plecoptera,  100 

Pleistocene  period,  476,  477,  509,  513, 
553,  554,  555,  557,  560,  565,  637 
Plesippus,  511 
Pleurococcus,  169 
Plicae  circulares,  284,  291 
Pliocene  period,  476,  511,  513,  555 
Pliohippus,  511 
Ploidy,  457 
Plovers,  114 
Poebrotherium,  509 
Poisons,  honey  bee,  212 
Polar  bodies,  430 
Polarity,  381 
Poles,  561 
Poliomvehtis,  620 
Pollen  basket,  207 
Pollen  brush,  207 
Pollen  comb,  207,  208 
Pollen  grains,  177 
Pcjlygordius,  93 
Polvnesians,  562 
Polyps,  81 

Obelia,  185 
Polysiphonia,  71 


Polystoma,  85 

Pons  varolii,  350 

Populations,  factors  in  changing,  34-38, 

41 
Porcupines,  505 
Porifera,  79,  103 
Portuguese,  561 

Posterior  lobe,  pituitary,  400,  402 
Posterior  nares,  287 
Posterior  peduncles,  351 
Post-glacial  period,  476,  513 
Potassium,  277 
Poultry,  515 
Precipitins,  626 
Preformation,  431 
Premolars,  286 
Pressure 

effect  on  adaptations,  490,  491 

oxygen,  316 
Pressure  receptors,  362,  363 
Priestley,  312 
Primary  oocytes,  430 
Primary  spermatocvte,  429 
Primates,  115,  233,\534,  540,  541 
Primitive  streak,  420 
Primitive  vascular  plants,  75 
Primordial  germ  cells,  428 
Primroses,  436 
Proboscidea,  117 
Proboscis,  bee,  204,  205 
Procamelus,  509 
Procellariiformes,  113 
Prochordates,  340 
Progestin,  399 
Proglottid,  226 
Prophase,  141,  142 
Propolis,  bee  glue,  213 
Prosecretin,  392 
Prosencephalon,  346 
Prostomium,  188 
Protection  of  embryo,  422 
Protective  coloration,  492 
Protein  elimination,  321 
Proteins,  276,  282 

in  plant  synthesis,  262 

split,  621  ■ 
Froteocephalus  ambloplitis,  229,  230 
Proteoses,  294 

Proterozoic  era,  475,  476,  513 
Protonema,  moss,  176 
Protoplasm 

colloidal  nature  of,  132-133 

composition  of,  128,  131-132 

in  cell,  128-130 
Protoplasmic  extensions,  335 
Protoplasmic  strands,  338 
Protoplast,  169 
Protopterus,  107 
Prototheria,  115 


664 


INDEX 


Protozoa,  77,  103,  151-163,  328,  334, 
335,  342,  411 

disease-causing,  582 

parasitic,  220 
Protozoans,  234,  485,  511,  531 
Protracheates,  315 
Protylopus,  509 
Psalterium,  288 
Pseudopodia,  334,  342 
Psilotum,  75 
Psychozoic  era,  476,  513 
Pterodactyls,  513 
Pteropsida,  75 
Ptyalin,  293 
Puff  balls,  73 

Pulmonary  artery,  308,  317 
Pulmonary  circulation,  309 
Pulp  cavity,  287 
Pulvini,  function  of,  374 
Pupa,  bee,  212,  213 
Pupil,  369 
Pygmies,  561,  562 
Pylorus,  289 
Pyramidal  tracts,  351 
Pyramids,  351 
Pyrenoids,  170 

Quarantine,  reasons  for,  635 
Queen  bee,  201,  205,  211,  212,  214 
Queen  cell,  212 

Rabbit  brush,  50 

Rabbits,  493,  499,  514,  515 

Rabies,  632 

Raccoons,  115,  546 

Race  horses,  453 

Races,  559 

Radial  canals,  336 

Radial  nerve  cords,  343 

Radial  symmetry,  343,  531 

Radio-activity,  use  of,  for  estimating 

geological  formations,  554 
Radiolaria,  77,  328 
Radius,  332,  544 
Rails,  114 
Rana,  109 

Rancho  La  Brea,  477 
Rathke's  pocket,  399 
Ratios,  modified,  450 
/Rats,  115,  545 
Ray,  John,  64 

