Mendelism

R. B. HINMAN
COLLECTION

PROFESSOR OF ANIMAL HUSBANDRY
1921-1943

New York
State College of Agriculture
At Cornell University
Ithaca, N. Y.

Cornell University Library
- QH 431.P83 1912

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mann

Cornell University

Library

The original of this book is in
the Cornell University Library.

There are no known copyright restrictions in
the United States on the use of the text.

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MENDELISM

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GREGOR MENDEL

ABBOT OF BRUNN

Frontispiece

MENDELISM

BY

R. C. PUNNETT, F.R.S.

FELLOW OF GONVILLE AND CAIUS COLLEGE
PROFESSOR OF BIOLOGY IN THE UNIVERSITY OF CAMBRIDGE

FOURTH EDITION

MACMILLAN AND CO., LIMITED
ST. MARTIN’S STREET, LONDON

1912

COPYRIGHT

First Published 1905.
Second Edition (revised) 1907.
Reprinted 1908, 1909.
American Edition 19c9.
German Edition 1910.
Swedish Edition 1911.
Third Edition (entirely rewritten and much enlarged) 1911.
Fourth Edition (revised) 1912.

NOTE TO FOURTH EDITION

THE chief alteration in the present edition concerns
the Appendix to Chapter IX., which has been
revised and extended in accordance with recent
research. At the same time I have been able to
correct sundry errors, typographical and other, which
appeared in the last edition. To those who from
time to time have been kind enough to point these
out I would take this opportunity of expressing

my thanks.
R. C. P.

CAMBRIDGE, August 1912.

PREFACE

A FEW years ago I published a short sketch of
Mendel’s discovery in heredity, and of some of the
recent experiments which had arisen from it. Since
then progress in these studies has been. rapid, and
the present account, though bearing the same title,
has been completely rewritten. A number of illustra-
tions have been added, and here I may acknowledge
my indebtedness to Miss Wheldale for the two
coloured plates of sweet peas, to the Hon. Walter
Rothschild for the butterflies figured on Plate VI,
to Professor Wood for photographs of sheep, and to
Dr. Drinkwater for the figures of human hands. To
my former publishers also, Messrs. Bowes and Bowes,
I wish to express my thanks for the courtesy with
which they acquiesced in my desire that the present
edition should be published elsewhere.

As the book is intended to appeal to a wide
audience, I have not attempted to give more experi-
mental instances than were necessary to illustrate
the story, nor have I burdened it with bibliographical
reference. The reader who desires further informa-

tion may be referred to Mr. Bateson’s indispensable
vii

viii MENDELISM

‘volume on Mendel’s Principles of Heredity (Cam-
bridge, 1909), where a full account of these matters
is readily accessible. Neither have I alluded to
recent cytological work in so far as it may bear
upon our problems. Many of the facts connected
with the division of the chromosomes are striking
and suggestive, but while so much difference of
opinion exists as to their interpretation they are
hardly suited for popular treatment.

In choosing typical examples to illustrate the
growth of our ideas it was natural that I should give
the preference to those with which I was most
familiar. For this reason the book is in some
measure a record of the work accomplished by the
Cambridge School of Genetics, and it is not unfair
to say that under the leadership of William Bateson
the contributions of this school have been second to
none. But it should not be forgotten that workers
in other European countries, and especially in
America, have amassed a large and valuable body.
of evidence with which it is impossible to deal in a
small volume of this scope.

It is not long since the English language was
enriched by two new words—Eugenics and Genetics
—and their similarity of origin has sometimes led to
confusion between them on the part of those who
are innocent of Greek. Genetics is the term applied
to the experimental study of heredity and variation
in animals and plants, and the main concern of its

PREFACE ix

students is the establishing of law and order among
the phenomena there encountered. Eugenics, on the
other hand, deals with the improvement of the
human race under existing conditions of law and
sentiment. The Eugenist has to take into account
the religious and social beliefs and prejudices of
mankind. Other issues are involved besides the
purely biological one, though as time goes on it is
coming to be more clearly recognised that the
Eugenic ideal is sharply circumscribed by the facts
of heredity and variation, and by the laws which
govern the transmission of qualities in living things.
What these facts, what these laws are, in so far as
we at present know them, I have endeavoured to
indicate in the following pages; for I feel convinced
that if the Eugenist is to achieve anything solid it is
upon them that he must primarily build. Little
enough material, it is true, exists at present, but that
we now see to be largely a question of time and
means. Whatever be the outcome, whatever the
form of the structure which is eventually to emerge,
we owe it first of all to Mendel that the foundations

can be well and truly laid.
R. C. P.

CAMBRIDGE, March 1911.

CONTENTS

CHAPTER I

THE PROBLEM P ‘ 2 ‘ ‘ ‘ I

CHAPTER II

HISTORICAL . ; 5 5 : : ‘ 7

CHAPTER III

MENDEL’S WORK ; : . P 4 15

CHAPTER IV

THE PRESENCE AND ABSENCE THEORY . ‘ 26

CHAPTER V

INTERACTION OF FACTORS : F : : . 39

CHAPTER VI

REVERSION... ‘ : ‘ , , ; ep ibe

CHAPTER VII

DOMINANCE . ; : : ; " : . 63
x1

xii MENDELISM

CHAPTER VIII

WILD FORMS AND DOMESTIC VARIETIES .

CHAPTER Ix

REPULSION AND COUPLING OF FACTORS .

CHAPTER X

SEX

CHAPTER XI

SEX (continued)

CHAPTER XII

INTERMEDIATES

CHAPTER XIII

VARIATION AND EVOLUTION

CHAPTER XIV

ECONOMICAL

CHAPTER XV

MAN

APPENDIX
INDEX .

PAGE

73

81

96

III

130

146

161

175
179

ILLUSTRATIONS

PLATE

xiii

PLATES
PAGE
Gregor Mendel ‘ : . , Frontispiece
. Rabbits. ’ ‘ , To face 55
. Sweet-Peas » 58
. Sheep » 72
Sweet-Peas a | a
. Fowls » 102
. Butterflies . » 139
FIGURES IN TEXT

Scheme of Inheritance in simple Mendelian Case . 19
Feathers of Silky and Common Fowl . 27
Single and Double Primulas 28
. Fowls’ Combs 30
. Diagram of Inheritance of Fowls’ Combs 34
. Fowls’ Combs 36

Diagram of F, Generation resulting from Cross
between two White Sweet-Peas 43
Diagram illustrating 9: 3:4 Ratio in Mice . 48
. Sections of Primulas . : : 51

xiv MENDELISM

FIG, a PAGE
to, Small and Large-eyed Primulas . ; : ‘ 2
11, Diagram illustrating Reversion in Pigeons . : 61
12. Primula sinensis x Primula stellata. x B 64

13. Diagram illustrating Cross between Dominant and

Recessive White Fowls . : ; ; 68
14. Bearded and Beardless Wheat . : F 69
15. Feet of Fowls . ‘ ; ‘ . . ; 71
16, Scheme of Inheritance of Horns in Sheep. : Wel
16A. The “ Cretin” Sweet-Pea : ‘ : go
17. Abraxas grossulariata and var, lacticolor . ‘ 97
18, Scheme of Inheritance in Adraxras . : : 99

19. Scheme of Inheritance of Silky Hen x Brown Leg-
horn Cock . . : , : ; 102

20, Scheme of Inheritance of Brown Leghorn Hen x

Silky Cock ‘ : : . . 103
21. Scheme of F, (ex Brown Leghorn x Silky Cock)

crossed with pure Brown Leghorn . ‘ F 104
22. Scheme for Silky Hen x Brown Leghorn Cock 105
23. Scheme for Brown Leghorn Hen x Silky Cock 106
24. Diagram illustrating Nature of Offspring from

Brown Leghorn Henx F, Cock. 3 . 107
25. Scheme to illustrate Heterozygous Nature of Brown

Leghorn Hen : : ‘ : , ‘ 108
26. Scheme of Inheritance of Colour-blindness . . 113
27, Single and Double Stocks . : 3 : . 118
28. F, Generation ex Silky Hen x Brown Leghorn

Cock . : er . 5 : - 123

29. Pedigree of Eurasian Family . : 3 » 126

FIG.

30.
31.
32.
33.
34.
35-

ILLUSTRATIONS

Curve illustrating Influence of Selection

Curve illustrating Conception of pure Lines .

Brachydactylous and Normal Hands
Radiograph of Brachydactylous Hand
Pedigree of Brachydactylous Family

Pedigree of Hemophilic Family .

XV
PAGE
152

154
162
163
164
166

For although it be a more new and dif-
ficult way, to find out the nature of things,
by the things themfelves ; then by read-
ing of Books, to take our knowledge up-
on truft from the opinions of Philofo-
phers: yet muft it needs be confef-
fed, that the former is much more open,
and leffe fraudulent, efpecially in the Se-
crets relating to Natural Philofophy.

Witiiam Harvey,
Anatomical Exercitations, 1653.

CHAPTER I
THE PROBLEM

A CURIOUS thing in the history of human thought,
so far as literature reveals it to us, is the strange
lack of interest shown in one of the most interesting
of all human relationships. Few if any of the more
primitive peoples seem to have attempted to define
the part played by either parent in the formation of
the offspring, or to have assigned peculiar powers
of transmission to them, even in the vaguest way.
For ages man must have been more or less con-
sciously improving his domesticated races of animals
and plants, yet it is not until the time of Aristotle
that we have clear evidence of any hypothesis to
account for these phenomena of heredity. The pro-
duction of offspring by man was then held to be
similar to the production of a crop from seed. The
seed came from the man, the woman provided the
soil. This remained the generally accepted view for
many centuries, and it was not until the recognition
of woman as more than a passive agent that the
physical basis of heredity became established. That
recognition was effected by the microscope, for only
with its advent was actual observation of the minute
I B

2 MENDELISM CHAP.

sexual cells made possible. After more than a
hundred years of conflict lasting until the end of the
eighteenth century, scientific men settled down to
the view that each of the sexes makes a definite
material contribution to the offspring produced by.
their joint efforts. Among animals the female con-
tributes the ovum and the male the spermatozoon ;
among higher plants the corresponding cells are borne
by the ovules and pollen grains.

As a general rule it may be stated that the re-
productive cells produced by the female are relatively
large and without the power of independent move-
ment. In addition to the actual living substance
which is to take part in the formation of a new
individual, the ova are more or less heavily loaded
with the yolk substance that is to provide for the
nutrition of the developing embryo during the early
stages of its existence. The size of the ova varies
enormously in different animals. In birds and
teptiles, where the contents of the egg form the sole
resources of the developing young, they are very
large in comparison with the size of the animal
which lays them. In mammals, on the other hand,
where the young are parasitic upon the mother
during the earlier stages of their growth, the eggs
are minute and only contain the small amount of
yolk that enables them to reach the stage at which
they develop the processes for attaching themselves
to the wall of the maternal uterus. But whatever
the differences in the size and appearance of the
ova produced by different animals, they are all
comparable in that each is a distinct and separate
sexual cell which, as a rule, is unable to develop

1 THE PROBLEM 3

into a new individual of its species unless it is
fertilised by union with a sexual cell produced by
the male.

The male sexual cells are always of microscopic
size and are produced in the generative gland or
testis in exceedingly large numbers. In addition
to their minuter size they differ from the ova in
their power of active movement. Animals present
various mechanisms by which the sexual elements
may be brought into juxtaposition, but in all cases
some distance must be traversed in a fluid or semi-
fluid medium (frequently within the body of the
female parent) before the necessary fusion can
occur. To accomplish this latter end of its journey
the spermatozoon is endowed with some form of
motile apparatus, and this frequently takes the form
of a long flagellum, or whip-like process, by the
lashing of which the little creature propels itself
much as a tadpole with its tail.

In plants as in animals the female cells are
larger than the male cells, though the disparity
in size is not nearly so marked. Still they are
always relatively minute since the circumstances of
their development as parasites upon the mother
plant render it unnecessary for them to possess any
great supply of food yolk. The ovules are found
surrounded by maternal tissue in the ovary, but
through the stigma and down the pistil a potential
passage is left for the male cell, The majority of
flowers are hermaphrodite, and in many cases they
are also self-fertilising. The anthers burst and the
contained pollen grains are then shed upon the
stigma. When this happens, the pollen cell slips

4 MENDELISM CHAP.

through a little hole in its coat and bores its way
down the stigma to reach an ovule in the ovary.
Complete fusion occurs, and the minute embryo of a
new plant immediately results. But for some time
it is incapable of leading a separate existence, and,
like the embryo mammal, it lives as a parasite upon
its parent. By the parent it is provided with a
protective wrapping, the seed coat, and beneath this
the little embryo swells until it reaches a certain
size, when as a ripe seed it severs its connection with
the maternal organism. It is important to realise
that the seed of a plant is not a sexual cell but a
young individual which, except for the coat that it
wears, belongs entirely to the next generation. It is
with annual plants in some respects as with many
butterflies. During one summer they are initiated
by the union of two sexual cells and pass through
certain stages of larval development—the butterfly
as a caterpillar, the plant as a parasite upon its
mother. As the summer draws to a close each
passes into a resting-stage against the winter cold—
the butterfly as a pupa and the plant as a seed, with
the difference that while the caterpillar provides its
own coat, that of the plant is provided by its mother.
With the advent of spring both butterfly and plant
emerge, become mature, and themselves ripen germ
cells which give rise to a new generation.

Whatever the details of development one cardinal
fact is clear. Except for the relatively rare instances
of parthenogenesis a new individual, whether plant
or animal, arises as the joint product of two sexual
cells derived from individuals of different sexes.
Such sexual cells, whether ova or spermatozoa,

1 THE PROBLEM 5

are known by the general term of gametes, or
marrying cells, and the individual formed by the
fusion or yoking together of two gametes is spoken
of as a zygote. Since a zygote arises from the
yoking together of two separate gametes, the indi-
vidual so formed must be regarded throughout its
life as a double structure in which the components
brought in by each of the gametes remain intimately
fused in a form of partnership. But when the
zygote in its turn comes to form gametes, the
partnership is broken and the process is reversed.
The component parts of the dual structure are
resolved with the formation of a set of single struc-
tures, the gametes.

The life cycle of a species from among the higher
plants or animals may be regarded as falling into
three periods: (1) a period of isolation in the form
of gametes, each a living unit incapable of further
development without intimate association with another
produced by the opposite sex; (2) a period of
association in which two gametes become yoked
together into a zygote, and react upon one another
to give rise by a process of cell division to what we
ordinarily term an individual with all its various
attributes and properties; and (3) a period of
dissociation when the single structured gametes
separate out from that portion of the double structured
zygote which constitutes its generative gland. What
is the relation between gamete and zygote, between
zygote and gamete? how are the properties of the
zygote represented in the gamete, and in what
manner are they distributed from the one to the
other ?—these are questions which serve to indicate

6 MENDELISM CHAP. I

the nature of the problem underlying the process of
heredity.

Owing to their peculiar power of growth and
the relatively large size to which zygotes attain,
many of their properties are appreciable by observa-
tion. The colour of an animal or of a flower, the
shape of a seed, or the pattern on the wings of a
moth, are all zygotic properties, and all capable of
direct estimation. It is otherwise with the properties
of gametes. While the difference between a black
and a white fowl is sufficiently obvious, no one by
inspection can tell the difference between the egg
that will hatch into a black and that which will hatch
into a white. Nor from a mass of pollen grains can
anyone to-day pick out those that will produce white
from those that will produce coloured flowers.
Nevertheless, we know that in spite of apparent
similarity there must exist fundamental differences
among the gametes, even among those that spring
from the same individual. At present our only way
of appreciating those differences is to observe the
properties of the zygotes which they form. And as
it takes two gametes to form a zygote, we are in the
position of attempting to decide the properties of
two unknowns from one known. Fortunately the
problem is not entirely one of simple mathematics.
It can be attacked by the experimental method, and
with what measure of success will appear in the
following pages.

CHAPTER II
HISTORICAL

To Gregor Mendel, monk and abbot, belongs the
credit of founding the modern science of heredity.
Through him there was brought into these problems
an entirely new idea, an entirely fresh conception
of the nature of living things. Born in 1822 of
Austro-Silesian parentage, he early entered the
monastery of Briinn, and there, in the seclusion
of the cloister garden, he carried out with the common
pea the series of experiments which has since become
so famous. In 1865, after eight years’ work, he
published the results of his experiments in the
Proceedings of the Natural History Society of Briinn,
in a brief paper of some forty pages. But brief as
it is, the importance of the results and the lucidity
of the exposition will always give it high rank
among the classics of biological literature. For
thirty-five years Mendel’s paper remained unknown,
and it was not until 1900 that it was simultaneously
discovered by several distinguished botanists. The
causes of this curious neglect are not altogether
without interest. Hybridisation experiments before
Mendel there had been in plenty. The classificatory
work of Linnaeus in the latter half of the eighteenth

7

8 MENDELISM CHAP.

century had given a definite significance to the word
species, and scientific men began to turn their
attention to attempting to discover how species were
related to one another. And one obvious way of
attacking the problem was to cross different species
together and see what happened. This was largely
done during the earlier half of the nineteenth
century, though such work was almost entirely con-
fined to the botanists. Apart from the fact that
plants lend themselves to hybridisation work more
readily than animals, there was probably another
reason why zoologists neglected this form of investi-
gation. The field of zoology is a wider one than
that of botany, presenting a far greater variety of
type and structure. Partly owing to their importance
in the study of medicine, and partly owing to their
smaller numbers, the anatomy of the vegetable was
far better known than that of the animal kingdom.
It is, therefore, not surprising that the earlier part of
the nineteenth century found the zoologists, under
the influence of Cuvier and his pupils, devoting their
entire energies to describing the anatomy of the
new forms of animal life which careful search at
home and fresh voyages of discovery abroad were
continually bringing to light. During this period the
zoologist had little inclination or inducement to
carry on those investigations in hybridisation which
were occupying the attention of some botanists.
Nor did the efforts of the botanists afford much
encouragement to such work, for in spite of the
labour devoted to these experiments the results
offered but a confused tangle of facts, contributing
in no apparent way to the solution of the problem

rt HISTORICAL 9

for which they had been undertaken. After half a
century of experimental hybridisation the determina-
tion of the relation of species and varieties to one
another seemed as remote as ever. Then in 1859
came the Origin of Species, in which Darwin pre-
sented to the world a consistent theory to account
for the manner in which one species might have
arisen from another by a process of gradual evolution.
Briefly put, that theory was as follows:—In any
species of plant or animal the. reproductive capacity
tends to outrun the available food-supply, and the
resulting competition leads to an inevitable struggle
for existence. Of all the individuals born, only a
portion, and that often a very small one, can survive
to produce offspring. According to Darwin’s theory,
the nature of the surviving portion is not determined
by chance alone. No two individuals of a species
are precisely alike, and among the variations that
occur some enable their possessors to cope more
successfully with the competitive conditions under
which they exist. In comparison with their less
favoured brethren they have a better chance of
surviving in the struggle for existence and, conse-
quently, of leaving offspring. The argument is
completed by the further assumption of a principle
of heredity, in virtue of which offspring tend to
resemble their parents more than other members
of the species. Parents possessing a favourable
variation tend to transmit that variation to their
offspring, to some in greater, to others in less degree.
Those possessing it in greater degree will again have
a better chance of survival, and will transmit the
favourable variation in even greater degree to

10 MENDELISM CHAP.

some of their offspring. A competitive struggle for
existence working in combination with certain
principles of variation and heredity results in a slow
and continuous transformation of species through
the operation of a process which Darwin termed
natural selection.

The coherence and simplicity of the theory, sup-
ported as it was by the great array of facts which
Darwin had patiently marshalled together, rapidly
gained the enthusiastic support of the great majority
of biologists. The problem of the relation of species
at last appeared to be solved, and for the next forty
years zoologists and botanists were busily engaged
in classifying, by the light of Darwin’s theory, the
great masses of anatomical facts which had already
accumulated, and in adding and classifying fresh ones.
The study of comparative anatomy and embryology
received a new stimulus, for with the acceptance of
the theory of descent with modification it became
incumbent upon the biologist to demonstrate the
manner in which animals and plants differing widely
in structure and appearance could be conceivably
related to one another. Thenceforward the energies
of both botanists and zoologists have been devoted
to the construction of hypothetical pedigrees suggest-
ing the various tracks of evolution by which one
group of animals or plants may have arisen from
another through a long-continued process of natural
selection. The result of such work on the whole
may be said to have shown that the diverse forms
under which living things exist to-day, and have
existed in the past so far as palaeontology can tell
us, are consistent with the view that they are all

II HISTORICAL II

related by the community of descent which the
accepted theory of evolution demands, though as
to the exact course of descent for any particular
group of animals there is often considerable diversity
of opinion. It is obvious that all this work has
little or nothing to do with the manner in which
species are formed. Indeed, the effect of Darwin’s
Origin of Species was to divert attention from the
way in which species originate. At the time that it
was put forward his explanation appeared so satisfy-
ing that biologists accepted the notions of variation
and heredity there set forth and ceased to take any
further interest in the work of the hybridisers. Had
Mendel’s paper appeared a dozen years earlier it is
difficult to believe that it could have failed to attract
the attention it deserved. Coming as it did a few
years after the publication of Darwin’s great work,
it found men’s minds set at rest on the problems
that he raised and their thoughts and energies
directed to other matters.

Nevertheless, one interesting and noteworthy
attempt to give greater precision to the term
heredity was made about this time. Francis Galton,
a cousin of Darwin, working upon data relating to
the breeding of Basset hounds, found that he could
express on a definite statistical scheme the proportion
in which the different colours appeared in successive
generations. Every individual was conceived of as
possessing a definite heritage which might be .ex-
pressed as unity. Of this, $ was on the average
derived from the two parents (ze. + from each parent),
4 from the four grandparents, § from the eight great-
grandparents, and so on. The Law of Ancestral

12 MENDELISM CHAP.

FTeredity, as it was termed, expresses with fair accu-
racy some of the statistical phenomena relating to
the transmission of characters in a mixed population.
But the problem of the way in which characters are
distributed from gamete to zygote and from zygote
to gamete remained as before. Heredity is essen-
tially a physiological problem, and though statistics
may be suggestive in the initiation of experiment,
it is upon the basis of experimental fact that progress
must ultimately rest. For this reason, in spite of its
ingenuity and originality, Galton’s theory and the
subsequent statistical work that has been founded
upon it failed to give us any deeper insight into the
nature of the hereditary process.

While Galton was working in England the
German zoologist, August Weismann, was elabora-
ting the complicated theory of heredity which
eventually appeared in his work on The Germplasm
(1885), a book which will be remembered for one
notable contribution to the subject. Until the pub-
lication of Weismann’s work it had been generally
accepted that the modifications brought about in the
individual during its lifetime, through the varying
conditions of nutrition and environment, could be
transmitted to the offspring. In this biologists were
but following Darwin, who held that the changes in
the parent resulting from increased use or disuse of
any part or organ were passed on to the children.
Weismann’s theory involved the conception of a sharp
cleavage between the general body tissues or somato-
plasm and the reproductive glands or germplasm.
The individual was merely a carrier for the essential
germplasm whose properties had been determined

WI HISTORICAL 13

long before he was capable of leading a separate
existence. As this conception ran counter to the
possibility of the inheritance of “ acquired characters,”
Weismann challenged the evidence upon which it
rested and showed that it broke down wherever
it was critically examined. By thus compelling
biologists to revise their ideas as to the inherited
effects of use and disuse, Weismann rendered a
valuable service to the study of genetics and did
much to clear the way for subsequent research.

A further important step was taken in 1895, when
Bateson once more drew attention to the problem of
the origin of species, and questioned whether the
accepted ideas of variation and heredity were after
all in consonance with the facts. Speaking generally,
species do not grade gradually from one to the other,
but the differences between them are sharp and
specific. Whence comes this prevalence of discon-
tinuity if the process by which they have arisen is
one of accumulation of minute and almost imper-
ceptible differences? Why are not intermediates of
all sorts more abundantly produced in nature than
is actually known to be the case? Bateson saw
that if we are ever to answer this question we must
have more definite knowledge of the nature of varia-
tion and of the nature of the hereditary process by
which these variations are transmitted. And the
best way to obtain that knowledge was to let the
dead alone and to return to the study of the living.
It was true that the past record of experimental
breeding had been mainly one of disappointment.
It was true also that there was no tangible clue by
which experiments might be directed in the present.

14 MENDELISM CHAP. 11

Nevertheless in this kind of work alone seemed there
any promise of ultimate success.

A few years later appeared the first volume of
de Vries’ remarkable book on The Mutation Theory.
From a prolonged study of the evening primrose
(Oenothera) de Vries concluded that new varieties
suddenly arose from older ones by sudden sharp
steps or mutations, and not by any process involving
the gradual accumulation of minute differences. The
number of striking cases from among widely different
plants which he was able to bring forward went far
to convincing biologists that discontinuity in varia-
tion was a more widespread phenomenon than had
hitherto been suspected, and not a few began to
question whether the account of the mode of evolu-
tion so generally accepted for forty years was after
all the true account. Such, in brief, was the outlook
in the central problem of biology at the time of the
rediscovery of Mendel’s work.

CHAPTER III
MENDEL’S WORK

THE task that Mendel set before himself was to
gain some clear conception of the manner in which
the definite and fixed varieties found within a species
are related to one another, and he realised at the
outset that the best chance of success lay in working
with material of such a nature as to reduce the
problem to its simplest terms. He decided that
the plant with which he was to work must be
normally self-fertilising and unlikely to be crossed
through the interference of insects, while at the
same time it must possess definite fixed varieties
which bred true to type. In the common pea
(Pisum. sativum) he found the plant he sought.
A hardy annual, prolific, easily worked, Pzsum
has a further advantage in that the insects which
normally visit flowers are unable to gather pollen
from it and so to bring about cross feftilisation.
At the same time it exists in a number of strains
presenting well-marked and fixed differences. The
flowers may be purple, or red, or white; the plants
may be tall or dwarf; the ripe seeds may be yellow
or green, round or wrinkled,—such are a few of the

15

16 MENDELISM CHAP,

characters in which the various races of peas differ
from one another.

In planning his crossing experiments Mendel
adopted an attitude which marked him off sharply
from the earlier hybridisers. He realised that their
failure to elucidate any general principle of heredity
from the results of cross fertilisation was due to
their not having concentrated upon particular
characters or traced them carefully through. a
sequence of generations. That source of failure
he was careful to avoid, and throughout his ex-
periments he crossed plants presenting sharply
contrasted characters, and devoted his efforts to
observing the behaviour of these characters in
successive generations. Thus in one series of ex-
periments he concentrated his attention on the
transmission of the characters tallness and dwarf-
ness, neglecting in so far as these experiments were
concerned any other characters in which the parent
plants might differ from one another. For this
purpose he chose two strains of peas, one of about
6 feet in height, and another of about 14 feet.
Previous testing had shown that each strain bred
true to its peculiar height. These two strains were
artificially crossed’ with one another, and it was
found to make no difference which was used as the
pollen parent and which was used as the ovule
parent. In either case the result was the same.
The result of crossing tall with dwarf was in every
case nothing but talls, as tall or even a little taller
than the tall parent. For this reason Mendel
termed tallness the dominant and dwarfness the

1 Cf. note on p. 171,

11 MENDEL’S WORK 17

recessive character. The next stage was to
collect and sow the seeds of these tall hybrids.
Such seeds in the following year gave rise to a
mixed generation consisting of talls and dwarfs dz
no intermediates. By raising a considerable number
of such plants Mendel was able to establish the fact
that the number of talls which occurred in this
generation was almost exactly three times as great
as the number of the dwarfs. As in the previous
year, seed were carefully collected from this, the
second hybrid generation, and in every case the seeds
Jrom each individual plant were harvested separately
and separately sown in the following year. By this
respect for the individuality of the different plants,
however closely they resembled one another, Mendel
found the clue that had eluded the efforts of all his
predecessors. The seeds collected from the dwarf
recessives bred true, giving nothing but dwarfs. And
this was true for every dwarf tested. But with the
talls it was quite otherwise. Although indistinguish-
able in appearance, some of them bred true, while
others behaved like the original tall hybrids, giving a
generation con-
sisting of talls |
and dwarfs in

. F,
the proportion of
three of the for-
mer to one of the *
latter. Counting | -——r— 1 i m
showed that the T TT) TD) DTT(D) TD)D D---F
number of the iL p-—-F,
talls which gave

dwarfs was double that of the talls which bred true.
Cc

T(D) T(D) D---Fs

18 MENDELISM CHAP.

If we denote a dwarf plant as D, a true breeding
tall plant as T, and a tall which gives both talls
and dwarfs in the ratio 3:1 as T(D), the result of
these experiments may be briefly summarised in the
foregoing scheme.’

