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Cornell University

Library

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

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

http://www.archive.org/details/cu31924003049339

AtFrepD RussELL Wa Lace, in the Contemporary Review of
August, 1908, p. 140, in an article on ‘The Present Posi-
tion of Darwinism,’ writes as follows:

‘In conclusion, I would suggest to those of my readers
who are interested in the great questions associated with the
name of Darwin, but who have not had the means of study-
ing the facts either in the field or the library, that in order
to obtain some real comprehension of the issue involved in
the controversy now going on they should read at least one
book on each side. The first I would recommend is a
volume by Mr. R. H. Lock on “ Variation, Heredity and
Evolution” (1906) as the only recent book giving an account
of the whole subject from the point of view of the Mendelians
and Mutationists. When they have mastered this, I ask
them to read my own book on “ Darwinism” (1901), which,
though published before Mendelism became prominent,
gives some idea in popular language of the vast range of
subjects which Darwinism explains, and adduces a sufficient
body of facts to show the inadequacy of the whole series of
phenomena yet made public.

‘Having read these two works and again considered the
arguments adduced in this article, I leave them to form
their conclusions as to whether Darwinism is or is not an
“unsuccessful hypothesis.”’’

‘ALFRED R. WaLLace.’

VARIATION, HEREDITY, AND
EVOLUTION

First Epition, December, 1906
Reprinted, April, 1907

35 March, 1909
Seconp EDITION, June, 1909
Tuirp Epition, September, 1911

Fiontispiece|

ee Sar ca.

RECENT PROGRESS IN THE
STUDY OF VARIATION,
HEREDITY, AND EVOLUTION

By ROBERT HEATH LOCK, M.A,

FELLOW OF GONVILLE AND CAIUS COLLEGE, CAMBRIDGE

NEW YORK
E, P. DUTTON AND COMPANY
Ig1t

PREFACE TO THE THIRD EDITION

In this edition it has been found necessary to make a
further correction of the account of the yellow coat
colour in mice. Problems connected with the yellow
colour still remain unsolved in more than one group
of animals.

I have taken the opportunity of introducing into
the concluding chapter a brief statement of the recent
discoveries relating to so-called-‘ graft-hybrids.’ These
probably represent the most important advance in our
knowledge of the hereditary processes of plants which
has been made within the past two years.

Other corrections and alterations are of minor
importance.

R. H. LOCK.

PERADENIYA, CEYLON,
May 17, 1911.

PREFACE TO THE SECOND EDITION

Tue following are the principal alterations and addi-
tions which have been made in the present issue :

A short list of references has been added at the end
of each chapter. These include only a small number
of works which may be regarded as essential for every

Vv

vi PREFACE TO THE SECOND EDITION

serious student of genetics. For a fuller bibliography
reference must be made elsewhere (see end of
Chapter I.).

The account of the evidences of evolution in
Chapter II. has been somewhat expanded.

In Chapter III. a brief note on Isolation has been
inserted, and the remarks on Mimicry have been
modified.

In Chapter IV. a definition of the Co-efficient of
Variability has been added, and a more definite attitude
has been adopted with regard to the theory of the
pure line.

To Chapter V. there have been added an account of
the effect of external conditions upon plants and a
brief allusion to the artificial production of muta-
tions.

The most important changes in Chapter VIII. are
as follows :

An account of partial gametic coupling has been
added ; the description of the inheritance of coat colour
in mice has been revised ; a classification of the different
forms of latency has been inserted. Further additions
are a discussion of the case of supposed non-segregating
hybrids, and some remarks on purity of strain.

The problem of sex determination has been further
discussed in Chapter IX.

Chapter X. as a whole is new.
R. H. LOCK.

PERADENIYA, CEYLON,
May 17, 1909.

PREFACE TO THE FIRST EDITION

THE idea of writing this little book occurred to me
whilst reading Mr. W. C. D. Whetham’s volume on
‘The Recent Development of Physical Science.’ I
found the story of the modern progress of physics so
interesting as to encourage the belief that a similar
account of the subjects with which I was myself more
particularly familiar might prove of a like interest to
other people. I did not, indeed, suppose for a moment
that I could vie with Mr. Whetham in the power of
literary expression which renders his book so eminently
readable. I rather hoped that the peculiar interest
and importance of the theme might outweigh the
present author’s deficiencies in this respect.

For the group of subjects of which I intended to give
a brief account Mr. W. Bateson has recently proposed
the term ‘ genetics,’ an expression which sufficiently
indicates their scope to the initiated. Since, however,
the meaning of the word ‘genetics’ is not yet clearly
understood by everybody, it seemed better to adopt
in the present instance a somewhat more descriptive
title.

viii PREFACE TO THE FIRST EDITION

The rediscovery of Mendel’s law some seven years
ago led to a complete change in our attitude towards
the problems of variation, heredity, and evolution ;
and the new method of study thus introduced has
rendered possible a renewal of the attack upon these
problems with increased vigour and with remarkable
results. At the present time this activity may be
said to have its centre in the school of genetic research
founded at Cambridge by the independent energy of
Mr. Bateson. So far-reaching are the results already
arrived at by Mr. Bateson and others, both in their
scientific interest and in their probable influence upon
human affairs, that it seemed desirable to give an
immediate account of these and of kindred lines of
recent study, even though the rapid progress which is
a characteristic manifestation of this department of
science must render any such attempt a more or less
transitory one.

Whilst I was still engaged upon my task, the first
volume of Dr. Lotsy’s admirable ‘ Vorlesungen iiber
Descendenztheorien’ made its appearance. But for
the fact that most of the following pages had then
already been written, I might have hesitated to pursue
my project, since a book not altogether unlike the
present might be produced by the comparatively simple
process of making a series of judicious extracts from
Dr. Lotsy’s work. The latter is, however, in the
German language,'and on a considerable scale, so that
there seemed still to be room for an introduction to

PREFACE TO THE FIRST EDITION ix

the science of genetics of the more modest dimensions
which I had contemplated. I should wish, however,
particularly to recommend Dr. Lotsy’s lectures to any
reader who wishes to go further into these matters.

I am indebted to several friends for assistance during
the course of my work. Mr. R. P. Gregory kindly
read through the proof of the chapter on cytology ; and
I wish here to record my thanks to Mr. J. Stanley
Gardiner, to Mr. C. T. Regan, to Mr. W. S. Perrin, and
to Mr. R. C. Punnett for information on special points.
To the last-named I owe the photograph which appears
as Fig. 15. I am particularly grateful to Mr. R. H.
Biffen and to Mr. G. Udny Yule for access to work
which has not hitherto appeared in print.

Adequately to acknowledge Mr. Bateson’s influence
upon these pages is a more difficult matter, and not
the less so because I have deliberately refrained as
far as possible from consulting him whilst the book
was in course of preparation, in order that it might
retain if possible some traces of individuality. It is
therefore clear that he is in no way responsible for its
deficiencies. But, apart from the fact that I am
conscious of having quoted his ideas at more points
than could possibly be acknowledged seriatim, I owe
to Mr. Bateson both my first introduction to the science
of genetics, and a continual fund of encouragement in
the prosecution of studies connected with it.

I have to thank Mr. Francis Darwin for kind per-
mission to reproduce a portrait of his father; Professor

x PREFACE TO THE FIRST EDITION

de Vries for the present of an excellent portrait ; and
Mr. Francis Galton for the loan of a photograph weli
known from earlier reproductions. The portrait of
Mendel is reproduced from the frontispiece to Mr.
Bateson’s ‘ Defence,’ by the permission of the Syndics
of the Cambridge University Press. Messrs. Macmillan
and Co. have kindly allowed the reproduction of the
diagram which occupies p. 87, and of the table and
figure on pp. 89 and 90. The figures facing pp. 143
and 150 are from de Vries’ ‘ Mutationstheorie,’ pub-
lished by Messrs. Veit.

The attempt has been made to render the following
pages intelligible to the general reader, as well as to
the more scientific public, to which they are primarily
addressed. A short glossary has been added, which
may be found useful by those who have no previous
acquaintance with biological terms.

CAMBRIDGE,
October 23, 1906.

CONTENTS

CHAPTER I
INTRODUCTION

Subject-matter—The study of evolution—Dependence
on other sciences—The problem of the nature and
origin of species—Linnzan species—Jordan’s species
—tThe discontinuity of species—Methods of study—
Variation—Different theories as to the origin of
species: Lamarck’s theory; Darwin’s theory; the
sree theory—Recent study of species Biometry ;
Mendelism - - I

PAGE

CHAPTER'II
EVOLUTION

Definition—Evolution of matter—Evolution in geology
—The age of the earth—Organic evolution—Evi-
dences of organic evolution—The__ theory _ of
Lamarck. - = 7 : - - 22

4 CHAPTER IL
THE THEORY OF NATURAL SELECTION

Historical—Darwin’s attitude to variation—Artificial
selection—Malthus and the geometrical rate of
increase of living things—Natural selection—Adapta-
tion—Protective resemblance and mimicry—Parallel
evolution — Regeneration — Sexual selection — The
inheritance of acquired characters—Herbert Spencer’s
arguments—Weismann and the purity of the germ

cells - - - - - 7 42
sa

xii CONTENTS

CHAPTER IV
BIOMETRY
PAGB

Biology and_ statistics—Normal variations—Definite
differences — Biometrical methods—The normal
curve—Probable error—The biometrical treatment
of heredity—Correlation—Structural and mental
characters—The law of ancestral heredity—The
effect of selection—Johannsen’s theory of the pure
line—Selection in pure lines and in populations - 80

CHAPTER V
THE THEORY OF MUTATION

Objections to the theories of Lamarck and of Darwin—
Aristotle and Dr. Thomas Brown on mutation—
Galton and organic stability—Bateson and discon-
tinuous variation—Symmetry—Merism—Similar and
simultaneous variation—Homcosis—The views of
de Vries — The effect of selection — Mutation
in C£nothera — Eversporting varieties — Different
methods of mutation—The survival of useless struc-
tures - - - - - [21

CHAPTER VI
THE OLDER HYBRIDISTS

K6élreuter — Knight — Herbert — Gaertner — Naudin —
Millardet - - - - . - 159

CHAPTER VII
MENDELISM

The double nature of animals and of the higher plants—
Unit characters—The inheritance of definite char-
acters in cross-bred maize—Mono-hybrids and di-
hybrids—Mendel’s law—Gregor Johann Mendel—
Experiments with peas—Dominance—Heterozygote
forms—tThe case of the Andalusian fowl—The nature
of allelomorphs - - - > - 194

CONTENTS

CHAPTER VIII
MENDELISM (continued)

Coupling : complete and partial—Reversion on crossing :

The

Sir

The

masked characters—Examples in peas, mice, and
sweet-peas—Various Mendelian characters—Doubt-
ful cases; normal fertilization The supposed
‘ swamping ’ of new species on crossing with the old—
Summary of Mendelian ideas-— Mendelism and
biometry : researches of Yule and Pearson—Practical
applications : Biffen’s work with cereals ; ‘ strength ’
of wheat-flour and rust-immunity - - -

CHAPTER IX
RECENT CYTOLOGY

cell—Cell-division and cell-fusion—The nucleus—

Nuclear division — The nature and properties of
chromosomes—The reducing division—The study of
the chromosomes in the light of Mendel’s discovery—
Chromomeres — The heredity of sex— The germ-
plasm theory - - - -

CHAPTER X
EUGENICS

Francis Galton, pioneer—Genetics and biometry—
Effect of civilization and humanitarianism—Twins—
Hereditary nature more important than training—
Consequent legislation—Restraint of propagation—
Graduation of incomes—Karl Pearson—Eugenics
Education Society - - -

CHAPTER XI
CONCLUDING CHAPTER

science of genetics—Mendel’s law—The #-generation
and the 2#-generation—Discontinuous variation—
The mutation theory—Continuous variations—Gene-
tic variations and acquired variations—Johannsen’s
theory of the pure line—The modern view of the
method of origin of new species—The science of
genetics applied to human affairs—George Bernard
Shaw’s view of the situation - - - -

xili

PAGE

197

239

282

298

LIST OF ILLUSTRATIONS

PORTRAITS
CHarLES DARWIN - - - - Frontispiece
Huco DE VRIES - - - To face page 121
JosEPH GOTTLIEB KOLREUTER - - 2 ‘ 159
GREGOR JoHANN MENDEL - - = ” 174
FRaNcIs GALTON - - - - os 282
DIAGRAMS \

PAGE PAGE

Fig. 1 - 7 - 74 | Fig. 14. Mendelian Pro-

Fig.2 - - - 84 portions in Maize -
Fig.3 - - - 87 To face 186

Fig. 4 - - 89 | Fig. 15. Inheritance in
Fig. 5 - - go Primula - To face 194
Figs. 6 and 7 - - 94 | Fig. 16 - - - 212
Fig. 8 - - - 96 | Figs. 17 and 18 - - 241
Fig.9 - - - 103 | Figs. 19to26 - - 249
Fig. 10 - is - 126] Figs.27to31 - - 252
Fig. 11 - - - 138 | Figs.32t037 - - 259
Fig. 12. Mutation in Figs. 38 to 42 - - 260
Cinothera -To face 143 | Fig. 43 - - - 262
Fig. 13. Trifolium Pra- Figs. 44 and 45 - - 266
tense Quinquefolium Fig. 46 - . - 268
To face 150 | Fig. 47- - - 272

RECENT PROGRESS
IN THE STUDY OF VARIATION,
HEREDITY, AND EVOLUTION

CHAPTER I

INTRODUCTION

THE present volume deals with variation and inheri-
tance in plants and animals, especially in so far as
those subjects bear upon the problem of the origin of
species. By inheritance we mean those methods and
processes by which the constitution and characteristics
of an animal or plant are handed on to its offspring,
this transmission of characters being, of course, asso-
ciated with the fact that the offspring is developed by
the processes of growth out of a small fragment de-
tached from the parent organism. The term ‘ varia-
tion,’ on the other hand, includes a number of different
phenomena which will be described at greater length
as the work proceeds ; but; broadly speaking, we may
say that the study of variation is concerned with the
circumstance that members of the same species are
not all alike, and more particularly with the fact that
I

2 INTRODUCTION

differences are to be found between different members
of the same family. Some of these differences arise
comparatively late in life, and may be the result of
circumstances or of education. It is the first duty of
the student of variation to distinguish as far as may
be possible between differences of this kind on the one
hand, and those differences on the other which depend
upon the fact that the different detached fragments,
as we have termed them, of the parent organism—its
germ-cells, in fact—show greater or smaller differences
among themselves.

The facts of variation have this very special impor-
tance, that the whole theory of organic evolution is
based upon them. The fact that members of the same
species are not all alike, depending upon the further
fact that offspring may differ from their parents, makes
it possible in the course of generations for progressive
changes to take place, so that from the offspring of
different members of the same species different new
species may arise. But for this fact of variation it
would have been quite impossible for Darwin to have
overthrown the former crude belief in a special crea-
tion of each separate species, since there would have
been no material for his great factor—natural selection
—to work upon. It is with variation, then, and with
the manner in which characters appear in the succes-
sive generations of living things, that we are here
concerned.

Ever since the publication of Darwin’s ‘Origin of
Species’ in 1859, these subjects, and especially the
theoretical aspects of them, have been received even

THE STUDY OF EVOLUTION 3

by the general public with all the signs of a genuine
enthusiasm ; and none; moreover, can be more fas-
cinating to the professional naturalist. But since the
time of Darwin the more popular accounts have dealt
almost exclusively with theoretical considerations and
with matters of opinion. Highly abstruse contro-
versies have raged freely between Neo-Lamarckians
and Neo-Darwinians, and these have found a place
in the pages of works ostensibly intended for the in-
struction of all and sundry; whilst only a bare re-
siduum of actual matters of fact has seen the light of
popular publication. If the truth must be told, the
experimental method was given up for a long time by
the majority of specialists themselves in favour of the
controversial, and, indeed, this tendency has by no
means yet died out from among the habits of some
professed evolutionists. On the other hand, during
the last fifteen to twenty years, a few scattered workers
have diligently applied themselves to a study of the
facts of variation and inheritance, with results which
already more than justify the anticipation in which
their work was begun—namely, that by such methods
alone can any real progress in our knowledge of the
processes of evolution be brought about. —

The science of organic evolution is by no means the
simple and isolated study it might be supposed to be
from a perusal of some of the more popular accounts.
Its footing rests immediately upon the widest founda-
tions which zoology, botany, and physiology can afford ;
and these in their turn are ultimately based upon the
results of chemical and physical science. But some

I—2

4 INTRODUCTION

of the most fundamental parts of physical science, as
I think we may fairly call the branches of electricity
and molecular physics, seem at present to be under-
going modifications which bid fair to bring about a
complete revolution in current ideas upon these sub-
jects. It is highly probable that these results will
ultimately lead to a considerable modification in pre-
vailing notions about living things; but the new
developments have yet to reach biology through the
channels of organic chemistry, physiology; cytology,
and the like, and at present we do not know what the
result of this influx is likely to be. These considera-
tions need not, however, detain us, for the new know-
ledge of variation and inheritance, of which it is pro-
posed to give some account, is largely concerned with
the grosser characters of organisms, so that ultraminute
structures may be left alone for the present until the
stream of physical knowledge stirs them into greater
prominence. So much is this the case with the study
of variation and inheritance by experimental methods
at the present day, that this science is treated by some
with a fine contempt, because its tools are those of the
breeder and gardener, and because the assistance of
the compound microscope may often be laid aside for
days together. Yet this applies only to one aspect of
the subject, and the microscopic study of the embryonic
rudiments of organisms, going hand in hand with the
experimental observation of adult structures, is rapidly
leading to a clearer understanding of the processes of
heredity.

The problem which those who are engaged in this

THE PROBLEM OF SPECIES 5

kind of work have set themselves for solution is that
of the nature and method of origin of the existing
differences between certain groups of organic beings
—namely, species. Basing their studies on the doc-
trine that the present species have arisen through the
modification of pre-existing species, they endeavour
to observe how modifications of existing species do
actually arise in Nature, as well as under domestica-
tion ; and they watch the hereditary transmission of
the modified forms when like is bred with like, and
when different types are crossed together. For the
theory of uniformity, now universally accepted, teaches
us that the organisms with which we are now familiar
owe their present characteristics to the accumulation
of a series of changes similar to those which are still
in progress. It has, therefore, appeared likely to a
few that a further understanding of the processes of
evolution might best be obtained by a closer study,
firstly, of variation, or the ways in which offspring
differ from their parents ; and, secondly, of inheritance,
or the ways in which the resemblances between parents
and their offspring are perpetuated from one genera-
tion to another.

It may be well to point out at once that the further
study of the method of origin of new species, admitting,
as it does, that this process is not yet by any means
fully understood, does not for this reason imply that
the theory of organic evolution itself is open to criti-
cism. The evidence that new species arise by the
modification of pre-existing species is quite indepen-
dent of the evidence that this process invariably occurs

6 INTRODUCTION

by the action of natural selection upon minute differ-
ences, in the manner which Darwin described, and
which has been claimed by others as the sole means
by which the origin of new forms takes place. The
evidences of evolution are much more numerous and
more weighty than the evidences of the survival of
the fittest. The theory of evolution, as opposed to
the creation hypothesis, is supported by innumerable
facts of classification, of morphology, and of embryo-
logy, by the geographical distribution of animals and
plants, and by their succession in the geological strata,
as well as by direct observation of the actual occurrence
of changes in the case of domestic productions as well
as under Nature, and many of these facts have no
direct bearing upon the theory of natural selection.

Before discussing the problem of the origin of species,
it is necessary to arrive at some idea as to what the
term ‘species’ means. And this is not altogether an
easy matter, since a precise definition has not been,
and cannot be, agreed upon. The idea of species is,
indeed, of great antiquity and very gradual growth.
Primitive men doubtless recognised certain plants or
animals as being like one another, and different from
others, and they gradually came to distinguish such
forms by giving a different name to each. The names
first used must have applied as a rule to genera rather
than to species. Thus, such common names of plants
as rose, bramble, vetch, nettle, dock, crowfoot, are
names of genera—groups of greater extent than species,
and often more easily defined than the latter. Later

LINNAN SPECIES 7

on civilized men paid closer attention to tne different
kinds of plants, and the old herbalists discovered and
described a number of different sorts of roses, of butter-
cups, and of other plants, and distinguished each by a
descriptive sentence.

As more and more species came to be described, this
method of designation became very cumbersome, until
Linnzus, about the middle of the eighteenth century,
adopted the idea of a binomial nomenclature (originally
suggested by Bachmann), in which every species of
each known genus received a separate name of its own
to distinguish it, so that the different kinds of butter-
cups were now known as Ranunculus acris, R. bulbosus,
R. sceleraius, and so on.

Linnzus himself appears to have had a very definite
idea of what constituted a species, and in accordance
with the view then current, he defined a species as
being a group of organisms which owed its origin to a
separate act of creation. From the nature of the case
this definition could be of little use in practice. Prac-
tically, then, species were defined as groups of animals
or plants, the members of which resembled one another
in definite morphological characteristics—that is to
say, in constant features of form and structure. This
definition has survived the downfall of the dogma of
the constancy of species, and at the present day species
as defined by Linnzus are found to be groups of much
merit both for naturalness and for convenience—at
any rate in the case of plants. The fact that inter-
mediate forms and minor groups do sometimes and to
some extent bridge over the gap which separates a

8 INTRODUCTION

pair of species thus defined seems to have caused dis-
quiet in the mind of Linnzus himself, and he recom-
mended his disciples to have no dealings with these
inferior varieties, as being beneath the dignity of a
botanist to notice. Of late years these minor species
have excited much attention, and it is to a study of
this kind of species in particular that.the mutation
theory of de Vries owes its origin, as will be told in a
later chapter.

Such minor groups, occurring within the limits of a
single Linnzan species, and subdividing it into smaller
collections of individuals, were made the object of
special study in the case of plants by the French
botanist, Jordan ; and for this reason they are some-
times referred to as Jordan’s species. Jordan, for
example—though the example is indeed an extreme
one—described more than two hundred different types,
all of which would formerly have been included in
the single Linnzan species, Dvaba verna. To take a
more familiar instance. We find in the ‘British
Flora’ of Bentham and Hooker the primrose, the
cowslip, and the true oxlip, all described as varieties
of one and the same species ; yet these three kinds of
plants are now almost universally recognised to be
as good species as any in nature.* In a similar way,
on closer investigation, it has been found necessary to
split up a considerable number of Linnzan species,
and to subdivide each into several species of smaller
range.

* A contrary opinion is, however, expressed in the Journal
of Botany for July, 1906.

JORDAN’S SPECIES 9

It has already been pointed out that Linnzus him-
self distinctly deprecated this process of splitting.
‘Varietates levissimas non curat botanicus,’ said
Linneus. Jordan, however, applied the method of
experiment to many of the species of his own defini-
tion, and having transplanted them from a variety
of localities to the uniform soil of a garden, found that
they preserved their distinctive characters and came
perfectly true to seed.

It appears then that Jordan’s species are just such
true and constant groups as those of Linneus. They
are separated from one another by definite features of
form and structure, only these differences are not so
wide as those which separate Linnzan species. The
latter are, indeed, to be looked upon as more or less
artificial groups or aggregates of these physiological
species, as Jordan’s species have also been called. The
problem of the origin of the smaller groups is clearly
to be placed before that of the origin of the larger
species.

It is true that in the case of certain groups of
animals and plants there would appear to be no possi-
bility of drawing hard and fast lines between the
species, which thus seem to shade gradually one into
the other. There is, however, a great difference
between the admission that certain nearly related
species are difficult or impossible to separate definitely,
and the statement that there is no true distinction
between them, and the latter statement is one which
few are bold enough to make. The case stands thus.
We know that great numbers of large groups (classes

10 INTRODUCTION

and families) of animals and plants exist, in which
the most nearly related species are quite definitely
distinct from one another. In other classes systema-
tists have so far found great difficulty in framing
definitions of specific groups. We shall see later on,
though at first sight it may appear almost paradoxical,
that it is quite possible for groups to be perfectly
distinct, although individual members of them may
have deviated so far, each from its proper type, as to
render impossible the task of deciding from their
appearance which group any of these individuals
belong to.

Let us next consider a particular example of a class
of animals in which the discrimination of species is
difficult or impossible. This is said to be the fact
with the majority of sessile animals—such animals
as resemble plants in their stationary habit, and in
no case are the problems of species separation more
difficult than in the class of the stony corals. Now,
attempts to determine the species of corals have so
far been made almost entirely from a study of what
may be called vegetative characters—usually from
details of the shape and structure of the stony skeleton
ofthe animals. How far these features may be affected
by external circumstances has not been determined,
but it must be noted that the so-called skeleton is
entirely external to the living organism. Now we
know that in the case of many of the higher plants
vegetative characters are extremely liable to become
modified owing to the action of the environment.
Differences of moisture, light, soil, climate, and alti-

DISCONTINUITY OF SPECIES II

tude, are all capable of changing the general appear-
ance of a plant so as to render it scarcely recognisable.
Fortunately, in the case of the higher plants, the
floral organs, which are the ones chiefly made use of
for purposes of specific discrimination, are very little
liable to modification by external conditions; but
in the corals a similarly stable set of organs does
not appear to have been discovered. It seems, there-
fore, hardly fair to regard the example of the corals
as affording an established exception to what we
must look upon as the general rule—namely, that
species are on the whole definite and discontinuous
groups.

As a tule, then, the species riddle presents itself
definitely as the problem of the existence of a series
of discontinuous groups of creatures, sharply marked
off the one from the other, and often, too, existing
among surroundings which afford no corresponding
discontinuity, though each is well enough fitted for
the life which it has to lead.

The problem which we have to face has been
enunciated by Bateson in the form of the two following
propositions :

‘1. The forms of living things are various, and on
the whole are discontinuous or specific.

‘2. The specific forms on the whole fit the places
they have to live in.

‘How,’ he continues, ‘have these discontinuous
forms been brought into existence, and how is it they
are thus adapted ? This is the question the naturalist
is to answer. To answer it completely he must find

12 INTRODUCTION

(x) the modes and (2) the causes by which these things
have come to pass.’

The differences between existing species are open
to study in more than one way. By way of limiting
the discussion for the present, we shall consider the
case of plant species only ; but the methods of study
which are applicable to animal species are of quite
similar kinds.

Four methods at least are available. Firstly, that
of comparison ; secondly, the method of statistical
examination ; thirdly, the method of cultural experi-
ment ; and lastly, that of cross-breeding.

The method of comparison is the one to which the
ordinary worker in descriptive botany is almost of
necessity confined. In this way plants which closely
resemble one another are grouped together as belong-
ing to the same species, and separated from others,
the appearance of which is different. By appearance
is not meant simply the general habit of the plants ;
all morphological features whatever may be used for
purposes of comparison, and the most minute are
often of the greatest importance. But the systema-
tist who works only in this way knows nothing of the
real relationships between the plants with which he
is dealing.

When a sufficient number of specimens is available,
the methods of statistics can be applied. These
involve the making of a series of accurate measure-
ments or countings of the parts upon which depend
the supposed differences or resemblances of the plants

METHODS OF STUDY _ 13

under consideration. The resulting numbers are then
ranged in order so that a precise view of the numerical
characters of a large number of specimens can be
readily obtained. By the use of such methods valu-
able information is often to be arrived at. But the
same limitation affects them as in the preceding case.

So that the only way in which we can come to a
definite decision as to whether a given putative
species does or does not represent a definite and con-
stant type is by resorting to our third method, that
of sowing its seeds and actually rearing its progeny.
And this is not so simple a matter as might appear
at first sight, for a great many precautions have to
be taken. Thus we must separately sow the seeds of
many different individuals of the species which we
are examining, so as not to base our conclusions upon
a few experiments only. But in many cases, even
when this has been done, we should only know one of
the parents of our seedlings—that is to say, in cases
where the pollen for fertilization may possibly have
been conveyed by natural agencies from a different
plant. In such a case we must either ensure self-
pollination by isolating our plants, or we must arti-
ficially provide pollen from a separate known parent.
If under these circumstances a particular group of
plants preserves the characteristic differences which
distinguish it from another group which has also been
grown for a number of generations under the same
conditions, we have at last reasonable grounds upon
which to base the opinion that we are dealing with
two distinct physiological species; even though the

14 INTRODUCTION

visible differences between them may seem very small
to an untrained eye.

Lastly, evidence of a confirmatory nature may be
obtained by observing the results of cross-fertilization
between a pair of closely allied species. Much, too,
may be made out from the failure of such experiments,
since the refusal of two plants to breed together is
generally regarded as clear evidence of their specific
distinctness. But for this reason the method of cross-
breeding is more particularly adapted for the examina-
tion of forms somewhat nearly related to one another
—for example, different members of the same species.

As the result of the methods presently to be
described, the fact has been established that two
entirely distinct sorts of divergencies may appear
among members of a single family. Variations, that
is to say, may be of at least two different kinds. In
the first place we have those slight differences which
invariably distinguish all the members of every family
—individual variations which affect every part and
every character. Such differences are known as
fluctuating, normal, or continuous variations. As an
example we may cite the variations in size or stature
shown by the various members of any purely-bred race.
When a large number of individuals are compared in
respect of a character of this kind, they are found to
fall into a continuous series ranging from a certain
extreme of shortness on the one hand to an extreme
of tallness on the other. Individuals of a medium
height, however, are usually more numerous than either

VARIATION 15

of the extreme forms. Some further account of the
study of continuous variations will be given in
Chapter IV.

A second kind of variation is variously known as
abnormal, definite, and discontinuous variation, and
includes what are known as sports and mutations.
Such variations, as is indicated by the terms applied
to them; involve definite differences usually of con-
siderable amplitude. A good example of a discon-
tinuous variation would be afforded by the appear-
ance of a child having six fingers in a family in which
this abnormality was not previously known to occur.
We shall pursue the discussion of discontinuous varia-
tion and of the methods of perpetuation of the types
which thus arise in Chapters V. and VII.

A short account of the historical development of the
theory of organic evolution is given in Chapter II. As
alternatives to the theory of the special creation of
each individual species, a number of more or less crude
speculations were indulged in by the philosophers of
ancient Greece. Passing to more recent times, three
distinct accounts of the method of origin of specific
differences have been proposed almost within the last
century, and each of these theories still finds a number
of supporters.

1. The view of Lamarck, published first in 1801, and
in an enlarged form in 1809, was briefly as follows:
Noticing that the organs of men and other animals are
increased and strengthened by use, and particularly
by conscious use, Lamarck assumed that this effect

16 INTRODUCTION

could be passed on by inheritance from parent to off-
spring, and so accumulated from generation to genera-
tion. In the case of animals Lamarck conceived the
production of a new specific form to take place in the
following way : Owing to some change of external con-
ditions, the desire to perform some new kind of action
was set up in the parent species, and by the hereditary
effect of the striving occasioned by this desire a modi-
fication of the organs affected into forms better fitted
to carry out the new function was gradually achieved.

Thus Lamarck supposed that snakes were evolved
from a pre-existing type of animal which was of a much
less attenuated shape, and which possessed two pairs
of limbs like any other vertebrates. And he supposed
this evolution to have taken place owing to the con-
stant striving of these animals to pass through narrow
crevices ; the effect of such striving being inherited, and
so accumulated from one generation to another.

In the case of plants, in which conscious effort is
precluded, a similar result was supposed to have been
attained by an hereditary accumulation of the effects
of the environment.

2. The explanation of Darwin, or at least the Neo-
Darwinian form of it, as interpreted by Wallace,
Weismann, and others, and as opposed to and exclud-
ing the view of Lamarck, was as follows : Two separate
factors are primarily concerned : (1) the fact of fluc-
tuating variation—the fact that no two members of
the same family ever resemble one another exactly ;
and (2) the occurrence of a struggle for existence
between organisms owing to the geometric rate of

THEORIES OF THE ORIGIN OF SPECIES 17

increase of living things. From these two facts it
follows that when a change of environment takes
place, certain members of an existing species will be
somewhat better adapted than others to withstand
the new conditions, and the former will tend to survive
to the exclusion of the latter. It is assumed that
during a long series of generations this process will .
cause a steady change in the character of the species in
the direction of better adaptation to the new conditions.
Thus we might suppose that among the ancestors of
the snakes those which happened to possess the longest
and thinnest bodies and the smallest limbs had the
advantage over their fellows that they were able to
crawl through narrower holes, and that for this reason
a greater number of them survived to produce off-
spring. Here we have a better basis for reasoning
than the supporters of Lamarck’s doctrine, because
we actually know that longer parents, in whom this
quality was apparently not the result of taking thought,
do tend to produce on the average longer offspring.
3. The view of the mutationists, already fore-
shadowed by Aristotle, and in recent years especially
associated with the names of Bateson and de Vries,
expresses the conclusion that the evolution of new
species has taken place principally by the help of
variations of the discontinuous kind. By this process
there can arise at a single step new forms which have
already the complete and definite character usually
associated with a species specially adapted to particular
conditions. Of these new forms, those which happen
to be fitted for their surroundings as well as or better

2

18 INTRODUCTION

than their predecessors will survive, whilst those which
are worse will be destroyed by the action of natural
selection.

Thus it would be an appropriate use of this concep-
tion to seek in a mutation the explanation of the final
loss of the much reduced limbs presumably exhibited
by comparatively recent ancestors of the family of
snakes. This final loss is especially difficult to under-
stand on the Darwinian theory. Moreover, changes of
a closely similar nature are not hypothetical, but have
actually been observed to take place. At the same
time it must not be supposed that mutations are con-
fined to the loss of pre-existing organs ; indeed, the
origin of a totally new organ is quite inexplicable on
either of the two preceding theories. The very first
inception of such an organ must, it would seem, of
necessity be sudden.

After giving some account of the earlier theories of
evolution, we shall next proceed to treat of those sub-
jects with which we are more properly concerned—
that is to say, the recent experimental observations
on variation and natural inheritance, together with
their bearing on the theories of evolution. And in the
first place we shall describe some recent studies which
are not strictly experimental, but which nevertheless
deal to some extent with actual facts—namely, the
statistical study of variations, particularly of con-
tinuous variations. This subject has been dignified
by a special name, and is new described as the science
of biometry.

MENDEL’S METHOD 19

Of even greater interest, however, are the more
strictly experimental researches which have been pub-
lished within the last five or six years. In the first
place, we have the observations of de Vries, who has
introduced anew method of study—that of cultivating
great numbers of seedling plants with the object of
discovering definite new forms or mutations among
their number. Lastly, and in its results much
the most important of all, we have the method of
Mendel, published half a century ago, but only re-
‘cently brought into prominence owing to its redis-
covery and confirmation by three independent workers
—Correns, Tschermak, and de Vries. This method
consists in the cross breeding of strains of plants or
animals which differ in definite characters, and in the
statistical examination of the proportions in which
these characters appear among the offspring obtained
from the crosses.

Further experiments on the lines which Mendel in-
dicated bid fair to revolutionize within a few years the
arts of the breeders of plants and animals. This is
due to the fact that such experiments are leading to
the introduction into these pursuits of a degree of
scientific exactness which was previously altogether
unforeseen. The change in our ideas regarding the
method of hereditary transmission of characters, which
has resulted from these experiments, has been aptly
compared with the change brought about in men’s
understanding of the science of chemistry by
Dalton’s conception of the atom. For the rest the
new experiments tend on the whole to confirm the

2—2

20 INTRODUCTION

experience of practical breeders; only the elucida-
tion of one simple rule of inheritance has brought into
order a host of phenomena, which were previously quite
incapable of a coherent explanation.

The experimental results with which it is the pro-
vince of this book to deal are, then, firstly those of
biometry, or the statistical study of variations, and
particularly of continuous variations ; secondly, the
results of direct observations bearing upon the origin
of species by the discontinuous method ; and thirdly,
the results of experimental observations on heredity
by the methods of scientific breeding. By these
methods results of the utmost moment to mankind
have been, and are being, arrived at, quite apart from
their interest as bearing upon the problems of evolu-
tion. From a biologist’s point of view, however, the
latter is, of course, paramount. And so it has been
thought fitting to begin with a brief discussion of the
problems of evolution, and of the various solutions of
them which have been from time to time suggested.

In a later chapter some of the more prominent recent
results of the kindred science of cytology—the micro-
scopic study of the minute constituent parts of organ-
isms—will be briefly described, on account of the very
close connection which recent progress in this subject
bears to the experimental study of the inheritance of
the grosser characters.

Finally, we have essayed a brief account of the
science to which the name of ‘ Eugenics’ has recently
been applied by Sir Francis Galton. In the chapter
dealing with Eugenics an attempt is made to show

i

EUGENICS 2

how far our recently acquired knowledge of variation
and heredity can be applied to mankind, and to ‘ the
study of agencies under social control that may im-
prove or impair the racial qualities of future genera-
tions.’

BIBLIOGRAPHY.

The two following works will be found specially useful on
account of the extensive bibliographies which they contain :

Morean, T. H.: Experimental Zoology, 1907.
THomson, J. A.: Heredity, 1908.

These may be consulted for further references beyond those
given at the end of each of the following chapters.
As an introduction to the whole subject of Heredity :

Doncaster, L.: Heredity in the Light of Recent
Research, 191I.

CHAPTER II

EVOLUTION

EVOLUTION may be defined as progress involving dif-
ferentiation, an ever-growing complication of things
which accompanies almost all the operations of Nature.
The idea of a differentiation of this kind may be en-
forced by a homely and quite imaginary illustration
of such a process. Imagine the proper ingredients of
a plum cake to be very finely minced and intimately
mixed together, so as to form a more or less homo-
geneous material. Then, if by any means the separate
particles of currants, raisins, peel, and so forth, could
be made to segregate out in such a way as to give rise
to the ordinary structure of this pleasant confection,
we should have arrived at the structure of a plum cake
by a process of evolution involving considerable dif-
ferentiation.

The progressive increase in complexity which is
characteristic of so many natural processes is in great
part occasioned by the fact that a single ‘cause’ is
followed as a general rule by more than one ‘ effect.’
This apparently simple circumstance was pointed out

22

EVOLUTION OF MATTER 23

by Herbert Spencer,* who has perhaps done more than
any other to establish and emphasize the general ap-
plicability of the evolution idea. For the law of origin
by evolution is by no means exclusively confined to
the method of coming into existence of the species of
animals and plants. On the contrary, it was equally
well applied by Spencer himself to describe the manner
in which are supposed to have arisen the stars and
other heavenly bodies, the geological strata and geo-
graphical configuration of the earth, and the various
gradations of human society.

The discovery that certain chemical elements exist
which are themselves not immutable has been made
since Spencer’s time. Quite recently ‘ the phenomena
of radio-activity have forced us to believe that radium
is passing continuously into helium,’t and something
more than a suspicion has been aroused that radium
is itself derived from uranium. Thus the dreams of
the alchemists are shown to have been not wholly
without foundation, for the probability is strong

* Spencer gives the following illustration: Regarding the
striking together of two bodies as a ‘cause,’ he points to the
following possible ‘effects’: A sound; other vibrations or
movements in the surrounding air ; a disarrangement of the
particles of the two bodies in the neighbourhood of the point
of collision ; the production of heat, and possibly of a spark—
2.€., of light.

Two words ‘in this sentence are placed between inverted
commas, to indicate that they are used in a strictly popular
sense. The use of the words ‘cause’ and ‘effect,’ though
seldom strictly scientific, is often convenient, and if used with
caution, there is no reason why they should lead to misunder-
standing. See Whetham, ‘The Recent Development of
Physical Science,’ chapter i.

t Whetham.

24 EVOLUTION

that under suitable conditions other matter may be
observed to behave in the same way as radium. More
than this, Professor Sir J. J. Thomson has been able to
describe the atoms of the elements as different aggrega-
tions of a single kind of corpuscles, and to show that
a progressive change in the number of corpuscles
making up the atom is accompanied by a progressive
alteration in the properties of the atom itself, so that
it has now become possible to establish a theory of
the evolution of the chemical elements themselves.
Passing from the almost immeasurably small to the
almost immeasurably great, we may briefly consider
the probable mode of origin of the solar system from
an extremely diffuse cloud of material substance, ac-
cording to the famous nebular hypothesis of Laplace.
By a long-continued process of contraction under the
influence of gravity the nebular substance came to be
of varying density, and acquired a rotary movement
in one plane. As the mass continued to contract
owing to the mutual attraction of its particles, the
velocity of rotation increased, until at last the increas-
ingly rapid motion of the outermost ring of the now
lens-shaped nebula gave rise to a centrifugal force great
enough to counteract the tendency to contraction, and
in the further condensation of the mass this ring was
left behind. The ring next broke down at one point,
and contracting on itself gave rise to a single spheroidal
body which acquired a movement of rotation in the
same direction as that of the parent nebula. This
body was the outermost planet Neptune, and the rest
of the planets were produced in a similar manner,

EVOLUTION IN GEOLOGY 25

until at last a central mass was left, and this became
the sun. Satellites were thrown off from several of
the planets just in the same way as the planets them-
selves arose from the original nebula, and Saturn’s
rings are pointed to as showing this process even now
in course of operation.

Such a description as this may appear fanciful at
first sight, but it was worked out quantitatively as
well as qualitatively by its author, and was shown to
explain in detail a multitude of phenomena. Spencer
points out that when we have, worked out by one of
the first of mathematicians, a definite theory of plane-
tary evolution based on established mechanical laws:
and one which accounts in a satisfactory way for all
the known phenomena, the conclusion that the solar
system really did arise by a process of evolution is, to
say the least, difficult to avoid.

The establishment and propagation of the idea that
the present condition of the earth’s surface arose
through a course of gradual evolution, by the agency
of such processes only as are known to be in operation
at the present day, is the great contribution of Sir
Charles Lyell to the science of geology. We may
briefly trace the evolution of the idea itself, beginning
with the speculations of Werner, who, from observa-
tions of the geological formations of a limited tract of
country, came to the conclusion that the successive
strata were precipitated one by one from an universal
ocean. Here we see the first germ of the idea of
evolution embodied in the notion that the stratified

26 EVOLUTION

rocks came into existence gradually and through the
operation of a supposed natural cause.

A great advance upon Werner’s theory was made
by Hutton, who, observing the formation of strata
at the present day from the sediment washed down
by rivers, concluded that the ancient strata were
deposited in the same manner. Since, by the long
continuation of this process the continents must
gradually become reduced to the level of the sea,
Hutton supposed that at long intervals of time the
action of subterranean heat came into play, and
fresh continents were upheaved, a process accom-
panied by the outpouring of the igneous rocks,
the true origin of which he had duly recognised.
In this theory a hypothetical cause still survives, since
we have no actual experience of vast upheavals of the
kind which Hutton supposed to have taken place.
Lyell showed that such slight changes of level as are
known to be in progress at the present day, especially
in association with the phenomena of earthquakes,
might, if continued over a long series of ages, give
rise to the necessary amounts of elevation. Lyell also
pointed out a number of subsidiary causes of dis-
integration and deposition of strata of the. kind which
can still be seen in operation at different parts of the
earth’s surface. At the present time it is sometimes
thought that Lyell went a little too far in his cham-
pionship of the cause of uniformity. Lyell supposed
that the agencies which may now be everywhere ob-
served, in operation, such as rain and rivers, the sea,
volcanoes and earthquakes, were sufficient to account

AGE OF THE EARTH 27

for all the phenomena which the crust of the earth
exhibits. It is now more generally supposed that in
very early times forces similar in kind to those in
action at the present day may have exhibited con-
siderably greater violence.

To produce the present condition of the surface of
the earth by the action, gradually accumulated, of
such processes of denudation and upheaval as are now
going on around us, vast ‘periods of time are clearly
necessary. The early evolutionists, having once got
rid of the idea that the date given by Bishop Usher
as that of the creation of the world is a necessary and
integral part of religion, immediately allowed their
imaginations to run riot with regard to the amount
of time at their disposal. Since this question of the
extent of geological time has an important bearing on
the problem of organic as well as upon that of inorganic
evolution, it will be well to pay some attention to
more recent views upon the subject.

Some years ago the generous ideas of biologists as
well as of geologists were to a great extent shattered
by the calculations of Lord Kelvin. These were based
upon three separate sets of data, which we may enume-
rate without entering into a lengthy explanation of
the calculations involved. The evidence made use of
consisted of (1) the rate of the earth’s rotation, as
affected by tidal retardation ; (2) the rate of secular
cooling of the earth, as deduced from the rate at which
the temperature of the earth’s crust rises on passing
inward from the surface ; and (3) the rate of cooling of
the sun by radiation. The three calculations were

28 EVOLUTION

found to show a very fair measure of agreement, and
they led to the conclusion that considerably less than
a hundred million years has elapsed since the first for-
mation of seas upon this planet, an event which must
have preceded the possibility of aqueous geological
action and the existence of living organisms

Allowing for the circumstance that geological pro-
cesses may have gone forward with considerably
greater rapidity during the earlier periods of the
earth’s history than is the case at the present day, the
time thus allowed by the physicist is generally regarded
by geologists as too little. Reckoning from the known
rate of denudation, which is, of course, the same as
the rate at which the same material is deposited
beneath the sea, Geikie, who admitted, however, that
such data are only of a very rough description,
concluded that the space of a hundred million years
would afford sufficient time for the laying down
of the known aqueous strata. But there can be
little doubt that the lower metamorphosed rocks
represent a much longer period of time than
the primary, secondary, and tertiary epochs added
together; consequently, the respective estimates of
Lord Kelvin and the geologists appear to be contra-
dictory. The recent discovery of the enormous quan-
tities of energy stored up in radio-active substances
introduces a serious modification into the mathematical
argument from astronomical data, and Sir George
Darwin ‘sees no reason for doubting the possibility
of augmenting the estimates of solar heat, as derived
from the theory of gravitation, by some such factor

ORGANIC EVOLUTION 29

as ten or twenty,’ on the supposition that a consider-
able proportion of the sun’s substance was made up of
tadio-active material.

The above remarks may serve to illustrate the im-
portance of the theory of evolution as applied to the
two sciences of astronomy and geology. We pass next
to a brief historical consideration of the development
of the evolution theory as a method of describing the
origin of the species of animals and plants.

The views of the ancient Greeks cannot be said to
have much more than a purely speculative interest.
Some rudiments of the idea of evolution have been
attributed to Empedocles as well as to several other
early writers, and in the writings of Aristotle, for
whom the too great faith of his successors for many
ages has been followed by a somewhat unmerited
degree of contempt in modern times, we find that the
evolution idea had reached quite a respectable degree
of development.

In the Middle Ages the adoption of the Jewish cos-
mogony by the Christian Churches effectually annihi-
lated all useful thought upon the subject of species,
since the hypothesis of separate creation affords no
scope for further speculation or experiment, and it is
not until the end of the seventeenth century that we
find thoughtful men beginning to struggle against the
ecclesiastical bondage. Thus Erasmus Darwin de-
rived the idea of -generation rather than creation of
the world from David Hume, and himself waxes
enthusiastic over the thought :

‘That is, it (the world) might have been gradually

30 EVOLUTION

produced from very small beginnings, increasing by
the activity of its inherent principles, rather than by
a sudden evolution of the whole by the almighty fire.
What a magnificent idea of the infinite power of THE
GREAT ARCHITECT! The Cause of causes. Parent of
parents. Ens entium,

De Maillet, writing in 1735, showed a definite idea
of the production of existing species by the modifica-
. tion of their predecessors. At the beginning of the
nineteenth century similar speculations were published
by Goethe and by Treviranus, and the latter was the
first to apply the term ‘ biology ’ to the science of the
phenomena of life. Lamarck about the same time pro-
vided a definite theory as to the method by which the
modification of species takes place.

Before discussing Lamarck’s hypothesis and the
alternative theories more recently proposed, it will be
well to pass in review the evidence upon which is
based our belief that the species of animals and plants
have arisen through the modification of pre-existing
species, and to show that the greater part of this evi-
dence is quite independent of any views which we may
adopt as to the actual method by which a particular
species came into existence. And in the first place we
may point out the entire absence of any evidence, direct
or indirect, in favour of the alternative supposition of
a special creation of each separate species.

The evidence for evolution falls naturally into a
number of fairly well defined sections; these have
been so admirably summarized by Huxley in his
essay on ‘Evolution in Biology,’ that we cannot well

EVIDENCES OF ORGANIC EVOLUTION 31

do better than recapitulate, with only slight modifi-
cations, the arguments there given :

_I. THE GRADATION OF ORGANISMS.—Both in the
animal and vegetable kingdoms we may trace, in spite
of certain gaps, a long series of gradations in com-
plexity of structure, so that between the simplest and
the most complicated of living things a great number
of intermediate stages are to be found. When we
pass to the lower end of the scale in either case, we
come upon a group of creatures of comparatively
simple organization. Among them we find members
with regard to which we cannot definitely say that
they are either animals or plants. Moreover, these
unicellular organisms resemble in many ways the
egg-cell from which every individual among the higher
animals and plants originates.

It is true that we now know it to be quite impossible
to dispose all the members of the animal kingdom in
a single linear series, such as was formerly suggested,
passing in orderly sequence from the amceba up to
man. ‘ Instead of regarding living things as capable
of arrangement in one series, like the steps of a ladder,
the results of modern investigation compel us to dis-
pose them as if they were the twigs and branches of a
tree. The ends of the twigs represent individuals,
the smallest groups of twigs species, larger groups
genera, until we arrive at the source of all these
ramifications of the main branch, which is represented
by a common plan of structure.’

2. EMBRyYOLoGY.—AII the members of a particular
group of animals or plants as a rule resemble one

32 EVOLUTION

another more closely in the early stages of their indi-
vidual development than they do in the adult condi-
tion, and in the earliest stages of all they are often
indistinguishable. These facts are explained if we
suppose that such individuals have a common origin,
that they are descended from a common ancestor, and
that traces of their pedigree are still to be observed in
the developmental stages through which each one
passes. We do not find a complete parallelism be-
tween the development of the individual and the
history of the race, nor should we expect to do so,
since embryonic as well as adult stages may be modified
in the course of evolution ; what we should expect is
a more or less vague historical sketch, and this is what
is usually found remaining.

“It is not true, for example, that a fish is a reptile
arrested in its development, or that a reptile was ever
a fish ; but it is true that the reptile embryo, at one
stage of its development, is an organism which, if it
had an independent existence, must be classified
among fishes ; and all the organs of the reptile pass, in
the course of their development, through conditions
which are closely analogous to those which are per-
manent in some fishes.’

3. MorPHOLoGy.—On comparing together the dif-
ferent members of one of the great groups or classes
of animals or plants, we find the same fundamental
plan of organization running through all of them.
Series of corresponding organs are often to be made
out which are built upon the same general scheme,
although their functions may be quite dissimilar ; so

EVIDENCES OF ORGANIC EVOLUTION 33

that, for instance, in the hand of a man, the paw of a
dog, the wing of a bat, and the paddle of a whale,
almost identically the same series of bones can be
traced. An obvious explanation is to be found in the
supposition that these parts have arisen by the
divergent modification of parts which were originally
identical.

4. GEOGRAPHICAL DISTRIBUTION. — Observation
shows that groups of closely allied creatures are often
found living in neighbouring districts, and that when
such a barrier as an ocean or a range of lofty moun-
tains is passed an entirely new fauna and flora are
usually to be met with. These facts may be explained
by the hypothesis that allied groups of species origi-
nated by a process of descent in the same countries
which they now inhabit, and they can be explained
by no other known hypothesis. The alternative sup-
position that each species was specially created and
placed in the locality in which it was best adapted
to dwell is singularly in disagreement with the well-
known facts that animals and plants transported into
entirely new regions often thrive better than in their
original homes. The examples of rabbits in Australia
and of cardoons and thistles on the Pampas of La
Plata are familiar to all from the writings of Darwin.

