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MENDEL'S

PRINCIPLES OF HEREDITY

CAMBRIDGE UNIVERSITY PRESS

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A a rights reserved

GREGOR MENDEL, 1866

Enlarged fro7n a group of the brethren of the Konigskloster

MENDEL'S

PRINCIPLES OF HEREDITY

W. BATESON, M.A., F.R.S., V.M.H.

HONORARY FELLOW OF ST JOHn's COLLEGE,
DIRECTOR OF THE JOHN INNES HORTICULTURAL INSTITUTION

Cambridge :

at the University Press

Published March 1909

Reprinted August 1909

Third Impression with Additions 19 13

PREFACE

THE object of this book is to give a succinct account
of discoveries in regard to Heredity made by the
application of Mendel's method of research. Following the
clue which his long lost papers provided we have reached a
point from which classes of phenomena hitherto proverbial
for their seeming irregularity can be recognized as parts of
a consistent whole. The study of Heredity thus becomes
an organised branch of physiological science, already abun-
dant in results, and in promise unsurpassed.

A translation of Mendel's two papers, together with
a biographical note, is appended. The translation of the
first, based on a draft prepared for the Society by Mr C. T.
Druery, was printed in the Royal Horticultural Society's
Journal, 1901. With modifications I published it separately
in 1902, giving a brief summary of Mendelism as then
developed, under the title Menders Pri7iciples of Heredity :
A Defence. The object of that publication was to put
Mendel's work before the English speaking peoples and to
repel the attack which the late Professor W^eldon had
recently made on Mendelian methods and the conclusions
drawn from them. The edition was at once sold out, but
I did not reprint the book. As a defence it had served
its purpose. Moreover the progress of experiment with
the extension of Mendelian conceptions was rapid, and the
account of those conceptions there given was in some
important respects soon out of date. In particular my
view of the nature of compound factors was shown to be
unnecessarily complex and largely incorrect. Though

vi Preface

obviously in a subject fast extending under the influence
of many workers there can be no finaHty, yet at the present
time our knowledge of the main facts has reached a
definite stage, and a useful and relatively permanent pre-
sentation of the phenomena can be attempted.

The range and diversity of facts, zoological and botani-
cal, from which the material is drawn are so wide that it
has been difficult to present them adequately within a
moderate compass. Many of the types studied might
singly provide the subject of a treatise, and the temptation
to annotative excursion has been very great ; but the course
which seemed most useful was to admit only such detail as
had a clear significance in the exposition of principle, or as
a suggestion for further inquiry. The reader therefore will
understand that if he turns to the original records specified
he will almost always find information, perhaps important,
which is omitted here.

In the original plan of the book it was intended to
discuss somewhat fully the bearing of the new facts on the
great problems of Biology, but it is perhaps more fitting
that these theoretical considerations should be detached
from a presentation of the concrete phenomena. In 1907
I had the honour of delivering the Silliman Lectures in
Yale University, and I then took these wider aspects of
Genetics as my theme, showing the bearing of the new
knowledge on current theory, especially on that of Evolu-
tion, and the nature of Variation. The substance of these
lectures I propose to publish separately with amplifications,
and on the present occasion allusion to these matters has
been restricted to the briefest possible indication of the lines
of thought which Mendelism inevitably suggests.

A chapter dealing with practical applications of Mendelian
principles has been introduced. Such applications will
probably far exceed any limits we can yet perceive. Among
them we must foresee not merely advances in the art of

Preface vii

breeding animals and plants, but a control over the destiny
of our own species. These things are spoken of in their
place. To prevent disappointment, however, it must be at
once admitted that for fanciers Mendelism can as yet do
comparatively little. ''Fancying" provides the chief interest
in life for thousands of persons in this country. It is an
occupation with which the scientific naturalist should have
more sympathy than he has commonly evinced. If the
scientific world had kept in touch with the operations of the
*' fancy " much nonsense which has passed into scientific
orthodoxy would never have been written. The study of
Mendelian phenomena will do something to bring about a
fruitful interchange of experience. But for the '' fancy "
our work can as yet do two things only. First, in the
study of the workings of the Mendelian system it will
provide a most fascinating pursuit, which if followed with
assiduous care may at any moment lead to some consider-
able advance in scientific knowledge. Secondly, the prin-
ciples already ascertained will be found of practical assistance
in the formation of new breeds and may save many mistakes
and waste of time. But applied to the business of breeding
winners in established breeds they cannot materially help,
for almost always the points which tell are too fine to be
dealt with in our analysis.

In a work of this kind an author must necessarily speak
of various subjects on which his knowledge can be super-
ficial only, and I trust that if inaccuracies have been intro-
duced, readers will be good enough to send me corrections.

Much and varied assistance has been given me by
many persons. Such help on special points has been
acknowledged in the text, but a fuller and more prominent
acknowledgment is due to my colleagues. Without their
cooperation there would have been, so far as Cambridge is
concerned, but meagre contributions to record. In the
early days of Mendelism, and before, Miss E. R. Saunders

viii Preface

collaborated with me. A beautiful series of results, especially
relating to the heredity of Stocks (Matthiola), has been the
fruit of her labours exclusively. Not only have these
results greatly advanced our knowledge of genetic pheno-
mena, but I think that at a time when Mendelism was, in
England at least, regarded with suspicion, the obvious
precision of her work and the persistence of her advocacy
did much to convince the scientific world of the reality of
our assertions.

In 1904 I had the good fortune to gain Mr R. C. Punnett
as a partner. Since that date we have worked in close
collaboration, and the work that we have thus done has
been in every sense a joint product, both as regards design,
execution, and interpretation of results. Though for the
presentation of the views contained in this book I am solely
responsible, those that apply to the subjects of our own
work are often his, or have been arrived at in consequence
of interchange of ideas with him.

On some points of general physiology I have received
useful suggestions and criticism from Mr F. F. Blackman,
and in this respect I am also especially beholden to
Miss F. M. Durham.

The Plates of Sweet Peas and Mice are photographic
reproductions, on the whole very accurate, of coloured
drawings most kindly made for me by Miss Wheldale.
The Plate of Primula flowers is taken from an excellent
coloured photograph by Mr Waltham^. For Fig. 9 I am
obliged to the New Phytologist.

For several years past I have had an exceptional
opportunity of seeing breeding conducted on a large scale

■^ Since the word " magenta," often used in English for' the description
of a colour, is not understood on the Continent, I may say that it means a
purplish or bluish red, as distinguished from a crimson or pink red. On
Plate VI, Figs. 8, 9, 14, 15, 20, 21 represent shades of magenta, while
Figs. 2, 7, 13, 19 are true reds.

Preface ix

through the great kindness of Messrs Sutton of Reading,
who have given me the privilege of watching such parts of
their work in raising varieties as seemed especially in-
structive, with unrestricted access to their pedigree books.
From this I have derived much profit, and many hints
which have formed the starting point for fuller experiment.
My hearty thanks are due to them for this important
assistance.

W. BATESON.

Grantchester, Cambridge.
February, 1909.

Note to the third impression.

In the past three years the progress of Mendellan
analysis has been very rapid, and certain chapters of this
book, especially those dealing with Coupling and Repulsion,
and with the Heredity of Sex, are in essential respects out
of date. Knowledge of these subjects is at present in a
transitional stage, and I have endeavoured in a series of
brief Appendixes to acquaint the reader with the nature
of the principal advances made, while awaiting an oppor-
tunity of rewriting the book.

I am obliged to Professor Arnold Lang and to
Mr C. C. Hurst for calling my attention to errors, which,
with some others, have been corrected.

W. B.

November, 19 12.

CONTENTS

PART I.

CHAPTER PAGE

I. Introductory. Mendel's Discovery . . . . i

Introductory — Some pre-Mendelian Writings — Mendel's Discovery —
Dominant and Recessive— Segregation. Allelomorphism — Homo-
zygote and Heterozygote. Purity of Type.

II. The Material Investigated . . . . . .18

List of Structural Characters in Plants and Animals — List of Types in
which the inheritance of Colour has been studied — Preliminary
Deductions — Dominance and heterozygous characters — Mendel's
system distinguished from that of Galton. — "

III. Numerical Consequences and Recombinations . . 57

Representations of the Fg Generation and Novelties due to Re-combina- .
tion of Factors — Compound Characters — Combs of Fowls — Hetero-
stylism — White Flowers from Red x Cream.

IV. Heredity of Colour 74

Factors determining Colours : the Ratio 9:3- 4— The " Presence and
Absence " Hypothesis. Epistatic and Hypostatic Factors — Colours
of Mice — Pied Types — A Dominant Piebald.

V. Heredity of Colour {continued) . . . . . 88

Albinos giving Coloured Offspring ; Reversion on Crossing — Various
Kinds of Whites — Stocks— Orchids— Pigeons— Fowls — Primula.

VI. Heredity of Colour {continued) ....

Eye-Colours. Variations in Colour of the Iris— Deficiency of Eye
Pigments in some Coloured Types.

VII. Heredity of Colour {cofitinued) . . • •

The Genetics of Yellow Pigments in certain Animals. Yellow Mice not
breeding true— The Case of Basset Hounds and the " Law of An
cestral Heredity." Relation of this Principle to Mendelian Rules.

VIII. Heredity of Colour {continued) ....

Various Specific Phenomena in Colour-Inheritance. Relation of Colour
to Hoariness in Stocks. Miscellaneous Cases. Colour of a Special
Part controlling that of other Parts— Summary and Discussion—
Sublraction-Slages.

107

115

132

Contents

— Interfering Factors — Fluctuational Forms — "Pure Lines."

XIV. Miscellaneous Exceptional and Unconformable Phe-
nomena .........

Crosses breeding true without Segregation. Parthenogenetic or Apo-
gamic Forms. Hieracmm — Sexual Forms — Numerical Aberrations
- — Irregularities of Dominance — Alternation of Generations — Mater-
nal Characters in certain Seeds.

XV.

XVI.

Biological Conceptions in the Light of Mendelian
Discoveries

Nature of Units — Nature of Segregation — Moment of Segregation —
Differentiation of Parts compared with Segregation — Reversion and
Variation. " Bush " and " Cupid " Sweet Peas — Mendelian Segre-
gation and Species — Discontinuity in Variation — Mendelism and
Natural Selection.

Practical Application of Mendelian Principles

Meaning of Pure-bred — Rogueing — Raising Novelties — A Practical Ex-
ample— Unfixable Types — Technical Methods — Sociological Appli-
cation.

164

chapter page

IX. Gametic Coupling and Spurious Allelomorphism . 148

Pollen-Shape and Flower-Colour. Axil-Colour and Sterile Anthers —
Hooded Standard and Flower-Colour in Sweet Peas.

X. Heredity and Sex ........

Evidence from Breeding Experiments. ^;7£'w/«— Sex-limited Heredity.
The Horns of Sheep — Colour-Blindness — Sex and Spurious Allelo-
morphism. The Currant Moth — The Cinnamon Canary — The Silky
Fowl — Aglia tau — Cytological Evidence — Summary.

XI. Double Flowers ........

Miscellaneous Cases. Recessive and Dominant Doubling — " Hose-in-
Hose" Flowers — The Special Case of Double Stocks.

■4i XII. Evidence as to Mendelian Inheritance in Man

Normal Characters — Diseases and Malformations. Dominants — Sex-
limited Dominants — Recessives — Notes on collecting Evidence.

XIII. Intermediates between Varieties and the "Pure
Lines" of Johannsen

Intermediates as Heterozygous Forms — Subtraction-Stages of Dominants

196

205

235

245

266

291

Appendixes ........

PART II.

'^)\ Biographical Notice of Mendel

2. Translation of the Paper on Hybridisation

3. Translation of the Paper on Hieracmm .
Bibliography . ' .

Index of Subjects

Index of Authors

307

327

335

380

387
403
411

LIST OF ILLUSTRATIONS

In 1866
About 1862
About x88o

PORTRAITS OF MENDEL.

Frontispiece
to face p. 309
to face /. 3 1 7

COLOURED PLATES.

Plate I.
IL
III.
IV.
V.
VI.

Lepidoptera .
Mice ....
Reversion in Sweet Peas
Fowls ....

to face p. 44-

between pp. 80-81

betweefi pp. 93-94

to face p. 103

Spurious Allelomorphism in Sweet Peas between pp. 154-155
Heredity of Colour in Primula Sinensis between pp. 294-295

FIGURES.

FIGURE

1. Tall and "Cupid" dwarf Sweet Peas

2. Diagram showing consequences of Segregation .

3. Inheritance of seed-characters in Pea

4. Branched and unbranched forms in Stocks (Alatthiola)

5. Hooded and normal Barley .....

6. Heredity in Wheat .......

7. Fern-leaf and palm-leaf in Primula Sinensis

8. Two-row and six-row Barley .....

9. Starch-grains in Peas .......

10. Round and wrinkled seed in Maize ....

11. Down-colour in Chickens ......

12. Types of combs in Fowls ......

13. Combs of newly-hatched Chickens ....

14. Descent of "homostyle" character in Primula .

PAGE

9
12

20
21

1 ->

24
27
29

51
61

XIV

List of Ilhtstrations

34.

35-
36.

37.
38.

FIGURE

15. Diagram of Fo showing ratio 9:3:4

16. Diagram of Fg showing ratio 9:7

17. Diagram of Fo in Sweet Pea showing ratio 27:9

18. Pedigrees of eye-colour in Man

19. Pollen grains of Sweet Peas ....

20. Heredity of horns in Sheep ....

21. Heredity of a peculiar form of curly hair

22. Descent of congenital lock of white hair .

23. Brachydactylous hands .....

24. Skiagram of hands ......

25. Hands of brachydactylous child

26. Pedigree of brachydactylous family .

27. Descent from brachydactylous members

28. Drinkwater's pedigree of brachydactyly

29. Descent of prae-senile cataract . . - .

30. Another cataractous family .....

31. Descent of stationary night-blindness

32. Descent of Colour-blindness .....

33. Ideal Scheme of descent of simple sex-limited condition

e.g. horns of Sheep ......

Tentative representation of descent of Colour-blindness
Polish X Rivet Wheat .......

Seeds of Polish x Rivet Wheat .....

Reversion in height of Sweet Peas ....

Two types of dwarf Sweet Peas ....

between pp.

PAGE

77
89

108

150
171

207

211

212

213

214

215
216

217

220-221

223

230
231

259
260

282

PART I

CHAPTER I

INTRODUCTORY. MENDEL'S DISCOVERY.

Introductory — Some p7x-Mendetia7i Writings — Mendel's
Discovery — Dominant and Recessive — Segregation.
Allelomorphism — Homozygote and Heterozygote.
Purity of Type,

Among the biological sciences the study of heredity
occupies a central position. Whether we be zoologists,
botanists, or physiologists, the facts of heredity concern
us. Upon this physiological function all the rest in some
degree depend. Every advance in knowledge of that
central function must affect the course of thouoht alonof
each several line of biological inquiry.

Moreover though, as naturalists, we are not directly
concerned with the applications of science, we must perceive
that in no region of knowledge is research more likely to
increase man's power over nature. The science of sociology,
and in many of its developments the science of medicine
also, must of necessity form working hypotheses respecting
the course of heredity, and we cannot doubt that a percep-
tion of the truth in reQfard to the function of transmission
will greatly contribute to the progress of these sciences.
Lastly, to the industrial arts of the breeder of plants or
animals, the knowledge we are attempting to provide is of
such direct importance that upon this consideration no
special emphasis is required. In studying heredity, there-
fore, we are examining a vital problem of no mean
consequence, and those who engage in that work are
happy in the thought that they are assisting one of the
main advances in natural knowledge.

But though we may approach this study oi genetics —
to use the modern designation — from so many different
sides, it is especially in their bearing on the problem of

B. H. I

2 Introductory [ch.

the evolution of species that the facts have hitherto been
most profitably investigated. It was in the attempt to
ascertain the interrelationships between species that experi-
ments in genetics were first made. /The words "evolution"
and " origin of species " are now so intimately associated
with the name of Darwin that we are apt to forget that the
idea of a common descent had been prominent in the minds
of naturalists before he wrote, and that, for more than half
a century, zealous investigators had been devoting them-
selves to the experimental study of that possibility. Promi-
nent among this group of experimenters may be mentioned
Koelreuter, John Hunter, Herbert, Knight, Gaertner, Jordan,
Naudin, Godron, Lecoq, Wichura — men whose names are
familiar to every reader of Animals and Plants tender
Donieslication. If we could ask those men to define the
object of their experiments, their answer would be that they
were seeking to determine the laws of hereditary trans-
mission with the purpose of discovering the interrelationships
of species. In addition to the observation of the visible
structures and habits of plants and animals they attempted
by experiment to ascertain those hidden properties of living
things which we may speak of as genetic, properties which
breeding tests can alone reveal. The vast mass of
observation thus accumulated contains much that is of
permanent value, hints that if followed might have saved
their successors years of wasted effort, and not a few
indications which in the light of later discovery will greatly
accelerate our own progress.

Yet in surveying the work of this school we are
conscious of a feeling of disappointment at the outcome.
There are signs that the workers themselves shared this
disappointment. As we now know, they missed the clue
without which the evidence so laboriously collected remained
an inscrutable medley of contradictions.

While the experimental study of the species problem
was in full activity the Darwinian writings appeared.
Evolution, from being an unsupported hypothesis, was at
length shown to be so plainly deduclble from ordinary
experience that the reality of the process was no longer
doubtful. With the triumph of the evolutionary idea,
curiosity as to the significance of specific difierences was

i] Introductory 3

satisfied. The Origin was published in 1S59. During
the following decade, while the new views were on trial,
the experimental breeders continued their work, but before
1870 the field was practically abandoned.

In all that concerns the problem of species the next
thirty years are marked by the apathy characteristic of an
age of faith. Evolution became the exercising-ground of
essayists. The number indeed of naturalists increased ten-
fold, but their activities were directed elsewhere. Darwin's
achievement so far exceeded anything that was thought
possible before, that what should have been hailed as a
long-expected beginning was taken for the completed work.
I well remember receiving from one of the most earnest
of my seniors the friendly warning that it was waste of
time to study variation, for '' Darwin had swept the field."

Parenthetically we may notice that though scientific
opinion in general became rapidly converted to the doctrine
of pure selection, there was one remarkable exception.
Systematists for the most part kept aloof. Everyone was
convinced that natural selection operating in a continuously
varying population was a sufficient account of the origin of
species except the one class of scientific workers whose
labours familiarised them with the phenomenon of specific
difference. From that time the systematists became, as
they still in great measure remain, a class apart.

A separation has thus been effected between those who
lead theoretical opinion and those who by taste or necessity
have retained an acquaintance with the facts. The con-
sequences of that separation have been many and grievous.
To it are to be traced the extraordinary misapprehensions
as to the fundamental phenomena of specific difference
which are now prevalent.

If species hacl really arisen by the natural selection for
impalpable differences, intermediate forms should abound,
and the limits between species should be on the whole
indefinite. As this conclusion follows necessarily from the
premisses, the selectionists believe and declare that it
represents the facts of nature. Differences between species
being by axiom indefinite, the differences between varieties
must be supposed to be still less definite. Consequently
the conclusion that evolution must proceed by insensible

4 Introductory [ch.

transformation of masses of Individuals has become an
established dogma. Systematists, entomologists or botan-
ists for example, are daily witnesses to variation occurring
as an individual and discontinuous phenomenon, but they
stand aside from the debate ; and whoever in a discussion
of evolutionary theory appeals to the definiteness of varietal
distinctions in colour for instance, or in form, as recognizable
by common observation without mechanical aid, must be
prepared to meet a charge of want of intelligence or candour.
This is no doubt a passing phase and will end so soon as
interest in the problems of evolution is combined with some
knowledge of variation and heredity.

Genetic experiment was first undertaken, as we have
seen, in the hope that it would elucidate the problem of
species. The time has now come when appeals for the
vigorous prosecution of this method should rather be based
on other grounds. It is as directly contributing to the
advancement of pure physiological science that genetics
can present the strongest claim. We have an eye always
on the evolution-problem. We know that the facts we are
collecting will help in its solution ; but for a period we shall
perhaps do well to direct our search more especially to the
immediate problem* of genetic physiology, the laws of
heredity, the nature of variation, the significance of sex and
of other manifestations of dimorphism, willing to postpone
the application of the results to wider problems as a task
more suited to a maturer stao^e. When the magnitude
and definiteness of the advances already made in genetics
come to be more generally known, it is to be anticipated
that workers in various departments of biology will realise
that here at last is common ground. As we now know, the
conceptions on which both the systematists and the specula-
tive biologists have based their methods need complete
revision in the light of the new facts, and till the possibilities
of genetic research are more fully explored the task of
reconstruction can hardly be begun. In that work of
exploration all classes of naturalists will alike find interest.
The methods are definite and exact, so we need not fear
the alienation of those systematists to whom all theoretical
inquiry is repulsive. They are also wide in their scope, ^
and those who would turn from the details of classification

Pre-Me}ideliaii IVritin^s

as offering matter too trivial for their attention may engage
in genetic inquiries with great confidence that every frag-
ment of solid evidence thus discovered will quickly take
its place in the development of a coordinated structure.

Some pre-Mendelian Writings.

Of the contributions made during the essayist period
three call for notice : Weismann deserves mention for his
useful work in asking for the proof that " acquired
characters " — or, to speak more precisely, parental ex-
perience— can really be transmitted to the offspring. The
occurrence of progressive adaptation by transmission of the
effects of use had seemed so natural to Darwin and his
contemporaries that no proof of the physiological reality
of the phenomenon was thought necessary. Weismann's
challenge revealed the utter inadequacy of the evidence
on which these beliefs were based. There are doubtless
isolated observations which may be interpreted as favouring
the belief in these transmissions, but such meagre indications
as exist are by general consent admitted to be too slight to
be of much assistance in the attempt to understand how the
more complex adaptative mechanisms arose. Nevertheless
it was for the purpose of elucidating them that the appeal
to inherited experience was made. Weismann's contribution,
though negative, has greatly simplified the practical investi-
gation of genetic problems.

Though it attracted little attention at the time of its
appearance, an honourable place in the history of our
science must be accorded to the paper published by
de Vries (1889) under the title Int^-aceliulare Pangenesis.
This essay is remarkable as a clear foreshadowing of that
conception of tmit -characters which is destined to play so
large a part in the development of genetics.

The supreme importance of an exact knowledge of
heredity was urged by Galton in various writings published
during the period of which I am speaking. He pointed
out that the phenomena manifested regularity, and he made
the first comprehensive attempt to determine the rules they
obey. It was through his work and influence that the
existence of some order pervading the facts became generally

6 Pre-Mendelian IVritings [en.

recognized. In 1897 he definitely enunciated his now
I famous " Law " of heredity, which declared that to the total
heritage of the offspring the parents on an average contribute
^, the grandparents ^, and the great-grandparents |-, and so
on, the total heritage being taken as unity. To this
conclusion he had been led by several series of data, but
the evidence upon which he especially relied was that of
the pedigrees" of Basset Hounds furnished him by the late
Sir Everett Millais."^ In that instance the character con-
sidered was the presence or absence of black in addition to
yellow and white. The colours were spoken of as tri-colour
and non-tri-colour, and the truth of the law was tested by
the average numbers of the respective colours which resulted
from the various matings of dogs of known ancestral
composition. These numbers corresponded so well with
the expectations given by the law as to leave no reasonable
doubt that the results of calculation were in general har-
monv with natural fact.

There are features in this important case w^hich need
special consideration, and to these I will return. Meanwhile
we may note that though there was admittedly a statistical
accord between Galton's theory and some facts of heredity,
yet no one familiar with breeding or even with the literature
of breeding could possibly accept that theory as a literal or
adequate presentation of the facts. Galton himself in pro-
mulgating it made some reservations ; but in the practice of
breeding, so many classes of unconformable phenomena
were already known, that while recognizing the value of his
achievement, we could not from the first regard it as more
than an adumbration of the truth. As we now know,
Galton's method failed for want of analysis. His formula
should in all probability be looked upon rather as an
occasional consequence of the actual laws of heredity than
in any proper sense one of those laws.

Of the so-called investigations of heredity pursued by
extensions of Galton's non-analytical method and promoted
by Professor Pearson and the English Biometrical school
it is now scarcely necessary to speak. That such work
may ultimately contribute to the development of statistical
theory cannot be denied, but as applied to the problems of
heredity the effort has resulted only in the concealment of

I] Rediscovery of Mendel 7

that order which it was ostensibly undertaken to reveal.
A preliminary acquaintance with the natural history of
heredity and variation was sufficient to throw doubt on the
foundations of these elaborate researches. To those who
hereafter may study this episode in the history of biological
science it will appear inexplicable that work so unsound in
construction should have been respectfully received by the
scientific world. With the discovery of segregation it
became obvious that methods dispensing with individual
analysis" of the material are useless. The only alternatives
open to the inventors of those methods were either to
abandon their delusion or to deny the truth of Mendclian
facts. In choosing the latter course they have certainly
succeeded in delaying recognition of the value of Mendelism,
but with the lapse of time the number of persons who have
themselves witnessed the phenomena has increased so much
that these denials have lost their dangerous character and
may be regarded as merely formal.

Rediscovery of Mendel : his Method.

With the year 1900 a new era begins. In the spring
of that year there appeared, within a few weeks of each
other, the three papers of de Vries, Correns, and Tschermak,
giving the substance of Mendel's long-forgotten treatise.
Each of these three writers was able from his own ex-
perience to confirm Mendel's conclusions, and to extend
them to other cases. There could therefore, from the first,
be no question as to the truth of the facts. To appreciate
what Mendel did the reader should refer to the original
paper^, which is a model of lucidity and expository skill.
His success is due to the clearness with which he thought
out the problem. Being familiar with the w^orks of Gaertner
and the other experimental breeders he surmised that
their failure to reach definite and consistent conclusions
was due to a want of precise and continued analysis. In
order to obtain a clear result he saw that it was absolutely
necessary to start with pure-breeding, homogeneous materials,
to consider each character separately, and on no account to
confuse the different generations together. Lastly he realised

* See Part II.

8 Menders Method [ch.

that the progeny from distinct individuals must be separately
recorded. All fhese ideas were entirely new in his day.
When such precautions had been observed he anticipated
that a regular result would be attainable if the experiments
were carried out on a sufficient scale.

^ CAfter several preliminary trials he chose the edible Pea
[Pistmt sativum) for his subject. Varieties in cultivation
are distinguished by striking characters recognizable with-
out trouble. The plants are habitually self-fertilised, a
feature which obviates numerous difficulties^
y [__Following his idea that the heredity of each character
must be separately investigated, he chose a number oi pairs
of characters, and made crosses between varieties differing
markedly in respect of one pair of characters. The case
which illustrates Mendelian methods in the simplest way
is that in which heredity in respect of height was studied.
Mendel took a pair of varieties of which one was tall, being
6 — 7 feet high, and the other was dwarf, f to i-|- feet.
These two were then crossed together. In peas this is
an easy operation. The unbroken anthers can be picked
out of a bud with a pair of fine forceps and the pollen of
the plant chosen for the father may be at once applied to
the stigma of the emasculated flower. The cross-bred seeds
thus produced grew into plants which were always tall,
having a height not sensibly different from that of the pure
tall variety. In our modern terminology such a cross-bred,
the first filial generation, is called F^. From the fact that
the character, tallness, appears in the cross-bred to the
exclusion of the opposite character, Mendel called it a
dominant character ; dwarfness, which disappears in the F^
plant, he called recessiver~\

y r The tall cross-bred, so produced, in its turn bore seeds
by self-fertilisation. These are the next generation, F^.
When grown up they prove to be mixed, many being
tall, some being short, like the tall and the short grand-
parents respectively. Fig. i shows such an 7% family in
the Sweet Pea. Upon counting the members of this F,
generation it was discovered that the proportion of tails to
shorts exhibited a certain constancy, averaging about three
tails to one short, or in other words, 75 per cent, dominants
to 25 per cent, recessives^

Mendel s Method

^ £ These F^ plants were again allowed to fertilise them-
selves and the offspring of each plant wa^ separately sown.
It was then found that the offspring, F^, of the recessives

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consisted entirely of recessives. Further generations bred
from these recessives again produced recessives only, and
therefore the recessives which appeared in /% are seen to

lo Segregation [ch.

be pure to the recessive character, namely, in the case we
are considering, to dwarfness.^

But the tall i% dominants when tested by a study of
their offspring {F^, instead of being all alike (as the dwarfs
or recessives were), proved to be of two kinds, viz.

(a) Plants which gave a mixed F.^ consisting of both
tails and dwarfs, the proportion showing again an average
of three tails to one dwarf

(<5) Plants which gave tails only and are thus pure to
tallness.

The ratio of the impure (a) plants to the pure (I)) plants
was as 2 to i.

The whole F^ generation therefore, formed by self-
fertilisation of the original hybrid consists of three kinds
of plants :

pure dominants impure dominants pure recessives
or 3 dominants : i recessive.

Segregation. Allelomorphism,

The conclusion which Mendel drew from these observa-
tions is one which will suggest itself to any one who reflects
on the facts. The result is exactly what would be expected
if both male and female germ-cells of the cross-bred F^ were
in equal numbers bearers of either the dominant (D) or
recessive i^R) character, but not both. If this were so, and
if the union of the male and female germ-cells occurs at
random, the result would be an F^ family made up of

2^DD : 2^DR : 2^RD : 2sRR.
sB : iR.

But, as the first cross showed, when D meets R in
fertilisation the resulting individual is in appearance D ;
therefore F^ appears as 3/^ : iR. The results of the F^

I] and its Consequences 1 1

generation are in exact agreement with this suggestion :
for the R plants give R only ; and of the D plants one-
third give D only, while two-thirds give the same mixture,
2^D : \R, which was produced by /^^ (Fig. 2. I).

The descent may be represented diagrammatically
thus :

Parents Tall ( TT) x Short [tt]

F^ Tall ( Tt)

F^ Tall Tall Tall Short

TT Tt tT tt

p2L7'e talc I I pttre short

F, TT tt tY U TF~¥t 1 1 U

Now since the fertilised ovum or zygote, formed by the
original cross, was made by the union of two germ-cells or
gametes bearing respectively tallness and dwarfness, both
these elements entered into the composition of the original
F^ zygote; but if the germ-cells which that zygote eventually
forms are bearers of either tallness or dwarfness, there
must at some stage in the process of germ-formation be a
separation of the two characters, or rather of the ultimate
factors which cause those characters to be developed in the
plants. This phenomenon, the dissociation of characters
from each other in the course of the formation of the
germs, we speak of as segregation, and the characters which
segregate from each other are described as alleloniorphic,
i.e. alternative to each other in the constitution of the
gametes (Fig. 2).

That this is the true account was proved by further
experiments which Mendel made by crossing the F^ with
pure dominants and with pure recessives. For DR x DD
gave an offspring all dominant in appearance, though in
reality consisting of both DR plants and DD plants, on an
average in equal numbers. On the other hand DR x RR
gives an equal number of dominants and recessives. of
which the dominants are all DR plants, and the recessives
are all pure recessives. These various experiments illus-
trate the composition of the four simple types of Mendelian
families, which may be set out thus :

Segregation

[CH.

DD X RR gives all DR

Z^T? X 7^7? gives \DR

DR X Z^T^ gives \DD : \DR
DR X DR gives iZ^Z^ : 2/^7?

appearing as 3 dominants : i recessive.

F ^

•2 R
Batio

IRR
IR

D
R

JicuUo ID
II

R
R

cdlD
III

Fig. 2. Diagrams showing numerical consequences of segregation.

I. The mating DD x RR, and DR x DR. 11. The mating DR x i?i?.

III. The mating DR x Z>Z).

The way in which these ratios are produced may be
easily represented by means of a number of draught-men.
Pairs of draughts then represent zygotes ; single draughts
represent germ-cells. That there is a propriety in repre-
senting zygotic or somatic cells as double structures and
germ-cells as single structures will be evident to biologists ;
for we know that each somatic nucleus in plants and
animals is a double structure, containing twice the number
of chromosomes present in each mature germ-cell. Two
black draughts may then be taken to represent a pure black
individual, two white draughts a white individual. When
they are crossed together F, is represented by a black

I] and its Consequences 13

draught and a white one (Fig. 2. I). Supposing the black
to be a dominant the fact may be represented by putting it
on the top. When segregation of the allelomorphs, black-
ness and whiteness, takes place in gameto-genesis, the
germ-cells of the cross-bred are again bearers of blackness
or of whiteness, and it may readily be shown experimentally
that the results of their various random combinations give
rise to the ratios stated above.
)^ fTrhe fact of segregation was the essential discovery
which Mendel made. As we now know, such segregation is
one of the normal phenomena of nature. It is segregation
which determines the regularity perceptible in the here-
ditary transmission of differences, and the definiteness or
discontinuity so often Conspicuous in the variation of animals
and plants is a consequence of the same phenomenon.
Segregation thus defines the tAiits concerned in the consti-^
tution of organisms and provides the clue by which an
analysis of the complex heterogeneity of living forms may_^
be begun. JJ

There are doubtless limits beyond which such analysis
cannot be pursued, but a vast field of research must be
explored before they are reached or determined. It is likely
also that in certain cases the units are so small that no
sensible segregation can be proved to exist. As yet, how-
ever, no such example has been adequately investigated ;
nor, until the properties and laws of interaction of the
segregable units have been much more thoroughly examined,
can this class of negative observations be considered with
reat prospect of success.

HThe dominance of certain characters is often an impor-
tant but never an essential feature of Mendelian heredity.
Those who first treated of Mendel's work most unfortu-
nately fell into the error of enunciating a *' Law of Domi-
nance" as a proposition comparable with the discovery of
seereofation. Mendel himself enunciates no such law.
Dominance of course frequently exists. The consequences
of its occurrence and the complications it introduces must
be understood as a preliminary to the practical investigation
of the phenomena of heredity, but it is only a subordinate
incident of special cases, and Mendel's principles of inherit-
ance apply equally to cases where there is no dominance

14 Segregation [cn.

and the heterozygous type is intermediate in character
between the two pure types. Q -"v^) • ^^ '

^ C To the detection of the gene^i^^stem of any given case
it is however necessary that the results of combinations
should be sensibly regular. When, as occasionally happens,
a character may sometimes behave as a dominant and
sometimes not, we have as yet no satisfactory means of
further analysis. These irregularities in dominance may
confidently be attributed to the disturbing effects of other
factors or of conditions, but the detection of such unknown
factors must be a long and perhaps impossible task. J

>f C Mendel applied his method to the following seven
distinct pairs of characters in peas, and found that in each
the inheritance was similar. The dominant character is
put first^

1. Height : whether tall or short.

2. Distribution of flowers on the stem : whether

arranged along the axis of the plant, or bunched
together at the top so as to form a false umbel '^.

Colour of unripe pod : whether a shade of green
or bright yellow.

Shape of pod : whether simply inflated, or deeply
constricted between the seeds, i.e. as in "sugar-
peas" or *'Pois sans parchemin."

Colour of seed-skin : whether various shades of
grey or brown, with or without violet spotting,
or white. The '' grey " skins are always asso-
ciated with coloured flowers and almost always
with a purple or red mark in the axils.

Colour of cotyledons : whether yellow or green.

7. Shape of seeds : whether rounded or wrinkled.

^ It will be observed that the first five 2.r^ plant-characters.
In order to see the result of crossing, the seeds must be
sown and allowed to grow into plants. The last two
characters belong to the seeds themselves. The seeds of
course are members of a generation later than that of the
plant which bears them. Thus when a cross is made the

* This is a fasciated and semi-monstrous form.

and its Consequences

resultant seeds are F^, showing the dominant character
yellowness or roundness, but the seed-skins are maternal
tissue. Such F^ seeds grow into F^ plants and bear F.,
seeds which show the typical mixture of dominants and
recessives in the pods (Fig. 3). In each case Mendel's

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Fig.

3. Inheritance of seed-characters in Pea. The seed of a green
round variety fertilised by pollen of a yellow wrinkled variety are
yellow and round (i^j). The reciprocal cross would give the same
result. Two pods of F^ seed borne by the F^ plant are shown. There
were 6 yellow round, 3 green round, 3 yellow wrinkled, i green
wrinkled.

observations have been substantially confirmed by later
observers, and the operation of similar processes has now
been recognized in a long series of most diverse characters
in both animals and plants. H

/} Consequences of Segregation : Homozygote and

Heterozygote,

Before considering the various extensions of Men-
delian research, it may be well to indicate in general terms
the chief significance of the facts. The first conception
to which we are led is that of tinit-charactc7's, units
because they may be treated as such in the cell-divisions
of gametogenesis. It is evidently upon some process of

1 6 Purity of Type [en.

qualitative segregation occurring in one or more of these
cell-divisions that allelomorphism depends. The opposite
members of each pair of characters being allelomorphic to
each other, every zygote "^j or individual produced in ferti-
hsation, must, in respect of any such pair, be either a
komozygote, that is to say, a zygote formed by the union of
two gametes each bearing the same allelomorph, as AA
and aa, or a heterozygote formed by the union of two germs
bearing different allelomorphs, as Aa. Therefore in respect
of any pair of allelomorphic characters, the tndividtmls
composing the whole population are of three kinds only :

1. Homozygotes of the form ^y4,

2. Homozygotes of the form aa,

3. Heterozygotes of the form Aa.

The gametes are of two kinds only, A and a. Each
kind of homozygote is pure to the character of the gametes
which compose it. ""^

Purity of Type.

Purity of type thus acquires a precise meaning. It is
dependent on gametic segregation, and has nothing to do
with a prolonged course of selection, natural or artificial.

All this is of course consonant with the visible facts
that have been discovered by the cytologists, in so far as
the nucleus of each somatic cell is a double structure, while
the nucleus of each gametic cell is a single structure. It is,
in my judgment, impossible as yet to form definite views
as to the relations of the various parts of the cell to the
function of heredity. The details of cytology and their
interpretation are beyond our present province, but this
much is certain: that when in these discussions we idealize
the characters as borne by the gamete in an unpaired state
and by the zygote in a paired state, we make no assumption
which is not in full accord with histological appearances.

From the fact that the development of characters in
animals, or plants depends on the presence of definite units

* In botany the term zygote is usually restricted to the single cell
which results from the process of fertilisation, but by a natural extension
the word may be used for the individual which develops by somatic
divisions from that cell.

I] Pttrity of Type 17

or factors in their germ-cells, the paradox at once follows
that an organism may be pure-bred in respect of a given
character though its parents were cross-bred in the same
respect. Purity depends on the meeting of two gametes
bearing similar factors, and when two similarly-constituted
gametes do thus meet in fertilisation, the product of their
union is pure. The belief, so long prevalent, that purity
of type depends essentially on continued selection is thus
shown to have no physiological foundation.

Similarly it is evident that an individual may be pure in
respect of one character and cross-bred or impure in respect
of others. ^, ^.. /^

As a consequence of the application lof Mendel's prin-
ciples, that vast medley of seemingly capricious facts which
have been recorded as to heredity and variation is rapidly
being shaped into an orderly and consistent whole. A new
world of intricate order previously undreamt of is disclosed.
We are thus endowed with an instrument of peculiar
range and precision, and we reach to certainty in problems
of physiology which we might have supposed destined to
continue for ages inscrutable.

After such a discovery it is obvious that old ideas must
be revised. Systematists debating the limits of "specific
rank " or the range of variability, morphologists seeking to
reconstruct phylogenetic history, physiologists unravelling
the interaction of bodily functions, cytologists attempting
to interpret the processes of cell-division — each of these
classes of naturalists must now examine the current con-
ceptions of his study in the light of the new knowledge.
The practical breeder of animals or plants, basing his
methods on a determination of the Mendelian units and
their properties, will in many of his operations be able to
proceed with confidence and rapidity. Lastly, those who
as evolutionists or sociologists are striving for wider views
of the past or of the future of living things may by the use
of Mendelian analysis attain to a new and as yet limitless
horizon.

B. II,

CHAPTER II

THE MATERIAL INVESTIGATED.

List of Structural Characte7^s in Plants and Animals —
List of Types in which the inheritance of Colour has
been studied — Preliminary Deductions — Dominance
and heterozygoiis characters — Mendel's system distin-
guished from that of Gallon,

Heredity following the general rules described In the
last chapter has been witnessed in a great diversity of
animals and plants. The characters already proved to
follow such rules show an equal diversity. The following
is a list of some of them. Adequately to represent the facts
respecting each of these cases lengthy description w^ould be
needed. In regard to several of them occurrences which
do not readily fall into line have been recorded. Of these
some are probably due to errors of observation or mistakes
of various kinds, but a few will doubtless prove to be
genuine exceptions to rule and may constitute points of
departure for fresh and more extended research. In the
outline of the phenomena, w^hlch Is all that this book can
profess to offer, it seemed best to restrict as far as possible
the enumeration of these details, w^hich can only be
thoroughly appreciated by reference to the original papers ;
but such annotations as appeared necessary either in elucida-
tion of the phenomena or by way of incentive to further
work are briefly given with references to the original
sources. These annotations will be better understood after
the later chapters have been read.

In the following list when one character is conspicuously
dominant it is put first, but in several cases the dominance
is imperfect.

Plants,

I. Tallness and dwarfness. Peas [Piszim) and Sweet
Peas [Lathyrus odoratus). Runner and French Beans
(JPhaseolus).

As regards Peas the facts have been recorded by Mendel (195), ;
Tschermak (269, 270, &c.), R.E.C. * (20). When varieties differing \

* R.E.C. stands throughout for Reports to the Evolution Committee of '
the Royal Society, giving an account of the experiments of W. Bateson, .
E. R. Saunders and R. C. Punnett. Other contributors to these Reports
are mentioned by name.

CH. ii] Structural Characters : Plants 19

greatly in height are used, dominance is complete, and the two parent forms
are represented as three to one in F^. No clear exception has yet been
observed. Peas {Pisiim) exist in a vast number of distinct horticultural
varieties which can roughly be classified as tall (about 5 — 6 ft.), half-dwarfs
(about 4 ft), dwarfs (about 9 ins. to 3 ft.). The genetic relations of the
half-dwarfs to the others are not fully explored, and further investigation
will probably lead to the discovery of important facts. The cross half-
dwarf X tall giving tall as dominant has produced some extreme dwarfs in
i^o) doubtless by recombination {q-V.)y R.E.C. 20, p. 69. The cross
half-dwarf X dwarf has given intermediates in F-^ {i/nd).

The cross between tall and dwarf " Cupid " Sweet Peas gives complete
dominance of tallness and simple segregation in Fn, " Cupids " indis-
tinguishable from the original "Cupid" parent reappearing (Fig. i).

Phaseolus has been investigated especially by Tschermak (278) who
records some apparently anomalous results, de Vries (298, 11. p. 76)
states that he found that extracted F^_ dwarf Antirrhinum did not breed
true, but threw plants of various heights. The experiment should be
repeated.

2. Branching habit and the unbranchcd habit. Sun-
flower {HeliantJms, Shull, 241) and Cotton (Balls, 6). The
branched form of Stock i^Matthiola incana) is dominant to
the unbranched Brompton type. In /^ the unbranched
type reappears, but the ratio has not been determined
(Fig. 4). E. R. Saunders (unpublished).

3. The straggling habit of both the tall and dwarf
" Cupid " Sweet Peas, and the much-branched erect habit of
the " Bush" Sweet Peas (R.E.C. 22).

The relation of these two types to each other is not altogether simple.
As described {q.v-^ F^ from Cupid x Bush is a 7'eversionary for?n exactly
like the 7iormal tall variety. Neither the tall varieties nor the Cupids show
the profuse branching of the Bush Sweet Peas which gives them their
peculiar appearance. This is evidently recessive to the unbranched
condition, and the fact thus stands out in contrast to those observed in
the case of Sunflower and Cotton. But in the Sweet Pea we have the
additional complication that the factor which represses the excessive
branching by its presence gives increase of height. The tall and the Bush
differ from each other in respect of this factor only. It is present in the
tall but absent from the Bush. In the cross between Bush and Cupid two
pairs of factors are concerned as explained in the passage referred to.

4. Hairiness and glabrousness. Lychnis. Alatthiola
(Stocks). Wheat.

The case of Lydmis has been studied by de Vries (288) and R.E.C.
(19). In crosses between fully hairy and glabrous strains the discon-
tinuity is complete. Various forms intermediate in hairiness may
nevertheless be found wild and are by no means rare. Silene injiata

-^2—2

Structural Characters : Plants

[CH.

often exists in two forms, hairy and glabrous, growing side by side, and
doubtless their genetic relations are the same as those found for the.
corresponding varieties of Lychnis. In this species a third form is found
with hairs on the edges only (12).

The case of Matthiola is important and presents features of special
interest, R.E.C. (19, 20, 21, see also Correns, 61). Between thoroughly
hoary and glabrous strains the discontinuity is absolute, and the glabrous

Fig. 4. Mafthiola, Branched and unbranched forms in F^. A photo-
graph of Miss Saunders' plants, the leaves removed. (Supplied by
Miss Killby.)

are entirely destitute of hairs. The dominance is complete and homo-
zygotes cannot be distinguished from heterozygotes. A third, or "half
hoary" form exists, which is glabrous or nearly so on the upper surface
only. Its behaviour has not been fully investigated (19, p. 33.)

The genetics of hairiness in wheat have been studied by Spillman (247),
I'schermak (270), Biffen (27). The heterozygotes are sometimes inter-
mediate in hairiness.

The Peach and the Nectarine are probably related to each other as
hairy dominant and glabrous recessive.

Peculiar results are recorded in Cotton (Balls, 6).

Structural Characters : Plants

5. Prickllness and smoothness of fruits. Datura.
(R.E.C. 19, 20.) Ranunculus arvensis (20).

The case of Datura is interesting from the fact that it sometimes has
mosaic fruits, one quarter or one half being prickly and the rest smooth.
This is perhaps to be regarded as indicative of segregation occurring
among zygotic cells (see Chap. xv.).

Ranunculus arvensis has three types, spiny, tuberculated, and smooth.
The first is a simple dominant. Tuberculated x smooth gave F^ partially
spiny (21, p. 55).

6. Absence of glands i^Matthiola incana) on leaves was
dominant to presence of glands [M, simtata) (R.E.C. 20,
p. 40).

^^g- 5- Cross between a normally awned Barley and a variety with
"hooded" awns. P^ P^ the parents. F^ shows partial dominance of
hoods. The increase in length of ear is noticeable. The case also
illustrates the result of crossing a 2-row type with a 6-row type,
showing dominance of the former. (From Professor Bifien's specimens,)

22 Structural Characters : Plants Lcii.

7. Rough and smooth foliage. Wheat. BIffen (27).

8. Keeled glumes and rounded glumes. Wheat. Ibid.

9. Beardless and bearded ears. Wheat. Ibid. Also
Spillman (247) and Tschermak (270).

Most, if not all, of the "beardless" varieties exhibit a slight and variable
amount of awn especially on the uppermost spikelets (Fig. 6).

10. The "hoods" or '' Kapuzen'' characteristic of
certain Barleys show a partial dominance over the normal
type. These hoods, Professor Biffen states, are, structurally,
aborted florets (Fig. 5). Tschermak (270), Biffen (30).

11. Hollow and solid straw. Wheat. Biffen (27).

This is a structural character of an interesting kind, and one upon
which the commercial value of straw very largely depends. It was shown
that many factors were concerned in the production of the stem-characters;
and in F^ by the recombination of these factors a great variety of straws
appeared.

12. Blunt and pointed pods. Pisttm. Tschermak
(271), R.E.C. (20). Phaseolus. Tschermak (272).

The dominance in this case is complete. Some varieties exist in both
a blunt and a pointed type {e.g. Sutton's Continuity). The nature of these
cases is discussed later.

13. Lax and dense ears of Wheat and Barley give
different results according to the varieties used. Sometimes
F^ is lax, sometimes it is intermediate (Spillman, 247;
Biffen, 27, 28). See Fig. 6. In Barley an increase in ear-
length has been observed (Fig. 5).

14. Development of fibrous parchment-like lining to
pods, and the absence of the same which constitutes the
"sugar peas." Pisum. In Phaseolus (kidney-beans), where
similar types occur, the evidence is that the dominance is
reversed (Emerson, 120, 121).

This is one of the features originally Investigated by
Mendel. He regarded the parchmented type as a dominant.
In our experiments F^ has always had some parchment but
the quantity is so much reduced as to cause the heterozygote
to have a very distinct appearance (R.E.C. 20).

15. Much serrated and litde serrated edges of leaves.
Urtica (cp. Phyteitma, Correns, 70, p. 197). This cross

11]

Struchtral Characters : Plants

was described by Correns {y"]) who gives a striking diagram
representing his results. The cross was made between
two forms known as Dodartii and pilulife^^a, which were

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regarded by Linnaeus as distinct species. The ahiiost
entire-leaved Dodartii has been treated by later authors
as a variety oi pihilifera.

structural Characters : Pla^tts

[CH.

1 6. Palmatifid or "palm-leaf" and pinnatlfid or
** fern-leaf." Primula Sinensis (Fig. 7).

The fern-leaved form arose in English horticulture about
i860 as a variation from the normal type. I have had
opportunities of seeing its genetic behaviour on a large
scale at Messrs Sutton's, and many experiments have been
made with it by Mr R. P. Gregory in conjunction with me.
Dominance is usually complete, but at Messrs Sutton's I
have seen on two occasions strains containing plants of
intermediate leaf-shape, which were presumably hetero-
zygous, for the two types occurred on sister-plants. The
leaf-shape is entirely independent of the colours and other
features of the plant, and can be transferred bodily from
one colour-type to another. Messrs Sutton's varieties
" Mont Blanc" and "Sirdar," for example, are sold both in
the palm-leaved and in the fern-leaved forms.

Fern-leaf {R) Palm-leaf (Z>)

Fig. 7. The two types of leaf found in Pi-imula Sinensis.

17. Leaves and petals normal or laciniated. Cheli-
doniu7n majus. de Vries (290) and (298), i. p. 134.

This case is interesting in comparison with No. 15. In the Nettle,
serration is a dominant, while here laciniation is a recessive. A careful
study of the physiological distinction between the two processes would
probably lead to important results (cp. Leake, 170, on leaves of Cotton).

II] structural Characters : Plants 25

18. Certain leaf characters in Capsella bitrsa-pastoris.
Shepherd's Purse. (Unpublished work of Shull ; about to
appear as a publication of the Carnegie Institution.)

19. Various characters in the seed of Cotton. Balls (6).

Many of these are of great commercial importance. Balls (6) gives the
following list :

Dominant. Recessive.

Long staple. Short staple.

Regular distribution. Irregular distribution.

Coloured lint. White lint.

Silky lint. Harsh lint.

More fuzz. Less fuzz.

He says that all the desirable characters are dominant, and that hence
the chances of picking out a stable form by common selection are very
small. Individual selection must be adopted.

20. Biennial habit and annual habit. Hyoscyavius.
Correns (j'^).

More research on the relations of annuals to biennials is greatly to be
desired. Points of the highest physiological interest are involved. In
connection with root crops also some questions of commercial importance
are raised. In R.E.C. (19, p. 135) I ventured to suggest that the persist-
ence of " runners " which go to seed in such plants as Beet and Mangel
may be due to want of individual selection of pure dominants, and in view
of Correns' observation the probability of this suggestion is increased.

21. Normal stem and fasciated stem associated with
peculiar distribution of inflorescences. Peas [Pisiwi).
Mendel (195), R.E.C. (20). In our experience various
intermediates occur in F^,

22. Susceptibility to rust-disease [Puccinia ghimariuii)
and resistance to the same. Wheat. Biffen (27, 29).

This is perhaps one of the most important instances to which
Mendelian method has yet been applied. Using a variety very susceptible
to rust and another practically immune to its attacks Biffen found that F^
was not perceptibly less attacked than the rusty type. F^. showed ordinary
segregation, and the green, resistant plants, standing among the yellow
rusty ones, formed a very striking spectacle. The recessives bred true and
their progeny has remained rust-proof. It has not yet been shown to what
the resistance is due. Working with Professor Biffen, Miss Marryat (193)
found that the rust-hyphae are checked after entering the stomata of the
resistant plants. If, as may be suspected, the resistance is due to the
presence of some anti-toxin, the dominance of "susceptibility" must be
taken to indicate that the formation of the anti-toxin is prevented by the
presence of a factor in the dominant forms, a conclusion which may lead to
definite progress in the physiology of disease-resistance.

26 structural Characters : Plants [ch.

23. Flat standard and hooded standard In the flower of
Sweet Pea. R.E.C. (22). See Plate V. Some very curious
phenomena have been observed in this case, which are
described In Chap. ix. The type known as "Snapdragon,"
perhaps an extreme form of hood, is also a recessive to the
flat type. R.E.C. (20, p. %i).

24. Imbricated petals and stellate or "star "-type.
Pri7nula Sine7isis. Observations made at Messrs Sutton's
and experiments of R. P. Gregory with W. Bateson. (See
Fig. 14.)

25. The monstrous condition of the calyx In which it
resembles the corolla, seen In " hose-In-hose " Campanula is
an imperfect dominant to the normal. Associated with this
homoeotic variation, the female organs are more or less
completely sterile in certain strains. Correns (76). This
subject is discussed in Chap. xi.

26. Abortion of the female organs in the lateral florets
of Barley, as found in the 2 -rowed types, and the complete
or hermaphrodite development of the florets, as in the
6-rowed types (Fig. 8). Tschermak (270) and (275), p. 11.
Biffen (30).

This case is somewhat complex. There are three types, (i) Six-row,
in which 3 perfect hermaphrodite florets are developed in each spikelet.
All set seed and the result is that the ear has 6 rows of seeds. (2) Types
in which the lateral florets have anthers but no female organs. (3) The
"Abyssinian" type in which the lateral florets contain neither male nor
female organs. The types (2) and (3), being able to make seeds only in
the central florets of each spikelet, alike develop two rows of seeds.
When (i) is crossed with (3), F^ is like (2) ; and (2) crossed with (3) gives
F-^ also like (2). Some further complexities have been observed, but in
general it appears that the dominant factor has the power of partially
preventing the formation of the reproductive organs in the lateral florets.
The facts may perhaps be interpreted as bearing on the phenomenon of
Sex.

Tschermak (275) describes crosses between a 2-row and a "4-row"
type. From Professor Bifl'en I understand that the latter is in reality a
lax-eared 6-row type. F-^ is 2-row, and in 7^2 the ratio is 12 2-row :
3 "4-row": I 6-row. This is a special case of the ratio 9:3:3:1, lax-
ear and 2-row being dominants. Tschermak and Shull (242) regard it as
an illustration of the effects of a latent factor.

Striicttiral Characters : Plants

27. The two-celled type of fruit is dominant to the
many-celled type in Tomato. Price and Drinkard (221).

The case is one of the few in which the genetic behaviour of a meristic
or divisional feature has been investigated apart from any complexity
introduced by differentiation.

Fig. 8. Cross between Abyssinian 2-row Barley and a club-headed 6-ro\v
type. The middle figure shows F-^. The length of ear is increased.
(From Professor Biffen's specimens.)

28. Style short, associated with large pollen grains,
constituting the "thrum" type, and style long, associated
with small pollen grains constituting the *' pin-eyed" type.
Prbimla Sinensis and acaulis. Bateson and Gregory (17).

The short styled type has been found in the homozygous condition in
P. Sinensis but not yet in P. acaulis. For the relations of these types to
the "homostyled" form, see p. 68. Dominance is complete.

28 structural Characters : Plants [ch.

29. Long style and short style In Oenothera. (This
difference is probably quite distinct in nature from ordinary
heterostylism as seen in Primula, &c.) de Vries (290).
The same fact has been observed by Balls (6) in Cotton.

30. Normal long pollen grains with three pores, and
rounded pollen grains usually with two pores. Sweet Pea
{Lathyrus odoratus). R.E.C. (20, 21, 22), See Fig. 19.

31. Normal anthers and sterile anthers. Sweet Pea.
R.E.C. (20, 21, 22).

With regard to these two last features numerous complications occur,
which are described in later chapters.

32. Roundness of seed connected with the presence of
starch in large elongated simple grains, and wrinkledness
of seed connected with the presence of peculiar compound
starch-grains. Piswn.

This is one of the most familiar of Mendel's original
examples (see Fig. 3). It has been re-investigated by
many observers. Correns (60) ; Tschermak (269, &c.) ;
R.E.C. (20); Hurst (155); Lock (172, 173). The F^ seeds
made by fertilising an emasculated flower of a wrinkled
variety with pollen from a round variety, or vice versa, are
generally ordinary round seeds, and F ^ shows the common
ratio 3 round : i wrinkled, the two types being mixed in
the pods of the F^ plants.

Among the multitude of varieties of peas now cultivated there is a great
diversity both of rounds and of wrinkleds. The interrelations of these
several types, even as regards seed-shape, have as yet been imperfectly
explored. The degree to which the wrinkles are formed is fairly uniform
for any one type, but the various types show different degrees of wrinkling.
The differences obviously depend chiefly on the chemical and physical
properties of the reserve-materials in the cotyledons, and an analysis of
these peculiarities might lead to further discoveries.

Gregory (134) found that the starch in round peas occurs chiefly as
large elongated simple grains, whereas in wrinkled peas it is in the form of
small grains of irregular shape which are often compoimded together (Fig. 9).
Darbishire (94) added the interesting fact that In F^ the grains are
intermediate, many being large and simple, but round instead of elongated,
with an admixture of compound grains. He confirmed also Denaiffe's
observation* that wrinkled take up more water than round, but he found
that F-^ is intermediate in this respect, and he suggests that the size, the

* Denaiffe, Les Pois poiagers^ p. 9.

Structural Characters : Plants

shape of the grains, and their simple or compound nature, may be
governed by distinct factors. He regards the absorptive power as again
separable from these features.

^oM

(PO

■^cJ

"Cr

(?/

c9.??;(g)

000 o-„

o 0

Round and Indent.

Wrinkled.

Fig. 9. Outlines of starch grains in the different types of peas. The
wrinkled contains many compound grains. (From Gregory.) Magni-
fication the same in both figures.

A third type of pea, of which the purple sugar-peas (sa/ts parcJicmin)
are a good instance, may be described as "indent." These also have
large, simple starch grains. Such seeds are of irregular flattened shape and
may be confounded with true wrinkled peas. Their properties are entirely
different, and the two sorts must be carefully distinguished. One of their
special properties will be discussed in a later chapter, but here it must
suffice to say that their genetic properties are essentially those of round
peas. Much confusion has been introduced by want of care in distinguish-
ing these types.

Intermediates, which on casual sorting, cannot be classed either as
round or wrinkled, sometimes occur. Some of the round types {e.g.
Victoria Marrow) contain a large proportion of such seeds. Their
peculiarity is almost certainly due to environmental influence, though
obviously the liability to this affection may be transmitted. When such
seeds are found in F^ from a cross between thoroughly round and wrinkled
varieties, the pitting, when it exists, generally affects all the round seeds of
Xh^pods in which it occurs. With experience such pitting can immediately
be recognized as distinct from the true genetic wrinkling, and in our
experiments the results of a further sowing have repeatedly confirmed the
judgment made by inspection of the seeds.

A complete account of all the phenomena would run to great length.
The interrelations of round and wrinkled seeds are to be recommended as
offering perhaps the most favourable example for an investigation of the
chemical nature of a genetic factor. The wrinkling is evidently the
consequence of a particular method of drying, and this must depend on the
nature of the reserve-materials. A first step would be to determine the
relative amounts of sugar and starch in the two chief types. It is natural

Strtictttral Characters : Plants

[CH.

to suppose that the wrinkled peas are those in which the transformation of
sugar into starch has gone less far than in the round peas ; but, as much
starch is formed in the wrinkleds, one ferment having this transformative
power must be present in them. Hence we are led to suppose that in the
round pea a second ferment is present which can carry the process further.
As offering an attractive problem in physiological chemistry the phenomena
are recommended to those who have the requisite skill to investigate them.

'^'^. Starchy endosperm giving a full, rounded seed,
and sugary endosperm giving a shrivelled and wrinkled
seed. Maize. Fig. lo. de Vries (290); Correns (63); Lock

(172, 174)-

Fig. 10. A cob of Maize [Zea mays) borne by an F^ plant from the cross
round x wrinkled, fertilised with its own pollen, showing the mixture of
round (dominant) and wrinkled (recessive) seeds. (From a specimen
given by Dr Webber.)

Of the various Mendelian experiments this is one of the most demon-
strative. Dominance is perfect so far as external observation goes. Correns
records a remarkable excess of round seeds as recurring with great
constancy in certain families when F^ is self-fertilised (see later).

It often happens that pollen from one variety of maize is blown by the
wind to the stigmas of another variety. If this pollen possesses a dominant
factor capable of affecting the seed, seeds exhibiting it are formed. If for
instance pollen from a round maize is blown on to a wrinkled or sugar-corn,
round seeds will be formed among the normally wrinkled seeds. When
formerly it was supposed that the endosperm, which contains the reserve-
materials, was a maternal structure, the change in the seed was regarded as
an influence exerted by the embryo on the maternal tissues. The effects
of such influences were called by Focke '''' Xenia.^'' There are a few
examples of such influence which may with probability be regarded as
genuine*; but since the discovery of the fact that the endosperm of maize
results from a double fertilisation effected by the second nucleus of the

* The phenomena are discussed by Darwin, A71. and Fits., ed. 11. 1885,
I. pp. 428-433. It seems likely that in some of these instances the factor
introduced by the pollen-grain can influence or infect tissues in contact
with the embryo.

II] structural Characters : Plants 31

pollen-tube, cases like that of maize are not strictly to be classed as Xenia
(see Correns, 58).

34. Glutenous and starchy endosperms. Wheat. Biffen
(27).

Professor Biffen's researches respecting these important features are
not yet completed. The glutenous, translucent, hard type has definite
dominance over the opaque, soft, starchy type..

35. Single flowers usually behave as dominants to
doubles, as in Stocks, Primula, &c. In Carnations the
doubleness dominates.

The most extensive researches on the genetics of double-
ness are those of Miss Saunders in the case of Stocks
{Matthiola), R.E.C. (20-23). The peculiar phenomena
discovered are discussed in a separate chapter {q.v.).

36. In Phaseolus hypo-geal cotyledons are dominant
to epi-geal. Various intermediates in F^. Tschermak
(278, p. 54).

This list and that which follows make no pretension to
completeness. Those features are enumerated which either
seem of special interest, or have been studied with some
thoroughness. Indications respecting many more are to be
found in the original papers (see especially for Peas and
Phaseolus the writings of Tschermak and Lock ; for
Cotton, Balls; for Oe7iothera, &c., de Vries, and jMacdougal
(186); for Wheat and Barley, Biffen, and Tschermak; for
Maize, Correns, and Lock; for various plants, Correns, and
de Vries).

In the orenus Brassica numerous crosses have been
studied by Sutton (262). In his experiments it was found,
among other important results, that the bulbing of the
Swede, Turnip, and Kohl Rabi disappeared completely in
crosses with non-bulbing Kales, and that in /% imperfect
bulbing reappeared. Professor Biffen, who is continuing
work on the same lines, tells me that in regard to these
and similar characters cultural conditions play a great part,
and lead to curious and conflicting results.

32 Strttctural Characters : Aiiimals [ch.

Animals. Structural Characters,

Man.

A considerable number of diseases and malformations
have been shown to behave usually as dominants. A few
conditions may be said, more doubtfully, to behave as
recesslves. The subject of human inheritance is discussed
in Chap. xii. Of normal characteristics, eye-colour is the
only one yet studied (Hurst, i6i)^ sufficiently to justify a
positive statement as to the existence of a Mendellan system
of descent.

Cattle.

37. Absence of horns in polled breeds of Cattle Is
dominant to the presence of horns (R.E.C. 19; SpIUman,

246).

In sheep the Inheritance of horns Is sex-limited {g.v.),
and from evidence given me by Mr E. P. Boys-Smith I
suspect that this is true in the case of Goats also.

Horse.

38. There is little doubt that the gait known as
"pacing" is recessive to the ordinary trotting gait In the
American trotters. Trotters bred together may produce
pacers, but hitherto I have found no authentic instance of
genuine natural pacers, when mated together, producing
trotters. Correspondents have sent me word of several
apparent exceptions to this rule, but all on inquiry have
proved to be erroneous. In the pacing gait the two legs of
the same side of the body are moved together or nearly so,
while in trotting the foreleg of one side moves almost with
the hind leg of the other. Horses may be trained with
more or less success to adopt either gait, but the distinction
between natural pacers and natural trotters is a fairly sharp
one (16). The physiological nature of the difference is
quite obscure, but presumably it is of nervous origin.

Mouse.

39. From time to time mice are found hairless, with the
skin thrown up into corrugated folds. Experimenting with
such mice Mr Archibald Campbell found the condition to

* See also Davenport (107).

II] structural Characters : Aniuials 33

be a recessive, the presence of normal fur being- a dominant.
The fur grows at first normally and falls off as maturity is
reached. Of 12 /^o mice 3 lost their hair. I am indebted
to Mr Campbell for information respecting this interesting
case, and for living specimens. The attempt to breed the
recessives together failed, but in Gaskoln's case* naked
parents produced young like themselves. From his account
it appears that the young which he observed never grew
their hair, but the fact is not absolutely certain from the
description. [See also Campbell, Zoologist, 1907, p. i, PI]

40. The normal condition and the "waltzing" habit in
Japanese mice. The waltzers exhibit a peculiar vertiginous
movement of the head when they come out into the light,
and spin often with extreme rapidity, running after their
tails till apparently exhausted.

Our knowledge of this case is derived from Von Guaita
(135) and Darblshire (90). The dominance of the normal
type is complete, and in F ,^ waltzers reappear. The F ..
numbers obtained by Darbishire were 458 normals, 97
waltzers, where the expectation is 416 normals, 139 waltzers.
The deficiency may perhaps indicate a complication, but
m(3re probably it is due to the greater delicacy of the
abnormal mice, which was so great that all attempts to
breed them together were unsuccessful.

Rabbit, Guinea-pig.

41. Normal short hair and the long "Angora" hair
Rabblt, Guinea-pig, and doubtless Cat (see Hurst, 157;
Castle, 45 and 48; SoUas, ttnpublished; Castle and
Forbes, 55).

Castle (48), p. 64, gives important details as to the physiological nature
of the distinction between the normal and "Angora" hair, which he regards
as resulting from a special method of growth.

42. The rough or rosetted condition of the coat in the
Guinea-pig dominates over the normally smooth condition
(Castle, 48 ; Sollas, itnpublisJicd).

Castle found occasionally that animals partially rosetted occurred in F.^.

* For references see Bateson. Materials for Study of Variation^ 1S94.
p. 56. A good figure is given by Gaskoin, Froc. Zool. Soc. 1856.

B. H. 3

34 Strticttiral Characters : Animals [ch.

43. Polydactylism occurred in a Guinea-pig, the off-
spring of normal parents, and ran an irregular course in
its subsequent descent (Castle, 49).

Cat.

44. The abbreviated tail of the Manx Cat is a dominant
(more or less imperfect) to the normal tail (see Anthony, 2 ;
Hind, 151; Davenport, 98; Kennel, 166,^). Godron (J//;;/.
Ac. Stanislas, 1865) records a similar observation in the Dog.

45. Polydactylism is almost certainly dominant ; but,
as in other types, irregularities doubtless occur.

Fowls.

For the study of heredity Fowls are especially well
suited. In addition to their many colour-characteristics the
various breeds present a great range and variety of struc-
tural features.

Among the long series of offspring which hens of the
more fertile breeds produce, the descent of these charac-
teristics can be watched in families of ample length. The
chief papers dealing with Fowls are R.E.C. (19-22); Hurst
(156); Davenport (loi). The following is a list of the
principal facts already elicited as to the behaviour of these
structural features but much remains to be done.

46. Various shapes of comb, for example the rose comb
and the pea comb, are both dominant to the single comb.
The double or longitudinally split condition is also dominant
to the unsplit.

See pp. 61-7. Many of the finer details in regard to the heredity
of comb-shapes are not yet clear. The classification of the comb-types in
the newly-hatched chickens is generally very easy, but in occasional strains
forms intermediate between the pea and the single occur in F^, which may
probably be due to subtraction-stages of the pea factor {q.v.). Some of the
singles extracted in F<2, from various crosses have lateral "sprigs" — as
fanciers say. It is not impossible that these irregular processes are due to
additional minor factors, but they are subject to so much fluctuation that
their descent would be very difficult to trace. The comb of the Silky fowl
is a rose, + a trifid element which causes its posterior end to be divided into
three irregular points. In F^ from Silky x Single, regular rose combs are
produced in those individuals which have the rose factor without this trifid
element.

Attention may be called to the dominance of the median splitting of
the comb found in certain breeds, for the facts may have a bearing on the
genetics of meristic characters. Splitting of the comb may occur in one of

II] structural Characters : Animals 35

several distinct ways. It may affect mainly the anterior portion, or the
posterior. The split combs of established breeds have possessed ordinary
dominance; but a form of posterior splitting somewhat like that of the F-^
from Breda x Single occurred apparently as a mutation among extracted
singles, and exhibited a curious genetic behaviour suggesting irregularity of
dominance (20, pp. 108 and 113).

47. The normally webbed feathers are dominant to
the peculiar feathers of the Silky fowl.

48. Extra toe is usually dominant to the normal four-
toed condition, but exceptions occur.

This irregularity of dominance is exhibited by all cases of polydactylism
yet studied in birds or mammals. It seems to be a property of certain
strains. Some families run a perfectly regular Mendelian course, others
contain members with only the normal four toes, which are yet capable of
transmitting the extra toe. The numbers in such families are not favour-
able to the suggestion that the irregularity is caused by a definite disturbing
factor.

49. Crest is dominant to no crest.

F^ may contain individuals with crests far larger than those of the
parent crested breed, a fact which suggests that in breeds with small crests
{e.g. Silky) the full development of the crest is kept in check by some other
factor.

50. Feathered leg partially dominates over clean leg.

Both Hurst (156) and Davenport found dominance very irregular. F^
is intermediate, and traces of leg-feathering are occasionally seen in the
offspring of clean-legged birds.

51. "Frizzling," or turning back of the feathers, is
dominant to the plain straight feathers of the normal.

52. Normal size of feathers on the hocks, or tibio-tarsal
region, is dominant to elongation of these feathers to form
quills — the "Vulture-hock" of fanciers.

53. Muff, or tuft of feathers at sides of the bill and throat,
as in Faverolles, is dominant to no muff, as In ordinary breeds.

54. Imperfect development of coccyx and tail-feathers
with absence of tail, as In " Rumpless " fowls, is dominant
to the normal development of those parts. Davenport (101)
and Amcr. Nat. xliv. p. 134.

The case is exactly comparable with that of the Manx Cat. Davenport's
later paper shows that this is the right interpretation of the facts. It is also in
harmony with the observation given by Dai win in An. and Fits. cd. 2, 11. p. 4.

3—2

36 Striicttiral Characters : Animals .[ch.

55. Certain breeds (Houdan, Polish, Breda) have an
extraordinary development of the nostril, which is patulous,
with alae horizontal instead of curving downwards. This
peculiarity is recessive to the normal (Davenport, 10 1).
Hurst has observed the same thing and Mr Punnett and I
have similar evidence from the Breda. Davenport states
that in his experience the ''high" nostril is never combined
with a fully developed comb.

56. The tendency to go broody and sit on eggs
dominates over the absence of this instinct, characteristic
of several Mediterranean breeds. There is probably segre-
gation in regard to these two dispositions, but this cannot
yet be asserted positively.

In regard to fertility as measured by egg-production there is as yet no
clear evidence.

57. The loud and penetrating shrieks which the cocks
(and to a less degree the hens) of an Egyptian breed give
out when caught, were reproduced almost exactly by the
F^ generation from a cross with a non-shrieking breed.
Though numerical data in regard to such a character are
scarcely attainable, there is little doubt of the segregation
as evidenced by /%.

Pigeons.

58. The normal foot is dominant to the webbed con-
dition of the toes which sometimes occurs as an abnormality
(Staples-Browne, 254).

Mr J. L. Bonhote tells me that in his experiments webbed birds have
produced normal offspring. He is making further experiments with this
family.

59. The ''shell," or turning-back of the head-feathers
of the Nun is dominant to the normal plain head {ibid.).

60. Birds with normal, 12-feathered tails crossed with
the many-feathered Fantall give Intermediate numbers in
F^. In F^ 12-feathered tails reappear, but, so far, no real
Fan has come from the cross-breds. Mr Staples- Browne, to
whom I am indebted for this Information, will publish a
complete account of his evidence. He tells me that the
extracted 12-feathered birds do not breed true, but may
throw birds with 13 or 14 feathers.

II] Structural Characters : Anijuals 37

Canaries.

61. Crest is dominant to plain-head, as the non-crested
condition is called by fanciers (R.E.C. 19, p. 131 ; Daven-
port, 105).

The type of crest which fanciers admire consists of feathers neatly
laid down over the head. To produce such birds crested individuals are
bred with plain-heads^ and it is clear that the exhibition type of crest is
a heterozygous form. When crested birds are bred together it is said
that an ugly, standing crest frequently is produced, and presumably this
is the homozygous type of crest. The mating of two crested parents is
by several authors said to give rise to some bald birds. Other writers
{e.g. Blakston) have ridiculed this statement, and formerly I was inclined
to regard it as a mere exaggeration, but Davenport in his recent paper
mentions bald heads as sometimes occurring among his crested birds.
He has kindly supplemented his published account with the statement
that the bald patch is an area "on the back of the head varying from four
to six millimetres in diameter practically without feathers and remaining
featherless throughout life. The crest, however, on top of the skull remains
perfectly evident, and often baldness can only be detected by blowing the
feathers." In no case was such a bald patch found in a plain-head.

The bald patch on the occiput is recognized by Blakston ( Cage Birds,
p. 104) as a property of crested birds, and presumably the "balds" alleged
to come from the mating of two crests are birds homozygous for crest-
factor, in which the crest stands up and allows the bald patch to be seen.
Davenport had a crested bird without any bare patch, and he found that
the feathering in this region was due to a separate dominant factor.

Animals and Plants in which Colottr-Characters have been
shown to have a Mendelian Inheritance.

The phenomena of colour-inheritance are complicated
in several ways. Some of these complications which are
of great importance and interest will be considered in
subsequent chapters. It is, however, convenient to enu-
merate the genera in which Mendelian heredity has been
observed in order to illustrate the scope of the principle.
The followino- list of ^renera contains the chief of those in
which heredity according to a Mendelian system has
been shown to occur. In some of them as the result of
extensive research many Mendelian features of colour have
been discovered, and the existence of numerous colour-
factors is demonstrated. In others only one such factor
for colour has been detected.

Colours of Plants

[CH.

Plants.

Antirrhinum (Snapdragon).

Atropa.

Brassica (Turnips and Swedes).

Clarkia.

Coreopsis.

Datura.

Gossypium (Cotton).

Helianthus (Sunflower).

Hordeum (Barley).

Hyoscyamus (Henbane).

Lathyrus (Sweet Pea).

Lychnis.

Matthiola (Stocks).

Mirabilis.

Orchids (several genera).

Papaver.

Phaseolus.

Phyteuma.

Pisum.

Polemonium.

Primula.

Salvia.

Solanum (Tomato).

Triticum (Wheat).

Verbascum (Mullein).

Viola.

Zea (Maize, Indian Corn).

Animals.

Man.

Mice.

Rats.

Rabbits.

Guinea-pigs.

Horse.

Pigs.

Sheep.

Cattle.

Cats.

Fowls.

Piejeons.

Canaries.

Axolotl.

Lepidoptera, various (Silkworm ; A-

braxas grossulariata ; Anger ona

prunaria, &c.).
Coleoptera {Li?ta ; Leptinotarsa ;

Crioceris).
Helix.

For the convenience of readers acquainted with the phenomena in
outline and desirous of pursuing the subject further the following brief
annotations are placed here. Until the chemistry of pigmentation is
better understood, a comparison between the behaviour and properties of
the several types cannot be instituted with much confidence.

Antirrhinum. Wheldale (303) has shown that the lowest or hypostatic
factor dominant to albino gives yellow in the "lips" of the flower; the
addition of various other factors produces anthocyan reds which superposed
on the yellow give deep crimsoti red colour. A second series of reds, more
purplish or magenta in tint (colour of wild A. majus), results from addition
of a factor which in absence of anthocyans gives an ivory colour. This
ivory is epistatic to yellow. It is remarkable that the lowest anthocyan
factor gives red in the tube with a tinge in the lips, while the addition of
the next above it gives the self-coloured flower.

There is also a wkiU-tuheA type of each colour-combination (''Delila"
of de Vries, 298).

All the factors except that for yellow and ivory can be carried by the
albino. Among the reds several heterozygous combinations can be recog-
nized. The heredity of striping is still under investigation.

Atropa Bellado7ina. The normal dark-fruited type is dominant to
yellow-fruited (de Vries, 290; Saunders, 19).

II] Colours of Plants 39

Brassica. White chromoplasts dominant to yellow in Swedes and
Turnips (Sutton, 262).

Clarkia elegaiis. Common magenta-red dominant to salmon pink
(Bateson and Punnett).

Coreopsis thidoria. Ordinary yellow type dominant to var. britn7ica
with brown flowers (de Vries, 290). The brown flowers like those of
C/ieirant/ms (Wall-flower) are no doubt due to presence of much dark
anthocyan, and the case is probably one in which the development of little
anthocyan dominates over the development of much anthocyan (cp.
Lathyrus, Prhmtla^ &c.).

Datura. Purple in flower or stem dominant to white flower and green
stem (de Vries, 290; Saunders, 19).

Gossyphi^n (Cotton). Dominance of many colour-characters in plant,
flower, and seed (Balls, 6). F^ details not yet published.

Helianthus, Purple disk dominant to yellow disk (Shull, 241).

Hordeum (Barley). Black pigment in paleae dominant to its absence
(Tschermak, 270; Biffen, 30).

Hyoscyamus niger anniius x H. iiiger pallidtis were found by Correns (69)
to give F-y flowers of intermediate tint.

Lathyrus {^sveQt Pea). Anthocyan colours dominant. Purples dominant
to reds. Colour depends on two complementary factors. Yellow chromo-
plasts recessive to colourless. Facts fully described in later chapters.
Plants with coloured flowers have dark seed-coats. Whites have seed-coats
colourless.

Lychnis. F^ between L. diurna and Z. vespertina has flowers of inter-
mediate tint ranging through many grades (see de Vries, 290 ; Correns, 69;
R.E.C. 19). Segregation imperfectly studied.

Matthiola (Stocks). Colours as in Sweet Pea (R.E.C. 19-21 .
Tschermak, 278; Correns, 61). For colours of seeds see R.E.C. 19.

Mirabilis. Colours consist of a complex series of reds and yellows, the
interrelations of which are not yet clear (see Correns, 67, 74, 77). Miss
Marryat's experiments (unpublished) prove the existence of a number of
heterozygous forms.

Orchids. Dominance of anthocyan colour in Cypripedium is clear.
In that genus it results from union of two complementary factors (Hurst,
Card. Chron. 1908, i. p. 173). As regards distribution of colour the facts
are complex, but several indications of Mendelian distribution have been
recognized (Hurst, 153, 160). See p. 96.

Papaver. Presence of dark purple spot at base of petals dominant to
the absence of such colour (de Vries, 290).

Phaseolus. The elaborate researches of Tschermak (271-3, 275, 278)
have demonstrated the existence of numerous factors controlling the colour
of the flowers and seed-coats in P. vulgaris, P. muliiftorus and their hybrids.
The flower-colours are purples, reds, and white, with a bicolour form of the
red ("Painted Lady"). Colour has not yet been produced by union of

40 Colours of Plajits [ch.

two whites, but on the analogy of the Sweet Pea such a result may be
attainable. Otherwise the same rules apply generally. At least two sets of
pigments take part in coloration of seed coats : (i) brown, (2) purple. White-
flowered plants have seed-coats unpigmented, and the bicolour flowers go
with parti-coloured half-white seeds. The development of purple in the
coats and the pattern in which it is deposited depend on various factors
which can be carried by the albino. Various complications were met with
(see originals). The cross between the two species showed some degree of
sterility.

Similar results were obtained by Emerson (120, 121). The seed-coats
of heterozygous plants in some cases were distinguishable. He also found
green pods dominant \.o yellow pods (cp. Pisum, p. 14).

Further facts^ with a scheme elucidating some of the curious ratios
which the seed-colours may exhibit {e.g. 18 : t8 : 6 : 6 : 16) in F2, are
given by Shull (242).

Phyteiima Halleri (dark violet) x P. spicatum (white) gave two types in
7^1, 5 plants being bright blue with violet tinge, 4 violet (Correns, 70).

Pisum (Edible Peas). Flower-colours of three types, (i) Purple.
Standard a pale purplish white ; wings deep chocolate purple. (2) Pink.
Standards pinkish white ; wings a fine salmon-pink. (3) White. P,
containing all three types is the usual 9 : 3 : 4 in order named. Mark in
axils of leaves, if present, is purple in (i), red in (2), absent in whites.
Tschermak experimented with a purple strain without the axil-mark, and
found that, as in Sweet Peas, the factor for that character can be carried by
the albino.