Mendel experimented with other pairs of con-
trasted characters and found that in every instance
they followed the same scheme of inheritance. Thus
coloured flowers were dominant to white, in the ripe
seeds yellow was dominant to green, and round
shape was dominant to wrinkled, and so on. In
every case where the inheritance of an alternative
pair of characters was concerned the effect of the
cross in successive generations was to produce three
and only three different sorts of individuals, viz.
dominants which bred true, dominants which gave
both dominant and recessive offspring in the ratio
3: 1, and recessives which always bred true. Having
determined a general scheme of inheritance which
experiment showed to hold good for each of the
seven pairs of alternative characters with which he
worked, Mendel set himself to providing a theoretical
interpretation of this scheme which, as he clearly
realised, must be in terms of germ cells. He con-
ceived of the gametes as bearers of something capable
of giving rise to the characters of the plant, but he
regarded any individual gamete as being able to
carry one and one only of any alternative pair of
characters. A given gamete could carry tallness or
dwarfness, but not both, The two were mutually

1 It has been found convenient to denote the various generations
resulting from a cross by the signs Fj, Fo, Fy, etc. F, on this system
denotes the first filial generation, F, the second filial generation pro-
duced by two parents belonging to the F, generation, and so on.

HI MENDEL’S WORK 19

exclusive so far as the gamete was concerned. It
must be pure for one or the other of such a pair,
and this conception of the purity of the gametes is
the most essential part of Mendel’s theory.

We. may now proceed with the help of the
accompanying scheme (Fig. 1) to deduce the re-
sults that should

flow from Mendel’s parent eS ©- naBeant
conception of the sanets@ © OQ czametes
nature of the gam- /

etes, and to see ‘ ¥

how far they are AD----- en
in accordance with Pe i

the facts. Since kK @----> & <---@&

the original tall PA :
plant belonged to 3 e---- © ee

a strain which bred Tore © ay
true, all the gam- a 2
etes produced by it i Rese ©

must bear the tall
character. Simi-
Fic. 2.

larly all the gam- Scheme of inheritance in the cross of tall with dwarf
etes of the original pea. (Gametes represented by small and zygotes

by larger circles.

dwarf plant must

bear the dwarf character. A cross between these
two means the union of a gamete containing
tallness with one bearing dwarfness)s Owing ©
to the completely dominant nature of the tall
character, such a plant is in appearance indis-
tinguishable from the pure tall, but it differs
markedly from it in the nature of the gametes to
which it gives rise. When the formation of the
gametes occurs, the elements representing dwarfness

2).
generation

20 MENDELISM CHAP.

and tallness segregate from one another, so that half
of the gametes produced contain the one, and half
contain the other of these two elements. For on
hypothesis every gamete must be pure for one or
other of these two characters. And this is true for
the ovules as well as for the pollen grains. Such
hybrid F, plants, therefore, must produce a series of
ovules consisting of those bearing tallness and those
bearing dwarfness, and must produce them in equal
numbers. And similarly for the pollen grains. We
may now calculate what should happen when such
a series of pollen grains meets such a series of ovules,
ze. the nature of the generation that should be pro-
duced when the hybrid is allowed to fertilise itself.
Let us suppose that there are 4x ovules so that 2%
are “tall” and 2% are “dwarf.” These are brought
in contact with a mass of pollen grains of which
half are “tall” and half are “dwarf.” It is obvious
that a “tall” ovule has an equal chance of being
fertilised by a “tall” or a “dwarf” pollen grain.
Hence of our 2% “tall” ovules, x will be fertilised by
“tall” pollen grains and x will be fertilised by
“dwarf” pollen grains. The former must give rise
to tall plants, and since the dwarf character has
been entirely eliminated from them they must in
_ the future breed true. The latter must also give
rise to tall plants, but since they carry also the
recessive dwarf character they must when bred from
produce both talls and dwarfs. Each of the 2%
dwarf ovules, again, has an equal chance of being
fertilised by a “tall” or by a “dwarf” pollen grain.
Hence +x will give rise to tall plants carrying the
recessive dwarf character, while x will produce

I MENDEL’S WORK 21

plants from which the tall character has been
eliminated, ze. to pure recessive dwarfs. Conse-
quently from the 4% ovules of the self- fertilised
hybrid we ought to obtain 34% tall and « dwarf
plants. And of the 3 talls + should breed true to
tallness, while the remaining 2%, having been formed
like the original hybrid by the union of a “tall”
and a “dwarf” gamete, ought to behave like it
when bred from and give talls and dwarfs in the
ratio 3:1. Now this is precisely the result actually
obtained by experiment (cf. p. 17), and the close
accord of the experimental results with those deduced
on the assumption of the purity of the gametes as
enunciated by Mendel affords the strongest of
arguments for regarding the nature of the gametes
and their relation to the characters of the zygotes in
the way that he has done.

It is possible to put the theory to a further test.
The explanation of the 3:1 ratio of dominants and
recessives in the F, generation is regarded as due
to the F, individuals producing equal numbers of
gametes bearing the dominant and recessive elements
respectively. If now the F, plant be crossed with
the pure recessive, we are bringing together a series
of gametes consisting of equal numbers of dominants
and recessives with a series consisting solely of
recessives. We ought from such a cross to obtain
equal numbers of dominant and recessive individuals,
and further, the dominants so produced ought all to
give both dominants and recessives in the ratio
3:1 when they themselves are bred from. Both
of these expectations were amply confirmed by
experiment, and crossing with the recessive is now a

22 MENDELISM CHAP,

recognised way of testing whether a plant or animal
bearing a dominant character is a pure dominant or
an impure dominant which is carrying the recessive
character. In the former case the offspring will be
all of the dominant form, while in the latter they
will consist on the average of equal numbers of
dominants and recessives.

So far we have been concerned with the results
obtained when two individuals differing in a single
pair of characters are crossed together and with
the interpretation of those results. But Mendel also
used plants which differed in more than a single pair
of differentiating characters. In such cases he found
that each pair of characters followed the same definite
rule, but that the inheritance of each pair was
absolutely independent of the other. Thus, for
example, when a tall plant bearing coloured flowers
was crossed with a dwarf plant bearing white flowers
the resulting hybrid was a tall plant with coloured
flowers. For coloured flowers are dominant to white,
and tallness is dominant to dwarfness. In the
succeeding generation there are plants with coloured
flowers and plants with white flowers in the pro-
portion of 3:1,and at the same time tall plants
and dwarf plants in the same proportion. Hence the
chances that a tall plant will have coloured. flowers
are three times as great as its chance of having
white flowers. And this is also true for the dwarf
plants. As the result of this cross, therefore, we
should expect an F, generation consisting of four
classes, viz. coloured talls, white talls, coloured
dwarfs, and white dwarfs, and we should further
expect these four forms to appear in the ratio of

ur MENDEL’S WORK 23

9 coloured talls, 3 white talls, 3 coloured dwarfs,
and 1 white dwarf. For this is the only ratio
which satisfies the conditions that the talls should
be to the dwarfs as 3:1, and at the same time the.
coloured should be to the whites as 3:1. And
these are the proportions that Mendel found to
obtain actually in his experiments. Put in a more
general form, it may be stated that when two indi-
viduals are crossed which differ in two pairs of
differentiating characters the hybrids (F,) are all
of the same form, exhibiting the dominant character
of each of the two pairs, while the F, generation
produced by such hybrids consists on the average
of g showing both dominants, 3 showing one
dominant and one recessive, 3 showing the other
dominant and the other recessive, and 1 showing both
recessive characters. And, as Mendel pointed out,
the principle may be extended indefinitely. If, for
example, the parents differ in three pair of characters
A, B, and C respectively dominant to a, 4, and c¢, the
F, individuals will be all of the form ABC, while
the F, generation will consist of 27 ABC, 9 ABe,
g AbC, 9 aBC, 3 Abe, 3 aBc, 3 abC, and 1 abe.
When individuals differing ina number of alternative
characters are crossed together, the hybrid generation,
provided that the original parents were of pure
strains, consists of plants of the same form; but
when these are bred from, a redistribution of the
various characters occurs. Taat redistribution follows
the same definite rule for eacn character, and if the
constitution of the original parents be known, the
nature of the F, generation, ze. the number of
possible forms and the proportions in which they

24 MENDELISM CHAP.

occur, can be readily calculated. Moreover, as
Mendel showed, we can calculate also the chances
of any given form breeding true. To this point,
however, we shall return later.

Of Mendel’s experiments with beans it is sufficient
to say here that they corroborated his more ample
work with peas. He is also known to have made
experiments with many other plants, and a few of
his results are incidentally given in his series of letters
to Nageli the botanist. To the breeding and cross-
ing of bees he also devoted much time and attention,
but unhappily the record of these experiments
appears to have been lost. The only other published
work that we possess dealing with heredity is a brief
paper on some crossing experiments with the Hawk-
weeds (/zeracium), a genus that he chose for working
with because of the enormous number of forms under
which it naturally exists. By crossing together the
more distinct varieties, he evidently hoped to pro-
duce some of these numerous wild forms, and so
throw light upon their origin and nature. In this
hope he was disappointed. Owing in part to the
great technical difficulties attending the cross-fer-
tilisation of these flowers he succeeded in obtaining
very few hybrids. Moreover, the behaviour of those
which he did obtain was quite contrary to what he
had found in the peas. Instead of giving a variety
of forms in the F, generation, they bred true and
continued to do so as long as they were kept under
observation. More recent research has shown that
this is due to a peculiar form of parthenogenesis
(cf. p. 123), and not to any failure of the characters
to separate clearly from one another in the gametes.

ree MENDEL’S WORK 25

Mendel, however, could not have known of this, and
his inability to discover in Hzeracium any indication
of the rule which he had found to hold good for
both peas and beans must have been a source
of considerable disappointment. Whether for this
reason, or owing to the utter neglect of his work by
the scientific world, Mendel gave up his experimental
researches during the latter part of his life. His
closing years were shadowed with ill-health and
embittered by a controversy with the Government on
a question of the rights of his monastery. He died
of Bright’s disease in 1884.

NVote.—Shortly after the rediscovery of Mendel’s paper
a need was felt for terms of a general nature to express the
constitution of individuals in respect of inherited characters,
and Bateson accordingly proposed the words homozygote
and heterozygote. An individual is said to be homo-
zygous for a given character when it has been formed by
two gametes each bearing the character, and all the gametes
of a homozygote bear the character in respect of which it is
homozygous. When, however, the zygote is formed by
two gametes of which one bears the given character while
the other does not, it is said to be heterozygous for the
character in question, and only half the gametes produced
by such a heterozygote bear the character. An individual
may be homozygous for one or more characters, and at
the same time may be heterozygous for others.

CHAPTER IV
THE PRESENCE AND ABSENCE THEORY

IT was fortunate for the development of biological
science that the rediscovery of Mendel’s work found
a small group of biologists deeply interested in the
problems of heredity, and themselves engaged in
experimental breeding. To these men the extra-
ordinary significance of the discovery was at once
apparent. From their experiments, undertaken in
ignorance of Mendel’s paper, de Vries, Correns, and
Tschermak were able to confirm his results in peas
and other plants, while Bateson was the first to
demonstrate their application to animals. Thence-
forward the record has been one of steady progress,
and the result of ten years’ work has been to
establish more and more firmly the fundamental
nature of Mendel’s discovery. The scheme of in-
heritance, which he was the first to enunciate, has
been found to hold good for such diverse things as
height, hairiness, and flower colour and flower form
in plants, the shape of pollen grains, and the
structure of fruits; while among animals the coat
colour of mammals, the form of the feathers and of
the comb in poultry, the waltzing habit of Japanese
26

cH. 1v PRESENCE & ABSENCE THEORY 27

mice, and eye colour in man are but a few examples
of the diversity of characters which all follow the
same law of transmission. And as time went on
many cases which at first seemed to fall without the
scheme have been gradually brought into line in the
light of fuller knowledge. Some of these will be

FIG. 2.

A wing feather and a contour feather of an ordinary and a silky fowl. The peculiar
ragged appearance of the silky feathers is due to the absence of the little hooks
or barbules which hold the barbs together. The silky condition is recessive.

dealt with in the succeeding chapters of this book.
Meanwhile we may concern ourselves with the single
modification of Mendel’s original views which has
arisen out of more ample knowledge.

As we have already seen, Mendel considered that
in the gamete there was either a definite something
corresponding to the dominant character or a definite
something corresponding to the recessive character,

28 MENDELISM CHAP,

and that these somethings whatever they were could not
coexist in any single gamete. For these somethings
we shall in future use the term factor. The factor,
then, is what corresponds in the gamete to the unit-
character that appears in some shape or other in
the development of the zygote. Tallness in the pea
is a unit-character, and the gametes in which it is

TiG. 4,

Two double and an ordinary single primula Mower. This form of double is recessive
to the single.

represented are said to contain the factor for tallness.
Beyond their existence in the gamete and _ their
mode of transmission we make no suggestion as to
the nature of these factors.

On Mendel’s view there was a factor correspond-
ing to the dominant character and another factor
corresponding to the recessive character of each
alternative pair of unit-characters, and the characters
were alternative because no gamete could carry more

vo =©PRESENCE & ABSENCE THEORY 29

than one of the two factors belonging to the alter-
native pair. On the other hand, Mendel supposed
that it always carried either one or the other of such
a pair. As experimental work proceeded, it soon
became clear that there were cases which could
not be expressed in terms of this conception. The
nature of the difficulty and the way in which it was
met will perhaps be best understood by considering
a set of experiments in which it occurred. Many
of the different breeds of poultry are characterised
by a particular form of comb, and in certain cases
the inheritance of these has been carefully worked
out. It was shown that the rose comb (Fig. 4, B)
with its flattened papillated upper surface and back-
wardly projecting pike was dominant in the ordinary
way to the deeply serrated high single comb (Fig. 4, C)
which is characteristic of the Mediterranean races.
Experiment also showed that the pea comb (Fig. 4, A),
a form with a low central and two well-developed
lateral ridges such as is found in Indian game, behaves
as a simple dominant to the single comb. The inter-
esting question arose as to what would happen when
the rose and the pea, two forms each dominant to
the same third form, were mated together. It seemed
reasonable to suppose that things which were alter-
native to the same thing would be alternative to one
another—that either rose or pea would dominate in
the hybrids, and that the F, generation would consist
of dominants and recessives in the ratio 3.1. The
result of the experiment was, however, very different.
The cross rose x pea led to the production of a comb
quite unlike either of them. This, the so-called
walnut comb (Fig. 4, D), from its resemblance to

30 MENDELISM CHAP.

the half of a walnut, is a type of comb which is
normally characteristic of the Malay fowl. Moreover,
when these F, birds were bred together, a further
unlooked-for result was obtained. As was expected,

FIG. 4.

Fowls’ combs. A, pea; B, rose; C, single ; D, walnut.

there appeared in the F, generation the three forms
walnut, rose, and pea. But there also appeared
a definite proportion of single combed birds, and
among many hundreds of chickens bred in this way
the proportions in which the four forms walnut, rose,
pea, and single appeared was 9:3:3:1. Now this,

i PRESENCE & ABSENCE THEORY 31

as Mendel showed, is the ratio found in an F,
generation when the original parents differ in two
pairs of alternative characters, and from the propor-
tions in which the different forms of comb occur we
must infer that the

: Rose X Pea
walnut contains both
‘dominants, the rose and —
the pea one dominant Se a
each, while the single
- puns tor bets es Walnut Rose Pea Single
sive characters. This (9) (3) (3) (z)
accorded with subse- -
quent breeding experiments, for the singles bred
perfectly true as soon as they had once made
their appearance. So far the case is clear. The
difficulty comes when we attempt to define these
two pairs of characters. How are we to express the
fact that while single behaves as a simple recessive
to either pure rose or to pure pea, it can yet
appear in F, from a cross between these two pure
forms, though neither of them should, on Mendel’s
view, contain the single? An explanation which
covers the facts in a simple way is that which has
been termed the “Presence and Absence” theory.
On this theory the dominant character of an alter-
native pair owes its dominance to the presence of
a factor which is absent in the recessive. The tall
pea is tall owing to the presence in it of the factor
for tallness, but in the absence of this factor the pea
remains a dwarf. All peas are dwarf, but the tall
is a dwarf plus a factor which turns it into a tall.
Instead of the characters of an alternative pair being
due. to two separate factors, we now regard them as

32 MENDELISM CHAP.

the expression of the only two possible states of a
single factor, viz. its presence or its absence. The
conception will probably become clearer if we follow
its application in detail to the case of the fowl’s
combs. In this case we are concerned with the
transmission of the two factors, rose (2), and pea (P),
the presence of each of which is alternative to its
absence. The rose-combed bird contains the factor
for rose but not that for pea, and the pea-combed
bird contains the factor for pea but not that for rose.
When both factors are present in a bird, as in the
hybrid made by crossing rose with pea, the result is
a walnut. For convenience of argument we may
denote the presence of a given factor by a capital
letter and its absence by the corresponding small
letter. The use of the small letter is merely a
symbolic way of intimating that a particular factor
is absent in a gamete or zygote. Represented thus
the zygotic constitution of a pure rose-combed bird
is RRpp; for it has been formed by the union of
two gametes both of which contained # but not P.
Similarly we may denote the pure pea-combed bird
as rrPP. On crossing the rose with the pea, union
occurs between a gamete Ap and a gamete 7#P,
resulting in the formation of a heterozygote of the
constitution RrPp. The use of the small letters
here informs us that such a zygote contains only
a single dose of each of the factors R and P, although,
of course, it is possible for a zygote, if made in a
suitable way, to have a double dose of any factor.
Now when such a bird comes to form gametes a
separation takes place between the part of the
zygotic cell containing R and the part which does

1 PRESENCE & ABSENCE THEORY 33

not contain it (vy). Half of its gametes, therefore,
will contain R and the other half will be without
it (7). Similarly half of its gametes will contain P
and the other half will be without it (pf). It is
obvious that the chances: of R being distributed to
a gamete with or without P are equal. Hence the
gametes containing & will be of two sorts, PR and
Rp, and these will be produced in equal numbers.
Similarly the gametes without R will also be of two
sorts, ~P and 7, and these, again, will be produced
in equal numbers. Each of the hybrid walnut-
combed birds, therefore, gives rise to a series con-
sisting of equal numbers of gametes of the four
different types RP, Rp, rP, and rp; and the breeding
together of such F, birds means the bringing together
of two such series of gametes. When this happens
an ovum of any one of the four types has an equal
chance of being fertilised by a spermatozoon of any
one of the four types. A convenient and simple
method of demonstrating what happens under such
circumstances is the method sometimes termed the
“chessboard” method. For two series each con-.
sisting of four different types of gamete we require
a square divided up into 16 parts. The four terms
of the gametic series are first written horizontally
across the four sets of four squares, so that the
series is repeated four times. It is then written
vertically four times, care being taken to keep to
the same order. In this simple mechanical way all
the possible combinations are represented and in
their proper proportions. Fig. 5 shows the’ result
of applying this method to our series RP, Rd, rP, rp,
and the 16 squares represent the different kinds of
D

3.4 MENDELISM CHAP,

zygotes formed and the proportions in which they
occur. As the figure shows, 9 zygotes contain both
R and P, having a double or a single dose of either
or both of these factors. Such birds must be all
walnut combed. Three out of the 16 zygotes contain

R but not P, and
RP RP RP RP these must be
RP Rp P ™P rose-combed birds.
Walnut! Walnut] Walnut! Walnut] Three, again, con-

Rp Rp Rp Rp tain P but not &,
RP Rp rP rp ‘and must be pea-
Walnut Rose} Walnut} Rose combed birds.
'P rP =) rP Finally one out
RP Rp rP rp _of the 16 contains
Walnut} Walnut Pea Pea neither K nor P.
me = 5 = It cannot be rose
RP Rp rP rp —it cannot be
Walnut Rose Pea | Single pes It must,

: therefore, be some-
Fic. 5. _ thing else. Asa
ae ee tomer of Get 1

is single. Why
“it should be single and not something else follows from
what we already know about the behaviour of these
various forms of comb. For rose is dominant to
single ; therefore on the Presence and Absence theory
a rose is a single plus a factor which turns the single
into a rose. If we could remove the “rose” factor
from a rose-combed bird the underlying single would
come into view. Similarly a pea comb is a single plus
a factor which turns the single into a pea, and a walnut
is a single which possesses two additional modifying
factors. Singleness, in fact, underlies all these combs,

= PRESENCE & ABSENCE THEORY 35

and if we write their zygotic constitution in full
we must denote a walnut as RRPPSS, a rosé as
RRppSS, a pea as rr PP SS, and a single as rrppSS.
The crossing of rose with pea results in a reshuffling
of the factors concerned, and in accordance with the
principle of segregation some zygotes are formed in
which neither of the modifying factors R and P are
present, and the single character can then become
manifest. :

The Presence and Absence theory is to-day
generally accepted by students of these matters.
Not only does it afford a simple explanation of the
remarkable fact that in all cases of Mendelian in-
heritance we should be able to express our unit-
characters in terms of alternative pairs, but, as we
shall have occasion to refer to later, it suggests a
clue as to the course by which the various domesti-
cated varieties of plants and animals have arisen
from their wild prototypes.

Before leaving this topic we may draw attention
to some experiments which offer a pretty confirma-
tion of the view that the rose comb is a single to
which a modifying factor for roseness has been
added. It was argued that if we could find a type
of comb in which the factor for singleness was
absent, then on crossing such a comb with a rose
we ought, if singleness really underlies rose, to obtain
some single combs in F, from such a cross. Such
a comb we had the good fortune to find in the
Breda fowl, a breed largely used in Holland. This
fowl is usually spoken of as combless for the place
of the comb is taken by a covering of short bristle-
like feathers (Fig. 6, D). In reality it possesses the

36 MENDELISM CHAP.

vestige of a comb in the form of two minute lateral
knobs of comb tissue. Characteristic also of this
breed is the high development of the horny nostrils,
a feature probably correlated with the almost com-

Fic. 6.

Fowls’ combs. A and B, Fy hen from rose x Breda; C, an Fy cock from the cross
of single X Breda; D, head of Breda cock.

plete absence of comb. The first step in the
experiment was to prove the absence of the factor
for singleness in the Breda. On crossing Breda
with single the F, birds exhibit a large comb of
the form of a double single comb in which the two

tv PRESENCE & ABSENCE THEORY 37

portions are united anteriorly, but diverge from one
another towards the back of the head (Fig. 6, C).
The Breda contains an element of duplicity which
is dominant to the simplicity of the ordinary single
comb. But it cannot contain the factor for the
single comb, because as soon as that is put into it
by crossing with a single the comb assumes a large
size, and is totally distinct in appearance from its
almost complete absence in the pure Breda. Now
when the Breda is crossed with the rose duplicity
is dominant to simplicity, and rose is dominant to

Rose X Breda

Duplex y, Duplex
Rose | Rose

TT T | T : 1
Duplex Rose Duplex Single Breda

Ro: Single (Duplex.
ce & and Simplex)

lack of comb, and the F, generation consists of birds
possessing duplex rose combs (Fig. 6, A and B).
On breeding such birds together we obtain a genera-
tion consisting of Bredas, duplex roses, roses, duplex
singles, and singles. From our previous experiment
we know that the singles could not have come from
the Breda, since a Breda comb to which the factor for
single has been added no longer remains a Breda.
Therefore it must have come from the rose, thus
confirming our view that the rose is in reality a
single comb which contains in addition a dominant
modifying factor (R) whose presence turns it into

38 MENDELISM CHAP. IV

a rose. We shall take it, therefore, that there is
good experimental evidence for the Presence and
Absence theory, and we shall express in terms of it
the various cases which come up for discussion in
succeeding chapters.

CHAPTER ¥
INTERACTION OF FACTORS

WE have now reached a point at which it is possible
to formulate a definite conception of the living
organism. A plant or animal is a living entity
whose properties may in large measure be expressed
in terms of unit-characters, and it is the possession
of a greater or lesser number of such unit-characters
renders it possible for us to draw sharp distinctions
between one individual and another. These unit-
characters are represented by definite factors in the
gamete which in the process of heredity behave as
indivisible entities, and are distributed according to
a definite scheme. The factor for this or that unit-
character is gither present in the gamete or it is
not present. It must be there in its entirety or
completely absent. Such at any rate is the view to
which recent experiment has led us. But as to the
nature of these factors, the conditions under which
they exist in the gamete, and the manner in which
they produce their specific effects in the zygote, we
are at present almost completely in the dark.

The case of the fowls’ combs opens up the im-
portant question of the extent to which the various
factors can influence one another in the zygote.

39

40 MENDELISM CHAP.

The rose and the pea factors are separate entities,
and each when present alone produces a perfectly
distinct and characteristic effect upon the single
comb, turning it into a rose or a pea as the case
may be. But when both are present in the same
zygote their combined effect is to produce the
walnut comb, a comb which is quite distinct from
either and in no sense intermediate between them.
The question of the influence of factors upon one
another did not present itself to Mendel because
he worked with characters which affected different
parts of the plant. It was unlikely that the factor
which led to the production of colour in the flower
would affect the shape of the pod, or that the height
of the plant would be influenced by the presence’ or
absence of the factor that determined the shape of
the ripe seed. But when several factors can modify
the same structure it is reasonable to suppose that
they will influence one another in the effects which
their simultaneous presence has upon the zygote.
By themselves the pea and the rose factors each
produce a definite modification of the single comb,
but when both are present in the zygote, whether as
a single or double dose, the modification that results
is quite different to that produced by either when
present alone. Thus we are led to the conception
of characters which depend for their manifestation
on more than one factor in the zygote, and in the
present chapter we may consider a few of the
phenomena which result from such interaction be-
tween separate and distinct factors.

