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MENDEL'S 
PEINCIPLES    OF   HEEEDITY 


§^3  9 

aiontion:    C.   J.    CLAY   and   SONS, 

CAMBRIDGE    UNIVERSITY   PRESS  WAREHOUSE, 

AVE   MARIA  LANE, 

AND 

H.  K.  LEWIS,  136,  GOWER  STREET,  W.C. 


©lasgoto:    50,  WELLINGTON  STREET. 

ILtipjtg:    F.  A.  BROCKHAUS. 

i^cto  Horfe:  THE  MACMILLAN  COMPANY. 

Bomfeag  anti  (ITalcutta:   MACMILLAN  AND  CO.,  Ltd. 


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GREGOR  MENDEL 

Abbot  of  Briinn 
Born  1822.     Died  1884. 

From  a  photog7-aph  kindly  supplied  by  the  Very  Rev.  Dr  Janeischek, 
the  p}-esent  Abbot. 


MENDEL'S 
PRINCIPLES  OF  HEPEDITY 

A   DEFENCE 

BY 

W.    BATESON,    M.A.,    F.KS. 


WITH  A    TRANSLATION  OF  MENDEL'S  ORIGINAL 
PAPERS  ON  HYBRIDISATION 


CAMBRIDGE  : 
AT    THE    UNIVERSITY    PRESS. 

1902 


aLRmbxiagt: 


PRINTED    BY    J.   AND    C.    P.    CLAY, 
AT   THE   UNIVERSITY   PRESS. 


PREFACE. 

TN  the  Study  of  Evolution  progress  had  well- 
J-  nigh  stopped.  The  more  vigorous,  perhaps  also 
the  more  prudent,  had  left  this  field  of  science 
to  labour  in  others  where  the  harvest  is  less  pre- 
carious or  the  yield  more  immediate.  Of  those  who 
remained  some  still  struggled  to  push  towards  truth 
through  the  jungle  of  phenomena:  most  were  content 
supinely  to  rest  on  the  great  clearing  Darwin  made 
long  since. 

Such  was  our  state  when  two  years  ago  it  was 
suddenly  discovered  that  an  unknown  man,  Gregor 
Johann  Mendel,  had,  alone,  and  unheeded,  broken  oflP 
from  the  rest — in  the  moment  that  Darwin  was  at 
work — and  cut  a  way  through. 

This  is  no  mere  metaphor,  it  is  simple  fact.  Each 
of  us  who  now  looks  at  his  own  patch  of  work  sees 
Mendel's  clue  running  through  it :  whither  that  clue 
\Yi\\  lead,  we  dare  not  yet  surmise. 

It  was  a  moment  of  rejoicing,  and  they  who  had 
heard  the  news  hastened  to  spread  them  and  take  the 


vi  Preface 

instant  way.     In  this  work  I  am  proud  to  have  borne 
my  little  part 

But  every  gospel  must  be  preached  to  all  alike. 
It  will  be  heard  by  the  Scribes,  by  the  Pharisees,  by 
Demetrius  the  Silversmith,  and  the  rest.  Not  lightly 
do  men  let  their  occupation  go ;  small,  then,  would 
be  our  wonder,  did  we  find  the  established  prophet 
unconvinced.  Yet,  is  it  from  misgiving  that  Mendel 
had  the  truth,  or  merely  from  indifference,  that  no 
naturalist  of  repute,  save  Professor  Weldon,  has  risen 
against  him  ? 

In  the  world  of  knowledge  we  are  accustomed  to 
look  for  some  strenuous  effort  to  understand  a  new 
truth  even  in  those  who  are  indisposed  to  believe. 
It  was  therefore  with  a  regret  approaching  to  in- 
dignation that  I  read  Professor  Weldon's  criticism*. 
Were  such  a  piece  from  the  hand  of  a  junior  it 
might  safely  be  neglected ;  but  coming  from  Professor 
Weldon  there  was  the  danger — almost  the  certainty — 
that  the  small  band  of  younger  men  who  are  thinking 
of  research  in  this  field  would  take  it  they  had  learnt 
the  gist  of  Mendel,  would  imagine  his  teaching  ex- 
posed by  Professor  Weldon,  and  look  elsewhere  for 
lines  of  work. 

In  evolutionary  studies  we  have  no  Areopagus. 
With  us  it  is  not — as  happily  it  is  with  Chemistry, 

*  Biometrika,  i.,  1902,  Pt.  ii. 


Preface  vii 

Physics,  Physiology,  Pathology,  and  other  well- 
followed  sciences — that  an  open  court  is  always 
sitting,  composed  of  men  themselves  workers,  keenly 
interested  in  every  new  thing,  skilled  and  well  versed 
in  the  facts.  Where  this  is  the  case,  doctrine  is  soon 
tried  and  the  false  trodden  down.  But  in  our  sparse 
and  apathetic  community  error  mostly  grows  un- 
heeded, choking  truth.  That  fate^  must  not  befall 
Mendel  now. 

It  seemed  imperative  that  MendeFs  own  work 
should  be  immediately  put  into  the  hands  of  all  who 
will  read  it,  and  I  therefore  sought  and  obtained  the 
kind  permission  of  the  Royal  Horticultural  Society  to 
reprint  and  modify  the  translation  they  had  already 
caused  to  be  made  and  published  in  their  Journal. 
To  this  I  add  a  translation  of  Mendel's  minor  paper 
of  later  date.  As  introduction  to  the  subject,  the 
same  Society  has  authorized  me  to  reprint  with 
alterations  a  lecture  on  heredity  delivered  before 
them  in  1900.  For  these  privileges  my  warm  thanks 
are  due.  The  introduction  thus  supplied,  composed 
originally  for  an  audience  not  strictly  scientific,  is  far 
too  slight  for  the  present  purpose.  A  few  pages  are 
added,  but  I  have  no  time  to  make  it  what  it  should 
be,  and  I  must  wait  for  another  chance  of  treating 
the  whole  subject  on  a  more  extended  scale.  It  wall 
perhaps    serve    to    give    the    beginner    the    slight 


viii  Preface 

assistance  which  will  prepare  him  to  get  the  most 
from  Mendel's  own  memoir. 


The  next  step  was  at  once  to  defend  Mendel  from 
Professor  Weldon.  That  could  only  be  done  by 
following  this  critic  from  statement  to  statement  in 
detail,  pointing  out  exactly  where  he  has  gone  wrong, 
what  he  has  misunderstood,  what  omitted,  what  in- 
troduced in  error.  With  such  matters  it  is  easy  to 
deal,  and  they  would  be  as  nothing  could  we  find  in  his 
treatment  some  word  of  allusion  to  the  future  ;  some 
hint  to  the  ignorant  that  this  is  a  very  big  thing ; 
some  suggestion  of  what  it  all  may  mean  if  it  he 
true. 

Both  to  expose  each  error  and  to  supply  effectively 
what  is  wanting,  within  the  limits  of  a  brief  article, 
written  with  the  running  pen,  is  difficult.  For  sim- 
plicity I  have  kept  almost  clear  of  reference  to  facts 
not  directly  connected  with  the  text,  and  have  foregone 
recital  of  the  now  long  list  of  cases,  both  of  plants 
and  animals,  where  the  Mendelian  principles  have 
already  been  perceived.  These  subjects  are  dealt 
with  in  a  joint  Report  to  the  Evolution  Committee  of 
the  Royal  Society,  made  by  Miss  E.  R.  Saunders  and 
myself,  now  in  the  Press.  To  Miss  Saunders  who 
has  been  associated  with  me  in  this  work  for  several 
years  I  wish  to  express  my  great  indebtedness.   Much 


Prefam  ix 

of  the  present  article  has  indeed  been  written  in 
consultation  with  her.  The  reader  who  seeks  fuller 
statement  of  facts  and  conceptions  is  referred  to  the 
writings  of  other  naturalists  who  have  studied  the 
phenomena  at  first  hand  (of  which  a  bibliography  is 
appended)  and  to  our  own  Report. 

I  take  this  opportunity  of  acknowledging  the 
unique  facilities  generously  granted  me,  as  repre- 
sentative of  the  Evolution  Committee,  by  Messrs 
Sutton  and  Sons  of  Reading,  to  watch  some  of  the 
many  experiments  they  have  in  progress,  to  inspect 
their  admirable  records,  and  to  utilise  these  facts  for 
the  advancement  of  the  science  of  heredity.  My 
studies  at  Reading  have  been  for  the  most  part 
confined  to  plants  other  than  those  immediately  the 
subject  of  this  discussion,  but  some  time  ago  I  availed 
myself  of  a  kind  permission  to  examine  their  stock  of 
peas,  thus  obtaining  information  which,  with  other 
facts  since  supplied,  has  greatly  assisted  me  in  treating 
this  subject. 

I  venture  to  express  the  conviction,  that  if  the 
facts  now  before  us  are  carefully  studied,  it  will  be- 
come evident  that  the  experimental  study  of  heredity, 
pursued  on  the  lines  Mendel  has  made  possible,  is 
second  to  no  branch  of  science  in  the  certainty  and 
magnitude  of  the  results  it  ofiers.    Tliis  study  has 


X  Preface 

one  advantage  which  no  other  line  of  scientific  inquiry 
possesses,  in  that  the  special  training  necessary  for 
such  work  is  easily  learnt  in  the  practice  of  it,  and 
can  be  learnt  in  no  other  way.  All  that  is  needed  is 
the  faithful  resolve  to  scamp  nothing. 

If  a  tenth  part  of  the  labour  and  cost  now  devoted 
by  leisured  persons,  in  this  country  alone,  to  the 
collection  and  maintenance  of  species  of  animals  and 
plants  which  have  been  collected  a  hundred  times 
before,  were  applied  to  statistical  experiments  in 
heredity,  the  result  in  a  few  years  would  make  a 
revolution  not  only  in  the  industrial  art  of  the  breeder 
but  in  our  views  of  heredity,  species  and  variation. 
We  have  at  last  a  brilliant  method,  and  a  solid  basis 
from  which  to  attack  these  problems,  ofibring  an 
opportunity  to  the  pioneer  such  as  occurs  but  seldom 
even  in  the  history  of  modern  science. 

We  have  been  told  of  late,  more  than  once,  that 
Biology  must  become  an  exact  science.  The  same  is 
my  own  fervent  hope.  But  exactness  is  not  always 
attainable  by  numerical  precision :  there  have  been 
students  of  Nature,  untrained  in  statistical  nicety, 
whose  instinct  for  truth  yet  saved  them  from  perverse 
inference,  from  slovenly  argument,  and  from  misuse 
of  authorities,  reiterated  and  grotesque. 

The  study  of  variation  and  heredity,  in  our  ignor- 
ance of  the  causation  of  those  phenomena,  must  be 


Preface  xi 

built  of  statistical  data,  as  Mendel  knew  long  ago  ; 
but,  as  he  also  perceived,  the  ground  must  be  pre- 
pared by  specific  experiment.  The  phenomena  of 
heredity  and  variation  are  specific,  and  give  loose  and 
deceptive  answers  to  any  but  specific  questions.  That 
is  where  our  exact  science  will  begin.  Otherwise  we 
may  one  day  see  those  huge  foundations  of  "biometry '^ 
in  ruins. 

But  Professor  Weldon,  by  coincidence  a  vehement 
preacher  of  precision,  in  his  haste  to  annul  this  first 
positive  achievement  of  the  precise  method,  dispenses 
for  the  moment  even  with  those  unpretending  forms 
of  precision  which  conventional  naturalists  have  use- 
fully practised.  His  essay  is  a  strange  symptom  of 
our  present  state.  The  facts  of  variation  and  heredity 
are  known  to  so  few  that  anything  passes  for  evidence ; 
and  if  only  a  statement,  or  especially  a  conclusion,  be 
negative,  neither  surprise  nor  suspicion  are  aroused. 
An  author  dealing  in  this  fashion  with  subjects  com- 
monly studied,  of  which  the  literature  is  familiar  and 
frequently  verified,  would  meet  with  scant  respect. 
The  reader  who  has  the  patience  to  examine  Professor 
Weldon's  array  of  objections  will  find  that  almost  all 
are  dispelled  by  no  more  elaborate  process  than  a 
reference  to  the  original  records. 

With  sorrow  I  find  such  an  article  sent  out  to 
the  world  by  a  Journal  bearing,  in  any  association, 


xii  Preface 

the  revered  name  of  Francis  Galton,  or  under  the 
high  sponsorship  of  Karl  Pearson.  I  yield  to  no  one 
in  admiration  of  the  genius  of  these  men.  Never 
can  we  sufficiently  regret  that  those  great  intellects 
were  not  trained  in  the  profession  of  the  naturalist. 

Mr  Galton  suggested  that  the  new  scientific  firm 
should  have  a  mathematician  and  a  biologist  as 
partners,  and — soundest  advice — a  logician  retained 
as  consultant*.  Biologist  surely  must  one  partner  be, 
but  it  will  never  do  to  have  him  sleeping.  In  many 
well-regulated  occupations  there  are  persons  known 
as  "  knockers-up,"  whose  thankless  task  it  is  to  rouse 
others  from  their  slumber,  and  tell  them  work-time  is 
come  round  again.  That  part  I  am  venturing  to  play 
this  morning,  and  if  I  have  knocked  a  trifle  loud,  it  is 
because  there  is  need. 

March,  1902. 


Biometrilca,  i.  Pt.  t.  p.  5. 


CONTENTS. 


INTEODUCTION. 

The  Problems  of  Heredity  and  their  Solution,  pp.  1 — 39. 

Preliminary  statement  of  Mendel's  principles,  8.  Re- 
lation of  Mendel's  discovery  to  the  law  of  Ancestral 
Heredity,  19.  Heterozygote  and  Homozygote^  23.  New 
conceptions  necessitated  by  Mendel's  discovery,  26.  Simple 
alternative  characters,  or  allelomorphs,  27.  Compound 
allelomorphs  and  their  components,  29.  Analytical  Varia- 
tions, 29.  Relation  of  Mendel's  principle  to  continuous 
variation,  32.  Dominance,  32.  Non-Mendelian  pheno- 
mena, 33.  False  hybrids  of  Millardet,  34.  Brief  historical 
notice,  36. 

MENDEL'S  EXPERIMENTS  IN  PLANT  HYBRIDISATION, 

pp.  40—95. 

Introductory  Remarks,  40.  Selection  of  Experimental 
Plants,  42.  Division  and  Arrangement  of  Experiments,  44. 
Characters  selected,  45.  Number  of  first  crosses,  47. 
Possible  sources  of  error,  47.  Forms  of  the  Hybrids,  49. 
Dominant  and  recessive,  49. 

First  generation  bred  from  the  Hybrids,  51.  Numbers 
of  each  form  in  ofispring,  52.  Second  generation  bred  from 
the  Hybrids,  55.  Subsequent  generations  bred  from  the 
Hybrids,  57. 

Offspring  of  Hybrids  in  which  several  differentiating 
characters  are  associated,  59.  The  reproductive  cells  of 
the  Hybrids,  66.  Statement  of  Mendel's  essential  deduc- 
tions, 67.  Experiments  to  determine  constitution  of  germ- 
cells,  68.     Statement  of  purity  of  germ-cells,  72. 

Experiments  with  Phaseolus,  76.  Compound  characters, 
80.     Concluding  Remarks,  84. 

MENDEL'S  EXPERIMENTS  WITH  HIERACIUM,  96—103. 


xiv  Contents 

A  DEFENCE   OF   MENDEL'S   PRINCIPLES   OF 
HEREDITY,   104—208. 
Introductory,  104. 

I.  The  Mendelian  Principle  of  Purity  of  Germ-cells 

AND  THE  Laws  of  Heredity  based  on  Ancestry,  108. 

II.  Mendel  and  the  critic's  version  of  him. 

The  Law  of  Dominance,  117. 

III.  The  facts  in  regard  to  Dominance  of  Characters  in 

Peas,  119. 
The  normal  characters  :  colours  of  cotyledons  and  seed- 
coats,  120.  Shape,  122.  Stability  and  variability,  124. 
Results  of  crossing  in  regard  to  seed-characters  :  normal  and 
exceptional,  129.  Analysis  of  exceptions,  132.  The  "mule" 
or  heterozygote,  133. 

IV.  Professor  Weldon's  collection  of  "Other  evidence 

concerning  Dominance  in  Peas." 

A.  In  regard  to  cotyledon  colour:  Preliminary,  137. 
Xenia,  139.  (1)  Gartner's  cases,  141.  (2)  Seton's  case,  143. 
(3)  Tschermak's  exceptions,  145.  (3  a)  Buchsbaum  case,  145. 
(3  h)  Telephone  cases,  146.     (3  c)  Couturier  cases,  147. 

B.  Seed-coats  and  Shapes.  1.  Seed-coats,  148.  2.  Seed- 
shapes  :  (a)  Rimpau's  cases,  150.  (6)  Tschermak's  cases,  152. 
3.  Other  phenomena,  especially  regarding  seed-shapes,  in 
the  case  of  "  grey  "  peas.     Modern  evidence,  153. 

C.  Evidence  of  Knight  and  Laxton,  158. 

D.  Miscellaneous  cases  in  other  plants  and  animals  : 

1.  Stocks  {Matthiola).     Hoariness,  169.     Flower- 
colour,    170. 

2.  Datura,  172. 

3.  Colours  of  Rats  and  Mice,  173. 

V.  Professor  Weldon's  quotations  from  Laxton,  178. 

Illustration  from  Primula  sinensis,  182. 

VI.  The  argument  built  on  exceptions,  183. 

Ancestry  and  Dominance,  185. 
Ancestry  and  purity  of  germ-cells,  193. 
The  value  of  the  appeal  to  Ancestry,  197. 

VII.  The  question  of  absolute  purity  of  germ-cells,  201. 
Conclusion,  208. 


ERRATA. 

p.  22,  par.  3,  line  2,  for  "  falls"  read  "  fall." 

p.  63,  line  12,  for  ''AabbC"  read  '' AaBbc.'' 

p.  66,  in  heading,  for  "  of  hybrids  "  read  "  of  the  hybrids." 


Note  to  p.  125.  None  of  the  yellow  seeds  produced  by  Laxton's 
Alpha  germinated,  though  almost  all  the  green  seeds  sown  gave 
healthy  plants.  The  same  was  found  in  the  case  of  Express,  another 
variety  which  bore  some  yellow  seeds.  In  the  case  of  Blue  Peter,  on 
the  contrary,  the  yellow  seeds  have  grown  as  well  as  the  green  ones. 
Few  however  were  wholly  yellow.  Of  nine  yellow  seeds  produced  by 
crossing  green  varieties  together  (p.  131),  six  did  not  germinate, 
and  three  which  did  gave  weak  and  very  backward  plants.  Taken 
together,  this  evidence  makes  it  scarcely  doubtful  that  the  yellow  colour 
in  these  cases  was  pathological,  and  almost  certainly  due  to  exposure 
after  ripening. 


2  The  Problems 

It  is  in  the  hope  of  inducing  others  to  follow  these 
lines  of  investigation  that  I  take  the  problems  of  heredity 
as  the  subject  of  this  lecture  to  the  Eoyal  Horticultural 
Society. 

No  one  has  better  opportunities  of  pursuing  such 
work  than  horticulturists  and  stock  breeders.  They  are 
daily  witnesses  of  the  phenomena  of  heredity.  Their 
success  also  depends  largely  on  a  knowledge  of  its  laws, 
and  obviously  every  increase  in  that  knowledge  is  of 
direct  and  special  importance  to  them. 

The  want  of  systematic  study  of  heredity  is  due 
chiefly  to  misapprehension.  It  is  supposed  that  such 
work  requires  a  lifetime.  But  though  for  adequate  study 
of  the  complex  phenomena  of  inheritance  long  periods 
of  time  must  be  necessary,  yet  in  our  present  state  of 
deep  ignorance  almost  of  the  outline  of  the  facts,  obser- 
vations carefully  planned  and  faithfully  carried  out  for 
even  a  few  years  may  produce  results  of  great  value.  In 
fact,  by  far  the  most  appreciable  and  definite  additions 
to  our  knowledge  of  these  matters  have  been  thus 
obtained. 

There  is  besides  some  misapprehension  as  to  the 
kind  of  knowledge  which  is  especially  wanted  at  this 
time,  and  as  to  the  modes  by  which  we  may  expect  to 
obtain  it.  The  present  paper  is  written  in  the  hope  that 
it  may  in  some  degree  help  to  clear  the  ground  of  these 
difiiculties  by  a  preliminary  consideration  of  the  question, 
How  far  have  we  got  towards  an  exact  knowledge  of 
heredity,  and  how  can  we  get  further? 

Now  this  is  pre-eminently  a  subject  in  which  we 
must  distinguish  what  we  can  do  from  what  we  want 
to  do.  We  want  to  know  the  whole  truth  of  the  matter ; 
we   want  to   know   the   physical  basis,   the   inward    and 


of  Heredity  3 

essential  nature,  "the  causes,"  as  they  are  sometimes 
called,  of  heredity:  but  we  want  also  to  know  the  laws 
which  the  outward  and  visible  phenomena  obey. 

Let  us  recognise  from  the  outset  that  as  to  the  essential 
nature  of  these  phenomena  we  still  know  absolutely 
nothing.  We  have  no  glimmering  of  an  idea  as  to  what 
constitutes  the  essential  process  by  which  the  likeness 
of  the  parent  is  transmitted  to  the  offspring.  We  can 
study  the  processes  of  fertilisation  and  development  in 
the  finest  detail  which  the  microscope  manifests  to  us, 
and  we  may  fairly  say  that  we  have  now  a  considerable 
grasp  of  the  visible  phenomena  ;  but  of  the  nature  of 
the  physical  basis  of  heredity  we  have  no  conception 
at  all.  No  one  has  yet  any  suggestion,  working  hypo- 
thesis, or  mental  picture  that  has  thus  far  helped  in 
the  slightest  degTee  to  penetrate  beyond  what  we  see. 
The  process  is  as  utterly  mysterious  to  us  as  a  flash  of 
lightning  is  to  a  savage.  We  do  not  know  what  is  the 
essential  agent  in  the  transmission  of  parental  characters, 
not  even  whether  it  is  a  material  agent  or  not.  Not  only 
is  our  ignorance  complete,  but  no  one  has  the  remotest 
idea  how  to  set  to  work  on  that  part  of  the  problem. 
We  are  in  the  state  in  which  the  students  of  physical 
science  were,  in  the  period  when  it  was  open  to  anyone 
to  believe  that  heat  was  a  material  substance  or  not,  as 
he  chose. 

But  apart  from  any  conception  of  the  essential  modes 
of  transmission  of  characters,  we  can  study  the  outward 
facts  of  the  transmission.  Here,  if  our  knowledge  is 
still  very  vague,  we  are  at  least  beginning  to  see  how 
we  ought  to  go  to  work.  Formerly  naturalists  were 
content  with  the  collection  of  numbers  of  isolated  instances 
of  transmission — more    especially,   striking    and    peculiar 

1—2 


4  The  Problems 

cases — the  sudden  appearance  of  highly  prepotent  forms, 
and  the  like.  We  are  now  passing  out  of  that  stage. 
It  is  not  that  the  interest  of  particular  cases  has  in 
any  way  diminished — for  such  records  will  always  have 
their  value — but  it  has  become  likely  that  general  ex- 
pressions will  be  found  capable  of  sufficiently  wide  appli- 
cation to  be  justly  called  "laws  "  of  heredity.  That  this 
is  so  was  till  recently  due  almost  entirely  to  the  work  of 
Mr  F.  Galton,  to  whom  we  are  indebted  for  the  first 
systematic  attempt  to  enuntiate  such  a  law. 

All  laws  of  heredity  so  far  propounded  are  of  a 
statistical  character  and  have  been  obtained  by  statistical 
methods.  If  we  consider  for  a  moment  what  is  actually 
meant  by  a  "law  of  heredity"  we  shall  see  at  once  why 
these  investigations  must  follow  statistical  methods.  For 
a  "law"  of  heredity  is  simply  an  attempt  to  declare 
the  course  of  heredity  under  given  conditions.  But  if 
we  attempt  to  predicate  the  course  of  heredity  we  have 
to  deal  with  conditions  and  groups  of  causes  wholly 
unknown  to  us,  whose  presence  we  cannot  recognize, 
and  whose  magnitude  we  cannot  estimate  in  any  par- 
ticular case.  The  course  of  heredity  in  particular  cases 
therefore  cannot  be  foreseen. 

Of  the  many  factors  which  determine  the  degree 
to  which  a  given  character  shall  be  present  in  a  given 
individual  only  one  is  usually  known  to  us,  namely, 
the  degree  to  which  that  character  is  present  in  the 
parents.  It  is  common  knowledge  that  there  is  not  that 
close  correspondence  between  parent  and  offspring  whicH 
would  result  were  this  factor  the  only  one  operating ; 
but  that,  on  the  contrary,  the  resemblance  between  the 
two  is  only  an  uncertain  one. 

In  dealing  with  phenomena  of  this   class   the   study 


of  Heredity  5 

of  single  instances  reveals  no  regularity.  It  is  only  by 
collection  of  facts  in  great  numbers,  and  by  statistical 
treatment  of  the  mass,  that  any  order  or  law  can  be 
perceived.  In  the  case  of  a  chemical  reaction,  for  instance, 
by  suitable  means  the  conditions  can  be  accurately  repro- 
duced, so  that  in  every  individual  case  we  can  predict 
with  certainty  that  the  same  result  will  occur.  But  with 
heredity  it  is  somewhat  as  it  is  in  the  case  of  the  rainfall. 
No  one  can  say  how  much  rain  will  fall  to-morrow  in 
a  given  place,  but  we  can  predict  with  moderate  accuracy 
how  much  will  fall  next  year,  and  for  a  period  of  years 
a  prediction  can  be  made  which  accords  very  closely  with 
the  truth. 

Similar  predictions  can  from  statistical  data  be  made  as 
to  the  duration  of  life  and  a  gTeat  variety  of  events,  the 
conditioning  causes  of  which  are  very  imperfectly  under- 
stood. It  is  predictions  of  this  kind  that  the  study  of 
heredity  is  beginning  to  make  possible,  and  in  that  sense 
laws  of  heredity  can  be  perceived. 

We  are  as  far  as  ever  from  knowing  why  some  characters 
are  transmitted,  while  others  are  not ;  nor  can  anyone  yet 
foretell  which  individual  parent  will  transmit  characters  to 
the  offspring,  and  which  will  not ;  nevertheless  the  progress 
made  is  distinct. 

As  yet  investigations  of  this  kind  have  been  made  in 
only  a  few  instances,  the  most  notable  being  those  of 
Galton  on  human  stature,  and  on  the  transmission  of 
colours  in  Basset  hounds.  In  each  of  these  cases  he  has 
shown  that  the  expectation  of  inheritance  is  such  that  a 
simple  arithmetical  rule  is  approximately  followed.  The 
rule  thus  arrived  at  is  that  of  the  whole  heritage  of  the 
offspring  the  two  parents  together  on  an  average  contribute 
one   half,   the   four  grandparents    one-quarter,   the   eight 


6  The  Problems 

great-grandparents  one-eighth,  and  so  on,  the  remainder 
being  contributed  by  the  remoter  ancestors. 

Such  a  law  is  obviously  of  practical  importance.  In 
any  case  to  which  it  applies  we  ought  thus  to  be  able  to 
predict  the  degree  with  which  the  purity  of  a  strain  may 
be  increased  by  selection  in  each  successive  generation. 

To  take  a  perhaps  impossibly  crude  example,  if  a 
seedling  show  any  particular  character  which  it  is  desired 
to  fix,  on  the  assumption  that  successive  self-fertilisations 
are  possible,  according  to  Galton's  law  the  expectation  of 
purity  should  be  in  the  first  generation  of  self-fertilisation 
1  in  2,  in  the  second  generation  3  in  4,  in  the  third  7  in  8, 
and  so  on"^. 

But  already  many  cases  are  known  to  which  the  rule  in 
any  simple  form  will  not  apply.  Galton  points  out  that 
it  takes  no  account  of  individual  prepotencies.  There  are, 
besides,  numerous  cases  in  which  on  crossing  two  varieties 
the  character  of  one  variety  almost  always  appears  in  each 
member  of  the  first  cross-bred  generation.  Examples  of 
these  will  be  familiar  to  those  who  have  experience  in  such 
matters.  The  offspring  of  the  Polled  Angus  cow  and  the 
Shorthorn  bull  is  almost  invariably  polled  or  with  very 
small  loose  "scurs."  Seedlings  raised  by  crossing  Atropa 
helladonna  with  the  yellow-fruited  variety  have  without 
exception  the  blackish-purple  fruits  of  the  type.  In  several 
hairy  species  when  a  cross  with  a  glabrous  variety  is  made, 
the  first  cross-bred  generation  is  altogether  hairy  t. 

Still  more  numerous  are  examples  in  which  the  characters 
of  one  variety  very  largely,  though  not  exclusively,  pre- 
dominate in  the  offspring. 

*  See  later.    Galton  gave  a  simple  diagrammatic  representation  of 
his  law  in  Nature,  1898,  vol.  lvii.  p.  293. 

+  These  we  now  recognize  as  examples  of  Mendelian  '  dominance.' 


of  Heredity  7 

These  large  classes  of  exceptions — to  go  no  further — 
indicate  that,  as  we  might  in  any  case  expect,  the  principle 
is  not  of  universal  application,  and  will  need  various 
modifications  if  it  is  to  be  extended  to  more  complex  cases 
of  inheritance  of  varietal  characters.  No  more  useful  work 
can  be  imagined  than  a  systematic  determination  of  the 
precise  "law  of  heredity"  in  numbers  of  particular  cases. 

Until  lately  the  work  which  Galton  accomplished  stood 
almost  alone  in  this  field,  but  quite  recently  remarkable 
additions  to  our  knowledge  of  these  questions  have  been 
made.  In  the  year  1900  Professor  de  Vries  published 
a  brief  account"^  of  experiments  which  he  has  for  several 
years  been  carrying  on,  giving  results  of  the  highest  value. 

The  description  is  very  short,  and  there  are  several 
points  as  to  which  more  precise  information  is  necessary 
both  as  to  details  of  procedure  and  as  to  statement  of 
results.  Nevertheless  it  is  impossible  to  doubt  that  the 
work  as  a  whole  constitutes  a  marked  step  forward,  and 
the  full  publication  which  is  promised  will  be  awaited  with 
great  interest. 

The  work  relates  to  the  course  of  heredity  in  cases 
where  definite  varieties  differing  from  each  other  in  some 
one  definite  character  are  crossed  together.  The  cases  are 
all  examples  of  discontinuous  variation  :  that  is  to  say, 
cases  in  which  actual  intermediates  between  the  parent 
forms  are  not  usually  produced  on  crossing!.  It  is  shown 
that  the  subsequent  posterity  obtained  by  self-fertilising 
these  cross-breds  or  hybrids,  or  by  breeding  them  with  each 
other,  break  up  into  the  original  parent  forms  according  to 
fixed  numerical  rule. 

*  Comptes  Rendus,  March  26,   1900,  and   Ber.  d.  Deutsch.  Bot. 
Ges.  xviii.  1900,  p.  83. 

t  This  conception  of  discontinuity  is  of  course  pre-Mendelian. 


8  The  Problems 

Professor  de  Vries  begins  by  reference  to  a  remarkable 
memoir  by  Gregor  Mendel"^,  giving  the  results  of  his 
experiments  in  crossing  varieties  of  Pisum  sativum.  These 
experiments  of  Mendel's  were  carried  out  on  a  large  scale, 
his  account  of  them  is  excellent  and  complete,  and  the 
principles  which  he  was  able  to  deduce  from  them  will 
certainly  play  a  conspicuous  part  in  all  future  discussions 
of  evolutionary  problems.  It  is  not  a  little  remarkable 
that  Mendel's  work  should  have  escaped  notice,  and  been 
so  long  forgotten. 

For  the  purposes  of  his  experiments  Mendel  selected 
seven  pairs  of  characters  as  follows  : — 

1 .  Shape  of  ripe  seed,  whether  round  ;  or  angular  and 
wrinkled. 

2.  Colour  of  "  endosperm  "  (cotyledons),  whether  some 
shade  of  yellow  ;  or  a  more  or  less  intense  green. 

3.  Colour  of  the  seed-skin,  whether  various  shades  of 
grey  and  grey-brown  ;  or  white. 

4.  Shape  of  seed-pod,  whether  simply  inflated ;  or 
deeply  constricted  between  the  seeds. 

5.  Colour  of  unripe  pod,  whether  a  shade  of  green  ;  or 
bright  yellow. 

6.  Nature  of  inflorescence,  whether  the  flowers  are 
arranged  along  the  axis  of  the  plant ;  or  are  terminal  and 
form  a  kind  of  umbel. 

7.  Length  of  stem,  whether  about  6  or  7  ft.  long,  or 
about  f  to  Ij  ft. 

Large  numbers  of  crosses  were  made  between  Peas  dif- 
fering in  respect  of  one  of  each  of  these  pairs  of  characters. 

*  '  Versuche  iib.  Pflanzenhybriden '  in  the  Verli.  d.  Naturf.  Ver. 
Brilnn,  iv.  1865. 


of  Heredity  9 

It  was  found  that  in  each  case  the  offspring  of  the  cross 
exhibited  the  character  of  one  of  the  parents  in  ahnost 
undiminished  intensity,  and  intermediates  which  could  not 
be  at  once  referred  to  one  or  other  of  the  parental  forms 
were  not  found. 

In  the  case  of  each  pair  of  characters  there  is  thus 
one  which  in  the  first  cross  prevails  to  the  exclusion  of  the 
other.  This  prevailing  character  Mendel  calls  the  dominant 
character,  the  other  being  the  recessim  character*. 

That  the  existence  of  such  "dominant"  and  "recessive" 
characters  is  a  frequent  phenomenon  in  cross-breeding,  is 
well  known  to  all  who  have  attended  to  these  subjects. 

By  letting  the  cross-breds  fertilise  themselves  Mendel 
next  raised  another  generation.  In  this  generation  were 
individuals  which  showed  the  dominant  character,  but  also 
individuals  which  presented  the  recessive  character.  Such 
a  fact  also  was  known  in  a  good  many  instances.  But 
Mendel  discovered  that  in  this  generation  the  numerical 
proportion  of  dominants  to  recessives  is  on  an  average  of 
cases  approximately  constant,  being  in  fact  as  three  to  one. 
With  very  considerable  regularity  these  numbers  were 
approached  in  the  case  of  each  of  his  pairs  of  characters. 

There  are  thus  in  the  first  generation  raised  from  the 
cross-breds  75  pei^  cent,  dominants  and  25  per  cent, 
recessives. 

These  plants  were  again  self-fertilised,  and  the  offspring 
of  each  plant  separately  sown.  It  next  appeared  that  the 
offspring  of  the  recessives  remained  pure  recessive,  and 
in  subsequent  generations  never  produced  the  dominant 
again. 

But  when   the   seeds   obtained  by   self-fertilising   the 

*  Note  that  by  these  novel  terms  the  complications  involved  by 
use  of  the  expression  "  prepotent  "  are  avoided. 


10  The  Problems 

dominants  were  examined  and  sown  it  was  found  that 
the  dominants  were  not  all  alike,  but  consisted  of  two 
classes,  (1)  those  which  gave  rise  to  pure  dominants,  and 
(2)  others  which  gave  a  mixed  offspring,  composed  partly 
of  recessives,  partly  of  dominants.  Here  also  it  was  found 
that  the  average  numerical  proportions  were  constant,  those 
with  pure  dominant  offspring  being  to  those  with  mixed 
offspring  as  one  to  two.  Hence  it  is  seen  that  the  75  per 
cent,  dominants  are  not  really  of  similar  constitution,  but 
consist  of  twenty-five  which  are  pure  dominants  and  fifty 
which  are  really  cross-breds,  though,  like  the  cross- breds 
raised  by  crossing  the  two  original  varieties,  they  only 
exhibit  the  dominant  character. 

To  resume,  then,  it  was  found  that  by  self-fertilising 
the  original  cross-breds  the  same  proportion  was  always 
approached,  namely — 

25  dominants,  50  cross-breds,  25  recessives, 

or  \D  :  2DR  :  \R. 

Like  the  pure  recessives,  the  pure  dominants  are 
thenceforth  pure,  and  only  give  rise  to  dominants  in  all 
succeeding  generations  studied. 

On  the  contrary  the  fifty  cross-breds,  as  stated  above, 
have  mixed  offspring.  But  these  offspring,  again,  in  their 
numerical  proportions,  follow  the  same  law,  namely,  that 
there  are  three  dominants  to  one  recessive.  The  recessives 
are  pure  like  those  of  the  last  generation,  but  the  dominants 
can,  by  further  self-fertilisation,  and  examination  or  culti- 
vation of  the  seeds  produced,  be  again  shown  to  be  made 
up  of  pure  dominants  and  cross-breds  in  the  same  proportion 
of  one  dominant  to  two  cross-breds. 

The  process  of  breaking  up  into  the  parent  forms  is 
thus  continued   in   each   successive  generation,  the  same 


of  Heredity  11 

numerical  law  being  followed  so  far  as  has  yet  been 
observed- 
Mendel  made  further  experiments  with  Pisum  sativum, 
crossing  pairs  of  varieties  which  differed  from  each  other 
in  two  characters,  and  the  results,  though  necessarily  much 
more  complex,  showed  that  the  law  exhibited  in  the  simpler 
case  of  pairs  differing  in  respect  of  one  character  operated 
here  also. 

In  the  case  of  the  union  of  varieties  AB  and  ah 
differing  in  two  distinct  pairs  of  characters,  A  and  a, 
B  and  h,  of  which  A  and  B  are  dominant,  a  and  h 
recessive,  Mendel  found  that  in  the  first  cross-bred  gene- 
ration there  was  only  one  class  of  offspring,  really  AaBh. 

But  by  reason  of  the  dominance  of  one  character  of 
each  pair  these  first  crosses  were  hardly  if  at  all  distin- 
guishable from  AB. 

By  letting  these  AaBb'^  fertilise  themselves,  only  four 
classes  of  offspring  seemed  to  be  produced,  namely, 

AB  showing  both  dominant  characters. 
Ah         „        dominant  J.  and  recessive  h. 
aB        „        recessive  a  and  dominant  B. 
ah         „        both  recessive  characters  a  and  h. 

The  numerical  ratio  in  which  these  classes  appeared 
were  also  regular  and  approached  the  ratio 

^AB  '.^Ah:  MB  :  lah. 

But  on  cultivating  these  plants  and  allowing  them  to 
fertilise  themselves  it  was  found  that  the  members  of  the 

Ratios 

1  ah  class  produce  only  a6's. 

1     aB  class  may  produce  either  all  a^'s, 
,2  or  both  aB'^  and  a6's. 


12  The  Problems 

Ratios 

1     Ah  class  may  produce  either  all  J.6's, 
^2  or  both  Ah'^  and  ah'^. 

1  AB  class  may  produce  either  all  J.^'s, 

2  or  both  ^5's  and  AU^, 
H  2  or  both  AB'^  and  a^'s, 

or  all  four  possible  classes  again,  namely, 
AB'&,  J.^'s,  aB^y  and  a6's, 

and  the  average  number  of  members  of  each  class  will 
approach  the  ratio  1  :  3  :  3  :  9  as  indicated  above. 

The  details  of  these  experiments  and  of  others  like 
them  made  with  three  pairs  of  differentiating  characters  are 
all  set  out  in  Mendel's  memoir. 

Professor  de  Vries  has  worked  at  the  same  problem  in 
some  dozen  species  belonging  to  several  genera,  using  pairs 
of  varieties  characterised  by  a  great  number  of  characters  : 
for  instance,  colour  of  flowers,  stems,  or  fruits,  hairiness, 
length  of  style,  and  so  forth.  He  states  that  in  all  these 
cases  Mendel's  principles  are  followed. 

The  numbers  with  which  Mendel  worked,  though  large, 
were  not  large  enough  to  give  really  smooth  results  * ;  but 
with  a  few  rather  marked  exceptions  the  observations  are 
remarkably  consistent,  and  the  approximation  to  the  num- 
bers demanded  by  the  law  is  greatest  in  those  cases  where 
the  largest  numbers  were  used.  When  we  consider,  besides, 
that  Tschermak  and  Correns  announce  definite  confirmation 
in  the  case  of  Pisum,  and  de  Vries  adds  the  evidence  of  his 
long  series  of  observations  on  other  species  and  orders, 
there  can  be  no  doubt  that  Mendel's  law  is  a  substantial 

*  Professor  Weldon  (p.  232)  takes  great  exception  to  this  state- 
ment, which  he  considerately  attributes  to  "  some  writers."  After 
examining  the  conclusions  he  obtained  by  algebraical  study  of  Mendel's 
figures  I  am  disposed  to  think  my  statement  not  very  far  out. 


of  Heredity  13 

reality ;  thougli  whether  some  of  the  cases  that  depart 
most  widely  from  it  can  be  brought  within  the  terms  of 
the  same  principle  or  not,  can  only  be  decided  by  further 
experiments. 

One  may  naturally  ask,  How  can  these  results  be 
brought  into  harmony  with  the  facts  of  hybridisation 
hitherto  known ;  and,  if  all  this  is  true,  how  is  it  that 
others  who  have  carefully  studied  the  phenomena  of  hy- 
bridisation have  not  long  ago  perceived  this  law?  The 
answer  to  this  question  is  given  by  Mendel  at  some  length, 
and  it  is,  I  think,  satisfactory.  He  admits  from  the  first 
that  there  are  undoubtedly  cases  of  hybrids  and  cross-breds 
which  maintain  themselves  pure  and  do  not  break  up. 
Such  examples  are  plainly  outside  the  scope  of  his  law. 
Next  he  points  out,  what  to  anyone  who  has  rightly 
comprehended  the  nature  of  discontinuity  in  variation  is 
well  known,  that  the  variations  in  each  character  must  be 
separately  regarded.  In  most  experiments  in  crossing, 
forms  are  taken  which  differ  from  each  other  in  a  multi- 
tude of  characters — some  continuous,  others  discontinuous, 
some  capable  of  blending  with  their  contraries,  while  others 
are  not.  The  observer  on  attempting  to  perceive  any 
regularity  is  confused  by  the  complications  thus  intro- 
duced. Mendel's  law,  as  he  fairly  says,  could  only  appear 
in  such  cases  by  the  use  of  overwhelming  numbers,  which 
are  beyond  the  possibilities  of  practical  experiment.  Lastly, 
no  previous  observer  had  applied  a  strict  statistical  method. 

Both  these  answers  should  be  acceptable  to  those  who 
have  studied  the  facts  of  variation  and  have  appreciated 
the  nature  of  Species  in  the  light  of  those  facts.  That 
different  species  should  follow  different  laws,  and  that  the 
same  law  should  not  apply  to  all  characters  alike,  is  exactly 
what   we   have   every  right  to   expect.     It   will  also  be 


14  The  Problems 

remembered  that  the  principle  is  only  explicitly  declared 
to  apply  to  discontinuous  characters*.  As  stated  also 
it  can  only  be  true  where  reciprocal  crossings  lead  to  the 
same  result.  Moreover,  it  can  only  be  tested  when  there 
is  no  sensible  diminution  in  fertility  on  crossing. 

Upon  the  appearance  of  de  Vries'  paper  announcing  the 
"rediscovery"  and  confirmation  of  Mendel's  law  and  its 
extension  to  a  great  number  of  cases  two  other  observers 
came  forward  almost  simultaneously  and  independently 
described  series  of  experiments  fully  confirming  Mendel's 
work.  Of  these  papers  the  first  is  that  of  Correns,  who 
repeated  Mendel's  original  experiment  with  Peas  having 
seeds  of  different  colours.  The  second  is  a  long  and  very 
valuable  memoir  of  Tschermak,  which  gives  an  account  of 
elaborate  researches  into  the  results  of  crossing  a  number 
of  varieties  of  Pisum  sativum.  These  experiments  were  in 
many  cases  carried  out  on  a  large  scale,  and  prove  the 
main  fact  enuntiated  by  Mendel  beyond  any  possibility  of 
contradiction.  The  more  exhaustive  of  these  researches 
are  those  of  Tschermak  on  Peas  and  Correns  on  several 
varieties  of  Maize.  Both  these  elaborate  investigations 
have  abundantly  proved  the  general  applicability  of  Mendel's 
law  to  the  character  of  the  plants  studied,  though  both 
indicate  some  few  exceptions.  The  details  of  de  Vries' 
experiments  are  promised  in  the  second  volume  of  his  most 
valuable  Mutationstheorie.  Correns  in  regard  to  Maize 
and  Tschermak  in  the  case  of  P.  sativum  have  obtained 
further  proof  that  Mendel's  law  holds  as  well  in  the  case  of 
varieties  differing  from  each  other  in  two  pairs  of  characters, 
one  of  each  pair  being  dominant,  though  of  course  a  more 
complicated  expression  is  needed  in  such  cases  f. 

*  See  later. 

t  Tschermak's  investigations  were  besides  directed  to  a  re-exami- 


of  Heredity  15 

That  we  are  in  the  presence  of  a  new  principle  of  the 
highest  importance  is  manifest.  To  what  further  con- 
clusions it  may  lead  us  cannot  yet  be  foretold.  But  both 
Mendel  and  the  authors  who  have  followed  him  lay  stress 
on  one  conclusion,  which  will  at  once  suggest  itself  to 
anyone  who  reflects  on  the  facts.  For  it  will  be  seen  that 
the  results  are  such  as  we  might  expect  if  it  be  imagined 
that  the  cross-bred  plant  produced  pollen  grains  and  egg- 
cells,  each  of  which  bears  only  one  of  the  alternative  varietal 
characters  and  not  both.  If  this  were  so,  and  if  on  an 
average  the  same  number  of  pollen  grains  and  egg-cells 
transmit  each  of  the  two  characters,  it  is  clear  that  on  a 
random  assortment  of  pollen  grains  and  egg-cells  Mendel's 
law  would  be  obeyed.  For  25  per  cent,  of  "dominant" 
pollen  grains  would  unite  with  25  per  cent,  "dominant" 
egg-cells ;  25  per  cent.  "  recessive "  pollen  grains  would 
similarly  unite  with  25  per  cent.  "  recessive "  egg-cells ; 
while  the  remaining  50  per  cent,  of  each  kind  would  unite 
together.  It  is  this  consideration  which  leads  both  Mendel 
and  those  who  have  followed  him  to  assert  that  these  facts 
of  crossing  prove  that  each  egg-cell  and  each  pollen  grain 
is  pure  in  respect  of  each  character  to  which  the  law 
applies.  It  is  highly  desirable  that  varieties  differing  in 
the  form  of  their  pollen  should  be  made  the  subject  of 
these  experiments,  for  it  is  quite  possible  that  in  such  a 
case  strong  confirmation  of  this  deduction  might  be  ob- 
tained. [Preliminary  trials  made  with  reference  to  this 
point  have  so  far  given  negative  results.  Eemembering 
that  a  pollen  grain  is  not  a  germ-cell,  but  only  a  bearer  of 

nation  of  the  question  of  the  absence  of  beneficial  results  on  cross- 
fertilising  P.  sativum,  a  subject  already  much  investigated  by  Darwin, 
and  upon  this  matter  also  important  further  evidence  is  given  in 
great  detail. 


16  The  Problems 

a  germ-cell,  the  hope  of  seeing  pollen  grains  differentiated 
according  to  the  characters  they  bear  is  probably  remote. 
Better  hopes  may  perhaps  be  entertained  in  regard  to 
spermatozoa,  or  possibly  female  cells.] 

As  an  objection  to  the  deduction  of  purity  of  germ-cells, 
however,  it  is  to  be  noted  that  though  true  intermediates 
did  not  generally  occur,  yet  the  intensity  in  which  the 
characters  appeared  did  vary  in  degree,  and  it  is  not  easy 
to  see  how  the  hypothesis  of  perfect  purity  in  the  repro- 
ductive cells  can  be  supported  in  such  cases.  Be  this, 
however,  as  it  may,  there  is  no  doubt  we  are  beginning  to 
get  new  lights  of  a  most  valuable  kind  on  the  nature  of 
heredity  and  the  laws  which  it  obeys.  It  is  to  be  hoped 
that  these  indications  will  be  at  once  followed  up  by 
independent  workers.  Enough  has  been  said  to  show  how 
necessary  it  is  that  the  subjects  of  experiment  should  be 
chosen  in  such  a  way  as  to  bring  the  laws  of  heredity  to  a 
real  test.  For  this  purpose  the  first  essential  is  that  the 
differentiating  characters  should  be  few,  and  that  all  avoid- 
able complications  should  be  got  rid  of.  Each  experiment 
should  be  reduced  to  its  simplest  possible  limits.  The 
results  obtained  by  Galton,  and  also  the  new  ones  especially 
described  in  this  paper,  have  each  been  reached  by  restricting 
the  range  of  observation  to  one  character  or  group  of  char- 
acters, and  it  is  certain  that  by  similar  treatment  our 
knowledge  of  heredity  may  be  rapidly  extended. 


To  the  above  popular  presentation  of  the  essential  facts, 
made  for  an  audience  not  strictly  scientific,  some  addition, 
however  brief,  is  called  for.  First,  in  regard  to  the  law  of 
Ancestry,  spoken  of  on  p.  5.  Those  who  are  acquainted  with 
Pearson's  Grammar  of  Science,  2nd  ed.  pubHshed  early  in 


of  Heredity  17 

1900,  the  same  author's  paper  in  Proc.  R.  S.  vol.  66,  1900, 
p.  140,  or  the  extensive  memoir  (pubd.  Oct.  1900),  on  the 
inheritance  of  coat-colour  in  horses  and  eye-colour  in  man 
(Fkil.  Trans.  195,  a,  1900,  p.  79),  will  not  need  to  be  told 
that  the  few  words  I  have  given  above  constitute  a  most 
imperfect  diagram  of  the  operations  of  that  law  as  now  de- 
veloped. Until  the  appearance  of  these  treatises  it  was, 
I  believe,  generally  considered  that  the  law  of  Ancestral 
Heredity  was  to  be  taken  as  applying  to  phenomena  like 
these  (coat-colour,  eye-colour,  &c.)  where  the  inheritance 
is  generally  alternative,  as  well  as  to  the  phenomena 
of  hlended  inheritance. 

Pearson,  in  the  writings  referred  to,  besides  withdrawing 
other  large  categories  of  phenomena  from  the  scope  of  its 
operations,  points  out  that  the  law  of  Ancestral  Heredity 
does  not  satisfactorily  express  the  cases  of  alternative 
inheritance.  He  urges,  and  with  reason,  that  these  classes 
of  phenomena  should  be  separately  dealt  with. 

The  whole  issue  as  regards  the  various  possibilities  of 
heredity  now  recognized  will  be  made  clearer  by  a  very  brief 
exposition  of  the  several  conceptions  involved. 

If  an  organism  producing  germ-cells  of  a  given  constitu- 
tion, uniform  in  respect  of  the  characters  they  bear,  breeds 
with  another  organism*  bearing  precisely  similar  germ- 
cells,  the  offspring  resulting  will,  if  the  conditions  are 
identical,  be  uniform. 

In  practice  such  a  phenomenon  is  seen  in  jl?^«rg-breeding. 
It  is  true  that  we  know  no  case  in  nature  where  all  the 
germ-cells  are  thus  identical,  and  where  no  variation  takes 
place  beyond  what  we  can  attribute  to  conditions,  but  we 

*  For  simplicity  the  case  of  self-fertilisation  is  omitted  from  this 
consideration. 

B.  2 


18  The  Problems 

know  many  cases  where  such  a  result  is  approached,  and 
very  many  where  all  the  essential  features  which  we  regard 
as  constituting  the  characters  of  the  breed  are  reproduced 
with  approximate  certainty  in  every  member  of  the  pure- 
bred race,  which  thus  closely  approach  to  uniformity. 

But  if  two  germ-cells  of  dissimilar  constitution  unite 
in  fertilisation,  what  offspring  are  we  to  expect*?  First 
let  us  premise  that  the  answer  to  this  question  is  known 
experimentally  to  differ  for  many  organisms  and  for  many 
classes  of  characters,  and  may  almost  certainly  be  in  part 
determined  by  external  circumstances.  But  omitting  the 
last  qualification,  certain  principles  are  now  clearly  detected, 
though  what  principle  will  apply  in  any  given  case  can  only 
be  determined  by  direct  experiment  made  with  that  case. 

This  is  the  phenomenon  of  ^ ross-breeding.  As  generally 
used,  this  term  means  the  union  of  members  of  dissimilar 
varieties,  or  sj^cies  :  though  when  dissimilar  gametes  t  pro- 
duced by  two  individuals  of  the  same  variety  unite  in 
fertilisation,  we  have  essentially  cross-breeding  in  respect 
of  the  character  or  characters  in  which  those  gametes  differ. 
We  will  suppose,  as  before,  that  these  two  gametes  bearing 
properties  unlike  in  respect  of  a  given  character,  are  borne 
hy  different  individuals. 

In  the  simplest  case,  suppose  a  gamete  from  an  in- 
dividual presenting  any  character  in  intensity  A  unite  in 
fertilisation  with  another  from  an  individual  presenting 
the  same  character  in  intensity  a.     For  brevity's  sake  we 

*  In  all  the  cases  discussed  it  is  assumed  that  the  gametes  are 
similar  except  in  regard  to  the  ' '  heritage  "  they  bear,  and  that  no 
original  variation  is  taking  place.  The  case  of  mosaics  is  also  left 
wholly  out  of  account  (see  later). 

t  The  term  "gamete"  is  now  generally  used  as  the  equivalent  of 
"germ-cell,"  whether  male  or  female,  and  the  term  "zygote"  is  here 
used  for  brevity  to  denote  the  organism  resulting  from  fertilisation. 


of  Heredity  19 

may  call  the  parent  individuals  A  and  a,  and  the  resulting 
zygote  Aa.  "What  will  the  structure  of  Aa  be  in  regard  to 
the  character  we  are  considering  ? 

Up  to  Mendel  no  one  proposed  to  answer  this  question 
in  any  other  way  than  by  reference  to  the  intensity  of  the 
character  in  the  progenitors,  and  'primarily  in  the  parents, 
A  and  a,  in  whose  bodies  the  gametes  had  been  developed. 
It  was  well  known  that  such  a  reference  gave  a  very  poor 
indication  of  what  Aa  would  be.  Both  A  and  a  may  come 
from  a  population  consisting  of  individuals  manifesting  the 
same  character  in  various  intensities.  In  the  pedigree  of 
either  A  ot  a  these  various  intensities  may  have  occurred 
few  or  many  times.  Common  experience  leads  us  to  expect 
the  probability  in  regard  to  Aa  tQ>  be  influenced  by  this 
history.  The  next  step  is  that  which  Gait  on  took.  He 
extended  the  reference  beyond  the  immediate  parents  of 
Aa,  to  its  grandparents,  great-grandparents,  and  so  on,  and 
in  the  cases  he  studied  he  found  that  from  a  knowledge  of 
the  intensity  in  which  the  given  character  was  manifested 
in  each  progenitor,  even  for  some  few  generations  back,  a 
fairly  accurate  prediction  could  be  made,  not  as  to  the 
character  of  any  individual  Aa,  but  as  to  the  average 
character  of  Aa'^  of  similar  parentage,  in  general. 

But  suppose  that  instead  of  individuals  presenting  one 
character  in  differing  intensities,  two  individuals  breed 
together  distinguished  by  characters  which  we  know  to  be 
mutually  exclusive,  such  as  A  and  B.  Here  again  we  may 
speak  of  the  individuals  producing  the  gametes  as  A  and 
B,  and  the  resulting  zygote  as  AB.  What  will  ^^  be 
like  ?  The  population  here  again  may  consist  of  many  like 
A  and  like  B.  These  two  forms  may  have  been  breeding 
together  indiscriminately,  and  there  may  have  been  many 
or  few  of  either  type  in  the  pedigree  of  either  A  or  B. 

2—2 


20  The  Problems 

Here  again  Galton  applied  his  method  with  remarkable 
success.  Referring  to  the  progenitors  of  A  and  B^  deter- 
mining how  many  of  each  type  there  were  in  the  direct 
pedigree  of  A  and  of  B,  he  arrived  at  the  same  formula  as 
before,  with  the  simple  difference  that  instead  of  expressing 
the  probable  average  intensity  of  one  character  in  several 
individuals,  the  prediction  is  given  in  terms  of  the  probable 
number  of  4's  and  5's  that  would  result  on  an  average 
when  particular  J.'s  and  J5's  of  known  pedigree  breed 
together. 

The  law  as  Galton  gives  it  is  as  follows  : — 
"It  is  that  the  two  parents  contribute  between  them 
on  the  average  one-half,  or  (0'5)  of  the  total  heritage  of 
the  offspring;  the  four  grandparents,  one-quarter,  or  (0*5)^; 
the  eight  great-grandparents,  one-eighth,  or  (0*5)^,  and  so 
on.  Then  the  sum  of  the  ancestral  contributions  is  ex- 
pressed by  the  series 

{(0-5)  +  (0-5)^  +  (0-5)^&c.}, 

which,  being  equal  to  1,  accounts  for  the  whole  heritage." 

In  the  former  case  where  A  and  a  are  characters  which 
can  be  denoted  by  reference  to  a  common  scale,  the  law 
assumes  of  course  that  the  inheritance  will  be,  to  use 
Galton' s  term,  blended ^  namely  that  the  zygote  resulting 
from  the  union  of  A  with  a  wdll  on  the  average  be  more 
like  a  than  if  A  had  been  united  with  A  ;  and  conversely 
that  dji.  Aa  zygote  will  on  the  average  he  more  like  A  than 
an  aa  zygote  would  he. 

But  in  the  case  of  J.'s  and  jS's,  which  are  assumed  to 
be  mutually  exclusive  characters,  we  cannot  speak  of 
blending,  but  rather,  to  use  Galton's  term,  of  alternative 
inheritance. 

Pearson,  finding  that  the  law  whether  formulated  thus, 


of  Heredity  21 

or  in  the  modified  form  in  which  he  restated  it*,  did  not 
express  the  phenomena  of  alternative  inheritance  known 
to  him  with  sufficient  accuracy  to  justify  its  strict  appli- 
cation to  them,  and  also  on  general  grounds,  proposed  that 
the  phenomena  of  blended  and  alternative  inheritance 
should  be  treated  apart — a  suggestion  t  the  "vvisdom  of 
which  can  scarcely  be  questioned. 

Now  the  law  thus  imperfectly  set  forth  and  every 
modification  of  it  is  incomplete  in  one  respect.  It  deals 
only  Avith  the  characters  of  the  resulting  zygotes  and 
predicates  nothing  in  regard  to  the  gametes  which  go  to 
form  them.  A  good  prediction  may  be  made  as  to  any 
given  group  of  zygotes,  but  the  various  possible  constitu- 
tions of  the  gametes  are  not  explicitly  treated. 