Seed-coats colourless or greenish in white-flowered plants. In plants
with coloured flowers one or more of three distinct kinds of pigments always
present: (i) a purple, occurring in spots, (2) a brown, distributed either
generally over the surface, or in bands (as in Maple peas), (3) an insoluble
greenish grey, distributed over the whole testa. Neither (i) nor (2) can be
developed in the absence of (3), but traces of (2) may sometimes be seen
in white-flowered plants. There are separate factors for (i), (2), and (3),
of which (i) and (2) may be carried by the whites (Lock, 176).

Cotyledon-colours are yellow, and green. Yellow is a dominant, and
heterozygotes are indistinguishable from homozygous dominants. In rare
cases green has been seen as exceptions in F^^ but these are probably
due to abnormal conditions. Many modern varieties have cotyledons
patched with green and yellow. Genetically these are greens which show
a special liability to bleaching. If protected while ripening they remain
green.

Colours of Pisum have been chiefly studied by Mendel (195); Tscher-
mak (269, 271-3); Correns (60); R.E.C. (20); Lock (172, 173, 175-6);
Hurst (155).

Polemoniiwi. Correns (70) found the white var. of P. coeruleum
dominant to the yellow of P. flavum ; and the blue type oi coeruleum y^fiavum
gave 7^1 blue. It may be inferred that the yellow of flavum is a chromo-
plast colour, and that the blue anthocyan dominated as usual. Hybrids
sterile.

Primula. P. Sinensis exists in a long series of colour-types the
relations of which are still being investigated by R. P. Gregory in con-

II] Colours of Plants 41

junction with me. Some of the more striking facts are referred to in
later chapters. White flowers with green stem constitute an albino,
recessive to all colours. The magenta shades have a factor epistatic
to crimson and pink. Blue is hypostatic to all the red shades. Tlic
whites which have red or reddish ste?ns are do?ninant whites, showing only a
pale shade or tinge of colour in F^. Deep colours cannot appear on
stems that are not red except in the white-edged "Sirdar" {q.7K).

Salvia Horminum. Purple, red, white, related as in Fisuni^ &c.
Saunders, R.E.C. (20).

Triticum (Wheat). Red chaff is dominant to white chaff. Grey chaff
is epistatic to red and dominant to white. Tschermak (270) ; Biffen (27);
Spillman (247).

Verbascum blattaria. Yellow (a sap-colour) dominant to white. Shull
(241).

Viola. White is recessive to colour (de Vries, 290) in V. conmta.
The brown seed-colour of V. papilionacea is dominant to buff of V.
hirsutula, and the purple of the capsule of hirsiitula to the absence of
purple vci papilio7iacea (Brainerd, 41).

Zea (Maize). Yellow endosperm dominant to white. Blue in aleurone
layer an irregular dominant to absence of blue. (Definite exceptions are
frequent.) Red pericarp, a plant-character, dominant to absence of red.
The relations of the striped types have not been clearly determined.
Correns {(i'^\ Lock (172, 174).

Colours of Animals.
Man.

Eye-colour (^.z'.), Hurst (161), Davenport (107).

Albinism {q.v.) is doubtless recessive, but in man its descent is complex
and has not yet been elucidated.

Red hair is recessive to dark hair and perhaps to ordinary brown (see
Hurst, 162).

Cattle.

Red -roan is a heterozygote of red and white (Wilson, 311); and blue-
roan is similarly related to black and white, Spillman (249) suggests that
black is dominant to red.

Cats.

Red is dominant to black in males. Tortoiseshell is the corresponding
form of the heterozygote in females. Doncaster (109). Dilution-types,
blue, and cream, recessive to saturated colours.

As to eye-colour see Przibram (224).

Mice, Rats, Rabbits, Guinea-pigs.

Colours fully discussed in later chapters. Chief papers by Allen (i) ;
Bateson (10); Castle (48, 53); Crampe (83, «); Cuenot (84-9); Darbishire
(90); Durham (it6); Hurst (157); Mud.ce (204). Albino recessive in all
cases. A piebald type dominant to self-colour exists in mice (Durham,
116) and in rabbits, the so-called "Enghsh" variety (Hurst).

42 Colours of Animals [ca

Horse.

Chestnut recessive to bays and browns. Relations of these two domi-
nants to each other not clear. Hurst (158).

Pigs.

Several notes published by Spillman (249, 251-2). White is usually a
dominant to colour in domesticated races* but piebaWs are frequent in F-^.
The relation of black to red is not yet clear. The white belt, characteristic
of certain breeds, is, according to Spillman, due to a complementary pair of
factors which may be separately carried by self-blacks. He makes the
interesting suggestion that the appearance of the belt may be a "reversion"
to a condition like that of the Indian Tapir (251).

The colour of the wild boar is dominant to the red of Tamworth and
segregates normally from it (252). The wild colour is presumably due co
an " agouti " factor like that of the rodents.

Sheep.

From such fragments of evidence as I can find it seems that the white
of ordinary sheep is not, as in the pig, a dominant to colour, but a
recessive. From Darwin's record (Afi. and Pits. 11. p. 4) of the appearance
of all black sheep from a cross between white Southdown ewes and a
Spanish ram with two black spots, it may perhaps be inferred that the
black colour is due to complementary factors.

Black face and white face give a speckled face in the heterozygote.
The dark ring round the eyes depends on a separable factor (Wood; 312).

Fowls.

Colours very complicated and genetics imperfectly understood. Whites
are of various kinds, one being dominant and at least two recessive.
Colour depends on complementary factors which may be borne by whites.
Black is an imperfect dominant to black-red. Brow^n-red a dominant to
black-red (Fig. 11). Blue is a heterozygous colour of black and a splashed
white. The red and yellow pigments of the black-red cock may be replaced
by white, thus giving the Silver Duckwing, but in the hen the red of the
bi-east is not thus replaced, and the Duckwing hen differs from the black-
red in having the yellow of hackle and mantle replaced by w^hite. These
replacements may occur as consequence of recombination in F^ from
crosses between white and coloured breeds, whence it is to be inferred that
the replaced reds and yellows depend on a special factor. Pencilling is a
dominant to its absence, and various mottlings are also dominant. The
descent of colour is influenced in some cases by sex in ways not yet clear,
and in both sexes heterozygous types occur.

The relations of the buff of Cochin (and of other breeds derived from
it) to other colours are not yet known.

As regards colour of the dow^n, the brown striped condition is dominant
to the pale brown down associated, lor example, with Wheaten. Both
types of down may occur in the same breed ie.g. Indian Game), and the
fact seems to have no relation to the adult plumage. R.E.C. (21, 22).

* Mr Staples-Browne has given me confirmatory evidence.

II] Colours of Animals 43

The black body-pigmentation of the Silky is a dominant, but may be
inhibited by another factor, the descent of which is sex-limited. See Cha[). x.

Another sex-limited descent is to be found in the relations of Cuckoo
to black, but the details have not been ascertained (Sjjillman, 249, 253, a).

The daw-eye is recessive to red. The dark iris is usually a dominant
to red.

Red ear-lobe is an imperfect dominant to white. R.E.C. (19).

Principal papers dealing with these features are R.E.C. (19-22),
Hurst (156); Davenport (loi).

Pigeons.

Black is dominant to blue. Relations of red and yellow not clear.
Black and blue are dominant to white of Fantail ; heterozygotes generally,
if not always, having some white. Chequering dominant to its absence.
The white rump of the Rock-pigeon is dominant to blue rump (Staples-
Browne, 255).

Cana?ies.

Presence of black, as in green and pied types, dominant to absence of
black as in the various yellows and cinnamons. The pink eye of cinnamons
is recessive to black eye, with a sex-limited inheritance. There are probably
several heterozygous colours, but in order to determine these the genetic
interrelationships of the various Yellows, Jonque, Mealy, must be worked
out. The "cap" and lacing of the Lizard are dominants. (Noorduijn,
213-5; Davenport, 105; F. M. Durham, impiiblished^

Axolotl.

Crosses between normal and albino gave dominance of pigmentation.
Subsequent generations showed remarkable and as yet unique features. In
F^ dark larvae to white larvae were 3:1; but the white F^ larvae, though
remaining red-eyed, acquired a certain amount of pigment, sometimes
distributed as a metameric chequering. No thorough albino occurred in F^.
When however these chequered albinos were bred with a true albino, the
latter was found dominant and true albinos were produced (Hacker,
143-4). Hacker compares this case with the phenomena seen in Mice,
&c., but there is an essential distinction on the fact that in all other
instances true albinos come in F^ and in the dominance of the true
albinism over the chequered character. It would be interesting to see
whether the development of pigment in the F^ whites is in any way
dependent on conditions.

Lepidoptera.

Bombyx mori (Silkworm). Brown colour seen in a dark variety of the
moth was proved to be an imperfect dominant (Coutagne, 83, p. 122).

The larvae have many colour-types. Coutagne used a dark '^fnoricautf'
variety, a variety with transverse stripes, and ordinary white larvae. Both
the coloured types are dominant to w^hite, but when the dark self-colour
factor and the stripe-factor are present in the same larva the stripes show
on the dark ground-colour {ibid. p. 142). Toyama (268) also made many
experiments with the colours of the larvae. He found striping a dominant
over plain white. In certain Fc^ families from striped x white a new pale

44 Colours of Animals [ch.

form was produced (p. 349). The ratio of striped : common marked
white : pale unmarked white is as ShuU points out (242) 12:3:1 (actually
1463, 363, 126). I incline to interpret this as signifying that the 12 striped
were in reality of two kinds, in the ratio 9 : 3, but that the distinction
between the common and the pale was not easy to detect in the striped
class. On this view the striped parent was a "pale." Shull regards the
paleness as " latent " in both parents.

As regards the colour of silk there is a complication. In Toyama's
experiments yellow was always a dominant to white. Coutagne sometimes
obtained this result, but (/. c. p. 123) the white of a race called "Blanc des
Alpes " proved to be dominant to yellow.

Abraxas grossidariata (Currant Moth). The type is dominant to var.
ladicolor {q.v.), Doncaster (iii, 114). See Plate I, figs, i, 2. The pecu-
liarities of this case are discussed in connection with Sex.

Mr L. W. Newman has been good enough to send me information as
to a cross between A. grossulariata and the var. varleyata. This is a nearly
black suffused form (see Porritt, Ent. Rec. xv. p. 10). F^ was typical
grossula?'iata, and in /^2 there were 24 typical and 7 varleyata (46", 3 ? )•

Ajigerona priuiarta. The dark-banded var. sordiata dominant to the
normal, reticulated type. In the heterozygotes the lighter bands are
more or less reticulated. Doncaster (m). See Plate I, figs. 7-10.

Xanthorhoe ferrugata. The form with purplish band is dominant to that
with black band. Prout, L. B. (223).

Honerophila abruptaria-. the dark var. may be inferred to be a dominant
to the type, from the experiments of Harris (147). Plate I, figs. 5, 6.

Amphidasys betulaiia (Peppered Moth). The normal is almost
certainly recessive to the black, or doubledayaria form (see, for example, the
records of Main and Harrison, 192). Plate I, figs. 3, 4.

Triphaena co?7ies : the reddish form is recessive to the melanic (see
Bacot, 3, and Prout, 222).

CalliiJiorpha domiiiula : red of the hind wings is dominant to yellow.
Standfuss (253, ^, p. 222). Mr L. W. Newman has kindly given me in-
formation that he bred 34 reds and 10 yellows in 7^2-

Aglia tau: type is recessive to the dark form bigens. Standfuss (253, b,
p. 311), (see Chap, x, for the sex-distribution of these varieties).

Lasiocampa quercus (Oak-egger). The heredity of colours of the hairs
of the larvae has been investigated by Bacot (4) and Warburg (302).
Several varieties were studied and their genetic interrelationships are not
altogether certain, but it appeared that red fur of var. sicula was dominant
to the white of the var. me?'idw7talis. \\'hen English and French races were
crossed, various blend-forms were produced in F^'

COLEOPTERA.

Most of the observations thus far made relate to Phytophaga. Com-
plication's were met with in all the cases investigated by Miss McCracken.
I have not been able clearly to understand the exact procedure followed in
the matings and must refer the reader to the original papers.

Plate I

• •

^' *

# ^

I. Abraxas grossiilariata. i. Ditto var. laiticolor. 3. The var. nouhlcdayana
»f 4. AinpJiidasys betularia. 5. Dark \ar. of 6. Ifcnwrop/iila abrufflaria.

and 8. Male and female var. sordiata of 9. and 10. Afij^t'roNa pninafia, male and
emale

II] ■ Colours of Animals 45

Lt'na lapponica. Two forms occur, one having elytra spotted with
black on a brown ground, while in the other the elytra are entirely black.
The latter is recessive, and the formation of the brown pigment in the
ground is thus due to a dominant factor. McCracken (189, 199).

Melasoma {Lino) scripta. A totally black form was found to be re-
cessive to the spotted type. Two intermediate conditions may occur, one
of which may be a homozygous type (see original). McCracken (191).

Gastroidea dissimilis. Two forms, either deep blue-black, or shiny
bright green. The latter is recessive. Curious complication as regards
numerical results. McCracken (190).

Leptinotarsa decemlitieata (Colorado potato beetle). Evidence obtained
by Tower (266, pp. 275-9) indicates that a variety called by him pallida
behaves as a recessive. The var. melaiiothorax of the species Z. viulti-
taeniata also proved to be a recessive to its type {ibid. pp. 284, 292, 293).
Various other more complex phenomena are recorded {q. v.).

Crioceris asparagi (Asparagus beetle). Each elytron has three yellow
areas or spots on a blue-black ground. The upper spot is sometimes
united to the middle one. This condition proved to be recessive to that
in which the spots are separate, but all intermediate conditions occur
[being presumably heterozygous]. Lutz (182).

MOLLUSCA.

Helix hortetisis and H. netnoralis. The u?ibanded variety is dominant
to the banded types in both the species, sometimes completely, some-
times partially. Generally also red ground-colour is dominant to yellow or
brown, but this effect may diminish with age of the hybrid individual.
Lang (167-9). For details as to hybrids between the two species see
(169).

As regards dominance of colours very little in the way
of general rule can yet be predicated, nor till the chemistry
of pigments is much better understood is it likely that such
general rules will be discovered. It may, however, be
remarked that actual albinism, the total absence of pig-
mentation, is alw^ays, so far as we know, a recessive
character in both animals and plants. Curious cases never-
theless are known both in animals and plants where a
partial whiteness, which we should a priori imagine to be
a kind of albinism, behaves as a dominant"^. Another fact
of a somewhat paradoxical nature is to be seen in the
behaviour of some of the very deep colours, red and purples,
characteristic of the flowers and other parts of some garden
strains. These more intense colours both in Primula
Sinensis^ in the Stock, and in the Sweet Pea (and doubtless

* See Chap. v.

46 Preliminary Deductions [ch.

Cyclamen) are recessive to the paler and more commonplace
tmts^.

Though in the case of colours in plants which are clue
to the development of pigmented sap, albinos are recessive
to the coloured types, the yellow or cream colour due to
the presence of yellow ch7^omoplasts is recessive to the
colourless condition of the chromoplasts. Hence we find,
what at first seems paradoxical, that white flowers are
dominant over cream-coloured fiowers. Yellow dependent
on sap-Q.o\o\xx is dominant to the corresponding white.

With regard to the behaviour of black pigment, which
might naturally be supposed to have similar genetic pro-
perties in the various animals, no quite satisfactory general
rule can be laid down. The presence of black pigment is
commonly dominant to the absence of black, as in the race-
horse, where chestnut, namely the absence of black ''points"
is recessive to the presence of such "points" as in bays and
browns. Most cases, however, such as that of the mouse,
and other animals in which black pigment exists intimately
mixed with other pigments are not so simple as this and
involve special problems. In so far as the features of those
cases can be expressed in the simple terminology hitherto
used, these blacks must be classed as recessive to the normal
colours. Further particulars will be given in the chapters on
Colour.

P7^eliminary Deditctions from Mendelian Phenomena.

It will be observed that animals and plants, as such,
do not show any difference in their manner of heredity.
Inheritance on simple Mendelian lines may be followed
by characters of very diverse kinds, such as height, shape,
chemical constitution, colour, and several structural features.
In view of such a list the important question arises whether
there is any distinct category or class of characters to which
the Mendelian system does not apply. Various possible
limitations may be discovered when the phenomena have
been more fully examined, but it may be stated at once
that no such class of characters has hitherto been identified.

* In Antirrhmum Miss Wheldale finds that the deeper magentas are
recessive to the ordinary magentas, but in the crhnson-rid series the paler
are recessive to the deeper tints.

II] Meristic Cases

As yet only one example of a character which can at
all readily be interpreted as meristic in nature has been
shown to have a Mendelian inheritance. This Is the case
of the reduction in number of the human phalanges in
brachydactyly. We speak of a character as " meristic "
when it manifests itself in respect of the number of parts
into which the body or one of its organs is divided. Meristic
characters are in several ways distinguishable from other
features of bodily organisation. The physiological occur-
rences which result In meristic variations are In all likelihood
distinct from those which produce substantive changes, and
exceptional Interest would attach to any Investigation of
the genetic properties of such variations. Polydactylism Is
of course a meristic feature, but It may involve something-
more than a divisional change, pure and simple, since
change in the number of digits is usually accompanied by
chano-e In the distribution of differentiation. A case In
which the disturbance of differentiation is not so evident Is
provided by the cross between Oxalis tetraphylla^ much
cultivated In Germany as Glilcksklee, and one of the forms
with three leaflets. This cross was partially investigated
by Hildebrand"^, who used O. latifolia. He found that
the 3-fold character was an imperfect dominant, the leaves
being 3-fold with the exception of occasional 4-fold leaves
which appeared for the most part at the flowering period.
The hybrids were fully fertile, but their progeny has not
been studied. Satisfactory meristic cases from which all
confusing elements are eliminated must be rare, but it is
greatly to be hoped that they will now be searched for. It
is most desirable that cases of difference in the ground-
plan numbers of some radial type will be found amenable
to experimental tests.' Here the problem may be seen in
a somewhat simplified form on account of the elimination
of serial differentiation t.

* Hiklebiandj/cv/^^T//^ Ztsch. f. Natiirw. 18S9, xxiii. N. F. xvi. p. 56.

t Since this paragraph was set up Price and Drinkard's (221) evidence
has been published showing the dominance oi two chambers in the fruit of
the tomato over the many-chambered condition. More evidence as to
such cases would be welcome.

Drinkwater's recent discovery as to the bones of the brachydactylous
fingers, showing that the middle phalanx is actually formed as a distinct
bone which afterwards unites with the distal phalanx, raises considerable
doubt whether the variation in that case is meristic after all.

48 Rights and Lefts [ch.

Respecting the genetics of one most interesting class
of variations evidence is scanty. This is right- and left-
handedness. From Mayer's^ observations on Partula
(Gastropod) we learn that parents of either twist may bear
young of either twist. The numbers in the uteri were so
small that the absolute numbers are insignificant, and it
may be an accident that no mixture of types was found in
anyone uterus. Langf bred numerous left-handed Helix-
pomatia with each other and obtained thousands of young,
all right-handed, which in their turn again produced ex-
clusively right-handed offspring. Direction of twist is a
fundamental meristic phenomenon, being, as Crampton and
Conklin have proved, determined as early as the first
cleavage-plane in the ^<g^ ; and great light on the problems
of cell-division might perhaps be obtained if the inheritance
of these differences could be determined. The only case
we have attempted to study, that of Medicago, in which the
fruits are right- or left-handed spirals according to species,
proved unworkable, perhaps on account of the minute size
of the flower and the roughness of the manipulations.

Lutz (181) has collected facts as to the inheritance of
the mode in which the hands are clasped, whether the right
or left thumb is placed uppermost. No definite result was
obtained, but effects of heredity were somewhat marked,
though neither condition bred true. Lutz kindly tells me
that a full analysis was made, taking families separately.

When the Mendelian principles were first rediscovered
the suggestion was made that though the system might
apply to the unions of pure races, there was no certainty
that such rules apply to the uncontrolled matings of natural
forms. The objection was not one which was likely to
have weight with those who had -an acquaintance with
genetic phenomena, but it had undoubtedly an effect in post-
poning general recognition of the importance of Mendel's
discovery. Categorical proof of the invalidity of this ob-
jection is now provided by one of the cases referred to
above — that which concerns the heterostylism of Primula.
It is scarcely doubtful that in the Primrose nearly every
plant arises by the ** legitimate " union of long- and short-

* Mayer, A. G., Mem. Miis. Comp. Zool. Harvard, xxvi. No. 2, 1902.
i" Lang, A., Vierteljahrs. d. ?iat. Ges. Zurich, 1896, and 168, p. 42,
together with information kindly sent in a letter.

II] Cases in Wild Types 49

styled individuals. Yet the long--styled are always pure.
Moreover, all the short-styled plants hitherto tested have
proved to be simple heterozygotes, giving equality of longs
and shorts when bred with longs. Hitherto no pure DD,
viz. short-styled plant, has been found in the case of the
Primrose, but no difficulty has been met with in raising
pure short-styled plants of Pri7nula Sinensis. Besides this
example of Mendelian heredity manifested by a wild type
several of the examples of colour-inheritance in insects
relate to wild species.

The circumstance that a character has not been pre-
viously bred pure does not, so far as is known, in any way
influence the mode of transmission of that character. For
instance, in the breeding of thoroughbred race-horses the
heredity of chestnut colour is that of an ordinary recessive"^,
though the various colours, bay, brown, and chestnut have
been indiscriminately united together in the breed. No
difference is manifested between colour-inheritance of chest-
nuts which have had many chestnut ancestors in recent
generations, and those that have no chestnut progenitor in
the nearer degrees. The same is true for some of the
colour-cases seen in Lepidoptera which had not been the
subject of any previous selection. A remarkable example
of an obviously Mendelian inheritance in a wholly wild
form is that of the eye-colour of the Owl — Athene
noctua\.

Abundant examples of characters breeding true, though
newly-constituted, will be provided by those cases in which
a novelty of structure is brought suddenly into existence by
the occurrence of fresh combinations. In spite of their
recent origin, such new combinations have just the same
genetic properties and powers of transmission that are
possessed by the types of long-selected breeds.

The suggestion hazarded by several writers that a dis-
tinction may be drawn between inter-racial and intra-racial
heredity has no foundation in fact.

* Mr Hurst, who first elucidated the colour-inheritance of race-horses,
found that according to the records, chestnuts of various ancestries have
exclusively chestnut offspring with about i °/^ of exceptions, wiiich are
very possibly due to error in the returns (see later).

t Giglioli, Ibis^ i903> P- i* (See later, p. iio.)

B. H. 4

50 Nature of Dominance [ch.

Dominance : the Heterozygote Character.

The character of the heterozygote, the ''hybrid character"
of Mendel, gives no indication as to the system by which
the parental characters are transmitted. The expressions,
'* blended" inheritance, "particulate" inheritance and so on,
terms formerly devised by Galton for describing the zygotic
appearances, are now seen to be descriptive not so much of
the mode of transmission as of the consequences of certain
groupings of special allelomorphs ; and as it is obviously
preferable in all possible cases to use the ultimate descrip-
tions reduced to terms of gametic composition, such terms
are now seldom requisite. Dominance must be discussed
more fully when other facts have been set forth, and in this
preliminary notice of the more salient features of the
phenomena it will be enough to point out that dominance
is no inseparable attribute of Mendelian inheritance. The
essential phenomenon is segregation.

The occurrence of dominance is often an assistance to
the investigator and may greatly simplify the analysis of
the various generations. Seldom however is dominance
uniformly complete, and in certain cases, as those of the
combs in poultry, where dominance is quite definite, it is
still possible for an observer thoroughly familiar with the
material to distinguish the homozygous dominants from the
heterozygous with fair certainty. Provided the recessives
as a class can be identified the application of Mendelian
analysis is almost equally easy whether the heterozygotes
show definite dominance or some intermediate condition.

The statement made by de Vries that dominance is an
attribute of the phylogenetically older character has not been
borne out by more extended investigation. In the lists
given above many examples to the contrary occur. No one,
for instance, can doubt that the various types of dominant
comb (rose, pea, &c.) in fowls and the colour called " Brown- fl
breasted" have arisen since domestication. This colour- \
example is illustrated by Fig. ii, where the distinction
between the striped Black-red type and the almost uni- t
colorous Brown-red, or " Brown-breasted " type is shown.
The striped type is practically that of the wild Gallus
bankiva, but the unicolorous type of down-colour is a com-

,n]

Nature of Dominance

plete dominant. Males raised as F^ between the two pure
breeds are in their ad nit plumage almost intermediate, but
the F^ hens are indistinguishable from the pure Brown-red
hens.

,■.•■■' '^m

"^ i^wBB

i ^^^^^^^

'i^^^^^^^H

. ».

A B

Fig. II. Two newly-hatched chickens in F^ generation from the cross
Brown-red Game Bantam x Black-red. A is the Black-red type
having dark stripes on a light ground. B is the Brown-red (or
"Brown-breasted") type, a dark, almost unicolorous, blackish brown.
A is the recessive and B is the dominant. The whole F. family
consisted of 58 like B and 18 like A.

As an example in which the heterozygotes are Inter-
mediate the inheritance of colour in the Andalusian fowl
may be taken. Andaluslans are in general colour what
fanciers call blue — namely a diluted black. In the cocks
the hackles and saddle-feathers are full black, and the
feathers of the breast are edged or '' laced" with black.
The hens are blue, laced with black more or less, all over.
This breed is recognized by the fanciers as never breeding
true to colour. When blue is bred with blue three colours
are produced, blacks, blues, and a peculiar white* splashed
with grey. Experimenting with this breed we have found

* These splashed whites are quite distinct from actual whites. They
are in reality coloured birds as regards composition, and their down-colour
is a faint bluish, very like that of the White Rosecomb bantam.

4—2

Andalusian Fowl

[CH.

that the numbers from blue x blue average about i black :
2 blues : i splashed white. Both the blacks and the whites
extracted from blues breed true to their respective types,
black X black giving all blacks ; white x white giving
all whites again. When however black is bred with one of
these whites the offspring are all blues. There is thus no
doubt that the blue is the heterozygous form, while the
gametes bear either the blackness, or the whiteness. Ob-
viously in such a case, continued selection of blues will not
make them breed true. This can only come to pass if it
shall be found possible to get a blue bird in the gametes
of which the blue or intermediate character is carried as
a definite factor. These results may be represented in
tabular form thus :

Blue X Blue

n Blacks

2n Blues

n Splashed whites

Blacks only n Blacks 2n Blues n Whites Splashed whites

only

Blacks only,
&c.

Splashed whites
only, &c.

Black X Splashed white

I
Blues only

n Blacks

2n Blues n Splashed whites

Such a case as this shows well what Mendel meant by
the *' hybrid-character." It is that character, or appearance,
or quality, which is produced by the meeting of the opposite
allelomorphs of the same pair in one zygote or individual.
The hybrid-character is a thing apart, which must always
be separately determined by experiment. Sometimes it is
indistinguishable from the dominant, sometimes, as here,
it is an appearance recognizably distinct from that of either
dominant or recessive.

Prof J. Wilson (311) has shown that a similar rule pro-
bably holds in the case of shorthorn colours, where red-

II] Imperfect Dominance 53

roan is a heterozygous character, caused by the meeting of
the factors for red and white ^. The blue-roan so often
seen in the cross between black Aberdeen-Angus cattle and
white shorthorns is presumably the corresponding hetero-
zygote form for black and white.

With further knowledge of the details and closer exami-
nation of material probably many such cases will be found.
Darbishire, for example, has lately shown that though it is
usually impossible in the case of peas to tell pure round
seed from the heterozygous rounds, by external appearances,
yet on microscopical examination the two classes can be
distinguished at once by the different structure of the starch
grains (94).

Many such cases where dominance is imperfect are now
known. This phenomenon has no bearing on the more
important question of the degree of perfection with which
segregation is accomplished. The supposition that domi-
nance was an essential phenomenon of Mendelism was of
course a delusion. Imperfection of dominance does not
even obscure the application of Mendelian analysis. The
cases in which difficulty does arise are those in which
dominance is irregular and the recessive class cannot be
distinguished with certainty. In the fowl, for instance, the
extra toe is usually a dominant, but in some strains there is
irregularity, and birds without the extra toe may neverthe-
less transmit it. So also the blue colour of maize seeds,
though usually a dominant, may sometimes be carried on
by seeds which appear white (Lock, 174). Even in these
examples, however, there is no reason to think that such
irregularities are indications of imperfect segregation.
It is not impossible that they may be ascribed to inter-
ference caused by the presence of other factors in various
combinations, and sometimes, no doubt, to disturbance by
, external conditions.
^%^ All observations point to a conclusion of great import-
ance, namely that a dominant character is the condition

* Further information based on a long series of observations of the
Sittytown herd of Shorthorns has since been pabHshed by Mr Robert Bruce
in Bj-eeder's Gazette^ 25 Nov. 1908. Full statistics are given, aftbrding with
rare exceptions evidence strongly confirmatory of Prof Wilson's views. I
am obliged to Mr Alexander Bruce for a copy of this paper.

54 Prese7tce and Absentee [ch.

due to the p^^esence of a definite factor, while the corre-
sponding recessive owes its condition to the absence of the
same factor. This generahsation, which so far as we yet
see, is applicable throughout the whole range of Mendelian
phenomena, renders invaluable assistance in the interpreta^
tion of the phenomena of Heredity. The green pea, for
instance, owes its recessive greenness to the absence of the
factor which, if present, would turn the colouring matter
yellow, and so forthTj With the examination of further
evidence the significance of this principle will become
readily apparent.

Menders System distinguished from that of Galton.

From the outline of the evidence now set forth the
essential aims and methods of Mendelian inquiry will have
been understood. By this method we reach reality and
concrete fact among phenomena that had become almost
,^ proverbial for their irregularity. {^^The key to the problems
of genetics and, as we confidently believe, to that of Species
also, lies in the recognition of the character-units, or factors
as we often call them. Their allelomorphism is a pheno-
menon of gametogenesis, and is a consequence of those
attractions and repulsions by which the germinal cell-
divisions are effected. Discontinuity in variation — to use
the word variation in its old, comprehensive sense — results
from the existence of these units. We recognize therefore
that this discontinuity — Galton's "Organic Stability" — is
ultimately dependent on the physiology of gametogenesis,
• and not as we formerly supposed on some feature in the
physiology of zygotes^ How this simple conclusion was
missed we may in vam surmise. The discovery at one
stroke replaces all previous disquisitions regarding the laws
of inheritance. The magnitude of the discovery and the
novelty of its consequences have indeed delayed general
recognition of its truth. To this may have been due the
curious fact that the famous Nageli failed altogether to
realise the importance of Mendel's work. Nageli was of
course especially devoted to the study of heredity, and even
made it the subject of elaborate mathematical treatment.
As we now know, he was in correspondence with Mendel,

II] Galton's Sy stern compared 55

from whom he received a considerable series of letters and
illustrative specimens (197). These must have utterly failed
to arouse his interest, for when in 1884, the year of Mendel's
death, he published his great treatise on heredity, no refer-
ence was made to Mendel or his work. That this neMect
was due to want of comprehension is evident from a passage
where he describes an experiment or observation on cats,
which as it happens, gave a simple Mendelian result. The
Angora character (recessive) disappeared in a cross with a
certain common cat whose hair-character is, as we now
know, dominant. The cross-breds were mated together
and the Angora character reappeared in one individual
among a litter of common cats^. This typically Mendelian
fact was thus actually under Nageli's own observation, but
from the discussion which he devotes to the occurrence it
is clear that Mendel's work must have wholly passed from
his memory, having probably been dismissed as something
too fanciful for serious consideration.

C It may be useful to specify the distinctive features of
Mendelian inheritance which differentiate the cases ex-
hibiting it from those to which Galton's system of calculation
— or any other systems based on ancestral composition —
can apply.

(i) In Mendelian cases, in which the characters behave
as units, the types of individuals considered with respect to
any pair of allelomorphic characters are three only, two
being homozygous and one heterozygous ; while according
to such a system as Galton's the number of possible types
is regarded as indefinite.

(2) The Mendelian system recognizes that purity of
type may be absolute, and that it may arise in individuals
of the 7% or any later generation bred from heterozygotes.
The views based on ancestry regard purity of type as
relative, and arising by the continued selection of numbers
of individuals.

(3) In Galton's system no account is taken of domi-
nance, a phenomenon which plays so large a part in the
practical application of any true scheme of heredity.

These distinctions are so definite and striking that at
first sight it seemed likely that the two methods might be

* C. Nageli, MechaniscJi-J>hysioiugisc/ie Theorie der Abstammun^sUhre,
1884, P- 199-

56 Zygotes are Double Strticfures [ch. h

applicable to two physiologically distinct classes of pheno-
mensCTJ It was anticipated that some characters, or possibly
even some forms of life, might follow the one system and
others the other.

The results of further researches make this supposition
increasingly improbable ; and though undoubtedly there
are cases which cannot yet be subjected to Mendelian
analysis, it is fairly certain that there is no large group of
facts in heredity to which the Galtonian system or any
modification of it exclusively applies.

There are however numerous examples where the arith-
metical results predicable by either system are nearly or
quite the same, though further breeding would of course
reveal that even in these cases the applicability of the
Galtonian method was only superficial.

j^ ^ The first aim of genetics must now be to determine the
magnitude, number and ultimately the nature of those units
which together make up the visible fact we call heredity;
and so to discover the consequences of their several com-
binations in zygosis or fertilisation. For the power thus to
formulate our purpose and for the development of a method
by which it may be successfully pursued we are beholden
to Mendel's genius.JJ

^^ CThe difficulty which some feel in realising the signifi-
cance of Mendelism arises from the habit of looking on the
bodies of animals and plants as single structures. So soon
as the mind becomes thoroughly accustomed to the fact
that all individuals, at least those of the higher and more
familiar types, are doicble, it becomes easy to think in
Mendelian terms, and the world of gametes, whose pairings
have brought into existence the individuals we see, comes
naturally and persistently before the mind. Henceforth we
have to penetrate behind the visible appearances of the
individual, and endeavour to reconstruct first those pro-
cesses of cell-division which produced the germ-cells or
gametes, distributing the characters or factors among them
according to definite systems ; and then the subsequent
process of union of those gametes pair by pair, in fertilisa-
tion to form zygotes, each developing and manifesting in
its development those properties of structure, instinct and
conduct conferred upon it by that particular complement
i of factors which its two original gametes contained/^

CHAPTER III

NUMERICAL CONSEQUENCES AND RECOMBINATIONS.

Representations of the /% Generation and Novelties due
to Re- combination of Factors — Co77zpotind Characters — ^•
Combs of Fowls — Heterostylism — White Flowers from
Red X Cream.

The unity of characters being recognized, we may next
examine some of the statistical consequences of this pheno-
menon. In order to determine the number of units and
allelomorphic pairs which are concerned in any practical
case, we have to be guided first by the visible statistical
composition of F^ ; and next by such tests of the gametic
constitution of the several F^ individuals as can be made by
breeding from them.

To those who are familiar with algebraical methods, the
employment of F.-, numbers to discover the number of terms
in the gametic series may present small difficulty, but to
others the following graphic method of demonstration
may be of service. For the introduction of this system,
which greatly simplifies difficult cases, I am indebted to
Mr Punnett.

Take the simplest example, of one pair of allelomorphs,
say Tall (7") and Dwarf if). The parent zygotes of the
pure strains are then TT and tt. Their gametes are T, Z,
and t, ty respectively. The F^ heterozygote is Tt, and its
gametes are all either T or t in equal numbers. As this
is true both of the female germs and the male germs, there
are four possible combinations. Make therefore four

Representation of F^ Generation [ch.

squares, representing the female gametes or germ-cells,
similar cells being placed vertically, thus :

Similarly the male germs may be represented by four
squares :

Here the similar germs are placed horizontally instead
of vertically. If therefore the first set of squares be
superposed on the second, all the four possible zygotic
combinations are represented, thus :

Thus there are iTT-\-iTt-\- \tt\ and since 7" is domi-
nant, the visible appearance of F.^ is

We may next deal similarly with the case of two allelo-
morphic pairs, Aa and Bb. We shall now require i6
squares. Writing each set of four similar female germs in
vertical rows and the corresponding sets of male germs in
horizontal rows, keeping to the same order, the sixteen
possible combinations are represented. In each square the

Ill]

Representation of F^ Generation

upper expression Indicates the nature of the female gamete,
the lower one that of the male orametc :

AB
AB

Ab
AB

aB
AB

ab
AB

Ab
Ab

ab
Ab

AB
aB

Ab
aB

aB
aB

ab
aB

Ab
ab

ab
ab

The constitution of each of the sixteen types which are
produced in F^ is thus displayed, and since A is dominant
over a, and B over b, the visible appearance of F^ is

(^AB : 2iAb : ^aB : lad.

Similarly when three pairs of factors are concerned,
Aa, Bd, Cc, the F^ type will exhibit them all, and be in
appearance ABC. In /% there must then be eight visibly
distinct types, and the ratios in which they severally appear
will be as follows :

2jABC+gaBC+^ABc^()AbC+2>^bc + 2)^Bc

-\- 2i^dC + labc.

I am obliged to a mathematical friend for the following
scheme by which the number of types and the ratios in
which each will appear are given for any number of pairs
of factors, one factor of each pair being dominant and the
other recessive.

4 = 3 + 1

i6 = (3 + 1)' = 3' + 3 + 3 + 1=9 + 3 + 3 + 1

64 = (3 + i)' = 3' + 3-3' + 3. 3+1 =27 + 27 + 9+^

256 = (3+1/ =3' + 4 -3' + 6. 3"' + 4. 3 + I

= 81 +27 + 27 + 27 + 27

+9+9+9+9+9+9

+3+3+3+3
+ 1.

6o Compound Characters [ch,

So in general

+ 3''"^ + 3''"^ + n times

+ 3""'+3''~^+ i^ (^^ 0 times

+ 3^^"^ + 2>'~^ + "6 ^ (^^ ^ 0 (^ — 2) times

+ &c.

Compound Characters and Novel Types produced by

Re-combinations.

Thus far we have dealt only with cases in which the
characters of each allelomorphic pair have independent
effects on the visible appearance of the zygotes. In peas,
for instance, we have seen a pair of characters, tallness and
dwarfness, producing their effects quite independently of
other pairs of characters, such as those which determine
the flower colour or the seed-shapes. Each can be
separately perceived by its effects, and the presence of the
one in no way influences the development of the other.
Such are the imaginary characters A, a, and B^ b, whose
distribution is represented above, and the four types pro-
duced by their several combinations are each distinguished
without difficulty, as AB, aB, Ab, ab.

We now pass to a class of cases manifesting greater
complexity. The essential phenomenon in these cases is
that definite characters are produced by the mutual inter-
action of factors belonging to distinct allelomorphic systems.
Such interactions, as we now know, are of the greatest
importance in heredity, and the progress of genetics will
consist largely in disentangling the elements to which these
combination-effects are due.

We may speak of characters thus produced as compound
characters. The nature of such compound characters is
well exemplified by phenomena which have been observed
respecting the inheritance of several types of combs seen
in various breeds of poultry, and as these cases are illustra-
tive of many others I propose to consider them in some
detail.

Ill]

Compound Characters

The Combs of Foivls,

In the first place we are concerned with the followinrr
types :

Single comb. The high, serrated comb which is famiHar
to everybody is called by fanciers a single comb (Fig. 1 2 A).
It is the type found in Galhts bankiva and I believe all the
wild species, and we are fairly safe in regarding it as the
primitive or original form from which all the others have
been derived. It is the characteristic comb of Leghorns,
Minorcas, and many other breeds.

Fig. 12. Various types of combs in Fowls.

A. Single Comb : cock.

B. Pea Comb : cock.

C. Pea Comb : hen.

D. Rose Comb : (Bantam) cock.

E. Walnut comb in a young cock. This is the type in ^Malays, and

can be produced by crossing Rose x Pea.

Pea Cojub. This comb (Fig. 12 B, C) differs from the
single in being much lower and closer to the head. It has
no spike-shaped serrations, but merely rounded lumpy pro-
jections along the middle line. In addition to these there
is a characteristic development of similar lumps or tubercles
on each side usually uniting to torm a more or less definite

Combs of Fowls

[CH.

lateral ridge. These two lateral ridges together with the
median one constitute the three ridges commonly spoken of
as the essential feature of the pea comb. The size and
details of development of these combs differ a good deal
with individuals and with strains. Pea comb is especially
characteristic of Indian Game, Aseel, and the Brahma
breeds.

The F^ from pea x single is pea, that character mani-
festing a definite dominance. The heterozygous pea combs
are generally higher than the pure pea and may usually,
though not always, be distinguished from them. Sometimes
the heterozygous pea comb is so large and has the ridges
so ill-defined that it approaches the single type, but combs
which cannot at once be referred to one class or the other
are extremely rare. The distinction is especially sharp in
the case of the newly hatched chicks, becoming somewhat
less marked with later development. F^ from this cross is
of the usual form, 3 pea : i single.

^^H|M||^H

^HP^'^H

^^^^^^^^^^^^PiP|' . \W ■•/ ^^^^^H

^^■^^^^^^^^R ^ff i ^^H^ ^^^^^^^^1

^^^B^Hrnl^^^S^i 5i]^HUK^Bu w #%i^^^^^^^^B^^I^^^V^

^^^^^^^H

^i^^^^^^^^^H

I^^H^I

^^^^^^^^mT .^^^^^^^^^^^^^^^^^^^^BK w^^^^^^^^^^H

Fig. 13. The combs as they appear in newly hatched chickens.

In the top row from left to right : — Walnut comb in a light-
coloured bird, showing the peculiar band of hairs ; ditto in a dark-
coloured bird ; Rose comb.

In lower row from left to right : — Pea comb; ditto ; Single comb ;
ditto.

iiij Combs of Fowls 63

Rose Comb. The next type to be taken into account is
the rose comb, which consists of a triangular mass of small
spikes or papillae. The apex of the triangle, called the
peak or pike, points backwards and is free from the head.
Such a comb is characteristic of Hamburghs, Rose-combed
Dorkings and many other breeds. A particular form of rose
comb with the pike curved downwards is a peculiarity of
Wyandottes.

F^ from rose x single is rose. The dominance of the
rose comb is very definite, and it is frequently quite im-
possible to distinguish pure rose from the heterozygous
type containing single. /% consists of the usual 3 rose :
I single.

Rose X Pea, When a pure rose-combed bird is crossed
with a pure pea the resulting comb is very different from
either. It has no distinct papillae like the rose, or ridges
like the pea. In the newly hatched chick the region of the
comb is covered with a nearly flat or somewhat warty-
looking skin. At the beginning of the posterior third there
is generally a most curious band of bristles or hairs crossing
the comb. Some F^ birds have hairs scattered over the
posterior part of the comb, either with or without a definite
transverse band. The hairs usually increase in quantity
and definiteness as maturity approaches. It seldom happens
that a rose-pea ("walnut") bird has none of these hairs,
though with age they may get worn off As the chicken
grows, the skin of the comb itself Increases in size and
becomes more or less corrugated. Such corrugations may
become very large in males and are especially developed
anteriorly. In this region the comb is often widened so as
to form a lobe on each side, but the part behind the band
remains single, so that the whole comb has a 3-lobed appear-
ance when seen from above. One of the corrugations very
often appears as a furrow separating the flatter posterior lobe
Irom the anterior and more elevated part of the comb. It
is from the corrugated surface of this comb that it is called
by fanciers a "walnut" comb. The only pure breed in
which such a comb occurs Is the Malay. In Fig. 13 the
hairs can be seen fairly well in the top left-hand bird, which
has light plumage. In the bird next to it the plumage is dark
and the band of hairs is not so distinct in the photograph.

64 The Walnut Comb [ch.

It was not a little surprising to see so striking and
characteristic a structure as the walnut or Malay comb
appear with strict regularity as the product of two such
dissimilar parents as the rose and the pea. Not only is
the general appearance of the walnut quite distinct from
these, but the presence of the hairs constitutes a feature of
absolute difference, for no hairs are found on combs of the
usual types. Were the walnut, rose, pea, and single combs
found as characteristics of wild birds no naturalist would
hesitate to regard them as four distinct specific characters ;
and even as the special properties of domesticated birds I
suppose they would by many be regarded as evidence ol
long-continued selection. Nevertheless, as will be seen,
these four forms stand to each other in a simple genetic
relation and the fact suggests wide possibilities in regard
to many hitherto unexceptionable differences ** of specific
value " recognized among animals and plants.

The interpretation of the facts was at first by no means
easy, and I am sorry to have been responsible for the
promulgation of a quite erroneous suggestion regarding
them. Further knowledge of kindred phenomena has,
however, made the elucidation of this case now perfectly
clear and simple.

To return to the experimental results. Having found
that rose x pea gives walnut, the next thing to be done
was to test the genetic properties of birds thus produced.
This was done in two ways (i) by breeding the walnuts
together, (2) by breeding them with singles. In what
follows, the names may be abbreviated thus : R, rose ;
P, pea ; RP, walnut ; and 5*, single.

Experiment showed that RP x RP gave an F^ family
RP, R, P, and S. The appearance of S, which was not
known to have been put in, is not at first sight intelligible.
Repeated trials proved that the ratio in which these combs
appeared was

gRP : sR : 2,^ : iS.

It was further proved by experiment that the R birds
were either pure R or contained the recessive S, but gave
no more P or RP ; that the P birds similarly could only
give P, or P and S ; while the 6^ were all pure to that
character.

Ill]

The IValniU Comb

Consistent with this result were the offspring obtained
from first-cross RP x S, for this mating gave the ratio

iRP : iR : iP : i^.

As it was already established that R and P were each
dominant to S, the inference was certain that the gametes
produced by the F^ {RP) birds were RP, R, P, and S in
equal numbers.

The whole series of phenomena may be represented as
due to the combinations of two pairs of allelomorphic
characters or factors, namely

1. Rose (domt) R, absence of rose (rec.) r.

2. Pea (domt) P, absence of pea (rec.)/.

walnut

pure

giving rose

giving pea

giving all 4

Rp
RP

Rp
Rp

Rp
rP

Rp
rp

walnut

rose

walnut

rose

giving rose

pure

giving all 4

giving single

walnut

pea

giving pea

giving all 4

pure

giving single

rp
RP

rp
Rp

rp
rP

walnut

rose

pea

single

giving all 4

giving single

pure

Reference to the diagram of combinations shows that
the two "absences" of rose and pea respectively will meet
once on an average in 16 times, and to such a meeting
without doubt the appearance of the single combs as a
novelty, in /%, is to be ascribed.

In the early years of this experiment, knowing that R

B. H.

66 The Breda Comb [ch.

and P were allelomorphic to S, I came to regard them as
also allelomorphic to each other. This idea led to con-
fusion, but we know now that no case justifies such an
application of the principle of allelomorphism. A rose
comb is not due to an elemental factor which can segregate
from the pea comb factor. The two factors belong to
distinct allelomorphic pairs and each in the gametogenesis
of the heterozygote segregates from its own allelomorph,
which is simply the absence of the factor in question. The
single comb contains neither R nor P The rose comb is
a single comb modified by the presence of R, while the pea
comb is produced by the presence of P. We may therefore
describe the rose as R no P, and the pea 2iS P no R. It is
convenient to use capital letters for dominants and small
letters for recessives, the rose being thus written Rp, and
the Pea, rP, The walnut comb is the RP^ while rp gives
the single.

The allelomorphism of the elements which go to the
constitution of the shapes of combs in fowls may without
doubt be carried very much further. For example there
are indications that the size of the comb depends to some
extent at least on other pairs of factors. Another curious
set of phenomena, perhaps worth investigating further, may
be studied in a cross between a single comb breed and the
"Breda" fowl. The Breda is usually said to have ''no
comb." As a matter of fact it has two very minute
tubercles which represent the comb. When this breed is
crossed with the single comb, P-^ has what may be called a i
"double" single comb. It consists of two large lobes or
leaves diverging outwards from a common base^. Such a
comb is evidently due to the introduction by the Breda of a
factor which may be called ** bifidity." This factor acts on '
the large comb brought in by the single-combed parent and
the result of the combination is a large, double comb. P^
from this cross has not yet been raised, but there can be no
doubt that it will contain members having the '' no comb "
of the Breda and the "absence of bifidity" of the single-
combed breed. Such birds probably have only a minute
tubercle at the posterior end of the comb region.

* The two lobes sometimes unite anteriorly to a greater or less extent.

ni] The Breda Comb 67

Mr Hurst suggested to us that an excellent confirmation
of the truth of the analytical method by which the composi-
tion of the rose and pea combs has been represented could
be obtained by a cross between the rose comb and the
Breda, which, as has been stated, has the bifidity-factor but
practically no comb at all. The elements involved are

Rose, R, No rose, r.

Comb present, C Comb absent, c.

Bifidity, B, No bifidity, b.

The F^ generation has the composition Rr, Cc, Bb,
namely a double rose comb. F^ generation contains a great
variety of forms, of which those having B and C, but no R,
have the high bifid comb like that of the F^ raised in the
cross Breda x Single, and those which contain only r and b
in combination with C have single combs of the ordinary
high type (Fig. 12). Both these kinds occurred in /%, and
their appearance is entirely confirmatory of the scheme of
representation adopted. Since neither the rose nor the
Breda have outwardly any suggestion of single comb in
their appearance, were it not for a knowledge of Mendelian
analysis such a result must have seemed utterly unaccount-
able.

When therefore we look at such an organ as the comb
of a fowl and attempt to conceive its genetic properties, we
have to remember that the structure as a whole may be
composite in origin, and that the visible appearances and
properties may result from the interaction of a number of
distinct elements each transmitted independently in gameto-
genesis.

Everything however points to the conclusion that the
number of these elements is finite, and that their properties
are not beyond the reach of orderly analysis.

In the example of the walnut combs the interaction of
two dominant factors is such that the 9 members ot the F.,
series of 16 have a feature, the walnut comb, distinct from
those of the original parents, while the i member of the
series which contains neither dominant factor, also has a
feature, the single comb, distinct from anything visibly

5--2

68 Heterostylism [ch.

introduced. The next case also illustrates the appearance
of a novelty in F.^, but, as will be seen, it is one of the
groups of 3 members which manifests it.

Hete7'ostylism in Primula.

The dimorphic, heterostyled condition of Primula plants
is well known and need not be described in detail. The
two types are distinguished thus :

A. Thrum, or Sho7't-styled type,

1. Style short, the stigma standing at the level of

constriction of the tube.

2. Anthers at the mouth of the tube.

3. Pollen grains large.

B. Pin, or Loitg-styled type.

1. Style long, the stigma usually standing in the mouth

of the tube.

2. Anthers at the level of constriction.

3. Pollen grains small.

Experiments made by Mr R. P. Gregory in conjunction
with me showed that the inheritance of these two types
is Mendelian^, the short-styled or thrum behaving as
dominant. The long-styled, being recessive, always breed
true to that type on self-fertilisation or when bred inter se.

In connection with this fact it is interesting to observe

* I know no authentic case of the presence of both long- and short-
styled flowers on the same plant. Such occurrences are frequently
announced, but so far as I can discover the records are based on
mistakes. In occasional flowers on long-styled plants, especially in the
beginning of the flowering period, the style does not attain its proper length.
Such flowers are through carelessness sometimes misdescribed as short-
styled. The anthers however are at the lower level, and the pollen-grains
are small, so the essentially long-styled nature of these plants is quite clear.

The statement is also sometimes made that pin plants have produced
thrum-eyed off'spring without the intervention of a cross. This mistake is
due to the appearance of a type with '* exsert " anthers. Such anthers
project from the mouth of the tube and give a thrum-like look to the
flower. But careful examination shows that the anther-filaments are
inserted at the lower level, and the pollen-grains are small. Such plants
are therefore long-styled.

in] lieterostylisin 69

that Primula Sinensis, which Is preferred by fanciers in the
pin form, was easily bred true to that type, and is always so
maintained in good strains. For our experiments it was
with considerable difficulty that we procured any thrum
plants. On the other hand it was decided long ago that
the Auricula and the Polyanthus for exhibition purposes
must always be thrums, but though thrums have thus been
largely selected for breeding, pin-eyed plants are continu-
ally reproduced, as must be expected, for the thrum is a
dominant, and therefore liable to contain the recessive type.
The facts about to be described relate to an experiment
made with a peculiar race of P. Sinensis grown by Messrs

Fig. 14. Some of the types of flowers in F. from the cross short-style
(thrum) ; small eye x homostyle ; large eye.

A. The long-styled flower : with small eye.

B. The homostyled : large eye.

C. The short-styled : small eye.

D. The short-styled : large eye.

In the two upper flowers the corolla is of the " star " type. D is
the ordinary, imbricated type of Si?iensis. C is more or less inter-
mediate in corolla-shape. This shape is the usual heterozygote formed
between star type and Sinensis type. The corolla-shapes are of course
quite independent of the style and " eye " characters.

yo Heterostylism [ch.

Sutton under the name '' Primrose Queen." To the casual
observer this race differs from an ordinary Primula in the
fact that the yellow *' eye," instead of forming a small and
well-defined pentagon, is continued as a yellow flush ex-
tending far over the limb of the corolla. The anthers of
these flowers stand at the lower level and the pollen-grains
are small ; but the style instead of projecting into the mouth
of the tube stops at the anther-level, being thus practically
the same length as in the short-styled type. Such a form
of flower was called by Darwin homostyled.

Crosses were made between this homostyle race with
the yellow flush and an ordinary thrum with the pentagonal
or small eye. F^ is thrum with the small eye, showing that
the yellow flush is recessive. F^ gave the following series :

9 thrum or short-style with small eye,

3 ,, ,, with the large eye,

3 ordinary /m or long-style with small eye,

I homostyle with the large eye like Primrose Queen.

The long-styled or pin type, which apparently was not
put in, is evidently due to the re-combination of the charac-
ters. The two pairs of characters are

Thrum type (domt). Pin type (rec).

Small eye (domt). Large eye (rec).

The homostyle is the form which the pin type assumes
when the large eye is developed ; but when in F^ the pin
type meets the small eye, the ordinary pin or long-styled
form is produced.

/% gave a simple and confirmatory result ; for of the
F^ long-styled plants, some proved pure to the long-styled
character, while others threw the recessive homostyle.

White Flowers in F^ from Red x Cream.

Exactly comparable with the foregoing case is the
paradoxical appearance of w/^//^-flowered individuals in the
F^ from the cross of a sap-coloured variety with a variety
having cream-coloured flowers. For example, in Sweet
Peas or Stocks [Afattkiola) when a red- (lowered type is
crossed with a cream, F^ is red, without any cream-colour.

Ill] Novelties by Re-combination 71

F. consists of 9 reds without cream, 3 reds with cream,
3 whites, I cream.

When the allelomorphs are correctly distinguished the
significance of this series is obvious. The operations may
be shown in a tabular form, thus •

Parents Red variety x Cream variety

Allelomorphs I ^^^ '^P ^^^ , Colourless sap {R)

^ (Colourless corps. (Z>) Yellow corpuscles (i?)
rp f Red sap

^ (Colourless corpuscles

rp j Red sap Red sap Colourless sap Colourless sap

- (Colourless corps. Yellow corps. Colourless corps. Yellow corps.

Appearance 9 Red 3 Red Cream 3 White i Cream

These cases of novelties resultino^ throuoh a re-combi-
nation of the factors brought in by the original pure types
are striking because it is not at first sight evident how the
novelty has been produced. Generally speaking, however,
the re-combinations form in /% 3- series of types many of
which are obviously new combinations of features which
could be recognized on inspection as present in the pure
parents. Thus the cross between a bearded, rough chaff,
red wheat, and a beardless, smooth chaff, white wheat give
in /^l a beardless, rough chaff, red. But in /% ^11 the
different possible combinations occur, such as bearded,
smooth, red ; beardless, rough, white ; bearded, rough,
white, &c., each in their appropriate numerical proportions.
In the Guinea-pig, starting from albino, smooth coat, long
hair, and crossing it with coloured, rough coat, short hair,
/^l is coloured, rough coat, short hair. But F._ contains the
various re-combinations of these three pairs of characters,
such as albino, rough coat, short hair ; coloured, smooth coat,
long hair, &c. Thus by selecting any desired type in F^
any of these new combinations can be fixed and perpetuated.
Basing his procedure on a knowledge of the dominance or
recessiveness of each character the breeder may thus guide
his operations with certainty.

That this has been the mode by which most of the new-
breeds of domesticated plants and animals have been created
is obvious. The traces of it remain in many cases. For

72 Practical Examples [ch.

instance, Sutton's " Nonpareil," one of the marrow peas,
consists in about equal numbers of yellow-cotyledon seeds
and green-cotyledon seeds. Like all the new peas it must
have arisen at some definite moment bv the selection of
an individual yellow-seeded plant which was true for the
various good qualities of Nonpareil — being homozygous for
them in other words — but in cotyledon-colour it was
heterozygous. As the diversity of colour was not thought
objectionable it persists. If any one wishes to make an
exclusively green-seeded Nonpareil, all he has to do is to
take green seeds from a sack of that variety and sow them,
saving the seeds they bear. If he desires the yellow type
pure, he may similarly sow yellow seeds from the same sack.
Most of these by now will presumably be pure to yellow,
but some may not. By keeping the seeds of any plant
which gives only yellow seeds a pure yellow Nonpareil will
be constituted.

Similarly Sutton's ** Continuity " is a pea which is
allowed to be either pointed in pod or blunt. The variety
is true in other respects and it is clear that its original
progenitor was a plant homozygous for the peculiarities of
Continuity but heterozygous in respect of the pod-shape.
The pointed-pod plants would be found true for that
character, since it is recessive, while the blunt-pod plants
might or might not be true for it.

So in the Chinese Primrose, several varieties, e.g.
Sutton's Mont Blanc, Sirdar, &c., distinguished by peculi-
arities of colour have been fixed both in a palm-leaved and
in a fern-leaved form, these having of course been saved
in F^ or later generations from a heterozygote in which the
palm and fern-leaved characters were combined. In Sweet
Peas, after the original dwarf " Cupid " was found — accord-
ing to tradition a chance seedling among tall plants — it was
easy to transfer the characteristic colour of the various tall
types on to a Cupid foundation, and now any colour almost
can be had either as a tall or as a Cupid.

Possible Limits to Re -combination.

These illustrations might be extended indefinitely. It
will probably occur to many that there are limits to these
possibilities of transference, and so undoubtedly there are.

in] Limits to Interchangeability 73

The detection of these limits Is one of the more Important
tasks still awaiting us. Though on this head little can yet
be asserted with confidence it is likely that such limitations
are constituted in two distinct ways : First, from all we
know of the capacities of animals and plants we must
anticipate that some characters are incompatible in the
same individual. For example in cattle the highest milk-
production is not to be found in the breeds which make the
best beef. Meat-production and milk-production are to some
extent alternative and can only be combined by compro-
mising one quality or both. That such an alternative
distribution Is merely a result of allelomorphism seems on
the whole unlikely, though certainly not impossible.

Then again we must surely expect that these transferable
characters are attached to or implanted upon some basal
organisation, and the attributes or powers which collectively
form that residue may perhaps be distinguishable from the
transferable qualities. The detection of the limits thus set
upon the interchangeability of characters would be a dis-
covery of high importance and would have a most direct
bearing on the problem of the ultimate nature of Species.

CHAPTER IV

HEREDITY OF COLOUR.

Factors determining Colours: the Ratio 9 : 3 : 4. — The
''Presence and Absence'' Hypothesis. Epistatic and
Hypostatic Factors — Colours of Mice — Pied Types —
A Domi7iant Piebald,

With regard to the application of the Mendellan system
to problems of colour inheritance the evidence is now-
considerable. The fact that in both animals and plants
albinism behaved as a recessive to colours was soon dis-
covered. Several examples among plants are mentioned by
de Vries in his first paper on this subject, and shortly after
similar facts were recorded in regard to animals. Since in
the course of a large range of experiments with many species
of animals and plants no case to the contrary has been met
with,' it may perhaps be asserted as a general truth that
pigmentation is always dominant to total absence of pig-
ment"^. When however we proceed to the investigation of
the genetic properties of varieties which are so far deficient
in pigment as to be called sometimes partial albinos, we
find that various specific rules are followed and no uni-
formity of behaviour has yet been discovered. White fowls
for instance are thus commonly spoken of as partial albinos,
but the pigmentation of their eyes sharply distinguishes
them from albinos which are destitute of pigment, and
many of their genetic properties are found experimentally
to be quite distinct from those of real albinos. The same
is true of certain varieties of plants, which though varying
from the specific type by possessing white flowers have
yet some red or purple sap in the stem or elsewhere. With-

* The Axolotl is perhaps an exception. See p. 43. The fact that in
plants colourless chromoplasts are dominant to yellow chromoplasts scarcely
constitutes an exception, for the yellow of the chromoplasts is not pigment
in the usual acceptation of that term.

CH. IV] The Ratio 9:3:4 75

out experiment no prediction can be made with confidence
as to the behaviour of such types in their crosses.

Albinism being recessive in all ordinary cases, /^, from the
cross colour x albino contains i albino to 3 coloured members.
As regards the characters of the dominant or coloured mem-
bers various complications have to be considered.
-*=; In the simplest cases the coloured F^ individuals are all
of the same colour. For example on crossing a grey rabbit
with an albino, F^ is grey and F^ may be 3 greys : i albino.
But frequently it is found that in addition to the greys and
albinos blacks appear in F^. Repeated experiments, for
example those of Hurst, have shown that in such families
the i^o ratio is

9 greys : 3 blacks : 4 albinos

12 : 4

3 '- 1

The relation of this ratio to the ordinary 9:3:3:1
was first pointed out by Cuenot (86)*. As represented by
him two pairs of allelomorphs are concerned, namely :

Do77tina7it. Recessive.

1. Colour (C). Albinism {A).

2. Grey determiner (6^). Black determiner {B).

The presence of one or other of the determiners G ox B
is only perceptible when it exists in combination with the
colour-factor. If (^ is present together with C, the colour
is grey ; \{ B \s present with C but without G, the colour is
black. If a coloured individual contain both G and B,
being thus heterozygous in the second pair of factors, the
colour is grey, for the effects of grey dominate. But since
In the absence of colour (C) neither determiner produces a-
perceptible effect, albinos may exist of the forms AAGG,
AAGB, or A ABB, and without breeding tests. It will not
be possible to distinguish between these several forms. On
crossing w^ith a black of course each albino can be known
by its effects. For GG will then give greys only, GB will
give equal numbers of greys and blacks, while BB albinos
will give only blacks.

* Shortly before the publication of Cuenot's paper Mr R. H. Lock
wrote to me from Ceylon with the same suggestion.

76 Presence and Absejtce [cti.

The ''Presence and Absence'' Hypothesis applied to the

Case of Colour.

So lonof as attention is restricted to crosses like these
involving only two sorts of colours besides the albinos, the
system suggested by Cuenot is adequate, but when a third
colour has to be considered, as in the case of mice, some
modification is required. The simplest notation by which
these and other complex Mendelian phenomena can be
expressed is provided by what is spoken of as the Presence
and Absence hypothesis already illustrated in the case of the
combs of fowls.

Mendel himself probably conceived of allelomorphism
as depending on the separation of a definite something
responsible for the dominant character from another some-
thing responsible for the production of the recessive
character. It is however evidently simpler to imagine
that the dominant character is due to the presence of
something which in the case of the recessive is absent.
As yet there is no absolute proof that this mode of de-
scribing the facts is correct, but everything points that
way, and no phenomena have yet been encountered which
cannot be thus formulated when their nature is understood.
In cases where the pure dominants are recognizably distinct
from the heterozygous dominants, it must naturally be sup-
posed that two *' doses " of the active factor are required, one
from the paternal, and another from the maternal side, in
order to produce the full effect.

Applying the presence and absence system to the case
of the colours of rabbits, the first pair of allelomorphs can
obviously be represented as

Dominant. Recessive,

I. Presence of Colour (C). Absence of Colour (c).

The second pair we have so far spoken of as the grey
determiner and the black determiner, regarding these two
as allelomorphic to each other. But it is equally possible
to describe them thus

2. Grey determiner (G), Absence of ditto i^g).

IV]

Presence and Absence

Then in the case where grey x albino gives in /%

9 grey : 3 black : 4 albino,

we simply have to regard B, the black determiner, as
common to both parents, and the same numerical result
is produced. Such a case may usefully be represented In
a tabular form, thus :

Parents Grey x Albino

Gametic Composition . . . CGB cgB

Grey
CcGgBB

F^ 9 Grey

12
3

3 Black

4 Albino

CGB
CGB

giey

cGB
CGB

grey

CgB
CGB

grey

CgB
CGB

grey

CGB
cGB

grey

cGB
cGB

albino

CgB
cGB

grey

CgB
cGB

albino

CGB
CgB

grey

cGB
CgB

grey

CgB
CgB

black

CgB
CgB

black

CGB

CgB

grey

cGB
CgB

albino

CgB

black

CgB
CgB

albino

Fig. 15. Distribution of grey, black, and albino individuals in I\ from
the cross grey, CGB, with albino cgB, showing the meaning of the
ratio 9 grey : 3 black : 4 albino.

j8 Presence and Absence [ch.

In this diagram the 9 squares containing C, Gy B, are
the 9 greys, the 3 squares containing C and B only are the

3 blacks and the 4 squares containing no C at all are the

4 albinos.

Proceeding to the case of mice we write the composition
as follows :

Grey C, G, B, Ch.

Black C, g, B, Ch.

Chocolate C, g, d, Ch.

We thus regard the black mouse as one from which the
grey determiner, (9, has been removed. In the chocolate
mouse the process of removal has been carried further and
the black determiner, B^ is also gone.

A proof that this system of representation is so far
correct is obtained by crossing the grey mouse with the
chocolate. Such a cross, if G is not allelomorphic to B, must
give blacks in /%. This experiment has been lately carried
out by Miss F. M. Durham, to whose work our knowledge
of the genetics of mice is largely due. The result is that,
as expected, /% does contain blacks, and though the num-
bers as yet obtained are small, there can be little doubt
that the F^ ratio is

12 grey : 3 black : i chocolate.

Some interesting questions arise in regard to the greys.
Obviously we shouia expect 9 greys containing G and B
+ 3 greys without B. Now fanciers are well aware of a dis-
tinction between two kinds of greys or ''agoutis" as they are
called. These are known as " golden agoutis " and *' cinna-
mon agoutis," the former containing black pigment, the
latter being without it. In the /% from grey x chocolate
both these kinds of agoutis appear, and evidently the
cinnamon agoutis are the expected greys wanting in the
determiner B.

Thus far all is clear. Certain difficulties however
remain unexplained. These will be described later. At
this stage in the discussion it is convenient to notice that
in view of the facts now stated the use of the term domi-
nance must be more carefully restricted than has hitherto
been necessary. When we speak of the colour as being
dominant over the absence of colour we mean that if the
colour is present it will appear, and that if the factor lor

IV] Epistatic and Hypostatic 79

colour is absent the Individual will be devoid of colour.
The term is thus used correctly to denote the relation
between allelomorphic features belonging to the same pair.
But confusion will be introduced if we extend the same
term to the relationship between various determining factors
which belong to distinct allelomorphic pairs.

Hitherto we have spoken of the determiner for such a
colour as grey in rabbits and mice as "dominant" over
the colours lower in the scale, such as black or chocolate.
Nevertheless we are here dealing with a relationship quite
different In order from that subsisting between the coloured
and the albino. Pending a more precise knowledge of the
nature of this relationship it will be enough to regard those
factors which prevent others from manifesting their effects
as higher, and the concealed factors as lower. In accord-
ance with this suggestion the terms epistatic and hypostatic
may conveniently be introduced. We shall then speak of
the determiner for grey as epistatic to that for black ; that
for black as epistatic to the determiner for chocolate, and
so on.

When the facts are thus clearly represented we perceive
that the variation by which, for example, a black mouse came
originally into existence, consisted in the omission of the de-
terminer for grey. The chocolate mouse similarly owed Its
origin to the successive omission of the determiner for black.

The important question what the effect of the grey
determiner, for example, actually is, remains undecided.
A further serious difficulty also arises in regard to the
relation of the coXowv yellow to the other colours. Neither
of these points is yet satisfactorily understood In the case
of mice. The recent papers of Castle (53) and of Hurst^
have made the phenomena in rabbits comparatively clear,
though even there, however, an unexplained difficulty
remains. The special problems raised by the behaviour of
yellow pigment in these animals will be discussed in a sub-
sequent section! (see Chap. vii).

* Read at the Internal. Congr. Zool. Boston, 1907 : not yet published,
t According to the number of factors involved and to the definiteness
with which their several combinations can be distinguished, an indefinite
variety of ratios may of course be produced in T^o families. Some of the
I most interesting are those in which some of the heterozygous combinations
can be distinguished from the homozygous dominants. (See for examples
Shull, 242.)

8o Saturated and Diltite Colotirs [ch.

The F^ ratio 9:3:4, the significance of which we
have been considering, is one which very frequently recurs
in Mendelian analysis. For example, as Tschermak found,
when a pink-and-white flowered eating pea [Pistcm sativum)
is crossed with a white flowered type, F^ is often of the
original purple flowered type. Then /% will be

9 purple : 3 pink-and-white : 4 white.

Similarly pink Salvia HorrniuMin x white may give F^
purple, and /% 9 purple : 3 pink : 4 white. In these cases
the factor for the purpleness is of course brought in by the
albino, but exactly the same 7% i^^-Y result from a cross
between the purple type and an albino not carrying the
factor for purpleness. All that is essential for the produc-
tion of this ratio in F^ is that F^ should be heterozygous
for two factors, of w^hich one is perceptible whenever
present, while the other needs the presence of the first in
order that its own effects may be manifested. Such cases
are very numerous and in practical breeding are to be
looked out for continually. Care must be taken to distin-
guish them from families like those of the Andalusian fowl
(p. 52) in which the commonest term in the F^ series is a
heterozygous type. There the numbers will be 1:2:1,
which in a practical example may give results not obviously
distinguishable from 3 19:4. To decide between the two
possibilities it is necessary to breed the /% types again. If
neither of the scarcer types when bred inter se can throw
the other, and the commoner type cannot be bred pure, the
latter is a heterozygous type; but if one of the scarcer types
can throw the other, then the ratio is presumably 9:3:4,
and in such a case it will be possible to raise a breed true
to the type occurring as 9.

Satu7^ation and Dilution of Colours,

Omitting yellow from our consideration, we thus re-
cognize that in the mouse the colours, grey {agouti), black,
chocolate, which the fur visibly presents, result from the
interaction of several factors, and that these factors can in
great measure be shown to be distributed in gametogenesis
according to Mendelian allelomorphic systems. The ex-

<rt..

I- A.s^outi.

2. Black.

3- Chocolate.

Plate I

4. Cinnamon Agouti, viz. Agouti without black.

5. Blue : = dilute black.

;iy.<*i»->>.'*»*^:*^qipg;::r

6. Silver fawn := dilute chocolate.

IV] Colours of Mice 8i

periments of Miss Durham (ii6) have shown that not
only the particular pigment is thus constituted, but also
that the intensity or degree of saturation in which it is
formed can be represented as determined by similar factors.

For example the black colour may exist in the saturated
condition, when the. mouse is called black, or in a more
dilute form, when it gives the ''blue" appearance. Similarly
the chocolate colour when diluted gives what fanciers call
''silver- fawn." Experiment shows that the cross black x
silver fawn gives exactly the same result (in F,^ and /%) as
blue X chocolate. (See Plate II.)

The following are experimental results illustrating these
points.

The allelomorphs concerned may be represented as B,
the black determiner, b the absence of B, leaving the
colour chocolate. D the dense or saturated condition of
the colour, d, the absence of D, leaving the colour dilute.
(In the case of the introduction of the albino we should
have also to take cognizance of C, the presence of colour, c,
its absence.)

The actual results may then be expressed in a tabular
form.

Blue X Chocolate
{Bd) I {pD)
Ft, Black

{BbDd)

^ ,

i^,... Black Blue Chocolate Silver-fawn

{BD) {Bd) {bD) {bd)

Observed 44 17 17 8

Calculated ^c?-^ 16' i i&i S'4

Black X Silver-fawn
{BD) ! {bd)

Bj_ Black

{Bb£>d)

1 I ,

i?2... Black Blue Chocolate Silver-tawn

{BB>) {Bd) {bD) {bd)

Observed 67 21 20 5

Calculated dj'd 212 212 71

It is thus immaterial whether the factor for saturation
is brouQ-ht in top^ether with the black determiner or with

B. H. 6

82 Colours of Mice [cii.

the chocolate (more strictly, with the ''absence of the black
determiner"). So long as the same factors are introduced,
the consequences in F^ and the results of re-comxbination in
/% are the same. But when the dilution is introduced from
each side, F^ is of course the usual 3 dominants : i recessive,
thus :

Blue X Silver-fawn
{Bd) I {bd)

F, Blue

{Bbdd)

F^...B\ue'{Bd) Silver-f^wn {bd)

Observed 46 17

Calculated 47 -2^ 15 75

It is evident that an extracted albino cannot be carrying
a determiner for a colour higher in the scale than that of
its coloured parents. Moreover if the parents from which
an albino is extracted are alike, and if they throw no
offspring with colours other than their own (besides the
albinos), then the albinos so extracted must be all bearers
of the determiner for their parental colour. If such albinos
are crossed with forms of a colour lower in the scale than
that borne by the albinos, F^ must be of the colour deter-
mined by the albinos. For example, Miss Durham obtained
the following :

Silver-fawn x Albino (extracted from chocolates)
{Cbd) I {cbD)

Fi Chocolate

{CcbbDd)

r -^ n .

i^2 • • • Chocolate Silver-fawn Albino

{CbD) {Cbd) (various)

Observed 19 4 6

Calculated 16-3 34 7' 2

Black X Albino (extracted from chocolates)

{CBD) I {cbD)

F^ Black

{CcBbDD)

r ^ 1 .

F2... Black Chocolate Albino

Observed 76 24 27

Calculated yi'4 23-8 3i'7

IV] Colours of Mice 83

Blue X Albino (extracted from chocolates)
{CBd) I {cbD)

■*■ 1 •

{CcBbDd)

Obs.
Calc.

..Black
{CBD)

27-4

Blue
{CBd)

Chocolate

{CbD)

9-1

Silver-fawn
{Cbd)
2
30

1
Albino

(various)

162

On microscopical examination the dilution of the pig-
ment seems to consist in a diminution in the number of
the pigment granules, and not in a reduction of their size.
It is interesting to notice that many different animals have
varieties in which the dilution has proceeded to the same
extent. For example the particular dilution of black which
we call blue is known as characterizing definite varieties in
both rabbits, cats and mice, not to mention other cases less
certainly comparable.

As was mentioned in the description of the Andalusian
fowl and its (genetic features the blue colour of that breed is
not comparable with the blues we have been discussing.
In the mouse the blue is gametic, being a condition which
can be carried by the germ cells, while in the Andalusian
the blue is zygotic and depends on the collocation in one
individual of one oerm-cell bearinof black with another
which does not bear black. It is interesting, as exemplify-
ing the danger of reasoning from analogy where genetic
phenomena are concerned, that the blue-roan of cattle should
not follow the same rules as the other mammalian blue
varieties, but should conform rather to the Andalusian
system. Of course blue roan is, even to the eye, not the
homoQfeneous blue of the blue cat or mouse, but a mixture
of white or whitish hairs amono- blue and black ones. Still
on analogy we might have expected the blue of cattle to
be capable of representation in the germ-cells, but the
facts, so far as I can discover, afford no support to that
supposition"^.

* From such meagre evidence as I have obtained it is likely that the
blue, or blue dapple, of Dachshunds and other hounds is also a heterozygous
combination. As to the blue of Greyhounds and Great Danes I have no
iniormation, but I suspect it to be a dilute black capable of being bred
true.

6—2

84 Colours of Mice [cii.

Wholly Coloured and Pied Varieties,

In the analysis of the relationship between the whole
or self-coloured forms and the varieties which differ from
them in having an admixture of white many curious and
specific phenomena are met with. It is probably true to
say that generally the whole-colour is dominant to the pied,
but several examples to the contrary are already known.
In all the cases yet studied the genetic properties of the
pied types can be represented factorially by regarding the
pattern or distribution of the colour as due to a distinct
factor or to its absence. Where the whole-colour is a
dominant, the presence of the factor must be taken as
causing that distribution, so that in the absence of that
factor the individual is pied. Conversely if the pied type is
dominant the presence of the pattern-factor acts by re-
straining the distribution of the colour, and in the absence
of the restraining factor the whole-colour prevails.

One of the clearest cases is that studied by Hurst in
the rabbit, where the pattern known as ''Dutch-marked"
was proved to be a recessive. In the Dutch rabbit the
hind quarters are coloured, for example with grey or black,
while the front half of the body is white except for a patch
of colour — grey or black as the case may be — surrounding
the eyes and covering the ears. This pattern though
fluctuating in minor respects is fairly definite and is at once
distinguishable both from the self-colours and from the
various other pied forms. The cross between a pure self-
colour and a Dutch gives F^ nearly self-colour with /^
consisting of 3 selfs : i Dutch in the ordinary way. An
interesting feature is however to be observed in the fact
that the heterozygotes between self and Dutch generally
(? always) have some small amount of white collar, marked
especially on the back behind the head. Apart from
Mendelian experiment it might have been supposed that
such a white mark showed that the aiiimal contained some
albino blood. Experiment shows on the contrary that the
actual self-colours may be carrying albino as a recessive,
while the small white mark is an indication of heterozygosis
with Dutch pattern only.

The behaviour is in all respects as if the whole-colour

IV] Pied Varieties 85

pattern depended on the existence of a dominant. So also,
exactly as in the case of dilution and saturation, an albino
may carry either the whole-colour factor, or its absence.
Consequently when a Dutch rabbit is crossed with an
albino bearing self-colour, F^ is self-colour and /% gives
9 self-colour : 3 Dutch : 4 albinos.

In rats there is a type of colouring which rather closely
corresponds to the Dutch of rabbits. This has a "hood"
of colour over the back of the head and shoulders continued
down the back in a stripe which may either be entire or
broken into spots. This behaves towards the self-colour
just as does the Dutch in the rabbit ; and just as in the
rabbit the heterozygote between the self-pattern and this
hooded type always has some white. Crampe {^^2)^ ^)
observed this fact before the days of Mendelian analysis.
He noticed that when a wild grey rat was crossed with an
albino, F^^ as we call it, might be a real self-colour, or might
have some white; but that subsequently hooded rats only
occurred as offspring of those which had some white. Such
rats, which are nearly whole-colour but have a little white,
are known in the fancy as the "Irish" type. Experiments
made by Doncaster and also by Mudge indicate that these
may be again divided into two subordinate classes, distin-
guished by the amount and distribution of the white, and
there is some evidence to show that these two types of
Irish rats have distinct gametic compositions.

Though the Dutch rabbit and the hooded rat are each
such clearly recognizable types, yet within these types there
is great fluctuation, and it is practically certain that the
fanciers ideal Dutch-pattern rabbit, with the demarcation
between the colour and the white passing in a sharp trans-
verse line across the middle of the animal, does not exist as
a gametic entity. Such individuals of course come into
existence from time to time, but selection will not fix their
type. As Castle and MacCurdy (183) have shown in the
case of rats, selection may nevertheless to a considerable
extent be effective in producing hooded types with more
colour, and with less colour, which are evidently gametic
possibilities.

In mice no pied type exists which is quite so definite
as the Dutch pattern of rabbits or the hooded type of rats.

86 Pied Varieties [ch.

and by taking pains every gradation in amount of white
could be found represented among fancy mice. Experience,
however, soon shows that some at least of these are gametic
types, while presumably others are the consequence of
various heterozygous combinations. Cuenot's work with
mice led him to the conclusion that in mice the several
degrees of piedness are recessive to each other in the order
of the amount of white, those with more white being re-
cessive to those with less. In general terms this is a true
account, but we have not found the rule to apply quite
strictly even in mice, perhaps through the existence of the
complication next to be considered. It should be remarked
also that no general statement can be made as to dominance
of self-colours over pied which is applicable to animals in
general, and on a wide survey of the results of breeding
many paradoxical occurrences are met with. Especially
curious are the cases, by no means very rare, in which a
cross between a domesticated and a wild animal, e.g, dog
and wolf — has produced a partially pied offspring.

