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HORMONES AND HEREDITY

HOKMONES AND HEREDITY

A DISCUSSION OF THE EVOLUTION OF

ADAPTATIONS AND THE EVOLUTION

OF SPECIES

BY

J. T. CUNNINGHAM, M.A. (OxoN), F.Z.S,

SOMETIME FELLOW OF UNIVERSITY COLLEGE, OXFORD

LECTURER IN ZOOLOGY AT EAST LONDON COLLEGE

UNIVERSITY OF LONDON

LONDON

CONSTABLE AND CO. LTD.
1921

Printed in Great Britain

PREFACE

MY chief object in writing this volume was to discuss
the relations of modern discoveries concerning
hormones or internal secretions to the question of
the evolution of adaptations, and on the other hand
-to the results of recent investigations of Mendelian
heredity and mutations. I have frequently found,
from verbal or written references to my opinions,
that the evidence on these questions and my own
conclusions from that evidence were either im-
perfectly known or misunderstood. This is not
surprising in view of the fact that hitherto my only
publications on the hormone theory have been a
paper in a German periodical and a chapter in an
elementary text-book. The present publication is
by no means a thorough or complete exposition of
the subject, it is merely an attempt to state the
fundamental facts and conclusions, the importance
of which it seems to me are not generally appreciated
by biologists.

I have reviewed some of the chief of the recent
discoveries concerning mutations, Mendelism, chromo-
somes, etc., but have not thought it necessary
to repeat the illustrations which are contained in
many of the volumes to which I have referred. I
have made some Mendelian experiments myself,
not always with results in agreement with the strict

vi HORMONES AND HEREDITY

Mendelian doctrine, so that I am not venturing to
criticise without experience. I have not hesitated
to reprint the figure, published many years ago, of a
Flounder showing the production of pigment under
the influence of light, because I thought it was
desirable that the reader should have before him this
figure and those of an example of mutation in the
Turbot for comparison when following the argument
concerning mutation and recapitulation.

I take this opportunity of expressing my thanks to
the Councils of the Royal Society and the Zoological
Society for permission to reproduce the figures in
the Plates. I also desire to thank Professor Dendy,
F.R.S., of King's College for his sympathetic interest
in the publication of the book, and Messrs.
Constable and Co. for the care they have taken in

its production.

J. T. CUNNINGHAM.

LONDON, June 1921.

CONTENTS

INTRODUCTION

PAGE

HISTORICAL SURVEY OF THEORIES OR SUGGESTIONS OF

CHEMICAL INFLUENCE IN HEREDITY . . xi

CHAPTER I

CLASSIFICATION AND ADAPTATION ... 1

CHAPTER II

MENDELISM AND THE HEREDITY OF SEX . . .39

CHAPTER III

INFLUENCE OF HORMONES ON DEVELOPMENT OF SOMATIC

SEX-CHARACTERS . . . . .67

CHAPTER IV

ORIGIN OF SOMATIC SEX-CHARACTERS IN EVOLUTION . .107

CHAPTER V

MAMMALIAN SEXUAL CHARACTERS. EVIDENCE OPPOSED TO

THE HORMONE THEORY . . . .134

vii

viii HORMONES AND HEREDITY

CHAPTEE VI

PAOB

ORIGIN OF NON-SKXUAL CHARACTERS: THE PHENOMENA

OF MUTATION . . . . . .170

CHAPTER VII

METAMORPHOSIS AND RECAPITULATION . . .198

INDEX 243

LIST OF PLATES

PLATE I. RECESSIVE PILE FOWLS . . . to face p. 47
„ II. ABNORMAL SPECIMEN OF TURBOT . . „ 206

„ III. FLOUNDER, SHOWING PIGMENTATION OF LOWER

SIDE AFTER EXPOSURE TO LIGHT 212

i.v

INTRODUCTION

HISTORICAL SURVEY OF THEORIES OR SUGGESTIONS
OF CHEMICAL INFLUENCE IN HEREDITY

WEISMANN, strongly as he denied the possibility of
the transmission of somatic modifications, admitted
the possibility or even the fact of the simultaneous
modification of soma and germ by external conditions
such as temperature. Yves Delage l in 1895, in
discussing this question, pointed out how changes
affecting the soma would produce an effect on the
ovum (and presumably in a similar way on the sperm).
He writes : —

' Ce qui empeche 1'ceuf de recevoir la modification
reversible c'est qu'etant constitue autrement que les
cellules differenciees de I'organisme il est influence
autrement qu'elles par les memes causes pertur-
batrices. Mais est-il impossible que malgre la
difference de constitution physico-chimiques il soit
influence de la meme fagon ? '

The author's meaning would probably have been
better expressed if he had written ' ce qui parait
empecher.5 By ' modification reversible ' he means
a change in the ovum which will produce in the next
generation a somatic modification similar to that by
which it was produced. It seems natural to think
of the influence of the ovum on the body and of the
body on the ovum as of similar kind but in opposite
directions, but it must be remembered always that

1 Yves Delage, VHeredite (Paris, 1895), pp. 806-812.

xi

xii HORMONES AND HEREDITY

the development of the body from the ovum is not
an influence at all but a direct conversion by cell-
division and differentiation of the ovum into the body.

