The Cambridge Manuals of Science and
Literature

HEREDITY

CAMBRIDGE UNIVERSITY PRESS

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First Edition 1910

Reprinted 1911

Second Edition 1912

/FzV/5 /)4(? exception of the coat of arms at
the foot, the design on the title page Is a
reproduction of one used by the earliest known
Cambridge printer John Slberch 1 5 2 1

PREFACE

IN a book of the size to which the Cambridge
Manuals of Science and Literature are limited, it
is plainly impossible to treat in detail every aspect of
a subject like Heredity. One of the chief difficulties,
therefore, which I have encountered in preparing this
little book has been to decide what to leave out. To
some it will doubtless seem that parts of the subject
have been treated too fully, and other important
branches omitted or barely mentioned, but my aim
has been to give the reader a sketch of the most
important lines in which recent advances have been
made. There are many excellent works dealing with
the older theories — and in this subject age is measured
by very few years, — but our knowledge has increased
so greatly and is still progressing so quickly that
books become out of date almost as soon as they are
published. My attempt, then, has been to deal chiefly
with the quite modern developments of the subject,
and in order that the reader who is not very familiar
with the matter may feel he is on fairly sure ground,

vi PREFACE

and not confuse fact with speculation, I have tried to
avoid purely speculative questions in the body of the
book, and have devoted a few pages to one of the most
interesting of these in an appendix, together with a
historical summary of Theories of Heredity. There
are many, of course, who will regard parts of the
chapters dealing with Mendelism as consisting largely
of speculation ; I can only reply that I regard the
facts referred to as established, and the theoretical
deductions from them as the only ones that have
yet been offered which can fit them adequately.

No attempt has been made to quote authorities
for every statement, but a list of books and papers is
given in which a further account will be found of the
subjects treated. The numbers in square brackets [ ]
in the text refer to this list. I have also followed
the example set by Mr Lock in providing a glossary
of unfamiliar terms of which the use has been
unavoidable.

For the chapter on Statistical Study of Inherit-
ance my chief sources of information have been
Prof. Pearson's Grammar of Science and his numerous
papers on the subject published by the Royal Society
and in Biometriha. I have tried to summarise in
words the results of his work, and of that of other
workers on similar lines, and if the inadequacy of my
mathematical knowledge has led me into any serious
errors in the attempt, I owe them my apologies.

PREFACE vii

Chiefly of course I am indebted to Prof. Bateson's
recent work on Mendel's Principles of Heredity as
the most complete and authoritative account of the
subject, from the acknowledged leader of the Men-
delian school. For permission to reproduce several
figures from this book I tender my thanks. In
addition to information from many original papers, I
have also not hesitated to make use of Lock's Recent
Progress in the Study of Variation, Heredity and
Evolution, Thomson's Heredity, and some other
books dealing with the general aspects of the subject ;
to these authors, and to several friends who have
been kind enough to give me written information
on matters with which they are especially conversant,
I wish to record my indebtedness. I wish also
especially to thank Mrs A. C. Seward for drawing
the sections of Primula reproduced in fig. 10
(p. 81).

L. DONCASTEPu

Cambridge,
June 1910.

CONTENTS

CHAP.

I. Introduction. Relation of Heredity to other branches of

knowledge.— The questions to be answered . page 1

II. Variation. Occurrence and kinds of Variation. — Con-

tinuous variation and methods of study. — Discontinuous
variation. — Inborn and acquired characters . 7

III. The Causes of Variation. Mutation. — Action of en-

vironment on body and germ-cells. — Variation on cross-
ing.— Relative importance of 'inherent' and 'acquired'
characters 22

IV. The Statistical Study of Heredity. Two methods of

studying Heredity. —The biometrical method. — Corre-
lation and Regression. — Parental correlation and the
Law of Ancestral Heredity. — Heredity in 'pure lines'
(Johannsen etc.). — Inheritance of 'mental and moral'
characters in Man. — Eugenics .... 32

V. Mendelian Heredity. Mendel's Law illustrated.— Segre-

gation and Allelomorphism. — Crosses concerning more
than one pair of characters. — Examples of Mendelian
characters in Plants and Animals. — Combs of Fowls. —
The Andalusian Fowl 52

VI. Mendelian Heredity {continued). The Inheritance

of Colour. Concurrence of two factors in the pro-

X CONTENTS

CHAP.

duction of Colour.— Colour in Animals ; some coloure
'epistatic' over others.— Flower-colours ; Reversion on
crossing. — The nature of albinism. — More complex cases
of interrelation ; Stocks, Primulas . . . 71

VII. Some Disputed Questions. Mendelian segregation.—

Inheritance of acquired characters. — Indirect and ex-
perimental evidence. — Telegony. — Maternal Impres-
sion 85

VIII. Heredity in Man. Physical and Mental Characters.—

Diseases.— Mendelian Characters; Eye-colour, Brachy-
dactyly, abnormalities of the Bye. — Non-Mendelian
characters; Skin and Hair-colour. — Importance of
Heredity in relation to Sociology . . . 101

IX. Heredity and Sex. Secondary sexual characters.—
Dominance altered by Sex. — Sex-limited inheritance;
the Currant Moth, Fowls, the &j DrosophUa.— Sex-limited
affections in Man ; Colour-blindness, Night-blindness and
Haemophilia. — Sex inherited as a Mendelian character. —
Sexual Dimorphism 119

Appendix I. Historical Summary of Theories of Heredity.
Lamarck.— Darwin and the Theory of Pangenesis.—
Weismann's Theory of Germ-Plasm . . .135

Appendix II. The Material Basis of Inheritance. The
Nucleus and Chromosomes as possible 'bearers' of
Heredity. Behaviour of Chromosomes in Germ-cell
formation 145

Literature List l-j'

Glossary 1^^

Index ^^^

CHAPTER I

INTRODUCTION

During the whole history of scientific enquiry,
one of the most fascinating and at the same time one
of the most baffling of the problems which confront
mankind has been the cause of the resemblances and
differences between parents and children. In general,
the facts are common knowledge; the essence of
Heredity and Variation is expressed in the proverbs
' Like begets like ' and * Nature never uses the same
mould twice.' Yet clearly the two proverbs are con-
tradictory, for if like really begets like Nature must
use the same mould for all the members of a family.
Our object therefore is to investigate, first, how
the characters of a parent actually are distributed
among the children, and how the offspring of the same
parentage may differ among themselves ; and secondly,
if possible, what is the mechanism by which the re-
semblances and diversities are brought about.

These problems are interesting from various points
of view. They attract us for their own sake, as does

D. I

2 HEREDITY [ch.

anything mysterious or unexplained ; tbey have a
deep human and practical importance, for not only
do they affect us all individually, but upon their
solution depends, to an extent as yet only dimly
realised, the answer to some of our most pressing
social questions ; and finally they lie at the very root
of all theories of organic evolution, so that they form "
as it were the basis of philosophical biology. The
relation of the study of Heredity and Variation to
sociology must be left to a later chapter, but before
proceeding further we must shortly consider its bearing
on theories of evolution.

The fact of organic evolution is admitted by all
schools of biology, but about the causes of the pro-
cess and the manner in which it takes place there is
still wide diversity of opinion. To some of the more
important theories of evolution it will be necessary
to refer again later, but however great may be the
difference of opinion with regard to them, all biologists
agree that evolution depends ultimately on Variation
and Heredity. Darwin called his great book The
Origin of Species because the unit step, so to speak,
on the scale of evolution is the transition from one
species to another. But if a species A is to give rise
to a species B, in the first place some individuals of A
must vary in the direction of B, and then the variation
must be inherited, for otherwise no permanent change
can take place. The differences with regard to the cause

I] INTRODUCTION 3

and method of evolution arise therefore partly from
our ignorance of the laws of variation and heredity,
and partly from different ideas as to the causes which
lead to progression in certain directions rather than
in others. This latter source of disagreement is to a
large extent outside the province of this book, but
the subjects of Heredity and Variation are so inti-
mately bound together that one cannot be adequately
treated without the other. If, however, we can come
to any definite decision with regard to the nature of
Heredity and Variation, we shall have made a long
step towards understanding the method by which
evolution has taken and is taking place.

One other point must be mentioned. The study
of heredity brings us face to face with perhaps the
most fundamental problem of biology — the ultimate
nature of living matter. For if an ovum, barely
visible to the eye, or the much smaller spermatozoon
which is visible only with high magnification, can
bear potentially all the parental characters which
may be inherited by the offspring, it is clear that
any hypothesis of the nature of living matter must
take these things into account ; and though we
cannot unravel or even imagine it, we can at least
get some idea of the amazing complexity of the
substances which in thoughtless moments we group
together under the single name of 'protoplasm.'

1—2

4 HEREDITY [ch.

We will now attempt, by means of a few examples,
to illustrate some of the questions which must be
answered, and some of the facts which must be
brought into relation, by any consistent account
of the process of heredity. A tall man on the
average has taller children than a short man, but
if all the sons of a number of tall men were measured,
it would be found that they showed every gradation
in height between the tallest and shortest ; some
would be taller than the fathers, others shorter, but
every gradation between them would occur. Also,
if a tall man marries a short wife, the sons are neither
all as tall as the father, nor divided sharply into a tall
group and a short group ; again they make a graded
series from short to tall. But if we cross a tall
variety of the sweet-pea with a dwarf variety, all
the offspring are as tall as the tall parent, and among
the offspring of these crossed tails, some are tall and
some short, but none are intermediate. Here then
we get two distinct modes of inheritance, and also
two kinds of variation ; in the first case the character
varies in such a way that all intermediates are found
between the extreme conditions, and in the second
the individuals can be classified sharply into two
groups. Again, we cross a white mouse or rabbit
with a black one, and all the offspring may have the
grey-brown colour of the wild animal — we have pro-
duced what is called reversion to the wild type, and

I] INTRODUCTION 6

have obtained a form different from either parent.
But if we mate the same black parent with another
white individual, it may happen that all the offspring
are black, and instead of reverting to the wild form
they all follow one parent. If either the greys or
blacks produced in this way are mated together^
some of their young will be white ; although none
of the children of the original white individual
resembled their white parent in colour, yet the white
has appeared again among the grandchildren after
skipping a generation. In man, a colour-blind father
rarely has colour-blind children, but some of his
nephews and male grandchildren through the female
line are usually affected ; that is to say, the disease
appears in males but is transmitted by females.

It is clear from this short list of examples that
there are a number of different forms of hereditary
transmission, and our object must be, first to classify
them into groups in which the behaviour is similar,
and next to attempt to bring them under a common
scheme. And it is also clear that the different kinds
of heredity are associated with different kinds of
variation ; for example variation in height in man is
inherited diffferently from variation in colour-vision,
and both differ fi'om variation of coat-colour in
rabbits, in their inheritance.

A question of a different kind is the cause of
inherited diff'erences, and whether differences due to

6 HEREDITY [ch. i

the action of circumstances are inherited. Does a
man, for instance, who develops certain muscles by
frequent use, or who injures his health by excessive
drinking, have children with larger muscles or poorer
health in consequence? The question is frequently
answered in the affirmative, but it is part of the
province of the study of heredity to investigate the
matter, and in these and all other cases to decide
not only whether a character is inherited, but, if it is,
to what extent and in what manner it will appear in
the offspring.

CHAPTER II

VARIATION

We have seen that the subjects of Heredity and
Variation are so closely connected that one cannot
be considered apart from the other, for without
variation all the offspring of the same parents would
be exactly alike, and the study of heredity would
resolve itself into an investigation of the cause of
this likeness. But the actual problem is much less
simple ; it includes the questions how and why the
members of a family may differ from one another,
and according to what rules and by what means
these differences are transmitted to later generations.
In practice therefore the study of heredity is the
study of the manner and cause of the inheritance of
variations, and hence the nature ^of variation must
be examined before enquiry into its transmission.

Before the time of Darwin variations were fre-
quently regarded as abnormalities, inconvenient to
the systematist and of relatively small importance.
Every species was supposed to conform to the type

8 HEREDITY [ch.

originally created, and divergences from this type
were regarded as imperfections. But it was obvious
that there was always more or less fluctuation about
the type in different individuals, and breeders of
plants and animals made use of this want of uni-
formity to select the best specimens and so to
improve the race. The Natural Selection theory
of Darwin and Wallace supposes that a process
comparable with this takes place in nature, and so
brings about the adaptations of natural species.

Of the causes which induce variation nothing
definite was known, but Darwin's belief was generally
accepted that it is due to changes in environment
acting directly or indirectly on the organism. He
regarded the action of such changes as cumulative
through a number of generations, so that its effect
in producing variation might not be visible until the
change had acted on several generations. This belief
was founded on the observation that animals bred
in captivity appear to be much more variable than in
the wild condition, and the changed conditions of
life are supposed to induce the variation. But species
in nature are not by any means subject to uniform
environment, and thus their variability was ascribed
to similar causes.

Darwin and Wallace pointed out that variation
occurs in all parts of every species, that it appears
to occur in every possible direction, and to every

a] VARIATION 9

extent from very small to considerable range. They
therefore founded their theory on this type of
variability rather than on the occurrence of con-
siderable 'occasional variations' which are not
connected with the type by a series of intermediates.
It was not, however, until after the theory of
Natural Selection had obtained general recognition,
that any detailed study was undertaken of the actual
frequency and extent of variation, and its mode of
occurrence.

The accurate investigation of variation has thus
been in progress only for some twenty or twenty-five
years, and according to the methods adopted students
have become divided into two somewhat distinct
schools. One of these has devoted itself rather to
the attempt to observe and classify the different
kinds of variation, and the other, generally called
the ' biometrician ' school, to measure its frequency
and range. It will be convenient to consider the
results obtained by the second method first

If a character is chosen which can be accurately
measured, such as human stature, and a sufficiently
large number of individuals are observed, it will
commonly be found that there is considerable range
of variation, and that every gradation in size occurs
between the smallest and largest. Such variation is
spoken of as 'continuous/ as opposed to 'discontinu-
ous' Tariation in which individuals of two kinds occur,

10

HEREDITY

[CH.

which are not connected by intermediates. Further,
in cases of continuous variation it will appear that
one size is more common than any other, and, in the
simplest cases, that the individuals are progressively
rarer as the size of the structure considered diverges
more and more from the most frequent value. The
most frequent condition is named the ' mode,' and its

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700

800

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Fig. 1. Curve illustrating stature, the vertical scale representing
heights above 60 inches, the horizontal scale numbers of indi-
viduals who are supposed to be placed side by side in order of
their height.

size the 'modal value' for the character. For ex-
ample, if the heights of a large number of men were
measured, it might be found that they ranged by
every gradation from 60 to 7Q inches. If the measure-
ments were taken to the nearest inch, it might then
be found that a greater number had a stature of
68 inches than any other height, that the next most

II] VARIATION 11

frequent heights were 67 and 69 inches, and that the
more the stature differed from 68 inches in either
direction, the fewer would be the men having that
measurement. This could be represented graphically
by arranging vertical lines representing the heights
of every man in order of their height; a line joining
their tops would then rise rapidly at the lower end,
would be nearly flat as it passed over the men having
heights near the 'modal value' of 68 inches, and
would rise again steeply to the exceptionally tall
men at the upper end of the row (fig. 1). [13]^

A more instructive method of graphically repre-
senting the distribution of variation is to take a base-
line and divide it into equal parts, each representing
an equal increment in the structure measured. From
each division of the base-line a vertical line is drawn
representing by its length the number of individuals
having that measurement.

In the imaginary case taken above, the base-line
would have 17 divisions representing successive
heights of from 60 to 7Q inches; at each division
a vertical line is drawn which by its length repre-
sents the percentage of the population which have
that height (fig. 2). By joining the tops of the per-
pendiculars (ordinates) a curve, or more strictly a
polygon, is obtained which graphically represents the

1 For references see the end of the Volume.

12

HEREDITY

[CH.

distribution of the variation among the population
measured. The highest point of the curve represents
the mode for the character, and the extremes of
variation are where it touches the base-Hne. The
more numerous are the subdivisions into wliich the

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Stature in Inches

Fig. 2. Curves showing distribution of stature in women (mothers)
—dotted line ; and in men (fathers)— continuous liue. The curves
approach the 'normal curve.' (Data from Pearson. )

variable character is classified, tlie more nearly the
line joining the tops of the ordinates will approxi-
mate to a smooth curve ; e.g. if the population were
measured to the nearest quarter of an inch instead
of to the nearest inch there would be four times

II] VARIATION 13

as many ordinates and the curve would be nearly
smooth.

It is clear that a curve of this kind can be used
for comparing the variability of different characters,
for the greater the variability of the population the
wider Avill be the base ; consequently the curve for
a very variable character will be relatively low and
wide, that for a slightly variable one measured in the
same scale will be tall and steep. A curve of this
kind, which is quite similar on either side of the
longest perpendicular ('median,' representing the
modal value), may be obtained by plotting any
measurements which vary fortuitously around a most
frequent value, and such a curve is called a ' normal
curve.' For example, if a large number of beans in-
cluding equal numbers of white ones and black ones
were placed in a sack, and drawn out ten at a time
without selection of colour, most frequently five white
and five black would be drawn, less often six of one
colour and four of the other, more rarely seven and
three and so on to the rarest case of ten of one colour.
If the numbers of white beans in a draw are plotted
along the base-line, and the ordinates represent the
number of draws for each combination, a polygon
approaching the normal curve will be obtained.
Variation which gives a normal curve when plotted
in this way is spoken of as normal variation.

As mentioned above, the steepness of the curve is

14 HEREDITY [oh.

a measure of variability, and this can be expressed
by taking a point in the curve, the perpendicular
from which to the base-line divides the area, enclosed
by the curve, the median and the base-line, into two
equal parts. Or, differently expressed, the perpen-
dicular divides the curve in such a way that the
number of individuals between it and the mean is
the same as that between it and the extreme. The
distance of this perpendicular may be used as a
measure of the variability of the character considered,
for clearly the greater the variability (and thus the
flatter the curve), the further this perpendicular will
be from the median \

In many variable characters, the frequency of
variation below the mode is not exactly equal to
that above it, in which case the curve will be steeper
on one side of the mode than on the other, and the
average value for the character ('mean') will not be
identical with the mode. For example, if the varia-
tion in the number of children in a family were
plotted in this way, the sizes of families would range
from 0 to about 20, but the most frequent number
would perhaps be four. Four would then be the

^ In practice, not this perpendicular, bat another rather further
from the median is used, which for practical purposes is more con-
venient. The distance of this perpendicular, measured in units of
the horizontal scale, is called the 'standard deviation' and is regu-
larly employed as a measure of variability.

11]

VARIATION

15

modal value, but the average or meau might be about
six ; the curve would rise steeply to the mode, and
fall away more gradually to the maximum number
(fig. 3). Such a curve is described as 'skew.' In

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Size of Family
Fig. 3. Curve showing distribution of size of 3837 families in
America containing deaf-mutes. (After Schuster, ' Hereditary
Deafness.' Biometrika, Vol. iv. 1906, p. 474.)

extreme cases the mode is at one end of the curve,
when variation takes place only on one side of it,
e.g. in the marsh-marigold, the most frequent number
of 'petals' is five, but there may rarely be six, seven
or eight, but practically never less than five, so that
in plotting the frequency a ' half-curve ' is obtained
(fig. 4).

16

HEREDITY

[CH.

Another rather frequent condition is that the
curve has two maxima or modes (fig. 5), indicating
that a large number of individuals have a low measure-
ment, a less number are intermediate, and again a
larger number have a higher measurement Species
which vary in this way are called ' dimorphic/ or if

300

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100

5 6 7 8

Eig. 4. ' Half-cnrve ' representing the number of ' petals ' on 416
flowers of the marsh-marigold {Caltha palustrls). (After de Vries.)

there are more than two peaks to the curve, 'poly-
morphic' Further, it is possible that the two parts
of the curve should be entirely separate, if inter-
mediates between the low and high groups are
completely wanting. This type of variation is spoken
of as ' discontinuous ' in contrast to the ' continuous '

n]

VARIATION

17

variation hitherto considered. It is possible that
dimorphic cases in which intermediates exist are
really essentially discontinuous, but that the two
groups into which the species is divided each exhibit
continuous variation about the mode for the group,
to such an extent tliat the higher members of one

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Lengths of Forceps in Millimetres

Fig. 5. Curve with two modes, representing frequency of lengths
of forceps of male Earwigs from the Fame Islands. (After
Bateson.)

group overlap the lower of the other. For example,
if the modal (most frequent) stature for a race of
men were 68 inches, and for the women 62 inches, it
might happen that on plotting a frequency curve for
the stature of adults including both sexes, a curve
D. 2

18 HEREDITY [ch.

would be obtained having two maxima, one near 62
and another near 68. Yet the stature might be a
definite sexual character, and hence essentially as
discontinuous as the sexes themselves. This distinc-
tion between continuous and discontinuous variation
may seem unimportant in itself, but when its in-
heritance is considered the distinction becomes of the
first importance.

Clearer examples of discontinuous variation are
given by such characters as colour, or by organs
which are repeated in series, such as vertebrae and
ribs, the segments of a worm or the petals of a
flower. When variation occurs in this latter group
it is generally complete, so that the different forms
are visibly discontinuous. In the case of colour in
the skin or hair in animals, or petals of flowers,
discontinuity is sometimes less apparent, and grading
frequently occurs, but even in apparently graded
cases the inheritance of the character may often
reveal discontinuity. For example, a piebald animal
might be thought to be intermediate between the
fully-coloured and albino (white with no pigment),
but breeding tests would at once show that piebald-
ness was an independent character, which cannot be
regarded as in any sense intermediate between the
other two conditions except in general appearance.
The same applies to such cases as the ' silver ' cat or
rabbit, or the pale purple sweet-pea ; the cause of

II] VARIATION 19

the pale colour is entirely distinct from the cause
of the absence of pigment in the white varieties of
those species.