Nevertheless  a  definite  assumption  is  implicitly  made 
regarding  the  gametes.  It  is  not  in  question  that  differences 
between  these  gametes  may  occur  in  respect  of  the  heritage 
they  bear  ;  yet  it  is  assumed  that  these  differences  will  be 
distributed  among  the  gametes  of  any  individual  zygote  in 
such  a  way  that  each  gamete  remains  capable,  on  fertilisa- 
tion, of  transmitting  all  the  characters  (both  of  the  parent- 
zygote  and  of  its  progenitors)  to  the  zygote  which  it  then 
contributes  to  form  (and  to  the  posterity  of  that  zygote)  in 
the  intensity  indicated  by  the  law.  Hence  the  gametes  of 
any  individual  are  taken  as  collectively  a  fair  sample  of  all 
the  racial  characters  in  their  appropriate  intensities,  and  this 
theory  demands  that  there  shall  have  been  no  qualitative 
redistribution  of  characters  among  the  gametes  of  any 
zygote  in  such  a  way  that  some  gametes  shall  be  finally 
excluded  from  partaking  of  and  transmitting  any  specific 

*  In  Pearson's  modification  the  parents  contribute  0*3,  the  grand- 
parents O'lo,  the  great-grandparents  -075. 
t  See  the  works  referred  to  above. 


22  The  Problems 

part  of  the  heritage.  The  theory  further  demands — and 
by  the  analogy  of  what  we  know  otherwise  not  only  of 
animals  and  plants,  but  of  physical  or  chemical  laws, 
perhaps  this  is  the  most  serious  assumption  of  all — that 
the  structure  of  the  gametes  shall  admit  of  their  being 
capable  of  transmitting  any  character  in  any  intensity 
varying  from  zero  to  totality  with  equal  ease ;  and  that 
gametes  of  each  intensity  are  all  equally  likely  to  occur, 
given  a  pedigree  of  appropriate  arithmetical  composition. 

Such  an  assumption  appears  so  improbable  that  even 
in  cases  where  the  facts  seem  as  yet  to  point  to  this 
conclusion  with  exceptional  clearness,  as  in  the  case  of 
human  stature,  I  cannot  but  feel  there  is  still  room  for 
reserve  of  judgment. 

However  this  may  be,  the  Law  of  Ancestral  Heredity, 
and  all  modifications  of  it  yet  proposed,  falls  short  in  the 
respect  specified  above,  that  it  does  not  directly  attempt 
to  give  any  account  of  the  distribution  of  the  heritage  among 
the  gametes  of  any  one  individual. 

Mendel's  conception  differs  fundamentally  from  that 
involved  in  the  Law  of  Ancestral  Heredity.  The  relation 
of  his  hypothesis  to  the  foregoing  may  be  most  easily 
shown  if  we  consider  it  first  in  application  to  the  pheno- 
mena resulting  from  the  cross-breeding  of  two  pure 
varieties. 

Let  us  again  consider  the  case  of  two  varieties  each  dis- 
playing the  same  character,  but  in  the  respective  intensities 
A  and  a.  Each  gamete  of  the  A  variety  bears  A,  and 
each  gamete  of  the  a  variety  bears  a.  When  they  unite  in 
fertilisation  they  form  the  zygote  Aa.  What  will  be  its 
characters  ?  The  Mendelian  teaching  would  reply  that 
this  can  only  be  known  by  direct  experiment  with  the  two 
forms  A  and  a,  and  that  the  characters  A  and  a  perceived 


of  Heredity  23 

in  those  two  forms  or  varieties  need  not  give  any  indication 
as  to  the  character  of  the  zygote  Aa.  It  may  display  the 
character  J.,  or  a,  or  a  character  half  way  between  the  two, 
or  a  character  beyond  A  or  below  a.  The  character  of  J. a 
is  not  regarded  as  a  heritage  transmitted  to  it  by  A  and  by 
«,  but  as  a  character  special  and  peculiar  to  Aa^  just  as 
NaCl  is  not  a  body  half  way  between  sodium  and  chlorine, 
or  such  that  its  properties  can  be  predicted  from  or  easily 
stated  in  terms  of  theirs. 

If  a  concrete  case  may  help,  a  tall  pea  A  crossed  with 
a  dwarf  a  often  produces,  not  a  plant  having  the  height  of 
either  A  or  a,  but  something  taller  than  the  pure  tall 
variety  A. 

But  if  the  case  obeys  the  Mendelian  principles — as  does 
that  here  quoted — then  it  can  be  declared  first  that  the 
gametes  of  Aa  will  not  be  bearers  of  the  character  proper  to 
Aa ;  but,  generally  speaking,  each  gamete  will  either  bear 
the  pure  A  character  or  the  pure  a  character.  There  will 
in  fact  be  a  redistribution  of  the  characters  brought  in  by 
the  gametes  which  united  to  form  the  zygote  Aa,  such  that 
each  gamete  oi  Aa  is  pure,  as  the  parental  gametes  were. 
Secondly  this  redistribution  will  occur  in  such  a  way  that, 
of  the  gametes  produced  by  such  Jla's,  on  an  average 
there  will  be  equal  numbers  of  A  gametes  and  of  a 
gametes. 

Consequently  if  J.a's  breed  together,  the  new  A  gametes 
may  meet  each  other  in  fertilisation,  forming  a  zygote  A  A, 
namely,  the  pure  A  variety  again  ;  similarly  two  a  gametes 
may  meet  and  form  aa,  or  the  pure  a  variety  again.  Bat  if 
an  A  gamete  meets  an  a  it  will  once  more  form  Aa,  with 
its  special  character.  This  Aa  is  the  hybrid,  or  "mule" 
form,  or  as  I  have  elsewhere  called  it,  the  heterozygote,  as 
distinguished  from  A  A  ov  aa  the  komozygotes. 


24  The  Problems 

Similarly  if  the  two  gametes  of  two  varieties  distin- 
guished by  characters,  A  and  B,  which  cannot  be  described 
in  terms  of  any  common  scale  (such  as  for  example  the 
"  rose  "  and  "  single  "  combs  of  fowls)  unite  in  fertilisation, 
again  the  character  of  the  mule  form  cannot  be  predicted. 
Before  the  experiment  is  made  the  "mule"  may  present  ani/ 
form.  Its  character  or  properties  can  as  yet  be  no  more 
predicted  than  could  those  of  the  compounds  of  unknown 
elements  before  the  discovery  of  the  periodic  law. 

But  again — if  the  case  be  Mendelian — the  gametes  borne 
by  AB  will  be  either  A's  or  ^'s*,  and  the  cross-bred 
^^'s  breeding  together  will  form  AA's,  AB's  and  ^^'s. 
Moreover,  if  as  in  the  normal  Mendelian  case,  AB's  bear  on 
an  average  equal  numbers  of  A  gametes  and  B  gametes,  the 
numerical  ratio  of  these  resulting  zygotes  to  each  other  will  be 

lAA  :2AB  -.IBB. 

We  have  seen  that  Mendel  makes  no  prediction  as  to 
the  outward  and  visible  characters  of  AB,  but  only  as 
to  the  essential  constitution  and  statistical  condition  of  its 
gametes  in  regard  to  the  characters  A  and  B.  Nevertheless 
in  a  large  number  of  cases  the  character  of  AB  is  known 
to  fall  into  one  of  three  categories  (omitting  mosaics). 

(1)  The  cross-bred  may  almost  always  resemble  one 
of  its  pure  parents  so  closely  as  to  be  practically 
indistinguishable  from  that  pure  form,  as  in  the 
case  of  the  yellow  cotyledon-colour  of  certain  varieties 
of  peas  when  crossed  with  green-cotyledoned  varieties  ; 
in  which   case   the  parental  character,  yellow,  thus 

*  This  conception  was  clearly  formed  by  Naudin  simultaneously 
with  Mendel,  but  it  was  not  worked  out  by  him  and  remained  a  mere 
suggestion.  In  one  place  also  Focke  came  very  near  to  the  same  idea 
(see  Bibliography). 


of  Heredity  25 

manifested  by  the  cross-bred  is  called  "dominant" 
and  the  parental  character,  green,  not  manifested,  is 
called  recessive. 

(2)  The  cross-bred  may  present  some  condition 
intermediate  between  the  two  parental  forms,  in 
which  case  we  may  still  retain  the  term  "blend" 
as  applied  to  the  zygote. 

Such  an  "intermediate"  may  be  the  apparent  mean 
between  the  two  parental  forms  or  be  nearer  to  one 
or  other  in  any  degree.  Such  a  case  is  that  of  a 
cross  between  a  rich  crimson  Magenta  Chinese  Prim- 
rose and  a  clear  White,  giving  a  flower  of  a  colour 
appropriately  described  as  a  "washy"  magenta. 

(3)  The  cross-bred  may  present  some  form  quite 
different  from  that  of  either  pure  parent.  Though, 
as  has  been  stated,  nothing  can  be  predicted  of  an  un- 
known case,  we  already  know  a  considerable  number 
of  examples  of  this  nature  in  which  the  mule-form 
approaches  sometimes  with  great  accuracy  to  that  of 
a  putative  ancestor,  near  or  remote.  It  is  scarcely 
possible  to  doubt  that  several — though  perhaps  not 
all — of  Darwin's  "reversions  on  crossing"  were  of 
this  nature. 

Such  a  case  is  that  of  the  "wild  grey  mouse "  produced 
by  the  union  of  an  albino  tame  mouse  and  a  piebald 
Japanese  mouse*.  These  "reversionary"  mice  bred 
together  produce  the  parental  tame  types,  some  other 
types,  and  "reversionary"  mice  again. 

From  what  has  been  said  it  will  now  be  clear  that  the 
appHcabihty  of  the  Mendelian  hypothesis  has,  intrinsically, 

*  See  von  Guaita,  Ber.  natnrf.  Gen.  Freilmrg  x.  1898  and  xi.  1899, 
quoted  by  Professor  Weldon  (see  later). 


26  The  Problems 

nothing  whatever  to  do  with  the  question  of  the  inheritance 
being  blended  or  alternative.  In  fact,  as  soon  as  the  relation 
of  zygote  characters  to  gamete  characters  is  appreciated,  it  is 
difficult  to  see  any  reason  for  supposing  that  the  manifes- 
tation of  characters  seen  in  the  zygotes  should  give  any 
indication  as  to  their  mode  of  allotment  among  the  gametes. 
On  a  previous  occasion  I  pointed  out  that  the  terms 
"Heredity"  and  "Inheritance"  are  founded  on  a  mis- 
application of  metaphor,  and  in  the  light  of  our  present 
knowledge  it  is  becoming  clearer  that  the  ideas  of  "trans- 
mission "  of  a  character  by  parent  to  offspring,  or  of  there 
being  any  "contribution"  made  by  an  ancestor  to  its  pos- 
terity, must  only  be  admitted  under  the  strictest  reserve, 
and  merely  as  descriptive  terms. 

We  are  now  presented  with  some  entirely  new  con- 
ceptions : — 

(1)  The  purity  of  the  gametes  in  regard  to  certain 
characters. 

(2)  The  distinction  of  all  zygotes  according  as  they  are  or 
are  not  formed  by  the  union  of  like  or  unlike  gametes. 
In  the  former  case,  apart  from  Variation,  they  breed 
true  when  mated  with  their  like ;  in  the  latter  case 
their  offspring,  collectively,  will  be  heterogeneous. 

(3)  If  the  zygote  be  formed  by  the  union  of  dissimilar 
gametes,  we  may  meet  the  phenomenon  of  (a)  domi- 
nant and  recessive  characters  ;  (b)  a  blend  form ; 
(c)  a  form  distinct  from  either  parent,  often 
reversionary'^. 

*  This  fact  sufficiently  indicates  the  difficulties  involved  in  a 
superficial  treatment  of  the  phenomenon  of  reversion.  To  call  such 
reversions  as  those  named  above  "  returns  to  ancestral  type  "  would 
be,  if  more  than  a  descriptive  phrase  were  intended,  quite  misleading. 


of  Heredity  27 

But  there  are  additional  and  even  more  significant  de- 
ductions from  the  facts.  We  have  seen  that  the  gametes  are 
differentiated  in  respect  of  pure  characters.  Of  these  pure 
characters  there  may  conceivably  be  any  number  associated 
together  in  one  organism.  In  the  pea  Mendel  detected  at 
least  seven — not  all  seen  by  him  combined  in  the  same 
plant,  but  there  is  every  likelihood  that  they  are  all  capable 
of  being  thus  combined. 

Each  such  character,  which  is  capable  of  being  dissociated 
or  replaced  by  its  contrary,  must  henceforth  be  conceived 
of  as  a  distinct  unit-character ;  and  as  we  know  that  the 
several  unit-characters  are  of  such  a  nature  that  any  one 
of  them  is  capable  of  independently  displacing  or  being  dis- 
placed by  one  or  more  alternative  characters  taken  singly, 
we  may  recognize  this  fact  by  naming  such  unit-characters 
allelomorphs.  So  far,  we  know  very  little  of  any  allelomorphs 
existing  otherwise  than  as  pairs  of  contraries,  but  this  is 
probably  merely  due  to  experimental  limitations  and  the 
rudimentary  state  of  our  knowledge. 

In  one  case  (combs  of  fowls)  we  know  three  characters, 
pea  comb,  rose  comb  and  single  comb ;  of  which  pea  and 
single,  or  rose  and  single,  behave  towards  each  other  as  a 
pair  of  allelomorphs,  but  of  the  behaviour  of  pea  and  rose 
towards  each  other  we  know  as  yet  nothing. 

We  have  no  reason  as  yet  for  affirming  that  any 
phenomenon  properly  described  as  displacement  of  one 
allelomorph  by  another  occurs,  though  the  metaphor  may 
be  a  useful  one.  In  all  cases  where  dominance  has  been 
perceived,  we  can  affirm  that  the  members  of  the  allelo- 
morphic  pair  stand  to  each  other  in  a  relation  the  nature 

It  is  not  the  ancestral  type  that  has  come  back,  but  something  else 
has  come  in  its  guise,  as  the  offspring  presently  prove.  For  the  first 
time  we  thus  begin  to  get  a  rationale  of  "  reversion." 


28  The  Problems 

of  whicli  we  are  as  yet  wholly  unable  to  apprehend  or 
illustrate. 

To  the  new  conceptions  already  enumerated  we  may 
therefore  add 

(4)  Unit-characters  of  which  some,  when  once  arisen  by 
Variation,  are  alternative  to  each  other  in  the  consti- 
tution of  the  gametes,  according  to  a  definite  system. 

From  the  relations  subsisting  between  these  characters, 
it  follows  that  as  each  zygotic  union  of  allelomorphs  is  re- 
solved on  the  formation  of  the  gametes,  no  zygote  can  give 
rise  to  gametes  collectively  representing  more  than  two  cha- 
racters allelomorphic  to  each  other,  apart  from  new  variation. 

From  the  fact  of  the  existence  of  the  interchangeable 
characters  we  must,  for  purposes  of  treatment,  and  to  com- 
plete the  possibilities,  necessarily  form  the  conception  of  an 
irresoluble  base,  though  whether  such  a  conception  has  any 
objective  reality  we  have  no  means  as  yet  of  determining. 

We  have  now  seen  that  when  the  varieties  A  and  B 
are  crossed  together,  the  heterozygote,  AB,  produces 
gametes  bearing  the  pure  A  character  and  the  pure  B 
character.  In  such  a  case  we  speak  of  such  characters  as 
simple  allelomorphs.  In  many  cases  however  a  more 
complex  phenomenon  happens.  The  character  brought  in 
on  fertilisation  by  one  or  other  parent  may  be  of  such  a 
nature  that  when  the  zygote,  AB,  forms  its  gametes,  these 
are  not  individually  bearers  merely  of  A  and  B,  but  of  a 
number  of  characters  themsehes  again  integral,  which  in, 
say  A,  behaved  as  one  character  so  long  as  its  gametes 
united  in  fertilisation  with  others  Hke  themselves,  but  on 
cross-fertilisation  are  resolved  and  redistributed  among  the 
gametes  produced  by  the  cross-bred  zygote. 

In  such  a  case  we  call  the  character  A  a  compound 


of  Heredity  29 

allelomorph,  and  we  can  speak  of  the  integral  characters 
which  constitute  it  as  hypallelomorpJis.  We  ought  to  write 
the  heterozygote  {AA'A"...)  B  and  the  gametes  produced 
by  it  may  be  of  the  form  A,  A',  A",  A"\...B.  Or  the 
resolution  may  be  incomplete  in  various  degrees,  as  we 
already  suspect  from  certain  instances ;  in  which  case  we 
may  have  gametes  A,  A'A'\  A!" A"" ,  A'A'A\...B,  and 
so  on.  Each  of  these  may  meet  a  similar  or  a  dissimilar 
gamete  in  fertilisation,  forming  either  a  homozygote,  or  a 
heterozygote  with  its  distinct  properties. 

In  the  case  of  compound  allelomorphs  we  know  as  yet 
nothing  of  the  statistical  relations  of  the  several  gametes. 

Thus  we  have  the  conception 

(5)  of  a  Compound  character^  borne  by  one  gamete, 
transmitted  entire  as  a  single  character  so  long  as 
fertilisation  only  occurs  between  like  gametes,  or  is, 
in  other  words,  ''symmetrical,"  but  if  fertilisation 
take  place  with  a  dissimilar  gamete  (or  possibly  by 
other  causes),  resolved  into  integral  constituent- 
characters,  each  separately  transmissible. 

Next,  as,  by  the  union  of  the  gametes  bearing  the 
various  hypallelomorphs  with  other  such  gametes,  or  with 
gametes  bearing  simple  allelomorphs,  in  fertilisation,  a 
number  of  new  zygotes  will  be  formed,  such  as  may  not  have 
been  seen  before  in  the  breed  :  these  will  inevitably  be 
spoken  of  as  varieties ;  and  it  is  difficult  not  to  extend  the 
idea  of  variation  to  them.  To  distinguish  these  from  other 
variations — which  there  must  surely  be — we  may  call  them 

(6)  Analytical  variations  in  contradistinction  to 

(7)  Synthetical  variations,  occurring  not  by  the 
separation  of  pre-existing  constituent-characters  but 
by  the  addition  of  new  characters. 


30  The  Problems 

Lastly,  it  is  impossible  to  be  presented  with  the  fact 
that  in  Mendelian  cases  the  cross-bred  produces  on  an 
average  equal  numbers  of  gametes  of  each  kind,  that  is  to 
say,  a  symmetrical  result,  without  suspecting  that  this  fact 
must  correspond  with  some  symmetrical  figure  of  distribu- 
tion of  those  gametes  in  the  cell-divisions  by  which  they  are 
produced. 

At  the  present  time  these  are  the  main  conceptions — 
though  by  no  means  all — arising  directly  from  Mendel's 
work.  The  first  six  are  all  more  or  less  clearly  embodied 
by  him,  though  not  in  every  case  developed  in  accordance 
with  modern  knowledge.  The  seventh  is  not  a  Mendelian 
conception,  but  the  facts  before  us  justify  its  inclusion  in 
the  above  list  though  for  the  present  it  is  little  more  than 
a  mere  surmise. 

In  Mendelian  cases  it  will  now  be  perceived  that  all 
the  zygotes  composing  the  population  consist  of  a  limited 
number  of  possible  types,  each  of  definite  constitution, 
bearing  gametes  also  of  a  limited  and  definite  number  of 
types,  and  definite  constitution  in  respect  of  pre-existing 
characters.  It  is  now  evident  that  in  such  cases  each 
several  progenitor  need  not  be  brought  to  account  in 
reckoning  the  probable  characters  of  each  descendant ; 
for  the  gametes  of  cross-breds  are  differentiated  at  each 
successive  generation,  some  parental  (Mendelian)  characters 
being  left  out  in  the  composition  of  each  gamete  produced 
by  a  zygote  arising  by  the  union  of  bearers  of  opposite 
allelomorphs. 

When  from  these  considerations  we  return  to  the 
phenomena  comprised  in  the  Law  of  Ancestral  Heredity, 
what  certainty  have  we  that  the  same  conceptions  are  not 
applicable  there  also  ? 


of  Heredity  31 

It  has  now  been  shown  that  the  question  whether  in  the 
cross-bred  zygotes  in  general  the  characters  blend  or  are 
mutually  exclusive  is  an  entirely  subordinate  one,  and 
distinctions  with  regard  to  the  essential  nature  of  heredity 
based  on  these  circumstances  become  irrelevant. 

In  the  case  of  a  population  presenting  continuous 
variation  in  regard  to  say,  stature,  it  is  easy  to  see  how 
purity  of  the  gametes  in  respect  of  any  intensities  of 
that  character  might  not  in  ordinary  circumstances  be 
capable  of  detection.  There  are  doubtless  more  than 
two  pure  gametic  forms  of  this  character,  but  there  may 
quite  conceivably  be  six  or  eight.  When  it  is  remem- 
bered that  each  heterozygous  combination  of  any  two 
may  have  its  own  appropriate  stature,  and  that  such  a 
character  is  distinctly  dependent  on  external  conditions, 
the  mere  fact  that  the  observed  curves  of  stature  give 
"chance  distributions"  is  not  surprising  and  may  still  be 
compatible  with  purity  of  gametes  in  respect  of  certain 
pure  types.  In  peas  {P.  sativum),  for  example,  from 
Mendel's  work  we  know  that  the  tall  forms  and  the  ex- 
treme dwarf  forms  exhibit  gametic  purity.  I  have  seen 
at  Messrs  Sutton's  strong  evidence  of  the  same  nature 
in  the  case  of  the  tall  Sweet  Pea  {Lathyrus  odoratus) 
and  the  dwarf  or  procumbent  "Cupid"  form. 

But  in  the  case  of  the  Sweet  Pea  we  know  at  least  one 
pure  form  of  definitely  intermediate  height,  and  in  the 
case  of  P.  sativum  there  are  many.  When  the  extreme 
types  breed  together  it  will  be  remembered  the  heterozygote 
commonly  exceeds  the  taller  in  height.  In  the  next 
generation,  since  there  is,  in  the  case  of  extremes,  so  much 
margin  between  the  types  of  the  two  pure  forms,  the  return 
of  the  offspring  to  the  three  forms  of  which  two  are  homo- 
zygous and  one  heterozygous  is  clearly  perceptible. 


32  The  Problems 

If  however  instead  of  pure  extreme  varieties  we  were  to 
take  a  pair  of  varieties  differing  normally  by  only  a  foot  or 
two,  we  might,  owing  to  the  masking  effects  of  conditions, 
&c.,  have  great  difficulty  in  distinguishing  the  three  forms 
in  the  second  generation.  There  would  besides  be  twice  as 
many  heterozygous  individuals  as  homozygous  individuals 
of  each  kind,  giving  a  symmetrical  distribution  of  heights, 
and  who  might  not — in  pre-Mendelian  days — have  accepted 
such  evidence — made  still  less  clear  by  influence  of  con- 
ditions— as  proof  of  Continuous  Variation  both  of  zygotes 
and  gametes  ? 

Suppose,  then,  that  instead  of  two  pure  types,  we  had 
six  or  eight  breeding  together,  each  pair  forming  their  own 
heterozygote,  there  would  be  a  very  remote  chance  of  such 
purity  or  fixity  of  type  whether  of  gamete  or  zygote  being 
detected. 

Dominance,  as  we  have  seen,  is  merely  a  phenomenon 
incidental  to  specific  cases,  between  which  no  other  common 
property  has  yet  been  perceived.  In  the  phenomena  of 
blended  inheritance  we  clearly  have  no  dominance.  In  the 
cases  of  alternative  inheritance  studied  by  Galton  and 
Pearson  there  is  evidently  no  universal  dominance.  From 
the  tables  of  Basset  hound  pedigrees  there  is  clearly  no 
definite  dominance  of  either  of  the  coat-colours.  In  the  case 
of  eye-colour  the  published  tables  do  not,  so  far  as  I  have 
discovered,  furnish  the  material  for  a  decision,  though  it  is 
scarcely  possible  the  phenomenon,  even  if  only  occasional, 
could  have  been  overlooked.  We  must  take  it,  then,  there 
is  no  sensible  dominance  in  these  cases  :  but  whether  there 
is  or  is  not  sensible  gametic  purity  is  an  altogether  different 
question,  which,  so  far  as  I  can  judge,  is  as  yet  untouched. 
It  may  perfectly  well  be  that  we  shall  be  compelled  to 
recognize  that  in  many  cases  there  is  no  such  purity,  and 


of  Heredity  33 

that  the  characters  may  be  carried  by  the  gametes  in  any 
proportion  from  zero  to  totahty,  just  as  some  substances 
may  be  carried  in  a  solution  in  any  proportion  from  zero 
to  saturation  without  discontinuous  change  of  properties. 
That  this  will  be  found  true  in  some  cases  is,  on  any 
hypothesis,  certain ;  but  to  prove  the  fact  for  any  given 
case  will  be  an  exceedingly  difficult  operation,  and  I  scarcely 
think  it  has  been  yet  carried  through  in  such  a  way  as  to 
leave  no  room  for  doubt. 

Conversely,  the  absolute  and  iinivei^sal  purity  of  the 
gametes  has  certainly  not  yet  been  determined  for  any 
case  ;  not  even  in  those  cases  where  it  looks  most  likely 
that  such  universal  purity  exists.  Impairment  of  such 
purity  we  may  conceive  either  to  occur  in  the  form  of 
mosaic  gametes,  or  of  gametes  with  blended  properties. 
On  analogy  and  from  direct  evidence  we  have  every  right 
to  believe  that  gametes  of  both  these  classes  may  occur  in 
rare  and  exceptional  cases,  of  as  yet  unexplored  nature*, 
but  such  a  phenomenon  will  not  diminish  the  significance 
of  observed  purity. 

We  have  now  seen  the  essential  nature  of  the  Mendelian 
principles  and  are  able  to  appreciate  the  exact  relation  in 
which  they  stand  to  the  gToup  of  cases  included  in  the  Law 
of  Ancestral  Heredity.  In  seeking  any  general  indication 
as  to  the  common  properties  of  the  phenomena  which  are 
already  know^n  to  obey  Mendelian  principles  we  can  as  yet 
point  to  none,  and  whether  some  such  common  features 
exist  or  not  is  unknown. 

There  is  however  one  group  of  cases,  definite  though 
as  yet  not  numerous,  where  we  know  that  the  Mendelian 

*  It  will  be  understood  from  what  follows,  that  the  existence  of 
mosaic  zygotes  is  no  proof  that  either  component  gamete  was  mosaic. 

B.  3 


34  The  Problems 

principles  do  not  apply.  These  are  the  phenomena  upon 
which  Mendel  touches  in  his  brief  paper  on  Hieracium. 
As  he  there  states,  the  hybrids,  if  they  are  fertile  at  all, 
produce  offspring  like  themselves,  not  like  their  parents. 
In  further  illustration  of  this  phenomenon  he  cites  Wichura's 
Salix  hybrids.  Perhaps  some  dozen  other  such  illustrations 
could  be  given  which  rest  on  good  evidence.  To  these 
cases  the  Mendelian  principle  will  in  nowise  apply,  nor  is  it 
easy  to  conceive  any  modification  of  the  law  of  ancestral 
heredity  which  can  express  them.  There  the  matter  at 
present  rests.  Among  these  cases,  however,  we  perceive 
several  more  or  less  common  features.  They  are  often, 
though  not  always,  hybrids  between  forms  differing  in 
many  characters.  The  first  cross  frequently  is  not  the 
exact  intermediate  between  the  two  parental  types,  but 
may  as  in  the  few  Hieracium  cases  be  irregular  in  this 
respect.  There  is  often  some  degree  of  sterility.  In  the 
absence  of  fuller  and  statistical  knowledge  of  such  cases 
further  discussion  is  impossible. 

Another  class  of  cases,  untouched  by  any  hypothesis  of 
heredity  yet  propounded,  is  that  of  the  false  hybrids  of 
Millardet,  where  we  have  fertilisation  without  transmission 
of  one  or  several  parental  characters.  In  these  not  only 
does  the  first  cross  show,  in  some  respect,  the  character  or 
characters  of  one  parent  only,  but  in  its  posterity  no  re- 
appearance of  the  lost  character  or  characters  is  observed. 
The  nature  of  such  cases  is  still  quite  obscure,  but  we  have 
to  suppose  that  the  allelomorph  of  one  gamete  only  developes 
after  fertilisation  to  the  exclusion  of  the  corresponding  alle- 
lomorph of  the  other  gamete,  much — if  the  crudity  of  the 
comparison  may  be  pardoned — as  occurs  on  the  female  side 
in  parthenogenesis  without  fertilisation  at  all. 


of  Heredity  35 

To  these  as  yet  altogether  unconformable  cases  we  can 
scarcely  doubt  that  further  experiment  will  add  many  more. 
Indeed  we  already  have  tolerably  clear  evidence  that  many 
phenomena  of  inheritance  are  of  a  much  higher  order  of 
complexity.  When  the  paper  on  Pisum  was  written 
Mendel  apparently  inclined  to  the  view  that  with  modi- 
fications his  law  might  be  found  to  include  all  the  phenomena 
of  hybridisation,  but  in  the  brief  subsequent  paper  on 
Hieracium  he  clearly  recognized  the  existence  of  cases  of 
a  different  nature.  Those  who  read  that  contribution  will 
be  interested  to  see  that  he  lays  down  a  principle  which 
may  be  extended  from  hybridisation  to  heredity  in  general, 
that  the  laws  of  each  new  case  must  be  determined  by 
separate  experiment. 

As  regards  the  Mendelian  principles,  which  it  is  the 
chief  aim  of  this  introduction  to  present  clearly  before  the 
reader,  a  professed  student  of  variation  will  easily  be  able 
to  fill  in  the  outline  now  indicated,  and  to  illustrate  the 
various  conceptions  from  phenomena  already  familiar.  To 
do  this  is  beyond  the  scope  of  this  short  sketch.  But 
enough  perhaps  has  now  been  said  to  show  that  by  the 
application  of  those  principles  we  are  enabled  to  reach  and 
deal  in  a  comprehensive  manner  with  phenomena  of  a 
fundamental  nature,  lying  at  the  very  root  of  all  con- 
ceptions not  merely  of  the  physiology  of  reproduction 
and  heredity,  but  even  of  the  essential  nature  of  living 
organisms ;  and  I  think  that  I  used  no  extravagant  words 
when,  in  introducing  Mendel's  work  to  the  notice  of  readers 
of  the  Royal  Horticultural  Society's  Journal,  I  ventured  to 
declare  that  his  experiments  are  worthy  to  rank  with  those 
which  laid  the  foundation  of  the  Atomic  laws  of  Chemistry. 


3—2 


36  Brief  Historical  Notice 

As  some  biographical  particulars  of  this  remarkable 
investigator  will  be  welcome,  I  give  the  following  brief 
notice,  first  published  by  Dr  Correns  on  the  authority 
of  Dr  von  Schanz  :  Gregor  Johann  Mendel  was  born  on 
July  22,  1822,  at  Heinzendorf  bei  Odrau,  in  Austrian 
Silesia.  He  was  the  son  of  well-to-do  peasants.  In  1843 
he  entered  as  a  novice  the  "Koniginkloster,"  an  Augustinian 
foundation  in  Altbriinn.  In  1847  he  was  ordained  priest. 
From  1851  to  1853  he  studied  physics  and  natural  science 
at  Vienna.  Thence  he  returned  to  his  cloister  and  became 
a  teacher  in  the  Realschule  at  Brlinn.  Subsequently  he 
was  made  Abbot,  and  died  January  6,  1884,  The  experi- 
ments described  in  his  papers  were  carried  out  in  the 
garden  of  his  Cloister.  Besides  the  two  papers  on  hybridi- 
sation, dealing  respectively  with  Pisum  and  Hieracium, 
Mendel  contributed  two  brief  notes  to  the  Verh.  Zool.  hot. 
Verein^  Wien,  on  Scopolia  margarltalis  (1853,  iii.,  p.  116) 
and  on  Bruchus  pisi  {ibid.  1854,  iv.,  p.  27).  In  these 
papers  he  speaks  of  himself  as  a  pupil  of  KoUar. 

Mendel  published  in  the  Briinn  journal  statistical 
observations  of  a  meteorological  character,  but,  so  far 
as  I  am  aware,  no  others  relating  to  natural  history. 
Dr  Correns  tells  me  that  in  the  latter  part  of  his  life 
lie  engaged  in  the  Ultramontane  Controversy.  He  was 
for  a  time  President  of  the  Briinn  Society^. 

For  the  photograph  of  Mendel  which  forms  the  frontis- 
piece to  this  work,  I  am  indebted  to  the  Very  Rev.  Dr 
Janeischek,  the  present  Abbot  of  Briinn,  who  most  kindly 
supplied  it  for  this  purpose. 

So  far  as  I  have  discovered  there  was,  up  to  1900,  only 
one  reference  to  Mendel's  observations  in  scientific  literature, 
namely  that  of  Focke,  Pflanzenmischlinge ,  1881,  p.  109, 
*  A  few  additional  particulars  are  given  in  Tschermak's  edition. 


Brief  Historical  Notice  37 

where  it  is  simply  stated  that  Mendel's  numerous  experi- 
ments on  Pisum  gave  results  similar  to  those  obtained 
by  Knight,  but  that  he  believed  he  had  found  constant 
numerical  ratios  among  the  types  produced  by  hybridisation. 
In  the  same  work  a  similar  brief  reference  is  made  to  the 
paper  on  Hieracium. 

It  may  seem  surprising  that  a  work  of  such  importance 
should  so  long  have  failed  to  find  recognition  and  to  become 
current  in  the  world  of  science.  It  is  true  that  the  journal 
in  which  it  appeared  is  scarce,  but  this  circumstance  has 
seldom  long  delayed  general  recognition.  The  cause  is 
unquestionably  to  be  found  in  that  neglect  of  the  experi- 
mental study  of  the  problem  of  Species  which  supervened 
on  the  general  acceptance  of  the  Darmnian  doctrines.  The 
problem  of  Species,  as  Kolreuter,  Gartner,  Naudin,  Wichura, 
and  the  other  hybridists  of  the  middle  of  the  nineteenth 
century  conceived  it,  attracted  thenceforth  no  workers.  The 
question,  it  was  imagined,  had  been  answered  and  the 
debate  ended.  No  one  felt  much  interest  in  the  matter. 
A  host  of  other  lines  of  work  were  suddenly  opened  up,  and 
in  1865  the  more  original  investigators  naturally  found 
those  new  methods  of  research  more  attractive  than  the 
tedious  observations  of  the  hybridisers,  whose  inquiries 
were  supposed,  moreover,  to  have  led  to  no  definite  result. 

Nevertheless  the  total  neglect  of  such  a  discovery  is 
not  easy  to  account  for.  Those  who  are  acquainted  with 
the  literature  of  this  branch  of  inquiry  will  know  that  the 
French  Academy  offered  a  prize  in  1861  to  be  awarded  in 
1862  on  the  subject  ^^  Etudier  les  Hyhrides  vegetaucc  au 
point  de  vue  de  leur  fecondite  et  de  la  perpetuite  de  leurs 
caracteresy  This  subject  was  doubtless  chosen  with 
reference  to  the  experiments  of  Godron  of  Nancy  and 
Naudin,  then  of  Paris.     Both  these  naturalists  competed. 


38  Brief  Historical  Notice 

and  the  accounts  of  the  work  of  Godron  on  Datura  and 
of  Nan  din  on  a  number  of  species  were  published  in  the 
years  1864  and  1865  respectively.  Both,  especially  the 
latter,  are  works  of  high  consequence  in  the  history  of  the 
science  of  heredity.  In  the  latter  paper  Naudin  clearly 
enuntiated  what  we  shall  henceforth  know  as  the  Mendelian 
conception  of  the  dissociation  of  characters  of  cross-breds 
in  the  formation  of  the  germ-cells,  though  apparently  he 
never  developed  this  conception. 

In  the  year  1864,  George  Bentham,  then  President  of 
the  Linnean  Society,  took  these  treatises  as  the  subject  of 
his  address  to  the  Anniversary  meeting  on  the  24  May, 
Naudin's  work  being  known  to  him  from  an  abstract,  the 
full  paper  having  not  yet  appeared.  Referring  to  the 
hypothesis  of  dissociation  which  he  fully  described,  he  said 
that  it  appeared  to  be  new  and  well  supported,  but  required 
much  more  confirmation  before  it  could  be  held  as  proven. 
{J.  Linn.  Soc,  Bot.,  viii.,  Froc,  p.  xiv.) 

In  1865,  the  year  of  Mendel's  communication  to  the 
Briinn  Society,  appeared  Wichura's  famous  treatise  on  his 
experiments  with  Salicc  to  which  Mendel  refers.  There  are 
passages  in  this  memoir  which  come  very  near  Mendel's 
principles,  but  it  is  evident  from  the  plan  of  his  experiments 
that  Mendel  had  conceived  the  whole  of  his  ideas  before 
that  date. 

In  1868  appeared  the  first  edition  of  Darwin's  Animals 
and  Plants,  marking  the  very  zenith  of  these  studies,  and 
thenceforth  the  decline  in  the  experimental  investigation 
of  Evolution  and  the  problem  of  Species  has  been  steady. 
With  the  rediscovery  and  confirmation  of  Mendel's  work 
by  de  Vries,  Correns  and  Tschermak  in  1900  a  new  era 
begins. 

That  Mendel's  work,  appearing,  as  it  did,  at  a  moment 


Brief  Historical  Notice  39 

when  several  naturalists  of  the  first  rank  were  still  occupied 
with  these  problems,  should  have  passed  wholly  unnoticed, 
wdll  always  remain  inexplicable,  the  more  so  as  the  Briinn 
Society  exchanged  its  publications  with  most  of  the 
Academies  of  Europe,  including  both  the  Royal  and 
Linnean  Societies. 

Nau din's  views  were  well  known  to  Darwin  and  are 
discussed  in  Animals  and  Plants  (ed.  1885,  ii.,  p.  23);  but, 
put  forward  as  they  were  without  full  proof,  they  could  not 
command  universal  credence.  Gartner,  too,  had  adopted 
opposite  views;  and  Wichura,  working  with  cases  of 
another  order,  had  proved  the  fact  that  some  hybrids  breed 
true.  Consequently  it  is  not  to  be  wondered  at  that 
Darwin  was  sceptical.  Moreover,  the  Mendelian  idea  of 
the  "hybrid-character,"  or  heterozygous  form,  was  unknown 
to  him,  a  conception  without  which  the  hypothesis  of  dis- 
sociation of  characters  is  quite  imperfect. 

Had  Mendel's  work  come  into  the  hands  of  Darwin,  it 
is  not  too  much  to  say  that  the  history  of  the  development 
of  evolutionary  philosophy  would  have  been  very  different 
from  that  which  we  have  witnessed. 


EXPERIMENTS    IN  PLANT-HYBRIDISATION*. 
By  Gregor  Mendel. 

{Read  at  the  Meetings  of  the  Sth  February 
and  8th  March,  1865.) 

Introductory  Remarks. 

Experience  of  artificial  fertilisation,  such  as  is  effected 
with  ornamental  plants  in  order  to  obtain  new  variations 
in  colour,  has  led  to  the  experiments  which  will  here  be 
discussed.  The  striking  regularity  with  which  the  same 
hybrid  forms  always  reappeared  whenever  fertilisation  took 
place  between  the  same  species  induced  further  experiments 
to  be  undertaken,  the  object  of  which  was  to  follow  up  the 
developments  of  the  hybrids  in  their  progeny. 

To  this  object  numerous  careful  observers,  such  as 
Kolreuter,  Gartner,  Herbert,  Lecoq,  Wichura  and  others, 
have  devoted  a  part  of  their  lives  with  inexhaustible 
perseverance.  Gartner  especially,  in  his  work  "Die  Bas- 
tarderzeugung  im  Pflanzenreiche"  (The  Production  of 
Hybrids  in  the  Vegetable  Kingdom),  has  recorded  very 
valuable  observations ;  and  quite  recently  Wichura  published 
the  results  of  some  profound  investigations  into  the  hybrids 

*  [This  translation  was  made  by  the  Eoyal  Horticultural  Society, 
and  is  reprinted  with  modifications  and  corrections,  by  permission. 
The  original  paper  was  published  in  the  Verh.  naturf.  Ver.  in  Brunn, 
Abhandlungeii,  iv.  1865,  which  appeared  in  1866.] 


Menders  Experiments  in  Hybridisation      41 

of  the  Willow.  That,  so  far,  no  generally  applicable  law 
governing  the  formation  and  development  of  hybrids  has 
been  successfully  formulated  can  hardly  be  wondered  at  by 
anyone  who  is  acquainted  with  the  extent  of  the  task,  and 
can  appreciate  the  difficulties  with  which  experiments  of 
this  class  have  to  contend.  A  final  decision  can  only  be 
arrived  at  when  we  shall  have  before  us  the  results  of 
detailed  experiments  made  on  plants  belonging  to  the  most 
diverse  orders. 

Those  who  survey  the  work  done  in  this  department 
will  arrive  at  the  conviction  that  among  all  the  numerous 
experiments  made,  not  one  has  been  carried  out  to  such  an 
extent  and  in  such  a  way  as  to  make  it  possible  to  determine 
the  number  of  different  forms  under  which  the  offspring  of 
hybrids  appear,  or  to  arrange  these  forms  with  certainty 
according  to  their  separate  generations,  or  to  definitely 
ascertain  their  statistical  relations*. 

It  requires  indeed  some  courage  to  undertake  a  labour 
of  such  far-reaching  extent ;  it  appears,  however,  to  be  the 
only  right  way  by  which  we  can  finally  reach  the  solution 
of  a  question  the  importance  of  which  cannot  be  over- 
estimated in  connection  with  the  history  of  the  evolution 
of  organic  forms. 

The  paper  now  presented  records  the  results  of  such 
a  detailed  experiment.  This  experiment  was  practically 
confined  to  a  small  plant  group,  and  is  now,  after  eight 
years'  pursuit,  concluded  in  all  essentials.  Whether  the 
plan  upon  which  the  separate  experiments  were  conducted 
and  carried  out  was  the  best  suited  to  attain  the  desired 
end  is  left  to  the  friendly  decision  of  the  reader. 

*  [It  is  to  the  clear  conception  of  these  three  primary  necessities 
that  the  whole  success  of  Mendel's  work  is  due.  So  far  as  I  know 
this  conception  was  absolutely  new  iu  his  day.] 


42  Menders  Experiments 


Selection  of  the  Experimental  Plants. 

The  value  and  utility  of  any  experiment  are  determined 
by  the  fitness  of  the  material  to  the  purpose  for  which  it  is 
used,  and  thus  in  the  case  before  us  it  cannot  be  immaterial 
what  plants  are  subjected  to  experiment  and  in  what  manner 
such  experiments  are  conducted. 

The  selection  of  the  plant  group  which  shall  serve  for 
experiments  of  this  kind  must  be  made  with  all  possible 
care  if  it  be  desired  to  avoid  from  the  outset  every  risk  of 
questionable  results. 

The  experimental  plants  must  necessarily — 

1.  Possess  constant  differentiating  characters. 

2.  The  hybrids  of  such  plants  must,  during  the 
flowering  period,  be  protected  from  the  influence  of  all 
foreign  pollen,  or  be  easily  capable  of  such  protection. 

The  hybrids  and  their  offspring  should  suffer  no  marked 
disturbance  in  their  fertility  in  the  successive  generations. 

Accidental  impregnation  by  foreign  pollen,  if  it  oc- 
curred during  the  experiments  and  were  not  recognized, 
would  lead  to  entirely  erroneous  conclusions.  Reduced 
fertility  or  entire  sterility  of  certain  forms,  such  as  occurs  in 
the  offspring  of  many  hybrids,  would  render  the  experiments 
very  difficult  or  entirely  frustrate  them.  In  order  to  dis- 
cover the  relations  in  which  the  hybrid  forms  stand  towards 
each  other  and  also  towards  their  progenitors  it  appears  to 
be  necessary  that  all  members  of  the  series  developed  in 
each  successive  generation  should  be,  without  exception, 
subjected  to  observation. 

At  the  very  outset  special  attention  was  devoted  to  the 
Leguminosce  on  account  of  their  peculiar  floral  structure. 


in  Hybridisation  43 

Experiments  which  were  made  with  several  members  of  this 
family  led  to  the  result  that  the  genus  Pisum  was  found  to 
possess  the  necessary  conditions. 

Some  thoroughly  distinct  forms  of  this  genus  possess 
characters  which  are  constant,  and  easily  and  certainly 
recognisable,  and  when  their  hybrids  are  mutually  crossed 
they  yield  perfectly  fertile  progeny.  Furthermore,  a  dis- 
turbance through  foreign  pollen  cannot  easily  occur,  since 
the  fertilising  organs  are  closely  packed  inside  the  keel  and 
the  anther  bursts  within  the  bud,  so  that  the  stigma 
becomes  covered  with  pollen  even  before  the  flower  opens. 
This  circumstance  is  of  especial  importance.  As  additional 
advantages  worth  mentioning,  there  may  be  cited  the  easy 
culture  of  these  plants  in  the  open  ground  and  in  pots,  and 
also  their  relatively  short  period  of  growth.  Artificial 
fertilisation  is  certainly  a  somewhat  elaborate  process,  but 
nearly  always  succeeds.  For  this  purpose  the  bud  is 
opened  before  it  is  perfectly  developed,  the  keel  is  removed, 
and  each  stamen  carefully  extracted  by  means  of  forceps, 
after  which  the  stigma  can  at  once  be  dusted  over  with  the 
foreign  pollen. 

In  all,  thirty-four  more  or  less  distinct  varieties  of  Peas 
were  obtained  from  several  seedsmen  and  subjected  to  a 
two  years'  trial.  In  the  case  of  one  variety  there  were 
remarked,  among  a  larger  number  of  plants  all  alike,  a  few 
forms  which  were  markedly  different.  These,  however,  did 
not  vary  in  the  following  year,  and  agreed  entirely  with 
another  variety  obtained  from  the  same  seedsmen ;  the 
seeds  were  therefore  doubtless  merely  accidentally  mixed. 
All  the  other  varieties  yielded  perfectly  constant  and 
similar  offspring ;  at  any  rate,  no  essential  difference  was 
observed  during  two  trial  years.  For  fertilisation  twenty- 
two  of  these  were  selected  and  cultivated  during  the  whole 


44  Mendel's  Experiments 

period  of  the  experiments.    They  remained  constant  without 
any  exception. 

Their  systematic  classification  is  difficult  and  uncertain. 
If  we  adopt  the  strictest  definition  of  a  species,  according 
to  which  only  those  individuals  belong  to  a  species  which 
under  precisely  the  same  circumstances  display  precisely 
similar  characters,  no  two  of  these  varieties  could  be  re- 
ferred to  one  species.  According  to  the  opinion  of  experts, 
however,  the  majority  belong  to  the  species  Pisum  satimim  ; 
while  the  rest  are  regarded  and  classed,  some  as  sub-species 
of  P.  sativum,  and  some  as  independent  species,  such  as 
P.  quadratum,  P.  saccharatum,  and  P.  umhellatum.  The 
positions,  however,  which  may  be  assigned  to  them  in  a 
classificatory  system  are  quite  immaterial  for  the  purposes 
of  the  experiments  in  question.  It  has  so  far  been  found 
to  be  just  as  impossible  to  draw  a  sharp  line  between  the 
hybrids  of  species  and  varieties  as  between  species  and 
varieties  themselves. 


Division  and  Arrangement  of  the  Experiments. 

If  two  plants  which  differ  constantly  in  one  or  several 
characters  be  crossed,  numerous  experiments  have  demon- 
strated that  the  common  characters  are  transmitted  un- 
changed to  the  hybrids  and  their  progeny  ;  but  each  pair  of 
differentiating  characters,  on  the  other  hand,  unite  in  the 
hybrid  to  form  a  new  character,  which  in  the  progeny  of  the 
hybrid  is  usually  variable.  The  object  of  the  experiment 
was  to  observe  these  variations  in  the  case  of  each  pair  of 
differentiating  characters,  and  to  deduce  the  law  according 
to  which  they  appear  in  the  successive  generations.  The 
experiment  resolves    itself    therefore   into   just  as   many 


in  Hybridisation  45 

separate  experiments  as  there  are  constantly  differentiating 
characters  presented  in  the  experimental  plants. 

The  various  forms  of  Peas  selected  for  crossing  showed 
difterences  in  the  length  and  colour  of  the  stem ;  in  the 
size  and  form  of  the  leaves  ;  in  the  position,  colour,  and 
size  of  the  flowers ;  in  the  length  of  the  flower  stalk ;  in  the 
colour,  form,  and  size  of  the  pods  ;  in  the  form  and  size  of 
the  seeds ;  and  in  the  colour  of  the  seed-coats  and  the 
albumen  [cotyledons].  Some  of  the  characters  noted  do 
not  permit  of  a  sharp  and  certain  separation,  since  the 
difference  is  of  a  "  more  or  less "  nature,  which  is  often 
difficult  to  define.  Such  characters  could  not  be  utilised 
for  the  separate  experiments  ;  these  could  only  be  confined 
to  characters  which  stand  out  clearly  and  definitely  in  the 
plants.  Lastly,  the  result  must  show  whether  they,  in 
their  entirety,  observe  a  regular  behaviour  in  their  hybrid 
unions,  and  whether  fi'om  these  facts  any  conclusion  can 
be  come  to  regarding  those  characters  which  possess  a 
subordinate  significance  in  the  type 

The  characters  which  were  selected  for  experiment  relate : 

1.  To  the  difference  in  the  form  of  the  ripe  seeds.  These 
are  either  round  or  roundish,  the  wrinkling,  when  such  occurs 
on  the  surface,  being  always  only  shallow ;  or  they  are 
irregularly  angular  and  deeply  wrinkled  (P.  quadratum). 

2.  To  the  difference  in  the  colour  of  the  seed  albumen 
(endosperm)*.  The  albumen  of  the  ripe  seeds  is  either 
pale  yellow,  bright  yellow  and  orange  coloured,  or  it 
possesses  a  more  or  less  intense  green  tint.  This  diff"erence 
of  colour  is  easily  seen  in  the  seeds  as  their  coats  are 
transparent. 

*  [Mendel  uses  the  terms  "albumen"  and  "endosperm"  somewhat 
loosely  to  denote  the  cotyledons,  containing  food-material,  within  the 
seed.] 


46  MendeVs  Experiments 

3.  To  the  difference  in  the  colour  of  the  seed-coat. 
Tliis  is  either  white,  with  which  character  white  flowers 
are  constantly  correlated ;  or  it  is  grey,  grey-brown,  leather- 
brown,  with  or  without  violet  spotting,  in  which  case  the 
colour  of  the  standards  is  violet,  that  of  the  wings  purple, 
and  the  stem  in  the  axils  of  the  leaves  is  of  a  reddish  tint. 
The  grey  seed-coats  become  dark  brown  in  boiling  water. 

4.  To  the  difference  in  the  form  of  the  ripe  pods.  These 
are  either  simply  inflated,  never  contracted  in  places ;  or 
they  are  deeply  constricted  between  the  seeds  and  more  or 
less  wrinkled  (P.  saccharatum). 

5.  To  the  difference  in  the  colour  of  the  unripe  pods. 
They  are  either  light  to  dark  green,  or  vividly  yellow,  in 
which  colouring  the  stalks,  leaf-veins,  and  calyx  par- 
ticipate*. 

6.  To  the  difference  in  the  position  of  the  flowers. 
They  are  either  axial,  that  is,  distributed  along  the  main 
stem ;  or  they  are  terminal,  that  is,  bunched  at  the  top  of 
the  stem  and  arranged  almost  in  a  false  umbel ;  in  this 
case  the  upper  part  of  the  stem  is  more  or  less  widened  in 
section  (P.  umhellatun%)\ . 

7.  To  the  difference  in  the  length  of  the  stem.  The 
length  of  the  stem  J  is  very  various  in  some  forms;  it  is, 

*  One  species  possesses  a  beautifully  brownish-red  coloured  pod, 
which  when  ripening  turns  to  violet  and  blue.  Trials  with  this 
character  were  only  begun  last  year.  [Of  these  further  experiments 
it  seems  no  account  was  published.  Correns  has  since  worked  with 
such  a  variety.] 

t  [This  is  often  called  the  Mummy  Pea.  It  shows  slight  fasciation. 
The  form  I  know  has  white  standard  and  salmon-red  wings.] 

X  [In  my  account  of  these  experiments  {R.H.S.  Journal,  vol.  xxv. 
p.  54)  I  misunderstood  this  paragraph  and  took  "  axis  "  to  mean  the 
floral  axis,  instead  of  the  main  axis  of  the  plant.  The  unit  of 
measurement,  being  indicated  in  the  original  by  a  dash  ('),  I  care- 


in  Hybridisation  47 

however,  a  constant  character  for  each,  in  so  far  that  healthy- 
plants,  grown  in  the  same  soil,  are  only  subject  to  unim- 
portant variations  in  this  character. 

In  experiments  with  this  character,  in  order  to  be  able  to 
discriminate  with  certainty,  the  long  axis  of  6 — 7  ft.  was 
always  crossed  with  the  short  one  of  f  ft.  to  Ij  ft. 

Each  two  of  the  differentiating  characters  enumerated 
above  were  united  by  cross-fertilisation.     There  were  made 

for  the 

1st  trial  60  fertilisations  on  15  plants. 


2nd 

f) 

58 

55        10 

3rd 

55 

35 

,5        10 

4th 

)) 

40 

,5        10 

5th 

55 

23 

5,         5 

6th 

J5 

34 

,5        10 

7th 

55 

37 

55        10 

From  a  larger  number  of  plants  of  the  same  variety  only 
the  most  vigorous  were  chosen  for  fertilisation.  Weakly 
plants  always  afford  uncertain  results,  because  even  in  the 
first  generation  of  hybrids,  and  still  more  so  in  the  sub- 
sequent ones,  many  of  the  offspring  either  entirely  fail  to 
flower  or  only  form  a  few  and  inferior  seeds. 

Furthermore,  in  all  the  experiments  reciprocal  crossings 
were  effected  in  such  a  way  that  each  of  the  two  varieties 
which  in  one  set  of  fertilisations  served  as  seed-bearers  in 
the  other  set  were  used  as  pollen  plants. 

The  plants  were  grown  in  garden  beds,  a  few  also 
in  pots,  and  were  maintained  in  their  naturally  upright 
position  by  means  of  sticks,  branches  of  trees,  and  strings 
stretched  between.  For  each  experiment  a  number  of  pot 
plants  were  placed  during  the  blooming  period  in  a  green- 
house, to  serve  as  control  plants  for  the  main  experiment 

lessly  took  to  have  been  an  inch,  but  the  translation  here  given  is 
evidently  correct.] 


48  MendeVs  Experiments 

in  the  open  as  regards  possible  disturbance  by  insects. 
Among  the  insects*  which  visit  Peas  the  beetle  Bruchus 
pisi  might  be  detrimental  to  the  experiments  should  it 
appear  in  numbers.  The  female  of  this  species  is  known 
to  lay  the  eggs  in  the  flower,  and  in  so  doing  opens  the 
keel ;  upon  the  tarsi  of  one  specimen,  which  was  caught  in 
a  flower,  some  pollen  grains  could  clearly  be  seen  under  a 
lens.  Mention  must  also  be  made  of  a  circumstance  which 
possibly  might  lead  to  the  introduction  of  foreign  pollen. 
It  occurs,  for  instance,  in  some  rare  cases  that  certain  parts 
of  an  otherwise  quite  normally  developed  flower  wither, 
resulting  in  a  partial  exposure  of  the  fertilising  organs.  A 
defective  development  of  the  keel  has  also  been  observed, 
owing  to  which  the  stigma  and  anthers  remained  partially 
uncovered!.  It  also  sometimes  happens  that  the  pollen 
does  not  reach  full  perfection.  In  this  event  there  occurs 
a  gradual  lengthening  of  the  pistil  during  the  blooming 
period,  until  the  stigmatic  tip  protrudes  at  the  point  of  the 
keel.  This  remarkable  appearance  has  also  been  observed 
in  hybrids  of  Phaseolus  and  Lathyrus. 

The  risk  of  false  impregnation  by  foreign  pollen  is, 
however,  a  very  slight  one  with  Pisum,  and  is  quite 
incapable  of  disturbing  the  general  result.  Among  more 
than  10,000  plants  which  were  carefully  examined  there 
were  only  a  very  few  cases  where  an  indubitable  false 
impregnation  had  occurred.  Since  in  the  greenhouse  such 
a  case  was  never  remarked,  it  may  well  be  supposed  that 
Bruchus  pisi,  and  possibly  also  the  described  abnormalities 
in  the  floral  structure,  were  to  blame. 

*  [It  is  somewhat  surprising  that  no  mention  is  made  of  Thrips, 
which  swarm  in  Pea  flowers.  I  had  come  to  the  conclusion  that  this 
is  a  real  source  of  error  and  I  see  Laxton  held  the  same  opinion.] 

+  [This  also  happens  in  Sweet  Peas.] 


in  Hyhridisation  49 


The  Forms  of  the  Hybrids.* 

Experiments  whicli  in  previous  years  were  made  witli 
ornamental  plants  have  already  afforded  evidence  that  the 
hybrids,  as  a  rule,  are  not  exactly  intermediate  between 
the  parental  species.  With  some  of  the  more  striking 
characters,  those,  for  instance,  which  relate  to  the  form 
and  size  of  the  leaves,  the  pubescence  of  the  several  parts, 
&c.,  the  intermediate,  indeed,  was  nearly  always  to  be 
seen ;  in  other  cases,  however,  one  of  the  two  parental 
characters  was  so  preponderant  that  it  was  difficult,  or 
quite  impossible,  to  detect  the  other  in  the  hybrid. 

This  is  precisely  the  case  with  the  Pea  hybrids.  In 
the  case  of  each  of  the  seven  crosses  the  hybrid-character 
resembles!  that  of  one  of  the  parental  forms  so  closely  that 
the  other  either  escapes  observation  completely  or  cannot 
be  detected  with  certainty.  This  circumstance  is  of  great 
importance  in  the  determination  and  classification  of  the 
forms  under  which  the  offspring  of  the  hybrids  appear. 
Henceforth  in  this  paper  those  characters  which  are  trans- 
mitted entire,  or  almost  unchanged  in  the  hybridisation, 
and  therefore  in  themselves  constitute  the  characters  of 
the  hybrid,  are  termed  the  dominant,  and  those  which 
become  latent  in  the  process  recessive.  The  expression 
"recessive"  has  been  chosen  because  the  characters  thereby 
designated  withdraw  or  entirely  disappear  in  the  hybrids, 

*  [Mendel  throughout  speaks  of  his  cross-bred  Peas  as  "  hybrids," 
a  term  which  many  restrict  to  the  offspring  of  two  distinct  species. 
He,  as  he  explains,  held  this  to  be  only  a  question  of  degree.] 

t  [Note  that  Mendel,  with  true  penetration,  avoids  speaking  of 
the  hybrid-character  as  "transmitted"  by  either  parent,  thus  escap- 
ing the  error  pervading  modern  views  of  heredity.] 