Delage argues that if the egg contains the sub-
stances characteristic of certain categories of cells of
the organism it ought to be affected at the same time
as those cells and by the same agents. He thinks
that the egg only contains the substances or the
arrangements characteristic of certain general
functions (nervous, muscular, perhaps glandular
of divers kinds) but without attribution to localised
organs. In his view there is no representation of
parts or of functions hi the ovum, but a simple
qualitative conformity of constitution between the
egg and the categories of cells which in the body are
charged with the accomplishment of the principal
functions. Thus mutilations of organs formed of
tissues occurring also elsewhere in the body cannot
be hereditary, but if the organ affected contains the
whole of a certain kind of tissue such as liver, spleen,
kidney, then the blood undergoes a qualitative modi-
fication which reacts on the constitution of the egg.

Suppose the internal secretion of a gland (e.g.
glucose for the liver, glycolytic ferment for the
pancreas) is the physiological excitant for the gland.
If the gland is removed in whole or in part the pro-
portion of its internal secretion in the blood will be
diminished. Then the gland, if the suppression is
partial, will undergo a new diminution of activity.
But in the egg the specific substance of the gland
will also be less stimulated, and in the next genera-
tion a diminution of the gland may result. Thus
Delage states Massin found that partial removal of
the liver in rabbits had an inherited effect. In the
case of excretory glands the contrary will be the case,

INTRODUCTION xiii

for their removal causes increase in the blood of the
exciting substances urea and uric acid.

The effects of disuse are similar to those of mutila-
tions and of use vice versa. Delage, as seen above,
does not consider that increase or decrease of par-
ticular muscles can be inherited, but only the
muscular system in general. If, however, in con-
sequence of the disuse of a group of muscles there
was a general diminution of the inherited muscular
system, the special group would remain diminished
while the rest were developed by use in the in-
dividual : there would thus be a special heredity
produced indirectly. With regard to general con-
ditions of life, Delage states that there are only
two of which we know anything — namely, climate
and alimentation — and he merely suggests that tem-
perature and food act at the same time on the cells
of the body and on the similar substances in the egg.

H. M. Vernon (Variation in Animals and Plants,
1903, pp. 351 seq.) cites instances of the cumulative
effects of changed conditions of life, and points out
that they are not really instances of the inheritance
of acquired characters, but merely of the germ-
plasm and the body tissues being simultaneously
affected. He then asks, Through what agency is
the environment enabled to act on the germ-plasm ?
And answers that the only conceivable one is a
chemical influence through products of metabolism
and specific internal secretions. He cites several
cases of specific internal secretions, making one
statement in particular which seems unintelligible,
viz. that extirpation of the total kidney substance of
a dog leads not to a diminished secretion of urine
but to a largely increased secretion accompanied by
a rapid wasting away which soon ends fatally.

xiv HORMONES AND HEREDITY

Whenever a changed environment acts upon the
organism, therefore, it to some extent affects the
normal excretions and secretions of some or all of
the various tissues, and these react not only on the
tissues themselves, but also to a less degree upon the
determinants representing them in the germ-plasm.
Thus the relative size of the brain has decreased in
the tame rabbit. This may be due to disuse ; the
excretions and secretions of the nervous tissues would
be diminished, and the corresponding determinants
less stimulated. Another instance is afforded by
pigmentation of the skin in man ; which varies with
the amount of light and heat from the sun to which
the skin is exposed. Specific excretory products of
pigment in the skin may stimulate the pigment
determinants in the germ-plasm to increased vigour.
But only those characters of which the correspond-
ing tissues possess a specific secretion or excretion
could become hereditary in this way. For instance,
the brawny arm of the blacksmith could not be
transmitted, as it is scarcely possible that the arm
muscles can have a secretion different from that of
the other muscles.

In 1904, P. Schiefferdecker 1 made the definite
suggestion that the presence of specific internal
secretions could be very well used for the explanation
of the inheritance of acquired characters. When
particular parts of the body were changed, these
modifications must change the mixture of materials
in the blood by the substances secreted by the
changed parts. Thereby would be f ound a connexion
between the modified parts of the body and the
germ-cells, the only connexion hi existence. It is to

1 P. Schiefferdecker, Ueber Symbioae. S.B. d. Niederrhein. Gescllsch.
zu Bonn. Sitzung der Mnlirinischrn Soktion, 13 Juni 1904.

INTRODUCTION xv

be assumed, according to this author, that only a
qualitative change in the nutritive fluid of the germ-
cells could produce an effect : a quantitative change
would only cause increased or decreased nourish-
ment of the entire germ cells.