Dominant Pied Types,

So far we have been considering the behaviour of
pied patterns recessive to the whole-coloured types.
Though several points remain for investigation the genetic
relations of these patterns are fairly clear. A remarkable
complication has next to be mentioned. Both in the rabbit
and the mouse it is now known that in addition to the pied
types which are recessive there are others which are domi-
nant to the whole-coloured form. Hurst has proved this
for the variety of rabbit called *' English pattern." This
animal is white with spots of colour (black, grey, or other-
wise) generally of small size on the sides of the body, a
patch over the eyes, and a " chain " of spots sometimes
nearly continuous down the middle of the back. In ideal
specimens the spots should be of a special form and have a
definite distribution, but neither of these features seems to
be gametic. The English type is not now much in fashion
and looks uncommon to those used to modern rabbits, but
formerly it was very abundant.

iv] A Domina7it Piebald Type 87

Hurst's experiments (160) showed that this pattern is
an ordinary dominant to self-colour — a definite but most
unexpected result.

More lately Miss Durham (116) has found a dominant,
and doubtless analogous, pied type in mice. So far no
criterion has been discovered which distino^uishes this
dominant pied pattern externally from the recessive ones,
but in breeding the distinction is perfectly sharp. It is
likely that the factor for this new dominant was brought
into Miss Durham's strains by the introduction of the type
called ''black-eyed white," but the evidence is not perfectly
clear on this point. As may be supposed, the combination
of the dominant pied with the recessive pied in the same
strain gave results which it was impossible to disentangle,
though when each of these types was isolated the course of
descent was perlectly clear. The case is interesting not
merely as exemplifying a new kind of factor, but as illus-
trating a type of complication that may very possibly have
to be reckoned with in other difficult and as yet incom-
prehensible sets of phenomena.

Another example of a pied condition dominant to self-
colour has been seen in our poultry experiments (22) ; but
since in fowls some of the wholly white breeds are prac-
tically dominant in whiteness, it is only to be expected that
some of the partial whites should also show dominance.

In plants, as in animals, no general rule can be laid
down as to the dominance of self-coloured or parti-coloured
flowers. In the Sweet Pea, for example, the old-fashioned
" Painted Lady" (see Plate V) has a red standard and
wings nearly white. It is thus a bi-colour type, but it is
dominant to the self-coloured reds and pinks. On the
other hand in Antirrhi7i2tm the self-coloured types are
always dominant to the '' Delilah " forms which have the
lips coloured and the tube or throat white, as was first
shown by de Vries. Miss Wheldale who has since worked
on a large scale with Anhi^rJiimun {^o^ found that lor
each shade of flower which exists as a self-colour, a corre-
sponding Delilah or white-tubed type can be made which
behaves to the corresponding self-colour as a simple reces-
sive.

CHAPTER V

HEREDITY OF COLOUR— continued.

Albinos giving Coloured Offspring; Reversion on Crossing —
Various Kinds of Whites — Stocks — Orchids — Pigeons —
Fowls — Primula.

We have seen that albinos, both animal and vegetable,
though devoid of pigments, may yet bear factors which are
capable of determining the quality and distribution of colour
when they meet with colour in the zygote. Particular
colours may thus be due to the co-existence of several
distinct determining factors, each with an independent
distribution among the germ-cells. The grey of the rabbit
for instance is caused by the presence (i) of a colour-
element or elements ; (2) of a factor which determines them
to be the mixture we call grey, and not for instance, black
or yellow. In the case of certain plants this analysis can
be carried a step further, and the formation of colour at all
in the flowers can be proved to depend on the co-existence
o{ two complementary factors in the individual.

The first indication of this phenomenon was found in the
fact that two plants each totally devoid of colour in the
flowers and stems, and each breeding true to albinism, may
when crossed together give purple flowers in F^, The two
white parents each contain a factor which, alone, is incap-
able of forming colour. Each of these factors is indepen-
dently transmitted in gametogenesis, and thus in F^ the
ratio of coloured individuals to whites is 9 : 7. This pro-
portion depends on the fact that a series of 16 individuals
is necessary to exhibit all the possible combinations of
germ-cells, for, as in any example of hybridisation involving
two pairs of allelomorphs, there will be tour types of female
cell and four types of male cell produced by F^, Of these

CH. V]

Reversion in Colour

sixteen Individuals 9 will contain both the dominant or
present factors, while of the remaining 7 individuals, 3 will
contain the one dominant, 3 will contain the other, and i
will contain neither. There will therefore be 9 which are
coloured and 7 which are albino. In the diagrammatic
scheme C and R are the symbols representing the two
complementary factors, c and r being their respective allelo-
morphic absences.

cR
cR

or
cR

Fig. 16. Composition of the 9 coloured and 7 albino offspring in F<.
from the cross between albino Cr with albino cR, showing the ratio
9 coloured : 7 albinos.

Tvv^o examples of this phenomenon have been studied
in detail. The first is that provided by the Sweet Pea
{Lathyrus odoratus) which has formed the subject of ex-
periments carried on jointly by Mr Punnett and myself for
some years.

The work was begun by crossing two white sweet peas
belonging to the variety Emily Henderson. These plants
were alike in every respect so far as could be perceived,
excepting that the shapes of their pollen grains differed,
the one having the normal long pollen grains of the species,
while the other had roundish grains. The object of the
experiment was to trace the descent of the pollen-character
and at the beginning no question of colour was entertained.
When F^ was grown however it was clear that here was a

90 Reversion in Colour [ch.

remarkable opportunity of studying a reversion in colour
due to crossing, for these plants instead of being white
were purple like the wild Sicilian plant from which our
cultivated sweet peas are descended.

The facts respecting the colour inheritance will now be
given. With regard to the pollen-shape it must suffice at
present to state that the long shape is a dominant and the
round a recessive. The details as to the distribution of
these two shapes among the F^ individuals, which are
interesting and have greatly aided the development of
genetic theory, are given in the chapter dealing with the
phenomenon of *' gametic coupling." The present section
is concerned with colour only.

When the reversionary F^ generation was first seen its
nature was entirely mysterious. When 7% was raised from
these F^ plants the series consisted of a mixture of plants,
some coloured and some white. In some cases the series
of coloured plants consisted of two kinds only, purples like
7^,, and a red bicolour type, the well-known old variety
called '* Painted Li^dv.''

In other cases F^ contained besides those mentioned,
two quite distinct additional types of purples and two
corresponding additional types of reds.

The phenomena, though, as will be shown, in reality
very simple, presented superficially an appearance of great
complexity. Further difficulties were met with in the fact
which was soon discovered, that the cross between long-
pollened whites and round-pollened whites does not always
give the coloured types, but may result in ordinary whites
only.

It is unnecessary to go through the long series of steps
by which the analysis of the phenomena was carried out.
The meaning of the facts is now perfectly clear and they
can all be arranged in one consistent scheme.

Of the two white parents originally used the one
possessed one of the two factors we have called C and R,
the other introducing the other complementary factor. .The
meeting of these two elements produces colour in the fiower.
If no other epistatic factor is present their colour is red. As
a matter of experiment however one of the parents, proved
afterwards to be that which had the long pollen, did carry

Colours of Sweet Peas

such an eplstatic factor whose property is to make the
colour purple, just as the factor B, in the mouse, makes the
coat colour black. F^ was therefore purple, and F._ con-
sisted of 27 purples : 9 reds : 28 whites, as shown below.
The factor which determines the colour to be purple is
represented as B, the blue factor.

The diagram (Fig. 17) exhibits in a tabular form the
composition of the various F^ plants. The distribution of the

Fig.

57:7

57 -.7

15:42

17. The F^ family from the cross white Emily Henderson Sweet
Pea, long pollen x white ditto, round pollen. The 27 cross-hatched
squares represent purple plants, all containing B. The 9 single-
hatched squares without B are the reds. The 27 squares lacking
eitlier C, or R, or both, are all whites of various compositions. The
numbers at the foot show the rat'o of long to round pollens in the
types of individuals represented 'in the columns above.

92 Extracted Whites [ch.

pollen-shapes among them will be the subject of further
consideration (Chap. ix).

White (long-pollened) x White (round-pollened)
CrB cRb

F^ Purple

CcRrBb
i^2 • • • 2 7 purple : 9 red : 28 whites

CRB CRb of various compositions, but

none containing both Cand R

36 : 28

9 : 7

When these facts are made out we have no longer any
difficulty in understanding how it can be that various results
may follow the union of two white plants which would
breed true to whiteness in perpetuity if left to self-fertili-
sation. For example, white x white will always give only
whites unless one of the complementary factors C and R is
present in each parent. Whites of the form Crb and cRb
when crossed together will give reds only, with 9 reds to
7 whites in /%. Whites of the form CrB and cRB crossed
together will give purple in F^, and in /% 9 purples :
7 whites, no reds being present in such a family because
the blue factor B occurs on each side of the parentage.

Extracted Whites.

We have so far represented each type of white used for
a parent as if it were homozygous for those factors, but of
course it may be heterozygous for one or more of them, in
which case F^ will contain a mixture of types. For example,
CCrrBb x ccRRbb will give in F^ purples and reds in equal
numbers. Ccrrbb x ccRrbb will give in F^ a family consist-
ing of 3 whites : i red. Various other combinations are
possible and most of them have now been met with in the
course of experiment.

The fact that the extracted whites, that is to say, those
which appear in F^, have not in all cases the same pro-
perties as the original parental types is readily intelligible.
In the early stages of the research it seemed strange that
whereas the original long-pollened white crossed with the
round-pollened white gave a coloured result, it was possible
and indeed more usual to find whites exclusively produced
by the cross of two extracted F^ whites, long-pollened

V] Colours of Sweet Peas

and round-pollened respectively, which outwardly were
indistinguishable from the two original parents. As we
now know, by the redistribution of factors a great variety
of whites is in reality produced in /%, and only those pairs
which bear the complementary factors C and R can cjve
coloured offspring. This apparent dissimilarity between
the behaviour of the extracted forms and that of the pure
types from which they are derived has been adduced as
being inconsistent with Mendelian principles. When how-
ever the factorial composition of the various individuals is
correctly represented it is evident that this multiplicity of
composition in F^ is what must be expected, and there is
abundant evidence that such complications exist in all the
plants and animals that have been experimentally studied
on an adequate scale '^.

Subordinate Types in F^.

We have now to consider the meaning of the appearance
of subordinate types in the purple and in the red classes
which is a feature of some F^ families. Of these there
are frequently two. The first is characterized by having
the wing-petals much more fully coloured than those of the
F^ purple, or of the Painted Lady. These distinctions are
due to the operation of an epistatic factor for the lighter
wing-colour. When this factor is absent the wings are
dark, and the flower, if a purple, has the wrings deep purple

* Factors which may thus exist without making their presence visible
have been named by Tschermak "Cryptomeres." The term is open to
the objection which zoologists especially will feel, that it may cause con-
fusion owing to the fact that the series of words containing "mere" are
now universally understood to refer to phenomena of division — or Meristic
features. When the study of that part of genetics which is concerned with
meristic variation is more fully developed this confusion will be aggravatea.
For the present moreover the expression "factor," qualified, if necessary,
as unseen, seems sufficiently precise.

The words "latent" and "latency" must be applied in these cases
with great care, if at all. Various writers have, for instance, spoken of the
purple colour as "latent" in a white flower. This description is quite
misleading, for the colour need not be present in such plants. The term
"latent" is only admissible in application to X\\q factors (as here, to the
factor which can turn the colouring matters purple) not to the diaracUrs^
except loosely to those which are actually present but hidden owing to the
operation of some epistatic tactor.

94 Colours of Sweet Peas [ch.

instead of a light purplish blue. Similarly reds having the
factor for light-wing are Painted Lady, if without it are of
the type called Miss Hunt, with wings of a full pink.

Another pair of subordinate classes is due to the
absence of a factor for whole-colour. When this factor is
absent the purples become purple picotees, with an
edging of colour on a white ground, and the reds become
what we have called " tinged whites " ; a type with a little
pink in the standard only. The purple picotees, w^hen
the bud is just opening, have a purple edge to the
standard and a blue edge to the wings, these edges being
all that remains of the colours proper to these parts
in whole-coloured purple. The ''tinged white," which is
really a red picotee, has the red of the Painted Lady
standard reduced to a few pink lines in the centre of the
standard. Both these picotee forms, like the picotees of
tulips and most other flowers, acquire a more general flush
of colour as the flower ages. These various types are
illustrated in the coloured plate (Plate III), from which their
peculiarities will be easily appreciated. Among the picotees
of course there are two types of purples and two types of
reds, differing in the presence or absence of the factor for
light w^ing, but in forms so nearly white these differences
are evasive, and we have not found it possible to classify
the picotee class in respect of these distinctions.

It is to be observed that the simultaneous appearance
of the subordinate types in each of the classes is a charac-
teristic and necessary feature of the normal Mendelian
processes. It would be impossible, for instance, to make a
plant which could throw purple picotees in F^ without
red picotees, if the family contain both purples and reds.
Conversely, if the plants throw purples and reds, and also
whole-colours and picotees, then both kinds must be repre-
sented in both classes. In any examination of F^ families,
so soon as one pair of large classes can be recognized and
one subordinate type perceived in one of them, search
should be made in the other class in order to find the
corresponding subordinate type in it also. If in one class
a type exists which is never to be found in the other class
though a long series has been examined, some complication
is to be suspected, but whether the phenomenon is due to

Cff.

^ - ?.

l^ X

•— ^ f— ■

. ^-

-m*

r->

•

«^

c3 —

o •-

C — c o

rr\ • — , —

, ' r" '_ **<

iP^

" ■

r""

-T

...

•^

— — ^ /5

^' '5 J 1.

"5 vii ^ y

V] Colours of Stocks 95

coupling or to some other cause can only be decided by the
circumstances of the special case (see, for example, the case
of Stocks, in which plants with hoary leaves are wanting in
the classes without sap-colour In the flower (Chap, viii),
and the distribution of the hooded standard among Sweet
Peas (Chap. ix)).

Colottrs of Stocks {MattJiioIa),

The experiments of Miss E. R. Saunders have revealed
a closely similar inter-relationship between the various
colour-types In Stocks. There also coloured F^^ is produced
by crossing two types both destitute of sap-colour. The
two types giving this result are white and cream. The
pale yellow of the cream Is due not to sap-colour but to
chromoplasts, and thus the two types are as regards sap-
colour both albinos. In this case also F^ has the common
purple flower which we may regard as reversionary*, and in
F^ there appear both purples and reds. These results can
be represented exactly like those in the Sweet Pea case.
The sap-colour is due to the meeting of complementary
factors, C and R, derived from the two parents respectively.
In the absence of the "blue" factor B^ the colour produced
by CR is red, but when B is present the colour is purple.
As a matter of fact it can easily be proved that B is intro-
duced by the original white parent ; for when a red is
crossed with the white, F^ is purple, but when a red is
crossed with a cream, F^ is red, showing that the cream is
devoid of B, but that the white possesses it.

The numbers in F^^ are also on the plan described for
the Sweet Pea. The total of sap-coloured plants to the
total of non-sap-coloured (whites -f creams) averages 9 : 7,
and when the proportion of purples to reds is also repre-
sented the ratio is

27 purples : 9 reds : 28 whites and creams.

Just as In the Sweet Pea again there is a series of
subordinate types among the purples in /%, and a corre-
sponding series of types among the reds. For example,
there is a dilute form of purple which the horticulturists

* As will be described later, in this case when both parents have
glabrous leaves, F^ reverts to the hoary leaves of the original type.

96 Reversion in Orchids [ch.

call **plum," and a dilute form of red which is known as J
"■ copper," &c. Any of these colours may be on a white
ground, or on a cream ground '^.

Albino Orchids giving Colouj^ed I^^,

In Orchids a series of facts has been observed which
are exactly comparable with those described in the Sweet
Peas and Stocks. There are several albino orchids in the
genus Cypripediitm. Of these some when crossed together
give coloured offspring, and others crossed together give
albinos only. Mr Hurst f most kindly furnished me with a
complete list of the results of crossing albino Orchids so far
as they have been published. This list covers a very wide
range of cases to which no reference can be made here. As
regards Cyp7npeci '^tm most of the experiments were made
by Mr N. C. Coc 1 of Wylam. I am much indebted to
him and to his gaidener Mr H. J. Chapman for information
about them. For example :

C. callostun Sanderae x C. bellatuhtm album gives all
offspring coloured (25 plants bred by Mr Cookson).

C, Lawrenceamun Hyeanum x C bellatuhtm album, all
offspring coloured (Cookson).

On the other hand

C Lawrencea7ium Hyeanum x C. callosum Sanderae
gives offspring all albinos (viz. C. ^^ Maudiae'') (raised
by Messrs Charlesworth and independently by Baron
Schroder).

If therefore we suppose that bellatulum album is carrying
one of the complementary factors, say C, and that the other
factor R is present in callosum Sanderae and in Lazvrence-
anum Hyeanum, the res ^ are correctly and consistently
represented. ^^"4

As regards the he the pure types however there

is one occurrence to i dv d which cannot be explained.
Of the albinos named al o v ^ iC, callosttm Sanderae has been
self-fertilised and gave albi s only, as must be expected.

* Though all the various c* ^ itions occur it is probable that the

distribution of white and cream ' no means a simple one, but the

analysis of the inter-relationship ^n white and cream has not yet
been worked out completely.

t See also Hurst, Gard. Chron. 6, 1909, p. 8i.

V] Reversion in Orchids c^^

i
Laivrenceamim Hyeamim when selfed, gave 14 albinos, but

in addition, one colonized plant, which ]\Ir Cookson tells me

cannot be thought to have resulted from error. Such an

occurrence is as yet unaccountable, but otherwise all is clear.

In addition to the cases given above there are many in
which C. insigne Smideriamim and insigne Sanderae were
used, but Mr Hurst tells me that of these the latter is
certainly a tinged type, while the former has some dark
hairs on its petals which may contain sap-colour. Some of
the crosses with these types gave reversionary offspring,
and others gave various whites.

The genus Cattleya provides another list of comparable
cases. For instance

C. Mossiae Wagenerix C. Gaskelliana alba gave 4 albinos

(Hye).

„ „ X C. intermedia u^^.s^ gave 14 albinos

(Holford).

C. Schrdderae alba x C. iritermedia alba gave coloured

offspring (Cookson).

C Gaskelliana alba x C. Ha7'7'iso7iiana alba gdiVe/i^colovw^d

plants (Thwaites).

X C Warneri alba gave 5 albinos and
2 coloured plants (Peeters).

x C. Mendelii alba gave 2 F^ pink
flush (Thwaites, Gard. Chiton.
1910, I. p. 62).

From these statements it may with great probability be
inferred that the albinism of Mossiae Wageneri, Gaskelliana
alba and intermedia alba is du^^ he absence of one factor
(say C) ; that in Hannsoniaii ' ■" '^nd \hrdderae alba it
is the other complementary fi 'c^ s wanting (sa)' R)\

and that Wariieri alba is hetc^S v. ^ for the presence of
"ne of them (being Cc, on thhr nenie). It is to be hoped
^ome Orchid grower will 'lake the various unknown
'ons and extend the '<^.

^ besides the cas iumerated, many instances
in Cypi ^ Cattleya, other genera, of various

tinged type. '^ true -n self-fertilised and giving

reversionary coi jftsp'^ when crossed.

B. H. 7

>> >J

» ))

98 Comparison between [cii.

The Case of Antirrhmtmi [Snapdragon).

The experiments of Miss Wheldale (303) on Antir-
rhinum show that in that plant the appearance of the red
and magenta flower-colours is similarly dependent on the
presence of two complementary factors. In one respect,
however, this case differs from those which we have been
considering ; for whereas in them either of the two colour-
factors, spoken of as C and R, might be present in a truly
white-flowered plant, it appears that in Antirrhinum one of
the two factors produces either a bright yellow or a very pale
*^ ivory " yellow in the flowers. The other complementary
factor may be present in actual whites. Hence magentas
or reds may be produced by the cross between white and
yellowy or white and ivory, but hitherto crosses between
actual whites have not given coloured ofl'spring. If in the
Sweet Pea for instance the presence of C caused the flowers
to be ivory instead of white the case would be like that of
Antirrhinum. At first sight it might be supposed that the
phenomenon seen iwMatthiola (Stocks), where white x cream
gives F^ with anthocyan colours, was more strictly parallel,
but in the Stock the cream is a plastid-colour, whereas in
Antirrhinum the ivory is a true sap -colour.

It is a noticeable and rather unexpected feature In the
case of Antirrhimtm that the pale ivory is due to a factor
epistatic on the bright yellow. The ivory is so pale that it
might on casual examination be described as white ; never
theless it is definitely and completely dominant to the bright
yellow type.

Ivory crossed with white gives F^ magenta-red but
yellow crossed with the same white gives F^ crimson-red.
The distinction between the two kinds of red is due to the
factor which turns bright yellow Into ivory.

A comparison between the Sweet Peas or Stocks on the
one hand and A^itirrhhium on the other, may be represented
in tabular form. Using a terminology applicable to both
cases let us designate the factors thus"^ :

Presence and absence of the *' ferment." F, f.

"Chromogen." C, c.
Epistatic factor. E, e.

* In the previous discussion of the Sweet Pea the factors 7^ and E are
designated R and B respectively.

3) »» >) >}

Sweet Pea

Fee

white

fCe

white

fcE

white

fCE

white

FCe

red

FCE

purple

V] Antirrhiman and Sweet Peas 99

The following appearances will then be produced in the
flowers by the various combinations.

AntirrJiinmn

white

yello%v

white

ivory

crimson-red

magenta-red.

As regards the distribution of the pigments over the
parts of the flower, many facts of importance have been
discovered (see original papers). Several distinct factors
are involved, and by careful analysis Miss Wheldale has
been able to show that almost all the heterozygotes can be
recognized and distinguished from the corresponding homo-
zygotes. The degree to which it has been found possible
to effect the genetic analysis of the types in Anth-rhimnn is
very remarkable. At first sight the series of forms appears
to consist of numberless intergrading tints, but by continued
experiment Miss Wheldale has succeeded in disentangling
the various o^enetic combinations and showinof the factorial
composition of almost all. Even as regards the inheritance
of striping an approximate factorial representation has been
worked out, though from the fact that striped types may
occasionally have self-coloured offspring this part of the
analysis must still remain incomplete.

Needless to say there is as yet no system of colour-
reproduction, whether by lithography or photography, which
can be applied with accuracy sufficient to represent the
various tones of colour, and for the most part the work
is unpublished.

Reversion on Ci^ossing : the Nature of Variation.

From the facts now before us a clear conception may be
formed of the meaning of reversion occurring in conse-
quence of a cross. This phenomenon Is due to the meeting
together of factors which are complementary to each other,
and must be together present in order that the original or
reversionary type may reappear. We have seen how In
the mouse or rabbit the wild grey type is reproduced when

7—2

lOO Reversion in Pigeons [ch.

a black is crossed with an albino bearing the factor G,
greyness. Here the reversion is due to the meeting of
this factor with the colour-element present in the black
animal. The case of the Sweet Pea or the Stock, when
two colourless types crossed together produce coloured /^,
is of exactly the same nature, except in so far as both of
the complementary factors are in this case imperceptible
until they meet each other. The occurrence of the ratio 9
coloured viz. reversionary : 7 uncoloured is a proof of the
correctness of this mode of representing the facts.

When reversion is correctly represented we are led on
to form some mental picture of the essential nature of a
variation. It must occur by the omission or by the intro-
duction of a factor.

Reversion in Pigeons.

In connection with the reversion produced by crossing
two white individuals the famous case of pigeons may be
considered, though in that example one parent was coloured.

Darwin was the first to draw attention prominently to
such phenomena, and the experiments which he made
consequent on a hint given by Boitard and Corbie have
long been classical. In these experiments the blue colour
with black bars resembling the colour of the wild rock
pigeon {Columba livia) was reproduced as a result of cross-
ing birds quite free from such characteristics. Darwin's
experiments were very complicated"^. He was concerned
only with the question of origin, and to provide a qualitative
demonstration that the peculiarities of the blue rock pigeon
could be reproduced from modern varieties which had
ostensibly lost them.

Mr Staples- Browne (255) has recently investigated the
matter afresh in a simpler case. He crossed a Black Barb
with a White Fantail, producing F^ black with some white
feathers. From such F^ birds /% was produced, consisting
of

Blacks. Blacks Blues. Blues Whites,

with white with white

feathers. feathers.

* Ajwnals ajid Plants, ed. 2, 1885, i. p. 207.

V] Dominmtt and Recessive Wliites lor

The numbers were not sufficient to give the ratio
definitely, but it was estabHshed by subsequent breedino-
that the whites bred true ; that the blues did not throw
blacks ; but some at least of the blacks could throw blues.
The blues breed true except in so far as they may throw
whites^

We have then the fact that a reversionary type was
produced, but that it did not appear till F._, The meaning
of this is obviously that the blue cannot appear in F^
because black is epistatic to it. The elements necessary to
produce blue are all present in F^, but in it black also is
present which conceals the blue. After the re-combinations,
some /^2 forms contain the blue group of elements without
black, and these are therefore blue. We do not know in
this case that the Black Barb itself did not contain all that
is necessary to the production of blue, and it is thus possible
that the reversion on crossing may here be only a pheno-
menon of re-combination. This could only be decided by a
long statistical analysis.

Genetic Properties of White Types ivhich are not Albinos
[Foiuls ajid Primula). Dominant and Recessive Whites.

Albinism, namely total deficiency of colour, as we have
seen, is always recessive to the presence of colourf. In
contrast with real albinos, the white forms which contain
pigment In some part of their organisation show various
phenomena of colour-Inheritance. Among both animals
and plants such types are known. For example, races of
pigs, cats, mice, cattle, dogs and fowls exist in which the
skin, hair or feathers are white, or nearly so, though the
eye is pigmented. The " white" races of mankind perhaps
belong to the same category in some respects. Our know-
ledge of the genetic behaviour of most of these types as
regards pigmentation is still fragmentary, but what little is
known points to the existence of considerable complications.

Respecting white poultry, however, several features have
been ascertained which are interestlnQ^. In them we meet

■^ Apparently only those blacks and blues wliich had some white
feathers were capable of throwing whites.

t In Axolotl (see p. 43) an alleged exception is recorded.

I02 . White Fowls [ch.

a series of facts which perhaps, better than any other,
illustrate the impossibility of understanding the significance
of genetic phenomena without minute individual analysis.

In poultry we know at least four different sorts of white
plumage, each with its own special properties. As far as
external appearances of the adults go there is little or
nothing which would lead an observer to suspect that the
genetic powers or capabilities of these four types were
entirely distinct in respect of colour.

In the first type the white acts as a dominant. Generally
speaking when a purely white variety such as White Leg-
horn is crossed with a coloured variety, such as Indian
Game or Brown Leghorn, F^ is, in the newly hatched
condition, white with a few specks of black. The black
then present may be confined to a single hair, but we have
met very few instances of total absence of such a speck or
''tick" as it is called. As the dov/n is replaced by feathers
similar ticks of black and brown appear in them. The
position and amount of the ticks in the feathers bear no
obvious relation to the down-ticks. These F^ birds, however,
at all ages are substantially white birds, the amount of
colour though definite, being almost always insignificant.
Exceptions occur in the case of cocks, which sometimes
have enough red in the wings to bring them into the
category called by fanciers "pile," though they are of the
usual white in the downy stage.

We may thus speak of White Leghorn as a do7ninant
white. F^ from such an F^ generation consists of a great
variety of colours, the relations of which have not been
worked out.

But among the derivatives from various matings"* which
Mr Punnett and I have carried out white birds have been
produced which are, so far as their whiteness is concerned,
simple recessives to colour. Sometimes, but not always, they
have ticks of grey colour. These recessive whites breed
true to whiteness as the White Leghorns do, and no one
looking at them would doubt for a moment that they were
moderate specimens of that breed.

In addition to these the white of White Rosecomb

^ These birds arose from a mixture of Brown Leghorn, White Dorking
and Indian Game. See Rep. Evol. Ctee^ ni. p. 19.

■H,

Plate IV

'k t

•» *

1. Hen of Recessive White strain.

2. Hen of Silky breed.

v] White Fowls 103

Bantams is also a recessive. Strictly speaking perhaps
this breed should not be called white. Though appearing
white on casual inspection, more careful examination always,
I believe, reveals the presence of one or more ticks of grey
colour. These flecks of grey colour may be extremely
minute, but in our experience are never absent. The down,
too, of the breed is of a faint bluish or smoky colour, though
there is no such smokiness in the adult plumage.

Lastly we have to consider the white of White Silky
fowls, which is again a recessive to colour. The adult
Silky is pure white as a rule, but like the White Rosecomb,
the down of the chickens has some colour usually (though
perhaps not always). The colour in this case however is
buff, not blue or black, and it occurs on the sides of the
head, in the region of the pale stripes of striped chickens,
and on the rump.

To resume : there are four kinds of whites.

(i) Dominant white of White Leghorns. A pure
white,

(2) Recessive white of our own derivative strain.

(3) Recessive white of Rosecomb Bantams.

(4) Recessive white of Silky fowls.

Crossed with coloured breeds such as Brown Leghorn

(i) gives white birds with a little colour.

(2), (3) and (4) give wholly coloured birds with the
pigmentation of the plumage as fully de-
veloped as in any coloured breeds.

The next point concerns the relations of the various
Recessive whites with each other, and is of considerable
interest. Our experiments have shown that the cross
between Recessive whites (2) and the Silky (4) gives ex-
chisively whole-coloured birds in /\. In /% such birds
give 9 whole-coloured to 7 whites. Of the 7 whites some
have white down like the Recessive white (2), while others
have buff in the down like the Silky (4)"^. The appearance
of coloured F^ from two whites, with 9 coloured : 7 white in
/^2, is obviously a phenomenon comparable with that which
has been described in the Sweet Peas and Stocks. The
two white types (2 and 4) evidently contain complementary

* The numerical relations of these two forms are not yet clear.

I04 JVhite Fowls [ch.

factors which must be together present in the zygote in
order that colour should be produced. The colour of the
F^ birds is practically Black-red, approaching closely to the
plumage of Gallus bankiva (see Plate IV).

It might a priori be expected that since the White
Rosecomb (3) has very distinct though light pigmentation
in its down, a cross between this and the Silky (4) would
be more likely to give a coloured F^, This, nevertheless,
is not the case. The cross between these two breeds is an
ordinary white, showing that no reliance whatever can be
placed on such considerations, and that experiment alone
can decide what properties the several types possess.

The whiteness of the recessive whites is thus due to
the absence of one of the elements needed for the de-
velopment of colour. Analytical experiments show on the
contrary that the white of the dominant white (i) is not
due essentially to the absence of an element but to the
presence of a factor which prevents the development of
colour. Thus, if X and Y are the two complementary
factors which produce colour, the one recessive white is
Xy and the other is xY, The dominant white (i) has the
suppressing factor S, and as the result of many experiments
it appears that the individuals of that breed have various
compositions in respect of X and K, though always
homozygous for S.

Hence it follows that when Dominant white (i) is

crossed with Recessive white (2), F^ is white; but F^ may,

and in our experience always does, give some coloured

birds. The proportion in which the coloured F^ birds

appear must of course depend on the composition of the

individual dominant white which was originally introduced,

and various ratios occur. The numbers obtained show

that of the dominant whites some are Xx Yy SS, another

contained only one of the colour-factors and so may be

written Xxyy SS. Probably all the possible combinations

occur, but the experiment has not been carried far enough

to prove that they do*.

* The difference in size between Recessive white (2) and the White
Rosecomb is so considerable that the direct cross between these two
cannot be made. But jFi from White Rosecomb x Silky has been crossed
with (2) and the offspring were in about equal numbers whites and browns.
There is therefore no reasonable doubt that the factor carried by the
Silky and by the Rosecomb is the same. Either both have X, or both V.

V] Dominant Whites \o^

The facts thus take their places in a consistent scheme,
and when the factors are correctly determined, all the
observed properties of the several types can be represented.
Some curious consequences follow which would have ap-
peared very surprising both to evolutionists and to fanciers
a few years ago. For example, if a pair of coloured birds
throw very few whites the chances are large that these
whites if mated together will breed true. On the other
hand if a pair of coloured birds throw ma7iy whites, the
chances are that such whites if bred together will have
some coloured offspring, and it is not very unlikely that all
their offspring will be coloured ! For in the first case where
the whites are few, they are probably i in 4, indicating
that both parents are homozygous in either X or K, say X,
and heterozygous in respect of the other factor Y. There-
fore all the whites which come will be carrying X and none
will have Y. Therefore they will breed true to whiteness
if mated together.

In the second case however, where the whites are
many, they are probably 7 whites : 9 coloured. In such
a case the parents must be heterozygous both in X and in
Y. Consequently some of the whites will be bearing X
and others K, and there will be a good chance that if such
whites are bred together these two complementary factors
will again come into combination and coloured offspring be
produced.

Among plants only one example of dominance of white
flower-colour over the ordinary sap-colours has yet been
investigated. This occurs in Primula Shiensis. In this
species colour may be present either in the flowers, or in
the stems and leaves, or in both. When white flowers
occur on stems green and devoid of pigment, the white is
recessive as in any other case of total albinism. There are
however white varieties which have more or less red in
the stems, and these when crossed with varieties having
coloured flowers give F^ with flowers white or nearly so.
There is almost but not quite always a slight tinge of
magenta-red in the petals of such F^ plants, and the in-
tensity of this colour increases with low temperatures but
diminishes if the house be kept warm.

Such dominant whites, when crossed with the green-

io6 Dominant Whites [ch. v

stemmed recessive whites, give a white-flowered F^ with a
reddish stem. The F^ generation from this cross has
contained a small number of plants with coloured flowers,
but the ratio has not yet been determined. There is an
obvious general agreement between this case and that
described in fowls, and we may feel fairly sure that the
Primula colours similarly depend on two colour-factors, and
that the absence of colour in the dominant whites is due to
the super-imposition of a third factor^.

* The white-flowered variety of Matthiola incana (Stock) is as regards
its genetic behaviour, a coloured form. Moreover its flowers tinge on
fading and its embryos have the deep green colour characteristic of purple-
flowered types. Crossed with a self-coloured type it gives F^ with purple
flowers. Clearly therefore it is not, as some of the tinged forms of P?'i?nula
Sinensis are, a dominant white. Miss Wheldale has suggested with some
plausibility that the white M. incana is comparable with the picotee types
of Sweet Peas.

CHAPTER VI

HEREDITY OF COLOUR— continued.

Eye-Colo?crs. Variations in Colour of the Iris — Deficiency
of Eye-Pigments in some Coloicred Types.

The colours of eyes present some special genetic pro-
blems which are worth examining in detail.

Pigments are formed in several parts of the vertebrate
eye, but chiefly in the choroid and the iris. In all normal
eyes the choroid contains much deep black pigment, and to
this fact the blackness of the pupil is of course due. The
common differences in eye-colour with which we are familiar
in man are not caused by changes in the colour of the pupil
but by variations in the structure and pigmentation of the
iris alone. Such differences, being conspicuous factors in
individual and racial differentiation, have long attracted the
attention of anthropologists. With the object of elucidating
the heredity of eye-colours extensive pedigrees were
collected by Galton, and biometrical studies of a similar
kind have been published by Professor Pearson and his
assistants. Various conclusions of a statistical nature have
been based on these data, but as no critical analysis was
attempted, such results are devoid of genetic importance.
As we now know, moreover, the method of classification
employed in collection was unfortunately defective. The
categories were not sufficiently precise, and the material
would give no reliable indications if analysis were attempted.

As the result of comprehensive investigation carried
out in a single district on Mendelian lines, Hurst has
succeeded in making an important advance*. Recognizmg
that such descriptions as "light," "dark," and so forth are

* Similar evidence has also been published by Davenport (107).

io8

Eye-Colours

[CH,

too vague he instituted a more critical classification, which
led to the discovery that the presence of drown pigment on
the anterior surface of the iris behaves as a dominant to the
absence of such pigment, which is a recessive character.

Parents without the pigment in question have exclusively
children who are similarly without it, whereas persons
possessing the brown pigment may be either homozygous
or heterozygous in that respect, and their offspring follow
the numerical rules with fair exactitude.

GTxQ

Q^cr C

cr cfcr cr 9 a

^ GfO'0'9 gtctq

>fg .

cr>

QxcTl

(If

9 9 9 9 9 9 9

Dupleoi Sel/*" ao lour*
O Dupleoc Runted/

DonunxLnjts

RjecessiAre.

Fig. 1 8. Two pedigrees showing the descent of eye-colour (after Hurst).

''Brown" eyes are no doubt all dominants, and ''blue"
eyes are very often recessives; but, on the other hand, many
eyes which would be called ''light" have some of the

VI] Eye-Colours 109

pigment, and many eyes which would be called "dark"
are without it. The differences between the non-pigmented
''dark" iris and the non-pigmented "light" iris are caused
by structural differences in the iris itself.

Apart from its scientific importance it will be evident
that this discovery is of considerable practical interest as a
contribution to human genetics. Much nevertheless remains
to be done in the application of precise methods to the
problem of human eye-colours. For example, amongst the
dominants examined by him Hurst recognized two distinct
types: (i) self -coloured ixiA^'s in which the pigment is distri-
buted over the whole surface of the iris, (2) ringed i rides
in which the pigment was absent from the periphery but
existed as a ring round the pupil. The relations between
these two types could not be satisfactorily determined. x'\ll
that the evidence showed was that the "self" behaves as a
dominant to the "ringed." But on analogy with other such
cases we of course should expect that these distinctions
were caused by distinct determining factors, either of which
might be borne by the non-pigmented recessive without
revealing its presence. If this were so, some of the non-
pigmented should, on breeding with the ringed, have off-
spring with self-coloured irides, just as some albino rabbits
on breeding with Dutch-marked rabbits may have self-
coloured offspring. Curiously enough, how^ever, in an
ample series of observations, Hurst met with no such
occurrence, though examples of self parents producing the
three kinds of offspring, self, ringed, and non-pigmented,
were not rare. Here, at present, the matter rests ^^

Hurst's observations relate entirely to the population
of a small English village where naturally only a limited
range of types would be met with. The inter-relations of
the eye-colours of strongly marked racial types doubtless
will provide an excellent field for the application of a similar
analysis. It is likely too that such work would result in the
discovery of the principles governing the association ot
certain eye-colours with particular colours of the hair, and

* Hurst has suggested with some plausibility that the absence of any
self-coloured irides among the offspring of ringed x non-pigmented may be
attributed to the fact that the offspring examined were mostly school-
children. He thinks it not improbable that some of the ringed may
subsequently develop into self-coloured irides.

no Eye-Colours [ch.

other characteristics. In all such Inquiries the first step is
to distinguish the critical differences which are treated as
units in the constitution of the germ-cells.

In addition to these observations on man we have a
few indications as to the heredity of iris-colours in birds.

The iris of the domestic breeds of fowls presents at least
three types of coloration. The common colour in the full
grown birds is a bright red, apparently due to the formation
of a red pigment when the period of adult life is approached,
the colour in earlier stages being a dull blackish green.

Malay fowls are peculiar in having a pale, yellowish
white iris — the "daw-eye" of fanciers — which behaves as a
recessive to the red iris. This eye may be found in birds
of various colours and probably it always indicates an
admixture of Malay blood '^. I have seen, for instance, a
daw-eye in a White Leghorn of a good strain. Here it
undoubtedly cropped out as a recessive character among the
normal red-eyed birds, and I happen to know that White
Malays were used by a prominent White Leghorn breeder
to increase the size of his strain.

Another peculiar type is seen In the nearly black iris
of Andalusians and Silkies. In Andalusians we have found
this eye imperfectly dominant to the red. The case of the
Silkies is much more complex and the inheritance is dis-
turbed by sex in the manner hereafter to be described.

In the Little Owl [Athene noctud), Giglioli has described
the occurrence of a remarkable variation in eye-colour which
is evidently an example of the appearance of a recessive in
the wild state. The irides of the normal birds are yellow,
but in the variety, of which several specimens were found
in one district, the irIdes were black. The circumstances
of the discovery which are related by Giglioli in detail t
leave little doubt that the condition was recessive. It is a
curious fact that, in all the aberrant individuals found, the
distribution of the brown markings on the feathers also
differed from that of the type. So distinct was the variety
from the ordinary A. noctua that a new specific name was
suggested for it until the fact that it could be bred from the
type was established.

* Sir J. Bowring {Philipphie Islands^ 1859, p. 151) says that "white eyes"
are preferred in fighting cocks. Probably these are the origin of our Malays
and Aseels. t Giglioli, Ibis^ i903> P- i*

VI] Pink Eyes in Coloured Types 1 1 1

Deficiency of Eye-Pigment in some Coloured Types.

In the eyes of actual albinos, destitute of pigment, the
eye has the well-known pink appearance. This is clue to
the admission of light through the partially transparent
iris. Generally speaking such great deficiency of ocular
pigments occurs only in total albinos, but to this rule there
are a few remarkable and interesting exceptions.

The best known example is the so-called Himalayan
Rabbit"^. In it the eyes look like those of the pure albino
though having a trace of pigment. At birth HImalayans
are white or nearly so, but soon the muzzle, ears, tail, and
feet assume a deep chocolate-black. This type behaves as
a recessive to the normal grey, and to black, and it Is a
normal dominant to the pure albino. Hurst however found
that HImalayans, though obviously pigmented, may carry
the factors for black and for self-colour, without any in-
dication of such factors being manifested in their own
colour or In its distribution. Recent experiments made
by Punnett have given the same result.

The next example of the pink eye present In an animal
not devoid of colour Is seen in certain *' Japanese Waltzing "
mice.

The "waltzing" is a peculiar vertiginous spinning to
which those animals are subject. I understand that the
exact physiological nature of this movement has never been
fully elucidated, but the condition has been shown to be
often, if not always, associated with malformation of the
auditory labyrinth. As already mentioned, the waltzing
character behaves as a Mendellan recessive to the normal
habit.

Such waltzing mice exist in various colours, some having
ordinary black eyes, others pink eyes. Various colours also
may occur in the coats and we have not yet complete

* Darwin gives {Animals and Plants^ ed. 2, 18S5, i. p. 113) tacts as to
the production of Himalayans by crossing Silver-greys. In the hght of
present knowledge it is no longer possible to accept these experiments as
evidence of an original production of the variety. Presumably the case
was only one of the reappearance of a recessive form, and indeed Darwin
adds the remark that he had "recently been assured the pure Silver-greys
of any such breed occasionally produce Himalayans."

112 Eye-Colours of Mice [ch.

knowledge as to which of these colours are compatible with
the pink eye and which are not.

Elaborate experiments were made by Darbishire (90)
to test the inheritance of these mice. The waltzers used
were all pink-eyed and bred true to that character. In
general terms the coat-colour was fawn-and-white, but no
detailed account of the pigments present has been published.

Crosses were made between these waltzers and ordinary
albinos. The F^ generation so produced were usually, as
regards coat-colour, grey like wild mice, the shade being
sometimes darker, sometimes lighter, together with more or
less white. In a few families the colour was black. The
albino parents being of miscellaneous origin, such diversity
in /^l colour is what we should now expect.

In respect of eye-colour the remarkable fact was ob-
served that the F^ mice were always black-eyed. \xv F^
various coat-colours occurred, including a peculiar new type
with exceedingly dilute pigment in the hair, spoken of as
"lilac." Among these coat-colours the eye-colours were
distributed according to systems not yet ascertained. All
that can be clearly perceived is that the mice with agouti
or full black in the coat-colour (whether pied with white
or not) always had black eyes ; the completely white indi-
viduals always had pink eyes ; and lastly, the eyes of the
''lilac" mice were always pink^. The facts were un-
fortunately presented by Darbishire in a form which makes
further analysis impossible. A proper investigation of this
series of phenomena would have greatly increased our
knowledge of the genetics of pigmentation.

The important conclusion may nevertheless be con-
fidently drawn that at least two factors are concerned in
producing the black colour of the eye. Cu^not (88, p. 11),
as the result of his own experiments, identifies these with
(i) colour in the coat; (2) a factor determining the black-
ness of the eye, which needs the coat-colour factor as its
complement. This is no doubt a part of the truth, but

* Miss Durham (116) has shown that the pink eyes of these "lilac"
mice and of the original pink-eyed waltzers are not wholly devoid of
pigment, but contain traces both in the iris and choroid visible in
microscopic sections. The eyes of the albino mice are absolutely without
pigment.

vi] Eye- Colon ys 113

there are suggestions that it is not the whole, and that
further compHcations have to be met. To discuss these in
detail is beyond the scope of this work, but it seems likely
that the deo^ree of saturation or dilution of the coat-colour
has to be considered.

There is evidently some intimate relation between the
colour of the eye and the colour of the coat ; for no mice
have the coat-colour wild-grey ('* agouti " of fanciers),
black, or even blue, unless the eyes are black. The
interesting fact has also been discovered that the actual
nature of the pigment of the choroid differs in the different
varieties'^. Miss Durham finds that the chocolate mice
always have chocolate eyes, not black, no black pigment
being present in either iris or choroid. The same fact has
been published by Castle (53) for the guinea-pig and was
also noticed independently by Miss Sollas in guinea-pigs.
From these observations it is to be inferred that either the
coat-colour controls the eye-colour, or the colour of the eye
controls that of the coat, but for various reasons it is not
possible yet to declare positively which account is the right
one.

Guinea-pigs differ from other animals in that their
albino strains generally have small '* smudges " of blackish
pigment on some of the extremities, especially the tips of
the ears, though the eyes are entirely without pigment. It
is said that albino guinea-pigs occur without these smudges,
but I have not seen one.

The Cinnamon Canary must also be noticed here.
This variety on hatching has pink eyes, though as develop-
ment proceeds, pigmentation supervenes. Many fanciers
believe that they can recognize a difference between the
eyes of Cinnamons and those of other varieties, but the
distinction is not readily obvious. In plumage Cinnamons
differ from other breeds in that the black pigment is
absent and the feathers consequently have a brownish tint.
Miss Durham, from her observations, concludes that the
pigment which is developed in both eyes and feathers is
chocolate, and that the Cinnamon is in fact a chocolate

* John Hunter has an important paper on this subject : '' On the
Colour of the Pigmentum of the Eye in different Animals" {Obs. on
certain Parts of the Animal Economy y 1786, p. 199).

B. H. 8

114 Eye-Colours [ch. vi

variety of the Canary. The heredity of these characters
has not yet been investigated by precise methods. Fanciers
however are agreed that some very remarkable phenomena
of sex-hmitation occur, which will be described in con-
nection with other instances of the same kind.

The only other example I know of a race which com-
bines an albino eye with external colour is the breed of
cats called " Siamese." These animals, which breed per-
fectly true, were introduced from Siam, where they have
been kept for an indefinite period as pets of the royal
household. Like the Himalayan rabbit, Siamese cats are
born almost white, but the fur becomes a curious fawn with
darker chocolate points on the ears and extremities. The
eyes are more or less deficient in pigment throughout life.
Some pigment however is formed in the eye, as is
evidenced by the fact that the iris has a blue colour and is
partially opaque.

From the little that is known respecting this group of
cases it is clear that the genetics of eye-pigments offer
many features of interest. We have seen first that varia-
tions in the colour of the iris may exhibit simple Mendelian
heredity. Secondly when the general pigmentation of the
eye is considered, it appears that the presence or absence
of such pigment may follow Mendelian rules ; but there is
an inter-dependence between the factor or factors which
produce the pigmentation of the eye and those which
govern the pigmentation of the fur or feathers, such that
the eye-colour must be regarded in certain cases (e.g, mice)
as determined by two, or perhaps more, complementary
factors. The actual nature of the pigment in the eye is
also variable, black being replaced by chocolate in several
types destitute of black in their hair. Lastly, there is a
noticeable association of chocolate pigment, in hair or
feathers, with the more or less non-pigmented eye, as in the
Himalayan rabbit, the Siamese cat, and the Cinnamon
canary.

The peculiar phenomena connected with Albinism in
Man are mentioned later in connection with human heredity
in general.

CHAPTER VII

HEREDITY OF COLOUR— continued.

The Genetics of Yellow Pigments in certain Animals.
Yellow Mice not breeding true — The Case of Basset
Hounds and the ''Law of Ancestral Heredity''
Relation of this Principle to Mendelian Rules.

The genetic relations of yellow and black pigments
present so many difficulties that a separate chapter is
needed for their consideration. There is one comparatively
simple case, that of the thorough-bred or race horse, in
which the presence of black behaves as a simple dominant
to the colour known as chestnut. In practically all the
other forms that have been investigated there is some
complication. One of these is the celebrated example of
the Basset Hounds, which played so prominent a part in
connection with Galton's enunciation of the "Law of
Ancestral Heredity.'*

In examining this section of the evidence which Mende-
lian analysis has provided we meet three facts of surprising
novelty. First, that the relations of yellow to black and
other colours may be entirely different in animals so nearly
related as the rabbit and the mouse. Secondly, that in mice
yellows never breed true, though in the rabbit, for instance,
no such difficulty occurs. Thirdly, that in rabbits the pre-
sence of the factor for the grey or agouti type of coloration
alters the appearance of the yellow animal, giving it a white
belly and tail, whereas yellows destitute of this factor have
these parts of a bluish colour. Such facts cannot of course
be properly interpreted until the chemistry of pigmentation
is more fully understood, but taken together they ilkistratc
the extraordinary multiplicity of specific rules which genetic
research reveals.

8—2

ii6 Yellow Rabbits [ch.

When the hair of the wild house-mouse, for instance, is
examined microscopically three kinds of pigment-granules
can be distinguished, black, chocolate, and yellow. As
Miss Durham pointed out (lo, p. 72), in a solution of potash
the yellow granules dissolve at once ; the chocolate dissolve,
but more slowly ; the black not at all. The pigments may
co-exist in the same parts of a hair, but in the wild ''grey"
or agouti colour there are bands in the individual hairs,
where only yellow pigment can be seen.

The black mouse has black and also chocolate pigment,
but no yellow. The chocolate has chocolate alone.

The yellow mouse may have exclusively yellow pig-
ment, but in some yellows heterozygous with black, granules
of the other pigments may be mixed with the more abundant
yellow ones.

In the blue, the silver-fawn and the cream, the pigments
are as in the black, the chocolate, or the yellow, respec-
tively, but the number of granules is fewer, giving the well-
known appearance of dilution in colour.

The colours of rats, rabbits, guinea-pigs and horses,
appear to be similarly made up of three presumably com-
parable sets of pigments. There are differences however
both in the number of distinct pigmented types which exist,
and in their genetic behaviour.

Rats.

In rats the only colours known are agouti, and black.
Both types contain chocolate pigment as well as black, and
agouti also contains yellow pigment ; but no distinct choco-
late or yellow varieties are known, and none of the dilute
forms (blue, &c.) have been recorded.

Rabbits,

Of the cases in rodents w^here yellow exists as a separable
variety that of the rabbit is the simplest. Numerous
colour-types of the rabbit are known, but in our present
Inquiry we are immediately concerned with the following ;
I. Agouti, or wild grey, 2. Black, 3. Yellow. The question
whether a chocolate rabbit can be formed is important. It
appears that no such type is known to fanciers but Mr
Punnett saw, in the possession of a French breeder, rabbits

VII] Yellow Rabbits 117

which seemed to him to be chocolate. No examination of
the pigments of these rabbits has been made.

Agouti, as we know from the experiments of Hurst and
others, is epistatic to black. Next, Castle (53) has shown and
Hurst independently discovered^ the fact that the yellows
are of two distinct types. A. Yeltozvs with white betlics and
tails, which always bear the agouti factor, G. B. Yellows
with blue bellies and tails ^ which do not contain G. The
latter have been spoken of as sooty yellows, and to English
fanciers they are known as tortoise-shells. Respecting the
pigments of these types full information is wanting. Miss
Durham found that agouti certainly contains black, choco-
late and yellow ; that black contains black and chocolate ;
but hitherto the yellows have not been studied on a com-
prehensive scale, and all that we know is that yellows may
contain both black and chocolate granules in addition to
yellow. None have yet been seen to contain yellow only.

Following Castle, the genetic composition and behaviour
of the several types may be represented thus :

Agouti (wild grey) GBY,

Black gBY.

Yellow A, white belly GbY.

Yellow B, blue belly gb Y.

This scheme is so far satisfactory, but two difficulties
arise when an attempt is made to compare the phenomena
seen in the rabbit with those observed in mice and guinea-
pigs ; for first the scheme takes no account of chocolate ;
and secondly, in the mouse yellow is dominant to black,
not recessive as in the rabbit. There is however no doubt
as to the essential facts, that yellow A crossed with black
gives agouti, while yellow B crossed with black gives
simply black dominant. A further confirmation of this
symbolic representation is found in the fact observed by
Castle that agouti x yellow B gives blacks in F^_ as the
scheme demands t. The nature of the factor G is obscure.
Castle regards it as a pattern-factor governing the distribu-
tion of the colours ; but a grey rabbit dififers from a black

* Communicated to the Internat. Congress Zool. 1907 : as yet un-
published.

t Compare the similar phenomena in mice, p. 78.

ii8 Red Guinea-pigs [cii.

in more than pattern, for actual bands of yellow pigment
are formed in the hairs. G therefore is a factor which can
make pigment yellow that would have been black in the
absence of G, so that in a restricted sense it is a dominant
yellow factor, but the relation of this yellow of agoutis to
the yellow pigment of yellow varieties is quite uncertain.

There is no suggestion that yellows of both types
cannot be bred pure (cp. Mice).

Guinea-pigs.

In guinea-pigs no true yellow exists, but it is apparently
represented by red. The work of Castle (53) and the
experiments of Miss Sollas, as yet unpublished, show that
when red is crossed with black the offspring are either
(i) black-reds, viz. patched with black and red; or (2)
agoutis, in which the black and the red are intimately mixed
in the banded hairs. Families of three kinds may thus be
produced. Red bred with black may give

(A) all agoutis,

(B) all black-reds,

It seems practically certain that the agouti-factor, G, as
we have called it, may be carried by the red, not by the
black. Consequently case (A) is that in which the red
is homozygous for this factor. In case (B) the red is homo-
zygous for its absence, and in case (C) the red is hetero-
zygous for the same factor. There is no difficulty in
breeding reds as a true strain, and the peculiar phenomenon
about to be described in yellow mice does not occur in
guinea-pigs.

In guinea-pigs a distinct chocolate type exists, but such
animals according to Miss Sollas always have some red
hairs. The relation of these to the other types is not yet
fully understood.

Mice.

The facts seen in rabbits and guinea-pigs are in fair
agreement with each other, but when these are compared
with the phenomena now well ascertained in the case of
mice, fundamental differences are perceived which make it

vn] Yellow Mice 1 1 g

impossible to bring the whole series of observations into
one consistent scheme. For first, Cuenot {%^) has shown
that in the mouse yellow is dominant to black, instead of
being recessive to it as in the rabbit. The dominance is so
far incomplete that yellows heterozygous with black have
often — perhaps always — more or less black pigment in their
hairs, but in general appearance they are obviously yellow.

Next according to Cuenot's experience none of the
yellow individuals in /% from a cross of yellow x black or
yellow X agouti are pure to yellow"^. All such extracted
yellows when bred together throw agoutis or blacks respec-
tively. This fact has led him to suggest that there may
be some incompatibility which prevents two yellow-bearing
gametes from uniting in fertilisation. These " impure "
yellows bred together gave 232 yellows and 86 agoutis, a
fairly close approximation to 3 : i. The expectation on
that ratio is 238*5 : 79*5. Cuenot, commenting on the
numbers produced, remarks that since pure yellows are not
formed, he would anticipate a diminution in the relative
numbers of yellows, and that such a diminution actually
occurs. But, as Mr Punnett has pointed out to me, there
seems to be no valid reason for expecting a departure from
the ratio 3:1; for the spermatozoa may be regarded as
unlimited in number. Of the "non-yellow" ova half would
be fertilised by "non-yellow" sperm, producing agoutis, the
other half being fertilised by "yellow" sperm and pro-
ducing yellows. The "yellow" ova would all, on Cuenot's
hypothesis, be fertilised by "non-yellow" sperm, and give
rise to yellows. The nett result would then be still
3 yellows : i agouti.

Of 81 F^ yellows tested by Cuenot, all proved to be
heterozygous, so that the reality of the abnormal pheno-
menon must be regarded as established. Observations made

* A fact of a different order, though perhaps having a bearing on the
problem, was observed by Hurst in crosses between Belgian Hares
(Rabbits) and Angoras. The Belgian is in colour an '* agouti," or grey,
but it is much yellower than the wild rabbit of the warrens. Hurst found
that 7^1 from such Belgians and Angoras (bearing G) was of the wild or
warren type of grey, and that in A i^o rabbit quite of the Belgian colour
ever came. The greys in F^ varied a little in the amount Oi yellow, antl it
is possible that, as in other cases in which a parental type fails to recur in
F^^ the absence may be ascribed to the rarity of a particular combination.

I20 Pile Fowls [ciL

by Miss Durham agree entirely with those of Cuenot She
finds it impossible to obtain any mice pure to yellow. The
experience of fanciers seems to be universal that yellows
cannot be bred in a pure strain. Those with which she
has experimented always throw either agoutis, blacks, or
chocolates, and we may take it that yellows of both sexes
are always heterozygotes formed by the meeting of a
yellow and a non-yellow gamete"^. In spite of their genetic
composition the yellows which throw chocolates have only
yellow pigment in their hairs, and no chocolate.

Cats.

In cats we meet a new complication in the inheritance
of yellow, namely that it is disturbed by sexual dimorphism.
The rarity of tortoise-shell males is w^ell known, and Don-
caster (109) has produced evidence which makes it prac-
tically certain that in the immense majority of instances
the female heterozygote of orange x black is tortoise-shell,
namely, patched with orange and black (like the guinea-pig),
but that the male heterozygote similarly produced is orange
(see Chap, x, dealing with the heredity of Sex). The
dilute types cream and blue are similarly related to blue
tortoise-shell.

Fowls.

There are further indications of peculiarity in the genetics
of yellow pigments in the case of birds. The coloration
known as " Pile " in fowls is seldom bred for exhibition
from two pile birds. The colour consists, in cocks, of
orange-yellow or red in the hackles and wing-coverts com-
bined with a white ground. The hens are white, with a
chestnut or reddish yellow breast. In the down the chickens
of both sexes have longitudinal stripes of light chestnut.
In the adults, and generally in the chickens too, there
is a small and variable amount of blackish grey ticking,
which is of course considered a fault. Pile is a colour
known in several breeds, but especially in Game, Game

* Besides this curious peculiarity, yellows show another remarkable
feature in their frequent tendency to become excessively fat. Miss Durham
has met with several such specimens and the fact is well known to fanciers.
In her experience also yellows are more liable to sterility than other mice.

vii] Pec^Ularities of Yellow Types 121

Bantams, and Leghorns. It is said never to breed true,
throwing ordinary black-reds in unascertained proportions.
Presumably pile is a heterozygous combination, black-red
being one of the constituents, but what the other pure form
(or forms) involved may be, is quite uncertain. The whole
question is still very obscure, and from preliminary experi-
ments made by Mr C. Fryer it seems probable that two
pairs of factors are concerned in addition to black-red.

For exhibition purposes piles are often crossed with
black-reds, fanciers being under the impression that a
brighter pile results from this mating. The fact suggests
that there may be piles of several different compositions.
Taking the known facts together the one clear deduction
is that pile, or yellow if we may so call it, is dominant to
black-red"^.

Recapitulation and Discussion of the Foregoing

Facts.

The outstanding peculiarities of this group of pheno-
mena are as follows :

I. The difference between the genetics of similar
colours in allied forms.

In rabbits black is epistatic to yellow, giving (in the
absence of G, the agouti factor) simply a black heterozygote.
In guinea-pigs (where red represents yellow), the hetero-
zygote of black and red is a patch-work, viz. tortoise-shell.
In both rabbits and guinea-pigs when G is present this
heterozygote is agouti, the factor G being introduced by
the yellow parent.

In mice yellow is epistatic to black, giving, in the
absence of G, a yellow heterozygote. We can form no
surmise as to the nature and causation of this discrepancy
between these types. It is to be observed that the recessive
yellow of the rabbit, viz. the blue-bellied type lacking both
G and B, nevertheless has some black pigment in its blue
fur, while the yellow mouse does not necessarily have any.

* The whole-coloured buff of Cochins and some other breeds derived
from them is quite distinct from pile. The genetics ot this pigmentation
are complex and little known as yet.

122 Peculiarities of Yellow Types [ch.

2. The fact that in mice no yellow homozygote exists.
It is certainly a most surprising fact that mice cannot be

bred true to yellow — the more so since there is no difficulty
in producing pure yellow rabbits and pure red strains of
guinea-pigs. Among the ordinary phenomena of heredity
no quite parallel case is known, though it is possible that
pile (yellow and white) fowls are in the same condition^.
Cuenot's suggestion of the incompatibility between yellow-
bearing gametes female and male, is the only one yet
offered in elucidation of the phenomenon!. While admitting
that this account has great plausibility, I think that we
must not place complete reliance on a hypothesis to which
no adequate or thorough test can as yet be applied. The
idea of incompatibility between gametes has more than once
been introduced to deal with the genetic phenomena of
Sex [q.v) and in that connection the case of yellow mice
should be remembered.

3. The behaviour of the agouti factor G.

The operation of this factor varies to some extent in
the different types. In all it can be carried by the yellow
variety, but in the rabbit alone are yellows which bear G
obviously different (having white bellies) from those which
do not (having blue bellies). It seems difficult to suppose
that this factor actually causes the appearance of white on
the belly and tail, yet the fact is well-established that such
yellow rabbits are really agoutis, or wild greys, wanting in
black.

The relation of the yellow, due to the presence of the
yellow factor K, to the yellow in the bands, caused by the
agouti factor G, is at present quite problematical, but micro-
scopically these yellows are indistinguishable.

Until the relations of chocolate to the other pigments in
the rabbit and guinea-pig have been more fully explored it
is scarcely possible to draw up a scheme of symbolic notation
representing the comparative compositions of the different
animals ; but adopting the gametic formulae given (p. 78)
for the various colours in mice we may tentatively suppose
that yellows exist of all compositions which would be pro-
duced by adding Y to each of those groups of symbols.

* See also Basset Hounds (p. 128).

t For more recent evidence see Appendix to Part I.

VII] Colours of Thorough-bred Horses 123

Thus the gametes for each of the four non-yellow colours
are as follows :

Agouti (with black) G,B . Ck.

Cinnamon agouti (without black) G .5 . C/i.

Black g ' ^ ' Ch,

Chocolate g - ^ - Cli.

Yellows being ahvays heterozygous, all zygotes con-
taining yellow are to be represented by adding Yy to the
zygotic formula produced by compounding two of the above
gametic formulae. Thus the yellows which throw chocolate
only are

Yy . gg . bb . CJiCh.

The yellows throwing black only are

Yy . gg . BB . ChCh,

and so on. This representation must in any case approxi-
mate to the truth, but there is still some doubt whether the
relation of yellow to non-yellow is quite so simple as here
suggested"^.

We now pass to two cases which are of some interest
on account of the somewhat conspicuous part that they
have played in general discussions of heredity. The first
is that of Race horses, where the phenomena, in so far as
they have been adequately studied, are remarkably simple.
The second example, that of the Basset Hounds, presents a
feature of complexity.

Race Horses.

In '* thorough-bred*' or race horses the behaviour of the
yellow and black pigments In descent is comparatively
simple and the facts thus form a remarkable contrast with

* Attention should be called to the fact that if the two kinds of agoutis
in the mouse might be supposed to represent the two kinds of yellow in
the rabbit then yellow of the rabbit might be sui)posed to correspond with
chocolate of the mouse. This natural suggestion is however negatived by
the fact that a definite chocolate pigment can be seen in the hairs of
rabbits. In the pink-eyed Himalayan rabbit the pigment according to
Miss Durham is exclusively chocolate.

124 Colours of Thorough-bred Horses [ch.

the complicated phenomena elsewhere observed in regard
to pigments ostensibly the same. The colours of race
horses are recorded with great accuracy in Weatherby's
General Sttid Book. By a careful analysis of these returns
Hurst was able to show that chestnut on the one hand is
recessive to bays and browns, which are dominant. Chest-
nuts are distinguished from bays and browns by the fact
that in their hair no black pigment is developed. Bays
always have black in the mane, tail and fetlocks. Browns
have more or less black in the same parts, and generally
black pigment is distributed to some extent over the whole
coat.

As the records show, there are only about i per cent,
of alleged exceptions to the rule that chestnut x chestnut
produces chestnut exclusively. Of the dominants some
are pure dominants and give — again with about i per cent,
of exceptions — bays and browns only, whether the other
parent is chestnut, or bay or brown. Other dominants are
shown also to be DR in constitution, giving, when bred
with chestnuts, equal numbers of dominants and recessives.

With regard to the few exceptions of both kinds ap-
pearing in the Stud Book records some are demonstrably
mistakes, and the actual frequency of exceptions must be
considerably less than i per cent., if indeed there are any
genuine exceptions at all. The relation of bays to browns
has not yet been made out, and as the two classes grade
into each other somewhat, the detection of their relation-
ship would demand observations of a sort somewhat more
critical than those which the Stud Book provides. So far
however as the segregation of black from the absence of
black is concerned, the case is simple and regular^.

* The inheritance of coat-colour in horses was the subject of an extensive
investigation by Professor K. Pearson and his assistants (219). The records
of the Stud Books were tabulated and investigated by the application of bio-
metrical methods. Various propositions have been enunciated as the result
of this inquiry, amongst others the statement that nothing corresponding to
Mendel's principles appears in the case of horse-colours. The key to the
phenomena was of course the fact that chestnuts mated with chestnuts
breed true — with rare and dubious exceptions. It would seem at first
sight impossible to devise a system of tabulation which could fail to
disclose so prominent a feature. Nevertheless Professor Pearson's correla-
tion-tables, which were compiled from the records of more than 6000
horses, were made in such a way that the colours of sire and dam could

VII] Colours of Thoroitgli-bred Horses 12s

The time has not arrived for any attempt to analyse
the relations of horse-colours in general. By microscopical
methods Miss Durham has found in horses the three pio--
ments, black, chocolate, and yellow seen in other types.
The term chestnut is used somewhat loosely in describing
colours, and though usually a chestnut is a horse possessing
yellow pigment only, there is a dark type of chestnut,
sometimes spoken of as liver-chestnitt, which is actually
chocolate^. Nothing is known of the genetic relation of
this to the yellow chestnut, or of the respective properties
of those various chestnuts distinguished by the colour of
their manes and other subordinate differences.

Among thorough-bred or race horses, types intermediate
between chestnut and the dominant bays and browns must
be exceedingly rare, for though entries indicating doubt
between bay and brow^n are rather common in the Stud
Books, alternative designations are scarcely ever given in
regard to chestnuts.

Among common horses and hackneys such animals
though exceptional can be found by looking out for them,
and one or two may usually be seen in a day's walk through
London streets. I have no information as to their genetic
capacities, but presumably they are due to dilution-stages
of the black pigment, corresponding to those which in the
mouse &c. constitute the blue varieties. They may also be
cases of imperfect dominance.

not be taken into account together. The tables thus provide answers to
questions as to the probable colour of a foal by a chestnut sire, the dam's
colour being taken as unknown ; as to the probable colour of the brother
or half-brother of a chestnut foal, when the colours of both sire and dam
are taken as unknown ; with solutions of other problems of equal signifi-
cance. Since however the colours of both sire and dam are recorded, and
must indeed have been actually extracted from the Slud Book for the
purposes of the tabulations, the investigators, by refraining from an
mspection of these data till they had been separated, placed themselves at
a gratuitous disadvantage. The true nature of the inheritance was therefoie
not discovered.

The failure was due to want of analysis. The similar failure of bio-
metrical methods to find the plain rule of inheritance in the case of human
eye-colour was due to the same defect of method, though in that case
further obscurity arose from the use of faulty and uncritical observations.

* When very dense this type of colour might carelessly be mistaken
for black.

126 Basset Hounds [ch.

^^ Tricolour''^ and ^* Non-tricolour'' in Basset Hounds:
The Law of Ancestral Heredity.

The question of the relation of the yellow and black
pigments is raised in the celebrated case of the colours of
Basset Hounds. The importance of that subject is due to
the fact that it was from a study of the evidence in regard
to Bassets that Mr F, Galton was led to enunciate his
"Law of Ancestral Heredity" with confidence as one which
"appears to be universally applicable to bi-sexual descent."
The publication of that paper played a considerable part in
the history of modern genetic research and it is necessary
that we should consider the facts in some detail.

The colours of Bassets are two, the first spoken of as
tricolour^ consisting of black and yellow marks on a white
ground ; the second, non-tricolour, which differs from the
first in having no black. It is said that dogs which cannot
be easily referred to one or other of these two types do not
occur, and they must certainly be very rare if they exist at all.

Mr Galton's investigation was based on data supplied to
him by the late Sir Everett Millais, a keen fancier of the
breed. This evidence consisted in records giving the
number of offspring of each type which had occurred in
families of various compositions. It was thus possible to
compare the number of tricolour and non-tricolour dogs
produced in the families with the number of the respective
types distributed among their pedigrees. Galton's figures
indicated that there was a close correspondence between
these two numbers, so that it was possible, given the
ancestral composition of the families, to predict with con-
siderable accuracy the numerical proportions in which the
respective types would appear. According to Galton's system
the family was regarded as the production of all the ancestors.
Each ancestor was supposed to contribute in his or her
degree to this total heritage, the more immediate progenitors
contributing more, and the remoterprogenitors less, according
to a definite arithmetical rule. This rule was that the average
contribution of each ancestor was to be reckoned

for each parent 1/4

for each grandparent 1/16

for each great-grandparent 1/64

vit] Basset Hounds 127

and so on, the total heritage being thus reckoned as unity.
It will be observed that this scheme differs entirely from
those based on Mendelian principles, inasmuch as every
ancestor is, according to the Law of Ancestral Heredity,
supposed to have some effect on the composition of each
family in its posterity, and each recent progenitor is re-
garded as having a very sensible influence on these
numbers.

Though no one with a knowledge of practical breeding
could entertain the supposition that Galton's Law had the
universality of application claimed for it, there was on the
other hand no doubt that the Law had successfully expressed
a variety of facts in which no order at all had been pre-
viously detected.

We have now to consider the meaning of this evidence
in the light of modern knowledge. At the time that
Galton's views were promulgated nothing was known of
segregation. The supposition that any individual, whatever
its own characters, was capable of carrying on and trans-
mitting to its posterity any of the characters exhibited by
its immediate progenitors, at all events, was generally
received without question by biologists. According to that
idea the number of classes of individuals differing in respect
of their ancestral composition and transmitting powers is to
be regarded as indefinitely large, whereas in all cases of
sensible allelomorphism the number of classes of individuals
is three only, two being homozygous and one heterozygous.
The difference between the two schemes is thus absolute
and irreconcileable.

When Mendelian phenomena were first recognized it
was naturally supposed that some classes of cases would be
found to conform to the Mendelian scheme and others to
the Law of Ancestral Heredity. With the progress of
research however almost all the cases to which precise
analytical methods have been applied have proved to be
reducible to terms of Mendelian seQ^ree^ation ; and of those
which have not already been so elucidated some, we may
feel confident, if not all, will be eventually shown to be
governed by similar rules. \\\ discussing aberrant pheno
mena like those alleq^ed in reo^ard to the Bassets the first
question to be settled is whether the lacts are correctly

128 . Basset Hounds [ch.

reported. If either type is recessive we should naturally
expect this to be the non-tricolour, which is without black.
U nfortunately as the non-tricolours are not fashionable there
were comparatively few matings between two parents of
that colour. Nevertheless 41 dogs, offspring of such matings,
are given, of these 20 being tricolour. Though the records
were not made by scientific men or with a scientific purpose
directly in view it is almost impossible to imagine that all
these cases can depend on mistakes, and pending the pro-
duction of new and direct evidence we must take the records
as correct.

In the Theory and Practice of Ratio7ial Breeding
(London, 1889), pp. 26 and 27, Sir Everett Millais gives one
or two more notes bearing on this question. He says that
in England there were then two strains of Bassets, the
Couteulx and the Lane. "The Couteulx is as a rule a very
perfectly marked tricolour, with the tan and black markings
deeply accentuated. The Lane hounds, on the other hand,
are very weak in markings if they happen to be tricolour,
but as a fact they are far more generally found to be lemon
and white." In another place he mentions that "in nearly
every litter of pure Couteulx there is generally a lemon and
white puppy."

If it were not that the genetic relations of yellow and
black pigments are, as we have seen, so complicated and
uncertain in other types, we might be inclined to attribute
the alleged production of tricolours by non-tricolours to
imperfect classification of **weak" tricolours, but in dealing
with this group of phenomena no such suggestion can be
hazarded with any confidence

The strange fact that yellow mice are never pure natur-
ally occurs to the mind in connection with the evidence as to
Bassets. A comparison between the two cases cannot
nevertheless be instituted at all easily ; for in the Bassets
yellow is evidently not usually a dominant, which it should
be if the impurity of the non-tricolour is to be attributed to
a state of things comparable with that existing in mice.
At present the Basset phenomena must be regarded as
definitely unconformable. Perhaps the most probable view
of their nature is that they are an illustration of irregular
dominance, but this cannot be asserted with much confidence.

vir] Law of Ancestral Heredity 129

It is curious that the one example to which a partially correct
system of analysis was applied before Mendelian methods
were rediscovered, should have been of this remarkably
exceptional order.

There Is little reason to anticipate, as we once did, that a
distinct group of cases obeying a Law of Ancestral Heredity
will have to be recognized. That principle in certain cases
gives an epitome of the consequences of the Mendelian
process, and in all likelihood its applicability to any pheno-
mena of natural inheritance is due to this fact. The Law
of Ancestral Heredity takes of course no account of domi-
nance, or of segregation with all the consequences it entails;
but as describing the results to be witnessed among a
population interbreeding at random, its predictions would
frequently approximate to the truth. In particular it is to
be observed that the arithmetical results of DR x R and of
DR X DR are correctly predicted by the Law of Ancestral
Heredity. Some of the phenomena of blending are also ex-
pressed with accuracy. Besides this, Inasmuch as dominant
individuals which have both parents dominants will in a
mixed population frequently be pure dominants, statistical
phenomena which could be mistaken for an effect of
ancestral composition will often occur^.

For instance if purple Sweet Peas in F. were bred from
promiscuously, F^ would consist of purples and other colours,
and the excess of purples in the mass would be greater than
it was in F.. If the F^ purples were again separately saved,
the proportion of purples in F^ would be still greater, and
so on. This gradual increase in the proportion of purples
might carelessly be mistaken for a consequence of the fact
that each generation had more purples in its ancestry. As
we now know, that conclusion would be quite incorrect.
The increase is in reality due to the appearance of actually
pure purples hi F, and in subsequent generations, and to
the effect which their presence has on the composition of
the population. The impure purples in each generation

* Darbishire's conclusions in regard to mice {Biometrika, 1904, iii.
pp. 23-5) were obviously based on a mistake of this kind, as he lias
himself since admitted. (See Jihfn. Manchester Lit. Fhil. Soc. 1905,
No. 6, p. 7.)

B. H. 9

130 Law of Ancestral Heredity [cii.

remain exactly in the same condition as at their first appear-
ance, and the selection has merely resulted in a reduction of
their number. If the purples were saved individually and
their seeds separately sown, it would immediately be seen
that some were pure, giving purples only, and that others
were impure, the latter consisting of the various gametic
types with which we are now familiar.

Since then we know that ancestral composition does not
decide the constitution of such a population, and since
individuals of identical parentage may have most divergent
genetic properties, it is absurd to attempt to trace the
workings of any Law of Ancestral Heredity among these
phenomena

The suggestion that methods based on unanalysed
statistics have scientific value in the study of heredity can
scarcely mislead those who have examined the facts.
Professor Pearson and others committed to these methods
have of late defended their position by arguing that there is
no fundamental incompatibility between Laws of Ancestral
Heredity and the conclusions of Mendelian analysis. The
matter would not be worth notice were it not that the same
proposition is being freely repeated by several writers seek-
ing some convenient shelter of neutrality^. It is to be
observed however that the supposition of an underlying
harmony between Mendelian and biometrical results was
not put forward by the biometricians until every possible
means of discrediting the truth of Mendelian facts had been
exhausted. Those attacks having failed, we are asked to
observe that the Law of Ancestral Heredity was meant as
a statement of a statistical consequence, and is not concerned
with physiological processes. Mr Galton's views on this
point are well shown in the following passage in which he
explicitly appeals to the physiological process of gameto-
genesis as apparently occurring in the way which his Law
requires. For in introducing the Law as applicable to
Bassets (125, p. 403) he wrote;

"It should be noted that nothing in this statistical law
contradicts the generally accepted view that the chief, if not

^ See for example Darbishire, Mem. Manchester Lit. and Phil. Soc.
1905, No. 6 and 1906, No. 11; and Professor J. A. Thomson, Heredity ^
K^o'], passim.

VII] Law of Ancestral Heredity 131

the sole, line of descent runs from germ to germ and not
from person to person The person may be accepted on
the whole as a fair representative of the germ, and bcino- so,
the statistical laws which apply to the persons would apply
to the germs also, though with less precision m individual
cases. Now this law is strictly consonant with the observed
binary subdivisions of the germ cells, and the concomitant
extrusion and loss of one-half of the several contributions
from each of the two parents to the germ-cell of the off-
spring. The apparent artificiality of the law ceases on these
grounds to afford cause for doubt ; its close agreement with
physiological phenomena ought to give a prejudice m favour
of its truth rather than the contrary."

Had segregation been known to Mr Galton the Law of
Ancestral Heredity would not have been promulgated.
It is obvious that so soon as that phenomenon is recog-
nized and appreciated, all question of useful or direct
applicability of the Law of Ancestral Heredity is at an end.
That method of representing the phenomena of Heredity
and all modifications of it are based on the false assumption
that any individual can transmit the characteristics of any
ancestor, and especially of any recent ancestor. When this
conception was shown to be untrue, the structure which the
biometricians have offered to the world as a scientific study
of Heredity ceased to have meaning or value. Statistical
examination of ancestral composition may, as we have seen,
occasionally give a prediction in good correspondence with
fact, but this is due to coincidence and not to any elements
of truth in the ratiocination by which the prediction was
reached.

As an attempt to compass the solution of an intricate
problem by labour and ingenuity without proper data or
equipment Mr Galton's work deserves long to be remem-
bered. It stands out as a significant and stimulating event
in the history of biology.

9—2

CHAPTER VIII

HEREDITY OF COLOUR— continued.

Various Specific Phenomena in Colour-Inheritance. Re-
lation of Colour to Hoariness in Stocks. Miscellaneous
Cases. Colottr of a Special Part controlling that of
other Parts. — Sum7na7y and Discussion. — Subtraction-
Stages.

Again and again in tracing the genetic properties of
colours in animals and plants we encounter the phenomenon
of a specific connection between certain colours and their
modes of hereditary transmission on the one hand, and
various apparently distinct physiological properties on the
other. Colour, which S3^stematists have often spoken of as
one of the superficial or impermanent properties of organ-
isms, seems thus to be bound up with fundamental pheno-
mena of chemical economy. To treat this part of genetics
with any fulness is not yet possible. As an illustration may
be mentioned the curious result discovered by Miss Saunders
in Mat thio la, using the varieties known as ''ten-week Stocks."
These may be either ''hoary," viz. covered with branching
hairs forming a tomentum, or glabrous and destitute of
hairs. When the hoary are crossed with the glabrous,
hoariness is an ordinary dominant, giving 3 hoary : i
glabrous in /^.

But when certain glabrous strains are crossed together
the P^ form is hoa^y, reverting to the primitive type. This
reversion never occurs when any of the many red or purple
varieties are crossed together, but is universal when any of
them are crossed with either the white or the cream-coloured
glabrous strains. The purples and reds owe their colours
to the presence of coloured sap. This coloured sap is not
present in the whites, nor in the creams, whose colour is
due to the existence of yellow plastids in the cells of their
petals.

CH. VIII] Colour and Hoariness in Stocks i

F. from the cross, for instance, of purple x white con-
tains sap-coloured and non-sap-coloured plants, and of these
some are hoary and some glabrous. Bui none of the plants
which come without coloured sap have any hairs on their
leaves. A consideration of the case shows that the factor
for hoariness is really Introduced by the ei/Zz/'/e'-ilowered
glabrous plant, and that the glabrousness is due to the
inability of the hoariness-factor to make the hairs grow in
the absence of the factors for sap-colour. The facts may be
represented thus, C and R representing, as before, the factors
for sap-colour, and H the factor for hoariness.

Purple glabrous x White glabrous
CRh I cRH

Fx Purple hoary

CcRRHh

r -L n

/^...9 Purple hoary 3 Purple glabrous 4 White glabrous

all containing all containing all containing

C, R, H C,R,h c, R, and i7, or A

If cream glabrous be substituted for white glabrous the
result is the same so far as sap-colour and hoariness are
concerned, and in /% only those plants can be hoary which
also have coloured sap.

Finally wdien cream and white glabrous types are crossed
together, jF^ is purple and hoary, thus showing reversion in
colour, owing to the meeting of the two complementary
factors C and R, one coming in from the cream and one
from the white ; and also reversion to hoariness because the
hoariness-factor was really present all the while in both the
cream and the white types, but was unable to show Itself
because one of the sap-colour elements was absent in each
type. The heterozygosis of the two types brings together
all the three elements C, R^ and //, so the Fx plants are
both coloured and hoary.