The recognition of the importance of discontinuity
in variation, which we owe chiefly to the work of
Bateson in England and De Vries in Holland, is one
of the chief advances which the study of the subject
has made since the time of Darwin.

One other distinction between different kinds of
variation must be mentioned here, which will be
discussed more fully in subsequent chapters. The
kinds of variation mentioned above are all inborn, or
inherent in the individual and to a great extent in-
dependent of its manner of life. But it is well known
that the continued use of an organ or structure, or the
prolonged action upon it of some external stimulus,
may alter its form or cause it to assume a condition
different from that which it would have had if these
influences had not acted. In general, an organ tends
to adapt itself either to the uses to which it is put or
to the action of the environment which surrounds it.
The muscles of a limb used for strenuous work in-
crease in size and strength, or a part of the skin
continually exposed to bright light develops a deeper
colour than if it is covered. The converse process is
also true ; an organ which is not used or exposed to
its normal stimuli tends to diminish, and become less

2—2

20 HEREDITY [ch.

adapted to the use to which it is normally put. Such
characters as these, arising in response to a stimulus,
and not appearing in its absence, are technically
called ' asguired characters,' a phrase which it will be
necessary to use rather frequently in the following
pages. As a rule, such 'acquired characters' are
adaptive, that is, they render the organism or
structure better fitted to its surroundings than if
they had not been developed. The older students of
heredity never doubted that these acquired characters
were inherited as strongly as the inborn characters
discussed above, but since the publication of
Weismann's theory of heredity (see Appendix I) with
the great body of evidence which he has collected
on the other side, opinion has turned increasingly
towards the belief that acquired variations are not
transmitted. Weismann regards the germ-cells as
essentially distinct from the rest of the body, so
that acquired modifications of the body cannot be
transmitted because the germ-cells are not affected.
The germ-cells collectively, or rather that part of
them which is concerned with the transmission of
hereditary characters, he calls ' germ-plasm,' the rest
of the body consisting of ' body-plasm ' (or * soma '),
and he regards * acquired ' modifications as affecting
body-plasm only. A developing germ-cell gives
origin to both germ-plasm and body-plasm of a new

Ii] VARIATION 21

individual, and hence characters borne by the germ-
plasm appear in the body ; but since body-plasm
cannot be converted into germ-plasm, modifications
of the body cannot be transmitted to offspring.

The possible inheritance of acquired characters is
treated more fully in a later chapter.

r

CHAPTER III

THE CAUSES OP VARIATION

In the last chapter the distinction has been
explained between continuous and discontinuous
variation ; some confusion has however arisen with
regard to the terms used in describing these conditions.
Continuous variation about a mean (or more accur-
ately modal) condition is sometimes spoken of as
' fluctuation/ but as will be seen below this kind of
variability probably includes two very distinct groups
of facts. It may include inherent variability arising
in the germ-cells, or it may include differences in the
adult condition having their origin in different effects
of environment during growth. Some writers have
used the word ' fluctuation ' for this latter condition
only.

Discontinuous variation is sometimes called
'mutation,' a word which also has been used in
several senses. It may mean the appearance of a
form varying discontinuously from the type, or it
may be applied to the discontinuous character itself.

OH. Ill] CAUSES OF VARIATION 23

A more serious source of confusion is that the term is
used by some to denote any discontinuous variation
arising ' spontaneously,' by others for cases in which
the variety differs from the type in several apparently
distinct characters, and not only in one, so that the
new form constitutes an ' elementary species.' Since
in studying heredity it is usually important to con-
sider distinct characters separately, it may be per-
missible to use the word for the origin of a form
differing recognisably from the type and not connected
with it by true intermediates.

It has already been pointed out that very little is
accurately known about the causes of variation, and
it is not impossible that the different forms of varia-
tion have different origins. Most writers agree that
the ultimate cause must lie in the action of environ-
ment in some form, but as Darwin clearly stated in
the Origin of Species the environment may act
directly or indirectly. In variation of size for ex-
ample, it is clear that the supply of nourishment,
etc., during growth may have considerable influence
on the size of the adult, and such variation will
commonly be continuous owing to the evenly graded
action on different individuals. In these cases the
action is direct. If, however, Weismann's theory of
germ-plasm and body-plasm is correct, such action may
affect only the body and not be transmitted to off-
spring. It is also possible that the germ-cells may

24 HEREDITY [ch.

be indirectly affected, giving rise to variation in the
offspring; in such a case, however, there is no
necessity that the effect on the offspring should be in
any way similar to the direct effect of the conditions
on the parent. Nothing is know^n of the nature of
possible effects of environment on the germ-cells ;
the action may possibly be effective immediately and
give rise to variability in the next generation, or it
may be that the effects are cumulative and only cause
visible changes after several generations have been
exposed to the same influences. Galton [13] suggested
that the organism may have a certain * stability,' but
that influences acting for several generations may
have a cumulative effect which will gradually alter the
equilibrium until it is finally upset and falls into a
new condition of stability, giving rise to an apparently
sudden variation. A chemical analogy may make this
clearer. If litmus is added to an alkaline solution its
colour will be blue. Acid may now be added drop
by drop to neutralize the alkali, and suddenly, when
the solution becomes acid, the litmus turns red.
Examples of variation of which this may possibly be
an analogy will be given below.

With regard to the action of environment on the
body many facts are known, but it is not certain that
they really have any bearing on the question of the
origin of variation. For variations so produced are
'acquired characters,' and in many cases at least

Ill] CAUSES OF VARIATION 25

there is no evidence that they are inherited. For
example, many butterflies have two generations in
the year, one of which lives through its whole life-
history in the summer and the other passes the winter
as a pupa (chrysalis). In some cases the two genera-
tions are strikingly different, and it has been shown
that by freezing the pupae of the summer brood at
the right stage, specimens like the spring brood can
be obtained. The difference between the two genera-
tions is thus due to the action of cold on the pupa.
But the two forms regularly alternate in nature and
the effects of cold are not inherited. In plants, some
species produce quite different leaves according to
whether they are grown in water or in dry soil, but
the conditions act on the individual, and do not affect
its progeny. In such a case, what is inherited is
the faculty of making a certain definite response to
definite conditions, and this faculty is present whether
the conditions operate or not. In man such diseases
as tuberculosis are commonly called hereditary ; this
however does not mean that the child has the disease
because his parent had it, but that the parent had
a constitution liable to that disease, and the child
inherits a similar constitutional liability. If the
parent had never been exposed to infection the
child would still inherit the liability, for what is
transmitted is not the disease or its effects, but the
faculty of acquiring it if exposed. It will be found

26 HEREDITY • [ch.

that most cases which at first sight seem to support
the theory of the inheritance of acquired characters
are equally explicable in the view that both parent
and offspring are susceptible to the action of the ex-
ternal factor ; what is inherited is not the character
acquired, but the innate power of acquiring it.

But it is always possible that some_ forms of
external conditions may act on both the body-cells
and germ-cells concurrently, and produce similar
efiects in each. For example, it may happen that
extremes of temperature produce striking colour-
variations in certain butterflies. Weismann has
pointed out that, according to his theory, in a
developing butterfly the determinants for producing
colour not only exist in the germ-cells which will
transmit the character to the offspring, but also in
the embryonic cells of the body which go to produce
the coloured parts of the perfect insect. If extremes
of heat or cold cause changes in the colour-deter-
minants in the developing wings, so that abnormal
colours result, it is possible that the determinants
in the germ-cells which transmit the colour-pattern
to the next generation will be similarly modified, so
that the offspring will show similar abnormalities.
This would not be the transmission of an 'acquired
character ' in the strict sense of the expression, but
the simultaneous modification of body and germ-cells
in the same manner.

Ill] CAUSES OF VARIATION 27

But as mentioned above, it is possible that the
same factor acting on body and germ-cells may pro-
duce different results in the two cases, so that the
individual on which the influences have acted may
show one modification and its offspring another. It
is also possible that not aU the germ-cells will be
affected alike, and so among the progeny some will
show modification and others not, or some may be
differently affected from others ; for the conditions of
stability of different germ-cells may conceivably be
different. Certain experiments on insects give reason
for supposing that this is so. The results obtained by
Fischer, Standfuss and others from exposing pupae
of butterflies and moths to abnormal temperatures,
while not entirely concordant among themselves, on
the whole indicate that moderate degrees of heat and
cold tend to alter in the same way the whole batch of
insects treate<l, often in the direction of varieties of
the species naturally occurring in warmer or colder
climates. But excessive heat or cold causes extreme
variations among only a small proportion of the
insects treated, and among the offspring of these ab-
normal specimens only a small fraction are abnormal,
and some of these have not the same abnormality
as the parents. These observations, together with
the fact that the variations produced by heat, cold,
and other disturbing factors, may all be similar,
suggest that extreme conditions may upset the
stability of the type, causing abnormalities to appear.

28 HEREDITY [CH.

and that some of the germ-cells may also be altered,
but not necessarily in the same manner as the body-
cells.

An American zoologist, Tower, describes the pro-
duction of mutations by the action of environment in
a beetle (Leptinotarsa). In nature he found about
one such variation among 6000 specimens ; when bred
in captivity -they were more frequent, but when the
fuU-grown beetles were exposed to extremes of heat,
humidity, etc., during the maturation of the eggs, the
offspring may include a large proportion (over 80 per
cent.) of * mutations.' These were of several distinct
kinds, like those rarely found in nature, and when
bred together they are stated to breed true. In this
case the abnormal conditions produced no effect on
the individuals exposed to them, for they already had
their final form, but as their eggs were matured
under these conditions the action took effect on the
eggs, and mutation resulted among the offspring.
When part of the eggs of an individual were matured
under abnormal, another part under normal con-
ditions, mutation occurred only among the offspring
in the first case, all the beetles in the second being
normal. It should be noted that as in the experi-
ments with butterflies the effect of changed conditions
was not specific ; the same conditions may produce
more than one kind of mutation in the same batch of
eggs, and some eggs were not affected at all. In
both cases the abnormal environment seems to upset

Ill] CAUSES OF VARIATION 29

the equilibrium, but the eifects may differ in different
individuals. It is the nature of the organism or
germ-cell affected which determines whether and
to what extent the change shall take place ; the
environment merely supplies the stimulus.

It will be seen that our knowledge of the causes
of variation, in so far as these are connected with
environment, is very incomplete and unsatisfactory,
for although it is fairly clear that conditions may
sometimes disturb the equilibrium of the germ-ceUs
and provide a stimulus to variation, yet we have no
knowledge of the way in which the stimulus acts
and can make no prediction as to the direction the
variation will take. Before leaving the subject, one
other cause of variability must be mentioned — the
effect of crossing different races in producing varia-
tion, it frequently happens that the result of
crossing distinct races is that the crossed individuals
differ from either parent ; sometimes in the direction
of increased vigour, as was pointed out by Darwin,
and other more recent observers ; sometimes by the
development of characters apparently not possessed
by either parent, as in the case of 'reversion on
crossing.' The cause of this latter phenomenon will
be discussed in a later chapter. In the subsequent
generations from the cross great diversity may often
appear, and Darwin supposed that the mingling of
two distinct germinal stocks had an effect in dis-

30 HEREDITY [ch.

turbing the equilibrium similar to tbat produced by
change of environment. To some extent this is
doubtless true, but recent developments of the
theory of heredity have afforded a more exact ex-
planation, m the recombination of the different
characters of the two races which are crossed. A
fuller account of 'variation induced by crossing'
must therefore be postponed until the principles of
heredity have been discussed.

One further question should be mentioned before
proceeding to the subject of heredity, namely, the
relative importance of 'inherent' and 'acquired'
characters in making up the sum of characters of a
mature individual It is often assumed, especially in
human cases, that the environment has a prepon-
derating influence in shaping the individual. In a
certain sense this is true, for many characters can
only develop in a suitable environment ; muscles
must be exercised to be properly formed and the
mind cannot develop its full powers if it is never
used. But the study of variation leads inevitably
to the conclusion that the inherent characteristics
are all-important, and that the effect of environment
is not much more than to give them opportunity to
develop. This is perhaps most impressively seen in
the case of 'identical twins,' as has been shown by
Galton [12]. There is reason to believe that such
twins are produced by the division of one ovum, and

Ill] CAUSES OF VARIATION 31

even if exposed to different conditions they remain
through life much more alike than ordinary brothers
who may be brought up under precisely similar
surroundings. The same fact is still further em-
phasised by the study of heredity.

CHAPTER IV

THE STATISTICAL STUDY OF HEREDITY

In studying heredity, either of two methods may
be adopted. We may either choose a character and
observe or measure its development in a large number
of parents and in their childi'en, and so deduce the
average extent of resemblance between parents and
children for that character ; or we may consider a
number of individual cases separately, and endeavour
to discover the manner in which the character appears
in the children who have parents or ancestors pos-
sessing it. With regard to the first method Prof.
Pearson has written * We must proceed from inheri-
tance in the mass to inheritance in narrower and
narrower classes, rather than attempt to build up
general rules on the observation of individual in-
stances.' And ', . .the very nature of the distribution
...seems to indicate that we are dealing with that
sphere of indefinitely numerous small causes, which
in so many other instances has shown itself only
amenable to the calculus of chance, and not to

OH. IV] STATISTICAL STUDY 33

the analysis of the individual instance' [25, 'JNIath.
Contrib. III.' Phil. Trans. Roy. Soc. A, 1896, p. 255].
The second method on the other hand has been used
in cases where the causes of variation appear to be
few and definite, and seeks to isolate these causes.
The first method is thus clearly adapted especially to
characters which vary continuously and which can
be measured ; the second to characters which vary
discontinuously and can be sharply separated into
classes. The first method gives on the whole the
average intensity of inheritance, but little information
with regard to its probable development in individual
cases ; the second attempts to answer the question
in what manner the character wiU be distributed
among the offspring in any family.

The founder of the modern statistical, or as it is
now often called, the biometrical study of heredity was
Sir Francis Galton, and its leading exponents have
been Professor Karl Pearson and the late Professor
Weldon. In this chapter an attempt will be made to
explain the fundamental principles on which the
biometric methods rest, and to outline the chief
results obtained ; the methods themselves frequently
require mathematics of an advanced order, and for
the study of them the reader is referred to the books
and papers dealing with the subject mentioned in the
bibliography.

It has already been seen that in the case of a

D. 3

34 HEREDITY [ch.

character which varies continuously about a mean or
mode, the greater the divergence from the mode in
either direction, the fewer will be the individuals
showing that divergence. In the case of human
stature, if the modal height of a population is 68
inches, there will be fewer men of 64 or 72 inches
than of 66 or 70, and still fewer of 63 or 73 inches.
If now the sons of all the men having a given diver-
gence were measured, and it were found that they
averaged as gi'eat a divergence from the mode as
their fathers, it is clear that on the average the
height of the sons would equal that of their fathers.
This does not mean that every son would exactly
resemble his father in stature, but the sons would
vary about the paternal stature equally above and
below it, and when plotted in a curve their statures
would make a curve having the paternal stature as
its mode. The average stature of the sons would
then be completely determined by the stature of the
fathers. If on the other hand the stature of the
father had no relation with that of his sons, it is
clear that the statures of the sons of fathers of any
height would vary about the mean of the general
population considered. In practice it is found that
the modal value for sons of fathers of a given height
is between the height of their fathers and the mode
of the general population. That is to say, if the fathers
diverge a given amount from the general mode, their

IV] STATISTICAL STUDY 35

sons will on the average diverge less ; they will vary
about a modal value lying between the general mode
and the fathers' measurement. This fact is called
* regression.' It sometimes seems paradoxical to those
who have not considered it that the mean deviation
of children from the general mode is always less than
that of their parents. But of course it does not mean
that all sons of tall fathers will be shorter than their
fathers ; some will be as tall or taller, but the sons of
a number of fathers of given stature will vary about
a mode lying between the fathers' stature and the
mode of the whole population.

Now it is plain that the amount of regression is a
measure of the intensity of inheritance ; if the modes
for sons of fathers of every deviation have deviations
nearly as great as those of the fathers, the intensity of
inheritance would be high ; if the modes for the sons
deviate but slightly from the general mode, whatever
be the deviation of the fathers, the intensity would
be low. A deJQnite case may make this clearer.
Suppose the modal stature of the population is 68
inches ; it might then be found that for fathers of 64
inches (i.e. deviating 4 inches below), the height of
the sons ranged from 61 to 72 inches. If, however,
their modal value had a deviation only slightly less
than the fathers' deviation, say with a mode at 65
inches, the regression would be slight and the intensity
of inheritance high ; if the sons' mode had a deviation

3—2

36 HEREDITY [ch.

much less than the fathers', say at 67 inches, the
regression on the general mean would be considerable
and the intensity of inheritance low. If then we can
find means of determining the ratio between the
deviation of sons in general and the deviation of their
parents, we shall have a measure of the intensity of
inheritance for the character considered. This ratio
is called the 'coefficient of correlation' between
father and son for that character. It should be
noticed that correlation simply means that two
quantities vary in relation to each other ; the corre-
lation between parents and children is a convenient
method of estimating the intensity of inheritance,
but correlation exists between any two related
variables, e.g. between the measurements of two
limbs in the same individual, such as an arm and a
leg, or between the numbers obtained in successive
throws of dice, if not all the dice are picked up for
the second throw. The correlation between the same
measurement in brothers may be used as a measure
of inheritance, for two brothers resemble each other
more than two chance individuals because they are
children of the same parents.

The principle of obtaining a coefficient of corre-
lation between father and sons is as follows. It will
be convenient to assume that the variability of the
character considered is normal, i.e. that the frequency
curve falls evenly on either side of the mode, so that

IV]

STATISTICAL STUDY

37

the mode is identical with the mean. Stature in
inches may be taken as an example. A large number
of fathers and a son of each are measured to the
nearest inch ; it can then be found what is the
average measurement of sons for fathers of each

«~ u
§^

3"-

go

W a
e "

u u

60
61
62
63
64
65
63
67
68
69
70
71
^2
73
74
75
76
77

k

\

\

r

\

U

^

;

\

)

^
)

^

\

Vj

S'

<\

;

"^

^

C

0

v..

<

D

\

'^

s

\

"v

^N*

J

\

V,

■F

\

\

\

\

B

^

60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77

Statures of Fathers
Fig. 6. Diagram of correlation between fathers and sons.

height from the lowest value to the highest. It will
be found that the mean deviation of the sons from
the mean of the population is less than the deviation
of the fathers for each class of fathers. The average
ratio between the mean deviations of the sons to the

38 HEREDITY [ch.

deviations of the fathers is then the coefficient of
correlation between father and son for this character^
This is more clearly seen in diagram form.

If a square is made with its sides divided into
equal lengths corresponding to equal increments in
stature from 60 to 76 inches, the top may represent
the scale of statures of fathers and the side the scale
of mean statures of sons for each class of fathers. If,
then, there were complete correlation between fathers
and sons, the mean stature of sons of fathers 62 inches
high would be 62, of fathers of 63 inches, 63, of 64,
64 and so on. If on the other hand there were no
correlation, the means of the sons of every class of
father would be the mean of the population (68).

In the first case the line joining the points repre-
senting the means of the sons would be a diagonal
running from corner to corner (AB), in the second
case a horizontal line running across the middle (CD).
But if the correlation is between these extremes the
line would lie between the diagonal and the horizontal
(EF), and the greater the correlation the steeper
would be the slope of EF. The steepness of this line
is thus a measure of correlation, and since all these
lines pass through 0 in the middle of the square, the

1 It is assumed throughout that the variahiHty of the sons is similar
to that of the fathers. If their variability were different this would
have to be allowed for. The variation is also assumed to be normal,
so that the mode in each case coincides with the mean.

IV]

STATISTICAL STUDY

39

slope is measured by the size of the angle EOC. The
angle made by the diagonal at 0 is 45°, the tangent
of which is 1 (unity). If there were no correlation
the angle would vanish, EF coinciding with CD, and
the correlation coefficient would be 0. Intermediates
are represented by the value of the tangent of the
angle EOC. In practice it rarely happens that the
points representing the means of the sons for each
class of fathers lie in a perfectly straight line ; when
they approach it closely the correlation is called
'linear'; when they depart from it considerably, it
is called ' skew.'

Table of i/ntensitj/ of parental inheritance in different
species. [Froin Pearson.)

Species

Character

Mean
value

Numbej

of Pain

used

Man

Stature

•506

4886

Span

•459

4873

Forearm

•418

4866

Eye Colour

•495

4000

Horse

Coat Colour

•522

4350

Basset Hound

Coat Colour

•524

823

Greyhound
Aphis

Coat Colour
Eatio of right Antenna

•507
•439

9279
368

(Hyalopterus
trirhodus)
Water-flea

to Frontal Breadth

(non-sexual reproduction)

Ratio of Basal Joint of

466

96

(Daphnia magna)

Antenna to Body length
(non-sexuaJ reproduction)

40 HEREDITY [ch.

Prof. Pearson and his collaborators have worked
out the correlation between parent and child for a
number of measurable characters in Man, Animals,
and Plants, and they find that the numbers group
themselves about a value not far from 0*48, varying
from 0-42 to 0-52. That is to say, on the average the
offspring deviate from the mean about half as much
as the parent.

The parental correlation hitherto discussed has
taken no account of the second parent, for if in-
dividuals mate at random the one parent may be
considered alone, and the second will on the average
have the mean value for the general population. But
it is clear that one may take for the parental value
in each class the mean of the two parents (making
allowance for any difference in measurement due to
sex), and plot the means of the sons (or daughters)
against the classes so produced. The value derived
from taking the mean of father and mother is called
the * mid-parent,' and the correlation so arrived at
would give the measure of resemblance between
children and their mid-parents. This is naturally
higher than the correlation observed when only one
parent is considered; for if both parents deviate in
the same direction from the mode of the population,
the children will average a gi'eater deviation than if
only one does so, and still more than if one deviates
in one direction, the other in the opposite. We thus

IV] STATISTICAL STUDY 41

obtain a measure of the amount contributed to the
offspring by the two parents together, but even now
we do not find the correlation complete (I'O) because
the contributions from previous ancestors have also
to be taken into account.