B.  4 


50  MendeVs  Experiments 

but  nevertheless  reappear  unchanged  in  their  progeny,  as 
will  be  demonstrated  later  on. 

It  was  furthermore  shown  by  the  whole  of  the  experi- 
ments that  it  is  perfectly  immaterial  whether  the  dominant 
character  belong  to  the  seed-bearer  or  to  the  pollen  parent ; 
the  form  of  the  hybrid  remains  identical  in  both  cases.  This 
interesting  fact  was  also  emphasised  by  Gartner,  with  the 
remark  that  je<5n  ^tayinost  practised  expert  is  not  in  a 
position  tOide^CTmine  in\  hybrid  which  of  the  two  parental 
species  wm^e  se^  orTheNpollen  plant*. 

Of  tha  ^fferei^^tint  gharacters  which  were  used  in  the 
experimelffi  tne  fQiipwiifc  ^i  dominant : 

1.  Tn^r&ind^r  riJugngiish  form  of  the  seed  with  or 
without  sA^ow  d^^essiofes.1 

2.  The^y^llowQoloumig  of  the  seed  albumen  [coty- 
ledons], v^o         '•' / 

3.  The  gr^y^J^sji^brown,  or  leather-brown  colour  of 
the  seed-coat,  in  connection  with  violet-red  blossoms  and 
reddish  spots  in  the  leaf  axils. 

4.  The  simply  inflated  form  of  the  pod. 

5.  The  green  colouring  of  the  unripe  pod  in  connection 
with  the  same  colour  in  the  stems,  the  leaf-veins  and  the  calyx. 

6.  The  distribution  of  the  flowers  along  the  stem. 

7.  The  greater  length  of  stem. 

"With  regard  to  this  last  character  it  must  be  stated 
that  the  longer  of  the  two  parental  stems  is  usually  exceeded 
by  the  hybrid,  which  is  possibly  only  attributable  to  the 
greater  luxuriance  which  appears  in  all  parts  of  plants 
when  stems  of  very  diff"erent  length  are  crossed.  Thus,  for 
instance,  in  repeated  experiments,  stems  of  1  ft.  and  6  ft. 
in  length  yielded  without  exception  hybrids  which  varied 
in  length  between  6  ft.  and  7  J  ft. 

*  [Gartner,  p.  223.] 


in  Hybridisation  51 

The  hybrid  seeds  in  the  experiments  with  seed-coat  are 
often  more  spotted,  and  the  spots  sometimes  coalesce  into 
small  bluish-violet  patches.  The  spotting  also  frequently 
appears  even  when  it  is  absent  as  a  parental  character. 

The  hybrid  forms  of  the  seed-shape  and  of  the  albumen 
are  developed  immediately  after  the  artificial  fertilisation 
by  the  mere  influence  of  the  foreign  pollen.  They  can, 
therefore,  be  observed  even  in  the  first  year  of  experiment, 
whilst  all  the  other  characters  naturally  only  appear  in  the 
following  year  in  such  plants  as  have  been  raised  from  the 
crossed  seed. 


The  First  Generation  [Bred]  from  the  Hybrids. 

In  this  generation  there  reappear,  together  with  the 
dominant  characters,  also  the  recessive  ones  with  their  full 
peculiarities,  and  this  occurs  in  the  definitely  expressed 
average  proportion  of  three  to  one,  so  that  among  each 
four  plants  of  this  generation  three  display  the  dominant 
character  and  one  the  recessive.  This  relates  without 
exception  to  all  the  characters  which  were  embraced  in 
the  experiments.  The  angular  wrinkled  form  of  the  seed, 
the  green  colour  of  the  albumen,  the  white  colour  of  the 
seed-coats  and  the  flowers,  the  constrictions  of  the  pods, 
the  yellow  colour  of  the  unripe  pod,  of  the  stalk  of  the 
calyx,  and  of  the  leaf  venation,  the  umbel-like  form  of  the 
inflorescence,  and  the  dwarfed  stem,  all  reappear  in  the 
numerical  proportion  given  without  any  essential  alteration. 
Transitional  fm^ms  were  not  observed  in  any  experiment. 

Once  the  hybrids  resulting  from  reciprocal  crosses  are 
fully  formed,  they  present  no  appreciable  difference  in  their 

4—2 


52  MendeVs  Expei^hnents 

subsequent  development,  and  consequently  the  results  [of 
the  reciprocal  crosses]  can  be  reckoned  together  in  each 
experiment.  The  relative  numbers  which  were  obtained  for 
each  pair  of  differentiating  characters  are  as  follows  : 

Expt.  1.  Form  of  seed. — From  253  hybrids  7,324  seeds 
were  obtained  in  the  second  trial  year.  Among  them  were 
5,474  round  or  roundish  ones  and  1,850  angular  wrinkled 
ones.     Therefrom  the  ratio  2*96  to  1  is  deduced. 

Expt.  2.  Colour  of  albumen. — 258  plants  yielded  8,023 
seeds,  6,022  yellow,  and  2,001  green  ;  their  ratio,  therefore, 
is  as  3*01  to  1. 

In  these  two  experiments  each  pod  yielded  usually  both 
kinds  of  seed.  In  well-developed  pods  which  contained  on 
the  average  six  to  nine  seeds,  it  often  occurred  that  all  the 
seeds  were  round  (Expt.  1)  or  all  yellow  (Expt.  2);  on  the 
other  hand  there  were  never  observed  more  than  five  angular 
or  five  green  ones  in  one  pod.  It  appears  to  make  no 
difference  whether  the  pods  are  developed  early  or  later  in 
the  hybrid  or  whether  they  spring  from  the  main  axis  or 
from  a  lateral  one.  In  some  few  plants  only  a  few  seeds 
developed  in  the  first  formed  pods,  and  these  possessed 
exclusively  one  of  the  two  characters,  but  in  the  subse- 
quently developed  pods  the  normal  proportions  were  main- 
tained nevertheless. 

As  in  separate  pods,  so  did  the  distribution  of  the 
characters  vary  in  separate  plants.  By  way  of  illustration 
the  first  ten  individuals  from  both  series  of  experiments 
may  serve*. 

*  [It  is  much  to  be  regretted  that  Mendel  does  not  give  the 
complete  series  individually.  No  one  who  repeats  such  experiments 
should  fail  to  record  the  individual  numbers,  which  on  seriation  are 
sure  to  be  full  of  interest.] 


in  Hybridisation  53 


Experiment  1. 
Form  of  Seed. 

Exp( 
Colour 

3riment  2. 
of  Albumen. 

Plants. 

1 

Eound. 
45 

Angular. 
12 

Yellow, 
25 

Green. 
11 

2 

27 

8 

32 

7 

3 

24 

7 

14 

5 

4 

19 

10 

70 

27 

5 

32 

11 

24 

13 

6 

26 

6 

20 

6 

7 

88 

24 

32 

13 

8 

22 

10 

44 

9 

9 

28 

6 

50 

14 

10 

25 

7 

44 

18 

As  extremes  in  the  distribution  of  the  two  seed  charac- 
ters in  one  plant,  there  were  observed  in  Expt.  1  an  instance 
of  43  round  and  only  2  angular,  and  another  of  14  round 
and  15  angular  seeds.  In  Expt.  2  there  was  a  case  of  32 
yellow  and  only  1  green  seed,  but  also  one  of  20  yellow 
and  19  green. 

These  two  experiments  are  important  for  the  determ- 
ination of  the  average  ratios,  because  with  a  smaller 
number  of  experimental  plants  they  show  that  very  con- 
siderable fluctuations  may  occur.  In  counting  the  seeds, 
also,  especially  in  Expt.  2,  some  care  is  requisite,  since  in 
some  of  the  seeds  of  many  plants  the  green  colour  of  the 
albumen  is  less  developed,  and  at  first  may  be  easily 
overlooked.  The  cause  of  the  partial  disappearance  of  the 
green  colouring  has  no  connection  with  the  hybrid-character 
of  the  plants,  as  it  likewise  occurs  in  the  parental  variety. 
This  peculiarity  is  also  confined  to  the  individual  and  is 
not  inherited  by  the  offspring.  In  luxuriant  plants  this 
appearance  was  frequently  noted.  Seeds  which  are  damaged 
by  insects  during  their  development  often  vary  in  colour 
and  form,  but,  w  ith  a  little  practice  in  sorting,  errors  are 


54  Menders  Experiments 

easily  avoided.  It  is  almost  superfluous  to  mention  that  the 
pods  must  remain  on  the  plants  until  they  are  thoroughly 
ripened  and  have  become  dried,  since  it  is  only  then  that 
the  shape  and  colour  of  the  seed  are  fully  developed. 

Expt.  3.  Colour  of  the  seed-coats. — Among  929  plants 
705  bore  violet-red  flowers  and  grey-brown  seed-coats  ;  224 
had  white  flowers  and  white  seed-coats,  giving  the  proportion 
3-15  to  1. 

Expt.  4.  Form,  of  pods. — Of  1,181  plants  882  had  them 
simply  inflated,  and  in  299  they  were  constricted.  Resulting 
ratio,  2"95  to  1. 

Expt.  5.  Colour  of  the  unripe  pods. — The  number  of 
trial  plants  was  580,  of  which  428  had  green  pods  and  152 
yellow  ones.    Consequently  these  stand  in  the  ratio  2*82  to  1. 

Expt.  6.  Position  of  flowers. — Among  858  cases  651 
blossoms  were  axial  and  207  terminal.     Ratio,  3*14  to  1. 

Expt.  7.  Length  of  stem. — Out  of  1,064  plants,  in  787 
cases  the  stem  was  long,  and  in  277  short.  Hence  a  mutual 
ratio  of  2 "84  to  1.  In  this  experiment  the  dwarfed  plants 
were  carefully  lifted  and  transferred  to  a  special  bed.  This 
precaution  was  necessary,  as  otherwise  they  would  have 
perished  through  being  overgrown  by  their  tall  relatives. 
Even  in  their  quite  young  state  they  can  be  easily  picked 
out  by  their  compact  growth  and  thick  dark-green  foliage. 

If  now  the  results  of  the  whole  of  the  experiments  be 
brought  together,  there  is  found,  as  between  the  number 
of  forms  with  the  dominant  and  recessive  characters,  an 
average  ratio  of  2 "98  to  1,  or  3  to  1. 

The  dominant  character  can  have  here  a  double  signi- 
fication— viz.  that  of  a  parental-character,  or   a   hybrid- 


in  Hybridisation  55 

character*.  In  which  of  the  two  significations  it  appears 
in  each  separate  case  can  only  be  determined  by  the  follow- 
ing generation.  As  a  parental  character  it  must  pass  over 
unchanged  to  the  whole  of  the  offspring ;  as  a  hybrid- 
character,  on  the  other  hand,  it  must  observe  the  same 
behaviour  as  in  the  first  generation. 

The  Second  Generation  [Bred]  from  the  Hybrids. 

Those  forms  which  in  the  first  generation  maintain  the 
recessive  character  do  not  further  vary  in  the  second 
generation  as  regards  this  character ;  they  remain  constant 
in  their  offspring. 

It  is  otherwise  with  those  which  possess  the  dominant 
character  in  the  first  generation  [bred  from  the  hybrids]. 
Of  these  two-thirds  yield  offspring  which  display  the 
dominant  and  recessive  characters  in  the  proportion  of 
3  to  1,  and  thereby  show  exactly  the  same  ratio  as  the 
hybrid  forms,  while  only  o??^- third  remains  with  the  domi- 
nant character  constant. 

The  separate  experiments  yielded  the  following  results: — 

Expt.  1. — Among  565  plants  which  were  raised  from 
round  seeds  of  the  first  generation,  193  yielded  round  seeds 
only,  and  remained  therefore  constant  in  this  character  ; 
372,  however,  gave  both  round  and  angular  seeds,  in  the 
proportion  of  3  to  1.  The  number  of  the  hybrids,  therefore, 
as  compared  with  the  constants  is  1'93  to  1. 

Expt.  2. — Of  519  plants  which  were  raised  from  seeds 
whose  albumen  was  of  yellow  colour  in  the  first  generation, 
166  yielded  exclusively  yellow,  while  353  yielded  yellow 

*  [This  paragraph  presents  the  view  of  the  hybrid-character  as 
something  incidental  to  the  hybrid,  and  not  "  transmitted  "  to  it— a 
true  and  fundamental  conception  here  expressed  probably  for  the 
first  time.] 


56  MendeVs  Exj^eriments 

and  green  seeds  in  the  proportion  of  3  to  1.  There  resulted, 
therefore,  a  division  into  hybrid  and  constant  forms  in  the 
proportion  of  2  "13  to  1. 

For  each  separate  trial  in  the  following  experiments 
100  plants  were  selected  which  displayed  the  dominant 
character  in  the  first  generation,  and  in  order  to  ascertain 
the  significance  of  this,  ten  seeds  of  each  were  cultivated. 

Expt.  3.— The  offspring  of  36  plants  yielded  exclusively 
grey-brown  seed-coats,  while  of  the  offspring  of  64  plants 
some  had  grey-brown  and  some  had  white. 

Expt.  4. — The  offspring  of  29  plants  had  only  simply 
inflated  pods  ;  of  the  offspring  of  71,  on  the  other  hand, 
some  had  inflated  and  some  constricted. 

Expt.  5. — The  offspring  of  40  plants  had  only  green 
pods ;  of  the  offspring  of  60  plants  some  had  green,  some 
yellow  ones. 

Expt.  6. — The  offspring  of  33  plants  had  only  axial 
flowers  ;  of  the  offspring  of  67,  on  the  other  hand,  some 
had  axial  and  some  terminal  flowers. 

Expt.  7. — The  offspring  of  28  plants  inherited  the  long 
axis,  and  those  of  72  plants  some  the  long  and  some  the 
short  axis. 

In  each  of  these  experiments  a  certain  number  of  the 
plants  came  constant  with  the  dominant  character.  For 
the  determination  of  the  proportion  in  which  the  separation 
of  the  forms  with  the  constantly  persistent  character  results, 
the  two  first  experiments  are  of  especial  importance,  since 
in  these  a  larger  number  of  plants  can  be  compared.  The 
ratios  1*93  to  1  and  213  to  1  gave  together  almost  exactly 
the  average  ratio  of  2  to  1.  The  sixth  experiment  has  a 
quite  concordant  result ;  in  the  others  the  ratio  varies  more 
or  less,  as  was  only  to  be  expected  in  view  of  the  smaller 


i7b  Hybridisation  57 

number  of  100  trial  plants.  Experiment  5,  which  shows 
the  greatest  departure,  was  repeated,  and  then  in  lieu  of 
the  ratio  of  60  and  40  that  of  65  and  35  resulted.  The 
average  ratio  of  2  to  1  appears,  therefore,  as  fixed  with 
certainty.  It  is  therefore  demonstrated  that,  of  those  forms 
which  possess  the  dominant  character  in  the  first  generation, 
in  two-thirds  the  hybrid  character  is  embodied,  while  one- 
third  remains  constant  with  the  dominant  character. 

The  ratio  of  3  to  1,  in  accordance  with  which  the 
distribution  of  the  dominant  and  recessive  characters 
results  in  the  first  generation,  resolves  itself  therefore  in 
all  experiments  into  the  ratio  of  2  :  1  :  1  if  the  dominant 
character  be  differentiated  according  to  its  significance  as 
a  hybrid  character  or  a  parental  one.  Since  the  members 
of  the  first  generation  spring  directly  from  the  seed  of  the 
hybrids,  it  is  now  dear  that  the  hybrids  form  seeds  having 
one  or  other  of  the  two  differentiating  characters,  and  of 
these  one-half  develop  again  the  hybrid  form,  while  the  other 
half  yield  plants  which  remain  constant  and  receive  the  domi- 
nant or  recessive  characters  [f^espectively]  in  equal  numbers. 

The  Subsequent  Generations  [Bred]  from  the  Hybrids. 

The  proportions  in  which  the  descendants  of  the  hybrids 
develop  and  split  up  in  the  first  and  second  generations 
presumably  hold  good  for  all  subsequent  progeny.  Experi- 
ments 1  and  2  have  already  been  carried  through  six 
generations,  3  and  7  through  five,  and  4,  5,  and  6  through 
four,  these  experiments  being  continued  from  the  third 
generation  with  a  small  number  of  plants,  and  no  departure 
from  the  rule  has  been  perceptible.  The  offspring  of  the 
hybrids  separated  in  each  generation  in  the  ratio  of  2  :  1  :  1 
into  hybrids  and  constant  forms. 


58 


MendeVs  Experiments 


If  A  be  taken  as  denoting  one  of  the  two  constant 
characters,  for  instance  the  dominant,  a,  the  recessive, 
and  A  a  the  hybrid  form  in  which  both  are  conjoined,  the 
expression 

A + 2Aa + a 

shows  the  terms  in  the  series  for  the  progenj^  of  the  hybrids 
of  two  differentiating  characters. 

The  observation  made  by  Gartner,  Kolreuter,  and  others, 
that  hybrids  are  incHned  to  revert  to  the  parental  forms,  is 
also  confirmed  by  the  experiments  described.  It  is  seen 
that  the  number  of  the  hybrids  which  arise  from  one 
fertilisation,  as  compared  with  the  number  of  forms  which 
become  constant,  and  their  progeny  from  generation  to 
generation,  is  continually  diminishing,  but  that  never- 
theless they  could  not  entirely  disappear.  If  an  average 
equality  of  fertility  in  all  plants  in  all  generations  be 
assumed,  and  if,  furthermore,  each  hybrid  forms  seed  of 
which  one-half  yields  hybrids  again,  while  the  other  half 
is  constant  to  both  characters  in  equal  proportions,  the 
ratio  of  numbers  for  the  offspring  in  each  generation  is 
seen  by  the  following  summary,  in  which  A  and  a  denote 
again  the  two  parental  characters,  and  Aa  the  hybrid 
forms.  For  brevity's  sake  it  may  be  assumed  that  each 
plant  in  each  generation  furnishes  only  4  seeds. 


Katios. 


Generation 

A 

Aa 

a 

A 

Aa 

a 

1 

1 

2 

1 

1 

2 

1 

2 

6 

4 

6 

3 

:  2 

3 

3 

28 

8 

28 

7 

2 

7 

4 

120 

16 

120 

15 

2 

15 

5 

496 

32 

496 

31 

2 

31 

n 

2"-l 

2 

2» 

in  Hybridisation  59 

In  the  tenth  generation,  for  instance,  2**- 1  =  1023. 
There  result,  therefore,  in  each  2,048  plants  which  arise  in 
this  generation  1,023  with  the  constant  dominant  character, 
1,023  with  the  recessive  character,  and  only  two  hybrids. 


The   Offspring  of  Hybrids  in  which   Several 
Differentiating  Characters  are  Associated. 

In  the  experiments  above  described  plants  were  used 
which  differed  only  in  one  essential  character*.  The  next 
task  consisted  in  ascertaining  whether  the  law  of  develop- 
ment discovered  in  these  applied  to  each  pair  of  differen- 
tiating characters  when  several  diverse  characters  are  united 
in  the  hybrid  by  crossing.  As  regards  the  form  of  the 
hybrids  in  these  cases,  the  experiments  showed  throughout 
that  this  invariably  more  nearly  approaches  to  that  one  of 
the  two  parental  plants  which  possesses  the  greater  number 
of  dominant  characters.  If,  for  instance,  the  seed  plant  has 
a  short  stem,  terminal  white  flowers,  and  simply  inflated 
pods ;  the  pollen  plant,  on  the  other  hand,  a  long  stem, 
violet-red  flowers  distributed  along  the  stem,  and  con- 
stricted pods;  the  hybrid  resembles  the  seed  parent  only  in 
the  form  of  the  pod  ;  in  the  other  characters  it  agrees  with 
the  pollen  parent.  Should  one  of  the  two  parental  types 
possess  only  dominant  characters,  then  the  hybrid  is 
scarcely  or  not  at  all  distinguishable  from  it. 

*  [This  statement  of  Mendel's  in  the  light  of  present  knowledge  is 
open  to  some  misconception.  Though  his  work  makes  it  evident  that 
such  varieties  may  exist,  it  is  very  unlikely  that  Mendel  could  have 
had  seven  pairs  of  varieties  such  that  the  members  of  each  pair 
differed  from  each  other  in  only  one  considerable  character  {loesentliches 
Merkmal).  The  point  is  probably  of  little  theoretical  or  practical 
consequence,  but  a  rather  heavy  stress  is  thrown  on  ^^ivesentlich.^'] 


60  Mendel's  Experiments 

Two  experiments  were  made  with  a  larger  number  of 
plants.  In  the  first  experiment  the  parental  plants  differed 
in  the  form  of  the  seed  and  in  the  colour  of  the  albumen ; 
in  the  second  in  the  form  of  the  seed,  in  the  colour  of  the 
albumen,  and  in  the  colour  of  the  seed-coats.  Experiments 
with  seed  characters  give  the  result  in  the  simplest  and 
most  certain  way. 

In  order  to  facilitate  study  of  the  data  in  these  experi- 
ments, the  different  characters  of  the  seed  plant  will  be 
indicated  hj  A,  B,  C,  those  of  the  pollen  plant  by  a,  b,  c, 
and  the  hybrid  forms  of  the  characters  by  A  a,  Bh,  and  Cc. 

Expt.  1. — J. ^,  seed  parents;         a^,  pollen  parents  ; 

A,  form  round  ;  a,  form  angular  ; 

B,  albumen  yellow.      b,  albumen  green. 

The  fertilised  seeds  appeared  round  and  yellow  like  those 
of  the  seed  parents.  The  plants  raised  therefrom  yielded 
seeds  of  four  sorts,  which  frequently  presented  themselves 
in  one  pod.  In  all  556  seeds  were  yielded  by  15  plants, 
and  of  these  there  were  : — 

315  round  and  yellow, 
101  angular  and  yellow, 
108  round  and  green, 
32  angular  and  green. 

All  were  sown  the  following  year.  Eleven  of  the  round 
yellow  seeds  did  not  yield  plants,  and  three  plants  did  not 
form  seeds.     Among  the  rest : 

38  had  round  yellow  seeds       .         .         .  AB 

65  round  yellow  and  green  seeds     .         .  ABb 

60  round  yellow  and  angular  yellow  seeds  AaB 
138  round  yellow  and  green,  angular  yellow 

and  green  seeds         .         .         .         .  '  AaBb. 


in  Hybridisation 


61 


From  the  angular  yellow  seeds  96  resulting  plants  bore 
seed,  of  which  : 

28  had  only  angular  yellow  seeds  aB 

68  angular  yellow  and  green  seeds        aBh. 

From  108  round  green  seeds  102  resulting  plants  fruited, 
of  which  : 

35  had  only  round  green  seeds  Ah 

67  round  and  angular  green  seeds  Aab. 

The  angular  green  seeds  yielded  30  plants  which  bore  seeds 
all  of  like  character ;  they  remained  constant  ab. 

The  offspring  of  the  hybrids  appeared  therefore  under 
nine  different  forms,  some  of  them  in  very  unequal  numbers. 
When  these  are  collected  and  co-ordinated  we  find  : 

38  plants  with  the  sign  AB 


35 

Ah 

28 

,          aB 

30 

,           ab 

65 

ABh 

68 

aBh 

60 

AaB 

67 

,           Aah 

38 

AaBh 

The  whole  of  the  forms  may  be  classed  into  three 
essentially  different  groups.  The  first  embraces  those  with 
the  signs  AB,  Ah,  aB,  and  ab  :  they  possess  only  constant 
characters  and  do  not  vary  again  in  the  next  generation. 
Each  of  these  forms  is  represented  on  the  average  thirty- 
three  times.  The  second  group  embraces  the  signs  ABh, 
aBh,  AaB,  Aab  :  these  are  constant  in  one  character  and 
hybrid  in  another,  and  vary  in  the  next  generation  only 
as  regards  the  hybrid  character.     Each  of  these  appears  on 


62  MendeVs  Experiments 

an  average  sixty-five  times.  The  form  AaBb  occurs  138 
times  :  it  is  hybrid  in  both  characters,  and  behaves  exactly 
as  do  the  hybrids  from  which  it  is  derived. 

If  the  numbers  in  which  the  forms  belonging  to  these 
classes  appear  be  compared,  the  ratios  of  1,  2,  4  are  un- 
mistakably evident.  The  numbers  32,  65,  138  present  very 
fair  approximations  to  the  ratio  numbers  of  33,  66,  132. 

The  developmental  series  consists,  therefore,  of  nine 
classes,  of  which  four  appear  therein  always  once  and  are 
constant  in  both  characters ;  the  forms  AB,  ah^  resemble 
the  parental  forms,  the  two  others  present  combinations 
between  the  conjoined  characters  A,  a,  B,  h,  which  com- 
binations are  likewise  possibly  constant.  Four  classes 
appear  always  twice,  and  are  constant  in  one  character 
and  hybrid  in  the  other.  One  class  appears  four  times, 
and  is  hybrid  in  both  characters.  Consequently  the 
offspring  of  the  hybrids,  if  two  kinds  of  differentiating 
characters  are  combined  therein,  are  represented  by  the 
expression 

AB+Ab  +  aB  +  ah+2ABh+2aBh  +  2AaB+'lAah  +  4.AaBh. 

This  expression  is  indisputably  a  combination  series  in 
which  the  two  expressions  for  the  characters  A  and  a,  B 
and  6,  are  combined.  We  arrive  at  the  full  number  of  the 
classes  of  the  series  by  the  combination  of  the  expres- 
sions : 

J.  +2  Aa^a 

B+2  Bh  +  h. 
Second  Expt. 

ABC,  seed  parents  ;  abc,  pollen  parents  ; 

A,  form  round  ;  a,  form  angular  ; 

B,  albumen  yellow  ;  h,  albumen  green  ; 
G,  seed-coat  grey-brown,    c,  seed-coat  white. 


in  Hybridisation 


63 


This  experiment  was  made  in  precisely  the  same  way  as 
the  previous  one.  Among  all  the  experiments  it  demanded 
the  most  time  and  trouble.  From  24  hybrids  687  seeds 
were  obtained  in  all  :  these  were  all  either  spotted,  grey- 
brown  or  grey- green,  round  or  angular^.  From  these  in 
the  following  year  639  plants  fruited,  and,  as  further 
investigation  showed,  there  were  among  them : 


8  plants 

ABC. 

22 

plants  ABCc. 

45 

plants 

ABbCc. 

14       „ 

A  Be. 

17 

5) 

AbCc. 

36 

aBbCc. 

9       „ 

AhC. 

25 

M 

aBCc. 

38 

AaBCc. 

11       » 

Abe. 

20 

15 

abCc. 

40 

AabCc. 

8       „ 

aBC. 

15 

5) 

ABbO. 

49 

AabbC. 

10       „ 

aBc. 

]8 

55 

ABbc. 

48 

AaBbc. 

10       „ 

abC. 

19 

55 

aBbC. 

7       „ 

abc. 

24 

55 

aBbc. 

14 

55 

AaBC. 

78 

55 

AaBbCc 

18 

55 

AaBc. 

20 

55 

AabC. 

16 

15 

Aabc. 

The  whole  expression  contains  27  terms.  Of  these  8 
are  constant  in  all  characters,  and  each  appears  on  the 
average  10  times ;  12  are  constant  in  two  characters,  and 
hybrid  in  the  third;  each  appears  on  the  average  19  times  ; 
6  are  constant  in  one  character  and  hybrid  in  the  other 
two  ;  each  appears  on  the  average  43  times.  One  form 
appears  78  times  and  is  hybrid  in  all  of  the  characters. 
The  ratios  10,  19,  43,  78  agree  so  closely  with  the  ratios 
10,  20,  40,  80,  or  1,  2,  4,  8,  that  this  last  undoubtedly 
represents  the  true  value. 

The   development   of   the   hybrids   when   the   original 

*  [Note  that  Mendel  does  not  state  the  cotyledou-colour  of  the 
first  crosses  in  this  case ;  for  as  the  coats  were  thick,  it  could  not 
have  been  seen  without  opening  or  peeling  the  seeds.] 


64  MendeVs  Experiments 

parents  differ  in  three  characters  results  therefore  according 
to  the  following  expression  : 

ABC  +  ABc  +  AhC  +  Abe  +  aBC  +  aBc  +  ahC  +  abc  + 
2  ABCc  +  2  AbCc  +  2  aBCc  +  2  abCc  +  2  ABbC+  2  ABbc  + 
2  aBbC+  2  aBbc  +  2AaBC  +  2  AaBc  +  2  AabC+  2  Aabc  + 
4  ABbCc+A  aBbCc  +  4.  AaBCc+4.  AabCc  +  A  AaBbC  + 
4  AaBbc  +  8  AaBbCc. 

Here  also  is  involved  a  combination  series  in  which  the 
expressions  for  the  characters  A  and  a,  B  and  b,  C  and  c, 
are  united.     The  expressions 

A+2Aa+a 
B-¥2Bb  +  b 

C+2CC  +  C 

give  all  the  classes  of  the  series.  The  constant  combinations 
which  occur  therein  agree  with  all  combinations  which  are 
possible  between  the  characters  A,  B,  C,a,b,c\  two  thereof, 
ABC  and  abc,  resemble  the  two  original  parental  stocks. 

In  addition,  further  experiments  were  made  with  a 
smaller  number  of  experimental  plants  in  which  the  re- 
maining characters  by  twos  and  threes  were  united  as 
hybrids :  all  yielded  approximately  the  same  results.  There 
is  therefore  iw  doubt  that  for  the  whole  of  the  characters 
involved  in  the  experiments  the  principle  applies  that  the 
offspring  of  the  hybrids  in  which  several  essentially  different 
characters  are  combined  represent  the  terms  of  a  series  of 
combinations,  in  which  the  developmental  series  for  each  pair 
of  differentiating  characters  are  associated.  It  is  demon- 
strated at  the  same  time  that  the  relation  of  each  pair  of 
different  characters  in  hybrid  union  is  independent  of  the 
other  differences  in  the  two  original  parental  stocks. 

If  n  represent  the  number  of  the  differentiating  charac- 
ters in  the  two  original  stocks,  3*^  gives  the  number  of  terms 


in  Hyhridisation  65 

of  the  combination  series,  4^^  the  number  of  individuals 
which  belong  to  the  series,  and  2"'  the  number  of  unions 
which  remain  constant.  The  series  therefore  embraces,  if 
the  original  stocks  differ  in  four  characters,  3'*  =  81  of  classes, 
4'' =256  individuals,  and  2^^=16  constant  forms  ;  or,  which 
is  the  same,  among  each  256  offspring  of  the  hybrids  there 
are  81  different  combinations,  16  of  which  are  constant. 

All  constant  combinations  which  in  Peas  are  possible  by 
the  combination  of  the  said  seven  differentiating  characters 
were  actually  obtained  by  repeated  crossing.  Their  number 
is  given  by  2'=  128.  Thereby  is  simultaneously  given  the 
practical  proof  that  the  constant  characters  which  appear  in 
the  several  varieties  of  a  group  of  plants  may  he  obtained  in 
all  the  associations  which  are  possible  according  to  the 
[mathematical]  laws  of  combination^  by  means  of  repeated 
artific  ial  fertilisation. 

As  regards  the  flowering  time  of  the  hybrids,  the  ex- 
periments are  not  yet  concluded.  It  can,  however,  already 
be  stated  that  the  period  stands  almost  exactly  between 
those  of  the  seed  and  pollen  parents,  and  that  the  con- 
stitution of  the  hybrids  with  respect  to  this  character 
probably  happens  in  the  same  way  as  in  the  case  of  the 
other  characters.  The  forms  which  are  selected  for  experi- 
ments of  this  class  must  have  a  difference  of  at  least  twenty 
days  from  the  middle  flowering  period  of  one  to  that  of  the 
other;  furthermore,  the  seeds  when  sown  must  all  be  placed 
at  the  same  depth  in  the  earth,  so  that  they  may  germinate 
simultaneously.  Also,  during  the  whole  flowering  period, 
the  more  important  variations  in  temperature  must  be  taken 
into  account,  and  the  partial  hastening  or  delaying  of  the 
flowering  which  may  result  therefrom.  It  is  clear  that  this 
experiment  presents  many  difficulties  to  be  overcome  and 
necessitates  great  attention. 

B.  5 


66  MendeVs  Experiments 

If  we  endeavour  to  collate  in  a  brief  form  the  results 
arrived  at,  we  find  that  those  differentiating  characters 
which  admit  of  easy  and  certain  recognition  in  the 
experimental  plants,  all  behave  exactly  alike  in  their 
hybrid  associations.  The  offspring  of  the  hybrids  of  each 
pair  of  differentiating  characters  are,  one-half,  hybrid  again, 
while  the  other  half  are  constant  in  equal  proportions  having 
the  characters  of  the  seed  and  pollen  parents  respectively. 
If  several  differentiating  characters  are  combined  by  cross- 
fertilisation  in  a  hybrid,  the  resulting  offspring  form  the 
terms  of  a  combination  series  in  which  the  permutation 
series  for  each  pair  of  differentiating  characters  are  united. 

The  uniformity  of  behaviour  shown  by  the  whole  of  the 
characters  submitted  to  experiment  permits,  and  fully 
justifies,  the  acceptance  of  the  principle  that  a  similar 
relation  exists  in  the  other  characters  which  appear  less 
sharply  defined  in  plants,  and  therefore  could  not  be 
included  in  the  separate  experiments.  An  experiment 
with  peduncles  of  different  lengths  gave  on  the  whole  a 
fairly  satisfactory  result,  although  the  differentiation  and 
serial  arrangement  of  the  forms  could  not  be  effected  with 
that  certainty  which  is  indispensable  for  correct  experiment. 

The  Reproductive  Cells  of  Hybrids. 

The  results  of  the  previously  described  experiments 
induced  further  experiments,  the  results  of  which  appear 
fitted  to  afford  some  conclusions  as  regards  the  composition 
of  the  Qgg  and  pollen  cells  of  hybrids.  An  important  matter 
for  consideration  is  afforded  in  Pisum  by  the  circumstance 
that  among  the  progeny  of  the  hybrids  constant  forms 
appear,  and  that  this  occurs,  too,  in  all  combinations  of  the 
associated  characters.     So  far  as  experience  goes,  we  find 


in  Hybridisation  67 

it  in  every  case  confirmed  that  constant  progeny  can  only 
be  formed  when  the  egg  cells  and  the  fertilising  pollen  are 
of  like  character,  so  that  both  are  provided  with  the  material 
for  creating  quite  similar  individuals,  as  is  the  case  with  the 
normal  fertilisation  of  pure  species*.  We  must  therefore 
regard  it  as  essential  that  exactly  similar  factors  are  at  work 
also  in  the  production  of  the  constant  forms  in  the  hybrid 
plants.  Since  the  various  constant  forms  are  produced  in 
one  plant,  or  even  in  one  flower  of  a  plant,  the  conclusion 
appears  logical  that  in  the  ovaries  of  the  hybrids  there  are 
formed  ,as  many  sorts  of  egg  cells,  and  in  the  anthers  as 
many  sorts  of  pollen  cells,  as  there  are  possible  constant 
combination  forms,  and  that  these  egg  and  pollen  cells 
agree  in  their  internal  composition  mth  those  of  the 
separate  forms. 

In  point  of  fact  it  is  possible  to  demonstrate  theoretically 
that  this  hypothesis  would  fully  sufiice  to  account  for  the 
development  of  the  hybrids  in  the  separate  generations,  if 
we  might  at  the  same  time  assume  that  the  various  kinds 
of  egg  and  pollen  cells  were  formed  in  the  hybrids  on  the 
average  in  equal  numbers!. 

In  order  to  bring  these  assumptions  to  an  experimental 
proof,  the  following  experiments  were  designed.  Two  forms 
which  were  constantly  difterent  in  the  form  of  the  seed  and 
the  colour  of  the  albumen  were  united  by  fertilisation. 

If  the  differentiating  characters  are  again  indicated  as 
A,  B,  a,  b,  we  have  : 

AB,  seed  parent ;  ab,  pollen  parent ; 

A,  form  round;  a,  form  angular; 

B,  albumen  yellow.  b,  albumen  green. 

*  ["  False  hybridism  "  was  of  course  unknown  to  Mendel.] 
t  [This  and  the  preceding  paragraph  contain  the  essence  of  the 
Mendeliau  principles  of  heredity.] 

5—2 


68  Mendel's  Experiments 

The  artificially  fertilised  seeds  were  sown  together  with 
several  seeds  of  both  original  stocks,  and  the  most  vigorous 
examples  were  chosen  for  the  reciprocal  crossing.  There 
were  fertilised  : 

1.  The  hybrids  with  the  pollen  of  AB. 

2.  The  hybrids         ,,  „         ab. 

3.  AB  „  „         the  hybrids. 

4.  ab  „  „         the  hybrids. 

For  each  of  these  four  experiments  the  whole  of  the 
flowers  on  three  plants  were  fertilised.  If  the  above  theory 
be  correct,  there  must  be  developed  on  the  hybrids  egg  and 
pollen  cells  of  the  forms  AB,  Ab,  aB,  ab,  and  there  would 
be  combined  : — 

1.  The  Qgg  cells  AB,  Ab,  aB,  ab  with  the  pollen 
cells  AB. 

2.  The  Qgg  cells  AB,  Ab,  aB,  ab  with  the  pollen 
cells  ab. 

3.  The  Qgg  cells  AB  with  the  pollen  cells  AB,  Ab, 
aB,  ab. 

4.  The  Qgg  cells  ab  with  the  pollen  cells  AB,  Aby 
aB,  ab. 

From  each  of  these  experiments  there  could  then  result 
only  the  following  forms  : — 

1.  AB,  ABb,  AaB,  AaBb. 

2.  AaBb,  Aab,  aBb,  ab. 

3.  AB,  ABb,  AaB,  AaBb. 

4.  AaBb,  Aab,  aBb,  ab. 

If,  furthermore,  the  several  forms  of  the  Qgg  and  pollen 
cells  of  the  hybrids  were  produced  on  an  average  in  equal 
numbers,  then  in  each  experiment  the  said  four  combinations 


in  Hyhridisation  69 

should  stand  in  the  same  ratio  to  each  other.  A  perfect 
agreement  in  the  numerical  relations  was,  however,  not  to 
be  expected,  since  in  each  fertilisation,  even  in  normal 
cases,  some  &gg  cells  remain  undeveloped  or  subsequently 
die,  and  many  even  of  the  well-formed  seeds  fail  to 
germinate  when  sown.  The  above  assumption  is  also 
limited  in  so  far  that,  while  it  demands  the  formation  of 
an  equal  number  of  the  various  sorts  of  Qgg  and  pollen 
cells,  it  does  not  require  that  this  should  apply  to  each 
separate  hybrid  with  mathematical  exactness. 

The  first  and  second  experiments  had  primarily  the 
object  of  proving  the  composition  of  the  hybrid  Qgg  cells, 
while  the  third  and  fourth  experiments  were  to  decide  that  of 
the  pollen  cells*.  As  is  shown  by  the  above  demonstration 
the  first  and  second  experiments  and  the  third  and  fourth 
experiments  should  produce  precisely  the  same  combinations, 
and  even  in  the  second  year  the  result  should  be  partially 
visible  in  the  form  and  colour  of  the  artificially  fertilised 
seed.  In  the  first  and  third  experiments  the  dominant 
characters  of  form  and  colour,  A  and  B,  appear  in  each 
union,  and  are  also  partly  constant  and  partly  in  hybrid 
union  with  the  recessive  characters  a  and  h,  for  which 
reason  they  must  impress  their  peculiarity  upon  the  whole 
of  the  seeds.  All  seeds  should  therefore  appear  round  and 
yellow,  if  the  theory  be  justified.  In  the  second  and  fourth 
experiments,  on  the  other  hand,  one  union  is  hybrid  in 
form  and  in  colour,  and  consequently  the  seeds  are  round 
and  yellow  ;  another  is  hybrid  in  form,  but  constant  in  the 
recessive  character  of  colour,  whence  the  seeds  are  round 
and  green  ;  the  third  is  constant  in  the  recessive  character 
of  form  but  hybrid  in  colour,  consequently  the  seeds  are 

*  [To  prove,  namely,  that  both  were  similarly  differentiated,  and 
not  one  or  other  only.] 


70  MeyideVs  Experiments 

angular  and  yellow ;  the  fourth  is  constant  in  both  recessive 
characters,  so  that  the  seeds  are  angular  and  green.  In 
both  these  experiments  there  were  consequently  four  sorts 
of  seed  to  be  expected — viz.  round  and  yellow,  round  and 
green,  angular  and  yellow,  angular  and  green. 

The  crop  fulfilled  these  expectations  perfectly.  There 
were  obtained  in  the 

1st  Experiment,  98  exclusively  round  yellow  seeds  ; 
dru  ,,  y4:  ,,  '    jj  J)         j> 

In  the  2nd  Experiment,  31  round  and  yellow,  26  round 
and  green,  27  angular  and  yellow,  26  angular  and  green  seeds. 

In  the  4th  Experiment,  24  round  and  yellow,  25  round 
and  green,  22  angular  and  yellow,  27  angular  and  green 
seeds. 

A  favourable  result  could  now  scarcely  be  doubted ;  the 
next  generation  must  afford  the  final  proof  From  the  seed 
sown  there  resulted  for  the  first  experiment  90  plants,  and 
for  the  third  87  plants  which  fruited :  these  yielded  for  the — 

1st  Exp.  3rd  Exp. 

20  25     round  yellow  seeds   .         .         .         .         .  AB 

23  19     round  yellow  and  green  seeds          .         .  ABh 

25  22     round  and  angular  yellow  seeds       .         .  AaB 

22  21     round  and  angular  green  and  yellow  seeds  AaBb 

In  the  second  and  fourth  experiments  the  round  and 
yellow  seeds  yielded  plants  with  round  and  angular  yellow 
and  green  seeds,  AaBb. 

From  the  round  green  seeds  plants  resulted  with  round 
and  angular  green  seeds,  Aab. 

The  angular  yellow  seeds  gave  plants  with  angular 
yellow  and  green  seeds,  aBb. 

From  the  angular  green  seeds  plants  were  raised  which 
yielded  again  only  angular  and  green  seeds,  ab. 


in  Hybridisation  71 

Although  in  these  two  experiments  likewise  some  seeds 
did  not  germinate,  the  figures  arrived  at  already  in  the 
previous  year  were  not  affected  thereby,  since  each  kind  of 
seed  gave  plants  which,  as  regards  their  seed,  were  like  each 
other  and  different  from  the  others.  There  resulted  there- 
fore from  the 


2nd  Exp. 

4tli  Exp. 

31 

24 

plants  of  the  form  AaBh 

26 

25 

„                    „        Aab 

27 

22 

„                     „         aBb 

26 

27 

ah 

In  all  the  experiments,  therefore,  there  appeared  all  the 
forms  which  the  proposed  theory  demands,  and  also  in 
nearly  equal  numbers. 

In  a  further  experiment  the  characters  of  floral  colour 
and  length  of  stem  were  experimented  upon,  and  selection 
so  made  that  in  the  third  year  of  the  experiment  each 
character  ought  to  appear  in  half  of  all  the  plants  if  the 
above  theory  were  correct.  A,  B,  a,  h  serve  again  as 
indicating  the  various  characters. 

J.,  violet-red  flowers.  a,  white  flowers. 

B,  axis  long.  h,  axis  short. 

The  form  Ah  was  fertilised  with  ah,  which  produced  the 
hybrid  Aah.  Furthermore,  aB  was  also  fertilised  with  ah, 
whence  the  hybrid  aBh.  In  the  second  year,  for  further 
fertilisation,  the  hybrid  Aah  was  used  as  seed  parent,  and 
hybrid  aBh  as  pollen  parent. 

Seed  parent,  Aah.  Pollen  parent,  aBh. 

Possible  Qgg  cells,  Ahah.      Pollen  cells,  aBah. 

From  the  fertilisation  between  the  possible   egg  and 
pollen  cells  four  combinations  should  result,  viz. : — 
AaBh  +  aBb  +  Aah  +  ah. 


7^  MendeVs  Experiments 

From  this  it  is  perceived  that,  according  to  the  above 
theory,  in  the  third  year  of  the  experiment  out  of  all  the 
plants 

Half  should  have  violet-red  flowers  {Aa),  Classes  1,  3 

„         „         ,,      white  flowers  (a)  ,,  2,  4 

„         „         „      a  long  axis  {Bh)  „  1,  2 

„         „         „      a  short  axis  (b)  „  3,  4 

From  45  fertilisations  of  the  second  year  187  seeds 
resulted,  of  which  only  166  reached  the  flowering  stage  in 
the  third  year.  Among  these  the  separate  classes  appeared 
in  the  numbers  following : — 


Class. 

Colour  of  flower. 

Stem. 

1 

violet-red 

long 

47  times 

2 

white 

long 

40      „ 

3 

violet-red 

short 

38      „ 

4 

white 

short 

41      „ 

There  consequently  appeared — 

The  violet-red  flower  colour  (Aa)  in  85  plants. 
„     white  „  „       (a)     in  81      „ 

„     long  stem  (Bb)  in  87      „ 

„    short    „  (6)      in  79      „ 

The  theory  adduced  is  therefore  satisfactorily  confirmed  in 
this  experiment  also. 

For  the  characters  of  form  of  pod,  colour  of  pod,  and 
position  of  flowers  experiments  were  also  made  on  a  small 
scale,  and  results  obtained  in  perfect  agreement.  All 
combinations  which  were  possible  through  the  union  of  the 
differentiating  characters  duly  appeared,  and  in  nearly 
equal  numbers. 

Experimentally,  therefore,  the  theory  is  justified  that 
the  pea  hybrids  form  egg  and  pollen  cells  which,  in  their 


in  Hybridisation  73 

constitution^  represent  in  equal  numbers  all  constant  forms 
which  result  from  the  combination  of  the  characters  when 
united  in  fertilisation. 

The  difference  of  the  forms  among  the  progeny  of  the 
hybrids,  as  well  as  the  respective  ratios  of  the  numbers  in 
which  they  are  observed,  find  a  sufficient  explanation  in  the 
principle  above  deduced.  The  simplest  case  is  afforded  by 
the  developmental  series  of  each  pair  of  differentiating 
characters.  This  series  is  represented  by  the  expression 
A  +  2Aa  +  a,  in  which  A  and  a  signify  the  forms  with 
constant  differentiating  characters,  and  Aa  the  hybrid 
form  of  both.  It  includes  in  three  different  classes  four 
individuals.  In  the  formation  of  these,  pollen  and  Qgg 
cells  of  the  form  A  and  a  take  part  on  the  average  equally 
in  the  fertilisation ;  hence  each  form  [occurs]  twice,  since 
four  individuals  are  formed.  There  participate  consequently 
in  the  fertilisation — 

The  poUen  cells  A  +  A-^  a  +  a 
The  Qgg  cells  A  +  A  +  a  +  a. 

It  remains,  therefore,  purely  a  matter  of  chance  which 
of  the  two  sorts  of  pollen  will  become  united  with  each 
separate  Qgg  cell.  According,  however,  to  the  law  of 
probability,  it  will  always  happen,  on  the  average  of  many 
cases,  that  each  pollen  form  A  and  a  will  unite  equally 
often  with  each  Qgg  cell  form  A  and  «,  consequently  one 
of  the  two  pollen  cells  A  in  the  fertilisation  will  meet  with 
the  Qgg  cell  A  and  the  other  with  an  Qgg  cell  a,  and  so 
likewise  one  pollen  cell  a  will  unite  with  an  Qgg  cell  A^ 
and  the  other  with  egg  cell  a. 

Pollen  cells  A  A       a  a 

\ 


" 


Egg  cells  A  A       a  a 


74  MendeVs  Experiments 

The  result  of  the  fertilisation  may  be  made  clear  by 
putting  the  signs  for  the  conjoined  Qgg  and  pollen  cells  in 
the  form  of  fractions,  those  for  the  pollen  cells  above  and 
those  for  the  Qgg  cells  below  the  line.     We  then  have 

A      A      a     a 

—  +  —  +—  +-. 
A      a      A      a 

In  the  first  and  fourth  term  the  Qgg  and  pollen  cells  are  of 
like  kind,  consequently  the  product  of  their  union  must  be 
constant,  viz.  A  and  a\  in  the  second  and  third,  on  the 
other  hand,  there  again  results  a  union  of  the  two  different- 
iating characters  of  the  stocks,  consequently  the  forms 
resulting  from  these  fertilisations  are  identical  with  those 
of  the  hybrid  from  which  they  sprang.  There  occurs 
accordingly  a  repeated  hybridisation.  This  explains  the 
striking  fact  that  the  hybrids  are  able  to  produce,  besides 
the  two  parental  forms,  offspring  which  are  like  themselves ; 

—  and  -r  both  give  the  same  union  Aa,  since,  as  already 
a  A 

remarked  above,  it  makes  no  difference  in  the  result  of 
fertilisation  to  which  of  the  two  characters  the  pollen  or 
egg  cells  belong.     We  may  write  then — 

A      A      a      a      .      ^  . 

—  +  —  +-r+-  =  J.  +  2Aa  +  a. 
A      a      A      a 

This  represents  the  average  result  of  the  self-fertilisation 
of  the  hybrids  when  two  differentiating  characters  are 
united  in  them.  In  solitary  flowers  and  in  solitary  plants, 
however,  the  ratios  in  which  the  forms  of  the  series  are  pro- 
duced may  suffer  not  inconsiderable  fluctuations*.  Apart 
from  the  fact  that  the  numbers  in  which  both  sorts  of  Qgg 
cells  occur  in  the  seed  vessels  can  only  be  regarded  as  equal 
on  the  average,  it  remains  purely  a  matter  of  chance  which 

*  [Whether  segregation  by  such  units  is  more  than  purely  for- 
tuitous could  probably  be  determined  by  seriation.] 


in  Hybridisation  75 

of  the  two  sorts  of  pollen  may  fertilise  each  separate  egg 
cell.  For  this  reason  the  separate  values  must  necessarily 
be  subject  to  fluctuations,  and  there  are  even  extreme  cases 
possible,  as  were  described  earlier  in  connection  with  the 
experiments  on  the  form  of  the  seed  and  the  colour  of  the 
albumen.  The  true  ratios  of  the  numbers  can  only  be 
ascertained  by  an  average  deduced  from  the  sum  of  as 
many  single  values  as  possible ;  the  greater  the  number 
the  more  are  merely  chance  elements  eliminated. 

The  developmental  series  for  hybrids  in  which  two 
kinds  of  differentiating  characters  are  united  contains 
among  sixteen  individuals  nine  different  forms,  viz., 
AB  +  Ab  +  aB  +  ab  +  2ABb+2aBb  +  2AaB+2Aab+4.AaBb. 
Between  the  differentiating  characters  of  the  original  stocks 
Aa  and  Bb  four  constant  combinations  are  possible,  and 
consequently  the  hybrids  produce  the  corresponding  four 
forms  of  egg  and  pollen  cells  AB,  Ab,  aB,  ab,  and  each 
of  these  will  on  the  average  figure  four  times  in  the 
fertilisation,  since  sixteen  individuals  are  included  in  the 
series.  Therefore  the  participators  in  the  fertilisation  are — 
Pollen  cells  AB  +  AB  +  AB  +  AB  +  Ab  +  Ab  + Ab  + Ab  + 

aB  +  aB  +  aB  +  aB  +  ab  +  ab  +  ab  +  ab. 
Egg  cells     AB  +  AB  +  AB  +  AB  +  Ab  +  Ab  +  Ab  +  Ab  + 

aB  +  aB  +  aB  +  aB  +  ab  +  ab  +  ab  +  ab. 
In  the  process  of  fertilisation  each  pollen  form  unites  on  an 
average  equally  often  with  each  egg  cell  form,  so  that  each 
of  the  four  pollen  cells  AB  unites  once  with  one  of  the 
forms  of  egg  cell  AB,  Ab,  aB,  ab.     In  precisely  the  same 
way  the  rest  of  the  pollen  cells  of  the  forms  Ab,  aB,  ab 
unite  with  all  the  other  egg  cells.     We  obtain  therefore — 
AB     AB     AB     AB      Ah      Ab     Ab     Ab 
AB  ^  Ab^  aB  "^  ab   ^  AB^  Ab^  aB^  ab^ 
aB     aB     aB     aB      ab       ab       ab      ab 
AB'^Ab^^^lih^AB^Ab^^^ab' 


7Q  Mender s  Experiments 

or 

AB  +  ABh  +  AaB  +  AaBh  +  ABh  +  Ah  ■¥  AaBb  +  Aah  + 
AaB^-  AaBh  +  aB  +  ^56  +  A  aBh  +  J^a^  +  aBh  -^ah  =  AB  + 
Ah  +  aB  +  ah  +  2  ABh  +  2aBh  +  2^a5  +  2  Aah  +  4.AaBh^. 

In  precisely  similar  fashion  is  the  developmental  series 
of  hybrids  exhibited  when  three  kinds  of  differentiating 
characters  are  conjoined  in  them.  The  hybrids  form  eight 
various  kinds  of  egg  and  pollen  cells — ABC,  ABc,  AhC, 
Abe,  aBC,  aBc,  ahC,  ahc — and  each  pollen  form  unites 
itself  again  on  the  average  once  with  each  form  of  Qgg  cell. 

The  law  of  combination  of  different  characters  which 
governs  the  development  of  the  hybrids  finds  therefore  its 
foundation  and  explanation  in  the  principle  enunciated, 
that  the  hybrids  produce  egg  cells  and  pollen  cells  which 
in  equal  numbers  represent  all  constant  forms  which  result 
from  the  combinations  of  the  characters  brought  together 
in  fertilisation. 


Experiments  with  Hybrids  of  other  Species  of  Plants. 

It  must  be  the  object  of  further  experiments  to  ascertain 
whether  the  law  of  development  discovered  for  Pisum 
applies  also  to  the  hybrids  of  other  plants.  To  this  end 
several  experiments  were  recently  commenced.  Two  minor 
experiments  with  species  of  Phaseolus  have  been  completed, 
and  may  be  here  mentioned. 

An  experiment  with  Phaseolus  vulgaris  and  Phaseolus 
nanus  gave  results  in  perfect  agreement.  Ph.  nanus  had 
together  with  the  dwarf  axis  simply  inflated  green  pods. 
Ph.  vulgaris  had,  on  the  other  hand,  an  axis  10  feet  to 

*  [In  the  original  the  sign  of  equality  (  =  )  is  here  represented 
by  + ,  evidently  a  misprint.] 


in  Hybridisation  77 

12  feet  high,  and  yellow  coloured  pods,  constricted  when 
ripe.  The  ratios  of  the  numbers  in  which  the  different 
forms  appeared  in  the  separate  generations  were  the  same 
as  with  Pisum.  Also  the  development  of  the  constant 
combinations  resulted  according  to  the  law  of  simple  com- 
bination of  characters,  exactly  as  in  the  case  of  Pisum. 
There  were  obtained — 


Constant 

Axis 

Colour  of 

Form  of 

combinations 

the  unripe  pods. 

the  ripe  pods. 

1 

long 

green 

inflated 

2 

}j 

55 

constricted 

3 

j> 

yellow 

inflated 

4 

>j 

55 

constricted 

5 

short 

green 

inflated 

6 

jj 

55 

constricted 

7 

n 

yellow 

inflated 

8 

)j 

)j 

constricted 

The  green  colour  of  the  pod,  the  inflated  forms,  and  the 
long  axis  were,  as  in  Pisum,  dominant  characters. 

Another  experiment  with  two  very  different  species  of 
Phaseolus  had  only  a  partial  result.  Phaseolus  nanus,  L., 
served  as  seed  parent,  a  perfectly  constant  species,  with 
white  flowers  in  short  racemes  and  small  white  seeds  in 
straight,  inflated,  smooth  pods;  as  pollen  parent  was  used 
Ph.  multiflorus,  W.,  with  tall  winding  stem,  purple-red 
flowers  in  very  long  racemes,  rough,  sickle-shaped  crooked 
pods,  and  large  seeds  which  bore  black  flecks  and  splashes 
on  a  peach-blood-red  ground. 

The  hybrids  had  the  greatest  similarity  to  the  pollen 
parent,  but  the  flowers  appeared  less  intensely  coloured. 
Their  fertility  was  very  limited;  from  seventeen  plants, 
which  together  developed  many  hundreds  of  flowers,  only 
forty-nine   seeds  in   all   were   obtained.      These   were    of 


78  MendeVs  Experiments 

medium  size,  and  were  flecked  and  splashed  similarly  to 
those  of  Ph.  muUiflorus,  while  the  ground  colour  was  not 
materially  different.  The  next  year  forty-four  plants  were 
raised  from  these  seeds,  of  which  only  thirty-one  reached 
the  flowering  stage.  The  characters  of  Ph.  nanus,  which 
had  been  altogether  latent  in  the  hybrids,  reappeared  in 
various  combinations;  their  ratio,  however,  with  relation 
to  the  dominant  characters  was  necessarily  very  fluctuating 
owing  to  the  small  number  of  trial  plants.  With  certain 
characters,  as  in  those  of  the  axis  aud  the  form  of  pod,  it 
was,  however,  as  in  the  case  of  Pisum,  almost  exactly  1:3. 
Insignificant  as  the  results  of  this  experiment  may  be  as 
regards  the  determination  of  the  relative  numbers  in  which 
the  various  forms  appeared,  it  presents,  on  the  other  hand, 
the  phenomenon  of  a  remarkable  change  of  colour  in  the 
flowers  and  seed  of  the  hybrids.  In  Pisum  it  is  known 
that  the  characters  of  the  flower-  and  seed-colour  present 
themselves  unchanged  in  the  first  and  further  generations, 
and  that  the  offspring  of  the  hybrids  display  exclusively  the 
one  or  the  other  of  the  characters  of  the  original  stocks'^. 
It  is  otherwise  in  the  experiment  we  are  considering.  The 
white  flowers  and  the  seed-colour  of  Ph.  nanus  appeared,  it 
is  true,  at  once  in  the  first  generation  [/rom  the  hybrids] 
in  one  fairly  fertile  example,  but  the  remaining  thirty 
plants  developed  flower  colours  which  were  of  various 
grades  of  purple-red  to  pale  violet.  The  colouring  of  the 
seed-coat  was  no  less  varied  than  that  of  the  flowers.     No 

*  [This  is  the  only  passage  where  Mendel  can  be  construed  as 
asserting  universal  dominance  for  Pisum ;  and  even  here,  having 
regard  to  the  rest  of  the  paper,  it  is  clearly  unfair  to  represent  him  as 
predicating  more  than  he  had  seen  in  his  own  experiments.  More- 
over in  flower  and  seed-coat  colour  (which  is  here  meant),  using  his 
characters  dominance  must  be  almost  universal,  if  not  quite.] 


iri  Hybridisation  79 

plant  could  rank  as  fully  fertile;  many  produced  no  fruit 
at  all ;  others  only  yielded  fruits  from  the  flowers  last  pro- 
duced, which  did  not  ripen.  From  fifteen  plants  only  were 
well-developed  seeds  obtained.  The  greatest  disposition 
to  infertility  was  seen  in  the  forms  with  preponderantly 
red  flowers,  since  out  of  sixteen  of  these  only  four  yielded 
ripe  seed.  Three  of  these  had  a  similar  seed  pattern  to 
Pk.  multiflomis,  but  with  a  more  or  less  pale  ground  colour; 
the  fourth  plant  yielded  only  one  seed  of  plain  brown  tint. 
The  forms  with  preponderantly  violet  coloured  flowers  had 
dark  brown,  black-brown,  and  quite  black  seeds. 