The reason why hoary-leaved plants are never produced
by crossing two types possessing coloured sap is at once
apparent. For if the factor for hoariness were present in
these types, they would be hoary.

As may well be supposed the disentangling ot these
results was a long and tedious process. The occurrences
seemed at first contradictory, but after it had been ascer-

134 Swedes and Turnips [ch.

tained that each kind of family was produced with regularity
as the consequence of a particular kind of union, the work
of bringing all these into one analytical scheme was only a
matter of time. Much still remains to be done before the
analysis will be complete for Stocks in general. For
example, among the Brompton Stocks races occur which are
hoary though devoid of sap- colour, and as yet we are not
aware what condition or factor exists which there enables
the hoariness-factor to assert itself.

In cases like these we get glimpses of the strict specific
rules which govern the genetics of pigmentation. In the
Sweet Pea again we have found that both variation in the
pollen-shape and in the structure of the standard petal are
closely related with the distribution of the factor which
turns the colouring matter purple. There is every hope
that in our further analyses these apparently trivial
phenomena will serve as indications of the underlying
processes.

Apart however from these curious inter-relations be-
tween colours and structural peculiarities, there are several
remarkable specific phenomena to be seen in the genetic
behaviour of colours. Of these some examples may be
given as incentives to future experiment General rules
regarding colour-inheritance are scarcely to be expected as
yet, for very little is known of the pigments of either
animals or plants. Beyond the fact that albinism has always
been found to be recessive to colour in both animals and
plants no general proposition can be put forward with
confidence. We believe also that yellow chromoplast-colour
is always recessive to white or colourless chromoplast-colour,
though the cases studied are not numerous enough to justify
a general assertion. Stocks, Sweet Peas, Swedes and
Turnips, Verbascuni, may be cited as plants following this
rule and no clear exception is yet known.

Mr Arthur Sutton tells me it is w^ell known that when
Swedes are being grown for seed, Turnips must not be
allowed to flower near them, but that in growing turnip-
seed, no injury is done by the presence of flowering Swedes.
The meaning of this is now clear. The Swedes are in
general yellow-fleshed, their colour being due to yellow
plastids. Turnips as a rule are white. If therefore the

VIII] Black Fruits

135

pollen of Turnips is carried by insects to the Swedes, the
hybrid thus produced will be white-fleshed, and consequently
attract attention, spoiling the uniformity of the crop. But
as the white of the Turnip is a dominant, no visible effect
is produced even though Swede pollen is brought by the
insects to the Turnip flowers, for F^ is white. In /% of
course yellow Turnips would appear"^, but as the roots are
almost always eaten off, this result is scarcely ever reached
by the farmer.

It might be expected perhaps that the blue and purple
colours in flowers would always be dominant to the reds,
but this is not so. In Stocks, Sweet Peas, Peas, and Salvia
the purples are dominant. Probably the same is true for
the blues of De/pkinium, Cineraria and a good many more,
but when we come to Primiila Sinensis we find blue a
recessive to the reds and magentas. Doubtless the chemistry
of the blue pigment is there quite different.

In Solaman and Atropa black fruit is dominant to the
yellow fruit, but in BiyoJiia the red fruit of B. dioica is
dominant to the black fruit of B. alba. This paradox has
been elucidated satisfactorily by Miss Wheldale. She tells
me that the nature of the distinction between the tw^o types
is quite clear. In Atropa the black colour is due to the
presence of a dark purple anthocyanin which like other
pigments of the same kind is dominant to its absence. In
Bryonia alba the black colour is caused by the presence of
a little carotin in plastids, together Avith green chlorophyll
undecomposed. In the red-berried Bryonia dioica the
chlorophyll is decomposed (just as it is in the cotyledons of
yellow-seeded Peas) and much carotin is present. Conse-
quently, as may be expected, the presence of the decomposer
of the chlorophyll Is a dominant, as also is the abundant
development of the carotin, and thus the black colour of the
fruits is recessive to red.

In Rabbits, as has been stated above, yellow is a reces-
sive to black, while in Mice it is a dominant.

The yellow varieties of many red Lepidoptera [Zygacna,
Arctia, Sec.) are presumably recessive t, and the same is

"^ From Mr Sutton's experiments (262) it seems however tliat 7^, is
sterile.

t Proved lor Callimorpha dominida. See p. 44-

136 Melanic Types [cii.

apparently true of the yellow fruits, as compared with the
red fruits of their corresponding types, and of the yellow
flowers of some Composites {e.g. Ge7^bera) ^.

In many types of flowers, e.g. Stocks, Primula, Sweet
Pea, the very dark and more fully-coloured varieties are
regularly recessive to the less dark types, whether purple or
red. The same will almost certainly be proved for Cycla-
men, Rose, Hollyhock, Dahlia, Carnation, Sweet William,
and many more. In Antirrhinum Miss Wheldale finds that
among magentas the darker are recessive to the common
colours, but among the crimsons or reds the darker are
dominant to the lighter.

The difficulties which preclude general statements in
regard to the genetic relations of melanic types among
animals have been illustrated in much that has gone before.
The loose description ''melanic varieties," common in the
writings of systematists, covers a number of phenomena
essentially distinct. For example there are melanic forms
which owe their greater blackness to the presence of some
dominant factor responsible for a greater deposit of black, or
at least dark, pigment. In Fowls, for instance, black is at
least partially dominant over the bmikiva colour which
fanciers call " Black-red." The dark brown variety called
" Brown-breasted" is similarly a dominant. In Pigeons, as
Staples-Browne has proved, black is dominant to the blue
of the wild type. In the Horse the presence of black, as in
bays and browns, is dominant over the absence of blacks as
in chestnutst.

On the contrary in Rabbits, Rats, Mice, &c. the black
variety is produced by the omission of the agouti-factor, G,
from the wild type, and black thus is apparently a recessive.
Even here however the presence of black pigment is
dominant to its own absence. It would be interesting to
know to which group the Cat belongs.

In Insects again no rule of universal application to

* That in Tomato yellow fruit is recessive to red was established by
Hurst (160, p. 115). The case of Gerbera is given on the evidence of
crosses made by Mr R. I. Lynch between red Gerbera Jainesoni and the
yellow-flowered variety " Sir Michael."

t The genetic relation of the totally or self-coloured black to the other
horse-colours is not yet known.

VIII] Silkworms 137

melanic varieties can be given. Of the mclanic varieties of
Moths which have been tested several are apparently
dominant, more or less, to the normal or non-melanic types;
but in the black Chrysomelid Beetle, Lina iapponica, inves-
tigated by Miss McCracken, the evidence shows plainly
that the uniformly black type was recessive^ to the normal
which has black only in the form of spots. The common
melanic varieties of the 2-spot Lady bird {Coccinella bipimc-
tata) are probably also recessive to the ordinary red type.

The experiments of Standfuss Interpreted according to
the Mendelian system show that the dark variety higens is
dominant to the ordinary fulvous yellow type of Aglia taic
(a Saturniid Moth). This case comes up for consideration
in some detail with reference to the heredity of Sex [q.v.).

In Silkworms a melanic variety of the moth is an im-
perfect dominant to the normal, pale-coloured moth, giving
a blend-form in F^ (Coutagne, 83).

As regards the colour of the silk some interesting results
have been obtained. Yellow silk was always found by
Toyama (268) to be dominant to white, and this result was
obtained by Coutagne In certain cases t. For example the
yellow race called " Var " was dominant in silk-colour to the
white Japanese race used by Toyama and to the white
"Bagdad" used by Coutagne; but Coutagne found the same
yellow to be recessive to the white of two French races with
which he experimented. Presumably this distinction is due
to some Idiosyncrasy on the part of the whites, analogous to
what has been seen in fowls and Primula, but as to this
nothing is known.

In crossing a yellow Siamese race with a white Japanese
race, Toyama obtained a resolution-effect in /%. Yellow
X white generally gives yellow F^ with 3 yellow : i white in
F^ ; but in this special case there were two new forms in F.,,
a pale pinkish yellow, and a greenish white. This latter
white could not always be satisfactorily distinguished from
the pure whites, so the /% family has to be taken as 9 : 3 : 4.

^ There is a possible complication in this case.

t Toyama states that the colour of the silk always corresponds to that
of the abdominal legs of the larvae, and consequently it is not necessary
to rear all the larvae up to the spinning stage in order to ascertain the
colour of their cocoons.

138 Colours in Critical Parts [ch.

Observation, for example, gave 70:21 136, the expectation
being 72:24:32. In certain other cases this resolution-
effect did not occur, though from analogy it might have
been expected. Toyama regards the distinction as due to
differences in the whites used, but it seems not impossible
that it was really the yellows which possessed individual
differences in this case. In either event there are difficulties
to be faced, and on the evidence it is not clear which account
is actually the more probable.

There are some illustrations of a principle by which the
colour of one part of the organism may limit or control the
possible colours of other parts.

In animals it is fairly certain that the eye-colour may act
in this way, certain coat-colours being produced only if the
eye be black, and others only if the eye be chocolate, but
the facts are still somewhat obscure.

If the stem of the Chinese Primula be green and not red
the deeper flower-colours cannot be developed in self-colottred
types. A cross with a red-stemmed type, however pale in
flower-colour, at once reveals the presence of the factors for
the deep colours if they are there.

On the contrary, the white-edged types, such as Sutton's
''Sirdar," though their flowers may be of a deep shade of
purple or red, appear exclusively on stems which are green
throughout except for a development of red colour at the
collar or extreme base of the petioles. Such " Sirdars "
cannot exist on a wholly-coloured stem. The stem may be
parti-coloured in Primulas though the flower is whole
coloured, but these special types of parti-coloured flower
can only occur on a parti-coloured stem. In the Fc^ series
it is curious to see these deeply coloured, white-edged,
flowers on stems apparently green, while none of their
green-stemmed sisters with self-coloured flowers can bear a
flower darker than pale salmon-pink.

Another striking example of the same phenomenon is to
be seen in these Primulas, with the difference that there the
want of a particular colour in the critical or "controlling"
position is due to the dominance of a negative character, not
to the absence of a complementary one. Certain deep red
spots occur in some varieties, e.g. Sutton's "Crimson King"

VIII] Discussion of Co lour- Evidence 130

(a fine dark red), on the petals just external to the yellow
eye (see Plate VI). These spots are never formed unless
the stig77ia is red. When such a type is crossed with one
having a green stigma, F^ has a green stigma and no spots
on the petals. In /% there are of course some with green
stigmas and some with red, some with spots and some with
no spots. But the distribution of these two characters
shows that the combination green stigma + spot on petals
does not occur. The stigma may be red though no spot be
formed, but if the stigma be green, the spot is absent, though
the factor for it may exist in the individual.

Formerly such cases might have been regarded as
examples of *' correlation," but that term is only applicable
to them in a loose and quite incorrect sense.

Nothing so fully demonstrates the fundamental signifi-
cance of colour in the economy of plants and animals as the
strange series of phenomena that have been discovered in
regard to the complex inter-relations between the genetic
behaviour of certain kinds of pigmentation on the one hand
and certain structural features on the other. In the chapters
dealing with gametic coupling and with the heredity of Sex
it will be shown that not only the factors governing structure,
but also the factors which are the ultimate cause of sexual
differentiation, may be distributed among the germ-cells
according to systems which are modified and ordered in
inter-dependence on the distribution of the factors for colour.

SMmmary and Discussion of the Evidence as to the
Genetics of Colour and Colour-Patterns.

Since we have abundant proof that the development of
colour and even of particular colours may be bound up with
other features of morphological or physiological importance,
it is clearly impossible to regard the genetics ot colour-
characters as apart from the rest. A summary of the
chapters dealing with that subject will nevertheless be useful
at this point, and with this may be combined a brief dis-
cussion of essential points.

In many animals and plants colour has been shown
experimentally to behave as if due to a single allelomorphic

140 Discussion of Colour-Evidence [ch.

factor. In three cases among plants (Sweet Pea, Stock,
Orchids) however we already know that the production of
colour {sc. sap-colour) requires the fortuitous concourse of
two complementary factors which have independent distribu-
tions in gametogenesis, and individuals lacking either of
these factors are completely devoid of colour.

In the light of this discovery we naturally ask whether
it is not probable that the sap-colours of plants in general
may not in reality be produced by pairs of complementary
factors. It is tempting also to speculate on the possibility
that the colours of animals may have a similar nature. At
present however the objection holds that in no species of
animal have two pure albinos been found to produce coloured
offspring when mated together, and the F^ ratio from the
cross albino x coloured is always 3 coloured : i albino, never
9 : 7. But it may be suggested with great plausibility that
this simply indicates that every individual, coloured or
albino, contains one of the two factors, and the question
whether colour is a single — or a double — factor character
remains undecided. If a variation were to occur by which the
supposed common factor was omitted from the composition
of an albino, albinos bearing respectively each of the two
factors could be raised, and nothing would then preclude the
production of coloured individuals by crossing the two sorts
of albinos together.

At first sight some of the facts related in regard to fowls
seem to supply evidence of this kind. White Silky x a
recessive white strain produces F^ fully coloured. But
neither parent is an albino in any strict sense, for both have
eyes fully pigmented. As a matter of fact also the Silky
breed, though quite white in plumage when adult, often —
perhaps always — has some buff colour in its down. The
resemblance is therefore far from being complete.

Another case which suggests a similar interpretation is
that of eye-colour in the mouse, for there black eyes result
in F^ from crossing certain pink-eyed mice having coloured
coats with certain albinos. But here again one of the
parents is obviously not albino, and as we now know from
Miss Durham's observations, the eye of the coloured parent
though ostensibly pink, really contains a small but definite
amount of pigment.

VIII] Discussion of Colour-Evidence 141

Animals and plants are alike In the fact that their colours,
however produced, may be modified by the presence of
additional factors. In each case therefore we must conceive
of one lowest or hypostatic colour, and of epistatic factors
superimposed on this which produce their several effects.
In the Sweet Pea the lowest colour Is red, which is turned
to purple or blue If the factor having this power is present.
Similarly In the mouse the lowest colour is chocolate, which
becomes black If the black factor is added, and so on. The
intensity and also the distribution or pattern of colours
behave In descent as If they also were governed by such
superimposed factors, though as will shortly appear, It Is
not certain that this mode of representation is strictly
correct. However this may be, we are safe in regarding
the pigmentation of animals and plants as a character usually
resulting from the combined operations of several distinct
factors, transmitted separately in heredity.

Applying conceptions which have lately become current
in physiology Cuenot suggested that the determiners which
modify colour In the mouse, for Instance, may be distinct
diastases acting on a single ckromoge?i substance. In the
present state of physiological chemistry It is, I suppose,
Impossible to speak with confidence as to the nature of the
bodies concerned and we must keep an open mind. Nothing
yet precludes the possibility that there may be one diastase
responsible for the production of colour, and another set of
bodies which, acting In the presence of the diastase and of
ll the chromogen, determine the quality or shade of the
colour '^.

So In the mouse, the wild grey colour results from the
joint action of at least three factors: (i) the colour, which. If
no epistatic factor Is present, would be chocolate; (2) a dlack
determiner, which causes black pigment to appear ; (3) the
agouti-factor, G, which gives the hairs their banded appear-
ance and also causes some yellow pigment to be formed in
them. In rats a black variety exists because the factor G
may be absent, but no chocolate variety has been recorded
because the factor for blackness has not yet fallen out. It

* As pointed out above, Cuenot's suggestion that in the case of mice
the agouti-factor, G, is allelomorphic to the factor for blackness, B^ is not
an adequate representation of the phenomena. (See p. 76.)

142 Patterns of Colotir [ch.

a chocolate rat were to be produced, then by crossing it with
the wild grey type, blacks must occur in i%, just as they are
known to do in the case of the same mating in mice.

The facts compel the recognition of such a series of
determining elements, and it is perhaps simpler to imagine
these elements as distinct from the exciting cause, and
additional to it, while remembering the possibility that they
may in reality be only modifications of it.

Similarly in attempting to express the genetic inter-
relations of the several patterns of a colour as depending on
the existence of definite factors, we have to bear in mind
that we are only using a convenient symbolism. It is not
incumbent on us to believe that there are any physiological
substances which have the power of governing the distribu-
tion of the colour. Experiment shows that the power to
cause the colour to be uniformly distributed as in the
'' self" type, or to be restricted to special regions of the
body as in the Dutch rabbit, for instance, can be carried by
the gametes, and that when these two possibilities are com-
bined in heterozygosis, they segregate in gametogenesis.

This being so, the two possibilities may thus be repre-
sented symbolically as two factors, having regard to their
effects on the configuration of the resulting zygote ; but if
we must attempt to imagine an answer to the question,
wherein does the distinction between self pattern and Dutch
pattern physiologically consist, we should, I suppose, refer
it rather to differences in the distribution of one of the
chromogenic factors than to the presence or absence of an
additional element. In the self-coloured rabbit the two
colour-producing elements are generally distributed over the
skin, while in the Dutch rabbit either the chromogen or the
diastase — if these be the critical substances — is restricted to
certain areas. The colours in the pied animal thus come
out in certain patches just as do lithographic colours upon
the prepared parts of the stone when the ink is applied to
the whole surface.

As the black rabbit or mouse is an animal from which
the grey determiner, G, is absent, so the pied animal is one
from which the self-coloured distribution is absent. Never-
theless the essential distinction between the two forms must
surely be quantitative. In the self-coloured type one of the

VIII] Subtract ioji-St ages 1 43

substances, say the chromogen, is distributed over the whole
surface, but in the Dutch-marked, for example, it is reduced
in quantity. The reduction however occurs in a fairly
definite way, leading to the formation of a type havin<rr a
recognizably distinct pattern. It does not seem an un-
reasonable speculation to suppose that we have here to deal
with a condition in which the amount of the substance is
• insufficient to cover the whole region wdiich it occupies in
the self-coloured type, though why it should be restricted
to one special region more than another it is impossible
to say.

If the definite pied phases are to be thus regarded as
representing quantitative diminution in the development of
one of the determining substances, we may make a similar
supposition in regard to the diluted colorations already
mentioned in the case of mice. In the diluted colours the
reduction in quantity, instead of diminishing the coloured
area while keeping the intensity of the colour, is effected by
diminishing the intensity of the colour while the totality of
the distribution is retained. The black Dutch-marked
mouse may thus be imagined to be a mouse in which one
of the colour-factors exists In its full intensity, though there
is not enough of it to cover the skin, while in the blue mouse
the factor is generally distributed over the skin but in a
dilute condition. In both cases alike the subtraction-staoe
as we may call it is a fairly definite stage in the reduction of
the amount of plgment.

A physical analogy — doubtless imperfect, but neverthe-
less instructive — may be drawn from the way in which
various oils distribute themselves over the surface of a
liquid with which they do not mix, some forming circum-
scribed patches of greater thickness, which may be compared
with the patches on the Dutch rabbit, others spreading In
a thin layer over the whole surface, like the dilute colours
spread over the whole coat. The analogy breaks down
at the fact that in the oils the physical distinctions to which
the different behaviours are due cannot be transferred
from one oil to the other, whereas in the rabbit this is
accomplished — a fact which entitles us to represent the
several properties as distinct and transferable factors. Thus
the results of the cross between a black-and-white Dutch-

144 Pattern showing in Albino [en.

marked mouse and a self-coloured blue may be represented
as due to the re-combinations of the two pairs of factors.

1. 6", self colour. j, its absence, viz. Dutch-marked.

2. D^ dense colour. d, its absence, viz. dilute colour.

Black is common to both parents, and so need not be represented.

Dutch-marked black x Blue

sD Sd

F^ Black self colour

SsDd
7^... Black self colour Black Dutch Blue self colour Blue Dutch
^ SD 2>sD 2>Sd 1 sd

The recognition of these subtraction-stages becomes
important when we attempt to estimate the minimum
number of factors necessary to produce the results we per-
ceive. Symbolically the various subtraction-stages may be
represented as depending on the removal of distinct factors,
but physiologically they may be caused by special quantita-
tive subtractions from one of the factors causing the produc-
tion of colour, and consequently an economy of hypothesis
may be made.

In exceptional cases the pattern which an albino form is
carrying, if the expression be permitted, can be actually
recognized by inspection though no real colour is developed.
For example Lock noticed the following case in edible peas
[Pisum sativum). Certain varieties of peas have brown
anastomosing lines on the coats of the seeds. These peas
are called maples in England [pots perdrix of French seeds-
men). The maple skin occurs only in the seeds of strains
which have coloured flowers. Such plants crossed with an
ordinary white-flowered type having a plain seed-coat gave
this result :

Parents Flowers coloured x Flowers white

coats maple coats plain

!>] Flowers coloured

coats maple

Fl Flowers coloured Flowers white

^ ~^ — "^

9 coats maple 3 coats plain 4 coats plain : some showing

traces of mapling

Among the white-flowered group in F^ w^ere some plants
which bore seeds showing traces of the maple marking, not
as brown lines, for no actual pigment seemed to exist, but as
a damasked pattern showing where the mapling would have

viii] ''Latency'' 145

been if the plant had been a coloured one. Lock (176) has
spoken of this faint pattern as the "ghost" of the mapling.

Mudge (204) has observed a very similar phenomenon in
young albino rats. When the hair is short the coat may be
seen to be similarly damasked, those parts which would be
pigmented if the animal had pigment looking different in
consistency from the rest.

What the exact difference between the hairs in these
areas and the rest may be has not been ascertained, but
evidently it must be a modification due to the existence of
one of the factors for colour in those hairs. They are the
parts prepared to develop colour if the other element were
present in them. In black leopards and black kittens a
similar damask effect can often be seen, the parts which in
the spotted leopard or tabby cat would be light being dis-
tinguishable on careful examination. As it is not yet known
whether black is dominant or recessive in these cases the
exact meaninof of these marks is uncertain.

Both in animals and plants there is satisfactory proof
that whiteness, the absence of colour, may be due to partial
or complete suppression of the pigment-factors and not
merely, as in the albino, to their absence. This suppression
is caused by a dominant, epistatic factor. White individuals
containing such a factor are more or less totally do?mnant
whites, whereas whites due to the absence of one or more
pigmentation-factors are recessive whites. A cross between
a dominant and a recessive white may obviously cause
coloured individuals to appear in /^5, if the factors for pig-
mentation were introduced by the parental types ; for in F.^
there will be individuals lacking the suppressing factor.

Confusion has been introduced into these analyses by
the use of the term "latency" in application to those factors
which cannot be perceived without breeding tests. This
difficulty has occurred especially in regard to albinos, though
it pervades the whole system of factorial analysis. Albinos,
for instance, in any species may have the most diverse
factorial composition. All that is common to them is the
absence of colour, i.e., if we adopt Cuenot's suggestion, of
the chromogenic substance. The composition of each albino
may be ascertained by crossing it with a coloured type and
raising the F^ generation. If the coloured type chosen be

B. H. 10

146 ''Latency'' [ch.

in all its characters hypostatic — or recessive, to use the
simpler if less accurate term — to all the factors of the albino,
the composition of the albino may be seen even from /%.
For instance, when bred to a pure black, a GG albino will
give greys only ; a GB albino will give equal numbers of
greys and blacks ; while a BB albino will ^\v^ blacks only.
Conversely it is possible to manufacture by suitable matings
albinos of each composition. For example, albinos extracted
from chocolates can only bear the chocolate determiner.
Those from black mice must all bear the black determiner
if the families have been large and no chocolate has occurred.
So, those from greys must all bear the determiner G^ if in
sufficient numbers no blacks or chocolates have been pro-
duced. With regard to pattern and saturation or dilution
of colour the case is exactly the same. An albino from
Dutch-marked parents cannot bear the self-colour factor,
and one from blues cannot bear the saturation-factor.

It is this fact, that in most cases the albino will be
bearing the determiner proper to the colour of the last
coloured parent from which it was extracted, which has led
several writers to speak of these colours or patterns as
"latent" in the albino. This mode of expression is much
to be regretted. There is no "latency" of black, or grey,
or self-colour, as a whole in the albino. Certain factors
which are essential to the production of those features may
be present in any albino of unknown origin, but this fact
does not in any way touch the question of the purity of the
germ-cells, as has been quite erroneously suggested. Sul-
phate of copper is blue and chloride of copper is green, but
it would be incorrect to speak of blue as latent in sulphuric
acid, or of green as latent in hydrochloric acid ; nor has the
acid obtained from chloride of copper more of "greenness"
in it than has the same acid obtained from sodium chloride.

Taking the evidence respecting the genetics of colour as
a whole, though much remains which is obscure, as has
been stated, especially in the discussion of yellow in the
Rodents, there can be no reasonable doubt that with rare
exceptions it will be found possible to express the whole
series of phenomena as due to the combination and re-com-
bination of a limited number of recognizable factors which
are treated by the cell-divisions of gametogenesis as units.

VIII] The Nature of Variation 147

As the nature and properties of each of these units are
successively determined, we cannot doubt that additions will
be made to the number of examples, already not inconsider-
able, in which a fixed interrelation can be proved to exist
between the units which govern colour and those responsible
for form and other physiological attributes.

In any attempt to picture the process of Evolution the
group of genetic phenomena discovered in regard to colour
has extreme value and interest. We thus are at once pro-
vided with clear illustrations which enable us to see the
nature, if not as yet the causation, of Variation, and the
significance of those particular Variations which we call
reversionary. Such illustrations may well serve as test-
cases, by which the truth of evolutionary systems may be
gauged. Though the result of these trials may largely prove
destructive, the facts are not without a constructive bearing.
One positive deduction cannot be overlooked : that the
organism is so built up that definite additions to, or sub-
tractions from its totality may readily be made by Variation,
and that the consequence of such alteration of the ingredients
may be recognizably definite, or to use another term,
specific.

CHAPTER IX

GAMETIC COUPLING AND SPURIOUS ALLELOMORPHISM.

Pollen- Shape and Flower-Colour. Axil-Colour and Sterile
Anthe7^s — Hooded Standard and Flower- Co loitr in
.Sweet Peas.

We have now to consider one of the most curious and
interesting developments of Mendelian research. In all
the examples hitherto described the F^ numbers have shown
that when allelomorphs belonging to various pairs are simul-
taneously distributed among the gametes in the process of
gametogenesis, the distribution is random, so that all possible
combinations are represented by equal numbers of gametes.
For example, in the case of the double heterozygote formed
by crossing a yellow, round pea with a green, wrinkled
variety, the gametes produced by the F^ plant are in equal
numbers bearers of

(i) yellow, round,

(2) yellow, wrinkled,

(3) green, round,

(4) green, wrinkled.

This fact is proved by the numbers 9:3:3;! in
which the several types of zygotes appear in F.. The
phenomena now to be described indicate a system of segre-
gation taking place in such a way that gametes presenting
certain such combinations occur with greater frequency than
the others.

The example in which this state of things was first
detected is that of the pollen-shapes of the Sweet Pea
[Lathyrus odoratus). The experiments in that case were
begun, as has already been described (p. 89), by crossing
a white Emily Henderson having long pollen, with a white

CH. IX] Gametic Coupling 1 4^

Emily Henderson having romid ^^oW^Vi. The F^ generation
was a reversionary purple bicolor, with long pollen — for
the long pollen-shape is dominant — and from this the F ^
generation was raised which consisted of

27 purples : 9 reds • 28 whites.

^- , ^

36 : 28

9 7

It has been shown that the interrelations of these several
types proved that the colour in both purples and reds
is due to the simultaneous presence in the zygote of two
factors which we have called C and R, and that the white-
flowered plants are those in which one or both of these
factors are absent. The point, however, which now con-
cerns us more immediately is the distinction between the
purples and the reds. This, we have seen, is due to the
presence or absence of a factor B^ blue. Those coloured
plants which have the factor B are purple, those devoid
of this factor B are red-flowered.

We saw also that there were various subordinate classes
among the purples with corresponding subordinate classes
among the reds, each due to the possession or to the want
of some special factor. These differences are similarly
distributed among the purples and among the reds. For
example if the family is one which contains picotee types,
there are picotees among the purples and corresponding
types among the reds, and in each colour-group the pro-
portions are the same, averaging 3 fully-coloured to i
picotee in the case of the purples and the same in the case
of the reds.

When however the distribution of the pollen-characters,
long and round, in a family containing purple, red, and white
members is examined, it is found that taking the family as
a whole the long-pollened plants are to the round-pollened
as 3 to I in the usual way. Among the white-flowered
also there are 3 longs to i round. But when the purples and
the reds are separately studied, the numbers 3 long : i round
are not found. On the contrary, in the purples there is a
great excess of longs, which are to the rounds as about
12 to I, while among the reds there is an excess oj rounds,
which are to the longs as about 3*2 to i.

ISO

Gavicfic Coiipliiig

[CH.

The result of extensive counting shows that an approxi-
mation to the observed numbers would be produced by a
gametic system of such a kind that the combinations of long
pollen with blue factor, and round pollen with no blue tactor

Fig. 19. Pollen-grains of Sweet Pea.

The normal long (dominant) type is shown in II and IV : the
peculiar round type (recessive) in I and III. The upper figures
represent the dry condition. The lower figures show the appearance
in sulphuric acid, which makes the pores visible. Three-pored and
one-pored grains sometimes occur among the rounds, but they are
usually two-pored. In IV one of the grains is seen end-on, and in I
three of the disc-like grams are seen edge-wise.

IX] Gametic Coupling 151

occur seven times as often as the other two possible combi-
nations. We speak of this phenomenon as Gametic Coupling.

The term "coupHng" is strictly applicable, because the
association is between two dominant or " present " factors,
here those for blue colour and long pollen. Abnormal
distributions due to such coupling are to be carefully dis-
tinguished from those described later under the name
"Spurious Allelomorphism," to which the term "coupling"
should not be applied.

If the two pairs of factors are expressed thus :

Dominant Recessive

Blue colour B Red colour h

Long pollen L Round pollen /

the gametic series is not

\ BL^\ Bl-^ I bL+i 61,

but 7 BL + I Bl-\- I bL^-y bl,

or very nearly so.

Such a gametic series would give an /% family com-
posed thus :

177 blue-long : 15 blue-round : 15 red-long : 49 red-round.

Reference to Fig. 17 will show how these numbers are
arrived at.

These ratios agree very nearly with those observed in
actual experiments. For example the following series has
been produced.

Purple Red White

Long Round Long Round Long Round

Observed 1528 106 117 381 1199 394

Calculated 1448-5 1227 1227 4oi'5 1220-5 407'4

The correspondence between calculation and observa-
tion is sufficiently close. Further experiments in which
plants heterozygous in blue colour and in long pollen were
crossed with red-flowered plants having round pollen make
it practically certain that the series 7:1:1:7 expresses
both the male and the female gametic series with approxi-
mate correctness.

Study of the F^ families has proved conclusively that
the abnormal distribution occurs only among the gametes
of plants which are heterozygous both in the pollen characters

152 Gametic Coupling [ch.

and also as regards the factor B. Plants that are homozy-
gous in either of these allelomorphs have the normal distri-
bution of characters among their gametes, and they may
be heterozygous in C, R, or in any of the other factors
recognized in the Sweet Pea without any departure from
the ordinary ratios being produced.

The gametic series has been spoken of as 7 : i : i : 7
and these are the numbers which fit the observed result most
closely, but attention should at once be called to the possi-
bility that the series may in reality be 8 : i : i : 8. The
observed numbers are too small to enable us as yet to dis-
criminate between these two possibilities, though, as will be
seen when the nature of coupling is discussed, the signifi-
cance of the two series must be entirely different. It is
however to be noticed that the series of gametes necessary
to complete the whole system is thus either 16, or 16 + 2.

In the next two examples of such partial coupling the
association is in groups of 32, or 32 + 2. Both these also
occur in the Sweet Pea. The first concerns the peculiar
sterility or contabescence of the anthers which has already
been mentioned as a recessive character. The second factor
is again a colour-factor. Among the various factors which
control colour in the Sweet Pea is one which causes the
appearance of a reddish purple spot in the axils of the
leaves, referred to already as the dark-axil factor. When
this factor is present (and the flowers are coloured) the
axils are dark ; when it is absent the axils are simply green
as they always are in w/^ 2'/^- flowered plants'^. At an early
stage in the Sweet Pea investigation it was noticed that
when a family contained plants differing in respect of sterility
and fertility of anthers as well as in respect of dark and
light axils, the plants with sterility in the anthers (having
coloured flowers) were almost always light-axilled, and
conversely the dark-axilled plants were almost always fertile
in the anthers. In such families, among the white-flowered

* Dark axils sometimes exist in plants which have the flowers so
nearly white as to pass for real whites. Probably in all such flowers a
trace of colour is developed, and certainly in them the seed-coat is always
black as it is in all the Sweet Peas with coloured flowers.

Plants raised from wild Sicilian seed were all purple bicolour in flower-
colour, and nearly all had dark axils, but a few had light axils.

ix] spurious A lleloinorphis}n 153

plants, the sterile to the fertile were 3 to i without com-
plication.

Statistical examination of these families on a large scale
has shown that among the plants with coloured llowcrs
the ratio of fertile with dark axil : fertile with light axil :
sterile with dark axil : sterile with light axil, approaches
closely to that which would be produced if the series of
gametes bearing these four respective combinations were

15 . I : I : 15.

The actual numbers observed were

Dark axil Light axil

Fertile Sterile Fertile Sterile

627 27 17 214

Expectation 637 27 27 194

Repulsion or Spitrious Allelomorphism,

A third example of partial gametic coupling relates again
primarily to the blue factor (B) and the pollen-shapes,
but in order to make clear the circumstances in which it
occurs, another set of phenomena must first be described.

In the old types of Sweet Pea the standard is erect and
has a small notch in the middle of its upper border. This
is the natural shape of the wild flower. Of the modern
types many are what is called "" hooded." The standard in
hooded forms turns forward and downward in various de-
grees, the amount varying with the type and also to some
extent with the weather and the condition of the flower.
The hooded standard differs also from the erect one in
having little or no trace of the central notch. This differ-
ence causes the buds of the two types to be recognizably
distinct before the flower opens, for in the hooded type the
point of the folded standard projects sharply forward in
front of the wings, while in the erect type this tip is rounded
off" by reason of the notch. (Plate V.)

The hooded standard also is sometimes distinguished by
the existence of a sinus of variable size on each side of the
standard, which thus has lateral lobes more or less well
developed. These differences obviously point to a different
distribution of the strains produced by the growth in the
two types. The lateral sinus is not represented in the
hooded flowers shown in Plate V.

154 Spurious Allelomorphism [ch.

The hooded types may have a great diversity of colours,
and fixed hooded varieties now exist in the purple, blue,
red, pink, cream and other classes. It is nevertheless a
remarkable fact that, so far as I am aware, none of the
regular bicolour varieties ever have a really hooded standard.
There is for instance no hooded type having the colour of
the original purple, with its chocolate-purple standard and
blue wings, nor can Painted Lady with standard red and
wings nearly white be produced in a hooded shape. On
the contrary the hooded types always have the standard and
wings more nearly alike in colour, and there is the clearest
evidence that in families {F^ and later generations) which
contain original bicolour purples as well as hooded types,
the hooded types corresponding to them are of the uni-
colorous kind known as " Duke of Westminster^."

From these facts it is evident that there is here some
interdependence between the colour of the flower and its
form. This interdependence is of course somatic, but as
will be seen there is also a gametic connection between the
phenomena of shape and colour.

The experiments bearing on these questions originated
in a cross between the white, round-pollened Emily Hen-
derson and a white, long-pollened hooded type known as
Blanche Burpee. The Emily Henderson has an ordinary
erect standard with the central notch.

F^ produced from these two is a bicolour purple, with
erect standard and long pollen, indistinguishable from the
reversionary F^ previously described as the offspring of the
long and round whites. 7^, from such plants consists of
the following types :

Purple Red White

K ^

bicolour unicolorous bicolour erect hooded

erect " hooded erect

72 36 36 84 28

108 112

27 9

~~w-~

36 28

9 7

* Similarly if the bicolour purples with dark wings are present in the
class with an erect standard, they are represented by "Duke of Sutherland"
in the hooded class, viz. a deep unicolorous purple.

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.-^

^^^

• 14

■ M

■o

^—

•™

- ^

—^

■^

■ ■ ■

' 1

'v'

/—

:-»

k— 4

"~"

^ .

—

— *

•^

-^^

^—

•->

— ~ r^

IX] spurious AUelomorpIiisui 15:^

From these results It appears that the erect standard is
dominant to the hooded. Next we have the remarkable
feature that whereas the purples and the whites are both
represented In the two classes erect and hooded, the reds
are all erect. This fact indicates that those gametes which
bear the factor for erect standard do not bear the factor
B which causes the purple or blue colour ; and conversely
the gametes which do not bear E, the erectness-factor, bear
B. The gametes in respect of these two allelomorphic
pairs have thus the composition Be, or bE, and as regards
transmission of characters the effect is that which would be
produced if B were allelomorphic \,o E. In view of this
curious fact it seems not impossible that we may be obliged
hereafter to extend the conception of allelomorphism, and
to recognize that factors concerned with features of organisa-
tion which seem to have no special physiological association,
may be allelomorphic to each other.

Another curious result follows from the existence of
this repulsion or spuriotis allelomorphism. If the hooded
purples in F^ be bred from, the F^ families will be either
all hooded purples again or they will be such plants together
with hooded whites. Similarly the red bicolours will give
F^ consisting either of red bicolours, or of red bicolours and
erect whites. But the purple bicolours, must all by their
constitution be heterozygous In both the factor for erectness
and for blueness, and consequently their /% families contain
hooded purples, erect bicolour purples, and erect bicolour
reds In the ratio 1:2:1 (together with whites If the par-
ticular F^ happens to be heterozygous in either C or R, the
fundamental colour-factors).

Such bicolour purples thus present the anomaly of being
permanent heterozygotes, though in appearance they are
actually the original type of Sweet Pea. Families may thus
consist entirely of

Hooded Erect Erect

unicolorous purples bicolour purples bicolour reds

I : 2 : I

and the bicolour purples In such families will give similar
families again indefinitely.

On determining the pollen-characters of those famih'es

156 Discussioj'i of Coupling [ch.

which contain erect and hooded, coloured and white, long
and round pollens, it was found that in 7% the distribution
followed the system derived from the gametic series
7:1:1:7, but among the F^ families derived from them
several were found to exhibit coupling between the blue
factor and long pollen according to the system 15:1:1:15.
For instance, to take the group of plants in which this was
most evident, a certain /% plant with purple flower and
erect standard together with eight similar plants (its offspring,
in F^ gave collectively

Purple Red

long round long round

expectation on 583 26 24 170

15 : I : I : 15 basis 578 24 24 177

Here it is practically certain that the 15 : i formula
correctly expresses the gametic distribution. Nevertheless,
though some of the offspring of F^ gave such definite
indications of the 15 : i system others no less definitely
followed the 7 : 1 plan, and others again gave results so
uncertain that it was impossible to assign them to either
group with any confidence.

The facts are thus exceedingly complex, and all that can
be stated is that coupling between the blue factor and the
long pollen does certainly exist in certain families derived
from a cross which involved the factors for erect and hooded
standard ; but that inasmuch as the F^ distribution followed
the 7 : I plan^, the heterozygosis between erectness and
hood cannot be the direct cause of this change in the
coupling. Pending further analysis the distinction which
decides whether the coupling shall follow the 7 : i system
or the 15 : I system must be regarded as quite unknown,
for examination of the various families has not revealed any
consistent difference between them (see 22, pp. 10—13).

Since the B factor is alternative to E, the erect standard,
in the gametic composition, it follows that the zygotic com-

* F^ here was

Purple
long round

Red

long round

expectation on
7:1:1:7 basis

296 19
295 25

27 85
25 82

ix] and Spurious Allelomorphis)H

bination of hooded standard with round pollen must be
exceedingly rare in any of these families. If the gametic
coupling is

7 Blue long+ i Blue round + i Red long +7 Red round,

the zygotic expectation is, in the simpler case where all the
standards are erect,

I -j^ Blue long + 1 5 Blue round + 1 5 Red long + 49 Red round.

But when the standards may be either erect or hooded,
all the hooded plants are homozygous in B, and the expecta-
tion of a round-pollened plant occurring among the BB class
is only i in 64. Observation agreed with this expectation,
for in the F^ families which certainly all followed the 7 : i
system, there were 83 hooded purple plants and of them
one was round-pollened. The same expectation holds in
regard to the hooded plants with white flowers, which also
must all be BB, Unfortunately most of these were recorded
before the Importance of the question was appreciated,
and in them the hoods were not noted. Of 17 plants
of this class which were examined 16 were loner and
I was round.

Similarly, when the coupling is on the 15 : i : i : 15
system the hooded purples will be still rarer, and should
occur only as i in 256 of their class. Among the 9
families which definitely followed the 15 : i plan, one
hooded round occurred among 209 plants of the hooded
class.

Disctission of the Physiological Significance of Gametic
Conpling and Spimous AlleloniorpJiism^ .

The significance of the phenomena just described lies In
the fact that they demonstrate the existence of a complex
interrelation between the factorial units. This Interrelation
is such that certain combinations between factors may be
more frequent than others. The circumstances in which
this interrelation is developed and takes effect we cannot as
yet distinguish ; still less can we offer with confidence any
positive conception as to the mode in which it is exerted.

* For recent discoveries regarding these phenomena see Appendix to
Part I.

158 Discussion of Coupliiig [ch.

The time has not yet come for such an analysis to be
attempted. Nevertheless we can scarcely forbear from con-
sidering some of the possibilities which suggest themselves.

In spurious allelomorphism the outward facts are com-
paratively simple. Two dominant, or "present" factors,
behave as if in the cell-divisions of gametogenesis they
repelled each other, and we must suppose that this repulsion
is exerted at some definite cell-division, such that one factor
passes into one daughter-cell and the other factor into the
other. The dividing cell being AaBb, the daughter-cells
are respectively Ab and aB, Though as yet only one case
has been definitely proved to follow this system, the
evidence in that case is very positive. Moreover when the
facts of sexual inheritance come to be related, a group of
cases will be described which conform so precisely with this
type-example of spurious allelomorphism that it is practically
certain that this case is not a solitary example, but one
which typifies a category of genetic phenomena. It may
therefore be taken that repulsion — or, more strictly, a
relation which can be represented as repulsion — may exist
between factors belonging to distinct allelomorphic pairs.

The state of things which results in gametic coupling
is much more obscure. The association of characters here
is quite distinct from the association of characters produced
by spurious allelomorphism. In gametic coupling the
dominant factors are associated together, while in spurious
allelomorphism the dominant factors are dissociated from
each other. If the coupling were total, so that all the
gametes were either AB or ab, just as in spurious allelo-
morphism they are all either Ab or aB, we might naturally
suppose the one phenomenon to be the converse of the
other. The one might then be represented as an effect of
attraction just as the other may be represented as the
result of repulsion between the two dominant factors. So
far, however, as experiment has yet gone, we have no
certain case in which the coupling is complete. There are
no doubt instances of features apparently distinct which
are nevertheless transmitted in collocation. In the Sweet
Pea, for instance, the deep brown or blackish pigmentation
of the seed-coat occurs only in plants with some colour in
the flower, but these two features may thus be supposed to

IX] and Spurious Allelo)norpIiis}ii 159

depend on one allelomorph, not on two. To prove the
existence of complete coupling it would be necessary to
show that features elsewhere known to depend on separate
allelomorphs, could on occasion be linked in a complete
union. Whether such a state of things is possible we do
not know. There is no a priori reason for supposing that
it is impossible.

The arithmetical series in which the numbers occur is
the only guide as to the nature of the process, and obviously
this is quite insufficient. The existence of the 7 : i systems
and of the 15:1 systems naturally suggests the possibility
that a system based on 3 : i may exist. We might then
arrange the systems in a series thus"^ :

Gametes

Total in Series

No coupling

lAB

lAb

laB

\ab 4

3 : I

ZAB

lAb

laB

?>ab 8

7 •' I

TAB

\Ab

laB

^ab 16

15 : I

iSAB

lAb

laB

^^ab 32

Hitherto, though some dubious indications of such
a series have been seen, there is no clear case of coupling
on the system 3:1.

It is not easy to conceive any probable system of
symmetrical cell-divisions or dichotomies which would
produce the series 7 : i and 15:1 If the segregation
of characters were not all completed at one cell-division
we might of course imagine a scheme which would give
the system 8 + i + i -j- 8, thus •

AaBb

/ \
ABb aBb

/\ /\

AB Ab aB ab

after which, if the cells AB and ab each divided again

* The F^ numbers resulting from these couplings are as follows :

AB . Ab .aB .ab.

3:1:1:3 41 : 7 : 7 : 9

7:1:1:7 177 : 15 • 15 : 49

15 : I : I : 15 737 : 3^ ^ 3^ • 225.

If n be half the number of gametes needed to express the whole series of

couplings in a given case, then the four F^ numbers are given by the

foiniula

yi^ - {in - i) \ 2n - \\ 271 - I \?i^ -{in-i).

i6o Discussion of Coupling [ch.

three times, the series ZAB + \Ab + \aB + ^ab would
result. We cannot, on .the observed numbers, assert
quite positively that the system is not 8 : i, but as obser-
vations accumulate, this supposition becomes increasingly
improbable, for the numbers all point rather to 7 : i
than 8:1.

If, as many suppose, the whole process of segregation
is completed in the reduction-division, it is obvious that
any suggestion involving successive segregations fails. Still
it is worth noting that nothing yet limits us to the con-
ception of segregation as occurring all at once. We know
very little yet as to the cytological processes antecedent to
the reduction-division. Moreover it cannot yet be asserted
that all the gametes, or even all the gametes of one sex
(in hermaphrodite forms) are in the same cell-generation,
counting from the first cleavage-plane of the zygote.

It is to be noted also that where the germ-cells are
many, as in the testes of animals and the anthers of most
plants, it is not difficult to imagine the formation of even very
long series of couplings. The egg-cells, on the contrary,
are few, and in plants they are very often definitely grouped
in special organs which again are arranged on a definite
geometrical plan relatively to the gross anatomy of the
plant. Even if the various accessory cells of the plant
ovary are reckoned as belonging to the gametic series, the
number still seems insufficient to allow for the development
of a coupling which demands a long series for its expression.
The question may naturally be asked whether there is any
organised system of differentiation connecting the several
ovaries into a common plan. The differentiation among the
egg-cells might conceivably be distributed on a geometrical
plan like the differentiation among the somatic organs of the
plant. All the available evidence is however against this
suggestion, for in maize and peas, where indications of this
system might be found if they existed, all the evidence
is entirely negative.

There is still another direction in which we may look for
an elucidation of the nature of gametic coupling. If the
factors can act upon each other in such a way that certain
combinations do not occur, as we have already seen actually
happening in the case of Spurious Allelomorphism, it seems

IX] and Spurious AlleloiJiorpIiisiu i6i

possible that such a system as the 7:1:1:7 may be the
result of a complex series of repulsions exerted among a
number of factors. At present this sugc^estion is quite
unfounded. It could however be tested if breeding on a
really large scale could be undertaken, and supposing it to
be true, the evidence for its truth would appear in the
relative infrequency with which some types appeared. From
that evidence the missing gametic combinations could be
identified. As yet however it is quite premature to pursue
such an analysis, and we must be content to note that
when, as in these Sweet Peas, there is heterozygosis be-
tween a number of distinct allelomorphic pairs, the numerical
proportions in which the various combinations occur may
certainly be affected by the interactions exerted by allelo-
morphs of different pairs upon each other.

T/ie Possibility of Selective Mating between Gametes.

It has naturally occurred to many minds that as gametes
are now known to possess differentiating qualities, these
differentiations may affect the readiness with which various
classes of gametes may unite. We recognize that the simple
Mendelian numbers are produced when every kind of female
gamete has an equal probability of uniting with every kind
of gamete produced by the male. Conversely, when irregular
and unexpected numbers appear as the result of experi-
ment, the question may have to be considered whether the
irregularity is not due to a selective assortment taking
place among the gametes, such that certain types of unions
occur in fertilisation with greater readiness than others.
Hitherto it is doubtful whether any instance has been dis-
covered in which abnormal numbers can be proved to
occur with such regularity as to warrant a recourse to this
hypothesis. Correns observed several families of maize
where F^ from F^ round seed x wrinkled seed, self-fertilised,
contained a great excess of round seeds. The totals were
8975 round : 171 1 wrinkled where the expectation is S014
round : 2671 wrinkled. Thirty-five F^ plants contributed
to this total and the discrepancy between observed result
and expectation was fairly constant throughout.

B. H. ir

1 62 Possibility of Selective [ch.

To test whether the numerical output of gametes was
abnormal, reciprocal crosses were made between F^ plants
of the same breeding and recessives. In both cases the
normal equality between round and wrinkled seeds was
produced. Correns therefore concludes that some process
of selective mating was responsible for the aberrant F.,
numbers (65).

So far as I am aware, no case altogether similar to this
one has been observed, certainly none in which the numbers
available are so large. The proportions for maize seeds
are usually very regular in regard to the round and wrinkled
characters, as the records of both Correns and Lock testify.

Pending further acquaintance with phenomena of this
class there is no more to be said. The possibility of dis-
turbance by selective attraction between particular kinds
of gametes must be recognized, though without much more
definite evidence its occurrence can scarcely be regarded
as demonstrated.

In another instance of a different kind the same sug-
gestion was made by Cuenot. Of this case I have already
spoken "^ in describing the inheritance of yellow colour in
animals. Experimenting with mice he found it impossible
to find a yellow mouse pure to yellowness. Among mice
yellow behaves as a dominant, in the sense that agoutis or
blacks may be bred from two yellows. If the case were
an ordinary one, some of the yellows produced by the
mating of two yellows should be pure, and on breeding to
blacks or agoutis they would be expected to give all yellows.
Cuenot's experience is that this is never realized, and all
the yellows he has ever tested, amounting to 8 1 individuals,
also show, in such matings, some colours other than yellow
(cp. Basset Hounds, p. 128). Miss Durham has made
similar experiments with the same result. Yellows were
always found to give off either agoutis, or blacks, or
chocolates.

Cuenot interprets the peculiar result as meaning that
two gametes both bearing the determiner for yellow are
incapable of uniting in fertilisation. The numbers were

* The discussion of this remarkable case was given in another con-
nection at p. 119, but in view of its special niiportance the facts and
argument are repeated here.

IX] Unions between Gametes 163

232 yellows and ^6 agoutis, which is a near approach to
the normal 3 : i (238*5 : 79*5). Cuenot comments on this
as a difficulty in the way of his view, saying that he would
have expected the ratio 2:1; but as Mr Punnett pointed
out to me, if all the ova bearing the yellow factor were
fertilised by agouti spermatozoa, the number of these being
indefinite, the chances of the non-yellow ova being fertilised
by a spermatozoon bearing yellow cr non-yellow would
remain sensibly equal. .Thus the ratio 3 yellow : i agouti
would result.

Nevertheless the impression left on my mind by these
observations, and indeed by other strange phenomena which
yellows exhibit, is that the genetics of yellow mice are very
imperfectly investigated and that it is premature to formu-
late definite views as to their behaviour"^.

One of the peculiarities of yellow mice, well known
to fanciers, is their frequent tendency to excessive fatness.
Miss Durham, who has had considerable experience with
yellows, finds that this condition is not universal among
them, but shows itself in frequent individuals. She has
also found the genetic investigation of yellows very difficult
on account of the fact that they are often sterile, and the
I suggestion is perhaps worth considering that this sterility
may be responsible for some of the complications.

* Miss Durham has recently proved beyond all reasonable doubt that
the true account of this case is not that the two * yellow ' gametes are
incapable of uniting, but that zygotes so formed perish at some stage
before birth. For some physiological reason unknown, they are incapable
of living. From her own and other records the result of yellow x yellow
is now 151 1 yellows and 767 non-yellow, a close approach to 2 : i, which
gives the expectation 1518-6 : 759-3. The case, in fact, is analogous to
that of Baur's golden Aniirr]iiniiin (p. 253), in which the pure albinos are
missing, leaving a ratio of 2 golden : i green. (See Durham, Journ.
Genetics^ i. 1911, p. 167.)

CHAPTER X

HEREDITY AND SEX.

Evidence fro7ii Breeding Expevi7nents. Bryonia — Sex-
limited Heredity. The Horns of Sheep — Colour-
Blindness — Sex and Spurious Allelomorphism. The
Currant Moth — The Cinnamon Canary — The Silky
Foivl — Aglia tau — Cytological Evidence — Sum.77tary.

The facts of sexual dimorphism have to be considered
in any exposition of the laws of inheritance. There are two
aspects in which the phenomena of sex concern us: (i) the
nature and transmission of sex itself, (2) the influence which
sex has in deciding the development or suppression
of characters introduced into the zygote. The evidence
relating to these two questions is so closely interwoven
that they must be in practice treated together ; for the
facts concerninof the influence of sex on the distribution of
characters constitute, as will be seen, a most important
means of indirectly investigating the problem of the actual
nature of sex, and have provided already several clues which
will probably lead to the unravelling of that baffling mystery.
It need scarcely be said that in view of the great obscurity
still surrounding the genetics of sex any conclusions of a
positive kind can only be made tentatively. The phenomena
however are among the most interesting and important with
which the student of genetics is concerned, and every frag-
ment of evidence regarding them is at present worthy of
record.

Numerous essays dealing with the subject of the de-
termination of sex ^ have recently appeared, and the belief
is extending that sex will probably be found to be a result
of gametic differentiation. This conclusion rests partly on

* This chapter naturally makes no pretence to cover the whole ground.
I can treat only of those parts of the subject which come more immediately
within the scope of Mendel's principles.

CH. x] Nature of Sex 165

analogy. Mendelian experiments have shown that in all
cases which can be adequately investigated, a mixture of
distinct zygotic types occurring in one family is due to
gametic differentiation. Sex is a case of such mixture
of zygotes and the presumption is thus created that the
case is comparable in causation with those amenable to more
direct analysis. This suggestion was made in 1902 (Rep.
Evol. Ctee, i. p. 138), as a natural deduction from Mendelian
discoveries and it is interestinor to know that the same
possibility occurred to Mendel himself {197, p. 241).

The argument from analogy may perhaps be carried
a step further. If the distinction between the sexes is the
result of gametic differentiation, the fact that in ordinary
cases the two sexes are produced in equal numbers must
be taken as a strong indication that one sex is heterozygous
in respect of sex-character, and the other homozygous. That,
as we now well know, is the simplest way by which numerical
equality in the production of two types is brought about.

In his important paper on this subject Castle argued
that both sexes are to be regarded as heterozygous in sex.
But in order to apply this suggestion two serious assump-
tions are required. First, since male and female are each
regarded as heterozygous in sex, unions between ova and
male cells bearing similar sex-factors must be assumed to
be impossible. At the present time no fact can be adduced
which negatives this assumption, and there are indeed
general considerations which may be appealed to as render-
ing it somewhat probable.

The second assumption involved is more serious. As
both sexes are regarded as heterozygotes containing the
same factors, the nature of their dissimilaritv is still so far
unrepresented. In view of this difficulty Castle regards
dominance as a matter of chance. Doncaster, in respect
of a case to be discussed below, suggested that dominance
may belong exclusively to the cells coming from one parent
say, for example, those of the mother. These postulates
seem unsatisfactory and do not accord well with anything
that we know in regard to the nature of dominance else-
where observed ; for the applicability of all schemes hitherto
discovered lies in the fact that it has been found possible to
represent zygotic composition and structure as determined

1 66 Sex in Bryony [ch.

by the composition of the gametes of which the zygotes are
formed.

■ Naturally therefore we look for a simpler solution of
the problem of sex-determination. As was stated above, it
would a priori seem most probable that one sex is hetero-
zygous in sex and the other homozygous. I now propose
to consider the applicability of this simpler account.

Two lines of work have been followed, that of experi-
mental breeding, and that of cytological study of the germ-
cells. Both have led to very positive conclusions, but these
conclusions are diametrically opposed, as will be immediately
seen. From experimental breeding we are on the whole led
to conclude that the types used have females heterozygous
in sex (female being dominant) and males homozygous
recessives, while in all cases in which cytological evidence
is forthcoming it appears that the females are homozygous
and the males heterozygous. The meaning of this curious
discrepancy will be considered when the facts have been
related.

Evidence from Breeding Experiments.

The Case of Bryonia,

By the nature of the case, direct Mendelian experiment
cannot be applied. There are however certain indirect
lines by which the problem can be approached. The first
is that followed by Correns in cross-breeding monoecious
or hermaphrodite types with dioecious or bisexual types.
In this connection the most striking experiment is that
which he made by crossing Bryonia dioica with B. alba
which is monoecious. The reciprocal crosses gave with
much consistency a surprising difference in results. B.
dioica, female, fertilised with the pollen of B. alba gave F^
females, with or (usually) without occasional male flowers.
The observed numbers from this mating were 589 females,
with 2 males which must be regarded as exceptional. In
the reciprocal cross, alba (monoecious) used as female
X dioica t gave F^ consisting of males and females in
approximately equal numbers. In Cambridge we have
repeated both experiments and obtained the same results.
Unfortunately the hybrids, however produced, are absolutely ,

x] Sex in Bryony 167

sterile, forming no good pollen and failing to set seed
when fertilised with pollen of the types.

Correns (81, p. 27) interprets these facts as meaning
that the germ-cells of dioica $ are differentiated in regard
to sex, and respectively bear either malcness or femalencss.
The male is thus heterozygous in sex, malencss bei no-
dominant. The female dioica is consequently taken to be
homozygous in femaleness. The condition of the sexual
cells of the monoecious alba is not quite so readily repre-
sented. Since dioica $ x alba $ gives plants all $ or with
only traces of maleness, the male cells of alba are regarded
as all alike undifferentiated in respect of sex, bearing, that
is to say, the monoecious character, and presumably the
female cells of alba are in the same condition.

The facts would then be represented thus :

dioica % Is taken to be % %,

dioica ^ ,, ,, $ ^, male being dominant.

alba \ ,, ,, ^

alba Z M M ^

Thus as the results of breedi no- we have :

A. dioica % x dioica $ giving females $ $ and males ? ^.

B. alba ? x dioica $ giving females $ $ and males ^ § .

C. dioica $ x alba $ giving all females of the form ? § .

The difficulty in this scheme is that, if maleness were
dominant, it is not clear why the plants produced in case
C should be almost entirely females ; for it seems natural to
expect that some of the dominance of maleness would
attach to the gametes of monoecious character produced by
alba, and hence that male flow^ers should be produced with
frequency by the female hybrids. This nevertheless is not
the case. In our experiments, out of 37 plants thus bred,
34 were purely female and only 3 showed any male flowers.
These were in all cases at the lowest flowering nodes. In
one plant 4 male flowers .occurred in this position, and in
each of the other two plants a single male flower appeared.
The reciprocal cross, alba $ x dioica $ gave 6 pure females.
I lemale with a single male (lower, and 9 pure males.

Correns observed a similar occurrence ol male llowers
in the same position on the otherwise female hybrids raised

1 68 Sex in Bryony [ch.

by him, but the occurrence was quite exceptional. One
plant alone showed a distinct tendency to the monoecious
condition, almost resembling alba. If maleness were domi-
nant this prevailing absence of indications of the monoecious
condition is not easily accounted for.

In view of this difficulty it is worth considering whether
other schemes are not equally possible, and it seems to me
that there is another method of interpreting the facts which
is certainly not yet excluded. In Correns' scheme male-
ness is taken to be dominant, but if femaleness is taken to
be dominant we can then represent the appearances thus :

dioica % has egg-cells % and ^.

dioica $ has pollen all $.

alba g has egg-cells $ and ^ ; and pollen all $.

The matings will then stand as follows :

A. dioica $ x dioica $ gives females ? J and males $ $.

B. dioica % x alba ^ gives pure females % $ and hetero-

zygous females % $.

C. alba $ X dioica $ gives females ? $ and males $ $.

D. It follows that alba self- fertilised should give />zire
females as well as the ordinary monoecious plants. Whether

this is the case or not I do not know. Only three plants
were raised from seed here, and these were monoecious, but
there would be of course nothing unusual in a species, often
monoecious, producing female plants, for many such examples
are known.

The view that it is th^ female and not the male which is
heterozygous in sex is not improbable because, as will soon
be shown, there is a remarkable group of cases among animals
in which that interpretation is almost forced upon us.

Lastly, for the apparently anomalous representation of
the pollen-cells of alba as of a constitution dissimilar to that
of the egg-cells of the same plant, we can quote (Chap, xi)
a somewhat parallel case which is well established in regard
to Stocks [Matlhiola), where certain strains have the egg-
cells of two types and pollen-cells of one type. In that
case however the pollen-grains bear the recessive character,
not the dominant as they must be supposed to do here.

x] Heredity and Sex 169

Admittedly there are difficulties in the way of this method
of representation but they seem to be no greater than those
besetting the hypothesis that maleness is dominant*.

Correns (81, 72, 75) has observed another group of facts,
doubtless important though equally difficult of interpretation,
in regard to the results of breeding from species which
have female, and hermaphrodite or gyno-monoecious indi-
viduals. His experience is that when the females are used
as mothers and are fertilised with pollen from the her-
maphrodites, the offspring are almost exclusively female.
When however the hermaphrodites are used as mothers
the offspring are mostly, though not so exclusively, her-
maphrodites. From experiments of this kind it is likely
that a good deal of light will be obtained when statistics
on an ample scale are available. Those who may under-
take such work will of course remember that as the consti-
tution of the individuals may be dissimilar, each must be
separately tested.

The relation of dioecious to hermaphrodite and monoe-
cious forms will not in all probability be satisfactorily or
rapidly elucidated until some case can be found in which
the two types can be crossed together with a fertile result.
No evidence from such a case as that of Bryonia is free
from the suspicion that the sterility of F^ may itself be
introducing a complexity.

Heredity lifnited by Sex: the Horns of Sheep.

I now pass to the consideration of evidence as to
the part which sex plays in determining or limiting the
descent of certain characters in heredity. The manner in
which these effects are shown is well illustrated by the
following example investigated by Professor T. B. Wood
(312, 313) in the case of horned and hornless sheept- As
a horned breed he chose the Dorset Horned, in which the

* A fragment of evidence bearing on these problems is that contributed
by Gartner's experiment with Lychnis diiirna $ crossed with pollen from
Z. flos-cuculi ^ . This gave 4 males and 2 females ; but of these, one
only, a female which was totally sterile, gave any certain indication of
having resulted from the cross. Pending a repetition of the experiment no
conclusion can be drawn with much confidence from this account.

t As was stated before, hornlessness is a dominant in both sexes so far

1 70 Heredity of Horns [ch x

horns are well developed in both sexes. These he crossed
reciprocally with Suffolks, a breed without horns in either
rams or ewes. The F^ male lambs all developed horns of
fair size, but the F-^ ewes remained hornless. The horned
character may therefore be described as dominant in males
and recessive in females (Fig. 20).

The /% generation bred from these consisted of all the
four types, horned and hornless males, horned and hornless
females. The experiment was not carried out on a scale
sufficient to justify a statement that the numbers are simply

Males Females

3 horned : i hornless i horned : 3 hornless

but so far as the evidence went these simple ratios seem to
be followed. (For diagram of such a descent see Fig. 33.)

Subsequent experiment has confirmed the conclusion
indicated by these facts, namely that horned ewes in F., are
pure for the presence of horns, and that hornless 7'ams in /%
are pure for the absence of horns.

In order therefore that the female may possess horns,
she must be homozygous in that character. The factor for
hornedness must come in from both sides of the parentage.
Conversely, in order that the male should be hornless, he
must receive the deficiency from both sides of his parent-
age. A case that may be compared with this has been
observed in regard to the descent of wing-development in
a cross between two species of moths of which the one
Boarmia {Bistoii) hiiHaria has a winged female, while the
other, Boarmia .{Biston) pomonaria, has a wingless female.
Oberthiir"^ crossed these two species and obtained F^ males
all with fully developed wings and four females with wings
only half-formed, in a condition thus intermediate between

as horned cattle are concerned. The heterozygote is either quite hornless,
or has only small loose horny lumps — " scurs " as they are called in the north.

As to the descent in goats I have no thoroughly adequate evidence.
The Rev. E. P. Boys-Smith has kindly given me particulars of many
matings which he has made, but the details are complex and I have not
been able to extract a consistent scheme from them. There is probably
some intricacy due to gametic coupling comparable with that described in
the next section, or perhaps to sex-limitation.

* Bull. Soc. Ent. Fiance^ 1897? P- 256.

>l..

34.

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t O

c S

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f^ r-

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5 J"

o
o

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O ij wj

-^WSfffW^"

CX)

^ ^

*- 1> 1/5

V5 r) —

w. ^ '^

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QJ 7Z tA

^ so C

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172 Tortoise-Shell Cats [ch.

those of the two parent species. Unfortunately the hybrids
were completely sterile and the experiment went no further.
Here we see that the one "dose" of wingedness — as we
may call it — sufficed only to bring the wings to half the full
size, and two '' doses " are needed to develop them properly.

Doncaster has collected evidence about the inheritance
of tortoise-shell colour in cats which illustrates the same
phenomenon. It has long been known that tortoise-shells
are almost always females. The suggestion which Don-
caster made (109) is that this is the female form of the
heterozygote between the colours orange and black. The
facts, as he pointed out, show fairly clearly that the corre-
sponding heterozygote in the male is orange. Orange
colour is thus dominant in males, but in females the domi-
nance is imperfect and the patch-work form, tortoise-shell,
results. The same is true for cream and blue, which are
only the dilute forms of the colours orange and black re-
spectively"^. It is true that the exceptional tortoise-shell
males do occasionally exist, but they are exceedingly rare
and nothing as yet is known respecting their breeding.

More complex systems of sex-limited descent are fol-
lowed by certain abnormal conditions met with in Man.
Of these the most familiar is colour-blindness.

The ordinary form of that affection may roughly be
described as the inability to distinguish red from green.
Colour-blind persons are commonly male, and in European
countries it appears that at least 4 per cent, of the male
population are colour-blind (of females less than 0*5 per
cent). The children of colour-blind fathers are usually not
colour-blind, whether they be sons or daughters. The
datightei's however frequently transmit colour-blindness to
their sons again. The unaffected males in these families do
not transmit the condition, and their posterity are exempt
unless the colour-blindness be introduced afresh. Until
the facts were examined in the light of Mendelian dis-
coveries nothing could be more puzzling. The statement
however that colour-blindness is a condition dominant in
males but recessive in females will express a great part of

* The idea is sometimes met with that it is only tortoise-shells without
white which are almost exclusively female ; but there is no truth in this
supposition.

X] Heredity of Colour-BIijidiiess 1 73

the facts respecting the genetics of colour-bHndncss. The
transmitting daughters of colour-blind fathers are evidentlv
heterozygotes in whom the affection is, h'ke the horned
character in the F^ ewes, recessive ; but in the fact that
both the sons and their offspring are free from the affection
(unless of course it be introduced by the mother) we meet
a fresh complication the meaning of which is considered
later (p. 195, note). Here it must suffice to sa)' that the
most obvious test of the nature of the inheritance is pro-
vided by the families of colour-blind women. Accordincr
to the scheme the simplest expectation is that all sons of
such women should be colour-blind. Up to the present
time we have records of seven colour-blind women only who
had sons. In all they had 17 sons who lived to be tested,
and all were found to be colour-blind*. We may therefore
rest assured that the scheme provides at least a substantial
part of the truth, and that colour-blindness is a condition
produced by the addition of a dominant factor.

In the examples just considered sex itself acts as a
specific interference, stopping or inhibiting the effects of a
dominant factor^ and it is not a little remarkable that the
inhibition occurs always, so far as we know, in the female,
never in the male. When the effects of a factor fail to
appear in a zygote the failure is due to one of two causes.
Either some complementary element is absent which is
needed to produce the effect, or some other element is
I present which inhibits it. The facts scarcely enable us to
distinguish between these two possibilities but we may feel
some confidence that our cases belong not to the first
group but to the second. For since the condition can be
developed in females, it is evident that maleness itself is
not a necessary complement ; and it is not easy to suppose
that there is some other factor regularly coupled with male-
ness which has this property, though that possibihty cannot
be absolutely excluded. The suggestion however that the
female contains something which suppresses the effect ot the
otherwise dominant factor is consistent with the observation
that when these sex-limited conditions, as they are called,
do appear in females, they are developed to a somewhat
less degree than in males, just as in horned breeds of sheep
the ewes have horns smaller than those of the rams. The

* See p. 224.

174 The Case of the Ctirrant Moth [ch.

inhibiting factor may be thought of as able to suppress
completely the development of the character when that
character is heterozygous — introduced, to use that ex-
pression, as one "dose" only — but unable to suppress it
when it is homozygous, represented by a double "dose."
On the view that femaleness itself is the suppressing
factor one difficulty has to be considered. Just as disease
or removal of the ovaries may lead to the appearance of
male characters we should expect that such disease might
lead to the occasional appearance of colour-blindness in
females. I do not however know of such a case. It must
nevertheless be remembered that even the appearance of
male characters is itself by no means a regular consequence
of these lesions, and perhaps we need not regard the
difficulty specified as a serious one in the case of colour-
blindness.

Sex and Spiiriotis Allelonwrphism: the Currant Moth.

The next case of which I shall speak is one which has
been worked out in considerable detail, and I anticipate
that for a long time to come it must rank as a classical
experiment in all discussions as to the nature of sex. The
work in question was done by Doncaster and Raynor and
it relates to crosses made between the Geometrid moth
Abi^axas grossttlariata and its variety lacticotor (iii).

Lacticolor was originally known as an exclusively female
form (see Plate I). The experimental crossings gave the
following curious series of results:

1. Lacticolor % y. grossulariata $ produced /^^ $s and
$s all grossula^data.

2. F^ grossulariata % x F^grossulariata $ gave grossii-
la7nata ^s and $s and lacticolor $s ; no ^s of lacticolor being
formed.

3. Lacticolor $ x F^grossulariata Z gave all four possible
forms, namely gi^ossulariata $s and $s, and lacticolor $s
and $s. The lacticolor males were the first that had ever
been seen.

x] Sex and Spurioits Allelouiojphisin 175

4. F^ grossulariata % x lacticolor $ gave all the ^s
grossiilariata and all the ?s lacticolor.

Adopting Castle's view that both sexes are heterozygous
in sex, Doncaster showed that a consistent scheme could
be devised which represented the foregoing experimental
results. According to this scheme the following supposi-
tions are made

(i) Each sex is regarded as giving off ^-bearing
gametes and ^-bearing gametes.

(2) In females heterozygous ior grosstdariata and lacti-
color there is gametic coupling such that each gamete
bearing the grossitlariata factor bears maleness, and con-
versely each gamete bearing lacticolor bears femaleness
also.

(3) In the heterozygous male there is no coupling.

(4) There is selective mating between the gametes,
such that union can take place only between gametes of
opposite sex, namely such as bear maleness and femaleness
respectively.

(5) Dominance attaches to the sex which is brought
into the zygote by the egg.

On these assumptions the observed facts would be
produced. Inspection however shows that there is a sim-
pler solution, which avoids the need for assumption (4) that
selective mating occurs. On this scheme two assumptions
only are made.

(i) That the female is heterozygous in sex, femaleness
being dominant, and the male a homozygous recessive.

(2) That when in F^ the two dominant characters
femaleness and the z'^^ossulariata factor co-exist, there is
spurious allelomorphism or repulsion between them, such
that each gamete takes one or other of these factors, not
both.

176 Sex and Sptirioits Allelomorphism [ch.

The whole series of facts is then consistently represented
as follows :

lad. 9 >^ gross. $

gross. $
DRS i

gross. 9

composition
gametes

Fx gross. ^ X -Fi gross. ^
Z>R9 6 DR$$

[R ? \R6

2 gross.
DR$ $
DI)6 6

^ I gross. 9

DR^ 6

I lact. 9

RR^ 6

lact. 9 X ^1 gross. $
RR<i 6 \ DRS 6

I g?'OSS. $

DR6 6

I gP'OSS. 9

I lact. (^
RR6 6

I lact. 9
RR9 6

A gross. 9 >< ^^<^^- c?

DR9 6 \ RR6 ^

-~i

jDR6 6

lact. 9
RR^ ^

In Doncaster's first series of experiments the numbers
observed were very wild and irregular, but more recently
he has repeated the matings and got numbers according
sufficiently well with those predicted by the scheme. The
spurious allelomorphism supposed to exist between female-
ness and the grossulaj^iata factor would be a phenomenon
similar to that described already (p. 153) in the Sweet Pea,
where the factor for blueness and that for the erect standard
were shown to behave as if mutually repellent, and alterna-
tive to each other in the formation of the germ-cells. The
facts as to the limitations which sex introduces into the
descent of a varietal character are thus consistent with the
possibility that sex is a Mendelian character and that
femaleness is a dominant, depending for its development on
the presence of some definite factor, and that this factor has

x] Sex-limited Inheritance 177

properties like those elsewhere proved to attach to other
dominants.

One very curious observation made In the case of
grossulariata is that which remains to be stated. It was
communicated by Doncaster to the Dublin Meetlnc,^ of the
British Association (Sept. 1908). The experiments enu-
merated were, it will be observed, Incomplete in so far as
the mating \^'Adi gross, $ x lact. $ had not been made. The
results of this mating are now known. FamiHes thus bred
consist of mdAQS, grossulariata, ■Siwd females lacticolor\ In
other words, the ordinary wild grossulariata even in districts
where lacticolor is unknown, are in reality a race of which
the males are i^uvq grossulariata, though the females are in
reality hybrids of lacticolor, and so continue from generation
to generation. The normal female grossulariata and the
F^% grossulariata bred from lact. $ y. gross. $ are thus seen
to be Identical in composition. Whether a gross. $ has a
gross. ? for a mother, or a lact. $ for a mother makes no
difference to its composition and properties. This fact is
one of the most striking to which genetic research has yet
led. It affords strong confirmation of the Interpretation of
the series of phenomena given in the text, and enables us
to see in the evidence both as to the grossulariata case
and as to the other cases which follow, a consistent mass of
testimony all pointing in the same direction. In a recent
paper (114) Doncaster accepts the view here suggested.
It must of course still be remembered that attractive though
the present suggestion is, by reason of Its simplicity, we have
no proof that the natural scheme may not be more complex.

The two cases next to be considered resemble that
of grossttlariata in the fact that reciprocal crosses be-
tween pure types give dissimilar results. Though both
examples to be discussed are only imperfectly explored, the
facts elicited are so curious that some preliminary notice of
them is called for. We may confidently anticipate that
further search will discover other comparable instances. It
will be seen that these phenomena point very plainly in the
same direction as those previously described, to the con-
clusion, namely, that, in the types used, femaleness depends
on the presence of a definite dominant iactor.

p>. H, 12

lyS Sex-limited Inheritance [ch.

The Cin7tamon Canary,

The first case of a sex-limited distinction between
reciprocal crosses is provided by the evidence as to the
descent of what is called '* Cinnamon " in Canaries. The
characteristic of these canaries when adult is the presence
of the pale drab colour known as Cinnamon in the feathers,
evidently replacing the black pigment characteristic of
" green " canaries. The green is of course due to the black
pigment showing through the yellow. According to Miss
Durham's observations the cinnamon pigment is of the
same nature as the ''chocolate," of which we have spoken
in discussing the pigmentation of mammals, and it is
probable that the various aberrational forms of mammals
and birds so often recorded by systematists as " isabelline "
are all similar cases of the replacement of black by
chocolate.

When newly hatched, Cinnamons differ very strikingly
from ordinary canaries in the fact that they have pink 07^
unpigmented eyes, like the albinos of many animals. As
they grow up this distinction is scarcely, if at all, per-
ceptible on ordinary examination. The eyes become pig-
mented and come to look about as dark as those of ordinary
canaries. Some fanciers allege that they can distinguish
the eyes of Cinnamons throughout life, but the difference is
evasive and these determinations are unreliable. Micro-
scopically, however. Miss Durham finds that there is a real
difference in the fact that the pigment of the Cinnamon eye
is chocolate, like that of the chocolate mouse. Apparently
the cinnamon^ feathers are never developed in birds that
have black eyes, but the pink eyes can be transferred to
birds which are pure yellow and without any trace of
cinnamon in their feathers.

The inheritance with which we are concerned is that of
the pink eyes. It has long been declared by fanciers that
when Cinnamon hens, viz. pink-eyed, are bred with green,
viz. black-eyed, cocks the offspring of both sexes all come
black-eyed. When however green (black-eyed) hens are
bred to Cinnamon (pink-eyed) cocks, both greens and
Cinnamons may be bred, but these Cinnamons are
always hens. The F^ black-eyed cocks are said to have
again the power of producing pink-eyed offspring from

X] The Cinnaiuon Canary 179

black-eyed hens, but again all these pink-eyed birds are
hens. Statements to this effect are to be found in many of
the fanciers' books, but a particularly crood and lucid account
of the phenomena was given by ]\lr Noorduijn (213), of
Groningen, who has been good enough to answer many
questions on the subject.

Miss Durham has begun a series of experiments
designed to investigate the problem constituted by these
facts, but the work has not yet gone far enough to provide
a complete solution. She finds (118) that, as stated,

(i) Cinnamon ? x pure green ^ gives all offspring of
both sexes black-eyed.

,(2) Green % x Cinnamon ^ gives in general F^ cocks
black-eyed, and F^ hens pink-eyed. Two black-eyed hens
of unknown breeding mated to Cinnamon cocks have also
given, in addition, black-eyed hens as well as pink-eyed hens.
This result is exceptional.

(3) Cinnamon ^ x F^ black-eyed $ gives all four types,
black-eyed cocks and hens, and pink-eyed cocks and hens.

(4) Green ^ x F^ black-eyed $ gives cocks all black-
eyed, and hens of both types, black-eyed and pink-eyed.

The mating of F^ black-eyed hens (from Cinnamon $ x
black-eyed $) x Cinnamon ^ has not yet been made, but
there is little doubt that such a pair of birds will give the
same results that are obtained in mating (2).

As regards the descent of the pink-eye there seems to
be no difference in result whether green or yellow black-
eyed birds are used.

The general run of these experiments is now intelligible.
The case is evidently comparable with that of Ab7'axas
zrossulariata and lacticolor. Were it not for the occasional
production of black-eyed hens from green $ x Cinnamon ^
the whole series of results could be represented in one
simple scheme. That exceptional occurrence proves of
course that there is some further clement to be considered,
but neglecting that for the present, the scheme of descent is
as follows. As before, we take female as heterozygous,
femaleness being dominant, and we assume that there is
spurious allelomorphism between femaleness and the black-
eye factor.

12 — 2

i8o Sex-limited Inheritance [cii.

The allelomorphs are

?, Zi presence and absence of femaleness.

B, d, presence and absence of the factor for black eye

The *'pure" green hen is thus S(.$Bb, being, as experi-
ment proves, actually heterozygous for the black-eye-factor.
The gametes are BZ and b%.

The pure black-eyed green male is pure in maleness
(recessive) and in the dominant black-eye-factor, and his
composition is represented thus ZZB^-

The Cinnamon hen is %Zbb and the Cinnamon cock is

nbb.

The matings are then as follows.

1. Cinnamon pink-eyed $ x black-eyed ^ •
composition bb^ $ x BB ^ $

/^ black-eyed he?is black-eyed cocks

Bb^ $ Bb6 $

2. Black-eyed ? x Cinnamon pink-eyed S
composition Bb^ $ bb $ ^

gametes |^ ^°

F-^ pink-eyed hens black-eyed cocks

bb<^ 6 • Bb^ S

3. Pink-eyed $ x black-eyed ^1 S
composition bb^ $, Bb ^ $

iblack-eyed hens black-eved cocks

result \^''^ ^ ^^^'^

\pink-eyed hens pink-eyed cocks

\bb^ $ bbSS

4. Black-eyed 9 x black-eyed F^ ^
composition Bb^ $ Bb $ ^

{B$ B$

\b^ bS

black-eyed hens black-eyed cocks

_Bb^ 6 BBS 6

\pi7ik-eyed hens and also

\bb^ $ BbS$

This representation is therefore complete except in so
far as it takes no account of the production of black-eyed
hens together with pink-eyed, which Miss Durham has
twice seen in mating (2). These create a definite difficulty
which as yet there is no means of overcoming. The con-
jecture may be hazarded that they owe their origin to the

gametes

result

X] The Silky Foi.ul i8i

disturbing effects of some other dominant factor unde-
termined, which alters the normal distribution of the factors
B and ? in the gametogenesis of certain black-eyed hens,
but that is mere conjecture.

Attention must be drawn to the paradoxical conclusion
that black-eyed hen Canaries, zvhethei^ green or yellow^ are
normally heterozygous in respect of the black-eye character*,
being thus in common parlance "hybrids" of Cinnamon!
The males, on the contrary, are in general homozygous or
pure in the black-eye character. This discovery gives rise
to many reflections, some of which will be spoken of after
the facts of the next case are described.

