ie ee ee 7 . 4
ei bs eh ribiitebatitisitietale str ecisteeelntsribivlcbtineesshtetisseisl atl. 5 21et nip tah: 2212-0 cretei bens i titestelou ial itis -tcleiai gs slclalslei ie tinieisiaiets sisset ssbb Srtebhiscetosi sis ibid scsi sherei ta tiiethcsssene «i. SSEREETT CE Te iiss t rrtssteortatitecesee tape es eye tthe os eee eee epee so sib: SaseTere i Sraberseies oNb2S7s 755 -5-iSse8er~sasktlei= - intel ged Sethe ecessesasae eter HipepesiaeiecctcrT ss
ttyeesttae ataeieleis sigberbsleserdt station ? Spit ssi wiei shai . eletasbeieitie.s) a = 7 ses goa Siti ~ ial: ise si -grccte 7s atit lope statisisieieres, Sei eies ai sre. if Prasat me Seinbicasicas it. cibratepebgiata tional sisbcat ito sis ciserstariehe sei Ths oi it tyes? Ott tse 14 jeaeia leamiacceibits ty Ry etststr estat ert * e
peeNetera se alters es ptgicre riggs ssgiprisioseliserberehenetaleredslerpretatstsitieegesMs teeter iM piblsletsccirerce ster giekeheries crete sieneearleisieteletiter ees eele owiehe ter taatie tise i tie es ee ete
Hd) ry ibid atyerer at ee) ‘ Tiilevepeserstetan ei tet:
VAN retireree ae ielanpeistsdes tities Uelespleng satel
Given by. .grz.G<..L. e.. WOO
through pu
LI
*,* No book or pamphlet is to be removed from the Lab+
oratory without the permission of the Trustees.
i Cee ca nit eae on a
Me Ree cg) Wii Tot Suan:
My ve: ; 2. been a MiG?
a1 ' a i ; ; Pia yo &
‘ : - ‘ 1
’ .
N
a
i
'
'
U
id
‘
,
af i.
|
me
ANIMAL BIOLOGY
THE MACMILLAN COMPANY
NEW YORK -: BOSTON «+ CHICAGO * DALLAS
ATLANTA + SAN FRANCISCO
MACMILLAN AND CO., Limitep
LONDON + BOMBAY - CALCUTTA * MADRAS
MELBOURNE
THE MACMILLAN COMPANY
OF CANADA, LIMITED
TORONTO
! a ! } ; o 4 a4 : - _ ¥
OT Cr halal SP de er ae Tey
= as - : ¥ y y
‘fe 5 y 7 a ie
‘ ley i ‘
u s fn P
ni a = Yh : %,,
2 Se a Ye eee a i id ie
—* m, — )
a
=
ey
=
en |
ne
-
p
ZEA
ies, oor:
4]
HARRESON
=EDES
SAWN ic
ile
A Marine BriocoENos!Is
s, Molluscs, and Chordates on a wharf p
Coelenterate
ANIMAL BIOLOGY
BY
LORANDE LOSS WOODRUFF
Professor of Protozoology in Yale University
NEW YORK
THE MACMILLAN COMPANY
1938
SECOND EDITION COPYRIGHTED, 1938,
By THE MACMILLAN COMPANY
ALL RIGHTS RESERVED—NO PART OF THIS BOOK MAY BE
REPRODUCED IN ANY FORM WITHOUT PERMISSION IN WRITING
FROM THE PUBLISHER, EXCEPT BY A REVIEWER WHO WISHES
TO QUOTE BRIEF PASSAGES IN CONNECTION WITH A REVIEW
WRITTEN FOR INCLUSION IN MAGAZINE OR NEWSPAPER
Printed in the United States of America
Set up and electrotyped. Published April, 1938
First edition copyrighted and published, 1932,
By The Macmillan Company
PREFACE TO SECOND EDITION
This revised edition of the Animal Biology incorporates numer-
ous changes suggested by experience and the development of the
science, especially in the field of genetics. Growing interest in
man’s past has led to the introduction of a chapter on the human
background.
Withal, the page and figure numbers remain unaltered in the
sections referred to in Professor Baitsell’s Manual of Animal
Biology, so that book is still available for the details of the morphol-
ogy and physiology of selected types, as well as for laboratory
directions, which obviously would be out of place in the present
volume.
For suggestions in regard to the new chapter I am indebted to
my colleagues, Professor R. S. Lull and Doctor G. E. Lewis;
and my thanks are due Professor J. H. McGregor of Columbia
University, the University of Chicago Press, William Wood and
Co., and the Yale University Press for permission to reproduce
figures from their publications. Miss E. L. Gelback has assumed
efficiently much of the editorial work involved in seeing the book
through the press. And, finally, I wish to express my appreciation
of the continued hearty codperation afforded by The Macmillan
Company.
L. L. WoopRUFF
YaLe UNIVERSITY,
March, 1938.
FROM PREFACE TO FIRST EDITION
The present volume is published in response to a demand for a
special adaptation of the author’s Foundations of Biology, Fourth
Edition, designed especially for use in courses in animal biology
and general zoology in which plants are considered only inci-
dentally in their relations with animals. The essential plan as
well as much of the material is, with purpose, the same in both
books since the author is apparently far from alone in the convic-
tion that the general biological viewpoint affords the natural ap-
proach to an introductory survey of either animal or plant science.
The author continues to be indebted to his colleagues in the
Osborn Zoological Laboratory at Yale University and to many
others, including his wife, who by suggestions and constructive
criticism aided in the development of the book. Special mention
is gladly made of the interest expressed by Professor J. W. Bu-
chanan of Northwestern University, Professor D. B. Casteel of
the University of Texas, Professor W. B. Unger of Dartmouth
College, and by Professors W. R. Coe, G. A. Baitsell, J. S. Nicholas,
and D. A. Kreider of Yale.
The author has had again at his disposal the interest and skill
of Mr. R. E. Harrison who has drawn a large number of new
figures and revised old ones especially for this book. Acknowledg-
ments are also due to the authors and publishers of the following
works, who have supplied additional illustrations: Chapman’s
Handbook of Birds of Eastern North America (D. Appleton &
Co.); Folsom’s Entomology (P. Blakiston’s Sons & Co.); Hough
and Sedgwick’s The Human Mechanism, Linville and Kelly’s
General Zodlogy (Ginn & Co.); Martin’s Human Body, Sedgwick
and Wilson’s General Biology (Henry Holt & Co.); Metcalf and
Flint’s Destructive and Useful Insects, Noble’s Biology of the
Amphibia (McGraw-Hill Book Co.); Romanes’ Darwin and After
Darwin (Open Court Publishing Co.); Conklin’s Heredity and
Environment in the Development of Men (Princeton University
Press); Conn and Budington’s Advanced Physiology and Hygiene
(Silver, Burdett & Co.); Kudo’s Protozodlogy (C. C. Thomas);
Chandler’s Animal Parasites and Human Disease, Curtis and
Vi
FROM PREFACE TO FIRST EDITION vii
Guthrie’s General Zodlogy (John Wiley & Sons); Wilson’s Physical
Basis of Life (Yale University Press); and Hegner’s College Zo-
ology, Lindsey’s Evolution and Genetics, Lull’s Organic Evolu-
tion, Metcalf’s Organic Evolution, Newman’s Vertebrate Zoology,
Parker and Haswell’s Textbook of Zodlogy, Peabody and Hunt’s
Biology and Human Welfare, Scott’s The Theory of Evolution,
Shipley and McBride’s Zodlogy, Walter’s Genetics (The Macmillan
0.).
Furthermore, the author is indebted, of course, to innumerable
sources for the facts and principles outlined and for suggestions
for figures adaptable to present needs. Many of these references
are specifically mentioned in the preface to the Foundations of
Biology and are listed in the bibliography of the present volume.
Finally, to The Macmillan Company the author is grateful for its
hearty and generous cooperation in all the arrangements incident
to publication.
L. L. WoopruFF
YALE UNIVERSITY,
February, 1932.
CONTENTS
I. THE SCOPE OF BIOLOGY
A. Origin of Life Lore
B. Biological Sciences ;
C. Biology and Human Progress
Ly: a ee ORGANIZATION OF LIFE
. The Cell
B. Cell Division . 2
Ill. THE PHYSICAL BASIS OF LIFE .
A. The Protoplasm Concept
B. Unique Characteristics of Living M Matter .
. Organization
. Chemical Composition .
. Metabolism...
- Maintenance and Growth .
. Reproduction.
. Irritability and Adaptation
IV. METABOLISM OF ORGANISMS
A. Green Plants .
1. Protococcus .
2. Food Making
3. Respiration .
B. Animals ,
1. Amoeba...
2. Food Taking
3. Respiration and Excretion |
C. Colorless Plants
1. Bacteria .
2. Cycle of the Elements in Nature .
3. The Hay Infusion Microcosm
Nn ke whe
V. SURVEY OF UNICELLULAR ANIMALS .
AS Sarcodina . .
B. Mastigophora .
C. Sporozoa
D. Infusoria
VI. THE MULTICELLULAR ANIMAL .
A. Development .
B. Tissues .
C. Organs and Organ Systems
VII. SURVEY OF INVERTEBRATES.
A. Sponges
B. Coelenterates .
AGS ©
x CONTENTS
CHAPTER PAGE
CG Flat Worms . ... .) © 3 9° oes
Dy. Round: Worms: . . . .)\.0., 2. 3° aoe
E. Segmented Worms. La) a) en
F. Rotifers, Bryozoans, and Brachiopods oo ee emer
G: Echinoderms «°° ./2)° 4-*2.)°".. 4% .<! ee
T. Molluses’ < . .&)' 208" @. 7. 9s 4) 2 See 86
[.. Arthropods...) -.As-ai ler Mi TN ee
VIII. THE INVERTEBRATE: BODY |. 92). ioe
Ae Hydra 3 Ss" Sas ae nbs of
B. Earthworm —. "2 2... <0... oo. ae
1 Body Plan =. a a EI eS eee
2. Tissues and Organs og J eh Vee ee ee
C, 'Cravyiishivn Pe 6) eh 1
[X. SURVEY OF VERTEBRATES |. ~°2 <> 2 20 aeeenen
A. Fishes . . ob eB SS Se
1. Sharks and Rays . a ae 2 a Se ee
2. Bony Pishes. 2 2°.) “9.6! ee
3. Lume Fishes... . .. . « -. ==") 2) eee
B. Amphibians... atk vt pe
1. Salamanders and Newis . . . . . 2) een
2. Voads ‘and Frogs". = =. .. “.° .>)). eee
C. Reptiles .. . «oe ae ee
1. Turtles and Portoises: .. . 2 | aes neues
2. Crocodiles and Alligators . . ... : 7 3)
3. Lizards and Snakes . 2 2a gn 1;
Ds Birds a. oe og 2 § 5 See
E. Mammals .. fu tho eu ie os nS 7) ae
i Monotremes . ok oe? og OC 4 eae Te een
2; Marsupials’ «9.0. 0a ee eo eee
3: Placéntals 2 ..° = 2%: 2:4 4 2 eee
X. THE VERTEBRATE. BODY 2.2 = eee
A. Body Plan. ©. 2° «+ 4°"... 4,237 ee
B. Skin 2 a ee
Go Muscles (2 0h 2 oe
D. Skeleton... So fp ce ca a SE See
E. The Human Body LO SS a eee
F. Distinctive vibe Characters. . . . . . 146
Me NUTRILIONS = ~ o » oa
A. Buccal Cavity, Pharynx, and | Bsophagus ee
B. Stomach . . . 7 aes
C. Small Intestine .. «. \E tines Jo Se,
1. Liver and Pancreas Dg oe ee
2.. Absorption. (5 4% 5 ph) 2s 18
3: Distribution: °°. -. ©] > .c0 20 eee
D. Large Intestme, . 2°) 2 2D eee
E. Food Use Oe SS hoe ee
Fe Dachleetec a oo UP ieseee Th lise epee So as err er
CONTENTS
CHAPTER
XII. RESPIRATION
A... Longs
B: Rees Mechanism .
C. Respiratory Interchange .
XIII. CIRCULATION ;
A. Circulation in the Lower Ventcheates
B. Circulation in the Higher Vertebrates
XIV. EXCRETION
A. Gills and Pane:
Bokm:. 5.72%
C. Liver
D. Kidneys
1. Urme yee
2. Evolution of Kidneys ;
XV. REPRODUCTION
A. Invertebrates .
B. Vertebrates :
1. Uterine Dev elopment
2. Hormones :
3. Urogenital System
XVI. COORDINATION
A. Chemical Coardinaton :
B. Codrdination by the Nervous Sy stem
1. Brain and Spinal Cord . ;
2. Cranial and Spinal Nerves
3. Autonomic System .
C. Sense Organs ..
i, Cutaneous Senses
2. Sense of Taste
3. Sense of Smell
A Ears a
9. Eye
XVII. ORIGIN OF LIFE
A. Biogenesis and Abiogenesis .
B. Origin of Life on the Earth .
1. Cosmozoa Theory
. Pfliiger’s Theory
. Moore’s Theory .
. Allen’s Theory .
. Troland’s Theory
. Osborn’s Theory
. Huxley’s Statement .
XVIII. CONTINUITY OF LIFE
A. Reproduction ..
B. Origin of the Germ Cells
L. Mitosis
2. Chromosomes of the Germ Geils.
=
ID) U1 ee Co DO
xil CONTENTS
CHAPTER :
3. Spermatogenesis.
4, Odgenesis
5. Chromosome Cycle .
XIX. FERTILIZATION -
A. Gametes :
B. Union of Gametes.
1. Synkaryon .
2. Significance of Fertilization
Protozoa a ee
Metazoa
XX. DEVELOPMENT .
A. Embryology of the Eenthwonn:
B. Embryology and Metamorphosis of the Fr. Toe. es
C. Embryonic Membranes of the ale Vertetnieds
D. Problems of Development
XXI. INHERITANCE
A. Heritability of Neniacione
1. Modifications
2. Recombinations .
3. Mutations
B. Mendelian Principles .
1. Monohybrids
2. Dihybrids
3. Trihybrids
4. Summary.
C. Alterations of Mendelian Ratios
D. Mechanism of Inheritance
1. Sex Determination .
2. Linkage .
3. Crossing-over
4. Mutations
E. Nature and Nurture .
XXII. ORGANIC ADAPTATION .
A. Adaptations to the Physical Bnviroumeun
1. Adaptations a Sok Functional .
Food
Temperature .
Pressure.
2. Adaptations Essentially Structural
Adaptive Radiation of Mammals .
Animal Coloration
Legs of the Honey Bee .
B. Adaptations to the Living Environment
1. Communal Associations
2. Symbiosis
3. Parasitism
4. Immunity.
C. Individual Adaptability
CONTENTS
CHAPTER
XXIII. DESCENT WITH CHANGE
A. Evidences of Organic Evolution.
: Classification
. Comparative Anatomy
. Paleontology
: Embryology
. Physiology
. Distribution .
B. Factors of Organic Ev olution
iz Lamarckism . .
2. Darwinism .
3. Genetics and Ey olution
Selection
Method of Evolution
DN Ot Con
XXIV. BIOLOGY AND HUMAN WELFARE
A. Medicine
ik Microdrganisms and Disease
Malaria
Yellow Fever .
Syphilis
2. Parasitic Worms
Trematodes
Cestodes
Nematodes.
3. Health and Wealth .
B. Biology and Agriculture. .
1. Plant and Animal Food
2. Insects Injurious to Animals .
3. Insects Injurious to Plants
4. Beneficial Insects
C. Conservation of Natural Resources
D. Constructive Biology .
XXV. THE HUMAN BACKGROUND
A. The Prehuman Lineage .
B. Fossil Man... .
. Java Man ..
. Peking Man .
. Piltdown Man .
. Heidelberg Man
. Neanderthal Man .
Cré-Magnon Man .
C. Cultural Development
1. Paleolithic Culture .
2. Mesolithic Culture .
3. Neolithic Culture
4. Age of Metals
XXVI. DEVELOPMENT OF BIOLOGY .
A. Greek and Roman Science
NOE WN
B. Medieval and Renaissance Science
xiv CONTENTS
CHAPTER
C. The Microscopists
D. Development of the Subdivisions of Biology :
. Classification
. Comparative Anatomy .
. Physiology
. Histology
. Embryology .
. Genetics . ,
. Organic Evolution .
NHS WN
APPENDIX
I. A BRIEF CLASSIFICATION OF ANIMALS .
II. BIBLIOGRAPHY .
III. GLOSSARY .
INDEX
PAGE
452
455
455
457
459
464
466
468
ATL
AT7
481
492
515
ANIMAL BIOLOGY
ANIMAL BIOLOGY
CHAPTER I
THE SCOPE OF BIOLOGY
Science is, in its source, eternal; in its scope, unmeasurable; in its
problem, endless; in its goal, unattainable. — von Baer.
THE oldest as well as the most obvious classification of the ob-
jects composing the world about us is into non-living and living;
and the knowledge accumulated during many centuries in regard
to the former is to-day represented in the physical sciences, while
that of the latter comprises the content of BroLoGy, the science
of matter in the living state. Biology, like all science, has as its
ultimate object the description of its phenomena in terms of what
may be regarded as basic concepts — matter and energy acting
in space and time; but it is needless to say that the attainment of
this object is not imminent in any department of knowledge, and
least so in the science of living things. These exhibit a state of
matter and energy which altogether transcends the classifications
of physicist and chemist to-day —a condition which expresses
in its highest manifestations what we call our ‘life.’
Whether the ‘riddle of life’ will ultimately be solved is a question
which everyone would like to answer but only the rash would at-
tempt to predict. Suffice it to say that biologists who are on the
firing line of progress to-day are directing their attention solely
to the description and measurement of the phenomena exhibited
by living things — those phenomena which distinguish life from
lifeless — in an attempt to relate them to the familiar and more
readily accessible phenomena of which we have some exact know]-
edge in the realm of the non-living. But this should by no means
be taken to indicate that biologists do not recognize the stupen-
dous problems they face, or do not appreciate to the full — indeed
more fully than others — the enormous gap that separates even
the simplest forms of life from the inorganic world.
1
2 ANIMAL BIOLOGY
Our present interest, however, is not in discussing the theoretical
goal of biology, but in drawing in bold strokes an outline picture
of the present-day knowledge of the subject which represents the
cumulative results of the application of the scientific method
to problems of life. This method is not peculiar to science, but is
merely a perfected concentration of our human resources of obser-
vation, experimentation, and reflection. ‘Thus far this has been
a most productive method and certainly has given no evidence
that its usefulness is being exhausted. But, of course, “in ultimate
analysis everything is incomprehensible, and the whole object of
science is simply to reduce the fundamental incomprehensibilities
to the smallest possible number.”
A. OricGiIN oF LirE LoRE
The foundations of the scientific study of living nature were
laid by Aristotle and Theophrastus over 2000 years ago. On the
basis of collecting, dissecting, classifying, and pondering they
reached generalizations, many of which have but recently been
put on a firm hasis of fact. Indeed, they seem to have raised
nearly all the broad questions which are fundamental to-day;
but from the Greeks until about the fifteenth century there is
little to record. There were many additions to the body of
knowledge during this long slumber period, but fact and fancy
were so intermingled that the truth was largely obscured.
(Figs. 288, 289.)
The feeling that, though Man is of nature, he is still apart, was
expressed at the revival of learning, during the sixteenth century
and later, in the broad classification of all knowledge as history
of nature and history of Man; the former recording the “history
of such facts or effects of nature as have no dependence on Man’s
will, such as the histories of metals, plants, animals, regions, and the
like’; the latter treating of the voluntary actions of men in
communities. Thus all record of facts was either NATURAL HISTORY
or civil history. From this general field of natural history the
present-day sciences of astronomy, physics, chemistry, geology,
and biology became separated as relatively independent bodies of
facts as each gained content, clearness, and individuality. Astron-
omy, physics or natural philosophy, and chemistry were set apart
first owing to the fact that their material was more readily suscep-
tible to mathematical and experimental treatment, thus leaving
THE SCOPE OF BIOLOGY 5)
the histories of the Earth, animals, and plants, or so-called obser-
vational sciences, as the residue for natural history.
It remained, however, for Lamarck in particular, during the
opening years of the nineteenth century, to attain a vision of the
unity of animal and plant life and to express it in the term BIOLOGY.
But biology is something more than a union of plant science —
BOTANY — and animal science — zoOLOGY — under one name; for
it endeavors, in addition to describing the characteristics of plants
and animals, to unfold the general principles underlying both.
Accordingly ANIMAL BIOLOGY, the subject of the present volume,
is the study of the basic principles of life with especial reference
to animals, including Man.
B. BroLoGicaAL SCIENCES
Thus the biologist has as his field the study of living things —
what they are, what they do, and how they do it. He asks, how
this animal or that plant is constructed and how it works — and
this he attempts to answer. He would like to ask, and often does
ask, why it is so constructed and why it works the way it does,
but then he passes beyond the scope of science into the realm of
philosophy.
These queries of the biologist reflect the two primary viewpoints
from which biological phenomena may be approached: the mor-
phological in which interest centers upon the form and structure of
living things; and the physiological in which attention is concen-
trated upon the functions performed — the mechanical and chem-
ical engineering of living machines. Clearly, however, it is im-
possible to draw a hard and fast distinction between morphology
and physiology because in the final analysis structure must be
interpreted in terms of function, and vice versa. But again, the
fields of morphology and physiology naturally resolve themselves
into special departments of study, depending on the level of analy-
sis of structure or of function which is emphasized. Thus mor-
PHOLOGY stresses the general form of the animal or plant; ANATOMY,
the gross structure of individual parts, or organs; HISTOLOGY, the
microscopic structure of organs, or tissues; CYTOLOGY, the com-
ponent elements of tissues, or cells, and the physical basis of life,
or protoplasm. Similarly, PHysIoLoGy investigates the activities
of animals and plants, the functions of organs, the properties of
tissues, the phases of cell life, and finally the physico-chemical
4 ANIMAL BIOLOGY
characteristics of protoplasm. So much for the study of the adult
individual animal or plant — but this is not all. The origin and
development of the individual, GENETICS and EMBRYOLOGY; and
the origin and development of species, ORGANIC EVOLUTION, are
other wide fields which must be approached from both the struc-
SOCIOLOGY ANATOMY
THE STUDY OF |THE STUDY OF
Animal Gross
PSYCHOLOGY \ Societies Structure
Mental
Phenomena of
HISTOLOGY
Microscopic
Structure
ECOLOGY
Environmental
Relations
TAXONOMY
Classification
ZOOLOGY
ANIMAL
BIOLOGY
PLANT
PHYSIOLOGY
Function
EVOLUTION
Origin of
Species
PATHOLOGY
Abnormalities
GENETICS
inheritance
Development
Fic. 1. — The chief divisions of Biology.
tural and functional aspect if any real advance is to be made to-
ward a comprehensive appreciation of life. (Fig. 1.) !
Thus, just as the various physical sciences have expanded and
become specialized until they are beyond the grasp of a single man,
so biology and its subdivisions, or the BIOLOGICAL SCIENCES, are
now distributed among many specialists. Although specializa-
tion results in a narrowing and isolating of the fields of study, as
deeper levels of investigation have been reached in all the sciences
there has been a tendency for the basic phenomena to meet on the
common ground of the fundamental sciences, physics and chem-
istry — for in the last analysis the biologist must assume, as a
working hypothesis, that the properties of protoplasm are the re-
‘In order not to interrupt the continuity of the narrative, formal definitions of
technical terms are usually omitted from the text. See the GLossary, Appendix III.
THE SCOPE OF BIOLOGY 5)
sultant of the properties and interrelationships of the chemical
elements which compose it. But he must not suppose that physics
and chemistry when added up fulfil the rdle of biology. Rather
he must grip the cardinal fact that with new relations the proper-
ties of things change — the properties of protoplasm depend on and
emerge from those of its chemical constituents only when the
latter are actually in protoplasm.
Thus “in one direction, supported by chemistry and physics,
biology becomes biochemistry and biophysics. In another direc-
tion it becomes the basis of the psychical sciences which relate
to human nature, of psychology and sociology,” etc. Indeed,
it is not an exaggeration to regard all knowledge as really bio-
logical, since the process of knowing is a life process which is basal
to every art and its practice, to every science and its application,
and to every philosophy and its exposition.
C. BroLtogy AND HumMAN PROGRESS
Probably the value of a knowledge of biological principles, and
of the order of nature in general, cannot be better emphasized
than in the words of the founder of modern methods of biological
teaching. Huxley wrote: “Suppose it were perfectly certain that
the life and fortune of every one of us would, one day or other,
depend upon his winning or losing a game of chess. Don’t you
think that we should all consider it to be a primary duty to learn
at least the names and the moves of the pieces? . . . Do you not
think we should look with disapprobation upon the parent who
allowed his child, or the state that allowed its members, to grow
up without knowing a pawn from a knight? Yet it is a very plain
and elementary truth that the life, the fortune, and the happiness
of every one of us, and, more or less, of all who are connected with
us, do depend upon our knowing something of the rules of a
game infinitely more difficult and complicated than chess. It
is a game which has been played for untold ages, every man and
woman of us being one of the two players in a game of his or her
own. The chessboard is the world, the pieces are the phenomena
of the universe, the rules of the game are what we call the laws
of Nature. The player on the other side is hidden from us... .
To the man who plays well, the highest stakes are paid with an
overflowing generosity. And one who plays ill is checkmated.”’
(Fig. 298.)
6 ANIMAL BIOLOGY
The contributions of the biological sciences to human welfare —
the rules of the game of life — we shall consider later more fully.
At this point it is merely necessary to emphasize that biology
affords certain fundamental principles which are of universal
application — principles of so great importance that they have
revolutionized modern thought and action in nearly every field of
human endeavor, and color civilized Man’s entire mental outlook
on the world about him. Biology is the supreme agent of adjust-
ment of human life to human life-conditions, and life goes on
solely by reason of the adequacy of such adaptations. Specifically,
of course, biology forms the indispensable foundations of med-
icine — health, and of agriculture — food and raiment. Together
these spell wealth: without them we would be poor indeed.
Thus biology meets many of the physical needs of mankind and
so adds enormously to the basis of human welfare, but it also has
another equally important aspect. The appeal of biology for its
high place as a contributor to the progress of humanity combines
its practical gifts with the more subtle development of aesthetic
values which naturally flow from the pregnant thought of the unity
of nature — the oneness of life — based on the firm and ever-
increasing sense of control as knowledge grows, which ‘‘robs life
of none of its mystery but rather serves to link it securely with the
larger mystery of the universe and the Infinite back of it all.”
Man, though one with all living beings, has the unique and all-
important power consciously to study the ways, to direct the
forces of nature, and to adapt himself to them.
CHAPTER II
CELLULAR ORGANIZATION OF LIFE
Science never destroys wonder, but only shifts it, higher and
deeper. — Thomson.
WITH a synoptic view of the scope and importance of biology
before us, we now turn directly to the study of life itself in the
only form it is known — the bodies of plants and animals.
A thin slice of material from the surface of the skin of a Frog
or the leaf of a Buttercup when examined under the microscope
shows the same general structure. Each appears to be composed
of innumerable small bodies, no two of which are exactly alike
even in the same piece, though all are similar enough to be one and
Fic. 2. — Cells, highly magnified, from the surface of a Frog’s skin (A),
and a plant leaf (B).
the same type of unit. And if we extend our study to other parts
of the Buttercup or the Frog or, indeed, to any part of any familiar
plant or animal — or to the human body — we find essentially
similar units of structure in every case. In fact, the bodies of all
living things either consist of a single organic unit or of millions
of essentially similar units called cELLs. (Figs. 2, 3, 4.)
Each cell is itself the theater of all the fundamental vital proc-
esses — each is alive. This, of course, is obvious when a cell forms
the whole body of a UNICELLULAR plant or animal, but not so
apparent when it is only one of myriads forming a MULTICELLULAR
organism. But actually the life of the organism as a whole is, in
7
8 ANIMAL BIOLOGY
final analysis, the product of the life of the component cells: the
expression of their harmonious cooperation. Accordingly the cells
are the basic units of the actual living matter, termed PROTOPLASM.
age Poe
i oe
5 ues we 2 Socom
Ty
oe
.
zo)
YES
say ak
Fic. 3. — Vertical section (highly magnified) of a leaf to show its cellular
structure. a, guard cells, at opening (stoma) through epidermis; 6, cells con-
taining chlorophyll; c, upper and lower epidermal cells. (From Bailey.)
Such being the case, we reach the first great biological general-
ization: all animals and plants have the same elementary cellular
structure. Indeed, the specific local differentiations in the living
Fic. 4. — Transverse section (highly magnified) of a simple animal (Hydra)
to show the cellular structure. Outer layer, ectoderm; inner layer, endoderm;
central cavity, enteric cavity.
materials of the various parts of animals and plants are made
possible largely because the protoplasm is disposed in microscopic
unit masses, or cells.
CELLULAR ORGANIZATION OF LIFE .- 9
A. Toe CELL
With the diversity of gross structure of animals and plants in
mind, one is not surprised that there are considerable, even great,
variations in their component cells. In fact, the characteristics of
an organism or part of an organism are determined by those of
the cells. But there are certain fundamental cell characters which
are common to all cells — by virtue of which they are cells — and
it is important to emphasize these. (Fig. 5.)
In its simplest form a cell is a small, more or less spherical mass
of protoplasm. Such are the eggs of various animals and the com-
A
Fic. 5. — Diagram of a cell shown (A) as a solid body, and (B) in optical
section, through plane x~y, as it appears under the microscope.
plete body of some of the lowest plants and animals. Cells forming
the units of multicellular organisms, however, frequently exhibit
more or less hexagonal surfaces on account of stresses and strains
incident to their position among other cells; while specializations
and differentiations, for one purpose or another, produce forms
which are characteristic of different parts of the organism, as, for
example, the long spindle-shaped cells of certain muscles, and the
widely branching cells of parts of the brains of animals. Broadly
speaking, the greater diversity of cell form is found in animals,
while in plants, owing to the more general presence of rigid CELL
WALLS about the protoplasm, the cells more frequently present
symmetrical, angular outlines. (Figs. 6, 7.)
The term cell is a relic of the time when the cell wall was re-
garded as the most important part, and its protoplasmic contents,
if observed at all, were considered as only of secondary importance,
if not waste material. Now we recognize many cells which are
10 ANIMAL BIOLOGY
essentially naked masses of protoplasm, such as Amoeba and white
blood cells. In other words, the protoplasm is the actual living
part — the cell wall typically being a non-living accessory which
more or less sharply separates one unit mass of protoplasm from
another and lends rigidity and form to the group of cells as a whole.
The living material of cells is highly organized into various
complex structures, some of which are present in all cells and
others only in cells adapted for special functions. For the present,
Gastric vacuole
Nucleus
PSs os
tees
oameece® Cee
|
= sets ndoplasm
Contractile vacuole Ectoplasm
Fic. 6. — A simple animal (Amoeba proteus) which consists of a single cell
(highly magnified). Locomotion is by streaming of the protoplasm forming
temporary protrusions, or pseudopodia.
however, it is only necessary to emphasize that the protoplasm of
all typical cells is differentiated into two chief parts: the cyTo-
PLASM, or general groundwork which makes up the bulk of the cell;
and the NUCLEUS, a more or less clearly defined spherical body,
situated near the center of the cytoplasmic mass.
Cytoplasm and nucleus, looked at from the functional view-
point, represent a physiological division of labor within the con-
fines of the cell. Experiments have shown that they are mutually
necessary for cell life; the removal of the nucleus putting an end to
constructive processes — assimilation, repair, and growth — and
thus leading rapidly to death. Accordingly the nucleus may be
considered as the center of the synthetic activities of the cell, and
the cytoplasm, if not as the area in which destructive processes
are chiefly involved, at least as the region in which the results of
the nuclear activity become realized. It seems clear that the
nucleus is the controlling agent in cell activity, and hence a
CELLULAR ORGANIZATION OF LIFE 11
B Muscle
BY) BL ess
see, More
‘€
C Blood
- 5 Si
Nt
(Ff F Nerve
TCE:
Fic 7. — Various types of cells, highly magnified. A, egg and sperm of
Segmented Worm; B, muscle cells (unstriated) from bladder of Frog; C, one
white and three red blood cells of Frog; D, pigment cell from skin of Fish;
E, epithelial cells (ciliated), including a gland cell, from intestine of Dog; F,
nerve cell (neuron) from brain of Mouse.
12 ANIMAL BIOLOGY
primary factor in growth, development, and transmission of specific
qualities from cell to cell, and so from one generation to the next.
B. CELL DIvIsion
All the evidence indicates that, at the present time at least,
living matter never arises except under the influence of preéxist-
ing living matter. That is, protoplasm grows — cells grow and,
having attained a certain size, reproduce by dividing into two es-
sentially equal parts. Then there are two cells — the parent cell
Fic. 8. — An Amoeba in six successive stages of division. The dark body
surrounded by a clear area is the nucleus. (Modified, after Schultze.)
has lost its identity in its offspring. Cell division is reproduction.
Indeed, in final analysis reproduction is always cell division,
through this primary fact is largely obscured by accompanying
phenomena in higher animals and plants. (Fig. 8.)
The process of cell division involves the division of both cyto-
plasm and nucleus, and therefore we must enlarge our concep-
tion of a cell as a small mass of protoplasm differentiated into
cytoplasm and nucleus, by adding that both cytoplasm and
nucleus arise through the division of the corresponding elements
of a preéxisting cell.
CELLULAR ORGANIZATION OF LIFE 13
We shall later have occasion to make a study of the details of
cell division, known as mitosis, but from what has been said it
must occur to the reader that, since cells arise only by division,
those of the present day, whether complete free-living organisms
or units composing the bodies of higher plants and animals, in-
cluding Man, are actually lineal descendants in unbroken series
from the beginning of cellular life on the Earth. (Fig. 164.)
CHAPTER. Ft
THE PHYSICAL BASIS OF LIFE
Over the structure of the chemical molecule rises the structure of
the living substance as a broader and higher kind of organization.
— Hertwig.
THE realization that all animals and plants possess a funda-
mentally similar organization — the structural and physiological
units, or cells — leads quite naturally to an intensive study of the
material of which the cells are composed — the physical basis of
life itself. Accordingly we must now consider more specifically
the characteristics of this actual life-stuff — protoplasm.
The old saying that the materials forming the human body
change completely every seven years is a tacit recognition that
lifeless material, in the form of food, is gradually transformed into
living matter under the active influence of the body. Indeed, just
as a geyser retains its individuality from moment to moment
though it is at no two instants composed of the same molecules of
water identically placed, so the living individual is a focus into
which materials enter, play a part for a time, and then emerge to
become dissipated in the environment. But here the analogy stops.
For in the living organism the materials which enter as food, en-
dowed with POTENTIAL ENERGY, are arranged and rearranged until
specific molecular combinations result, which in turn are trans-
formed into integral parts of the organization of life itself. How-
ever, to live is to work, and to work means expenditure — the
transformation of the potential into KINETIC energy — with the
result that materials in relatively simple form and largely or en-
tirely devoid of energy are returned to the realm of the non-living.
And note, the living organism must continuously utilize energy
in order merely to maintain itself. Cessation is death.
Thus we reach a fact of prime importance: so far as we know,
living matter — protoplasm — is merely ordinary matter that has
assumed, for the time being, unique physico-chemical relationships
that display the remarkable series of phenomena which we recog-
nize as LIFE.
14
THE PHYSICAL BASIS OF LIFE 15
But non-living matter is always closely associated with living
matter in the bodies of animals and plants. Indeed, the two are
so intimately related that it is frequently difficult to distinguish
sharply between them. Obviously, in the human body, for in-
stance, the visible parts of hair and nails, a large part of bone, and
the liquid part of blood are non-living material. But the non-living
is not confined to gross structures, for the dead among the living
is still revealed until the penetrating power of the microscope
fails us.
A. Tur PROTOPLASM CONCEPT
Although there is a continuous stream of matter and energy
flowing through the living individual, nevertheless the physical
and chemical study of living matter from whatever source we take
it — Mold or Elm, Amoeba or Man — reveals a striking similarity
in its fundamental factors, and this is the basis of the protoplasm
concept held by modern biologists.
As the finer structure of animals and plants came within the
range of vision through improvements in microscope lenses, it was
gradually recognized that the ultimate living part appeared to be
a granular, viscid fluid. This started a long series of studies on the
materials of the bodies of unicellular organisms similar to Amoeba
and of the cellular elements of higher animals and plants, which
finally led, about the middle of the last century, to the complete
demonstration of the full morphological and physiological signifi-
cance of protoplasm. There is, in truth, an essentially similar,
fundamental, living material of both animals and plants — a com-
mon physical basis of life. This reduction of all life phenomena to
a common denominator laid the foundations for — indeed, actu-
ally established — the life-science, biology. (Figs. 6, 9.)
Although we speak of a common ‘physical basis of life,’ it is of
paramount importance to bear in mind that the protoplasm of no
two animals or plants or, indeed, of different parts of the same
animal or plant is exactly the same. Identity of protoplasm would
mean identity of structure and function — identity of life itself.
The concept protoplasm merely emphasizes that, after allowances
are made for all the variations, we still have the similarities far
outnumbering the dissimilarities in the ‘agent of vital manifesta-
tions.’
The physical chemists tell us that protoplasm consists of matter
16 ANIMAL BIOLOGY
in the colloidal state — a condition of matter that chemists have
long been familiar with in the inorganic world. A coLLorp has
been described as matter divided into particles larger than one
molecule and suspended in a medium of different matter. There-
fore butter and cream are each colloids: the former consisting of
water finely divided and suspended in oil, and the latter essentially
of finely divided oil in water. But protoplasm is a stupendously
more complex colloidal system. It comprises not two, but very
many substances, some in simple and others in highly complex
molecular form, so finely divided that they are invisible with the
ordinary microscope.
Now colloidal systems in general are characterized by tremen-
dous surface activity — the result of energy relations between the
contact surfaces of the particles of the different component sub-
stances. This being so, and protoplasm being a colloid composed
of very many different kinds of materials, the total surface area
between suspended substances and suspending media is very great,
and thus affords the requisite conditions for an exceedingly in-
tricate system of energy relations. And when we add to this the
fact that at such surfaces chemical changes, some involving
changes in electrical potential, occur; and also that mechanical
changes are induced by precipitation, coagulation, and constant
redistribution between the suspending media and the substances
in suspension, we begin to get at least a glimpse of the exceedingly
intricate and delicate energy-ltransforming system that protoplasm
really is. To work out these intricacies is one of the imposing tasks
still before the biologist, chemist, and physicist.
But the statement that protoplasm is a colloidal system —
roughly speaking, a rather fluid sort of jelly — leaves the reader
without any clear conception of the appearance of protoplasm.
As a matter of fact it is as difficult to describe the appearance of
protoplasm as it is to define it. Protoplasm must be seen under
the microscope to be appreciated. With a moderate magnification,
it presents a fairly characteristic picture, appearing like a trans-
lucent, colorless, viscid fluid containing many minute granules as
well as clear spaces or vacuoles. If it is examined in water it ex-
hibits no tendency to mix with the surrounding medium, though
investigations show that osmoric interchanges are constantly
going on. For this reason it is impossible to consider proto-
plasm except in connection with its surroundings, whatever
THE PHYSICAL BASIS OF LIFE i
they may be — variations in its environment and variations in
its activities being reflected directly or indirectly in its ap-
pearance. (Fig. 6.)
Under the highest magnifications, not only does the finer struc-
ture of protoplasm differ in various specimens, but also in the same
cell under slightly different physiological conditions. At one time
it presents the appearance of a fairly definite net-like structure, or
Ay, yy
yw 4
yng Oey ii
>
+
>
=
Se
ly magnified), exhibit-
ing alveolar, or foam-like, structure. This arises by the appearance, growth,
and crowding together of minute bodies in a homogeneous ground substance.
(From Wilson.)
reticulum, the meshes of which enclose a more fluid substance; at
another, a frothy, or alveolar, appearance due to a more liquid
substance scattered or emulsified as spherical bodies in a less
liquid medium. Again, at other times, the denser portion seems
to take the form of minute threads, or fibers, or of tiny granules
distributed in a somewhat fluid matrix. (Fig. 9.)
These appearances have given rise to various theories which
emphasize one or another as the universal formula for the physical
structure of protoplasm, from which the other appearances are
merely secondarily derived. But the trend of recent work has
been to indicate that although the general similarity of proto-
plasmic activity, wherever we find it, might lead us to expect to
find also a visible fundamental structural basis, such does not exist
within the range of magnifications at our command. Reticular,
alveolar, and other structures which our microscopes reveal are,
as it were, merely surface ripples from underlying physico-chemical
changes in the colloidal system which, thus far, are unfathomable.
18 ANIMAL BIOLOGY
B. Unique CHARACTERISTICS OF Livinc MATTER
Since the phenomena of life are without exception the results of
protoplasmic activity, it is obvious that we must look to proto-
plasm for the primary attributes of living matter. The properties
which are absolutely characteristic of living matter are its spe-
cific organization, chemical composition, metabolism involving the
power of maintenance, growth, and reproduction, and irritability
resulting in the power of adaptation.
1. Organization
It must be emphasized that living things are not homogeneous,
but possess structural and physiological ORGANIZATION. Animals
and plants are made up of various parts adapted for certain pur-
poses. They exhibit ‘a viable unity’ and so stand in sharp con-
trast with objects comprising the inorganic world as, for instance,
rocks and rivers. Accordingly animals and plants are referred to
as ORGANISMS. Moreover, as we have seen, the organizational
units of all living things are cells, and so it follows that cell struc-
ture is a direct or indirect expression of all the unique life char-
acteristics that we are about to survey. A few of the details of
cell structure are necessary for an appreciation of the organization
of organisms.
It will be recalled that the protoplasm of all typical cells is dif-
ferentiated into two chief parts: the cyfoplasm, or general ground-
work which makes up the bulk of the cell; and the nucleus, a more
or less clearly defined spherical body, situated near the center of
the cytoplasmic mass.
CyTopLasm. The cytoplasm may be considered the less special-
ized protoplasm of the cell, and its appearance and other charac-
teristics are those which have been outlined in our discussion of
protoplasm. With that in mind, for the sake of definiteness, we
may consider its basis as consisting of a meshwork, composed of
innumerable, minute granules which permeate an apparently homo-
geneous ground-substance, or HYALOPLASM. Distributed through-
out the cytoplasm are usually various lifeless inclusions such as
granules of food, droplets of water or oil, vacuoles of cell sap,
crystals, etc., representing materials which are to be, or have been,
a part of the living complex, or are by-products of the vital proc-
esses. This passive material is frequently referred to as META-
THE PHYSICAL BASIS OF LIFE 19
PLASM, but it is quite evident that such a term stands for no essen-
tial morphological part of the cell, and we have no absolute criterion
to distinguish between some granules which are regarded as meta-
plasmic in nature and others which are ordinarily considered active
elements of the cytoplasm. But there are various undoubtedly
active bodies besides the nucleus in the cytoplasm. Chief among
these are the CENTROSOME which plays an essential part in ceil
reproduction, and the PLASTIDS, MITOCHONDRIA, and GOLGI BODIES
which apparently are the seat of various special physiological
activities. (Fig. 10.)
The cytoplasm, since it forms the general groundwork, is that
part of the cell which comes most closely into relations with the
environment, and accordingly near the surface it is frequently
Cell wall
Plasma-membrane
Golgi bodies
Plastid Mitochondria
Vaenole Metaplasm
Fic. 10. — Diagram of a cell.
modified somewhat in texture and consistency so that a definite
outer region, Or ECTOPLASM, may be distinguished from an inner,
or ENDOPLASM. The ectoplasm is limited externally by a PLAsMaA-
MEMBRANE Just beneath the cell wall. The plasma-membrane is
certainly a part of the living cytoplasm, while the cell wall must
be regarded as non-living, though in many cases it is a direct
transformation of the living material which grows and plays,
in connection with the plasma-membrane, an important part in
controlling the flow of matter and energy to and from the cell and
its surroundings.
20 ANIMAL BIOLOGY
Nucieus. As already mentioned, within the cytoplasmic mass
there is an area of clearly differentiated material which typically
has a rounded form, bounded by a membrane, so that it appears
as a definite body of protoplasm called the nucleus. The struc-
tural basis of the nucleus consists of a homogeneous ground-sub-
stance, or KARYOLYMPH, which is permeated by a meshwork that
usually appears to consist of two substances, LININ and CHROMATIN,
which are probably chemically closely related. Chromatin is the
highly characteristic nuclear material which takes various forms
during different phases of cell activity but generally gives the ap-
pearance of a network of tiny granules with one or more dense
‘knots’ of chromatin. Frequently there are one or more conspicu-
ous, round bodies within the nucleus known as NUCLEOLI. Later it
will be necessary to describe some of the important changes in
chromatin arrangement that occur during various phases of cell
activity, especially during cell division, but it is sufficient at this
moment to emphasize that the nucleus is a differentiated area
of the cell protoplasm which is the arena of the chromatin.
Indeed, the nucleus probably represents the highest type of or-
ganization in the organism. (Fig. 164.)
2. Chemical Composition
It is impossible to make an analysis of living matter because the
disturbance of its molecular organization by chemical reagents
kills it. Therefore our knowledge of its chemical composition has
of necessity been derived from a study of dead protoplasm. How-
ever, since in the transformation from the living to the non-
living state there is clearly no loss of weight, it follows that the
complete material basis of life is still present for examination.
In other words, the death of protoplasm is a result of disor-
ganization.
Chemical analysis of protoplasm shows that it invariably com-
prises the elements OXYGEN, CARBON, HYDROGEN, NITROGEN,
PHOSPHORUS, SULFUR, CALCIUM, MAGNESIUM, SODIUM, POTASSIUM,
TRON, and CHLORINE. Probably other elements are always present;
certainly some others are found normally in the protoplasm of
certain parts of various species of animals and plants. Thus in
addition to the elements just mentioned which form by far the
larger part of the human body, there are also present traces of sev-
eral other elements, such as iodine, copper, manganese, and fluorine.
THE PHYSICAL BASIS OF LIFE at
The average composition of the human body, including cellular
and intercellular material, is about as follows:
CUBS be eC eas oer De ToT ca fs 65.00%
ESOP es, Sole os) Pee Cole eae ee 18.00
EAC cs na Re ee 10.00
IER EMIS cao a oS Fae ey eee 3.00
See Re See) soars spay vceciadn. a Read eed ede Ae OD
IESEIPSHOMUIS yee 52 cs So oi ey, as ae Se ae 1.00
AGUAS STII, 809k ba a ed eee 0.35
‘SUELO 2p a rr en REP Se 0.25
Bonner eos es oe onde Se eee 0.15
WiNrinet ie wae feud foils aialhba. ae eee 0.15
MUGISTTEST Ln (A a re rere Bee) 0.05
MER IP os sk oie 3:5 & dex, 23s FS 0.004
At first glance there is nothing very striking about this list of
elements. They are all common ones with which the chemist is
familiar in the non-living world. The materials of Man’s body are
worth less than one dollar! Furthermore, quantitatively the most
important compound is nothing more complex than WATER
(H,O). It composes more than two-thirds of the human body.
But there are combinations of the elements which are highly
significant and characteristic, and result from the capacity of
carbon, hydrogen, and oxygen, or carbon and hydrogen together,
to form the numerous complex compounds which in turn supply
the basis for intimate associations with other elements. As a matter
of fact, the bulk of protoplasm is composed of carbon, oxygen,
hydrogen, and nitrogen associated with each other in an apparently
infinite series of relationships, in which the carbon seems to play
the leading role — the indispensable bond that links all other ele-
ments in organic unity. Some of these compounds are relatively
simple, but the majority consist of elaborate atomic arrangements
and not a few represent molecular complexes of hundreds and even
thousands of atoms.
The COMPOUNDS OF CARBON which are characteristic of proto-
plasm fall into three chief groups: proteins, carbohydrates, and
fats.
PROTEINS invariably consist of the elements carbon, oxygen,
hydrogen, nitrogen, and usually sulfur, and frequently phosphorus,
iron, etc. Examples are ALBUMIN of the white of egg, CASEIN of
milk, GLIADIN of wheat, and MyosIn of lean meat. The nitrogen par-
ticularly distinguishes proteins from the other compounds of the
22 ANIMAL BIOLOGY
living complex and, as we shall see later when considering the
chemical processes in animals and plants, is largely responsible
for their commanding position as ““the chemical nucleus or pivot
around which revolve a multitude of reactions characteristic of
biological phenomena.’ Study of the relationship of nitrogen to
the other chemical elements of proteins has revealed that the
protein molecule is a huge complex of linked AMINO acips — an
amino acid being an organic acid in which one hydrogen atom is
replaced by the amino group, NH». The amino acids are the nitrog-
enous units with which organisms deal physiologically, rather than
the proteins themselves. An animal, for example, with various
proteins available in its food, chemically disrupts them into their
amino acid constituents, and then takes an amino acid here and
another there and synthesizes the specific proteins it demands.
And further, if individual amino acids are supplied, the animal
employs them.
Although there are less than two dozen amino acids, the num-
ber of proteins is legion. Furthermore, besides the so-called
simple proteins composed sclely of amino acids, there are many
which comprise in addition other radicals, such as the hemo-
globins which contain hematin and the nucleoproteins with nucleic
acid. It appears that the specific structure of an organism depends
upon the chemical specificity of its proteins, but for our purposes
it is sufficient to recognize that the presence of proteins and the
power of forming them is a prime chemical characteristic of living
matter. Apparently these huge, complex molecules with their
high lability and, therefore, tendency to chemical change are
fundamentally associated with the plasticity and responsiveness
of protoplasm.
CARBOHYDRATES consist of various combinations of carbon,
hydrogen, and oxygen, the latter elements typically being present
in the proportion found in water (H.O). Though more simple in
chemical structure than proteins, they range in complexity from the
simple sugars, or monosaccharides such as GLUCOSE and FRUCTOSE,
to polysaccharides such as STARCH, GLYCOGEN, and CELLULOSE.
Fats are composed of the same elements as the carbohydrates, .
but in quite different arrangements. The proportion of oxygen is
always less, and therefore they are more oxidizable and richer in
potential energy. Fats represent the union of an acid (fatty acid) and
glycerol. Examples are butter and oils of plant or animal origin.
THE PHYSICAL BASIS OF LIFE 23
Thus proteins, carbohydrates, and fats represent large classes
of substances which are distinctly characteristic of living matter,
not being found in nature except as the result of protoplasmic ac-
tivity; although biochemists now can artificially construct certain
fats and carbohydrates as well as the amino acid constituents of
some proteins. Proteins undoubtedly play the most important
part in the organization of protoplasm, while the carbohydrates
and fats contribute largely to the supply of available energy.
However, it is impossible to draw a hard and fast distinction in
regard to their respective contributions because, for example, as
we shall see later, carbohydrates form the foundation upon which
proteins are built by green plants.
Proteins, carbohydrates, and fats are frequently referred to as
the foodstuffs, but it will be recognized that while, in a way, they
constitute the chief groups, all the constituents of protoplasm must
be available. Accordingly, inorganic salts, water, and free oxygen
are really foodstuffs. Furthermore, recent investigation has dis-
closed another class of organic substances which are absolutely
necessary and are known as VITAMINS. These accessory food sub-
stances are referred to as vitamin A, B, C, etc., though now the
chemical constitution of some of them is known. Scurvy, for in-
stance, is a disease induced by the lack of vitamin C which has
proved to be a hexuronic acid. Finally must be mentioned a great
group of organic catalyzers, called ENzyMEs. These are not food
substances but. special proteins formed in organisms where they
play a major part in chemical processes. However, when all is said,
our knowledge of the chemical complexities of protoplasm affords
no adequate conception of how they are related to life.
3. Metabolism
We have emphasized that living matter is continually changing,
and this fundamental fact is reflected in nearly all attempts to
define life. Aristotle described life as “the assemblage of opera-
tions of nutrition, growth, and destruction’’; deBlainville, as a
“twofold internal movement of composition and decomposition ”’;
and Spencer, as “the continuous adjustment of internal relations
to external relations.”’
This interaction consists of chemical and physical processes in
which combustion, or oxidation, plays the chief role. Over a century
ago it was shown that animal heat results from a slow burning of
24 ANIMAL BIOLOGY
the materials of the body, involving the consumption of oxygen
and the liberation of carbon dioxide; and further, that for a given
consumption of oxygen and liberation of carbon dioxide, about
the same amount of heat is produced by an animal as by a
burning candle. In other words, the oxidation of the complex
compounds which enter the body as food is definitely proportional
to the amount of energy which the body gives out, just in the
same way as the amount of work performed by a steam engine
and the amount of heat it liberates bear a strict proportion to its
consumption of fuel.
This is an important discovery, because it goes far toward
establishing the fact that at least certain characteristic vital
phenomena are in accord with the laws which hold in the non-living
world. But the processes of METABOLISM are not so simple as per-
haps might be imagined from the results just mentioned. Heat
represents but one of the many energy transformations within the
organism, and biologists are at work trying to interpret one after
another in terms that are equally applicable in the realms of the
living and the lifeless. “* The symbol of the organism is the burning
bush of old.”’
One naturally asks whether living matter possesses some special
form of energy — ‘vital force’ — which is quite different from
chemical and physical energy. This is the philosophically impor-
tant question of viraLisM. From the standpoint of biology we
may say that no instrument ever devised has detected such en-
ergy, and until some unique vital energy can be made evident to
one of the human senses, it does not fall within the scope of science
— science can neither deny nor affirm its existence. Perhaps for
the present it is sufficient to realize that unique phenomena may
emerge from new relationships — relationships change the prop-
erties of things. The properties of molecules are those which the
atoms have when they are in the molecule, and the phenomena
of life depend on — emerge from — the physico-chemical con-
stituents of protoplasm when, and only when, they are in proto-
plasm. A living cell exhibits ““many unpredictable properties be-
yond those of the mere sum of its individual constituent molecules
and compounds, or the additive resultants to be derived from any
arrangement of them.”
However, it is important to note that many of the grosser phe-
nomena of life are being gradually restated in terms of the physical
THE PHYSICAL BASIS OF LIFE 25
sciences. So it appears clear that the organism is a system for
transforming energy into work performed — transforming the po-
tential energy stored in chemical complexes of its own substance
into the various vital processes of life. And it is in this transfer of
energy from one kind to another that we find exhibited the activi-
ties which are most distinctive of living things. In these processes
of metabolism many complex substances rich in potential energy,
which have entered as food and have been, in whole or part, added
to the protoplasmic system, are reduced to simpler and simpler
conditions and finally, with their energy content nearly or entirely
exhausted, are eliminated as EXCRETIONS. Obviously, if life is to
persist, this continual waste must be counterbalanced by a propor-
tionate intake of food in order to renew the supply of energy and
to provide the materials which, after preliminary changes, are made
into an integral part of the living organism.
4. Maintenance and Growth
Thus in living the animal or plant is partially consuming and
rebuilding itself continually — metabolism is a dual process.
When constructive metabolism, ANABOLISM, keeps pace with de-
structive metabolism, KATABOLISM, the individual remains essen-
tially unchanged — it balances its account physiologically — and
this MAINTENANCE is the normal condition of adult life. But one of
the most obvious results of metabolism is growth, or permanent
increase in the size of the individual. As a rule growth is most
rapid during the early part of the individual’s existence, or youth.
Indeed, at birth a child is about a billion times larger than the egg
cell from which it has developed. Later in life, when a certain phys-
iological balance, or maturity, is attained, growth chiefly occurs in
making good, in so far as may be, the wear and tear incidental to
living, and in providing for reproduction.
Growth, as well as maintenance, means that the organism takes
the materials which it receives in the form of food, transforms
them and fits them into the protoplasmic organization here and
there throughout as needed. This INTERSTITIAL GROWTH by chem-
ical synthesis is in striking contrast to the growth, for example, of
crystals that increase in size merely by the addition to the surface
of new material of the same kind from the saturated solution, the
mother liquid, in which they are suspended. Crystal growth is
passive; organic growth is active. Protoplasm, with materials
26 ANIMAL BIOLOGY
and energy taken from its environment, constructs more proto-
plasm — endows the non-living with its own unique organization.
It makes more life-stuff. And, if the available materials are ade-
quate, the living substance tends to increase indefinitely, or until
the specific limits of the cell or organism are attained.
5. Reproduction
So far as is known, living matter arises only by the activities
of preéxisting living matter. We have seen that this transforma-
tion is continually going on in anabolic processes in the animal
or plant, and brings about repair and growth of the individual;
but it is in reproduction that what may be termed the overgrowth
of the individual results in the production of a new one.
Thus reproduction and growth are phenomena which are in-
trinsically the same — both are the result of a proponderance of
the constructive phase of metabolism. The single cell, whether a
whole organism or a single unit of a complex body, increases in
volume up to a certain limit and then divides. In the former case
two new individuals replace the parent cell; in the latter, the com-
plex body has been increased to the extent of one cell. In both
cases cell division has resulted in cell reproduction. Thus cell
division is always reproduction, though it is customary and con-
venient to restrict the term reproduction to cell divisions which
result in the formation of new individuals — single cells or groups
of cells which sooner or later separate from the parent organism.
They are the new generation. This is a unique characteristic of
living things which provides for the continuation of the race.
(Figs. 8, 164.)
6. Irritability and Adaptation
The discussion of metabolism has emphasized the close interre-
lationship between the living organism and its surroundings, and
the dependence of life upon the interplay and interchange between
protoplasm and its environment. As a matter of fact the plant or
animal retains its individuality — lives — solely by its powers of
developing and maintaining exquisite adjustments to its surround-
ings. This results from the raRITABILITY of living substance: its
inherent capacity of reacting to environmental changes by changes
in the equilibrium of its matter and energy. The inciting changes,
known as STIMULI, may be chemical, electrical, thermal, photic,
THE PHYSICAL BASIS OF LIFE 27
or mechanical, but the nature of the response is determined rather
by the fundamental character of protoplasm itself than by the
nature of the stimulus. Thus muscle cells respond by contracting,
regardless of the nature of the stimulus.
The reactions of organisms usually result in MOVEMENT, one of
the most obvious manifestations of life. Movement depends in
every instance upon tumultuous ultramicroscopic physico-chemical
changes of protoplasm itself. Thus it is to these changes that, in
the last analysis, we must turn for the energy which brings about
the visible movements in animals and plants.
ay
Fic. 11. — Amoeboid movement. Successive changes in form assumed by
an Amoeba. The clear ectoplasm flows forward, followed by the granular
endoplasm with the nucleus (darker) and contractile vacuole (lighter). See
Fig. 6. (Modified, after Verworn.)
The obvious movement of the higher animals is, of course, the
result of the contraction (shortening and broadening) of the indi-
vidual muscle cells forming the muscular system, but movements
of other cells of the body occur as, for example, the flowing move-
ment of the white blood cells which is similar to that of certain
unicellular animals, the Amoebae, and so is known as AMOEBOID
movement. The protoplasm of an active Amoeba is one of the
most striking and beautiful sights under the microscope; the cell
ceaselessly changing its form as one outflowing, or PSEUDOPODIUM,
follows another and the whole cell advances in the direction of the
main stream. When particles of food are met, the pseudopodia
28 ANIMAL BIOLOGY
flow around them, and when they have been digested within the
cell, the protoplasm moves onward leaving the waste material
behind. In many other unicellular animals, such as Paramecium,
an active, internal circulation, or cycLosis, of the protoplasm is
visible, and the cells of certain multicellular plants, such as Nitella
and Tradescantia, afford remarkable exhibitions of rotation and
circulation of protoplasm.
Moreover, locomotion in Paramecium and related Infusoria is
effected by myriads of short, vibratile, thread-like prolongations
of the cytoplasm, termed crL1A; while other unicellular forms, such
as Euglena, move by long whip-like processes, or FLAGELLA. Fi-
nally, ciliated cells form membranes, or CILIATED EPITHELIA, that
serve various purposes in the bodies of higher animals. Thus the
food and respiratory currents of Molluscs, such as the Clam, are
induced by ciliary action, and certain internal passages of the
human body are provided with cilia for the transport of materials.
(Figs. 6,7, 11 22; 27)
These and all other reactions of living matter are results of its
irritability and involve not only response to a stimulus but also
conduction so that the cell, or group of cells, as a whole is directly
or indirectly influenced — a condition which attains its fullest ex-
pression in the higher animals with a nervous system. Every
organism responds as a coordinated unit — an individual. It adapts
itself structurally and functionally to the necessities of its existence.
This power of ADAPTATION, as exhibited in active adjustment be-
tween internal and external relations, overshadows every mani-
festation of life and contributes, more than any other factor, to
the enormous gap that separates even the lowest forms of life from
the inorganic world.
The characteristics which we have described — specific organiza-
tion, chemical composition, metabolism involving the power of
maintenance, growth, and reproduction, and irritability resulting
in the power of adaptation — individually and collectively are
characteristic of living matter. Any formal objections that may
be raised to the diagnostic character of one or another only serve
to emphasize the unique conditions which obtain in life.
In our discussion thus far, we have endeavored to describe the
characteristics of life in terms of its organizational units — cells,
and of its physical basis — protoplasm. But we have not attempted
THE PHYSICAL BASIS OF LIFE 29
formally to define ‘life’ or ‘protoplasm’ because, since they are
unique, it is impossible to resort to the trick of comparing them
with something else; and because the expressions ‘protoplasm’ and
‘life’ are generalizations. The former indicates that all animals
and plants have an essentially similar foundation, and the latter
that they exhibit certain characteristic actions and reactions. The
living organism exhibits a permanence and continuity of individ-
uality correlated with specific behavior, and this it transmits to
other matter which it makes a part of itself, and to its offspring at
reproduction. The organism regarded as a whole is, indeed, a
unique phenomenon: one whose fundamental nature is as essential
as any of the concepts of physics.
CHAPTER} 1.
METABOLISM OF ORGANISMS
Nature, which governs the whole, will soon change all things which
thou seest, and out of their substance will make other things ana
again other things from the substance of them, in order that the world
may be ever new. — Marcus Aurelius.
Lire is only known to us in the form of individual organisms,
so we turn now to concrete examples of unicellular plants and
animals which present, in relatively simple form within the con-
fines of a cell, the essentials of all the fundamental life processes,
many of which we shall later have occasion to study in their com-
plex expressions in the higher animals. Our present interest in
these simple forms is chiefly to illustrate the complex nutritional
interdependence of three great groups of organisms — green plants,
animals, and colorless plants. Animals cannot exist without plants.
A. GREEN PLANTS
Unicellular green plants are distributed all over the world and
adapted to a great variety of conditions. We find them, for ex-
ample, forming green coatings on the bark of trees, scums on
puddles and ponds, or being blown about in dust by wind. Of the
many hundreds of species we select Protococcus vulgaris because
of the simplicity of its structure and life history, and because it is
readily obtained for study.
1. Protococcus
A single Protococcus is invisible to the naked eye, but like many
another microscopic form, it makes up in numbers for the small
size of the individual, and frequently gives a greenish color to moist
surfaces of rocks, tree trunks, fence posts, and flower pots.
Examined under the microscope, the organism is seen to consist
of a spherical mass of protoplasm with a nucleus centrally located.
Most of the cytoplasm surrounding the nucleus is specialized to
form a PLASTID which contains a green pigment. The whole organ-
ism is enclosed within a distinct, rigid cell wall which has been
30
METABOLISM OF ORGANISMS 31
secreted by the protoplasm and is composed of CELLULOSE: a
carbohydrate especially characteristic of plants. It is evident that
the organism is a single cell. (Fig. 12.)
Since Protococcus is a single cell, we find reproduction pre-
sented in its simplest terms: under favorable conditions the cell
divides and the two resultant cells sooner or later separate. Some-
times, however, several divisions occur before the cells are sep-
arated and thus there is formed, as it were, a temporary multi-
Fic. 12. — Protococcus vulgaris, a unicellular green plant. Separate indi-
vidual and temporary cell groups formed by cell division.
cellular body, but one without any physiological division of labor
between the cells because all are independent in their vital ac-
tivities. Moreover, this tendency to form temporary cell groups
suggests the type of step that was taken when the multicellular
body was established during plant evolution.
We may now turn our attention to the point Protococcus was
chosen especially to illustrate — the characteristic life processes
of green plants. At first glance it may appear that a multicellular
plant, such as a tree or shrub, would afford a more suitable ex-
ample, but since the fundamental distinction between plants and
animals is chiefly a question of metabolism, there are advantages
in studying it in a single cell, where one’s attention is not dis-
tracted by root, stem, and leaf.
2. Food Making
Since Protococcus lives, grows, and multiplies in moisture ex-
posed to sunlight, it is to this environment that we must look for
the materials with which it constructs protoplasm, and the energy
which it employs in the process. And, furthermore, since the organ-
ism is enclosed within a cell wall, its income and outgo must be ma-
terials in solution in order to pass through to the living protoplasm.
In short, Protococcus takes materials from its surroundings in
the form of simple compounds, as carbon dioxide, water, and
mineral salts, which are relatively stable and therefore practically
32 ANIMAL BIOLOGY
devoid of energy, and, through the radiant energy of sunlight,
shifts and recombines their elements in such a way that products
rich in potential energy result. Protococcus thus exhibits the
prime distinguishing characteristic of green plants — the power
to construct its own foodstuffs.
The key to this power of chemical synthesis by light — pHoto-
SYNTHESIS — resides in a complex chemical substance called
CHLOROPHYLL which consists of two very similar but distinct pig-
ments. Chlorophyll is segregated in special cytoplasmic bodies,
the plastids, and gives to Protococcus during its active phases
and to the foliage of plants in general their characteristic green
color. Plastids bearing chlorophyll are known as CHLOROPLASTS.
The chlorophyll arrests and transforms a small part of the energy
of the sunlight which reaches it, in such a way that the protoplasm
can employ this energy for food synthesis.
The first great step in the constructive process is a combination
of carbon with hydrogen and oxygen to form a carbohydrate.
Protococcus gets these elements from carbon dioxide and water
by a process of molecular disruption. We know that when char-
coal, for instance, is burned, carbon and oxygen unite to form |
carbon dioxide, and energy in the form of light and heat is liber-
ated. Obviously Protococcus must employ an equal amount of
energy in separating the carbon and oxygen of carbon dioxide;
that is, in overcoming their chemical affinity. And this kinetic
energy which the plant employs is then represented in the chemical
potential which exists between the oxidizable carbon and free
oxygen — it has become potential energy. Thus the plant in sun-
light is continually separating the carbon from the oxygen of carbon
dioxide. The oxygen is liberated as free oxygen, while the carbon
which has been separated from the oxygen is combined with mole-
cules of water to form a carbohydrate — grape sugar (glucose). —
The conventional equation for this reaction is:
6 CO, a 6 H.O = C.Hi20¢ — 6 Oo
(carbon dioxide) (water) (glucose) (free oxygen)
However, the processes involved are by no means so simple as is
implied above. It is probable that a relatively simple compound,
such as formaldehyde (CH,O), is first produced from carbon
dioxide and water, and that molecules of this substance are then
united to form glucose. Although there is little conclusive data in
.
METABOLISM OF ORGANISMS 33
regard to the details of the intermediate stages, the equation stated
affords a satisfactory expression of the end result of photosynthesis
that is adequate for the present discussion.
The first great step in food synthesis, the formation of a sugar,
having been accomplished, the green plant usually transforms the
sugar and stores it as starch for future use as fuel or as the basis of
further synthesis. Starch is the first visible product of photo-
synthesis.
We have seen that the chief characteristic of proteins as com-
pared with carbohydrates (sugars, starches) is the presence of
nitrogen, and this element must be added to the CHO basis already
constructed as the next step toward protein synthesis. The green
plant not only can, but must employ nitrogen in simple combina-
tions, chiefly nitrates, and this is a fact of prime importance, for
typically, as will appear later, animals and most colorless plants
require nitrogen in more complex combinations. Thus by the
addition of nitrogen to the carbohydrate basis relatively simple
nitrogenous compounds, amino acids, are built, which in turn form
the foundation for the synthesis of the immense variety of proteins.
Little is known of the complex chemical processes involved in
protein construction, and nothing about the actual incorporation
of the proteins as an intrinsic part of the architecture of the living
matter itself. But it is evident that synthesizing enzymes play a
crucial rdle. These are special proteins which are known only as
products of living protoplasm and are the activating agents
(catalytic agents) for chemical transformations in which, however,
they themselves take no integral part.
Protococcus thus takes the raw elements, so to speak, of living
matter and by the radiant energy of sunlight, which its chlorophyll
traps, constructs carbohydrate, protein, protoplasm. In other
words, the green plant is a synthesizing agent, building up highly
complex and unstable molecular aggregates brimming over with
the energy received from the Sun. So the green plant, whether
Protococcus or Elm, by this AuroTropHIc method manufactures
its own food for itself — and incidentally, as it were, for the living
world in general, including Man.
3. Respiration
As we have already stated, protoplasm is always at work — to
live is to work — and this means expenditure of energy: the same
34 ANIMAL BIOLOGY
energy that chlorophyll has secured for the plant and stored away
in its food. Therefore the food must be oxidized — burned — in
order to release the energy, and for this the plant must have
available a supply of free oxygen. Protococcus obtains this
oxygen dissolved in water and also, in sunlight, from that liberated
through photosynthesis. The process involved, for the sake of
simplicity, may be represented by the equation:
CeH120¢ + 6 O- = 6 CO. 4 6 H.O
which, it will be noted, is the reverse of the equation for photo-
synthesis. This intake of free oxygen by the cell and outgo of
carbon dioxide and water, the chief products of combustion, is
known aS RESPIRATION. It is essentially the securing of energy from
food, involving the exchange of carbon dioxide for oxygen by
protoplasm. And this interchange of gases between the living
matter and its surroundings is not only characteristic of Proto-
coccus and all green plants, but of all living things. Plants respire _
just as truly as animals, though the more active life and complex
bodies of most of the latter require an elaborate respiratory
apparatus in order that an adequate gaseous interchange may be
effected with the necessary rapidity.
Thus the green plant may be regarded as a chemical machine
for the transformation of energy — the radiant energy from the
Sun — into lifework; the matter and energy which enters, forms,
and leaves the organism obeying, to the best of our knowledge, the
fundamental laws of matter and energy of the non-living world.
We have now obtained some idea of one living organism, Proto-
coccus vulgaris, a green plant reduced to the simplest terms — a
single cell provided with chlorophyll. And we have seen that this
chlorophyll is the key to the photosynthetic activity of the green
plant. In other words, the expression ‘green plant’ does not refer
specifically to the color of a plant (in some cases it may appear
red or brown), but to the fact that there is present a complex pig-
ment functionally similar to chlorophyll by virtue of which the
plant is a constructive agent in nature. It has the power to manu-
facture its own foodstuffs from relatively simple compounds largely
devoid of energy and, in particular, is able to utilize nitrogen in the
form of nitrates.
We pass now from the essential constructive agents in nature
METABOLISM OF ORGANISMS 39
to the destructive agents; from the collectors of energy to the
energy dissipators; from the green plants to animals and then to
‘colorless’ plants.
B. ANIMALS
There is probably no better introduction to the study of the
biology of an animal than that afforded by an Amoeba such as
Amoeba proteus, a common organism of ponds, ditches, and de-
caying vegetable infusions. Amoebae, frequently referred to as
the simplest animals, are representatives of the great group of
single-celled animals, or Protozoa. Members of this group are
found in almost every niche in nature and, like the Protophyta,
as the unicellular plants are sometimes called, are important be-
cause, although small in size, the number of individuals is incon-
ceivably large. Collectively they produce profound changes in
their environment.
1. Amoeba
In order to study Amoeba it is necessary to magnify it several
hundred times. This done, it appears as a more or less irregular
mass of granular jelly-like ma-
terial, rather slowly changing its
shape and thereby moving along.
As a matter of fact it is essen-
tially a naked bit of protoplasm,
without obviously specialized Gastric vacuole
parts. However, careful study
reveals that the organism really
consists of a single protoplasmic mm ES
unit differentiated into cyto- &)\ C8
plasm and nucleus —it is a i ;
cell: an animal. (Fig. 11.) S
But there are no specialized Contractile
locomotor organs—merely now V@c¥le
and again the clear outer layer of
protoplasm, or ECTOPLASM, flows
out, followed by the internal granular ENDOPLASM, so that a pro-
jection, or PSEUDOPODIUM, is formed. ‘There is no permanent
mouth, food being engulfed by the protoplasm flowing about it
as opportunity offers. There is no permanent digestive or excre-
tory apparatus. (Figs. 6, 13.)
Pseudopodium —_____
Ectoplasm (
Endoplasm
Nucleus
Fic. 13. — Amoeba proteus.
36 ANIMAL BIOLOGY
Amoeba, under favorable conditions, grows rapidly and, when
it has attained the size limit characteristic of the species, cell
division, termed BINARY FISSION, takes place, with the result that
from the single large cell there are formed two smaller individuals
which soon become complete in all respects. These, in turn, grow
and repeat the process so that, as in the case of Protococcus,
within a few days the original Amoeba has divided its individual-
ity, so to speak, among a multitude of descendants. (Fig. 8.)
Clearly Amoeba performs all the essential vital functions that
become an animal. Such being so, it is important to compare the
metabolism of Amoeba, the animal, with that of Protococcus,
the green plant.
2. Food Taking
The food of Amoeba is chiefly other microscopic animals and
plants that it meets in its environment of pond water or vegetable
infusion. Coming in contact with its prey, pseudopodia are extended
about it and soon the prospective food material is enclosed with
the endoplasm of its captor. Here the food is surrounded by a
droplet of fluid, a GASTRIC VACUOLE, into which the endoplasm se-
cretes chemical substances (enzymes, etc.) which gradually simplify
— digest — the complex proteins, carbohydrates, etc., of the food.
Finally, this material, which shortly before was the protoplasm of
another organism, is incorporated into Amoeba protoplasm —
matter and energy is supplied and the animal lives and grows.
This is, in most respects, a strikingly different condition from
that which we have seen in Protococcus. In Amoeba solid par-
ticles of food — tiny animals and plants — are taken into the
cell, and since the chief organic constituents of protoplasm are
proteins, associated with carbohydrates and fats, it is clear that
the income of the animal organism is, unlike that of the green
plant, chiefly ready-made complex foodstuffs. In other words,
Amoeba, like all animals, requires relatively complex chemical
compounds rich in potential energy: proteins, carbohydrates, and
fats. Of these, proteins or their constituent amino acids are ab-
solutely indispensable because it is only from this source that
nitrogen is available for the animal. But the green plant, through
its chlorophyll apparatus, is able to take materials largely devoid
of energy and to rearrange them and endow them with potential
energy which it has received in the kinetic form from sunlight.
METABOLISM OF ORGANISMS 37
3. Respiration and Excretion
Of course, during life the Amoeba, like Pleurococcus, is con-
tinually breaking down its food and its own protoplasm by a proc-
ess of combustion which involves an intake of free oxygen and
the liberation of carbon dioxide and water. Nitrogenous wastes,
chiefly URIC ACID or UREA, as well as inorganic salts are also ex-
creted. This interchange takes place over the entire surface of
the animal, aided by a CONTRACTILE VACUOLE that expels fluid
from the cell. So the animal, like the plant, returns to its environ-
ment the elements in simple combinations which are devoid or
nearly devoid of energy. We have stated that green plants are
essentially constructive and animals destructive agents in nature.
It is apparent, of course, that green plants are both constructive
and destructive, but the constructive processes of green plants are
necessary and sufficient not only for themselves but for all living
things.
A little consideration of the income and outgo of green plants
and animals will show that, although the animals are dependent
on the plants for their complex foodstuffs, they do not return, for
example, the nitrogen to the outer world in a form simple enough
to be available for green plants. For example urea, (NH2)2CO,
which still has a little energy left that the animal is unable to
extract, must be transformed into nitrates.
Furthermore, since plants die, which are not consumed by ani-
mals, and animals die, which are not devoured by other animals,
large stores of matter and energy are locked up in the complex
compounds of their dead tissues. Clearly, there must be some
way of completing the cycle of the elements, for if there were not,
life, as we know it, could not have continued long on the Earth.
This gap is filled by the so-called coLORLESS PLANTS; that is, plants
which, because chlorophyll is not present, lack the power of photo-
synthesis and so in most cases are dependent for food on more
complex substances than green plants demand, though not so
complex as animals require.
C. CoLorLeEss PLANTS
As representative of the diverse types of colorless plants which,
lacking chlorophyll or a functionally similar pigment, are without
the power of photosynthesis, we select the vast group known as
38 ANIMAL BIOLOGY
the Bacteria. For reasons that will appear later, it is not practical
to focus attention on one particular species of Bacteria, as we have
just done in considering green plants and animals. Instead we
shall discuss in very general terms the group as a whole, referring
now and then to special kinds of Bacteria to illustrate particular
points.
1. The Bacteria
The wide distribution of the Protozoa is exceeded by the Bac-
teria. Representatives are literally found everywhere: floating
with dust particles in the air, in salt and fresh water, in the water
of hot springs, frozen in ice, in the upper layers of the soil, and in
the bodies of plants and animals. Bacteria have received a con-
siderable notoriety under the names of ‘microbes’ and ‘germs,’
owing to the fact that certain types subsist within the human
body as parasites and bring about disturbances, chiefly chemical,
which we interpret as disease. But aside from these forms which,
though all too many, are relatively few in number, human life
and life in general on the Earth could not long continue without
their services. Indeed, the Bacteria have practically ruled the
world since they first secured a foothold when the Earth was young.
It is this aspect of the Bacteria which concerns us at present.
Among the Bacteria are the smallest organisms known. Some
species are less than one fifty-thousandth of an inch in length and
much less in breadth. None of the typical forms comes within
the range of unaided vision, while some are not revealed by the
highest powers of the microscope — indeed there is room and to
spare for thousands of millions of Bacteria to live in a thimblefull
of sour milk. The small size and similarity of structure of many
of the Bacteria render their study particularly difficult, and accord-
ingly they are grouped and classified largely on the basis of chem-
ical changes which they produce in their surroundings, rather
than on structural characteristics. However, there are three chief
morphological types: the rod-like forms or BACILLI: the spherical
forms or coccr; and the spiral forms or sprrmLuaA. Either bacilli
or cocci may be associated in linear, branching, or plate-like series,
or grouped together in colonies. (Fig. 14.)
The individual bacterium is regarded as a single cell, though in
most species there is no definite nuclear body; the chromatin
material being distributed in the form of granules throughout the
METABOLISM OF ORGANISMS 39
cytoplasm. A cell wall chemically similar to protein is usually
present. Some forms show active movements by means of thread-
like prolongations of the cytoplasm, or FLAGELLA, as in the case
of the common Spirillum of decaying vegetable infusions.
Reproduction is by a process of cell division which, under very
favorable conditions, may occur as often as every fifteen minutes.
Fic. 14. — Chief types of Bacteria. A, cocci; B, bacilli: C, spirilla.
The vast multitude of cells thus produced before long exhaust the
food supply and contaminate with excretion products the medium
in which they are living, so that further growth is inhibited. In
many species, under these circumstances the protoplasm within
the cell wall assumes a spherical form and secretes a protecting
coat about itself, and thus enters upon a resting state. In this
SPORE form the Bacteria can withstand drying, variations in tem-
perature, and other conditions —in certain cases even strong
carbolic acid — to which in the active state they would readily
succumb, and thereby the organisms tide over periods of unfavor-
able conditions and are ready to start active life again when the
opportunity occurs. It is certainly fortunate for Man that the
great majority of disease-producing Bacteria are unable to form
spores. (Fig. 200.)
2. Cycle of the Elements in Nature
We have seen that carbon dioxide is the source from which
green plants derive the carbon which they synthesize into carbo-
hydrates, fats, and proteins. Animals directly or indirectly feed on
plants, so that the ultimate source of the carbon of animals is
likewise the carbon dioxide of the atmosphere. Although both
plants and animals by their respiratory process are continually
40 ANIMAL BIOLOGY
returning to the outer world some of this carbon as carbon dioxide,
it is evident that relatively enormous amounts of carbon are ney-
ertheless being taken out of circulation and locked up in the bodies
of the plants and animals. For example, it has been estimated
that about one-half the weight of a dried tree trunk is contributed
by carbon.
The same general segregation is going on in regard to nitrogen.
The green plants take it in the form of nitrates, for instance, and
store it away in the proteins; and again animals get their nitrogen
from plant proteins, so that the ultimate source of the animal
nitrogen is the same. In a somewhat similar manner we might
trace the fate of the other chemical elements necessary for proto-
plasm, but that of carbon and nitrogen is particularly striking and
Dead
Organisms
ivi Bacterial
Living
Animals Decay
Carbohydrates,
ee Sa Fermentation
in Green Plants sad pm aan
Respiration
Intermediate
Decomposition
Fermentation Atmosphere Products
and Plant
Living Bare
es eS
eg
Fic. 15. — The Carbon Cycle. A schematic representation of the circulation
of carbon in nature.
instructive, and is sufficient to illustrate the fact that although
both green plants and animals are continually taking elements
from and returning them to their environment, nevertheless more
is taken away than is returned. (Figs. 15, 16.)
The agents which restore to the inorganic world the elements
removed by green plants and animals are the colorless plants, such
as the Bacteria, Molds, Yeasts, etc. As we know, when an animal
or plant dies, DECAY sets in almost immediately; that is, the com-
plex chemical compounds are slowly but surely reduced to simpler
METABOLISM OF ORGANISMS 4]
and simpler forms until ‘dust’ remains. Although undoubtedly
many of these compounds would automatically, so to speak, tend
to simplify, nevertheless this is not only hastened, but chiefly
carried out by organisms of decay such as the Bacteria. Through
Animal
Proteins
Bacterial
Decay
Proteins
of Green Plants
Nitrates Nitrate Nitrite Ammonia
N-Fixing Atmosphere Denitrifying
Bacteria Y Bacteria
N
Fic. 16.— The Nitrogen Cycle. A schematic representation of the circulation
of nitrogen in nature.
enzymes, or ferments, which they form, FERMENTATION occurs.
The carbohydrates and fats are resolved into carbon dioxide and
water, and the proteins are reduced to carbon dioxide, water, and
ammonia (NH;) or free nitrogen, while the nitrogenous waste
(urea, etc.) of animals is similarly broken down. (Fig. 17.)
Cc D
Fic. 17. — Yeast, a unicellular colorless plant, very highly magnified. A, cell
showing granular cytoplasm and a large vacuole; B, showing nucleus; C, cell
budding; D, mother cell and bud after division is completed.
Practically all of these long series of chemical reactions are
carried on by different kinds of Bacteria. Most green plants, how-
ever, take their nitrogen chiefly in the form of nitrates, and ac-
cordingly we find that another type of Bacteria (Nirrire Bac-
42 ANIMAL BIOLOGY
TERIA) acts upon the ammonia and transforms it into nitrous acid
(HNO,). After certain chemical reactions in the soil, forming, e.g.,
potassium nitrite or ammonium nitrite, still another type (NITRATE
BacrerRIA) oxidizes the nitrites into nitrates (e.g., KNO3 or
NH,NOQs), so that again this nitrogen is in a form which is avail-
able for green plants.
But, still confining our attention to the nitrogen, it is obvious
that there is a leak from this cycle, since some of the nitrogen in
the form of ammonia or free nitrogen escapes to the atmosphere.
The greatest loss, however, is brought about by a group of DENT-
TRIFYING BACTERIA whose activities are largely spent in changing
nitrates into gaseous nitrogen which escapes into the air, and so
is placed beyond the reach of green plants and animals. But
fortunately there are many kinds of NrirROGEN-FIXING BACTERIA
which rescue the nitrogen from the atmosphere and return it to
the cycle of elements in living nature. These organisms are widely
distributed, some living freely in the soil and others in tiny nodules
which they produce on the rootlets of higher plants, such as Beans,
Clover, and Alfalfa; and this accounts for the fact, long known
but not understood, that these plants when plowed under are
particularly efficient in enriching the soil.
In brief, there is a cycle of the elements in nature through green
plants and animals and back again to the inorganic world through
the Bacteria and other colorless plants. Such is the reciprocal
nature of the nutritive processes of living organisms.
It is hardly necessary to state that the chemical changes pro-
duced by the Bacteria are either the direct results of, or are in-
cidental to the process of nutrition in these organisms. Therefore
the material taken as food by certain groups is relatively complex:
for example, by those which bring about the early putrefactive
changes in proteins; while that employed by others is very simple
since they find adequate chemical combinations less complex than
those needed by green plants. Indeed, certain Bacteria are able to
utilize carbon dioxide and water just as do green plants, but instead
of obtaining energy for the synthesis from sunlight, these auTo-
TROPHIC forms derive it from chemical energy liberated by the
oxidation of inorganic substances in their environment. Such a
process possibly represents the most primitive method of nutri-
tion from which all the others have been derived in the evolution
of life. |
METABOLISM OF ORGANISMS 43
3. The Hay Infusion Microcosm
The importance of the complex nutritional interdependence of
organisms in general as well as the cosmical function of green
plants — the link they supply in the circulation of the elements
in nature — may be emphasized and summarized by a brief de-
scription of a ‘hay infusion.’
Probably nowhere is the ‘web of life’ more conveniently or
convincingly exhibited than in the kaleidoscopic sequence of events
— physical, chemical, and biological — which are initiated when
a few wisps of hay are added to a beaker of water. Apparently
the chief components of a hay infusion are hay and water, but
these merely supply the matter and energy for the interplay of
various forms of life. Most of these are beyond the scope of un-
aided vision though chiefly responsible for the obvious changes
which occur from day to day in their environment.
Ordinary tapwater, for instance, contains free oxygen and vari-
ous inorganic salts in solution, and not infrequently different
species of Bacteria, unicellular green plants, and Protozoa. The
hay soaking in the water contributes soluble salts, carbohydrates,
proteins, etc. It also supplies many microscopic animals and plants
which have adhered to it in dormant form and are only awaiting
suitable surroundings to assume active life again. (Fig. 18.)
A microscopical examination of an infusion when it is first made
shows very few active organisms, but within a day or so, depend-
ing largely on the temperature, it reveals countless numbers of
Bacteria which have arisen by division from the relatively small
number of dormant and active specimens originally present. At
first the Bacteria are fairly evenly distributed in the infusion, but
as conditions change, largely through the chemical and physical
transformations which they themselves bring about, those species
which can employ oxygen in combined form (that is, in chemical
compounds) find existence possible and competition less keen at
the bottom of the beaker, while those types of Bacteria which
are dependent upon free oxygen gather nearer the surface
where the supply is being replenished constantly from the
atmosphere.
Up to this point the life of our microcosm is largely bacterial —
unicellular sAPpROPHYTIC plants which employ as food the complex
decomposition products of the proteins, etc., of the hay. The proc-
AA ANIMAL BIOLOGY
ess is essentially destructive and the simplified products are rep-
resented in the relatively simple excretions of thé Bacteria.
But during bacterial ascendancy another factor has been grad-
ually intruding itself almost imperceptibly into the drama. This
is the microscopic animal life which has been multiplying with
increasing rapidity as conditions became more favorable, and
forthwith assumes the dominant life phase in the infusion. Among
the animal forms, the first to appear are exceedingly minute
flagellated Protozoa, known as Monads, many species of which
absorb products of organic disintegration brought about by the
Bacteria, while others exhibit HoLozorIc nutrition, ingest solid
Myoneme fibres
Myoneme of stalk
Fic. 18. — Some types of Protozoa found in infusions. A, two species of
flagellated Monads (Mastigophora); B, Colpoda, a small Ciliate (Infusoria) ;
C, Vorticella, one of the most complex Ciliates.
food — the Bacteria themselves. Then tiny ciliated Protozoa,
probably Colpidium and Colpoda, appear in untold numbers
and feed upon the Bacteria. The dominance of these smaller
Ciliates is brought to an end after a few days by the ascendency
of larger Ciliates which, though feeding to a certain extent upon
the already greatly depleted bacterial population, obtain most
of their food by eating the smaller Ciliates. And so the cycle of
life continues — saprophytic forms gradually being replaced in
dominance by herbivorous and these in turn by carnivorous or-
ganisms. In truth, nothing lives or dies to itself. (Figs. 18,
23, 26.)
METABOLISM OF ORGANISMS 45
But obviously this chain of events must sooner or later come to
an end through the dissipation of energy brought about by the
metabolic processes of the colorless plants and animals. Sooner or
later the supply of potential energy stored up in the chemical
compounds of the hay will have become nearly or completely ex-
hausted — transformed into the kinetic form and expressed in
the life activities of the plant and animal population.
Thus, after a few weeks, the hay infusion world has reached a
standstill — extermination faces the population and inevitably
occurs unless microscopic green plants, close relatives of Proto-
coccus, find opportunity to develop in the energy-exhausted en-
vironment and proceed to entrap the kinetic energy of sunlight,
store it up in carbohydrates and proteins, and thus restore energy
in the potential form to the hay infusion.
If such occurs, the hay infusion werld is a microcosm indeed
— green plants, colorless plants, and animals gradually become
reciprocally adjusted so that a self-perpetuating condition of
practically stable equilibrium is established; in other words, what
is termed a ‘balanced aquarium.’ The rise and fall of teeming
populations, made possible by the rapidly changing environmental
conditions which the bringing together of hay and water initiated,
is replaced by an apparently harmonious interdependence of or-
ganisms demanding different food conditions, such as we are
familiar with in the world at large.
CHAPTER V
SURVEY OF UNICELLULAR ANIMALS
The most important discoveries of the laws, methods, and progress
of Nature have nearly always sprung from the examination of the
smallest objects which she contains, and from apparently the most
insignificant enquiries. — Lamarck.
THE invention of aids to the human senses, in particular the
microscope, has made us aware of a “world of the infinitely little,”
whose representatives — Protococcus, Bacteria, and Amoeba —
have been used in our study of the nutritional interdependence of
organisms: animals cannot live without plants. We turn now to
a brief survey of the close allies of Amoeba in the great group of
unicellular animals, the Protozoa.
The Protozoa are the simplest forms of animal life, each in-
dividual comprising typically but a single unit of living matter,
a cell. But it does not follow that they are devoid of complex
organization. Indeed some Protozoa exhibit a complexity of struc-
ture within the confines of a single cell that is not exceeded, per-
haps not equalled, in the cells of higher animals. The Protozoa
are the simplest, but by no means simple animals.
Because these microscopic forms afford the nearest available
approach to primitive animal life, innumerable studies on their
physiology have been made in the hope that they would more
readily supply the key to life processes — e.g., nutrition, growth,
reproduction, sex — that find such complex and bewildering ex-
pression in higher animals. Furthermore, not a few of the Protozoa
live as parasites in Man and beast and cause some of the most
serious diseases and greatest economic loss. So on both the theo-
retical and practical side, PROTOZOOLOGY claims attention.
In any survey of animals or plants, first of all it is necessary to
arrange the various kinds, or SPECIES, in some order — to classify
them. To place nearer together those that appear related in struc-
ture and function and separate farther those that seem to be less
related. And if relationship is the basis of classification, we should
46
SURVEY OF UNICELLULAR ANIMALS 47
know what the biologist means by relationship. The answer is:
‘blood relationship,’ descent with change from a common an-
cestor — organic evolution. But this large subject must be left
until much later in our study; merely stating now that although
the object of a natural classification is to express the pedigrees of
the organisms, most classifications are, at best, still very far from
realizing this ideal. With probably a million known species of
animals on the Earth to-day, classification is a stupendous problem;
it is not a small problem even with the fifteen thousand known
species of Protozoa. For our purpose, however, classification can
be reduced to a minimum — to a mere skeleton outline, — to facili-
tate a synoptic view of the diversities of animals. (See page 352
and Fig. 313.)
The first great subdivision, or PHYLUM, of the Animal Kingdom
is the Protozoa. All of the Protozoa, since they are single cells,
demand for active life a more or less fluid medium, and are typi-
cally aquatic animals. However, different species exhibit all
gradations of adaptation to variations in moisture from those that
thrive in oceans and lakes, or pools and puddles, to those which
find sufficient the dew on soil or grass blade, or the fluids within
the tissues and cells of higher animals and plants.
The phylum Protozoa is divided, largely on the basis of the
locomotor organs, into four chief groups, or CLASSES: the Sar-
codina, Mastigophora, Sporozoa, and Infusoria. In general, we
may regard the Sarcopina as forms, like Amoeba, that move
about by means of pseudopodia; the MasticopHora as cells with
flagella as locomotive organs, such as Euglena; and the [NrusoRIA
as organisms, like Paramecium, that swim by cilia. The Sporozoa,
all of which are parasitic, such as the organisms causing malaria,
possess no characteristic type of organ for locomotion though
many are motile. (Figs. 13, 22, 27, 223.)
A. SARCODINA
Amoeba, which we have already studied, may be visualized as
the type of the Sarcodina since all members of this class are,
broadly speaking, ‘amoebae,’ but there are numerous species of
the Genus Amoeba itself. The common, relatively large fresh-
water species, usually called Amoeba proteus, has as associates
many other fresh-water and some salt-water species. Again, numer-
48 ANIMAL BIOLOGY
ous species of Amoeba, such as those comprising the genus End-
amoeba, live within the bodies of Man and other animals. Fur-
7
Fic. 19. — A, Arcella showing the protopiasm through the transparent shell
and also protruding as pseudopodia; B, Difflugia showing the same.
thermore, there are many com-
mon fresh-water genera, such as
Arcella and Difflugia, that have
resistant protective coverings,
or shells. The shells have an
opening through which the
pseudopodia are protruded so
that locomotion, securing of
food, etc., can be performed.
But all of these animals, whether
free-living or parasitic, naked
or provided with a shell, are
creeping cells with more or less
broad or blunt finger-form pseu-
dopodia and comprise the first
ORDER of the Sarcodina known
as the AMOEBAEA. (Figs. 6, 11,
19, 245.)
An immense group of chiefly
marine Protozoa, the Foramt-
NIFERA, constitutes the second
order of the Sarcodina. Most
species have quite complex
shells of calcium carbonate, with
one or many openings through
which delicate pseudopodia
emerge, branch, and flow to-
Fic. 20.— Allogromia, one of the
few fresh-water Foraminifera. Note
the pseudopodial mass emerging from,
and surrounding the shell. (Modified,
after Cambridge Natural History.)
SURVEY OF UNICELLULAR ANIMALS 49
gether so that the shell becomes essentially an internal struc-
ture. The almost incalculable number of Foraminifera constitutes
an important source of marine food for small animals which,
in turn, are the food of economically important fishes. The more
resistant Foraminifera shells sinking to the sea-bottom cover vast
areas with the so-called Globigerina ooze, accumulation of which
in the geologic past is evidenced to-day by the chalk cliffs of Eng-
land. The Pyramids and the Sphinx are built of Foraminiferous
rock. (Figs. 20, 244.)
The final two orders of the Sarcodina, known as the Hetiozoa
and RapioLariA, are characterized by unbranched, radiating
pseudopodia, each supported by a core of more dense protoplasm.
Most species are floating, spherical forms and the pseudopodia,
Fic. 21. — A, Heliozoon, Actinophrys sol; B, Radiolarian, Thalassicolla
nucleata. (From Kudo.)
protruding from the entire surface of the cell, give the appearance
of the conventional figure of the Sun. Accordingly the fresh-water
forms are commonly called Sun Animalcules, or Heliozoa. The
Radiolaria are all marine and are more complex than the Heliozoa,
the protoplasm being differentiated into several layers, enclosing
a dense, perforated CENTRAL CAPSULE, and usually supported by
an elaborate skeleton of silica. The Radiolaria vie in numbers and
importance with the Foraminifera in the economy of the sea; the
deposits of their skeletons forming the Radiolarian ooze. Cer-
tain rock strata hundreds of feet thick are contributions to the
Karth’s surface made by the Radiolaria during bygone ages.
(Fig. 21.)
50 ANIMAL BIOLOGY
B. MASTIGOPHORA
The popular phrase “‘from Amoeba to Man” would lead one to
believe that Amoeba and its allies clearly represent the lowest
Stigma
Contractile
vacuole
Reservoir-+
Chromato-
phores
Fic. 22. — Euglena viridis. A, free-swimming individual showing details of
structure (from Doflein); B, reproduction by longitudinal binary fission;
C, two stages of fission within a cyst.
sroup of Protozoa. However, biologists are not sure that such is
the case because the great group of flagellated Protozoa, the class
Mastigophora, includes many forms that are not only exceedingly
simple in structure but also are plant-like in that they possess
chlorophyll and so are able to perform photosynthesis. For in-
SURVEY OF UNICELLULAR ANIMALS 51
stance, Euglena is an organism that is claimed by both zodlogists
and botanists: by the former because it can employ complex food
materials, and by the latter because in the presence of sunlight it
can manufacture its own food. Such forms attest the fact that it
is impossible to distinguish sharply the so-called animal and plant
kingdoms — one merges into the other when the primitive forms
of life are approached. Our classifications fail at the lowest level
H BCS . ‘ :
Trachelomonas Phacus Monosiga Trichomonas Cercomonas
Fic. 23. — Common flagellated Protozoa (Mastigophora).
of life: all living nature is one. Moreover it is not possible to draw
a hard and fast line between the Mastigophora and the Sarcodina
because certain organisms possess both flagella and pseudopodia
during various phases of their life. (Figs. 18, 22, 23.)
The Mastigophora are very widely distributed in sea, pond, and
infusions of organic matter. An immense order, the DINOFLAGEL-
LIDA, constitutes not a small part of the
microscopic life of the sea, competing
with the Sarcodina and microscopic
plants in variety of species and number
of individuals — numbers so immense
that wide areas of the sea may become
discolored or appear phosphorescent.
Others, such as the TRYPANOSOMEs, have
invaded the field of parasitism, living in
the digestive tract and blood stream of
higher animals. (Figs. 24, 224.)
The versatility of the Mastigophora in
mode of life and nutrition apparently im-
plies a high potential of adaptation and evolution. Indeed, it is
among them that certain interesting associations, or COLONIES, of
Fic. 24. — A Dinoflagellate,
Gymnodinium.
22 ANIMAL BIOLOGY
individuals occur that suggest a possible method of origin of the
multicellular body of higher animals. (Figs. 29, 30.)
C. SPOROZOA
Although parasitic species are found in all four classes of Pro-
tozoa, the Sporozoa have the distinction of living solely at the
Fic. 25. — Life cycle of a Sporozoon, Monocystis. A, spore consisting of a
spore case enclosing eight sporozoites; B, transverse section of same; C and D,
liberated sporozoites; E, sporozoite after entering “sperm-sphere’ of Earth-
worm; F and G, growth until fully developed trophozoite is formed surrounded
by the degenerate remains of sperm-sphere with flagella of sperm; H, two
trophozoites that have become free from degenerate sperm-sphere and united
as gametocytes; I, encystment of gametocytes; J, division of nuclei and cyto-
plasm to form gametes; K, union of the gametes to form zygotes, residual
cytoplasm of gametocytes in center of cyst; L, cyst containing many sporo-
zoites formed by secretion of a spindle-shaped spore case around each zygote
which then divides to form eight sporozoites. These become arranged as in A®
and are ready to be transferred to another host. (From Curtis and Guthrie.)
expense of other organisms, their HosTs, in which they create dis-
turbances of more or less severity which frequently result in dis-
ease. The successful attempt to get a living with little expenditure
SURVEY OF UNICELLULAR ANIMALS 53
of energy has in the Sporozoa, as elsewhere in the Animal Kingdom,
resulted in a degeneration of structures necessary for a free life,
and an elaboration of the reproductive processes to ensure that
the parasite secures access to the proper host. For, in general,
each Sporozodn is adapted to live in one (or two) particular
species of animal—indeed probably every species of higher
animal has at least one Sporozoon specially fitted to live at its
expense.
Monocystis is a common Sporozoén that spends its entire life
in the body of an Earthworm. The ‘adult’ Monocystis is an
elongated cell living in a part of the reproductive system (seminal
vesicles) and securing its nourishment from the developing germ
cells of the worm. Here food is plentiful, and accordingly much is
stored for use during the complex reproductive changes which
terminate in the production of resistant spores. These eventually
are discharged from the body of the worm, and trust to chance
that entrance may be gained to the body of another worm in order
that the life cycle may be repeated. (Fig. 25.)
Malarial fevers would seem to be of more importance than dis-
eases of the Earthworm, though our knowledge of the Sporozoa
that are responsible for these human maladies has been made
possible by basic studies on the Protozoa as a whole. Malaria is
transmitted to Man solely by diseased female Mosquitoes that
inoculate into the blood stream a Sporozodn of the genus Plas-
modium. Once in the human blood, the parasite enters a red blood
cell and begins its complicated life history. Multiplication of the
parasite until thousands are present results in the periodic libera-
tion of poisonous material in the blood which produces the alter-
nate chills and fever. In order to complete its life history the
parasite must again be taken into the body of a Mosquito by the
latter biting an infected person. (Figs. 223, 246, 247.)
D. INFUSORIA
The ciliated Protozoa, constituting the class Infusoria, probably
represent the most complex development of the unicellular plan
of animal structure. Infusoria have afforded ready material for
the study of various physiological problems, not only because some
of the species are relatively large, but also because, in general,
they lend themselves most readily to experiment. Most of the
Infusoria are free-living in fresh and salt water, though not a few
54 ANIMAL BIOLOGY
are parasitic: there is a highly complex fauna in the digestive
tract of horses and cattle, and Man is not immune. (Figs. 18, 26.)
The organization of the group may be illustrated by Paramecium
which is a giant among the Protozoa, being just visible to the naked
eye as a whitish speck if the water in which it is swimming is
EG
:
Z
23
JY
4
LZ
Euplotes Stylonichia
Lacrymaria
Spirostomum
Lionotus
Fic. 26. — Common ciliated Protozoa (Infusoria) from fresh water. (From
Curtis and Guthrie.)
properly illuminated. When magnified several hundred times it
appears as a more or less slipper-shaped organism which one would
not consider, at first glance, a single cell because it shows highly
specialized parts. However, careful study shows that it really
consists of a single protoplasmic unit differentiated into cytoplasm
and nucleus. (Fig. 27.)
The nuclear material in Paramecium, instead of forming a
single body as it does in most cells, is distributed in two parts: a
larger body, or MACRONUCLEUS, and a smaller body, or MICcRONU-
CLEUS.! Strictly speaking, the macronucleus and micronucleus
1 The several species of Paramecium differ in regard to micronuclear number;
e.g., P. caudatum has one micronucleus, P. aurelia and P. calkinsi have two, and
P. polycaryum and P. woodruffi have several micronuclei.
SURVEY OF UNICELLULAR ANIMALS 59
together constitute the nucleus of the cell, and represent a sort of
physiological division of labor of the chromatin complex.
But it is in the cytoplasm that specialization is most conspicuous.
Not only are there general differentiations into ectoplasm and
endoplasm, but these regions also have local specializations such
as thousands of hair-like,
vibratile cra for loco-
motion and securing
food, TRICHOCYSTS fOr Contractile vacuole
defense, PERISTOME, with canals
MOUTH, and GULLET for
the intake of solid food,
GASTRIC VACUOLES for
digestion, and CONTRAC-
TILE VACUOLES for ex-
cretion. And withal, re-
cent investigations indi- Gastric
cate that various parts “#4
of the cell may be coor-
Macronucleus
Micronuclei
Contractile
dinated by a NEUROMO- vacuole
TOR apparatus. (Figs. 27, |
} , Trichocyst.
£355, Zo, 226.) Cilia Pi . ricnhocysts
Paramecium, under
favorable conditions,
grows rapidly and, when it has attained the size limit character-
istic of the species, cell division takes place. This process of
multiplying by dividing can go on indefinitely under favorable
environmental conditions. But periodically Paramecium under-
goes an internal nuclear reorganization process (ENDOMIXIS). Also
now and then individuals temporarily fuse in pairs and inter-
change nuclear material (CONJUGATION) — an expression of funda-
mental sex phenomena, involving FERTILIZATION, which we shall
have occasion to consider later. (Figs. 28, 170, 171.)
Indeed the Infusoria seem, so to speak, to have made the most
of their unicellular plan of structure, for Paramecium is fairly
representative: it is not the most simple nor yet the most complex.
Specialization of one part and another of the cell has produced in
the Infusoria a group of animals that, judged by distribution and
numbers, is highly successful in the microscopic world of life.
Fic. 27. — Paramecium aurelia.
56 ANIMAL BIOLOGY
In this necessarily cursory survey of the Protozoa, at least one
major point must be forced upon our attention — the versatility
of form and function exhibited by these unicellular animals —
their ADAPTATION to many and varied modes of life in the most
diverse environmental conditions. But their unicellular plan of
structure, permitting only cytological differentiation, has obvious
Contractile
Micronucleus
vacuole
Macronucleus
Gullet
=~ Contractile
vacuole
Contractile
vacuole
Micronucleus
Gullet
Macronucleus
Contractile
vacuole
Fic. 28. — Paramecium aurelia dividing by binary fission. (From Lang.)
inherent limitations which adaptation cannot surmount, while
the multicellular type of organization characteristic of all other
animals affords, as we shall presently see, opportunities which
almost transcend imagination. It makes possible both cytological
and histological — tissue — differentiation. However, the potential
of evolution of the protozoan type was expressed, it is believed,
when it gave rise in the geological past to the stock from which
the higher animals have developed. This allowed the powers of
adaptation pent up, so to say, in the single cell to find expression
in specialization and cooperation in the individual of another and
higher order, which multicellular animals represent.
CHAPTER VI
THE MULTICELLULAR ANIMAL
Over the structure of the cell rises the structure of plants and ani-
mals, which exhibit the yet more complicated, elaborate combinations
of millions and billions of cells coordinated and differentiated in the
most extremely different ways. — Hertwig.
Ir has been pointed out that all organisms consist either of one
free-living cell or of many cells, and some idea has been gained of
unicellular animals from our survey of the Protozoa, so we are now
in a position to consider the origin and organization of the individual
in the Merazoa, as the multicellular animals are sometimes called.
Every Metazoan individual, with exceptions to be noted later,
begins its existence as a single cell which has been set free as such
from the parent, or which has been formed by the fusion of two
cells, or GAMETES, each typically derived from a separate parent
individual: one MALE, the other FEMALE. The former method is
known as UNIPARENTAL, Or ASEXUAL, reproduction and the latter
aS BIPARENTAL, Or SEXUAL, reproduction. The union of male and
female gametes (SPERM and EGG) in sexual reproduction to form
the zYGOTE is termed FERTILIZATION. Both asexual and sexual
methods are common in many of the lower animals, but in higher
forms, including Man, only sexual reproduction prevails, so em-
phasis at present will be placed on the latter. (Fig. 133.)
The most remarkable fact about the zygote (fertilized egg) is its
power to develop into an organism similar to the parents from
which its components, the gametes, have separated. The zygote is
set, one may say, to go through a series of changes which trans-
form an apparently simple cell into an obviously complex multi-
cellular animal with all the characteristics of the species. It is
important, at this point, to review the typical method by which
the development of the adult is accomplished.
A. DEVELOPMENT
Briefly, the general method of animal development is cell divi-
sion accompanied by growth and differentiation. The zygote
a7
58 ANIMAL BIOLOGY
by a succession of cell divisions, termed CLEAVAGE, passes from
the single-cell stage to a two-cell stage and then, with more or
less regularity, to four-cell, eight-cell, sixteen-cell stages, etc. If
these cells separated after each division, the same general condi-
tion would occur here which has been seen in the Protozoa, where
each organism is a complete free-living cell. Or again, if cleavage
merely resulted in a group of so many exactly similar cells, there
would arise a COLONY of unicellular individuals rather than a
multicellular organism.
Such colonial forms are, in fact, numerous among the lower
plants and animals, and show nearly all grades of complexity from
simple associations of a few identical cells,
as for example in Spondylomorum, to groups
of many thousands of cells in which some of
the individuals are specialized for certain func-
tions. Volvox affords an instructive example
of the latter condition. The majority of the
cells, ten thousand or more, that form the
relatively large spherical colony are flagel-
lated, Euglena-like individuals, each of which
Fic. 29. — A simple lives a practically independent existence in
edo ace ae organic union with its fellows. The chief con-
se Sean ca tribution of each of these cells to the economy
each of which carries of the whole results from the lashing of its
on all the functions of flagella, which helps to propel the colony
eee “Pro” through the water. But, under certain condi-
tions, some of the cells become specialized
for reproduction, both asexual and sexual, and form new colonies
which sooner or later are set free. Thus we have a differentiation
of reproductive (GERM) cells from body (SOMATIC) cells, and a fore-
shadowing of that further specialization and physiological division
of labor between cells which is the most characteristic feature of the
higher organisms. (Figs. 29, 30, 154.)
Indeed, the complex bodies of multicellular organisms are made
possible by cell differentiation and cell cooperation. All protoplasm
possesses, for instance, the primary attribute of contractility, but
in the muscle cells of animals we find the capacity for contraction
very greatly developed, and to a certain extent at the expense of -
other powers. But this differentiation would be ineffective were
not innumerable similarly specialized cells grouped together —
THE MULTICELLULAR ANIMAL 59
marshalled to perform a certain function. The power of a muscle
results from the combined action of its component muscle cells.
Differentiation without codperation between cells we have seen
in unicellular organisms. Paramecium has highly specialized
parts; other Protozoa have still more — perhaps the limit of possi-
Fic. 30. — Volvor globator, a large colony of flagellated unicellular organ-
isms in which the various cells have become organically connected, and certain
cells specialized for reproduction. (Highly magnified.) A, mature colony
(highly magnified) showing sperm, <, and eggs, 2, in various stages of de-
velopment; B, four cells (more highly magnified) showing the connections
between three ‘somatic’ cells, and the early differentiation of a reproductive
cell, rp. cv, contractile vacuole; sf, ‘eyespot’ or stigma. (From Kolliker.)
bilities of the single cell. But the multicellular body solves the
difficulty by removing the limit — by assigning special functions
to groups of cells rather than to parts of one cell.
Thus cell division (cleavage) involving growth, differentiation,
and its attendant physiological division of labor is the keynote
of development in the higher animals and plants. Among animals,
the cells which arise from the cleaving egg (zygote) usually become
60 ANIMAL BIOLOGY
arranged so that they form the surface of a hollow sphere of cells
known as a BLASTULA. All the cells at first appear essentially
similar, but soon those at one side of the blastula become in-
RY
<i a
s
Fic. 31.— Early stages in the development of the egg of an animal.
A-F, cleavage and formation of the blastula; G, section of blastula showing the
beginning of gastrulation; H-I, early and later gastrula stages. a, ectoderm;
b, endoderm; c, blastocoel; d, blastopore, leading into the enteric cavity;
e, cells, arising from the endoderm, destined to form the mesoderm.
vaginated until the central cavity, termed the BLASTOCOEL, is
largely obliterated. Accordingly there results the GASTRULA stage,
which may be roughly compared to a sac, composed of two layers
of cells with an opening to the exterior termed the BLASTOPORE.
The outer layer is known as the ECTODERM, and the inner, which
THE MULTICELLULAR ANIMAL 61
lines the gastrula cavity (ENTERIC CAVITY), as the ENDODERM.
The ectoderm comprises cells which are already somewhat differen-
tiated among themselves for special purposes, but which, as a
whole, form a primary tissue with general functions of its own,
chiefly sensory and locomotor. Similarly the endoderm consists
of cells which, as a group, form the nutritive cells of the embryonic
animal. (Fig. 31.)