The  experiment  was  continued  through  two  more  genera- 
tions under  similar  unfavourable  circumstances,  since  even 
among  the  offspring  of  fairly  fertile  plants  there  were  still 
some  which  were  less  fertile  or  even  quite  sterile.  Other 
flower-  and  seed-colours  than  those  cited  did  not  sub- 
sequently present  themselves.  The  forms  w^hich  in  the 
first  generation  [bred  from  the  hybrids]  contained  one  or 
more  of  the  recessive  characters  remained,  as  regards  these, 
constant  without  exception.  Also  of  those  plants  which 
possessed  violet  flowers  and  brown  or  black  seed,  some  did 
not  vary  again  in  these  respects  in  the  next  generation; 
the  majority,  however,  yielded,  together  with  ofi"spring 
exactly  like  themselves,  some  which  displayed  white  flowers 
and  white  seed-coats.  The  red  flowering  plants  remained 
so  slightly  fertile  that  nothing  can  be  said  with  certainty 
as  regards  their  further  development. 

Despite  the  many  disturbing  factors  with  which  the 
observations  had  to  contend,  it  is  nevertheless  seen  by  this 
experiment  that  the  development  of  the  hybrids,  with 
regard  to  those  characters  which  concern  the  form  of  the 
plants,  follows  the  same  laws  as  does  Pisum.  With  regard 
to  the  colour  characters,  it  certainly  appears  difficult  to 


80  MendeVs  Experiments 

perceive  a  substantial  agreement.  Apart  from  the  fact 
that  from  the  union  of  a  white  and  a  purple-red  colouring 
a  whole  series  of  colours  results,  from  purple  to  pale  violet 
and  white,  the  circumstance  is  a  striking  one  that  among 
thirty-one  flowering  plants  only  one  received  the  recessive 
character  of  the  white  colour,  while  in  Pisum  this  occurs 
on  the  average  in  every  fourth  plant. 

Even  these  enigmatical  results,  however,  might  probably 
be  explained  by  the  law  governing  Pisum  if  we  might 
assume  that  the  colour  of  the  flowers  and  seeds  of  Ph. 
multiflorus  is  a  combination  of  two  or  more  entirely 
independent  colours,  which  individually  act  like  any  other 
constant  character  in  the  plant.  If  the  flower  colour  A 
were  a  combination  of  the  individual  characters  A^  +  A^+  ... 
which  produce  the  total  impression  of  a  purple  colouration, 
then  by  fertilisation  with  the  differentiating  character, 
white  colour,  a,  there  would  be  produced  the  hybrid  unions 
A^a  +  A^a  + ...  and  so  would  it  be  with  the  corresponding 
colouring  of  the  seed-coats'*.  According  to  the  above 
assumption,  each  of  these  hybrid  colour  unions  would  be 
independent,  and  would  consequently  develop  quite  inde- 
pendently from  the  others.  It  is  then  easily  seen  that 
from  the  combination  of  the  separate  developmental  series 

*  [It  appears  to  me  clear  that  this  expression  is  incorrectly  given, 
and  the  argument  regarding  compound  characters  is  consequently  not 
legitimately  developed.  The  original  compound  character  should  be 
represented  as  A-^A^A^...  which  when  fertilised  by  a-^  gives  A-j^A^A^..^ 
as  the  hybrid  of  the  first  generation.  Mendel  practically  tells  us 
these  were  all  alike,  and  there  is  nothing  to  suggest  that  they  were 
diverse.  When  on  self-fertilisation,  they  break  up,  they  will  produce 
the  gametes  he  specifies ;  but  they  may  also  produce  A-^A-^  and  A^A^y 
A^A^a,  &G.,  thereby  introducing  terms  of  a  nature  different  from  any 
indicated  by  him.  That  this  point  is  one  of  the  highest  significance, 
both  practical  and  theoretical,  is  evident  at  once.  ] 


in  Hybridisation  81 

a  perfect  colour-series  must  result.  If,  for  instance, 
A  =  A^+  A^,  then  the  hybrids  A^a  and  A^a  form  the 
developmental  series — 

Ai  +  2A^a  +  a 
A^+  2A^a-¥a. 

The  members  of  this  series  can  enter  into  nine  different 
combinations,  and  each  of  these  denotes  another  colour"^ — 

1  A^A^  2  A^aA^  1  A^a 

2  A^A^a        4  A^aA^a        2  A^aa 
1  A^a  2  Aiaa  1  aa. 

The  figures  prescribed  for  the  separate  combinations 
also  indicate  how  many  plants  with  the  corresponding 
colouring  belong  to  the  series.  Since  the  total  is  sixteen, 
the  whole  of  the  colours  are  on  the  average  distributed 
over  each  sixteen  plants,  but,  as  the  series  itself  indicates, 
in  unequal  proportions. 

Should  the  colour  development  really  happen  in  this 
way,  we  could  offer  an  explanation  of  the  case  above 
described,  viz.  that  the  white  flowers  and  seed-coat  colour 
only  appeared  once  among  thirty-one  plants  of  the  first 
generation.  This  colouring  appears  only  once  in  the  series, 
and  could  therefore  also  only  be  developed  once  in  the 
average  in  each  sixteen,  and  with  three  colour  characters 
only  once  even  in  sixty-four  plants. 

It  must,  however,  not  be  forgotten  that  the  explanation 
here  attempted  is  based  on  a  mere  hypothesis,  only  supported 
by  the  very  imperfect  result  of  the  experiment  just  de- 
scribed. It  would,  however,  be  well  worth  while  to  follow 
up  the  development  of  colour  in  hybrids  by  similar  experi- 

*  [It  seems  very  doubtful  if  the  zygotes  are  correctly  represented  by 
the  terms  A^aA^a,  A/xa,  A-^^aa ;  for  in  the  hybrids  A^a,  &c.  the  allelo- 
morphs Ai  and  a,  &c.  should  by  hypothesis  be  separated  in  the  gametes.] 

B.  6 


82  Mender s  Experiments 

ments,  since  it  is  probable  that  in  this  way  we  might  learn 
the  significance  of  the  extraordinary  variety  in  the  colouring 
of  our  ornamental  flowers. 

So  far,  little  at  present  is  known  with  certainty  beyond 
the  fact  that  the  colour  of  the  flowers  in  most  ornamental 
plants  is  an  extremely  variable  character.  The  opinion  has 
often  been  expressed  that  the  stability  of  the  species  is 
greatly  disturbed  or  entirely  upset  by  cultivation,  and 
consequently  there  is  an  inclination  to  regard  the  develop- 
ment of  cultivated  forms  as  a  matter  of  chance  devoid  of 
rules  ;  the  colouring  of  ornamental  plants  is  indeed  usually 
cited  as  an  example  of  great  instability.  It  is,  however, 
not  clear  why  the  simple  transference  into  garden  soil 
should  result  in  such  a  thorough  and  persistent  revolution 
in  the  plant  organism.  No  one  will  seriously  maintain 
that  in  the  open  country  the  development  of  plants  is  ruled 
by  other  laws  than  in  the  garden  bed.  Here,  as  there, 
changes  of  type  must  take  place  if  the  conditions  of  life  be 
altered,  and  the  species  possesses  the  capacity  of  fitting 
itself  to  its  new  environment.  It  is  willingly  granted  that 
by  cultivation  the  origination  of  new  varieties  is  favoured, 
and  that  by  man's  labour  many  varieties  are  acquired 
which,  under  natural  conditions,  would  be  lost ;  but  nothing 
justifies  the  assumption  that  the  tendency  to  the  formation 
of  varieties  is  so  extraordinarily  increased  that  the  species 
speedily  lose  all  stability,  and  their  offspring  diverge  into 
an  endless  series  of  extremely  variable  forms.  Were  the 
change  in  the  conditions  of  vegetation  the  sole  cause  of 
variability  we  might  expect  that  those  cultivated  plants 
which  are  grown  for  centuries  under  almost  identical  con- 
ditions would  again  attain  constancy.  That,  as  is  well 
known,  is  not  the  case,  since  it  is  precisely  under  such 
circumstances  that  not  only  the  most  varied  but  also  the 


in  Hybridisation  83 

most  variable  forms  are  found.  It  is  only  the  Leguminosce , 
like  Pisum,  Phaseolus,  Lens,  whose  organs  of  fertilisation 
are  protected  by  the  keel,  which  constitute  a  noteworthy 
exception.  Even  here  there  have  arisen  numerous  varieties 
during  a  cultural  period  of  more  than  1000  years ;  these 
maintain,  however,  under  unchanging  environments  a  sta- 
bility as  great  as  that  of  species  growing  wild. 

It  is  more  than  probable  that  as  regards  the  variability 
of  cultivated  plants  there  exists  a  factor  which  so  far  has 
received  little  attention.  Various  experiments  force  us  to 
the  conclusion  that  our  cultivated  plants,  with  few  ex- 
ceptions, are  members  of  various  hybrid  series,  whose 
further  development  in  conformity  with  law  is  changed  and 
hindered  by  frequent  crossings  inter  se.  The  circumstance 
must  not  be  overlooked  that  cultivated  plants  are  mostly 
grown  in  great  numbers  and  close  together,  affording 
the  most  favourable  conditions  for  reciprocal  fertilisation 
between  the  varieties  present  and  the  species  itself  The 
probability  of  this  is  supported  by  the  fact  that  among  the 
great  array  of  variable  forms  solitary  examples  are  always 
found,  which  in  one  character  or  another  remain  constant, 
if  only  foreign  influence  be  carefully  excluded.  These  forms 
develop  precisely  as  do  those  which  are  known  to  be  members 
of  the  compound  hybrid  series.  Also  with  the  most  sus- 
ceptible of  all  characters,  that  of  colour,  it  cannot  escape 
the  careful  observer  that  in  the  separate  forms  the  inclination 
to  vary  is  disj^layed  in  very  different  degrees.  Among 
plants  which  arise  from  one  spontaneous  fertilisation  there 
are  often  some  whose  offspring  vary  widely  in  the  constitution 
and  arrangement  of  the  colours,  while  others  furnish  forms  of 
little  deviation,  and  among  a  greater  number  solitary  examples 
occur  which  transmit  the  colour  of  the  flowers  unchanged 
to  their  offspring.      The   cultivated  species  of  Dianthus 

6—2 


84  Menders  Experiments 

afford  an  instructive  example  of  this.  A  white-flowered 
example  of  Diantkus  caryophyllus,  which  itself  was  derived 
from  a  white-flowered  variety,  was  shut  up  during  its 
blooming  period  in  a  greenhouse ;  the  numerous  seeds 
obtained  therefrom  yielded  plants  entirely  white-flowered 
like  itself  A  similar  result  was  obtained  from  a  subspecies, 
with  red  flowers  somewhat  flushed  with  violet,  and  one 
with  flowers  white,  striped  with  red.  Many  others,  on  the 
other  hand,  which  were  similarly  protected,  yielded  progeny 
which  were  more  or  less  variously  coloured  and  marked. 

Whoever  studies  the  colouration  which  results  in  orna- 
mental plants  from  similar  fertilisation  can  hardly  escape 
the  conviction  that  here  also  the  development  follows  a 
definite  law  which  possibly  finds  its  expression  in  the 
combination  of  several  independent  colour  characters. 

Concluding  Remarks. 

It  can  hardly  fail  to  be  of  interest  to  compare  the 
observations  made  regarding  Pisum  with  the  results  arrived 
at  by  the  two  authorities  in  this  branch  of  knowledge, 
Kolreuter  and  Gartner,  in  their  investigations.  According 
to  the  opinion  of  both,  the  hybrids  in  outer  appearance 
present  either  a  form  intermediate  between  the  original 
species,  or  they  closely  resemble  either  the  one  or  the  other 
type,  and  sometimes  can  hardly  be  discriminated  from  it. 
From  their  seeds  usually  arise,  if  the  fertilisation  was 
effected  by  their  own  pollen,  various  forms  which  differ 
from  the  normal  type.  As  a  rule,  the  majority  of  individuals 
obtained  by  one  fertilisation  maintain  the  hybrid  form, 
while  some  few  others  come  more  like  the  seed  parent, 
and  one  or  other  individual  approaches  the  pollen  parent. 
This,  however,  is  not  the  case  with  all  hybrids  without 
exception.      With  some  the   offspring  have   more   nearly 


in  Hyhridisation  85 

approached,  some  the  one  and  some  the  other,  original 
stock,  or  they  all  incline  more  to  one  or  the  other  side ; 
while  with  others  they  remain  perfectly  like  the  hybrid  and 
continue  constant  in  their  offspring.  The  hybrids  of  varieties 
behave  like  hybrids  of  species,  but  they  possess  greater  varia- 
bility of  form  and  a  more  pronounced  tendency  to  revert  to 
the  original  type. 

With  regard  to  the  form  of  the  hybrids  and  their 
development,  as  a  rule  an  agreement  with  the  observations 
made  in  Pisum  is  unmistakable.  It  is  otherwise  with  the 
exceptional  cases  cited.  Gartner  confesses  even  that  the 
exact  determination  whether  a  form  bears  a  greater  resem- 
blance to  one  or  to  the  other  of  the  two  original  species 
often  involved  great  difficulty,  so  much  depending  upon 
the  subjective  point  of  view  of  the  observer.  Another 
circumstance  could,  however,  contribute  to  render  the 
results  fluctuating  and  uncertain,  despite  the  most  careful 
observation  and  differentiation;  for  the  experiments  plants 
were  mostly  used  which  rank  as  good  species  and  are 
differentiated  by  a  large  number  of  characters.  In  addition 
to  the  sharply  defined  characters,  where  it  is  a  question  of 
greater  or  less  similarity,  those  characters  must  also  be 
taken  into  account  which  are  often  difficult  to  define  in 
words,  but  yet  suffice,  as  every  plant  specialist  knows,  to 
give  the  forms  a  strange  appearance.  If  it  be  accepted 
that  the  development  of  hybrids  follows  the  law  which  is 
valid  for  Pisum,  the  series  in  each  separate  experiment 
must  embrace  very  many  forms,  since  the  number  of  the 
components,  as  is  known,  increases  with  the  number  of 
the  differentiating  characters  in  cubic  ratio.  With  a 
relatively  small  number  of  experimental-plants  the  result 
therefore  could  only  be  approximately  right,  and  in  single 
cases  might  fluctuate  considerably.     If,  for  instance,  the 


86  MendeVs  Experiments 

two  original  stocks  differ  in  seven  characters,  and  100  and 
200  plants  were  raised  from  the  seeds  of  their  hybrids  to 
determine  the  grade  of  relationship  of  the  offspring,  we  can 
easily  see  how  uncertain  the  decision  must  become,  since 
for  seven  differentiating  characters  the  combination  series 
contains  16,384  individuals  under  2187  various  forms ; 
now  one  and  then  another  relationship  could  assert  its 
predominance,  just  according  as  chance  presented  this  or 
that  form  to  the  observer  in  a  majority  of  cases. 

If,  furthermore,  there  appear  among  the  differentiating 
characters  at  the  same  time  dominant  characters,  which 
are  transferred  entire  or  nearly  unchanged  to  the  hybrids, 
then  in  the  terms  of  the  developmental  series  that  one  of 
the  two  original  stocks  which  possesses  the  majority  of 
dominant  characters  must  always  be  predominant.  In  the 
experiment  described  relative  to  Pisum,  in  which  three 
kinds  of  differentiating  characters  were  concerned,  all  the 
dominant  characters  belonged  to  the  seed  parent.  Although 
the  terms  of  the  series  in  their  internal  composition 
approach  both  original  stock  plants  equally,  in  this  experi- 
ment the  type  of  the  seed  parent  obtained  so  great  a 
preponderance  that  out  of  each  sixty-four  plants  of  the 
first  generation  fifty-four  exactly  resembled  it,  or  only 
differed  in  one  character.  It  is  seen  how  rash  it  may  be 
under  such  circumstances  to  draw  from  the  external  resem- 
blances of  hybrids  conclusions  as  to  their  internal  nature. 

Gartner  mentions  that  in  those  cases  where  the  develop- 
ment was  regular  among  the  offspring  of  the  hybrids  the 
two  original  species  were  not  reproduced,  but  only  a  few 
closely  approximating  individuals.  With  very  extended 
developmental  series  it  could  not  in  fact  be  otherwise. 
For  seven  differentiating  characters,  for  instance,  among 
more  than  16,000  individuals — offspring  of  the  hybrids — 


in  Hybridisation  87 

each  of  the  two  original  species  would  occur  only  once.  It 
is  therefore  hardly  possible  that  these  should  appear  at  all 
among  a  small  number  of  experimental  plants  ;  with  some 
probability,  however,  we  might  reckon  upon  the  appearance 
in  the  series  of  a  few  forms  which  approach  them. 

We  meet  with  an  essential  difference  in  those  hybrids 
which  remain  constant  in  their  progeny  and  propagate 
themselves  as  truly  as  the  pure  species.  According  to 
Gartner,  to  this  class  belong  the  remarlzably  fertile  hybrids 
Aquilegia  atropmyurea  canadensis,  Lavatera  pseudolbia 
thuringiaca,  Geum  urhano-rivale,  and  some  Dianthus 
hybrids  ;  and,  according  to  Wichura,  the  hybrids  of  the 
Willow  species.  For  the  history  of  the  evolution  of  plants 
this  circumstance  is  of  special  importance,  since  constant 
hybrids  acquire  the  status  of  new  species.  The  correctness 
of  this  is  evidenced  by  most  excellent  observers,  and  cannot 
be  doubted.  Gartner  had  opportunity  to  follow  up  Dianthus 
Armeria  deltoides  to  the  tenth  generation,  since  it  regularly 
propagated  itself  in  the  garden. 

With  Pisum  it  was  shown  by  experiment  that  the 
hybrids  form  egg  and  pollen  cells  of  different  kinds,  and  that 
herein  lies  the  reason  of  the  variability  of  their  offspring. 
In  other  hybrids,  likewise,  whose  offspring  behave  similarly 
we  may  assume  a  like  cause  ;  for  those,  on  the  other  hand, 
which  remain  constant  the  assumption  appears  justifiable 
that  their  fertilising  cells  are  all  alike  and  agree  with  the 
foundation-cell  [fertilised  ovum]  of  the  hybrid.  In  the 
opinion  of  renowned  physiologists,  for  the  purpose  of 
propagation  one  pollen  cell  and  one  ^gg  cell  unite  in 
Phanerogams*   into   a   single   cell,    which   is    capable    by 

*  In  Pisum  it  is  placed  beyond  doubt  that  for  the  formation  of  the 
new  embryo  a  perfect  union  of  the  elements  of  both  fertilising  cells 
must  take  place.     How  could  we  otherwise  explain  that  among  the 


88  MendeVs  Experiments 

assimilation  and  formation  of  new  cells  to  become  an 
independent  organism.  This  development  follows  a  con- 
stant law,  which  is  founded  on  the  material  composition 
and  arrangement  of  the  elements  which  meet  in  the  cell 
in  a  vivifying  union.  If  the  reproductive  cells  be  of  the 
same  kind  and  agree  with  the  foundation  cell  [fertilised 
ovum]  of  the  mother  plant,  then  the  development  of  the 
new  individual  will  follow  the  same  law  which  rules  the 
mother  plant.  If  it  chance  that  an  Qgg  cell  unites  with  a 
dissimilar  pollen  cell,  we  must  then  assume  that  between 
those  elements  of  both  cells,  which  determine  the  mutual 
differences,  some  sort  of  compromise  is  effected.  The 
resulting  compound  cell  becomes  the  foundation  of  the 
hybrid  organism,  the  development  of  which  necessarily 
follows  a  different  scheme  from  that  obtaining  in  each  of  the 
two  original  species.  If  the  compromise  be  taken  to  be  a 
complete  one,  in  the  sense,  namely,  that  the  hybrid  embryo 
is  formed  from  cells  of  like  kind,  in  which  the  differences 
are  entirely  and  'permanently  accommodated  together,  the 
further  result  follows  that  the  hybrids,  like  any  other  stable 
plant  species,  remain  true  to  themselves  in  their  offspring. 
The   reproductive  cells  which  are  formed  in  their  seed 


offspring  of  the  hybrids  both  original  types  reappear  in  equal  numbers 
and  with  all  their  peculiarities  ?  If  the  influence  of  the  egg  cell  upon 
the  pollen  cell  were  only  external,  if  it  fulfilled  the  role  of  a  nurse 
only,  then  the  result  of  each  artificial  fertilisation  could  be  no  other 
than  that  the  developed  hybrid  should  exactly  resemble  the  pollen 
parent,  or  at  any  rate  do  so  very  closely.  This  the  experiments  so  far 
have  in  no  wise  confirmed.  An  evident  proof  of  the  complete  union 
of  the  contents  of  both  cells  is  afforded  by  the  experience  gained  on 
all  sides  that  it  is  immaterial,  as  regards  the  form  of  the  hybrid, 
which  of  the  original  species  is  the  seed  parent  or  which  the  pollen 
parent. 


in  Hybridisation  89 

vessels  and  anthers  are  of  one  kind,  and  agree  with  the 
fundamental  compound  cell  [fertilised  ovum]. 

With  regard  to  those  hybrids  whose  progeny  is  'variable 
we  may  perhaps  assume  that  between  the  differentiating 
elements  of  the  Q^g  and  pollen  cells  there  also  occurs  a 
compromise,  in  so  far  that  the  formation  of  a  cell  as 
foundation  of  the  hybrid  becomes  possible;  but,  never- 
theless, the  arrangement  between  the  conflicting  elements 
is  only  temporary  and  does  not  endure  throughout  the  life 
of  the  hybrid  plant.  Since  in  the  habit  of  the  plant  no 
changes  are  perceptible  during  the  whole  period  of  vege- 
tation, we  must  further  assume  that  it  is  only  possible  for 
the  differentiating  elements  to  liberate  themselves  from  the 
enforced  union  when  the  fertilising  cells  are  developed.  In 
the  formation  of  these  cells  all  existing  elements  participate 
in  an  entirely  free  and  equal  arrangement,  in  which  it 
is  only  the  differentiating  ones  which  mutually  separate 
themselves.  In  this  way  the  production  would  be  rendered 
possible  of  as  many  sorts  of  egg  and  pollen  cells  as  there 
are  combinations  possible  of  the  formative  elements. 

The  attribution  attempted  here  of  the  essential  difference 
in  the  development  of  hybrids  to  a  permanent  or  temporary 
union  of  the  differing  cell  elements  can,  of  course,  only 
claim  the  value  of  an  hypothesis  for  which  the  lack  of 
definite  data  offers  a  wide  field.  Some  justification  of  the 
opinion  expressed  lies  in  the  evidence  afforded  by  Pisum 
that  the  behaviour  of  each  pair  of  differentiating  characters 
in  hybrid  union  is  independent  of  the  other  differences 
between  the  two  original  plants,  and,  further,  that  the 
hybrid  produces  just  so  many  kinds  of  egg  and  pollen 
cells  as  there  are  possible  constant  combination  forms. 
The  differentiating  characters  of  two  plants  can  finally, 
however,  only  depend  upon  differences  in  the  composition 


90  MendeVs  Experiments 

and  grouping  of  the  elements  which  exist  in  the  foundation- 
cells  [fertilised  ova]  of  the  same  in  vital  interaction*. 

Even  the  validity  of  the  law  formulated  for  Pisum 
requires  still  to  be  confirmed,  and  a  repetition  of  the  more 
important  experiments  is  consequently  much  to  be  desired, 
that,  for  instance,  relating  to  the  composition  of  the  hybrid 
fertilising  cells.  A  differential  [element]  may  easily  escape 
the  single  observer!,  which  although  at  the  outset  may 
appear  to  be  unimportant,  may  yet  accumulate  to  such 
an  extent  that  it  must  not  be  ignored  in  the  total  result. 
Whether  the  variable  hybrids  of  other  plant  species  observe 
an  entire  agreement  must  also  be  first  decided  experiment- 
ally. In  the  meantime  we  may  assume  that  in  material 
points  a  difference  in  principle  can  scarcely  occur,  since  the 
unity  in  the  developmental  plan  of  organic  life  is  beyond 
question. 

In  conclusion,  the  experiments  carried  out  by  Kolreuter, 
Gartner,  and  others  with  respect  to  the  transformation  of 
one  species  into  another  hy  artificial  fertilisation  merit 
special  mention.  A  special  importance  has  been  attached 
to  these  experiments,  and  Gartner  reckons  them  among 
"the  most  difficult  of  all  in  hybridisation." 

If  a  species  A  is  to  be  transformed  into  a  species  B, 
both  must  be  united  by  fertilisation  and  the  resulting 
hybrids  then  be  fertilised  with  the  pollen  of  B ;  then,  out 
of  the  various  offspring  resulting,  that  form  would  be 
selected  which  stood  in  nearest  relation  to  B  and  once 
more  be  fertilised  with  B  pollen,  and  so  continuously  until 
finally  a  form  is  arrived  at  which  is  like  B  and  constant  in 

*  ^^  Welche  in  den  Grundzellen  derselben  in  lebendiger  Wechsel- 
ivirkung  stehen.^' 

+  "  Dem  einzelnen  Beobachter  kann  leicTit  ein  Differenziale  ent- 
geheny 


i7i  Hybridisation  91 

its  progeny.  By  this  process  the  species  A  would  change 
into  the  species  B.  Gartner  alone  has  effected  thirty  such 
experiments  vnth.  plants  of  genera  Aquilegia,  Dlanthus, 
Geum,  Lavatera,  Lychnis,  Malva,  Nicotiana,  and  QEnothera. 
The  period  of  transformation  was  not  alike  for  all  species. 
"While  with  some  a  triple  fertilisation  sufficed,  with  others 
this  had  to  be  repeated  five  or  six  times,  and  even  in  the 
same  species  fluctuations  were  observed  in  various  experi- 
ments. Gartner  ascribes  this  difference  to  the  circumstance 
that  "the  specific  \typische\  force  by  which  a  species,  during 
reproduction,  effects  the  change  and  transformation  of  the 
maternal  type  varies  considerably  in  different  plants,  and 
that,  consequently,  the  periods  mthin  which  the  one  species 
is  changed  into  the  other  must  also  vary,  as  also  the  number 
of  generations,  so  that  the  transformation  in  some  species 
is  perfected  in  more,  and  in  others  in  fewer  generations." 
Further,  the  same  observer  remarks  "that  in  these  trans- 
formation experiments  a  good  deal  depends  upon  which  type 
and  which  individual  be  chosen  for  further  transformation." 
If  it  ma)^  be  assumed  that  in  these  experiments  the 
constitution  of  the  forms  resulted  in  a  similar  way  to  that 
of  Pisum,  the  entire  process  of  transformation  would  find 
a  fairly  simple  explanation.  The  hybrid  forms  as  many 
kinds  of  egg  cells  as  there  are  constant  combinations 
possible  of  the  characters  conjoined  therein,  and  one  of 
these  is  always  of  the  same  kind  as  the  fertilising  pollen 
cells.  Consequently  there  always  exists  the  possibility  with 
all  such  experiments  that  even  from  the  second  fertilisation 
there  may  result  a  constant  form  identical  with  that  of  the 
pollen  parent.  Whether  this  really  be  obtained  depends  in 
each  separate  case  upon  the  number  of  the  experimental 
plants,  as  well  as  upon  the  number  of  differentiating 
characters  which  are  united  by  the  fertilisation.     Let  us, 


92  MeiideVs  Experiments 

for  instance,  assume  that  the  plants  selected  for  experiment 
differed  in  three  characters,  and  the  species  ABC  i^  to 
be  transformed  into  the  other  species  ahc  by  repeated 
fertilisation  with  the  pollen  of  the  latter ;  the  hybrids 
resulting  from  the  first  cross  form  eight  different  kinds  of 
Qgg  cells,  viz. : 

ABC  J  A  Be,  AbC,  aBC,  Abe,  aBc,  abC,  abc. 

These  in  the  second  year  of  experiment  are  united  again 
with  the  pollen  cells  abc,  and  we  obtain  the  series 

AaBbCc  +  AaBbe  +  AabCe  +  aBbCe 

+  A  abc  +  aBbe  +  abCe  +  abc. 

Since  the  form  abc  occurs  once  in  the  series  of  eight 
components,  it  is  consequently  little  likely  that  it  would  be 
missing  among  the  experimental  plants,  even  were  these 
raised  in  a  smaller  number,  and  the  transformation  would 
be  perfected  already  by  a  second  fertilisation.  If  by  chance 
it  did  not  appear,  then  the  fertilisation  must  be  repeated 
with  one  of  those  forms  nearest  akin,  Aabc,  aBbc,  abCc. 
It  is  perceived  that  such  an  experiment  must  extend  the 
farther  the  smaller  the  number  of  experimental  'plants  and 
the  larger  the  number  of  differentiating  characters  in  the 
two  original  species;  and  that,  furthermore,  in  the  same 
species  there  can  easily  occur  a  delay  of  one  or  even  of  two 
generations  such  as  Gartner  observed.  The  transformation 
of  widely  divergent  species  could  generally  only  be  completed 
in  five  or  six  years  of  experiment,  since  the  number  of 
different  Qgg  cells  which  are  formed  in  the  hybrid  increases 
in  square  ratio  with  the  number  of  differentiating  characters. 

Gartner  found  by  repeated  experiments  that  the  respec- 
tive period  of  transformation  varies  in  many  species,  so  that 
frequently  a  species  A  can  be  transformed  into  a  species  B 


in  Hybridisation  93 

a  generation  sooner  than  can  species  B  into  species  A .  He 
deduces  therefrom  that  Kolreuter's  opinion  can  hardly  be 
maintained  that  "  the  two  natures  in  hybrids  are  perfectly 
in  equilibrium."  It  appears,  however,  that  Kolreuter  does 
not  merit  this  criticism,  but  that  Gartner  rather  has  over- 
looked a  material  point,  to  which  he  himself  elsewhere 
draws  attention,  viz.  that  "  it  depends  which  individual  is 
chosen  for  further  transformation."  Experiments  which  in 
this  connection  were  carried  out  with  two  species  of  Pisum 
demonstrated  that  as  regards  the  choice  of  the  fittest 
individuals  for  the  purpose  of  further  fertilisation  it  may 
make  a  great  difference  which  of  two  species  is  transformed 
into  the  other.  The  two  experimental  plants  differed  in 
five  characters,  while  at  the  same  time  those  of  species  A 
were  all  dominant  and  those  of  species  B  all  recessive. 
For  mutual  transformation  A  was  fertilised  with  pollen  of 
B^  and  B  with  pollen  of  A,  and  this  was  repeated  with 
both  hybrids  the  following  year.    With  the  first  experiment 

-J   there  were  eighty-seven  plants  available  in  the  third 

year  of  experiment  for  the  selections  of  individuals  for 
further  crossing,  and  these  were  of  the  possible  thirty-two 

A 

forms ;  with  the  second  experiment  -j:  seventy-three  plants 

resulted,  which  agreed  throughout  perfectly  in  habit  with 
the  pollen  parent;  in  their  internal  composition,  however, 
they  must  have  been  just  as  varied  as  the  forms  of  the 
other  experiment.  A  definite  selection  was  consequently 
only  possible  with  the  first  experiment ;  with  the  second 
some  plants  selected  at  random  had  to  be  excluded.  Of 
the  latter  only  a  portion  of  the  flowers  were  crossed  with 
the  A  pollen,  the  others  were  left  to  fertilise  themselves. 
Among   each  five    plants   which   were    selected    in    both 


94  Mendel's  Experiments 

experiments  for  fertilisation  there  agreed,  as  the  following 
year's  culture  showed,  with  the  pollen  parent: — 


1st  Experiment. 

2nd  Experiment. 

2  plants 

— 

in  all  characters 

3       „ 

— 

))         '^                        5J 

— 

2  plants 

?j     3             „ 

— 

2       „ 

u     2             „ 

— 

1  plant 

,,     1  character 

In  the  first  experiment,  therefore,  the  transformation 
was  completed;  in  the  second,  which  was  not  continued 
further,  two  more  fertilisations  would  probably  have  been 
required. 

Although  the  case  may  not  frequently  occur  that  the 
dominant  characters  belong  exclusively  to  one  or  the  other 
of  the  original  parent  plants,  it  will  always  make  a  difference 
which  of  the  two  possesses  the  majority.  If  the  pollen  parent 
shows  the  majority,  then  the  selection  of  forms  for  further 
crossing  will  afford  a  less  degree  of  security  than  in  the 
reverse  case,  which  must  imply  a  delay  in  the  period  of 
transformation,  provided  that  the  experiment  is  only 
considered  as  completed  when  a  form  is  arrived  at  which 
not  only  exactly  resembles  the  pollen  plant  in  form,  but 
also  remains  as  constant  in  its  progeny. 

Gartner,  by  the  results  of  these  transformation  experi- 
ments, was  led  to  oppose  the  opinion  of  those  naturalists 
who  dispute  the  stability  of  plant  species  and  believe  in  a 
continuous  evolution  of  vegetation.  He  perceives  in  the 
complete  transformation  of  one  species  into  another  an 
indubitable  proof  that  species  are  fixed  within  limits 
beyond  which  they  cannot  change.  Although  this  opinion 
cannot  be  unconditionally  accepted  we  find  on  the  other 
hand  in  Gartner's  experiments  a  noteworthy  confirmation 


in  Hyhridisatio7i  95 

of   that   supposition    regarding  variability   of    cultivated 
plants  which  has  already  been  expressed. 

Among  the  experimental  species  there  were  cultivated 
plants,  such  as  Aquilegia  atropurpurea  and  canadensis, 
Dianthus  caryophyllus,  chinensis,  and  japonicus,  Nicotiana 
rustica  and  paniculata,  and  hybrids  between  these  species 
lost  none  of  their  stability  after  four  or  five  generations*. 

*  [The  argument  of  these  two  last  paragraphs  appears  to  be  that 
though  the  general  mutability  of  natural  species  might  be  doubtful, 
yet  among  cultivated  plants  the  transference  of  characters  may  be 
accomplished,  and  may  occur  by  integral  steps  until  one  species  is 
definitely  "transformed"  into  the  other.] 


ON   HIERACIUM-HYBRIDS   OBTAINED   BY 
ARTIFICIAL  FERTILISATION 

By  G.  Mendel. 

{Communicated  to  the  Meeting  9  June,  1869*.) 

Although  I  have  already  undertaken  many  experiments 
in  fertilisation  between  species  of  Hieracium,  I  have  only 
succeeded  in  obtaining  the  following  6  hybrids,  and  only 
from  one  to  three  specimens  of  them. 

H.  Auricula  2  x  H.  aurantiacum  c? 

H.  Auricula  ^  ^  H.  Pilosella  $ 

H.  Auricula  ^  x  H.  pratense  S 

H.  eckioidesf  ^  x  H.  aurantiacum  S 

H.  prcealtum  ^  y-  H.  flagellare  Rchb.  $ 

H.  prwaltum  ^  ^  H.  aurantiacum  $ 

The  difficulty  of  obtaining  a  larger  number  of  hybrids 
is  due  to  the  minuteness  of  the  flowers  and  their  peculiar 
structure.  On  account  of  this  circumstance  it  was  seldom 
possible  to  remove  the  anthers  from  the  flowers  chosen  for 

*  [Published  in  Verh.  naturf.  Ver.  Brilnn,  Abhandlungen,  viii.  1869, 
p.  26,  which  appeared  in  1870.] 

t  The  plant  used  in  this  experiment  is  not  exactly  the  typical 
H.  echioides.  It  appears  to  belong  to  the  series  transitional  to 
H.  prcealtum,  but  approaches  more  nearly  to  H.  echioides  and  for 
this  reason  was  reckoned  as  belonging  to  the  latter. 


MendeVs  Experiments  with  Hieracimn       97 

fertilisation  without  either  letting  pollen  get  on  to  the 
stigma  or  injuring  the  pistil  so  that  it  withered  away. 
As  is  well  kno^vn,  the  anthers  are  united  to  form  a  tube, 
which  closely  embraces  the  pistil.  As  soon  as  the  flower 
opens,  the  stigma,  already  covered  with  pollen,  protrudes. 
In  order  to  prevent  self-fertilisation  the  anther-tube  must 
be  taken  out  before  the  flower  opens,  and  for  this  purpose 
the  bud  must  be  slit  up  with  a  fine  needle.  If  this 
operation  is  attempted  at  a  time  when  the  pollen  is  mature, 
which  is  the  case  two  or  three  days  before  the  flower  opens, 
it  is  seldom  possible  to  prevent  self-fertilisation:  for  with 
every  care  it  is  not  easily  possible  to  prevent  a  few  pollen 
gTains  getting  scattered  and  communicated  to  the  stigma. 
No  better  result  has  been  obtained  hitherto  by  removing 
the  anthers  at  an  earlier  stage  of  development.  Before  the 
approach  of  maturity  the  tender  pistil  and  stigma  are  ex- 
ceedingly sensitive  to  injury,  and  even  if  they  are  not  actually 
injured,  they  generally  wither  and  dry  up  after  a  little 
time  if  deprived  of  their  protecting  investments.  I  hope 
to  obviate  this  last  misfortune  by  placing  the  plants  after 
the  operation  for  two  or  three  days  in  the  damp  atmosphere 
of  a  greenhouse.  An  experiment  lately  made  with  H. 
Auricula  treated  in  this  way  gave  a  good  result. 

To  indicate  the  object  with  which  these  fertilisation 
experiments  were  undertaken,  I  venture  to  make  some 
preliminary  remarks  respecting  the  genus  Hieracium.  This 
genus  possesses  such  an  extraordinary  profusion  of  distinct 
forms  that  no  other  genus  of  plants  can  compare  with  it. 
Some  of  these  forms  are  distinguished  by  special  peculiarities 
and  may  be  taken  as  type-forms  of  species,  while  all  the 
rest  represent  intermediate  and  transitional  forms  by  which 
the  type-forms  are  connected  together.  The  difficulty  in 
the  separation  and  delimitation  of  these  forms  has  demanded 

B.  7 


98  MendeVs  Experiments 

the  close  attention  of  the  experts.  Regarding  no  other 
genus  has  so  much  been  written  or  have  so  many  and  such 
fierce  controversies  arisen,  without  as  yet  coming  to  a 
definite  conclusion.  It  is  obvious  that  no  general  under- 
standing can  be  arrived  at,  so  long  as  the  value  and 
significance  of  the  intermediate  and  transitional  forms  is 
unknown. 

Regarding  the  question  whether  and  to  what  extent 
hybridisation  plays  a  part  in  the  production  of  this  wealth 
of  forms,  we  find  very  various  and  conflicting  views  held 
by  leading  botanists.  While  some  of  them  maintain  that 
this  phenomenon  has  a  far-reaching  influence,  others,  for 
example.  Fries,  will  have  nothing  to  do  with  hybrids  in 
Hieracia.  Others  take  up  an  intermediate  position;  and 
while  granting  that  hybrids  are  not  rarely  formed  between 
the  species  in  a  wild  state,  still  maintain  that  no  great 
importance  is  to  be  attached  to  the  fact,  on  the  ground 
that  they  are  only  of  short  duration.  The  [suggested] 
causes  of  this  are  partly  their  restricted  fertility  or  complete 
sterility ;  partly  also  the  knowledge,  obtained  by  experiment, 
that  in  hybrids  self-fertilisation  is  always  prevented  if 
pollen  of  one  of  the  parent-forms  reaches  the  stigma.  On 
these  grounds  it  is  regarded  as  inconceivable  that  Hieracium 
hybrids  can  constitute  and  maintain  themselves  as  fully 
fertile  and  constant  forms  when  growing  near  their  pro- 
genitors. 

The  question  of  the  origin  of  the  numerous  and  constant 
intermediate  forms  has  recently  acquired  no  small  interest 
since  a  famous  Hieracium  specialist  has,  in  the  spirit  of 
the  Darwinian  teaching,  defended  the  view  that  these 
forms  are  to  be  regarded  as  [arising]  from  the  trans- 
mutation of  lost  or  still  existing  species. 

From  the  nature  of  the  subject  it  is  clear  that  without 


with  Hieracium  99 

an  exact  knowledge  of  the  structure  and  fertility  of  the 
hybrids  and  the  condition  of  their  offspring  through  several 
generations  no  one  can  undertake  to  determine  the  possible 
influence  exercised  by  hybridisation  over  the  multiplicity 
of  intermediate  forms  in  Hieracium.  The  condition  of 
the  Hieracium  hybrids  in  the  range  we  are  concerned  with 
must  necessarily  be  determined  by  experiments ;  for  we  do 
not  possess  a  complete  theory  of  hybridisation,  and  we  may 
be  led  into  erroneous  conclusions  if  we  take  rules  deduced 
from  observation  of  certain  other  hybrids  to  be  Laws  of 
hybridisation,  and  try  to  apply  them  to  Hieracium  without 
further  consideration.  If  by  the  experimental  method  we 
can  obtain  a  sufficient  insight  into  the  phenomenon  of 
hybridisation  in  Hieracium,  then  by  the  help  of  the  ex- 
perience which  has  been  collected  respecting  the  structural 
relations  of  the  wild  forms,  a  satisfactory  judgment  in 
regard  to  this  question  may  become  possible. 

Thus  we  may  express  the  object  which  was  sought  after 
in  these  experiments.  I  venture  now  to  relate  the  very 
slight  results  which  I  have  as  yet  obtained  with  reference 
to  this  object. 

1.  Respecting  the  structure  of  the  hybrids,  we  have 
to  record  the  striking  phenomenon  that  the  forms  hitherto 
obtained  by  similar  fertilisation  are  not  identical.  The 
hybrids  H.  pra^altum  9  x  H.  aurantiacum  $  and  H.  Auri- 
cula 2  ^  H.  aurantiacum  S  are  each  represented  by  two, 
and  H.  Auricula  2  ^  H  pratense  6  by  three  individuals, 
while  as  to  the  remainder  only  one  of  each  has  been 
obtained. 

If  we  compare  the  individual  characters  of  the  hybrids 
with  the  corresponding  characters  of  the  two  parent  types, 
we  find  that  they  sometimes  present  intermediate  structures, 

7—2 


100  MendeFs  Experiments 

but  are  sometimes  so  near  to  one  of  the  parent  characters 
that  the  [corresponding]  character  of  the  other  has  receded 
considerably  or  almost  evades  observation.  So,  for  instance, 
we  see  in  one  of  the  two  forms  of  H.  Auricula  ?  x  ZT. 
aurantiacum  6  pure  yellow  disc-florets ;  only  the  petals 
of  the  marginal  florets  are  on  the  outside  tinged  with  red 
to  a  scarcely  noticeable  degree  :  in  the  other  on  the  contrary 
the  colour  of  these  florets  comes  very  near  to  H.  aurantiacum, 
only  in  the  centre  of  the  disc  the  orange  red  passes  into  a 
deep  golden-yellow.  This  diff'erence  is  noteworthy,  for  the 
flower-colour  in  Hieracium  has  the  value  of  a  constant 
character.  Other  similar  cases  are  to  be  found  in  the 
leaves,  the  peduncles,  &c. 

If  the  hybrids  are  compared  with  the  parent  types  as 
regards  the  sum  total  of  their  characters,  then  the  two 
forms  of  H.  prwaltum  ^  ^  H.  aurantiacum  S  constitute 
approximately  intermediate  forms  which  do  not  agree  in 
certain  characters.  On  the  contrary  in  H.  Auricula  ^  x  ff. 
aurantiacum  c?  and  in  If.  Auricula  '^  x  H.  pratense  S  we 
see  the  forms  widely  divergent,  so  that  one  of  them  is 
nearer  to  the  one  and  the  other  to  the  other  parental  type, 
while  in  the  case  of  the  last-named  hybrid  there  is  still  a 
third  which  is  almost  precisely  intermediate  between  them. 

The  conviction  is  then  forced  on  us  that  we  have  here 
only  single  terms  in  an  unknown  series  which  may  be 
formed  by  the  direct  action  of  the  pollen  of  one  species  on 
the  egg-cells  of  another. 

2.  With  a  single  exception  the  hybrids  in  question 
form  seeds  capable  of  germination.  H.  echioides  ^  x  H. 
aurantiacum  6  may  be  described  as  fully  fertile ;  *H.  prw- 
altum ^  X  H.  flagellare  $  as  fertile  ;  H.  prwaltum  ^  x  H. 
aurantiacum  $  and   H.   Auricula  ^  x  H.  pratense  6  as 


with  Hieraciiim  101 

partially  fertile  ;  H.  Auricula  ^  x  ff.  Pilosella  $  as  slightly 
fertile,  and  H.  Auricula  ^  ^H.  aurantiacum  $  as  unfertile. 
Of  the  two  forms  of  the  last  named  hybrid,  the  red-flowered 
one  was  completely  sterile,  but  from  the  yellow-flowered 
one  a  single  well-formed  seed  was  obtained.  Moreover  it 
must  not  pass  unmentioned  that  among  the  seedlings  of  the 
partially  fertile  hybrid  H.  pra?altum  ^  ^  IT.  aurantiacum  ^ 
there  was  one  plant  which  possessed  full  fertility. 

[3.]  As  yet  the  ofl'spring  produced  by  self-fertilisation 
of  the  hybrids  have  not  varied,  but  agree  in  their  characters 
both  with  each  other  and  with  the  hybrid  plant  from  which 
they  were  derived. 

From  H.  proealtum  2  y-  H.  flagellare  S  two  generations 
have  flowered ;  from  H.  echioides  2  ■><  H.  aurantiacum  6 , 
H.  prwaltum  '^  ^  IT.  aurantiacum  6 ,  H.  Auricula  ^  x  ff. 
Pilosella  S  one  generation  in  each  case  has  flowered. 

4.  The  fact  must  be  declared  that  in  the  case  of  the 
fully  fertile  hybrid  H.  echioides  ^  x  H.  aurantiacum  $  the 
pollen  of  the  parent  types  was  not  able  to  prevent  self- 
fertilisation,  though  it  was  applied  in  great  quantity  to  the 
stigmas  protruding  through  the  anther-tubes  when  the 
flowers  opened. 

From  two  flower-heads  treated  in  this  way  seedlings 
were  produced  resembling  this  hybrid  plant.  A  very 
similar  experiment,  carried  out  this  summer  with  the 
partially  fertile  H.  prwaltum  ^  x  H.  aurantiacum  6  led  to 
the  conclusion  that  those  flower-heads  in  which  pollen 
of  the  parent  type  or  of  some  other  species  had  been 
applied  to  the  stigmas,  developed  a  notably  larger  number 
of  seeds  than  those  which  had  been  left  to  self-fertilisation 
alone.  The  explanation  of  this  result  must  only  be  sought 
in  the  circumstance  that  as  a  large  part  of  the  pollen -grains 


102  Mender s  Experiments 

of  the  hybrid,  examined  microscopically,  show  a  defective 
structure,  a  number  of  egg-cells  capable  of  fertilisation  do 
not  become  fertilised  by  their  own  pollen  in  the  ordinary 
course  of  self-fertilisation. 

It  not  rarely  happens  that  in  fully  fertile  species  in  the 
wdld  state  the  formation  of  the  pollen  fails,  and  in  many 
anthers  not  a  single  good  grain  is  developed.  If  in  these 
cases  seeds  are  nevertheless  formed,  such  fertilisation  must 
have  been  effected  by  foreign  pollen.  In  this  way  hybrids 
may  easily  arise  by  reason  of  the  fact  that  many  forms 
of  insects,  notably  the  industrial  Hymenoptera,  visit  the 
flowers  of  Hieracia  with  great  zeal  and  are  responsible  for 
the  pollen  which  easily  sticks  to  their  hairy  bodies  reaching 
the  stigmas  of  neighbouring  plants. 

From  the  few  facts  that  I  am  able  to  contribute  it 
will  be  evident  the  work  scarcely  extends  beyond  its  first 
inception.  I  must  express  some  scruple  in  describing  in 
this  place  an  account  of  experiments  just  begun.  But  the 
conviction  that  the  prosecution  of  the  proposed  experiments 
will  demand  a  whole  series  of  years,  and  the  uncertainty 
whether  it  will  be  granted  to  me  to  bring  the  same  to  a 
conclusion  have  determined  me  to  make  the  present 
communication.  By  the  kindness  of  Dr  Nageli,  the 
Munich  Director,  who  was  good  enough  to  send  me  species 
which  were  wanting,  especially  from  the  Alps,  I  am  in  a 
position  to  include  a  larger  number  of  forms  in  my 
experiments.  I  venture  to  hope  even  next  year  to  be  able 
to  contribute  something  more  by  way  of  extension  and  con- 
firmation of  the  present  account. 

If  finally  we  compare  the  described  result,  still  very 
uncertain,  with  those  obtained  by  crosses  made  between 
forms  of  Pisum,  which  I  had  the  honour  of  communi- 
cating in  the  year  1865,  we  find  a  very  real  distinction. 


with  Hieradmn  103 

In  Pisum  the  hybrids,  obtained  from  the  immediate 
crossing  of  two  forms,  have  in  all  cases  the  same  type, 
but  their  posterity,  on  the  contrary,  are  variable  and 
follow  a  definite  law  in  their  variations.  In  Hieracium 
according  to  the  present  experiments  the  exactly  opposite 
phenomenon  seems  to  be  exhibited.  Already  in  describing 
the  Pisum  experiments  it  was  remarked  that  there  are 
also  hybrids  whose  posterity  do  not  vary,  and  that,  for 
example,  according  to  Wichura  the  hybrids  of  Salix 
reproduce  themselves  like  pure  species.  In  Hieracium 
we  may  take  it  we  have  a  similar  case.  Whether  from 
this  circumstance  we  may  venture  to  draw  the  conclusion 
that  the  polymorphism  of  the  genera  Salix  and  Hieracium 
is  connected  with  the  special  condition  of  their  hybrids  is 
still  an  open  question,  which  may  well  be  raised  but  not 
as  yet  answered. 


A  DEFENCE   OF   MENDEL'S   PRINCIPLES 
OF   HEREDITY. 

"  The  most  fertile  men  of  science  have  made  blunders,  and  their 
consciousness  of  such  slips  has  been  retribution  enough;  it  is 
only  their  more  sterile  critics  who  delight  to  dwell  too  often 
and  too  long  on  such  mistakes?^     Biometrika,  1901. 

Introductory. 

On  the  rediscovery  and  confirmation  of  Mendel's  Law  by 
de  Vries,  Correns,  and  Tschermak  two  years  ago,  it  became 
clear  to  many  naturalists,  as  it  certainly  is  to  me,  that  we 
had  found  a  principle  which  is  destined  to  play  a  part  in 
the  Study  of  Evolution  comparable  only  with  the  achieve- 
ment of  Darwin — that  after  the  weary  halt  of  forty  years 
we  have  at  last  begun  to  march. 

If  we  look  back  on  the  post-Darwinian  period  we 
recognize  one  notable  effort  to  advance.  This  effort — 
fruitful  as  it  proved,  memorable  as  it  must  ever  be — was 
that  made  by  Galton  when  he  enuntiated  his  Law  of 
Ancestral  Heredity,  subsequently  modified  and  restated 
by  Karl  Pearson.  Formulated  after  long  and  laborious 
inquiry,  this  principle  beyond  question  gives  us  an 
expression  including  and  denoting  many  phenomena  in 
which  previously  no  regularity  had  been  detected.     But 


A  Defence  of  Mendel's  Prhiciples  of  Heredity    105 

to  practical  naturalists  it  was  evident  from  the  first  that 
there  are  great  groups  of  facts  which  could  not  on  any 
interpretation  be  brought  within  the  scope  of  Galton's 
Law,  and  that  by  no  emendation  could  that  Law  be 
extended  to  reach  them.  The  existence  of  these  phen- 
omena pointed  to  a  different  physiological  conception  of 
heredity.  Now  it  is  precisely  this  conception  that  Mendel's 
Law  enables  us  to  form.  Whether  the  Mendelian  principle 
can  be  extended  so  as  to  include  some  apparently  Galtonian 
cases  is  another  question,  respecting  which  we  have  as  yet 
no  facts  to  guide  us,  but  we  have  certainly  no  warrant  for 
declaring  such  an  extension  to  be  impossible. 

Whatever  answer  the  future  may  give  to  that  question, 
it  is  clear  from  this  moment  that  every  case  which  obeys 
the  Mendelian  principle  is  removed  finally  and  irretrievably 
from  the  operations  of  the  Law  of  Ancestral  Heredity. 

At  this  juncture  Professor  Weldon  intervenes  as  a 
professed  exponent  of  Mendel's  work.  It  is  not  perhaps 
to  a  devoted  partisan  of  the  Law  of  Ancestral  Heredity 
that  we  should  look  for  the  most  appreciative  exposition  of 
Mendel,  but  some  bare  measure  of  care  and  accuracy  in 
representation  is  demanded  no  less  in  justice  to  fine  work, 
than  by  the  gravity  of  the  issue. 

Professor  Weldon's  article  appears  in  the  current  number 
of  Biometrika,  Vol.  L  Pt.  ii.  which  reached  me  on  Saturday, 
Feb.  8.  The  paper  opens  with  what  purports  to  be  a 
restatement  of  Mendel's  experiments  and  results.  In  this 
"restatement"  a  large  part  of  Mendel's  experiments — 
perhaps  the  most  significant — are  not  referred  to  at  all. 
The  perfect  simplicity  and  precision  of  Mendel's  own 
account  are  destroyed  ;  with  the  result  that  the  reader  of 
Professor  Weldon's  paper,  unfamiliar  with  Mendel's  own 
memoir,  can  scarcely  be  blamed  if  he  fail  to  learn  the 


106  A  Defence  of  MendeVs 

essence  of  the  discovery.  Of  Mendel's  conception  of  the 
hybrid  as  a  distinct  entity  with  characters  proper  to  itself, 
apart  from  inheritance — the  most  novel  thing  in  the 
whole  paper — Professor  Weldon  gives  no  word.  Upon  this 
is  poured  an  undigested  mass  of  miscellaneous  "facts" 
and  statements  from  which  the  reader  is  asked  to  conclude, 
first,  that  a  proposition  attributed  to  Mendel  regarding 
dominance  of  one  character  is  not  of  "general""^  application, 
and  finally  that  "all  work  based  on  Mendel's  method"  is 
"vitiated"  by  a  "fundamental  mistake,"  namely  "the 
neglect  of  ancestry!." 

To  find  a  parallel  for  such  treatment  of  a  great  theme 
in  biology  we  must  go  back  to  those  writings  of  the  orthodox 
which  followed  the  appearance  of  the  "  Origin  of  Species." 

On  17th  December  1900  I  delivered  a  Report  to  the 
Evolution  Committee  of  the  Royal  Society  on  the  experi- 
ments in  Heredity  undertaken  by  Miss  E.  R.  Saunders  and 
myself.  This  report  has  been  offered  to  the  Society  for 
publication  and  will  I  understand  shortly  appear.  In  it  we 
have  attempted  to  show  the  extraordinary  significance  of 
Mendel's  principle,  to  point  out  what  in  his  results  is 
essential  and  what  subordinate,  the  ways  in  which  the 
principle  can  be  extended  to  apply  to  a  diversity  of  more 
complex  phenomena — of  which  some  are  incautiously  cited 

*  The  words  "general"  and  ''universal"  appear  to  be  used  by 
Professor  Weldon  as  interchangeable.  Cp.  Weldon,  p.  235  and 
elsewhere,  with  Abstract  given  below. 

t  These  words  occur  p.  252  :  "The  fundamental  mistake  which 
vitiates  all  work  based  upon  Mendel's  method  is  the  neglect  of 
ancestry,  and  the  attempt  to  regard  the  whole  effect  upon  offspring  pro- 
duced by  a  particular  parent,  as  due  to  the  existence  in  the  parent  of 
particular  structural  characters,  &c."  As  a  matter  of  fact  the  view 
indicated  in  these  last  words  is  especially  repugnant  to  the  Mendelian 
principle,  as  will  be  seen. 


PrincijjUs  of  Heredity  107 

by  Professor  Weldon  as  conflicting  facts — and  lastly  to 
suggest  a  few  simple  terms  without  wliich  (or  some  equi- 
valents) the  discussion  of  such  phenomena  is  difficult. 
Though  it  is  impossible  here  to  give  an  outline  of  facts  and 
reasoning  there  set  out  at  length,  I  feel  that  his  article 
needs  an  immediate  reply.  Professor  Weldon  is  credited 
with  exceptional  familiarity  with  these  topics,  and  his  paper 
is  likely  to  be  accepted  as  a  sufficient  statement  of  the  case. 
Its  value  will  only  be  known  to  those  who  have  either 
worked  in  these  fields  themselves  or  have  been  at  the 
trouble  of  thoughtfully  studying  the  original  materials. 

The  nature  of  Professor  Weldon's  article  may  be  most 
readily  indicated  if  I  quote  the  summary  of  it  issued  in  a 
paper  of  abstracts  sent  out  with  Review  copies  of  the  Part. 
This  paper  was  most  courteously  sent  to  me  by  an  editor 
of  Biometriha  in  order  to  call  my  attention  to  the  article 
on  Mendel,  a  subject  in  which  he  knew  me  to  be  interested. 
The  abstract  is  as  follows. 

"Few  subjects  have  excited  so  much  interest  in  the  last 
year  or  two  as  the  laws  of  inheritance  in  hybrids.  Professor 
W.  F.  R.  Weldon  describes  the  results  obtained  by  Mendel  by 
crossing  races  of  Peas  which  dififered  in  one  or  more  of  seven 
characters.  From  a  study  of  the  work  of  other  observers,  and 
from  examination  of  the  'Telephone'  group  of  hybrids,  the 
conclusion  is  drawn  that  Mendel's  results  do  not  justify  any 
general  statement  concerning  inheritance  in  cross-bred  Peas.  A 
few  striking  cases  of  other  cross-bred  plants  and  animals  are 
quoted  to  show  that  the  results  of  crossing  cannot,  as  Mendel 
and  his  followers  suggest,  be  predicted  from  a  knowledge  of  the 
characters  of  the  two  parents  crossed  without  knowledge  of  the 
more  remote  ancestry." 