The Silky Foivl

The example of heredity limited by sex that we are
about to consider exhibits a complication which at first
sight suggests that the phenomena are the converse of
those last detailed. In the Canaries when the two sexes
raised from a cross differed from each other it was the
females which presented the recessive feature and the males
which showed the dominant. We shall here see that when
the sexes differ it is the female which ostensibly shows the
dominant. Nevertheless the difference is apparent, not
real. In the case of the Cinnamon Canary the observed
results were due to the combinations of two pairs of factors,
while those about to be described are due to the combina-
tions of thi^ee pairs. The presence of the third dominant
cannot however be told by inspection. Breeding tests alone
reveal its existence.

Mr Punnett and I have been enoag-ed for some years in
experiments with Silky fowls. The most interesting results
were obtained in crosses between Silkies and certain ordin-
ary fowls with unpigmented shanks.

Silkies are remarkable in many ways, but the peculiarity
with which we are here concerned is the intense black
pigmentation of the mesoblastic membranes. The perios-
teum, pia mater, somatopleure, parts of the splanchnopleure,
the sheaths of some blood-vessels, and the connective tissue

* Miss Durham has so fnr tested 7 yellow hens and 6 green hens, and
all of these have thrown pink-eyed hens when mated to pmk-eyed cocks,
thus proving their heterozygous nature.

1 82 Sex-limited Inheritance [ch.

beneath the skin, are deeply pigmented with black^. The
skin of the bird has thus a deep purple or blue colour, due
to the black pigment showing through the uncoloured
epithelium of the skint. Ordinary fowls, Brown Leghorns
for instance, the breed chiefly used in our work, have no
pigment in these parts beyond dubious traces in rare
exceptions. Such fowls we may call by contrast non-
pigmented, and the phenomena to be described occur as the
results of crossing them with SilkiesJ.

The essential facts are as follows. When the Silky hen
is bred with Brown Leghorn cock all the F^ offspring, both
male and female, are either destitute of the pigmentation, or
only show a small amount of it in certain parts, especially
in the ribs, on the vertex of the skull, and on the iris. In
the case of the iris it shows in adult birds as minute dots
of black on the red ground. The skin of such F^ birds is
scarcely different from that of unpigmented breeds. Both
the skin and the periosteum sometimes have well-defined
patches of pigment.

When however the Brown Leo^horn hens are bred with
the Silky cock, the F^ males are indistinguishable from
those bred in the reciprocal mating, bid the F^ hens are
almost as much pigmented as the pure Silky hens.

Of the many possible matings into which the F~^ birds,
male or female, can be introduced, few have yet been made
on a scale sufficient to justify very confident statements. In
most of the derivative families thus produced there is also a
good deal of grading, and the analysis and classification of
these intermediate types have not yet been adequately
carried out.

The main facts however are

(i) That when the F^ hens, whether of the deeply
pigmented sort or of the slightly pigmented kind formed in
the reciprocal mating, are bred with Brovvn Leghorn cocks,

* The liver and the lungs are little if at all invaded by the pigmentation,
and curiously enough, the allantois is entirely unpigmented.

t When the pigmentation occurs in birds having a yelloiv skin the
general appearance is greenish. White skin is dominant to yellow skin,
but the transmission of these characters is independent of that of the
peculiar pigmentation of the Silky.

X The silkiness of the feathers is an ordinary recessive to the hard
plumage of common fowls. This character seems to be distributed inde-
pendently of the rest.

x] The Silky Fozul 183

their offspring, whether male or female, are never more
than moderately pigmented. Just as in the making of F^
therefore, the pigmentation must come in from the male
side in order that it may appear to any full degree in the
offspring.

(2) The F^ males from both kinds of mating are
identical in composition so far as pigmentation and their
powers of transmitting it are concerned. When they are
bred with Brown Leghorn hens they produce one bird in
eight, on an average, deeply pigmented, and these are always

females.

This result was exceedingly definite and regular. A
long series of matings between F^ ^s from Brown Leghorn
hen X Silky cock as fathers and unpigmented hens gave
205 birds with various degrees of pigmentation from
moderate to nil, and 31 fully pigmented hens. Using F^ ^s
from Silky % x Brown Leghorn ^ the numbers of these two
classes were 170 and 23, the 23 fully pigmented birds being
again all hens. The total, 375 to 54, is 7 to i.

(3) The F^ cock has only been tried once with a Silky
hen, and the offspring consisted of 7 males and 8 females,
showing all the different degrees of pigmentation, but from
so few no quantitative conclusions can be drawn.

(4) When the deeply pigmented F^ hens are bred with
the pure Silky cock, all the offspring of both sexes are
deeply pigmented.

(5) From a slightly pigmented F^ hen (offspring
namely of Silky ? x Brown Leghorn f) bred with a pure
Silky cock only a few birds have yet been bred.

As a test of the distribution of the factors among the
gametes of the hen this mating is one of the most important
and many critical questions can be answered by it. The
numbers are as yet too small to be of much significance.
They are 8 hens all deeply pigmented, and 5 cocks ranging
from deeply pigmented to intermediate.

The F^ results are so complicated that until they h.ue
been obtained on a very large scale it would be premature
and useless to describe them in any detail. In general
terms the F„ families contain both males and females of
the deeply-pigmented, the slightly pigmented, and the

184 Sex-limited Inheritance [ch.

non-plgmented kinds, with many puzzling intergradatlons
between them.

Leaving aside all that is obscure in this experiment for
future consideration one fact stands out clearly as an almost
inevitable conclusion from the data, namely that In addition
to the sexual differentiation into male and female, and the
racial differentiation into pigmented and non-pigmented, the
operation and interference of some unknown third kind of
differentiation has to be reckoned with. In no other way
can the results from the reciprocal crosses between the pure
types, and those from the non-pigmented hens bred to the
F^ male, be brought into harmony. It is the appearance of
the one deeply-pigmented female in eight birds which gives
the clue.

Since half the gametes of the F^ male must be bearing
pigmentation, and since from results of the mating Brown
Leghorn x Silky male we know that a pigment-bearing
gamete from the male may dominate in the female, it would
be expected that half the female offspring would be deeply
pigmented ; for certainly half of them contain the necessary
element. But as only a q2mrter of the female offspring are
of the deeply-pigmented class there must be some other
element present which obliterates the pigmentation, or holds
it in check in the missing quarter. In several cases we are
well aware of the existence of such inhibiting factors, for
example that which causes the flower of a Chinese Primula
to be white though the factors for colour are present in it
(see Chap, v, p. 105).

This factor may be spoken of as Z^. In the Silky it is
evidently not present and therefore it must come from the
Leghorn. Since Silky % x Brown Leghorn ^ gives no
deeply-pigmented offspring we must consider the Brown
Leghorn cock to be homozygous in D , But as the Brown
Leghorn $ x Silky ^ gives the female offspring deeply
pigmented, the Brown Leghorn hen must be heterozygous
in D, and there must be some system or mechanism by
which this factor D descends to her male offspring and not
to her female offspring"^.

* A priori it might be thought possible that the dominance of the
pigmentation-element in the F-^ 9 was due to some special differentiation
of half the gametes of the male. On the hypothesis that each sex is
heterozygous for sex such a system might without improbability be con-

X] The Silky Fowl 185

Now if femaleness be a dominant factor and can repel
D, forminof a spurious allelomorphism with it in the way
suggested for the case of Ab^^axas grossulariata x lacticolor
and the Canaries, a system would be provided which would
fulfil all the chief conditions marked out by the experimental
data.

Taking- then the following allelomorphs, the occurrences
in the three matings which have given clear results may be
represented in a tabular form.

$, ^, presence and absence of femaleness, a dominant
factor without which the zygote developes into a
male.

P, p, presence and absence of the black pigmentation.

D, d, presence and absence of a factor which can sup-
press or mask the development of P,

1. Silky $ X Brown Leghorn $
composition PPdd^ $ ppDD ^ $

gametes |^^^ ^^^

F^ females slightly males slightly

pigmented pigmented

composition PpDd% $ PpDd $ S

2. Brown Leghorn $ x Silky $
composition ppDd^ $ PPdd$, $

gametes \^^^^ ^^ ^

F^ females deeply males slightly

pigmented pigmented

Ppdd^ cJ PpDdS 6

3. Brown Leghorn ^ -/. F-^S
composition ppDd^ ^ PpDd $ $

<rametes [P^ ^ [PD$.Pd$

^<^^^^tes 1^^^^ \pD$pd$

Fcfnales Males

offspring pd^.PD$ pD$.PD$

*pd^.Pd$ pD$.Pd$

pd^.pD$ pD$.pD$

pd^.pd$ pD^.pdS

According to this analysis one bird in eight, namely the

female marked *, will be of the deeply-pigmented type.

ceived. It would, however, fail to represent the i deeply-pigmented $ in
8 birds from Brown Leghorn 9 x t^^ o , and woukl increase the expectation
to 2 9 in 8 birds, and this is negatived by the results of experiment.

1 86 Sex-limited Inherit mice [ch.

The above tabulation represents so well the main outline
of the results of experiment that it is probably a close
approximation to the truth. With regard to the somewhat
meagre results of the other matings it may be said that, as
far as they go, they are fairly consistent with the schematic
representation, though some discrepancies occur, and un-
questionably much remains still to be cleared up in this
remarkable case. The distribution of the several factors
among the gametes of the two types of F-^ females cannot
yet be satisfactorily represented, and there is some evidence
that the repulsion between femaleness and D is not always
so complete as the scheme demands.

In introducing the evidence as to Silkies attention was
called to the fact that the difference between reciprocal
matings occurred when the non-pigmented parent was of a
breed which had unpigmented shanks. There are of course
many breeds which though they have nothing corresponding
to the general pigmentation of the Silky, are black or bluish
in the shanks from a deposition of black pigment in the skin
of that part"^. When such hens are crossed with the Silky
cock the pigmentation is developed not only in the female
offspring, but also in the males. Davenport made crosses
between Silkies and Spanish (black-shanked) and Frizzles
(slaty shanks), and found no distinction between reciprocal
matings, both sexes of the offspring being deeply pigmented.
We crossed white rose-combed Bantam hens with a Silky
cock with the result that the F^ males and females were
both deeply pigmented on hatching, though as they became
adult the males lost much of their pigmentation. The white
rose-comb is slightly bluish in down-colour and the shanks
are slightly pigmented in varying degrees.

From many signs we know that there exists some com-
plex relation between the colour of the shanks in fowls
generally, and sexual differentiation. Some years ago we
described (19, p. 95) a case of this kind in which Indian
Game $ x White Leghorn ^ always gave F^ yellow-shanked
like both the parent breeds. But White Leghorn % x Indian
Game ^ gave cocks yellow-shanked like the parents, while

* This pigment is confined to the skin. There is none in the
periosteum. When the general skin-colour is yellow these pigmented
shanks have a greenish tinge, the "willow" of fanciers.

X] The Moth, AgUa tait 187

the hens came with a good deal of pigment in the shanks
ranging to nearly a full black.

Another case illustrating this relationship between sex
and shank-colour is to be seen in the newly made breed
called Black Leghorn. According to the fanciers' ideal both
sexes should have full yellow shanks. There is no difficulty
in getting this quality in the cocks, but hitherto clear
yellow-shanked hens have been very rare, and the same
difficulty is encountered in breeding Black Wyandottes.

Lastly, though the D factor seems to be aKvays present
in Brown Leghorns, the cocks of which are homozygous
and the hens heterozygous in respect of it, there is evidence
that in an Egyptian breed of various nondescript brown
colours, with which we have worked, the D factor may be
absent from both sexes, though the shanks are not dark in
colour. Search among the breeds of our own country will
probably lead to the discovery of other such material.

The Case of the Moth Aglia tatc and
its Variety htgens.

There is one very remarkable group of facts which
cannot, so far as I see, be brought into harmony with the
system proposed for the three cases already considered.
These are the results recorded by Standfuss (253, b) as the
outcome of experiments with the Moth Aglia taic- and its
dark variety htgens. These experiments were brought
into prominence by the discussion which Castle devoted to
them in his important paper on sex (46).

The facts were briefly these. We have no explicit state-
ment of a comprehensive kind as to the results of the cross
between pure htgens and pure tan, but from subsequent
results it is clear that, as usual in moths, the dark form liigens
is dominant over the light form tan. With heterozygous

hiQeiis five matinc>"s were made as follows :

Resu/t

/;/»•. iau ///:,^ fau

1. ta2i {RR) ? X h/ge?is (BR) 6

2. h^orens (BR) 9 X /au (RR) }

3. /i^ge/is {BR) $ X /uoe/is {BR) <^
4" >> >>

5* >» "

Total of BR X BR

1 1

A 6

129

i()

__94

1 88 Cytological Evidence as to Sex [ch.

In his discussion of these curious numbers Castle calls
attention to the fact that the results of DR x RR are very
nearly 2D^ : iR$ : iD^ : 2R%, and developing his view that
each sex is heterozygous in sex, he suggests that the
gametes of DR females and males may bear the sex-
characters and the colour-characters coupled in this way,
forming a series 2 + i + i + 2. Assuming also that in
fertilisation union can only take place between gametes of
opposite sex, the F^ numbers would be '^D'^ : \R^\6D% 13^$,
a series which, as he points out, fits the total of the observed
numbers extraordinarily well.

Recognizing the great interest of the case I feel, never-
theless, that so long as it stands entirely alone, we are
justified in treating it as of somewhat doubtful significance.
We know from divers sources that the sex-ratios of the
Lepidoptera are liable to astonishing fluctuations. Very
large families consisting of all, or nearly all, females, or
males, as the case may be, having been not rarely witnessed,
and until experiments with tait and htgens are repeated with
a full understanding of the importance that may attach to
them, w^e may postpone positive conclusions.

The Cytological Evidence.

We have now to mention a group of facts which, though
agreeing with the general conclusion that one sex is hetero-
zygous and the other homozygous, suggests that in the
types concerned, the roles are reversed.

From the cytological side a remarkable advance in the
problem of sex has been made in the discovery of the
accessory chromosome in the spermatogenesis of certain
Insects. McClung, studying this structure, originally ob-
served by Henking, was the first to insist on its importance.
He showed that in certain Insects half the sperms have
it and half are without it. This fact led him to make
the natural suggestion that the structure might be con-
cerned in the differentiation of sex. This suggestion has
been shown by E. B. Wilson to be correct, but the accessory
body proves to be the peculiarity of the sperms which are
destined to form females^ not of those which will form
males, as had been previously supposed. The evidence for
this is the fact that while the number of chromosomes in
the male cells of the species concerned is either n, or n— \,

X] Cytological Evidence as to Sex 189

the number In the female cells is n. The male cells there-
fore which have n chromosomes on uniting with an ovum
in fertilisation make up the zygotic number to 2n ; but the
n— I spermatozoa can only make up the zygotic number tu
211— I, forming thus a male. The germ-cells of the male
again divide in gametogenesis to form cells with ;/, and
cells with n — i chromosomes as before.

In such cases it can scarcely be stated yet that the
accessory chromosome is the cause of femaleness, for con-
ceivably it may be a feature associated with that cause ; but
the evidence must be taken with confidence as the long-
expected proof that sex is determined, in this case at least,
by gametic differentiation. The female in these Insects
must then be regarded as homozygous in sex, and may
be represented as DD, while the male is heterozygous and
may be represented as DR. Such a result accords well
with the general conclusions to which breeding experiments,
on the whole, point. For though great disparities between
the numerical proportions of the sexes occur in certain
matings, these disparities seem to be obliterated in suc-
ceeding generations. If the one sex were homozygous and
the other heterozygous such impermanence of the numerical
divergences is what we might naturally expect. Neverthe-
less the extraordinary fact remains that while the cytological
evidence suggests that the male is heterozygous, equally
cogent evidence — from breeding in other t}pes — indicates
\h^ female as the heterozygous sex.

T. H. Morgan (202)'^ has lately carried the discussion a
stasfe further. It is well known that in the case of animals
having a series of parthenogenetic female generations, such
as Aphis, Dapliiiia, &c., when fertilisation does take place,
the result of such fertilisation is always a female. In a
Phylloxei^a Morgan has found that the spermatids are of
two kinds, those which contain an accessorv chromosome,
and those which do not. The spermatids lacking the acces-
sory body degenerate, and consequently only those provided
with it take part in fertilisation. The fact that the results
of fertilisation are all females, taken in connection with the
degeneration of half the sperms, points evidently to the
view that the accessory chromosome is here responsible lor

* Since confirmed by von Baehr (7) for Aphidae.

IQO Summary of Evidence as to Sex [cii.

the femaleness. It is of especial interest to know that in
this case the accessory chromosome goes through a stage
in which it is partially divided into two, though after this
partial division the two parts reunite and then the whole
passes into one of the daughter-cells. The point that
remains obscure is the nature of the cell-division by which
the males are produced. For at one stage in the cycle
males are of course born parthenogenetically from the
females, and it would be of the greatest importance to
know something of the cytological process by which the ova
are destined to become males — or male-bearing females
if there are two lines (as in the Aphides studied by Miss
Stevens, 256, 260)^. Many other very interesting ques-
tions arise in connection with this part of the evidence, but
further discussion must be postponed pending the accumu-
lation of further evidence.

SMmmary of Expe^^imental Evidence as to the Heredity

of Sex.

In a full discussion of the problem of sex-inheritance
many other kinds of fact would need to be considered. To
these no reference can now be made. All that I have
attempted is to provide a sketch of the new evidence on the
problem which Mendelian experiments contribute. The
main conclusion to which several quite distinct lines of
inquiry unmistakably point, is that in the two Vertebrates
and in the Currant moth the female is a sex-heterozygote,
with femaleness dominant. The female is a hybrid, "female-
male," while the male is pure male, or ''male-male." The
eggs of the female are thus females and males respectively'!',
while the spermatozoa are all male. In other words, the
female contains a factor which makes her female, but the
male is male because he is without this factor. The
phenomena can, as has been shown, with certain exceptions,
be represented symbolically by a system based on this
conclusion. The exceptions are real, but they are manifestly
exceptional, and for the present we may be content to deal
with the main course of the descent. It may conceivably

* These papers contain an account of the beginning of a very valuable
experiment on the descent of colour in Aphis and its relation to sex.
t As those of Dinophilus apatris are.

x] Swnmary of Evidcjice as to Sex igi

be through some extraordinary coincidence that the facts
fall into a system which can be represented by this sym-
bolism, but the convergence of the several sets of facts
strongly supports the belief that the symbolical system gives
a true representation of the physiological basis of sex in
those forms at least to which the system applies.

The evidence from the descent of the dominant sex-
limited diseases, such as colour-blindness, and of the horns
in Sheep is also consistent with the same view, namely that
femaleness is due to the presence of a dominant factor. For
in these examples there is evidently some additional element
present in the female which inhibits or suppresses the opera-
tions of the sex-limited dominant, and that additional element
may not improbably be the factor for femaleness.

At the same time we should be wrong in supposing that
the recognition of femaleness as a definite, allelomorphic
factor is more than a first step towards an understanding of
the phenomenon of sexual dimorphism. There are abundant
signs that this representation expresses only a part of the
truth. There seems to be no reason, a priori, why the
gametic constitution of the two sexes should be the same in
all types. In the various forms on which our conclusion is
founded, the arrangement, we have seen, is probably for the
females DR, and for the males RR. I n the I nsects studied by
E. B. Wilson and Morgan the cytological evidence suggests
DD for the females and DR for the males. All that can be
claimed is that Mendelian analysis provides a hint of the
way in which we may proceed in the attempt to unravel
these intricacies.

It is, I think, on the lines indicated in the last paragraph
that we must look for a reconciliation between the cytological
and the experimental evidence. On the one hand the
cytologists show that in most orders of Insects proof that
the male is heterozygous can be obtained. From breeding
experiments we find that in Vertebrates and in one Lepi-
dopteran xh^ fe^naie must almost certainly be regarded as
heterozygous. The cytological evidence shows extraordinary
differences even in nearly allied forms, some having a
distinct unpaired chromosome, while in others this body is
either fully or imperfecdy paired. In Lepidoptera. as it
happens, the accessory chromosome has not been found.

192 Summary of Evidence as to Sex [ch.

Improbable as at first sight it may appear, the view that
most commends itself to me is that in different types Sex
may be differently constituted. From the results of castra-
tion-experiments we are led to a similar view ; for castration
of the male Vertebrate on the whole leads merely to the
non-appearance of male features, while injury or disease of
the ovaries may lead to the assumption of male characters by
the female. In Crustacea the evidence of Geoffrey Smith ^
and Potts shows that these consequences are there reversed.
The great diversity of cytological features in allied Insects
is also consistent with the expectation that the phenomena
are specific rather than universal.

Then again we may feel fairly sure that the allelo-
morphism between the sex-characters must be a relation of
no common order. The curious numbers that so often
occur in collections of sex-ratios are evidence of this. The
records given in the case Aglia tati and liigens bring us at
once to difficulties with which as yet we cannot deal.

From time to time evidence has been advanced to show
that the production of the sexes can be influenced by special
modes of nutrition or other environmental influences. An
adequate discussion of this evidence would run to great
length. Some of the evidence has been found faulty in
various respectsf, and I am not aware that any example has
been confirmed by successive observers in such a w^ay as to
warrant definite belief in its validity!. It is not impossible,

* My attention has been called to the fact that in his monograph of
Rhizocephala {Fauna u. Flora d. Golf. Neapel, xxix. 1906, p. 89) Geoffrey
Smith explicitly suggested, probably for the first time, the view here adopted,
that one sex, sometimes the female, sometimes the male, is heterozygous
in sex.

t See for instance Punnett (227) on sex-determination in Hydatina
with a criticism of the evidence of Nussbaum on the positive effects of
nutrition.

% The most striking case in which positive results are said to have
been attained is that lately published by Russo (236). This observer
claims to have demonstrated in the rabbit the difference between the ova
destined to become females and those destined to become males. His
conclusion is so far in harmony with that to which genetic experiments
have led us, that the female is heterozygous in sex. Heape, however, has
described {Froc. Roy. Sac. vol. 76, B, 1905) in the rabbit processes by
which ovarian ova frequently degenerate, apparently as a normal occurrence.
Mr Heape very kindly gave me an opportunity of examining his prepara-
tions, and it was impossible to avoid being impressed with the general

x] Cert am Females iiorinally Hybrid 193

nevertheless, to imagine that the output of crametcs repre-
senting respectively one or other of a pan* of allelomorphs
may be influenced by circumstances. We have not yet any
proof that such a phenomenon may occur; but the long runs
of unexpected numbers which we from time to time witness
in the course of Mendelian experiments are suggestive of
some definite disturbance of the normal equality in the out-
put of the two kinds of gametes. Data for an adequate
statistical examination of this question scarcely exist as yet,
but if it be found that the long runs are too frequent to be
accepted at all readily as chance aberrations, the search for
environmental causes of disturbance will have to be under-
taken.

The second conclusion that we have reached is one
which no previous experience of nature could have led us to
anticipate. Naturalists are so accustomed to considering the
males and females of a true-breedino- strain as of identical
composition, except in so far as their sex is concerned, that
those who have no practical acquaintance with genetic
phenomena may find a difficulty in realizing that the females
of a breed can be hybrid in some important respect though
the males are not. The evidence however leaves no doubt
of the reality of this conclusion. The consequences of such
a discovery are not easy to foresee, but as in our very
limited range of experimental study three such cases have
already been encountered, we may feel fairly sure that this

resemblance which such degenerating ova bore to those which Russo
regards as destined to become males. Consequently before that view of
their nature is adopted, the relation of the so-called "male" ova to the
degenerating ova will need very careful study ; for it seems as yet not
unlikely that those differences which Russo has taken to indicate maleness
may prove to be due to incipient degeneration.

In addition to the detection of the distinction between male and
female ova Russo states that he has, by administration of lecithin,
succeeded in greatly increasing the proportion of female births. He
gives figures to show that in rabbits, normally bred, male births are largely
in excess of female births, and another series of figures sliowing a great
excess of female births from females treated with lecithin. Both lists of
figures are however declared to be selected from a larger number in ortier
to illustrate the author's thesis. It is to be hoped that the full lists will
soon be published. Meanwhile I may mention that Mr Hurst in his very
considerable experience of breeding rabbits under normal conditions has
found the male and female births to be sensibly equal in number.

B. H. 13

194 Certain Females normally Hybrid [ch.

special heterozygosis of females is no rarity^ Bred with
their own corresponding male type these females would in
the ordinary course never reveal that they were hetero-
zygous ; for the spurious allelomorphism subsisting between
the femaleness and the dominant factor which it repels
would preserve the males homozygous and the females
heterozygous, and so the impurity would be hidden in
perpetuity. The real condition could only be brought to
light either by a cross with a distinct race; or by a variation
which through the introduction of some new factor or the
omission of one previously existing, upsets the equilibrium
hitherto maintained among the several elements through
the cell-divisions of gametogenesis.

The recognition of these facts brings us a step, if a small
one, nearer to the discovery of the nature of variation. In
such a case as that of the Canary, since the ordinary green
hens are " hybrids " of Cinnamon, the Cinnamon variety is
perceived to have been already in existence, ready to appear
if the critical event which could release the variety should
occurf . What that critical event may have been we still
cannot suggest.

The minute analysis of such cases as these will almost
certainly disclose systems of interrelationship between the
factors more complex than that we have been considering.
In particular, it will be well to keep in view the obvious
possibility that sexual dimorphism may be, sometimes at
least, a phenomenon in which a compound character plays
a part, so that for the production of femaleness, for instance,
it may be necessary that two factors should coincide in the
same zygote.

Finally, In a preliminary survey of the subject, the
attractiveness of Castle's suggestion is not to be denied.
It may well be that the unions of spermatozoa with ova

* Amongst much evidence which Professor C. O. Whitman most
kindly gave me in 1907 concerning the resuUs of his long-continued
experiments in crossing the species of Pigeons, was an example which
is strongly suggestive of the same heterozygosis of the female. When
Turtur hiimilis $ is crossed with a white T. risorius $ , white young may
come in T^i and are then always females.

t The same remarks apply of course also to the moth Abraxas grossu-
lariata of which the males are pure to the type, while the females are
hybrids of lacticolor.

x] Gametes in Groups of Four 195

are not all equally possible, and that fertilisation can only
take place between gametes dissimilar in respect of sex-
factors'"'. A fact perhaps favourable to this conception is the
almost universal arrangement of gametic systems in groups
o{ fo2tr\ The maturation-processes of male cells, and, with
very rare exceptions, those of ova which are to undergo
fertilisation, take place in such a way that a group oi four,
not two, reduced nuclei results. I3oth pollen-grains and
spermatozoa are arranged in tetrads. The ovum prepared
for fertilisation by the maturation-processes has ejected,
except in a few special cases, three nuclei comparable with
itself (though very often two of these may by an equation-
division be extruded in combination). Such differentiation
by groups of four is strongly suggestive of the possibility
that the four parts are not all comparable. It may be that
the simple allelomorphism we so often find is reall)- a
phenomenon of simple cases only, and that the fundamental
differentiation is in reality dimorphic for each sex. The
gametic series for the heterozygote may not be A, a, A, a,
but A, a, A', a' both on the female and male sides; and this
may be the meaning of the grouping into sets of four, not
sets of two. Naturally however very cogent evidence must
be produced in order to establish such a proposition as this.

* There is one piece of direct evidence strongly suggestive of di-
morphism among spermatozoa. It was mentioned (p. 172) that the
daughters of colour-bhnd men can transmit the affection, while the so?is
of these same fathers are free from it and cannot transmit it. Until
proper statistics are forthcoming it is not possible to build with complete
confidence on this fact, but it is a clear indication of dimorphism among
the sperms, such that those destined to take part in the production of
females bear the colour-blindness factor, while those destined to fertilise
the male ova are free from this factor. We do not yet know that all the
daughters of colour-blind men can transmit, but the records are I thmk
consistent with the belief that they may. While holding to the view
expressed in the text that the female is heterozygous in femalcness {F) we
may perhaps suppose that the male is heterozygous for maleness (J/). We
thus avoid the difficulties entailed by the theory that both sexes are hetero-
zygous in "sex" (see p. 165), for two allelonwrphic pairs are now involved,
i^and its absence, J/ and its absence. The eggs may be represented as
(?) and O; the sperms as ^ and 0> so that in fertilisation the union
is always between 7^ and a blank sperm or between J/ and a blank ovum.
The spurious allelomorphisms described in the canary. Abraxas grossu-
lariata and the silky fowl are cases in which /'^repels certain factors, while
in colour-blindness there may be an exactly similiar spurious allelomorphism
between M and the factor for colour-blindness.

13 — 2

CHAPTER XI

DOUBLE FLOWERS.

Miscellmieous Cases, Recessive and Dominant Dotthling —
^' Hose-in-Hose'' Flowers — The Special Case of Double
Stocks.

The inheritance of doubling in flowers has been only
studied with success in a few instances. In one of these,
however, that of Stocks [Matthiola), a feature of such great
physiological significance has been discovered that we may
be sure the subject will before long assume considerable
importance. A special chapter, though a brief one, must
be devoted to it.

Doubling, the multiplication, that is to say, of the
conspicuous parts of flowers, especially the petals, may
occur as the result of a number of various and apparently
quite distinct physiological processes. The different sorts
of doubling have been often described in the treatises on
plant teratology"^. In the commonest kind the stamens
are bodily transformed into petals, as a manifestation of
that phenomenon which I have called Homoeotlcf variation,
viz. the transformation of a part into the likeness of
another with which it stands in a series. This is the kind
of doubling which occurs so conspicuously in the Rose,
Ranuncuhcs, Godetia, &c. In other cases doubleness is

* A good general account of the phenomena will be found in Masters'
Teratology^ 1869. For a more minute description see K. Goebel,
" Beitrage zur Kenntniss gefiillter Bliithen," Pringsheim's Jahrbilcher^ xvii.
1886.

t Materials for the Study of Variatio7i, 1894, p. 85.

CH. XI] Double Flowers igy

attained by an actual multiplication or division of the
petals, the stamens and other parts remaining apparently
unchanged. Of this some of the double Fuchsias, Hya-
cinths and several Liliaceae afford very clear examples.
These two processes, however, very often, in fact most
usually, occur in combination with each other, and it is not
generally possible to distinguish how much of the change in
number is due to the one process and how much to the
other.

In some flowers, especially those with gamopetalous
corollas, the reduplication can occur in such a way that the
corolla is simply repeated, two or more corollas standing
in the place of one, but sometimes when two corollas
are thus formed it may be seen that the outermost is in
reality formed by a homoeotic variation of the sepals Into
the likeness of the petals. The most familiar examples of
this " hose-in-hose " arrangement are known in Pinmula,
Campanula and Minndus. Another kind of doubling is
due to what is termed proliferation or prolification (Masters)
of the floral envelopes. The best illustration of this is the
common double Arabis albida, in which the corolla and
calyx are repeated tier above tier on an elongation of the
axis. The different forms of increase in the number of
the petals may commonly occur in varying degrees of
perfection, and many grades of doubling are often to be
seen on the same individual plant. There is also evidence
that, in certain cases at least, high feeding and generous
cultivation greatly promote the doubling. In some forms
too it is known that the amount of doubling undergoes a
change with the age of the plant, showing what has been
called '' Periodicity'^," the most extensively doubled flowers
appearing on the strongest stems and at the height of the
flowering period.

To students of genetics the interest of the doubling of
flowers arises partly from the fact that it is a character the
heredity of which can be readily investigated, but especially
from the obvious suggestion that the phenomenon is or at
least may be associated with disturbance in the sexuality of
the plants.

*■ See Correns, yB. TV/ss. Bot. xli. 1905, p. 465, do Vries, Bcr.
Dent. hot. Ges. xvii. 1899, p. 45.

198 Do2tble Flowers [ch.

In many fully double flowers fertility Is obviously im-
paired through the conversion of the reproductive organs
into vegetative parts. It must not however be assumed that
the sterility so often accompanying doubling Is solely due to
this comparatively definite circumstance. I n the hose-in-hose
Campamtla, which has the sepals petalold, the well-formed
anthers contain plenty of pollen (some may be petalodic),
but the female organs are in some way influenced by the
variation of the sepals so that they are In some strains
sterile. In the double Arabis albida mentioned above
there are neither male nor female organs, but the tier-
upon-tler structure, which is here the form the doubleness
assumes, plainly shows that something more than a simple
homoeosis of the stamens and carpels has occurred.

In the Stock two kinds of doubling occur which are of
quite distinct nature. The ordinary double Stocks much
used in gardens are fully double, possessing an Immense
number of petals but no sexual organs, male or female^.
Such doubles must therefore, as we shall see, always be
bred from singles, a phenomenon which furnishes one of
the most curious problems that the study of heredity has
to elucidate. Besides these real doubles, plants are occasion-
ally seen with one or two extra petals, but the experience
of breeders makes it probable that these plants are not more
prone than ordinary singles to produce the real doubles.

In Petunia another problem is presented. The doubles
have an immense mass of petals, apparently formed at the
expense of the stamens. A few anthers are nevertheless
formed which contain good pollen. The female organs on
the contrary are abortive and the double flowers set no
seedt-

The case of Begonia oflers some points of interest. The
plants are of course monoecious and it is only the male
flowers which are double as a rule. When the plants flower
for their first season the doubleness is frequently extreme,
no anthers being formed. But In their second season
such plants bear male flowers which contain good anthers,
especially, according to the experience of practical breeders,

* Goebel states that rudiments of anthers are very rarely produced,
t See Vilmorin, Fleurs de pleine Terre^ 1886, p. 669, and various
horticultural authorities.

XI] Double Primula

T99

if the plants are starved'^. The female flowers are crenerally
normal, but sometimes extra petals are formed in them also.
Such female flowers have singular malformations which have
been often described, the most noticeable being an opening
of the ovary which causes the ovules to lie freely exposed.

Of the hereditary descent of doubleness we know little,
but from what is known it appears that several distinct
systems are followed.

In Primula Sinensis doubleness is an ordinary recessive,
singleness being completely dominant. The doubleness of
Primula is of a very unusual kind. It may be primarily a
petalody of the anthers, but I have never fully satisfied my-
self of this, nor do I know any critical observations on the
subject. When a large collection of Primulas is examined,
strains can usually be found, the members of which exhibit
the lower degrees of petalody in some or all of their flowers.
In the fully double flowers there is a complete series of
petals inside the normal ones and arising from them. These
are formed as images of the outer petals, so placed that
their inner surfaces correspond with the outer surfaces of
the normal petals, and the two adjacent surfaces are struc-
turally both alike inner surfaces. The bizarre colour of
double Primulas is due to this circumstance. Inside the
corolla the stamens stand properly formed, and in their
normal relations. The appearance of these fully double
flowers is strongly suggestive of a delamination in the petals
themselves ; but as in those partially double flowers which
have one or two extra petals imperfectly formed and showing
their staminal origin, the petaloid tissue similarly faces out-
wards, it becomes impossible to distinguish any boundary
between the two phenomena. The morphological problems
which these facts create must be left to the expert botanist.

The segregation of the single character from the fully
double is, in some families, clearly quite sharp, and doubles
always breed true when fertilised inter se. The inherit-
ance of the partial or petalodic doubles has been investigated
by Mr R P. Gregory, and his experiments show that when
such a strain is crossed with a pure single, F^ is single,
and in P.. singles, partial petalodics, and some doubles

* For information on this point 1 am obliged to Mr Leonard Sutton
and to Mr Wootten.

200 Hose-m-Hose Flowers [ch.

appear, but owing to the great fluctuation in the degree of
doubleness exact counts are impossible.

In Carnation, where the doubHng is of the ordinary
kind, due to petalody combined with sub-division of parts,
there is some reason for suspecting doubleness to be a
dominant. Everyone who has grown Carnations from seed
is aware that a proportion, often considerable, of the seed-
lings come single. It is most improbable that any large
number of these can owe their singleness to cross-fertilisa-
tion with single plants, for breeders would not keep such
plants wittingly. Till critical experiments are made, how-
ever, the point cannot be regarded as certain beyond
question.

In Poppies, on the contrary, the dominance of the single
type appears to be quite complete. In the annual Larkspur
(DelphiniMm consolida) also, from the fact that pink doubles
exposed to the pollen of blue singles produce among their
seedlings some blue singles^, there is no doubt that the
single type is dominant. Instances of this sort could
probably be multiplied without difficulty, and from what we
know of plant-breeding in general, there is no practical
doubt that doubling of the ordinary type is usually recessive
to sinorleness.

The Hose-in-Hose or Calycanthemoics Campanula and

Mimulus.

The inheritance of this well-known variation or mon-
strosity was studied by Correns (76). The calyx, as
described above, is petaloid in many degrees, varying from
a condition in which the sepals are still partially green up
to the full development of a second corolla. In Correns'
experience this variation in the calyx is accompanied by a
marked diminution in the fertility of the female organs.
This in Campanula persicifolia amounted to total sterility,
though in C media a few seeds were set.

The abnormal character proved to be a partial and
somewhat irregular dominant, considerable fluctuation occur-
ring on the individual plants. Owing to complete failure of ||
F^ to set seed F^ could not be raised, but when the normal
type was fertilised with pollen from F^ a mixture of the two

* 1 have observed this in my own garden.

XI] Double Stocks

20 1

forms resulted*. The Campanulas are plants well suited
for this kind of experiment and it is to be hoped their
genetic properties may be fully explored. Interesting
experiments, for example, could be made with the white
double form of persicifolia known in horticulture as C.
Moerhehnii. In it there is extreme doubling, apparently
of the ordinary kind, due to petalody of stamens and split-
ting of the parts.

In the hose-in-hose Mmitclus Correns found also that
that variety is an irregular dominant. Here however there
was no sign of sterility in the female organs.

Double Stocks [Mat thiol a).

Elaborate experiments on the heredity of doubleness in
Stocks have been made by Miss E. R. Saunders, and though
the research must still be regarded as in an incipient stage,
some facts of quite unusual interest have been discovered.

Single Stocks in general breed true to singleness. As a
rarity an extra petal may appear, but there is no evidence
to connect such an appearance with the extreme and most
definite kind of doubling characteristic of the ordinary
double Stocks of horticulture. In double Stocks both carpels
and stamens are wholly absent so far as our observations
have gone, though, according to Goebel, rudimentary anthers
are sometimes formed. From time immemorial these
doubles have been bred entirely as the offspring of special
strains of single Stocks which are maintained for that
purpose. Since all doubles are absolutely sterile the

succession is represented thus : —

singles

singles doubles

* Correns states that seed sold for "hose-in-hose" or "cup-and-saucer"
Campanula gives a majority of plants showing the variation. I'he sterility
of the ovules is perhaps an accompaniment of the petalody when strongly
pronounced. Campanula ?nedia, with considerable though not complete
calycanthemy, that I have examined had no obvious reduction in fertility.

202 Double Stocks [ch.

The numerical proportion in which the doubles appear
cannot be confidently stated, but it is certain that this pro-
portion differs greatly in different strains. There is some
reason for supposing the ratio 9 doubles : 7 singles to be
the common one in some families, but the experience of
breeders points clearly to the fact that the proportion of
doubles may be much higher than this in specially good
strains, while in others the proportion again may be much
lower. Generally speaking, however, we have the certain
fact that plenty of strains consist of singles which throw a
great excess of doubles.

Many writers have recommended special cultural devices
for increasing the output of doubles. Starvation, drying the
plants off, keeping the seeds from the lower ends of the
pods, are among the expedients advised. I do not know
that any of these suggestions have been properly tested,
and it would be rash to deny that they may have some
effect. On the other hand it is practically certain that
horticultural bad treatment will not cause a double Stock to
produce stamens or carpels. Even in the weakest flowers
on the doubles, which often may be so reduced as to have
only 4 or 5 petals, no sexual organs are formed.

The morphological nature of the doubleness of Stocks is
by no means clear. When such flowers are examined it is
seen that the 4 sepals are normal. Above them the floral
axis is continued for some length, and on it are set the
crowded petals in imbricated fashion. There is no repetition
of the sepals as in the case of Arabis albida, and the
variation cannot readily be described as a strobilisation.

Since the doubles are totally sterile, the problem of the
hereditary transmission of the condition must be investigated
by making crosses with the single-flowered parents which
produce these doubles. As the doubleness appears in the
offspring of these singles the condition is evidently present
either in the male cells of such flowers or in the lemale cells,
or is produced by the combination (in fertilisation) of the
factors with which these cells are endowed.

Miss Saunders has made many crosses between different
strains of double-throwing singles, and as was to be expected,
such crosses have given a mixture of doubles and singles
just as either parent would have done if self-fertilised.

XI] Double Stocks

203

The numbers are Irregular and obviously need a further
analysis.

The crosses which are instructive are those which were
made between the double-throwers and the pure sino-le
strains. Whichever way the cross is made, F, is always
single. When F, is raised from these plants the curious
fact appears that the resiclt differs acco7'ding io tJie way in
which the original cross was made. When the cross is in
the form pure single % x double-throwing ^, all''' the F^
plants give a mixture of doubles and singles in F.. ; but
when the reciprocal cross is made, namely double-thrower
used as $ X pure single used as ^, it is found that the F^
plants are of two kinds, (i) those which throw doubles
mixed with singles in F^, and (2) those which throw only
singles in F^.

The conclusion to be drawn is evidently that the pollen-
cells of the double-throwers are all (perhaps nearly all)
bearinor the double character, but that the eesf-cells of these
same plants are of two kinds, those which bear singleness
and those which bear doubleness (see 22, pp. 5 and 36).

For the first time therefore we have a proof that, in a
hermaphrodite form, there may be a substantial difference
between the factors borne by the male and female cells of
the same plant. It is the existence of this instance which
leads me to hazard the suQfQrestion introduced in the discus-
sion of the facts of sex-inheritance in Bryony, that the
female cells of Bryonia alba may be of two kinds and male
cells of one kind (see p. 168).

This discovery, though it may prove to be the clue to
the problem of the double Stocks, leaves the main difticulty
still unsolved. We have to find the scheme whereby it
comes to pass that the doubles, though bred from singles,
are nevertheless as a rule in the majority, and the difficulty
of offering a plausible suggestion is not diminished by a
knowledge of the fact that the male cells may be all double,
for this fact Implies that a majority of the egg-cells must
also be bearing doubleness, and ultimately that part of the
smgleness must have, as it were, disappeared in gameto-
genesis. We are at present quite unable to otter any

* No exception has yet been met with. There are however some
reasons for anticipating that exceptions may exist.

204 Dotible Stocks [ch. xi

symmetrical or probable scheme of character-distribution by
which such a phenomenon can be represented '^.

The various difficulties of the case culminate in the
fact which seems well established, that though the double-
throwers generally give 7 singles : 9 doubles — with certain
departures from this ratio which are probably not fortuitous —
yet F^ raised from single x double-thrower, and from the
reciprocal cross, always on self-fertilisation gives 3 singles :
I doublet.

* For further information on this subject see the Appendix to Part I.

t The relation of white to cream-colour in the strain known as "Sulphur"
is approximately the same as that of single to double described above.
"Sulphurs" are single whites throwing singles all white, and doubles which
are usually creams with whites as exceptions. The pollen-grains of "Sulphur"
all bear cream, while of the ovules some are creams and some whites.

CHAPTER XII

EVIDENCE AS TO MENDELIAN INHERITANCE IN MAN.

Normal Characters — Diseases and Malformations. Domi-
nants— Sex-limited Dominants — Recessivcs — Note on
Collecting Evidence.

Of Mendelian inheritance of normal characteristics in
man there is as yet but Httle evidence. Only a single case
has been established with any clearness, namely that of
eye-colour. The deficiency of evidence is probably due to
the special difficulties attending the study of human heredity.
Human families are small compared with those of our
experimental animals and plants, and the period covered by
each generation is so long that no observer can examine
many. Now that the critical methods of study are under-
stood we may have every confidence that progress will be
made.

In human inheritance there is however one somewhat
peculiar feature, the complexity of the transmission of the
various colour-characters^. In our experimental studies of
animals and plants w^e have rarely met with examples of a
descent so complex as that which the colour of hair and
complexion in the mixed populations of western Europe
certainly presents. If the colours that we see in our own
population followed in their descent rules so simple as those
traced m the mouse, or the sweet pea, or even as those
which a little study would undoubtedly detect in regard to
the colour of cats, the essential facts of JMendclism must
have long ago been part of the common stock of human
knowledge

The case of eye-colour is comparatively simple. As
Hurst has shown by examining the children in a Leicester-

* See also later with regard to albinism in man.

2o6 Mendelian Heredity in Man [ch.

shire village and comparing them with their parents, the
type in which pigment is present on the front of the iris is
a dominant, the absence of such pigment being recessive.
When the pigment is present in some quantity the eye is
called brown or black, while when it is absent the eye is
called blue or grey. In general terms therefore it may be
said that brown eyes are dominant to blue eyes. Careful
examination however shows that many eyes which might
on a hasty glance be called blue really have some of the
characteristic pigment. Casual descriptions therefore made
by use of the popular names for the colours are quite
unreliable, and the propositions based on them can only be
received with great caution. Some further details and
pedigrees are given in the Chapter on Eye-Colour (see
p. io8).

With respect to hair-colour in our own population
nothing can yet be said with much confidence. The segre-
gation of red hair from black hair may be seen in many
families and this red is presumably a recessive, but to work
out the interrelations of hair-colours in general would be
a very difficult undertaking"^. Just as in the case of eye-
colours, so here, the attempt to force the various colours
into a continuous colour-scale and to classify the material
by reference to that scale is useless ; for though probably
intermediates could be found existing in such gradations as
to bridge the gaps between the more distinct types, there
cannot be the least doubt that so soon as a strict method of
analysis is instituted the various intermediates will be shown
to be caused by the interactions of a limited number of
definite factors. (See Intermediates, Chap, xiii.) In the
analysis of such phenomena research must proceed by the
detection of the pairs of factors, beginning with the more
obvious, and when their behaviour and powers are thoroughly
understood, a search for the remainder may be attempted.

The fact of continuous descent through many genera-
tions creates a probability that several notorious family
characteristics, such as the Hapsburg lip and many more
which could be cited, would prove on examination to be
• dominants, but the evidence for a proper analysis has not

* Cp. Hurst (162).

XIl]

Mendelian Heredity in Man

207

hitherto been compiled. Two such cases may be given.
The first is that of a pecuHar form of short woolly hair,
resembling that of the negro (Gossage (132), on the

^ o

~r
o

1^
o

"1
o

Fig. 21. Descent of a peculiar form of curly hair recorded by Dr Walter
Bell. (After Gossage, 132.) The black symbols are the affected
members.

authority of Dr Walter Bell). Family tradition attributed
this peculiarity to a "Mexican" ancestor several generations
back. The peculiarity was transmitted directly through
those who exhibited it, as shown in the diagram (Fig. 21).

The second is a similar inheritance of a lock of congeni-
tally white hair recorded by Rizzoli, which clearly behaved
as an ordinary dominant (see Fig. 22).

r
o

«r •

I I' i I II II I rrn
o#ooo®@ (g)omo

cf #•

~r~i

GOO

Fig. 22.

Descent of a congenital lock of white hair, recorded by
Rizzoli. (After Gossage, 132.)

There can be litde doubt that if pains were taken to
record the descents of striking features many such pedigrees
could be compiled. It is of course necessary, if oral testi-

2o8 Mulattos [CH.

mony and tradition have to be appealed to, that the
characteristic should be a definite one about which mistakes
could scarcely occur. It Is necessary also that the feature
should not be dependent on external influences. For
example, in many families a crooked little finger Is common.
This peculiarity may descend both through the affected and
through the unaffected, but the affection, I believe, is like
that seen in chickens whose feet may be deformed through
weakness. In some families such chickens occur commonly,
but with much greater frequency when the incubators have
been working badly, a fact which suggests that it Is the
disposition to the peculiarity which Is transmitted, not the
peculiarity itself Probably the crooking of the little finger
is an expression of a certain weakness, which if circum-
stances are favourable need not become apparent. Pedigrees
of such peculiarities cannot be expected to give results of
much positive value.

As to the results of inter-crossing between distinct races
of mankind there exist, so far as I am aware, no records of
that critical and m.inute sort which are alone of value for the
adequate study of Mendelian inheritance. For example,
the statement has been repeatedly made that the mulatto
formed between the European and the Negro breeds true
to an intermediate type, but we still await precise data in
support of this statement. It may well be that there is no
sensible segregation In regard to colour In that case, and
certainly many intergrading colours exist, but there is no
material yet upon which a definite pronouncement can be
made. As regards length and curliness of hair, Indeed, I
anticipate from casual observation that the consequences of
segregation could be made evident without very much
difficulty. With regard to skin-colour the general trend of
evidence is in favour of the conclusion that If definite
determining factors are responsible for the colour seen, the
number of such factors or of their subtraction-stages must
be considerable. A point not to be forgotten is the diversity
not only of the negro races, but of the white population,
from which the mulattos arise.

From such information as I have been able to glean
from travellers I am disposed to think that in the crosses
between white races and the Inhabitants of India there are

xii] Htnnan Stature 2og

signs of a segregation more complete than exists among
mulattos^.

Mr Mudge has pubHshed also reports of the existence of
segregation in regard to several characteristics in the cross-
bred offspring of Canadian Red Indians and Europeans
{Nature, Nov. 7, 1907).

Pending further research we must take it that the
mulatto is probably a genuine exception, in so far at least
as no obvious segregation normally occurs.

Respecting the genetics of other normal characteristics
in man, such as stature, little progress has been made. The
case of human stature is interesting as it furnished material
for Galton's original determination t of the Law of Ancestral
Heredity spoken of above (p. 6). We may feel assured
that this deduction is to be interpreted as meaning that the
number of factors involved in deciding human stature is
large — which indeed is evident, if it be remembered how
many kinds of physiological difference must contribute to
the determination of the length of the body. Something
must be decided by the number of cells in the leg-bones,
vertebrae and various cartilages, by the size of these cells,
by the amount and density of the bony substance secreted,
by the shape of the skull, by the obliquity of the neck of the
femur, by the curvature of the spine, and by many elements
of bodily proportions, some easily influenced by conditions,
which interact with each other to produce the single
''character" we call height. Such a character could only
show segregation if it were studied in families made by the
inter-crossing of extreme forms each breeding sensibly true
to type. It is perhaps a little remarkable that in the pea
and the sweet pea segregation in height should be so
marked, but the reason is evidently to be found partly in
the immense differences there available for study, and in
the existence of one predominant contributing factor, the
internodal length.

* There is a sood deal of scattered information which tends to show
that even among negro-mulattos segregation sometimes occurs. See, for
instance, W. Lawrence, Lectures on FhysioL, 1823. p. 259.

t N^atural Inheritancef 1889.

B. II. 14

2IO Hufnan Heredity [ch.

Dominant Hereditary Diseases and Malformations

Though knowledge of genetics of normal characters in
man has advanced so little, we have now clear evidence as
to the laws of descent followed by many striking peculiarities
which are of the nature of deformity or disease. It is
somewhat singular that nearly all the abnormal features
(except those which are sex-limited) that have been yet
positively shown to follow Mendelian rules in man are
dominant to the normal. There are indications that
certain abnormal conditions are recessive, but in two of
these only is there much evidence. Dominants are of course
much easier to trace, as the peculiarity then descends
directly from parent to offspring, and so a continuous
history is provided. Probably it is to this circumstance
that the comparative plenty of evidence respecting the
dominants is due.

Brachydactyly.

A good example of these dominants, which indeed was
the first Mendelian case to be demonstrated in man, is that
described by Farabee. The peculiarity consists in a short-
ening of the fingers and toes, which had only one phalangeal
articulation like the thumbs instead of two (Figs. 23 and
25). The condition is said to have been the same in all
the fingers and toes of the affected members of the family.
The descent is represented in the diagram. As there indi-
cated, the peculiarity descended solely through the aftected,
and the children of the unaffected did not in any single
instance reproduce it, for they were evidently pure reces-
sives. The offspring of the affected, on the contrary,
consist of affected and unaffected, in approximately equal
numbers (36 : ^io)- The affected parents in each case
married with normal persons, so these unions are all of the
form DR x RR, and the equality of affected and unaffected
is in accordance with Mendelian expectation.

Farabee's families are American, being centred in Penn-
sylvania, and the diagram shows records referring to five
generations. Recently Drinkwater has published a full and

XII]

Brachydactyly

2X1

excellent account of the same peculiarity as manifested in
an English family extending over seven generations. In
his cases also the same law of inheritance is strictly fol-
lowed, the normal offspring of abnormals being always free
of the abnormality ; while the abnormals marrying with
normal persons produce on an average equal numbers of
affected and unaffected children (39 : 32, countmg those only
whose condition is definitely known).

There are several physiological questions of importance
arising out of this case. First, as to the exact nature of

Fig. 23. Brachydactylous hands. (After Farabee, 122.)

the malformation, it seems to be clear that in all abnormal
individuals examined the fingers and toes are all alike in
being short and in having only one phalangeal articulation.
Farabee from his radiographs concluded that a phalanx was
definitely absent in all cases,but Drinkwater in the extensive
series studied by him proved that in some individuals there
is in digits III and IV a distinct representation of the
middle phalanx as well as the proximal and terminal. Mis
view is that the basal epiphysis of phalanx 1 1 is absent, and
that this absence constitutes the essential malformation.

14—2

212

Brachydactyly

[CH.

Perhaps the epiphysis of phalanx III is sometimes also
unrepresented and it may be that phalanges II and III are
sometimes originally one cartilage, but there is not always
complete union between these phalanges even in adult life
(compare Figs. 24 and 25).

Another point of exceptional interest is the fact that
all the abnormal persons were unusually sho7^t in stattire
as well as short in the fingers and toes. This association
of characters was equally pronounced in Farabee's and in

■it*,^f^

A B

Fig. 24. Skiagrams of normal {A) and brachydactylous {B) members
of Drinkwater's family. Both are adult. (From his photographs.)

Drinkwater's families. The latter author gives many details
of measurement from which it appears that the average
height of the normals exceeds that of the abnormals in
males by 8J inches and in females by 4f inches. The dis-
proportion between the head-lengths was relatively much
larger. Upon what structural peculiarity the reduction in
stature depended was not discovered. There was no indi-
cation that the length of the limb-bones was out of pro-
portion to that of the trunk. The whole series of facts has

XII]

Brachydactyly

213

obviously a close bearing on the nature of meristic varia-
tion, but a discussion of that problem is beyond m)- [present
scope^.

If we were dealing with natural species or varieties a
debate might arise on the question whether it is readily
to be imagined that so definite a variation could have arisen
independendy in the two sets of families, in America and
England respectively. To those who have experience of
variation and who know how large and well defined discon-
tinuous variations may frequently be, it will not seem much
more difficult to conceive of the repetition of the variation

Fig. 25. Hands of brachydactylous woman, age 32. A separate ossification
can be seen in digit III between the 1st and 3rd phalanges. In the
other digits union of this element with the 3rd phalanx has already
taken place. (From Drinkwater's unpublished photograph.)

than of its first occurrence. There is however some plausi-
bility in the suggestion that these two families may in
reality be one in origin, for it is known that a male abnormal
of the 4th generation in Drinkwater's strain did emigrate
to America. On the other hand the abnormal first recorded
in Farabee's strain was a female in the 5th generation from

* It must be observed that in view of Drinkwater's facts the variation
is not simply meristic in the sense that the digit divides into two joints
instead of three. As he himself is inclined to suppose, the case is more
probably to be regarded as a homoeotic variation of the digits into the
likeness of the hallux and poUex.

214

Brachydadyly

[CH. XII

the youngest. There Is consequently no very probable
correspondence between the two, but their connection is not
impossible on our present evidence.

Fig. 26. The pedigree of Farabee's brachydactylous family.

N^ normal. A^ abnormal.

Another type of brachydactyly has also been recorded by
Walker ^. In it there was an imperfect union of the phalanges
affecting especially the first and second phalanges of the
middle and ring fingers. The degree of the affection varied

i i i ^ cf T T t

? ?

5 T

^ ?

T ? <? f i

9 i

T ! n T^T ? f ? ?!

LLl J_ 1

Fig. 27. Pedigree of affected members of Farabee's brachydactylous
family. Black symbols are atfected persons.

"^ Walker, G., /o/ins Hopkins Hasp. Bnll., xn. 1901.
author 2 unaffected in family of John B. are deleted.

In copy sent by

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2l6

split Hand

[CH.

in different individuals. The pedigree shows that this con-
dition descended through the affected in four generations,
but the descendants of the unaffected are not recorded.
According to this table the families of the affected add up to
Affected 8 ; Unaffected 15 ; not known 2.

A correspondent has sent me a pedigree of a family in
which the fingers were almost entirely aborted. The descent
was that of an ordinary dominant. The families of the
affected add up to 19 affected and 16 unaffected. The
children of the unaffected were normal in all cases.

The condition known as "split hand" and ** split foot,"
in which these extremities have a monstrous, claw-like

mZTH"!-^;?

Fig. 29. Descent of prae-senile cataract. (Condensed from Nettleship's

chart.)

appearance also descends as a dominant"^. Partial suppres-
sion of the digits is one of the features of this abnormality.
The degree of malformation varies greatly among the
affected individuals of the same family, and though there is
generally a rough correspondence between the two hands
and the two feet of the same person the symmetry is only
partial.

A considerable list of abnormalities following a course
of heredity similar to that of the short digits can now be
given. Of these a few only are mentioned here.

* For full collection of evidence see Lewis, T., and Embleton, D. (170).

XIl]

Cataract

217

^ Cataract, A study of the extensive collection of
evidence made by Nettleship shows that several forms of
cataract are usually transmitted as dominants, the descent
passing through the affected persons. This is shown most
clearly in regard to the congenital varieties. In these the
cataract is present and generally recognized very early in
life. Some of the family records exemplifying descents of
this kind are very extensive. Occasionally there is descent

i^Jii

^^ M^^f^ <:^

Fig. 30. Descent of prae-senile cataract. The condition was here
transmitted, exceptionally, through one individual who was probably
unaffected. (Condensed from Nettleship's chart.)

of the cataract through a parent recorded as unaffected, but
the course of the transmission is usually normal. Whether
in the exceptional cases the apparently normal persons who
transmitted were in reality slightly affected cannot yet be
said, but it is noticeable that several of these alleged excep-
tions occur in the case of progenitors whose state is only
known by tradition (Fig. 30).

Cataracts acquired later in life, and even the senile forms,
seem, so far as the evidence goes, to follow the same rule in

2i8 Cataract [ch.

their descent. But in dealing with this evidence difficulty
arises from the fact that the age at which the cataract
appears is liable to great variation and consequently the
negative record, that a person was not affected, is of doubt-
ful value, for the affection might have been due to appear
later in life. As we should expect, however, it is in respect
of cataracts of this kind that transmission through the
unaffected is recorded with considerable frequency^.

There are several examples of families showing even
congenital cataract in several members where the affection
seems to have arisen de novo, no history of cataract being
discoverable in the parentage. With regard to the sub-
sequent transmission of such cataracts no very definite
information at present exists.

As there is obvious doubt of the completeness of the
records in many instances it is not possible to give numeri-
cal data with exactitude. A fair idea of the general run of
the numbers may be gathered from the following list of the
more complete families. These are somewhat arbitrarily
selected from Nettleship's collection, but all the more
extensive families are included f. Some of the more
aberrant numbers even in these more perfect records are
almost certainlv due to the sources of error named above.

Recorder

Nettleshif s case-number

Affected

Unaffected

Not hiown

Berry

Fukala

Nettleship

25 +

Bandon

Froebelius

Gjersing

Fisher

108

Zirm

112

Nettleship and

Ogilvie [

212

T48

155 +

* It should nevertheless be remarked that the appearance of the
cataract at earlier ages in successive generations — "anticipation," as
Nettleship calls it — has been observed with some frequency.

t Except No. 98, Green's family, where the cataract appeared at
various ages, sometimes not till advanced life. The numbers in it are
18 affected, and 37 unaffected. Transmission here occurred twice through
the unaffected. Since this table was made Nettleship has published some
new and important pedigrees (210). One of these gives a isixnily from two
DR parents with senile cataract. Up to time of writing their children
were 7 affected, 2 unaffected, and i died at 45 with sight good.

XII] Tylosis 2 1 9

A valuable collection of pedio^rees illustrating Mendelian
descent chiefly of several affections of the skin and hair has
been published by Gossage. All the conditions there con-
sidered behave as dominants, and respecting several of these
the evidence is fairly adequate (132).

Respecting tylosis palijiaris et plantaris the records are
especially clear. There is a condition in which the skin of
the palms and soles is abnormally thick in varying degrees ''^.
From a study of over thirty family groups affected with
this peculiar thickening of the skin it appears that trans-
mission was always through affected members except in the
case of one family historyf. Adding together all the
available recorded numbers of children born to affected
parents there were 220 affected and 184 unaffected. Since
from the nature of such records it is practically certain that
affected members of families would be less likely to be
missed or forgotten than the unaffected, we may regard
these numbers as not an impossible representation of an
actual equality. In view of the complete escape of the
ll offspring of the unaffected except in one family tree the fact
that tylosis (called also keratosis) almost always behaves
as a simple dominant may be regarded as established.

One family group was unique in the fact that the
ancestor first showing the disease came from parents
believed not to have had it. This person had a family of
12, all affected In general we know nothing definite as to
the origin of such variations, and whether the unitorm

* Compare the famous family of " porcupine-men " often alluded to by
the earlier anthropologists. A good account of this family is given by Sir
W. Lawrence (Z(?^/z^;'<?i', 1823, p. 385), who remarks : — ''Let us suppose that
the porcupine family had been exiled from human society, and had been
obliged to take up their residence in some solitary spot or desert island. By
matching with each other, a race would have been produced, more widely
different from us in external appearance than the Negro. If they had been
discovered at some remote period, our philosophers would have explained to
us how the soil, air, or climate, had produced so strange an organization ;
or would have demonstrated that they must have sprung from an originally
different race ; for who would acknowledge such bristly beings for brothers?"

t This is the family recorded by Ballantyne, Pediatrics, 1896, i. p. 337.
This family was peculiar not only inasmuch as transmission occurred
through the unaffected, but also in the fact that the condition appeared
in females only. This sex-limitation suggests that the etiology of the
condition was in this family distinct from that of the ordinary tylosis.

220 Various Human Diseases [cii.

affection of all the family of the first case is in any way
connected with the sudden appearance of the condition
cannot of course be said.

The other conditions which Gossage finds to be domi-
nants as a rule are :

Epidermolysis bullosa : in which the skin is liable to
blister for trifling causes.

Xanthoma : the presence of yellow patches on the skin.

Multiple Teleangiectasis : small naevus-like spots.

Hypotrichosis congenita familiaris : the loss of hair at
or soon after birth.

Monilithrix : a nodose condition of the hair.

Porokeratosis : a curious disease in which a raised horny
ridge appears on the skin, spreading centrifugally, leaving
behind it a patch in which the constituents of the skin
undergo partial atrophy.

The spontaneous origin of porokeratosis has several
times been observed. In epidermolysis transmission through
unaffected persons occurred in some of the strains but was
exceptional.

Besides these skin-diseases Gossage suggests that en-
larged spleen is generally transmitted as a dominant.
There are also four pedigrees showing the same pheno-
menon in the case of the condition called diabetes insipidus
or polyuria.

To these may be added with some probability certain
forms of inherited oedema and some of the kinds of hare-
lip.

The most extensive pedigree that has yet been compiled
for any disease in man is that for congenital stationary
night-blindness, of which the first part was published by
Cunier in 1838*. Working on this foundation Nettleship
succeeded in obtaining material for a most elaborate
genealogy including 21 16 persons (Fig. 31). The aftection
consists in a marked inability to see in a dim light. In this
family the transmission is through the affected, and no

* Annates Soc. Med. de Gafid, 1838, p. 383. [Not seen : quoted from
Nettleship.]

cii.

ay
311

Fig. 31. Descent of a form of stationary night-blindness. (Condensed fro
Only those families which contain affected members are here set out in
by the 16^^ black symbol from the left in generation vii.

Black symbols show the night-blind individuals. The descent is
According to the records there is a great excess of normals over the affe
theless the table gives a remarkable illustration of the permanence and i

r~i

LTl

~7K-

?-■ V

JJ^sulS

6^44 smUlS 4632 4 p4^^2ii^

6<>33 51

3 4

49 '^''^^fri^fm V

6 583^5 2574^3, m^ i 19 /sViliJ 2 5 39 W

4 24

2666 47 5 6 5

2 3

* ^iH

published by Nettleship based on Ciinier's records with later additions )
-ffected man {I?J^) who married the affected woman (D/^) is represented

,^h the afifected, showing that the condition is due to a dominant factor,
not De said that the responsible factor is a simple allelomorph. Nevcr-
nt ot a do m man t variety.

\^o vin

XII] Diseases of the Eye 221

departure from this rule has occurred In the ten orenerations

that have elapsed since the birth of the earliest known case

(born 1637). The people are peasants living in a group of

somewhat isolated villaoes in the south of France. Addino-

... '^

together all the families which can be regarded as the off-
spring of affected and presumably heterozygous {^DR)
individuals mated with normals the numbers are 130 affected
and 242 unaffected*. These figures depart very widely from
the expected equality. Nettleship thmks there are reasons
for supposing the number of affecteds somewhat understated,
through a disposition to conceal the infirmity This cannot
however account for the whole discrepancy, since error
introduced from this cause would lead to the production of
affecteds from alleged normals which has never occurred.
It is of course possible that several who died before their
condition was ascertained may have been counted as normals
In past generations.

Qualitatively the descent is evidently that of a dominant,
and though perhaps these aberrant numbers point to a
genuine complication such a conclusion can scarcely be
drawn positively from materials of this kind.

It Is somewhat remarkable that stationary night-blindness
usually follows a sex-limited descent, and the absence of any
sex-limitation in the genealogy just spoken of, suggests that
the physiological nature of the aftection in this family may
be distinct. There is, I understand, nothing which yet
differentiates the one condition from the other unless it
be that the sex-limited kind is usually associated with
myopia which was not conspicuous here.

In the recorded pedigrees there are indications that, in
addition to those already mentioned, the following ophthalmic
diseases or malformations may be transmitted as dominants
sometimes, though the evidence does not justify a compre-
hensive general statement and many exceptional cases are
known.

Distichiasis : development of eye-lashcs in place of
glands on inside of eye-lids (perhaps a homoeotic variation).

* In one case an affected pair married and their two daughters were
affected. Of these one (who may have been £fD) had two affected
children. The totals given above are arrived at after deduction for this
family.

222 Sex-limited Diseases [ch.

Ptosis : droop of the upper lid.

Coloboma or h^ideremia : a congenital defect in or
absence of the iris.

Ectopia lentis : dislocation of the lens.

Glaucoma is a condition arising from various distinct
pathological causes. From the pedigrees I have examined
it is likely that in some of its forms the dominance rule is
followed^.

The heredity of these various conditions could only be
discussed adequately at great length. To do this the time
is scarcely ripe. Owing to the interest now taken in the
problems of genetics evidence is being rapidly accumulated
and detailed analysis is best postponed till this new informa-
tion becomes available.

Dominant Diseases or Variations in Man following a

Sex-limited Descent.

In discussing the phenomenon of Sex allusion was
made to the sex-limited diseases (p. 172). The best known
of these are :

Haemophilia : the liability to bleed profusely from trifling
cuts or abrasions.

Colour-blindness : in its commonest form an inability to
distinguish certain reds and greens.

P setido- hyper tr op Jiic m^uscular paralysis, or Gowers
disease.

Nizht-blindness of certain kinds.

The peculiarities in the descent of these conditions are
that :

(i) They affect males much more commonly than
females.

(2) They may be transmitted by the affected males t,
but are not — save in very rare exceptions (haemophilia) —
transmitted by unaffected males.

* See for instance Howe, Arch, of Ophth., xvi. p. 72.
t Patients sufifering from Gowers' disease rarely survive to adult life
and practically nothing is known of their powers of transmission.

XIl]

Colour-Bliiiduess

223

(3) They are nevertheless transmitted by the tinaffected
females. Apparently normal women, dauo^hters or sisters
of the affected males, thus may transmit the condition to
some of their so7is (Fig. 32).

Such a system of heredity has long been a physiological
paradox, and one of the most curious and interesting de-
ductions from Mendelian research is the clue which it has
provided to the solution of this problem of sex-limited
descents (cp. p. 173).

The experiment dealing with the inheritance of horns
in sheep showed very clearly the probable lines on which
an explanation was to be found. Just as in the sheep the
horned character is dominant in males and recessive in
femaleSy so with these sex-limited conditions. If the males

i rrtn

^W^^iU^mk t^mrk

Fig. 32. Pedigree illustrating descent of colour-blindness. This family
was found by Dr VV. H. R. Rivers among the Todas, a hill-tribe of
Southern India.

contain the factor for the condition, they exhibit it ; conse-
quently the affected males can transmit, while the unaffected
males cannot. In the females, on the contrary, something —
almost certainly the presence of some other factor — prevents
or inhibits the development of the condition, and then they
may possess the factor without its making itself apparent.
Such females may then transmit it to their offspring, but
it will only be visible in the males, except in tlie rare case
of a union between a heterozygous female and an affected
male. Then the female children also may be aftected,
because they may be homozygous to the factor, receiving
two " doses " of it, one from their father and on^ from
their mother.

224 Colour-Blindness [ch.

According- to the simple plan thus sketched, all the sons
of the affected females will have the affection and exhibit it.

The only one of the notoriously sex-limited conditions
which is available for testing this rule is Colour-blindness.
Haemophilia and Gowers' disease are too fatal, and night-
blindness is too rare and too little known. Regarding Colour-
blindness even, the evidence is not enough yet to provide a
statement as to the quantitative results. A search undertaken
by Mr Nettleship has however been successful in so far that
he has obtained evidence of five colour-blind women with
sons, in all eleven, who are all colour-blind. In three cases
also there is, in addition, evidence that the women's fathers
were colour-blind, as required by the scheme ; and in one
case also there is a record that the woman came from a
strain having colour-blind members. Besides these there
are two published accounts of families of two colour-blind
women, each with three sons, all colour-blind. In all there-
fore the seven colour-blind women had ly sons, all colour-
blind. We can I think feel no further doubt that the
scheme must so far present an approximation to the actual
facts. In dealing with such phenomena exceptional cases
must be expected, but hitherto they have not been found ''\

Colour-blindness is not, therefore, as might have been
imagined, a condition due to the omission of something
from the total ingredients of the body, but is plainly the
consequence of the addition of some factor absent from
the normal. We can scarcely avoid the surmise that this
added element has the power of paralysing the colour-sense,
somewhat as nicotln-poisoning may do.

The other sex-limited diseases in all probability follow
similar rules, but in regard to haemophilia there are
various difficulties. The records are too heterogfeneous
for satisfactory tabulation as yet, and it is to be suspected
that more than one condition may pass by the same name.
One remarkable feature must be mentioned, namely that
the records show with great constancy that too many males
are affected and too many females transmit, the excess being
far greater than any which could readily be ascribed to
recorders' errors. This excess is also strongly exemplified

* By the kindness of Miss J. E. Downey of Wyoming University I
have since been informed of a case which is probably a real exception — a
woman with defective colour-sense having; a child with normal colour-vision.

XII] Recessives in Man 225

in the case of another sex-hniited disease — a peculiar
paralysis of a "peroneal" type, studied by Herrinohani*
Some definite disturbing complication must be looked for
in these phenomena, and the statement that the factor is
dominant in males, and recessive in females, can only be
taken as giving a qualitative description of its behaviour.

In Gowers' disease we have perhaps also to deal with
more than one condition, and the evidence suggests that
the recessiveness in females is not universal.

Tabular representations of the most probable expecta-
tions in the case of a sex-limited condition are given in
Figs, II and 34.

Recessive Variations in Man.

As so many abnormalities are known to behave as domi-
nants with some consistency it is perhaps surprising that
we have no quite positive case of pathological conditions
behaving as recessives in man. The evidence regarding
the normal light eye-colour has already been given, but this
as yet stands practically alone. Hurst (162) has given
some further facts suggesting that the musical sense is a
recessive, but great and obvious difficulties make it very
hard to obtain convincing proofs in that case.

In regard to diseases that may be recessive, there
are several records which are suggestive, but little that
amounts to proof. Naturally, as evidence of direct trans-
mission is not to be expected, the likeliest place to look
for recessives will be among those conditions which have
been noticed as coming with special frequency in families
resulting from consanguineous matings. In such matings,
and particularly in those of first cousins, bearers of similar
recessive characters may come together, and thus by the
meeting of two similar germs in fertilisation offspring
exhibiting the recessive character may be formed. The
comparatively frequent appearance of a variation among
the children of such unions is \\\w'^ prima facie a suggestion
that it is a recessive to the normal. This has been observed
very noticeably in regard to retinitis pioDientosa, a degenera-
tive disease of the retina. In Herrlinger'sf colleciion of

^ nerringham, W. P., Brain, xi. 1889, p 230.

t Herrlinger, " Ueb. d. Aetiologie d. Retinitis Pigmentosa," Jnaug,
Diss., Tubingen, 1899.

B. H. '5

226 Httina7i Albinism [ch.

records among 761 cases, 228 are said to have been offspring
of consanguineous marriages. It is not in dispute that the
condition may be produced by various specific causes also,
but the heredity through consanguineous marriages creates
a presumption that a group of cases may be of a recessive
nature.

It should perhaps be pointed out categorically that
nothing in our present knowledge can be taken with any
confidence as a reason for regarding consanguineous mar-
riages as improper or specially dangerous. All that can
be said is that such marriages give extra chances of the
appearance of recessive characteristics among the offspring.
Some of these are doubtless bad qualities, but we do not
yet know that among the recessives there may not be
valuable qualities also.

From w^hat is known of the genetics of albinism in other
types we should on analogy expect it to be recessive in
man. There is no reasonable doubt that this description
is true so far as it goes, but obvious complications are
met with. No union of two albinos is on record so far
as I know, but the frequency with which the albinos have
proved to be ofTspring of related parents, especially of first
cousins, justifies us in definitely inclining to the view that
albinism is in man a recessive character. The families
available for numerical comparison are scarcely well enough
reported for a proper analysis to be made, but it is clear
that they exhibit again the difficulty met with before in
human pedigrees, namely, that the affected are far in excess
of expectation (on the hypothesis that they are ordinary
recessives). For example, from a set of figures derived
from various sources'^ I get the numbers 115 albinos and
174 normals, where the expectation is only 72 to 216.
There are reasons for thinking these records of albinos too
high, and those of normals too low, but the discrepancy is
too larofe to be accounted for thus.

Human albinism differs from that of our domestic
animals in the fact that it shows many gradational forms
which connect it with the normal. There are also several

* I am especially grateful to Dr V. Magnus of Christiania for a series
of 10 albino pedigrees. One of these contains an extraordinary family of
which seven are stated to be albinos and only one pigmented.

XII] Human Albinism 227

records of congenital albinos acqiiirlnci^ pigmentation more
or less complete. Lastly, albinism in man differs greatly
from that in other forms in the fact that it is very often
associated with disease, especially of the nervous system.
Even nystagmus, the oscillating movement of the eyes so
generally associated with human albinism, is not, so far as
I know, met with in the pink-eyed rabbits, guinea-pigs,
rats or mice. In the cat however somethinor more like
human albinism is to be seen, for in that animal we find
the association of certain types of albinism with deafness,
and in it also several degrees of pigmentation in the iris
occur. Careful pedigrees of crosses with albino cats might
help to a solution of this problem. But in studying the
subject of human albinism and also that of retinitis pigmen-
tosa one meets not infrequently features indicative of a
widespread and multiform degeneration in the affected family
not at all resembling the simple course of Mendelian in-
heritance where natural variations are concerned. It is at
least doubtful whether there may not be some distinction
between albinism thus appearing and that less definitely
associated with disease.

An interesting observation is recorded by Stedman
{Stirinaui, 1806, 11. p. 260) to the effect that an albino
negress married to a European had children all mulattos.
Hence we may infer that the factor determining the black-
ness of the negro may be carried by the albino. The great
Irequency of albinos among several coloured races of men
has often been remarked on by anthropologists.

A rare condition known as Alkaptoiiuria in which the
urine is red from the presence of the substance alkapton
must surely be a recessive. The facts published by Garrod*
make it likely that the disease follows recessive lines, for
of 17 families in which cases have been seen, 8 were
offspring of first cousins. On the other hand Garrod gives

* Garrod, A. E., Lancet, 1902, Dec. 13, and Arch. f. Gcs. Physiolo^^ie,
Bd. 97, 1903, p. 410. The inheritance of cystinuria would be equally
interesting ; see Garrod, ibid, and Abderhalden, Ztsch. f. Physiol. Chcmic,
1903, xxxvni. p. 557. For a valuable discussion of the evidence see also
Garrod (127). Since that paper was published he has called my attention
to a fresh tamily discovered by Fromher/, Imiug. Diss., Strassburg, 1908,
containing 3 alkaptoniirics and 8 or 9 normals.

15-2

228 Irregular Phenomena [ch.

one case of direct transmission from an alkaptonuric
parent, and it is of course improbable that this extremely
rare condition should have existed on the other side of
the parentage. Though the evidence is scarce, it is very
important, since the question of the actual mode of
descent followed by such a "chemical sport," as Garrod
has called it, is of absorbing interest. Anyone who has
an opportunity should not fail to use every effort to trace
the descent of alkaptonuria.

Miscellaneous irregular Phenomena of Inheritance

hi Man.

There remain great numbers of pedigrees of malforma-
tions and diseases which in our present ignorance seem
altogether irregular. Of these some, for example, poly-
dactylism, are perhaps to be regarded as due to dominant
factors which can be inhibited or suppressed as a result of
the presence of other factors. In poultry we know by
experiment that the presence of extra toe may behave as a
dominant, following the simple rule with fair regularity, but
in other families the number of dominants produced is too
small and transmission may occur through normals destitute
of extra toes Such facts point to the existence of some
unknown complication in those families.

Several pedigrees of ectrodactylism, the deformity of
hands or feet by the absence of digits, are recorded in
medical literature. Transmission in these cases usually
occurs through the affected members, but the degree of
malformation is exceedingly irregular, and the number of
affected persons is higher than expectation. On the
evidence it is improbable that any simple Mendelian scheme
will express all these descents'^. In Fotherby's "split-hand"
family a case of polydactylism also occurred, and the con-
dition is not very rarely recorded as associated with the
more extreme forms of human monstrosity.

A vast literature exists relating to the heredity of various

"^ See for instance Fotherby, Brit. Med.Jour.^ 1886 (i), p. 975; Tubbv;
Lancet, 1894 (i), p. 396 ; for a full and more recent collection, also Lewis
and Embleton (171).

XII] Nervous Disease 229

forms of paralysis, of deaf-mutism, and of mental disease.
From the analysis of considerable collections of such evidence
it is clear that in the present state of knovvledire anv
reduction of the phenomena into a simple scheme is im-
possible. The first difficulty is that the pathology of thdse
diseases is obscure and their diagnosis often imperfect,
various dissimilar affections passing by one name.

Deaf-mutism, for Instance, though at first sight a very
definite phenomenon, may obviously be due to congenital
deafness arising from many distinct causes, and hence the
genealogies of deaf-mutism cannot be treated as though
they all related to a single physiological condition*.

Next In dealing with diseases of the nervous system it
must be remembered that many of them depend for their
appearance on the presence of external stimuli. Forms of
Insanity which appear when the individual Is subjected to
various strains and excitements may not appear at all if
these causes be absent. The element transmitted is evi-
dently the liability, not necessarily the developed condition.
The descent of such peculiarities Is therefore beyond the
range of our analysis.

The peculiar form of Insanity knowm as Hereditary
Chorea Is exceptional, in that It very clearly follows the
course of an ordinary dominant with few complications.
Adding all the families apparently from heterozygous {DR)
parents recorded as mating with unaffected persons 1 get
the totals 117 affected, 99 unaffected, as nearly approaching
the normal equality as we can expect when the nature of
the evidence Is remembered.

Note on Collecting Evidence as to Human Dcscoit.

As some persons may read this chapter who have not
leisure for study of the more elaborate phenomena ot
Mendelism a few words as to the collection of evidence may
usefully be introduced here. The one essential point in
such collections is that the normal members of families

* Dr C. J. Bond, Bn'f. Med. Jour., Oct. 28, 1905, calls attention to
evidence suggesting sex-limitation in certain deaf-mute families, sometimes
the male, sometimes the female being the affected sex.

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CH. XIl]

Collecting Evidence

231

should be recorded with as much care as the abnormals.
In all cases, where possible, inquiry should be made re-
specting the children of the normals. The sex and age as
far as possible of each individual should be noted. If the
condition studied be not a congenital one, the age at which
it appeared in each individual should be entered.

Dominant characters will in general be recognized as
such from the fact that they are transmitted through affected
persons only. The dominants will as a rule have had one
parent affected with the peculiarity and one parent free
from it. It is then to be expected that the children of such
dominants, resulting from their marriages with unaffected
persons, will be in equal numbers ^^^^r/ev/ and nornial.