5. THE GEOLOGICAL SUCCESSION OF ORGANISMS.—
The general facts regarding the distribution of allied
species of animals and plants in time point in pre-
cisely the same direction as those relating to their
distribution in space. In a few cases, notably in that
of the extinct horse of North America, a long chain of

3

34 EVOLUTION

possibly ancestral types has been found leading back
to a remote and very different progenitor. This sup-
posed ancestor of the horse was a creature little larger
than a moderate-sized dog. It had four separate toes
to each fore-limb, and three to each hind-limb, and its
teeth were much simpler and less specialized than
those of existing horses. We say possibly ancestral,
advisedly. Geological evidence is naturally insufficient
to establish the actual relationship of the series of
types which has been described, and Professor Sedg-
wick has recently criticized the view that this series
of forms constitutes a demonstrative historical proof
of the doctrine of organic evolution. It is, therefore,
preferable to claim this group of fossils as an illustra-
tion of the possible geological ancestry of an existing
species rather than as affording concrete proof of an
actual pedigree. Even with this reservation, we claim
that such a series constitutes a valuable collateral proof
in favour of evolution. The general distribution of
organisms throughout the geological strata agrees
moreover, in a remarkable way with what is to be
expected on the evolution theory.

6. CHANGES UNDER DomESTICATION.—Among do-
mesticated animals and plants we know of numerous
cases in which the actual origin of new forms has been
observed. These have often differed from their pre-
decessors by amounts quite comparable with the dif-
ferences by which natural species or even genera are
separated. A notable example of this process is afforded
by the numerous breeds of pigeons known to have arisen
under domestication from a single wild species. We

EVIDENCES OF ORGANIC EVOLUTION 35

have no reason whatever for supposing that domesti-
cated species are more mutable than wild species, and
there is consequently every reason to believe that
changes of a similar character take place in Nature.
Mr. Alfred Russell Wallace has quoted with approval
Sir W. Thistelton Dyer’s criticism that if there is an
equal chance of the occurrence or origin of new forms
in nature and under cultivation, then their appearance
—t.e., their survival until a stage at which they can
be readily recognized as distinct from the original type
—should be more frequent in nature than in cultiva-
tion, because the former has a larger population to
work with. The reply to this argument is obvious.
In the first place, the much greater facilities for obser-
vation under cultivation may fairly be set against the
greater numbers stated to exist in nature ; but, in the
second place, Mr. Wallace may well be challenged to
cite a natural species of which a larger number of
individuals has passed under man’s observation than
is the case with cultivated wheat, for example. Buta
third line of argument is much more conclusive than
either of these. The modifications which occur under
cultivation are in most cases decidedly weakly as com-
pared with the original forms, as every gardener knows
to his cost. They are only enabled to survive to a
recognizable stage, because cultivation consists in the
removal of competition ; all are given an equal chance.
This is not soin nature. There, competition (according
to the exponents of the Wallacian doctrine) is so intense
that even very slight variations may determine success
or failure. According to the doctrine of natural selec-
3-2

36 EVOLUTION

tion, then, decidedly weakly specimens, if they occurred
in nature, would have practically ~o chance of sur-
vival, and would consequently never be seen. This
attempt to undermine one of the strongest evidences
of organic evolution, therefore, falls to the ground.

4. THE OBSERVED Facts or Mutation.—As a
matter of fact, novel types are seen in nature not in-
frequently, and are specially common in some groups
of plants, as Mr. C. T. Druery has shown for the case
of the British ferns; and isolated specimens of par-
ticular wild species belonging to other families are fre-
quently found, which, if they had occurred as con-
stant features of a considerable group of individuals,
would afford a basis for the description of a new species.
The study of mutation will, however, require a special
chapter of its own.

We see, therefore, that the evidence in favour of the
existing species of animals and plants, having arisen
by a process of evolution, is of a most ample and con-
vincing kind. The theory of organic evolution is,
however, incomplete until we have arrived at a true
account of the method or methods by which new
species arise from old ones. The earliest definite
explanation, as already stated, was that given by
Lamarck, and we may next proceed to consider the
Lamarckian theory of the origin of species.

Earlier writers had already supposed that species
‘became modified through the action of the external
conditions to which they were exposed. Lamarck laid
special stress upon the observed facts that the organs
of individuals become increased and developed through

THE THEORY OF LAMARCK 37

use, and that disuse is followed by a dwindling and
loss of the power of action. By the inherited effects
of use and disuse, and of modifications caused by ex-
ternal conditions, Lamarck supposed all evolution of
species to have come about.

Reference has already been made to Lamarck’s de-
scription of the method of origin of the characteristic
form of snakes, owing to the endeavours of the snakes’
ancestors to creep through narrow passages. Lamarck
was quite consistent inasmuch as he explained the
different types which have arisen among domesticated
species by the same theory as he applied to the origin
of species in a state of nature. Thus he supposed the
differences between race-horses and heavy cart-horses
to be the direct result of the different kinds of enforced
exercise to which the ancestors of these races were
respectively subjected. Similarly, all the different
breeds of dogs were supposed to have arisen owing to
the different habits which the various successors of the
first domesticated dogs acquired, small changes being
accumulated by inheritance in each successive genera-
tion.

Turning now to species in a state of nature, the case
of the giraffe is one of those most often quoted.
Lamarck supposed a comparatively short - necked
ancestor of the giraffes to have taken up the habit of
browsing upon the leaves of trees, owing to the diffi-
culty of obtaining other food in an arid region. In
order to obtain their new food the animals were obliged
to be continually stretching upward, and the effort to
elongate their necks was attended with some small

38 EVOLUTION

measure of success in each individual. This increase,
being accumulated by inheritance in every succeeding
generation, ultimately led to the great stature exhibited
by the giraffes of the present day.

The stilt-like legs of many wading birds were ascribed
by Lamarck to the result of the continued attempts of
ancestors which had shorter extremities to obtain their
food in shallow water without wetting their feathers.
The long-continued endeavours of these birds to stretch
and elongate their legs had the same effect as the
similar efforts made by the ancestors of the giraffes.
It has been suggested, however, by a critic of Lamarck’s
position that such birds would be likely to eschew fish
dinners long before any notable increase in the length
of their legs was arrived at.

If some of the above cases appear a little ludicrous,
there are other instances in which the Lamarckian
hypothesis seems to afford a perfectly adequate and
natural explanation. Such a case is afforded by the
family of the flat fishes, including such well-known
species as the sole and plaice. In the adult condition
these fishes lie flat on one side; and during their
development from the young condition, that eye which,
if it remained in its original position, would look
directly downwards travels round the head until it
comes to lie quite upon the upper surface. As Darwin
pointed out, agreeing in this with Mivart, a sudden
spontaneous transformation in the position of the eye
is hardly conceivable, and it is equally impossible to
explain the origin of this remarkable feature by the
action of natural selection, because a slight change

THE THEORY OF LAMARCK 39

in the position of the eye could be of no advantage
so long as this organ remained upon the under surface.
The very young fish, whilst still symmetrical, are
known sometimes to fall upon one side, and when in
this position to twist the lower eye forcibly upwards.
Darwin himself therefore supposed that the origin of
the adult structure is to be attributed to the ened
effect of efforts of this kind.

The interest of the last case lies in the fact that it
relates to a structure, the origin of which does not
appear explicable on the theory of natural selection ;
its bearing will therefore be better understood when
we come to discuss that theory in the next chapter.

The inherited effects of voluntary striving can clearly
have no application to the case of plants. Lamarck
therefore supposed that evolution in the vegetable
kingdom had taken place entirely through the action of
external agencies upon plants. The soil, for example;
in which a plant grows has a direct influence upon its
form. Altitude, moisture, heat, and light are other
important factors, and the effect of their influence upon
the plant was supposed by Lamarck to be inherited.
The shape of irregular flowers was regarded as having
been directly caused by the strains and pressures
occasioned by bees and other insects whilst making
their visits in search of honey or pollen.

Lamarck’s theory turns entirely upon the question
whether acquired characters are inherited, and if so,;
to what extent, sime, if such inheritance is shown to
be extremely slight, the cause, though a true one,
may be insufficient to explain the effects attributed

40 EVOLUTION

to it. Now, theories of heredity apart, and leaving
aside the results of minute observations which had
not been made in Lamarck’s time, the natural supposi-
tion undoubtedly is that acquired characters are
inherited just as much as any others. Given the ob-
served fact that offspring resemble their parents more
closely than they do other members of the same species,
it is natural to believe that the child will take after
the forms exhibited by its parents at the time of its
conception rather than after those shown by them at
any previous period of their lives. This seems to be
the natural view in the absence of any other evidence
for or against, and so accurate a thinker as Herbert
Spencer, writing before the publication of the * Origin
of Species,’ regarded the term inheritance as neces-
sarily implying inheritance of this particular kind.
For this reason it has sometimes been thought that
Darwin scarcely accorded to Lamarck the appreciation
which he deserved ; and yet Darwin himself fell back
upon the Lamarckian explanation on the few occasions
when natural selection seemed to have failed him.
When, however, we come to know more of the actual
facts of sexual generation, we find that it is very
difficult, if not impossible, to imagine any kind of
mechanism by which this supposed transmission of
acquired modifications can take place. We shall defer
the further discussion of this subject, as well as the
question of the existence of direct and other evidence
of use inheritance, until the latter half of the next
chapter, where we shall refer briefly to the contro-

THE THEORY OF LAMARCK 41

versy upon these subjects which followed the estab-
lishment of the principle of natural selection.

BIBLIOGRAPHY.

Wuetuam, W. C. D.: The Recent Development of Physical
Science, 1904.

SPENCER, HERBERT: Egsays—Scientific, Political, and Specu-
lative, 1868.

Hux tey, T. H. : Collected Essays, vol. ii., 1899.

Romanes, G. J. : Darwin and After Darwin, vol. i., 1897.

Lamarck, J. B.: Philosophie Zoologique, 1809.

LYELL, SIR CHARLES, Principles of Geology, eleventh edition,
1872.

Buttar, S.: Evolution Old and New, 1879.

CHAPTER III

THE THEORY OF NATURAL SELECTION

In 1813 a communication was read before the Royal
Society by Dr. W. C. Wells upon the differentiation
which exists between certain races of mankind. In
Dr. Wells’s paper this differentiation was explained
from the facts that, since no two individuals are alike,
some would be better fitted than others to resist the
diseases proper to a particular country, and would
consequently tend to survive, whilst their less fortunate
neighbours would perish in greater numbers. Wells
supposed the dark races of mankind to be better
adapted to warm climates than white races are, and
he thus applied to the particular case of the human
species the true Darwinian principle of a gradual
evolution through the survival of the fittest.

A similar view was applied to the origin of species
in general by Patrick Matthews in a book on naval
timber and arboriculture published in 1831.

Both these works were unknown to Darwin at the
time of the first publication of the ‘ Origin of Species,’
and it is quite unnecessary to point out that their
existence does not in the least prejudice the value or
originality of that great work. Their interest at the

42

HISTORICAL 43

present time is merely historical, as showing the
‘direction in which thought was tending in the earlier
half of the nineteenth century.

Before the ‘Origin of Species’ was published,
A. R. Wallace communicated to Darwin a paper in
which the bearing of the same idea was worked out at
some length, and this paper was read, together with
an abstract of Darwin’s own views, at a meeting of
the Linnean Society in July, 1858.

With this notice of other claimants to the idea of
natural selection we may proceed to give an account
of the theory as it is developed in the earlier chapters
of the ‘ Origin of Species.’

We must first glance at Darwin’s method of using
the term variation. Darwin applied this term to every
kind of difference which is found to occur between
parents and their offspring, or between members of
the same family, no matter whether these differences_
were great or small. It has since been shown that a
number of quite distinct phenomena were in this way
regarded from a single standpoint, without a proper
discrimination being made between them. But the
differences between continuous and discontinuous
variation, quantitative and qualitative variation, and
the rest, were not pointed out until long subsequent to
1859. Thus, beyond recognising a distinction between
sports and individual differences, and attaching greater
weight to the latter kind of changes, as being those
which chiefly led to the origin of new species, Darwin
made no further analysis of the facts of variation, but
accepted all sorts of differences between individuals as

44 THE THEORY OF NATURAL SELECTION

affording the material upon which natural selection
might be expected to operate.

The idea that a selective influence exists in Nature
arose from a study of the remarkable effects produced
in the case of domestic animals and plants by the action
of artificial selection. Darwin seems, however, to have
been a little credulous in accepting the statements of
certain breeders with regard to their power of producing
any desired new type to order. Now that scientific
men are themselves beginning to make experiments
in breeding, with the check of exact records to act
as a drag upon the exuberance of the imagination,
they are becoming somewhat sceptical as to the mystic
and almost miraculous powers attributed to the old-
fashioned breeders, though, indeed; Mr. Luther Bur-
bank would seem to be a survival from the period
we speak of, if the statements of his recent enthusiastic
biographer are to be credited.* Less gifted but more
methodical observers find that they have no creative
powers worth speaking of, and that all they can do is
to keep a sharp look-out for the novelties which Nature
may send them.

~~ Selection, whether natural or artificial; can indeed

of itself have no power in the direction of creating

_anything new ; its influence is ucti reserva-
tive, but nothing more than this. The breeder keeps
the new forms which take his fancy, and destroys the
rest _; that-is the whole story.

* Harwood, ‘ New Creations in Plant Life.’ Mr. Burbank

certainly seems to have a really wonderful instinct for the dis-
covery of curious and useful novelties.

ARTIFICIAL SELECTION 45

Yet a remarkable number of new kinds of creatures
are known to have arisen in this way, and their diver-
sity is no less astonishing, as a visit to any great show
of domestic plants or animals will at once demonstrate.
Here may be seen varieties of pigeons, for example,
like the carrier, pouter, fantail, and tumbler, which,
if they were found existing in a wild condition, would
be placed in separate genera by any ornithologist.
The domestic races of fowls, dogs, horses, sheep, and
cattle show scarcely less divergence, and modifications
no less remarkable have been perpetuated in the case
of many cultivated species of plants. Whilst these
types have survived, being deliberately preserved on
account of their use or beauty or curious appearance, a
still greater number have doubtless been exterminated,
either because they did not attract the breeder’s
favourable attention, or on account of their having
passed out of fashion.

Darwin sought in Nature a substitute for the baleful
judgment of the breeder, and found it in an extension
of the Malthusian doctrine to organic beings in general.
The idea which is identified with this expression
did not, however, originate with Malthus, nor does
that author claim it as his own, as the following extract
from the first chapter of the ‘Essay on Population ’
will show :

‘It is observed by Dr. Franklin that there is no bound
to the prolific nature of animals and plants but what
is made by their crowding and interfering with each
other’s means of subsistence. Were the face of the
earth, he says, vacant of other plants, it might be

46 THE THEORY OF NATURAL SELECTION

gradually sowed and overspread with one kind only,
as, for instance, with fennel ; and were it empty of other
inhabitants, it might in a few ages be replenished from
one nation only, as, for instance, with Englishmen.’

Malthus’ ‘ Essay ’ was first published in 1798, and
was subsequently much enlarged. Its author proved
incontrovertibly, by a survey of facts gathered from
almost all the countries of the world, that human popu-
lation tends to increase in a geometrical ratio, and
that, consequently, after a time, the less gifted classes
of any community are bound to suffer from a stress of
poverty, only partly relieved by a high infant mortality,
periodic famines, and similar factors, or in less civilized
countries by infanticide and other artificial checks.

Among animals and plants in a state of nature the
rate of increase is often very much greater than in the
case of the human family, and even where it is not so,
unchecked breeding would in a comparatively few years
lead to the overpeopling of the earth with the de-
scendants of a single pair. As an example of the rate
of increase shown by a wild species, we may consider
the case of the elephant, instanced by Darwin himself,
since this is regarded as being one of the slowest
breeders among all known animals. Darwin assumes
that the elephant begins breeding at thirty years, and
continues to do so until it reaches the age of ninety,
bringing forth six young in the interval, and surviving
to the age of a hundred. Then, if there were no
casualties, he calculates that after from 740 to 750
years there would be nearly nineteen million elephants
alive descended from the first pair.

RATE OF INCREASE 47

Let us also consider the case of a minute rapidly
breeding animal of a typical kind. My friend Mr.
R. C. Punnett has recently been engaged upon an
experiment which involved the breeding of rotifers,
a kirid of animal barely visible to the naked eye,
They were bred for sixty-seven generations, and
each individual produced on the average thirty eggs.
The whole experiment occupied less than a year, yet
Mr. Punnett calculated that if he had been able to
rear all the animals which, at this rate of breeding, for
this number of generations, were theoretically obtain-
able, he would have become the possessor of a solid
sphere of organic material with a radius greater than
the probable limits of the known universe.

This geometrical rate of increase is common in a
greater or less degree to all living organisms. -Since
the space and food-supply available for the support of
any species has no corresponding tendency to in-
crease, it follows that a large proportion of the
individuals born must perish before they reach the
adult state, or at least without producing offspring.
Darwin’s contention is that there will be a strong
tendency for those individuals which show slight
modifications in the direction of a better adaptation
to their environment to survive at the expense of
those of their brethren which do not exhibit similar
modifications. This is the principle called natural
selection by Darwin, and by Herbert Spencer the
survival of the fittest. Let us quote Darwin’s own
summary of the process :

“Tf under changing conditions of life organic beings

48 THE THEORY OF NATURAL SELECTION

present individual differences in almost every part of
their structure, and this cannot be disputed ; if there
be, owing to their geometrical rate of increase, a severe
struggle for life at some age, season, or year, and this
certainly cannot be disputed ; then, considering the
infinite complexity of the relations of all organic
beings to each other and to their conditions of life,
causing an infinite diversity in structure, constitu-
tion, and habits, to be advantageous to them, it would
be a most extraordinary fact if no variations had ever
occurred useful to each being’s own welfare, in the same
manner as so many variations have occurred useful to
man. But if variations useful to any organic being ever
do occur, assuredly individuals thus characterized will
have the best chance of being preserved in the struggle
for life ; and from the strong principle of inheritance,
these will tend to produce offspring similarly charac-
terized. This principle of preservation, or the survival
of the fittest, I have called Natural Selection. It leads
to the improvement of each creature in relation to its
organic and inorganic conditions of life, and, conse-
quently, in most cases, to what must be regarded as an
advance in organisation. Nevertheless, low and
simple forms will long endure, if well fitted for their
simple conditions of life.’*

We have here a very definite and concise statement
of the way in which Darwin believed the principle of
natural selection to take effect in the production of new
kinds of organisms. It will be our business in this and
in succeeding chapters to show how far the modern

* ‘Origin of Species,’ sixth edition, p. 96.

NATURAL SELECTION 49

study of the nature of individual differences and of
other kinds of variations, as well as of the manner of
operation of ‘ the strong principle of inheritance,’ has
confirmed this view as to the method of origin of
species, or has led to the introduction of modifications.

Let it be remembered that this suggestion of a
natural means of modification had, within a few years,
the effect of convincing practically the whole thinking
world of the truth of the theory of organic evolution—
an effect which all the other arguments recited in the
last chapter were quite unable to produce, so strong
was the then existing prejudice in favour of the doc-
trine of special creation.

The truth of the general principle of the survival of
the fittest is quite untouched by recent criticism ;
but a great deal of argument has been expended over
the questions: (x) how much fitness is sufficient to
lead to survival, and (2) whether very small advan-
tages in the way of fitness, even if they lead to the sur-
vival of the individuals which exhibit them, will be
followed to an indefinite extent in succeeding genera-
tions by further improvements in the same direction.
We shall find that a good deal of evidence has accu-
mulated tending to show that the second of these
questions must be answered in the negative, although
the point is not yet settled to the satisfaction of
everyone. The remainder of the present chapter will
be occupied in discussing some of the arguments which
bear upon this question.

An important factor which must probably be added
to the factors of variation, selection, and heredity, in

4

30 THE THEORY OF NATURAL SELECTION

order to complete the Darwinian account of the origin
of species, is that of segregation or isolation. If the
selection of minute fortuitous variations in different
directions are capable of breaking up a species into a
number of new species, it seems clear that this can
only happen when the members of the different branches
are prevented from interbreeding ; since otherwise the
effect of selection would be counteracted by the
mingling or blending of characters which may be sup-
posed to result from free intercrossing. Further, many
zoologists, and more especially the systematists among
them, believe that isolation in itself has a most im-
portant function in modifying species. This isolation
may be either geographical, as when distance or some
physical barrier separates different members of the
same species; or it may be physiological, as when
structural or temperamental differences, or mutual
distaste, prevent the mating of certain individuals.

The researches of Gulick upon the species of snails
found in the Haiwaian Islands showed that the differ-
ences between the species correspond in amount with
their degree of separation in space—t.e., with their
isolation. The characters which separate these species
could not be shown to have any relation to differences
in the environment, since adjoining valleys, which
differed considerably in vegetation and rainfall, pos-
sessed closely related species ; whilst in valleys further
apart, but more similar in the environment offered
to the snails, the characteristic species showed much
greater differences. The case of the Haiwaian snails,
therefore, appears to afford an exception to the prin-
ciple next to be described.

NATURAL SELECTION 51

The fact that organic beings on the whole are, as a
general rule, very closely fitted for the conditions in
which they have to pass their lives is clearly shown
by the study of adaptations. This is a subject which
those followers of Darwin who believe in the all-
sufficiency of natural selection have brought into con-
siderable prominence. For a full account of many
supposed beautiful adaptations, from the point of view
of the most prominent member of the school in ques-
tion, reference may be made to Weismann’s recently
published book, ‘ The Evolution Theory.’

On the theory of natural selection in its extreme
form, all the parts of an animal or plant—or, at any rate,
all the points in which one species differs from another
nearly related species—are supposed to have arisen on
account of their usefulness to the creatures possessing
them. Every detail of structure is thus regarded as
being more or less closely adapted to the circum-
stances which attend the life of the animal or plant in
question. This adaptation is never, indeed, regarded
as perfect, because natural selection is always in
progress, and its work is never absolutely done;
but the point is that the features of every part are
aimed at some useful purpose; or, if they are not,
then they have been useful in former times and under
different circumstances, and are now undergoing a
process of gradual] removal, because the individuals in
which the useless structure is least developed will now
have the best chance of surviving. That the form and
structure of an animal or plant is in general closely
fitted to its environment is of course true ; otherwise

4—2

52 THE THEORY OF NATURAL SELECTION

the creature would very soon cease to cumber the earth.
But the student of adaptation goes into details, and
endeavours to find a use for every minute point of
structure, on the assumption, which we shall presently
see to be open to criticism, that but for their useful-
ness these details would not exist. We may proceed
to glance at one or two examples of the kind of thing
which is meant when it is said that an animal or plant
exhibits very marked adaptative features.

The order Cetacea belongs to the class of mammals
of which the more typical members are land animals’
possessing four legs, and having their bodies covered
with hair. The true whalebone whales, a sub-order
which includes the Greenland whale, are in many ways
the most specialized members of the group.

The Greenland whale* has a spindle-shaped body
like that of a fish, and its fore limbs are modified into
flippers resembling the pectoral fins of fishes. The
hind legs are only represented by a few rudimentary
bones, which are completely hidden within the body
wall, and the function of propulsion, which is performed
by the hind legs in such less completely aquatic animals
as seals, is here taken over by a great tail-fin which
resembles that of a fish, except that it is placed hori-
zontally. Hair is absent, but under the skin a thick
layer of blubber is developed, which prevents a too
rapid loss of heat, and at the same time adjusts the
specific gravity of the body to that of the surrounding
water. External ears are entirely wanting, and the

* Weismann, ‘The Evolution Theory,’ English edition,
41. 313.

ADAPTATION 53

waves of sound are apparently transmitted to the
drum of the ear directly through the bones of the head.
The external openings of the nostrils are placed quite
on the upper surface of the head, so that the animal
can breathe whilst almost completely submerged ; and
the larynx is so modified that the function of swallowing
does not interfere with that of breathing. Perhaps the
most remarkable feature of all is the enormous develop-
ment of the head, and especially of the mouth. The
huge jaws, in combination with the extraordinary plates
of whalebone which fringe the edges of the mouth and
act as a sieve, enable the animal to get its nutriment
from the minute free-swimming creatures with which
the surface waters of the ocean abound. Associated
with this special method of feeding is the fact that
teeth are only to be recognised in the embryo, and
afterwards entirely disappear.

The whales differ in all these points from any other
mammals, and failing almost any of these differences,
would not be able to live in the special conditions in
which they find themselves. It must therefore be
admitted that we have here a case of very close adapta-
tion of an animal to its natural surroundings, and one
which extends to almost every detail of its structure.
Darwin himself, moreover, has been at special pains to
show how some of the most remarkable of these
structural adaptations may possibly have arisen
through natural selection.

One of the most remarkable cases of mutual adapta-
tion, in which an animal and a plant are associated
together, is shown by the method of fertilization

54 THE THEORY OF NATURAL SELECTION

observed to take place in the flowers of the Yucca
plant of the Southern United States. The act of
pollination is performed by a moth—Pronuba—which
possesses special organs particularly adapted for this
purpose, in the shape of peculiar maxillary tentacles
which are found in no other kind of moth. The female
has also a long ovipositor with which she can pierce
the tissues of the ovary of the plant, and so lay her
eggs within it. With the aid of her peculiar tentacles the
female moth collects from several flowers a ball of pollen
of considerable size, which she kneads into a firm pellet.
She then carries this to a different flower, and after
depositing a few eggs in the ovary she climbs to the
top of the style and presses the ball of pollen into the
stigma. Thus the ovules of the flower are fertilized,
and whilst some are eaten by the larve of the moth,
others develop into seeds and reproduce the plant.

The foregoing are perhaps two of the most remarkable
cases known of animals having peculiar habits, and
possessing at the same time special organs which render
them well fitted for these habits and no others; but
many other cases of scarcely less wonderful adapta-
tions have been pointed out.

Darwin himself indicated the direction in which the
study of adaptation was to proceed, and his books on
‘Insectivorous Plants’ and on the ‘Fertilization of
Orchids’ afford us a delightful insight into a number of
adaptive contrivances which are to be seen in plants.
Another very interesting series of adaptive characters
are those which have been gathered together under
the heads of Protective Resemblance and Mimicry, and

MIMICRY 55

these have a special interest for us, because they illus-
trate the way in which the zeal of the seeker after adap-
tive contrivances may run away with him if not kept
wellin hand. For there is scarcely any limit to the num-
ber of problematical cases which have been described
as adaptive resemblances, and so explained as having
arisen through natural selection, whilst the evidence
in favour of such a supposition is in many cases highly
questionable. On the other hand, in a number of
well-marked instances, the theory of mimicry certainly
seems to afford an adequate explanation of the way
in which many curious characters and structures may
possibly have come into existence.

The families of the Mantidg and Phasmida, includ-
ing the walking-stick and leaf insects, afford many
examples of animals which both in their colour and
configuration show a very close resemblance to sur-
rounding inanimate objects. This resemblance must
have the effect of concealing them from their enemies,
and more particularly from their prey, as, indeed, a
study of their habits indicates quite clearly.

Phyllopteryx, an ‘Australian fish allied to the well-
known sea-horse (Hippocampus), is provided with a
number of irregular appendages of ragged skin
resembling the seaweed amongst which this animal
is found. In this way the characteristic symmetrical
appearance of a live animal is got rid of, and the
creature is rendered extremely difficult of observation.
Here, again, the concealment afforded is probably
useful in leading to the deception of the smaller
organisms upon which the creature feeds.

56 THE THEORY OF NATURAL SELECTION

Examples of this kind in which the shape of an
animal leads to its concealment are less numerous
than those in which protection is afforded by an obscure
pattern or inconspicuous colour. Indeed, some re-
semblance between the colour of an animal and its
surroundings is to be traced in the majority of the
members of many groups. Familiar examples are
afforded by the white colour of animals which live in
snow, the tawny grey colour of most desert species,
the green of grass-frequenting animals, and so on. It
is perhaps not quite certain that in some of these cases
the peculiar colour is not evoked by the direct action
of some cause which affects different species in the same
way; but such a cause awaits discovery, and in the
meantime natural selection has certainly a strong
claim to be regarded as the proper explanation.

A more strict use of the term mimicry, however, is
to restrict it to cases where one species apes the colour
pattern or other external character proper to another
species which inhabits the same region ; and the idea
of mimicry has been put forward as especially appro-
priate in cases where the mimicked species is common,
and can be thought to possess some special means of
protection. Numerous supposed examples of this
phenomenon have been described among insects, espe-
cially in the case of various butterflies from Africa,
Malaya, and South America. It would be beyond the
scope of this work to do more than call attention to the
fascinating subject, the literature of which includes a
large number of papers to be found in the Proceedings
of the Linnean Society and elsewhere. For a general

MIMICRY 57

account of this work, the reader is referred to Professor
Poulton’s recent ‘ Essays on Evolution.’

Manystudents of evolution in its more recent develop-
ments are disposed to attach greater importance than
does Professor Poulton to the difficulties which beset
the theory of mimicry, in so far as the theory consists
in explaining these resemblances by natural selection
accumulating minute variations in the proper direction.
Indeed, the power of this evolutionary factor seems
here to be stretched to its utmost limits of tension.
The independent evolution of a similar external appear-
ance has certainly taken place in some cases in which
any suggestion of mimicry is excluded, and there is
nothing to prove that colour-patterns of the same type
may not have arisen from the same causes in widely
different groups. In cases where the environment to
which the different forms were exposed was similar—
as would be the case especially in any single locality—
such a process of parallel evolution might be thought
to be all the more likely.

It is not to be supposed that we intend for a moment
to impugn the reality of these marvellous resem-
blances. The smallest acquaintance with the facts
must show the absurdity of any such suggestion, just
as the multiplicity of the cases described renders any
suggestion of coincidence ridiculous. It is only the
current explanation of these resemblances to which we
take exception, for the brain reels before the task of
picturing the gradual building up of such a resemblance
by the successive additions of small differences, each
one useful to the possessor of it.

58 THE THEORY OF NATURAL SELECTION

Resemblances can only be properly explained as
representing cases of mimicry when both the species
concerned—the mimic and the mimicked—inhabit the
same locality ; but plenty of cases of matching between
the colour patterns of insects which live in quite
different parts of the world could also be pointed out.
Let us take a concrete example. Everyone is familiar
with the flower-frequenting flies (Syrphid@) which are to
be seen hovering about plants in sunny weather. These
insects closely mimic the appearance of various small
bees and wasps, the habits of which are similar. Here,
then, is surely a case where the deceptive resemblance
to an animal well armed in its sting must cause pro-
spective enemies to let these flies alone. In Southern
Japan, as Dr. Andrez pointed out to me, flies of this
kind are surprisingly numerous, and their resemblance
to bees particularly noticeable. So abundant are they
that, from the point of view of the flowers which they
visit, these flies doubtless provide an efficient substitute
for the bees of other countries, which are here con-
spicuous by their absence. But if real stinging insects
are wanting, or even very scarce, the supposed enemies
of the flies can have no experience of the ill-effects
produced by catching them. How, then, can these
flies benefit from their resemblance to bees ?

This kind of thing must make us somewhat sus-
picious of supposed cases of mimicry even between
species possessing the same range.

When the ideas of mimicry and protective resem-
blance are carried into the vegetable kingdom, as they
have been by some writers, absurdities are soon found

PARALLEL EVOLUTION 59

to arise. Thus it has been suggested that the leaves
of dead nettles resemble those of the common nettle
for the sake of the protection so afforded, and that the
mottled stems of certain tropical herbaceous plants
gain a similar immunity on account of their resemblance
to snakes.

In plants a great number of fanciful resemblances
between different species can be detected, and some
between plants and animals, very few of which can be
supposed to be of any possible utility to the species
which exhibit them. They must be regarded as cases
of parallel evolution, the causes of which are quite
unknown. Such resemblances as that between the
shoots of Casuarina indica and those of the common
horse-tail, between Saxifraga hypnotdes and certain
mosses, between the horse- and Spanish-chestnut, be-
tween the seed of a pine and the fruit of an ash-tree,
are so frequent in the vegetable kingdom as to be the
delight of malicious examiners in elementary botany.
It is impossible to believe that in such cases the
resemblance is in itself of any value to either species,
and few people will be found to maintain that the
likeness of a bee- or spider-orchis to an insect is of any
utility to either animal or plant.

But if resemblances can arise which are useless, and
which, consequently, cannot be explained through
natural selection, it becomes uncertain whether this
principle can hold good as the true description of the
origin of any sort of resemblance. On the other hand,
resemblances which are useful will tend to survive
through natural selection in whatever way they may

60 THE THEORY OF NATURAL SELECTION

have arisen. This last consideration will account for
the frequency with which apparently adaptive like-
nesses are to be found in nature, even if we suppose
that their origin was ‘accidental,’ or simply due to
the operation of similar external causes. The same
criticism applies to all cases of adaptation of whatever
kind, so far as concerns their supposed method of origin
by the action of natural selection upon individual
differences.

Perhaps a still more serious criticism of the methods
of those who spend their time in seeking out or devising
cases of adaptation has been made by Bateson, who
points out the logical difficulty that we can never
make any quantitative estimate of the amount of
benefit or the reverse which any particular structure
may afford to its possessor. It is easy enough to
imagine particular circumstances in which an organ
developed in a particular way may be of undoubted
service, but whether the net amount of such service
throughout the life of the creature considered is
greater or less than the strain upon its resources caused
by the development of such an organ is quite beyond
our powers of determination.

‘ The students of adaptation forget that even on the
strictest application of the theory of selection it is
unnecessary to suppose that every part an animal has,
and everything which it does, is useful and for its good.
We, animals, live not only by virtue of, but also in
spite of what we are. It is obvious from inspection
that any instinct or organ may be of use; the real
question we have to consider is how much use it is.

REGENERATION 61

To know that the presence of a certain organ may lead
to the preservation of a race is useless if we cannot tell
how much preservation it can effect, how many indi-
viduals it can save that would otherwise be lost ; unless
we know also the degree to which its presence is
harmful; unless, in fact, we know how its presence
affects the profit and loss account of the organism.’*

A great many other criticisms and objections have
been brought at various times against the theory of
the origin of adaptations by the action of natural
selection, and many of these were considered and
replied to by Darwin in the later editions of the ‘ Origin
of Species.” We shall only consider here a few which
have been put forward more or less recently. Before
doing so it will be well to point out once more that no
one questions the validity of natural selection as a means
of exterminating types which are unfitted for their
environment—there is clearly a tendency for the
fittest types to survive once they have come into
existence. Nor can there be any doubt that species
in general are well adapted to the conditions which
their environments present. But when this is admitted
it does not necessarily follow that natural selection,
directing the accumulation of minute differences, has
been the method by which these adapted forms have
originated.

The power of regenerating a lost part must clearly
often be of service to the creatures which possess it.
Such a power may in many cases be considered to be
a well-marked adaptation. But, as Morgan has well

* W. Bateson, ‘ Materials for the Study of Variation,’ p. 12;

62 THE THEORY OF NATURAL SELECTION

pointed out, there are insuperable difficulties in the
way of adopting the belief that such a power can have
been acquired through the action of natural selection.
Many crustaceans, when they have lost a claw or limb,
proceed straightway to grow a new one. To account
for this power by natural selection we should have to
suppose, firstly, that every stage in the growth of a
partly regenerated claw, even its first small rudiment,
was useful to the animal; and, secondly, that there
was so much competition between lobsters which had
lost their claws, that those which could regenerate
them a little better would survive rather than the
others. The first of these suppositions as to the utility
of a partly regenerated claw is in the highest degree
improbable; but against the second there is an
entirely fatal argument, since, if the lobsters which
regenerated badly were exterminated owing to com-
petition with those which had better powers of re-
generation, much more would all the injured lobsters
be exterminated in competition with those which had
escaped injury.

The theory of sexual selection constitutes an im-
portant branch of the Darwinian account of the origin
of specific structures. We are here concerned with
this hypothesis only in so far as it leads to a criticism
of the efficacy of natural selection from another point
of view. By the theory of sexual selection Darwin at-
tempted to explain the origin of two sorts of characters
in particular, one or other of which frequently appears
in the male sex only of many of the higher animals.

SEXUAL SELECTION 63

In the first place, we have to notice the presence of
special weapons, such as horns or tusks, developed
exclusively or to a special extent in the males of
those species in which it is the habit of the members
of this sex to strive together for the possession of the
females. In such cases the stronger and better-armed
males are supposed to survive, and to leave a greater
number of offspring than their weaker rivals; so that
this form of competition is regarded as acting in
quite a similar way to natural selection.

In a second set of cases, of which many remarkable
instances are to be seen among birds, the males are
found to exhibit brilliant and varied colours, or to
possess special decorations in the form of plumes or
other appendages, or to be gifted with the power of
song. It is to cases such as these that the term sexual
selection more properly applies, since the females are
supposed to bestow their favours upon the most
beautiful males, and to reject the advances of those
among their suitors which are less lavishly provided
with ornament. ,

In these cases, where the development of brilliant
colours or other ornamental arrangements is believed
to have taken place owing to the choice of the females
—particularly in such a case as is represented by the
peacock’s tail or the wings of the Argus pheasant—
the supposed change must have come about in direct
opposition to the action of natural selection, since the
latter would favour a production of colours resembling
those of the natural environment for the sake of con-
cealment, and would hinder the formation of such

64 THE THEORY OF NATURAL SELECTION

exaggerated appendages on account of the loss of
activity which they must entail. We are, therefore,
obliged to conclude that natural selection is much less
rigorous in its action than some-people have supposed,
for if this principle is inadequate to prevent such an
exuberance of form and colour in these particular
instances, its action becomes open to question in
other cases as well.

Similarly, Morgan finds a difficulty in understanding
why natural selection has not led to the extermination
of species which are handicapped by the existence of
internecine strife between the males, in favour of other
species which faced the battle of life with united
strength. But in this argument it seems to be for-
gotten that examples of the kind of strife in question
are most frequent among herbivorous animals, where
the struggle for existence must be chiefly determined
by the quantity of vegetable food which the individuals
can obtain, so that the loss of the weaker males may
not bea disadvantage. Moreover, Darwin’s conclusion
that natural selection is most rigorous between members
of the same species is left out of account.

The preceding arguments seem to show that in par-
ticular cases certain structures and phenomena asso-
ciated with species cannot be explained as having
arisen through the unaided action of natural selection.
When weighed against the great mass of evidence
which Darwin accumulated in support of his theory,
these few considerations cannot be said to be in any
way fatal to the belief that natural selection of minute
differences has played an important part in the origin

INHERITANCE OF ACQUIRED CHARACTERS 65

of species. Still, they add in some measure to the
weight of recent evidence which points to the con-
clusion that many specific structures have had a
different method of origin. We have already pointed
out that there are two alternative methods, each of
which has its adherents. Before passing to a con-
sideration of the now prevalent view of mutation,
something still remains to be said with regard to the
remaining theory—the theory of Lamarck.

Darwin himself, as we have seen; admitted the
minor importance of the inheritance of acquired
characters, as well as that of the phenomenon of
sporting, regarding both these processes as causes of
the origin of new species subsidiary to the action of
natural selection upon individual differences, whilst he
looked upon the latter as the main process in organic
evolution.

Later writers, however; have asserted that natural
selection is the sole cause of the origin of species, and
in particular they have denied any effect to the in-
heritance of acquired characters—the Lamarckian
factor—asserting that there is not, and cannot be,
any such inheritance. Among the most distinguished
opponents of the theory of use-inheritance were A. R.
Wallace, the co-discoverer of natural selection; and
Professor Weismann, who has argued the case with
particular ability. Much the most able defender of
the principle of use-inheritance was Herbert Spencer,
who was one of the few who had thoroughly convinced
themselves of the truth of the theory of evolution
years before the ‘ Origin of Species ’ made its appear-

66 THE THEORY OF NATURAL SELECTION

ance. Since all arguments in favour of the evolution
of species were incomplete unless some means by
which such an evolution could take place had been
suggested, Spencer adopted the Lamarckian theory of
modification, and to this he always firmly adhered,
though admitting the validity of natural selection as
an additional factor in the process. Some of Spencer’s
arguments in favour of a belief in the inheritance of
acquired characters are well worth repetition, since
they have never been altogether refuted.

Herbert Spencer’s argument consisted mainly in the
enumeration of structures the origin of which cannot
be explained by natural selection. On the other hand,
the inheritance of acquired characters, if this form of
inheritance could be proved to have a real existence
—as Spencer believed it could—was shown to be a
perfectly adequate explanation of the origin of the
structures in question. In 1893, when Spencer up-
held his opinion for the last time, Bateson had not yet
pointed out that the facts of definite and discontinuous
variation afford an alternative way out of some of
these difficulties. In the absence, therefore, of any
other effective cause, the result of the argument
pointed strongly to the conclusion that the inheritance
of acquired characters must be a reality.

The first of Spencer’s arguments was based upon
the different powers of tactua] discrimination which
are to be found in different parts of the human body-
The degree of this sensitiveness may De estimated by
the use of a pair of compasses, the points of which can
be set at different distances apart. It is then found

DISCRIMINATION BY TOUCH 67

that with the tip of the forefinger the points can be
distinctly recognised as two when they are separated
by no more than j, inch. When applied to the middle
of the back, on the other hand, the points must be
opened to a distance of 24 inches before the sensation
of a single touch becomes resolved into two distinct
sensations.

The distribution of this power of discrimination over
the surface of the body is approximately as follows :

Tip of tongue: “ ‘ie -» gy inch.

Tip of finger. . : ae 55
Inner surface of second ‘joint of

finger .. i os a ery
Tip of nose .. 4 oss
Cheek, palm of hand, and end of

great-toe 4 54
Forehead .. we: OE ay
Back of hand, crown of head ee i Ap
Breast Id ons
Middle of back, middle of thigh,

middle of forearm aa 2k;

Now, it is out of the question to suppose that natural
selection can account for all these differences. An
increased sensitiveness of the tips of the fingers might,
indeed, be of so much use as to give the individual
possessing it a definitely increased chance of survival.
But it is hard to believe that it can be important for
a man to have the tip of his tongue twice as sensitive
as the tips of his fingers. And why should the tip of
‘ the nose be more sensitive than the cheek, or the
cheek than the top of the head, or the breast than

the back ? In the last case it might even be suggested
5—2

68 THE THEORY OF NATURAL SELECTION

that in a savage, since the sense of touch is the only
one with which his back is provided, it might be
useful for that surface to have acquired a more deli-
cate sense of touch than the anterior surface, which
is guarded by the power of vision, as well as being
more readily explored by the sensitive finger-tips.
If such an argument is regarded as far-fetched, so in
an equal degree must be any attempt to explain the
actually observed distribution through the action of
natural selection.

On the other hand; Spencer points out that the
series of parts enumerated in the above table stands
in almost exactly the order of the frequency with
which the members composing it are actually exposed
to tactual experience.

The tongue is perpetually in contact with the minute
unevennesses afforded by the surfaces of the teeth.

The palm of the hand and the lower joints of the
fingers are used chiefly in grasping, and not in the
more minute manipulations for which the finger-tips
are employed. And the experience of the back of the
hand in coming into contact with various irregular
bodies is not to be compared with that of the palmar
surface, yet it is very much greater than that of so
unexposed a part as the middle of the forearm.

For the carrying on of his argument, Herbert Spencer
has shown that increased use of the power of dis-
criminating small objects by touch is accompanied by
an increased degree of sensitiveness in individuals.
Blind people use their finger-tips in this way to a much
greater extent than those whose sight is unimpaired.

CO-ORDINATED STRUCTURES 69

Two blind boys examined by Spencer were both found
to be able to distinguish with the tips of their fingers
points separated by only ;; inch. And two skilled
compositors could both distinguish in this way points
placed no more than 4, inch apart, so that a person
with a trained sense of touch acquires a considerably
finer development of this faculty than an ordinary
individual.

If, then, acquired characters of this kind are in-
herited, even to an extremely minute extent, such as
would be scarcely perceptible in a single generation,
the account of the origin of the observed phenomena
would be complete.

As a second argument, Herbert Spencer points out
the difficulty of accounting for the development of
co-ordinated sets of structures by the action of natural
selection upon separate minute variations of the several
parts concerned.

The enormous horns of the ancient Irish elk, weigh-
ing in some cases over a hundredweight, required
specially strong neck muscles, bones, and ligaments,
and strong fore legs for their support. But an increase
in the strength of a single muscle following increased
weight of the horns would be useless if unaccompanied
by a corresponding increase in many other structures,
and, if useless, could not be selected. The chance of
all the parts concerned varying simultaneously in a
corresponding direction is very small if these variations
are really independent, and the chance of their doing
so repeatedly is in such a case infinitesimal.

Let us take another case of a similar nature. The

70 THE THEORY OF NATURAL SELECTION

hind legs of such an animal as a cat are admirably
adapted for the purpose of making a spring. In order
to arrive at such a structure by the modification of
limbs previously adapted only for running, changes
must occur in almost all the bones, muscles, and
ligaments of the limbs, and these changes must keep
pace with one another so that one part may not grow
out of proportion with the rest. It is quite impossible
to suppose that this can be effected by the natural
selection of minute fortuitous variations of the various
parts, each occurring independently. But simultane-
ously with these changes the fore legs have become
modified in a totally opposite direction. They have
become straight, firm, and pillar-like for receiving the
weight of the body in the downward leap. Compare,
says Herbert Spencer, the silence of a cat’s leap up
on to a table with the thud made by the fore legs as it
jumps down upon the floor.

Modification of the fore legs and of the hind must
thus have proceeded in almost exactly opposite direc-
tions in the two cases, and in each a great number of
parts are separately co-ordinated. For natural selec-
tion to have had any effect, all the co-ordinated parts
of one pair of legs must have varied in one direction,
whilst similar parts in the other pair of legs varied
simultaneously in another direction. It is out of the
question to suppose that this could have happened
simply by chance.

‘What, then, is the only defensible interpretation ?
If such modifications of structure produced by modifi-
cations of function as we see take place in each indi-

INHERITANCE OF ACQUIRED CHARACTERS 71

vidual are in any measure transmissible to descendants,
then all these co-adaptations, from the simplest up to
the most complex, are accounted for. In some cases
this inheritance of acquired characters suffices by itself
to explain the facts; and in other cases it suffices when
taken in combination with the selection of favourable
variations. An example of the first class is afforded
by the change just considered ; and an example of the
second class is furnished by the case, before named,
of development in a deer’s horns. If, by some extra
massiveness spontaneously arising, or by the formation
of an additional “ point,” an advantage is gained either
for attack or defence, then, if the increased muscularity
and strengthened character of the neck and thorax,
which wielding of these somewhat heavier horns pro-
duces, are in a greater or less degree inherited, and in
several successive generations are by this process
brought up to the required extra strength, it becomes
possible and advantageous for a further increase in the
horns to take place, and a further increase in the
apparatus for wielding them, and so on continuously.
By such processes only in which each part gains
strength in proportion to function can co-operative
parts be kept in adjustment, and be re-adjusted to meet
new requirements. Close contemplation of the facts
impresses me more strongly than ever with the two
alternatives—either there has been inheritance of
acquired characters, or there has been no evolution.’*
As we pointed out in the last chapter, there seems at

* Herbert Spencer, ‘The Inadequacy of Natural Selection,’
p- 29.

72 THE THEORY OF NATURAL SELECTION

first sight to be no inherent difficulty in the way of
acquired characters being inherited. Weismann has,
however, pointed out a very serious difficulty, which
is brought into prominence on making a study of the
minute anatomy of the cells of organisms during the
earlier stages of their development.

In the ordinary course of events every one of the
higher animals and plants begins its existence in the
form of a single minute cell—the fertilized ovum or
egg. This cell exhibits no trace of the complicated
series of organs which will develop from it when it is
subjected to the proper conditions. When the egg
is placed in favourable circumstances with regard
to warmth, moisture, food-supply, and the like, it
first divides into two equal portions; and microscopic
study shows that elaborate precautions are taken to
insure the equal bipartition of its minute constituent
parts. Each of the two cells thus formed divides
again into two further cells, and by a series of repeated
bipartitions of this kind the cells which constitute the
adult body are at last brought into existence. Since
the body soon becomes differentiated into a number of
unlike organs, it is clear that at certain stages of the
process the two cells arising from a division must come
to differ slightly from one another; and the cells ulti-
mately produced show very considerable differences
of form, structure, and size. Among all the cells which
finally arise those which have undergone the least
modification from their original condition are those
from which are developed the sexual reproductive cells,
or germ-cells, of the organism. Indeed, Weismann con-

PURITY OF THE GERM-CELLS 73

cludes that there is no reason for supposing that these
have undergone any modification at all.

If we consider the cells which build up an adult organ,
and for the moment regard each separate cell as an
individual, we see that each of these individuals
possesses an ancestry of cells stretching right back to
the fertilized ovum—the single cell in which the whole
organism originated. So far as the later cell divisions
are concerned, the cell-lineage of a particular organ is
separate and distinct from that of the cells of any
other organ. At a certain distance back in the history
of the organism we shall come across a common cell-
ancestor for the cells belonging to a pair of neighbour-
ing organs, and the more widely separate the parts to
which the cells we are considering belong the further
back must we go before we find their ancestry merging
in a single cell. In asimilar way as with other organs,
so it is found that the sexual cells or germ-cells of an
adult organism have a history quite distinct from that
of the cells of any other part of the body; and these
cells are the only ones which are concerned in the
formation of the offspring. Thus we see that the
particular cell-lineage leading up to the germ-cells is
the only one which is continued into another genera-
tion; all the others terminate with the death of the
individual creature of which they form a part. From
this point of view we may consider the nature of a
given series of animals as being determined only by
the particular series of cells which constitute the
direct ancestry of the germ-cells in each individual ;
the cells which make up the bodily structure are the

74 THE THEORY OF NATURAL SELECTION

result of so many offshoots which come to an end at
the death of the organism, and leave no progeny of

their own.

Wilson has expressed this view of Weismann’s very
clearly : ‘It is a reversal of the true point of view to

G
G )
ee
Ss
GO ;
yr
Ss
nh
GO 3
| &
ite
ae a
BR "5
Bs 9, S
og
Be >
Be
“9
FIG. I.

G, Germ cells; S, Somatic
cells,

regard inheritance as taking place
from the body of the parent to that
of the child. The child inherits
from the parent germ-cell, not from
the parent body, and the germ-cell
owes its characteristics not to the
body which bears it but to its
descent from a pre-existing germ-
cell of the same kind. Thus the
body is as it were an offshoot from
the germ-cell. As far as inheri-
tance is concerned the body is
merely the carrier of the germ-cells
which are held in trust for coming
generations.’ (The diagram illus-
trating Weismann’s theory of in-
heritance is a modification of that
given by Wilson.*)

In the light of this conception
it may be seen that the idea of the
inheritance of a modification ac-
quired by an adult bodily organ

is comparable with the supposition that if a man
develops his muscles by exercise his brother’s children
will be thereby modified.

* ‘The Cell in Development and Inheritance,’ p. 13.