Reactions  to  stimuli,  366,  368 
Recapitulation  theory,  501 
Receiving  neurons,  375 
Receptor-efifector  system,  343 
Receptor  neuron,  341 
Receptors,  376 

Recessive  characters,  445,  454 
Rectum,  292 


Redi,  406 
Rediae,  413 
Reduction  division,  429 

moss,  176 

Oedogonium,  173 

Spirogyra,  111-172 
Reed,  Walter,  619 
Reflex  actions,  342,  376 
Reflex  arcs,  341 
Regeneration,  408 

in  arthropods,  410 

in  Hydra,  Jf09 

in  Planaria,  409 

in  starfish,  409 

in  vertebrates,  410 
Reindeer,  559 
Rejuvenescence,  417 
Relations 

between  different  species,  50-51 

between  flowers  and  insects,  61-&2, 
Renal  corpuscle,  323,  324 
Renal  pyramids,  323 
Renal  tubules,  322,  323 
Repetition  theory,  502,  503 
Reproduction,  410 

bee,  211-212 

plants,  61,  62,  270-272 
Reproductive  organs,  earthworm,  196 
Reproductive  polyp,  Obelia,  186,  186 
Reproductive  system,  earthworm,  195- 

196 
Reptilia,  103,  111,  329,  487,  497,  501, 
513,  533 

fossil  armored,  334 
Resistance  to  bacteria,  623 
Respiration 

external,  311 

Hydra,  182 

internal,  311 

plant,  265-266 
Respiratory  center,  318 
Respiratory  papillae,  313 
Respiratory  trees,  313 
Responses 

causes  of,  366 

to  gravity,  plants,  371-373,  372 

to  stimuli,  nature  of,  369-370 

to  water,  roots,  370,  373,  374 
Reticulum,  288 
Retina,  369,  360 
Retinular  cells,  358 
Rheas,  113 
Rheiformes,  113 
Rhinoceros,  117,  508,  556,  557 
Rhizoids,  mold,  173 
Rhodophyceae,  71 
Rhombencephalon,  346 
Rhynchocephalia,  111 
Rhynia,  75 


INDEX 


665 


Rhythms  of  plant  Hfe,  23 
Ribs,  331 
Ricca,  73,  371 
Riddle,  Oscar,  467 
Rind,  250 
Ring  canal,  336 
Ritter,  W.  E.,  528 

Rockefeller  Sanitary  Commission,  con- 
trol of  hookworm  by,  616 
Rocky  Mountain  spotted  fever,  222 
Rodentia,  115 

Rods  and  cones,  358,  359,  360 
Rogers,  391 
Root 

dicotyledonous,  243 

perceptive  region  of,  373 

tooth,  286 

work  of,  243-245 
Root  hair,  24^-245 
Rosa,  312 

Roses,  S39,  505,  515 
Ross,  406 

Rotifers,  89,  103,  413 
Roundworms,  87,  103,  219 
Roux,  Wilhelm,  527,  632 
Rubner,  312 
Rugae,  289 
Rumen,  28S 

Ruminant  stomach,  288 
Russian  thistle,   adaptations  for  seed 

scattering,  39 
Rusts,  228 

Saccharomyces,  71 
Saccharomycetes,  71 
Sacculina,  500 
Sagitta,  93 

Salamanders,  103,  109 
Salivary  glands,  293 
Salmon 

egg-laying  habits  of,  596-597 

depletion  of,  597 
Sanborn,  453 
Saprolegnia,  71 
Saprophytic  nutrition,  281 
Saprophytism,     effect     on     biological 

adaptations,  492 
Sap-wood,  246,  247 
Sarcodina,  77 
Sarcolemma,  338 
Sauropsida,  507 
Scab  mites,  216 
Scandinavians,  562 
Scaphopoda,  97 
Scarlet  fever,  633-634 
Scavengers,  52-53 
Schaefifer,  A.  A.,  382 
Schick  test,  633 
Schizogony,  227 