In the gastrula stage of most animals, a third layer of cells arises
from the endoderm and becomes disposed between the ectoderm
and endoderm. This new middle layer is the MESODERM. In this
way the so-called three PRIMARY GERM LAYERS are established
which are characteristic of the developing animal, and from these
are derived the specialized tissues which compose the various
organs of the adult. For example, the ectoderm by cell division
and differentiation gives rise to the outer skin and central nervous
system; the mesoderm to muscular and supporting tissues and
the blood vascular system; while the endoderm forms the layer
of cells which lines the alimentary canal of the adult organism.
B. TIssukEs
This grouping of more or less similar cells into functional sys-
tems, or TISSUES, is at the basis of the architecture of multicellular
organisms, and thus we have now reached another level in the
analysis of their structure. Although the unit of organization is
the cell, these are associated in groups, or tissues, which represent
a morphological unit of a higher order — a division of labor among
the cells that makes possible the multicellular body with its attend-
ant complexity. A tissue may be defined as a group of essentially
similar cells specialized to perform a certain function. It is con-
venient to distinguish six main groups of animal tissues: epithelial,
supporting, muscular, nervous, circulating, and germinal. (Figs. 4,
32, 34.)
The importance of EPITHELIAL TISSUES is evident from the fact
that the body is covered and lined with these cellular membranes
so they form the point of contact between the organism and its
environment. For example, food before it is really inside the body
must pass through an epithelium lining the digestive tract, and
before it can do that it must be digested by enzymes secreted by
epithelial cells. Furthermore, the waste products of metabolism
must be excreted and pass from the body through epithelial mem-
62 ANIMAL BIOLOGY
AMI CC
. . VD
B_ Connective tissue ss
A Nia ftir :
Bs F
te
Se IRSA
C Cartilage
Q Core > Ss
Re Sel Cece. willie
ee :
\ >
{lita
nh 3 NN)
eee UU LLIN Vere cssnan
i wun vit fc Mee
1 al 1
taut) HME IN iii! |
‘ Be : Pin ) “a Hb iW
{5% NEY Sani
‘ HH]
t
lj
ANSLEY NN
" Param t
| ARUN | Figg. 8, MUAH HOM UY
ium ' ; funn )
\/
dl imCieiituty
Fe iui ,
RUIN
K iu (ist AA,
ATT Aes OPAL A eg
UE Ap PA ~\ AN
i i]
|
vit wes LORI NE Ue M/A ug Hann ni
Pf fil Mh sui
tll (Sin > iit
i
;
/
‘Jk
a | Nica P| \ yiiitibillt
anu YY tae Jandy | wl
Quin (sid youu AA iii x
NERC Peer LULL ea
Vault,
, mL ; Yi ill fn ‘
finn Litt? Atay
WuyOlid
gM
F Nervous tissue
Fic. 32. — Various Vertebrate tissues. A, Epithelium (simple ciliated)
from human intestine; a, surface view; 6, longitudinal section; B, Connective
tissue (subcutaneous) from Rabbit, showing cells and fibers (elastic and
fibrillated); C, Cartilage, showing cells in spaces in hyaline matrix; D, Bone:
section of human humerus showing many spaces in matrix in which the bone
cells lie; E, Muscle (striated) from Man; a, longitudinal section; 6b, cross
section; F, Nervous tissue: bipolar nerve cells from ganglion of auditory nerve
of Cat.
THE MULTICELLULAR ANIMAL 63
branes. Specialization for secretion and excretion leads to GLAND
formation. Glands may be unicellular and scattered here and there
in the surface of the cellular membrane, or they may be grouped
at certain points of vantage. Just as often, however, many cells
combine to form multicellular glands, which sink, as it were, below
the surface as simple or complex tubes or sacs of secreting cells, and
thus amplify many-fold the effective surface within a given space.
And finally, specialized epithelial cells form important elements of
sense organs — the outposts of the nervous system. (Fig. 33.)
Obviously the larger and more complex the organism, the greater
is the necessity for sustaining and binding material; and therefore
as we ascend the animal scale we find, in general, an increase in
SUPPORTING TISSUE. Whereas in epithelia the cells themselves
form the major part of the tissue, in supporting tissues it is direct
or indirect products of the cells, known as intercellular material,
Epithelium
A
Fic. 33. — Diagram of glands.
or matrix, that gives character to the tissue. Thus the function
of connective tissue is performed chiefly by intercellular bundles of
fibers, and the same principle is true for the cartilage and bone form-
ing the internal skeletons of higher animals.
MuScuLAR TISSUE is responsible for the power of motion and
locomotion so characteristic of most animals, and also for the
necessary movements performed by the internal organs in carry-
ing on the various life processes. Muscle cells have in a highly
developed and specialized form a fundamental property of all
protoplasm, contractility, which they exhibit by shortening and
broadening when stimulated by impulses coming chiefly through
the nervous system. A muscle is a cooperating group of muscle
cells, usually bound together by connective tissue and richly sup-
plied with blood vessels and nerves. (Figs. 7, 32, 98, 99.)
64 ANIMAL BIOLOGY
All protoplasm is irritable — it responds to stimuli — but ob-
viously the larger and more complex the body becomes, the more
necessary it is that the actions and reactions of its component cells
be instantly codrdinated so that the parts act as a whole — an
organism. This function is performed by NERVOUS TISSUE com-
posed of nerve cells, or NEURONS, whose long fibers are bound to-
gether by connective tissue into cables, or NERVES. (Fig. 138.)
Longitudinal muscle--2!s"yR =
Peritoneal membran€ Qo 2 yey
< ~
Blood vessel-
Fic. 34. — Portion of a transverse section of the small intestine of the Frog,
highly magnified, to show cellular differentiation, and tissues combined to
form an organ.
One seldom thinks of BLoop, LympH, and other fluids that trans-
port materials in the body, as tissues, but in truth they must be
regarded as CIRCULATING TISSUE in which the living cells, or
CORPUSCLES, are suspended in a fluid matrix, the PLASMA. Cir-
culating tissues minister to, and are in intimate contact with,
every tissue and cell of the organism.
THE MULTICELLULAR ANIMAL 65
And finally, within the body — though perhaps not of the body
—is the GERMINAL TISSUE destined to contribute not to the in-
dividual but to the propagation of the race during reproduction.
So the multicellular animal — the organism as a whole — is a
multitude of codperating protoplasmic units: upward of one hun-
dred thousand billions in Man. However, we must not overlook
the whole which is greater than its parts. The cells merge their
individuality in that of the entire organism, and as long as its cells
remain associated they are to be regarded, not as individuals, but
as specialized centers of action and reaction of the living body
itself, by means of which physiological division of labor is made
possible.
C. ORGANS AND ORGAN SYSTEMS
Since the similar cell components of multicellular organisms
are grouped to form tissues, it follows that the major working
units, or ORGANS, of the animal body as a whole are formed of
tissues. In other words, an organ is a complex of tissues which
has assumed a definite form for the performance of a certain func-
tion — a major division of the body which allows the tissues and,
therefore, the cells devoted to a special function to play their
part under the most suitable relations to internal or external con-
ditions: for example, the human hand composed of bone, muscle,
nerve, etc. (Figs. 34, 125.)
As one would naturally expect, among the lowest Metazoa there
are forms in which the body is relatively simple, without highly
specialized tissues and organs, but in most animals specialization
is carried still another step forward by the grouping of organs de-
voted to the performance of some one general function into an
ORGAN SYSTEM.
The organ systems may be classified as the INTEGUMENTARY
and SUPPORTING SYSTEMS which constitute the covering and the
framework of the individual; the ALIMENTARY, RESPIRATORY, CIB-
CULATORY, and EXCRETORY SYSTEMS which directly or indirectly
are concerned with nutrition; the NERVOUS SYSTEM which, in
cooperation with the system of SENSE ORGANS, the MUSCULAR SYS-
TEM, etc., not only codrdinates the various parts of the individual,
but also orients the whole with respect to its environment; and,
finally, the REPRODUCTIVE SYSTEM which makes possible the con-
tinuation of the race. The fundamental life processes for which
66 ANIMAL BIOLOGY
these systems provide must be carried on by all animals, and the
chief differences in the structure of animals, from the lowest to
the highest, are the result of the means adopted to serve these
essential functions under different conditions imposed by the
environment and mode of life. It is on the basis of these funda-
mental structural characteristics that the Metazoa are classified.
The Metazoa may be divided into two large groups known as
INVERTEBRATES and VERTEBRATES. The former group, frequently
referred to as the lower animals, comprises nearly a million known
species and exhibits an enormous variety of form and complexity
of structure ranging from the lowly Sponges to the highly suc-
cessful Insects. On the other hand, the Vertebrates, or higher ani-
mals, form a relatively homogeneous group of about fifty thou-
sand species, including the Fishes, Amphibians, Reptiles, Birds,
and Mammals. It is to a survey of the Metazoa that we now turn.
(Fig. 297.)
CHAPTER VII
SURVEY OF INVERTEBRATES
It seems as if Nature had essayed one after the other every possi-
ble manner of living and moving, as if she had taken advantage of
every permission granted by matter and its laws. — Gide.
THE stupendous group of multicellular animals constituting
the Invertebrates presents a bewildering array of forms adapted
to nearly every conceivable environment. Some are smaller than
many of the Protozoa and others much larger than certain Ver-
tebrates. Some are aquatic, others are terrestrial, and still others
spend most of their life in the air. Most possess the power of
locomotion, but many are fixed, or sessile. And not a few — per-
haps the majority — live as parasites in or on the bodies of other
animals. (See Appendix I.)
The fact that there are nearly a million known species, and
probably twice as many yet to be discovered, at once suggests that
our survey must be confined to a relatively few representatives
from each of the major Invertebrate phyla; enough to place the
three forms we have selected for special study later — Hydra,
Earthworm, and Crayfish — in proper perspective.
A. SPONGES
Cne is not surprised that the Sponges which constitute the
PoriFERA, the lowest phylum of multicellular animals, show certain
relationships with the Protozoa. Specialization of cells and physio-
logical division of labor, though carried far beyond the most com-
plex colonial forms, such as Volvox, nevertheless have not sup-
pressed the individualities of the cooperating cells to the extent that
occurs in most higher forms. Witness the fact that when certain
Sponges are gently squeezed through the meshes of fine silk cloth,
so that the tissues are resolved into separate cells, these cells will
gather together in small groups and each group will grow into a
Sponge. In brief, the Sponge is an individual animal, but its
organization is loose and the dependence of one part upon another
67
68 ANIMAL BIOLOGY
is relatively slight. However, the Sponge dominates its tissues.
Just so long as the component cells of the Sponge remain associated,
they are not to be regarded as individuals, but as specialized
centers of action and reaction of the Sponge’s body, by means of
which physiological division of labor is made possible.
At first glance, indeed, one would not recognize a Sponge as
an animal at all. Thus the common Leucosolenia of the New
England coast appears to be a group of tiny tubes, or sacs, per-
vy, Osculum
Buds ae S
KGELF
Flagellum
Collar ————
B
Fic. 35. — A, small colony of Leucosolenia; B, a collar cell; C, Grantia.
manently attached at the base to a rock just below low-tide
mark. It is without the power of locomotion and does not visibly
respond in any way when touched. However, if it is examined
under a lens, it will be found that the whole surface of each sac-like
body is dotted with innumerable pores, or osT1A, through which
water is being drawn into the GASTRAL CAVITY and passed on out
through a sieve-like membrane at the top, or oscuLum. The name
of the phylum, Porifera, refers to the pores. (Fig. 35.)
A section of the body wall of Leucosolenia shows that it is
supported by a skeleton, composed of a network of three-pronged
SURVEY OF INVERTEBRATES 69
SPICULES of calcium carbonate, embedded in the tissue. The latter
consists of two chief layers of cells: an outer, or DERMAL EPITHE-
LIUM, and an inner, or GASTRAL EPITHELIUM, separated by a jelly-
like material containing many amoeboid wandering cells.
The gastral epithelium is of particular interest because it con-
sists chiefly of a layer of COLLAR CELLS, each provided with a flagel-
lum, and resembling certain flagellated Protozoa from which,
indeed, it is probable that the Sponges have descended. It is the
constant lashing of the flagella that creates the current of water,
laden with food and oxygen, through the pores into the gastral
cavity, and on out through the osculum, bearing excretions. It is
also the collar cells that capture, engulf, and digest particles of food
in typical unicellular fashion (intracellular digestion) and pass the
products on to the other cells. Similarly, respiration and excretion
are carried on by the individual cells.
Such is the essential plan of structure of a simple Sponge, but
there are more complex forms which, in general, can be derived
from this so-called AScon type by a thickening of the body wall,
and then either the restriction of the collar cells to canals em-
bedded in it in close proximity to the gastral cavity (SYCON type),
or their segregation in tiny cavities more remotely situated in the
tissues but still in communication with the gastral cavity by long
canals (RHAGON type). The sycon type is represented by Grantia,
and the rhagon type by the common Fresh-water Sponge, Spon-
gilla, and the Bath Sponge, Euspongia. (Fig. 36.)
But one must not gain the idea that all Sponges are quite
similar in outward appearance. As a matter of fact some are no
larger than a pin head, others are six feet or more tall. Some are
branched, fan-shaped, or cup-shaped. Most are white or gray
though many contribute brilliant colors to the picture presented
by the sea floor.
Among all this variety of Sponges, however, certain species,
chiefly of the genus Euspongia, stand out as of high economic
importance because their flexible, fibrous skeleton of SPONGIN,
when cleaned and dried, is the familiar bath sponge of commerce.
These Sponges, long gathered by diving and dredging, are now
also farmed. Live Sponges are cut into very small pieces, wired to
cement plates and then sunk to favorable places for sponge growth.
After several years the pieces have become Sponges of sufficient
size for the crop to be marketable.
70 ANIMAL BIOLOGY
Sponges reproduce both asexually and sexually: asexually by
Bubs and by the liberation of groups of cells called GEMMULES
possessing the power to form new individuals; and sexually by
eggs and sperm. The zygote develops into a tiny ciliated embryo
which after a short free-swimming existence settles down, becomes
permanently attached, and soon attains the adult structure.
Osculum :
& + aa
ay \ A | ow?
fy 3 My Be
= 3 - < a 2) Gastral S
if a ProsopyleZ ' 5 cavity SS
ie e) ~sS a8 bea tots
i * Pent:
it Ns 3 p= elect Incurrent
tg . Gastral * g:~ S eo oe
Hs cavity 3 OB SEE ered ;
‘A bs a
bach DRS Flagellated
s gee chamber
Openings of excurrent canals
Dermal pores
| Mit &, Subdermal
~,
LP
Di. ‘gy l Daa
sa
Flagellated uw
chambers
Incurrent
canals Se —
(i Ce ro] Bee =
cc nna en Minick
tneceerent canals
Cc
Fic. 36. — Types of canal systems of Sponges. A, ascon type (Leucosolenia)
B, sycon type (Grantia); C, rhagon type (Spongilla, Euspongia). The arrows
indicate the direction of the current of water. The thick black line in A and B
represents the gastral layer; the dotted portion, the dermal layer. (From
Minchin, and Parker and Haswell.)
The relationship of the Porifera to the series of Invertebrates
raises an interesting problem. The tissues of Sponges are, as we
have seen, not only too complex to be considered colonies of Pro-
tozoa, but they also are much less complex than those of the higher
multicellular animals. And they are organized in a radically differ-
ent way as well, including an apparent reversal of the two primary
SURVEY OF INVERTEBRATES a
cell layers which are not homologous with the ectoderm and
endoderm of higher animals. All considered, Sponges probably
represent a side branch of the Animal Kingdom that went up, so
to say, an evolutionary blind alley, and remained only Sponges!
(Fig. 313.)
B. CoELENTERATES
With the large group of aquatic animals comprising the CoELEN-
TERATA — the Polyps, Jellyfish, Sea-anemones, and Corals — we
reach the basic phylum of the Invertebrates because, with a mouth
opening into a digestive cavity and with specialized body parts
coordinated by a simple nervous system, they institute the plan of
structure that proves to be the fruitful one for the derivation of
certain features exhibited in all higher animals. Our review of the
phylum will merely serve to indicate some of the chief forms as-
sumed by the potyp type of individual in the highly diversified
classes known as Hydrozoa, Scyphozoa, and Anthozoa.
Hyprozoa. One of the few Coelenterates inhabiting fresh water
is the common polyp, Hydra. Like all polyps its body is essentially
a sac composed of two cell layers — ectoderm and endoderm —
separated by a jelly-like non-cellular layer known as the MEso-
GLOEA. The opening of the sac is the mouth which leads into the
digestive cavity, or ENTERIC CAVITY. The mouth is surrounded by
a circle of TENTACLES well supplied with nettle cells, or NEMATO-
cysts. Obviously the polyp exhibits RADIAL symmetry — right and
left sides do not exist. It has no internal organs and so, of course,
no organ systems. The adult Hydra now and again produces on its
body-surface male and female sexual organs that liberate sperm
and eggs and, after fertilization, the zygote develops ints another
Hydra. Furthermore, Hydra reproduces asexually by BupDs: small
outpocketings from the body wall grow into small Hydras and
then separate as independent polyps. (Figs. 57, 135-137, 155.)
However, the life history of Hydra is not representative of the
many kinds of marine Hydrozoa that commonly grow on sub-
merged objects, such as the piles of piers, because most of them
consist of large colonies of Hydra-iike individuals organically
connected; somewhat as though a Hydra formed many buds that
remained attached. Moreover, the colony is only one phase of
the life history of a Hydroid, for it develops special buds, known
aS MEDUSAE, which become separated from the colony as inde-
712 ANIMAL BIOLOGY
pendent free-swimming individuals. At first glance a medusa bears
little resemblance to a polyp, though it is essentially a sexual polyp
that has been produced on the colony by budding and then liber-
ated. Accordingly the complete life history of a typical Hydrozoon
involves an ALTERNATION OF GENERATIONS consisting of the asexual
Wi
Gi Ne _.. Statocyst
IZZ- _- Radial canals
<= .. Reproductive organs
a. = “rentacles
Ss
SS
‘s =~
S++ + Mouth B
Hydrotheca,
-
-—-
-
as
meme ee
ye H | ..Gonotheca
petodern: tray if
Entoderm /
Enteron ime
Perisarc™
Ly. - Medusa-bud
|
q
AY
If
“yy
UWA) |
\ f |
-e4
4
By
Coenosarc.-"
Fic. 37. — Life history of a Hydroid, Obelia. A, portion of the asexual
colony showing three hydranths and a gonangium; B, free-swimming sexual
medusa, oral view; C, medusa, side view; D, ciliated larva. (C and D of closely
related species.) (From Hegner, after Allman, and Hargitt.)
colony and the sexual medusae that swim away and disseminate
the species. Hydra is apparently a specialized form in which the
medusa generation is suppressed. (Figs. 37, 38.)
The Hydroid colonies exhibit some interesting examples of
physiological division of labor between the component individuals.
Thus in Bougainvillea there are two kinds of polyps: the typical
SURVEY OF INVERTEBRATES if:
feeding polyps, or HYDRANTHS, and the MEDUSOID polyps which
later, with some structural changes, are set free. Again in Obelia,
another common Hydroid, there are also highly modified individ-
uals, or GONANGIA, that produce by budding the medusa generation.
In still other Hydrozoa this POLYMORPHISM is carried even further.
Thus Physalia, the Portuguese Man-of-War, is a floating Hydroid
colony consisting of an air-filled bag, or float, with a sail-like crest,
from which are suspended a large number of polyps. These indi-
viduals are very diverse: some are nutritive, others are tactile,
and still others bear batteries of nematocysts. Furthermore, there
are male reproductive polyps and others that give rise to egg-
Ectoderm,
——
Enteric
cavity
Radial canal
)\ Circular
Tentacle —#% canal
Nerve Velum
rings
“Hypestema
A B
Fic. 38. — Diagram showing the fundamentally similar structure of Hydra
or a hydranth of Obelia (A) and of a medusa (B). (From Parker.)
Tentacle |'@
producing medusae. Obviously an animal such as Physalia sug-
gests that in many of the lower forms the INDIVIDUAL is not so
sharply defined, and is more difficult to define, than in the more
familiar animals. Indeed, individuality in the Animal Kingdom
is a problem in itself; one the reader might well keep in mind as
we ascend the scale of animal life.
ScypHozoa. The second great class of the Coelenterates com-
prises chiefly large medusae, such as the common saucer-shaped
Aurelia of the Atlantic coastal waters or its giant relative, Cyanea,
which may attain a diameter of eight feet; each and all well-
named Jellyfish since their soft tissues are more than 99 per cent
water.
The structure of Aurelia is basically the same as that of the
medusa generation of the Hydroids; the most obvious difference
being an excessive development of the mesogloea between ecto-
74 ANIMAL BIOLOGY
derm and endoderm so that the body wall is relatively thick. The
mouth is in the center of the under (oral) surface, opening into
a large enteric cavity which branches into RADIAL CANALS that run
to the circumference, or ‘rim,’ where they merge into a CIRCULAR
CANAL. Thus digested food is carried directly to all parts of the
animal without the necessity of a special circulatory system. Sur-
rounding the mouth are four ribbon-like ORAL ARMs which, to-
Enteric
Gonad cavity
ii
ill
|
i
tatise
HT]
laiiats
ante -#
tee
ib
gIRAI!]
i
ti }
efeeda gy!
p> : a 4
An eB:
S24 ie
EEG
<( Oral arm
Fic. 39. — Aurelia, lateral view; one quadrant shown in section. (From
Parker and Haswell.)
gether with the fringe of tentacles about the rim of the animal and
certain filaments in the radial canals, are well supplied with nema-
tocysts for stinging the prey. (Fig. 39.)
Among the tentacles on the rim are small rounded SENSE ORGANS
which control, through the underlying nerve cells and muscle
bands, the movements of the animal. Contraction of the rim of
the bell, slowly and rhythmically, presses the water within the
bell against that without and so forces the animal along. But
Aurelia is largely at the mercy of wind and wave — few being
destined to reproduce before dissolution, and all being destroyed
by winter.
The reproductive organs of Aurelia consist of four large U-
shaped GONADS which shed their products, either eggs or sperm,
into the radial canals and so to the outer world through the mouth.
Fertilization takes place in the open water to form a zygote that
develops into a free-swimming larva. This soon settles to the
bottom, becomes attached, and assumes a polyp-like form which
is comparable to the hydroid generation of the Hydrozoa. After
SURVEY OF INVERTEBRATES 75
wintering in this form, however, the fixed individual proceeds, as
it were, to slice itself up in orderly fashion into a series of saucer-
like embryo medusae which, one by one, separate, swim away,
and eventually attain the adult, sexual Aurelia form. (Fig. 40.)
a Vey
ee = F
3 J ne D E =~
Gastra] filament Stomach
Gonad pi tat ee
. RX) “0 x.
CMe T TSN
an
fe
NSS
i
Fic. 40. — Aurelia, life-history. A, B, C, longitudinal sections through
gastrula stages; D, scyphistoma; E, F, strobila; G, H, ephyra, I, vertical
section through adult. A, B, and C are more highly magnified than the other
figures. (From Hegner, after Kerr.)
ANTHOZOA. So far our survey of the Coelenterata has shown that
all are polyp-like animals, and the final class to be considered, the
Anthozoa, is no exception, comprising, as it does, the Sea Anemones
and the Corals.
Metridium, the well known Sea-anemone of the North Atlantic
coast, is common in tide pools or attached to piers of wharves;
its slight powers of creeping being seldom exerted. It is apparently
a ruggedly built polyp, nearly cylindrical in form, with the upper
end disclosing the mouth in the midst of a crown of numerous
tentacles. (Fig. 41.)
The mouth opens into a large gullet which leads down into
the enteron. The latter is quite complex because its walls give
rise to several series of vertical, radially arranged partitions, or
MESENTERIES, thereby increasing the functional digestive and
respiratory surface. Within the enteric cavity are peculiar, long,
76 ANIMAL BIOLOGY
coiled, nematocyst-bearing threads that can be protruded in pro-
fusion through both the mouth and numerous pores in the body
wall, and so are efficient offensive and defensive weapons. Further-
more, the tentacles of Metridium are not only well supplied with
nematocysts, but also with a coat of cilia that plays a crucial part
in the ingestion of food. Once the prey has been paralyzed by the
Gullet _
leading into enteron
\ Lip about mouth
Tentacles
\ :
Nt | p \ - End of a
1 i (1, - \ \Cmesenterial
KO i Gn filament
\ Lx
oe Primary mesentery
Reproductive organ
A Edge of mesentery
Fic. 41. — Sea Anemone, Metridium marginatum. A, View of polyp with
one quadrant removed; B, Diagram of transverse section showing mesenteries.
discharge of the nematocysts, the tentacles bend inward and over
so that ciliary action sweeps the food slowly but surely toward the
mouth. Here other cilia, aided by muscular contractions, carry it
on down into the enteron.
Metridium typically reproduces sexually, though asexual repro-
duction by budding may also occur. The reproductive organs are
within the enteric cavity where, in the case of females, the eggs
are fertilized, and the embryo develops into a ciliated larva before
making its exit to settle down and assume the adult characters.
Metridium, then, may serve to illustrate the general type of
SURVEY OF INVERTEBRATES fi
polyp structure exhibited by the Anthozoa, and it is only necessary
to relate this to the Corals which are essentially Metridium-like
polyps that secrete, under and about themselves, more or less
complex skeletons, chiefly of carbonate of lime. As the polyp
continues to secrete the coral, it actually pushes itself farther and
farther away from the surface to which it became attached. This
growth process, together with multiplication by budding, grad-
ually builds up considerable masses of coral about larger and larger
colonies of polyps — the arrangement of the polyps and the dis-
: Endoderm uf
‘ Ea Mesentery rw
Theca — Wary Hin |S/
Basal plate Et J NS _) a:
Columella 75 no
Fic. 42.— Coral. A, Skeleton of a young colony of Organ-pipe Coral,
Tubipora musica; B, small branch of Red Coral, Corallium rubrum, showing
living polyps; C, Sea Pen, Pennatula phosphorea; D, diagrammatic section of
a single coral polyp.
position of the coral being characteristically different in the
numerous species of Coral animals. Certain kinds of Corals, acting
through long periods of time, are responsible for building not only
atolls and islands but also fringing reefs and barrier reefs; the
Great Barrier Reef of Australia is over a thousand miles long and
fifty broad. (Fig. 42.)
From the Hydroid polyps to polyps building coral islands, from
the tiny medusae of Obelia to the giant Cyanea, we gain at least a
78 ANIMAL BIOLOGY
glimpse of the basic group of Coelenterates and what it has accom-
plished with the simple radially symmetrical body plan, with two
primary tissue layers, and an enteric cavity. The polyp type is
successful in its way, but its chief contribution was apparently in
affording the stem from which higher forms started along their
main path of ascent. (Fig. 297.)
C. FLATWORMS
Passing over a smail phylum of marine Coelenterate-like ani-
mals, the CTENOPHORA, popularly called Sea-walnuts and Comb-
jellies, that have made a somewhat abortive attempt to establish
a body on the three primary layer plan, we come directly to the
first great group of TRIPLOBLASTIC animals, the Flatworms, con-
stituting the phylum PLATYHELMINTHES. (Fig. 43.)
On the lower surface of submerged stones near the edges of ponds
are usually many tiny black, gray, or white Flatworms, creeping
by cilia, known as Planaria. They are obviously radically different
in structure from a polyp since they exhibit BILATERAL instead of
radial symmetry: they have a broad anterior end with simple
eyes and brain, and a narrower posterior end, and so have right
and left sides. Strange to say, however, the mouth is situated not
in the ‘head,’ but behind the middle of the ventral surface of the
body, and functions both for the intake of food and the exit of
waste. The mouth, instead of opening into a sac-like enteron,
leads into a long, protrusible pharynx and this, in turn, into an
extremely branched intestine which extends throughout the body.
Somewhat similarly extend the excretory system, the male and
female reproductive systems, as well as the web-like nervous sys-
tem; each and all embedded in a continuous spongy mass of tissue
which is derived from a third primary germ layer, the mesoderm.
Thus the organ systems do not actually lie in a definite body
cavity. Since nearly every organ system extends throughout the
body, no circulatory system is required, and apparently oxygen
sufficient for the animal’s need can diffuse through the tissues.
(Figs. 156, 161.)
The genus Planaria is a member of the first class of the phylum,
known as the TURBELLARIA, the other classes being the TRE-
MATODA, or Flukes, and the Crestopa, or Tapeworms. Both of
the latter have departed superficially somewhat widely in struc-
ture from the free-living Planarians and also have developed ex-
SURVEY OF INVERTEBRATES 79
traordinarily complex life histories, involving alternation of gen-
erations, in becoming adapted to various parasitic modes of life.
The study of Flukes and Tapeworms constitutes a major part of
the science of PARASITOLOGY because they inhabit Man and beast
_ “Eye iGenital pore
ee
weed
8 Proboscis
Side of head
A
Intestine} 3 F Yolk glands
aes
Lat oe cee
ia Seve
Sos
pe)
Pharynx
Intestine—-s SE Se
= at: my, a ~~ =
Testis aN S EE .
OS p> BLED | Intestine
\ SCF ys ao PS
Ee zy isa
Uterus
Genital poré
Oviduct
Fic. 43. — A, Planaria polychora, a fresh-water Flatworm; B, diagram of
internal organs of a Flatworm. (From Hegner, after Shipley and MacBride,
and von Graff.)
and produce various important diseases. Accordingly we can more
advantageously consider their life histories later when discussing
the relations of biology to human welfare. (Figs. 251-253.)
D. RouNDWoRMS
If either number of species or number of individuals were the
sole criterion of a phylum’s importance, unquestionably the lowly
NEMATHELMINTHES, or Roundworms, would rank high in the
80 ANIMAL BIOLOGY
Animal Kingdom. They live literally everywhere from hot springs
to Arctic ice, from desert sand to bottom mud of lakes and seas,
from the roots of plants to the blood of Man. A thimbleful of soil
may contain hundreds, even thousands, of Roundworms. Esti-
mates indicate that there are upward of twenty thousand species
parasitic on Vertebrates alone, and twice as many more if all the
other parasitic and free-living species were catalogued.
The Roundworms are slender, cylindrical worms as is indicated
by other common names, such as Threadworms and Hairworms.
Their body plan shows, in general, considerable advance over
that of the Flatworms, because the simple intestine has two open-
ings to the exterior, mouth and anus; the nervous system consists
of a nerve ring about the pharynx from which arise large nerves,
extending the length of the body; the excretory system is well
Uteri
Y
Excretory pore
Anus Intestine Genital pore Mouth
Fic. 44. — Roundworm, Ascaris lumbricoides: diagram of dissection from
right side.
developed: and the male and female reproductive systems are
usually in separate individuals. Furthermore the various organs
lie in a spacious body cavity. However, there are no special cir-
culatory or respiratory systems. (Fig. 44.)
Naturally the Roundworms that have been most studied are
parasites of Man and domestic animals. The Guinea worm is some-
times several feet long, and spends its adult life just under the
human skin and its youth in a Water-flea. The various species of
Ascaris are relatively large intestinal worms, the females attaining
a length of more than a foot and producing some fifteen thousand
eggs a day. But two of the most dreaded parasites are the tiny
Trichinella, the cause of the often fatal TRICHINOSIS contracted
by eating infected pork, and Necator, notorious as the Hookworm.
All considered, the Roundworms are hardly second to the Flat-
worms from the standpoint of medical zodlogy and, like them,
tax the ingenuity of biologists in ferreting out their complex re-
productive processes and life histories that have been evolved
SURVEY OF INVERTEBRATES 81
to insure entrance to the proper host. Before a parasite can enjoy
Utopia, it must get there. (Figs. 44, 254, 255.)
E. SEGMENTED Worms
The Segmented Worms form a relatively straightforward phy-
lum, the ANNELIDA, that carries us on apace in the development
of the more complex animal body: conspicuously by definitely
introducing the principle of SEGMENTATION. Well-known repre-
sentatives of the chief groups are the marine Sandworms and
Tubeworms, or Potycuaeta; the Earthworms, or OLIGOCHAETA,
of soils the world over; and the Leeches, or HrruDINEA, such as
the Medicinal Leech, so popular when blood-letting was in vogue.
(Figs. 45, 60.)
The body of the Sandworm, Nereis, consists of a linear series
of some fifty units, or SEGMENTS. Several of the anterior ones
form the HEAD, with sense organs, notably rather complex EYEs,
mouth, and chitinous jaws; while most of the rest are provided
with paddle-like, bristled PARAPopIA that act both as respiratory
and swimming organs. The anus opens on the terminal segment.
The Earthworms show a simplification of the external structures
present in Sandworms, because the head is far smaller and simpler
and lacks specialized sense organs. Moreover the bristles (SETAE)
are all that remain of the conspicuous locomotor organs of the
Sandworm. The largely sedentary and nocturnal life of Earth-
worms renders highly specialized locomotor, respiratory, and
sense organs unnecessary. But the internal organs of both follow
the same general plan, and since we shall have occasion later
to study those of the Earthworm somewhat in detail, it will
suffice now merely to emphasize the essential progressive features.
(Fig. 45.)
The principle of segmentation, as already mentioned, is probably
the most significant advance made by the phylum. At all events,
it is the plan of organization exemplified by the most successful of
the higher groups — the Arthropods and Vertebrates. Apparently
the segment is an adaptable unit of organization that makes possible
local specialization in higher forms, and so contributes in an im-
portant way to their response to various environmental changes.
Furthermore, in Segmented Worms the coelom first attains high
structural significance and affords ample space for the disposal of
more elaborate organ systems. Thus we find a complete circula-
82 ANIMAL BIOLOGY
tory system of blood vessels that propels and distributes blood to
all parts of the body: an efficient transport system for carrying ab-
sorbed food to every organ, waste products to the excretory organs,
and carbon dioxide to, and oxygen from the respiratory surface.
And when we add the advances in digestive, nervous, and excretory
systems, the Segmented Worms approach more nearly the popular
Prostomium bearing
Tentacles S ; four eyes
_—
S
Fic. 45. — A, Sandworm, Nereis virens; B, Medicinal Leech, Hirudo medicinalis.
concept of an animal than the Invertebrates we have heretofore
met.
And withal, in variety and numbers the phylum is second to
few. Some idea of the importance of Earthworms may be gathered
by Darwin’s estimate that in many soils they annually bring to
the surface some eighteen tons of earth per acre. Not unimpor-
tant transformations of the surface soil is thus effected: indeed
Earthworms, unseen and unheard, perform a vitally important
part in loosening and aérating — in plowing — the soil.
F. Rotirers, Bryozoans, AND BRACHIOPODS
At this point in our survey of the Invertebrates it is opportune
to take merely a passing glance at several aberrant groups whose
position in the series is decidedly uncertain, and which contribute
little of theoretical or practical importance.
SURVEY OF INVERTEBRATES 83
The Rotirera, or Wheel-animalcules (TROCHELMINTHES), con-
stitute an immense group of tiny animals commonly found in
fresh and salt water, in association with the Protozoa. Micro-
scopic though they are, they possess a highly complex series of
internal organs, in spite of which they can withstand slow drying
in mud. In this condition Rotifers may be blown about unti! they
happen again upon moisture, whereupon they gradually ‘swell
up’ and assume active life and reproduction.
The Bryozoa are frequently referred to as Moss-animals be-
cause most of them are mat-like fixed colonies growing on sub-
merged objects in sea or pond. A superficial examination of the
common fresh-water Plumatella, for example, might suggest that
it is a Hydroid, but closer scrutiny reveals a very different struc-
tural plan, including complex internal organs. In addition to
reproducing sexually and by typical budding, Plumatella develops
peculiar internal buds, or STATOBLASTS, enclosed within a chitinous
shell. In the event that the pond dries up or the colony is frozen,
the resistant statoblasts survive to start a new colony upon the
return of favorable conditions.
The Bracuiopopa, or Lamp-shells, at first glance look like
Clams, and so bear no obvious resemblance to the colonial Bryozoa.
However, Brachiopods are actually related to the Bryozoa, as is
evidenced by their internal structure which approaches rather
closely to that of Plumatella. Brachiopods constitute one of the
older marine groups, their shells constituting conspicuous and
characteristic fossils in the more ancient rock strata. Those living
to-day are merely little-changed survivors of a successful group
of yesterday — perhaps nearly a billion years ago in the early
Paleozoic seas. Indeed, the genus Lingula has persisted without
change, to be well dubbed the “senior genus of the world of
animal life.” (See: Appendix I.)
G. EcHINODERMS
Everyone who has spent a summer at the seashore is certainly
familiar with the Starfish and Sea Urchin, common examples of
the marine phylum of spiny-skinned animals, the ECHINODERMATA.
All members of the group have radially symmetrical bodies — a
condition we have not seen since we left the Coelenterata. How-
ever, the symmetry indicates no direct relationship with the
Coelenterates, because during early embryonic life an Echinoderm
84 ANIMAL BIOLOGY
is actually bilaterally symmetrical and the radial form is only
secondarily assumed — it masks the basic structure. Indeed
‘‘Nature’s pentagonal experiment’”’ has produced a series of bizarre
forms that have been successful from early geologic time to the
present, though they bear little resemblance to other animals.
Witness the structure of a common Starfish. (Fig. 46.)
The body of the Starfish consists of a CENTRAL DISK from which
radiate five Arms. It is protected by calcareous plates embedded
in the tissue, and by short, blunt spines. About the latter are tiny
pincers, or PEDICELLARIAE, that keep the surface free from debris
ha
= ALES: oe Teen,
..\
WE CIree,.
a
ae Tube feet
SATS
SS a
> 5 As
SENS Marginal spines ea
(33 ---/Adambulacral spines &¥
iy 2 fate ane
ER aa
: Ss LP hae Fe Cray
A KS. eat
ge tT PIT ALE < .
ee 4 S Ce te ia ’
Bes S = p .
Maat a, i
care: e SW AINE
4
Fic. 46. — Starfish, Asterias. A, oral view; B, devouring a Clam.
(From Cambridge Natural History.)
and protect the delicate, protruding DERMAL BRANCHIAE. ‘The
mouth is situated in the disk on the surface that is ventral as the
animal crawls along, and the anus on the opposite surface. Near
the anus is a small, porous plate, the MADREPORITE, that admits
water to a system of tubes, or WATER-VASCULAR SYSTEM, that ter-
minates along the AMBULACRAL GROOVE on the ventral surface in
myriads of TUBE FEET: the unique hydraulic locomotor and food
capturing organs.
The mouth opens into a large stomach that leads into the Py-
LORIC SAC from which large, glandular PyLORIC CAECA extend into
each arm. Above the stomach is a small rectum that passes to the
aboral surface, but waste materials are ejected through the mouth.
SURVEY OF INVERTEBRATES 85
The nervous system is essentially a nerve ring encircling the mouth,
with a branch extending into each arm, but there is no central,
dominating nerve mass that is comparable to a brain, and only
NaH Aa t ley ”
yr. AWA ey \
we ze OLAS Can wa
5
dyer
nated
IRS
1 Ygs
“Sz axe } Se
LEA Oe ; i
mF YS = !
eS Spee Ny
em NES ly
HP ‘=
bas (ic Yih " eer A\ i
en CUO ee 7, ;
WN ce
BE VV EAN LEAN i
LAV FS LING AIO 8 VG 3
NT WAND Hi) Kes S\N NA j F
Lt} . sy N44 YY (i Weak ) TANT fx ts nye : - z
PENN SA ARE a7! :
44333 Zz:
A is
Ss ie =
Sh BARS
4 Se
ie ASP
: SYR
Zi
tn aay
7 re:
“SA
YSZ
ny Os aia
ATO ggplly °
soe eS
md
NA
B
Fic. 47. — A, Sea Urchin, Asthenosoma, oral view; B, Sea Cucumber, Cucu-
maria; C, Sea Lily, Metacrinus. (From Thomson, Ludvig, and Carpenter.)
the simplest sense organs, the tube feet acting largely in the latter
capacity. The functions of a circulatory and respiratory system
are carried on by the coelomic fluid — mostly sea-water in which
amoeboid cells float — that fills the large body cavity, or coelom,
circulates through the dermal branchiae, and bathes all the organs,
including the male or female reproductive organs.
86 ANIMAL BIOLOGY
The Starfish is a member of the first class, the ASTEROIDEA, of
the phylum, and serves to give some idea of the fundamental
anatomical plan of the several other classes. True, members of
most of the other classes bear little obvious resemblance to the
Starfish. Most similar are the Brittle Stars (class OPHTUROIDEA)
with flattened central disk and long, slender, and sometimes
branched arms that are fragile and readily discarded when injured.
But the Sea Urchins and Sand Dollars (class EcHINOImDEA) are
without arms; the more or less spherical body being enclosed within
a hard shell composed of a multitude of closely fitting plates and
covered with a forest of spines. Then the Sea Cucumbers (class
HoLoTHuUROIDEA) are essentially elongated, flexible, muscular sacs
with contractile tentacles, representing modified tube feet, about
the mouth. They seem to be little inconvenienced if they eject
most of their internal organs, because a period of rest suffices for
their regeneration. And finally, the Sea Lilies (class CRINomEA),
apparently the antithesis of the Sea Cucumbers, are temporarily
or permanently attached, usually by a joimted stalk from which
extend their much-branched arms in plume-like fashion, and so
some are called Feather Stars. (Fig. 47.)
Still, with all this diversified array of Echinoderms, it is, we
repeat, true that all are basically similar in structure and develop-
ment — they have almost surely been derived in a round-about
way from a primitive worm-like ancestor.
H. Mo.uuscs
The great phylum Motuusca, which includes not only such well-
known edible ‘ shell-fish’ as the Clams, Oysters, Scallops, and Snails,
but also the Cuttle-fish, Devil-fish, Nautili, etc., presents a consid-
erable departure in bodily plan from that exhibited by the Seg-
mented Worms, and constitutes another large and remarkable
branch of the Invertebrate ‘tree.’ However, from certain structural
features exhibited by the lowest class, the AMPHINEURA represented
by Chiton, and particularly the developmental stages of various
Molluscs, it appears clear that, like the Echinoderms, they have
arisen from a worm-like ancestral type which, instead of adopting
segmentation, became otherwise specialized to form a unique and
highly successful group. (Fig. 48.)
The most characteristic structures of Molluscs are the external
skeleton, or SHELL; a fleshy muscular organ, the Foot, typically
SURVEY OF INVERTEBRATES 87
for locomotion; and a MANTLE CAVITY between the main body and
an enclosing envelope, the MANTLE. Among the five classes of
Molluscs only three are of sufficient general interest to command
our attention here: the Gastropoda, Pelecypoda, and Cephalopoda.
GASTROPODA. One usually thinks of Molluscs as sea-dwelling
animals but among the some sixteen thousand species of Snails,
Slugs, and other similar Gastropods, more than one-third are
terrestrial. Everyone is familiar with the spirally-coiled Snail’s
shell into which the animal can completely retire when disturbed,
but many close relatives, such as the Limpets, have shells that are
merely simple flattened cones, while most of the Slugs have no
shell at all. The shell when present is secreted chiefly by the in-
| // Head
Intestine
Respiratory aperture B
Fic. 48. — A, Chiton, ventral view; B, Snail, diagrammatic side view.
conspicuous mantle. The common Snails and Slugs glide along on a
path of slime by muscular contractions of the foot which forms the
entire ventral part of the body, but some of the marine species ac-
tively swim by means of delicate, undulating expansions of the foot.
The Snails and their allies have a well-developed head with
tentacles and eyes, and a mouth, supplied with a unique rasping
tongue-like organ, the RADULA, that leads into a complicated di-
gestive tract. Add to the digestive organs the complex blood vascu-
lar system, excretory system, nervous system, and reproductive
system, and it becomes evident that even the lowly Garden Slug
belies its soft slimy body. (Fig. 48.)
Petecypopa. However, the Mollusca is a phylum of surprises,
for the Pelecypoda is a class of headless animals that for the most
part have taken to a sedentary life within a shell composed of two
VALVES hinged together: they are Bivalves. The adult Oysters
are permanently attached and therefore footless; the Clams are
88 ANIMAL BIOLOGY
sluggish, sand-burrowing creatures; the Shipworms riddle wharves
with their tunnels, while the Scallops swim about by rapidly open-
ing and clapping together the valves of the shell. (Fig. 49.)
The most distinctive feature of the Bivalves, aside from the
shell, is their peculiar method of securing food from a current of
water that is kept in motion by the activity of cilia on the GILLs,
mantle surface, and about the mouth. In brief, water laden with
oxygen and microscopic animals and plants is drawn through an
opening, the INHALENT SIPHON, into the mantle cavity where the
Pericardial sac Ventricle
Auricle
Rectum
Kidney
Bulbus arteriosus
= Bae, adductor
Umb of foot
mbo fp
Upper branchial
TGA, chamber
Anterior adductor Gill
muscles of foot
Labial palp Posterior
Mouth adductor
muscle
Anus
Anterior
adductor
muscle
Exhalent
siphon
Inhalent
siphon
Foot Lower branchial
Mantle Gonad chamber
Liver Intestine
Fic. 49. — Hard-shell Clam, or Quahaug, Venus mercenaria. Dissection
showing structures visible when left valve, mantle, and gills are removed.
sieve-like gills are suspended. Passing through the gills the food is
picked out and carried by ciliary action to the mouth, while at the
same time the blood in the gills is aérated. From here the water
current passes on out by the EXHALENT SIPHON, carrying with it
various waste products.
The reproductive process varies considerably in different Bi-
valves. Oysters spawn in the spring, liberating the sperm and
eggs, and the zygotes develop into microscopic free-swimming
larvae. Within a week these sink to the bottom and become at-
SURVEY OF INVERTEBRATES 89
tached to whatever object they happen to touch and, if fortunate,
gradually develop into adult Oysters. Fortunate, because it is esti-
mated that a larva has less than one chance in a million of surviv-
ing to attain maturity and the qualities that appeal to the human
palate. But one female may contain nearly half a billion eggs.
The eggs of the Fresh-water Clams, or Mussels, are fertilized
by sperm entering the mantle cavity with the food current, and
the larvae develop in the gills which act as temporary brood-
Dorsal
Posterior
Edge of mantle
--Siphon
-Head
~ Edge of mantle.
Eye-
ail gf i
tig? =F €. vt ;
Y
with suckers
Ventral
Fic. 50. — A, B, Octopus, at rest and in motion. f, siphon. C, Squid, side
view. (From Hegner, after Merculiano and Williams.)
pouches. Eventually as tiny clams, known as GLocuip14, they
escape, settle on the bottom of the pond or river, and die unless
a Fish rubs against them. In this event each glochidium becomes
attached to the fish and as a parasite obtains free food and trans-
portation for several weeks until it has developed sufficiently to
shift for itself.
The economic importance of the Bivalves hardly need be men-
tioned. Oyster-farms in America alone produce an annual crop
valued at many millions of dollars. Fresh-water Mussels are the
basis of the pearl button industry of the Mississippi Valley. And,
90 ANIMAL BIOLOGY
finally, pearls produced by the irritated mantle tissue of several
species of Bivalves are perhaps the most highly prized and priced
jewels.
CEPHALOPODA. The Squids, Cuttle-fish, Devil-fish, and their
close allies present in many ways a marked contrast with the rest
of the Molluscs, being relatively active, aggressive animals that
have in most cases practically discarded the shell and developed
a highly specialized ‘head’ apparently by combining head and
foot — hence the class name Cephalopoda. (Fig. 50.)
The head, surrounded by arms, or tentacles, is provided not
only with parrot-like beak and rasping tongue, but also with a
rather large brain, and efficient eyes that superficially are very
like those of Fishes. As a matter of fact some of the Cephalopods |
are in many respects more capable than some of the lower Verte-
brates. They apparently exhaust the possibilities of the Molluscan
body plan both in complexity and in size. Indeed, the Giant Squid
is the largest Invertebrate, for when measured to the tip of the ex-
tended arms it exceeds in length any living Vertebrate except the
largest Whales.
I. ARTHROPODS
From our detour through the unsegmented Molluscan phylum,
we now turn to the more orthodox — because dominant — phylum of
segmented Invertebrates with jointed appendages, the ARTHROP-
opA. The most important classes of the Arthropoda are the Crusta-
cea, Myriapoda, Insecta, and Arachnoidea: the first chiefly aquatic
and breathing by means of GILLs, and the rest typically terrestrial
with TRACHEAE or equivalent air-breathing organs. (Fig. 116.)
CrusTAcEA. A varied multitude of marine and a relatively
small number of fresh-water animals constitute the class Crustacea.
Among its approximately twenty thousand species, probably the
best known, because they are among the largest and some are
edible, are the Lobsters, Crayfishes, and Crabs. The latter appear
unique because the posterior part of the body (ABDOMEN) is
permanently bent forward under the CEPHALOTHORAX. Then
at the other extreme, as it were, are the little-known Wood-lice,
or Pill-bugs, that live in damp places, even in our gardens. All of
these and their many close relatives form one of the two Crustacean
subclasses, the MaAnLacostraca. The other great subclass, the
ENTOMOSTRACA, includes even greater diversity of structure.
SURVEY OF INVERTEBRATES 91
There are the Cirripedes, or Barnacles, that early in their youth
settle down permanently to a sedentary existence, and thereby
foul the hulls of ships and encrust driftwood and stones on the
seashore. Then unrecognized by other than specialists are the
many kinds of microscopic Crustaceans, such as Daphnia, Cyclops,
and their allies. These so-called Water-fleas vie in numbers with
the Protozoa and microscopic plants in the vastness of open
seas as well as in many lakes. Thus they form a crucial part of
the food of larger animals, including Fishes, and so indirectly of
Man. (Figs. 51, 62-64.)
Fic. 51. — Crustaceans. A, Edible Blue Crab, Callinectes; B, Fiddler
Crab, Gelasimus; C, Caprella; D, Daphnia, a Water-flea. (From Paulmier
and Claus.)
Myrrapopa. Passing to the Myriapoda we reach the terrestrial
Arthropods with long serpentine bodies, such as the more or less
flattened Centipedes and the rounded Millipedes, the latter with
the body segments united in pairs, so that there seem to be four
legs on each segment. But, as their names indicate, legs are plenti-
ful, in some Millipedes reaching well on toward two hundred.
Centipedes are carnivorous forms with poisonous jaws but Milli-
pedes are destructive vegetarians. (Fig. 52.)
Insecta. Everyone knows various members of the Insecta,
familiarly represented by that most domestic of animals, the House
Fly, but probably few realize that the species of Insects outnumber
all the other species of the Animal Kingdom. (Figs. 53, 258.)
Fic. 52. — Myriapods. A, Centipede, Lithobius forficatus; B, Millipede
Julus. (Modified, after Koch.)
—~
tied!
Leann oud
y
y
. MY
i
u
‘! ' L
. Ls +
“3
é }
“Lf: Me
( 4
2 Un
i é € : }
aif >
a
£
€
Fic. 53. — Primitive Insects. A, Silver-fish, Lepisma; B, Springtail, Podura.
(From Parker and Haswell.)
92
SURVEY OF INVERTEBRATES 93
A typical Insect is characterized by a body divided into three
major parts: head, thorax, and abdomen. The head bears a pair
of compound eyes, usually one to three simple eyes (OCELLI), a
pair of ‘feelers’ (ANTENNAE), and the mouth parts: the LABRUM,
MANDIBLES, MAXILLAE, and LABIUM. (Figs. 54, 116, 215.)
The thorax is composed of three segments, each of which typi-
cally bears a pair of legs. The legs of Insects in most cases per-
form many functions in addition to locomotion: they are really
a set of tools. Witness the legs of the Honey Bee. Usually two
of the three thoracic segments each bear a pair of wings, but the
House Fly, of course, has but one pair and the Flea none at all,
though the Fly’s ‘balancers’ are remnants of its missing pair. The
wings of Insects are entirely dissimilar in origin and structure
from the legs and therefore bear no relation to the paired append-
Head Thorax Abdomen
| : |
Antouiae | Auditory organ
ee fies eye |
XQ aN Zs
Ni Femur ‘ Spiracles
Tibia
Fig. 54A. — Grasshopper, or Locust, Melanoplus differentialis.
ages of other Arthropods. They are new structures that confer upon
them the honor of being the only Invertebrates to conquer the
air. Indeed, their adaptive radiation to all sorts of habitats and
modes of life exhibits a versatility that somewhat parallels that
of the highest Vertebrates, the Mammals. (Figs. 53, 208, 257,
258, 261.)
And moreover, certain Insects excel all the rest of the whole
living world, except Man, in the remarkable development of com-
munal organization. This involves specialization of individuals
for definite contributions to the economy of the social unit, such
as the Ant nest or the Bee hive. (Figs. 214-219, 222.)
94 ANIMAL BIOLOGY
ZA XXI. COLEOPTERA
XXII. HYMENOPTERA
\ LB
XIV. LEPIDOPTERA
IV. DERMAPTERA
Vv.
II. COLLEMBOLA
L THYSANURA Vil. EPHEMEROPTERA
ANNELID-LIKE ANCESTOR
Fic. 54B. — General relationships of the chief orders of the class INSEcTA.
I, Silver-fish; II, Springtail; III, Praying mantis; IV, Earwig; V, Termite; VI,
Stonefly; VII, Mayfly; VIII, Dragonfly; IX, Lacewing fly; X, Scorpion fly;
XI, Caddice-fly; XII, Stable fly; XIII, Flea; XIV, Butterfly; XV, Bird louse;
XVI, Cootie; X VII, Book louse; X VIII, Thrips; XIX, Cicada; XX, Triatoma
kissing bug; X XI, Potato beetle: XXII, Bumble bee. In general, the classi-
fication is based on the presence or absence of wings, the structure of the mouth
parts, and the character of the life history. See page 478. (From Hegner.)
SURVEY OF INVERTEBRATES 95
ARACHNOIDEA. We conclude the Arthropod phylum with the
class Arachnoidea: the Spiders, Ticks, Mites, Scorpions, and their
close relatives. These are frequently confused with Insects, but the
more common forms, such as the Spiders and Ticks, are readily
distinguished by the possession of eight legs. (Fig. 55.)
Spiders are carnivorous animals that capture their prey by
elaborately constructed webs, or by stalking and pouncing upon
/ : it. Some Spiders are poisonous and their repu-
| | tation has served to malign many harmless
relatives. But the Scorpions are in general
Hand
ee en
Chelicera \\_,Pedipalp
Lateral eyes
“Median eyes
Cc
Fic. 55. — A, Spider, Epeira verrucosa (from Emerton); B, Scorpion, Buthus
occitanus (from Krapelin); C, King-crab, Limulus polyphemus.
poisonous, and the Mites and Ticks are injurious to Man in many
ways. Some bite and others burrow into our bodies and those of
our domestic animals; the common Rabbit frequently harbors
several thousand Ticks. Even our garden crops and forests suffer.
Unfortunately certain species infect their hosts with various Bac-
teria and Protozoa and so produces serious diseases. But there
are many harmless forms, represented by the well-known red
Harvest Mites.
And then appended to the Arachnoidea is the peculiar marine
King-crab, or Limulus, that is of considerable theoretical interest
to students of evolution. (Fig. 55, C.)
96 ANIMAL BIOLOGY
This welter of Arthropod forms, as already suggested, is built
on the plan of a chain of segments; two or more of the anterior
segments constituting the head, with the mouth, and the posterior
one containing the anus. In the simplest Arthropods there is
relatively little differentiation between either the segments or
the characteristic pair of jointed appendages that each bears; but
proceeding to more complex forms, one finds a progressive union
and specialization of segments in certain regions of the body and
a shifting and transformation of their appendages and internal
organs for one function or another. Indeed it would seem that
all the possible changes are rung on the pervading segmental
chain: a fact to be illustrated later when we study the Crayfish.
(Figs. 62, 63.)
Another conspicuous feature of the phylum is the presence of
a hard, unyielding external armor, or EXOSKELETON, with flexible
joints moved by attached muscles. This skeleton hampers the
increase in size of the inhabitant, so periodically it is shed — the
animal MOULTS. Seizing the opportunity, so to speak, the animal
rapidly increases in size at the expense of material stored for this
purpose, and also secretes a new skeleton. Of course, a ‘soft-shelled’
Crab is one which has recently mculted and has been taken at a
disadvantage before the newly secreted skeleton has had time to
harden.
The life history of many of the Arthropods is so surprisingly
complex, involving such radical form changes, that it is termed a
METAMORPHOSIS. Thus the embryo of certain Crustacea may be
hatched as an unsegmented larva, then after moulting assume a
segmented larval form, and so on until the adult state is attained.
In other Crustacea one or more of these stages may be briefly
summarized, as it were, in the egg before hatching. Finally,
animals like the Crayfish hatch with essentially the adult form.
And this series of metamorphic stages in the development of the
higher Crustacea is of considerable theoretical interest, because
they are very similar to the larval or adult forms of certain other
Crustacea that are regarded as more primitive in organization.
Thus it would seem that individual development in the higher
Crustacea briefly and very broadly and incompletely summarizes
— RECAPITULATES — the ancestral, or evolutionary, history of the
race.
However, metamorphosis is called to our attention more promi-
SURVEY OF INVERTEBRATES 97
nently in the Insects. It is common knowledge that caterpillars are
the larval, worm-like feeding forms of Butterflies and Moths. The
winged adult condition is attained by a final moult that takes place
while the larva is in a ‘resting’ condition, the PUPA, made necessary
by the radical structural changes which are involved. Perhaps it
is not such common knowledge that even Locusts, or Grasshoppers,
undergo metamorphosis, because no one moult results in such
marked changes. The young Locust is similar in form to the adult,
but after each moult it is larger than before, and finally is a fully
winged adult. Such a transformation not involving a pupal
stage is frequently referred to as incomplete metamorphosis.
(Figs. 257-260.)
So we conclude for the present our necessarily limited view of
the Arthropods, but as we proceed with our study we shall fre-
quently have occasion to discuss special representatives. Judged
by the stupendous number of species and individuals, or by variety
Fic. 56. — Peripatus, Peripatus capensis. An interesting Arthropod that seems
to link the Arthropods with the Segmented Worms. (From Sedgwick.)
of form, or by sheer success in competition with both lower and
higher animals in air, water, and on the ground, they are “ Nature’s
most successful Invertebrate experiment.” The segmental plan
begun in a small way in some of the lower worms, is definitely
established in the Segmented Worms, and is made the most of
in the Arthropods. (Fig. 56.)
However, Arthropods are hampered by inherent structural limi-
tations. The hard, dead, external skeleton imparts certain me-
chanical restrictions on size and freedom of action that are re-
moved in Vertebrates by an internal living skeleton. And small
size precludes a constant body temperature greater than the sur-
roundings, so they cannot achieve the constancy of living possible
to the warm-blooded Birds and Mammals. Nevertheless it has
been suggested as not beyond the range of possibility that Insects
may yet dominate the Earth!
CHAPTER VIII
THE INVERTEBRATE BODY
Nature is so varied in her manifestations that many must unite
their knowledge and efforts in order to comprehend her. — Laplace.
From our survey of the Invertebrates it is obvious that the
highly complicated and varied organization of animals renders
it impossible to present a concise and adequate plan of a typical
animal body. It is therefore necessary in the present work to
select one group of animals as the basis of study and then to com-
pare with this, in so far as comparisons are possible without con-
fusion, a few of the most significant morphological and physio-
logical variations presented by other groups. We naturally select
the group of Vertebrates for chief consideration not only because
its relative homogeneity renders it the most available, but because
it includes Man. However, even before we focus attention on the
Vertebrates, it is necessary to bring into clear relief certain struc-
tural principles that we have seen exhibited among the Inverte-
brates — selecting as types the Hydra, Earthworm, and Cray-
fish — in order to afford a background for the consideration of
Vertebrate structure and function.
A. Hypra
In discussing the development of animals, it was pointed out
that the dividing egg typically forms first a blastula which, in
turn, becomes transformed into the gastrula stage. The gastrula
is essentially a sac composed of two layers of cells: an outer, or
ectoderm, and an inner, or endoderm, layer. Although no adult
animals retain this simple gastrula form, those composing the
group known as the Coelenterates are to all intents and purposes
permanent gastrulae since their bodies are built on the plan of a
two-layered sac. This is well exhibited in Hydra, the almost micro-
scopic, fresh-water polyp which is commonly found attached to
submerged vegetation or stones in brooks and ponds. (Fig. 31.)
The body of Hydra somewhat resembles a long narrow sac, the
base constituting the foot, and the opening at the opposite end
98
THE INVERTEBRATE BODY 99
forming the mouth. Surrounding the mouth is a circle of outpocket-
ings of the body wall, termed tentacles. The main axis of the body
extends from foot to mouth, and every plane passing through this
axis divides the body into symmetrical halves. In other words, the
parts of the body are symmetrically disposed about, or radiate
from the main axis, and so Hydra affords an example of RADIAL
SYMMETRY. (Figs. 38, 57, 155.)
The body wall of Hydra is composed of two distinct cell layers,
ectoderm and endoderm, separated by a thin non-cellular support-
Ectoderm
Endoderm
Testis
ei Enteric cavity
Pe
2
fo,
O
Older bud -Se ck
CS
Ay)
Py shNaD
wien lela ores Ovary
oy
ie cs
‘Wy; f a
Zils
Fic. 57. — Hydra. A, two specimens with buds, one contracted; B, dia-
grammatic longitudinal section. (From Newman, after Pfurtscheller, and
Parker.)
NY
ing layer of jelly-like material (WESOGLOEA) secreted by the cells
of both ectoderm and endoderm. Hydra thus illustrates a simple
type of Metazoan structure in which but two primary tissues
exist; such specializations as are necessary for the performance of
the essential life functions being confined to the various cells that
compose these layers. The majority of the cells of the endoderm
which line the ENTERON, enclosing the ENTERIC CAVITY, are con-
cerned with the digestion of solid food taken in through the mouth,
100 ANIMAL BIOLOGY
while those of the ectoderm are variously modified for protection,
and the other relations of the individual to its surroundings, as
well as for reproduction.
In short, in the organization of Hydra the primary tissues (ecto-
derm and endoderm) have not become differentiated into secondary
v Ectoderm
Mesogloea
‘Endoderm
Enteric cavity
a
alec
Las bs
& :
io
Paden
} a4
\ oX(\
s
Ss rite bs,
)
es A j . eo: , 0)
aot
i} A i>
Fic. 58. — Hydra. Transverse section highly magnified.
“i {
He
g s
a
<<
specialized tissues (muscular tissue, nerve tissue, etc.) for one
function or another — the simple life processes of the animal are
adequately provided for by the specialization of isolated cells or
small cell groups within ectoderm and endoderm. (Fig. 58.)
B. EARTHWORM
The bodies of all animals above the Coelenterates are built of
three primary layers, which, as development of the individual
proceeds, give rise to the secondary tissues and thereby form a
relatively complex body. This third primary layer (tissue), the
MESODERM, typically is developed, as we have described earlier,
from the endoderm and comes to occupy the position held by the
mesogloea of Hydra; that is, between the ectoderm and the endo-
derm.
The development of the mesoderm is the key to the advance in
body organization of higher animals, because it makes possible
a radical change in plan that involves the establishment of a body
THE INVERTEBRATE BODY 101
cavity, or COELOM, in which are disposed many of the chief organs.
Accordingly the Coelenterates, with an enteron but no coelom,
are referred to as ACOELOMATES, and the animals above the Coelen-
terates, since they possess the coelom, are known as the CoELo-
MATES. The difference in structure can best be made clear by
comparing the body plan of a higher Invertebrate, such as the
common Earthworm, with that of Hydra. (Fig. 313.)
1. Body Plan
Whereas the Hydra body is essentially a sac composed of two
layers of cells surrounding the enteric cavity, the body of the
Earthworm is built on the plan of a tube within a tube — the
Capillaries
Dorsal blood vessel
/
Mouth Ventral Ovary | Sub-intestinal
ganglia Qviduet blood vessel Nephridia
Fic. 59. — Diagrams of the body plan of the Earthworm. A and C, Jongitu-
dinal sections; B, transverse section. (From Sedgwick and Wilson.)
outer tube forming the body wall, and the inner, the wall of the
digestive tract, or alimentary canal. The walls of these tubes
merge into each other at both ends, and thus together they enclose
a space, the coelom. Or, to state it another way: the outer tube,
or body wall, surrounds a space, the coelom, in which is suspended
a second tube, the alimentary canal, which opens to the exterior
at either end forming the mouth and anus. (Figs. 59, 60.)
The coelom of the Earthworm is divided by a large number of
transverse partitions, called septa, which extend from the inner
surface of the body wall to the outer surface of the alimentary
canal. The result is that the worm’s body cavity is not a continuous
102 ANIMAL BIOLOGY
Cerebral ganglion: brain Cavity of pharynx
Subesophageal ganglion
Wall of pharynx
Body wall of fifth segment Retractor muscle of pharynx
Beginning of esophagus
Ventral blood vessel tree
——Seventh segment
Subneural blood vessel
Internal opening
of a nephridium
End of nephridium
opening to exterior
First aortic loop
Septum
Lateral blood vessel
Seminal receptacles
Basal part of Ss
seminal vesicles ——
Calciferous gland
207 (/
Fifth aortic loop
Parietal blood vessel
Ovary Dorsal blood vessel
Oviduct Posterior seminal vesicle
- Posterior end of esophagus
Crop
Ventral] nerve cord
Gizzard
Beginning of intestine
Fic. 60.— Earthworm. Diagram of a dissection, lateral view. (Modified,
after Linville and Kelly.)
THE INVERTEBRATE BODY 103
space running from one end of the animal to the other, but con-
sists of a linear series of chambers through the center of which
runs the alimentary canal. The limits of these chambers are in-
dicated on the outside of the worm by a series of grooves which
encircle the body wall. In short, the body is made up of a series
of essentially similar units known as SEGMENTs, and thus affords
a simple example of SEGMENTATION, which is a characteristic ex-
pressed in varying degrees in nearly all the higher animals.
Many of the chief organs of the Earthworm are developed as
outgrowths from the walls enclosing the coelom, so that it is in
Dorsal blood vessel Longitudinal muscle
Circular muscle
wels<
CAKE YY F. .
Typhlosole PSS
Ly SNE
Intestine
Chloragogen cells Ce ety
NS
Epidermis
Cuticle
=
(ogni
SS
™
o
or
9
@
a) \;
(
aw
“a
Coelom = i = aN 14 Menhendi
Ventral vessel , OIG a WV ARE bis ain
(AEE LLNS
YA \ fr THU MAUUNY NON WiiviinninssSesE ETE > '
ee ute i neraran Nephrostome
Setae aa
Nephridiopore
Nerve cord Subneural vessel
Fic. 61. — Transverse section through the middle region of the body of
an Earthworm.
this cavity that we find, for example, the main organs devoted to
circulation, excretion, codrdination, and reproduction. Moreover,
the organs are symmetrically arranged with respect to the long
axis of the body which passes from mouth to anus. For instance,
the chief blood vessels and the nerve cord lie in the long axis
and extend from end to end, while the excretory and reproductive
organs are disposed in pairs on either side of this axis. Thus
there may be passed through the main axis a single plane which
divides the body into symmetrical halves, each of which is a * mirror
image’ of the other. The main axis, therefore, extends from the
mouth (anterior end) to the anus (posterior end), and the plane
104 ANIMAL BIOLOGY
which divides the body into right and left sides passes through the
upper (dorsal) and lower (ventral) side: the body exhibits BILAT-
ERAL SYMMETRY which is characteristic of higher animals. (Fig. 61.)
2. Tissues and Organs
Bilateral symmetry practically implies the existence of definitive
parts of the body, or organs and organ systems, and these we find
highly developed in the Earthworm. Again, the presence of organs
demands a much greater differentiation of tissues than occurs in
Hydra where local modifications of ectoderm and endoderm serve
the purposes of its relatively simple organization. Accordingly in
the Earthworm and in all higher forms the mesoderm is added to
the two primary cell layers, and from these three there is developed
a great variety of special tissues: epithelial, supporting, muscular,
circulating, nervous, and germinal. Finally, the codperation of
tissues to form organs demands the further codperation of organs
to form organ systems, each of which plays its part in the economy
of the whole organism. (Figs. 32, 34.)
Thus it is clear that the body plan of the Earthworm and all
higher forms is radically different from that of Hydra, exhibiting
as it does such essential new features as mesoderm, coelom, bilateral
symmetry, segmentation, specialized tissues, definitive organs, and
complex organ systems. The persistence and development of this
basic plan from Earthworm to Man is interpreted by biologists as
evidence of evolution.
C. CRAYFISH
Bearing in mind the general plan of the body of the Earthworm,
we must next consider briefly the main principle underlying the
Intestine Dorsal blood
live Cerebral ganglion vessel _~Chitinous exoskeleton
Antenna 7 eet eer =
LILES
appendages : | _ — =
Subesophageal Ventral nerve Jointed
ganglion cord ganglion appendages
Fic. 62. — Diagrammatic representation of the structure of an ideal primi-
tive Arthropod in which very little specialization of the segments has occurred.
(From Schmeil.)
changes in this plan which give rise to many of the diverse forms
among the higher animals.
uos|a,
snuy
soSepuadde
=
jeurwopqy
ajosnul
[bul wopqy
f
Araqae [esog
ee |
Sorte;
euljsaquy]
uolsurs
jeeseydosaqns
Aqoyie
Teague spadi[ixe]{]
yonp weds p109
jo SuruedgQ dATOU LaPUr IN
[exquaA,
Arde
jeusa3s
a
qarey
oyut
suiuadg
is2)
dp
Q youwo}s % e
He Towed ¢ Ss oS
iS) = © ©
3 IOWI9}SOq © 2. ve
3.
A1dAB =
uvipeul
JOWaUy
—
ulBiq
‘uolduesd
[Agata
%,
yInoy
v/
hou
Ba] d1IvL0YY
wnipiuydau
jo sutuodg
?
7]
—— sa]nuuejuy
1 eS
i]
yu
=
vuuaUy
eq,
wns SOY
EYNG |
Fic. 63. — Crayfish, Cambarus affinis. Dissection, lateral view-
105
106 ANIMAL
2 3 Protopodite
.Exopodite
Endopodite
M.
ee Endopodite
Mx, 1 a
Mp. 1
Epipodite ~
Endopodite
Protopodite,.. a °
Protopodite
Epipodite »
BIOLOGY
The body of a primitive Ar-
thropod differs from that of the
Earthworm chiefly in the reduc-
tion of the number of segments
and the development of paired
jointed appendages as outgrowths
from the body in each segment.
From such a primitive type all
the multitude of diverse forms of
Arthropod bodies can be derived.
For instance, in the Crayfish,
which is essentially a fresh-water
Lobster, the body consists of
twenty-one segments, of which
segments 1 to 6 together form
the HEAD; segments 7 to 14, the
THORAX; and segments 15 to 21,
the ABDOMEN. In other words,
by the union or complete fusion
of certain segments, the body has
become divided into more or less
distinct regions. (Figs. 62, 63.)
Furthermore, the primitive lo-
comotor appendages of the re-
spective segments have become
modified into organs for widely
different functions: those of the
head, as sensory organs, jaws,
etc.; those of the thorax, as organs
for grasping, offense and defense,
and walking; and those of the
abdomen for swimming, etc. Thus
change in structure has gone on
Fic. 64. — Typical appendages of a
Crayfish. All have been derived from a
simple biramous appendage. Protopo-
dite, endopodite, and exopodite are ho-
mologous throughout the series. A. 1,
antennule; A2, antenna; L. 4, fourth walking leg; M., mandible; Mp. 1, first
maxilliped; Mp. 2, second maxilliped; Mp. 3, third mazxilliped: Mx. 1, first
maxilla; Mx. 2, second maxilla.
(From Hegner, after Kerr.)
THE INVERTEBRATE BODY 107
hand in hand with change in function, so that although there is no
obvious resemblance between the jaws of the Crayfish and the legs
employed for swimming, nevertheless a study of their develop-
ment shows beyond doubt that they owe their origin to modifica-
tions of one primary type. Accordingly the various appendages
are said to be HOMOLOGOUS, signifying a fundamental similarity
of structure based on descent from a common antecedent form.
(Figs. 64, 65.)
On the other hand, organs of fundamentally dissimilar structure,
which nevertheless perform the same function, are called ANALO-
Dorsal abdominal artery Abdominal muscles
Intestine
Tergum Ventral
a4 nerve cord
Muscles of
Pleura appendage
Protopodite
Endopodite WF
Exopodite i\ WASH Ventral abdominal artery
Fic. 65. — Crayfish. Transverse section of fifth abdominal segment.
cous. In Insects the series of head appendages and the legs are
homologous with those of the primitive Arthropod type, while the
wings are new, unrelated structures and not modifications of the
primitive serial appendages of the ancestral form. However, as
we shall see later, the wing of a Bird and the arm of Man are
homologous, while the wing of an Insect and the wing of a Bird
are analogous structures. One of the chief tasks of the branch of
biology known as COMPARATIVE ANATOMY is to determine the
various parts of animals which are homologous, and to study the
adaptive changes which are associated with change of function.
(Fig. 227.)
108 ANIMAL BIOLOGY
We have considered the principle of specialization and fusion of
the segments of the higher Arthropods in so far as it affects external
structures, but profound modifications of the internal organs also
occur. In the first place, the partitions between the various seg-
ments which are present in the Earthworm have disappeared in the
Crayfish. Again, the alimentary canal of the Earthworm is a
nearly straight tube extending through the body, with relatively
slight modifications in certain segments for the elaboration of the
food material as it passes along from
mouth to anus; while in the Crayfish
we see the accentuation of such modi-
fied regions, and the development of
large outpocketings, or glands, which
are specialized for the formation of
chemical substances to DIGEST the food
material. That is, to change the food
into a soluble form so that it can pass
through the cellular membrane which
lines the digestive tract and thus ac-
tually pass to the circulatory system
for distribution to the tissues of the
animal.
As a final illustration we may take
the nervous system. In the Earthworm
Fic. 66. — Diagram of the
anterior portion of the central
nervous system of an Earth-
worm (A) and a Crayfish (B).
a, brain (cerebral, or supra-
esophageal, ganglion); b, nerve
commissures, encircling the
pharynx (shown in section);
c, subesophageal ganglion; d,
ganglia of the ventral nerve
cord, with nerves emerging.
this consists of a NERVE CORD which
runs along the body in the mid-ven-
tral line below the digestive tract. At
the anterior end, it divides into two
branches which encircle the digestive
tract and unite above in a relatively
large body of nervous tissue which
constitutes the cerebral ganglion, or BRAIN. In each segment
the nerve cord also is somewhat enlarged to form masses of
nerve tissue (GANGLIA) from which NERVES pass to the organs
in the vicinity. The nervous system of the Crayfish exhibits the
same general plan as that of the Earthworm, but certain modifica-
tions have been brought about by the uniting of segments in the
region of the head and thorax. This process has resulted in the
fusion of the segmental ganglia in this region into larger ganglionic
masses. The brain of the Crayfish, for example, comprises the
THE INVERTEBRATE BODY 109
primitive ganglia of the segments which have united to form the
head. (Fig. 66.)
We have now reviewed and emphasized the body plan of Hydra,
Earthworm, and Crayfish as representative Invertebrate types
that illustrate several of the fundamental structural principles
which are to be found in the Vertebrate body. But it will be re-
called that certain other Invertebrates exhibit body plans that
superficially at least depart very widely from the types described,
although it is believed that these forms do not break the general
evolutionary continuity of the Animal Kingdom.
‘SoUIT} 90IY} JHOGeR peyuseyy “PpoAOUd [TBM Apoq 1J?]
-I9 A JSOMOJ IY} JO BAIPBOI Ivo w ‘$N}D]0dIUD) DUO}SOYIUDLE ‘snxorydur
dq} JO JSOUL YPIM MOIA [BIO] Sd} RIG}
y ‘oyepsoyy oArjtuntid wv jo Kurozeue pesos) — *L9 “OT
snuy, a10do1yy WIN[NOIEAIpP IOAVT xudaeyd Jo
uy [epne QWI0J OAT
sieq [vIyouRIg
sy[s [11D Lalo [eoong
peuoy, worry | } sepoejuey re[9A
oD Ssh a * ; mK | (hes Vay aS 3
Seger ae RN Bg emerges ee ak es ie ome RSI : NS . WY n ~ \! my: as Lop
rn TTS 7 CoO cue KS % = * st Spore aece * ie . ; 5 1% Ce por een - a Gpiratts “an Re aireies :
eee mr, . SiG Orns dD Seed rice tL etseet tian RON TU
SS LLM OE
EERE WO CULL Fevowrrry voreverecverrvvveveemnUrcTaTTICTCo@ or
paoo jeurds UleIg,
p10yooj0ON teas Apog shel UL] au0z04]ff yno” = pooy [HQ
uly [ess0g
110
CHAPTER IX
SURVEY OF VERTEBRATES
The wise man wonders at the usual. — Emerson.
Now that we have viewed the important phyla of the lower
animals, it remains to survey the highest and concluding phylum
of the Animal World, technically known as the CHorpbaATa, which
for all practical purposes is synonymous with the VERTEBRATA.
The only Chordates that are not Vertebrates, or BACKBONED
animals, are a few lowly creatures apparently having Invertebrate
and certainly Vertebrate affinities; the latter chiefly evidenced by
the presence of a NOTOCHORD which is the forerunner of the back-
bone, a dorsal NERVE TUBE, and GILL SLITs for respiration. Among
these primitive Chordates the most interesting is Amphioxus be-
cause it is most closely related to the Vertebrates. (Fig. 67.)
It will suffice for us, then, to proceed directly to the true Ver-
tebrates, and since emphasis is later to be placed on the anatomy
and physiology of the Vertebrate body because Man is a Verte-
brate, our immediate attention can be largely confined to their
classification.
The Vertebrates include all the larger and more familiar animals
— Fishes, Amphibians, Reptiles, Birds, and Mammals — so that
in the popular mind the words animal and Vertebrate are essen-
tially synonymous. A Fish, as everyone knows, is an aquatic back-
boned animal which breathes by means of gills and moves by fins.
An Amphibian, such as a Frog, may, in a general way, be thought
of as a Fish which early in life — at the end of the tadpole stage —
discards its gills, develops lungs, substitutes five-toed limbs for
fins, and takes up a terrestrial existence. Similarly, a Reptile, say
a Lizard, may be pictured as an Amphibian which has relegated,
as it were, the tadpole stage to the egg, and therefore emerges
with limbs and lungs. Birds and Mammals may be regarded as
separate derivatives of the reptilian stock which have transformed
the scales of the Reptile into feathers and hair respectively, and
have developed a special care for their young: the Birds by incu-
bation of the eggs and the Mammals by retention of the young
ig
112 ANIMAL BIOLOGY
essentially as parasites within the body of the female until birth
occurs. (See: Appendix I.)
It will be appreciated, of course, that other important charac-
teristics — many of which will be apparent as we proceed —
delineate these chief Vertebrate groups; but, in fact, the Verte-
brates as a whole are remarkably homogeneous both structurally
and functionally, the most obvious external differences to the
contrary. Some of the outstanding characters typical of Vertebrates,
in addition to the unique notochord, living endoskeleton, dorsal
nerve tube (spinal cord), and gill slits, are bilateral symmetry,
traces of segmentation, coelom, red blood corpuscles, brain encased
in a skull, paired appendages (fins or limbs), and a tail. (Fig. 94.)
Vertebrates are fhe modern animals: the “athletes of the Animal
Kingdom.” ‘They have parcelled out, as it were, the available
environment amongst themselves. The Fishes dominate the waters,
the Birds, the air, and the Mammals, the land. To be sure, the
Amphibians waver between water and land, and the Reptiles are
chiefly terrestrial; but both are minor groups to-day: the suprem-
acy of the Reptiles passed to the Mammals in the geological yes-
terday. Man is a Mammal.
A. FISHES
Living as they do in an aquatic environment, Fishes find at
least two problems of large-bodied, active, terrestrial animals con-
siderably simplified. In the first place, the density of water makes
less necessary either supports to raise the body or sturdy muscles
to move them. Thus the paired appendages, FrNns, and the tail of
Fishes are adapted solely for propulsion and steering. In the
second place, although an efficient respiratory apparatus is re-
quired, no special provision is needed to maintain the respiratory
membranes moist. The water merely passes into the mouth, over
the gills, and then cut through the gill slits. (Fig. 117.)
Fishes are cold-blooded since they possess no mechanism to
maintain a constant body temperature — a character they share
with the Amphibians and Reptiles. Most species are ovIPAROUS —
the eggs are shed; but some, such as the well-known Guppy, are
VIVIPAROUS — the eggs develop within the mother’s body and the
young are born.
If we neglect the primitive fish-like creatures devoid of true jaws
and paired fins, known as CycxLosromes, and the peculiar
SURVEY OF VERTEBRATES 113
LuNG-FISHES to be mentioned later, the Fishes fall into two main
groups: the Sharks and Rays with an internal skeleton of gristle,
after Dean.)
skeleton is largely replaced by one of BonE. The latter group com-
prises the dominant Fish population of the Earth to-day, repre-
sented, for example, by the Mackerel and Perch, Goldfish and
Guppy, and a Goby less than one-third of an inch long — the
smallest known Vertebrate. (Figs. 68, 75.)
1. Sharks and Rays
The most primitive of the true Fishes are the Sharks and Rays,
or ELASMOBRANCHS: a small remnant of a once dominant group
Fic. 69. —Sharks. A, Frilled Shark, Chlameidoselachus anguineus; B, Thresher
Shark, Alopecias vulpes; C, Angel Shark, Rhina squatina. (From Newman.)
of Vertebrates. They differ from higher Fishes chiefly by a car-
tilaginous skeleton, by gills communicating directly with the
114 ANIMAL BIOLOGY
body surface by several gill slits, and by a skin roughened by
small tooth-like projections.
Sharks are confined to marine waters and are most abundant
in the tropics. The most common species off our coasts are the
ZEN, ONG
(uy Waa
- ——
I
Ltr
ee _ = Le d Pia
re ae = eee eve, = ~
ZE, igre tie : sece, SS ed
“Wy, ; \\)
“nd Me
Ly \S\wr
Uff Ny
Fic. 70. — A, Sting Ray, Stoasodon narinari; B, Eagle Ray, Myliobatis
aquila. (From Newman, after Jordan and Evermann, and Bridges.)
small Dogfish Sharks, notorious pests to fishermen but favorites
for dissection in zo6logical laboratories. (Figs. 69, 120.)
Whereas the Sharks have the typical stream-line body of swift
swimmers, the Rays, or Skates, are bottom-dwellers, and have a
0
4
Fic. 71. — Mackerel, Scomber scombrus.
greatly flattened body with eyes on the dorsal, and mouth and
gill slits on the ventral surface. The most famous of the Rays are
the Torpedoes, so-called because they are able to give a severe
electric shock. (Fig. 70.)
SURVEY OF VERTEBRATES 115
2. Bony Fishes
All of the fresh-water Fishes, as well as the great majority of
those dwelling in the sea — indeed, what we usually think of as
Fishes — typically have a skeleton of
bone, and a body-covering of scales.
They are Bony Fishes, or TELEosTs.
True, the more primitive forms have
partially cartilaginous skeletons, and ,
bony plates instead of scales, as rep-
resented by the Garpikes and Stur-
geons, but they are exceptions form-
ing less than five per cent of the
group. All Teleosts have the external
openings of the gill slits covered by a
protecting flap, or OPERCULUM, so that
water bathing the gills leaves the body
by a single opening at the posterior
edge of the operculum on either side of
the head. (Figs. 71, 100, 106.)
Modifications of the typical fish-like
form of swift-swimming Fishes that Mie (2. ee nO ete
pocampus antiquorum. Male
have been assumed by various species showing brood pouch formed
in adaptation to different habitats and ftom combined pelvic fins.
: : : (From Doflein.)
modes of life are legion. One imme-
diately thinks of the snake-like body of the Eels, the grotesque
form of the Sea-horses, and the compressed body of the Flat-
fishes, such as the Flounders and Halibuts. But Flat-fishes are
ee hatched with typical fish
K GG AN form; and it is only as
Ses ° the animals settle down
on one side that the ‘un-
der’ eye moves up and
Ns oS over so both are on top.
4) YT Even the bizarre form ex-
Bid. ea latch. hibited by the Filying-
fishes that jump and sail
above the water is exceeded by the denizens of the ocean’s deepest
reaches, where sunlight never penetrates, no plant life grows,
the pressure is tremendous, and the temperature is only slightly
116 _- ANIMAL BIOLOGY
above the freezing point. In such surroundings some possess
luminous organs, some have immense eyes to make the most
of little light, some are blind and so depend upon tactile organs,
some have an immense mouth and an enormous stomach capa-
Fic. 74. — A, a Flying-fish; B, a deep-sea Fish. (From Gunther and Lull.)
ble of digesting a fish nearly their own size, and so on and on —
adaptations seemingly endless — a Fish for every condition of life
in water. (Figs. 72-74.)
3. Lung-fishes
Finally, mention must be made of a group that flourished in
the geological past but is represented to-day by only five fresh-
water species. They are known as Lung-fishes, or DIPNOANS,
a one ee as Wemricc: -
\ er
—
ee ae ha
DARL
Fic. 75. — African Lung-fish, Profopterus annectens. (From Dean.)
because an AIR SAC, which in most other Fishes acts as a hydrostatic
organ, here opens into the pharynx and functions as a LUNG when
sufficient water is lacking during a drought. Lung-fishes have been
regarded as intermediate between Fishes and Amphibians, but
they have many structures that are similar to those of the lower
Fishes. They are an interesting and puzzling remnant. (Fig. 75.)
SURVEY OF VERTEBRATES 117
So much for the Fishes — a group that is of such great economic
importance that the governments of progressive countries spend
vast sums for their study, protection, and propagation. The im-
portant food and game Fishes are almost without exception repre-
sentatives of the higher Bony Fishes — relatively modern forms
that did not exist during the Age of Fishes, when Fishes were the
only Vertebrates, but appeared later during the Age of Reptiles.
(Fig. 232.)
B. AMPHIBIANS
The members of the class AMpPHIBIA, commonly represented
by the Frogs and Salamanders, made a great forward step in Verte-
brate evolution by adopting —if somewhat falteringly — the
land-habit. This opened up to them a vast environment closed to
Fishes and demanded lungs and supporting limbs. The limbs
apparently were derived from the paired fins of Fishes and
built on the plan that persists in all the higher Vertebrates.
(Figs. 102, 103.)
True, most Amphibians are cold-blooded, slimy-skinned animals
that spend the early part of their life with fins, tail, and gills and
only substitute, or add, limbs and lungs when finally they emerge
on dry land. But they do make the change, and during this meta-
morphosis from the larval to the adult form they apparently re-
capitulate broadly their evolutionary history.
Nearly all Amphibians return to water to breed, and many
spend the cold months in a dormant condition buried in mud at
the bottom of ponds and streams. During this HIBERNATION period
the metabolic processes are greatly reduced, and the temperature
is little above the surroundings. However, Frogs cannot survive
being actually frozen although they may remain alive when em-
bedded in a solid block of ice.
Those Amphibians that retain the tail throughout adult life
constitute the order CaupaTa, and those deprived of this struc-
ture during metamorphosis, the order SALIENTIA.
1. Salamanders and Newts
The tailed Amphibians, or Caudata, such as the Salamanders
and Newts, though hatched as aquatic larvae, known as TADPOLES,
undergo a relatively inconspicuous metamorphosis that varies
considerably in different species. Thus some retain their gills
118 ANIMAL BIOLOGY
throughout life, though functional lungs are developed; others
resorb the gills but retain the gill slits; and still others lose all
traces of both gills and gill slits. In fact, some even go to the ex-
treme and lose their lungs, thus depending solely upon the moist
skin to act as a respir-
atory membrane. Obvi-
ously the lung-breathing
method is not consistently
adopted.
Common tailed Am-
phibians are Necturus (the
‘Mud-puppies’), Cry p-
tobranchus (the ‘Hell-
? benders’), Amblystoma
Fic. 76. — A, Amblystoma, Amblystoma (the Blunt-nosed and
tigrinum; B, Necturus, Necturus maculosus. ~..
(From Noble.) Tiger Salamanders), and
Triturus (the Newts).
Several species of Amblystoma have recently proved a boon to
biologists interested in fundamental problems of growth, vieing in
this field with the lowly Flatworms, such as Planaria. From
tadpole to adult they possess remarkable powers of REGENERATION:
they repair minor and major
mutilations, restoring excised
eyes and amputated limbs
and cven appropriating the
limbs of other species that
are grafted. Ingenious experi-
ments have given an entirely
new conception of the marvel-
lous plasticity possessed by at ta
Fic. 77.—A Newt, Triturus cristata.
least some of the Oe Verte- A, female; B, male during the breeding
brates. (Figs. 76, 77.) season. (From Gadow.)
2. Toads and Frogs
The majority of Amphibians, some nine hundred species of the
order Salientia, are Toads and Frogs with a relatively clear-cut
metamorphosis from tadpole to limbed, lung-breathing, tailless
adult. The common Toads, such as Bufo americanus, hop about
chiefly after dusk devouring Worms, Snails, and Insects and so
render a considerable service. In fact, someone has estimated that
EE AY ur
ail
ae 5 = —
Ze dnd an a — BE w=,
]
SURVEY OF VERTEBRATES 119
the gardener owes a Toad on his premises nearly twenty dollars
at the end of the season. Moreover, the Toad is much maligned
by having attributed to it the power to produce warts on the
human skin.
Tree Frogs and Tree Toads are tiny arboreal forms with soft,
adhesive pads on the toe tips. Many of them, such as the common
Fic. 78.— A, Toad, Bufo americanus, stalking prey; B, Leopard Frog,
Rana pipiens; C, Javan Flying Frog, Rhacophorus pardalis; D, Tree Frog,
Hyla versicolor. (From Newman, after Dickerson and Lydekker.)
Tree Frog, Hyla versicolor, are able to change their color through
various shades of gray, brown, and green, and so are rendered
inconspicuous in their natural surroundings. (Fig. 78.)
The true Frogs are represented by several well-known species
in the United States: among them the Leopard Frog (Rana pipiens),
the Bull Frog (Rana catesbeiana), and the Green Frog (Rana clami-
tans). More need not be said about Frogs at this point because
they will be referred to again. As a matter of fact they are in
many ways ideal subjects for anatomical and physiological in-
vestigations and therefore have contributed considerably to the
advancement of biological science. (Figs. 107, 175.)
120 ANIMAL BIOLOGY
C. REPTILES
Apparently descended from primitive Amphibians, the Reptiles
met new and more favorable land conditions with progressive
structural and physiological features: for example, they skipped
metamorphosis and so started out from the egg with functional
lungs and on four feet. So the Reptiles very soon, geologically
speaking, became the dominant Vertebrates on the Earth, flourish-
ing both in number of individuals and variety of species adapted
a
Fic. 79. — Reptiles of the past. A, a Dinosaur, Branchiosaurus (length
about 80 feet); B, a Pterodactyl, Rhamphorhynchus. (From Lull.)
to all sorts of land and swamp conditions, and even, secondarily,
to aquatic and aérial life. Probably the best known representa-
tives of the extinct population of the Age of Reptiles are some of
the giant Dinosaurs. (Figs. 79, 232.)
Although the supremacy of the Reptiles eventually passed to
the Mammals, there are still some five thousand species living
to-day. These are arranged in three chief orders: the TESTUDINATA,
CROCODILIA, and SQUAMATA, represented by the Turtles and
Tortoises, the Crocodiles and Alligators, and the Lizards and
Snakes, respectively.
1. Turtles and Tortoises
Typically encased in a shell composed of bony plates firmly
fixed to the backbone and to the ribs, the Turtles and Tortoises
SURVEY OF VERTEBRATES 121
— some land-dwellers, others aquatic — depend upon this rather
than speed for protection. In fact a few, like the Box Tortoise,
can completely seal themselves up, as it were, between the dorsal
CARAPACE and the ventral pLAsTRON. The tortoise-shell of com-
merce is the horny outer layer of the carapace of the Hawk’s
Fic. 80. — Turtles. A, Box Tortoise enclosed within carapace and plastron;
B, Tortoise-shell Tortoise, Eretmochelys imbricata; C, Snapping Turtle,
Chelydra serpentina; D, Mud Turtle, Cinosternum pennsylvanicum. (A from
Bamford; B, C, D from Newman, after Lydekker.)
bili, or Tortoise-shell, Turtle. Probably the protective shell also,
in part, accounts for the fact that the jaws of Turtles and Tor-
toises are toothless. Neverthless, many can inflict severe wounds
— witness the beaked jaws of the Snapping Turtle. (Fig. 80.)
2. Crocodiles and Alligators
Predatory inhabitants of tropical rivers, Crocodiles and Alli-
gators are lizard-like in form with long, gaping jaws well armed
with teeth, and with a thick, leathery skin covered with horny
scales. Alligator skin has long been popular for the manufacture
of leather goods.
3. Lizards and Snakes
The Lizards form a highly diversified group, typically with
scaly skin and well-developed limbs and long tail. Representative
Lizards are the common Iguana, the Gila-monster, and the Horned-
122 ANIMAL BIOLOGY
sof
Fic. 81. — Chameleon, Chamaeleon vulgaris. (From Gadow.)
é : :
SI i
= “AS
2 See
cy y GPRET GF:
) L
F v2
. A a
ie 1) TA
fn iy
: 2
,
Fic. 82. — Lizards. A, Horned-toad, Phrynosoma cornutum; B, European
Lizard, Lacerta viridis; C, Flying Dragon, Draco volans; D, Iguana, Iguana
tuberculata. (From Newman, after Gadow and Lydekker.)
SURVEY OF VERTEBRATES 123
‘toad.’ Closely related to the true Lizards are the Chameleons,
famous for their ability to change color rapidly in response to
their surroundings. (Figs. 81, 82.)
Snakes are essentially limbless Lizards in which even the in-
ternal supporting structures of the fore limbs have disappeared.
Most species of Snakes, in common with the great majority of
Vertebrates except the Mammals, are oviparous (egg-laying),
but a few are viviPAROUS (bring forth ‘living’ young). And it is
hardly necessary to say that a few have poison glands associated
with special teeth, or fangs. The Rattlesnakes, Copperheads,
(From Newman, after Lydekker.)
Water-moccasins, and Cobras are among the most notorious
in this respect. However, many species crush their prey as do
the Boa-constrictors, Pythons, and Kingsnakes. (Figs. 83, 230,
Zo.)
D. Brirps
The Birds, constituting the class AvEs, are the warm-blooded
(HOMOTHERMAL) animals that have made the air their own by the
development of fore limbs into wings, scales into an insulating
blanket of feathers, and other bodily adaptations. And not the
least of their progress is probably due to instinctive care of their
eges and young. That Birds are an offshoot from the Reptilian
stock, probably the Dinosaurs, is attested by the fossil remains
of a Bird, known as Archaeopteryx, with characteristic feathers
but lizard-like tail and teeth. (Figs. 232, 233.)
The Birds to-day form a remarkably homogeneous group, prob-
ably due to restrictions imposed by the mechanical problems in-
124 ANIMAL BIOLOGY
volved in flight. Sustained exercise in the air necessitates an
exceptionally efficient heart to rush supplies to the various parts
of the body, and codperating lungs that communicate with a
system of air spaces among the viscera and in the hollow bones.
Withal, the plumage not only provides an efficient heat-retaining
coat but the feathers of wings and tail also are well adapted for
propulsion and steering. Minimum weight with maximum strength
characterizes these living heavier-
than-air flying machines.
Birds usually are arranged in
two very unequal divisions, the
RATITAE and CARINATAE. The first
includes a few species without a
keel-like breast-bone to support
strong wing muscles, as_ illus-
trated by the flightless Apteryx
and Ostriches; and the second divi-
\ Wi
NY
Fic. 84. — A, Kiwi, Apteryr australis; B, Ostrich, Struthio camelus.
(From Newman, after Evans.)
sion comprises those with the ‘keel’ which is all the rest of the
bird population — about twenty thousand species. These are dif-
ferentiated by relatively minor anatomical variations, particularly
in regard to wings, feet, and horny beaks ensheathing tooth-
less jaws, in adaptation to various habitats and ways of life.
(Figs. 84-86.)
A consideration of the classification of the Carinatae would
carry us too far into details, but oRNrTHOLOGY is of very high
interest and greatest economic value. Birds contribute in large
measure, both directly and indirectly, to the human food
supply: directly as domestic and game birds, and indirectly be-
cause they make successful agriculture possible by eating almost
SURVEY OF VERTEBRATES 125
Fic. 85. — Representative adaptations of the beaks and feet of Birds.
a, Flamingo; b, Spoonbill; c, Bunting; d, Thrush; e, Falcon; f, Duck; g, Pelican;
h, Ostrich (running); j, Duck (swimming); k, Avocet (wading); /, Grebe
(diving); m, Coot (wading); n, Tropic Bird (swimming); 0, Stork (wading);
p, Kingfisher (grasping). (From Hegner, after several authors.)
126 ANIMAL BIOLOGY
inconceivable numbers of destructive Insects and weed seeds.
(Figs. 236, 239, 266.)
[flv :
Hi vt Na Se
SEA
WL TN SS
SND
Ny
So
p
y
ATS
ANS
Fic. 86.— Carinate Birds. A, Penguin, Eudyptes chrysocoma; B, Stork,
Ciconia alba; C, Hummingbird, Eulampis jugularis; D, Woodcock, Scolopar
rusticula; E, Parrot, Psittacus erithacus. (From Newman, after Lydekker
and Evans.)
E. MAMMALS
With the class MAMMALIA we reach the highest forms of life
on the Earth, culminating in Man, so here naturally our interest
is chiefly focussed. But since considerable attention is to be given
a little later to their anatomy and physiology with special reference
to the human body, only the essential Mammalian characters and
classification are now in point.
In brief, Mammals are warm-blooded, lung-breathing, hairy
Vertebrates. The young of all but the very lowest Mammals
develop before birth at the expense of food derived from the
mother’s blood. All immediately after birth receive milk from
special MAMMARY GLANDS. Mammals are classified under three
main subdivisions: Monotremes, Marsupials, and Placentals.
SURVEY OF VERTEBRATES 127
1. Monotremes
The primitive egg-laying Mammals living to-day, known as
MoNOTREMES, quite evidently point to a Reptilian ancestry for
the class. Although they are oviparous, the young when hatched
‘iD Ws SAW NS
> i Ad} Ve Ve .
wr VN
seed fips SEN AS SSS
Se HERS A WORSE Es
ap oe SUNT ny, nO I TN NAS EN x = SS
paps eas Nh OSES
Fic. 87. — Monotremes. A, Duckbill, Ornithorhynchus anatinus; B, Echidna,
Echidna aculeata. (From Newman.)
are nourished by milk. There are only three species, each about
the size of a Rabbit: the Duckbill (Ornithorhynchus) and the
Spiny Anteaters (Praechidna and Echidna) found in Australia,
Tasmania, and New Guinea. (Fig. 87.)
2. Marsupials
The pouched Mammals, or MArsuPIA.s, occur chiefly in Aus-
tralia and neighboring islands where they are the characteristic
; Y
AY
| | ) { i]
laby, Petrogale ranthopus; C, Koala, Phascolarctos cinereus. B and C carrying
young. (From Newman, after Vogt and Specht, and Brehm.)
Mammalian fauna: a primitive one that has flourished there
isolated from keen competition, but is now rapidly dying out since
Man has imported the higher Mammals. The Kangaroos and
128 ANIMAL BIOLOGY
Wallabies are the best known examples of the Australian Mar-
supials, and the Virginia Opossum is one of the few scattered
survivors in America of a group once widely distributed over the
Earth. (Fig. 88.)
The method of reproduction of the Marsupials is unique. The
eggs hatch, as it were, within the mother’s body where development
proceeds for a short time, nourishment being provided through
an atypical or very simple placenta. Then the young are born in an
exceedingly immature condition and make their way to a pouch on
the abdomen of the mother. Here they attach themselves to the
teats of the mammary glands and are nourished by milk. Even
after the young are well-developed the pouch serves as a refuge.
3. Placentals
The PLACENTALIA, or Eutheria, comprises all the rest of the
Mammals from the lowly Insectivora, such as Gymnura and
Hedgehog, to the Primates, including Man. All nourish their
young before birth by means of a highly complex PLACENTA that
makes possible the protracted development of the embryo under
3
oo er ts
OE Ete
E> — tr
B2ssean
‘ \ Ot
S=aS=S=:=::,
i} F = ay
nS ssa
fo — F
1\
VY)
Araravananat
\i
ANANGUANANAN
‘arenas
Cray
jn tiainiy
VANat
Nat
NF
te
aig
way
; ia < , eo
cee : — —— OLE eee ee
ze <<
‘4
“
——
Fic. 89. — An Edentate. Texas Nine-banded Armadillo, Dasypus novem-
cinctus teranus. (From Newman.)
ideal conditions for nutrition and prctection within the mother’s
body: conditions necessary for the establishment of niceties of
structure and function as exemplified, for instance, by a larger and
better brain. Thus it is fair to say that the placenta and associated
embryonic membranes are in no small degree responsible for the
commanding position of the group in competition with other forms
of life. (Fig. 134.)
The adaptive radiation of Placentals to nearly all types of
environments and modes of life — from Whales to Bats, and Moles
SURVEY OF VERTEBRATES 129
to Men — is an expression of their success which elsewhere in the
animal world is not reached even by the Insects. But it makes
their classification a difficult problem. However, for our purpose
the Placental Mammals may conveniently be grouped in four
great LEGIONS, chiefly on the basis of the structure and function
of their limbs and teeth.
The first is the immense assemblage of clawed mammals, or
UneuicunaTes. This is represented by the INSEcTIVoRA, or
Fic. 90. — Sulfur-bottom Whale, Sibbaldus sulfurous, and African Elephant,
Lozodonta africana, drawn to scale. (Modified, after Lull.)
insect-eating mammals — such as Gymnura, the Hedgehogs, and
Moles (Figs. 201, 205); the EDENTATEs, or toothless mammals —
Sloths, Anteaters, and Armadillos (Figs. 89, 204); the CxHrrop-
TERA, or flying mammals — the Bats (Figs. 207, 227); the Ro-
DENTIA, or gnawing mammals — Squirrels, Rabbits, Guinea-pigs,
Fic. 91. — Florida Manatee, Manatus americanus.
Rats, Mice, Porcupines, and Beavers (Figs. 108, 187); and finally
the Carnivora, or beasts of prey — Cats, Dogs, Bears, Seals,
etc. (Fig. 104.)
Another legion includes the completely aquatic CETACEANS —
Whales, Porpoises, and Dolphins. (Figs. 90, 206, 227.)
The hoofed mammals, or UNGuLATES, form a great legion of
herbivorous animals: important subdivisions being the ARrTIo-
130 ANIMAL BIOLOGY
DACTYLS, or even-toed Pigs, Hippopotami, Camels, Tapirs, Sheep,
Deer, Giraffes, etc.; the PeErIssoDACTYLS, or odd-toed Horses,
Tapirs, and Rhinoceroses; and the ProposciprAns, or Elephants.
Closely related are the aquatic SIRENIANS, or Manatees. (Figs. 91,
92, 227, 238.) i
SS
== ——LNNN
7 = NN
.
i
y
\
: - Ht!
Ay, Macs: UU, (/ iy
VA Vegtod No
By j tK
SSN
Fic. 92. — Ungulates. A, Giraffe, Giraffa camelopardalis; B, American
Tapir, Tapirus terrestris; C, African Rhinoceros, Rhinoceros bicornis. (From
Newman, after Beddard and Lydekker.)
The Primates form the concluding legion and include Lemurs,
Monkeys and Apes, and Man. They are predominant chiefly by
virtue of their mobile, grasping hands, and their intelligence.
Quite appropriately Primates have been called “the inquisitive
Mammals.” (Figs. 93, 228, 272-274.)
So is completed our glance at the chief types — phyla — in the
varied panorama of animal life from Protozodn to Mammal.
Necessarily brief, it is adequate for our purpose if we are impressed
with certain outstanding facts, not the least significant being the
versatility and prodigality of life. Nature has tried, as it were, one
experiment after another: some phyla have prospered and then
waned; some have gone up blind alleys and stayed there; some
have met the conditions of life to the full and have flourished: two
in outstanding fashion — Arthropods and Vertebrates. Only some,
then, and not all, have made a real contribution, but this has been
SURVEY OF VERTEBRATES 131
tenaciously conserved and appears again and again in ‘higher’
phyla. So there is a trace of structural and physiological continuity
woven in the picture of animal life that is interpreted as evidence
——_~=
SS
————
Gi
J SZ
[(ffaaetZ/
HA
Sf)!
|
Mpyit
1
ig
ode
1)
a of.
M/Z
oF) ig
eo
1
Fic. 93. — Primates. A, Dwarf Lemur, Microcebus smithii; B, Spider
Monkey, Afeles ater; C, Baboon, Papio leucophoeus; D, Gibbon, Hylobates
lar. (From Newman, after Beddard and Lydekker.)
of descent with change, or evolution. The appreciation of this unity
in diversity will contribute toward the proper perspective for a
more detailed consideration of the Vertebrate body and a pres-
entation of certain general biological principles.
CHAPTER X
THE VERTEBRATE BODY
If we contemplate the method of Nature, we see that everywhere
vast results are brought about by accumulating minute actions.
— Spencer.
As we know from our survey of the Animal Kingdom, the
Vertebrates form one of the most clearly defined divisions and in-
clude all the larger and more familiar forms — Fishes, Amphib-
ians, Reptiles, Birds, and Mammals. There is, in fact, less
diversity in structure among the Vertebrates as a whole than is
present, for example, in the one subdivision of the Arthropods,
the Crustacea, of which the Crayfish is a member. Accordingly
we shall confine our attention largely to a description of the
structure and physiology of an ‘ideal’ Vertebrate, and mention
incidentally some of the chief modifications of general signifi-
cance which appear in the different groups, and specifically in
Man.
A. Bopy PLAN
The ideal Vertebrate body is more or less cylindrical in form,
and is bilaterally symmetrical with respect to a plane passed ver-
tically through the main axis which extends from the anterior to
the posterior end. Three regions of the body may be distinguished,
HEAD, TRUNK, and TAIL. Frequently there is a narrow NECK be-
tween the head and trunk. (Figs. 67, 94, 95.)
The head forms the anterior end and contains the brain, eyes,
ears, and nostrils, or ANTERIOR NARES, as well as the mouth and
throat, or PHARYNX. On either side of the head, behind the mouth,
is a series of openings, or GILL SLITS, leading into the pharynx
which, however, in air-breathing Vertebrates disappear before the
adult condition is attained.
The trunk forms the body proper and contains the coelom, and
the major part of the alimentary canal leading posteriorly to the
exterior by the anus, as well as the chief circulatory, excretory, and
reproductive organs.
132
THE VERTEBRATE BODY 133
In most aquatic Vertebrates the trunk very gradually merges
into a large muscular TAIL: the region posterior to the coelom and
anus. Thus the Vertebrate tail is a unique structure — the tail
Lung Spinal cord Mesonephros
sy
Oral cavity
y
Internal gill slits 7 Urinary duct
Cloaca
Genital duct
Liver Bile duct ‘Spleen Urinary bladder
Heart Stomach
Fic. 94. — Body plan of an ideal Vertebrate. Longitudinal section of female.
of Invertebrates terminating with a segment bearing the anus. In
many terrestrial Vertebrates the tail has become practically an
inconsequential unpaired appendage.
The Vertebrate coelom comprises two chief parts — a large AB-
DOMINAL chamber and a small anterior chamber. The latter con-
Dorsal fin
Neural arch Fin ray
Spinal cord ah Dorsal muscles
Centrum of wa) ee x Dorsal aorta
Transverse process
Cardinal vein
Intermuscular rib
Subperitoneal rib | Mesonephros
Ventral muscles Mesonephric duct
Gonad Pronephric duct .
——— _——J;
Mesentery 3 = —— / ~~ Lateral vein
Intestine a Coelom
Peritoneum-visceral layer Peritoneum- parietal layer
Fic. 95. — Body plan of an ideal Vertebrate. Transverse section.
stitutes the PERICARDIAL chamber in Fishes but in higher forms
it is divided into two parts, one (pericardial) investing the heart
and the other (pleural) investing the lungs. The lining membrane
134 ANIMAL BIOLOGY
of the coelom, known as the PERITONEUM, forms the innermost
layer of the body wall, covers the organs, and in certain regions
forms broad folds, or MESENTERIES, In which they are suspended.
In the Mammals the organs of the chest, or THORAX, are separated
from those of the abdomen by a muscular partition, or DIAPHRAGM.
(Figs. 106-109.)
In aquatic forms thin extensions from the trunk and tail form
median FINS. Paired fins, developed from the trunk, comprise the
PECTORAL fins, situated near the junction of head and trunk, and
the PELVIC fins, just lateral to the anus. The pectoral and pelvic
fins, or the FORE LIMBS and HIND LIMBS which replace them in all
forms above the Fishes, are the only lateral appendages typically
found in Vertebrates.
B. SKIN
The surface of the body which comes in direct contact with the
environment is covered by an integument, or SKIN, which, though
Hair ve Opening of sweat gland
Bpidermis { S — Horny layer
jist ips ——— Epithelium’
Deri ‘ > Nerve ending
Blood vessel
pepacos =
an
“ Muscle
to hair
Connective
tissue
Subcutaneous Fat
tissue
Sweat gland
Hair follicle
D f x) is
- * yt, 1% 5
SFiS “Jat Cee / <
, re MeL he EF: RA
“ R43 SOON
5 5 RENT 7h
zs iS ar Rarrisen
—
Fic. 96. — Diagram of section through the human skin, highly magnified.
primarily protective and sensory in function, takes part to a greater
or less degree in respiration, excretion, and secretion. Scales,
feathers, claws, horns, hoofs, nails, teeth, etc., are derivatives of
the skin. The skin, unlike that of the Invertebrates, is formed of
THE VERTEBRATE BODY 135
two chief layers: an outer epithelial tissue, the EPIDERMIS, derived
from the ectoderm, and an inner DERMIS from the mesoderm of the
embryo.
The epidermis itself always consists of several layers of cells: the
lower comprising actively dividing cells whose products gradually
are moved up to form the superficial layers of flattened, horny cells.
<A, Enamel
ce aes
tnt
a
\\e
Crown IG if |
» i a
WY
\\
MW
Uy,
Mf
Wh
Wp
Msi
Yy, Dentine
Z
WM
WY,
Y]
y
Uf
Periosteum
Cement
Root canal
Fic. 97. — Human tooth, longitudinal section.
Thus this layer is not directly converted into the cuticle, as is the
case, for example, in the Arthropods. The dermis is a connective
tissue layer, with glands, blood vessels, etc., between the epidermis
and the muscular layer of the body wall. (Figs. 96, 97.)