In my own volume on Sexual Dimorphism in the
Animal Kingdom, published in 1900, I attempted to
explain the limitation of secondary sexual characters
not only to one sex, but usually to one period of the
individual life, namely, that of sexual maturity;
and in some cases, as in male Cervidae, to one season
of the year in which alone the sexual organs are active.
It had been known for centuries that the normal
development of male sexual characters did not take
place in castrated animals, but the exact nature of
the influence of the male generative organs on that
development was not known till a year or two later
than 1900, when it was shown to be due to an
internal secretion. My argument was that all
selection theories failed to account for the limitation
of secondary sexual characters in heredity, whereas
the Lamarckian theory would explain them if the
assumption were made that the effects of stimulation
having been originally produced when the body and
tissues were under the influence of the sexual organs
in functional activity, these effects were only
developed in heredity when the body was in the same
condition.

About the year 1906, when preparing two special
lectures in London University on the same subject,
I became acquainted with the work of Starling and
others on internal secretions or hormones, and saw
at once that the hormone from the testes was the
actual agent which constituted the ' influence '
assumed by me in 1900. In these lectures I

xvi HORMONES AND HEREDITY

elaborated a definite Lamarckian theory of the origin
of Secondary Sexual Characters in relation to
Hormones, extending the theory also to ordinary
adaptive structures and characters which are not
related to sex. Having met with many obstacles in
endeavouring to get a paper founded on the original
lectures published in England, I finally sent it to
Professor Wilhelm Roux, the editor of the Archiv fur
Entwicklungsmechanik der Organismen, in which it
was published in 1908.

In his volume on the Embryology of the Inverte-
brata, 1914 (Text-Book of Embryology, edited by
Walter Heape, vol. i.), Professor E. -W. MacBride in
his general summary (chapter xviii.j puts forward
suggestions concerning hormones without any
reference to those who have discussed the subject
previously. He considers the matter from the point
of view of development, and after indicating the
probability that hormones are given off by all the
tissues of the body, gives instances of organs being
formed in regeneration (eye of shrimp) or larvae
(common sea-urchin) as the result of the presence
of neighbouring organs, an influence which he thinks
can only be due to a hormone given off by the organ
already present. He then states that Professor
Langley had pointed out to him in correspondence
that if an animal changes its structure in response to
a changed environment, the hormones produced by
the altered organs will be changed. The altered
hormones will circulate in the blood and bathe the
growing and maturing genital cells. Sooner or later,
he assumes, some of these hormones may become
incorporated in the nuclear matter of the genital
cells, and when these cells develop into embryos
the hormones will be set free at the corresponding

INTRODUCTION xvii

period of development at which they were originally
formed, and reinforce the action of the environ-
ment. In this way MacBride attempts to explain
recapitulation in development and the tendency to
precocity in the development of ancestral structures.
His idea that the hormones act by ' incorporation '
in the genital cells is different from that of stimulation
of determinants put forward by myself and others,
but it is surprising that he should refer to un-
published suggestions of Professor Langley, and not
to the publications of authors who had previously
discussed the possible action of hormones in con-
nexion with the heredity of somatic modifications.

Dr. J. G. Adami in 1918 published the Croonian
Lectures, delivered by him in 1917 under the title
* Adaptation and Disease,' together with reprints of
previous papers, in a volume entitled Medical Con-
tributions to the Study of Evolution. In this work
(footnote, p. 71) the author claims that he preceded
Professor Yves Delage by some two years in offering
a physico-chemical hypothesis in place of deter-
minants, and also asserts that ' the conclusions
reached by him in 1901 regarding metabolites and,
as we subsequently became accustomed to term
them, hormones, and their influence on the germ-
cells, have since been enunciated by Heape, Bourne,
Cunningham, MacBride, and Dendy, although in
each case without note of his ( Adami' s) earlier
contribution.' These somewhat extensive claims
deserve careful and impartial examination. The
paper to which Dr. Adami refers was an Annual
Address to the Brooklyn Medical Club, published
in the New York Medical Journal and the British
Medical Journal in 1901, and entitled ' On Theories
of Inheritance, with special reference to Inheritance

b

xviii HORMONES AND HEREDITY

of Acquired Conditions in Man.' The belief that this
paper had two years' priority over the volume of
Delage entitled L'Heredite appears to have arisen
from the fact that Adami consulted the biblio-
graphical list in Thomson's compilation, Heredity,
1908, where the date of Delage' s work is given as
1903. But this was the second edition, the first
having been published, as quoted above, in 1895, six
years before the paper by Adami.

Next, with regard to the claim that Adami' s
views as stated in the paper to which he refers were
essentially the same as those brought forward by
myself and others many years later, we find on
reading the paper that its author discussed merely
the effect of toxins in disease upon the body-cells and
the germ-cells, causing in the offspring either various
forms of arrested and imperfect development or
some degree of immunity. In the latter case he
argues that the action of the toxin of the disease has
been to set up certain molecular changes, certain
alterations in the composition of the cell-substance,
so that the latter responds in a different manner when
again brought into contact with the toxin. Once
this modification in the cell-substance is produced,
the descendants of this cell retain the same pro-
perties, although not permanently. Inheritance of
the acquired condition has to be granted, he says, in
the case of the body-cells in such cases. But this is
not the question : inheritance in the proper sense of
the word means the transmission to individuals of
the next generation.