One of the most interesting and instructive cases
in which the interaction between separate factors has

¥ INTERACTION OF FACTORS 4I

been demonstrated is a case in the sweet-pea. All
white sweet-peas breed true to whiteness. And
generally speaking the result of crossing different
whites is to produce nothing but whites whether in
F, or in succeeding generations. But there are
certain strains of white sweet-peas which when
crossed together produce only coloured flowers. The
colour may be different in different cases, though for
our present purpose we may take a case in which the
colour is red. When such reds are allowed to self-
fertilise themselves in the normal way and the seeds
sown, the resulting F, : ;
generation consists of reds White Xx White

and whites, the former being
rather more numerous than
the latter in the proportion Ria White-__F
of 9:7. The raising of a (9) (7) =
further generation from the

seeds of these F, plants shows that the whites always
breed true to whiteness, but that different reds may
behave differently. Some breed true, others give reds
and whites in the ratio 3:1, while others, again, give
reds and whites in the ratio 9:7. As in the case
of the fowls’ combs, this case may be interpreted in
terms of the presence and absence of two factors.
Red in the sweet-pea results from the interaction of
two factors, and unless these are both present the
red colour cannot appear. Each of the white
parents carried one of the two factors whose inter-
action is necessary for the production of the red
colour, and as a cross between them brings these
two. complementary factors together the F, plants
must all be red. As this case is of considerable

42 MENDELISM CHAP.

importance for the proper understanding of much
that is to follow, and as it has been completely
worked out, we shall consider it in some detail.
Denoting these two colour factors by A and B
respectively we may proceed to follow out the
consequences of this cross. Since all the F, plants
were red the constitution of the parental whites
must have been AAJ&d and aaBB respectively, and

their gametes

White pe consequently Ad
4 and aB. The

F g % a ‘canst a ae of
a of parents the B,. plants

ae must, therefore,

jek be AaBb. Such

; : a plant being
Sk > Bs heterozygous for
ac Ab Ab fii two factors pro-
2% io nee 2 3 duces a series of

3 = gametes of the

four kinds AB,
Ab, aB, ab, and produces them in equal numbers
(cf. p. 33). To obtain the various types of zygotes
which are produced when such a series of pollen
grains meets a similar series of ovules we may make
use of the same “chessboard” system which we
have already adopted in the case of the fowls’
combs. An examination of this figure (Fig. 7)
shows that g out of the 16 squares contain both
A and B, while 7 contain either A or B alone, or
neither. In other words, on this view of the nature
of the two white sweet-peas we should in the F,
generation look for the appearance of coloured and

v INTERACTION OF FACTORS 43

white flowers in the ratio 9:7. And this, as we
have already seen, is what was actually found by
experiment. Further examination of the figure
shows that the coloured plants are not all of the
Same constitution, but are of four kinds with respect
to their zygotic constitution, viz. 4AABB, AAB2,

AaBB,and AaBo.
os Ab

Since AABB is
_
7
yo

homozygous for

both A and B, all
Fic. 7.

the gametes which
it produces must
contain both of
these factors, and
such a, plant must
therefore breed
true to the red
colour. A plant
of the constitution
AABéb is homo-
zygous for the _ : ;
factor A; tut” "Bon ae toe ont sven pear ati prea
coloured Fj.

heterozygous for

B. All of its gametes will contain A, but only one-
half of them will contain B, ze. it produces equal
numbers of gametes AB and Ad. Two such series
of gametes coming together must give a generation
consisting of + AABB, 2x AABb, and x AAbé,
that is, reds and whites in the ratio 3:1. Lastly
the red zygotes of the constitution A@Bb have the
same constitution as the original red made from the
two whites, and must therefore when bred from give
reds and whites in the ratio 9:7. The existence

44 MENDELISM CHAP,

of all these three sorts of reds was demonstrated by
experiment, and the proportions in which they were
met with tallied with the theoretical explanation.

The theory was further tested by an examination:
into the properties of the various F, whites which
come from a coloured plant that has itself been
produced by the mating of two whites. As
Fig. 7 shows, these are, in respect of their constitu-
tion, of five different kinds, viz. AAdé, Aabb, aaBB,
aaBb, and aabb. Since none of them produce any-
thing but whites on self-fertilisation it was found
necessary to test their properties in another way,
and the method adopted was that of crossing them
together. It is obvious that when this is done we
should expect different results in different cases.
Thus the cross between two whites of the constitution
AAbb and aaBB should give nothing but coloured
plants ; for these two whites are of the same ‘con-
stitution as the original two whites from which the
experiment started. On the other hand, the cross
between a white of the constitution aabb and any
other white can never give anything but whites.
For no white contains both A and B, or it would
not be white, and a plant of the constitution aabd
cannot supply the complementary factor necessary for
the production of colour. Again, two whites of the
constitution Aadé and aaBb when crossed should
give both coloured and white flowers, the latter
being three times as numerous as the former.
Without going into further detail it may be stated
that the results of a long series of crosses between
the various F, whites accorded closely with the
theoretical explanation.

v INTERACTION OF FACTORS 45

From the evidence afforded by this exhaustive
set of experiments it is impossible to resist the
deduction that the appearance of colour in the sweet-
pea depends upon the interaction of two factors
which are independently transmitted according to
the ordinary scheme of Mendelian inheritance.
What these factors are is still an open question.
Recent evidence of a chemical nature indicates that
colour in a flower is due to the interaction of two
definitive substances: (1) a colourless “ chromogen,”
or colour basis; and (2) a ferment which behaves as
an activator of the chromogen, and by inducing
some process of oxidation, leads to the formation of
a coloured substance. But whether these two bodies
exist as such in the gametes, or whether in some
other form we have as yet no means of deciding.

Since the elucidation of the nature of colour in
the sweet-pea phenomena of a similar kind have
been witnessed in other plants, notably in stocks,
snapdragons, and orchids. Nor is this class of
phenomena confined to plants. In the course of a
series of experiments upon the plumage colour
of poultry, indications were obtained that different
white breeds did not always owe their whiteness to
the same cause. Crosses were accordingly made
between the white Silky fowl and a pure white
strain derived from the white Dorking. Each of
these had been previously shown to behave as a
simple recessive to colour. When the two were
crossed only fully coloured birds resulted. From
analogy with the case of the sweet-pea it was
anticipated that such F, coloured birds when bred
together would produce an F, generation consisting

46 MENDELISM CHAP.

of coloured and white birds in the ratio 9:7, and
when the experiment was made this was actually
shown to be the case. With the growth of know-
ledge it is probable that further striking parallels
of this nature between the plant and animal worlds
will be met with.

Before quitting the subject of these experiments,
attention may be drawn to the fact that the 9:7
ratio is in reality a 9:3:3:1 ratio in which the
last three terms are indistinguishable owing to the
special circumstances that neither factor can produce
a visible effect without the co-operation of the other.
And we may further emphasise the fact that although
the two factors thus interact upon one another they
are nevertheless transmitted quite independently
and in accordance with the ordinary Mendelian
scheme.

One of the earliest sets of experiments demon-
strating the interaction of separate factors was that
made by the French
zoologist Cuénot on the
coat colours of mice. It

Agouti X Agouti was shown that in cer-

tain cases agouti, which

Aglutt ack Albino |S the colour of the
(9) (3) (4) ordinary wild grey
mouse, behaves as a

dominant to the albino variety, ze. the F, generation
from such a cross consists of agoutis and albinos in
the ratio 3:1. But in other cases the cross between
albino and agouti gave a different result. In the F,
generation appeared only agoutis as before, but the
F, generation consisted of three distinct types, viz.

Agouti X Albino

Vv INTERACTION OF FACTORS 47

agoutis, albinos, avd blacks. Whence the sudden
appearance of the new type? The answer is a simple
one. The albino parent was really a black. But it
lacked the factor without which the colour is unable to
develop, and consequently it remained an albino. If
we denote this factor by C, then the constitution of an
albino must be cc, while that of a coloured animal may
be CC or Cc, according as to whether it breeds true to
colour or can throw albinos. Agouti was previously
known to be a simple dominant to black, ze. an
agouti is a black rabbit plus an additional greying
factor which modifies the black into agouti. This
factor we will denote by G, and we will use B for
the black factor. Our original agouti and albino
parents we may therefore regard as in constitution
CCGGBS8 and ceggBB respectively. Both of the
parents are homozygous for black. The gametes
produced by the two parents are CGB and cg, and
the constitution of the F, animals must be CcGgBB.
Being heterozygous for two factors they will produce
four kinds of gametes in equal numbers, viz. CGB,
CgB, cGB, and cgB. The results of the mating of
two such similar series of gametes when the F,
animals are bred together we may determine by the
usual “chessboard” method (Fig. 8). Out of the
16 squares 9 contain both C and G in addition to
B. Such animals must be agoutis. Three squares
contain C but not G. Such animals must be
coloured, but as they do not contain the modifying
agouti factor their colour will be black. The remain-
ing four squares do not contain C, and in the absence
of this colour-developing factor they must all be

albinos. Theory demands that the three classes

48 MENDELISM CHAP.

agouti, black, and albino should appear in F, in the
ratio 9 :3:4; experiment has shown that these are
the only classes that appear, and that the proportions
in which they are produced accord closely with the
theoretical ex-
pectation. Put
briefly, then, the
explanation of
this case is that
all the animals
are black, and that
we are dealing
with the presence
and absence of
two factors, a
colour developer
Albino| (C), and a colour
modifier (G), both

_ acting, as it were,
Diagram to illustrate the nature of the F, generation
which may arise from the mating of agouti with upon asubstratum

albino in mice or rabbits. of hack. ‘The F,
generation really consists of the four classes agoutis,
blacks, albino agoutis, and albino blacks in the ratio
9:3:3:1. But since in the absence of the colour
developer C the colour modifier G can produce no
visible result, the last two classes of the ratio are
indistinguishable, and our F, generation comes to
consist of three classes in the ratio 9: 3: 4, instead
of four classes in the ratio 9: 3:3: 1.

This explanation was further tested by experi-
ments with the albinos. In an F, family of this
nature there ought to be three kinds, viz. albinos
homozygous for G (ccGGBB), albinos heterozygous

Fic. 8.

v INTERACTION OF FACTORS 49

for G (ccGgBB), and albinos without G (ceggBB).
These albinos are, as it were, like photographic plates,
exposed but undeveloped. Their potentialities may
be quite different, although they all look alike, but
this can only be tested by treating them with a colour
developer. In the case of the mice and rabbits the
potentiality for which we wish to test is the presence
or absence of the factor G, and in order to develop
the colour we must introduce the factor C. Our
developer, therefore, must contain C but not G. In
other words, it must be a homozygous black mouse
or rabbit, CCggBB. Since such an animal is pure
for C it must, when mated with any of the albinos,
produce only coloured offspring. And since it does
not contain G the appearance of agoutis among its
offspring must be attributed to the presence of G
in the albino. Tested in this way the F, albinos
were proved, as was expected, to be of three kinds:
(1) those which gave only agouti, ze. which were
homozygous for G; (2) those which gave agoutis
and blacks in approximately equal numbers, ze.
which were heterozygous for G ; and (3) those which
gave only blacks, and therefore did not contain G.
Though albinos, whether mice, rabbits, rats, or
other animals, breed true to albinism, and though
albinism behaves as a simple recessive to colour, yet
albinos may be of many different sorts. There are
in fact just as many kinds of albinos as there are
coloured forms—neither more nor less. And all
these different kinds of albinos may breed together,
transmitting the various colour factors according to
the Mendelian scheme of inheritance, and yet the
visible result will be nothing but albinos. Under
E

50 MENDELISM CHAP.

the mask of albinism is all the while occurring that
segregation of the different colour factors which would
result in all the varieties of coloured forms, if only
the essential factor for colour development were
present. But put in the developer by crossing
with a pure coloured form and their variety of con-
stitution can then at last become manifest.

So far we have dealt with cases in which the
production of a character is dependent upon the
interaction of two factors. But it may be that some
characters require the simultaneous presence of a
greater number of factors for their manifestation,
and Miss Saunders has shown that there is a
character in ten-week stocks which is unable
to appear except through the interaction of three
distinct factors. Coloured stocks may be either
hoary with the leaves and stem covered by small
hairs, or they may lack the hairy covering, in which
case they are termed glabrous. Hoariness is dominant
to glabrousness ; that is to say, there is a definite
factor which can turn the glabrous into a hoary plant
when it is present. But in families where coloured
and white stocks occur the white are always glabrous,
while the coloured plants may or may not be hoary.
Now colour in the stock as in the sweet-pea has
been proved to be dependent upon the interaction of
two separate factors. Hence hoariness depends
upon three separate factors, and a stock cannot be
hoary unless it contains the hoary factor in addition
to the two colour factors. It requires the presence
of all these three factors to produce the hoary
character, though how this comes about we have not
at present the least idea. ;

v INTERACTION OF FACTORS 51

A somewhat different and less usual form of inter-
action between factors may be illustrated by a case
in primulas recently worked out by Bateson and
Gregory. Like the common primrose, the primula
exhibits both pin-eyed and thrum-eyed varieties. In
the former the style is long, and the centre of the
eye is formed by the end of the stigma which more
or less plugs up the opening of the corolla (cf. Fig.
9, A); in the latter the style is short and hidden by

Fic. 9.

Sections of primula flowers. The anthers are shown as black. A, “‘ pin” form with
long style and anthers set low down; B, ‘‘thrum” form with short style and
anthers set higher up; C, homostyle form with anthers set low down as in
“pin,” but with short style. This form only occurs with the large eye.

the five anthers which spring from higher up in the
corolla and form the centre of the eye (cf. Fig. 9, B).
The greater part of the “eye” is formed by the
greenish-yellow patches on each petal just at the
opening of the corolla. In most primulas the eye
is small, but there are some in which it is large
and extends as a flush over a considerable part of
the petals (Fig. 10). Experiments showed that
these two pairs of characters behave in simple Men-
delian fashion, short style (= “thrum”) being dominant
to long style (=“ pin”) and small eye dominant to

52 MENDELISM CHAP.

large. Besides the normal long and short styled
forms, there occurs a third form, which has been
termed homostyle. In this form the anthers are
placed low down in the corolla tube as they are in
the long-styled form, but the style remains short
instead of reaching up to the corolla opening (Fig.
9, C). In the course of their experiments Bateson

FIG. Io.

Two primula flowers showing the extent of the small and of the large eye.

and Gregory crossed a large-eyed homostyle plant
with a small-eyed thrum (=short style). The F,
plants were all short styled with small eyes. On
self-fertilisation these gave an F, generation consist-
ing of four types, viz. short styled with small eyes,
short styled with large eyes, Jong styled with small
eyes, and omostyled with large eyes. The notable
feature of this generation is the appearance of long-
styled plants, which, however, occur only in associa-
tion with the small eye. The proportions in which
these four types appeared shows that the presence
or absence of but two factors is concerned, and at

v INTERACTION OF FACTORS 53

the same time provides the key to the nature of the
homostyled plants. These are potentially long styled,
and the position of the anthers is that of normal long-
styled plants, but owing to some interaction between
the factors the style itself is unable to reach its full
development unless the factor for the small eye is
present. For this reason long-styled plants with

Short style x Homo style
small eye large eye

Short style
small eye

|

I T T e 7
Shortstyle Short style Long style Homostyle
smalleye largeeye (“pin”) large eye

~ (9) (3) (3) (1)

the large eye are always of the homostyle form.
What the connecting-link between these apparently
unrelated structures may be we cannot yet picture
to ourselves, any more than we can picture the relation
between flower colour and hairiness in stocks, It is
evident, however, that the conception of the inter-
action of factors, besides clearing up much that is
paradoxical in heredity, promises to indicate lines of
research which may lead to valuable extensions in
our knowledge of the way in which the various parts
of the living organism are related to one another.

CHAPTER VI
REVERSION

AS soon as the idea was grasped that characters in
plants and animals might be due to the interaction
of complementary factors, it became evident that this
threw clear light upon the hitherto puzzling pheno-
menon of reversion. We have already seen that in
certain cases the cross between a black mouse or
rabbit and an albino, each belonging to true breeding
strains, might produce nothing but agoutis. In other
words, the cross between the black and the white in
certain instances results in a complete reversion to the
wild grey form. Expressed in Mendelian terms, the
production of the agouti was the necessary conse-
quence of the meeting of the factors C and G in the
same zygote. As soon as they are brought together,
no matter in what way, the reversion is bound to
occur. Reversion, therefore, in such cases we may
regard as the bringing together of complementary
factors which had somehow in the course of evolution
become separated from one another. In the simplest
cases, such as that of the black and the white rabbit,
only two factors are concerned, and one of them is
brought in from each of the two parents. But in

54

PLATE

SMOTTAEA ‘S ‘nosy ‘Pb

i za ‘sadéy

‘uedepeunpy ‘8 ‘]Teysesioyioy ‘4 !xoeTg ‘9

aA ‘g- Suorssaaar ‘yf (Aai3=) nosy ‘€ tuedepennpy ‘2 Syqqey yong MoTOR

2 *

I

CHAP. VI REVERSION 55

other cases the nature of the reversion may be more
complicated owing to a larger number of factors
being concerned, though the general principle remains
the same. Careful breeding from the reversions will
enable us in each case to determine the number and
nature of the factors concerned, and in illustration
of this we may take another example from rabbits.
The Himalayan rabbit is a well-known breed. In
appearance it is a white rabbit with pink eyes, but
the ears, paws, and nose are black (Pl. I, 2). The
Dutch rabbit is another well-known breed. Generally
speaking, the anterior portion of the body is white,
and the posterior part coloured. Anteriorly, how-
ever, the eyes are surrounded by coloured patches
extending up to the ears, which are entirely coloured.
At the same time the hind paws are white (cf.
Pl. I., 1). Dutch rabbits exist in many varieties of
colour, though in each one of these the distribution
of colour and white shows the same relations. In
the experiments about to be described a yellow
Dutch rabbit was crossed with a Himalaya. The
result was a reversion to the wild agouti colour
(Pl. I., 3). Some of the F, individuals showed
white patches, while others were self-coloured. On
breeding from the F, animals a series of coloured
forms resulted in F, These were agoutis, blacks,
yellows, and sooty yellows, the so-called tortoise-
shells of the fancy (PI. I., 4-7). In addition to these
appeared Himalayans with either black points or
with lighter brownish ones, and the proportions in
which they came showed the Himalayan character
to be a simple recessive. A certain number of the
coloured forms exhibited the Dutch marking to a

56 MENDELISM CHAP.

greater or less extent, but as its inheritance in this
set of experiments is complicated and has not yet
been worked out, we may for the present neglect it
and confine our attention to the coloured types and
to the Himalayans. The proportion in which the
four coloured types appeared in F, was very nearly
g agoutis, 3 blacks, 3 yellows, and 1 tortoiseshell.
Evidently we are here dealing with two factors: .(1)
the grey factor (G), which modifies black into agouti,
or tortoiseshell into yellow; and (2) an intensifying
factor (7), which intensifies yellow into agouti and

Yellow X Himalayan

a

Agouti X Agouti

vr T v T 1
Agouti Yellow Black co Himalayan

(7) (9) i) (16)

tortoiseshell into black. It may be mentioned here
that other experiments confirmed the view that the
yellow rabbit is a dilute agouti, and the tortoiseshell
a dilute black. The Himalayan pattern behaves as
a recessive to self-colour. It is a self-coloured black
rabbit lacking a factor that allows the colour to
develop except in the points. That factor we may
denote by XX, and as far as it is concerned the
Himalayan is constitutionally zr. The Himalayan
contains the intensifying factor, for such pigment
as it possesses in the points is full coloured. At
the same time it is black, ze. lacking in the factor G.
With regard to these three factors, therefore, the con-
stitution of the Himalayan is g¢//ar. The last char-

vI REVERSION 57

acter which we have to consider in this cross is the
Dutch character. This was found by Hurst to
behave as a recessive to self-colour (S), and for our
present purpose we will regard it as differing from
a self-coloured rabbit in the lack of this factor.' The
Himalayan is really a self-coloured animal, which,
however, is unable to show itself as a full black
owing to its not possessing the factor X. The results
of breeding experiments then suggest that we may
denote the Himalayan by the formula gg//arSS and
the yellow Dutch by GGzXXss. Each lacks two
of the factors upon the full complement of which the
agouti colour depends. By crossing them the com-
plete series G/XS is brought into the same zygote,
and the result is a reversion to the colour of the
wild rabbit.

Most of the instances of reversion yet worked out
are those in which colour characters are concerned.
The sweet-pea, however, supplies us with a good
example of reversion in structural characters. A
dwarf variety known as the “ Cupid” has been exten-
sively grown for some years. In these little plants
the internodes are very short and the stems are
few in number, and attain to a length of only 9-10
inches. In course of growth they diverge from one
another, and come to lie prostrate on the ground (PI.
II., 2). Curiously enough, although the whole plant
is dwarfed in other respects, this does not seem to
affect the size of the flower, which is that of a normal
sweet-pea. Another though less-known variety is
the “Bush” sweet-pea. Its name is derived from

1 Hurst’s original cross was between a Belgian hare and an albino
Angora which turned out to be a masked Dutch.

53 MENDELISM CHAP.

its habit of growth. The numerous stems do not
diverge from one another, but all grow up side by
side giving the plant the appearance of a compact
bush (PI.IL, 1). Under ordinary conditions it attains
a height of 34-4 feet. A number of crosses were
made between the Bush and Cupid varieties, with
the somewhat unexpected result that in every
instance the F, plants showed complete reversion
to the size and habit of the ordinary tall sweet-
pea (PI. IL, 3), which is the form of the wild plant
as it occurs in Sicily to-day. The F, generation from

Bush X Cupid

I T T ]
Tall Bush Cupid Cupid- -- - - I,
(procumbent) (erect)
(9) (3) (3) (1)

these reversionary talls consisted of four different
types, viz. talls, bushes, Cupids of the procumbent
type like the original Cupid parent, and Cupids
with the compact upright Bush habit (PI. II. 4).
These four types appeared in the ratio 9: 3:3: 1,
and this, of course, provided the clue to the nature
of the case. The characters concerned are (1) long
internode of stem between the leaves which is
dominant to short internode, and (2) the creeping
procumbent habit which is dominant to the erect
bush-like habit. Of these characters length of inter-
node was carried by the Bush, and the procumbent
habit by the original Cupid parent. The bringing
of them together by the cross resulted in a_pro-

PLATE Il.

1, Bush Sweet Pea; z, Cupid Sweet Pea; 3, F, reversionary Tall;
4, Erect Cupid Sweet Pea; s, Purple Invincible ; 6, Painted Lady;
7, Duke of Westminster (hooded standard).

VI REVERSION 59

cumbent plant with long internodes. This is the
ordinary tall sweet-pea of the wild Sicilian type,
reversion here, again, being due to the bringing
together of two complementary factors which had
somehow become separated in the course of evolution,

To this interpretation it may be objected that
the ordinary sweet-pea is a plant of upright habit.
This, however, is not true. It only appears so
because the conventional way of growing it is to
train it up sticks. In reality it is of procumbent
habit, with divergent stems like the ordinary Cupid,
a fact which can easily be observed by any one who.
will watch them grow without the artificial aid of
prepared supports,

The cases of reversion with which we have so
far dealt have been cases in which the reversion
occurs as an immediate result of a cross, ze. in the
F, generation. This is perhaps the commonest
mode of reversion, but instances are known in
which the reversion that occurs when two pure
types are crossed does not appear until the F,
generation. Such a case we have already met
with in the fowls’ combs. It will be remembered
that the cross between pure pea and pure rose gave
walnut combs in F,, while in the F, generation a
definite proportion, I in 16, of single combs
appeared (cf. p. 30). Now the single comb is the
form that is found in the wild jungle fowl, which is
generally regarded as the ancestor of the domestic
breeds. If this is so, we have a case of reversion in
F,; and this in the absence of the two factors
brought together by the rose-comb and pea-comb

parents. Instead of the reversion being due to the

60 MENDELISM CHAP.

bringing together of two complementary factors, we
must regard it here as due to the association of two
complementary absences. To this question, how-
ever, we shall revert later in discussing the origin of
domesticated varieties.

There is one other instance of reversion to which

Black Barb x White Fantail Black Barb x Spot!
|
Dark x Dark

Among the offspring one very similar
to the wild blue rock.

we must allude. This is Darwin’s famous case of
the occasional appearance of pigeons reverting to
the wild blue rock (Columba livia) when certain
domesticated races are crossed together. As is
well known, Darwin made use of this as an

Black x White
Barb | Fantail

Black % Black
(White Splashed) (White Splashed)

{ T I T i
Black Black Blue Blue White
(White Splashed) (White Splashed)

©) ao

argument for regarding all the domesticated varieties
as having arisen from the same wild species. The
original experiment is somewhat complicated, and
is shown in the accompanying scheme. Essentially

1 This is an almost white bird, the colour being confined to the
tail and the characteristic spot on the head.

vI REVERSION 61

it lay in following the results flowing from crosses
between blacks and whites. Experiments recently
made by Staples-Browne have shown that this case
of reversion also can be readily interpreted in
Mendelian terms. In these experiments the cross
was made between black barbs and white fantails.
The F, birds were all black with some white
splashes, evidently due to a separate factor intro-

duced by the fan-
CB CB CB CB
CB Cb cB cb

tail. On breeding
BLACK | BLACK| BLACK] BLACK

these blacks to-
a age
CB cB
BLACK JB BLACK

gether they gave
cB cB

(with or without
white splashes),
blues (with or
without white
splashes), and
whites in the ratio
9:3:4. Thefactors
concerned are

an F, generation,
consisting of blacks
Cb
BLACK BLACK

: Fic, 11.
colour (C), in the Diagram to illustrate the appearance of the rever-
absence of which sionary blue pigeon in F. from the cross of black

‘ . ‘ with white.
a bird is white, and

a black modifier (B),in the absence of which a coloured
bird is blue. The original black barb contained both
of these factors, being in constitution CCBB. The
fantail, however, contained neither, and was con-
stitutionally ccbé. The F, bitds produced by
crossing were in constitution CcBé, and being
heterozygous for two factors produced in equal
numbers the four sorts of gametes CB, Cd, cB, cb.

62 MENDELISM CHAP, VI

The results of two such series of gametes being
brought together are shown in the usual way in Fig.
11. A blue is a bird containing the colour factor
but lacking the black modifier, z.e. of the constitution
CCo6, or Ccbb, and such birds as the figure shows
appear in the F, generation on the average three
times out of sixteen. Reversion here comes about
in F,, when the redistribution of the factors leads to
the formation of zygotes containing one of the two
factors but not the other.

CHAPTER VII
DOMINANCE

IN the cases which we have hitherto considered the
presence of a factor produces its full effect whether
it is introduced by both of the gametes which go to
form the zygote, or by one of them alone. The
heterozygous tall pea, or the heterozygous rose-
combed fowl cannot be distinguished from the
homozygous form by mere inspection, however close.
Breeding tests alone can decide which is the
heterozygous and which the homozygous form.
Though this is true for the majority of characters
’ yet investigated, there are cases known in which the
heterozygous form differs in appearance from either
parent. Among plants such a case has been met
with in the primula. The ordinary Chinese primula
(P. sinensis) (Fig. 12) has large rather wavy petals
much crenated at the edges. In the Star Primula
(P. stellata) the flowers are much smaller, while the
petals are flat and present only a terminal notch
instead of the numerous crenations of P. szxenszs.
The heterozygote produced by crossing these forms
is intermediate in size and appearance. When self-
fertilised such plants behave in simple Mendelian
63

64 MENDELISM CHAP.

fashion, giving a generation consisting of szzenszs,
intermediates, and s¢el/ata in the ratio 1:2:1.
Subsequent breeding from these plants showed that
both the szzens¢s and stellata which appeared in the
F, generation bred true, while the intermediates

FIG. ¥2,

Primula flowers to illustrate the intermediate nature of the Fy flower when sinensis
is crossed with stellata.

always gave all three forms again in the same
proportion. But though there is no dominance
of the character of either parent in such a case as
this, the Mendelian principle of segregation could
hardly have a better illustration.

Among birds a case of similar nature is that of
the Blue Andalusian fowl. Fanciers have long

VII DOMINANCE 65

recognised the difficulty of getting this variety to
breed true. Of a slaty blue colour itself with
darker hackles and with black lacing on the

Sinensis X Stellata

Intermediate_. ____...-.--- F,

I T T }
Sinensis _ Inter. Inter. Stellata----F2
Sinensis Sin, Int. Int. Stell. Stellata- - - -F,
Sinensis Stellata- ---F,

feathers of the breast, it always throws “wasters”
of two kinds, viz. blacks, and whites splashed with
black. Careful breeding from the blues shows that
the three sorts are always produced in the same

Blue X Blue

CT T | 7
Black Blue i Blue i
|
,

i] T 1
Black Black Blue Blue White White

Black Xx ‘White

Blue
(all)

definite proportions, viz. one black, two blues, one

splashed white. This at once suggests that the

black and the splashed white are the two homozy-

gous forms, and that the blues are heterozygous, ze.
F

66 MENDELISM CHAP.

producing equal numbers of “black” and “ white
splashed” gametes. The view was tested by breed-
ing the “wasters” together—black with black, and
splashed white with splashed white—and it was
found that each bred true to its respective type.
But when the black and the splashed white were
crossed they gave, as was expected, nothing but
blues. In other words, we have the seeming paradox
of the black and the splashed white producing twice
as many blues as do the blues when bred together.
The black and the splashed white “wasters” are
in reality the pure breeds, while the “ pure” Blue
Andalusian is a mongrel which no amount of selec-
tion will ever be able to fix.