C.. MuscLes
The body wall proper is chiefly composed of muscular tissue,
commonly spoken of as flesh, which varies in thickness in dif-
ferent parts of the body. In the mid-dorsal region it surrounds the
brain and spinal cord (CENTRAL NERVOUS SYSTEM) and the axial
supporting structure (NoTOCHORD), while ventrally it forms the
wall of the coelom. In the lower Vertebrates and the embryonic
136 ANIMAL BIOLOGY
Submentalis
Hyoglossus
Mylohyoid
Geniohyoid
Deltoid Posterior cornu of hyoid
Ommosternum
Pectoralis
Pectoralis Z|
Rectus abdominis
“}> Obliquus internus
Obliquus externus
Linea alba
Inscriptio tendinea
Adductor longus
Adductor brevis
Pectineus
; Vastus internus
Vastus internus~
Femur
: 34 Adductor |
. be \ \dductor longus
Adductor
magnus
Sartorius
Rectus internus
minor
Sartorius
Rectus internus
major
Extensor eruris’
Tibiofibula '
Semi-tendinosus
Gastrocnemius
Tibialis
posticus
Peronaeus
Tibialis anticus,
s y Tibiofibula
Extensor tarsi
Fic. 98.— Muscles of the Frog, ventral view. (From Hegner, after Parker
and Haswell.)
THE VERTEBRATE BODY 137
stages of higher forms the muscular layer is composed of segments
known as MYOTOMES. But in the adult stage of the latter this
evidence of Vertebrate segmentation largely disappears, since the
muscular tissue for the most part assumes the form of highly
LE
= $ E \
Lg se
SS: LATE
2 SARTORIUS
ADDUCTOR
QUADRICEPS
FEMORIS
QUADRICEPS
FEMORIS
PERONEUS \
Mi
Ni
ANNULAR
LIGAMENT
Fic. 99. — Muscles of Man. (Courtesy William Wood and Co.)
complex longitudinal bands, extensions from which pass into the
paired appendages. (Figs. 94, 95.)
Muscles, such as those attached to the bones, in which contrac-
tion can be brought about at will, are termed VOLUNTARY muscles,
while those which cause most of the movements of the viscera are
138 ANIMAL BIOLOGY
known as INVOLUNTARY muscles. From the standpoint of their
microscopic structure, muscle cells are of three kinds. Voluntary
muscles consist of STRIATED muscle cells, and involuntary muscles,
except those of the heart, are composed of UNSTRIATED muscle
cells. The cells of the heart approach somewhat in structure those
of voluntary muscles and are known as CARDIAC muscle cells.
(Figs. 7, 32, 98, 99.)
D. SKELETON
The form of the Vertebrate body is maintained by a system of
supporting and protecting structures, termed the SKELETON.
Although various outgrowths of the skin, such as scales, feathers,
Notochord lt re mAs as fa
{
G
i \ |
Notochordal sheath ea ae ie |
Blood vessel
in connective tissue qi wD E
Fg 4 = ‘.
Growing cartilage fo ees
Bone developing to VsGn ms -
form vertebra
i
3
‘
Petree Air
. s mnie
wes ’
BAQIAJAVA DUO JO JUOJXY
Hg
id
Cy
Muscle tissue 4, SE
Fic. 100. — Diagram of a longitudinal section through a developing verte-
bral column to show the invasion of the notochordal sheath by cartilage to
form the centra of the vertebrae.
and hair, form a part of the skeletal system known as the Exo-
SKELETON which is comparable to the protective coverings of the
Invertebrates, it is a bony ENDOSKELETON which is characteristic
of the higher animals. This internal skeleton, which is largely mes-
odermal in origin, exhibits such great diversity and complexity
THE VERTEBRATE BODY 139
that its study, known as osTEOLOGY, forms an important sub-
division of comparative anatomy.
In the lower Fishes the endoskeleton is composed of a firm elastic
tissue, CARTILAGE, or gristle, but from the higher Fishes to Man
most of the cartilage becomes oOssIFIED: that is, impregnated
with lime salts and transformed into BONE. The human skeleton
is formed of about 200 separate bones, but the number varies at
different periods of life, because some bones which at first are
distinct later become fused. (Figs. 103-105.)
While it is true that the bones constitute the main supporting
framework of the body, they are entirely inadequate to knit to-
gether the organism into a working unit. We find therefore various
kinds of connective tissue interwoven between the integral parts
of the body. These tissues form sheaths about most of the organs
and also supply the connecting links between muscle and muscle,
muscle and bone — TENDONS; and bone and bone — LIGAMENTS.
Supporting tissues, of which bone, cartilage, and connective tissue
form the chief groups, are characterized by the development of
large amounts of resistant non-living material in or between the
Neural spine
Transverse Vertebral
process
em *
Anterior
articulating
surfaces
Centrum
Fic. 101. — A typical human vertebra (tenth thoracic).
component cells themselves; the character of the tissue being de-
termined chiefly by the nature of this matrix. (Fig. 32.)
The primitive axis of the skeleton consists of a cylindrical cord
or rod of cells (NorocHorD), which lies in the mid-dorsal line of
the body wall just below the brain and spinal cord and above
the coelom. In most Vertebrates, however, the notochord in its
140 ANIMAL BIOLOGY
original form is only a temporary structure, being partially or
completely replaced during later development by a linear series of
cartilaginous or bony elements, known as VERTEBRAE, which form
the VERTEBRAL COLUMN, or backbone. This is one of the most
characteristic structures of Vertebrates as compared with Inverte-
brates, or backboneless animals. (Figs. 94, 95, 100.)
A typical vertebra of the higher animals consists of a basal
portion, known as the CENTRUM, and a NEURAL ARCH which it
supports. These form a protecting ring of bone about the spinal
Scapula | }
Tlium
} — Glenoid cavity Acetabulum
Clavicle J £7] Humerus
# Femur
f) Pubis
Procoracoid . Ischium
Coracoid
Radius Ulna
Tibia Fibula
ees Ae toe ts I di
ae centralia “OC CO— Ulnare ee eee) a
ive dista ibulare
lia ~O§ Five aistalia—_-2 Oo,
Five ef
Five metatarsals
Taf
Phalanges
Il
A IIl
Fig. 102. — Diagram of the plan of the Vertebrate Hieabs. A, fore limb and
pectoral girdle; B, hind limb and pelvic girdle. (From Hegner, after Parker
and Haswell.)
cord. From various parts of the vertebra as a whole arise PROC-
ESSES for movable articulation with its neighbors, the attachment
of muscles, etc. Between the vertebrae of the Mammals are
cushions of cartilage which absorb shock. (Fig. 101.)
In some forms, RIBS are attached to the transverse processes of
certain vertebrae. ‘These extend outward and downward within
the body wall, and usually are attached in the ventral line to the
breast bone (STERNUM). Thus, in the adult of the higher Verte-
brates, the series of centra of the vertebrae come to occupy the
position formerly held by the notochord; while above, the neural!
THE VERTEBRATE BODY 141
arches form the VERTEBRAL CANAL containing the spinal cord;
and below, the transverse processes, ribs, and sternum surround
the anterior portion of the coelom. (Figs. 95, 101, 105.)
Premaxilla
Olfactory capsule ys Wake
Sphenethmoid mies ee
Fontanelles_ . oy
Antertor y YS A Frontal partetals Iv
y ; : \
«YF f Quadrato
re MAT \| CAN) oY \
y; -_ at a Posterior cornu
of hyoid I
Exoccipital XQ
Supra-scapula, ay
My
ical
ay le a)
Transverse fo
process
- 2
\N
Humerus Radio-uina,
girdle and the left fore and hind limbs are removed, as also are the membrane
bones on the left side of the skull. Permanently cartilaginous parts dotted.
(After Howes, slightly altered.)
The Vertebrate head, containing the anterior end of the ali-
mentary canal and respiratory passages, and also the brain and
chief sense organs, is protected in the lower Fishes by a case of
cartilage. In higher forms the cartilage is replaced by a bony SKULL
which articulates with the first vertebra of the backbone. JAws, or
supporting structures of the mouth, are attached to the skull.
|
"yery Jo uozopOYS — “POL YA
was Sasuveyd [esiej}eyaN
aA | / REX
q “f By ” ~ INS FAB =
room DEM Sse —= f s[esue]. \
Ss \ ~
gpedizg—, LR 2 RN
ae a cae
snipey- 3
smiawny !
Bl[aI@d-|}
Youle d1yVw0sAz,
UINULO}S
aTeuee | slopny — eindeog
1199} IPO!
|
Yja0} Iwjowlasg
y}00} eulued BK) a
yj99} Jostouy — Y, . | \
# MSS auids [einen » “A
. uw " Vs ~ ce -
Wy PEN BAGa}1IA jepned
BIQGI}IOA SIXW B.1G9}19A OM4-A}UIMY
BAQO}IAA TeqUI (sayy ees Aq pomor[oy
VAQIWOA SEY — 0 winsjua9 “Jo sasseooid ‘[elovs volY],
Surqzepnoiyly
Teqvond jesowenbs
ALTRI
[ejaeg eyidiosovadng
142
THE VERTEBRATE BODY 143
The skull and vertebral column form the main SKELETAL AXIS
from which is suspended the APPENDICULAR skeleton, or bony
framework of the paired appendages (FINS or LIMBS) and their
supporting structures (GIRDLES). This is relatively simple in the
anterior (pectoral) and posterior (pelvic) paired fins of Fishes,
which merely act as paddles; but when these are modified into
paired limbs for progression on land, the mechanical problems
involve the development of complex limb skeletons to support the
body, and to act as levers for the limb muscles to move in locomo-
tion. In response to this need an elaborate series of bones is de-
veloped which, in all cases, however, may be referred to a common
plan, known as the PENTADACTYL LIMB in allusion to the five digits
(FINGERS and TOES) in which it usually terminates. The limbs are
attached directly or indirectly to the vertebral column by groups
of bones which form respectively the PECTORAL and PELVIC GIR-
DLEs. (Figs. 102-104, 227, 228.)
E. THE Human Bopy
Considering specifically the human body, we find that its out-
standing characters are largely the result of Man’s erect posture.
True it is that the body is not perfectly adapted to its upright
position, but this is more than compensated for by the complete
division of labor between the upper and lower limbs that liberated
the former from contributing to locomotion and gave the oppor-
tunity for the pentadactyl plan to attain its highest development
in the human HAND. With its completely opposable thumb, the
hand is directly or indirectly responsible for more of Man’s unique
characters than one usually realizes. It is an efficient grasping
organ — a battery of tools that makes possible the use of artificial
tools which, in a way, may be regarded as accessory organs, de-
vised by the brain and appropriated or discarded at will.
But the hand is also a delicate ‘sense organ’ since touch is “the
great confirmatory sense” underlying many of our sensory ex-
periences with the world about us. ‘Tactile fingers are continually
learning.” Indeed, it is largely upon a basis of conscious and sub-
conscious tactile sensations that much of the superstructure of
the higher mental processes is reared. It seems clear that the erect
posture and the facile hand have contributed in no small way to
the supremacy of the brain and so to Man’s outstanding position
above the beasts. (Figs. 105, 125.)
144 ANIMAL BIOLOGY
Cranium
Superior maxillary bone . . Gk ‘we Malar (cheek) bone
Inferior maxillary bone
Cervical
Clavicle (collar bone) vertebrae
Thoracic
vertebrae
ertebral
column
Lumbar
vertebrae
Sacrum
Carpals
Metacarpals
Phalanges
Pelvis
Pelvic opening Femur
Patella
Tibia
Fibula
Tarsals
Metatarsals
Phalanges
Richard E
Harrison
Fic. 105. — Skeleton of Man.
THE VERTEBRATE BODY 145
F. DISTINCTIVE VERTEBRATE CHARACTERS
As a summary of this general outline of the structure of the
Vertebrate body, we may emphasize three characters which are
of prime diagnostic importance.
In the first place, whereas the skeletal structures of Inverte-
brates typically consist, as in the Crayfish, of an exoskeleton of
hard non-living materials deposited on the surface of the body,
the chief function of which is protection, the Vertebrate skeleton
is primarily a living endoskeleton. It is an organic part of the
organism which, although it affords protection for delicate parts,
provides adequately for support and supplies muscle levers, and
thus makes practicable the relatively large bodies of the higher
animals. The notochord is at once the foundation and axis of the
Vertebrate internal skeleton and either persists throughout life
as such, or simply long enough to function as a scaffolding about
which the vertebral column is built. In recognition of the prime
importance of the notochord, the Vertebrates and their nearest
allies (e.g., the Tunicates and Amphioxus) are technically known
as CHorRDATES. (Fig. 67; Appendix I.)
In the second place, it will be recalled that the central nervous
system of the Earthworm and Crayfish consists of a ventral nerve
cord running along in the coelom below the digestive tract, except
at the anterior end where it encircles the pharynx to form a
dorsal brain. The position of the Vertebrate brain is similar, though
the spinal cord is not a ‘cord’ but a nerve tube which lies in the
vertebral canal embedded in the muscles of the body wall above the
digestive tract and, of course, outside of the coelom. Thus the
spinal cord itself and its location are highly characteristic.
The third fundamental characteristic is a series of perforations or
slits through the throat and body wall. In the lower forms the
gill slits provide an exit for the current of water entering by the
mouth and, being richly supplied with blood, afford the chief
means of respiratory interchange between the animal and the
surrounding medium. In the higher Vertebrates the gill slits are
present merely during a transient phase in the development of
the individual, since the function of aérating the blood is taken
over by the lungs. (Figs. 94, 117, 235.)
"suaosaanyf DIA “YJ MOTE oy ‘Ysty Auoqg e Jo UoTassIC] — “OOT “OL
uundeevd o110] Ag
19}9IN JY BUY uaalds yonp Ig
SSBUL 3E J yoruio}s
Joppelq Areuip uly o1aqed Wy SKyy rede es
snuy our}SaqUy , pa
910d a : - S[[L pue Sz]S [IID
; Sa: 7 J afoluny | B40V
qreay JO a[d1IWUI A,
xuAreyd
RAS
4
,
2 NESS
Bits : Chto é
iN \ “tins Tas Z y Wp iff , va) HM) yi Hy Hee " A We iM ii (3 Pom ? chs She Cf DZ
A py ] es Prep van . 2 Bran é YY d b y ; aieu aa ee aoe ~ ape san 5 FY
afd JYBII JO UOIzISOg
ED be SA kf Fe /; f sAIOU A10}98I[O
9 A YN NY fa” a sArou 911dQ
: , Je hoy cs WIN1qe19/)
canna i ey aqoy o13do yJarT
Zi.
psoo yeuids y y,
wNn][aqa.105
wINIUeI
paoo rappeyq ary
Tappryq AV
auoqg [eulds1aqUy uly [es10q
146
‘suppUDjo DUDY ‘SOIT OU} ‘ueiqrydury ue jo uorzossiq — “LOL ‘I
SUI}SO}UT [EUS
svoloutg Jappe[d led
usae]d5 Yyoeuloys
K19} ase qaary VeOY JO 9[O1IIUI A,
{
OUI}SOqU Bd1e'T
gplne WYSIY
apne aT
Joppeyq Aueup) Wee ey0y
AEE: $1}}0
82] JO sopsnyy _Aome \ AMID
1 he xuAreyg ansuoy,
j
3 REGS we
apis atalod Cw
Ww
EY Se eee SSS a)
Kaeulan Jo suruedQ ee SS mel JaMoy
EE cae aqn} ueiyoejysn
eavojo Jo uiusdg GS : jo sutuedg
gaqyaan yy 31a Jo Buruadd. aS BD -Y}29L
a 7p 7
197219 Sa aot spams auog [BSeN
9448021) Zee ze 5 4 Bs a sdaru jeusezU]
‘£poq yeuad : Buny 3J°T a eqo] Ar0joe
Cua Sun anea On) euid dh oe
ae qusry peers as
° ; wn [aqasa9
ECL ENT umniuely
BIN pet
147
CATE pue o][[AUrT Joye ‘polyIpoy|) *S1SUIUI)OUDI SNUINIIGY ‘jarambg Kearny oq} ‘PULUIB TA B Jo UOTPOSSIGT — “BOT “YI
YOeVUloysS SBo10Ue T
aurjsaqut peug FPMNe IJoT
puels syadmog —- Aloquasoyy : wnIpreowag
arta P ML BTOLIQUIA JOT
“Hea j a0 WINUAI}S
OP ™
SS ys /
GY fpr snw Ay],
| pues prosAyy,
wnj019
ay si} ops1dq
SI}S0} 4JO'T
Teppeq Areutsp
eoyordy,
Soleu [BUI]
yi00],
purjd o}e\so1g ansuoy,
S[lJSON
mcr
SE See fs
SOP AY | = os
B1Q2}19 A
ee
OUI}SOPUL 9dAL] gonp waedg| |
qot £1099" J[O
Yih wWINnIUe ID
elyjad
pe cams z UUNIyoIOseyT =| Aaupty 4JeT CGE he -
nd Ny Wea
I 311 aoraiih pod, pelleany: IOLI9}SOg Yi parou ado
Sider UIdA el[Npo dAIOU [BULWUASIIT,
Woeyeetd reuowjng xusaeyg saaes
Agoqare Areuowyjndg winyjaqe4a7)
BALD BUGA AOLIIJUY
snseydosa
148
THE VERTEBRATE BODY
Cranium
Internal nares <2?
Nostrils &
= \ : / j :
Mouth Ey
Pharynx: Wy p |
Le
Abdominal cavity +
Liver 7
Stomach 4
SS
soos FP SQ
Large intestine a
Kidney 4
Small intestine!
Ureter 122
Urinary bladder
s
Urethra ANUS
Fic. 109. — Diagrammatic median section of the human body.
149
CHAPTER XI
NUTRITION
It is a great satisfaction for me to know when regaling on my humble
fare that I am putting in motion the most beautiful machinery with
which we have any acquaintance. — Dickens.
Amone the single-celled animals, such as Amoeba and Para-
mecium, nutrition is reduced to its simplest terms. The food ma-
terial enters the cell and is acted upon by substances formed by
the protoplasm in its vicinity: the food is chemically changed,
or digested, so that it becomes available for the use of the cell.
In Hydra a special layer of cells, the endoderm, is largely devoted
to digestion. Although some of the endoderm cells actually en-
gulf small particles of food and digest them within the cell (INTRA-
CELLULAR DIGESTION), the major part of digestion is brought
about within the enteric cavity by secretions from the endoderm
cells. Digestion of the latter type (INTERCELLULAR) is characteris-
tic of the Earthworm and all higher animals.
We have considered the form and supporting structures of the
body wall of a typical Vertebrate — the outer tube which sur-
rounds and contains the viscera — and therefore we recall that
through this outer tube, just as in the case of the Earthworm and
Crayfish, there runs from mouth to anus a second or inner tube,
the digestive tract, or ALIMENTARY CANAL.
The alimentary canal is essentially a tubular chemical labora-
tory which passes the food on by its own muscular activity from
one part to another. Each of these regions, in turn, supplies the
chemical reagents which it uses both for changing the food into
a soluble form so that it can pass through the walls and be dis-
tributed to the cells of the organism as a whole, and also for mak-
ing it suitable for use by these cells. Indeed, the complex food
materials which enter the human mouth run the gauntlet of a
whole series of digestive fluids.
Although the various kinds of food eaten by animals differ
widely in their chemical composition, nevertheless the process
150
NUTRITION 151
of digestion is basically similar in every case: it is a process of
HYDROLYSIS. This is a chemical reaction in which a molecule of
the substance to be digested combines with a molecule of water to
form a new compound. Then this splits into two or more simpler
molecules and, by repeated hydrolyses, exceedingly complex food
substances become relatively simple ones. Hydrolyses are brought
about through the activities of special catalytic agents, the fer-
ments, or ENZYMES — a special enzyme for each kind of chemical
reaction being supplied by the alimentary canal. Moreover, the
diverse enzymes, carrying out chemical simplification of various
foodstuffs, produce just a few relatively simple substances: AMINO
ACIDS from proteins, FATTY ACIDS and GLYCEROL from fats, and
SIMPLE SUGARS from carbohydrates. (Fig. 113.)
The wall of the alimentary canal consists of three chief cellular
layers: a lining epithelium, a connective tissue layer, and a muscu-
lar layer. The epithelium which lines the alimentary canal and
its derivatives is the digestive tract proper in the sense that it is
of basic functional importance in secreting the digestive fluids
and in absorbing the products of digestion. The other layers per-
form accessory functions such as support, conduction of blood
vessels, and movements of the canal. (Fig. 34.)
A. BuccaL Cavity, PHARYNXx, AND ESOPHAGUS
The entrance to the alimentary canal is the mouth, a transverse
aperture in the head, which leads into the mouth-chamber, or
BUCCAL CAVITY, supported by the jaws. The buccal cavity gradu-
ally merges into the throat, or PHARYNX, which in the Vertebrates
acts as a passage both for the food and the respiratory gases. The
respiratory current of water in aquatic forms soon passes to the
exterior by a series of perforations, the GILL sits, through the
pharynx and body wall; while the respiratory current of air in
higher forms enters the LuNGs. On the other hand, in all Verte-
brates the preparation of the food for its passage through the
alimentary canal starts in the buccal cavity. (Figs. 110, 117.)
The human buccal cavity is lined with a membrane, continu-
ous at the Lips with the outer skin, which is provided with uni-
cellular glands that secrete mucus. This and sativa, secreted
by three pairs of large SALIVARY GLANDS, lubricate the food so
that it may be more readily moved about by the TONGUE for
mastication by the teeth and passed on toward the ESOPHAGUS.
152 ANIMAL BIOLOGY
Furthermore, saliva contributes to the digestion of starches by
an enzyme termed PTYALIN, which in the alkaline medium con-
verts starch into a sugar, MALTOSE. But only a small proportion
of the starch is digested by ptyalin during the rapid passage of
: = Mouth
Salivary glands SF 32,
Pharynx i
fA Ay
: YE Al y
Thyroid gland MEH
A
i Trachea
zi
Thymus gland -
Lungs Esophagus
——
Diaphragm—© ~
eee — Stomach
Gall bladder \ ——Pancreas
Small intestine
Large intestine —
Vermiform appendix
\ Rectum
Fic. 110. — Diagram of the human alimentary canal and its derivatives.
The pharynx shows the embryonic position of five pairs of gill pouches, the
second pair probably giving rise to the tonsils, and the third and fourth to
the thymus glands.
food through the mouth region, and the activity of the enzyme
ceases soon after it reaches the stomach where the alkaline reac-
tion of the mouth gives place to an acid reaction.
So the mouthful of food, masticated, moistened, and with some
of its starch digested, is rolled by the tongue and passed along the
pharynx into the EsopHAGUS. Henceforth it is beyond voluntary
recovery, because it is pushed along by involuntary rhythmical
NUTRITION 153
contractions, PERISTALSIS, of the digestive tract wall. The esoph-
agus is a muscular tube which passes posteriorly through the
thorax, and rapidly delivers the food without further digestion
to the STOMACH.
B. SToMAcH
The stomach, really the first stopping place of food that has
been swallowed, is a thick-walled sac situated just below the dia-
phragm in Mammals. In common with most of the viscera, the
stomach is suspended in the abdominal cavity by broad loops of a
membrane, the MESENTERY, which is continuous with the perito-
neal membrane lining the cavity. Within the mesentery, blood
vessels, nerves, etc., pass to the stomach. (Figs. 106-110.)
Here the work of the digestive tract actively progresses by the
action of specific chemical substances present in the GASTRIC JUICE
which is secreted by innumerable GASTRIC GLANDS. The latter
are tiny pits in the stomach wall lined with special glandular cells.
Human gastric juice is a complex fluid comprising water — over
99 per cent; a protein-splitting enzyme, PEPSIN; a milk-curdling
enzyme, RENNIN; common salt, NaCl; and a free acid, HCl. This
array of components of gastric juice softens the food mass, gives
it an acid reaction, curdles the milk, and simplifies the proteins —
transforms, with the assistance of slow churning movements of
the stomach wall, the average meal in the course of an hour or
so into CHYME which gradually passes through the PYLORIC VALVE
into the INTESTINE.
C. SMALL INTESTINE
The human intestine is a much coiled tube, nearly twenty-five
feet in length, that extends from the stomach to the anus, and it is
in the upper part, known as the SMALL INTESTINE, that not only the
most radical changes in the food take place — digestion is essen-
tially completed; but also most of the products of digestion pass
through its walls into the body proper — absorption occurs. Thus
we find various glands to elaborate and secrete the digestive fluids
— cells that take from the circulatory system not only the materials
necessary for their own life but also other substances which they
transform chemically for the use of the organism as a whole. Some
are unicellular or simple multicellular tubular glands embedded in
the intestinal wall; others are highly complex and far removed
154 ANIMAL BIOLOGY
from the intestine into which they pour their products through
long ducts. But, as we know, even in the latter case the glands
are really derivatives of the intestine — cellular areas sunk, as it
were, below the membrane to which they really belong — because
fo ng
aoa \. Duct
ober rer
Fic. 111. — Diagram of a gland, in section, together with the surrounding
tissues. Highly magnified. See Fig. 33. (From Hough and Sedgwick.)
they arise during development as outpocketings of its wall: the
ducts being the sole remaining connection with the point of
origin. (Figs. 111, 112.)
1. Liver and Pancreas
The largest gland in the body, the LivEr, and an equally im-
portant one, the PANCREAS, pour their secretions into ducts which
unite to form a single duct. This carries the secretions to the upper
part of the small intestine. The secretion of the liver, termed
BILE, which is constantly formed but may be stored until needed
in the GALL BLADDER, is a highly complex mixture of substances.
Some of them are waste products on their way out of the body
through the intestine, while others contribute to digestion by
cooperating with an enzyme from the pancreas. Thus bile salts
aid in the emulsification of fats and the absorption of fatty acids.
However, the liver is still more versatile, as will appear beyond.
NUTRITION 155
The pancreas, or sweetbread, may be regarded as the chief
digestive gland in the Vertebrate body though it also performs
additional functions. The gland lies just below the stomach in
Man, and each day secretes into the small intestine nearly two pints
Common bile duct
cy SS _ Hepatic ee
Liver Q\ duct / \
NV \ \\ x | Aa ( tf iver
it = SS
Vs SZ
\ ) yy a KS LO =F
IX \ is: NS SS
Cystic ducts
De — <= i Cs it SF
{i}
Gall bladder Si
i) a
VY i ; fi
YN
SSS
a
SS
BSS
Me
x A Pancreatic
F duct
Liver =
— ¥. 4
Pancreas = Z
S. ZF Stomach
Sa
Ss
e ~ —
Bile duc -=
Ss
=
\
Small intestine y .
my S Pylorus
Fie. 112. — Liver and pancreas of the Frog, showing their ducts. Lobes of
liver turned forward. The cystic ducts unite with hepatic ducts and finally
lead into the common bile duct. The latter passes through the pancreas,
receives further hepatic ducts and the pancreatic duct and leaves the pan-
creas, opening into the upper part of small intestine.
of strongly alkaline PANCREATIC JUICE containing three enzymes —
TRYPSIN acting on proteins, AMYLASE on starches, and LIPASE on
fats. (Fig. 113.)
Thus food that has run the gauntlet of the enzymes of the upper
digestive tract is now attacked by the pancreatic juice. But this
is not all: the process of progressive simplification of the food is
carried on by the secretions of innumerable minute glands embedded
in the intestinal wall. This mvrestrnaL JUICE supplies several
enzymes — the EREPSIN group to change the protein products into
amino acids, and the others to convert complex sugars into simple
156 ANIMAL BIOLOGY
sugars. And, as in the stomach, digestion is facilitated by muscular
contractions of the intestinal wall. There are slow swayings of
entire loops of the intestine; there are local ‘segmentation’ con-
tractions; and finally there are peristaltic waves that pass the ma-
terial along for absorption or elimination.
One naturally wonders how it is that the living tissues of the
digestive tract, and especially of the glands themselves, withstand
INTER- PRopuctTs
SUBSTANCES
LocaTION |SECRETIONS| ENZYMES MEDIATE | READY FOR
CHANGED
Propucts | ABSORPTION
Mouth Saliva Ptyalin Starch Maltose
Stomach | Gastric Pepsin Protein Proteoses
juice Rennin Protein of Peptones
milk Casein
Small Pancreatic | Amylase Starch Maltose
intestine juice ——— —- a
Lipase Fats Fatty acids
and
Glycerol
Trypsin Proteins Amino
acids
Intestinal | Erepsin Proteoses | Amino
juice group Peptones | acids
Casein
Maltase Maltose
Sucrase Cane sugar } | Simple
Lactase Milk sugar | sugars
Fic. 113. — Chief chemical activities of the human digestive tract.
the digestive activities of their own enzymes, although the stomach
or intestine of an animal killed during active digestion will begin
self-digestion. A partial explanation of immunity during life
appears to be that the enzymes are not in an active form while
they are within the glands, but are later activated by certain
conditions that they meet in the digestive tract when food is
actually present.
NUTRITION 157
2. Absorption
The purpose of digestion is, first, to make the food soluble so
that it may enter the body proper: pass through the epithelium
lining the digestive tract wall; and, second, to put it into a chemical
form that can be used by the cells. The passage into the body, or
ABSORPTION, occurs chiefly in the
AER
small intestine, the walls of which wy,
are lined with millions of minute is) e pee,
projections, or vii, that bring = { we
. . ° . = 2 Z, \2 =
tiny vessels into intimate contact SAN Ole
with the absorptive membrane Vein as \ EF
while greatly increasing the effec- 2B My | =8
‘ ‘ Cee if les Artery
tive absorptive surface. Appar- =) i st
. . Ss ane = ae
ently absorption is not merely the ; any — 2
. : Z 7) i — a o. 3 =
result of simple physical processes “7° ay \ a
. . . %| W2F. lo _& 2
such as diffusion and osmosis, but : AAs. fe
is largely the function of the Le Wee
actual living cells forming the Jag \e =
. . . . . 2 = AG Sac a a
epithelium of the villi. (Figs. 114, = a\ Vis ee
144.) Se) NX |
é i we =
3. Distribution VWwi=
\e)
\\
NUN
The transportation system of
vessels in the villi consists of BLOOD
oe “Artery
vessels which take up the absorbed
Fic. 114. — Diagram of a sec-
products of protein and carbohy- ton through a villus. (From Pea-
drate digestion, and tympu vessels, body and Hunt.)
BSG
$ ASN
os
2 e
here called LAcTEALS, which receive the derivatives of the fats.
Both also take absorbed water and salts.
Accordingly the path through which the proteins and carbohy-
drates are transported differs from that of the fats. The blood
vessels returning blood from the digestive organs finally merge
to form a large vessel, the PORTAL VEIN, which proceeds to the
liver to allow that organ to regulate certain of the blood constitu-
ents — in particular to store up sugar, in the form of GLYCOGEN,
after a meal and later dole it out to the blood as conditions de-
mand. On the other hand, the lacteals merge into larger and larger
lymph vessels and finally into the THorRAcIC DUCT which empties
into the blood vascular system — the fats being switched, as it
158 ANIMAL BIOLOGY
were, around the liver so that they do not directly enter the blood
supply to that organ. (Fig. 115.)
Left jugular vein
Left subclavian vein
Superior vena cava
Lb iy,
DOLEA
OXy
Dy >
7 %, %
<i
Thoracic duct
Liver
Vein
ein i. .
Portal vein: Pe from Stomach
from Spleen —~ ’ ) iy
q
we Lacteals or
Le sess lymph vessels
in lumbar regions
\ Intestine
Fic. 115. — A, diagram of paths of absorbed food from the human digestive
tract. Proteins and carbohydrates by veins; fats by lymphatics. B, plan of
distribution of the chief lymphatic vessels in the human body.
D. LarcE INTESTINE
During the passage of food through the small intestine, digestion
is practically completed and absorption has progressed far. The
remaining material is carried through a constriction into the
LARGE INTESTINE, or COLON, where the activities of Bacteria bring
about various chemical changes apparently incidental to the slow
passage of the residue. Furthermore, water is gradually absorbed
— water that up to this point has been necessary to keep the ma-
terials fluid to facilitate digestion. Then the useless undigested
NUTRITION 159
materials, or FECES, are carried to the exterior either through a
terminal cavity, the CLOACA, into which also open the urogenital
ducts, or directly out through the anus as in most Mammals.
E. Foop UssE
Digestion, absorption, and distribution completed, the actual
food supplying matter and energy is at the disposal of the various
cells for carrying on the essential metabolic processes. Since the
daily energy output of the average man not carrying on heavy
physical labor is about 3000 calories, and the energy yield per gram
for proteins and carbohydrates is about 4.1 calories, and for fats
about 9.3 calories, it is not difficult to determine from these fuel
values how much of each foodstuff, or mixture of them, is re-
quired to supply the necessary energy. But, of course, provision
must also be made for tissue maintenance and this draws upon the
proteins with their nitrogen constituent. In general it may be
said that, since carbohydrates and fats adequately supply the
energy demands, protein consumption need not greatly exceed the
nitrogen required for tissue maintenance.
Moreover the body also requires small amounts of certain so-
called accessory food substances, or VITAMINS, which are usually
present in sufficient quantity in a normal mixed diet. Vitamins
are organic substances, not related chemically to one another, that
do not supply energy or structural material, but are necessary for
cell metabolism. The exact chemical constitution of most vitamins
is unknown.
ViTAMIN A has an effect on many of the membranes of the body,
a deficiency resulting in glandular disturbances and in lowered
resistance to infection. Rich sources are cod liver oil, carrots, and
butter. Viramrn B, prevents an inflammation of the peripheral
nerves and paralysis known as beri-beri, as well as disturbances of
the functions of the intestine and kidneys. Excellent sources are
whole grain cereals, milk, and liver. VITAMIN Bz is usually asso-
ciated with B;. Inadequate amounts give rise to the disease called
pellagra, of which brown pigmentation of the skin, general weak-
ness, digestive disturbances, and paralysis are symptoms. It is
readily available in egg-white, milk, lean meat, and green vege-
tables. Viramin C prevents scurvy, a disease marked by loosening
of the teeth and hemorrhages in the joints. It has proved to be a
hexuronic acid that is present in most citrus fruits and green
160 ANIMAL BIOLOGY
vegetables. Vitamin D in inadequate amounts results in rickets, a
disease characterized by various skeletal deformities. Oil from the
liver of the cod and halibut are rich sources, and the action of ultra-
violet rays on certain sterols produces the vitamin, so the exposure
of the human body to sunlight, within reasonable limits, is beneficial.
F. DucTLEss GLANDS
Finally, we must not overlook certain accessories of the ali-
mentary canal which lose all direct connection with it as develop-
ment proceeds — really glands that have carried, as it were, the
process of outpocketing from the digestive tract to the breaking
point and become DUCTLESS GLANDS. Such, for instance, are the
THYROID and THYMUS glands near the anterior end of the esoph-
agus. The thymus in Man regresses during early childhood,
while the thyroid delivers its secretion, a HORMONE, directly into
the blood stream as an INTERNAL, OF ENDOCRINE SECRETION.
We shall have occasion later to discuss the important coordinating
functions carried out by hormones secreted by ductless glands
and other endocrine organs. At the moment we may merely
remark that the pancreas is stimulated to secrete its digestive
enzymes by a hormone, known as SECRETIN, brought to it by the
blood. Secretin is liberated into the blood by special gland cells
in the wall of the small intestine when food enters. (Fig. 110.)
Certainly, at first glance, the complicated digestive system of the
Vertebrate may seem to have little in common with that of the
Earthworm, but as a matter of fact the fundamental plan is the
same. The differences which are present are chiefly the result of an
increase of the area of the alimentary canal, not only to afford
greater seeretive and absorptive surface and a larger variety
and amount of digestive substances, but also to prolong the length
of time the food is subjected to treatment. This increase in area
has been effected by folds and elevations of the inner surface of
the tract; by outpushings of limited areas of the tube to form large
glands which in most cases contribute their products to their point
of origin through ducts; and by increasing the length of the mner
tube as compared with the outer tube, or body wall, which results
in throwing the intestine into various convolutions within the body
cavity. Thus is met the increasingly complex nutritional demands
of more highly organized animals.
CHAPTER XII
RESPIRATION
The living body is the theatre of many chemical and physical cper-
ations in line with those of the inorganic domain. — Thomson.
As we have seen, the essential factor of respiration is an inter-
change of gases between protoplasm and the environment: an
intake of free oxygen for combustion, and an outgo of the waste
products, chiefly carbon dioxide and water. In the unicellular or-
ganisms, such as Protococcus and Amoeba, and in simple multi-
cellular animals like Hydra, this appears to be a relatively simple
process since an elaborate mechanism is not necessary to facilitate
the interchange. But with the establishment of a highly differen-
tiated multicellular body, fewer and fewer cells are in direct con-
tact with the aérating medium and so various provisions are
necessary to transfer the gases to and from the outer world and
the individual cells themselves.
In all forms the skin functions to some extent; in the Earth-
worm, in fact, it acts as the chief respiratory membrane since a
profuse supply of blood vessels to the moist surface of the body
effects a sufficiently rapid gaseous interchange for the relatively
inactive life of the organism. The Crayfish meets the problem of
respiration by finger-form outpocketings of the body wall, the
GILLS: a method of bathing a large area of the respiratory mem-
brane in the respiratory medium, the surrounding water. Insects,
however, instead of bringing the blood to the surface, develop a
network of tubes, or TRACHEAE, which ramify throughout the body
tissues and conduct air directly to the cells. (Fig. 116.)
Among the lower Vertebrates, as has been indicated, the an-
terior end of the digestive tract functions as a common food and
respiratory passage. In Fishes, the respiratory water current which
enters the mouth makes its exit by way of the gill pouches and gill
slits; the lining of the pouches — outpocketings of the lining of
the alimentary canal — functioning as the respiratory membrane.
(Fig. 117.)
Among the air-breathing Vertebrates there are the added
161
162 ANIMAL BIOLOGY
problems of protecting and keeping moist. the greatly increased
respiratory surface which their more active metabolism — pro-
portionally greater energy requirements— demands. Accordingly
the gill slits persist merely as transient embryonic reminders of
evolutionary history; the function of the gill pouches being taken
over by a huge outpocketing of the ventral wall of the pharynx
into the anterior portion of the body cavity, which constitutes the
Fic. 117. — Diagram of a
vertical section through the
head region of Fish (above)
Fic. 116. — Diagram of the and Reptile or Bird (below)
respiratory (tracheal) system to show the paths of the re-
of an Insect. The other in- spiratory currents (a) and
ternal organs are omitted. food (b). See Fig. 109.
LUNGS. Thus, even in Man, the respiratory membrane which lines
the lungs is, from the standpoint of development, a specialized
part of the epithelium of the alimentary canal. Furthermore, the
establishment of lungs entails, in turn, a complex respiratory
mechanism so that the air within them may be changed at frequent
intervals. (Fig. 235.)
A. Lunes
In the human respiratory process, air after entering the nostrils,
ANTERIOR NARES, passes along the nasal passages and out the
POSTERIOR NARES into the lower part of the pharynx. Air may
also enter through the mouth. From the pharynx it passes over
the EPIGLOTTIS, and through the slit-like GLoTTIs into the LARYNX,
or so-called Adam’s apple, and then down the windpipe, or
TRACHEA. Within the larynx are the vocAL corps which vibrate in
response to air currents; the amplitude of the vibrations and the
tension of the cords being responsible for the vorce. (Fig. 109.)
RESPIRATION 163
As the lower end of the trachea enters the chest, or THORAX, it
divides into right and left branches, the BRONCHIAL TUBES, which
thereupon directly enter the lungs, each of which is a bag of elastic,
spongy tissue. Within the lungs, the bronchial tubes divide into
smaller and again smaller branches, until finally they form micro-
scopic twigs, each ending in one or more tiny air sacs, or ALVEOLI.
Thus there are many thousands of alveoli in the human lungs,
Alveolus
Right bronchial tube
Connective tissue
GY,
Yi,
Body wall
Pleura
WD}
Pleura 2 SOL BES
covering of s
lung y \/
] : : ‘
Py Space occupied Diaphragm
by heart, ete.
Right lung cut open Left lung intact
Fic. 118. — Diagram of a vertical section through the human thorax,
showing lungs and associated structures.
everyone in direct communication with the outer air. Further-
more, each alveolus is profusely supplied with tiny thin-walled
blood vessels, or CAPILLARIES, through which flows blood sent
by the heart and soon to return to the heart so that it may be
distributed to every part of the body. It is while the blood is in
the capillaries of the alveoli that it gives up to the air in the alveoli
carbon dioxide, water, and heat taken from the tissues, and at the
same time receives oxygen. This is effected through the delicate
walls of the capillaries and alveoli. So the alveoli are really the
effective surface of the lungs. (Fig. 118.)
io4 ANIMAL BIOLOGY
B. RESPIRATORY MECHANISM
In order for the lungs to play their part in respiration, it is
evident that the air within them must be periodically renewed,
and this rhythmical process of INHALATION and EXHALATION 1s
what one usually refers to as breathing. The complex mechanism
involved and its method of operation may be briefly outlined.
The lungs are elastic sacs suspended in an air-tight cavity, the
thorax, which can be enlarged by raising the rrps and lowering
the DIAPHRAGM, a muscular partition between the thoracic and
Fic. 119A. — Diagram to illustrate the mechanism of diaphragm breathing.
The lungs of a Mammal are enclosed in a bell-jar. As the rubber membrane
below (representing the diaphragm) is pulled down enlarging the cavity, air
enters through the tube (trachea) and expands the lungs. (From Conn and
Budington, after Tigerstedt.)
abdominal cavities. The sole entrance to the lungs is through the
trachea, and accordingly an atmospheric pressure of approximately
fifteen pounds to the square inch is exerted down through the
trachea on the inner walls of the lungs and keeps them constantly
in close contact with the walls of the thoracic cavity — otherwise
there would be a vacuum between the lungs and the thoracic wall.
Therefore when the thoracic cavity is enlarged by contraction of
the muscles of the ribs and diaphragm, the elastic lungs expand in
maintaining contact with its walls — inspiration takes place; and
when the thoracic cavity is decreased, by relaxation of the same
muscles, expiration occurs. The lungs play an entirely passive
RESPIRATION 165
part in the process. Of course, if through injury the thorax is
punctured, then there is equal atmospheric pressure on both sides
of the walls of the lungs, and they collapse. (Fig. 119A.)
Since the rhythmical respiratory movements are due to the
contractions of muscles, it follows that stimuli reach the muscles
from the nervous system; because muscles, excepting those of the
heart and alimentary canal, do not contract automatically. Now
we know that nerve impulses arise in the so-called RESPIRATORY
CENTER in the lower part (MEDULLA) of the brain, just before it
merges into the spinal cord, and pass by nerves to the muscles
involved. Usually it is the oxygen-carbon dioxide content of the
blood reaching the respiratory center which determines the rate
of the respiratory movements that it induces; though there are
nerves that carry nerve impulses from the lungs which contribute
to the rhythm of breathing by holding in check the activities of
the center itself. Indeed, the center can be influenced by stimuli
Fic. 119B. — Diagram to illustrate the respiratory movements of the Frog.
At left, the external nares are open and air enters the buccal cavity. At right,
the external nares are closed and the floor of the buccal cavity raised, forcing
air into the lungs. Note that absence of diaphragm and air-tight thoracic
cavity necessitates an entirely different mechanism from that in Mammals.
(From Hegner.)
from nearly any part of the body: witness the effect of a cold
plunge on breathing. The action of the center is, of course, chiefly
involuntary since we breathe when we are asleep; but obviously it
can be controlled to a considerable extent by the will because we
can ‘hold the breath’ for some time by sending impulses to the
respiratory center which inhibit its discharge. If this center is
destroyed or the nerves are severed, respiratory movements imme-
diately and permanently cease.
166 ANIMAL BIOLOGY
C. RESPIRATORY INTERCHANGE
The respiratory mechanism has attained its objective when air
and blood are brought into such close relationship in the lungs
that gaseous interchange can occur, and heat be transferred.
Inhaled air varies very widely in temperature but in our homes is
perhaps most frequently about 20° C. (70° F.). Exhaled air is
about 36° C., or very nearly the same as that of the human body.
Thus under usual circumstances the exhaled air is warmer than
when it entered the lungs — the blood has lost heat. Again, the
amount of water in the inhaled air is variable, being low on a dry
day and high on a wet day; but the exhaled air is practically
saturated with water vapor — the blood has lost water. Further-
more, the inhaled air contains merely a trace of carbon dioxide,
while when it leaves the lungs it bears about 4 per cent — the
blood has given up carbon dioxide. And finally, air entering the
lungs comprises approximately 20 per cent oxygen, while when
exhaled there is only about 16 per cent — the blood has received
oxygen. In short, the blood by its traffic with the air in the lungs
gives up heat, moisture, and carbon dioxide, and takes up oxygen.
One naturally is interested to know how the blood while passing
through the lungs acquires the oxygen, since this is the element
demanded by every cell in its life processes. At least two phe.
nomena are involved. In the first place, the air contains a con-
siderable amount of oxygen under relatively high pressure and
therefore some oxygen passes into the liquid plasma of the blood
where the oxygen conditions are just the opposite. But the amount
of oxygen gained in this way by the blood is by no means equal to
the demands of the tissues, and so the emergency is met by special
blood cells, known as RED BLOOD CORPUSCLES, of which there are
many trillions in the human body. These carry a complex chemical
substance, HEMOGLOBIN, which has a high chemical affinity for
free oxygen: it is oxidized to form an unstable chemical compound,
OXYHEMOGLOBIN. Accordingly the millions of red blood corpuscles
leave the lungs with the oxygen affinity of the hemoglobin satisfied
and return to the heart to be distributed throughout the body.
The oxygen is actually delivered to the tissue cells through the
capillaries in the tissues where the oxygen content is low — just
the opposite of the condition in the lungs — because the various
cells use the oxygen nearly as rapidly as it is received. So the
RESPIRATION 167
corpuscles give up their oxygen — oxyhemoglobin is reduced to
hemoglobin — and the blood receives in exchange, as it were,
carbon dioxide, and also water and heat resulting from oxidative
processes — combustion — in the tissue cells. The carbon dioxide
is carried by the red blood corpuscles, and also by the blood plasma
in combination with sodium as sodium bicarbonate. (Fig. 126.)
And now the blood, after its passage — lasting about two sec-
onds — through the capillaries, proceeds on its way back to the
heart which sends it to the lungs so that it can transfer to the air
in the alveoli water, heat, and carbon dioxide. The latter passes
to the air because it is under higher tension in the blood than in
the air. The respiratory cycle is complete.
Such, in brief, is the elaborate apparatus present in the higher
animals, and Man, to provide for the new conditions arising be-
cause of the removal of many of their component cells further and
further from the source of oxygen, and the demand for more and
more facilities for securing it as their life processes increased in
activity. We shall have occasion later to consider the attendant
changes in the blood vessels; but now it is only necessary to be
sure that the mechanism does not obscure its object — to reiterate
that, although one ordinarily thinks of the movements involved
in the renewal of the air in lungs as respiration, it is neither in-
halation and exhalation, nor the interchange of gases between
blood and air or between blood and tissue cells. The essential
feature of respiration is the same here as it is in unicellular plants
and animals: the protoplasm of each and every cell of the body
securing energy from food by combustion, involving the appro-
priation of oxygen and the liberation of carbon dioxide. All else
is accessory — though necessary.
CHAPTER XIII
CIRCULATION
I finally saw that the blood, forced by the action of the left ventricle
into the arteries, was distributed to the body at large, and its several
parts, in the same manner as it is sent through the lungs, impelled by
the right ventricle into the pulmonary artery, and that it then passed
through the veins and along the vena cava, and so round to the
left ventricle . . . which motion we may be allowed to call circular.
— Harvey, 1628.
In the Protozoa and many of the lowest Metazoa, the transport
of materials to and from the various parts of the organism is ob-
viously a simple problem compared to that presented by animals
with deeply-hidden tissues, each and every cell of which must be
served. Indeed a complex body is impossible without a complex
CIRCULATORY SYSTEM, and we now proceed to a summary of how
the problem is met, particularly in the Vertebrates.
The crucial points of contact between the higher animal and its
environment, in so far as the intake of matter and energy is con-
cerned, are the membranes which line the digestive tract and a
large diverticulum from it, the lungs. Through the former must
pass all the materials which are to be assembled as integral parts of
the organism and the fuel which is to supply the energy for the vital
processes, while through the latter must pass the oxygen which is
to release this energy. Only when these membranes have been
passed are the materials really within the body and at its disposal
for distribution by the circulatory system to the individual cells
of the various organs which are to use them.
In addition to carrying the fuel and the oxygen, the circulatory
system must remove the wasie products of metabolism from the
cells and deliver them to the proper excretory organs, such as
the lungs or kidneys, to be passed to the outside world. The cir-
culatory system is therefore the essential connecting link between
the points of intake, utilization, and outgo of materials.
And incidentally, as it were, the circulatory system is also a
coordinating agent of crucial importance, because it distributes
168
CIRCULATION 169
complex chemical substances, known as HORMONES, from their
specific points of origin in the various endocrine glands to the
particular tissue or organ where their regulatory influence is to be
effected. So the circulatory system is a distributing system which
not only maintains a suitable environment for the myriads of
cells of the body, but also, in codperation with the nervous system,
unifies the organs into an organism. (Fig. 126.)
Various stages in the development of a circulatory system can
be traced in the Invertebrates. In some it consists merely of a
single cavity or several connected cavities filled with a fluid con-
taining various types of cells, while in others more and more
of the spaces are replaced by definite tubes, or VESSELS, for the con-
duction of the fluid. With the establishment of closed vessels, the
contractions of various organs and the movements of the body asa
whole can no longer be entirely depended on for the movement of
the fluid, and accordingly, in certain regions, a muscular layer is
developed in the walls of the vessels, which by rhythmic pulsation
forces the fluid along. Thus, for example, in the Earthworm there
is a fluid (CORLOMIC FLUID) within the body cavity, which is forced
about by the movements of the worm and bathes most of the
internal organs; and also a system of vessels (VASCULAR SYSTEM),
a part of which contracts rhythmically and distributes the BLoop
to the individual cells. (Figs. 59, 60.)
In the Vertebrates circulation is effected by two systems, the
BLOOD VASCULAR and the LYMPHATIC systems. The blood vascular
system consists of vessels which distribute the blood composed of
a liquid plasma, in which float various formed elements, chiefly
RED and WHITE Cells. The lymphatic system comprises spaces,
channels, and vessels in the lower Vertebrates, but in the Mam-
mals, including Man, it is essentially a network of vessels so that
in higher animals the so-called closed circulatory system gradually
takes the ascendency over the predominately open circulatory sys-
tem of lower forms. The lymphatics carry LympH which consists
of a liquid plasma with white cells. Both systems are closely asso-
ciated, but the lymphatic plays a relatively passive role, so it is the
blood vascular system that one ordinarily has in mind when speak-
ing of ‘the circulatory system.’ (Figs. 7, 114, 115, 120, 125, 126.)
The essential elements of the blood vascular system are, first, a
muscular organ for propulsion of the blood, the HEART, which lies
near the mid-ventral line in the anterior part of the coelom; and,
“MO]J POOTA JO UOTPOoIIP oPeOIpUL SMOITY “OVUM UT ‘( oandunt,) poyeuasXxo-uou *yoe|[q Ul UMOYS
pooyq (,oand,) poyeuaskxo SurkaTeo sjasso A ‘(ysy3oq) YreyS B Jo WSs IB[NSBA BY} JO MOLA [B1]L] orjyeuueIselqg — “071 “1
youwo}s UIVA UBIAP[IGNS g[OLIyUe A
uIdA [eyt0d oIyedey \ Az]9z7e URIAB|qGNS s[ANY
UIAA DVI] 4 Snsolia}1e SNUOD
uaa[dg UIaA d1}ECOH e108 [VASA
fx0}18 del] UIDA [e10}e'T ae aa Aro IV [CIYOUBIG UIIITJY
9ulysoU] WS [19 ISA
JOA,
airae " eS py See PANG R mea ws SDH a : — EIR
aces) ene ta NSN ee hee : Ke aie SHED)
vos ern 3S eehe ’ S é or J ‘ oe Ul’ ER eS
cI GCEOS ag F Cs t J ~~ ee See po
rere eR ROURE G co, VeOr nar r Te BED MS SG enor
je ae ie Spas reas 9 : R ey liye Saas eke secs
UIdA AP[NSNL
Krayre pryosey
UIA [epneDn
Arejre pepneg
uIeA [e}10d eudy
Axojre |VuIy
Aouplry
ULDA [eUeyY
Ki9y1e peryouesqg JUS JA
oe ore snsouaa snulg
peuop a0 Pe svaioueg ULdA [BARDOT
Aaoz1e d119]U9S9] faajyae 281[909 ej108 |es10q
UI9A [EUIPACD
170
CIRCULATION 171
second, tubes which convey the blood to the heart, the vErNs, and
away from the heart, the ARTERIES. The arteries divide and sub-
divide to form smaller and smaller arteries which finally merge
into exceedingly delicate tubes (CAPILLARIES) that permeate the
tissues of the body. The capillaries, in turn, deliver the blood to
tiny veins which pass it on through larger and larger veins to the
heart. Consequently the blood flows in a circle from heart to
heart again, through a closed system of vessels. Indeed, in the
meshes of a network of blood-streams all the life of our bodies
goes on. About one-twentieth of the weight of the normal human
body is blood. (Figs. 120, 124.)
A. CIRCULATION IN THE LOWER VERTEBRATES
The heart represents that part of the vascular system in which
the power of rhythmic contraction is concentrated, and it can be
regarded as a blood vessel whose walls have become highly modified
by an excessive development of muscular tissue. In the lowest
Vertebrates and in embryonic stages of higher forms the heart
consists typically of two chief chambers, an AURICLE and a VEN-
TRICLE, fitted with muscular flaps, or VALVES, which allow the
blood to flow in one direction only; that is, from auricle to ventricle.
An enlargement, the SINUS VENOSUS, connects the veins (VENOUS
SYSTEM) with the auricle, and there is frequently another, called
the CONUS ARTERIOSUS, in a similar position at the arterial end.
The heart is thus essentially a linear series of chambers. The
sinus venosus and auricle function mainly as reservoirs to fill
rapidly the especially muscular ventricle. The latter, acting both
as a suction and force pump, passes the blood on to the conus
arteriosus and from there to the ARTERIAL SYSTEM as a whole.
For our purposes, however, we may consider the heart in the lowest
Vertebrates (Fishes) as composed of the two chambers, auricle
and ventricle. (Fig. 121.)
The arterial system is the distributing system of vessels which
carries the blood to all regions of the body. Soon after its origin
at the heart, the circuit in the aquatic forms is temporarily inter-
rupted to allow the blood to pass through the Grits and exchange
carbon dioxide for a supply of oxygen. To facilitate this gaseous
interchange, the arteries (AFFERENT BRANCHIAL) as they enter
the gill membrane break up into smaller and smaller vessels which
finally are of microscopic caliber and consist of but a single layer
“MOTJ POOTT JO UONoIIP 9}BOIPUL SMOITY “ofIIVUOA YJoy (2 +O Ul
gPljyUOA JYsI ‘gq pue VY Ul ofolyUOA ‘z fgprme yoy ‘A {9 pue g Ul aplune YS “y Ur oprline ‘f {snsoudA snuIs ‘x SUOlsel peey
oy} WOIJ JUOIIND SNOUDA UINJoI ‘MN SULPA SNOGUBINO ‘a sULYS Jo soreyides ‘n ‘yreoy 0} Apoq jo syred Jotsoysod wWody YUOLINO
snousa ureul ‘7 /UIDA OLVedoY ‘s SdoAr[ JO solster[iIdeo “y SsuloA [BUOT ‘b Sura jeurope ‘d ‘skoupry oy} Jo sotseypideo ‘o Sutoa [eyJod
jeuod ‘U tseljtuiedyxo JOLIoysod Jo sormeyjideo ‘wu {sotoysie peuar Kq sXoupry 07 Ajddns pooyq ‘7 sutoa [eyod *y :*oJo ‘ouNsozU! ‘yor
-wo}s Jo souerides ‘f soo ‘ourysozuL ‘yoruroys 07 Ayddns poo [eWoye “7 SOAT 0} Agoqae ormedoy ‘y ‘kpoq Jo syed sotsaysod 04
(ey10e [esdop) Adoqre Joryo “H SULYS OF OplIPUOA WOIY Kaoqie ‘f sopoume yyoy 07 UloA Aueuowynd ‘(> pue g Ul) a :(UMOYS JOU ‘solo}IB
yerpouesq JUoJoya pue JUdJoyye SULooUUOS ‘s[[LB Jo soreyideo) sotseyse peryouRsq JUosayo Jo uoluN Kq pouldOy S]|[19 WO Adore
‘(y ul) a ‘sduny 07 Asoqae Areuowynd ‘(9 pue q Ul) Pp ‘s]]L3 JO solsoqre PeryouRIg JUOJEye oY} OJUT dn SuUIpLAIp JBoY WOTJ TOSSoA
‘(y Ul) p :Ssnsolio}1e SMUOD “Dd ‘sSuny Jo soteypideo ‘9 Suolsed peoy 0} SuIpeo] SoldoyIe Joryo “q ‘uoIse1 peoy Jo solseypides “pb ‘popeys
‘poojq ,oanduil, ‘popeysun ‘poojq ,eind, ‘“euuweyy pue ‘ueiqrydury ‘Yysty 8 JO UloysAs JepnoseA oY} Jo Ue[q — “Tél “OY
| ‘IVAWVW J V
12
CIRCULATION 173
of cells. These capillaries, in turn, merge into larger vessels (EF-
FERENT BRANCHIAL ARTERIES) which finally lead into the chief
artery of the body, the porsAL AorTA. This extends along the
median dorsal line of the body, just below the vertebral column,
and sends branches to the various organs.
The branches of the dorsal aorta, on reaching the location which
they supply with arterial blood, break up into capillaries similar
to those in the gills, so the blood can deliver food, oxygen, etc., to
the tissues. The blood receives in return various waste products
of metabolism, including carbon dioxide and, in certain cases,
absorbed food materials from the intestine, and special secretions
chiefly from endocrine glands. The fine capillaries lead into vein-
lets and these into veins of constantly increasing caliber which
sooner or later complete the circuit by returning the blood to the
heart.
The return current, however, is not quite so simple as would
appear from the above statement because, just as all the outgoing
stream is interrupted for the respiratory interchange in the gills,
so a part of the return current is temporarily side-tracked through
the liver. The veins returning blood from the digestive organs
merge to form the PORTAL VEIN which proceeds to the liver, where
it resolves into capillaries to allow that organ to regulate certain
of the blood constituents. From the capillaries the blood then
passes into the HEPATIC VEIN which conveys it toward the heart.
Thus the liver receives blood from two sources: an artery providing
blood primarily for the use of the organ itself, and a vein (portal
vein) delivering blood, containing a large amount of food material,
solely to receive special treatment before being sent back to the
heart and then all over the body. This special arrangement for a
venous blood supply to the liver is known as the HEPATIC PORTAL
SYSTEM.
Moreover, in Vertebrates lower than the Birds, the venous blood
from the posterior part of the body makes a detour through the
capillaries in the kidneys on its way back to the heart. This con-
stitutes what is termed the RENAL PORTAL SYSTEM. ‘Therefore
in these forms the kidneys as well as the liver receive blood from
two sources, an artery and a vein. It will be noted that both the
hepatic portal vein and the renal portal vein arise in capillaries
and terminate in capillaries. (Figs. 115, 120, 121.)
Such is the general plan of the blood vascular system of the
174 ANIMAL BIOLOGY
lower Vertebrates. The modifications of this which occur in higher
forms are related chiefly to changes in the respiratory mechanism
necessitated by abandoning an aquatic for a terrestrial mode of
life, with consequent dependence on the free oxygen of the atmos-
phere instead of that dissolved in the water.
B. CrrkcuLATION IN THE HIGHER VERTEBRATES
We may now note some of the far-reaching changes that the
blood vascular system undergoes as a result of the substitution of
lungs for gills. In the first place the series of paired branchial
arteries, which formerly supplied the gills, no longer break up into
capillaries, but instead lead directly into the dorsal aorta, and
accordingly are termed AORTIC ARCHES. ‘Thus Fishes bequeath,
as it were, to higher forms a series of pairs of aortic arches which,
though they are no longer of use in their former capacity, appear
in the developmental stages. Some disappear at that time and
others are modified and diverted to various uses in the adult.
(Fig. 122.)
For our purpose it is sufficient to emphasize that in Man’s
body one aortic arch continues to carry blood directly from the
heart to the dorsal aorta, while parts of another deliver blood
from the heart to the lungs and back again to the heart. Thus
there is established a second current of blood through the heart,
which necessitates a median partition in both the auricle and ven-
tricle in order to keep the two currents separate.
In this way a four-chambered heart arises which consists of right
and left auricles and ventricles. The RIGHT AURICLE receives blood
from the venous system of the body and passes it through the
TRICUSPID VALVE into the RIGHT VENTRICLE to be pumped through
the PULMONARY ARTERY to the lungs. After traversing the capil-
laries of the lungs, the blood is returned by the PULMONARY VEIN
to the LEFT AURICLE, thence through the MITRAL VALVE into the .
LEFT VENTRICLE, which forces it into the AORTA and so on its way
about the body as a whole. To all intents and purposes, the higher
Vertebrates have two hearts which act in unison — a right, or
pulmonary, heart receiving non-aérated blood from the entire
body and pumping it to the lungs, and a left, or systemic, heart
receiving aerated blood from the lungs and delivering it to the
body as a whole. Thus the blood vessels of the primitive aquatic
respiratory apparatus are transformed by graduai additions and
CIRCULATION 175
subtractions into the PULMONARY SYSTEM of the higher Verte-
brates, including Man. (Figs. 121, C; 123.)
The vascular system is, in truth, a highly efficient apparatus.
Day in and day out throughout life the human heart, beating
PRIMITIVE ‘ FISH AMPHIBIAN
BIRD MAMMAL
Fic. 122. — Diagram to show the transformation of the six pairs of primitive
gill arteries (aortic arches) in the ascending series of Vertebrates. a, dorsal
aorta; b, ventral artery from heart; c, internal carotids; d, external carotids;
e, e’, right and left aortic arches; f, pulmonary arteries; g, g’, subclavian arteries
to fore limbs.
rhythmically at an average rate of 70 times per minute, does about
300,000 foot-pounds of work. This power is expended in moving
the weight of the blood, in imparting to it the velocity of its mo-
tion, and in maintaining pressure in the aorta and pulmonary
artery. In its circulation through the body of a man, the blood
176 ANIMAL BIOLOGY
passes through a series of vessels that, if they could be arranged
continuously, would, it has been estimated, encircle the Earth!
The RATE of flow is greatest when the blood leaves the heart and
gradually diminishes until, in the capillaries of both the pulmonary
and systemic systems, it is
reduced to a minimum. On
the return trip from the cap-
illaries through the veins the
Superior :
verse ae rate of flow gradually in-
Yi UW: . ee
ele = | creases, though it reénters
the heart at a slower rate
than it departed. Thus of
the 23 seconds which it takes
a unit of blood to make the
auricle
- ees
SSS els: Oe WD
=A aM, Sy ‘
wy mY WAZ
Inferior Wa complete circuit in Man,
vena cava A
A Left about two seconds are spent
Right ventricle ventricle
Aorta
in the capillaries — a _ rela-
tively long time when it is
Fic. 123. — Diagram of human heart realized that the average
and associated blood vessels. The direc- length of the capillary path
tion of blood flow is indicated by arrows. . E
(From Peabody and Hunt.) is about one-fiftieth of an
inch. The chief factor un-
derlying the change in rate is simple. The blood, driven through-
out its course by the same force — the heart beat — varies in rate
with the width of the bed
through which it is flowing.
Although the area afforded
individually by the arteries
and veins is greater than
that by any single capillary,
nevertheless the total area
afforded by the capillary sys-
Es
Lymph tubes”
: Fic. 124.— Diagram of the intimate
tem is enormously greater relations between capillaries, lymphatics,
than that by either the arte- and tissue cells. (From Peabody and
rial or venous system. The Hunt.)
total surface of the capillaries of a man has been stated to be
about equal to the area of a city block.
Moreover, since a liquid in a closed system of tubes must flow
from a region of high to one of low pressure, the blood PRESSURE
continuously diminishes from heart back to heart again. But it
CIRCULATION 177
should be noted that although the pressure in the capillaries of
any region as a whole is greater than in the veins which they sup-
ply, nevertheless the pressure in a single capillary is very low, as
is demanded by its delicate wall.
Thus in the capillaries the blood moves very slowly under low pres-
sure and here the blood does its work — contributes to the TISSUE
FLUID and interchanges materials with it. All the rest of the vascu-
lar system — heart, arteries, and
veins —is arranged to give the
blood just this opportunity in the
capillaries.
The tissue fluid is essentially
some of the blood plasma, with
white cells, that has passed
through the walls of the capil-
laries, carrying along food mate-
rials, oxygen, etc., to be exchanged
for the various waste products of
metabolism of the cells which it
bathes. Thus there is a continuous
drainage of fluid from the capil-
laries into intercellular spaces.
Some of this tissue fluid, with
waste products, etc., passes imme-
diately into the capillaries again,
but the excess passes into small p,, 95 The supertalk and
LYMPH VESSELS. Thus lymph is _ some of the deeper lymphatics of the
essentially excess tissue fluid with human hand. (From Hough and
white cells augmented from lymph EEUU)
glands, etc., which passes through larger and larger lymphatic
vessels until it is finally delivered to veins in the region of the
neck, and the materials are restored to the blood. (Figs. 115,
124-126.)
With such a marvellously complex transportation system, clearly
some provision must exist for regulating the blood flow in order to
meet the varying local demands of the organs of the body under
different physiological conditions. This is attended to chiefly by
nerve impulses which are conducted by a system of VASOMOTOR
nerves and bring about the dilation or contraction of the smaller
arteries leading to an organ, and also by chemical substances in the
ferry
PN COR
q
ashes
Mi Ae NSSYj
WT PEE ARV
NANPA
a ) Whew
A \ Vic)
W >
178 ANIMAL BIOLOGY
blood inducing similar changes in the capillaries that penetrate the
tissues. Since the total volume of blood in the body is practically
constant, an extra supply to one part necessitates a slightly reduced
supply to another. So it happens, for instance, that after a hearty
meal more blood is sent to points where digestion is going on,
leaving less for other organs — the reduced supply to the brain
probably resulting in the proverbial drowsiness at such times.
Moreover, of course, the blood itself undergoes various changes
during its course through the body as it receives or delivers food
substances, secretions, and excretions. And furthermore, for in-
Body Cells
Hormones Food Oxygen
Waste Products
R 1 Station
jor oxyeen [9] ol el[elololelolo
(Lungs) Exchange Station
(Blood and Tissue Fluid)
Removal Station
Carbon Dioxide Pump to Keep
Blood in Motion
Renewal Station (Heart)
Hormones
(Endocrine Glands)
Renewal Station Removal Station Removal Station
Waste Products Excess Heat
(Digestive Tract) (Kidneys) (Skin)
Fic. 126. — Diagram illustrating how a suitable environment is maintained
by the flow of biood. (From Martin.)
stance, its supply of red blood cells is replenished from the bone
marrow and stabilized, in part, by a vascular organ, the SPLEEN,
which is situated in the abdomen behind the stomach. (Figs. 106—
108, 114.)
The elaborate mechanism in HOMOTHERMAL animals (Birds and
Mammals) that maintains a practically constant body tempera-
ture is largely dependent upon heat distribution, heat loss, and
heat conservation by the blood vascular system. At rest, a gentle
CIRCULATION 179
stream of blood flows through the capillaries of a man’s muscles;
in violent exercise, a great torrent, impelled by increased heart
action, brings food and oxygen and carries away waste products
and heat. This superfluous heat is carried by the blood to the
lungs and skin for elimination. The skin becomes flushed by the
distension of its blood vessels and heat is radiated, usually aided
by perspiration. Thus the body is losing heat, but we ‘feel warm’
because the sense organs that make us aware of temperature are
situated in the skin. We ‘feel cold’ when blood is withdrawn
from the skin to the internal organs and heat is being conserved.
This makes clear the apparent paradox — one is apt to ‘feel warm
when catching cold.’
So much for the paths and duties of the blood and lymph that
circulate through the body — just enough, perhaps, to emphasize
that with increase in size and complexity of the animal body there
goes hand in hand an elaboration of the transportation system.
It is gradually transformed to meet the new demands made upon
it, and so leaves in higher animals evidence of their origin.
CHAPTER XIV
EXCRETION
The mathematically accurate end-reaction of a chain of known and
unknown causes and effects. — Noyes.
Provistons for eliminating from the organism the waste products
of metabolism are not less important than those for supplying the
matter and energy by which the vital processes are carried on.
In many of the unicellular forms the whole surface of the organism
functions as an excretory organ, but it will be recalled that even
in some of these, such as Amoeba and Paramecium, contractile
vacuoles facilitate the removal of useless metabolic products. In-
deed, in all but a few of the lowest Metazoa there are highly spe-
cialized organs for excretion. In the Vertebrates we find the
KIDNEYS and the GILLs or the LUNGs devoted largely to excretion,
and the SKIN and LIVER acting in subsidiary capacities. Each
receives a profuse blood-supply from which it takes the waste
products that are to leave the body as an excretion. Usually
the effective surface of an excretory organ consists of gland cells.
(Migs, 6.27, 1915)
There is therefore an essential distinction between an excre-
tion, which represents chemical waste from the vital processes,
and the major part of the material which is ejected from the di-
gestive tract as feces. The latter is almost entirely indigestible
material taken in with the food which has not directly contrib-
uted to the metabolic processes of the organism, though some of it
may have acted temporarily in an accessory capacity. Accord-
ingly the digestive tract is not included in the list of excretory
organs, though it will be recalled that certain waste products
excreted by the liver reach the outside world with the feces.
The nature and amount of the material eliminated by the excre-
tory organs is, of course, determined by what is brought to them
by the blood, and this, in final analysis, is dependent upon the
food — the fuel that has been burned — and the disintegration
of protoplasm from the wear and tear of life. Carbohydrates and
fats yield carbon dioxide and water, while proteins give in addi-
180
EXCRETION 181
tion UREA, URIC ACID, CREATININE, AMMONIA, etc., the products of
nitrogenous metabolism.
A. GiIL~s AND Lunes
We have already emphasized the elimination of carbon dioxide
by the gills or the lungs. Here the cells of the respiratory mem-
branes play essentially a passive réle in excretion, since the carbon
dioxide, which is under higher tension in the blood than in the
water or air, follows the physical laws of diffusion of gases and
passes from the blood. In addition to carbon dioxide, the blood
of homothermal animals loses a large amount of water and heat;
the amount depending on the moisture and temperature of the
air which enters the lungs. When the air is exhaled, its tempera-
ture is very nearly that of the body and it is saturated with water
vapor. In Man about one-third of the
water eliminated is excreted by the lungs.
(Fig. 118.)
B. Skin
The skin of some of the lower Vertebrates,
for instance the Frog, is an exceedingly im-
portant excretory organ, because more car-
bon dioxide is eliminated through it than
through the lungs; but in most of the higher
forms, including Man, excretion by the skin
is confined to the SWEAT GLANDS. There are
nearly three millions of them, with a total
length of several miles, opening on the
surface of the human body. They take from
the blcod, in addition to large quantities of
water, traces of nitrogenous waste or urea,
fatty acids, and salts, which form a residue
on the surface of the skin when the PER- Capillaries
SPIRATION evaporates. However, most of Fic. 127. — Diagram
the water may be regarded as a secretion ofe surest eee
rather than an excretion because it is of use to the body, being
employed in regulating the body temperature. Everyone knows
that evaporation of perspiration accelerates the loss of heat by
the skin. In addition to sweat glands, the skin is provided with
SEBACEOUS GLANDS which open, as a rule, at the base of hairs
BP ss
182 ANIMAL BIOLOGY
and deliver a true secretion — a lubricant for hair and skin, and a
conserver of body heat. (Figs. 96, 127.)
CC. SLIVER
The liver, in addition to its various other functions, aids in no
small way in excretion. On the one hand, the liver removes delete-
rious compounds of ammonia from the blood and converts them
into urea. Then it secretes the urea into the blood from which it
is later removed by the kidneys. On the other hand, the liver col-
lects other waste products, etc., from the blood, which form the
bile. This passes to the GALL BLADDER for temporary storage, or
directly to the intestine. (Fig. 112.)
D. KiIpNEys
The kidneys, in codperation with the liver, are the chief excre-
tory organs, and any serious interference with their activity leads
to a poisoning of the body with waste products. However, excretion
is but one function of the kidneys — they act as a blood-regulator
to maintain a proper balance of the chemical constituents of the
blood plasma. A large amount of water and various salts, urea, etc.,
pass from the blood through the GLOMERULI into the TUBULEs.
Here such materials as are of value in the economy of the organism
are absorbed by the tubules and returned to the blood, while the
rest passes to the pelvis of the kidney and eventually out of the
body. In Man about 90 per cent of the nitrogenous waste is elim-
inated as urea. (Figs. 130-132.)
1. Urine
The total excretion of the kidneys, known as URINE, passes from
the kidneys through the URETERS to the URINARY BLADDER where
it is stored temporarily until passed to the exterior through the
URETHRA. Since urine is the medium for the elimination not only
of nearly all the normal products of katabolism, but also of the
majority of abnormal substances that may enter or be formed by
the organism, there is no better indication of the general metabolic
condition of the human body than that afforded by a chemical
and microscopical analysis of the urine. Thus in Bright’s disease
protein appears; in diabetes, glucose (grape sugar) and other ab-
normal substances; while in gout, uric acid is present in abnormal
quantity in the urine.
EXCRETION 183
However, the interpretation of the analysis is of first importance
in every case because the urine rapidly reflects normal changes in
the physiological condition as well as those that are abnormal:
even the nervous tension of an examination may well be evidenced
by the appearance of grape sugar. Again, the amount of urine
excreted depends upon many factors. Under normal conditions
with a given intake of water, the volume of urine varies chiefly
with the temperature and moisture of the atmosphere. For in-
stance, when the atmosphere is hot and dry, a relatively large
amount of water is eliminated as perspiration. Accordingly the
intake of water should be greater in order to carry off readily the
waste products of metabolism through the kidneys. One is apt to
think of the kidneys as essentially passive organs that merely
drain materials from the blood. But, as a matter of fact, a
grain of kidney tissue consumes, on the average, more oxygen
per unit of time than the same weight of the beating heart. The
kidneys work.
2. Evolution of Kidneys
Aside from their functional importance, the kidneys are of
considerable interest to the comparative anatomist because of
their complicated evolutionary his-
tory throughout the Vertebrate se-
ries. Indeed the basic elements of the
vertebrate kidney may be most
readily interpreted with the excre-
tory system of the Earthworm in
mind. (Figs. 59-61, 128.)
The chief excretory organs of the
Earthworm consist of pairs of coiled
tubes, or NEPHRIDIA, segmentally A
arranged in the coelom on either Fy. 128. — Diagram to show
side of the alimentary canal. Each the general structural plan of a
nephridium begins as an open fun- wage es ARE
nel in the coelom of one segment, ternal opening of nephridium; },
passes through the partition to the external opening; c, capillary net-
next posterior segment and there, WoTK about the coiled, glandular
a portion in the coelom.
after coiling, passes to the ventral
surface and opens to the exterior by a pore. Thus, reduced to its
simplest terms, a nephridium is a tube communicating between
‘601-901 ‘S8tq 99g “snsoyn ‘f SyonprAo 47 Syonp o1sydoueRyour ‘y :sorydouRyour ‘6 S[eIsI{s0A “yonp ormqdouosout ‘¢/’
gonp unads se Ajojos Suyoe yonp o1ydouosow “f ‘yonp useds se osye suroe yonp olydouosour “Tf: Gonp o1ydouoid jo
uorjsod Jotieysod <]peursiso0) yonp o1sydouoseut ‘f :sorydouosour ‘a -]RLST]SOA ‘oures oY} “'p “Joup ouydouoid ‘p tyeIsiysea
‘gues oy ‘'2 ‘soaydouodd ‘9 tyeueo AueyUoUTT]e Jo uotjsod yeutus9] 10 BOBOLD ‘q Sopeuey UL AUBAO ‘ofeUL UT Stysoq :peuos ‘D
*(§ ) oyeuroy ‘oures oy} “yy “SpeuUey, pue spilg ‘sogdoyy qynpe ur uroysXks oAtjonpoddod (©) opeut pue ‘uweysks onmydou
“RVI “AE (6 ) opeuoy ‘oues oy} ‘q *(s04q) suBIqrydury pue soysly qynpe Ul waysks oArj~onpoddoed (2 ) apeur oy} pue
wiaysks o1mydouosayyy ‘A *(SoWI0ySOPAT)) SoyeIqoyJoA JSOMOT OY} UL odAY YNpe puke ‘soyeIqo}OA ][B Ul ody ormuokaquia
— wiaysks onydouosayy ‘gq ‘ojodpey Soa oy} ur peuorjouny ‘soyerqoyJoA [fe UL odAy oruoXuquia Ajseo pue oATyud
— woysks oydouoig ‘y ‘so}e1qoyJoA JO Woysks yeyUosOIN oyy Jo ued yeyUOWRpUNy oy} JO WeIseIG| — “6ZT “OM
| a d ©) da Vv
q q
184
EXCRETION 185
the coelom and the outer world, and affording a path of egress for
the waste matter in the coelomic fluid. But in addition, the blood
vascular system carries nitrogenous waste, inorganic salts in solu-
tion, etc., to the coiled part of the nephridial tube where special
cells take them from the blood and deliver them to the interior
of the tube to be passed out of the body. Thus the nephridia
of the Earthworm remove waste material from both the coelomic
fluid and the blood. Artery Vein
Although the primitive
segmentation of the coe-
lom has disappeared in
the Vertebrates, neverthe-
less there are good grounds
for believing that the
ancient, segmentally ar-
ranged nephridia are the
basis of the essential excre-
tory elements of the kid-
neys. Thus in the lowest
Vertebrates the primitive
type of kidney, or PRO- Ureter Ureter
NEPHROS as it Is called, con-
sists of a series of segmen-
tally arranged nephridia in
the dorsal part of the ante-
rior end of the coelom.
These, however, instead of
opening independently to
the exterior, discharge their
products into a common
tube (PRONEPHRIC DUCT)
which passes them to the Fic. 130. — Diagram of the human urinary
outside. In higher forms system, posterior view. See Fig. 134B.
the pronephros disappears, and its function is taken over by an-
other series of nephridia which appears in the coelom posterior
to the pronephros. This series constitutes the MESONEPHROS, and
opens into the pronephric duct which accordingly now is called
the MESONEPHRIC DUCT. Finally, in still higher Vertebrates this
second urinary organ is replaced by a third, the kidney proper
(METANEPHROS) and its special duct, the ureter. (Fig. 129.)
Kidney Kidney
186 ANIMAL BIOLOGY
Thus as we ascend the Vertebrate series three distinct kidney
systems appear, in each case by the development and grouping of
nephridium-like elements into a definitive organ. In this process
the primitive communication of the individual nephridia with the
body cavity is lost and the functions of the tubular portion in-
creased, until, in the higher forms, all the waste products are taken
solely and directly from the blood. It is therefore apparent that
each of the relatively large, compact kidneys of the higher Verte-
brates consists essentially of an enormous number of nephridium-
Tunic apices
7 From renal
arter
Cortex — y
Glomerulus
within
7 Renal artery ae
To renal vein
Medullary Collecting
repion tubules
Tip of pyramid
Pyramid of medullary
region
- B
Fic. 131. — Human kidney. A, longitudinal section; B, diagram of the
course of the tubules in the kidney. The cortex is the region in which the
tubules come into functional association with the capillaries. Tubules extend
through the medullary region to open on the summits of the pyramids.
like elements, the tubules, bound together by connective tissue and
covered with a protective coat. The tubules within the kidney de-
liver the materials taken from the blood, the urine, to the pelvis of
the kidney, from which it passes down the ureter, into the urinary
bladder, and finally to the exterior. (Figs. 130, 131.)
Such, in broad outline, is the historical viewpoint from which
the kidneys of Man must be interpreted. As a matter of fact,
however, the evolutionary transformation is still further compli-
cated by anatomical, though not physiological, relations with the
EXCRETION 187
reproductive system. As will be pointed out later, this neighbor-
ing system now and again foists, as it were, some of its accessory
responsibilities upon parts of the excretory (urinary) system, and
even takes over portions and makes them integral parts of its own
when they have been permanently abandoned by the urinary
system in its development.
CHAPTER XV
REPRODUCTION
Let us first understand the facts, and then we may seek the cause.
— Aristotle.
REPRODUCTION, as we know, is in the final analysis cell division,
whether it is binary fission in unicellular forms such as Amoeba
and Paramecium, or the setting free by the multicellular organism
of cells with the power of going through a complex series of changes
by which they rapidly become transformed into the complex in-
dividual, similar to the parent. In most animals this process is
complicated at the start by sexual phenomena: the fusion of
two germ cells, the male and female GAMETES, to form the ferti-
lized egg, or zyGoTE. Indeed, sex is fundamentally a physiological
difference between gametes, which, however, so profoundly in-
fluences the body that we recognize it as male or female. (Figs. 8,
31, 168.) |
Disregarding for the time being the actual origin of the germ
cells in the body, we find in the Metazoa special organs in which
the germ cells reside and undergo changes preparatory to their
liberation. Such reproductive organs, or GONADS, ordinarily con-
tain germ cells of one kind, and accordingly are either OVARIES
(egg-producing organs) or TESTES (sperm-producing organs).
A. INVERTEBRATES
In many of the simpler animals, the gonads are merely tempo-
rary structures which appear during certain seasons of the year
when conditions favor sexual reproduction. In some species the
same individual produces both eggs and sperm, in which case the
sexuality of the germ cells is not reflected back, so to speak, to
the organism as a whole, which accordingly is known as an HER-
MAPHRODITE. Such frequently is the condition in Hydra where
the testes appear as small swellings in the ectoderm a little below
the circle of tentacles, and the ovary, which is usually single, is a
somewhat larger projection near the opposite end of the animal.
Both the testis and the ovary at first appear to be a group of
1838
REPRODUCTION 189
ectoderm cells, which in one case gives rise to many sperm and in
the other to a single egg. The mature sperm are set free from the
testis and swim about in the water, but sooner or later one enters
the now ruptured covering of the ovary and fuses with the egg.
With the conclusion of fertilization the zygote begins to divide
and forms an embryo which at an early stage becomes de-
tached from the parent. Thus in Hydra there is no complicated
apparatus for sexual reproduction; merely now and again the
temporary development of the primary sex organs, ovaries and
testes. (Fig. 57.)
The complex bodies of most animals, however, demand more or
less permanent gonads, as well as means for transferring the gam-
etes directly or indirectly to the exterior. This is brought about
by the fact that in coelomate animals the gonads come to lie, not
on the outside of the body, but within the coelom. In the Earth-
worm, which also is hermaphroditic, the testes and ovaries are
permanent organs attached to the partitions between certain seg-
ments. The sexual products are set free in the coelom where they
are taken up by SPERM DUCTS and ovipucTs and carried to the
outside. Although each Earthworm possesses both male and fe-
male reproductive organs, two worms pair and exchange the sperm
which are stored in the respective sperm receptacles. Later, when
the eggs pass to the exterior, the ‘foreign’ sperm are shed on them.
Thus cross-fertilization is insured in this hermaphroditic form. In
the Crayfish the sexes are represented by separate individuals,
males and females, and the appendages of the first and second
abdominal segments of the male are modified into copulatory
organs for the transfer of the sperm to the body of the female,
where they are retained until egg-laying. (Figs. 60, 63.)
B. VERTEBRATES
Throughout all the chief Vertebrate groups the sexes are dis-
tinct, although in rare instances abnormal hermaphroditic in-
dividuals occur. The definitive primordial germ cells first appear
as localized areas of the epithelium lining the coelom, on either
side of the vertebral column. As the germ cells develop they be-
come associated with connective tissue, blood vessels, and nerves
and form the paired gonads. In the most primitive Vertebrates a
condition more simple than in the Earthworm is found, for both
male and female gametes merely break out of the gonads and
190 ANIMAL BIOLOGY
find their way to the exterior by a pair of minute ABDOMINAL
poRES. In higher forms, however, special sperm ducts and ovi-
ducts are developed in close relationship with the urinary system.
(Fig. 132.)
In some aquatic, and most terrestrial Vertebrates, fertilization
occurs while the eggs are still within the oviducts; the copulatory
organ transferring the sperm directly to the terminal portion of
Opening of oviduct into coelom
Dorsal aorta
Posterior vena cava
Oviduct
Adrenal bodies
Kidneys
Mesonephric ducts
or ureters
Uterine dilation of oviduct
Vestigial oviduct in male Opening of
oviduct into
Openings of ureters © cloaca
Clout FEMALE
Fic. 132. — Urogenital organs of the Frog.
the ducts through which they make their way up to meet the de-
scending eggs. After fertilization the zygote may soon pass to the
exterior; usually, as in the case of the familiar hen’s egg, after
being wrapped up in nutritive and protective coats secreted about
it during its passage down the oviduct. Or, as is the case rarely
among lower forms and the rule among all Mammals except the
Monotremes, the zygote on reaching the lower part of the oviduct
becomes attached to the wall of an enlargement of the oviduct,
or of a chamber formed by the union of the two oviducts, called
REPRODUCTION 191
the uTERus. Here the embryo proceeds far along in development
before birth occurs. (Figs. 133, 134, 169.)
1. Uterine Development
In the human body, the attachment of the fertilized egg in the
uterus is followed by the very rapid development of what may
be regarded as a new uterine lining, profusely supplied with blood
Fic. 133. — Human egg and sperm. A, four sperm (left); and egg (right)
just removed from the ovary, surrounded by follicle cells of the ovary and a
clear membrane. The central part of the egg contains metaplasmic bodies and
the large nucleus. Superficially there is a clear ectoplasmic region. (Magnified
about 400 times.) B, two views of the human sperm, c, centrosome; h, head
consisting of the nucleus surrounded by a cytoplasmic envelope; m, ne, middle
piece; ¢, tail or flagellum. (Magnified about 2000 times.)
vessels. This soon surrounds the embryo and, as pregnancy pro-
ceeds, the embryo protected by EMBRYONIC MEMBRANES projects
into, and finally completely fills the gradually increasing uterine
cavity. As the uterus enlarges, it exerts increasing pressure on the
adjacent organs, and later shifts higher up in the abdominal cavity
where the flexible body wall more readily accommodates it.
Throughout the entire period of pregnancy the embryo leads
essentially a parasitic existence at the expense of the mother’s
192 ANIMAL BIOLOGY
body which supplies it with food and oxygen, and removes carbon
dioxide, urea, and other metabolic wastes. This, of course, imposes
a considerable amount of extra work on the various maternal
organs, especially the lungs, kidneys, and digestive system, and
therefore special provisions must be made for this intimate inter-
change of materials between the blood vascular systems of mother
and offspring. (Fig. 134A.)
The embryo at first is nourished by materials absorbed from
the rich blood supply of the uterine wall, but soon this proves in-
adequate for the rapidly increasing demands of the growing em-
bryo, and a new structure, the PLACENTA, is formed jointly by the
tissues of the uterus and embryo to meet the need. The connection
of the embryonic body with the placenta is by the UMBILICAL CORD.
The blood of the embryo passes by its arteries through the umbil-
ical cord to capillaries in the placenta, and after interchanging
wastes for food and oxygen by diffusion with the maternal blood,
returns by veins to the embryo. Accordingly there is no direct
intermixture of maternal and embryonic blood: the embryo from
the beginning is a separate organism whose blood supply inter-
changes materials with that of the mother through the placenta.
This temporary dependence is terminated when the child is ex-
pelled from the uterus, or born.
2. Hormones
Thus, except in the simplest animals, there is a special repro-
ductive system: a series of organs connected with the reproductive
function. But it must be emphasized that the essential organs are
the gonads themselves and all the rest are accessory. Furthermore,
in relation to the sexual differentiation of male and female in-
dividuals, many so-called sECONDARY SEXUAL CHARACTERS arise
which are not directly connected with the reproductive organs,
but nevertheless depend very largely for their development upon
HORMONES liberated by the gonads. For example, early castration
of the Stag inhibits the growth of a distinctive male secondary
sexual character, the antlers; while if performed later when the
antlers are full grown, they are shed and abnormal ones take their
place.
Indeed, the sexual life of the Vertebrates, including Man, is
largely controlled by hormones. Thus after the release of the egg
from the human ovary, its former location is filled by the corpus
REPRODUCTION 193
LUTEUM which secretes several hormones that prepare the uterus,
both structurally and physiologically, for the reception of the egg
and the attachment of the embryo. Moreover, certain hormones
secreted by the ovary during pregnancy directly influence the
PITUITARY GLAND which in turn induces the development and func-
( A\\
a\\\\\ \\\) \
Fic. 134A. — Diagrammatic section of the human uterus with developing
embryo. The embryo (h) is suspended in a fluid-filled cavity (c) surrounded
by embryonic membranes (e) and by tissue (f) from the uterus itself. The sole
path of communication between embryo and mother is by blood in vessels
passing up through the umbilical cord (i), spreading out into capillaries in the
placenta (6) and there coming into close relations with the maternal blood
supply. The openings of the oviducts (d) into the uterus become closed during
the development of the embryo. a, dorsal wall of uterus; b, placenta; c, fluid-
filled cavity of amnion; d, openings of oviducts (Fallopian tubes); e, embryonic
membranes; f, uterine tissue; g, uterine cavity; h, embryo; t, umbilical cord.
tioning of the MAMMARY GLANDS. At least two hormones are in-
volved; one directly stimulates the development of the mammary
glands, while another prevents their functioning until it is inacti-
vated at the birth of the offspring.
3. The Urogenital System
We must now outline the structural interrelations of the urinary
and reproductive organs forming the UROGENITAL SYSTEM. It has
194 ANIMAL BIOLOGY
been pointed out that the nephridia, which combine to form the
kidneys in some of the lower Vertebrates, retain their funnel-like
openings into the coelom and therefore afford a direct exit for
waste material in the coelomic fluid. It is some of these nephridia
which are employed in the lower Fishes for the transfer of the
germ cells to the outside. The testes of the male, which lie close to
the kidneys, become connected with the nephridia (mesonephros)
by a series of short delicate tubes. Through these tubes the spER-
MATIC FLUID, containing the sperm from the testes, is transferred
to the nephridia and by them to the kidney (mesonephric) ducts
and so to the exterior with the urinary waste. In this way, during
Urinary bladder “Ureter ride bladder
Vas deterens 4’ Oviduct << T
‘lll \\e
ih
IK
2 Ampulla
J of/vas
deferens
BaN\)) i
A : PN) — Seminal
Testis Nt Wz vesicle
x a Kee a P *) / /
yr Prostate gland
3--- Cowper’s gland
\ ACS
4--- Urethra
Seminiferous 1 _ A 2 B
tubule Epididymis
Fic. 134B.— Diagrams of the human urogenital organs. Posterior view.
A, male; B, female. (From Hegner.)
the period of sexual activity of the male, the kidney tubules satis-
factorily perform two functions, and the mesonephric ducts be-
come UROGENITAL CANALS. (Fig. 129, C.)
Turning to the female, we find that the ovaries, which are sit-
uated in about the same position with relation to the kidneys
as the testes in the male, do not enter into communication with a
set of nephridia of the kidneys (mesonephros); probably because
the eggs are too large to pass through the tubules. Instead, what
appears to be the coelomic opening of a single nephridium-like
structure on either side (which fails, so to speak, to enter the
kidney complex) enlarges and becomes the funnel which connects
REPRODUCTION 195
up with a new duct opening into the cloaca. Thus there arises from
the female urinary system a pair of entirely distinct oviducts. An
egg, liberated from the ovary into the coelom, finds its way into
one of the oviducts and descends directly to the outside, or into an
enlargement (uterus) of the terminal portion of the duct where
development proceeds until birth occurs. (Figs. 129, D; 132.)
The female reproductive system, though derived from the
mesonephric system, has become entirely independent of it. Ac-
cordingly the disappearance of the mesonephros and duct in higher
Vertebrates, when it is replaced by the metanephros and the ureter
as the functional urinary system, has little effect on the female
reproductive system. As a matter of fact, the abandoned meso-
nephros and duct degenerate and disappear in the female, while
in the male the mesonephric duct remains and becomes completely
appropriated by the reproductive system. ‘The sperm now pass
directly into the former mesonephric duct, which thereby becomes
solely a sperm duct. (Fig. 129, E, F.)
Such is the historical origin of the foundations of the reproduc-
tive system as it occurs in the Reptiles, Birds, and Mammals.
Each of these groups, building on this foundation, has developed
modifications and additions demanded by its special lines of evolu-
tion. It appears again that, whenever possible, structures at hand
are employed to construct what is to be, and thus is woven in the
woof and warp of higher forms a partial record of their ancestry.
(Fig. 134B.)
CHAPTER XVI
COORDINATION
It seems that Nature, after elaborating mechanisms to meet partic-
ular vicissitudes, has lumped all other vicissitudes into one and made
a means of meeting them all. — Mathews.
SINCE a primary attribute of protoplasm is irritability — the
power of responding to environmental changes by variations in the
equilibrium of its own matter and energy — it is not strange that
the cells of an organism mutually influence each other’s activities
and reciprocal interrelationships have been established during their
long evolutionary history. The various cells, tissues, organs, and
organ systems are unified into an organism by what may be called
the chemical interplay between its various parts, which is made
possible by the facilities for distribution afforded by the circulatory
system; and also by the directing influence of the nervous system
which supplies a central station with lines for instantaneous inter-
communication with every part of the body.
A. CHEMICAL COORDINATION
It is only with the recent increase in knowledge of the general
problem of metabolism that the far-reaching importance of the
chemical control of bodily processes has gradually been brought to
the fore. Although we may properly think of the various chemical
regulators, or HORMONES, as forming a codrdinating system in so
far as their collective action has such a result, in the present stage
of our knowledge it is possible to do little more than cite the
specific action of individual hormones as examples of the general
method of chemical regulation which their study, ENDOCRINOLOGY,
is revealing. (Pp. 159, 169, 192.)
Certain hormones are elaborated by special cells embedded in
organs, such as the pancreas, intestine, and reproductive organs,
largely devoted to other functions. Other hormones are secreted
by organs whose sole function is their production, such as the
various ductless, or endocrine, glands.
As an example of a hormone secreted by specialized cells within
the tissue of an organ devoted primarily to other functions, we may
196
COORDINATION 197
select INSULIN which reaches the blood directly from the pancreas,
and is not delivered with the other secretions of this organ to the
intestine through the pancreatic duct.
D1aBETEs has long been known to be the result of a deficiency
of a pancreatic function leading to a lack of codrdination of the
chemical processes by which the body uses carbohydrates to sup-
ply energy. The storage of sugar by the liver is unregulated. Evi-
dences of this disease are chiefly an increase in the sugar content
of the blood and the presence of sugar in the urine. Now we know
that a close approach to the normal metabolic condition can be
attained by administering to diabetics the hormone insulin ex-
tracted from the pancreas of sheep or other animals. Insulin thus
takes the place of the secretion which the pancreas fails to afford,
and so removes the pall of hopelessness from many of the most
acute and desperate cases of diabetes.
Turning now to a gland devoted solely to the secretion of a
hormone, we may select the thyroid which, as has been seen, arises
as an outpocketing of the digestive tract in the neck region and
finally loses all connection with its point of origin and becomes
a ductless gland. (Fig. 110.)
The general effect of the thyroid hormone, THYROXINE, on
metabolism is a regulation of the rate of oxidation in the body to
meet changing physiological demands. An excess of thyroxine
induces such vigorous fuel consumption that no surplus remains
in the body to be stored as fat; while a deficiency in the glandu-
lar secretion results in a tendency toward fat formation. The ad-
ministration of thyroid extract or thyroxine is often an efficient,
though dangerous, means of reducing fat by increasing the oxida-
tive processes of the body. A deficiency of the hormone during
adult life frequently results in a pathological condition called myx-
EDEMA. Children in whom the development of the thyroid is sup-
pressed become dwarfish idiots known as CRETINS, while over-
development of the gland induces increased nervous activity and
mental disorders. Feeding with thyroid material prevents or re-
tards the development of cretinism and cures myxedema, while
a surgical removal of part of the gland may cure the nervous in-
stability and other symptoms due to an excessive amount of the
hormone. GorrTeEr is a pathological enlargement of the thyroid due
to a deficiency in iodine needed to manufacture its iodine-contain-
ing thyroxine.
198 ANIMAL BIOLOGY
The almost uncanny potency of hormones in general wil! be
evident from the fact that about 1/1,000th of a gram of thyroxine
is sufficient to induce a 2 per cent increase of the total oxidation
of the resting adult human body. The amount of thyroxine re-
quired by the body during a whole year is probably about 23 grams,
while the amount in use at any one time is approximately 2/10th
of a gram. “But this pinch of material spells all the difference be-
tween complete imbecility and normal health’? —a fact that
should give pause not only to the biologist but also to the sociolo-
gist.
Moreover, as a further indication of the nicety of the reciprocal
adjustments within the organism, it may be mentioned that the
thyroid gland itself is affected by regulating stimuli reaching it
through the nervous system, and also by a hormone derived from
the PITUITARY GLAND which is another endocrine organ situated in
conjunction with the lower surface of the brain.
Finally, the ADRENAL GLANDS, one situated in close proximity to
each of the kidneys, are endocrine organs of high significance that
secrete at least two hormones. One, known as ADRENALINE, iS
poured into the blood when the glands are stimulated by the ner-
vous system. Adrenaline has its most marked effect when muscu-
lar exertion is at a premium. It accelerates heart action, increases
blood pressure, reduces muscular fatigue, stimulates the liver to
give up its stored glycogen, retards the activities of the alimentary
canal, dilates the eyes, etc. It is essentially a chemical whip which
makes various organs play their part in the general mobilization.
Glimpses of such interrelationships are being gradually afforded as
one hormone after another is discovered and studied — chemical
coordination is indeed a fact. (Figs. 107, 108, 132.)
B. CoORDINATION BY THE NERVOUS SYSTEM
Although hormones are indispensable as a means of regulating
many of the processes of the organism, they are entirely inadequate
for the instantaneous correlation of diverse parts of an animal and
also for the adjustment of the whole animal to its surroundings.
This need is supplied in the Metazoa by the NERVOUS SYSTEM: a
complicated arrangement of cellular elements in which irritability
and conduction are highly developed. The study of the nervous
system constitutes the science of NEUROLOGY.
COORDINATION 199
Even in some unicellular organisms certain portions of the proto-
plasm are especially differentiated for receiving and conducting
stimuli, and others for making effective such stimuli by contrac-
tions of the whole or parts of the cell. Indeed, Paramecium and
other Infusoria possess a NEUROMOTOR system which apparently
consists of a codrdinating center, or MoToRIUM, from which con-
ductile paths extend through the cell to the cilia, ete. But
it is in the lower Metazoa, such as Hydra
and its allies, that we find the establishment
of definite NERVE CELLS, some of which are
specialized for re-
fee eB kha ae
er
ZT ie __ Fic. 137. — Diagram of
Fic. 136. — Diagram
a more complex type of
AGRA USE
Fic. 135.— Nerve
cells in the ectoderm of
Hydra. The parallel
lines represent muscle
fibers. (After Schnei-
der.)
of a simple type of recep-
tor-effector system found
in some Hydra-like ani-
mals. It comprises recep-
tors (b), or sense cells,
reaching to the body sur-
face (a), with basal nerve
net (c) connecting with
muscle cells (d). (Slightly
modified, after Parker.)
receptor-effector system,
found in some Hydra-like
animals. It comprises, in
addition to the receptor
(b) with nerve net (c) and
the muscle cells (e), an-
other nerve (ganglion) cell
(d) interpolated in the
nerve net. a, body surface.
(From Parker.)
ceiving stimuli and others for conducting the excitation to cells
specialized for contracting (muscle cells), etc. Thus a simple
RECEPTOR-EFFECTOR system arises which may be regarded as the
basis for the development of the elaborate NEUROMUSCULAR
MECHANISM of higher forms, with RECEPTORS, or sense organs,
CONDUCTORS, or nerves, and EFFECTORS, or muscles and glands.
Although from the functional point of view it is difficult to differ-
entiate the receptors, conductors, and effectors in the economy
of the organism, from the standpoint of anatomy the conductors
constitute a definite entity, the nervous system proper. (Figs. 135-
137, 144, 145.)
The structural elements of the nervous system of all animals
consist of cells known as nerve cells, or NEURONS. In the lower
200 ANIMAL BIOLOGY
forms these cells are apparently united so that they form NERVE
NETS which surround and permeate the tissues which they stimu-
late to action. In more highly developed animals the net arrange-
ment is relegated to the control of relatively minor functions,
while the main nervous system consists of numerous neurons ar-
ranged in groups, or GANGLIA, and prolongations of the neurons, or
nerve FIBERS, bound together into cables, or NERVES. The neurons,
which are embedded in protective sheaths of connective tissue in
Coverings
of axon
Terminal
D branches
Fic. 138. — Neurons. Stages in the differentiation of nerve cells. A, prim-
itive neuron from the nerve net of Hydra-like animals; B, motor neuron of the
Earthworm; C, D, primary motor neurons of a Vertebrate.
the ganglia, are in physiological continuity one with another by
‘transmitting contacts, or SYNAPSES; but each neuron, it is be-
lieved, remains structurally distinct. (Figs. 138, 139, 143.)
1. Brain and Spinal Cord
It will be recalled that the first great differentiation during the
development of a multicellular animal establishes an outer ecto-
derm and inner endoderm, and thus separates the functions of
protection and general reactions to the environment from that of
nutrition. It is natural therefore that the ectoderm should be-
COORDINATION 201
come the seat of those specializations which have evolved into
the nervous system and sense organs. Such is the case in all forms
from the lowest to the highest, and thus the development and
comparative anatomy of the nervous system of Vertebrates, in
particular, affords strong evidence of the genetic continuity of
the whole series.
In the development of a Vertebrate, the first indication of the
nervous system is a longitudinal groove in the ectoderm along the
dorsal surface, which soon becomes converted into a tube by the
Ventral nerve cord
at ganglion Motor neuron cell body
Motor fiber ending in
longitudinal muscle Sensory fibers
—§—= Body cavity
Fic. 139. — Diagram of primary sensory and motor neurons of the ventral
nerve cord of an Earthworm, showing their connections with the skin and the
muscles to form a simple reflex arc. See Fig. 61.
apposition and, finally, the fusion of its edges. This NEURAL TUBE
then becomes separated from, and sinks below the surface ecto-
derm, and in time forms the CENTRAL nervous system consisting
of the BRAIN and SPINAL CoRD. As development proceeds, out-
growths from the central nervous system establish the PERIPHERAL
and the AUTONOMIC nervous systems, so that structurally as well
as physiologically the whole nervous system represents a unit;
a single organ, as it were, which secondarily becomes closely iden-
tified here and there with sense organ, muscle, or gland, as the
case may be. (Figs. 142, 174.)
The first marked structural modifications in the developing
central nervous system of Vertebrates are two constrictions of
the enlarged anterior end of the neural tube, which establish the
three primary brain vesicles: FORE-BRAIN, MID-BRAIN, and HIND-
BRAIN. Thus very early in embryonic development, one end of the
neural tube is molded into the brain, leaving the rest to become
the spinal cord. (Fig. 140.)
202 ANIMAL BIOLOGY
Fore-brain Mid-brain Hind-brain
A
Diencephalon
Telencephalon Mid-brain Cerebellum
i ae
B trereys weet Ly? pil
Cerebral hemisphere Pineal body
| Cerebellum
Mid-brain
Cc A Medulla
Meleneeonalone ~ ae =
Diencephalon Infundibulum
Cerebral hemispheres
Olfactory
lobes Mid-brain (oa
pe
Fic. 140. — Diagrams to illustrate the general method of transformation of
the anterior end of the neural tube into the brain. A, B, C, median vertical
sections; D, dorsal view of C.
COORDINATION 203
The three-vesicle brain now becomes transformed into one of
five vesicles by a hollow outpocketing from the anterior end of the
fore-brain and a dorsal outpocketing from the hind-brain. In the
lowest forms the brain throughout life consists essentially of these
divisions, known as TELENCEPHALON, DIENCEPHALON, MID-BRAIN,
CEREBELLUM, and MEDULLA OBLONGATA; the latter merging into
the spinal cord. Usually, however, the telencephalon gives rise to
a pair of CEREBRAL HEMISPHERES which are destined gradually to
overshadow in size and significance all the other parts of the brain.
Then the development from the telencephalon or its derivatives,
the cerebral hemispheres, of a pair of OLFACTORY LOBES completes
the establishment of the chief brain chambers.
The further changes which transform the more or less linear
series of vesicles into the increasingly complex and compact brain
of higher forms are due to bendings, or FLEXURES, and to unequal
thickenings and outgrowths of the chamber walls. For instance,
the upper and lower surfaces of the diencephalon give rise to the
PINEAL BODY and the INFUNDIBULUM respectively, while from
similar regions of the mid-brain are developed the opTIcC LOBES
and CRURA CEREBRI. Hand in hand with these changes the pri-
mary cavities of the chambers undergo a gradual restriction, but
throughout all there persists at least a remnant of the original
tubular cavity which is continuous with that of the spinal cord.
(Figs. 109, 141.)
The CEREBRUM, or cerebral hemispheres, is considerably the
largest and most important part of the human brain since it is
the center of perceiving, thinking, voluntary motion, and even
consciousness — the seat of the higher mental life in general. These
primary activities are performed by neurons, the cell bodies being
located in the corTEx, the outer layer of GRAY MATTER, while the
nerve fibers from the neurons extend deeper to form the inner
WHITE MATTER. These fibers transmit nerve impulses to and from
the cell bodies in the cortex.
Next in importance is the cerebellum which, of course, also con-
sists of neurons. Their functions are subsidiary to those of the
cerebrum since initiative resides in the cerebrum, but messages
from this director are codrdinated by the cerebellum on their way
to various parts of the body. Thus, one may consciously extend
an arm, but the various compensating movements of other parts
of the body that are necessary to maintain equilibrium are at-
‘paoo jeurds ‘b :eyjnpour “f
‘uunpJoqeseo ‘a ‘saqoy, odo ‘p :kpoq jeourd ‘9 ‘soroydsturay Pesqodeo *q +soqo] Kaoqoryjo ‘p (yer) euUTeTAy “5p (UOOsTq)
pag ‘q ‘(sovesyTy) edoy “OD ‘(#014
cl
) ueiqrydury ‘gq ‘(yoseg) Yysty Auog ‘y Jo uresq oy) JO MOLA Testo] — “THE “OM
204
COORDINATION 205
tended to by the cerebellum. It is discriminating but not con-
scious.
Finally, in the medulla, nerve fibers are in the ascendancy since
all nerve impulses to and from the cerebrum and cerebellum
and the spinal cord are trans-
mitted through the medulla. The
crura cerebri are large bundles of
fibers extending from the medulla
to the cerebrum. However, nerve
cells are also prominent in the
medulla: some of them give rise
to CRANIAL NERVES, and some
form functional groups, or centers
of actions for regulating the
respiratory and circulatory or-
gans, known as the RESPIRATORY
CENTER, VASOMOTOR CENTER, etc.
2. Cranial and Spinal Nerves
The brain and spinal cord are,
as we know, protected and iso-
lated by a cartilaginous or bony
tube, formed by the skull and
neural arches of the vertebrae,
which is embedded in the muscles
forming the dorsal part of the
body wall. The sole paths of
nervous communication between
the central system and the rest of
the organism and its surroundings
are a series of pairs of CRANIAL
and SPINAL NERVES. ‘These arise
at fairly regular intervals from
one end of the brain and cord to
the other, and pass out through
openings in the skull and between
or through the vertebrae to con-
stitute the peripheral nervous
system. (Figs. 109, 142.)
i
Fic. 142.— Ventral view of the
nervous system of the Frog. Br, sec-
ond and third spinal nerves (brachial
plexus); Js, sciatic nerve leading from
the sciatic plexus; O, eye; Ol, olfac-
tory nerve; Op, optic nerve; Sg 1-10,
ten ganglia of autonomic system;
Spn 1, first spinal nerve; Sp 4, fourth
spinal nerve; Vg, trigeminal ganglion;
Xg, ganglion of 10th cranial nerve,
or vagus. (From Ecker.)
206 ANIMAL BIOLOGY
It is usually considered that the primitive segmental condition
of the Vertebrate body is well exhibited in the arrangement of the
cranial and spinal nerves, and that the origin of the cranial nerves
from the brain affords a partial index to the primary series of seg-
ments which apparently have been merged to form the Vertebrate
head. Conditions as they exist at the present time can perhaps be
most readily understood by imagining a simple, ancestral, seg-
mented worm-like form in which the dorsal neural tube gives off
a pair of nerves to each segment of the body. As the result of a
gradual shifting forward, union, and finally complete fusion of
certain segments near the anterior end, there is formed a head
region with its brain, battery of sense organs, and skull, more or
less distinct from a trunk region with its spinal cord, vertebral
column, paired limbs, etc. This CEPHALIZATION naturally involves a
shifting and modification of the primitive condition of the paired
nerves; especially since the innervation of a group of cells in nor-
mal development is apparently rarely changed — a nerve following
the part which it originally supplied through many of the trans-
formations and even migrations of the latter.
If this point of view is accepted, the cranial and spinal nerves
are, historically considered, similar structures. But the former,
synchronously with the changes in the head region, have departed
somewhat widely from their ancestral condition and have even been
augmented by nerves of diverse origin. The spinal nerves, on the
other hand, continue to issue from the cord at about equal intervals
and in segmental arrangement as indicated by muscle segments
and skeletal structures, although those of certain regions unite in
the body cavity to form groups, or PLEXUSES, to afford an adequate
nerve supply to the appendages. (Fig. 142.)
From the standpoint of function the nerves are of three classes:
SENSORY, MOTOR, and MIXED. Sensory nerves are the paths over
which excitations (NERVE IMPULSES) due to stimuli are conducted
to the cord and brain, while motor nerves are the paths for dis-
tributing impulses from the brain and cord to muscle cells, gland
cells, etc., and thus induce the response of the organism. But the
great majority are mixed nerves which afford paths for sensory as
well as for motor impulses and so perform both functions.
It is important to note that a nerve is actually a bundle of nerve
fibers; the fibers themselves in turn being prolongations of nerve
cells, the cell bodies of which are usually in groups, or ganglia.
COORDINATION 207
Moreover, nerve impulses are not transmitted by a nerve as a
whole, but by one component cell process, a nerve fiber; that is, by
way of a definite cell path through the nerve. The same is equally
true of the cord and the brain, which differ from nerves largely in
Cortex
Motor fiber
Sensory fiber
Sensory fiber
Ganglion cell
Sensory nerve
/iiber ending in
<=> sense cell
My)
Ventral root “Motor nerve
fiber ending
in muscle
Motor fiber
/N
SPINAL NERVE
SOUR A yond)
rp ye
NUALAADD SMD!
Synapse
Mt
AW iba
ANTM
Fic. 143. — Diagram of the paths of sensory and motor nerve fibers. A
reflex arc from sense cell via spinal cord to muscle cell is shown at lower right.
that they comprise more cell processes and also the cell bodies
themselves. In other words, the brain and cord comprise the ele-
ments of both ganglia and nerves.
A mixed nerve conducts impulses both to and from the central
organ because it contains both sensory and motor cell paths, or
fibers. All peripheral nerves are primarily mixed nerves, because
typically they arise by two roots from the central organ; the DORSAL
ROOT containing only sensory (afferent) fibers and the VENTRAL
208
ANIMAL BIOLOGY
ROOT only motor (efferent) fibers. This condition is preserved by
the spinal nerves of higher forms since each arises by two roots.