PURITY OF THE GERM CELLS fe

The minute study of the germ-cells, taken in connec-
tion with modern experimental work on the methods
by which inheritance takes place, shows a strong
tendency to confirm Weismann’s view, so far as the case
of distinct and definite characters is concerned. But
if we regard such definite characters as having arisen
by definite steps or mutations according to the view
now gaining ground, the study of them will have no
bearing upon the question of use-inheritance, since
use does not lead to large and definite changes in the
individual, but to comparatively small changes of a
quantitative kind.

There are some, including de Vries, who regard all
fluctuating variations (individual differences) as being
of the nature of acquired characters, and as being at
the same time capable of hereditary transmission,
although de Vries believes the amount of progress
possible in this way to be strictly limited. Let us see
if there is any way in which a transmission of such
characters can be conceived of.

It must be pointed out that the cells which make
up an organism are not completely marked off and
separated from one another ; on the contrary, it seems
impossible to doubt that reactions may take place
between them long after their first formation. Indeed,
Sedgwick has shown that in a number of diverse kinds
of animals there is never any sharply limiting barrier
between cells at all, and this writer has gone so far as
to speak of animals in general as being built up of a
continuous network of protoplasm with nulcei at the
nodes. In plants, too, though at first sight their con-

76 THE THEORY OF NATURAL SELECTION

stituent cells seem to be cut off from one another like
so many closed boxes, it has been shown that there is
almost universal communication between the proto-
plasmic masses so enclosed, in the shape of minute
fibrils of living substance which traverse the interven-
ing walls.

It would thus seem possible for liquid or easily
soluble substances to pass freely from one part of the
body of an organism to another. It is possible, for
example, supposing the enlargement and strengthening
due to the exercise of a particular muscle to be associ-
ated with an increased production of some definite
chemical substance, to imagine that an increased
amount of the same substance might become enclosed in
the germ-cells, so that this substance would be present
in the offspring in greater abundance than would have
been the case if the muscle of the parent had not been
exercised. And this might facilitate a further develop-
ment of the same muscle by exercise in the next
generation. Ina similiar way increased bulk following
upon better nutrition might be inherited, and this
de Vries seems to have succeeded in showing to be
actually the case in plants. Such changes might
normally be so slight as to be almost imperceptible in
a few generations, and yet after many generations
might accumulate to an important extent. It would
be impossible in practice to distinguish changes of this
kind from what are known as accidental individual
differences, and, indeed, there is no evidence at hand
to disprove de Vries’ assertion that all continuous
variations are of the nature of acquired characters—

INHERITANCE OF ACQUIRED CHARACTERS 77

and we know that continuous variations are in-
herited.

On the other hand, several lines of inquiry have
separately led to the conclusion that a great number
of the visible characteristics of organisms are of a
definite kind, and are inherited definitely, their appear-
ance being determined by the presence of definite
structures or substances in the germ-cells. The evi-
dence, as we shall see later on, points to the conclusion
that such characters have arisen suddenly at a single
step, and we must conclude that in such a case a definite
change in the germinal structure has been followed by
a definite alteration in the character of the organism
arising from the germ ; since no one can suppose that a
large and definite structural alteration can be first
acquired by the adult organism and then inherited by
its offspring—such a process is unthinkable.

Thus we see that the inheritance of acquired char-
acters, if such inheritance really takes place at all,
must be confined to the transmission of changes of an
indefinite and quantitative kind—to the case, in fact, of
continuous variations or individual differences. More-
over; there is nothing to show that all continuous
variations are not of the nature of acquired characters.*

* We know, at any rate, that continuous variations are not
invariably due to the cause which Weismann supposed—namely,
to the mingling together of characters derived from the two
parents—a supposition which is of fundamental importance
to his theory—because, as Karl Pearson has pointed out in
this connection, parthenogenetically reproduced organisms,
in which no such mingling has taken Place, may be just as

variable as those which owe their origin to the process of
sexual generation.

78 THE THEORY OF NATURAL SELECTION

It is possible that variations of this nature may gradu-
ally lead to important and even to specific changes,
but whether this is the case still remains to be proved.
On the other hand, we shall see that specific differences
do sometimes arise at a single step, and there is strong
but indirect evidence to show that this is the way in
which a very great number of specific differences have
actually arisen. Indeed, some have contended that
this is the universal process by which such differences
originate, but this again is not proved, nor is it
altogether likely. In any case the inheritance of
acquired characters can have nothing to do with that
of definite and discontinuous differences.

This is a problem to which we shall return in the
concluding chapter, in the light of further evidence con-
cerning continuous and discontinuous variations and
their manner of inheritance, which will be by that
time available.

BIBLIOGRAPHY.

Darwin, CHarLes: The Origin of Species, sixth edition, 1872.
i * The Variation of Animals and Plants
under Domestication, 1868.
The Descent of Man, 1871.
The Various Contrivances by which
Orchids are Fertilized by Insects.
Different Forms of Flowers.
The Movements and Habits of Climbing
Plants.
oo 4 Insectivorous Plants.
Life and Letters of Charles Darwin.
ag More Letters of Charles Darwin.
Mautuus, T. R.: An Essay on Population.
Wa ttace, A. R.: Darwinism.

BIBLIOGRAPHY 79

Weismann, A.: Essays upon Heredity, 1889.
7 The Evolution Theory, 1906.
SPENCER, HERBERT: The Inadequacy of Natural Selection,
1893.
“% pe A Rejoinder to Professor Weismann,
1893.
Cope, E. D. : The Primary Factors of Organic Evolution, 1896.
Morean, T. H. : Evolution and Adaptation, 1903.
Romanes, G. J. : Darwin and After Darwin, vols. ii. and iii.
Poutton, E. B.: Essays on Evolution, 1908.
Darwin, F.: Presidential Address to the British Association,
1908.
But ar, S.: Life and Habit, 1877.
‘ip Unconscious Memory, 1880.
= Luck or Cunning, 1886.
WItiey, A. : Convergence in Evolution, 1911.

CHAPTER IV

BIOMETRY

In the present chapter we have to consider in some
detail the manner in which purely statistical methods
have been applied to certain biological data; a proceed-
ing to which the term biometry has been attached
by Professor Karl Pearson. Before concluding our
account we shall give a brief sketch of some of the
more important evidence bearing upon the problems
of evolution which has been brought to light by the
methods of biometrical science.

The first investigator to apply the methods of
statistics to the solution of biological problems was the
Belgian astronomer, Quetelet. In 1845, in the form
of a series of letters addressed to the Grand Duke of
Saxe-Coburg and Gotha, Quetelet published an admir-
able account of the theory of probability and its
relation to human affairs, and one in which the use of
advanced mathematics was avoided. The pioneer of
biometry in this country is Francis Galton, whose book
on ‘Natural Inheritance’ embodies an extremely
lucid introduction to the statistical study of variation
and inheritance. From these two works are derived

most of the ideas submitted in the present chapter.
80

BIOLOGY AND STATISTICS 81

The more recent advances in biometry are mostly
the result of work published by Professor Karl Pearson ;
they consist largely in the elaboration of mathematical
methods of dealing with statistical problems, and as
such it would be inappropriate to give any further
account of them here.

The mention of the word ‘ statistics’ at once raises a
certain prejudice in the ordinary mind; in common
parlance, the unreliability of arguments based upon
statistics is sometimes treated as proverbial, and as
used in biology they have, as a matter of fact, one very
serious danger at least. Statistics deal with groups
and not with individuals, and there is a real difficulty
involved in the fact that the average of a group may
represent something quite different from any individual
which the group contains, whilst at the same time a
group may include individuals of very diverse natures.
Nevertheless, when used without prejudice to the future
examination of individual inheritance by more detailed
investigations, the methods of biometry have un-
doubtedly yielded information of great value to the
evolutionist, particularly in the case of such material
as that afforded by the human race, since the applica-
tion of precise experiments to this particular species is
at present out of the question.

Some students of biometry, however, would go very
much further than this, for it is their professed opinion
that their own form of study is the only method by
which any real advance in our understanding of the
processes of evolution can be brought about. This
opinion is based upon the assumption, of me proof

82 BIOMETRY

is wanting, that new species have arisen exclusively
through the accumulation by natural selection of
variations of a strictly indefinite, fluctuating, or normal
kind. We have already seen reasons for believing that
this is very far from being the case, and future chapters
will be found to add considerably to the force and
quantity of the evidence already adduced.

Normal variations, strictly speaking, are individual
differences which can be supposed to depend upon a
large number of small factors or causes—factors so
numerous and so minute that the numerical distribu-
tion of the individuals examined, when ranged in order
according to the feature chosen for examination, is
found to conform closely to that which would be
expected on the mathematical theory of chance.
Such a distribution will only result when the differences
considered can be strictly regarded as lying upon a
linear scale, and when they are also evenly distributed
along that scale. That is to say, the biometrician
deals with continuous variations of a quantitative kind.
It is to be hoped that these somewhat obscure sayings
will be more easily understood in the light of what
follows.

The facts of variation have not been found readily
amenable to precise definition, but we shall endeavour
to make plain by the aid of a few examples what kinds
of variations do and what kinds do not appear to be
legitimate objects for the application of biometrical
methods. Thus it may be thought that the biome-
trician is outrunning his license when he ranks the
colours shown by the iris of the human eye in a con-

DEFINITE DIFFERENCES 83

tinuous series of eight shades, because in doing so he
groups together a number of probably definite factors
_with others which are of an indefinite kind. When the
colours of the human eye come to be studied in greater
detail, there can be little doubt that they will be found
to depend upon some such factors (among others) as
the following :

I. (a) Definite differences in structure, and (b) the
definite presence and absence of pigment in certain
definite positions ; as well as—

2. (a) Indefinite variations (individual differences) in
structure, and (0) in quantity of pigment—if, indeed,
the quantitative differences are not found to be also
definite.

In the above example a suitable and legitimate
object for biometrical investigation would be the
differences in amount of a particular pigment.

But definite differences may also exist in the case of
an apparently simple quantitative character. The
accompanying figure (Fig. 2) shows the variations in
length of the fruits of three different but closely allied
species of evening primrose, as measured by de Vries.
In this diagram the vertical distances are in each case
proportional to the number of individuals having
particular lengths of fruit, and the actual length of the
fruit is in each case proportional to the horizontal
distance from an imaginary vertical line some way to
the left of the figure; the points thus plotted are
joined by straight lines, so that a polygonal figure is
obtained representing the nature of the variation in

each particular case. The diagram shows at once that
6—2

84 BIOMETRY

the species A and C have each a characteristic mean
size of fruit, and the existence of this definite mean
is not affected by the fact that the range of variation

FIG 2.

overlaps in all three cases. Species B, on the other
hand, seems to show signs of division into at least two
separate groups.

Differences of a similar kind are sometimes to be
found among the progeny of the same individuals.
Races of garden peas may be selected which, amongst
other differences, are characterized by the presence of
large and of small seeds respectively. In each case
there is variation of a normal kind about a mean value,
but in each case the mean is quite distinct. There is
evidence that if a race of large-seeded peas is crossed
with a small-seeded variety, and the resulting cross-bred
plants are self-fertilized, their progeny in the second
generation will be separable into different groups, and
some of these will show almost exactly the same size

DEFINITE DIFFERENCES 85

characteristics as those which were exhibited by the
two original parental strains. The only difficulty in the
way of invariably distinguishing the two original kinds,
after their segregation in the offspring of the cross, lies
in the fact that the smallest seeds of the large type
may be smaller than the largest seeds of the smaller
strain, and this is a difficulty which applies equally
to the original strains before crossing, as well as to the
case of the evening primrose fruits just mentioned.

Now, it is clear that if we mixed together the seeds
of several different races of peas in the proper pro-
portions, the result might lead to a normal distribution
of the kind presently to be described. The several
races, however, would none the less be perfectly distinct,
even though we could not separate the individual
seeds belonging to each by any direct method.* Sucha
mixture of races would constitute a decided pitfall for
the unwary statistician, and it is well to remember
that, after even the most elaborate mathematical
analysis, the final result cannot be clothed with any
greater amount of certainty than the facts from which
the calculation set out. Those who have made a large
expenditure of intellectual effort in such processes have,
unfortunately; a natural tendency to overlook this
elementary fact.

Prior to the application of statistical methods to a
particular case of normal variation a number of pre-
liminary processes have to be gone through.

* It would generally be possible to decide which strain a
particular seed belonged to by sowing it and observing the
variation of its offspring.

86 BIOMETRY

Having selected a particular character for investiga-
tion, we must make a quantitative estimate of its
development in each member of a fair sample of
individuals which show the character in question.
What is to be understood as a fair sample was well
expressed by Quetelet when he wrote that statistics
must be collected without any preconceived ideas,
and without neglecting any numbers. We shall find
that in this point the biometrical method differs
from the method introduced by Mendel, since in the
latter careful discrimination of data is an essential
feature.

The quantitative determination of a character may
be made either by counting or by measurement.
That is to say, we must proceed by measurement if
the character we are dealing with is one of size or weight,
and by counting if the character shows a series of
numerical values of its own—e.g., if it is such a
character as the number of veins in a leaf or the
number of stigmatic bands on a poppy capsule.
Before we make any determinations we ought to be
quite certain that we are dealing with the same
character in each individual, and that the individuals
themselves are truly comparable with one another.
Thus we might make a series of measurements of a
particular bone in a particular limb of a particular
race of human beings with some assurance that we
should be dealing with homogeneous material.

Our measurements or countings will fall either
naturally or artificially into groups. In the case of
countings the groups are naturally limited by the

BIOMETRICAL METHODS

10

16 20

26

10

80

88 BIOMETRY

numbers which represent the character of each indi-
vidual, whilst measurements are artificially limited
through the fact that they have to be made in units
of some kind—e.g., to the nearest inch or some other
value. Such groups, characterized’ by equality of
range—each, that is to say, covering an equal number
of units—are technically known as classes.

Thus if we are dealing with human stature, and if our
measurements were made only to the nearest inch, all
the individuals of 6 feet in height would fall into one
class, those of 6 feet 1 inch into another class, and so
on. If, on the other hand, we were engaged in count-
ing the number of ray florets in the heads of daisies,
a class would include all those heads which possessed
a particular number of rays.

Without division into classes, however, a survey of
a comparatively small number of measurements may
be facilitated by ranging the values in some kind of
order. This is done, for example, in the accompanying
figure for the measurements to hundredths of an inch
of the lengths of the body, wing, and tail of thirty-one
specimens of a North American bird. The diagram
is taken from A. R. Wallace’s ‘ Darwinism.’

Even with this small number of measurements the
diagram brings out two points very clearly. In the
first place, there is no close correspondence between the
variations in length of body, wing, and tail. Secondly,
in the case of body-length, in respect of which the
specimens are ranged in order, the number of indi-
viduals of a medium size is seen to be greater than the
number of those which show extreme values. This

NORMAL VARIABILITY 89

excess of mediocre individuals comes much more
prominently into view as soon as a larger number of
measurements can be considered, and the results
artanged in a different way.

per cent. 407
30} iat
20} —
1of a F |
A ese
9 20 40 60 80 too Ibs.

FIG. 4.

The above diagram is constructed from the entries in
the third column of the accompanying table, which is
taken from Galton’s ‘ Natural Inheritance.’ It repre-
sents the variations in the strength of pull (as exerted
by an archer in drawing a bow) shown by 519 men
as recorded at the International Health Exhibition in
1884. Here equal distances measured off along the
base line represent equal increments in the strength
of pull of the right hand, and the vertical heights of the
rectangles erected upon these bases represent the
percentage numbers of the men examined which
exhibited each value of the character under con-
sideration. In this example it is easy to see that the
central class is the largest, whilst the extreme classes
contain a comparatively small number of individuals.

Oo BIOMETRY

TABLE I. (FROM GALTON).
STRENGTH OF PULL (519 MALES, AGED 23-26).

From Records made at the International Health Exhibition

in 1884.
Percentages.
Number of Cases

Strength of Pull. observed. Number of Cases Sums from
observed. beginning.

Under 50 lbs. 10 2 2

5 «©6605 42 8 10

” 79 5 140 27 37

» 80 4 168 33 79

» 90 » 113 21 91

» 100 4, 22 4 95

Above Ioo ,, 24 5 100

Total 519 100

Finally, we may display in a somewhat more detailed
fashion the result of a still larger number of measure-
pean Individuals. ° ,

700
600
Sool
400)
300
200

100

aoe at ra
62 63 64 65 66 67 68 69 70 7 #472. 73 +44 +75 + +76
Stature In Inches,

FIG. 5.

ments. Fig. 5 shows the variation in stature of a large
number of members of Cambridge University of Briti h

NORMAL VARIABILITY QI

extraction; and exhibits in a concise form the result of
4,426 measurements recorded by the Cambridge Anthro-
pometric Society. In this figure the stature in inches is
indicated on the base line, whilst the perpendicular
distances indicate the number of cases in which each
particular height was recorded. The separate classes
in this case include those who were found to fall within
the limits of 4 inch on either side of each consecutive
integral inch of stature, measurements which fell
exactly half-way between two classes—e.g., one of
693 inches—being reckoned as a half to each of the
classes in question. The continuous line in the diagram
represents the form of the ‘normal curve’ which
approximates most nearly to the line obtained by
joining together the points actually plotted.

There seems to be good evidence that in such a case
as that of human stature the figure obtained in this way
will approximate more and more closely to the shape
of what is known as a normal curve, according as the
number of individuals measured and the accuracy of the
measurements increase.

In order to arrive at a proper understanding of this
fact, we must consider the derivation of the ‘normal’
curve from another point of view—namely, from the
point of view of the mathematical theory of proba-
bility, which it will be our endeavour to present in as
simple a manner as possible.

Let us consider the result of tossing up a number
of similar coins simultaneously. If we toss up two
coins only we may get any of the following results :
(1) Head head, (2) head tail, (3) tail head, (4) tail tail.

92 BIOMETRY

And it is clear that any one of these combinations is
equally likely to appear on any given occasion, if the
coins are supposed to be strictly symmetrical, and are
tossed up entirely at random. Now, the second and
third results are the same unless the two coins are indi-
vidually distinguishable. So we may write the most
likely result of tossing up two pennies four times in the
following way :
1HH+2HT+1TT.

And in a similar way we may discover that the most
likely result of tossing up three coins eight times is :

IHHH+3HHT+3HTT+1TTT.

In the first case H T is twice as likely to appear as
H Hat any single throw, and in the second case H H T
is three times as likely as H H H in any single toss.

It is possible to work out the most probable relative
frequency of the various possible combinations in the
case of any number of coins. Thus for ten coins the
sequence of numbers runs :

TABLE II.
Heads. Tails. Prbatilty.

10 ° I

9 I Io

8 2 45

7 3 120

6 4 210

5 5 252

4 6 210

3 7 120

2 8 45

I 9 10

l fe} 10 I

NORMAL VARIABILITY 93

These values are plotted in the accompanying
diagram (Fig. 6) as vertical distances above a base
line. The figure obtained by joining together the
points thus arrived at may be observed to show some
resemblance to the previous Figs. 4 and 5.

The three series of numbers already given are those
which: are obtained on expanding the expressions
(I+1)%, (I+1)3, (t+1)*. In general the probabilities
of the various possible combinations when » coins are
tossed simultaneously are given by the expanded value
of (r+1)*.

Quetelet has worked out the relative probabilities of
the most frequent combinations in the case of 999 coins
simultaneously tossed—.e., the expanded value of

(r+1). A few of these values are given in the .
following table ;

TABLE III.

Heads. Tails. Preatilty.
500 499 ro
501 498 0°996
502 497 0'988
503 496 0°976
504 495 o'961
505 494 0°942
510 489 0803
520 479 0°432
530 469 _ 0155
540 459 0'037
550 449 0'006
560 439 0°0006

It may thus be seen that the likelihood that a result
appearing in any given throw will show a still greater

94 BIOMETRY

difference in the relative number of heads and tails
than 560 ; 439 becomes very small indeed. Although

Tails 1 2 3 4 5 6 7 8 9 10
Fic. 6.

a throw of all heads or all tails is possible, the odds
against such a result being ever actually seen are

1 1 ry n n 1 4
460 470 480 490 500 S10 $20 $30” 540

FIG. 7.

almost inconceivably great. In Fig. 7 the middle
values for (I+1) are plotted—those combinations

THE NORMAL CURVE 95

being included which lie between 550 : 449 and 449 : 550.
The points thus obtained are so close together that the
eye can scarcely distinguish whether they are joined by
straight or curved lines. We have, in fact, arrived at
a close approximation to the normal curve.

The curve thus approximately indicated may be
seen to be closely similar to the one shown in Fig. 5 ;
in fact, the two curves are of such a kind that by
altering the vertical and horizontal scales in one of
the figures in a suitable ratio their form could be made
practically identical.

The figure arrived at in this way approximates to a
mathematical curve which is intelligible to the mathe-
matician from the formula y=e-**. The theoretical
curve is really arrived at by supposing m in the ex-
pression (I +1)* to become indefinitely great. Prac-
tically, by making » very large we can get as near an
approximation as we may wish to the normal curve of
theory. Even in the case of relatively small values of
» the approximation to the normal curve is fairly close,
as may be seen by comparing together Figs. 6 and 7.

The example of tossing up coins was only taken
as a means of illustrating the more general assumption
of an event or a magnitude depending upon a number
of causes of equal strength, which in the long-run act
with equal frequency in two opposite directions. We
can understand that human stature may afford a
comparable case, when we consider the large number
of bones and cartilages the lengths of which must be
added together in order to make up the total stature
of any individual, and that the separate length of each

96 BIOMETRY

one of these elements depends upon factors which we
have no means of classifying exactly.

It now becomes necessary to mention one or two
technical terms which are used in connection with the
normal curve. The mode of such a curve is the
longest perpendicular which can be drawn from the

/\\

Q M Q’
Fic. 8.—NORMAL CURVE.

base-line to meet the curve itself (M in the above
figure). The curve is symmetrical on either side of the
mode—that is to say, any two perpendiculars drawn
from the base to the curve on either side of the mode
and at the same distance from it will be equal in
length.

When dealing with a symmetrical curve the position
of the mode is identical with that of the median—the
perpendicular line which divides the area of the
curve into two equal halves, and the foot of this
perpendicular also represents the mean or average of

THE NORMAL CURVE 97

all the values from which the curve is constructed. In
any actual case obtained by practical methods the
position of the mode, the median, and the mean will
only be approximately the same, because such a curve
is never perfectly symmetrical.

The same curve can always be reconstructed if the
position and magnitude of the mode are known, and,
in addition, any one other point on the curve itself.
A convenient point to take for this purpose is the point
at which the curve is met by a straight line erected
perpendicular to the base at such a distance from the
median that it divides the area enclosed by the median,
the base, and half the curve into two equal parts.
The distance of such a perpendicular from the median
is known as the quartile. Any given curve will have
two quartiles one on either side of the median ; they
are shown at Q and Q’ in Fig. 8.

In practice an approximation to the normal curve of
variability is constructed by plotting the values of a
number of separate measurements or other determina-
tions made upon different individuals. A variate is
one of the separate numerical values from which a
curve of variability can be constructed ; the biome-
trician usually deals with some such number as
I,000 variates. The total number of variates is
represented by the area enclosed by the curve, and
it will be seen that half the total number of variates
falls between the two quartiles and half outside
them.

A class (cf. p. 88) may be defined as a group of

variates all of which show a particular value or a value
. 7

98 BIOMETRY

falling between certain limits. The frequency of a class
is the number of variates which it contains.

The amount of variation shown by a particular
group of variates is measured by the degree of slope
of the curve. A flat curve indicates greater variability
and a steep curve denotes less variability. The flatter
the curve—supposing the area (the number of
variates) to remain the same—the further from the
mode will be the position of the quartile, so that the
distance of the quartile from the mode may be taken
as a convenient measure of variability. In a theoreti-
cally perfect curve the distance of Q and Q’ from M
is equal. A curve obtained from an actual series of
variates is never perfectly symmetrical, so that in
practice the distance of Q and Q’ from M may not be
quite the same. In such a case the average of the
two distances is taken as the measure of the variability
of the material in question, and this value may be
briefly denoted by the letter g.

In the example of variability of stature represented
by Fig. 5, q is equal to 1°6 inches. This amount of
variability can therefore be compared with other
values representing the variability in stature and in
other characters shown by various other groups of
individuals. This, then, is the first important biome-
trical result which we have arrived at—the determina-
tion of a numerical value representing the amount of
normal variability in any given case.

A measure of variability more often used than the
quartile, especially in recent work, is what is known
as the standard deviation of a normal curve, and may

PROBABLE ERROR 99

be expressed shortly aso. o represents a distance from
the mode equal to g+0'6745. Thus if o is known, g
can be readily determined, and vice versa. The reason
for the more frequent use of o is that it happens to be
determinable with greater accuracy from an actual
series of variates.*

We have still to find a measure which will enable us
to compare the variability of parts or organisms so
different that they require to be expressed in units of
quite different magnitudes. For this purpose what is
known as the coefficient of variability is used. This is
a purely abstract number obtained by dividing the
standard deviation by the magnitude of the mean in
any particular case, and multiplying the result by roo.
In this way a measure of variability is arrived at inde-
pendent of the particular kind of units of measurement
which were employed in obtaining it, and variabilities
previously expressed in terms of different units can
thus be compared together.

The circumstance that half the total number of
variates lies outside the limits of the quartiles and half
within leads us to the consideration of what is known
as the probable error. The probable error of any
statistical determination is obtained by finding a pair

* ¢ is found by multiplying the square of the deviation of each
class from the mean (or mode) by the frequency of the class,
adding together the series of products so obtained, dividing
this number by the total number of variates, extracting the
square root of the result, and multiplying by the number of
units in the class range (this last number is very often unity).
For further details with regard to the properties of the normal
curve Davenport’s ‘Statistical Methods’ may be consulted,

7—2

100 BIOMETRY

of values lying one above and one below the true value
required—e.g., the average stature of the whole of a
race—such that it is an even chance that the value
actually found will lie between them. Or the same
thing may be expressed in another way. If we plot in
the form of a curve a long series of actual determina-
tions of a particular value, the probable error of a
single determination will be nearly equal to the
quartile of the curve so obtained. We may illustrate
this state of things from our example of tossing coins,
or still better by the essentially similar case of draw-
ing balls out of a bag which contains a very large
number of balls—black and white in equal numbers.
Here the value to be determined experimentally is
the relative number of black balls to white, which we
know as a matter of fact to be equality; and our
single determination may consist in drawing out a
hundred balls, which are afterwards returned to the
bag. If we do this 1,000 times, and plot the number
of black balls drawn each time, we shall arrive approxi-
mately at a curve having its mode at 50, and possessing
a standard deviation which it is possible to determine
from the instructions given in the footnote to p. 99.
Multiplying o by 0°6745 gives us the quartile, which
represents the probable error of a single determination.
That is to say, it is an even chance whether any single
determination differs from 50 by more or less than g.
In this particular example the quartiles would be found
to lie very nearly at 46°6 and 53°4,so that the value of
the probable error is 3:4.

The properties of the normal curve tell us a number

PROBABLE ERROR IOI

of useful things about the probable error. In the first
place its value varies inversely as the square root of
the number of variates—that is to say, that in such a
case as we have just described the probable error varies
inversely as the square root of the number of balls
drawn each time. We can realize this point more
clearly when we remember that the linear dimensions
of a curve vary with the square root of its area (the
number of variates) ; the accuracy of our determination
varies in fact with the quartile, which is the linear
distance from the mode of a certain perpendicular.

We have seen that it is an even chance whether a
single determination differs from the proper value by
more or less than the amount of the probable error,
an amount which we may denote by the letter e.
The chance that any particular determination differs
from the true value by more than twice the probable
error is 4°5 to I against.

The chance that it differs by more than 3¢is 21:1 against.

” ” ” ” 4@,, 42:1 ”
»” ”» ” ” 5é,, 1,310: 1 ”

This is clearly very valuable information to possess
when we are dealing with any kind of statistics.

We must now pass on to consider what methods are
available to the biometrician for dealing with the
problems of heredity. His way is to take a large
number of pairs of relations, each pair consisting, say,
of a father and a son, and to find out how much more
like the members of such a pair are to one another on
the average than the members of similar pairs of
individuals would be, if taken at random and without

BIOMETRY

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CORRELATION 103

regard to relationships from among the general
population to which these fathers and sons belonged.
Now we shall see later on that this is not the only
way of looking at the phenomenon of heredity, nor is
it the way which is most familiar to biologists. But

62 64 66 68 70 72 74 76

F
72ab

B
74/
76} D

Fic. 9.—D1AGRAM OF CORRELATION,

it is important to remember that what the biometrician
means by amount of inheritance is a numerical value
which expresses the average degree of likeness between
a particular pair of relatives—for example, fathers and
sons.

104 BIOMETRY

In the ‘correlation table’ given on p. 102—a
purely imaginary illustration—there are tabulated the
statures of 4,503 fathers, and those of one son of
each of them. Thus 14 fathers, each 62 inches high,
are supposed to have had 14 sons, whose heights are
given in the first column. The series of heights of sons
corresponding to a particular class of fathers is known
as an array. Thus each column of the table represents
an array of sons, and similarly each line represents an
array of fathers. The mode of each array of sons is
given in the bottom line of the table.

Now if sons were on the average exactly the same
height as their fathers, the modal value of each array
of sons would be the same as the height of the corre-
sponding class of fathers. If, on the other hand, there
were no correlation between the heights of sons and
those of their fathers the mode of every array of sons
would be the same, and this value would be identical
with the mode of the heights of all the sons taken
at once. The actual result is found to be intermediate
between these two possible extremes. Thus we see
that sons tend to be like their fathers in respect of
stature, but not exactly like, and if the example given
were a real one the fundamental fact of a positive
resemblance or correlation between the statures of
fathers and sons would at once be clearly established.

The way in which a numerical value is attached to
this correlation can be shown graphically.

In the diagram opposite, the dots indicate the values
of the modes of the several arrays of sons as read off on
the vertical scale to the left of the figure, the heights

CORRELATION 105

of the corresponding classes of fathers being read off
on the horizontal scale. It will be seen that this series
of dots lies nearly in a straight line which is inclined
at a certain angle to the horizontal.

Now if there were perfect correlation between the
heights of fathers and sons, and no tendency existed
for sons to be more like the general mode of the popula-
tion than their fathers are, the inclination of the line
obtained in the above manner would be one of 45
degrees, as in the case of the line CD which passes
through the points at which the values as read off
in the vertical and horizontal scales are identical. If,
on the other hand, there were no correlation the line
would be horizontal, as EF.

The value taken to represent the amount of correla-
tion is the degree of slope of the line AB. This is
expressed mathematically as tan a, a being the angle
which the line in question makes with the horizontal.

When there is positive correlation this angle falls
between o and 45 degrees, and tan a between 0 and I.
In the present instance tan a is 0°5. This value is
known as the coefficient of correlation, and affords
the basis of a numerical comparison with other similar
coefficients obtained for other characters besides
stature, and in the case of other pairs of relatives
besides fathers and sons.

It ought now to be clearly understood that a com-
plete resemblance between each class of fathers of a
particular stature and the average stature of the
corresponding array of sons would be indicated by
the close approximation of our plotted points to a

106 BIOMETRY

straight line making an angle of 45 degrees with the
base line—a line, that is to say, having a slope of
I in I, or unity; whilst the entire absence of correlation
would be represented by a line having no slope—that
is to say, a horizontal line. The actual result in the
example given is represented by a line having a slope
of nearly I in 2, or 0°5.*

In the following table there are set down the corre-
lation coefficients for stature in the case of seven pairs
of relations, as obtained from actual data of a similar
character to that already given by way of illustration.

TABLE V. (FROM PEARSON).
CORRELATION COEFFICIENTS FOR HUMAN STATURE.

Father and son . eee O'F14

Father and daughter ... ees O'510
Mother and son wie ees O'494
Mother and daughter... eae 0°507
Brother and brother ... eos O'5IL
Sister and sister _ ees 0°537
Brother and sister «+. eee 0°553

Of the above, the first four values, representing
correlation between parents and children, are seriously

* Correlated Variability.—A precisely similar method is
employed to measure the correlation of two parts or organs of
the same individual. For example, the lengths of the right
and left arms of men are very closely correlated. In order to
attach a numerical value to this correlation the lengths of
the right arms of a number of men are treated in the same
way as the statures of fathers in the example given, and the
lengths of their left arms in the same way as the statures of
sons. The proper correlation coefficient can then be found
by plotting the result; or the labour of plotting may be
obviated by a-process of calculation.

CORRELATION 107

affected, as regards their value for representing quanti-
tatively a hereditary relationship between two indi-
viduals, by the fact that the statistics from which they
are derived show the existence of a marked correlation
between husbands and wives in point of stature,
amounting, indeed, to as much as 0:28—the result
of what is technically described as selective mating.
In the absence of such a relation between the statures of
the parents, the correlation between parent and child
might be expected to be distinctly less than that
between pairs of brothers or sisters.

The term correlation replaces to some extent the
older term yvegression employed by Galton. When
speaking of regression the facts already described are
regarded from a slightly different point of view. It
is sometimes found convenient to speak of the regres-
sion of the mean stature of an array of sons toward
the mean of the general population, instead of speaking
of the correlation between the filial mean and the
value of the parental class.

Regression represents the extent to which the
average son is more like the mean of the general
population than his father is. Correlation, on the
other hand, indicates the amount by which the son is
more like his parent than he is to the average of the
general population. Thus, instead of being exactly
like their parents, children are said to show regression
towards the mean of the general population to which
both parents and children belong.

In a case where the mean height of the fathers is
identical with the mean height of the sons examined,

108 BIOMETRY

and both are the same as the mean height of the general
population, the coefficient of regression is simply equal
to the reciprocal of the correlation coefficient between
fathers and sons. In actual practice this condition is
seldom realized, and it is then necessary to use a more
elaborate method in order to determine the value of
the regression coefficient.

Professor Pearson has extended the idea of correla-
tion to the case of characters which are not capable of
exact quantitative measurement. This extension is
based upon the assumption that such characters follow
a normal law of distribution in their variation, just in
the same way as such a character as human stature was
found to do. There is considerable doubt as to how
far this assumption is justified, so that at the outset
we may feel disposed to attach less importance to the
actual values arrived at in this way than we should in
the case of characters which can be shown to vary
normally. The method of calculation actually em-
ployed involves somewhat complicated mathematical
processes, but on Professor Pearson’s authority we
may assume both the validity of the method and the
accuracy of the results obtained—so far as the actual
process of computation is concerned. For the purpose
of making the necessary calculations the data were
arranged in such a form as in the table on p. 109.

By the suitable treatment of these figures the
value 0°45 was obtained as representing the coefficient
of correlation between sire and filly.

The amount of reliance which is to be placed in the

CORRELATION I09

above method of determining the value of a correlation
coefficient was tested by arranging in a similar manner
data with regard to stature which had already been
treated in the form of a complete correlation table.
The whole number of fathers was divided into two
groups containing the individuals above and below a
certain stature, and the same was done in the case
of the sons. And the separation into two groups was
made in several different ways by taking the dividing

PARENTAL CORRELATION OF COAT COLOURS IN HORSES,

FIiuigs. SrrgEs.
Total.
Colour. and Toker: ne
Bay and darker ... eae 631 125 756
Chestnut and lighter... 147 147 294
Total eas nes 778 272

line between the groups at various heights. By
applying to the statistics disposed in these various
arrangements the same method as was applied to the
statistics of horse colour already referred to, values
varying between 0°52 and o°6 were obtained for
parental correlation ; whereas the value arrived at by
the more usual and reliable method was 0'514. It
would therefore appear that there is with this method
a tendency to obtain too high a figure, as compared with
that derived from the method of the complete correla-
tion table. When this source of inaccuracy is taken

IIo BIOMETRY

into consideration, in combination with the doubtful-
ness of the assumption upon which the method is
based, it seems clear that its use will only give us a
roughly approximate view.of the correlation actually
existing in the cases to which it is applied. Having
made this reservation, we may compare the values
given in the following table with those which appeared
in Table V.:

TABLE VI. (FROM PEARSON).

AVERAGE PARENTAL CORRELATION.

Human eye colour... vec eee 0°495
Horse, coat colour... ae eee O'522
Basset hound, coat colour see vee O°524
Greyhound, coat colour ose es O°507

AVERAGE FRATERNAL CORRELATION.

Human eye colour... we see 01475
Horse, coat colour... ey) «+ 0°633
Basset hound, coat colour ae eee O'524
Greyhound, red in coat ser +++ 0°700
Greyhound, black in coat ae +. 0°660

Thus if we use the term inheritance at present simply
to express the fact that a more or less definite
numerical value can be attached to the average amount
of resemblance between any specified pair of relatives,
we see that a considerable number of physical characters
appear to be inherited at approximately the same rate
in men and in animals.

More than this, Professor Pearson has shown, by
the use of the same method as was applied to the case
of physical characters not quantitatively measurable,
that the average resemblance in mental characteristics

MENTAL CHARACTERS III

between pairs of brothers, pairs of sisters, and pairs
made up of a brother and a sister, can be expressed
by the values given in the following table :

TABLE VII. (FROM PEARSON).

Character. Brothers. Sisters. eee
Vivacity ae ae 0°47 0°43 o'49
Assertiveness ies 0°53 0'44 o'52
Introspection bai 0°59 0°47 0°63
Popularity... as 050 0°47 0°49
Conscientiousness ... 0°59 0°64 0°63
Temper owe ane O's! 0°49 o's!
Ability sas re 0°46 0°47 0'44
Handwriting... site 0°53 0°56 0°48

Mean ... ae O52 O'5r o°52

A sample of the collected facts from which this
information is arrived at is given in the following
table :

CONSCIENTIOUSNESS : BROTHER—-BROTHER.

First BroTuHEr.
Seconp BroTHeER. Total.
Keen. Dull.
Keen ... aoe we 970 216°5 1,186°5
Dull ... tas bee 216°5 287 503°5
Total ... wee 1,186°5 503°5 | 1,690

Every child was classified in this way as being either
above or below an average standard in respect of each

II2 BIOMETRY

character. The estimations were made by teachers
having at least six months’ experience of the children
in question.

The method of statistical treatment was, as we have
said, the same as that employed in the case of physical
characters not capable of quantitative measurement,
and there is little doubt that it is equally valid in the
present case. We may well feel, however, some
hesitation in accepting as sound the data to which the
method is applied. At the best this data can only be
of a roughly approximate kind. The evidence is,
however, undoubtedly sufficient to establish the con-
clusion that mental characters are inherited in man,
and that they are probably inherited at a rate not
greatly different from that at which physical characters
are inherited. For it will be observed that the values
given in Table VII. are in close agreement with one
another, and that they also agree with the average
value of fraternal correlation as found for a variety of
physical characters both in men and in other animals.
Assuming—and the assumptionseems to beareasonable
one—that equal fraternal correlationsindicate the exist-
ence of equal correlations between parents and children,
we arrive at the conclusion that the resemblance
between parents and their offspring is of much the
same kind and amount in the case of mental as it is in
the case of bodily characteristics.

What we may perhaps describe as the main general-
ization so far arrived at by biometricians is known
as the Law of Ancestral Heredity. This hypothesis
supposes, or at least in its original form supposed, that

LAW OF ANCESTRAL HEREDITY 113

every ancestor of a particular individual contributes
its quota to the heritable qualities displayed by that
individual. The law also states that the average
amount of resemblance between an individual and any
particular ancestor is capable of definite numerical
expression. Thus the mean amount of correlation
between (x) the two parents and the offspring, (2) the
four grandparents and the offspring, (3) the eight
great-grandparents and the offspring, and so on, is
believed to diminish in a geometrical series, which is
the same for all organisms and for all characters. The
actual amounts of these correlations were expressed by
Galton in the form of the series 0°50, 0°25, 0125, etc.
Pearson regards them as being more nearly represented
by the more rapidly diminishing series 0°6244, 0°1988,
0'0630, etc.

Now, there can be no doubt that the law as stated
above has been disproved in specific instances, and was
indeed disproved by the work of Gregor Mendel before
ever it was enunciated, although Mendel’s work was
not generally known untillater. According to Mendel’s
theory of inheritance, certain ancestors contribute
nothing to the constitution of certain offspring in respect
of certain characters. Furthermore, the modification of
the law of ancestral heredity which applied to alterna-
tive inheritance, and which was assumed in working
out the inheritance of coat colour in thoroughbred
horses, has recently been shown not to apply to that
particular case.

Unfortunately, most of the further biometrical
generalizations which concern students of heredity

8

II4 BIOMETRY

are based upon the assumption that the law of ancestral
heredity is strictly true. So that whilst we have
spent some time in considering the facts of normal
variability and of correlation between relatives,
because these facts are quite independent of any
theoretical assumption, the remainder of our review
must be passed over at a more rapid rate. Until
the theoretical conclusions now to be described have
been revised by their authors in the light of recent
knowledge, it is difficult to say how much reliance is to
be laid upon them, but it seems quite likely that they
will hold good as approximations. Indeed, though
not applying to individual cases, the law of ancestral
heredity does seem to hold good as a statistical state-
ment of general results, so that there would be no objec-
tion to it on either theoretical or practical grounds if
only it had been enunciated in some such terms as ‘a
law of average ancestral resemblance.’ Thus it is quite
possible that the total contribution of the eight great-
grandparents of an individual may be on the average
correctly represented by Pearson’s fraction, even
though their individual contributions are not always
the same.

Let us, then, briefly examine some of the further
conclusions which have been drawn from the data of
the biometricians.

Assuming the law of ancestral heredity, Pearson has
arrived at very interesting conclusions with regard to
the effects of artificial selection when the correlation
coefficients have those values which have been actually
found for them in the case of the human race. In the

EFFECT OF SELECTION II5

statement which follows, ancestors are supposed to
have been selected showing in each generation a devia-
tion h from the general mean of the population. Thus,
suppose the character selected to be stature: suppose
the mean height of the population to be 6 feet, and
the selected individuals to be 6 feet 6 inches high; h is
then 6 inches, and only individuals of a height of
6 feet 6 inches would be selected as parents in each
generation, so that after three generations of selection
we should be dealing with children whose parents,
grandparents, and great-grandparents were all of this
particular height.

Pearson calculates that after one generation of
selection the immediate offspring will show 0'62 of
the character selected (0°62 h). After two generations
they will show 0°82 h, after three 0-89 h, and after a
great number of generations 0:92 4. Thus in a com-
paratively small number of generations the development
of a character may be raised to within 90 per cent.
of the value selected, but, after this, further selection has
very little effect. If selection is stopped after one
generation, and the selected stock is then inbred, it
was calculated that the first generation of inbred stock
would show 0'59 h, the second 0°56 h, the third 0°52 h,
and the tenth 0-35 &. If, on the other hand, in-
breeding was started after the selection had continued
for a large number of generations, the first generation
of inbred stock will show 0°86 fh, the second o-81 &h,
the third 0-77 h, and the tenth 0°51 &. So that in-
breeding of a selected stock is followed by a very

8—2

116 BIOMETRY

gradual return towards the mean character of the
original race.*

It must be remembered that in the calculation which
led to this result perfect normal variability was assumed,
and the contribution of every ancestor of the same
degree to the hereditary endowment of the offspring
was supposed to be exactly equal. Since both these
assumptions are very unlikely to be realized in any
actual case, the statement here given must only be
regarded as an approximate indication of what is
likely to take place.

Some remarkable observations have recently been
published by Professor Johannsen, of Copenhagen, and
from them are drawn conclusions which seem likely to
lead to a distinct advance in our understanding of
the process of so-called continuous variation, and of
the way in which such variations are transmitted.
Johannsen’s conclusions have recently received remark-
able confirmation from the work of Jennings on pro-
tozoa, so that we are now justified as accepting the
theory of the pure line as a well-established working
hypothesis. We shall confine our present account to
Johannsen’s now classical observations.

The experiments in question were made upon plants
which could be self-fertilized for a series of generations.
In this way many complications were avoided which
are inevitably introduced in the case of biparental

* From this it seems necessarily to follow that it is impos-

sible to establish a permanent breed simply by a process of
selection. Professor Pearson, however, avoids this conclusion,

THEORY OF THE PURE LINE 117

inheritance. Barley and kidney beans were among
the plants examined, and the simplest character con-
sidered was the size of the seeds of the latter as
measured by weighing. In this particular series of
experiments each plant was regarded as _ being
characterized by the average weight of the seeds
which it produced.

All the descendants arising from a single plant by self-
fertilization are spoken of by Johannsen as making up
a ‘pure line.” And the members of such a line showed,
in respect of the weight of their seeds, normal varia-
bility about a mean or type value. The general
population of bean plants, made up of a great number
of such pure lines, also exhibited a normal curve when
the weights of the seeds were plotted. The pure
lines composing such a population showed various
types, some of them close to the modal value of the
population, but others differing widely from it. If
now a somewhat widely deviating member of a par-
ticular line was selected for propagation, its off-
spring showed regression to the type of this par-
ticular line, and not to the mean value of the general
population.

The case is indeed precisely similar to the supposed
example of a mixture of races of peas, which was made
use of as an illustration at the beginning of the present
chapter. In other words, a pure line consists of a
group of individuals which has a normal variability of
its own, and the offspring of which by self-fertilization
breed true to the type of their own particular group,

118 BIOMETRY

and show no regression towards the type of the general
population to which the group belongs.

If we were to carry on this conception to the case of
bisexual inheritance, we should find that the different
pure lines would become crossed and confused together
in a way which would be very difficult to disentangle.
There is no reason to doubt that statistical treatment
of such a population would yield similar results to
those actually obtained by biometricians from the
data at their disposal; and we may notice that a for-
tuitous mixture of a considerable number of pure
lines, having slightly different types, would admirably
fulfil the conditions we have seen to be necessary in
the case of material, to which methods based upon
the theory of chance are to be applied. The phe-
nomena which follow upon the crossing together of
two or more pure lines have been found, in the majority
of cases so far studied, to conform to those laws of
heredity associated with the name of Mendel which
are explained in Chapter VII. This being the case,
there appears to be every probability that the theory
of pure lines, in combination with the method of in-
heritance referred to, may adequately serve to describe
those phenomena to account for which the law of
ancestral inheritance was called into existence.

The conclusions to which Professor Johannsen’s
experiments lead him may be summed up as follows :
Individuals which differ (in size, for example) from
the mean of a population give rise to offspring which
differ from that mean value in the same direction
but to a smaller extent. Selection, therefore, will

THEORY OF THE PURE LINE IIg

produce a change in the average character of a popula-
tion taken as a whole. Selection within a pure line
produces no effect of this kind. The average character
of the offspring of typical members of the line is the
same as that of the offspring of members which show
the widest deviations from the type.

Selection in a population consists in the partial
separation of those lines the types of which differ in
the required direction from the average character of
the population. This effect must of necessity come to
an end when the most eccentric line is completely
isolated. The great complications introduced when
the lines are intermingled through mixed breeding may
make this process of isolation a very tedious one.

It will be seen that the values calculated by Pearson
to represent the result of selection in a population
agree quite well with Johannsen’s explanation of the
constitution of such a population out of a number
of pure lines. The result of Professor Johannsen’s
further experiments will therefore be awaited with
great interest by biologists and biometricians alike.

On the theory of pure lines it is to be noticed that
the personal character of a particular ancestor has
no influence upon his descendants; it is only the
type of the line to which he belongs which influences
the offspring, so that this theory is in perfect agreement
with Weismann’s theory of inheritance as described
on p. 74.

It is also to be observed that the principle of the
pure line applies only to quantitative characters—
such characters of size, or of weight, or of proportion,

I20 BIOMETRY

as are very seldom made use of by systematists for
the distinction of natural species.

BIBLIOGRAPHY.

Gatton, F.: Natural Inheritance, 1889.

Pearson, K.: The Grammar of Science, second edition, 1900.

Vernon, H. M.: Variation in Animals and Plants, 1903.

Davenport, C. B. : Statistical Methods, Igo4.

BIOMETRIKA, 190I—

JoHaNnnsEeNn, W.: Ueber Erblichkeit in Populationen und in
reinen Linien, 1903.

Elemente der Exakten Erblichkeitslehre,
1909.

HuGo DE VRIES.

[To face p.

CHAPTER V

THE THEORY OF MUTATION

MotaTIon is the term applied by de Vries to express
the process of origination of a new species, or of a new
specific character, when this takes place by the dis-
continuous method at a single step—a process which
he regards as the most important if not the sole
method by which new species or specific characters
arise. We shall see that although de Vries has
recently done much to forward the propagation of
this idea, the belief that such a discontinuous process
is the normal method by which new species come into
existence has been developing for a considerable time.

We have seen that those who accept the idea of
evolution by the action of natural selection upon a
series of minute and almost imperceptible variations
are confronted with the difficulty of explaining how
by this method there could arise a number of different
structures or parts so co-ordinated as to share in a
common function. Moreover, a closer examination
of the actual processes of variation and inheritance
render it doubtful whether the selection of continuous
variations of even a simple characteristic can ever lead
to the development of a permanent new race. The

121

122 THE THEORY OF MUTATION

result of Pearson’s calculations, described in the
preceding chapter, seems to indicate that the selection
of a certain value of a particular character for many
generations will never lead to the formation of a race
in which the mean value of the character is as high
as the selected value. But, says the selectionist, it
will happen in Nature that as the standard of the
race is raised by selection, the value selected will be
still further raised, and so on, and in this way an
indefinite amount of improvement is rendered possible.
If Johannsen’s conclusions are well founded, this is
clearly not the case; on the contrary, there is a
perfectly definite limit to the effect which selection
can produce.

The question whether or not a gradual method of
evolution is possible has not yet been absolutely
decided for any single species or character, but it cer-
tainly seems that now for the first time the possibility
of a definite decision is within sight. At the same
time it is impossible to prove a universal negative. If
we look at the other side of the problem we shall find
that the evidence in favour of an alternative process
has multiplied even faster than the evidence against
the continuous accumulation of minute differences ;
and the present tendency is certainly to look for other
sources of specific distinctness than that which is
offered by the natural selection of continuous varia-
tions.

Even before the new evidence which we have briefly
outlined was available, Herbert Spencer found the
difficulties in the way of accepting the purely

COORDINATED STRUCTURES 123

Darwinian explanation to be so great, that he adopted
the hypothesis of the inheritance of acquired char-
acters, as being the only adequate explanation of the
phenomena which was in his time available.

Unfortunately, satisfactory evidence that such a
form of inheritance ever actually takes place has never
been forthcoming in sufficient amount to lead to
universal conviction. Indeed, at the present day the
consensus of opinion among experts is undoubtedly
to the effect that acquired characters are not inherited
at all, except in so far as better nutrition of the parent
may lead to the production of more vigorous off-
spring. And it seems clear that such an effect as the
latter cannot go on accumulating for more than a few
generations.

Thus we see that in the purely Darwinian view there is
something wanting, whilst the Lamarckian explanation
is ruled out of court for the present for lack of direct
evidence. If, at this point, we find that in Nature a
co-ordinated set of structures can and does arise in
an already perfected condition at a single step, and
that such phenomena take place with sufficient fre-
quency to give ample opportunities for the survival
of the new type so arising, we have at once discovered
an alternative way out of the difficulty. Such a
discovery must throw abundant light on the obscurity
overshadowing the methods by which evolution has
taken place, even though we may not yet have arrived
at any kind of explanation of the cause of this phe-
nomenon of co-ordinated and definite variability.