In such cases as this it is obvious that we cannot
speak of dominance. And with the disappearance
of this phenomenon we lose one criterion for deter-
mining which of the two parent forms possesses the
additional factor. Are we, for example, to regard
the black Andalusian as a splashed white to which
has been added a double dose of a colour-intensifying
factor, or are we to consider the white splashed
bird as a black which is unable to show its true
pigmentation owing to the possession of some
inhibiting factor which prevents the manifestation
of the black. Either interpretation fits the facts
equally well, and until further experiments have
been devised and carried out it is not possible to
decide which is the correct view.

Besides these comparatively rare cases where the
heterozygote cannot be said to bear a closer re-
semblance to one parent more than to the other,
there are cases in which it is often possible to draw

Vil DOMINANCE 67

a visible distinction between the heterozygote and
the pure dominant. There are certain white breeds
of poultry, notably the White Leghorn, in which the
white behaves as a dominant to colour. But the
heterozygous whites made by crossing the dominant
white birds with a pure coloured form (such as the
Brown Leghorn) almost invariably show a few
coloured feathers or “ticks” in their plumage. The
dominance of white is not quite complete, and renders
it, possible to distinguish the pure from the impure
dominant without recourse to breeding experiments.

This case of the dominant white fowl opens up
another interesting problem in connection with
dominance. By accepting the Presence and Absence
hypothesis we are committed to the view that the
dominant form possesses an extra factor as com-
pared with the recessive. The natural way of
looking at this case of the fowl is to regard white
as the absence of colour. But were this so, colour
should be dominant to white, which is not the case.
We are therefore forced to suppose that the absence
of colour in this instance is due to the presence of
a factor whose property is to inhibit the production
of colour in what would otherwise be a pure coloured
bird. On this view the dominant white fowl is a
coloured bird plus a factor which inhibits the de-
velopment of the colour. The view can be put to
the test of experiment. We have already seen that
there are other white fowls in which white is reces-
sive to colour, and that the whiteness of such birds
is due to the fact that they lack a factor for the
development of colour. If we denote this factor by
C and our postulated inhibitor factor in the dominant

68 MENDELISM CHAP.

white bird by /, then we must write the constitution
of the recessive white as cczz, and the dominant white
as CC//. We may now work out the results we
ought to obtain when a cross is made between these
two pure white breeds. The constitution of the F,
bird must be Ce/z. Such birds being heterozygous
for the inhibitor factor, should be whites showing
some coloured “ticks.” Being heterozygous for
both of the two factors C and /, they will produce
in equal numbers the four different sorts of gametes
CT, Cz, cl, ct. The result of bringing two such
similar series of gametes together is shown in Fig,
13. Out of the sixteen squares, twelve contain /;
these will be white birds either with or without a few

coloured ticks.

Cl Cl Three contain C
er = but not /; these

must be coloured
Ci Ci i birds. One con-
CI Ci SSSR

tains neither C nor
7; this must be a
white. From such
a mating we ought,
therefore, to obtain
both white and
coloured birds in
the ratio 13:3.
The results thus
theoretically de-

x
SS

y
RS

FIG. 13.

Diagram to illustrate the nature of the Fy generation
from the cross between dominant white and duced were found

recessive white fowls.

to accord with the
actual facts of experiment. The F, birds were all
“ticked” whites, and in the F, generation came white

VII DOMINANCE 69

and coloured birds in the expected ratio. There seems,
therefore, little reason to doubt that the dominant
white is a coloured bird in which the absence of
colour is due to the action of a colour-inhibiting
factor, though as to the nature of that factor we can

Fic. 14.

Ears of beardless and bearded wheat. The beardless condition is dominant
to the bearded.

at present make no surmise. It is probable that
other facts, which at first sight do not appear to be
in agreement with the “Presence and Absence”
hypothesis, will eventually be brought into line
+ + through the action of inhibitor factors. Such a

70 MENDELISM CHAP.

case, for instance, is that of bearded and beardless
wheats. Though the beard is obviously the addi-
tional character, the bearded condition is recessive to
the beardless. Probably we ought to regard the
beardless as a bearded wheat in which there is an
inhibitor that stops the beard from growing. It is
not unlikely that as time goes on we shall find
many more such cases of the action of inhibitor
factors, and we must be prepared to find that the
same visible effect may be produced either by
the addition or by the omission of a factor. The
dominant and recessive white poultry are indistin-
guishable in appearance. Yet the one contains a
factor more and the other a factor less than the
coloured bird.

A phenomenon sometimes termed irregularity of
dominance has been investigated in a few cases, |
In certain breeds of poultry such as Dorkings there
occurs an extra toe directed backwards like the hallux
(cf. Fig. 15), In some families this character behaves
as an ordinary dominant to the normal, giving the
expected 3:1 ratio in F, But in other families
similarly bred the proportions of birds with and with-
out the extra toe appear to be unusual. It has
been shown that in such a family some of the birds
without the extra toe’ may nevertheless transmit the
peculiarity when mated with birds belonging to
strains in which the extra toe never occurs. Though
the external appearance of the bird generally affords
some indication of the nature of the gametes which
it is carrying, this is not always the case. Nevertheless
we have reason to suppose that the character segregates
in the gametes, though the nature of these cannot

vil DOMINANCE 71

always be decided from the appearance of the bird
which bears them.

There are cases in which an apparent irregularity
of dominance has been shown to depend upon
another character, as in the experiments with sheep
carried out by Professor Wood. In these experi-
ments two breeds were crossed, of which one, the
Dorset, is horned in both sexes, while the other,
the Suffolk, is without horns in either sex. Which-

FIG. 15.

Fowls’ feet. On the right a normal, and on the left one with an extra toe.

ever way the cross was made the resulting F,
generation was similar; the rams were horned, and
the ewes were hornless. In the F, generation raised
from these F, animals both horned and hornless
types appeared in both sexes but in.very different pro-
portions. While the horned rams were about three
times as numerous as the hornless, this relation was
reversed among the females, in which the horned
formed only about one-quarter of the total. The
simplest explanation of this interesting case is to

72 MENDELISM CHAP. VII

suppose that the dominance of the horned character
depends upon the sex of the animal—that it is
dominant in the male, but recessive in the female. A
pretty experiment was devised for putting this view to
the test. Ifit is true, equal numbers of gametes with
and without the horned factor must be produced by the
F, ewes, while the factor should be lacking in all the
gametes of the hornless F, rams. A hornless ram,

Dorset Suffolk Suffolk Dorset
Ram Ewe Ram Ewe

ca ee 35x?
Se ee

é 5 @
Fic. 16.

Scheme to illustrate the inheritance of horns in sheep. Heterozygous males shown
dark with a white spot, heterozygous females light with a dark spot in the centre.

therefore, put to a flock of F, ewes should give rise to
equal numbers of zygotes which are heterozygous
for the horned character, and of zygotes in which it is
completely absent. And since the heterozygous
males are horned, while the heterozygous females
are hornless, we should expect from this mating
equal numbers of horned and hornless rams, but only
hornless ewes. The result of the experiment con-
firmed this expectation. Of the ram lambs g were
horned and 8 were hornless, while all the 11 ewe
lambs were completely destitute of horns.

PLATE LT

‘quUT aM Sso[U

oy Fr

‘quvy awq pouopy “f

‘eT NI

‘OMG YOSIOd ‘S

ON

‘query we

TYVAddV SAdA LT

A sso WO YY

‘ue YOHNS ‘1

“SLNAUVd

‘t

‘quevy we

YT powioY

‘ury Ty ce

I

CHAPTER VIII
WILD FORMS AND DOMESTIC VARIETIES

In discussing the phenomena of reversion we have
seen that in most cases such reversion occurs when
the two varieties which are crossed each contain
certain factors lacking in the other, of which the
full complement is necessary for the production
of the reversionary wild form. This at once suggests
the idea that the various domestic forms of animals
and plants have arisen by the omission from time
to time of this factor or of that. In some cases
we have clear evidence that this is the most
natural interpretation of the relation between the
cultivated and the wild forms. Probably the species
in which it is most evident is the sweet-pea (Lathyrus
odoratus). We have already seen reason to suppose
that as regards certain structural features the Bush
variety is a wild lacking the factor for the pro-
cumbent habit, that the Cupid is a wild without
the factor for the long internode, and that the
Bush Cupid is a wild minus both these factors. Nor
is the evidence less clear for the many colour
varieties. In illustration we may consider in more
detail a case in which the cross between two whites

73

74. MENDELISM CHAP,

resulted iri a complete reversion to the purple colour
characteristic of the wild Sicilian form (PLIV.). In
this particular instance subsequent breeding from the
purples resulted in the production of six different
colour forms in addition to whites. The proportion
of the coloured forms to the whites was 9:7
(cf. p. 41), but it is with the relation of the six
coloured forms that we are concerned here. Of
these six forms, three were purples and three were
reds, The three purple forms were (1) the wild
bicolor purple with blue wings known in cultivation
as the Purple Invincible (Pl. IV., 4); (2) a deep
purple with purple wings (PI. 1V., 5); and (3) a very
dilute purple known as the Picotee (Pl. IV., 6). Cor-
responding to these three purple forms were three reds:
(1) a bicolor red known as Painted Lady (PI. IV., 7) ;
(2) a deep red with red wings known as Miss Hunt
(Pl. IV., 8); and (3) a very pale red which we have
termed Tinged White’ (Pl. IV., 9). In the F,
generation the total number of purples bore to the
total number of reds the ratio 3:1, and this ratio
was maintained for each of the corresponding classes.
Purple, therefore, is dominant to red, and each of the
three classes of red differs from its corresponding
purple in not possessing the blue factor (8B) which
turns it into purple. Again, the proportion in which
the three classes of purples appeared was g bicolors,
3 deep purples, 4 picotees. We are, therefore, con-
cerned here with the operation of two factors:
(1) a light wing factor, which renders the bicolor

1 The reader who searches florists’ catalogues for these varieties will
probably experience disappointment. The sweet-pea has been much
‘‘improved” in the past few years, and it is unlikely that the modern
seedsman would list such unfashionable forms,

PLATE IV.

10°

1, 2, Emily Henderson; 3, F, reversionary Purple; 4-10, Various F,

forms: 4, Purple; 5, Deep Purple; 6, Picotee ; 7, Painted Lady;
8, Miss Hunt; 9, Tinged White; 10, White.

vir WILD AND DOMESTIC VARIETIES 75

dominant to the dark winged form ; and (2) a factor
for intense colour, which occurs in the bicolor and in
the deep purple, but is lacking in the dilute picotee.
And here it should be mentioned that these con-
clusions rest upon an exhaustive set of experiments
involving the breeding of many thousands of plants.
In this cross, therefore, we are concerned with the
presence or absence of five factors, which we may
denote as follows :—

A colour base, 2.

A colour developer, C.

A purple factor, 2.

A light wing factor, Z

A factor for intense colour, 7

On this notation our six coloured forms are :—

(1) Purple bicolor. : . CRBIL}

(2) Deep purple : ‘ . CRBI.

(3) Picotee . CRBLIi or CRBU1.
(4) Red bicolor ( = Painted Lady) CRELI.

(5) Deep red (=Miss Hunt) . CRO//.

(6) Tinged white . : . CRbLi or CRO.

It will be noticed in this series that the various
coloured forms can be expressed by the omission of
one or more factors from the purple bicolor of the
wild type. With the complete omission of each
factor a new colour type results, and it is difficult
to resist the inference that the various cultivated
forms of the sweet-pea have arisen from the wild
by some process of this kind. Such a view tallies
with what we know of the behaviour of the wild

1 It is to be understood that wherever a given factor is present the
plant may be homozygous or heterozygous for it without alteration in

its colour.

76 MENDELISM CHAP,

form when crossed by any of the garden varieties.
Wherever such crossing has been made the form of
the hybrid has been that of the wild, thus supporting
the view that the wild contains a complete set of all
the differentiating factors which are to be found in the
sweet-pea.

Moreover, this view is in harmony with such
historical evidence as is to be gleaned from botanical
literature, and from old seedsmen’s catalogues. The
wild sweet-pea first reached this country in 1699,
having been sent from Sicily by the monk Franciscus
Cupani as a present to a certain Dr. Uvedale in the
county of Middlesex. Somewhat later we hear of
two new varieties, the red bicolor, or Painted Lady,
and the white, each of which may be regarded as
having “sported” from the wild purple by the
omission of the purple factor, or of one of the two
colour factors. In 1793 we find a seedsman offering
also what he called black and scarlet varieties. It is
probable that these were our deep purple and Miss
Hunt varieties, and that somewhere about this time
the factor for the light wing (Z) was dropped out in
certain plants. In 1860 we have evidence that the
pale purple or Picotee, and with it doubtless the
Tinged White, had come into existence. This time
it was the factor for intense colour which had
dropped out. And so the story goes on until the
present day, and it is now possible to express by
the same simple method the relation of the modern
shades, of purples and reds, of blues and pinks, of
hooded and wavy standards, to one another and
to the original wild form. The constitution of many
of these has now been worked out, and to-day it

vir. WILD AND DOMESTIC VARIETIES 77

would be a simple though perhaps tedious task to
denote all the different varieties by a series of letters
indicating the factors which they contain, instead of
by the present system of calling them after kings
and queens, and famous generals, and ladies more or
less well known.

From what we know of the history of the various
strains of sweet-peas one thing stands out clearly.
The new character does not arise from a pre-existing
variety by any process of gradual selection, conscious
or otherwise. It turns up suddenly complete in
itself, and thereafter it can be associated by crossing
with other existing characters to produce a gamut
of new varieties. If, for example, the character of
hooding in the standard (cf. Pl. IL, 7) suddenly
turned up in such a family as that shown on Plate
IV., we should be able to get a hooded form corre-
sponding to each of the forms with the erect stan-
dard; in other words, the arrival of the new form
would give us the possibility of fourteen varieties
instead of seven. As we know, the hooded char-
acter already exists. It is recessive to the erect
standard, and we have reason to suppose that it
arose as a sudden sport by the omission of the factor
in whose presence the standard assumes the erect
shape characteristic of the wild flower. It is largely
by keeping his eyes open and seizing upon such
sports for crossing purposes that the horticulturist
“improves” the plants with which he deals. How
these sports or mutations come about we can
now surmise. They must owe their origin to a
disturbance in the processes of cell division through
which the gametes originate. At some stage or

78 MENDELISM CHAP,

other the normal equal distribution of the various
factors is upset, and some of the gametes receive
a factor less than others. From the union of two
such gametes, provided that they are still capable
of fertilisation, comes the zygote which in course of
growth develops the new character.

Why these mutations arise: what leads to the
surmised unequal division of the gametes: of this
we know practically nothing. Nor until we can
induce the production of mutations at will are we
likely to understand the conditions which govern
their formation. Nevertheless there are already
hints scattered about the recent literature of experi-
mental biology which lead us to hope that we may
know more of these matters in the future.

In respect of the evolution of its now multi-
tudinous varieties, the story of the sweet-pea is
clear and straightforward. These have all arisen
from the wild by a process of continuous loss.
Everything was there in the beginning, and as the
wild plant parted with factor after factor there came
into being the long series of derived forms. Exquisite
as are the results of civilisation, it is by the degrada-
tion of the wild that they have been brought about.
How far are we justified in regarding this as a
picture of the manner in which evolution works ?

There are certainly other species in which we
must suppose that this is the way that the various
domesticated forms have arisen. Such, for example,
is the case in the rabbit, where most of the colour
varieties are recessive to the wild agouti form. Such
also is the case in the rat, where the black and albino
varieties and the various pattern forms are also reces-

vir WILD AND DOMESTIC VARIETIES 79

sive to the wild agouti type. And with the excep-
tion of a certain yellow variety to which we shall
refer later, such is also the case with the many fancy
varieties of mice.

Nevertheless there are other cases in which we
must suppose evolution to have proceeded by the
interpolation of characters. In discussing reversion
on crossing, we have already seen that this may not
occur until the F, generation, as, for example, in the
instance of the fowls’ combs (cp. p. §9). The rever-
sion to the single comb occurred as the result of the
removal of the two factors for rose and pea. These
two domesticated varieties must be regarded as each
possessing an additional factor in comparison with
the wild single-combed bird. During the evolution
of the fowl, these two factors must be conceived of
as having been interpolated in some way. And the
same holds good for the inhibitory factor on which,
as we have seen, the dominant white character of
certain poultry depends. In pigeons, too, if we
regard the blue rock as the ancestor of the domesti-
cated breeds, we must suppose that an additional
melanic factor has arisen at some stage. For we
have already seen that black is dominant to blue,
and the characters of F,, together with the greater
number of blacks than blues in F,, negatives the
possibility that we are here dealing with an inhibitory
factor. The hornless or polled condition of cattle,
again, is dominant to the horned condition, and if,
as seems reasonable, we regard the original ancestors
of domestic cattle as having been horned, we have
here again the interpolation of an inhibitory factor
somewhere in the course of evolution.

80 MENDELISM CHAP. VIII

On the whole, therefore, we must be prepared to
admit that the evolution of domestic varieties may
come about by a process of addition of factors in
some cases and of subtraction in others. It may be
that what we term additional factors fall irf€o distinct
categories from the rest. So far, experiment seems
to show that they are either of the nature of melanic
factors, or of inhibitory factors, or of reduplication
factors as in the case of the fowls’ combs. But while
the data remain so scanty, speculation in these
matters is too hazardous to be profitable.

CHAPTER IX
REPULSION AND COUPLING OF FACTORS

ALTHOUGH different factors may act together to
produce specific results in the zygote through their
interaction, yet in all the cases we have hitherto
considered the heredity of each of the different factors
is entirely independent. The interaction of the
factors affects the characters of the zygote, but makes
no difference to the distribution of the separate
factors, which is always in strict accordance with
the ordinary Mendelian scheme. Each factor in
this respect behaves as though the other were not
present.

A few cases have been worked out in which the
distribution .of the different factors to the gametes
is affected by their simultaneous presence in the
zygote. And the influence which they are able to
exert upon one another in such cases is of two kinds.
They may repel one another, refusing, as it were, to
enter into the same gamete, or they may attract one
another, and, becoming linked together, pass into
the same gamete, as it were, by preference. For the
moment we may consider these two sets of pheno-
mena apart.

81 G

82 MENDELISM CHAP.

One of the best illustrations of repulsion between
factors occurs in the sweet-pea. We have already
seen that the loss of the blue or purple factor (B)
from the wild bicolor results in the formation of the
red bicolor known as Painted Lady (Pl. IV., 7).
Further, we have seen that the hooded standard
is recessive to the ordinary erect standard. The
omission of the factor for the erect standard (£)
from the purple bicolor (Pl. II., 5) results in a
hooded purple known as Duke of Westminster (PI.
II., 7). And here it should be mentioned that
in the corresponding hooded forms the difference in
colour between the wings and standard is not nearly
so marked as in the forms with the erect standard,
but the difference in structure appears to affect the
colour, which becomes nearly uniform. This may
be readily seen by comparing the picture of the
purple bicolor on Plate II. with that of the Duke
of Westminster flower.

Now when a Duke of Westminster is mated with
a Painted Lady the factor for erect standard (£) is
brought in by the red, and that for blue (4) by the
Duke, and the offspring are consequently all purple
bicolors. Purples so formed are all heterozygous
for these two factors, and were the case a simple one,
such as those which have already been discussed,
we should expect the F, generation to consist of the
four forms erect purple, hooded purple, erect red,
and hooded red in the ratio 9: 3: 3:1. Such, how-
ever, is not the case. The F, generation actually
consists of only three forms, viz. erect red, erect
purple, and hooded purple, and the ratio in which
these three forms occur is 1:2:1. No hooded red

Ix REPULSION AND COUPLING 83

has been known to occur in such a family. More-
over, further breeding shows that while the erect
reds and the hooded purples always breed true, the
erect purples in such families zever breed true, but
Painted Lady X Duke of Westminster
(erect red). (hooded purple)

Purple Invincible
(erect purple)

I I |
Painted Purple Invincible Duke of
Lady ‘Westminster

(x) (2) (1)

always behave like the original F, plant, giving the
three forms again in the ratio 1:2:1. Yet we
know that there is no difficulty in getting purple
bicolors to breed true from other families; and we
know also that hooded red sweet-peas exist in other
strains.

On the assumption that there exists a repulsion
between the factors for erect standard and blue in
a plant which is heterozygous for both, this peculiar
case receives a simple explanation. The constitutions
of the erect red and the hooded purple are EA6d
and eeBB respectively, and that of the F, erect
purple is He5s. Now let us suppose that in such a
zygote there exists a repulsion between £ and 3B,
such that when the plant forms gametes these two
factors will not go into the same gamete. On this
view it can only form two kinds of gametes, viz. Ed
and @B, and these, of course, will be formed in equal
numbers. Such a plant on self-fertilisation must
give the zygotic series HEG)+2 EeBb+eeBB, te.

84 MENDELISM CHAP,

I erect red, 2 erect purples, and 1 hooded purple.
And because the erect reds and the hooded purples
are respectively homozygous for & and BS, they
must thenceforward breed true. The erect purples,
on the other hand, being always formed by the
union of a gamete £4 with a gamete eB, are always
heterozygous for both of these factors. They can,
consequently, never breed true, but must always give
erect reds, erect purples, and hooded purples in the

EEbb eeBB Parents
Eb Eb eB eB gametes
EeBb F,

2 Pa TG =

‘a ( Zo —> EEbb<— 25)" *

Q ) x —> EeBb <—eB | 3

# | eB —> EeBb <— 2b | &

g eB —>eeBB <—eB a

er eeipeiey a

F, generation

ratio 1:2:1. The experimental facts are readily
explained on the assumption of repulsion between
the two factors B and £& during the formation of the
gametes in a plant which is heterozygous for both.
Other similar cases of factorial repulsion have
been demonstrated in the sweet-pea, and two of
these are also concerned with the two factors with
‘1 In this account the repulsion between the two factors in question
has been assumed to be complete. Recent work, however, has shown
that this is probably not quite true. It is likely that the gametes ZB

and ed are formed though only very rarely. For further discussion
the reader is referred to the Appendix at the end of this chapter.

Ix REPULSION AND COUPLING 85

which we have just been dealing. Two distinct
varieties of pollen grains occur in this species, viz.
the ordinary oblong form and a rather smaller
rounded grain. The former is dominant to the
latter.. When a cross is made between a purple
with round pollen and a red with long pollen the
F, plant is a long pollened purple. But the F,
generation consists of purples with round pollen,
purples with long pollen, and reds with long pollen
in the ratio 1:2:1. No red with round pollen
appears in F, owing to repulsion between the factors
for purple (4) and for long pollen (Z). Similarly
plants produced by crossing a red hooded long with
a red round having an erect standard give in
F, long pollened reds with an erect standard, and
these } in F, produce the three types round pollened
erect, long pollened erect, and long pollened hooded
in the ratio 1:2:1. The repulsion here is between
the long pollen factor (Z) and the factor for the
erect standard (4).

Yet another similar case is known in which we
are concerned with quite different factors. In some
sweet-peas the axils whence the leaves and flower-
stalks spring from the main stem are of a deep red
colour. In others they are green. The dark
pigmented axil is dominant to the light one. Again,
in some sweet-peas the anthers are sterile, setting no
pollen, and this condition is recessive to the ordinary
fertile condition. When a sterile plant with a dark
axil is crossed by a fertile plant with a light axil, the

1 It should be mentioned that as the shape of the pollen coat,
like that of the seed coat, is a maternal character, all the grains of any
given plant are either long or else round. The two kinds do not
occur together on the same plant.

86 MENDELISM CHAP.

F, plants are all fertile with dark axils. But such
plants in F, give fertiles with light axils, fertiles
with dark axils, and steriles with dark axils in the
ratio 1:2:1. No light axilled steriles appear from
such a cross owing to the repulsion between the
factor for dark axil (D) and that for the fertile
anther (7).

These four cases have already been found in the
sweet-pea, and similar phenomena have been met
with in primulas and in peas. To certain seemingly
analogous cases in animals where sex is concerned
we shall refer later.

Now all of these four cases present a common
feature which probably has not escaped the attention
of the reader. In all of them ‘the original cross
was such as to introduce one of the repelling factors
with each of the two parents. If we denote our two
factors by A and B, the crosses have always been
of the nature AdAddxaaBs. Let us now consider
what happens when both of the factors, which in
these cases repel one another, are introduced by one
of ‘the parents, and neither by the other parent.
And in particular we will take the case in which we
are concerned with purple and red flower colour,
and with long and round pollen, ze. with the factors
Band L. When a purple long (BSLL) is crossed
with a red round (dd//) the F, (BOLD) is a purple with
long pollen, identical in appearance with that produced
by crossing the long pollened red with the round
pollened purple. But the nature of the F, generation
is in some respects very different. The ratio of
purples to reds and of longs to rounds is in each case
3:1, as before. But instead of an association

= REPULSION AND COUPLING 87

between the red and the long pollen characters the
reverse is the case. The long pollen character is
now associated with purple and the round pollen
with red. The association, however, is not quite
complete, and the examination of a large quantity of
similarly bred material shows that the purple longs
are about twelve times as numerous as the purple
rounds, while the red rounds are rather more than three
times as many as the red longs. Now this peculiar
result could be brought about if the gametic
series produced by the F, plant consisted of
7 BL+1 Bl+1 6L+7 & out of every 16 gametes.
Fertilisation between two such similar series of 16
gametes would result in 256 plants, of which 177
would be purple longs, 15 purple rounds, 15 red
longs, and 49 red rounds—a proportion of the
four different kinds very close to that actually
found by experiment. It will be noticed that in
the whole family the purples are to the reds as 3:1,
and the longs are also three times as numerous as
the rounds. The peculiarity of the case lies in the
distribution of these two characters with regard to
one another. In some way or other the factors for
blue and for long pollen become linked together
in the cell divisions that give rise to the gametes,
but the linking is not complete. This holds good
for all the four cases in which repulsion between the
factors occurs when one of the two factors is intro-
duced by each of the parents. When both of the
factors ave brought into the cross by the same parent
we get coupling between them instead of reputszon.
The phenomena of repulsion and coupling between
separate factors are intimately related, though

88 MENDELISM CHAP,

hitherto we have not been able to decide why this
should be so.

Nor for the present can we suggest why certain
factors should be linked together in the peculiar
way that we have reason to suppose that they are
during the process of the formation of the gametes.
Nevertheless the phenomena are very definite, and it
is not unlikely that a further study of them may
throw important light on the architecture of the
living cell.

APPENDIX TO CHAPTER IX

As it is possible that some readers may care, in spite of
its complexity, to enter rather more fully into the peculiar
phenomenon of the coupling of characters, I have brought
together some further data in this Appendix. In the case
we have already considered, where the factors for blue
colour and long pollen are concerned, we have been led to
suppose that the gametes produced by the heterozygous
plant are of the nature 7 BZL:1 Bl:1 6L£:7 b. Sucha
series of ova fertilised by a similar series of pollen
grains will give a-generation of the following com-
position :—

49 BBLL+7 BBLi+7 BOLL +49 BoL/
+7 BBLI+7 BOLL+ Boll

+ Boll
+49 BbL/
—

177 purple, long
+ BBU+7 Bbll+ bLL+7 b6Li4+ 49 bbu

+7 Boul +7 O6L1
-_—_—,_ ————’ ——
15 purple, 15 red, 49 red,
round long round

and as this theoretical result fits closely with the actual
figures obtained by experiment we have reason for

Ix REPULSION AND COUPLING 89

supposing that the heterozygous plant produces a series of
gametes in which the factors are coupled in this way. The
intensity of the coupling, however, varies in different cases.
Where we are dealing with another, viz. fertility (7) and
the dark axil (D) the experimental numbers accord with
the view that the gametic series is here 15 #D:1 Fa:
1fD:15 fd. The coupling is in this instance more
intense. In the case of the erect standard (Z) and blue-
ness (#) the coupling is even more intense, and the
experimental evidence available at present points to the
gametic series here being 127 £B:1 £b:1 eB:127 eb.
There is evidence also for supposing that the intensity of the
coupling may vary in different families for the same pair of
factors. The coupling between blue and long pollen is
generally on the 7:1:1:7 basis, but in some cases it may
be on the 15:1:1:15 basis. But though the intensity
of the coupling may vary it varies in an orderly way. If
A and B are the two factors concerned, the results obtained
in F, are explicable on the assumption that the ratio of the
four sorts of gametes produced is a term of the series—

3 AB+Ab+aB+ 3 ab
7 AB+Ab+aB+ 7 ab
15 A4B+Ab+aB+15 ab, etc., etc.

In such a series the number of gametes containing 4 is
equal to the number lacking 4A, and the same is true for
&. Consequently the number of zygotes formed contain-
ing A is three times as great as the number of zygotes which
do not contain 4; and similarly for &. The proportion
of dominants to recessives in each case is 3:1. It is
only in the distribution of the characters with relation to
one another that these cases differ from a simple Mendelian
case.