Galton was the first to introduce the idea of the
* mid-parent,' and he went on to attempt to estimate
the average contribution to the children from each
generation of ancestors. Since the correlation be-
tween offspring and mid-parent is not complete,
part of the heritage, which is not visibly present in
the parents, must be contributed from more distant
ancestors. Galton concluded from the data he collected
that on the average half the heritage of an individual
may be taken as derived from the two parents, one
quarter from the four grandparents, one eighth from
the great grandparents, and so on, the whole series
(I? h h tV — ) adding up to unity. Pearson estimates
the average correlation between offspring and one
parent, as about '5, of offspring with a grandparent
as '33, with a great grandparent as '22, the correlation
coefficient with an ancestor of each generation being
I of that of the next below ; these numbers, however,
are not in any way comparable with Galton's series
•5, "25, '125, etc. Galton attempted to estimate the
amount of the heritage received from the 'mid-
ancestor ' of each generation independently of what
was received from other generations ; but in the

42 HEREDITY [ch.

metaphor of bequests of property, he calculated that
of the total heritage of an individual, half on the
average was bequeathed by the parents, one quarter
by the grandparents direct to the grandchild and so
on, Pearson's series '5, '33, '22 etc. gives the average
measure of resemblance between children and an
ancestor of each generation, which is clearly a totally
different thing. From this series he has worked out
figures corresponding to Galton's, making the series
•6244, -1988, "0630, i.e. he finds that the parental
bequest is greater and the ancestral bequests less
than Galton estimated. From the results obtained
first by Galton and later by Pearson has been formu-
lated the 'Law of Ancestral Heredity,' which has
been stated in various forms, perhaps the most
general being 'the mean character of the ofispring
can be calculated with the more exactness, the
more extensive our knowledge of the corresponding
characters of the Ancestry ' (Yule [44]). But it should
be noted that there is an important diflbrence between
Galton's original statement of the law, and the later
statements of Prof. Pearson. Galton wrote that ' the
two parents between them contribute on the average
one-half of each inherited faculty, each of them
contributing one-quarter. The four grandparents
contribute between them one-quarter, or each of
them one-sixteenth ; and so on.' He regarded this
as a physiological statement of the way faculties

TV] STATISTICAL STUDY 43

were transmitted, while Pearson, in his later writings
at least, regards the law simply as a statistical
description of what is found when large numbers
are observed in mass.

It has been mentioned that the characters which
especially lend themselves to statistical treatment
are those which vary continuously and which can be
accurately measured, but Prof. Pearson has applied
similar methods to discontinuous characters, which
can be classified into groups but not measured, for
example coat-colour in horses. He finds as the
results of his enquiries that the inheritance of such
characters can be stated in terms similar to those
obtained with measurable characters, so that the
principle of ancestral correlation leading up to the
law of ancestral heredity may be applied to these
characters also. But whatever may be the case with
characters which vary continuously, it will be seen
below that discontinuous characters are commonly
alternative in their inheritance, i.e. there is no
blending, but the ofispring exhibit one or other only ;
and in some at least of these cases, the character of
the offspring cannot be calculated with any more
exactness if the ancestry is known than if it is not.
Such instances show clearly that although the law
may be statistically true when applied to considerable
populations, it gives us no clue to the physiological

44 HEREDITY [ch.

processes which determine the transmission of char-
acters from one generation to another.

Another argument that has been used against the
physiological validity of the law of ancestral heredity
is based on the work of Johannsen and others who
have obtained results similar to his in other cases.
Johannsen worked at the inheritance of weight of
seeds in beans and in barley, and self-fertilised the
plants investigated for a series of generations so as
to isolate what he calls ' pure lines.' He found that
in beans, for example, the seed-weights of a mixed
population gave a normal frequency curve — the
weights varied continuously and evenly about a mean
value. The beans on an individual plant when the
flowers are self-fertilised also form a normal curve
about a mean, but this mean is not necessarily identical
with that of the race in general. If now the flowers
on such an individual are self-fertilised, and the beans
produced are sown, the mean weight of the beans op
all the daughter plants will be identical with the
mean of the beans on the parent, i.e. among the
ofispring produced by self-fertilisation there is no
regression towards the mean of the race. It thus
makes no difiference whether large or small seeds are
chosen within the pure livie ; the mean weight of the
seeds on plants grown from the smallest and largest of
the parental beans (seeds) is in each case equal to the

ivj STATISTICAL STUDY 46

parental mean. Selection, therefore, within the pure
line has no effect in altering the mean weight of the
seeds, for the differences in seed weight within the
line are not inherited. The probable cause of this
is that the differences between the seeds on a self-
fertilised plant are due to the action of external
circumstances ; the position of the beans in the
pod or the position of the pods on the plant cause
differences in the nutrition which allow some beans
to grow larger than others. These differences are
'acquired characters,' and we have here additional
evidence that such are not inherited. It is to variation
of this type that the term ' fluctuation ' is applied by
some authorities.

It is clear then that if selection is made among
beans harvested from a mixed population, on the
whole the larger beans will belong to pure lines
having a higher mean, and thus selection for a few
generations will isolate pure lines having a high
value, and the mean of successive generations will
rise until the largest pure lines have been isolated.
Beyond that point further selection will have no
effect. This is precisely the result arrived at by
Prof. Pearson from a study of selection within a
mixed population ; the mean will rise rapidly on the
first selection, more slowly later, until in very few
generations it reaches a point at which selection
has no appreciable effect. Pearson calculates that if

46 HEREDITY [ch.

selection now ceases, the selected race will very
slowly revert towards the mean of the general popu-
lation. But, as has been seen, this conclusion is based
on the assumption that continuous variation is due to
the concurrent action of an indefinite number of small
independent causes. If, however, Johannsen is correct,
we may divide these causes into two classes : the
causes which induce ' fluctuation ' as explained above,
which agree with Pearson's requirements, and the
cause or causes which give rise to the difference
between one pure line and another. Now this second
group may conceivably consist in a single factor of
the nature of a small ' mutation,' and if so, by isolating
the pure line this factor is also isolated, and no return
towards the mean of the general population need
take place. According to Johannsen this isolation
can be efibcted in one generation by selecting the
self-fertilised plants which have the highest average
yield, instead of selecting the heaviest beans them-
selves.

We thus obtain by experiments such as those of
Johannsen a new conception of the possible nature of
continuous variation ; it may be due partly to ' fluctua-
tion' brought about by the action of environment
and not inherited, partly to a series of small step-
wise 'mutations,' each of which owing to fluctuation
overlaps the next, and can only be isolated when it is
possible to breed pure lines. It should be said that

IV] STATISTICAL STUDY 47

there is as yet no certainty that this account of
continuous variation is sufficient to cover all cases ;
it is a suggestion of possibility rather than a state-
ment of fact.

We have seen that there is reason to believe that
the Law of Ancestral Inheritance is true only when
applied to a large number of individuals considered
in mass, or, as it has been put, that it is a statistical
rather than a physiological law. In individual cases
it is not true that the offspring need be influenced
by ancestors beyond the parents, but in other cases,
as will be seen in dealing with Mendelian heredity,
these ancestors have important effects, so that
statistically it is possible to say what is the average
influence of the ancestors of any generation upon
the offspring. Now in cases where it is possible to
define rigidly single characters, much more is learned
from the physiological than from the statistical
method, but where no such rigid separation of
characters is possible the statistical law is the only
one that can be applied. This is particularly the
case in characters which vary continuously, or where
the categories into which the character falls overlap
one another, as for example in Johannsen's beans.
Further, the statistical method is frequently the only
one which is available when experiment is impossible
and when our knowledge of the facts is based solely
on numerical data from observed cases, and this of

48 HEREDITY [CH.

course applies especially to inheritance in Man,
where experimental evidence is not available.

By collecting family histories of distinguished
men, Galton showed long ago [15] that exceptional
mental qualities were inherited; and this work
has recently been much extended and made more
definite by Professor Pearson and his school. It
is commonly believed that exceptionally gifted men
do not have distinguished sons, but this like many
other popular beliefs is only partly true. It
has been seen that if an individual deviates a
certain amount from the general mean, his children
will on the average deviate less, because when the
whole ancestry is taken into account, the eflect of
previous generations is to cause regression on the
mean of the population. And since the theory of
regression depends on the assumption that variation
is due to the existence of a large number of inde-
pendent causes acting concurrently, it is unlikely
that among the limited number of offspring of one
exceptional man any one child will unite in himself
the same combination of factors as went to make up
the father's character. Further, it is improbable that
an unusually gifted man will marry a wife equally
gifted in the same manner, and the mother's influence
on the children is closely similar to that of the father.
It cannot therefore be expected that all great men
should have equally great sons, but they are far more

IV] STATISTICAL STUDY 49

likely to have exceptional sons than are mediocre
men, and if the mother is also exceptional in the
same direction this probability is greatly increased.
In the last feAV years the intensity of inheritance
in such characters has been given numerical ex-
pression. Professor Pearson, after working out the
statistical laws of inheritance in many physical char-
acters of man, animals and plants, has applied the
same methods to what are called the mental and
moral attributes. Characters were chosen such as
vivacity, popularity, conscientiousness, temper, ability,
hand-writing, which were estimated by reports from
school-teachers on the children in their schools ;
and also intellectual ability as shown in university
examinations or by the position in a public school
at a particular age (Schuster [10])\ All these, when
investigated by the same methods as were devised
for the coat-colour of horses or eye-colour in man,
are found to give results closely in accord with those
obtained for physical features. The conclusion is
therefore reached .that not only bodily characters,
but also those of the mind are essentially determined
by the hereditary endowment received from the
parents. This result is of great importance practi-

^ In these characters the resemblance between parent and child
cannot of course be estimated directly, but it has been pointed out
above that the resemblance between brothers may be used as a test
of the intensity of heredity.

D. 4

50 HEREDITY [CH

cally ; it shows how little room is left in the
development of the individual for the effects of
environment even on the intellect or mind in the
broadest sense of the word ; no doubt the direction
which intellectual development takes is to a con-
siderable extent determined by circumstances, but
the kind of mind is irrevocably decided before the
child is bom. Still less is there room for the
inheritance of the mental acquirements made by the
individual during his life, and hence the hopes held
out of improving the race by education and by
special care of the dull or feeble-minded are illusory,
except in so far as they improve the tradition. Just
as the welfare of the race may be increased by an
invention which is handed on from generation to
generation, so the good effects of education or other
improved conditions may be handed on, but this is
not heredity. The father may educate his children
because he himself was educated, but the mental
powers of his children will be the same whether he
had a good education or none\ And the effects of
special care given to the weakly or feeble-minded
may be absolutely harmful to the race, if the im-
provement so effected leads to more frequent

1 Of course education is a necessary condition for the full develop-
ment of the mental powers, hut at present we have no evidence that
it can add potentialities not present at birth. The subject is more
fully discussed in Chapters vii and viii.

IV] STATISTICAL STUDY 61

marriage among such unfortunates than would other-
wise be the case, for then an increased number of
defective cliildren may be born, and the race-average
be lowered. Hence has arisen the study known as
'Eugenics,' the study, that is, of the methods by
which the race may be improved both physically and
mentally. The whole trend of the results obtained
is that in order to produce exceptionally gifted men
in both body and mind, those with high development
of the characters desired should be encouraged to
marry ; and that to prevent the production of the
weakly and feeble-minded, the only method is to
prevent such from having oflFspring. It is admitted
that at present these things hardly come within
* practical politics,' but there is little doubt that the
nation which first finds a way to make them practical
will in a very short time be the leader of the world.

4—2

CHAPTEE V

MENDELIAN HEREDITY

In the last chapter the distinction has more
than once been referred to between the statistical
rules of inheritance discovered by observing great
numbers of cases taken together, and the physio-
logical laws which determine the actual manner of
transmission in individual cases. The province of
the present chapter is to indicate the methods by
which one at least of these physiological laws has been
investigated, and the results to which such work
has led. In studying this part of the subject it is
necessary to consider, at least in the first place,
characters which vary and are inherited discontinu-
ously, so that they may be sharply marked into
distinct categories. The foundation of the study
was laid by Johann Gregor Mendel, a monk of the
monastery of Briinn in Bohemia. His most important
paper was published in 1866 [2], but perhaps owing
to the fact that the biological world was then

CH.V] MENDELIAN HEREDITY 53

occupied almost solely with the discussion of the
* Origin of Species,' his work attracted no attention
at the time, and only became celebrated on its
rediscovery in 1900. One cannot avoid speculating
on the possible effects on biological thought, had
the experiments and conclusions of his now famous
contemporary ever come to the knowledge of
Darwin.

The method which led Mendel to his great
discovery was to experiment with plants exhibiting
discontinuous characters, and to consider each char-
acter separately. Previous workers in the same
field had made many laborious experiments in
crossing difterent races of plants or animals [7], but
had always regarded the individual as the unit, and
hence arose the belief that mongrels or hybrids were
usually intermediate between the parents, resembling
one in some features, the other in others, but with
no regular rule ; and further, that when hybrids were
bred together the oiFspring were often almost infinitely
variable, extending in a series from some closely
approaching one original parent through a diversity
of intermediate or new forms to others like the second
parent. So grew up the belief that the crossing
of distinct races or breeds is a potent cause of
variability, which, however, except when 'reversion
on crossing' took place, seemed to fall under no
ascertainable law.

54 HEREDITY [ch. v

Mendel's most important experiments were made
with races of the edible pea, which he grew in the
garden of his monastery. He found in peas several
characters which vary and are inherited discon-
tinuously, and he crossed together races which
differed in one or more of such characters, but in the
oflfspring and later generations he considered the
distribution of each character by itself, quite apart
from the other characters of the plant. As an
example we may take the character height or
tallness. Certain varieties of peas grow stems some
six feet in height, others are short and do not exceed
about two feet. The heights fluctuate about a mode,
but the smallest individuals of one race (grown under
proper conditions) are taller than the largest of the
other, and each race breeds true. Similar tall and
short races exist in the sweet-pea (fig. 7), the short
race being called 'Cupid' sweet-peas. When the
two races are crossed — and reciprocal crosses give
identical results — the offspring are not intermediate
but all are tall, perhaps taller than the tall parents.
When now these hybrid tails are self-fertilised,
among the plants produced some are tall and others
short, but again none are intermediate. Mendel
regarded the tallness or shortness as distinct alter-
native characters, and since tallness alone appears in
the fii'st cross, he spoke of it as ' dominant,' and the
shortness, which disappeared when crossed with the

56 HEREDITY [ch.

dominant tallness, he called 'recessive. More recent
work has indicated that a dominant character pos-
sesses some factor which is absent in its recessive
alternative ; in the present example the stem has
the power of continued growth which is absent in
the short pea. Dominance and recessiveness may
thus be regarded as presence and absence respectively
of the factor in question ; but since the presence or
absence of the factor may often give rise to the
appearance of an alternative pair of characters,
such a pair have been named by Bateson a pair of
'allelomorphs.' When a tall pea is crossed with
a short, the factor tallness is introduced from the
tall parent, and thus all the offspring are tall. These
are called the first filial generation, or more shortly
the generation JPj. Wlien these hybrid (F^) tails are
self-fertilised, their offspring (second filial or F2
generation) consist of tails and shorts. Now it has
been seen that if the factor tallness is present it
makes itself visible, and therefore the short peas in
Fi should contain no tall factor. And in fact when
self-fertilised, or fertilised with the original short
stock, they give only short offspring for as many
generations as the experiment has been carried to.
The tall factor has thus apparently been completely
eliminated from these short peas.

Fm-ther, Mendel found that among the tails in the
Fi generation, some breed true to tallness when self-

vj MENDELIAN HEREDITY 67

fertilised, while others again give a mixture of tails
and shorts. The whole result may be clearer in
symbolic form. If T stands for the tall factor, t for
its absence (shortness), the following results appear.
(The "2Tt' in Fi will be explained immediately.)

Original parents

Txt

1
Tt

Ft

r
TT

1
TT

2Tt

1

1

tt

1

Fs

1 1 1
TT 2Tt tt

1
tt

It is thus clear that among the offspring of the Fi
(hybrid) generation, some (tt) have eliminated the
taU factor altogether and show no difference from
their short ancestors ; others (TT) have nothing but
the tall factor and thus breed true to tallness ; and
a third group, which Mendel found was twice as
numerous as either of the others (therefore marked
2Tt), proved, by giving mixed offspring when selfed,
that it is hybrid like its jPi parent.

The explanation offered by Mendel of these facts
was as follows. The original tall plant produces
germ-cells ('gametes') bearing taUness ; the short
plant produces gametes bearing shortness (absence
of tallness). The Fi (hybrid) thus contains both
conditions ; its cells, resulting from the union of two
gametes, may be regarded as double structures, con-

58 HEREDITY [ch.

taining- a double set of determinants for the various
characters of the plant, one determinant of each
pair being derived from the male parent, the other
from the female. An individual produced by union
of two germ-cells (gametes) and having this double
character is called a ' zygote/ The F^ zygote thus
contains a determinant for tallness derived from one
parent, and a corresponding determinant in which
the tall factor is absent derived from the second
parent. Now Mendel's hypothesis to account for the
observed facts was that although the zygote produced
by union of tall-bearing and short-bearing gametes
contains both factors, yet when this hybrid zygote
gives rise to gametes, it produces some bearing
tallness and others bearing shortness, but none
bearing both determinants ; i.e. that the alternative
characters segregate from each other in the forma-
tion of the gametes, and that gametes bearing one
or other of the two conditions are formed in equal
numbers. Since large numbers of gametes of each
kind are formed, and since they meet indiscriminately
in fertilisation, a tall will equally often meet a tall or
a short, and a short will equally often meet a tall and
a short, and the combinations will thus be in the ratio
of \TT, \Tt, UT, Ut, or ITT, 2Tt, \U. If this
hypothesis is true, it can be tested by fertilising the
Fi hybrid zygote with the pure parental types ; the
Fi zygote produces equal numbers of T and t gametes,

v] MENDELIAN HEREDITY 69

the pure short race produces only t, so the offspring
of the hybrid and the original short should give equal
numbers of hybrid tails and pure shorts. Similarly
the hybrid zygote crossed with the pure tall should
give equal numbers of pure tails {TT) and hybrid
tails {Tt), Mendel found that this expectation was
in fact verified by experiment. The whole series
may be made clearer by a diagram (p. 60), in which
the zygotes are represented by squares, the gametes
by circles.

The middle part of this diagram represents .the
production of the Fi zygote and its ofispring when
self-fertilised, producing equal numbers of T and t
gametes (four of each being represented) and thus
giving offspring in the ratio of \TT, 2Tt, lit; the
sides of the diagram represent the results of crossing
back the Fi zygote with the parental types TT and tt.

At this point it is necessary to explain certain
convenient technical terms introduced by Bateson.
It has already been mentioned that a pair of alter-
native characters which segregate in the gametes, as
described, are called allelomorphs. When an indi-
vidual is produced by two gametes bearing different
allelomorphs, so that it contains both members of
a pair, it is called a ' heterozygote,' or is said to be
* heterozygous ' in respect of the character considered,
e.g., an individual of constitution Tt is heterozygous
in respect of talluess. If it contains only one kind

60

[CH.

zy

-W

X

h

9 ^

-I

h-

H

to

s:

CD

5

00

CO

so

^

CO

S "^

o

CO

o

s-1 03
CO V

2r>

!3>

N

s

N

«a

uT

C3

uT

v] MENDELTAN HEREDITY 61

of allelomorph of a pair it is a * homozygote/ e,g.
individuals of composition 2Tand tt are 'homozygous'
for tallness and shortness respectively. As will be
seen immediately, it is possible for an individual to
be heterozygous for one pair of allelomorphs and
homozygous for another. The essence of Mendel's
theory is that owing to the segregation of allelomorphs
from each other in the production of the gametes of
a heterozygote, the homozygous offspring, when self-
fertilised or mated with others of like constitution,
breed true to the character in question irrespective
of their ancestry. As far as observation can show,
the homozygous individuals TT and tt in the genera-
tion Fi breed as true to tallness or shortness as did
their pure-bred grandparents, in spite of the fact
that they are the offspring of a cross.

Hitherto the original parents have been considered
as differing from each other in only one pair of alter-
native characters (allelomorphs), but Mendel found
that in the pea there were several such pairs of
characters. For example, some races of peas have
purple flowers, others white ; these behave quite
similarly to tallness and shortness. The purple
flower contains a factor lacking in the white ; when
therefore purple is crossed with white, the purple
colour is dominant and the heterozygote {F^ hybrid) is
purple. Such a heterozygous purple if self-fertilised
yields 75 per cent, of purple oflspring and 25 per cent.

62 HEREDITY [ch.

of white ; the whites and one in every three of the
purples so produced are 'extracted' homozygotes,
being pure for whiteness or purpleness respectively,
and therefore breeding- true, while the remaining
purples are heterozygous and when ' selfed ' will give
both colours among their ofispring.

If now a tall purple-flowered pea is crossed with
a short white-flowered, the heterozygous offspring
will be tall with purple flowers, for both these
characters are dominant. In the production of
their gametes (pollen-cells and egg-cells) segregation
will take place between tallness and shortness, and
between purpleness and whiteness, but as these pairs
of characters are totally independent of one another
they may be associated in any combination as long as
both members of a pair do not occur in the same
gamete. Gametes will thus be produced of four
kinds ; if P represents purple, p its absence (white) ;
T tallness and t its absence (shortness), the gametes
produced by an individual heterozygous in both
characters will be PT,pT, Pt, pt, with equal numbers
of each. Since these will meet one another at random
in fertilisation, the F^ generation will consist of
individuals (zygotes) made up of all possible com-
binations of these four types of gametes, viz. in the
proportion of 4.PpTt, 2PpTT, 2PPTt, \PPTT;
2Pptt, \PPtt ; 2p27Tt, \ppTT ; \pptt

Since purple is dominant over white and tall over

v] MENDELIAN HEREDITY 63

short, the first four types of zygote, which all contain
both P and T, will be purple tails ; the next two
containing P but no T will be purple short ; the two
containing T but not P will be tall white, and the
last with neither P nor T will be short white. The
F2 offspring will thus appear in the ratio of 9 purple
tall, 3 purple short, 3 white tall, 1 white short.
Further, of the first group one will be homozygous
in both characters (PPTT), four homozygous in one
and heterozygous in the other (2PPTt, 2PpTT) and
four heterozygous in both (PpTt). Of the remainder,
one in each class will be homozygous in both char-
acters, and the others heterozygous in one, the
homozygous (pure) types being PPtt, ppTT and
pptt.