Schizomycetes,  71 

Schlerenchyma,  247 

Schuchert,  estimate  of  geological  time 

by,  553 
Schwann's  sheath,  340 
Sclerotic  coat,  359 
Scolex,  226 
Scorpions,  500,  514 
Scott,  W.  B.,  510 
Scurvy,  278 
Scyphozoa,  81 
Sea-anemones,  81 
Sea-cows,  117 
Sea-cucumbers,  95 
Sea-lilies,  328 
Sea-urchins,    95,    103,    334,   459,    485, 

505,  531 
Sea-walnuts,  83 
Secondary  oocyte,  430 
Secondary  sexual  characters,  398 
Secondary  spermatocyte,  430 
Secretin,  310,  392 
Sedges,  506 

Sedimentary  rocks.  475 
Seed,  435,  490 

formation  of,  62,  270-272 

methods  of  scattering,  39 

uses  of,  272 
Segmentation,  294 
Segregation,  447,  450 
Selaginella,  75 
Selection,  452 

artificial,  452 

germinal,  527 

mass,  453 

natural,  443,  452 

progeny,  453 

sexual,  527 
Sella  turcica,  350 
Semicircular  canal,  361 
Semilunar  valves,  304,  308 
Seminal  receptacles 

bee,  212 

earthworm,  196 
Seminal  vesicles 

bee,  212 

earthworm,  196 
Semper,  436 
Sense  organs,  498 

Sensitive  plant,  responses  in,  575-374 
Sensitivity,  definition  of,  127 
Sepals,  497 

Septa,  earthworm,  188,  189 
Serology,  514 
Serosa,  290 
Serum,  311 
Setae,  188,  193,  334 
Sex,  465,  470 

beginnings  of,  in  algae,  169-172 


666 


INDEX 


Sex  chromosomes,  429 

Sex  determination,  467,  471 

Sex  hormones,  471 

Sex  reversal,  471 

Sexual  reproduction 

invertebrates,  411 

vertebrates,  414 
Sexual  selection,  527 
Shaler,  531 
Shark  sucker,  56 
Shaw,  G.  B.,  527 
Sheep,  509,  515,  519 

effect  of,  grazing  on  trees,  20 
Shelford,  V.  E.,  6,  27 
Shellfish,  destruction  of,  598 
Shells,  493 
Shin-bone,  332 
Shrews,  534 
Shrubs,  240,  493 
Sight,  insect,  205,  206 
Silicon,  277 

Silurian  period,  476,  513 
Simian  shelf,  551 
Simple  reflex  arc,  341 
Sinanthropus,  556 
Siphonaptera,  102 
Sirenia,  117 
Skates,  423 
Skeletal  devices,  327 
Skeletons,  234,  498 

appendicular,  329,  332 

axial,  329,  332 

for  protection,  334 

for  support,  333 

functions  of,  333 

human,  331 

kinds  of,  328 

use  in  movement,  334 

visceral,  329 
Skin,  322 
Skin  color,  563 
Skull,  329,  345 

bones  of  human,  330 
Slime  fungi,  71 
Slime  molds,  71 
Sloth,  508 
Slugs,  97,  103 
Small  intestine,  291 
Smallpox,  mS-629 
Smell,  357,  545 
Smiley  and  Gould,  611 
Smith,  Wm.,  480 
Smuts,  73,  228 

Snails,  97,  103,  328,  334,  436,  523,  531 
Snakes,  103,  111,  490,  499,  533 
Snake  venom,  antitoxin  against,  634 
Snapdragon,  239 
Social  life,  492 
Sodium,  277 


Soft  palate,  284 
Soil,  435 

acid  and  alkali,  effects,  15 
Solar  plexus,  354 
Sollas,  estimate  of  geological  time  by, 

553,  554 
Soma  cells,  412 
Somatic  characters,  450 
Somatoplasm,  438,  453 
Somatopleure,  426 
Somites,  425  (See  Metameres) 
Space,  473 
Spallanzani,  406 
Spaniards,  561 
Sparrows,  520 
Species,  506 

concept  of,  66-69 

history  of,  64-69 
Species  Plantarum,  65 
Speech,  551 
Speman,  432 
Sperm,  466 
Spermary,  412 
Spermatids,  430 
Spermatocyte,  primary,  429 
Spermatogenesis,  428 
Spermatogonia,  428 
Spermatozoa,  415 
Spermists,  467 
Sphagnum,  73 
Sphenisciformes,  113 
Sphenodon,  111 
Sphenopsida,  75 
Sphincter,  pylorus,  289 
Spicules,  334 
Spiders,  48,  99,  102,  103,  500,  514.  520. 