But some of the cranial nerves, in response to the profound modi-
Fic. 144. — Diagram of a
section (highly magnified) of
the wall of the intestine of a
Vertebrate to show its intrin-
sie nervous organization
which brings about the move-
ments of the tube. The two
plexuses consist essentially of
simple neurons arranged as
nerve nets. a, food absorbing
surface of the intestine; },
mucous layer; c, plexus of
neurons (submucous); d, cir-
cular muscle; e, plexus of
neurons (myenteric); f, lon-
gitudinal muscle; g, serous
layer. (From Parker, after
Lewis.)
fications that have been wrought in the
head region, have only one root, and so
are either solely sensory, as those to the
sense organs, or only motor, as those
innervating the muscles which move
the eye. (Fig. 143.)
Many nerve impulses set up by sen-
sory stimuli are, in part, shunted di-
rectly from sensory to motor nerve
paths in the spinal cord itself. One
removes his finger from the pin-point
before he is conscious of the prick.
Thus so-called REFLEX ARCS. bring
about the multitude of REFLEXES
which relieve the brain of much unnec-
essary labor, and are the basis of the
behavior of animals. Many reflexes
apparently are inherited, but others,
known as CONDITIONED REFLEXES, are
established as the result of experience.
(Figs. 139, 143.)
3. Autonomic System
So far we have considered the central
system — the brain and spinal cord;
and its lines of communication with
the body as a whole, the peripheral
system — the cranial and spinal nerves.
In point of fact, however, the periph-
eral system gives rise to an auxiliary
series of ganglia and nerves which are
charged with the regulation of prac-
tically all of the functions of the body
that are not under voluntary control, such as the circulatory
system and alimentary canal.
This AUTONOMIC SYSTEM in the
higher Vertebrates consists essentially of a double nerve chain,
situated chiefly in the body cavity just ventral to the verte-
COORDINATION 209
bral column, from which branches proceed to innervate the nearby
organs. It communicates with the central system by way of the
sensory roots of the spinal and some of the cranial nerves.
(Figs. 142, 144.)
Such, in essence, is the distribution throughout the body of the
nervous system which, although it arises as an infolding of the
ectoderm and therefore is primarily external, comes to be internal
and so chiefly dependent upon more or less isolated groups of
SENSORY Cells for the reception of stimuli. Some of these, termed
EXTERNAL RECEPTORS, remain at the surface to receive stimuli
from the outer world, while others, known as INTERNAL RECEPTORS,
are situated within the body for the reception of stimuli arising
there. The external receptors are what one ordinarily thinks of
as sense organs.
C. SENSE ORGANS
Although among some of the Protozoa certain regions of the
cell are specialized so that they are more sensitive to one or another
kind of stimulation, the great majority show no trace of sense or-
gans. Nevertheless all forms, in common with all protoplasm,
possess the power of receiving and responding to environmental
changes. Thus Paramecium reacts to mechanical, thermal, chem-
ical, and electrical stimulation: the entire surface of the cell is
sensitive to stimuli, and the excitations are conducted from one
part to another essentially by the protoplasm as a whole, aided
apparently by the neuromotor apparatus. In some Invertebrates,
such as Hydra and the Earthworm, the entire surface of the body
is still depended upon as a receiving organ for all kinds of stimuli,
and only simple sense cells are developed. In the majority of ani-
mals, however, although all the cells, of course, retain to some ex-
tent their power of irritability, environmental changes exert their
influence chiefly upon complex receptors which are specialized to
respond most readily to particular forms of energy. The energy,
for example of heat or light, is transformed by appropriate
mechanisms into the energy of a nerve impulse, and accord-
ingly the sense organs constitute the outposts of the nervous
system. (Figs. 22, 135-137, 139, 225.)
Since we necessarily gain our knowledge of the outside world
solely through the data afforded by our sense organs, it follows
210. ANIMAL BIOLOGY
that we judge the capacity of the sense organs of other animals
merely by comparing them with our own. This is a safe procedure
only in the case of sense organs that more or less correspond
in structure to those which we possess.
In the Crayfish, for example, we find com-
plex sense organs which, without doubt,
are eyes, and others which are ears, or
at least perform one of the functions of
our ears, equilibration; while some of the
head appendages are particularly adapted
to receive sensations of touch. The senses
of smell and taste are also probably pres-
ent, but here we are on less sure ground.
It is, indeed, almost certain that environ-
mental changes which are without effect
on the sense organs of the human body,
and so play no recognizable part in the
‘world’ of Man, may stimulate receptors
Pee rer in lower organisms. But Man’s ingenuity
stages in the differentiation as in certain cases devised apparatus to
of sense cells. A, primitive minimize his limitations: witness the radio
sensory neuron of Hydra- receiver. (Figs. 54, 63.)
like animals; B, sensory ; { :
neuron of a Mollusc; C, Lhe simplest form of sense organ in
primary sensory neuron of Vertebrates is a single epithelial cell for
7 . . . .
a Vertebrate. In each case the reception of stimuli, connected with a
the sensory surface is rep- :
resented below, and there: Merve fiber for the conduction orgie
fore the nerve impulse nerve impulse to a _ sensory center.
ae ) pward. (From {jsyally, however, many associated cells are
; arranged to respond and are aided by ac-
cessory structures for intensifying the stimulus, protection, etc., so
that the whole forms a highly complex sense organ. (Figs. 145, 150.)
A B
1. Cutaneous Senses
Confining our attention to the Vertebrates, we find that prac-
tically the entire surface of the body constitutes a sense organ, be-
cause the skin is permeated with a network of sensory nerves.
Certain regions are supplied with special PRESSURE RECEPTORS,
which may take the form of a regular system of sense organs, such
as the LATERAL LINE ORGANS Of Fishes and Amphibians, or of
groups of TACTILE CORPUSCLES as in Man. In addition to pressure
COORDINATION 211.
receptors, most of the surface of the human body is provided with
PAIN, HEAT, and COLD SENSE SPOTS.
2. Sense of Taste
In the higher Vertebrates the sense of taste is restricted to the
cavity of the mouth, particularly to the tongue, where special
receptors known as TASTE BUDS are in communication with the
zat B\ | RAL ROO
ES We Pe
Supporting
tli ff
cells My Y,
Sense Y
cells & Word
Ss
SSS
ne
Y,
Te
Fic. 146. — A, Cells of olfactory epithelium from human nose. B, Cells of
a taste bud in epithelium of tongue. The sensory cells terminate externally in
hair-like processes which are activated by the chemical stimuli that produce
odor or taste.
brain by two of the cranial nerves; but in some Fishes similar
organs are scattered quite generally over the surface of the body.
(Fig. 146.)
3. Sense of Smell
The special sense organs of smell, or OLFACTORY CELLS, reside in
the membrane which lines a pair of invaginations of the anterior
end of the head, termed oLFACTORY POUCHES. The cells are in
communication with the brain by the olfactory, or first pair of
cranial nerves. The pouches constitute relatively simple sacs
in the lower Vertebrates, but in the air-breathing forms, and
especially in the Mammals, the walls of the pouches are thrown
into folds, ridges, and secondary pouches. This is necessitated
by the concentration of the olfactory surface to the air passages
of the NosE which lead to the lungs. However, the human ol-
factory apparatus has fallen somewhat from the complexity which
it attains in the lower Mammals, as is attested not only by its
212 ANIMAL BIOLOGY
structure in the adult but also by transient remnants in the
human embryo. (Fig. 146.)
4. The Ear
The EARS, or organs of hearing and equilibration, arise as paired
depressions of the ectoderm of the head, which, in all Vertebrates
above the lower Fishes, lose their connection with the exterior and
form the so-called INNER EAR, Or LABYRINTH. This becomes divided
into two chief parts, the saccuLus and the UTRICULUS, from which
are developed three SEMI-
CIRCULAR CANALS, one in
ee each plane of space. The
sacculus is largely devoted
to the reception of vibra-
tions of the surrounding
medium, that is to hearing
in the usual sense of the
word. Accordingly the sac-
Ampulla culus becomes progressively
differentiated as we ascend
the Vertebrate scale—a
Fic. 147. — Diagram of the left mem- complex derivative in the
branous labyrinth of a lower Vertebrate, M li foe th
showing the sacculus, utriculus, and the ammanan car Nem. €
three semicircular canals. The lagena isa COCHLEA. (Fig. 147.)
derivative of the sacculus which becomes On the other hand, the
the cochlea in higher Vertebrates.
Semicircular
canals
Utriculus
at
Sacculus —— = Lagena
utriculus and the semicircu-
lar canals provide for sensations of loss of equilibrium, or orientation
of the body in space, and show far less change. It is probable that
equilibration is the chief function of the entire labyrinth in Fishes,
as it is of the so-called auditory organs of many Invertebrates, such
as the Crayfish. With the progressive specialization of the laby-
rinth, the essential sensory cells, which are in communication with
the brain by the eighth, or AUDITORY NERVE, become limited to a
few definite areas. These sensory cells are provided with auditory
hairs which project into the cavity of the labyrinth and so are
stimulated by movements of the fluid which fills it.
The ears of Fishes lie immediately below the skull roof where
they are readily accessible to vibrations transmitted by the water.
But with the substitution of air for water as the surrounding
medium, there arises the necessity of a more delicate method for
COORDINATION 213
conducting and also for collecting and augmenting the sound waves.
The result is that, in ascending the Vertebrate series, we find the
ear proper receding farther and farther below the surface.
Soon, between the labyrinth, or inner ear, and the surface of the
head, a simple resonating chamber is added which is provided with
a vibrating TYMPANIC membrane, or EAR DRUM, situated just under
the skin. Then this is improved by the development of a bony
transmitting mechanism between the tympanic membrane and the
inner ear. This consists of a single bone until we reach the Mam-
mals, when two more bones are added by being diverted from their
Incus
Malleus
Semicircular
Vestibule
— Auditory nerve
Eustachian
tube
Tympanic chamber
: or middle ear
Auditory passage Stapeg
or outer ear Tympanum
Fic. 148. — Front view of a vertical section of the human ear, right side.
Note the transmitting mechanism of three bones: malleus, incus, and stapes.
Cartilage
earlier function of articulating the jaws with the skull. Finally,
the resonating (tympanic) chamber recedes farther below the sur-
face and becomes the MIDDLE EAR to which sound waves are con-
ducted through a tubular passage, the oUTER EAR. In some forms
the latter is provided with an external funnel-like expansion, the
PINNA. Apparently much is accomplished by accumulating minute
changes during the ages. (Fig. 148.)
5. The Eye
The organs of sight are the most complex sense organs of ani-
mals and reach a very high degree of specialization even in some
of the Invertebrate forms. Among the latter the essential sensory
element (RETINA) of the eye usually arises by the invagination of
214 | ANIMAL BIOLOGY
Fic. 149. — Diagrams illustrating the method of formation of the eye of an
Invertebrate (A) and a Vertebrate (B, C, D, E, — successive stages). Note that
the opposite surface of the retinal cells is exposed to the light rays in the Verte-
brates as compared with the Invertebrate eye. a, ectoderm; b, retinal area; c,
future position of optic nerve; d, cavity of the diencephalon; e, optic vesicle;
jf, stalk of optic vesicle, later replaced by the optic nerve; g, vitreous chamber
within optic cup; h, developing lens.
COORDINATION 215
a limited area of ectoderm, the cells of which become differentiated
for receiving the photic stimuli that produce nerve impulses to
be transmitted to the central nervous system. Among Vertebrates
the sensory cells are also of ectodermic origin, but only secondarily
so, since the OPTIC VESICLES arise as outpocketings directly from
the sides of the diencephalon. (Fig. 149.)
A retina alone, such as exists in some of the lower Invertebrates,
can afford no visual sensations other than light and darkness, and
perhaps in some cases the ability to distinguish light of one color
Muscle to
upper lid
Posterior chamber.
Anterior chamber
(both filled with
aqueous humor)
Eyelid Fy
Byelah 97 /
Pupil
Conj unctiva———}|_
Cornea *»
x
4 Optic
nerve
Iris
Lens muscles
__ Suspensory
ligament of lens
Choroid coat
Sclerotic coat
Muscle to eyeball
Fic. 150. — The Vertebrate eye (human). Vertical section of the eye and
associated structures.
from that of another. In order that not merely degrees of the
intensity of light may be perceived, but that objects may be seen,
many of the higher Invertebrates have developed various kinds
of complicated apparatus for bringing the rays from a given point
to a focus at one point on the retina. These culminate, on the one
hand, in the compound eye of the Arthropods; and on the other,
in the ‘camera’ eye of certain Molluscs such as the Squid. In the
latter case the mechanism is very much like that found in the
Vertebrates, but since it occurs in Molluscs which cannot be con-
sidered in the direct evolutionary line of the Vertebrates, it
216 ANIMAL BIOLOGY
affords an example of similar responses of different organisms to
similar needs giving rise to analogous structures. (Figs. 50, 150.)
The wall of the Vertebrate eye, or EYEBALL, forms a more or less
hollow sphere which can be rotated by several relatively large
muscles. The anterior exposed part of the eyeball consists of two
transparent layers: the delicate CONJUNCTIVA, continuous with the
inner lining of the eyelid, and the rigid CORNEA beneath. The sides
Pigmented ~"|~>.} * A ee Ge ae
epithelium [2022S eGuhset Lae
Nervous
layer
Fibers to
optic nerve
Fic. 151. — Diagram of a vertical section of the Vertebrate retina (human).
The pigmented epithelium is the retinal layer farthest from the vitreous
chamber, and in contact with the choroid coat.
and posterior part are also composed of two layers, the outer
SCLEROTIC coat and the inner CHOROID coat. Suspended within
the eyeball is the LENS which divides the cavity into two chief
parts; the one posterior to the lens, known as the vITREOUS
CHAMBER, being lined by the retina whose nerve cells supply the
nerve fibers forming the opTic, or second cranial nerve.
The Vertebrate eye is an optical apparatus that may be com-
pared roughly with a camera, but this conspicuous difference
COORDINATION 217
must be noted. A camera is focussed by altering the distance
between the lens and the film, whereas the eye is focussed by
changing the curvature of the lens. Thus light waves passing
through the conjunctiva, cornea, and an opening (PUPIL) in a
regulating diaphragm (irIs) reach the lens and are focussed on
the retina. The sensory stimulation of the Rops and cones of the
retina thus brought about is transmitted by the optic nerve to
the brain. The brain itself interprets the nerve impulses and com-
poses — sees — the picture. How this is done, nobody knows.
(Fig. 151.)
A broad survey of the sense organs of Vertebrates from the low-
est to the highest impresses one with the fact that, taken by and
large, the improvements, though considerable, are not so marked
as one might expect when the great development of the nervous
system, particularly the brain, is considered. The brain increases
enormously in volume and complexity from Fish to Man. In many
Fishes it seems to be little more than a slight modification of the
anterior end of the spinal cord, while in the Frog the brain and
cord weigh about the same. But the human brain weighs about
three pounds, nearly fifty times as much as the cord, and com-
prises many billions of nerve cells. So it would seem that we must
look to the general influence of the sensory stimuli themselves for
the underlying factors in the development of the brain during its
long evolutionary history — the brain, in turn, being enabled to
make more out of the same stimuli and create in Man the higher
mental life with all that it implies. It is, indeed, an appalling
thought that all human mental states are represented by a few
thimblefuls of cells constituting the cerebral cortex.
CHAPTER XVII
ORIGIN OF LIFE
The mystery of life will always remain. Science is not the death,
but the birth of mystery, awe, and reverence. — Donnan.
A GENERAL background of biological facts and principles has
now been established, and we are therefore in a position to take up
from an advantageous viewpoint some of the broad questions
raised by the science. For its antiquity as well as its universal
interest, the problem of the origin of life takes precedence.
A. BIOGENESIS AND ABIOGENESIS
It must seem strange to the reader, with some of the complexities
of organisms before him, that the best minds up to the seventeenth
century saw nothing more remarkable in the spontaneous origin
of plants and animals of all kinds from mud and decaying
matter, than does the boy of to-day who believes that horse
hairs soaked in water are transformed into worms. As a mat-
ter of fact, we find that even Aristotle, who laid such broad
foundations for the science and philosophy of the organism,
believed that certain of the Vertebrates, such as eels, arose
spontaneously.
Similar ideas are voiced repeatedly through more than twenty
centuries by scientists and philosophers, poets and theologians.
Van Helmont, one of the founders of chemical physiology during
the seventeenth century, gives particularly specific directions for
the experimental production of scorpions and mice; while Kircher
actually figures animals that he states arose under his own eyes
through the influence of water on the stems of plants. The follow-
ing ironical reflections, aroused by Sir Thomas Browne’s doubts
in regard to mice arising by putrefaction, are quite typical of opin-
ion of the time: ““So we may doubt whether, in cheese and tim-
ber, worms are generated or if beetles and wasps in cow dung,
or if butterflies, shellfish, eels, and such life be procreated of pu-
trefied matter. To question this is to question reason, sense, and
experience. If he doubts this, let him go to Egypt, and there he
218
ORIGIN OF LIFE 219
will find the fields swarming with mice begot of the mud of the
Nile, to the great calamity of the inhabitants.”
Naturally, with the gradual increase in knowledge of the com-
plexity of organisms, the idea of ABIOGENESIS, Or SPONTANEOUS
GENERATION, was restricted more and more to the lower forms. It
remained, however, for Francisco Redi during the latter part of the
seventeenth century to question seriously the general proposition,
and to substitute direct experimentation for discussion and hear-
say. By the simple expedient — it seems simple to-day — of pro-
tecting decaying meat from contamination by flies, he demon-
strated that these insects are not developed from the flesh and
that the apparent transformation of meat into maggots is due
solely to the development of the eggs deposited thereon by flies.
One may imagine that the practical man of affairs scoffed at
Redi toiling under the Italian sun with meat and maggots to satisfy
a scientific curiosity, and little dreamed that the practical results
which germinated from this ‘folly’ would be among the most
important factors in twentieth-century civilization. Indeed, it is
difficult to overestimate the importance of Redi’s conclusion from
either the theoretical or practical viewpoint, for with it was def-
initely formulated the theory that matter does not assume the
living state, at the present time at least, except from preéxisting
living matter.
The influence of this work gradually became evident in schol-
arly literature. One author during the next century states that
“spontaneous generation is a doctrine so generally exploded that
I shall not undertake to explode it. It is so evident that all animals,
yea and vegetables, too, owe their production to parent animals and
vegetables, that I have often admired the sloth and prejudices of
ancient philosophers in taking it upon trust.’’ Another writes
that he “would as soon say that rocks and woods engender stags
and elephants as affirm that a piece of cheese generates mites.”
But it is not to be supposed that the time-honored doctrine of
spontaneous generation actually had been so easily relegated to
the myths of the past, Redi’s work and these eighteenth-century
opinions to the contrary. Indeed, the history of the establishment
of BIOGENESIS — all life from life — extends down almost to the
present time, for no sooner had experiment apparently disposed of
spontaneous generation than it arose again with fresh vigor in a
slightly different form demanding further investigation.
220 ANIMAL BIOLOGY
The difficulties came from two chief sources. In the first place,
Redi himself had been baffled by the presence of parasites within
certain internal organs of higher animals, such as the brains of
sheep. How did they get there if not by spontaneous generation?
The answer to this had to await the working out of the marvel-
lously complex life histories of the parasitic worms and allied
forms which showed that they all arise from parents like them-
selves. In the second place, improvements in microscope lenses
contributed to the discovery of smaller and smaller living creatures.
Countless numbers and myriads of kinds of “animalcules” ap-
pearing almost overnight in decaying organic infusions aroused
widespread interest and amazement, and proved to be the chief
riddle. The plausible explanation seemed to be spontaneous gen-
eration. (Figs. 14, 18, 26, 251, 252.)
Among others, Needham studied the problem and believed that
he had demonstrated the spontaneous origin of minute organisms
in infusions that he had boiled and sealed in flasks. His results
attracted considerable attention because the famous French nat-
uralist, Buffon, found in them support for his theory that the bodies
of all organisms are composed of indestructible living units, which
upon the death of the individual are scattered in nature and later
brought together again to form the units of new generations.
Needham’s and similar results, however, were shown by Spal-
lanzani and others to be inconclusive because they were obtained
by insufficient sterilization and sealing of the flasks containing the
infusions. But at this point objections came from another source:
the chemists who had recently discovered the important part
played by free oxygen for life processes and for the putrefaction
and fermentation of organic substances. They argued that the
treatment to which Spallanzani and other experimenters subjected
the infusions might well have changed the organic matter, ex-
cluded oxygen, etc., so that it was impossible for life to be produced.
This objection was met by a long series of experiments by various
investigators during the first half of the last century, which showed
conclusively that thoroughly sterilized infusions never developed
living organisms even when air was admitted, provided the latter
had been rendered sterile by heat or by having all suspended
‘dust’ particles removed. Thus the chemists were answered —
the infusions possessed all the conditions necessary to support life
— but life did not arise. The biologist who contributed most to
ORIGIN OF LIFE 221
the establishment of this conclusion was Pasteur. His masterly
and comprehensive work was not only convincing and final, but
it also demonstrated the source of the life that so rapidly appeared
in infusions that are exposed to the air. It is the air. A consider-
able part of so-called dust is made up of
microorganisms in a dormant state ready
to resume active life when moisture and
other suitable conditions are encountered.
Furthermore, these organisms are not the
result, but the cause of
decay — their own ac-
tivities bring about
chemical changes: putre-
fac tEO D, fermentation, Fic. 152. — Flask used by Pasteur in his ex-
and, in the bodies of periments on spontaneous generation. After
higher organisms, dis- the contents were poured in, the top of the
a Aaadithas 4s anol flask was sealed. The opening at the end was
oe: Ie te Stay. sealed against the entrance of germs, but not
attested by the methods oxygen, etc., by the water of condensation
now universally used in that accumulated at the bend. Life did not
develop in the flask.
food preservation and
aseptic surgery—to mention but two instances. (Figs. 152,
155/-218).
But, though highly improbable, of course it is not impossible
that simple life is even at the present time arising spontaneously
under special environmental conditions, perhaps in the ocean
depths, though unable to come to fruition in competition with
existing highly specialized protoplasm of ancient pedigree. In-
deed, if such living matter is arising, it must be very simple com-
pared with protoplasm as we know it to-day; so simple, in fact,
that we would not recognize it as such, because the protoplasm
of even the simplest organisms has had a long evolutionary history.
So we may consider it firmly established that, so far as human
observation and experiment go, no form of life arises to-day ex-
cept from preéxisting life by reproduction. The evidence from in-
numerable sources converges overwhelmingly to the support of
biogenesis. There is no evidence in support of abiogenesis.
B. Oricin oF LIFE ON THE EARTH
If we accept the testimony of astronomer and geologist, the
Earth was at one time in a condition in which life could not exist,
222 ANIMAL BIOLOGY
and so we are face to face with the problem of how it came to be
established on the Earth in the past — the remote past, since the
geological record affords convincing proof that life has existed
continuously on the Earth for some hundreds of millions of years.
Accordingly, unless one is willing to ascribe life’s origin to
SPECIAL CREATION — which at once removes it from the sphere of
(yp
———
Fic. 153. — Apparatus used by Tyndall in his experiments on spontaneous
generation. The front and side windows (w, w) of the cabinet are made of
glass. Air can enter through the two tubes (a, b). The optical test for the purity
of the atmosphere within the cabinet was made by passing a powerful beam
of light from the lamp (/) through the side windows. When the atmosphere
contained no suspended ‘dust’ particles, the tubes (c) within were filled,
through the pipet (p) with sterile culture medium suitable for the growth of
germs, but none developed. (After Tyndall.)
science and so beyond the present discussion — we have the fol-
lowing alternatives: either life came to this planet from some other
part of the universe; or it arose spontaneously from non-living
matter at one period at least in the past as a natural result of the
evolution of the Earth and its elements. With these in mind, a
review may be made of several modern theories of the origin of
life. It is nearly, if not quite, as important to define our area of
ignorance as to extend our area of knowledge.
ORIGIN OF LIFE 223
1. Cosmozoa Theory
The establishment of biogenesis and the dawning realization
of the unique complexity of the structure of matter in the living
state have led several scientists to suggest that life has never
arisen de novo on the Earth but has been carried hither from else-
where in the universe — the so-called cosmozoa theory.
On the assumption that some of the heavenly bodies have al-
ways been the abode of life, and from the fact that small solid
particles, which presumably have been a part of such bodies, are
moving everywhere in space, these particles have been pictured
as the vehicles which disseminate the simplest forms of life through
interstellar space to find lodgment and development upon such
planets as afford a suitable environment. Clearly, from this point
of view, life may be as old as the universe itself.
The plausibility of the cosmozoa theory depends on two assump-
tions: that life exists elsewhere in the universe, and that life can
be maintained during the interstellar voyage. Neither assumption
has, of course, any foundation of established fact whatsoever,
though the second offers at least something tangible for discussion.
As we know, many of the Bacteria and Protozoa, especially
when under the influence of unfavorable surroundings, have the
power of developing protective coverings about themselves and
of assuming a dormant condition in which all the metabolic ac-
tivities are reduced to the lowest ebb. In this spore or encysted
state they can endure extremes of temperature and dryness which
would quickly prove fatal during active life. For instance, it
seems clear that the spores of certain kinds of Bacteria can survive
a temperature approaching that below which no chemical reactions
are known to occur, and others can endure as high as 140° C. for a
short time. The cysts of some relatively highly specialized Pro-
tozoa can retain their vitality for at least half a century, while the
seeds of some plants have been found to retain the power of germi-
nation for nearly a century; though statements that grain from
ancient Egyptian tombs still maintains its power of growth has
been positively disproved. (Fig. 200.)
But the hardships to which living matter would be exposed
when started on its interstellar journey are not to be minimized.
Meteors in their fall through the Earth’s atmosphere become
heated to incandescence and, if they are the vehicle of transfer,
224 ANIMAL BIOLOGY
it would only be conceivable for life to survive far below their
surface where the temperature is lower. To avoid this and other
difficulties it has been suggested that the radiant pressure of light
is sufficient to overcome the attraction of gravitation for particles
of the extraordinary minuteness of some of the lower forms of life,
and that isolated germs might make the journey to the Earth.
But on the assumption that an organism were forced out into
space by the mechanical pressure of light waves from the sun of
the nearest solar system, it would require many thousand years
for it to reach the Earth. However, it has been suggested that,
owing to the exceedingly low temperature and absence of water
vapor which must prevail in cosmic space, there is no reason why
spores should lose appreciably more of their germinating power in
ten thousand years than in six months.
Without further discussion, it is apparent that the cosmozoa
theory is one which cannot be proved or disproved. It removes the
origin of life to a “conveniently inaccessible corner of the universe
where its solution is impossible.” Although at first thought it
seems almost absurd, with its strictly scientific formulation by
recent physicists and biologists, and especially in view of our in-
creasing knowledge of the powers of life in the latent state, we are
justified, perhaps, in seriously wondering whether after all life
has ever arisen, whether it may not be as old as matter, and whether
its germs, passing from one world to another, may not have de-
veloped where they found favorable soil.
But the majority of biologists undoubtedly would agree that,
“knowing what we know, and believing what we believe, as to the
part played by evolution in the development of terrestrial matter,
we are, without denying the possibility of the existence of life in
other parts of the universe, justified in regarding cosmic theories
as inherently improbable.’ Accordingly we may turn to theories
which attempt to picture the evolutionary origin of life from the
inorganic upon the Earth.
2. Pfluger’s Theory
Assuming that the Earth was at one time in an incandescent
condition, Pfliiger imagines that there arose from this superheated
mass a combination of carbon and nitrogen to form the radical
CYANOGEN, CN. This union involves the taking up of a large
amount of energy in the form of heat, and therefore cyanogen
ORIGIN OF LIFE 220
contributes energy to organic compounds and, in particular, to
proteins of which he believes it to be the unique life-principle.
Accordingly the problem of the origin of life is essentially the
origin of cyanogen, and since cyanogen and its compounds arise
only at exceedingly high temperatures, Pfliiger holds that life is
essentially derived from fire.
Thus, if Pfliiger’s hypothesis is valid, once the cyanogen has
energized organic compounds they are on their way to proteins
and protoplasm, and finally to the evolution of the highly spe-
cialized protoplasm of organic life to-day.
3. Moore’s Theory
Moore essays to picture with rather bold strokes the origin of
life from the inorganic elements of the cooling Earth, by a continua-
tion of the process of complexification which he sees inherent in
the nature of matter. This he expresses in a general way as a “law
universal in its application to all matter, and holding throughout
all space as generally as the law of gravitation —a law which
might be called the LAW OF COMPLEXITY — that matter, so far as
its energy environment will permit, tends to assume more and
more complex forms in labile equilibrium. Atoms, molecules,
colloids, and living organisms arise as a result of the operations
of this law, and in the higher regions of complexity it induces
organic evolution and all the many thousands of living forms.”’
In this manner he conceives that the chasm between non-living
and living things can be bridged over, and that life arose as an
orderly development, which comes to every earth in the universe
in the maturity of creation when the conditions arrive within
suitable limits.
4, Allen’s Theory
Allen maintains that it is simplest to believe that life arose when
the physical conditions of the Earth came to be nearly what they
are at present, and therefore does not attempt to trace it to actual
Earth beginnings. If life formerly existed actively outside of the
range of the freezing and boiling points of water, it must, he says,
have been quite different from life as we know it. Allen imagines
some such reactions as the following to have occurred: solar energy,
acting on the water or damp earth containing the raw materials,
caused dissociation and rearrangement of the atoms; the nitrogen
226 ANIMAL BIOLOGY
abstracting oxygen from its compounds with carbon, hydrogen,
sulfur, and other elements and delivering it to the atmosphere.
Not much energy would be absorbed by a transparent liquid;
but such reactions would occur particularly in water containing
compounds of iron in solution or suspension since these compounds
would absorb the solar energy. In this way compounds of nitro-
gen, carbon, etc., accumulated in the water or damp earth; and
further reactions, anabolic and katabolic, occurred among them
by virtue of the lability of the nitrogen compounds. Life at this
stage was of the humblest kind since there were no definite organ-
isms, only diffuse substances trading in energy, and between this
stage and the evolution of cellular organisms an immense period
elapsed.
5. Troland’s Theory
With the increasing realization of the importance of enzymes in
the economy of organisms, it is not strange that in these chemical
bodies has been sought the key to life’s origin, and accordingly we
find Troland stating that life is something which has been built up
about the enzyme. This author assumes that, at some moment in
Earth history, a small amount of a certain autocatalytic enzyme
suddenly appeared at a definite point within the yet warm ocean
waters which contained in solution various substances reacting very
slowly to produce an oily liquid which did not mix with water. If,
when this occurred, the enzyme became related to the reaction in
such a way as to greatly increase its rate, Troland believes it is ob-
vious that the enzyme would become enveloped in the oily material
resulting from the reaction, and the little oil drop would increase
until it was split into smaller globules, provided the original sub-
stances which combined were soluble in oil as well as in water. Thus
arose, according to Troland, the first and simplest life-substance,
possessing the power of indefinitely continued growth.
6. Osborn’s Theory
Starting with the assumption that the primal earth, air, and
water contained all the chemical elements and three of the more
simple but important chemical compounds — water, nitrates,
and carbon dioxide, Osborn suggests that an initial step in the
origin of life was the bringing of these elements into combined ac-
tion. This took place when the Earth’s surface and waters had
ORIGIN OF LIFE 227
temperatures between 6° and 89° C. and before the atmospheric
vapors admitted a regular supply of sunlight. The earliest func-.
tion of living matter, he thinks, was to capture and transform the
electric energy of the chemical elements characteristic of proto-
plasm, and this power probably developed only in the presence of
heat energy derived from the Earth or the Sun.
An early step then, in the organization of living matter, was the
assemblage of several of the ‘life-elements,’ and next their group-
ing in a state of colloidal suspension — “they were gradually
bound by a new form of mutual attraction, whereby the actions
and reactions of a group of life elements established a new form
of unity in the cosmos, an organic unity, or organism, quite dis-
tinct from the larger and smaller aggregations of inorganic matter
previously held or brought together by the forces of gravity.”
7. Huzley’s Statement
Such is an outline of some of the foremost attempts of scientists
to conceive the origin of the living state of matter from the ele-
ments of the Earth. It will be noted that all, except the cosmozoa
theory, have one assumption in common — the ‘chance’ assem-
blage of the various elements of protoplasm: an assumption re-
garded by some as not unreasonable when the stupendous dura-
tion of time and the almost infinite variations in conditions that
were at the disposal of nature are appreciated. According to the
statistical theory of probability, if we wait long enough, anything
that is possible, no matter how improbable, will happen. Ob-
viously this leaves out of the picture the marvellous ‘order of
nature’ which many modern biologists and physicists insist cannot
be thought of as emerging from the fortuitous. But the statements
of the respective theories necessarily have been presented so briefly
here as hardly to be fair to their authors. However, our purpose is
attained if they provide an instructive illustration not only of the
evolutionary trend which biological thought follows in this prob-
lem, but also of the divergent results reached by the scientific
imagination when it has few or no facts to guide it.
So we may more profitably turn to a consideration of the present-
day manifestations of life, and dismiss the insolvable problem of
the origin of life on the Earth with the conservative statement
penned more than half a century ago by Huxley: “Looking back
through the prodigious vista of the past, I find no record of the
228 ANIMAL BIOLOGY
commencement of life, and therefore I am devoid of any means of
forming a definite conclusion as to the conditions of its appearance.
Belief, in the scientific sense of the word, is a serious matter, and
needs strong foundations. To say, therefore, in the admitted ab-
sence of evidence, that I have any belief as to the mode in which
existing forms of life have originated, would be using words in a
wrong sense. But expectation is permissible where belief is not;
and if it were given to me to look beyond the abyss of geologically
recorded time to the still more remote period when the Earth was
passing through physical and chemical conditions, which it can
no more see again than a man can recall his infancy, I should ex-
pect to be a witness of the evolution of living protoplasm from not
living matter. ... That is the expectation to which analogical
reasoning leads me; but I beg you once more to recollect that I
have no right to call my opinion anything but an act of philo-
sophical faith.” (Fig. 298.)
CHAPTER XVIII
THE CONTINUITY OF LIFE
Owing to the imperfection of language the offspring is termed a new
animal, but is in truth a branch or elongation of the parent.
— Erasmus Darwin.
SINCE so far as is known all life now arises from preéxisting life
and has done so since matter first assumed the living state, it
apparently follows that the stream of life is continuous from the
remote geological past to the present and that all organisms of
to-day have an ancient pedigree. It is to the establishment of this
as the reasonable conclusion from the data accumulated during
recent years, that from now on our attention is somewhat more
particularly directed; and accordingly it is necessary first of all to
consider in some detail the relation of parent to offspring in present-
day forms as exhibited by reproduction.
A. REPRODUCTION
The power of producing new individuals specifically similar to
the parent is, as has been seen, one of the most important character-
istics of living in contrast with lifeless matter. Furthermore, re-
production is typically cell division. This is quite evident in uni-
cellular plants and animals, but by no means so obvious in higher
organisms where, as we know, special gonads and highly complex
accessory organs are developed in furtherance of reproduction.
It will be recalled that in Paramecium, for example, the nucleus
and cytoplasm divide into two parts, so that by cell division, here
called BINARY FISSION, the identity of the parent organism is
merged into the two new cells. Simple as this seems, the fission
of Paramecium actually involves considerably more than the halvy-
ing of the original cell, because, as a matter of fact, each half
must reorganize into a complete new individual with all parts
characteristic of the parent. (Figs. 8, 28.)
Among some unicellular animals (e.g., the Sporozoa) the parent
cell, instead of merely forming two cells by binary fission, becomes
resolved into many cells by a series of practically simultaneous
229
230 ANIMAL BIOLOGY
divisions known aS MULTIPLE FISSION, Or SPORULATION. This is
usually preceded by a considerable growth of the parent cell and
its enclosure in a protective covering, or cyst, which ruptures to
liberate the spores. Other unicellular forms, such as the Yeasts —
colorless plants chiefly responsible for alcoholic fermentation —
exhibit a modified form of fission in which the parent cell forms
one or several outgrowths, or BUDS, which gradually assume the
characteristic adult form and sooner or later become detached as
complete similar individuals. (Figs. 17, 25, 200, 223.)
In a considerable number of instances, however, the cells arising
by multiple fission or budding remain closely associated or organi-
cally connected so that they form a coLtony. In some colonial
organisms the component cells are all alike and each retains its
A. Paramecium.
CELL DIvIsIon TEMPORARY Ceti Divis1on CELL DIvIsIon
(Binary fission) CoNnJUGATION (Period of re- (Binary fission)
An indefinite number (Fertilization) construction) An indefinite
of generations. Each cell fer- Each fertilized number of gen-
tilizes the other. cell gives rise erations,
to typical etc.
animals.
B. Volvox.
on @)
Crit Division CELL Division CELL Division PERMANENT CELL Drvision
(Colony for- (Asexual re- (Gamete for- Consuea- (Colony forma-
mation) production) mation) TION tion)
Zygote (z)de- Germ cells (g.c.) Certain germ (Fertilization) Zygote develops
velops into a_ give rise to new cells produce One sperm into a colony,
colony. colonies. eggs (e), others fuses with one etc.
sperm (sp.). egg, forming
a zygote (z.).
THE CONTINUITY OF LIFE
231
G. Hydra.
ty omen
—————— cee
CELL DIvISION BupDDING Ceuu Division PERMANENT CELL DIVISION
(Embryological (Asexual re- (Gamete for- ConsuGA- (Embryologica]
development) production) mation) TION development)
Zygote (z) Part of animal Certain germ _ (Fertilization) Zygote (z)
produces ani- separates from cells produce one sperm produces ani-
mal contain- parent and eggs (e); others unites with mal, ete.
ing germ cells
(g.c.) and two
layers of spe-
cialized somatic
cells, the
ectoderm (ec.)
and endoderm
(en.).
leads separate produce sperm
; one egg, form-
existence. (sp.).
ing a zygote
(z).
D. Earthworm.
AL. ©
a 9
y
=<
z2
4
‘
‘
‘
‘
‘
UJ
(e}----»
4
We
oe
Sp.
OO nd
CELL DIVvIsION CELL DIVISION PERMANENT CELL DIVISION
(Embryological (Gamete forma- CoNnsJUGATION (Embryological
development) tion) (Fertilization) development)
Zygote (z) pro-
duces animal
containing germ
cells (g.c.) and
three layers of
specialized
somatic cells:
the ectoderm,
mesoderm, and
endoderm.
Certain germ
cells produce
eggs (e); others
produce sperm
(sp.).
One sperm unites
with one egg,
forming a zygote
(z).
Zygote (z) pro-
duces animal,
etc.
Fic. 154. — Diagrams to illustrate the general reproductive cell cycle in
A, a unicellular organism (Paramecium); B, a colony of cells (Volvor); C, a
simple Metazodn (Hydra); and D, a more complex Metazo6n (Earthworm).
(Modified, from Hegner.)
232 ANIMAL BIOLOGY
individuality, while in others certain cells are restricted more or
less in their functions, so that a physiological division of labor is
established which involves the shifting of individuality from the
cells to the colony as a whole. This specialization is exhibited
chiefly with regard to reproduction and reaches its highest expres-
sion among colonial Protozoa in VoLvox, where among ten thou-
sand or so cells, perhaps a score are specialized for reproduction
and the rest are somatic. Usually each of the reproductive cells
(germ cells) divides to form a group which is set free as a mini-
ature colony; but in certain cases some of the reproductive cells
become transformed into male and others into female gametes.
After fertilization of the eggs, usually by sperm from another
colony, the zygotes develop into new colonies which eventually
are liberated from the parent colony. (Figs. 29, 30.)
As has been previously suggested, the physiological division of
labor in the colonial Protozoa, involving, as it does, a segregation
of reproductive from somatic structures, affords a logical transi-
tion from the unicellular condition to that characteristic of the
multicellular forms. These, to all intents and purposes, may be
considered highly complex colonies of cells in which specialization,
no longer confined merely to demarking germinal and somatic
regions, has transformed the latter into a complex of tissues and
organs, the body (soma) of the individual, while the germinal tis-
sue (GERM) is confined to the essential reproductive organs.
It is customary, therefore, to draw a more or less sharp distinc-
tion between the soma and germ — to consider the soma the
individual which harbors, as it were, the germ destined to continue
the race. This theory of GERMINAL CONTINUITY, which is chiefly
associated with the name of Weismann, recognizes that the germ
contains living material which has come down in unbroken con-
tinuity ever since the origin of life and which is destined to persist
in some form as long as life itself. On the other hand, the soma may
be said to arise anew in each generation as a derivative or offshoot
of the germ; and, after playing its part for a while as the vehicle
of the germ, to pass the germ on at reproduction, and then die.
The germinal continuity concept has altered the attitude of biolo-
gists toward certain fundamental questions in heredity and evolu-
tion, as will be apparent when these subjects are considered.
(Figs. 154, 162, 180, 305.)
Though Volvox and other colonial forms afford a glimpse of the
THE CONTINUITY OF LIFE 233
conditions which probably prevailed when the evolutionary bridge
from unicellular to multicellular organisms was crossed, the varied
methods of reproduction of the lat-
ter by no means indicate the early
establishment of a hard and fast
boundary between soma and germ. J
Many of the Invertebrates, such as
—
Hydra and various types of worms, NN -
reproduce not only sexually by eggs er
and sperm, but also by strictly
ASEXUAL processes which are known
as FISSION and BUDDING. These -
processes are comparable merely in, 155, — Hoda, reneedue
a superficial way with the similarly K celitz.)
named methods in the Protozoa. In
some forms the whole complex body divides into two or more parts,
each of which reforms — REGENERATES — what was lost and so
becomes a complete though a smaller individual. In other species,
Fic. 156.
A Flatworm,
Planaria, in
the process of Fig. 157. — Reproduction of a Flatworm, Lineus socialis,
fission.(From by fission. A, mature worm; B, same, in nine parts; C, regen-
Child.) eration of each resulting in normal smaller worms. (From Coe.)
as well as in Hydra itself, buds arise as outgrowths from the body
and assume the form of the parent either before or after becoming
detached. (Figs. 155-157.)
234. ANIMAL BIOLOGY
In many of the nearest allies of Hydra, it will be recalled that
the buds remain permanently attached so that eventually a large
colony of organically connected polyps is developed. Moreover,
this condition leads to a physiological division of labor between
the various polyps which may become more or less changed in
structure so that, for instance, feeding, protective, and repro-
ductive individuals are established, and thereby the Hydroid
colony exhibits polymorphism. Our present interest is confined
to the reproductive polyps, which in many of the Hydroids are so
modified that they are dependent upon the colony as a whole for
all the necessities of life and are merely bodies which form asexually
by budding other individuals known as MEDUSAE. The medusae
liberate their sexual products in the water where fertilization
occurs, and the zygote gives rise to a free-swimming embryo which
soon becomes attached to some submerged object and develops
into a hydroid colony. (Figs. 37, 57.)
Thus the common Hydroids, such as Obelia, exhibit two dis-
tinct phases, or generations, in their life history — the fixed, poly-
morphic colony of polyps which is produced sexually but is itself
asexual; and the free-swimming medusae which are produced
asexually but are themselves sexual. The asexual and sexual gen-
erations alternate with each other in regular sequence, so that an
ALTERNATION OF GENERATIONS occurs.
Alternation of asexual and sexual methods of reproduction,
attended by more or less difference in structure of the individuals
of the generations, is fairly widespread among the Invertebrate
groups, particularly in forms which have adopted a parasitic mode
of life as, for example, the Liver Fluke. Frequently the life his-
tories are exceedingly complicated: several asexual, sexual, and
parthenogenetic generations succeeding one another in response
to the special conditions imposed by adaptation to a life within
another animal or series of animals. (Fig. 251.)
It is clear from such life histories that the conception of special
germ cells early set aside, as it were, from the somatic cells must
not be taken too literally. The same point is emphasized by the
power exhibited by plants and animals in restoring parts lost by
mutilations of one kind or another. Among many plants, pieces
of the root, stem, or, in special cases, of the leaf may give rise to
individuals complete in every respect. Until the middle of the
eighteenth century this was considered a property peculiar to
THE CONTINUITY OF LIFE 235
plants, and soon after Hydra was discovered experiments were
made to determine whether the organism was a plant or an animal.
Specimens were cut into several pieces and it was found that each
piece developed into a complete Hydra. This result, from the ideas
of the time, should have led to the conclusion that Hydra is a plant,
but additional characteristics were observed which outweighed all
other considerations. Accordingly Hydra was recognized as an ani-
mal with the power of replacing lost parts. (Fig. 158.)
Since the classic work on Hydra the power of regeneration has
come to be recognized as a fundamental property of all animals.
It is exhibited to the greatest degree among the lower animals,
while in the higher Vertebrates it is confined chiefly to the replace-
Changes in a
piece of normal Hydra Later changes in
the same piece
Fic. 158. — Regeneration in Hydra.
ment of cells which especially suffer from wear and tear, such as
those forming the outer layers of the skin. Regeneration is one
phase of a fundamental property of protoplasm, namely growth,
whether it consists in restoring a part of a Paramecium, trans-
forming a bit of a Flatworm into a complete animal, or replacing
half of an Earthworm, the head of a Snail, the claw of a Crayfish,
or the leg of a Salamander. But, it will be recognized that associ-
ated with growth there are complex processes of simplification
(dedifferentiation) of tissues and organs, and later a rebuilding
(differentiation) in order that a part may become again a normally
organized whole. Witness certain marine Flatworms that can be
cut down to less than one two-hundred-thousandth of their origi-
nal size and still become miniature worms like the original. —
The experimental study of regeneration phenomena has opened
236 ANIMAL BIOLOGY
up a new vista of the regulatory powers of living things from
Protozoon to Vertebrate and from egg to adult, and has afforded a
means of approach to some fundamental biological problems. And
withal it has a practical value. The surgeon now knows more about
the regeneration of tissues in general and of nerves in particular in
wound healing, and the oysterman knows — or should know —
that his attempt to destroy Starfish by tearing them up and throw-
ing the pieces overboard may serve merely to increase this enemy
of the Oyster. (Figs. 46, 159-161.)
Fic. 159. — Regeneration in a Flatworm, Lineus socialis. A, normal worm;
B, B’, section cut from body and its appearance after twelve days; C, same
after thirty days, cut in planes 3-3 and 4-4. Successive cuttings and regenera-
tions indicated by arrows. M, M’, similar experiments and results on posterior
end of body. (From Coe.)
The power of fragments of distinctively somatic tissue, as in
many lower animals and plants, to form a complete organism
including the reproductive organs and germ cells, indicates that
we must postulate at least a potential supply of the germ residing
in the somatic tissue, which can make good the definitive germ
cells when they are lost. At first glance this may seem to be a far
cry to save an idea, but it is a fact that there is a continuity of the
nuclear complex (GERM PLASM) whether the germ cells are set aside
early in individual development, or later by the transformation
of what seem to be typical somatic cells. That this is really the
crux of the question will be appreciated after the details of cell
clivision have been discussed.
THE CONTINUITY OF LIFE 237
A B
Fig. 160. — Regeneration and gra‘ting in the Earthworm. A, regeneration
of removed anterior segments by the posterior piece. B, regeneration of pos-
terior segments by the posterior part, so that the worm has a ‘tail’ at either
end. C, regeneration of removed posterior end by the anterior piece. D, three
pieces grafted together to make a long worm; E, two pieces grafted to form a
worm with two ‘tails’; F, short anterior and posterior pieces grafted together.
Regenerated portions are dotted. (From Morgan.)
Fic. 161. — Regeneration of a Flatworm, Planaria maculata. A, normal
worm; cut across at line indicated. B, B’, and C, C’, regeneration of anterior
and posterior parts of A to form complete worms. D, piece cut from a worm;
D1, D?, D*, D4, successive stages in the regeneration of D. E, ‘head’ from
which rest of animal has been cut off. E!, E?, E*, successive stages in the re-
generation by E of a complete body. F, similar experiment to E, but a new
‘head’ in reversed position is regenerated instead of a body, F!. (From
Morgan.)
238 ANIMAL BIOLOGY
B. OrIGIN OF THE GERM CELLS
Among the Vertebrates, as previously described, the germ cells
reside during adult life in definite organs, the ovaries and testes, and
upon these cells the power of reproduction of the individual is
solely dependent. It seems clear, however, that the primary germ
cells do not arise as such by division in the tissues which during
Gametes \@ 2
Prim, Sez Cells
Fic. 162. — Diagram of the lineage of the body cells and germ cells in a
Worm or Mollusc. Lineage of germ cells shown in black, of ectoderm in white,
and of endoderm and mesoderm in shaded circles. (From Conklin.)
development form the ovaries and testes. Just when the germ cells
are set aside in Vertebrates is uncertain, but it would seem to occur
very early in embryonic life, perhaps during the cleavage of the
egg. Then by shiftings of the tissues during growth, and possibly
also by amoeboid movements of the germ cells themselves, they
finally reach definite positions in the epithelium lining the dorsal
wall of the coelom, which becomes an integral part of the gonads
as development proceeds. (Fig. 162.)
With regard to the fate of the PRIMORDIAL GERM CELLS, once they
THE CONTINULTY OF: EEE 239
have reached testis or ovary, we are on surer ground and can trace
with considerable exactness their divisions and transformations
which give rise to the gametes: sperm and eggs. In the first place
the primordial germ cells proceed to divide in the testis and ovary so
that they produce a large number of germ cells known as SPERMA-
TOGONIA and OOGONIA respectively. (Fig. 165, A, B.)
Thus, for instance, the ovary of an adult female Frog shows
oogonia in various stages of development. The eggs of the next
breeding season are the largest cells; those of intermediate size
Early stage in development of egg
Second stage
Third stage eee
Blood vessel
)
Wall of eae
{
Fourth stage
Epithelium lining
the coelom
Fifth stage
Fic. 163. — Diagram of a section through a lobe of the ovary of a Frog,
showing several stages in egg development.
represent approximately those of the following year; and many
smaller cells are the odgonia from which the eggs of later years
will arise. Furthermore there are other cells that form a matrix
of supporting and nutritive tissue (FOLLICLE CELLS, etc.) in which
the germ cells are embedded. The Mammalian ovary presents a
similar picture, except that since fewer eggs are produced at each
breeding period, the supply of odgonia is less numerous. Never-
theless, even in the human ovary there are potentially many thou-
sands of eggs. The testis shows a similar condition, but since so
many more sperm than eggs are produced, the spermatogonia
divide much more actively. (Figs. 133, 163.)
1. Mitosts
Before taking up the origin of the gametes by division from the
spermatogonia and odgonia, it will be necessary to describe in
240 ANIMAL BIOLOGY
some detail the complicated internal process involved in all typical
cell divisions, known as MITOSIS, which was dismissed when con-
sidering the origin of cells until the reader would be in a position
to appreciate to the full its significance. (Fig. 10.)
Reduced to its simplest terms, a typical resting cell, that is one
which is not dividing, consists of a mass of CYTOPLASM surrounding
a NuCLEUs; the latter with its chromatin distributed so that it
presents a net-like appearance. In addition to the nucleus, it will
be recalled that there is present another important cell organ, the
CENTROSOME, which appears like a tiny body enclosing a granule
and is situated in the cytoplasm near the nucleus. For practical
purposes we may consider the cytoplasm as the arena in which
mitosis takes place, the centrosome as the dynamic agent, and the
nucleus, or more specifically its CHROMATIN, as the essential element
which the complicated process is to distribute with exactness to
the daughter cells that are about to be formed. With this in mind
we may proceed to an outline of the chief stages of mitosis, though
perhaps it should be emphasized that variations in the details are
as numerous as the different types of cells, and that any general
account can do no more than present the fundamental plan of
operations.
Broadly speaking, mitosis can be divided into four chief stages:
PROPHASE, METAPHASE, ANAPHASE, and TELOPHASE, during each of
which characteristic changes take place in the nucleus, cytoplasm,
and centrosome. (Fig. 164.)
At the beginning of the prophase, or earlier, the centrosome
divides to form two, each of which becomes surrounded by what
appears to be a halo (asTER) of radiating fibers, the nature of
which is unknown, that are the visible expression of physico-chemi-
cal forces. The centrosomes and asters now proceed to move apart,
take up positions at opposite sides of the nucleus, and the astral
fibers between lengthen until they form a CENTRAL SPINDLE. While
these changes are going on, the nucleus is not inactive. The nuclear
membrane gradually disappears and the chromatin granules, orig-
inally presenting a net-like appearance, now become visibly re-
solved into a number of split threads of chromatin, termed CHRO-
MOSOMES, which by chromatin concentration gradually become
shorter and thicker and so distinctly individual. The number of
chromosomes varies greatly in different species, but is typically
an even number and the same for all the cells of a given species.
THE CONTINUITY OF LIFE 241
When the chromosomes have assumed definitive form, the pre-
liminary events which constitute the prophase of mitosis are
Fic. 164. — Typical stages of mitosis (somatic) in which the chromosome
number is assumed to be eight. A, prophase (start): chromatin still exhibiting
net-like appearance, centrosome divided and surrounded by aster; B, prophase
(early): chromosomes visible as long split threads of chromatin, centrosomes
moving apart and spindle arising between; C and D, prophases (later): nuclear
membrane disappearing, chromosomes shorter and thicker; E and F, metaphase
and anaphase (early): chromosomes arranged in equatorial plate and each
separating into two along the longitudinal split; G, anaphase (later): a set of
eight chromosomes approaching each aster; H, telophase: gradual loss of
visibility of distinct chromosomes, asters and spindle disappearing, division
of cytoplasm beginning; I, mitosis completed, two cells.
brought to a close by the chromosomes being drawn to the center of
the spindle. Here they are arranged in a plane at right angles to
the long axis of the central spindle, midway between the two cen-
trosomes, and form the EQUATORIAL PLATE.
242 ANIMAL BIOLOGY
And now the stage is set for what is apparently the climax of
mitosis, designated the metaphase. Each of the chromosomes sep~
arates into two parts along the line of the longitudinal split already
present, in such a manner that each of the thousands of chromatin
granules which make up a chromosome is equally divided. Two
sets of similar daughter chromosomes are thus formed.
With chromosomal division consummated, the metaphase
merges into the anaphase which is devoted to a shifting of a
daughter set of chromosomes along the fibers to either end of the
spindle. In this way each centrosome becomes associated with
one set of daughter chromosomes.
The last stage, or telophase, is one of nuclear reconstruction and
division of the cytoplasm. The chromosomes become indistinct
as they spin out to form the net-like appearance of the chromatin
in the nucleus of each daughter cell; a nuclear membrane arises;
and the nucleus again assumes the form of a definite spherical
body characteristic of the resting cell. It must be emphasized,
however, that although the chromosomes usually disappear from
view as definitive entities in the resting nucleus, nevertheless the
individuality of each persists and the same chromosomes emerge
from the nuclear complex at the next division period.
Simultaneously with these nuclear changes, and before the
spindle and asters — the machinery of mitosis — disappear, the
division of the cytoplasm is initiated as indicated by an indenta-
tion of the cell wall, encircling the cell at the equator. This be-
comes deeper as it gradually extends through the cytoplasm in the
same plane which the equatorial plate formerly occupied, until
the cytoplasm is cut into two separate masses, each containing
one of the daughter nuclei and centrosomes. Thus one cell has
merged its individuality into two daughter cells by mitotic divi-
sion. Cell division — reproduction — has occurred.
What is the main thought that we carry away from this brief
view of a phenomenon that has been going on for untold ages;
is going on in various cells of our own bodies this very instant?
Surely it seems that whereas the mitotic process apparently re-
sults in merely a mass division of the cytoplasm, the chromatin
material is rearranged and distributed in a manner which makes
it possible for each cell to receive a very definite share. Each
daughter cell receives the same number of chromosomes, although
in many cases there is a very great difference in the size of the
THE CONTINUITY OF LIFE 243
resulting cells. Indeed, exactness of chromatin distribution ap-
pears to be the primary object of mitosis.
The significance of the nicety of chromatin distribution lies
in the fact that not only are the various chromosomes qualitatively
different but also each chromosome is qualitatively different from
one end to the other, and these different parts of the chromosomes,
known as GENES, are the determiners of characters which are
handed on from cell to cell. And since cell division is reproduction,
the chromosomes are the chief agents in the transmission of char-
acters from parent to offspring in inheritance. We shall consider
this important fact when we discuss heredity, but now we must
return to the origin of the gametes.
2. Chromosomes of the Germ Cells
It is clear that the spermatogonia and odgonia in the reproduc-
tive organs, together with all the cells forming the body proper,
are direct descendants by mitotic cell division from the fertilized
egg which gave rise to the individual organism. This, we have
just seen, is equally true of the chromosomes and, therefore, every
cell of the animal body has the same number of chromosomes as
the fertilized egg. Furthermore, since fertilization always consists
in the fusion of two gametes — a fusion of nucleus with nucleus
and cytoplasm with cytoplasm to form a zygote — one of two
things must happen. Either the zygote, which is one cell recon-
structed from two, must have double the chromosome number,
that is, a set contributed by both egg and sperm; or some method
must exist by which the chromosomes of the gametes are reduced
in number to one-half that characteristic of the somatic cells. As
a matter of fact, a reduction of the number of chromosomes always
does take place in animals during the final stages in the develop-
ment, or MATURATION, of the gametes.
The maturation or ‘ripening’ of the germ cells of animals in-
volves two cell divisions by which each spermatogonium gives
rise to four sperm, and each odgonium to one functional egg and
three tiny, abortive cells known as poLocyTEs; each and all with
one-half the number of chromosomes of the somatic cells and of the
germ cells up to this point in their development. Consequently
these two divisions, termed MATURATION DIVISIONS, must be ex-
amined in some detail if we are to appreciate the nicety of the
process by which the chromosome number is reduced one-half
244° ANIMAL BIOLOGY
without impairing the chromatin heritage from cell to cell. We
shall describe first the origin of the sperm, or SPERMATOGENESIS,
and then proceed to the fundamentally similar origin of the egg,
or OOGENESIS. (Fig. 165.)
3. Spermatogenesis
A given SPERMATOGONIUM, with, let us say, eight chromosomes
characteristic of the species, proceeds to increase in size prepara-
tory to the first maturation mitosis, and is designated a PRIMARY
SPERMATOCYTE. At the close of the growth period, when this cell
is preparing to divide, the chromosomes are arranged in pairs by
a process termed SsyNApsis. The number of such pairs will obviously
be half that of the chromosome number. The SYNAPTIC PAIRS are
then distributed in the equator of the spindle exactly as the single
chromosomes are in ordinary mitosis. But in the early anaphase
the members of each pair are separated, one synaptic mate going
to each pole of the spindle. Thus each of the daughter cells —
SECONDARY SPERMATOCYTES — receives half the total number of
chromosomes that were present in the primary spermatocyte. It
will be noted that the essential difference between this type of
mitosis (REDUCTION DIVISION, or MEIOSIS) and that of typical
nuclear divisions lies in the separation of entire chromosomes
(SYNAPTIC MATES) instead of the splitting of each chromosome.
Reduction thus involves the SEGREGATION of synaptic mates in
separate cells.
Both the secondary spermatocytes now divide by typical mito-
sis, including chromosomal division, and so each of the resulting
cells (SPERMATIDS) receives half the somatic number of chromo-
somes. The spermatids are presently transformed into sperm, and
thus each spermatogonium with eight chromosomes (diploid group)
gives rise to four sperm with four chromosomes (haploid group)
apiece. (Fig. 165.)
It may be mentioned in passing that the chromosomal division
just described as taking place in the secondary spermatocytes
usually occurs precociously in the primary spermatocyte while
the chromosomes are in synapsis. Thus each synaptic pair is
resolved into a group known as a fetrad, the four components of
which are thereafter distributed by the two maturation divisions,
and accordingly either or both of these divisions may be involved
in segregation. However, for simplicity of exposition we may dis-
THE CONTINUITY OF LIFE 245
A
Primordial germ cells
(> SS
SSS Pp
3 AS Ue
ee one yest x
Gy VCO VF
q an Primary spermatocyte i)
and primary oocyte 8
D
Secondary spermatocytes 2
and secondary oocytes =y .
\ E
Spermatids and =
OOOG mx ( Qoeee
v 3
— — |
Sperm
Division of zygote
Fic. 165. — Diagram of the general plan of spermatogenesis and odgenesis
in animals. Tetrad formation is disregarded. The somatic, or diploid, number
of chromosomes is assumed to be eight. Male, to the left; female, to the right.
A, primordial germ cells; B, spermatogonia and odgonia, many of which arise
during the period of multiplication; C, primary spermatocyte and odcyte, after
the growth period, with chromosomes in synapsis; D, secondary spermatocytes
and odcytes; E, spermatids (which become transformed into sperm) and egg
and three polocytes, each with the haploid number of chromosomes; F’, union
of sperm and egg (fertilization) to form zygote with diploid number of chro-
mosomes; G, chromosome complex of cells after first division of the zygote, and
of all subsequent somatic cells, and germ cells until maturation.
246 ANIMAL BIOLOGY
regard tetrad formation since it in no wise alters the basic results
attained by spermatogenesis or odgenesis.
4. Odgenesis
The maturation of the egg, as already intimated, follows the
same plan as that of the sperm, and the reduction of the chromo-
somes is the same. Such modifications as occur are related to the
fact that the egg is usually a relatively large, passive cell stored
with nutritive materials for use during the developmental process,
while the sperm is among the smallest of cells — essentially a nu-
cleus surrounded with a delicate envelope of cytoplasm. Accord-
ingly it is only necessary to emphasize that the growth period of
egg formation, in which the oO0GoNIUM becomes transformed
into the PRIMARY OOCYTE, is characterized by a much greater
increase in size than is the case in the corresponding period in
spermatogenesis; and that the following two cell divisions (mat-
uration divisions) involving chromosome reduction result in very
unequal division of the cytoplasm. Thus one SECONDARY OOCYTE
is very large, while the other is a tiny cell termed the First POLO-
CYTE:
Both the large secondary odcyte and first polocyte now divide
again; the former giving rise to a large cell, the mature EGG, and
a tiny SECOND POLOCcYTE; while the first polocyte divides equally
to form two polocytes. In this way arise the four cells, compa-
rable to the four sperm in spermatogenesis, each with half the
somatic number of chromosomes. But only one of these, the egg,
functions as a gamete. The three polocytes, although possess-
ing a similar chromosome complex, are sacrificed in providing one
cell, the egg, with its special cytoplasmic equipment. The polo-
cytes get just enough cytoplasm to be regarded as cells, and soon
degenerate and disappear. (Figs. 165, 166.)
Such is the outline of the essentials of spermatogenesis and
oogenesis in animals; processes which involve at one stage a modi-
fication of ordinary mitosis to give each gamete half the somatic
number of chromosomes characteristic of the species. It is clear
that this is not merely a mass reduction of chromatin material,
but is a separation and segregation after synapsis of definite chro-
matin entities, the chromosomes, so that the gametes receive the
reduced number.
THE CONTINUITY OF LIFE 247
Nucleolus Nuclear membrane Oil droplet
Cytoplasm
Sperm Sperm
nucleus
Fic. 166. — Maturation and fertilization of the egg of the Sandworm,
Nereis. A, egg (o6cyte) before start of maturation; B, first polocyte spindle
forming, sperm just entering; C, first polocyte spindle established; D, first
polocyte formed, second polocyte spindle near; spindle with sperm nucleus;
E, second polocyte formed, union of egg and sperm nucleus; F, spindle for
first division of fertilized egg. Note that in Nereis, as in many other animals,
maturation of the egg is deferred until the time of fertilization. (From Wilson.)
248 ANIMAL BIOLOGY
AbcD
Aa BbCc Dd :
V VI Vil
Fic. 167. — Diagram of the chromosome cycle of an animal. Somatic (dip-
loid) chromosome number assumed to be eight. Paternal chromosomes (from
sperm)= A BCD; maternal (from egg) = a bcd. I, union of nuclei of gametes,
each with a simplex group (haploid number) of chromosomes, in the zygote
at fertilization to form a duplex group (diploid number) of chromosomes.
Il, III, IV, somatic divisions or divisions of germ cells before maturation
(duplex groups of chromosomes). V, synapsis, involving pairing of homol-
ogous paternal and maternal chromosomes to give the haploid number of
paired chromosomes. VI, reduction division — separation of pairs into single
chromosomes again. VII, two gametes, with simplex groups (haploid number)
of chromosomes; there are 14 more possible combinations of the chromosomes,
or types of gametes, which are not shown. See: Fig. 189. (After Wilson.)
THE CONTINUITY OF LIFE 249
Throughout the animal kingdom, wherever sexual reproduction
occurs, phenomena which can be interpreted as nuclear reduction
have been observed in the formation of gametes. In some of the
Protozoa this seems to be merely an extrusion of a certain amount
of chromatin, but since whenever chromosomes can be observed
and counted the process has been found to follow in principle
essentially the same lines described above, we have every reason
to believe that it is never a haphazard mass reduction, and that
the ripe gametes emerge with a definite chromatin heritage, rela-
tively simple as this may be in the lowest forms.
5. The Chromosome Cycle
We have now surveyed the germ cell cycle from the fertilized
egg through the germ plasm in the adult to the gametes again,
but before proceeding to consider the details of the fusion of egg
and sperm — the fertilization process —it may clarify matters
to glance back to the chromosome condition in the fertilized egg
at the beginning of the cycle that has just been considered.
Obviously this fertilized egg (zygote) contained two groups of
chromosomes, one of which belonged to the egg and therefore
may be termed MATERNAL, and one which was derived from the
sperm and thus is PATERNAL. When the zygote divided by mitosis
to form the body and germ, every cell received two groups of
chromosomes directly derived from these two original groups
in the zygote. It logically follows, and all observations indicate,
that each and every cell, both of the body and of the germinal
tissue, possesses two groups of chromosomes, one of maternal and
one of paternal origin — in other words, direct lineal descendants
of the combined set formed at fertilization.
So it happens that each body cell really has a double set (diploid
number) — two complete sets — of chromosomes, and the same
is true of the germ cells until maturation. Then at synapsis corre-
sponding (HOMOLOGOUS) maternal and paternal chromosomes pair
and, after the maturation divisions the gametes have a single set
(haploid number). (Fig. 167.)
Thus far we have emphasized chromosome reduction as the
main result of the complicated maturation phenomena. The
question now arises: Is this chromatin distributed so that all the
gametes receive the same heritage?
As already stated, the evidence indicates not only that chromo-
250 ANIMAL BIOLOGY
somes differ qualitatively one from another, but also that the var-
ious parts of each chromosome are qualitatively distinct. And
further that these qualitative differences are the physical basis of
inheritance — the determiners (genes) of characters which will
be realized in the individual or the race to which the cell containing
them contributes. Such being the case, the chromosomal complex
of each of the nuclei which arises after synapsis — the nuclei of
the gametes — depends on how the various chromosomes happen
to be distributed during the two maturation divisions. As a mat-
ter of fact, all the chromosomal combinations occur that are
mathematically possible with the available number of chromo-
somes in a given species, but with one limitation: every cell must
receive one member of each synaptic pair of chromosomes, so that
each and every gamete receives a complete haploid group of
chromosomes, but rarely the same groups (maternal and paternal)
which existed before maturation. For example, if the somatic
(diploid) number of chromosomes is eight, sixteen different types
of gametes are possible. In Man with 48 somatic chromosomes
and after synapsis 24 pairs of paternal and maternal chromosomes,
there are 2 **, or about seventeen million possible types of gametes
in each sex; and since these combine at random at fertilization,
the possible number of different types of zygotes from one parental
pair mounts far up in the trillions. No wonder the children of a
family differ — there is variation! (Fig. 193.)
In a way, therefore, fertilization is not consummated, so far as
its influence on the race is concerned, until the maturation of the
gametes in the new generation to which it has given rise. We must
defer until later the consideration of the significance of these facts
In BIPARENTAL inheritance, and merely emphasize again that the
continuity of life implies not only the continuity of cells but also
of their nuclear elements, the chromosomes — the genes.
CHAPTER XIX
FERTILIZATION
The entire organism may be compared to a web of which the warp
is derived from the female and the woof from the male. — Huzley.
Now that we are familiar with the method of gamete formation
and its contribution to the continuity of life, it is in order to con-
sider some important details of the structure of the gametes them-
selves, and the significance of the complex series of phenomena
that they initiate at fertilization. The biological importance of
fertilization and the part it plays in the life of the individual or-
ganism and the race has aroused the interest of philosophers and
scientists since the time of Aristotle, but it is only within the past
half-century that at least a partial answer has been forthcoming
from a critical analysis of the gametes and their product, the
zygote.
A. GAMETES
The gametes, while exhibiting in certain cases peculiar adapta-
tions to special conditions, are remarkably similar in general struc-
ture throughout the animal series. It is possible to arrange a
series of lower forms which shows various stages in sex differen-
tiation. Beginning with those in which both gametes are struc-
turally similar, we pass by slow gradations to others in which the
egg is a relatively large, passive, food-laden cell and the sperm a
minute, active, flagellated cell.
As a matter of fact, the egg is subject to somewhat more varia-
tion in size and general appearance than the sperm, for after
fertilization it must be adapted to meet the special conditions of
development peculiar to the species. Thus, for instance, the actual
size of the egg in animals is determined chiefly by whether the de-
veloping embryo is in the main dependent upon food stored in the
cytoplasm of the egg itself, or upon some outside source, such as
the sea water in which it floats, or the tissues of the parent. The
first case is well illustrated by a Bird’s egg in which the so-called
YOLK is the egg cell proper, hugely distended by stored food, and
251
2902 ANIMAL BIOLOGY
surrounded by nutritive and protective envelopes consisting of
the ‘white of the egg,’ shell membranes, and shell which are formed
by secretion from the walls of the oviduct during the passage of
the egg to the exterior. On the other hand, the eggs of Mammals,
for instance of the Rabbit and Man, are very small — the human
egg being less than 1/125th of an inch in diameter — since their
Granular ‘polar’ cytoplasm
First polocyte
Second polocyte
spindle
acuolated ectoplasm
Dense endoplasm
SP gee —_ Vacuolated
5s endoplasm
ad
2
oft age oe
a a
as
Inner membrane
Outer membrane
Fig. 163. — A, section through the egg of a primitive Vertebrate, the Lam-
prey. B, sperm of the same species, drawn to scale. (From Kellicott, after
Herfort.)
essentially parasitic method of development in the uterus renders
superfluous the storage of any considerable amount of food ma-
terial in the egg cytoplasm. (Figs. 7, 133, 168, 177.)
With the specialization of the egg along lines which render it
non-motile, it has devolved upon the sperm to assume the func-
tion of seeking out the egg for fertilization. It does this in most
cases by active lashing of its flagellum. This necessitates a fluid
medium in which the sperm can swim, and such is provided by the
environment in which the organism lives or, in the case of most
higher animals, where fertilization takes place within the oviduct,
by special fluids secreted for the purpose.
A question of much interest is how the actual meeting of the
FERTILIZATION 2953
gametes is brought about. In many cases it is undoubtedly merely
by chance: the random swimming of the sperm sooner or later
bringing one in contact with an egg. In other cases the move-
ments of the sperm seem to indicate a definite attraction by the
egg. Thus the sperm of some of the lower animals apparently
are attracted by substances eliminated by the egg at matura-
tion. In such instances there can be but little doubt that chemical
stimulation of the sperm by specific substances plays a part in
bringing the gametes together. This is an example of CHEMO-
TROPISM: a phenomenon of considerable importance, especially in
the behavior of free-living cells.
B. UNIoN oF GAMETES
Once a single sperm has come into functional contact with the
egg, it initiates a chain of events which constitutes fertilization.
Albumen Blastoderm
Shell memb.. x < SN
\e : \\\
Chalaza UR '
ee ELASL, Shell memb. 1
/ f /') Shell memb. 2
Albumen iY Air space
= Yolk
Fig. 169. — Diagram of the egg of the domestic Fowl, before incubation.
Although, as might be expected, the variations in details are legion,
they do not obscure the main facts. The first reaction on the part
of the egg is to prevent the entrance of other sperm and thereby
to insure a free field for the operations of the first arrival. Fre-
quently a jelly-like layer is formed about the egg, or if a mem-
brane is already present this may be rendered impermeable or still
another formed. In cases where the egg is surrounded originally
by a dense and resistant wall, the tiny opening provided for the
entrance of the sperm is closed. However, the accessory wrappings
about certain eggs, such as those of Birds, have no relation to the
present subject since they are secreted, not by the egg itself, but
254 ANIMAL BIOLOGY
by glands in the wall of the oviduct, some time after fertilization
has occurred, when the egg is passing down. (Fig. 169.)
The reactions of the egg cytoplasm that exclude accessory sperm
are overshadowed in importance by others which upset the stable
equilibrium of the egg and render its surface permeable, so that
extensive osmotic interchanges take place between the cytoplasm
of the egg and its surroundings. Most often this is visible merely
in a shrinkage of the cytoplasm due to loss of water, but sometimes
contractions, amoeboid movements, or flowing of special cyto-
plasmic materials to definite regions of the egg are visible. In
any event it is certain that profound changes occur in the cyto-
plasm — its organization as a gamete soon gives place to a reor-
ganization that establishes the general outlines of its subsequent
development as a new individual. (Fig. 177, A, B.)
1. Synkaryon
Turning now to the nuclei, known as male and female GAMETIC
NUCLEI, the union of which to form the single nucleus (SYNKARYON)
of the zygote is the climax of fertilization. Disregarding the flagel-
lum of the sperm, which disappears as it enters the egg, we find
that the sperm nucleus moves through a quite definite path toward
the center of the egg where it is met by the egg nucleus. Both the
gametic nuclei now become resolved into chromosomes which lie
free in the cytoplasm, while two centrosomes, each surrounded by
an aster, appear and take up positions on either side of the chromo-
somes to form a typical mitotic figure. The two sets of chromo-
somes form an equatorial plate at the center of the spindle, thus
establishing at once not only the mitotic apparatus for the first
division of the egg, but also the intimate association on equal terms
of chromosomes, with their potentialities from the two parents,
to form a common structure — the nuclear complex of the new
individual. (Figs. 166, 167, I, II.)
Such are the outstanding facts of fertilization which a host of
investigators have brought to light chiefly within the past sixty
years. It was not until 1839 that Schwann, with the establishment
of the cell theory, recognized the egg as a cell, and sixteen years
more before the sperm was similarly understood; while the first re-
alization that fertilization is an orderly fusion of two cells to form
one came during the seventies of the past century. Then it be-
came evident that in sexual reproduction each individual con-
FERTILIZATION 299
tributes to the formation of the offspring a single cell, in which
must be sought the solution of the problems of sex, fertilization,
development, and inheritance. However, the concentration of at-
tention on the cell has not simplified the solution of these funda-
mental problems; but rather it has contributed to an ever-
increasing appreciation of the complexities of cell phenomena and
the difficulties of formulating them in general terms. (Fig. 303.)
2. Significance of Fertilization
Quite naturally the original view was that fertilization funda-
mentally is reproduction — the mature egg pauses in develop-
ment and usually comes to naught unless a sperm enters. How-
ever, as we know, reproduction is cell division or the detachment
of a portion of a living organism to form another, whereas fertiliza-
tion is the union of two cells to form one cell. The erroneous idea
that fertilization is reproduction is due to the fact that in higher
organisms, if fertilization is to occur at all, it must take place at
the period in the life history when the individual is but a single
cell detached from the parent — that is, at reproduction. With
this point clear, we may briefly discuss the significance of ferti-
lization, first on the basis of evidence derived from the Protozoa.
Protozoa. The life histories of nearly all Protozoa that have
been carefully studied include a period in which fertilization occurs.
Under favorable environmental conditions, Paramecium, for in-
stance, reproduces by binary fission two or three times a day so that
in aremarkably short period the one cell is replaced by a host of de-
scendants. Sooner or later, however, the individuals exhibit a
tendency to unite temporarily in pairs, or CONJUGATE. During
conjugation complicated changes take place in the nuclei of the
cells, involving chromosome reduction and the formation of two
gametic nuclei in each individual of the pair of conjugants. Then
one of the gametic nuclei in each conjugant migrates over and
fuses with the stationary gametic nucleus of the other to form
a synkaryon, or fertilization nucleus, in each cell. After this the
two Paramecia separate, reconstruct their characteristic vegeia-
tive nuclear apparatus, and proceed to reproduce by division as
before. (Fig. 170.)
This is fertilization in Paramecium, and on the assumption
that the primary significance of synkaryon formation should be
most evident in unicellular forms, of which, of course, this animal
256 ANIMAL BIOLOGY
is an example, a large amount of experimental breeding has been
carried out on Paramecium and its allies. The earlier results seemed
to demonstrate conclusively that Paramecium can divide only a
limited number of times, say a couple of hundred, after which the
cells die from exhaustion or SENILE DEGENERATION unless fertiliza-
tion takes place. In other words, it was believed that periodic
REJUVENATION by fertilization is a necessity for the continuance
of the life of the race. And therefore, so the natural conclusion
ran, protoplasm is unable to grow indefinitely; there is an inherent
tendency for the destructive phases of metabolism to gain ascend-
ancy over the constructive, and fertilization serves to maintain or
restore the youthful condition and thus secure the continuance of
the race.
In this connection, the life history of Paramecium from one
period of fertilization to the next is often compared to the life of a
multicellular organism from its origin as the fertilized egg, through
youth and adult life to old age. The striking difference is that, in
the case of Paramecium, the products of division of an animal
which has conjugated (EXCONJUGANT) separate as so many inde-
pendent cells, all of which are alike and, in later generations,
capable of fertilization; while all the products of division of the
fertilized egg of multicellular forms remain together as a unit and
become differentiated for particular functions in the individual,
except a few, the germ cells, which retain the power of forming
new individuals. Pushing this comparison a little further, it is
stated that after fertilization in Paramecium we have the period of
greatest cell vigor, or youth, followed by maturity when the cells
are ripe for fertilization again, and in the absence of fertilization —
and only then — the onset of old age, and death. Thus death has
no normal place in the life history of Paramecium, for all the cells
at the period of maturity are capable of fertilization. On the other
hand, in multicellular forms only some of the cells, the germ cells,
retain this power — the somatic cells have paid the penalty of
specialization and must die. Thus death of the individual except
by accident does not occur among unicellular forms because ferti-
lization ‘rejuvenates’ the cell, and the cell and the individual are
one and the same. With the origin of multicellular forms, involvy-
ing the segregation of soma from germ, death became possible, and
was established — it is the ‘price paid for the body.’ (Figs. 154,
C.D arog.)
FERTILIZATION 2957
Fic. 170. — Diagram of the nuclear changes during fertilization (conjuga-
tion) in Paramecium aurelia. A, union of two individuals along the peristomal
region; B, degeneration of macronucleus and first division of the micronuclei;
C, second division of micronuclei; D, seven of the eight micronuclei in each
conjugant degenerate (indicated by circles) and disappear; E, each conjugant
with a single remaining micronucleus; F, this nucleus divides into a stationary
micronucleus and a migratory micronucleus — the gametic nuclei. The mi-
gratory micronuclei are exchanged by the conjugants and fuse with the re-
spective stationary micronuclei to form the synkarya. This is fertilization.
G, conjugants, with synkarya, separate (only one is followed from this point);
H, first division of synkaryon to form two micronuclei; I, second reconstruc-
tion division; J, transformation of two micronuclei into macronuclei; K, divi-
sion of micronuclei accompanied by cell division; L, typical nuclear condition
restored.
258 ANIMAL BIOLOGY
Suggestive as is this comparison and contrast — and it is not
without some justification — the cardinal fact remains that recent
work has demonstrated that Paramecium and some closely related
forms, when bred under favorable environmental conditions, can
continue reproduction indefinitely, at least in one case for more
than thirty years and some twenty thousand generations, without
fertilization and without any signs of degeneration. Moreover in
many unicellular forms fertilization has never been observed and
perhaps does not occur in the life history. In other words, fertiliza-
tion is not a necessary antidote for inherent senescence, and this,
taken in connection with other data which point in the same direc-
tion, such as the unlimited reproduction of many plants by asexual
processes, and the recent discovery that certain tissue cells re-
moved from the Vertebrate body will live, grow, and divide appar-
ently indefinitely if given favorable conditions, renders it fairly
safe to make the general statement that senescence is not inherent
in protoplasm — the need of fertilization is not a primary attribute
of living matter. Reproduction and fertilization are intrinsically
separate processes which, however, have become closely associated,
especially in higher forms.
So far our conclusion is entirely negative — fertilization is not
reproduction and is not intrinsically necessary for reproduction.
What then is its significance? Though fertilization may not be
necessary in the life of simple organisms under favorable condi-
tions, this does not indicate that it may not be a stimulus to proto-
plasmic activity when it does occur — perhaps a very important
factor under special environmental conditions. Indeed it appears
certain that conjugation in many cases directly results in stimulat-
ing the vital processes of the cell, including reproduction. But it
would seem that the essential factor in this stimulation is not
the essence of fertilization, which is synkaryon formation. In
Paramecium, for example, an internal nuclear reorganization proc-
ess known aS ENDOMIXIS occurs periodically, which is carried on
by each individual, without a nuclear contribution from another.
Nevertheless it frequently effects a physiological stimulation similar
to that which follows synkaryon formation during fertilization.
Accordingly the factor common to both fertilization and endomixis,
that is general nuclear reorganization, apparently is responsible
for the ‘dynamic’ effects. (Fig. 171.)
Mertazoa. Turning from Paramecium and its allies, we may
FERTILIZATION 259
ee, -
:
C
D
es SES
Sas? aaa
Ce ee
25 H
Fic. 171. — Diagram of the nuclear changes during endomixis in Parame-
cium aurelia. A, typical nuclear condition; B, degeneration of macronucleus
and first division of micronuclei; C, second division of micronuclei; D, degener-
ation of six of the eight micronuclei; E, division of the cell; F, first reconstruction
micronuclear division; G, second reconstruction micronuclear division; H,
transformation of two micronuclei into macronuclei; I, micronuclear and cell
division; J, typical nuclear condition restored.
260 ANIMAL BIOLOGY
consider some evidence among higher forms in regard to the
dynamic influence of fertilization. Although fertilization is usu-
ally necessary for the resumption of the series of cell divisions
which paused after the maturation divisions, and which are to
transform egg into adult, there are many exceptional but entirely
normal cases where the egg proceeds to divide of its own accord.
Such PARTHENOGENETIC eggs are formed like other eggs, though
sometimes without chromosome reduction. Thus the eggs of the
Honey Bee, to cite the most interesting case, develop either with
or without fertilization — fertilized eggs forming females and un-
fertilized eggs, males. Certain species of Rotifers and Roundworms
apparently reproduce solely by parthenogenesis, males not being
known. Leaving out of the question the effect on the chromosome
complex, it is at once apparent that the mere fact that an egg di-
vides without the influence of a sperm indicates clearly that, in such
cases at least, neither structural additions nor physiological in-
fluences of the sperm are necessary to initiate deveiopment.
It may with justice be urged, however, that such cases of normal
parthenogenesis are special adaptations to peculiar conditions in
which the egg has usurped, as it were, the usual sperm function,
and that therefore the evidence is of little weight in determining
the primary significance of fertilization. Accordingly the data
from so-called ARTIFICIAL PARTHENOGENESIS are particularly co-
gent. Within recent years it has been found that the eggs of a con-
siderable number of Invertebrates and even of Vertebrates, such
as some Fishes and Frogs, which normally require fertilization,
can be induced to start development parthenogenetically by var-
ious artificial means such as subjection to certain chemicals,
unusual temperature changes, shaking, or the prick of a needle
— the effective stimulus varying with different species.
Just what happens in the egg as a result of such treatment is
open to discussion, but for our purposes it is sufficient to know
that the egg begins to divide in normal fashion. This shows con-
clusively that even eggs which normally require fertilization are
intrinsically self-sufficient at least to start to develop, and there-
fore this strongly indicates that an incidental and not the main
function of fertilization is to stimulate cell division.
Restating the evidence in its bearings on the meaning of fertili-
zation, we may say that fertilization is not fundamentally an
FERTILIZATION 261
indispensable event in the life history of the Protozoa living under
favorable environmental conditions. Certain species have been
bred for thousands of generations without conjugation, and, in-
deed, without endomixis. Similarly in the Metazoa, both normal
and artificial parthenogenesis indicate that the egg itself comprises
a mechanism which is capable of initiating and carrying on develop-
ment. From this viewpoint, fertilization may be satisfactorily
interpreted as a means of insuring under special or unfavorable en-
vironmental conditions in unicellular organisms, and under usual
conditions in the eggs of multicellular forms, a suitable stimulus
which otherwise might be unavailable at the proper time.
Granting then that fertilization may afford a stimulus to develop-
ment, is this its chief significance? Many lines of evidence surely
converge toward the view that the opportunities which fertilization
affords for changes in the complex of the germ are of paramount
importance. Fertilization establishes new diploid groups of heredi-
tary characters by combining diverse haploid groups from the two
gametes. It makes possible the shuffling of germinal variations so
that they are presented in new combinations. It is the pooling of
the germinal changes of two lines of descent. Some of the new
combinations may more effectually meet — be better adapted to —
the exigencies of the environment, and so have a survival value
for the organism in the struggle for existence. So whatever the
primary meaning of fertilization may be, its importance in estab-
lishing the essentially dual nature of every sexually produced
organism is settled beyond dispute, and it is the cardinal fact of
heredity. No wonder is it that from the lowest to the highest ani-
mals provisions are made for a process which multiplies many-fold
the opportunities for descent with change. (Fig. 189.)
In passing, it should be emphasized that provisions to ensure
fertilization have had a profound influence on the morphology and
physiology of organisms. Sex of the gametes and sex of the indi-
vidual body are, of course, radically different, although the latter
is indirectly an outcome of the former. The evolution of the gam-
etes themselves is relatively simple: from those alike so far as struc-
ture is concerned, though physiologically different, to those in
which one sex is smaller and more motile and the other larger and
usually non-motile. But the sexual evolution of the individual
body in the Metazoa presents amazing phenomena. Witness the
262 ANIMAL BIOLOGY
biological and physiological contrast of individuals that bear sperm
and eggs respectively. Sex, indeed, becomes largely a dominating
factor in the life of the lower animals, and even of Man, where
the primary function of somatic sexual differentiation to ensure
fertilization appears to become largely submerged. “It is as though
variety and beauty had become ends in themselves in the evolu-
tion of secondary sex characters, as exemplified in the plumage of
birds, and in the strife and amenities of human social relations.”
CHAPTER XX
DEVELOPMENT
The student of Nature wonders the more and is astonished the less,
the more conversant he becomes with her operations; but of all the
perennial miracles she offers to his inspection, perhaps the most
worthy of admiration is the development of a plant or animal from its
embryo. — Huzley.
THE new individual, established by the orderly merging of a cell
detached from each parent in sexually reproducing species, has
before it first of all the problem of assuming the adult form by a
complicated developmental process. As we have seen, this in-
volves cleavage of the egg, followed, in the Metazoa, by blastula
and gastrula stages during which the primary germ layers are
established — the fundament out of which the definitive form,
organs, and organ systems of the adult are evolved. The descrip-
tion and comparison of these processes in different organisms con-
stitute the content of one aspect of EMBRYOLOGY. (Fig. 288.)
It is unnecessary — indeed, it is impossible — for us to survey
the immense field included under embryology. We must be satis-
fied with the realization that animal development, though it varies
widely in producing the immensely diverse body forms, exhibits
throughout a thread of similarity in its fundamental features; and
the appreciation of the marvellous intricacy of the developmental
process even in the lowly animals. This perhaps may be gained
by concrete examples — first the embryological development of
the Earthworm from the zygote to the establishment of the gen-
eral body plan.
A. EMBRYOLOGY OF THE EARTHWORM
The egg of the Earthworm, after fertilization, proceeds to divide
first into two cells, then four cells, eight cells, and so on, with more
or less regularity, until a condition is attained in which many rela-
tively small cells are arranged about a central cavity. This stage
of the embryo will be recognized as the blastula.
The various cells of the blastula appear essentially the same ex-
cept that those at one end are somewhat larger than at the other.
263
264 ANIMAL BIOLOGY
The larger cells now sink into and nearly obliterate the central
cavity of the blastula, thus forming a typical gastrula stage com-
posed of two layers of cells, ectoderm on the outside and endoderm
on the inside. The infolded enteric pouch, or enteron, enclosing
«99
“ASOD, aX »
PRE RLY
?.
, KW) IQ
ete <4 $
Y}
pt
ae
Fic. 172.— General plan of the early development of the egg of an animal.
A-F, cleavage and formation of the blastula; G, section of blastula showing
the beginning of gastrulation; H-I, early and later gastrula stages. a, ecto-
derm; 6, endoderm; c, blastocoel; d, blastopore, leading into the enteric cavity;
e, cells, arising from the endoderm, destined to form the mesoderm.
the enteric cavity, eventually becomes the main part of the ali-
mentary canal of the worm; its present opening to the exterior,
or blastopore, forming the mouth. So the developing worm has
now reached a transient state which is broadly comparable to
the permanent adult condition of Hydra. (Figs. 172, 173.)
DEVELOPMENT 265
While these two primary germ layers are being established, the
developing embryo shows the rudiments of the third primary germ
layer (mesoderm) in the form of two MESOBLAST CELLS which
leave their original position in the wall of the embryo and take
up a place between the ectoderm and endoderm; that is, in the
remnant of the cavity of the blastula which the invagination proc-
ess during gastrulation has not completely obliterated. Here the
two cells, by division, form on either side of the enteric pouch a
linear series, or band, of mesoderm cells. These mesoderm bands
gradually increase in size and spread out until finally they unite
above and below, that is encircle, the enteric pouch. Thus they
form a continuous mesoderm layer between ectoderm and endo-
derm. Simultaneously with the growth of the mesoderm bands to
form a definite middle layer, a linear series of spaces appears in
each band which presages the future segmentation of the worm’s
body. These cavities increase in size and, when the bands unite
around the enteric pouch, the corresponding cavities of each band
also become continuous in the same regions. (Fig. 173, C—H.)
In this way the mesoderm itself becomes divided into what are
essentially two cellular layers, an outer, or SOMATIC LAYER, next
to the ectoderm, and an inner, or SPLANCHNIC LAYER, In contact
with the endoderm. The space between these layers of the meso-
derm is the body cavity, or coelom. The coelom, however, is not
a continuous cavity from one end of the embryo to the other, be-
cause the mesodermal cells which separate the linear series of
cavities in the respective mesodermal bands persist. These cells
form a regular series of connecting sheets of tissue between the
somatic and splanchnic mesoderm layers and thus divide the body
of the worm into a series of essentially similar segments, the limits
of which are indicated on the outside by a series of grooves which
encircle the worm’s body. (Fig. 173, I, J.)
While these processes are transforming the two-layered gastrula
into an embryo composed of three primary layers, and exhibiting
segmentation, coelom, etc., — in short, the ‘tube within a tube’
body-plan characteristic of higher forms — the embryo is gradu-
ally increasing in size and elongating. The mouth, representing
the blastopore, remains at one end, which is therefore designated
as anterior, while growth is chiefly in the opposite direction or
toward the posterior. At this end (the blind end of the enteric
pouch formed at gastrulation) an opening to the exterior, the
266 ANIMAL BIOLOGY
Biastocoel
Mesoblast 4
“ cell A) £0)
Enteron = Ectoderm
ri geoaee sooo we
a0 arise
Coelom
Fae
6016:
Enel Ma
tury nt
a.?
umn
aytteee
stag =:
a
Ard
a
LT]
sa
Bt EA
; i Splanchnic
mesoderm
le
LIT)
eaves
STITT
‘
Fic. 173. — Diagrams of stages in the development of the Earthworm.
A, blastula (surrounded by a membrane); B, section of a blastula showing
blastocoel and one of the primary cells (mesoblast cells) of the mesoderm;
C, later blastula with developing mesoderm bands; D, start of gastrulation;
KE, lateral view of gastrula showing invagination, which as it proceeds leaves
the mesoderm bands on either side of the body as indicated by the cells repre-
sented with dotted outline; F, section of E, to show mesoblast cells, meso-
derm bands, and enteric cavity. G, later stage showing cavities in the meso-
derm bands; H, the same (G) in transverse section; I, diagram of a longitudinal
section of a young worm after formation of mouth and anus; J, the same in
cross section; K, later stage in transverse section. (After Sedgwick and Wilson.)
DEVELOPMENT 267
anus, is formed so that the enteric pouch now communicates with
the exterior at both ends and becomes the alimentary canal. Thus
antero-posterior differentiation is clearly established.
A cross section perpendicular to the main axis of the developing
worm at this stage presents the appearance of a circle within a
circle. The smaller circle surrounds the enteric cavity and is the
wall of the alimentary canal. It is separated by a space, the coelom,
from the larger circle, or body wall. Moreover, each of these
circles is composed of two tissue layers: the alimentary canal,
formed internally of endoderm and externally of splanchnic meso-
derm; and the body wall, internally of somatic mesoderm and ex-
ternally of ectoderm. Thus the coelomic cavity is entirely enclosed
by mesoderm. (Fig. 173, K.)
It is from these three primary layers of cells (ectoderm, somatic
and splanchnic mesoderm, and endoderm) that all of the tissues
and organs. of the adult worm arise through later differentiation,
thickenings, foldings, outgrowths, etc. For example, the nervous
system is formed by the ingrowth of a thickened region of the
ectoderm; the blood vascular system develops by a specialization
of cells throughout the mesoderm; while the reproductive system
first appears as thickenings of the somatic mesoderm which, as
development proceeds, become largely separated from it as inde-
pendent organs in the coelom. (Figs. 60, 61.)
B. EmMBpryoLocy AND METAMORPHOSIS OF THE FROG
As an example of Vertebrate development we may take that of
the Frog, though it must be borne in mind that just as other
Invertebrates differ in their embryology from the Earthworm, so
the embryology of other Vertebrates departs widely from that of
the Frog — chiefly fundamental and highly significant similarities
persisting.
The fertilized egg of the Frog contains a large amount of stored
food material, or yolk, which influences the character of the cleav-
age of the egg. Thus cell division is progressively more rapid, and
accordingly the cells smaller, in the region with little yolk, the
upper (animal) pole, than in the yolk-laden lower (vegetal) pole.
The first and second division planes are from pole to pole, and
give rise to four cells of equal size. The third division plane
is just above the equator of the egg, at right angles to the former
planes, and establishes four dark, pigmented cells above, and four
268 ANIMAL BIOLOGY
Animal
pole
Vegetal pole
Blastocoel
Blastopore
Gill arches Notochord Spinal cord io Midbrain
urenteric AE :
ne Z ie & Forebrain
canal
Anal 7
region
Rectum ie Enteron
BOR “Sucker plate
Yolk ; eS Endoderm
tacit J Ectoderm K Mesoderm
Fic. 174. — Early development of the Frog. A, B, C, egg at two-, four-,
and eight-cell stage; D, early blastula; E, section of D; F, late blastula; G, early
gastrula: overgrowth of ectoderm (cells of ectoderm now too small to be
shown in figure); H, section of G showing germ layers, etc. (Blcel, blastocoel);
I, late gastrula: formation of neural groove and folds representing the founda-
tions of the nervous system; J, older embryo, with neural groove closed, as-
suming tadpole form; K, section of J. See Fig. 175.
DEVELOPMENT 269
larger pale, yolk-laden cells below the equator. As division pro-
ceeds, soon there are many cells arranged in the form of a hollow
sphere which will be recognized as the blastula. (Fig. 174.)
The transformation, in due course, of the blastula into the
gastrula by typical invagination is somewhat obscured by the
large amount of yolk in the prospective endoderm cells. In fact,
the endoderm is formed by a flat infolding of cells, just below the
edge of the small dark cells, that finally results in a crescentic
groove on the surface, which is the edge of the blastopore. But
gastrulation is not completed until all the large endoderm cells
are enclosed by ectoderm cells, and this is accomplished chiefly
by the latter gradually creeping and folding over the exposed,
pale endoderm cells (YoLK PLUG) until merely a small blastopore
remains leading into the enteron.
The development of the third germ layer, or mesoderm, takes
place by the ingrowth of a layer of cells between ectoderm and
endoderm along the edge of the blastopore where these two layers
merge. When the mesoderm has grown forward and spread out
between the ectoderm and endoderm, the lower portion (LATERAL
PLATE) splits into a somatic and splanchnic layer and thus gives
rise to the coelomic cavity between. The upper portion of the
mesoderm (VERTEBRAL PLATE) exhibits traces of primitive seg-
mentation as it forms a series of muscle plates, or MYOTOMES, on
either side of the NoTocHoRD which, in the meantime, has arisen
from an axial, dorsal strip of cells above the enteron.
During the later stages of gastrulation, the foundations of the
central nervous system are established by the differentiation of a
plate of ectoderm cells, the MEDULLARY PLATE, on the dorsal sur-
face of the embryo. Then a groove appears in the medullary plate
which is finally completely enclosed by the growth upward, over,
and then fusion of the edges of the medullary plate. Thus the
open groove is converted into the NEURAL TUBE which is soon to be
differentiated into fore-brain, mid-brain, hind-brain, and spinal
cord.
Simultaneously with the establishment of the central nervous
system, the differentiation of the enteron into the alimentary canal,
opening by mouth and anus, and also various other internal trans-
formations have proceeded apace. Furthermore, the embryo has
been gradually elongating so that it is nearly twice as long as
broad by the time it begins locomotion by means of cilia distributed
270 ANIMAL BIOLOGY
over the body surface. And soon thereafter the larva, or TADPOLE,
assumes a somewhat fish-like form, with a vertically flattened
tail edged by a fin which provides for locomotion during the
rest of the animal’s purely aquatic life. (Fig. 175.)
Pronephros
6) Bogmnent
c/- of eye
: &~ Olfactory pit
Gill slits
Oral ion
©e eo,
Ce SE oe
“Richard Edes- Harrison -1931-
Fic. 175. — Development and metamorphosis of the Frog. A, B, stages
closely following K in Fig. 174. C-L, stages from egg to adult drawn to scale.
Independent existence demands sense organs, and these are
already functioning in the head region. It also demands the in-
take of food which earlier was supplied by the yolk stored in the
egg, and so the mouth develops a hard rim for scraping food mate-
DEVELOPMENT 271
rial from the surface of aquatic plants. Moreover, increased res-
piration is necessary, and this is met by the appearance of branched
external gills on the sides of the head, which are the first of a
series of three different respiratory organs that succeed one another
during the life history. Indeed, these external gills are soon coy-
ered over by a fold of the skin, the oPERCULUM, which finally
leaves but a single opening, the SPIRACLE, to the exterior. And
no sooner is the operculum fully developed, than the external
gills are resorbed and a new set of fish-like internal gills take their
place in the gill slits. Then hind legs slowly make their appear-
ance, followed by the fore legs, already developed under the
operculum.
Now the larva is ready for METAMORPHOSIS — its transforma-
tion from a gill-breathing tadpole to a lung-breathing juvenile
Frog. During metamorphosis one stage melts rapidly into the
next: the tail is resorbed, the legs increase in size, the long coiled
intestine becomes shorter and specialized to digest animal food,
the internal gills are resorbed, and lungs are developed which
make it necessary for the tadpole to come to the surface of the
water for air. Finally, as a juvenile Frog, the animal transfers
its abode largely to land, and grows.
The process of metamorphosis varies in length from a few
weeks to several years in different species of Frogs. In all cases it is
some time after metamorphosis before sexual maturity arises. Pre-
ceding the first breeding season, the gonads develop rapidly, their
ducts become fully differentiated, and adult male and female
individuals are established. (Figs. 132, 163.)
C. Empryronic MEMBRANES OF THE HIGHER VERTEBRATES
The embryological development of the higher Vertebrates de-
parts rather widely in certain ways from that of the Earthworm
and the Frog. Thus the eggs of Reptiles and Birds contain much
more yolk than the egg of the Frog, with a consequent greater
obscuring — though not obliteration — of the characteristic blas-
tula, gastrula, etc. (Fig. 169.)
Furthermore, in the Frog the whole egg becomes converted
into the body of the tadpole, whereas in Reptiles, Birds, and
Mammals a part of the egg forms a hood-like membrane, the
AMNION, about the embryo. This is cast off at birth and with it
another membrane, the ALLANTOIS, which extends into the am-
Outer germ layer (Ectoderm)
Embryonic ue nee
: mbryo
-—~— J Middle germ layer (Mesoderm)\ —~— “Asaninge
0)
<0 Inner germ layer (Endoderm) aay fold
ee: Blastodermic vesicle XA an
oS ee
later becoming
yolk sac
B
Closing amniotic folds
Sx Allantois joining with
\==:=X chorionic membrane g
Amniotic cavity
Body stalk,
umbilicus
Degenerating
yolk sac
Extra-embryonic space
Fic. 176. — Diagrammatic sections showing the development of the egg and
embryonic membranes of a Mammal. A, blastula showing fundament of
embryo at top, before the appearance of the amnion. B, embryo outlined, with
developing amnion and yolk sac. C, embryo with amnion further developed
and allantois appearing. D, embryo with amnion closing, and allantois joining
with outer membrane, or chorion. E, F, embryos in which the vascular layer
of the allantois is applied to the chorion and growing into the villi of the latter
to form the fetal placenta; yolk sac reduced; amniotic cavity increasing;
mouth and anus established.
272
DEVELOPMENT 273
nion and has served temporarily as a respiratory membrane. The
development of these two EMBRYONIC MEMBRANES, to meet new
conditions of embryonic existence, makes possible not only a
more rapid and sure interchange of materials between the embryo
and its surroundings, but also affords greater protection for the
growth of complicated structures. (Fig. 176.)
Fic. 177A. — Photographs of early stages in the development of the egg
of the Rabbit. A, two-cell stage, 24 hours after fertilization, in thick surround-
ing membrane (zona pellucida); B, four-cell stage, 29 hours; C, eight-cell
stage, 32 hours. Highly magnified. (After Streeter.)
Finally, the eggs of typical Mammals, including Man, though
not provided with so large an amount of yolk because food is
A B
Fic. 177B.— Human embryos. A, one month old (6.7 mm.), showing arm
and leg buds, caudal end, umbilical cord, heart, gill slits, olfactory pit, and
eye; B, six weeks old (19 mm.), showing developing hands and feet, elbow and
knee, nose, eye, and ear. See Fig. 235. (After Streeter.)
supplied to the developing embryo by the blood vascular system
of the mother, nevertheless inherit from lower forms the embryonic
274 ANIMAL BIOLOGY
membranes. These contribute to the formation of the PLACENTA
and are modified and diverted to meet new conditions demanded
by the uterine life of the Mammalian embryo. (Figs. 133, 134.)
With merely this outline of some of the chief features of the
embryology of the Earthworm, Frog, and higher forms before us,
it is possible to gain some appreciation of the similarity of the
basic method of development which is ever present in the Animal
Kingdom — cleavage, blastula, gastrula, primary germ layers, etc.
In general, it may be said that in all the higher animals the ecto-
derm forms the outer skin and nervous system; the endoderm sup-
plies the lining membrane of the major part of the alimentary tract;
while the mesoderm contributes muscles, blood vessels, reproduc-
tive organs, and the membrane lining the coelom. This similarity
in origin of the organ systems from the primary germ layers,
throughout the animal series above the Coelenterates, is of the
highest significance because it indicates a fundamental structural
similarity in the body plan of all these forms. It is exhibited in
the developmental process in each generation, even though the
adult body in the various groups differs widely in form and arrange-
ment of organs. Such a state of affairs clearly suggests an heredi-
tary relationship throughout the animal series — the origin of the
diverse forms by gradual change. (Figs. 172, 174.)
D. PROBLEMS OF DEVELOPMENT
Embryology is something more than the description of the ka-
leidoscopic series of stages which seem to melt one into the other
as development progresses. It attempts, especially at the present
time, to look below and beyond structure to the processes in-
volved, and to determine how the sequence of events is brought
about. This is but a repetition of the stages of progress in all
science; a passage from the descriptive to the experimental. The
results thus far secured have raised many, and answered some prob-
lems of development of great practical importance and theoretical
interest. The outline of one broad problem may serve as an
example. |
From what the pioneer students of embryology during the
seventeenth and eighteenth centuries saw, or thought they saw,
with simple lens and newly invented compound microscope, there
were gradually formulated two opposing views of development
DBErVELOPMENT 275
which, though long since swept aside in their original form as a
result of the increase of knowledge, raised a problem that is still
before the embryologist to-day.
In brief, one view virtually denied development by maintaining
that the adult organism is nearly or completely formed within the
germ, either in the egg or the sperm, which merely by expansion,
unfolding, and growth gives rise to the new generation. In this
first crude form the PREFORMATION theory demanded the ‘ encase-
ment’ of all future generations one within another in the germ of
existing organisms, so that when it was computed that the pro-
genitor of the human race must have contained some two hundred
million homunculi (a conservative estimate, to say the least) the
reductio ad absurdum was irresistible.
The other view was reached by careful studies on the transforma-
tion of the Hen’s egg into the chick which soon made it clear that
the chick is not preformed in the egg. The embryo arises by a
gradual process of progressive differentiation from an apparently
simple fundament — it is a true process of development, or EPI-
GENESIS. But the upholders of epigenesis versus preformation were
before long beyond their depth and in danger of attempting to
get something out of nothing — lost in the miraculous.
A statement in such succinct form tends to accentuate the cru-
dities of these two conflicting views — “ preformation explaining
development by denying it and epigenesis explaining development
by reaffirming it’’ — and it may be well to remark that the early
embryologists with the means at their command faced a stupendous
task of which only recent work has brought a full appreciation.
The path to progress cleared by the realization that adult
structures are not preformed as such in the egg, and that develop-
ment is not an expansion but the formation — the ‘becoming ’—
by an orderly sequence of events of structures of great complexity
out of apparent simplicity, the problem of the embryologist was to
determine what the egg structure actually is, and how it is related
to that of the adult. To trace the development of these studies
would involve the history of embryology since the formulation of
the cell theory. We must confine ourselves to the bare statement
of the new guise in which the old theories of preformation and
epigenesis confront us to-day as a result of recent research.
The reader already recognizes the fertilized egg as a cell, with
its nucleus comprising a complex of quite definite elements — the
276 ANIMAL BIOLOGY
chromosomes — contributed jointly by the two gametes. To this
extent, then, the nucleus and therefore the egg exhibits a ready-
formed structural basis which (as we have already suggested, and
will have occasion to elaborate later) certainly is definitely related
to characters which appear in the offspring.
Turning to the egg cytoplasm, we are confronted with conditions
which are not so uniform but nevertheless highly suggestive. In
Fic. 178A. — Egg of a Mollusc, Dentalium, showing cytoplasmic differentia-
tion. A, egg, shortly after being extruded and before maturation is completed,
showing three differentiated regions; B, section through an egg after fertiliza-
tion, showing cytoplasmic rearrangement involving the segregation of clear po-
lar lobe at p; C, normal sixteen-cell stage, with materials of polar lobe now in
cell X. Removal of the polar lobe results in an abnormal embryo. (After
Wilson.)
the first place, before fertilization the egg possesses a definite
POLARITY, expressed, for example, by the position of the nucleus
and the distribution of food material (yolk), pigment granules,
and vacuoles. This polarity is traceable, in part at least, to the
polarity of the odgonia, and through them to the germinal epi-
thelium. In brief, the egg as a whole is organized; the invisible
organization of the fundamental matrix of the cytoplasm being
revealed, in part, by the disposition of various elements of the
cell. Now this cytoplasmic organization undergoes more or less
profound changes in establishing that of the new individual. In
some cases the reorganization occurs at fertilization, while in
others it is somewhat deferred. And herein, apparently, is to be
sought the explanation of the difference in behavior — in potential-
ities — of various types of eggs during cleavage stages. Two con-
trasting examples will serve to bring the main facts before us.
The first type is well illustrated by the egg of a Mollusc, Den-
DEVELOPMENT 201
talium, and a primitive Chordate, Styela. The egg of the latter
shows at the first division five clearly differentiated cytoplasmic
regions. For the sake of simplicity these may be described as
white, light and dark gray, and light and dark yellow. As cleavage
proceeds, these substances are distributed with great regularity
to definite cell groups, which in turn form special organs or organ
systems of the animal. Thus cells which receive the white region
form the ectoderm; those which receive the dark gray, the endo-
derm; while the cells with light or dark yellow form mesodermal
structures, and so on. And further, the experimental removal of a
cell or cell group in which a certain substance is segregated results
in an embryo deficient in the very structures which this normally
forms. In other words, the egg cytoplasm seems to be a mosaic of
organ-forming substances which possibly themselves directly, but
probably through more fundamental conditions of which they
are but the visible expression, have a causal relation to definite
adult structures. Just in so far as this is true, the adult is pre-
delineated in bold lines, though not actually preformed, in the egg.
(Fig. 178.)
Passing now to the second type, represented, for example, by the
eggs of some Sea Urchins, the results which we obtain seem to be
diametrically opposite. Although more or less clearly differentiated
cytoplasmic regions appear to exist, frequently the removal of a
part of the egg before division, or the separation of the cells at
the two-cell stage, and sometimes even at the four-cell stage, has no
permanent effect on the structural integrity of the developing
embryo. Each of the cells has the power to develop into an embryo
complete in every respect, but smaller than the normal. Or, to
put it another way: at the four-cell stage, a single cell which nor-
mally forms, let us say, one-fourth of the embryo, if isolated with
one other cell, may form one-half of a normal embryo and, if isolated
with two other cells may form one-third. And apparently the same
phenomenon occurs in the case of human identical twins. The egg
becomes separated into two parts during early development and re-
sults in two individuals with identical hereditary basis. Indeed iden-
tical quintuplets, all from one zygote, have become famous. In all
such cases one may ask, what has become of the egg organization?
At first glance the behavior of these two classes of eggs seems
to afford results which are irreconcilable — the former supporting
the doctrine of preformation in a refined form, and the latter its
2738 ANIMAL BIOLOGY
antithesis, epigenesis. But an explanation is not far to seek. The
difference apparently depends, as already suggested, upon the time
when chemical differentiation of the egg cytoplasm occurs and the
products are localized in special regions. If this occurs before or
at fertilization, so that the early divisions give rise to dissimilarly
organized cells, then each of the cells is not TOTIPOTENT and the
mosaic type of development results; but if the initial differentiation
and localization is delayed until later, or is relatively slight so that
the cells of the early stages are all essentially similar, then during
this period each cell is totipotent — the whole forms an EQUI-
POTENTIAL SYSTEM — as exhibited by the early stages of the Sea
Urchin. Thus we may bring under one viewpoint the apparently
contradictory behavior of the two classes of eggs, for it turns out
to be reducible to a common factor: the time of differentiation and
localization of the products. In one case this has progressed fur-
ther than in the other during the early embryonic stages. In both
cases, therefore, development is epigenetic in its obvious features.