It is clear then that by crossing two races which
differ in two allelomorphic characters, and self-
fertilising (or mating together) the crossed individuals,
in the F^ generation a definite proportion of neiv
pure combinations are produced. In the above
example, by crossing tall purple with short white,
in the second generation not only these types are
produced, but also short purple and tall white, and
by selecting the pure (homozygous) individuals pure
races of these new types are immediately established.
We thus obtain a new conception of organic char-
acters, as factors which can be replaced by alternative
characters without otherwise altering the constitution

64 HEREDITY [ch.

of the organism. The process is comparable with
a chemical reaction, where one element may replace
another in a compound ; for example, by mixing silver
nitrate with sodium chloride, silver chloride and
sodium nitrate are produced. Or a grosser analogy
may be taken from bricks in a wall ; a red brick may
be removed and replaced by a blue or a yellow one
without altering the rest of the wall, and similarly in
pea-plants by the process described white flowers
may be replaced by purple, or yellow seed by green.
After the fact of the segregation of allelomorphic
characters in the production of the germ-cells of
a heterozygote, the most striking result of Mendcljan
investigation is this discovery of the independence of
ohav^o.tf-.r'fi hftlnnffvpfy to different pairs.

That these results are not of merely academic
interest is shown by the work of Prof. Biflen on
wheat. Some valuable wheats are liable to the
attacks of a fungus giving rise to the disease called
' rust,' other less valuable races are immune. Biffen
has found that by crossing the two races together,
fertilising the hybrids (Fi) among themselves, and
selecting the homozygous plants in the Fo generation,
wheat can be produced which combines the valuable
features of one race with the immunity to rust of the
other, and so a new and most useful variety of wheat
is produced. This is only one out of many examples
that could be given of the possibility of combining

V]

MENDELIAN HEREDITY

65

into one race of wheat the characters previously found
in different varieties.

The chief reason that breeders of plants and
animals believe that tlie race is permanently con-
taminated by crossing different breeds is that
commonly two breeds differ in several or many
pairs of characters. If two pairs of allelomorphic

Fig. 8. A cob of Maize borne by an Fi plant from the cross smooth
(starchy) seed x wrinkled (sugary) seed, fertilised with its own
pollen, showing about three smooth (dominant) to one wrinkled
(recessive) seeds on the same cob. (From Bateson.)

characters are combined in the heterozygote, we
have seen that only one in sixteen of its offspring is
homozygous for any particular combination ; if three
characters, one in 64, if four characters, one in 256,
so that it is clear that Mendel's method of consider-
ing distinct characters separately must be followed, if

D. 5

66 HEREDITY [ch.

any rules are to be arrived at for the distribution of
characters among the offspring of hybrids.

Before proceeding to consider some of the further
applications of Mendelian inheritance, a few examples
will be given of characters in animals and plants
which are found to be inherited according to this law.

In plants, flower-colour, seed-colour (due to either
seed-coat or the contained embryo) ; production of
starch or sugar in seeds (maize, see fig. 8 in which both
forms of seed are shown on the same cob) ; hairiness
or smoothness (stocks. Lychnis, etc.); 'bearded' or
'beardless' ears (wheat); 'palm-leaf or 'fern-leaf
(Primula) ; long or short styles ('pin-eye' and 'thrum-
eye ' of Primula) ; pollen-shape, and also fertility or
sterility of anthers (sweet-pea). Many other examples
could be given ; it should be noted that several of
these normally occur in nature, e.g. the two flower-
types of the primrose.

In animals, coloured coat and albino (many
mammals) ; and many other colour-characters in
mammals and birds ; normal and long or ' Angora '
hair in rabbit, guinea-pig, etc. (some doubt as to
completeness of segregation) ; comb-characters in
fowls ; leg-feathering in pigeons ; horned and horn-
less condition (sheep and cattle) ; colour-characters
in moths, beetles, and snails. In man, several
abnormal conditions, and presence or absence of
brown pigment in the iris of the eye.

v] MENDELIAN HEREDITY 67

As in plants, several of these cases are not in
any way connected with domestication, and the wide
diversity of species and characters in which Mendelian
inheritance has been discovered shows that the phe-
nomena are not rare or exceptional, but universally
distributed.

It has been mentioned that of a pair of allelo-
morphic characters, one is regarded as containing some
factor absent from the other, and it may be well to
give an example of the kind of evidence that leads
to this conclusion. In fowls there are three chief
forms of comb; 'single' with a median serrated
ridge, 'rose' with a broad upper surface covered
with papillae, and 'pea' with a shape consisting
essentially of three parallel low ridges. Rose and
pea each behave as dominants to single, but when
rose is crossed with pea a fourth type, 'walnut'
results, which in the adult is swollen and dimpled,
and, in the young at least, is crossed by a transverse
band of bristles. In the Malay breed such ' walnut '
combs breed true, but when made by crossing ' rose '
by 'pea,' and mated together, the resulting chicks
appear in the ratio of 9 walnut, 3 rose, 3 pea, 1 single.
The appearance of singles in the F^ generation from
pure rose by pure pea is explained by the ' presence
and absence ' hypothesis. Rose (R) and pea (P) are
each allelomorphic with their absence (r, p). A rose-
combed bird is thus Rp, and a pea-combed rP,

B

D

Fig. 9. Types of combs in Fowls. A. Single Comb (cock). B. Pea
Comb (cock). C. Pea Comb (hen). D. Eose Comb (cock).
E. Walnut Comb (young cock). (From Bateson.)

CH. v] MENDELTAN HEREDITY 69

and the walnut combs produced by crossing them
have constitution RrPp. They produce four kinds
of germ-cells, RP, Rp, rP, rp, giving the normal
ratio in i^a of 9 birds containing R and P, 3 with
R and p, 3 with r and P, 1 rp. This rjJ, containing
neither rose nor pea is single, which may be regarded
as the normal comb with no other factor superposed
upon it.

In conclusion, one further fact should be noted.
Although the members of an allelomorphic pair differ
from each other in that one contains a factor lacking
in the other, and this present factor is commonly
dominant over its absence, yet a number of cases are
known in which the introduction of a factor from one
parent only is not sufficient to cause its full develop-
ment in the heterozygote. The crossed offspring are
then different from both the parental types, and are
commonly intermediate between them. But when
such heterozygous forms are mated together or self-
fertilised, both the homozygous parental types are
produced in addition to the heterozygous form, as in
the offspring of a heterozygous tall pea there occur
homozygous tails and shorts in addition to hetero-
zygous tails. The classical example of this condition
is the blue Andalusian fowl. This breed cannot be bred
true ; when blues are paired together about half the
chickens are blues and the remainder evenly divided
between blacks and dirty-whites. By many genera-

70 HEREDITY [ch. v

tions of selection breeders have tried without success
to eliminate these black and white 'wasters/ but it
remained for Bateson and Punnett to show that if
a black and a white are paired together, only blues
are produced. The two homozygotes are black and
white respectively ; when these are paired together
the single black factor introduced from one parent
is insufficient to cause the crossed chicks to be full
black, and a dilute black or 'blue' results. Such
incomplete dominance, in which a single factor
introduced from one parent is insufficient to bring
about the same effect in the heterozygote that is
produced by the 'double dose' present in the
homozygote, has been observed in a number of cases,
some of which must be referred to later.

CHAPTER VI

MENDELIAN HEREDITY (rontinued)
The Inheritance of Colour

In the simple Mendelian cases discussed in the
last chapter the separate allelomorphic pairs were
described as wholly independent of one another, and
in the manner of their inheritance this description is
correct for allelomorphic pairs in general except in
special cases, of which examples will be given later.
But although allelomorphs of distinct pairs are in-
herited independently, yet not infrequently they may
react upon one another so as to give an apparently
combined effect in the individual bearing them.
This is especially, but by no means exclusively,
seen in the colour-characters of animals and plants.
In the list of examples- of Mendelian characters it
was mentioned that coloured coat in animals or
coloured flowers in plants behave as an alternative
to whiteness (albinism, i.e. the absence of pigment).
But further analysis shows that the appearance of
colour depends upon the presence of at least two

n

HEREDITY

[CH,

factors, in the absence of either of which no colour
is produced. An actual example will make this
clearer. A white rat is mated with a wild (brown
or *gi'ey') rat, and since colour dominates over its
absence the F^ heterozygotes are all grey, like wild
rats. These gi'ey heterozygotes mated together give

grey x white

grey x grey

3 grey

\ white
GGpp

X black

BBPP

grey

GBPp

X grey

GBPp

9 grey

1GGPP
2GB PP
2GG Pp
4 GBPp

3 black

1 BBPP
2BBP0

4- white

7GG pp
2 GB pp
IBB pp

F4

coloured and albino in the ratio of three to one. If
now one of these extracted albinos is mated with
a black rat, the offspring may not be black but grey,
and such grey individuals paired together will give
young in the ratio of 9 grey, 3 black, 4 white.

The explanation is as follows. For the production
of colour, two factors must be present, one for the

VI] MENDELIAN HEREDITY 73

production of pigment in general (P) and the other
for the determination of the actual colour of that
pigment (6^ = grey, J5 = black). Neither G nor B
can produce any visible effect in the absence of P ;
a rat without P (represented by p) is thus an albino.
The extracted albinos in Pg from the cross wild grey
X albino then contain G derived from their wild
grandparent. These mated with black give grep
offspring because grey is dominant^ over black, and
the black individual introduces the factor P which
was absent in the albino. These grey rats (generation
Fs in the diagram) are thus heterozygous in the pair
of factors grey and black (G and B) and in the factors
presence and absence of P (P and p). They will thus
produce gametes GP, Gp, BP, Bp, which in meeting
at random will give 9 zygotes containing G and P, 3
containing B and P, 3 containing G and p, 1 con-
taining B and p. But the combinations Gp and Bp,
not having P, are albinos, and so we get 9 greys,
3 blacks, 4 whites.

If in the example just given nothing were known
of the origin of the white rat which was crossed with
the black (in the generation marked F^), it would be
said that a white variety crossed with a black had

1 This explanation has been simplified by the omission of the fact
that G and B do not represent factors for separate pigments, but that
G consists in the addition of a pigment to hairs already containing B.
A character dominant in this way is called ' epistatic,' see below p. 75.

74 HEREDITY [ch.

produced ' reversion on crossing ' and tlie young had
reverted to the ancestral wild form. It is not of
course necessary that the albino used to produce
such a 'reversion' should itself be the offspring of
a grey ; such grey-bearing albinos may be bred
together for an indefinite number of generations,
and still carry the factor (? ; or if they were ori-
ginally derived from a black stock they would bear
the factor B. When such stocks are crossed together
heterozygous GB albinos are produced, and G and B
segregate from one another in the albino just as in
the coloured rats in which the colour-factor P is
present. The fact that colour in animals and plants
depends on the concurrent action of distinct factors
thus explains the phenomena of 'reversion on crossing'
which have so long been a puzzle to biologists.

Among the varieties of the brown (grey) rat
only two colour types occur, grey (wild-colour) and
black, but in the rabbit, mouse and other animals
more are found. In the mouse there are four funda-
mental colour-types, yellow, grey, black and chocolate.
The behaviour of yellow is complicated and not yet
thoroughly understood, but of the others, grey crossed
with either black or chocolate gives grey ; black with
chocolate gives black, and chocolate can only appear
in the absence of all the others. This was formerly
described by saying that grey was dominant over
black and chocolate, and black over chocolate, but

VI] MENDELIAN HEREDITY 75

this is inconsistent with the hypothesis that allelo-
morphs exist always in pairs, one possessing a factor
lacking in the other. More correctly, then, each colour
is allelomorphic with its absence, but the presence of
a higher member of the series obscures or prevents
the development of the lower. This is expressed by
saying that grey is 'epistatic' over black and chocolate,
and black over chocolate. Since chocolate is the
lowest member of the series, it has been suggested
that its factor is indeed the pigment factor represented
in the case of the rats described above by the symbol
P, and that in other colours the special factors are
present in addition \ In grey mice yellow, black and
chocolate pigments are all present in the hairs, but
the factor for 'greyness' causes the yellow to be
restricted to certain parts of the hair. In black mice
both black and chocolate pigments are present, but
the black obscures the chocolate, and in chocolate
mice this pigment alone is present.

The object of this rather special digression is to
show how the hypothesis of a series of colour-factors
acting together can completely coordinate the pheno-
mena of colour-inheritance, which very few years ago
seemed hopelessly confused and subject to no definite
rules. It is now possible to forecast with accuracy the
results of a pairing between individuals of diflPerent

^ More recently a still lower member of the series, producing
' orange ' in the absence of the other colour-factors, has been described
hy Hagedoorn.

0

76 HEREDITY [ch.

colours, if the constitution of the parents with respect
to the colour-factors carried by them is known. Some
of these cases have been exceedingly difficult to elu-
cidate because it is often impossible by inspection to
determine the constitution of a given individual. This
must be tested by suitable matings with individuals
of colour lower in the series, and it is then found that
the results observed agree closely with expectation.

A more surprising instance of 'reversion on
crossing' was discovered by Bateson in sweet-peas.
He found that within the white variety known as
* Emily Henderson ' two distinct types exist, indis-
tinguishable in appearance, which when crossed
together give a purple closely resembling the wild
sweet-pea of Southern Europe. The purple rever-
sionary form in the first cross, (Fi), self-fertilised,
gives in the next generation, (F2), 9 coloured to 7
whites. The explanation is that some plants of the
white form lack one colour factor (called by Bateson
'C'); others lack the complementary factor 'R,'
which if present with C, would produce red pigment.
Since colour can only appear when both O and R are
present, each parental form is white, but when crossed
together O and R are combined in one plant and
coloured flowers result. The allelomorphic pairs are
C and its absence (c), and R and its absence (r) ; the
purple heterozygote is thus CcRr, and produces four
kinds of gametes CR, Cr, cR, cr. These mating at

VI] MENDELIAN HEREDITY H

random give offspring in the ratio of 9 with C and /?,
3 with c and R, 3 with G and r, 1 with c and r. But
only those containing both G and R can produce colour
and therefore 9 coloured appear to 7 white. Further,
among the coloured individuals of F<i, both purple
and red appear, because the factors G and R together
produce only red ; to get purple a third factor for
blue (5) must also be present, which can only take
effect in the presence of both G and R. Since B
was introduced by one only of the original whites,
the i^i purples were heterozygous for blue as well
as for G and R (with fully represented constitution
GcRrBh) and hence among the Fo plants one quarter
contain no B and in the presence of G and R are
red\

1 In this account, the production of colour (red) is described as
being due to two factors (C and R). The recent work of Miss
Wheldale [42] on the chemical nature of flower-colours indicates
that the essential bodies are an organic base or ' chromogen ' and
an oxidising ferment. The work of Chodat and Bach, however,
indicates that such oxidising ferments must contain two components,
neither of which alone is able to oxidise the chromogen and produce
the coloured derivative — anthocyanin. Both kinds of white sweet-
pea contain the chromogen, but it seems probable that one component
of the oxidising ferment is present only in one, and the other com-
ponent only in the other. Hence no colour can be produced in either.
But on mating the two whites together, the mechanism for the
oxidation of the chromogen is again complete, and red colour
(anthocyanin) is formed. The purple colour (represented by the
additional ' factor ' B) is due to a further stage of the oxidation
of the chromogen than when only red is produced. In some white

78 HEREDITY [CH.

In the account given above of the colour-factors
in the sweet-pea it has been shown that at least two
separate elements are required to produce colour (in
this case red), and a third if blue is to be present in
addition. But for the production of the various shades
or distribution of colour further factors are known,
e.g. for the intensification or dilution of colour, and for
making the wing-petals of the same or difierent colour
from the standard. Similar phenomena are concerned
with colour in animals, of which domestic varieties
of the rat provide a simple instance. Rats, other
than albinos, are in general either 'self-coloured,'
with little or no white (this, if present, is confined
to the ventral surface), or 'hooded,' i.e. white with

flowers (snapdragon) experiment shows that the chromogen itself
may be absent.

As the purple colour in sweet-peas is due to more complete
oxidation of a chromogen than red, so in animals colour-physiologists
find that the series yellow, brown, black, may represent successive
oxidation-stages of the same chromogen by the same ferment. The
various colours of mice, for instance, are not therefore to be regarded
as necessarily produced by different ferments, but the inherited
'colour-factors' determine to what stage the oxidation of the
chromogen shall be carried. Some confusion has arisen from
the assumption that the ' factors ' postulated by students of heredity
are actual specific colour-ferments, while they may be rather
determinants which cause the oxidation of the chromogen to
proceed to a particular stage, and may be compared with the
factors which determine the production of a rose-comb or single-
comb in fowls.

VI] MENDELIAN HEREDITY 79

coloured head and shoulders and a coloured stripe
along the spine. The self and hooded factors are an
allelomorphic pair independent of colour, so that
a hooded rat may be black or grey. The factors may
also be borne by albinos, and when very young an
albino bearing the factor for the hood may be dis-
tinguished by the different texture of the hair on
the head and shoulders, giving the appearance of
a water-mark or 'ghost-hood.' The heterozygote
between self-coloured and hooded patterns differs from
either parent, being black above and white below —
the so-called ' Irish ' type of the fancy. Such * Irish '
rats bred together always give both self-coloured and
hooded rats in addition to Irish among their progeny.
A similar case in rabbits is that of the well-known
Dutch marking, which seems to correspond with the
hooded condition in rats. In flowers the number of
such characters determining the nature and distribu-
tion of colour may be considerable, so that among
the offspring of a cross between two varieties of
Chinese primulas or snapdi-agons a very large
colour-series may be produced, which on first in-
spection may seem a continuous series from the
darkest to the palest ; but careful analysis of these
cases has shown that the different factors may be
recognised and isolated, and the series of colours
falls strictly within the rules of Mendelian inherit-
ance when each factor is considered alone.

80 HEREDITY [ch.

Hitherto in discussing the interaction of distinct
pairs of factors (allelomorphs), colour alone has been
considered, but cases are known where colour and
a structural character are interdependent in the
same way. Interesting examples of this are known
in stocks and primulas. When a certain smooth-
leaved cream-flowered stock is crossed with a smooth
white, the F^ plants are purple and hoary, i.e. they
revert to the ancestral wild purple and hoary-leaved
stock. The purple colour appears for the same reason
that the two forms of white sweet-pea gave purple ;
one colour-factor is introduced by the white parent
and its complement by the cream \ But the hoari-
ness appears because the parents contain a factor for
hoariness, which can only take efiect in plants with
purple, flowers. The parents are therefore smooth
although they contain the hoary factor. When the
Fx hoary purples are crossed together, the F^ genera-
tion consists of purple, white and cream-flowered
plants in the expected proportions, but only the
purples are hoary. Smooth-leaved purple strains do
exist, but these are plants lacking the hoary factor
altogether ; if it were present, it would appear when-
ever the flowers contain purple sap.

^ The cream-colour is due to a quite distinct factor, and the
pigment is in special bodies (plastids) in the cells of the petals.
The purple colour is due to a pigment dissolved in the sap, and
is independent of the cream plastid-colour.

VI]

MENDELIAN HEREDITY

81

B

Fig. 10. Sections of Chinese Primula Flowers,

A. Long-style (' pin-eye ').

B. Short-style ('thrum-eye').

C. Homostyle.

I>.

6

82 HEREDITY [ch.

In Chinese Primulas a curious case of inter-
relation between flower-colour and structure has
been investigated by Bateson and Gregory. They
find that the long-styled and short-styled types of
flowers, so well known from Darwin's work, are an
allelomorphic pair, short-styled being dominant. But
when the long-styled factor is associated with a con-
dition in which the yellow eye of the flower is enlarged
to cover about half the area of the petals, the style
remains short, although the anthers occupy the typical
long-styled position in the tube of the flower. This con-
dition is called 'homostyle' (fig. 10 C, p. 81). When a
short-styled small-eyed plant is crossed with homostyle
large-eyed, all the (Ft) ofi*spring are short-styled and
small-eyed, these characters being dominant. But in
the second generation, (F2), obtained by breeding to-
gether these Fi plants, the following types appear : —

9 short-style with small eye,
3 short-style with large eye,
3 long-style with small eye,
1 homostyle with large eye.

The long-styled form has appeared in Fo_ from short-
style X homostyle, because homostyle is a condition
of long-style modified by association with the large
eye. When this association is broken, the long-style
appears.

From these examples of the interaction of distinct

VI]

MENDELIAN HEREDITY

83

allelomorpliic pairs, many more of which are now
known, it will be seen that many of the * exceptions '
to the Mendelian rule which have been recorded may

Fig. 11. Some of the types of flowers in generation F» from the
cross short-style (thrum) small eye x homostyle large eye.

A. Long-style, small eye. B. Homostyle, large eye. C. Short-
style, small eye. D. Short-style, large eye.

In A and B the flowers are of the ' star ' type. This character
is inherited independently of the style and ' eye ' characters.
(From Bateson.)

R— 2

84 HEREDITY [ch. vi

be explicable on the assumption that what appears
to be a simple character is really dependent on two
or more distinct factors, which become separated on
crossing with a different form.