In the account given in the foregoing chapter we treated
of repulsion between two factors as being complete, and
for practical purposes this is generally true. Nevertheless
the most recent work indicates that repulsion like coupling
is partial, and the reasons for taking this view may best be

go MENDELISM CHAP.

given in a brief account of the experiments that have led
to it. There exists a monstrous form of sweet-pea in
which the flowers are malformed and the stigma protrudes
owing to the cleft condition of the keel which normally
curves it (cf. Fig. 16a). From its fancied resemblance to

FIG. 16A.

The “‘cretin ”” sweet-pea.

a gaping mouth with a protruding tongue, such a plant has
been termed a cretin. The pollen of a cretin was used to
fertilise a sterile plant with normal flowers, and all the
resulting F, plants were normal and fertile. Fertility (7)
is dominant to sterility (7), and the normal flower (/V) is
dominant to the cretin (7). The cross was so made that
one factor went in from each parent. An F, generation
was raised from 9 F, plants and consisted of —

IX REPULSION AND COUPLING gI

336 normal fertile,

150 normal sterile,

143 cretin fertile,
II cretin sterile.

Had it not been for the small proportion of sterile cretins
the case might have been considered as one in which com-
plete repulsion occurred between the factors # and VV
during the formation of the gametes of the F, plant. The
presence of these sterile cretins may, however, be explained
on the assumption that the repulsion is not complete, but
that the gametes produced by the F, plant form the series
NF: 3 Nf:32F:nf The union of such a series of female
gametes with a similar series of male gametes as occurs
when the F, plants are selfed would result in a generation
consisting of—

33 normal fertile,

15 normal sterile,

15 cretin fertile,

I cretin sterile,

proportions which are almost precisely those actually found
by experiment. From this and other experiments there is
reason for supposing that in cases where repulsion occurs
between two factors 4 and B the repulsion is not absolute,
but that the gametes are produced in one or other of the
series—

AB:3 Ab:3 aB:ab,

AB:7 Ab:7 aB: ab,

AB: 15 Ab: 15 aB: as, etc.

Such series are in a sense the converse of those in which
partial coupling occurs, for in the one case it is the two
terms Aé and aB that are reduplicated, while in the other
it is in the terms AZ and aé where this reduplication takes
place. A point of importance in connection with this view
of the nature of repulsion is that it enables us to bring the
phenomenon more closely into line with that of coupling.
Whatever may be the meaning of repulsion and coupling,

g2 MENDELISM CHAP.

and this at present is by no means clear, it is now evident
that they will both ultimately take their place in some
common scheme. The relations between the two types of
series may perhaps be more clearly shown in the subjoined
table which brings out several points of interest in connec-
tion with them. The series marked with an asterisk are
those which have already been demonstrated experimentally.
The middle term in the table, in which all four kinds of
gametes are produced in equal numbers, is, of course, that
of a simple Mendelian case where neither repulsion nor
coupling occur. .

n
‘S 2 Distribution of S 3
oes Factors in Gametic 6 hs Form of F2 Generation.
Zz Ss Series. Ag 2
AB. Ab. aB. ab AB. Ab, ab. ab
2n i (a-xr (m-1) 1 4n2 2n2+r net n2—1 i
256 THIS 3227) I 65536 32769 16383 16383 I
128 r: 63: 63 I 16384 8193 4095 4095 I
64 RE SEs gt I 4096 2049 1023 1023 I
32 Eh AS A ag I 1024 513 255 255 I
16 RE OF 7 1 256 129 63 63 1*
8 ri 3 3: 4. 64 33 15 15 1*
4 Ps LS fs = 16 9 3 3 I
8 22 £s FE 3, 64 4 7 7 oF
16 7 SER OR Boy 256 1977 15 15 49*
32 5 em aa ot 1024 737 3r 3r 225*
64 35 Hae or 4096 3009 63 63 g6r
128 63 ce £2 163 16384 12161 127 127 3060"
256 12 Li 8 Te! 65536 88 2 255 1607
by nd Ie 2 ee 1) re) WP Gh 1) (p22) ns) Ga 1)

As the table shows it is possible to give generalised expres-
sions by means of which the nature of the generations
resulting from any of the series can readily be calculated.
The table brings out clearly the reason why repulsion at
first appeared to be absolute, for as its intensity increases
the zygote exhibiting both recessive characters rapidly
becomes exceedingly rare. Even for a series of such com-
paratively low intensity as 4B:15 44:15 aB: ab the ex-
pected proportion of aaéé zygotes is only 1 in 1024, while
for a series on the 1: 127 basis it is only 1 in 65,536.

A question of interest is whether the coupling and re-

Ix REPULSION AND COUPLING 93

pulsion for the same pair of factors is of the same intensity
—whether for instance two characters coupled on the 15:1
basis will exhibit repulsion on the 1:15 basis. The existing
evidence certainly favours such a view. Experiments made
during the present year (1912) render it practically certain
that in the cretin case the coupling between the normal
flower and the fertile anther is on a 3: 1:1: 3 basis; and
there is strong reason for supposing that the repulsion
between long pollen and purple is on a 1:7:7:1 basis.
In each of these two instances the coupling and repulsion
appear to be of the same intensity. In the higher series
very large numbers are needed to determine the intensity
of the repulsion. Thus for the series 1 4B:127 4b: 127
aB:1 ab the double recessive is expected only once in
65,536 plants. It is unlikely that such cases will ever be
decided by direct experiment, but it may be asserted that
where the intensity of coupling is high, the evidence avail-
able certainly points to the intensity of repulsion being
high also.

One more point of interest may be noted in connection
with these series. As the table shows it is possible where
coupling is concerned to express the various gametic series
by the general formula (x-1)AB:Ab:aB:(n- 1) ab,
where 2 is the total number of the gametes in the series
A plant producing such a series of gametes gives rise to a
family of zygotes in which 37” — (22-1) show both of the
dominant characters and 2?—(22—1) show both of the
recessive characters, while the number of the two classes
which each show one of the two dominants is (27-1).
When in such a series the coupling becomes closer the
value of increases, but in comparison with 7? its value
becomes less and less. The larger x becomes the more
negligible is its value relatively to ~*. If, therefore, the
coupling were very intense, the series 3m*—(2m—1):
(2m —1):(22-—1):2*-—(22-—1) would approximate more
and more to the series 377:0:0: 7%, ze. toa simple 3:1
ratio. Though the point is probably of more theoretical
than practical interest, it is not impossible that some of the

94 MENDELISM CHAP.

cases which have hitherto been regarded as following a
simple 3 : 1 ratio will turn out on further analysis to belong
to this more complicated scheme.

This account of coupling and repulsion is based upon
the evidence from the sweet-pea, which has been more
carefully studied in this connection than any other species,
and it is not pretended that it offers more than a pre-
liminary account of these interesting and peculiar phen-
omena. Facts are known which are not entirely covered
by the scheme suggested. Baur,! for instance, working
with Antirrhinum, has adduced evidence that in a case
where coupling in F, resulted from the cross AA BB x aabd,
the converse cross 4444 x aaBB behaved in a normal
Mendelian manner without showing repulsion between
A and B.

Lastly, we may ask whether these phenomena are con-
fined to plants or whether they are also to be found among
animals. That in certain animals something in the nature
of repulsion takes place between certain factors and the
sex factor will appear in the next chapter. And evidence
is beginning to accumulate suggesting that coupling and
repulsion may occur in animals independent of the sex
factor. Morgan’s recent work on the inheritance of eye
colour and wing characters in the little pomace fly ? (Droso-
phila) is strongly suggestive of the operation of coupling,
while Hagedoorn ® has described the repulsion between the
factor for agouti (G) and that for colour (C) in a certain
strain of mice. Quite recently I have myself obtained
experimental evidence for the coupling of certain factors
influencing the coat colour of rabbits. Such evidence then
as we possess suggests that as we learn more of these
matters we shall find these phenomena under some form
or other playing an important part in the heredity of
animals. And, indeed, this is what, on general considera-
tions, we might have been led to expect. Among the

1 Zeit. f. ind. Abst. u. Vererb., 1912.

2 Journ. Exp. Zool,, 1911.
3 Zeit. f. ind. Abst. u. Vererb., 1912.

IX REPULSION AND COUPLING 95

phenomena of heredity in man few things are more striking
than the remarkable way in which children so often re-
semble this or that parent, not in one or two, but simul-
taneously in a number of characters. The human species
shows so many heritable features, and man is such a
mongrel, that it would be reasonable to look for relatively.
few points of resemblance between parent and offspring.
The fact that these are frequently so numerous suggests that
in man characters are transmitted as it were in bunches,
and it is not improbable that the study of cases in which
coupling occurs will eventually render clearer what is
evidently an important feature in the heredity of our own
kind.

CHAPTER X
SEX

IN their simplest expression the phenomena ex-
hibited by Mendelian characters are sharp and clean
cut. Clean cut and sharp also are the phenomena
of sex. It was natural, therefore, that a comparison
should have been early instituted between these two
sets of phenomena. As a general rule, the cross
between a male and a female results in the produc-
tion of the two sexes in approximately equal
numbers. The cross between a heterozygous domin-
ant and a recessive also leads to equal numbers of
recessives and of heterozygous dominants. Is it
not, therefore, possible that one of the sexes is
heterozygous for a factor which is lacking in the
other, and that the presence or absence of this factor
determines the sex of the zygote? The results of
some recent experiments would appear to justify
this interpretation, at any rate in particular cases.
Of these, the simplest is that of the common currant
moth (Adraxas grossulariata), of which there exists
a pale variety (Fig. 17) known as Jacticolor. The
experiments of Doncaster and Raynor showed that
the variety behaved as a simple recessive to the
96

x SEX 97

normal form. But the distribution of the dominants
and recessives with regard to the sexes was peculiar.
The original cross was between a J/acticolor female
and a normal male. All the F, moths of both sexes

FIG, 17.

Abraxas grossulariata, the common currant moth, and (on the right) its paler
lacticolor variety.

~were of the normal grossulariata type. The F,
insects were then paired together and gave a
generation consisting of 3 normal : 1 /actzcolor. But
all the J/acticolor were females, and all the males

Lacticolor X Grossulariata

3

Lact.d x Gr.? 4 Gr.d xX Lact.

| aed 1 q T T i] t T T 1
Gr.d Lact.? Gr.d Gr.d Gr.@ Lact.? Gr.d Lact.o Gr.? Lact.2

were of the normal pattern. It was, however, found

possible to obtain the J/acticolor male by mating a

lacticolor female with the F, male. The family result-

ing from this cross consisted of normal males and

normal females, dactzcolor males and dactzcolor females,
H

98 MENDELISM CHAP.

and the four sorts were produced in approximately
equal numbers. In such a family there was no
special association of either of the two colour
varieties with one sex rather than the other. But
the reverse cross, F, female by /acticolor male, gave
a very different result. As in the previous cross,
such families contained equal numbers of the normal
form and of the recessive variety. But all of the
normal grossulariata were males, while all the éact:-
cq@lor were females. Now this seemingly complex
collection of facts is readily explained if we make
the following three assumptions :—

(1) The grossularzata character (G) is dominant
to the éactzcolor character (g). This is obviously
justified by the experiments, for, leaving the sex
distribution out of account, we get the expected
3:1 ratio from F, x F,, and also the expected ratio
of equality when the heterozygote is crossed with
the recessive.

(2) The female is heterozygous for a dominant
factor (/) which is lacking in the male. The con-
stitution of a female is consequently //, and of a
male f/f This assumption is in harmony with the
fact that the sexes are produced in approximately
equal numbers.

(3) There exists repulsion between the factors
G and F in a zygote which is heterozygous for
them both. Such zygotes (F/Gg) must always be
females, and on this assumption will produce gametes
Fg and /G in equal numbers.

We may now construct a scheme for com-
parison with that on page 92 to show how these
assumptions explain the experimental results. The

x SEX 99

original parents were J/acticolor female and grossu-
lariata male, which on our assumptions must be
Ffgg and /fGG respectively in constitution. Since
the female is always heterozygous for F, her gametes
must be of two kinds, viz. #g and fg, while those of
the pure grossulariata male must be all f/G. When
an ovum F¢ is fertilised by a spermatozoon /G, the

Figg [2] 6G [¢]
gametes { ie \ j te

[J] feg FiGg(¢@] Gel ¢] Figg [2]
Bee = Z

I T T i, t T T 1

Figg ffGg Figg FiGg ffGg ffGG FiGg Figg fiGg figg
[2] [8] [9] [9] [6] [6] Ce] [9] [6] [3]

Fic. 18.

Scheme of inheritance in the F, and Fy, generations resulting from the cross of
lacticolor female with grossulariata male. The character of each individual is
represented by the sex signs in brackets, the black being grossudariata in
appearance and the light ones dacticolor.

resulting zygote, F/Gg, is heterozygous for both F
and G, and in appearance is a female gvossularzata.
The zygote resulting from the fertilisation of an
ovum /g by a spermatozoon /G is heterozygous for
G, but does not contain F, and therefore is a male
grossulariata. Such a male being in constitution
JfGg must produce gametes of two kinds, /G and /g,
in equal numbers. And since we are assuming
repulsion between F and G, the F, female being in
constitution, F/Gg must produce equal numbers of

100 MENDELISM CHAP,

gametes /g and fG. For on our assumption / and
G cannot enter into the same gamete. The series
of gametes produced by the F, moths, therefore, are
SG, fg by the male and Fg, fG by the female. The
resulting F, generation consequently consists of
the four classes of zygotes F/gg, F/Gg, ffGg, and
J/GG in equal numbers. In other words, the sexes
are produced in equal numbers, the proportion of
normal grossulariata to lacticolor is 3:1, and all of
the J/actzcolor are females; that is to say, the reé-
sults worked out on our assumptions accord with
those actually produced by experiment. We may
now turn to the results which should be obtained
by crossing the F, moths with the /actzcolor variety.
And first we will take the cross Jactzcolor female x F,
male. The gametes produced by the J/actzcolor
female we have already seen to be /g and fg, while
those produced by the F, male are f/G and fg. The
bringing together of these two series of gametes must
result in equal numbers of the four kinds of zygotes
FfGg, Ffee, ffGg, and ffyg, 2.e. of female grossulariata
and /acticolor, and of male grossulariata and lactécolor
in equal numbers. Here, again, the calculated results
accord with those of experiment. Lastly, we may
examine what should happen when the F, female is
crossed with the Jactzcolor male. The F, female,
owing to the repulsion between F and G, produces
only the two kinds of ova #g and /G, and produces
them in equal numbers. Since the /actécolor male
can contain neither / nor G, all of its spermatozoa
must be fg. The results of such a cross, therefore,
should be to produce equal numbers of the two
kinds of zygote Ffgg and //Gg, te. of Jacticolor

x SEX IOI

females and of grossulariata males. And this, as we
have already seen, is the actual result of such a cross.

Before leaving the currant moth we may allude
to an interesting discovery which arose out of these
experiments. The /actzcolor variety in Great Britain
is a southern form and is not known to occur in
Scotland. Matings were made between wild Scotch
females and dactzcolor males. The families resulting
from such matings were precisely the same as those
from dactzcolor males and F, females, viz. grossulariata
males and /acticolor females only. We are, therefore,
forced to regard the constitution of the wild grossu-
lariata female as identical with that of the F, female,
ze. aS heterozygous for the grossulariata factor as
well as for the factor for femaleness. Though from
a region where éactzcolor is unknown, the “ pure” wild
grossulaviata female is nevertheless a permanent
mongrel, but it can never reveal its true colours
unless it is mated with a male which is either
heterozygous for G or pure dacticolor. And as all
the wild northern males are pure for the grossu-
farviata character this can never happen in a state
of nature.

An essential feature of the case of the currant
moth lies in the different results given by reciprocal
crosses. Lactzcolor female x grossulariata male gives
grossulariata alone of both sexes. But grossulariata
female x /actzcolor male gives only grossulariata males
and J/actécolor females. Such a difference between
reciprocal crosses has also been found in other
animals, and the experimental results, though some-
times more complicated, are explicable on the same
lines. An interesting case in which three factors

102 MENDELISM CHAP.

are concerned has been recently worked out in
poultry. The Silky breed of fowls is characterised
among other peculiarities by a remarkable abundance
of melanic pigment. The skin is dull black, while
the comb and wattles are of a deep purple colour
contrasting sharply with the white plumage (PI.
V., 3). Dissection shows that this black pigment
is widely spread throughout the body, being especially
marked in such membranes as the mesenteries, the
periosteum, and the pia mater surrounding the brain.
It also occurs in the connective tissues among the
muscles. In the Brown Leghorn, on the other
hand, this pigment is not found. Reciprocal crosses
between these two breeds gave a remarkable differ-
ence in result. A cross between the Silky hen and

Silky BrowaLesthan the Brown Leghorn

exc cock produced F, birds,
in which both sexes
ete exhibited only traces

Q@ x @------- F, of the pigment. On
| casual observation they

fs ae might have passed for
ieee oy : :
SSS PLL LE unpigmented birds, for
FIG. 19. with the exception of
Scheme illustrating the result of crossing a

Silky hen with a Brown Leghorn cock. an occasional fleck of
Black sex signs denote deeply pigmented

birds, and light sex signs those without pigment their skin,
Gigck dot in the centredienate bias with Comb, and wattles were
i aa as clear as in the Brown
Leghorn (Pl. V., 1 and 4). Dissection revealed the
presence of a slight amount of internal pigment.
Such birds bred together gave some offspring with
the full pigmentation of the Silky, some without any

pigment, and others showing different degrees of

PLATE V.

3 4

1, 2, F, Cock and Hen, ex Brown Leghorn Hen x Silky Cock;
3, Silky Cock; 4, Hen ex Silky Hen x Brown Leghorn Cock.

x SEX 103

pigment. None of the F , male birds, however,
showed the full deep pigmentation of the Silky.

When, however, the cross was made the other way,
viz. Brown Leg-

horn hen x Silky Brown Leghorn Silky

cock, the result ex é

was different. ad ee

While the F, male Se igheeassrss F,
birds were almost

destitute of pig- = —-——-"7.— >. Tr
ment as in the € 6 6 6 @ 2? @ QE
previous cross, the FIG. 20.

F, hens, on the Scheme illustrating the result of crossing a Brown
Leghorn hen with a Silky cock (cf. Fig. 19).

other hand, were

nearly as deeply pigmented as the pure Silky (Pl. V.,
2). The male Silky transmitted the pigmentation,
but only to his daughters. Such birds bred together
gave an F, generation containing chicks with the full
deep pigment, chicks without pigment, and chicks
with various grades of pigmentation, all the different
kinds in both sexes.

In analysing this complicated case many other
different crosses were made, but for the present it
will be sufficient to mention but one of these, viz.
that between the F, birds and the pure Brown
Leghorn. The cross between the F, hen and the
Brown Leghorn cock produced only birds with a
slight amount of pigment and birds without pigment.
And this was true for both the deeply pigmented
and the slightly pigmented types of F, hen. But
when the F, cock was mated to a Brown Leghorn
hen, a definite proportion of the chicks, one in eight,
were deeply pigmented, and these deeply pigmented

104 MENDELISM CHAP.

birds were always females (cf. Fig. 21). And in
this respect all the F, males behaved alike, whether
they were from the Silky hen or from the Silky
cock. We have, therefore, the paradox that the F,
hen, though herself deeply pigmented, cannot trans-
mit this condition to any of her offspring when she
is mated to the unpigmented Brown Leghorn, but
that, when similarly mated, the F, cock can transmit

1
this pigmented condition to a quarter of his female

(Brown Leghorn) Silky
Xx

(BrownLegh.)g x ? 6 x Q (Brown Leghorn)

$322 883852229
FIG. 21,

Scheme to illustrate the result of crossing F, birds (e.g. Brown Leghorn x Silky)
with the pure Brown Leghorn.

offspring though he himself is almost devoid of
pigment.

Now all these apparently complicated results, as
well as many others to which we have not alluded,
can be expressed by the following simple scheme.
There are three factors affecting pigment, viz. (1)
a pigmentation factor (P); (2) a factor which
inhibits the production of pigment (/); and (3) a
factor for femaleness (/), for which the female birds
are heterozygous, but which is not present in the
males. Further, we make the assumptions (a) that
there is repulsion between F and J in the female
zygote (///z), and (4) that the male Brown Leghorn

x SEX 105

is homozygous for the inhibitor factor (/), but that
the hen Brown Leghorn is always heterozygous for
this factor just in the same way as the female of the
currant moth is always heterozygous for the grossu-
lariata factor. We may now proceed to show how
this explanation fits the experimental facts which
we have given.

The Silky is pure for the pigmentation factor,
but does not contain the inhibitor factor. The
Brown Leghorn, on the
other hand, contains [@] FfPPii ffppli[d ]
the inhibitor factor, but be ee:

‘ ik gametes gametes
not the pigmentation FPi . fp
factor. In crossing a {Pi fpl
Silky henwith a Brown
Leghorn cock we are
mating two birds of the FfPpli ffPpli
constitution &fPPiz [?] é [3]

1G. 22.
and ST¢pll, and all the Scheme to illustrate the nature of the Fy
F. birds are conse- generation from the Silky hen and Brown
1 Leghorn cock (cf. Fig. aay.

quently heterozygous
for both P and 7 In such birds the pigment is
almost but not completely suppressed, and as both
sexes are of the same constitution with regard to
these two factors they are both of similar appearance.
In the reciprocal cross, on the other hand, we are
mating a Silky male (/fP 22) with a Brown Leghorn
hen which on our assumption is heterozygous for the
inhibitor factor (/), dnd in constitution therefore is
Ffppli. Owing to the repulsion between F and /
the gametes produced by such a bird are Fpz and fp/
in equal numbers. All the gametes produced by
’ the Silky cock are fPz Hence the constitution of

106 MENDELISM CHAP,

the F, male birds produced by this cross is /fPplz
as before, but the female birds must be all of the
constitution #fPpz7. The Silky cock transmits the
fully pigmented condition to his daughters, because
the gametes of the Brown Leghorn hen which con-
tain the factor for femaleness do not contain the
inhibitory factor owing to the repulsion between

these factors. The

[2] Ffppli Pa [8] nature of the F, genera-
an Pat tion in each case is
Fpi {Pi in harmony with the

fpl \ x { above scheme. As,

however, it serves to

illustrate certain points

FfPpii ffPpli in connection with in-
[9] [3] termediate forms we
ape shall postpone further

Scheme to illustrate the nature of the Fy . 2 . .
generation from the Brown Leghorn hen consideration of it till

and Silky cock (cf. Fig. 22). .

we discuss these
matters, and for the present shall limit ourselves to
the explanation of the different behaviour of the F,
males and females when crossed with the Brown
Leghorn. And, first, the cross of Brown Leghorn
female by F, male. The Brown Leghorn hen is on
our hypothesis //pp/z, and produces gametes Fz and
Spl. The F, cock is on our hypothesis /fPp/7, and
produces in equal numbers the four kinds of gametes
SPI, fPi, fol, fot. The result of the mecting of
these two series of gametes is given in Fig. 24.
Of the eight different kinds of zygote formed only
one contains P in the absence of /, and this is a
female. The result, as we have already seen, is in
accordance with the experimental facts.

Xx SEX 107

On the other hand, the Brown Leghorn cock is
on our hypothesis //#s/7. All his gametes conse-
quently contain the inhibitor factor, and when he
is mated with an

F, hen all the |Fpi Fpi Fpi Fpi
zygotes produced | PI! ae fpl fpi
must contain J. g ? ? "4
None of his off- | fpr fpI fpl fpl
spring, therefore, | fP! fPi fp! fpi
can be fully pig- 3 3 3 3
mented, for this ;

ee Fic. 24.
condition only Diagram showing the nature of the offspring from a
occurs in the ab- Brown Leghorn hen and an F, cock bred from

a Silky hen x Brown Leghorn cock, or vice versa.
sence of the in-

hibitor factor among zygotes which are either homo-
zygous or heterozygous for P.

The interpretation of this case turns upon the
constitution of the Brown Leghorn hen, upon her
heterozygous condition with regard to the two factors
Fand J/, and upon the repulsion that occurs between
them when the gametes are formed. Through an
independent set of experiments this view of the
nature of the Brown Leghorn hen has been con-
firmed in an interesting way. ‘There are fowls which
possess-neither the factor for pigment nor the in-
hibitory factor, which are in constitution ppzz. Such
birds when crossed with the Silky give dark pig-
mented birds of both sexes in F,, and the F, genera-
tion consists of pigmented and unpigmented in the
ratio 3:1. Now a cock of such a strain crossed
with a Brown Leghorn hen should give only com-
pletely unpigmented birds. But if, as we have
supposed, the Brown Leghorn hen is producing

108 MENDELISM CHAP.

gametes /z and /p/, the male birds produced by
such a cross should be heterozygous for /, ze. in
constitution //pplz7, while the hen birds, though
identical in appearance so far as absence of pig-
mentation goes, should not contain this factor but
should be constitutionally F/ppzz7. Crossed with the
pure Silky, the F, birds of opposite sexes should
give an entirely different result. For while the hens

[Q] Fippli ffppii [d]
gives gives A
gametes gametes
Fpi fpi
fp } * Ce
[él fppri [2] Féppii fppli[6] FfPPi[?]
gives gives gives gives
gametes gametes gametes gametes
fPi Fpi fpl { FPi
cae { fpi fpi {Pp
ital — T T 1
FEPpii , ffPpii FfPpli FfPpii ffPpli ffPpii
[9] [6] [@] [9] [1 []
FIG. 25.

Scheme to illustrate the heterozygous nature of the pure Brown Leghorn hen.
For explanation see text.
should give only deeply pigmented birds of both
sexes, the cocks should give equal numbers of deeply
pigmented and slightly pigmented birds (cf. Fig.
25). These were the results which the experiment
actually gave, thus affording strong confirmation of
the view which we have been led to take of the
Brown Leghorn hen. Essentially the poultry case
is that of the currant moth. It differs in that the

x SEX 109

factor which repels femaleness produces no visible
effect, and its presence or absence can only be deter-
mined by the introduction of a third factor, that for
pigmentation.

This conception of the nature of the Brown
Leghorn hen leads to a curious paradox. We have
stated that the Silky cock transmits the pigmented
condition, but transmits it to his daughters only.
Apparently the case is one of unequal transmission
by the father. Actually, as our analysis has shown,
it is one of unequal transmission by the mother, the
father’s contribution to the offspring being identical
for each sex. The mother transmits to the daughters
her dominant quality of femaleness, but to balance
this, as it were, she transmits to her sons another
quality which her daughters do not receive. It is
a matter of common experience among human
families that in respect to particular qualities the
sons tend to resemble their mothers more than the
daughters do, and it is not improbable that such
observations have a real foundation for which the
clue may be provided by the Brown Leghorn hen.