The actual observation of variations of this kind is

124 THE THEORY OF MUTATION

of quite recent date, and their recognition is largely
due to the exertions of Bateson. But the idea that
this is the way in which evolution takes place is very
ancient, and seems to have been guessed at by Aristotle.
Perhaps the earliest use of the actual word ‘muta-
tion’ in this sense is to be found in ‘ Pseudodoxia
Epidemica,’ by Dr. Thomas Browne. I quote from
Book VI., chapter x., ‘Of the Blackness of Negroes ’*
(second edition, 1650) : ‘ We may say that men became
black in the same manner that some Foxes, Squirrels,
Lions, first turned of this complection, whereof there
are a constant sort in diverse Countries; that some
Chaughes came to have red legges and bills, that Crows
became pyed; All which mutations, however they
began, depend upon durable foundations, and such
as may continue for ever.’

The experiments upon cross-breeding, which are
described in a later chapter, will be found fully to bear
out the idea that ‘ mutations,’ or definite character-
istics which have arisen in a definite way, do depend
upon durable foundations.

The late Professor Huxley’s emphatic approval of
the ‘ Origin of Species,’ as signalized in his reviews of
the first edition of that work, was tempered by the
following mild criticism: ‘Mr. Darwin’s position
might, we think, have been even stronger than it is
if he had not embarrassed himself with the aphorism
‘Natura non facit saltum,” which turns up so often
in his pages. We believe... that Nature does make
jumps now and then, and a recognition of the fact is

* I am indebted to my friend Mr. R. C. Punnett for this
reference.

ORGANIC STABILITY 125

of no small importance in disposing of many minor
objections to the doctrine of transmutation.’*

The first person to formulate a more or less precise
view upon the subject of definite variation was Francis
Galton, although this author never entered into the
question at any great length. Galton’s attitude
towards the problem in its early stages may be gathered
from the following quotation from his ‘ Natural In-
heritance ’: ‘The theory of natural selection might
dispense with a restriction for which it is difficult to
see either the need or the justification—namely, that
the course of evolution always proceeds by steps that
are severally minute, and that become effective only
through accumulation. That the steps may be small,
and that they must be small, are very different views ;
it is only to the latter that I object, and only when the
indefinite word “‘ small” is used in the sense of “‘ barely
discernible,” or as small compared with such large sports
as are known to have been the origins of new races.’

But more than this, the idea of the existence of
stable forms, such as may be supposed to have arisen
by large and sudden variations, is very well expressed
by Galton in his division of varieties into the three
groups of primary types, subordinate types, and mere
deviations from the latter. A most luminous analogy
is afforded by the three types of public vehicles which
at the end of the nineteenth century were character-
istic of the streets of London ; and it is impossible to
resist quoting Galton’s account of them. These three

* ‘Collected Essays,’ vol. ii., p. 77.
ft ‘ Natural Inheritance,’ p. 32.

126 THE THEORY OF MUTATION

kinds of carriages, ‘namely, omnibuses, hansoms,
and four-wheelers, are specific and excellent illustra-
tions of what I wish to express by mechanical types
as distinguished from subtypes. Attempted im-
provements in each of them are yearly seen, but none
have as yet superseded the old familiar patterns,
which cannot, as it thus far appears, be changed with
advantage, taking the circumstances of London as
they are. Yet there have been numerous subsidiary
and patented contrivances, each a distinct step in
the improvement of one or other of the three primary

B A Cc B 18) Cc
FIG. Io.

types, and there are or may be an indefinite number of
varieties in details, too unimportant to be subjects of
patent rights.’*

More recently Galton might have pointed out the
introduction of motor traffic as illustrating a distinct
mutation.

The distinction between primary and subordinate
positions of stability is further excellently illustrated
by the model which is here represented, and which
is known as Galton’s polygon (Fig. 10).

* ‘Natural Inheritance,’ p. 26.

A PHYSICAL ANALOGY 127

The first position of the model, resting upon the side
AB, may be taken to represent the condition of a
type or stable form. A comparatively small push
(variation) will lead to the production of the subtype
illustrated by the position BC. When in this new
position, it is easier to cause the model to return to its
original position A B than it is to make it pass on to the
new and more modified position resting upon the side
CD. Astrong push (mutation) may force the model
to pass through the position C D until it comes to rest on
the side opposite to A B. This fresh position represents
a new stable form, and it is now once more surrounded
by positions of subordinate stability—subtypes.

One more analogy before we pass on to consider the
more recent observations upon discontinuous varia-
tions or mutations. We may compare the difference
which exists between deviations and stable forms,
arising by fluctuating and by definite variation respec-
tively, with the behaviour of the atoms of chemistry,
as expressed in the account of their structure recently
given by Professor Sir J. J. Thomson. Such an atom
is regarded as being made up of a number of electrons
or corpuscles bearing definite relations to one another
in space. In certain circumstances it seems that it
may be possible to remove a series of these corpuscles
from the atom one at a time, in which case every such
successive removal would be accompanied by a com-
paratively gradual and progressive change in the
properties of the atom so modified. But after a
certain time a point would be reached at which the
removal of one more electron would necessitate a

128 THE THEORY OF MUTATION

complete rearrangement of the remaining corpuscles
in order to arrive at a new position of equilibrium,
and this change would be accompanied by a marked
alteration in the chemical properties of the atom itself.
In like manner the chemical composition of the living
substance of a race of organisms may be conceived to
alter step by step, every such step being accompanied
by comparatively unimportant changes in its visible
characters, until the time arrives when any further
alteration must be associated with a deep-seated revo-
lution in the constitution of the living substance, and
with a corresponding marked mutation in the external
features of the members of the race.

The first really definite attempt to collect and co-
ordinate the facts of discontinuous variation was made
by Bateson in his book entitled ‘Materials for the
Study of Variation,’ published in 1894. The intro-
duction and concluding remarks at least of this volume
ought to be read by everyone who is interested in
these subjects. The bulk of the book contains a mass
of materia] of great value to specialists.

After pointing out the difficulties which prevent his
acceptance of the orthodox belief in the origin of dis-
continuous and apparently adaptative types of animals
and plants through the action of natural selection on
minute variations, difficulties to which we have
already paid some attention, Bateson records his con-
viction that the facts of discontinuous variation afford
a way out of the difficulty. He shows (rz) that differ-
ences of the kind which are generally used to dis-
tinguish separate species may arise as single variations ;

SYMMETRY 129

{2) that such a form of variation is by no means so
uncommon a phenomenon as was formerly supposed ;
and (3) that variations of this kind may occur in
every description of organ and part in a number of
different plants and animals. The facts with which
the main bulk of the book is concerned have reference
to the animal kingdom.

We shall find it profitable to consider the views
expressed in this book a little more closely, though it
would occupy too much space to give even a brief
summary of the facts upon which they are based, and
for which reference must be made to the original.

In the first place Bateson calls attention to the
phenomenon of symmetry as being a characteristic
feature common to almost all organisms. This sym-
metry may manifest itself in a number of different
ways. In bilateral and radial symmetry the parts
symmetrically disposed are related to one another in
the same kind of way as are an object and its image
reflected in a plane mirror. Such symmetry, as,
indeed, every kind of symmetry, is usually associated
with a repetition of parts. In the present instances
the parts are repeated in pairs, as with the two eyes
in the human face ; or in a radial series, like the arms
of a star-fish, or the petals of a buttercup. To this
phenomenon of the repetition of parts, generally
occurring in such a way as to produce a symmetry or
pattern, the term merism is applied. .

Symmetry may affect the proportions and shape
of the body of an animal or plant as a whole, or, on

the other hand, separate parts or organs may show a
9

130 THE THEORY OF MUTATION

separate symmetry of their own. For the phenomena
thus distinguishable separate terms are proposed. A
major symmetry is a form of pattern which includes the
body as a whole, as in the case of most animals where
the two sides of the body closely resemble one another.
A minor symmetry is a pattern completed in a separate
organ or part—for instance, in the flower of a plant or
the limb of an animal.

Once more we may lay stress upon the universal
existence of pattern among living things. Bateson
points out that in collecting any kind of living creature
it is the symmetry of it which, as a general rule, first
catches the eye and distinguishes the organized body
from surrounding inanimate objects.

The phenomenon of merism or repetition of parts
being understood, we are in a position to consider the
subdivision of variations into meristic variations and
substantive variations respectively.

Meristic variations are variations in symmetry and
in the number of repeated parts. A change in the
number of organs in a series may conceivably take
place gradually by the addition or subtraction of suc-
cessive fractions of a part. But, as a matter of fact,
this is very seldom the case. The increase or decrease
usually involves one whole member at a time and some-
times more, so that this kind of variation is, as a rule,
discontinuous. Abundant illustrations of this fact are
to be found in the case of changes in the number of
such parts as the teeth or vertebre of mammals ; and
a particularly good instance is afforded by the variations
which take place in the number of ray florets in various

MERISTIC VARIATIONS 131

composite plants—e.g., the daisy and chrysanthemum.
It is suggested that meristic variations are connected
with definite changes in the mechanical relations of
dividing parts, and that it is in the mechanics of cell-
division that the explanation of their discontinuous
appearance is to be sought for.

Thus when, for example, a tulip-flower appears
having its parts perfectly developed in sets of four
instead of in sets of three, it is suggested that the
arrangement in fours, like the arrangement in threes,
fulfils certain conditions of equilibrium among the
forces which affect the cell-divisions in the rudiment
of the flower, and that these conditions of stability
would not be equally well provided for by any inter-
mediate arrangement.

Substantive variations are changes in the actual
constitution or substance of the parts themselves.
For example, a plant with coloured flowers may give
rise to offspring the flowers of which are white. There
seems to be no mechanical necessity for such varia-
tions to be discontinuous rather than continuous; it
is quite possible to imagine a gradual dilution of colour
taking place throughout a long series of generations.
Discontinuous substantive variations are, however, not
infrequent, and in such cases it is suggested that they
may be associated with definite changes in chemical
composition. Thus, for example, definite alterations
in the colour of offspring as compared with their
parents are almost necessarily of a chemical nature.

The further evidence contained in the book we are
considering refers entirely to meristic variation.

9Q—z

132 THE THEORY OF MUTATION

An important point with regard to repeated parts
is to be observed in the fact that in a pair of allied
species, in which a series of repeated organs in the one
is clearly comparable with a similar series in the other,
all the parts in one form may differ from those in the
second by the same kind of distinction, whether this
be qualitative or numerical. The facts suggest strongly
that such cases are to be accounted for by all the
parts in question in one or both species having varied
in a similar way at the same time rather than in suc-
cession. The occurrence of such a similar and simul-
taneous process of variation of repeated parts clearly
simplifies in a marked degree the process of evolution,
and greatly reduces the time which would be required
for this process, if similar changes in repeated parts
always took place successively. If we take an ex-
treme case the latter supposition becomes absurd. In
the albino or pure white types which occur as varia-
tions in many species of birds and mammals it is
obvious that every hair or feather has taken on the
white colour at the same time and for the same reason,
whatever that reason may have been. Hairs or
feathers are very good examples of repeated parts of
the kind of which we have been speaking. It appears,
too, that colour patterns may originate and change in
a similar manner. In the case of such a bird as the
peacock we should expect on this view that the pattern
varied in all the tail feathers simultaneously, nor is it
necessary to suppose that even this process took place
by a very long series of minute steps. If we find that
the splendid coloration of the peacock’s tail arose

HOMCOSIS 133

by a few marked variations, each of which occurred
simultaneously in all the feathers at once, several
serious difficulties are avoided, and on the analogy of
similar known cases we have every reason to believe
that this was so. And similar changes may take place
in cases where the pattern depends on the coloration
of a group of feathers or hairs. Indeed, if we con-
sider, we shall find it very difficult to picture such a
process as taking place in any other way. We can
scarcely suppose the spots of the leopard, for instance,
to have arisen one at a time.

An important kind of discontinuous variation is
that to which Bateson has applied the term homeosts.
The same sort of change had previously been described
by Masters in the case of plants under the name
‘metamorphy,’ but the latter expression has also
been employed in other senses. Homeeosis consists
in the assumption by one member of a meristic series
of the form or character proper to another member
of the same series ; for example, the modification of
the petal of a flower into a stamen, or of the eye of a
crab into an antenna-like organ.

‘In these cases a limb, a floral segment, or some
other member, though itself a group of miscellaneous
tissues, may suddenly appear in the likeness of some
other member of the series, assuming at one step the
condition to which the member copied attained pre-
sumably by a long course of evolution.’ *

The phenomenon of homeeosis is frequently to be
seen among the parts of flowers. Double flowers in

* ‘Materials for the Study of Variation,’ p. 570.

134 THE THEORY OF MUTATION

many cases—for instance, in the case of the rose—arise
by the development of petal-like organs in the position
which would properly be occupied by stamens. A
parallel process is‘ to be seen in the heads of com-
posite flowers, such as the chrysanthemum. In a
double chrysanthemum the florets of the disc develop
in the likeness of ray florets. Both these cases would
be classed as examples of outward homeosts, because
the parts concerned resemble organs normally de-
veloped in a whorl exterior to themselves. A case of
inward homeosis, on the other hand, is afforded by
the appearance of a petaloid calyx—for example, in
a tobacco-plant—the outermost whorl of the flower
taking on the appearance of a whorl internal to itself.

In cases such as these we observe once more the
occurrence of a marked and definite change, which,
though at first sight quite distinct from the method of
similar and simultaneous variation, yet bears a certain
resemblance to that process in the fact that the direc-
tion in which a particular part varies is not wholly
unrelated to the behaviour of other parts of the same
organism. The process thus briefly described seems
likely to have had considerable importance in evolu-
tion, notably in the origin of differences in the numerical
relations of the bones in various parts of the spinal
column in different vertebrate animals.

The preceding account of the conclusions drawn
from Bateson’s laborious study of variation has in-
volved a good deal of technicality, but this is, un-
fortunately, unavoidable. The point chiefly to be
emphasized is the frequent occurrence in Nature of

EFFECT OF SELECTION 135

variations of a definite or discontinuous type—the
fact that differences of the kind which are constantly
used to distinguish natural species can and do arise
in Nature at a single step, so that it is not necessary
for such differences to be built up gradually by the
action of natural selection.

De Vries, in his ‘ Mutations Theorie,’ goes further
than this, and attacks the position held by those who .
accept the doctrine that natural selection of individual
differences can ever lead to definite and permanent, or
specific, distinctions. Indeed, one of the chief contri-
butions of this author to the species controversy is to
point out that the belief that artificial selection acts
in this way upon domestic plants is based upon a mis-
apprehension. De Vries himself has carried out a
number of experiments in selection, and he comes to
the conclusion that selection of ordinary individual
differences has no permanent effect at all.* The actual
effect of this kind of selection is well illustrated by the
results of the processes employed in the sugar-beet
industry, in which elaborate care is taken to select
those roots which contain the highest percentage of
sugar for the purpose of propagation. This process
was followed at first by a rapid improvement, but the
rate at which the percentage of sugar increased soon
fell off, until at the present day all that selection can
effect is to keep up the standard of excellence already
attained. Moreover, that this process of improvement

* Compare, however, Johanssen’s more recent conclusions
see p. 118).

136 THE THEORY OF MUTATION

was a very gradual one is to be accounted for in part,
at least, from the fact that the methods of selection
themselves gradually improved from year to year.
There is no reason to doubt that a thoroughly efficient
method of selection would have worked its full effect
in a few generations. A similar state of things is said
to be the case with the cereals, such as wheat and
barley, which have been selected largely for the size
of the grains. From his own experiments, de Vries
has come to the conclusion that, when selection is
really efficient, the full possible effect of this process
is exhausted in quite a small number of generations,
and that then the only further effect of selection is to
keep up the standard already arrived at.

We have seen that the theoretical conclusions of the
biometricians are in agreement with the opinions here
expressed, so long as selection is understood to be con-
fined to the choosing out of parents which show a
definite standard value of the character under con-
sideration, this value being the same in each genera-
tion. Under these circumstances, Professor Pearson
concludes that in the first two or three generations
a marked advance in the desired direction will take
place, but that further selection (in this sense) will
have comparatively little effect. But the believer
in continuous evolution maintains in addition that
selection will be followed to an indefinite extent by
further variations in the direction of selection, since
otherwise selection could never lead to important
changes in organization. In the face of the strong
contrary evidence, and of the fact that alternative

EFFECT OF SELECTION 137

methods of evolution are now known to be available,
the burden of proof of this proposition seems to
lie with those who maintain the all-important influ-
ence of continuous variation and selection. At present
we are free to reply in the words of Malthus, who long
ago protested against the extravagant powers which
were ascribed to the selection of small differences.

‘I have been told,’ Malthus writes, ‘that it is a
maxim among some of the improvers of cattle that
you may breed to any degree of nicety you please,
and they found this maxim upon another, which is,
that some of the offspring will possess the desirable
qualities of the parents in a greater degree. In the
famous Leicestershire breed of sheep, the object is to
procure them with small heads and small legs. Pro-
ceeding upon these breeding maxims, it is evident that
we might go on until the heads and legs were evan-
escent quantities ; but this is so palpable an absurdity
that we may be quite sure the premises are not just,
and that there really is a limit, though we cannot see
it or say exactly where it is.’ *

The only recorded example I am aware of in the
case of animals, which shows the result of long-con-
tinued selection acting upon a quantitative character,
is afforded by the case of the American trotting-horse.
In this case it appears highly probable that we are
dealing with a character which varies in a strictly
continuous fashion. In his book upon ‘ The Trotting
and Pacing Horse in America,’ Hamilton Busbey
gives a table from which the diagram on p. 138

* ‘Essay on Population,’ 6th ed., vol. ii., p. 11.

138 THE THEORY OF MUTATION

is constructed. The entries in this table show the
fastest times recorded for the feat of trotting a measured
mile in various years beginning with 1818. The ver-
tical scale contains the times, which vary from three
minutes down to one minute fifty-six seconds, and the
horizontal scale shows the year in which the record was

3,004

Year 1820 “30 ‘40 © ©6"50 60 ‘70 "80 °90 1900
FIG, 11.

The figures to the left of the diagram are to be read as minutes and
seconds.

made. Some part of the improvement shown is clearly
to be associated with better tracks, improved methods
of training, etc., but these will scarcely affect the
general character of the improvement due to selec-
tion. As may readily be seen from the diagram, the
improvement is at first rapid, but afterwards becomes

EFFECT OF SELECTION 139

gradually slower and slower. At the end of the series
two sudden steps upward break the general regularity
of the series of records. But on examination of the
evidence it is found that these are associated with
special conditions, and are not really exceptional. The
first of these breaks—that which occurs in 1892—is
coincident with the introduction of a new type of sulky,
having ball bearings and other improvements ; whilst
the record of 1903 was accomplished behind a pace-
maker carrying a wind-shield. Neither of these records,
therefore, is strictly comparable with the rest of the
series.

The observations in this case do not, indeed, seem
to be sufficient to afford the basis for a final decision
against the theory of the indeterminate power of selec-
tion. Yet Malthus’ criticism clearly applies very
definitely to such a case—i.e., there must be a limit
beyond which the speed of the trotting-horse will
never improve without a fundamental change taking
place in his organization. It seems, therefore, safe to
conclude that the curve to which the series of records
approaches is of the character of a parabola—+.e., one
which continually becomes more and more nearly
horizontal—as the speed of the horse gradually ap-
proaches its highest possible limit.

De Vries, then, contends that all new domestic breeds
have arisen by the discontinuous method as definite
novelties. Darwin himself was perfectly aware that
this is usually the case, but the conclusion which he
drew from the fact was a different one, as the following
passage shows :

140 THE THEORY OF MUTATION

‘He (man) often begins his selection by some half-
monstrous form, or at least by some modification pro-
minent enough to catch the eye or to be plainly useful
tohim.’ But he goes on: ‘ Under Nature, the slightest
differences of structure or constitution may well turn
the nicely-balanced scale in the struggle for life, and
so be preserved.’ *

Of the origin of a new type of plant in this definite
and sudden fashion, the Shirley poppies afford an
excellent example. These originated in a mutation
of the common wild field-poppy (Papaver rheas). In
1880 the Rev. W. Wilks, Vicar of Shirley, near Croydon,
noticed among a patch of this plant growing in a waste
corner of his garden a solitary flower, the petals of
which showed a very narrow border of white. The
seeds which this flower produced were sown, and next
year, out of about two hundred plants, there were four
or five upon which all the flowers showed the same
modification. From these, by further horticultural
processes, the strain of Shirley poppies originated.
We may point out in passing that if the original plant
had been self-pollinated, a much larger proportion of
the new type might have been expected to appear in
the next generation.

In the course of his own experiments, de Vries has
obtained quite a number of new types of plants by
methods like the above. It is to be observed that the
novelty in these cases usually shows a considerable
range of normal variability of its own, and that its
first appearance is generally in the form of an extreme

* “Origin of Species,’ 6th ed., p. 60.

EFFECT OF SELECTION I41

negative variation* from the proper type of the new
variety. In this way the novelty may not appear to be
very far removed from a high normal variation of the
original type. The behaviour of the progeny of the two
types, however—types which might thus in themselves
be readily confused—is entirely different, and a ready
means of distinguishing them is thereby provided.
Each set of offspring shows regression to its own proper
modal value ; so that the offspring of the novelty show
a further marked development of the new features,
whilst the offspring of an extreme normal variation
resemble the type of the original form more closely
than they do their own immediate progenitor.

If new types are not produced among domesticated
productions by the action of artificial selection, and all
that selection can effect is to pick out definite novelties
when they occur, the analogy between natural selec-
tion and artificial selection breaks down, and a large
and important section of the evidence in favour of the
production of natural species by the action of natural
selection is destroyed. In the place of this explana-
tion de Vries would put the theory of mutation, ac-
cording to which new species arise by single steps as
definite novelties, just in the same way as we find that
domestic varieties are produced. More than this, de
Vries believes that he has discovered a set of new
species in the very act of originating from an old
one in this way, a discovery which affords the basis

* I.e., a variant belonging to a class situated some dis-
tance from the mode of normal variability of the novelty,
and on the side of it nearest to the mode of the original type.

142 THE THEORY OF MUTATION

and groundwork of the views which he puts for-
ward.

The plant which afforded the material for this dis-
covery is known as nothera Lamarckiana—that is to
say, this is the name of the old species from which the
new species were found to be arising.| O. Lamarckiana
is an American plant, but the specimens which de Vries
found to be in a state of mutation had made their
escape from a garden, and were running wild over a
disused potato-field near a town called Hilversum, in
Holland. On examining these plants, de Vries found
two distinct new forms, which were quite unlike the
remainder. Each kind occurred in an isolated patch,
as if it had arisen from the seed of a single plant.

No description of either of these forms was to be
found in botanical literature, nor were there ‘specimens
of them in any of the great herbaria. But when de
Vries took seeds from some of the plants and sowed
them in his garden, he found that the new forms came
true to type—the plants produced resembled the
parents from which the seeds were taken, and not the
normal form of O. Lamarckiana.

Here, then, we have a case in which two new species
had originated from an old one in a state of nature.
But de Vries went further than this, and took measures
for observing the actual origin of new forms in the
cultivated offspring of the semi-wild Gnothera.

For this purpose he transplanted a number of roots
from the field where they were growing, and also took
seed from a number of other plants, and from these he
cultivated large numbers of seedlings for a series of

FIG. 12.—MUTATION IN CENOTHERA.

(From de Vries.)

Toprow .. .. Lam. lata
Second row .. subovata albida
Third row.. .. albida albida
Fourth row... _~— oblonga lata
Fifth row .. .. Lam. Lam.

Lam
Lam.
lata
Lam.

vubvinervis,

[Zo face p. 743

MUTATION IN (ENOTHERA 143

generations. The net result of his experiments was
this : out of about 50,000 individuals which were grown
to a recognisable stage, more than 800 showed muta-
tion—that is to say, they differed specifically from
the parent O. Lamarckiana. The 800 individuals
belonged to about fifteen new kinds, most of which
appeared repeatedly, though some were more frequent
than others. The process of mutation had, therefore,
taken place in about 14 per cent. of the seedlings which
were grown, and owing to various reasons this estimate
is probably considerably too low. For example, many
of the new forms were very weakly, and often died
before it was possible to distinguish them. Others,
again, could not be recognised until an advanced stage
of their growth had been reached, whereas only a small
proportion of the seedlings raised could be grown after
they had reached any considerable size, owing to con-
siderations of space.

We cannot now follow de Vries very far into his
elaborate account of his new species and of the way
in which they originated ; a few general remarks only
must suffice. Many of the new forms were recognis-
able as quite young seedlings, notably O. albida,
others not until a much later period of their growth.
O. gigas was the finest and strongest of the new forms,
but only made its appearance on two occasions.
O. lata also appeared to be as strong as the parental
type, whilst two other forms were able to survive in
nature in competition with the original species, as
has been already described. Other forms which were
grown and flowered were plainly less well fitted for

144 THE THEORY OF MUTATION

the battle of life than O. Lamarckiana, and only reached
the flowering stage by the help of careful cultivation,
and others, again, were never got to flower at all.
Some of the latter, however, were readily distinguish-
able by the strikingly original types of radical leaves
which they exhibited.

When they had once made their appearance, the
majority of the new types came true to seed. Some-
times new mutations appeared among their offspring,
but these always appeared in smaller numbers than
among the offspring of the parent O. Lamarcktana, and
some of the commoner mutations were usually omitted,
so that it appeared as if the process of mutation was
accompanied by a tendency towards a fresh stability.
Some of the most marked new forms came quite true
so far as the observations were carried.

Speaking generally, the nature of the differences
which distinguished the new forms from the parental
species was just of the same type as that of those which
distinguish Jordan’s species when found in nature.
The differences were not, as a rule, of the sort shown
when new garden varieties arise as sports. An example
of this latter kind occurred, however, in the case of the
new form O. nanella, which was a dwarf or permanently
stunted form, but in other respects closely resembled
the parent type. Apart from this, the new forms
appeared to be given off quite at random, without
showing any definite tendency towards progress in a
particular direction. One of the new species was
almost sterile as far as its ovules were concerned,
though producing good pollen, whilst in another the

MUTATION IN ENOTHERA 145

formation of the pollen was very defective. None of
the others was lacking in either of these respects. Each
new form was distinguished by certain definite features
which affected almost all its parts, not by one new
character only; and these features were never separable,
but always appeared in common on the same plant.

The new species, of course, showed normal fluctuat-
ing variability, and, as an extreme result of this varia-
bility, forms occasionally appeared midway between
one of the new species and the parental type. In such
cases, when the self-fertilized seed of the plant showing
such an intermediate character was sown, the offspring
were found to group themselves round the normal
form of the new species oy round that of the parent
Lamarckiana, thus affording evidence as to the true
nature of their parent.

Whether or not we are prepared to accept the whole
of de Vries’ conclusions from his experiments, we can
see at least that from one point of view they are of the
very greatest importance. For before de Vries pub-
lished this work it had been supposed to be quite im-
possible to make direct observations upon the manner
of origin of new species in Nature. De Vries has now
shown that such observations can be made, and this
is in itself a most valuable piece of information. He
has introduced an entirely new method into the domain
of species research, and one by the use of which it is
to be hoped that before long a definite answer will be
obtained to the question whether species in general
arise by definite steps, or with an imperceptible degree

of slowness.
Bae)

146 THE THEORY OF MUTATION

When results of the novelty and importance of those
which have been published by de Vries are brought
to our notice, we are naturally disposed to reserve our
acceptance of the conclusions which they seem to indi-
cate until observations have been made in confirma-
tion of them by some competent observer. This has
now been done by Professor MacDougal at the New
York Botanic Garden. MacDougal has carried out
observations similar to those above described upon
the offspring of seeds sent by de Vries from Holland,
and with closely similar results. Thus he has observed
all the new forms which de Vries described, as well as
some additional ones ; and he has obtained an even
higher percentage of ‘mutants’ than de Vries him-
self—namely, about 3 per cent. of the total number
of seedlings grown. This last result is probably only
due to the application of more thorough methods of
investigation, and to a smaller mortality of the weakest
plants, arrived at by greater care, and rendered pos-
sible by the warmer summer climate and by American
efficiency in method. De Vries himself, in one of his
later generations, when particular care was applied to
the methods of cultivation, obtained nearly 3 per cent.
of new forms. MacDougal also states that he has
observed undoubted cases of mutation taking place .
in other species besides Eenothera Lamarckiana.

It appears, then, that there can be no doubt about
the genuineness of the phenomenon described by de
Vries. But it is, of course, quite a different thing to
assert that all natural species arise in this fashion, and
this is what de Vries’ theory, as distinguished from

MUTATION IN G@NOTHERA 147

his facts, amounts to. De Vries made observations
upon a large proportion of the plants of his district
by the method of growing great numbers of their
seedlings, but he failed to find the same phenomenon
going on in any of them. He therefore supposes that
species are subject to comparatively short periods of
mutability which recur at relatively long intervals,
and that all the species he examined except the
Cnothera were in this intermediate stable period of
their existence. Direct proof of this suggestion is
naturally out of the question.

It will be well to summarize briefly the conclusions
at which de Vries has arrived, as the result of his
observations upon Cnothera.

The following are the points to which he attaches
chief importance :

i. The new species arise suddenly at a single step,
without transitional forms.

2. They are usually fully constant from the first
moment of their origin.

3. The distinctive characters of the new forms agree
in kind with those which distinguish from one another
such old and established species allied to Gnothera
Lamarckiana as O. biennis and O. muricata. Only one
of the new forms—namely, O. nanella, a dwarf type—
is analogous with any ordinary kind of variety of
garden origin.

4. A considerable number of individuals of the same
sort usually make their appearance at the same period.

5. Although the new types vary in a normal fashion,

and frequently transgress the limits dividing them
1o—2

148 THE THEORY OF MUTATION

from the parental type, yet their first appearance has
nothing to do with normal or continuous variability.

6. The mutations take place indefinitely, showing
no special tendency in any particular direction.

7. The tendency to mutate recurs periodically. But,
as was previously stated, there is no direct evidence
of this last supposition.

In addition to what has already been said with
reference to the method of origin of garden varieties in
general, de Vries has described a number of special
phenomena regarding the behaviour of garden varieties
of plants, some of which are of considerable interest.
Taken together, the facts substantiate to a great extent
the view that selection does not of itself lead to the
production of specific characters. But de Vries also
introduces certain new conceptions which require to
be briefly described on account of their great general
interest to practical breeders and gardeners. They
consist in the idea of races existing intermediate be-
tween a species and a complete variety or sub-type of
it. Such between-races are of two kinds, of which it is
unusual to find both in the case of the same species ;
moreover, either of them may occur even when the
complete variety is quite unknown. In the case of a
haljf-race a small percentage only of seedlings is found
to produce plants which show the racial character, the
remainder being of the original specific type; and even
if the racial type is selected for several generations, the
percentage of plants of this type which is produced
does not notably increase. A mid-race, on the other
hand, can readily be improved by selection, and when

HALF RACES 149

best developed as a rule either shows the racial char-
acter in about half of the seedlings produced, or else
exhibits in the great majority of its members a com-
bination of the character of the species with that of
the race. As an example, we may take the case of
variegated plants, in which the leaves show streaks
or patches of a yellow colour owing to the want of
development of the proper green tint. An ordinary
variegated plant, then, is looked upon as showing a
combination of the green type with the yellow char-
acter of a completely modified race—the aurea variety,
although the latter exists as such only in a few rare
cases, in which the plants bear leaves showing no
green pigment at all. On the other hand, many
species of plants produce a small proportion of varie-
gated individuals at each sowing, as is often the case,
for example, with Indian corn ; and this circumstance,
according to de Vries, indicates the existence of the
corresponding half-race.

The relative development of the two coexisting
characters in such cases is highly variable, as anyone
may observe for himself in variegated grasses and
similar plants.

It might be supposed that it would be possible to
pass from the species to the half-race, thence to the
mid-race, and so on to the complete race simply by
selection. De Vries shows that this is very rarely, if
ever, the case. He regards the passage from a half-
race to a mid-race, for example, as a mutation, and his
observations seem to show that this transition is not
more frequent than any other mutations.

150 THE THEORY OF MUTATION

As a further illustration of what is meant by a
between-race, mention may be made of the five-leaved
race of purple clover (Trifolium pratense) obtained by
de Vries, and developed by a process of selection. It
would appear that the plants occasionally found grow-
ing wild, which bear a single four-lobed leaf, usually
belong only to a half-race. De Vries was fortunate
enough to find two plants upon each of which several]
of the leaves showed this anomaly, and from these, by
an elaborate process of selection extending over several
years, a race was obtained, the leaves of which in the
majority of cases showed five lobes, whilst some had
six or seven. Since, however, it appeared impossible
to get rid of a certain proportion of three-lobed leaves,
and equally so, on the other hand, to obtain leaves
with more than seven lobes, de Vries concluded that
his experiment exemplified the development of a mid-
race, and not that of a constant race or true variety.

The phenomena described by de Vries under the
head of ‘ Between Races’ are difficult to understand
until the reader has arrived at some appreciation of
the very wide differences which even slight changes in
external conditions may make in the growth and
habit of plants. We may briefly describe one very
striking example.

The little water ranunculus, which often covers areas
of stagnant water in the spring with a sheet of white
blossom, possesses two kinds of leaves, the appearance
of which entirely depends upon the stimulus of external
circumstances. In the young state the whole plant is
submerged beneath the surface of the water, and bears

FIG. 13.—TRIFOLIUM PRATENSE QUINQUEFOLIUM.
(From de Vries.)

[To face p. 150.

BETWEEN RACES I5I

leaves so finely divided or dissected into minute seg-
ments as to resemble a camel’s-hair pencil when re-
moved from the water. Sooner or later the growing
terminal bud reaches the surface, and rises above it
into the air. As soon as this happens, the rudimentary
leaves just beginning to swell within the bud entirely
‘change their course of development. They grow now
into flat-lobed blades, which float upon the surface of
the water. The change of environment from water to
air has worked such an alteration in form that no one
who was not in the secret would suppose that these
two kinds of leaves could possibly have been borne
upon the same plant.

De Vries would say that the tendency to produce
the floating kind of leaf was latent in the submerged
plant. In other words, the appearance of any given
plant, or that of any given part of it, depends partly
upon its hereditary qualities. and partly to the external
circumstances to which it is submitted. Many other
examples of similar changes could be alluded to, and
one recently described is of rather special interest to
students of genetics. This relates to a variety of
Primula sinensis, which, if kept at a temperature of
30° C., in a moist greenhouse, produces red flowers
only ; whilst under similar conditions, but at a tem-
perature of 20° C., bears only pure white blossoms.
Another variety of the same species exists which only
produces white blooms when exposed to any condi-
tions under which it will flower at all. In describing
the hereditary difference between these two varieties,
we cannot say that it consists in the former having red

152 THE THEORY OF MUTATION

flowers and the latter having white. The difference is
that the former variety has the power of producing red
flowers under certain circumstances which can be more
or less rigidly defined, whilst the contrasted type has
no such power. Thus, a few degrees difference in
temperature may determine a marked change in the
colour of a flower.

It is not absolutely certain what it is that deter-
mines the difference between the successive leaves on
a plant of the five-leaved clover, but a strong hint is
afforded by the fact that the leaves with the maximum
number of lobes only appear when the plant is at the
height of its vegetative activity. Early in the season,
and again towards the close of the growing period,
leaves with fewer lobes are produced. It would there-
fore seem as if the change in the number of lobes were
intimately connected with changes in the vigour and
rate of growth of the plant.

In the present chapter we are dealing particularly
with the variations of plants, and, indeed, de Vries
himself has never applied his views to the case of the
animal kingdom. Although animals show some direct
response in structure and functions to changes in their
surroundings, these are not usually nearly so extensive
or definite as the changes which we have just described.
The statement has even been made that one of the
fundamental differences between plants and the higher
animals lies in the much greater susceptibility of the
former to environmental changes.

The views of de Vries with regard to the actual
origin of new species may be summed up as follows:

THE THEORY OF MUTATION 153

Bioadly speaking, species arise by mutation, by a
sudden step in which either a single character or a
whole set of characters together become changed. In
the former case a new variety in the strict sense of the
word is the result ; in the latter a new species (accord-
ing to Jordan’s definition) is produced.

But mutation may be of several kinds. In the first
place, an entirely new character or set of characters
may make its appearance. To such a phenomenon
de Vries applies the term of progressive mutation, and
it is by steps of this kind that he believes the main
divisions of the vegetable kingdom to have been built
up. In the case of such mutations the new character
is supposed to come into existence first in a latent or
hidden condition, and it may be only after many
generations that it makes its appearance visibly. On
this view the period of mutation is preceded by a
premutation period, during which the appearance of
the new character is being prepared for.

A second method of species formation, entitled
by de Vries degressive mutation, is indicated when a
change takes place in the partial latency of a character.
A completely latent character is, indeed, unrecognisable
as such. But characters may also be only partially
latent, and in these cases they exhibit themselves from
time to time in rare individuals in the form of sports
or abnormalities—a phenomenon which we havealready
seen to be characteristic of half-races ; indeed, a half-
race might have been defined as a strain in which the
character of the complete race is usually latent, and
only rarely appears. An active character, on the other

154 THE THEORY OF MUTATION

hand, is apparent in the great majority of the indi-
viduals of a race. If, now, a change from latency to
activity occurs suddenly, this is a form of mutation.
The reverse case, too, may occur—a character pre-
viously active may become latent ; the character then
appears to be lost, and the mutation is said to be
retrogressive. De Vries regards the great variety of
allied species which is to be found in many groups as
being to a large extent the result of retrogressive muta-
tion. This type of mutation is also frequent among
cultivated plants. Thus, the appearance of a white
variety of a species previously only known to produce
coloured flowers may constitute a good example of a
retrogressive change. Many of the phenomena some-
what vaguely described by de Vries as cases of latency
have now received a more precise interpretation in
terms of the interaction of invisible factors in perfectly
definite ways ; these are more fully described in the
chapters on Mendelism. Finally, new and distinct
types may arise by the intercrossing of separate
species, but this is not regarded by de Vries as being
an important source of permanent new forms.
Without following de Vries into all the niceties of
his theory as to the particular kinds and methods of
mutations, we must admit that his experiments go
far to establish the doctrine, in support of which a
considerable amount of evidence had previously been
accumulated, especially by Bateson, that the origin of
species in Nature is generally a definite process, and
takes place by steps of considerable amplitude. What,
then, is the meaning of individual differences, of that

THE THEORY OF MUTATION 155

continuous variability which is often so considerable,
and of the inheritance of this kind of differences which
the biometricians have been at so much pains to prove ?
De Vries points out that for no two plants are the con-
ditions of life exactly the same ; a considerable degree
of diversity among the plants themselves is therefore
advantageous, even when these belong to the same
specific type. Upon continuous variability depend
local races, forms adapted to wetter and drier situa-
tions, highland and lowland races, and the like, but
none of these are permanent. As regards the cause
of this variability, apart from the effect of sexual
reproduction, which combines the tendency to vary
of two separate parents, de Vries believes that indi-
vidual variability depends entirely upon nutrition ;
but under this head he includes practica]ly the whole
environment of plants—light, space, soil, moisture,
and the like. Characters acquired in a similar way by
previous generations are inherited, and the effect of
conditions upon the developing seed whilst still borne
upon the parent plant may be considerable. Thus
easily does de Vries dispose of the puzzling question
of the inheritance or non-inheritance of acquired
characters. Acquired characters are inherited; they
are not of any importance in the origin of species.
With regard to the causes of mutations, little is
known. Still, it is no longer incumbent upon us, as it
was a few years ago, to admit that we know nothing at
all about the means by which this form of variation
can be produced. W. L. Tower, in his ‘ Evolution in
Chrysomelid Beetles,’ has shown that variations due

156 THE THEORY OF MUTATION

to the influence of the environment upon the larval
stages of the beetle are not inherited ; but if the female
is subjected to abnormal conditions for a few days at
the time when the eggs are maturing, the eggs and
larve being afterwards allowed to develop in the
normal environment, a greatly increased number of
mutations is obtained, the majority of which are the
same as those found much more rarely in Nature.

MacDougal, too, has met with some success in the
attempt to produce mutations artificially in plants.
In one or two cases, after injecting weak solutions of
different chemical substances into the young ovaries
of Raimannia and (Enothera, seedlings were obtained
which differed from anything previously seen. Up to
the present time these successes seem to be too few in
number to allow of any definite conclusions being based
upon them.

Blaringhem has also recently published observations
which seem to show that in the maize-plant injuries to
the parent occurring previously to the differentiation
of the germ cells may lead to permanent modifications
in the offspring. In neither of these three sets of
experiments did the modification produced in the off-
spring show any trace of an adaptive relation to the
exciting cause which operated upon the parent.

So much may be stated in order to indicate the
direction in which research is proceeding. In the
course of another decade we may reasonably hope to
find out something more about the natural and artificial
production of mutations.

THE THEORY OF MUTATION 157

According to the view upheld by Wallace, Weismann,
and others, the actual origin of specific distinctions
takes place by natural selection acting upon individual]
differences ; and in this case it is to be observed that
it is the struggle between individuals of the same species
which is of primary importance. On the mutation
theory it is only the competition between allied species
which interests us from the point of view of evolution.
Natural selection is thus regarded as having no influ-
ence in the formation of species themselves. On the
other hand, the gaps existing between genera and still
larger groups, such as families and classes, are still sup-
posed to be due to the destructive action of natural
selection determining the survival of the fittest species,
so that this principle is by no means ousted from its
prominent position in the philosophy of evolution even
when the latter is expounded by the mutationist.

One further point. On the theory of mutation the
survival of useless structures becomes readily com-
prehensible. Indeed, a structure which is actually of
the nature of a handicap to its possessor may fail to
cause extinction if it is either combined with a vigorous
constitution, or correlated with other characteristics
which are sufficiently useful to make up for the dis-
advantages entailed. The survival of many apparently
useless and some apparently harmful structures is very
difficult to understand on the hypothesis of a con-
tinuous evolution by the survival of the fittest indi-
viduals. This is an argument upon which de Vries
lays considerable stress, although it may be pointed

158 THE THEORY OF MUTATION

out that it is usually very difficult to form a judgment
as to the real usefulness or otherwise of organs.

BIBLIOGRAPHY.

Bateson, W.: Materials for the Study of Variation, 1894.
Vries, H. bE: Die Mutationstheorie, 1901.
J Species and Varieties, their Origin by Mutation,
IQ05.
MacDoucaL, D. T.: Various papers published by Carnegie
Institution.
Tower, W.L.: An Investigation of Evolution in Chrysomelid
Beetles of the Genus Leptinotarsa, 1906.

JOSEPH GOTTLIEB KGLREUTER, 1733-1806.

(After an engraving by J. CEDERQUIST. )

[To face p. 159.

CHAPTER VI

THE OLDER HYBRIDISTS

THERE is one side of the practical study of heredity
-which dates back to the middle of the seventeenth
century—namely, that branch of the subject which is
concerned with the hybridizing or artificial cross-
breeding of different species and varieties of plants.
Quite recently the great importance which attaches
to this method of study has been realized once more,
and the interest thus awakened has led to a closer
examination of the accounts of experiments under-
taken a century or more ago, with the result of showing
that much of the work then carried out in this direc-
tion had attained to quite an astonishing degree of
excellence, In the brief sketch of the history of
hybridizing work here following, account will be taken
almost exclusively of experiments of which the in-
terest is not historical only, but which possess an
actual scientific value. Amongst other matters of
interest, it will be found that more than one observer
came very near to anticipating Mendel’s epoch-
making discovery, and thus arriving at the clue which
should unravel almost all the complex problems which
beset the early hybridizers.
159

160 THE OLDER HYBRIDISTS

Following the modern usage, we shall apply the
term ‘hybrid’ to all individuals arising from a cross
between parents which belong to distinct groups, no
matter whether these groups are separated as distinct
genera or species, or whether they are regarded as
representing only different races or varieties. This
wide interpretation of the term hybrid has only re-
cently been reintroduced. The use to which it has
returned is, indeed, the original one ; but many inter-
mediate writers, including Darwin, confined the em-
ployment of this expression to cases of crossing between
species, and applied the word ‘ mongrel’ to the off-
spring of crosses between races or varieties of the same
species. Darwin, however, did not regard species as
differing in kind from varieties, and he even particu-
larly emphasized the smallness of the distinction which
can be drawn between the behaviour and properties
of hybrids and mongrels respectively. Indeed, he
came to the highly important conclusion that the laws
of resemblance between parents and their children are
the same, whatever may be the amount of difference
between the parents in question—whether, that is to
say, they are distinguished only by individual differ-
ences, or whether they belong to separate varieties or
even species. We have already seen that the more
recent facts of biometry point strongly towards the
conclusion that individual and race differences are
inherited at approximately the same rate. It seems,
however, to be at present somewhat doubtful whether
all sorts of specific differences follow the same law of
propagation on cross-breeding.

KOLREUTER 16

Between 1760 and 1766 Joseph Gottleib Kélreuter
carried out the first series of systematic experiments
in plant hybridization which had ever been under-
taken. These experiments not only established with
certainty for the first time the fact that the seeds of
plants are produced by a sexual process comparable
with that known to occur in animals, but also led to a
knowledge of the general behaviour of hybrid plants,
which was scarcely bettered until Mendel made his
observations a century afterwards.

Kélreuter found that the hybrid offspring of two
different plants generally took as closely after the
plant which yielded the pollen as after that upon
which the actual hybrid seed was born. Indeed, he
found that it made little or no difference to the ap-
pearance of the hybrid which of the parental species
was the pollen-parent (male), and which the seed-
parent (female)—that is to say, in the case of plants
the result of reciprocal crosses is usually identical.
Thus, for the first time it was definitely shown that
the pollen-grain plays just as important a part in
determining the characters of the offspring as does the
ovule which the pollen-grain fertilizes. This was a
wholly novel idea in Kélreuter’s time, and the fact was
scarcely credited by his contemporaries.

Kélreuter had no means of discovering that the
contents of a single pollen-grain unite with the con-
tents of a single ovule in fertilization. But he ascer-
tained by experiments that more than thirty seeds
might be made to ripen by the application of between
fifty and sixty pollen-grains to the stigma of a par-

II

162 THE OLDER HYBRIDISTS

ticular flower, so that, if he had had any hint of the
actual microscopic processes of fertilization, he would
have been quite prepared for the more fundamental
discovery.

KGlreuter, indeed, believed that the act of fertiliza-
tion consisted in the intimate mingling together of two
fluids, the one contained in the pollen-grain, and the
other secreted by the stigma of the plant. The mingled
fluids, he supposed, next passed down the style into
the ovary of the plant, and arriving at the unripe
ovules, initiated in them those processes which led to
the formation of seeds. In this belief Kélreuter simply
followed the animal physiologists of his time, who
looked upon the process of fertilization in animals as
taking place by a similar mingling together of two
fluids. Now that we know that fertilization consists
essentially in the intimate union of the nuclei of two
cells, one of which, in the case of plants, is the ovum
contained within the ovule, whilst the other is repre-
sented by one of a few cells into which the contents
of the pollen-grain divide, we can understand more
clearly the bearing of KGélreuter’s observation. And
it is greatly to this eminent naturalist’s credit that he
succeeded in carrying out his observations with so
much accuracy, when the full meaning of those
observations was of necessity hidden from his com-
prehension.

Kélreuter was the first to observe accurately the
different ways in which pollen can be naturally con-
veyed to the stigma of a flower. This may take place
either by the pollen-grains falling directly upon the

KOLREUTER 163

stigma, or by the agency of the wind, or, lastly, the
pollen may be carried by insects visiting the flowers.
And he recognised many features characteristic of
flowers apt to be fertilized in one or other of these
ways in particular. Thus he was aware, for example,
of the nature and use of the nectar which so many
flowers produce—namely, that it is the substance from
which the bees—by far the most diligent visitors of
flowers—obtain their honey.

Curiously enough, Kolreuter was not aware of the
existence of any natural wild hybrid plants. But he
was quite right in contending that supposed examples
of such hybrids required for their substantiation the
experimental proof, which could only be afforded by
making actual artificial crosses between the putative
‘parent species.

The first hybrid made artificially by Kélreuter was
obtained in 1760 by applying the pollen of Nicotiana
paniculata to the stigma of Nicotiana rustica. The
hybrid offspring of this cross showed a character inter-
mediate between those of the two parent species in
almost every measurable or recognisable feature, with
a single notable exception. This exception was
afforded by the condition of the stamens and of the
pollen grains produced by the hybrids. These organs
were so badly developed that in all the earlier experi-
ments self-fertilization of the hybrid plants yielded no
good seed at all, nor were the pollen grains of the
hybrid any more effective when applied to the stigmas
of either of the parent species. On the other hand,

when pollen from either parent was applied to the
II—2

164 THE OLDER HYBRIDISTS

stigmas of the hybrid plants, a certain number of seeds
capable of germination was obtained, although this
number was much smaller than in the case of normal
fertilization of either parent species. This partial
sterility, affecting in particular the stamens and the
pollen which they produce, is a feature common to the
majority of hybrids between different natural species.
Many such hybrids, indeed, are altogether sterile, so
that a further generation cannot in any way be ob-
tained from them. On the other hand, the members
of different strains or varieties which have arisen under
cultivation yield, as a rule, when crossed together off-
spring which are perfectly fertile.

In subsequent years Kélreuter was able to obtain a
very few self-fertilized offspring from hybrids of the
same origin as the above. The resulting plants were
described as resembling their hybrid parent so closely
as to be practically indistinguishable from it.

The offspring obtained by crossing the hybrid plants
with pollen from either parent showed in each case a
form more or less intermediate between that of the
original hybrid and that of the parent species from
which the pollen was derived. But the plants were
not all alike in this respect, some of them being much
more like the parent species than others, and some,
again, varying in other directions. There were also
considerable differences between the different indi-
viduals in respect of fertility, so that some of the
plants were more and some less sterile than the original
hybrids. Also, there was some tendency to the produc-
tion of malformations of the flowers and other parts.

KOLREUTER 165

One of the most noted of Kélreuter’s experiments
was that which consisted in repeatedly recrossing a
hybrid plant with one of the parent species from which
the hybrid was derived. By continuing to pollinate
the members of one generation after another with the
pollen of the same parent species, plants were at last
arrived at which were indistinguishable from the parent
in question. We shall return to this fact later on,
when the reader will be in a position to appreciate its
importance more fully.

Koélreuter found that the result of reciprocal crosses
is usually identical—that is to say, the offspring ob-
tained by fertilizing a plant A with pollen from a
plant B are not to be distinguished from those ob-
tained when B is fertilized with the pollen of A. But
the two opposite processes of fertilization are not
always equally easy to carry out. An extreme instance
of this circumstance was met with in the case of the
genus Mirabilis. Mirabilis jalapa was easily fertilized
with pollen from M. longiflora. During eight years
K6lreuter made more than two hundred attempts to
effect the reverse cross, but without success.

It was shown by Kélreuter that hybrids between
different races or varieties of the same species are
usually much more fertile than hybrids obtained by
crossing distinct species. Indeed, he believed that
varieties of a single species were in all cases perfectly
fertile together, whilst hybrids between species always
showed some degree of sterility. But in this case Kél-
reuter based his definition of a species upon the very
point at issue, and when he found forms, which other

166 THE OLDER HYBRIDISTS

botanists regarded as good species, to be perfectly
fertile together, he immediately regarded them as
being only varieties of a single species.