Sex-limited domi^iant characters such as colour-blindness
and haemophilia affect one sex, generally the male, most
often.

c^f f f f cT cf cf

^f ?

cf ci

cf d

^ ^ ^ oT b-^ oT

Fig. 34. Tentative representation of the descent of coloiir-blintlness,
drawn up on the hypothesis stated p. 195, note. Symbols as in
Fig. 33. On this scheme a Jioviozy^:^ous colour-blind male cannot be
produced even by the union of two colour-blind parents.

232 Heredity and Pathology [ch.

The simplest kind of sex-limitation is illustrated in
Fig. ^iZ' -No example is yet known in man which exactly
follows this system, though perhaps haemophilia and night-
blindness may approximate to it. Colour-Blindness, as
stated above (pp. 172 and 195), shows remarkable com-
plications in the fact that not merely the development of
the peculiarity but also the actual transmission of the
responsible factor is affected by sex. Fig. 34 gives a ten-
tative representation of the system which is perhaps followed
by such a condition, but it is given rather as a hint to
collectors of evidence than as a positive statement of as-
certained fact.

Recessive characters will be recognized by the fact that
they may appear in the children of parents not exhibiting
such characters, and especially among children born of
consanguineous marriages. Complete proof of the recessive
nature of a characteristic will only be obtained by evidence
that all the children of affected parents exhibit the charac-
teristic.

In all these descents occasional exceptions are to be
expected, which in the light of present knowledge cannot
be interpreted; but when the habitual course of the in-
heritance has been determined, an inquiry into the nature
of these exceptions may perhaps be undertaken with
success.

A correct knowledge of the system which a hereditary
disease follows in the course of its descent will, it may be
anticipated, contribute to a proper understanding of the
pathology of these affections, and thus make a definite
advance in the general study of the physiology of disease.
If, for example, a disease descends through the affected
persons, as a dominant, we may feel every confidence that
the condition is caused by the operation of a factor or
element added to the usual ingredients of the body. In
such cases there is something present, probably a definite
chemical substance, which has the power of producing the
affection. Thus the abortion of the digits, or the inability
to distinguish certain colours, must be due to such added
factors. On the contrary, when the disease is recessive we
recognize that its appearance is due to the absence of some
ingredient which is present in the normal body. So, for

XII] Heredity and Pathology 233

example, albinism Is almost certainly due to the absence
of at least one of the factors, probably a ferment, which is
needed to cause the excretion of pigment ; and, as Garrod
has shown, alkaptonuria must be regarded as due to the
absence of a certain ferment which has the power of de-
composing the substance alkapton. In the normal body
that substance is not present in the urine, because it has
been broken up by the responsible ferment ; but when the
organism Is deficient in the power to produce that ferment,
then the alkapton Is excreted undecomposed and the urine
is coloured by it.

Similarly when we find that a condition such as retinitis
pigmentosa sometimes descends in one way and sometimes
in another, we may perhaps expect that a fuller knowledge
of the facts would show that more than one pathological
state may be included under the same name.

It need scarcely be remarked that when a disease, such
as tuberculosis, which is due to a pathogenic organism,
affects certain families or strains with special frequency, the
hereditary or transmitted property is either the presence of
something which renders the organism specially liable, or
the absence of something which confers a higher degree
of resistance. From the nature of the case pedigrees are
not of much service In the analysis of these examples, for it
cannot be asserted that an individual who escapes under-
went the same risk of infection as those who took the
disease. Though it cannot be doubted that the study of
the descent of disease-resistance is of the highest import-
ance, both theoretical and practical, progress must here
be made by careful breeding experiments with animals and
plants which can be tested by uniform methods of infection.
As yet we know only one clear example in which such a
rule of descent has been ascertained, namely that discovered
by Biffen (29), who proved that, in wheat, resistance to
Rust-disease is a recessive. In that classical example there-
fore we may suppose that this resistance is due to the absence
of some ingredient which is present in common wheats
and enables the rust-fungus to attack them successfully.
Varieties in several degrees possessing the property of re-
sisting disease are known in many orders of plants, and
though regarding animals less is known, there are some

234 Heredity mid Pathology [ch. xii

indications of a similar character. An attractive field of
research is thus provided. We may surmise that various
types of resistance will be found, some dominant as well as
others like that of wheat, which are recessive. Whatever
answers are obtained in regard to these critical distinctions
and their physiological nature, the evidence must in no
ordinary mieasure contribute to the advance of general
pathology.

CHAPTER XIII

INTERMEDIATES BETWEEN VARIETIES AND THE
"PURE LINES" OF JOHANNSEN.

Misunderstanding in regard to the physiological sig-
nificance of intermediates has been a fertile source of error,
and more than any other perhaps has tended to obscure the
true interpretation of genetic phenomena. The existence
of intermediates has been often alleged as a proof that
segregation does not occur in regard to the particular
variations towards which the gradational forms seem to
lead, and the misuse of statistical method so frequent in
biometrical attempts to investigate heredity has come about
chiefly through misinterpretation of the nature of such
gradational forms.

These errors are of course a legacy from the period of
the essayists, when evolution at large was the chief object
of study, and an examination of variation in detail as
occurring in specific instances had not been undertaken.

Biologists committed to the proposition that varieties
arise through the transformation of masses of individuals
by the selective accumulation of minute differences saw that
with each new case in which discontinuous variation could
be proved to occur, the scope for their views was reduced,
and the existence of intermediates constituted the most
promising line of defence. When intermediate and gra-
dational forms could be found, the contention mi^ht always
be hazarded that the definite and extreme forms had been
reached by transition through them.

Now the application of analytical methods to those

236 Intermediates [ch.

cases where much gradation occurs is usually troublesome,
but it is becoming clear that the difficulty is often a practical
one only. To analyse the intermediates and to determine
the various elements that have taken part in their pro-
duction may be a very laborious process, still it can be
shown in many examples that the intermediate character is
only a superficial or nett result of the interaction of factors
which are transmitted as units The several classes under
which such intermediate types present themselves are worth
enumerating.

I. Intermediates as heterozygous fo7n7is. Sometimes,
as is the case of the blue Andalusian fowl, the whole group
of heterozygotes forms a recognizable, distinct class which
might nevertheless be regarded as intermediate between
the two pure types black and white, with splashes of grey
and black. Similar intermediates occurring as heterozygous
forms have now been often seen and described. In the
case of P^dmitla Sinensis the heterozygote between the
star-shaped flower of the modern stellata varieties and the
common type with imbricated petals is always an inter-
mediate^.

In fowls though no perfectly distinct heterozygous type
of comb exists, yet with experience it is usually possible to
distinguish the pure {DD) pea combs from the DR group,
and, though with much less confidence, the same distinction
can sometimes be made among the rose-combs.

In Lychnis the crosses between v/hite vespertina and
red dioica are, so far as I know, always of an intermediate
shade of pink. In Piston the crosses between the races I
have called ''half-dwarf," standing about 3 to 4 feet high,
and actual dwarfs, about i foot high, are usually intermediate
in heio^ht. The researches of Miss Wheldale on Antirrhi-
nuni have shown that in that species numerous intergrading
forms are definite heterozygotes, and Miss Marry at has
proved that the same is true for Mirabilis.

Such instances may be multiplied indefinitely« Domi-
nance indeed is not often so pronounced that a practised

■^ Whether any such intermediate flower shape exists as a gametic form
which can be bred true I do not know for certain. Mr Gregory and I have
not met one among our derivatives from various crosses between stellata
and Sinensis.

xiii] hitennediates 2'x

observer cannot distinguish the DD types from the DRs
with fair certainty by a thorough and minute examination.
As examples in which heterozygotes are indistinguishable
from pure dominants, may be mentioned tall and dwarf in
Peas and Sweet Peas, coloured flowers and white flowers
in Sweet Peas and Stocks, hoary and glabrous in Stocks.
In all these the one " dose " of a dominant factor is sufficient
to produce the full effect.

2. Intermediates due to siibtraction-stages of dominant
factors. This is also a large and important class, one, too,
which has been perhaps more especially a source of the
confusion alluded to in the introduction to this section.
Of such intermediates the colour of the Dutch rabbit may
be taken as a type. The Dutch rabbit has the posterior
half of the body coloured, while the anterior half is white
except for patches including the eyes and ears. Such a
rabbit may be described as in a sense intermediate between
the self-coloured rabbit and the albino. Its particoloured
nature is due to the restriction of one (or conceivably more)
of its pigment-factors to certain areas. In the self-coloured
animal this factor has a more extended distribution. But
in speaking of the Dutch pattern as an intermediate, an
essential point, the definiteness of the Dutch pattern, is
missed. It is intermediate only in a strictly limited sense.
It is not a transition-stage between a self-colour and an
albino, nor would there be any better chance of breeding
albinos from genetically pure Dutch than there would be
from genetically pure self-coloured rabbits. The Dutch
pattern in fact constitutes a definite type and depends for
its appearance on the presence of a certain factor in a
fairly definite stage, and it does not appear when self-
colours are crossed with albinos unless the particular
necessary factor in its appropriate stage is introduced in
one of the parents.

Within the Dutch class there are subordinate types
again, the interrelations of which have not yet been worked
out. Some of these are very probably gametic types which
could be bred pure, while others are probably caused by
fluctuations of an irregular kind or by the interactions
between the factors. It will need very elaborate research

238 h^termediates [ch.

to determine the interrelations of these finer differences.
As one of the few points that are clear may be mentioned
the impossibility of fixing a form with the sharply-cut
division between the white and the colour which is the
fanciers ideal for Dutch. From this negative evidence it
may be inferred that that particular distribution is not one
of those represented as a gametic type.

Intermediates of this sort are commonly met with in
breeding. The "half-dwarf" peas are an instance. In
Stocks Miss Saunders has described a *' half-hoary " race
(19, p. ^'^ in which the lower surfaces of the leaves were
hoary while the upper surfaces were glabrous or nearly
so. No such ''intermediates" have occurred among the
thousands of plants raised by Miss Saunders except when
the definite '' half-hoary " type was originally introduced as
a parent. In the Sweet Pea one of the most definite and
unchanging types is the nearly white flower with a pink
streak, that comes next to the pure white if judged by the
criterion of amount of deficiency of colour. In the absence
of breeding experiments an ignorant person having raised
such a plant as offspring of a purple or a red might consider
that he had made an advance towards pure white. Never-
theless he would be no more likely to raise true whites from
such a tinged white than if he began with a wild Sicilian
purple pea. Indeed if I required to raise a white Sweet
Pea de novo, I should think the chance of getting it from
the wild pea much better than from the tinged white, for
again and again wild types have been found to throw off
albinos soon after their introduction into cultivation — by the
occurrence, of course, of a new genetic variation.

3. Intermediates pi^oduced by the interference of other
factors. There are many intermediates of this kind also.
In both animals (Fowl, Rabbit, Mouse) and plants {^Primula')
we have seen that a dominant factor may exist which has
the power, for example, of suppressing the development of
colour, leaving those parts white which in the absence of
that factor would otherwise be coloured. This suppressing
or inhibiting effect may be total, but when it is not total
some puzzling intermediate types may be constituted. Only
most careful breeding experiments can reveal the true

xiii] Intermediates

239

nature of these cases. To the ordinary observer the
" Painted Lady " Sweet Pea with its whitish wini^s and red
standard might appear intermediate between a white and a
totally red type. But as experiment shows, the bicolour
type with its whitish wings is dominant to the self-coloured
forms, and its wings are white, not as the Dutch rabbit is,
owing to the omission of something, but on account of the
addition of another- distinct element which suppresses the
pink colour in that particular part more or less completely.
The real white is of course due to the absence of one or
more of the colour-factors, an altogether different cause, and
it is therefore evident that to speak of the bicolour form as
in any way intermediate between self-colour and white
would be a complete misrepresentation of the facts.

Similarly in the Rabbit, Hurst has shown that the
curious type known as '' English " pattern — something like
a Dalmatian dog but for the more or less regular disposition
of the spots — is a dominant to self-colour, though a careless
observer might guess it to be a transition-stage towards
albinism.

In such cases again there is every likelihood that careful
selection might succeed in isolating subordinate types in
which -the suppression attains particular degrees of com-
pleteness ranging within well defined limits, but the evidence
distinctly negatives any attempt to treat these several forms
as a continuous series in which any member is capable of
reproducing any other among its offspring.

4. Intermediates as fl2tct7iational forms. Lastly there
are intermediates due to the disturbing effects of many
small causes not of genetic but presumably of environmental
orio'in. Such are the fluctuations in the weio^ht of individual
seeds which Johannsen has studied with success in beans
i^PJiaseohis).

When the weiofhts of the seeds of a sinole varietv of
bean are marshalled statistically they arrange themselves in
a normal curve round the weight of greatest frequency.
Similarly when the seeds are sown and the seeds from the
self-fertilised individual plants which grow from them are
harvested separately, the crops from each individual again
can be grouped according to their weights in normal curves

240 Phictuatton [ch.

round the most frequent or modal weight characteristic of
each individual. There will then be seen that some rouo-h

correspondence exists 'between the modes for the individual
plants and the weight of the individual seeds from which
they sprang. The heavier families on the whole come from
the heavier seeds, and the lighter from the lighter seeds.
Such correspondence is nevertheless very imperfect, and the
weight of the seed chosen from the original mass gives only
a vague indication of what the modal weight of that plant
will be.

But when any one family raised from a single seed was
taken, and the heavier and the lighter of its self-fertilised
seeds chosen and separately sown, Johannsen found that
the modal weights were approximately the same for the
produce of both the heavier and the lighter seeds. Selection
inside the family 7'aised fro7n a single seed did not alter the
7nodal weight, which ive7tt back or regressed to that of the
individual common pareiit. Such a family thus raised from
the single seed constitutes what Johannsen has called a
pzcre line. Within the genetically pure line there are
fluctuations in weight, but these fluctuations are due to
interference which Is external, or environmental in the
wide sense, and selection of those extremes which are due
to such interference produces no effect on the result, for the
differences in the weights of the seeds of a pure plant do
not indicate differences between the germ-cells which pro-
duced them (164).

It is to variation of this type that the name fluctuation
is given.

Johannsen has made a similar series of experiments In
regard to the proportion of defective grains present in
certain barleys and has reached a similar conclusion.

Whether the distinctions characteristic of the pure lines
segregate or not cannot yet be said. By the nature of such
cases the distinctions themselves are fine, and even if
segregation occurred the proof of its existence would involve
many and serious practical difficulties.

When the complex diversity of Intermediates is thus
appreciated it will be understood that extreme caution is
needed before the statement is made that in any specific

XIII] Shirley Poppies 241

case segregation does not occur. Even a sensible con-
tinuity between varieties is no proof that there is not segre-
gation which with careful work could be demonstrated. To
take ■ for instance the case of piebald mice, it would be
possible with industry to collect from the results of miscel-
laneous breeding specimens ranging from a self-colour to a
black-eyed white. From such series any number of false
conclusions could be drawn respecting the continuous
variation of mice. By appealing to the "ruby" eyes of
certain chocolates an uncritical observer miorht even aro-ue
that a series of intermediates connected the black and the
pink eyes.

Analytical breeding immediately shows the fallacy of
such deductions. For it would be found that the so-called
intermediates consisted of numbers of genetically distinct
types with distinct genetic properties depending on the
factors which constituted them. Some would carrv the
colour- factors for the Dutch pattern, others those for more
complete or less complete pigmentation, while others would
owe their partial whiteness to the presence of the dominant
factor which can suppress pigmentation in several stages of
completeness.

In general it is to be concluded that when the inter-
breeding of recognizably distinct types produces only those
types again without intermediates or with only few indi-
viduals that can be so regarded, the fact points strongly to
the existence of gametic segregation ; but on the other hand
the appearance of even a complete series of intergrading
forms is not to be accepted as proof that gametic segregation
does not occur.

Misconception of the nature and significance of inter-
mediates has deprived the work of the biometrical school
of scientific value as a contribution to the study of heredity.
This is well seen in the case of the colours of Shirley
Poppies, one of the subjects with reference to which copious
statistics have been amassed and published*. The colours
are red, pink, and white, of many shades, and mixtures ot
these tints variously disposed and graded. The problem
was to discover the laws of inheritance of the colours. The

* Biometrika, ii. i, 1902, and iv. 4, 1906.

B. H. 16

242 Shirley Poppies [cii.

Mendellan method would be to institute breeding experi-
ments with individual plants, self-fertilising and crossing
these individuals together, and recording the offsprino-
produced by each plant and by each combination. True
that by sufficient search flowers can be found which when
arranged in order provide a series of intergrades from the
darkest red to the whites so nearly complete as to persuade
an observer of moderate experience that there is full con-
tinuity. Nevertheless, from what we know of every such
case to which Mendelian analysis has been extensively
applied, there cannot be the smallest doubt that, as in
Antir7^hirmm, the Mouse, and many other examples, the
apparent continuity is misleading. It will, when analysis
is undertaken, unquestionably be shown that there are
definite, pure homozygous types, containing factors which
display the usual phenomena of dominance, and segregate
in an orderly fashion ; and that though, between these,
intermediates may occur, yet they again will be found for
the most part to be various consequences of heterozygosis.
There may, not improbably, be a residuum of intermediates
which cannot be thus represented, but it will be surprising
if these are more than insignificant fractions of the whole,
owing their peculiarities to various fluctuating influences.
The ground will then be approximately mapped, and the
laws of colour-inheritance approximately established for the
Shirley Poppy.

The method of Professor Pearson and his assistants
dispenses with all analysis. No attempt is made to discover
the factors concerned, to distinguish the pure types, or
the properties of dominance which they possess. No
artificial crosses are undertaken^. The flowers are left
uncovered, open to the insects which may introduce the
pollen of any other flower which they happen to be carrying.
The records show — probably with much accuracy — the
colours of the oflspring derived from mothers of recorded
colours, and no more. In this attempt to study the laws
of heredity of colour, the fathers of the progeny examined
are in every case unknown.

* Self-fertilisation was subsequently tried, but, owing to self-sterility,
gave no results.

XIII] Particoloured Peas 243

According to the biometrical system, if the characters of
one parent and of the offspring are known, the material can,
after mathematical treatment by means of the correlation-
table, be used to deduce a law of inheritance. The theoretical
conceptions upon which this method is based were formed
and elaborated first in ignorance and subsequently in dis-
regard of Mendelian facts; but improbable as their validity
was in pre-Mendelian times, it must now be obvious that
these methods can merely obscure the essential phenomena
they were devised to discover.

There are moreover abundant examples of other fluctua-
tions more obviously depending on environmental influences.
In studying the inheritance of the cotyledon-colours in the
seeds of peas such cases are met with. The cotyledons of
many varieties are in all ripe seeds a full yellow, while those
of many others are a full green. The contrast between the
yellow and the green seeds in /% after segregation may be
perfectly sharp and clear. Some varieties, especially the ^
more modern sorts, have many seeds which are in various
degrees partly yellow and partly green, and occasionally
such particoloured seeds are the commonest type of the
variety. In his criticism of Mendel's work the late Professor
Weldon called attention to these particoloured seeds, con-
tending that they showed failure of segregation. Before
looking into the facts I was disposed to admit the justice
of this contention. After experimenting with such kinds
however I found that the particoloured appearance was
not caused by any admixture of the yellow character, but
by exposure to sun and weather. All those seeds which
tinge are by nature ^r^^;^ and breed true to greenness. It
in a pod several are found yellowed, this bleaching affects
those sides of the seeds which face the same wav and is
obviously due to effects of light just like the reddening of
the exposed sides of many apples.

The hereditary property characteristic of these tinging
, sorts is the power of bleaching readily if exposed, but the
bleaching is checked and can generally be prevented if the
pods are gathered as soon as they are ripe. It may be
thought that the power of bleaching to which the ordinary
yellow varieties owe their colour is the same faculty inten-
sified. It may be so, but there are very marked distinctions ;

16 — 2

244 Particoloured Peas [ch. xiii

for seeds of yellow varieties turn yellow before they harden,
while the particoloured sorts bleach after they have become
ripe.

In some yellow varieties greenish seeds occur. This
phenomenon, the converse of the last, occurs in wet
years with many kinds, some of course being specially
liable.

The fact that such differences are fiuctuational and not
genetic appears at once when breeding is begun ; for if in
a green which bleaches {e.g American Wonder) the yel-
lowest and the greenest are taken from the same plant and
sown, there will be no corresponding difference between
their respective produce. By selecting the greenest and
yellowest from a large mass of commercial seeds of a variety,
not harvested plant by plant, some correspondence may be
obtained occasionally \e.g. Telephone) — but the occurrence
merely means that the mass, though called one variety, had
not really been selected down to a ''pure line," but was in
fact a mixture of sorts. In such a type as "Nonpareil," for
instance, where the difference between yellows and greens
is actually genetic, the yellows and the greens will give dis-
tinct results. "Nonpareil," in fact, is a form which consists
of true yellows and true greens in about equal numbers, and
it is simply a mixture of two varieties not sensibly different
except in seed-colour. No doubt it arose from a plant
saved in one of the later generations derived from a cross,
which plant was homozygous in other respects but hetero-
zygous in seed-colour.

CHAPTER XIV

MISCELLANEOUS EXCEPTIONAL AND UNCONFORMABLE

PHENOMENA.

Crosses breedinc^ true witJiozU Seo-rezation. Part he )io (genetic
(7r Apogamic Fonns. Hieraciitni — Sexual Forms —
Nitmerical Aberrations — Iri-egitlarities of Doviiiiance
— Alternation of Generations — Alaternal Characters
in certain Seeds.

Of the various cases alleged as exceptional, or declared
to be incompatible with Mendelian principles, few have
any authenticity. Several rest on errors of observation or
of interpretation and some have even been created by a
mistranslation or a misprint. With this class of exception
I have dealt on previous occasions. At a time when the
Mendelian idea was novel it was perhaps natural that a
doctrine so utterly at variance with received opinions should
have been regarded as suspect, and that from the conven-
tional standpoint the mere fact that a record was incom-
patible with Mendelism should have raised a presumption
in favour of its grenuineness. That staoe of the debate is
over, and we have now learnt that phenomena consistently
departing from Mendelian rules are so rare that announce-
ments of discoveries irreconcileable with the principle of
factorial composition may be safely disregarded, unless they
are made by observers experienced in this class of investi-
gation, or supported by evidence of exceptional strength.

The progress of research has gone steadily to show that
facts of heredity which at first seemed hopelessly compli-
cated can be represented in terms of a strict Mendelian
system. This simplification of the problem has far exceeded
our earlier anticipations, and I have to regret that iii
dealing with several sets of phenomena I countenanced

246 Parthenogenesis in Hieracimn [ch.

non-Mendelian interpretations which in almost every case it
has been found possible to replace by simple Mendelian
formulae. Where this reduction to a common plan has
not been yet effected, the difficulty, we feel fairly confident,
is ge^ierally created rather by the disturbance of environ-
mental causes or by the influence of undetermined factors
than by any more profound aberration in physiology.

It is the object of the present chapter to discuss some
of the more prominent of the phenomena which are really
or apparently unconformable. As will be seen, they are of
a miscellaneous nature.

I. Ci^osses breeding true without Segregation,
A. Parthenogenetic Cases,

Of crosses breeding true there are two types quite
distinct in nature and significance. In the first we now
know that the absence of segregation is due to the fact that
the reproduction is parthenogenetic, or apogamic, as it is
more often called by botanists. The famous example of
this kind of reproduction is provided by Hieracium (Hawk-
weeds). Mendel himself investigated the inheritance of
Hieracium experimentally, and his paper is given in trans-
lation at the end of this book. This o^enus beino- one of
the most strikingly polymorphic, he chose it after his
discovery regarding the inheritance of peas, as the subject
of further research. We may surmise that he expected to
find in it illustrations of the new principles. The technical
difficulties were extreme. The minuteness and the delicacy
of the flowers made the operation of castration almost
impossible to carry out on a large scale. It is recorded
that in the course of this work he injured his eyesight ;
and after all precautions were taken he was mortified by
what seemed to be frequent failures. Seeds that were
supposed to be hybrid gave plants of pure maternal type,
which he could, in those days, only attribute to accidental
pollination from anthers of the mother-plant, caused by
imperfect emasculation. In a few cases however he did
succeed in raising genuine and obvious hybrids. These
hybrids were partially sterile, but the seeds which they did
give reproduced the hybrid form again.

xivj Parthenogenesis in Hieraciu))i 247

In the earlier discussions which followed the rediscovery
of these papers we all were inclined to follow Mendel in
supposing- that Hiei^aciiiui illustrated a distinct kind of
sexual inheritance in which seo^regation was absent, and ii
seemed natural to suspect that the association of this jjheno-
menon with partial sterility was not accidental. As we
now know, there is a distinction between the inheritance
exemplified by these true breeding hybrids, but it is not a
speciality of Hieraciwn, or in any sense to be rec^arded as
an exception to the general rule that in sexual heredity
segregation of characters occurs. The meaning of the facts
was first discovered by Ostenfeld (219) following up an
experiment of Raunkiaer (228). These two observers
found that both Tm^axaawi and many Hicracia have the
power of setting seeds parthenogenetically without any
fertilisation. Such plants may set seed profusely when all
the anthers and stigmas are cut off in an unopened bud.
The cytology has been investigated by Rosenberg (233),
and he has shown that in the ovules which are capable of
parthenogenesis no reduction-process occurs. In the strict
sense therefore this method of reproduction is not sexual,
but asexual, being comparable with the reproduction from
buds or cuttings in which, as is well known, the parent type
almost always reappears without modification.

This fact explains not only w^hy Mendel's hybrid
Hieracia bred true, but also why he so often failed to
produce a hybrid when all precautions were taken to avoid
self-fertilisation. The plants which were supposed to be
due to accidental self-fertilisation were in reality partheno-
genetic. From Ostenfeld's experiments it was shown that
some only of the ovules are capable of fertilisation ; and it
is practically certain that the ovules with unreduced nuclei,
though they may give rise to plants by parthenogenesis or
apogamy, are incapable of being fertilised. The evidence
that the unreduced egg-cells merely reproduce the maternal
type is of course a strong support to the view that it is
in the reduction-division that the segregation of factors is
efiected.

It is by no means alone in the Compositae that partheno-
genesis has been observed. Fresh cases are continually
being' discovered among flowering |)lants. and in any

248 Monolepsis in Orchids [ch.

discussion of unexpected genetic phenomena this possibility
must be taken into account"^.

There is a further difficulty to be considered in this
connection. The fact that no seed is set when fertilisation
is excluded is in itself no proof that the inheritance is not
of a parthenogenetic type. A small but very definite and
well-ascertained group of cases occurs among Orchids in
which, though no seed is set unless the flowers are pollinated,
yet the offspring exhibit no trace of the paternal characters.
The facts are well known to orchid-growers, but they were
first made known to students of genetics by Hurst (152,
p. 104). The most notorious example is that of Zygopetahun
Mackayi, which when fertilised with pollen of various other
orchids has given nothing but Zygopetaltun Mackayi. Four
species of Odontoglossiim, a Lycasta, and an Oncidium, used
as father all gave this curious result. Hurst quotes other
records showing that several other orchids may behave in
the same way as Z. Mackayi, giving purely maternal plants
when fertilised with pollen of other types. In some of these
cases it has also been shown that self-fertilisation of the
supposed cross-bred again gives nothing but the same type.

Pending cytological investigation it is not possible to
form a satisfactory rationale of these cases, but we cannot
avoid the conclusion that fertilisation has acted as a stimulus
to development without any admixture of paternal character.
The result is, as Hurst has pointed out, exactly as if
parthenogenesis had in reality occurred, although pollination
had taken place and is indeed necessary to effect production
of seed. I have proposed to distinguish this phenomenon
as Mo7iolepsis, in which characters are taken from one
parent only, in contrast to Ampkilepsis, in which they are
brought in from both sides.

In the earlier staofes of these discussions we were dis-
posed to attach much importance to an observation of
Millardet (198), to the effect that in certain crosses among
strawberries the supposed hybrid was of purely paternal
type, showing no maternal features, and breeding true.
The term ''false hybrids" was suggested by him as descrip-
tive of such plants. No exactly similar example has been

* For important summary of new evidence see Rosenberg, Svensk Bot.
Tidsk., 1909, HI. p. 159.

XIV] Hybrids breeding tnte 249

found in the course of contemporary experiments. It is to
be hoped that the original experiment may be repeated, for
Millardet's note is very brief, and we have no means of
judging whether the various possibihties of error were
excluded {e.g. heterozygosis of the mother-plant).

Whatever the future may reveal as to the significance of
these ambiguous occuri-ences it is certain that in all cases
where irregular results are observed we may have to reckon
with two possibilities, (i) actual parthenogenesis, or the
developm.ent of unfertilised ova without fertilisation ; (2) a
phenomenon tantamount — as regards heredity — to partheno-
genesis, occurring after fertilisation"^. In both events the
offspring are purely maternal. The latter exemplifies the
conception of Strasburger and Boveri, that fertilisation ma\-
consist of two distinct processes, the stimulus to develop-
ment, and the union of characters in the zygote.

B. Sexual Types.

The literature of hybridisation and heredity abounds
with examples of hybrids or cross-breds which are said
to have bred true without splitting up, or as w^e should
now say, without segregation. Some of these, so far as
they refer to plants at least, are open to the criticism
made in the last section, that either actual parthenogenesis
or monolepsis may be occurring. But an even simpler and
more probable account is hardly ever excluded, for we have
rarely if ever any means of knowing that the plants studied
were in reality first crosses and not members of derivative
o^enerations. Moreover even in cases such as the famous
series of Salix hybrids made by Wichura (304), where this
objection does not apply, we know almost nothing as to the
numbers of individuals raised. These remarks probably
apply to all the illustrations given by Focke (123) so far as
I have been able to ascertain. In Mutatiojistheo7'ie, 11. p. 66,

* On a former occasion (i8, p. 156) I suggested that in mosaic forms
the presence of the recessive character in patches might be thought of as
due to failure of fertilisation in those areas. More accurate knowledge of
the facts has practically disposed of this suggestion. It was chiefly based
on evidence relating to the mosaic seeds of Peas, patched with green and
yellow in the cotyledons, but we may now feel fairly sure that the
abnormalities seen in their behaviour are due to disturbance caused by
external influences.

250 Hybrids breeding trnte [ch.

de Vrles mentions some cases which he accepts as satis-
factory, the most definite being that of Oenothera niuricata
X biennis raised by himself. The hybrids were partially
sterile in a high degree, but the subsequent generations
raised from them showed no definite departure from the F^
type. The evidence, as It stands, must be taken as consti-
tuting a definite exception. Nevertheless In view of the
great sterility exhibited by the hybrids, and the fact that
all that we know of the Oenothera crosses points to the
existence of very unusual features in their genetic physio-
logy, the significance of this curious observation is still
somewhat problematical"^

Another case given by de Vrles Is that of Anemone
silvestrisY^magellanica on the authority of Janczewsklf-
As this case well illustrates the anomalous and uncertain
behaviour of partially sterile hybrids, the facts are worth
giving in some detail. In the first account the hybrids are
said to have been either totally sterile, or fertile In a very
slight degree. The pollen Is said to have been all bad,
and in flowers containing about 100 pistils, one or rarely
two good akenes formed. Janczewski thinks that these
were fertilised from one of the parents. But the hybrid
sometimes gave rise by an adventitious bud, from the root,
to a plant having the female parts perfectly fertile, pro-
ducing akenes of intermediate shape. The pollen of these
flowers was not examined, and It was supposed that fertilisa-
tion was effected by pollen of A. sylvestris growing near by

In the later account It is recalled that the hybrids some-
times produced good akenes among a multitude of abortive
ones, and it Is said that the pollen, though very bad, contained
occasional good grains which could effect fertilisation without
foreign pollen. Some stems of this hybrid — not the whole
plant — gave rise suddenly to flowers perfectly fertile, with
all the akenes good. These flowers had mixed pollen-grains,
of which about three-fourths were good and effected self-
fertilisation. The second and also the third generations
raised from the hybrid, whether from the sterile or from the
fertile flowers, reproduced precisely the characters of the
original hybrid, exhibiting the complete fertility of the female

* See Appendix to Part I.

t Bull. Ac. Cracovie, 1889, June, No. 6, p. xxiv, and ibid. 1892, p. 228.

XIV] Abseiice (^f Segregation 251

organs of the fertile stems, and the mixed pollen of those
stems. Janczewski concludes that in this way a new specific
type could arise which, but for its mixed pollen-grains,
would be indistino'uishable from a natural and oricrinal
species.

This extreme irregularity and sportiveness in the degree
of fertility manifested is a frequent concomitant of partial
sterility in hybrids. The fact that the sterility once lost
does not return, obviously suggests that it is due to the
presence of some dominant factor or factors, and that they,
or the compound they make, can be eliminated by bud-
variation. Other reflections will occur to the reader which
we cannot now consider, but I feel it difficult to accept
such an example without further comment as a proof that
in ordinary sexual reproduction segregation may fail as a
normal event.

It is most desirable that search should be made for
genuine cases of failure of segregation amongst animals.
The example of the human mulatto is as yet one of the
best which ordinary experience suggests"^. In experimental
breeding perhaps the single clear instance is provided by
Castle's experiment (48) on the lengths of ears in rabbits.
He crossed the long-eared lop rabbit with ordinary short-
eared individuals. F^ had ears of intermediate length, and
the subsequent generations retained this character very
definitely. The later details have not yet been published,
but Professor Castle very kindly showed me many of his
records which plainly established the fact that no return to
the parental types took place.

In some experiments made with the butterfly Paj^ar^e
egeria I obtained a comparable result. In Spain and the
plains of Southern France ege^na has a bright fulvous brown
colour (intersected wdth darker brown), while in England
our representative form egeriades has the bright brown re-
placed by primrose yellow, In the Loire valley and in
Brittany a race almost exactly intermediate between cgeria
and egeriades is tound. By crossing the two extremes the
intermediate type was produced with great exactitude. /%
was not raised in any adequate numbers, but those that

* See p. 208.

252 Absentee of Segregation [ch.

were bred showed only dubious traces of segregation. On
the other hand F^ was crossed with both parent types and
some large families raised. The general aspect of these
families might be described by the statement that they
came on the whole intermediate between F^^ and the pure
parents respectively. Pending a repetition of the experi-
ment, and especially before F^ has been properly investi-
gated, the negative cannot be asserted with complete
confidence, but I am disposed to regard the case as one in
which segregation is really absent.

Experiments made by Bacot and Prout with Acidalia
virgularia (Geometrid moth) and a light variety from
Hyeres called canteneneraria showed that F^ was always
intermediate. In subsequent generations no segregation
could be detected. A good deal of fluctuation occurred,
but the evidence went to show that either extreme could
produce the other. The facts were described by Mr Bacot
in a lecture at the London Hospital in 1908, and I am
indebted to him for further information. A full account is
shortly to appear.

2. Departures from Numerical Expectation,

The material for a proper investigation of this important
question is still quite insufficient. It is the experience of
all who have made breeding experiments that from time to
time individual families show great aberration from the
numbers which are predicable by the simple rules. Not
till a laro-e mass of otherwise homoo^eneous statistics are
available will it be possible to ascertain whether any of
these aberrations are really indicative of the eflects of
special causes of disturbance or are to be accepted as
random departures from normality. From the point of
view of the statistician it may seem a comparatively simple
matter to procure such material, but in practice there are
many difficulties to be overcome. The impression formed
on my own mind is that the output of allelomorphic gametes,
where equality is expected, does in certain types show in-
dividual departures which are not purely fortuitous, but in
any collection of statistics large enough to give reliable
conclusions these individual aberrations would be completely

XIV] Numerical Abeynitions 253

masked ; for though they may be really Indicative of
physiological disturbance, these disturbances must be rather
of the nature of sporadic events than the consequence
of predominating large causes affecting long series of
records.

Occasionally however the effects of such definite causes
can be traced In our numbers. One excellent example Is
provided by the work of E. Baur (24), who Investigated
the Inheritance of the colour of the leaves In Antirrhinum.
He found that the green type breeds true, but that the
golden-leaved or '' aurea'' form gives, on self-fertilisation,
a mixture of goldens and greens. The resulting numbers
were 573 golden and 286 green, showing very clearly the
proportion 2:1, the golden being a majority. The question
why are the goldens not 3 to i of the greens is, as he points
out, readily answered. The missing group of plants are
those which would have been pure to the yellow character.
Such plants would contain no chlorophyll and consequently
would perish. A study of the germinating seeds sub-
sequently proved that this was the true account, for such
an examination gave "]"] green, 160 golden, 51 almost or
quite without green. The last group all died. Such an
observation throws a clear llorht on the meanino- of some at
least of those " ever-sporting " types to which de Vries drew
attention"^. As stated In a note on p. 163 Miss Durham
has now proved the condition of yellow mice to be exactly
comparable with that of these varietates aureae. It would
be a matter of very great interest to determine what Is the
exact cause of the non-vlablllty of the pure yellows, and
what the physiological action of the " yellow factor " may
be.

In the light of this experiment, and Indeed from a priori
consideration. It is clear that the non-viability of zygotes or
even of gametes bearing special combinations of factors
may play a large part In genetic phenomena. Nevertheless
with the exception of some cases in Pri))iu!a and the
peculiar phenomenon recorded in regard to maize (p. 161)
I know no larofe series of numbers which show a consistent
departure from expectation. There are however suggestions

* Mutationstheorie, i- P- 597-

254 Numerical Aberixitioiis [ch.

that In some types of animals and plants the amplitude of
the fluctuations about normality is greater than in others.
This is of course a point well adapted to statistical study
which sooner or later must be undertaken. Similarly there
are indications that in cases where the point can be tested,
the seeds of individual pods may give strangely aberrant
numbers though the plants as a whole may give regular
results. Miss Saunders has often been struck with this
peculiarity in studying the genetics of Stocks, but as yet
the figures have not been examined by statistical methods.
Knowing what we do as to the capacity of individual flowers
to sport, the suggestion obviously arises that such indi-
viduality may be manifested by the collective oflspring
of one ovary even though the flower In which it was
formed may show no peculiarity.

The very wide departures from expectation shown in
many pedigrees of human diseases and defects are certainly
in part attributable to the imperfections of the records, but
I cannot doubt that the discrepancies are In part due to
genuine physiological causes. In regard to some of these
it is I think still open to question whether the transmission
is a process comparable with that which we ordinarily
designate as Heredity. Some element is obviously handed
on from individual to individual, but it seems to me possible
that this element or poison is distributed irregularly among
the germ-cells, spreading among them by a process which
is mechanical, like the spread of an oil-stain in a heap of
paper, or of a fungus in a heap of seeds. In the present
state of pathological knowledge it is premature to make any
suggestion as to the possible nature of such poisons. I am
told by competent authorities that in the cases, for example,
of the various polymorphic hereditary paralyses it is very
improbable that pathogenic organisms can be the exciting
cause ; nevertheless, from a study of the inheritance In an
ample series of families, I am inclined to suppose that the
element transmitted is something apart from the normal
organism, and that it is handed on by a process independent
of the gametic cell-divisions. In such cases I do not an-
ticipate that any "law" of inheritance can be discovered, for
if my view is correct, the process is not heredity in the
naturalist's sense at all.

XIV] Irregular Dominance 2

3. Irregularities of Dominance.

It must be anticipated that irregularity or fluctuation
in dominance will prove a frequent source of exceptions.
Neither among animals nor plants is material often to be
found so homogeneous that the hetcroz)'gotes formc^d from
various individual pairs are quite identical, and the domi-
nant factors thus produce their effects in varying degrees.
Reference has already been made (p. 236) to the fact that
dominance may sometimes be so complete that the hetero-
zygote may pass for the pure homozygous type, while in
other individuals the distinction can be recognized. Such
cases present no real difficulty in analysis, for the two
classes can be taken as one in the counts of /%. Serious
difficulty is caused however in a few examples by the fact
that a dominant factor may be present without producing
any effect by which its presence can be perceived. Such
dominant individuals are indistinofuishable from recessives,
and analysis thus becomes difficult or impossible. Con-
spicuous examples of this ambiguous phenomenon have
occurred in two of the subjects investigated on a consider-
able scale. The first is the heredity of the extra toe in
fowls. In our work we have found the extra toe usually to
be a dominant. Such a 5-toed breed as Dorking or Houdan
crossed with a normal 4-toed breed commonly gives F^ birds
with extra toes developed to an extent somewhat less than
that found in the normal 5-toed type, and F. from such birds
is of the usual composition, averaging 3 birds with extra
toes more or less developed, to i normally 4-toed. But
it is by no means rare to meet with birds of the 4-toed type
"which when crossed with an extra-toed breed will throw
many F^ with 4-toed feet, or with traces only of the extra
digit. The two feet of the same bird may also differ, pre-
senting every combination. Such birds have in several
instances been proved by their progeny to be ordinary
heterozygotes. At first one might be disposed to attribute
the peculiarity to interference caused by some epistatic
factor repressing the development of the dominant charac-
teristic ; but this account cannot very readily be reconciled
with the actual numbers, for there is no consistent diminution
in the number of the dominant oft'spring of such birds. The

256 Irregular Dominance [ch.

nature of these unconformable families mnst be reearded
at present as altogether obscure. The records of human
polydactylism indicate that very similar disturbances of the
usual course of descent operate in regard to man.

Another example almost exactly similar has been en-
countered both by Correns and by Lock in the heredity of
the blue or black colour of the aleurone layer of the seeds
of maize. The blue is generally a dominant appearing in
various degree, but some white F^ seeds proved able to
carry on the blue character. The problem raised by this
defect of dominance is discussed at length by Lock (174),
but no altogether satisfactory elucidation has been found.

It may be pointed out that when, as in these examples,
the abnormal result is clearly perceptible in F^, no question
arises as to the occurrence of an imperfect segregation.
The peculiarity Is evidently zygotic, and Is caused either by
some feature of zygotic organisation, or by the influence of
external circumstances. This conclusion, so plainly dedu-
cible from results obtained in experimental crosses between
pure races, may be appealed to when, as In the descent of
some human characteristics, we witness as an exception
the handing down of a normally dominant factor through an
unaffected member of the family. In view of such occur-
rences the reality of the segregation may be doubted. No
experiment with pure types is possible in dealing with those
phenomena but the analogy of the genetics of extra toe in
fowls supports the view that the peculiarity of these cases is
zygotic and not due to failure of segregation.

4. Alternation of Generations : an outstanding

difficulty.

The question may be asked whether, on a general
survey of the facts of Natural History, any large classes
of phenomena are encountered which are altogether incon-
sistent with the Mendelian system to such a degree as to
suggest that they are quite incompatible with any scheme
of factorial analysis. Various exceptional cases have been
enumerated and considered. It is not in dispute that, for
example, in the phenomena of hybridisation between the
races of mankind, the facts (fairly well authenticated) must

XIV] Alternation of Generations 257

be taken as pointing at all events to the existence of special
features in them. In most of these, however, there is
nothing that can be construed as indicative of some
wholly different system of genetic physiology. There is
nevertheless one phenomenon, alternation of generations,
which stands out as at present incapable of factorial repre-
sentation. This is best exemplified by the case of seasonal
dimorphism so frequently met with in insects, especially
Lepldoptera. For example many species exist in two types
of which one emerges after the winter as a spring form.
This breeds and lays its eggs. The larvae feed up rapidly
and emerge in summer as the second or summer form. In
their turn individuals of the summer type give rise to the
larvae which are destined to become the spring type again.
A similar sequence of distinct types is a common feature of
many tropical species in which the wet and dry season
forms correspond to the winter and summer forms of
temperate countries. The cycle may be complicated in a
variety of ways, by the Intercalation of additional genera-
tions and otherwise. Of these more complex cycles the
Aphldae and Coccldae afford many striking Illustrations.
In none of these examples can we conceive that the distinc-
tions between the recurring types are due to addition or
removal of factors. The only other suggestion which can
be made is that these distinctions are ultimately referable
to the effects of external conditions. As yet very little If
any evidence can be adduced In support of this suggestion.
The experiments of Standfuss, Fischer, Merrlfield, and
others have shown that the form which the Individual
assumes can within rather narrow limits be determined by
altering the temperature to which It is exposed, but in
order to obtain a satisfactory elucidation of the phenomena
I am here considering this evidence Is Insufficient. In the
known examples the experimental influences were brought
to bear on the individuals themselves. It Is not im{)os-
slble that something might be accomplished by changing
the conditions to which the parents are exposed. It is, lor
example, a familiar experience of lepidopterists that when a
one-brooded type Is bred In captivity a second brood is
very commonly produced in the autumn, and it is natural
to refer this variation to some influence of the abnormal
B. H. 17

258 Maternal Characters in Seeds [ch.

conditions. But we here meet the old difficulty that the
distinction between the living types is often sharp, whereas
that between the environments is one of degree. We know
also that from the same parents a brood of larvae may be
raised some of which will feed up fast and emerge as the
autumn form while others will feed up slowly and emerge
with all the characteristics of the spring form after the
winter, and we can form no idea as to the circumstance
which determines the distinction. Though I can give no
hint as to the rationale of these occurrences which in
the cases of the sex of Aphidae &c. have an extreme im-
portance to the progress of genetics, I call attention to
them as constituting what must be regarded as the group
of phenomena least in accord with the conception that the
characteristics of living things are an expression of their
factorial composition^.

5. Maternal Characters in Embryos,

A. Seeds of Wheat,

In certain plants a curious and at present quite unin-
telligible phenomenon has been met with. Of this four
examples are known. The simplest is that seen when a
wheat having long glumes and seeds is crossed with a
variety having short seeds in short glumes. This case has
been studied by Biffen. The F^ seed, resulting from the
cross-fertilisation, is unchanged, and resembles the normal
seeds of the mother-plant. When this F^ seed is sown the
plant in due course flowers. Its glumes are of intermediate
length (Fig. 35) and the seeds they contain (/%) are all
alike of intermediate length (Fig. 36). Apparently there-
fore segregation has not taken place. But when these
7% seeds are sown, the plants which they become are
respectively longs, intermediates (like F^, and shorts, as
regards their glumes and seeds. Hence it is clear that
segregation had occurred among the gametes produced by
F^, but owing to some control exercised by the mother-
plant, all the seeds have a similar length and appearance.

* It is not impossible that the difficulties which have been met with
in the attempt to pursue Mendelian analysis in the double-brooded
Lepidoptera may be connected with the phenomena spoken of above.

xiv]

Maternal Characters in Seeds

259

This case at first sight does not seem altogether para-
doxical, for it may appear natural that the size of the
seeds should — at all events as regards the upper limit — be
governed by the size of the maternal envelopes. The next
group of cases however, though showing that the peculiarity
is in some way caused by the maternal tissues, suggests that
the influence may be of a more recondite nature.

Fig.

Polish wheat, with long glumes, crossed with a «^hort glumed
Rwet wheat. F^ has glumes intermediate in length. (BifTen's
specimens.)

B. The Indent Peas.

This example is that of the Peas {Pisum) known as
'' Indent." The crosses of this type of pea give rise to an
intricate series of results. For our knowledge of^ these
facts we are indebted in the first place to Tschermak* : and

* Tschermak's account is sometimes difficult to follow, because his
terminology does not always show whether he is referring to indent or to

wrinkled peas.

17—2

26o Maternal Characters in Seeds [ch.

several experiments have also been carried out both by
Lock, and by myself assisted by Miss Killby. In the
following account only the essential features are related.

As the type of an *' indent " pea I take the French
Purple Sugar pea"^ {Graue Riesen of German seedsmen). Its
flowers are purple. The seeds have deep, irregular inden-
tations, but with experience of peas it is fairly easy to
distinguish these ** indents " from all wrmkled types. As
always on plants with coloured flowers, the seed-coats are
coloured, being here a grey brown (with some purple spots).
In connection with the colour of the seed-coats we meet the
first complication. So far as is known, the thoroughly
indent type of seed never appears except m coloured coats,
but round or wrinkled seeds may exist in coats of all sorts,

Fig. 36. Upper row : Seeds of Rivet. Lower row : Seeds of Polish
wheat. Middle row : /^2 seeds borne by F^ plant, all similar and
of intermediate length, though segregation has occurred. (Biften's
specimens.)

whether coloured or uncoloured. As regards the nature
of the reserve-materials in these peas Gregory found that
the starch grains of indents are like those of round peas
(see Fig. 9), being large and simple.

For simplicity's sake I take first the results of crossing

"^ This pea has soft ^^ sans parchemiii^'' pods, but the characters of the
pods are not involved in the problem discussed in the text.

XIV] Maternal Characters in Seeds 261

indent with wrinkled (e.g. Satisfaction, Laxton's Alpha),
for the consequences are there comparatively regular. In-
dent fertilised by wrinkled gives the F^ seed indent and
unchanged. Wrinkled fertilised by indent gives the A,
seed rotmd. This is evidently in consequence of the fact
that the wrinkled variety used was a white-llowered plant,
with of course uncoloured seed-coats, in which the indenta-
tion cannot develop. Such an F^ seed becomes an F^ plant
with coloured flowers and coats, and the /% seeds it bears
show normal segregation, being indents and wrinkleds* in
the usual proportion 3:1. The wrinkled seeds if sown give
normal results, viz. 3 wrinkled with coloured flowers : i
wrinkled with white flowers. The indents if sown also oive
3 coloured-flowered : i white-flowered, but all seeds of
''round" nature on the coloured plants are of course iJidents
(w^hether all indents, or 3 indents : i wrinkled as before),
while all the corresponding seeds on the white-flowered
plants are true rozinds (being either all rounds, or 3 rounds : i
wrinkled).

Thus far all Is intelligible, but when we proceed to the
next combination, indent crossed with round, the real diffi-
culties are reached. Indent fertilised by a round gives of
course the F^ seed indent; and reciprocally, the round ferti-
lised by indent gives the F^ seed round. When these seeds
are sown, the F-^ plants grow up (with coloured flowers),
but the condition of their F^ seeds differs according to the
round variety used as original parent. I take first the
result which has been most commonly observed. Tschermak
observed it in many crosses made with " Victoria," a white-
flowered variety, of which more will be said later. Lock
saw the same thing with a round-seeded native Ceylon pea
having coloured flowers. I have similar results from Fill-
basket, Express and Blue Peter, all white-flowered types.
In all these cases the F., seeds, borne by F^ plants, are all
alike indent, none being round.

On inspection of such seeds it seems imposslbk? to sup-
pose that segregation has occurred. On sowing it is never-
theless found that the resulting /% plants are respectively

* In such a case the sorting oi' living seeds can only be approximate,
for to the eye the distinction is not quite sharp. They can be sorted
immediately by microscopical examination of the starch.

262 Maternal Characters ijt Seeds [ch.

coloured and uncoloured in the proportion 3:1, the coloured
all have indent seeds and the white-flowered all have round
seeds. Segregation is therefore normal, and the fact that
all the F^ seeds are indent is in some way brought about by
the nature of the maternal envelopes in which the seeds
develop. How this influence is exerted we cannot suggest, 1
but perhaps there is some quality in these seed-coats which
causes the loss of water on ripening to take place irregularly
and so induces an irregular shrinking of the cotyledons.
Such an account is difficult to apply, for the seed-coats seem
uniform and homogeneous, and as the next case proves, the
influence of the seed-coat, whatever it be, operates in an
extraordinarily capricious and specific manner.

Until recently the account given above was supposed to
apply to all crosses of round with indent. Going over
seeds which were harvested in 1904 and 1905 without
examination at the time I find that one case is altogether
different from the rest. In this the round parent was
Nain de Bretagne, a small, very round, white-flowered
variety. Indent fertilised by this variety gave F^ seeds
indent. These when sown became coloured F^ plants, but
their /% seeds, instead of being all indent as in the examples
described, are quite definitely indents and rounds, in the
usual 3 : I ratio ^. Moreover among the few F^ plants of
which the seeds were harvested, was one which had only
round seeds (in coloured coats) and it is evident that this
plant came from a round F^ seed which was sown without
note being taken of its character.

We are thus presented with the exceedingly definite and
specific fact that one round-seeded variety, by virtue of its
intrinsic nature, behaves quite differently from the others
that have been tried. Presumably its reserve-materials
(and by inference the ferments which lead to their formation)
have some distinctive property such that though ripening
in a seed-coat which would make the other round peas
shrink as indents, they are still able to retain their own
characteristics. Such a fact may well be remembered in
any discussion of the nature of specificity.

* Several were intermediate in appearance, as in such cases must be
expected. Three plants, for instance, had 339 indent, 119 round, and
39 uncertain.

XIV] Maternal Characters in Seeds 263

One further phenomenon of importance has been dis-
covered respecting indent peas. It was said above that
the true indents are in coloured coats, but it is obvious that
a coloured coat, as such, is not sufficient to mak(* peas
indent, for there are many round varieties with coloured
coats. It is clear from the experiments of Tschermak (273)
and Lock (173, 175), as they point out, and from results of
my own, that the properties of the indenting- coats depend on
two factors, viz. (i) the indenting factor proper, (2) a requisite
pigmentation. For crossing coloured, round varieties with
"Victoria," a white variety commonly round, F^ bore F,
seeds indent. Lock had a similar result from a round,
coloured, native Ceylon pea crossed with Satisfaction
(white, wrinkled), but as wrinkledness was introduced, F.,
seeds were indent and wrinkled. I also by crossing Maple
(round, purple flowers) wdth Victoria Marrow, found all /%
seeds indent.

In these cases it is evident that the indentinor factor
was introduced in the white-flowered plant, and this, meeting
its complementary pigmentation-factor from the coloured
side, was able to indent the /% seeds. It is worth noting
that ''Victoria," which I take to be our "Victoria Marrow,"
though generally a round type, produces sporadically a good
many seeds with some degree of indentation or pitting*.
These (and William I) come nearer to the true indent than
those of any white-flowered variety known to me. Probably
this is due to an imperfect and occasional manifestation of
the powers of the indenting factor, which cannot produce its
full effect as the pigment-element is missing ; but when that
element is brought in by the cross the compound factor is
complete and the seeds are indented.

An experiment of Tschermak's (273, p. 30, Case 9)
contributes one more important and instructive' fact, which
gives a clear indication as to the identity of this pigmen-
tation-factor. He crossed two round-seeded varieties both
having coloured flowe7's, and the F., seeds were indented.
Now one of these was an ordinary purple type, but the
other was of the pink (salmon-pink) colour well known tor

* I found by sowing that tlie most indented and tlie roundest give
identical results in the next generation. The distinction is lluctuational,
and is unaffected by selection.

264 Maternal Characters in Seeds [ch.

Instance in the " Mummy-pea" of our gardens. The
difference between these two is that the purple has the
blueing factor which in discussing Sweet Peas &c. we have
called B, and there can thus scarcely be a doubt -that this
B factor is the complement of the indenting factor which
here came in from the pink side.

Analogous observations have been made In regard to
Maize, especially by Lock. According as the seeds are
opaque or semi-transparent, the varieties are distinguished
as "Dent" or ''Flint." These distinctions are also maternal
characters, and though segregation is normal, its effects
cannot be seen by examining the cob of F^ seeds. Con-
ditions however, in particular the degree of ripeness, cause
a complication in this case, for the seeds, at the top and
bottom especially, may remain comparatively flinty after the
rest have assumed the dent character.

In Wheat the relation between hard and soft endo-
sperms is probably similar, but Professor Biffen tells me
that in that case also complications occur.

This group of cases introduces us to several points of
Interest. We have first the remarkable fact that the mother-
plant can Impress varietal characters on her offspring by
Influences which are not heredity in the ordinary sense.
Seeds are in botany what larvae are in zoology, and no
example is yet known in which the maternal Impress extends
beyond the seed-stage. But without any serious stretch of
Imagination we may suppose that a maternal impress may
be such as to produce an effect lasting at least for the life-
time of the immediate offspring ; and It w^ould not be
altogether surprising if such results were actually detected
in the cases enumerated ; for the difference in food-materials
between those provided by a dent seed and a flint, a
glutenous and a starchy, may, for aught we know, influence
the later life of the plant, just as the nature of the milk
supplied to the human infant is believed to do. Such
influences may probably enough be limited and perhaps
trifling in comparison with those that are in the strict
sense genetic, but we do not yet know that they are neg-
ligible.

XIV] Maternal Characters hi Seeds 265

The next feature of Interest Is the specific behaviour of
the round pea Nain de Bretagne, which distinguishes it
from the other round peas, creatine^ a problem In the
chemistry of the reserve-materials well worth investigating.

Lastly we have one more illustration of the special
properties of the '' B factor." This factor, it will be
remembered, was concerned both in producing some of the
phenomena of coupling, and spurious allelomorphism in the
Sweet Pea, and we may anticipate that Its presence will
be shown to have profound effects on the constitution of
plants^.

* The seed-shapes of Sweet Peas have not been studied. There is a
great diversity in size. Some also are quite spherical, others being some-
what elongated. The self-coloured vwlel-dowered kinds alone (Countess
Radnor, &c.) have shrivelled seeds. In jF^ seeds from these crossed
with common round sorts no mixture of shapes occurs, and the shape
is evidently a plant-character.

CHAPTER XV

BIOLOGICAL CONCEPTIONS IN THE LIGHT OF
MENDELIAN DISCOVERIES.

Nature of Units — Natttre of Segregation — Moment of
Segregation — Differentiation of Parts compared with
Segregation — Reversion and Va^'iation. '* Bush " and
*' Ctcpid'' Sweet Peas — Mendelian Segregation and
Species — Discontinuity in Variation — Mendelism and
Natttral Selection,

The purpose of the preceding chapters has been to
show the method of Mendehan analysis in apphcation to a
variety of problems, and to describe the concrete discoveries
to which that method has already led. To discuss the
bearino^ of the new facts on biolos^ical science is now a
considerable undertaking and all that will be attempted
within the limits of this volume is a slight sketch of the
possibilities which these facts suggest.

Natttre of the Units.

With the recognition of unit-characters our general
conceptions of the structure and properties of living things
inevitably undergo a change. We begin to perceive outlines
where previously all was vague, nor can we doubt that
those outlines will very soon become clearer. What the
physical nature of the units may be we cannot yet tell, but
the consequences of their presence is in so many instances
comparable with the effects produced by ferments, that with
some confidence we suspect that the operations of some
units are in an essential way carried out by the formation
of definite substances acting as ferments.

CH. XV] Nature of Units 267

The conception of dominant characters as each due
to Xh& presence of something which is absent from the corre-
sponding recessive may prove of use as assisting towards
the identification of these problematical bodies. To most
of the cases of allelomorphism between characters yet
detected, this method of representation may be very readily
applied. The round seed of peas, or of maize, is one which
contains something possessing the power of turning most of
the reserve-materials into starch. If the dominant factor
endowed with this power is absent, much of the sugar
remains sugar, and the seed wrinkles on ripening. The
actual physiological processes involved are doubtless more
complex than this, but there is no mistaking the essential
nature of the distinction between the round and the wrinkled
seed. So also it is easy to understand that an albino is an
organism from which a ferment responsible for the pro-
duction of colour has been omitted. No great strain is put
on this hypothesis of dominance even by the suggestion
that the production of hairs on the leaves of a Stock, or of
the disposition to go broody in a hen, may be directly
caused by specific substances, for it would not be difficult to
adduce pathological parallels for the production of bodily
and mental chanoes such as these in cases where the chanofe
is plainly the work of a specific poison. Again it is easy to
imagine that the presence or absence of a ferment can confer
a greater power of resistance to the attack of a fungus. I
suppose also that the dominant whites met with in some
animals and plants may reasonably be represented as
organisms possessing a substance which has the power of
suppressing the development of pigment, whether by pre-
venting its excretion or by destroying it when formed.

On the other hand it must be admitted that in appli-
cation to those examples in which the dominant factor
operates by inducing or suppressing the division of a certain
organ, the analogy with ferment-action is more difficult to
maintain. The double comb of fowls is nevertheless a
single comb made bifid by the addition of a dominant
factor. To suppose also that the suppression of a phalanx
in the digits of a brachydactylous man, or the development
of a rabbit's fur in the normal and not in the Anoora manner
is thus decided by a specific substance, is to contemplate a

268 Nature of Segregation [ch.

serious extension of the powers of specific substances as
usually imagined.

But disregarding for the present these more obscure
cases in which meristic phenomena play the chief part, we
may draw from Mendelian observations the conclusion that
in at least a large group of cases the heredity of characters
consists in the transmission of the power to produce some-
thing with properties resembling those of ferments. It is
scarcely necessary to emphasise the fact that the ferment
itself must not be declared to be the factor or thing trans-
mitted, but rather, the power to produce that ferment, or
ferment-like body.

Nature of Segregation.

Next we have to recognize that this antecedent power
must be of such a nature that in the cell-divisions of oameto-
genesis it can be treated as a unit, being included in one
daughter-cell and excluded from the other at some definite
cell-division. As we have no knowledge as to the actual
nature of the factor — and only a conjecture as to whether it
is a material substance, or a phenomenon of arrangement —
we are not in a position to hazard so much as a guess re-
specting the physical process of segregation. We may be
fairly confident that segregation is not a process of chemical
separation. I ts features point rather to mechanical analogies.
If some mental picture be demanded, I would for the purpose
of illustration suggest a comparison with the separation of
a fairly heavy precipitate from a filtrate by decanting. The
precipitate is to represent the factor, and the filtrate the
recessive product of division, lacking the factor. The
analogy is probably quite erroneous, and small purpose
would be served by developing it further.

Whatever that process may be, it must be one which is
applicable to an extraordinary variety of factors. No con-
clusion to which genetic research has led is so surprising
as the fact that the same system of transmission should be
follow^ed by characters which, by whatever test they are
judged, must be supposed to be most diverse in physio-
logical causation. Even if it were fairly easy to conceive
of all the dominant characters of animals and plants as due

XV] Moment of Segregation 269

to the operation of ferments their diversity must be anyhow
very great, and it seems strange that all these multifarious
potentialities should exhibit gametic allelomorphism. Let
us take an illustration. Colour, as we can prove in regard
to several plants and as we suspect in the case of animals,
is due to the meeting of two complementary factors. One
is presumably a ferment. Recent research strongly suggests
that it may be a tyrosinase'^. The other is referred to
sometimes as " chromogen." But whatever they are, the
two bodies — and surely the factors which produce them —
must be of utterly different nature, and yet genetically the
two potentialities are treated similarly. Each is allelo-
morphic to the absence of such a power.

• Similarly in regard to the anthocyanin colours of
Antirrhmtim Miss Wheldale has given reasons for the
belief that they are formed by the meeting of a tannin-
like body with a ferment, perhaps an oxydase. Both these
factors, whatever be their nature, are allelomorphic to their
absences.

How much more astounding is it, that when we pass
to qualities such as length of stalk, and shape of flower, the
I shape of a cock's comb, or the development of interlocking
barbules on the webs of its feathers t, we still fmd the same
rules in undeviating operation 1

Moment of Segregation.

At what particular cell-division of the many by which
the germinal cells are brought to their maturity does this
process of segregation happen } The question is of extreme
interest, but no positive answer can yet be made. Natu-
rally the common expectation we all share is that the
reduction-division is the critical moment. At that division
the number of chromosome-elements, of w^hich the nucleus
is formed, can be seen to be halved. Up to this point the
nucleus of each daughter-cell seems to be simply a repe-
tition of the nuclei of the body and may be supposed to
contain all the elements which they contain. But when
the number of these elements is halved, the germ-cell

* See F. M. Durham, Proc. Roy. Soc. 1904, vol. 74, p. 311.
t Distinguishing them from the loose, non-cohering webs of the feathers
in the Silky breed.

270 Moment of Segregation [ch.

begins to acquire its own special features, and we may
without much difficulty imagine that if two daughter-cells
are to be differentiated allelomorphically from each other,
the differentiation will come about at this reduction, or
meiotic division, as it is called. When however we seek for
proof, there is as yet none which Is quite convincing. The
one observation bearing immediately on the problem is
that to which Correns has appealed"^. In the maize we
know that those F^ seeds which have wrinkled or sugary
endosperms will give rise to plants with similar sugary
seeds. But the seed of maize is formed by a dottble fertili-
sation. It consists of two parts, an embryo, and an endo-
sperm, and it is the sugary condition of the reserve-material
in the endosperm which causes the wrinkled appearance of
the seed. From the fact that wrinkled seeds always give
rise to plants bearing wrinkled seeds, it is clear that the
same elements have entered into the composition of both
the embryo and the endosperm of the same seed. But the
embryo is formed by the union of one nucleus of the egg-
cell with one from the pollen-tube, and the endosperm is
similarly formed by the union of the united polar nuclei
with another from the pollen-tube. Hence we may infer
that the two nuclei brought in from the pollen-grain bear
similar allelomorphs, and that all the nuclei of the egg-
cell must also be similar in composition. Therefore the
segregation of characters cannot in the maize take place
after the division by which the pollen-grain was formed,
for both nuclei to which it gives rise are of the same
composition ; and the same argument applies to the egg-cell.

Chromosomes as the possible Bearers of Factors.

These are, I believe, at present, the only facts which
positively limit the inquiry. It has been pointed out by the
cytologlsts that the details of the processes by which nuclear
reduction is accomplished may be readily construed as
effecting the operation of segregation, but while admitting
this as a fair and even probable interpretation, nothing
in my judgment yet compels us to accept it as proved.

* The same conclusion is strongly supported by the evidence from
Hieraciiim (see p. 247).

XV] Chromosomes and Heredity 271

Much that Is known of chromosomes seems Inconsistent
with the view that they are the sole effective Instruments
in heredity.

Without presuming to a definite opinion on this question,
I venture to state what seem to me formidable difficulties
in the way of this expectation. If the chromosomes were
directly responsible as chief agents in the production of
the physical characteristics, surely we should expect to find
some degree of correspondence between the differences
distinguishing^ the types, and the visible differences of
number or shape distinguishing the chromosomes. So far
as I can learn, no indication whatever of such a corre-
spondence has ever been found. Besides this, although no
very thorough investigation of the chromosomes of somatic
structures has yet been made on an extensive scale, I
believe that consistent cytologlcal distinctions between the
nuclei of the various tissues of the same body have not been
detected. If chromosomes were the chief ofovernors of
structure, surely we should find great differences between
the chromosomes of the various epithelia, which differ
greatly in their structure and properties. As these cyto-
loglcal differences have not been found consistently there,
the prospect of successfully tracing them among the specific
types does not look very hopeful.

Again, no correspondence between the chromosome
numbers and complexity of structure has ever been asserted
to exist. Low forms may have many ; highly complex types
may have few.

Then, on the contrary, very closely allied types may
show great differences in these respects. As Is well known,
Rosenberg has shown that one species of Drosei'a has 20,
while another has 10. Again, Miss Lutz, and, independ-
ently, Gates, have found remarkable div^ersltles in Oenothera,
especially \}i\2X gigas has 28, while lata has 14*. Obviously
this doubllnor means somethlnor definite, but It is not su^r-
gestlve of the determination of specific difference.

In Aphis Miss Stevens, on the other hand, has shown
how wide a diversity may be presented by the chromosomes
of forms so alike as to have passed for one species. These

* Important evidence as to variations in chromosome numbers has
been published by R. R. Gates, Botanical Gazette, July, 1908.

272 Bud-Sports [CH.

differences prove both too little and too much. I cannot
but believe that all this evidence points to the conclusion
that we are about to find among the chromosomes one more
illustration of the paradoxical incidence of specific difference,
not the fundamental phenomena on which that difference
depends. Among coleopterists punctulation is sometimes
a feature of great systematic importance. To dipterists
neuration and chaetotaxy sometimes give useful critical
data. In certain orders of Lepidoptera, the Hesperidae, for
example, the structure of the gonapophyses sharply dis-
tinguishes the species where all outward tests fail. But
proceeding farther with each of these criteria, we are sure
to come upon other groups where for a long series of
diverse types the critical feature, so important elsewhere,
may show no differences, or, on the contrary, may show
marked instability.

There remains the suggestive fact that all that has been
witnessed regarding the behaviour of the chromosomes is
in fair harmony with the expectations which our Mendelian
experience would lead us to form respecting the hypothetical
*' bearers " of varietal differences^. On the other hand, with
one striking exception, nobody has been able to connect
a cytological difference with a character-difference in any
instance. The exception, of course, is the case of the
accessory chromosome which the researches, especially of
Wilson and Morgan, have proved to be definitely connected
with the development of femaleness (see p. i88j.

The Case of Bud-sports.

There is another circumstance which must be taken
into account in any attempt to see the facts of segregation
in proper relation to other biological phenomena. This is
the obvious fact that when a bud-sport occurs on a plant,
the difference between the sport and the plant which pro-
duced it may be exactly that which in the case of a seminal
variety is proved to depend on allelomorphism. This subject
may best be discussed in reference to a practical illustration.
All naturalists remember the passage in Ani7nals and Plants

* The recent work of Godlewski gives however strong reason to beHeve
that heredity in Echini may be governed by the cytoplasm of the egg.

XV] Bud-Sports 273

tinder Domestication^ in which Darwin collected the evi-
dence about the relation between peaches and nectarines.
The records proved abundantly, (i) that the seeds of
peaches may come up nectarines ; (2) that conversely the
seeds of nectarines may give rise to peaches ; (3) that
peach-trees may by bud-sports produce nectarines, the sport
either involving a part so large that a definite nectarine-
bearing branch is formed, or so small that only a segment
of a fruit is affected, part of the fruit being peach and part
nectarine ; (4) in one instance, the (Carclew) nectarine is
said to have produced a branch which bore peaches.

With the exception of the last fact (4) the significance
of this series of observations is now clear. The nectarine
is essentially a glabrous variety of the peach, and on the
analogy of other cases, the hoariness of the peach is pre-
sumably a dominant character. Thus, when from the seeds
of peaches, nectarines spring, we perceive that this is the
ordinary phenomenon of a recessive variety arising from a
dominant hybrid. When, on the contrary, peaches are
produced from the seeds of nectarines, the fact plainly
suggests that the nectarine has been pollinated from a
peach. To come next to the cases of the bud-sports, that
in which nectarines appear on peaches must be interpreted
as meaning that in the formation of that bud or cell from
which the branch, or fruit, or part of a fruit, derived its
separate existence, the element or factor for the peach-
character was omitted. Therefore at some cell-division,
evidently a somatic division, segregation of the allelomorph
for hoariness must have taken place, and we are thus obliged
to admit that it is not solely the reduction-divisions which
have the power of effecting segregation.

Case (4) remains unelucidated. The record is most
circumstantial, and its truth can scarcely be called in question.
Nevertheless it stands, so far as I know, as an isolated
instance. Whether the case is, as we might perhaps be
tempted to suppose, one of actual, de novo origin of a
dominant feature ; or whether, as seems more probable,
this particular tree was in reality a monstrosity due to
imperfect segregation of that character in the germ of a

* Vol. I. p. :i^(i2.

B. II. 18

274 Heredity mtd Repetition of Parts [ch.

heterozygous parent'^, we cannot say with any confidence ;
but in considering the significance of the phenomenon of
bud-sporting that special problem is of subordinate conse-
quence, for in either event there must have been a process
of allelomorphic segregation at some somatic division.

In our Sweet Pea cultivations a phenomenon precisely
comparable occurred in two individuals of similar breeding.
The plants were purples of the dark type with purple
wings (Plate III, fig. 7), and were heterozygous for the
blue factor, B. After a few hot days they stopped flowering.
Then wet weather succeeded and much secondary growth
was made, young flowering shoots springing in the axils of
the older stems. On two individuals one of these young
shoots bore a flower of the red, or Miss Hunt type
(Plate III, fig. 8), showing that the factor B had been
omitted in one of the cell-divisions by which they were
formed.

The Differentiation of Repeated Parts compared

with Segregation.

Such facts raise a theoretical question of fundamental
importance. If upon the same individual, parts may as an
abnormal occurrence present the same differentiation which
is known to be characteristic of dominant and recessive,
may not the differentiation normally existing between re-
peated parts of the same individual be a phenomenon of
segregation ? Why, for instance, may not the differentiation
normally existing between petal and leaf, or between the
appendages of arthropods, or any other meristically repeated
parts, be due to a segregation acting amongst somatic parts
as amongst gametes ? Evidently, as morphologists have
often argued, the relationship between individuals is com-
parable to that existing between repeated parts. By tracing
the comparison in one direction we reach the fact that
hereditary rese^nblance is the same phenomenon as that
which we elsewhere know as symmetry : for if a cell divides
into two similar halves, and each half undergoes similar

■* Comparable, for instance, with gynandromorphous insects, half male,
half female ; or with white flowers showing a well-defined patch of some
coloured variety.

XV] Heredity and Repetition of Parts 275

changes and developments, while remaining attached to
the other half, we call the resemblance between the two
halves Symmetry ; but if the division is one by which two
new individuals are formed, and the two halves separate
and lead independent lives, then we ascribe the resemblance
between the two individuals to heredity. On a previous
occasion (11) I pointed out that when the comparison is
followed in the other direction it appears that if

I, Symmetrical Repetition of Parts is comparable
with Heredity,

then 2. Differentiation between Parts is comparable with

Variation,

That there must be limits beyond which the comparison
fails, is clear enough, but I do not think they have been
yet satisfactorily defined. The discovery of a true delimi-
tation of the properties and attributes of individuals, which
distinguish them from parts, would constitute a great advance
in biological theory. Perhaps the nearest we can get to
such a distinction is a recognition of the fact that ordinary
somatic differentiation is ^/i"?^^//)/ distributed in a symmetrical
manner about one or more axes, while among gametic
tissues such axes are not tcstially perceptible ; but to both
statements there are some notable exceptions. Bud-sports,
however, never, so far as I know, are distributed symmetri-
cally about the axis of the individual producing them, and
thus are distinguishable from ordinary somatic differentia-
tions. Dr S. F. Harmer in his Presidential Address to
Sect. D of the British Association (Dublin, 1908), describing
the various forms of avicularla"^ found on the same colony
of certain Polyzoa, made the interesting suggestion that
there may be an allelomorphic relationship between these
parts. Sometimes one special type of avicularia characterises
a species, sometimes another type ; while again both types
may occur together on the same colony. Now this is a
case to which such a suggestion is especially applicable ;
for the different types of avicularia are not distributed In
a symmetrical pattern, but apparently at random on the

* These structures are prehensile organs of various patterns, somewhat
resembhng minute crab's claws or the heads of birds. Morphologically
they are regarded as much modified " individuals."

18—2

276 Symmetry [ch.

colony just as bud-sports are on plants, and hence It is by
no means unlikely that the differences between the avicularia
may be due to allelomorphic segregation.

Asymmetry and Variation.

Stripped of all that is superfluous and of all that is
special to particular cases, genetics stand out as the study
of the process of cell-division. If we had any real know-
ledge of the actual nature of the processes by which a cell
divides, the rest would be largely application and extension.
It is in cell-division that almost all the phenomena of
heredity and variation are accomplished.

Heredity being a special case of symmetrical division,
genetic variation is the consequence of asymmetrical division.
The cause of the asymm.etry may lie far back in the history
of the tissue or of the germs. The germ-series may for
instance be represented as

1. A

/ \

2. A A

T^. A a A A

where the division in which the variant, a, first appears is
actually an asymm.etrical one ; or we may imagine the
process occurring in such a way that the first appearance of
a is produced by the division of an A cell into a and a
thus :

1. A

/ \

2. A A

3. a a A A

In the latter case the asymmetry was introduced into
the series at some division antecedent to that at which the
a form actually appeared. The question whether one of
these schemes is correct as a representation of the natural
processes, to the exclusion of the other, is a not unimportant
one, but there is no adequate ground for a positive answer
as yet.

In comparing the somatic differentiations with genetic

XV] Symmetry 277

variations it may perhaps be found hereafter of use to bear
in mind that just as the divisions of germ-cells are of two
kinds,

(i) Symmetrical, producing the resemblance called
Heredity,

(2) Asym^netrical, producing the difference called
Variation,

so are somatic cell-divisions recognizably of two kinds. For
there also we meet divisions by which similar parts are
divided from each other, and differentiating divisions by
which parts with distinct characters and properties are
separated. It is evident that when the conception of
symmetry is applied to such phenomena it must be under-
stood to include the case of production of like parts by
division which result in the formation of a successive series.

In this connection reference should be made to a work
of more than ordinary suggestiveness lately published by
Jennings (163). Speaking of the fission of Paramoecium
which results in the formation of two individuals by a
transverse division, he truly says that *' it is evident that
even in Protozoa heredity is not a mere result of sub-
division," for in the new individuals the head-end produces
— regenerates, perhaps we might say — a tail, and the new
tail-end grows a head.

That there must be a real difference between the
mechanical processes by which this repetition comes about,
and that in which two halves are formed as optical images
is clear. The latter is the obvious case of real geometrical
symmetry. But when two similar individuals are formed,
so that one is placed in succession to the other, this result
may be described as symmetrical in so far as the two pro-
ducts are similar, but the homologous parts instead of being
adjacent to each other, are arranged in alte^mating series.

Jennings' observations relate to a remarkable case. He
found a Paramoecium with a monstrous outgrowth, or spine
of protoplasm, and as the animal successively divided, this
spine was handed on to one not both of the products of
division. Obviously there is here something which may be
interpreted as providing a rough model of the process of

278 Reversion and [ch.

segregation. The spine may be taken as a thing added
and "present," or in our terms, a dominant character. In
each division there is found a dominant half which has the
spine, and a recessive half without it. If only we could
know what would happen as the result of conjugation
between two Paramoecia possessing similar "spines," it is
possible that the manner in which segregation occurs in the
gametogenesis of the Metazoa would be elucidated. In
them, as mentioned above (p. 195), the gametes are almost
always formed in sets oi fotir, and a presumption is thus
created that the members of each set of four are not all
equivalent to each other. If they were equivalent w^e could
represent the set as AAA A, supposing the zygote which
produced them were homozygous. If the organism were
heterozygous we should then imagine the series to be
AaAa. But if the differentiation is not by pairs but by
fours, the series must be represented by AaA'a' (or
AA'aa') for the heterozygote, and as AA'AA' for the
homozygote.

Reversio7i and Variation.

It follows from what has been said of allelomorphism
that variation must now be regarded in the mam as a
phenomenon due to the addition or omission of one or more
definite elements. \^ hen, as in the case of the combs of
fowls, the types which have obviously arisen later in the
evolution of the species, for example, rose, pea, and walnut,
dominate over the primitive type, in this case the single,
the variation by which those dominant varieties came into
existence must have consisted in the addition of an element
(or in the case of the walnut-comb, two elements) to the
original stock of the species. When, on the contrary, the
new variety is recessive, it is clear that the variety occurs in
consequence of the omission of an element. (The suggestion
made by de Vries, that the do^ntnant is always the phylo-
genetically older form, has not been substantiated by further
investigation.) The problem of the causation of variation
is thus to some extent narrowed down. The "cause" of a
variation is the event which brings about the addition or
omission of a factor.

XV] Variation 2^g

In reply to the question so often asked, Has modern
Investigation given evidence as to the nature of these
causes? the answer must still be, Almost nothingr. Siens
there are however that the search for such evidence Is
beginning at last to be not altogether unfruitful, but the
detailed consideration of this part of the subject must be
postponed to another occasion.

Reversion occurs when the sum total of the factors
returns to that w^hlch It has been in some original type.
Such a return may be brought about by the omission of an
element or elements, as when the rose-comb fowl for any
reason has a single-combed offspring. Conversely, the
return may occur by the addition of some missing element
needed to complete the original type. As yet no means are
known by which the omission or addition of elements can
be made at will, except by crossing. Reversion on ci'ossing
is thus the particular case In which one or more missing
factors are brought in by the parents of the cross-bred.
The most striking cases of such reversion on crossing are
those in which neither parent seems to the observer to
contain anything specially reminiscent of the original type,
and yet the offspring of the cross are all of that type. Such
cases are those of the two white Sweet Peas which, though
each severally breeding quite true to whiteness, when
crossed together have a reversionary offspring ; or of the
two breeds of Pigeon, which though neither has the blue-
barred plumage of the Rock Pigeon, yet contain materials
from which blue-barred birds may be compounded.

Not often can we hope to be able to specify the comple-
mentary elements which must meet each other in order that
a certain compound character may be produced. Neverthe-
less, by the co-operation of physiological chemistry with
genetics there is every hope that In favourable cases of a
simple order actual demonstrations of these elements may
be carried out. Perhaps the nearest approach to such an
achievement is that made by Miss Wheldale in her experi-
ments on Antirrkimini (Snapdragon). Crossing the very
pale yellow, known as " Ivory," with a white variety, she
obtained the F^ generation all of the dull red of common
Snapdragons (303)"^. The work of Overton and others had
* F^ was of the normal kind expected in such cases.

28o Reversion and [ch.

indicated, as she points out, that the red anthocyan is
probably a glucoside formed by a combination of tannic
acid with a sugar. Now in this case Miss Wheldale found
that the "ivory" flowers do contain a glucoside and that
this body is absent from the white flowers. The breeding
experiment proves that the white variety introduces a red-
dening factor which is absent from the "ivories." According
to her view the contribution which the "ivory" makes is
the glucoside, and the complementary contribution made by
the white is an oxidase which acts upon the glucoside to
make anthocyan.