On this point Adami says we must logically admit
the action of the toxins on the germ-cells, and the
individuals developed from these must, subject to the
law of loss already noted, have the same properties.

INTRODUCTION xix

He admits that inherited immunity is rare, but says
that it has occasionally been noted. Here we have
again merely the same influence, chemical in this case,
acting simultaneously on somatic cells and germ-
cells, which" is not the inheritance of acquired char-
acters at all. Adami remarks that Weismann
would make the somewhat subtle distinction that
the toxins produce these results not by acting on
the body-cells but by direct action on the germ-cells,
that the inheritance is blastogenic not somatogenic,
and calls this ' a sorry and almost Jesuitic play
upon words.' On the contrary, it is the essential
point, which Adami fails to appreciate. However,
he goes further and refers to endogenous intoxica-
tion, to disturbed states of the constitution, due to
disturbances in glandular activity or to excess of
certain internal secretions. Such disturbances he
says, acting on the germ-cells, would be truly soma-
togenic. In the case of gout he considers that defect
in body metabolism has led to intoxication of the
germ-cells, and the offspring show a peculiar liability
to be the subjects of intoxications of the same order.
Now, however important these views and conclusions
may be from the medical point of view, in relation to
the heredity of general physiological or pathological
conditions, they throw no light on the problems
considered by myself and other biologists — namely,
the origin of species and of structural adaptations.

There is no mention anywhere in Adami' s short
paper of the evolution or heredity of structural
characters or adaptations such as wing of Bird or Bat,
lung of Frog, asymmetry of Flat-fish or of specific
characters, still less of secondary sexual characters,
which formed the basis of the hormone theory in my
1908 paper. He does not even consider the evolu-

XX

HORMONES AND HEREDITY

tion of the structural adaptations which enable man
to maintain the erect position on the two hind-limbs.
He does not consider the action of external stimula-
tion, whether the direct action on epidermal or other
external structures or the indirect action through
stimulation of functional activity. All his examples
of external agents are toxins produced by bacteria
invading the body, except in the case of gout, for
which he suggests no external cause at all.

Only once in the last part of the paper considered
does Adami mention internal secretions. His actual
words are : c We recognise yearly more and more the
existence of auto-intoxications, of disturbed states
of the constitution due to disturbances in glandular
activity or to excess of certain internal secretions or
of the substances ordinarily neutralised by the same.'
The only example he gives is that of gout. How
remote this is from the discoveries concerning the
specific action of hormones on the growth of the body
or of special parts of the body, or on the function of
glands, and from a definite hormone theory of
heredity as proposed by myself, is sufficiently
obvious.

CHAPTER I

CLASSIFICATION AND ADAPTATION

THE study of the animals and plants now living on
the earth naturally divides itself into two branches,
the one being concerned with their structure and
classification, the other with their living activities,
their habits, life histories, and reproduction. Both
branches are usually included under the terms
Natural History, or Zoology, or Botany, and a
work on any group of animals usually attempts
to describe their structure, their classification, and
their habits. But these two branches of biological
science are obviously distinct in their methods and
aims, and each has its own specialists. The pursuit,
whose ultimate object is to distinguish the various
kinds of organisms and show their true and not
merely apparent relations to one another in structure
and descent, requires large collections of specimens
for comparison and reference : it can be carried on
more successfully in the museum than among the
animals or plants in their natural surroundings.
This study, which may be called Taxonomies,
deals, in fact, with organisms as dead specimens,
and it emphasises especially the distinguishing
characters of the ultimate subdivisions of the vari-
ous tribes of animals and plants — namely, species
and varieties. The investigation, on the other
hand, of the different modes of life of animals or

A

2 CLASSIFICATION AND ADAPTATION

plants is based on a different mental conception
of them : it regards them primarily as living active
organisms, not as dead and preserved specimens,
and it can only be carried on successfully by observ-
ing them in their natural conditions, in the wide
spaces of nature, under the open sky.

The object of this kind of inquiry is to ascertain
what are the uses of organs or structures, what they
are for, as we say in colloquial language, to discover
what are their functions and how these functions
are useful or necessary to the life of the animals or
plants to which they belong. For example, some
Cuttle-fishes or Cephalopoda have 'eight arms or
tentacles and others ten. The taxonomist notices
the fact and distinguishes the two groups of
Octopoda and Decapoda.