532 
Spinal  column,  329 
Spinal  cord,  341,  353 
Spinal  nerves,  353 
Spines,  493 

Spiny-headed  worm,  219 
Spiracles,  315 

insect,  210 
Spiral  valve,  283 
Spireme,  141,  1^3 
Spirillum,  71,  165 
Spirogyra,  69,  170-171 

conjugation,  170 
Splanchnic  layer,  419 
Splanchnopleure,  426 
Splenic  fever,  622 

Sponges,  79,  103,  328,  334,  342,  491,  531 
Spongilla,  79 

Spontaneous  generation,  406 
Sporangia,  mold,  173,  174 
Sporangiophores,  173,  174 
Spores,  490 
Sporocysts,  414 


INDEX 


667 


Sporophyte   generation,    evolution    of, 

178 
Sporophytes,  498 
Sporozoa,  77 
Sporoz(Mte,  227,  228 
Squamata,  111 
Squids,  97 
Squirrels,  550 
Stackman,  E.  C,  582 
Stamens,  497 
Staphylococcus,  71 
Starch,  manufacture  of,  in  green  leaves, 

253 
Starfish,  67,  95,  103,  328.  334,  335,  336, 

343,  344,  549 
Starling,  392,  519 
State  conservation  departments,   602, 

603 
Statoblasts,  491 
Statocyst,  361,  372 
Stegocephalia,  109 
Stem 

passage  of  liquid  through,  245 

structure  and  function  of,  245-251 
St.  Martin,  Alexis,  290 
Stentor,  77 
Sterility,  471 
Sternberg,  482 
Sternum,  331 
Stickleback,  47 
Stigma,  61 
Stimuli 

classification  of,  367 

definition  of,  366 
Sting,  bee,  212 
Stingers,  493 
Stolens.  mold,  173,  174 
Stomach,  288,  289 
Stomata 

cross  section  of,  267 

movement  of  guard  cells,  ^67-268 

structure  of,  251,  252,  267 
Stone  canal,  336 
Stone  cells,  247 
Stone  warts,  71 
Stony  corals,  334 
Storks,  113,  493 
Strains,  molds,  174 
Stream  association,  3,  4 
Strepsiptera,  101 
Streptococcus,  71 

Struggle  for  existence,  334,  518,  520 
Struthioniformes,  113 
Stvela,  432 
Style,  61 

Stylonychia,  77.  335,  342 
Suberin,  247,  264 
Subesophageal  ganglion,  344 

bee,  211 


Submaxillary  gland,  293 

Submuco.sa,  289 

Subnasale,  563 

Successions,  32-34 

Succus  entcricus,  297 

Sullivan,  562,  563,  564 

Sulphur,  277 

Sumner,  experiments  of,  with  flounders, 

370 
Sundew,  54 
Sunflowers,  505 

Superior  vena  cava  (precava),  307 
Suprae.sophageal  ganglion,  344 

bee,  211 

earthworm,  194 
Survival,  518,  521 
Swarming,  bees,  214 
Swiss,  561 

Symbiosis,  54-56,  490 
Symmetry,  67,  69 

bilateral,  485,  532 

radial,  485,  531 
Synapse,  341 
Synapsis,  460,  467,  459 
Syrians,  561 
Systema  Naturae,  65 
Systemic  circulation,  309 
Systems,  506 

Taenia,  85 

Taenia  saginata,  226 

Taenia  solium,  226 

Tail,  329 

Tail  fold,  426 

Tail  piece,  415,  416 

Takamine,  393 

Tapeworms,  85,  103,  219,  229 

bass,  229 

beef,  226 

broad,  of  man,  229 

pork,  226 

taenioid,  226 
Tapir,  508 
Tarsal  bones,  332 
Tarsius,  535,  536 
Tarsus,  insect,  206,  207 
Taxonomy,  64-117,  504 
Taste  bud,  356 