With the development of the inquiry it has become clear
that variation, in so far as it consists in the omission of
elementary factors, is the consequence of a process of
'^unpacking." The white Sweet Pea was created in the
variation by which one of the colour-factors was dropped out.
Such variation is not, as it was formerly supposed that all
variation must be, a progress from a lower degree of com-
plexity to a higher, but the converse. When from a single
wild type, man succeeds in producing a multitude of new
varieties, we may speak of the result as a progress in
differentiation : but we must recognize that the term is only
applicable loosely, and that the obvious appearance of
increased complexity may in reality be the outcome of a
process of simplification. The facts nevertheless preclude
the suggestion that all variation even under domestica-
tion is of this nature, nor till experimental research has
developed far beyond its present limits, can we make any
confident estimate whether it is the one process or the other
which has played the larger part in the formation of the
diversity of living forms.

It is legitimate to conclude from what is known of
reversionary cases that when reversion in characteristics
other than colour results from crossing, similar processes
are operating. One such instance was described in con-
nection with the peculiar properties of colour-factors (p. 133),
a reversion to the hoary leaf resulting from the cross of two
glabrous types of Stock {MattJiiold). As yet however the
only other example of reversion in a structural character is
that furnished by the crosses between two varieties of Sweet
Peas known as Bush and Cicpid, The Cupids are the

XV] V ay i at ion 281

little prostrate plants shown in Fig. i. Crossed with
ordinary tall plants they ^\v^ a normal Mendelian result,
the tall being dominant. The Bush is a distinct type. It
is only half or two-thirds the height of the taller, and its
habit of growth is peculiar. The stems are erect, thin and
wiry, branching profusely, whence the description Busk is
derived. Bush again is an ordinary recessive to tall. But
Mr Punnett and I found, a good deal to our surprise, that
the cross Bush x Cupid gives F^ tali, i.e. reversionary
(Fig. 2il)' Though we cannot venture on any surmise as to
the chemical causation of this phenomenon it is obvious
that once again the reversionary character, here the tallness,
is a compound character due to the meeting of comple-
mentary elements. The allelomorphs are

Do7ninant. Recessive.

1. Tallness {T), Dwarfness (/),

2. Prostrate habit : Erect : branching (/).

non-branching {F),

In 7^2 therefore a new type occurs, namely an erect or
Bush-like Cupid (Fig. 38), and the F^ series is

9 Tall ( TP) : 3 Bush ( Tp) : 3 prostrate Cupid (tP) : i erect

Cupid {tp).

The "height" of Sweet Peas is evidently therefore a
compound character and m part depends on the existence
of a factor which suppresses or inhibits that stimulus which
otherwise would compel it to branch. A successful analysis
of the physiological processes concerned in this series of
phenomena would be a significant addition to plant
physiology.

The analysis of compound structural characters may
confidently be expected to lead to the recognition of
numerous examples comparable with these two.

With the progress of such analysis other examples will
certainly be encountered illustrating those curious inter-
relations between the factors spoken of under the names
Coupling and Spurious Allelomorphism. In the existence
of such phenomena we meet evidence that the central
problem of genetics is in part a geometrical one. Perhaps

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CH. XV]

Segregation and Species

283

the greatest advance that can be foreseen In this department
of physiology will be made when the nature of the inter-
action between the chemical and the geometrical phenomena
of heredity is ascertained.

Bearing on the Theory of Evolution.

The consequences of these discoveries to the general
theory of Evolution must be examined in reference to
concrete examples in order that the extent to which they
reach, and the limitations by which they are beset, may
rightly be apprehended. It is only by the detailed study
of actual cases of interrelation between kindred species that
the scope of Mendelism in this region of inquiry can be
measured and illustrated, I hope subsequently to publish
a separate volume'^ in which this part of the subject will
be considered. Two features which emerge salient on such
a survey may be named at once.

II.

Fig. 38. The two "Cupid" types in F^ from Cupid x Bush (Sweet Peas).
I. The ordinary, prostrate Cupid. II. The erect Cupid.

I. Mendelian Segregation and Species.

First it is certain that segregation in countless instances
plays a part in the constitution and maintenance of charac-
teristics held by systematists to be diagnostic of species.
One has only to glance over trays of birds' skins, the

* This volume which is designed to be in some respects a continuation
of the present book will be based on lectures given as Siliiman Lecturer to
Yale University in 1907, under the title "The Problems of Genetics."

284 Segregation and Species [ch.

portfolios of a herbarium, or drawers of butterflies and
moths to discover abundant *' species" which are analytical
varieties of others. The principles of heredity we trace in
our experimental breeding are operating throughout the j
natural world of species. They may apply to the inter- j
relations of allied forms which are species in the strictest
acceptance of that term. Remarkably clear and un-
controvertible evidence of this fact has been obtained by
Dr Ezra Brainerd in his studies of the American Violets.
The whole series of observations is full of significant details
which I must be content to omit from this summary. The
essential facts are as follows. Finding wild in nature plants
which from his knowledge of the genus Viola he determined
as accidental hybrids between certain species, he removed
these plants to a garden and kept them under observation.
They proved to be very nearly, but not quite completely,
sterile, thus manifesting one of the attributes of crosses
between genuine species in the strictest sense. Some small
quantity of seed was however produced in the cleistogamic
capsules, and therefore may be taken as certainly the result
of self-fertilisation. This seed gave rise to plants showing
obvious segregation in regard to many features — the colour
of the stems, the fruits, and the seeds, and also, though not
quite so palpably, in the shapes of the leaves. In some at
least of these derivative plants there was — as Herbert and
others have found in similar studies of the offspring of sterile
hybrids — some return of fertility. Such an experiment raises
the hope that successful investigation of the nature even of
the sterility consequent on crossing, the most obscure of all
genetic phenomena, may become one of the possibilities of
Mendelian research.

It is scarcely necessary to insist that plenty of the
characters which are now known to segregate would be far
more than sufficient to constitute specific differences in the
eyes of most systematists, were the plants or animals in
question brought home by collectors. We may even be
certain that numbers of excellent species universally recog-
nized by entomologists or ornithologists, for example, would
if subjected to breeding tests be immediately proved to be
analytical varieties, diftering from each other merely in the
presence or absence of definite factors.

XV] Segregation and Species 285

But this Is not enough. We must eventually go further;
and, supposing such tests to be applicable on a compre-
hensive scale to great numbers of natural forms, we must
ask whether the result of such an investigation will show
first that certain kinds of differences seQreo-ate and that certain
other kinds do not segregate; and secondly whether we shall
then recognize that it is to the non-segregating that the
conception of species attaches with the greater propriety.
Since of the non-segregating characters we know as yet
almost nothing, I am loth to attempt an answer, but I
cannot imagine upon what evidence anyone would rely
who should maintain that the answer must be affirmative.
De Vries has, as it seems to me, incautiously, defined with
some strictness the differences between varietal and specific
distinctions, declaring that it is the property of varietal
characters alone to exhibit Mendelian heredity (295, &c.).
Though I agree with him in perceiving that genetic research
may ultimately provide some approach to a valid distinction
between species and variety, I am reluctant to accept any
evidence yet attained as an adequate basis for so vast a
generalisation. Of the consequences of specific crosses —
in the stricter sense — little is known, and no case has been
fully explored. Even as to the results of crosses between
petites especes differing in several characters, present
information is most imperfect. Such cases as that of the
Violets, mentioned above, must be thoroughly investigated
by critical methods; and when a good number of these
examples taken from an ample range of types have been
submitted to factorial analysis it wqll perhaps be possible to
come to a more confident decision. But before any decision
at all is pronounced or even contemplated, the laws which
govern the Incidence of sterility must be most carefully
determined.

Feeling thus the Impossibility of now defining the
segregating from the non-segregating, I am unable to
follow de Vries in the further step which he has taken (299)
in assigning a definite physiological reason for the difierence
between these classes. His suggestion is that in Mendelian
heredity there is a process, spoken of as "bisexual," in
which each determining factor i^Anlage) meets a corre-
sponding opponent; while in the other, or "unisexual" case.

286 Disco7ttiniiity [ch.

the factor in question remains unpaired in the hybrid. The
latter description, "unisexual," is applied to characters which
breed true in the cross-bred. This suggestion is the out-
come of an attempt to incorporate the facts of Mendelian
inheritance with the conclusions previously drawn from the
mutations studied in Oenothera, But as knowledge of
Mendelian cases has increased, the applicability of what is
here spoken of as the "presence and absence" hypothesis
becomes more and more clear. We now, in fact, feel fairly
sure that a heterozygote is properly represented as one
which contains an unpaired factor. Hence the doubt may
be expressed whether if de Vries' terminology is to be
maintained, its application should not be reversed.

2. Discontimnty in Variation,

When some years ago I published a collection of facts
illustrating the phenomenon of Variation in animals^, I
pointed out that variation is frequently definite, or Discon-
tinuoMS. That conclusion is one which cannot fail to strike
an observer who makes a study of this part of physiology.
Inasmuch as the discontinuity of variation is manifested
again and again in respect of exactly those differences
which we are accustomed to recognize as distinguishing
specific forms from each other, the further conclusion
followed that the diversity of species may be regarded as
having come about very largely by the occurrence of these
discontinuous variations

The materials then put forward related almost entirely
to a restricted group of phenomena in animals, those which
are known as meristic, exemplifying the processes of change
in the number of parts and the relation of repeated parts
to each other. Had the field of inquiry been widened by
the Inclusion of variations in other characteristics of animals,
or in those of plantsf, a body of evidence more clearly
demonstrating the truth of this thesis could have been
presented. There were, however, reasons which led to the

* Materials for the Study of Variation^ 1894.

t A useful collection of facts of this nature in plants has been published
by Korschinsky, under the title "Heterogenesis und Evolution," Flora^ 89,
1901.

XV] Mtitatioii 287

preference of the one special group of facts — the merlstic —
as beinor in their nature more fundamental and homooeneous,
and my object was rather to map out the ground than to
erect a definite proposition upon it. The book was to have
been fohowed by similar collections dealing with the other
manifestations of variation ; but with the development which
genetics almost immediately underwent, it became clear that
the method of miscellaneous collection was no longer the
most direct, and that by experimental investigation of special
cases progress of a far more valuable order was possible.

Views somewhat similar to those that I had formed from
a general survey of the facts of variation were shortly
afterwards published by de Vries in his famous book Die
M^ttationstheorie, 1 901-3. Having at command a mass of
evidence far larger and more coherent than mine he was
at last successful in bringing workers of many schools to
eive these sua-Qfestions a serious consideration. For the
first time he pointed out the clear distinction between the
impermanent and non-transmissible variations which he
speaks oidJ^fliLctuatio^is, and the permanent and transmissible
variations which he calls imitations. Of his proofs, the
most striking, to many the. most convincing, Is that provided
by his study of Oe^iothera, in which he witnessed the actual
occurrence of sudden departures from type — not one but
several — by which at one step In descent distinct and
frequently pure-breeding types were produced. Whatever
be the true interpretation of these particular observations,
they manifestly provide examples of something so like the
generation of new species that in any future discussion of
Evolution they cannot possibly be passed over.

We may be doubtful of the validity of the superstructure
which de Vries has created, and yet in full agreement with
him in recoenlzlno- the fundamental truth, that there Is a
natural distinction between fluctuational variations and actual
genetic variations ; that the latter are those alone by which
permanent evolutionary change of type can be effected ; and
that commonly, though, as it seems to me, not always, the
steps by which such changes occur are so discontinuous as
to merit the name Mutations.

It is at this point that Mendellan discovery aids.
Whereas formerly, though the fact of Discontinuity was not

288 Natural Selection [ch.

doubtful, there was nothing to indicate how or when it was
determined. We now see that the discontinuous variations
are in the main the outward manifestations of the presence
or absence of corresponding Mendehan factors, and we
recognize that the unity of those factors is a consequence
of the mode in which they are treated by the cell-divisions
of gametogenesis. With the discovery of these factors
precise analytical treatment can at length be applied to the
problem of Evolution.

3. Mendelism and Natural Selection.

Knowledge of the physiology of heredity thus abolishes
an old difficulty often admitted to be an obstacle in the
way of any affirmation of Evolution by the process of natural
variation. The notion that a character once appearing in
an individual is in danger of obliteration by the intercrossing
of that individual with others lacking that character proves
to be unreal ; because in so far as the character depends
on factors which segregate, no obliteration takes place.
The factors are permanent by virtue of their own properties,
and their permanence is not affected by crossing.

If the acquisition of a new factor or the loss of an old
one is so damaging as sensibly to impair the chances of
life of the variety thus constituted, that variety must surely
be extinguished. On the contrary, the addition of a new
factor contributing sensibly to the success of its possessor,
or the omission of a detrimental element, will aid in the
preservation of those which exhibit that variation.

Mendelism, though aiding us greatly in showing how
the diversity of species and varieties may arise and be
maintained, provides no fresh clue to the problem of Adapta-
tion ; except in so far as it is easier to believe that a
definite, integral change In attributes can make a per-
ceptible difference to the prospect of success than that an
indefinite and impalpable change should entail such conse-
quences. Definite variational changes are being continually
offered, each giving an opportunity to natural or to artificial
selection, and we need not hesitate to declare that of such
materials the diversity of nature has been compiled. If
anywhere in such a province as evolutionary science certainty

XV] Natural Selection 289

may be reached, it is here. The conception of Evolution as
proceeding through the gradual transformation of masses
of individuals by the accumulation of impalpable changes
is one that the study of genetics shows immediately to be
false. Once for all, that burden so gratuitously undertaken
in ignorance of genetic physiology by the evolutionists of
the last century may be cast into oblivion. For the facts
of heredity and variation unite to prove that genetic varia-
tion is a phenomenon of individuals. Each new character
is formed in some germ-cell of some particular individual,
at some point of time. More we cannot assert. That the
variations are controlled by physiological law, we have now
experimental proof; but that this control is guided ever so
little in response to the needs of Adaptation there is not
the smallest sign. If chance variation was an improbable
source of the adapted diversity which living things exhibit,
the improbability remains, undiminished perhaps, but cer-
tainly not increased, by the recognition of that control.

There is also nothing in Mendelian discovery which
runs counter to the cardinal doctrine that species have
arisen "by means of Natural Selection, or the preservation
of favoured races in the struggle for life," to use the defini-
tion of that doctrine inscribed on the title of the Origin.
By the arbitrament of Natural Selection all must succeed
or fail. Nevertheless the result of modern inquiry has
unquestionably been to deprive that principle of those
supernatural attributes with which it has sometimes been
invested. The scope of Natural Selection is closely limited
by the laws of variation. How precise and specific are
those laws we are only beginning to perceive. In the light
of the new knowledge various plausible, but frequently
unsatisfying, suggestions put forward, especially by Wallace,
Weismann, and their followers, as probable accounts of
evolutionary progress, must be finally abandoned. It
cannot in candour be denied that there are passages in the
works of Darwin which in some measure give countenance
to these abuses of the principle of Natural Selection, but
I rest easy in the certainty that had Mendel's paper come
into his hands, those passages would have been immediately

revised.

For Darwin, indeed, Mendelism would have provided

B. H. 19

290 Natural Selection [ch. xv

sound reasons for a return to his own earlier views. In
abandoning his beHef in the importance of individual varia-
tions, which previously he had held in a form not incompatible
with that now demonstrated to be right, he took a step in the
wrong direction. The criticism before which he then gave
way has proved invalid '^. To him, most of all men, would
the knowledge have come as a delight, that progress, even
if in a direction unexpected by himself, had been made
with that problem the solubility of which he was the first
to make apparent to the world.

* As to this change of opinion see Darwin's letters to A. R. Wallace
{Life a?id Letters, 1888, ni. p. 108).

CHAPTER XVI

PRACTICAL APPLICATION OF MENDELIAN PRINCIPLES.

Meaning of Pttre-bred — Rogueing — Raising iVoveliies —
A Practical Example — Unfixable Types — Technical
Methods — Sociological Application.

No one who is acquainted with Mendelian method will
doubt that by its use practical breeders of animals and
plants may benefit. In so far as they are concerned with
the fixation of desirable varieties, or with the creation of new
types by re-combination of pre-existing characters, their
operations may now be greatly accelerated.

* ' Pnre-bred " and * ' Cross-bred.

II But apart from these obvious advantages which it may

confer, the new^ knowledge of heredity will react most
profoundly on the art and practice of the breeder by
introducing a new standard of precision. We at length
understand the physiological meaning of "pure-bred" and
"cross-bred." We know that these ideas must be applied
to the several characters of the animal or the plant, rather
than to the individual as a whole. For the individual to
be altogether pure-bred it must be homozygous in all
respects. In current parlance, dogs, for example, derived
from a cross a few generations back have been spoken of
as "I Bulldog, or -]^ Pointer blood, and so forth. Such
expressions are quite uncritical, for they neglect the fact
that the characters may be transmitted separately, and that
an animal may have only -z]-^ of the " blood " of some pro-
genitor, and yet be pure in one or more of his traits.

19 — 2

292 Practical Hints [ch.

Rogiteing.

The Introduction of these ideas will help much by showing
what is to be expected of a pure variety. I may give an
example. Hitherto, when in growing seed-crops, unde-
sirable " rogues " recur continually through long periods of
years, the fact has been accepted as part of the natural
perversity of the variety. The grower devotes much time
and expense in keeping the rogues down, but the idea that
they can be got rid of altogether does not generally occur
to his mind. Nevertheless in many such cases Mendelian
observation at once provides the means of carrying out
this radical treatment with success. I cannot here discuss
the intricate question of the reality and signification of that
degeneration of cultivated varieties which is believed to
occur generally and certainly occurs sometimes when selec-
tion is suspended. All that we can insist on at this stage
of the inquiry is the fact that much of the Irregularity of
crops which passes for such natural degeneration is readily
preventlble.

The rogue-plants may be of various kinds, and their
nature must be separately determined in each case. For
example, they may be recesslves merely, and if so they can
be eliminated by breeding from pure dominant individuals,
according to the system now well understood. It is possible
also that they may owe their existence — If the plants are
fertilised by insects — to special combinations of comple-
mentary characters. In that case, to exclude the possibility
of their production must be a more difficult, though not
necessarily a hopeless task. But in the light of present
knowledge one definite and conspicuous conclusion has
been attained, that for the appearance of each type in a
crop there must be some specific and usually ascertainable
cause ; and an aim of practical seed-growers should in future
be to search carefully for such causes. When this search
is made, the guess may even be hazarded with some con-
fidence that in numerous examples the cause of Impurity in
seed-crops will often be found to be nothing more recondite
than an unsuspected admixture of another variety, the seeds
of which are overlooked as apparently belonging to the
selected type. In a certain strain of eating peas I have

XVI] Practical Hints 293

seen a case of this kind. The true variety has a wrinkled
seed. The "rogues" had round seed. Every year the
variety was picked over by hand and the round seed rejected.
Nevertheless a pretty constant proportion of the rogues
persisted. Knowing that it was"" in the highest degree
unlikely that a zvrinkled p&ci, being recessive in that respect,
could give off a rotcnd-s^^d^d form, I examined the seed
with care and found that the seeds at the ends of the pods
of the rogues were liable to be shrivelled so much as to
pass for the true wrinkled type, and being thus admitted
into the selected seed, perpetuated the "rogues." This is
only one of many sources of error. Hitherto, through the
prevalence of incorrect views of the nature of variation and
heredity it has been thought more natural and likely that
plants should throw rogues than that they should not.
With the attainment of exact knowledge we see that the
opposite expectation is more probable, and in that hope all
operations of this sort should now be guided.

Raising Novelties.

When crossings between varieties are made, either with
the definite purpose of producing a combination of two
desirable qualities, or in the general expectation that some
novelty will turn up, it is scarcely necessary nowadays
to insist that the appearance and attributes of F^, the
first cross, o-ive no indication as to the failure or success
of the attempt. That outstanding fact is at length very
generally known and appreciated, with the result that first
crosses are now preserved which a few years ago would
have been rejected. More important is it to lay stress
on the necessity for sowing a really large sample of the
seed from which Fc^ is to be raised, for there must be
enough to give a chance of seeing the rarer combinations.
Since by the nature of the case most of the obvious crosses
between the familiar varieties of cultivated plants have been
tried in horticultural practice, the novelties are likely to be
found more often among these less frequent combinations.

A Practical Exainple,

As a good example of an F., family consisting of a
long series of types, that derived from one of the crosses

294 Practical Hints [ch.

between a white and a coloured P^'imtda Sinensis is figured
in Plate VI. The results of this mating have for several
years been studied by Mr R. P. Gregory in collaboration
with me. Though the quantitative relations between the
types, and their factorial composition are still not completely
worked out, the case forms a good illustrative example of
the operation of Mendelian processes. The coloured parent
is Sutton's Crimson King, a well-known very dark red sort,
with red stigma, red spots round the eye and full red stalks.
The white parent used was Primrose Queen, a stellate or
Star Primula, with a white flower and reddish stem. This
white type has the large yellow eye spoken of in connection
with the inheritance of heterostylism (p. 70), and as there
described, the style does not reach above the level of the
anthers, this being the condition known as homostyle.
Both types breed absolutely true to their respective
characters.

The plants, F^, produced by crossing these tw^o types
are quite uniform. In habit they are intermediate betw^een
the star type and the compacter shape of Crimson King.
The flow^er shape is also intermediate. The petals are
whitish, with a slight tinge of magenta, especially on the
lateral edges. When kept warm they lose the colour
almost entirely, but when they are kept cool, the colour
increases somewhat in amount.

By the self-fertilisation of such F^ plants a very complex
F^ series is produced. Plate VI gives a fair idea of some
of the more conspicuous types which appear, though it will
be understood that only the colour and the shapes of the
individual flowers can be represented there. In the mode
of growth, the shapes of the inflorescence and many other
features there is an equal diversity, so that an observer not
accustomed to the results of crossing may well find it
difficult to believe that this heterogeneous assemblage of
plants can all be the offspring of a single individual. A
systematist might make ten or even twenty species out of
such a family, w^ere the several types found isolated in
nature, and no one could accuse him of excessive ''splitting."

The magenta colours are evidently due to a factor
epistatic to the crimson or claret-coloured reds, and this
factor was obviously introduced by the white parent. In

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XVI] Practical Hints 295

F^ those whites which are quite devoid of colour, are pure
to whiteness. The crimson-reds are pure to redness. The
magenta types may be pure to magenta, or may throw the
hypostatic reds. The darker colours are recessive to the
Hghter, in the magenta classes, though the mutual relations
of the light to the dark among the reds are not so certain.

The large eye, combined with a homostyle structure, is
recessive to the small or long-styled type. i\s mentioned
(p. 139) the dark blotches round the eye can appear only
in those plants which have coloured stigmas. Another
curious point is to be noticed in this respect : that when the
dark blotches are developed in a large-eyed type, the blotch
extends over the whole area included in the "eye." If the
stigma be green the eye is therefore yellow (Plate VI, fig. 20).
The stigma may be coloured, though the eye is yellow ;
but if the stigma be coloured and the factor for the blotches
is present, then the appearance of the deep red and deep
magenta flowers figured in the bottom row (Plate VI, figs. 19
and 21) is produced. Such types are exceedingly distinct,
and might most naturally pass for different species. More-
over, of these singular varieties the red would always breed
true in both shape and colour, so that the illusion in that
case would be complete. The deep magenta type might also
be homozygous and breed true, but some individuals would
throw the red variety also. All the offspring of these
dark-eyed types would of course have the large dark eye.

As regards the relations of the various types all that
can be positively asserted is that the chief classes are
(i) pure white, (2) tinged white, (3) crimson-reds, and
(4) magenta-reds. There is some grading between the two
red classes, and between the pure and the tinged whites.
The distinction between the two kinds of red also cannot
be followed among the tinged whites. The numerical pro-
portions are therefore not quite certain, but presumably
the various whites collectively are 12, magenta-reds 3, and
the crimson-reds i.

In the plate no attempt is made to indicate these numerical
proportions and the flowers there shown are simply chosen
as representing the most distinct types which occur. Fhe
polymorphism of such an /% tamily is greatly increased by
the existence of extreme diversity in the shapes of the

2g6 Practical Application [ch.

umbels, the size of the calyx, of the Involucre of bracts,
and many other structural features the inheritance of which
has not been studied in detail.

Such a series will illustrate the ordinary practice of the
horticulturist who is engaged in the production of novelties.
He crosses together two types and picks out those novelties
which are produced in /% by the re-combination of pre-ex-
isting factors. This has been the method which has led to
the creation of nearly all our modern varieties of vegetables
and flowers. From Mendelian discovery the practical
breeder learns two lessons, both of importance. The first is
that he must not discard the F^ generation merely because
it does not give him anything he wants. This generation
may be uniform, and indeed must always be uniform if both
parental types were homozygous in all their several factors.
It may also be quite uninteresting from the horticultural
standpoint, exhibiting some old-fashioned or reversionary
type which is reproduced because all the factors which
constitute it happen to have been brought together into
one individual. But from the appearance and properties of
F^ no guess can be made as to the possibilities of /%. A
vast amount of valuable material has again and again been
discarded by practical horticulturists through ignorance of
Mendelian principles. The uninteresting types produced
by crossing, though no improvement on the old and familiar
varieties would, if their seed had been saved, have given
abundant novelties in the next ofeneration.

Fixing the Type.

The second practical lesson is more important. If the
plant is of a kind which is habitually propagated by budding
grafting, cuttings, or other asexual mode of division, no
doubt the object of the breeder is attained at once with the
appearance of his novelty. All that he has to do is to
multiply it. But If, as is generally the case with vegetables,
and very oft^n nowadays with flowers also, he requires to
work up a strain true from seed, Mendelian analysis shows
him how to accomplish this in the shortest time. He must
breed fro7n each individual separately. Take this case of
Primula. Suppose he wishes to get a pure strain of the
dark magenta {e.g. Plate VI, figs. 9, 15, 21). If he saves all

XVI] Practical Application 297

these together, some will be impure and throw the reds
(Plate VI, figs. 7, 13, 19), and he may go on for some
years saving collectively from the dark magentas but still
find a proportion of reds produced. The first time he saves
from each individual separately he will find that so77ze only
are impure and others pure. Then by saving from the pure
plants, his pure strain is immediately established.

When the desired combination has appeared, the readiest
way to perpetuate it is by self-fertilisation. But if, as
happens more often than is commonly supposed, there is
any considerable degree of self-sterility, the plant must be
fertilised by some other individual of the same type if that
can be obtained, or by one which is suspected of being a
heterozygous form containing the selected variety.

The advice to attend very carefully to this matter of
self-fertilisation may seem superfluous, but I know actual
cases where practical men have attempted for several years
to fix a new variety of a plant growing in the open ground
by merely leaving the individuals uncovered, exposed to the
visits of insects, though in an adjacent bed were plants of
the original type from which the novelty had been derived.
Year by year the proportion of plants which came true to
the new variety continued very small, and the fact was
accepted as a symptom of especial difficulty in fixing that
particular variety. A few yards of muslin arranged as a
cage over the plants and a few minutes spent in pollinating
the covered flowers would have saved much further trouble,
and the variety could have been raised true or " fixed " in
one season. In reality, of course, the supposed "tendency
to throw back " to the parent type was due simply to the
pollination of the variety by insects which had visited the
adjacent dominants.

The animal breeder, as he cannot self-fertilise his pro-
ductions, must follow a rather more complicated procedure,
but by the use of Mendelian methods he also can work with
certainty. He must take his birds or other animals and
test them for purity individually, usually by breeding each
first with a recessive, and then having found a pure
individual of each sex, he can by breeding these two together
create material for building up a pure strain.

298 Practical Application [ch.

Mendellan discovery at once abolishes the old delusion
that time and continued selection are needed in order to
make a variety breed true ; for the homozygous individuals,
which are the only ones that will breed true, may appear
in F.. It is the business of the breeder to find such indi-
viduals. By continued selection he may perhaps succeed
ultimately, for at each selection he somewhat increases
his chance of finding them, but by following Mendelian
method he can go straight to the desired end, obviating
many years' work.

Unfixable Types.

There are of course certain types which cannot be fixed
at all, for the reason that their special character is not
represented in their gametes, but is a special consequence
of the meeting of dissimilar gametes. In animals the Anda-
lusian fowl is a case in point.

The colours of Canaries are mostly of this class, and
in order to obtain the requisite shades of yellow various
crosses between pure coloured varieties are made, scarcely
any being bred pure for exhibition. The *' Golden Duck-
wing" of Game fowls is another heterozygous colour, and
can be produced by crossing Silver Duckwing with Black-
red. The only structural feature of this kind that I can name
is the crest of the crested Canaries, which is alwavs bred
for shows by mating cresteds with plain-headed birds (R. E.C.,
19 ; Davenport, 105). The admired crest consists of long,
drooping feathers radiating symmetrically from the top of
the head in one of several approved patterns. This neatly-
laid appearance is only produced when the bird is hetero-
zygous for the crest-factor.

Among plants there are doubtless many examples^. For
instance, among their pedigree strains of Pri^nula Sinensis
Messrs Sutton have met with two which are clearly of this
nature. One is a large-flowered type known as '' Giant
Lavender," having a pale magenta flower. This never
comes true from seed, throwing always a number of bright
magenta-reds, and a corresponding number of whites more

* Compare Baur's case of Antirrhiinwi (p. 253), which like many other
variegated plants proved unfixable because it is a heterozygous type.

XVI] Practical Application 299

or less tinged with magenta, which evidently exhibit the
two gametic elements that must be combined in one zygote
in order to produce the Lavender.

In this case the bright magenta-reds immediately breed
true when self-fertilised. In reo-ard to the tincred whites
there is a complication which might repay further study*.

There is another colour in Prinmla Sinensis which
apparently cannot exist in a pure form. This is a peculiar
shade of *' crushed strawberry," and the two pure forms by
the union of which it is formed are the deep crimson of
"Crimson King," and the white with a bright pink eye
brought out by Messrs Sutton under the name " Duchess."
By crossing these two together, the peculiar heterozygous
colour can at once be produced.

But though such examples are not rare, they are in a
minority, and speaking generally w^e may feel fairly con-
fident that a given type can be made to breed true, and
to perpetuate its good qualities indefinitely. A doubt
should perhaps be expressed as to the possibility of fixing
permanently such a property as a high degree of fertility.
Respecting the transmission of that character and the con-
ditions governing its existence little is positively known, and
to discuss the various indications properly is not possible
within present limits ; the remark must suffice that the
evidence on the whole suggests that hopes of fixing per-
manently such a quality as excessive egg-production of

* When such tinged whites are self-fertilised, most of them are found
to breed true. Some, however, in addition to their own type throw some
lavenders, and occasionally even a magenta-red. This result has occurred
both at Messrs Sutton's and also in the sowings which Mr R. P. Gregory
and I have made. The fact is probably due to imperfect classification of
the lavenders and tinged whites. These shades of colour are liable to vary
somewhat, especially with temperature, colder temperatures causing more,
higher temperatures less colour in the flowers. From our general experience
of Primula colours we may feel assured that the white of the tinged whites
m this case is due to the presence of some dominant inhibiting factor,
which we may call D. The factor for colour, C, is common to all these
plants. The bright magentas are CCdd, the lavenders are CCDd, and
the tinged whites are CCDD. In all probability there is an irregularity in
the dominance of D, for which we cannot at present account, such that a
lavender may occasionally be so pale in tint as to pass for a tinged white.
Further analysis would doubtless show that the number of these dubious
plants is in fact definite, and possibly that their abnormal behaviour is due
to the possession of some distinct element.

300 Practical Application [ch.

fowls or excessive fertility in pigs should not be entertained
with great confidence.

To avoid raising false expectations it should also be
said that many of the small fancy points which distinguish
individuals of the same breed from each other are rather of
the nature of fluctuations than definite transmissible at-
tributes. In the regulation of these finer details it is
improbable that heredity plays any very prominent or at
least assignable part. A Dutch rabbit, for instance, having
the transverse division between the coloured and the white
parts of the trunk exactly disposed as the fancier desires is
scarcely if at all more likely to have offspring correctly
marked than an average specimen of the same strain. In
such cases all the fancier can do is to use strains otherwise
good. A knowledge of genetic physiology will only help
him here in so far as it may warn him not to pay ex-
travagant prices for animals whose qualities are not genetic
or transmissible.

The domesticated animals and plants of European
countries have already been brought to such perfection
that in them the scope for new combination is reduced.
Nevertheless the line of progress which Professor Biffen
has inaugurated, by combining the resistance to rust-disease
of one variety of wheat, w^ith productiveness and other
qualities derived from another variety, is one which should
be capable of indefinite extension. It may be that resistance
to disease is incompatible with some of the valuable qualities
of animals and plants, but the attempt to produce these
combinations should be made on the largest possible scale.
The search for disease-resisting strains of animals and plants
is hardly begun.

In its application to the improvement of the domesticated
animals and plants of tropical regions the aid of Mendelian
method may be expected to be more immediate and direct.
Apart from the actual creation of new types by re-combina-
tion much can undoubtedly be accomplished, as Mr W. L.
Balls has indicated in the case of the Egyptian Cotton crop,
by purification of the cultivated sorts. According to the
traditional practice the ''variety" used consists in reality of
an immense number of distinct strains which under ordinary

XVI] Practical Application 301

conditions are continually being crossed together by Insect
agency. To purify such crops by " rogueing " is an in-
terminable and hopeless task. The proper course is to
identify, the factors governing the various characters, and
having ascertained their relations to each other, to build up
a desirable strain by individual selection. In former times
the confusion of types in the crop would have passed for
*' variability." Mendelian analysis shows that representation
of the facts to be entirely mistaken.

Technical Methods.

The technique of Mendelian experimentation Is usually
very simple. In crossing plants together the anthers of the
plant to be used as female must of course be picked out
with forceps before they dehisce. The other parts should
be injured as little as possible. The flower is then covered
to exclude insects. Muslin bags may be used for this
purpose, but they are neither so convenient nor so safe
as rain-proof bags made of parchment-paper, which most
manufacturing stationers can now supply in any required
size. The bag is put over the flower, the mouth being
crushed up so as to fit to the stem, and it is fixed in place
with a small bent piece of copper wire. If the stem be
delicate a thin bamboo stuck in the ground must be also
held in the copper clip. The use of the clip obviates
all tying and untying. Muslin bags are objectionable for
various reasons, especially because unless they are con-
tinually readjusted, flowers are sure to touch the sides and
there is the risk of fertilisation by insects. Bees will often
visit flowers covered by muslin, and I suspect that nocturnal
moths may do the same. If it is desired to give ventilation,
holes may be punched in the top of the paper bags, above
the flowers. The flower from which pollen is to be taken
must also be covered before it opens, in order to keep its
pollen from pollution. There are indications that some of
the results obtained by the older hybridists were confused
by neglect of this precaution.

In transferring pollen the use of brushes is to be
deprecated, as tending to the introduction of errors. The
best plan is to pick out with fine forceps an anther from

302 Practical Application [ch.

the flower to be used as male, and with it to touch the
stiema of the female flower. The anther is then thrown
away, unless the supply of pollen is very limited. Absolute
cleanliness is of course most essential. The fingers and
forceps should be continually cleaned with spirit in order to
kill any pollen-grains adhering to them before proceeding
to the next fertilisation. In operating on plants with much
pollen like Oenothera, Poppies, &c., care should be taken
to avoid including in the bag leaves on which pollen has
already fallen, and many other small precautions of this kind
will occur to the experimenter in practice. For example,
if muslin bags are used, they should not be laid on the
greenhouse stage where the plants have been standing, for
obviously the fallen pollen may adhere to them and be
passed on to the stigmas.

In sowing fine or light seed in pans the presser with
which the soil is patted down must be cleaned as each pan
is finished. Otherwise it is liable to carry on seed to the
next pan sown.

In recording results the capsule of seed resulting from a
cross should have a register-number under which it is sown.
Each plant which comes up must then be numbered sepa-
rately, and its number is written as an index-number to the
original number of the capsule. Thus, a pod of Emily
Henderson Sweet Pea % x Blanche Burpee ^ may, when
sown in 1904, have the number 20. If seven plants come
up, they are numbered 20^~^ In 1905 each of these is
sown under a fresh number. 20^ may for instance become
115, 20^ becomes 116, &c. The families 115, 116, &c., will
each contain a great number of individual plants, and each
of these from which the breeder intends to save seed must
then receive an index-number 115^ 115^ &c., under which
its characters are recorded in the register. In 1906 the
offspring of each of these plants receives a fresh number for
the year. The family of 115^ may become 305, and so on.
Reference is thus made easy, and the history of any in-
dividual plant can be rapidly traced. The same system can
of course be easily modified so as to adapt it to the case of
animals where the mating must be bi-parental.

XVI] Practical Application 303

Sociological Application.

It may be anticipated that a general recognition of
the chief results of Mendelian analysis will brine about a
profound change in man's conceptions of his own nature
and in his outlook on the world. Many have in all ages
held the belief that our powers and characteristics are
directly dependent on physical composition ; but when it
becomes known that the dependence is so close that the
hereditary descent of certain attributes can be proved to
follow definite predicable formulae, these ideas acquire a
solidity they never possessed before, and it is likely that
the science of sociology will pass into a new phase. The
evidence at our disposal already proves that in many simple
cases of defects and abnormalities the descent is of this
definite order, and it is scarcely doubtful that future search
will reveal comparable examples in abundance. As regards
more complex phenomena of human inheritance, the descent
of characters involving the coincidence of several factors,
and effects due to interference between factors, a complete
analysis may be unattainable ; but even in some of these
more obscure examples a close scrutiny will probably discover
positive traces of regularity in descent of such a kind as to
indicate in them also that the bodily or mental characteristic
considered is a consequence of definite factorial composition.
It is not in dispute that the appearance or non-appearance
of a characteristic may be in part decided by environmental
influences. Opportunity given may decide that a character
manifests itself which without opportunity must have lain
dormant. The question of opportunity and of the degree
to which the conditions of life are operative in controlling
or developing characters will some day demand attention,
but in order to answer such questions successfully it is the
first necessity that a knowledge of the genetic behaviour of
the factors should be obtained. They are the fundamental
elements, and the consequences of environmental inter-
ferences are subordinate to them. The previous attempts,
experimental and statistical, to determine the results of
changed conditions have led to quite inconclusive results
because no pains were taken to ascertain that the material
subjected to these various influences was genetically similar.

304 Practical Application [ch.

In the pre-Mendelian period, indeed, such an expression
had no definite meaning. From a knowledge of the
physiology of descent under uniform conditions it may be
possible to proceed to a determination of the consequences
followingf a changfe in those conditions. We cannot in-
vestigate both unknowns at once. In the analysis of these
wider problems we must begin with the more tractable of
the two, and it has become obvious that this is to be found
in the genetic aspect of the phenomena.

The outcome of genetic research is to show that human
society can, if it so please, control its composition more
easily than was previously supposed possible^. Whether
such control should be exercised, or the form which it is to
take, scarcely falls within the province of this text-book to
discuss. Nevertheless, as many are already becoming urgent
in advocating the practical application of genetic science to
human affairs, some few words on that subject may be
appropriate. Whatever course civilisations like those of
Western Europe may be disposed to pursue, there can be
little doubt that before long we shall find that communities
more fully emancipated from tradition will make a practical
application of genetic principles to their own population.

The power is in their hand and they will use that
power like any other with which science can endow them.
The consequence of such action will be immediate and
decisive. For this revolution we do well to prepare.

Interference may take one or both of two courses.

* Mr F. Galton's long-continued efforts have at length been successful
in directing public attention in some degree to the overwhelming im-
portance of Eugenics. Some of the earlier attempts in the same direction
are worth remembering. For example, Sir W. Lawrence frequently adverts
to the subject in language almost identical with that now current. "The
hereditary transmission of physical and moral qualities, so well understood
and familiarly acted on in the domestic animals, is equally true of man.
A superior breed of human beings could only be produced by selections
and exclusions similar to those so successfully employed in rearing our
more valuable animals. Yet in the human species, where the object is of
such consequence, the principle is almost entirely overlooked. Hence all
the native deformities of mind and body, which spring up so plentifully in
our artificial mode of life, are handed down to posterity, and tend, by their
multiplication and extension, to degrade the race." (W. Lawrence, Lectures
on Physiology^ Zoology, afid the Natural History of Man, London, 3rd Ed.,
1823, p. 393. See also ibid. pp. 260 and 389.)

XVI] Sociological Application 305

Measures may be taken to eliminate strains regarded as
unfit and undesirable elements in the population, or to
encourage the persistence of elements regarded as desirable.
From the standpoint of the sociologist these two kinds of
interference may seem merely complementary to each other,
but in the light of genetic physiology they are entirely
different.

To the naturalist it Is evident that while the elimination
of the hopelessly unfit Is a reasonable and prudent policy
for society to adopt, any attempt to distinguish certain
strains as superior, and to give special encouragement to
them would probably fail to accomplish the object proposed,
and must certainly be unsafe.

Comprehensive discussion of these questions would be
quite out of place here. It must suffice to point out that
the distinction is created partly by the fact that, whereas
our experience of what constitutes the extremes of unfitness
Is fairly reliable and definite, we have little to guide us In
estimating the qualities for which society has or may have
a use, or the numerical proportions In which they may be
required. But specially Important are the indications that
in the extreme cases, unfitness Is comparatively definite in
its genetic causation, and can, not unfrequently, be recognized
as due to the presence of a simple genetic factor. There is
as yet nothing In the descent of the higher mental qualities
to suggest that they follow any simple system of trans-
mission. It is likely that both they, and the more marked
developments of physical powers, result rather from the
coincidence of numerous factors than from the possession
of any one genetic element.

Some serious physical and mental defects, almost certainly
also some morbid diatheses, and some of the forms of vice
and criminality could be eradicated if society so determined.
That however is the utmost length to which the authority
of physiological science can In the present state of know-
ledge be claimed for interference. ]\Iore extensive schemes
are already being advocated by writers who are neither
Utopians nor visionaries. Their proposals are directed in
the belief that society is more likely to accept a positive
plan for the encouragement of the fit than negative Inter-
ference for the restraint of the unfit. Genetic science, as I

B. H. 20

3o6 Sociological Application [ch. xvi

have said, gives no clear sanction to these proposals. It
may also be doubted whether the guiding estimate of popular
sentiment is well-founded. Society has never shown itself
averse to adopt measures of the most stringent and even
brutal kind for the control of those whom it regards as its
enemies.

Genetic knowledge must certainly lead to new concep-
tions of justice, and it is by no means impossible that in
the light of such knowledge public opinion will welcome
measures likely to do more for the extinction of the criminal
and degenerate than has been accomplished by ages of penal
enactment.

APPENDIXES

Since this book first appeared advances have been
made which extend and in some cases modify several
statements contained in the previous chapters. Pending
an opportunity of re-writing the whole, some of these
researches are mentioned very briefly in the following
paragraphs. I have arranged them in the order followed
in the book, so that they may be read as annotations to
the several chapters.

The work done in the past three years with a bearing
on Mendelian Principles has been of vast extent, and
the contributions here enumerated are those only which
either for extent or for novelty seem especially important.

Appendix to Chaptei" II. Structtu^al Characters.

Peas. As to inheritance of Stature and the Time of
flowering see Keeble, F. and Pellew, C.,Jour. Gen., i. 1910,
p. 47. The following characters were found to be domi-
nants, Thick stem. Long internode, Late flowering.

Potato. Elaborate investigation of numerous characters.
Salaman, R. ^.,Jour. Gen., i. 19 10, p. 7.

Capsella. Several leaf-characters analysed by Shull,
G. H., ''Bursa bitrsa-pastoris and Bicrsa Hecgei'i biotypcs
and hybrids,'' Publication No. 112 of Carnegie Institution,

1909.

Beta. Kajanus {^Zts. f. indnkt. Abstam. u. Verer-
bungsleh^e, 191 1, vi. p. 137) describes experiments on the
inheritance of sugar-beets and mangels. Besides colours,
which were found to be controlled by very numerous factors,
some structural characters were investigated, especially the
shape of the bulb and the number of leaves. The results
were complex and some apparent inconsistencies were met
with.

20 — 2

3o8 Appendixes

Brassica. Kajanus has investigated the genetics of
seme species of this genus, particularly napits and rapa and
their hybrids. He confirms older observations as to the
production of lumpy outgrowths (resembling "finger and
toe") as the result of certain such crosses, and in this
respect he found remarkable differences between reciprocals.
Zts. f. tnd. Abstain., 191 2, vi. p. 217.

Cotton. Leake found the sympodial habit partially
dominant over the monopodial, with many forms of blending
between the two types. The inheritance of the leaf-shapes
was determined .by a special method of measurement and
it was found that certain shapes were always heterozygous.
Leake, H. yi., Jour. Gen., 191 1, p. 205.

Tomato. Genetic properties of several characters,
especially leaf-shape, Investigated by Groth, A., Bulletins
238 and 239. New Jersey Agricultural Station, 191 1.

Cattle. Evidence as to inheritance of long and short
legs, and red or black colour in Dexter Kerry. Wilson, J.,
Sci. Proc. R. Dublin Soc., xii. 1909, p. i.

Dogs. Genetics of several characters investigated by
A. Lang, Zts. f. hid. Abstammungslehre, iii. 19 10, p. i.

Colour- Characters^ Plants.

Antirrlmtzcm. Full and minute analysis of colours, for
the most part confirming Miss Wheldale's results, and
adding a remarkable case of coupling (see later, p. 318).
Baur, E., Zts. f. ind. Abst., 1910, iii. p. 34. An account
of Mendelian heredity in the case of a cross between distinct
species of this genus was given by Baur at the Genetics
Conference, Paris, 191 1, but publication has not yet been
made.

Cotton. Curious case of complete correlation between
small size of petals and white colour, and large size and
ye llozv colour. Leake, H. ^l., Jotcr. Gen., i. 191 1, p. 241.

Digitalis. Analysis of flower-colour, with evidence that
white may be a dominant. Keeble, Pellew, and Jones,
W. N., New Phytologist, ix. 19 10, p. 6%.

Helianthus. Discovery of a new red-flowered variety.
The heterozygote with the yellow type is intermediate in
colour. Cockerell, T. D. A., Pop. Sci Monthly, 191 2,

p. zn-

GREGOR MENDEL

About the year 1862

Appendixes 309

Mirabilis. Full analysis of the self-coloured types,
correcting the account previously given by Correns.
Marryat, D. C. E., Rep. Evol. Ctee. Roy. Soc.y 1909, p. 32.

Pinmtda Sinensis. Genetics of colour-characters in
flowers and stems ; also of several structural characters.
Gregory, R. Y.,Jour. Gen. i. 191 1, p. 'j'}^.

Colour- Cha7^acte7^s. Animals.

Cattle. Wilson, J., -'The colours of Highland Cattle,"
Sci. Proc. R. Dublin Sac, 1909, xii. (N. S.) p. 66, contains
evidence as to the relations between dun, brindled, and
other colours hitherto little studied.

Mice. Analysis of the coat-colours of mice having pink
eyes. Durham, F. '^., Jonr. Gen., i. 191 1, p. 159.

Analysis of coat-colours of mice in general with discussion
of the relation of the phenomena to those seen in other
rodents. Incidentally (p. 126) evidence is given as to a
case in which the factor for colour seemed to repel the
agouti factor. Hagedoorn, A., Zts. f. ind. Abstani., 191 1,
VI. p. 99.

Horse. Factors for the various colours provisionally
worked out. Sturtevant, A. W., Jonr. Gen. 11. 1912, p. 41.
Walther, A. R., Beitr. zitr Kenntniss d. Vei^erbnng d.
Pferdefa7'ben. Hannover, M. and R. Schaper, 191 2.
Wilson, J., Sci. P7^oc. R. Dublin Soc, 19 10, xiii. (N. S.)
p. 331 and ibid. xiii. (N. S.) 191 2, p. 184 (especially
regarding dun colour).

From these researches it seems to be established that
in the horse chestnut is the lowest colour in the series, and
that white is epistatic to all. As to the order of the other
factors there is not complete agreement. Sturtevant gives
reasons for the view that black stands next above chestnut,
and bay-or-brown taken as one indivisible type next above
black. Grey and roan come above these and below white.
Walther has much useful information as to the behaviour
of the peculiar piebald and spotted types.

Pigeons. Relations of black, blue, dun and silver.
R. Staples- Browne, Jonr. Gen. 11. 191 2, p. 131. Further
notes on colours of Pigeons ; Doncaster, ibid. p. 89. Inter-
relations of various colours, especially concerning Grizzles
and Mealies ; Bonhote, J. S. and Snialley, V . \V., P.Z.S.,

3IO Appendixes

191 1, p. 601. The authors disagree with some of Staples-
Browne's conclusions. He had held that blue was a dilute
form of black, and that silver is similarly a dilute form of
dun ; but Bonhote and Smalley came to the conclusion that
silver is dilute blue. It is not in dispute that blues do not
throw blacks, and that silvers do not throw duns, and that
both blue and dun are recessive to black. As however
there is no microscopical or chemical test by which the
nature of the pigments can be certainly distinguished the
point in question cannot be positively decided at present.

Lepidoptera.

[For sex-limited cases in Colzas, &c. see Appendix to
Chap. X.]

Aglia tau and the dark vars. lugens, &c. have been
investigated again by Standfuss, see especially Iris, 1910,
XXIV. p. 155, and Deut. Entom. National-Bibliothek,
1 9 10, I. In these experiments the peculiar sex-distribution
before recorded was not seen again. For an interpretation
of Standfuss's results, see Plate L, Arch. Rass. u. Gesel.,
1910, Hft. 6, p. 678.

Pygaera. Important paper on species-hybrids. Federley,
H., Aixh. Rassen u. Gesel.-BioL, 191 1.

Bombyx mori. Extensive series of experiments on
genetics of Silkworms. Kellogg, V. L., Leland Stanford
Univ. Publ., No. i, 1908. As regards the colours of
cocoons many special idiosyncrasies of strains and of
individuals were found, which have not yet been analysed
successfully. The larval colours were more regular in their
behaviour.

DiPTERA.

Drosophila ampelophila has been the subject of elaborate
investigation by Castle and his assistants (see especially
Proc. Amer. Acad. Arts and Sci., xli. 1906) with special
regard to the genetics of fertility ; by T. H. Morgan (chief
paper m Jour. Exp. Zoology, 191 1, Vol. xi. p. 365), who
has discovered many recessive forms, and shown that
remarkable cases of sex-limited inheritance occur (see
Appendix to Chap. X.) ; and by Lutz (Carnegie Institution,

Appendixes 3 1 1

Publication No. 143, 191 1), who has investigated the
inheritance of abnormal venation on a very large scale,
chiefly with negative results which suggest that these
features are largely fluctuational.

MOLLUSCA.

Lang, Zts. indukt. Ahstavi., 191 1, v. p. 97, has continued
and extended his researches on the genetics of H. 110)10-
ralis and Iioi^tensis, and has found that in some cases, as
previously recorded, when these two species are crossed,
genuine hybrids are produced ; but in other instances the
results of the cross are purely viaternal. This evidence
naturally suggests a comparison with the observations made
by those who have experimented with Echinoderm hybrids.
The literature of this latter subject is too extensive for
consideration on this occasion, and as no one has yet
succeeded in rearing the products to maturity the genetics
of the various species are still unknown.

Appendix to Chapter IV.

The analyses made by Nilsson-Ehle of the genetic
properties of Cereals have led him to the conclusion that
several factors all possessing the same property may co-
exist in one individual. For example, he gives evidence
that a certain wheat has in it three distinct factors having
the power of producing the red colour. Obviously such
possibilities must be remembered in future analyses, and
to them may perhaps be due various numerical aberrations
hitherto unelucidated. See especially, H. Nilsson-Ehle,
Kreiczungstinters2cchungen an Hafer 7tnd Weizen, Lund,
1909, p. 69.

Some new types of ratio, brought about by the inter-
action of various factors leading to the masking of some of
the terms in the series, are discussed and illustrated by
Shull, G. H., Amer. AT at., 1908, p. 43,

■1

»0'

Appendix to Chapter VII.

As stated above (p. 309) the analysis of the colours of
Horses has now been carried somewhat beyond the point
reached in this discussion.

3 1 2 Appendixes

Appendix to Chapter VI 11.

In this chapter an attempt Is made to discuss the
relation of self-coloured to pled types In the simplest
possible cases, such as that of the self-coloured and pled
rabbits. All further researches on this subject tend to
show that it Is one of great complexity. Even in animals
the Interrelations of the various degrees of pledness are not
established satisfactorily for any one example, and In plants
there are evidently phenomena to be explored which as yet
evade our analysis. In the case of the flaked flowers of
Mirabilis for example the experiments of Miss Marryat
showed that though the numbers suggest orderly behaviour,
it was not possible yet to express them in factorial terms,
and the same Is true for PiHrnula Sinensis. In resfard to
variegation of the leaves of Mii^abilis Correns has published
some very remarkable evidence. In an earlier paper, Zts.f.
indiikt. Abstam., 1909, i. p. 291^, he showed that a peculiar
variegated type which he called variegata usually behaved
as a recessive to the normal green. Sometimes however
this variegated form bears a branch entirely green. The
self-fertilised offspring of the variegated and of the green
branches were separately studied ; and it w^as found that
the variegated branches gave families consisting almost
entirely of variegated -f a small and irregular percentage
of greens; but the green branches gave normal Mendelian
families, of 3 greens : i variegated, 25 per cent, of the
greens being pure greens, the rest heterozygous. The
variegateds and the greens, of whatever origin, behaved in
the same way.

As regards the striping of the flowers in Mirabilis
Correns has also provided some new evidence. When
a plant bears both striped branches and unstrlped branches,
each type produces offspring which in the great majority
resemble Itself. The striped offspring then throw a small
and irregular proportion of self-coloured, but the self-coloured
consist of heterozygotes and homozygotes approximately In
the ratio 2:1. The facts in Correns's opinion indicate that
a homozygous branch can by some change pass Into a

*' For further discussion of cognate phenomena see Correns, ibid. 11.

1909. p. ZZ-^'

Appendixes 3 1 3

heterozygous condition, and he aroues that observations of
de Vries on Veronica, and of MacDougal on Oe^wthera
indicate the possibihty that this process may be reversed,
a heterozygous branch becoming a homozygous dominant.
(Correns, Ber. Deut. Bot. Ges., xxviii. 19 10, p. 433.)

The whole problem is evidently one of great obscurity,
but the facts suggest that a clear understanding of the
genetics of these variegated forms may give light on the
nature of segregation in general.

This paper of Correns should be read in connexion with
that of Erwin Baur on the periclinal variegation of Pelar-
gonium [Zts. ind. Abstani., i. 1909, p. 330)'*, in which one
of the most notable of recent advances in genetic physiology
is made. Plants with green inside and white epide^-mis and
sitb-epidennal layers have exclusively albino offspring, while
plants in which the inside is white and external layers green
have exclusively green offspring. Similarly when the
external layers are golden the offspring are greens and
goldens, just as when derived from wholly golden plants
(see p. 253). [The last two statements I have myself con-
firmed experimentally, but with the albo-nia^^ginatae I have
hitherto failed to obtain offspring.] The characters of the
cells of the sub-epidermal layer thus indicate the characters
of the germ-cells. In view of this discovery of Baur's we
are led to speculate whether the sub-epidermal layer of any
heterozygote is not in reality a patchwork of cells, bearing
or not bearinof a oriven factor, and that accordincr as this
patchwork is coarse, or fine (as in an emulsion) we obtain
aberrant or expected numbers in our ratios. It must be
remembered that external appearance is only an imperfect
guide to the genetic properties. For instance TropacoliDJi
with variegated leaves breeds nearly if not quite true in
this respect, though we might expect its germ-cells to
be green, yellow, or mosaic in nature. If in the
mixtures of substances capable of forming coarse or fine
emulsions we have a true picture of the heterozygous
state, such phenomena as those observed by Correns for
the green and variegated branches become capable of
representation without any great straining of the facts.
Segregation on this view is regarded as a phenomenon

* See also ibid. iv. 1910, p. 8i.

314 Appendixes

capable of occurring at any cell-division, and not merely in
gameto-genesis. The moment of segregation will thus
probably be found to vary with different types and with
different factors, and for some it is likely that great
irregularity in this respect will be found as a common
occurrence. In this way can we best hope to reconcile the
facts that with certain types such as wheat and barley the
expected ratios are almost always approximately realised,
whereas in other types, such as Matthiola there is great
fluctuation in the numbers from individual pods, as
Miss Saunders assures me, though when large series are
taken the normal ratios are usually approached.

Appendix to Chapter IX.

The result of further experiment has led to a great
simplification of the views as to coupling and repulsion
given in the text. We have now proof that coupling and
repulsion are consequences which may affect the same pair
of factors. If AB x ab gives an F^ in which A and B are
coupled, then Ab x aB will give F^ in which A and B are
''repelled." It is moreover established in three cases that
the appearance of '' repulsion " is in reality a consequence
of the fact that the gametes Ab and aB are represented in
numbers larger than those in which AB and ab occur. In
other words, if AB x ab gives F^ AaBb having gametes

"jAB : \Ab : laB : : yab ;

then Ab x aB will give F^, AaBb having gametes

lAB : yAb : yaB : lab,

or at least displaying some similar coupling. We have as
yet no sufficient data for asserting that the actual numbers
will be the same in the two cases, though this is perhaps
the simplest expectation.

7^/ie essential phenomenon in each case is the fomentation
by the hetei'ozygote of a greater number of germ-cells repre-
senting those combinations of factors which were possessed
by the original parents^ and fewer germ-cells representing
the new combincitions'^ .

* I have only lately seen Emerson, R. A., "Genetic Correlation and
Spurious Allelomorphism in Maize," 24M A?in. Rep. Nebraska Agric.
Exp. Sta. 191 1, in which this possibility is pointed out already.

Appendixes

To distinguish the two kinds of F^ we ought to write
AB X ab as AB . ab ; and aB x Ab ought to be written
aB . Ab. We must in fact recognize that in the dividino-
zygotic cell, at some time before segregation is effected —
probably from the moment of fertihsation — a polarity is
estabHshed ; and that the constituents derived from the two
parents are not, in these cases at least, placed indifferently,
but are grouped according to their parental origin.

Hence we may construct the following table :

Gametic series

Partial

repulsion

from zygote

of form

AbY. aB

Partial

coupling

from zygote -^

of form

ABy.ab

(

AB
I
I
I
I
I
I
3

Ab aB

{n-\) («-i)

15
7

15
31
63

V{«-i)

3
I

(/z-i)

Number

of gametes

111 series

3^
16

8
16

32
64

12S
2n

Number of
zygotes
formed

4;.'^
4096
1024

■2^6

1024
4096

16384

Nature of zygotic series

2n-+ I

7049

513
129

177

737

3009

12161

n- - I
1023

255

7
15

127

;/- - I
1023

255

15
31
63

12:

4«'^ 3«^- (2;/ - i) 2;/ - I 2« - I

;;^

9
49

961

3969

- (2// -

For details as to these phenomena see Bateson and
Punnett, Proc. Boy. Soc. 191 1, vol. 84 b, p. 3; de Vilmorin
and Bateson, ibid. ; Gregory, ibid. ; Bateson and Punnett,
Jour. Gen., i. 191 1, p. 293.

The evidence has been obtained from observation of
various characters in Sweet Pea ; the relations of tendrilled
leaves to the ''Acacia" form in Pisum, and of Magenta
to Short-style in Primula Sinensis (Gregory).

Now that we may regard the formation of four cells
of composition AB, Ab, aB, ab, as the foundation both
of the coupling- and of the repulsion-series the problem
is manifestly somewhat simplified. The time, excluding
gametogenesis, at which we can most readily imagine
four such definite quadrants to be formed is during the
delimitation of the embryonic tissues. It is then that the
plant is most clearly a single geometrical system. Moreover
the excess of gametes of parental composition characterising
the coupling- and repulsion-series must certainly mean that
the position of the planes of division by which the lour
quadrants are constituted is determined with regard to the
gametes taking part in fertilisation. Though the relative

3i6

Appeiidixes

positions of the constituents of the cells may perhaps be
maintained throughout the history of the tissues, It is
easier to suppose that the original planes of embryonic
division are determined according to these positions than
that their influence can operate after complex somatic
differentiation has been brought about.

At some early stage in the embryonic development or
perhaps In later apical divisions we can suppose that the
n— I cells of the parental constitution are formed by
successive perlcllnal and anticlinal divisions of the original
quadrants which occupy corresponding positions. The
accompanying diagram gives a schematic representation of
the process as we Imagine it. Obviously It does not pre-
tend to give more than a logical or symbolic presentation

AB X ab

Ab X aB

n-1

n- 1

n -

lAb

Iba

Sab

3Ab

IBa

lAB

Fig. 39-

GREGOR MENDEL AS PRALAT

Taken about the year 1880

Appendixes 3 1 7

of the phenomena. If such a system of segrei^^ation is
actually formed at the apex, it must be supposed that the
axes of the system revolve with the generatino^ spiral.

Whatever hypothesis be assumed, the following points
remain for consideration.

1. We are as yet unable to imagine any simple system
by which the four original quadrants can be formed by
two similar divisions. Evidently there must be two cell-
divisions, and if in one of them we suppose AB to separate
from ab, we cannot then represent the formation of Ab and
aB. Therefore we are almost compelled to suppose that
the original zygotic cell forms two similar halves, each
AaBb, and that the next division passes differently through
each of these two halves, in the one half separating AB
from ab, and in the other half separating Ab from aB. The
formation of these four quadrants must take place in every
case in which there is segregation in respect of two pairs
of factors. (For three pairs there must similarly be eight
segments, and so on.) The axes of this system may well
be determined by the position of the constituent parental
gametes. Reduplication or proliferation resulting in n — i
gametes may then take place in either of the opposite pairs
of quadrants according to the parental composition.

2. If in the gametes of any plant some factors are
distributed according to one of the reduplicated series and
other factors accordino^ to the normal Mendelian svstem —
as we know they may be — the segregations by which such
a system is brought about cannot have happened simulta-
neously. Moreover if various reduplications can take place
very early in some individuals and not in others, we cannot
imagine how the normal form of the plant remains un-
changed, unless these reduplications affect tissues originall)'
set apart as germinal.

As possibly significant we note here the fact that in
the embryonic development of plants the order of the
various divisions is known to be subject to great variation
and it is not inconceivable that such disturbances of the
order in which the planes of division occur may indicate
variations in the process of segregation*.

* See Coulter and Chamberlain, Morphology of Angiospcnns, 1903,
p. 187.

3 1 8 Appendixes

3. We do not yet know whether independent redupli-
cated systems can be formed in the same individual. In
the sweet pea for instance we have not yet seen the
consequences of combining blue, erect standard, and long
pollen with the fertile-sterile, dark-light axil series, and
much may be discovered when such families come to be
examined.

The phenomena seen in animals may well be produced
by the segmentations in which the parts of the ovary or
testis are determined. Hitherto no case of coupling has
been found in animals. Among the phenomena of repulsion,
however, of which many examples exist, certain suspicious
cases have been observed which may mean that in animals
reduplicated systems exist like those of the plants. Never-
theless at present it seems not impossible that the two
forms of life are really distinguished from each other in
these respects.

Lastly, in view of what we now know. It Is obvious
that the terms "coupling" and "repulsion" are misnomers.
" Coupling " was first introduced to denote the association
of special factors, while '' repulsion " was used to describe
dissociation of special factors. Now that both phenomena
are seen to be caused not by any association or dissociation,
but by the development of certain cells in excess, those
expressions must lapse. It is likely that terms indicative
of differential multiplication or proliferation will be most
appropriate. At the present stage of the inquiry we
hesitate to suggest such terms, but the various systems
may conveniently be referred to as examples of reduplica-
tion, by whatever means the numerical composition of the
gametic series may be produced.

It remains to be seen whether systems of reduplication
not contemplated by this scheme will be proved to exist,
but it should be mentioned that Baur has already shown
the probability that other ratios, especially 6:1:1.6,
occur In A^itii^rhinum (see Zts. f. ind. Abstam., 1912, vi.
p. 201) ; and once It is realized that the phenomenon is
probably one of reduplication of certain terms, there seems
no reason for supposing that various less simple series will
not be found.

Appendixes 3 1 g

Appendix to Chapter X,

With respect to sex-limitation a qreat deal of recent
evidence has been discovered. Several characters in Poultry-
have been shown to follow the same system of descent as
the black colour of Canaries. Especially may be mentioned
the barred markings of Plymouth Rock (Pearl and Surface,
Arch. Entzvm. xxx. 19 10). This barrinc;- is a dominant,
males being homozygous and females heterozyoous. The
females crossed with black males give black females and
barred males.

Staples-Browne {Jour. Gen., 191 2, 11. p. 158) gives
additional evidence that the brown colour of the Barbary
Dove is similarly sex-limited, crosses with a white male
giving usually dark males and white females.

Gerould \Amer. Nat., 191 1, p. 257) has investigated
the case of the white females of Colias, showing that white
is probably dominant in the female, recessive in the male ;
and much work has been done in reference to the poly-
morphic females of tropical butterflies, several of which
have been shown with great probability to follow a simple
Mendelian scheme in their descent (see especially Punnett,
Mendelisnt, ed. 3, 191 1, p. 134; Spolia Zeyianica, 1910,
VII. in which the case of Papilio polytes, aristolochiae, and
hector is elucidated).

The subject upon which the most extensive work has
been done is Drosophila avipelophila, the pomace-tly,
especially by T. H. Morgan (chief paper in Jour. Exp.
Morph., 191 1, vol. xi). The eyes are of several colours,
dark red, vermilion, orange, white, &c., and complex rules
of sex-limitation have been found governing their descent.
The simplest case is that of red eye and white eye, which
follow the same rules as colour-blindness in man, ivJiite eye
being the equivalent of colour-blindness. The whole subject
is full of interest, but could only be made clear at great
length.

In regard to colour-blindness and analogous phenomena
Doncaster has made a suggestion of great importance {Jour.
Gen., 191 1, I. p. zn)' Hitherto we have supposed, following
the analogy of horns in sheep, that the colour-blindness
must be the dominant, for the reason that the normal male

320

Appejtdixes

cannot transmit it. Doncaster shows that by assuming
both sexes to be heterozygous for sex-determiners it is
possible to construct a scheme on which normal vision is
the dominant, and colour-blindness the recessive, which
will express all the facts, and is in my judgment on the
whole the most probable account of these phenomena yet
suggested. Substituting white eye for colour-blindness,
the same scheme expresses Morgan's observations for red
and white eye in Drosophila.

Doncaster's scheme is thus expressed : '' Since the
male transmits the factor for colour-blindness only to his
dauohters, it must be assumed that the male in this case
is heterozygous for the sex-determiner. In former papers
I have suggested that if maleness is determined by a
factor ^, femaleness by a factor % epistatic to ^ when both
are present, then a male individual may be represented ^ O,
a female ^ $ ; i.e. that both sexes are heterozygous for sex-
determiners, with selective fertilisation between ^-bearing
eggs and 6^-bearing spermatozoa, and between ^-bearing
eggs and ^-bearing spermatozoa. If we adopt this scheme

gametes

Parents
gametes

gametes

n$ O

(affected male)

N$ O
(normal male)

N6 iV?
(normal female)

(normal female
heterozygous)

n$0
(affected male)

n$ O
(affected male)

n$,0

N$ O
(normal male)

(normal female (normal female)
heterozygous)

JVS n ?

(heterozygous

female)

n6 O N$ O

(affected male) (normal male)

(affected female) (normal female

heterozygous)

* Doncaster, ibid. p. 378.

Appendixes 32 1

as a working h3'pothesis, and then represent normal sight
by N, colour-bhndness by absence or modihcation of
N ( = ?0' ^^^ further suppose that N can only be borne
by gametes containing a sex-determiner (^ or $, not O), we
obtain the observed results."

Facts bearing on the general theory of sex-determination
have accumulated so fast that they would require a separate
treatise for adequate enumeration and discussion. Reference
can here be made to one group of experiments alone, those
of Goldschmidt (^Zts, indukt. Abstain., 191 2, vii. p. i), in
which phenomena of extraordinary novelty and significance
are recorded. The most striking experiment was that in
which the moths Lyniantria dispar and japoiica were
reciprocally crossed.

L. Japonica $ x dispa7^ $ gives constantly both sexes
hybrid, showing mixture of the parental characters.

The reciprocal, L. dispar $ y. japonica ^, gives families
which at first sight seem to be all males, but on closer
examination it is found that in reality they consist of nonual
males and gynandromorplis having the characters of the
sexes mixed in various degrees. From these gynandro-
morphs it was however possible to breed, and subsequent
generations were obtained from both types of crosses. The
results were complex and difficult to interpret, but an
analysis of great interest is suggested by Goldschmidt.

In connexion with this paper reference should be made
to another curious series of results obtained by Kuttner
{Intern. Rev. Gesamt. HydrobioL, 11. 1909) in an investi-
gation of the descent of sex In Daphnia. In this case
also the descent from certain peculiar forms called "pseudo-
hermaphrodites" was studied. Actual hermaphrodites re-
garded as originally females and others regarded as originally
males occurred in these series. The numbers obtained were
considerable, but as yet no satisfactory analysis has been
made.

Appendix to Chapter XL

Two important contributions to the Investigation of the
genetics of doubleness have been made by Miss Saunders.
A factorial analysis is provided by which the descent of

B. H. 21

322 Appendixes

doubleness in Stocks is approximately represented. Taking
X and Y as two factors, either of which is sufficient to
make the flower single, and representing the ever-sporting
type of single as XxYy, it is shown that the inheritance
from such a plant is given by a scheme in which the ovules
are arranged in a system

I'^XY : \Xy : \xY • i^xy

and t\\Q pollen is all xy.

To represent the descent from such a plant, if it be also
heterozygous for white and cream plastids, much more
complex expressions are provided. In these it is suggested
that the factors X, Y, and W (the factor for white plastids)
or any two of them may behave as inseparable in certain
ovules, and as separable in others. In the light of the new^
facts as to coupling and repulsion this representation should
be regarded as provisional only. (For the evidence see
Saunders, E. R., yo2ir. Gen., 191 1, i. p. 303.)

In the same Journal, i. 19 10, p. 57, Miss Saunders
describes experiments with Pehtnia, which show that the
pollcji of singles is always single so far as has been deter-
mined, but the ovules of singles, of whatever source, consist
of a mixture of singles and doubles. Thus, any mixture
fertilised by a double, gives a mixture of singles and doubles,
but the singles thus produced will not give doubles, whether
self- fertilised or crossed inter se. It is not as yet clear why
the interbreeding of singles should give no pure singles,
but that nevertheless seems to be the fact.

Appendix to Chapter XII.

In regard to inheritance of sex-limited conditions, see
Appendix to Chap. X.

The whole subject of the inheritance of human diseases
and malformations is now being rapidly explored, and for
extensions of knowledge on this branch of inquiry the
reader should consult the Treasury of Human Inheintance,
and especially the Transactions of the Ophthalmological
Society, in which the papers of E. Nettleship appear.
Among the latter is a collection of evidence as to hereditary
nystagmus {ibid., xxxi. 191 1), a condition following lines
of descent similar to those of colour-blindness.

Appendixes 323

Appendix to Chapter XIV.

Pa7'tIienogenetic cases. To the cases In the text must
be added the evidence of Przibram as to Alan I is, yirch.
Entwm., 1909, XXVIII. pp. 602 and 612, and of Lang, as to
Helix 7ievioralis and hortensis (^Zts. inditkt. Abstavi., 191 1,

V. p. 135)-

Sexual cases. Reference was made In the text to

de Vrles's observation that the hybrid Oenot/iera viuricata

X biennis breeds true. In a recent paper {Biol. Cbltt. 191 i,

XXXI. p. 97) he has briefly pubHshed a statement of the

very remarkable results which will probably provide an

elucidation of this and other paradoxical cases. It appears

that reciprocal crosses between these two species give

different hybrid types, and that both breed true. Next,

calling the reciprocal hybrids MB and BM, it was found

that MBxBM gives only muricata, and that BMxAIB

gives only bien7iis ! De Vries conjectures that this behaviour

may be due to abortion ot a class of gametes on each side ;

but I am Inclined to suspect that we should rather compare

the phenomenon with that seen In those Stocks and Petunias

in which the male and female parts of the same plant have

a different genetic constitution. However that may be, we

must anticipate that the study of the results of reciprocal

crosses will lead to a great extension of genetic science.

As to lop-eared Rabbits, and the alleged absence of
segregation in crosses between long and short-eared races,
see A. Lang, ibid., 19 10, iv. p. i, who argues that the
observed data are not inconsistent with the possibility that
there Is segregation of numerous factors.

Much information additional to that contained In this
volume will be found in the three text-books which have
appeared since :

Johannsen, W. Elemente der exaktcn ErblicJikcits-

lehre. Fischer, Jena, 1909.
Baur, E. EinfuJirnno- in die Expcri)nc}i telle Verer-

bnns'slehre. Berlin, loii.
Punnett, R. C. Mcndelisni, ed. 3, 191 1.

21 — 2

PART II

1. BIOGRAPHICAL NOTICE OF MENDEL

2. TRANSLATION OF THE PAPER ON HYBRIDISA-

TION

3. TRANSLATION OF THE PAPER ON HIERACIUM

I i

BIOGRAPPnCAL NOTICE OF MENDEL^

Gregor Johann Mendel was born on July 22, 1822,
at Heinzendorf bei Odrau, in the " Kuhland " district of
Austrian Silesia. His father was a small peasant proprietor,
being the first of the family to raise himself to that degree,
and he held his land by a kind of socage, performing
" Robot " (agricultural labour) for the lord.

The name Mendel suggests a Jewish origin, but it is
practically certain that the suggestion is incorrect. The
family appears in the Church Register of the seventeenth
century — the earlier ones were burnt by the Hussites —
usually under the name M^^ndel, whereas it was not till the
reign of Joseph H (1765-1790) that the Jews in Austria
assumed definite surnames. At the time of the Thirty
Years' War Kuhland was a protestant district, and several
of Mendel's ancestors were of that persuasion. His four
grandparents were all of the local Heinzendorf stock, which
may be described as a German colony surrounded by a

* I regret that the short paragraph which I pubHshed in 1902 respecting
Mendel's career contained several inaccuracies. I'he materials supplied to
me were meagre and in many respects incorrect. Somewhat fuller sources
of information are before me now. Of these the chief is an annotated
report of the oration delivered by Mendel's nei)liew, Dr Alois Scliindlcr, in
July, 1902, at the unveiling of a memorial tablet at Heinzendorf. Other
facts are to be gathered from Mendel's letters to Nageli dated 1866-1S73
(Bibliography, 197); an article by Dr E. von Proskowetz in N(ue Frete
Presse, 24 July, 1902; a similar notice by Dr Wiesner in the IViefier
Abendpost^ November, 1901 ; and from a somewhat I'liller account published
by Dr H. von litis in Tagesbote aus Miihren, 1906. I liave to thank
Dr Janetschek and Dr von Niessl for assistance given in tlie course of
inquuies which I made in Briinn, and both Dr Ferdinand Schindler and his
brother, Dr Alois Schindler, for letters giving many interesting particulars
respecting their uncle. I understand that Dr von litis has a fuller biograi)hy
in hand. My most serious misstatement was to the efiect that in his later
years Mendel devoted himself to the Ultramontane Controversy. Tiiis was
a complete mistake. The dis])ute in which he engaged was, as is described
above, of a totally different nature.

Q28 Biographical Notice

Slavonic population. It Is recorded of his father that he took
special interest in fruit-culture, initiating his son at an
early age into the methods of grafting. Mendel's maternal
uncle, Anton Schwirtlich, was evidently a man of Intellectual
tastes, which is shown by the fact that he started private
classes for the children of Heinzendorf who could not walk
so far as the neighbouring village, for in Heinzendorf itself
there was at that time no regular school. Mendel was thus
able to say with some pride that he came from an educa-
tional family.

On the death of Schwirtlich a government-school was
established which Mendel attended as a young boy. His
talent was noticed and encouraged by the master. At this
time also two older boys who had gone away to the school
at Leipnik fell in with Mendel during their holidays, and
excited his ambition, with the result that he asked his
parents to let him study, and eventually he too was sent to
Leipnik at 1 1 years old, though this involved considerable
sacrifice on the part of the family. Here he distinguished
himself so much that it was decided to continue his educa-
tion at the gymnasium at Troppau, a course which finished
with a year at Olmiitz. The parental resources were
severely taxed by such expenses, and Mendel was only
enabled to complete his course through the generosity of a
younger sister, who voluntarily contributed a part of her
dowry for this purpose. In after years he repaid her
advance many times over, himself providing the education
of her three sons, his nephews.

At Troppau one of the teachers was an Augustlnlan,
and It is surmised that perhaps his description of the scho-
larly tranquillity of the cloister may have turned Mendel's
thoughts towards a monastic life. However that may have
been, when his time at the gymnasium was ended he
became a candidate for admission to the Augustlnlan house
of St Thomas in Briinn, an Institution generally spoken
of as the Konigskloster. His application was successful,
and he was elected with a view to his taking part in the
educational work which then devolved on the institution.
On admission he took the name of Greeor '' in reli^rion,"
Johann being his baptismal name. In 1847 he was ordained
a priest.

Biograplncal Notice 329

At the expense of the cloister he was sent in 1851 to
the University of Vienna, where he remained till 1853,
studying mathematics, physics, and natural sciences'.
Returning to Brunn he became a teacher, especially of
physics, in the Realschule. He appears to have taken
great pleasure in teaching and to have been extraordinarily
successful in interesting his pupils in their work. He
continued this occupation till 1868, when he was elected
Abbot, or more strictly, Pralat of the Konigskloster.

The experiments which have made his name famous
throughout the world were carried on in the large garden
of the cloister. From the time of his novitiate he began
experimental work, introducing various plants into the
earden and watch incf their behaviour under treatment. He
was fond of showino- these cultures to his friends. Dr von
Niessl relates how on one occasion he was taken to see
Ficmna calthaefolia and Ficaria ramuiculoides (two forms
now regarded as varieties of Rannncuhts Ficaria) which
had for some years been cultivated side by side without
manifesting any noticeable change. Mendel jokingly said:
"This much I do see, that nature cannot get on further
with species-making in this way. There must be something
more behind."

With the views of Darwin which at that time were
coming into prominence Mendel did not find himself in full
agreement, and he embarked on his experiments with peas,
which as we know he continued for eight years. The
results were communicated to the Brlinn Society in 1865
and published in 1866, but they passed unheeded The
subsequent paper on Hiei^aciiim appeared in 1869, meeting
a similar fate.

During his period of scientific work Mendel, as we now
know, was engaged on a great variety of cognate researches.
In his letters to Nageli (197) there are allusions to some oi
these subjects, but unhappily few statements of results.
His largest undertaking besides the work on Pisuni was an
investio-ation of the heredity of bees. He had 50 hives

* To this period belong two notes which he published in the Vcrh zool.
Verein, Wien, on Scopolia mari^aritalis (1853, iii. p. 116) and Bruchus
pisi {ibid. 1854, iv. p 27). In these papers he speaks of himself as a pupil
of KoUar.

*
bot.

230 Biographical Notice

under observation. He collected queens of all attainable
races, European, Egyptian, and American, and effected
numerous crosses between these races, though it is known
that he had many failures. Attempts were made to induce
the queens to mate in his room, which he netted in with
gauze for the purpose, but it was too small or too dark, and
these efforts were unsuccessful. We would give much to
know what results he obtained. In view of their genetic
peculiarities a knowledge of heredity in bees would mani-
festly be of great value. The notes which he is known to
have made on these experiments cannot be found, and it is
supposed by some that in the depression which he suffered
before his death they were destroyed.

In 1905 I had the pleasure of visiting the Konigs-
kloster, hoping that some trace of the missing books might
be discovered. I was most courteously received by the
present Pralat and the brethren of the cloister. My thanks
are due in particular to Dr Janetschek for the assistance he
gave me. It is to him that I owe the photographs of
Mendel given in this volume. I saw the hives which had
been used standing in their places, but the note books are
gone^. A rich harvest of discovery awaits those who may
successfully repeat the work.

With his appointment as Pralat his researches may be
said to have ended. To Nageli he wTote that he hoped
that after an interval his elevation might enable him to find
better opportunities for study, but it was not to be. In
1872 the Government passed a law imposing special taxes
on the property of religious houses. This enactment
Mendel conceived to be unjust and he decided to resist,
claiming that all citizens should be equal in law, and that
if these taxes were imposed on one class of institution they
should be imposed on all. He thus took up a position which
in England we should call that of a " Passive Reslster." At
first several monasteries stood out with the Konigskloster,
but gradually they conformed, Mendel alone remaining firm.
The quarrel involved him In protracted trouble and litiga-
tion. High emissaries are said to have visited him pro-

* On chance of finding something I obtamed a file of the local bee-
journal of Biiinn, but beyond the fact that Mendel was a Vice-President of
the Verein, whose organ it is, I could discover in it nothing relating to him.