But it is also of interest to ascertain what is the
use of the two additional arms in the Decapoda.
They differ from the other arms in being much
longer, and provided with sockets into which they
can be retracted, and suckers on them are limited
to the terminal region. In the majority of zoo-
logical books in which Cephalopoda are described,
nothing is said of the use or function of these two
special arms. Observation of the living animal
in aquaria has shown that their function is to
capture active prey such as prawns. They act as
a kind of double lasso. Sepia, for instance, ap-
proaches gently and cautiously till it is within
striking distance of a prawn, then the two long ten-
tacles are suddenly and swiftly shot out from their
sockets and the prawn is caught between the suckers
at the ends of them. Another example is afforded
by the masked crab (Corystes cassivelaunus). This

CLASSIFICATION AND ADAPTATION 3

species has unusually long and hairy antennae. These
are usually tactile organs, but it has been found that
the habit of Corystes is to bury itself deep in the
sand with only the tips of the antennae at the
surface, and the two are placed close together so as
to form a tube, down which a current of water,
produced by movements of certain appendages,
passes to the gill chamber and provides for the re-
spiration of the crab while it is buried to a depth of
two or three inches. The results of the investiga-
tion of habits and functions may be called Bionomics.
It may be aided by scientific institutions specially
designed to supplement mere observation in the
field, such as menageries, aquaria, vivaria, marine
laboratories, the objects of which are to bring the
living organism under closer and more accurate
observation. The differences between the methods
and results of these two branches of Biology may
be illustrated by comparing a British Museum
Catalogue with one of Darwin's studies, such as
the ' Fertilisation of Orchids' or 'Earthworms.'

Other speculations in Biology are related to
Taxonomies or Bionomics according as they deal
with the structure of the dead organism or the
action of the living. Anatomy and its more theo-
retical interpretation, morphology, are related to
Taxonomies, physiology and its branches to Bio-
nomics. In fact, the fundamental principles of
physiology must be understood before the study of
Bionomics can begin. We must know the essential
nature of the process of respiration before we can
appreciate the different modes of respiration in a
whale and a fish, an aquatic insect and a crustacean.
The more we know of the physiology of reproduction,

4 CLASSIFICATION AND ADAPTATION

the better we can understand the sexual and parental
habits of different kinds of animals.

The two branches of biological gtudy which we
are contrasting cannot, however, be completely
separated even by those whose studies are most
specialised. In Bionomics it is necessary to dis-
tinguish the types which are observed, and often
even the species, as may be illustrated by the fact
that controversies occasionally arise among amateur
and even professional fishermen on the question
whether dog-fishes are viviparous or oviparous, the
fact being that some species are the one and others
the other, or the fact that the harmless slow-worm
and ring-snake are dreaded and killed in the belief
that they are venomous snakes. Taxonomies, on
the other hand, must take account of the sex of its
specimens, and the changes of structure that an
individual undergoes in the course of its life, and of
the different types that may be normally produced
from the same parents, otherwise absurd errors are
perpetrated. The young, the male, and the female
of the same species have frequently been described
under different names as distinct species or even
genera. For example, the larva of marine crabs
was formerly described as a distinct genus under
the name of Zoaea, and in the earlier part of the
nineteenth century a lively controversy on the
question was carried on between a retired naval
surgeon who hatched Zoaea from the eggs of crabs,
and an eminent authority who was Professor at
Oxford and a Fellow of the Royal Society, and who
maintained that Zoaea was a mature and inde-
pendent form. In the end taxonomy had to be
altered so as to conform with the fact of develop-

CLASSIFICATION AND ADAPTATION 5

ment, and the name Zoaea disappeared altogether
as that of an independent genus, persisting only as a
convenient term for an important larval stage in the
development of crabs.

These two kinds of study give us a knowledge
of the animals now living. But we find it a universal
rule that the individual animal is transitory, that
the duration of life, though varying from a few weeks
to more than a century, is limited, and that new
individuals arise by reproduction, and we have no
evidence that the series of successive generations
has ever been interrupted ; that is to say, the series
in any given individual or species may come to an
end ; species may be exterminated, but we know of
no instance of individuals coming into existence
except by the process of reproduction or generation
from pre-existing individuals. Further, we know
from the evidence of fossil remains that the animals
existing in former periods were very different from
those existing now, and that many of the exist-
ing forms, such as man, mammals, birds, bony
fishes, can only be traced back in the succession
of stratified rocks to the later strata or to those
about the middle of the series, evidence of their
existence in the periods represented by the most
ancient strata being entirely absent. Existing types
then must have arisen by evolution, by changes
occurring in the succession of generations.

These three facts — namely, the limited duration
of individual life, the uninterrupted succession of
generations, and the differences of the existing
animals and plants from those of former geological
periods whose remains are preserved in stratified
rocks — are sufficient by themselves to prove that

6 CLASSIFICATION AND ADAPTATION

evolution has taken place, that the history of
organisms has been a process of descent with modi-
fication. If the animals and plants whose remains
are preserved as fossils, or at any rate forms closely
related to these, were not the ancestors of existing
forms, there are only two other possibilities : either
the existing forms came into existence by new
creations after the older forms became extinct,
or the ancestors of existing forms, although they
coexisted with the older forms, never left any fossil
remains. Each of these suppositions is incredible.