Nor is this the only reflection that the Brown
Leghorn suggests. Owing to the repulsion between
the factors for femaleness and for pigment inhibition,
it is impossible by any form of mating to make a
hen which is homozygous for the inhibitor factor.
She has bartered away for femaleness the possibility
of ever receiving a double dose of this factor. We
know that in some cases, as, for example, that of the
blue Andalusian fowl, the qualities of the individual
are markedly different according as to whether he
or she has received a single or a double dose of a

IIO MENDELISM CHAP. X

given factor. It is not inconceivable that some of
the qualities in which a man differs from a woman
are founded upon a distinction of this nature. Certain
qualities of intellect, for example, may depend upon
the existence in the individual] of a double dose of
some factor which is repelled by femaleness. If this
is so, and if woman is bent upon achieving the results
which such qualities of intellect imply, it is not
education or training that will help her. Her problem
is to get the factor on which the quality depends
into an ovum that carries also the factor for female-
ness,

CHAPTER XI
SEX (continued)

THE cases which we have considered in the last
chapter belong to a group in which the peculiarities
of inheritance are most easily explained by supposing
that the female is heterozygous for some factor that
is not found in the male. Femaleness is an addi-
tional character superposed upon a basis of maleness,
and as we imagine that there is a separate factor
for each the full constitutional formula for a female
is F{MM, and for a male ff. Both sexes are
homozygous for the male element, and the difference
between them is due to the presence or absence of
the female element /. ;
There are, however, other cases for which the
explanation will not suffice, but can be best inter-
preted on the view that the male is heterozygous
for a factor which is not found in the female. Such
a case is that recently described by Morgan in
America for the pomace fly (Drosophila ampelophila).
Normally this little insect has a red eye, but white-
eyed individuals are known to occur as rare sports.
Red eye is dominant to white. In their relation to
sex the eye colours of the pomace fly are inherited
IIr

112 MENDELISM CHAP.

on the same lines as the grossulariata and lacticolor
patterns of the currant moth, but with one essential
difference. The factor which repels the red-eye
factor is in this case to be found in the male,
and here consequently it is the male which must
be regarded as heterozygous for a sex factor that
is lacking in the female.

In order to bring these cases and others into
line an interesting suggestion has recently been
put forward by Bateson. On this suggestion each
sex is heterozygous for its own sex factor only, and
does not contain the factor proper to the opposite

sex. The male is of the

Matt Bana constitution Mimff and the
gives gives

gametes gametes female Ffmm. Each sex
pay pied

Mf ——> fn (peas produces two sorts of
mf ———><Frd } fertiluations gametes, JZ and my in
as pean 5 eee the case of the male, and
Fim, fm in that of the

female. But on this view a further supposition is
necessary. If each of the two kinds of spermatozoa
were capable of fertilising each of the two kinds of
ova, we should get individuals of the constitution
MmFf and mmff, as well as the normal males and
females, Wmff and Ffmm. As the facts of ordinary
bisexual reproduction afford us no grounds for
assuming the existence of these two classes of indi-
viduals, whatever they may be, we must suppose that
fertilisation is productive only between the sperma-
tozoa carrying MM and the ova without F/, or between
the spermatozoa without 47 and the ova containing
F. In other words, we must on this view suppose that
fertilisations between certain forms of gametes, even

XI SEX 113

if they can occur, are incapable of giving rise to
zygotes with the capacity for further development.
If we admit this supposition, the scheme just
given will cover such cases as those of the currant
moth and the fowl, equally as well as that of the
pomace fly. In the moth there is repulsion between
either the grossulariata factor.and F, in the fowl
between the pigment inhibitor factor and F in the
female, while in the fly there is repulsion between
the factor for red eye and M in the male!
Whatever the merits or demerits of such a
scheme it certainly does offer an explanation of a
peculiar form of sex limited gx?
inheritance in man. It has
long been a matter of COMMON ig 56:3 @ x
knowledge that colour-blind-
ness is much more common 5}? 6€h62Q
among men than among Tie oe
women,and also that unaffected Scheme to illustrate the probable
women can transmit it to their Plime The dalcaete ne
sons. At first sight the case, Bers, ateded, individuals,
is not unlike that of the WP tSpetic of aacsmieng ie
sheep, where the horned char- — ndition to her sons.
acter is apparently dominant in the male but recessive
in the female. The hypothesis that the colour-blind

1 Still more recently Doncaster (Journal of Genetics, 1911) has
proposed that the female should be regarded as A/A7/Z and the male
as //Mm, each sex being heterozygous for its own sex factor, while
the female is also homozygous for that of the male. The view has
much to commend it, for it offers a basis of explanation for the not
infrequent assumption of male characters by females which are old or
suffering from ovarial disease. In such cases it may be supposed that
the factor F in some way loses its potency and allows the underlying
M factors to come into fuller play. In its general working the hypo-
thesis resembles that given in the text.

I

114 MENDELISM CHAP.

condition is due to the presence of an extra factor
as compared with the normal, and that a single dose
of it will produce colour-blindness in the male but
not in the female, will cover a good many of the
observed facts (cf. Fig. 26). Moreover, it serves to
explain the remarkable fact that a// the sons of colour-
blind women are also colour-blind. For a woman
cannot be colour-blind unless she is homozygous for
the colour-blind factor, in which case all her children
must get a single dose of it even if she marries a
normal male. And this is sufficient to produce
colour-blindness in the male though not in the female.

But there is one notable difference in this case as
compared with that of the sheep. When crossed
with pure hornless ewes the heterozygous horned
ram transmits the horned character to half his
male offspring (cf. p. 71). But the heterozygous
colour-blind man does not behave altogether like a
sheep, for he apparently does not transmit the colour-
blind condition to any of his male offspring. If,
however, we suppose that the colour-blind factor is
repelled by the factor for maleness, the amended
scheme will cover the observed facts. For, denoting
the colour-blind factor by .Y, the gametes produced
by the colour-blind male are of two sorts only, viz.
Mfx and mfX. If he marries a normal woman
(Ff/mmxx), the spermatozoa Mx unite with ova fur
to give normal males, while the spermatozoa mfX
unite with ova Fmzx to give females which are
heterozygous for the colour-blind factor. These
daughters are themselves normal, but transmit the
condition to about half their sons.

The attempt to discover a simple explanation of

XI SEX 115

the nature of sex has led us to assume that certain
combinations between gametes are incapable of
giving rise to zygotes which can develop further.
In the various cases hitherto considered there is no
reason to suppose that anything of the sort occurs,
or that the different gametes are otherwise than
completely fertile one with another. One peculiar
case, however, has been known for several years in
which some of the gametes are apparently incapable
of uniting to produce offspring. Yellow in the mouse
is dominant to agouti, but hitherto a homozygous
yellow has never been met with. The yellows from
families where only yellows and agoutis occur pro-
duce, when bred together, yellows and agoutis in
the ratio 2:1. If it were an ordinary Mendelian
case the ratio should be 3:1, and one out of every
three yellows so bred should be homozygous and
give only yellows when crossed with agouti. But
Cuénot and others have shown that a// of the yellows
are heterozygous, and when crossed with agoutis
give both yellows and agoutis. We are led, there-
fore, to suppose that an ovum carrying the yellow
factor is unproductive if fertilised by a spermatozoon
which also bears this factor. In this way alone does
it seem possible to explain the deficiency of
yellows and the absence of homozygous ones in the
families arising from the mating of yellows together.
At present, however, it remains the only definite
instance among animals in which we have grounds
for assuming that anything in the nature of unpro-
ductive fertilisation takes place.

1 For the most recent discussion of this peculiar case the reader is
referred to Professor Castle’s paper in Scdece, December 16, 1910.

116 MENDELISM CHAP.

If we turn from animals to plants we find a more
* complicated state of affairs. Generally speaking, the
higher plants are hermaphrodite, both ovules and
pollen grains occurring on the same flower. Some
plants, however, like most animals, are of separate
sexes, a single plant bearing only male or female
flowers. In other plants the separate flowers are
either male or female, though both are borne on the
same individual. In others, again, the conditions are
even more complex, for the same plant may bear
flowers of three kinds, viz. male, female, and herma-
phrodite. Or it may be that these three forms
occur in the same species but in different individuals
—female and hermaphrodites in one species ; males,
females, and hermaphrodites in another. One case,
however, must be mentioned as it suggests a possi-
bility which we have not hitherto encountered. In
the common English bryony (Bryonia dioica) the
sexes are separate, some plants having only male
and others only female flowers. In another Euro-
pean species, &. alba, both male and female flowers
occur on the same plant. Correns crossed these
two species reciprocally, and also fertilised B. dzozica
by its own male with the following results :—

dioica 9 x dioicagd gave 9 9 and 6 6
45 xalba ¢ 4, 92 @ only
alba @ xdioicad ,, 9 @ and 36.

The point of chief interest lies in the striking differ-
ence shown by the reciprocal crosses between dzozca
and alba. Males appear when alba is used as the
female parent but not when the female dozca is
crossed by male alba. It is possible to suggest

XI SEX 117

more than one scheme to. cover these facts, but we
may confine ourselves here to that which seems
most in accord with the general trend of other
cases. We will suppose that in adzozca femaleness
is dominant to maleness, and that the female is
heterozygous for this additional factor. In_ this
species, then, the female produces equal numbers of
ovules with and without the female factor, while
this factor is absent in all the pollen grains. Alba?
x dioicad gives the same result as dzorca? x
dioicad , and we must therefore suppose that alba
produces male and female ovules in equal numbers.
Albas x dioica? , however, gives nothing but females.
Unless, therefore, we assume that there is selective
fertilisation we must suppose that all the pollen
grains of alba carry the female factor—in other
words, that so far as the sex factors are concerned
there is a difference between the ovules and pollen
grains borne by the same plant. Unfortunately
further investigation of this case is rendered im-
possible owing to the complete sterility of the F,
plants.

That the possibility of a difference between the
ovules and pollen grains of the same individual must
be taken into account in future work there is
evidence from quite a different source. The double
stock is an old horticultural favourite, and for centuries
it has been known that of itself it sets no seed, but
must be raised from special strains of the single
variety. “You must understand withall,”’ wrote
John Parkinson of his gilloflowers,' “that those
plants that beare double flowers, doe beare no seed

1 Paradisus Terrestrts, London, 1629, p. 261.

118 MENDELISM CHAP.

at all . . . but the onely way to have double flowers
any yeare is to save the seedes of those plants of
this kinde that beare single flowers, for from that
scede will rise, some that will beare single, and some
double flowers.” With regard to the nature of these
double-throwing strains of singles, Miss Saunders
has recently brought out some interesting facts. She

Fic. 27.

Single and double stocks raised from the same single parent.

crossed the double-throwing singles with pure
singles belonging to strains in which doubles never
occur. The cross was made both ways, and in
both cases all the F’, plants were single. A distinc-
tion, however, appeared when a further generation
was raised from the F, plants. All the F, plants
from the pollen of the double throwing single behaved
like double throwing singles, but of the F, plants

XI SEX 119

from the ovules of the double throwers some behaved
as double throwers, and some as pure singles. We
are led to infer, therefore, that the ovules and pollen
grains of the double throwers, though both produced
by the same plant, differ in their relation to the
factor (or factors) for doubleness. Doubleness is
apparently carried by all the pollen grains of such
plants, but only by some of the ovules. Though
the nature of doubleness in stocks is not yet clearly

Single

—

Single Double

x oe
Pollen of Ovule of
pure single X Ovule Pollen X pure single

Single Single Single
Single Single Double Single Double
Single Single Double Single Double

understood, the facts discovered by Miss Saunders
suggest strongly that the ovules and pollen grains
of the same plant may differ in their transmitting
properties, probably owing to some process of
segregation in the growing plant which leads to an
unequal distribution of some or other factors to
the cells which give rise to the ovules as compared
with those from which the pollen grains eventually
spring. Whether this may turn out to be the true
account or not, the possibility must not be over-
looked in future work.

120 MENDELISM CHAP, XI

From all this it is clear enough that there is
much to be done before the problem of sex is solved
even so far as the biologist can ever expect to solve
it. The possibilities are many, and many a fresh
set of facts is needed before we can hope to decide
among them. Yet the occasional glimpses of clear-
cut and orderly phenomena, which Mendelian
spectacles have already enabled us to catch, offer a
fair hope that some day they may all be brought
into focus, and assigned their proper places in a
general scheme which shall embrace them ll.
Then, though not till then, will the problem of the
nature of sex pass from the hands of the biologist
into those of the physicist and the chemist.

CHAPTER XII
INTERMEDIATES

So far as we have gone we have found it possible to
express the various characters of animals and plants
in terms of definite factors which are carried by the
gametes, and are distributed according to a definite
scheme. Whatever may be the nature of these
factors it is possible for purposes of analysis to treat
them as indivisible entities which may or may
not be present in any given gamete. When the
factor is present it is present as a whole. The
visible properties developed by a zygote in the course
of its growth depend upon the nature and variety of
the factors carried in by the two gametes which
went to its making, and to a less degree upon
whether each factor was brought in by both gametes
or by one only. If the given factor is brought in by
one gamete only, the resulting heterozygote may be
.more or less intermediate between the homozygous
form with a double dose of the factor and the
homozygous form which is entirely destitute of the
factor.’ Cases in point are those of the primula
flowers and the Andalusian fowls. Nevertheless
these intermediates produce only pure gametes as is
121

122 MENDELISM CHAP,

shown by the fact that the pure parental types
appear in a certain proportion of their offspring.
In such cases as these there is but a single type of
intermediate, and the simple ratio in which this and
the two homozygous forms appear renders the
interpretation obvious. But the nature of the F,
generation may be much more complex and, where
we are dealing with factors which interact upon one
another, may even present the appearance of a series
of intermediate forms grading from the condition
found in one of the original parents to that which
occurred in the other. As an illustration we may
consider the cross between the Brown Leghorn and
Silky fowls which we have already dealt with in
connection with the inheritance of sex. The offspring
of a Silky hen mated with a Brown Leghorn are in
both sexes birds with but a trace of the Silky
pigmentation. But when such birds are bred
together they produce a generation consisting of
chicks as deeply pigmented as the original Silky
parent, chicks devoid of pigment like the Brown
Leghorn, and chicks in which the pigmentation
shows itself in a variety of intermediate stages.
Indeed from a hundred chicks bred in this way it
would be possible to pick out a number of indi-
viduals and arrange them in an apparently continuous
series of gradually increasing pigmentation, with the
completely unpigmented at one end and the most
deeply pigmented at the other. Nevertheless, the case
is one in which complete segregation of the different
factors takes place, and the apparently continuous
series of intermediates is the result of the interaction
of the different factors upon one another. The con-

XI INTERMEDIATES 123

stitution of the F,¢ is a ffPplz, and such a bird
produces in equal numbers the four sorts of gametes
SPI, fPi, fol, foi. The constitution of the F,@ in
this case is FfPplz. Owing to the repulsion between
F and / she produces the four kinds of gametes
FPi, Hpi, fPI, fol, and produces them in equal
numbers. The

result of bringing ree el ig
two such series of

gametes together ? iJ ? ?
is shown in Fig. |Fpi Fpi Fpi Fpi
28. Out of the |‘! i fpl fpi
sixteen types of : . y ?
zygote formed one | fPI {PI {PI {PI
(F/PPii)ishomo- |! | fPi_— | PT | ft
zygous for the pig- 3 é 3 3
mentation factor, | fpI fpI fpl fpl
and does not con- | ‘PI {Pi fpl fpi
tain the inhibitor é é rei 3
factor. Such a

Fic. 28.

bird is as
i t deeply Diagram to illustrate the nature and composition of
pigmented as the the F, generations arising from the cross of Silky

hen with Brown Leghorn cock.

pure Silky parent.

Two, again, contain a single dose of P in the absence
of 7. These are nearly as dark as the pure Silky.
Four zygotes are destitute of P though they may or
may notcontain /. These birds are completely devoid
of pigment like the Brown Leghorn. The remaining
nine zygotes show various combinations of the two
factors P and J/, being either PP/i, PPI/, Pell, or
Pplt, and in each of these cases the pigment is more
or less intense according to the constitution of the
bird. Thus a bird of the constitution PP

124 MENDELISM CHAP.

approaches in pigmentation a bird of the constitution
Ppit, while a bird of the constitution Pp// has but
little more pigment than the unpigmented bird. In
this way we have seven distinct grades of pigmenta-
tion, and the series is further complicated by the fact
that these various grades exhibit a rather different
amount of pigmentation according as they occur in a
male or a female bird, for, generally speaking, the
female of a given grade exhibits rather more pigment
than the corresponding male. The examination of
a number of birds bred in this way might quite well
suggest that in this case we were dealing with a
character which could break up, as it were, to give a
continuous series of intergrading forms between the
two extremes. With the constant handling of large
numbers it becomes possible to recognise most of
the different grades, though even so it is possible
to make mistakes. Nevertheless, as breeding tests
have amply shown, we are dealing with but two
interacting factors which segregate cleanly from one
another according to the strict Mendelian rule. The
approach to continuity in variation exhibited by the
F, generation depends upon the fact that these two
factors interact upon one another, and to different
degrees according as the zygote is for one or other
or both of them in a homozygous or a heterozygous
state. Moreover, certain of these intermediates will
breed true to an intermediate condition of the
pigmentation. A male of the constitution //PP/I
when bred with females of the constitution A/PP/7
will produce only males like itself and females like
the maternal parent. We have dealt with this case
in some detail, because the existence of families

XII INTERMEDIATES 125

showing a series of intermediate stages between two
characters has sometimes been brought forward in
opposition to the view that the characters of
organisms depend upon specific factors which are
transmitted according to the Mendelian rule. But,
as this case from poultry shows clearly, neither
the existence of such a continuous series of inter-
mediates, nor the fact that some of them may breed
true to the intermediate condition, are incompatible
with the Mendelian principle of segregation.

In connection with intermediates a more cogent
objection to the Mendelian view is the case of the
first cross between two definite varieties thence-
forward breeding true. The case that will naturally
occur to the reader is that of the mulatto, which
‘results from the cross between the negro and the
white. According to general opinion, these mulattos,
of intermediate pigmentation, continue to produce
mulattos. Unfortunately this interesting case has
never been critically investigated, and the statement
that the mulatto breeds true rests almost entirely
upon information that is general and often vague. It
may be that the inheritance of skin pigmentation
in this instance is a genuine exception to the normal
rule, but at the same time it must not be forgotten
that it may be one in which several interacting
factors are concerned, and that the pure white and
the pure black are the result of combinations which
from their rarity are apt to be overlooked. But
until we are in possession of accurate information it
is impossible to pronounce definitely upon the nature
of the inheritance in this case.

On the other hand, from the cross between the

126 MENDELISM CHAP.

darkly pigmented Eastern races and the white
segregation seems to occur in subsequent genera-
tions. Families are to be found in which one parent is
a pure white, while the other has arisen from the cross
between the dark and light in the first or some
subsequent generation. Such families may contain

é o Qxd a 3 ie
3 5x9 QS SSSELS? QP
| |
3 3 Several children
al © or O
FIG. 29.

Pedigree of a family which originated from a cross between a Hindu and a European.
Black signs denote individuals as dark as average Hindus. Plain signs denote
quite fair members, while those with a dot in the centre are intermediate. R

children indistinguishable from pure blonds as well
as children of very dark and of intermediate shades.
As an example, I may give the following pedigree,
which was kindly communicated to me by an Anglo-
Indian friend (Fig. 29). The family had resided in
England for several generations, so that in this case
there. was no question ofa further admixture of black.
Most noticeable is the family produced by a very dark

xu INTERMEDIATES 127

lady who had married a white man. Some of the
children were intermediate in colour, but two were
fair whites and two were dark as dark Hindus.
This sharp segregation or splitting out of blacks and
whites in addition to intermediates strongly suggests
that the nature of the inheritance is Mendelian,
though it may be complicated by the existence of
several factors which may also react upon one
another. Nor must it be forgotten that in so far as
these different factors are concerned the whites them-
selves may differ in constitution without showing
any trace of it in their appearance. Before the case
can be regarded as settled all these different
possibilities will have to be definitely tested. With
the dark Eastern races as with the negro we cannot
hope to come to any conclusion until we have
evidence collected by critical and competent
observers.

Though for the present we must regard the case
of the negro as not proven, there are nevertheless
two others in which the heredity would appear not
to follow the Mendelian rule. Castle in America
crossed the lop-eared rabbit with the normal form,
and found that the F, animals were intermediate
with respect to their ears. And subsequent experi-
ment showed that, on the whole, they bred true to
this intermediate condition. The other case relates
to Lepidoptera. The speckled wood butterfly
(Pararge egerta) has a southern form which differs
from the northern one in the greater brightness and
depth of its yellow-brown markings. The northern
form is generally distinguished as var. egerzades.
Bateson crossed the southern form from the south

128 MENDELISM CHAP.

of France with the paler British form, and found
that the offspring were more or less intermediate in
colour, and that in subsequent generations the parental
types did not recur. These cases at present stand
alone. It is possible that further research may
reveal complications which mask or interfere with
an underlying process of segregation. Or it may
be that segregation does not occur owing to some
definite physiological reason which at present we do
not understand. ;

And here it is impossible not to recall Mendel’s
own experiences with the Hawkweeds (Hzeracium).
This genus of plants exhibits an extraordinary pro-
fusion of forms differing from one another sometimes
in a single feature, sometimes in several. The
question as to how far these numerous forms were
to be classified as distinct species, how far as varieties,
and how far as products of chance hybridisation, was
even at that time a source of keen controversy among
botanists. There is little doubt that Mendel under-
took his experiments on the Hawkweeds in the hope
that the conception of unit-characters so brilliantly
demonstrated for the pea would serve to explain the
great profusion of forms among the Hieraciums.
Owing to the minute size of their florets, these
plants offer very considerable technical difficulties
in the way of cross-fertilisation. By dint of great
perseverance and labour, however, Mendel succeeded
in obtaining a few crosses between different forms.
These hybrids were reared and a further generation
produced from them, and, no doubt somewhat to
Mendel’s chagrin, every one of them proved to breed
true. There was a complete absence of that segrega-

Xu INTERMEDIATES 129

tion of characters which he had shown to exist in
peas and beans, and had probably looked forward
with some confidence to finding in Hzeracium. More
than thirty years passed before the matter was
cleared up. To-day we know that the peculiar
behaviour of the hybrid Hieraciums is due to the
fact that they normally produce seed by a peculiar
process of parthenogenesis. It is possible to take
an unopened flower and to shear off with a razor all
the male organs together with the stigmata through
which the pollen reaches the ovules. The flower,
nevertheless, sets perfectly good seed. But the cells
from which the seeds develop are not of the same
nature as the normal ova of a plant. They are
not gametes, but retain the double structure of the
maternal cells. They are rather to be regarded as
of the nature of buds which early become detached
from the parent stock to lead an independent exist-
ence, and, like buds, they reproduce exactly the
maternal characteristics. The discovery of the true
nature of this case was only rendered possible by
the development of the study of cytology, and it
was not given to Mendel to live long enough to learn
why his hybrid Hieraciums all bred true.

CHAPTER XIII
VARIATION AND EVOLUTION

THROUGH the facts of heredity we have reached a
new conception of the individual. Hitherto we have
been accustomed to distinguish between the members
of a family of rabbits like that illustrated on Plate
I. by assigning to each an individuality, and by
making use of certain external features, such as the
coat colour or the markings, as convenient outward
signs to express our idea that the individuality of these
different animals is different. Apart from this, our
notions as to what constituted the individuality in
each case were at best but vague. Mendelian analysis
has placed in our hands a more precise method of
estimating and expressing the variations that are
to be found between one individual and another.
Instead of looking at the individual as a whole,
which is in some vague way endowed with an indi-
viduality marking it off from its fellows, we now
regard it as an organism built up of definite char-
acters superimposed on a basis beyond which for the
moment our analysis will not take us. We have
begun to realise that each individual has a definite
architecture, and that this architecture depends
130

cu.xnr1 VARIATION AND EVOLUTION 131

primarily upon the number and variety of the
factors that existed in the two gametes that went
to its building. Now most species exhibit consider-
able variation and exist in a number, often very
large, of more or less well-defined varieties. How
far can this great variety be explained in terms
of a comparatively small number of factors if the
number of possible forms depends upon the number
of the factors which may be present or absent ?

In the simple case where the homozygous and
heterozygous conditions are indistinguishable in
appearance the number of possible forms is 2,
raised to the power of the number of factors con-
cerned. Thus where one factor is concerned there
are only 2'= 2 possible forms, where ten factors are
concerned there are 2‘°= 1024 possible forms differ-
ing from one another in at most ten and at least one
character. Where the factors interact upon one
another this number will, of course, be considerably
increased. If the heterozygous form is different in
appearance from the homozygous form, there are
three possible forms connected with each factor ;
for ten such factors the possible number of indi-
viduals would be 3°=59,049; for twenty such
factors the possible number of different individuals
would be 37°= 3,486,784,401. The presence or
absence of a comparatively small number of factors
in a species carries with it the possibility of an
enormous range of individual variation. But every
one of these individuals has a perfectly definite con-
stitution which can be determined in each case by
the ordinary methods of Mendelian analysis. For
in every instance the variation depends upon the

132 MENDELISM CHAP.

presence or absence of definite factors carried in by
the gametes from whose union the individual results.
And as these factors separate out cleanly in the
gametes which the individual forms, such variations
as depend upon them are transmitted strictly accord-
ing to the Mendelian scheme. Provided that the
constitution of the gametes is unchanged, the heredity
of such variation is independent of any change in
the conditions of nutrition or environment which may
operate upon the individual producing the gametes.

But, as everybody knows, an individual organism,
whether plant or animal, reacts, and often reacts
markedly, to the environmental conditions under
which its life is passed. More especially is this
to be seen where such characters as size or weight
are concerned. More sunlight or a richer soil may
mean stronger growth in a plant, better nutrition
may result in a finer animal, superior education may
lead to a more intelligent man. But although the
changed conditions produce a direct effect upon the
individual, we have no indisputable evidence that
such alterations are connected with alterations in
the nature of the gametes which the individual pro-
duces. And without this such variations cannot be
perpetuated through heredity, but the conditions
which produce the effect must always be renewed
in each successive generation. We are led, there-
fore, to the conclusion that two sorts of variations
exist, those which are due to the presence of specific
factors in the organism and those which are due to
the direct effect of the environment during its life-
time. The former are known as mutations, and are
inherited according to the Mendelian scheme; the

xm VARIATION AND EVOLUTION 133

latter have been termed fluctuations, and at present
we have no valid reason for supposing that they are
ever inherited. For though instances may be found
in which effects produced during the lifetime of the
individual would appear to affect the offspring, this
is not necessarily due to heredity. Thus plants
which are poorly nourished and grown under adverse
conditions may set seed from which come plants
that are smaller than the normal although grown
under most favourable conditions. It is natural
to attribute the smaller size of the offspring to the
conditions under which the parents were grown, and
there is no doubt that we should be quite right in
doing so. Nevertheless, it need have nothing to do
with heredity. As we have already pointed out, the
seed is a larval plant which draws its nourishment
from the mother. The size of the offspring is
affected because the poorly nourished parent offered
a bad environment to the young plant, and not
because the gametes of the parent were changed
through the adverse conditions under which it grew.
The parent in this case is not only the producer of
gametes, but also a part of the environment of the
young plant, and it is in this latter capacity that it
affects its offspring. Wherever, as in plants and
mammals, the organism is parasitic upon the mother
during its earlier stages the state of nutrition of the
latter will almost certainly react upon it, and in this
way a semblance of transmitted weakness or vigour
is brought about. Such a connection between mother
and offspring is purely one of environment, and it
cannot be too strongly emphasised that it has nothing
to do with the ordinary process of heredity.

134 MENDELISM CHAP.

The distinction between these two kinds of varia-
tion, so entirely different in their causation, renders
it possible to obtain a clearer view of the process of
evolution than that recently prevalent. As Darwin
long ago realised, any theory of evolution must be
based upon the facts of heredity and variation.
Evolution only comes about through the survival
of certain variations and the elimination of others.
But to be of any moment in evolutionary change
a variation must be inherited. And to be inherited
it must be represented in the gametes. This, as we
have seen, is the case for those variations which we
have termed mutations. For the inheritance of
fluctuations, on the other hand, of the variations
which result from the direct action of the environ-
ment upon the individual, there is no indisputable
evidence. Consequently we have no reason for
regarding them as playing any part in the pro-
duction of that succession of temporarily stable forms
which we term evolution. In the light of our present
knowledge we must regard the mutation as the basis
of evolution—as the material upon which natural
selection works. For it is the only form of variation
of whose heredity we have any certain knowledge.