Biographical Notice 331

posino- a compromise, and even offering honours in case of
submission. Old friends and acquaintances tried to influ-
ence him, but it was all in vain. He attended neither to
cajolement nor menace. The property of the house was
eventually distrained upon, but he did not c;;ive in. He
became also involved in the racial controversies which are
often rife in this part of Austria, and it is only too certain
that the last ten years of his life were passed in disappoint-
ment and bitterness. From being a cheerful, friendly man
he became suspicious and misanthropic. During this period
he fell into ill-heath, contracting a chronic nephritis, of
which he died January 6, 1884.

As to the propriety of his action in the great quarrel
with the Government I have no means of forming: an
opinion. It is nevertheless interesting to know that a few
years after his death the tax was removed without debate
or dispute.

For many years he attended closely to meteorology and
published his records annually in the BrLinn AbJiaudlnugen.
He also took a great interest in sunspots, making such
observations on them as he could by simple means'^*, drawing
them and recording the frequency of their occurrence. He
was amonof those who incline to the view that there is a
connection between the appearance of spots on the sun and
meteorological events on the earth. His notes on this
subject are also lost. He served a term as President of
the Naturforscher Verein in BrLinn. That he was credited
with good faculties for business is shown by the fact that
he was chosen to be Chairman of the Moravian Hypo-
theken-Bank in that city. He is said also to have attained
considerable skill as a chess-player, and he composed a
good many problems which however were not published.
This faculty reappears in one of his nephews.

His handwriting is remarkable for its extreme neatness,
every letter being formed with meticulous precision.

In Heinzendorf, his native village, he is remembered
as havino- been the oroaniser of a hre-brigade. When he
eventually became famous, the erection ot a new fire-station
was used as an opportunity of commemorating him, and a
memorial tablet was placed over the building in his honour.
•»«■ He used one of Fntsch's '' biachytelescopes."

332 Biographical Notice

The types of the great discoverers are most various.
To the naturahst the fact is full of meaning. The wild,
uncertain, rapid flash of genius, the scattered, half-focussed
daylight of generalisation, the steady, slowly-perfected ray
of penetrative analysis, are all lights in which truth may be
seen. Mendel's faculty was of the latter order. From the
fragmentary evidence before us w^e can in all probability
form a fairly true notion of the man, with his clear head,
strong interest in practical affairs, obstinate determination,
and power of pursuing an abstract idea.

The total neglect of his work is known to have been a
serious disappointment to him, as well it might. He is
reported to have had confidence that sooner or later it would
be noticed, and to have been in the habit of saying " Meine
Zeit wird schon kommen ! " This episode in the history of
science is not a very pleasant one to contemplate. There
are of course many similar examples, but there must be few
in which the discovery so long neglected was at once so
significant, so simple, and withal so easy to verify. The
scientific world may comfort itself with the thought that
in this case it sinned through inadvertence. With the ex-
ception of Nageli"^ perhaps none of the leading naturalists
ever saw the paper on peas. We would like to know
whether Mendel made any other attempt to interest his
contemporaries in his discovery. Probably having tried
Nageli and failed, he gave up further efforts.

So far as I have discovered there was, up to 1900, only
one reference to Mendel's observations in scientific litera-
ture t. namely that of Focke, Pflaiizejunischliiige, 1881,
p. 109, where it is simply stated that Mendel's numerous
experiments on Pisinn 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 Hiei^acium. For these references we may now
be grateful since it was through them that the papers were
rediscovered.

The fact that the Briinn journal is rather scarce does

* See p. 54.

t The Hierachim paper is referred to by Peter, Engler's bot. Jahrb.
Bde. V and vi, 1884, but only in its systematic bearings.

Biographical Notice 333

not In itself explain why the work was not noticed. Such
a circumstance has seldom long delayed general recognition.
The cause is unquestionably to be found in that neglect of
the experimental study of the problem of Species which
supervened on the general acce[)tance of the Darwinian
doctrines. The problem of Species, as Kolreuter, Gartner,
Naudin, Wichura, and the other hybridists conceiv^ed 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 1S65 the more
original investigators naturally found those new methods of
research more attractive than the tedious observations ot
the hybridisers, whose inquiries were supposed, moreover,
to have led to no definite result.

Nevertheless the total neglect of such a discoverv 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 '' Iitudier les Hybiades vSgdlaux an point de vite
de leiir fdconditd et de la perpdtiiitd de leurs ca^^acteresy
This subject was doubtless chosen with reference to the
experiments of Godron of Nancy and Naudin, then of Paris.
Both these naturalists competed, and the accounts of the
work of Godron on Datura and of Naudin 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 enunciated what we shall
henceforth know as the Mendelian conception of the disso-
ciation of characters of cross-breds in the formation of the
germ-cells, though apparently he never developed this con-
ception.

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 24th May,
Naudin's work being known to him fn^m 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 recjuired
much more confirmation before it could be held as i)roven.
(/. Linn. Soc. Bot. viii. Proc. p. xiv.)

334 Biographical Notice

In 1865. the year of Mendel's communication to the
Briinn Society, appeared Wichura's famous treatise on his
experiments with Salix 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 w^ork
by de Vries, Correns and Tschermak in 1900 a new era
beoins.

That Mendel's work, appearing as It did at a moment
when several naturalists of the first rank were still occupied
with these problems, should have passed wholly unnoticed,
will always remain inexplicable, the more so as the Briinn
Society exchanged its publications with most of the Acade-
mies of Europe, including both the Royal and Linnean
Societies.

Naudln's views were w^ell 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. Darwin took the objection
that Naudln's Ideas were not compatible with cases of
reversion, though as we now know, such cases are perfectly
consistent with the phenomenon of segregation. 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. Sioreover, the Mendellan
idea of the '' hybrid-character," or heterozygous form, was
unknown to him, a conception without which the hypothesis
of dissociation 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 difierent
from that which we have witnessed.

EXPERIMENTS IN PLANT-
HYBRIDISATION*.

By Gregor Mendel.

{Read at the Meetifigs of the 2)th February and ZtJi MarcJi^ 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 per-
severance. Gartner especially, in his work " Die Bastard-
erzeugung im Pflanzenreiche" (The Production of Hybrids
in the Vegetable Kingdom), has recorded very vakiable
observations ; and quite recently Wichura published the
results of some profound investigations into the hybrids
of the Willow. That, so far, no generally apph'cable 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 ot

* [This translation was made by the Royal Horticultural Society, and '^-
is reprinted with modifications and corrections, by permission. The original
paper was published in the Verh. nalurf. Vcr. in Bninn, Abhandlungtn.
IV. 1865, which appeared in 1866.]

336 Mendel's Experiments

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 definitely to
ascertain their statistical relations^.

It requires indeed some courage to undertake a labour
of such far-reaching extent ; this appears, however, to be
the only right way by which we can finally reach the solu-
tion 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.

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.

* [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 in his day.]

in Hybridisatioji 337

The experimental plants must necessarily —

1. Possess constant differentiatinir characters.

2. The hybrids of such plants must, durini^ the flower-
ing period, be protected from the Inlluence 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 occurred
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, witJiout exception,
subjected to observation.

At the very outset special attention was devoted to the
Leguminosae on account of their peculiar floral structure.
Experiments which were made with several members of
this family led to the result that the genus Pisum was
found to possess the necessary qualifications.

Some thoroughly distinct forms of this genus possess
characters which are constant, and easily and certainly
recognizable, 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.

B. H. 2 2

338 Mender s Experiments

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
noticed, 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 seedsman ;
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 w^ere selected and cultivated during
the whole 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 sativum ;
while the rest are regarded and classed, some as sub-species
of P. sativ^in^, and some as independent species, such as
P. qtiadrahim, P. sacckarahim, and P. U7nbellatum, 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

in Hybridisation 339

differentiating characters, and to deduce the law according
to which they appear in the sucicessive generations. The
experiment resolves itself therefore into just as many
separate experiments as there are constantly differentiat-
ing characters presented in the experimental plants.

The various forms of Peas selected for crossing showed
differences 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 of 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 applied
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 from these facts anv 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 afe either round or roundish, the depressions, if any,
occur on the surface, being always only shallow ; or they are
irregularly angular and deeply wrinkled (/^. quadrat uni).

2. To the difference i^i the colour of the seed albumen
(endosperm) '^. The albumen of the ripe seeds is either
pale yellow, bright yellow and orange coloured, or it pos-
sesses a more or less intense green tint. This difference
of colour is easily seen in the seeds as [ = if] their coats
are transparent.

3. To the difference in the colour of the seed-coat.
This 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,

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

22 — 2

340 Menders Experiments

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 differe7ice in tJie forin of the ripe pods.
These are either simply inflated, not contracted in places ;
or they are deeply constricted between the seeds and more
or less wrinkled {P. sacckaratum).

5. To the difference in the colour of the ttn7dpe pods.
They are either light to dark green, or vividly yellow, in
which colouring the stalks, leaf-veins, and calyx participate^.

6. To the diffei'-ei'ice 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. tnnbellatitni)^.

7. To the difference 171 the length of the stein. The
length of the stem| is very various in some forms ; it is,
however, a constant character for each, in so far that
healthy plants, grown in the same soil, are only subject
to unimportant variations in this character.

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

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

for the

I St trial 60 fertilisations on 15 plants,
2nd „ 58

3rd » 35
4th „ 40

.=ith „ 23
6th „ 34

7tl^ 5. 37
* One species possesses a bea

5, 10 „

>» 5 55
.. 10

itifuUy 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.]

% [In my account of these experiments {R.H.S. Jourtial^ vol. xxv.
p. 54) I misunderstood this paragraph and took *'axis" to mean X\\^ floral
axis, instead of the main axis of the plant. The unit of measurement,
being indicated in the original by a dash ( ' ), I carelessly took to have
been an iiich^ but the translation here given is evidently correct.]

in Hybridisation 341

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-bearer in
the other set was used as the pollen plant.

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
In the open as regards possible disturbance by Insects.
Among the Insects'^ which visit Peas the beetle Briuliiis
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
uncoveredf. 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 Phaseobts and Lathyrus.

The risk of false impregnation by foreign pollen is,

* [It is somewhat surprising that no mention is made of Tlirips, \vhi('h
swarm in Pea flowers. I had come to the conckision that tliis is a real
source of error and I see Laxton held the same opinion.]

t [This also happens in Sweet Peas.]

342 Menders Experiments

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
Brtichus fizsi, and possibly also the described abnormalities
in the floral structure, were to blame.

[F^ The Forms of the Hybrids'^,

Experiments which in previous years were made with
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, is nearly always to be seen ;
in other cases, however, one of the two parental characters
is so preponderant that it is 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 f 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 quite, 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 escaping the error
pervading the older views of heredity.]

hi^ Hybridisation 343

but nevertheless reappear unchanged in their pro'^^eny, 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-[)arent ;
the form of the hybrid remains identical in both cases.
This interesting fact was also emphasised by Gartner, with
the remark that even the most practised expert is not in a
position to determine in a hybrid which of the two parental
species was the seed or the pollen plant*.

Of the differentiating characters which were used in the
experiments the following are dominant :

1. The round or roundish form of the seed with or
without shallow depressions.

2. The yellow colouring of the seed albumen [coty-
ledons].

3. The grey, grey-brown, or leather-brown colour of
the seed-coat, in association 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 association
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, a fact which is possibly only attributable to
the greater luxuriance which appears in all parts of plants
when stems of very different length are crossed. Thus,
for instance, in repeated experiments, stems of i ft. and
6 ft. in length yielded without exception hybrids which
varied in length between 6 ft. and ']\^ ft.

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 t-

The hybrid forms of the seed-shape and of the allnmun
[colour] are developed immediately after the artificial terti-

* [Gartner, p. 223.]

t [This refers to the coats of the seeds borne by F^ plants.]

344 Menders Experiments

lisatlon by the mere influence of the foreign pollen. They
can, therefore, be observed even in the first year of experi-
ment, 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
peculiarities fully developed, 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 investi-
gated 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.
Transitio7ial forms were not observed in any expe7^inie7tt.

Since the hybrids resulting from reciprocal crosses are
formed alike and present no appreciable difference in their
subsequent development, consequently the results [of the
reciprocal crosses] can be reckoned together in each experi-
ment. The relative numbers which were obtained for each
pair of differentiating characters are as follows :

Expt. I. 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 i 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*oi to I.

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 happened that all the

in Hybridisation 345

seeds were round (Expt. i) or all yellow (Expt. 2) ; on the
other hand there were never observed more than five
wrinkled 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
subsequently developed pods the normal proportions were
maintained 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.

Experiment

Form of Seed.

Colour

of Alb

umen

Plants

Round

Angular

Yellow

Green

As extremes in the distribution of the two seed characters
in one plant, there were observed in Expt. i 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 i green seed, but also one of 20 yellow
and 19 green.

These two experiments are important for the determina-
tion of the average ratios, because with a smaller number
of experimental plants they show that very consider-
able 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 overkx^ked.
The cause of this partial disappearance of the green colour-
ing has no connection with the hybrid-character of the

346 MendePs Experiments

^plants, as it likewise occurs in the parental variety. This
peculiarity [bleaching] 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, with a little practice in sorting, errors are 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 propor-
tion 3*15 to I.

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 i. .

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 I.

Expt. 6. Position of flowers. — Among 858 cases 651
had inflorescences axial and 207 terminal. Ratio, 3*14 to i.

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 i. 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 i, or 3 to i.

* [This is true also of the dwarf or "Cupid " Sweet Peas.]

in Hybridisation 347

The dominant character can have here a double signi-
ficaho7t — viz. that of a parental character, or a hybrid-
character"^. In which of the two significations it appears in
each separate case can only be determined by the following
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 maintain the same
behaviour as in the first generation [i^J.

[F.^ The Second Generation [bred] from

THE Hybrids.

Those forms which in the first generation [/^] exhibit
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 tivo-th'wds yield offspring which display the domi-
nant and recessive characters in the proportion of 3 to i.
and thereby show exactly the same ratio as the hybrid
forms, while only one-ihir:d remains with the dominant
character constant.

The separate experiments yielded the following results:

Expt. I. 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 wrinkled seeds, in the
proportion of 3 to i. The number of the hybrids, therefore,
as compared with the constants is 1*93 to i.

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
and gi-een seeds in the proportion of 3 to i. There resulted,
therefore, a division into hybrid and constant forms in the
proportion of 2*13 to i.

[This paragraph presents the view of the hybrid-character as some-
incidental to the hybrid, and not "transmitted" to it— a true and
fund'amental conception here expressed probably tor the first time.]

thing

348 Menders Experiments

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 fiowers.

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 num.ber 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 i and 2*13 to i gave together almost exactly
the average ratio of 2 to i. The sixth experiment gave a
quite concordant result ; in the others the ratio varies more
or less, as was only to be expected in view of the smaller
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 \ appears^ therefore, as fixed with
certainty. It is therefore demonstrated that, of those forms
which possess the dominant character in the first genera-
tion, two-thirds have the hybrid-character, while one-third
remains constant with the dominant character

The ratio of 3 to i, in accordance with which the

in Hybridisation 349

distribution of the dominant and recessive characters resuhs
in the first generation, resolves itself therefore in all exi)eri-
ments into the ratio of 2 : i : i if the dominant character
be differentiated according- to its significance as a hybrid-
character or as a parental one. Since the nK^nbers of the
first generation [/%] spring directly from the seed of the
hybrids [/s], it is now clear that the Jiybrids form seeds 1

having one or other of the two difj'e7'entiating characters, ajtd j

of these one-half develop again the hybrid form, while the
other half yield plants which remain constant and 7'eceive
the dominant or the recessive characters [_respectively'] in [

equal members.

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 I 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 v^dth 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.

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

A-\'2Aa-\-a

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

The observation made by Gartner, Kolreuter, and others,
that hybrids are inclined 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
o-eneration, is continually diminishing, but that never-
theless they could not entirely disappear. If an average

350 Mender s Experiments

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.

Ratios
Generation A Aa a A

1121 I

2646 3

3 28 8 28 7

4 120 16 120 15

5 496 2>^ 496 31
n 2'^-!

Aa : a

2 : I

2 : 3

2 : 7

2 : 15

2 : 31

2 : 2"'- I

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 tv.o 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

* [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 {weseiiiliches Merhnal). The
point is probably of little theoretical or practical consequence, but a rather
heavy stress is thrown on '■'■ wese7itlich.^^\

in Hybridisation o-

a short stem, terminal white flowers, and simply inflated
pods ; the pollen plant, on the other hand, a long stem,
violet-red tiowers distributed alonu the stem, and con-
Striated 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.

Two experiments were made with a considerable 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 hy A, B, C, those of the pollen plant by a, b, c,
and the hybrid forms of the characters by Aa, Bd, and Cc.

Expt. I. — AB, seed parents; ad, pollen parents;

A, form round ; ^, form wrinkled ;

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,
loi wrinkled and yellow,
108 round and green,
32 wrinkled 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 wrinkled yellow seeds Aa/^
138 round yellow and green, wrinkled yellow

and screen seeds .... AaBb.

352 Menders Experiments

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

28 had only wrinkled yellow seeds aB

68 wrinkled yellow and green seeds aBb.

From 108 round green seeds 102 resulting plants fruited,

of which :

35 had only round green seeds Ab

67 round and wrinkled green seeds Aab.

The wrinkled 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 tind :

38 plants with the sign AB

Y:^

;»

)»

, •

ABb

<■>

aBb

■)•)

>»

AaB

Aab

AaBb.

The whole of the forms may be classed Into three
essentially different groups. The first includes those with
the signs AB, Ab, 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 includes the signs ABb,
aBb, 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
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 i, 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, ab, resemble

in Hybridisation 353

the parental forms, the two others present combinations
between the conjoined characters A, a, B, b, which com-
binations are hkewise 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 + ab-\- 2ABb + 2aBb + lAaB

-\-2Aab + /\AaBb.

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

A-^ 2Aa + a
B + 2Bb + b.
Expt 2.

ABC, seed parents ; abc, pollen parents ;

A, form round ; a, form wrinkled ;

B, albumen yellow ; b, albumen green ;

C, seed-coat grey-brown. r, seed-coat white.

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 w^ere all either spotted, grey-
brown or grey-green, round or wrinkled"*. From these in
the following year 639 plants fruited, and, as further
investigation showed, there were among them :

plants

ABC

A Be

AbC

Abe

aSC

aBe

abC

abe

plants

ABCc

45 I

:)lants

ABbCc

»)

AbCe

))

aBbCe

aBCe

AaBCe

abCe

AabCt

ABbC

AaBbC

ABbc

AaBbc

aBbC

aBbe

'»

AaBC

»»

AaBbC

AaBe

AabC

)»

Aabc

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

C. H. 23

354 * MeiideVs Experiments

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 I, 2, 4, 8, that this last undoubtedly
represents the true value.

The development of the hybrids when the original
parents differ in three characters results therefore according
to the following expression :

ABC + ABc + AbC + Abe + aBC 4- aBc + abC 4- abc 4-
2ABCc-\- 2AbCc-\- 2aBCc^ 2 abCc^2 ABbC+ 2 ABbc-h
2 aBbC + 2 aBbc + 2 AaBC + 2 AaBc + 2 AabC+ 2 Aabc 4-
4 ABbCc + 4 aBbCc 4- 4 AaBCc 4- 4 AabCc + 4 AaBbC +
j^AaBbc^"^ AaBbCc.

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

A + 2Aa + a
B + 2Bb-hb

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 tw^o 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 no doubt that for the whole of the characters
involved in the experiments the principle applies that ^/le
offspring of the hybrids in which several essentially diff'ej'ent
characters are combined exhibit the terms of a series of
combinations, in which the developmental series for each pair
of differentiating characters are tmited. It is demon-
strated at the same time that the 7^elation of each pair of

in Hybridisation o

030

different cJiaracters in Jiybrid union is independent of tti
otiier differences in tlie two 07'iginat parentat stoc/cs.

Un represent the number of the difterentiating characters
in the two original stocks, 3" gives the number of terms 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 contains, if the
original stocks differ in four characters, 3' = 81 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 bv
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 t/iat the C07istant characters wliich appeal' in
the severat varieties of a gronp of plants inay be obtained in
all the associations wJiich are possible accor diner (q the
\inatJie7natical^ laws of co7}ibination, by means of repeated
artificial fertilisation .

As regards the flowering time of the hybrids, the ex-
periments are not yet concluded. It can, however, already
be stated that the time stands almost exactly between those
of the seed and pollen parents, and that the constitution of
the hybrids with respect to this character probably follows
the rule ascertained in the case of the other characters.
The forms which are selected for experiments of this class
must have a difference of at least twenty days from the
middle flowering period of one to that of the other; further-
more, the seeds when sown must all be placed at the same
depth in the earth, so that they may germinate simul-
taneously. 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 difliculties to be overcome and

necessitates great attention.

If we endeavour to collate in a brief form the results
arrived at, we And that those differentiating characters,
which admit of easy and certain recognition in the exj:>eri-
mental plants, all behave exactly alike in their hybrid

23—2

356 Menders Experiments

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 combination
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 in-
cluded 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 the Hybrids.

The results of the previously described experiments
led to further experiments, the results of which appear
fitted to afford some conclusions as regards the composition
of the ^^^ and pollen cells of hybrids. An important clue
is afforded in Pisitm by the circumstance that among the
progeny of the hybrids constant forms appear, and that
this occurs, too, in respect of all combinations of the
associated characters. So far as experience goes, we find
it in every case confirmed that constant progeny can only
be formed when the ^^<g 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 certain that exactly similar factors must be at
work also in the production of the constant forms in the
hybrid plants. Since the various constant forms are pro-
duced in 07ie plant, or even in one flower of a plant, the
conclusion appears logical that In the ovaries of the hybrids
there ^re formed as many sorts of ^^^ cells, and in the

in Hybridisation 357

anthers as many sorts of pollen cells, as there are possible
constant combination forms, and that these et^i^ and pollen
cells agree in their internal composition with those of the
separate forms.

In point of fact it Is possible to demonstrate theoretically
that this hypothesis would fully suffice 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 ^"g^ 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 different in the form of the seed and
the colour of the albumen were united by fertilisation.

If the differentiatino- characters are as^ain indicated as
A, B, a, b, we have :

AB, seed parent ; ab, pollen parent ;

A, form round ; a, form wrinkled ;

B, albumen yellow. b, albumen green.

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

1. The hybrids with the pollen oi AB.

2. The hybrids ,, „ ab.

3. AB „ o 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 it^gg and
pollen cells of the forms AB, Ab, aB, ab, and there would
be combined :

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

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

* [This and the preceding paragraph contain the essence of the
MendeUan principles of heredity.]

358 Menders Expermients

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

4. The ^^g cells ab with the pollen cells AB, Ab,
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 eg^ and pollen
cells of the hybrids were produced on an average in equal
numbers, then in each experiment the said four combinations
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 ^g^ cells remain undeveloped or subsequently
die, and many even of the well-formed seeds fail to ger-
minate 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 ^gg 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 ^gg 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 third experiments and the second 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 b, for which
reason they must impress their peculiarity upon the whole

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

in Hybridisation 359

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 luiion 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 oreen ; the third is constant in the recessive character
of form but hybrid in colour, consequently the seeds are
wrinkled and yellow ; the fourth is constant in both recessive
characters, so that the seeds are wrinkled and green. In
both these experiments there were consequently four sorts
of seed to be expected — viz. round and )ellow, round and
green, wrinkled and yellow, wrinkled and green.

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

ist Experiment, 98 exclusively round yellow seeds ;
S^d „ 94 ,, ,, ,, „

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

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

There could scarcely be now any doubt of the success
of the experiment ; the next generation must afford the
final proof. From the seed sown there resulted for the
first experiment 90 plants, and for the third %-] plants
wdiich fruited : these yielded for the

ist Exp. 3rd Exp.

20 25 round yellow seeds ..... AB

23 19 round yellow and green seeds . . . ADb

25 22 round and wrinkled yellow seeds . . AaB

22 21 round and wrinkled green and yellow seeds AaBh

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

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

The wrinkled yellow seeds gave plants with wrmkled
yellow and green seeds, aBb.

360 Menders Experiments

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

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
therefore from the

2nd Exp.

4th Exp.

plants of the form AaBb

„ Aab

„ „ aBb

» ab

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

In a further experiment the characters of flower-colour
and length of stem were experimented upon, and selection
was 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, b serve again as
indicating the various characters.

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

B, axis long. b, axis short.

The form Ab was fertilised with ab, which produced the
hybrid Aab. Furthermore, aB was also fertilised with ab^
whence the hybrid aBb. In the second year, for further
fertilisation, the hybrid Aab was used as seed parent, and
hybrid aBb as pollen parent.

Seed parent, Aab. Pollen parent aBb.

Possible ^gg cells, Abab. Pollen cells, aBab.

From the fertilisation between the possible ^gg and
pollen cells four combinations should result, viz. :

AaBb + aBb + Aab + ab.

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

in Hybridisation 361

-1

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

„ „ „ white flowers {a) ,, 2, 4

„ „ „ a long axis {Bd) „ 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 subsequently appeared

The violet-red flower-colour {Aa) in 85 plants.
,, white „ ,, {a) in Si ,,

,, long stem (Bb) in 87 ,,

,; short ,, [b) 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 agreenient. All
combinations which were possible through the union of
the differentiating characters duly appeared, and in nearly
equal numbers.

Experimentally, therefore, the theory is confirmed that
the pea hybrids form egg and pollen cells wlu'ch, in their
constitntion, represent in eqnal nnmbers all constant forms
which result from the combination of the characters 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 iorm

362 Menders Experiments

of both. It includes in three different classes four indi-
viduals. In the formation of these, pollen and ^%^ 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 pollen cells A •\- A -\- a -\- a
The ^g^ 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 egg 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 egg cell form A and a, consequently one
of the two pollen cells A in the fertilisation will meet with
the egg cell A and the other with an egg cell a, and so
likewise one pollen cell a will unite with an egg cell A,
and the other with egg cell a.

Pollen cells A A a a

1 X 1

Egg cells A A a a

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

A^ A a a
A'^'^'^A'^a'

In the first and fourth term the egg 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 differen-
tiating characters of the stocks, consequently the forms
resulting from these fertilisations are identical with those
of the hybrid from which they sprang. T/iere occurs
accordingly a repeated hyb7Hdisation. This explains the
striking fact that the hybrids are able to produce, besides

in Hybridisation 363

the two parental forms, offspring which are Hke themselves;

A a .

— and —. both give the same union Aa, since, as already

(t y^t

remarked above, it makes no difference in the result of

fertilisation to which of the two characters the pollen or

^Z% cells belong. We may write then

A A a a ^ ,

-77 + — + -;^ ^- = A-^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 individual flowers and in individual 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 ^<g^
cells occur in the seed vessels can only be regarded as equal
on the average, it remains purely a matter of chance which
of the two sorts of pollen may fertilise each separate ^"g^
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 effects 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 ^ lABb ^ 2aBb ^ 2AaB -\- 2Aab ^

^AaBb.

Between the differentiating characters of the original stocks
Aa and Bb four constant combinations are possible, and
consequendy the hybrids produce the corresponding four
forms of ^g<g and pollen cells AB, Ab, aB, ab, and each
of these will on the average figure four times in the

* [Whether segregation by such units is more than purely fortuitous
may perhaps be determined by seriation.]

364 Menders Experiments

fertilisation, since sixteen individuals are included in the
series. Therefore the participators in the fertilisation are

Pollen cells AB -V AB -V AB ^ AB -V Ab-\- Ab^ Ab^ Ab^
aB + aB + aB + aB -h 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 ^g^ cell form, so that each
of the four pollen cells AB unites once with one of the
forms of ^^g 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 ^^^ cells. We obtain therefore

AB AB AB AB Ab Ab Ab Ab
'AB^ Ab '^ aB'^ ~^^ AB"^ Ab^ aB'^ ab^

aB aB aB aB ab ab ab ab
+ -:r7 + -Ti + — r + -^rr. + -^; + -^ +

AB ' Ab aB ' ab ' AB ' Ab aB ab'
or

AB + ABb + AaB + AaBb + ABb +Ab-\- AaBb + Aab +
AaB + AaBb -\-aB + aBb + AaBb + Aab + aBb -\-ab=^ AB +
Ab-\-aB + ab + 2 ABb + 2aBb + 2 AaB + 2 Aab + ^AaBb^,

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 ^gg and pollen cells — ABC, ABc, AbC,
Abe, aBC, aBc, abC, abc — and each pollen form unites
itself again on the average once w^ith each form of ^gg 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 ^gg cells and pollen cells which
in equal numbers represent all constant forms which result
from the combinations of the characters brought together
in fertilisation.

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

in Hybridisation

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 Pliaseohis have been completed,
and may be here mentioned.

An experimimt with Pliaseolus vulgaris and Phascolus
nantis gave results in perfect agreement. Pli. nanus had
together with the dwarf axis, simply inflated, green pods.
Ph, vulgaris had, on the other hand, an axis lo feet to
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
combinations

I
2

5
6

7
8

Axis

long

short
>»

Colour of
the unripe pods

green

yellow

green

yellow
>»

Form of
the ripe pods

inflated
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. Pliaseolus nanus, L.,
served as seed parent, a perfectly constant species, with
white flowers in short racemes and small white seeds m
straight, inflated, smooth pods ; as pollen parent was used
PL 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-blossom-red ground.

366 Menders Experiments

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
medium size, and were flecked and splashed similarly to
those of Ph. imdtiflorus, 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 plants was necessarily very fluctuating
owing to the small number of trial plants. With certain
characters, as In those of the axis and 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 Pistun It is known
that the characters of the flower- and seed-colour present
themselves unchanged in the first and further generations,
and that the oflspring of the hybrids display exclusively the
one or the other of the characters of the original stocks;
It is otherw^ise in the exp.eriment we are considering. The
white flowers and the seed-colour of Ph. nanus appeared, it
is true, at once In the first generation \_fr07n 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
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 w^Ith preponderantly
red flowers, since out of sixteen of these only four yielded
ripe seed. Three of these had a similar seed pattern to
Ph. mMltiflorits, but with a more or less pale ground colour;
the fourth plant yielded only one seed of plain brown tint.

in Hybridisation 367

The forms with preponderantly violet-coloured (lowers 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 came again
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 which 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 offspring
exactly like themselves, some which displayed white llowers
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
reo-ard to those characters which concern the form of the
plants, follows the same laws as in Pisiim. With regard
to the colour characters, it certainly appears difficult to
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 [in /^], 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 Pisiun 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 (lowers and seeds of Ph.
multiflorics is a combination of two or more entirely in-
dependent colours, which individually act like any other
constant character in the plant. If the flower-colour A
were a combination of the individual characters .-/, -f--/,+ ...
which produce the total impression of a purple colora-
tion, then by fertilisation with the differentiating character,
white colour, a, there would be produced the hybrid unions

368 Mender s Experime^tts

A^a-\- A.a-V "' and so would it be with the correspondinor
colouring of the seed-coats^. According to the above
assumption, each of these hybrid colour unions would
be independent, and would consequently develop quite
independently from the others. It is then easily seen
that from the combination of the separate developmental
series a complete colour-series must result. If, for instance,
A=A^-\-A^, then the hybrids A^a and A.;^a form the
developmental series —

A^ + 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.^ I A^a

2 A^A.^a 4 A^aA^a 2 A^aa
I A^a 2 A^aa \ 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, nevertheless, not be forgotten that the explana-
tion here attempted is based on a mere hypothesis, only
supported by the very imperfect result of the experiment
just described. It would, however, be well worth while to
follow up the development of colour in hybrids by similar

* [As it fails to take account of factors introduced by the albino this
representation is imperfect. It is however interesting to know that Mendel
realized the fact of the existence of compound characters, and that the
rarity of the white recessiveb was a consequence of this resolution.]

in Hybridisation 369

experiments, since It is probable that in this way we mifht
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 the sole cause of variability we
might expect that those cultivated plants which are grown
for centuries under almost Identical conditions 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 most variable forms are found.
It is only the Legitminosae, like Pi stun, PJiascohis^, 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 under most various conditions; these
maintain, however, under unchanging environments a sta-
bility as great as that of species growing wild.
* \Fhaseolus nevertheless is insect-fertilised.]

B. H. -4

370 Menders Experiments

It is more than probable that as regards the variablh'ty
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
exceptions, are ^nembers of various hybrid series, whose
further development in conformity with law is varied and
interrupted 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 behave pre-
cisely as do those which are known to be members of the
compound hybrid series. Also with the most susceptible
of all characters, that of colour, it cannot escape the careful
observer that in the separate forms the inclination to vary
is displayed in very different degrees. Among plants which
arise from one spontaneous fertilisation there are often some
whose offspring vary widely in the constitution and arrange-
ment of the colours, while that of others shows 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
afford an instructive example of this. A white-flowered
example of Dianthus ca^yophyllus^ 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 sub-species,
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 coloration 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 zn the
combination of several independent coloitr characters.

in Hybridisatioi ^li

Concluding Remarks.

It can hardly fail to be of interest to compare the
observations made regarding Pisuni 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 outward 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 h\ brids without
exception. Sometimes the offspring have more nearly
approached, some the one and some the other of the two
original stocks, or they all incline more to one or the other
side ; while in other cases they remain pcrjectly like the
hybrid diXid continue constant in their offspring. The hybrids
of varieties behave like hybrids of species, but they possess
greater variability of form and a more pronounced tendency
to revert to the original types.

With regard to the form of the hybrids and their
development, as a rule an agreement with the observations
made in Piszim 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 cir-
cumstance could, however, contribute to render the results
fluctuating and uncertain, despite the most careful observa-
tion and differentiation. For the experiments plants were
mostly used which rank as good species and are difler-
entiated 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

24—2

372 Menders Experiments

taken into account which are often difficult to define in
words, but yet suffice, as every plant specialist knows, to
give the forms a peculiar 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 contain very many forms, since the number of the
terms, as is known, increases with the number of the
differentiating- characters as the powers of three. 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
two original stocks differ in seven characters, and loo 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 differentiatinof 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 doininant characters, which
are transmitted entire or nearly unchanged to the hybrids,
then in the terms of the developmental series that one of
the two original parents which possesses the majority of
dominant characters must always be predominant. In the
experiment described relative to Pisimi, in which three
kinds of differentiatinsf 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 parents equally, yet in this experiment 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 must be under such
circumstances to draw from the external resemblances of
hybrids conclusions as to their internal nature.

Gartner mentions that in those cases where the develop-
ment was regular, among the oftspring of the hybrids the
two original species were not reproduced, but only a few
individuals which approached them. With very extended

in Hybridisation 373

developmental series It could not in fact be otherwise.
For seven dilTerentiatinor characters, for instance, amon<^
more than 16,000 individuals — offspring of the hybrids
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 di/fer-ence In those hybrids
which remain constant in their progeny and j)roijagate
themselves as truly as the pure species. According to
Gartner, to this class belong the reniarkabty fe^'tilc Jiybrids
Aqznlegia atropurpiirea canade)isis, Lavatera pseudotbia tJiu-
riiio^iaca, Geum urbano-rivate, and some Diaiitlius hvbrids;
and, according to Wichura, the hybrids of the Willow
family. 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 the
facts Is guaranteed by eminent observers, and cannot be
doubted. Gartner had an opportunity of following up
DiantJms Armeida deltoides to the tenth generation, since
it regularly propagated itself in the garden.

With Pisum it was shown by experiment that the
hybrids form ^^<g 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 reproductive 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 ^g^ cell unite* in
Phanerogams^^ into a single cell, which Is capable by

* In Pisiim it is placed beyond doubt that for the formation of the
new embryo a perfect union of the elements of both rcproduciivc cells must
take place. How could we otherwise explain that among the ort>pring ot
the hybrids both original types reappear in equal numbers and with all
their peculiarities? If the inllucnce of the egg cell upon the pollen ce 1
were only external, if it fulfilled ihe 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 m no wise conhrined. .An

374 Me7iders Experimetits

assimilation and formation of new cells to become an in-
dependent organism. This development follows a constant
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 indi-
vidual will follow the same law which rules the mother
plant. If it chance that an ^^^ cell unites with a dissimilar
pollen cell, we must then assume that between those
elements of both cells, which determine opposite characters
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 two similar cells, in which the differences
are entirely and permanently accommodated together, the
further result follows that the hybrids, like any other stable
plant species, reproduce themselves truly in their offspring.
The reproductive cells which are formed in their seed
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 ^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

evident proof of the complete union of the contents of both cells is afiforded
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 375

in an entirely free and equal arrangement, by which it
is only the differentiating ones which mutually separate
themselves. In this way the production would he rendered
possible of as many sorts of ^^^ 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 pcrmanc7it or U'Diporary
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 scope. Some justification of the
opinion expressed lies in the evidence afforded by Pision
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
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 Pisjim
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 observerf, 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 ex[)eriment-
ally. In the meantime we may assume that in material
points an essential <lifterence can scarcely occur, since the
unity in the developmental plan of organic life is beyond

question.

In conclusion, the experiments carried out by Kdlreuter,
Gartner, and others with respect to the transformation of
one species into another by artificial fertilisation merit
special mention. Particular importance has been attached

* ''Welche i?i de?t Grundzellcn derselben in lebnidi^er WWhsthrirkun^
t ''Dem emzeliwi Beobachkr kann leicht ein Diffcrcriziale ent^chntr

376 Menders Experiments

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
its progeny. By this process the species A would change
into the species B. Gartner alone has effected thirty such
experiments with plants of genera Aquilegia, Dianthus,
Geuifiy Lavatera, Lychnis, Malva, Nicotiana, and Oenothera.
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^ power 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 within 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 may 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 ^gg cells as there are constant combinations possible
of the characters conjoined therein, and one of these is
always of the same kind as that of 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

in Hybridisation yj-j

characters which are united by the fertilisation. Let us,
for instance, assume that the plants selected for experiment
differed in three characters, and the species ABL is to
be transformed into the other species abc by repeated
fertilisation with the pollen of the latter ; the hybridn
resulting from the first cross form eight different kinds
of e^g cells, viz. :

ABC, ABc, AbC, aBC, Abc, aBc, abC, abc.

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

AaBbCc + AaBbc + AabCc + aBbCc

+ ^labc + aBbc + abCc + abc.

Since the form abc occurs once in the series of eio-ht
terms, 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 miviber of experimental plants and
the largx^^ 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
o-enerations such as Gartner observed. The transforma-
tion of widely divergent species could generally only be
completed in five or six years of experiment, since the
number of different ^<g^ cells which are formed in the hybrid
increases as the powers of two with the number of difieren-
tiating 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
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

378 Menders Experiments

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

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

year of experiment for selection of the individuals for

further crossing, and these were of the possible thirty-two

A
forms ; with the second experiment ^ 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 in the
other experiment. A definite selection was consequently
only possible with the first experiment ; with the second
the selection had to be made at random, merely. 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
experiments for fertilisation there agreed, as the following
year's culture showed, with the pollen parent :

ist Experiment 2nd Experiment

2 plants — in all characters

3 »> ' )5 4 j>

— 2 plants „ 3

2 55 5? 2 ,,

— I plant ,, I 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 in which

in Hybridisation

the dominant characters belong exckisively to one or the
other of the orig-inal parent plants, it will always make
a difference which of the two possesses the majority of
dominants. If the pollen parent has the majority, then
the selection of forms for further crossincr will afford a less
degree of certainty 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 exacdy 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 mto 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
Gartners experiments a noteworthy confirmation of that
supposition regarding variability of cultivated plants which
has already been expressed.

Among the experimental species there w^ere cultivated
plants, such as Aqiiilegia atropurpui'ca and caiiadcnsis,
Diantlms c ar y op Jiy litis, chiiiensis^ and japonicus, Nicotiana
rustica and paniadata, and hybrids between these species
lost none of their stability after four or five generations.

* ["Es sieht " in the original is clearly a misprint for •' Er sieht"]

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 :

U. Auricula $ x H. attrajitianim t
H. Auricula %y.H. Pilose I la $
H. Auricula % x H. p7'atense $
H. echioides\ % x H. aurantiacum ^
H. praealtum % x H. flagellare Rchb. Z
H. praealtum % x H. aurantiacu7n $

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
fertilisation without either letting pollen get on to the
stigma or injuring the pistil so that it withered away.
As is well known, the anthers are united to form a tube,

* [Published in Verh. 7iaturf. Ver. Brimn, 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. praealtum,
but approaches more nearly to H. echioides and for this reason was
reckoned as belonging to the latter.

Menders Expei^iinejits luit/i Hieraciu))! 381

which closely embraces the pistil. As soon as the flower
opens, the stigma, already covered with pollen, j^rotrudes.
In order to prevent self-fertilisation the anther-tube must
be taken out before the flow^er 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 ojjens,
it is seldom possible to prevent self-fertilisation ; for with
every care it Is not easily possible to prevent a few pollen
erains o-ettino- scattered and communicated to the sti'mia.
No better result has been obtained hitherto by reniovmg
the anthers at an earlier stage of develoj:)ment. IJefore
the approach of maturity the tender pistil and stigma are
exceedingly 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 //.
Aurimla 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 ///Vr^r//^;;/. Ihis
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 m
the separation and delimitation of these forms has demanded
the close attention of the experts. Regarding no cnher
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 torms are

unknown.

Regardinq; the question whether and to what extent
hybridfsationVays a part in the production of this wc-alth
of forms, we find very various and conllicting views held
by leading botanists. While some ot them mamtam that

382 Menders Experiments

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-
ofenitors.

The question of the origin of the numerous and constant
Intermediate forms has recently acquired no small interest
since a famous Hieraci7iin 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
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 Hieraciujn without
further consideration. If by the experimental method we
can obtain a sufficient insight Into the phenomenon of
hybridisation in Hie^^acium, 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

with Hiemcinni 383

slight results which I have as yet obtained with reference
to this object.

T. Respecting the structure of the hybrids, we have
to record the striking phenomenon thai the forms hitherto
obtained by similar fertilisation are not identical. Tlie
hybrids H. praealltcm $ x H. aiirantiacum $ and //. Auri-
cida $ X H. aurantiantin $ are each represented by twr),
and //. AMrictila $ x //. pratense $ 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 an intermediate structure,
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 //.
aurantiactmi $ pure yellow disc-florets ; only the petals
of the mareinal florets are on the outside tinoed with red
to a scarcely noticeable degree : in the other on the contrary
the colour of these florets comes very near to //. aiirantiacuw,
only in the centre of the disc the orange red passes into a
deep golden-yellow. This difference is noteworthy, for the
flower-colour in Hierachun 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. praealtttm % x //. atu^antiacum Z constitute
approximately intermediate forms which do not agree in
certain characters. On tlie contrary in //. Auricula % x //.
aurantiacMm Z and in H. Auricula $ x //. pratense Z 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 o\\ us thai we have here
only single terms in an unknown series which may be
formed by the direct action of the i)olIen of one species on
the egg-cells of another.

384 Menders Experimejtts

2. With a single exception the hybrids in question
form seeds capable of germination. H. echioides % x H,
aurantiacum $ may be described as fully fertile ; //. prae-
altum ^x H. flagellare $ as fertile ; //. praealtum ^x H.
aMrantiacmn $ and //. Atiricula % x H. pratense ^ as
partially fertile ; H. Auricula %x H. Pilosella $ as slightly
fertile, and //. Aurmi/a ^ x //. aurantiacum $ as infertile.
Of the two forms of the last-named hybrid, the red-fiowered
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. praealtum % x H. aurantiacuin $
there was one plant which possessed full fertility.

J-

As yet the offspring 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 //. praealt2i7n '^x H. flagellare $ two generations
have flowered; from H. echioides %xH. aurantiacum $,
H. praealttmi %xH. aurantiacum ^, H. Auricula %xH.
Pilosella $ 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 par-
tially fertile 7/. praealttim %x H. aurantiacum $ 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
of the hybrid, examined microscopically, show a defective
structure, a number of egg-cells capable of fertilisation do

with Hieraciiiiii 385

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
wild state the formation of the pollen fails, and \\\ 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 Hiei^acia 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. Hut 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 com-
munication. 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 confn-mation 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 communicating
in the year 1865, we find a very real distinction. 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 HieracUcm according to the present
experiments the exactly opposite phenomenon seems to be
exhibited. Already in describing the Pisuni 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 Hieraciitin we may take it we have a similar case.

B. H. 25

386 Me7iders Experiments with Hieraciiim

Whether from this circumstance we may venture to draw
the conclusion that the polymorphism of the genera Salix
and Hieraciujn is connected with the special condition of ^k
their hybrids is still an open question, which may well be ^
raised but not as yet answered. i

[The discovery of Ostenfeld and Raunkiaer that Hiera-
cittm is frequently parthenogenetic, or apogamous, of course
puts an entirely new construction on the results of these
experiments. See p. 247.]

BIBLIOGRAPHY

\T/iis list does not include works published since the appearance of the first
edition. References to the more iinportant of these will b^ found in the
Appendixes^

1. Allen, G. M. The heredity of coat colour in mire. Proc. Am.
Acad. Arts and Sci., Vol. XL. No. 2. 1904. [See also Castle.]

2. Anthony, R. Ball. Soc. Anthr., 1899, p. 303. [Heredity in
Manx Cat.]

3. Bacot, A. Experiments with Triphaena comes and melanic form.
Proc. Ent. Soc. Lond. 1905, p. Ixvii.

4. . Larvae of Lasiocanipa quercus and its vars. and of cross-
pairings between these races. Ent. Rec. xiii. p. 114.

5 Balls, W. L. The wheat problem in Egypt. Year-book of
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195. Mendel, Gregor Johann. Versuche iiber Pflanzen-Hybriden.
Verb. Naturf. Ver. in Briinn, Bd. 10, 1865, Abh. p. i ; reprinted in
Flora, 1 90 1, and in Ostwalds Klassiker d. exakten Wissensch. English
Trans, in Journ. R. Hort. Soc. 1901, xxvi.

196. . Uber einige aus kiinstlicher Befruchtung gevvonnene Hiera-

cium-Bastarde. Ibid. viii. 1869, Abh. p. 26.

197. . Briefe an Carl NageH. [See Correns (78).]

198. Millardet, A. Note sur I'hybridation sans croisement ou tausse
hybridation. Mem. Soc. des Sci. phys. et nat. de Bordeaux, iv. 4""^
Serie, 1894.

199. Morgan, T. H. The Assumed Purity of the Germ-Cells in
Mendelian Results. Science, 1905, xxii. p. 877.

Bibliogyapliy 3gy

200. Morgan, T. H. Are the Germ-Cells of Mendelian Hybrids

"Pure"? Biol. Cblt. 1906, xxvi. p. 2S9.

201. . The cause of Gynandromorphism in Ins(v t-^. Amer. Nat.

XLi. 1907, p. 715.

202. -. Production of two kinds of Spermatozoa in Phylloxerans —

Functional *' Female-producing and Rudimentary Spermatozoa." Pror.
Sec. Exp. Biol, and Med. v. 1908, p. 56.

203. . Some Experiments in Heredity in Mice. Science, N. S.

XXVII. 1908, p. 493.

204. Mudge, G. P. On some features in the Hereditary Transinissioi)
of the Self-black and the "Irish" Coat Characters in Rats. Proc.
Roy. Soc. B., Vol. 80, 1908, p. 97. Also Ibid. p. 388.

205. . On Intravascular Coagulation in Albinoes, <S:c. ibid. Vol. 79,

1907, p. 103.

206. Nettleship, E. On heredity in the various forms of Cataract.
Rep. Roy. Lond. Ophth. Hosp. xvi. 1905, p. i.

207. . Additional cases of Hereditary Cataract. Ibid. xvi.

208. . On Retinitis pigme?itosa and allied diseases. Ibid. xvii.

Pts. I, II, and III.

209. . Cases of colour-blindness in women. Ophth. Soc. Trans.

XXVI. 1905.

210. . Three new Pedigrees of Eye Disease. Ophth. Soc. Trans.

XXVIII. 1908, p. 220.

211. . A history of congenital stationary night-blindness in nine

consecutive generations. Ibid, xxvii. 1907.

212. and Ogilvie, F. M. A peculiar form of hereditary con-
genital cataract. Ibid. xxvi. 1906.

213. Noorduyn, C. L. W. lets over Kleuren, Kleurverandering der
Vogels en paring van varieteiten. Album der Naluur, Dec. 1903.
p. 71.

214. . Een Tegenspraak van Mendel's Wet der Erfclijkhcid. Ibid.

1906. [Wild male Canaries bred with some domestic varieties gave
some offspring more or less pied with white or yellow.]

215. . Die Erblichkeit der Farben bei Kanarienvogeln. Ard..

Rassen u. Gesell. Biol., 5 Jahrg. 190S, p. 161.

[Ogilvie, F. M., see Nettleship, E.]

216. Orton, W. A. The Wilt disease of Cotton and its control. U.S.
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217. Ostenfeld, C. H. Zur Kenntnis der Apogamie in der Gatlung
Hieracium. Ber. Deutsch. Bot. Gcsellsch. 1904, xxii. 7.

218. . Castration and Hybridisation Ivvperiments with some si>ccics

of Hieracia. Bot. Tidsk. xxvii. 3. Kjobenhavn, 1906, p. 225.

39^ Bibliography

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220. . On a generalised Theory of Alternative Inheritance, with

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229. Rosenberg, O. Das Verhalten der Chromosomen in einer
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232. . Zur Kenntnis der Reduktionsstellung in Pflanzen. Bot.

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233. . Uber die Embryobildung in der Gattung Hieracium. Ber.

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237. Schuster, E. H.J. Results of crossing Grey (Housc'i Mice with
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244. . The Significance of Latent Characters. Ibid. p. 792.

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253. . Origin of Varieties in Domesticated Species. Ibid., N. S.,

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256. Stevens, N. M. Study of the Germ-cells of Aphis rosae and
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263. Sutton, W. S. On the Morphology of the Chromosome group
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266. Tower, W. L. An Investigation of Evolution in Chrysomelid
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274. . Der gegenwartige Stand der Mendel'schen Lehre. Ibid.

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275. . Die Theorie der Kryptomerie und des Kryptohybridismus.

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278. . Weitere Kreuzungsstudien an Erbsen, Levkojen und Bohnen.

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302. Warburg, J. C. On some races of Lasiocampa quercus. Ent.
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308. . Studies on Chromosomes. III. The sexual differences of

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309. . Note on the Chromosome-groups of Metapodius and Banasa.

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310. . The Case of A?iasa Tristis. Science, N. S., xxv. 1907,

p. 191.

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312. Wood, T. B. Note on the inheritance of horns and face-colour
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314. Woods, F. A. Mendel's Law and some records in Rabbit
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315. . Non-inheritance of Sex in Man. Biometrika, v. 1906,

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INDEX OF SUBJFXTS

88, 96

314

Abraxas giossulariata

colours 44

relations to var. lacticolor 174
Acidalia virgiilaria 252
Acquired characters 5
Adaptation 288
Aglia tau 44, 310

sex-limited descent 187, 310
Agouti-colour 80, 117, 119
ALBINOS

axolotl, exceptional 43

cats 227

giving reversionary offspring

guinea-pigs, smudged 113

man 41, 226

orchids 96

rabbit 75

recessive 45
Alkaptonuria 227, 233
Allelomorphism 10, 11, 16

spurious 151, 153, 160, 195,
Alternation of generations 257
Amphidasys betularia 44
Amphilepsis 248

Ancestry, theory of heredity based on,
contrasted with Mendelian system
55, 130

applied to Basset hounds
Anemone, hybrid 250
Angerona prunaria 44
Annual and biennial habit
Anthers 28
Anthocyan 98, 280
Anticipation, in age at

appears 218
Anth-rhimwi

colours 19, 38, 269, 30S

coupling 318

"Delilah" types 87

dwarf 19

golden-leaved 253

heredity compared with that of sweet
pea 98

intermediate colours 236
ApJiis

alternation 257

chromosomes 271

colour 190

sex 189, 258
Apogamy 246

126

which disease

Aquilegia, hybrid said to breed tnic 373

Aruiiis all'ida, double 197

Asymmetry compared with variation 276

Athene noctiia 49, no

Airopa Belladonna 3S, 135

Axils of sweet peas 153

Axolotl albino 43

BARLEY

abortion of florets 26

colour 39

ears 22, 27

hoods 2 r
Basset hounds 6, 126
Bean, see Phaseohts
Bees, Mendel's experiments with 3:9
Beet, "runners" 25

various characters 307
Begonia, doubling iy8
Beta 25, 307

Biennial and annual habit 25
Biometry 6

misuse of statistics 235
" Bisexual" heredity 285
Black

fowls 1 36

pigeons 100

insects 137
Blue colour

in animals 83

in plants, dominant and recessive 135

in maize 256
Boar mi a, wings 170
Bombyx viori, see Silkworm
Brachydactyly 210, 314
Brassica 31, 39, 30S
Bryonia dioica and alba

colour of fruits 135

sex 166, 203
Bud-sports 272
Bulbing, of turnip 31

of Beta 307
Bursa 25, 307

Callimorpha dominula 44, 135
C;mipanula hosc-in-hosc 26, 200
CANARY

baldness 37

cinnamon 113, 178

colours 43

26 -2

404

Index of Subjects

Qt^^KKY—contimied

crest 37

lizard 43

sex-limited inheritance of 178
Capsella bursa pastoris 25, 307
Carnation 31

double 200
Carotin 135
CAT

albino 227

Angora 33, 55

Manx 34

polydactylism 34, 41

Siamese 114

tortoiseshell 120, 172
Cataract

prae-senile 216

senile 217
CATTLE

Aberdeen- Angus 53

colours 41, 308, 309

horns 32

Kerry 308

length of legs 308

roan 53
C at t ley a 97
Chelidonium majits 24
Chorea, hereditary 229
Chromogen 98, 14 1, 269, 2 Co
Chromoplasts, see Plastids
Chromosomes 270

accessory 188
Cineraria 135

Cinnamon canary, heredity of 177
Clarkia elegans 39
Coleoptera 44
Colias 310, 319
Coloboma 222
Colorado beetle 45
Colour-blindness 172, 195, 223, 231, 3x9

diagram of descent 231

a dominant 173, 222

expectation in offspring 231

possibly recessive 320
COLOURS

Antirrhinuvi 98

Aphis 190

canaries 43

dark and light in flowers 136

fowls 42, 102

from complementary factors 88,

discussion of 139

hex-edity 74-163

103

horse 42

interrelation

9I' 98
list of cases

of colours in mice
of colours ixi

78
sweet peas

of one part controlling colour of other

parts 138
pigeons 43

plastids 98 ...

saturated and dilute 82, 143

C O L O U V.^—contimied

stocks 95

various specific phenomena 132
Combs of fowls 34, 61

Breda 67

walnut 63
Complexnentary factors 88
Coxiipound characters 60

Mendel's view of 368, 370
Consanguineous matings 225
Coreopsis tinctoria 39
Corn, see Maize
Cotton

branching habit 19, 20

colour 39

various characters 25, 30S
Cotyledoxi

colour of, peas 14
,, stocks 39

hypo-geal 31
Coupling, gaxiietxc 151, 159, 314, 315
Cousins, marriages of 225
Crest

canary 37

fowls 34
Crioceris asparagi 45

Cross-bred, meaning of 291
Cryptomeres 93
Cypripcdiiim 96
Cystinuria 227
Cytology

evidence as to sex 188

diversity of 192

Dachshund 83
Daphnia, sex 189, 321
Datura 39

pricklincss of fruit 21
Deaf-mutes 229
Delphinium 135, 200
Diabetes insipidus 220
Diagrams of F^ gexxexation 58, 59, 89, 91
Dianthtis, colour-heredity 370

Ar/neria-delloides, said to breed true

373
Diastases 141

Differentiation compared with variation

274

Digitalis 308

Dilution of colours 80, 143

Dinophilits, eggs of 190

Discontinuity in variation 286

Diseases

hereditary 210

resistance to 233, 300
Distichiasis 221
Dogs 308

Basset hounds 126

colour 6, 83

taillessness 34
Doxnestication, effects of 329, 369
Dominance 8, 342

of epistatic factors 79

Index of Subjects

405

Dominance

imperfect 53

incident of special cases 13, 50

irregularities of ?55 •

and phylogeny 278

recognition of -231

in sex 165
Dominant malformations and diseases
210

sex-limited 222
Dominant whites loi, 104

in plants 105, 299
DOUBLE FLOWERS 31, 196

Arabis 197

carnation 200

Petunia 198

Prinnila 199

stocks 201
Drosera, chromosomes 271
Drosoph ila 310, 319
Dutch pattern 84, 85, 142
Dwarfness 8, 18, 23S, 281

Echinoderm hybrids 311
Ectopia lentis 222
Ectrodactylism 228
Environmental disturbance 246
Epidermolysis 220
Epistatic iactors 79, 98, loi
Eugenics 304
"Ever-sporting" types 253
Evolution 283
Exceptional cases 245

in pigeon 36
Extracted types 92
EYE-COLOUR

canaries 113

cats 41

fowls 43

guinea-pig 113

heredity, general account of 107

man 41, 106

mice III

owl 49

Siamese cats 114
Eye-lashes, reduplication of 221

p\ and Fc, generations defined 8, 57
FACTORS

carried by albino 77

for colour 75

complementary 88

epistatic and hypostatic 79

latent 93, 146
"False-hybrids" 248
Fancy points, often Iructualional 300
Fasciation 14

Fat, excessive in yellow mice 163
Ferments 98, 233
Fertilisation

double 270

nature of 249
Ficaria 329

Fingers

abortion of 216

crooked 208

short 210
Fixing ty|)cs 296
FUu luaiional variation 239, 287
FOWLS 34

Andalusians 5 1

,, eye-colour ito

Aseel 102

barring 319

Brahma 102

IJreda 35, 36

broodiness 36

brown-breasted dominant ij6

buft' 121

colours 42
comb shapes 34, 61
crest 35
Dorking 63, 255
down colour 42, 51
duckwing 42
Egyptian 36, 187
eye-colour 43, iio
Faverolles 35
feathered leg 35
fertility 36
frizzling 35
Hamburgh d},
Houdan 36, 255
Indian game 62, 186
Leghorn, black 187

brown 10:, 182

white 102, 186
,, eyecoluur 1 10
lobe 43
Malay 63

,, eye-colour no
muff 35
pile 120

Plymouth Rock 319
Polish 36

polydactylism 35, 22S, 255
reversion in 103
rose-comb, white 51, 103
rumpless 35

sex-limited descent 43, 319
shrieking 36

silky 34, 35. 43. '03

,, eye-ct>lour no

,, heredity of pigmentation iSi
skin yellow or while i8a
Spanish 186
"vulture-hock" 35
white types dominant and recessive

102
Wyandolles,

black 187

Callus bankiva

colour 104

comb 61
Gallon's theory of heredity comparcvl with
Mendelism 55, 1:9

4o6

Index of Subjects

Gametes ii, i6

compared with zygotes 56

in groups of four 195
Gastroidea dissiviilis 45
Genetics

geometrical aspect 281

study of I
Ge7-he7-a Jamesoni, red and yellow 136
Geiim ip-bano-rivale, S2i\diohxt&d\.x\XQ 355
" Ghost "-markings in peas 145
Glands, Maithiola 21
Glaucoma 222
Gossypmm, see Cotton
Gowers' disease 222, 225
Gradational forms 235
GUINEA-PIG

albino 113

Angora hair 33
" chocolate ij8

colour 41

eye-colour 113

polydactylism 34

re- combinations in 71

rosetted 33
Gynandromorphs 321

Haemophilia 222, 224
Hair

curly 207

white lock 207
Hands

malformations of 211, 216

mode of clasping 48
Hapsburg lip, probably dominant 206
Hare-lip 220
Hawkweeds 246
Height

Anlirrhintirji i3

man 209

peas 8

sweet peas 9
Heliantlms

branching habit 19

colour 39

red variety 308
Helix hortensis and 7ienioralis 45, 311
Hemerophila abj-iiptai ia 44
Heterostylism 27, 48, 68
Helerozygote 15, 16

permanent 51, 155, 181, 253, 298

in sex 190
Heterozygous forms 37, 52, 299
Hierachwi 246

Mendel's experiments with 380
Homoeosis 196, 213, 221
Homozygote 15, 16
Hooded standard in sweet pea, genetics

of 153
Hordeum, see Barley
Horns

cattle 32

goats 170

sheep 169

Horse, race-
chestnut, a recessive 124
colour 42, 309, 311
trotters and pacers 32

Hose-in-hose flowers 26, 197, 200

"Hybrid-character" 347

Hybrids

breeding true 246, 249, 323, 355
some females permanently 193
sterility of 251

Hydatina 192

Hyoscyamiis
annual 25
colour 39

Hypo-geal cotyledons, Phaseohis 31

Hypostatic factors 79

Incompatibility of characters 73

Insanity 229

Intermediates

misunderstanding in regard to 235, 241

various kinds of 236-239
Inter-racial heredity not different from

intra-racial 49
Irideremia 222
Iris, colours of 108

Lasiocampa querctis 44
, "Latency" 93, 145
Lathyrus odoratus, see Sweet Pea
Lavatera 373

Leaf-characters 22, 24, 25, 72
Lecithin, alleged effects of 193
Leopard, black, damask marks in 145
Lepidoptera colours 43, 310

gynandromorphs 321

polymorphic females 319
Leptiiiotarsa 45
Lina lapponica 45
Lychnis

colour 39

hairiness 19

sex 169
Lymanti'ia dispar 321

Maize

colour 41, 256

dent and flint 264

maternal characters 264

starchy endosperm 30
MAN 32, 41, 204

eye-colour 106, 206

hair-colour 206

interracial crossing 208

irregular numbers 254

mulatto 208

musical sense 225

recessive variations 225

stature 209

various dominants 207
Mangel, "runners" 25, 307
Maternal characters

in orchids 248

Index of Subjects

407

Maternal characters in seeds 258

in snails 31 1
Matthiola, see Stocks
,, simiata 21

Me dial go, twist of fruits 48
Alegachile., crosses sweet peas 151
Mclanic moths 44, 137, 187
JMelasoiiia scripta 45
Mendelian system contrasted with Gallon's

55, J29
Meristic characters 47
MICE

black-eyed white 87

colour 41, 309

dominant pied 87

eye-colour iii, 309

excessive fatness [13

hairless 32

interrelation of colours 78

reversion 112

sterility of 120

wahzing habit 33, iii

yellow 79, 118
Mimulus, hose-in-hose 201
Mirabilis

colours 39, 309, 312

intermediate colours in 236
Mollusca 45, 48
Monililhrix 220
Monolepsis 248, 323
Mosaic seeds 249
Mulatto 208
Musical sense 225
Mutation 287
Myopia 221

Nectarine 20, 273
Negro 208

Nicotiana hyl^rids 371
Night-blindness

sex-limited 222

stationary 221
Novelties

made by re-combinations 60

raising 293
Numbers, aberrant 252, 345
Nystagmus 227, 322

Oak-egger 44

Oats 311

Oedema, nervous 220

Oenothera 28, 31, 250, 286, 323

chromosomes 271
Orchids

reversion in 39

monolepsis 248
Owl, eye-colour 49,
Oxalis 47

1 10

Papaver 39

colour 241

double 200
Papilio, polymorphic females

.^'9

Paralysis, familiar J29

peroneal 225
Pai amoecium 277
Pixrarge egtria 251
rarlhcnogenesis 246, 323
Par tula 48
PEA, edil^le

' ' Acacia " 315

"American Wondci J44

blunt puds 2 2, 72

colour of cotyledon 14, 40. ;:, 344
blcacliing 346
parlicolouicd 243

colour of flower 40
,, of pod 14, 346
,, of seed-coat 14.40,144,362,343

"Continuity" 72

dwaifness 18, 236

fasciated stem 25

fasciation 14

height 14, 236, 307

indent 29, 259

maple 144, 263

Mendel's experiment 8, 337

"mummy" 263

"Nain de Bretagne" 263

"Nonpareil " 72, 244

pod

colour of 14
parchment 22
pointed 22
shape of 14

. soft 22

seed

particoloured 243
shape 14, 28, 260. 267

starch in seed 28

sugar peas 22, 260

tallness 18

thickness of stem 307

time of flowering 307

♦'Victoria" 263
Peach 20, 273
Pela)goniuin, leaf colour 313
Periodicity 197
Petalody 197
Petals

imbricated 26

laciniated 24
Petunia, doubling 198
Phascolus

colours 39

cotyledons 31

Mendel's experiments with y^\

pods 22

sterility in hyl)rid.«. 343

tall and dwarf iS

weight of seeds 239
Phylloxera 1S9
Phyteuma HalUri 40
Ph'ytophaga 44 . • • .

Pied varieties, dominant p»cu in mice and
fowls 86

4o8

Index of Stibjects

Pied varieties, relation to whole-colouied

84
Pigs, colour 42
PIGEONS

colour 43, 309

fantail 36

nun 36

reversion 100

sex-limited descent 194, 319

shell 36

webbed foot 36
Pigments

of horses 125

of mice 116
Pisiim sativum, see Peas, edible
Plant-characters 14
Plastid colours 95, 98, 204

white dominant to yellow 134
Polarity of zygote cell 315
Foiemonhiin 40
Pollen grains of sweet pea 2S, 91, 150

figured I ^o
POLYDACTYLISM

cat 34

fowls 35, 228, 255

guinea-pig 34

imperfect dominance of 53

man 228
Polyzoa 275
Poppies 39, 200, 24T
"Porcupine men" 219
Porokeratosis 220
Potato 307

Practical hints 231, 291
Pre-Mendelian writings 5
Presence and absence, hypothesis of 54,

bearing on pathology 232
Prickliness 21
PRIMULA 40, 309

colour in F-i 72, 294

colour in F2 from two whites 105

coupling 315

crossing 294

dominant and recessive whites 105

double 31, 199

heterostylism 27, 68, ,295

"lavender" 299

palm-leaf and fern-leaf 24

special phenomena of colour in 138

stellate and imbricate petals 26, 236

striping 312

white edged flowers 138
Pseudo-hypertrophic muscular paralysis

222, 225
Ptosis 222

Puccinia gliimariim 25, 233
"Pure lines" of Johannsen 239
Purity of type

critical meaning of 17, 291
, how produced by Selection 129

of germ -cells 146
Pygaera 310

RABBIT

Angora hair 33

Belgian hare 1 19

colour 41

a dominant pied form 86

Dutch-marked 84, 85, 142, 237

"English" pattern 41, 86, 239

Himalayan iii, 123

interrelations of colours 75

lop-eared 251, 323

sex statistics 193

tortoiseshell 117

yellow 116
Ranunculus arvensis, fruits 21

,, fie aria 329

Ratios in Fi

9 : ^ : 4 explained 77, 80; i^ : 5 :i, 78;

9:7, 89 ; 18 : 18 : 6 : 6 : 16, 40 ;

27 :9 : 28, 91

aberrant 252, 318

various 79, 311
RATS

colours 41

" ghost "-markings in albino 145

hooded 85

Irish colour 85

no yellow known ir6
Recessive characters 8, 342

recognition of 232

in man 225
Reciprocal crosses giving different results

167, 174, 186, 203, 323
Re-combination

novelties through 60

source of new breeds 71
Red Indians 209

Reduplication in gametic series 318
Repetition of Parts and Heredity 275
Repulsion 151, 155, 314
Retinitis pigmentosa 225
REVERSION
nature of 99, 278

fowls 103

mice 99, III

orchids 96

pigeons 100

stocks 95

sweet peas 89
Right- and Left-hand edness 48
Rogueing crops 292
Round seed 28
Rust disease 25, 233

Salix 249, 385

Salvia 4 1

Sap- colour 98

Saturation of colours 80, 143

Seasonal forms 257

Seed shape peas 14

character i 5

maize 30
Seed skin, Pisum, colour of 14
Segregation 7, 10, 11, 13

Index of Subjects

409

Segregation — continued

absence of 246, 256, 373

consequences of 15, 356

naiiue of 268

time of 269, 313

somatic 273, 313

and species 283
Selection 240

natural 288
Selective mating between gametes 119,

i6i, 195
Serrated leaf, Urtua, Phyteuma 22
SEX

and chromosomes 188

and horns in sheep 169

and double flowers 197

and Spurious Allelomorphism 174, 195,

319.

and wings 170

determination of 164, 321
of Bryonia 166
Lychnis 169
of rabbits 192
summary of evidence 190
Sex-limited descent 169, 221, 222, 310,

319
in deaf- mutism 229

in tylosis 219

dominants 231
Sheep

colour 42

horns 169
Shirley poppy 241
Silene injiaia 19
Silk, colours 137, 310
Silkworm colours 43, 137, 310
Silky fowl

comb 34

feathers 35

inheritance of pigmentation 185

white colour of 103
Single flowers 31
Smooth foliage 22 ,
Snapdragon, see Antirrhimim
Sociological application 303
Somatic segregation 273, 312
Species

basal organisation perhaps not trans-
ferable 73

hybrids of 283, 371

problem of 4, 2S3

segregation between 284
Spleen, enlarged 220
Sports, bud 272
Standard

erect and hooded 26, 153-157

shape of, in sweet pea 153
Stature

man 209

peas 8, 281
Stem, colour of, in Primula 138
Sterility of anthers in sweet pea 28,
152

Sterility — conlinttfd

affecting descent 169

of Hoarmia hybrids 173

of double flowers 19S. ;oi

of hybrids 251, 366

of yellow mice 163

self- sterility 242
Stigmas, colour of, in I'rimula i lo
STOCKS

hianclung 10, 20

" lirompton,'*^ |)eculiaritics of 134

colours 95, 98

cream 204

double 201

glands 2 1

'•half-hoary" 238

hoariness 19, 95, 238

„ relation to colours 133

ovules and pollen dissimilar in doubles
166

pods, individuality of 254

ratios 314

reversion 95, 133

"sulphur" 204

white, peculiarity of 106
Si raw 22
Strawberry 248
Striping, in Anlirriiinutn 99

in Muabilis 3 1 2
Structural characters in animals 52

,, „ in plants 18

Style

Pri nulla 27

Oenotiiaa 28
Subtraction-stages of factors 143, 237
Swedes, colour 13^
SWEET PEA 8, 9. 39

bicolours dominant 87

bud-sport 274

'* bush " variety 19
crossed with cupid " 2S1

colour in axils 152

colours, relation of 91

••cupid" 19, 2S1

gametic coupling in 14O

height 18, 281

heredity compared with that of Aulir'
rhinum 98

pollen grains 28, 89, 91. 150, 156

reversion in 89, 281

seed-shape 265

*' snapilragon" 26

spurious allelomorphism 153, 155
Symmetry and heredity 374

Taillessness, Cat, Dog 34

Fowl 35
Tallness 8, iS
Taraxacum 247
Techni<iue 301
Teleantjiectasis 220
Terms in Ft, numlxr of 59
Todas, colour- blindness in 2:3

4IO

Index of Subjects

Tomato 26, 308
Tortoiseshell

cats 120

rabbits i r 7
Transformation, Gartner's views on
Iriphaena comes 44
Tritiaim, see Wheat
Tropaeolii7}i^ variegated 313
Turnip

bulbing of 31

colour of 135
Turtiir, sex-limited descent 194
Tylosis 219

Unfixable types 298
"Unisexual" heredity
Unit- characters 5, 15

nature of 266
Urtica

28/

Variation

nature of 100,
confusion as to
Variegation 312
Verbascu7n., colour
Viola 41, 284

I47>
301

280

41. '34

Waltzing mice 33, 11 1
Weight of seed 239
WHEAT

beard 22

colour 41, 311

ears 22

foliage 22

V^U'EP^T—coutinued
glutenous 31, 258
keel 22

maternal characters in seed 258
376 Polish 259

re-combination of characters 71
rust 25, 233
White flowers in F2 from cream x red

White types, not albinos

dominant loi, 104, 145

in poultry, various types 102

properties of loi

recessive 145
Wild species, Mendelian phenomena in

49
Wrinkled seed 28

Xanthoma 220
Xanthorhoe ferrugata 44
Xenia 30

YELLOW

in certain animals 115

in fowls 120

discussion of 121

mice, peculiar phenomena in 118

silk 137

varieties of red moths 135

Zea, see -Maize
Zygopetaluni 248
Zygote II, 16

a double structure 56

Jf^^-

INDEX OF AUTHORS

Abderhalden 227
Allen, mice, rats 41
Anthony, Manx cat 34

Bacot

Triphaena cofnes 44

Lasiocanipa qitercus 44

Acidalia virgniaria 252
Baehr, von, sex of Aphidae 189
Ballantyne 219

Balls, cotton 19, 20, 25, 28, 39, 300
Baur, E. 253, 308, 313, 318
Bell, W. 207
Bentham 333
Biffen

barley 22, 26, 39

Bras ska 31, 233

gluten 31

straw 22

wheat 20, 22, 41

maternal character in 258

rust in 25
Blakston, canary 37
Bond, C. J. 229
Bonhote, pigeons 36, 309
Boveri 249

Boys-Smith, goats 32, 170
Brainerd, Viola 41, 284
Bruce, R. 53

Campbell, A., hairless mice 32
Castle

Drosoph ila 310, 311

guinea-pig 33, 34, 118

mice 41

rabbit 33, 79, 117
lop-eared 251

rats 85

sex 165, 187
Chapman, H. J. 96
Charlesworth, Messrs 96
Cockerell 308
Conklin 48

Cookson, N. C, orchids 96
Correns

Bryonia 166

Cam pallida 26, 200

Hyoscyaniiis 25, 39

Lychnis 39

Alatthiola 20, 39

maize 30, 41, 162, 256

Mendel 7

Miimilits 200

Mirabilis 309, 312

Phyteuma 23, 40

Correns — continucii

Pisuin 28, 40

Poleinonium 40

segregation 270

sex 167, 169
Coutagne, silkworm 43, 137
Crampe, rats 41, 85
Crampton 48
Cuenot

mice 41, 76, 86, 112

yellow mice 1 19
Cunier, night-blindness 220

Darbishire 129

mice 33, 41, 112

starch in peas 28, 53
Darwin, C. 2, 35, 70, 2S9

reversion in pigeons 100

Himalayan rabl^its iii

peaches and nectarines 273

on segregation 334
Davenport

canary 37, 43

eye-colour 41. 106

fowls 34, 35, 36, 43

Manx cat 34
Denaiffe, Pisuni 28
Doncaster

cats 41

colour-blindness 320

Abraxas gi-ossulanata 44. i 74

Anger oua priinaria 44

rats 85

tortoiscshell cats 120, 172
Downey 224
Drinkard, tomato 26, 47
Diinkwater, brachydaclyly :io
Duiliam

action of tyrosinase 169

mice 41, 78, 87, 309
,, eye-colour 113

canaries 43, 177

,, eye-colour 113

pigments, idcntilicatiou of 116, 1 25

yellow mice 1 20

Embk'ton, D. 216, 328
Emerson, Phaaolus 22, 40

Farabee, brachyd?.clyly 210
Fedcrlcy 310
P'ischer, E. 257
Eockc 249, 332

Xenia 30
Forbes, rabbit anu guinea-pig 33

412

Index of Authors

227,

^-11

Fotherby 228
Fromherz 227
Fryer, C. 121

Gaertner 7, 331, 371

on Lychnis 169
Gal ton 5
Basset hounds 126
eugenics 304
eye-colour 106
law of heredity 6, 54, 55
stature 209
Garrod, A. E
Gaskoin 33
Gates 271
Gerould 319
Giglioli 49, no
Godron 2, 34, 333
Goebel, double Howers 196,
Goldschmidt 321
Gossage, heredity of disease 20

220
Gregory

doubles 109

Primula 24, 26, 27, 40, 309
starch m Pea seed 28, 68
Groth 308
Guaita, von t^-^

!OI

2I<

315

106

Hacker, axolotl 43
Hagedoorn 309
Harmer, S. F. 275
Harris 44

Harrison, Amphidasys betidaria
Heape 192
Henking 188
Herringham 225
Plerrlinger 225
Hildebrand 47
Hind, Manx cat
Hunter, J. 113
Hurst
eye-colour 41,

fowls 34, 43

horse 42, 124
mice 41

musical sense 225
orchids 39, 96, 248
Pisiiin 28, 40
Himalayan rabbit m
yellow rabbit 117, no
'babbit ^^, 75, 79, 84, 86
sex of 193

Janczewski 250
Jennings 277
Johannsen, pure lines 2^0
Jones 308

Kajanus 307, 308
Keeble 307, 308
Kellogg 310
Killby, E. 260

Kolreuter 349
Korschinsky 286
Kuttner 321

Lang, A., Helix 45, 308, 311-323
Lavvrence, Sir W. 209, 219, 304^
Eeake 308 ^' 0 -^

Lewis, T. 216, 228

Lock

^^i^e 30, 41, 53, 256, 264
Pisum 28, 40, 260

"ghost "-markings on seed-coats i
■Lutz, A. M. 271

Lutz, F. E.

Crioceris 45

Drosophila 310

rights and lefts 48

McClung 188

McCracken, Coleoptera 44, 4^ 1:57
MacCurdy 85 ^^ ^'

Magnus, V. 226
Main, Amphidasys aa
Marryat ^ -^ ^'^ •

rust in wheat 25

Mirabilis 236, 309, 312
Masters, double flowers iq6
Mayer, A. G. 48
Mendel

biography 327

experiments 10, 11, 335

fasciation 25

Hieracium 380

Fisum 18, 40, 339

rediscovery of method 7

round seed 28

segregation 13

sugar-pea 22, 348
Merrifield 257

Millais Sir E., Basset hounds 6, 126

Millardet 248

Morgan, T. H., Drosophila 210

sex of Aphis 189, 191
Mudge 41, 85, 209

Jf^gel.i 54. 329, 332
JNaudm 333

Kettleship, E.

cataract 217; "anticipation" in 218

colour-bhndness 224

night-blindness 220
^nystagmus 322
Newman —

Abraxas grossulariata a a

Calliiiwrpha 44
Nilsson-Ehle 311
Noorduijn, canaries 43
Nussbaum 192

Oberthur, Boamiia hybrids

Ogilvie 2r8
Ostenfeld 247
Overton 279

170

•

Index of ^liitliors

4'3

192

26,

Pearl 3 1 9
Pearson, K.

biometry 6

eye colour 106

horse-colours 124

colours of poppies 242
Pellew 307
Plate 310
Porritt 44
Potts, F. A.
Price, tomato
Prout 44, 252
Przil)ram, eye colour in cats
Punnett

chocolate rabbit

Clarkia elegans

coupling 315

fowls 36, 102

I limalayan rabbit 1 1 1

IJydatina 192

pt)lymorphic females 319

silky fowls 181

sweet pea 89

yellow mice 119

Raunkiaer 247
Raynor, G. H. 174
Rivers, W. H. R. 223
Rizzoli 207
Rosenberg 247, 271
Russo, sex of rabbits 192

44. »37. '**7-3'0

116
39

Salaman 307
Saunders

Atropa belladomta

Datura 39

Petunia 322

stocks {Matthiold)

314
double 201, 322

Ranunculus 21

Salvia 4 1
Schroder, Baron 96
Shull 79, 307, 311

Capsella 2 5

Htlianthus 19, 39

Phaseolus 40

ratios 79

Verbasciim 41
Smalley 309
Smith, GeoHrey 192
Sollas

guinea-pig

19, 31, 132, 254,

Spillnian

cattle 41

pigs 42

wheat 20, 22,
'"'"idfuss 257

33 —

coat-colours of 118

eye-colour of 113

43, 100, 3CM;, 319

190

3'» 39. 'i*

Standfuss, Aglia fan

Callimorpha 44
Stn pics- Browne

pigeons 36,

pigs 42
Stcdman 227
Stevens 271

sex of Apliis
Strasburgcr 249
Sturlevanl 309
Surface 319
Sutton, A. W., firassica
Sutton, L. 199

Thomson, J. A. 130
Toyama, silkworm 43,
Tschcrmak

barley 22, 26, 39

cryptomercs 93

Matthiola 39

Mendel 7

riiaseoliis 22, 31, 39

Pisum 18, 22, 28, 259

wheat 20, ::, 41

de Vilmorin 315
de Vries

Antirrhinum 3S, 87

Atropa belladonna 38

Chelidoniuni 24

Coreopsis 39

Datura 39

dominance 278

hybrids breeding true 249, 313

Lychnis 19, 34

maize 30

Mendel 7

mutation and fluctuation 287

Oenothera 28, 249, 313

Papaver 39

species and variety 2S5

unit characters 5

I ^iola 4 1

Walker, G., brachy<lactyly 214

Wallace, A. R. 289

Walt her 309

Warburg, Lasiocamfa quercus 44

Weismann 5, 2S9

Wcldon 243

Wheldalf, Antirrhinum 38, 87, 98, jSo,
308
colour of fruits 135

Wliitman, C. O., pigeons 194

Wicluira 249, 335

Wilson. K. B.. sex and accessory chromo-
some 187. 191

WiKon. J., cattle 4I, ^i, 308

Wood, sheep 4:, i^»9

WoBften 199

'«:: PRINTED BY JOHN CLAY, M.A. AT THE UNIVERSITY PRESS.

80 308 0

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