In view of these plain facts and their logical
conclusion it is curious to notice how -Darwin in his
Origin of Species constantly mingles together argu-
ments to prove the proposition that evolution has
occurred, that the structure and relations of existing
animals can only be explained by descent with
modification, with arguments and evidence in
favour of natural selection as the explanation and
cause of evolution. In the great controversy about
evolution which his work aroused, the majority of
the educated public were ultimately convinced of the
truth of evolution by the belief that a sufficient cause
of the process of change had been discovered,
rather than by the logical conclusion that the
organisms of a later period were the descendants
of those of earlier periods. Even at the present
day the theory of natural selection is constantly
confused with the doctrine of evolution. The
fact is that the investigation of the causes of evolu-
tion has been going on and has been making progress
from the time of Darwin, and from times much earlier
than his, down to the present day.

Bionomics show that every type must be adapted

CLASSIFICATION AND ADAPTATION 7

in structure to maintain its life under the conditions
in which it lives, the primary requirements being
food and oxygen. Every animal must be able to
procure food either of various kinds or some special
kind — either plants or other animals; it may be
adapted to feed on plants or to catch insects or fish
or animals similar to itself ; its digestive organs must
be adapted to the kind of food it takes ; it must
have respiratory organs adapted to breathe in air or
water; it must produce eggs able to survive in
particular conditions, and so on.

One of the most interesting results of the study
of the facts of evolution is that each type of animal
tends to multiply to such an extent as to occupy
the whole earth and adapt itself to all possible
conditions. In the Secondary period reptiles so
adapted themselves : there were oceanic reptiles,
flying reptiles, herbivorous reptiles, carnivorous
reptiles. At the present day the Chelonia alone
include oceanic, fresh-water, and terrestrial forms.
Birds again have adapted themselves to oceanic
conditions, to forests, plains, deserts, fresh waters.
Mammals have repeated the process. The organs
of locomotion in such cases show profound modifica-
tions, adapting them to their special functions. One
thing to be explained is the origin of adaptations.

It is, however, necessary to distinguish between
the adapted condition or structure of an organ
and the process by which it became adapted in
evolution ; two ideas which are often confused.
The eye would be equally adapted for seeing whether
it had been created in its actual condition or gradually
evolved. We have to distinguish here, as in other
matters, between being and becoming, and, further,

8 CLASSIFICATION AND ADAPTATION

to distinguish between two kinds of becoming —
namely, the development of the organ in the in-
dividual and its evolution in the course of descent.
The word * adaptation ' is itself the cause of much
fallacious reasoning and confusion of ideas, inas-^
much as it suggests a process rather than a condition,
and by biological writers is often used at one time
to mean the former and at others the latter. We
may take the mammary glands of mammals or
organs adapted for the secretion of milk, whose
only function is obviously the nourishment of the
offspring. Here the function is certain whatever
view we take of the origin of the organs, whether we
believe they were created or evolved. But if we
consider the flipper or paddle of a whale, we see
that it is homologous with the fore-leg of a terrestrial
mammal, and we are in the habit of saying that in
the whale the fore-limb is modified into a paddle and
has become adapted for aquatic locomotion. This,
of course, assumes that it has become so adapted
in the course of descent. But the pectoral fin of a
fish is equally ' adapted ' for aquatic locomotion,
but it is certainly not the fore-leg of a terrestrial
mammal adapted for that purpose. The original
meaning of adaptation in animals and plants, of
organic adaptation to use another term, is the
relation of a mechanism to its action or of a tool
to its work. A hammer is an adaptation for knock-
ing in nails, and the woodpecker uses its head and
beak in a similar way for making a hole in the bark
of trees. The wings and the whole structure of a
bird's body form a mechanism for producing one of
the most difficult of mechanical results, namely,
flight. Then, again, there are stationary conditions,

CLASSIFICATION AND ADAPTATION 9

such as colour and patterns, or scales and armour,
which may be useful in the life of an animal or
flower, but are not mechanisms of moving parts
like a bird's wing, or secreting organs like mammary
glands. Unless we choose or invent some new
term, we must define adaptations apart from all
questions of evolution as any structures or characters
in an organism which can be shown either by their
mere presence, or by their active function, to be
either useful or necessary to the animal's existence.
We must be on our guard against assuming that the
word ' adaptation ' implies any particular theory or
conclusion concerning the method and process
by which adaptations have arisen in the course of
evolution. It is that method and process which we
have to investigate.

On the other hand, when we look primarily at
differences of structure we find that not only are
there wide and distinct gaps between the larger
categories, such as mammals and birds, with few
or no intermediate forms, but the actual individuals
most closely similar to one another naturally and
inevitably fall into distinct groups which we call
kinds or species. The conception of a species is
difficult to define, and authorities are not agreed
about it. Some, like Professor Huxley, state that a
species is purely a mental conception, a generalised
idea of a type to which actual individuals more or
less closely conform. According to Huxley, you
cannot lock the species ' horse ' in a stable. Others
regard the matter more objectively, and regard the
species merely as the total number of individuals
which possess a certain degree of resemblance,
including, as mentioned above, all the forms which

10 CLASSIFICATION AND ADAPTATION

may be produced by the same parents, or which are
merely stages in the life of the individual. There
are cases in which the limits of species or the
boundaries between them are indistinct, where there
is a graduated series of differences through a wide
range of structure, but these cases are the exception ;
usually there are a vast majority of individuals
which belong distinctly to one species or another,
while intermediate forms are rare or absent. The
problem then is, How did these distinct species arise ?
How are we to explain their relations to one another
in groups of species or genera ; why are the genera
grouped into families, families into orders, orders
into classes, and so on ?