Tea,  annual  production  of,  574 
Teeth,  286,  488,  496 

garpike,  285 

shark,  285 
Telencephalon.  346,  347 
Telophase,  142,  143 
Teleostei,  107 
Temperature,  490 

optimum,  9 
Tendon,  338 
Termites,  100 


668 


INDEX 


Testis,  415 
Tetanus,  396,  634 
Tetrads,  176 
Tetrapoda,  68,  109 
Texas  fever,  620 
Thallophyta,  69 
Thallophytes,  168 
Theophrastus,  570 
Theridium,  520 
Thistles,  493,  505 
Threadworms,  87 
Thoracic  basket,  329,  330 
Thoracic  cavity,  318 
Thoracic  duct,  298,  304 
Thoracicolumbar,  354 
Thoracic  segments,  insect,  201 
Thorax,  543 

bee,  202,  206-208 
Thymus,  398 
Thyone,  95 

Thyreotropic  hormone,  401 
Thyroid,  S94,  395,  396,  397 
Thyroxin,  394 
Thysanoptera,  101 
Thysanura,  99 
Tibia,  332 

insect,  206,  207 
Tick,  620 

Tidal  shore  habitat,  18 
Time,  473,  553 
Time-scale,  476 
Tissue  formation,  422 
Tissues 

animal,  ^4^-148 

circulatory,  U6,  147,  148 

classification  of,  144 

conducting,  H5,  146 

epithelial,  146,  147 

fundamental,  144,  145 

meristemic,  145,  146 

muscular,  146,  147 

nervous,  I46,  148 

plant,  144,  I45,  146 

protective,  144,  145 

reproductive,  I46,  148 

supporting,  I46,  147 
Tmesipteris,  75 
Toad,  492 

economic  value  of,  577 
Toadstool,  519 
Tools,  546,  550 
Totipotent  eggs,  432 
Touch,  545 
Toxin-antitoxin,  633 
Toxoid,  633 
Trachea,  317 
Tracheae,  insects,  210 
Tracheal  system,  302 

of  worker  bee,  210 


Tracheophyta,  75 
Tracks,  479 
Traits,  640 
Translocation,  457 
Transpiration,  266-268 

experiments  to  show,  266 

loss  of  water  by,  267 
Transverse  commissure,  344 
Tree,  240 

Tree  shrews,  534,  540 
Trees,  493 
Trematoda,  85,  219 
Trematode,  complex  cycle  of,  231 
Triassic  period,  476,  513 
Triceps,  338 
Trichamophora,  71 
Trichinella,  87,  615 
Trichinella  spiralis,  225 

life  cycle  of,  226 
Trichocysts,  158 
Trichoptera,  102 
Tricuspid  valve,  307 
Trihybrids,  447,  448,  449 
Trilobites,  513 
Triploblastic,  68 
Trochanter,  206,  207 
Trochelminthes,  89,  103 
Tropical  rain  forest,  28 
Tropisms,  364,  367-369 
Trypanosoma,  77,  103 
Trypanosomes,  220,  620 
Trypsin,  295 
Tschermak,  443 
Tsetse  fly,  511 
Tube-feet,  336,  344 
Tuberculosis 

bovine,  623 

cause  of,  623 

cure  for,  624 

death  rate  of,  623,  624 
Tuna,  545 

Tunicates,  105,  329,  499  ] 
Tupaia,  534 
Turbellaria,  85 
Turks,  561 
Turk's  saddle,  350 
Turkeys,  114 

Turtles,  103,  111,  328,  334,  514 
Twins,  identical,  471 
'Twixt-brain,  346,  348 
Tympanic  membrane,  362 
Tyndall,  407 

Typhlosole,  earthworm,  189,  283 
Typhoid,  629-630 
Typhoid  Mary,  490,  630 
Typhus,  619-620 

Ulna,  332 
Ulopteryx,  71 


INDEX 


669 


Ulothrix,  69 

structure  of,  171 

Ulva,  69 

Umbilical  cord,  428 

Undulating  membrane,  159 

Unguiculata,  115 

Ungulata,  117 

Ungulates,  487,  514,  534 

Unguligrade  feet,  333 

Unit  characters,  450 

United  States  Bureau  of  Fisheries,  598- 
599 

United  States  Department  of  Agricul- 
ture, 571,  573,  577,  578,  585,  587 