In conclusion, it must be mentioned that a number
of cases are now known in which a pair of Mendelian
characters are closely associated with Sex. In some
cases the sex of the individual determines whether
a character is dominant or recessive ; for example, if
a horned race of sheep is crossed with a hornless, the
male offspring are horned and the females hornless ;
and in cats, it appears that yellow crossed with black
gives yellow males but tortoiseshell females. In other
cases certain Mendelian characters can be borne only
by germ-cells which will give rise to one or the other
sex. This aspect of the subject, which includes some
of the most interesting recent advances in our know-
ledge of Heredity, will be considered in a subsequent
chapter (ix), after some other questions in which the
sex-factor is not directly concerned have been dis-
cussed.

CHAPTER VII

SOME DISPUTED QUESTIONS

In this chapter will be briefly considered certain
questions which either are still quite unsettled, or
upon which there is stiU active disagreement among
biologists. It will be convenient to take first some
which are closely connected with the Mendelian theory
of heredity, and pass on later to others which are
related equally to any theory of inheritance which
may be adopted.

One of the chief lines of attack on the Mendelian
theory has been the proposition that the absolutely
complete segregation of allelomorphic characters in
the germ-cells, postulated by that theory, has not
been proved. If the theory is rigidly true, then in
the case of a tall pea crossed with a short (Chap, v)
the homozygous tails and shorts among the ofispring
of the cross should be &» pure for tallness or short-
ness as the original parents ; neither character should

86 HEREDITY [ch.

have been influenced in any way by its association
with the other. It has been maintained that the
Mendelian categories are not sufficiently definite to
allow such a statement to be made with certainty.
The Mendelian can only reply, that in the great
majority of cases the ' extracted ' pure individuals in
the F2 generation do not differ recognisably from
the original parents in the characters considered,
and that no signs of impurity can be found in later
generations.

There are however instances in which it appears
that Mendelian segregation may not be perfect. It
has been maintained that an instance of this is
provided by hair-length in guinea-pigs. When a
long-haired ('Angora') guinea-pig is mated with
a short-haired, the F^ offspring are short-haired,
shortness being dominant, owing perhaps to the
presence of a factor which prevents the growth of
the hair after reaching a certain length. But when
such Fi (heterozygous) short-hairs were mated
together, in addition to apparently pure longs and
shorts, animals with hair of intermediate length
were produced, and these crossed back with pure
long-hairs gave no short-haired young. It is suggested
that the long and short characters have become fused
in some germ-cells, segregation being incomplete or
non-existent, so that germ-cells bearing the mixed
character are produced. Again, in a cross between

VII] DISPUTED QUESTIONS 87

lop-eared and short-eared rabbits, young with ears of
intermediate length are produced, and these mated
together give no evidence of segregation in the next
generation. From these and some other similar
observations it must be concluded, either that in
some cases there is incomplete segregation or even
complete fusion of alternative characters, or that
what appear to be simple characters are really com-
plex, and that the true-breeding intermediates are
formed by a new combination of elementary factors.
An instance, which is perhaps similar, will be men-
tioned in the next chapter in discussing the inheritance
of pigment in Man.

A second question with regard to Mendelian
segregation has at present received hardly any
answer, namely, whether the apparently 'continuous'
variations such as were illustrated by the example of
Johannsen's beans (Chap, iv) segregate according to
the Mendelian rule. It was seen that each 'pure
line,' derived by self-fertilisation from an individual
plant, has its own type about which the size of the
beans borne by the plant fluctuates ; but it is not
known whether, when two plants having different
types (modal sizes) are crossed, segregation takes place
between the two types in the formation of the germ-
cells of the crossed individual. To determine this
would naturally require a long and difficult series of
experiments, and at present very little is known of

88 HEREDITY [ch.

the subject, but when it is remembered how wide-
spread and in what varied characters segregation has
been found, it would not seem improbable that it
should occur in such cases also.

Another subject which Mendelian investigation
has brought into prominent notice, and which has led
to much controversy, is the kind of variation which
has been effective in the process of evolution. Darwin
assumed that evolution takes place by the preservation
of very small ' continuous ' variations which occur in
a direction favourable to the species, but even among
his immediate followers, for example Huxley, doubt
was expressed whether larger step-like variations or
'mutations' may not have been operative. Darwin
rejected this idea chiefly on the ground of the rarity
of such mutations, which makes it inevitable that the
mutating individual should generally mate with one
of the normal type, and so it was supposed that the
mutation would be diluted and rapidly lost. But
Mendelian work shows that this dilution does not
occur in simple cases ; the offspring of the cross
between the mutation and the type produces half its
germ-cells bearing the mutation to its full extent,
and these will transmit the mutation until the race
may become widely infected with it, and not infre-
quently individuals both of which possess it will mate
together. If the mutation be dominant, as in the
case of the well-known black variety of the 'Peppered

VII] DISPUTED QUESTIONS 89

Moth' (Amphidasys hetvlaria), it may spread rapidly
until it becomes common, and if recessive it will
equally often be represented in the germ-cells of
many individuals and will appear when two which
bear it mate together. In either case if the mutation
be advantageous it may be preserved at the expense
of the type by natural selection, until it obtains a
firm footing. But the difficulty has naturally been
felt that the marvellously perfect adaptations which
are so frequent in nature cannot be imagined to
have arisen by large steps, but must have been ac-
quired gradually, and therefore many naturalists
reject the suggestion that mutations can have been
largely operative in evolution. The fallacy here is
the assumption that all discontinuous variations
must be large ; the case of Johannsen's beans shows
that essentially stable variations occur, which pro-
bably differ from mutations only in their small extent,
and by the selection of such 'minute mutations'
the wonderfully perfect structures of living things
might be produced. It may perhaps be regarded as
hair-splitting to distinguish between minute mutations
and fluctuating variability, but the distinction lies in
the nature of their inheritance, which is the essential
thing in evolution. It has been seen that no selection
within the 'pure line' in the case of the beans has
any effect ; for progress to be made, a new mutation,
small though it might be, is necessary.

90 HEREDITY [CH.

The problem in heredity which has probably
given rise to more controversy than any other is that
alluded to more than once previously, of the inherit-
ance or non-inheritance of acquired characters, that
is, characters produced in the individual during its
life by the action of some sort of stimulus. Some
aspects of the question have already been considered,
and from what has been shown of the very definite
nature of the inheritance of germinal (inborn) cha-
racters, it will be understood why students of heredity
are increasingly disposed, a priori, to disbelieve in
the transmission of acquirements ; for if these were
transmitted to any considerable extent, this fact
must interfere, one would suppose, with the orderly
appearance in the offspring of the characters repre-
sented in the germ-cells of the parents. But at the
present time no treatment of heredity could be re-
garded as complete without some mention of the evi-
dence which has been adduced in favour of the
transmission of such characters. Unfortunately, the
evidence is almost always capable of interpretation
in more than one sense. The supporters of the belief
in transmission rely largely on indirect evidence,
especially on the difficulty of imagining any cause
of evolution in certain directions if the eflfects of
acquirement are excluded. A vast literature has
grown up around this question, of which only
illustrative examples can be given. In animals which

VII] DISPUTED QUESTIONS 91

live in the dark the pigment in the skin is frequently
absent, as it is also in flat-fish on the side of the body
which lies protected from light on the sea-floor. It is
said that pigment in such cases cannot be harmful, and
so its disappearance is not due to natural selection.
But pigment very generally appears in response to
the action of light, and so it is supposed that the
absence of the stimulus to production, acting for
many generations, has caused the pigment to dis-
appear. This is illustrated by the well-known
experiment of Cunningham on flat-fish. The young
fish is pigmented on both sides of the body ; it
then settles on one side and the pigment on that
side disappears. Cunningham reared such young
fish in an aquarium lighted from below : when they
settled on the bottom the pigment disappeared, but
if kept still longer exposed to light from beneath,
the pigment began to come back again. The dis-
appearance of the pigment, although exposed to
light, proves that the loss is hereditary ; its return
on continued exposure to light is interpreted by
Cunningham to mean that its disappearance was
due to absence of light, and has gradually become
hereditary, but that the process can be reversed by
again exposing the lower side to the action of the
stimulus. The same argument has been used with
regard to the colourless skins and vestigial eyes of
animals living in caves ; where the structure is use-

92 HEREDITY [ch.

less it disappears, and the most obvious cause to
assume is lack of use, which, acting cumulatively
through many generations, has become hereditary.
Such evidence, however, is only presumptive, it does
not amount to proof, and on the other side may be
adduced the pigments of birds' eggs. Birds which
nest in holes or dark places usually have colourless
or slightly coloured eggs, while those which lay in
open places have eggs more or less matching their
surroundings. This appears closely comparable with
the condition of skin-colour in fishes and amphibians,
and yet it is impossible that the action of light could
have any direct efiect on the production of pigment
in birds' eggs. That the loss of pigment in each case
is connected with its uselessness is probable, but the
birds' egg case seems to show that it is not due to
* use-inheritance.'

A second instance of the indirect evidence for the
inheritance of acquired characters may be given, that
of instinct. Instincts are very similar to firmly rooted
habits, and have been regarded as habits which from
being performed through many generations have
become hereditary. There can be no doubt that, in
the higher animals especially, instincts may be rein-
forced and perfected by habit, but many cases can be
adduced in which it seems impossible that habit has
played a part in the evolution of an instinct. Many
insects have exceedingly perfect and complex in-

viij DISPUTED QUESTIONS 93

stincts in connexion with egg-laying, yet the process
may last only a few hours, and the eggs may all be fully
formed and ready for laying before the insect hatches
from the pupa. In the worker-bee, too, there are many
admirably developed instincts, and also structural
features which might be thought to have originated
by the transmission of acquired adaptations, and yet
the worker-bee, except in rare cases, never repro-
duces itself, but is produced by a queen and a drone
with structures and instincts diiFerent from its own.
If in these cases we find perfect instincts which
cannot have arisen by the inheritance of acquire-
ments, it seems unreasonable to assume that instincts
in other species must have arisen in this way. These
two cases are given merely as examples of the pre-
sumptive evidence that has been brought forward-
It is admitted that the process of evolution would
be more easily comprehensible if the inheritance
of acquired characters were a fact, but it is clear
that no absolute proof of its existence can be based
on cases of this kind.

Exact experiments on the possible inherited
efiects of acquirements are difficult to devise so as
to be unequivocal, and most have given negative
results \ A case which at first sight seems to prove

1 Experiments on moths and butterflies have been mentioned in
Chapter in ; in Bome, notably those of Fischer with the Tiger Moth,

94 HEREDITY [ch.

the inherited eiFects of conditions is the experiment
of Kellogg in starving silkworms, in which he found
that when the caterpillars were starved for two
generations, the third generation, even if well fed,
were below the normal size. But there is here a
possible source of error, that the eggs produced by
starved females may have been lacking in yolk, so
that the resulting caterpillars would be weakly from
the beginning and never overtake the normal size.
If so, the apparent effect of inheritance of bad
conditions would be due really to poor embryonic
nourishment, not to germinal difference. The same
explanation might apply to the apparent cumulative
effects of under-feeding in man, if the mother cannot
adequately nourish the infant before birth. The
famous experiments of Brown-Sequard on the in-
heritance of artificial injuries in guinea-pigs must
be mentioned. He found that when the parents
were subjected to operations of various kinds, some
of the young showed corresponding abnormalities,
especially in the case of the effects of certain injuries
to the nervous system. Subsequent experiments
however have not completely confirmed his results,
and there is reason to believe that where they have

Arctia cnja, definite evidence for the transmission of modifications
was obtained, but this may have been due to direct modification of
the germ-cells themselves.

VII] DISPUTED QUESTIONS 95

been confirmed there are other possible explanations
of the apparent transmission of the effects of injury.
For example, Brown-Sequard found that when the
chief nerve of the leg is severed, the toes become
morbid and the animals frequently nibble them away.
A small percentage of the offspring of guinea-pigs
lacking toes from this cause also had toes missing.
But it has been pointed out that rodents in captivity
sometimes eat off the toes or tails of their young, and
if the mother had acquired the habit of nibbling her
own toes, she might bite off those of her young
shortly after birth and give the appearance of the
inheritance of a mutilation [20, 29]. Evidence has
also been brought forward that the mutilation of the
parents may cause the production of a toxin, which
is transmitted directly to the offspring, and causes
abnormalities to appear in them in the same organs
that were injured in the parent.

Quite recently Kammerer [19] in Vienna has made
some remarkable observations on salamanders and
a species of toad which seem to support the idea of
the inheritance of acquired characters. For example,
among other experiments, he finds that the animals
can be accustomed to lay their eggs in water instead
of on land, and tlie young become modified to suit
their new surroundings, and the modifications are
progressively increased in later generations. He
points out however that most of his results, like

96 HEREDITY [ch.

those obtained by Tower (Chap. Ill), may have been
brought about by the action of environment on the
eggs at the time of maturation, but they differ from
Tower's in the regularity of their appearance and in
being adaptive. Further work in this direction will
be awaited with interests

• Keference must also be made to the work of Sumner [32 a] who
finds that rats kept at a high temperature differ in several particulars
(proportions of the body, etc.) from those kept in a cold room. These
characteristics are inherited to a measurable extent, and the ex-
planation offered in the case of insects, that the germ-cells were
directly affected by the temperature, is not easy to apply to a warm-
blooded animal. There may of course be indirect effects, brought
about by a change in general constitution, but the admission of this
hardly differs from accepting the inheritance of acquired characters.
A considerable list of cases of the transmission of environmental
modification, chiefly but not entirely in plants, is given by M^Dougall
[22 a]. An important contribution to the subject is the recently
published work of Boas, reviewed in Nature, Nov. 3, 1910, p. 11. He
finds from a study of the children of European immigrants into New
York that whatever the racial characters of the parents, the children
born after their arrival in America tend to approach a certain type,
and approach it more nearly, the longer the parents have lived in
New York. For example, the children of a long-headed race have
progressively shorter and rounder heads, the longer the parents have
lived in America before their children are born, while those of round-
headed parents become longer so as to approach the same type.
In this case there is no question of the inheritance of 'acquirements'
in the ordinary sense, for the shape of the parents' heads is not
affected ; it appears that either the germ-cells or the young childien
themselves are affected in such a way as to cause them to approach
a characteristic American type in head-shape and other characters.

VII] DISPUTED QUESTIONS 97

On the whole, the hypothesis of the inheritance
of acquired characters must be regarded as 'not
proven/ and our increasing knowledge of the definite-
ness of many germinal characters makes it doubtful
whether it can be a factor of great importance in the
constitution of the individual, or to the course of
evolution. Some further evidence in this direction
will be given in the next chapter \

A few minor questions remain. One of these,
which has played a considerable part in biological
literature, is the alleged phenomenon called Telegony.

^ The recent evidence which has been brought forward on the
subject warns us against a dogmatic denial of the possibility of the
inheritance of acquired modifications. The number of cases recorded
is now considerable, in which adaption to changed environment,
either of structure or instinct, appears to be transmitted to the next
generation. The tendency of biological thought is certainly towards
a recognition of the unity of the organism as a whole, including
its germ-cells, and especially where the organism adapts itself to
change, it seems possible that this adaptation is transmissible. The
belief that 'somatic' changes could not be transmitted rests largely
on the idea that every character is determined by a 'factor' or
determinant in the germ-cell, but it is clear that any character is not
developed directly from the germinal determinant, but by the relation
existing between the determinant and its surroundings, viz. the body
of the organism. If the surroundings are changed, this relation may
be altered, and the altered relation may be transmitted to the off-
spring, so bringing about a corresponding change in the character
as it appears in the next generation. How far such changes are of
real importance, and whether they are permanent or, so to speak,
temporary expedients to meet changed conditions, is still an open
question. Students of heredity generally would take the latter view.

D. 7

98 HEREDITY [CH.

It was formerly believed, and the belief is still firmly
held by fanciers and animal breeders, that if a female
of one breed bears young by a male of another breed,
and is then mated with a male of her own kind, the
ofispring of this second mating will in some cases
show the influence of the first sire, and instead of
being pure-bred will in some respects be mongrels
resembling the mongrel offspring of the first mating.
The instance of this made classical by Darwin is
'Lord Morton's Mare,' in which a chestnut mare bore
a colt by a quagga, and afterwards two colts by a
black Arab stallion, both of which were dun-coloured,
and bore stripes on the legs and in one colt on the
neck also [7]. But it is known that dun horses are
frequently striped to some extent, and Ewart's
well-known work with zebras [11], in which it was
attempted to repeat this experiment, gave negative
results. The belief in telegony is widely held among
dog-fanciers, and many cases could be quoted, but
whenever properly controlled experiments have been
made, no evidence of telegony has been forthcoming.
The belief in it is almost certainly due to the habit
of generalising from individual instances ; whenever
a case occurs which appears to favour the belief, it is
adduced as proof, even though other causes may have
been operative, and matings in which no evidence for
it appears are passed over in silence. If it were
a genuine phenomenon, it is almost certain that

VII] DISPUTED QUESTIONS 99

conclusive evidence for it would have been obtained
in the numerous breeding experiments recorded in
recent years.

Another idea very widely held, but apparently
resting on no better evidence, is the belief in maternal
impressions, especially in the case of mankind. By
maternal impression is meant the influencing of the
child by events affecting the mother during pregnancy.
It is commonly believed that if a pregnant woman
is injured in any part, or even sees an injury to
another person so as to excite her imagination, the
corresponding part in the child may be abnormally
developed, or may bear some mark, caused, it is
supposed, by an impression conveyed from the
mother. More general still is the belief that the
temperament of the child is influenced by the mother's
mental condition during pregnancy. This latter belief
is scarcely susceptible of accurate investigation, but
the belief in bodily marks or malformations being
due to corresponding injury to the mother, or to her
attention being strongly attracted to that part, is
almost certainly based on coincidence. A large
number of children are born with some abnormality,
and a very large proportion have some mark on the
skin. Many mothers during pregnancy undergo some
slight accident or see some deformed person, and thus
it must happen that a mark on the child will often

7—2

100 HEREDITY [CH. vii

roughly coincide in position with the part affected in
the parent. If every coincidence of this kind is
quoted as proof of the reality of maternal impression,
and the cases are left unheeded in which no relation
can be found between abnormality in the child and
events affecting the mother, it is natural that a
belief in the phenomenon will easily take firm root.
The evidence available however is probably insuffi-
cient to support any other view than that of accidental
coincidence.

CHAPTER VIII

HEREDITY IN MAN

In the chapters dealing with the various aspects
of heredity in general, a number of instances have
been given illustrating inheritance of various cha-
racters in man, and the province of this chaptei*
will be to collect and add to these cases, so as to
sketch the general outlines of what is known of
human inheritance. It has been seen that as man
differs in no important way in his bodily characters
from the other mammalia, so the laws governing the
variation and transmission of those characters are
the same as are found throughout the animal and
vegetable kingdoms wherever they have been in-
vestigated ; and further that the 'mental and moral'
attributes of man, which presumably are correlated
with physical structures, are inherited no less strongly
than the bodily features themselves. When investi-
gated by the biometric methods, the stature, span,
length of fore-arm, eye-colour, and certain other
physical characters or measurements, are found to

102 HEREDITY [ch.

give a parental correlation and thus an intensity of
inheritance closely similar to those obtained from
the study of various animals and plants. When
various non-measurable and less definite characters
such as intellectual ability, hand-writing, etc. are
investigated by the same methods, a similar intensity
of heredity is found, and finally the same is true when
the character chosen is liability to certain diseases,
notably tuberculosis and insanity, or such abnormal
conditions as congenital deafness. Since these latter
conditions have been only briefly alluded to, and are
of such fundamental importance for the well-being
of mankind, the evidence may be referred to rather
more fully here. The case of insanity is especially
convincing, for it is not open to the objection some-
times made with regard to infectious diseases that
the cause of the apparent inheritance of the condition
is the exposure of the child to infection from the
parent. It must be remembered that there are
many kinds of insanity, in one of which at least
(chorea), the inheritance appears to be Mendelian,
and that of two men with equal tendency to mental
aberration, one who is not exposed to strain may
remain normal through life, while another under
more arduous conditions may break down. But the
data of occurrence of insanity among tainted stocks
make it certain that ' the insane diathesis is inherited
with at least as great an intensity as any physical or

VIII] HEREDITY IN MAN 103

mental character in man. It forms... probably no
exception to an orderly system of inheritance in
man, whereby on the average about one-half of the
mean parental character, whether physical, mental or
pathological, will be found in the child. It is accord-
ingly highly probable that it is in the same manner
as other physical characters capable of selection or
elimination by unwise or prudential mating in the
course of two or three generations.' (Heron [10]).
Similarly for congenital deafness, Schuster writes
'...that striking confirmation has been obtained of
previous work on widely different characters, at any
rate with regard to the correlation between father
and children, and mother and children.' [31].

These examples, which might be added to, of
results obtained by 'biometric' methods, make it
sufficiently clear that a knowledge of the facts of
inheritance is of importance to mankind, and that
the further collection of accurate data is one of the
most needed social requirements. Before passing on
to other aspects of the question one other subject
may be mentioned. The measure of resemblance
in these characters has not only been worked out
between parent and child, but between brothers
and sisters, between children and grandparents and
uncles and aunts, and between cousins. Some esti-
mate can therefore be made of the probability of an
individual being afiected if his relatives are known,

104 HEREDITY [ch.

a thing which should not only be useful to insurance
offices, but to all thinking men, for it may ultimately
become the basis for deciding on the propriety of
marriage by members of tainted families. In general
it appears that the resemblance of a child to its grand-
parent is rather more than half of that to its parent,
and that the resemblance between uncle and nephew,
or between first cousins, is very slightly less than
between grandchild and grandparent.