However, since cytoplasmic differentiation is a fact whether it
appears early or late, we have merely pushed the solution of the
problem further back and the question becomes: Is there a primary
differentiation and, if so, where? It is not possible to present here
the specific evidence on this point, but the reader’s knowledge of
the nucleus, and particularly its definite chromosomal architec-
ture, will lead him to anticipate that modern research tends more
and more to emphasize the gene as representing a material con-
figuration — apparently it is a protein molecule — which is trans-
mitted, in a way, ‘preformed’ from generation to generation
and determines the cytoplasmic characteristics of the cells. As to
how the specific physical basis of inheritance, the genes constitut-
ing the chromosomes, is related to cytoplasmic organization and
to characters which arise later, we can offer no satisfactory expla-
nation or even guess. We must be content with a discussion, in
the next chapter, of some of the facts of heredity which show that
certain chromosomes are causally related to the inheritance of
certain characters.
But in so far as the nucleus possesses an organization which is
definitely related to differentiations of the cytoplasm, organ-
forming substances, or characters of embryo and adult, we may
look upon the chromatin to this extent as representing a sort of
primary preformation which is realized by a process of building
DEVELOPMENT 279
up — epigenesis — as one character after another becomes estab-
lished in the development of the individual. This is the guise in
which the old problem of preformation versus epigenesis faces the
biologist to-day.
So the early embryologists were right when, studying the egg
of the Frog or Hen, they maintained that development is develop-
DQ
WD
Fic. 178B. — Diagram to illustrate how the character of the first division
of an egg may influence the distribution of the products of cytoplasmic differ-
entiation and therefore the potentialities of the resulting cells. A, immature
egg, assumed to have no definite segregation of cytoplasmic stuffs; B, mature
egg, with cytoplasmic zones established; C, first division of egg; D and E, two
types of two-cell stages; D, type with one cytoplasmic zone entirely distributed
to one of the cells, and therefore each of the two cells, if separated, gives rise
to an abnormal larva; E, type with equal distribution of the zones to both cells,
and therefore, if separated, each of the two cells gives rise to a normal larva.
(From Wilson.)
ment and not merely the unfolding of an organism already fash-
ioned in more or less definite adult form. But it took two centuries
of research to reveal the fact that, below and beyond its super-
ficial aspects, there is a germ of truth in the principle of preforma-
280 ANIMAL BIOLOGY
tion deeply hidden in the nuclear architecture, the enormously
complex physico-chemical structure of the genetic basis, or genes,
and therefore the origin of the individual, though obviously through
epigenesis, is fundamentally from a sort of preformed basis. We
no longer bother ourselves with the old conundrum as to which is
more complex, the egg or the adult, but recognize that each is com-
plex in its way — the simplicity of the egg being more apparent
than real, as is convincingly attested by every endeavor to analyze
cytoplasm, nucleus, chromosomes, genes, and beyond.
CHAPTER XXI
INHERITANCE
So careful of the type .. .
So careless of the single life. — Tennyson.
Tue old adage that ‘like begets like’ expresses the general fact
of HEREDITY. Everyone recognizes that parent and offspring agree
in their fundamental characteristics: they “belong to the same
species. And everyone realizes that the resemblance may be
strikingly exact even in details of form or behavior. Family traits
reappear. The mere statement of striking resemblances among the
individuals of a family is a tacit admission that no two individuals
are exactly alike; in other words, heredity is organic resemblance
based on descent — inheritance of the characters exhibited by the
parents is not complete, there is VARIATION. Indeed “variation is
the most invariable thing in nature,” but one must guard against
the impression that there is an antithesis between heredity and
variation. “Living beings do not exhibit unity and diversity, but
unity in diversity. Inheritance and variation are not two things,
but two imperfect views of a single process.”
We may now address ourselves to the problems of heredity and
variation which are at the basis not only of what organisms have
been in the past and are at the present, but also of whatever the
future may have in store for them. Variations are the raw materials
of evolutionary progression or regression. From a broad point of
view, the origin of species and the origin of individuals are essen-
tially the same question. If we can solve the relations of parent
and offspring, the origin of species will largely take care of itself.
As a matter of fact, historically the question of species origin was
approached first, and through the work of Darwin became of
paramount interest in the latter half of the nineteenth century.
The twentieth century finds the individual — the hereditary re-
lation of parent and offspring — the center of investigation, and
it forms the science of genetics. ORGANIC EVOLUTION attempts to
establish the general fact that all organisms are related by descent;
GENETICS attempts to show how specific individuals are related.
281
282 ANIMAL BIOLOGY
Even further has the pendulum swung from the general to the
particular. To-day the most intense investigation is centered not
on the heritage of the individual as a whole, but on particular
characters of the individual. An immense amount of experimental
work has demonstrated that, for practical purposes, the individual
may be regarded as an aggregate of essentially separate characters,
both structural and physiological, that are relatively stable and
may be inherited more or less as units. But the analysis does not
stop even at this level. Each character is regarded as represented in
Fic. 179. — The evolution of the Game Cock. Results produced largely by
selection before our present knowledge of the mechanism of inheritance.
(From Metcalf, after Wright.)
the chromosomes of the germ cells by one or more determining
factors, or GENES; and whether or not a given character wil! be
present in a tree or a man depends upon whether the genes for this
particular character entered into the nuclear complex of the fer-
tilized egg which formed the individual. Therefore, geneticists are
now studying the relative positions which the genes occupy on
certain chromosomes; how they may cross-over from one chro-
mosome to the other of a synaptic pair; how they mutually influence
one another, etc. (Fig. 196.)
At present we are witnessing great advances in knowledge of
the underlying factors of heredity, and the data recently accumu-
lated are so vast that we can attempt here no more than to indicate
the character and promise of the principles already discovered.
We may glimpse the field before us by a concrete example.
INHERITANCE 283
About thirty-five years ago, just at the opening of the modern
concentrated attack on genetic problems, an association of British
millers awoke to the fact that some active means must be taken
to offset the increasingly great deficiency in quantity and quality
of the wheat yield. Accordingly they commissioned a specially
trained biologist to investigate the matter. He collected many
different varieties of domestic and foreign Wheat, each known to
have one or more good qualities, and studied how these were in-
herited. Making use of the data thus secured, in the course of a
few years he produced a wheat which combined the good qualities
of several varieties; including high content of gluten, beardless-
ness, immunity to Rust, and large yield, and this ‘made to order’
wheat proved successful in the British Isles. But with the open-
ing up of new territory in western Canada another obstacle was
encountered: the growing season was too short for the finest vari-
eties of wheat. This condition was quickly met by transferring
the quality of early ripening from an inferior grade of wheat to
a wheat possessing several other valuable characters.
In a similar fashion, a host of workers have performed the im-
possible of a few years ago. Corn of desirable percentage content
of starch or sugar; cotton with long fibers of foreign varieties and
quick maturing qualities to escape insect ravages; Sheep com-
bining choice mutton qualities of one breed with the fine wool of
another and the hornlessness of a third; and so on almost without
end. Furthermore, there is no limit in sight to the new stable
races of plants and animals which are forthcoming as the principles
already known are applied, and subsidiary ones are discovered.
And last but not least, Man has begun to study himself as a
product of heredity and the process of evolution — to determine
the distribution of characters in the family, and the consequences
of their combinations in the physical and mental make-up of the
individual.
A. HERITABILITY OF VARIATIONS
What then are the basic principles of heredity which are to-day
at the command of the scientific breeder? To answer this question
it is necessary to go into some details because no real appreciation
of the underlying principles involved is otherwise forthcoming.
Most of these details have been acquired through patient inves-
tigations made from the standpoint of so-called pure science — one
284
ANIMAL BIOLOGY
more proof of the indebtedness of the ‘practical man of affairs’
to the biological laboratory.
In the Protozoa the problems of heredity confront us in their sim-
plest, though by no means simple form. Amoeba or Paramecium,
Soma
Germ
*®
Fic. 180. — Scheme to illustrate the con-
tinuity of the germ. Each triangle repre-
sents an individual composed of germ
(dotted) and soma (clear). The beginning
of the life cycle of each individual is at the
apex of the triangle where both germ and
soma are present. In biparental (sexual)
reproduction the germ cells of two individ-
uals become associated in a common stream
which is the germ and gives rise to the soma
and germ of the new generation. This con-
tinuity is indicated by the heavy broken line
and the collateral contributions at each suc-
ceeding generation by light broken lines.
(From Walter.)
as we know, divides into
two cells which through
growth and reorganization
soon are to all intents and
purposes exactly similar to
the parent cell. The par-
ent has merged its individ-
uality into that of its off-
spring. Thus stated, one
does not wonder that par-
ent and offspring are alike
— each is composed of es-
sentially the same proto-
plasm. But when we come
to multicellular forms in
which reproduction is re-
stricted to special germ
cells which involve ferti-
lization, confusion is apt
to arise unless one keeps
clearly in mind — and per-
haps exaggerates for the
sake of concreteness — the
distinction between germ
and soma which has been
previously discussed. Since
in higher forms, to which
brevity demands that our
attention be confined, the
sole connection between parent and offspring is through the germ
cells, it follows that they must be the sole path of inheritance.
In other words, whatever characters the body actually inherits
must have been represented by genes in the fertilized egg from
which it arose; and furthermore, any characters which the in-
dividual can transmit must be represented in its germ cells.
(Figs. 8, 28, 180.)
INHERITANCE 285
1. Modifications
Every individual organism — a man, for instance — is a com-
posite not only of inherited characters, but also of MODIFICATIONS
of the soma produced by external conditions during embryonic
development or later. The individual’s environment, food, friends,
enemies, the world as he finds it, on the one hand, and on the other
his education, work, and general reactions to this environment,
all have their influence on body and mind and determine to a
considerable extent the realization of the possibilities derived from
the germ — what he makes of his endowment. He acquires, let
us say, the strong arm of the blacksmith, the sensitive fingers of
the violinist, or the command of higher mathematics. In other
words, what he is depends on his heritage and what he does with
it. Now, if he does develop an inherited capacity, can he transmit
to his offspring this talent in a more highly developed form than
he himself received it? Or must his children begin at the same
rung of the ladder at which he started and make their own way
up in the world? This is the old question of the inheritance of
modifications, or so-called ACQUIRED CHARACTERS. (Fig. 198.)
Is the great length of the Giraffe’s neck, to take a classic though
crude example, due to a stretching toward the branches of trees
during many successive generations, with the result that a slightly
longer neck has been gained in each generation and inherited by
the following? If so, it is a result of the inheritance of modifications
because the changes were somatic in origin. Or is the length of
the neck the result of the survival in each generation of those in-
dividuals which “happened to be born’ with longer necks and ac-
cordingly were better adapted to foliage conditions than those
which varied toward shorter necks) If so, it is not the result of
modifications but of changes having their origin in the germ cells.
(Fig. 92.)
To-day biologists almost unanimously deny the former and
accept the latter interpretation — the consensus of opinion is
certainly that modifications, or changes in the individual body
due to nurture, use, and disuse, are not transmitted as such. This
conclusion is held chiefly because there is no positive evidence
of the inheritance of modifications while there is much negative
evidence; and also because there is no known mechanism by which
a specific modification of the soma can so influence the germ com-
286 ANIMAL BIOLOGY
plex that this modification will be reproduced as such or in any
representative degree. However, it should be emphasized that
biologists in general recognize the potent influence of environment
and the organism’s reactions to the environment on the destinies
of the race, even though they see, at present, no grounds for a
belief that any specific modification can enter the heritage and
so be reproduced. (Fig. 199.)
In this connection the question of the inheritance of disease will
undoubtedly arise in the reader’s mind. But this is really not a
special case. If the disease is the result of a defect in the germinal
constitution, it may be inherited just as any other character,
physiological or morphological, that has a germinal basis. But if
the disease is a disturbance set up in the body by some accident
of life or through infection by specific microorganisms, before birth
or later, it is a modification and inheritance does not occur. Of
course, the well known fact that susceptibility or immunity to
disease-producing organisms — the ‘soil’ for their development —
may be inherited is not an exception to this statement. It may,
however, be suggested in passing that from the standpoint of the in-
dividual born malformed, structurally or mentally, as a result of
parental alcoholism, syphilis, or other obliquities, it probably will
not appear of the first moment that the sins have been visited
otherwise than by actual inheritance. (Fig. 250.)
The whole question of the non-heritability of modifications, or
acquired characters, is a relatively new point of view which has
been fostered by the elusiveness of crucial experimental evidence,
by an ever-increasing knowledge of the details of the chromosome
mechanism of inheritance, and by the general influence of Weis-
mann’s contrast of the soma and germ. Indeed, Lamarck did not
question the inheritance of acquired characters and made it the
corner-stone of his theory of evolution, while some have even
gone so far as to say that either there has been inheritance of
acquired characters, or there has been no evolution. However,
the question is not so serious as that, as will be seen later on;
though it obviously is profoundly important from many view-
points, biological, educational, and sociological. In passing, it
may be mentioned for those who would like to believe that ac-
quired characters are inherited, that if desirable modifications
were inheritable, undesirable ones would be also. Perhaps Nature
is merciful! (Figs. 305, 309.)
INHERITANCE 287
2. Recombinations
Turning from modifications, which appear to be useless to the
geneticist, we find that the most common inherited differences
which appear in offspring are RECOMBINATIONS which owe their
origin to new groupings of the germinal factors, or genes.
Everyone is familiar with some of the more obvious hereditary
differences following fertilization which are the result of new
combinations of parental characters represented in the egg and
sperm: that is, cases in which nothing is apparent which is not
clearly related to the conditions expressed in the ancestors. In
the first place the offspring may exhibit a character, eye color, let
us say, of one parent to the exclusion of that of the other — the
character appearing unmodified. This is termed ALTERNATIVE
inheritance. Or the offspring may seem to be a sort of Mosaic
of the characters of its progenitors, each parent contributing a
certain character but not to the exclusion of that of the other.
Sometimes the parental traits seem to fuse so that the progeny
exhibit a more or less intermediate and different condition, as
in the color of the skin of mulattoes, frequently called BLEND-
ING inheritance. And as a final example, characters of grand-
parents or more remote ancestors may crop out, and constitute
REVERSION.
3. Mutations
But quite different results now and then occur. Characters
which have no place in the ancestry appear and are transmitted
to the descendants. Sometimes these new inherited variations,
or MUTATIONS, are only slight departures from the parental con-
dition, while in other instances they are quite abrupt. But the
significant fact is that mutations result from relatively radical al-
terations in the gene complex and so afford new opportunities for
variation.
Thus combinations and mutations contrast sharply with modi-
fications which are not transmitted to the offspring; the latter
being merely the results of environing conditions on the soma
during embryonic development or later. The importance of this
distinction can hardly be over emphasized because it makes
comprehensible many of the inconsistencies of earlier work on
genetics.
288 ANIMAL BIOLOGY
B. MENDELIAN PRINCIPLES
The statistical treatment of biological data as a method of
studying inheritance was first brought prominently to the attention
of biologists by the work of Galton, a cousin of Darwin, during the
closing decades of the last century and started the widespread
investigation of genetic problems. In particular, his work on the in-
heritance of characters in Man, such as stature and intellectual
capacity, is a biological classic judged by the momentous conse-
quences which followed from the discussion it evoked. But it
was reserved for Mendel to apply statistical methods to facts ob-
served in the progeny derived from carefully controlled experi-
ments in breeding. Mendel’s studies that founded the modern
science of genetics actually were made more than a score of years
before Galton’s, but failed to reach the attention of the biological
world engrossed in the evolution theory; in fact were not known
by Darwin to whom they would have meant so much in his work
to secure experimental data on heredity. We can with advantage
introduce the survey of genetic principles by a study of examples
from Mendel’s own work. (Fig. 290.)
Mendel chose seven pairs of ALTERNATIVE characters which
he found were constant in certain varieties of edible Peas, such
as the form and color of the seeds, whether round or wrinkled,
yellow or green; and the length of the stem, whether tall or dwarf,
and these he studied in the HYBRIDS. One ordinarily thinks of a
hybrid as a cross between two species or, at least, two characteristi-
cally distinct varieties of animals or plants; but as a matter of
fact the offspring of all sexually reproducing organisms are really
hybrids because two parents seldom, if ever, are exactly the same
in all of their germinal characters. Consequently the offspring are
hybrids with respect to the characters in which the parents differ;
but in the following exposition the terms hybrid and pure are used
solely in regard to the particular characters under analysis.
1. Monohybrids
Mendel found, in crossing pure tall and dwarf varieties of Peas,
that all of the progeny of this PARENTAL (P) generation, in the
FIRST FILIAL (F\) generation, were tall like one parent, there being
no visible evidence of their actual hybrid character. Accordingly
tallness was designated a DOMINANT (D) and dwarfness a RECES-
SIVE (d) character.
INHERITANCE 289
Mendel’s next step was to follow the behavior of these charac-
ters in succeeding generations. Therefore the tall hybrids (F;)
were inbred (self-fertilized) and their offspring, the SECOND FILIAL
(F.2) generation, were found to be tall and dwarf in the ratio of
three to one (3D : 1d). This is nowa broadly established Mendelian
ratio. But, of course, in dealing with a small number of individ-
uals this ratio is merely approximate; the greater the number of
offspring, the closer it is approached. In this particular case Mendel
obtained 787 dominant and 277 recessive individuals: a ratio ap-
proximating 3 : 1. (Fig. 181A.)
aa s 3
S s
oO x e
F,
ee
ats é e 3 &
Fic. 181A. — Inheritance of size in a cross between a tall and a dwarf race
of the edible garden Pea. The small circles represent the genes involved.
Continuing the work, Mendel found that the dwarfs (recessives)
when inbred gave only recessives generation after generation,
and accordingly were pure. On the other hand, the tall plants
(dominants) when inbred proved to be of two kinds: one-third
pure dominants which bred true indefinitely, and two-thirds hy-
brids like their parents, giving when inbred the same ratio of
three dominants to one recessive in the THIRD FILIAL (F3) genera-
tion.
290
ANIMAL BIOLOGY
Aside from his masterly foresight in realizing that success de-
pended on simplifying the problem by dealing with definite al-
CO) @
7
Fic. 181B. — Diagram of a Men-
delian monohybrid. Results of cross-
ing tall (S) and dwarf (s) Pea plants.
The circles represent the zygotes and
the characters of the soma (pheno-
type); the letters within the circles,
the germinal constitution (genotype).
The letters outside the recombination
Square represent the gametes. Note
that each of the parents (P) repre-
sents a different phenotype and gen-
otype; all the F; (one shown) belong
to the same phenotype and genotype;
while the F. represent two pheno-
types and three genotypes. The rel-
ative number of individuals compos-
ing the F, phenotypes is 3:1.
ternative characters, Mendel’s
claim to fame lies chiefly in his
discovery of a simple principle by
which the results may be ex-
plained. Since the hybrids when
inbred always give rise to hybrids
and also to each of the parental
types in a pure form, it must be
that the factors (genes) which de-
termine the characters in question
are sorted out, or SEGREGATED, 1n
the ripe germ cells, or gametes.
Assuming for illustrative purposes
that a single gene determines a
given character, it follows: that
after segregation the genes are
F distributed so that some gametes
bear the gene for one character
and other gametes bear the gene
for the other character, but no
gamete ever bears the genes for both
characters. If the gametes of the
original tall parent contained the
gene for tallness (S), and those
of the dwarf parent the gene for
dwarfness (s) — then the hybrids
will arise from a zygote which
combines both genes (Ss), and
since tallness is dominant over
dwarfness all will be tall. Further,
when the germ cells of this hybrid
(Ss) mature, and these genes are
segregated so that half of the
gametes bear S and half bear s,
then when such plants, each with
this germinal constitution, are inbred there will be equal chances
for gametes bearing the same and for gametes bearing different
genes to meet in fertilization.
INHERITANCE 291
The zygotes are 1 SS :2Ss:1ss. But, since S is dominant, the
resulting organisms will be in the ratio of 3 tall to | dwarf, which
is the familiar 3 : 1 Mendelian ratio of dominants to recessives in
the F, generation. The important point, however, is that these
tall plants, although they all appear alike and therefore belong to
the same PHENOTYPE, are actually different with respect to their
germinal constitution; because one-third bear gametes all of which
contain the gene 8, and two-thirds bear gametes half of which con-
tain S and the other half s. Consequently the tall phenotype is
and white (albino) Guinea-pigs. The small circles represent the genes involved.
composed of two GENOTYPES which are distinguishable only by
what they produce. (Figs. 181, 182.)
It is thus apparent why the pure tall plants always breed true,
and why the dwarfs (necessarily pure) do the same — all the
gametes of one bear S and those of the other, s. The pure plants
are HOMOZYGOUS with respect to the characters in question. It is
also clear why the hybrids give rise to hybrids and pure dominants
and recessives — half of their gametes bear S, and half bear s. The
hybrid plants are HETEROZYGOUS.
The real difference then between the F. hybrids (Ss) and the
pure dominants (SS) is that the former are heterozygous and the
latter are homozygous. In order to tell which is which, since they
are phenotypically the same, it is necessary to breed them. When
self-fertilization can be practiced, as in the case of most plants,
292 ANIMAL BIOLOGY
we get the result directly; that is an individual’s progeny are either
all dominants or dominants and recessives in 3 : | ratio, and thus
the gametic constitution of the parent is immediately known.
However, in the case of animals, where self-fertilization is impos-
sible, the determination can be made by mating the dominants
with recessives; for a homozygous (pure) dominant then will give
all dominants, while a heterozygous (hybrid) dominant will give
half dominants and half recessives. Thus:
Gametes = p< xi
Gametes = d d d d
Possible zygotes = 100% Dd 50% Dd+50% dd
So far we have considered inheritance in MONOHYBRIDS, that is
cases involving one pair of alternative characters that can be
interpreted as the resultant of one pair of genes termed AL-
LELOMORPHS, but now we proceed to cases where two, three,
or more pairs of genes are concerned, known as DIHYBRIDS, TRIHY-
BRIDS, etc.
2. Dihybrids
Mendel investigated inheritance in dihybrids by crossing, for
example, a Pea producing yellow round seeds with one producing
green wrinkled seeds. The plants in the F; generations bear only
yellow round seeds, and therefore yellow and round are each
dominant characters when paired with green and wrinkled. After
self-fertilization such hybrid plants produce offspring (F2) with
seeds showing all the possible combinations of the four characters,
and in the ratio of 9 yellow round to 3 yellow wrinkled to.3 green
round to | green wrinkled. (Fig. 183A.)
This logically can only be interpreted as indicating that one of
the original parent plants bore gametes all containing the genes
for yellow and for round peas (YR), while the other parent plant
bore gametes all containing the genes for green and for wrinkled
(yr). Such being the case, the resulting zygote is YRyr, and the
hybrid which it forms develops gametes with all the possible com-
binations of these genes (except, of course, Rr and Yy) which are
YR, Yr, yR, and yr — there is an INDEPENDENT ASSORTMENT of
the genes as evidenced by the new combinations Yr and yR. Now,
INHERITANCE 293
in turn, at fertilization there are sixteen possible combinations of
gametes. since there are four different kinds of sperm and four dif-
ferent kinds of eggs
with respect to the
characters in question.
Accordingly the F»
generation, which is
produced by the union
of these gametes, is
represented by one
pure dominant
(YRYR), one pure re-
cessive (yryr), four
homozygotes — includ-
ing the two just men-
tioned, and twelve het-
erozygotes. These six-
teen individuals form
nine genotypes but,
since only the domi-
nant character is ex-
pressed when dominant
and recessive genes
combine, they are re-
solvable into four
phenotypes (YR, Yr,
yR, yr) in the ratio
PVives Lr so ylx : 1 yr.
Tins the 923 :3 : 1
Mendelian ratio for
two pairs of alterna-
tive characters is
merely the monohy-
brid 3:1 expanded.
Both rest on the same
fundamental assump-
tion that the genes for
alternative characters
Qi@
|
;
Fy
Fic. 183A. — Diagram of a Mendelian dihy-
brid. Results of crossing yellow round seeded
(YR) Peas with green wrinkled seeded (yr). The
circles represent the zygotes and the characters
of the soma (phenotype); the letters within the
circles, the germinal constitution (genotype). The
letter groups outside the recombination square
represent gametes. The hybrids of the F; gen-
eration are all yellow round seeded since green
and wrinkled are recessive. The F; plants form
four types of gametes which afford sixteen pos-
sible types of zygotes, representing four pheno-
types (shown graphically) and nine genotypes
(numbered). There is one pure dominant (1) and
one pure recessive (9). The zygotes numbered 4
are identical with the F; generation. Four are
homozygotes (1, 7, 8, 9) and the rest are hetero-
zygotes. Those numbered 7 and 8 are new homo-
zygous combinations resulting from independent
assortment. The relative number of individuals
composing the phenotypes is 9 :3:3: 1.
are segregated during gamete formation — both members of a pair
of allelomorphs never occur in the same gamete. (Fig. 183B.)
294. ANIMAL BIOLOGY
F,
9
F,
: (! rt Ke
pends ; A : ;
ay \y 4 ‘
8 asks Gln LiL
Fic. 183B. — Result of crossing pure smooth black with rough white Guinea-
pigs. Rough and biack (pigmented) are dominant. Note that each parent
(P) bears both a dominant and a recessive character. The IF, illustrates the
principle of independent assortment.
3. Trihybrids
Similarly, Mendelian trihybrids, for example the cross between
pure tall Peas bearing yellow round seeds and dwarfs bearing green
wrinkled seeds, or the cross between pure Guinea-pigs with long,
rough, black hair and those with short, smooth, white hair give
in the F. generation 27 genotypes and 8 phenotypes; the number
of individuals in the phenotypes being in the ratio 27:9:9:9:
3:3:3:1. Of course, in nature there are few instances in which
parents and offspring differ by only one, two, or three characters,
but since characters arising from each pair of allelomorphs can
usually be treated singly, convenience demands that the analysis
be made with respect to one or two pairs at a time, which there-
fore is the usual method of procedure. (Figs. 184, 185.)
4. Summary
Before passing to certain extensions of these established hered-
itary principles, it may serve to clarify the subject if we restate
INHERITANCE 295
Tall Dw
Yell x eer arf
Round | US led =
4 R
ase els
aleelaelalnle
. SSOOQOOO
- GeQeaQoe@el ,
Sqooodeace
- Gla@leleans
- S2QOOSHOO
- JeOODeOS
Fic. 184. — Diagram of a Mendelian trihybrid. Results of crossing tall
Peas bearing yellow round seeds (SYR) with dwarf Peas bearing green wrinkled
seeds (syr). The circles represent the zygotes and the characters of the soma
(phenotype); the letters within the circles, the germinal constitution (geno-
type). The letter groups outside the recombination square represent the gam-
etes. The F, hybrids form eight types of gametes, giving sixty-four pos-
sible types of zygotes, representing eight phenotypes (shown graphically) and
twenty-seven genotypes. There is one pure dominant (in upper left corner)
and one recessive (in lower right corner). Eight are homozygotes (diagonal
from upper left to lower right corner) and the rest are heterozygotes. The
zygotes in the diagonal from upper right to lower left are identical with the
F, generation. The relative number of individuals composing the phenotypes
VST flees! CL! ELS ies Tab ides aaa I
296 ANIMAL BIOLOGY
it in slightly different form and thus emphasize the essential facts
thus far discussed chiefly on the basis of Mendel’s own work.
Every cell of the soma of an individual may be regarded as
bearing a pair of genes for each alternative character (e.g., size in
ete
ae
f Z
Boss Wie
ay spr
Fic. 185.— The eight phenotypically different kinds of Guinea-pigs in
the F, generation of a trihybrid. S = short hair, s = long hair, P = pig-
mented coat, p = non-pigmented coat or albino, R = rough coat, r = smooth
coat. The hybrid parents (F,) were phenotypically SPR.
the case of the garden Pea), one member of each pair having been
derived from each gamete which contributed to the individual’s
make-up. When both genes are identical (e.g., either SS or ss)
they are expressed in the soma (e.g., the plant is tall or dwarf).
The individual is homozygous with respect to size. But when the
two genes are not identical (e.g., Ss), the one, the dominant (S),
INHERITANCE 297
is expressed in the soma (the plant is tall), while the other, the
recessive (s), is not expressed. The individual is heterozygous with
respect to the character in question (e.g., size).
After synapsis during the maturation of the germ cells of the
individual, segregation of the genes occurs with the result that each
gamete receives only one gene for each character. Thus the gametes
of homozygous individuals are all alike with respect to the gene in
question (e.g., all bear either S or s), while the gametes of hetero-
zygous individuals are of two numerically equal classes (e.g.,
50 per cent bear S and 50 per cent bear s).
Finally, there is an independent assortment of the genes for dif-
ferent characters, as evidenced by new combinations of characters
in the progeny of dihybrids, etc. For example, size and color are
independently inherited. This depends, as we shall see later, upon
the genes involved being in different pairs of chromosomes.
The principles of segregation and independent assortment are
usually known as Mendel’s laws.
C. ALTERATIONS OF MENDELIAN RATIOS
The immense amount of experimental breeding that has been
carried on since Mendel’s time has accentuated the significance of
the principles of segregation and independent assortment, but has
revealed that dominance is by no means universal. A few examples
will bring the main facts before us.
The seven pairs of alternative characters in Peas which Mendel
studied showed essentially complete dominance of one character
in each pair, but we now know a great many cases in which the
hybrid (F;) shows a different condition from either of the parents.
For instance, on crossing homozygous red and white races of the
Four-o’clock, all the progeny in the heterozygous (F,) generation
bear pink flowers, or, we may say, flowers intermediate in color
between the two parents. Neither red nor white is dominant: the
result is BLENDING inheritance. But inbreeding the hybrids gives a
F, of | red : 2 pink : 1 white. Thus the typical Mendelian 3 : | ratio
is, So to speak, automatically resolved into the 1 : 2 : 1 ratio which,
when one character is dominant, is evident only on further breed-
ing. Segregation actually occurs as usual, because the homozygous
progeny of the hybrid exhibit the original parental characters
unmodified. (Fig. 186.)
In certain other cases, the hybrids, instead of being true inter-
298 ANIMAL BIOLOGY
mediates, really exhibit the characters of both parents: neither
character is recessive. Thus in Shorthorn cattle, red and white
when mated give roan, a color effect resulting from a close inter-
mingling, or MOSAIC, of red and white hairs in the coat. Accordingly
roan Shorthorns are always heterozygous, but their offspring give
Fic. 186. — Diagram to illustrate the results from crossing white and red
flowered races of Four-o’clocks, Mirabilis jalapa. The somatic condition
(phenotype) is shown graphically; the small circles represent the genes which
are involved.
the expected ratio of 1 red : Z roan : 1 white which is clear evidence
of segregation. Another example is the well-known blue Andalusian
fowl. This will not breed true — it is a hybrid in which the charac-
ters of both parents are exhibited, apparently not blending though
giving a somewhat intermediate effect. Its offspring show the
ratio of 1 black :2 blue Andalusian : 1 white-splashed-with-blue.
In order to obtain all blue Andalusians — the type recognized by
poultrymen — it is necessary to mate black with white-splashed-
INHERITANCE 299
with-blue birds. So again it is clear that segregation is involved, as
it is in innumerable instances where no sharp distinction can be
made between complete and incomplete dominance. (Fig. 187.)
Illustrations of some of the complications are afforded by cases
of blending inheritance that result from the CUMULATIVE action of
Le
-Ruchard- Edes’Harrisons
Fic. 187. — Cross of white-splashed-with-blue and black fowls, giving in
the F,; all blue Andalusians, and in the F2 one white-splashed-with-blue to two
blue Andalusians to one black.
several pairs of genes (MULTIPLE GENES) as in the cross of white and
black human races. The mulatto (F;), from a cross between an
individual homozygous for white and an individual homozygous
for black, is intermediate in skin color between the parental types,
and in the F. and later generations gives a series of gradations
between white and black but rarely pure white or black offspring.
Assuming that three pairs of genes for color are involved, the
genetic constitution of the black race may be represented by
AABBCC and that of the white race by aabbcc. Accordingly the
hybrid, or mulatto, has the genetic constitution AaBbCe and is
intermediate in color since only half of the genes for black pig-
300 ANIMAL BIOLOGY
mentation are present. Furthermore, the progeny (F2) of mulattoes
show different degrees of color ranging from pure white to pure
black owing to the sixty-four possible recombinations of genes
aE
ae | SE | | | HE
ee ae |e | | | FE
Fic. 188. — The distribution
of the sixty-four possible recom-
binations in the F. generation
when three pairs of similar genes,
cumulative in action, produce a
character; e.g., the offspring of
mulattoes. The figures indicate
the number of genes (e.g., for
black) present in the individ-
uals represented by the column
above. (From Walter.)
according to the trihybrid formula:
the more ‘black’ genes present, the
darker the pigmentation of the skin.
The infrequent appearance of pure
whites or blacks in the F, generation
is because the chances are slight that,
through segregation and independent
assortment, all the separate genes for
black or white will be brought to-
gether in a single gamete and, fur-
ther, that such a gamete at fertiliza-
tion will meet another similarly en-
dowed. (Fig. 188.)
From these few examples, selected
almost at random from the wealth
of data at hand, it is apparent that
the various types of inheritance can
be satisfactorily interpreted on the
fundamental principles of segregation
and independent assortment. It is
merely necessary to bear in mind
that it is the genes, which condition
the development of characters, and
not the characters themselves that
behave as units; for now we know
that most characters are determined
by multiple genes, and that even in
cases where one gene seems to pro-
duce a character, other MODIFYING
genes, COMPLEMENTARY genes, etc.,
may affect its development. Genes
are units in inheritance but are not
units in development.
Within the past few years genet-
icists have been able by the multiple
gene, modifying gene, and similar
concepts to bring the inheritance of
INHERITANCE 301
a large number of characters into line with the principles pre-
sented. Thus stature, proportions of the parts of the body, build,
as well as nearly all of the physiological and mental characteristics
in Man are evidently dependent upon multiple genes. In certain
Fruit Flies eye color may be influenced by more than forty pairs
of genes and the wings by upward of ninety. Thus it is becoming
increasingly clear that what a given gene will produce is determined
by the constitution of the gene plus its interaction with many,
if not all of the other genes of the complex, although, of course,
as we have seen, the single gene pair, in many or most cases, does
have its most conspicuous effect on a certain character of the
organism. And, furthermore, what the gene complex will produce
bears an intimate relationship to the environment. For instance,
in Fruit Flies the abnormal condition of extra legs is inherited in
typical Mendelian manner when the flies are reared at a low
temperature; whereas supernumerary legs do not appear in flies
with the same gene heritage when bred at a higher temperature.
In short, the environment, in certain cases at least, may act as
a differential intensifying or diminishing gene action.
So it happens, as is usually the case, the more a problem is stud-
ied the more complex it appears to become. Suffice it to say that
although our knowledge of inheritance is to-day very much broader
than Mendel conceived on the basis of his classic experiments, it
is evident that he supplied us with basic principles which are
affording a common denominator for an ever-increasing number of
facts in genetics.
D. MEcHANISM OF INHERITANCE
With this general outline of genetic principles before us, it is
now necessary to bring them into relation with the facts so far
discovered in regard to the structure of the germ cells. In other
words, we have assumed, on the basis of the experimental results
derived from breeding plants and animals, the existence of genes,
the occurrence of segregation, etc., but has the actual study of
cells (CYTOLOGY) by means of the microscope given any evidence
of the physical basis of genes and of a segregating mechanism?
The reader will at once answer this in the affirmative on the basis
of our discussion of the origin and structure of the germ cells and
their behavior in fertilization. Accordingly the essential facts may
now be restated from this viewpoint. (Fig. 165.)
302 ANIMAL BIOLOGY
The egg and sperm each carry a definite number of chromosomes
and consequently after fertilization the zygote contains a double
set. For each chromosome contributed by the sperm there is a
corresponding, or HOMOLOGOUS, chromosome contributed by the
egg. In other words, there are two chromosomes of each kind
which may be considered as pairs. When division of the zygote
takes place each chromosome splits into two chromosomes, so that
each daughter cell receives a daughter chromosome derived from
each of the original ones. Since all the cells of the organism are
lineal descendants by similar mitotic cell divisions, all of its cells
contain the double set of chromosomes — half paternal and half
maternal; and since the primordial germ cells have a similar origin,
they also have a double set of chromosomes. But during the mat-
uration process SYNAPSIS occurs: that is, homologous chromosomes
of paternal and maternal origin unite in pairs — the process of
fertilization which gave rise to the individual being consummated
in the ripening of its own germ cells. But this union is only tem-
porary; during the maturation process the maternal and paternal
chromosomes of each synaptic pair are separated and one of each
(though very rarely all of the same maternal or paternal set) passes
to the daughter cells — segregation occurs. Thus each mature
germ cell, or gamete, contains one member of every chromosome
pair and the number of chromosomes is reduced one-half. (Figs.
167, 189.)
It has been assumed that the genes for alternative characters
segregate in the formation of the gametes of hybrids so that a single
gamete bears one and not both genes of a pair of allelomorphs.
That is'the genes, which come together in the zygote that forms
the hybrid, separate again in the formation of its own gametes.
This is just what cytological studies show. Chromosome behavior
exactly parallels the typical behavior of the ‘Mendelian’ gene,
because after synapsis, during spermatogenesis and oogenesis,
each chromosome of paternal origin separates from the correspond-
ing chromosome of maternal origin. Moreover, since the genes
similarly situated on homologous maternal and paternal chromo-
somes are HOMOLOGOUS GENES, or allelomorphs, it follows that
homologous genes are segregated in separate gametes during mat-
uration — the two members of a pair of allelomorphs pass to
different gametes. This is the basis of the so-called purity of the
gametes.
INHERITANCE 303
Furthermore, in considering dihybrids we found, for instance,
that genes for yellow and round, and green and wrinkled seeds
were inherited in a fashion which indicated that yellow and round
are segregated independently of each other — there is an inde-
pendent assortment because all possible combinations with green
and wrinkled occur. This clearly is fully accounted for, provided
the gene for color and the gene for form are not in the same pair of
Body Primordial
Cells Germ Cells
Homologous
Chromosomes Synapsis
Paired
Ge AB aB
Ge ec D cD
AB aB aB
cd ae ae Grd ed
the union of haploid groups of either the
chromosomes or of the genes of the gametes to form the diploid condition of
the zygote, body cells, and primordial germ cells. Finally their pairing at
synapsis, and segregation in the gametes. With four pairs of chromosomes or
of genes (Aa, Bb, Cc, Dd) there are sixteen possible types of gametes.
chromosomes. Moreover, following synapsis the gametes secure
one of each pair of homologous chromosomes (a haploid group),
but not necessarily — indeed very rarely — all of maternal or
paternal origin. (Figs. 183A, 189.)
In short, when two gametes unite, each contributes to the zygote
a homologous haploid group of genes with the result that the off-
spring is of diploid gene constitution. Similarly, each gamete
contributes a homologous haploid chromosome group so that the
zygote is of diploid chromosome constitution. Thus both the
chromosomes and the genes (characters) are in the haploid condi-
304 ANIMAL BIOLOGY
tion in the gametes and the diploid in the zygote. This close paral-
lelism of character and chromosome behavior affords further
proof that the chromosomes through their constituent genes de-
termine the physical basis of inheritance, and that segregation
and related phenomena are facts. For all practical purposes,
A, B, C, D, and a, b, c, d, in Figures 167 and 189 may be inter-
preted either as chromosomes or as characters (genes).
Turning now to the inheritance of characters whose genes are
borne by the same chromosome: these would seem to be indissolv-
O
°O Zygote, O
Spernt \"O
ees ag 1) \ Maturation
@ Hit 1
Spermatogonium ,
‘Mature Egg He)
Fic. 190. — Diagram to show the relation of the two classes of sperm in
fertilization. The formation of gametes in the male is shown at the left, in the
female at the right; fertilization, producing the male or female zygote, in the
center. Somatic chromosome number assumed to be six. X chromosome
(large) and Y chromosome (small) in black.
ably linked together. And since the chromosome number is usually
not large — there are twenty-four chromosomes in the gametes of
Man — compared with that of heritable characters, we would
expect sometimes to find characters linked together. That is, not
separately inherited, or independently assorted, as are yellow and
round in our example. In reality many cases are known in which
characters are usually inherited together. The inheritance of sex
and sex-linked characters will make the main point clear, and at
the same time serve to bring before us the essential facts in regard
to the determination of sex.
1. Sex Determination
Intensive studies of the chromosomes in the somatic cells and
in the germ cells before the maturation divisions have shown that
INHERITANCE 305
usually in male animals two members of a certain pair of homol-
ogous chromosomes (synaptic mates) differ recognizably one
from the other in size or form or both. For example, one mem-
ber, referred to as the X chromosome, may be similar in size to
the other chromosomes, while its mate, called the Y chromo-
some, may be atypical in form, or much
smaller or, indeed, not present at all in
certain species. Furthermore, it has been
found that in corresponding cells of
females there are two X chromosomes.
Thus the chromosome groups of males
possess an X—Y pair, and those of fe-
males, an X—X pair. This difference
between the chromosomes of the sexes
naturally suggests that the X chromo-
some bears essential determiners for sex,
and this appears to be the case. (Fig. 190).
During spermatogenesis the matura-
tion divisions following synapsis segre-
gate the synaptic mates (X—Y) in the
regular way, so that two classes of
sperm result: half the sperm bear the X
chromosome and half bear the Y chro-
mosome. Furthermore, in odgenesis the
‘ ar ae Fic. 191. — A, Fruit Fly,
maturation divisions distribute an X Drosophila, the study of
chromosome (from the X—X pair) to which has contributed greatly
each cell, and accordingly every egg iets seg bans
hears an chromosome... ‘Uhustin Wan male and female: the lower
the somatic number of chromosomes is_ Pair in the female is the X-X,
48; males having 46 plus an X—Y pair, Aa es pea:
and females 46 plus an X—X pair. Half
the sperm bear 23 plus X, and half bear 23 plus Y; while all the
eggs bear 23 plus X. (Figs. 191, 193.)
Since there are equal numbers of sperm with and without the X
chromosome, on the average as many eggs will be fertilized by one
class of sperm as the other, with the result that half of the zygotes
will contain one X and half two X chromosomes. The former will
develop into males and the latter into females, since the somatic
cells of males have one X chromosome and similar cells of females
have two.
306 ANIMAL BIOLOGY
So it seems clear that sex is typically determined in many
animals, including Man, at the time of fertilization by the same
fundamental mechanism that controls inheritance in general.
Moreover, multiple genes are involved. The decision is given
by certain genes in the X chromosomes, acting in connection with
genes in other chromosomes. The genes in the X chromosomes
J\\ «|\ fe
x xX
A B C
Fic. 192. — Influence of the balance between X chromosomes and the other
chromosomes on sex in Drosophila. A, “super female’; B, ‘intersex’; C, ‘super
male.’ See Figs. 191, 197.
MC ‘ ‘ Y
vo
turn the balance under the usual conditions of development so
that a series of processes is initiated, involving the action and
interaction of hormones, nutritional factors, and various envi-
ronmental conditions, that lead to the sex differentiation of the
adult.
But many unusual cases, some normal and others abnormal,
occur particularly among the lower animals. Thus it has been
found that INTERSEXES, indi-
IL S907 5259 viduals exhibiting varying de-
CU fice 3) 9 grees of male and female
characters, may result from
abnormal sets of chromosomes
in which the balance between
B 7
Tied ART cocen atest the X and the other chromo-
nium with 48 chromosomes; B, chromo- SOMEesS is upset, as is well il-
somes arranged to show the 24 pairs of lustrated by studies on Dro-
synaptic mates. The X—Y pair is at sophila. Or hormones may
lower right. (From Painter.) Lave :
have a modifying influence.
Thus a male twin in cattle may render abnormal the develop-
ment of its female twin in the uterus, so that a sterile ‘free-martin ’
results. Or still again, SEX REVERSAL may occur through environ-
mental factors acting on the early embryo, as in the case of the
Frog. And finally, the sex of the adult may change, sometimes
8098 Pe cere ec ee
CCL CCee ceet gs
INHERITANCE 307
periodically as in the Oyster. Such marked departures from typical
sex differentiation serve to emphasize that the final establishment
of sex is indeed a resultant of many complex factors. (Fig. 192.)
2. Linkage
Since the basic mechanism that regulates sex is the same
as that which determines the distribution of other characters
P |&J} <Oy XY XX
9 3 9
©)
es @)
? Or
¥ XQ
an
nop > ©> GB xXx HX XY XY
Q Q 3} 3 2 2 3 ze
Fic. 194. — Diagram to show the inheritance of color-blindness from the
male. A color-blind man (shown in black) transmits the character through his
daughters (carriers) to half of his grandsons. % indicates the X chromosome
with the gene for color-blindness.
in inheritance, it might be supposed that the genes of other
characters as well are carried in the X chromosome. As
a matter of fact the behavior in inheritance of certain characters is
such that it can only reasonably be explained on this assumption.
Accordingly such characters are known as SEX-LINKED. This brings
us again to the point at which we digressed to consider sex — the
discussion of genes associated in the same chromosome.
The best known examples of sex-linked characters in Man are
COLOR-BLINDNESS in which the affected individual is unable to
distinguish red from green, and HEMOPHILIA in which the individ-
ual’s blood has little tendency to clot so that bleeding from even
a slight wound may be serious. Both abnormalities have long been
known to be inheritable, and in the same peculiar criss-cross way.
Thus color-blindness is usually transmitted from a color-blind man
308 ANIMAL BIOLOGY
through his daughters, who are normal, to half of his grandsons;
and from a color-blind woman to all of her sons and none of her
daughters. When both parents are color-blind, all the children
show the defect. This behavior is readily accounted for if we as-
sume that the gene for color-blindness when present is associated
with the factors for sex on the X chromosome, and that color-
i blindness develops in males
|
when it is received from one
I Ila IIc
parent, and develops in fe-
males when it is received from
both parents. Thus a color-
Fic. 195. — Diagram showing a pos-
sible mechanism of crossing-over during
synapsis of homologous paternal and
blind man is always hetero-
zygous for the character, while
maternal chromosomes. ‘The segments
indicate the assumed linear arrange-
a color-blind woman is ho-
ment of the genes with allelomorphic
C—_ Ea Biexunmeana BES
It
mozygous. A woman who is
heterozygous has normal vi-
genes opposite each other. I, pair of
chromosomes which have entered and
sion, but is a ‘carrier,’ that is,
produces gametes half of which
emerged from the synaptic state with-
out any crossing-over; Ila, chromo-
carry the gene for color-blind-
somes winding about each other at
ness. It is obvious why color-
blindness is very much less fre-
synapsis; IIb, separation of these chro-
mosomes, involving breaking at the
points of crossing; IIc, their emergence
quent in women than in men.
(Fig. 194.)
Color-blindness and hemo-
philia thus serve to illustrate
the association of genes of
different characters in the
same chromosome and_ the
from synapsis with the members of the
pairs of allelomorphic genes inter- association later of their re-
Se te spective characters in the
adult — independent assortment does not occur. Incidentally,
it also clearly shows that whatever characters are borne by the
X chromosome are not transmitted by a father to his sons, and so
perhaps minimizes, from the standpoint of heredity, the importance
usually ascribed to descent in the direct male line.
3. Crossing-over
However, the presence of different genes in the same chromo-
some — we know that chromosomes are really linkage groups of
genes — by no means indicates that such genes must always be
INHERITANCE 309
distributed together. Thus during synapsis homologous genes
(allelomorphs) often reciprocally CrRoss-ovER from one synaptic
mate to the other, and so become separated from their former gene
associates in the same chromosome. When such occurs the ex-
changed genes are segregated independently of their former asso-
ciates in the same chromosome, and accordingly a greatly in-
creased flexibility is afforded the genetic mechanism. (Fig. 195.)
13.7
"
!
cv
Fic. 196. — Cyto-genetic map. Terminal portion of the X chromosome
from the salivary gland of Drosophila, with the locations of some of the
genes indicated. ac, achaete; br, broad; cv, cross-veinless; ec, echinus; fa, facet;
N, notch; pn, prune; rb, ruby; sc, scute; w, white; y, yellow. (After Painter.)
In addition to its great importance in bringing about genetic
change, crossing-over affords the geneticist an opportunity to
determine the relative positions of different genes in a chromosome.
It has been found that the distance between two genes in a chromo-
some is, in general, proportional to the percentage of crossing-over
which occurs between these genes at synapsis — the longer the
distance, the more likely is crossing-over to occur. Accordingly,
in very extensive breeding experiments it is possible to construct
so-called chromosome maps by plotting the relative positions of
the various genes in the chromosomes. In the case of Drosophila
the genes for more than six hundred characters have already been
mapped. (Fig. 196.)
4. Mutations
But the possibilities of genetic change are not necessarily limited
by the typical chromosome groups or to crossing-over that usually
310 ANIMAL BIOLOGY
afford the material for recombinations. Not infrequently relatively
radical alterations, or MUTATIONS, occur, usually just before or
during the maturation of the germ cells, which thereupon are at
the disposal of the usual genetic mechanism for segregation, etc.
Mutations may be broadly classified as chromosomal aberra-
tions and intrinsic changes in the individual gene. Indeed some
geneticists restrict the term mutation to gene changes. A few
illustrations from the wealth of available data must suffice.
CHROMOSOMAL ABERRATIONS consist of departures from the
normal number and arrangement of the chromosomes, and of
their parts. Thus many cases are known in which the whole nor-
mal chromosome set (diploid number) has been increased to some
multiple of the typical haploid number. Such symmetrical changes
in the chromosome groups are termed PLOIDY, and appear to be
more frequent in plants than in animals. Thus the haploid sets of
three well-known varieties of Wheat consist of 7, 14, and 21 chro-
mosomes respectively.
Some marked instances of the origin of new types have recently
been observed in the Jimson weed (Datura). In one case the plants
differ in size and several other characters that clearly are attribut-
able to the doubling of the diploid number of chromosomes. They
have 24 pairs of chromosomes instead of the typical 12 pairs.
Equally interesting are certain observations on Drosophila which
typically has 4 pairs of chromosomes. Individuals have appeared
that differ markedly from the normal flies and possess 12 chromo-
somes as a result of the addition of another haploid set.
Another type of chromosome mutation, known as HETERO-
PLOIDY, involves only one, or rarely two, chromosomes, and not an
entire set. Thus again in Drosophila, the failure of the X chro-
mosomes to separate after synapsis (NON-DISJUNCTION) gives rise
to individuals with nine chromosomes, including one Y and two X
chromosomes. This may be regarded as a ‘male’ complement of
chromosomes plus another X chromosome, with the result that
flies so endowed are females with an altered heredity visible chiefly
as larger size. (Fig. 197.)
Still other irregularities may involve only a portion of one chro-
mosome. Thus part of a chromosome may be lost, or duplicated,
or attached to another chromosome. For example, a race of
Drosophila has been obtained in which a part of the Y chromo-
some has become transferred (TRANSLOCATED) to the end of an X
INHERITANCE dll
the X chromo-
some in Dro-
sophila as a
result of non-
chromosome; and another race in which one of the X chromosomes
has been broken into two nearly equal parts — one part being
united to one of the other chromosomes. Moreover, recent work
emphasizes the significance of the order of arrangement of the
genes in a chromosome; alterations of the usual order resulting in
hereditary changes referred to as POSITION EFFECTS.
All such chromosomal aberrations, rare and radical as most of
them are, may be regarded, in a way, as a broad extension of the
principal of recombination. They exert their influ-
ence by new relations and proportions of the genetic Py) CG
material, and offer new hereditary possibilities in the NG
event that they are not lethal. fl? 0d.
GENE MUTATIONS apparently involve intrinsic al- J “
terations in the individual genes themselves or even at eg a
var uplication of
the origin of new genes, and probably are the most
significant changes in the hereditary complex. They
presumably are a result of an alteration in the
physico-chemical constitution of the gene; although disjunction.
some would assign them mostly to position effects. See Fig. 191.
It appears that usually only one of a pair of homol- eos
ogous genes mutates at a given time so the change ,
is extremely localized, and most frequently takes place just be-
fore or during maturation. Although gene mutations occur rel-
atively rarely, several hundred examples have been identified
in Drosophila, chiefly by the elaborate studies of Morgan and his
collaborators — studies involving upward of 25 million fruit flies.
‘These experiments have made this tiny insect the greatest contribu-
tor to our knowledge of genetics since Mendel’s experiments with
peas, and justified the award of a Nobel Prize to Professor Morgan.
Perhaps the best-known example of gene mutation in carefully
pedigreed animals is the sudden appearance, at long intervals, of
a single white-eyed Drosophila in a true-breeding red-eyed stock.
The white-eyed mutant breeds true from its origin, and the genetic
data indicate that a specific point on one chromosome — it has
been mapped — suddenly changed so that the developmental
processes that formerly gave rise to the usual red eyes thereafter
produced white eyes. And similar evidence of so-called ‘point
changes’ has been obtained in a number of other kinds of animals
and in plants that have been bred under controlled experimental
conditions.
312 ANIMAL BIOLOGY
Such are a few of the types of mutations in the nuclear complex
of the germ cells that we now know give rise to genetic variations.
Chromosomal aberrations afford new relations and proportions
of their constituent genes, whereas gene mutations actually deter-
mine the nature of the chromosomal elements themselves. In
many instances, probably the majority, mutations produce lethal
combinations in the gametes or zygotes — the altered hereditary
constitution renders development or survival impossible. In other
cases, individuals with the mutant characters become established
and produce offspring with these new characters and so supply the
material for descent with change.
Finally, in passing, it should be mentioned that mutations may
also occur in somatic cells. Such SOMATIC MUTATIONS give rise
to changes in the individual body which, of course, cannot be
transmitted by the germ cells, but may be perpetuated by vegeta-
tive reproduction. Thus many desirable types of fruit are produced
solely by tissue, descended from the original plants in which the
mutations occurred, that has been grafted on other plants.
Important as is the recently acquired knowledge of some of the
nuclear changes at the basis of mutations, we still do not know
the fundamental factors underlying their origin. However, recent
experiments have afforded a most valuable clue. It has been found
possible to induce mutations in certain animals and plants by sub-
jecting their germ cells to unusual conditions — the most effective
so far employed being irradiations (X-rays, etc.) and certain tem-
perature changes. Thus since mutations can be induced by con-
trolled and measured external agents, the way seems to be open-
ing for an experimental attack on the problem of the origin of
mutations in nature that apparently is at the basis of organic
evolution.
E. Nature AND NURTURE
Even after making due allowance for the possibilities of genetic
change involved in mutations, the individual still may be considered
as a composite of very many characters which usually behave in
a definite way in inheritance. Expressed somewhat fancifully,
individuals may be regarded as temporary kaleidoscopic recom-
binations of the various genes belonging to the species; the act of
reproduction, especially the maturation divisions, involving segre-
INHERITANCE ols
gation, and subsequent fertilization, providing the new turn of the
kaleidoscope.
But since the life of an organism is one continuous series of re-
actions with its surroundings, it follows that nurture plays an
immensely important part in molding the individual on the basis
of its heritage. Indeed we are apt to overlook the fact, already
mentioned, that every character is a product both of factors of the
—
3
. 2 Be ae Bib Tl is REO ca
Fic. 198. — Corn of a single variety (Leaming dent): at the left, grown well
spaced: at the right, badly crowded. The heredity of each piot ef corn is
the same; the striking differences in growth are therefore solely due to en-
vironment. They are modifications. (From Blakeslee.)
heritage and of the environment and can be reproduced only when
both are present. Those characters that appear regularly in suc-
cessive generations are those whose development depends upon
factors always present in the normal surroundings. Other charac-
ters, potentially present, do not become realized unless the un-
usual environmental conditions necessary for their development
happen to be met. Heredity and environment are collaborating
artists with different rdles to play as molders of the individual.
To disentangle the closely interwoven influences of heredity and
314 ANIMAL BIOLOGY
environment is one of the most important and perplexing problems
of the science of genetics. This is especially true in the case of
Man. Development is a form of behavior, and how a child develops
physically and mentally is determined not by its heritage alone nor
by its environing conditions alone, but by both in intricate com-
bination. (Fig. 198.)
Although apparently we do not inherit the effects on our fore-
bears of their surroundings and training, nevertheless we are the
m= heirs to their customs — each gen-
ey. oN 74 ° . .
nS eens /~} eration builds upon the intellectual
\ ~S = / \ A yo e .
= ee \-« ~~ / and material foundations of all of
X SS ab aa x ee / . . .
\ ee Ge / its predecessors — and this entails
wy KK U / ee
\ pe oe NS added responsibilities as well as
\ / c ~ ‘8
\ ip A seed Pe SS ne y yz iH e, @ .
ry eL ee opportunities for each succeeding
we “A generation. Already in certain
HERITAGE fields the applications of science to
Fic. 199. — Scheme to illustrate human affairs tax the ability of
the contributions of nature and M t th cal Th
nurture to the make-up of the in- | sk , eae Koon Se ae us
dividual. The triangles represent ‘social heredity’ bids fair to out-
various types of individuals which strip our conservative and essen-
are 7 Lh PSs to eos tially unchanging inherited nature.
training are variable. The foun- —The EUTHENIST emphasizes nur-
dation of the ‘triangle of life’ is ture, the EUGENIST emphasizes na-
heritage. (After Conklin.) 3
ture. As is so often the case, how-
ever, when doctrines are opposed, the truth combines both; though
we cannot doubt, knowing what we know of the genetic constitu-
tion of organisms, that from the standpoint of permanent advance
— racial rather than individual — the path to progress is chiefly
through EUGENICS, the science of being well born. (Fig. 199.)
This distinction between heritage and acquirements leaves a
fatalistic impression in many minds, which to a slight extent is
justified. We cannot get away from inheritance. On the other
hand, although the organism changes slowly in its heritable or-
ganization, it is very modifiable individually; and this is Man’s
particular secret — to correct his internal organic inheritance by
what we may call his external heritage of material and spiritual
influences. It is therefore clear that the problem of human im-
provement has two aspects: in the first place, the effects of
culture on the individual which, though not inherited, are cum-
ulative from generation to generation through training; and
INHERITANCE 315
secondly, racial betterment through breeding the best. (Pages 420-—
425, 437, 444.)
Summarizing our survey of inheritance, in the first place, it is
evident that the basis of inheritance is in the germinal rather than
in the somatic constitution of the individual. A character to be
inherited must be represented by one or more genes in the germ
cells, although the environment is not unimportant in the develop-
ment of the character from the gene complex. Secondly, there is
no satisfactory evidence that modifications of the body, ‘acquired
characters,’ can be transferred from the body to the germ complex
and so be inherited. And thirdly, the germinal basis of characters,
genes, may be dealt with essentially as units. The chromosomes —
linkage groups of genes — undergo segregation and independent
assortment during the development of the gametes of an indi-
vidual, so that paternal and maternal contributions may be re-
adjusted in all the possible combinations. Finally, mutations
afford still further changes in the gene complex for distribution by
the genetic mechanism, and so provide crucial opportunities for
variation — for descent with change.
CHAPTER XXII
ORGANIC ADAPTATION
Every creature is a bundle of adaptations. Indeed, when we take
away the adaptations, what have we left? — Thomson and Geddes.
SINCE organisms are dependent for their life processes upon
energy liberated by physico-chemical processes in protoplasm, any
and all influences which induce changes in the structure or func-
tions of an organism must initially modify the underlying meta-
bolic phenomena. In other words, organic response is a problem
of metabolism. Although it is highly important that this cardi-
nal fact be clearly grasped, the science of biology to-day is not
in a position to interpret the responses of organisms in these
fundamental terms. Accordingly we can merely present some
representative instances to illustrate the fact that the response
of organisms, as exhibited in active adjustment — ADAPTATION —
of internal and external relations, overshadows in uniqueness all
other characteristics of life and at one stroke differentiates even
the simplest organism from the inorganic.
Overwhelmingly striking as is the fitness of organisms to their
physical surroundings, we must not lose sight of the fact that the
environment itself presents a reciprocal fitness. This results from
the “unique or nearly unique properties of water, carbonic
acid, the compounds of carbon, hydrogen, and oxygen. . . . No
other environment consisting of primary constituents made up of
other known elements, or lacking water and carbonic acid,
could possess a like number of fit characteristics, or in any
manner such great fitness to promote complexity, durability,
and active metabolism in the organic mechanism which we call
life.’ (Henderson. )
A. ADAPTATIONS TO THE PHysICAL ENVIRONMENT
In any consideration of the reciprocal relations which must
exist between organisms and their surroundings, of first importance
is the inconstancy of the latter. Uncertainty is the one certainty
in nature and accordingly the response of living things — their
316
ORGANIC ADAPTATION 317
adaptability to environmental change — is at once the most strik-
ing and indispensable adaptation.
1. Adaptations Essentially Functional
Although the changes of the environment are almost incon-
ceivably complex — witness the kaleidoscopic series of events ex-
hibited in the hay infusion microcosm — there are certain general
conditions which every environment must supply, and without
which life cannot exist. These are food, including water and oxygen,
and certain limits of temperature and pressure. (Fig. 18.)
Foop. As we know, food represents the stream of matter and
energy which is demanded for the metabolic processes of living
matter. And each and every element which forms an integral
part of protoplasm must be available. Since all protoplasm con-
sists chiefly of a dozen chemical elements, these, of course, must
be present; and further, since protoplasm is a colloidal complex in
which water plays a fundamental role, life processes without water
are impossible. But the old adage that what is food for one is
another’s poison has a broader significance than is immediately
apparent. Although it is true there are general ‘food-elements’
which all life demands, it is equally true that the combinations
in which these elements must be presented to the organism, in
order to be available for its metabolic processes, are subject to
the widest variation.
We have emphasized and contrasted the nutrition of a typical
animal, green plant, and colorless plant, and have seen the re-
ciprocal part which they play in the circulation of the elements
in nature; so it is hardly necessary, with these facts in mind, to cite
special cases in order to illustrate the adaptation of organisms to
special nutritional conditions. Perhaps the demands of the Yeasts,
that affect human life from so many angles, will suffice. (Fig. 17.)
The Yeasts include a host of microscopic colorless plants which
play an important part in the simplification of organic compounds.
An ounce of “brewers’ yeast”? contains about five billion cells.
Since they are devoid of chlorophyll, Yeast cells, of course, lack
photosynthetic powers, though like many other colorless plants
they are not dependent upon proteins for nitrogen but obtain it
in less complex form. But the essential fact of interest at present is
the chemical changes associated with Yeast metabolism — the
transformation of a large proportion of the sugar content of the
318 ANIMAL BIOLOGY
medium in which they live into alcohol and carbon dioxide. This
process of ALCOHOLIC FERMENTATION may be approximately ex-
pressed by the formula:
C.H1.0, (sugar) + Yeast = 2 C;H;OH (alcohol) + 2 CO,
The explanation is not far to seek. Deprived of an adequate sup-
ply of air, Yeasts resort to the energy released when, with the de-
composition of the sugar, the carbon and oxygen unite as COQ.
The formation of alcohol by the remnants of the sugar molecules
is, from the standpoint of the Yeasts, a mere incidental factor
which is, so to speak, unavoidable. On the other hand, from the
broad viewpoint, the waste products of the action of the Yeast
plants’ enzymes represent an important phase in the general
simplification of organic compounds in nature. And Man turns
to account in numerous ways both products of the Yeasts’ de-
structive powers — the alcohol and the carbon dioxide.
Thus the Yeasts are practically independent of free oxygen and
in this they agree with many kinds of Bacteria as well as some
animals, chiefly parasitic worms, which are able to secure the
necessary energy by the rearrangement of the atoms within, or
the disruption of molecules containing oxygen. Indeed, certain
species of Bacteria not only do not need free oxygen at all, but are
killed when it is present in any considerable amount. All such
organisms are termed ANAEROBES. A common example is Clos-
tridium tetant which inhabits garden soil and street dust and pro-
duces TETANUS, or lockjaw, in Man and certain domesticated ani-
mals when it gains entrance and develops in the tissues.
TEMPERATURE. Although protoplasmic activity is restricted to
ranges of temperature which do not seriously interfere with the
chemico-physical processes involved, it is a commonplace that
various species are adapted to different degrees of temperature.
The great majority of organisms, however, find their optimum
temperature between 20° C. and 40° C., though species inhabiting
the polar and tropical regions show adaptations to the temperature
extremes of their surroundings. Asa matter of fact, it is not possible
to state the upper and lower limits beyond which active life ceases,
but some Protozoa are known to multiply in the water of hot
springs, certainly at temperatures higher than 50° C., and others
in water until freezing actually occurs.
Many of the Bacteria and Protozoa develop protective cover-
ORGANIC ADAPTATION 319
ings and assume a resting condition in which the metabolic proc-
esses are reduced to the lowest terms. In this spore or encysted
state they are immune to extremes of temperature and of dry-
ness to which they succumb during active life. Thus some
Bacteria can withstand nearly — 200° C. for six months, and
about — 250° C. for shorter periods, while others can endure
140° C. for a short time. (Fig. 200.)
A BG
Fic. 200. — A-E, Bacillus biitschlii: A, cell structure; B, C, spore forma-
tion; D, E, germination of a spore; F, various types of spore formation occur-
ring among bacilli. (From Smith and others; A-E after Schaudinn.)
It is clear that the great majority of organisms are at the mercy
of environmental temperatures. This is true of all except the Birds
and Mammals. These homothermal animals possess a complex
mechanism which maintains their body temperature practically
constant; e.g., in Man at 37° C.
The heat regulatory mechanism represents, so to speak, the final
result of the assembling and elaborating, throughout Vertebrate
evolution, of elements that appear in the Fishes. In the Mammals
it comprises insulation by the skin, a closed blood vascular system,
power of rapid oxidation, endocrinal and other glandular products,
evaporation surface of the lungs and skin, ‘trophic’ and ‘tempera-
ture’ nerves, codrdinating centers, etc.,— the whole complex
rendering its possessors largely independent of the surrounding tem-
perature and making possible the carrying on of the various func-
tions with such nicety as the life of these forms demands. Indeed,
this mechanism makes it possible for a man to stay in a hot dry
chamber sufficiently long to see a chop cooked. It is hardly prob-
able that the human brain could have developed to function as it
does if its cells were subject to wide temperature variations.
320 ANIMAL BIOLOGY
Pressure. The metabolism of organisms, in common with
chemical processes in general, is influenced by the surrounding
mechanical pressure. Therefore it is evident that the pressure
of either the water or air plays an important part in the operation
of the life functions. We find organisms adapted to the greatest
depths of the ocean where the pressure is several tons to the
square inch —so great that some forms burst when rapidly
brought to the surface; while others are adapted to live at high
altitudes where the air pressure is relatively low. And again, the
higher Vertebrates present an adaptive mechanism which ren-
ders them less dependent on a constant atmospheric pressure.
These few examples must suffice to emphasize the general en-
vironmental conditions which are necessary for life, as we know
it, to exist, and to suggest that within these broad limits organisms
are adapted to special environmental conditions so that there is
scarcely a niche in nature untenanted.
2. Adaptations Essentially Structural
We may now broaden our view of the plasticity of organisms
by a brief consideration of adaptations which are essentially struc-
y
Zi
Z
GLE Za ==
A ee EE——
| WWE
y/ YL
Fic. 201. — Gymnura. (From Horsfield and Vigors.)
tural. But here as elsewhere it is absolutely impossible to distin-
guish sharply between structure and function which, obviously,
are only reciprocal aspects of the fitness of living creatures.
ADAPTIVE RADIATION OF MAmmats. In the group of Mammals,
forms are to be found which are extraordinarily modified in adapta-
tion to the most diverse environmental conditions. From a more
or less primitive type, or focus, there radiate, as it were, types
which are specialized for very different habitats and modes of
ORGANIC ADAPTATION 321
life. We may select from the Placental Mammals a small Malayan
insectivorous animal known as Gymnura, which is allied to the
Hedgehogs, as most similar among living Mammals to the gen-
eralized or focal type of terrestrial Mammal. Gymnura has rela-
tively short pentadacty! limbs with the entire palms and soles rest-
ing flat upon the ground (PLANTIGRADE) and therefore essentially
adapted for comparatively slow progression. (Figs. 201, 202.)
Ill. ARTIODACTYLA
iad
VIII. EDENTATA IX. PRIMATES,
Fic. 202. — Diagram of parallelism in evolution and adaptive radiation in
Mammals. I, Gymnura; II, Bear; III, Camel; IV, Badger; V, Anteater;
VI, Seal; VII, Dolphin; VIII, Sloth; IX, Gibbon; X, “Flying” Squirrel;
XI, Bat; XII, Kangaroo; XIII, Jerboa. (From Hegner, after Newman and
others.)
Radiating from this focus, adaptations for rapid running (cUR-
SORIAL adaptations) are chiefly evident in a lengthening of the
limbs. Thus, for example, in the Dogs, Foxes, and Wolves, the
effective limb length is increased by raising the wrist and heel
from the ground and walking merely upon the digits (DIGITIGRADE) ;
while in Antelopes, Horses, and hoofed runners in general, the
o22 ANIMAL BIOLOGY
chief limb bones themselves are lengthened, subsidiary ones are
suppressed, and the wrist and ankle are raised still further from
the ground, so that merely the tips of one or two digits of each
Fra. 203. — Foot postures of Mam-
mals. A, plantigrade; B, digitigrade;
C, unguligrade. (From Lull, after
Pander and D’ Alton.)
limb support the animal (UNGULI-
GRADE). Thus the typical cur-
sorial forms represent the culmi-
nation of Mammalian adaptation
to plains and steppes; regions in
which long distances must fre-
quently be traversed in quest of
food, and safety is to the swift.
(Fig. 203.)
Another line of adaptive radia-
tion is presented by the tree
dwellers: ARBOREAL forms which
make their own the world of
foliage high above the ground.
Such are, for instance, the Sloths
which are really tree climbers
that walk and sleep upside down
suspended from branches; the
tailless Apes that swing among
the boughs chiefly by their arms;
and the Squirrels that scamper
along the branches. Some Squir-
rels and the so-called Flying Le-
murs take long soaring leaps sup-
ported by wide folds of skin
between the sides of the body
and the extended limbs. But the
Mammals have not left the air
untenanted, for truly VOLANT
forms are represented by the
Bats in which the fore limbs with
greatly elongated fingers form
the framework of true wings.
(Figs. 204, 207.)
Passing below the surface of the
earth, FossoRIAL Mammals are
found such as the Woodchucks,
ORGANIC ADAPTATION 323
Gophers, and especially the Moles, which are adapted to a subter-
ranean existence by bodily modifications which facilitate digging.
r¥
TaN x hh \ |
nN
Fic. 204. — A Sloth, Choloepus, walking suspended from a branch.
(From Allen.)
\
\
Furthermore, the gap between terrestrial and aquatic Mammals is
bridged by the Muskrats, Beavers, Otters, and Seals which are
more or less equally at home on land and in the water. (Fig. 205.)
> AML ELEESS
= pl eg =
hf Ss =
~
Fic. 205. — Skeleton of a Mole, Talpa europaea. (From Pander and D’ Alton.)
The truly aquatic Mammals, such as the Porpoises and Whales,
have completely abandoned the land of their ancestors of the
geological past and to-day approach, in adaptations to a marine
Fic. 206. — Skeleton of a Porpoise. The vestigial pelvic bones are shown
imbedded in the flesh. (From Pander and D’Alton.)
life, the general contour of the primitiveiy adapted aquatic Ver-
tebrates, the Fishes. (Fig. 90, 91, 206.)
Thus the various lines of adaptive radiation of the Mammals from
a generalized terrestrial type, such as Gymnura, have provided
324 ANIMAL BIOLOGY
Mammals fitted for all sorts and conditions of the environment —
representatives are competing with members of other groups be-
neath, on, and above the earth and in the water. Somewhat sim-
Fic. 207. — A, ‘Flying Lemur,’ Galeopithecus volans; B, Bat, Vespertilio
noctula. (From Lull.)
ilar adaptive radiations are traceable in other animal groups, espe-
cially the Insects, though there seems no doubt that the adapta-
bility of the Mammalian stock — its potential of evolution — is in
Aquatic-Volant
Fossorial a
Fossorial-Volant ie e forms
en re Wate Cstriders
Vo eke
Ambula ry -Volant
oe
CNon-volant)
Ambulatory
CNon-volant)
Fic. 208. — Diagram of the adaptive radiation of a group of Insects, the
Hemiptera, or Bugs. (From Lindsey.)
no small degree responsible for the dominant position which the
Mammals hold in the animal world of to-day. Man is a Mammal.
(Figs. 85, 208.)
ANIMAL COLORATION. Perhaps the most generally striking
characteristic of organisms is their color and color pattern. Among
ORGANIC ADAPTATION 320
plants this applies chiefly to the flowers and fruit of the higher
forms, though here and there throughout the plant series the
typical green color is replaced or rendered inconspicuous by others.
But the absence of photosynthetic pigments in animals and their
relatively active life have permitted more latitude in body color,
and accordingly it is in the animal world that color adaptations
are more numerous and varied. Some colors and color patterns
are, of course, merely incidental to the chemical composition of
the whole or parts of the body. Others, however, irresistibly arouse
our interest and seem to demand a less simple explanation because
they are apparently of special service to their possessors. A few
examples will serve to bring the problem before us and indicate
the class of facts involved.
The color and color patterns of many animals are such that
they harmonize or fuse with the usual surroundings of the crea-
MEE EE
At EE ERS) >. ae
Tes ESTRELA é ce
— = Sle sm =a ia EN . Lis SE
re /> =
M Es Sar ] 4
= Se ee Kp Py 14—F =
SQA \
Sane — > N oo
Fic. 209. — The common green Katydid, Microcentrum. (From Riley.)
tures and render them practically indistinguishable from their im-
mediate environment. Every frequenter of the open knows innu-
merable instances. The song of the green Katydid readily guides
one to its immediate vicinity, but it is quite another matter to dis-
tinguish its leaf-green wings among the foliage of its retreat. Again,
one is attracted by the striking colors of an Underwing Moth while
in flight, but is at a loss to find the insect when scarlet or orange is
obscured by the overlapping, grayish-mottled fore-wings blending
with the tree trunk where it has come to rest. (Figs. 209, 210.)
The white of the Foxes, Hares, and Owls of alpine and arctic
regions; the green color of foliage-dwelling Toads and Frogs; the
tendency toward fawn and gray of desert Insects, Reptiles, Birds,
and Mammals; the olive upper surface of the bodies of brook
Fishes; the steel gray above and white below of sea Birds that
harmonize with sea and sky when viewed from above and below
respectively — the number of such cases is legion. (Gazelles living
326 | ANIMAL BIOLOGY
on the lava fields of volcanic regions are dark gray, while those
of the great stretches of sand plains are white — the same species |
exhibiting regional variations in color which blend with the sur-
roundings. Furthermore, the same individual may vary in color
with the seasonal changes in its environment, or present different
color schemes in different localities. Thus the summer coat colors
a Sots gig’ <a ¥
BE: y! vey A eS pte : m7
Fig. 210. — Underwing Moth. A, wings expanded, exposing the highly
colored hind wings; B, resting on bark. (From Folsom.)
of the Arctic Fox and the Weasel harmonize with the browns of
rocks; and the winter coat of white, with snow-clad nature. And
the Chameleons are by no means unique in their ability to change
color very rapidly in response to that of their immediate surround-
ings. (Fig. 81.)
But confusion is worse confounded when to harmonizing color
is added harmonizing form, striking examples of which are the
Leaf Butterflies of the East Indian region, the familiar Walking-
ORGANIC ADAPTATION 327
sticks, and the caterpillars (‘inch-worms’) of Geometrid Moths.
(Figs. 211, 212.)
Although the general tendency in nature is for sympathetic col-
oration — indeed, it is frequently possible to infer from the color
of an animal its habitat — there are numerous cases in which
the colors and color schemes seem to be in striking contrast with
the animal’s usual background. Sometimes, however, the con-
trast which is so striking with the bird in the hand, proves to be
( Harrison
Fic. 211. — Leaf Butterfly, Kallima.
obliterative with the bird in the bush —a conspicuous color pat-
tern, expressing gradations of light and shadow, and counter
shading, fuses with a background of light and shadow afforded
by foliage.
But examples of color patterns which by the most liberal stretch
of the imagination cannot be interpreted as harmonious with the
animal’s usual surroundings are not far to seek. Brilliant yellows
and reds render, for instance, many Wasps, Bees, Butterflies, and
various species of Snakes actually conspicuous. And it is suggestive
that very many of these forms are provided with special means of
ANIMAL BIOLOGY
328
defense, such as poison glands and formidable jaws, or special
secretions which render them unpalatable.
still more interesting, many animals possessing this protective
conspicuousness, which renders them easily identified and adver-
\
Moreover, what is
wy y
ag :
LS ties
ore Ne a)
‘itt — J Ve ,
Pa i Og ga PMN cates, Uh Fr -
Lyi, }) ‘( R Re ve Wy) ys
Lyla Suv al, OP ms Se
a i Genes 1g y
é See O®
a LY RO Ae ,
r CY, 4 Ye fp
AES sy
Fic. 212. — A, Walking-stick Insect on a twig; B, larva of a Geometrid
Moth resting extended from a twig. (Modified, after Jordan and Kellogg.)
tises that they are to be avoided by their foes, are frequently
‘mimicked’ in color pattern and form by defenseless creatures.
Thus commonly associating with the various species of Bees hover-
ing about flowers are defenseless Flies which are so bee-like in
appearance that they are usually mistaken for Bees, and avoided
ORGANIC ADAPTATION 329
accordingly by human, and presumably by other enemies also.
(Figs. 213, 218.)
Now, what is the significance of such phenomena of animal
coloration and form? This problem has attracted much attention
and appears by no means so simple to-day as it did a generation
Fic. 213. —‘ Protective mimicry.’ A, drone Honey Bee; B, a Bee-fly,
Eristalis tenax. (From Folsom.)
ago. Biologists to-day are not so ready to interpret individual
cases as ‘protective,’ ‘aggressive,’ ‘alluring,’ ‘confusing,’ or “mi-
metic.” But nowhere else is the plasticity — adaptability — of or-
ganisms better illustrated, and, taken by and large, many such
adaptations are of crucial importance in the life and strife of species.
Apparently the origin of such adaptive variations must be sought
in mutations — the unadaptive mutations being eliminated in the
struggle for existence.
Worker
Drone
Fic. 214. — The Honey Bee, Apis mellifica. (From Bureau of Entomology.)
Tue Lecs oF THE Honey Bree. From time immemorial the
Honey Bee (Apis mellifica) has been the subject of wonder and
study, and to-day there is no more interesting and instructive ex-
ample of adaptation than that exhibited by the Bee in relation to
the highly specialized community life of the hive. (Fig. 214.)
330 ANIMAL BIOLOGY
An average hive comprises some 65,000 Bees of which one is
a QUEEN, several hundred are DRONES, and the rest WORKERS.
The queen is the only fertile female and accordingly she is the
mother of nearly all the other members of the hive. Throughout
her life of a few years she is tended and fed by her numerous off-
spring. The drones, or males, contribute nothing to the work of
the hive but, after the old queen departs at the swarming, one of
them during the nuptial flight mates with a virgin queen which
then is the queen of the hive. Thus queen and drones represent
Gonreound Ocelli an adaptation of the colony to
communal life —a physiological
division of labor in the hive which
involves a_ specialization of a
class solely for reproduction, while
the daily work and strife of the
colony devolves upon the workers.
The latter are sexually undevel-
oped females which do not lay
| eggs but spend their time carry-
TLabraur Nj Antenna ing water, collecting nectar and
Med ead | 8° gpipharynx pollen, secreting wax, building
EA the comb, preparing food, tending
the young, and cleaning, airing,
Labial palp = and defending the hive.
The worker is a ‘bundle of
adaptations’ for its varied duties.
The primitive insect appendages
mo) have become specialized in the
eas Labellum ‘
eee aan a ee worker Bee, so that collectively
Honey Bee. they constitute a battery of tools
adapted with great nicety to the
uses for which they are employed. This applies to all of the
appendages of the insect’s body, but we shall neglect those of the
head and consider only the specializations of the three pairs of
legs. These, as in all Insects, arise from the THORAX; the anterior
pair from the first segment of the thorax (prothorax) ; the second,
or middle, pair from the second thoracic segment (mesothorax) ;
and the posterior pair from the third, or last, thoracic segment
(metathorax). A typical insect leg consists of several parts: the
coxa, which forms the junction with the body, followed in order
‘00g Aouoyy JoysIom oy) JO ssa] — ‘oTZ ‘o1g
SAT OOVYOHLVLAW
Oss sary
omer Sapy
Cs
Ay ee
“Paeyriy
vy sy)
SAT OIOVYOHLOSAN
SnsIv,
snsivy,
ie
apts JOUUI UO
BY, SQUIOD UdT[Og
== | as ca
SNSIe}B}OI]
ip
4, YSNAQ UdaT[Og
i
Od OIOVYOHLOYA
epluny
ey By snsivy,
i! X Wy,
4 \4
CEE erat \,
Uspoog SS a , | f s,
AINWO J
NS
AZ — ysniq ahq
ty EN Na) i} | i an
goyseq SE
@ Ur[Od Xs
WA
yt
ysniq ualjog
i =
JaLUBYIOIT, >
if
et Wy
bisa p ae
Suluveo
udYyM 89]
Jo WOI}Isog
331
332 ANIMAL BIOLOGY
by the TROCHANTER, FEMUR, TIBIA, and five-jointed TARSUS, or
foot. (Figs. 215, 216.)
The worker Bee’s PROTHORACIC LEGS show the following special-
izations. The femur and tibia are covered with long, branched
FEATHERY HAIRS which aid in gathering pollen when the Bee
visits flowers: the tibia, near its junction with the tarsus, bears a
group of stiff bristles (POLLEN BRUSH) which is used to brush to-
gether the pollen grains that have been dislodged by the hairs of
b = A,
CRRA
XG WWW
(QQ (QQ QQ Q rxsN
Fic. 217. — Foot of the Honey Bee in the act of climbing, showing the
‘automatic’ action of the pulvillus. A, position of foot on a slippery surface;
B, position of foot in climbing on a rough surface; C, section of a pulvillus just
touching a flat surface; D, the same applied to the surface. an, claw; er, curved
rod; fh, tactile hairs; pv, pulvillus; ¢, last segment of tarsus. (From Packard,
after Cheshire.)
the upper leg-segments. On the opposite side of the leg is a com-
posite structure, the ANTENNA CLEANER, formed by a movable
plate-like process (vELUM) of the tibia which fits over a circular
notch in the upper end of the tarsus. The notch is provided with
a series of bristles which form the teeth of the ANTENNA COMB.
The antennae, or ‘feelers,’ which are important sense organs of
the head, are cleaned by being placed in the toothed notch and,
after the velum is closed down, drawn between the bristles and
the edge of the veltum. On the anterior face of the first segment
of the tarsus is a series of bristles (EYE BRUSH) which is used to
remove pollen and other particles adhering to the hairs on the
head about the large compound eyes and interfering with their
operation.
The terminal segment of the tarsus of each leg is provided with
a pair of notched cLaws, a sticky pad (PULVILLUS), and a group
of TACTILE HAIRS. When the Bee is walking up a rough surface,
the points of the claws catch and the pulvillus does not touch,
but when the surface is smooth, so that the claws do not grip,
they are drawn beneath the foot. This change of position applies
the pulvillus, and it clings to the smooth surface. Thus the charac-
ORGANIC ADAPTATION 333
ter of the surface automatically determines whether claw or pul-
villus shall be used. But there is another adaptation equally re-
markable. ‘‘The pulvillus is carried folded in the middle, but
opens out when applied to a surface; for it has at its upper part
an elastic and curved rod, which straightens as the pulvillus is
pressed down. The flattened-out pulvillus thus holds strongly while
pulled along the surface by the weight of the Bee, but comes up
at once if lifted and rolled
off from its opposite sides,
just as we should pull a wet
postage stamp from an enve-
lope. The Bee, then, is held
securely till it attempts to
lift the leg, when it is freed
at once; and, by this exquisite
yet simple plan, it can fix
and release each foot at least
twenty times per second.”
(Fig. 217.)
The characteristic struc-
tures of the middle (MESO-
THORACIC) legs of the Bee are
a small POLLEN BRUSH and a ne
long spine, or POLLEN SPUR. 2 B \.7 — Palpus of sting
The METATHORACIC LEGS 1:
exhibit four remarkable
adaptations to the needs of
the insect. known as the Fic. 218. — Sting of a worker
° Honey Bee.
POLLEN COMBS, PECTEN,
AURICLE, and POLLEN BASKET. The pollen combs comprise a
series of rows of bristle-like hairs on the inner surface of the first
segment of the tarsus: the pecten is a series of spines on the distal
end of the tibia which is opposed by a concavity, the auricle, on
the proximal end of the tarsal segment; while the pollen basket
is formed by a depression on the outer surface of the tibia which
is arched over by rows of long curved bristles arising from its
edges.
Thus the worker is fully equipped. Flying from flower to flower,
the Bee brushes against the anthers laden with pollen, some of
which adheres to the hairs on its body and legs. While still in
Poison sac
Levers to
move barb
334 ANIMAL BIOLOGY
the field, the pollen combs are first brought into play to comb the
pollen from the hairs, while the pectens scrape the pollen from
the combs. Then the auricles are manipulated so that the ac-
cumulating mass of pollen is pushed up into the bristle-covered
pollen baskets. This process is repeated until the baskets are full
and then the insect returns to the hive, where the contents of the
Fic. 219. — Pollination of an Orchid, Cypripedium, by a Bumble Bee.
A, Bee forcing its way into the flower; B, Bee obtaining nectar in the flower;
C, Bee starting to escape brushes pollen upon the stigma of the flower; D, be-
fore finally escaping the Bee receives another load of pollen from the anther.
pollen baskets are removed by the aid of the spurs with which the
mesothoracic legs are provided. (Figs. 218, 219.)
Moreover, the structural adaptations of the worker Bee are but
one aspect of a reciprocal fitness. Many of the flowers which the
Bee visits show remarkable adaptations for the reception of the
Bee and for dusting it with pollen, because Bees are effective
agents in transferring pollen from flower to flower and thus insur-
ORGANIC ADAPTATION 339
ing cross-fertilization. And so the Bee which has been given as our
final example of adaptation to the physical environment, serves
also as an introduction to the consideration of adaptation to the
living environment. No better proof could be asked of the futility
of attempting to classify the adaptations of organisms — the
organism is a unit: a complex of adaptations to any and all of
its surroundings, inanimate and animate, otherwise it could not
exist.
B. ADAPTATIONS TO THE LIVING ENVIRONMENT
We now turn more specifically to some striking interrelations
of organism with organism, in order to make possible an apprecia-
tion of the devious means to which they have recourse — to what
Mussels ( Physae
Bullhead
AS~{ Sphaeridae
a __|
Ree. Small aquatic insects
Protozoa
Decaying Vegetation
Black bass adults ae |
Pickerel
Black bass young EES
Fic. 220. — Diagram of the web of life and the equilibrium of nature, as
illustrated by the food relations in a pond community. Arrows point from the
organisms eaten to those doing the eating. The task of ecology is to decipher
the patterns of the web of life. (From Shelford.)
extent the strands of the web of life become entangled — in the
competition for a livelihood.
The mutual biological interdependence of organisms is, in the
final analysis, the result of the primary demands of all creatures —
proper food, habitat, reproduction, defense against enemies and
the forces of nature. The web of life is an expression of the coodpera-
tion, jostling, and strife of individual with individual, and species
with species for these primary needs; and the activities which
follow from them form the foundations of life in the lowest as well
336 ANIMAL BIOLOGY
as the highest. A little patch of meadow soil two feet square has
revealed, within about an inch from the surface, over a thousand
animals and three thousand plants. There is a struggle for existence.
Take a single example. A common food Fish, the Squeteague,
captures the Butter-fish or the Squid, which in turn have fed on
young Fish, which in their turn have fed on small Crustacea, which
themselves have utilized microscopic Algae and Protozoa as food.
Thus the food of the Squeteague is actually a complex of all these
factors, and such a nutritional chain is no stronger than its single
links. Circumstances which modify or suppress the food and
thereby reduce the abundance of the Algae and Protozoa of the
sea are reflected in correlative changes in the abundance of eco-
nomically important food Fishes. And this same principle is
true throughout living nature, though only occasionally is it pos-
sible to trace it. “‘Nature is a vast assemblage of linkages.”
(Figs. 220, 266.)
1. Communal Associations
Perhaps the simplest associations of organisms are represented
by GREGARIOUS animals, such as Wolves which hunt in packs, and
Buffaloes and Horses which herd for protection. Here the associa-
tion is more or less temporary and there is no division of labor
between the members, other than leadership by one animal.
COMMUNAL animals, however, exhibit highly complex associa-
tions in which the members merge, as it were, their individuality
in that of the community. This is well exhibited, for example,
among the Ants, in which all of the various species, about 5,000 in
number, are communal, and the Wasps and Bees in which all
gradations exist from solitary to hive-dwelling species. In the
case of the Bees, and still more in the Ants, the division of labor
has developed to the extent that structural differentiations have
given rise to classes of individuals specially adapted for the per-
formance of certain functions in the economy of the hive.
But the differentiations of various members of a colony of Ants
or Bees are limited to their bodies and are practically fixed and
irreversible, while in human society, differentiations are no longer
confined to the bodies of individuals. Man’s ingenuity has de-
vised what are to all intents and purposes artificial, accessory
organs — tools and machines. Accordingly it is in Man that we
find the highest expression of communal codperation, because in-
ORGANIC ADAPTATION 337
creased intelligence, in particular, makes flexible the stereotyped
life as exhibited in the lower forms — the human individual is
adaptable to the various community tasks.
2. Symbiosis
Associations are not confined to members of the same species,
nor are all an expression of cooperative adaptations. All grada-
tions occur from those which are mutually beneficial to the parties
in the pact, to those in which one member secures all the advantage
at the expense of the other.
The most intimate associations in which the organisms involved
are mutually benefited, if not absolutely necessary for each other’s
Fic. 221. — The formation of a Lichen, Physcia paratina, by the combina-
tion of an Alga and a Fungus. A, germination of a Fungus spore (sp), whose
filaments are surrounding two cells (a) of the unicellular Alga, Cystococcus
humicola. B, later stage in which spores have formed a web of filaments
(mycelium), enveloping many algal cells. Magnified about 400 times. (From
Bonnier.)
existence, are termed symBiotic. A familiar case is the common
green Hydra (Chlorohydra viridissima) that owes its color to the
presence of a large number of unicellular green plants which live
in its endoderm cells. The products of the photosynthetic activity
of the plant cells are at the disposal of the Hydra, and the latter
in return affords a favorable abode and the material necessary
for the life of the plants.
338 ANIMAL BIOLOGY
A far more striking example of symbiosis is afforded by Lichens
which represent intimate combinations of various species of color-
less plants (Fungi) and simple green plants (Algae). In each case
the Fungus supplies attachment, protection, and the raw materials
of food, while the Alga performs photosynthesis. Each can live
independently under favorable conditions, but in partnership they
are superior to hardships with which many other plants cannot
cope, and thus some Lichens become the vanguard of vegetation
in repopulating rocky, devastated areas. (Fig. 221.)
From the practical standpoint of agriculture the symbiotic
nitrogen-fixing Bacteria are of first importance. It will be recalled
KE
: 77K" f
M8
;
Wig N ’ ~
a 2 Y, .
aA ae Ss
y , GOES PL
, 3S:
=
sql
é! pi
i Wy
REHarfison
Fic. 222. — Rose Aphids visited by Ants.
that these Bacteria form small nodules on the rootlets of higher
plants, such as Beans and Clover, and make atmospheric nitrogen
available to the latter — return it to the cycle of the elements in
living nature. (Fig. 16.)
Still another type of association in which both partners profit is
ORGANIC ADAPTATION 339
represented by the relation that occurs between Ants and Plant
Lice, or Aphids. The defenseless Aphids are protected, herded, and
‘milked’ by the Ants to supply their demand for honeydew, a
secretion of the Aphids which the Ants greedily devour. (Fig. 222.)
3. Parasitism
But associations in which one organism, the PARASITE, secures
the sole advantage, and in most cases at the expense of the help-
less second party, the Host, are far more numerous — it has been
estimated that nearly half the animal kingdom are parasites. And
these justly receive considerable notoriety because many human
diseases are the result of Man’s unwilling partnership in such
associations. Indeed, PARASITOLOGY has become an important
subdivision of biology, both practical and theoretical. Practical,
as a cornerstone of public health; and theoretical, because many
of the most remarkable functional and structural adaptations are
exhibited by parasites in becoming fitted for this apparently
highly successful method of gaining a livelihood, and by the hosts
in bearing the burden with the least outlay. Generally speaking,
the effect on the parasite consists in a simplification of the various
organs of the body devoted to food-getting, locomotion, etc.,
since their duties are performed by the host; while the organs and
methods of reproduction are highly specialized and elaborated,
owing to the necessity of producing enough offspring to compen-
sate for the hazards involved in reaching a proper host. For in
the majority of cases a parasite is adapted to live in a specific
host, and death ensues if this is not attained at the proper time.
(Pies25, 251, 252, 260.)
Probably the most generally interesting example of parasitism
is the cause of the disease known as MALARIA. Man is subject to
at least three types of malaria, each the result of infection by a
different malarial organism. The malarial parasites are all uni-
cellular animals, Protozoa, with complicated life histories which
are adaptations to the specific demands of their parasitic exis-
tence. One part of the life history, the asexual, is passed in the
red blood corpuscles of Man; while the other, the sexual, occurs
in the digestive tract of certain species of Mosquitoes. A single
parasite inoculated into the human vascular system by the bite
of an infected Mosquito enters a red blood corpuscle and multi-
plies. The progeny, liberated from the destroyed corpuscle, sim-
340 ANIMAL BIOLOGY
ilarly attack other corpuscles and multiply until a very large
number of blood corpuscles are destroyed. And poisonous prod-
ucts of the life processes of the parasites provoke the chills and
fevers characteristic of the disease. (Fig. 223.)
But the parasites must make their escape before the human
host successfully combats the toxic substances, kills the parasites
Schizogony
(rapid increase
of animals
Sporozoite entering 7 x in blood)
uman red blood Merozoites
corpuscle 3)”
(} aa Os Male Ge
Transmission CYCLE IN MAN : Te
pel l CYCLE IN MOSQUITO Mea
: Maturation
Sporozoites enter / in stomach
salivary glands —_ of Mosquito
\W (@ ‘
Microgametes“D> Macrogamete
ie (Sperm) )\ 7 (Ege
Odcyst ruptures,
releasing sporo-
zoltes into
Fertilization
body cavity \ fas i
in stomach
Ookinete (Zygote)
penetrates epithelium
i of stomach
Oodcyst
yy in wall of stomach
“*// Odcyst developing sporozoites
Odcyst with fully /[{4@
developed sporozoites \WNwiy,
Fic. 223. — Life history of a Malarial Parasite, Plasmodium malariae.
by taking quinine, or succumbs to them. The getaway is accom-
plished, if at all, by a Mosquito biting the host and taking with
the blood certain sexual stages of the parasite which can develop
in the cold-blooded insect. And now the Mosquito is the host.
In its stomach the sexual phase of the life history of the malarial
parasite takes place, fertilization occurs, and finally the numerous
products of the zygote work their way to the mouth parts of the
Mosquito, where they await an opportunity to enter the human
blood.
ORGANIC ADAPTATION 341
To cerebrospinal fluid
causing sleeping sickness and death
Trypanosomes
in human blood
causing Trypanosome fever
Man, Antelope, etc.
Transmission by Transmission by
bite of tsetse fly bite of tsetse fly
/ Tsetse Fly
Y :
Forms in Salivary glands
ready for re-infection
(20th-30th day)
NES Crithidial forms in
salivary glands Forms in mid gut
Ls (2 or 3 days later) (48 hrs. after infective meal)
oye
Newly arrived form
in salivary gland
(12th to 20th days)
Long, slender forms in proventriculus
(about l0th to 15th days)
Fic. 224. — Life history of the Trypanosome, Trypanosoma gambiense,
that produces African Sleeping Sickness in Man. Magnified about 1500 times.
(Modified, after Chandler.)
342 ANIMAL BIOLOGY
The life history of malarial parasites exhibits a continuous series
of adaptations to parasitic life: the nicety of the adjustment being
especially well illustrated at the transfer from Man to Mosquito,
since all the parasites which enter the stomach of the latter are
digested except those sexual forms which are ready to initiate the
sexual part of the cycle in the new host. (Figs. 246, 247.)
But the acme of parasitic associations is only attained when
the adaptations of parasite and host have become so complete that
the latter ‘pays the price’ without any ill effects. Thus the Ante-
lopes and similar Mammals of certain regions of Africa harbor
in their blood various species of Protozoan parasites, known as
TRYPANOSOMES, without any apparent discomfort. But if the in-
termediate hosts, which are biting Flies, transfer certain species
of Trypanosomes to the blood of imported Horses or Cattle, or
of Man, serious diseases result which are usually fatal. Indeed, the
opening up of large regions of Africa has been greatly retarded
by the ravages of Trypanosomes in new hosts to which they are
not adapted. Generally speaking, pathogenic species may be re-
garded as aberrant forms which are not yet adapted to their hosts
or are not in their normal hosts. And these are the parasites which
are especially forced upon our attention, though there are few
organisms without their specially adapted parasites — the parasites
themselves not excepted. (Figs. 224, 245.)
4. Immunity
At best, however, the part played by the host cannot be re-
garded as ideal, and devious types of adaptations against parasites
exist which, in so far as they are effective, bring about IMMUNITY.
Usually among the higher animals, including Man, immunity
to the ravages of pathogenic microdrganisms seems to depend
chiefly upon the activities of the white blood cells and upon spe-
cific chemical substances in the blood, termed ANTIBODIES.
The white blood cells have been called the policemen of the
body because, under the influence of invading organisms, some
of them make their way through the walls of the capillaries in the
region of the infection and, in amoeboid fashion, engulf and digest
the intruders. When acting in this capacity they are referred to as
PHAGOCYTES. Similar phagocytic cells are found in groups, PEYER’S
PATCHES, scattered in the intestinal wall. Apparently they are
to forestall invasion of the tissues by Bacteria that swarm there.
ORGANIC ADAPTATION 343
Among the various classes of antibodies are the ANTITOXINS
that neutralize the poisonous products (toxins) of certain Bac-
teria; the PRECIPITINS that act upon foreign proteins of bacterial
or other origin; the Lystns that actually destroy foreign cells; and
the opsontns that render Bacteria vulnerable to the attacks of
the phagocytes.
Various specific antibodies may be naturally present in the
blood —a part of the heritage — so that an individual is immune
to certain diseases due to pathogenic organisms. Or the anti-
bodies may be produced in response to the parasites themselves,
and the individual acquires immunity only after undergoing the
disease. Again, immunity may be artificially acquired by various
means, such as VACCINATION, which stimulate the production of
antibodies so that the individual is prepared in the event of an in-
fection.
Indeed, the subject of immunity has become a science in itself
(IMMUNOLOGY) within the past few years — a science which has as
its fundamental basis the investigation of the marvelous power of
adaptation of protoplasm as exemplified in coping with disease-
producing parasites, and even with those ultramicroscopic agents
of disease, the so-called FILTERABLE VIRUSES, such as produce
smallpox, measles, rabies, etc. The viruses are so small that they
pass through porcelain filters and, in one case at least, consist of a
single protein molecule. But they have the power to multiply
when in the protoplasm of host cells and so exhibit one funda-
mental characteristic of life. The viruses appear to be on the
border line between the lifeless and the living but to include them
with the latter would necessitate an extension of our present con-
ception of life. Their further study is certain to be of immense
practical and theoretical importance.
C. INpIvIDUAL ADAPTABILITY
We may now turn to a survey of the highest expression of ad-
aptation evolved by Nature, which appears as TROPISMS and other
elements of BEHAVIOR in the lower organisms, gains definiteness
and content as we ascend the animal series, and becomes the basis
of INTELLIGENCE and all that the mental life of Man involves. It
is the adaptation which renders Man essentially superior to adapta-
tion — enables him to a large extent to control, instead of being
controlled by his environment. “It seems that Nature, after elab-
344 ANIMAL BIOLOGY
orating mechanisms to meet particular vicissitudes, has lumped
all other vicissitudes into one and made a means of meeting them
all’? — the nervous mechanism.
That organisms respond to environmental changes, we are well
aware. Life itself is the result of — in fact, is — a continuous flow
of physico-chemical actions, interactions, and reactions with the
surroundings. But by the behavior of the organism we refer
specifically to the reactions of the organism as a unit, rather than
to the internal processes in the economy of its life. And surveyed
from a broad viewpoint, there is discernible in the behavior of
animals, just as in their structure in general and in their nervous
system in particular, from the lowest to the highest, a great
though gradual increase in complexity. The behavior of Amoeba
(Cs
‘Co
Fic. 225. — Diagram to illustrate the avoiding reaction of Paramecium.
A, asolid object or other source of stimulation. 1—6, successive positions taken
by the animal. The rotation on its long axis is not indicated. (From Jennings.)
or Paramecium is an expression of the primary attributes of pro-
toplasm — irritability, conductivity, and contractility. So is the
behavior of Hydra and Earthworm in which special cells constitute
a definite codrdinating, or nervous system. And so is the complex
behavior of the higher animals, including Man, with their elaborate
series of sense organs and highly developed sensorium, or brain.
‘Let us now try to form a picture of the behavior of Parame-
cium in its daily life under natural conditions. An individual
is swimming freely in a pool, parallel with the surface and some
distance below it. No other stimulus acting, it begins to respond
to the changes in distribution of its internal contents due to the
fact that it is not in line with gravity. It tries various new positions
until its anterior end is directed upward, and continues in that
ORGANIC ADAPTATION
direction. It thus reaches the surface film.
To this it responds by the avoiding reac-
tion (Fig. 225), finding a new position and
swimming along near the surface of the
water. . . . Swimming forward here, it ap-
proaches a region where the sun has been
shining strongly into the pool, heating the
water. The Paramecium receives some of
this heated water in the current passing
from the anterior end down the oral groove.
(Fig. 226.) Thereupon it pauses, swings its
anterior end about in a circle, and finding
that the water coming from one of the
directions thus tried is not heated, it pro-
ceeds forward in that direction. This course
leads it perhaps into the region of a fresh
plant stem which has lately been crushed
and has fallen into the water. The plant
juice, oozing out, alters markedly the chemi-
cal constitution of the water. The Parame-
cium soon receives some of this altered
water in its ciliary current. Again it pauses,
or if the chemical is strong, swims back-
ward a distance. Then it again swings the
anterior end around in a circle till it finds
a direction from which it receives no more
of this chemical; in this direction it swims
forward. ...
“In this way the daily life of the animal
continues. It constantly feels its way about,
trying in a systematic way all sorts of con-
ditions, and retiring from those that are
harmful. Its behavior is in principle much
like that of a blind and deaf person, or one
that feels his way about in the dark. It isa
continual process of proving all things and
holding to that which is good.” (Jennings.)
The behavior of Paramecium leaves one
with the impression that the animal is
largely at the mercy of its surroundings —
Fic. 226. — Diagram
to show the rotation on
the long axis, and the
spiral path of Parame-
cium. 1-4, successive
positions assumed. The
dotted areas with small
arrows represent the cur-
rents of water drawn
from in front. (From
Jennings.)
346 ANIMAL BIOLOGY
that the environment rather than the organism itself is the domi-
nant factor, and this is true to a considerable degree. But Parame-
cium is not merely an automaton. Its behavior is modifiable and,
in the long run, is adapted to the usual changes of its surroundings.
It forms immense aggregations where food and other conditions
are favorable. That the reactions are adequate for the simple life
and methods of reproduction of Paramecium is attested by its
success — it is one of the most common and widely distributed
animals. (Fig. 27.)
In such simple beginnings, then, must be sought the largely
automatic responses of animals to the changes in external con-
ditions, known as REFLEXES and INsTINCTs. Both apparently are
chiefly the result of inherited nervous structure and therefore may
be regarded as inherited behavior. And increase in the complexity
of life processes has involved at the same time an increase in the
number and complexity of reflexes and instincts. The primitive
reflexes and instincts of Hydra lead it to seize small organisms
within reach of its tentacles and pass them to its mouth; the
Earthworm, to swallow decaying leaves as it burrows through the
soil; the Crayfish, to grasp its prey with its large claws, tear it
into pieces by means of certain appendages about the mouth
which are adapted just for the purpose — and so on to the higher
Vertebrates where the feeding instincts reach their maximum of
complexity. The remarkable behavior of Ants and Bees is essen-
tially a complex of instincts. Moreover, instincts of fear, self-
defense, play, care of the young, etc., render a considerable part
of the behavior of even the higher organisms more automatic than
is perhaps, at first thought, apparent. (Figs. 139, 143.)
But just as the behavior of Paramecium and its allies is modi-
fiable, so reflexes and instincts which seem the most fixed show at
least a slight degree of adaptability to unusual conditions. Indeed
new reflexes, Known as CONDITIONED reflexes, may be established
as the result of experience during the life of the individual. And
it is this ever-present power of modifiability, which is in man called
‘choice,’ that leavens the whole and becomes the dominant factor
in the behavior of the highest animals; while reflex action and in-
stinct are relegated to a subsidiary though by no means unimpor-
tant role. A large part of human education consists in establishing
other fixed adaptive responses called HABITS which join the earlier
reflexes and instincts in relieving the conscious life of innumerable
ORGANIC ADAPTATION 347
simple factors of behavior, and leave it more or less free for the
higher intellectual processes. The cerebrum, regarded as the organ
of the MIND, is superimposed upon the system of automatic,
machine-like responses of the reflex centers. It is the executive
that reacts to the state of affairs as a whole and coordinates or
alters responses when the routine responses are inadequate. The
progress of modifiability toward conscious choice-responses to ex-
ternal conditions constitutes a gradual and ill-defined transition
from instincts to ASSOCIATIVE MEMORY, or newly conditioned
responses, to learning and the highest intellectual processes.
(Fig. 143.)
Although it is necessary to emphasize that mind and intelligence,
in the biological sense, are expressions for that integration of
nervous states and actions which makes possible a nicety of adapta-
tion of behavior to environmental conditions that otherwise would
be impossible — that it is our chief means of adaptation; it is a
serious mistake to minimize the importance of the vast gulf be-
tween Man’s nature and that of the most highly developed lower
animals. In no respect are these differences more marked than in
the various forms of learning that collectively form the means
of education. While associative memory, or conditioning, will
account for the various non-instinctive actions of Man’s animal
associates, human racial history and the individual’s experience
contain much that baffles explanation in such terms. Indeed, in
the highest reaches of conscious life we appreciate that it is able
to form strange conceptions; that it has not only memory of the
past, but also anticipation of the future. We can brood and medi-
tate and understand in part. We are guided by the past, present,
and future in making adaptations. “The largest fact in the story
of evolution is the growing dominance of the mental aspect of
life.”
Thus it is clear that, with all the variations in structure and
function, organisms all possess irritability in common: they all
exhibit adaptive responses which enable them to exist in spite of
surrounding changes. “‘ Adaptability appears to be the touchstone
with which nature has tested each kind of organism evolved; it has
been the yard-stick with which she has measured each animal type;
it has been the counterweight against which she had balanced
each of her productions . . . the general course of evolution has
348 ANIMAL BIOLOGY
been always in the direction of increasing adaptability or increas-
ing perfection of irritability.” The individual’s heritage affords
the cumulative result of the adaptations of the race — including
adaptability.
CHAPTER XXIIf
DESCENT WITH CHANGE
Thoughtful men will find in the lowly stock whence Man has
sprung, the best evidence of the splendor of his capacities; and will
discern in his long progress through the past, a reasonable ground
of faith in his attainment of a nobler future. — Huzley.
From the time of the Greek natural philosophers there always
have been men who have sought a naturalistic explanation of the
origin of the diverse forms of animals and plants, and who have
suggested that the present ones arose from earlier forms by a
long process of descent with change, or EVOLUTION. But with the
revival of natural history studies after the Middle Ages, the pre-
vailing ideas in regard to creation led the majority, perhaps al-
most unconsciously, to assume that there is merely a limited
number of kinds of organisms, all of which were created at one time.
And this is not so strange when one considers that nearly all of
the important facts which we have reviewed in the preceding
pages were yet to be discovered, and that the number of known
kinds of animals totalled but a thousand or so instead of about a
million as to-day.
The pioneer work of the early Renaissance naturalists consisted
principally of collecting and describing animals and plants. This
involved making a catalog of the different kinds — classifying them
in some way — and consequently some basis of CLASSIFICATION
was sought. Thus attention was focussed on the kinds, or
SPECIES, and for practical, if for no other reasons, the species as-
sumed a prominence which overshadowed the individuals which
composed it.
Indeed, to-day biologists are hard put to it to define a species.
Of course everyone recognizes not only that there are many kinds
of animals and plants, but also that many individuals are essen-
tially the same. Groups may be formed of individuals which differ
less among themselves in the sum of their characters than they
do from the members of any other group of individuals. And
further, the members of a group produce other individuals which
343
350 ANIMAL BIOLOGY
are essentially similar. It is such a group of similar individuals
that is regarded by the biologist as a species. But it is difficult to
formulate a satisfactory brief definition of a species, unless perhaps
it be “a group of individuals that do not differ from one another in
excess of the limits of ‘individual diversity,’ actual or assumed.”’
So with but slight exaggeration it may be said that a species is
largely a concept of the human mind: a somewhat arbitrary con-
venience. The real unit in nature is the individual animal or plant,
and an understanding of the differences between individuals should
give us the key to the differences between species. In the final
analysis, the problem of the origin of species is a problem in
genetics. (Figs. 236, 280.)
This seemingly obvious point of view has but relatively recently
been clearly grasped by biologists, and the species rather than
the individual has loomed large in the discussions of how plants
and animals-came to be what they are to-day. As a matter of
fact, during the eighteenth century the greatest student of plant
and animal classification, Linnaeus, emphasized the idea that
each species represents a distinct thought of the Creator, and
that the object of classification is to arrange species in the order of
the Creator’s consecutive thoughts. This viewpoint is somewhat
whimsically expressed by the old naturalist who, finding a beetle
which did not seem to agree exactly with any species in his collec-
tion, solved the difficulty by crushing the unorthodox individual
under his foot. His credulity surely would have been strained by
the estimate of modern entomologists that if all the species of In-
sects were known they would total upward of three million.
We may consider, then, that the consensus of opinion up to the
middle of the last century was overwhelmingly on the side of
SPECIAL CREATION and FIXITY OF SPECIES, and therefore against
the idea occasionally advanced by men, as it now appears, ahead
of their times, that DESCENT WITH CHANGE is the true explana-
tion of the origin of the diverse forms of plants and animals. But,
as nearly everyone knows, a complete reversal of opinion has oc-
curred since 1860 — to-day professional scientists and most edu-
cated laymen accept ORGANIC EVOLUTION. And we have accepted
it in the preceding sections of this work; but if this appears to
have been prejudging the question, the explanation is that the
genetic connection of organisms is the guiding principle of all
modern biology. The mere fact that an unbiased presentation of
DESCENT WITH CHANGE 351
the data seems to prejudge the question is the most cogent pre-
sumptive evidence for evolution. It is true that there are wide
differences of opinion among biologists in regard to the factors
which have brought about the evolutionary change — but there
are none in regard to the fact of evolution itself. It will be con-
venient, therefore, first to summarize a few of the evidences of evolu-
tion, and then to present certain modern views in regard to the
methods of evolution.