Urea,  296,  310,  322 

Ureter,  323 

Urethra,  324 

Urochordata,  104,  105 

Urodela,  109 

Use  and  disuse,  516 

Uvula,  284 

Vaccination,  smallpox,  628 

Vaccines,  631-632 

Vacuoles,  129 

Vagina,  bee,  212 

Vagus,  318,  352 

Vampire,  508 

Van  Helmot,  241 

Van  Leeuwenhoek,  151-152,  153 

Vasa  efferentia,  earthworm,  196 

Vascular  rays,  ^45-249 

Vas  deferens,  earthworm,  196 

Vasomotor  center,  310 

Variation,  417,  442,  518,  519 

Vaucheria,  69 

Vectors,  insect,  618-620 

Veddahs,  561 

Veins,  302,  303,  304,  305 

leaf,  250,  251-^5^ 
Venereal  diseases,  634-635 
Ventral  diaphragm,  insect,  210 
Ventricle,  306,  348,  350,  351 
Venules,  303 
Vermis,  350 
Vesalius,  305 
Vessels,  lymph,  304 
Vestigial  structures,  499,  501 
Vermiform  appendix,  499 
Vertebrae,  330 

caudal,  330,  331 

cervical,  331 

lumbar,  331 

sacral,  331 

thoarcic,  331 
Vertebral  column,  328,  345 
Vertebrata,  105 

Vertebrates,  103,    105,  233,   334,   336, 
532 


Vicuna,  509 
Villus,  291,  428 
Vincent,  611 
Violets,  506 
Vision,  545 
Vitamin  A,  278 
Vitamin  B,  278 
Vitamin  C,  278 
Vitamin  D,  278,  279 
Vitamin  E,  278 
Vitamin  G,  278 
Vitamins,  265,  277 

antineuritic,  278 

antipellagric,  278 

antirachitic,  279 

antiscorbutic,  278 

antisterility,  279 
Vitreous  humor,  360 
Voit,  312 
Volvox,  67,  179 
Von  Baer,  502 
Von  Bering^  632-633 
Von  Frisch,  204 
Von  Hohnel,  267 
Von  Mering,  397 
Von  Sachs,  241,  269 
Vorticella,  77 

Walcott,  estimation  of  geological  time 
by,  553 

Wallace,  A.  F.,  518,  519 

Walter,  H.  E.,  385 

Ward,  Henshaw,  486,  529 

Ward,  H.  B.,  10 

Warm  receptors,  363 

Warning  colors,  492 

Wart  hog,  508 

Water 

amount  in  living  things,  133,  254 

rise  of,  in  plants,  269 

role  of,  in  plant  life,  255,  256 

Water  fleas,  413 

Water  vascular  systems,  335,  336 

Weapons,  547,  550 

Weaver,  243 

Weed  seeds,  40 

Weeds,  40,  519 

Weismann,  517,  523,  526 

Wells  553 

Wells!  Huxley,  and  Wells,  383 

Whale,   117,  490,   498,  499,  514,  521, 
534,  535,  545 

Wlieat,  454,  573 

Wheat  rust,  73,  228 

White  matter,  350,  353 

Widal  test,  626,  627 

Wiedersheim,  497 

Willughby,  64 

Wings,  bee,  206 


670 


INDEX 


Winter  eggs,  491 
Witch-hazel,  505 
Wolf,  477 
Wood,  Dr.,  611 
Woodruff,  417 
Worker  bee 

lateral  view,  202 

mouth  parts,  205 
Worms,  499,  531 

round,  219 

spiny-headed,  219 

tape,  219 
Wrist,  544 

Xanthophyll,  258 
X-chromosome,  468 
Xerophytes,  6 
Xiphidium,  467 
Xylem,  243,  245,  2^8,  250 


Y-chromosome,  468,  469 
Yeast,  71 
Yellow  fever,  619 
Yellow  spot,  360 
Yerkes,  R.  M.,  387,  538 
Yolk  sac,  423,  4^4,  488,  501 
Yolk  stalk,  426 

Zebra,  508 

Zonal  distribution,  4^ 
Zoochlorellae,  180 
Zoology,  systematic,  64 
Zoospores,  47 

Ulothrix,  171 
Zucker,  279 
Zygnion,  563 

Zygospores,  Spirogyra,  170 
Zygotes,  417 

Closterium,  164-165 

Spirogyra,  171