One curious point is worth quoting in this connec-
tion. Five varieties of Nicotiana tabacum were found
to be perfectly fertile with one another, but when
crossed with Nicotiana glutinosa one of them was
found to be distinctly less sterile than the rest.

Another interesting point observed by Kolreuter
was the fact that hybrid plants often exceed their
parents in luxuriance of growth. Upon this fact, as
we shall see later on, Knight and afterwards Darwin
based theoretical conclusions of considerable impor-
tance in connection with the problem of sex.

Thomas Andrew Knight, who was also a botanist of
high reputation in other fields, was the earliest observer
to lay stress upon the practical aspect of the study of
hybrids, and he occupied himself to a considerable
extent with the improvement of useful races of plants
by cross-breeding. Breeders of animals had already
made important improvements by the method of inter-
crossing different races, and selecting the most notable
types which made their appearance in consequence,
when Knight bethought him of applying the same
principles to the improvement of plants, and particu-
larly of fruit-trees.

Knight also carried out a series of experiments with
domestic peas, the results of which were published in
1779. These experiments have a particular interest
from the historical point of view, since it was by dint
of similar experiments upon the same kind of plants

KNIGHT 167

that Mendel’s law was afterwards discovered. This
very discovery might even have been made by Knight
himself, if he had only realized the importance of ascer-
taining on a large scale the numerical proportions in
which the different kinds of plants, arising in the
second generation from the crosses, made their appear-
ance. Unfortunately, this particular form of inquiry
never seems to have occurred to him.

Knight’s experiments were made with a different
object in view—namely, that of discovering whether
a cross with a distinct race would provide the stimulus
necessary to restore its lost vigour to a strain of plants
which was supposed to have become debilitated, owing
to its members having been bred exclusively by self-
pollination for a long series of generations.

The result of the experiments undoubtedly estab-
lished the fact that in some cases the hybrid offspring
of two distinct races shows a more vigorous habit of
growth than either of the parental types. The follow-
ing extract from Knight’s own account will indicate
the nature of the experiments upon which his con-
clusions rest : .

‘ By introducing the farina of the largest and most
luxuriant kinds into the blossoms of the most diminu-
tive, and by reversing this process, I found that the
powers of the male and female, in their effects on the
offspring, are exactly equal. The vigour of the growth,
the size of the seeds produced, and the season of
maturity, were the same though the one was a very
early, the other a very late variety. I had in this
experiment a striking instance of the stimulative effects

168 THE OLDER HYBRIDISTS

of crossing the breeds, for the smallest variety, whose
height rarely exceeded two feet, was increased to six
feet, whilst the height of the large and luxuriant kind
was very little diminished.’

We shall see, however, that the phenomenon last
alluded to admits of a different interpretation.

It was upon the somewhat slender basis afforded
by this experiment that the generalization known as
the Knight-Darwin law was originally established.
Knight’s own expression of this idea was to the effect
that ‘Nature intended that a sexual intercourse should
take place between neighbouring plants of the same
species.” And the same conclusion was expressed still
more forcibly by Darwin in the aphorism : ‘ Nature
abhors perpetual self-fertilization.’ But although it
may be true that in a considerable number of cases
advantages are gained from the process of cross-
fertilization between different members of the same
species, which do not accrue when self-fertilization
takes place, yet several cases are now known in which
self-fertilization really does seem to be indefinitely
continued.

Knight crossed a pea having white flowers and seed-
coats, and green stems, with one in which the flowers
and stems were coloured purple, and the seeds grey.
The seeds immediately resulting from the cross were
unchanged in appearance, but the plants arising from
these took closely after their coloured male parent.
On crossing the cross-bred plants once more with a
white strain a certain proportion of white plants was
again obtained, though what that proportion was

KNIGHT 169

Knight failed to notice. He observed, however, that
white crossed by a purple strain invariably gave purple,
whilst the cross-bred purples, when crossed again with
white, yielded some white and some purple plants.

In 1822 John Goss recorded the fact that a ‘ blue’
pea crossed with a ‘white’ yielded from the crossed
flowers pods with white seeds only, the seeds contained
in other pods upon the same plant being, of course,
blue. The plants produced from the white seeds bore
some pods with all blue, some with all white, and many
pods with both white seeds and blue ones; and a
coloured plate is given which shows one of the latter
pods together with its contents. The blue seeds, when
sown separately, yielded plants which produced blue
seeds only, but plants arising from the white seeds
yielded a mixture of blue and white seeds.

Knight pointed out quite correctly that the colours
of the seeds which are here referred to are occasioned
by the colour of the cotyledons or seed-leaves of the
pea, which are visible through the semitransparent
seed-coat. Green cotyledons give rise in this way to
a bluish appearance, whilst, when the cotyledons are
yellow, the resulting appearance of the seed is described
as whitish.

The Hon. and Rev. W. Herbert was another observer
who made many important experiments in hybridiza-
tion towards the beginning of the nineteenth century.
These led him to the conclusion that Ké6lreuter and
Knight were wrong in their assertion that hybrids
between distinct species were always sterile. Herbert
considered that only generic or family types were

170 THE OLDER HYBRIDISTS

constantly sterile, and this led him to the further
conclusion, now believed to be erroneous, that the
separate genera or families were those which were
originally created, whilst he believed that the separate
species of the same genus arose from a single original
type by a genuine process of evolution.

The most prolific in work of all the hybridists, how-
ever, was undoubtedly Carl Friedrich v. Gaertner
(1772-1850). Gaertner made a great number of
crosses between species belonging to all sections of
the natural system, and his book, published in 1849,
contains a great mass of valuable information. Gaert-
ner’s theoretical conclusions, for the most part, only
amplify and confirm those of Koélreuter, upon whom
in this direction he made but little advance.

C. Naudin’s essay, entitled ‘New Researches on
Hybridity in Plants,’ made its appearance in 1862.
The author pointed out that the facts of the return
of hybrids to the specific forms of their parents, when
repeatedly crossed with the latter, are naturally ex-
plained by the hypothesis of the disjunction of the
two specific essences in the pollen grains and ovules
of the hybrid. The idea may, perhaps, be made some-
what clearer as follows : Let us consider the case of a
species A crossed with another species B. Naudin
supposes that some of the pollen grains and ovules of
the hybrid plant will be potentially* of the exact

* When it is said that a pollen grain or ovule potentially
resembles the species A, it is meant that the germ-cell in
question is of such a kind that, when united with one derived
from an ovule or pollen grain of similar constitution, it would
give rise to a plant exactly resembling A.

NAUDIN 171

character of one species (A), whilst others will bear
no potential resemblance to A, but will be precisely
similar in nature to the ovules and pollen grains of
the pure species B. In cases where this separation of
the materials representing the two types in a potential
condition is complete, forms exactly resembling the
parents might be obtained. As we shall see, this
hypothesis makes a remarkably near approach to that
of Mendel ; and the importance of the fact that the
first hybrid generation is generally uniform, as con-
trasted with the diversity of types often appearing
in the second generation, is clearly recognised by
Naudin. This observer considered the hybrid in the
adult state to consist of an aggregate of particles,
homogeneous and characteristic of a single species
when taken separately, but mingled in various pro-
portions in the organs of the hybrid, which is thus
looked upon as a kind of living mosaic.

The only other discovery of first-class importance,*
in addition to that of Mendel, made during the nine-
teenth century in the domain of hybridization, was
that published by Millardet in 1894.

Millardet’s principal experiments were made upon
strawberries, of which plants he crossed together a
number of different species and varieties. Contrary
to what had been observed in the majority of such
crosses between other specles of plants, in which the
offspring was usually more or less intermediate be-
tween the two parents from which it arose, Millardet

* That is to say, if it is really genuine. The phenomena
do not appear to have been seen by anyone else.

172 THE OLDER HYBRIDISTS

found in a considerable number of cases that the off-
spring resembled one parent only, from which it was
indeed indistinguishable, whilst no trace of likeness
to the second parent could be detected in it. In some
cases the resemblance was to the paternal species
(pollen-parent), and in others to the maternal species
(seed-parent). In several instances the hybrid off-
spring, on being self-fertilized, bred true to the type
which they already exhibited, so that the second
generation, like the first, seemed to derive its whole
constitution from one parent, to the total exclusion
of the other.

The precise meaning of this remarkable phenomenon
is not clearly understood. There is some doubt as to
whether Millardet’s experiments were really sufficient
to establish it as a scientific fact. Moreover, Millar-
det’s observations have never been confirmed by later
workers. In the absence of directly contradictory
evidence it seemed necessary to draw attention to the
facts as they have been described.

Great numbers of observations upon the character-
istics and behaviour of hybrid plants and animals have
been from time to time recorded, and the preceding
pages contain only a brief selection of such facts as
are most necessary for a proper understanding of
modern work in hybridization. Until quite recently
the laws of transmission of characters in hybrids were
still completely hidden. The facts were wonderful
enough, but they showed no signs of falling into an
orderly arrangement. In the next chapter it will be
our business to describe the remarkable discovery

THE OLDER HYBRIDISTS 173

which has introduced order into this previously chaotic
region, and which has enabled a few workers to estab-
lish in half a dozen years the foundations of a great

science, the importance of which is not at all generally
realized.

BIBLIOGRAPHY.

K6LREUTER, J. G.: Vorlaufige Nachricht von einigen das
Geschlecht der Pflanzen betreffenden Versuchen und
Beobachtungen, 1761.

GAERTNER, C. F. von: Bastarderzeugung, 1849.

Focke, W. O.: Die Pflanzen-Mischlinge, 1881.

Wicuura, M.: Die Bastardbefruchtung im Pflanzenreiche,
1865.

Dasa C.: The Effects of Cross and Self Fertilization.

CHAPTER VII

MENDELISM

WE have already had occasion to point out how im-
portant it is, when engaged upon questions of heredity,
not to treat whole animals or plants as units, but to
deal with their separate characters one at atime. In
the course of the present chapter the reason for pro-
ceeding in this way will appear more clearly, and we
shall find that the adoption of this method is fully
justified by the results which it enables us to obtain,
and which could not have been arrived at in any other
way. We shall also find reasons for believing that
this method is the correct one from a theoretical point
of view.

Naturally, considerable care is necessary in deter-
mining what are and what are not separable characters.
At the outset it is not always possible to make this
discrimination with certainty, but during the course
of the experiments which follow it is almost always
possible to arrive at a clear definition of each character,
and in many cases the distinction of characters is quite
obvious from the beginning.

Up to the present time the experimental study of
heredity by the methods of definite breeding has yielded

174

GREGOR JOHANN MENDEL.

[Lo face p. 174.

MENDELISM 175

clear and definite information only when applied to
cases where clearly definable characters have distin-
guished the parental forms examined. This, however,
is in great part due to the fact that the experimental
method has scarcely yet been used in dealing with
characters of a less definite nature. The science is
still in its infancy, and attention has naturally been
first paid to the simpler problems which it affords.
The difficulties of treatment which confront those who
would deal with highly variable characters and those
of a ‘more or less’ nature are considerable, although
there is no reason for supposing that such problems are
insuperable. As we have seen, however, the majority
of characters which distinguish species or races from
one another appear to be of a perfectly definite descrip-
tion, so that the limitation just referred to is not so
serious as might appear at first sight. The recent
revival of work upon the subject of inheritance by
the use of breeding methods has, as a matter of fact,
already been rewarded. with results as valuable and
as clear as could possibly have been anticipated—
results which are sufficient in themselves to show that
the discovery made by Mendel was of an importance
little inferior to those of a Newton or a Dalton.

It is important to remember that every animal or
plant, which has come into existence in the ordinary
way through sexual generation, owes its individuality
to the mingled natures of two separate parents. The
following lines, in which the poet Goethe speaks of
his own hereditary endowment, have been quoted
more than once in this connection ;

176 MENDELISM

Vom Vater hab’ ich die Statur,

_ Des Lebens ernstes Fuhren,
Vom Mutterchen die Frohnatur
Und Lust zu fabulieren.’

In such a case we must always look upon the corre-
sponding character of the second parent as existing
in the offspring side by side with the character which
finds expression, only the former is overpowered by
the latter, and forced to remain invisible. That the
hidden character is nevertheless actually present is
shown by the fact that a feature characteristic of a
particular grandparent, which did not appear in the
parent, may reappear in the child. For instance, a
characteristic masculine feature of the maternal grand-
father may thus make its appearance in the son.

It is found that any individual may be looked upon
as being to a large extent an aggregation of separate
characteristics. In a pair of allied races a great
number of the separate characters are the same in
the two cases, the distinction between the two forms
depending upon a few definite features only. The
majority of salient characteristics are identical in such
a pair, but some of the corresponding characters are
opposed. Thus in different races of mankind com-
plexions may be dark or fair, eyes blue or brown,
hair straight or curly, and the like. Now the off-
spring of parents which had smooth and curly hair
respectively might exhibit smooth or curly or inter-
mediate (wavy) hair, according as one or the other
character, or both in combination, made its presence
obvious ; for in the simplest cases both will necessarily

UNIT CHARACTERS 177

be present, though one may be hidden. What will
happen in the grandchildren ?

The manner in which characters comparable with
the above are actually transmitted has been worked
out in the case of many races of animals and plants,
and in cases where experimental matings can be
readily carried out, and a large number of offspring
reared, it is found that a simple rule applies which
holds good in every example thoroughly examined
hitherto. This law was discovered by Mendel about
the year 1865, and has since been called by his name.
Before enunciating it we shall consider the informa-
tion afforded by the case of a single pair of simple
characters. Afterwards we shall endeavour to show
the application of the law to the more complex cases
in which combinations of characters are concerned.

A grain of Indian corn or maize contains a germ or
embryo, which under suitable conditions will give rise
to the future plant. The embryo is surrounded by a
certain amount of reserve food material constituting
the endosperm—a store which is made use of by the
young plant during its germination. The embryo
arises as the result of a process of fertilization which
takes place in the following manner: The ovum, or
female cell hidden in a flower, contains a nucleus, and
this on fusion with one of the nuclei derived from a
grain of pollen initiates the vital processes which lead
to the development of an embryo plant.

Nuclei are the central and, from the point of view of

heredity, the most important parts of cells—the con-
I2

178 MENDELISM

stituent units of the plant body. The cells which,
together with their nuclei, take part in the process of
fertilization are known as gametes, or germ-cells—male
and female respectively, the latter being the ovum.

It is less generally known that the endosperm of a
grain of Indian corn arises by a very similar process
to the one which gives rise to the embryo itself. A
second nucleus derived from the same pollen grain
fuses with a nucleus situated near the ovum, and to
the product of this fusion the endosperm owes its
origin. It is further found, so far at least as those
characters are concerned to which we shall at present
confine our attention, that these two important nuclei
hidden in the same female flower are exactly alike in
hereditary constitution, and so are the two generative
nuclei derived from a single pollen grain. In conse-
quence of this fact, the observed character of the endo-
sperm may be regarded as a true guide to the nature
of the plant into which the associated embryo will
afterwards develop. The hereditary qualities of the
two are exactly the same.

It is not difficult to find a variety of Indian corn in
which the endosperm is yellow, and another in which
the colour of this tissue is white, owing to the absence
of any visible yellow pigment. If a female flower of a
white variety is fertilized with pollen taken from a
yellow variety, the resulting grain shows its hybrid
nature by the presence of the yellow colour in its endo-
sperm. This is found to be a regular rule. Grains
upon a plant of a white strain which has been pollinated
with ‘white pollen’ are white, but if pollinated from

INHERITANCE IN MAIZE 179

a yellow strain the grains are yellow. On the other
hand, the grains upon a plant belonging to a yellow
strain retain their yellow colour even if the flowers
which produce them have been pollinated from a white
variety.

These facts are expressed in technical language by
saying that yellowness is dominant over whiteness, and
the latter is said to be recessive.

Let us now suppose that we have sown a number
of the yellow grains derived from the cross yellow x
white* or white x yellow, and that we have exposed
the female flowers of the resulting plants at the proper
stage of their existence to the influence of pollen
derived from a pure white strain, taking care that
none of their own hybrid pollen falls upon them at
the same time. The result of this experiment takes
us at once to the very heart of the Mendelian theory.
Half the total number of grains obtained in this way
—from the cross (white x yellow) x white—are white,
and half are yellow.

Thus in an experiment carried out in the manner
described there were obtained upon ninety-five plants :
Yellow grains 26,792, or 50°03 per cent.

White grains 26,751, 5, 49°97 iy

But we must go further than this. On sowing the
white grains obtained in this second generation (F,),
and allowing the plants obtained from them mutually
to pollinate one another, cobs were obtained bearing
exclusively white grains without any trace of yellow-
ness.

* x is to be read ‘ fertilized with pollen from.’
I2—2

180 MENDELISM

Half the grains, then, of the parentage (white x
yellow) x white are pure white in colour, and not to be
distinguished from grains of the parentage white x white
even after an extensive examination of their offspring,
which is the most rigorous test we are able to apply.

The yellow grains born upon the same hybrid plants
(F,)* had clearly each of them one white parent—
namely, the plant from which the white pollen was
derived. On sowing these yellow grains and once
more pollinating by pure white, a precisely similar
result was observed to that obtained in the preceding
generation—that is to say, these plants, derived from
yellow grains, produced once more 50 per cent. of
white grains and 50 per cent. of yellow. We are,
therefore, led to suppose that the yellow grains born
upon the hybrid plants are of precisely the same nature
as the original yellow hybrid grains (white x yellow),
since their behaviour when pollinated from the same
white strain is identical. We may express the result
so far obtained in the form of the following diagram ;

p* white x yellow
F, yellow x white
| | ;
F, white x yellow (50 %) (50 %) white x white
| ) |
F,; white(50%) yellow (50 %) white (exclusively).

* The following shorthand expressions are adopted to denote
the different generations in cross-breeding experiments: P is
the generation of the original parents ; F, is the first genera-
tion of offspring—the cross-bred seeds and the plants to
which they give rise. To the F, generation belong the seeds
produced upon the F, plants, and the plants to which they
give rise, and so on.

INHERITANCE IN MAIZE 181

The pollen of the F, plants (¢.e., those plants which
were derived from the yellow cross-bred grains)—when
applied to the female flowers of the same pure white
strain of maize, caused in like manner the appearance
of white and yellow grains in equal numbers. This
result is equally well expressed by the above diagram
on simply regarding the yellow in F, as the male parent
(pollen-parent) instead of as the female parent (seed-
parent) of F,.

What is, then, the meaning of these results? The
case is really very simple. The germ-cells (ova and
pollen-nuclei) of the cross-bred plants (white x yellow)
must be potentially either pure white or pure yellow,
with no blending of these characters. Further, the
two kinds (yellow and white) of male germ-cells or
pollen-nuclei must arise in equal numbers, and the
same must be true of the female germ-cells or ova.
By this supposition only can the observed facts be
explained. If the supposition is true, then, when the
cross-bred plant (F,) is crossed again with the pure
white form, its white germ-cells give rise to white
grains which are of the nature (white x white), and
are therefore pure. Its yellow germ-cells give rise to
yellow grains which are of the nature (yellow x white).
And, since the number of yellow- and white-bearing
germ-cells is equal, the number of yellow and of white
grains produced in this way is approximately the same.
The yellow grains are of the same composition as the
original cross-bred grains obtained by crossing pure
white with pure yellow, and we have seen that they
behave in exactly the same way on further cross-

182 MENDELISM

breeding. This conclusion is at least so far firmly
established that no alternative hypothesis has been
put forward which will explain the facts.

We have next to consider what will be the result of
crossing our cross-bred plants with one another instead
of with the pure white form. The following possi-
bilities present themselves :

A yellow female gamete may pair with a yellow male gamete.
” ” ” ” white ”

”
A white 3 ss ” » yellow -

” ” ” ” ” white ”

All these combinations are equally likely to occur,
because in each plant there are the same number of
yellow and white female gametes as well as of yellow
and white male gametes. In the long-run, therefore,
each of the above pairings will be found to have taken
place in an equal number of cases. The grains which
we shall obtain, then, will be yellow and white in colour,
and the two kinds will occur in the following propor-
tions: I pure white; 2 white x yellow or yellow x
white, which, as we have already seen, will be yellow
in appearance ; and 1 pure yellow. Altogether, we
shall expect a ratio of 3 yellow grains to 1 white.

In an actual experiment the following result was
obtained :

Yellow grains 16,592, or 74'5 per cent.
White 4, 5,681, 4, 25°5 ‘a
—that is to say, a ratio of 2:9 yellow to 1 white.

The expression 14 :2Aa:i1a, in which A represents
the dominant character (yellow) and a the recessive
character (white), may be spoken of as a Mendelian

MONO-HYBRID CROSSES 183

formula. It indicates the proportion in which the
two pure types and their hybrid brethren will appear,
on breeding together the offspring of a simple or
mono-hybrid cross—t.e., one in which attention is
paid to the behaviour of a single pair of characters only,

So far we have been dealing with a pair of characters
consisting in the presence and absence respectively of
a particular pigment. Precisely similar results are to
be obtained in the case of a pair of structural char-
acters. The endosperm, or reserve substance, of cer-
tain varieties of Indian corn shows a smooth surface,
and contains an essentially starchy reserve material,
whilst in other races the surface of the endosperm is
wrinkled and the reserve product is of a sugary nature.
This sugary endosperm is characteristic of the kinds
of corn largely used in the United States of America
as a table vegetable.

On crossing together a variety with smooth starchy
grains and one with wrinkled sugary grains, the grains
immediately resulting are smooth and starchy, no
matter whether the starchy strain is used as the seed-
parent oras the pollen-parent—that is to say, thestarchy
character is dominant, a dominant character being one
which appears in F, to the complete or almost com-
plete exclusion of the corresponding character ex-
hibited by the other parent, which is spoken of as
recessive. Inthe present case the sugary character is
recessive.

The further behaviour of the cross between smooth
and wrinkled is precisely the same as that of yellow

184 MENDELISM

crossed with white. Thus, if the hybrid plants are bred
together or self-fertilized, the resulting cobs will exhibit
a proportion of three smooth grains to one wrinkled
grain. Inan actual example there were obtained 5,310
smooth grains and 1,765 wrinkled, or 75°06 per cert.
of the former and 24°94 per cent. of the latter.

In a further generation the wrinkled grains breed
true. On the average one out of every three smooth
grains does thelike. Theremaining twosmooth grains
are of hybrid nature, and on self-fertilization yield
again the same proportion of three smooth to one
wrinkled. Such hybrid grains and the plants into
which they develop are spoken of as heterozygotes.

Thus, if we write B for smooth and 0 for wrinkled,
the following scheme will express the result of crossing
together plants which bear these characters, and after-
wards self-fertilizing the offspring obtained ;

P (dominant character) B x 4 (recessive character)
Fy

Bob
(heterozygote looking like 2)
on self-fertilization yields

F, BB Bb 6B bb
(extracted (heterozygotes looking like BB) (extracted
dominant) on self-fertilization each gives recessive)

| | | |

Bs BB BB Bb 6B bb bb

(extracted —S (extracted
dominant) and so on recessive)

Fy BB BB bb bb

and so on, and so on, and soon, andsoon,
breeding breeding breeding breeding

true. true. true. true.

DI-HYBRID CROSSES 185

So far we have seen that both a pair of structural
characters and a pair of colour characters can
‘Mendelize,’ according to the phrase coined by the
Germans—that is to say, the germinal representa-
tives of such pairs of characters remain perfectly
distinct in the hybrid plant, and separate completely
at the formation of its gametes, in such a way that
an equal number of gametes arises containing either
character.

The members of a pair of characters which behave
in this way on crossing are called allelomorphs. When
a pair of gametes fuse together in the process of fer-
tilization the resulting cell is known as a zygote. A
zygote formed by the conjunction of two like gametes
is called a homozygote. When the gametes contain
opposite members of a pair of allelomorphs the result
is called a heterozygote. The same terms may also be
applied to the adult multicellular organisms into which
these fertilized egg-cells develop.

We have still to consider what happens when parents
are crossed which differ in more than one pair of allelo-
morphs. The actual result is as follows:

Suppose a smooth yellow type of maize to be crossed
with a wrinkled white variety, both smoothness and
yellowness being dominant. The grains produced in
F, are therefore yellow and smooth. Let the F, plants,
arising from the smooth yellow heterozygote grains,
be crossed with the wrinkled white parent, which
is recessive in respect of both these characters.
In this way the true nature of every germ cell
produced by the heterozygote will be able to manifest

186 MENDELISM

itself in the visible character of the grain produced
from it.

The following result was actually obtained in this
way :

Smooth yellow grains 2,869, or 25°3 per cent.
Smooth white grains 2,933, or 25°7 5
Wrinkled yellow grains 2,798, or 24°5 yy
Wrinkled white grains 2,803, or 24°54,

Thus we see that a nearly equal number of the germ
cells of the double heterozygote bears each of the four
possible combinations of characters—that is to say, it
is an even chance whether a particular gamete, which
bears the allelomorph yellowness, bears also smoothness
or wrinkledness. In other words, the two pairs of
allelomorphs segregate in entire independence the one
of the other. It is particularly to be noticed that we
arrive in this way at two perfectly new combinations
of characters, which were not shown by the original
parent strains. We have synthesized two new sorts
of maize with smooth white and wrinkled yellow grains
respectively. In a precisely similar way, if the cross
is made between strains of which the grains are re-
spectively smooth white and wrinkled yellow, we
should obtain in F, the new combinations smooth
yellow and wrinkled white.

The result obtained on self-fertilizing the hybrid
plant is somewhat more complicated.

If we write A for yellowness, a for whiteness, B for
smoothness, and 6 for wrinkledness as before, AB x ab
gives the heterozygote ABab. Equal numbers of the
germ cells of the heterozygote will be of the composi-
tions AB, Ab, aB, and ab.

Be,

ff 'p " fy e+ a

é
Ue ivetpy:

~
.
*

ee

6 OF

bait

Fic. 14.--MENDELIAN PROPORTIONS IN MAIze.

Cobs born by heterozygote plants pollinated with the recessive, showing

equality of smooth and wrinkled and of coloured and white grains.
(To face p. 186.

DI-HYBRID CROSSES 187

All the following zygotic combinations are, then,
equally likely :

ABAB ABAb ABaB ABab
ALAB AbAb AbaB Abab
aBAB aBAb aBaB aBab

aAB abAb abaB abab

Altogether there are sixteen combinations. The
result can be expressed more shortly in the form
(A +2Aa+a) (B+2Bb +b),* which will be found to give
the above terms when expanded. Thus the combina-
tion of the Mendelian formule for F, when each of
the pairs of allelomorphs is considered separately,
gives us the formula for the two pairs of allelomorphs
considered simultaneously.

The same result may also be written in the form :

4A IB 2 Bb 16
8 Aa [ combined with } 2B 4 Bb 26

4a 1B 2 Bo 1d
or
AB 2 ABS Ab
2 Aab 4 AaBb 2 Aad
aB 2 aBb ab*®

* It is customary to condense these expressions as far as
possible by never repeating the same letter more than once
in each term. Thus, A stands for AA, B for BB, and so on.
On expansion, 7.¢., multiplying together the contents of the two
brackets, AxB gives ABAB, AxBb gives ABA}, and so on
for all the other terms of the expression.

188 MENDELISM

Let us consider the external appearance of these
various types in the particular example before us.

Nine of the above sixteen terms include A and B,
and are therefore smooth yellow in appearance. (We
need not stop to consider whether a or b or both are
present in addition, since these are recessive.)

Three terms include A and 0, B being absent. These,
therefore, appear wrinkled yellow.

Three include a and B, A being absent. These,
therefore, appear smooth white.

One contains 4 and 0 only, and is, therefore, wrinkled
white.

With regard to internal constitution :

The nine individuals of appearance AB include the
following types :

One pure, ABAB, breeding true to the smooth
yellow type on self-fertilization.

Two ABAD, heterozygous in respect of the
pair B-b, but pure yellow.

Two ABaB, heterozygous in respect of A-a,
but pure smooth.

Four ABab, heterozygous in respect of both
pairs of characters.

The three individuals of appearance Ab include the
following types :

One pure, AbAd, breeding true to the (new)
wrinkled yellow type.

Two Abad, giving both wrinkled yellow and
wrinkled white.

DI-HYBRID CROSSES 189

The three individuals of appearance aB include the
following types :

One pure, aBaB, breeding true to the (new)
smooth white type.

Two aBab, giving both smooth white and
wrinkled white.

The remaining individual is ab in appearance and
abab in constitution, and breeds true to the wrinkled
white type.

The expected behaviour of all these different types
can be followed out by the aid of suitable breeding
experiments, and not only has this been done in the
case of the cross which we have been considering, but
precisely similar phenomena have been shown to be
taking place for a large number of other characters in
many different species of plants and in a good many
animals as well.

We are now in a position to state the important
proposition known as Mendel’s law, which is to the
following effect :

The gametes of a heterozygote bear the pure parental
allelomorphs completely separated from one another,
and the numerical distribution of the separate allelo-
morphs in the gametes is such that all possible com-
binations of them are present in approximately equal
numbers. (Note that it is impossible for more than
one member of the same pair of allelomorphs to occur
together in the same gamete.)

This is the essence of the great discovery made by
Gregor Mendel, Abbot of Brunn, and published by

190 MENDELISM

him in the Transactions of the Brunn Natural History
Society in 1866. By some extraordinary chance
Mendel’s paper was entirely lost sight of until the
same facts were independently rediscovered in 1899
by de Vries working in Holland, by Correns in Germany,
and by Tschermak in Austria.

Gregor Johann Mendel was born on July 22, 1822,
at Heinzendorf, near Odrau, in Austrian Silesia. In
1843 he entered as a novice the Augustine Convent at
Altbrunn, and was ordained priest in 1847.

Mendel was a teacher of natural science in the Brunn
Realschule from 1853 to 1868, when he was appointed
Abbot of his monastery. During this time he was
largely occupied with experiments in cross-breeding a
great variety of plants, and some idea of his activity
in this line of scientific work is to be gathered from a
perusal of his letters to the German biologist Nageli,
a correspondence which has recently been published
by Professor Correns. Mendel himself only published
the result of his work with peas, and that of a few of
his experiments with Hieracium.

After 1873 the cares associated with the position of
Abbot of Brunn appear to have prevented further
biological work. His death took place in 1884, two
years after that of Charles Darwin, to whom Mendel
was thirteen years junior.

Mendel’s own experiments—that is to say, the
chief ones published by him—were made with peas, a
kind of plants which were found to be remarkably
well suited to this kind of work. Seven pairs of
characters in these plants were found to behave in

GREGOR MENDEL IgI.

precisely the same manner as those characters of the
maize-plant which have been already described, and
in all of them the phenomenon of dominance also
appeared. The characters dealt with by Mendel were
as follows, the dominant member of the pair being in
each case placed first :

Smooth seeds, and wrinkled seeds.

Yellow, and green reserve material—+.e., cotyledons.

Deeply coloured (grey), and nearly colourless testas
or seed-coats.

Inflated or stiff, and wrinkled or soft pods.

Green, and yellow pods.

Flowers scattered up the stem, and flowers in a
terminal bunch or umbel.

Tall, and dwarf stems.

As the result of these experiments Mendel came to
the conclusion with which his name is now closely
associated—that the male and female germ-cells of
hybrid plants contain each of them one or the other
member only of any pair of differentiating characters
exhibited by the parents, and that each member of
such a pair of characters is represented in an equal
number of germ-cells of both sexes. Furthermore,
separate pairs of differentiating characters (allelo-
morphs) conform to this law in complete independence
of one another.

Although in Mendel’s own experiments one member
of each pair of differentiating characters was always
dominant, dominance is by no means an universal
phenomenon when different varieties of plants are
crossed together. In a considerable number of in-

192 MENDELISM

stances the heterozygote is found to exhibit an appear-
ance which is more or less intermediate between the
types of character shown by the parents. It may be
almost exactly intermediate, or the appearance of the
cross-bred form may be nearer to that of one parent
than to that of the other. Dominance is clearly only
an extreme case of this latter phenomenon. The
term ‘dominance’ is applied to those cases in which
the appearance of the hybrid offspring is so near to
that of one parent as to be no longer clearly distin-
guishable from it.

In other cases, still of a simple Mendelian nature,
the appearance of the heterozygote may be quite
different from that of either parent homozygote. An
excellent example which is almost certainly of this
nature is afforded by the Andalusian fowls studied
by Messrs. Bateson and Punnett. And this will also
serve as our first illustration of the application of these
principles to animals as well as to plants. The facts
of the case are as follows :

The ‘blue’ type of Andalusian appears to be a
heterozygote form which has never been got to breed
true. When a pair of these birds are mated together
only about half their offspring are like themselves, the
remainder being entirely different. Half these re-
maining ‘wasters’ are black, and half are nearly
white, showing only a few black ‘splashes.’ If, now,
a pair of the black wasters are mated together, they
breed perfectly true, yielding only black offspring like
themselves. Similarly the splashed whites mated
together give rise to splashed white, and nothing else.

HETEROZYGOTE FORMS 193

Both these forms, then, the black and the splashed
white, are clearly pure homozygotes. On mating a
black and a splashed white together, black-bearing
gametes and white-bearing gametes will meet together
in fertilization. In every case in which this form of
mating was carried out the resulting chicks were
invariably blue.

The gametes of the blue type of Andalusians, then,
according to our supposition, do not bear the blue
character at all. Half of them contain the black and
half of them the splashed white allelomorph. Such
gametes, meeting by chance when a pair of blue An-
dalusians are mated together, give rise to the zygotes
—one black-black, two black-white, one white-white—
the black-whites being, of course, blue in appearance
as before.

Now, we may put this explanation to the test by a
very simple experiment—namely, by mating the sup-
posed heterozygote blues with the black and with
the splashed white types respectively. Both these
forms of mating were examined by Bateson and
Punnett, and the results were as follows : It was found
that blues crossed with blacks gave rise to equal
numbers of blue and of black offspring, whilst when
blues were crossed with splashed whites there ap-
peared blue and splashed white chicks in equal numbers.
And by a repetition of the process it could be shown
that the blues so obtained were heterozygotes as before.
Here, then, we have clear evidence that equal numbers
of the germ-cells produced by the blue birds bear the
pure black allelomorph and the pure splashed white

- 33

194 MENDELISM

allelomorph respectively, since half the offspring ob-
tained on mating the blue birds with black are black,
and half the offspring obtained on mating them with
splashed white are splashed white. The following
scheme of inheritance illustrates the phenomena
described :

1 (zygote) blue heterozygote
I (gametes) black and white

| | |
2 (zygote) 1 black-black 2 black-white 1 white-white

(blue heterozygote)

2 (gametes) black black white
black white white

| | . |

|

3 (zygote) black black 2blue 1white white
black white
5 (gametes) black a white

4 (zygote) all blue heterozygotes

A case which is closely similar to that of the An-
dalusian fowl is afforded by the cross between Primula
sinensis and Primula stellata.

P. sinensis crossed with P. stellata gives rise to a
type which is different from either parent, being in
some respects intermediate between the two. The
hybrid is so distinct that a special name has been given
to it, and the new type is known as P. pyramidalis.
So far it has been found impossible to obtain a strain
of P. pyramidalis which will breed true. On self-
fertilization the offspring are found to show the types

Fic 15.—PRIMULA SINENSIS CROSSED WITH P. STELLATA.

Above, the parents. In the middle, the heterozygote offspring
—P. pyvamidalis, Below, the result of self-pollinating P. pyramidalis :
1. P, sinensis: 2. P. pyvamidalis ; 1. P, stellata,

(To face p. 104.

MENDELISM 195

of P. sinensis, P. pyramidalis, and P. stellata in the
ratio of 1: 2:1.

Cases like the above illustrate the essential part of
Mendel’s law even better than those in which domi-
nance is present, the characteristic proportion of one
of each homozygote type to two of the heterozygote
being at once recognisable in such a case without the
necessity for further breeding ; whereas, in cases where
there is dominance, further study is necessary in order
to distinguish, among the individuals of dominant
appearance, those which are pure dominant and those
which are heterozygous in constitution.

In concluding our account of the simpler forms of
Mendelian phenomena we may consider one further
point with regard to the nature of the two allelomorphs
making up any particular pair. In what is probably
a majority of the cases hitherto examined the dominant
and recessive allelomorph seem to represent respec-
tively the presence and absence of something. Thus
the dominance of colour to absence of colour, or white-
ness, is a very frequent phenomenon. And in some of
the more complex cases to be described in the next
chapter we shall find the presence and absence of a
particular factor very often behaving as a pair of Men-
delian allelomorphs. The question arises as to how
far this conception should be extended. It seems, for
instance, somewhat far-fetched to speak of dwarfness
as being simply determined by the absence of the factor
for tallness, though it is not impossible that this may
be the correct way of looking at the facts. Be this as

it may, it is to be remembered that a Mendelian pair
I3—2

196 MENDELISM

often represents the presence and absence respectively
of a particular feature.

BIBLIOGRAPHY.

MENDEL, G. J.: Versuche ueber Pflanzenhybriden, 1865.
= Ueber einige aus kunstlicher Befruchtung
gewonnen Hieracium-Bastarde, 1869.
Punnett, R. C.: Mendelism, second edition, 1907.
Bateson, W.: Mendel’s Principles of Heredity, 1902.
- Address to the Zoological Section, British
Association, 1904.
Lock, R. H.: In Annals of the Royal Botanic Gardens,
Peradeniya, 1904.

CHAPTER VIII

MENDELISM (continued)

MENDEL’s law, as stated in the preceding chapter,
has already been found to hold good in a very large
number of cases—cases in which all kinds of characters
are concerned, belonging to many different species of
animals and plants. In certain instances, however,
complications arise, and these may be treated of in
two main sections.

The first kind of complication arises from the pheno-
menon known as coupling. The essence of this pheno-
menon consists in the existence of some kind of affinity
occurring in the same individual between allelomorphs
which belong to distinct pairs. In consequence of such
an affinity exceptions are found to the rule that sepa-
rate pairs of allelomorphs segregate independently.

The closeness of the connection between the char-
acters concerned shows a series of gradations in dif-
ferent cases. In the simplest cases of all, what are
loosely spoken of as separate characters are found on
closer examination to be only different aspects of one
and the same characteristic feature. These cases, then,
offer no real exception to the rule, for only one pair of
allelomorphs is actually concerned. Asan example, we

197

108 MENDELISM

may take the case of the wrinkled sugary type of maize
already contrasted with the smooth starchy variety.
The essential difference between the two kinds depends
upon the fact that in the former the reserve product
laid down in the endosperm is different, being largely
of a sugary nature instead of being starchy. With
this circumstance is associated the presence of a larger
proportion of water in the unripe grain. And the
result of this is that, when the grain dries, its surface
falls into folds. The sugary nature of the grains also
causes them to take on a more hyaline or semi-
transparent appearance than the grains of the starchy
variety. All these characters, if they can be so called,
behave on crossing as a single Mendelian allelomorph,
and are doubtless represented in the germ cells by a
single substantive representative.

A simple example of what may probably be regarded
as a real case of coupling is afforded by certain colour
characters exhibited by pea-plants. In these plants
coloured flowers, a red or purple colouration in the
axils of the leaves, and a marked pigmentation of the
testas, or seed-coats, are always associated together on
the same plants ; so that, if we find a plant which has
green leaf axils, we may be sure that its flowers will
be white, and the testas of its seeds only slightly pig-
mented. On crossing plants bearing coloured axils,
coloured flowers, and pigmented testas, on the one
hand, with plants bearing green axils, white flowers,
and unpigmented seed-coats, on the other, the two sets
of characters are found to behave as a simple pair of
allelomorphs, and the simultaneous appearance of

COUPLING 199

colour in these different situations doubtless depends
upon the presence of a particular pigment in the plant
which exhibits it. Nevertheless, we can scarcely
fail to look upon these three separate manifestations
of the pigment as representing distinct characters, and
this being so, we suppose their germinal representa-
tives to be coupled together in such a way that they
remain associated at the time when, during the forma-
tion of the germ-cells of the heterozygote, other allelo-
morphs become independently segregated.

And this way of looking at the facts is further
justified by the behaviour of the characters in ques-
tion in another species of plant. For in the sweet pea
it is possible for the coupling between these characters
to be broken down, so that a plant which exhibits
green leaf axils may, under certain circumstances, bear
coloured flowers. In such a plant the leaf-axil-colour
and the flower-colour must clearly be represented by
independent allelomorphs.

In other cases, again, there may be coupling between
characters which have no obvious relation to one
another at all. In illustration we may take the case
of a cross between two strains of peas, one of which had
white flowers and opened its buds severa] days earlier
than the second, the blossoms of which were purple.

The F, plants (with purple blossoms) came into flower
at a period intermediate between those of the parents.
In F, 506 plants were grown successfully. Some of
these flowered as early as the white parent, and others
as late as the purple parent ; but the majority of the
plants ranged between these two extremes, so that it

200 MENDELISM

was impossible to rank the individuals into definite
classes in respect of so indefinite a character as time
of flowering. On making a perfectly arbitrary division,
however, it was found that 175 purple and 104 white
plants were in flower on a certain day, and that 208
purple and 19 white plants did not open their buds
until afterwards. There is, therefore, clearly some
coupling between the presence of white blossoms and
early flowering on the one hand, and between lateness
and purple flowers on the other. Two characters
more diverse than colour of the flowers and time of
flowering could at first sight scarcely be imagined.

In this last example the two characters concerned
do not appear to be completely, but only partially
coupled. In some cases it is found that the degree
of partial coupling can be expressed numerically.
This phenomenon of partial gametic coupling has only
been discovered very recently, and experiments have
not been carried far enough to enable us to speak
about it with any great degree of certainty. But as
it seems clear that this phenomenon will play an
important part in the immediate future of genetic
discovery, it seems necessary to devote some space
to it, whilst making the proviso that the account may
require modification within a comparatively short time.

The phenomenon in question has been observed by
Bateson and Punnett in crosses between different
strains of sweet-peas. In the first example the char-
acters concerned were the shape of the pollen grains,
whether oval or spherical, and the colour of the flowers,
whether blue or red.

COUPLING 201

When blue-oval is crossed with red-spherical, the
F, plants are all blue-oval. But in F,, instead of
getting g blue-oval, 3 blue-spherical, 3 red-oval, and
I red-spherical, the numbers of the different kinds of
plants obtained closely approached the following
proportion—I77 : 15:15: 49.

Such a series would be produced if the allelomorphs
concerned were associated in the gametes in the
following proportion, 7 blue-oval, 1 blue-spherical,
r red-oval, 7 red-spherical, as may easily be verified by
multiplication. Enormous numbers of plants must
naturally be examined before it can be asserted that
the series actually chosen is really the correct one ;
in fact, mere statistics are hardly capable of proving so
complicated a proportion as this in the absence of
independent considerations.

In a second case, in which the characters concerned
were fertility and sterility of the anthers, and presence
and absence of pigment in the axils of the leaves,
there was evidence to show that the gametic series
was I5:1:1:15, instead of being 7:1:1:7. In fact, a
series such as the following appears to be suggested :

AB. aB. Ab, ab. Total.

No coupling ans 9 3 3 I 16
3Z:1:1: 3 ae 41 7 7 9 64
7 aoe te tee ore 177 15 15 49 256
I5:1:1:15 oie 737 31 31 225 1,024

It is remarkable that coupling of the type 3: 1:1:3
has not yet been observed. The search has, however,

202 MENDELISM

only just commenced, and it is not unlikely that
additional members Of the series still remain to be
discovered.

The converse of coupling is sometimes shown in
what is known as gametic repulsion. Thus certain
F 2 ratios have been observed which are most readily
interpreted on the supposition that a particular mem-
ber of one pair of allelomorphs is unable to exist in
the same gamete with a particular member of a
distinct pair.

The second class of complications that we have to
deal with—although the term complication may be
to a certain extent justified in connection with it—
does not involve any exception to Mendel’s law of
segregation. The phenomenon of so-called reversion
on crossing has long been familiar to biologists. Its
meaning, however, was totally obscure, and even the
Mendelian was at first unable to offer any explanation.
The phenomenon consists in the appearance, in the
offspring of a cross, of a character which was not
visibly present in either parent, and in many cases this
character can properly be regarded as ancestral—it is
a character which has been lost by both parents in
the course of their divergent evolution from a common
primitive form. Now, these cases differ entirely from
those of the appearance of a heterozygote form on
crossing, such as are due to the combined action of the
two parental allelomorphs in the cross-bred offspring,
because in true cases of reversion a certain proportion
of the reversionary individuals of F, are found to breed
true, which a simple heterozygote will never do.

MASKED CHARACTERS 203

It has been found that the essential part of this
phenomenon of reversion on crossing consists in the
existence in the parents of certain hereditary factors—
allelomorphs, in fact—which, although by themselves
invisible, .yet, when combined in cross-breeding with
certain other allelomorphs, belonging to independent
pairs, lead to the appearance of new visible characters.

The term reversion cannot properly be applied to
these phenomena as a class, because, in the first place,
characters may arise in this way which cannot be
regarded as ancestral, and, secondly, because reversions
may take place in other ways; for example, the
reappearance of a simple recessive character would
legitimately be ranked among reversions. The best
general name for the class of phenomena we are about
to describe is perhaps masking of characters, or crypto-
merism, the latter being the term employed by Tscher-
mak, who was the first to describe these phenomena
in connection with Mendelian ratios.

In the simpler cases an invisible or masked factor
derived from one patent, on becoming associated with
a different factor born by the other parent, and already
visibly represented among the external features of this
second parent, makes itself apparent among the visible
characteristics of the heterozygote. In such a case
the characteristic appearance exhibited by the hetero-
zygote may subsequently become permanent, owing
to the building up of a type which is a homozygote in
respect of both the necessary factors.

This may be made clearer by a definite illustration.

A pea-plant characterized by the presence of a greyish

204 MENDELISM

or brownish testa to its seeds (grey) was crossed with
a plant having nearly colourless testas (white). The
testas of the F, plants were marked with bright purple
dots on a grey ground (purple). These hybrid plants
were self-pollinated, and in F, the three types appeared
in the following proportions : 9 purple, 3 grey, 4 white.
What is the meaning of this ratio? In order to
complete the ordinary expectation for a simple Men-
delian case in which two pairs of allelomorphs are
concerned (di-hybridism) we must write down the
following expression :

ope} ss (gm) ss (Bom, } + (ae)
But it would seem that the purple character cannot
appear when the grey colour, or some factor con-
stantly associated with this colour, is absent, as is
the case in the original white parent from which the
factor for purple spots was derived. Consequently, the

three { aga plants are indistinguishable from the
no grey

no purple
{ no grey
the result which was described as being the one actually
obtained—namely, 9 purple : 3 grey : 4 white.

In other respects this example is precisely like the
case of two pairs of allelomorphs described on p. 187.
We may write A for presence of grey pigment, a for
absence of grey pigment, B for presence of purple, and
6 for its absence. Then the original cross was of the
form Ab xaB, from which AaBod resulted in F,. And

} plants or whites, and we thus arrive at

MASKED CHARACTERS 205

the visible characters of the types which appeared
in F, would be represented by 9AB+3Ab+(3aB +
tab). On referring to the account given on p. 176 it
will be seen that one in nine of the purple plants is of
the constitution ABAB, and may be expected to
breed true.

A precisely similar result may be obtained in F, in
cases where there is no reversion in F,. In the
following example a white pea, which did not contain
the masked purple factor, was crossed with a ‘ maple-
seeded’ pea. The characteristic feature of maple is a
marbling of brown spots on a grey ground colour. In
F, the marbling was dominant, and the seeds resembled
the maple parent.

In F, there appeared 9 maple : 3 grey : 4 white—
t.€., the same ratio as in the previous case, this time
without reversion. This ratio is brought about by the
simple combinations of two pairs of allelomorphs A-a,
and C-c, C being unable to manifest itself unless A is
present in the same zygote. As a matter of fact, in
this particular case C does sometimes just manage
to appear in the absence of A, the result being a
white seed with a sort of faint ‘ghost’ of a maple
marking.

When a strain bearing both maple marking and
purple spots is crossed with a white in which neither
of these factors is latent, we can easily calculate the
ratio to be expected in F, by using the formula
(4+2Ada+a) (B+2Bb+b) (C+2Cc+c). The result
works out as follows (writing m for maple, # for purple,

and g for grey): 27mpg, 9mg, opg, 3g, (gmp, 3m,

206 MENDELISM

3p, Iw). Since g is absent from all the members of
the series enclosed in the bracket, these appear white,
or nearly so, the total number of whites being thus 16.
And the numbers obtained in an actual experiment
accorded closely with the expected ratio 27: 9: 9: 3: 16.

Among the sixteen whites, some will be bearing the
factors for m and #, others that for # only, others that
for m only, whilst one in sixteen will contain neither of
these factors. Until such invisible differences between
the different white plants are actually proved to be
present the whole account so far given will remain more
or less hypothetical. The proof is obtainable by cross-
ing the different whites with a pure grey strain. The
grey factor being thus introduced, the whites which
contain a # or an m factor will exhibit the same in
their offspring. A number of the whites obtained in
F, and in Jater generations were actually crossed with
the same grey-seeded plant. Some of the offspring
showed both the maple and the purple character,
others the maple without the purple, others the purple
without the maple, and others, again, showed neither ;
the seeds of these last being exactly like those of the
grey parent owing to simple dominance of the grey
allelomorph over white.

The first example of this kind of phenomenon to
be observed in the case of animals was one described
by the French zoologist Cuénot. Cuénot’s original
account has had to be somewhat modified in view
of more recent work by Miss Durham, and the latest
account of the facts runs as follows. For the sake
of simplicity we shall deal in the first instance with

EPISTATIC CHARACTERS 207

only four types of colour—the ‘ agouti’ or wild grey
colour, black, ‘ chocolate,’ and albino. The behaviour
of these colours in heredity can be described in terms
of three pairs of allelomorphs :

Gg: The presence and absence of the factor which
gives the ‘ agouti’ or grey pattern in the hairs.

Bb: Presence and absence of the black determiner.

Cc: Presence and absence of colour.

Where C is present without G or B, the colour is
chocolate, the proper formula for such an animal
being CCggbb or Cceggbb. Black mice may be CCggBB,
etc., and grey mice CCGGBB, etc.

All albino mice are to be represented as those from
which C—+.e., the chocolate colour—is absent ; but
either G or B, or both, may be present (but masked)
in an albino individual.

When B and C are both present, the colour is black,
and not chocolate. We cannot, however, speak of
black as being dominant to chocolate, since these two
factors belong to independent allelomorphic pairs. A
new term is therefore required for this relationship,
and also for the relationship between grey and black.
Bateson’s suggestion for the required terminology may
be given in his own words: ‘ We can, perhaps, best
express the relation between the grey and the black
by the use of the metaphore “ higher and lower,” and
I therefore suggest the term epistatic as applicable to
characters which have to be, as it were, lifted off in
order to allow the lower or hyfostatic character to
appear.” Thus grey is epistatic to black, and black
is epistatic to chocolate.‘

208 MENDELISM

A curious and not fully explained phenomenon
appears in the case of yellow mice, which must be
briefly mentioned here on account of its bearing upon
a subject discussed in the next chapter. Yellow
appears to be epistatic to grey as well as to black,
but yellow mice, so far as the evidence goes, are always
heterozygous. Cuénot’s experiment to demonstrate
this fact was as follows :

When YyGGCC is crossed with yyGGCC, equal
numbers of yellow and grey offspring are to be ex-
pected, since G is hypostatic to Y. In various crosses
of this nature Cuénot actually obtained 177 yellows
and 178 greys, from which we may deduce that the
heterozygote yellow was giving off the expected pro-
portion of gametes bearing the yellow character (i.e.,
50 per cent.).