Such  is  the  judgment  a  fellow-student  passes  on  this 
mind 

"  Voyaging  through  strange  seas  of  thought  aloneJ^ 


108  A  Defence  of  MendeVs 

The  only  conclusion  which  most  readers  could  draw 
from  this  abstract  and  indeed  from  the  article  it  epitom- 
izes, is  that  Mendel's  discovery  so  far  from  being  of 
paramount  importance,  rests  on  a  basis  which  Professor 
Weldon  has  shown  to  be  insecure,  and  that  an  error  has 
come  in  through  disregard  of  the  law  of  Ancestral  Heredity. 
On  examining  the  paper  it  is  perfectly  true  that  Professor 
Weldon  is  careful  nowhere  directl}^  to  question  Mendel's 
facts  or  his  interpretation  of  them,  for  Avhich  indeed  in 
some  places  he  even  expresses  a  mild  enthusiasm,  but  there 
is  no  mistaking  the  general  purpose  of  the  paper.  It  must 
inevitably  produce  the  impression  that  the  importance  of 
the  work  has  been  greatly  exaggerated  and  that  supporters 
of  current  views  on  Ancestry  may  reassure  themselves. 
That  this  is  Professor  Weldon's  own  conclusion  in  the 
matter  is  obvious.  After  close  study  of  his  article  it  is 
evident  to  me  that  Professor  Weldon's  criticism  is  baseless 
and  for  the  most  part  irrelevant,  and  I  am  strong  in  the 
conviction  that  the  cause  which  will  sustain  damage  from 
this  debate  is  not  that  of  Mendel 


I.     The  Mendelian  Principle  of  Purity  of  (jerm- Cells 
AND  THE  Laws  of  Heredity  based  on  Ancestry. 

Professor  Weldon's  article  is  entitled  "Mendel's  Laws 
of  Alternative  Inheritance  in  Peas."  This  title  expresses 
the  scope  of  Mendel's  work  and  discovery  none  too 
precisely  and  even  exposes  him  to  distinct  miscon- 
ception. 

To  begin  with,  it  says  both  too  little  and  too  much. 
Mendel  did  certainly  determine  Laws   of  Inheritance   in 


Princijyles  of  Heredity  109 

peas — not  precisely  the  laws  Professor  Weldon  has  been 
at  the  pains  of  drafting,  but  of  that  anon.  Having  done 
so,  he  knew  what  his  discovery  was  worth.  He  saw,  and 
rightly,  that  he  had  found  a  principle  which  must  govern 
a  wide  area  of  phenomena.  He  entitles  liis  paper  therefore 
*'  Versuchs  ilber  Pflanzen- Hybrid  en, ^^  or,  Experiments  in 
Plant-Hybridisation. 

Nor  did  Mendel  start  at  first  with  any  particular 
intention  respecting  Peas.  He  tells  us  himself  that  he 
wanted  to  find  the  laws  of  inheritance  in  hybrids,  which 
he  suspected  were  definite,  and  that  after  casting  about 
for  a  suitable  subject,  he  found  one  in  peas,  for  the  reasons 
he  sets  out. 

In  another  respect  the  question  of  title  is  much  more 
important.  By  the  introduction  of  the  word  "Alternative  " 
the  suggestion  is  made  that  the  Mendelian  principle  applies 
peculiarly  to  cases  of  "  alternative  "  inheritance.  Mendel 
himself  makes  no  such  limitation  in  his  earlier  paper, 
though  perhaps  by  rather  remote  implication  in  the  second, 
to  which  the  reader  should  have  been  referred.  On  the 
contrary,  he  wisely  abstains  from  prejudicial  consideration 
of  unexplored  phenomena. 

To  understand  the  significance  of  the  word  "alternative" 
as  introduced  by  Professor  Weldon  we  must  go  back  a 
little  in  the  history  of  these  studies.  In  the  year  1897 
Galton  formally  announced  the  Law  of  Ancestral  Heredity 
referred  to  in  the  Introduction,  having  previously  "stated 
it  briefly  and  with  hesitation"  in  Natural  Inheritance, 
p.  134.  In  1898  Professor  Pearson  published  his  modifi- 
cation and  generalisation  of  Galton's  Law,  introducing  a 
correction  of  admitted  theoretical  importance,  though  it  is 
not  in  question  that  the  principle  thus  restated  is  funda- 


110  A  Defence  of  Mendel's 

mentally  not  very  different  from  Galton's*.  It  if^  an 
essential  part  of  the  Galton-Pearson  Law  of  Ancestral 
Heredity  that  in  calculating  the  probable  structure  of  each 
descendant  the  structure  of  each  several  ancestor  must  be 
brought  to  account. 

Professor  Weldon  now  tells  us  that  these  two  papers 
of  Galton  and  of  Professor  Pearson  have  "given  us  an 
expression  for  the  effects  of  blended  inheritance  which 
seems  likely  to  prove  generally  applicable,  though  the 
constants  of  the  equations  which  express  the  relation 
between  divergence  from  the  mean  in  one  generation,  and 
that  in  another,  may  require  modification  in  special  cases. 
Our  knowledge  of  particulate  or  mosaic  inheritance,  and  of 
alternatim  inheritance,  is  however  still  rudimentary,  and 
there  is  so  much  contradiction  between  the  results  obtained 
by  different  observers,  that  the  evidence  available  is  difficult 
to  appreciate." 

But  Galton  stated  (p.  401)  in  1897  that  his  statistical 
law  of  heredity  "appears  to  be  universally  applicable  to 
bi-sexual  descent."  Pearson  in  re-formulating  the  principle 
in  1898  made  no  reservation  in  regard  to  "alternative" 
inheritance.  On  the  contrary  he  writes  (p.  393)  that  "if 
Mr  Galton's  law  can  be  firmly  established,  it  is  a  complete 
solution,  at  any  rate  to  a  first  approximation,  of  the  whole 
problem  of  heredity"  and  again  (p.  412)  that  "it  is  highly 
probable  that  it  [this  law]  is  the  simple  descriptive  state- 

*  I  greatly  regret  that  I  have  not  a  precise  understanding  of  the 
basis  of  the  modification  proposed  by  Pearson.  His  treatment  is  in 
algebraical  form  and  beyond  me.  Nevertheless  I  have  every  confidence 
that  the  arguments  are  good  and  the  conclusion  sound.  I  trust  it 
may  not  be  impossible  for  him  to  provide  the  non-mathematical  reader 
with  a  paraphrase  of  his  memoir.  The  arithmetical  differences  between 
the  original  and  the  modified  law  are  of  course  clear. 


Princij^les  of  Heredity  111 

ment  which  brings  into  a  single  focus  all  the  complex 
lines  of  hereditary  influence.  If  Darwinian  evolution  be 
natural  selection  combined  with  heredity,  then  the  single 
statement  which  embraces  the  whole  field  of  heredity  must 
prove  almost  as  epoch-making  as  the  law  of  gravitation 
to  the  astronomer*." 

As  I  read  there  comes  into  my  mind  that  other  fine 
passage  where  Professor  Pearson  warns  us 

"There  is  an  insatiable  desire  in  the  human  breast 
"to  resume  in  some  short  formula,  some  brief 
"statement,  the  facts  of  human  experience.  It  leads 
"  the  savage  to  '  account '  for  all  natural  phenomena 
"by  deifying  the  wind  and  the  stream  and  the  tree. 
"  It  leads  civilized  man,  on  the  other  hand,  to  express 
"his  emotional  experience  in  works  of  art,  and  his 
"physical  and  mental  experience  in  the  formulae  or 
"so-called  laws  of  science f." 

No  naturalist  who  had  read  Galton's  paper  and  had 
tried  to  apply  it  to  the  facts  he  knew  could  fail  to  see 
that  here  was  a  definite  advance.  We  could  all  perceive 
phenomena  that  were  in  accord  with  it  and  there  was  no 
reasonable  doubt  that  closer  study  would  prove  that  accord 
to  be  close.  It  was  indeed  an  occasion  for  enthusiasm, 
though  no  one  acquainted  with  the  facts  of  experimental 
breeding  could  consider  the  suggestion  of  universal  applica- 
tion for  an  instant. 

*  I  have  searched  Professor  Pearson's  paper  in  vain  for  any  con- 
siderable reservation  regarding  or  modification  of  this  general  state- 
ment. Professor  Pearson  enuntiates  the  law  as  "  only  correct  on 
certain  limiting  hypotheses,"  but  he  declares  that  of  these  the  most 
important  is  "  the  absence  of  reproductive  selection,  i.e.  the  negligible 
correlation  of  fertility  with  the  inherited  character,  and  the  absence 
of  sexual  selection."     The  case  of  in-and-in  breeding  is  also  reserved. 

t  K.  Pearson,  Grammar  of  Science,  2nd  ed.  1900,  p.  36. 


112  A  Defence  of  Mendel's 

But  two  years  have  gone  by,  and  in  1900  Pearson 
writes  *  that  the  values  obtained  from  the  Law  of  Ancestral 
Heredity 

"  seem  to  fit  the  observed  facts  fairly  well  in  the  case  of 
^^  blended  inheritance.  In  other  words  we  have  a 
"certain  amount  of  evidence  in  favour  of  the 
"conclusion  :  That  whenever  the  sexes  are  equipotent, 
^''hlend  their  characters  and  mate  pangamously,  all 
^^  characters  will    he    inherited    at    the  same  rate^' 

or,  again  in  other  words,  that  the  Law  of  Ancestral  Heredity 
after  the  glorious  launch  in  1898  has  been  home  for  a 
complete  refit.  The  top-hamper  is  cut  down  and  the  vessel 
altogether  more  manageable ;  indeed  she  looks  trimmed 
for  most  weathers.  Each  of  the  qualifications  now  intro- 
duced wards  off  whole  classes  of  dangers.  Later  on  (pp. 
487 — 8)  Pearson  recites  a  further  list  of  cases  regarded  as 
exceptional.  "  All  characters  will  be  inherited  at  the  same 
rate  "  might  indeed  almost  be  taken  to  cover  the  results  in 
Mendelian  cases,  though  the  mode  by  which  those  results 
are  arrived  at  is  of  course  wholly  different. 

Clearly  we  cannot  speak  of  the  Law  of  Gravitation  now. 
Our  Tycho  Brahe  and  our  Kepler,  with  the  yet  more  distant 
Newton,  are  appropriately  named  as  yet  to  comet. 

But  the  truth  is  that  even  in  1898  such  a  comparison 
was  scarcely  happy.  Not  to  mention  moderns,  these  high 
hopes  had  been  finally  disposed  of  by  the  work  of  the 
experimental  breeders  such  as  Kolreuter,  Knight,  Herbert, 
Gartner,  Wichura,  Godron,  Naudin,  and  many  more.  To 
have  treated  as  non-existent  the  work  of  this  group  of 
naturalists,  who  alone  have  attempted  to  solve  the  problems 

*  Grammar  of  Science,  2nd  ed.  1900,  p.  480. 
t  Phil.  Trans.  1900,  vol.  195,  A,  p.  121. 


Principles  of  Heredity  113 

of  heredity  and  species — Evolution,  as  we  should  now  say — 
by  the  only  sound  method — experimental  hreeding — to 
leave  out  of  consideration  almost  the  whole  block  of 
evidence  collected  in  Animals  and  Plants — Darwin's  finest 
legacy  as  I  venture  to  declare — was  unfortunate  on  the 
part  of  any  exponent  of  Heredity,  and  in  the  writings  of  a 
professed  naturalist  would  have  been  unpardonable.  But 
even  as  modified  in  1900  the  Law  of  Ancestral  Heredity 
is  heavily  over-sparred,  and  any  experimental  breeder  could 
have  increased  Pearson's  list  of  unconformable  cases  by  as 
many  again.    . 

But  to  return  to  Professor  Weldon.  He  now  repeats 
that  the  Law  of  Ancestral  Heredity  seems  likely  to  prove 
generally  applicable  to  blended  inheritance,  but  that  the 
case  of  alternative  inheritance  is  for  the  present  reserved. 
We  should  feel  more  confidence  in  Professor  "Weldon's 
exposition  if  he  had  here  reminded  us  that  the  special 
case  which  fitted  Galton's  Law  so  well  that  it  emboldened 
him  to  announce  that  principle  as  apparently  "  universally 
applicable  to  bi-sexual  descent"  was  one  of  alternative 
inheritance — namely  the  coat-colour  of  Basset-hounds. 
Such  a  fact  is,  to  say  the  least,  ominous.  Pearson,  in 
speaking  (1900)  of  this  famous  case  of  Galton's,  says  that 
these  phenomena  of  alternative  inheritance  must  be  treated 
separately  (from  those  of  blended  inheritance)^,  and  for 
them  he  deduces  a  proposed  ''''law  of  reversion"  based  of 
course  on  ancestry.  He  writes,  "In  both  cases  we  may 
speak  of  a  law  of  ancestral  heredity,  but  the  first  predicts 
the   probable  character   of  the  individual  produced  by  a 

*  "  If  this  be  done,  we  shall,  I  venture  to  think,  keep  not  only  our 
minds,  but  our  points  for  observation,  clearer  ;  and  further,  the  failure 
of  Mr  Galton's  statement  in  the  one  case  will  not  in  the  least  afifect 
its  validity  in  the  other."     Pearson  (32),  p.  143. 

B.  8 


114  A  Defence  of  Menders 

given  ancestry,  while  the  second  tells  us  the  percentages 
of  the  total  offspring  which  on  the  average  revert  to  each 
ancestral  type*." 

With  the  distinctions  between  the  original  Law  of 
Ancestral  Heredity,  the  modified  form  of  the  same  law, 
and  the  Law  of  Reversion,  important  as  all  these  considera- 
tions are,  we  are  not  at  present  concerned. 

For  the  Mendelian  principle  of  heredity  asserts  a 
proposition  absolutely  at  variance  with  all  the  laws  of 
ancestral  heredity,  however  formulated.  In  those  cases  to 
which  it  applies  strictly,  this  principle  declares  that  the 
cross-breeding  of  parents  need  not  diminish  the  purity  of 
their  germ-cells  or  consequently  the  purity  of  their  off- 
spring. When  in  such  cases  individuals  bearing  opposite 
characters,  A  and  B,  are  crossed,  the  germ-cells  of  the 
resulting  cross-bred,  AB,  are  each  to  be  bearers  either 
of  character  A  or  of  character  B,  not  both. 

Consequently  when  the  cross-breds  breed  either  together 
or  with  the  pure  forms,  individuals  will  result  of  the  forms 
A  A,  AB,  BA,  BBt  Of  these  the  forms  A  A  and  BB, 
formed  by  the  union  of  similar  germs,  are  stated  to  be  as 
pure  as  if  they  had  had  no  cross  in  their  pedigree,  and 
henceforth  their  offspring  will  be  no  more  likely  to  depart 
from  the  A  type  or  the  B  type  respectively,  than  those  of 
any  other  originally  pure  specimens  of  these  types. 

Consequently  in  such  examples  it  is  7iof  the  fact  that 
each  ancestor  must  be  brought  to  account  as  the  Galton- 
Pearson  Law  asserts,  and  we  are  clearly  dealing  with  a 
physiological  phenomenon  not  contemplated  by  that  Law 
at  all. 

*  Grammar  of  Science,  1900,  p.  494.    See  also  Pearson,  Proc.  Roy. 
Soc.  1900,  Lxvi.  pp.  142-3. 

t  On  an  average  of  cases,  in  equal  numbers,  as  Mendel  found. 


Principles  of  Heredity  115 

Every  case  therefore  which  obeys  the  Mendelian  principle 
is  in  direct  contradiction  to  the  proposition  to  which  Pro- 
fessor Weldon's  school  is  committed,  and  it  is  natural  that 
he  should  be  disposed  to  consider  the  Mendelian  principle 
as  applying  especially  to  "alternative"  inheritance,  while 
the  law  of  Galton  and  Pearson  is  to  include  the  phenomenon 
of  blended  inheritance.  The  latter,  he  tells  us,  is  "the 
most  usual  case,"  a  view  which,  if  supported  by  evidence, 
might  not  be  without  value. 

It  is  difficult  to  blame  those  who  on  first  acquaintance 
concluded  Mendel's  principle  can  have  no  strict  application 
save  to  alternative  inheritance.  Whatever  blame  there  is 
in  this  I  share  with  Professor  Weldon  and  those  whom  he 
follows.  Mendel's  own  cases  were  almost  all  alternative ; 
also  the  fact  of  dominance  is  very  dazzling  at  first.  But 
that  was  two  years  ago,  and  when  one  begins  to  see  clearly 
again,  it  does  not  look  so  certain  that  the  real  essence  of 
Mendel's  discovery,  the  purity  of  germ-cells  in  respect  of 
certain  characters,  may  not  apply  also  to  some  phenomena 
of  blended  inheritance.  The  analysis  of  this  possibility 
would  take  us  to  too  great  length,  but  I  commend  to  those 
who  are  more  familiar  with  statistical  method,  the  consider- 
tion  of  this  question  :  whether  dominance  being  absent, 
indefinite,  or  suppressed,  the  phenomena  of  heritages 
completely  blended  in  the  zygote,  may  not  be  produced 
by  gametes  presenting  Mendelian  purity  of  characters. 
A  brief  discussion  of  this  possibility  is  given  in  the 
Introduction,  p.  31. 

Very  careful  inquiry  would  be  needed  before  such  a 
possibility  could  be  negatived.  For  example,  we  know 
that  the  Laws  based  on  Ancestry  can  apply  to  alternative 
inheritance  ;  witness  the  case  of  the  Basset-hounds.  Here 
there  is  no  simple  Mendelian  dominance ;  but  are  we  sure 

8—2 


116  A  Defence  of  MendeVs 

there  is  no  purity  of  germ-cells  ?  The  new  conception  goes 
a  long  way  and  it  may  well  reach  to  such  facts  as  these. 

But  for  the  present  we  will  assume  that  Mendel's 
principle  applies  only  to  certain  phenomena  of  alternative 
inheritance,  which  is  as  far  as  our  warrant  yet  runs. 

No  close  student  of  the  recent  history  of  evolutionary 
thought  needs  to  be  told  what  the  attitude  of  Professor 
Weldon  and  his  followers  has  been  tow^ards  these  same 
disquieting  and  unwelcome  phenomena  of  alternative 
inheritance  and  discontinuity  in  variation.  Holding  at 
first  each  such  fact  for  suspect,  then  treating  them  as  rare 
and  negligible  occurrences,  he  and  his  followers  have  of 
late  come  slowly  to  accede  to  the  facts  of  discontinuity  a 
bare  and  grudging  recognition  in  their  scheme  of  evolution*. 

Therefore  on  the  announcement  of  that  discovery  which 
once  and  for  all  ratifies  and  consolidates  the  conception  of 
discontinuous  variation,  and  goes  far  to  define  that  of 
alternative  inheritance,  giving  a  finite  body  to  what  before 
was  vague  and  tentative,  it  is  small  wonder  if  Professor 
Weldon  is  disposed  to  criticism  rather  than  to  cordiality. 

We  have  now  seen  what  is  the  essence  of  Mendel's 
discovery  based  on  a  series  of  experiments  of  unequalled 
simplicity  which  Professor  Weldon  does  not  venture  to 
dispute. 

*  Eead  in  this  connexion  Pearson,  K.,  Grammar  of  Science,  2nd 
ed.  1900,  pp.  390—2. 

Professor  Weldon  even  now  opens  his  essay  with  the  statement — 
or  perhaps  reminiscence — that  *'  it  is  perfectly  possible  and  indeed 
probable  that  the  difference  between  these  forms  of  inheritance 
[blended,  mosaic,  and  alternative]  is  only  one  of  degree."  This  may  be 
true ;  but  reasoning  favourable  to  this  proposition  could  equally  be 
used  to  prove  the  difference  between  mechanical  mixture  and  chemical 
combination  to  be  a  difference  of  degree. 


Principles  of  Heredity  117 

11.    Mendel  and  the  Critic's  Version  of  him. 
The  ^^  Law  of  Dominance." 

I  proceed  to  the  question  of  dominance  which  Professor 
Weldon  treats  as  a  prime  issue,  almost  to  the  virtual  con- 
cealment of  the  great  fact  of  gametic  purity. 

Cross-breds  in  general,  AB  and  BA,  named  above, 
may  present  many  appearances.  They  may  all  be  indis- 
tiuguishable  from  A,  or  from  B ;  some  may  appear  J.'s 
and  some  ^'s  ;  they  may  be  patchworks  of  both  ;  they  may 
be  blends  presenting  one  or  many  grades  between  the  two  ; 
and  lastly  they  may  have  an  appearance  special  to  themselves 
(heing  in  the  latter  case^  as  it  oft  en  happens^  ^Reversionary"), 
a  possibility  which  Professor  Weldon  does  not  stop  to 
consider,  though  it  is  the  clue  that  may  unravel  many 
of  the  facts  which  mystify  him  now. 

Mendel's  discovery  became  possible  because  he  worked 
with  regular  cases  of  the  first  category,  in  which  he  was  able 
to  recognize  that  one  of  each  of  the  pairs  of  characters 
he  studied  did  thus  prevail  and  was  "dominant"  in  the 
cross-bred  to  the  exclusion  of  the  other  character.  This 
fact,  which  is  still  an  accident  of  particular  cases,  Professor 
Weldon,  following  some  of  Mendel's  interpreters,  dignifies 
by  the  name  of  the  "Law  of  Dominance,"  though  he 
omits  to  warn  his  reader  that  Mendel  states  no  "Law  of 
Dominance  "  whatever.  The  whole  question  whether  one  or 
other  character  of  the  antagonistic  pair  is  dominant  though 
of  great  importance  is  logically  a  subordinate  one.  It 
depends  on  the  specific  nature  of  the  varieties  and  in- 
dividuals  used,   sometimes   probably  on  the   influence  of 


118  A  Defence  of  Mendel's 

external  conditions  and  on  other  factors  we  cannot  now 
discuss.  There  is  as  yet  no  universal  law  here  perceived 
or  declared. 

Professor  Weldon  passes  over  the  proof  of  the  purity 
of  the  germ-cells  lightly  enough,  but  this  proposition  of 
dominance,  suspecting  its  weakness,  he  puts  prominently 
forward.  Briefest  equipment  will  suffice.  Facing,  as  he 
supposes,  some  new  pretender — some  local  Theudas — 
offering  the  last  crazy  prophecy, — any  argument  will  do 
for  such  an  one.  An  eager  gathering  in  an  unfamiliar 
literature,  a  scrutiny  of  samples,  and  he  will  prove  to 
us  with  small  difficulty  that  dominance  of  yellow  over 
green,  and  round  over  wrinkled,  is  irregular  even  in  peas 
after  all ;  that  in  the  sharpness  of  the  discontinuity  ex- 
hibited by  the  variations  of  peas  there  are  many  grades ; 
that  many  of  these  grades  co-exist  in  the  same  variety; 
that  some  varieties  may  perhaps  be  normally  intermediate. 
All  these  propositions  are  supported  by  the  production 
of  a  collection  of  evidence,  the  quality  of  which  we 
shall  hereafter  consider.  "Enough  has  been  said,"  he 
writes  (p.  240),  "  to  show  the  grave  discrepancy  between  the 
evidence  afforded  by  Mendel's  own  experiments  and  that 
obtained  by  other  observers,  equally  competent  and  trust- 
worthy." 

We  are  asked  to  believe  that  Professor  Weldon  has 
thus  discovered  "a  fundamental  mistake"  vitiating  all  that 
work,  the  importance  of  which,  he  elsewhere  tells  us,  he 
has  "no  wish  to  belittle." 


Principles  of  Heredity  119 

III.     The  Facts  in  regard  to  Dominance  of 
Characters  in  Peas. 

Professor  Weldoii  refers  to  no  experiments  of  his  own 
and  presumably  has  made  none.  Had  he  done  so  he  would 
have  learnt  many  things  about  dominance  in  peas,  whether 
of  the  yellow  cotyledon-colour  or  of  the  round  form,  that 
might  have  pointed  him  to  caution. 

In  the  year  1900  Messrs  Vilmorin-Andrieux  &  Co.  were 
kind  enough  to  send  to  the  Cambridge  Botanic  Garden  on 
my  behalf  a  set  of  samples  of  the  varieties  of  Fisum  and 
Phaseolus,  an  exhibit  of  which  had  greatly  interested  me 
at  the  Paris  Exhibition  of  that  year.  In  the  past  summer 
I  grew  a  number  of  these  and  made  some  preliminary 
cross-fertilizations  among  them  (about  80  being  available 
for  these  deductions)  with  a  view  to  a  future  study  of 
certain  problems,  Mendelian  and  others.  In  this  work 
I  had  the  benefit  of  the  assistance  of  Miss  Killby  of 
Newnham  College.  Her  cultivations  and  crosses  were 
made  independently  of  my  own,  but  our  results  are  almost 
identical.  The  experience  showed  me,  what  a  naturalist 
would  expect  and  practical  men  know  already,  that  a  great 
deal  turiis  on  the  variety  used ;  that  some  varieties  are 
very  sensitive  to  conditions  while  others  maintain  their 
type  sturdily ;  that  in  using  certain  varieties  Mendel's 
experience  as  to  dominance  is  regularly  fulfilled,  while  in 
the  case  of  other  varieties  irregularities  and  even  some 
contradictions  occur.  That  the  dominance  of  yellow 
cotyledon-colour  over  green,  and  the  dominance  of  the 
smooth  form  over  the  wrinkled,  is  a  general  truth  for 
Pisum  sativum  appears  at  once  ;  that  it  is  a  universal 
truth  I  cannot  believe  any  competent  naturalist  would 
imagine,    still   less   assert.      Mendel   certainly   never   did. 


120  A  Defence  of  Menders 

When  he  speaks  of  the  "law"  or  "laws"  that  he  has 
established  for  Pisuni  he  is  referring  to  his  own  discovery 
of  the  purity  of  the  germ-cells,  that  of  the  statistical 
distribution  of  characters  among  them,  and  the  statistical 
grouping  of  the  different  germ -cells  in  fertilization,  and 
not  to  the  "Law  of  Dominance"  which  he  never  drafted 
and  does  not  propound. 

The  issue  will  be  clearer  if  I  here  state  briefly  what,  as 
far  as  my  experience  goes,  are  the  facts  in  regard  to  the 
characters  cotyledon-colour  and  seed-shapes  in  peas.  I  have 
not  opportunity  for  more  than  a  passing  consideration  of 
the  seed-coats  of  pure  forms*;  that  is  a  maternal  character, 
a  fact  I  am  not  sure  Professor  Weldon  fully  appreciates. 
Though  that  may  be  incredible,  it  is  evident  from  many 
passages  that  he  has  not,  in  quoting  authorities,  considered 
the  consequences  of  this  circumstance. 

The  normal  characters:  colour  of  cotyledons 
and  seed-coats. 

Culinary  peas  (P.  sativum,  omitting  purple  sorts)  can 
primarily  be  classified  on  colour  into  two  groups,  yellow^ 
and  green.  In  the  green  certain  pigmentary  matters 
persist  in  the  ripe  seed  which  disappear  or  are  decomposed 
in  the  yellow  as  the  seed  ripens.     But  it  may  be  observed 

*  The  whole  question  as  to  seed-coat  colour  is  most  complex. 
Conditions  of  growth  and  ripening  have  a  great  effect  on  it.  Mr 
Arthur  Sutton  has  shown  me  samples  of  Ne  Plus  Ultra  grown  in 
England  and  abroad.  This  pea  has  yellow  cotyledons  with  seed-coats 
either  yellow  or  "  blue."  The  foreign  sample  contained  a  much 
greater  proportion  of  the  former.  He  told  me  that  generally  speaking 
this  is  the  case  with  samples  ripened  in  a  hot,  dry  climate. 

Unquestionable  Xenia  appears  occasionally,  and  will  be  spoken  of 
later.  Moreover  to  experiment  with  such  a  plant-charsicter  an  extra 
generation  has  to  be  sown  and  cultivated.  Consequently  the  evidence 
is  meagre. 


Principles  of  Heredity  121 

that  the  "green"  class  itself  is  treated  as  of  two 
divisions,  green  and  blue.  In  the  seedsmen's  lists  the 
classification  is  made  on  the  external  appearance  of  the 
seed,  without  regard  to  whether  the  colour  is  due  to  the 
seed-coat,  the  cotyledons,  or  both.  As  a  rule  perhaps 
yellow  coats  contain  yellow  cotyledons,  and  green  coats 
green  cotyledons,  though  yellow  cotyledons  in  green  coats 
are  common,  e.g.  Gradus,  of  which  the  cotyledons  are  yellow 
while  the  seed-coats  are  about  as  often  green  as  yellow  (or 
"  white,"  as  it  is  called  technically).  Those  called  "blue" 
consist  mostly  of  seeds  which  have  green  cotyledons  seen 
through  transparent  skins,  or  yellow  cotyledons  combined 
with  green  skins.  The  skins  may  be  roughly  classified  into 
thin  and  transparent,  or  thick  and  generally  at  some  stage 
pigmented.  In  numerous  varieties  the  colour  of  the  coty- 
ledon is  wholly  yellow,  or  wholly  green.  Next  there  are 
many  varieties  which  are  constant  in  habit  and  other 
properties  but  have  seeds  belonging  to  these  two  colour 
categories  in  various  proportions.  How  far  these  pro- 
portions are  known  to  be  constant  I  cannot  ascertain. 

Of  such  varieties  showing  mixture  of  cotyledon-coXoViYii 
nearly  all  can  be  described  as  dimorphic  in  colour.  For 
example  in  Sutton's  Nonpareil  Marrowfat  the  cotyledons 
are  almost  always  either  yellow  or  green,  with  some  piebalds, 
and  the  colours  of  the  seed-coats  are  scarcely  less  distinctly 
dimorphic.  In  some  varieties  which  exist  in  both  colours 
intermediates  are  so  common  that  one  cannot  assert  any 
regular  dimorphism*. 

*  Knowing  my  interest  in  this  subject  Professor  Weldon  was 
so  good  as  to  forward  to  me  a  series  of  bis  peas  arranged  to 
form  a  scale  of  colours  and  shapes,  as  represented  in  his  Plate  I. 
I  have  no  doubt  that  the  use  of  such  colour-scales  will  much  facilitate 
future  study  of  these  problems. 


122  A  Defence  of  MendeVs 

There  are  some  varieties  which  have  cotyledons  green 
and  intermediate  shading  to  greenish  yellow,  like  Stratagem 
quoted  by  Professor  Weldon.  Others  have  yellow  and 
intermediate  shading  to  yellowish  green,  such  as  McLean's 
Best  of  all^.  I  am  quite  disposed  to  think  there  may  be 
truly  monomorphic  varieties  with  cotyledons  permanently 
of  intermediate  colour  only,  but  so  far  I  have  not  seen 
onef.  The  variety  with  greatest  irregularity  (apart  from 
regular  dimorphism)  in  cotyledon-colour  I  have  seen  is  a 
sample  of  ''''mange-tout  a  rames,  a  grain  vert,^^  but  it  was  a 
good  deal  injured  by  weevils  {Bruchus),  which  always  cause 
irregularity  or  change  of  colour. 

Lastly  in  some  varieties  there  are  many  piebalds  or 
mosaics. 

From  what  has  been  said  it  will  be  evident  that  the 
description  of  a  pea  in  an  old  book  as  having  been  green, 
blue,  white,  and  so  forth,  unless  the  cotyledon-colour  is 
distinguished  from  seed-coat  colour,  needs  careful  con- 
sideration before  inferences  are  drawn  from  it. 

Shape. 

In  regard  to  shape,  if  we  keep  to  ordinary  shelling  peas, 
the  facts  are  somewhat  similar,  but  as  shape  is  probably 
more  sensitive  to  conditions  than  cotyledon-colour  (not 
than  seed-coat  colour)  there  are  irregularities  to  be  perhaps 
ascribed  to  this  cause.  Broadly,  however,  there  are  two 
main  divisions,  round  and  wrinkled.  It  is  unquestioned 
that  between  these  two  types  every  intermediate  occurs. 

*  I  notice  that  Vilmorin  in  the  well-known  Plantes  Potageres, 
1883,  classifies  the  intermediate- coloured  peas  with  the  green. 

t  Similarly  though  tall  and  dioarf  are  Mendelian  characters,  peas 
occur  of  all  heights  and  are  usually  classified  as  tall,  half -dwarfs,  and 
dwarfs. 


Princijyles  of  Heredity  123 

Here  again  a  vast  number  of  varieties  can  be  at  once 
classified  into  round  and  wrinkled  (the  classification 
commonly  used),  others  are  intermediate  normally.  Here 
also  I  suspect  some  fairly  clear  sub-divisions  might  be 
made  in  the  wrinkled  group  and  in  the  round  group  too, 
but  I  would  not  assert  this  as  a  fact. 

I  cannot  ascertain  from  botanists  what  is  the  nature  of 
the  difference  between  round  and  wrinkled  peas,  though  no 
doubt  it  will  be  easily  discovered.  In  maize  the  round 
seeds  contain  much  unconverted  starch,  while  in  the 
wrinkled  or  sugar-maize  this  seems  to  be  converted  in 
great  measure  as  the  seed  ripens ;  with  the  result  that, 
on  drying,  the  walls  collapse.  In  such  seeds  we  may 
perhaps  suppose  that  the  process  of  conversion,  which  in 
round  seeds  takes  place  on  germination,  is  begun  earlier, 
and  perhaps  the  variation  essentially  consists  in  the  pre- 
mature appearance  of  the  converting  ferment.  It  would  be 
most  rash  to  suggest  that  such  a  process  may  be  operating 
in  the  pea,  for  the  phenomenon  may  have  many  causes ; 
but  however  that  may  be,  there  is  evidently  a  difference  of 
such  a  nature  that  when  the  water  dries  out  of  the  seed  on 
ripening,  its  walls  collapse* ;  and  this  collapse  may  occur 
in  varying  degrees. 

*  Wrinkling  must  of  course  be  distinguished  further  from  the 
squaring  due  to  the  peas  pressing  against  each  other  in  the  pod. 

In  connexion  with  these  considerations  I  may  mention  that 
Vilmorin  makes  the  interesting  statement  that  most  peas  retain  their 
vitality  three  years,  dying  as  a  rule  rapidly  after  that  time  is  passed, 
though  occasionally  seeds  seven  or  eight  years  old  are  alive;  but 
that  lorinkled  peas  germinate  as  a  rule  less  well  than  round,  and 
do  not  retain  their  vitality  so  long  as  the  round.  Vilmoi'in-Andrieux, 
Plantes  Potageres,  1883,  p.  423.  Similar  statements  regarding  the 
behaviour  of  wrinkled  peas  in  India  are  made  by  Firminger,  Gardening 
for  India,  3rd  ed.  1874,  p.  146. 


124  A  Defence  of  Mendel's 

In  respect  of  shape  the  seeds  of  a  variety  otherwise 
stable  are  as  a  rule  fairly  uniform,  the  co-existence  of 
both  shapes  and  of  intermediates  between  them  in  the 
same  variety  is  not  infrequent.  As  Professor  Weldon  has 
said,  Telephone  is  a  good  example  of  an  extreme  case  of 
mixture  of  both  colours  and  shapes.  William  /,  is  another. 
It  may  be  mentioned  that  regular  dimorphism  in  respect 
of  shape  is  not  so  common  as  dimorphism  in  respect 
of  colour.  Of  great  numbers  of  varieties  seen  at  Messrs 
Suttons'  I  saw  none  so  distinctly  dimorphic  in  shape  as 
William  I.  which  nevertheless  contains  all  grades  commonly. 

So  far  I  have  spoken  of  the  shapes  of  ordinary  English 
culinary  peas.  But  if  we  extend  our  observations  to  the 
shapes  of  large-seeded  peas,  which  occur  for  the  most  part 
among  the  sugar-peas  (mange-touts),  of  the  "grey"  peas 
with  coloured  flowers,  etc.,  there  are  fresh  complications 
to  be  considered. 

Professor  Weldon  does  not  wholly  avoid  these  (as 
Mendel  did  in  regard  to  shape)  and  we  will  follow  him 
through  his  difliculties  hereafter.  For  the  present  let  me 
say  that  the  classes  round  and  wrinhled  are  not  readily 
applicable  to  those  other  varieties  and  are  not  so  applied 
either  by  Mendel  or  other  practical  writers  on  these 
subjects.  To  use  the  terms  indicated  in  the  Introduction, 
seed-shape  depends  on  more  than  one  pair  of  allelomorphs — 
possibly  on  several. 

Stability  and  Variability. 

Generally  speaking  peas  which  when  seen  in  bulk  are 
monomorphic  in  colour  and  shape,  will  give  fairly  true  and 
uniform  offspring  (but  such  strict  monomorphism  is  rather 
exceptional).  Instances  to  the  contrary  occur,  and  in  my 
own  brief  experience  I  have  seen  some.     In  a  row  of  Fill- 


Principles  of  Heredity  125 

basket  grown  from  selected  seed  there  were  two  plants  of 
different  habit,  seed-shape,  etc.  Each  bore  pods  with  seeds 
few  though  large  and  round.  Again  Blue  Peter  (blue  and 
round)  and  Laxtoii s  Alpha  (blue  and  wrinkled),  grown  in 
my  garden  and  left  to  nature  uncovered,  have  each  given 
a  considerable  proportion  of  seeds  with  yellow  cotyledons, 
about  20  7o  ill  the  case  of  Laxtoii  s  Alpha.  The  distribution 
of  these  on  the  plants  I  cannot  state.  The  plants  bearing 
them  in  each  case  sprang  from  green-cotyledoned  seeds 
taken  from  samples  containing  presumably  unselected  green 
seeds  only.  A  part  of  this  exceptional  result  may  be  due 
to  crossing,  but  heterogeneity  of  conditions*  especially  in 
or  after  ripening  is  a  more  likely  cause,  hypotheses  I  hope 
to  investigate  next  season.  Hitherto  I  had  supposed  the 
crossing,  if  any,  to  be  done  by  Britchus  or  Thrips,  but 
Tschermak  also  suspects  Megachile,  the  leaf-cutter  bee, 
which  abounds  in  my  garden. 

Whatever  the  cause,  these  irregularities  may  undoubtedly 
occur ;  and  if  they  be  proved  to  be  largely  independent  of 
crossing  and  conditions,  this  will  in  nowise  vitiate  the  truth 
of  the  Mendelian  principle.  For  in  that  case  it  may  simply 
be  variability.  Such  true  variation,  or  sporting,  in  the 
pea  is  referred  to  by  many  observers.  Upon  this  subject  I 
have  received  most  valuable  facts  from  Mr  Arthur  Sutton, 
who  has  very  kindly  interested  himself  in  these  inquiries. 

*  Cotyledon-colour  is  not  nearly  so  sensitive  to  ordinary  changes 
in  conditions  as  coat-colour,  provided  the  coat  be  uninjured.  But 
even  in  monomorphic  green  varieties,  a  seed  which  for  any  cause  has 
burst  on  ripening,  has  the  exposed  parts  of  its  cotyledons  ijelloio. 
The  same  may  be  the  case  in  seeds  of  green  varieties  injured  by 
Bruchus  or  birds.  These  facts  make  one  hesitate  before  denying  the 
effects  of  conditions  on  the  cotyledon-colour  even  of  uninjured 
seeds,  and  the  variation  described  above  may  have  been  simply 
weathering.  The  seeds  were  gathered  very  late  and  man}'  were 
burst  in  Laxton's  Alpha.     I  do  not  yet  know  they  are  alive. 


126  A  Defence  of  MendeVs 

He  tells  me  that  several  highly  bred  varieties,  selected  with 
every  possible  care,  commonly  throw  a  small  but  constant 
proportion  of  poor  and  almost  vetch-like  plants,  with  short 
pods  and  small  round  seeds,  which  are  hoed  out  by  experi- 
enced men  each  year  before  ripening.  Other  high-class 
varieties  always,  wherever  grown,  and  when  far  from  other 
sorts,  produce  a  small  percentage  of  some  one  or  more 
definite  "  sports."  Of  these  peculiar  sports  he  has  sent  me 
a  collection  of  twelve,  taken  from  as  many  standard  varieties, 
each  "sport"  being  represented  by  eight  seeds,  which  though 
quite  distinct  from  the  type  agree  with  each  other  in  almost 
all  cases. 

In  two  cases,  he  tells  me,  these  seed-sports  sown 
separately  have  been  found  to  give  plants  identical  with 
the  standard  type  and  must  therefore  be  regarded  as  sports 
in  seed  characters  only ;  in  other  cases  change  of  plant-type 
is  associated  with  the  change  of  seed-tjrpe. 

In  most  standard  varieties  these  definite  sports  are  not 
ver)^  common,  but  in  a  few  they  are  common  enough  to 
require  continual  removal  by  selection"^. 

I  hope  before  long  to  be  able  to  give  statistical  details 

*  It  is  interesting  to  see  that  in  at  least  one  case  the  same — or 
practically  the  same — variety  has  been  independently  produced  by 
different  raisers,  as  we  now  perceive,  by  the  fortuitous  combination 
of  similar  allelomorphs.  Sutton's  Ringleader  and  Carter's  First  Crop 
(and  two  others)  are  cases  in  point,  and  it  is  peculiarly  instructive  to 
see  that  in  the  discussion  of  these  varieties  when  they  were  new,  one 
of  the  points  indicating  their  identity  was  taken  to  be  the  fact  that 
they  produced  the  same  ^^ rogues."  See  Gard.  Chron.  1865,  pp.  482  and 
603;  1866,  p.  221;  1867,  pp.  546  and  712. 

Eimpau  quotes  Blomeyer  [Kultur  der  Landw.  Nutzpfianzen,  Leipzig, 
1889,  pp.  357  and  380)  to  the  effect  that  ^wrpZe-flowered  plants  with 
wrinkled  seeds  may  spring  as  direct  sports  from  peas  with  lohite 
flowers  and  round  seeds.  I  have  not  seen  a  copy  of  Blomeyer's 
work.     Probably  this  "wrinkling"  was  "indentation." 


Princi2jles  of  Heredity  127 

and  experiments  relating  to  this  extraordinarily  interesting- 
subject.  As  de  Vries  writes  in  his  fine  work  Die  Muta- 
tionstheorie  (i.  p.  580),  "  a  study  of  the  seed-differences  of 
inconstant,  or  as  they  are  called,  '  still '  unfixed  varieties,  is 
a  perfect  treasure-house  of  new  discoveries." 

Let  us  consider  briefly  the  possible  significance  of  these 
facts  in  the  light  of  Mendelian  teaching.  First,  then,  it  is 
clear  that  as  regards  most  of  such  cases  the  hypothesis  is 
not  excluded  that  these  recurring  sports  may  be  due  to  the 
fortuitous  concurrence  of  certain  scarcer  hypallelomorphs, 
which  may  either  have  been  free  in  the  original  parent 
varieties  from  which  the  modern  standard  forms  were 
raised,  or  may  have  been  freed  in  the  crossing  to  which  the 
latter  owe  their  origin  (see  p.  28).  This  possibility  raises 
the  question  whether,  if  we  could  make  "pure  cultures  "  of 
the  gametes,  any  variations  of  this  nature  would  ever  occur. 
This  may  be  regarded  as  an  unwarrantable  speculation,  but 
it  is  not  wholly  unamenable  to  the  test  of  experiments. 

But  variability,  in  the  sense  of  division  of  gonads  into 
heterogeneous  gametes,  may  surely  be  due  to  causes  other 
than  crossing.  This  we  cannot  doubt.  Cross-fertilization 
of  the  zygote  producing  those  gametes  is  one  of  the  causes 
of  such  heterogeneity  among  them.  We  cannot  suppose  it 
to  be  the  sole  cause  of  this  phenomenon. 

"When  Mendel  asserts  the  purity  of  the  germ-cells  of 
cross-breds  he  cannot  be  understood  to  mean  that  they  are 
more  pure  than  those  of  the  original  parental  races.  These 
must  have  varied  in  the  past.  The  wrinkled  seed  arose 
from  the  round,  the  green  from  the  yellow  (or  vice  versa, 
if  preferred),  and  probably  numerous  intermediate  forms 
from  both. 

The  variations,  or  as  I  provisionally  conceive  it,  that 
differentiant  division  among  the  gametes  of  which  variation 


128  A  Defence  of  MendeVs 

(neglecting  environment)  is  the  visible  expression,  has  arisen 
and  can  arise  at  one  or  more  points  of  time,  and  we  have 
no  difficulty  in  believing  it  to  occur  now.  In  many  cases 
we  have  clear  evidence  that  it  does.  Crossing, — dare  we 
call  it  asymmetrical  fertilization  ? — is  one  of  the  causes  of 
the  production  of  heterogeneous  gametes — the  result  of 
divisions  qualitatively  differentiant  and  perhaps  asjon- 
metrical"^. 

There  are  other  causes  and  we  have  to  find  them. 
Some  years  ago  I  wrote  that  consideration  of  the  causes 
of  variation  was  in  my  judgment  premature!.  Now  that 
through  Mendel's  work  we  are  clearing  our  minds  as  to  the 
fundamental  nature  of  "gametic"  variation,  the  time  is 
approaching  when  an  investigation  of  such  causes  maybe 
not  unfruitful. 

Of  variation  as  distinct  from  transmission  why  does 
Professor  Weldon  take  no  heed  ?     He  writes  (p.  244)  : 

"  If  Mendel's  statements  were  universally  valid,  even  among 
Peas,  the  characters  of  the  seeds  in  the  numerous  hybrid  races 
now  existing  should  fall  into  one  or  other  of  a  few  definite 
categories,  which  should  not  be  connected  by  intermediate 
forms." 

Now,  as  I  have  already  pointed  out,  Mendel  made  no 
pretence  of  universal  statement :  but  had  he  done  so,  the 
conclusion,  which  Professor  Weldon  here  suggests  should 
follow  from  such  a  universal  statement,  is  incorrectly 
drawn.     Mendel  is  concerned  with  the  laws  of  transmission 

*  The  asymmetries  here  conceived  may  of  course  be  combined  in 
an  inclusive  symmetry.  Till  the  differentiation  can  be  optically 
recognized  in  the  gametes  we  shall  probably  get  no  further  with  this 
part  of  the  problem. 

t  Materials  for  the  Study  of  Variation,  1894,  p.  78. 


Principles  of  Heredity  129 

of  existing  characters,  not  with  variation,  which  he  does 
not  discuss. 

Nevertheless  Professor  Weldon  has  some  acquaintance 
with  the  general  fact  of  variability  in  certain  peas,  which 
he  mentions  (p.  236),  but  the  bearing  of  this  fact  on  the 
difficulty  he  enuntiates  escapes  him. 

Results  of  crossing  in  r^egard  to  seed  characters : 
normal  and  exceptional. 

The  conditions  being  the  same,  the  question  of  the 
characters  of  the  cross-bred  zygotes  which  we  will  call 
AB'^  depends  primarily  on  the  specific  nature  of  the 
varieties  which  are  crossed  to  produce  them.  It  is  un- 
necessary to  point  out  that  if  all  ^^'s  are  to  look  alike, 
both  the  varieties  A  and  B  must  be  pure — not  in  the 
common  sense  of  descended,  as  far  as  can  be  traced, 
through  individuals  identical  with  themselves,  but  pure  in 
the  Mendelian  sense,  that  is  to  say  that  each  must  be  at  that 
moment  producing  only  homogeneous  gametes  bearing  the 
same  characters  A  and  B  respectively.  Purity  of  pedigree 
in  the  breeder's  sense  is  a  distinct  matter  altogether.  The 
length  of  time — or  if  preferred — the  number  of  generations 
through  which  a  character  of  a  variety  has  remained  pure, 
alters  the  probability  of  its  dominance,  i.e.  its  appearance 
when  a  gamete  bearing  it  meets  another  bearing  an  antago- 
nistic character,  no  more,  so  far  as  we  are  yet  aware,  than 
the  length  of  time  a  stable  element  has  been  isolated  alters 
the  properties  of  the  chemical  compound  which  may  be 
prepared  from  it. 

Now  when  individuals  (bearing  contrary  characters), 
pure  in  the  sense  indicated,  are  crossed  together,  the 
question  arises.  What  will  be  the  appearance  of  the  first 

B.  9 


130  A  Defence  of  MendeVs 

cross  individuals  ?  Here  again,  generally  speaking^  when 
thoroughly  green  cotyledons  are  crossed  with  thoroughly 
yellow  cotyledons,  the  first-cross  seeds  will  have  yellow 
cotyledons  ;  when  fully  round  peas  are  crossed  with  fully 
wrinkled  the  first  result  will  generally  speaking  be  round, 
often  with  slight  pitting  as  Mendel  has  stated.  This  has 
been  the  usual  experience  of  Correns,  Tschermak,  Mendel, 
and  myself*  and,  as  we  shall  see,  the  amount  of  clear 
and  substantial  evidence  to  the  contrary  is  still  exceed- 
ingly small.  But  as  any  experienced  naturalist  would 
venture  to  predict,  there  is  no  universal  rule  in  the 
matter.  As  Professor  Weldon  himself  declares,  had  there 
been  such  a  universal  rule  it  would  surely  have  been 
notorious.  He  might  further  have  reflected  that  in 
Mendel's  day,  when  hybridisation  was  not  the  terra 
incognita  it  has  since  become,  the  assertion  of  such  uni- 
versal propositions  would  have  been  peculiarly  foolish. 
Mendel  does  not  make  it ;  but  Professor  Weldon  perceiving 
the  inherent  improbability  of  the  assertion  conceives  at 
once  that  Mendel  must  have  made  it,  and  if  Mendel 
doesn't  say  so  in  words  then  he  must  have  implied  it. 
As  a  matter  of  fact  Mendel  never  treats  dominance  as 
more  than  an  incident  in  his  results,  merely  using  it  as 
a  means  to  an  end,  and  I  see  no  reason  to  suppose  he 
troubled  to  consider  to  what  extent  the  phenomenon  is  or 
is  not  universal — a  matter  with  which  he  had  no  concern. 

*  The  varieties  used  were  Express,  Laxton's  Alpha,  Fillbasket, 
McLean's  Blue  Peter,  Serpette  nain  blanc,  British  Queen,  tres  nain 
de  Bretagne,  Sabre,  mange-tout  Debarbieux,  and  a  large  "grey" 
sugar-pea,  pois  sans  parchemin  geant  a  tres  large  cosse.  Not  counting 
the  last  two,  five  are  round  and  three  are  wrinkled.  As  to  cotyledons, 
six  have  yellow  and  four  have  green.  In  about  80  crosses  I  saw  no 
exception  to  dominance  of  yellow ;  but  one  apparently  clear  case  of 
dominance  of  wrinkled  and  some  doubtful  ones. 


Principles  of  Heredity  131 

Of  course  there  may  be  exceptions.  As  yet  we  cannot 
detect  the  causes  which  control  them,  though  injury, 
impurity,  accidental  crossing,  mistakes  of  various  kinds, 
account  for  many.  Mendel  himself  says,  for  instance,  that 
unhealthy  or  badly  grown  plants  give  uncertain  results. 
Nevertheless  there  seems  to  be  a  true  residuum  of  ex- 
ceptions not  to  be  explained  away.  I  will  recite  some 
that  I  have  seen.  In  my  own  crosses  I  have  seen  green  x 
gTeen  give  yellow  four  times.  This  I  incline  to  attribute 
to  conditions  or  other  disturbance,  for  the  natural  pods  of 
these  plants  gave  several  yellows.  At  Messrs  Suttons'  I  saw 
second-generation  seeds  got  by  allowing  a  cross  of  Sutton's 
Centenary  (gr.  wr.)  x  Eclipse  (gr.  rd.)  to  go  to  seed ;  the 
resulting  seeds  were  both  green  and  yellow,  wrinkled  and 
round.  But  in  looking  at  a  sample  of  Eclipse  I  found 
a  few  yellow  seeds,  say  two  per  cent.,  which  may  perhaps 
be  the  explanation.  Green  wrinkled  x  green  round  may 
give  all  wrinkled,  and  again  \vrinkled  x  wrinkled  may  give 
round'^.  Of  this  I  saw  a  clear  case — supposing  no  mistake 
to  have  occurred — at  Messrs  Suttons'.  Lastly  we  have 
the  fact  that  in  exceptional  cases  crossing  two  forms — 
apparently  pure  in  the  strict  sense — may  give  a  mixture 
in  the  first  generation.  There  are  doubtless  examples  also 
of  unlikeness  between  reciprocals,  and  of  this  too  I  have 
seen  one  putative  caset. 

Such  facts  thus  set  out  for  the  first  cross-bred 
generation  may  without  doubt  be  predicated  for  subsequent 
generations. 

What  then  is  the  significance  of  the  facts  ? 

*  Professor  Weldon  may  take  this  as  a  famous  blow  for  Mendel, 
till  he  realizes  what  is  meant  by  Mendel's  '*  Hybrid-character." 

t  In  addition  to  those  spoken  of  later,  where  the  great  difference 
between  reciprocals  is  due  to  the  maternal  characters  of  the  seeds. 

9—2 


132  A  Defence  of  MendeVs 

A  nalysis  of  exceptions. 

Assuming  that  all  these  "contradictory"  phenomena 
happened  truly  as  alleged,  and  were  not  pathological  or 
due  to  error — an  explanation  which  seems  quite  inadequate 
— there  are  at  least  four  possible  accounts  of  such  diverse 
results — each  valid,  without  any  appeal  to  ancestry. 

1.  That  dominance  may  exceptionally  fail — or  in  other 
words  he  created  on  the  side  which  is  elsewhere  recessive. 
For  this  exceptional  failure  we  have  to  seek  exceptional 
causes.  The  artificial  creation  of  dominance  (in  a  character 
usually  recessive)  has  not  yet  to  my  knowledge  been  demon- 
strated experimentally,  but  experiments  are  begun  by  which 
such  evidence  may  conceivably  be  obtained. 

2.  There  may  be  what  is  known  to  practical  students 
of  evolution  as  the  false  hybridism  of  Millardet,  or  in  other 
words,  fertilisation  with — from  unknown  causes — transmis- 
sion of  none  or  of  only  some  of  the  characters  of  one  pure 
parent.  The  applicability  of  this  hypothesis  to  the  colours 
and  shapes  of  peas  is  perhaps  remote,  but  we  may  notice  that 
it  is  one  possible  account  of  those  rare  cases  where  two 
pure  forms  give  a  mixed  result  in  the  first  generation,  even 
assuming  the  gametes  of  each  pure  parent  to  be  truly 
monomorphic  as  regards  the  character  they  bear.  The 
applicability  of  this  suggestion  can  of  course  be  tested  by 
study  of  the  subsequent  generations,  self-fertilised  or  ferti- 
lised by  similar  forms  produced  in  the  same  way.  In  the 
case  of  a  genuine  false-hybrid  the  lost  characters  will  not 
reappear  in  the  posterity. 

3.  The  result  may  not  be  a  case  of  transmission  at  all 
as  it  is  at  present  conceived,  but  of  the  creation  on  crossing 


Princijyles  of  Hei^ecUty  1 33 

of  something  new.  Our  ^^'s  may  have  one  or  more 
characters  ijeculiar  to  themselves.  We  may  in  fact  have 
made  a  distinct  "  mule  "  or  heterozygote  form.  Where  this 
is  the  case,  there  are  several  subordinate  possibilities  we 
need  not  at  present  pursue. 

4.  There  may  be  definite  variation  (distinct  from  that 
proper  to  the  "mule")  consequent  on  causes  we  cannot 
yet  surmise  (see  pp.  125  and  128). 

The  above  possibilities  are  1  believe  at  the  present  time 
the  only  ones  that  need  to  be  considered  in  connexion  with 
these  exceptional  cases*.  They  are  all  of  them  capable 
of  experimental  test  and  in  certain  instances  we  are 
beginning  to  expect  the  conclusion. 


The  "  mule  "  or  heterozygote. 

There  can  be  little  doubt  that  in  many  cases  it  is  to 
the  third  category  that  the  phenomena  belong.  An  indication 
of  the  applicability  of  this  reasoning  will  generally  be  found 
in  the  fact  that  in  such  "mule"  forms  the  colour  or  the 
shape  of  the  seeds  will  be  recognizably  peculiar  and  proper 
to  the  specimens  themselves,  as  distinct  from  their  parents, 
and  we  may  safely  anticipate  that  when  those  seeds  are 
grown  the  plants  will  show  some  character  which  is 
recognizable  as  novel.  The  proof  that  the  reasoning  may 
apply  can  as  yet  only  be  got  by  finding  that  the  forms  in 

*  I  have  not  here  considered  the  case  in  which  male  and  female 
elements  of  a  pure  variety  are  not  homolofjous  and  the  variety  is  a 
permanent  monomorphic  "  mule."  Such  a  phenomenon,  when  present, 
will  prove  itself  in  reciprocal  crossing.  I  know  no  such  case  in 
peas  for  certain. 


134  A  Defence  of  MendeVs 

question  cannot  breed  true  even  after  successive  selections, 
but  constantly  break  up  into  the  same  series  of  forms*. 

This  conception  of  the  "mule"  form,  or  "hybrid- 
character"  as  Mendel  called  it,  though  undeveloped,  is 
perfectly  clear  in  his  work.  He  says  that  the  dominant 
character  may  have  two  significations,  it  may  be  either  a 
parental  character  or  a  hybrid-character,  and  it  must  be 
differentiated  according  as  it  appears  in  the  one  capacity 
or  the  other.  He  does  not  regard  the  character  displayed 
by  the  hybrid,  whether  dominant  or  other,  as  a  thing 
inherited  from  or  transmitted  by  the  pure  parent  at  all,  but 
as  the  peculiar  function  or  property  of  the  hybrid.  When 
this  conception  has  been  fully  understood  and  appreciated 
in  all  its  bearings  it  will  be  found  to  be  hardly  less  fruitful 
than  that  of  the  purity  of  the  germ-cells. 

The  two  parents  are  two — let  us  say — substances  f 
represented  by  corresponding  gametes.  These  gametes 
unite  to  form  a  new  "substance" — the  cross-bred  zj-gote. 
This  has  its  own  j)roperties  and  structure,  just  as  a  chemical 
compound  has,  and  the  properties  of  this  new  "  substance  " 
are  not  more  strictly  traceable  to,  or  "inherited"  from, 
those  of  the  two  parents  than  are  those  of  a  new  chemical 
compound  "inherited"  from  those  of  the  component 
elements.  If  the  case  be  one  in  which  the  gametes  are 
pure,  the  new  "  substance "  is  not  represented  by  them, 
but  the  compound  is  again  dissociated  into  its  components, 
each  of  which  is  separately  represented  by  gametes. 

*  It  will  be  understood  that  a  "  mule  "  form  is  quite  distinct  from 
what  is  generally  described  as  a  "blend."  One  certain  criterion  of 
the  "  mule "  form  is  the  fact  that  it  cannot  be  fixed,  see  p.  25. 
There  is  little  doubt  that  Laxton  had  such  a  "  mule  "  form  when  he 
speaks  of  "the  remarkably  fine  but  unfixable  pea,  Evolution."  J.  R. 
Hort.  Soc.  XII.  1890,  p.  37  {v.  infra). 

t  Using  the  word  metaphorically. 