A. EvipENCES oF ORGANIC EVOLUTION
To one who has thoughtfully followed the preceding pages
there must immediately occur many facts which are readily and
reasonably interpreted from the point of view of descent of one
species from another, but which are entirely obscure from that
of the special creation of species. For instance, one will recall the
cellular structure of all organisms; the method of origin and the
fate of the germ layers in animals; the interrelationship of the
urinary and reproductive systems in the Vertebrates; the com-
parative anatomy of the vascular and skeletal systems of Verte-
brates; the similarity of the physical basis of inheritance in
animals and plants: in a word —the ‘unity in diversity’
that pervades the world of living things. (Figs. 122, 129,
wAly 227.)
In general, such are the types of data which support the eve-
lution theory. Although the evidence, from the nature of the
case, must be indirect, it is none the less impressive, chiefly because
the facts for evolution are from such diverse sources and all con-
verge toward the same conclusion. The theory of evolution reaches
the highest degree of probability, since in every branch of botany
and zoology all the data are most simply and reasonably explained
on the basis of descent with change. It is a cardinal principle of
science to accept the simplest conceptions which will embrace all
the facts.
Assuming the reader’s familiarity with the contents of this
volume up to the present point, it is now necessary to summarize
some of the most important evidence from various subdivisions of
biology. But, as will soon appear, it is impossible to arrange the
facts in natural groups because the evidence from one merges into
that from another — the evidence interlocks.
352 ANIMAL BIOLOGY
1. Classification
When the serious study of biological classification was well
under way, biologists found increasing evidence of the similarity,
or affinity, of various SPECIES of animals and plants. Not only is
it possible to arrange animals, for example, in an ascending series
of increasingly complex forms, but also in many cases it is difficult
or impossible to decide just where one species ends and the next
begins. That is, the most divergent individuals within a given
species frequently approach those of a closely similar species. There
are intergrades. (Fig. 236.)
Furthermore it is found that species themselves can be naturally
arranged in more comprehensive groups to which the name GENUS
is applied. For example, the common Gray Squirrel represents the
species carolinensis, and the Red Squirrel, the species hudsonicus.
Both are obviously Squirrels, and therefore both species are grouped
under the genus Sciurus. Accordingly, each animal is given a name
composed of two words: the first, generic and the second, specific.
The Gray Squirrel is Sciturus carolinensis and the Red Squirrel is
Sciurus hudsonicus. Thus to give a scientific name to an animal or
plant is really to classify it, because the first word of the name indi-
cates that it possesses some fundamental characteristics in common
with the other species of the genus — in fact, is more like them than
it is like any other group of organisms.
But again, the members of the genus Sciurus have many char-
acteristics in common with other animals which obviously are not
true squirrels. The Chipmunks or Ground Squirrels, for instance,
differ not only in certain obvious features, but in the possession of
internal cheek pouches, etc. This dissimilarity and similarity is
expressed by placing them in a different genus, Tamias, but in the
same FAMILY, Sciuridae. The familiar eastern Chipmunk is Tamias
striatus.
Moreover, while the Beaver (Castor americana) differs still more
from the Squirrels than do the Chipmunks, and therefore is placed
in a distinct family, the Castoridae, it nevertheless agrees with both
in many fundamental ways so that it is placed in the ORDER Ro-
dentia, which also includes the Squirrels and Chipmunks, as well as
many other families and genera. Other orders, such as the Ungulata
(Horses, Cattle, etc.), the Carnivora (Cats, Dogs, Bears, etc.), and
the Primates (Monkeys, Apes, etc.), while they differ widely from
DESCENT WITH CHANGE 393
the Rodents, still agree with them in possessing hair, and milk
glands for suckling the young. This basic likeness is expressed by
including all under the cLass Mammalia.
The Mammals in turn are readily distinguished from Birds,
Reptiles, Amphibians, and Fishes (each of which forms a separate
class), but nevertheless are constructed on the same basic plan,
comprising a dorsal central nervous system surrounded by skeletal
elements forming the skull and vertebral column. Therefore, all are
comprehended in the larger group Vertebrata which, with certain
minor groups, comprises the PHyLUM Chordata and stands in con-
trast with all the Invertebrate phyla which include Hydra, Earth-
worm, Crayfish, etc. The classification of the Gray Squirrel,
Sciurus carolinensis (Fig. 108), may be outlined as follows:
Kincpom — Animalia
SUBKINGDOM — Metazoa
PuyLtum — Chordata
SUBPHYLUM — Vertebrata
Cxiass — Mammalia
OrbDER — Rodentia
Famity — Sciuridae
GENUs — Sciurus
SPECIES — carolinensis.
This classification of the Gray Squirrel, although it incidentally
serves to illustrate the general method of classification of all or-
ganisms, is important because it places concretely before us the
fact that organisms show such fundamental similarities with ob-
vious dissimilarities. In short, the mere fact that animals and
plants naturally arrange themselves, as it were, in classes, orders,
families, genera, species, etc., raises the question of the origin of
species. Is special creation implying fixity of species, or is descent
with change the more plausible explanation? (See Appendix:
Classification.)
The unavoidable answer is descent with change — evolution —
because the principle in accordance with which the groups of
increasing comprehensiveness are formed is solely the greater or
less similarity in the structural features of the organisms. It is
much more reasonable to assume that the thread of fundamental
similarity which runs through all the Vertebrates, for instance, is
the result of inheritance, while the differences of orders, families,
304 ANIMAL BIOLOGY
genera, etc., are due to changes brought about under different
unknown conditions, than it is to assume that each is the result
of a special creative act. Especially so when we realize that in
a very large number of cases it is difficult or impossible to decide
the limits of a species, owing to variations among the individuals
comprising it, and it is necessary to resort to SUBSPECIES and
VARIETIES in Classification. Again, among genera, intergrading
forms demand SUBGENERA; among orders, SUBORDERS; among
classes, SUBCLASSES; and so on. If we admit the origin by descent
with change of the subspecies and varieties, there is no logical
reason for denying the same origin of species, orders, and higher
groups. The difference is one of degree and not of kind. Be-
fore the recognition of evolution, classification was a groping after
an elusive ideal arrangement which naturalists felt but were un-
able to express except in artificial form and in transcendental terms.
Under the influence of the evolution theory, classification became
the natural expression of biological pedigrees. (Figs. 236, 297.)
2. Comparative Anatomy
The evidence from taxonomy is, as has just been seen, really
evidence from comparative anatomy, since modern classifications
are based chiefly on anatomical characters. The various groups —
classes, orders, families, genera, species, etc. — are founded not
on a single difference, nor on several differences, but on a large
number of similarities. For instance, the differences exhibited
throughout the five classes of the Vertebrates are relatively slight
in comparison with the basic resemblances. This similarity in
dissimilarity is brought out by the science of comparative anatomy.
A few concrete examples, some of which we are already familiar
with, will serve to bring the main facts clearly before us.
The fore legs of Frogs and Lizards, the wings of Birds, the fore
legs of the Horse, and the arms of Man are built on the same basic
plan. The same is true of the hind limbs. Clearly all are HomoL-
OGOUS structures, such variations as exist being brought abovt
chiefly by the transformation or absence of one part or another. In
short, all the chief parts of both the fore limbs and the hind limbs
are homologous throughout the series. All are composed of the same
fundamental materials disposed in practically the same way —
nearly all the bones, muscles, blood vessels, and nerves are homol-
ogous. Or compare the digestive systems of the same forms, or
("49009 WOOL) “sytsrp ‘A—] Seuyn
‘Q ‘snipes “y ‘spedreoejour ‘op ssnaoumny ‘FY ‘spedseo “ry ‘ue YYST ‘UeTA, ‘so, OOF YYSII ‘osIOFT So] o10F YYSIA ‘XQ ‘sodduy
BJ] “OTPUAA + SUIM YYSTI “Veg 'SUIM Jo] ‘Pilg ‘SoMMyoNAYS [eJo[9YS sNoSojouoYy MOYS 07 SqUIT] BOF oFeIGQoI0A — "LZZ “OL
359
306 ANIMAL BIOLOGY
the execretory and reproductive systems. One has but to recall
that, on an earlier page, it was possible to describe in general
terms these systems as they exist throughout the Vertebrate
series — in forms as obviously different as Fish and Man. They
are all fundamentally the same. (Figs. 102-110, 121, 129, 141,
227, 226.)
Turning to the Invertebrates, we may remind the reader that
all the appendages of the Crayfish are built on the same simple
UV
Q 5,
i
IN
Gow
A
4
i) —
‘
Fic. 228. — Skeletons of Man and of Gorilla. (From Lull.)
plan as exhibited in the swimming legs (swimmerets) of the ab-
domen. The highly specialized walking legs, great claws, jaws,
and feelers (antennae and antennules) are all reducible to modifi-
cations of the simple swimmeret type. In short, all are homologous
structures, though differing widely in function. This is a most
striking example of SERIAL HOMOLOGY, though we have seen the
same principle exhibited in the Vertebrates where the fore limbs
and the hind limbs of each animal are homologous. Moreover,
DESCENT WITH CHANGE BbY¢
the appendages of the Crayfish are not only serially homologous
among themselves, but are also homologous with the appendages
of all the other members of the class Crus-
tacea — just as the limbs of one Vertebrate
are homologous with those of all other
Vertebrates. (Figs. 64, 65, 227.)
Another class of facts presented by
comparative anatomy is derived from Nittsting membrane
the so-called VESTIGIAL organs. In Man
AAW
there are nearly a hundred structures RSS KN i
. 5 TSS SA) })
which apparently are useless and some- Sy a OG 7
times are harmful. One thinks at once =
of the VERMIFORM APPENDIX of the large Zz Sto Bird
intestine, apparently a remnant of an or- ETM en
gan that serves a useful purpose in certain Fic. 229. — The nicti-
vegetable-feeding (herbivorous) Mammals. t@ting membrane, or third
But equally suggestive are the muscles ache
of the ear, which in some individuals are sufficiently developed
to move the external ear; or the so-called third eyelid at the in-
ner angle of the eye which corresponds to the lid (NICTITATING
MEMBRANE) that moves laterally across
the eye of Reptiles and Birds; or the
terminal vertebrae (coccyx) of the human
spinal column. Other animals likewise
possess many such structures. Porpoises
have vestiges of hind limbs enclosed
within the body, and certain species of
Snakes bear tiny useless hind legs. The
splint bones of the Horse are remnants of
lost toes. (Figs. 109, 206, 229, 230, 234.)
In another class of cases, the organs, or
remnants of organs of a lower form are
Hic 230-— Veatiaial altered or completely made over, as it
hind limbs of a Snake, were, into new organs of the higher form.
Python. f, femur or thigh The milk glands of Mammals are trans-
aaa oe une formed sweat glands of the skin, while the
poison glands of Snakes are specialized
salivary glands. During the embryonic life of Vertebrates there
are gill slits, all of which vanish in higher forms except one
pair which remains as passages (HUSTACHIAN TUBES) connect-
308 ANIMAL BIOLOGY
ing each middle ear with the pharynx. Gill arches, which func-
tion as supports for the gills in the aquatic Vertebrates, persist
in highly modified form as skeletal structures associated with
the tongue and entrance to the lungs (LARYNX) in terrestrial
forms. Finally, in this connection the reader will recall the trans-
formations of the blood vessels in the Vertebrates which occur
with the substitution of lungs for gills, and also the variations
and interrelationships of the excretory and reproductive systems
in the ascending series of Ver-
tebrate classes. (Figs. 121,
122,127,129, 14372515)
One may, of course, con-
clude from all these facts that
brates have each been inde-
Fic. 231. — Head of a Rattlesnake,
with the skin and part of the muscles pendently created according to
removed. The long oval mass in the the same preconceived plan —
upper jaw which is connected by a duct and likewise all the great num-
with the curved tooth, the fang, is the bers of orders, families, genera,
poison gland. (From Smallwood.) 5;
species, etc., of each of the five
classes that these forms represent. Or one may conclude that all
have arisen by descent with change from a primitive Vertebrate
organism which possessed the fundamental similarities exhibited
from Fish to Man. The latter is the conclusion accepted by biolo-
gists to-day.
3. Paleontology
Huxley once said that if zodlogists and embryologists had not
put forward the theory of evolution, it would have been necessary
for paleontologists to invent it. What then are the main facts
offered by PALEONTOLOGY, the study of the FossiL remains of ex-
tinct animals and plants?
In the first place it must be made clear that geologists are able
to determine, with remarkable accuracy in most cases, the se-
quence in time, or CHRONOLOGICAL SUCCESSION, of the rock strata
composing the Earth’s surface. The main outline of this scheme
of geological chronology was understood long before the evolution
of organisms was a crucial question; so that we may consider the
evidence which it affords of the chronological succession of the
fossil remains exhibited by the various strata, as impartial testi-
DESCENT WITH CHANGE 399
THE GEOLOGICAL TIME-TABLE 1?
PRESENT TIME.
Psycuozoic Era: AGE OF MAN OR AGE OF REASON.
The present or ‘Recent time,’ and the time during which Man at-
tained his highest civilization; estimated to be less than 10,000
years.
GEOLOGIC TIME.
Crnozoic oR MopeERN ERA: AGE OF MAMMAL AND SEED PLANT DOMI-
NANCE.
Glacial or Pleistocene epoch. Last great ice age. Rise of Man.
Pliocene epoch. Rise of types transitional to Man.
Miocene and Oligocene epochs. Rise of Apes.
Eocene epoch. Rise of higher Mammals.
Mesozoic on Meprevat Era: AGE OF REPTILE DOMINANCE.
Cretaceous period. Rise of primitive Mammals and Seed Plants.
Jurassic period. Rise of Birds and flying Reptiles.
Triassic period. Rise of Dinosaurs, and Mammalian stock.
PaLeEozoic OR ANCIENT ERA: AGE OF PRIMITIVE ANIMALS AND PLANTS.
Permian period. Rise of Reptiles and Conifer forests. Another great
ice age.
Carboniferous period. Age of Amphibia. Rise of Insects. Marked
coal accumulation.
Devonian period. Age of Fishes. First known marine Fishes, and
Amphibians.
Silurian period. First known land floras.
Ordovician period. First fresh-water Fishes.
Cambrian period. First shell-bearing marine animals, and dominance
of Trilobites.
Proterozoic ERA: AGE OF INVERTEBRATE DOMINANCE.
An early and a late ice age.
ARCHEOZOIC ERA: ORIGIN OF PROTOPLASM AND OF SIMPLEST ORGANISMS.
COSMIC TIME.
ForMATIVE ERA: ORIGIN OF THE EARTH AS A RESULT OF SOLAR DISRUP-
TION.
Beginnings of the atmosphere and hydrosphere, and of continental
platforms and oceanic basins. No known geological record.
1 From Schuchert, somewhat modified.
360 ANIMAL BIOLOGY
mony to the order of appearance on the Earth of the different
types of animals and plants. (Fig. 232.)
The geological time-table (page 359) summarizes the panoramic
succession of life as it is seen by the paleontologist. It is of little
value to discuss the absolute duration of geologic time, because
the estimates vary so greatly, though there are fairly reliable data
in regard to the relative length of the various eras. Perhaps the
conservative estimate of two billion years — much more than half
ANTES —— Les BIRDS or
AGE OF MAN 7 7
AGE
oF Z
MAMMALS Z
CRETACEOUS
CRETAC
EOMANCHEAN
WY AAA
AB.
3
AGE
OF
REPTILES
JURASSIC
MESOZOIC
\ AAA AAA A MM \
AGE
OF
AMPHIBIANS
MISSISSIPPIAN
CARDONIFEROUS)
SILURIAN :
ae SE Seer
ORDOVICIAN
ee ee a ee ee ee
CAMBRIAN
Fic. 232. — Chart showing the origin and degree of development of the
chief groups of Vertebrates, correlated with the geological time-table. (From
Lindsey, after Osborn.)
PALAEOZOIC
(H}D-PALAEOZOIC
AGE
OF
@SVERTEBRATES
EARLY PALAEOZOIC
of which was before the Cambrian period — will serve to spell the
Earth’s unfathomable past and to afford some idea of the immen-
sity of time available for evolutionary changes.
Even a casual survey of this history — natural history — of the
Earth and its inhabitants cannot but impress one with the fact
that, taken all in all, there has been a continuous, though not
always a uniform, advance in the complexity of organisms from
the most ancient times, and that the older types seem gradually
to melt into modern forms as the remoter geological eras merge
into the more recent. ‘‘Only the shortness of human life allows us
DESCENT WITH CHANGE 361
to speak of species as permanent entities.” Invertebrates appear
in the Proterozoic Era; Fishes, Amphibians, and Reptiles in the
Paleozoic; Birds and primitive Mammals in the Mesozoic; higher
Mammals and Man in the Cenozoic. Mosses and Ferns arise
before Conifers and the latter before the familiar Seed Plants.
Just in proportion to the completeness of the geological record
is the unequivocal character of its testimony to the truth of the
Fic. 233. — Reptilian Bird, Archaeopteryx (A), compared with Pigeon,
Columba livia (B). (From Lull.)
evolution theory. For the sake of concreteness we may select two
examples from the wealth of material offered by the paleontologist.
At first glance there seems to be little but contrasts between a
typical Reptile and a typical Bird; between a cold-blooded, scaly-
skinned Lizard, let us say, and a warm-blooded, feathered Pigeon.
And yet the zodlogist is convinced that Birds have evolved from
a reptilian stock, because, in spite of superficial dissimilarities,
there are fundamental structural resemblances not only between
adult Reptiles and Birds, but also between their embryological
stages. And further, because the fossil remains of a very primitive
Bird, Archaeopteryx, have been found which form, in many ways,
362 ANIMAL BIOLOGY
a connecting link between the Reptiles and Birds as we know them
to-day. (Fig. 233.)
Archaeopteryx was undoubtedly a bird about the size of a
Pigeon, but one with jaws supplied with many small teeth; with
a long lizard-like tail formed of many vertebrae, each bearing a
pair of quill feathers; with a four-fingered reptilian hand; and
so on. In brief, just such a creature as the imagination of an evolu-
tionist would picture for a primitive Bird has been actually dis-
covered in the lithographic stone quarries of Bavaria, representing
the later Jurassic period.
The ancestry of the modern Horse has been the most impressive
fossil pedigree, ever since Professor Marsh collected the famous
series of fossil skeletons from the western United States and ar-
ranged them in the Yale University Museum. They have been
referred to as the “first documentary record of the evolution of a
race.” Huxley studied this collection and regarded it as conclusive
evidence of evolution.
The essential facts of the evolution of the Horse are these.
Horse-like animals probably arose from an extinct group, similar
to the Condylarthra, which had five toes on each foot and a large
part of the sole resting on the ground. However, the first unques-
tionable horse-like forms found in North America are little animals
about a foot in height, known as Eohippus, from rocks of the
EocEnE epoch. The fore foot of Eohippus has four complete toes
(digits 2, 3, 4, and 5) but no trace of the inner digit, or thumb,
while the hind foot has three complete digits (2, 3, and 4) with
vestigial remains, or splints, of the first and fifth. Later in the
Eocene appears Orohippus showing a somewhat larger central
digit in the fore foot and the disappearance of the splints in the
hind foot. Passing up to the OLIGOCENE epoch, Mesohippus, an
animal about the size of a wolf, occurs with fore and hind feet
three-toed, but with the side toes much smaller than the central
one, and just a trace of the fifth digit in the fore foot as a splint.
Then Merychippus appears in the MiocENE epoch, with still
shorter side toes (digits 2 and 4) so that they do not reach the
ground, and the weight is borne solely on the hoofed tip of the third
digit. This same general reduction of the lateral digits and advance
in size and functional importance of the middle digit is carried
further during the PLIOCENE epoch in Pliohippus, which is usually
regarded as the first type of ‘one-toed’ Horse, and leads finally
THE EVOLUTION OF THE HORSE.
Hind Foot
Fore Foot
Formations in Western United States and Characteristic Type of Horse in Each
One Toe
* Splints of
24 ond 4athdigits
One Toe
Splints of
2 ond 4! digits
Pliohippus
Three Toes
Three Toes
Side toes
=
e 2
“ = =
2we oo
o,¥ < o,? 2
Flfy | BY
Vio = vIE =
Lis oo = 5
"= — oe
a = ia
Jers
=
|
|
z:
“ =
2285 n
irl rt)
al aa e
Vlzwe =
4 Pe =
Eas: 2
e ‘ote
GA Eohippus
The Premolar Teeth
become more and more
Hypothetical Ancestors with Five Toes on Each Foot
and Teeth like those of Monkeys etc,
like true molars
Cretaceous
Age of
(From Matthew.)
Jurassic
Triassic
Reptiles
Fic. 234. — Graphic presentation of the evolution of the Horse.
364 ANIMAL BIOLOGY
to the genus Equus which has continued from the PLEISTOCENE
epoch to the present. This genus includes the modern Horse,
Equus caballus, with one functional digit on each foot and vestiges
of two more (digits 2 and 4) as the splint bones. (Fig. 234.)
In this outline of what must be interpreted as the fossil ancestors
of the Horse of to-day, we have merely selected several representa-
tive forms to emphasize changes in foot structure. But the reader
will realize that many other equally significant changes were in-
volved in the transformation — during perhaps ten million gen-
erations — of an Eohippus type into that of Equus. This much
appears certain to the biologist: ““In early Eocene times there
lived small five-toed hoofed quadrupeds of generalized type, that
the descendants of these were gradually specialized throughout
long ages along similar but by and by divergent lines, that they
lost toe after toe till only the third remained, that they became
taller and swifter, that they gained longer necks, more complex
teeth, and larger brains. So from the short-legged splay-footed
plodders of the Eocene marshes there were evolved light-footed
horses running on tiptoe on the dry plains.”
Truly, the stupendous and ever increasing record of ancient
forms of life is not that of a disordered multitude. Newly dis-
covered fossil remains, one after another, fall into the scheme
of a common tree of descent — descent with change.
4. Embryology
If evolution is a fact, one would expect to find evidences of the
genetic relationships of organisms in their embryological develop-
ment from egg to adult. Under former headings we have inciden-
tally mentioned embryological data which point toward evolution,
so that now attention may be confined to an attempt to make clear
a fact of first importance — the history of the individual frequently
corresponds in broad outlines to the history of the race as indicated
by evidence from comparative anatomy, etc. If we have in mind
the earlier discussion of Vertebrate anatomy, just a few examples
will suffice to suggest the type of evidence which supports this
so-called RECAPITULATION THEORY, Or BIOGENETIC LAW. (Page 96.)
Lower Vertebrates, such as the Fishes, have a heart composed of
two chief chambers: an auricle which receives blood from the
body as a whole and a ventricle which pumps it to the gills on its
way to supply all parts of the body. Among the members of the
DESCENT WITH CHANGE 365
next higher group, the Amphibia (Frogs, Toads, etc.), the auricle
is divided into two parts, while the ventricle remains as before.
Thus these forms have a three-chambered heart. Passing to the
Reptiles, we find that most of the Lizards, Snakes, and Turtles
have the ventricle partially divided into two chambers, while
the more specialized Crocodiles and Alligators have a complete
partition and therefore a four-chambered heart. This is the con-
vr yy
SELL
Ba on
On! tan
ye
BH
i ia ap
Uti Greet
Mid d
rire THR
ere
SBSH
Fic. 235. — Embryos in corresponding stages of development. A, Fish
(Shark); B, Bird; C, Man. g, gill slits. (From Scott.)
dition in all adult Birds and Mammals, but the significant fact
is that, in the development of the heart of the individual Bird and
Mammal, embryonic stages succeed each other which parallel in
a general though remarkable way this sequence from a two-
chambered to a four-chambered condition as exhibited in the
adults of the lower Vertebrates. (Figs. 120-122.)
Or take the development of the brain in the Vertebrate series.
Even in the human embryo the fundament of the brain arises by
simple transformations of the anterior end of the neural tube,
366 ANIMAL BIOLOGY
which at first are nearly indistinguishable from the conditions
which exist in the lowest Vertebrates. Then the changes become
progressively more complex along lines broadly similar to those
occurring from Fish to Mammal, until finally the complex human
brain is formed. (Figs. 140, 141.)
The same picture is presented by a study of the development of
the excretory system, the reproductive system, the skull, and so
on. One cannot avoid the fact that the organs of higher animals
during development pass through stages which correspond with
the larval or adult condition of similar organs in lower forms. The
correspondence is far from exact — to be sure, there are gaps and
blurs — but it is not an exaggeration to say that embryological
development is parallel to that which anatomical study leads us
to expect. A knowledge of the anatomy of an animal actually
gives a sound basis of facts from which to predict in broad out-
lines its embryological development. (Figs. 129, 235.)
What are the bearings of these facts on the evolution theory?
It is perfectly logical to conclude that it is an architectural neces-
sity, let us say, for the four-chambered heart to arise from a two-
and three-chambered condition — and undoubtedly if this were
the only example of ‘ontogeny repeating phylogeny’ the conclusion
would be justified. But when one considers the widespread general
correspondence of the developmental stages in higher forms with
conditions as they exist in the adults of lower forms, the facts al-
most overwhelmingly force us to go further and conclude that the
similarity has its basis in inheritance, in actual blood relationship
between the higher and lower forms, in descent with change — evo-
lution.
5. Physiology
Fundamental structural similarities throughout a series of or-
ganisms implies fundamental physiological similarities — struc-
ture and function go hand in hand, each being an expression of
the other: function alone gives permanence to structure — and
this is strikingly corroborated by the data accumulated which
show that species are characterized not only by morphological
attributes, but by their specific biochemical constitution as well.
But this physiological evidence of the relationships of organisms
is less readily presented in brief form, so we may confine attention
to the most significant examples.
DESCENT WITH CHANGE 367
It will be recalled that chemical! control by hormones plays an
important part in the coordination of the multicellular animal
into a working unit, especially in the case of such functions as
digestion, growth, and reproduction. Now it is significant that
the hormones seem to be largely if not completely interchangeable
from one species of Vertebrate to another. Thus a deficiency in
the insulin hormone in Man may be supplied from the pancreas
of a Fish or a Sheep. Obviously this strongly suggests that at least
certain of the chemical regulators have been common factors
from a remote ancestral period.
But chemical differences as significant as anatomical ones have
been revealed by the type of crystals formed by the hemoglobin
of the blood. When orders, families, genera, or species are clearly
separated by anatomical criteria, the crystals are correspondingly
markedly differentiated. Thus from crystal form it is evident
that the common White Rat is the albino of the Norway Rat
(Mus norvegicus) and not of the Black Rat (Mus rattus); and that
Bears are more nearly related to the Seals than they are to Dogs.
Again, there are other important chemical differences, not de-
terminable by ordinary chemical analysis, between the blood even
of closely related species, long known by the fact that the TRANS-
FUSION of the blood of one species into another is usually attended
by physiological disturbances and often by death. It has been
found by innumerable transfusions and also by so-called PRECIPITIN
TEsts of the blood in vitro, that is outside the body, that the degree
of the reaction is in many cases proportional to the degree of re-
lationship of the species involved, as indicated by their classifica-
tion on the basis of anatomical structure.
Thus, as one would expect, human blood shows by the precipitin
test closer chemical relationships with the blood of the highest
Apes than it does with that of the Old World Monkeys; closer re-
lationships with the blood of the latter than it does with that of
the New World Monkeys; and closer with the blood of these than
with that of the Lemurs; and so on. Or, descending to the Reptiles:
paleontology indicates that there is a close relationship between
Lizards and Snakes and also between Turtles and Crocodiles, while
the reptilian ancestor of the Birds was probably more closely allied
with the latter than the former groups. These same relationships
are indicated by blood tests.
Thus aside from a few startling exceptions, which further study
368 ANIMAL BIOLOGY
perhaps may bring into line, all the data warrant the conclusion that
the chemical characteristics of the blood are almost as constant
as structural similarities of the blood vessels. Indeed, the inorganic
salts present in the various circulating fluids of animals correspond
in nature and relative amounts to what we have good reason to
believe was the composition of the ocean some hundred million
years ago. So in evolutionary terms, a common property has per-
sisted in the bloods of animals throughout the ages which have
elapsed during their evolution from a common ancestor: of all
the systems, the blood perhaps is the most conservative in retain-
ing its ancestral condition. Blood relationship is a fact.
6. Distribution
Everyone recognizes that there are wide variations in the fauna
and flora of different parts of the Earth. There is a characteristic
life on mountain, plain, and seashore, and in the sea — as well as
in pond and puddle — and also in arctic, temperate, and tropical
regions. But the problem of animal and plant distribution is by
no means so simple as this statement might seem to imply, because
the study involves the investigation of both the relations of the
various organisms to the general environing conditions, and the
interrelations of the species with each other. It forms a part of the
sciences of plant and animal EcoLoay. (Figs. 220, 236.)
Confining attention merely to the geographical distribution of
animals — which forms the science of zoOOGEOGRAPHY — let us take
a couple of clear-cut examples and see whether or no evolution
offers a reasonable explanation of the facts.
A characteristic genus of Mammals, known as the Tapirs, is
represented to-day by distinct species in two widely separated
regions: Central and South America and southern Asia and ad-
jacent islands. But distribution in the past proves to be the key
to the present distribution. Paleontological studies show that in
the Pliocene epoch Tapirs were distributed over nearly all of North
America, Europe, and northern Asia, and thereafter gradually
became extinct so that by the close of the Pleistocene epoch the
remnants were distributed as we find them to-day. In brief, the
present discontinuous distribution represents the remnants of a
world-wide Tapir population, and the differences between the
existing species are such as one might expect to find among the
members of a genus long isolated in different environments by
DESCENT WITH CHANGE 365
Fic. 236. — Geographical distribution of Song Sparrows. Each different
number indicates habitat of a subspecies. ‘‘We find complete integradation in
color and in size. Nowhere can one draw the line. As the climatic conditions
under which the birds live change, the birds keep pace. Here we have a species
in flower, as it were, a single Song Sparrow stalk with its twenty-nine blos-
soms, any one of which might make an independent growth as a species if it
were separated from the parent stem. Doubtless some day the separation will
come, when we shall have several species, each with its groups of races, but
at present we have only one species, divided into some twenty-nine sub-
species, or species in process of formation.”” (Chapman.)
370 ANIMAL BIOLOGY
geographical barriers. We know, for example, that a litter of
European Rabbits was introduced on the small island of Porto
Santo during the fifteenth century and by the middle of the last
century its descendants had become so distinct from the parent
form that they were described as a new species. (Fig. 92.)
Fic. 237. — Successive forms of a Snail, Paludina, from the Tertiary deposits
of Slavonia. (From Lull, after Neumayr.)
As a matter of fact the characteristic fauna of islands was what
impressed Darwin with the need of some interpretation of their
origin other than by special creation. During his famous three
years’ voyage around the world on the “ Beagle,’ he stopped at the
Galapagos Islands, situated about 600 miles off the west coast of
South America, and was astonished to find that although the fauna
as a whole resembled fairly closely that of the mainland, neverthe-
less the species for the most part not only were different, but even
those of the separate islands were distinct — the islands nearest to
DESCENT WITH CHANGE 371
Fic. 238. — Evolution of the head and molar teeth of Elephants. A, A’,
Elephas, Pleistocene; B, Stegodon, Pliocene; C, C’, Mastodon, Pleistocene;
D, D’, Trilophodon, Miocene; E, E’, Palaeomastodon, Oligocene; F, F’, Moeri-
therium, Eocene. (From Lull.)
372 ANIMAL BIOLOGY
each other having species most similar. Darwin wrote, “ My atten-
tion was first thoroughly aroused by comparing together the
numerous specimens, shot by myself and several others on board,
of Mocking Thrushes, when, to my astonishment, I discovered
that all those from Charles Island belonged to one species
(Mimus trifasciatus); all from Aibemarle Island to M. parvulus;
and all from James and Chatham Islands (between which
two other islands are situated as connecting links) belonged to
M. melanotis.”
Darwin’s observations of such facts as these have been cor-
roborated in the Galapagos and extended to isolated island faunas
and floras all over the world. For instance, half of the species of
Insects and four-fifths of the species of Seed Plants that occur on
St. Helena are found nowhere else. And further, Darwin’s ex-
planation of the phenomena is the most plausible extant. Conti-
nental islands secure their life from the mainland before they are
cut off, and oceanic islands after their formation by volcanic action
alone or aided by coral growth. In either event the organisms in-
habiting islands are isolated from the main stock of the species,
and they diverge, in proportion to the length of time and the degree
of isolation, until they constitute separate races and _ species.
Isolation promotes divergence apparently to a considerable extent
by preventing new types from being swamped by interbreeding
with the old, and by allowing many mutations to become established
that would not survive in a more competitive field. Furthermore,
new conditions may afford new problems to be met by mutations,
and so favor their persistence. We see evolution as a response of
life to its environment. Each species peculiar to each isolated
island can reasonably be interpreted as having arisen by descent
with change. (Figs. 236-238.)
We have now summarized a few concrete examples of the chief
types of evidence that organisms — species — have come to be
what they are to-day through a long process of descent with change.
This evidence, taken with that presented, so to speak, on and be-
tween the lines throughout this work, should place the reader in a
position to form a more or less independent judgment of the ques-
tion. It is only necessary to remind him again that, although the
evidence, from the nature of the case, must inevitably be indirect,
its force is tremendously increased by its amount. And the reader,
with only a very limited amount of the data before him, cannot
DESCENT WITH CHANGE 373
appreciate the overwhelming impressiveness of all the concordant
evidence for organic evolution.
B. Factors oF ORGANIC EVOLUTION
Taking for granted the fact of evolution, what are the factors
which have brought about evolution? That is quite a different
question, but one which has often brought confusion to the popular
mind. Although biologists are in general agreement on the basic
factors involved, there is much debate in regard to their relative
importance and method of operation. And the layman has mis-
taken the questioning of one factor or another for a questioning
of the fact.
No purpose will be served by a long historical account of the
origin of the present-day point of view. Suffice it to say that the
evolution idea is a generalization which has crept from science to
science — from astronomy to geology, from geology to biology,
thereupon becoming ORGANIC evolution. The idea in one form or
another is as old as history, but for all practical purposes the biol-
ogist Lamarck, during the early part of the nineteenth century,
formulated the first consistently worked out theory of organic
evolution. (Fig. 293.)
1. Lamarckism
The evidence for organic evolution offered by Lamarck was
necessarily limited in amount, and in some cases neither happily
selected nor convincingly presented, so it was laughed out of court
by biologists and laymen alike. His evolution factor was essentially
the change of the organism through the use and disuse of parts;
the physiological response of the organism to new needs offered by
new conditions of life. And these changes, somatic in origin, he
believed were transmitted to the progeny. His first statement in
1809, freely translated, is as follows:
“First Law: In every animal which has not exceeded the term
of its development, the more frequent and sustained use of any
organ gradually strengthens this organ, develops, and enlarges it,
and gives it a strength proportioned to the length of time of such
use, while the constant lack of use of such an organ imperceptibly
weakens it, causing it to become reduced, progressively diminishes
its faculties, and ends in its disappearance.
“Second Law: Everything which nature has caused individuals
374 ANIMAL BIOLOGY
to acquire or lose by the influence of the circumstances to which
their race may be for a long time exposed, and consequently by the
influence of the predominant use of such an organ, or by that of
the constant lack of use of such part, it preserves by heredity and
passes on to the new individuals which descend from it, provided
that the changes thus acquired are common to both sexes, or to
those which have given origin to these new individuals.”
Lamarck’s first law is, in general, sound, but the second — the
inheritance of acquired characters — is highly questionable to say
the least, because, as we have seen, there is no evidence that modi-
fications are heritable. But this weak point was not the one which
caused the rejection of the theory by Lamarck’s contemporaries.
The various antagonistic influences can be summed up by saying:
the time was not ripe for evolution.
2. Darwinism .
Then a generation later appeared Charles Darwin in England.
With a better background prepared for him, in part by headway
being made by the evclution theory in geology, he did two things
in his Origin of Species which was published in 1859. He presented
an overwhelming mass of facts which could be explained most
reasonably by assuming the origin of existing species by descent
with change from other species. And he offered as an explanation
of the origin of species the theory of “NATURAL SELECTION, or the
preservation of favoured races in the struggle for life.” It was the
combination of the facts and the theory to account for the facts
that won the thinking world to organic evolution — a common
height from which we view the whole world of living beings.
(Fig. 296.)
What, in brief, was the theory? In the first place, without
attempting to determine the cause of variations, Darwin showed
the great amount of VARIATION in nature. And any and all kinds
of heritable variations were, broadly speaking, important — though
he somewhat grudgingly admitted the inheritance of acquired
characters.
The universality of variations established, Darwin emphasized
the fact that the POWER OF REPRODUCTION of organisms far ex-
ceeds space for the offspring to live in and food for them to eat.
Some recent data will illustrate this point. A microscopic Para-
mecium possesses the power to eat, grow, and reproduce — to
DESCENT WITH CHANGE 375
transform the materials of its environment into Paramecium proto-
plasm — at the rate of 3000 generations in five years. And all the
descendants (if they actually existed) would equal 2 raised to the
3000th power, or a volume of protoplasm approximately equal to
101° times the volume of the Earth! The Plant Lice, or Aphids,
may produce a dozen generations in a year. The final brood, as-
suming the average number of young produced by each female to
be one hundred and that every individual produced its full com-
plement of young, would consist of ten sextillion individuals —
a procession, if it could be marshalled, that ““would extend from
the Earth out into space far beyond the furthest star that has
ever been discerned by the telescope.”” The common House Fly
under favorable conditions may lay as many as six batches of eggs,
of about one hundred and forty eggs each, during its short life
of approximately three weeks. Assuming that all the progeny
survived and multiplied at the same rate, “the progeny of a single
pair, if pressed together into a single mass, would occupy some-
thing like a quarter of a million cubic feet, allowing 200,000 flies to
a cubic foot.” And the all too familiar Mosquito may have nearly
two hundred billion descendants during one summer. Indeed, the
common Rat will afford astounding figures. (Figs. 222, 246, 258.)
The number of individual organisms on the Earth is essentially
infinite. If it is assumed that the average life span of an individual
is a year —a day would probably be nearer the truth — then
one must grip the fact that this infinitude of individuals is each
year wiped out, and replaced by reproduction. Such an almost
explosive expansion of a population under favorable conditions is
appalling, though true, and serves to afford an appreciation of
the enormous realized and unrealized potentialities of living mat-
ter to make more living matter — to reproduce. ‘The problem
of organic evolution is that of the evolution of an organic mass
consisting of an infinitude of individuals reproduced during an
infinitude of generations. ”’
Something must — does — suppress the inherent power of each
species to overpopulate the Earth, and Darwin emphasized the
STRUGGLE FOR EXISTENCE between the individuals of species.
Since the struggle is so keen, a variation, however slight, which
fits — adapts — an individual better to its surroundings than its
neighbors are adapted, will, more cften than not, give its possessor
an advantage in the struggle, and accordingly the latter will
376 ANIMAL BIOLOGY
Fic. 239. — A few varieties of domestic Pigeons. Over one hundred and
fifty different breeds have been derived by selection from the wild Blue-rock
Pigeon, some of which “differ fully as much from each other in external
characters as do the most distinct natural genera.””’ (Darwin.) 1, Blue-rock
Pigeon, Columba livia, ancestral form; 2, homing; 3, common mongrel; 4, arch-
angel; 5, tumbler; 6, bald-headed tumbler; 7, barb; 8, pouter; 9, Russian
trumpeter; 10, fairy swallow; 11, black-winged swallow; 12, fantail; 13, car-
- rier; 14, 15, bluetts; bird between 14 and 15, a tailed turbit. (From photo-
graph of an exhibit in the United States National Museum.)
DESCENT: WITH CHANGE ott
tend to survive and to pass on the favorable variation to its prog-
eny. Thus by NATURAL SELECTION is brought about THE SURVIVAL
OF THE FITTEST — the survival of those individuals, and there-
fore species, which are best adapted to the peculiar conditions
of their environment and mode of life. And note, this offers an
explanation of the fact of adaptation itself — perhaps the most
striking phenomenon which organisms exhibit.
This is all so simple from one point of view and so confusingly
complex from others that it may well be restated in a couple of
sentences by Darwin himself: ““As many more individuals of each
species are born than can possibly survive, and as, consequently,
there is frequently recurring struggle for existence, it follows that
any being, if it vary however slightly in any manner profitable
to itself, under the complex and sometimes varying conditions of
life, will have a better chance of surviving, and thus be naturally
selected. From the strong principle of inheritance any selected
variety will tend to propagate its new and modified form.”
Nothing succeeds like success, and once started Darwin’s theory
gradually swept nearly all opposition away. Indeed, some of its
advocates in their enthusiasm extended Darwin’s theory to a
point not justified by his own conservative statements. Then,
as was to be expected, the reaction came. One objection after
another was raised as the problem was studied from nearly every
standpoint by biologists the world over. But it is unnecessary to
obscure the main issue by entering into these controversies. What
is the status of the theory of natural selection to-day) The answer
must be sought in the light of genetics. (Figs. 180, 239.)
3. Genetics and Evolution
Evolution is not a closed book — an event which has been com-
pleted in the past — but a process which is actively going on now.
It may well be an accelerating process that is gaining momentum.
Perhaps it is even to-day but little beyond the beginning of its
revelations. ‘‘ Nothing endures save the flow of energy and the ra-
tional order that pervades it.’’ And there is every reason to be-
lieve that the factors involved in present evolution are the same
as those which have operated in the past. This UNIFORMITARIAN
doctrine has proved productive in explaining the evolution of the
Earth, and all the available evidence indicates that this view-
point will prove — is proving — equally valuable in understand-
378 ANIMAL BIOLOGY
ing the origin of the diverse inhabitants of the Earth. We now
realize that organic evolution is a bird’s-eye view of the results
of heredity since the origin of life— the facts of inheritance
hold the key to the factors of evolution. Therefore we shall
consider the relations of recent discoveries in genetics to the
evolution problem — to the origin of the fitness of organisms.
SELECTION. The process of selection has long been successfully
practiced by man to establish desirable types of domestic animals
and plants, and, as we know, Darwin assumed that a somewhat
similar but automatic selective process determines the survival
of the better adapted wild forms in nature. Darwin clearly
recognized that selection in itself can produce nothing — its efficacy
depends on the materials afforded by variation. But he did not
and, of course, could not make the modern sharp distinction be-
tween modifications, recombinations, and mutations. In general
he accepted all variations as at the disposal of selection, but em-
phasized the importance of small, finely-graded fluctuating varia-
tions in gradually producing, through many generations, a cumu-
lative effect in the direction of selection — variations that to-day
we know are, in part, modifications.
The modern approach to the critical analysis of significant varia-
tions was opened by the work of two botanists, deVries and
Johannsen. DeVries laid stress on the importance of discontinuous
variations which he called MUTATIONS — a class of variations that
we have already discussed; while Johannsen made clear that in a
homozygous germ complex, or
PURE LINE, selection is ineffective,
as will appear beyond.
Some of the problems of selec-
tion will be clear from an example.
Take, say, a quart of beans and
sort them into groups according
1 at Ae to the weight of each bean. Then
Fic. 240. — Diagram to illus- ;
trate a quart (population) of beans put each group into a separate
assorted according to weight. cylinder and arrange the cylinders
in a_ series according to the
weight of the enclosed beans. Now if we imagine a line connecting
the tops of the bean piles in the cylinders, it takes the form of a
normal curve of probability, or variability curve. A similar figure
DESCENT WITH CHANGE 379
would be obtained by the statistical treatment of nearly all fluctuat-
ing characters among the members of any large group of organisms,
or of the size of the grains in a handful of sand, or the deviations
of shots from the bull’s-eye in a shooting match. Therefore the
variations with respect to a given character very closely approxi-
mate the expectation from the mathematical theory of probability,
or chance, and the reasonable conclusion is that such finely-graded
fluctuating variations are a resultant of a large number of factors,
each of which contributes its slight and variable quota to the ex-
pression in a given individual. (Figs. 240, 241.)
The question is, what results are obtained by breeding from
individuals which exhibit such a fluctuating variation to, let us
Inches 54 55 56 57 58 59 60 Gl 62 63 64 65 G6 67 68 69 70 71
Persone 2 S$ 7 18 $4 80 135 163 183 163 115 78 41 146 8 & 2
Fic. 241. — Normal variability curve plotted from measurements of the
height of 1052 women (population). The height of each rectangle is propor-
tional to the number of individuals of each given height. (From Kellicott,
after Pearson.)
say, a greater degree than that of the mean of a mixed POPULA-
TION? One will perhaps expect, and rightly, that the offspring
usually will exhibit the character to a less degree than the parents
but to a greater degree than the population. The top (mode) of the
curve will have moved, so to speak, slightly in the direction of selec-
tion. Now, by continuing generation after generation to select as
parents the extreme individuals, is it possible, with due allowance
for some regression, to take one step after another indefinitely, or
until the character in question is expressed to a degree which
did not exist previously? The experience of practical breeders
380 ANIMAL BIOLOGY
gives a partial answer, since the continual selection of the best
animals for mating and the best plants for seed has been a profit-
able procedure. But it has long been known that after a certain
amount of selection has been practiced it may cease to be effec-
tive, and thenceforth serves chiefly to keep the character at the
higher level attained. (Fig. 242.)
The crux of the matter is in regard to exactly what the varia-
tions are. Both modifications and recombinations are usually in-
cluded, and this mixture of non-heritable and heritable variations
1 2 3 4
Fic. 242. — Schematic representation of the effect of selection from the
viewpoint of Galton’s ‘law of filial regression.’ 1, Mode before selection;
2, 3, 4, new (successive) modes, the results of selections of individuals at
2’, 3’, 4’. The mode has been shifted in the direction of selection (toward the
right). But there has been each time an amount of regression indicated by
the length of the arrows.
is what makes confusion. If we rule out recombinations by inbreed-
ing or by self-fertilization of homozygous individuals, soon we
establish puRE LINES. Then the variations are all modifications
and selection is ineffectual with characters which are not inherited.
The importance of this point was discovered by Johannsen in
careful experiments on the inheritance of characters in single pure
lines of a brown variety of the common garden Bean. For example,
by keeping the progeny of each individual bean separate from that
of all the rest, he was able to isolate a number of pure lines which
differed in regard to the average weight of the beans. Thus selec-
tion resolved the bean population with which he began into its con-
stituent ‘weight types,’ or lines, each of which exhibited a charac-
teristic variability curve of its own with a mode departing more
or less from that of the population. But when Johannsen selected
within a pure line (ruled out recombinations) nothing at all re-
sulted; he was unable to shift the mode because he was dealing
with non-heritable characters. In other words, selection sorts out
preéxisting pure lines (lines with homogeneous germinal constitu-
DESCENT. WITH CHANGE 381
tion) from a population and then stops — though if selection is
stopped the isolated lines usually merge soon again into the original
population. A mutation must occur in the pure line for selection
to be effective — but by
: : lt
the mutation the single —
pure line becomes two. :
(Fig. 243.)
Thus the pure line con-
cept has served to clarify
our ideas in regard to 2
selection by focussing at-
tention on the actual na-
ture of the variations be-
ing dealt with — to make 8
a sharp discrimination
between modifications,
which are a result of en-
vironmental influences
recurrent in each genera-
tion, and variations that
are heritable because they 3
are the result of changes
in the germ plasm.
However, it will be
recognized at once that,
in general, the animal
breeder, as well as Nature, Population
deals with hybrid stock,
heterozygous in regard to
m
a characters, rather Fic. 243. — Diagram to illustrate a popula-
than pure lines. Even tion of beans and its five component pure
pure lines do not stay pure lines. The beans are assorted according to
—mutations occur. Here weight. Tubes containing beans of the same
: weight are placed in the same vertical row.
selection has ample ma- g¢e¢ Fig. 240. (From Walter, after Johannsen.)
terial at its disposal so it
can and does isolate new combinations and accumulate mutations
in the direction of selection. If it is carried on sufficiently long,
the extent of the change may be very great: a more or less steady
change in the direction of selection when mutations are available.
Although selection is not ‘creative,’ it is effective: the appre-
382 ANIMAL BIOLOGY
ciation of its limitations has but accentuated its possibilities.
NATURAL SELECTION may automatically act as a ‘sieve’ and sort
out the new combinations and mutations presented — retain the
fit and discard the unfit — and so afford a natural explanation of
adaptation. (Figs. 179, 239.)
MetTHOD OF EvoLuTiIon. A synoptic view of some of the essen-
tial facts, presented from a different angle, may serve to clarify
our view of evolution as fundamentally a complex problem in
genetics.
In the first place, we have seen that though variations are the
rule and not the exception, some are of importance for evolution
and some are not. All the evidence indicates that the effective
variations are germinal and not somatic. Changes arising in the
soma — modifications — are unable to attain representation in the
germ so that they are ‘born again,’ although modifications re-
newed by the soma in each generation may enable a race to survive
until appropriate recombinations or mutations appear. Evolution
must be brought about by changes in the germinal complex — by
the evolution of the germ plasm itself. Accordingly selection must
operate to eliminate the unfit germ plasm (genotype) rather than
the unfit soma (phenotype), though as a matter of fact the fitness
of an individual is determined largely by its somatic characters.
Dominant genes are directly within the reach of natural selection,
whereas recessive genes may slip by because they are frequently
concealed by dominants. The latter is a much more select, and
selected, group: recessives may be the “skeleton in the nuclear
cupboard of the race.” We know that inbreeding, e.g., cousin-
marriages in man, often reveals recessives because relatives are
likely to carry the same recessives and so afford more chance for
them to meet and become expressed in the offspring. This presents
a complication of the mental picture of the operations of selection
which did not exist before our modern concept of phenotype and
genotype. Since individuals frequently belie their genotypic con-
dition — what they can pass on to their progeny — natural selec-
tion has, so to speak, a more devious though not less sure path.
(Fig. 182.)
Secondly, how does the germ plasm change? It will be recalled
that germinal alterations result from the usual processes of re-
combination and crossing-over, as well as from the more radical
mutations — chromosomal aberrations and intrinsic gene changes.
DESCENT WITH CHANGE 383
These afford a wealth of opportunities for alterations in the germ
plasm and so for the appearance of new characters in organisms.
Indeed the significance of fertilization, which, of course, is at the
basis of hybridization, can hardly be overemphasized at this point.
It provides new combinations not only of established germinal
factors but also of such mutations as occur, and so affords oppor-
tunity for relatively rapid germinal change. (Figs. 167, 189, 196.)
True it is that mutations seem to be infrequent in comparison
with non-inheritable changes of somatic origin, nevertheless it
must be borne in mind not only that somatic changes are more
readily apparent, but also that the majority of the mutations
which occur lead to a decrease in vitality or even to death. This
is to be expected, for a random change in a highly complicated
mechanism such as a living organism, which has long survived
in a severely competitive environment, is far more apt to upset
than to improve the nicety of its internal and external adaptation;
natural selection is conservative unless a changing environment is
presenting new conditions to be met.
Relatively little information is available in regard to the basic
factors that induce mutations, but we have seen that recent ex-
perimental work indicates that environmental factors, such as
irradiation, etc., acting directly on the genetic complex are not
without influence — mutations have been produced. Indeed it
seems probable that both external and internal environmental
conditions, particularly the new cellular environment of the genes
following hybridization, are potential inducers of genetic change
and so of new variations at the disposal of natural selection. How-
ever, when all is said, we are far from any appreciation of the
physico-chemical changes in the germinal material itself that are
responsible for the new characters. Characters may emerge that,
at least, are not recognizable as the computable or additive result
of newly associated genes — the expressions of the genes may
change, new properties may emerge — “emergent evolution.” But
witness the properties of water that emerge from a certain associa-
tion of hydrogen and oxygen!
One may well inquire whether geneticists in their extensive ex-
periments during the past two decades have succeeded in ‘creat-
ing’ a new species. And the answer is largely determined by one’s
concept of a species — a problem we have already discussed. It is
fair to say that some biologists hold that new species have been
384 ANIMAL BIOLOGY
‘created,’ while others who are more conservative prefer to con-
sider the new forms as ‘artificial species.’ Probably all would
agree that some of the new types would be regarded as true species.
were their origin not actually known. The question, however, is
not so important as it may, at first glance, appear. The essential
fact is that we now understand, at least to a considerable extent,
the mechanism of inheritance and variation that is at the basis of
similarity and dissimilarity of individuals, parents and offspring —
the mechanism that surely is crucially involved in the differentia-
tion of groups of similar individuals, or species.
To epitomize — these facts from genetics, taken in connection
with the wealth of data from geographical distribution, the suc-
cession of types in the geologic past, and so on, give us the modern
background for attempting to form an opinion of the method of
evolution. The opinion of most biologists is that natural selection
in general is a guiding principle underlying the establishment
of the adaptive complexes of organisms. Evolution is the result
of mutations, germinal variations, largely, though not entirely,
independent of environing conditions. Many of these variations
give rise to characters which neither increase nor decrease the
adaptation of the organism, and consequently are neutral from
the standpoint of its survival. With regard to such characters
natural selection is essentially inoperative. Other germinal changes
occur, some of which produce adaptive and others unadaptive
characters, and here natural selection is effective. It may elim-
inate the unadaptive and leave the adaptive variations and so
make possible the survival value of the latter in the struggle for
existence. The germ plasm never ceases to experiment, or natural
selection to discover. Variability affording opportunity for adapta-
bility is expressed in evolvability — perhaps the most profoundly
significant characteristic of life.
So, it will be noted, this is essentially a clarified view of Darwin’s
idea of natural selection that has been made possible by recent
intensive studies of the intrinsic nature and the origin of varia-
tions. Natural selection still affords the most satisfactory explana-
tion of that coordinated adaptation which pervades every form of
life: it shows how nature can be self-regulating in establishing
adaptations. But it is probable — indeed, positive — that there
are more factors involved than are dreamt of in our biology.
In the words of Thomson: ‘The process of evolution from in-
DESCENT WITH CHANGE 385
visible animalcules has a magnificence that cannot be exaggerated.
It has been a process in which the time required has been, as it
were, of no consideration, in which for many millions of years there
has been neither rest nor haste, in which broad foundations have
been laid so that a splendid superstructure has been secured, in
which, in spite of the disappearance of many masterpieces, there
has been a conservation of great gains. It has its outcome in per-
sonalities who have discerned its magnificent sweep, who are
seeking to understand its factors, who are learning some of its
lessons, who cannot cease trying to interpret it. It looks as if
Nature were Nature for a purpose’’ — but this thought carries us
beyond the accepted sphere of science into the great fields of phi-
losophy and theology.
CHAPTER XXIV
BIOLOGY AND HUMAN WELFARE
Man is part of a web of life which he continues to fashion, and the
success of his weaving depends upon his understanding. — Thomson.
Now that we have made a general survey of the foundations of
biology, it is important to consider some of the outstanding con-
tributions of biology to human welfare — contributions made, for
the most part, within a century, but already so interwoven with
our everyday life that they have become indispensable.
Strange as it may at first glance appear, usefulness is not the
basic standard of value adopted by most scientific men: the dis-
covery of truth is their aim, lead where it will. Although their
controlling motive is increase of knowledge and enlargement of our
outlook on nature, it is nevertheless a fact that the practical ap-
plication of their discoveries is responsible for most of the conditions
which constitute the environment of modern life. “Science brings
back new seeds from the regions it explores, and these seem to be
nothing but trivial curiosities to the people who look for profit from
research, yet from these seeds come the mighty trees under which
civilized man has his tent, while from the fruit he gains comfort
and riches.”” Indeed, the supreme test of the intellectual life of a
community is the importance which it attaches to research and
creative intellectual effort. Unless research is held in high esteem,
with adequate facilities for its maintenance and adequate rewards
for those who devote themselves to it, the development of applied
science will be retarded.
One of the most surprising illustrations of the way in which
seemingly useless biological research often reveals itself almost
overnight as of the greatest utility to business, is afforded by the
present interest of the oil industry in certain Protozoa known as
FORAMINIFERA: not living Foraminifera but fossil forms from the
geological past. A short time ago oil companies were wasting large
sums in sinking drills from which no oil came: frequently such a
drilling cost as high as sixty thousand dollars, eventually to be
paid chiefly by the motor-car owner. Then it was noted that
386
BIOLOGY AND HUMAN WELFARE 387
Foraminifera were brought to the surface with the drilling, and the
problem was to determine whether the species found in material,
from relatively near the surface in drills that proved to be oil-
bearing, could be distinguished from those from dry drills. Accord-
ingly the companies turned to the United States National Museum
Fic. 244. — Shells of several species of Foraminifera, considerably magnified.
1, Cyclammina pauciloculata, two views; 2, Globotertularia anceps; 3, Margin-
ulina ensis; 4, Vagulina spinigera; 5, Nodosaria filiformis; 6, Rhabdammina
abyssorum; 7, Chilostomella grandis.
which has made a practice of gathering and preserving samples of
microscopic life from all parts of the world: material which the
layman would undoubtedly regard as refuse and throw away.
Thanks to a lifetime spent in studying Foraminifera, one expert
was able to make the necessary determination, and his knowledge
when applied in the oil fields resulted in saving the industry many
millions of dollars. Whereas the small governmental appropriations
388 ANIMAL BIOLOGY
for this biologist’s work had been considered by many an economic
waste, to-day it is reported that the annual income tax on the
salary his services command from the oil industry repays many-
fold to the Government the annual grants that formerly were
made to him. (Figs. 20, 244.)
Hence it is far from true that so-called pure science and applied
science are distinct and independent activities. There is only one
kind of science — science aspiring for truth and knowledge, and
there could be no application of science unless that knowledge pre-
viously existed. (Great innovations are chiefly facile applications
of truths which their authors have pursued for their own sake,
and in our most theoretical moods we are frequently nearest to the
most practical applications.
Indeed, some of the great biological generalizations with which
we have become familiar on previous pages have in everyday
affairs a profound practical significance which is easily overlooked.
For example, the cellular structure of all living things — imply-
ing the existence of a fundamental similarity in organization
throughout the living world. Again, the basically similar life-stuff,
protoplasm — demonstrating that all living nature is united by a
common bond not only of cellular organization but also of proto-
plasmic basis to which all life phenomena are referable. Still
again, the transformation by protoplasm of non-living material
into living material — proving that living matter is ordinary
matter peculiarly organized. And finally, organic evolution. All
nature is one.
These and other great biological truths have a far-reaching im-
port to everyone, because collectively they unmistakably lead to
the grand conclusion that human life must be interpreted in terms
of all life. Man must conform to the general order of living nature
of which he is an integral but dominant part. Remove one of the
essentials of life and he perishes like the beasts. But he differs in
capacity to understand and to take advantage of circumstances.
Human welfare, therefore, demands that. Man must ‘control’
nature by consciously adapting himself to it. Indeed, the chief
purpose of education is the adaptation of the individual and the
promotion of adaptability — adjustment to the basic internal and
external conditions of life without a loss of plasticity. Thus biology
affords the natural foundation of the science and art of right living
which human welfare demands.
BIOLOGY AND HUMAN WELFARE 389
A. MEDICINE
Health — the adaptation supreme — is a priceless possession
whether it be estimated from the standpoint of the well-being of
the individual or in terms of national wealth. Accordingly, medi-
cine, in the broadest sense of the word, is without doubt the most
important aspect of applied biology. Human anatomy and physi-
ology, on which the foundations of medicine rest, are merely special
parts of the general sciences of anatomy and physiology of all
organisms. In fact the interpretation of human anatomy is impos-
sible except in the light of the comparative anatomy of Verte-
brates, while human physiology owes its present state of develop-
ment to the fundamental principles derived from experimentation
on the lower animals. And hope for further advance is chiefly de-
pendent upon similar investigations on animals which have been
rendered insensible to pain by anesthetics. To mention one ex-
ample: experimental surgery practiced on animals has demon-
strated the possibility of innumerable operations which no con-
scientious surgeon would have ventured to perform for the first
time on Man. In the words of Darwin who gave up the sport of
hunting on account of his great sympathy with the suffering of
animals: ‘‘ Physiology can make no progress if experiments on liv-
ing animals are suppressed, and I have an intimate conviction that
to retard the progress of physiology is to commit a crime against
humanity.”
1. Microdrganisms and Disease
No one will gainsay that discoveries in preventive and curative
medicine rank amongst the most important contributions of sci-
entific research to civilization, and nearly all have as their founda-
tion studies by generations of biologists. Though Pasteur’s first
investigations were in chemistry, his subsequent work, which
pointed out the way of preventing and eradicating diseases due to
microorganisms, followed naturally from his discovery that the
souring of wine and milk is the result of the activities of organisms
from the air which induce chemical changes. Injure the skin of a
grape, and organisms irom the atmosphere enter and fermentation
begins. Exclude air or sterilize it, and fermentation is prevented.
Lister immediately saw the importance of this for surgery, and
modern aseptic surgery — one of the greatest blessings of man-
kind — was born. (Figs. 152, 278.)
390 ANIMAL BIOLOGY
Proceeding on the theory that since fermentation is the result
of the activities of microdrganisms, certain diseases of plants and
animals are likewise caused by the invasion of the body by similar
germs, Pasteur’s early success in preventive medicine came from
the study of cholera in French Fowls and anthrax in Sheep and
Cattle. The treatment he employed reduced the death rate of the
animals from about ten per cent to less than one per cent, and saved
the French nation in twenty years not less than the amount of the
war indemnity of 1871. Then Pasteur devised the treatment for
A B
Fic. 245. — Endamoeba histolytica, a parasitic Amoeba of the human in-
testine, which gives rise to amoebic dysentery. A, active Amoeba showing
nucleus and three ingested human red blood corpuscles; B, encysted Amoeba
with four nuclei, preparatory to division into four individuals. (Redrawn,
after Dobell.)
rabies. The human fatalities from this disease, usually arising by
infection from the bite of a ‘mad’ Dog, fell almost at once from
nearly one hundred per cent to less than one per cent.
During the past half-century a host of investigators, following
the lead of Pasteur, have secured undreamed-of results in discover-
ing preventive measures and curaiive treatments for a long series
of diseases of Man, domestic animals, and plants. One thinks im-
mediately of diphtheria, tuberculosis, bubonic plague, typhus
fever, malaria, yellow fever, syphilis, amoebic dysentery, and
African sleeping sickness — all the results of the infection of Man
BIOLOGY AND HUMAN WELFARE 391
by microscopic organisms. It is hard to realize that so recently as
1884, Koch, probably the greatest successor of Pasteur, proved
that tuberculosis is caused by a specific type of Bacteria, and
thereby revolutionized the treatment of this disease which has
been estimated to cause about one-seventh of all the deaths in the
world each year.
Only second in importance to the prevention of diseases of Man
that are due to microdrganisms, is the suppression of diseases of
COMMON EPIDEMIC
DISEASES
Note Recent Years When Microbic Causes
were Established —
A Anthrax Uplenic Rver) 1876 || | Malta Fever 1887
m Asiatic Cholera 1883 || Mcningius 1887
# Bubonic Plague 1894 | Pncumonia 1884
8 Diphtheria 1884 | Relapsing Fever 1873
: YcAntitoxin discovered 1890)
H Dysentcry 1g98 || SYPhilis su
H. Glanders 1882 || TCTANUS «Lockjaw) 1889
Gonorrhea. , 1879 || Tuberculosis 1884
Infantile Paralysis 1909 |
Typhoid Fever 1884
Influenza. 1892 Hite. zing Vaccine
Leprosy 1892 ; | established. 1896.)
Malaria. ,,,. 1880 [cae Fever
2 (Transmission by Mosquitoes (Transmission by Mosquitoes
4 established 1897.) | established /900)
bubonic plague by rats, ana sleeping sickness
by the tsetse fly. This knowledge has shown how
Several fevers are transmitted by lice or other insects ~
4: prevent the spread of these diseases,
ea on Medical Education and Hospitals
AMERICAN MEDICAL ASSOCIATION
domestic animals. For example, the Chief of the Bureau of Animal
Industry estimates that the Bacteria which produce infectious
abortion in Cattle are responsible for an animal loss of approxi-
mately fifty million dollars each year in the United States. And
the study of this disease is proving of increasing significance from
the recognition that undulant fever in Man is caused by a member
of the same group of Bacteria — an excellent instance of how
knowledge leads to further knowledge.
Indeed, the way interlocking data from several biological fields
are frequently necessary to determine the causative agent of a
392 ANIMAL BIOLOGY
disease can, perhaps, be best illustrated by a brief outline of the
development of our knowledge of malaria, yellow fever, and
syphilis.
MatartiA. From ancient times malaria, as the name indicates,
was supposed to be due to foul air, especially from swampy regions,
but the first step toward the correct explanation was made in 1880
when Laveran found certain microscopic parasites always present
in the blood of malarial patients. Nearly two decades later Ross
demonstrated similar parasites in the body of a Mosquito, and
“Richard -Edes'Harrison-
ANOPHELES
Fic. 246. — Mosquito life histories. Mosquitoes of the genus Anopheles,
which transmit malarial parasites, differ from the common Culer in every
stage. When at rest the adult Culer holds its body parallel to the surface,
whereas Anopheles holds it nearly perpendicular.
then a long series of studies by various investigators, among whom
Grassi stands foremost, showed that when a mosquito of the genus
Anopheles bites a malarial patient, it secures with the blood: some
of the parasites such as Laveran had discovered, and thereupon
the mosquito becomes the host of the Malarial organism. Within
the mosquito, the parasite undergoes a complicated series of
changes, including rapid reproduction, This finally results in myri-
ads of parasites located in the salivary glands of the insect, ready
to be injected into the blood of the next individual bitten and to
begin the other phase of its life history in Man. (Fig. 223.)
So stated, it appears simple enough, but years of study by spe-
cialists on Insects (ENTOMOLOGISTS), by specialists on Protozoa
BIOLOGY AND HUMAN WELFARE 393
(PROTOZOOLOGISTS), and by physicians highly trained in general
medical zoology are behind the scenes in making clear the way to
eradicate a disease which, it is estimated, each year costs the United
States not less than one hundred million dollars and the British
Empire three times that amount. In India alone it is respon-
DEATHS _ FROM
MALARIA
=, “V LOUISIANA IN 1918-19/9-1920
1 AM THE ANOPHELES MOSOU!
' 1 CAUSE THESE SLAP eg
THOUSAND CAS$
OF SICKNESS AND COST LOUISIANA
ABWT $4,000,00022 A YEAR
LOUISIANA
STATE BOARD OF HEALTH
Fic. 247. — Map used in anti-malaria campaign in Louisiana. Each dot
represents a death from malaria.
sible for a death list of more than a million people annually.
(Figs. 246, 247.)
YELLOW FEverR. Proof that malaria is due to a Protozoon
which can only be transmitted to Man by members of a certain
genus of Mosquitoes, was largely the work of English, French, and
Italian biologists; but the demonstration that another kind of
mosquito, Aedes, is responsible for the transmission of the agent
which causes yellow fever is due to Reed, Lazear, and other mem-
bers of the United States Yellow Fever Commission working in
Cuba in 1900. This was followed by the investigations of many
biologists, with the result that to-day one may be vaccinated with
a weakened virus of yellow fever and probably be rendered immune
for several years.
394 ANIMAL BIOLOGY
Most inspiring is the long story of heroism and hard work which
has made it possible to cope with yellow fever; made possible the
building of the Panama Canal, since the earlier attempt by France
was unsuccessful largely on account of its ravages. We even have
to be reminded that half a million cases occurred in the United
States during the past century: the epidemic of 1793 took a total
of one-tenth of the population of Philadelphia, and that of 1878
killed more than thirteen thousand in the Mississippi Valley alone.
Fic. 248. — Results of a quarter century of yellow fever control.
(From the Rockefeller Foundation.)
However, while this and more is true, the recent discovery of
new sources of infection in the interior of South America and
Africa has made the problem much more complex and the complete
control of the fever less certain than it appeared to be a decade
ago. (Fig. 248.)
SypHiuis. The brilliant investigations, chiefly of the proto-
zoologist Schaudinn in 1905, revealed the unicellular parasite,
Treponema pallidum, that is the cause of syphilis — one of the
greatest scourges of mankind since it became widespread during
the sixteenth century. The ravages of the parasite produce many
symptoms — frequently a type of paralysis, or paresis, with a
BIOLOGY AND HUMAN WELFARE 395
gradual loss of the mental faculties and death. The discovery of
the cause has made possible intensive studies of therapeutic meas-
ures to combat it, and the test of upward of a thousand chemical
substances has resulted in the discovery of certain organic arsenic
compounds of considerable specific value if employed in the early
stages. This knowledge, supported by the enlightened attitude
that is gradually being taken toward the disease, offers a brighter
outlook for the future. While syphilis, of course, is not inherited,
much of it in the world today is due to the infection of infants
before or at the time of birth. (Fig. 249.)
Fic. 249. — Treponema pallidum (the spiral bodies) in liver of child with
congenital syphilis. Highly magnified.
2. Parasitic Worms
Thus far we have been considering causative agents of disease
which are popularly called microbes, and we must now turn from
this “‘“world of the infinitely little’ to somewhat larger organisms
which form a most important part of medical zodlogy. This field
may be illustrated by various kinds of parasitic worms. (Fig. 250.)
TREMATODES. There are many parasitic Flatworms, related to
the free-living Planaria, that comprise the group TREMATODA.
Parasitic species exhibit, for the most part, complicated life his-
tories which have taxed the patience and ingenuity of biologists
to unravel. Among the numerous species, the Liver Fluke is per-
haps of most interest. (Figs. 43, 156, 161, 251.)
396 ANIMAL BIOLOGY
The adult Liver FLUKE is a worm about an inch long which
lives in the bile ducts of the liver of Sheep, Cattle, Pigs, etc., and
occasionally in Man. It is hermaphroditic, each individual pos-
sessing both male and female reproductive organs, and in its iso-
lated position is almost continually producing fertilized eggs. In
fact, one Fluke may liberate over five hundred thousand eggs
__{ Paragonimus westermant
i Ascaris lumbricoides (larva)
lasmodi lei -— { Echinococcus granulosus
Plasmodium falciparum ate sip te
Leishmania donovani
! Ascaris lumbricoides
Giardia lamblia \ \ Ancylostoma duodenale
3 Taenia solium
Isos, hominis }
oc acs i Diphyllobothrium latum
Balantidium coli }
Endamoeba histolytica }
Endamoeba coli !
Schistosoma mansont
Enterobius vermicularis
Trichomonas hominis —~ ye
Trichuris trichiura
5 Wuchereria bancroftt
Trichomonas vaginalis — j (lymph nodes)
ic Phthirius pubis
Ye Schistosoma haematobium
Fig. 250. — The chief points of jocalization of some of the parasites in the
organs of the human body. At the left are Protozoa; at the right, Worms.
(After Hegner.)
which pass down the bile ducts of the host (sheep) into the intes-
tine and finally leave the body with the feces. An egg that happens
tc reach moisture develops into a ciliated larva, or MIRACIDIUM,
which escapes from the egg-shell and swims about. For further
development to occur the larva must encounter within a few hours
a certain species of fresh-water Snail, otherwise death results.
But once it has bored into a snail’s body, the parasite receives
BIOLOGY AND HUMAN WELFARE
Proboscis (extruded)
AP Sf>
iO
a :
1 -
i
G ve)
Mice
Ny,
WET} rn)
Intestine. __
ry
Ventral sucker__/¢ iy
Nephridium +4,
397
»\ ~
° es, —Germ-cells
Fic. 251. — Life history of the Liver Fluke, Fasciola hepatica. A, ‘egg’;
B, miracidium; C, sporocyst; D, E, rediae; F, cercaria; G, encysted stage;
H, adult (nervous and reproductive systems omitted). (From Hegner, after
Kerr.)
398 ANIMAL BIOLOGY
a new lease of life involving a series of changes. After about
two weeks it has become a sac-like creature, or SPOROCYST, which
in turn proceeds to develop within itself a brood of another larval
stage, the REDIA. Each redia liberated from a ruptured sporocyst
usually gives rise to one or more generations of redia, and the
final generation of these produces a third kind of larva, known as a
CERCARIA.
All these stages have been arising in the snail’s body, but now
the swarm of cercariae emerges from the snail, and each swims
about in the water and finally encysts on a blade of grass. Here
again the life of the parasite hangs in the balance, for death fol-
lows unless the grass with the cyst is eaten by a sheep, and
the cyst reaches the animal’s intestine. This location success-
fully attained, the cercaria escapes from the cyst, and makes
its way to the bile ducts where it soon develops into a mature
Liver Fluke, the cause of liver-rot in sheep. The life history is
completed.
The large number of eggs produced by a single Fluke increases
the chances of a ciliated larva meeting the proper kind of snail,
while the various generations within the snail multiplies many-
fold the number of cercariae from a single egg, and just to that
extent increases the opportunities for at least one to reach another
sheep. This life history, while remarkable, is by no means unique,
and is presented as a type which is broadly representative of a
large group of parasitic Flatworms. No wonder that years of study
are required by specialists in different branches of zoology to
discover the various stages of the different species and determine
their relationships.
CresTopEs. The group of Flatworms known as the CESTODA
comprises the numerous species of TAPEWworMsS which infest the
lower animals and Man. The best known species are Taenia
solium and Taenia saginata. both living as adults in the human
digestive tract, while the larvae of the former infest Pigs, and
those of the latter, Cattle.
Taenia is a long ribbon-like worm comprising a small knob-like
head, or SCOLEX, which is an organ for attachment to the lining
of the human digestive tract, and a large number of similar seg-
ments, Or PROGLOTTIDES. These are formed by growth just be-
hind the scolex so that the oldest and largest proglottides are
at the posterior end of the animal. (Fig. 252.)
BIOLOGY AND HUMAN WELFARE 399
The adult Tapeworm is hermaphroditic and each of the older
proglottides contains both male and female reproductive organs,
while the terminal ‘ripe’ ones are almost completely filled with
eggs which have already developed into embryos. One by one the
ripe proglottides become detached from the worm and pass from
the host with the feces. For development to proceed further an
embryo must be swallowed by a pig, whereupon it bores through
the walls of the animal’s intestine, passes to the voluntary muscles
and there encysts. In this position it develops into the BLADDER-
Rostellum
—
Fic. 252. — Tapeworm, Taenia solium. A, Anterior and posterior parts of
a specimen about 8 feet long comprising some 900 proglottides. Uteri filled
with eggs are shown in the last two proglottides. B, Scolex more highly mag-
nified. (From Hegner.)
WORM, Or CYSTICERCUS, stage. To complete the life history, in-
fected meat, insufficiently cooked, must be eaten by Man. [If this
transfer is successfully accomplished, upon attaining the human
digestive tract the parasite gradually assumes the adult form, the
scolex becomes attached, and a series of proglottides begins to
develop. (Fig. 253.)
Since Tapeworms which live as adults in Man and the higher
animals secure their food by absorbing that of their host, they
seriously interfere with nutrition, but larval stages are still more
dangerous. Thus, the larvae of a tapeworm (Echinococcus), which
lives as an adult in the intestine of the dog and other carni-
vores, form in the brain, liver, etc., of man, pig, and sheep large
400 ANIMAL BIOLOGY
vesicles, or HYDATIDS, usually with fatal results. Such larvae in the
brains of sheep were a stumbling-block for the early exponents of
A B
Fic. 253. — Stages in the development of a Tapeworm. A, egg with embryo;
B, bladder worm (cysticercus) before head (scolex) is protruded; C, same
after protrusion. (From Hegner.)
biogenesis, since, with the life history unknown, they could not
account for the larvae except on the theory of spontaneous gen-
eration.
NeMATODES. Passing now to the NeEmMatopa, or ROUNDWORMS,
we come to a group which, from the standpoint of medical zoology,
is of as much importance as the Flatworms. Free-living forms are
found literally everywhere in water, soil, and air, and blown about
by the wind. Most of these are harmless, but some are of great
economic interest because of their destructive action on the roots
and other parts of plants. Among the species parasitic in Man and
the higher animals, Trichinella and the Hookworm will serve as
examples. (Fig. 44.)
TRICHINELLA is the cause of a serious disease in Man, Pigs, and
Rats known as TRICHINOSIS. Man becomes parasitized by eating
infected pork, insufficiently cooked, and pigs contract the disease
by eating offal or infected rats. The larvae from the meat quickly
mature in the human intestine, and each female worm produces
nearly ten thousand larvae which bore through the intestinal wall,
migrate throughout the voluntary muscles, and encyst there to
await a possible getaway from the body at death. Since thou-
sands of resistant cysts may occur in a single gram of muscle, the
riddling of the tissues is not only very serious, but incurable.
(Figs. 250, 254.)
BIOLOGY AND HUMAN WELFARE 401
Ip
i
“
My
S
nu
i
’
=S=Ss>>
seS=>>
eS 5
Fic. 254. — Trichinella spiralis. A, larvae free among muscle fibers; B, a
single larva encysted among fibers; C, piece of pork containing many encysted
worms, natural size; D, adult worm, highly magnified. (From Leuckart.)
The widespread distribution of the several species of Hooxk-
worms and their insidious effects make them also of great prac-
tical importance. Biological studies have demonstrated the
process by which the tiny worms, just visible to the naked eye
as whitish threads, are hatched in a warm moist soil, make their
way through the skin of the human foot, enter the circulatory
system, are carried to the lungs, and finally work to the intestine.
Fic. 255. — The American Hookworm, Necator americanus,
highly magnified.
Here, as adults, they become attached, feed upon the blood of
their host and liberate eggs. These pass out with the host’s feces
to become the source of infection for others. The spread of knowl-
edge of the essential facts and of vermifuges to expel the parasites
has been an important contribution of the International Health
Board which has carried on a campaign in over fifty countries. It
has been estimated that not less than two million persons are af-
flicted with the disease, but the recently acquired facts in regard
to the parasite should result eventually in its almost complete
eradication. (Fig. 255.)
402 ANIMAL BIOLOGY
3. Health and Wealth
It has been aptly said that health and wealth are essentially
synonymous, and this is amply shown by the fact that medical
progress is reputed to have added at least twenty years to the
average life span of Man during the past century — mainly years
of the highest efficiency from the age of thirty-five to fifty-five
years. It is conservative to say that the increase in longevity has
effected a saving of more money than has ever been expended in
support of every kind of scientific investigation, and this without
taking into account the economic value of the lives or the im-
mensely greater factor of human happiness which follows from
healthful and unbroken family life.
The same can also be said for the leading rdle which medical
zoology has played in rendering vast regions of the tropics almost
as safe for human habitation as the temperate regions of the Earth
—regions which must offer an outlet for the rapidly increasing
human population. Statistics make clear that the population of
the world has more than doubled during the last century: what
it will do during the next century experts in vital statistics are
actively computing. But this much is certain: it is merely a matter
of time before regions now untenanted by civilized Man must be
encroached upon more and more, not only for food and other ma-
terials but also for a place of abode; and the first step necessary
to make this possible is the survey of the innumerable biological
competitors in the form of parasites, etc., which Man must en-
counter in adjusting himself to this environment.
Again, knowledge is power — the best investment from the
standpoint of health and wealth is in support of research. It is
easy to forget that combined studies on the life history of Bacteria,
Fleas, Rats, and the rest have made impossible to-day such epi-
demics as have many times in the past swept over the world.
During the Christian era more people have succumbed to the
Plague than constitute the total population of the Earth to-day.
It is easy to forget that the biological forces of disease are costing
the United States nearly four billion dollars annually —a loss
largely preventable by an efficient dissemination of knowledge
and an efficient application of biological principles already well es-
tablished. Civilizations in the past have succumbed to the on-
BIOLOGY AND HUMAN WELFARE 403
slaughts of pestilences, and the hope of our immensely more com-
plex community life depends upon the development of knowledge
of these living agents of disease.
B. BioLocy AND AGRICULTURE
True it is that Man cannot live by bread alone, but it is equally
true that the fundamental urge of all living things to secure food
and to multiply is, in the final analysis, at the basis of human en-
deavor. Modern agriculture represents the body of knowledge ac-
cumulated by mankind during its slow progress toward civiliza-
tion, involving increasingly exacting food demands as community
life became more and more complex. Agriculture is, of course,
dependent upon many fundamental biological sciences — indeed,
agriculture is one aspect of applied biology — but merely a few
examples must suffice to bring the most significant points before
us.
1. Plant and Animal Food
As we know, all animals, including Man, are absolutely de-
pendent for their food upon the photosynthetic activities of plants.
Green plants must manufacture enough food for themselves, and
to spare for the rest of the living world as well. Animals must
have ready-made food which, after they have used it, is useless both
for animals and for green plants. Here, it will be recalled, the
Bacteria and other colorless plants come in and make possible the
completion of the biological cycle of the elements in nature: put
the materials in a condition in which they are again available to
green plants. (Figs. 15, 16, 17.)
This intimate food interrelationship of all living organisms, which
has been demonstrated by interlocking data accumulated by thou-
sands of biochemical studies, is not only of profound theoretical
interest, but also of incalculable practical importance in all prob-
lems of soil fertility, including soil composition and maintenance,
crop rotation, etc. To coax greater productivity from the soil —
‘civilization rests upon the soil’’ — is a problem of no mean im-
portance. One of its most crucial factors is the demand for the
economical production of food for plants themselves; in particular,
for an adequate supply of nitrogen in a form readily available for
the use of cultivated plants.
A natural source of nitrogenous plant food is, of course, the
404 ANIMAL BIOLOGY
nitrogen of the atmosphere, trapped by Nitrogen-fixing Bacteria;
but the artificial conversion of this gaseous element into solid
forms suitable for fertilizers has taxed to the utmost the ingenuity
of chemists. Finally after years of intensive laboratory study and
experimentation, they have succeeded in developing industrial
processes for accomplishing this result, which have created an
enormous industry of world-wide extent and supplied agriculture
with a cheaper source of this indispensable nitrogenous food for
plants.
Soil investigations involving the codperation of chemist and
biologist lay the foundations for studies on seed planting, germina-
tion, and growth, which in turn lead to others on transplanting,
grafting, and pruning, and on pollination, hybridizing, and develop-
ing new varieties of plants. Nearly every cultivated plant that is
important for food or other purposes has been improved. The great
body of practical information which the human race has accumu-
lated from centuries of toil has been multiplied a hundred fold
within merely the past. generation, through the intensive work of
investigators at Agricultural Experiment Stations, Colleges, and
Universities throughout the world. (Page 283.)
But this indispensable work is only a small part of all that must
be accomplished. Man’s conquest of the plant kingdom has hardly
begun, because the number of species already brought into his
service is insignificant in comparison with the wealth of available
kinds. Relatively few of about two hundred thousand known spe-
cies of higher plants are cultivated, and most of these in merely an
incidental way. There is every reason to believe that many plants
not as yet employed possess intrinsic value at least equal to those
under cultivation — “‘some neglected weed in the hands of a skilled
botanist may one day revolutionize agriculture.’’ Furthermore, the
botanist must develop varieties of important plants which not
only afford the largest yield, but also are most resistant to unfavor-
able climatic conditions and to disease; the forester must develop
timberland both for the materials it supplies and the indirect effect
it has on soil erosion and on water conservation for agricultural and
other purposes; the entomologist and plant pathologist must devise
means for holding in check destructive insects, as well as bacterial
and related microscopic parasites of plants. All these and others
must cooperate. For what does it profit us if we are robbed of our
crops? (Fig. 268.)
BIOLOGY AND HUMAN WELFARE 405
2. Insects Injurious to Animals
The former Chief of the United States Bureau of Entomology
informs us that Insects alone in this country continually nullify
the labor of a million men, in spite of the annual expenditure of
between two and three hundred million dollars in fighting insects,
and if human beings are to continue to exist they must first win the
war. This can only be accomplished by the labors of an army of
patient and skilled investigators, and will occupy very many years,
possibly all time to come. This is not only because the insect
complex is enormous — there are possibly three million species
of which only about six hundred thousand have been described —
but also because insects achieved an important place on this globe
many millions of years before Man came into existence, and to-day
are probably the most perfectly adapted of all creatures to live
under all sorts of environmental conditions. If this statement
appears extreme, the following examples will serve to make clear
some of the cardinal facts which are necessary for an appreciation
of the stupendous problems involved.
Among this teeming insect population, probably the Bottflies,
Fleas, and Lice stand preéminent as parasites of Man and beast.
Botflies of various kinds infest domesticated animals but rarely
human beings. The most common Horse Botrty attaches its eggs
to the Horse’s hair where the eggs can be licked off and swallowed.
Then the larvae spend nearly a year attached to the lining of the
stomach, and if present in considerable numbers cause irritation
and serious digestive disturbances. When full grown the larvae pass
out with the feces, pupate in the ground and emerge as adult bot-
flies.
Again, the common Ox Bortr.y deposits its eggs chiefly on the
legs of Cattle, but when the larvae emerge, they penetrate the hide,
and then wander through the tissues until the following spring.
Finally they come to rest just under the hide of their host; which
they puncture to get air. When the larvae are ready to assume the
pupal state they burrow out, drop to the ground and there com-
plete their life history. It is estimated that the monetary loss from
the Ox Botfly alone in the United States is about one hundred
million dollars annually. (Fig. 256.)
Fleas and Lice of various species are common parasites of the
higher animals throughout most of the world. The JigcerR FLEA
406 ANIMAL BIOLOGY
of warmer climates is frequently a serious human pest because the
female flea burrows into the skin when ready to deposit eggs. The
Cat Flea, Dog Flea, and House Flea we usually consider merely a
nuisance, but they are potential carriers of disease-producing
B, larva; C, larva just beneath air-hole in skin of Ox; D, adult.
microorganisms. One might think that a life devoted to the study
of fleas and lice could be more profitably spent, until we recall that
expert knowledge of these animals was essential to discover that
Trench fever in the World War was transmitted by lice; was es-
Fic. 257. — Dog Flea, Ctenocephalus canis. A, larva in cocoon; B, pupa;
C, adult. (From Howard.)
sential to make clear that Bubonic plague, or ‘Black Death,’
is carried by fleas. (Fig. 257.)
- It was long known that rats die in great numbers during a plague
epidemic, and accordingly biologists set out to determine whether
BIOLOGY AND HUMAN WELFARE 407
there is any relation between the disease of the Rat and of Man,
and found that Man is infected with the plague bacillus, Bacillus
pestis, by being bitten by a Flea from an infected rat. Extermina-
tion of rats and fleas means the practical eradication of the disease,
but in California the Ground Squirrels have become infected with
the bacillus so the problem has become somewhat greater. Bu-
bonic plague is doomed, though it has already taken an incalculable
toll of human lives: even during the first four years of the present
century it destroyed about two million people in India. Still more
recently San Francisco has been fighting an outbreak of the
f
\
Eggs Larva Pupa Adult
Fic. 258. — Life history of the House Fly, Musca domestica.
plague that not long ago would have been a national calamity;
but it was immediately stamped out with the loss of compara-
tively few lives.
In brief, numerous parasitic Insects not only actually develop
at the expense of animal tissue, but others act as the transmitting
agents of Bacteria, Protozoa, etc., which are the actual parasites, as
we have already seen in the case of malaria, yellow fever, and Afri-
can sleeping sickness. And last, but not least, we know that HousE
Furies, which we tolerate as uninvited guests at our tables, have
been shown to carry the germs of typhoid fever, tuberculosis,
dysentery, and several other scourges. (Figs. 224, 248, 258.)
A08 ANIMAL BIOLOGY
3. Insects Injurious to Plants
It has been stated, and truly, that it costs the American farmer
more to feed his Insect foes than to educate his children: in fact,
more than is expended for all the educational institutions in the
United States, nearly twice as much as for our military and naval
forces, and more than twice the loss by fire. And we all pay the
bill. Every kind of plant supports many species of insects, although
usually certain ones are especially destructive. Thus Oak trees are
attacked by no less than a thousand kinds of insect pests; Apple
trees by about four hundred, and Clover and Corn by some two
hundred insect enemies. A few random examples obviously must
suffice for our view of the field.
The ArmMy-woprsM is the larva of a brown Moth which sometimes
becomes so numerous in regions east of the Rocky Mountains
that the caterpillars have to migrate in search of food. Immense
armies crawl along totally destroying the crops over large areas.
Fortunately, the pest has its own insect enemies, the chief being
certain Tachina Flies which lay their eggs on the caterpillars, and
the larvae of the flies burrow into their bodies and finally destroy
them. (Fig. 266.)
Of equal interest is the CABBAGE BUTTERFLY which was ac-
cidentally introduced from Europe into Canada in 1868, and has
Chrysalis
Adult
Fic. 259. — Life history of the Cabbage Butterfly, Pieris rapae.
gradually made the whole of the United States its field, even oust-
ing a related native species. Many of the caterpillars of the Cab-
bage Butterfly are destroyed by parasites; one being a Brachonid
Fly which was imported from its old home in Europe by entomol-
ogists for this special purpose. (Fig. 259.)
BIOLOGY AND HUMAN WELFARE 409
The Porato BEETLE first began to attract attention about
eighty years ago when it transferred its activities from certain weeds
in the Colorado region to the recently introduced Potato plant,
and since that time it has spread all over the United States and
has emigrated to Europe to become one of the serious insect pests.
Large masses of yellow eggs are deposited by the beetles on the
under surface of Potato leaves which serve as food for the cater-
pillars until they are full grown and ready to pupate in the ground.
Two broods of adults are usually produced annually to carry on
the depredation.
Among the most destructive parasites of Wheat, Rye, and Bar-
ley, nearly the world over, is the HEsstan Fiy which was intro-
duced into America toward the end of the eighteenth century.
The life history is especially adapted to the growth of wheat,
and two or three broods of the insect develop in one year. For-
tunately, it has numerous parasites of its own that hold it some-
what in check.
The EurRoPEAN Corn Borer has long been distributed over
a large part of the Old World but only recently has reached Amer-
ica. Starting in New England, it is rapidly moving westward
and bids fair before long to infest the entire corn-raising area of
the continent. The destructive stage, of course, is the caterpillar
which, throughout most of the insect’s range, spends the winter
in the stem of its food plant and gives rise to the adult moth
early the following summer. Unfortunately, however, in New Eng-
land there are two generations annually, one of which winters in
the larval state.
The JAPANESE BEETLE was accidentally introduced into New
Jersey from Japan about twenty years ago and since has spread
rapidly through many of the eastern States, defoliating trees and
shrubs and destroying lawns and golf greens. The larva spends the
winter underground and the adult emerges the following summer.
It seems safe to say that the destruction wrought by the Corron
Boi WEEVIL exceeds even its notoriety. During the first thirty-
five years after its invasion of the United States from Mexico, it
had to its account a wastage of upward of three billion dollars,
not to mention other immense financial losses due to depreciated
land values, etc. Probably each person in the United States pays
annually ten dollars more for cotton fabrics than he would if this
weevil did not exist. The injury to the Cotton plant is caused
410 ANIMAL BIOLOGY
both by the adults and larvae: the former by feeding and boring
holes for their eggs, and the latter by injuring the developing
Peo
Fic. 260. — Life history of the Cotton Boll Weevil, Anthonomus grandis.
On the right, a Cotton plant attacked by the Weevil, showing a, a dry in-
fested ‘square’; b, a ‘flared square’ with punctures; c, a cotton boll sectioned
to show attacking Weevil and larva in its cell; g, adult female with wings
spread as in flight; d, adult viewed from the side; h, pupa, ventral view; e, larva.
(From Metcalf and Flint, after U. S. Department of Agriculture.)
flowers so that they either fail to bloom, or produce seeds with
few cotton fibers. (Fig. 260.)
While ScALE-INSECTS are so small and obscure that they are
only a name to all except specialists, they constitute economically
one of the most important groups of the insect world. Scale-insects
infest almost all kinds of trees and shrubs; in some cases doing
merely temporary damage and in others actuaily killing the hosts.
Among the myriads of species, the San Jos& SCALE is probably the
most important, and since being brought to California from China
BIOLOGY AND HUMAN WELFARE 411
has spread all over the United States. The adult female insect
lies permanently attached by its beak to the bark, underneath
a tiny waxy scale which it secretes. Here eyes, legs, and antennae
are lost and the sac-like creature sucks the plant sap and repro-
duces. It is estimated that the progeny of a single individual
during one season would number thirty million if all were to sur-
vive. (Fig. 264.)
About forty years ago the vineyards of France, and later those
of California, appeared to be doomed to destruction by the attacks
of a species of minute plant lice, or ApHips. The French govern-
ment offered a large reward for an effective remedy, and many
entomologists and botanists devoted all their time to the study
of the problem. Eventually it was discovered that certain Ameri-
can wild Grapes were naturally immune to the pest. Accordingly
by grafting the cultivated grape upon the resistant wild stock a
combination was effected which saved the vineyards of both coun-
tries. (Figs. 222, 265.)
The MErpITERRANEAN FRuIT FLy appeared a few years ago in
certain Florida orchards but the invasion apparently has been
repulsed by the vigilance of the eee ae . 557 hes.
United States Bureau of Ento- | . 2
mology and Plant Quarantine. fo ,
It is the larvae of the fly that
are the mischief-makers, because
when they develop from eggs de-
posited in the fruit they soon ren-
der it unfit for human food. When
we realize that the annual fruit
and vegetable crop of Florida
amounts to well over a hundred
million dollars, it is no wonder
that this fly is one of our most aS Se
notorious foreign emigrants. But — Fie. 261. — Mediterranean Fruit
we maintain a defense on the Rio ae Ceratitis cap 2 Se (Brom Ui:
Grande against the Mexican Fruit Coen seoubace)
Fly, and others throughout the country against our many native
species of Fruit Flies, though the latter are held in check to a
considerable extent by their own insect enemies. (Fig. 261.)
Another great problem is the preservation of forest and shade
trees from native and also imported pests. The enormity of the
A12 ANIMAL BIOLOGY
loss attributed to this army of silent tree-killers is staggering.
They destroy two-thirds as much of the nation’s wealth each year
as do forest fires — timber equal to one-fifth of the wood produced
annually in the United States. At the present time these destruc-
tive insects recognize neither limits nor boundaries, for their march
has received relatively little resistance. And resistance is not easy
when we recall that some insects may travel hundreds of miles on
the wings of the wind — aviators have trapped them more than
two miles aloft.
The Mountatn Pine Bark Beette, after invading the forests
of the Northwest, now is threatening those of the Yellowstone
Fic. 262. — Gypsy Moth, Porthetria dispar. A, larva; B, pupa; C, adult
female. (From Howard.)
National Park. The Gypsy Mora, accidentally introduced near
Boston, has spread throughout a large part of southern New
England and is besieging the New York State line. Entomologists
have made intensive studies of its European enemies, left behind
when it came to America, and the introduction of some of these
BIOLOGY AND HUMAN WELFARE 413
into New England gives hope that it may eventually be conquered.
In one year alone nearly three million enemies, representing eight
species, were liberated. The Brown-rarm Morn is another im-
portation from Europe whose activities thus far have been con-
fined to New England as the result, in part, of control measures.
These three examples may stand as representative of the legions
of destructive forest insects. (Fig. 262.)
Finally we should be reminded, if necessary, that our households
are not immune to insect marauders that take an immense ag-
Fic. 263. — A, Carpet Beetle, Anthrenus scrophulariae; a, larva of Carpet
Beetle; B, Clothes Moth, Tinea pellionella; b, larva of Clothes Moth. (From
Riley.)
gregate toll each year. Carpet BEETLES, popularly called Buf-
falo Moths, and CirorHes Morus are all too familiar examples.
(Fig. 263.)
4. Beneficial Insects
Although we have mentioned incidentally the part played by
certain insects in suppressing other noxious kinds, it would be
unfair to the insect world not to emphasize the existence of mem-
bers which are serviceable to Man: those thousands which prey
upon our enemies or supply us with materials. It has been well
said that “if insects would quit fighting among themselves, they
would overwhelm all Vertebrate animals”; though sometimes
long biological investigations are necessary to keep them fighting
when we have upset the natural conditions; e.g., moved them to a
new environment away from their natural enemies. Thus Acacia
plants brought from Australia introduced the Corrony CUSHION
SCALE which soon spread to the great California orange and lemon
groves, and entailed enormous losses. The fruit growers finally sent,
at their own expense, an expert entomologist to study in Australia
the native enemies of the Scale-insect. As a result some Ladybird
414 ANIMAL BIOLOGY
Beetles were eventually discovered which offered hope of meeting
the needs, and these were sent to California and reared until they
could be colonized in the infected groves. Here they multiplied
and ever since have held the Scale in check. (Figs. 264-266.)
ses =
Fic. 264. — Australian Ladybird Beetle, Rodolia cardinalis, and Fluted
Scale-insect; Icerya purchasi. a, larvae of beetle feeding on scale; b, pupa of
beetle; c, adult beetle; d, Orange twig showing scales and beetles. (From
Marlatt.)
Furthermore we must not forget that such insects as the SILK-
worm Mors and the Honey BEzE are really domesticated animals:
each is at the basis of an enormous wor!d industry. One of Pas-
teur’s most important studies was on the PEBRINE disease of
Fic. 265. — A, An Ichneumon Fly inserting egg in an Aphid. B, emergence
of the parasite that has developed from the egg. (From Webster.)
Silkworms, and not only saved the silk industry of France, but also
paved the way for the study of infectious diseases in higher animals
BIOLOGY AND HUMAN WELFARE 415
ee
ave
16)
Fic. 266. — Army-worm Moth, Cirphis unipuncta, and its ecological rela-
tionships with other Insects. a, adult Moth; 6, full-grown larva; c, eggs;
d, pupa in soil; e, parasitic Fly, Winthemia, laying eggs on an Army-worm;
f. Ground Beetle, Calosoma, preying upon an Army-worm, and, at right,
Calosoma larva emerging from burrow; g, a Digger Wasp, Spher, carrying an
Army-worm to its burrow; h, a wasp-like parasite of the Army-worm. (From
U.S. Department of Agriculture.)
416 ANIMAL BIOLOGY
and Man. The cause of pebrine proved to be a Protozoan parasite,
Nosema bombycis. (Figs. 214, 267.)
The dependence of many plants upon insects for pollination is
well illustrated by the difficulties in establishing the Smyrna Fic
in California. The fruit would not mature, and studies by botanists
and entomologists showed that in the plant’s native land pollina-
tion was effected by a certain tiny insect. Importation and es-
Fic. 267. — Silkworm Moth, Bombyr mori. A, caterpillar; B, cocoon;
C, male moth; D, female moth. (From Shipley and MacBride.)
tablishment of the insect carrier in California created there the
immense fig-growing industry. (Fig. 219.)
As a matter of fact an amazing number of plants that we most
highly prize would be unable to reproduce were it not for pollina-
tion by insects: for instance, there would be no pears, apples,
peaches, plums, oranges, or strawberries. So it is perhaps not
unreasonable that an entomologist has asked whether insect dep-
redations may not be regarded as a twenty per cent commission
we pay for the invaluable services that “friendly insects” render.
It may, but it is economical, if not generous, to reduce the tax
to the lowest limit!
Insects touch human affairs in other vital but less direct ways.
It will be recalled that Darwin emphasized the importance of
Earthworms in aérating and plowing the soil; but various insects
BIOLOGY AND HUMAN WELFARE 417
probably contribute at least as greatly to this indispensable work.
Ground burrowing insects are still more widely distributed than
Earthworms and in most regions they are more numerous and more
active. Moreover, not only do they carry decaying leaves beneath
the soil, but also rich nitrogenous plant food such as manure and
the dead bodies of animals. (Fig. 266.)
Finally, it is not an exaggeration to wonder how land plants
could have arisen without the direct or indirect services of insects.
Indeed geological history indicates that land plants did not flour-
ish and Seed Plants did not exist before insects became a well-
established part of the Earth’s fauna.
Enough, perhaps, has been said to indicate the struggle for
knowledge which Man must maintain in order to cope with the
biological forces that would rob him, are robbing him, of what he
considers his heritage. But it is only fair to add that biologists
who have given the most thought to the problem are by no means
certain that the struggle will eventually be successfully terminated;
it is possible that insects and allied enemies will gain the upper
hand in the warfare for food when the human population has in-
creased beyond a certain limit. This seems incredible, though
it is a conservative statement by men who are specialists and not
pessimists. In any event, it is clear that the most urgent need to-
day is more knowledge of the life habits of insects and other de-
structive organisms. Generous Federal and State appropriations
must be made so that through research effective methods of con-
trol may be developed. Experience has shown that the research
dollar is not only invested in a gilt-edge security, but one at the
same time producing a national dividend almost beyond com-
putation. (Figs. 270, 271.)
C. CONSERVATION OF NATURAL RESOURCES
We are slowly awakening to the fact that we have been very
shortsighted. Conservation of natural resources has, until re-
cently, given very little concern, although it is one of the greatest
problems which biologists of the present generation face, and it
must be solved now or it will be too late. What happened in
America is being repeated in many other parts of the world. Our
forefathers came to a land of fertile soil covered with primeval
forests, abounding with large and small Birds and Mammals, and
418 ANIMAL BIOLOGY
with waters richly supplied with Fish. These they necessarily
and rightly drew upon for their livelihood. It was their wealth —
Nature’s generous bonus.
But the apparently inexhaustible supply has already become
alarmingly reduced and conservation must be the watchword, as
was recognized nearly a half century ago by Theodore Roosevelt
who considered it “the weightiest problem now before the nation,
as nobody can deny the fact that the natural resources of the
United States are in danger of exhaustion, if the old wasteful
methods of exploiting them are permitted longer to continue.”
Yet in spite of this, the conditions are still such that a prominent
legislator is more than justified in stating that “it is time that the
national conscience be awakened to the necessity of preserving
what is left of the outdoor heritage of our fathers, and of restoring
some of that which has been destroyed and defiled.”’
Only about one-eighth of the virgin forest of the United States
remains to-day. It seems incredible for a civilized nation — but is
only too true. Approximately one-half of this is held by the Govern-
ment but the rest is being destroyed far more rapidly than unaided
nature can restore it. And there is nowhere in the world a suffi-
cient supply of the kinds of timber we use to take their place. We
have continuously treated our forests, except those under public
control, not as a farm on which to produce crops, but as a mine
whose useful product is to be gathered once for all. The axe has
held almost unregulated sway, but with ideas of conservation
becoming increasingly widespread it appears that hope for a better
future for our forests is well founded.
It seems hardly necessary to state that forests are of inestimable
value in many ways entirely aside from the lumber they supply.
We are, perhaps, prone to forget that under nature’s stabilizers
of forests, shrubbery, and grass the blowing and washing away of
the soil progresses but slowly, while with their removal by Man this
erosion is increased tremendously. Witness the great dust storms
during recent years in the Southwest. The devastating floods
that swept down the Mississippi in 1927, the Yangtze in 1931, and
the Ohio River in 1937 are in no small part attributable to defor-
estation. China’s affliction is the product of millenniums, ours of
little more than a century. Scientific forestry is crucial for our
future. (Fig. 268.)
Many of the larger animals have been exterminated and some of
BIOLOGY AND HUMAN WELFARE 419
the smaller ones are fast approaching the same fate. The Bison is
extinct in the United States except for the few hundreds preserved
in reservations; the Elk is restricted in numbers and range; the
Elephant Seal remains only in one small colony; the Bowhead
Whale and Right Whale are threatened with extermination; and
the Beaver has disappeared from most of its former haunts. The
demand for furs is estimated to be responsible for the destruction
Se Rie naa > 2 a ig
Fic. 268. — Soil erosion due to removal of all the trees.
(From U.S. Forest Service.)
of thirty million Mammals throughout the world each year, and this
number is nearly doubled if all the wild Mammals destroyed for
commercial purposes are included. While there is life there is hope,
but unless immediate steps are taken to reduce the slaughter, the
fur-bearing animals of the world at large are doomed.
Birds are now faring somewhat better owing to the heroic efforts
started a generation ago by the Audubon Societies, so that a partial
restoration of our former bird population seems probable. Of what
use are the Birds? Even if usefulness were the only question in-
420 ANIMAL BIOLOGY
volved, the answer would still be clear. Experts tell us that without
the services of insect-eating and seed-eating Birds, successful
agriculture would soon be impossible, and the destruction of the
greater part of our vegetation would also result. One caterpillar
during its lifetime of less than two months can consume three-
quarters of a pound of leaves, or nearly ninety thousand times its
original weight. Birds are “the winged wardens of our farms.”
Their help is needed in the struggle.
In truth, it is dangerous for Man to upset the intricate balance
of the economy of nature by the reckless destruction of plant or
animal without taking thought for the morrow — without inten-
sive study of the far-reaching consequences which may follow from
the breaking of one link in the chain of the interrelationships of
organisms. The destruction even of certain Protozoa and other
microscopic life may seal the fate of Fish valuable for food.
(Figs. 24, 220, 266.)
D. ConstructivE BioLoGcy
The great living heritage which we have received will be per-
manently impaired for posterity, even though useless waste is
stopped, unless the highly complex problem of conservation is at-
tacked constructively in the light of modern biological knowledge.
Merely to hold Nature’s bonus unimpaired, crucial as that is, will
not adequately meet the requirements of increasing populations
with the attendant demands of complex civilized life. Few seem to
realize that the whole of our business life takes root in nature. All
of our progress and prosperity is predicated on the abundance of
our natural resources and the manner in which we develop them for
Man’s use. Methods of raising crops and domestic animals which
were sufficient for primitive communities are entirely inadequate
to satisfy modern conditions.
Indeed, the state of civilization of a people is closely related to its
success in developing plants and animals for particular needs. One
hears of new ‘creations,’ but often fails to recall that Man can
merely direct the laws of inheritance, and this he can do only
by intensive investigations of the principles underlying heredity.
Certainly the most important recent contribution of biology is the
discovery of the general method of transmission of characters from
generation to generation, common to all living things, which has
established the new biological science, genetics. To-day, as we
BIOLOGY AND HUMAN WELFARE
(N) Norma/ man QI755
® Normal women
feet/e-minded man
@ Feeble-ninded woman
_
77 P
Undetermined man Nomeless
a ink Died 'n infancy
Jeeble-minded girl
421
@
D770 §
NS
LS
NS Normal wife
After the Revolution
F LN (N)
§|. 2457
Ss
During the Revolution
WIR 2 a} 8 SIS s s
CISS} ul Gl & Sg 8 $
poles GOES IRS st ah is Sl Rl el Slopes
F ize’? |I@@ (- @WOW “% a] S} a} a} 3] S$} &
WN) WN) @®) @& @ IN] WD
a——G NN——®
Justin
IN
Martha
Deborah
Fic. 269. — The Kallikak family.
Of the 480 descendants, in five
generations, of this branch, 143 are
known to have been feeble-minded,
36 illegitimate, 33 sexually immoral
mostly prostitutes, 24 alcoholic, 3
epileptic, and 3 criminal. 82 died
in infancy.
Of the 496 descendants, in five
. generations, of this branch, none were
feeble-minded, and all but 2 were nor-
mal mentally. 2 were alcoholic and 15
died in infancy. Thus nearly all were
good citizens. Among them were edu-
cators, physicians, lawyers, judges,
traders, landowners — men and women
prominent in every phase of social life.
(After Goddard.)
422 ANIMAL BIOLOGY
have seen, biologists the world over are developing and applying
these principles in plant and animal breeding. What was impossible
a few years ago is now being accomplished almost as a mere matter
of routine work in many biological laboratories.
Important as the application of these principles is in the mastery
of agricultural problems, a far more profound power remains to be
realized when, as nations, mankind becomes awake to the fact that
these same basic principles apply equally to human inheritance.
If it be true that the human race has not improved in bodily and
mental characteristics since the time of the ancient Greeks, the
responsibility rests with Man himself. He has studied and applied
selective breeding of animals and plants. He has in general kept
the best for his purposes that nature offered and eliminated the
rest. He has depended chiefly on the stock and only secondarily
on the environment for permanent improvement. But it has been
otherwise with himself. Much of the worst human stock has
continued to survive and multiply, and disproportionately so as
civilization has advanced. Reliance has been placed almost solely
on the improvement of the conditions of life and not on breeding
from the best. We may fairly say that humanity is what it is to-
day in spite of the continual violation of many of the biological
principles which would improve the race. (Figs. 179, 269.)
This, of course, is merely a statement of fact: not a reproach.
Man could not have done otherwise until it had been demonstrated
by biological investigation that he is a part of, and not apart from
the rest of living nature — the most profoundly important fact that
biology has contributed to human welfare. With this fully grasped,
new import is given to the study of general biological principles and
no plant or animal is too insignificant to throw light on life prob-
lems. And this has been the chief source of the stupendous po-
tential for biological control which has within less than a century
come to mankind. Potential for control, we say, because years of
research by generations of investigators is still necessary before
we shall be prepared to solve the problems we now face and which
the saturation point of human population will immensely augment.
(Figs. 188, 194, 270, 271.)
The natural method of securing a healthy — adapted — race is,
of course, the gradual process of adjustment throughout the ages by
response to environmental stimuli. But mankind has the power,
BIOLOGY AND HUMAN WELFARE 423
which we believe is denied to the lower animate beings, of conscious
response — of choice. Human volition and action can retard or ac-
celerate nature’s response. Human volition may not only decide,
within limits, the response to surrounding conditions, but also not
infrequently may directly change the conditions so as to render
unnecessary individual or racial adaptation to them. Thoughts
such as these make plain the enormous complexity of the situation
and reveal the possibility of danger lurking where it may be least
Besse ame
Fic. 270. — The Marine Biological Laboratory, Woods Hole, Massachu-
setts. One of the laboratories where biologists spend the summer in research
covering many branches of the science.
expected — danger lest in acting as we think humane from the
point of view of particular individuals, we may be rendering a dis-
service to the race.
The complex problems of eugenics, the science of being well-born,
are problems in genetics so intricately interwoven with those of all
the other sciences of human life and relationships, in particular
sociology and psychology, that propaganda in eugenics not funda-
mentally grounded in basic interlocking data from these sciences is
not only premature, but fraught with insidious possibilities. Eu-
genics seeks improvement through nature, euthenics seeks im-
provement through nurture. [Each is a partial view and before
significant progress can be made the proper balance between these
two aspects of one problem must be grasped. Organism and envi-
ronment are one and inseparable. ( Fig. 198.)
A424 ANIMAL BIOLOGY
The crying need of the present is for more and still more knowl-
edge which is secured in the laboratory rather than on the lecture
platform. When we realize that if the infants Darwin and Lincoln,
who were both born on the same day, had been exchanged by their
parents, almost certainly neither would have produced epoch-
making contributions to science or civilization, it gives us pause.
The Danes have a proverb that it does no harm to be born in a
duckyard if you are laid in a Swan’s egg — thus emphasizing hered-
Fic. 271. — The National ae of sae and the National Research
Council of the United States, Washington, D. C. The focus of American
scientific research.
ity, though heredity implies not a repetition in kind, but in possi-
bilities. We cannot hope to be born equal, but we may ask to be
born with an equal opportunity to develop what is in us. At least
we can say at present, without fear of contradiction, that Man
owes it to himself not to be less mindful of his own stock than he is
of that of his domestic animals.
From every standpoint the mere pittance Man casts to biolog-
ical research returns to him many-fold in health and wealth, in
comfort and power, and most of all in a broader and more ap-
preciative outlook on a congenial world. “*‘Man becomes more in-
telligible, and therefore more controllable, when he recognizes his
affiliations and the ancestral strands that linger in his fabric. It
BIOLOGY AND HUMAN WELFARE 425
is encouraging to know that we have behind us not a descent, but
an ascent, and that there is some appreciable momentum in the
right direction.” Far from depriving life of its mystery, biology
affords a sublime picture of the interrelatedness of living things
and still more inextricably interweaves human life with that of all
Nature.