When such heterozygous yellows are bred together,
the expected result would be as follows :

YyGGCC x YyGGCC = YYGGCC + 2 YyGGCC + yyGGCC

3 yellow I grey

Eighty-one yellow mice were actually obtained in
this way. Among them some twenty-seven would
naturally be expected to be pure dominant, and to give
yellow only when crossed with black or grey indi-
viduals. To Cuénot’s astonishment, he found on making
the necessary crosses that every one of these eighty-
one yellows gave some black or grey among its off-
spring; not one of them was a pure homozygous

yellow.

YELLOW MICE 209

The facts are explicable in one of two ways. On the
earlier hypothesis there was supposed to be some
obstacle in the way of the fertilization of a yellow-
bearing egg by a spermatozoon bearing the same
character. In this case, since spermatozoa are in
great numerical excess as compared with eggs, it is
still possible for every Y-bearing egg to be fertilized
by a spermatozoon lacking Y, as well as half the non-
yellow eggs by Y-bearing sperms, giving a ratio of
3:1 in F2 from yellow by yellow. On the other
hand, it is possible that pure yellow zygotes are formed
in fertilization, but for some unknown cause are in-
capable of development. In this case a third of the
yellow progeny would be wanting, and the expected
ratio would be 2:1. Cuénot’s figures led to the
belief that the first hypothesis was the correct one,
but in further experiments by Castle and by Miss
Durham the ratio was found to approach 2:1 ina
majority of cases.

We have still to describe a case in which two latent
factors, one derived from each parent, give rise, by
their simultaneous presence in the zygote produced,
to the appearance of an entirely new character. The
following example is the first one of the kind to be
completely elucidated, and is one of those studied by
Messrs. Bateson and Punnett and Miss Saunders.

The white-flowered variety of sweet-pea known as
Emily Henderson was found to exist in two forms,
only to be distinguished from one another by the
shape of the pollen grains which they produced. In
one of the two the shape of the pollen is elliptical

14

210 MENDELISM

(long pollen), in the other it is approximately spherical
(round pollen). Sweet-peas normally undergo self-
pollination, so that the two types naturally remain
distinct. Let us see what happened when the long-
and the round-pollined forms were crossed together.

The cross-bred plants (F,) had coloured flowers —
flowers of the old-fashioned purple type known to
florists as Purple Invincible, which is characterized by
a purple standard and blue wings. The pollen pro-
duced by these plants was of the long type. Thus as
regards the shape of the pollen grains there was simple
dominance. But the union of two white-flowered
types has given rise to a series of plants all possessing
a definite colour character—purple with blue wings.
This character is very probably the same as that
exhibited by the common ancestor of all our cultivated
sweet-peas. Here, then, we seem to have a clear case
of reversion to the ancestral type on crossing. We
shall find that the Mendelian principles will enable us
to arrive at a clear conception of the mechanism of
this process.

The cross-bred plants were self-pollinated, and in F,
the following types made their appearance in approxi-
mately the proportions given ;

Purple Invincible ... 81 3
or
Picotee _ wee a
Painted Lady ee =. 277) 3
} or
Tinged white ee 9
White... see we 112 7

LATENT CHARACTERS PAS |

Painted Lady is a well-known colour type which is
characterized by the presence of a red standard and
white wings. Picotee and tinged white are also forms
well known to the sweet-pea fancy. They appear to
be diluted forms of the purple and Painted Lady types
respectively, their appearance depending upon the
presence of a definite diluting factor in addition to the
factor for the colour in question, or perhaps more
properly upon the absence of the proper strengthening
factor which converts Picotee into purple, and tinged
white into Painted Lady.

The following explanation of the result so far
déscribed has now been well established by further
experiment. In the first place, we may consider all the
coloured forms together as a single group opposed to
white. It is now clear that the coloured type of F, is
due to the meeting together of two factors, one of them
born by one white parent and the other by the second,
and it is necessary for both these factors to be simul-
taneously present in order that any colour may make
its appearance. We may call these two factors C and
R, denoting the absence of either by c and 7 respectively.
By the simple Mendelian behaviour of these two pairs
of factors C-c and R-r, the ratio of nine coloured
plants to seven white appearing in F, is readily
explicable, and the way in which this happens is shown
in the diagram on the opposite page.

To explain the presence of the four different types of
coloured plants which make their appearance in F,,
two further pairs of allelomorphs are called in. The

dominant member (B) of one of these, when present
14—2

212 MENDELISM

in combination with C and R, produces the purple
or Picotee colour (blue), whilst its absence (0) in pre-
sence of C and R is accompanied by the appearance
of the red colours—Painted Lady and tinged white.
Purple Invincible and Painted Lady are regarded as
intensified forms of Picotee and tinged white respec-
tively. The presence of the second factor (T) is attended
by the <a o, of the full colours purple and

26

crROe

c

“cr
Cr

cr
cer

Fic 16.

The shaded squares represent coloured plants, the blank squares
white plants.

Painted Lady ; its absence (¢) causes the appearance of

the diluted forms Picotee and tinged white.

B and T may be present when either C or R or both
are absent ; the resulting plant has then white flowers.
And it is interesting to notice that the ultimate recessive
white, containing c 7 b ¢, occurs only once among 256
individuals of F,. The whole apparently complex system
of floral colours is thus explained by the simple Men-
delian behaviour of four separate pairs of allelomorphs.

Bateson and his collaborators have, therefore, provided

MENDELIAN CHARACTERS 213

a complete account of the phenomenon of reversion
on crossing, an account which has already been demon-
strated to hold good in other instances besides that
of the sweet-pea. The facts are expressed in the
following manner by their discoverers : ‘ “ Reversion ”
is thus seen to be a simple and orderly phenomenon
due to the meeting of factors belonging to distinct
though complementary allelomorphic pairs, which at
some moment in the phylogeny of the varieties have
each lost their complement.’*

Dr. G. H. Schull has classified the different known
types of latency in the following manner :

xr. Latency due to separation. In this form of
latency an allelomorph, when acting alone, has no
external manifestation, and is only rendered visible
by combination with another allelomorph belonging
to a distinct pair. In this way such ratios as 9: 3: 4
or 9 : 7 may arise. We have already observed an
example of this phenomenon in the case of the two
factors required to produce colour in the sweet-pea.

2. Rare cases have been described of latency due
to combination. This, instead of being due to separa-
tion, is the result of a union in the same zygote of two
dominant allelomorphs, either of which alone will
produce a manifest character. When both allelo-
morphs are present the character fails to appear.

3. Latency due to hypostasis, or the masking of:
one dominant character by another character also
dominant and epistatic to the masked character, has
already been described in the case of the coat colours

* Proceedings of the Royal Society, B. vol. 77, p. 238.

214 MENDELISM

of mice. The simplest typical ratio thus produced is
12¢3 2%.

4. The last type of latency described by Dr. Schull
is of a totally different kind. It is represented by the
phenomenon to which the term ‘ latency ’ was origin-
ally applied by de Vries. This latency consists in the
disappearance of certain characters under the influence
of poor nutrition, or other changes of conditions.
Examples of this phenomenon described in the chapter
on Mutation were the submerged and floating leaves
of the water-ranunculus, and the red and white colour
of the flowers of a species of primula.

We may now proceed to pass in rapid review a
selection of the more remarkable instances of Mendelian
inheritance which have been so far demonstrated.

The ease with which characteristics of colour can be
distinguished and defined has naturally led to a good
deal of attention being paid to the phenomena of their
inheritance. In this way many cases of simple domi-
nance have been discovered in plants and in animals,
as well as several examples of reversion in F,, followed
in both cases by a Mendelian segregation of characters.

Thus the colours of many flowers afford perfectly
simple phenomena, whilst other cases, like the sweet-
peas and the closely similar case of stocks studied by
Miss Saunders, have required long and arduous ex-
periment for their elucidation. No case of this kind
hitherto examined has been definitely proved to be
non-Mendelian.

Colour characters which follow Mendel’s law have
been observed in mice, rats, rabbits, guinea-pigs,

MENDELIAN CHARACTERS 215

pigeons, fowls, cats, and so on. In butterflies and
other insects, and even in snails, similar phenomena
have been described. The study of the larger domestic
animals awaits for the present the proper endowment
of these researches. When this takes place, the
inheritance of far more important characters than
colour will be adequately studied to the great profit
of all who are concerned in the breeding industry.
Hurst has already shown from an examination of
the stud-book that the bay and brown colours of
thoroughbred horses are Mendelian dominants to
chestnut.

Other characters of the most diverse kinds are also
similarly inherited. We have already referred to
structural characters in maize and in peas. Stature
is a character which is definitely inherited in many
plants. Among more subtle characters a similar mode
of transmission has been found in the case of differences
in chemical composition, and in that of immunity
from and susceptibility to the attacks of certain
diseases. The thrum-eyed condition of the primrose
has been shown by Bateson and Gregory to be a
Mendelian dominant to the pin-eyed. condition, so that
we have here a partial solution of a problem which is
rendered specially interesting from the fact that it
baffled Darwin.

A study of numerous pedigrees has enabled Bateson to
show that there is great probability that in the case
of the human race certain congenital diseases are simply
transmitted from parent to offspring in accordance
with Mendel’s law.

216 MENDELISM

How far the influence of the Mendelian principles
may extend we do not yet know. But it is certain that
very few, if any, cases have so far been discovered in
which differentiating characters do not behave in this
way when the types which exhibit them are crossed
together. Experiments have now been made upon a
great variety of plants and animals, involving a con-
siderable diversity of kinds of characters. Nevertheless
it is scarcely possible to cite a case in which it is
definitely and certainly known that Mendel’s law,
subject to the modifications already described, does
not hold good. Cases of various kinds are, indeed,
recorded, but these records are derived from experi-
ments either carried out before the bearings of the
Mendelian phenomena were at all fully appreciated, or
—and this is the most frequent case—without any
knowledge at all of Mendej’s discovery.

Thus a considerable number of cases were formerly
described in which the first cross or heterozygote of F,
bred true instead of segregating in F,. There is some
doubt whether any case of this kind will really stand
criticism; Millardet’s case, for example, which was
described at the end of the last chapter but one, has
never been confirmed. It is quite certain that among
all the numerous crosses studied during the last six
years no example of the kind has been substantiated.
The most recent cases to be described of a first cross
breeding true are those of de Vries, and at these we
are bound to pause, because de Vries is surpassed by
no recent observer in weight of authority. Neverthe-
less, de Vries’ cases are of so complex a kind that we

MENDELIAN CHARACTERS 217

have some hesitation in accepting them without further
study.

It must be admitted that the suggestion that Men-
delian segregation may be a universal phenomenon
accompanying the formation of the germ-cells of all
animals and plants is one which will be disputed by
many biologists. There is ample evidence, they will
say, of first crosses breeding true in the case of
numerous plant and animal hybrids. But there are
several points to be considered before we can accept
the mere fact of a uniform progeny as sufficient evidence
that there was nothing in the nature of segregation
amongst the gametes of the hybrid.

To prove a negative is proverbially difficult, and
the attempt to show that hybrid characters exist
which do not segregate in the germ cells of the hybrid
is no exception to this rule. In favour of the opposite
contention the following considerations may be
alleged :

The characters for which non-segregation has been
asserted are generally complex and difficult to define.
Such characters, for instance, are the general habit
of a plant or the general shape of a flower. Characters
such as these may reasonably be supposed to owe
their appearance to the interaction of a considerable
number of independent allelomorphs.

Where two such factors are concerned, in cases
where the heterozygote form is intermediate, we
should expect one out of sixteen F, individuals to
resemble each parent form, the remaining fourteen
plants being more or less intermediate.

218 MENDELISM

With three such factors we should expect only one
of each parent type to appear among sixty-four
individuals.

There seems to be no reason for doubting that the
total number of visible factors, even, which go to
make up the total shape of a flower may be consider-
ably greater than this; and the number of invisible
allelomorphs upon which these depend may be pro-
portionately more numerous still. In such a case, in
an experiment where the total number of offspring
grown was limited, types like the original parents
might never be seen, simply because the sample taken
was not sufficiently large.

Again, we have to consider the possibility of a
differential fertility among the various allelomorphic
combinations. This might lead to an intermediate
form, actually breeding true in spite of the fact that
segregation was going on in its germ-cells. The
possibility has to be borne in mind that among the
offspring of two widely different species those indi-
viduals which more nearly resemble their immediate
hybrid parent may have a better chance of survival
than the forms which have more characters in common
with a single pure grandparent. The only piece of
evidence we have bearing upon this point is the fact
that we know the hybrid form itself to be capable of
surviving.

De Vries has recently published a very interesting
observation, which ought to be mentioned in this
connection. It appears that two forms of Cnothera,
which originated in a cross, and are distinguished as

NORMAL FERTILIZATION 219

Leia and Veluttna respectively, possess female gametes
half of which can be shown to bear the tall and half
the dwarf character. All the functional pollen grains
of Leta bear the tall character, and all the functional
pollen grains of Velutina bear the dwarf character.
De Vries does not offer the obvious suggestion that
half of the total number of pollen grains in each case
are impotent. If this suggestion should prove to
represent the truth, Cnothera lata would afford a
case of a hybrid which breeds true in spite of the fact
that typical segregation is taking place among its
germ-cells. In the light of this discovery it is clear
that non-segregation cannot properly be asserted in
any given case until the hybrid has been crossed with
each of the parent forms on a considerable scale.

We may now turn for a brief space to some of the
cases in which we have as yet no certain knowledge of
the manner in which inheritance proceeds.

The most obvious extension of Mendel’s law to
processes where it cannot be directly shown to hold
good is to suppose that the same rule applies to cases
of normal fertilization as to hybrid fertilizations. We
should then picture the former process as taking
place in somewhat the following way. Every visible
character of the individual which can be separately
distinguished, and which on cross-breeding would be
inherited on ordinary Mendelian lines, must be repre-
sented in the gametes by a definite factor of some kind,
possibly by a definite substance or combination of sub-
stances. The pair of parental factors for a particular
character would combine on fertilization, and at the

220 MENDELISM

formation of the gametes in the offspring its members
would separate as perfectly definite entities, to re-
combine when these gametes meet once more with their
corresponding mates. Such a definite segregation of
characters taking place within a pure strain would be
very difficult of absolute demonstration, but it is hard
to avoid the conclusion that this is a true deduction
from the facts observed when cross-breeding takes
place. Such a segregation would formerly have been
thought a very small assumption in comparison with
that of the segregation of pairs of allelomorphs of
which no trace is externally visible, and yet the latter
assumption has now been shown to be perfectly well
established.

This idea of unit characters, capable of being inherited
independently of one another, is one of the most
important conceptions which has ever been introduced
into the science of biology, and the introduction of it
has followed as the direct result of Mendel’s work. It
is a conception which has led to a complete change in
our ideas of heredity, since we no longer look upon
the individual as a unit, but find ourselves compelled
to study separately the independent characters of
which the individual is built up. The idea of the
individual as a living mosaic—an idea put forward long
ago by Naudin with only a partial realization of its
significance—has thus returned to us. In this con-
nection a curious problem presents itself. What
would be left if we could imagine all the separable
characters of a living creature as having been taken
away? Would there, or would there not, be any

x

ESTABLISHMENT OF NEW SPECIES = 221

residuum ? Upon this knotty point there is a disagree-
ment among authorities, and so we may be content
to leave it, since the question is hardly one which is
capable of a practical solution.

A phenomenon to which it is scarcely doubtful that
Mendelian principles will ultimately be found to apply,
although as yet the precise proof is wanting, is that of
sex. In the male and female sexes of the majority of
animals we have a very clear example of a pair of
definite differentiating characters. And the fact that
in the majority of forms the two sexes make their
appearance in nearly equa] numbers, may be thought
to point clearly to the conclusion that the separation
of the sexes depends upon some quite simple gametic
process. Light has recently been thrown upon this
question from the side of the study of the minute
structure of the gametes, and we shall defer the further
discussion of the problem to the chapter which deals
with microscopic phenomena within the cell.

A proper understanding of Mendel’s law enables us
to escape certain theoretical difficulties which have
long been prominent in the minds of students of
evolution. Many evolutionists were accustomed to
argue that a new form suddenly arising in the midst of
an old-established species could not give rise to a new
and permanent variety or elementary species, because
it would immediately be ‘swamped’ by intercrossing
with the parent species from which it was derived.
If, however, the character distinguishing the new
type is allelomorphic to the corresponding character,
or absence of a character, shown by the parent form,

222 MENDELISM

this difficulty disappears. For suppose as an extreme
case that the new type arises as a single individual
only, which is therefore compelled to mate with a
member of the original species. If the new character
is recessive it will disappear in the immediate offspring
of this cross. But half the germ-cells produced by
the cross-bred form will bear the new character pure
and undiluted. If any of these cross-breds mate to-
gether the new type will appear in a quarter of their
offspring. Even if all of them mate with members of
the original type, half the offspring of such matings
will be heterozygous, and sooner or later the hetero-
zygotes will be sure to mate with one another, and
give rise once more to the novel type of individuals.
If the new form has any structural or other advantage
over the old species, the former will tend to survive at
the expense of the parent type, and it may survive
if it is only equally well fitted for the battle of life. In
the case of dominance of the new form the same
process will take place, only it will be apparently
more rapid in the early stages because the cross-breds
will themselves exhibit the new character. In this
case, even if the new type has a very marked advantage
over the parent form, the process of completely sup-
planting the latter will be delayed, because the old
type of character can survive concealed in heterozygote
individuals.

Let us pause for a moment to sum up the novel ideas
which have so far been presented in this and the
preceding chapter.

We found in the first place that from the point

SUMMARY 223

of view of heredity we must look upon an animal or a
plant as a composite being, made up of a great number
of unit characters, each capable of separate description,
and all inherited independently of one another.

When a pair of nearly-related animals or plants
are mated together, when, in fact, like is bred with
like, and with still greater certainty in cases of self-
fertilization such as are not uncommon among plants,
every unit character born by one gamete finds a
corresponding mate among the characters born by the
second gamete. It naturally follows that a series of
characters similar to those of the parent or parents
make their appearance in the offspring.

When a pair of individuals belonging to distinct
varieties or races are mated together, the result is the
same in the case of the majority of characters exhibited
by each of them. For separate varieties of the same
species differ from one another in a small number of
units only, and organisms which differ in more than a
few unit characters refuse altogether to unite for the
production of offspring. From the study of the precise
behaviour of those characters in which a pair of
parental organisms differ, a flood of light has been
thrown upon the phenomena of inheritance.

We find, as a rule, that opposed to every differen-
tiating unit character of one parent there exists a
corresponding but different character in the other
parent. One parent may have smooth seeds and the
other wrinkled seeds, for example. Very frequently
the corresponding feature consists in the absence—or
failure to appear—of a particular character, as, for

224 MENDELISM

instance, when the non-development of pigment leads
to the appearance of white flowers.

We can now realize how necessary it is, in order to
avoid hopeless confusion, to follow the behaviour of
each pair of characters in the offspring separately.

The result of the meeting between the two opposed
characters of the same pair we saw to be different in
different cases. There may arise in the offspring
(1) the appearance of a simple blend of the two parental
characters. Or (2) one character may be more or less
dominant over the other. Or (3) the combination of
the two parental characters in the offspring may give
rise to an appearance quite different from that of either
of them, very much in the same way as in chemistry
oxygen and hydrogen when combined give rise to water.
Or (4) we may get further complications in which un-
suspected characters, present in an invisible condition
in one or both parents, take a part, often giving rise
to the appearance of a supposed reversion.

The most important phenomenon of all, however,
is that which is found to occur at the formation of the
germ cells of the heterozgyote plant or animal. What-
ever the appearance of the hybrid form may have been,
at this stage in its history the determining factors for
each member of the pair of parental allelomorphs
reappear in their entirety in certain cells which by
their division give rise to the gametes, and at one
of the divisions in question the parental characters (in
a potential condition) separate completely from one
another, so that half the gametes bear one allelomorph
and half of them the other. In cases where more than

SUMMARY 225

one pair of allelomorphs has taken part in the cross, the
members of each pair are found, as a rule, to undergo
this process of segregation quite independently of all
the other pairs.

The result of this phenomenon of segregation is
that we find our conception of what constitutes purity
in a strain of animals or plants to be completely
altered. We now know that purity does not depend
upon the length of time during which the race has
exhibited a constant character. A strain of absolute
purity may arise from the second generation of a cross.
Such a pure strain may show an entirely new com-
bination of the parental characters. But this is so
far the only kind of novelty which we can produce at
will. We know almost nothing as to the method by
which entirely new characters arise. We can only
take advantage of such characters when they happen
to make their appearance.

I would draw special attention to the definiteness of
the characters with which we deal. We do not evoke
improved features by gradual selection ; the characters
are either there or they are not. Let it be further
remembered that every process of this kind which has
been worked out in the case of a plant can be paralleled
by similar phenomena taking place in some one or
other of the higher animals.

On the mind of a biologist familiar with what was
known of heredity only ten years since these facts
must fall with a sense of complete novelty. The
ideas current even so short a time ago are not so much
extended, or even altered, as replaced by an entirely

15

226 MENDELISM

new set of ideas. And it may be remarked in passing
that the biologist of fifty years ago and more was much
nearer to our present line of inquiry.

We have seen enough to enable us to recognise
very clearly the vital importance of an understanding
of the constitution of the gametes in all questions of
heredity. There must exist in the gametes, in an
uncombined condition, those units which by their
combination in zygotic organisms lead to the appear-
ance of the characters which we can recognise. But
we have seen that, owing to the appearance of domi-
nance and other kindred phenomena, the visible external
characters of an organism are not a complete guide to
the nature of its gametes. It is only by careful
breeding that we can distinguish the heterozygote from
the pure dominant form—to take the simplest possible
example of this difficulty. For this reason it has now
become the chief business of the student of heredity
to determine by experiment what combinations of
allelomorphs are present in the gametes of the indi-
viduals with which he is working.

The behaviour of these allelomorphs has now been
disentangled in many cases of very considerable com-
plexity ; and all such cases as have been so far examined
in detail have proved explicable in terms of a larger or
smaller number of allelomorphic pairs, all of which
obey Mendel’s law—with the single exception of those
cases in which coupling between the allelomorphs of
different pairs introduces a slight further complication.
Although it is perhaps scarcely probable that Mendel’s
law will ultimately prove universal in its application,

MENDELISM AND BIOMETRY 227

nevertheless the few exceptions recorded by competent
observers still require further examination before they
can be accepted as invalidating the law in any single
instance.

The question naturally arises as to how far the Men-
delian rule of inheritance agrees with or contradicts
those estimations of hereditary values which have been
arrived at by the labours of the biometricians.

So long ago as 1902 Mr. G. Udny Yule endeavoured
with some apparent success to reconcile the Mendelian
results with those of biometry. Progress has been
rapid during the last four years, and what we have now
before us is rather the question of reconciling the bio-
metrical conclusions with the firmly established facts
of Mendelian inheritance. More recently Mr. Yule
seems to have succeeded in performing this service for
science, although the comments of other biometrical
students upon his work have still to be awaited.

In 1902 Yule considered the case of a pair of simple
Mendelian characters, A and a, exhibited in a mixed
population breeding together at random, in such a way
that the total number of germ cells bearing A and a
respectively might be regarded as equal in any genera-
tion. In such a case it will always be an even chance
whether a recessive parent will produce a dominant
or a recessive child, because the chance of its gamete
(a2) mating with A or a is the same. A knowledge
of the ancestry of the recessive parent makes no
difference to the result. Consequently the case of
the pure recessive does not fall in with any possible

theory of ancestral heredity.
15—2

228 MENDELISM

But on turning to the dominant parent, the case is
found to be different. For such an one may be either
a pure dominant homozygote giving off A-gametes
only, or it may be a heterozygote giving off equal
numbers of A- and a-gametes. Yule shows that if both
the parents of the A individual exhibited the character
A, the proportionate number of its offspring which may
on the average be expected to show the A character is
greater than would have been the case if one of its
parents exhibited the character a. And in a similar
way a knowledge of the characters shown by the grand-
parents adds something to the certainty of the pre-
diction as to the proportionate numbers of offspring of
the two kinds which are to be expected, when the
average of a number of cases is taken according to the
usual statistical method.

Yule therefore regarded the case of the dominant
character as showing conformity with the law of
ancestral heredity, according to his own statement of
that generalization, which was to the following effect :
The law that ‘ the mean character of the offspring can be
calculated with the more exactness, the more extensive
our knowledge of the corresponding characters of the
ancestry, may be termed the law of ancestral heredity.’*

It may be remarked in passing that Yule’s dis-
tinction of the problems of genetics into those of
intra-racial heredity and those of hybridization cannot
now be regarded as holding good, unless the term
hybridization is to be extended to many cases—e.g.,
that of the inheritance of coat colour in thoroughbred

* “New Phytologist,’ vol. i., p. 202.

MENDELISM AND BIOMETRY 229

horses, which would have been classed unhesitatingly
as instances of heredity by all biometricians in Igoz.
Bateson’s instinct did not fail him when he divided
these problems into those of continuity and those of
discontinuity respectively, although at the present time
the realm of continuous variation and inheritance is
being steadily encroached upon owing to the analysis
of complex characters into definite constituents.

In 1904 Karl Pearson struck a blow at the prospect
of conformity between biometrical and Mendelian
results in his memoir, ‘On a Generalized Theory of
Alternative Inheritance, with special reference to
Mendel’s Laws.’ Pearson’s treatment of the subject
involved advanced mathematical reasoning, and we
can, therefore, only give a brief summary of his main
results. Pearson proposes special terms for the A and
the a elements respectively of a couplet or pair of
allelomorphs. He proposes to call the A element a
protogene, and the a element an allogene, and he thus
distinguishes between the two sorts of homozygotes
by calling AA a protozygote and aa an allozygote.

Pearson considered the case of a population breeding
together at random, in which a single measurable
character, suchasstature, is determined by the combined
action of an indefinite number of pairs of allelomorphs,
and he proceeded to work out the value of parental
correlation which was to be expected under these
circumstances. This value he found to be exactly
one-third, a value which happens to be identical with
Galton’s original determination of parental correlation
from his statistics of human stature. A considerable

230 MENDELISM

number of determinations of parental correlation
have, however, since been made in the case of al]
kinds of characters. The values show considerable
variation, but the average which they indicate is much
nearer to o°5 than to 0°33. Pearson therefore con-
cluded that in none of these cases could anything
resembling Mendelian inheritance be taking place,
and that the latter is, in fact, the exception rather than
the rule.

Mendelians, aware of the certainty of their own
results, and being convinced that these facts must
have a very wide application, were thereupon driven
reluctantly to the conclusion that something was
seriously wrong with the methods adopted by biome-
tricians for determining the coefficients of correlation.
It seems, however, that this conclusion may have been
arrived at with undue haste.

In August, 1906, Mr. Yule read before the Inter-
national Congress of Hybridization assembled in
London a very interesting paper on ‘ The Theory of
Inheritance of Quantitative Compound Characters on
the basis of Mendel’s Laws.’ Though some difficulty
was then experienced in following his argument by
an audience unaccustomed to statistical methods,
Yule’s conclusion is really very simple.

Yule points out that the only character dealt with
in Pearson’s memoir is the number of protogenic or
allogenic couplets present in the individual, and it is the
proportionate number of these couplets present in the
parent and in the offspring respectively which is taken
as determining the value of the correlation coefficient.

MENDELISM AND BIOMETRY 231

Consequently Pearson’s treatment of the subject does
not justify his statement that the Mendelian theory
gives a rigid value for the coefficients of parental cor-
relation for all races and characters—a conclusion
which he regards as fatal to this theory, because the
coefficients for different characters and races, as found
statistically, show considerable individual differences,
and seem to cluster round a value considerably higher
than that indicated by his elaboration of the theory of
the pure gamete. Yule thereupon discusses a somewhat
more general case, and considers the inheritance of a
length made up of a number of distinct segments, each
of which is determined by an independent pair of
allelomorphs. Supposing each segment to take the
length a, b, or c, according as the corresponding proto-
zygote, heterozygote, or allozygote is present, Yule
arrives at an equation from which the correlation
between parent and offspring may be found. From
that equation the following results are deducible :

If there is dominance—+.e., if a=), or b =c, the corre-
lation coefficient is the same as that found by Pearson
—i.é., one-third.

But if the heterozygote always gives rise to a
length exactly intermediate between those due to
the respective homozygotes, the correlation is found
to be one-half.

Cases of partial dominance will give an intermediate
value. Consequently, according to the degree of
imperfection of dominance, and without assuming any
other disturbing circumstances, values of parental
correlation varying from 0°33 to 0°5 are to be expected

232 MENDELISM

on the Mendelian theory of inheritance when applied
to populations. These figures are calculated on the
supposition that there is random mating of the parents,
but if there were a tendency for like to mate with like
the correlation values would become still higher. Yule
therefore concludes that ‘there is therefore no diffi-
culty in accounting for a coefficient of 0°5 on the
theory of segregation, but such a value probably
indicates an absence of the somatic phenomenon of
dominance. In the case of characters like stature,
span, etc., in man this does not seem very improb-
able.’

It is impossible to bring the present chapter to a con-
clusion without some reference to the practical aspects
of the Mendelian discovery. The progress of experi-
mental research in this field during the last half-dozen
years has been so rapid, that there is little ground for
astonishment in the fact that only a small proportion
of those to whom the discovery of the Mendelian
method is of the very highest importance from a com-
mercial point of view have yet arrived at any serious
appreciation of it. The improvement of the breeds of
cultivated plants and domestic animals is a subject of
vital importance to the whole human race, quite apart
from the question of the commercial profit which it
represents for those whose business it is to be directly
concerned with the process—the actual plant- and
animal-breeders themselves.

Hitherto the methods of amelioration which have
been adopted have depended largely upon guess-work,

PRACTICAL APPLICATIONS 233

or at the best upon the result of practical experience.
We are now within sight of the day when a complete
system of precise scientific methods will have been
elaborated. The time required for the development
and application of these methods must chiefly depend
upon the apathy or enterprise of those in whose hands
rests the means of subsidizing this kind of work, for
without proper resources the progress of any such
study must of necessity be slower than it would be
if properly - equipped establishments were at the
disposal of duly trained experimenters receiving an
adequate remuneration.

The practical application of Mendelism cannot be
better illustrated than by an account of Prof. R. H.
Biffen’s work upon the improvement of cereals, particu-
larly of wheat—work which exhibits an extraordinary
contrast in point of scientific exactness with everything
of the kind which has been previously undertaken.
This contrast was remarkably displayed at one of the
morning sessions of the recent International Congress
on Hybridization and Plant Breeding, held under the
auspices of the Royal Horticultural Society. On that
occasion a series of communications upon the subject
of cereals culminated in an admirable account given by
Prof. Biffen of the way in which the problems of their
improvement have been overcome at the experimental
farm of the Cambridge University Department of
Agriculture. And it was a gratifying sign of better
times to observe the enthusiastic interest with which
practical men greeted his communication.

As a preliminary measure Biffen has worked out the

234 MENDELISM

inheritance of a number of comparatively simple
characters, many of which have little practical impor-
tance. But the fact of their strictly Mendelian be-
haviour showed the possibility of readily obtaining any
desired combination of them, and at the same time
rendered it highly probable that characters of a more
practical value to the farmer would prove similarly
amenable to the breeder’s art.

Thus Biffen found that the following pairs of
characters, among others, exhibited simple Mendelian
phenomena, the one placed first being in each case the
dominant :

Beardless ears. Bearded ears.
Keeled glumes. Round glumes.
Felted glumes. Glabrous glumes.
Red chaff. White chaff.
Red grain. White grain.

Thick and hollow stem. Thin and solid stem.

And so on. In other cases, again, the F, generation
showed a character intermediate between those of
the parents, and in F, there appeared a ratio corre-
sponding to A: 2Aa:a.

Thus when Polish wheat (early) was crossed with
Rivet wheat (late), the time of ripening of the F,
generation was intermediate between those of the
parents. In F,, 103 early, 210 intermediate, and
too Jate plants, were counted. Time of ripening is,
moreover, clearly a character which may be of con-
siderable practical importance.

In further illustration of what can be done from a com-
mercial point of view, we will consider the case of two
other characters only—rust immunity and ‘strength.’

PRACTICAL APPLICATIONS 235

There is a quality of wheat grains known as strength
which is essential for the production of a flour such as
can be baked into the kind of loaf which is at present
the only one saleable in England. This quality un-
fortunately happens to be wanting in all the strains of
wheat which it has hitherto been possible to grow at
a profit in this country. For this reason imported
American and Canadian hard wheats, which possess
this quality of strength, are worth in England some
shillings a quarter more than home-grown wheats.

When such strong American varieties are grown in
this country the majority of them are rapidly found to
lose this quality, and to become after a short time as
‘weak’ as ordinary English wheats. Some of them do,
however, retain their strength, and after several seasons
—in one case fourteen—show no signs of deterioration.
An example of a wheat of this latter type is afforded
by Red Fife, which is the basis of the mixed wheat
known commercially as Manitoba Hard, the latter
consisting, as a matter of fact, of a mixture of several
different varieties. Unfortunately these permanently
hard wheats do not yield so large a crop as the com-
monly cultivated English varieties, and so their higher
price does not make up for the smaller number of
bushels per acre obtained when they are grown.

Biffen therefore set to work upon the problem of
combining hardness or strength with the power of
yielding a good crop, and with the other desirable
qualities characteristic of the home-grown varieties.
With this end in view Manitoba Hard was crossed with
a typical English wheat—Rough Chaff.

236 MENDELISM

The F, plants produced grains all of which were
fully as hard as those of the Manitoba variety.

These grains were sown, and it was found that some of
the resulting plants produced strong grains and others
weak ones, and that the former were to the latter very
nearly in the numerical ratio of 3:1. Actually they
were as 152: 48 in a sample of 200 taken at random.

In order to obtain confirmation of this most impor-
tant result, Biffen sent samples of the grains borne
by the F, plants to a well-known authority on milling
wheats, requesting his judgment upon them, but with-
out telling him their manner of origin. The answer
was even more satisfactory than could possibly have
been anticipated. Certain of the samples were stated
by the expert to belong to the variety Red Fife,
which is the name of the particular strain of Manitoba
Hard originally made use of in the experiments,
whilst others were assigned to a definite strain of
ordinary weak English wheat. The segregation of
these characters was, therefore, complete, strength
being a Mendelian dominant to weakness.

In the next generation certain of the dominant
plants, as was to be expected, bred true, and amongst
them were individuals which combined with strength
of grain the other desirable qualities of the second
parent. The problem has, therefore, been completely
solved, and there can be little doubt that when these
new types are brought into general cultivation the
profit obtainable from the growing of wheat in this
country will be increased by several shillings to the
acre of crop grown.

PRACTICAL APPLICATIONS 237

We may next turn to an even more important
achievement. In many countries the annual loss of
crop due to the attacks of yellow rust, Puccinia
glumarum, amounts on a moderate estimate to a con-
siderable number of millions of pounds sterling.
Certain strains of wheat exist, indeed, which are more
or less completely immune to the ravages of this fungus,
but these are usually wanting in other qualities which
are indispensable to the farmer. If it should be found
that immunity to rust is a simple Mendelian allelo-
morph, it would be possible to combine this quality
with any other useful character which obeyed the same
law of inheritance—as several useful characters have
already been shown to do. At one time it must have
been thought that a similar method of inheritance of
the character rust-immunity was too excellent a boon
to be reasonably hoped for.

Among a great number of strains of wheat grown
on the Cambridge experimental farm, several types
showed marked differences in the degree of their
immunity from, or susceptibility to, the attacks of
Puccina glumarum. Among them Mr. Biffen found
one which was apparently quite immune, and, though
grown in the midst of numbers of rusted plants, itself
never showed a trace of infection. Of another type,
known as Michigan bronze, no single individual ever
escaped the rust, and so badly were the plants of this
strain diseased that very few ripe grains could ever be
obtained from them.

Biffen crossed these two types together. In the
first generation every nlant without exception was

238 MENDELISM

badly rusted, but fortunately a considerable number
of ripe grains was obtained, and these were sown
to produce the second generation. When the plants
of this generation had grown up it was observed
that among a majority of badly-rusted plants certain
individuals stood out fresh and green, being entirely
free from infection. On examination it was found that
every plant could be placed in one or other of two
categories—either it was badly rusted or it was
entirely free from rust ; and the numbers of the two
kinds of plants were as follows: 1,609 infected, 523
immune.

It is clear, then, that immunity and susceptibility
to the attacks of yellow rust behave as a simple pair of
Mendelian characters, immunity being recessive. And
it is, therefore, possible to obtain by crossing, in three
generations, a pure rust-free strain containing any other
desired quality which is similarly capable of definite
inheritance.

BIBLIOGRAPHY.

MENDEL, G. J.: Briefe an Carl Nageli.

Bateson, W.: Mendel’s Principles of Heredity, 1909.

Bateson, W.; PuNNETT, R. C.; and SaunpERs, Miss E. R.:
Reports to the Evolution Committee of the Royal Society,
1902-1909.

THE JOURNAL OF GENETICS, I910—

CHAPTER IX

RECENT CYTOLOGY

Every living creature may be regarded as being built
up of a number of structural units which are known
as cells. In the case of some of the simplest animals
and plants, indeed, the whole body of the organism is
composed of a single cell—a small mass of living proto-
plasm, containing, as a rule, only one nucleus. But
in all the higher animals and plants the adult body is
made up of a great number of such cells living in
intimate association with one another.

The living material of which the cell is composed is
known as protoplasm. Protoplasm is a highly com-
plicated and unstaple combination of substances,
amongst the constituents of which the chemical
elements, carbon, oxygen, hydrogen, nitrogen, and
sulphur, play the chief parts. Its consistency is slimy
and semifluid.

Concerning the nucleus—the most essential and
characteristic of cell organs—more will have to be said
later on. Other important organs of cells are a wall
or membrane which externally surrounds them, one
or more vacuoles or cavities containing a watery fluid,
or sometimes a gas, and a certain number of more solid

239

240 RECENT CYTOLOGY

bodies or flastids. Certain plastids present in the
majority of plants are of particular importance as con-
taining the green substance chlorophyll, which plays
an essential part in the fixation of carbon from the
atmosphere.

Amongst unicellular organisms—the creatures
already mentioned as being made up of a single cell
only—those which contain chlorophyll and are pro-
vided with a firm cell wall, built up of a material
known as cellulose, are usually regarded as simple
plants; whilst those in which chlorophyll and a cell
wall are absent are looked upon as simple types of
animals. Similarly slight differences distinguish the
cells which build up the fabric of the higher plants
from those of which the bodies of the more complicated
animals are composed, so that in almost all essential
points an account of the behaviour of the cells of the
members of one kingdom will apply equally well to
those of the other. After a few further preliminary
remarks we shall, therefore, for the sake of simplicity,
speak of a generalized type of cell, the behaviour of
which, except in points of detail, will resemble that of
the actual cells of plants or animals indifferently. But
in order to convey a more definite idea of an uni-
cellular animal to those who are unfamiliar with the
rudiments of Elementary Biology, we may refer briefly
to the well-known form Ameba, which will serve as
an excellent type of an animal consisting of a single
free-living cell.

This little creature consists of a mass of protoplasm
enclosing a nucleus which is more or less centrally

THE CELL 241

situated and approximately spherical in form. The
protoplasm is divided into an outer hyaline and an
inner granular portion, the former being limited exter-
nally by a very delicate membrane. The shape of the
animal is irregular, and, moreover, undergoes gradual
alteration owing to the characteristic amceboid move-
ments. These consist in a slow protrusion and with-
drawal of processes of the body, enabling the animal
to change its position by a kind of flowing movement,
and also to engulf its food, which consists of various

Fic. 17.—AMCEBA. Fic. 18.—PLEUROCOCCUS,

n, Nucleus; f, food particle. n, Nucleus; w, cell-wall ;
chl, chloroplast.
minute organic particles, by the simple process of
flowing around it.

In contrast with Ameba the unicellular plant Pleuro-
coccus is motionless, and is surrounded by a firm wall
of cellulose. In addition to a central nucleus, the
plant contains, embedded in its peripheral protoplasm,
several plastids which bear the chlorophyll concerned
in the assimilation of carbon from the gases of the
atmosphere. This chlorophyll lends a green colour to
the whole contents of the cell, and in its natural habitat
the plant is quite conspicuous. The green powdery

substance often to be seen on the bark of trees,
16

242 RECENT CYTOLOGY

especially on the side turned to the north, and in
similar shady situations, consists, as a rule, of great
numbers of minute Pleurococcus plants, although the
size of a single specimen may be represented by a
diameter of little more than the two-thousandth part
of an inch.

We are more particularly concerned, however, with
the higher animals and plants, the bodies of which are
built up of a great number of separate cells. Some
of these cells may be modified in various ways, but they
all conform, at least in the youthful condition, to types
not far removed from those of Ameba and Pleurococcus
respectively. Certain parts of these higher organisms,
indeed, such as the bones of vertebrate animals and
the wood of trees, do not consist solely of living cells,
but are composed to a great extent of dead material
excreted or built up by the activity of living cells.
These latter have, then, either ceased to live, or they
may continue to exist in the interstices of the hard
skeletal framework.

New cells come into existence in only one way—
namely, by a process of division which takes place in a
pre-existing cell. In comparatively rare cases a cell
may give off a small bud which forthwith develops
into a new cell like the old one. In such a case we may
speak of the cell which gives off the bud as the mother-
cell, and of the cell into which the bud develops as
the daughter-cell. But by far the most frequent
method of cell-reproduction, and the only one which
is characteristic of the higher animals and plants, takes .
place by the equal division of an old cell into two

THE CELL 243

new ones. In this case, it is only by a stretch of
Janguage that we can speak of parent- and daughter-
cells, for the individuality of the pre-existing cell is
completely lost, and two fresh individualities have now
taken its place.

Since all the cells of the animal or plant body arise
by the bipartition of pre-existing cells, it is clear that if
we follow these processes far enough back, in the case
of any individual organism, we may arrive at a period
at which only one cell was present. And under
ordinary circumstances this is actually the case.
Every individual among the higher animals and plants,
arising by the ordinary sexual method, existed at the
earliest stage of its embryonic history in the form of a
single cell, the fertilized ovum. And the first obvious
process in the development or embryology of the young
organism consisted in the division of this primitive
cell into two new cells. Each of these new cells then
divided again in like manner, and the multiplication of
cells continued until all the innumerable cells which
build up the organs of the adult body had finally
come into existence. When growth is completed cell-
divisions continue more slowly, producing new cells to
make good the wear and tear of the bodily tissues.

As the number of cells increased, their relation to
one another in space was constantly changing. Dif-
ferent cells, too, became modified in different ways ;
for instance, the cells on the outside of the young
embryo took on a different form from those within, in
accordance with the different conditions to which

they were exposed, and a host of other changes took
16—2 .

244 RECENT CYTOLOGY

place too numerous for us to follow in detail. Thus
the complicated structure of the adult organism was
gradually arrived at by a process of development in
which cell-multiplication played a most prominent and
essential part.

We have next to inquire what is the method of origin
of the original embryonic cell—the fertilized ovum—
from which the new animal or plant develops.

As is indeed implied by the expression ‘ fertilized
Dvum,” this cell arises by the fusion together of two
independent cells, such fusion constituting the process
of fertilization or impregnation. One of the cells
which took part in the fusion was derived from one
parent organism, and bore the distinguishing character-
istics of the cells which composed that parent—or at
least some part of those characteristics—whilst the
other was in like manner derived from the second
parent.

It is to be observed that this fusion together of a
pair of cells, derived (in the case we are considering—
namely, that of ordinary biparental reproduction)
from two separate individuals, results in the formation
of a complete new individuality, which arises definitely
at that point of time at which the fusion of the two
conjugating cells takes place. In this way the cells
of the offspring are seen to be of double origin, and it
is found that traits and characters derived from both
the father and the mother can co-exist in them side by
side.

The cells which take part in the above-mentioned
fusion are known as gametes, or germ-cells—male and

THE CELL 245

female respectively, according to the sex of the parent
from which each is derived. In animals the female
gamete is known as the ovum, and the male as the
spermatazoon, and the product of their fusion, as already
said, is called the fertilized ovum. Germ-cells of a
similar kind arise in a slightly different way in plants.
The germ-cells are produced in special parts of the
organism known as the generative organs, which in
flowering plants are represented by the pistils and
stamens.

A more convenient expression for the fertilized ovum
is that of zygote, a term which we have previously
encountered in fhe shape of the homo- and hetero-
zygotes of the Mendelian. By an expansion of meaning
the term zygote is also used to express the whole
organism which ultimately arises from the product of
fusion of a pair of gametes, and by this use the impor-
tance of the gamete, as opposed to the zygotic organism
as a whole, is brought into due prominence.

We find, then, that the succession of generations in
the higher animals and plants, according to the common
use of this expression, depends upon the succession
of a much larger number of cell-generations. By
repeated divisions, each giving rise to a new generation
of cells, the fertilized ovum gradually develops into
the adult organism. By the division of certain
members of the later generations of cells which compose
this organism the gametes are produced. By the
conjugation of a pair of gametes a zygote of the
second generation arises, and the same processes are
continually repeated.

246 RECENT CYTOLOGY

Each of the cells hitherto referred to possesses a single
nucleus, which is usually a more or less spherical body
occupying a central position within the cell. Nuclei,
like the cells which contain them, arise only by the
division of pre-existing nuclei. Thus the history of
the nuclei is in every way similar to the history of the
cells, of which they constitute so important a part.
In fertilization the nuclei of the conjugating cells or
gametes fuse together to form the single nucleus of
the fertilized ovum, and every division of this cell, as
well as of its cell-progeny, is preceded by a division
of the nucleus into two similar portions.

We may forthwith concentrate our attention upon
the nucleus as being that part of the cell which is of
primary importance from the point of view of heredity,
for it is now generally recognised that the nucleus
is the part of the cell in which hereditary features
are in some way carried. And we may next consider
a little more closely the structure of the nucleus as
seen under high powers of the microscope. :

In what is somewhat improperly called its resting
condition—a condition which is characteristic of nuclei
at all times when they are not actually undergoing
division, or preparing for that process—the nucleus may
be seen to be bounded by a more or less definite nuclear
membrane. The internal structure of such a nucleus is
described as reticular—that is to say, at least two
different substances are differentiated within the
nucleus, one of them forming a reticulated meshwork,
the interspaces of which are occupied by the other

(Fig. 19, p. 232).

THE NUCLEUS 247

In entering into a detailed description of the changes
which take place in the finer structure of the nucleus,
it must be clearly understood that the more minute
features alluded to are only to be seen with any degree
of definiteness in dead cells which have been killed
practically instantaneously by the action of some
powerful chemical poison. Under suitable conditions
it is believed that treatment of this kind fixes the con-
stituent parts of the nucleus in very nearlv the same
relative positions as they occupied in life at the
moment immediately preceding the death of the cell.
The tissues containing the cells to be examined are then
usually cut into very thin sections, and other chemicals
are applied to them, the result of this treatment being
to stain different parts of the nucleus of different
colours and with different degrees of intensity. “It is to
the behaviour of the structures thus made visible that
our description applies, since it is impossible to follow
these changes in actually living cells except to a very
imperfect extent. It may be pointed out, however,
that we have every reason for believing that the
differential effect produced by the processes of fixing
and staining only serves to render more clearly visible
rea] differences which actually existed during the life
of the cell, and some indications of many of these
differences have even been actually seen in living cells
under exceptionally favourable conditions.

The nucleus, when treated in the manner described,
is seen to be built up of a network of branching fibrils,
the meshes of which enclose a comparatively clear and
hyaline substance. The fibrils of the network are made

248 RECENT CYTOLOGY

up of a material of comparatively weak staining
capacity ; embedded in this substance are numerous
granules of a very intensely staining material which
is known as chromatin. There are strong reasons for
believing that the chromatin of the nucleus is of
special importance from the point of view of the
mechanism of heredity. This reticular structure of
the nucleus is indicated in a diagrammatic fashion in
Fig. Ig.

Further light is thrown upon the detailed structure
of the nucleus by the changes which become visible
during the process of nuclear division. This process,
which is known as mitosts, we must now proceed to
describe.

In the description of mitosis which follows, the
account of this process has been somewhat generalized
and simplified, and Figs. 19 to 26, which illustrate the
phenomena, are purely diagrammatic. It is hoped
that the most important features of this complicated
process may be in this way rendered comprehensible ;
and although in different organisms considerable
variations in the details of the process are to be met
with, yet in their general features all ordinary mitoses
in animals and plants are believed to conform to the
essential type of our description.

The first change in the appearance of the nucleus
which indicates that a division is about to take place
consists in a rearrangement of the chromatin network,
which now takes on the appearance of a tangled
thread (Fig. 20). The outwardly-directed loops of
this skein often correspond to the separate portions

CHROMOSOMES 249

into which the thread eventually breaks up. The
thread gradually grows shorter and thicker, and
presently becomes divided into a number of pieces
which are known as chromosomes. In the chromo-
somes the shortening and thickening process is con-
tinued until these bodies arrive finally at the form of
stumpy rods, each of which often becomes bent into

FIG. 19. FIG. 20. FIG. 21. FIG. 22.

FIG. 23. FIG. 24. FIG. 25. Fic. 26,

the form of a horseshoe. Meanwhile the nuclear
membrane breaks down, so that the hyaline substance
of the nucleus becomes continuous with that of the
cell body surrounding it. A fresh phenomenon now
becomes visible. A spindle-shaped arrangement makes
its appearance consisting of a number of minute fibrils
which connect together two points—the poles of the
spindle — situated at opposite ends of the cell. The

250 RECENT CYTOLOGY

chromosomes now change their position so that they
come to lie in the plane of the equator of the spindle,
and about this time, but sometimes earlier, each
chromosome splits longitudinally into two equal
portions (Figs. 22, 23). This splitting in the case of
each chromosome takes place in the equatorial plane
of the spindle, so that one member of each pair of
daughter chromosomes faces towards one pole of the
spindle, and the second towards the other pole. The
members of each pair of daughter chromosomes now
begin to move away from one another towards the
two poles of the spindle, and as they do so the first
indication of a dividing wall between the two new
cells begins to make its appearance in the equatorial
plane.

Arriving at the poles, the daughter chromosomes
begin to elongate, and to put out processes which
finally meet and fuse with those of their neighbours
to form the chromatin reticulum of the new nuclei
(Fig. 25). Surrounding each new nucleus, thus
developing at either pole of the now rapidly dis-
appearing spindle, a new nuclear membrane makes its
appearance ; the dividing wall in the position of the
equator of the spindle develops into a complete
partition (at least in the case of plants, in which,
however, a number of minute passages are left pene-
trating the cell-wall and preserving the communication
between the protoplasmic contents ofthe separate
cells); and the division into two new cells is thus
completed (Fig. 26). Each new cell is provided with
a nucleus into which has entered precisely its fair

CHROMOSOMES 251

share of the chromatin which was present in the
parent nucleus.