Principles  of  Heredity  135 

The  character  of  the  cross-bred  zygote  may  be  anything. 
It  may  be  something  we  have  seen  before  in  one  or  other  of 
the  parents,  it  may  be  intermediate  between  the  two,  or  it 
may  be  something  new.  All  these  possibilities  were  known 
to  Mendel  and  he  is  perfectly  aware  that  his  principle  is 
equally  applicable  to  all.  The  first  case  is  his  "  dominance." 
That  he  is  ready  for  the  second  is  sufficiently  shown  by  his 
brief  reference  to  time  of  flowering  considered  as  a  character 
(p.  &o).  The  hybrids,  he  says,  flower  at  a  time  almost 
exactly  intermediate  between  the  flowering  times  of  the 
parents,  and  he  remarks  that  the  development  of  the 
hybrids  in  this  case  probably  happens  in  the  same  way  as 
it  does  in  the  case  of  the  other  characters*. 

That  he  was  thoroughly  prepared  for  the  third  possibility 
appears  constantly  through  the  paper,  notably  in  the 
argument  based  on  the  Phaseolus  hybrids,  and  in  the 
statement  that  the  hybrid  between  tails  and  dwarfs  is 
generally  taller  than  the  tall  parent,  having  increased 
height  as  its  "  hybrid-character." 

All  this  Professor  Weldon  lias  missed.  In  place  of  it 
he  off'ers  us  the  sententia  that  no  one  can  expect  to 
understand  these  phenomena  if  he  neglect  ancestry.  This 
is  the  idle  gloss  of  the  scribe,  which,  if  we  erase  it  not 
thoroughly,  may  pass  into  the  text. 

Enough  has  been  said  to  show  how  greatly  Mendel's 

■  conception   of  heredity   was   in   advance  of  those   which 

pass  current  at  the  present  day  ;  I  have  here  attempted 

*  "  Ueber  die  BlUthezeit  der  Hybriden  sind  die  Versuche  noch  jiicht 
abfjeschlossen.  So  viel  kann  indessen  schon  anrjecjeben  werden,  dass 
diescdbe  fast  genau  in  der  Mitte  zwischen  jener  der  Samen-  nnd 
Pollenpjianze  steht,  und  die  Enhvicklmig  der  Hybriden  bezuglich 
dieses  Merkviales  xcaltrscheinlich  in  der  namlichen  Weise  erfolgt,  loie  es 
fiir  die  iibriyen  Merkmale  der  Fall  ist."     Mendel,  p.  23. 


136  A  Defence  of  MendeVs 

the  barest  outline  of  the  nature  of  the  "hybrid-character," 
and  I  have  not  sought  to  indicate  the  conclusions  that  we 
reach  when  the  reasoning  so  clear  in  the  case  of  the  hybrid 
is  applied  to  the  pure  forms  and  their  own  characters. 

In  these  considerations  we  reach  the  very  base  on  which 
all  conceptions  of  heredity  and  variation  must  henceforth 
rest,  and  that  it  is  now  possible  for  us  to  attempt  any  such 
analysis  is  one  of  the  most  far-reaching  consequences  of 
Mendel's  principle.  Till  two  years  ago  no  one  had  made 
more  than  random  soundings  of  this  abyss. 

I  have  briefly  discussed  these  possibilities  to  assist  the 
reader  in  getting  an  insight  into  Mendel's  conceptions. 
But  in  dealing  with  Professor  Weldon  we  need  not  make 
this  excursion  ;  for  his  objection  arising  from  the  absence  of 
uniform  regularity  in  dominance  is  not  in  point. 

The  soundness  of  Mendel's  work  and  conclusions  would 
be  just  as  complete  if  dominance  be  found  to  fail  often 
instead  of  rarely.  For  it  is  perfectly  certain  that  varieties 
can  be  chosen  in  such  a  way  that  the  dominance  of  one 
character  over  its  antagonist  is  so  regular  a  phenomenon 
that  it  can  be  used  in  the  way  Mendel  indicates.  He  chose 
varieties,  in  fact,  in  which  a  known  character  was  regularly 
dominant  and  it  is  because  he  did  so  that  he  made  his 
discovery^.  When  Professor  Weldon  speaks  of  the  exist- 
ence of  fluctuation  and  diversity  in  regard  to  dominance  as 
proof  of  a  "  grave  discrepancy  "  between  Mendel's  facts  and 
those  of  other  observers!,  he  merely  indicates  the  point  at 
which  his  own  misconceptions  began. 

*  As  has  been  already  shown  the  discovery  could  have  been 
made  equally  well  and  possibly  with  greater  rapidity  in  a  case  in 
which  the  hybrid  had  a  character  distinct  from  either  parent.  The 
cases  that  would  not  have  given  a  clear  result  are  those  where  there 
is  irregular  dominance  of  one  or  other  parent. 

t  Weldon,  p.  240. 


Principles  of  Heredity  137 

From  Mendel's  style  it  may  be  inferred  that  if  he  had 
meant  to  state  universal  dominance  in  peas  he  would 
have  done  so  in  unequivocal  language.  Let  me  point  out 
further  that  of  the  34  varieties  he  collected  for  study,  he 
discarded  12  as  not  amenable  to  his  purposes*.  He  tells 
us  he  would  have  nothing  to  do  with  characters  which 
were  not  sharp,  but  of  a  "  more  or  less  "  description.  As 
the  34  varieties  are  said  to  have  all  come  true  from  seed, 
we  may  fairly  suppose  that  the  reason  he  discarded  twelve 
was  that  they  were  unsuitable  for  his  calculations,  having 
either  ill-defined  and  intermediate  characters,  or  possibly 
defective  and  irregular  dominance. 


IV.     Professor  Weldon's  collection  of  "  Other 

EviDEITCE   concerning   DOMINANCE   IN   PeAS." 

A.     In  regard  to  cotyledon  colour:  Preliminary. 

I  have  been  at  some  pains  to  show  how  the  contradictory 
results,  no  doubt  sometimes  occurring,  on  which  Professor 
Weldon  lays  such  stress,  may  be  comprehended  without 
any  injury  to  Mendel's  main  conclusions.  This  excursion 
was  made  to  save  trouble  with  future  discoverers  of 
exceptions,  though  the  existence  of  such  facts  need 
scarcely  disturb  many  minds.  As  regards  the  dominance 
of  yellow  cotyledon-colour  over  green  the  whole  number  of 
genuine  unconformable  cases  is  likely  to  prove  very  small 
indeed,  though  in  regard  to  the  dominance  of  round  shape 
over  wrinkled  we  may  be  prepared  for  more  discrepancies. 
Indeed  my  own  crosses  alone  are  sufficient  to  show  that 
in  using  some  varieties  irregularities  are  to  be  expected. 

*  See  p.  43. 


138  A  Defence  of  MendeVs 

Considering  also  that  the .  shapes  of  peas  depend  un- 
questionably on  more  than  one  pair  of  allelomorphs  I 
fully  expect  regular  blending  in  some  cases. 

As  however  it  may  be  more  satisfactory  to  the  reader 
and  to  Professor  Weldon  if  I  follow  him  through  his 
"contradictory"  evidence  I  Avill  endeavour  to  do  so.  Those 
who  have  even  a  slight  practical  acquaintance  with  the 
phenomena  of  heredity  will  sympathize  with  me  in  the 
difficulty  I  feel  in  treating  this  section  of  his  arguments 
with  that  gravity  he  conceives  the  occasion  to  demand. 

In  following  the  path  of  the  critic  it  will  be  necessary 
for  me  to  trouble  the  reader  with  a  number  of  details  of  a 
humble  order,  but  the  journey  will  not  prove  devoid  of 
entertainment. 

Now  exceptions  are  always  interesting  and  suggestive 
things,  and  sometimes  hold  a  key  to  great  mysteries.  Still 
when  a  few  exceptions  are  found  disobeying  rules  elsewhere 
conformed  to  by  large  classes  of  phenomena  it  is  not  an 
unsafe  course  to  consider,  with  such  care  as  the  case  permits, 
whether  the  exceptions  may  not  be  due  to  exceptional 
causes,  or  failing  such  causes  whether  there  may  be  any 
possibility  of  error.  But  to  Professor  Weldon,  an  exception 
is  an  exception — and  as  such  may  prove  a  very  serviceable 
missile ;  so  he  gathers  them  as  they  were  "smooth  stones 
from  the  brook." 

Before  examining  the  quality  of  this  rather  miscellaneous 
ammunition  I  would  wish  to  draw  the  non-botanical  reader's 
attention  to  one  or  two  facts  of  a  general  nature. 

For  our  present  purpose  the  seed  of  a  pea  may  be 
considered  as  consisting  of  two  parts,  the  embryo  with  its 
cotyledons,  enclosed  in  a  seed-coat.  It  has  been  known  for 
about  a  century  that  this  coat  or  skin  is  a  maternal  structure, 
being  part  of  the  mother  plant  just  as  much  as  the  pods 


Principles  of  Heredity  139 

are,  and  consequently  not  belonging  to  the  next  generation 
at  all.  If  then  any  changes  take  place  in  it  consequent  on 
fertilisation,  they  are  to  be  regarded  not  as  in  any  sense  a 
transmission  of  character  by  heredity,  but  rather  as  of  the 
nature  of  an  "infection."  If  on  the  other  hand  it  is  desired 
to  study  the  influence  of  hereditary  transmission  on  seed- 
coat  characters,  then  the  crossed  seeds  must  be  sown  and 
the  seed-coats  of  their  seeds  studied.  Such  infective  changes 
in  maternal  tissues  have  been  known  from  early  times,  a 
notable  collection  of  them  having  been  made  especially  by 
Darwin ;  and  for  these  cases  Focke  suggested  the  convenient 
word  Xenia.  With  this  familiar  fact  I  would  not  for  a 
moment  suppose  Professor  Weldon  unacquainted,  though  it 
was  with  some  surprise  that  I  found  in  his  paper  no  reference 
to  the  phenomenon. 

For  as  it  happens,  xenia  is  not  at  all  a  rare  occurrence 
with  certain  'varieties  of  peas  ;  though  in  them,  as  I  believe 
is  generally  the  case  with  this  phenomenon,  it  is  highly 
irregular  in  its  manifestations,  being  doubtless  dependent 
on  slight  differences  of  conditions  during  ripening. 

The  coats  of  peas  differ  greatly  in  different  varieties, 
being  sometimes  thick  and  white  or  yellow,  sometimes 
thick  and  highly  pigmented  with  green  or  other  colours, 
in  both  of  which  cases  it  may  be  impossible  to  judge  the 
cotyledon-colour  without  peeling  off  the  opaque  coat ;  or 
the  coats  may  be  very  thin,  colourless  and  transparent,  so 
that  the  cotyledon-colour  is  seen  at  once.  It  was  such  a 
transparent  form  that  Mendel  says  he  used  for  his  experi- 
ments with  cotyledon-colour.  In  order  to  see  xenia  a  pea 
with  a  pigmented  seed-coat  should  be  taken  as  seed-parent, 
and  crossed  with  a  variety  having  a  different  cotyledon- 
colour.  There  is  then  a  fair  chance  of  seeing  this 
phenomenon,  but  much  still  depends  on  the  variety.     For 


140  A  Defence  of  MendeVs 

example,  Fillbasket  has  green  cotyledons  and  seed- coat 
green  except  near  the  hilar  surface.  Crossed  with  BerpetU 
nain  blanc  (yellow  cotyledons  and  yellow  coat)  this  variety 
gave  three  pods  with  17  seeds  in  which  the  seed-coats  were 
almost  full  yellow  (xenia).  Three  other  pods  (25  seeds), 
similarly  produced,  showed  slight  xenia,  and  one  pod  with 
eight  seeds  showed  little  or  none. 

On  the  other  hand  Fillbasket  fertilised  with  nain  de 
Bretagne  (yellow  cotyledons,  seed-coats  yellow  to  yellowish 
green)  gave  six  pods  wdth  39  seeds  showing  slight  xenia, 
distinct  in  a  few  seeds  but  absent  in  most. 

Examples  of  xenia  produced  by  the  contrary  proceeding, 
namely  fertilising  a  yellow  pea  with  a  green,  may  indubitably 
occur  and  I  have  seen  doubtful  cases  ;  but  as  by  the  nature 
of  the  case  these  are  negative  phenomena,  i.e.  the  seed-coat 
remaining  greenish  and  not  going  through  its  normal 
maturation  changes,  they  must  always  be  equivocal,  and 
would  require  special  confirmation  before  other  causes  were 
excluded. 

Lastly,  the  special  change  (xenia)  Mendel  saw  in  "grey" 
peas,  appearance  or  increase  of  purple  pigment  in  the  thick 
coats,  following  crossing,  is  common  but  also  irregular. 

If  a  transparent  coated  form  be  taken  as  seed-parent 
there  is  no  appreciable  xenia,  so  far  as  I  know,  and  such  a 
phenomenon  would  certainly  be  paradoxical*. 

In  this  connection  it  is  interesting  to  observe  that 
Giltay,  whom  Professor  Weldon  quotes  as  having  obtained 
purely  Mendelian  results,  got  no  xenia  though  searching 
for  it.  If  the  reader  goes  carefully  through  Giltay's 
numerous  cases,  he  will  find,  almost  without  doubt,  that 
none  of  them  were  such  as  produce  it.     Reading  Giant,  as 

*  In  some  transparent  coats  there  is  pigment,  but  so  little  as  a 
rule  that  xenia  would  be  scarcely  noticeable. 


Pmnciples  of  Heredity  141 

Giltay  states,  has  a  transjparent  skin,  and  the  only  xenia 
likely  to  occur  in  the  other  cases  would  be  of  the  peculiar 
and  uncertain  kind  seen  in  using  "grey"  peas.  Professor 
AYeldon  notes  that  Giltay,  who  evidently  worked  with  ex- 
treme care,  peeled  his  seeds  before  describing  them,  a  course 
which  Professor  Weldon,  not  recognizing  the  distinction 
between  the  varieties  with  opaque  and  transparent  coats, 
himself  wisely  recommends.  The  coincidence  of  the  peeled 
seeds  giving  simple  Mendelian  results  is  one  which  might 
have  alarmed  a  critic  less  intrepid  than  Professor  Weldon. 

Bearing  in  mind,  then,  that  the  coats  of  peas  may  be 
transparent  or  opaque  ;  and  in  the  latter  case  may  be 
variously  pigmented,  green,  grey,  reddish,  purplish,  etc. ; 
that  in  any  of  the  latter  cases  there  may  or  may  not  be 
xenia ;  the  reader  will  perceive  that  to  use  the  statements 
of  an  author,  whether  scientific  or  lay,  to  the  effect  that  on 
crossing  varieties  he  obtained  peas  of  such  and  such  colours 
without  specifying  at  all  whether  the  coats  were  transparent 
or  whether  the  colours  he  saw  were  coat-  or  cotyledon-colours 
is  a  proceeding  fraught  with  peculiar  and  special  risks. 

(1)  Gartner  s  cases.  Professor  Weldon  gives,  as  ex- 
ceptions, a  series  of  Gartner's  observations.  Using  several 
varieties,  amongst  them  Pisum  sativum  macrospermum, 
a  "grey"  pea,  mth  coloured  flowers  and  seed-coats*, 
he  obtained  results  partly  Mendelian  and  partly,  as 
now  alleged,  contradictory.  The  latter  consist  of  seeds 
"  dirty  yellow "  and  "  yellowish  green,"  whereas  it  is 
suggested  they  should  have  been  simply  yellow. 

Now  students  of  this  department  of  natural  history  will 

know  that  these  same  observations  of  Gartner's,  whether 

rightly  or  wrongly,  have  been  doing  duty  for  more  than 

half  a  century  as  stock  illustrations  of  xenia.     In   this 

*  Usually  correlated  characters,  as  Mendel  knew. 


142  A  Defence  of  MendeVs 

capacity  they  have  served  two  generations  of  naturalists. 
The  ground  nowadays  may  be  unfamiliar,  but  others  have 
travelled  it  before  and  recorded  their  impressions.  Darwin, 
for  example,  has  the  following  passage"^  : 

"These  statements  led  Gartner,  who  was  highly  sceptical  on 
the  subject,  carefully  to  try  a  long  series  of  experiments ;  he 
selected  the  most  constant  varieties,  and  the  results  conclusively 
showed  that  the  colour  of  the  shin  of  the  pea  is  modified  when 
pollen  of  a  differently  coloured  variety  is  used."  (The  italics  are 
mine.) 

In  the  true  spirit  of  inquiry  Professor  Weldon  doubtless 
reflected, 

'"Tis  not  Antiquity  nor  Author^ 
That  makes  Truth  Truth,  altho'  Time's  Daughter  ^^  ; 

but  perhaps  a  word  of  caution  to  the  reader  that  another 
interpretation  exists  would  have  been  in  place.  It  cannot 
be  without  amazement  therefore  that  we  find  him  appro- 
priating these  examples  as  referring  to  cotyledon-colour, 
with  never  a  hint  that  the  point  is  doubtful. 

Giltay,  without  going  into  details,  points  out  the 
ambiguity!.  As  Professor  Weldon  refers  to  the  writings 
both  of  Darwin  and  Giltay,  it  is  still  more  remarkable 
that  he  should  regard  the  phenomenon  as  clearly  one  of 
cotyledon-colour  and  not  coat-colour  as  Darwin  and  many 
other  writers  have  supposed. 

*  Animals  and  Plants,  2nd  ed.  1885,  p.  428. 

+  "  Eine  andere  Frage  ist  jedoch,  oh  der  Einfluss  des  Pollens  auf 
den  Keini  schon  dusserlich  an  diesen  letzteren  sichtbar  sein  kann. 
Darwin  fuhrt  mehrere  hierher  gehorige  Fdlle  an,  und  ivahrscheinlich 
sind  auch  die  Resultate  der  von  Gartner  ilber  diesen  Gegenstand  aus- 
gefiihrten  Experimente  hier  zu  erwdhnen,  ivenn  es  auch  nicht  ganz 
deutlich  ist,  oh  der  von  Gartner  erwdhnte  directe  Einfluss  des  Pollens 
sich  nur  innerhalh  der  Grenzen  des  Keimes  merklich  macht  oder  nicht.^^ 
p.  490. 


Principles  of  Heredity  143 

Without  going  further  it  would  be  highly  improbable 
that  Gartner  is  speaking  solely  or  even  chiefly  of  the 
cotyledons,  from  the  circumstance  that  these  observations 
are  given  as  evidence  of  "  the  influence  of  foreign  pollen  on 
the  female  organs^^ ;  and  that  Gartner  was  perfectly  aware  of 
the  fact  that  the  coat  of  the  seed  was  a  maternal  structure 
is  evident  from  his  statement  to  that  effect  on  p.  80. 

To  go  into  the  whole  question  in  detail  would  require 
considerable  space ;  but  indeed  it  is  unnecessary  to  labour 
the  point.  The  reader  who  examines  Gartner's  account 
with  care,  especially  the  peculiar  phenomena  obtained  in 
the  case  of  the  "gxey"  pea  {macrospermuni),  with  specimens 
before  him,  will  have  no  difficulty  in  recognizing  that 
Gartner  is  simply  describing  the  seeds  as  they  looked  in 
their  coats,  and  is  not  attempting  to  distinguish  cotyledon- 
characters  and  coat-characters.  If  he  had  peeled  them, 
which  in  the  case  of  "grey"  peas  would  be  absolutely 
necessary  to  see  cotyledon-colour,  he  must  surely  have 
said  so. 

Had  he  done  so,  he  would  have  found  the  cotyledons 
full  yellow  in  every  ripe  seed ;  for  I  venture  to  assert  that 
anyone  who  tries,  as  we  have,  crosses  between  a  yellow- 
cotyledoned  "grey"  pea,  such  as  Gartner's  was,  with  any 
pure  green  variety  will  see  that  there  is  no  question 
whatever  as  to  absolute  dominance  of  the  yellow  cotyledon- 
character  here,  more  striking  than  in  any  other  case. 
If  exceptions  are  to  be  looked  for,  they  will  not  be  found 
there ;  and,  except  in  so  far  as  they  show  simple  dominance 
of  yellow,  Gartner's  observations  cannot  be  cited  in  this 
connection  at  all. 

(2)  Seton's  case.  Another  exception  given  by  Pro- 
fessor Weldon  is  much  more  interesting  and  instructive. 


144  A  Defence  of  MendeFs 

It  is  the  curious  case  of  Seton"^.     Told  in  the  words  of 
the  critic  it  is  as  follows  : — 

"  Mr  Alexander  Seton  crossed  the  flowers  of  Dwarf  Imperial^ 
'a  well-known  green  variety  of  the  Pea,'  with  the  pollen  of 
'a  white  free-growing  variety.'  Four  hybrid  seeds  were  ob- 
tained, '  which  did  not  differ  in  appearance  from  the  others 
of  the  female  parent.'  These  seeds  therefore  did  not  obey  the 
law  of  dominance,  or  if  the  statement  be  preferred,  greenness 
became  dominant  in  this  case.  The  seeds  were  sown,  and 
produced  plants  bearing  '  green '  and  '  white '  seeds  side  by 
side  in  the  same  pod.  An  excellent  coloured  figure  of  one  of 
these  pods  is  given  {loc.  cit.  Plate  9,  Fig.  1),  and  is  the  only 
figure  I  have  found  which  illustrates  segregation  of  colours  in 
hybrid  Peas  of  the  second  generation." 

Now  if  Professor  Weldon  had  applied  to  this  case  the 
same  independence  of  judgment  he  evinced  in  dismissing 
Darwin's  interpretation  of  Gartner's  observations,  he  might 
have  reached  a  valuable  result.  Knowing  how  difficult  it 
is  to  give  all  the  points  in  a  brief  citation,  I  turned  up  the 
original  passage,  where  I  find  it  stated  that  the  mixed 
seeds  of  the  second  generation  "  were  all  completely  either 
of  one  colour  or  the  other,  none  of  them  having  an  inter- 
mediate tint,  as  Mr  Seton  had  expected."  The  utility  of 
this  observation  of  the  absence  of  intermediates,  is  that  it 
goes  some  way  to  dispose  of  the  suggestion  of  xenia  as  a 
cause  contributing  to  the  result. 

Moreover,  feeling  perfectly  clear,  from  the  fact  of  the 
absence  of  intermediates,  that  the  case  must  be  one  of 
simple  dominance  in  spite  of  first  appearances,  I  suggest 
the  following  account  with  every  confidence  that  it  is 
the    true    one.      There    have    been    several   ^'' Imperials ^^ 

*  Appendix   to  paper   of  Goss,  Trans.  Hort.  Soc.  v.  1822,  pub. 
1824  {not  1848,  as  given  by  Professor  "Weldon),  p.  236. 


Principles  of  Heredity  145 

though  Dwarf  Imperial,  in  a  form  which  I  can  feel  sure 
is  Seton's  form,  I  have  not  succeeded  in  seeing ;  but 
from  Vilmorin's  description  that  the  peas  when  ripe  are 
^franchement  verts^^  I  feel  no  doubt  it  was  a  green  pea 
with  a  green  skin.  If  it  had  had  a  transparent  skin  this 
description  would  be  inapplicable.  Having  then  a  green 
skin,  which  may  be  assumed  with  every  probability  of  truth, 
the  seeds,  even  though  the  cotyledons  were  yellow,  might, 
especially  if  examined  fresh,  be  indistinguishable  from  those 
of  the  maternal  type.  Next  from  the  fact  of  the  mixture 
in  the  second  generation  we  learn  that  the  semi-transparent 
seed-coat  of  the  paternal  form  was  dominant  as  a  plant- 
character,  and  indeed  the  coloured  plate  makes  this  fairly 
evident.  It  will  be  understood  that  this  explanation  is 
as  yet  suggestive,  but  from  the  facts  of  the  second  genera- 
tion, any  supposition  that  there  was  real  irregularity  in 
dominance  in  this  case  is  out  of  the  question*. 

(3)  Tschermak's  exceptions.  These  are  a  much  more 
acceptable  lot  than  those  we  have  been  considering. 
Tschermak  was  thoroughly  alive  to  the  seed-coat  question 
and  consequently  any  exception  stated  as  an  unqualified 
fact  on  his  authority  must  be  accepted.  The  nature  of  these 
cases  we  shall  see.  Among  the  many  varieties  he  used, 
some  being  not  monomorphic,  it  would  have  been  sur- 
prising if  he  had  not  found  true  irregularities  in  dominance. 

(3  a)  Buchsbaum  case.  This  variety,  growing  in  the 
open,  gave  once  a  pod  in  which  every  seed  hut  one  was  green. 
In  stating  this  case  Professor  Weldon  refers  to  Buchsbaum 

*  Since  the  above  passage  was  written  I  find  the  "  Imperials  ^^ 
described  in  "Report  of  Chiswick  Trials,"  Proc.  R.  Hort.  Soc.  1860, 
I.  p.  340,  as  "skin  thick  "  ;  and  on  p.  360  "  skin  thick,  blue  "  ;  which 
finally  disposes  of  this  "  exception." 

B.  10 


146  A  Defence  of  MendeVs 

as  "a  yellow-seeded  variety."  Tschermak"^,  however,  de- 
scribes it  as  having  '^  gelbes,  qfters  gelblich-grunes  Speicker- 
gewehe  "  (cotyledons) ;  and  again  says  the  cotyledon-colour 
is  ^''  aller dings  gerade  hei  Buchsbaum  zur  Spontanvariation 
nach  gelb-grun  neigend!"  The  (!)  is  Tschermak's.  There- 
fore Professor  Weldon  can  hardly  claim  Buchsbaum  as 
"yellow-seeded"  without  qualification. 

Buchsbaum  in  fact  is  in  all  probability  a  blend-form 
and  certainly  not  a  true,  stable  yellow.  One  of  the  green 
seeds  mentioned  above  grew  and  gave  15  yellows  and  three 
greens,  and  the  result  showed  pretty  clearly,  as  Tschermak 
says,  that  there  had  been  an  accidental  cross  with  a  tall 
green. 

On  another  occasion  Telephone  ?  (another  impure 
green)  x  Buchsbaum  gave  four  yellow  smooth  and  two  green 
wrinkled,  but  one  [?both:  the  grammar  is  obscure]  of  the 
greens  did  not  germinate!. 

(3  b)  Telephone  cases.  Telephone,  crossed  with  at  least 
one  yellow  variety  (Auvergne)  gave  all  or  some  green  or 
greenish.  These  1  have  no  doubt  are  good  cases  of 
"  defective  dominance  "  of  yellow.  But  it  must  be  noted 
that  Telephone  is  an  impure  green.  Nominally  a  green,  it 
is  as  Professor  Weldon  has  satisfied  himself,  very  irregular 
in  colour,  having  many  intermediates  shading  to  pure  yellow 
and  many  piebalds.  It  is  the  variety  from  which  alone 
Professor  Weldon  made  his  colour-scale.  /  desire  therefore 
to  call  special  attention  to  the  fact  that  Telephone,  though 

*  (36),  p.  502  and  (37),  p.  663. 

t  Professor  Weldon  should  have  alluded  to  this.  Dead  seeds 
have  no  bearing  on  these  questions,  seeing  that  their  characters  may 
be  pathological.  The  same  seeds  are  later  described  as  "  wie 
Telephone  selbst,"  so,  apart  from  the  possibility  of  death,  they  may 
also  have  been  self-fertilised. 


Principles  of  Heiedity  147 

not  a  pure  green^  Tschermak^s  sample  being  as  he  says 
""  gelhlichweiss  gr'im^'  a  yellowisJi-icliite-green  in  cotyledon- 
colour^  is  the  variety  which  has  so  far  contributed  the 
cleai^est  evidence  of  the  green  colour  dominating  in  its 
crosses  with  a  yellow  \  and  that  Buchsbaum  is  probably  a 
similar  case.  To  tbis  point  we  sball  return.  It  may  not 
be  superfluous  to  mention  also  that  one  cross  between 
Fillbasket  (a  thorough  green)  and  Telephone  gave  three 
yellowish  green  seeds  (Tschermak,  (36),  p.  501). 

(3  c)  Couturier  cases.  This  fully  yellow  variety  in 
crosses  with  two  fully  green  sorts  gave  seeds  either  yellow 
or  greenish  yellow.  In  one  case  Fillbasket  ?  fertilised  by 
Couturier  gave  mixed  seeds,  green  and  yellow.  For  any 
evidence  to  the  contrary,  the  green  in  this  case  may  have 
been  self-fertilised.  Nevertheless,  taking  the  evidence 
together,  I  think  it  is  most  likely  that  Couturier  is  a 
genuine  case  of  imperfect  dominance  of  yellow.  If  so,  it  is 
the  only  true  "exception"  in  crosses  between  stable  forms. 

We  have  now  narrowed  down  Professor  Weldon's 
exceptions  to  dominance  of  cotyledon-colour  to  two  varieties, 
one  yellow  {Couturier),  and  one  yellow  "tending  to  green" 
{Buchsbaum),  which  show  imperfect  dominance  of  yellow ; 
and  one  variety.  Telephone,  an  impure  and  irregular  green, 
which  shows  occasional  but  uncertain  dominance  of  green. 

What  may  be  the  meaning  of  the  phenomenon  shown 
by  the  unstable  or  mosaic  varieties  we  cannot  tell ;  but  I 
venture  to  suggest  that  when  we  more  fully  appreciate  the 
nature  and  genesis  of  the  gametes,  it  will  be  found  that 
the  peculiarities  of  heredity  seen  in  these  cases  have  more 
in  common  with  those  of  ''false  hybridism"  (see  p.  34) 
than  with  any  true  failure  of  dominance. 

Before,  however,  feeling  quite  satisfied  in  regard  even 

10—2 


148  A  Defence  of  MendeVs 

to  this  residuum  of  exceptions,  one  would  wish  to  learn 
the  subsequent  fate  of  these  aberrant  seeds  and  how  their 
offspring  differed  from  that  of  their  sisters.  One  only  of 
them  can  I  yet  trace,  viz.  the  green  seed  from  Telephone  ? 
X  Buchshaum  3 ,  which  proved  a  veritable  "green  dominant." 
As  for  the  remainder,  Tschermak  promises  in  his  first 
paper  to  watch  them.  But  in  his  second  paper  the  only 
passage  I  can  find  relating  to  them  declares  that  perhaps 
some  of  the  questionable  cases  he  mentioned  in  his  first 
paper  ^^  are  attributable  to  similar  isolated  anomalies  in 
dominance;  some  proved  themselves  by  subsequent  cultivation 
to  be  cases  of  accidental  self -fertilisation ;  others  failed  to 
germinate^ y  I  may  warn  those  interested  in  these  ques- 
tions, that  in  estimating  changes  due  to  ripening,  dead 
seeds  are  not  available. 

B.     Seed-coats  and  shapes. 

1.  Seed-coats.  Professor  Weldon  lays  some  stress  on 
the  results  obtained  by  Correnst  in  crossing  a  pea  having 
green  cotyledons  and  a  thin  almost  colourless  coat  {grune 
spate  Erfurter  Folger-erbse)  with  two  purple-flowered 
varieties.  The  latter  are  what  are  known  in  England 
as  "grey"  peas,  though  the  term  grey  is  not  generally 
appropriate. 

In   these  varieties  the  cotyledon-colour  is  yellow  and 

*  '■^Vielleicht  sind  einige  der  I.e.  507  bis  508  erwdhnten  fraglichen 
Fdlle  auf  dhnliche  vereinzelte  Anomalien  der  MerJcmalswerthigkeit 
zu  beziehen  ;  einige  eriviesen  sich  allerdings  beini  Anbau  ah  Producte 
ungeioollter  Selbstbefruchtung,  andere  keimten  nicht." 

t  Kegarding  this  case  I  have  to  thank  Professor  Correns  for  a 
good  deal  of  information  which  he  kindly  sent  me  in  response  to  my 
inquiry.  I  am  thus  able  to  supplement  the  published  account  in 
'■some  particulars. 


Princiijles  of  Heredity  149 

the  coats  are  usually  higlily  coloured  or  orange-brown. 
In  reciprocal  crosses  Correns  found  no  change  from  the 
maternal  seed-coat-colour  or  seed-shape.  On  sowing  these 
peas  he  obtained  plants  bearing  peas  which,  using  the 
terminology  of  Mendel  and  others,  he  speaks  of  as  the  "first 
generation." 

These  peas  varied  in  the  colour  of  their  seed-coats 
from  an  almost  colourless  form  slightly  tinged  with  green 
like  the  one  parent  to  the  orange-brown  of  the  other 
parent.  The  seeds  varied  in  this  respect  not  only  from 
plant  to  plant,  but  from  pod  to  pod,  and  from  seed  to  seed, 
as  Professor  Correns  has  informed  me. 

The  peas  with  more  highly-coloured  coats  were  sown  and 
gave  rise  to  plants  with  seeds  showing  the  whole  range  of 
seed-coat-colours  again. 

Professor  Weldon  states  that  in  this  case  neither  the 
law  of  dominance  nor  the  law  of  segregation  was  observed ; 
and  the  same  is  the  opinion  of  Correns,  who,  as  I  under- 
stand, inclines  to  regard  the  colour-distribution  as  indi- 
cating a  "mosaic"  formation.  This  is  perhaps  conceiv- 
able ;  and  in  that  case  the  statement  that  there  was  no 
dominance  would  be  true,  and  it  would  also  be  true  that 
the  unit  of  segregation,  if  any,  was  smaller  than  the  in- 
dividual plant  and  may  in  fact  be  the  individual  seed. 

A  final  decision  of  this  question  is  as  yet  impossible. 
Nevertheless  from  Professor  Correns  I  have  learnt  one 
point  of  importance,  namely,  that  the  coats  of  all  these 
seeds  were  thick,  like  that  of  the  coloured  and  as  usual 
dominant  form.  There  is  no  "mosaic"  of  coats  like  one 
parent  and  coats  like  the  other,  though  there  may  be  a 
mosaic  of  colours.  In  regard  to  the  distribution  of  colour 
however  the  possibility  does  not  seem  to  me  excluded  that 
we  are  here  dealing  with  changes  influenced  by  conditions. 


150  A  Defence  of  MendeVs 

I  have  grown  a  "grey  "  pea  and  noticed  that  the  seed-coats 
ripened  in  my  garden  differ  considerably  and  not  quite 
uniformly  from  those  received  from  and  probably  ripened 
in  France,  mine  being  mostly  pale  and  greyish,  instead 
of  reddish-brown.  We  have  elsewhere  seen  (p.  120)  that 
pigments  of  the  seed-coat-colour  may  be  very  sensitive  to 
conditions,  and  slight  differences  of  moisture,  for  example, 
may  in  some  measure  account  for  the  differences  in  colour. 
Among  my  crosses  I  have  a  pod  of  such  "  grey  "  peas  ferti- 
lised hy Laxton  s  Alpha  (green  cotyledons,  coat  transparent). 
It  contained  five  seeds,  of  which  four  were  red-hrown  on 
one  side  and  grey  with  purple  specks  on  the  other.  The 
fifth  was  of  the  grey  colour  on  both  sides.  I  regard  this 
difference  not  as  indicating  segregation  of  character  but 
merely  as  comparable  with  the  difference  between  the  two 
sides  of  a  ripe  apple,  and  I  have  little  doubt  that  Correns' 
case  may  be  of  the  same  nature"^.  Phenomena  somewhat 
similar  to  these  will  be  met  with  in  Laxton's  case  of  the 
*' maple"  seeded  peas  (see  p.  161). 

2.  Seed-shapes.  Here  Professor  Weldon  has  three  sets 
of  alleged  exceptions  to  the  rule  of  dominance  of  round 
shape  over  wrinkled.  The  first  are  Rimpau's  cases,  the 
second  are  Tschermak's  cases,  the  third  group  are  cases  of 
*'  grey  "  peas,  which  we  will  treat  in  a  separate  section  (see 
pp.  153  and  158). 

{a)  Rimpau's  cases.  Professor  Weldon  quotes  Rimpau 
as  having  crossed  wrinkled  and  round  peasf  and  found 

*  Mr  Hurst,  of  Burbage,  tells  me  that  in  varieties  having  coats 
green  or  white,  e.g.  American  Wonder,  the  white  coats  are  mostly 
from  early,  the  green  from  later  pods,  the  tints  depending  on 
conditions  and  exposm'e. 

t  In  the  first  case  KnighVs  MaiToiv  with  Victoria,  both  ways  ;  in 
the  second  Victoria  with  Telephone,  both  ways. 


Principles  of  Heredity  151 

the  second  hybrid  generation  dimorphic  as  usual.  The 
wrinkled  peas  were  selected  and  sown  and  gave  wrinkled 
peas  and  round  peas,  becoming  "true"  to  the  wrinkled 
character  in  one  case  only  in  the  fifth  year,  while  in  the 
second  case — that  of  a  Telephone  cross — there  was  a  mixture 
of  round  and  wrinkled  similarly  resulting  from  wrinkled 
seed  for  two  years,  but  the  experiment  was  not  continued. 

These  at  first  sight  look  like  genuine  exceptions.  In 
reality,  however,  they  are  capable  of  a  simple  explanation.  It 
must  be  remembered  that  Kimpau  was  working  in  ignorance 
of  Mendel's  results,  was  not  testing  any  rule,  and  was  not 
on  the  look  out  for  irregularities.  Now  all  who  have 
crossed  wrinkled  and  round  peas  on  even  a  moderate  scale 
will  have  met  with  the  fact  that  there  is  frequently  some 
wrinkling  in  the  cross-bred  seeds.  Though  round  when  com- 
pared with  the  true  wrinkled,  these  are  often  somewhat  more 
wrinkled  than  the  round  type,  and  in  irregular  degrees. 
For  my  own  part  I  fully  anticipate  that  we  may  find  rare 
cases  of  complete  blending  in  this  respect  though  I  do  not 
as  yet  know  one. 

Rimpau  gives  a  photograph  of  eight  peas  (Fig.  146) 
which  he  says  represent  the  wrinkled  form  derived  from 
this  cross.  It  is  evident  that  these  are  not  from  one  pod 
but  a  miscellaneous  selection.  On  close  inspection  it  will 
be  seen  that  while  the  remainder  are  shown  with  their 
coti/ledo7i-sur{&ce^  upwards,  the  two  peas  at  the  lower  end 
of  the  row  are  represented  with  their  hilar-snYhces 
upwards.  Remembering  this  it  will  be  recognized  that 
these  two  lower  peas  are  in  fact  not  fully  wrinkled  peas 
but  almost  certainly  round  "  hybrids,"  and  the  depression 
is  merely  that  which  is  often  seen  in  round  peas  (such  as 
Fillbasket),  squared  by  mutual  pressure.  Such  peas,  when 
sown,  might  of  course  give  some  round. 


152  A  Defence  of  MendeVs 

As  Tschermak  writes  ((37),  p.  658),  experience  has 
shown  him  that  cross-bred  seeds  with  character  transitional 
between  "round"  and  "wrinkled"  behave  as  hybrids,  and 
have  both  wrinkled  and  round  offspring,  and  he  now  reckons 
them  accordingly  with  the  round  dominants. 

Note  further  the  fact  that  Rimpau  found  the  wrinkled 
form  came  true  in  the  fifth  year,  while  the  round  gave  at 
first  more,  later  fewer,  wrinkleds,  not  coming  true  till  the 
ninth  year.  This  makes  it  quite  clear  that  there  was 
dominance  of  the  round  form,  but  that  the  heterozygotes 
were  not  so  sharply  distinguishable  from  the  two  pure 
forms  as  to  be  separated  at  once  by  a  person  not  on  the 
look-out  for  the  distinctions.  Nevertheless  there  was 
sufficient  difference  to  lead  to  a  practical  distinction  of 
the  cross-breds  both  from  the  pure  dominants  and  from 
the  pure  recessives. 

The  Telephone  case  may  have  been  of  the  same  nature  ; 
though,  as  we  have  seen  above,  this  pea  is  peculiar  in  its 
colour-heredity  and  may  quite  well  have  followed  a  different 
rule  in  shape  also.  As  stated  before,  the  wrinkled  off- 
spring were  not  cultivated  after  the  third  3^ear,  but  the 
round  seeds  are  said  to  have  still  given  some  wrinkleds  in 
the  eighth  year  after  the  cross,  as  would  be  expected  in  a 
simple  Mendelian  case. 

(h)  TschermaFs  cases.  The  cases  Professor  Weldon 
quotes  from  Tschermak  all  relate  to  crosses  with  Telephone 
again,  and  this  fact  taken  with  the  certainty  that  the 
colour-heredity  of  Telephone  is  abnormal  makes  it  fairly 
clear  that  there  is  here  something  of  a  really  exceptional 
character.  What  the  real  nature  of  the  exception  is,  and 
how  far  it  is  to  be  taken  as  contradicting  the  "law  of 
dominance,"  is  quite  another  matter. 


Principles  of  Heredity  153 

3.  Other  phenomena,  especially  regarding  seed-shapes, 
in  the  case  of  ^^  grey^^  peas.  Modern  evidence.  Professor 
Welclon  quotes  from  Tschermak  the  interesting  facts  about 
the  "grey"  pea,  Graue  Rlesen,  but  does  not  attempt  to 
elucidate  them.  He  is  not  on  very  safe  ground  in  adducing 
these  phenomena  as  conflicting  with  the  "law  of  dominance." 
Let  us  see  whither  we  are  led  if  we  consider  these  cases. 
On  p.  124  I  mentioned  that  the  classes  round  and  wrinkled 
do  not  properly  hold  if  we  try  to  extend  them  to  large- 
seeded  sorts,  and  that  these  cases  require  separate  con- 
sideration. In  many  of  such  peas,  which  usually  belong 
either  to  the  classes  of  sugar-peas  {mange-touts)  or  "  grey  " 
peas  (with  coloured  flowers),  the  seeds  would  be  rather 
described  as  irregularly  indented,  lumpy  or  stony*,  than  by 
any  use  of  the  terms  round  or  wrinkled.  One  sugar-pea 
{Debarhieux)  which  I  have  used  has  large  flattish,  smooth, 
yellow  seeds  with  white  skins,  and  this  also  in  its  crossings 
follows  the  rules  about  to  be  described  for  the  large-seeded 
"grey  "peas. 

In  the  large  "grey"  peas  the  most  conspicuous  feature 
is  the  seed-coat,  which  is  grey,  brownish,  or  of  a  bright 
reddish  colour.  Such  seed-coats  are  often  speckled  with 
purple,  and  on  boiling  these  seed-coats  turn  dark  brown. 
They  are  in  fact  the  very  peas  used  by  Mendel  in  making 
up  his  third  pair  of  characters.     Regarding  them  Professor 


*  Gartner's  macrospermum  was  evidently  one  of  these,  though 
from  the  further  account  (p.  498)  it  was  probably  more  wrinkled. 
There  are  of  course  mange-touts  which  have  perfectly  round  seeds. 
Mendel  himself  showed  that  the  mange-tout  character,  the  soft 
constricted  pod,  was  transferable.  There  are  also  mange-touts  with 
fully  wrinkled  seeds  and  "grey"  peas  with  small  seeds  (see  Vilmorin- 
Andrieux,  Plantes  Potageres,  1888). 


154  A  Defence  of  Mendel's 

Weldon,  stating  they  may  be  considered  separately,  writes 
as  follows: — 

"Tschermak  has  crossed  Graue  Riesen  with  five  races  of 
P.  sativum,  and  he  finds  that  the  form  of  the  first  hybrid  seeds 
follows  the  female  parent,  so  that  if  races  of  P.  sativum  with 
round  smooth  seeds  be  crossed  with  Graue  Riesen  (which  has 
flattened,  feebly  wrinkled  seeds)  the  hybrids  will  be  round  and 
smooth  or  flattened  and  wrinkled,  as  the  P.  sativum  or  the 
Graue  Riesen  is  used  as  female  parent"^.  There  is  here  a  more 
complex  phenomenon  than  at  first  sight  appears ;  because  if  the 
flowers  of  the  first  hybrid  generation  are  self-fertilised,  the 
resulting  seeds  of  the  second  generation  invariably  resemble 
those  of  the  Graue  Riesen  in  shape,  although  in  colour  they 
follow  Mendel's  law  of  segregation!" 

From  this  account  who  would  not  infer  that  we  have 
here  some  mystery  which  does  not  accord  with  the 
Mendelian  principles?  As  a  matter  of  fact  the  case  is 
dominance  in  a  perfectly  obvious  if  distinct  form. 

Graue  Riesen,  a  large  grey  sugar-pea,  the  pois  sans 
parchemin  geant  of  the  French  seedsmen,  has  full-yellow 
cotyledons  and  a  highly  coloured  seed-coat  of  varying  tints. 
In  shape  the  seed  is  somewhat  flattened  with  irregular 
slight  indentations,  lightly  wrinkled  if  the  term  be  preferred. 
Tschermak  speaks  of  it  in  his  first  paper  as  "  Same  flach, 
zusammengedruckt " — a  flat,  compressed  seed ;  in  his  second 
paper  as  '"''flaclie,  oft  sckwach  gerunzelte  Cotyledonen-form" 
or  cotyledon-shape,  flat,  often  feebly  wrinkled,  as  Professor 
Weldon  translates. 

First-crosses  made  from  this  variety,  each  with  a  differ- 
ent form  of  P.  sativum,  are  stated  on  the  authority  of 
Tschermak's  five  cases,  to  follow  exclusively  the  maternal 
seed-shape.  From  ^^schwach  gerunzelte,^'  "feebly  wrinkled," 
Professor  Weldon  easily  passes  to  "  wrinkled,"  and  tells  us 

*  Correns  found  a  similar  result. 


Principles  of  Heredity  155 

that  according  as  a  round  sativum  or  the  Graue  Riesen  is 
used  as  mother,  the  first-cross  seeds  "will  be  round  and 
smooth  or  flattened  and  wrinkled." 

As  a  matter  of  fact,  however,  the  seeds  of  Graue  Riesen 
though  slightly  wrinkled  do  not  belong  to  the  "  wrinkled  " 
class;  but  if  the  classification  "wrinkled"  and  "round"  is 
to  be  extended  to  such  peas  at  all,  they  belong  to  the  round. 
Mendel  is  careful  to  state  that  his  round  class  are  "  either 
spherical  or  roundish,  the  depressions  on  the  surface,  when 
there  are  any,  always  slight"  ;  while  the  "wrinkled"  class 
are  "irregularly  angular,  deeply  wrinkled"^." 

On  this  description  alone  it  would  be  very  likely  that 
Graue  Riesen  should  fall  into  the  round  class,  and  as  such 
it  behaves  in  its  crosses,  being  dominant  over  wrinkled 
(see  Nos.  3  and  6,  below).  I  can  see  that  in  this  case 
Professor  Weldon  has  been  partly  misled  by  expressions 
of  Tschermak's,  but  the  facts  of  the  second  generation 
should  have  aroused  suspicion.  Neither  author  notices 
that  as  all  five  varieties  crossed  by  Tschermak  with  Graue 
Riesen  were  round,  the  possibilities  are  not  exhausted. 
Had  Tschermak  tried  a  really  wrinkled  sativum  with  Graue 
Riesen  he  would  have  seen  this  obvious  explanation. 

As  some  of  my  own  few  observations  of  first-crosses  bear 
on  this  point  I  may  quote  them,  imperfect  though  they  are. 

I  grew  the  purple-flowered  sugar-pea  ''  Pois  sans  par- 
chemin  geant  a  tres  large  cosse,'^  a  soft-podded  "mange- 
tout"  pea,  flowers  and  seed-coats  coloured,  from  Vilmorin's, 
probably  identical  with  Graue  Riesen. 

1.  One  flower  of  this  variety  fertilised  with  Pois  trh 
nain  de  Bretagne  (very  small  seed;  yellow  cotyledons  ;  very 

*  ^'Entweder  kugelrund  oder  rundlich,  die  Einsenkungen,  wenn 
welche  an  der  Oherfinche  vorkommen,  inimer  nur  seicht,  oder  sie  sind 
unregelmnssig  kantig,  tief  runzlig  (P.  quadratum).^' 


156  A  Defence  of  3IendeVs 

round)  gave  seven  seeds  indistinguishable  (in  their  coats) 
from  those  of  the  mother,  save  for  a  doubtful  increase  in 
purple  pigmentation  of  coats. 

2.  Fertilised  by  Laxtons  Alpha  (green ;  wrinkled ;  coats 
transparent),  two  flowers  gave  11  seeds  exactly  as  above, 
the  purple  being  in  this  case  clearly  increased. 

In  the  following  the  purple  sugar-pea  ws^s  father. 

3.  Laxton's  A  Ipha  (green ;  wrinkled ;  coats  transparent) 
fertilised  by  the  purple  sugar-pea  gave  one  pod  of  four 
seeds  with  yellow  cotyledons  and  round  form. 

4.  Fillhashet  (green ;  smooth  but  squared ;  coats 
green)  fertilised  by  the  purple  sugar-pea  gave  one  pod 
with  six  seeds,  yellow  cotyledons  * ;  Fillhasket  size  and 
shape  ;  but  the  normally  green  coat  yellowed  near  the  hilum 
by  xenia. 

5.  Express  ("  blue  "-green  cotyledons  and  transparent 
skins ;  round)  fertilised  with  purple  sugar-^Qd,  gave  one 
pod  with  four  seeds,  yellow  cotyledons,  shape  round,  much 
as  in  Fillhasket. 

6.  British  Quee7i  (yellow  cotyledons,  wrinkled,  white 
coats)  9  X  purple  sugar-pea  gave  two  pods  with  seven  seeds, 
cotyledons  yellow,  coats  tinged  greenish  (xenia  ?),  all  round. 

So  much  for  the  ''Purple^'  sugar-pea. 

I  got  similar  results  with  Mange-tout  Deharhieux.  This 
is  a  soft-podded  Mange-tout  or  sugar-pea,  with  white  flowers, 
large,  flattish,  smooth  seeds,  scarcely  dimpled ;  yellow  coty- 
ledons. 

*  The  colour  is  the  peculiarly  deep  yellow  of  the  "grey"  mange- 
tout. 


Principles  of  Heredity  157 

7.  Deharhieux  fertilised  hj  Serpette  nain  hlanc  (yellow 
cotyledons ;  wrinkled ;  white  skin ;  dwarf)  gave  one  pod 
with  six  seeds,  size  and  shape  of  Deharhieux,  with  slight 
dimpling. 

8.  Deharhieux  by  nain  de  Bretagne  (very  small ;  yellow 
cotyledons;  very  round)  gave  three  pods,  12  seeds,  all 
yellow  cotyledons,  of  which  two  pods  had  eight  seeds  iden- 
tical in  shape  with  Debarhieux,  while  the  third  had  four 
seeds  like  Debarhieux  but  more  dimpled.  The  reciprocal 
cross  gave  two  seeds  exactly  like  nain  de  Bretagne. 

But  it  may  be  objected  that  the  shape  of  this  large 
grey  pea  is  very  peculiar* ;  and  that  it  maintains  its  type 
remarkably  when  fertilised  by  many  distinct  varieties 
though  its  pollen  effects  little  or  no  change  in  them  ;  for, 
so  long  as  round  varieties  of  sativum  are  used  as  mothers, 
this  is  true  as  we  have  seen.  But  when  once  it  is  under- 
stood that  in  Graue  Biesen  there  is  no  question  of  wrinkling, 
seeing  that  the  variety  behaves  as  a  round  variety,  the 
shape  and  especially  the  size  of  the  seed  must  be  treated 
as  a  maternal  property. 

Why  the  distinction  between  the  shape  of  Graue 
Biesen  and  that  of  ordinary  round  peas  should  be  a  matter 
of  maternal  physiology  we  do  not  know.  The  question  is 
one  for  the  botanical  chemist.  But  there  is  evidently  very 
considerable  regularity,  the  seeds  borne  by  the  cross-breds 
exhibiting  the  form  of  the  "grey"  pea,  which  is  then  a 
dominant  character  as  much  as  the  seed-coat  characters 

*  It  is  certainly  subject  to  considerable  changes  according  to 
conditions.  Those  ripened  in  my  garden  are  without  exception  much 
larger  and  flatter  than  Vilmorin's  seeds  (now  two  years  old)  from 
which  they  grew.  The  colour  of  the  coats  is  also  much  duller.  These 
changes  are  just  what  is  to  be  expected  from  the  English  climate — 
taken  with  the  fact  that  my  sample  of  this  variety  was  late  sown. 


158  A  Defence  of  Menders 

are.  And  that  is  what  Tschermak's  Graue  Riesen  crosses 
actually  did,  thereby  exhibiting  dominance  in  a  very  clear 
form.  To  interject  these  cases  as  a  mystery  without  pointing 
out  how  easily  they  can  be  reconciled  with  the  "law  of 
dominance"  may  throw  an  unskilled  reader  into  gratuitous 
doubt. 

Finally,  since  the  wrinkled  peas,  Laxtons  Alpha  and 
British  Queen,  pollinated  hy  a  large  flat  mange-tout,  witness 
Nos.  3  and  6  above,  became  round  in  both  cases  where  this 
experiment  was  made,  we  here  merely  see  the  usual  domin- 
ance of  the  non- wrinkled  character  ;  though  of  course  if  a 
roz^Tzc^-seeded  mother  be  used  there  can  be  no  departure 
from  the  maternal  shape,  as  far  as  roundness  is  concerned. 

Correns'  observations  on  the  shapes  of  a  "grey"  pea 
crossed  with  a  round  shelling  pea,  also  quoted  by  Professor 
Weldon  as  showing  no  dominance  of  roundness,  are  of 
course  of  the  same  nature  as  those  just  discussed. 

C.     Evidence  of  Knight  and  Laxton. 

In  the  last  two  sections  we  have  seen  that  in  using 
peas  of  the  "grey"  class,  i.e.  with  brown,  red,  or  purplish 
coats,  special  phenomena  are  to  be  looked  for,  and  also 
that  in  the  case  of  large  "  indented  "  peas,  the  phenomena 
of  size  and  shape  may  show  some  divergence  from  that 
simple  form  of  the  phenomenon  of  dominance  seen  when 
ordinary  round  and  wrinkled  are  crossed.  Here  the  fuller 
discussion  of  these  phenomena  must  have  been  left  to  await 
further  experiment,  were  it  not  that  we  have  other  evidence 
bearing  on  the  same  questions. 

The  first  is  that  of  Knight's  well-known  experiments, 
long  familiar  but  until  now  hopelessly  mysterious.  I  have 
not  space  to  quote  the  various  interpretations  which  Knight 
and  others  have  put  upon  them,  but  as  the  Mendelian 


Principles  of  Heredity  159 

principle  at  once  gives  a  complete  account  of  the  whole, 
this  is  scarcely  necessary,  though  the  matter  is  full  of 
historical  interest. 

Crossing  a  white  pea  with  a  very  large  grey  purple- 
flowered  form  Knight  (21)  found  that  the  peas  so  produced 
"were  not  in  any  sensible  degree  different  from  those 
afforded  by  other  plants  of  the  same  [white]  variety ; 
owing,  I  imagine,  to  the  external  covering  of  the  seed  (as 
I  have  found  in  other  plants)  being  furnished  entirely  by 
the  female*."  All  grew  very  tallf,  and  had  colours  of 
male  parent  :j:.    The  seeds  they  produced  were  dark  grey§. 

"  I  had  frequent  occasion  to  observe,  in  this  plant  [the 
hybrid],  a  stronger  tendency  to  produce  purple  blossoms, 
and  coloured  seeds,  than  white  ones  ;  for  when  I  introduced 
the  farina  of  a  purple  blossom  into  a  white  one,  the  whole 
of  the  seeds  in  the  succeeding  year  became  coloured  [viz. 
DR  X  D  giving  DD  and  DK\ ;  but,  when  I  endeavoured 
to  discharge  this  colour,  by  reversing  the  process,  a  part 
only  of  them  afforded  plants  with  white  blossoms ;  this 
part  sometimes  occupying  one  end  of  the  pod,  and  being  at 
times  irregularly  intermixed  with  those  which,  when  sown, 
retained  their  colour  "  [viz.  DR  x  R  giving  DR  and  RR^^ 
(draws  conclusions,  now  obviously  erroneous ||). 

In  this  account  we  have  nothing  not  readily  intelligible 
in  the  light  of  Mendel's  hypothesis. 

The  next  evidence  is  supplied  by  an  exceptionally 
complete  record  of  a  most  valuable  experiment  made  by 

*  Thus  avoiding  the  error  of  Seton,  see  p.  144.  There  is  no  xenia 
perhaps  because  the  seed-coat  of  mother  was  a  transparent  coat. 

t  As  heterozygotes  often  do. 

X  Dominance  of  the  purple  form. 

§  Dominance  of  the  grey  coat  as  a  maternal  character. 

II  Sherwood's  view  {J.  R.  Hort.  Soc.  xxii.  p.  252)  that  this  was  the 
origin  of  the  "Wrinkled"  pea,  seems  very  dubious. 


160  A  Defence  of  MendeVs 

Laxton*.  The  whole  story  is  replete  with  interest,  and  as 
it  not  only  carries  us  on  somewhat  beyond  the  point 
reached  by  Mendel,  but  furnishes  an  excellent  illustration 
of  how  his  principles  may  be  applied,  I  give  the  whole 
account  in  Laxton's  words,  only  altering  the  paragraphing 
for  clearness,  and  adding  a  commentary.  The  paper  ap- 
pears in  Jour.  Hort.  Soc.  N.S.  iii.  1872,  p.  10,  and  very 
slightly  abbreviated  in  Jour,  of  Hart,  xviii.  1870,  p.  86. 
Some  points  in  the  same  article  do  not  specially  relate  to 
this  section,  but  for  simplicity  I  treat  the  whole  together. 

It  is  not  too  much  to  say  that  two  years  ago  the 
whole  of  this  story  would  have  been  a  maze  of  be- 
wildering confusion.  There  are  still  some  points  in  it 
that  we  cannot  fully  comprehend,  for  the  case  is  one  of  far 
more  than  ordinary  complexity,  but  the  general  outlines 
are  now  clear.  In  attempting  to  elucidate  the  phenomena 
it  will  be  remembered  that  there  are  no  statistics  (those 
given  being  inapplicable),  and  the  several  offspring  are 
only  imperfectly  referred  to  the  several  classes  of  seeds. 
This  being  so,  our  rationale  cannot  hope  to  be  complete. 
Laxton  states  that  as  the  seeds  of  peas  are  liable  to  change 
colour  with  keeping,  for  this  and  other  reasons  he  sent  to 
the  Society  a  part  of  the  seeds  resulting  from  his  experi- 
ment before  it  was  brought  to  a  conclusion. 

"  The  seeds  exhibited  were  derived  from  a  single  experiment. 
Amongst  these  seeds  will  be  observed  some  of  several  remarkable 
colours,  including  black,  violet,  purple-streaked  and  spotted, 
maple,  grey,  greenish,  white,  and  almost  every  intermediate  tint, 
the  varied  colours  being  apparently  produced  on  the  outer  coat 
or  envelope  of  the  cotyledons  only. 

*  It  will  be  well  known  to  all  practical  horticulturalists  that 
Laxton,  originally  of  Stamford,  made  and  brought  out  a  large  number 
of  the  best  known  modern  peas.     The  firm  is  now  in  Bedford. 