There are thus two main problems of evolution:
first, how have animals become adapted to their
conditions of life, how have their organs become
adapted to the functions and actions they have to
perform, or, at least, which they do perform ? The
power of flight, for example, has been evolved by
somewhat different modifications in several different
types of animals not closely related to one another :
in reptiles, in birds, and in mammals. We have no
reason to believe that this faculty was ever uni-
versal, or that it existed in the original ancestors.
How then was it evolved ? The second great prob-
lem is, How is it that existing animals, and, as the
evidence of the remains of extinct animals shows,
these that existed at former periods of time also,
are divided into the groups or types we call species,
naturally classified into larger groups which are
subdivisions of others still larger, and so on, in what
we call the natural system of classification ? The
two problems which naturalists have to solve, and

CLASSIFICATION AND ADAPTATION 11

which for many recent generations they have been
trying to solve, are the Origin of Species and the
Origin of Adaptations.

Former generations of zoologists have assumed
that these problems were the same. Lamarck
maintained that the peculiarities of different animals
were due to the fact that they had become adapted
to modes of life different to those of their ancestors,
and to those in which allied forms lived, the change
of structure being due to the effect of the conditions
of life and of the actions of the organs. He did not
specially consider the differences of closely allied
species, but the peculiarities of marked types such
as the long neck of the giraffe, the antlers of stags,
the trunk of the elephant, and so on; but he con-
sidered that the action of external conditions was
the true cause of evolution, and assumed that in
course of time the effects became hereditary.

Lamarck's views are expounded chiefly in his
Philosophic Zoologique, first published in 1809, and
an excellent edition of this work with biographical
and critical introduction was published by Charles
Martins in 1873. Although his conception of the
mode in which structural changes were produced is
of little importance to those now engaged in the
investigation of the process of evolution, since it
was naturally based on the physiological ideas of his
time, many of which are now obsolete, for the sake
of accuracy it is worth while to cite his principal
propositions in his own words : —

' II sera en effet evident que Petat ou nous voyons
tous les animaux, est d'une part, le produit de la
composition croissante de P organisation, qui tend
a former une gradation reguliere, et de Pautre part

12 CLASSIFICATION AND ADAPTATION

qu'il est celui des influences d'une multitude de
circonstances tres differentes qui tendent continuelle-
ment a detruire la regularite dans la gradation de la
composition croissante de 1' organisation.

6 Ici il devient necessaire de m'expliquer sur le sens
que j' attache a ces expressions : Les circonstances
influent sur la forme et 1' organisation des animaux,
c'est-a-dire qu'en devenant tres differentes elles
changent avec le temps et cette forme et P organisa-
tion elle-meme par des modifications proportionnees.

' Assurement si Ton prenait ces expressions a la
lettre, on m'attribuerait une erreur ; car quelles
que puissent etre les circonstances elles n'operent
directement sur la forme et sur F organisation des
animaux aucune modification quelconque. Mais
de grands changements dans les circonstances a-
menent pour les animaux de grands changements
dans leurs besoins et de pareils changements dans les
besoins en amenent necessairement dans les actions.
Or, si les nouveaux besoins deviennent constants ou
tres durables, les animaux prennent alors de nou-
velles habitudes qui sont aussi durables que les
besoins qui les ont fait naitre. II en sera resulte
Fernploi de telle partie par preference a celui de
telle autre, et dans certains cas le defaut total
d'emploi de telle partie qui est devenue inutile.'

The supposed effect of these changes of habit is
definitely stated in the form of two ' laws ' : —

PREMIERE Loi

' Dans tout animal qui n'a point depasse le terme
de ses developpements I'emploi plus frequent et
soutenu d'un organe quelconque, fortifie peu a peu cet
organe, le developpe, Fagrandit et lui donne une

CLASSIFICATION AND ADAPTATION 13

puissance proportionnee a la duree de cet emploi ;
tandis que le defaut constant d'usage de tel organe
1'affaiblit insensiblement, le deteriore, diminue pro-
gressivement ses facultes, et finit par le faire dis-
paraitre.

DEUXIEME Loi

6 Tout ce que la nature a fait acquerir ou perdre
aux individus par 1' influence des circonstances ou
leur race se trouve depuis longtemps exposee, et par
consequent, par P influence de P emploi predominant
de tel organe, ou par celle d'un defaut constant
d'usage de telle partie, elle le conserve par la genera-
tion aux nouveaux individus qui en proviennent,
pourvu que les changements acquis soient communs
aux deux sexes, ou a ceux qui ont produits ces
nouveaux individus.'