A great deal of evidence has recently accumulated
to show that chromosomes are very definite and
important organs. In the first place, the number of
chromosomes which make their appearance at each cell
division is the same in all the cells of any given |
creature, and this numerical constancy further extends
to the cells of all the members of a particular species,
though in members of allied species the number of
chromosomes may be different. In widely separated
species the number of chromosomes varies consider-
ably ; thus from 2 to 200 have been counted in the
case of various different members of the animal and
vegetable kingdoms. One of the commonest numbers
found fs twelve, and this number occurs in a con-
siderable variety of different animals and plants.

Next it has been shown that the chromosomes
which arise at the beginning of a nuclear division are
identical with those daughter chromosomes of the
preceding division which originally entered into the
nucleus now about to divide. An example of the
kind of evidence upon which this conclusion is based
may next be given.

Figs. 27, 28, and 29 show the three possible arrange-
ments of the four chromosomes which are found in the
cells of the worm-like animal Ascaris, as seen from the
direction of the pole of the spindle in the dividing
nucleus. Of these arrangements, that: shown in
Fig. 29 is much the least common. Now in this par-
ticular case the chromosomes, when they first make

252 RECENT CYTOLOGY

their appearance immediately before the process of
division, are found with their extremities situated in
little pockets or bulgings of the nuclear membrane,
so that their exact position is very definitely marked ;
and the arrangement of the chromosomes may be any
one of those already indicated. Boveri observed that

BLE YK

FIG. 27. FIG. 28. FIG. 29.

in the case of two neighbouring cells which had
originated by the division of the same mother-cell, the
chromosomes made their appearance in both cases in
the uncommon position of Fig. 29. Figs. 30 and 31
indicate their actual arrangement. The conclusion to
be drawn from this observation is that the same

chromosomes have preserved their individuality right
through the resting stage of the nucleus, to reappear
in the same position at the outset of a new phase of
division.

It is believed, then, that the same stages which the
chromosomes passed through at the close of one

CHROMOSOMES 253

nuclear division, giving rise to the nuclear reticulum
in the daughter nucleus, are repeated in the reverse
order at the outset of the next division; the same
processes are withdrawn into the same chromosomes,
and these shorten into structures identical with those
which passed into the nucleus at its first formation,
except that they have increased in bulk during the
interval.

Boveri, in fact, concludes that the separate chromo-
somes are to be looked upon as distinct individuals—
almost as separate simple organisms—which preserve
their individuality throughout the history of the cell,
and reproduce themselves, just as cells and nuclei do,
by a process of bipartition. As far as the chromo-
somes themselves are concerned, their typical or
resting form is that of the short simple rods seen in
mitosis. The branched anastomozing character seen
during the stage of the nuclear reticulum is associated
with the active co-operation of the chromosomes in the
physiological processes going forward within the
nucleus. For this reason the term ‘resting stage’
applied to this condition of the nucleus is a particularly
inappropriate one.

Boveri illustrates the amount of credence which he
would attach to this theory of the individual persis-
tence of the chromosomes throughout the resting
condition of the nucleus, by means of the following
analogy : ‘ We make water from oxygen and hydrogen,
and from this water we can obtain oxygen and hydrogen
again in the same proportions. Just in the same way
as the chemist on the evidence of these facts regards

254 RECENT CYTOLOGY

water as containing oxygen and hydrogen, although
the properties of these substances are completely in
abeyance, so I believe it to be with equally good
reason that our theory regards the individual chromo-
somes as being preserved in the resting nucleus.’*

Since Boveri expressed this opinion Rosenberg has
produced further evidence of an equally convincing
kind. He finds that in the case of certain plants the
chromosomes do not pass over into a continuous
reticulum during the resting condition of the nucleus,
but remain separate, so that the same number of
chromatic bodies can be counted during this stage as
during the actual process of mitosis.

Boveri has also produced evidence to show that
different chromosomes play different parts in the
economy of the organism. For example, when dif-
ferent chromosomes were artificially removed from
the nucleus of an embryonic cell by taking advantage
of certain abnormal methods of division, the embryos.
which arose from these cells developed to different
extents and in different abnormal ways.

This result is of particular interest, because it gives
full corroboration to the suspicion, previously enter-
tained, that the chromosomes are specially concerned
with hereditary processes—with the building up of
particular parts of the developing organism into shapes
which resemble those of the corresponding parts dis-
played by other members of the same species; and it
seems further to show that particular chromosomes

* Dr. T. Boveri, ‘Ergebnisse iiber die Konstitution der
Chromatischen Substanz des Zellkerns,’ p. 22.

CHROMOSOMES 255

may be specially concerned in the development of
particular parts.

Sutton has recently shown that the different chromo-
somes contained in the same nucleus of a particular
animal may be of different shapes and sizes, so that each
is individually recognisable. It was thus possible to
demonstrate that an identically similar set of chromo-
somes appeared at each of several successive cell
divisions. In this way additional evidence is afforded
of the individual persistence of the chromosomes and
of their separate identity.

We have already pointed out how, in the process of
fertilization, the two conjugating germ-cells, as well
as the nuclei which they contain, become completely
fused together to form a single cell containing only one
nucleus. It might have been expected that the sepa-
rate chromosomes contained in the conjugating nuclei
would also fuse together in pairs during this process, but
this is not the case. The paternal and maternal chromo-
somes remain separate, so that the nucleus of the zygote
contains twice as many chromosomes as does that of
either of the gametes by. the fusion of which it arose.
This double number of chromosomes reappears at every
cell division during the embryonic history of the
zygote, and thus the fact is accounted for that the
number of chromosomes in a somatic nucleus is always
even.* Thus we see that the chromosomes derived
from the two parents are present side by side in the
nuclei of the offspring, and reproduce themselves by
bipartition at every nuclear division which takes place

* See, however, p. 270 for an exceptional case.

256 RECENT CYTOLOGY

in the zygote. In this way every somatic nucleus of
the latter contains a double set of chromosomes, half
of them being descended from the chromosomes intro-
duced by one parent, whilst the other half came from
the second parent.

There is reason to believe that the set of chromo-
somes derived from one parent is complete in itself,
containing everything necessary for the development
of a normal individual. Indeed, in some cases of
parthenogenesis (development of the unfertilized egg),
egg cells have been known to develop which contained
only a single set of chromosomes. Boveri proved
very prettily that the paternal set of chromosomes is
equally adequate for complete development. By dint
of violent shaking Boveri contrived to remove the
nucleus from the egg-cells of a sea-urchin, and he
afterwards allowed a sperm-nucleus to enter the
enucleated egg, which presently developed into a
complete embryo. Thus it was shown that the
paternal as well as the maternal set of chromosomes
is sufficient by itself to determine the proper production
of all the organs of the embryo. But Boveri also
showed that if any chromosome of the paternal (or
maternal) set were wanting in such a case, normal
development of the embryo could no longer take place.
Let it once more be emphasized that the somatic cells
of an ordinary organism contain a double complement
of essential nuclear material.

Since the gametes contain only half as many chromo-
somes as the somatic cells, and since the number of
chromosomes present in the latter is constant for each

CHROMOSOMES © 257

species, it follows that either during the formation of
the gametes, or at some one or other of the cell
divisions leading up to their formation, there must occur
a reduction in the number of chromosomes to one-half
of their former number. In the case of the higher
animals this reduction takes place during the two cell
divisions which directly lead up to the formation of the
gametes themselves. In plants, on the other hand, the
reduction takes place during the formation of those cells
which are known as spores. From these, after a certain
number of intervening cell generations, the gametes
take their origin. These intervening cell divisions in
plants are characterized in every case by the appear-
ance of the reduced number of chromosomes. In
the higher plants, in fact, a generation is, as it were,
interposed between the reducing division and the
actual formation of the gametes. For the spores are
themselves unicellular reproductive bodies like the
gametes, but differ from the latter in the fact that
they develop without undergoing conjugation, and
give rise to a larger or smaller mass of tissue consisting
of cells with the reduced number of chromosomes.
From the fact that the cells of this gamete-bearing
generation contain half as many chromosomes as
those of the spore-bearing generation with which it
alternates, the generation produced from the spores
has been spoken of as the x-generation in contrast
with the ordinary, or 2x-, generation. In animals the
x-generation is reduced to a single generation of cells
only, which is represented by the gametes themselves.

We must next proceed to examine the actual
17

258 RECENT CYTOLOGY

method by which the reduction in the number of the
chromosomes is brought about.

The simplest type of the process of reduction of the
chromosomes takes place at the formation of the male
germ-cells, or spermatozoa, of animals. For the sake of
clearness we shall consider the case of an animal in
which the somatic cells contain four chromosomes only,
and in which the reduced number characteristic of the
gametes is therefore two.

The reduction in number of the chromosomes takes
place during two successive cell divisions which
immediately lead up to the formation of the germ-
cells. A particular mother-cell divides twice in rapid
succession, and the four cells thus arising develop into
spermatozoa without further subdivision. During
these two nuclear divisions the somatic number of
chromosomes becomes halved, giving rise to the
number characteristic of the gametes.

Immediately before the first of these divisions the
chromosomes become closely associated together in
pairs, and in certain cases it has been shown that one
member of each pair is very probably the descendant
of a chromosome derived from the male parent, whilst
the other member of the pair is the descendant of the
corresponding maternal chromosome.

This association of the chromosomes in pairs may
be so close, and may take place so early, that when
these bodies are first visibly differentiated only half
the usual number of them is to be seen. But in these
cases, too, it is reasonable to believe that each of
the chromosomes actually visible consists of a maternal

THE REDUCING DIVISION 259

and a paternal member fused together. Each of the
visible chromatic bodies next divides into four parts,
the set of four deeply staining bodies being known as a
tetrad. Thus when there are four somatic chromosomes
the number of tetrads appearing will be two (Fig. 32).
A mitosis now ‘takes place, during which there is no
further division of chromosomes, but half of each

Fig.32 Fig.33 Fig.34 *

Fig.37

tetrad passes to either pole of the nuclear spindle,
so that each daughter nucleus comes to contain two
half-tetrads, each consisting of a pair of deeply-staining
bodies (Fig. 34). This division is not followed by the
production of a resting nucleus, for before any nuclear
reticulum is formed, and while the half-tetrads still
retain their definite appearance, the daughter nuclei
divide again. At this second division in each nucleus
17—2

260 RECENT CYTOLOGY

the separate members of each of the two half-tetrads
pass to opposite poles (Figs. 35, 36). In the nucleus
of each of the four cells which thus arise there is, there-
fore, present one quarter of each of the four chromo-
somes which originally appeared—one member, that
is to say, of each tetrad (Fig. 37). Each of the cells of
which we have thus traced the origin develops directly
into a single spermatozoon.

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The method of development, or maturation, of the
ova, or egg-cells, of animals is in all essential respects
similar to the process by which the spermatozoa arise.
It differs, however, in the fact that of the four cells
which result from the corresponding divisions, one is
very large and constitutes the ovum, whilst the other
three are very minute, and are apparently of no further
importance. In the accompanying diagrams (Figs. ‘40
to 42), the smaller cells, or polar bodies, have been

THE REDUCING DIVISION 261

enormously exaggerated relatively to the size of the
ovum itself.

The original tetrad is believed in all cases, and has
been actually observed in a few cases, to arise by a
separation of the two fused chromosomes, followed by
a division of each of these bodies into two. In cases
where the chromosomes retain their rod-like appear-
ance throughout these changes there would seem to be
some doubt as to whether the first of the divisions
giving rise to the ‘ tetrad ’ is transverse or longitudinal
in direction, and it is possible that the process may be
different in different cases. But it is generally agreed
that the first division separates the two original
chromosomes, and that at the first of the two nuclear
divisions which ensue the members of a pair of parental
chromosomes pass into separate nuclei. The second
division, on the other hand, like an ordinary mitosis,
separates halves of chromosomes. This agreement
among authorities is explained by the circumstance
that those observers who have seen a longitudinal
first division believe that the parental chromosomes
conjugated side by side, whilst those who describe a
transverse division describe also an end-to-end con-
jugation of the chromosomes.

The first of these two ideas is the one illustrated in
the accompanying diagram. (Fig. 43), representing the
behaviour of a single pair of parental chromosomes
during the two nuclear divisions which give rise to
four sperm cells. The chromosome derived from one
parent is shaded, whilst the other is left blank.

Thus the first of the two gamete-producing divisions

262 RECENT CYTOLOGY

differs from all other mitoses in the fact that in it an
actual separation of whole chromosomes takes place ;
it is a qualitative and not only a quantitative division.
It is to this mitosis that the term reducing division is
properly applied.

We have to notice that at one stage of the process
now described the chromosomes derived from the two
parents are in a close state of fusion. It would seem
as if the actual conjugation of chromosomes, which
failed to take place when the conjugating gametes and
their nuclei fused together in the formation of the
zygote, was only delayed, and now occurs hundreds or

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thousands of cell generations after the actual process
of fertilization, and immediatély before the production
of those cells which are to give rise to the new
individual.

It may be pointed out that, although the chromo-
somes which emerge from this fusion seem to be
identical with those which entered into it, yet it is
difficult to believe that they have not undergone some
change, or exercised some mutual influence upon one
another. If no such influence has been exerted, it is
difficult to imagine any possible reason for the process
of fusion taking place at all.

In the higher plants a similar reducing division

FIG. 43.

THE REDUCING DIVISION 263

takes place at the formation of the spores, which arise
in sets of four, each set corresponding to a group
of four spermatozoa, or to the ovum and the three
polar bodies of an animal. In the case of flowering
plants the nuclei contained in the spores make a few
further divisions, at each of which the reduced number
of chromosomes is to be observed, and one or more of
the cells thus finally produced take on the character of
germ-cells. The spores are of two kinds, large and
small, the latter being the pollen grains. The larger
spores give rise to female gametes and the smaller to
male, and fertilization takes place in the ordinary
manner bya fusion between the nuclei of these germ-
cells.

We have seen so far that the number of chromosomes
contained in the somatic nuclei of a given species is
always the same, and is always even. We have also
seen that this number is made up of two separate sets
derived respectively from the two parents, and that
the members of the two sets preserve their separate
individuality right through the long series of nuclear
divisions which take place during the development of
the individual zygote. A fusion of chromosomes of
paternal and maternal origin respectively takes place
only in the direct line of ancestry of the germ-cells
which are destined to give rise to new members of the
species. This process of fusion takes place in animals
immediately before the formation of the actual germ-
cells, but in plants a larger or smaller number of cell
generations earlier. After fusion the paternal and
maternal chromosomes apparently separate, and the

264 RECENT CYTOLOGY

nuclear division which ensues differs from all other
mitoses in the fact that instead of merely dissevering
halves of chromosomes, the actual somatic chromo-
somes separate and become distributed equally between
the resulting nuclei; so that in these nuclei, and in the
germ nuclei which arise by their division, the number
of chromosomes is reduced to half the somatic number.
When fertilization takes place the somatic number of
chromosomes is restored by the union of nuclei, each
of which contains half that number.

Is it possible to throw any further light upon the
meaning of these facts regarding the behaviour of the
minute constituent parts of organisms ?

Let us return to Mendel’s experimental discovery,
of which an account was given in the last two chapters,
and let us consider the case of a cross between parents
which differ in respect of two pairs of allelomorphs.
Expressing these pairs as A-a and B-b, Mendel
showed that the germ-cells of the cross-bred or hetero-
zygote bear in equal numbers the combinations AB, Ab,
aB,and ab. Now, it seems clear from this behaviour
that the allelomorphs must be represented in the cells
of the organism by some kind of definite particles,
which remain distinct from one another throughout
all the cell divisions of the body, since we know that
at the formation of the germ-cells these characters are
capable of becoming completely segregated. Let us,
then, trace the behaviour of the allelomorphs in a
diagrammatic way, regarding each as a distinct par-
ticle. These particles we may distinguish by certain

AND MENDEL’S LAW 265

letters. A and a are the allelomorphs of one pair,
B and b those of the other, and we will suppose that
one of the parents exhibits the characters A and b
and the other the characters a and B (Fig. 44). Then,
in the zygote resulting from fertilization, A, a, B, aut b
will all be present.

Since all the cells, at least in the direct line of
ancestry of the gametes, must contain every allelo-
morph, it will be necessary for the particle representing
each allelomorph always to divide into two before a
cell division takes place, for only in this way can some-
thing corresponding to each allelomorph pass into each
of the two cells produced by the division. And a
similar process will be repeated at each somatic
mitosis (Fig. 44). At the formation of the germ-cells,
however, or at some preceding cell division, the two
members of each pair of allelomorphs must become
separated from one another in such a way that the
particles originally derived from different parents
pass over into different cells. When two pairs of
allelomorphs are concerned, this process of separation
can take place in either of the two ways shown in
Fig. 45. And the experimental evidence shows that
the two methods occur with equal frequency in the
formation, of the germ-cells of the same heterozygote.

Anyone who has succeeded in following the above
account of the behaviour of the supposed particles
representing Mendelian allelomorphs in the cells of a
hybrid organism, on comparing it with the preceding
description of the behaviour of chromosomes in the

RECENT CYTOLOGY

266

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AND MENDEL’S LAW 267

somatic and reducing divisions respectively, can
scarcely fail to be struck by the extraordinary simi-
larity between the two processes. It seems quite
clear that there must be some real connection between
the behaviour of chromosomes as seen microscopically
on the one hand, and the behaviour of allelomorphic
characters as deduced from the results of experiment
on the other ; and that the evidence derived from these
two forms of study is bound to be of considerable
mutual benefit.

At first sight it might be thought that the chromo-
somes are the actual bearers of Mendelian characters,
in the sense that each chromosome represents a single
allelomorph ; and, indeed, there is no fundamental
difference between the behaviour of chromosomes and
that of our supposed character-bearing particles. But
there is, at least in some cases, a fatal objection to
this belief in the fact that in certain plants the number
of separate allelomorphic pairs which may be born by
a hybrid is greater than the reduced number of chromo-
somes which the germ-cells of this hybrid contain.
For instance, in the case of the pea the reduced
number of chromosomes is seven, and Mendel himself
described the behaviour of seven independent pairs of
allelomorphs in peas. Recent study has revealed the
presence of at least four additional pairs of allelo-
morphs in these plants, all of which are probably equally
independent of one another.

We must, therefore, seek a different explanation,
and de Vries has recently suggested one which up to

268 RECENT CYTOLOGY

the present time appears the most likely to represent
the true account of the phenomena. De Vries’ ex-
planation is associated with the finer structure of the
chromosomes themselves, a subject upon which we
have not hitherto entered. Under high powers of the
microscope, and after very careful preparation, it is
possible to observe that each chromosome contains a
number of separate darkly-staining granules which are
known as chromomeres. When the pairs of parental
chromosomes fuse together previous to the reducing
division, the chromomeres which they contain appear
to meet together in corresponding pairs. The members
of each pair fuse together completely, afterwards
separating as the chromosomes separate;

Fic. 46.

De Vries supposes the Mendelian allelomorphs to
be contained in the chromomeres, and that when these
granules fuse together an exchange of allelomorphs
takes place between the chromosomes. This ex-
change proceeds in such a way that when the chromo-
somes separate after fusion, it is a matter of simple
chance whether a particular allelomorph has remained
in the chromomere which originally contained it, or
has passed over into the other member of the pair.
Thus, in a sufficient number of cases we should get
all possible chance distributions of allelomorphs be-

CHROMOMERES 269

tween the two chromosomes, except that, of course, the
two members of the same pair of allelomorphs would
never coexist in the same chromosome. Since the
‘two chromosomes of a pair pass into different germ-
cells, precisely that chance distribution of allelomorphs
which is required on the Mendelian theory would thus
be arrived at.

De Vries’ explanation throws light on one pheno-
menon which is not accounted for on the supposition
that each chromosome represents a separate allelo-
morph. In the diagrams previously given of the
behaviour of Mendelian characters within the cells we
have given no indication of a conjugation in pairs
previous to the reducing division. Such a process of
fusion is, however, one of the most marked phenomena
in the behaviour of the chromosomes at the parallel
stage of their existence. On the chromosome-allelo-
morph view, the phenomenon of mitosis as bringing
about an equal division of hereditary particles between
the cells, and the process of reduction in the number
of the chromosomes, are both accounted for, but there
is no explanation of the fusion between the pairs of
chromosomes. On de Vries’ view, however, this pro-
cess is necessary in order to bring about the necessary
redistribution of allelomorphs between the chromo-
somes, and so between the germ-cells into which the
latter pass.

In cases where the phenomenon of correlation or
coupling has been observed we must suppose that there
is some mechanism which causes the representative

270 RECENT CYTOLOGY

particles of the respective characters concerned to
remain in company during the process by which the
other allelomorphs are being reassorted between the
chromosomes. Of this process of coupling the cytolo-
gists have not yet been able to observe any visible
indication in the behaviour of the chromosomes, any
more than they can really see the redistribution of the
supposed factors carried by the chromomeres. But
apart from this it must be allowed that the facts of
experiment and of microscopic observation fit in with
one another in a remarkable way, and that the Men-
delian theory throws considerable light on the minute
features of cell anatomy.

The possibility still remains that in certain cases
particular characters may be associated with par-
ticular chromosomes as a whole, and we shall next
proceed to describe what actually seems to be an
example of this sort.

The case we have to describe is directly concerned
with one of the most interesting and elusive of bio-
logical problems—namely, the problem of the heredity
of sex. Until recently opinion has been largely
dominated by the view that sex-production is in
general controlled by the influence of external con-
ditions. A large number of the earlier researches,
and some of the later ones, have, in fact, seemed to
show that sex is thus determined. The most recent
knowledge appears, however, to point conclusively to
the belief that sex is already determined in the fer-
tilized ovum. The fact that, so far as the evidence

HEREDITY OF SEX 271

goes, where more than one individual develops from
the same fertilized egg all are of the same sex seems
to point conclusively in this direction ; and further
valuable evidence has recently been adduced. Prof.
E. B. Wilson has recently investigated the behaviour
of the chromosomes in the somatic cells and in the
germ-cells of a particular species of insect known as
Protenor belfragi. The case afforded by this animal
is remarkable inasmuch as the somatic cells in the
male, and only in the male, contain an odd number
of chromosomes. An irregularity is accordingly intro-
duced into the process of fusion of the chromosomes
in pairs, which, as already described, always precedes
the formation of the germ-cells with their reduced
number of chromosomes. In the case of the male
Protenor all the chromosomes fuse in pairs except one,
which is, of necessity, left over. This odd chromosome
is described as the heterotropic chromosome. The
female Protenor has one more chromosome in its
somatic cells than the male, thus making up an even
number; that is to say, in the female the pair to
the odd chromosome of the male is present, so that
there are two heterotropic chromosomes, or idio-
chromosomes. These fuse and separate in the reducing’
division, which thus proceeds in the normal manner
in this sex. In the male, on the other hand, when
the reducing division occurs, the heterotropic chromo-
some passes complete into one of the resulting cells.
In the second gamete-producing division, every chro-
mosome present having divided into two, the products

272 RECENT CYTOLOGY

of this division pass into different gametes. These
latter divisions are of two kinds, since in one of them
the heterotropic chromosome takes part, whilst in the
other it is wanting; consequently, two out of the
four spermatozoa eventually produced contain the
heterotropic chromosome and two do not. (Only one

a

Somatic cells
Fusion of
Chromosomes

&
v
o~
/,
@ Spermatazo
i

+O
oS
Qee
ee)
er)
9 DM
x
+
@
i}

Fig. 47.

ov, ovum ; #b, polar body; sp, spermatozoa (a and b the two kinds).

spermatozoon of each kind is shown in Fig. 47.) Thus
there is a differentiation of the spermatozoa into two
different kinds, and one of these kinds contains a
chromosome less than the other. On the other hand,
every egg (as well as every polar body) contains an
idiochromosome.

In fertilization some of the eggs become impregnated

HEREDITY OF SEX 273

by spermatozoa containing the heterotropic chromo-
some. Such eggs invariably develop into females
having a pair of idiochromosomes in each somatic
cell. Other eggs are fertilized by spermatozoa lacking
the heterotropic chromosome, and these become males,
their somatic cells containing only the single hetero-
tropic chromosome derived from the egg. The ac-
companying diagram illustrates the behaviour of the
chromosomes during these processes. The hetero-
tropic chromosomes are represented as black, whilst
the remaining chromosomes are left white, and for the
sake of simplicity only two pairs of the latter are
indicated in the somatic cells.

When the above facts are taken into consideration,
it is scarcely possible to doubt that there is a causal
relationship between the characteristics of the female
sex and the presence of two heterotropic chromo-
somes, and that a similar connection exists between
maleness and the presence of only one. Let us trace
this relationship a little further.

The facts clearly prove, in the first place, that the
unpaired heterotropic chromosome alternates between
the two sexes in alternate generations, passing from
the male to the female in the production of females,
and from the female to the male in the production of
males (see the diagram).

Assuming that these particular chromosomes are
really concerned in the determination of sex, Wilson
suggests the following interpretation on Mendelian
lines. Since the heterotropic chromosome is the only

18

274 RECENT CYTOLOGY

one present in the male, it must represent the male
determinant. But, since spermatozoa which contain
this chromosome produce only females, the maternal
mate of the male heterotropic chromosome, already
present in the egg, must be a dominant female de-
terminant. And in the process of fertilization which
gives rise to males the heterotropic chromosome
derived from the egg must represent the male deter-
minant. Two different sorts of eggs are therefore
produced—presumably in equal numbers—which con-
tain the male and female determinant respectively ;
the former are fertilized only by spermatozoa lacking
the heterotropic chromosome and vice verséd. The
combinations which arise in this way may be repre-
sented as (m)f and m. A selective process of fertiliza-
tion is therefore a sime quad non for this explanation—
it must be impossible for a spermatozoon bearing the
male determinant to fertilize an egg in which a male
determinant is already present—in other words, only
eggs containing the female determinant can be fertilized
by sperms which contain a heterotropic chromosome.
In another species of insect closely allied to Protenor
the somatic cells of the male, like those of the female,
contain each a pair of idiochromosomes; but in the
male one member of the pair is much larger than the
other, whilst in the female they are of equal size. The
behaviour of the larger member of the unequal pair of
chromosomes, in the various nuclear processes which
occur during the life-history, is precisely like that of
the single heterotropic chromosome of Protenor. It is

HEREDITY OF SEX 275

still possible to regard this chromosome as representing
a recessive male determinant, and to suppose that the
process of sex determination is precisely similar in
the two cases. On this supposition, the smaller idio-
chromosome is regarded as being without function so
far as sex is concerned.

In a third insect belonging to the same natural group
both male and female sexes bear alike a pair of idio-
chromosomes of equal size. Here, again, it is possible
to apply the same theory of sex determination by.
simply disregarding one of the idiochromosomes of the
male as unimportant. We may suppose, in fact, that
one of these chromosomes corresponds to the smaller
idiochromosome of the preceding case, and that it
takes no essential part in these phenomena. The fact
that this chromosome takes no active part in these
processes may, indeed, have led to its reduction in the
second of the three species, and to its final disappear-
ance in the first.

Thus, by dint of a good deal of speculation, Wilson
has arrived at a possible Mendelian description of the
phenomenon of sex in a species in which the chromo-
somes of male and female are alike ; and it is a descrip-
tion which has its basis in actual phenomena observed
in two other related animals.

The view here suggested, then, is that the female is
heterozygous for the female factor, whilst the male is a
homozygous recessive so far as that factor is concerned.

Various facts relating to different groups of animals
can further be adduced in favour of this interpreta-

276 RECENT CYTOLOGY

tion. Amongst these are the observations made by
Doncaster on certain moths, which gave curiously
complex results explicable on the above supposition
as to the nature of sex.

In the human race colour-blindness is common in
men, but rare in women. Men with normal sight
cannot transmit colour - blindness, but women with
normal sight can. According to Bateson, only seven
cases are known of colour-blind women, and these had
in all seventeen sons, all of whom were colour-blind.
Colour-blindness must therefore be supposed to be due
to the presence of a dominant allelomorph, and its
non-appearance in women must be caused by the
presence of another dominant factor which is epistatic
to the factor for colour-blindness. It has been sug-
gested that the counteracting element is none other
than the factor for femaleness itself.

Old and impotent females among the higher animals
are known sometimes to assume some of the secondary
characters of the male sex. This power suggests the
presence of the male element as a previously latent
factor in their constitution. On the other hand, the
supposed female characters assumed by castrated
males can probably be accounted for as the result
of arrested development, without any necessity for
assuming the presence of a female factor.

It is not necessarily to be supposed that the above
description of the facts of sex-determination will hold
good for every kind of organism. In fact, Correns
has produced evidence which seems to show that in

HEREDITY OF SEX 207

the case of certain plants it is the male which is hetero-
zygous, whilst the female is homozygous. On the
other hand, the first Mendelian interpretation of sex
ever put forward—namely, that due to Castle—
supposed that both sexes were heterozygous. Many
further facts must be obtained and discussed’ before
the problem of sex-determination can be regarded as
in any degree settled.

By way of further illustrating the far-reaching im-
portance of the information which has been rendered
available by the combined use of experimental and
cytological methods, we may here briefly criticise the
celebrated theory of inheritance put forward by Weis-
mann in 1892 under the name of the ‘Germ Plasm
Theory.’ Some notice of this theory, which might
otherwise have been permitted to go the way of similar
valuable provisional hypotheses, is rendered almost
necessary by the circumstance of its having been
recently revived in a prominent manner in the English
translation of Weismann’s book, ‘The Evolution
Theory.’ In this book, published in 1904, the bearing
of the Mendelian evidence upon the subject of inheri-
tance is practically ignored ; although, in the face of the
definite experimental information now rendered avail-
able, the younger biologists, at least, are beginning
to realize that the circumstantial evidence, formerly
so much relied upon, will. in future constitute a much
less prominent feature in these discussions.

Weismann’s theory of inheritance, and the Theory

278 RECENT CYTOLOGY

of Ancestral Heredity in its original form, are based
upon a common assumption, which is now shown by
Mendel’s discovery to have been unfounded. This is
the assumption that all ancestors of the same degree—
é.g., grandparents—make a substantially equal con-
tribution to the hereditary qualities of the offspring.
Mendel has shown that in the case of particular
hereditary characteristics this is not the case.

But if we venture to criticise Weismann’s conception
in the light of more recent knowledge, it must not be
forgotten that biology, and especially modern cytology,
owes a great debt to Weismann. To Weismann is due
the conception of the isolation of the germ-cells from
somatic influences, a view which is in complete accord-
ance with the Mendelian view of the inheritance of
definite characters. And it was Weismann who first
emphasized the belief that the chromosomes represent
those parts of the nucleus which are specially concerned
in the processes of heredity. These conceptions—
which, indeed, constitute an essential part of his own
theory of heredity—have stood the test of time in an
admirable manner. ,

Let us turn our attention, then, for a short space to
the Germ Plasm Theory of inheritance. On Weis-
mann’s theory, as in most other theories of heredity
from the time of Darwin and Nageli downwards, the
separate parts of the living organism are supposed to
be represented by separate material particles in the
germ-cells. These representative particles are known
as determinants. A complete set of determinants in

THE GERM-PLASM THEORY 279

which every part of the organism is thus represented
constitutes an td. So far Weismann’s hypothesis is in
close agreement with the idea of representative particles
which we are driven to adopt by the facts of Mendelian
inheritance, except that, following de Vries, we should
speak of separate characters rather than parts as
being thus represented ; for there seems to be no doubt
that the same character-determinant can affect the
development of a number of different parts. But at
the next step the Mendelian parts company with
Weismann. The latter assumes that the cells of an
organism contain a large number of tds, or complete
sets of determinants, half of the total number being
derived from either parent, and that, although at the
reducing division which precedes the formation of the
gametes the total number of ids is reduced to half of
what it was in the somatic cells, still, several ids
derived from each parent are present in every germ-
cell.

Thus the reduced number of chromosomes in the
germ-cells is regarded as containing all the primary
constituents of both parents. And it is an essential
point in Weismann’s theory that he regards a given
germ-cell as containing a considerable number of ids
derived from its ancestors, all near ancestors being
thus represented.

But Mendel’s experiments and others of the same
kind show, in the case of a great number of different
characters, that although every essential character is
represented in every germ-cell, yet each Mendelian

280 RECENT CYTOLOGY

character is represented by a paternal or a maternal
determinant only, and not by both. Thus, not
only are all immediate ancestors not represented
in the germ-cells in respect of any particular
character, but only one of the parents is so repre-
sented—to the complete exclusion, so far as we can
tell, of the other parent. In fact, we are led to
believe that the germ-cells contain one set of de-
terminants only—a single 7d—whilst the somatic cells
contain two ids only. The Mendelian theory is thus
seen to be considerably simpler than the germ-plasm
theory, which it replaces. At the same time it must
not be forgotten that many of the conceptions used in
the Mendelian expression of the facts are borrowed
from Weismann’s theory, and that but for Weismann’s
work it would have been impossible for us to have got
so far in the co-ordination of the facts derived from
experiment and microscopic observation respectively.
The preceding sketch may serve to show how Mendel’s
observations have been found to throw light upon many
of the facts of cytology the meaning of which was pre-
viously obscure ; and how it affords at the same time
a criterion by which may be tested the truth of theories
based upon the interpretation of minute phenomena
only made visible by the highest powers of the micro-
scope. The disinterment of Mendel’s discovery took
place only eleven years ago; and the rapid manner
in which the facts of cytology have been found to
fall into line with Mendelian conceptions augurs well
for the future progress of discovery in these fields.

THE GERM-PLASM THEORY 281

BIBLIOGRAPHY,

Witson, E. B.: The Cell in Development and Inheritance
(2nd ed.). 1900.

3 is Recent Researches on the Determination
and Heredity of Sex. Science, January 8,
1909.

Boveri, T.: Ergebnisse itiber die Konstitution der chroma-
tischen Substanz des Zellkerns. 1904.

WEIsMann, A.: The Germ-Plasm: a Theory of Heredity
(English translation by W. N. Parker and H. Rénnfeldt),
1893.

CHAPTER X
EUGENICS

Eucenics is defined as ‘ the study of agencies under
social control that may improve or impair the racial
qualities of future generations, either physically or
mentally.’ Sir Francis Galton, the pioneer of this
study, may well be regarded as one of the foremost
figures in recent science. His claim to this regard is
based upon a pre-eminence in many different branches
of knowledge—a claim which those to whom his work
is as yet unknown will find substantiated with all
becoming modesty in his recently-published ‘ Memories
of My Life.’ Much of Galton’s work in other fields
may be forgotten, overshadowed as it is by his founda-
tion of the science we are about to describe. It will
be for future generations to discover that niche in the
temple of fame which is fitting for the man who first
pointed out a feasible direction in which humanity
might set about its own improvement, instead of
resting content in the vain expectation of being im-
proved vicariously.

We have neither the mathematical equipment, nor
have we here the space, to enter into the statistical

inquiries upon which Galton’s conclusions are mainly
282

FRANCIS GALTON.

282.

To face p

C

GALTON’S RESEARCHES 283

based. For an account of these the reader must be re-
ferred to the admirable popular treatises enumerated at
the close of the present chapter. In spite of the rather
wide divergence of these studies from those with which
this book is mainly concerned, the agreement between
the conclusions on race-improvement drawn by the
students of Genetics, on the one hand, and by those
of Biometry, on the other, is a remarkable one, and
may perhaps be taken to indicate that both these
methods are right in their several directions.

Consequently we propose attempting a summary of
the line of researches and arguments which have led
Galton to his present conclusion that the human race
is capable of vast improvements in physique, in
beauty, in character, and in intellect. The importance
of this conclusion is augmented by the corollary that
acquisition of these improvements leads to a keener
appreciation of their value, and, incidentally, to the
greater happiness of mankind. But the student of
Eugenics does not rest satisfied with conclusions. He
proposes to utilize the great forces of fashion and
public opinion as agents of modification and improve-
ment by diverting their influence into the right
direction and out of their present remarkably wrong
direction.

The first link in the chain of evidence was forged
long ago by Darwin, when he showed how far man
had already risen from a simpler and lower type of
animal. The evolution of man is now a part of the
ordinary intellectual creed of most educated men, and
yet few politicians or charitable people pause to apply

284 EUGENICS

this belief to the present or to the future. Few
realize the full significance of the inferences that, if
man has risen in the past, he may still rise higher in
ages yet to come; if the type has undergone modifica-
tion in the past, it may even now be changing. Those
who move the forces of money or of popular opinion
should take heed how their actions affect the rate
and direction of this possibly momentous change.

The evolution of man, like that of other animals,
is believed to have been largely due to the effect of
selection. Whether the variations selected were great
or small makes little difference to the argument. In
former days this selective action was exerted by fierce
competition, which led to the survival of individuals
endowed with certain qualities and to the extinction
of other individuals differently constituted.

Modern civilization and humanitarianism have
effectually set aside the action of natural selection.
The result at the present day indicates that the upward
progress of the race has probably almost ceased, and
that there is every danger of finding that a downward
journey has begun. This is an inevitable conclusion
to those who accept the well-established facts (1) that
men are not born equal, but differ widely in their
birthright in respect of every physical and mental
character, and (2) that, although every man is free to
become the father of a family, yet the tendency to
bring up large families is becoming relatively smaller
among those classes of society which we must regard
as the best endowed, both physically and mentally.

Are there any practicable means by which this

PROGRESS OF THE RACE 285

tendency to degenerate can be combated and changed
into an upward bias? In order to discover such a
means it is needful to gather all possible information
with regard to the primary factors of organic evolution
in the human race—that is to say, variation and
heredity. Galton was the first to show that the laws
of variation and heredity in man are closely similar
to the same laws in other organisms. He showed, too,
that mental and moral characters are inherited just
as strongly as physical characters—a fact which is
by no means so generally appreciated as it should
be, in spite of the further evidence adduced by Prof.
Karl Pearson.

Many people believe that the progress of the race
can be directly and permanently affected by improve-
ments in education and the amelioration of social
conditions. It is certain that the qualities of any
person— health, character, efficiency, and so forth,
depend upon his environment and upbringing, as well
as upon his hereditary endowments. It is therefore
necessary, before we proceed farther, to arrive at some
estimate of the relative importance of inherent qualities
and of education—of nature and nurture respectively.
Practically the only piece of good evidence upon this
point is one which we owe to the researches of Galton.

Galton’s data are derived from the history of twins.
Now, human twins are of two kinds. One of these
kinds corresponds to the young of those animals which
normally bear two or more at a birth, each being
derived from a separate ovum, whilst the second kind
is the result of the development of two embryos from

286 EUGENICS

the same fertilized ovum. In the former case the
twins bear no more resemblance to one another than
any other ordinary pair of brothers or sisters, and they
are often of opposite sex ; in the latter case the twins
are known as ‘ identical,’ and are always of the same
sex. Our present knowledge of Genetics—not, of
course, available to Galton when he first wrote upon
this subject—leads us to believe that such twins are
indeed identical, and bear precisely the same hereditary
endowment. It is as though a single individual were
divided into two parts, and each part grew into a
complete person. Galton quotes numerous stories
of the frequent confusion between identical twins.
‘ I have one case,’ he writes, ‘ in which a doubt remains
whether the children were not changed in their bath,
and the presumed A is not really B, and vice versa.’
In the records thus collected there is excellent material
for discovering how far identical twins can come to
differ from one another when exposed to different
conditions, and, on the other hand, for ascertaining
how far distinct twins brought up under similar con-
ditions can come to resemble one another. Galton
obtained information with regard to eighty cases of
probably identical twins. In many of these cases the
twins remained closely alike in temper and character,
as well as in appearance, up to an advanced age.
When differences arose in later life, these were generally
due to some illness or accident which affected one of
the twins. The gradual influence of a number of small
causes seemed to have very little effect in this respect.
‘In not a single instance,’ Galton writes, ‘ have I met

EFFECT OF ENVIRONMENT 287

with a word about the growing dissimilarity being due
to the action of the firm, free will of one or both the
twins which had triumphed over natural tendencies ;
and yet a large proportion of my correspondents happen
to be clergymen, whose bent of mind is opposed, as I
feel assured, to a necessitarian view of life.’ The only
conclusion which can be drawn from these observations
is that the relative influence of nurture as opposed to
nature is very much smaller than has been generally
supposed; and this inference is confirmed by the
history of dissimilar twins. The descriptions of the
latter agreed without exception in showing an entire
absence of convergence of character in cases where
the whole training and environment were closely
similar. From this evidence it seems right to conclude
that the hereditary nature of a man is more important
than his training and circumstances in determining
his adult mental and physical equipment, and the
result of common observation may be said to be in
agreement with this conclusion. +
You may educate generation after generation, and
yet the starting-point from which each individual has
to begin his struggle upwards may remain the same,
even though each may struggle a little farther than the
one who came before him. On the other hand, we
have all of us met a few of those happy people to
whom it seemed second nature to do the right thing,
and for whom the difficulties of life appear to have
no menace. These qualities are those of nature, and
not of nurture, and their children will inherit them.
Some are born great, some achieve greatness, and

288 EUGENICS

some have greatness thrust upon them. But those
who are born great—meaning by this not those in
high position, but great in themselves—are the men
and women to whose descendants we must look for
the future greatness of mankind.

If this view be justified, we shall be obliged to revise
very carefully our ideas of what is desirable in social
legislation. Important as education, sanitation, and
the like may be, their effects are strictly limited.
The relative birth-rate of good and bad stock, on the
other hand, is the fundamental factor. Its influence
in a single generation may be so small as to be barely
recognizable, but its effect increases from generation
to generation, and, moreover, it is an effect which, once
produced, is quite irrevocable. We have to consider,
therefore, how the relative incidence of the birth-rate
falls at the present time, and what are the causes
which affect it. We have to consider especially
whether existing and proposed legislation of which the
intention is to improve the education and condition
of upbringing of children has any effect, direct or in-
direct, upon the relative birth-rate of different classes
of society. For if such legislation be found to favour
the rapid reproduction of the less-efficient, it will
become a matter for serious consideration whether the
advantages of mental and physical improvement in the
individual are not being purchased too dearly at the
expense of posterity.

Individuals, as well as Governments, will do well
to look closely into the possible results of the best-
intentioned proceedings. To quote Galton once more:

SIZE OF FAMILIES 289

‘It is known that a considerable part of the huge
stream of British charity furthers by indirect and
unsuspected ways the production of the Unfit ; it is
most desirable that money and other attention be-
stowed on harmful forms of charity should be diverted
to the production and well-being of the Fit. For
clearness of explanation we may divide newly married
couples into three classes, with respect to the probable
civic worth of their offspring. There would be a small
class of ‘‘ desirables,” a large class of ‘“‘ passables,” of
whom nothing more will be said here, and a small
class of ‘‘ undesirables.” It would clearly be advan-
tageous to the country if social and moral support, as
well as timely material help, were extended to the
“ desirables,”’ and not monopolized, as it is now apt
to be, by the “ undesirables.”

Let us consider the relative birth-rates of different
classes of the community. This is in itself a very large
question. We have only space here to record a few
figures, taken from a recent paper by Prof. Karl Pearson,
than whom no better authority could be quoted.

Here are a few of the figures, in terms of the average
size of family, childless marriages being excluded :

Group I.
Criminals .. “e a -. 66
English deaf-mutes .. oe ia) 2
London, mentally defective .. -. 70
¢ ‘
Group II.
English middle class we a &2
London normal artisan ate aie RI

English intellectual class 4°7

19

290 EUGENICS

This means, if one takes into consideration the fact
that members of the first group probably marry several
years earlier, on the average, than members of the
second group, that the mentally defective class is
reproducing itself about twice as rapidly as the intel-
lectual class. There can be no doubt as to what the
result of such a proportion as this must be. Let us
quote Prof. Pearson’s own conclusion : ‘ The progress
of the race inevitably demands a dominant fertility
in the fitter stocks. If that principle be not recognized
as axiomatic by the mentally and bodily fit themselves,
if the statesman do not accept it as a guide in social
legislation, then the race will degenerate until, sinking
into barbarism, it may rise again through the toilsome
stages of purification by crude natural selection.’ Or,
it may be added, until it be annihilated by a more
efficient neighbour.

With regard to the cause of these differences in the
birth-rate of different sections of the community, there
can be very little doubt that they are largely due to
deliberate restraint on the part of the more prudent
and intelligent classes. When a person has attained
by his own industry to a certain standard of comfort,
and even luxury, he naturally desires to give his
children a fair start, and to insure for them an equal
degree of ease and security ; and if he finds that this is
more easily done when the number of his family is
limited, it is difficult to blame him for taking’advantage
of the fact. If this class of men had more children
on the average than are produced by people in in-
ferior stations, it would mean that the children of the

CARE OF INFERIOR STOCKS 291

comfortably situated would have to be contented with

positions somewhat inferior, on the average, to those

of their parents. This is precisely the condition of

affairs most desirable from the point of view of race-.
improvement and from that of national efficiency,

since any given position would thus be recruited from a

better and not from a worse class than the one which

previously occupied it. The individual may be par-

doned if this is not what he desires to happen in the

case of his own children.

National education and the proposed feeding and
care of the children of inferior stocks at the cost of the
State are measures which will have certain definite
effects upon the relative birth-rates of different classes.

It is proposed to do all this at the expense of the
fitter stock, which is thus rendered still less capable
of raising, as well as still less disposed to raise, large
families of healthy children. Such measures can only
be justified by making at the same time every possible
effort to correct these dangerous differences.in the
incidence of the birth-rate. Legislation in these two
directions ought to go hand in hand. Indeed, the
improvement in the supply of children ought for every
reason to precede the improvement in the care and
education of children; for if the State cares for the
children, it has a right to insist that the supply of
children shall be the best possible, and this is far from
being the case at present.

Remedies for the existing condition of things have
been proposed by would-be philanthropists from Plato
downwards. But against all suggestions for running

19—z2

292 EUGENICS

the human race on the principles of the stud farm the
objection holds good which was put forward by Huxley:
“Who is competent to do the necessary selecting ?
How can the pigeons be their own Sir John Sebright ?’
‘ The points of a good or of a bad citizen,’ says Huxley,
‘are really far harder to discern than those of a puppy
or a short-horn calf. Many do not show themselves
before the practical difficulties of life stimulate man-
hood to full exertion. And by that time the mischief
is done. The evil stock, if it be one, has had time to
multiply, and the selection is nullified.’ And there is
another objection. . The ruthlessness necessary for the
carrying out of the method of deliberate selection is
in itself so unsocial a quality that, if it were ever to
arise, society would probably be far worse off than
before. The method is in itself directly opposed to
the development of the higher social qualities.

The student of Eugenics must therefore endeavour
to devise other methods, both for encouraging the
fertility of the better stock and for discouraging that
of the inferior stock. A considerable number of
specific suggestions have already been made. ‘ Not
a few medical men,’ writes Heron, ‘ are urging that
propagation among the obviously unfit—those affected
with definite hereditary taints: the imbeciles, the
idiotic, the sufferers from syphilis and tuberculosis,
should be authoritatively restrained.’ Can it be urged
that such a proceeding would be unduly tyrannous ?
Surely if these people understood the irrevocable laws
of heredity—if they only knew—they would be them-
selves unwilling to hand on a tainted existence to

OBJECT OF EUGENICS 293

future generations! If there are people so debased
that this argument does not appeal to them, surely
such a crime against Society as a marriage of this kind is
at least as open to coercive treatment as many of the
acts which are treated as criminal by existing laws ! We
elaborately prevent and punish paltry offences against
property, and yet deliberate crimes like marriages
between the Unfit are not recognized as criminal.

Various suggestions for encouraging the multiplica-
tion of the Fit have also been made.

Mr. Sidney Webb, in his * Decline of the Birth-Rate,’
has suggested indiscriminate help to all parents, since
this should afford encouragement to those who limit
their families for prudential reasons and at the same
time leave the thriftless where they are. But here
it is necessary to point out that the existence of an
unlimited population must of necessity bring want
and misery to the lowest strata of society. The first
object of Eugenics, Galton tells us, ‘is to check the
birth-rate of the unfit, instead of allowing them to
come into being, though doomed in large numbers to
perish prematurely. The second object is the improve-
ment of the race by furthering the productivity of
the Fit by early marriages and healthful rearing of
their children. Natural selection rests upon excessive
production and wholesale destruction ; Eugenics upon
bringing into the world no more individuals than can
properly be cared for, and those only of the best
stock.’

In the ideal socialistic community, in which, in
addition to all the present varieties of civil servants,

294 EUGENICS

all those engaged in education, all members of the
Legislature, all doctors and all lawyers, would no doubt
derive their incomes from the State, a recent sugges-
tion of Mr. McDougall’s might be put into practice.
This is to graduate incomes according to the number
of children. Thus the position of married people
would be made much fairer on some such scheme as
the following: Supposing the salary of a particular
post under existing conditions to be £700 a year, a
bachelor occupying the post would be paid only £400
a year (say). On his marriage an addition of £100 per
annum would be made to his income, and a similar
increment would again take place at the birth of
every child. Under the conditions here postulated
this system would apply to the bulk of the more
intellectual members of the community, and, incident-
ally, it would have a special advantage, to which we
may now make allusion.*

Although a nation’s welfare depends to a very great
extent upon the mental and bodily health of the rank
and file of its citizens, yet the birth of an occasional
genius makes an enormous difference to the progress
of the world. Now, Galton has shown quite con-
clusively that there is a much greater chance of a
genius appearing among the children of eminently
intelligent parents than in an average family. There-
fore, if the fertility of the more intellectual classes is
encouraged, the chance of obtaining a genius now and
again is much increased.

* It is not to be inferred from this paragraph that the
writer is an advocate of socialism.

CRIMINAL TAINT HEREDITARY 295

It must not be supposed that the writer is a special
advocate of all or any of the suggestions which have
been mentioned above. They are only alluded to in
order to indicate the directions in which the problem
of race-improvement may be attacked.

The point of view which has been adopted in the
present chapter is very well summed up in the following
paragraph from a paper by Prof. Karl Pearson :

“As we have found conscientiousness is inherited,
so I have little doubt that the criminal tendency
descends in stocks. To-day we feed our criminals up,
and we feed up the insane; we let both out of the
prison or the asylum “‘ reformed ” or “ cured,” as the
case may be, only after a few months to return to
State supervision, leaving behind them the germs of
a new generation of deteriorants. The average number
of crimes due to the convicts in His Majesty’s prisons
to-day is ten apiece. We cannot reform the criminal
nor cure the insane from the standpoint of heredity ;
the taint varies not with their moral or mental conduct.
These are the products of the somatic cells ; the disease
lies deeper in their germinal constitution. Education
for the criminal, fresh air for the tuberculous, rest and
food for the neurotic—these are excellent. They may
bring control, sound lungs, and sanity to the individual;
but they will not save the offspring from the need of
like treatment, nor from the danger of collapse, when
the time comes. They cannot make a nation sound in
mind and body ; they merely screen degeneracy behind
a throng of arrested degenerates. Our highly developed
human sympathy will no longer allow us to watch

296 EUGENICS

the State purify itself by aid of crude natural selection.
We see pain and suffering only to relieve it, without
inquiry as to the moral character of the sufferer, or as
to his national or racial value. And this is right. No
man is responsible for his own being ; and nature and
nurture, over which he had no control, have made him
the being he is, good or evil. But here science steps
in, crying, “ Let the reprieve be accepted, but next
remind the social conscience of its duty to the race.
No nation can preserve its efficiency unless dominant
fertility be associated with the mentally and physically
fitter stocks. The reprieve is granted, but let there
be no heritage if you would build up and preserve a
virile and efficient people.’