Principles  of  Heredity  161 

The  peas  were  selected  for  their  colours,  &c.,  from  the  third 
year's  sowing  in  1869  of  the  produce  of  a  cross  in  1866  of  the 
early  round  white-seeded  and  white-flowered  garden  variety 
"  Ringleader,"  which  is  about  2|^  ft.  in  height,  fertilised  by  the 
pollen  of  the  common  purple-flowered  "maple"  pea,  which  is 
taller  than  "  Ringleader,"  and  has  slightly  indented  seeds. 
I  effected  impregnation  by  removing  the  anthers  of  the  seed- 
bearer,  and  applying  the  pollen  at  an  early  stage.  This  cross 
produced  a  pod  containing  five  romid  white  peas,  exactly  like 
the  ordinary  "  Ringleader "  seeds'^. 

In  1867  I  sowed  these  seeds,  and  all  five  produced  tall 
purple-flowered  purplish-stemmed  plants  t,  and  the  seeds,  with 
few  exceptions,  had  all  maple  or  brownish-streaked  envelopes 
of  various  shades  ;  the  remainder  had  entirely  violet  or  deep 
purple-coloured  envelopes  i :  in  shape  the  peas  were  partly  in- 

*  A  round  white  ?  x  grey  c?  giving  the  usual  result,  round,  "  white  " 
(yellow)  seeds. 

t  Tall  heterozygotes,  with  normal  dominance  of  purple  flowers. 

X  Here  we  see  dominance  of  the  pigmented  seed-coat  as  a  maternal 
character  over  white  seed-coat.  The  colours  of  the  seed-coats  are 
described  as  essentially  two  :  maple  or  brown-streaked,  and  violet,  the 
latter  being  a  small  minority.  As  the  sequel  shows,  the  latter  are 
heterozygotes,  not  breeding  true.  Now  Mendel  found,  and  the  fact 
has  been  confirmed  both  by  Correns  and  myself,  that  crossing  a  grey 
pea  which  is  capable  of  producing  purple  leads  to  such  production  as 
a  form  of  xenia. 

We  have  here  therefore  in  the  purple  seeds  the  union  of  dissimilar 
gametes,  with  production  of  xenia.  But  as  the  brown-streaked  seeds 
are  also  in  part  heterozygous,  the  splitting  of  a  compound  allelomorph 
has  probably  taken  place,  though  without  precise  statistics  and 
allotment  of  offspring  among  the  several  seeds  the  point  is  uncertain. 
The  colour  of  seed-coats  in  "  grey  "  peas  and  probably  "  maples  "  also 
is,  as  was  stated  on  p.  150,  sensitive  to  conditions,  but  the  whole 
difference  between  "maples"  and  purple  is  too  much  to  attribute 
safely  to  such  irregularity.  "  Maple  "  is  the  word  used  to  describe 
certain  seed-coats  which  are  pigmented  with  intricate  brown  mottliugs 
on  a  paler  buff  ground.     In  French  they  are  perdrix. 

B.  11 


162  A  Defence  of  MendeVs 

dented  ;  but  a  few  were  round*  Some  of  the  plants  ripened  off 
earlier  than  the  "maple,"  which,  in  comparison  with  "Ring- 
leader," is  a  late  variety ;  and  although  the  pods  were  in  many 
instances  partially  abortive,  the  produce  was  very  large  t. 

In  1868  I  sowed  the  peas  of  the  preceding  year's  growth,  and 
selected  various  plants  for  earliness,  productiveness,  &c.  Some 
of  the  plants  had  light-coloured  stems  and  leaves ;  these  all 
showed  white  flowers,  and  produced  round  white  seeds  |.  Others 
had  purple  flowers,  showed  the  purple  on  the  stems  and  at  the 
axils  of  the  stipules,  and  produced  seeds  with  maple,  grey, 
purple-streaked,  or  mottled,  and  a  few  only,  again,  with  violet- 
coloured  envelopes.  Some  of  the  seeds  were  round,  some  partially 
indented  §.  The  pods  on  each  plant,  in  the  majority  of  instances, 
contained  peas  of  like  characters  ;  but  in  a  few  cases  the  peas  in 
the  same  pod  varied  slightly,  and  in  some  instances  a  pod  or 
two  on  the  same  plant  contained  seeds  all  distinct  from  the 
remainder II .    The  white-flowered  plants  were  generally  dwarfish, 

*  This  is  not,  as  it  stands,  explicable.  It  seems  from  this  point 
and  also  from  what  follows  that  if  the  account  is  truly  given,  some 
of  the  plants  may  have  been  mosaic  with  segregation  of  characters  in 
particular  flowers  ;  but  see  subsequent  note. 

t  As,  commonly,  in  heterozygotes  when  fertile. 

Ij:  Eecessive  in  flower-colour,  seed-coat  colour,  and  in  seed-shape 
as  a  maternal  character  :  pure  recessives  as  the  sequel  proved. 

§  These  are  then  a  mixture  of  pure  dominants  and  cross-bred 
dominants,  and  are  now  inextricably  confused.  This  time  the  round 
seeds  may  have  been  all  on  particular  plants — showing  recessive  seed- 
shape  as  a  maternal  character.  It  seems  just  possible  that  this 
fact  suggested  the  idea  of  "round"  seeds  on  the  coZoure^  plants  in 
the  last  generation.  Till  that  result  is  confirmed  it  should  be 
regarded  as  very  doubtful  on  the  evidence.  But  we  cannot  at  the 
present  time  be  sure  how  much  difference  there  was  between  these 
round  seeds  and  the  normal  maples  in  point  of  shape ;  and  on  the 
whole  it  seems  most  probable  tbat  the  roundness  was  a  mere  fluctua- 
tion, such  as  commonly  occurs  among  the  peas  with  large  indented 
seeds. 

II  Is  this  really  evidence  of  segregation  of  characters,  the  flower 


Prmcii^les  of  Heredity  163 

of  about  the  height  of  "  Kingleader  "  ;  but  the  coloured-flowered 
sorts  varied  altogether  as  to  height,  period  of  ripening,  and 
colour  and  shape  of  seed*.  Those  seeds  with  violet-coloured 
envelopes  jDroduced  nearly  all  maple-  or  parti-coloured  seeds, 
and  only  here  and  there  one  with  a  violet-coloured  envelope  ; 
that  colour,  again,  appeared  only  incidentally,  and  in  a  like 
degree  in  the  produce  of  the  maple-coloured  seeds  f. 

In  1869  the  seeds  of  various  selections  of  the  previous  year 
were  again  sown  separately ;  and  the  white-seeded  peas  again 
produced  only  plants  with  white  flowers  and  round  white  seeds  %. 
Some  of  the  coloured  seeds,  which  I  had  expected  would  produce 
pm'ple-flowered  plants,  produced  plants  with  white  flowers  and 
round  white  seeds  only§  ;  the  majority,  however,  brought  plants 
with  purple  flowers  and  with  seeds  principally  marked  with 
purple  or  grey,  the  maple-  or  brown-streaked  being  in  the 
minority Ij.  On  some  of  the  purple-flowered  plants  were  again 
a  few  pods  with  peas  differing  entirely  from  the  remainder  on 
the  same  plant.  In  some  pods  the  seeds  were  all  white,  in 
others  all  black,  and  in  a  few,  again,  all  violet  IT  ;  but  those  plants 
which  bore  maple-coloured  seeds  seemed  the  most  constant  and 
fixed  in  character  of  the  purple-flowered  seedlings**,  and  the 
purplish  and  grey  peas,  being   of  intermediate  characters,  ap- 

being  the  unit?  In  any  case  the  possibility  makes  the  experiment 
well  worth  repeating,  especially  as  Correns  has  seen  a  phenomenon 
conceivably  similar, 

*  Being  a  mixture  of  heterozygotes  (probably  involving  several 
pairs  of  allelomorphs)  and  homozygotes. 

t  This  looks  as  if  the  violet  colour  was  merely  due  to  irregularity 
of  xenia. 

%  Pure  recessives. 

§  Pure  recessives  in  coats  showing  maternal  dominant  character. 

II  Now  recognized  as  pure  homozygotes. 

IT  This  seems  almost  certainly  segregation  by  flower-units,  and  is 
as  yet  inexplicable  on  any  other  hypothesis.  Especially  paradoxical 
is  the  presence  of  "  white  "  seeds  on  these  plants.  The  impression  is 
scarcely  resistible  that  some  remarkable  phenomenon  of  segregation 
was  really  seen  here. 

**  Being  now  homozygotes. 

11—2 


164  A  Defence  of  MendeVs 

peared  to  vary  most"^.  The  violet-coloured  seeds  again  produced 
almost  invariably  purplish,  grey,  or  maple  peas,  the  clear  violet 
colour  only  now  and  then  appearing,  either  wholly  in  one  pod  or 
on  a  single  pea  or  two  in  a  pod.  All  the  seeds  of  the  purple- 
flowered  plants  were  again  either  round  or  only  partially  in- 
dented ;  and  the  plants  varied  as  to  height  and  earliness.  In 
no  case,  however,  does  there  seem  to  have  been  an  intermediate- 
coloured  flower ;  for  although  in  some  flowers  I  thought  I  found 
the  purple  of  a  lighter  shade,  I  believe  this  was  owing  to  light, 
temperature,  or  other  circumstances,  and  applied  equally  to  the 
parent  maple.  I  have  never  noticed  a  single  tinted  white  flower 
nor  an  indented  white  seed  in  either  of  the  three  years'  produce. 
The  whole  produce  of  the  third  sowing  consisted  of  seeds  of  the 
colours  and  in  the  approximate  quantities  in  order  as  follows, — 
viz. :  1st,  white,  about  half ;  2nd,  purplish,  grey,  and  violet 
(intermediate  colours),  about  three-eighths;  and,  3rd,  maple, 
about  one-eighth. 

From  the  above  I  gather  that  the  white-flowered  white- 
seeded  pea  is  (if  I  may  use  the  term)  an  original  variety  well 
fixed  and  distinct  entirely  from  the  maple,  that  the  two  do  not 
thoroughly  intermingle  (for  whenever  the  white  flower  crops  out, 
the  plant  and  its  parts  all  appear  to  follow  exactly  the  characters 
of  the  white  pea),  and  that  the  maple  is  a  cross-bred  variety 
which  has  become  somewhat  permanent  and  would  seem  to 
include  amongst  its  ancestors  one  or  more  bearing  seeds  either 
altogether  or  partly  violet-  or  purple- colo  ured ;  for  although 
this  colour  does  not  appear  on  the  seed  of  the  "maple,"  it  is 
very  potent  in  the  variety,  and  appears  in  many  parts  of  the 
plant  and  its  offspring  from  cross-fertilised  flowers,  sometimes 
on  the  external  surface  or  at  the  sutures  of  the  pods  of  the 
latter,  at  others  on  the  seeds  and  stems,  and  very  frequently  on 
the  seeds;  and  whenever  it  shows  itself  on  any  part  of  the 
plant,  the  flowers  are  invariably  purple.  My  deductions  have 
been  confirmed  by  intercrosses  effected  between  the  various 
white-,  blue-,  some  singularly  bright  green-seeded  peas  which  I 
have  selected,  and  the  maple-  and  purple-podded  and  the  purple- 
flowered  sugar  peas,  and  by  reversing  those  crosses. 
*  Being  heterozygotes  exclusively. 


Pri7iciples  of  Heredity  165 

I  have  also  deduced  from  my  experiments,  in  accordance 
with  the  conclusions  of  the  late  Mr  Knight  and  others,  that  the 
colours  of  the  envelopes  of  the  seeds  of  peas  immediately 
resulting  from  a  cross  are  never  changed*.  I  find,  however, 
that  the  colour  and  probably  the  substance  of  the  cotyledons 
are  sometimes,  but  not  always,  changed  by  the  cross  fertilisation 
of  two  different  varieties ;  and  I  do  not  agree  with  Mr  Knight 
that  the  form  and  size  of  the  seeds  produced  are  unaltered  t; 
for  I  have  on  more  than  one  occasion  observed  that  the  coty- 
ledons in  the  seeds  directly  resulting  from  a  cross  of  a  blue 
wrinkled  pea  fertilised  by  the  pollen  of  a  white  round  variety 
have  been  of  a  greenish-white  colour  |,  and  the  seeds  nearly 
round  §  and  larger  or  smaller  according  as  there  may  have  been 
a  difference  in  the  size  of  the  seeds  of  the  two  varieties  1 1 . 

I  have  also  noticed  that  a  cross  between  a  round  white  and 
a  blue  wrinkled  pea  will  in  the  third  and  fourth  generations 
(second  and  third  years'  produce)  at  times  bring  forth  blue 
round,  blue  wrinkled,  white  round  and  white  wrinkled  peas  in 
the  same  pods,  that  the  white  round  seeds,  when  again  sown, 
will  produce  only  white  round  seeds,  that  the  white  wrinkled 
seeds  will,  up  to  the  fourth  or  fifth  generation,  produce  both 
blue  and  white  wrinkled  and  round  peas,  that  the  blue  round 
peas  will  produce  blue  wrinkled  and  round  peas,  but  that  the 
blue  wrinkled  peas  will  bear  only  blue  wrinkled  seeds  IT.     This 

*  The  nature  of  this  mistake  is  now  clear ;  for  as  stated  above 
xenia  is  only  likely  to  occur  when  the  maternal  seed-coat  is  pigmented. 
The  violet  coats  in  this  experiment  are  themselves  cases  of  xenia. 

t  Knight,  it  was  seen,  crossed  round  ?  x  indented  s  and  conse- 
quently got  no  change  of  form. 

X  Cotyledons  seen  through  coat. 

§  Ordinary  dominance  of  round. 

II  This  is  an  extraordinary  statement  to  be  given  as  a  general 
truth.  There  are  sometimes  indications  of  this  kind,  but  certainly 
the  facts  are  not  usually  as  here  stated. 

^  If  we  were  obliged  to  suppose  that  this  is  a  matured  conclusion 
based  on  detailed  observation  it  would  of  course  constitute  the  most 
serious   "exception"  yet   recorded.     But   it   is   clear   that   the   five 


166  A  Defence  of  MendeVs 

would  seem  to  indicate  that  the  white  round  and  the  blue 
wrinkled  peas  are  distinct  varieties  derived  from  ancestors 
respectively  possessing  one  only  of  those  marked  qualities;  and, 
in  my  opinion,  the  white  round  peas  trace  their  origin  to  a 
dwarfish  pea  having  white  flowers  and  round  white  seeds,  and 
the  blue  wrinkled  varieties  to  a  tall  variety,  having  also  white 
flowers  but  blue  wrinkled  seeds.  It  is  also  noticeable,  that  from 
a  single  cross  between  two  different  peas  many  hundreds  of 
'  varieties,  not  only  like  one  or  both  parents  and  intermediate, 
but  apparently  differing  from  either,  may  be  produced  in  the 

statements  are  not  mutually  consistent.  We  have  dominance  of 
round  white  in  first  cross. 

In  tbe  second  generation  blue  wrinkled  give  only  bhie  wrinkled, 
and  blue  round  give  blue  wrinkled  and  round,  in  accordance  with 
general  experience.  But  we  are  told  that  white  round  give  only 
white  round.  This  would  be  true  of  some  white  rounds,  but  not, 
according  to  general  experience,  of  all.  Lastly  we  are  told  wldte 
wrinkled  give  all  four  classes.  If  we  had  not  been  just  told  by 
Laxton  that  the  first  cross  showed  dominance  of  white  round,  and 
that  blue  wrinkled  and  blue  round  give  the  Mendelian  result,  I  should 
hesitate  in  face  of  this  positive  statement,  but  as  it  is  inconsistent  with 
the  rest  of  the  story  I  think  it  is  unquestionably  an  error  of  statement. 
The  context,  and  the  argument  based  on  the  maple  crosses  show 
clearly  also  what  was  in  Laxton's  mind.  He  plainly  expected  the 
characters  of  the  original  pure  varieties  to  separate  out  according  to 
their  original  combinations,  and  this  expectation  confused  his 
memory  and  general  impressions.  This,  at  least,  until  any  such 
result  is  got  by  a  fresh  observer,  using  strict  methods,  is  the  only 
acceptable  account. 

Of  the  same  nature  is  the  statement  given  by  the  late  Mr  Masters 
to  Darwin  {Animals  and  Plants,  i.  p.  318)  that  blue  round,  white  round, 
blue  wrinkled,  and  white  wrinkled,  all  reproduced  all  four  sorts  during 
successive  years.  Seeing  that  one  sort  would  give  all  four,  and  two 
would  give  two  kinds,  without  special  counting  such  an  impression 
might  easily  be  produced.  There  are  the  further  difficulties  due  to 
seed-coat  colour,  and  the  fact  that  the  distinction  between  round  and 
wrinkled  may  need  some  discrimination.  The  sorts  are  not  named, 
and  the  case  cannot  be  further  tested. 


Principles  of  Heredity  167 

course  of  three  or  four  years  (the  shortest  time  which  I  have 
ascertained  it  takes  to  attain  the  chmax  of  variation  in  the 
produce  of  cross-fertihsed  peas,  and  until  which  time  it  would 
seem  useless  to  expect  a  fixed  seedling  variety  to  be  produced*), 
although  a  reversion  to  the  characters  of  either  parent,  or  of 
any  one  of  the  ancestors,  may  take  place  at  an  earlier  period. 

These  circumstances  do  not  appear  to  have  been  known  to 
Mr  Knight,  as  he  seems  to  have  carried  on  his  experiments  by 
continuing  to  cross  his  seedlings  in  the  year  succeeding  their 
production  from  a  cross  and  treating  the  results  as  reliable; 
whereas  it  is  probable  that  the  results  might  have  been  materially 
affected  by  the  disturbing  causes  then  in  existence  arising  from 
the  previous  cross  fertilisation,  and  which,  I  consider,  would,  in 
all  cases  where  either  parent  has  not  become  fixed  or  permanent, 
lead  to  results  positively  perplexing  and  uncertain,  and  to  varia- 
ations  almost  innumerable.  1  have  again  selected,  and  intend 
to  sow,  watch,  and  report ;  but  as  the  usual  climax  of  variation 
is  nearly  reached  in  the  recorded  experiment,  I  do  not  anticipate 
much  fm'ther  deviation,  except  in  height  and  period  of  ripening — 
characters  which  are  always  very  unstable  in  the  pea.  There 
are  also  important  botanical  and  other  variations  and  changes 
occurring  in  cross-fertilised  peas  to  which  it  is  not  my 
province  here  to  allu.de;  but  in  conclusion  I  may,  perhaps,  in 
furtherance  of  the  objects  of  this  paper,  be  permitted  to  inquire 
whether  any  light  can,  from  these  observations  or  other  means, 
be  thrown  upon  the  origin  of  the  cultivated  kinds  of  peas, 
especially  the  "  maple  "  variety,  and  also  as  to  the  source  whence 
the  violet  and  other  colours  which  appear  at  intervals  on  the 
seeds  and  in  the  ofis})ring  of  cross-fertilised  purple-flowered  peas 
are  derived." 

The  reader  who  has  closely  followed  the  preceding 
passage  will  begin  to  appreciate  the  way  in  which  the  new 
principles  help  us  to  interpret  these  hitherto  paradoxical 
phenomena.  Even  in  this  case,  imperfectly  recorded  as  it 
is,  we  can  form  a  fairly  clear  idea  of  what  was  taking  place. 

*  See  later. 


168  A  Defence  of  Menders 

If  the  "round"  seeds  really  occurred  as  a  distinct  class,  on 
the  heterozygotes  as  described,  it  is  just  possible  that  the 
fact  may  be  of  great  use  hereafter. 

We  are  still  far  from  understanding  maternal  seed- 
form — and  perhaps  size — as  a  dominant  character.  So  far, 
as  Miss  Saunders  has  pointed  out  to  me,  it  appears  to  be 
correlated  with  a  thick  and  coloured  seed-coat. 


We  have  now  seen  the  nature  of  Professor  Weldon's 
collection  of  contradictory  evidence  concerning  dominance 
in  peas.  He  tells  us:  ''Enough  has  been  said  to  show  the 
grave  discrepancy  between  the  evidence  afforded  by  Mendel's 
experiments  and  that  obtained  by  observers  equally  trust- 
worthy." 

He  proceeds  to  a  discussion  of  the  Telephone  and 
Telegraph  group  and  recites  facts,  which  I  do  not  doubt 
for  a  moment,  showing  that  in  this  group  of  peas — which 
have  unquestionably  been  more  or  less  "blend"  or  "mosaic" 
forms  from  their  beginning — the  "laws  of  dominance  and 
segregation "  do  not  hold.  Professor  Weldon's  collection 
of  the  facts  relating  to  Telephone,  &c.  has  distinct  value, 
and  it  is  the  chief  addition  he  makes  to  our  knowledge 
of  these  phenomena.  The  merit  however  of  this  addition 
is  diminished  by  the  erroneous  conclusion  drawn  from  it,  as 
will  be  shown  hereafter.  Meanwhile  the  reader  who  has 
studied  what  has  been  written  above  on  the  general  questions 
of  stability,  "purity,"  and  "universal"  dominance,  will  easily 
be  able  to  estimate  the  significance  of  these  phenomena  and 
their  applicability  to  Mendel's  hypotheses. 


Pi'inciples  of  Heredity  169 


D.     Miscellaneous  cases  in  other  plants  and'  animals. 

Professor  Weldon  proceeds  : 

"  In  order  to  emphasize  the  need  that  the  ancestry  of  the 
parents,  used  in  crossing,  should  be  considered  in  discussing  the 
results  of  a  cross,  it  may  be  well  to  give  one  or  two  more  ex- 
amples of  fundamental  inconsistency  between  dififerent  competent 
observers." 

The  ''  one  or  two  "  run  to  three,  viz.  Stocks  (hoariness 
and  colour)  ;  Datura  (character  of  fruits  and  colour  of 
flowers) ;  and  lastly  colours  of  Rats  and  Mice.  Each  of 
these  subjects,  as  it  happens,  has  been  referred  to  in  the 
forthcoming  paper  by  Miss  Saunders  and  myself.  Datura 
and  Matthiola  have  been  subjected  to  several  years'  experi- 
ment and  I  venture  to  refer  the  reader  who  desires  to  see 
whether  the  facts  are  or  are  not  in  accord  with  Mendel's 
expectation  and  how  far  there  is  "fundamental  inconsist- 
ency "  amongst  them  to  a  perusal  of  our  work. 

But  as  Professor  Weldon  refers  to  some  points  that 
have  not  been  explicitly  dealt  with  there,  it  will  be  safer 
to  make  each  clear  as  we  proceed. 

1.  Stocks  {Matthiola).  Professor  Weldon  quotes 
Correns'  observation  that  glabrous  Stocks  crossed  with 
hoary  gave  offspring  all  hoary,  while  Trevor  Clarke  thus 
obtained  some  hoary  and  some  glabrous.  As  there  are 
some  twenty  different  sorts  of  Stocks*  it  is  not  surprising 
that  different  observers  should  have  chanced  on  different 
materials  and  obtained  different  results.     Miss  Saunders 

*  The  number  in  Haage  and  Schmidt's  list  exceeds  200,  counting 
colour-varieties. 


170  A  Defence  of  Menders 

has  investigated  laws  of  heredity  in  Stocks  on  a  large 
scale  and  an  account  of  her  results  is  included  in  our 
forthcoming  Report.  Here  it  must  suffice  to  say  that  the 
cross  hoary  ?  x  glabrous  c^  always  gave  offspring  all  hoary 
except  once  :  that  the  cross  glabrous  ?  x  hoary  ^  of  several 
types  gave  all  hoary ;  but  the  same  cross  using  other 
hoary  types  did  frequently  give  a  mixture,  some  of  the 
offspring  being  hoary,  others  glabrous.  Professor  Weldon 
might  immediately  decide  that  here  was  the  hoped  for 
phenomenon  of  "reversed"  dominance,  due  to  ancestry, 
but  here  again  that  hypothesis  is  excluded.  For  the 
glabrous  (recessive)  cross-breds  were  pu?^e,  and  produced 
on  self-fertilisation  glabrous  plants  only,  being  in  fact, 
almost  beyond  question,  "false  hybrids"  (see  p.  34),  a 
specific  phenomenon  which  has  nothing  to  do  with  the 
question  of  dominance. 

Professor  Weldon  next  suggests  that  there  is  discrepancy 
between  the  observations  as  to  flower-colour.  He  tells  us 
that  Correns  found  violet  Stocks  crossed  with  ''yellowish 
white"  gave  violet  or  shades  of  violet  flaked  together. 
According  to  Professor  Weldon 

"  On  the  other  hand  Nobbe  crossed  a  number  of  varieties  of 
M.  annua  in  which  the  flowers  were  white,  violet,  carmine- 
coloured,  crimson  or  dark  blue.  These  were  crossed  in  various 
ways,  and  before  a  cross  was  made  the  colour  of  each  parent  was 
matched  by  a  mixture  of  dry  powdered  colours  which  was  pre- 
served. In  every  case  the  hybrid  flower  was  of  an  intermediate 
colour,  which  could  be  matched  by  mixing  the  powders  which 
recorded  the  parental  colours.  The  proportions  in  which  the 
powders  were  mixed  are  not  given  in  each  [any]  case,  but  it  is 
clear  that  the  colours  blended*." 

*  The  original  passage  is  in  Landwirths.  Versuchstationen,  1888, 
XXXV.  [not  xxxiv.],   p.  151. 


Princiijles  of  Heredity  171 

On  comparing  Professor  Weldon's  version  with  the 
originals  we  find  the  missing  explanations.  Having  served 
some  apprenticeship  to  the  breeding  of  Stocks,  we,  here, 
are  perhaps  in  a  better  position  to  take  the  points,  but 
it  is  to  me  perfectly  inexplicable  how  in  such  a  simple 
matter  as  this  he  can  have  gone  wrong. 

Note  then 

(1)  That  Nobbe  does  not  specify  which  colours  he 
crossed  together,  beyond  the  fact  that  vjhite  was  crossed 
with  each  fertile  form.  The  crimson  form  {Karmoisinfarhe)^ 
being  double  to  the  point  of  sterility,  was  not  used.  There 
remain  then,  white,  carmine,  and  two  purples  (violet,  ''dark 
blue").  When  ivhite  was  crossed  with  either  of  these, 
Nobbe  says  the  colour  becomes  paler,  whichever  sort  gave 
the  pollen.  Nobbe  does  not  state  that  he  crossed  carmine 
with  the  purples. 

(2)  Professor  Weldon  gives  no  qualification  in  his 
version.  Nobbe  however  states  that  he  found  it  very 
difficult  to  distinguish  the  result  of  crossing  calamine  with 
vjhite  from  that  obtained  by  crossing  dark  blue  or  violet 
with  ivhite"^,  thereby  nullifying  Professor  Weldon's  state- 
ment that  in  every  case  the  cross  was  a  simple  mixture  of 
the  parental  colours — a  proposition  sufficiently  disproved  by 
Miss  Saunders'  elaborate  experiments. 

(3)  Lately  the  champion  of  the  "  importance  of  small 
variations,"  Professor  Weldon  now  prefers  to  treat  the 
distinctions    between    established    varieties    as    negligible 

*  "£«  ist  sogar  sehr  schwierig,  einen  Untersehied  in  der  Farhe  der 
Kreuzungsprodukte  von  Karmin  und  Weiss  gegeniiber  Dunkelblau  oder 
Violett  und  Weiss  zu  erkennen.'' 


172  A  Defence  of  MendeVs 

fluctuations  instead  of  specific  phenomena"^.  Therefore 
when  Correns  using  ^'yellowish  white^'  obtained  one  result 
and  Nobbe  using  ''white''  obtained  another,  Professor 
Weldon  hurries  to  the  conclusion  that  the  results  are 
comparable  and  therefore  contradictory.  Correns  however 
though  calling  his  flowers  gelhlich-weiss  is  careful  to  state 
that  they  are  described  by  Haage  and  Schmidt  (the  seed- 
men)  as  ''  schwefel-gelh''  or  sulphur-yellow.  The  topics 
Professor  Weldon  treats  are  so  numerous  that  we  cannot 
fairly  expect  him  to  be  personally  acquainted  with  all ; 
still  had  he  looked  at  Stocks  before  writing,  or  even  at  the 
literature  relating  to  them,  he  would  have  easily  seen  that 
these  yellow  Stocks  are  a  thoroughly  distinct  formf  ;  and 
in  accordance  with  this  fact  it  would  be  surprising  if  they 
had  not  a  distinctive  behaviour  in  their  crosses.  To  use 
our  own  terminology  their  colour  character  depends  almost 
certainly  on  a  compound  allelomorph.  Consequently  there 
is  no  evidence  of  contradiction  in  the  results,  and  appeal  to 
ancestry  is  as  unnecessary  as  futile. 

2.  Datura.  As  for  the  evidence  on  Datura,  I  must 
refer  the  reader  again  to  the  experiments  set  forth  in  our 
Report. 

The  phenomena  obey  the  ordinary  Mendelian  rules  with 
accuracy.    There  are  (as  almost  always  where  discontinuous 

*  See  also  the  case  of  Buchsbaum,  p.  146,  which  received  similar 
treatment. 

+  One  of  the  peculiarities  of  most  double  "  sulphur  "  races  is  that 
the  singles  they  throw  are  white.  See  Vilmorin,  Fleurs  de  pleine 
Terre,  1866,  p.  354,  note.  In  Wien.  III.  Gartenztg.  1891,  p.  74, 
mention  is  made  of  a  new  race  with  singles  also  "sulphur,"  cp. 
Gartenztg.  1884,  p.  46.  Messrs  Haage  and  Schmidt  have  kindly 
written  to  me  that  this  new  race  has  the  alleged  property,  but  that 
six  other  yellow  races  (two  distinct  colours)  throw  their  singles  white. 


Principles  of  Heredity  173 

variation  is  concerned)  occasional  cases  of  "mosaics,"  a 
phenomenon  which  has  nothing  to  do  with  "ancestry." 

3.  Colours  of  Rats  and  Mice.  Professor  Weldon 
reserves  his  collection  of  evidence  on  this  subject  for  the 
last.  In  it  we  reach  an  indisputable  contribution  to  the 
discussion — a  reference  to  Crampe's  papers,  which  together 
constitute  without  doubt  the  best  evidence  yet  published, 
respecting  colour-heredity  in  an  animal.  So  far  as  I  have 
discovered,  the  only  previous  reference  to  these  memoirs  is 
that  of  Eitzema  Bos*,  who  alludes  to  them  in  a  consideration 
of  the  alleged  deterioration  due  to  in-breeding. 

Now  Crampe  through  a  long  period  of  years  made  an 
exhaustive  study  of  the  peculiarities  of  the  colour-forms  of 
Rats,  white,  black,  grey  and  their  piebalds,  as  exhibited  in 
Heredity. 

Till  the  appearance  of  Professor  Weldon' s  article 
Crampe's  work  was  unknown  to  me,  and  all  students  of 
Heredity  owe  him  a  debt  for  putting  it  into  general 
circulation.  My  attention  had  however  been  called  by 
Dr  Correns  to  the  interesting  results  obtained  by  von 
Guaita,  experimenting  with  crosses  originally  made  between 
albino  mice  and  piebald  Japanese  waltzing  mice.  This 
paper  also  gives  fall  details  of  an  elaborate  investigation 
admirably  carried  out  and  recorded. 

In  the  light  of  modern  knowledge  both  these  two 
researches  furnish  material  of  the  most  convincing  character 
demonstrating  the  Mendelian  principles.  It  would  be  a  use- 
ful task  to  go  over  the  evidence  they  contain  and  rearrange 
it  in  illustration  of  the  laws  now  perceived.  To  do  this  here 
is  manifestly  impossible,  and  it  must  suffice  to  point  out 
that  the  albino  is  a  simple  recessive  in  both  cases  (the 

*  Biol  Cblt.  XIV.  1894,  p.  79. 


174  A  Defence  of  MendeVs 

waltzing  character  in  mice  being  also  a  recessive),  and  that 
the  "wild  grey"  form  is  one  of  the  commonest  heterozygotes 
— there  appearing,  like  the  yellow  cotyledon-colour  of  peas, 
in  either  of  two  capacities,  i.e.  as  a  pure  form,  or  as  the 
heterozygote  form  of  one  or  more  combinations"^. 

Professor  Weldon  refers  to  both  Crampe  and  von 
Guaita,  whose  results  show  an  essential  harmony  in  the 
fact  that  both  found  albino  an  obvious  recessive,  pure 
almost  without  exception,  while  the  coloured  forms  show 
various  phenomena  of  dominance.  Both  found  hetero- 
zygous colour-types.  He  then  searches  for  something  that 
looks  like  a  contradiction.  Of  this  there  is  no  lack  in  the 
works  of  Johann  von  Fischer  (11) — an  authority  of  a  very 
different  character — whom  he  quotes  in  the  following 
few  words  : 

"  In  both  rats  and  mice  von  Fischer  says  that  piebald  rats 
crossed  with  albino  varieties  of  their  species,  give  piebald  young 
if  the  father  only  is  piebald,  white  young  if  the  mother  only  is 
piebald." 

But  this  is  doing  small  justice  to  the  completeness  of 
Johann  von  Fischer's  statement,  which  is  indeed  a  pro- 
position of  much  more  amazing  import. 

That  investigator  in  fact  began  by  a  study  of  the  cross 
between  the  albino  Ferret  and  the  Polecat,  as  a  means  of 
testing  whether  they  were  two  species  or  merely  varieties. 
The  cross,  he  found,  was  in  colour  and  form  a  blend  of  the 
parental  types.     Therefore,  he  declares,  the  Ferret  and  the 

*  The  various  "  contradictions  "  which  Professor  Weldon  suggests 
exist  between  Crampe,  von  Guaita  and  Colladon  can  almost  certainly 
be  explained  by  this  circumstance.  For  Professor  Weldon  "  wild- 
coloured"  mice,  however  produced,  are  "wild-coloured"  mice  and 
no  more  (see  Introduction). 


Princi])les  of  Heredity  175 

Polecat  are  two  distinct  species,  because,  "  as  everybody 
ought  to  know," 

"  The  result  of  a  ovss  between  albino  and  normal  [of 
one  species]  is  always  a  constant  one,  namely  an  offspring 
like  the  father  at  least  in  colour  "^^'^ 

whereas  in  crosses  (between  species)  this  is  7iot  the  case. 

And  again,  after  reciting  that  the  Ferret-Polecat  crosses 
gave  intermediates,  he  states  : 

"  But  all  this  is  not  the  case  in  crosses  between  albinos  and 
normal  animals  within  the  species,  in  which  always  and  without 
any  exception  the  young  resemble  the  father  in  colour  t." 

These  are  admirable  illustrations  of  what  is  meant  by 
a  ^^unicersar'  proposition.  But  von  Fischer  doesn't  stop 
here.  He  proceeds  to  give  a  collection  of  evidence  in  proof 
of  this  truth  which  he  says  ''  ought  to  be  known  to  every- 
one." He  has  observed  the  fact  in  regard  to  albino  mole, 
albino  shrew  {&orex  araneus),  melanic  squirrel  {Sciwus 
vulgaris),  albino  ground-squirrel  (Ifypudaeus  te?Testris), 
albino  hamster,  albino  rats,  albino  mice,  piebald  (grey- 
and- white  or  black-and-white)  mice  and  rats,  partially 
albino  sparrow,  and  we  are  even  presented  with  two  cases 
in  Man.     Xo  single  exception  was  known  to  von  Fischer  J. 

*  "Das  Eesultat  einer  Kreuzung  zwischen  Albino-  uud  Normal- 
form  ist  stets,  also,  constant,  ein  dem  Vater  mindestens  in  der 
Farbung  gleiches  Junge."  This  law  is  predicated  for  the  case  in 
which  both  parents  belong  to  the  same  species, 

t  "  Dieses  Alles  ist  aber  nie  der  Fall  bei  Kreuzungen  unter 
Leucismen  und  normalen  Thieren  innerhalb  der  Species,  bei  denen 
stets  und  ohne  jede  Ausnahme  die  Jungen  in  Farbung  dem  Vater 
gleichen.''^ 

X  He  even  withdraws  two  cases  of  his  own  previously  published, 
in  which  grey  and  albino  mice  were  alleged  to  have  given  mixtures, 
saying  that  this  result  must  have  been  due  to  the  broods  having 
been  accidentally  mixed  by  the  servants  in  his  absence. 


176  A  Defence  of  MendeVs 

In  his  subsequent  paper  von  Fischer  declares  that  from 
matings  of  rats  in  which  the  mothers  were  grey  and  the 
fathers  albino  he  bred  2017  pure  albinos  ;  and  from  albino 
mothers  and  grey  fathers  3830  normal  greys.  "Not  a 
single  individual  varied  in  any  respect,  or  was  in  any  way 
intermediate." 

With  piebalds  the  same  result  is  asserted,  save  that 
certain  melanic  forms  appeared.  Finally  von  Fischer 
repeats  his  laws  already  reached,  giving  them  now  in  this 
form  :  that  if  the  offspring  of  a  cross  show  only  the  colour 
of  the  father,  then  the  parents  are  varieties  of  one  species  ; 
hut  if  the  colour  of  the  offspring  be  intermediate  or  different 
from  that  of  the  father,  then  the  parents  belong  to  distinct 
species. 

The  reader  may  have  already  gathered  that  we  have 
here  that  bane  of  the  advocate — the  witness  who"  proves 
too  much.  But  why  does  Professor  Weldon  confine  von 
Fischer  to  the  few  modest  words  recited  above  ?  That 
author  has — so  far  as  colour  is  concerned — a  complete 
law  of  heredity  supported  by  copious  "  observations." 
Why  go  further? 

Professor  Weldon  "brings  forth  these  strong  reasons" 
of  the  rats  and  mice  with  the  introductory  sentence  : 

"  Examples  might  easily  be  multiplied,  but  as  before,  I  have 
chosen  rather  to  cite  a  few  cases  which  rest  on  excellent  authority, 
than  to  quote  examples  which  may  be  doubted.  I  would  only 
add  one  case  among  animals,  in  which  the  evidence  concerning 
the  inheritance  of  colour  is  affected  by  the  ancestry  of  the 
varieties  used." 

So  once  again  Professor  Weldon  suggests  that  his  laws 
of  ancestry  will  explain  even  the  discrepancies  between 
von  Fischer  on  the  one  hand  and  Crampe  and  von  Guaita 


Principles  of  Heredity  177 

on  the  other  but  he  does  not  tell  us  how  he  proposes  to 
apply  them. 

In  the  cross  between  the  albino  and  the  grey  von  Fischer 
tells  us  that  both  colours  appear  in  the  offspring,  but  always, 
without  exception  or  variation,  that  of  the  father  only,  in 
5847  individuals. 

Surely,  the  law  of  ancestry,  if  he  had  a  moment's 
confidence  in  it,  might  rather  have  warned  Professor 
Weldon  that  von  Fischer's  results  were  wrong  somewhere, 
of  which  there  cannot  be  any  serious  doubt.  The  precise 
source  of  error  is  not  easy  to  specify,  but  probably  careless- 
ness and  strong  preconception  of  the  expected  result  were 
largely  responsible,  though  von  Fischer  says  he  did  all  the 
recording  most  carefully  himself. 

Such  then  is  the  evidence  resting  "on  excellent 
authority" :  may  we  some  day  be  privileged  to  see  the 
"  examples  which  may  be  doubted  "  ? 

The  case  of  mice,  invoked  by  Professor  Weldon,  has 
also  been  referred  to  in  our  Report.  Its  extraordinary 
value  as  illustrating  Mendel's  principles  and  the  beautiful 
way  in  which  that  case  may  lead  on  to  extensions  of  those 
principles  are  also  there  set  forth  (see  the  present 
Introduction,  p.  25).  Most  if  not  all  of  such  "conflicting" 
evidence  can  be  reconciled  by  the  steady  application  of 
the  Mendelian  principle  that  the  progeny  will  be  constant 
when — and  only  when* — similar  gametes  meet  in  fertilisa- 
tion, apart  from  any  question  of  the  characters  of  the 
parent  which  produces  those  gametes. 

*  Excluding  *'  false  hybridisations." 


12 


178  A  Defence  of  Mendel's 

V.     Professor  Weldon's  quotations  from  Laxton. 

In  support  of  his  conclusions  Professor  Weldon  adduces 
two  passages  from  Laxton,  some  of  whose  testimony  we 
have  just  considered.  This  further  evidence  of  Laxton 
is  so  important  that  I  reproduce  it  in  full.  The  first 
passage,  published  in  1866,  is  as  follows  : — 

"  The  results  of  experiments  in  crossing  the  Pea  tend  to  show 
that  the  colom*  of  the  immediate  offspring  or  second  generation 
sometimes  follows  that  of  the  female  parent,  is  sometimes 
intermediate  between  that  and  the  male  parent,  and  is  sometimes 
distinct  from  both;  and  although  at  times  it  partakes  of  the 
colour  of  the  male,  it  has  not  been  ascertained  by  the  experimenter 
ever  to  follow  the  exact  colour  of  the  male  parent"^.  In  shape, 
the  seed  frequently  has  an  intermediate  character,  but  as  often 
follows  that  of  either  parent.  In  the  second  generation,  in  a 
single  pod,  the  result  of  a  cross  of  Peas  different  in  shape  and 
colour,  the  seeds  are  sometimes  all  intermediate,  sometimes 
represent  either  or  both  parents  in  shape  or  colour,  and 
sometimes  both  colours  and  characters,  with  their  intermediates, 
appear.  The  results  also  seem  to  show  that  the  third  generation 
or  the  immediate  offspring  of  a  cross,  frequently  varies  from  its 
parents  in  a  limited  manner — usually  in  one  direction  only, 
but  that  the  fourth  generation  produces  numerous  and  wider 
variations  f ;  the  seed  often  reverting  partly  to  the  colour  and 
character  of  its  ancestors  of  the  first  generation,  partly  partaking 
of  the  various  intermediate  colours  and  characters,  and  partly 
sporting  quite  away  from  any  of  its  ancestry." 

*  This  is  of  course  on  account  of  the  maternal  seed  characters. 
Unless  the  coat-characters  are  treated  separately  from  the  cotyledon- 
characters  Laxton's  description  is  very  accurate.  Both  this  and  the 
statements  respecting  the  "  shape  "  of  the  seeds,  a  term  which  as  used 
by  Laxton  means  much  more  than  merely  "wrinkled"  and  "smooth," 
are  recognizably  true  as  general  statements. 

t  Separation  of  hypallelomorphs. 


Principles  of  Heredity  179 

Here  Professor  Weldoii's  quotation  ceases.  It  is  un- 
fortunate he  did  not  read  on  into  the  very  next  sentence 
with  which  the  paragraph  conchides  : — 

"  These  sports  apj^ear  to  become 
fixed  and  permanent  in  the  next  and  succeeding  generations ; 
and  the  tendency  to  revert  and  sport  thenceforth  seems  to 
become  checked  if  not  absohitely  stopped*." 

Now  if  Professor  Weldon  instead  of  leaving  off  on  the 
w^ord  "ancestry"  had  noticed  tliis  passage,  I  think  his  article 
would  never  have  been  written. 

Laxton  proceeds  : — 

"  The  experiments  also  tend  to  show  that  the  height  of  the 
plant  is  singularly  influenced  by  crossing ;  a  cross  between  two 
dwarf  peas,  commonly  producing  some  dwarf  and  some  tall 
[?  in  the  second  generation];  but  on  the  other  hand,  a  cross 
between  two  tall  peas  does  not  exhibit  a  tendency  to  diminution 
in  height. 

"Xo  perceptible  difference  appears  to  result  from  reversing 
the  parents;  the  influence  of  the  pollen  of  each  parent  at  the 
olimax  or  fourth  generation  producing  similar  results  t." 

The  significance  of  this  latter  testimony  I  will  presently 
discuss. 

Professor  Weldon  next  appeals  to  a  later  paper  of 
Laxton's  published  in  1890.    From  it  he  quotes  this  passage  : 

"  By  means,  however,  of  cross-fertilisation  alone,  and  unless  it 
be  followed  by  careful  and  continuous  selection,  the  labours  of 
the  cross- breeder,  instead  of  benefiting  the  gardener,  may  lead 
to  utter  confusion," 

*  The  combinations  being  exhausted.  Perhaps  Professor  Weldon 
thought  his  authority  was  here  lapsing  into  palpable  nonsense ! 

t  Laxton  constantly  refers  to  this  conception  of  the  "climax"  of — 
as  we  now  perceive — analytical  variation  and  recombination.  Many 
citations  could  be  given  respecting  his  views  on  this  "climax"  (cp. 
p.  167). 

12—2 


180  A  Defence  of  MendeVs 

Here  again  the  reader  would  have  gained  had  Professor 
Weldon,  instead  of  leaving  off  at  the  comma,  gone  on  to 
the  end  of  the  paragraph,  which  proceeds  thus  : — 

"because,  as  I  have  previously  stated, 
the  Pea  under  ordinary  conditions  is  much  given  to  sporting 
and  reversion,  for  when  two  dissimilar  old  or  fixed  varieties 
have  been  cross-fertilised,  three  or  four  generations  at  least 
must,  under  the  most  favourable  circumstances,  elapse  before 
the  progeny  will  become  fixed  or  settled ;  and  from  one  such 
cross  I  have  no  doubt  that,  by  sowing  every  individual  Pea 
produced  during  the  three  or  four  generations,  hundreds  of 
different  varieties  may  be  obtained ;  but  as  might  be  expected, 
I  have  found  that  where  the  two  varieties  desired  to  be 
intercrossed  are  unfixed,  confusion  will  become  confounded"^, 
and  the  variations  continue  through  many  generations,  the 
number  at  length  being  utterly  incalculable." 

Professor  Weldon  declares  that  Laxton's  "experience 
was  altogether  different  from  that  of  Mendel."  The  reader 
will  bear  in  mind  that  when  Laxton  speaks  of  fixing  a 
variety  he  is  not  thinking  particularly  of  seed-characters, 
but  of  all  the  complex  characters,  fertility,  size,  flavour, 
season  of  maturity,  hardiness,  etc.,  which  go  to  make  a 
serviceable  pea.  Considered  carefully,  Laxton's  testimony 
is  so  closely  in  accord  with  Mendelian  expectation  that 
I  can  imagine  no  chance  description  in  non-Mendelian 
language  more  accurately  stating  the  phenomena. 

Here  we  are  told  in  unmistakable  terms  the  breaking 
up  of  the  original  combination  of  characters  on  crossing, 
their  re-arrangement,  that  at  the  fourth  or  fifth  generation 
the  possibilities  of  sporting  [sub-division  of  compound 
allelomorphs  and  re-combinations  of  them  ?]  are  exhausted, 
that  there  are  then  definite  forms  which  if  selected  are 

*  Further  subdivision  and  recombination  of  hypallelomorphs. 


Princi27les  of  Heredity  181 

thenceforth  fixed  [produced  by  union  of  similar  gametes  ?] 
that  it  takes  longer  to  select  some  forms  [dominants?] 
than  others  [recessives  ?],  that  there  may  be  "  mule " 
forms'*  or  forms  which  cannot  be  fixed  at  allt  [produced 
by  union  of  dissimilar  gametes?]. 

But  Laxton  tells  us  more  than  this.  He  shows  us  that 
numbers  of  varieties  may  be  obtained — hundreds — "in- 
calculable numbers."  Here  too  if  Professor  "Weldon  had 
followed  Mendel  with  even  moderate  care  he  would  have 
found  the  secret.  For  in  dealing  with  the  crosses  of 
Phaseolus  Mendel  clearly  forecasts  the  conception  of 
compound  characters  themselves  again  consisting  oj  definite 
units,  all  of  which  may  be  separated  and  re-combined  in 
the  possible  combinations,  laying  for  us  the  foundation  of 
the  new  science  of  Analytical  Biology, 

How  did  Professor  Weldon,  after  reading  Mendel,  fail 
to  perceive  these  principles  permeating  Laxton's  facts? 
Laxton  must  have  seen  the  very  things  that  Mendel  saw, 
and  had  he  with  his  other  gifts  combined  that  penetration 
which  detects  a  great  principle  hidden  in  the  thin  mist  of 
"exceptions,"  we  should  have  been  able  to  claim  for  him 
that  honour  which  must  ever  be  Mendel's  in  the  history  of 
discovery. 

When  Laxton  speaks  of  selection  and  the  need  for  it, 
he  means,  what  the  raiser  of  new  varieties  almost  always 
means,  the  selection  of  definite  forms,  not  impalpable 
fluctuations.  When  he  says  that  without  selection  there 
will  be  utter  confusion,  he  means — to  use  Mendelian  terms 

*  For  instance  the  tails  produced  by  crossing  dwarfs  are  such 
"  mules."  Tschermak  found  in  certain  cases  distinct  increase  in 
height  in  such  a  case,  though  not  always  (p.  531). 

t  "The  remarkably  fine  but  unfixable  pea  Evolution,''^  Laxton, 
p.  37. 


182  A  Defence  of  Menders 

— that  the  plant  which  shows  the  desired  combination  of 
characters  must  be  chosen  and  bred  from,  and  that  if  this 
be  not  done  the  grower  will  have  endless  combinations 
mixed  together  in  his  stock.  If  however  such  a  selection 
be  made  in  the  fourth  or  fifth  generation  the  breeder  may 
very  possibly  have  got  a  fixed  form — namely,  one  that  will 
breed  true*.  On  the  other  hand  he  may  light  on  one 
that  does  not  breed  true,  and  in  the  latter  case  it  may  be 
that  the  particular  type  he  has  chosen  is  not  represented 
in  the  gametes  and  will  7iever  breed  true,  though  selected 
to  the  end  of  time.  Of  all  this  Mendel  has  given  us  the 
simple  and  final  account. 

At  Messrs  Sutton  and  Sons,  to  whom  I  am  most 
grateful  for  unlimited  opportunities  of  study,  I  have  seen 
exactly  such  a  case  as  this.  For  many  years  Messrs  Sutton 
have  been  engaged  in  developing  new  strains  of  the  Chinese 
Primrose  (Primula  sinensis,  hort.).  Some  thirty  thoroughly 
distinct  and  striking  varieties  (not  counting  the  Stellata 
or  "Star"  section)  have  already  been  produced  which 
breed  true  or  very  nearly  so.  In  1899  Messrs  Sutton 
called  my  attention  to  a  strain  knowTi  as  "Giant  Lavender," 
a  particularly  fine  form  with  pale  magenta  or  lavender 
flowers,  telling  me  that  it  had  neVer  become  fixed.  On 
examination  it  appeared  that  self-fertilised  seed  saved  from 
this  variety  gave  some  magenta-reds,  some  lavenders,  and 
some  which  are  white  on  opening  but  tinge  with  very  faint 
pink  as  the  flower  matures. 

On  counting  these  three  forms  in  two  successive  years 
the  following  figures  appeared.  Two  separately  bred 
batches  raised  from  "Giant  Lavender"  were  counted  in 
each  year. 

*  Apart  from  fresh  original  variations,  and  perhaps  in  some  cases 
imperfect  homozygosis  of  some  hypallelomorphs. 


Princij^les  of  Heredity  183 


Magenta 

Lavender 

White 

red 

faintly  tinged 

1901  1st  batch 

19 

27 

14 

„      2nd     „ 

9 

20 

9 

1902  1st      „ 

12 

23 

11 

„      2nd     „ 

14 

26 

11 

54  96  45 

The  numbers  54  :  96  :  45  approach  the  ratio  1:2:1 
so  nearly  that  there  can  be  no  doubt  we  have  here  a  simple 
case  of  Mendelian  laws,  operating  without  definite  domi- 
nance, but  rather  with  blending. 

When  Laxton  speaks  of  the  "remarkably  fine  but 
unfixable  pea  Evolution^^  we  now  know  for  the  first  time 
exactly  what  the  phenomenon  meant.  It,  like  the  "Giant 
Lavender,"  was  a  "mule"  form,  not  represented  by  germ- 
cells,  and  in  each  year  arose  by  "self-crossing." 

This  is  only  one  case  among  many  similar  ones  seen  in 
the  Chinese  Primrose.  In  others  there  is  no  doubt  that 
more  complex  factors  are  at  work,  the  subdivision  of 
compound  characters,  and  so  on.  The  history  of  the 
"Giant  Lavender"  goes  back  many  years  and  is  not 
known  with  sufficient  precision  for  our  purposes,  but 
like  all  these  forms  it  originated  from  crossings  among 
the  old  simple  colour  varieties  of  sinensis. 

VI.    The  argument  built  on  exceptions. 

So  much  for  the  enormous  advance  that  the  Mendelian 
principles  already  permit  us  to  make.  But  what  does 
Professor  Weldon  offer  to  substitute  for  all  this  ?   Nothing. 

Professor  Weldon  suggests  that  a  study  of  ancestry 
will  help  us.     Having  recited  Tschermak's  exceptions  and 


184  A  Defence  of  Menders 

the  great  irregularities  seen  in  the   Telephone  group,  he 
writes  : 

"Taking  these  results  together  with  Laxton's  statements, 
and  with  the  evidence  afforded  by  the  Telephone  group  of 
hybrids,  I  think  we  can  only  conclude  that  segregation  of  seed- 
characters  is  not  of  universal  occurrence  among  cross-bred  peas, 
and  that  when  it  does  occur,  it  may  or  may  not  follow  Mendel's 
law." 

Premising  that  when  pure  types  are  used  the  exceptions 
form  but  a  small  part  of  the  whole,  and  that  any  supposed 
absence  of  "segregation"  may  have  been  variation,  this 
statement  is  perfectly  sound.     He  proceeds  : — 

"  The  law  of  segregation,  like  the  law  of  domi- 
nance, appears  therefore  to  hold  only  for  races  of  particular 
ancestry  [my  italics].  In  special  cases,  other  formulae  expressing 
segregation  have  been  offered,  especially  by  De  Vries  and  by 
Tschermak  for  other  plants,  but  these  seem  as  little  likely  to 
prove  generally  valid  as  Mendel's  formula  itself. 

"The  fundamental  mistake  which  vitiates  all  work  based 
upon  Mendel's  method  is  the  neglect  of  ancestry,  and  the 
attempt  to  regard  the  whole  effect  upon  offspring,  produced  by 
a  particular  parent,  as  due  to  the  existence  in  the  parent  of 
particular  structural  characters  ;  while  the  contradictory  results 
obtained  by  those  who  have  observed  the  offspring  of  parents 
identical  in  certain  characters  show  clearly  enough  that  not 
only  the  parents  themselves,  but  their  race,  that  is  their  ancestry, 
must  be  taken  into  account  before  the  result  of  pairing  them  can 
be  predicted." 

In  this  passage  the  Mendelian  view  is  none  too  precisely 
represented.  I  should  rather  have  said  that  it  was  from 
Mendel,  first  of  all  men,  that  we  have  learnt  not  to  regard 
the  effects  produced  on  offspring  "  as  due  to  the  existence 
in  the  parent  of  particular  structural  characters."  We 
have  come  rather  to  disregard  the  particular  structure  of 


Princiijles  of  Heredity  185 

the  parent  except  in  so  far  as  it  may  give  us  a  guide  as  to 
the  nature  of  its  gametes. 

This  indication,  if  taken  in  the  positive  sense — as  was 
sufficiently  shown  in  considering  the  significance  of  the 
"  mule  "  form  or  "  hybrid-character  " — we  now  know  may 
be  absolutely  wortliless,  and  in  any  unfamiliar  case  is  very 
likely  to  be  so.  Mendel  has  proved  that  the  inheritance 
from  individuals  of  identical  ancestry  may  be  entirely 
different :  that  from  identical  ancestry,  without  new 
variation,  may  be  produced  three  kinds  of  individuals 
(in  respect  of  each  pair  of  characters),  namely,  individuals 
capable  of  transmitting  one  type,  or  another  type,  or  both  : 
moreover  that  the  statistical  relations  of  these  three  classes 
of  individuals  to  each  other  will  in  a  great  number  of  cases 
be  a  definite  one  :  and  of  all  this  he  shows  a  complete 
account. 

Professor  Weldon  cannot  deal  with  any  part  of  this 
phenomenon.  He  does  little  more  than  allude  to  it  in 
passing  and  point  out  exceptional  cases.  These  he  suggests 
a  study  of  ancestry  will  explain. 

As  a  matter  of  fact  a  study  of  ancestry  will  give  little 
guide — perhaps  none — even  as  to  the  probability  of  the 
phenomenon  of  dominance  of  a  character,  none  as  to  the 
probability  of  normal  "purity"  of  germ-cells.  Still  less 
will  it  help  to  account  for  fluctuations  in  dominance,  or 
irregularities  in  "purity." 

Ancestry  and  Dominance. 

In  a  series  of  astonishing  paragraphs  (pp.  241-2)  Professor 
Weldon  rises  by  gradual  steps,  from  the  exceptional  facts 
regarding  occasional  dominance  of  green  colour  in  Telephone 
to  suggest  that  the  whole  phenomenon  of  dominance  may  he 


186  A  Defence  of  MendeVs 

attributable  to  ancestry,  and  that  in  fact  one  character  has  no 
natural  dominance  over  another,  apart  from  what  has  been 
created  by  selection  of  ancestry.  This  piece  of  reasoning, 
one  of  the  most  remarkable  examples  of  special  pleading  to 
be  met  with  in  scientific  literature,  must  be  read  as  a  whole. 
I  reproduce  it  entire,  that  the  reader  may  appreciate  this 
curious  effort.  The  remarks  between  round  parenthetical 
marks  are  Professor  Weldon's,  those  between  crotchets  are 
mine. 

"  Mendel  treats  such  characters  as  yellowness  of  cotyledons 
and  the  like  as  if  the  condition  of  the  character  in  two  given 
parents  determined  its  condition  in  all  their  subsequent  off- 
spring*. Now  it  is  well  known  to  breeders,  and  is  clearly  shown 
in  a  number  of  cases  by  Galton  and  Pearson,  that  the  condition 
of  an  animal  does  not  as  a  rule  depend  upon  the  condition  of  any 
one  pair  of  ancestors  alone,  but  in  varying  degrees  upon  the 
condition  of  all  its  ancestors  in  every  past  generation,  the 
condition  in  each  of  the  half-dozen  nearest  generations  having 
a  quite  sensible  effect.  Mendel  does  not  take  the  effect  of 
differences  of  ancestry  into  account,  but  considers  that  any 
yellow-seeded  pea,  crossed  with  any  green-seeded  pea,  will  behave 
in  a  certain  definite  way,  whatever  the  ancestry  of  the  green  and 
yellow  peas  may  have  been.  (He  does  not  say  this  in  words, 
but  his  attempt  to  treat  his  results  as  generally  true  of  the 
characters  observed  is  unintelligible  unless  this  hypothesis  be 
assumed.)  The  experiments  afford  no  evidence  which  can  be 
held  to  justify  this  hypothesis.  His  observations  on  cotyledon 
colour,  for  example,  are  based  upon  58  cross-fertilised  flowers, 
all  of  which  were  borne  upon  ten  plants ;  and  we  are  not  even 
told  whether  these  ten  plants  included  individuals  from  more 
than  two  races. 