We may now turn to definitely discontinuous cha-
racters in man, some of which are clearly Mendelian
in their inheritance. One of the most interesting
cases is that of eye-colour. Hurst [17] has shown that
complete absence of pigment in the front of the iris
is recessive to the presence of pigment ; that is to
say, that two pure blue-eyed people have only blue-
eyed offspring, but that a blue-eyed individual married
to one with any brown or yellow in the iris may have
children with pigmented eyes and that two pigmented
parents have pigmented children, with or without
a proportion of blue-eyed in addition. Within the
pigmented class there is great range of variation,
from a small yellow rim round the pupil to com-
pletely dark eyes, and the relation of the various
pigmented types to each other has not yet been
analysed. But that the characters ' pigmented ' and
' non-pigmented ' are a Mendelian pair, his evidence
leaves no doubt. This is thus a case in which the

VIII] HEREDITY IN MAN 105

occurrence of apparently continuous variation within
a discontinuous category is clearly shown. Of other
Mendel ian characters in man, colour-blindness, com-
plicated by its relation Avith sex, has already been

Fig. 12. Bracbydactylous bands. (From Bateson, after Farabee.)

mentioned and is further considered in Chapter ix.
Several cases are known in which an abnormality
behaves as a simple dominant, e.g. the condition of

106 HEREDITY [ch.

the fingers known as 'brachydactyly,' in which the
fingers have one joint less than the normal ; congenital
cataract, and probably other diseases of the eye.
Perhaps the most remarkable human pedigree ever
collected is one of 'night-blindness/ extending through
nine generations and going back to the seventeenth
century, which has been published by Nettleship
(see [1]). The condition is one in which the patient
cannot see in dull light, and it behaves as a Mendelian
dominant, probably, however, with some complication,
since the numbers aflected are less than the theore-
tical expectation. In all these cases in which the
abnormality is dominant, only affected individuals
can transmit it ; the normal members of the family
have only normal ofi*spring, a condition which is shortly
summarised as 'once free, always free.'

The rule that the affected alone transmits will
be followed only when the condition depends on a
single factor ; if it depends on more than one, or if
its dominance is modified by sex or other conditions,
then non-afiiected individuals may have affected ofi-
spring. This is possibly the case in many diseases in
which it appears that the affection is dominant, and
yet certain non-affected individuals have affected
offspring, and in such examples it must also be
remembered that the disease is probably not always
developed in people in whom the tendency is present ;
the tendency may be there but the conditions required

VIIl]

HEREDITY IN JNIAN

107

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Indicates consanguinity.

108 HEREDITY [CH.

to bring out the disease may be avoided, especially if
it is a condition not present at birth, but appearing
later in life. This kind of thing may perhaps be
illustrated by the pedigrees of Retinitis pigmentosa
taken from Nettleship (Bowman Lecture, 1909 [23])
on p. 105.

The disease is not usually present at birth, but
comes on at a varying age, sometimes during or after
middle life, and it will be seen that in the first
pedigree it is transmitted only by affected members
of the family, so behaving as a typical Mendelian
dominant. In the second pedigree, however, it com-
monly 'skips' a generation, the parents of affected
individuals usually being normal, but themselves
children or sisters of those who are affected. The fact
that in both families (as in most cases of this disease)
males are more frequently affected than females,
suggests that there is some complication, and this
is perhaps connected with the fact that in one family
the disease behaves as a simple dominant, while in
the other it is most frequently transmitted, like
colour-blindness, through normal females from affected
males. These pedigrees are given as examples of
the somewhat irregular inheritance of diseased con-
ditions such as may frequently be seen in the medical
journals ; many of them are probably explicable in
the ways suggested above.

A somewhat different group of phenomena is

VIII] HEREDITY IN MAN 109

illustrated by the iiilieritance of pigmentation in
man, in skin- and hair-colour. In the case of hair-
colour, Hurst has given evidence that bright red
behaves as a recessive to dark-coloured hair, and
that to some extent at least segregation takes place.
But the shades of haii'-colour graduate into one
another so continuously that it is impossible to
place them with confidence in Mendelian categories,
and further the colour alters so greatly between
infancy and maturity in many persons that classifi-
cation is difficult. Many children for example have
bright red hair, in whom during adolescence the
colour deepens to brown, while other members of
the same family, whose hair has hardly diflered from
the first during childhood, keep the bright red until
middle life. In families with red hair we may see
clear evidence of segregation between red and dark
brown hair-colour, but the differences between the
originally red-haired individuals show that some
contain a darkening factor which is absent in the
others. Probably then in human hair-colour there
are a number of factors which interact upon one
another in a way even more complex than in the
hair-colour of mice mentioned in a previous chapter ;
and where rigid experiment is impossible, the analysis
of these factors is almost hopeless. The same remarks
perhaps apply to the various eye-colours within the
class with pigmented iris, and the very frequent but

no HEREDITY [ch.

not absolutely perfect correlation between dark eye-
colour and dark hair suggests that similar factors
may perhaps be at work in both cases \

The inheritance of skin-colour in man is also one
of the cases which has hitherto defied Mendelian
analysis, and has been quoted more than once as
disproving the universality of Mendelian inheritance.
When a * white ' European marries a negro, the ofi-
spring are 'mulattoes/ intermediate between the
parents. Mulattoes however are not all alike, some
have brown skins and some yellowish. When they
marry among themselves they are said never to
produce full blacks or full whites, but again mulat-
toes, who however vary in the depth of their colour.
When a midatto marries a white, the 'quadroon'
offspring are lighter than mulattoes but usually
darker than Europeans ; there is evidence however
that they vary considerably, with possibly a certain
amount of discontinuity between the darkest and
lightest. Some evidence of segregation is also pro-
vided by the occasional instances of ' throw back *
to very dark skin and negi'oid features or hair among
children of apparently white people with some negro
ancestry. The whole problem however is very insuffi-
ciently known, and the difficulty of obtaining reliable

' In this connexion it may be mentioned that two recent writers
have maintained that the famous 'Habsburg lip' and the peculiar
Jewish physiognomy are Mendelian characters, the former dominant,
the latter recessive to the normal type.

VIII] HEREDITY IN MAN 111

data is doubtless increased by race-prejudice. Taken
in mass, the results of crossing white and black races
seem to give a blended inheritance with continuous
variation ; but as has been seen in the case of hair-
colour the accurate investigation of individual families
would possibly show that several factors were con-
cerned, and that in the later generations, only when
all these factors are combined in one individual
would the colour be identical with that of either of
the original races. In this respect crosses between
different races of mankind resemble hybrids between
different species of animals or plants, except that
there is usually no sterility. Most of the Mendelian
investigations have been made on varieties which
differ in few characters, for the sake of simplicity,
but when species are crossed and the offspring are
fertile so many diverse characters are concerned, of
which the relation to one another is not generally
known, that the offspring of the hybrids may con-
tain no individuals closely resembling either parent
species. This has been explained by saying that only
varietal and not specific characters segregate from
one another on the Mendelian scheme, but it is not
improbably due to the multiplicity of characters con-
cerned, and their complicated interrelations, which
makes analysis exceedingly difficult. It is also
not impossible, when germ-cells differing very con-
siderably in constitution combine in fertilisation,
that in the formation of the germ-ceUs of the next

112 HEREDITY [ch.

generation the machinery for segregation is inade-
quate. Extreme cases of this are possibly the cause of
the frequent sterility of hybrids, but it may be that
when the parental differences areinsufficient to prevent
the formation of fertile germ-cells, they may yet be
enough to interfere with normal Mendelian segregation.
Certain aspects of inheritance in mankind have
now been reviewed, and it remains briefly to indicate
the lines on which our knowledge may be of practical
importance. One of the things which is especially
prominent when the evidence is considered as a
whole is the exceeding definiteness or determinancy
of the process of heredity. Given parents of certain
constitution, it can be said with confidence that on
the average a certain proportion of their offspring
will have such and such characters. It matters not
whether the character considered is regarded from
the standpoint of the Biometrician or the Mendelian,
both agree that what is present in the germ-cell will
be present in the individual, and that external con-
ditions as a rule play but a small part in determining
its appearance. The Biometrician finds an average
value for the intensity of inheritance, and shows that
it is sensibly the same whether the character con-
sidered is stature, eye-colour, ability, or tendency to
congenital disease. When the character in question
is a simple case of presence or absence, the Men-
delian finds that it is present in a definite proportion
of the children of affected parents, so that he can say

VIII] HEREDITY IN MAN 113

with confidence that among the offspring of a parent
who has congenital cataract or abnormally jointed
fingers, about one-half will be similarly affected, and
there is no hope in such a case that the severity
of the affection will diminish in later generations.
Where the disease depends on several factors, it
may perhaps be eliminated by repeated marriage
with untainted stock, but in such cases as cataract
or colour-blindness there is no hope of this.

It is commonly supposed that inherited disease
arises largely from the cumulative effect of bad
conditions, drink and the like, but it has been seen
how doubtful it is whether the effects of such things
are really transmitted, and in any case it can be
proved that in comparison with the germinal con-
stitution, the effects of environment are relatively
insignificant. Galton was one of the first to illustrate
this by the study of twins. Human twins are of two
sorts ; in one case they arise by the simultaneous
development of two ova, as in the litters of lower
animals, and then they are no more alike than other
children of the same parents, and may be of different
sexes. Twins of the second kind are probably pro-
duced by division of one ovum, and are then of the
same sex and so alike as to be called 'identical.'
Such identical' twins remain through life, despite
differences of environment, more like one another
than successively born brothers commonly are, even
D. 8

114 HEREDITY [CH.

when brought up in precisely the same surroundings.
The same thing has been shown by an investigation of
school-children in relation to their home environment
and the habits of their parents. From a study of over
70,000 children in Glasgow, classified according to the
employment or non-employment of their mothers in
work outside the home, it was found that the rela-
tion of their height and weight to the employment or
non-employment of the mother was almost negligible
compared with the relation between the physical
characters of the mother and child. Still more
surprising, if correct, is the observation that no
regular relation could be found between drinking
habits in the parents and the health, intelligence
or physical development of some 1400 children in
the schools of Edinburgh. [Elderton, 10\] Investi-
gations of this kind are still in their infancy, and
perhaps more urgently needed than any other social
data — and it would be rash to make sweeping general
statements from the little that has been done.
Results like the examples quoted make one doubt

1 The conclusions arrived at from this investigation have been
severely criticised from both the medical and the statistical side, and
it is probable that the statistical material used is not capable of
yielding a decisive answer to the question whether drinking habits
in the parents cause deterioration of the children or not. Neverthe-
less, the fact that it was not found suggests that the habits of the
parents are relatively unimportant compared with the nature of the
stock in determining the character of the children.

VIII] HEREDITY IN MAN 115

whether the generally accepted statements about the
degeneration caused by unhealthy conditions or drink
are really at all reliable. It is easy where insanity
or other disease occurs, to say that in so many per
cent, of the cases there has been alcoholism in the
ancestors, and that therefore alcoholism is a cause of
insanity ; but in the first place it must be shown that
the alcoholism is not the result of nervous disorder,
which in the next generation appears as insanity^ ;
and in the second place, in order to prove a causal
connexion, in addition to this it must be shown that
insanity is actually more frequent in the descendants
of drunkards than in those of the sober. The un-
doubted evils of excessive drinking are many and
obvious enough, but it does not follow that physical
or mental degeneration of the descendants are among
them, and it may be a false hope to suppose that
these evils could be removed merely by the abolition
of drink.

The same sort of argument may apply to the
undoubted physical and mental inferiority of our
slum population. It is not yet proved whether this
is the effect of miserable surroundings, or whether
the ' unfit ' gravitate to the worst places because the
more fit occupy the better. These are problems
which society has as yet scarcely attempted to face,
and yet it is clear that on their correct solution

* In this connexion see Barrington and Pearson [10].

8—2

116 HEREDITY [ch.

depends the central question of social reform. If
man is to any appreciable extent the creature of his
environment, then improved conditions will improve
the race. But if, as the study of heredity suggests,
though it would be rash to say it is proved, man
is almost entirely the product of inborn factors
which are little aflFected by environment, then
improved conditions may only encourage the pro-
pagation of the degenerate, and the race as a
whole may go back rather than forward. Respon-
sible students are not lacking who maintain that this
is already taking place. It is said that the increase
of insanity which is believed to have taken place
in modern times is due to the provision of asylums
where the insane are properly cared for and fre-
quently discharged as 'cured.' When the insane
were treated on the ' strait jacket ' system no cure
could be eflfected, and so the unfortunates could not
recover to propagate their kind. But on the present
system it not infrequently happens that the insane
are enabled to bring into the world large families, so
that it is not improbable that the increase in number
may be due to this, rather than to the increased
strain of modern conditions. No one would advocate
a return to the old system, but some restriction
on the reproduction of the mentally deficient is
undoubtedly demanded by modern knowledge of
heredity.

VIII] HEREDITY IN MAN 117

It is even said that hospitals and the feeding of
destitute school-children are really working in the
direction opposite to what is intended, by enabling
the degenerate to live and beget families, who under
harder conditions would never have survived \ If a
child is to survive it is undoubtedly better that he
should be well fed and cared for, but looking at the
matter apart from all sentiment, it is quite possible
that posterity will be worse rather than better as a
result of such institutions. It is not improbable that
future generations will find that our methods for the
relief of distress are on wrong lines, and that other
means must be found for dealing with the problem,
which will cure the evil at its root instead of attempt-
ing to alleviate the symptoms.

Another point at which the study of heredity
touches social problems is the treatment of criminals.
It is becoming recognised that a large proportion of
criminals are in some way abnormal, and that their
crimes are due not to evil surroundings nor to wilful
perversity, but to inherited defects. If this is
actually the case, penal treatment of such is no less
cruel than similar treatment of the insane, but in
both cases efforts at reclamation or cure, followed

^ It is of course not suggested that all or even the majority of
those who receive such help are degenerate, but it can hardly be
doubted that a very high proportion of the ' unfit ' will take advan-
tage of it.

118 HEREDITY [ch. viii

by liberty and encouragement to marry, may only
lead to a repetition of the same evils in the next
generation. The present teaching of biology is per-
fectly clear, that in the case of the evils mentioned
above and many others, marriage of those afflicted,
and to a less extent of their near relatives, involves
a grave risk of transmitting the aflFection to descen-
dants, and 80 of inflicting serious injury upon society^

1 See [43J.

CHAPTER IX

HEREDITY AND SEX

In all the cases discussed hitherto the characters
considered have been transmitted independently of
the sex of the parents or offspring. The results of
reciprocal matings have been identical, and the
distribution of the characters among the offspring
has been independent of their sex. It is well
known, however, that many characters, in animals
especially, are confined to one sex or are developed
differently in males and females; this is most con-
spicuously so in 'secondary sexual characters,' that
is to say, features characteristic of one sex only but
not in any way directly connected with reproduction.
As examples may be mentioned the distinctive
plumage of many male birds, or patterns of butter-
flies, the horns of male mammals, and the hair on
the face in men. Darwin long ago showed that
such distinctively male characters may be transmitted
through the female, and it has recently been shown
that for Pheasants the converse is also true. Before

120 HEREDITY [ch.

entering in greater detail into the discussion of such
cases it will be simpler to consider some other instances
of 'sex-limited' or sex-modified inheritance which
have been worked out in recent years, and which
will probably throw light on sexual dimorphism in
general.

It has been seen that in Mendelian inheritance
one allelomorphic character is commonly dominant
over the other, but that many cases are known in
which dominance is partial, and it is probable that in
some instances a positive or 'present' character may
be recessive in the sense that it cannot appear unless
introduced into the individual (zygote) from both
parents. Now recent work has shown that some
characters are dominant in one sex but recessive or
only partially visible in the other (when heterozygous).
One of the first cases of this to be discovered was the
Tortoiseshell and Yellow Cat, and although there
are probably complications in this instance which are
not fully worked out, yet it is fairly certain that in
cats which are in constitution heterozygotes between
yellow and black, in males the yellow is completely
dominant, but in females the dominance is partial,
giving tortoiseshell. That is to say, in yellow-black
heterozygotes, in males the yellow colour completely
suppresses the black, but in females the black appears
with the yellow. Thus tortoiseshell cats are almost
invariably females, the corresponding males being

IX] HEREDITY AND SEX 121

yellow. A more completely known case is that of
horns in Sheep. If a sheep of a breed in whicli both
sexes bear horns is mated with one of a completely
hornless breed, among the F^ offspring the rams bear
horns and the ewes are hornless. Such F^ young bred
together give in F^ both horned and hornless males
and females, and all the horned ewes and hornless
rams are homozygous and breed true, while the horn-
less ewes and horned rams may be either pure or
heterozygous. Here we have a clear case of a character
which is dominant in one sex (male) and recessive in
the other (female).

In such a case as this it may be said that the
character is transmitted independently of sex, but its
appearance in heterozygous oifspring is modified or
controlled by the sex of the individual. Both males
and females receive the character alike, but it appears
in males while it is suppressed in females of the same
constitution.

In another important group of cases the actual
distribution of the character among the offspring is
determined by their sex, and a study of such throws
important light upon the vexed question of sex-deter-
mination and the nature of sex itself. As an instance
of this we will first consider one of the best-known
examples, that of the inheritance of the variety
lacticolor in the common Currant Moth {Abraxas
grossidariata). The very rare variety lacticolor is

122 HEREDITY [CH.

found wild almost solely in females. When mated
with a typical (grossulariata) male all the offspring
of both sexes are typical, i.e. the type is completely
dominant over the variety lacticolor. When two such
heterozygous individuals are paired together, lacticolor
appears as would be expected in about one quarter
of the offspring, but all the lacticolor si^ecimens are
females. When a lacticolor female is mated with a
heterozygous male, half the males and half the females

JFig. 13. Abraxas grossulariata and its var. lacticolor.

are lacticolor, that is to say, by this mating alone can
the lacticolor character be transferred to the male sex.
Lact. male x lact. female gives only lact. in both sexes,
as would be expected from two recessives paired to-
gether, but lact. male by heterozygous female gives all
the females lact., all the males typical (heterozygous)
grossulariata. Using the abbreviations L for variety
lacticolor, G for the typical grossulariata character,
G {L) for heterozygous grossulariata, ^ for male.

IX] HEREDITY AND SEX 123

and $ for female, these results may be summarised
thus:

1.

L^xG $

gives gross. (^ , gross. $ .

2.

G{L)2^G{L)$

gives gross. $ , gross. 9 , lact. $ .

3.

L^xG{L) $

gives gross. ^,lact. $, gross. 2,lact. $

4.

G(L) ?xZ cJ

gives gross. $, I act. $.

5.

L <^xL S

gives lact. ^, lacL ?.

The important points about these facts are that
lacticolor is clearly a recessive variety when crossed
with grossidariata, but the distribution of the char-
acter among- the sexes is dilJerent in the reciprocal
crosses, nos. 3 and 4 above. The only explanation
that has been offered to account for this is that
females lay two kinds of eggs, one destined to become
males and the other to become females, and that the
female-bearing eggs cannot carry the grossidariata
factor. Thus in mating no. 4 above (using G and L
for gross, and lact), the heterozygous {G (L)) female
produces male eggs bearing G and female eggs bearing
L ; all the spermatozoa (germ-cells) of the male bear
L, and thus the offspring are GL (= gross.) males
and LL (= lact.) females. But in the converse cross
no. 3, the lact. female (LL) produces male and female
eggs both bearing L, the heterozygous (GL) male
produces equal numbers of G and L spermatozoa,
and thus the offspring are GL and LL males, GL and
LL females. We thus arrive at the conclusion that

124 HEREDITY [CH.

in this species at least the sex is determined by the
egg of the female parent, even before fertilisation.

Perhaps the most remarkable fact in the whole
case remains to be mentioned, that, as far as experi-
ment has shown, pure (homozygous) females of the
moth A. grossulariata do not exist; all the wild and
apparently pure gross, females that have been tested
by pairing with lact. males have given lact. female
and gross, male offspring, so showing that they are in
constitution GL. Since, however, wild males are
pure gross. (GG) and since GL females produce
(j-bearing male eggs and iy-bearing female eggs, the
lact. (L) factor never normally becomes transferred
to the male sex. Similar cases have been discovered
in the inheritance of pink eyes in Canaries, and more
recently for more than one pair of characters in
Fowls. Of the latter, the most noteworthy is the
barring of the Plymouth Rock, for which all barred
hens are found to be heterozygous when mated
with an unbarred (black) cock.

The essential feature of the explanation given
above is that eggs of two kinds, male-producing and
female-producing, are laid in equal numbers, and that
the character for femaleness cannot be associated in
the same egg with the gross, factor, so that female-
bearing eggs must always bear lact. The case is
probably comparable with the 'gametic coupling'
which has been found between characters belonging

IX] HEREDITY AND SEX 125

to diiferent allelomorphic pairs in the sweet-pea and
other flowers, quite apart from sex. Sweet-pea
flowers are of two shapes, with 'erect' standards
or with 'hooded' standards. The erect character
is dominant to the hooded. Among the off^spring
of certain sweet-peas heterozygous for this pair
of characters and also for purple and red flowers, the
purple ofispring include both erect and hooded
standards, but the reds are nearly all erect. This
means that in the formation of the germ-cells the
purple factor and the erect factor go into diflferent
germ-cells, so that instead of four kinds of germ-
cells being produced in equal numbers, two are
produced in great excess, viz. purple-hooded and red-
erect. These meeting at random give purple-hooded,
purple-erect and red-erect, but hardly any red-hooded.

It has recently been discovered by Bateson and
Punnett that the purple and erect factors go chiefly
into difierent germ-cells if introduced into the
heterozygote by diflerent parents, but are ' coupled '
together and go into the same germ-cell if introduced
from the same parent. In the case of the Currant
Moth the G character is always introduced into the
female from the male and possibly for this reason is
never borne by female-bearing eggs.