CHAPTER XXV
THE HUMAN BACKGROUND
The process of evolution has its outcome in personalities who
have discerned its magnificent sweep. — Thomson.
As the culmination of our survey of the continuity of life, it is
important to consider briefly what is known in regard to human
origins — to bring Man more specifically into relationship with
the evolutionary process and to appreciate his affiliations as evi-
denced by the ancestral strands that persist in his fabric.
Although the general recognition of Man’s place in nature has
but recently been attained, more than two thousand years ago
Aristotle placed him at the summit of animal creation, emphasiz-
ing his God-like nature, but withal regarding him as only the
highest point of the Scale of Nature. Linnaeus, the founder of
our modern method of classification, during the middle of the
eighteenth century placed Man in the order Primates of the class
Mammalia, a position he has since held. (Figs. 288, 296.)
A. THe PREHUMAN LINEAGE
The order Primates to-day comprises three suborders: the LE-
MUROIDEA, or Lemurs; the TAarstormpEA, or Tarsiers; and the
ANTHROPOIDEA, or Monkeys, Apes, and Man. Most of the charac-
ters which distinguish them from the other groups of Mammals
represent adaptations to a tree-dwelling life: adaptations for
climbing, for subsisting on a simple and plentiful food supply, and
for taking advantage of changing environmental conditions by a
bigger and better brain. (Figs. 93, 272.)
The Anthropoidea, in which our interest centers, are widely
distributed throughout the tropical regions of both hemispheres,
and although those of the eastern and western hemispheres doubt-
less were derived from the same original stock, they have followed
somewhat different though, in general, parallel evolutionary paths
since their origin more than fifty million years ago, early in the Age
of Mammals.
The Old World branch of the Anthropoidea is of especial im-
426
THE HUMAN BACKGROUND 427
ye
ae
bases: "6
- nS).
Qoayi
—¢
— >
/),
“
CERCOPITHECIDAE
LEMUROIDEA
———
ANCESTRAL PRIMATES
Fic. 272. — General relationships of the Primates. 1, Lemur; 2, Tarsius;
3, New World Monkey; 4, Marmoset; 5, Old World Monkey; 6, Gibbon;
7, Orang-utan; 8, Chimpanzee; 9, Gorilla; 10, Neanderthal Man. See p. 480.
(Modified, after Hegner.)
428 ANIMAL BIOLOGY
portance because it is dignified by the inclusion of Man. It
comprises two families higher in rank than the tailed Monkeys,
or Macaques and Baboons. These are the SmmimpAg, or man-like
(anthropoid) Apes — the Gibbons, Orang-utans, Chimpanzees, and
Gorillas; and the Hominipag, or Man. However, all specialists are
Fic. 273. — Chimpanzee, Pan troglodytes. Note large
ears, long lips, ridge above eyes, long arms, nails on fingers
and toes, and hand resting on back of fingers. Height
AS feet; weight of male, 160 pounds. (From Hegner, drawn
by R. Bruce Horsfall.)
not agreed that this separation of anthropoid apes from Man is
justified, for, as one states it: ‘‘ between the Gorilla and Man, bar-
ring for the moment the mental and spiritual distinctions, there is
hardly more difference than there is between the horse and the ass,
and the degree of consanguinity is much the same.” (Page 480).
At all events, the inevitable inference is that these various forms
have evolved from a common stock, though it should be emphasized
THE HUMAN BACKGROUND 429
that no living species represents the direct human ancestor. The
chimpanzee-gorilla group is regarded as the nearest to Man, but
these apes have evolved along different lines since the divergence of
the human lineage in the geologic past. Apparently the common
ancestor was a rather large animal with a mode of life more similar
to the present-day Gibbons than to either the Chimpanzees or the
Gorillas, although anatomically the Gibbons to-day differ more
Fic. 274. — Gorilla, Gorilla gorilla. Note large head, small ears, short
lips, large canine teeth, ridges above eyes, and absence of a chin. The gorilla
walks on the backs of its fingers. Height about 5% feet, weight 500 pounds.
(From Hegner, drawn by R. Bruce Horsfall.)
markedly from Man than do any of the other anthropoid apes.
Gibbons are relatively small active animals with such very long
arms that the knuckles reach the ground even when the body is
erect. But they are strictly arboreal forms whose amazing adapta-
tions for progression through the foliage of trees has been suggested
as of prime importance in the development of their mentality; and
so possibly it was with the prehuman ancestors. (Figs. 93, 273, 274.)
Receding still further into the past, there were antecedent to the
common ancestor of the anthropoid apes and Man more and more
430 ANIMAL BIOLOGY
primitive forms — Tarsioids and Lemuroids — and so back, and
still further back to an insectivorous stem from which the Primates
originally emerged. It is perhaps somewhat in deference to Man
that the Primates are usually placed as the culminating order of
Mammals and so of the Vertebrate series, because in spite of their
larger brain — and the use they make of it — the Primates retain
many primitive characters that elsewhere are found only in the
lowly order [NSEcTIVoRA. (Figs. 201, 202.)
No useful purpose will be served at the present time by delving
further into the geologic past for still earlier antecedent forms,
because behind the Insectivores the actual record becomes in-
creasingly obscure. However, it seems certain that the latter were
evolved from still more primitive Mammals during the Mesozoic
Age — the Age of Reptiles; the earliest Mammals arising from
the reptilian stock.
Returning to Man as a Primate and his relationships with the
anthropoid apes, we are on more firm ground. This relationship
Wii, ine
a ee
(ila
Ad
GORILLA MAN
Fic. 275. — Feet of Anthropoid Apes and Man. (From Schultz, after
several authors. )
ORANG-UTAN
is supported by many independent but interlocking lines of evidence,
such as the similarities in structure of the brain and viscera, of the
musculature and the skeleton in general and of the hands and feet
in particular. The differences are almost entirely in proportions,
the structures are almost identical. Indeed, even the ridges on the
palms and soles, and the chemical properties of the blood indicate
affinity. And not the least important evidence is the structure of
the premolar and molar teeth, because those of Man are very un-
like those of any other animals except the great apes. Therefore
the teeth, especially since they withstand well the ravages of geo-
logic time, afford excellent clues in the search for the fossil an-
THE HUMAN BACKGROUND 431
thropoids at the root of the human family tree. (Figs. 228, 275.)
Numerous fossil remains have been found that give evidence
in regard to the origin of the anthropoids. Back in the Eocene
epoch are various Tarsioids and a monkey which bridge the gap to
higher Primates, and in the Oligocene epoch are several monkeys
and anthropoid apes that, apparently, are near to the main line of
ascent. And then late in the Miocene epoch, perhaps ten million
years ago, appears Ramapithecus with teeth that foreshadow
those of Man. Indeed, the number and arrangement of the teeth,
the bicuspid pattern of the premolars, as well as various characters
of the incisors, canine, and the milk dentition, are prophetic of
the human dentition to-day, particularly in certain primitive races.
It appears that the differences shown by the human teeth are to a
considerable extent the results of an omnivorous diet, and of
changes in the proportions of the jaws, following the great expan-
sion of the cranium in providing for the enlarging brain. (Page 359.)
So it seems reasonably clear that the prehuman ancestor arose
through or near the Ramapithecus stem, from animals adapted to
live in the vast forests of the Old World. But as geologic time
progressed, climatic changes of course occurred, the forests became
restricted, and we may assume that the Primates that had not
already made a retreat were impelled to renounce, in part, the
arboreal for a terrestrial habitat. Thus the precursor of Man
reached the ground: an environment that was provocative of many
adaptations, in particular the erect body with hind limbs support-
ing the entire weight. This necessitated considerable mechanical
readjustment, including the alternating curvature of the vertebral
column for the nicer balance of the larger cranium. Moreover, the
development of the brain, furthered by the emancipation of the
hands from their part in locomotion and by changes in the vocal
organs leading to speech, eventually paved the way to the
emergence of culture from the biosocial foundations of preman—to
invention, communication, and social habituation. In the course
of the ages Man arrived. (Figs. 105, 109.)
B. Fosstrr Man
The evolution of Man, unlike that of other organisms, presents
two clear-cut aspects: the physical and the mental. His physical
evolution was exceedingly slow, but his cultural development,
once started, proceeded with increasing momentum. Both can be
432 ANIMAL BIOLOGY
traced with some assurance from the actual fossil remains of pre-
historic man and the relics of his handiwork.
When one realizes how slight are the chances for the remains of
prehistoric man to become fossilized and, if preserved, to be un-
earthed to-day, it seems remarkable that the record is no more
fragmentary than it actually is, especially when the short time
that interest has centered in the problem is considered. Some of
the important fossil forms are of the greatest significance, though
experts are by no means unanimous in regard to the interpretation
of details. At present it is prema-
ture to attempt to reach a decision
in regard to the specific Early
Pleistocene ancestor of modern
man, but there is reasonable as-
surance that the problem will
eventually be solved. At all
events the emergence of man is
essentially a Pleistocene story.
Some representative ‘fossil men’
may be reviewed. (Page 359.)
Fic. 276. — Skull and face of the 1. The Java Man
Java Man, Pithecanthropus erectus. Ty deposits of the Middle Pleis-
Portion below irregular line restored.
(From Lull, adapted from McGregor.) tocene of Java there have been
discovered during the past cen-
tury various skeletal fragments which possess many of the attri-
butes of ‘missing links.’ The bones found probably represent several
species. The first discovered and the most famous consist of a skull-
cap, femur, and three teeth. With these fragments as a guide, ex-
perts have attempted to restore the chief features of this so-called
Java MAN, Pithecanthropus erectus. (Fig. 276.)
The face of Pithecanthrepus shows immense beetling brows.
The skull viewed from above is essentially human although the
brain itself has deficiencies in the parietal and prefrontal regions,
the latter in particular being the seat of the higher mental faculties.
The motor and auditory speech centers are significantly developed
so it seems probabie that Pithecanthropus had at least the rudi-
ments of articulate speech which differentiated him from the apes.
The volume of the brain, estimated at about 940 cc., is small com-
pared with that of modern man, though barely within the range of
THE HUMAN BACKGROUND 433
the human brain variation. It far exceeds that of a very large male
gorilla. The skull was supported by powerful muscles and ligaments
as in the great apes, and not nicely poised as in man, thus indicating
a stooping posture. The straight femur probably belongs to a more
erect and advanced type of Pleistocene man.
From several standpoints, Pithecanthropus affords an interest-
ing link with the past, but one that is based on somewhat confused
evidence. Indeed the importance of this Java man is hardly com-
mensurate with the fame which he has acquired, and he is now
overshadowed by the more
recent discoveries in regard to
Peking man, who actually
preceded him.
2. The Peking Man
At various times during the
past thirty-five years frag-
ments of fossil man have been
found near Peiping, China, in
deposits of Early Pleistocene
age, perhaps a million years Fic. 277. — Peking Man, Sinanthro-
- pus. The face and jaws are mainly re-
old. From the fragments dis- stored. (From Romer, after Weinert.)
covered — skulls, jaws, teeth,
etc. —it appears that the PEKING man, Sinanthropus pekinensis,
had very thick cranial walls but surprisingly large cranial ca-
pacity. The brain was loftier but narrower than in the Java
man, and the mastoid region of the temporal bone is sug-
gestive of that in the adult anthropoid apes and the human in-
fant. The teeth are somewhat primitive although essentially
human. The Peking man obviously represents a very primitive
type in the general line of advance, which probably is fairly
closely, but not directly related to the Java man. Moreover,
there is some evidence of the dawn of culture with this early man.
Abundance of carbonized material indicates the use of fire, and
pieces of crudely chipped stones suggest the use of tools. (Fig. 277.)
3. The Piltdown Man
Significant discoveries were made near Piltdown Common in
Sussex, England, from 1911 to 1913, that included parts of two
crania and a lower jaw, nasal bones, and several teeth. The enor-
434 ANIMAL BIOLOGY
mously thick cranial walls of this Prprpown MAN, Eoanthropus
dawsoni, resemble those of the Peking man, but the forehead and
vault of the skull approach nearer to that of modern man. How-
ever, the jaw and canine tooth are more ape-like than in the man
of Peking, and probably do not belong to the same skeleton. Al-
though geologically about contemporaneous, it is difficult from the
scanty data to bring the relatively large-brained Piltdown man
into relationship with the Java and Peking men. It has been sug-
gested that the human line comes from the Piltdown rather than
Fic. 278. — Skull and face of Pilt- Fic. 279. — Skull and face of Hei-
down Man, FEoanthropus dawsont. delberg Man, Homo heidelbergensis,
(From Lull, adapted from McGregor.) based upon McGregor’s restoration
of the skull, the lower jaw only being
known. (From Lull.)
the Peking line, but their relative significance from the standpoint
of the direct human lineage remains to be determined. (Fig. 278.)
4. The Heidelberg Man
A species of early Man is represented merely by a lower jaw
found during 1907 in a sand deposit of Eariy Pleistocene age near
Heidelberg, Germany. The quite massive jaw is of a primitive
type, but the teeth are essentially human both in relative size and
general appearance. Accordingly, the HrE1ipELBERG MAN is included
in the same genus with modern man, as Homo heidelbergensis.
However, the relationships of Heidelberg man are obscure, though
THE HUMAN BACKGROUND 435
possibly he is an ancestor of the men of Neanderthal, his succes-
sors. (Fig. 279.)
5. Neanderthal Man
The remains of NEANDERTHAL MAN, Homo neanderthalensis,
appear in the caverns or rock shelters of Europe several hundred
thousand years after the Heidel-
berg man. The history of man
during the vast interim has not
yet been revealed, but we must
suppose that he persisted precar-
iously through the intermittent
periods of glaciation during the
great ice ages. Indeed, the Nean-
derthal race may have diverged
early in the Pleistocene, but it
flourished in the last interglacial
and the early part of the last
glacial epoch. It appears to have
sprung from an earlier stock of
which the Java and Peking men
were members. (Fig. 272.)
Neanderthal man is known to
us from many skeletons, one of
the earliest in point of discovery
bemg found in the Neander cell |
Valley, near Diisseldorf, Ger- el
many, in 1857, and one of the Hate Plank atic et ited
most recent in the Cave of compared with that of a living native
Robbers near Jerusalem. The Australian (B), Homo sapiens; the
men of Neanderthal averaged aie the lowest existing race. (After
: E oodward.)
about five feet, four inches in .
height and were stocky and powerful. They probably walked with
a shuffling, slouching gait since curved thigh bones and imperfect
curvatures of the spine show that the limbs were habitually bent
at hip and knee. A large head with heavy jaws was supported
by powerful neck muscles. (Fig. 280.)
The skull is notable for its size but the cranium is low and the
forehead retreats from a continuous brow-ridge that is distinctive
of the race. The large brain is relatively simple compared with
436 ANIMAL BIOLOGY
that of modern man, especially in the regions devoted to the higher
mental activities. However, that these cave men were not without
ability is attested by the well-wrought stone hunting implements
found associated with their remains. But bestiality outweighs hu-
man features, according to modern
Fic. 281. — Skull and face of Nean- Fic. 282. — Skull and face of
derthal Man, Homo neanderthalensis. Cré-Magnon Man, Homo sapiens.
(From Lull, adapted from McGregor (From Lull, adapted in part from
and Boule.) McGregor.)
standards, though it seems that some higher human traits lurked
in their make-up because in certain instances there is evidence
of reverential burial, with all that it implies. (Figs. 281, 283.)
6. Cré-Magnon Man
Sometime in the last glacial epoch the supremacy of Nean-
derthal man was challenged by a superior race — the first that
is recognized as of the same species, Hemo sapiens, to which
modern man is assigned. This invading race of CRO-MAGNON MEN
seems to be of different immediate stock from the Neanderthal men
of Europe and, in large part at least, to be responsible for their ex-
tinction. Apparently Cr6-Magnon man came from an Asiatic source
about fifty thousand years before the dawn of history, after an ante-
cedent evolution of many more thousands of years from a Ne-
anderthaloid stem. If this is true, modern man may be, so to
speak, Neanderthal man’s progressive nephew, though not his
direct descendant.
THE HUMAN BACKGROUND 437
Although numerous remains of the Cro-Magnon race have been
discovered, those from a rock-shelter of Cr6-Magnon in western
France represent the type. The men were of large stature, averag-
ing at least six feet in height, while the women were much smaller.
The posture was entirely erect with the characteristically alternat-
ing curves of the human spine, and straight limbs. The cranium
Fic. 283. — A, Neanderthal Man; B, Cré-Magnon Man. Original models
by Professor J. H. McGregor, in the American Museum of Natural History.
was of very large capacity with high, vertical forehead and no brow-
ridges. The face was broad in comparison with the cranium, and
the somewhat prominent chin, narrow and pointed. (Figs. 282, 283.)
The Cr6d-Magnons were a splendid race physically, and the re-
markable Paleolithic art that still survives in certain caverns of
France and Spain attests their mental equipment. Thus from both
aspects they meet the standard of the species Homo sapiens, and
differ in no great degree from their successors, the so-called
Mesolithic and Neolithic races which, in turn, closely approach
the present-day races of man.
C. CuLtturRAL DEVELOPMENT
It seems evident that man’s position above the beasts is based
chiefly upon the fact that he alone possesses a genuine culture.
Man domesticated himself and so originated a culture involving
the basic tripod of invention, communication, and social ha-
438 ANIMAL BIOLOGY
bituation. Man created culture, and culture created Man. But, of
course, the biological basis constitutes the foundations upon
which the cultural superstructure rests — Man cannot get away
from Nature. Various culture patterns are impressed upon the indi-
vidual as habits of thought and action and this social conditioning
demands a complex organism with the ability to make environ-
mental adaptations.
Animals in general exhibit various BIOSOCIAL reactions, such as
the group life of Bees and Ants or the simple family life of the
anthropoid Apes, that are inherited from generation to generation
and are the outcome of evolution. But upon the hereditary biosocial
endowment of actions and reactions, Man has superimposed proc-
esses of a cultural order that are acquired in each generation by
the continuity of so-called group conditioning; t.e., habits of body
and mind are impressed upon the young by the elders. Although
these cultural processes are an addition to, and are dependent upon
the hereditary biosocial endowment, they are something more
than merely an elaboration of it. They are essentially untramelled
by the limitations of the slow process of organic evolution that is
dependent upon germinal variations and natural selection. Thus
the cultural aspect of human nature can and does forge ahead in
so far as the physical and mental heritage is adequate to meet the
emergency.
1. Paleolithic Culture
Most of the specimens of prehistoric man have been found in
Europe and this holds true also for the evidences of his culture,
so our attention may be confined to this region where the chro-
nology has been worked out most thoroughly.
The first artifacts appear in the Pliocene epoch or very early in
the Pleistocene epoch of periodic glaciation, and consist of small
chips of flint known as EOLITHS, or ‘dawn stones,’ that obviously
have been crudely shaped by man. This culture is evidence of the
exceedingly slow dawning of human mental life because it persisted
with little improvement for not less than several hundred thousand
years. It apparently represents the cultural scale of the Peking and
Piltdown men and possibly also of Heidelberg man. However, as
time passes we find that the artifacts increase in variety of form
and nicety of manufacture, and the so-called PALEOLITHIC culture
emerges. There are cleavers, axes, scrapers, drills, etc., some of
THE HUMAN BACKGROUND 439
them apparently shaped for convenience in grasping, and even-
tually we reach the work of Neanderthal man — the first Paleo-
lithic culture that can be assigned to a definite race of OLD STONE
AGE MEN.
The relics of Neanderthal man, unlike those of his precursors,
are found typically in rock shelters and caverns. The chief source
has been in western France, although a similar culture is wide-
Fic. 284. — Mousterian implements. Flint scrapers and points, and two bone
compressors from a rock shelter of La Ferrassie (Dordogne), France. Middle
Paleolithic Period. (From MacCurdy, after Capitan and Peyrony.)
spread in other suitable regions of Europe, and in Palestine and
Mongolia. The most famous cavern, at Le Moustier in France, is
believed to have served as a human abode for more than fifty
thousand years, and accordingly the culture of the Neanderthals
is known as MousTeRIAN. (Fig. 284.)
The stone implements of the Neanderthals show technical im-
provements in the methods of chipping as well as a greater variety
of form. In addition to the point and the scraper there are saws,
hammers, drills, and skinning implements. Also tools of bone were
used for dressing hides, but there is no evidence of implements
for sewing so it is assumed that clothing, such as it was, consisted
of single skins. The use of fire was known and burial of the dead
was practiced, but even the simplest pictorial art was not developed.
440 ANIMAL BIOLOGY
Apparently chipping flint tools and hunting demanded all the en-
ergy and ability of the Neanderthals until they were superseded
by the Cro-Magnons.
A wave of migration from Asia brought the Cro-Magnons to
western Europe where they met and to some extent mingled with
the Neanderthals, but to the eventual extinction of the latter.
Clearly it was the survival of the fit for the Cr6-Magnon was es-
sentially like modern man both physically and mentally, and “the
Fig. 285. — Harpoons of reindeer horn from France and Switzerland. Late
Paleolithic Period. (From MacCurdy, after Breuil.)
characters of their crania reflect their moral and spiritual poten-
tialities.”” It was a race of hunters and warriors, of sculptors and
painters that lived at the close of the long glacial epoch, some
fifty thousand years ago.
The marked line of cleavage between the Neanderthal and Cro-
Magnon cultures is attested by a newly developed kit of tools. No
longer are the implements confined to such as can be fashioned
merely by chipping, but include, for example, the harpoon of rein-
deer horn, the flat bone point with cleft base, the needle of bone or
ivory, the dart thrower, and the flint scratchers, knives, and gravers.
THE HUMAN BACKGROUND 441
The latter were used both for cutting bone, horn, and ivory and
also for engraving and sculpturing. Indeed the Cr6-Magnon artist
not only modeled in clay, but also was skillful with colors, first
simple, later blended, as is still attested by the drawings and fres-
coes on the walls and ceilings of his caverns. However, the art
that depicted the Wooly Rhinoceros and the Mammoth was to
fade as the diminishing ice sheets forced these animals north to
eventual extinction. The cause of this flowering of artistic ability
Fic. 286. — Bison incised on limestone. Rock shelter of Laugerie-Basse
(Dordogne), France. Late Paleolithic Period. (From MacCurdy.)
and its passing remains an enigma. It was not exhibited by the
immediate successors of Paleolithic man. (Figs. 285, 286.)
2. Mesolithic Culture
The so-called MrsouitruHic culture succeeded the Paleolithic and
represents, as it were, the Dark Ages of the prehistoric era. Char-
acteristic of the period are the huge shell heaps — refuse piles
that throw considerable light on the food habits and implements
of the people. These are found in Europe, Asia, and America.
Usually they were situated near water as is evidenced by the great
abundance of oyster and mussel shells, and of the bones of the
duck, goose, gull, and swan. Mammals are represented by bones
442 ANIMAL BIOLOGY
of the stag, boar, wolf, bear, beaver, etc. It is significant that re-
mains of domesticated animals are not present, except possibly of
the dog — the earliest companion of man. Among the implements
are arrow points, axes, adzes, and blade-like flints, while remnants
of pottery show that at least crudely fashioned bowls and jars
were employed.
Other interesting accessions of the period are the curious painted
pebbles that are found in stream beds. The pebbles bear symbols
of various kinds, some of them crudely resembling modern letters.
Indeed it has been seriously suggested that these symbols represent
a mode of writing and that some of them have had their influence
on our own alphabet.
3. Neolithic Culture
The culture of the NeouirHic, or NEw STONE AGE, is essentially
that of modern men who have deserted cavern life and taken to
the open. The animal life is also modern since no ‘prehistoric’
animals persisted and none have since become naturally extinct.
The human population appears to have been increasing in num-
bers, and the division of labor between individuals and communities
to have become more significant. Thus the Mesolithic hunter and
fisherman gave place to the Neolithic husbandman who, to some
extent at least, controlled his food supply and so made possible
the development of community life. From a mere food gatherer,
Man became a food producer.
Almost surely the relatively rapid transformation of the primi-
tive civilization was the outcome of the cultivation of plants — such
as wheat, barley, rye, flax, grape, apple, and pear; the domestica-
tion of animals — for example the dog, ox, sheep, and goat; and
the development of the art of making pottery and textiles. More-
over pottery and textiles afforded an outlet for artistic ability and
this is also evidenced, for instance, in the beautifully chipped flint
poignards and knives. The shaping and finishing of stone tools
and weapons by a process of polishing appears for the first time in
the Neolithic period which accordingly is frequently referred to
as the age of polished stone implements. (Fig. 287.)
Transportation on water by means of dugouts began in Neo-
lithic times, and the custom was developed of erecting habitations
on piles in rivers, lakes, and swamps. Such PILE VILLAGES, serving
both sanitation and safety, were widely distributed in Europe
THE HUMAN BACKGROUND 443
during the later Neolithic and survived into the following age.
The best known representatives are the Swiss Lake Dwellings.
Transportation by dugout apparently antedates the invention
of the wheel, but Neolithic man employed at least crude wheels
made from sections of logs, and the significance of this advance
can hardly be overestimated. The wheel is so inextricably woven
into the fabric of modern civilization that one is inconceivable
———— =
5 6
Fic. 287. — Implements typical of the Neolithic Period. 1, ax-hammer;
2, ax; 3, saw; 4, dagger; 5, knife; 6, arrow point. (From American Museum
of Natural History.)
without the other. “Take away fire and the wheel and the world
would suddenly revert to sub-Neolithic level.”
Although burial of the dead extends further into the past, it
reaches ceremonial significance with Neolithic man. Numerous
so-called MEGALITHIC monuments still remain— some of them
memorials to the dead, and others of unknown symbolic import.
Probably the most celebrated is the Stonehenge on Salisbury Plain
in England. And finally, to his other accomplishments, Neolithic
444 ANIMAL BIOLOGY
man developed some surgical skill as evidenced, in particular, by
skulls showing trepanation. So culture moved on apace to the
Age of Metals.
4. Age of Metals
The gradual transition from the Stone Age — Paleolithic, Meso-
lithic, and Neolithic — to the AGE or MErats effected perhaps the
most important step in the history of human culture. It meant a
release from the restrictions inherent in the very nature of stone
and the opening up of the almost infinite possibilities of metals in
the fabrication of newer and better instruments and utensils. In-
vention was stimulated.
This turning point in culture came with the development of the
art of extracting metals from their ores and of melting and casting
them. Copper was the first employed, probably because it was
available in its native condition, and led to a transition period, the
so-called AGE oF Copper. This later gave place to the great BRONZE
AGE, with the discovery of the many advantages of this alloy of
copper and tin. The Bronze Age extended approximately from
3000 to 1500 B.c. and shows gradual progression in the variety and
nicety of fabrication of tools, utensils, ornaments, etc., as well as
concomitant progress in many other aspects of human culture. It
eventually led to the IRon AGE which bridges the transition be-
tween prehistoric and historic times.
So Man has travelled far since culture emerged from the mere
biosocial pattern. He has gained an increment in each generation
and passed it on by so-called ‘social heredity’ until the cumulative
result is monumental. Witness the high order of endeavor and
social integration that has led Man not only to modern science and
art, but has created within him the aspirations and ideals that
make him unique in the world of life. It spells modern civilization
that, ideally at least, ministers to the health, wealth, and happiness
of mankind.
It is important to note that cultural evolution has given to
humanity greatly increased powers, although the hereditary physi-
cal basis apparently has remained essentially the same since the
origin of Homo sapiens. Increased cultural complexity has de-
pended upon the intelligent use of structures and capacities already
present and not upon the evolution of new ones. Indeed, the rela-
THE HUMAN BACKGROUND 445
tively static character of Man’s nature may possibly constitute
a crucial handicap to indefinite human progress. One may be a
confirmed optimist and still admit that the increasing momentum
of the stupendous cultural advance during the past century is to-
day taxing the adjustment capacity — the adaptability — of the
human biological heritage. Surely, Man must study himself more
intensively.
And so we may appropriately reiterate what was stated on an
early page: the most pregnant thought from the study of biology
in general and Man’s past in particular is the unity of nature —
the oneness of life — based on the ever-increasing background of
knowledge which “robs life of none of its mystery but rather serves
to link it securely with the larger mystery of the universe and the
Infinite back of it all.’”” But Man, though one with all living beings,
has the unique and all-important power consciously to study the
ways, to direct the forces of nature, and to adapt himself to them.
The knowledge of Man’s physical development through the ages
in no wise minimizes the other aspects of his nature on whose
origin biology is silent, and which constitute the enormous gap
that separates him from the beasts. When the grandeur of this
view of life to which biology leads is appreciated to the full, no
reassurance is necessary of Man’s commanding position — his op-
portunities and his responsibilities.
CHAPTER XXVI
DEVELOPMENT OF BIOLOGY
History must convey the sense not only of succession but also of
evolution. — New York Times.
Tue story of Man’s slow emergence from a condition in which he
was completely at the mercy of his environment, to his relatively
masterful position as exhibited in our present civilization, is the
inspiring history of science — the intellectual development of the
race. Indeed, as we have seen, knowledge spells power — power
to direct and become adapted to the forces of nature, and this
knowledge Man has acquired after much labor and safely treasured
with great pains as a result of scientific study. Truly “the succes-
sion of men during the course of many centuries should be consid-
ered as one and the same man who exists always and learns con-
tinuously’; but we, for the most part, forget the past whose heirs
we are — ‘‘the present is vocal and urging, the past silent and
patient.”” Let us for the moment turn to the works of some of the
outstanding contributors to biological history.
Some knowledge of hunting, agriculture, and husbandry was one
of the early acquirements of prehistoric Man, and at the dawn of
history, nearly 5000 years ago, systems of medicine apparently
found a place in Egyptian and Babylonian civilizations. So, on the
practical side, biology has a very ancient beginning. But biology as
the science of life in which emphasis is placed on the study of vital
phenomena for their own sake really begins with the Greeks.
A. GREEK AND ROMAN SCIENCE
Science reaching Greece from the South and East fell upon fertile
soil, and in the hands of the Hellenic natural philosophers was
transformed into coherent systems through the realization that
nature works by fixed laws — a conception foreign to the Oriental
mind but the corner-stone of all future scientific investigation. It is
not an exaggeration to say that to all intents and purposes the
Greeks laid the foundations of the chief subdivisions of natural
science and, specifically, created biology.
446
DEVELOPMENT OF BIOLOGY 447
ARISTOTLE (384-322 B.c.), the most famous pupil of Plato and
dissenter from his School, represents the highwater mark of the
Greek students of nature and is justly called the Father of Natural
History. Although Aristotle’s contributions to biology are numer-
ous, perhaps of most significance is the fact that he took a broad
survey of the existing data and welded them into a science. He did
this by relying, to a considerable extent, on the direct study of
organisms and by insisting that the only true path of advance lies
in accurate observation and description. The observational method
and its very modern development, the laboratory method of biolog-
Fic. 288. — Aristotle.
ical study, find their first great exponent in Aristotle. But mere
observation without interpretation is not science. Aristotle’s gen-
eralizations based on the facts accumulated and his elaboration
of broad philosophical conceptions of organisms give to his biolog-
ical works their lasting significance. (Fig. 288.)
While Aristotle’s biological investigations were devoted chiefly
to animals, his pupil and co-worker, THEOPHRASTUS (370-286 B.c.),
made profound studies on plants. Theophrastus not only laid
the foundations but also gave suggestions of much of the super-
structure of botany; an achievement which entitles him to rank
as the first great student of plant science. (Fig. 289.)
Before leaving the Greeks we must mention HIppocRATES
448 ANIMAL BIOLOGY
(460-370 B.c.), the Father of Medicine. Lecturing a generation
before Aristotle, at the height of the Age of Pericles, Hippocrates
crystallized the knowledge of medicine into a science and gave to
physicians a high moral inspiration.
The history of medicine and of biology as a so-called pure science
are so closely interwoven that consideration of the one involves
that of the other. Indeed the physicians form the chief bond of
continuity in biological history between Greece and Rome. The
chief interest of the Romans lay largely in practical affairs so it
would seem that the advantages to be gained from medicine should
Fic. 289. — Theophrastus of Eresus.
have led them to make important contributions. As it happened,
however, two Greek physicians were destined to have the most
influence: Dioscorides, an army surgeon under Nero, and Galen,
physician to the Emperor Marcus Aurelius.
DioscoripEs wrote the first important treatise on applied
botany. This was really a work on the identification of plants for
medicinal purposes but, gaining authority with age and being var-
iously transformed, it became the standard ‘botany’ for fifteen
centuries.
GALEN (131-201) was the most famous physician of the Roman
Empire and his voluminous works represent both a depository for
the anatomical and physiological knowledge of his predecessors,
DEVELOPMENT OF BIOLOGY 449
improved and worked over into a system, and also a large amount
of original investigation. Galen was at once a practical anatomist
and also an experimental! physiologist, inasmuch as he described
from dissections and insisted on the importance of vivisection and
experiment. Galen gave to medicine its standard ‘anatomy’ and
‘physiology’ for fifteen centuries.
Any consideration of the biological science of Rome would be
incomplete without a reference to the vast compilation of mingled
fact and fancy made by Puiny the Elder (23-79.) It was aside
from the path of biological advance, but long the recognized
Natural History, passing through some eighty editions after the
invention of printing.
B. MEDIEVAL AND RENAISSANCE SCIENCE
For all practical purposes we may consider that biology at the
_ decline of the Roman Empire was represented by the works of
- Aristotle, Theophrastus, Dioscorides, Galen, and Pliny. Even
these exerted little influence during the Middle Ages. Dioscorides,
Galen, and Pliny were in the hands of the scholars, but in so far
as science reached the people in general it was chiefly by collec-
tions of quotations from corrupt texts of these authors interspersed
with anecdotes and fables. From diverse sources gradually devel-
oped the oft-quoted PHysroLocus, found in many forms and lan-
guages, which is at once a collection of natural history stories, and
a treatise on symbolism and the medicinal use of animals. Here,
for instance, the mythical centaur and phoenix take their place
with the Frog and Lion in affording illustrations of theological
texts and in pointing out more or less far-fetched morals. Allu-
sions from the Physiologus are readily found in Dante, Cervantes,
and Shakespeare, while its illustrations are immortalized in the
gargoyles of medieval cathedrals.
Indeed, science was submerged to such an extent that the scien-
tific Renaissance owes its origin largely to the revival of classical
learning: in particular to the translation of Aristotle and Theo-
phrastus, and renewed study of Dioscorides and Galen. Their
works were so superior to the current science that, in accord with
the spirit of the times, to question their authority became almost
sacrilegious. The first studies were merely commentaries on the
writings of these authors, but as time went on more and more new
observations were interspersed with the old. In short, the climax
450 ANIMAL BIOLOGY
of the scientific Renaissance involved a turning away from the
authority of Aristotle and the past, and an adoption of the Aris-
totelian method of observation and induction.
Botany was the first to give visible signs of the awakening,
probably because of the dependence of medicine on plant products.
“ All physicians professed to be botanists and every botanist was
thought fit to practice medicine.”” In the Hersats published in
Germany during the sixteenth century we can trace the growth of
Fic. 290. — Andreas Vesalius.
plant description and classification from mere annotations on the
text of Dioscorides to well-illustrated manuals of the plants of
western Europe.
Meanwhile zodlogy began to emerge as a distinct science, but
the less obvious immediate utility of the subject, combined with
the greater difficulty of collecting and preserving animals, and
therefore the necessity of more dependence on travellers’ tales,
contributed to retard its advance. One group of naturalists, the
ENCYCLOPAEDISTS, so-called from their endeavor to gather all the
available information about living things, attempted the impos-
sible. However, this gleaning from the ancients and adding such
material as could be gathered led to the publication of huge volumes
of fact and fiction, which in the case of the best — Gesner’s History
DEVELOPMENT OF BIOLOGY 451
of Animals — served to popularize zoology and afforded the neces-
sary survey which must precede constructive work.
Although GESNER (1516-1565) of Switzerland was without
doubt the most learned naturalist of the period and probably the
best zoologist who had appeared since Aristotle, the direct path to
progress was blazed by men whose plans were less ambitious.
Contemporaries of Gesner, who confined their treatises to special
groups of organisms which they themselves investigated, really
Fig. 291. — William Harvey.
instituted the biological MONOGRAPH which has proved to be an
effective method of scientific publication.
While the HERBALISTS, ENCYCLOPAEDISTS, and MONOGRAPHERS
at work in natural history were making earnest endeavors to
develop the powers of independent judgment, long suppressed dur-
ing the Middle Ages, the emancipator of biology from the traditions
of the past appeared in the Belgian anatomist, VESALIUS (1514—
1564). Not content with the anatomy of the time, which consisted
almost solely in interpreting the works of Galen by reference to
crude dissections made by barbers’ assistants, Vesalius attempted
to place human anatomy on the firm basis of exact observation.
The publication of his great work On the Structure of the Human
Body made the year 1543 the dividing line between ancient and
modern anatomy, and thenceforth anatomical as well as biological
452 ANIMAL BIOLOGY
investigation in general broke away from the yoke of authority,
and men began to trust and use their own powers of observation.
(Fig. 290.)
The work of Vesalius was on anatomy, and physiology was
treated somewhat incidentally. The complementary work on the
functional side came in 1628 with the publication of the epoch-
making monograph On the Motion of the Heart and Blood in Ani-
mals by Harvey (1578-1657) of London. No rational conception
of the economy of the animal organism was possible under the
influence of Galenic physiology, and it remained for Harvey to
demonstrate by a series of experiments, logically planned and
ingeniously executed, that the blood flows in a circle from heart
back to heart again, and thus to supply the background for a
proper understanding of the physiology of the organism as a whole.
With the work of Vesalius and Harvey, biologists had again laid
hold of the great scientific tools — observation, experiment, and
induction — which since then have not slipped from their grasp.
(Figs. 121, 291.)
C. THe Microscopists
During this revival period, when collections and accurate de-
scriptions of plants and animals were being made and the study of
anatomy and physiology was going rapidly forward, optical inven-
tions occurred which were destined to make possible modern
biology. First came the development of the SIMPLE MICROSCOPE,
through an adaptation of the principles of spectacles, during the
sixteenth century; then the combination of lenses to form the
COMPOUND MICROSCOPE first effectively employed by Galileo in
1610; and by the middle of the century simple and compound
microscopes were being made by opticians in the leading centers of
Europe. Significant of the times is the clear appreciation of the
importance of studying nature with instruments, which increase
the powers of the senses in general and of vision in particular, ex-
pressed by Hooke (1635-1703) of London in a remarkable book,
the Micrographia, published in 1665. Using his improved com-
pound microscope, Hooke clearly observed and figured for the
first time the “‘little boxes or cells’ of organic structure, and his
use of the word cell is responsible for its application to the proto-
plasmic units of modern biology. (Fig. 10.)
Microscopical work was a mere incident among the varied inter-
DEVELOPMENT OF BIOLOGY 453
ests of Hooke, while LEEUWENHOEK (1632-1723) of Holland spent
a long life studying nearly everything which he could bring within
the scope of his simple lenses. With an unexplored field before him,
all of his observations were discoveries. Bacteria, Protozoa, Hydra,
and many other organisms were first revealed by his lenses. But
Leeuwenhoek’s discovery of the sperm of animals created the most
astonishment. His imagination, however, outstripped his observa-
tions for he thought he saw evidence of the organism preformed
within the sperm and so regarded it as the complete germ which
had only to be hatched by the female. (Figs. 14, 26, 292.)
Samet
Fic. 292. — Antony van Leeuwenhoek.
The patience and ingenuity of Leeuwenhoek was equalled, if not
exceeded, in the studies on insect anatomy made by SWAMMERDAM
(1637-1680) of Holland. Inspired largely by the desire to refute
the current notion that Insects and similar lower animals are with-
out complicated internal organs, Swammerdam spent his life in
studies on their structure and life histories. Revealing, as he did,
by the most delicate technique in dissection, the finest details
observable with his lenses, Swammerdam not only set a standard
for minute anatomy which was unsurpassed for a century, but also
dissipated for all time the conception of simplicity of structure in
the lower animals. He thus, quite naturally, added one more
argument to those of the Italian Rep1 (1626-1698) and others
454, ANIMAL BIOLOGY
against spontaneous generation — an erroneous theory that sur-
vived until the work of Pasteur (1822-1895) in the nineteenth
century. (Figs. 152, 294.)
Malpighi of Bologna and Grew of London, contemporaries of
Hooke, Leeuwenhoek, and Swammerdam, may be considered as
the pioneer histologists. GREW (1641-1712) devoted all his atten-
tion to plant structure, while Matpicui (1628-1694), in addition
to botanical studies which paralleled Grew’s, made elaborate inves-
tigations on animals. The versatility as well as the genius of
Fic. 293. — Marcello Malpighi.
Malpighi is shown by his studies on the anatomy of plants, the
function of leaves, the development of the plant embryo, the em-
bryology of the chick, the anatomy of the Silkworm, and the struc-
ture of glands. Skilled in anatomy but with prime interest in
physiology, his lasting contribution lies in his dependence upon the
microscope for the solution of problems where structure and func-
tion, so to speak, merge. This is well illustrated by his ocular
demonstration of the capillary circulation in the lungs, which is
not only his greatest discovery but also the first of prime impor-
tance ever made with a microscope, since it completed Harvey’s
work on the circulation of the blood by revealing what his experi-
ments predicted. (Fig. 293.)
DEVELOPMENT OF BIOLOGY 455
D. DEVELOPMENT OF THE SUBDIVISIONS OF BIOLOGY
The microscopists taken collectively created an epoch in the
history of biology, so important is the lens for the advancement of
the science. Broadly speaking, we find that its development along
many lines during the eighteenth and particularly the nineteenth
century went hand in hand with improvements in the compound
microscope itself and in microscopical technique. Again, the mi-
croscopists in general and Malpighi in particular opened up so
Fic. 294. — Louis Pasteur.
many new paths of advance that from this period on it is not pos-
sible, even in the most general survey, to discuss the development
of biology as a whole. The composite picture must be formed by
emphasizing and piecing together various lines of work, such as
classification, comparative anatomy of animals, embryology, phys-
iology of plants and animals, genetics, and evolution.
1. Classification
Classification has as its object the bringing together of organisms
which are alike and the separating of those which are unlike; a
problem of no mean proportions when a conservative estimate to-
day shows nearly a million known species of animals and about a
quarter of a million plants — leaving out of account the myriads
of forms represented only by fossil remains.
456 ANIMAL BIOLOGY
Naturally the earliest classifications were utilitarian or more or
less physiological — fowl of the air, beasts of the field; edible,
poisonous, etc. But as knowledge increased emphasis was shifted
to the anatomical criterion of specific differences, and thenceforth
classification became an important aspect of natural history —a
central thread both practical and theoretical. Practical, in that it
involved the arranging of living forms so that a working catalog
was made which required nice anatomical discrimination, and
therefore the amassing of a large body of facts concerning animals
and plants. Theoretical, because in this process zoologists and
Fic. 295. — John Ray.
botanists were impressed, almost unconsciously at first, with the
‘affinities’ of various types of animals and plants, and so were led
to problems of their origin. (Fig. 297.)
From Aristotle, who emphasized the grouping of organisms on
the basis of structural similarities, we must pass Over some seven-
teen centuries, in which the only work of interest was done by the
herbalists and encyclopaedists, to the time of Ray (1628-1705) of
England and Linnarus (1707-1778) of Sweden. Previous to Ray
the term species was used somewhat indefinitely, and his chief
contribution was to make the word more concrete by applying it
solely to groups of similar individuals which seem to exhibit con-
stant characters from generation to generation. This paved the
way for the great taxonomist, Linnaeus. (Figs. 295, 296.)
DEVELOPMENT OF BIOLOGY 457
First and foremost a botanist, Linnaeus published a practical
classification of the Seed Plants which afforded a great impetus
to plant study, particularly because he insisted on brief descriptions
and the scheme of giving each species a name of two words, generic
and specific, thereby establishing the system of BINOMIAL NOMEN-
CLATURE. Linnaeus’ success with botanical taxonomy led him to
extend the principles to animals and even to the so-called mineral
kingdom: the latter showing at a glance his lack of appreciation of
any genetic relationship between species. Although Linnaeus be-
lieved that species, genera, and even higher groups represented
Fic. 296. — Carolus Linnaeus.
distinct acts of creation, nevertheless his greatest works, the Species
Plantarum and Systema Naturae, are of outstanding importance in
biological history and by common consent the base line of priority
in botanical and zodlogical nomenclature. (Page 352.)
2. Comparative Anatomy
Owing to the less marked structural differentiation of plants in
comparison with animals, plant anatomy lends itself less readily
to descriptive analysis, so that an epoch in the study of comparative
anatomy is not so well defined in botany as in the sister science,
zoology.
Comparative anatomy as a really important aspect of zodlogical
work, in fact as a science in itself, was the result of the life-work of
458 ANIMAL BIOLOGY
CuviER (1769-1832) of Paris. It is true that some of his pred-
ecessors had reached a broad viewpoint in anatomical study but
Cuvier’s claim to fame rests on the remarkable breadth of his in-
vestigations — his survey of the comparative anatomy of the whole
series of animal forms. And not content merely with the living, he
made himself the first real master of the anatomy of fossil Verte-
brates, as his contemporary LAMARCK was of fossil Invertebrates.
(Figs. 232, 297, 309.)
Cuvier’s grasp of anatomy was due to his emphasizing, as Aris-
totle had done before him, the functional unity of the organism:
Fic. 297. — Georges Cuvier.
that the interdependence of organs results from the interdepend-
ence of function: that structure and function are two aspects of the
living machine which go hand in hand. Cuvier’s famous principle
of correlation — “Give me a tooth,” said he, “‘and I will construct
the whole animal’ — is realiy an outcome of this viewpoint. Every
change of function involves a change in structure and, therefore,
given extensive knowledge of function and of the interdependence
of function and structure, it is possible to infer from the form of one
organ that of most of the other organs of an animal. But Cuvier
undoubtedly allowed himself to exaggerate his guiding principle
until it exceeded the bounds of fact.
Among Cuvier’s immediate successors, OWEN (1804-1892) of
London perhaps demands special mention. He spent a long life
DEVELOPMENT OF BIOLOGY 459
dissecting with untiring patience and skill a remarkable series of
animal types, as well as reconstructing extinct forms from fossil
remains. Aside from the facts accumulated, probably his greatest
contribution was making concrete the distinction between homol-
ogous and analogous structures. This has been of the first impor-
tance in working out the pedigrees of plants as well as of animals;
though Owen himself took an enigmatical position in regard to
organic evolution — not unlike that of the great teacher and in-
vestigator of zoology in America, AGassiz (1807-1873), but quite
Fic. 298. — Thomas Henry Huxiey.
different from that of Huxitey (1825-1895), his famous English
contemporary comparative anatomist. (Figs. 227, 298, 299.)
3. Physiology
The functions of organisms were discussed by Aristotle with his
usual insight, though, as might be expected since physiology is
more dependent than anatomy upon progress in other branches of
science, with less happy results. Similarly Galen was hampered in
his attempt to make physiology a distinct department of learning,
based on a thorough study of anatomy, and the corner-stone of
medicine; though fate foisted upon uncritical generations through
fifteen centuries his system of human physiology.
Neither Vesalius nor Harvey made an attempt to explain the
workings of the body by appeal to so-called physical and chemical
460 ANIMAL BIOLOGY
laws; and for good reason. Chemistry had not yet thrown off the
shackles of alchemy and taken its legitimate place among the elect
sciences, while during Harvey’s lifetime, under the influence of
Galileo, the new physics was born. But by the end of the seven-
teenth century both physics and chemistry had forced their way
into physiology and split it into two schools. The physical school
was founded by BoreE.ur (1608-1679) of Italy, who, employing
incisive physical methods, attacked a series of problems with
brilliant results; while the chemical school developed from the
Fic. 299. — Louis Agassiz.
influence of Franciscus Sytvius (1614-1672) of Holland as a
teacher rather than as an investigator.
This awakening brought a host of workers into the field and
the harvest of the century was garnered and enriched by HALLER
(1708-1777) of Geneva. In a comprehensive treatise which at
once indicated the breadth of view and critical judgment of its
author, Haller established physiology as a distinct and important
branch of biological science. It was no longer a mere adjunct of
medicine. Perhaps the most significant advance in Haller’s cen-
tury consisted in setting the physiology of nutrition and of res-
piration — both of which awaited the work of the chemists — well
upon the way toward their modern form. (Fig. 300.)
REAUMUR (1683-1757) of Paris and SPALLANZANI (1729-1799) of
Pavia may be singled out for their exact studies of gastric digestion,
DEVELOPMENT OF BIOLOGY 461
which showed ‘solution’ of the food to be the main factor in diges-
tion; although it was not clear how these changes differ from or-
dinary chemical ones. It was left for nineteenth-century investi-
gators to establish the fact that food in passing along the digestive
tract runs the gauntlet of a series of complex chemical substances,
each of which has its part to play in putting the various constit-
uents of the food into such a form that they can pass to the various
cells of the body where they are actually used. (Fig. 113.)
pie a
Fic. 300. — Albrecht von Haller.
On the side of respiration, a closer approach was made toward a
true understanding of the process. In France Lavoisiter (1743-
1794) demonstrated that the chemical changes taking place in
respiration involve essentially a process of combustion, and it
chiefly remained for later work to show that this takes place in
the tissues rather than in the lungs. (Fig. 118.)
Most of the firm foundation on which the physiology of animals
rests to-day has been built up by the work on Vertebrates. But
since the middle of the nineteenth century, when the versatile
MUuzer (1801-1858) of Germany emphasized the value of study-
ing the physiology of higher and lower animals alike, there has
been an ever-increasing tendency to focus evidence, in so far as
possible, from all forms of life on general problems of function.
This has culminated in the science of GENERAL PHYSIOLOGY.
462 ANIMAL BIOLOGY
The less obvious structural and functional differentiation of
plants retarded progress in plant physiology as it did in plant
anatomy. Probably of most historical, and certainly of most gen-
eral interest is the development of our knowledge of the nutrition
of green plants. Aristotle’s notion that the plant’s food is pre-
pared for it in the ground was still prevalent during the seven-
teenth century when Malpighi, from his studies on plant histology,
gave the first hint of supreme importance — the crude ‘sap’ en-
ters by the roots and is carried to the leaves where, by the action
Fic. 301. — Stephen Hales.
of sunlight, evaporation, and some sort of a ‘fermentation,’ it is
elaborated and distributed as food to the plant as a whole.
It is Hates (1667-1761) of England, however, to whom the
botanist looks as the Harvey of plant physiology, because in his
Vegetable Staticks (1727) he laid the foundations of the physiology
of plants by making ‘plants speak for themselves’ through his
incisive experiments. For the first time it became clear that
green plants derive a considerable part of their food from the at-
mosphere, and also that the leaves play an active réle in the move-
ments of fluids up the stem and in eliminating superfluous water
by evaporation. Still the picture was incomplete, and so remained
until the biologist had recourse to further data from the chemist.
(Fig. 301.)
DEVELOPMENT OF BIOLOGY 463
In 1779, Priesttey (1733-1804) of England, the discoverer of
oxygen, showed that this gas under certain conditions is liberated
by plants. This fact was seized upon by a native of Holland,
INGENHOUSZz (1730-1799), who demonstrated that carbon dioxide
from the air is reduced to its component elements in the leaf during
exposure to sunlight. The plant retains the carbon and returns
the oxygen — this process of carbon-getting being quite distinct
from that of respiration in which carbon dioxide is eliminated. It
remained then for DE SAUSSURE (1767-1845) in Geneva to show
that, in addition to the fixation of carbon, the elements of water
are also employed, while from the soil various salts, including com-
binations of nitrogen, are obtained. But it was nearly the middle
of the last century before the influence and work of Liesic (1803-—
1873) at Giessen led to a clear realization of the fundamental part
played by the chlorophyll of the green leaf in making certain chem-
ical elements available to animals. The establishment of the cos-
mical function of green plants — the link they supply in the cir-
culation of the elements in nature — is an epoch in biological
progress. (Figs. 15, 16.)
Enough perhaps has been said to indicate the trend of physi-
ology away from the maze of Galenic “‘spirits” in which science
lost itself, toward the modern viewpoint of science which assumes
as its working hypothesis that life phenomena are an expression of
a complex interaction of physico-chemical laws which do not differ
fundamentally from the so-called laws operating in the inorganic
world, and that the economy of the organism is in accord with the
law of the conservation of energy — probably the most far-reaching
generalization attained by science during the past century.
However, it is important to emphasize that viTaALisM — the
conception that life phenomena are, in part at least, the resultant
of manifestations of matter and energy which transcend and differ
intrinsically in kind from those displayed in the inorganic world:
a denial, as it were, in the organism of the full sufficiency of known
fundamental iaws of matter and energy — has arisen many times
in the development of biological thought. This has been either as a
reaction against premature conclusions of the rapidly growing
science, or from an overwhelming appreciation of the staggering
complexity of life phenomena. (Page 24.)
Vitalism attained perhaps its most concrete formulation as a
doctrine during the early part of the eighteenth century, in opposi-
464 ANIMAL BIOLOGY
tion to the obviously inadequate explanations which chemistry
and physics could offer for the phenomena of irritability of living
matter then prominently engaging the attention of biologists.
The vitalists of that period abandoned almost completely all
attempts to explain life processes on a physico-chemical basis, and
assumed that an all-controlling, unknown, mystical, hyper-
mechanical force was responsible for all living processes. It is
apparent that such an assumption in such a form is a negation of
the scientific method, and at once removes the problem from the
realm of scientific investigation.
Of course, no biologist at the present time subscribes to vitalism
in this form; some uphold vitalism — if it must still be called vital-
ism — in its considerably limited modern form; while al! will un-
doubtedly admit that we are at the present time utterly unable
to give an adequate explanation of the fundamental life processes
in terms of physics and chemistry. Whether we shall ever be
able to do so is unprofitable to speculate about, though certainly
the twentieth century finds few scientists who really expect a
scientific explanation of life ever to be attained or who expect
that protoplasm will ever be synthesized. However that may be,
this much is positive: during the past fifty years some biologists
have now and then thought they were on the verge of artificially
creating life in the test tube, only to leave the problem, like the
alchemists of old, with more respect for the complexities of proto-
plasmic organization and the enormous gap which separates even
the simplest forms of life from the inorganic world.
4. Histology
Studies on the physiology of plants and animals naturally
involved the progressive analysis of the physical basis of the phe-
nomena under consideration, but the Aristotelian classification of
the materials of the body as unorganized substance, homogeneous
parts or tissues, and heterogeneous parts or organs, practically
represented the level of analysis until the beginning of the eight-
eenth century. In fact it was not until the revival of interest in
embryology early in the last century that the cell became a particu-
lar object of study, and attention began gradually to shift from
more or less superficial details to cell organization. This culminated
in the classic investigations of two German biologists, the botanist
SCHLEIDEN (1804-1881) and the zodlogist ScHWANN (1810-1882),
DEVELOPMENT OF BIOLOGY 465
published in 1838 and 1839. Together these studies clearly showed
that all organisms are composed of units, or cells, which are at once
structural entities and the centers of physiological activities. And
further that the development of animals and plants consists in the
multiplication of an initial cell to form the multitude of different
kinds which constitute the body of the adult. (Figs. 7, 32, 302,
303.)
Unquestionably the cell concept represents one of the greatest
generalizations in biology, and it only needed for its consummation
Fic. 302. — Matthias Jacob Schleiden.
the full realization that the viscid, jelly-like material which zoolo-
gists interpreted as the true living matter of animals, and the
quite similar material which botanists considered the true living
part of plants are practically identical. This viewpoint was crystal-
lized in the early sixties by ScnuttzE (1825-1874) of Germany in
the formulation of the protoplasm concept, and thenceforth not
only morphological elements — cells — but also the material of
which they are composed — protoplasm — were recognized as
fundamentally the same in all living beings. Indeed, the realiza-
tion of a common physical basis of life in both plants and ani-
mals — a common denominator to which all vital phenomena are
466 ANIMAL BIOLOGY
reducible — gave content to the term biology and created the Sci-
ence of life in its modern form. (Fig. 9.)
5. Embryology
The cell theory resulted, as we have seen, from combined studies
on the adult structure and on the development of plants and ani-
mals, and accordingly implies that the science of embryology has a
history of its own. As a matter of fact, Aristotle discussed the
wonder of the beating heart in the hen’s egg after three days’ in-
cubation, but there the subject practically rested until FaBricrus
Fic. 303. — Theodor Schwann.
(1537-1619) at Padua, early in the seventeenth century, published
a treatise which illustrated the obvious sequence of events within
the hen’s egg to the time of hatching. This beginning was built
upon by a pupil of Fabricius, the celebrated Harvey, who added
many details of interest, though little progress in embryology was
possible without the microscope.
The microscope was first turned on embryological problems by
the versatile MaALpicui in two treatises published in 1672, and
at one step animal development was placed upon a plane so ad-
vanced that for over a century it was unappreciated. One conclu-
DEVELOPMENT OF BIOLOGY 467
sion of Malpighi, however, was seized upon by contemporary
biologists. Apparently, unbeknown to him, some of the eggs which
he studied were slightly incubated, so that he thought traces of the
future organism were preformed in the egg. This error contributed
to the formulation of the preformation theory, which gradually
became the dominant question in embryology. (Page 274.)
As a matter of fact the time was not ripe for theories of develop-
ment. The preformationists were wrong, but so were Aristotle,
Harvey, and later supporters of epigenesis who went to the opposite
extreme and denied all egg organization and therefore tried to get
Fic. 304. — Karl Ernst von Baer.
something out of nothing. It remained, as we know, for the present
generation of embryologists to work out many of the details of the
origin and organization of the germ cells, and to reach a level of
analysis deep enough to suggest how “the whole future organism is
potentially and materially implicit in the fertilized egg cell”’ and
thus that “the preformationist doctrine had a well-concealed
kernel of truth within its thick husk of error.”
The next great advance came in the accurate and comprehensive
studies of the Russian, von Baer (1792-1876), published in the
thirties of the last century. Taking his material from all the chief
groups of higher animals, von Baer founded COMPARATIVE EMBRY-
oLocy. Among his achievements may be mentioned: the clear
468 ANIMAL BIOLOGY.
discrimination of the chief developmental stages, such as cleavage
of the egg, germ layer formation, tissue and organ differentiation;
the insistence on the importance of the facts of development for
classification; and the discovery of the egg of Mammals. His obser-
vations on the origin and development of the germ layers, which
afforded the key to many general problems of the origin of the
body form, and his emphasis on the resemblance of certain embry-
onic stages of higher and lower animals, were made by his suc-
cessors, under the influence of the evolution theory, the point of
departure for the development of the GERM LAYER THEORY and the
RECAPITULATION THEORY. (Figs. 176, 235, 304.)
From every point of view von Baer created an epoch in embry-
ology just when the cell theory began to exert its influence on
biological research, and thenceforth it became the problem of the
embryologist to interpret development in terms of the cell. It is
unnecessary to follow historically the establishment of the fact
that the egg and the sperm are really single nucleated cells; that
fertilization consists in the fusion of egg and sperm and the orderly
arrangement of their chief nuclear contents, or chromosomes; that
the new generation is the fertilized egg, since every cell of the body
as well as every chromosome in every cell is a lineal descendant by
division from the zygote, and so from the gametes which united at
fertilization to form it. Such, however, are the chief results of
cytological study since von Baer. But embryologists have not been
content to employ merely the descriptive method, and the dom-
inant note of the most modern research is physiological — the
experimental study of the significance of fertilization, the dynamics
of cell division, the basis of differentiation, the influence of environ-
mental stimuli, and so on. (Figs. 162, 164, 178.)
6. Genetics
The study of inheritance could be little more than a groping in
the dark until embryology, under the influence of the cell theory,
afforded a body of facts which clearly indicated that typically the
fertilized egg is the sole bridge of continuity between successive
generations. Indeed, the present science of genetics has a history
largely confined to this century.
Although clearly intimated by a number of workers, the concep-
tion of the continuity of the germ plasm was first forced upon the
attention of biologists and given greater precision by WEISMANN
DEVELOPMENT OF BIOLOGY 469
(1834-1914) of Germany in a series of essays culminating in 1892
in his volume entitled The Germ Plasm. He identified the chro-
matin material which constitutes the chromosomes of the cell
nucleus as the specific bearer of hereditary characters, and em-
phasized a sharp distinction between germ cells and somatic cells.
(Figs. 181, 305.)
While this viewpoint had been gradually gaining content and
precision, the science of genetics had been advancing not only by
exact studies on the structure and physiology of the germ cells, but
also by statistical studies of the results of heredity — the various
Fig. 305. — August Weismann.
characters of animals and plants as exhibited in parents and off-
spring. The studies of this type which first attracted the attention
of biologists were made by GALTon (1822-1911) of England. In
the eighties and nineties of the last century, he amassed a great
volume of data in regard to, for example, the stature of children
with reference to that of their parents, and derived his well-known
‘laws’ of inheritance.
But the work which eventually created the modern science of
genetics was that of MENDEL (1822-1884) of Briinn, Austria. Men-
del combined in a masterly manner the experimental breeding of
pedigree strains of piants and the statistical treatment of the data
470 ANIMAL BIOLOGY
thus secured in regard to the inheritance of certain characters, such
as the form and color of the seeds in Peas. His work was published
in 1865 in an obscure natural history periodical, and he abandoned
teaching and research to become the Abbot of his monastery. Thus
terminated prematurely the scientific work of one of the epoch-
makers of biology, and the now famous Mendelian laws of inherit-
ance were unknown to science until 1900, when other biologists,
Fic. 306. — Gregor Johann Mendel.
coming to similar results, unearthed his forty-year-old paper.
(Figs. 183, 306.)
We have already seen that the fundamental principle of the
segregation of the genes during the development of the gametes, |
which Mendel’s work indicated, has been extended to other plants
and to animals, and that instead of being, as at first thought, a
principle of rather limited application, appears to be the key to all
inheritance. And the present results are extremely convincing
because cytological studies on the architecture of the chromosome
complex of the germ cells keep pace with, and afford a picture of
the physical basis of inheritance — the mechanism by which the
segregation and independent assortment of characters by the Men-
delian formula takes place. Such is the deeply hidden kernel of truth
in the old preformation theories. (Fig. 167.)
DEVELOPMENT OF BIOLOGY 471
7. Organic Evolution
A question which has interested and perplexed thinking men of
all times is how things came to be as they are to-day. The historian
of human affairs attempts to trace the sequence and relationship
of events from the remote past to the present. Similarly, the geol-
ogist endeavors to formulate the history of the Earth; and the bi-
ologist, the history of plants and animals on the Earth. All rec-
Fic. 307. — Comte de Buffon.
ognize that the present is the child of the past and the parent of
the future, and that past, present, and future, though causally re-
lated, are never the same. It was the Greek natural philosophers
who introduced this idea of history into science and attempted to
give a naturalistic explanation of the Earth and its inhabitants,
and thus started the uniformitarian trend of thought which cul-
minated in the establishment of organic evolution during the past
century. (Page 349.)
Aristotle apparently held the general idea of the evolution of
life from a primordial mass of living matter to the higher forms,
and placed Man at the head of animal creation. “To him be-
longs the God-like nature. He is preéminent by thought and voli-
tion. But although all are dwarf-like and incomplete in comparison
with Man, he is only the highest point of one continuous ascent.”
And evolution is still going on — the highest has not yet been
472 ANIMAL BIOLOGY
attained. In looking for the effective cause of adaptation Aristotle
rejected the hypothesis of EMpEpocLes (495-435 B.c.), which em-
bodied in crude form the idea of the survival of the fittest, and
substituted secondary natural laws to account for the apparent
design in nature. This was a sound induction by Aristotle from
his necessarily limited knowledge of nature, but had he accepted
the idea to account for adaptations, perhaps it would not be an
exaggeration to regard him as “the literal prophet of Darwinism.”
Fic. 308. — Erasmus Darwin.
The thread of continuity in evolutionary thought is not broken
from Aristotle to the present, but from the strictly biological
viewpoint two Frenchmen, Buffon and Lamarck, and two English-
men, Erasmus Darwin and his grandson, Charles Darwin, stand
preeminent.
BuFron (1707-1788) was a peculiarly happy combination of
entertainer and scientist who found expression in each new volume
of his great Natural History. And it was largely, so to speak, be-
tween the lines of this work that Buffon’s evolutionary ideas were
displayed; apparently beyond the reach of the censor and dilettante.
It is not strange, therefore, that it is often difficult to decide just
how much weight is to be placed on some of his statements; though
certainly it is not exaggerating to ascribe to him not only the recog-
nition of the factors of geographical isolation, struggle for existence,
DEVELOPMENT OF BIOLOGY 473
artificial and natural selection in the origin of species, but also
the propounding of a theory of the origin of variations — that the
direct action of the environment brings about modifications in
the structure of animals and plants and these are transmitted to
the offspring. (Fig. 307.)
When Buffon’s influence had passed its height, ERasmus Dar-
win (1731-1802) expressed consistent views on the evolution of
organisms, in several volumes of prose and poetry, which lead
Fic. 309. — Jean-Baptiste Lamarck.
biologists to-day to recognize him as the anticipator of the La-
marckian doctrine that somatic variations arise through the reac-
tion of the organism to environmental conditions. “All animals
undergo transformations which are in part by their own exertions,
in response to pleasures, and pain, and many of these acquired forms
or propensities are transmitted to their posterity.” (Fig. 308.)
Lamarck (1744-1829) developed with great care the first com-
plete and logical theory of organic evolution and is the one out-
standing figure in biological uniformitarian thought between Aris-
totle and Charles Darwin. ‘‘For nature,” he writes, “time is
nothing. For all the evolution of the Earth and of living beings,
nature needs but three elements, space, time, and matter.” In
regard to the factors of evolution, Lamarck put emphasis on the
indirect action of the environment in the case of animals, and the
ATA ANIMAL BIOLOGY
direct action in the case of plants. The former are induced to
react and so adapt themselves, as it were; while the latter, without
a nervous system, are molded directly by their surroundings. And,
so Lamarck believed, such changes, somatic in origin — acquired
characters — are transmitted to the next generation and bring
about the evolution of organisms. (Fig. 309.)
Through the relative weakness of Lamarck’s successors the
French school of evolutionists dwindled to practical extinction;
Fic. 310. — Charles Lyell.
while in Germany, GOETHE (1749-1832), the greatest poet of
evolution, and TREvrRANUS (1776-1837) “‘brilliantly carried the
argument without carrying conviction,” for the man and the
moment must agree. Then in England the uniformitarian ideas
elaborated by LyEtui (1797-1875) in his Principles of Geology es-
tablished evolution in geology and the way was paved for CHARLES
Darwin (1809-1882) to do the same for the organic world. (Figs.
310, 312; pages 373-377.)
True, “the idea of developmeni saturated the intellectual at-
mosphere — nevertheless the elaborate and toilsome labor of think-
ing it through for the endless realm of nature was to be done”’ and
Darwin did it in his Origin of Species which appeared in 1859. By
his brilliant, scholarly, open-minded, and cautious marshalling of
the facts pointing toward the universality of variations and the
DEVELOPMENT OF BIOLOGY 475
mutability of species; and by the theory of natural selection on the
basis of slight adaptive variations resulting in the survival of the
fittest in the struggle for existence — which, strange to say, Darwin
and WALLACE (1822-1913) reached simultaneously and independ-
ently — Darwin “made the old idea current intellectual coin.”’
(Figs. 236; 239; SLY, 312.)
To-day, as we know, no representative biologist questions the
fact of evolution — “evolution knows only one heresy, the denial
Fic. 311. — Alfred Russel Wallace.
of continuity’ — though in regard to the factors involved, there is
much difference of opinion. It is possible that we shall have reason
to depart widely from Darwin’s interpretation of the effective prin-
ciples at work in the origin of species, but withal this will have
little influence on his position in the history of biology. The great
value which he placed upon facts was exceeded only by his demon-
stration that this ‘value is due to their power of guiding the mind
to a further discovery of principles.’’ The Origin of Species brought
biology into line with the other inductive sciences, recast prac-
tically all of its problems, and instituted new ones. Darwin beauti-
fully and conservatively expressed this new outlook on nature in
the historically important concluding paragraph of his epoch-
making work:
476 ANIMAL BIOLOGY
‘It is interesting to contemplate a tangled bank, clothed with
many plants of many kinds, with birds singing on the bushes, with
various insects flitting about, and with worms crawling through the
damp earth, and to reflect that these elaborately constructed
forms, so different from each other, and dependent upon each other
in so complex a manner, have all been produced by laws acting
around us. These laws, taken in the largest sense, being Growth
with Reproduction; Inheritance which is almost implied by re-
production; Variability from the indirect and direct action of the
Fig. 312. — Charles Darwin.
conditions of life, and from use and disuse: a Ratio of Increase so
high as to lead to a Struggle for Life, and as a consequence to
Natural Selection, entailing Divergence of Character and the
Extinction of less-improved forms. Thus, from the war of nature,
from famine and death, the most exalted object which we are
capable of conceiving, namely, the production of the higher ani-
mals, directly follows. There is a grandeur in this view of life, with
its several powers, having been originally breathed by the Creator
into a few forms or into one; and that, whilst this planet has gone
cycling on according to the fixed law of gravity, from so simple a
beginning endless forms most beautiful and most wonderful have
been, and are being evolved.”’
APPENDIX
I. A BRIEF CLASSIFICATION OF ANIMALS
The figures indicate the approximate number of known species.
Phylum 1. PROTOZOA. (20,000 species.)
Class I. Sarcopina: Amoeba, Foraminifera, Heliozoa, Radiolaria.
(Figs. 6, 8, 11, 13, 19-21, 244, 245.)
Class II. Masticopuora: Flagellates. Monads, Trypanosomes, Eu-
glena, Volvox. (Figs. 18, 22-24, 224.)
Class III. Sporozoa: Plasmodium, Monocystis. (Figs. 25, 223.)
Class IV. Inrusoria: Ciliates. Paramecium, Vorticella, Stentor.
(Figs. 18, 26-28, 135, 226.)
Phylum 2. PORIFERA: Sponges. (3000 species.) Leucosolenia, Grantia,
Euspongia. (Figs. 35, 36.)
Phylum 3. COELENTERATA. (10,000 species.)
Class 1. Hyprozoa: Hydra, Obelia, Gonionemus. (Figs. 37, 38, 57,
58, 155, 158)
Class II. Scypuozoa: Jellyfish. Aurelia. (Figs. 39, 40.)
Class III. ANtHozoa: Sea Anemones, Corals. (Figs. 41, 42.)
Phylum 4. CTENOPHORA: Sea Walnuts. (100 species.)
Phylum 5. PLATYHELMINTHES: Flatworms. (7000 species.)
Class I. Tursevuarta: Planaria. (Figs. 43, 156, 161.)
Class If. Trematopa: Liver Flukes. (Fig. 251.)
Class III. Cestropa: Tapeworms. (Figs. 252, 253.)
Class IV. Nemertinea. (Figs. 157, 159.)
Phylum 6. NEMATHELMINTHES: Roundworms. (3000 species.)
Class I. Nematopa: Ascaris, Trichinella, Hookworm. (Figs. 44,
254, 255.)
Class II]. Nematromorpua: Gordius.
Class III. Acanruocepuata: Echinorhynchus.
Phylum 7. ANNELIDA: Segmented Worms. (7000 species.)
Class J. ARCHIANNELIDA: Polygordius.
Class II. PotycHarta: Sandworms, Tubeworms. (Figs. 45, 166.)
Class III. Or1cocuarta: Earthworms, Naids. (Figs. 60, 160, 173.)
Class IV. GepuyRea: Sipunculus.
Class V. Hirupinea: Leeches. (Fig. 45.)
Phylum 8. ROTIFERA: Rotifers. (1800 species.)
Phylum 9. BRYOZOA: Bryozoans. (3000 species.)
Phylum 10. BRACHIOPODA: Brachiopods. (130 species.)
xb
A78 APPENDIX
Phylum 11. ECHINODERMATA. (5000 species.)
Class I. AstEromwea: Starfish. (Fig. 46.)
Class II. OpHiuroripEa: Brittle Stars.
Class III. EcurnoinEa: Sea Urchins. (Figs. 47, 172.)
Class IV. HoLotHurorpEaA: Sea Cucumbers. (Fig. 47.)
Class V. Crrinoipka: Feather Stars, Sea Lilies. (Fig. 47.)
Phylum 12. MOLLUSCA. (80,000 species.)
Class I. AmMpuIngeurRa: Chiton. (Fig. 48.)
Class II. ScapHopopa: Dentalium. (Fig. 177.)
Class III. Gastropopa: Snails, Slugs. (Fig. 48.)
Class IV. PELEcypopa: Oysters, Clams, Scallops, Shipworm. (Fig.
49.)
Class V. CEpHALOpoDA: Squid, Octopus, Nautilus. (Fig. 50.)
Phylum 13. ARTHROPODA. (700,000 species.)
Class I. Crustacka.
Subclass 1. Entomostraca. Daphnia, Cyclops, Barnacles. (Fig. 51.)
Subclass 2. Malacostraca. Crayfish, Lobsters, Crabs, Pill-bugs..
(Figs. 51, 63-65.) |
Class II. OnycHopuora: Peripatus. (Fig. 56.)
Class III. Mynrrapopa: Centipedes, Millipedes. (Fig. 52.3
Class IV. Insecta. (Fig. 54B.)
Commonly accepted orders are:
1. Thysanura: Silverfish, etc. 13. Anoplura: Sucking Lice.
2. Collembola: Springtails. 14. Hemiptera: Bugs.
3. Orthoptera: Grasshoppers, 15. Homoptera: Plant-lice, etc.
Crickets, Roaches, etc. 16. Dermaptera: Earwigs.
4. Isoptera: Termites. 17. Coleoptera: Beetles.
5. Neuroptera: Ant-lions, Lace- 18. Strepsiptera: Stylopids.
wings, etc. 19. Mecoptera: Scorpion-flies, etc.
6. E/phemeroptera: Mayflies. 20. Trichoptera: Caddice-flies.
7. Odonata: Dragonflies. 21. Lepidoptera: Moths and But-
8. Plecoptera: Stoneflies. terflies.
9. Psocoptera: Book-lice, etc. 22. Diptera: Flies, Mosquitoes.
10. Mallophaga: Bird-lice. 23. Siphonaptera: Fleas.
11. Embioptera: Embiids. 24. Hymenoptera: Bees, Wasps,
12. Thysanoptera: Thrips. Ants, Ichneumons, etc.
Class. V. ARACHNOIDEA: Scorpions, Spiders, Mites. (Fig. 55.)
Phylum 14. CHORDATA. (70,000 species.)
Subphylum A. ENTEROPNEUSTA: Dolichoglossus.
Subphylum B. TUNICATA: Tunicates, Sea-squirts. Styela.
Subphylum C. CEPHALOCHORDA: Lancelets. Amphioxus (Branchi-
ostoma). (Figs. 67, 278.)
A BRIEF CLASSIFICATION OF ANIMALS 479
Subphylum D. VERTEBRATA.
Class I. Cyciostomata: Hagfish, Lamprey. (Figs. 68, 168.)
Class IJ. ELasmMoprancuii: Sharks and Rays. Dogfish. (Figs. 69,
70, 120.)
Class III. Pisces. (30,000 species.)
Subclass 1. Teleostomi. Mackerel, Trout, Cod, Perch, Goldfish,
Guppy. (Figs. 71-74, 106.)
Subclass 2. Dipnot. Lungfishes. (Fig. 75.)
Class IV. Ampurpra. (2000 species.)
Order 1. Apoda: Coecilians.
Order 2. Caudata: Necturus, Salamander, Cryptobranchus, Am-
blystoma. (Figs. 76, 77.)
Order 3. Salientia: Frogs, Toads. (Figs. 78, 98, 103, 107, 174, 175.)
Class V. Repritia. (5000 species.)
Order 1. Testudinata: Turtles, Tortoises. (Fig. 80.)
Order 2. Rhynchocephalia: Sphenodon.
Order 3. Crocodilia: Crocodiles, Alligators.
Order 4. Squamata: Chameleons, Lizards, Snakes. (Figs. 81-83,
230, 231.)
Class VI. Aves: Birds. (15,000 species.)
Subclass 1. Archaeornithes: Archaeopteryx (extinct). (Fig. 233.)
Subclass 2. Neornithes.
Division A. Ratitae: Apteryx, Ostrich. (Fig. 84.)
Division B. Carinatae: All familiar Birds. (Figs. 85, 86, 239.)
Class VII. Mammatra. (10,000 species.)
Subclass 1. Prototheria: Monotremes. Duck-bill, Echidna. (Fig. 87.)
Subclass 2. Metatheria: Marsupials. Opossums, Kangaroos. (Fig.
88.)
Subclass 3. Eutheria: Placentals. All familiar Mammals. (Fig. 202.)
Order 1. Insectivora: Moles, Shrews, Hedgehogs, Gymnura.
(Figs. 201, 205, 207.)
Order 2. Edentata: Sloths, Anteaters, and Armadillos. (Figs. 89,
204.)
Order 3. Chiroptera: Bats. (Figs. 207, 227.)
Order 4. Rodentia: Rats, Mice, Rabbits, Squirrels, Beavers,
Porcupines, Guinea-pig. (Figs. 108, 187.)
Order 5. Carnivora: Cats, Dogs, Bears, Seals, Walruses. (Fig.
104.)
Order 6. Cetacea: Whales, Porpoises, Dolphins. (Figs. 90, 206.)
Order 7. Ungulata: Horses, Tapirs, Rhinoceroses, Camels, Oxen,
Antelopes, Giraffes, Pigs, Hippopotami, Elephants. (Figs. 90,
92, 234, 238.)
Order 8. Sirenia: Manatee. (Fig. 91.)
A480 APPENDIX
Order 9. Primates: (Fig. 272.)
Suborder 1. Lemuroidea: Lemurs. (Fig. 93.)
Suborder 2. Tarsioidea: Tarsiers. (Fig. 272.)
Suborder 3. Anthropoidea: Monkeys, Apes, Man. (Fig. 272.)
Series 1. Platyrrhini: New World Species.
Family 1. Hapalidae: Marmosets.
Family 2. Cebidae: Capuchins, Howler Monkeys, Spider
Monkeys, etc. (Fig. 93.)
Series 2. Catarrhini: Old World Species.
Family 3. Cercopithecidae: Tailed Monkeys. Macaques,
Baboons, etc. (Fig. 93.)
Family 4. Simiidae: Man-like or Anthropoid Apes. Gib-
bons, Orang-utans, Chimpanzees, Gorillas. (Figs. 93,
228, 273, 214.)
Family 5. Hominidae: Man.
Phylum Ctenophora
Phylum Coelenterata All other Phyta
ACOELOMATA COELOMATA
(Animals with (Animals with
enteric cavity ) enteric cavity
and coelom)
Phylum Porifera ENTEROZOA
PARAZOA
(Sponges)
Metazoa
(Multicellular animals)
Phylum Protozoa
(Unicellular animals)
Fic. 313. — Diagram of the relationships of the animal phyla.
i
@=oa
cS
ISK
ey
witLIBRARY }
z
Il. BIBLIOGRAPHY
Some easily available works in English which are suitable for reference
and collateral reading.
BaliTsELL, G. A. Manual of Animal Biology. Macmillan, 1932. Detailed
descriptions of the structure and life processes of a number of repre-
sentative animals, together with directions for their study in the lab-
oratory. Written especially to accompany WoopruFF’s Animal Biology.
CHAPTER I.— THE SCOPE OF BIOLOGY
Biological Abstracts of the World’s Literature in Biology. Issued monthly.
Vol. 1, 1926. Philadelphia.
CatTEi, J. McK. (editor). American Men of Science. A Biographical
Dictionary. 6th edition. Science Press, 1938.
DampierR-WHETHAM, W. C.D. A History of Science. Cambridge Univer-
sity Press, 1930.
Grecory, R. A., Discovery, or the Spirit and Service of Science. Macmillan,
1916.
Hawupane, J. S. The Philosophical Basis of Biology. Doubleday, Doran,
1931.
HeEnpeErsON, I. F., and HenpErson, W. D. A Dictionary of Scientific
Terms: Pronunciation, Derivation, and Definition of Terms in Biology,
Botany, Zodlogy, Anatomy, Cytology, Embryology, Physiology. 2d edition.
Van Nostrand, 1929.
Huxtey, T. H. “Educational Value of the Natural History Sciences.”
1854. Collected Essays, Vol. Science and Education. Appleton.
Huxtey, T. H. “On Our Knowledge of the Causes of the Phenomena of
Organic Nature.’ 1863. Collected Essays, Vol. Darwiniana.
Huxtey, T. H. “On the Study of Biology.” 1876. Collected Essays, Vol.
Science and Education.
JaEGER, E. C. A Dictionary of Greek and Latin Combining Forms Used in
Zoological Names. Thomas, 1930.
ME .anper, A. L. Source Book of Biological Terms. College of the City
of New York, 1937.
Tuomson, J. A. An Introduction to Science. Holt, 1911.
We ts, H. G., Huxtey, J. S., and Wetts, G. P. The Science of Life.
Doubleday, Doran, 1931.
Westaway, F. W. Scientific Method. 3d edition. Blackie & Son, 1923.
WuiteHeaD, A. N. Science and the Modern World. Macmillan, 1925.
481
482 APPENDIX
CHAPTER IJ.— CELLULAR ORGANIZATION OF LIFE
Cownry, E. V. (editor). General Cytology. University of Chicago Press,
1924.
Cowpry, E. V. (editor). Special Cytology. 2d edition. Hoeber, 1932.
Gray, J. Text Book of Experimental Cytology. Macmillan, 1931.
SHarp, L. W. Introduction to Cytology. 3d edition. McGraw-Hill, 1934.
Tuompson, D’Arcy W. On Growth and Form. Cambridge University
Press, 1917.
Witson, E. B. The Cell in Development and Heredity. 3d edition. Mac-
millan, 1925.
CHAPTER III.— THE PHYSICAL BASIS OF LIFE
Bautpwin, E. An Introduction to Comparative Biochemistry. Macmillan,
19357.
Barnes, T. C. Textbook of General Physiology. Blakiston, 1937.
Bayuiss, W. M. Principles of General Physiology. 4th edition. Longmans,
Green, 1924.
Bayuiss, W. M. The Nature of Enzyme Action. Longmans, Green, 1925.
HeEILBrunn, L. V. Colloidal Chemistry of Protoplasm. Borntraeger, 1928.
HerILprunn, L. V. General Physiology. Saunders, 1937.
Huxtey, T. H. “On the Physical Basis of Life.”” 1868. Collected Essays,
Vol. Method and Results. Appleton.
Logs, JacquEs. The Dynamics of Living Matter. Columbia University
Press, 1906.
Parsons, T. R. The Materials of Life. Norton, 1930.
Puiu, J. C. Physical Chemistry: Its Bearing on Biology and Medicine.
Longmans, Green, 1925.
Serrriz, W. Protoplasm. McGraw-Hill, 1936.
SHERMAN, H. C. Chemistry of Food and Nutrition. 5th edition. Macmillan,
1937.
SHERMAN, H. C., and Smitu, 8. L. The Vitamins. American Chemical
Society, 1931.
Witson, E. B. The Physical Basis of Life. Yale University Press, 1923.
CHAPTER IV.— METABOLISM OF ORGANISMS
GREAVES, J. E., and Greaves, E. O. Elementary Bacteriology. 3d edition.
Saunders, 1936.
Hotmes, E. The Metabolism of Living Tissues. Cambridge University
Press, 1937.
Huxtey, T. H. “On the Border Territory between the Animal and
Vegetable Kingdoms.” 1876. Collected Essays, Vol. Discourses Biological
and Geological. Appleton.
JORDAN, E. O. Bacteriology. 11th edition. Saunders, 1937.
BIBLIOGRAPHY 483
Lutman, B. F. Microbiology. McGraw-Hill, 1930.
MarsHa.u, C. E. Microbiology. 3d edition. Blakiston’s, 1926.
PLunkeEtt, C. R. Outlines of Modern Biology. Holt, 1930.
Waksman, S. A., Principles of Soil Microbiology. Williams & Wilkins, 1927.
Waxsman, S. A., and Starkey, R. L. The Soil and the Microbe. Wiley,
1931.
Wooprurr, L. L. “The Origin and Sequence of the Protozoan Fauna of
Hay Infusions.” Journal of Experimental Zoélogy, Vol. 12, 1912.
Wooprurr, L. L. Foundations of Biology. 5th edition. Macmillan, 1936.
CHAPTER V.— UNICELLULAR ANIMALS
Caxins, G. N. Biology of the Protozoa. 2d edition. Lea and Febiger, 1933.
Heener, R. W., and TauiaFerro, W. H. Human Protozodlogy. Macmil-
lan, 1924.
Jennincs, H. S. Genetics of the Protozoa. Nijhoff, 1929.
Kupo, R. R. Handbook of Protozoélogy. Thomas, 1931.
Mincutn, E. A. Introduction to the Study of the Protozoa. Arnold, 1912.
SurpLey, A. E. Hunting under the Microscope. Macmillan, 1928.
Wenricn, D. H. ‘Eight Well-defined Species of Paramecium.” Trans.
Amer. Microscopical Society, Vol. 47, 1928.
Wenyon, C. M. Protozodlogy. William Wood, 1926.
Wooprurr, L. L. ‘‘The Protozoa and the Problem of Adaptation,”’ in
Organic Adaptation to Environment. Yale University Press, 1924.
CHAPTER VI.— THE MULTICELLULAR ANIMAL
Bremer, J. L. Histology. 5th edition. Blakiston, 1936.
DauLGREN, Uric, and Kepner, W. A. Principles of Animal Histology.
Macmillan, 1908.
GacE, S. H. The Microscope. 16th edition. Comstock, 1936.
Guyer, M. F. Animal Micrology. Practical Exercises in Zoélogical Micro-
technique. 4th edition. University of Chicago Press, 1936.
Ke.uicotr, W. E. General Embryology. Holt, 1913.
Kincssury, B. F., and JoHANNSEN, O. A. Histological Technique. Wiley,
1936.
McCuung, C. E. (editor). Microscopical Technique. 2d edition. Hoeber,
1937.
CHAPTERS VII-XVI.— INVERTEBRATES AND
VERTEBRATES
Apams, L. A. An Introduction to the Vertebrates. Wiley, 1936.
BaitsEtL, G. A. Human Biology. Edwards, 1938.
Bepparp, F. E. Earthworms and Their Allies. Cambridge University
Press, 1901.
484 APPENDIX
BorrapDAILe, L. A., and Potts, F. A. The Invertebrata. 2d edition.
Macmillan, 1935.
Carson, A. J., and Jounson, V. The Machinery of the Body. University
of Chicago Press, 1937.
CRANDALL, L. A. An Introduction to Human Physiology. Saunders, 1934.
Drew, G. A. Invertebrate Zodlogy. 5th edition. Saunders, 1936.
Fotsom, J. W., and Warp.ig, R. A. Entomology. 4th edition. Blakiston,
1934.
Harmer, S. F., and Surptey, A. E. (editors). The Cambridge Natural
History. Ten volumes. Macmillan, 1895.
Heoner, R. W. College Zodlogy. Ath edition. Macmillan, 1936.
Heaner, R. W. Invertebrate Zodlogy. Macmillan, 1933.
Heoner, R. W. Parade of the Animal Kingdom. Macmillan, 1935.
Herrick, C. J. Introduction to Neurology. 5th edition. Saunders, 1931.
Homes, S. J. Biology of the Frog. 4th edition. Macmillan, 1927.
Hoskins, R. G. The Tides of Life. Norton, 1933.
Howe tt, W. H. Textbook of Physiology. 13th edition. Saunders, 1936.
Hux ey, T. H. The Crayfish. Appleton, 1880.
Hyman, L. H. A Laboratory Manual for Comparative Vertebrate Anatomy.
University of Chicago Press, 1922.
Hyman, L. H. “‘Hydras of North America.” Trans. Amer. Micr. Society,
Vol. 50, 1931.
JoHnson, M. E., and Snook, H. J. Seashore Animals of the Pacific Coast.
Macmillan, 1926.
Ket, AntHur. The Engines of the Human Body. 2d edition. Lippincott,
1926.
Kincs ey, J. S. Comparative Anatomy of Vertebrates. 3d edition. Blak-
iston, 1927.
Kinostey, J. S. The Vertebrate Skeleton. Blakiston, 1925.
LaANKEsTER, E. R. (editor). Treatise on Zodlogy. Eight volumes. Mac-
millan.
Martin, H. N. The Human Body. 12th edition. Holt, 1934.
MitcHett, P. H. Text Book of General Physiology. 2d edition. McGraw-
Hill, 1932.
Morean, A. H. Field Book of Ponds and Streams. Putnam, 1930.
Neat, H. V., and Ranp, H. W. Comparative Anatomy. Blakiston,
1936.
NeEeEDHAM, J. G. Guide to the Study of Fresh-water Biology. 2d edition.
Thomas, 1930.
NeEEpDHAM, J. G. Culture Methods for Invertebrate Animals. Comstock,
1937.
Newman, H. H. Vertebrate Zodlogy. 2d edition. Macmillan, 1929.
Nose, C. K. The Biology of the Amphibia. McGraw-Hill, 1931.
BIBLIOGRAPHY 485
Parker, T. J., and Haswetit, W. A. Textbook of Zodlogy. 4th edition.
Macmillan, 1927.
Pratt, H. 8S. A Manual of the Common Invertebrate Animals, Exclusive
of Insects. 2d edition. Blakiston, 1935.
Pratt, H. S. A Manual of Land and Fresh Water Vertebrate Animals of
the United States, Exclusive of Birds. 2d edition. Blakiston, 1935.
Rocers, C. G. Comparative Physiology. McGraw-Hill, 1927.
Water, H. E. Biology of the Vertebrates. A Comparative Study of Man
and His Animal Allies. Macmillan, 1928.
Wa ter, H. E. The Human Skeleton. Macmillan, 1918.
Warp, H. B., and Wurppete, G. C. Fresh-water Biology. Wiley, 1918.
Wiuiams, J. F., Atlas of Human Anatomy. Barnes & Noble, 1935.
CHAPTER XVII.— ORIGIN OF LIFE
Huxtey, T. H. “Biogenesis and Abiogenesis.’’ 1870. Collected Essays,
Vol. Discourses Biological and Geological. Appleton.
Moore, B. The Origin and Nature of Life. Holt, 1912.
Newman, H. H. (editor). The Nature of the World and Man. University
of Chicago Press, 1926.
Osporn, H. F. The Origin and Evolution of Life upon the Earth. Scribner,
1917.
Scorer, E. A. “Life: Its Nature, Origin and Maintenance.” Science,
Vol. 36, 1912.
Trouanp, L. T. “The Chemical Origin and Regulation of Life.”” The
Monist, Vol. 24, 1914.
Verworn, M. General Physiology. 2d edition. Macmillan, 1899.
Wooprurer, L. L. “The Origin of Life,” in Hvolution of Earth and Man.
Yale University Press, 1930.
CHAPTERS XVIII-XX.— CONTINUITY OF LIFE, FERTILIZA—
TION, AND DEVELOPMENT
Cuiip, C. M. Senescence and Rejuvenescence. University of Chicago Press,
1915.
Davenport, C. B. How We Came by Our Bodies. Holt, 1936.
Donps, G. S. Essentials of Human Embryology. Wiley, 1936.
Driescu, Hans. Science and Philosophy of the Organism. Gifford Lectures,
1907-08. Black.
GEpbpES, P., and THomson, J. A. Sez. Holt, 1914.
Hecner, R. W. The Germ-cell Cycle in Animals. Macmillan, 1914.
Jennincs, H. S. Life and Death, Heredity and Evolution in Unicellular
Organisms. Gorham, 1920.
Ke.uicott, W. E. Chordate Development. Holt, 1913.
486 APPENDIX
Liu, F. R. Problems of Fertilization. University of Chicago Press,
1919.
Morean, T. H. Experimental Embryology. Columbia University Press,
1927.
NEEDHAM, JOSEPH. Chemical Embryology. Cambridge University Press,
1931.
Ricuarps, A. Outline of Comparative Embryology. Wiley, 1937.
Suumway, W. Vertebrate Embryology. 3d edition. Wiley, 1935.
WEIsMANN, Aucust. The Germ Plasm. Scribner, 1892.
Wieman, H. L. An Introduction to Vertebrate Embryology. McGraw-Hill,
1930.
Witson, E. B. The Cell in Development and Heredity. 3d edition. Mac-
millan, 1925.
Wooprurr, L. L. ‘‘The Physiological Significance of Conjugation and
Endomixis in the Infusoria.”” American Naturalist, Vol. 69, 1925.
CHAPTER XXI.— INHERITANCE
ALLEN, EpcGar (editor). Ser and Internal Secretions. Williams & Wilkins,
1932.
Baur, E., Fiscner, E., and Lenz, F. Human Heredity. Macmillan,
1931.
Cast Le, W. E. Genetics and Eugenics. 4th edition. Harvard University
Press, 1930.
Conkuin, E. G. Heredity and Environment in the Development of ——
7th edition. Princeton University Press, 1927.
GALTON, Francis. Natural Inheritance. 1889.
Gates, R. R. Heredity in Man. Constable, 1929.
GotpscHmipT, R. Physiological Genetics. McGraw-Hill, 1938.
Guyer, M. F. Being Well-born. 2d edition. Bobbs-Merrill, 1927.
JENNINGS, H. S. Genetics. Norton, 1935.
JENNINGS, H. 8. Genetics of the Protozoa. Nijhoff, 1929.
JENNINGS, H. S. The Biological Basis of Human Nature. Norton,
1930.
Moraan, T. H. The Theory of the Gene. 2d edition. Yale University
Press, 1929.
RussgE.i, E. 8. The Interpretation of Development and Heredity. Oxford
University Press, 1930.
Sinnott, E. W., and Dunn, L. C. Principles of Genetics. 2d edition.
McGraw-Hill, 1932.
Snyper, L. H. The Principles of Heredity. Heath, 1935.
StockarD, C. R. The Physical Basis of Personality. Norton, 1931.
Watrer, H. E. Genetics. 4th edition. Macmillan, 1938.
Winters, L. M. Animal Breeding. 2d edition. Wiley, 1930.
BIBLIOGRAPHY 487
CHAPTER XXIT.— ORGANIC ADAPTATION
AuLEE, W. C. Animal Aggregations. University of Chicago Press, 1931.
ALVERDES, F. Social Life in the Animal World. Harcourt, Brace, 1927.
CuapMan, R. N. Animal Ecology with Especial Reference to Insects.
McGraw-Hill, 1931.
Cuitp, C. M. Physiological Foundations of Behavior. Holt, 1924.
Crite, G. W. Man — An Adaptive Mechanism. Macmillan, 1916.
Darwin, CHar.es. The Fertilization of Orchids. The Various Contriv-
ances by Which Orchids Are Fertilized by Insects. London, 1862.
Exton, C. Animal Ecology. Macmillan, 1927.
Haupang, J. S. Organism and Environment. Yale University Press,
1917.
Henverson, L. J. The Fitness of the Environment. Macmillan, 1913.
Hineston, R. W. G. Instinct and Intelligence. Macmillan, 1929.
Homes, S. J. The Evolution of Animal Intelligence. Holt, 1911.
Jennines, H. S. Behavior of the Lower Organisms. Columbia University
Press, 1906.
Luoyp, R. E. What Is Adaptation? Longmans, Green, 1914.
Loges, Jacques. Forced Movements, Tropisms, and Animal Conduet.
Lippincott, 1918.
Pearse, A. S. Environment and Life. Thomas, 1930.
Punnett, R. C. Mimicry in Butterflies. Cambridge University Press,
1915.
RoosEvELT, THEODORE. “‘Revealing and Concealing Coloration in the
Birds and Mammals.” Bull. Amer. Museum Nat. Hist., Vol. 30, 1911.
SHELFORD, V. E. Laboratory and Field Ecology. Williams & Wilkins,
1929.
Snoperass, R. E. The Anatomy and Physiology of the Honey Bee. McGraw-
Hill, 1925.
Symposium on Adaptation. Papers by M. M. Metcalf, B. E. Livingston,
G. H. Parker, A. P. Mathews, and L. J. Henderson. American Natural-
ist, Vol. 47, 1913.
TaLIAFERRO, W. H. The Immunology of Parasitic Infections. Century,
1930.
Tuayer, G. H. Concealing Coloration in the Animal Kingdom. Macmillan,
1909.
Tuompson, J. A. The System of Animate Nature. Holt, 1920.
Tuompson, J. A. Biology for Everyman. Dutton, 1935.
WasuBurn, M. F. The Animal Mind. A Textbook of Comparative
Psychology. 4th edition. Macmillan, 1937.
Wooprurr, L. L. ‘The Protozoa and the Problem of Adaptation,” in
Organic Adaptation to Environment. Yale University Press, 1924.
488 APPENDIX
WoopworntH, R. 8. Psychology. 3d edition. Holt, 1934.
ZinsseER, Hans. Resistance to Infectious Diseases. 4th edition. Macmillan,
1931.
CHAPTER XXIII.— DESCENT WITH CHANGE
BaiTsELL, G. A. (editor). Evolution of Earth and Man. Yale University
Press, 1929.
Bercson, Henri. Creative Evolution. English translation, 1911.
Berry, E. W. Paleontology. McGraw-Hill, 1929.
ConkuIn, E. G. Direction of Human Evolution. Scribner, 1921.
Creation by Evolution. A consensus of present-day knowledge as set forth
by twenty-five scientists. Macmillan, 1928.
Darwin, CHARLES. Voyage of the Beagle. (A Naturalist’s Voyage.)
London, 1839.
Darwin, CHARLES. The Origin of Species. London, 1859. 6th edition,
1880.
Darwin, CuHar_es. The Descent of Man. London, 1871.
Darwin, CuHares. Variation in Animals and Plants under Domestica-
tion. London, 1868.
DoszHansky, T. Genetics and the Origin of Species. Columbia University
Press, 1937.
Fisuer, R. A. The Genetical Theory of Natural Selection. Oxford Univer-
sity Press, 1930.
JENNINGS, H. 8. “Diverse Doctrines of Evolution, and Their Relation
to the Practice of Science and of Life.’’ Science, Vol. 65, 1927.
Linpsey, A. W. Problems of Evolution. Macmillan, 1931.
Luuti, R. 8. Organic Evolution. 2d edition. Macmillan, 1929.
Moraan, T. H. The Scientific Basis of Evolution. 2d edition. Norton,
1935.
Nutraui, G. H. F. Blood Immunity and Blood Relationships. Cambridge
University Press, 1904.
Osporn, H. F. The Origin and Evolution of Life. Scribner, 1917.
Parker, G. H. What Evolution Is. Harvard University Press, 1925.
ReEIcHERT, E. T., and Brown, A. P. The Differentiation and Specificity of
Corresponding Proteins and Other Vital Substances in Relation to Bi-
ological Classification and Organic Evolution. Carnegie Institution, 1909.
ScHENK, E. T., and McMasters, J. H. Procedure in Taxonomy. Stanford
University Press, 1936.
SCHUCHERT, CHARLES. “The Earth’s Changing Surface and Climate,”’ in
The Evolution of Earth and Man. Yale University Press, 1929.
Scott, W. D. The Theory of Evolution. Macmillan, 1911.
SHutz, A. F. Evolution. McGraw-Hill, 1936.
Txomson, J. A. Concerning Evolution. Yale University Press, 1925.
BIBLIOGRAPHY 489
DEVrIES, Huco. Species and Varieties. Their Origin by Mutation. 3d edi-
tion. Open Court, 1912.
Wattwaceg, A. R. Darwinism. 3d edition. Macmillan, 1905.
Watuace, A. R. The Geographical Distribution of Animals. London, 1876.
Warp, HensHaw. Evolution for John Doe. Bobbs-Merrill, 1925.
We tts, H. G. “The Evidence Furnished by Biochemistry and Immunol-
ogy on Biologic Evolution.”’ Archives of Pathology, Vol. 9, 1930.
CHAPTER XXIV.— BIOLOGY AND HUMAN WELFARE!
Bower, F. O. Plants and Man. Macmillan, 19235.
CHANDLER, A. C. Human Parasitology. 5th edition. Wiley, 1936.
Cowpry, E. V. (editor). Human Biology and Racial Welfare. Hoeber, 1936.
Darwin, Leonarp. What Is Eugenics? Galton, 1930.
Doane, R. W. Common Pests. Thomas, 1931.
FERNALD, H. T. Applied Entomology. 3d edition. McGraw-Hill, 1935.
Gopparb, H. H. The Kallikak Family. A Study in the Heredity of Feeble-
mindedness. Macmillan, 1912.
Hecner, R. W., and Taviarerro, W. H. Human Protozodlogy. Macmil-
lan, 1924.
HENDERSON, J. The Practical Value of Birds. Macmillan, 1927.
Homes, 8S. J. Human Genetics and Its Social Import. McGraw-Hill, 1936.
Howarp, L. O. The Insect Menace. Century, 1931.
Keen, W. W. Animal Experimentation and Medical Progress. Houghton
Mifflin, 1914.
Kirkpatrick, T. B., and Heutrner, A. F. Fundamentals of Health.
Ginn, 1931.
Kororp, C. A. “The Human Values of Biology.”” University of Texas
Bulletin, 1925.
pEKruir, Paut. Microbe Hunters. Harcourt, Brace, 1925.
pEKrRuiF, Paut. Hunger Fighters. Harcourt, Brace, 1928.
Marvin, F. S. (editor). Science and Civilization. Oxford University Press,
1923.
NEWSHOLME, A. Evolution of Preventive Medicine. Williams & Wilkins, 1927.
Pack, C., and Gitt, T. Forests and Mankind. Macmillan, 1930.
Parkins, A. E. (editor). Our National Resources and Their Conservation.
Wiley, 1937.
PopENoE, P. Practical Applications of Heredity. Williams & Wilkins, 1930.
Ritey, W. A., and Jowannsen, O. A. Medical Entomology. McGraw-
Hill, 1932.
SMITH, THEOBALD, Parasitism and Disease. Princeton University Press,
1934.
SwEET™AN, H. L. The Biological Control of Insects. Comstock, 1936.
1 Also see Bibliography for Chapter X XI.
490 APPENDIX
VeppeEr, E. B. Medicine: Its Contribution to Civilization. Williams &
Wilkins, 1929.
Waker, C. C. The Biology of Civilization. Macmillan, 1930.
Warp ie, R. A. Principles of Applied Zodlogy. Longmans, Green, 1929.
Wriept, C. Heredity in Live Stock. Macmillan, 1930.
CHAPTER XXV.— THE HUMAN BACKGROUND
ALVERDES, F. Social Life in the Animal World. Harcourt, Brace, 1927.
BalTsELL, G. A. (editor). The Evolution of Earth and Man. Yale Univer-
sity Press, 1929.
Coxe, F. C. The Long Road. Reynal & Hitchcock, 1933.
Darwin, C. The Expression of the Emotions in Man and Animals. London,
1872.
Grecory, W. K. Man’s Place among the Anthropoids. Oxford University
Press, 1934.
JenninGSs, H. 8. The Biological Basis of Human Behavior. Norton, 1930.
pDELAGuNA, G. A. Speech. Yale University Press, 1927.
Lut, R. 8. Ancient Man. Doubleday, Doran, 1929.
Luguet, G.-H. The Art and Religion of Fossil Man. Yale University
Press, 1930.
MacCurpy, G. G. Human Origins. Appleton, 1924.
MacCurpy, G. G. The Coming of Man. University Society, 1932.
MacCurpy, G. G. (editor). Early Man. Lippincott, 1937.
Ossporn, H. F. Men of the Old Stone Age. Scribner, 1915.
Ossorn, H. F. Man Rises to Parnassus. Princeton University Press, 1928.
Peake, H., and Fieurs, H. J. The Corridors of Time. Vols. 1-3. Yale
University Press, 1927.
Romer, A. S. Man and the Vertebrates. University of Chicago Press, 1933.
ScuucHEertT, C., and DunBar, C. O. Textbook of Geology. 3rd edition.
Wiley, 1933.
Sauitu, G. E. The Evolution of Man. Oxford University Press, 1924.
Sotias, W. J. Ancient Hunters. 3d edition. Macmillan, 1924.
Titney, F. The Brain from Ape to Man. Hoeber, 1929.
Wappen, C. J. The Evolution of Human Behavior. Macmillan, 1932.
Warpen, C. J. The Emergence of Human Culture. Macmillan, 1936.
Yerkes, R. M., and Yerkes, A. W. The Great Apes. Yale University
Press, 1929.
CHAPTER XXVI.— DEVELOPMENT OF BIOLOGY
Cray, R. S., and Court, T. H. The History of the Microscope. Griffin,
1932.
Coz, F.C. The History of Protozodlogy. University of London Press, 1926.
BIBLIOGRAPHY 491
Dana, E. S. and others. A Century of Science in America. Yale University
Press, 1918.
Darwin, Cuartes. Life and Letters, Including a Biographical Chapter.
Edited by Francis Darwin. Appleton, 1887.
Foster, MicuakEu. History of Physiology during the 16th, 17th and 18th
Centuries. Cambridge University Press, 1901.
GaRRISON, F. H. History of Medicine. 3d edition. Saunders, 1921.
Huxtey, T. H. “The Progress of Science, 1837-1887.” Collected Essays.
Vol. Methods and Results. Appleton.
Huxtuey, T. H. Life and Letters. Edited by Leonard Huxley. Appleton,
1901.
Jupp, J. W. The Coming of Evolution. The Story of a Great Revolution in
Science. Cambridge University Press, 1910.
Locy, W. A. Biology and Its Makers. 3d edition. Holt, 1915.
Locy, W. A. The Growth of Biology. Holt, 1925.
NationaL AcapEeMy. History of the National Academy of Sciences of the
United States of America, 1913.
NORDENSKIOLD, E. History of Biology. Knopf, 1928.
Ossorn, H. F. From the Greeks to Darwin. An Oulline of the Development
of the Evolution Idea. Revised edition. Scribner, 1929.
Sewarp, A. C. (editor). Darwin and Modern Science. Cambridge Univer-
sity Press, 1909.
SinceER, C. Short History of Medicine. Oxford University Press, 1928.
SincEr, C. The Story of Living Things. Harper, 1931.
Wooprurr, L. L. (editor). The Development of the Sciences. Yale Uni-
versity Press, 1923.
Youne, R. T. Biology in America. Gorham, 1922.
Ill. GLOSSARY
ABIOGENESIS. The abandoned idea that living matter may arise at the
present time from the non-living without the influence of the former.
See Biogenesis.
ApsorPTION. The passage of nutritive and other fluids into living cells.
AcoELOMATE. Not possessing a coelom, or body cavity; e.g., Hydra. See
Enterocoela.
AcgurRED CHARACTER. A modification of body structure or function
which arises during individual life.
ApaPTATION. The reciprocal fitness of organism and environment; a
structure or reaction fitted for a special environment; the process by
which an organism becomes fitted to its surroundings.
ADRENAL GLANDS. Ductless glands situated near the kidneys. Secretion
supplies the hormones adrenaline and cortin.
AgrosBeE. An organism requiring free oxygen. See Anaerobe.
AFFERENT Root. Dorsal, or posterior, root of certain cranial and all
spinal nerves through which sensory nerve impulses enter the brain and
spinal cord. See Efferent Root.
Axprno. An individual lacking normal pigment, e.g., a white rat. Al-
binism in the Rat and Man is a typical Mendelian recessive character.
AucaAg. A heterogeneous group of lower plants in which the body is uni-
cellular or consists of a thallus; e.g., Protococcus, Spirogyra, Seaweeds.
Auuantors. An embryonic membrane of higher Vertebrates, chiefly res-
piratory in function.
ALLELomMoRPHS. Alleles. Genes similarly situated on homologous chromo-
somes. Homologous genes. See Homologous Chromosomes.
ALTERNATION OF GENERATIONS. Typically the alternate succession of sex-
ual and asexual generations in the life history; e.g., Obelia.
ALTERNATIVE INHERITANCE. See Dominant Character.
Amino Acips. Components of proteins. Organic acids in which one hydro-
gen atom is replaced by the amino group (NHz2).
Amnion. A delicate membrane enclosing the developing embryos of
Reptiles, Birds, and Mammals.
Amogesorp. Usually applied to the flowing movements of a cell, as in
Amoeba and white blood corpuscles.
AmPuHIMIxis. The mingling of the germ plasm of two gametes in the zygote.
ANABOLISM. The constructive phase of metabolism. See Katabolism.
ANAEROBE. An organism not requiring free oxygen; e.g., certain Bacteria
and parasitic Worms. See Aerobe.
492
GLOSSARY 493
AnaLocy. Structural resemblance, usually superficial, due to similarity
of functions; e.g., wing of Butterfly and Bird. See Homology.
ANAPHASE. Period in mitosis during which the daughter chromosomes
move toward the respective centrosomes. See Telophase.
Anatomy. The structure of organisms, especially as revealed by dissec-
tion. See Morphology.
ANTENNAE. A pair of appendages of the Arthropod head, sensory in
function.
Anus. Terminal orifice of the alimentary canal.
Aorta. A great trunk artery carrying blood away from the heart. See
Dorsal Aorta.
Aortic ArcHEs. Arteries arising from the ventral aorta and supplying
the gills in aquatic Vertebrates. Undergo many modifications in the
ascending series of air-breathing Vertebrates.
Apuips. Small sucking Insects; e.g., the green Plant Lice of garden shrubs.
ApopyLes. Pores leading from the flagellated chambers to the gastral
cavity of Sponges.
Artery. A blood vessel carrying blood away from the heart.
ArtTHROPODA. Phylum of Invertebrates. Includes the Crustaceans, In-
sects, Spiders, etc.
AssocriaTIVE Memory. Representative cerebral activity of the higher
animals and Man, exclusive of reason which presumably is confined to
the latter.
Aster. Radiations surrounding the centrosome during cell division.
AUTONOMIC SysTEM. System of outlying ganglia and nerves which com-
municates with the central nervous system via the roots of the spinal and
cranial nerves. Regulates nearly all the involuntary functions of the
body. Sympathetic system.
Autotropuic. Power to synthesize food from inorganic substances. Green
plants (holophytic) secure the necessary energy from light, and certain
Bacteria by the oxidation of inorganic substances. See Holozoic.
Axon. A nerve fiber conducting impulses away from the nerve cell body.
Dendrites conduct toward the cell body. See Neuron.
Bite Duct. Tube which conveys the secretions (bile) of the liver to the
small intestine. Usually unites with the pancreatic duct to form a com-
mon duct which enters the intestine.
Binary Fission. The division of a cell, especially a unicellular organism,
into two daughter cells; e.g., in Paramecium.
Brvomrat NomMeEnNciature. The accepted scientific method of designating
organisms by two Latin or Latinized words, the first indicating the genus
and the other, the species; e.g., the Dog, Canis familiaris; Man, Homo
sapiens.
494 APPENDIX
BiocogEnosis. An association of diverse organisms forming a natural
ecological unit in which there is more or less interdependence.
BiocengEsi!s. The established doctrine that all life arises from preéxisting
living matter. See Abiogenesis.
BioGEnetTic Law. See Recapitulation Theory.
BroLtocy. Study of matter in the living state, and its manifestations.
BipaRENTAL. Involving two progenitors, male and female.
Brramous. Comprising two branching parts; e.g., abdominal appendages
(swimmerets) of the Crayfish.