"  The  many  thousands  of  individuals  raised  from  these  ten 

*  Mendel,  on  the  contrary,  disregards  the  "  condition  of  the 
character  "  in  the  parent  altogether  ;  but  is  solely  concerned  with  the 
nature  of  the  characters  of  the  gametes. 


Principles  of  Heredity  187 

plants  afford  an  admirable  illustration  of  the  effect  produced 
by  crossing  a  few  pairs  of  plants  of  known  ancestry  ;  but  while 
they  show  this  perhaps  better  than  any  similar  experiment, 
they  do  not  afford  the  data  necessary  for  a  statement  as  to  the 
behaviour  of  yellow-seeded  peas  in  general,  whatever  their 
ancestry,  when  crossed  with  green-seeded  peas  of  any  ancestry. 
[Mendel  of  course  makes  no  such  statement.] 

"  When  this  is  remembered,  the  importance  of  the  exceptions 
to  dominance  of  yellow  cotyledon-colour,  or  of  smooth  and 
rounded  shape  of  seeds,  observed  by  Tschermak,  is  much  in- 
creased; because  although  they  form  a  small  percentage  of  his 
whole  result,  they  form  a  very  large  percentage  of  the  results 
obtained  with  peas  of  certain  races.  [Certainly.]  The  fact  that 
Telephone  behaved  in  crossing  on  the  whole  like  a  green-seeded 
race  of  exceptional  dominance  shows  that  something  other  than 
the  mere  character  of  the  parental  generation  operated  in  this  case. 
Thus  in  eight  out  of  27  seeds  from  the  yellow  Pois  cP Auvergne 
$  X  Telephone  ^  the  cotyledons  were  yellow  with  green  patches ; 
the  reciprocal  cross  gave  two  green  and  one  yellow-and-green 
seed  out  of  the  whole  ten  obtained ;  and  the  cross  Telephone  $ 
X  (yellow-seeded)  Buchshaum^  ^  gave  on  one  occasion  two  green 
and  four  yellow  seeds. 

"So  the  cross  Couturier  (orange-yellow)  $  x  the  green-seeded 
Express  ^  gave  a  number  of  seeds  intermediate  in  colour.  (It 
is  not  clear  from  Tschermak's  paper  whether  all  the  seeds  were 
of  this  colour,  but  certainly  some  of  them  were.)  The  green 
Plein  le  Panier  [Fillbasket]  $  x  Couturier  (^  in  three  crosses 
always  gave  either  seeds  of  colour  intermediate  between  green 
and  yellow,  or  some  yellow  and  some  green  seeds  in  the  same 
pod.  The  cross  reciprocal  to  this  was  not  made ;  but  Express  $ 
X  Couturier  ^  gave  22  seeds  of  which  four  were  yellowish 
green  t. 

"These  facts   show  first  that   Mendel's  law   of  dominance 
conspicuously  fails  for  crosses  between  certain  races,  while  it 

*  Regarding  this  "exception"  see  p.  146. 
t  See  p.  148. 


188  A  Defence  of  Mendel's 

appears  to  hold  for  others ;  and  secondly  that  the  intensity  of  a 
character  in  one  generation  of  a  race  is  no  trustworthy  measure 
of  its  dominance  in  hybrids.  The  obvious  suggestion  is  that  the 
behaviour  of  an  individual  when  crossed  depends  largely  upon 
the  characters  of  its  ancestors'^.  When  it  is  remembered  that 
peas  are  normally  self-fertilised,  and  that  more  than  one  named 
variety  may  be  selected  out  of  the  seeds  of  a  single  hybrid  pod, 
it  is  seen  to  be  probable  that  Mendel  worked  with  a  very  definite 
combination  of  ancestral  characters,  and  had  no  proper  basis  for 
generalisation  about  yellow  and  green  peas  of  any  ancestry" 
[which  he  never  made]. 

Let  us  pause  a  moment  before  proceeding  to  the  climax. 
Let  the  reader  note  we  have  been  told  of  two  groups  of 
cases  in  which  dominance  of  yellow  failed  or  was  ir- 
regular. (Why  are  not  Gartner's  and  Seton's  "  exceptions  " 
referred  to  here  ?)  In  one  of  these  groups  Couturier  was 
always  one  parent,  either  father  or  mother,  and  were  it 
not  for  Tschermak's  own  obvious  hesitation  in  regard  to 
his  own  exceptions  (see  p.  148),  I  would  gladly  believe 
that  Couturier — a  form  I  do  not  know — may  be  an  ex- 
ceptional variety.  How  Professor  Weldon  proposes  to 
explain  its  peculiarities  by  reference  to  ancestry  he  omits 
to  tell  us.  The  Buchshaum  case  is  already  disposed  of, 
for  on  Tschermak's  showing,  it  is  an  unstable  form. 

Happily,  thanks  to  Professor  Weldon,  we  know  rather 
more  of  the  third  case,  that  of  Telephone,  which,  whether 
as  father  or  mother,  w^as  frequently  found  by  Tschermak  to 
give  either  green,  greenish,  or  patchwork-seeds  when  crossed 
with  yellow  varieties.  It  behaves,  in  short,  "like  a  green- 
seeded  pea  of  exceptional  dominance,"  as  we  are  now  told. 
For  this  dominant  quality  of  Telephone's  greenness  we  are 
asked  to  account  hy  appeal  to  its  ancestry.     May  we  not 

*  Where  was  that  "logician,"  the  ** consulting-partner,"  when 
this  piece  of  reasoning  passed  the  firm? 


Principles  of  Heredity  189 

expect,  then,  this  Telephone  to  be — if  not  a  pure-bred  green 
pea  from  time  immemorial — at  least  as  pure-bred  as  other 
green  peas  which  do  not  exhibit  dominance  of  green  at  all  ? 
Now,  what  is  Telephone  ?  Do  not  let  us  ask  too  much. 
Ancestry  takes  a  lot  of  proving.  We  would  not  reject  him 
^^ parce  qu^il  navait  que  soiccante  d:  onze  quartiers^  <£-  que  le 
reste  de  son  arhre  genealogique  avait  ete  perdu  par  V injure 
du  tems.^' 

But  with  stupefaction  we  learn  from  Professor  Weldon 
himself  that  Telephone  is  the  very  variety  which  he  takes 
as  his  type  of  a  permanent  and  incorrigible  mongrel,  a 
character  it  thoroughly  deserves. 

From  Telephone  he  made  his  colour  scale  I  Tschermak 
declares  the  cotyledons  to  be  "yellowish  or  whitish  green, 
often  entirely  bright  yellow*."  So  little  is  it  a  thorough- 
bred green  pea,  that  it  cannot  always  keep  its  own  self- 
fertilised  offspring  green.  Not  only  is  this  pea  a  parti- 
coloured mongrel,  but  Professor  "Weldon  himself  quotes 
Culverwell  that  as  late  as  1882  both  Telegraph  and 
Telephone  '^  will  always  come  from  one  sort,  more  especially 
from  the  gTeen  variety " ;  and  again  regarding  a  supposed 
good  sample  of  Telegraph  that  "  Strange  to  say,  although 
the  peas  were  taken  from  one  lot,  those  sown  in  January 
produced  a  great  proportion  of  the  light  variety  known  as 
Telephone.  These  were  of  every  shade  of  light  green  up  to 
white,  and  could  have  been  shown  for  either  variety,"  Gard. 
Ohron.  1882  (2),  p.  150.  This  is  the  variety  whose  green, 
it  is  suggested,  partially  *'  dominates  "  over  the  yellow  of 
Pois  d' Auvergne,  a  yellow  variety  which  has  a  clear  lineage 
of  about  a  century,  and  probably  more.  If,  therefore,  the 
facts  regarding  Telephone  have  any  bearing  on  the  signi- 

*  "  Speichergeivehe  gelhlich — oder  loeisslich — <7/vV»,  manclimal  auch 
volUtdndig  helUjelh.^'     Tschermak  (36),  p.  480. 


190  A  Defence  of  ^lendeVs 

ficance  of  ancestry,  they  point  the  opposite  way  from  that 
in  which  Professor  Welclon  desires  to  proceed. 

In  view  of  the  evidence,  the  conclusion  is  forced  upon 
me  that  the  suggestion  that  "ancestry"  may  explain  the 
facts  regarding  Telephone  has  no  meaning  behind  it,  but  is 
merely  a  verbal  obstacle.  Two  words  more  on  Telephone. 
On  p.  147  I  ventured  to  hint  that  if  we  try  to  understand 
the  nature  of  the  appearance  of  green  in  the  offspring  of 
Telephone  bred  with  yellow  varieties,  we  are  more  likely  to 
do  so  by  comparing  the  facts  with  those  of  false  hybridi- 
sation than  with  fluctuations  in  dominance.  In  this 
connection  I  would  call  the  reader's  attention  to  a  point 
Professor  Weldon  misses,  that  Tschermak  also  got  yelloivish- 
green  seeds  from  Fillbashet  {green)  crossed  with  Telephone. 
I  suggest  therefore  that  Telephone's  allelomorphs  may  be 
in  part  transmitted  to  its  offspring  in  a  state  which  needs 
no  union  with  any  corresponding  allelomorph  of  the  other 
gamete,  just  as  may  the  allelomorphs  of  "false  hybrids." 
It  would  be  quite  out  of  place  here  to  pursue  this  reasoning, 
but  the  reader  acquainted  with  special  phenomena  of 
heredity  will  probably  be  able  fruitfully  to  extend  it. 
It  will  be  remembered  that  we  have  already  seen  the 
further  fact  that  the  behaviour  of  Telephone  in  respect  to 
seed-shape  was  also  peculiar  (see  p.  152). 

Whatever  the  future  may  decide  on  this  interesting 
question  it  is  evident  that  with  Telephone  (and  possibly 
Buchsbauni)  we  are  encountering  a  specific  phenomenon, 
which  calls  for  specific  elucidation  and  not  a  case  simply 
comparable  with  or  contradicting  the  evidence  of  dominance 
in  general. 

In  this  excursion  we  have  seen  something  more  of  the 
"  exceptions."  Many  have  fallen,  but  some  still  stand, 
though  even  as  to  part  of  the  remainder  Tschermak  enter- 


Principles  of  Heredity  191 

tains  some  doubts,  and,  it  will  be  remembered,  cautions  his 
reader  that  of  his  exceptions  some  may  be  self-fertilisations, 
and  some  did  not  germinate*.  Truly  a  slender  basis  to 
carry  the  coming  structure  ! 

But  Professor  Weldon  cannot  be  warned.  He  told  us 
the  "law  of  dominance  conspicuously  fails  for  crosses 
between  certain  races."  Thence  the  start.  I  venture  to 
give  the  steps  in  this  impetuous  argument.  There  are 
exceptions  t — a  fair  number  if  we  count  the  bad  ones — there 
may  be  more — must  be  more — are  more — no  doubt  many 
more  :  so  to  the  brink.  Then  the  bold  leap  :  may  there 
not  be  as  many  cases  one  way  as  the  other  ?  We  have  not 
tried  half  the  sorts  of  Peas  yet.  There  is  still  hope. 
True  we  know  dominance  of  many  characters  in  some 
hundreds  of  crosses,  using  some  twenty  varieties — not  to 
speak  of  other  plants  and  animals — but  we  do  know  some 
exceptions,  of  which  a  few  are  still  good.     So  dominance 

""  In  his  latest  publication  on  this  subject,  the  notes  to  the 
■edition  of  Mendel  in  Ostwald's  Klassiker  (pp.  60 — 61),  Tschermak, 
who  has  seen  more  true  exceptions  than  any  other  observer,  thus 
refers  to  them.  As  to  dominance: — "Immerhin  kommen  vereinzelt 
nuch  zweifellose  Falle  von  Merkmalmischung ,  d.  h.  Ueherqangsformen 
zicischen  gelber  iind  griiner  Farhe,  runder  und  runzeliger  Form  vor, 
die  sich  in  iceiteren  Generationen  u'ie  dominantmerhnalige  Misolilinge 
verhalten."  As  to  purity  of  the  extracted  recessives: — Ganz  vereinzelt 
scheinen  Ausnahmsfdlle  vorzukommeny 

Kiister  (22)  also  in  a  recent  note  on  Mendelism  points  out,  with 
reason,  that  the  number  of  "exceptions"  to  dominance  that  Ave 
shall  find,  depends  simply  on  the  stringency  with  which  the  supposed 
*'law"  is  drawn.  The  same  writer  remarks  further  that  Mendel 
makes  no  such  rigid  definition  of  dominance  as  his  followers  have 
done. 

t  If  the  "  logiciau-consulting-partner  "  will  successfully  apply  this 
Fallacia  acervalia,  the  "  method  of  the  vanishing  heap,"  to  dominant 
peas,  he  will  need  considerable  leisure. 


192  A  Defence  of  Mendel's 

may  yet  be  all  a  myth,  built  up  out  of  the  petty  facts  those 
purblind  experimenters  chanced  to  gather.  Let  us  take 
wider  views.  Let  us  look  at  fields  more  propitious — more 
what  we  would  have  them  be  !  Let  us  turn  to  eye-colour  : 
at  least  there  is  no  dominance  in  that.  Thus  Professor 
Weldon,  telling  us  that  Mendel  "had  no  proper  basis  for 
generalisation  about  yellow  and  green  peas  of  any  ancestry," 
proceeds  to  this  lamentable  passage  : — 

"Now  in  such  a  case  of  alternative  inheritance  as  that  of 
human  eye-colour,  it  has  been  shown  that  a  number  of  pairs  of 
parents,  one  of  whom  has  dark  and  the  other  blue  eyes,  will 
produce  offspring  of  which  nearly  one  half  are  dark-eyed,  nearly 
one  half  are  blue-eyed,  a  small  but  sensible  percentage  being 
children  with  mosaic  eyes,  the  iris  being  a  patch-work  of 
lighter  and  darker  portions.  But  the  dark-eyed  and  light-eyed 
children  are  not  equally  distributed  among  all  families;  and  it 
would  almost  certainly  be  possible,  by  selecting  cases  of  marriage 
between  men  and  women  of  appropriate  ancestry,  to  demonstrate 
for  their  families  a  law  of  dominance  of  dark  over  light  eye-colour, 
or  of  light  over  dark.  Such  a  law  might  be  as  valid  for  the 
famines  of  selected  ancestry  as  Mendel's  laws  are  for  his  peas 
and  for  other  peas  of  probably  similar  ancestral  history,  but  it 
would  fail  when  apphed  to  dark  and  light-eyed  parents  in 
general, — that  is,  to  parents  of  any  ancestry  who  happen  to 
possess  eyes  of  given  colour." 

The  suggestion  amounts  to  this  :  that  because  there 
are  exceptions  to  dominance  in  peas  ;  and  because  by  some 
stupendous  coincidence,  or  still  more  amazing  incompetence, 
a  bungler  might  have  thought  he  found  dominance  of 
one  eye-colour  whereas  really  there  was  none*;  therefore 

*  I  have  no  doubt  there  is  no  universal  dominance  in  eye-colour. 
Is  it  quite  certain  there  is  no  dominance  at  all?  I  have  searched 
the  works  of  Galton  and  Pearson  relating  to  this  subject  without 
finding  a  clear  proof.     If  there  is  in  them  material  for  this  decision 


Principles  of  Heredity  193 

Professor  TVeldon  is  at  liberty  to  suggest  there  is  a  fair 
chance  that  Mendel  and  all  who  have  followed  him  have 
either  been  the  victims  of  this  preposterous  coincidence  not 
once,  but  again  and  again ;  or  else  persisted  in  the  same 
egregious  and  perfectly  gratuitous  blunder.  Professor 
Weldon  is  skilled  in  the  Calculus  of  Chance  :  will  he 
compute  the  probabilities  in  favour  of  his  hypothesis  ? 


Ancestri/  and  purity  of  germ-cells. 

To  what  extent  ancestry  is  likely  to  elucidate  dominance 
we  have  now  seen.  We  will  briefly  consider  how  laws 
derived  from  ancestry  stand  in  regard  to  segregation  of 
characters  among  the  gametes. 

For  Professor  Weldon  suggests  that  his  view  of  ancestry 
will  explain  the  facts  not  only  in  regard  to  dominance  and 
its  fluctuations  but  in  regard  to  the  purity  of  the  germ-cells. 
He  does  not  apply  this  suggestion  in  detail,  for  its  error 
would  be  immediately  exposed.  In  every  strictly  Mendelian 
case  the  ancestry  of  the  pure  extracted  recessives  or 
dominants,  arising  from  the  breeding  of  first  crosses,  is 
identical  with  that  of  the  impure  dominants  [or  impure 
recessives  in  cases  where  they  exist].  Yet  the  posterity  of 
each  is  wholly  different.  The  pure  extracted  forms,  in 
these  simplest  cases,  are  no  more  likely  to  produce  the 
form  with  which  they  have  been  crossed  than  was  their 
pure  grandparent ;  while  the  impure  forms  break  up  again 
into  both  grand-parental  forms. 

Ancestry  does  not  touch  these  facts  in  the  least.     They 

I  may  perhaps  be  pardoned  for  failing  to  discover  it,  since  the  tabula- 
tions are  not  prepared  with  this  point  in  view.  Eeference  to  the 
original  records  would  soon  clear  up  the  point. 

B.  13 


194  A  Defence  of  MendeVs 

and  others  like  them  have  been  a  stumbling-block  to  all 
naturalists.  Of  such  paradoxical  phenomena  Mendel  now 
gives  us  the  complete  and  final  account.  Will  Professor 
Weldon  indicate  how  he  proposes  to  regard  them? 

Let  me  here  call  the  reader's  particular  attention  to 
that  section  of  Mendel's  experiments  to  which  Professor 
Weldon  does  not  so  much  as  allude.  Not  only  did  Mendel 
study  the  results  of  allowing  his  cross-breds  (DE's)  to 
fertilise  themselves,  giving  the  memorable  ratio 

IBB  :  2BE  :  IBR, 

but  he  fertilised  those  cross-breds  (BE's)  both  with  the 
pure  dominant  (B)  and  with  the  pure  recessive  (B) 
varieties  reciprocally,  obtaining  in  the  former  case  the  ratio 

IBB  :  IBB 

and  in  the  latter  the  ratio 

IBB  :  IBB. 

The  BB  group  and  the  BB  group  thus  produced  giving 
on  self-fertilisation  pure  B  offspring  and  pure  B  offspring 
respectively,  while  the  BB  groups  gave  again 

IBB  :  2BB  :  IBB. 

How  does  Professor  Weldon  propose  to  deal  with  these 
results,  and  by  what  reasoning  can  he  suggest  that 
considerations  of  ancestry  are  to  be  applied  to  them  ? 
If  I  may  venture  to  suggest  what  was  in  Mendel's  mind 
when  he  applied  this  further  test  to  his  principles  it 
was  perhaps  some  such  considerations  as  the  following. 
Knowing  that  the  cross-breds  on  self-fertilisation  give 

IBB  :  2BB  :  1  BB 

three  explanations  are  possible  : 


Principles  of  Heredity  195 

(«)  These  cross-breds  may  produce  pure  D  germs  of 
both  sexes  and  pure  R  germs  of  both  sexes  on  an 
average  in  equal  numbers. 

(h)  Either  the  female,  or  the  male,  gametes  may  be 
alone  differentiated  according  to  the  allelomorphs, 
into  pure  i)'s,  pure  i^'s,  and  crosses  DR  or  RD,  the 
gametes  of  the  other  sex  being  homogeneous  and 
neutral  in  regard  to  those  allelomorphs. 

(c)  There  may  be  some  neutralisation  or  cancelling 
between  characters  in  fertilisation  occurring  in  such 
a  way  that  the  well-known  ratios  resulted.  The 
absence  of  and  inability  to  transmit  the  D  character 
in  the  RR's,  for  instance,  might  have  been  due 
not  to  the  original  purity  of  the  germs  constituting 
them,  but  to  some  condition  incidental  to  or  connected 
with  fertilisation. 

It  is  clear  that  Mendel  realized  (b)  as  a  possibility,  for 
he  says  DR  was  fertilised  with  the  pure  forms  to  test  the 
composition  of  its  egg-cells,  but  the  reciprocal  crosses  were 
made  to  test  the  composition  of  the  pollen  of  the  hybrids. 
Readers  familiar  with  the  literature  will  know  that  both 
Gartner  and  Wichura  had  in  many  instances  shown  that 
the  offspring  of  crosses  in  the  form  (axb)^  xc 6  were  less 
variable  than  those  of  crosses  in  the  form  a  ?  x  (6  x  c)  c^ , 
&c.  This  important  fact  in  many  cases  is  observed,  and 
points  to  differentiation  of  characters  occurring  frequently 
among  the  male  gametes  when  it  does  not  occur  or  is  much 
less  marked  among  the  maternal  gametes.  Mendel  of 
course  knew  this,  and  proceeded  to  test  for  such  a  possi- 
bility, finding  by  the  result  that  differentiation  was  the 
same  in  the  gametes  of  both  sexes*. 

*  See  Wichura  (46),  pp.  55-6. 

13—2 


196  A  Defence  of  MendeVs 

Of  hypotheses  (h)  and  (c)  the  results  of  recrossmg  with 
the  two  pure  forms  dispose ;  and  we  can  suggest  no 
hypothesis  but  {a)  which  gives  an  acceptable  account  of  the 
facts. 

It  is  the  purity  of  the  "  extracted  "  recessives  and  the 
"  extracted  "  dominants — primarily  the  former,  as  being 
easier  to  recognize — that  constitutes  the  real  proof  of  the 
validity  of  Mendel's  principle. 

Using  this  principle  we  reach  immediately  results  of 
the  most  far-reaching  character.  These  theoretical  de- 
ductions cannot  be  further  treated  here — but  of  the 
practical  use  of  the  principle  a  word  may  be  said.  Where- 
ever  there  is  marked  dominance  of  one  character  the 
breeder  can  at  once  get  an  indication  of  the  amount  of 
trouble  he  will  have  in  getting  his  cross-bred  true  to  either 
dominant  or  recessive  character.  He  can  only  thus  fore- 
cast the  future  of  the  race  in  regard  to  each  such  pair  of 
characters  taken  severally,  but  this  is  an  immeasurable 
advance  on  anything  we  knew  before.  More  than  this,  it 
is  certain  that  in  some  cases  he  will  be  able  to  detect  the 
"mule"  or  heterozygous  forms  by  the  statistical  frequency 
of  their  occurrence  or  by  their  structure,  especially  when 
dominance  is  absent,  and  sometimes  even  in  cases  where 
there  is  distinct  dominance.  With  peas,  the  practical 
seedsman  cares,  as  it  happens,  little  or  nothing  for  those 
simple  characters  of  seed-structure,  &c.  that  Mendel  dealt 
with.  He  is  concerned  with  size,  fertility,  flavour,  and 
numerous  similar  characters.  It  is  to  these  that  Laxton 
(invoked  by  Professor  Weldon)  primarily  refers,  when  he 
speaks  of  the  elaborate  selections  which  are  needed  to  fix 
his  novelties. 

We  may  now  point  tentatively  to  the  way  in  which 
some  even  of  these  complex  cases  may  be  elucidated  by  an 


Principles  of  Heredity  197 

extension  of  Mendel's  principle,  though  we  cannot  forget 
that  there  are  other  undetected  factors  at  work. 


The  value  of  the  appeal  to  Ancestry. 

But  it  may  be  said  that  Professor  Weldon's  appeal  to 
ancestry  calls  for  more  specific  treatment.  When  he 
suggests  ancestry  as  "one  great  reason"  for  the  different 
properties  displayed  by  different  races  or  individuals,  and 
as  providing  an  account  of  other  special  phenomena  of 
heredity,  he  is  perhaps  not  to  be  taken  to  mean  any 
definite  ancestry,  known  or  h)rpothetical.  He  may,  in 
fact,  be  using  the  term  "ancestry"  merely  as  a  brief 
equivalent  signifying  the  previous  history  of  the  race  or 
individual  in  question.  But  if  such  a  plea  be  put  forward, 
the  real  utility  and  value  of  the  appeal  to  ancestry  is 
even  less  evident  than  before. 

Ancestry,  as  used  in  the  method  of  Galton  and  Pearson, 
means  a  definite  thing.  The  whole  merit  of  that  method 
lies  in  the  fact  that  by  it  a  definite  accord  could  be  proved 
to  exist  between  the  observed  characters  and  behaviour 
of  specified  descendants  and  the  ascertained  composition 
of  their  pedigree.  Professor  Weldon  in  now  attributing 
the  observed  peculiarities  of  Telephone  &c.  to  conjectural 
peculiarities  of  pedigree — if  this  be  his  meaning — renounces 
all  that  had  positive  value  in  the  reference  to  ancestry. 
His  is  simply  an  appeal  to  ignorance.  The  introduction  of 
the  word  "ancestry"  in  this  sense  contributes  nothing. 
The  suggestion  that  ancestry  might  explain  peculiarities 
means  no  more  than  "we  do  not  know  how  peculiarities  are 
to  be  explained."  So  Professor  Weldon's  phrase  "peas  of 
probably  similar  ancestral  history*"  means  "peas  probably 
*  See  above,  p.  192. 


198  A  Defence  of  MendeVs 

similar  "  ;  when  he  speaks  of  Mendel  having  obtained  his 
results  with  "  a  few  pairs  of  plants  of  known  ancestry"^,"  he 
means  "a  few  pairs  of  known  plants"  and  no  more ;  when 
he  writes  that  "the  law  of  segregation,  like  the  law  of 
dominance  appears  to  hold  only  for  races  of  particular 
ancestry!,"  the  statement  loses  nothing  if  we  write  simply 
"  for  particular  races."  We  all  know — the  Mendelian,  best 
of  all — that  particular  races  and  particular  individuals 
may,  even  though  indistinguishable  by  any  other  test, 
exhibit  peculiarities  in  heredity. 

But  though  on  analysis  those  introductions  of  the  word 
"ancestry"  are  found  to  add  nothing,  yet  we  can  feel  that 
as  used  by  Professor  Weldon  they  are  intended  to  mean  a 
great  deal.  Though  the  appeal  may  be  confessedly  to 
ignorance,  the  suggestion  is  implied  that  if  we  did  know 
the  pedigrees  of  these  various  forms  we  should  then  have 
some  real  light  on  their  present  structure  or  their  present 
behaviour  in  breeding.  Unfortunately  there  is  not  the 
smallest  ground  for  even  this  hope. 

As  Professor  Weldon  himself  tells  us  J,  conclusions  from 
pedigree  must  be  based  on  the  conditions  of  the  several 
ancestors ;  and  even  more  categorically  (p.  244),  "  The 
degree  to  which  a  parental  character  affects  offspring  depends 
not  only  upon  its  development  in  the  individual  parent ,  hut 
on  its  degree  of  development  in  the  ancestors  of  that  parent.  ^^ 
[My  italics.]  Having  rehearsed  this  profession  of  an  older 
faith  Professor  Weldon  proceeds  to  stultify  it  in  his  very 
next  paragraph.  For  there  he  once  again  reminds  us  that 
Telephone,  the  mongrel  pea  of  recent  origin,  which  does  not 
breed  true  to  seed  characters,  has  yet  manifested  the  peculiar 
power  of  stamping  the  recessive  characters  on  its  cross-bred 

*  See  above,  p.  187.  f  See  above,  p.  184. 

\  See  above,  p.  186. 


Principles  of  Heredity  199 

offspring,  though  pure  and  stable  varieties  that  have 
exhibited  the  same  characters  in  a  high  degree  for 
generations  have  not  that  power.  As  we  now  know,  the 
presence  or  absence  of  a  character  in  a  progenitor  may  be 
no  indication  whatever  as  to  the  probable  presence  of  the 
character  in  the  offspring ;  for  the  characters  of  the  latter 
depend  on  gametic  and  not  on  zygotic  differentiation. 

The  problem  is  of  a  different  order  of  complexity  from 
that  w^hich  Professor  Weldon  suggests,  and  facts  like  these 
justify  the  affirmation  that  if  we  could  at  this  moment 
bring  together  the  whole  series  of  individuals  forming  the 
pedigree  of  Telephone,  or  of  any  other  plant  or  animal 
known  to  be  aberrant  as  regards  heredity,  we  should  have 
no  more  knowledge  of  the  nature  of  these  aberrations ;  no 
more  prescience  of  the  moment  at  which  they  would  begin, 
or  of  their  probable  modes  of  manifestation ;  no  more 
criterion  in  fact  as  to  the  behaviour  such  an  individual 
would  exhibit  in  crossing*,  or  solid  ground  from  which  to 
forecast  its  posterity,  than  we  have  already.  We  should 
learn  then — what  we  know  already — that  at  some  parti- 
cular point  of  time  its  peculiar  constitution  was  created, 
and  that  its  peculiar  properties  then  manifested  themselves : 
how  or  why  this  came  about,  we  should  no  more  compre- 
hend with  the  full  ancestral  series  before  us,  than  we  can 
in  ignorance  of  the  ancestry.  Some  cross-breds  follow 
Mendelian  segregation ;  others  do  not.  In  some,  palpable 
dominance  appears  ;  in  others  it  is  absent. 

If  there  were  no  ancestry,  there  would  be  no  posterity. 
But  to  answer  the  question  why  certain  of  the  posterity 
depart  from  the  rule  which  others  follow,  we  must  know, 
not  the  ancestry,  but  how  it  came  about  either  that  at  a 

*  Beyond  an  indication  as  to  the  homogeneity  or  "purity"  of  its 
gametes  at  a  given  time. 


200  A  Defence  of  Menders 

certain  moment  a  certain  gamete  divided  from  its  fellows  in 
a  special  and  unwonted  fashion ;  or,  though  the  words  are 
in  part  tautological,  the  reason  why  the  union  of  two  par- 
ticular gametes  in  fertilisation  took  place  in  such  a  way  that 
gametes  having  new  specific  properties  resulted*.  No  one 
yet  knows  how  to  use  the  facts  of  ancestry  for  the  elucida- 
tion of  these  questions,  or  how  to  get  from  them  a  truth 
more  precise  than  that  contained  in  the  statement  that  a 
diversity  of  specific  consequences  (in  heredity)  may  follow 
an  apparently  single  specific  disturbance.  Rarely  even  can 
we  see  so  much.  The  appeal  to  ancestry,  as  introduced  by 
Professor  Weldon,  masks  the  difficulty  he  dare  not  face. 

In  other  words,  it  is  the  cause  of  variation  we  are  here 
seeking.  To  attack  that  problem  no  one  has  yet  shown  the 
way.  Knowledge  of  a  different  order  is  wanted  for  that 
task ;  and  a  compilation  of  ancestry,  valuable  as  the 
exercise  may  be,  does  not  provide  that  particular  kind 
of  knowledge. 

Of  course  when  once  we  have  discovered  by  experiment 
that — say.  Telephone — manifests  a  peculiar  behaviour  in 
heredity,  we  can  perhaps  make  certain  forecasts  regarding 
it  with  fair  correctness ;  but  that  any  given  race  or 
individual  will  behave  in  such  a  way,  is  a  fact  not 
deducible  from  its  ancestry,  for  the  simple  reason  that 
organisms  of  identical  ancestry  may  behave  in  wholly 
distinct,  though  often  definite,  ways. 

It  is  ficom  this  hitherto  hopeless  paradox  that  Mendel 
has  begun  at  last  to  deliver  us.  The  appeal  to  ancestry  is 
a  substitution  of  darkness  for  light. 

*  May  there  be  a  connection  between  the  extraordinary  fertility 
and  success  of  the  Telephone  grou-p  of  peas,  and  the  peculiar  frequency 
of  a  blended  or  mosaic  condition  of  their  allelomorphs?  The  con- 
jecture may  be  wild,  but  it  is  not  impossible  that  the  two  phenomena 
may  be  interdependent. 


Principles  of  Heredity  201 

VII.     The  question  of  absolute  purity  of  germ-cells. 

But  let  us  go  back  to  the  cases  of  defective  "purity" 
aud  consider  how  the  laws  of  ancestry  stand  in  regard  to 
them.  It  appears  from  the  facts  almost  certain  that  purity 
may  sometimes  be  wanting  in  a  character  which  elsewhere 
usually  manifests  it. 

Here  we  approach  a  question  of  greater  theoretical 
consequence  to  the  right  apprehension  of  the  part  borne 
by  Mendelian  principles  in  the  physiology  of  heredity. 
We  have  to  consider  the  question  whether  the  purity  of 
the  gametes  in  respect  of  one  or  other  antagonistic  character 
is  or  is  likely  to  be  in  case  of  any  given  character  a 
universal  truth  ?  The  answer  is  unquestionably — No — ^but 
for  reasons  in  which  "ancestry"  plays  no  part*. 

Hoping  to  interest  English  men  of  science  in  the 
Mendelian  discoveries  I  offered  in  November  1900  a  paper 
on  this  subject  to  "  Nature."  The  article  was  of  some 
length  and  exceeded  the  space  that  the  Editor  could  grant 
without  delay.  I  did  not  see  my  way  to  reduce  it  without 
injury  to  clearness,  and  consequently  it  was  returned  to 
me.  At  the  time  our  own  experiments  were  not  ready  for 
publication  and  it  seemed  that  all  I  had  to  say  would 
probably  be  common  knowledge  in  the  next  few  weeks,  so 
no  further  attempt  at  publication  was  made. 

In  that  article  I  discussed  this  particular  question  of 
the  absolute  purity  of  the  germ-cells,  showing  how,  on 
the  analogy  of  other  bud-variations,  it  is  almost  certain 
that  the  germ-cells,  even  in  respect  to  characters  normally 
Mendelian,  may  on  occasion  present  the  same  mixture  of 
characters,  whether  apparently  blended  or  mosaic,  which 

*  This  discussion  leaves   "false   hybridism"  for   separate   con- 
sideration. 


202  A  Defence  of  MendeVs 

we  know  so  well  elsewhere.  Such  a  fact  would  in  nowise 
diminish  the  importance  of  Mendel's  discovery.  The  fact 
that  mosaic  peach-nectarines  occur  is  no  refutation  of  the 
fact  that  the  total  variation  is  common.  Just  as  there 
may  be  trees  with  several  such  mosaic  fruits,  so  there  may 
be  units,  whether  varieties,  individual  plants,  flowers  or 
gonads,  or  other  structural  units,  bearing  mosaic  egg-cells 
or  pollen  grains.  Nothing  is  more  likely  or  more  in 
accordance  with  analogy  than  that  by  selecting  an  in- 
dividual producing  germs  of  blended  or  mosaic  character, 
a  race  could  be  established  continuing  to  produce  such 
germs.  Persistence  of  such  blends  or  mosaics  in  asexual 
reproduction  is  well-known  to  horticulturists ;  for  example 
"bizarre"  carnations,  oranges  streaked  with  " blood "- 
orange  character,  and  many  more.  In  the  famous  paper  of 
Naudin,  who  came  nearer  to  the  discovery  of  the  Mendelian 
principle  than  any  other  observer,  a  paper  quoted  by 
Professor  Weldon,  other  examples  are  given.  These  forms, 
once  obtained,  can  be  multiplied  hy  division ;  and  there  is 
no  reason  why  a  zygote  formed  by  the  union  of  mosaic  or 
blended  germs,  once  arisen,  should  not  in  the  cell-divisions 
by  which  its  gametes  are  formed,  continue  to  divide  in  a 
similar  manner  and  produce  germs  like  those  which  united 
to  form  that  zygote.  The  irregularity,  once  begun,  may 
continue  for  an  indefinite  number  of  divisions. 

I  am  quite  willing  to  suppose,  with  Professor  Weldon 
(p.  248),  that  the  pea  Stratagem  may,  as  he  suggests,  be 
such  a  case.  I  am  even  willing  to  accept  provisionally  as 
probable  that  when  two  gametes,  themselves  of  mosaic  or 
blended  character,  meet  together  in  fertilisation,  they  are 
more  likely  to  produce  gametes  of  mosaic  or  blended 
character  than  of  simply  discontinuous  character.  Among 
Messrs  Sutton's  Primulas  there  are  at  least  two  striking 


Principles  of  Heredity  203 

cases  of  "flaked"  or  "bizarre"  unions  of  bright  colours 
and  white  which  reproduce  themselves  by  seed  with  fair 
constancy,  though  Mendelian  purity  in  respect  of  these 
colours  is  elsewhere  common  in  the  varieties  (I  suspect 
mosaics  of  "  false  hybridism  "  among  allelomorphs  in  some 
of  these  cases).  Similarly  Galton  has  shown  that  though 
children  having  one  light-eyed  and  one  dark-eyed  parent 
generally  have  eyes  either  light  or  dark,  the  comparatively 
rare  medium  eye-coloured  persons  when  they  mate  together 
frequently  produce  children  with  medium  eye-colour. 

In  this  connection  it  may  be  worth  while  to  allude  to  a 
point  of  some  practical  consequence.  We  know  that  when 
pure  dominant — say  yellow — is  crossed  with  pure  recessive 
— say  green — the  dominance  of  yellow  is  seen ;  and  we 
have  every  reason  to  believe  this  rule  generally  {not 
universally)  true  for  pure  varieties  of  peas.  But  we  notice 
that  in  the  case  of  a  form  like  the  pea,  depending  on 
human  selection  for  its  existence,  it  might  be  possible  in 
a  few  years  for  the  races  with  pure  seed  characters  to  be 
practically  supplanted  by  the  "  mosaicized  "  races  like  the 
Telephone  group,  if  the  market  found  in  these  latter  some 
specially  serviceable  quality.  In  the  maincrop  peas  I 
suspect  this  very  process  is  taking  place*.     After  such  a 

*  Another  practical  point  of  the  same  nature  arises  from  the  great 
variability  which  these  peas  manifest  in  plant-  as  well  as  seed- 
characters.  Mr  Hurst  of  Burbage  tells  me  that  in  e.g.  William  the 
First,  a  pea  very  variable  in  seed-characters  also,  tall  plants  may  be 
so  common  that  they  have  to  be  rogued  out  even  when  the  variety  is 
grqwn  for  the  vegetable  market,  and  that  the  same  is  true  of  several 
such  varieties.  It  seems  by  no  means  improbable  that  it  is  by  such 
I'oguing  that  the  unstable  mosaic  or  blend-form  is  preserved.  In  a 
thoroughly  stable  variety  such  as  Ne  Plus  Ultra  roguing  is  hardly 
necessary  even  for  the  seed-market. 

Mr  N.  N.  Sherwood  in  his  useful  account  of  the  origin  and  races 


204  A  Defence  of  Menders 

revolution  it  might  be  possible  for  a  future  experimenter  to 
conclude  that  Pismn  sativum  was  by  nature  a  "mosaicized" 
species  in  these  respects,  though  the  mosaic  character  may 
have  arisen  once  in  a  seed  or  two  as  an  exceptional 
phenomenon.  When  the  same  reasoning  is  extended  to 
wild  forms  depending  on  other  agencies  for  selection,  some 
interesting  conclusions  may  be  reached. 

But  in  Mendelian  cases  we  are  concerned  primarily  not 
with  the  product  of  gametes  of  blended  character,  but  with 
the  consequences  of  the  union  of  gametes  already  dis- 
continuously  dissimilar.  The  existence  of  pure  Mendelian 
gametes  for  given  characters  is  perfectly  compatible  with 
the  existence  of  blended  or  mosaic  gametes  for  similar 
characters  elsewhere,  but  this  principle  enables  us  to  form 
a  comprehensive  and  fruitful  conception  of  the  relation  of 
the  two  phenomena  to  each  other.  As  I  also  pointed 
out,  through  the  imperfection  of  our  method  which  does 
not  yet  permit  us  to  see  the  differentiation  among  the 
gametes  though  we  know  it  exists,  we  cannot  yet  as  a 
rule  obtain  certain  proof  of  the  impurity  of  the  gametes 
(except  perhaps  in  the  case  of  mosaics)  as  distinct  from 
evidence  of  imperfect  dominance.  If  however  the  case  be 
one  of  a  "mule"  form,  distinct  from  either  parent,  and 
not  merely  of  dominance,  there  is  no  a  priori  reason  why 
even  this  may  not  be  possible ;  for  we  should  be  able  to 

of  peas  {Jour.  R.  Hort.  Soc.  xxii.  1899,  p.  254)  alludes  to  the  great 
instability  of  this  class  of  pea.  To  Laxton,  he  says,  "we  are  indebted 
for  a  peculiar  type  of  Pea,  a  round  seed  with  a  very  slight  indent,  the 
first  of  this  class  sent  out  being  William  the  First,  the  object  being  to 
get  a  very  early  blue- seeded  indented  Pea  of  the  same  earliness  as  the 
Sangster  type  with  a  blue  seed,  or  in  other  words  with  a  Wrinkled  Pea 
flavour.  This  type  of  Pea  is  most  difficult  to  keep  true  on  account  of 
the  slight  taint  of  the  Wrinkled  Pea  in  the  breed,  which  causes  it  to 
run  back  to  the  Round  variety." 


PHndijles  of  Heredity  205 

distinguish  the  results  of  breeding  first  crosses  together 
into  four  classes  :  two  pure  forms,  one  or  more  blend  or 
mosaic  forms,  and  "  mule  "  forms.  Such  a  study  could  as 
yet  only  be  attempted  in  simplest  cases  :  for  where  we  are 
concerned  with  a  compound  allelomorph  capable  of  resolu- 
tion, the  combinations  of  the  integral  components  become 
so  numerous  as  to  make  this  finer  classification  practically 
inapplicable. 

But  in  many  cases — perhaps  a  majority — though  by 
Mendel's  statistical  method  we  can  perceive  the  fluctuations 
in  the  numbers  of  the  several  products  of  fertilisation,  we 
shall  not  know  whether  abnormalities  in  the  distribution  of 
those  products  are  due  to  a  decline  in  dominance,  or  to 
actual  impurity  of  the  gametes.  We  shall  have  further  to 
consider,  as  afi'ecting  the  arithmetical  results,  the  possibility 
of  departure  from  the  rule  that  each  kind  of  gamete  is 
produced  in  equal  numbers* ;  also  that  there  may  be 
the  familiar  difficulties  in  regard  to  possible  selection  and 
assortative  matings  among  the  gametes. 

I  have  now  shown  how  the  mosaic  and  blend-forms  are 
to  be  regarded  in  the  light  of  the  Mendelian  principle. 
What  has  Professor  Weldon  to  say  in  reference  to  them  ? 
His  suggestion  is  definite  enough — that  a  study  of  ancestry 
will  explain  the  facts  :  how,  we  are  not  told. 

In  speaking  of  the  need  of  study  of  the  characters  of 
the  race  he  is  much  nearer  the  mark,  but  when  he  adds 
"that  is  their  ancestry,"  he  goes  wide  again.  When 
Telephone  does  not  truly  divide  the  antagonistic  characters 
among  its  germ-cells  this  fact  is  in  nowise  simply  traceable 
to  its  having  originated  in  a  cross — a  history  it  shares  with 
almost  all  the  peas  in  the  market — but  to  its  own  peculiar 

*  In  dealing  with  cases  of  decomposition  or  resolution  of  compound 
characters  this  consideration  is  of  highest  importance. 


206  A  Defence  of  Menders 

nature.      In  such  a  case  imperfect  dominance  need  not 
surprise  us. 

What  we  need  in  all  these  phenomena  is  a  knowledge 
of  the  properties  of  each  race,  or  variety,  as  we  call  it  in 
peas.  We  must,  as  I  have  often  pleaded,  study  the  pro- 
perties of  each  form  no  otherwise  than  the  chemist  does  the 
properties  of  his  substances,  and  thus  only  can  we  hope  to 
work  our  way  through  these  phenomena.  Ancestry  holds 
no  key  to  these  facts  ;  for  the  same  ancestry  is  common  to 
own  brothers  and  sisters  endowed  with  dissimilar  properties 
and  producing  dissimilar  posterity.  To  the  knowledge  of 
the  properties  of  each  form  and  the  laws  which  it  obeys 
there  are  no  short  cuts.  We  have  no  periodic  law  to  guide 
us.     Each  case  must  as  yet  be  separately  worked  out. 

We  can  scarcely  avoid  mention  of  a  further  category  of 
phenomena  that  are  certain  to  be  adduced  in  opposition  to 
the  general  truth  of  the  purity  of  the  extracted  forms.  It 
is  a  fact  well  known  to  breeders  that  a  highly-bred  stock 
may,  unless  selections  be  continued,  "degenerate."  This 
has  often  been  insisted  on  in  regard  to  peas.  I  have  been 
told  of  specific  cases  by  Messrs  Sutton  and  Sons,  instances 
which  could  be  multiplied.  Surely,  will  reply  the  supporters 
of  the  theory  of  Ancestry,  this  is  simply  impurity  in  the 
extracted  stocks  manifesting  itself  at  last.  Such  a  con- 
clusion by  no  means  follows,  and  the  proof  that  it  is 
inapplicable  is  obtained  from  the  fact  that  the  "degenera- 
tion," or  variation  as  we  should  rather  call  it,  need  not 
lead  to  the  production  of  any  proximate  ancestor  of  the 
selected  stock  at  all,  but  immediately  to  a  new  form,  or  to 
one  much  more  remote — in  the  case  of  some  high  class  peas, 
e.g.,  to  the  form  which  Mr  Sutton  describes  as  "vetch- 
like," with  short  pods,  and  a  very  few  small  round  seeds, 
two  or  three  in  a  pod.     Such  plants  are  recognized  by  their 


Principles  of  Heredity  207 

appearance  and  are  rigorously  hoed  out  every  year  before 
seeding. 

To  appreciate  the  meaning  of  these  facts  we  must  go 
back  to  what  was  said  above  on  the  nature  of  compound 
characters.  We  can  perceive  that,  as  Mendel  showed,  the 
integral  characters  of  the  varieties  can  be  dissociated  and 
re-combined  in  any  combination.  More  than  that;  certain 
integral  characters  can  be  resolved  into  further  integral 
components,  by  analytical  variations.  What  is  taking 
place  in  this  process  of  resolution  we  cannot  surmise,  but 
we  may  liken  the  consequences  of  that  process  to  various 
phenomena  of  analysis  seen  elsewhere.  To  continue  the 
metaphor  we  may  speak  of  return  to  the  vetch-like  type  as 
a  synthetical  variation  :  well  remembering  that  we  know 
nothing  of  any  substance  being  subtracted  in  the  former 
case  or  added  in  the  latter,  and  that  the  phenomenon  is 
more  likely  to  be  primarily  one  of  alteration  in  arrangement 
than  in  substance. 

A  final  proof  that  nothing  is  to  be  looked  for  from  an 
appeal  to  ancestry  is  provided  by  the  fact — of  which  the 
literature  of  variation  contains  numerous  illustrations — 
that  such  newly  synthesised  forms,  instead  of  themselves 
producing  a  large  proportion  of  the  high  class  variety  which 
may  have  been  their  ancestor  for  a  hundred  generations, 
may  produce  almost  nothing  but  individuals  like  themselves. 
A  subject  fraught  with  extraordinary  interest  will  be  the 
determination  w^hether  by  crossing  these  newly  synthesised 
forms  with  their  parent,  or  another  pure  form,  we  may  not 
succeed  in  reproducing  a  great  part  of  the  known  series  of 
components  afresh.  The  pure  parental  form,  produced,  or 
extracted,  by  "  analytical "  breeding,  would  not  in  ordinary 
circumstances  be  capable  of  producing  the  other  components 
from  which  it  has  been  separated ;  but  by  crossing  it  with 


208    A  Defence  of  MendeVs  Principles  of  Heredity 

the  ''synthesised"  variety  it  is  not  impossible  that  these 
components  would  again  reappear.  If  this  can  be  shown 
to  be  possible  we  shall  have  entirely  new  light  on  the  nature 
of  variation  and  stability. 


Conclusion. 

I  trust  what  I  have  written  has  convinced  the  reader  that 
we  are,  as  was  said  in  opening,  at  last  beginning  to  move. 
Professor  Weldon  declares  he  has  "  no  wish  to  belittle  the 
importance  of  Mendel's  achievement "  ;  he  desires  "  simply 
to  call  attention  to  a  series  of  facts  which  seem  to  him  to 
suggest  fruitful  lines  of  inquiry."  In  this  purpose  I  venture 
to  assist  him,  for  I  am  disposed  to  think  that  unaided  he 
is — to  borrow  Horace  Walpole's  phrase — about  as  likely  to 
light  a  fire  with  a  wet  dish-clout  as  to  kindle  interest  in 
Mendel's  discoveries  by  his  tempered  appreciation.  If  I 
have  helped  a  little  in  this  cause  my  time  has  not  been 
wasted. 

In  these  pages  I  have  only  touched  the  edge  of  that  new 
country  which  is  stretching  out  before  us,  whence  in  ten 
years'  time  we  shall  look  back  on  the  present  days  of  our 
captivity.  Soon  every  science  that  deals  with  animals  and 
plants  will  be  teeming  with  discovery,  made  possible  by 
Mendel's  work.  The  breeder,  whether  of  plants  or  of 
animals,  no  longer  trudging  in  the  old  paths  of  tradition, 
will  be  second  only  to  the  chemist  in  resource  and  in 
foresight.  Each  conception  of  life  in  which  heredity  bears 
a  part — and  which  of  them  is  exempt? — must  change  before 
the  coming  rush  of  facts. 


BIBLIOGRAPHY. 


1.  CoRRENS,  C.     G.   Mendel's   Kegel   iiber  das  Verhalten   der 

Nachkommenscliaffc  der  Rassenbastarde,  Ber.  dent.  hot. 
Ges.,  xviiL,  1900,  p.  158. 

2.     Gregor  Mendel's  "  Versuche  iiber  Pflanzen-Hybriden  " 

und  die  Bestatigung  ihrer  Ergebnisse  durcb  die  neuesten 
Untersuchungen,  Bot.  Ztg.,  1900,  p.  229. 

3.     Ueber  Levkoyenbastarde  zur  Kenntniss  der  Grenzen 

der  Mendel'schen  Regeln,  Bot.  Chit.,  1900,  Vol.  lxxxiv., 
p.  97. 

4.     Bastarde  zwischen  Maisrassen,  mit  besonderer  Beriick- 

sichtigung  der  Xenien,  Bibliotheca  Botanica,  Hft.  5.3, 
1901. 

5.  Crampe.    Kreuzungen  zwischen  Wanderratten  verschiedener 

Farbe,  Landwirths.  Jahrh..,  vi.,  1877,  p.  384. 

6.     Zucht-Yersuche  mit  zabmen  Wanderratten,     1.    Re- 

sultate  der  Zucbt  in  Verwandtschaft,  ihid.,  xii.,  1883, 
p.  389.  2.  Resultate  der  Kreiizung  der  zahmen  Ratten 
niit  wilden,  ihid.,  xiii.,  1884,  p.  699. 

7.     Die   Gesetze   der   Vererbung   der   Farbe,   ihid.,  xiv,, 

p.  539. 

8.  Darwin,    C.       Variation    of   Animals    and    Plants    under 

Domestication,  ed.  2,  i.,  pp.  348  and  428. 

9.  Fischer,  Johann  von.    Die  Saugethiere  dcs  S'  Petcrsburger 

Gouvernements,  Zool.  Garten,  x.,  1869,  p.  336. 

10.     litis   {Mustela  putorius)   und   Frett  {Mustela  furo), 

ihid.,  XIV.,  1873,  p.  108. 

B.  14 


210  Bibliograjjhy 

11.  Fischer,  Johann  von.     Beobachtungen  iiber  Kreuzungen 

verschiedener  Farbenspielarten  innerhalb  einer  Species, 
ihid.,  XV.,  1874,  p.  361. 

12.  FocKE,  W.  0.     Die  PJlanzen-MischUnge,  Borntrager,  Berlin, 

1881. 

13.     Ueber  dichotype   Gewachse.      Oesterr.    hot.   Ztschr., 

XVIII.,  1868,  p.  139. 

14.  Galton.   F.      Natural    Inheritance,   Macmillan    and    Co., 

London,  1889.  - 

15.     The  Average  Contribution  of  each  several  Ancestor 

to  the  total  Heritage  of  the  Offspring,  Proc.  Roy.  Sac, 
LXL,  1897,  p.  401. 

16.  Gartner,  C.  F.  von.     Versuche  und  Beohachtungen  uber 

die  Bastarderzeugung  im  PJlanzenreich,  Stuttgart, 
1849. 

17.  GiTAT,  E,     Ueber  den  directen  Einfluss  des  Pollens  auf 

Frucht-  und  Samenbildung,  PringsheiirCs  JB.  d.  wiss. 
Bot.,  XXV.,  1893,  p.  489. 

18.  Godron,  D.  a.     Des  Hybrides  Veg^taux,  etc.     Ann.  Sci. 

Nat.  Bot.,  Ser.  4,  xix.,  1863,  p.  135,  and  a  series  of 
papers  in  Mem.  Acad.  Stanislas,  Nancy,  1864,  1865,  and 
especially  1872. 

19.  GuAiTA,   G.   VON.      Versuche   mit   Kreuzungen   von  ver- 

schiedenen  Rassen  der  Hausmaus,  Ber.  d.  naturf. 
Ges.  Freihurg,  x.,  1898,  p.  317. 

20.     Zweite  Mittheilung,  etc.,  ihid.,  XL,  1900,  p.  131. 

21.  Knight,  T.  A.     An  account  of  some  experiments  on  the 

Fecundation  of  Vegetables,  Phil.  Trans.,  1799,  Pt.  ii., 
p.  195. 

22.  KuSTER,  E.     Die  Mendel'schen  Regeln,  ihre  urspriingliche 

Fassung  und  ihre  moderne  Erganzungen,  Biol.  Chit., 
XXII.,  1902,  p.  129. 

23.  Laxton,  T.     Observations  on  the  variations  effected  by 

crossing  in  the  colour  and  character  of  the  seed  of  Peas, 
Internat.  Hort.  Exhih.  and  Bot.   Congr.,  Report,  1866, 
p.  156. 
24. Notes    on    some  Changes    and    Variations   in    the 


B^bliogra2^hy  211 

OfiPspriDg  of  Cross-fertilized  Peas,  Jour.  Hort.  Soc, 
N.S.  III.,  1872,  p.  10. 

25.  Laxton,  T.     Improvement  amongst  Peas,  Jour.  Rort.  iS'oc, 

1890,  XII.,  1,  p.  29. 

26.  Mendel,    Gkegor    Johann.      Versuche    iiber    Pflanzen- 

Hybriden,  Verh.  naturf.  Ver.  in  Briinii,  Band  iv.,  1865, 
Ahhandlungen,  p.  1 ;  reprinted  in  Flora,  1901,  and 
in  Ostwald's  Klassiker  d.  exaJcten  Wiss.  English  trans- 
lation in  Jour.  R.  Hort.  Soc,  1901,  xxvi. 

27.     Ueber  einige  aus  kiinstlicher  Befruchtung  gewonnenen 

Hieracium-Bastarde,  ibid.,  viii.,  1869,  Ahhandlungen, 
p.  26. 

28.  MiLLARDET.     Note  sur  I'hybridation  sans  croisement,  ou 

fausse  hybridation,  Mem.  Soc.  Sci.  Bordeaux,  Ser.  4, 
IV.,  1894,  p.  347. 

29.  C.  Naudin.     Nouvelles  recherches  sur  I'Hybridite  dans  les 

Vegetaux,  Nouv.  Arch.  Mus.,  i.,  1865,  p.  25. 

30.     Ann.  sci.  nat.,  Bot.,  Ser.  4,  xix.,  p.  180. 

31.  Pearson,  Karl.    On  the  Law  of  Ancestral  Heredity,  Proc. 

Roy.  Soc,  Lxii.,  1898,  p.  386. 

32.     On   the    Law   of    Keversion,   ibid.,   lxvi.,    1900,   p. 

140. 

33.     The   Grammar   of  Science,  second  edition,   London, 

A.  and  Charles  Black,  1900. 

34.  Mathematical  Contributions  to  the  Theory  of  Evolu- 
tion. VIII.  On  the  Inheritance  of  Characters  not 
capable  of  exact  Measurement,  Phil.  Trans.  Roy.  Soc, 

1900,  Vol.  195,  p.  79. 

35.  RiMPAU,       Kreuzungsprodukte      landw.      Kulturpflanzen, 

Landvj.  Jahrb.,  xx.,  1891. 

36.  TscHERMAK,   E.     Ucber  kiinstliche   Kreuzung   bei  Pisum 

sativum,  Ztschrft.  f.  d.  landwirths.  Versuchswesen  in 
Oesterr.,  1900,  in.,  p.  465. 

37.     Weitere   Beitrage  iiber  Verschiedenwerthigkeit   der 

Merkmale  bei  Kreuzung  von  Erbsen  und  Bohnen,  ibid., 

1901,  IV.,  641 ;  abstract  in  Ber.  deut.  bot.  Ges.,  1901, 
XIX,,  p,  35, 


212  Bibliography 

38.  TsCHERMAK,    E.     Ueber    Ziichtung    neuer    Getreiderassen 

mittelst  kiinstlicher  KreuzuDg,  ^6^'o?.,  1901,  iv.,  p.  1029. 

39.  Vilmorin-Andrieux  and  Co.   Les  Plantes  Potageres,  1st  ed. 

1883;  2nd  ed.  1891. 

40.  Vries,   H.   de.     Sur  la   loi   de   disjonction   des   hybrides, 

Comptes  Rendus,  26  March,  1900. 

41.     Das   Spaltungsgesetz   der   Bastarde,   Ber.   deut   hot. 

Ges.,  1900,  XVIII.,  p.  83. 

42.     Ueber  erbungleiche  Kreuzungen,  ihid.,  p.  435. 

43.     Sur   les   unites   des   caract^res   specifiques    et    leur 

application  h.  I'etude  des  hybrides,  Re'i).  Gen.  de  Bot.y 
1900,  XII.,  p.  257.  See  also  by  the  same  author,  Intra- 
cellular e  Pangenesis^  Jena,  1889,  in  which  the  conception 
of  unit-characters  is  clearly  set  forth. 

44.     Die  Mutationstheorie^  Vol.  i.,  Leipzig,  1901. 

45.  Weldon,    W.    F.    R.     Mendel's   Laws   of  Alternative    In- 

heritance in  Peas,  Biometrika,  i.,  Pt.  ii.,  1902,  p.  228. 

46.  WiCHURA,   Max.      Die   Bastardbefruchtung  im   Pflanzen- 

reich,  erlautert  an  den  Bastarden  der  Weiden,  Breslau, 
1865. 


Received  as  this  sheet  goes  to  press: — 

CoRRENS,  C.     Die  Ergebnisse  der  neuesten  Bastardforschungen 

fiir  die  Yererbungslehre,  Ber.  deut.  hot.  Ges.^  xix.,  General- 

versammlungs-Heft  1. 
Ueber  den  Modus  und  den  Zeitpunkt  der  Spaltung  der 

Anlagen  bei  den  Bastarden  vom  Erbsen-Typus,  Bot.  Ztg.y 

1902,  p.  65. 


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