It has been seen that the case of the Currant
Moth and the similar cases in Fowls and Canaries lead
to the inevitable conclusion that in these animals

126 HEREDITY [ch.

there are two kinds of eggs, male-bearing and female-
bearing. In other cases, however, exactly similar
evidence shows that the germ-cells of the male
(spermatozoa) are of two kinds. The example of this
which has been most fully worked out is the Fly
Drosophila, in which a series of remarkable dis-
coveries have been made in America by Morgan. He
has found several sex-limited characters, perhaps the
simplest of which is eye-colour. The normal flies
have red eyes ; a white-eyed male appeared and when
mated with a red-eyed female gave only red-eyed
oflfepring, both males and females. When, however,
these were mated together, all the female oflPspring
were red-eyed, but about half the males had white eyes.
A white-eyed male mated with a heterozygous red-
eyed female gave red and white eyes in both sexes,
but any red-eyed male mated with a white- eyed
female gave all the males white, all the females red-
eyed. This case then is in every respect precisely
the converse of what is found in the Currant Moth ;
the males must be heterozygous for a sex-determiner,
and the red factor in the male is transmitted only to
the female offspring.

Apparently • similar conditions are found in Man
in certain diseases of the eye, e.g. colour-blindness
and certain forms of night-blindness, and in the disease
known as Haemophilia. Here the affected male
appears to transmit the factor for the disease only to

IX] HEREDITY AND SEX 127

his daughters, for sons of aflected men are normal
and do not transmit, while the daughters of affected
men, though not themselves affected, transmit the
disease to some of their sons.

The suggested explanation is that ' normality ' is
represented by a factor for which the female is
homozygous (NN), the male heterozygous (Nil). If
in the male the normality factor N is replaced by the
factor for colour-blindness, for example (C), it will
cause the appearance of the disease in the male for
there is no second N to counteract C. The male
transmits N or C only to his daughters, but a woman
who receives C from her father will receive N from
her mother, and so will not be colour-blind. She will
transmit C, however, to half her children, so that
some of her sons will be colour-blind. This is illus-
trated in the following scheme.

? NN X ^ On

normal woman //\ colour-blind man

X \

%CN X ^ Nil

heterozygous /\ normal man
woman y \

%CN'%NN ^ Cn J^i

In the fly Drosophila and in Man, then, it is clear
that the germ-cells of the male must be of two kinds,
while we have seen that precisely similar evidence

128 HEREDITY [ch.

from a moth and two kinds of birds shows that in
them the female produces two kinds of germ-cells.

The conclusion has been drawn by some students
of the subject that in the Moth, Canary and Fowl the
egg determines the sex of the offspring, while in Man
and the fly Drosophila the spermatozoon determines
the sex. There is some evidence, however, that both
modes of inheritance may exist in Fowls, which would
contradict this supposition, and for this and other
reasons it is perhaps more likely that in animals
generally, and probably in plants also, there are two
kinds of egg-cells (germ-cells produced by the female)
and two kinds of spermatozoa (germ-cells of the male)
and that an egg-cell destined to produce a female is
fertilised by a spermatozoon of one kind, a male-
producing egg by a spermatozoon of the other kind.
The facts in any case indicate that the determiners
for maleness and femaleness are comparable with
Mendelian characters, which segregate from each
other in the formation of the germ-cells as do the
'factors' for Mendelian allelomorphs. For a more
detailed presentation of this hypothesis and the
evidence on which it rests, the reader must be referred
to the works cited in the bibliography [especially 2,
9a, 226, 43a].

It will be noticed that in the description of the
nature of sex-determination here barely outlined, the
sex of an individual is regarded as being no less

ixl HEREDITY AND SEX 129

irrevocably determined by the factors contained in
the germ-cells than is the appearance of an inherited
Mendelian character. This view is opposed to the
older belief that the sex is determined by nutrition
and other environmental conditions acting on the
embryo or larva. It may be well therefore to
summarise the evidence for the belief that sex is
essentially inherited, and not determined by conditions.
The evidence is of several distinct kinds. In the first
place, in a variety of animals (and in Man in the case
of 'identical twins'), whenever one egg divides to
form two or more individuals, these are always of tiie
same sex. Secondly, several animals belonging to
widely diverse groups produce two kinds of eggs, one
yielding males, the other females, and in these cases the
sex is clearly determined before development begins.
Thirdly, in the Bee and other forms, a fertilised egg
yields females, an unfertilised egg, males ; in this case
fertilisation appears to determine the sex, but condi-
tions operating later have no power to change it. In
all these widely distributed cases we have direct
evidence that sex is determined from the fertilisation
of the egg or before it, presumably by 'sex-deter-
minants' present in the germ-cells.

Evidence of a different nature has been afforded
by the study of the development of the germ-cells,
especially but not exclusively in Insects. In the
nucleus of the developing cell in a number of species

D. 9

130 HEREDITY [ch.

it is found that the bodies known as chromosomes (see
Appendix, pp. 140, 146) have an even number in the
female and one less in the male, for example in one
case 22 in the female and 21 in the male. In the
female two of these difler visibly from the rest, in the
male there is one odd one, the remaining twenty being
like the corresponding twenty of the female. At the
* maturation divisions,' when the chromosome number
is halved, 11 go into each mature egg, but in the
male the odd one goes into half the spermatozoa, leav-
ing the other half without it. All eggs thus contain
10 + 1, but spermatozoa are formed in equal numbers
having 10+1 and 10. In fertilisation then two kinds
of zygotes will be produced, those made by (10 + 1)
egg + (10 + 1) spermatozoon giving 20 + 2 (female),
and those made by (10 + 1) -,"- 10 giving 20 + 1 (male).
On the hypothesis of the nature of sex outlined above
it is supposed that the odd chromosomes are related
to the sex-determinants (if they are not themselves
the determinants), so that the sex is determined by
the combination of chromosomes in the nucleus.
In any case, since the numbers are regularly found,
external conditions can have no part in deciding the
sex of the individual.

Finally, the cases of relation between sex and in-
herited characters described above (lactieolor, colour-
blindness etc.) leave no room for the action of
environment after the individual has once begun to

IX] HEREDITY AND SEX 131

develop. But it should be pointed out that environ-
ment may influence the proportion of male-producing
and female-producing germ-cells which come to
maturity, and a considerable amount of evidence has
been collected showing that the varying proportions
of the sexes under different conditions may be due to
this cause. If some conditions favour the output of
male-producing eggs, others of female-producing, the
conditions will then indirectly affect the sex of the
offspring, not by changing it in the individual, but by
causing more individuals of one or the other sex to
be born.

One of the reasons which have led biologists to
assume that environment may modify the sex of the
individual is the fact that changed conditions may
influence the extent to which the sexual characters
develop. Worker bees for example are females in
which modified food and surroundings have prevented
the full development of the female characters. The
characters proper to one sex frequently are found
in a rudimentary condition in the other, and in
abnormal cases may develop. Or these characters may
be prevented from developing in the sex which nor-
mally exhibits them by certain conditions, especially
by removing the ovary or testis from the young
animal, for example the non-appearance of horns in
castrated deer or the disappearance of the distinctive
sex-features in Crabs in which the reproductive organs

9—2

132 HEREDITY [cH.

are destroyed by parasites \ Cases of this kind there-
fore bring us back to the question of 'secondary sexual
characters ' and sexual dimorphism which were briefly
touched upon at the beginning of this chapter.

In discussing the determination of sex, the nature
of sex itself must be sharply distinguished from the
secondary characters which are normally associated
with it. The essential difference between the sexes
is the presence of testes in the male and ovaries in
the female ; the associated characters, which may or
may not be directly connected with reproduction, are
certainly sometimes and may perhaps always be
clearly separated into a distinct category. It has
been seen that the evidence is conclusive that sex
often, and probably always, is determined by the
germ-cells, i.e. from the moment of fertilisation if not
before. But it is equally clear that the characters
associated with one or the other sex are not de-
termined in the same way ; their presence or the
extent to which they are developed may be to some
extent dependent upon environment. Tliis is shown
by the case of the worker-bee ; by the effects of cas-
tration in preventing the appearance of distinctively
sexual characters ; by the appearance in old or

1 In this particular instance it has been found that a male crab
affected by the parasite may even produce eggs, and it must be supposed
that femaleness is present in a recessive condition, so to speak, in the
male, and is caused to appear by the action of the parasite (Sacculina).

IX] HEREDITY AND SEX 133

sterile female birds of plumage resembling that of
the male ; and lastly by the occasional development
in one sex of characters which are normally rudi-
mentary in that sex, but are well developed in the
other, such as the breasts in man. If such characters
are not directly connected with reproduction, for
example the splendid plumage of some male birds or
the horns of deer, they are called secondary sexual
characters, but there is no sharp distinction between
these and accessory sexual structures such as the egg-
laying apparatus of female insects.

It was said above that the appearance of sexual
characters is to some extent dependent on environ-
ment, but in fact the only changes which can modify
their development are those Avhich aifect the essential
reproductive organs, the ovaries and testes. When
these are removed or suppressed, the sexual characters
may be modified ; their appearance indeed depends
in many cases upon the presence of functional sexual
organs, and frequently they do not appear until the
sexual organs are mature. Now it has been seen
above that certain characters are dominant in one sex
and recessive in the other, and the most typical case
of this is in an actual secondary sexual character —
horns of sheep, for when horns are present they are
always better developed in the male. Their 'recessive'
condition in a ewe produced by crossing a horned by
a hornless breed must probably be ascribed to a

134 HEREDITY [CH.

factor, confined to the female, which prevents horns
from developing even though the factor for horned-
ness is present. Some such inhibiting factor, confined
to one sex, may possibly explain sexual dimorphism
in general. It may be of course that in some cases the
inheritance of sexual characters is limited like that of
the variety lacticolor in the Currant Moth ; if the var.
lacticolor were dominant instead of recessive, we
should have an instance of ordinary sexual dimorphism
in this case, for all females are normally heterozygous
and would therefore show one character, all males
homozygous, and would show the other. Some con-
dition of this kind will probably be found to apply to
the remarkable cases of certain butterflies which have
two or three discontinuous and very distinct forms of
the female, but only one kind of male, and in which all
the different forms have been bred from eggs laid by
one female insect. The few breeding experiments that
have yet been made with these species (Papilios or
* Swallow-tails ' from Africa and India) suggest that
a combination of alternative dominance and sex-
limited inheritance will probably be found to explain
them. It seems reasonable to suppose, therefore, that
not only the determination of sex itself, but the
difficult problems of sexual dimorphism and the
inheritance of secondary sexual characters, will all be
found on analysis to fall into line with the simple
principles of Mendelian Heredity.

135

APPENDIX I

HISTORICAL SUMMARY OF THEORIES
OF HEREDITY

In the foregoing pages, Heredity has been regarded
as the relation between parents and offspring in
respect of their bodily characters, and it has been
shown that this relation has been brought about in
some way by the germ-cells, but very little has been
said about the mechanism by which this is effected.
This side of the question is very largely speculative,
and in order to keep speculation to some extent
apart from facts, an account of theories of hereditary
transmission, and of recent work on the supposed
material basis of hereditary characters, has been
postponed to appendices, which the reader who seeks
for facts and well-founded deductions alone, may
omit at will. First, a summary of the chief theories
of heredity will be given, and then a short account of
recent work on the subject.

In the introduction it was pointed out how closely
related are our ideas of Heredity and Variation with

136 HEREDITY [ap.

theories of Evolution, and thus the history of the two
subjects largely coincides. The first important theory
was that of Lamarck, published in 1809, and although
it had little influence at the time, in more recent
years Lamarck's main principle has found many sup-
porters. His theory was essentially that ' acquired '
modifications are being continually produced and
perfected by every organism during its life, and that
they are at least partially transmitted to its ofispring,
so that each generation will be rather better adapted
to its surroundings than its predecessor. In this way,
for example, the great length of the neck of the
girafie would be explained by the continual striving
through many generations to reach higher leaves on
the trees ; or the limbless condition of snakes and
slow-worms by the gradual loss of limbs through
disuse. But it has been seen that the assumption
that acquired characters are inherited is open to
grave doubt, and hence the followers of Lamarck
are fewer at the present time than formerly.

Darwin's great theory of Evolution by Natural
Selection of course depends on quite different prin-
ciples, but it, like Lamarck's, is based essentially upon
the laws of variation and heredity. Darwin himself
made astonishing progress in the investigation of
these laws, and although he would doubtless have
been the first to admit the incompleteness of our
knowledge, yet he collecLed sufficient evidence to

ij THEORIES OF HEREDITY 137

enable him to formulate one of the first really impor-
tant theories of heredity, which he called the Theory
of Pangenesis [7, (1868)]. The essence of his theory
was that every cell of an organism gives off minute
particles or ' Gemmules ' from itself, which circulate
in the body and finally come to rest in the germ-cells,
or in parts where buds may be developed. The
gemmules were regarded as being capable of multi-
plication, and of transmission to a future generation
in a dormant state. They were supposed to be given
off from all tissues at every stage of development, so
that every unit of the organism at every stage would
be represented in the germ-cells. On the develop-
ment of the germ-cell, the contained gemmules
would give rise to cells like those from which they
were derived, and so the characters of one generation
would be transmitted to those which follow.

By this hypothesis Darwin accounted for the
phenomena of sexual and non-sexual reproduction,
regeneration of lost parts, variability, inheritance
both of inborn and acquired characters, and lastly
of reversion to a previous ancestor. The hypothesis
was one of the first which attempted to bring all
these various groups of facts into line, but it had
the serious defect that there was no direct evidence
whatever for the existence of gemmules, and, assuming
their existence, to be accommodated in the germ-cells
they must be so exceedingly minute as to be almost

138 HEREDITY [ap.

unimaginable. The Theory of Pangenesis never gained
any very wide acceptance, but is of great importance
owing to its stimulating effect on later work and
thought. To a gi-eat extent it led to the formulation
of other theories of heredity \ any account of which is
prevented by limitation of space. It can only be
mentioned that the chief hypotheses which followed
Darwin's laid successively more and more emphasis
upon the idea that the germ-cells are not made up of
samples taken from the body, but have a certain
independence. So gi-ew up the conception of 'germinal
continuity,' that is, the idea that the germ-cell of one
generation gives rise not only to the body of the next,
but also directly to its germ-ceils, so that the body
does not produce germ-cells, but only contains them.
We must now turn to the theory in which this idea finds
its most celebrated expression, AVeismann's Theory
of the Germ-2)lasm (1885) [40, 8].

It is impossible in a short space to give an
adequate account of Weismann's great theory, which
he has worked out in fuller minuteness of detail than
has been done with any other theory of heredity, and
by which he has done more to stimulate discussion
and research than perhaps any biologist since Darwin.

^ For a summary of the more important theories of heredity,
especially those of Herbert Spencer (1863, i.e. before Darwin's theory
of Pangenesis), Galton (1875) and de Viied (1889), see Thomson's
Heredity [33]. See also [8].

I] THEORIES OF HEREDITY 139

Weismann was led by his work on the origin of the
germ-cells to a belief in germinal continuity as
explained above, but the facts of regeneration of
lost parts and other related phenomena caused him
to give up the idea that a sharp distinction could be
drawn between the cells of the body and the germ,
and to substitute for it the idea of a distinction
between body-substance and germ-substance, or as
he calls it, body-plasm and germ-plasm. According
to this hypothesis, the egg contains germ-plasm
derived from that of the parent, and as the egg
develops the germ-plasm increases and becomes
distributed among the cells, and gradually, as the
cells become specialised to form the different parts,
the germ-plasm becomes converted into body-plasm
and builds up the varied kinds of ceUs of the
body. But some cells continue to possess the full
complement of ancestral germ-plasm, and these will
go to form the germ-cells of the next generation.
When an organ remains capable of regenerating
lost parts, it is assumed that germ-plasm having
the power to develop such parts remains in the cells
and becomes active when required- Germ-plasm can
thus be converted into body-plasm, but body-plasm
cannot become germ-plasm, and hence Weismann
assumes that no change brought about in the body
(by environment, etc.) but not affecting the germ-cells,
can be inherited by subsequent generations. It is

140 HEREDITY [ap.

therefore impossible according to his theory, that
'acquired characters' in the technical sense should
ever be inherited. The germ-plasm of one genera-
tion gives origin to the germ-plasm of the next, and
no external conditions acting on the body which
contains and nourishes the germ-plasm can have
effects which are transmitted unless the germ-plasm
itself is altered.

Weismann in a series of books and papers has
built up a detailed and highly complicated and
speculative scheme of the nature and composition
of the germ-plasm, only a brief summary of which
can be given here. Much of it will doubtless not
stand the test of fuller investigation, and parts of
it are already discredited ; but it has had the merit
of stimulating an immense amount of valuable re-
search, and there are indications that some of his
fundamental ideas will form the foundations of a
true theory of the material basis of heredity.

Weismann assumes that the germ-plasm is con-
tained in the nucleus of the cell, and, in particular,
in the bodies known as chromosomes. Every nucleus
contains a number of these bodies, in the ordinary
condition of the nucleus distributed through its
substance so as to be unrecognisable, but when the
cell is about to divide they make their appearance
as rod-like bodies whose number in general is con-
stant in the nuclei of the same species of animal or

I] THEORIES OF HEREDITY 141

plant. Before the nucleus divides the chromosomes
split longitudinally so that they are accurately halved
and the two halves of each go into different daughter-
nuclei. Weismann supposes that the germ-plasm
is contained in the chromosomes, and consists of
numerous units with different properties. When the
chromosome splits, each unit is supposed to divide
into two similar halves, and thus each daughter-
nucleus receives a similar complement of germ-plasm.
In the union of male and female cells in fertilisa-
tion, the nucleus of each cell brings its complement
of chromosomes, and thus if there were no special
provision, the number of chromosomes would be
doubled in each generation. But it is actually found
that in the cell-divisions immediately preceding the
development of both male and female sex-cells, a
process occurs which results in the removal of half
the chromosomes from the nucleus, and thus when
the male and female nuclei unite the normal number
of chromosomes for the species is restored. Since
Weismann regards the chromosomes as consisting
of germ-plasm, and as made up of a vast number
of units, each of which is the determinant for one
hereditary character, he saw that, without some such
process of removal of chromosomes in the formation
of the sex-cells, the germ-plasm must in a few genera-
tions become infinitely complicated. He therefore
predicted that some process of 'reduction' of chromo-

142 HEREDITY [at.

somes must occur, either by elimination of complete
chromosomes or by transverse instead of longitudinal
splitting, before any complete observations had been
made showing that this actually happens.

Since Weismann supposes that the germ-plasm
is contained in the chromosomes of the germ-cells,
and since half the chromosomes are removed in the
'maturation' of these cells without preventing the
transmission of any part to the offspring by inherit-
ance, he concluded that each chromosome contains
aU the units (' determinants ') necessary to a complete
individual. (Later work has rendered this conclusion
doubtfid : see Appendix ii.) When fusion of male
and female sex-cells takes place, the resulting individual
will contain a mixture of the parental germ-plasms,
the paternal in some chromosomes, the maternal in
others. In the maturation of the sex-cells half
these germ-plasms will be removed and in the next
generation a fresh mixture will take place. It thus
follows that the different chromosomes contain germ-
plasms descended from different ancestors, and
different mixtures of these will occur in different
individuals. Here then we come to Weismann's
hypothesis of the origin of variation. Since different
individuals contain different combinations of an-
cestral germ-plasms, these wiU lead to varying
effects in the development of the body ; new com-
binations will be continually occurring in every

I] THEORIES OF HEREDITY 143

fertilised egg, and thus arises the variation between
separate individuals. Further, although by his theory
changes brought about in the body-plasm cannot be
transferred to the germ-plasm, yet influences acting
on the germ-plasm itself may modify it and so their
effects will be transmitted. The most important of
these influences is nourishment, which may favour
some units of the germ-plasm rather than others.
He further supposes that there may be competition
for nourishment among the difierent units ('deter-
minants') so that some increase at the expense of
others, and if this process should be continued
through a series of generations, certain characters
would show a steady increase while others corre-
spondingly decrease. Variation thus arises by changes
brought about in the germ-plasm, and by the
recombination of varied ancestral germ-plasms in
each generation. Such variations wiU be inherited,
and in this respect will differ entirely from changes
brought about in the body during its life by the
action of environment.

It has been shown that in the earlier theories
of heredity it was assumed that the germ-cells were
produced by the body, and that they must therefore
be supposed either to contain samples of all parts
of it, or at least some kind of units derived from
those parts and able to cause their development
in the next generation. Gradually, as the study

144 HEREDITY [ap. i

of heredity and of the actual origin of the germ-cells
has progressed, biologists have given up this view
in favour of a belief in germinal continuity, that is,
that the germ-substance is derived from previous
germ-substance, the body being a kind of offshoot
fi'om it. The child is thus like its parent, not
because it is produced from the parent, but because
both child and parent are produced irom. the same
stock of germ-plasm.

145

APPENDIX II

THE MATERIAL BASIS OF INHERITANCE

In Appendix i, it has been mentioned that
Weismann regarded the chromosomes of the nucleus
as the bearers of hereditary characters, and more
recent work has added to the probability of this
view, while not as yet providing anything which can
be called proof. That some, if not all, the hereditary
characters are determined by the nucleus of the
germ-cell is indicated by several facts. In the first
place, the spermatozoon consists of little else but a
nucleus with a vibrating tail, and the tail may be
shed as the spermatozoon enters the ovum. Secondly,
experiments in fertilising non-nucleated fragments
of sea-urchin eggs by sperm of a different species,
give evidence that the hereditary characters of the
resulting larvae are exclusively those of the paternal
species. This conclusion however has been disputed,
and can only be regarded as probable rather than
certain. Again, experiments in fertilising one egg
simultaneously by more than one spermatozoon, lead

D. 10

146 HEREDITY [ap.

Boveri [3] to believe that the subsequent develop-
ment of the cells of the embryo depends on the
distribution of the chromosomes in the abnormal
divisions consequent on double fertilisation. And
Herbst [16] has obtained sea-urchin larvae made by
crossing distinct species, which on one side of the
body resemble one parent, and on the other side
the other parent. He shows that these differences
are connected with differences in the size of the
nuclei of the two sides, and that probably the part
with maternal characters contains only maternal
nuclear substance, while the part showing the
paternal character has nuclei derived from both
parents.