It is evident that this view of the process of
evolution is in some respects at variance with that
generally held during the past half century. There
we were given the conception of an abstract type
representing the species, and from it most of the
individuals diverged in various directions, though,
generally speaking, only to a very small extent. It
was assumed that any variation, however small,
might have a selection value, that is to say, could be

xu1 VARIATION AND EVOLUTION 135

transmitted to the offspring. Some of these would
possess it in a less and some ina greater degree
than the parent. If the variation were a useful one,
those possessing to a rather greater extent would be
favoured through the action of natural selection at
the expense of their less fortunate brethren, and
would leave a greater number of offspring, of whom
some possessed it in an even more marked degree
than themselves. And so it would go on. The
process was acumulative one. The slightest variation
in a favourable direction gave natural selection a
starting-point to work on. Through the continued
action of natural selection on each successive genera-
tion the useful variation was gradually worked up,
until at last it reached the magnitude of a specific
distinction. Were it possible in such a case to
have all the forms before us, they would present the
appearance of a long series imperceptibly grading
from one extreme to the other.

Upon this view are made two assumptions not
unnatural in the absence of any exact knowledge of
the nature of heredity and variation. It was assumed,
in the first place, that variation was a continuous
process, and, second, that any variation could be
transmitted to the offspring. Both of these assump-
tions have since been shown to be unjustified. Even
before Mendel’s work became known Bateson had
begun to call attention to the prevalence of dis-
continuity in variation, and a few years later this
was emphasised by the Dutch botanist Hugo de
Vries in his great work on The Mutation Theory.
The ferment of new ideas was already working in
the solution, and under the stimulus of Mendel’s

136 MENDELISM CHAP,

work they have rapidly crystallised out. With the
advent of heredity as a definite science we have
been led to revise our views as to the nature of
variation, and consequently in some respects as to
the trend of evolution. Heritable variation has a
definite basis in the gamete, and it is to the gamete,
therefore, not to the individual, that we must look
for the initiation of this process. Somewhere or
other in the course of their production is added or
removed the factor upon whose removal or addition
the new variation owes its existence. The new
variation springs into being by a sudden step, not
by a process of gradual and almost imperceptible
augmentation. It is not continuous but discon-
tinuous because it is based upon the presence or
absence of some definite factor or factors—upon
discontinuity in the gametes from which it sprang.
Once formed, its continued existence is subject to
the arbitrament of natural selection. If of value in
the struggle for existence natural selection will
decide that those who possess it shall have a better
chance of survival and of leaving offspring than
those who do not possess it. If it is harmful to
the individual natural selection will soon bring about
its elimination. But if the new variation is neither
harmful nor useful there seems no reason why it
should not persist.

In this way we avoid a difficulty which beset the
older view. For on that view no new character
could be developed except by the piling up of
minute variations through the action of natural
selection. Consequently any character found in
animals and plants must be supposed to be of

xur VARIATION AND EVOLUTION — 137

some definite use to the individual. Otherwise it
could not have developed through the action of
natural selection. But there are plenty of characters
to which it is exceedingly difficult to ascribe any
utility, and the ingenuity of the supporters of this
view has often been severely taxed to account for
their existence. On the more modern view this
difficulty is avoided. The origin of a new variation
is independent of natural selection, and’ provided
that it is not directly harmful there is no reason
why it should not persist. In this way we are
released from the burden of discovering a utilitarian
motive behind all the multitudinous characters of
living organisms. For we now recognise that the
function of natural selection is selection and not
creation. It has nothing to do with the formation
of the new variation. It merely decides whether it
is to survive or to be eliminated.

One of the arguments made use of by supporters
of the older view is that drawn from the study of
adaptation. Animals and plants are as a rule re-
markably well adapted to living the life which their
surroundings impose upon them, and in some cases
this adaptation is exceedingly striking. Especially
is this so in the many instances of what is called
protective coloration, where the animal comes to
resemble its surroundings so closely that it may
reasonably be supposed to cheat even the keenest
sighted enemy. Surely, we are told, such perfect
adaptation could hardly have arisen through the
mere survival of chance sports. Surely there must
be some guiding hand moulding the species into the
required shape. The argument is an old one. For

138 MENDELISM CHAP.

John Ray that guiding hand was the superior
wisdom of the Creator: for the modern Darwinian
it is Natural Selection controlling the direction of
variation. Mendelism certainly offers no suggestion
of any such controlling force. It interprets the
variations of living forms in terms of definite physio-
logical factors, and the diversity of animal and plant
life is due to the gain or loss of these factors, to the
origination of new ones, or to fresh combinations
among those already in existence. Nor is there any
valid reason against the supposition that even the
most remarkable cases of resemblance, such as that
of the leaf insect, may have arisen through a process
of mutation. Experience with domestic plants and
animals shows that the most bizarre forms may arise
as sports and perpetuate themselves. Were such
forms, arising under natural conditions, to be favoured
by natural selection owing to a resemblance to some-
thing in their environment we should obtain a striking
case of protective adaptation. And here it must not
be forgotten that those striking cases to which our
attention is generally called are but a very small
minority of the existing forms of life.

For that special group of adaptation phenomena
classed under the head of Mimicry, Mendelism seems
to offer an interpretation simpler than that at present
in vogue. This perhaps may be more clearly ex-
pressed by taking a specific case. There is in Africa
a genus of Danaine butterflies known as Amaurzs,
and there are reasons for considering that the group
to which it belongs possesses properties which render
it unpalatable to vertebrate enemies such as birds or
monkeys. In the same region is also found the

PLATE VI.

iBraqryem eByesng

SnUBdIUTULOp STUNBULY

Risayda SlINRWy

xmt VARIATION AND EVOLUTION 139

genus Euralia belonging to the entirely different
family of the Nymphalidae, to which there is no
evidence for assigning the disagreeable properties of
the Danaines. Now the different species of Euralia
show remarkably close resemblances to the species
of Amaurzs, which are found flying in the same
region, and it is supposed that by “ mimicking” the
unpalatable forms they impose upon their enemies
and thereby acquire immunity from attack. The
point at issue is the way in which this seemingly
purposeful resemblance has been brought about.

One of the species of Euralia occurs in two very
distinct forms (Pl. VI.) which were previously re-
garded as separate species under the names &.
wahlbergt and E. mima. These two forms respec-
tively resemble Amauris dominicanus and A. echeria.
For purposes of argument we will assume that one of
these forms has been derived from the other and that
A. dominicanus is the more recent form of the two.
On the modern Darwinian view certain individuals
of A. echeria gradually diverged from the echerza type
and eventually reached the domznicanus type, though
why this should have happened does not appear to
be clear. At the same time those specimens of the
Euralia which tended to vary in the direction of A.
dominicanus in places where this species was more
abundant than A. echerza were encouraged by natural
selection, and under its guiding hand the form wahl-
bergé eventually arose from mma.

According to Mendelian views, on the other hand,
A. dominicanus arose suddenly from A. echeria (or vice
versa), and similarly wahlbergi arose suddenly from
mima. If wahlberg? occurred where A. dominicanus

140 MENDELISM CHAP.

was common and A, echerda was rare, its resemblance
to the more plentiful distasteful form would give it
the advantage over mzma and allow it to establish
itself in place of the latter. On the modern Dar-
winian view natural selection gradually shapes mzma
into the wahklbergd form owing to the presence of A.
dominicanus ; on the Mendelian view natural selec-
tion merely conserves the wahlberg¢ form when once
it has arisen. Now this case of mimicry is one
of especial interest, because we have experimental
evidence that the relation between mzma and
wahlbergi is a simple Mendelian one, mzma here
being the dominant and wahlbergd the recessive
form. The two have been proved to occur in
families bred from the same female without the
occurrence of any intermediates, and the fact that
the two segregate cleanly is strong evidence in
favour of the Mendelian view. On this view the
genera Amauris and Euralza contain a similar set
of pattern factors, and the conditions, whatever they
may be, which bring about mutation in the former
lead to the production of a similar mutation in the
latter. Of the different forms of Euvala produced
in any region that one has the best chance of
survival, through the operation of natural selection,
which resembles the most plentiful Amaurzs form.
Mimetic resemblance is a true phenomenon, but
natural selection plays the part of a conservative,
not of a formative agent.

It is interesting to recall that in earlier years
Darwin was inclined to ascribe more importance to
“sports” as opposed to continuous minute variation,
and to consider that they might play a not incon-

xr VARIATION AND EVOLUTION § 141

siderable part in the formation of new varieties in
nature. This view, however, he gave up later, be-
cause he thought that the relatively rare sport or
mutation would rapidly disappear through the
swamping effects of crossing with the more abund-
ant normal form, and so, even though favoured by
natural selection, would never succeed in establishing
itself. Mendel’s discovery has eliminated this diffi-
culty. For suppose that the sport differed from the
normal in the loss of a factor and were recessive.
When mated with the normal this character would
seem to disappear, though, of course, half of the
gametes of its progeny would bear it. By continual
crossing with normals a small proportion of hetero-
zygotes would eventually be scattered among the
population, and as soon as any two of these mated
together the recessive sport would appear in one
quarter of their offspring.

A suggestive contribution to this subject was
recently made by G. H. Hardy. Considering the
distribution of a single factor in a mixed population
consisting of the heterozygous and the two
homozygous forms he showed that such a population
breeding at random rapidly fell into a stable con-
dition with regard to the proportion of these three
forms, whatever may have been the proportion of
the three forms to start with. Let us suppose, for
instance, that the population consists of homozygotes
of one kind, y homozygotes of the other kind, and 2g
heterozygotes. Hardy pointed out that, other
things being equal, such a population would be in
equilibrium for this particular factor so long as the
condition g’=gr was fulfilled. If the condition is

142 MENDELISM CHAP.

fulfilled to start with the population remains in
equilibrium. If the condition is not fulfilled to start
with, Hardy showed that a position of equilibrium
becomes established after a single generation, and
that this position is thereafter maintained. The
proportions of the three classes which satisfy the
equation g*=yr are exceedingly numerous, and
populations in which they existed in the proportions
shown in the appended table would remain in stable
equilibrium generation after generation :—

p. 29. Te
I 2 I
I 4 4
I 6 9
I 8 16
I 20,000 100,000,000
I 20 ne

This, of course, assumes that all three classes are
equally fertile, and that no form of selection is
taking place to the benefit of one class more than
of another. Moreover, it makes no difference
whether / represents the homozygous dominants or
whether it stands for the recessives. A population
containing a very small proportion of dominants
and one containing a similar proportion of recessives
are equally stable. The term dominant is in some
respects apt to be misleading, for a dominant
character cannot in virtue of its dominance establish
itself at the expense of a recessive one. Brown
eyes in man are dominant to blue, but there is no
reason to suppose that as years go on the population
of these islands will become increasingly brown eyed.
Given equality of conditions both are on an equal

xur =VARIATION AND EVOLUTION 143

footing. If, however, either dominant or recessive
be favoured by selection the conditions are altered,
and it can be shown that even a small advantage
possessed by the one will rapidly lead to the
elimination of the other. Even with but a 5 per
cent selection advantage in its favour it can be shown
that a rare sport will oust the normal form in a few
hundred generations. In this way we are freed
from a difficulty inherent in the older view that
varieties arose through a long-continued process
involving the accumulation of very slight variations.
On that view the establishing of a new type was of
necessity a very long and tedious business involving
many thousands of generations. For this reason
the biologist has been accustomed to demand a very
large supply of time, often a great deal more than
the physicist is disposed to grant, and this has
sometimes led him to expostulate with the latter for
cutting off the supply. On the newer views, however,
this difficulty need not arise, for we realise that the
origin and establishing of a new form may be a very
much more rapid process than has hitherto been
deemed possible.

One last question with regard to evolution. How
far does Mendelism help us in connection with the
problem of the origin of species? Among the plants
and animals with which we have dealt we have been
able to show that distinct differences, often con-
siderable, in colour, size, and structure, may be
interpreted in terms of Mendelian factors. It is not
unlikely that most of the various characters which
the systematist uses to mark off one species from
another, the so-called specific characters, are of this

144 MENDELISM CHAP.

nature. They serve as convenient labels, but are
not essential to the conception of species. A
systematist who defined the wild sweet pea could
hardly fail to include in his definition such characters
as the procumbent habit, the tendrils, the form of
the pollen, the shape of the flower, and its purple
colour. Yet all these and other characters have
been proved to depend upon the presence of definite
factors which can be removed by appropriate cross-
ing. By this means we can produce a small plant
a few inches in height with an erect habit of growth,
without tendrils, with round instead of oblong pollen,
and with colourless deformed flowers quite different
in appearance from those of the wild form. Such a
plant would breed perfectly true, and a botanist to
whom it was presented, if ignorant of its origin,
might easily relegate it to a different genus. Never-
theless, though so widely divergent in structure,
such a plant must yet be regarded as belonging to the
species Lathyrus odoratus. For it still remains fertile
with the many different varieties of sweet-pea. It
is not visible attributes that constitute the essential
difference between one species and another. The
essential difference, whatever it may be, is that
underlying the phenomenon of sterility. The visible
attributes are those made use of by the’ systematist
in cataloguing the different forms of animal and
plant life, for he has no other choice. But it must
not be forgotten that they are often misleading.
Until they were bred together Euralia wahlbergi
and £. mima were regarded as perfectly valid species,
and there is little doubt that numbers of recognised
species will eventually fall to the ground in the same

xt VARIATION AND EVOLUTION § 145

way as soon as we are in a position to apply the
test of breeding. Mendelism has helped us to
realise that specific characters may be but incidental
to a species—that the true criterion of what con-
stitutes a species is sterility, and that particular form
of sterility which prevents two healthy gametes on
uniting from producing a zygote with normal powers
of growth and reproduction. For there are forms of
sterility which are purely mechanical. The pollen
of Mirabilis jalapa cannot fertilise M. longifiora,
because the pollen tubes of the former are not long
enough to penetrate down to the ovules of the latter.
Hybrids can nevertheless be obtained from the
reciprocal cross. Nor should we expect offspring
from a St. Bernard and a toy terrier without recourse
to artificial fertilisation. Or sterility may be due to
pathological causes which prevent the gametes from
meeting one another in a healthy state. But in
most cases it is probable that the sterility is due to
some other cause. It is not inconceivable that
definite differences in chemical composition render
the protoplasm of one species toxic to the gametes
of the other, and if this is so it is not impossible
that we may some day be able to express these
differences in terms of Mendelian factors. The very
nature of the case makes it one of extreme difficulty
for experimental investigation. At any rate, we
realise more clearly than before that the problem of
species is not one that can be resolved by the study
of morphology or of systematics. It is a problem
in physiology.

CHAPTER XIV
ECONOMICAL

SINCE heredity lies at the basis of the breeder’s
work, it is evident that any contribution to a more
exact knowledge of this subject must prove of service
to him, and there is no doubt that he will be able
to profit by Mendelian knowledge in the conduct of
his operations. Indeed, as we shall see later, these
ideas have already led to striking results in the
raising of new and more profitable varieties. In
the first place, heredity is a question of individuals.
Identity of appearance is no sure guide to repro-
ductive qualities. Two individuals similarly bred
and indistinguishable in outward form may never-
theless behave entirely differently when bred from.
Take, for instance, the family of sweet - peas
shown on Plate 1V. The F, generation here con-
sists of seven distinct types, three sorts of purples,
three sorts of reds, and whites. Let us suppose that
our object is to obtain, a true breeding strain of the
pale purple picotee form. Now from the pro-
portions in which they come we know that the
dilute colour is due to the absence of the factor
which intensifies the colour. Consequently the
146

CHAP. XIV ECONOMICAL 147

picotee cannot throw the two deeper shades of red
or purple. But it may be heterozygous for the
purpling factor, when it will throw the dilute red
(Tinged white), or it may be heterozygous for either
or both of the two colour factors (cf. p. 41), in
which case it will throw whites. Of the picotees
which come in such a family, therefore, some will
give picotees, tinged whites, and whites, others will
give picotees and tinged whites only, others will give
picotees and whites only, while others, again, and
these the least numerous, will give nothing but
picotees. The new variety is already fixed in a
certain definite proportion of the plants; in this
particular instance in 1 out of every 27. All that
remains to be done is to pick out these plants.
Since all the picotees look alike, whatever their
breeding capacity, the only way to do this is to save
the seed from a number of such plants zxdividually,
and to raise a further generation. Some of them
will be found to breed true. The variety is then
established, and may at once be put on the market
with full confidence that it will hereafter throw none
of the other forms. The all-important thing is to
save and sow the seed of separate individuals
separately. However alike they look, the seed from
different individuals must on no account be mixed.
Provided that due care is taken in this respect no
long and tedious process of selection is required for
the fixation of any given variety. Every possible
variety arising from a cross appears in the F,
generation if only a sufficient number is raised, and
of all these different varieties a certain proportion of
each is already fixed. Heredity is a question of

148 MENDELISM CHAP.

individuals, and the recognition of this will save the
breeder much labour, and enable him to fix his
varieties in the shortest possible time.

Such cases as these of the sweet-pea throw a
fresh light upon another of the breeder’s conceptions,
that of purity of type. Hitherto the criterion of a
“pure-bred” thing, whether plant or animal, has
been its pedigree, and the individual was regarded
as more or less pure bred for a given quality accord-
ing as it could show a longer or shorter list of
ancestors possessing this quality. To-day we realise
that this is not essential. The pure-bred picotee
appears in our F, family though its parent was a
purple bicolor, and its remoter ancestors whites for
generations. So also from the cross between pure
strains of black and albino rabbits we may obtain in
the F, generation animals of the wild agouti colour
which breed as true to type as the pure wild rabbit
of irreproachable pedigree. The true test of the
pure breeding thing lies not in its ancestry but in
the nature of the gametes which have gone to its
making. Whenever two similarly constituted gametes
unite, whatever the nature of the parents from which
they arose, the resulting individual is homozygous
in all respects and must consequently breed true.
In deciding questions of purity it is to the gamete,
and not to ancestry, that our appeal must hence-
forth be made.

Improvement is after all the keynote to the
breeder’s operations. He is aiming at the produc-
tion of a strain which shall combine the greatest
number of desirable properties with the least number
of undesirable ones. This good quality he must

XIV ECONOMICAL 149

take from one strain, that from another, and that,
again, from a third, while at the same time avoiding
all’ the poor qualities that these different strains
possess. It is evident that the Mendelian concep-
tion of characters based upon definite factors which
are transmitted on a definite scheme must prove of
the greatest service to him. For once these factors
have been determined their distribution is brought
under control, and they can be associated together
or dissociated at the breeder’s will. The chief
labour involved is that necessary for the determina-
tion of the factors upon which the various characters
depend. For it often happens that what appears to
be a simple character turns out when analysed to
depend upon the simultaneous presence of several
distinct factors. Thus the Malay fowl breeds true
to the walnut comb, as does also the Leghorn to the
single comb, and when pure strains are crossed all
the offspring have walnut combs. At first sight it
would be not unnatural to regard the difference as
dependent upon the presence or absence of a single
factor. Yet, as we have already seen,.:two other
types of comb, the pea and the rose, make their
appearance in the F, generation. Analysis shows
that the difference between the walnut and the single
is a difference of two factors, and it is not until this
has been determined that we can proceed with
certainty to transfer the walnut character to a single-
combed breed. Moreover, in his process of analysis
the breeder. must be prepared to’ encounter the
various phenomena that we have described under
the headings of interaction of factors, coupling, and
repulsion, and the recognition of these phenomena

150 MENDELISM CHAP.

will naturally influence his procedure. Or, again,
his experiments may show him that one of the
characters he wants, like the blue of the Andalusian
fowl, is dependent upon the heterozygous nature of
the individual which exhibits it, and if such is the
case he will be wise to refrain from any futile
attempt at fixing it. If it is essential it must be
built up again in each generation, and he will
recognise that the most economical way of doing
this is to cross the two pure strains so that all the
offspring may possess the desired character. The
labour of analysis is often an intricate and tedious
business. But once done it is done once for all.
As soon as the various factors are determined upon
which the various characters of the individual
depend, as soon as the material to be made use
of has been properly analysed, the production and
fixation of the required combinations becomes a
matter of simple detail.

An excellent example of the practical application
of Mendelian principles is afforded by the experi-
ments which Professor Biffen has recently carried
out in Cambridge. Taken as a whole English
wheats compare favourably with foreign ones in
respect of their cropping power. On the other
hand, they have two serious defects. They are liable
to suffer from the attacks of the fungus which causes
rust, and they do not bake into a good loaf.
This last property depends upon the amount of
gluten present, and it is the greater proportion of
this which gives to the “hard” foreign wheat its
quality of causing the loaf to rise well when baked.
For some time it was held that “hard” wheat with

x1v ECONOMICAL 181

a high glutinous content could not be grown in
the English climate, and undoubtedly most of the
hard varieties imported for trial deteriorated greatly
in a very short time. Professor Biffen managed to
obtain a hard wheat which kept its qualities when
grown in England. But in spite of the superior
quality of its grain from the baker’s point of view,
its cropping capacity was too low for it to be grown
profitably in competition with English wheats.
Like the latter, it was also subject to rust. Among
the many varieties which Professor Biffen collected
and grew for observation he managed to find one
which was completely immune to the attacks of the
rust fungus, though in other respects it had no
desirable quality to recommend it. Now as the
result of an elaborate series of investigations, he was
able to show that the qualities of heavy cropping
capacity, “hardness” of grain, and immunity to rust
can all be expressed in terms of Mendelian factors.
Having once analysed his material the rest was
comparatively simple, and in a few years he has
been able to build up a strain of wheat which com-
bines the cropping capacity of the best English
varieties with the hardness of the foreign kinds, and
at the same time is completely immune to rust.
This wheat has already been shown to keep its
qualities unchanged for several years, and there is
little doubt that when it comes to be grown in
quantity it will exert an appreciable influence on
wheat-growing in Great Britain.

It may be objected that it is often with small
differences rather than with the larger and more
striking ones that the breeder is mainly concerned.

152 MENDELISM CHAP.

It does not matter much to him whether the colour
of a pea flower is purple or pink or white. But it
does matter whether the plant bears rather larger
seeds than usual, or rather more of them. Even a
small difference when multiplied by the size of the
crop will effect a considerable difference in the profit.
It is the general experience of seedsmen and others
that differences of this nature are often capable of
being developed up to a certain point by a process

3 400 a SS

o 7 S.

8 Var IN

00

33 V7 7 \ .

la | / \ \

® 200 7 v

a } \

5 1 N

yz, 100 fr ‘ a
N

6 8 10 12 14 16 18 20
Weight of individual seeds

FIG. 30.

Curves to illustrate the influence of selection.

of careful selection each generation. At first sight
this appears to be something very like the gradual
accumulation of minute variations through the con-
tinuous application of a selective process. Some
recent experiments by Professor Johannsen of Copen-
hagen set the matter in a different light. One of
his investigations deals with the inheritance of the
weight of beans, but as an account of these experi-
ments would involve us in the consideration of a
large amount of detail we may take a simple
imaginary case to illustrate the nature of the con-

XIV ECONOMICAL 153

clusions at which he arrived. If we weigh a
number of seeds collected from a patch of plants
such as Johannsen’s beans we should find that they
varied considerably in size. The majority would
probably not diverge very greatly from the general
average, and as we approached the high or low
extreme we should find a constantly decreasing
number of individuals with these weights. Let us
suppose that the weight of our seed varied between
4 and 20 grains, that the greatest number of
seeds were of the mean weight, viz. 12 grains,
and that as we passed to either extreme at 4
and 20 the number became regularly less. The
weight relation of such a collection of seeds can be
expressed by the accompanying curve (Fig. 30).
Now if we select for sowing only that seed which
weighs over 12 grains, we shall find that in the
next generation the average weight of the seed is
‘raised and the curve becomes somewhat shifted to
the right as in the dotted line of Fig. 30. By con-
tinually selecting we can shift our curve a little more
to the right, ze. we can increase the average weight
of the seeds until at last we come to a limit be-
yond which further selection has no effect. This
phenomenon has been long known, and it was
customary to regard these variations as of a con-
tinuous nature, ze. as all chance fluctuations in a
homogeneous mass, and the effect of selection was
supposed to afford evidence that small continuous
variations could be increased by this process. But
Johannsen’s results point to another interpretation.
Instead of our material being homogeneous it is
probably a mixture of several strains each with its

154 MENDELISM CHAP.

own average weight about which the varying con-
ditions of the environment cause it to fluctuate.
Each of these strains is termed a pure line. If we
imagine that there are three such pure lines in our
imaginary case, with average weights 10, 12, 14
grains respectively, and if the range of fluctuation
of each of these pure lines is 12 grains, then our

3
i=}

w
{=}
i=)

Number of seeds

”
°
°

=

{

H

4 6 8 10 12 14 16 18 20
Weight of individual seeds

FIG. 31.

Curves to illustrate the conception of pure lines in a population.

curve must be represented as made up of the three
components

A fluctuating between 4 and 16 with a mean of 10
B ” ” 6 Prd 18 ” bE) 12
Cc 2 ” 8 ” 20 ” ” 14

as is shown in Fig. 31. A seed that weighs 12
grains may belong to any of these three strains. It
may be an average seed of B, or a rather large seed
of A, or a rather small seed of C. If it belongs to
B its offspring will average 12 grains, if to A
they will average 10 grains, and if to C they
will average 14 grains. Seeds of similar weight

XIV ECONOMICAL 155

may give a different result because they happen to
be fluctuations of different pure lines. But within
the pure line any seed, large or small, produces the
average result for that line. Thus a seed of line
C which weighs 20 grains will give practically the
same result as one that weighs Io grains.

On this view we can understand why selection
of the largest seed raises the average weight in the
next generation. We are picking out more of C
and less of A and B, and as this process is repeated
the proportion of C gradually increases and we get
the appearance of selection acting on a continuously
varying homogeneous material and producing a
permanent effect. This is because the interval
between the average weight of the different pure
lines is small compared with the environmental
fluctuations. None the less it is there, and the
secret of separating and fixing any of these pure
lines is again to breed from the individual separately.
As soon as the pure line is separated further selection
becomes superfluous.

Since the publication of Darwin’s famous work
upon the.effects of cross- and self-fertilisation, it has
been generally accepted that the effect of a cross is
commonly, though not always, to introduce fresh
vigour into the offspring, though why this should be
so we are quite at a loss to explain. Continued
close inbreeding, on the contrary, eventually leads
to deterioration, though, as in many self-fertilised
plants, a considerable number of generations may
elapse before it shows itself in any marked degree.
The fine quality of many of the seedsman’s choice
varieties of vegetables probably depends upon the

156 MENDELISM CHAP.

fact that they have resulted from a cross but a few
generations back, and it is possible that they often
oust the older kinds not because they started as
something intrinsically better, but because the latter
had gradually deteriorated through continuous self-
fertilisation. Most breeders are fully alive to the
beneficial results of a cross so far as vigour is con-
cerned, but they often hesitate to embark upon it
owing to what was held to be the inevitably lengthy
and laborious business of recovering the original
variety and refixing it, even if in the process it was
not altogether lost. That danger Mendelism has
removed, and we now know that by working on
these lines it is possible in three or four generations
to recover the original variety in a fixed state with
all the superadded vigour that follows from a cross.
Nor is the problem one that concerns self-fertilised
plants only. Plants that are reproduced asexually
often appear to deteriorate after a few generations
unless a sexual generation is introduced. New
varieties of potato, for example, are frequently put
upon the market, and their excellent qualities give
them a considerable vogue. Much is expected of
them, but time after time they deteriorate in a dis-
appointing way and are lost to sight. It is not
improbable that we are here concerned with a case
in which the plants lose their vigour after a few
asexual generations of reproduction from tubers, and
can only recover it with the stimulus that results
from the interpolation of a sexual generation. Un-
fortunately this generally means that the variety is
lost, for owing to the haphazard way in which new
kinds of potatoes are reproduced it is probable that

XIV ECONOMICAL 157

most cultivated varieties are complex heterozygotes.
Were the potato plant subjected to careful analysis
and the various factors determined upon which its
variations depend, we should be in a position to
remake continually any good potato without running
the risk of losing it altogether, as is now so often
the case.