Signs are not wanting that a few thoughtful people
outside the ring of scientific enthusiasts are beginning
to take a serious interest in the questions of race-
improvement and national efficiency. But before any
real progress is likely to be made, a knowledge of the
fundamental facts of heredity must have become very
much more widely distributed than is at present the
case. Valuable work is being done in this direction
by the Eugenics Education Society, the object of which
is to popularize and carry into practical effect the idea
of national Eugenics, as defined at the beginning of
the present chapter. The intention of this society to
make known generally the fact that biological principles
apply to the production of human beings is one which
cannot be too highly estimated. Since this is pre-
cisely the object with which the present sketch has been
added to a work otherwise almost purely theoretical

BIBLIOGRAPHY 297

in character, I have thought it proper to conclude
my chapter on Eugenics with a brief reference to the
aims and objects of this society.

BIBLIOGRAPHY.

Gatton, F.: Hereditary Genius. 1869.

”

English Men of Science. 1874.

Inquiries into Human Faculty and its Develop-
ment. 1883.

Natural Inheritance. 1889.

Eugenics: Its Definition, Scope, and Aims
(Sociological Society Papers, 1905).

Tuer EvuceEenics REVIEW, 1909—

CHAPTER XI

CONCLUDING CHAPTER

In the preceding chapters a considerable variety of
topics has been dealt with, and in spite of the fact that
all are more or less intimately connected with the
study of organic evolution, the nearly historical order in
which the subject-matter has been in great part pre-
sented has inevitably rendered the treatment a little
disjointed.

The method we have so far adopted serves to illus-
trate the state of transition in which our studies
stand, and which it is our first object to assist in
hastening to a close—the transition between the specu-
lative philosophy of evolution and the exact science of
genetics.

Future treatises on genetics will make a fair beginning
with the law of Mendel, and will then deal with the
application of this law in detail ; and in this concluding
chapter we may adopt the same method, and proceed
to show how Mendel’s discovery affords the connecting-
link between the various divergent branches which we
have already sketched in outline.

The central generalization, then, around which the

subjects considered in the preceding pages are found
298

THE SCIENCE OF GENETICS 299

naturally to group themselves is afforded by the law
of inheritance discovered by the Abbé Mendel about
the year 1865. This discovery has rendered possible
that rapid advance of the science of genetics, or the
study of the hereditary phenomena of organisms,
which has taken place during the first few years of the
twentieth century. It is the writer’s avowed opinion
that Mendel’s brief paper is the most important con-
tribution of its size which has ever been made to bio-
logical science. Little apology is therefore needed for
formulating once again the law based by Professor
Correns upon the conclusions which this paper contains.

Mendel’s law relates to the inheritance of certain
definite characters, which have since been called allelo-
morphs. It is a distinctive feature of allelomorphic
characters that they are found to group themselves
naturally into pairs of more or less antagonistic
qualities. In many cases the pair is represented by
the presence and absence respectively of a certain
definite feature. The two allelomorphs of a pair may
be conveniently written as A and a.

We have seen that the cells of zygotic organisms—
organisms, that is to say, which have arisen by the
process of sexual reproduction—contain a double
complement of hereditary qualities. Such cells may
contain A and A,aanda,orAanda. The forms AA
and aa are described as homozygotes, the form Aa as
a heterozygote. In the simpler cases we are enabled
to study the behaviour of such a single pair of allelo-
morphs by itself, without reference to any other features
which the animals or plants under consideration may

300 CONCLUDING CHAPTER

display. The demonstration that there exist definite
and separable unit characters of this kind is the first
great debt that science owes to Mendel.

Up to the present our certain knowledge of the
Mendelian behaviour of unit characters has been con-
fined to cases of cross-breeding. In the simplest case
which we have to consider, two homozygote forms, AA
and aa, are crossed together.

The external character or visible appearance of the
heterozygote Aa, produced in this manner, differs in
different cases. In the commonest case A represents
the dominant allelomorph, and in this case the appear-
ance of the heterozygote Aa is practically indistinguish-
able from that of the homozygote AA. In other cases
the heterozygote Aa is different in appearance from
either homozygote AA or aa. Sometimes Aa is inter-
mediate between AA and aa, in other cases it is to all
appearances totally distinct from either.

So much for the external appearance of homozygote
and heterozygote forms. In the production of the
gametes, or germ-cells, we arrive once more at the
simplest possible form of hereditary constitution, for
we believe each feature in the body to be represented
in the germ-cells by a single determining factor only.
Still confining our attention to the representatives otf
a single pair of allelomorphs, we find that all the germ-
cells of a homozygote contain only A or only a, as the
case may be. But in the case of the germ-cells derived
from a heterozygote, A and a are represented in an
equal number of the gametes produced by the same
individual. And the separation between the two

MENDEL’S LAW 301

allelomorphs is found in almost all cases to be perfectly
complete. ,

This complete segregation of the two allelomorphs in
equal numbers of the germ-cells of a heterozygote
constitutes the first and most important section of
the generalization known as Mendel’s law.

The second part of the law refers to the fact that,
as a general rule, separate pairs of allelomorphs segre-
gate quite independently of one another. To this rule
a few exceptions have been recorded in cases where
apparently distinct pairs of determining factors behave
in segregation like a single pair of allelomorphs. In
such cases we regard the members of the distinct pairs
of allelomorphs as being coupled together, although no
serious attempt has yet been made to picture the way
in which this coupling comes about. In other cases
the coupling is only partial ; in others, again, there is
repulsion between allelomorphs belonging to distinct
pairs.

The fact that in the great majority of cases separate
pairs of allelomorphs segregate independently of one
another leads to the possibility of new combinations of
the parental characters being formed in the germ-cells
of the cross-bred individuals ; in fact, this must always
happen when the parent types differ in more than
one pair of segregable characters. When two similar
germ-cells, each bearing the same new combination of
allelomorphs, meet together in fertilization, the result
is a new zygotic combination which is a pure type in
respect of the characters concerned, and henceforth
breeds true. Thus if AB. AB is crossed with ab. ab

302 ' CONCLUDING CHAPTER

the heterozygote AB .ab produces in equal numbers
the germ-cells AB, Ab, aB,and ab. Among the combina-
tions of these germ-cells which are represented by the
various offspring of the heterozygote there must appear
Ab.Ab and aB .aB—novel types which are pure in
constitution, and which may form the starting-points
for new strains or races.

Upon this fact depends the enormous importance of
Mendel’s law in the breeding of new and useful types
of animals and plants. When it is remembered that in
wheat, for example, resistance and non-resistance to
the attacks of disease, earliness and lateness of
ripening, good and bad milling quality, are all pairs of
Mendelian allelomorphs, and that it is now possible to
take a different example of these qualities from each
of three different strains, and to combine them together
in a single new variety with perfect certainty and in
four generations, it does not require much imagination
to foresee that every department of the animal and
plant breeding industries must sooner or later benefit
enormously from Mendel’s discovery.

So far we have only been dealing with the very simplest
of Mendelian phenomena, leading to the arithmetical
addition and subtraction of definite visible characters.
Other kinds of allelomorphs also exist which undergo
a similar process of segregation during gamete forma-
tion, following Mendel’s law in a perfect manner; but
which may remain entirely invisible and unsuspected
so long as certain other allelomorphs, belonging to
quite distinct pairs, are excluded from the zygotes in
which these invisible factors are concealed. When

MENDEL’S LAW 303

this other complementary allelomorph is introduced,
however, by crossing with an individual which contains
it, the feature previously hidden becomes visible, giving
rise to the phenomenon which has long been familiar
under the name of reversion on crossing. The demon-
stration of these invisible factors, and of the fact that
they also obey Mendel’s law with perfect regularity,
is surely one of the most remarkable discoveries which
have ever been made in the whole history of biology.
This, again, is a piece of knowledge which may be of
the very greatest importance, not only to breeders of
bright flowers, some of which are already known to
exhibit the phenomenon described, but also in all
classes of breeding work where similar facts doubtless
await discovery. ;

To the man of science, however, the practical aspect
of these achievements will be of little account in com-
parison with the importance of their application to the
advance of human knowledge in that most fascinating
of scientific studies—biology. Let us, then, turn to
consider the way in which Mendel’s discovery affects
other branches of biological science.

We have so recently had occasion to point to the
remarkable coalition between Mendelism and cytology
that little more need be said here upon the subject.
Mendel’s theory has, indeed, thrown a flood of light
upon the meaning of the microscopic phenomena
exhibited by the minute constituent parts of the cells
of living organisms, phenomena the meaning of which
could only be vaguely guessed at previously to the
introduction of the new method.

304 CONCLUDING CHAPTER

The intimate connection between Mendelism and
cytology rests to a large extent upon the close parallel
which exists between the behaviour of allelomorphic
characters on the one hand and that of chromosomes
on the other.

In the germ-cells of the higher animals the allelo-
morphs of the Mendelian become segregated, being
reunited in fertilization, and, as a consequence, the
cells of the zygote contain twice as many of these
factors as do the gametes or germ-cells themselves.

Similarly, in the cell processes upon which the vital
functions of the higher animals are founded, the
number of chromosomes characteristic of somatic or
zygotic cells becomes halved at the formation of the
gametes, the double number being restored by the
association of chromosomes derived from two separate
gametes in the process of fertilization. We have said
that in the higher animals the gametes are sometimes
spoken of as constituting an ‘x-’ generation, which
alternates with the ‘2x-’ generation represented by
the zygote. We may justify the use of these expressions
by a brief comparative statement of the facts relating
to the two so-called generations which recur in the
life-history of certain families of plants. In doing so
we shall begin our account with the most primitive
and simplest forms, and then pass on to other types
which are regarded as standing on higher planes of
evolution.

What are probably some of the most primitive
members of the vegetable kingdom belong to the
class of the green alge. This group includes a great

CYTOLOGY OF PLANTS 305

number of comparatively lowly organisms, the majority
of which dwell submerged beneath the surface of fresh
or salt water. In such members of the green alge as
have so far been examined from this point of view, it
would appear that the 2x-generation is exclusively
represented by the single cell which arises as the
actual product of conjugation between a pair of
gametes. Reduction takes place in the actual zygotic
cell, so that each of the products of this cell’s division
shows once more the reduced number of chromosomes.
Thus the great bulk—the vegetative mass—of the
species is constituted by the x-generation, and the
2x-generation is composed of a single cell only—a state
of things which is exactly the reverse of what is to be
seen in the higher animals.

In the vegetable kingdom evolution seems to have
been accompanied by a gradual increase of the 2x-
generation, and a corresponding reduction of the
x-generation in point of importance. Between the
two extremes afforded by the alge on the one hand,
and the flowering plants on the other, we can trace a
series of intermediate stages represented by types in
which many other features also must be regarded as
standing on intermediate planes of organization.

As an example of an intermediate condition of this
kind, we may take the case of the ferns.

The fern plant, as commonly understood, represents
the 2x-generation. The method by which the life-
history of the fern plant is continued is by the forma-
tion of unicellular reproductive bodies which are

known as spores. The formation of the spores takes
20

306 CONCLUDING CHAPTER

place in sets of four, and their production is preceded by
a reducing division, so that each spore nucleus contains
half as many chromosomes as the nuclei of the fern-
plant—the spores, in fact, represent the initiation of
the x generation.

Spores take no part in any process of conjugation.
They at once germinate and enter on an embryonic
development of their own, giving rise to a considerable
mass of cells, all of which contain the reduced number
of chromosomes. Thus in the case of the fern we have
a small but well-developed x-generation alternating
with a much larger 2x-generation. The mass of
cellular tissue making up the x-generation has been
named the prothallus.

Certain cells of the prothallus develop, without
change in the number of their chromosomes, into the
gametes. These are differentiated in the usual way
into male and female—ova and spermatozoids respec-
tively.

Fertilization of the ovum by the spermatozoid gives
rise to a zygote in which the double number of chromo-
somes is restored. In this way the 2x-generation or
fern plant is initiated, and by the usual processes of
cell multiplication and differentiation this body
becomes completed, developing its characteristic fronds
and so forth. Thus in the ferns the 2%-generation has
arrived at a high degree of development, and represents
the chief bulk of the plant. The x-generation, however,
still embodies a considerable mass of cells.

Turning to the higher plants, among which we may
include those which produce typical flowers with

CYTOLOGY OF PLANTS 307

stamens or with pistils, or more usually with both, we
find that the x-generation has become still further
reduced, so that it no longer occupies an independent
phase of the life-history, but has come to be entirely
dependent upon the 2x-generation for its support.

A plant which bears both stamens and pistils gives
rise to spores of two kinds, differing greatly in size.
The smaller spores are represented by the pollen-grains,
and in these, after one or two cell divisions, unaccom-
panied by growth, the one or two male gametes are
produced. The small association of cells arising in this
way is all that is left of the x-generation on the male
side.

The nucleus of the larger spore also divides a few
times, and one of the final products of division becomes
the ovum. Spore and ovum, as well as the few inter-
vening cells, bear the reduced number of chromosomes.
The x-generation thus represented is never set free,
but remains enclosed in the tissues of the 2x-generation
right up to the time of fertilization. In the process
of fertilization the double number of chromosomes
characteristic of the 2x-generation is once more
arrived at.

We can look upon the 2x-generation of the higher
plants as being formed by an expansion of the fertilized
ovum. The zygote, instead of comprising a single cell
only, by dint of delaying the reducing division, has
come to consist of a great mass of cells, all the nuclei
of which contain the double number of chromosomes.
This fact is also our excuse for applying the same term

of zygote to the cell produced by the conjugation of
20—2

308 CONCLUDING CHAPTER

gametes, as well as to the mass of cells to which the
zygote (in the strictest sense) eventually gives rise.
In the simplest forms, such as the green algz, the cell-
and nuclear-fusion constituting conjugation are imme-
diately followed by fusion of the chromosomes, an
event which we have seen to be the first step towards
a reduction in the number of these bodies. In the
higher plants, by delaying this fusion of chromosomes
until many cell generations later than the fusion of the
nuclei, the advantages associated with the possession
of a double nucleus have been obtained for a large and
complicated mass of cells. And this mass has gradu-
ally advanced in organization and relative importance,
until ultimately the x-generation has been reduced
almost to the vanishing point.

The sex-phenomena of the higher animals can most
readily be brought into line with those of the higher
plants if we consider that in animals the spore and the
gamete are identical; the x-generation is here con-
densed into the smallest possible limits—namely, those
of a single cell.

A female animal produces ova, and a male produces
spermatozoa. Similarly, we may regard as a female
plant one which produces only the larger variety of
spores from which ova arise ; and we may regard as a
male plant one which produces only pollen. It is
much more usual to find a flowering plant bearing both
pistils and stamens, and producing both large and small
spores. Such an organism is described as herma-
phrodite—bearing both sexes. Among animals ex-
amples of hermaphrodite species are also not infrequent,

CYTOLOGY OF PLANTS 309

and here, just as in the case of plants, whole families
may display this method of reproduction.

We see, then, that the course of evolution in the
vegetable kingdom would appear to have been accom-
panied by a gradual increase in the 2x-generation
at the expense of the x-generation. Starting with
lowly aquatic organisms, and passing upwards through
the mosses and ferns to the flowering plants, we find
a steady diminution in the %-generation, whilst the
vegetative labour of the plant is taken over by the
ax-generation. It is, therefore, proper to suppose that
organisms in which the main stage in the life-history
is of double origin, and bears a double complement of
hereditary factors, have some advantage over organisms
in which this is not the case. We cannot, of course, be
certain as to the exact nature of this advantage, but
we may point out that it is only in the former kind
of organisms that the operation of Mendel’s law can
lead to the production of new combinations of parental
characters in the body which represents the main stage
of the life-history ; and that this circumstance may
possibly lead to a greater power of adaptability to
external circumstances.

‘Perhaps the most interesting application of the infor-
mation afforded by Mendel’s discovery is shown in its
bearing upon the question of discontinuity in the origin
of species. The fact of the definite and discontinuous
inheritance of the differentiating features which dis-
tinguish cultivated varieties from one another would
point very plainly to a belief that such differences had
arisen in a definite and discontinuous manner, even if

310 CONCLUDING CHAPTER

we did not actually know from direct evidence that
the origin of new races under cultivation is usually
sudden and complete.

It is not necessary to repeat Darwin’s demonstration
of the close analogy between the origin of varieties
under cultivation and the origin of species in Nature.
It is more to the purpose to point out that Mendel’s
law has already been shown to hold good in the case
of many differences which have certainly not arisen
under cultivation, and that we have, moreover, sure
knowledge of the definite and spontaneous origin of
some natural species.

Here we arrive at a point at which the evidence is
not yet by any means complete. We do not know
whether all or even many specific differences obey
Mendel’s law on crossing, and a sharp limit is put to
our researches in this direction by the fact that so many
natural hybrids are sterile. Still less do we know
from direct evidence whether the majority of natural
species have arisen discontinuously, although there is
much circumstantial evidence which points to the con-
clusion that this must have been the case.

Clearly this discontinuous method of variation is
likely to repay some further discussion. That such
mutation, or definite variation, is a phenomenon of
the germ-cells follows from the fact that every germ-
cell normally bears the complete specific character.
Bateson has shown that we must regard mutation as
consisting in the production of new kinds of gametes,
which differ from those normally characteristic of the
species. Such a change is most readily pictured by

DISCONTINUOUS VARIATION 311

imagining an asymmetrical nuclear division taking
place immediately before the formation of the germ-
cells, and this would lead us to expect a mutating
species to give rise to more than one new kind of
offspring at the same time. Such was actually the case
with the Gnothera Lamarckiana studied by de Vries;
and this observation stands as the most complete piece
of evidence of a mutating species so far known to us.
We may be assured, then, that the complete potential
nature of new types as well as of old ones is already
laid down in the germ-cells previous to fertilization.
As Bateson puts it : ‘ For the first time in the history of
evolutionary thought Mendel’s discovery enables us to
form some picture of the process which results in
genetic variation. It is simply the segregation of a
new kind of gamete, bearing one or more characters
distinct from those of the type. We can answer one
of the oldest questions in philosophy. In terms of
the ancient riddle, we may reply that the owl’s egg
existed before the owl; or, if we hesitate about the
owl, we may be sure about the bantam.’*

Let us consider a little more closely the evidence of
mutation afforded by de Vries’ studies of CEnothera
Lamarckiana. Semi-wild specimens of this species,
when transplanted and carefully observed, were found
to yield nearly 3 per cent. of seedlings which differed
definitely from their parent, and among these mutants
some fifteen distinct new sorts were described. Some
of the new species equalled or even surpassed the parent

* British Association, Cambridge, 1904. Address to the
Zoological Section, p. 14.

312 CONCLUDING CHAPTER

O. Lamarckiana in vigour and prolific habit, and two
of them actually became established side by side with
the parent type without man’s assistance.

It is unfortunate from the point of view of de Vries’
interpretation of this case that the behaviour of
O. Lamarckiana should suggest in some respects, as
Bateson has pointed out, the phenomena of hybridiza-
tion. It must be observed in support of de Vries’
view that the species appears to exhibit the same
phenomenon in other localities, and, further, that it
has not been possible to make any suggestion as to the
second species with which the pure Lamarckiana might
be supposed to have been crossed.

From one point of view, as de Vries has himself
pointed out, mutation in (Enothera is clearly a
phenomenon of hybrids, and this circumstance of
itself introduces considerable complications into the
story.

We saw just now that there is every reason for the
conviction that mutation takes place in the germ-
cells, and not in the zygote after fertilization. Since
the number of mutants given off under the most
favourable circumstances did not exceed 3 per cent.
of the total offspring, the enormous majority of mutated
germ-cells (on de Vries’ view) must unite with germ-
cells bearing the ordinary specific character. Conse-
quently, the new types which appear will in most cases
have originated in the form of a cross between a mutated
germ-cell and an ordinary germ-cell. And since this
is not the final limit to the possible complications of the
case, we can easily recognise that the complete inter-

THE MUTATION THEORY 313

pretation of the behaviour of Ginothera Lamarcktana is
not by any means an easy matter.

As enunciated by de Vries, the theory of mutation
amounts to a very complete and definite hypothesis.
A large part of this author’s suggestions are, however,
almost purely speculative, and for this reason we have
treated the whole at somewhat less length than it per-
haps deserves. Some of de Vries’ speculations are,
indeed, more picturesque than convincing.

Thus, de Vries regards the number of unit characters
—each of which has arisen by a single mutation—to
be quite limited, even in the highest organisms.
Three or four thousand such characters, he thinks,
may go to build up the hereditary endowment of
the most complicated species. He further supposes
a period of mutation to recur about once in 4,000
years. Four thousand multiplied by 4,000 gives
16,000,000—the number of years required to evolve
the lords of creation from a ‘ primordial protoplasmic
atomic globule.’ And he points out that this estimate
is well within the limits of geological time as allowed
by the physicist. In this way de Vries believes that
his mutation theory removes a difficulty which besets
the selection hypothesis—the difficulty, namely, of
insufficient time. The selectionist may reasonably
reply that the amount of change necessary to produce
in 4,000 years, by the gradual method, a difference
equal to that represented by a single unit character,
might very well be quite imperceptible in a single
generation.

We may summarize our present conclusions as to

314 CONCLUDING CHAPTER

the discontinuous nature of species in the following
manner: A great number of specific characters are,
without doubt, definite ; they are inherited as definite
entities, and there can be no question that their first
coming into existence was a definite event. Every
year tends to increase the range of characters to which
the conception of discontinuity has to be applied.

Certain groups of characters do, however, seem to
exhibit the phenomena of continuity. Let us endeavour
to arrive at some closer idea as to the nature of these
characters.

A study of continuous variations very quickly leads
to the conclusion that the variable features are those
which are especially liable to modification during the
lifetime of the individual, owing to the action of ex-
ternal circumstances. Such quantitative features of
size and shape and number of parts are particularly
plastic in the case of plants.

The habit, or general form and appearance, of a
plant is a feature very characteristic of individual
species. The presence of a dwarf or of a tall habit
does, indeed, constitute a frequent distinction between
different strains of garden plants, and the inheritance
of these characters in many cases follows Mendel’s
law. But leaving aside this particular example, the
inheritance of habit is very little understood ; although
habit is a feature which is very liable to considerable
fluctuations. Habit seems, in fact, usually to afford
an example of continuous variability.

The habit of some species of plants when grown
under alpine conditions on mountain summits is so

CONTINUOUS VARIATIONS 315

different from that of the same species when growing
in the plains, that inexperienced persons might readily
suppose two such forms to belong to as many distinct
species. At intermediate levels the habit is more or
less intermediate. Bonniér made the experiment of
dividing individual plants into two portions, plant-
ing one part at a high elevation and the other near the
level of the sea. In a few years the plant grown on
the mountain had taken on the full alpine habit,
whilst that growa on the plain retained the ordinary
appearance of the species. In this way very con-
siderable differences in habit were shown to be directly
dependent on external conditions.

In some few cases the environment determines the
production of perfectly definite and discontinuous
features. The water ranunculus, when growing sub-
merged beneath the surface of a pond, produces leaves
the blades of which are cut up into a great number of
fine thread-like segments. As soon as the top of the
plant reaches the surface of the water those leaf rudi-
ments which are just commencing their existence pro-
ceed to develop in a totally different fashion. The
leaves to which they give rise possess a wide and undi-
vided blade, which floats upon the surface of the water.
The two sorts of leaves are as utterly different in
appearance as it is possible for leaves to be. Yet the
effect of external conditions upon the young leaf-
rudiment determines which of the two kinds is to
appear.

In this instance we see a discontinuous change in
conditions—the change from water to air as a sur-

316 CONCLUDING CHAPTER

rounding medium—giving rise to a discontinuous
change in structure. Such cases are, however, com-
paratively rare. Much more usually the changes in
external conditions are continuous, as changes of
altitude, moisture, or chemical composition of the soil,
and so on; and the changes induced by them in the
plant are similarly of a continuous kind.

In most animals changes in external circumstances
have a much smaller influence on the form and struc-
ture of the individual than is the case with plants. In
animals considerable modifications are, however,
brought about by exercise and the use of different
parts, as Lamarck long ago observed. But these
modifying factors usually affect all the members of a
single species in nearly the same manner. Neverthe-
less, some part of the differences between individuals in
respect of strength and of proportion, and possibly also
of stature, is undoubtedly associated with differences
of training and nutrition, as the example of the human
race is sufficient to show. Professor Cope has pointed
out how the proper development of such structures as
the joints of vertebrates depends to a very large
extent upon exercise ; and the effect of disuse may be
practically tested by anyone whom accident obliges
to keep a knee or other joint immovable for any length
of time. The so-called play in which the young of
many animals indulge—for example, lambs and kittens
—must have a great influence upon the perfection of
their locomotory functions.

We can now see more clearly the reason for that
great instability of vegetative type which sessile

ACQUIRED VARIATIONS 317

animals, like plants, exhibit. No necessity for definite
and co-ordinated movements involving their whole
structure forces the development of these animals
along certain definite paths. External circumstance
is, therefore, free to mould them into a host of slightly
different shapes. And thus the great variability of
the species of corals, for instance, is doubtless deter-
mined to a large extent by the influence of different
environmental conditions.

Strictly speaking, the term variability ought not to
be applied to modifications of this description. It will,
perhaps, be most convenient, however, to distinguish
true variations—having their origin in differences
among the germ-cells—as genetic variations, contrasting
them with the acquired variations which arise during
the development of individuals.

Enough has now been said to show that it is a very
difficult matter to distinguish in the case of continuous
variations between those which are genetic and those
which are acquired.

It is easy to understand how acquired variations
come to be continuous, and to obey the law of normal
variability. We saw that the normal distribution ot
characters was induced by the random operation of a
multitude of small causes. During the development
of the individual a great number of different external
influences come into play, leading to slight modifications
of every part, now in one direction, now in another.
This being so, we may be quite sure that a large pro-
portion of the normal variability which any species
exhibits is acquired.

318 CONCLUDING CHAPTER

Now we saw that there seems to be good evidence
that normal or continuous variations are inherited.
Logic does not, however, permit us to make the step:
Acquired variations are continuous variations; con-
tinuous variations are inherited; therefore acquired
variations are inherited. It seems, indeed, to be this
fallacy which has led to the long-continued belief in
the inheritance of acquired characters as an important
factor in organic evolution, in spite of so many argu-
ments to the contrary.

Formal disproof of this proposition is very difficult,
and in the meantime the confusion between continuous
acquired variations and continuous genetic variations,
which is always present in practice, constitutes a very
serious drawback to the biometric method of research.
At present Johannsen’s explanation of these phenomena
seems to afford so much the simplest solution that
we may once more repeat his statement of the case,
though with the proviso that the proof of his hypothesis
is still to be awaited.

Johannsen looks upon a population which, as a
whole, exhibits continuous or normal variability, as
being capable of analysis into a number of pure lines.
In a single pure line genetic variability is sensibly
absent. The members of such a pure line exhibit,
however, very considerable acquired variability, so
that in this way each line shows a normal variability
of its own. And the range of this variability may
greatly exceed the limits which separate two pure lines
from one another. The result is to give a completely
blurred picture when all the lines are looked at simul-
taneously. And thus the normal variability of the

PURE LINES 319

population as a whole is brought about by the com-
bination of these two separate factors.

This statement applies to the case of an organism
in which self-fertilization is the general rulé, so that in
this way the separate lines are kept distinct. Where
cross-fertilization takes place between the members of
different pure lines the case becomes enormously com-
plicated, and this is much the most frequent instance
which we have actually to deal with. It has been
suggested that the members of.different lines when
crossed together may display Mendelian phenomena,
but the existence of so large a proportion of acquired
variability renders the problem of analyzing the result
almost insuperable. We have seen, however, that the
numerical results obtained by the biometricians do
not appear to be inconsistent with the existence of
Mendelian inheritance in populations.

We find, then, that the questions of inheritance of
acquired characters and of evolution by the aid of
continuous genetic variations are not yet absolutely
settled. Recent discoveries by Winkler and Baur
regarding the nature of so-called ‘ graft-hybrids’ go
far to prove that acquired characters cannot be in-
herited in plants. The classical example of a ‘ graft-
hybrid’ is Cytasus Adamt, which was produced in 1825
by grafting Cytisus purpureus on the laburnum. This
plant, which is more or less intermediate between the
two parent species, has been reproduced by further
grafting, but its seeds always give rise to plants which
areindistinguishable from thelaburnum. Adventitious

320 CONCLUDING CHAPTER

shoots exactly resembling those of one or other of the
parents often appear upon the ‘ hybrid’ plants.

It now appears that in Cyttsus Adami the cells of the
two component species remain perfectly distinct, and
that its reproductive cells are always of the laburnum
type. In spite of the intimate association of the two
groups of cells which build up a common plant body,
the cells of Cyttsus purpureus are unable to transmit
any hereditary influence to the cells of the laburnum,
and these give rise to offspring which are pure laburnum.
The epidermis of the ‘ graft-hybrid ’ is said to consist
wholly of cells of the Cytisus purpureus type. It seems
fair to argue that if one species wrapped in the epi-
dermis of another receives no heritable influence what-
ever from its living integument, it is in the highest
degree unlikely that the germ cells will be able to
acquire transmissible modifications from an environ-
ment wholly external to the plant. The proof may
not be absolutely conclusive, but when it is combined
with all the other evidence pointing in the same direc-
tion, we think that the inheritance of acquired char-
acters may be disregarded as a practical factor in
evolution.

Meanwhile the number of cases in which discon-
tinuity of inheritance can be shown to hold good is
constantly increasing, and the analysis of some cases
of supposed continuous variation into discontinuous
Mendelian factors has already been made. It may
be safely concluded that a very large part, if not the
whole, of evolution has taken place by the discon-
tinuous method.

New little species—Jorddn’s species—arise, then,

CONCLUSION 321

from time to time, each at a single step, from pre-
existing species. Upon the material thus supplied
natural selection operates ; the weaker go to the wall,
the stronger survive. This is also, in all probability,
the way in which adaptations have arisen. Creatures
which came into existence displaying a particular new
structure, which happened to be fitted for a particular
new function or suited to a particular niche in Nature,
survived and flourished exceedingly. Those in which
undesirable organs appeared perished and were no
more seen. To take Aristotle’s example. If a man
were to be born with molars in front and incisors at
the back of his jaw he would die—at least, in the days
before dentistry. Having his teeth in the positions
in which they actually stand (although not for this
reason only), he survives and rules the world.

After all, the difference between the point of view
thus briefly indicated, and that of Darwin as expressed
in the -‘ Origin of Species,’ is only one of detail—of
detail as to the particular sort of variations by which
evolution chiefly proceeds. Darwin’s analogy between
the origin of species in Nature and the origin of races
under cultivation may be repeated with emphasis,
although Huxley’s famous criticism, to the effect that
races which are sterile together have not arisen in
cultivation, is not yet completely answered. But
this renders the discontinuous origin of such sterility
only the more likely ; and when we recall the Mendelian
behaviour of such characters as long and short style in
the primrose, or sterility of the anthers in the sweet-
pea, the solution of the problem does not seem very

far to seek.
21

322 CONCLUDING CHAPTER

Let us see how the principles of which an outline has
now been given affect the human race itself. The
question of improving the human stock in this country
has lately excited a good deal of attention. But
without a scientific knowledge of the factors upon
which improvement and degeneration depend the dis-
cussion is not likely to be of much profit, and in such
a case misdirected energy may be even worse than
apathy. Without venturing to make any very positive
suggestions, it may at least be pointed out that our
present practice in these matters is in almost every
case the very worst possible.

Professor Karl Pearson has lately shown how the
low birth-rate of the professional and middle classes—
the classes amongst which the intelligence of the
nation is to a large extent segregated—leads to the
recruiting of these classes from amongst the lower and
less intelligent strata of society. In other words, a
steady breeding out of intelligence is taking place.
Recognising that intelligence is an important factor in
national greatness, we proceed to remedy this defect by
endeavouring to reduce the infant mortality among the
less desirable classes, and by offering every inducement
to the production of large families by the lower strata
of society ; indeed, we propose to remove from them
all responsibility for the production of children, and to
feed and house the latter as we already educate them
at the expense of the State.

The principles of heredity teach us that education
and training, however beneficial they may be to indi-
viduals, have no material effect upon the stock itself.
If they have any effect at all, this is undoubtedly

CONCLUSION 323

unimportant in comparison with the effect which would
be produced by the selection of individuals who
exhibit desirable qualities. The demand for a higher
birth-rate ought to apply strictly to desirables.
Instead of this the cry is for education and physical
training, processes which can have no permanent
beneficial effect upon the race.

One writer who holds to some extent the attention
of the intelligent public has recognised the true state
of affairs—I mean Mr. Bernard Shaw. Unfortunately
the public does not take Mr. Bernard Shaw seriously,
wherein, when I recall Mr. Shaw’s published views on
such topics as vivisection and the medical profession,
the public has my sympathy. Nevertheless I know of
no better expression of the moral to be drawn from
the science of genetics than that which is embodied in
the following passage :

‘I do not know whether you have any illusions left
on the subject of education, progress, and so forth.
I have none. Any pamphleteer can show the way to
better things, but when there is no will there is no way.
My nurse was fond of remarking that you cannot make
a silk purse out of a sow’s ear, and the more I see of
the efforts of our churches and universities and literary
sages to raise the mass above its own level, the more
convinced I am that my nurse was right. Progress
can do nothing but make the most of us all as we are, ,
and that most would clearly not be enough even if
those who are already raised out of the lowest abysses
would allow the others a chance. The bubble of
heredity has been pricked, the certainty that acquire-
ments are negligible as elements in practical heredity

2I—2

324 CONCLUDING CHAPTER

has demolished the hopes of the educationists as well
as the terrors of the degeneracy-mongers, and we now
know that there is no hereditary “‘ governing class ” any
more than a hereditary hooliganism. We must either
breed political capacity or be ruined by democracy,
which was forced on us by the failure of the older
alternatives. Yet if despotism failed only for want of
a capable benevolent despot, what chance has demo-
cracy, which requires a whole population of capable
voters—that is, of political critics who, if they cannot
govern in person for lack of spare energy or specific
talent for administration, can at least recognise and
appreciate capacity and benevolence in others, and
so govern through capably benevolent representatives ?
Where are such voters to be found to-day ? Nowhere.
Promiscuous breeding has produced a weakness of
character that is too timid to face the full stringency
of a thoroughly competitive struggle for existence, and
too lazy and petty to organize the commonwealth
co-operatively. Being cowards, we defeat natural
selection under cover of philanthropy ; being sluggards,
we neglect artificial selection under cover of delicacy
and morality.’*

Mr. Shaw recognises, however, that our knowledge
is at present insufficient to prescribe for the breeding
of a ‘ Superman,’ even if we were able to come to any
agreement as to what qualities are the most desirable.
Nevertheless it is along the lines which we have en-
deavoured to indicate that such knowledge must be
sought in the future.

* ‘Man and Superman,’ p. xxiii.

GLOSSARY

[Many technical terms not included in this glossary ave
printed in italics on their first appearance in the body of the
book, and their meaning is then defined. Such definitions
may be discovered on a reference to the index.]

ADAPTATION. —A_ teleological explanation of the corre-
spondence often shown between the structure and habits of
a particular creature and the environment in which the
creature lives.

ALsBiIno,—An animal or plant characterized by the absence
of colouring matter from its external tissues.

Atc#.—A group of plants, mostly aquatic and of re-
latively simple organization.

ANTHER.—The upper part of a stamen, containing the
pollen.

Atom.—The smallest part of a chemical element which
can exist as such.

Axit.—The angle enclosed between the base or stalk of
a leaf and the stem upon which the leaf is borne.

BinoMIAL NOMENCLATURE.—The application of a double
name to an animal or plant, the first name being that of
the genus, the second that of the species. '

Brotocy.—The science of the phenomena of life.

Brometry. — The application of statistical methods to
biological problems.

Botany.—tThe scientific study of plants.

Catyx.—The outermost whorl of floral leaves, which in
the bud usually encloses the other organs of the flower.

CuHARACTER.—In heredity, a single definable attribute.

Crass.—One of the larger subdivisions of the animal
kingdom—e.g., mammals, birds.

325

326 GLOSSARY

Composirz.—A family of plants, including the daisy,
chrysanthemum, and many others.

ConyuGation.—The process of fusion of a pair of gametes.

CoroLita.—The second envelope of a flower, consisting of
petals—leaf-like organs—usually brightly coloured.

CorPuscLE.—A very minute particle.

CytoLocy.—The scientific study of the minute con-
stituent parts of organisms by the aid of the microscope.

DENUDATION.—The wearing away of the earth’s surface by
the action of rain, rivers, etc.

DIFFERENTIATION.—The separation or discrimination of
parts which were previously more or less united and uniform.

EmBryo.—A young plant or animal—usually one which is
still contained in the seed or the womb.

EmBryoLocy.—The history of the development of young
plants or animals from the egg.

ENVIRONMENT.—Natural surroundings.

EvoLutTion.—See p. 22.

Famity.—A group of allied genera, as the family of apes
(Anthropoida), the buttercup family (Ranunculacea).

Fauna.—The sum total of animals inhabiting a particular
region.

FERTILIZATION.—The union of male and female repro-
ductive cells or gametes. ;

FLorets.—The separate flowers of a crowded inflorescence.

GameETEs.—Sexual cells which unite in conjugation or
fertilization.

Genus.—A group of allied species.

GrEoLocy.—The study of the earth’s crust.

GrometTric RaTE oF INCREASE.—Progress consisting in
successive multiplications of the preceding number, instead
of simply in additions to it.

GERM-CELLS.—See GAMETES.

HERBALIST.—One who collects and studies herbs.

HEREDITY.—The transference of similar characters from
one generation of organisms to another, a process effected by
means of the germ-cells or gametes,

GLOSSARY 327

IcnEous.—Produced in connection with great heat.
INBREEDING.—The mating together of near relatives for a
number of generations.

Larva.—The young of an insect after it has emerged from
the egg—e.g., a caterpillar.

MANTIDAE.—A group of predatory insects.

Maxi_Ltary.—Connected with the mouth parts.

MorpHoLocy.—tThe study of form and structure.

Murtation.—The sudden origin of a new species at a single
step.

OrGanism.—A living creature.

ORNITHOLOGIsT.—A student of birds.

Ovary.—In animals the organ which produces ova. In
plants the organ which contains the ovules.

Ovum.—The female gamete.

OvuLre.—The structure surrounding the spore which gives
rise to the female gamete or ovum in the higher plants.

PrtTaLt.—One of the (usually) coloured leaves composing
the corolla.

PETALOID.—Resembling the corolla, usually in the circum-
stance of being coloured.

PuysioLocy.—The study of the functions of organisms.

PIN-EYED.—Having the stigma on a level with the throat
of the corolla, and the anthers lower down, enclosed within
the tube.

Pist1t.—The central organ of a flower, which contains the
ovules, and ultimately becomes the fruit, or the chief part
of it.

PoLt—EN.—Those spores of the flowering plants which
produce the male gametes.

POLLINATION.—The transference of pollen to the stigma of
a plant.

PRIMARY, SECONDARY, AND TERTIARY Epocus.—The three
great divisions of geological time during which the known
fossiliferous strata were deposited.

RapicaL Leaves.—Leaves arising immediately from the
root-stock in the form of a rosette.

328 GLOSSARY

REVERSION.—The reappearance in the offspring of a
character proper to a more or less remote ancestor, and not
exhibited by the immediate parents.

Rotirers.—A kind of minute aquatic animals.

SEGMENT.—One of a series of more or less similar trans-
verse divisions.

SESSILE.—Fixed and stationary, but (in the strict sense)
without a stalk.

SomaTic.—Belonging to the body of a zygote.

Species, LInNzan.—A group of organisms of closely
similar appearance.

SPECIES, JORDAN’S.—A group of organisms believed to have
arisen by a mutation. (Jordan himself did not, however,
suppose so.)

Sport.—A marked mutation—often one occurring under
domestication.

StTamMENs.—The organs of a flower which bear the pollen.

STANDARD.—The large, upright petal at the back of such
a flower as that of the sweet-pea.

StigmAa.—The uppermost part of the pistil, upon which the
pollen is received.

STRATUM.—A layer.

STYLE.—A stalk connecting the stigma with the ovary—
part of the pistil.

Txsta.—The skin or coat covering a seed.

THRUM-EYED.—Having the anthers situated at the throat
of the corolla, and the stigma lower down, enclosed in the
tube.

Tusz.—The basal tubular portion of a corolla in which
the separate petals are closely fused together, as is the case
with that of the primrose,

UNICELLULAR.—Consisting of a single cell.

VaRIATION, CONTINUOUS.—See Chapter IV.
VARIATION, DisconTINUOUS.—See Chapter V.

Wincs.—The lateral petals of a pea-flower.

ZooLocy.—The scientific study of animals,
ZyGotEe.—The organism produced by the fusion of a pair of
gametes,

INDEX

ACQUIRED characters, inher-
itance of, 65 et seq., 155
variations, 317
Adaptation, 51 et seg.
Algz, 304
Allelomorphs, 185, 195, 208
Allogene, 229
Allozygote, 229
Ameeba, 240
Ancestral heredity, law of,
34, 113, 228
Andalusian fowl, 192
Andrez, 58
Argus pheasant, 63
Aristotle, 124, 320
Array, 104
Ascaris, 251
Atoms, chemical, 127
Aurea variety, 149

Bachmann, 7

Basset-hound, I1ro

Bateson, W., 17, 61, 66, 128
ét ség., 192, 200, 215, 310,
311

Beetles, 155

Bentham and Hooker, 8

Between-race, 148 e¢ seq.

Biffen, R. H., 233 e¢ seq.

Biometry, 18, 80 ef seg., 227
et seq.

Blaringhem, 156
Bonniér, 315

Boveri, 252 et seq.
Browne, Dr. T., 124
Burbank, 44

Busbey, 137
Butterflies, tropical, 56

Casuarina indica, 59

Cat, 70

Cell, 239 ef seq.

Cereals, 233 e¢ seq.

Chromatin, 248

Chromomeres, 268

Chromosomes, 248 et seq.

Classes, 88, 97

Clover, 150

Coat colour of horses, 109,
IIO, 215

Coefficient of variability, 99

Coins, spinning of, 91 e¢ seq.

Colour, inheritance of, 213

of animals, 56

Comparison, 12
| Compositors, 69

Conscientiousness, III

Controversy, 3

Co-ordinated structures, 69
variability, 123

Cope, 316

Correlated variability, 106

329

330

Correlation, 102 e¢ seq.
coefficient, 106 e¢ seq.
Correns, 19, 190, 299
Coupling, 197 e¢ seq.
Cowslip, 8
Criminals, 295
Crustaceans, 62
Cryptomerism, 203
Cuénot, 206 et seq., 274
Cytology, 239 et seq., 303

Dalton, 19, 175
Darwin, Charles, 2, 3, 16, 33,
38, 40 et seqg., 54, 131,
160, 215, 310, 320
Erasmus, 29
Sir G., 28
Daughter-cell, 242
Davenport, 99
De Maillet, 30
Degressive mutation, 153
Determinants, 279
Deviation, 98, 125
de Vries, 17, 19, 75, 76, 83,
I2I, 135 ef seg., 153, I90,
216, 218, 267, 311
Di-hybrids, 186
Discontinuous variation, 128
et seq., 309 et seq.
Domestication, 34
Dominance, 192
Dominant, 179
Double flowers, 133
Druery, C. T., 36
Dyer, Sir W. Thistelton, 35

Elephant, 46
Embryo, 177
Embryology, 31, 243
Emily Henderson, 209
Empedocles, 29
Endosperm, 177

INDEX

Error, probable, 99
Eugenics, 20, 282 e¢ seq.
Evening primrose, 83
Evolution, 22 et seq.

parallel, 57, 59
Eye, colour of human, 83, IIo

Ferns, 305
Fertilization, 162, 244, 246
normal, 216

Flat fishes, 38

Fluctuating variations, 75

Formula Mendelian, 183

Franklin, 45

Fraternal correlation, 106 e?
seq.

Frequency, 98

Gaertner, 170

Galton, 80, 89, 107, 113, 125
et seq., 282 et seq.

Gametes, 178, 244

Geikie, 28

Genetics, 298

Genetic variations, 317

Geographical distribution, 33

Geological succession, 33

time, 27, 313

Germ-cells, 73, 178, 244

Germ-plasm theory, 276 et seq.

Giraffe, 37

Goethe, 30, 75

Goss, 169

Gradation of organisms, 31

Greenland whale, 52

Gregory, R. P., 215

Greyhound, coat colour of,
IIo

Gulick, 50

Habit of plants, 314
Half-race, 148 et seq.

INDEX

Hawaiian Islands, 50
Helium, 23
Herbert, 169
Heredity, 101, 174 et seg.
Heron, 292
Heterotropic chromosome,270
et seq.
Heterozygote forms, 192
Heterozygotes, 184 e¢ seq.
Hieracium, 190
Hippocampus, 55
Homeeosis, 133 e seq.
Homozygote, 185
Horns, 69
Horse, American trotting-,
138 e¢ seq.

coat colour in, 109, 215

extinct, 33
Hume, David, 29
Hurst, 215
Hutton, 26
Huxley, 30, 124, 292, 320
Hybrid, 159
Hypostasis, 213

Idiochromosomes, 271

Ids, 279

Immunity to rust, 237

Indian corn, 177 et seq.

Inheritance of acquired char-
acters, 65 et seq., 155

Irish elk, 69

Isolation, 50

Japan, 58

Jennings, 116

Johannsen, 116 ef seq., 318
Jordan, 8, 153

Jordan’s species, 9, 320

Kelvin, Lord, 27
Knight, 166

331

Knight-Darwin law, 168
KG6lreuter, 161 et seq.

Lamarck, 15 et seg., 30, 36 et
seq., 65, 316

Laplace, 24

Latent characters, 203

Leicestershire sheep, 137

Line, pure, 109 et seg., 318

Linneus, 7 et seq.

Lyell, Sir C., 25

Macdougal, 146, 156, 294

Maize, 177 et seg., 198

Malthus, 45, 46, 137

Manitobar Hard, 235

Mantide, 55

Maple pea, 205

Masters, 133

Matthews, 42

Mean, 96

Median, 96

Mendel, 19, 113, 189 ef seq.,

264, 299 et seq.

Mendel’s law, 189, 299 ef seq.

Mental characters, III

Merism, 129

Meristic variations, 130

Metamorphy, 133

Mice, 207, 274

Michigan bronze, 237

Mid-race, 148 et seq.

Millardet, 171 et seq.

Mimicry, 54 e¢ seq.

Mirabilis jalapa, 165
longiflova, 165

Mitosis, 248 e¢ seq.

Mivart, 38

Mode, 96

Mongrel, 160

Mono-hybrid, 183 e¢ seq.

Morgan, T. H., 61 e¢ seq.

332

Morphology, 32

Mother-cell, 242

Mutation, 36, 121 e¢ seg., 311
atavistic, 154
degressive, 153
progressive, 153
retrogressive, 154

Mutationist, 17

Nageli, 190, 279
Natural selection, 42 ef seq.,
48, 121, 155

Naudin, 170, 220

Nebular hypothesis, 24

Neo-Darwinian, 3, 16

Neo-Lamarckian, 3

Newton, 175

Nicotiana, 163, 166

Normal variations, 82 et seq.
curve, 95 ef seq.

Nuclear division, 248 eé seq.
membrane, 246

Nucleus, 246 ef seq.

Nutrition, 76, 155

Cnothera albida, 143
biennis, 147
gigas, 143
Lamarckiana, 142 et seq.,

311

leita, 219

lata, 143

muricata, 147

nanella, 144, 147

velutina, 219
Ovum, 177, 245
Oxlip, 8

Painted Lady, 210

Parental correlation, 1or ef
seq.

Partial Gametic coupling, 200

INDEX

Pea, garden, 84, 166 e¢ seq., ,
190, 198, 204
Peacock, 63, 132
Pearson, Karl, 77, 80 et seq.s
108, I10, 136, 229, 285, 321
Phasmide, 55
Phyllopteryx, 55
Physical science, 4
Picotee, 210
Pigeons, 34, 45
Plastids, 240
Pleurococcus, 241
Polish wheat, 234
Polygon, Galton’s, 126
Populations, 117
selection in, 119
Poulton, Professor, 57

'Primrose, 8, 215

Primula, 151, 194
Probability, theory of, 91
Progressive mutation, 153

‘Pronuba, 54
' Protenor belfragi, 270

Prothallus, 306

'Protogene, 229

Protoplasm, 239

Protozygote, 229

Puccinia glumarum, 237

Punnett, R.C., 47, 192, 200

Pure lines, 109 e¢ seq., 318
selection in, 119

Purple Invincible, 210

Quartile, 97
Quetelet, 80, 93

Radium, 22
Ranunculus, 7

water, 150, 315
Recessive characters, 179
Red Fife, 235
Reducing division, 258 e¢ seq.

INDEX

Regeneration, 61 et seq.

Regression, 107

Resemblance, protective, 56

Retrogressive mutation, 154

Reversion on crossing, 202 ef
Séq., 213

Rivet wheat, 234

Rosenberg, 254

Rotifers, 47

Rough Chaff, 235

Rust immunity, 237

Saunders, 209, 213
Saxifraga hypnoides, 59
Schull, Dr. G. H., 213
Sea-urchin, 256
Sedgwick, A., 75
Selection, artificial, 44, 115,
135
natural, 42 et seq., 48, 121
Selective fertilization, 209,
274
Sex, 221, 270 ef seq.
Sexual selection, 62
Shaw, G. B., 322
Sheep, Leicestershire, 137
Shirley poppies, 140
Snails, 215
Snakes, 37
Species, 5
idea of, 6
Jordan’s, 9, 320
Linnean, 7
Spencer, Herbert, 22, 40, 47,
65, 68, 122
Spermatozoa, 245
Spindle, nuclear, 250
Spores, 257, 305
Standard deviation, 98
Statistics, 12, 81
collection of, 86
Stature, 90, 106

333

Sterility, 320
Stocks, 213
Strawberries, 171
Strength of pull, 89
of wheat flour, 234
Structural characters,
heritance of, 183
Substantive variation, 131
Sugar-beet industry, 135
Survival of the fittest, 47
Swamping effect of crossing,
221
Sweet-peas, 209 ef seq.
Symmetry, 129
Syrphide, 58

in-

Tactual discrimination, 66
Tetrads, 259
Thomson, J. J., 24, 127
Throw-back, 154
Tinged white, 211
Tower, W.L., 155
Treviranus, 30
Trifolium pratense, 150
Tschermak, 19, 190, 203
Types, primary and
ordinate, 125

sub-

Unicellular animals, 240
Unit characters, 176, 220
Usher, Bishop, 27

Variability, acquired, 317
correlated, 106
genetic, 317
coefficient of, 99
Variate, 97
Variation, 14
continuous, 14, 82, 314
et seq.
discontinuous,
310 eft seg.

I4, 128,

334 INDEX

Variation, meristic, 130
similar and simultane-
ous, 132
substantive, 131

Wading birds, 38

Wallace, A. R., 16, 35, 43, 65,
88, 157

Webb, Sidney, 293

Weismann, 16, 51, 65, 72 et
Seq., 157, 278 et seq.

Wells, 42 ‘

Werner, 25

Whale, 52

Wheat, 233 et seq.
Wilks, 140

Wilson, 74, 270 et seq.

X-generation, 304

Yucca plant, 54
Yule, G. U., 227 et seq.

Zygote, 185, 245

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