BLasTocoEL. The cavity within the blastula. Segmentation cavity.
BuLasToporRE. The opening to the exterior from the enteric pouch of a
gastrula.
BuastosTyYLe. Central axis of an individual (gonangium) of a Hydroid
colony that buds medusae, e.g., in Obelia.
Buastuta. The stage following cleavage when the cells are arranged in a
single layer to form a hollow sphere.
BLENDING INHERITANCE. Apparent fusion of parental characters in the
offspring so that a more or less intermediate condition arises; e.g., skin
color of mulattoes.
Bioop Corpuscies. Detached cells present in the fluid plasma of the
blood. Two principal kinds, red and white.
Buccat Cavity. Mouth cavity.
CALCIFEROUS GLANDs. Glands opening into the esophagus of the Earth-
worm which secrete calcium carbonate, probably to neutralize acidity
of food.
Catorig£. The unit of heat energy, and therefore largely of fuel value.
Heat required to raise 1000 grams of water through 1° C. Large calorie.
CARBOHYDRATES. Compounds of carbon with hydrogen and oxygen, the
hydrogen and oxygen typically in the same proportion as in water (H.0).
CARDINAL VEINS. Pair of large veins returning blood from posterior part
of body, e.g., in Dogfish.
CasTRATION. Removal of the gonads, especially of the male.
Catatysis. The inducing or accelerating of a chemical reaction by a
substance (e.g., an enzyme) which itself remains unchanged.
Ceti. A structural and physiological unit mass of protoplasm, differ-
entiated into cytoplasm and nucleus.
CELLULOSE. A carbohydrate which characteristically forms the walls of
plant cells.
CENTRAL Capsute. Perforated partition that separates the endoplasm
from the ectoplasm in the Radiolaria.
CENTROSOME. A body, enclosing a minute granule, or centriole, situated
in the center of the aster and active during cell division.
GLOSSARY 495
CueirPeD. The first thoracic appendages of the Crayfish and its allies.
The ‘ pincer.’
CuemotropisM. A simple orienting response, either positive or negative,
to chemical stimuli; e.g., of Paramecium or sperm. Chemotaxis.
CHLORAGOGEN CELLS. Outer layer of intestine of Earthworm. Probably
excretory in function.
CHLOROPHYLL. The characteristic green coloring matter of plants, through
which photosynthesis takes place. Comprises two pigments.
Cutoropiasts. The special cytoplasmic bodies containing chlorophyll.
CHOLESTEROL. A complex monohydric alcohol of the lipid series. Closely
related to vitamin D, certain hormones, and cancer-producing sub-
stances. See Lipids.
CuorpaTe. An animal whose primary axial skeleton consists temporarily
or permanently of a notochord. All Vertebrates are Chordates, but the
lowest Chordates are not Vertebrates. See Appendix I.
Cuorion. External embryonic membrane of Mammals.
CuHromaTIN. A deeply staining substance characteristic of the nucleus,
forming chromosomes, etc. See Germ Plasm.
CHROMOSOME. One of the deeply staining bodies into which the chromatin
of the nucleus becomes visibly resolved during mitosis. A linkage
group of genes. See Germ Plasm.
Cit1a. Delicate protoplasmic projections from a cell, which lash in unison
and propel the cell in the water (e.g., Paramecium), or move particles
over the cell surface (e.g., cells lining various tubes in multicellular forms).
Cuass. In classification, a main subdivision of a phylum. See Order.
CLEAVAGE. Cell divisions which transform the egg into the blastula stage
during development.
Cioaca. A cavity at the posterior end of the Vertebrate body, into which
the intestine, urinary, and reproductive ducts open. Not present in
most Mammals.
CocuiEA. The portion of the ear, in communication with the sacculus,
which is the essential organ of hearing in the higher Vertebrates.
CoELom. The body cavity, enclosed by tissue of mesodermal origin.
CoELOMATE. Possessing a coelom, or body cavity; as in all the chief
groups of animals above the Coelenterates.
CoELomic EpirHetium. See Peritoneum.
Cornosarc. Tissue of the tubular branches of a Hydroid; e.g., Obelia.
CotiarR CEs. Cells with cytoplasmic flange, or collar, surrounding the
base of the flagellum. Represented by certain Protozoa, and in the
gastral epithelium of Sponges.
Cotto. A state of matter in which a substance is finely divided into
particles larger than one molecule and suspended in another substance,
semi-fluid or fluid.
496 APPENDIX
Cotony. An aggregation or intimate association of several or many
individuals to form a superior unit.
Compounp Eyr. One composed of numerous facets, or separate visual
elements. Supposed to afford mosaic vision; e.g., in Crayfish and
Locust.
ConsuGaTion. Union (usually temporary) of two cells, resulting in fer-
tilization; e.g., in Paramecium. See Endomixis.
CONSERVATION OF ENERGY. The ‘law’ that the total energy of the universe
is constant, none being created or destroyed but merely transformed
from one form to another.
CONTRACTILE Vacuo.Le. A reservoir in unicellular organisms (e.g., Para-
mecium) in which water and waste products of metabolism collect and
are periodically expelled to the exterior.
Cowper's GLAND. Small ovoid body associated with the prostate gland
and urethra in the male of Mammals.
CrantaL Nerves. Nerves which arise from the brain.
Cranium. The protective case enclosing the brain.
CREATININE. A nitrogenous waste product. Present in small quantity in
human urine.
Cretin. A defective individual, due to a deficiency of thyroid secretion.
Crossinc-ovEerR. The rearranging of linked characters as a result of the
exchange of homologous genes during synapsis of chromosomes.
Crura Ceresri. Thickenings of ventral surface of mid-brain.
Crustacea. A group of Arthropoda, including Crayfish, Crabs, ete.
Curaneous. Pertaining to the skin.
CuticLte. The outermost lifeless layer of the skin. See Epidermis.
Cyst. A resistant envelope formed about an organism (e.g., many Pro-
tozoa) during unfavorable conditions or reproduction.
Cyto.tocy. The science of cell structure and function.
Cytopiasm. Protoplasm of a cell exclusive of nucleus. Cytosome.
Darwinism. Charles Darwin’s theory of Natural Selection. Erroneously
used as synonymous with organic evolution.
Decay. Chemical decomposition involving putrefaction or fermentation.
See Putrefaction.
DENpRITE. See Axon.
DeNiITRIFYING BactEertA. Types of Bacteria which break down com-
pounds of nitrogen and set free the nitrogen to the atmosphere.
DerrMAL. Pertaining to the skin. The dermis is the inner layer of the
Vertebrate skin. See Epidermis.
DIFFERENTIATION. A transformation from relative homogeneity to heter-
ogeneity, involving the production of specific substances or parts from
a general substance or part. Specialization.
GLOSSARY 497
Dirrusion. Intermingling of two substances due to migration of their
molecules. Pressure exerted by molecules in diffusion through a semi-
permeable membrane is osmotic pressure. See Osmosis.
Dicestion. Chemical simplification of food so that it can be absorbed
and utilized.
Diaysrip. Progeny of parents differing in two given characters.
Drie.os.iastic. Composed of only two primary layers: ectoderm and endo-
derm, e.g., Hydra. See Triploblastic.
Drietow. Having two complete sets of homologous chromosomes. Max-
imum number of chromosomes in the life history of a given species. See
Haploid.
Division oF Lapor. Allocation of special functions to special parts which
codperate toward the unity of the whole.
Dominant CuaractTer. One of a pair of alternative characters, repre-
sented by homologous genes, which appears in the phenotype to the
exclusion of the other (recessive) character when both are present in
the genotype.
Dorsat Aorta. Chief artery distributing pure blood to the body. Ven-
tral aorta carries blood from heart to gill arteries in Fishes.
Ductiess Guanp. An organ whose function is to elaborate and secrete a
hormone directly into the blood. An endocrine gland.
Ecotocy. The study of the relations of the organism to environing condi-
tions, organic and inorganic.
Ecroperm. The primary tissue comprising the surface layer of cells in the
gastrula. See Germ Layer.
Ecropiasm. Modified surface layer of cytoplasm of a cell. See En-
doplasm.
EFFERENT Root. Ventral, or anterior, root of certain cranial and all spinal
nerves through which motor nerve impulses leave the brain and spinal
cord. See Afferent Root.
Emsryo.tocy. Study of the early developmental stages, or embryos, of
individual organisms.
EncystTMENT. The formation of a resistant covering, or cyst wall, about
an organism; e.g., Euglena.
ENpocrRINE GLAND. See Ductless Gland.
EnpopERM. The primary tissue comprising the inner layer of cells
in the gastrula, and in subsequent stages forming the lining of the
essential parts of the digestive tract and its derivatives. See Germ
Layer.
Enpomrxis. A nuclear reorganization process in Protozoa, e.g., Parame-
cium, which does not involve the codperation of two cells (as in conjuga-
tion) or synkaryon formation.
498 APPENDIX
EnpopiasM. The inner cytoplasm surrounding the nucleus; eg., in
Amoeba, Paramecium. See Ectoplasm.
EnpopopiTEe. The inner of the two distal parts of the typical biramous
Crustacean appendage. See Protopodite and Exopodite.
ENDOSKELETON. An internal living skeleton affording support and protec-
tion, as well as levers for the attachment of muscles. Characteristic of
Vertebrates.
Enercy. See Potential Energy.
Enteric Cavity. The digestive cavity of the gastrula stage, and of simple
Metazoa, e.g., Hydra.
ENTERON. Enteric pouch forming the wall of the enteric cavity.
EnTEROzoA. Animals with an enteron, and with or without a coelom.
All animals above the Sponges. See Parazoa.
Enzymes. Special chemical substances (apparently proteins) of organ-
isms, which bring about by catalytic action many of the chemical
processes of the body; e.g., digestion. See Catalysis.
Epwermis. The outer cellular layer of the skin.
EpicENnEsis. Development from absolute or relative simplicity to com-
plexity. See Preformation.
EpirHevium. A layer of cells covering an external or internal surface,
including the essential secreting cells of glands.
EquaToriIAL PLate. The equator of the spindle with its group of chromo-
somes during the metaphase of mitosis.
Esopuacus. Tubular passage from pharynx to stomach.
Kucenics. The system of improving the human race by breeding the best.
“The science of being well born.’ See Euthenics.
EustTacH1aAN Tue. Passage connecting the Vertebrate middle ear with
the pharynx. Remnant of the most anterior gill slit, represented in
present-day Sharks by the ‘blow-hole,’ or spiracle.
EutuHenics. The system of improving the human race by good environ-
ment. See Eugenics.
Eutuerta. The highest of the three subclasses of Mammals, including all
the familiar forms. Placentals. See Appendix I.
EvoLuTion, OrGcanic. Present-day organisms are the result of descent
with change from those of the past.
Excretion. The elimination of waste products of metabolism. A waste
product. See Secretion.
Exopopite. The outer of the two distal parts of the typical biramous
Crustacean appendage. See Protopodite and Endopodite.
EXOSKELETON. A non-living external skeleton chiefly for protection. The
characteristic skeleton of Invertebrates; e.g., Crayfish.
EXTERNAL Receptors. Sense organs upon the surface of the body. See
Internal Receptors.
GLOSSARY 499
Famity. In classification, a main subdivision of an order. See
Genus.
Fats. One of the chief groups of foodstuffs. Compound (esters) of glyc-
erol with a fatty acid; e.g., mutton tallow is chiefly the fat stearin
(Cs7H1190¢) = glycerin plus stearic acid. More oxidizable than carbo-
hydrates. See Lipids.
FERMENTATION. The transformation of carbohydrates by the activity of
ferments, or enzymes, derived from living organisms. See Putrefaction.
FertILizaTion. The union of male and female gametes, especially their
nuclei, by which the chromatin complex of each is arranged to form the
composite nucleus (synkaryon) of the zygote.
Fetus. An embryo of a Vertebrate, in egg or uterus. ;
FiaceEtitum. A whip-like prolongation of the cytoplasm, the movements
of which usually effect the locomotion of the cell; e.g., Euglena.
Fiuctuations. Relatively slight variations, usually forming a finely
graded series, always found in organisms; may be either modifications or
recombinations, but usually the former.
Funct. Colorless plants; e.g., Bacteria, Yeast, Mushrooms.
Gat BiappeEr. Receptacle near the liver for the temporary storage of
bile.
GameETE. A cell which unites with another at fertilization to form a zygote.
Egg or sperm.
Gametic Nucier. Nuclei of gametes that unite to form the synkaryon,
or nucleus of the zygote.
GANGLION. A group of nerve cells, chiefly the cell bodies, with supporting
cells.
Gastric VacuoLe. A droplet of fluid enclosing ingested food, in which
digestion occurs; e.g., in Amoeba and Paramecium. Food vacuole.
Gastrouitus. Calcareous bodies found at certain times in the lateral
walls of the stomach of the Crayfish. Probably represent the storage of
material for the exoskeleton.
GastRuLA. A stage in animal development in which the embryo consists
of a two-layered sac, ectoderm and endoderm, enclosing the enteric
cavity which opens to the exterior by the blastopore.
GeMMULE. An asexual reproductive body liberated by certain Sponges.
GENE. Independently inheritable factor or element in the chromosomes
which influences the development of one or more characters in the
organism. Presumably a protein molecule.
Genetics. The science of heredity.
GenotyPeE. The fundamental hereditary constitution of an organism or
group of organisms. The gene complex of an organism. See Phenotype.
Genus. In classification, a main subdivision of a family. See Species.
500 ; APPENDIX
GERMINAL Continuity. The concept of an unbroken stream of germ
plasm from the beginning of life, from which each generation is derived.
GERM Layer. A primary tissue (ectoderm, endoderm, or mesoderm) in
the embryo, from which the tissues and organs of the adult animal de-
velop.
Germ Layer THEeory. The doctrine that the germ layers are fundamen-
tally similar throughout the Metazoa and that homologous structures
in various animals are derived during development from the same germ
layer.
GERM Puiasm. The physical basis of inheritance. The chromatin (genes)
which forms the specific bond of continuity between parent and offspring.
Contrasted with soma or somatoplasm. Germ.
Gitu Suits. Paired lateral openings leading from the anterior end of the
alimentary canal to the exterior for the exit of the respiratory current
of water. Permanent or embryonic characters of Vertebrates. Bran-
chial clefts.
GLaNpb. One cell or a group of many epithelial cells which elaborate mate-
rials and secrete the product for the use of the organism.
Gwocuipium. A bivalved larva of certain fresh-water Mussels, that lives
temporarily as a parasite on a Fish.
Guiottis. The opening from the pharynx into the tube (trachea) leading
to the lungs.
GLYCOGEN. So-called animal starch. Sugar is stored as glycogen in liver
and muscle cells.
Gotar Bopres. Formed elements in the cytoplasm; apparently active
chemically.
Gonapb. Ovary or testis.
GonotHEcA. Transparent sheath, or exoskeleton, of the reproductive in-
dividuals (gonangia) of a Hydroid colony; e.g., Obelia.
Gray Crescent. Localized organizing substance in Frog’s egg.
GustaTory. Relating to the sense of taste.
Hapwiorp. The reduced (one-half) number of chromosomes. A complete
single set of chromosomes. See Diploid.
HemMoG.LoBIn. Complex chemical compound, in the red blood corpuscles
of Vertebrates, which enters into a loose combination with oxygen, be-
coming oxyhemoglobin. Respiratory pigment.
Hepatic Portrat System. Non-oxygenated but food-laden blood from
digestive tract passes to the liver by the hepatic portal vein. Oxygenated
blood reaches liver by the hepatic artery. Both leave by hepatic vein.
Thus there is a double blood supply to liver in all Vertebrates.
Herepiry. The transmission of characters from parent to offspring by
the germ cells.
GLOSSARY 201
HermMapuropite. An organism bearing both male and female reproduc-
tive organs; e.g., Earthworm.
Hererozycous. Hybrid. Formed by gametes dissimilar in regard to a
given character, or characters, and producing gametes dissimilar in
regard to the character, or characters, in question. See Homozy-
gous.
Histo.tocy. The science which treats of animal and plant tissues. Mi-
croscopic anatomy.
Hotozorc. Type of nutrition involving the ingestion of solid food. Char-
acteristic of animals. See Autotrophic and Saprophytic.
Homotocous CHromosomMes. The members of a pair of chromosomes, of
a diploid group, one paternal and the other maternal in origin, which
bear homologous genes. See Synaptic Mates.
Homotocous Genes. Genes similarly situated on homologous chromo-
somes, and contributing to the same or different expressions of a char-
acter. Allelomorphs. Alleles.
Homotocy. Fundamental structural similarity, regardless of function,
due to descent from a common form; e.g., wing of Bird and fore leg
of Dog.
HomoTHERMAL. Animals provided with a mechanism which maintains
the body at a practically constant temperature, usually higher than
that of the environment; e.g., the ‘warm-blooded’ animals, or Birds
and Mammals.
Homozycous. Pure. Formed by gametes the same in regard to a given
character, or characters, and producing gametes all the same in regard
to the character, or characters, in question. See Heterozygous.
Hormone. An internal secretion usually from a ductless gland. Secreted
directly into the blood which distributes it throughout the body where
it selectively influences tissues and organs.
Host. An organism in or on which a parasite subsists.
Hyatopiasm. The clear ground-substance of protoplasm.
Hysrip. The progeny of parents which differ in regard to one or more
characters. A heterozygote.
Hyprantu. A feeding polyp of a Hydroid colony; e.g., Obelia.
Hyproips. A group of animals (Coelenterates) exhibiting alternation of
generations; e.g., Obelia.
Hyprotysis. Decomposition of a chemical compound by reaction with
water; e.g., in digestion.
Hyprostatic OrGan. Organ for regulating the specific gravity of an
animal in relation to that of water; e.g., the air-bladder of certain
Fishes.
HyprotHeca. Vase-like expansion of the exoskeleton, or perisarc, about
a hydranth; e.g., of Obelia.
502 APPENDIX
Immunity. Resistance of the body to infection by disease-producing
organisms. Exemption from disease.
INDEPENDENT ASSORTMENT. Members of different pairs of genes, located
in different pairs of chromosomes, are distributed independently.
INFUNDIBULUM. A funnel-like outgrowth from the ventral wall of the
diencephalon., See Pituitary Gland.
INTERCELLULAR DiGEstTIOoN. Digestion by the secretion of enzymes into a
digestive cavity; e.g., in Earthworm and Man. See Intracellular Diges-
tion.
INTERNAL Receptors. Sense organs within the body. See External Re-
ceptors.
INTERNAL SECRETION. See Hormone and Ductless Gland.
IntTEsTINE. Portion of the alimentary canal. In higher forms, portion
from pyloric end of stomach to cloaca or anus. Usually divided into
small and large intestine.
INTRACELLULAR DicEstIon. Digestion of food within the cell itself; e.g.,
in Paramecium and to some extent in the endoderm cells of Hydra. See
Intercellular Digestion.
InvaGinaTIOoN. Sinking or growing in of a portion of the surface of a hol-
low body; e.g., during transformation of blastula into gastrula.
INVERTEBRATE. Animal without a notochord or a vertebral column.
IRRITABILITY. The power of responding to stimuli, exhibited by all proto-
plasm.
KaryotymMpH. The more fluid material of the nucleus in contrast with the
linin and chromatin.
Katapo.ism. The destructive phase of metabolism. See Anabolism.
Kinetic ENERGy. Energy possessed by virtue of motion; e.g., union of
C with O2 transforms chemical potential energy into kinetic energy,
i.e., heat, etc. See Potential Energy.
Lactreats. Lymphatic vessels of the small intestine.
Lamarckism. Essentially the doctrine of the inheritance of modifications,
or acquired characters, as a factor in evolution.
Larva. An immature stage in the life history of certain animals, usually
active and differing widely in appearance from the adult; e.g., cater-
pillar of Butterfly, tadpole of Frog.
Linin. Non-stainable portion of the nuclear reticulum, probably closely
related chemically to chromatin.
Linxace. The inheritance together of characters represented by genes in
the same chromosome. Independent assortment does not occur.
Liriws. Fatty substances including the true fats and such compounds as
cholesterol (C»;H,;0H) and the lecithins containing also phosphorus and
nitrogen. See Fats.
GLOSSARY 303
Lympu. Essentially excess tissue fluid passing through vessels on its way
back to the blood vascular system. See Tissue Fluid.
Macronuc.eus. The large ‘somatic’ nucleus in Infusoria with dimorphic
nuclei; e.g., in Paramecium. See Micronucleus.
Mapreporic Piatt. A small perforated plate on the aboral surface of
certain Echinoderms (e.g., the Starfish) that allows the passage of water
to the water-vascular system.
Matrtose. A double sugar derived from starch by hydrolysis during diges-
tion.
MANDIBLEs. Jaws.
Mantte. Layer of tissue that secretes the shell in Molluscs.
Maturation. Final stages in the formation of the germ cells, involving
chromosome reduction (meiosis).
Maxiuurreps. The three anterior pairs of appendages of the thorax of the
Crayfish.
Mecuanism. The doctrine that the phenomena of life are interpretable
in terms of the laws of matter and energy which hold in the realm of the
non-living. See Vitalism.
Mepusa. Sexual, gonad-bearing generation of hydra-like animals, the
Hydrozoa, and also the Scyphozoa.
Merosts. See Reduction.
MENDEL’s Laws. See Segregation and Independent Assortment.
MesenTeErRY. Fold of the peritoneal lining of the body cavity, suspend-
ing the alimentary canal. Also, radial partitions in the enteric cavity;
e.g., of Metridium.
MeEsopermM. A primary tissue, or germ layer, of animals, which develops
between the ectoderm and endoderm. See Germ Layer.
Mesocioea. The non-cellular layer between the two primary tissue
layers of Coelenterates.
Mesorcuium. Mesentery-like membrane supporting the testes.
Merasouism. The sum of the physico-chemical processes in organisms,
involving the building up, maintenance, and breaking down of the
living matter and its constituents. See Anabolism and Katab-
olism.
Meramorpuosis. A more or less abrupt transition from one developmen-
tal stage to another; e.g., transformation of larva into adult during the
life history of a Butterfly or Frog.
MerapnHase. Climax of mitosis involving the separation of the halves of
the longitudinally split chromosomes arranged in the equatorial plate.
See Anaphase.
Merapuyta. Multicellular plants.
Merapuasm. Lifeless inclusions in cytoplasm; e.g., yolk granules, ete.
504 APPENDIX
Merazoa. Multicellular animals with cells differentiated to form tissues.
Invertebrates and Vertebrates. See Protozoa.
Micronucteus. The small ‘germinal’ nucleus in Infusoria with dimorphic
nuclei; e.g., Paramecium caudatum has one, P. aurelia and P. calkinsi
have two, and P. woodruffi and P. polycaryum have several micronuclei.
See Macronucleus.
MitrocuonpriA. Bodies in the cytoplasm which apparently contribute to
specific chemical processes.
Mitosis. The typical process of cell division.
MopirFications. See Acquired Characters.
Mononysrip. The progeny of parents differing in regard to one given
character.
MorpuHocenssis. The embryological development of the form and struc-
ture of an organism.
Morpuoxocy. The science of the form of animals and plants.
Mosaic INHERITANCE. Inheritance of a character in part from each parent
but without blending.
Moutrt. To cast off the outside covering, such as the exoskeleton of
Arthropods. Ecdysis.
Mutation. A heritable variation due to a change in the constitution of
the chromosome (gene) complex, independent of the usual processes
of segregation and crossing-over. Chromosomal aberrations and in-
trinsic gene changes.
Myonemes. Contractile fibrils of certain Protozoa; e.g., Vorticella.
Myotomes. Muscle segments in body wall of lower Vertebrates and em-
bryos of higher forms.
NATURAL SELECTION. The processes occurring in nature which result in
the “survival of the fittest’ individuals and the elimination of those
less adapted to the conditions imposed by their environment and mode
of life. Essence of the Darwinian theory of evolution.
Nematocyst. A _nettle-cell or stinging-cell of Coelenterates; e.g.,
Hydra.
NEPHRIDIOSTOME. Funnel-like opening of a nephridium into the coelom.
Nepuripium. A tubular excretory organ; e.g., in Earthworm.
NerRvVE. Essentially a group or cable of parallel nerve fibers bound to-
gether. See Axon.
NeEuRAL Tuss. A tube derived from the ectoderm and forming the brain
and spinal cord in Vertebrates.
NEURENTERIC CANAL. Temporary passage between cavity of enteron
and neural tube in Vertebrate embryos; e.g., Frog.
Neuron. A nerve cell, comprising cell body and cytoplasmic processes.
See Axon.
GLOSSARY 505
Nirairyinc Bacteria. Nitrite Bacteria which, in the process of their
nutrition, change ammonia (NHs3) into compounds with the NO: radical
(nitrites), and Nitrate Bacteria which change nitrites Into compounds
with the NOs radical (nitrates).
NItROGEN-FIXING Bacteria. Types of Bacteria which take free atmos-
pheric nitrogen and combine it with oxygen so that nitrates available
for green plants are formed. Found in the soil and in tubercles on root-
lets of various leguminous plants such as Beans, Clover, Alfalfa.
Non-pissuNcCTION. Failure of homologous chromosomes to separate after
synapsis. Therefore they are not independently segregated during
maturation — both pass to the same gamete.
Norocuorp. An axial cord of cells characteristic of the Chordates, and
about which the vertebral column is formed in Vertebrates.
Nucreouus. A spherical, achromatic body in the nucleus. Plasmosome.
Karyosome is chromatic.
Nucteus. A specialized protoplasmic body in all typical cells. Most
characteristic element is chromatin. See Cytoplasm.
Ocrettus. Sense organ responsive to light, especially the simple eyes of
Insects. See Compound Eye.
Ouracrory. Relating to the sense of smell.
ONTOGENY. The developmental history of the individual. See Phylogeny.
Oocyst. Encysted zygote; e.g., of Malarial Parasite.
Oocyte. The ovarian egg before maturation.
OOcENEsIS. The development of the mature egg from a primordial germ
cell.
Optic Loses. Thickenings of the dorsal surface of the mid-brain.
Orver. In classification, a main subdivision of a class. See Family.
Orcan. A complex of tissues for the performance of a certain function;
e.g., the heart.
Osmosis. Diffusion of dissolved substances through a semi-permeable
membrane. Osmotic pressure may be considered as a result of the in-
hibited power of diffusion of a dissolved substance — inhibited because
the membrane is semi-permeable, i.e., permitting water but not the
substance in solution to pass through. The physical phenomena of dif-
fusion and osmosis are complicated in living cells by the fact that their
limiting membranes undergo changes in permeability. See Diffusion.
OstEoLtocy. The study of the Vertebrate skeleton.
Ostrum. In Sponges, the opening from the gastral cavity to the exterior.
Oviparous. Egg-laying. See Viviparous.
Ovum. Egg. Female gamete.
OxipaTion. The combination of any substance or its constituent parts
with oxygen. Combustion.
506 APPENDIX
PaLeEontoLocy. The science of extinct animals and plants represented
by fossil remains.
Parapopium. Locomotor and respiratory organ of marine worms; e.g.,
Nereis.
ParasiTE. An organism which secures its livelihood directly at the ex-
pense of another living organism, on or in whose body it lives.
Parazoa. Animals without an enteron or coelom. Sponges.
PARTHENOGENESIS. Development of an egg without fertilization.
PaTHOGENIC. Disease-producing, especially in regard to the relation of
a parasite to its host.
PENTADACTYL. Having five fingers or toes; typical Vertebrate limb.
PrericarRpiuM. Peritoneum lining the pericardial cavity containing the
heart.
PeriostEuM. Connective tissue sheath covering bone and contributing
to its growth.
PrrIsTALsis. Rhythmical contractions of the wall of the alimentary canal
which force the food along.
PERITONEUM. Membrane lining coelom of Vertebrates. Mesodermal in
origin.
Puarynx. Region of alimentary canal between buccal cavity, or mouth,
and esophagus. Throat.
PHENOTYPE. The somatic, or expressed, characters of an organism or
group of organisms irrespective of those potential in their germ cells.
See Genotype.
PHOTOSYNTHESIS. Process by which complex compounds are built up from
simple elements through the energy of sunlight absorbed by chlorophyll.
Puy.LoGeny. The ancestral history of the race. See Ontogeny.
Puytum. In classification, a main subdivision of the animal kingdom.
See Class.
PuystoLocy. The study of the functions of animals and plants. The
mechanical and chemical engineering of organisms.
PinEAL Bopy. An outgrowth from the upper wall of the diencephalon.
The vestige of an additional eye possessed by the ancestors of existing
Vertebrates. Possibly functions as an endocrine gland in Mammals.
Brow-spot of Frog.
Piruirary Guanp. A glandular body under the brain, formed of tissue
from the nervous system and from the alimentary canal. Secretes
several hormones.
PiaceNTA. A Mammalian organ adapted for the interchange of all nu-
tritive, respiratory, and excretory materials between the embryo (fetus)
and mother. It also serves as an organ of attachment. In the higher
Mammals it is composed of both fetal and maternal tissues. See Um-
bilical Cord.
GLOSSARY 207
Pxasma. Liquid portion of the blood, lymph, and tissue fluid.
PLASMA-MEMBRANE. Living cell membrane, as distinguished from the
cell wall which may also be present.
Piastip. A specialized cytoplasmic body. See Chloroplast.
Piexus. The intermingling of fibers from one nerve with those of an-
other to form a network; e.g., sciatic plexus.
PoLieN. The microspores of Seed Plants.
PoxturnaTtion. The transference of pollen to the stigma of the pistil in
Seed Plants. Eventuates in fertilization.
Potocytes. Tiny abortive cells arising by division from the egg during
maturation. Polar bodies.
PotymorpuHisM. Occurrence of several types of individuals during the
life history, or composing a colony; e.g., in some Hydroids.
Potype. Hydra, or a Hydra-like individual of Hydroids, Corals, and other
Coelenterates.
PoputaTIon. Entire group of individuals from which samples are taken
for genetical study. Usually comprises several pure lines.
PotrenTIAL Enerncy. Energy possessed by virtue of stresses, 1.e., two
forces in equilibrium. Criterion is work done against any restoring
force; e.g., kinetic energy of sunlight through agency of chlorophyll
separates CO, into C and O, and thereupon is represented by an equal
amount of chemical potential energy. Restoring force is here chemical
affinity. Similarly a raised weight possesses gravitational potential
energy in amount equal to kinetic energy expended in raising it. See
Kinetic Energy and Conservation of Energy.
PREFORMATION. The abandoned doctrine that development is essentially
an unfolding of an individual ready-formed in the germ. See Epigenesis.
Pronepuros. Primitive kidney of Vertebrates.
Propuase. Preparatory changes during mitosis leading to the disposition
of the chromosomes in the center of the cell (equatorial plate) ready
for division. See Metaphase.
Prosopy.es. Pores leading into the flagellated chambers of Sponges.
ProstaTE GLAND. An accessory male genital gland in Mammals.
Prostomium. A lobe which projects from the first segment of the body of
the Earthworm and forms an upper lip.
Protern. A class of complex chemical molecules, containing nitrogen,
which form the chief characteristic constituent of protoplasm.
Protopuyta. Unicellular plants.
ProtopiasM. The physical basis of life. Matter in the living state.
ProtopopiTe. The basal portion of the typical Crustacean appendage
from which arise the endopodite and exopodite.
Protozoa. Unicellular animals, or colonies of animal cells not differ-
entiated to form tissues; e.g., Amoeba, Volvox.
508 APPENDIX
ProtrozooLocy. The science of unicellular animals, or Protozoa.
Pseupopopium. Temporary protoplasmic projections for locomotion,
feeding, etc., as in Amoeba.
PupaTte. Assumption of a quiescent stage (pupa), in the life history of
Insects with a ‘complete’ metamorphosis, during which the larva is
reorganized as an adult; e.g., chrysalis of a Butterfly and in cocoon
of a Moth.
Pure Linge. A group of individuals bearing identical genes, derived from
a common homozygous ancestor.
PutreFactTion. The simplification of nitrogenous compounds, such as
proteins, chiefly through the action of enzymes of living organisms. See
Fermentation and Decay.
Pytoric Vatve. Muscular constriction between stomach and small intes-
tine.
Pyrenorp. Portion of chloroplast specialized for starch formation.
RECAPITULATION THEORY. Doctrine that individual embryonic develop-
ment (ontogeny) repeats in abbreviated and modified form the de-
velopment of the race (phylogeny). So-called biogenetic law.
RECESSIVE CHARACTER. See Dominant Character.
RECOMBINATION. Heritable variation due to the typical reassortment of
the chromosomes during maturation and fertilization. Includes crossing-
over of genes.
Repuction. The process in maturation, during spermatogenesis and
oogenesis, which separates synaptic mates and reduces the chromosome
number one-half. Meiosis. The mechanism of segregation.
Rervex. A relatively simple and essentially automatic response resulting
from the transmission of a sensory impulse to a nerve center and its
immediate reflection as a motor impulse independent of volition. A
conditioned reflex is one established by training.
REGENERATION. The replacement of parts which have been lost through
mutilations or otherwise.
RENAL Portat System. Blood (‘impure’) passes from posterior part
of the body to kidneys by the renal portal vein; oxygenated blood
to kidneys by the renal artery. Thus in animals with the renal
portal system there is a double blood supply to the kidneys.
Present in Fishes, Amphibians, and Reptiles; vestigial in Birds; absent
in Mammals.
Repropuction. The power of living matter to reproduce itself. Proto-
plasmic growth resulting in cell division.
ReEsprraTIon. Essentially the securing of energy from food by oxi-
dation, involving the exchange of carbon dioxide for oxygen by
protoplasm.
GLOSSARY 209
Response. Any change in the activity of protoplasm, and therefore of an
organism as a whole, as the result of a stimulus. See Irritability.
Restinc Cetu. One which is not undergoing mitosis.
Retina. Actual percipient part of the eye by virtue of a sensory layer
which is stimulated by light rays.
Reversion. The appearance of an ancestral character in an individual
after it has been ‘latent’ for one or many generations. Atavism.
Rorirera. Microscopic, aquatic, multicellular animals. Wheel animal-
cules.
Rusts. Fungi which are destructive parasites of the higher plants; e.g.,
the Wheat Rust.
SaccuLus. The anterior sac of the labyrinth of the ear, a derivative of
which becomes the cochlea in higher Vertebrates.
Sapropuytic. Type of nutrition involving the absorption of complex
products of organic decomposition; e.g., in many groups of Bacteria
and other Fungi, as well as various species of lower animals. Saprozoic.
See Holozoic and Autotrophic.
SeBAcEOUS GLANDs. Glands which elaborate a fatty substance (sebum)
and secrete it into the hair follicles. Oil glands.
SECONDARY SExuAL CuaractTers. Differences between the sexes, other
than those of the gonads and related organs.
SECRETION. A substance elaborated by glandular epithelium; or the
process involved. See Gland and Excretion.
SEGREGATION. The distribution of homologous chromosomes, and there-
fore of homologous genes (allelomorphs), to separate cells during the
formation of the gametes. The chief factor of Mendelian inheritance.
See Reduction.
SEMICIRCULAR CANALS. Portion of the Vertebrate ear devoted to the
maintenance of equilibrium.
SEMINAL ReEcEPTACLES. Sacs within the body cavity of certain animals
(e.g., Earthworm), which receive sperm from another individual and
retain them until fertilization is to occur.
Septa. The partitions which divide the coelom of the Earthworm into a
series of chambers, or segments.
SeRIAL Homotocy. Homology of a structure of an organism with another
of the same organism; e.g., appendages of the Crayfish, fore and hind
limbs of Vertebrates.
SessILE. Attached, sedentary; e.g., Sponges.
SeTak. Bristle-like structures which protrude from the body wall and
aid in locomotion; e.g., in Earthworm and Nereis.
SEX-LINKED CHARACTERS. Characters represented by genes in the X
chromosome.
510 APPENDIX
Soma. Body tissue (somatoplasm) in contrast with germinal tissue (germ
plasm).
SpEcIAL CREATION. Doctrine that each species was specially created. Im-
plies fixity of species. See Evolution.
Species. In classification, the main subdivision of a genus. A group of
individuals which do not differ from one another in excess of the limits of
‘individual diversity,’ actual or assumed.
SpeRM. Male gamete. Spermatozoon.
SpermMatTip. Male germ cells after the final maturation division but be-
fore assuming the typical form of the ripe sperm.
SPERMATOCYTES. Cells arising from the spermatogonia. Primary sperma-
tocyte arises by growth from the last generation of spermatogonia.
Primary divides to form two secondary spermatocytes.
SPERMATOGENESIS. The development of the sperm from a primordial
germ cell.
Sprnp.LeE. The fiber-like apparatus between the centrosomes during
mitosis.
Sprractes. Openings on the body surface leading into the tracheal system
of Insects.
SPLEEN. A vascular ductless organ of most Vertebrates, usually situated
near the stomach, that acts chiefly as a stabilizer of the supply of red
blood corpuscles.
Sponain. A horny substance, chemically allied to silk, forming the fibers
of the skeleton of certain Sponges; e.g., the Bath Sponge.
SPONTANEOUS GENERATION. See Abiogenesis.
Spore. A cell which gives rise without fertilization to a new individual.
Also the resistant phase assumed by certain unicellular organisms; e.g.,
Bacteria, Sporozoa.
SPORULATION. Occurrence of several simultaneous divisions by which a
unicellular organism is resolved into many smaller cells. Formation of
spores.
Sratocysts. Organs of equilibrium; e.g., in medusae.
Stimutus. Any condition which calls forth a response from living matter.
See Irritability.
Symsiosis. The association of two species in a practically obligatory and
mutually advantageous partnership; e.g., Lichens, Hydra (green).
SyMPATHETIC Nervous System. See Autonomic System.
Synapse. The contact of one nerve cell with another, which makes pos-
sible the conduction of a nervous impulse from cell to cell.
Synapsis. The pairing of homologous chromosomes during maturation
of the germ cells.
Synaptic Mates. Homologous chromosomes of maternal and paternal
origin that pair during synapsis.
GLOSSARY 511
SyNKARYON. The composite nucleus formed by the union of the nuclei of
two gametes. Male and female gametic nuclei united in the zygote.
See Zygote.
Tapir. A large herbivorous Mammal, having short stout limbs and flexible
proboscis with the nostrils near the end. New World species are brown-
ish-black, those of the Old World are black and white.
Taxonomy. The science of classification.
TELOPHASE. Final phase of mitosis during which the two daughter nuclei
are formed and cytoplasmic division is completed. See Prophase.
TetTrabD. Group of four chromosomes formed by a precocious division of
synaptic mates in the primary spermatocyte and primary o6cyte.
THorax. Portion of the trunk in Mammals, containing esophagus, lungs,
and heart. The middle portion of the body in the Arthropoda; e.g., in all
Insects. In the Crayfish the head and thorax are fused to form the
cephalothorax.
Tuymus. A glandular structure in the pharyngeal region of Vertebrates.
Regresses during early life in Man. Function obscure.
Tuyroip GLAND. An endocrine or ductless gland in the pharyngeal region
of Vertebrates.
TissuE. An aggregation of similar cells for the performance of a certain
function. See Organ.
TissuE Fiurp. Essentially plasma and white blood cells that have passed
through the capillary walls to supply the milieu of the tissue cells.
Intercellular fluid. See Lymph.
TRACHEAL SysTEM. Series of tubes that convey air throughout the tissues
of certain Arthropods; e.g., Insects.
Tricuocysts. Minute bodies, arranged in the outer part of the ectoplasm
of certain Infusoria (e.g., Paramecium), each of which upon proper
stimulation is transformed into a thread-like process protruding from
the cell surface. Apparently defensive structures.
Trinysrip. The progeny of parents differing in regard to three given
characters.
TRILOBITES. Crustacea dominant during the early Paleozoic era. Ex-
tinct.
TRIPLOBLASTIC. Composed of three primary germ layers: ectoderm, endo-
derm, and mesoderm, as in all animals above the Coelenterates.
TROPHOZOITE. Growing and feeding form developed from a Sporozoite;
e.g., in Monocystis and Malarial Parasite.
Tropism. Element of behavior of the lower organisms. Orientation with
respect to an external stimulus; e.g., chemotropism, phototropism, etc.
Tuse Feet. Locomotor organs of Echinoderms; e.g., Starfish and Sea
Urchin.
512 APPENDIX
Turcor. Pressure within a cell, largely due to the absorption of water,
which distends or holds rigid the cell wall.
TypHLosoLe. A median dorsal invagination along the entire length of
the intestine of the Earthworm which increases the area of the digestive
and absorptive surface.
UmepiticaL Corp. A Mammalian structure, commonly known as the
navel cord, by which the embryo is attached to the placenta. The blood
vessels from the embryo to the placenta pass through it. See Placenta.
UneuricunaTe. Provided with claws.
UNIFORMITARIAN Doctrine. An interpretation of the present condition of
the Earth on the assumption of similarity of factors at work during
past ages and to-day.
UNIPARENTAL. Derived from a single progenitor; e.g., in asexual reproduc-
tion. See Biparental.
Urea. Nitrogenous waste product of animal metabolism. Formed as such
in the liver, removed from the blood by the kidneys and eliminated from
the body chiefly in urine. CO(NH,)2. A major part of the nitrogen is
excreted in this form in human urine. See Creatinine and Uric Acid.
Ureter. A tube carrying urine from kidney to the cloaca or to the urinary
bladder.
Uric Aci. A nitrogenous waste product. Present in small quantity in
the urine of Man.
UroceEnirTau. Relating to the urinary and reproductive systems.
UrostyLe. Terminal rod-like bone of the vertebral column of the Frog.
Uterus. Lower portion of the oviduct (or oviducts) modified for the
retention of the eggs, temporarily or during development.
Urerus Mascuuinus. Remnant of the pronephric ducts in some male
Mammals.
Urricu.tus. The posterior sac of the labyrinth of the ear into which the
semicircular canals open.
VacIna. Passage leading from the uterus to the exterior.
VasomoTor NervEs. Nerves which regulate the caliber of small arteries
by bringing about relaxation or contraction of the muscular layer of
their walls.
VERMIFORM APPENDIX. Blind outpocketing of the large intestine near its
origin from the small intestine. Vestigial end of the caecum. Found
only in Apes and Man.
VERTEBRA. One of the series of elements forming the backbone, or ver-
tebral column.
VERTEBRATE. An animal with a backbone, or vertebral column. See
Chordate.
GLOSSARY 313
Viratism. The doctrine which attributes at least some of the phenomena
of life to an interplay of matter and energy which transcends the so-
called laws operable in the inorganic world. See Mechanism.
Vitamins. Indispensable accessory food substances whose importance
has but recently been realized. Chemical composition of most of them
is unknown.
Viviparous. Producing young that have developed to a relatively ad-
vanced stage in the uterus; e.g., most Mammals. See Oviparous.
WorkinG Hypotuesis. A basic assumption to guide the study of a sub-
ject, and to be proved or disproved by facts accumulated.
X CHromosoMe. The so-called sex chromosome.
Y Curomosome. The synaptic mate of the X chromosome when present.
Yeast. A group of unicellular colorless plants (Fungi) which are chiefly
responsible for alcoholic fermentation.
Youk. Food material stored within the cytoplasm of an egg. See Meta-
plasm.
Yok Sac. When a great quantity of yolk is present in an egg, the endo-
derm gradually grows over it to form the yolk sac which is usually of
enormous size in comparison with the embryo proper.
ZoOGEOGRAPHY. The science of the geographical distribution of animals.
Zycotr. The composite cell formed by the union of male and female
gametes. Fertilized egg. See Synkaryon.
INDEX
(Figures in italics designate pages on which illustrations occur)
Abdomen, 90, 93, 106, 133, 134, 149
Abdominal pore, 190
Abiogenesis, 218-221
Abortion, infectious, 391
Absorption, 153, 154, 157, 158
Academy of Sciences, National, 424
Acoelomate, 101, 480
Acquired character, 285, 286, 374;
See: Modifications
Actinophrys, 49
Adam’s apple, 162
Adaptability, 343-348
Adaptation, 18, 26, 28, 56, 316-337,
375-377, 384, 388, 405, 431, 445
Adaptive radiation, 320-324
Adrenal gland, 190, 198
Adrenaline, 198
Aedes, 393
Aerobe, 318
Afferent nerve, 208
African sleeping sickness, 341. 342,
390, 407
Agassiz, 459, 460
Age, old, 25, 256-259
Ages, geological, 360, 363
Agriculture, 6, 342, 403-417, 422
Air bladder, 146
Albino, 291, 296, 367
Albumin, 21, 253
Alcohol, 286, 318, 427
Algae, 335-337
Alimentary canal, 65, 150-160, 168,
180, 267-270
Allantois, 271, 272
Allelomorphs, 292, 293, 302, 308
Allen’s theory, 225, 226
Alligator, 120, 121, 365
Allogromia, 48
Alternation of generations, 72, 234.
Alternative characters, 288
Alveolus, 163, 164.
Amblystoma, 178
Ambulacral groove, 84
Amino acid, 22, 33, 36, 151, 156
Ammonia, 41, 181
Ammnion, 193. 271. 272
Amoeba, 10, 12, 15, 27, 28: 35-37
261, 344, 390, 477
Amoeboid movement, 27, 28, 238,
342
Amphibia, 111, 117-119, 147, 173,
359, 365; See: Frog
Amphineura, 86
Amphioxus, 1170, 111, 145, 478
Amylase, 155, 156
Anabolism, 25; See: Metabolism
Anaerobe, 318
Analogy, 107, 216, 459
Anaphase, 240-242
Anatomy, 3, 4, 354-388, 451-454,
457-459
Andalusian fowl, 298, 299
Animal, 35-37, 43, 46-56, 67-131;
body, 98-109, 132-149; coloration,
324-329; number, 47, 66, 67, 349
Animal biology, 3
Annelida, 81, 82, 457, 480; See:
Earthworm
Anopheles, 392, 393; See: Mosquito
Ant, 93, 336-339, 346
Anteater, 127, 321
Antelope, 321, 341, 342
Antenna, 93, 104-106, 330, 331, 356;
cleaner, 331, 332; comb, 3317, 332
Antennule, 105, 106, 356
Anterior chamber of eye, 150
Anther, 334
Anthozoa, 75-77
Anthrax, 390, 391
Anthropoidea, 426-431
Antibody, 342, 343
Antitoxin, 343
Antlers, 192
Anus, 80, 132, 159
Aorta, 174, 176, 185
Aortic arches, 173, 175; loop, 101, 102
Ape; 130; 131, 322, /396;(399) csor.
426-431
Aphid, 338, 339, 375, 411, 414
Apopyle, 70
a16
Appendages of crayfish, 96, 105, 106,
356
Appendicular skeleton, 142, 143
Apple, 408
Applied science, 386, 388
Aptera, 92, 94
Apteryx, 124
Aqueous humor, 275
Arachnoidea, 95
Arcella, 48
Archaeopteryx, 123, 361, 362
Aristotle, 2, 23, 188, 218, 251, 446,
449, 451, 456, 462, 464, 466, 471,
472
Armadillo, 728, 129
Army worm moth, 408, 4175
Arterial system, 171-177
Artery, 105, 170-179
Arthropoda, 90-97,
215, 478
Artificial parthenogenesis, 260, 261
Artificial selection, 378-384, 473-476;
See: Selection
Artiodactyl, 130
Ascaris, 80, 396
Ascon, 69, 70
Asexual reproduction, 57; See: Bud-
ding, Regeneration, Reproduction,
and Schizogony
Associative memory, 347
Aster, 240, 241
Asteroidea, 86
Astronomy, 2
Atlas, 142
Atmosphere, 320
Auditory nerve, 212, 213
Audubon Societies, 419
Aurelia, 73-75
Autonomic system, 201, 205, 208, 209
Autotrophic, 33, 42
Aves, 123-126
Avoiding reaction, 344
Axon, 200
104-109, 130,
Baboon, 131
Babylonian science, 446
Bacillus, 38, 39; See: Bacteria
Backbone, 111; See: Skeleton
Bacon, 452
Bacteria, 38-45, 158, 220-223, 318,
319, 342, 343, 391, 402-404, 407,
453; denitrifying, 45; nitrite, 45;
nitrogen-fixing, 42, 338
INDEX
Badger, 321
von Baer, 1, 467, 468
Balanced aquarium, 45; See: Web of
life
Balantidium, 396
Barley, 409
Barnacle, 91
Bat, 128, 129, 321,324, 355
Bath sponge, 69, 70
“Beagle,” voyage of, 372
Bean, inheritance, 380, 387
Bear, 129, 321, 352, 367
Beaver, 129, 323, 352
Bee, 93, 94, 327-336, 346, 414
Bee-fly, 329
Behavior, 253, 314, 343-348
Belief, 228
Beri-beri, 159
Bibliography, 481-491
Bilateral symmetry, 78, 104, 132
Bile, 154; duct, 133, 146, 148, 154,
155, 398
Binary fission, 50, 56, 188, 229, 230,
255
Binomial nomenclature, 353, 457
Biochemistry, 5, 366, 403
Biocoenosis, frontispiece, 494
Biogenesis, 218-228, 454
Biogenetic law, 364-366, 468
Biological sciences, 3, 4
Biology, 1-6, 466
Biophysics, 5
Biosocial reactions, 437, 444
Biparental, 274
Biramous, 106, 107
Bird, 111, 123-126, 204, 251, 253, 319,
325, 355, 361, 362, 365, 369737
376, 419, 420, 479
Birth, 191, 192, 194
Bison, 419, 441
Bivalve, 87-89
Bladderworm, 399, 400
de Blainville, 23
Blastocoel, 60, 264, 266, 268
Blastoderm, 253, 272
Blastopore, 60, 264, 266, 268
Blastostyle, 72
Blastula, 60, 98, 263, 264, 266, 268
Blending inheritance, 287, 297-301
Blind spot, 275
Blood, 11, 15, 64, 165, 166, 169-179,
182, 192, 367, 430, 452, 454; corpus-
cles, 11, 28, 64; pressure, 176, 177;
INDEX
rate, 176; relationship, 367, 368;
reservoir, 178; transfusion, 367;
vessels, 157; volume, 178; See: Ar-
tery and Vein
Blue crab, 91
Boa-constrictor, 123
Bodo, 51
Body, invertebrate, 98-109; verte-
brate, 132-149
Boll weevil, 409, 410
Bone, 15, 62, 63, 113, 124, 138, 139,
213, 215; See: Skeleton
Bony fishes, 115, 116
Borelli, 460
Botany, 3, 4, 404, 447, 450, 454, 456,
457, 462, 463
Botfly, 405
Bougainvillea, 72
Box tortoise, 127
Brachiopod, 82, 83, 477
Brachonid fly, 408
Brain, 104, 105, 108, 110, 143, 200-
217. 268, 269, 319, 344, 365, 366
Branchial artery, 170-173
Branchiostoma, 110; See: Amphioxus
Bright’s disease, 182
Brittle star, 86
Bronchial tubes, 163
Bronze age, 444
Browne, 218
Brown-tail moth, 413
Bryozoa, 82, 83, 477
Bubonic plague, 390, 391, 406, 407
Buccal cavity, 151, 152
Bud. 417,68; 71, 72, 77 -99; 230
Buffalo, 336
Buffalo-moth, 473
Buffon, 220, 471, 472
Bufo, 118, 179
Bumble bee, 94, 219
Buttercup, 7
Butterfish, 336
Butterfly, 94, 96, 327, 408
Cabbage butterfly, 94, 408
Caddice-fly, 94
Calciferous gland, 102
Calorie, 159
Cambrian period, 359, 360
Camel, 130, 327
Camera, 216, 217
Candle, 24
Cane sugar, 156
517
Capillaries, 101, 163, 170-179, 186,
198, 342, 454
Caprella, 97
Carapace, 121
Carbohydrate, 22, 23, 32, 33, 36, 39,
ESL, 156, 157, 159; 197
Carbon, 21-23; cycle, 40; dioxide,
165, 166, 181
Cardiac muscle, 138
Carinatae, 124, 126
Carnivora, 129, 352
Carpal, 142, 144, 355
Carpet beetle, 473
Carrier, 307, 308
Cartilage, 62, 63, 138, 139
Casein, 21, 156
Castration, 192
Gat, 129) 142. 204, 352
Catalysis, 23, 33
Caterpillar, 408, 416, 420
Caudata, 117, 118
Cell, 3, 7-12, 14, 18, 19, 28, 30, 31, 39,
254, 351, 388, 452, 465, 466, 468;
cycle, 230, 231, 238; division, 12,
13; 26: 31,41, 50, 56,60, 230° 262:
theory, 464, 466
Cellulose, 22
Cenozoic era, 359, 361
Centaur, 449
Centipede, 97, 92
Central capsule, 49
Central disk, 84
Central nervous system, 201; See:
Nervous system
Central spindle, 240, 241
Centrosome, 19, 240, 241, 254
Centrum, 133, 139, 140
Cephalization, 206
Cephalopoda, 89, 90
Cephalothorax, 90
Cercaria, 397, 398
Cercomonas, 51
Cerebellum, 146-149, 202-204
Cerebral, cortex, 207, 217; ganglion,
102; hemispheres, 202-205; See:
Brain
Cerebrum, 203
Cestode, 78, 398, 399
Cetacea, 129
Chalaza, 253
Chameleon, 122, 123, 326
Chapman, 369
Cheliped, 105
o18
Chemical coordination, 196-198
Chemical physiology, 5, 366, 403
Chemistry, 2, 5, 15, 20-23, 196-198,
403, 404, 460, 464
Chemotropism, 253, 343
Chess, 5
Chills and fever, 340
Chimpanzee, 427, 428, 431
Chipmunk, 352
Chiroptera, 129
Chiton, 86, 87
Chloragogen cells, 103
Chlorohydra, 337
Chlorophyll, 8, 34, 36, 325, 463
Chloroplast, 32, 33
Choice, 346, 347, 423
Cholera, 390, 391
Chordate, 111, 145, 478-480; See:
Vertebrate
Chorion, 272
Choroid, 275
Chromatin, 19, 20, 240-243; See:
Nucleus
Chromatophore, 50
Chromosome, 240-250, 275, 279, 280,
304-314, 468, 469; aberrations, 310,
311; combinations, 248-250; cycle,
248-250; human, 306-311; map,
309
Chronological succession, 358-361
Chrysalis, 408
Chyle, 158
Chyme, 153
Cicada, 94
Cilia, 4728; 55; 62
Ciliate, See: Infusoria
Circulating tissue, 64; See: Blood and
Lymph
Circulation, 65, 82, 108, 168-179, 196,
451, 452, 454
Cirripede, 91
Clam, 88
Class, 353
Classification, 46, 47, 349, 352-354,
455, 457, 477
Clavicle, 140, 144
Cleavage, 58-60, 468
Cloaca, 133, 147, 159, 190, 194
Clostridium, 318
Clothes moth, 473
Clover, 408
Clypeus, 330
Cobra, 123
INDEX
Coccus, 38, 39
Coccyx, 149, 357
Cochlea, 212, 213
Cocoon, 406
Coelenterate, 71-78, 98-100, 477
Coelom, 81, 701-103, 112, 132, 133,
145, 183, 185, 189, 194, 266, 267,
269, 480
Coelomate, 101, 169, 189, 480
Coelomic fluid, 169
Cenosarc, 72
Cold blooded, 112, 117
Cold sense spots, 211
Coleoptera, 94
Collar cell, 68, 69
Colloid, 16, 227, 317
Colon, 158
Colony, 51, 58,.71, 72, 230; 232520%4
330, 336 >
Coloration of animals, 324-329
Color-blindness, 307, 308, 383
Colorless plants, 37; See: Bacteria
Colpidium, 44
Colpoda, 44
Comb-jellies, 78
Combustion, 166, 167
Communal associations, 93, 336, 337
Comparative anatomy, 107, 354-358,
389, 457-459
Complementary genes, 300
Compound eye, 215, 330
Conditioned reflex, 208, 346, 437, 438
Condylarthra, 362
Conifer, 359, 361
Conjugation, 55, 230, 255-260; See:
Fertilization
Conjunctiva, 215-217
Connective tissue, 62-64, 134, 139
Consciousness, 347
Conservation, of energy, 463; of re-
sources, 417—420
Constructive biology, 420-425
Continuity of life, 229-250
Contractile vacuole, 10, 27, 35, 37,
50, 55, 56, 180
Coniractility, 63, 344
Conus arteriosus, 170, 171, 173
Coordination, 196-217
Cootie, 94
Copper, age of, 444
Copperhead, 123
Copulation, 189, 190
Coracoid, 140
INDEX 519
Coral, 77 Dante, 449
Corn, 283, 313, 408, 409 Daphnia, 97
Corn borer, 409
Cornea, 215, 216
Corpus luteum, 192, 193
Cortex of brain, 203
Cosmic time, 359
Cosmozoa theory, 223, 224, 227
Cotton boll weevil, 409, 410
Cousin-marriage, 383
Cowper’s gland, 148, 194
Coxa, 330, 331
Crab, 90, 91, 96
Cranial nerve, 205, 206, 211
Cranium, See: Skull
Crayfish, 90, 96, 104-109, 132, 145,
Pol 189) 210, 292, 239, 325, 346,
356, 357
Creatinine, 181
Cretin, 197
Crinoidea, 85, 86
Crithidia, 341
Crocodile, 120, 121, 365, 367
Cr6-Magnon man, 436-438, 440, 441
Crop, 102
Crossing-over, 282, 308, 309
Crura cerebri, 203, 205
Crustacea, 90, 917, 104-109; See:
Crayfish
Crystal, 26, 367
Ctenophora, 78, 477
Culex, 392
Cultural development, 431, 437-445
Cumulative genes, 299-301
Cursorial, 321
Curve of probability, 378, 379
Cutaneous senses, 210, 211
Cuticle, 702, 103, 135
Cuttle-fish, 86
Cuvier, 458
Cyanea, 73
Cyanogen, 224, 225
Cycle of elements, 39-45, 403
Cyclosis, 28
Cyclostome, 112, 113, 184, 479
Cyst, 50, 52, 223, 230, 319, 390
Cystic duct, 155
Cysticercus, 399, 400
Cyto-genetic map, 309
Cytology, 3, 301
Cytoplasm, 10, 18, 19, 240-242; See:
Protoplasm
Cytoplasmic organization, 276-27
Darwin, Charles, 82, 281, 290, 372,
376, 378, 384, 389, 416, 472-476
Darwin, Erasmus, 229, 472, 473
Darwinism, 374-377, 384, 472-476
Datura, 310
Death, 14, 20, 258
Decay, 40, 41, 221; See: Fermenta-
tion and Putrefaction
Dedifferentiation, 235
Dendrite, 200
Denitrifying bacteria, 41, 42
Dentalium, 276
Dermal branchia, 84.
Dermis, 134, 135
Descent with change, 349; See: Evo-
lution
Development, 57-61, 231, 263-280,
364-366; See: Embryology
Devil-fish, 86
Diabetes, 182, 197
Diaphragm, 134, 148, 149, 152, 163,
164
Dickens, 150
Didinium, 54
Diencephalon, 202, 203, 214
Differentiation, 56, 57, 235, 274, 276—
279, 460
Difflugia, 48
Diffusion, 157, 181, 497
Digestion, 36, 61, 69, 108, 150-160,
338, 460, 461
Digestive tract, 168; See: Alimentary
canal and Enteric cavity
Digit, 322, 355, 361-364.
Digitigrade, 321, 322
Dihybrid, 292-294, 303
Dinoflagellata, 51
Dinosaur, 120, 123, 359
Dioscorides, 448-450
Diphtheria, 390, 391
Diploid, 245, 248, 249, 303
Dipnoi, 116
Diptera, 94
Disease, 38; inheritance of, 286; mi-
croodrganisms and, 389-402
Distribution of animals, 368-373
Division of labor, 232, 234; See: Phys-
iological division of labor
Dog, 129, 321, 352, 367, 399, 442
Dogfish, 114, 171
Dog flea, 406
520
Dolphin, 129, 327
Dominant character, 290-295, 383
Donnan, 218
Dorsal aorta, 171-173; See: Aorta
Dorsal nerve root, 201, 207, 208
Dragonfly, 94
Drone, 330
Drosophila, 305, 306, 309, 311
Duckbill, 727
Duct, See: Gland
Ductless gland, 160, 196; See: Hor-
mone
Dust, 220, 222, 418
Dysentery, 391, 407
Ears 705-210, 272. 213, 357, 358
Barth;-221, (222, 227, 359, 360
Earthworm, 52, 81, 82, 100-1703, 145,
161, 783, 189, 207,. 209, 237, 235,
237, 344, 346, 416, 477; embry-
ology, 263-267; excretion, 183;
feeding instinct, 346; nerve cells,
201; regeneration and_ grafting,
237
Earwig, 94
Echidna, 127
Echinococcus, 396, 399
Echinoderm, 83-86; See: Sea urchin
Ecology, 4, 335, 368, 415, 420; See:
Web of life
Ectoderm, 8, 60, 99, 100, 134, 200,
214, 215, 231, 238, 272; See: Germ
layers
Ectoplasm, 10, 19, 27, 35, 252
Education, 5, 347, 388, 437, 444
Eel, 115, 218
Efferent nerve, 208; See: Nerve
Kog, 97-51, 59; 191, 250), 2a, 238;
247, 251, 252, 253, 340, 466-468;
bird, 253; frog, 190, 239, 268; hu-
man, 197, 239, 252, 273; mem-
brane, 252, 253; organization, 254,
275-280; rabbit, 273; sea urchin,
264, 279; shell, 252, 253
Egypt, 218, 446
HKjaculatory duct, 194
Elasmobranch, 113, 114
Elements, cycle of, 39-45
Elephant, 129, 370, 371
Fit, 278
Elk, 419
Embryo, 272, 365; human, 193, 273,
365
INDEX
Embryology, 4, 57-61, 231, 263-280,
364-366, 454, 466, 468
Embryonic membrane, 128, 191-193,
271-273
Emergent evolution, 384
Emerson, 111
Empedocles, 472
Encyclopaedist, 460, 461
Endamoeba, 48, 390, 396
Endocrinology, 160, 172,
319, 367
Endoderm, 8, 60, 99, 100, 231, 238,
QP aot
Endomixis, 55, 258, 259
Endoplasm, 19, 27, 35, 252
Endopodite, 106, 107
Endoskeleton, 138, 145; See: Skeleton
Energy, 114, 16, 24, 25, 27; 32, Bas
34, 37, 42, 159, 168, 197, 318; con-
servation of, 463; kinetic, 25, 32,
33, 36, 45; potential, 25, 325933.
36, 45, 498
Enteric cavity, 8, 60, 61, 71, 73, 99,
100, 264, 266, 268, 480
Enteric pouch, 264
Enteron, 76, 99, 264, 269; See: Enteric
cavity
Enterozoa, 480
Entomology, 350, 392, 416; See: In-
sects
Entomostraca, 90
Environment, 27, 285, 286, 301, 312—-
316, 372, 383, 473, See: Adapta-
tion
Enzyme, 23, 33, 41, 61, 151-159, 226,
318
Eoanthropus, 433, 434
Eocene epoch, 359-362, 430
Eohippus, 362-364
Eolith, 438
Ephyra, 75
Epidemic diseases, 391
Epidermis, 8, 103, 134, 135, 274, 280
Epigenesis, 274-280, 467
Epiglottis, 748, 162, 421
Epipharynx, 330
Epipodite, 106
Epithelium, 17, 28, 61-64, 70, 134,
151, 201, 208. 211, 216
Equatorial plate, 241
Equilibration, sense of, 210, 212
Equilibrium of nature, 335; See: Web
of life
196-198,
INDEX
Equipotential system, 278
Equus, 363, 364; See: Horse
Erepsin, 155
Erosion of soil, 479
Esophagus, 103, 146-149, 151-153
Ethics, 5
Eugenics, 314, 422-424
Euglena, 50, 477
Euplotes, 54
Euspongia, 69, 70
Eustachian tube, 273, 357
Euthenics, 314, 423
Eutheria, 128-131, 479
Evolution, 4, 131, 327, 348-385, 388,
459, 471-476
Exconjugant, 256, 257
Excretion, 25, 37, 39, 61, 134, 154,
180-187
Excretory system, 65, 164, 180-187
Exopodite, 106, 107
Exoskeleton, 96, 104, 105, 138, 145
Experimental method, 2, 219, 274,
452, 468
Eye, 79, 81, 82, 90, 93, 104, 105, 210,
2S 217, 330; 357; brush, 332:
color, 287
Eyelid, 215, 357
Fabricius, 466
Fallopian tube, 193, 194; See: Oviduct
Family, 352, 353
Fang, 358
Pat, 22. 20,.00; 39, 151, 155,156, 157,
159
Fat body, 190
Fatty acid, 22, 151, 156, 181
Feather, 97, 111, 123, 124, 134, 138
Feather star, 85, 86
Feces, 159, 180
Feeble-minded, 427
Female, 57; See: Egg and Sex
Femur, See: Vertebrate and Bee
Fermentation, 40, 41, 221, 389, 390
Fern, 361
Fertilization, 55, 57, 230, 231, 243,
249-262, 305, 340, 383, 468; signifi-
cance, 255-262
Fibula, 142, 144
Fiddler crab, 91
Fig, 416
Filaria, 80
Filial regression, 380
Filterable virus, 343
521
Fin, 110, 112, 133, 142, 143, 146, 270
Fire, 433, 439, 443
Fish, Lil, Y12=117, 1462 168. 773.
184, 194, 204, 212, 336, 359, 418,
420
Fission, 230, 233; See: Binary fission
Fixity of species, 350; See: Evolu-
tion
Flagellate, 477; See: Euglena, Mas-
tigophora
Flagellum, 28, 39, 51, 58, 59, 68, 69
Flatfish, 175
Flatworm, 78, 79, 233, 235, 396-400,
477
Flea, 93, 94, 402, 405, 406
Flight, 124
Flipper, 355
Flounder, 115
Flower, 334
Fluctuations, 379
Fluke, 78, 396-398
Fly, 219, 407-409, 411, 415
Flying dragon, 122
Flying-fish, 775, 116
Flying lemur, 322, 324
Flying squirrel, 321
Follicle, cells, 239; hair, 134
Food, 14, 26, 31-33, 36, 159, 221, 317,
318, 335, 336, 403-407; See: Me-
tabolism and Nutrition
Foodstuffs, 23, 32-36
Foot, 140, 431; See: Limb
Foraminifera, 48, 49, 386, 387, 477
Fore-brain, 201, 202, 215, 268, 269
Forest conservation, 404, 412, 418
Formaldehyde, 32
Fossil man, 431-437
Fossils, 83, 358-364, 431-437, 459
Fossorial, 327, 322
Four-o’clock, 297, 298
Fox, 321, 325,326
Free-martin, 306
Fresh-water sponge, 69, 70
Frog, 7, LY, V7-119; 18s 784,325:
circulation, 147, 173, 365; embry-
ology, 267—271; intestine, 147, 155;
metamorphosis, 267-271; muscular
system, 136; nervous system, 204,
205; pancreas and liver, 155; respir-
atory mechanism, 165; skeleton,
141; urogenital system, 190, 239
Fructose, 22
Fruit fly, 305, 306, 309, 311, 411
o22
Fuel value, 159
Fungus, 337, 338
Galapagos Islands, 372
Galen, 448, 452, 463
Galileo, 452, 462
Gall bladder, 146, 147, 152, 154, 155,
182
Galton, 288, 380, 469
Game cock, 282
Gamete 52757168, 230.) 234- 238.
243, 251-262, 291-305
Gametocyte, 52, 340
Ganglion, 101, 108, 199-216
Garpike, 115
Gastral cavity, 68-70
Gastric, glands, 153, 156; juice, 153,
156; mill, 705; vacuole, 10, 35, 36,
55
Gastropoda, 87
Gastrula, 60, 75, 87, 98, 263-268
Gazelle, 325
Geddes, 316
Gene, 243, 249, 250, 282, 290-315,
352, 353, 383, 384; mutation, 311
Genetics, 4, 281-305, 350, 377-384,
420-425, 468-470
Genital duct, See: Excretory and Re-
productive systems
Genotype, 291-305, 382
Genus, 47, 352, 353, 457
Geographical distribution, 368-373,
384
Geological succession, 358-364, 370-
371, 474
Geometrid moth, 327, 328
Germ, cells, 58, 59, 65, 188, 231, 232,
238-250, 284, 297, 397, 467, 469;
origin, 238-250; layers, 265, 268,
272,351; plasm,.232; 234. 236, 283,
284, 469
Germinal
plasm
Gesner, 450, 451
Giardia, 396
Gibbon, 321, 427-429
Gide, 67
Gill, 88, 90, 113, 114, 117, 119, 145,
146, 161, 162, 170-172, 180, 181,
270,. 364, 365: arch, 268, 358;
pouch, 161, 162; sht, 710, 111,
114) 115s 132) 133.2 145, 1465 151,
161, 162, 357, 364, 365
continuity, See: Germ
INDEX
Giraffe, 130, 285
Girdle, limb, 142-144; See: Skeleton
Gizzard, 102
Glacial epoch, 359, 435, 436, 440, 441
Gland, 63, 108, 126, 134, 148-153,
154, 159, 180, 181, 193, 32853345
357, ooo
Glanders, 391
Gliadin, 21
Globigerina, 49
Glochidium, 89
Glomerulus, 182, 186
Glossa, 330
Glossary, 492
Glottis, 147, 162, 165
Glucose, 22, 32, 182, 183
Gluten, 22
Glycerol, 23, 151, 156
Glycogen, 157, 198
Goby, 113
Goethe, 474
Goiter, 197
Goldfish, 113
Golgi bodies, 19
Gonad, 74, 171, 188-195, 229; See:
Ovary and Testis
Gonangium, 72, 73
Gonionemus, 477; See: Medusa
Gonorrhea, 391
Gonotheca, 72
Gopher, 323
Gorilla, 356, 427-429, 431
Gout, 182
Grafting, 118, 237
Grantia, 68
Grape, 389, 411; sugar, 182, 183
Grasshopper, 93, 97
Grassi, 392
Gray crescent, 268
Gray matter, 203, 207
Greeks, 2, 349, 422, 446-449, 471;
See: Aristotle
Green gland, 105
Green plant, 30-35, 463
Gregarious, 336
Grew, 454
Gristle, 139
Group conditioning, 438
Growth, 10, 25, 26, 28, 57; See: Anab-
olism
Guinea-pig, 129, 291, 294, 296
Guinea worm, 80
Gullet, 55, 56
INDEX
Gum, 135
Guppy, 113
Gustatory cell, 271
Gymnodinium, 57
Gymnura, 128, 129, 320, 321, 323, 479
Gypsy moth, 412
Habit, 347
Habituation, social, 437, 438
Hagfish, 113
Har. 15,°97, 111. 126, 134, 181
Hairworm, 80
Hales, 462
Halibut, 115
Haller, 460, 467
Hand, 130, 143, 177, 430, 431
Haploid, 245, 248, 249, 303
Hare, 325
Harpoon, 440
Harvest mite, 95
Harvey, 168, 451, 452, 454, 466, 467
Hay infusion, 43-45, 220, 317
Head, 132, 206, 330, 371
Health, 339, 389-403
Heart, 105, 133, 146-149, 158, 163,
169-179, 364, 365, 452
Heat, animal, See: Temperature
Heat sense spots, 211
Hedgehog, 128, 129, 321
Heidelberg man, 434, 438
Heliozoa, 49
Hematochrome, 32
Hemiptera, 94, 324
Hemoglobin, 166, 367; See: Blood
Hemophilia, 307, 308
Hen, 190, 251, 253, 274
Henderson, 316
Hepatic, artery, 170, 173; duct, 155;
portals 72... 173; vem, 172, 173;
See: Liver
Herbal, 450, 451
Herbivorous, 129, 357
Heredity, 3, 281-315, 377-384, 420-
425, 468-470; See: Inheritance
Hermaphrodite, 188, 189, 396, 399
Hertwig, 14, 57
Hessian fly, 409
Heteroploidy, 310
Heterozygous, 291-299, 382; See:
Hybrid
Hexuronic acid, 23, 159
Hibernation, 117
Hind-brain, See: Brain
323
Hippocrates, 448
Hippopotamus, 130
Hirudinea, 81, 82
Histology, 3, 4, 462, 464-466
History of biology, 2, 3, 218-221, 446-
476
Hive, 330, 334
Holothuroidea, 86
Holozoic, 44
Hominidae, 428, 480
Homo sapiens, 436; See: Man
Homologous chromosomes, 302-305,
308; See: Synapsis
Homologous genes, 302, 308
Homology, 107, 354-356, 459
Homothermal, 123, 126, 178, 319;
See: Temperature
Homozygous, 291-299
Honey bee, 260, 327-336, 414
Hoof, 134, 362-364
Hooke, 452
Hookworm, 80, 401
Hormone, 160, 169, 192, 193, 196-
198, 306, 367
Horn, 134
Horned-toad, 122
Horse, 321, 352, 355, 361-364
Horse botfly, 94, 405
Host, 52, 339, 340, 342; See: Parasite
House fly, 93, 375, 407
Human, body composition, 21; hand,
143, 431; heredity, 283, 285-288,
299-301, 306-315; origin, 426-445;
posture, 143, 431, 435, 436; skele-
ton, 356; welfare, 386-425; See: Man
Humerus, 141, 142, 144, 355
Hummingbird, 126
Huxley, 5, 227, 228, 251, 263, 349,
358, 361, 459
Hyaloplasm, 18, 19
Hybrid, 290, 314, 382-384
Hydatid, 400
Hydra, 8, 71, 73, 98-100, 150, 188,
189, 199, 209; 210, 237, 233,290;
337, 344, 477
Hydranth, 73
Hydrolysis, 151
Hydrozoa, 71-73, 234
Hymenoptera, 94, 478; See: Bee
Hyoid, 142
Ice age, 359; See: Pleistocene epoch
Ichneumon fly, 474
024
Ideal vertebrate, 132, 133
Iguana, 122
Iliac artery, 171
Ilium, 141, 142, 357
Imbecility, 197, 198, 421
Immunity, 286, 342, 343
Inbreeding, 289, 297, 383
Inch-worm, 327, 328
Incomplete dominance, 299
Incomplete metamorphosis, 97
Incubation, 253
Incus, 273
Independent assortment,
300, 315
Individual, 27, 29.73, 232, 350; 375
Infantile paralysis, 391
Influenza, 391
Infundibulum, 202, 203
Infusoria, 44, 47, 53-56, 477; See:
Paramecium
Ingenhousz, 463
Inhalation, 164
Inhalent siphon, 88
Inheritance, 281-315, 377-384, 420-
425, 468-470; See: Genetics
Inner ear, 272, 213
Insanity, 217
Insect, 91-94, 161, 162, 324-335,
340, 372, 392, 404-413, 453, 478;
beneficial, 413-417; injurious, 405-
413, 417, 420
Insectivora, 128, 129, 427, 430, 479
Instinct, 346
Insulin, 197, 367
Integumentary system, 65
Intelligence, 343, 347
Internal secretion, 159; See: Hor-
mone ,
Intersex, 306
Interstitial growth, 25
Intestinal juice, 155
Intestine, 64, 105, 146-149, 171, 441
Invention, 444
Invertebrates, 66-109, 188, 214; See:
Hydra, Earthworm, and Crayfish
Involuntary muscle, 139
Iodine, 197
Iris 2152 ta
Tron age, 444
Irritability, 26, 27, 64, 344, 347; See:
Nervous system
Ischium, 142
Islands, 372
292-297,
INDEX
Japanese beetle, 409
Java man, 432, 433
Jaws, 112, 141, 151; See: Skull
Jellyfish, 72-75
Jennings, 346
Jerboa, 327
Jigger flea, 405
Jimson weed, 310
Johannsen, 380
Jugular vein, 171
Julus, 92
Jurassic period, 359-361
Kallikak family, 427
Kallima, 326, 327
Kangaroo, 127, 321
Karyolymph, 20
Katabolism, 25, 182; See: Metabolism
Katydid, 325
Kidney, 146-149, 168, 172, 180, 182-
187, 192, 194; evolution, 183-187
Kinetic energy, 14, 25, 502; See:
Energy
King crab, 95
Kingsnake, 123
Kircher, 218
Kissing bug, 94
Kiwi, 124
Knee cap, 142, 144
Koala, 127
Koch, 391
Labellum, 330
Labial palp, 330
Labium, 93
Labrum, 93, 330
Labyrinth, 212, 213
Lacewing fly, 94
Lacrymaria, 54
Lactase, 156
Lacteal, 157, 158
Ladybird beetle, 414
Lagena, 212
Lamarck, 3, 46, 286, 373-375, 458,
472-474.
Lamprey, 113, 252
Lamp-shell, 83
Laplace, 98
Larva, 397, 398, 407, 475
Larynx, 149, 162, 358
Lateral, line, 210; plate, 269
Laveran, 392
Lavoisier, 461
INDEX 229
Leaf, 7, 8
Leaf butterfly, 326, 327
Leech, 81, 82
Leeuwenhoek, 453
Leg, of insects, 93, 330, 334; See:
Limb
Leishmania, 396
Lemur, 130, 131, 322, 324, 426, 427,
480
Lens, 274-216
Lepidoptera, 94; See: Butterfly and
Moth
Lepisma, 92
Leprosy, 391
Leucosolenia, 68
Lice, 405
Lichen, 337, 338
Liebig, 463
Life, 1, 130, 229, 402; charcteristics,
1, 18-28; continuity, 229-250; defi-
nition, 23, 29; origin, 218-228
Ligament, 139
Limb, 112, 117, 118, 134, 142-144;
structure, 327, 322, 354-357; See:
Leg
Limpet, 87
Limulus, 95
Lincoln, 424.
Lineus, 233, 236
Lingula, 83
Linin, 19, 20
Linkage, 307-309
Linnaeus, 350, 426, 456, 457
Lionotus, 54.
Lipase, 155, 156
Lister, 389
Lithobius, 92
Liver, 110, 146-149, 152, 154, 155,
174 NZ NSO? VB 2, 197
Liver fluke, 121-123, 234, 396-398,
477
Lizard, 111, 120-122, 365, 367
Lobster, 90, 106
Lockjaw, 318
Locomotion, See: Movement
Locust, 93, 97
Louse, 94
Lumbricus, See: Earthworm
Lung, 116, 133, 145, 147-149, 151,
152, 162-166, 168, 180, 181, 192,
358, 454, 461
Lung-fish, 113, 716, 479
Lyell, 474
Lymph, 64, 169, 176, 177; vessels,
157, 158. 177:
Lysin, 342
Macaque, 428
Mackerei, 113, 174
Macrogamete, 340
Macronucleus, 44, 54-56, 257-259
Madreporite, 84
Maggot, 219
Maintenance, 25
Malacostraca, 90
Malaria, 53, 339, 340, 390-393, 396,
407
Male, 57; See: Gamete and Sex
Malleus, 213 -
Malpighi, 454, 462, 466, 467
Malta fever, 391
Maltose, 151, 156
Mammal, 111, 126-131, 148, 173,
$93, 252, 319,. 353, 359-361, 365"
419, 426-431; adaptation, 320-324
Mammary gland, 193, 357
Mammoth, 441
Man, 130, 135, 143, 149, 163, 175, 177,
355, 365; embryo, 193, 273, 365;
fossil, 431-437; inheritance, 250,
307-313, 421-423; muscles, 137;
skeleton, 144; urogenital system,
186, 193, 194; See: Human, Mam-
mal, and Vertebrate
Manatee, 129
Mandible, 93, 105, 106, 142, 330;
See: Jaws
Mantis, 94
Mantle, 87, 88
Manubrium, 73, 75
Marmoset, 427
Marsh, 361
Marsupial, 727, 128, 479
Mastigophora, 44, 47, 50-52, 477
Mastodon, 371
Mathews, 196
Matrix, 62, 63, 139
Maturation, 243, 244-247, 249, 255,
304, 340
Maxilla, 93, 142, 144, 330
Maxilliped, 105, 106
Mayfly, 94
Measles, 343
Mechanism of inheritance, 301-312
Medicine, 6, 389-403, 448, 450
Medieval science, 449, 450
526
Mediterranean fruit fly, 477
Medulla, 147-149, 165, 202-205
Medullary plate, 269
Medusa, 71-74, 234
Megalithic monuments, 443
Meiosis, 244
Membrane bone, 141
Membranous labyrinth, 212, 213
Memory, 347
Mendel, 288, 297, 469, 470; See: In-
heritance and Genetics
Mendel’s laws, 297
Menengitis, 391
Mental life, 217, 347, 431, 437, 444
Merozoite, 340
Merychippus, 362, 363
Mesenterial filament, 76
Mesentery, 133, 134, 147, 148, 153
Mesoblast, 265, 266
Mesoderm, 60, 61, 100-103, 134, 237,
238, 264-269, 272; bands, 265, 266
Mesogloea, 99, 100
Mesohippus, 362, 363
Mesolithic culture, 441, 442
Mesonephric system, 133, 184-186,
190, 194
Mesozoic era, 359-361, 430
Metabolism, 18, 23-26, 159, 180-183,
316-318; See: Anabolism and Ka-
tabolism
Metacarpal, 142, 144, 355
Metals, age of, 444
Metamorphosis, of frog, 267-271; of
insects, 96, 97, 406-410, 412, 413,
415, 416
Metanephric system, 96, 117-120,
184-187
Metaphase, 240, 241
Metaplasm, 19; See: Yolk
Metatarsal, 742, 144
Metatheria, 479
Metazoa, 66, 258, 260, 480
Metridium, 75, 76
Microbe, 38, 396; See: Bacteria
Microcosm, 43-45, 317; See: Web of
life
Microgamete, 340; See: Sperm
Micronucleus, 44, 54-56, 257, 259
Microérganisms and disease, 38, 39,
389-396
Microscope, 15, 452-454, 466
Mid-brain, 201, 202, 268, 269
Middle Ages, 349, 449
INDEX
Middle ear, 273, 358
Milk, 128, 153, 156; souring, 389
Millipede, 91, 92
Mimicry, 328, 329
Mind, 217, 347
Miocene epoch, 359, 362, 363, 430
Miracidium, 396, 397
Mite, 95, 219
Mitochondria, 19
Mitosis, 13, 239-243, 254, 304
Mitral valve; See: Heart
Mixed nerve, 208
Moccasin, 123
Modifications, 285, 286, 288, 289,
313-315, 374, 380-384, 473, 474
Modifying genes, 300
Mold, 40
Mole, 323
Mollusc, 86-90, 210, 216, 276, 478
Monad, 44
Monkey, 130, 131, 367, 426-431, 480
Monocystis, 52
Monograph, 451
Monohybrid, 289-292
Monosiga, 57
Monotreme, 127, 190, 479
Moore’s theory, 225
Morgan, 311
Morphology, 3, 4; See: Anatomy
Mosaic inheritance, 287, 298-301
Mosquito, 53, 339, 340, 375, 392, 393
Moss, 361
Moss-animal, 83
Moth, 412-416
Motorium, 199
Motor nerve, 201, 206, 207
Moult, 96
Mouse, 129, 218
Mousterian culture, 439
Mouth, 132, 149, 152, 156, 162
Movement, 27, 28; See: Tropism
Mucus, 151
Mud-puppy, 118
Mud turtle, 727
Mulatto, 299, 300
Miller, 461
Multicellular organism, 7, 57-66; See:
Metazoa
Multiple genes, 299, 300
Muscle, 27, 62-64, 102, 103, 133,
135-138, 165, 201, 215; cells, 17,
28, 138
Muskrat, 323
INDEX
Mussel, 89
Mutation, 285, 287, 308-312, 315,
329, 372, 381-384
Mycelium, 337
Myoneme, 44
Myosin, 21
Myotome, 110
Myriapoda, 91, 92
Myxedema, 197
Nail, 15, 134
Nares; See: Nostril
National Academy of Sciences, 424
Natural history, 2, 3, 349
Natural selection, 285, 374-377, 381-
385, 438, 473-476; See: Selection
Nature and nurture, 312-315, 444;
See: Environment
Nautilus, 86
Neanderthal man, 427, 435, 436, 439,
440
Nectar, 334
Necturus, 178
Needham, 220
Negro, 299, 300
Nemathelminthes, 79, 80, 477
Nematocyst, 71, 76
Nematode, 400, 401, 477; See: Nema-
thelminthes
Neolithic culture, 442-445
Nephridiopore, 102, 103, 183
Nephridium, 102, 103, 183, 186, 194,
397
Nephrostome, 102, 103, 183
Nereis, 81, 82, 247
Nerve, 11, 64, 108, 134, 135, 200-216,
319; cell, 62, 64, 199-217; cord,
101-105, 108, 145; fiber, 200, 211,
216; impulse, 206; net, 199, 208
Nervous system, 65, 108, 168, 199-
217, 269, 344; See: Nerve
Neural, arch, 139, 140; groove, 201,
268, 269; spine, 139, 149; tube,
145, 201, 202
Neurenteric canal, 268
Neurology, 198 ‘
Neuromotor system, 55, 199
Neuromuscular mechanism, 199
Neuron, 11, 62, 64, 199-217
Neuroptera, 94
New stone age, 442
Newt, 117, 118
Nictitating membrane, 357
027
Nitella, 28
Nitrate, 33, 40, 41
Nitrite, 41, 42
Nitrogen, 22, 41, 42, 159, 226, 403,
404, 417
Nitrogen-fixing bacteria, 47, 42, 404.
Nitrogenous waste, 181-185
Non-disjunction, 310, 311
Nosema, 416
Nostril, 132, 147-149, 162, 165,
Pal Uf
Notochord, 110-112, 133, 135, 138,
139, 145, 268, 269
Noyes, 180
Nucleolus, 79, 20, 247
Nucleus, 10-12, 18, 19, 20, 27, 35, 41,
200, 239, 241, 247, 280, 390, 469
Nurture, 312-315, 423, 424
Nutrition, 30-45, 460; See: Metabo-
lism
Nutritional chain, 336; See: Web of
life
Oak, 408
Obelia, 72, 73, 234
Obliterative coloration, 327
Ocellus, 93, 330
Octopus, 89
Oil, 386, 388
Old stone age, 438-442
Olfactory, cells, 271; lobes, 147, 148,
202-205; nerves, 146; pouches, 211;
pit, 270
Oligocene epoch, 359, 362, 363, 430
Oligochaeta, 81, 477
Ontogeny, 366; See: Embryology
Odcyst, 340
Odcyte, 245-247
Odgenesis, 245-248, 305 ;
Odgonium, 239, 243-245, 275
Oodkinete, 340
Operculum, 114, 115, 270, 271
Opossum, 127
Opsonin, 343
Optic, cup, 214; capsule, 142: lobe,
146, 147, 203, 204; nerve, 146, 148,
205, 214-216; vesicle, 214, 215
Oral arm, 74
Orang-utan, 427, 428, 431
Orchid, 334
Order, 352, 353
Order of nature, 227, 388
Organ, 3, 65, 104; systems, 65, 104
528
Organ-forming substance, 277-279
Organic adaptation, 316-337
Organic evolution, See: Evolution
Organism, 14, 18, 29
Organization, 18-20
Origin, of life, 218-228; of germ cells,
238-250; of species, 349-385, 471-—
476
Ornithology, 124
Ornithorhynchus, 127
Orohippus, 362, 363
Orthoptera, 94
Osborn’s theory, 226, 227
Osculum, 70
Osmosis, 16, 157, 505
Osteology, 139; See: Skeleton
Ostium, 68, 70, 76
Ostrich, 124
Otter, 323
Outer ear, 213
Ovary, 99, 101, 102, 188-195, 238, 239
Oviduct, 102, 184, 189-195
Oviparous, 123
Owen, 458, 459
Owl, 325
Ox, 300
Ox botfly, 405, 406
Oxidation, 23, 24, 42, 165, 166, 197
Oxygen, 165, 166, 463
Oxyhemoglobin, 166
Oyster, 88, 89, 236, 306
Pain sense spots, 211
Paleolithic, art, 437, 440, 441; cul-
ture, 438-441; man, 433-441
Paleontology, 4, 358-364, 368
Paleozoic era, 359-361
Panama Canal, 394
Pancreas, 133, 146-148, 152, 154, 155,
171, 196, 197
Pancreatic juice, 155
Paralysis, 394
Paramecium, 53-56, 199, 229, 230,
235, 255-260, 477; behavior, 344—
346; conjugation, 55, 230, 255-260;
endomixis, 55, 258, 259; heredity
284; irritability, 344-346; repro-
duction, 56, 256, 375; species, 54;
structure and life history, 53-55
Parasite, 51, 53, 79, 220, 234, 339-
343, 396-401, 407
Parazoa, 480
Parrot, 126
INDEX
Parthenogenesis, 234, 255, 260, 261;
artificial, 260, 261
Pasteur, 227, 389, 390, 414, 454, 455
Patella, 142, 144
Pathology, 4
Pea, inheritance, 288-297
Peach, 416
Pear, 416
Pearl, 89, 90
Pebrine, 414
Pecten, 331, 333, 334
Pectoral girdle, 142-144
Pedicellaria, 84
Pedigreed race of Paramecium, 258
Peking man, 433, 438
Pelecypoda, 87, 88
Pellagra, 159
Pelvic girdle, 142-144, 323
Penguin, 126
Penis, 148, 194
Pentadactyl limb, 140-144, 321
Pepsin, 153, 156
Peptone, 156
Peranema, 51
Perch, 113, 146]
Pericardial cavity, 133
Pericardium, 133, 148
Periosteum, 135
Peripatus, 97, 478
Perisarc, 72
Peristalsis, 153, 156
Peristome, 55
Peritoneum, 64, 134
Perspiration, 178, 181, 183, 198
Petromyzon, 113
Peyer’s patches, 342
Pfliiger’s theory, 224, 225
Phacus, 57
Phagocyte, 342
Phalanges, 142-144, 355
Pharynx, 102, 110, 132, 133, 151, 152,
358
Phenotype, 291-296, 382
Philosophy, 5, 385, 447
Phosphorescence, 51
Photosynthesis, 32, 33 50, 317, 337,
338, 403, 463
Phylogeny, 366, 480
Phylum, 47, 353, 477-480
Physalia, 73
Physical basis of life, 14-29, 465; See:
Protoplasm
Physics, 1, 2, 5, 460, 464
INDEX
Physiological division of labor, 10,
58, 65, 232-234, 330, 336
Physiologus, 449
Physiology, 3, 4, 366-368, 389, 449,
454, 459-464
Pig, 130, 398, 400, 401
Pigeon, 362, 376
Pigment, 11
Pile village, 442
Pill-bug, 90
Piltdown man, 433, 434, 438
Pine bark beetle, 412
Pineal body, 202-204
Pinna, 213
Pisces, See: Fishes
Pithecanthropus, 432, 433
Pituitary gland, 193, 198
Placenta, 128, 192, 193, 272, 273
Placental, 128-131, 321, 479
Plague, 402, 406, 407
Planaria, 78, 79, 396; See: Flatworm
Plant, 30-35, 37-45, 404, 408-412,
417, 462, 463
Plantigrade, 321, 322
Plasma, 64, 166, 169
Plasma-membrane, 19
Plasmodium, 53, 340, 396, 477
Plastid, 19, 30, 32; See: Chloroplast
Plastron, 127
Plato, 447
Platyhelminthes, 78, 79, 477, 480
Pleistocene epoch, 359, 363, 364, 368,
432
Pleura, 163
Plexus, 205, 206
Pliny, 449
Pliocene epoch, 359, 362 363, 368
Pliohippus, 362, 363
Ploidy, 310
Plum, 416
Plumatella, 83
Pneumonia, 3917
Podura, 92
Point change, 311
Poison gland, 357
Polar lobe, 376
Pollen, 333, 334
Pollen basket, 337, 333, 334; brush,
331-333; comb, 331, 333
Pollination, 334, 416
Polocyte, 243, 245-247, 252, 304
Polychaeta, 81, 477
Polymorphism, 73, 234
229
Polyp, 71-78, 98, 99
Pond community, 335
Population, 378-381
Porcupine, 129
Porifera, 67-71, 477, 480
Pork, infected, 400, 407
Porpoise, 129, 323, 357
Portal vem, 157, 758, 172; 173
Porto Santo rabbit, 372
Portuguese man-of-war, 73
Position effect, 311
Potato beetle, 94, 409
Potential energy, 14, 22, 507; See:
Energy
Pottery, 442
Precipitin, 343, 367
Preformation, 274, 280, 453, 467, 470
Pregnancy, 191—193, 273
Prehistoric man, 431—437
Prehuman lineage, 426-431
Pressure, 320; receptors, 210
Preventive medicine, 389
Priestley, 463
Primary, germ layers, 61; tissues, 61,
104.
Primate, 128, 130, 131, 352, 426-431,
480
Primordial germ cells, 238, 305; See:
Germ cells
Probability, theory, 227
Proglottid, 398, 399
Pronephros, 133, 184, 185, 270
Prophase, 240, 241
Prorodon, 54
Prosopyle, 70
Prostate gland, 148, 194
Prostomium, 82
Protective mimicry, 328
Protein, 21—23, 33, 39, 151, 155-159,
220
Proteose, 156
Proterozoic era, 359-361
Prothorax, 330, 331
Protococcus, 30-35
Protophyta, 35
Protoplasm, 3, 10, 14-29, 388, 464,
465; appearance, 17; composition,
20-23; concept, 15; organization,
18-20, 254, 275-280; structure, 17
Protopodite, 106, 107
Prototheria, 479
Protozoa, 35-56, 209, 223, 229, 232,
244-246, 248, 255-261, 284, 318,
530
319, 335, 336, 341, 342, 386, 387,
390-394, 396, 407, 416, 420, 453,
477, 479; See: Amoeba and Para-
mecium
Protozodlogy, 46, 393; See: Protozoa
Pseudopodium, 10, 35, 48
Psychology, 4, 5, 423
Psychozoic era, 359
Pterodactyl, 120
Ptyalin, 152, 156
Pubis, 140, 142
Public health, 6, 339, 389-403
Pulmonary circulation, 173-176
Pulvillus, 332, 333
Pupa, 97, 392, 406, 407, 410, 412
Pupil, 215, 217
Pure line, 380-382
Pure science, 386, 388
Purity of gametes, 302; See: Segrega-
tion
Purpose in Nature, 385
Putrefaction, 21; See: Decay
Pyloric, caeca, 84; sac, 84; valve, 153
Pylorus, 155
Pyramids, 49
Python, 123, 357
Quahaug, 88
Queen bee, 329, 330
Quinine, 396
Rabbit, 129, 372
Rabies, 343, 390
Radial, canal, 72, 74; symmetry, 71,
18,99
Radio, 210
Radiolaria, 49
Radius, 142, 144, 355
Radula, 87
Ramapithecus, 430, 431
Rana, See: Frog
Rat, 129, 367, 401, 402, 407
Ratitae, 124.
Rattlesnake, 123, 358
Ray, 113, 114
Ray, John, 456
Reason, 347
Reaumur, 460
Recapitulation theory, 96, 364-366,
468
Receptaculum chyli, 158
Receptor, See: Sense organ
Receptor-effector system, 199
INDEX
Recessive character, 290-297, 383
Recombination, 287, 288, 314, 315,
380-384.
Rectum, 152, 268
Redi, 219, 220, 453
Redia, 397, 398
Reduction of chromosomes, 244, 245,
247, 305; See: Maturation
Reed, 393
Reef, 77
Reflex action, 201, 207, 208, 346,
347
Regeneration, 67, 118, 233, 235-237
Rejuvenation, 256-260
Relapsing fever, 391
Renaissance science, 349, 449-451
Renal, artery, 171, 185, 186; portal
system, 171-173; vein, 171, 185,
186
Rennin, 153, 156
Reorganization, 229
Repair, 10; See: Regeneration
Reproduction, 12, 26, 57, 65, 184,
188-195, 229-258, 262; power of,
aD
Reproductive organs, 184; See: Re-
production
Reptile, 111, 120-123, 204, 359, 360,
367, 430
Research, 383, 386, 387, 394, 402-
404, 417, 423-425
Resorption, 271
Respiration, 33, 34, 37, 134, 145, 161—
167, 181, 461, 463
Respiratory center, 165, 205; inter-
change, 165, 166; mechanism, 164,
165
Resting cell, 240
Retina, 213-216
Reversion, 287
Rhagon, 69, 70
Rhinoceros, 130
Rib, 133, 142, 144, 164
Rickets, 160
Roan cattle, 298
Rodentia, 129, 352, 353
Rods and cones, 2176, 217
Roman science, 448, 449
Roosevelt, 418
Ross, 392
Rotifer, 82, 83, 260, 477
Roundworm, 79, 80, 260, 400, 401
Rye, 409
INDEX
Sacculus, 272, 213
Salamander, 117, 118, 235
Salientia, 117-119
Saliva, 151, 156
Salivary gland, 151, 152
Sandworm, 81, 82, 247, 477
San José scale, 410, 411
Saprophyte, 43, 44
Sarcocystis, 396
Sarcodina, 47-49; See: Amoeba
de Saussure, 463
Scale-insect, 410-414
Scale of nature, 426
Scales, 134, 138
Scallop, 86, 87
Scapula, 142, 144
Schaudinn, 394
Schistosoma, 396
Schizogony, 340
Schleiden, 464, 465
Schultze, 465
Schwann, 254, 464-466
Sciatic plexus, 205
Science, scope, 1,
388
Scientific method, 2, 228, 464.
Sciurus, 148, 353
Sclerotic coat, 216
Scolex, 398-400
Scorpion, 95
Scorpion-fly, 94
Scrotum, 148
Scurvy, 23, 159
Scyphistoma, 75
Scyphozoa, 73-75
Sea anemone, 75, 76
Sea cucumber, 85, 86
Seahorse, 175
Seal; 129, 321, 323, 367, 419
Sea lily, 85, 86
Sea pen, 77
Sea urchin, 85, 86, 277, 279, 478
Sea walnut, 78
Sebaceous gland, 134, 181
Secondary sexual characters,
262; See: Sex
Secretin, 160
Secretion, 61, 63, 134, 159, 180
Seed plants, 221, 359, 361, 372, 417,
457
Segment, 81, 103, 707, 108
Segmentation, 81, 82, 90, 96, 97, 101-
108, 112, 185, 206
24, 385, 386,
192,
del
Segmented worm, 81, 82, 97; See:
Earthworm
Segregation, 244, 246, 290-297, 300,
302, 5315;,510
Selection, 282, 376-384, 473, 475, 476;
See: Natural selection
Self-digestion, 156
Semicircular canal, 272, 213
Seminal receptacle, 103; vesicle, 103,
194
Seminiferous tubule, 194
Senile degeneration, 256-259
Sense organ, 65, 74, 143, 199, 201, 207,
209-217, 344
Sensory nerve, 201, 206, 207
Septum, 101, 102
Serial homology, 356, 357; See: Ho-
mology
Setae, 81, 102, 103
Sex, 188, 189, 192, 254, 255, 261, 262,
284, 306-310; determination, 304—
307; differentiation, 306; linked,
307, 308; reversal, 306; See: Fer-
tilization and Reproduction
Shakespeare, 449
Shark, 173, 114, 171, 365
Sheep, 130, 283, 396-398
Shell, 86-88
Shell-fish, 86
Shell-heaps, 441
Shipworm, 88
Shorthorn cattle, 298
Shoulder blade, 144
Silkworm, 414, 416, 454
Silver-fish, 92, 94
Simiidae, 427-429
Simple eye, 330
Simplex group, 245, 248, 249, 261, 306
Sinanthropus, 433
Sinus venosus, 170, 171, 173
Siphon, 88, 89
Sirenia, 129, 130
Skate, 114
Skeleton, 63, 77, 138-144, 322, 323,
355-357, 435
Skin, 7, 134, 135, 161, 180, 181, 207,
319
Skull, 112, 141-149, 432-437
Sleeping sickness, 341, 391, 407
Sloth, 129, 321-323
Slug, 87
Small intestine, 752-154
Smallpox, 343
o32
Smell, sense of, 210, 277
Smyrna fig, 416
Snail, 86, 87, 235, 370, 396-398
Snake, 120-123, 327, 357, 358, 365,
367
Snapping turtle, 727
Social habituation, 431, 437
Social heredity, 314, 444
Sociology, 4, 5, 286
Soil, 82, 403, 416-419
Solar energy, 32-34, 45, 224, 226
Soma, 58, 59, 231, 232, 236, 243, 284,
383, 469; See: Germ plasm
Somatic, cells, “243; mesoderm, 265,
266; mutations, 312
Song sparrow, distribution, 369
Spallanzani, 220, 460
Span of life, 375, 402
Special creation, 222, 350, 351
Species, 46, 47, 349-354, 369, 376,
456, 457; number, 47, 477-479;
origin, 349-385, 471-476
Spencer, 23, 132
Sperm, 177 57, 59; 491. 230,231, 247,
251, 252, 340, 453, 468; See: Gam-
ete
Spermatic fluid, 194
Spermatid, 245
Spermatocyte, 244, 245
Spermatogenesis, 244-248, 305
Spermatogonium, 239, 243-245, 304,
306
Spicule, 68, 69
Spider, 95, 478
Spinal, cord, 170-112, 133, 135, 145,
146-149, 200-216; nerve, 205-207
Spiny anteater, 127
Spiracle, 93, 271
Spirillum, 38, 39
Spirostomum, 54
Splanchnic mesoderm, 265, 266
Spleen, 146-148, 178
Splint bone, 357, 362-364
Spondylomorum, 58
Sponge, 67-71, 477, 480
Spongilla, 69, 70
Spontaneous’ generation,
399, 454
POLE, 39, D2) Das 223; 200, 319, sav
Sporocyst, 397, 398
Sporozoa, 47, 52, 53, 229, 230, 340,
AT7
Sporulation, 230; See: Sporozoa
218-221,
INDEX
Springtail, 92, 94
Spur, 331, 333, 334
Squamata, 120, 127
Squeteague, 336
Squid, 89, 90, 215, 336
Squirrel, 129, 148, 321, 322, 352-354,
407
Stable fly, 94
Stag, 192
Stapes, 273
Starch, 22, 33, 155
Starfish, 17, 83, 84, 236, 478
Statoblast, 83
Statocyst, 72
Stature, human, 379
Stentor, 54
Sterilization, 220
Sternum, 139, 142, 144, 149
Stick-insect, 327, 328
Stigma, 50, 51, 59
Stimulus, 27, 64, 165, 209, 217; See:
Sense organs
Sting of bee, 333
Stomach, 105, 146-149, 152-156
Stone age, 438-444
Stonefly, 94
Stonehenge, 443
Stork, 126
Strawberry, 416
Stream of life, 229
Strobila, 75
Struggle for existence, 329, 336, 375,
376, 472-476; See: Web of life
Sturgeon, 115
Styela, 275, 279, 478
Stylonychia, 54
Subclass, 354
Subcutaneous tissue, 134
Subesophageal ganglion, 102, 105-108
Subgenital pit, 74
Subspecies, 354, 369
Sucrase, 156
Sugar, 22, 32, 33, 156, 182;°183, 197,
318
Sun animalcule, 49
Sunlight, 32, 33, 45
Supporting tissue, 62, 63, 65, 139;
See: Cartilage, Skeleton, etc.
Suprascapula, 741
Surgery, 221, 389, 444
Survival of the fittest, 377, 475, 476;
See: Web of life, Evolution
Swammerdam, 453
INDEX
Swan, 424
Swarming of bees, 330
Sweat gland, 181, 357
Sweetbread, 154
Swiss lake dwellings, 443
Sycon, 69, 70
Sylvius, 460
Symbiosis, 337, 338
Symmetry, 71, 99, 104, 132
Sympathetic coloration, 327
Synapse, 200, 207
Synapsis, 244, 245, 249, 302-304, 308
Synkaryon, 254-262
Syphilis, 286, 390, 394, 395
Tachina fly, 408
Tactile organ, 143, 210
Tadpole, 111, 117, 270, 271
Taenia, See: Tapeworm
ans? 133
Tapeworm, 78, 396, 398, 399, 400, 477
Tapir, 130, 368
Tarsius, 426, 427
Tarsus, 142, 144; of bee, 331-333
Taste, 210, 271
Taxonomy, 4, 455-459; See: Classifi-
cation
Teat, 128
Telencephalon, 202, 203
Teleost, 175, 116
Telophase, 240—242
Telson, 105
Temperature, 318, 319; body, 181,
223; on inheritance, 303; sense, 178
Tendon, 139
Tennyson, 281
Tentacle, 71-74, 76, 82, 87
Termite, 94
Testis, 99, 147, 148, 188-195, 238, 239
Testudinata, 120, 121
Tetanus, 318, 391
Tetrad, 244, 245
Textiles, 442
Thalassicolla, 49
Theophrastus, 2, 447, 448
Thomson, 7, 161. 316, 384, 386, 426
Thoracic duct, 157, 158, 177
Thorax, 93, 106, 134, 163, 330, 331
Thrips, 94
Throat, 151; See: Pharynx
Thrush, 372
Thumb, 143, 431
Thymus gland, 148, 152, 160
D333
Thyroid gland, 152, 160, 197
Thyroxine, 197, 198
Tibia, 142, 144, 331-333
Tick, 95
Times, New York, 446
Tissue, 3, 61-65, 100, 104
Tissue fluid, 176-178
Toad, 118, 119, 325, 365
Toe, 142, 361-364, 431
Tongue, 151, 211, 358
Tonsil, 152
Tool, 143, 330, 336, 433, 436, 438-444
Tooth, 129, 134, 135, 142, 144, 358,
363, 371, 430
Torpedo, 114
Tortoise, 120, 127; shell, 127
Totipotent, 278
Touch sense, 143, 210
Toxin, 343
Trachea, 90, 147-149, 161-164
Trachelomonas, 51
Tradescantia, 28
Training, 3173; See: Education
Transfusion, blood, 367
Translocation, 310
Transverse process, 139, 140
Tree, 412, 418
Tree frog, 119
Trematode, 78, 396-398
Trench fever, 406
Treponema, 394, 395
Treviranus, 474
Trial and error, 345
Triangle of life, 314
Triassic period, 359, 360, 363
Triatoma, 94
Trichinella, 80, 400, 401
Trichocyst, 55
Trichomonas, 57, 396
Trichurus, 396
Tricuspid valve, 174
Trigeminal ganglion, 205
Trihybrid, 294-296
Trilobite, 359
Triploblastic, 78
Triturus, 118
Trochanter, 331, 332
Trochelminthes, 83
Troland’s theory, 226
Trophozoite, 52
Tropism, 253, 343
Trypanosome, 51, 341, 342, 396
Trypsin, 155
o34.
Tsetse fly, 341, 342
Tube feet, 84
Tuberculosis, 390, 391, 407
Tubeworm, 81
Tunicate, 145, 478
Turbellaria, 78
Turtle, 120, 121, 367
Tympanic membrane, 273
Tyndall, 222
Typhlosole, 102
Typhoid, 390, 391, 407
Ulna, 142, 144, 355
Umbilical cord, 192, 193, 272
Underwing moth, 326
Undulant fever, 391
Unguiculate, 129
Ungulata, 129, 130, 352
Unguligrade, 321, 322
Unicellular, 7, 28, 30-56; See: Pro-
tozoa and Bacteria
Uniformitarian doctrine, 377, 471
Uniparental, 57; See: Asexual
United States National Museum, 387
Unity of life, 6, 131, 351, 388, 424,
425, 445
Urea, 37, 41, 181, 182, 192
Ureter, 147, 149, 182, 185, 186, 190,
194
Urethra, 148, 149, 182, 194
Uric acid, 37, 181, 182
Urinary bladder, 133, 146-149, 182,
185, 194
Urine, 183, 186, 197
Urogenital system, 159, 784, 193-195,
351, 358
Urostyle, 141, 147
Uterus, 184, 190-194, 252, 273, 399
Uterus masculinus, 148
Utriculus, 272, 213
Vaccination, 343
Vacuole, cell, 79; contractile, 10, 27,
Jd; SAStric: 10.750, 00
Vagina, 194
Valve, 87, 88, 170, 173, 174, 176
Van Helmont, 218
Variability curve, 378, 379
Variation, 250, 261, 281-315, 374—
385, 473-476
Varieties, 354.
Vascular system, 169-179, 185, 186,
192; See: Circulation
INDEX
Vas deferens, 194
Vasomotor, center, 205; nerve, 177
Vegetal pole, 267, 268
Vein, 148, 158, 170-179, 185, 186
Velum, 337, 332
Venous system, 170
Ventral, ganglion, 201; nerve root,
201, 207, 208
Ventricle, 170-173
Vermiform appendix, 149, 152, 357
Vertebra, 138, 139, 142, 144, 146-149,
BIT
Vertebral canal, 139, 141, 145, 149,
350
Vertebral column, 111;
eton
Vertebral plate, 269
Vertebrate, 66, 111-131, 252, 253;
body plan, 132-149, 185, 189, 353;
characters, 112, 145; See: Mammal
and Man
Vesalius, 450, 452, 459
Vestigial organs, 323, 357, 358, 362-
364.
Villus, 157, 208
Virginia opossum, 127, 128
Virus, 343, 393
Vitalism, 24,
464
Vitamin, 23, 159, 160
Vitreous, chamber, 274-216; humor,
215
Viviparous, 123
Vivisection, 449
Vocal cords, 162
Voice, 162
Volant, 322
Voluntary muscle, 137, 138
Volvox, 58, 59, 67, 230-232, 477
Vomer, 7141
Vorticella, 44
See: Skel-
227, 347, 385, 463,
Walking-stick insect, 327, 328
Wallaby, 128
Wallace, 475
Wasp, 327, 336, 415
Water, 21, 383
Water-flea, 80, 97
Water-vascular system, 84
Wealth, 389
Weasel, 326
Web of life, 43, 91, 335, 336, 386, 388,
394, 415, 420
INDEX
Weevil, 409, 410
Weismann, 232, 286, 468, 469
Whale, 90, 128, 129, 323, 355, 419
Wheat, 283, 310, 409
Wheel, 443
Wheel-animalcule, 83
White matter, 203, 207 .
White rat, 367
Wing, 107, 355
Wolf, 321, 336
Woodchuck, 322
Woodcock, 126
Wood-lice, 90
Woods Hole Marine Biological Lab-
oratory, 423
Working hypothesis, 4, 463
Worms, See: Earthworm, Flatworm,
etc.
030
X chromosome, 304-311, 383
X-rays, 312
Y chromosome, 304-311
Yale University Museum, 361
Yangtze, 418
Yeast, 40, 41, 230, 317, 318
Yellow fever, 391-395, 407
Yolk, 251-253, 267, 268; plug, 268,
269; sac, 272
Youth, 25
Zona pellucida, 273
Zodgeography, 368-373
Zoology, 3, 4
Zygomatic arch, 142
Zygote, 52, 57, 59, 188, 190, 230, 237,
238, 243, 245-247, 290-305, 340,
468
_ .
‘
a@
aa
s -
— ’
@e.
«
_
a
as
_—
~
I
= ee -
————— ae na %4
f ; f
a
roe at
La Be ng
se
x
* 4
o% F €
' “es » }
yy « a)
o :
. rl :
_ 4
“iejeiel4
piri.te:
4
“3
srotelal
4
ae
aeretareeis
3
Live Music Archive
Librivox Free Audio
Metropolitan Museum
Cleveland Museum of Art
Internet Arcade
Console Living Room
Books to Borrow
Open Library
TV News
Understanding 9/11