But probably the best evidence for regarding the
chromosomes as bearing the essential determinants
for hereditary characters is provided by the behaviour
of the chromosomes themselves in the maturation
divisions of the germ-cells. It has been pointed out
that at these nuclear divisions the chromosome
number is halved, and restored to the fuU number
again in the next generation by the union of two
germ-cells each bearing the half-number. Now it
has been found in certain cases that the chromosomes
are not all alike, but differ among themselves in size
and shape, and when this is so it can be seen that
the nucleus just before maturation contains two of
each kind. If the different kinds of chromosomes

II] MATERIAL BASIS 147

are represented by letters, A, B, C, D..., there will
then be two A'b, two ^'s, etc. in the nucleus. The
actual processes in the reduction division are some-
what complex, but briefly they consist in a pairing
together of the chromosomes, followed by a division
of the nucleus in which the two members of each
pair are separated into diflerent daughter-nuclei,
so that the daughter-nuclei each contain half the
full number. When the chromosomes differ among
themselves, it is seen that two similar ones always
pair together, i.e. A with A, B with B, etc. Thus
the daughter-nuclei each contain the whole series
A, B, C. . ., but have only one of each, instead of two.
If then it is imagined that each chromosome is
the bearer of the determinant (or 'factor') for a
Mendelian character, we may regard one individual
as having a double series of chromosomes A, B, C. . .,
etc., and another as bearing the allelomorphic cha^
racters a, b, c..., etc. When these individuals are
mated, the heterozygote will bear both series,
A and a, B and b, etc. In the formation of the
germ-cells, A will segregate from a, B from b in
exactly the way required by Mendelian theory. But
there is no reason to suppose the series A, B, C...
should all go into one germ-cell, and a, b,c... into the
other ; A may go into the first daughter-nucleus and
a into the second, but b may go with A into the first,
and B into the second. So in crossing races differing

10—2

148 HEREDITY [ap.

in more than one allelomorphic pair, all possible
combinations can be produced, except that no germ-
cell can contain both the members of one pair.

The suggestion that this segregation of chromo-
somes, which can be seen to take place, is the
mechanism by which the members of an allelo-
morphic pair of characters are segregated, is quite
speculative; but it seems exceedingly unlikely that
machinery so exactly adapted to bring it about should
be found in every developing germ-cell, if it had no
connexion with the segregation of characters that
is observed in experimental breeding. There is also
the further fact in support of the suggestion, that
it is known in many insects that one pair of chromo-
somes is closely connected with sex, for in the males of
these species one chromosome of the pair is absent or
much reduced, but in the female both are similar.
These sex-chromosomes separate from one another
like the others (when both are present), and it has
been seen that there is experimental evidence for the
view that the sex-determiners behave like Mendelian
allelomorphs. One serious diflSiculty however suggests
itself at once ; the chromosomes are limited in number,
and it is undoubted that more allelomorphic pairs of
characters may exist in a species than there are pairs
of chromosomes, although in such cases there is no
evidence that members of different pairs are always
associated together. Several suggestions have been

II] MATERIAL BASIS 149

made to meet this difficulty, of which perhaps the
most adequate is that the chromosomes are not in-
divisible entities, but are composed of smaller units,
each of which corresponds with one Mendelian factor.
The chromosomes are not permanently present in
the distinct form which is seen during cell-division,
but during the resting condition of the nucleus their
substance becomes diffused over a network of threads,
only to be collected again into definite chromosomes,
having the same number and form as before, pre-
paratory to the next division. If each chromosome
consists of a series of units having a definite arrange-
ment, and these units become scattered in the
'resting phase,' but are re-collected in the same
order when the chromosomes are re-formed, it does
not seem unlikely that a unit N may take the place
of the corresponding unit n from the other chromo-
some of the pair, so that if the chromosome A
consisted at one division of units M, N, O..., and the
corresponding chromosome a consisted of m, n, o...,
after the resting stage N and n might have exchanged
places, and chromosome A would consist of M, n, O. .
and a of m, N, o.... By some process of this kind
it seems probable that the observed phenomena of
chromosome reduction would account for all the
facts of Mendelian segregation.

It must be stated quite clearly, however, that
the study of the possible relation between chromo-

150 HEREDITY [ap. ii

somes and body-characters is as yet in its infancy ;
and this brief note can only sketch the lines on which
modern work seems to support Weismann's hypo-
thesis that the chromosomes are the physical basis
of inheritance. It will be seen that his suggestion
that all the chromosomes are on the whole similar
is not confirmed, but the evidence that chromosomes
do bear factors for inherited characters is consider-
ably stronger than when the idea was first put
forward \

^ The suggestion referred to in the note on p. 97 must also be
borne in mind, that it is not the • factor ' alone which determines the
development of a character, but a physiological relation between the
factor contained in a chromosome and the surrounding protoplasm.
If the latter is altered from any cause, the relation may be changed
and the character modified, just as the plants raised from one sample
of seed may be modified by growing them in different soils. If, as is
quite possible, the relation is a reciprocal one, the factor may in some
cases be permanently modified, and there would thus be a mechanism
for the transmission of acquired modifications. At present, however,
it must be admitted that experiment hardly provides sufiQcient basis
for speculations such as these.

151

LITERATURE LIST

The Works marked * are general treatises suitable
for further study.

1. Bateson, W. Materials for the Study of Variation. London,
1894.

*2. ■ Mendel's Principles of Heredity. 2nd Impression.

Cambridge, 1909. (Full bibliographyto date of publication.)

3. Boveri, T. Ergebnisse iiber die Konstitution der Chroma-

tischen Substanz des Zellkerns. Jena, 1904.

4. Castle, W. E. Heredity of Hair-length in Guinea-pigs, and

its Bearing on the Theory of Pure Gametes. Publ. Carnegie
Inst. Washington, No. 49, 1906.

5. Studies of Inheritance in Rabbits. Publ. Carnegie

Inst No. 114, 1909.

*5a. Darbishire, A. D. Breeding and the Mendelian Discovery.

Cassell, 1911.
*6. Darwin, C. The Origin of Species.

*7. Variation of Animals and Plants under Domestication.

*8. Darwin and Modern Science. Cambridge, 1909.
9. Davenport, C.B. Statistical Methods with Special Reference

to Biological Variation. New York and London, 1899.
9a. Doncaster, L. Recent work on the Determination of Sex.

Science Progress, July 1909: reprinted in Smithsonian

Report (Washington) for 1910.

152 LITERATURE LIST

10. Eugenics Laboratory. Publications of the Galton Laboratory

for National Eugenics, University of London : —

Especially, Elderton, E. M. Relative strength of Nature
and Nurture. Heron, D. A First Study of the Statistics
of Insanity. Schuster, E. Inheritance of Ability. Bar-
ring-ton, A. and Pearson, K. A Preliminary Study of
Extreme Alcoholism in Adults.

11. E wart, J. C, The Penycuik Experiments. London, 1899.
*12. Galton, P. Inquiries into Human Faculty and its Develop-
ment. Macmillan, 1883. (Cheap Edition, J. M. Dent.)

*13. Natural Inheritance. Macmillan, 1889.

14. Essays in Eugenics. Eugenics Education Soc. Lon-
don, 1909.
*15. Hereditary Genius. Macmillan, 1869.

16. Herbst, C. Vererbungsstudien, i — vi. Archiv fiir Entwick-

lungsmechanik, 1906 — 1909. Especially v — vi, 1907 and
1909.

17. Hurst, C. C. On the Inheritance of Eye-colour in Man.

Proc. Roy. Soc. (B) Vol. 80. 1908.

18. Johannsen, W. Ueber Erblichkeit in Populationen und

in Reinen Linien. Jena, 1903.
18a. Journal of Genetics, 1911 — A quarterly journal containing
irapoi-tant papei's on Heredity and allied subjects, edited
by Professors Bateson and Punnett. Camb. Univ. Press.

19. Kammerer, P. Vererbung erzwungener Fortpflanzungsan-

passungen. Arch, f Entwicklungsmechanik, Vols, xxv

(1907) and xxviii (1909).
*20. Kellogg, V. L. Darwinism Today. London and New York,

1907.
*21. Lock, R. H. Recent Progress in the Study of Variation,

Heredity and Evolution. 2nd Edition. Murray, 1909.
*22. Lotzy, J. P. Vorlesungeu iiber Descendenztheorien. Jena,

1906.

LTTimATlIRE LIST 153

22rt. M''Dougall, D. T. Organic Response. American Naturalist,

Vol. 45, p. 5, Jan. 1911.
22b. Morgan, T. 11. Nvnnerous papers in 'Science,' 'American

Natiu'alist' and 'Journal of Experimental Zoology,' iniO-

II.
23. Nettleship, E. Bowman Lecture. Trans. Ophthalm. Soc.

1909.
*24. Pearson, K. The Grammar of Science. 3rd Edition, Part ii.

London, 1912.

25. Mathematical Contributions to the Theory of Evolution.

Proc. and Trans. Roy. Soc. (A) 1896 — 1903; also numerous
papers in Biometrika, 1902 — , especially 'On the Laws
of Inheritance in Man,' 1903, 1904; and 'The Law of
Ancestral Heredity,' 1903.

26. Huxley Lecture of Anthropologicallnst. of Gt. Britain

and Ireland. Trans. Anthrop. Inst. 1903, p. 179.

*26«. Przbram, H. Exi^erimental Zoologie. Vol. in. Phylogenese.
*27. Punnett, R. C. Mendelism. 3rd Edition. Macmillan, 1911.
*28. Reid, Archdall. The Principles of Heredity. London, 1906.
*29. Romanes, J. G. Darwin and After Darwin. 3 vols. London,
1892—1897.

30. Royal Society. Reports to the Evolution Committee, i — v,

1902—1909.

31. Schuster, E. Hereditary Deafness. Biometrika, iv, 1906,

p. 465.

32. Standfuss. Handbuch der Palaarktischen Grossschmetter-

linge. Jena, 1896.
32a. Sumner, F. B. An Experimental Study of Somatic

Modifications. Arch. f. Entwicklungsmechanik, Vol. xxx,

Pt. II, p. 317, and American Naturalist, XLVi, p. 5.
*33. Thomson, J. A. Heredity. Murray, 1908. (Very good

bibliography.)
34. Tower. An Investigation of Evolution in Chrysomelid

154 LITERATURE LIST

Beetles of the Genus Leptinotarsa. Publ. Caniegie Inst.
Washington, No. 48, 1906.
35. Treasury of Human Inheritance. Publ. Gralton Laboratory
for Nat. Eugenics, London University.
*36. Vries, H. de. The Mutation Theory (Trans.). London, 1910.

*37. Species and Varieties, their origin through Mutation.

Chicago and Loudon, 1905.
*38. Wallace, A. K Darwinism. London, 1889.
39. Weisniann, A. Essays upon Heredity and Kindred Subjects
(Trans.). Oxford, 1891, 1892.

*40. The Germ Plasm (Trans. Parker and Ronnfeldt).

London, 1893.

*41. The Evolution Theory (Trans. J. A. and M. R.

Thomson). London, 1904.
42. Wheldale, Muriel. Plant Oxydases and the Chemical Inter-
relationships of Colour-varieties. Progressus Rei Botani-
cae, ni, p. 457. Jena, 1910.
*43. Whetham, W. C. D. and C. The Family and the Nation.
London, 1910.
43rt. Wilson, E. B. Studies on Chromosomes. Journal of Exp.

Zoology. 1905-1909.
44. Yule, G. U. Mendel's Laws and their probable relations
to intra-racial Heredity. New Phytologist. Vol. i, 1902,
p. 193.

155

GLOSSARY

Acquired Character. A feature developed during the life of the
individual possessing it, in response to the action of use or
environment.

Albino. An animal without pigment in the skin, hair or eyes.
The hair is white ; the eyes pink ovnng to the colour of the
blood. Among plants, white-flowered varieties are called
albinos. The condition is called albinism.

Allelomorph. One of a pair of alternative Mendelian characters.
When a pair of characters are alternative in their in-
heritance, and segregate from each other in the formation of
the germ-cells of an individual which contains both, they are
said to be allelomorphic with each other. See Segregation.

Anther. The part of the stamen in a flower which contains the
pollen.

Chromogen. A colourless substance which when oxidised gives
rise to a coloured body (pigment).

Chromosome. A body in the nucleus of a cell, which absorbs stains
and becomes clearly visible during nuclear division, but be-
comes dispersed through the nucleus during the resting phase.
During nuclear division each chromosome becomes accurately
halved, so that in general all cells of each species of animal
or plant contain an equal number of chromosomes.

Continuovis. See Variation.

Determinant. The hypothetical unit in a germ-cell which
determines the production of a i^articular character in the
individual derived from that germ-cell. See Factor.

156 GLOSSARY

Deviation. The amount by which an individual differs from the
mode in continuous variation.

Dimorphism. The condition in which a species exists in two
distinct types or sharply separable varieties. When the two
sexes differ thus, the condition is called Sexual Dimorphism.

Dominant. When two varieties, differing in one character, are
crossed together, and all the offspring have the character
borne by one parent, that character is dominant Applied
to one of a pair of Mendelian allelomorphs.

Egg-cell. The germ-cell produced by the female.

Epistatic. When one character A is superposed upon another B,
so that A prevents or obscures the appearance of B, al-
though they are not allelomorphic with each other, A is said
to be epistatic to B.

^Extracted' Hom,ozygote. When two heterozygous individuals
are mated together, their homozygous offspring are spoken of
as ' extracted ' homozygotes.

i^i, F^. The Symbol Fi is used to indicate the offspring (first
filial generation) of a mating between two differing individuals.
The later generations (second, third filial, etc.) are repre-
sented by Fi, F3, etc.

Factor. In Mendelian inheritance, the hereditary determinant
(q.v.) of a particular character is spoken of as the factor for
that character.

Ferment. A body which has the power of causing chemical
action between substances which in its absence are inactive
towards one another.

Fertilisation. The union of male and female germ-cells which
precedes the development of a new individual. It consists
essentially in the fusion of the nuclei of the gei-m-cells.

Gamete. A germ-cell, q.v.

Gemmules. Hypothetical bodies supposed to be given off by the
cells of the body, and entering the gei-m-cells, to transmit
heritable characters to the next generation.

GLOSSARY 157

Germ-cell. A reproductive cell, which, usually after union with
a germ-cell from another individual (fertilisation), develops
into a new individual. In animals the germ-cells of the male
are spermatozoa, those of the female ova {egg-cells). In
plants the male germ-cells are contained in the pollen ; the
female, egg-cells, in the ovules or embryo-seeds.

Germ-plasm. The germinal substance, which according to
Weismann is alone able to give origin to new individuals.

Heterozygote. An individual containing both members of an
allelomorphic pair of characters, i.e. which is hybrid in respect
of that pair of characters, and produces germ-cells bearing
one and the other respectively. Adjective — heterozygous.

Homozygote. An individual made by union of two germ-cells
each of which bears the same member of an allelomorpliic
pair of characters, so that it is 'pure' in respect of that
character, and all its germ-cells bear the same character.
Adjective — homozygous.

Mode. The most frequent condition of a character which varies
continuously. Its measurement is called the modal value.

Mutation. A variety which is not connected with the type by
intermediates. More strictly, the sudden origin of such a
variety.

Nucleus. A sharply defined body found in every cell, which seems
to control the activities of the cell.

Ovum. The germ-cell produced by the female, an egg-cell.

Pin-eye. In Primula (Primrose, Cowslip, etc.), the form in
which the style is long and the anthers low down in the
flower-tube. The other form, with short style and anthers
high up, is called Thrum-eye.

Pollen. The powder bearing the male germ-cells in a flowering
plant.

Polym,orphism. The condition in which a species exists in
several distinct forms or varieties.

Recessive. When two individuals are crossed, bearing diflcrent

158 GLOSSARY

members of an allelomorphic pair of characters, the member
of the pair which does not appear in the offspring is called
recessive.

Reversion. A ' throw-back ' to a previous ancestor, or to the
type of the species, when varieties are crossed.

Segregation. In Mendelian inheritance, the separation of the
two characters of an allelomorphic pair, in a heterozygote,
into distinct germ-cells, i.e. the formation of gametes each
bearing one character of a pair, by an individual which
contains both members.

Self -fertilisation. The fertilisation of a female gamete by a male
gamete produced from the same individual The process in
plants is spoken of shortly as selfing.

Somatic. Having reference to the body {'soma') considered as
distinct from the germ-cells. A character borne or exhibited
by the body but not represented in the germ-cells is called
somatic as contrasted with germ.inal.

Spermatozoon. See Germ-Cell.

Style. The part of a flower which receives the pollen, and con-
ducts the male germ-cell to the egg-cell.

Telegony. The supposed influence of a former sire upon young
born to a later sire by the same mother.

Type. The normal form of a species, which is regarded as
typical.

Variation, Variability. The differing among themselves of
individuals of the same species. When the extreme forms
are connected by a complete series of intermediates, the
variation is Continuous; when distinct forms occur, not
connected by intermediates, it is Discontinuous.

Zygote. An individual produced by the union of two gametes.

INDEX

Ability, Inheritance of, 48

Abraxas, 121

Acquired Clmracters, 19, 24, 30,

45, 90-99, 136, 140
Albinism, 66, 71
Allelomorph, 56, 61, 71, 147
Amphidasys betularia, 89
Ancestral Heredity, Law of, 42
Andalusian Fowl, 69
Bateson, W., 19, 70, 76, 82, 125
Bee, 93, 129
BiFFEN, R. H. , 64
Biometric Methods, 9, 32-51
Boas, F., 96 (note)
BovERi, T., 146
Brachydactyly, 106
Bkown-Sequard, 94
Canary, 124
Cat, 120
Cataract, 106

Chromosomes, 130, 140, 145-150
Colom'-blindness, 126, 127
Colour-inheritance, 71-83
Combs of Fowls, 66, 67
Correlation, 36

„ parental, 39, 103

Cunningham, J. T., 91
Currant Moth, 121
Darwin, 2, 8, 23, 24, 88, 136
Deafness, 103
Dihybridism, 62
Disease, Inheritance of, 102
Dominant Characters, 54
Drosophila, 126-128
Earwig, 17
Eldbbton, E. M., Hi

Environment, 24-20, 11*2-117

Epistatic, 73, 75

Eugenics, 51

Evolution, 2, 88, 136

Ewart, J. C, 98

Extracted Homozygote, 62, 86

Eye-colour, 66, 104, 109

Feeble-minded, 50

Fischer, E., 27, 93

Flatfish, 91

Flower-colour, 66, 74, 77, 79-83

Fluctuation, 22, 45

Fowls, 66, 67, 124

Galton, Sir F., 24, 30, 33, 41, 113

Gametes, 57 {see Germ-cells)

Gametic coupling, 125

Gemmules, 137

Germ-cells, 57, 137, 139, 145

Germinal Continuity, 138, 144

Germ -plasm, 20, 23-26, 139-144

Gregory, R. P., 82

Guinea-pig, 86, 94

Haemophilia, 126

Hair-colour, 66, 109

Hair-length, 66, 86

Herbst, C, 146

Heron, D., 103

Heterozygote, 59

Homostyle (Primula), 82

Homozygote, 61

Horns of Sheep, 84, 121

Hurst, C. C, 104, 109

Hybridisation, 53

Insanity, 102, 114

Instinct, 92

JOHANNSEN, W., 44, 87

160

INDEX

Kammekek, p., 06

Lamarck, 136

Leptinotarsa, 28

Lychnis, 66

Maize, 65, 66

Man, Heredity in, 101-117, 126

Marsh-marigold, 15

Maternal Impression, 99

Mendelian Heredity, 52-89

„ „ in Man, 102,

104-109
Mendel's Experiments, 53, 54
Mental and Moral Characters, 49
Mid-parent, 40
Mode, 10, 34
Morgan, T. H., 123
Mouse, 74
Mulatto, 110
Mutation, 22

Natural Selection, 8, 89, 136
Nettleship, E., 106, 108
Night-blindness, 106
Nucleus, 140, 145
Organic Stability, 24
Pangenesis, 137
Pearson, K., 32, 41, 115 (note)
Peas, 54
Pigeons, 66
Primula, 66, 79, 81-83
Protoplasm, 3
PUNNETT, R. C, 70, 125
'Pure Lines,' 44, 87
Rabbit, 74, 79, 87
Rat, 72, 79, 96 (note)
Recessive Characters, 56
Regression, 35
Retinitis pigmentosa, 107
Reversion, 74, 76
Rust in Wheat, 64

ScHtrsTEE, E., 49, 103

Sea-urchin eggs, 145, 146

Seed-colour, 66

Segregation, 58

Sex, 84, 105, 108, 119-134

Sheep, 84, 121

Silkworms, 94

Skew Curves, 15
„ Correlation, 39

Skin-colour, 110

Social Problems, 114

Standfuss, M., 27

Statistical Study of Variation, 32

Stature, 10, 34, 37

Sterility, 66, 112

Stocks, 66, 80

Sumner, F. B., 96 (note)

Sweet-pea, 54, 66, 76, 125

Telegony, 97

Temperature, effects of, 25, 27, 28

Tortoiseshell Cat, 84, 120

Tower, W. L., 28

Tuberculosis, 25

Twins, 30, 113, 129

Variation, 2, 5, 7

„ Causes of, 8, 23- 81, 142

Variation, Continuous and Dis-
continuous, 9-12, 17-19, 22, 88

Variation, Curves illustrating,
11-17

Variation induced by crossing, 29

Vries, H. de, 19

Wallace, A. R., 8

Weismann, a., 20, 23, 138-144

Weldon, W. F. R., 33

Wheat, 64, 66

Wheldale, Muriel, 77

Yule, G. U., 42

Zygote, 58

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D65
1912

Doncaster, Leonard

Heredity in the light of
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