The application of Mendelian principles is likely
to prove of more immediate service for plants than
animals, for owing to the large numbers which can
be rapidly raised from a single individual and the
prevalence of self-fertilisation, the process of analysis
is greatly simplified. Even apart from the circum-
stance that the two sexes may sometimes differ in
their powers of transmission, the. mere fact of their
separation renders the analysis of their properties
more difficult. And as the constitution of the indi-
vidual is determined by the nature and quality of its
offspring, it is not easy to obtain this knowledge
where the offspring, as in most animals, are relatively
few. Still, as has been abundantly shown, the same
principles hold good here also, and there is no reason
why the process of analysis, though more trouble-
some, should not be effectively carried out. At the
same time, it affords the breeder a rational basis for
some familiar but puzzling phenomena. The fact,
for instance, that certain characters often “skip a
generation” is simply the effect of dominance in F,
and the reappearance of the recessive character in the
following generation. “Reversion” and “atavism,”
again, are phenomena which are no longer mysterious,
but can be simply expressed in Mendelian terms
as we have already suggested in Chap. VI. The

158 MENDELISM CHAP.

occasional appearance of a sport in a supposedly
pure strain is often due to the reappearance of a
recessive character. Thus even in the most highly
pedigreed strains of polled cattle such as the Aber-
deen-Angus, occasional individuals with horns appear.
The polled character is dominant to the horned, and
the occasional reappearance of the horned animal
is due to the fact that some of the polled herd are
heterozygous in this character. When two such indi-
viduals are mated, the chances are 1 in 4 that the
offspring will be horned. Though the heterozygous
individuals may be indistinguishable in appearance
from the pure dominant, they can be readily separated
by the breeding test. For when crossed by the
recessive, in this case horned animals, the pure
dominant gives only polled’ beasts, while the hetero-
zygous individual gives equal numbers of polled and
horned ones. In this particular instance it would
probably be impracticable to test all the cows by
crossing with a horned bull. For in each case it
would be necessary to have several polled calves
from each before they could with reasonable certainty
be regarded as pure dominants. But to ensure that
no horned calves should come, it is enough to use a
bull which is pure for that character. This can
easily be tested by crossing him with a dozen or so
horned cows. If he gets no horned calves out of
these he may be regarded as a pure dominant and
thenceforward put to his own cows, whether horned
or polled, with the certainty that all his calves will
be polled.

Or, again, suppose that a breeder has a chestnut
mare and wishes to make certain of a bay foal from

XIV ECONOMICAL 159

her. We know that bay is dominant to chestnut,
and that if a homozygous bay stallion is used a bay
foal must result. In his choice of a sire, therefore,
the breeder must be guided by the previous record
of the animal, and select one that has never given
anything but bays when put to either bay or chest-
nut mares. In this way he will assure himself of a
bay foal from his chestnut mare, whereas if the record
of the sire shows that he has given chestnuts he will
be heterozygous, and the chances of his getting a
bay or a chestnut out of a chestnut mare are equal.

It is not impossible that the breeder may be
unwilling to test his animals by crossing them with
a different breed through fear that their purity may
be thereby impaired, and that the influence of the
previous cross may show itself in succeeding genera-
tions. He might hesitate, for instance, to test his
polled cows by crossing them with a horned bull for
fear of getting horned calves when the cows were
afterwards put to a polled bull of their own breed.
The belief in the power of a sire to influence sub-
sequent generations, or telegony as it is sometimes
called, is not uncommon even to-day. Nevertheless,
carefully conducted experiments by more than one
competent observer have failed to elicit a single shred
of unequivocal evidence in favour of the view. Until
we have evidence based upon experiments which are
capable of repetition, we may safely ignore telegony
as a factor in heredity.

Heterozygous forms play a greater part in the
breeding of animals than of plants, for many of the
qualities sought after by the breeder are of this
nature. Such is the blue of the Andalusian fowl,

160 MENDELISM CHAP. XIV

and, according to Professor Wilson, the roan of the
Shorthorn is similar, being the heterozygous form
produced by mating red with white. The characters
of certain breeds of canaries and pigeons again
appear to depend upon their heterozygous nature.
Such forms cannot, of course, ever be bred true,
and where several factors are concerned they may
when bred together produce but a small proportion
of offspring like themselves. As soon, however, as
their constitution has been analysed and expressed
in terms of Mendelian factors, pure strains can be
built up which when crossed will give nothing but
offspring of the desired heterozygous form.

The points with which the breeder is concerned
are often fine ones, not very evident except to the
practised eye. Between an ordinary Dutch rabbit
and a winner, or between the comb of a Hamburgh
that is fit to show and one that is not, the differences
are not very apparent to the uninitiated. Whether
Mendelism will assist the breeder in the production
of these finer points is at present doubtful. It may
be that these small differences are heritable, such as
those that form the basis ‘of Johannsen’s pure lines.
In this case the breeder’s outlook is hopeful. But it
may be that the variations which he seeks to per-
petuate are of the nature of fluctuations, dependent
upon the earlier life conditions of the individual, and
not upon the constitution of the gametes by which
it was formed. If such is the case, he will get no
help from the science of heredity, for we know of
no evidence which might lead us to suppose that
variations of this sort can ever become fixed and
heritable.

CHAPTER XV
MAN

THOUGH the interest attaching to heredity in man
is more widespread than in other animals, it is far
more difficult to obtain evidence that is both com-
‘plete and accurate. The species is one in which
the differentiating characters separating individual
from individual are very numerous, while the number
of the offspring is comparatively few, and the gener-
ations are far between. For these reasons, even if
it were possible, direct experimental work with man
would be likely to prove both tedious and expensive.
There is, however, another method besides the direct
one from which something can be learned. This
consists in collecting all the evidence possible, ar-
‘ranging it in the form of pedigrees, and. comparing
“it with standard cases already worked out in animals
and plants. In this way it has been possible to
demonstrate in man the existence of several char-
acters showing simple Mendelian inheritance. As
few besides medical men have hitherto been con-
cerned practically with heredity, such records as
exist are, for the most part, records of deformity or
of disease. So it happens that most of the pedigrees
at present available deal with characters which are
161 M

162 MENDELISM CHAP.

usually classed as abnormal. In some of these the
inheritance is clearly Mendelian. One of the cases
which has been most fully worked out is that of a

FiG. 32.

Normal and brachydactylous hands placed together for comparison.
(From Drinkwater.)

deformity known as brachydactyly. In brachy-
dactylous people the whole of the body is much
stunted, and the fingers and toes appear to have two
joints only instead of three (cf. Figs. 32 and 33).
The inheritance of this peculiarity has been carefully

XV MAN 163

investigated by Dr. Drinkwater, who collected all the
data he was able to find among the members of a
large family in which it occurred. The result is the
pedigree shown on p. 159. It is assumed that all
who are recorded as having offspring were married
to normals. Examination of the pedigree brings
out the facts (1) that all affected individuals have an
affected parent; (2) that none of the unaffected in-

Fic. 33.

Radiograph of a brachydactylous hand.

dividuals, though sprung from the affected, ever
have descendants who are affected ; and (3) that in
families ;' where both affected and unaffected occur,
the numbers of the two classes are, on the average,
equal. (The sum of such families in the complete
pedigree is thirty-nine affected and thirty-six normals.)
It is obvious that these are the conditions which are
fulfilled in a simple Mendelian case, and there is
nothing in this pedigree to contradict the assertion
that brachydactyly, whatever it may be due to,
M 2

)

(4 unknown)

é3
é

é

é

ex and type
uncertain

f

9°

MENDELISM CHAP.

behaves as a simple
dominant to the normal
form, ze. that it depends
upon a factor which the
normal does not con-
tain. The recessive nor-
mals cannot transmit the
affected condition what-
evér their ancestry.
Once free they are always
free, and can marry other
normals with full confi-
dence that none of their
children will show the
deformity.

te 3 The evidence avail-
ot 4 able from pedigrees has
& = 3 revealed the simplest
poss form of Mendelian in-
re —tee heritance in several
lee human defects and dis-
CS eases, among which may
be be mentioned presenile
ie cataract of the eyes, an
Leet abnormal form of skin
*0 thickening in the palms
sian hs of the hands and soles of

the feet, known as tylosis,
and epidermolysis bul-
losa, a disease in which
the skin rises up into
numerous bursting
blisters.

Pedigree of Drinkwater's brachydactylous family. The affected members are indicated by black and the normals by light circles.

XV MAN ' 165

Among the most interesting of all human pedi-
grees is one recently built up by Mr. Nettleship
from the records of a night-blind family living
near Montpellier in the south of France. In night-
blind people the retina is insensitive to light which
falls below a certain intensity, and such people
are consequently blind in failing daylight or in
moonlight. As the Montpellier case had excited
interest for some time, the records are unusually
complete. They commence with a certain Jean
Nougaret, who was born in 1637, and suffered from
night-blindness, and they end for the present with
children who are to-day but a few years of age.
Particulars are known of over 2000 of the descend-
ants of Jean Nougaret. Through ten generations and
nearly three centuries the affection has behaved as a
Mendelian dominant, and there is no sign that long-
continued marriage with folk of normal vision has
produced any amelioration of the night-blind state.

Besides cases such as these where a simple form
of Mendelian inheritance is obviously indicated, there
are others which are more difficult to read. Of some
it may be said that on the whole the peculiarity
behaves as though it were an ordinary dominant ;
but that exceptions occur in which affected children
are born to unaffected parents. It is not impossible
that the condition may, like colour in the sweet-pea,
depend upon the presence or absence of more than
one factor. In none of these cases, however, are
the data sufficient for determining with certainty
whether this is so or not.

A group of cases of exceptional interest is that
in which the incidence of disease is largely, if not

166 MENDELISM CHAP.

absolutely, restricted to one sex, and so far as is
hitherto known the burden is invariably borne by
the male. In the inheritance of colour-blindness
(p. 108) we have already discussed an instance in
which the defect is rare, though not unknown, in the
female. Sex-limited inheritance of a similar nature
is known for one or two ocular defects, and for
several diseases of the nervous system. In the
peculiarly male disease known as hemophilia the
blood refuses to clot when shed, and there is nothing

Ob
+O
+O 4

g
—Tt { T 1 tr T T 1 } T T TT
$dd22 2 FB eo¢g¢ 22? P&P dé
Mh ey te hh dite
ééeé eee odds?
Children ae Se
(*)o gd ¢ all healthy 76 3?) F
Fie. 35.

Pedigree of a hemophilic family. Affected (all males) represented by black,
and normals of both sexes by light circles, (From Stahel.)
to prevent great loss from even a superficial scratch.
In its general trend the inheritance of hemophilia is
not unlike that of horns among sheep, and it is possible
that we are here again dealing with a character which
is dominant in one sex and recessive in the other.
But the evidence so far collected points to a differ-
ence somewhere, for in hemophilic families the
affected males, instead of being equal in number to
the unaffected, show a considerable preponderance.
The unfortunate nature of the defect, however, forces
us to rely for our interpretation almost entirely upon
the families produced by the unaffected females who

xv MAN 167

can transmit it. Our knowledge of the offspring
of “bleeding” males is as yet far too scanty, and
until it is improved, or until we can find some
parallel case in animals or plants, the precise
scheme of inheritance for hemophilia must remain
undecided.

Though by far the greater part of the human
evidence relates to abnormal or diseased conditions,
a start has been made in obtaining pedigrees of
normal characters. From the ease with which it
can be observed, it was natural that eye-colour
should be early selected as a subject of investigation,
and the work of Hurst and others has clearly demon-
strated the existence of one Mendelian factor in
operation here. Eyes are of many colours, and the
colour depends upon the pigment in the iris. Some
eyes have pigment on both sides of the iris—on the
side that faces the retina as well as on the side that
looks out upon the world. Other eyes have pigment
on the retinal side only. To this class belong the
blues and clear greys ; while the eyes with pigment
in front of the iris also are brown, hazel, or green in
various shades according to the amount of pigment
present. In albino animals the pigment is entirely
absent, and as the little blood-vessels are not ob-
scured the iris takes on its characteristic pinkish-red
appearance. The condition in which pigment is
present in front of the iris is dominant to that in
which it is absent. Greens, browns, or hazels mated
together may, if heterozygous, give the recessive
blue, but no individuals of the brown class are to
be looked for among the offspring of blues mated
together. The blues, however, may carry factors

168 MENDELISM CHAP.

which are capable of modifying the brown. Just
as the pale pink-tinged sweet-pea (PI. IV., 9)
when mated with a suitable white gives only deep
purples, so an eye with very little brown pigment
mated with certain blues produces progeny of a
deep brown, far darker than either parent. The
blue may carry a factor which brings about intensi-
fication of the brown pigment. There are doubtless
other factors which modify the brown when present,
but we do not yet know enough of the inheritance of
the various shades to justify any statement other than
that the heredity of the pigment in front of the iris
behaves as though it were due to a Mendelian factor.

Even this fact is of considerable importance, for
it at once suggests that the present systems of classi-
fication of eye-colours, to which some anthropologists
attach considerable weight, are founded on a purely
empirical and unsatisfactory basis. Intensity of
colour is the criterion at present in vogue, and it is
customary to arrange the eye-colours in a scale of
increasing depth of shade, starting with pale greys
and ending with the deepest browns. On this
system the lighter greens are placed among the
blues. But we now know that blues may differ from
the deep browns in the absence of only a single
factor, while, on the other hand, the difference
between a blue and a green may be a difference
dependent upon more than one factor. To what
extent eye-colour may be valuable as a criterion of
race it is at present impossible to say, but if it is
ever to become so, it will only be after a searching
Mendelian analysis has disclosed the factors upon
which the numerous varieties depend.

xv MAN 169

A discussion of eye-colour suggests reflections of
another kind. It is difficult to believe that the
markedly different states of pigmentation which
occur in the same species are not associated with
deep-seated chemical differences influencing the
character and bent of the individual. May not these
differences in pigmentation be coupled with and so
become in some measure a guide to mental and
temperamental characteristics? In the National
Portrait Gallery in London the pictures of cele-
brated men and women are largely grouped accord-
ing to the vocations in which they have succeeded.
The observant will probably have noticed that there
is a tendency for a given type of eye-colour to pre-
dominate in some of the larger groups. It is rare
to find anything but a blue among the soldiers and
sailors, while among the actors, preachers, and
orators the dark eye is predominant, although for
the population as a whole it is far scarcer than the
light. The facts are suggestive, and it is not im-
possible that future research may reveal an intimate
connection between peculiarities of pigmentation and
peculiarities of mind.

The inheritance of mental characters is often
elusive, for it is frequently difficult to appraise the
effects of early environment in determining a man’s
bent. That ability can be transmitted there is no
doubt, for this is borne out by general experience,
as well as by the numerous cases of able families
brought together by Galton and others. But when
we come to inquire more precisely what it is that is
transmitted we are baffled. A distinguished son
follows in the footsteps ofa distinguished father. Is

‘170 MENDELISM CHAP.

this due to the inheritance of a particular mental apti-
tude, or is it an instance of general mental ability
displayed in a field rendered attractive by early
association? We have at present very little definite
evidence for supposing that what appear to be
special forms of ability may be due to specific
factors. Hurst, indeed, has brought forward some
facts which suggest that musical sense sometimes
behaves as a recessive character, and it is likely
that the study of some clean-cut faculty such as the
mathematical one would yield interesting results.
The analysis of mental characters will no doubt
be very difficult, and possibly the best line of attack
is to search for cases where they are associated with
some physical feature such as pigmentation. If an
association of this kind be found, and the pigmenta-
tion factors be determined, it is evident that we
should thereby obtain an insight into the nature of
the units upon which mental conditions depend.
Nor must it be forgotten that mental qualities, such
as quickness, generosity, instability, etc—dqualities
which we are accustomed to regard as convenient
units in classifying the different minds with which
we are daily brought into contact—are not neces-
sarily qualities that correspond to heritable units.
Effective mental ability is largely a matter of tem-
perament, and this in turn is quite possibly dependent
upon the various secretions produced by the different
tissues of the body. Similar nervous systems associ-
ated with different livers might conceivably result in
individuals upon whose mental ability the world
would pass a very different judgment. Indeed, it is
not at all impossible that a particular form of

XV MAN 171

mental ability may depend for its manifestation, not
so much upon an essential difference in the structure
of the nervous system, as upon the production by
another tissue of some specific poison which causes
the nervous system to react in a definite way. We
have mentioned these possibilities merely to indicate
how complex the problem may turn out to be.
Though there is no doubt that mental ability is
inherited, what it is that is transmitted, whether
factors involving the quality and structure of the
nervous system itself, or factors involving the pro-
duction of specific poisons by other tissues, or both
together, is at present uncertain.

Little as is known to-day of heredity in man,
that little is of extraordinary significance. The
qualities of men and women, physical and mental,
depend primarily upon the inherent properties of the
gametes which went to their making. Within limits
these qualities are elastic, and can be modified to a
greater or lesser extent by influences brought to bear
upon the growing zygote, provided always that the
necessary basis is present upon which these influences
can work. If the mathematical faculty has been
carried in by the gamete, the education of the zygote
will enable him to make the most of it. But if the
basis is not there, no amount of education can trans-
form that zygote into a mathematician This isa
matter of common experience. Neither is there any
reason for supposing that the superior education of a
mathematical zygote will thereby increase the mathe-
matical propensities of the gametes which live within
him. For the gamete recks little of quaternions. It is
true that there is progress of a kind in the world, and

172 MENDELISM CHAP.

that this progress is largely due to improvements in
education and hygiene. The people of to-day are
better fitted to cope with their material surroundings
than were the people of even a few thousand years
ago. And as time goes on they are able more and
more to control the workings of the world around
them. But there is no reason for supposing that
this is because the effects of education are inherited.
Man stores knowledge as a bee stores honey or a
squirrel stores nuts. With man, however, the hoard
is of a more lasting nature. Each generation in
using it sifts, adds, and rejects, and passes it on to
the next a little better and a little fuller. When
we speak of progress we generally mean that the
hoard has been improved, and is of more service to
man in his attempts to control his surroundings.
Sometimes this hoarded knowledge is spoken of as
the inheritance which a generation receives from
those who have gone before. This is misleading.
The handing on of such knowledge has nothing
more to do with heredity in the biological sense than
has the handing on from parent to offspring of a
picture, or a title, or a pair of boots. All these
things are but the transfer from zygote to zygote
of something extrinsic to the species. Heredity, on
the other hand, deals with the transmission of some-
thing intrinsic from gamete to zygote and from
zygote to gamete. It is the participation of the
gamete in the process that is our criterion of what
is and what is not heredity.

Better hygiene and better education, then, are
good for the zygote, because they help him to make
the fullest use of his inherent qualities. But the

XV MAN 173

qualities themselves remain unchanged in so far as
the gamete is concerned, since the gamete pays no
heed to the intellectual development of the zygote
in whom he happens to dwell. Nevertheless, upon
the gamete depend those inherent faculties which
enable the zygote to profit by his opportunities, and,
unless the zygote has received them from the gamete,
the advantages of education are of little worth. If
we are bent upon producing a permanent betterment
that shall be independent of external circumstances,
if we wish the national stock to become inherently
more vigorous in mind and body, more free from
congenital physical defect and feeble mentality, better
able to assimilate and act upon the stores of know-
ledge which have been accumulated through the
centuries, then it is the gamete that we must con-
sult. The saving grace is with the gamete, and
with the gamete alone.

People generally look upon the human species as
having two kinds of individuals, males and females,
and it is for them that the sociologists and legislators
frame their schemes. This, however, is but an im-
perfect view to take of ourselves. In reality we are
of four kinds, male zygotes and female zygotes, large
gametes and small gametes, and heredity is the link
that binds us together. Jf our lives were like those
of the starfish or the sea-urchin, we should probably
have realised this sooner. For the gametes of these
animals live freely, and contract their marriages in
the waters of the sea. With us it is different,
because half of us must live within the other half or
perish. Parasites upon the rest, levying a daily toll
of nutriment upon their hosts, they are yet in some

174 MENDELISM CHAP. XV

measure the arbiters of the destiny of those within
whom they dwell. At the moment of union of two
gametes is decided the character of another zygote,
as well as the nature of the population of gametes
which must make its home within him. The union
once effected the inevitable sequence takes its course,
and whether it be good, or whether it be evil, we, the
zygotes, have no longer power to alter it. We are in
the hands of the gamete; yet not entirely. For
though we cannot influence their behaviour we can
nevertheless control their unions if we choose to do
so. By regulating their marriages, by encouraging
the desirable to come together, and by keeping the
undesirable apart we could go far towards ridding
the world of the squalor and the misery that come
through disease and weakness and vice. But before
we can be prepared to act, except, perhaps, in the
simplest cases, we must learn far more about them.
At present we are woefully ignorant of much, though
we do know that full knowledge is largely a matter
of time and means. One day we shall have it, and
the day may be nearer than most suspect. Whether
we make use of it will depend in great measure upon
whether we are prepared to recognise facts, and to
modify or even destroy some of ‘the conventions
which we have become accustomed to regard as the
foundations of our social life. Whatever be the
outcome, there can be little doubt that the future of
our civilisation, perhaps even the possibility of a
future at all, is wrapped up with the recognition we
accord to those who live unseen and inarticulate
within us—the fateful race of gametes so irrevocably
bound to us by that closest of all ties, heredity.

APPENDIX

AS some readers may possibly care to repeat
Mendel’s experiments for themselves, a few words
on the methods used in crossing may not be super-
fluous. The flower of the pea with its standard,
wings, and median keel is too familiar to need
description. Like most flowers it is hermaphrodite.
Both male and female organs occur on the same
flower, and are covered by the keel. The anthers,
ten in number, are arranged in a circle round the
pistil. As soon as they are ripe they burst and shed
their pollen on the style. The pollen tubes then
penetrate the stigma, pass down the style, and
eventually reach the ovules in the lower part of the
pistil. Fertilisation occurs here. Each ovule, which
is reached by a pollen tube, swells up and becomes a
seed. At the same time the fused carpels enclosing
the ovules enlarge to form the pod. When this, the
normal mode of fertilisation, takes place, the flower
is said to be selfed.

In crossing, it is necessary to emasculate a
flower on the plant chosen to be the female parent.
For this purpose a young flower must be taken
in which the anthers have not yet burst. The

175

176 MENDELISM

keel is depressed, and the stamens bearing the
anthers are removed at their base by a pair of fine
forceps. It will probably be found necessary to tear
the keel slightly in order to do this. The pistil is
then covered up again with the keel, and the flower
is enclosed in a bag of waxed paper until the follow-
ing day. The stigma is then again exposed and
dusted with ripe pollen from a flower of the plant
selected as the male parent. This done, the keel is
replaced, and the flower again enclosed in its bag to
protect it from the possible attentions of insects until
it has set seed. The bag may be removed in about a
week after fertilisation. It is perhaps hardly necessary
to add that strict biological cleanliness must be
exercised during the fertilising operations. This is
readily attained by sterilising fingers and forceps
with a little strong spirit before each operation, there-
by ensuring the death of any foreign pollen grains
which may be present.

The above method applies also to sweet-peas,
with these slight modifications. As the anthers ripen
relatively sooner in this species, emasculation must
be performed at a rather earlier stage. It is generally
safe to choose a bud about three parts grown. The
interval between emasculation and fertilisation must
be rather longer. Two to three days is generally
sufficient. Further, the sweet-pea is visited by the
leaf-cutter bee, Megachile, which, unlike the honey
bee, is able to depress the keel and gather pollen.
If the presence of this insect is suspected, it is
desirable to guard against the risk of admixture of
foreign pollen by selecting for pollinating purposes
a flower which has not quite opened. If the

APPENDIX 177

standard is not erected, it is unlikely to have been
visited by Megachile. Lastly, it not infrequently
happens that the little beetle Meligethes is found
inside the keel. Such flowers should be rejected
for crossing purposes.

INDEX

Abraxas grossutariata, 96

‘« Acquired” characters, 13
Adaptation, 137

Agouti mice, 46

Albino mice, 46

Albinos, nature of, 49
Amauris, 138

Analysis of types, 149
Ancestral Heredity, Law of, 11
Andalusian fowls, 64

Axil colour in sweet-peas, 85

Bateson, W., 13, 25, 26, 51, 112,
127, 135

Biffen, R. H., 150

Blue Andalusian fowls, 65

Brachydactyly, 162 -

Bryony, 116

Bush sweet-peas, 57

Castle, 127

Cattle, horns in, 79, 158

Colour, nature of, in flowers, 45

Colour-blindness, 113

Combs of fowls, 29, 40

Correns, C., 26, 116

Coupling of characters in gametes,
86

Cuénot, 46, 115

‘Cupid ” sweet-peas, 57

Currant moth, 96

Darwin, C., 9, 60, 140, 155
De Vries, H., 14, 26, 135
Discontinuity in variation, 13
Dominant characters, 16
Doncaster, L., 96
Drinkwater, H., 163

Dutch rabbits, 55

Eggs, 2

Environment, influence of, 132
Euralia, 139

Evolution, 9, 78, 134

Eye, in primulas, 51
Eye-colour, in man, 167

Factor, definition of, 28

Factors, interaction of, 39
Fertilisation, 3

Fertilisation, self- and cross-, 155
Fixation of varieties, 146
Fluctuations, 133

Fowls, coloured from whites, 45, 68

Galton, 11, 169
Gametes, nature of, 5
Gregory, R. P., 51

Hemophilia, 166

Hardy, G. H., 141

Heterozygote, definition of, 25

Heterozygote, of intermediate
form, 63

Hieracium, 24, 128

Himalayan rabbits, 55

Homostyle primulas, 52

Homozygote, definition of, 25

Hooded sweet-peas, 82

Horses, bay and chestnut in, 158

Hurst, C. C., 57, 167, 170

Immunity in wheat, 151
Individuality, 130

Inhibition, factors for, 69, 104
Intermediates, 121

179

180
Johannsen, W., 152
Lop-eared rabbits, 127

Mendel, 7, 15, 24, 128

Mental characters, 170

Mice, inheritance of coat colour in,
46

Mimicry, 138

Mirabilis, 145

Morgan, T. H., 111

Mulattos, 125

Mutation, 77, 132

Nageli, C., 24

Natural selection, 10, 135, 137, 143
Nettleship, E., 165
Night-blindness, 165

Pararge egeria, 127
Parkinson, J., 117

Pea comb, 29

Peas, coloured flowers in, 22
Peas, tall and dwarf, 16

Pigeons, 79

Pin-eye in primulas, 51
Pisum, 15

Primulas, 28, 51, 63, 86
Pollen, 3

Pollen of sweet-peas, 84

Pomace fly, rr1

Population, inheritance of char-
acters in a, 141

Presence and Absence theory, 31

Pure lines, 154

Purity of gametes, 21

Purity of type, 148

Rabbits, 49, 55
Ratios, Mendelian—

ge a 31a

9: 3:3: 4, 23, 30
9: 3:4 48
9:7, 46

Ray, John, 138
Recessive characters, 17
Repulsion between factors, 83

MENDELISM

Reversion, 54, 157
in rabbits, 55
in sweet-peas, 57
in fowls, 59
in pigeons, 60
Rose comb, 29

Saunders, E. R., 50, 118
Seeds, nature of, 4
Segregation, 20
Selection, 155
Sheep, horns in, 71
Silky fowls, 27, 102
Single comb, 29
Species, nature of, 143
Species, origin of, 10
Speckled wood butterfly, 127
Spermatozoa, 3
Sports, 140
Staples-Browne, R., 61
Sterility, 145
Sterility in sweet-peas, 85
Stocks, double, 117
Stocks, hoariness in, 50
Sweet-pea, colour in, 41, 74
history of, 76
inheritance of hood in, 82
inheritance of size in, 57

Telegony, 159

Thrum-eye in primulas, 51
Toe, extra toe in poultry, 70
Tschermak, E., 26

Unit-character, definition of, 28

Variation, 13, 132, 134

Walnut comb, 29

Weismann, A., 12

Wheat, beard in, 69
experiments with, 150

White, dominant in poultry, 67

Wilson, J., 160

Yellow mice, 115

Zygotes, nature of, 5

Printed by R. & R. Crark, Limitep, Edinburgh.

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