It will be seen that this last condition excludes
the question of the origin of organs or characters
confined to one sex, or secondary sexual characters.
With regard to the expression ' emploi de telle
partie,' the explanation which Lamarck gives of the
evolution of horns and antlers is curious. He does
not attempt to show how the use or employment of
the head leads to the development of these out-
growths of bone and epidermic horn, but attributes
their development in stags and bulls to an ' interior
sentiment in their fits of anger, which directs the
fluids more strongly towards that part of their
head.5

Lamarck's actual words (Phil. ZooL, edit. 1873,
p. 254) are : ' Dans leurs acces de colere qui sont
frequents surtout entre les males, leur sentiment
interieurs par ses efforts dirige plus fortement les

14 CLASSIFICATION AND ADAPTATION

fluides vers cette partie de leur tete, et il s'y fait une
secretion de matiere cornee dans les uns (Bovidae)
et de matiere osseuse melangee de matiere cornee
dans les autres (Cervidae), qui donne lieu a des pro-
tuberances solides : de la Forigine des comes, et des
bois, dont la plupart de ces animaux ont la tete
armee.'

Darwin, on the other hand, definitely set before
himself the problem of the origin of species, which
the majority of naturalists, in spite of Lamarck and
his predecessor Buffon, regarded as permanent and
essentially immutable types established by the
Creator at the beginning of the world. This prin-
ciple of the persistence and fundamentally un-
changeable nature of species was regarded as an
article of religion, following necessarily from the
divine inspiration of the Bible. This theological
aspect of the subject is sufficiently curious when we
consider it in relation to the history of biological
knowledge, for Linnaeus at the beginning of the
eighteenth century was the first naturalist who
made a systematic attempt to define and classify the
species of the whole organic world, and there are few
species of which the limits and definition have not
been altered since his time. In fact, at the present
time there are very numerous groups, both in
animals and plants, on the species of which scarcely
any two experts are agreed.

In many cases a Linnaean species has been split
up till it became, first, a genus, then a family, and, in
some cases, an order. What one naturalist considers
a species is considered by another a genus containing
several species, and, vice versa, the species of one
authority is described as merely a variety by another.

CLASSIFICATION AND ADAPTATION 15

The older naturalists might have said with truth:
we do not know what the species are, but we are
quite certain that whatever they are they have
never undergone any change in their distinguishing
characters. At the same time we know that whether
we call related forms varieties or species or genera
in different cases, we find, whatever organisms
we study, whether plants or animals, definite types
distinguished by special characters of form, colour,
and structure, and that individuals of one species
or type never give rise by generation to individuals
of any other known species or type. We do not
find wolves producing foxes, or bulldogs giving
birth to greyhounds. As a general rule the dis-
tinguishing characters are inherited, and it is by
no means easy even in domesticated animals and
plants to obtain an exact and complete record of
the descent of a new variety from the original form.
Among species in a state of nature it is the exception
to find two recognised species which can be crossed
or hybridised. In the case of the horse and the ass,
although mules are the hybrid offspring of the two,
the mules themselves are sterile, and there are many
similar cases, so that some naturalists have main-
tained that mutual infertility should be recognised
as the test of separation in species.

Darwin founded his theory on the assumption
that differences of species were differences of adapta-
tion. His theory of natural selection is a theory
of the origin of adaptations, and only a theory of
the origin of species on the assumption that their
distinguishing characters are adaptations to different
modes and conditions of life, to different require-
ments. He pointed out that there is always a

16 CLASSIFICATION AND ADAPTATION

considerable range of variation in the specific
characters, that, as a rule, no two individuals are
exactly alike, even when produced by the same two
parents. The central principle of his theory was
the survival of individuals possessing those variations
which were most useful in the competition of species
with species and of individual with individual. He
thus explained adaptation to new conditions and
divergence of several species from a common
ancestor. Characters which were not obviously
adaptive were explained either by correlation or
by the supposition that they had a utility of which
we were ignorant. Darwin also admitted the direct
action of conditions as a subordinate factor.

Weismannism not only retained the principle
of utility and selection, but made it the only
principle, rejecting entirely the action of external
conditions as a cause of congenital modifications,
i.e. of characters whose development is pre-
determined in the fertilised ovum. It is to
Weismann that we owe precise and definite con-
ceptions, if not of the nature of heredity, at least of
the details of the process. From him we learned to
think of the ova or sperms, of the reproductive cells
or ' gametes ' of an individual, as cells which were
from an early stage of development distinguished
from the cells forming the organs and tissues ;
to regard the organism as consisting of soma on the
one hand and gametes on the other, both derived
from the original zygote cell, not the gametes from
the soma. Weismann saw no possibility of changes
induced by any sort of stimulation in the soma
affecting the gametes in such a way as to be re-
developed in the soma of the next generation. He

CLASSIFICATION AND ADAPTATION 17

attributed variation partly to the union of gametes
containing various determinants, which he termed
amphimixis : this, however, would introduce nothing
new. Then he proposed his theory of germinal
selection, determinants growing and multiplying
in competition, some perhaps disappearing alto-
gether, though this does not satisfactorily account
for entirely new characters. With Weismann,
however, every species was a different adaptation,
and natural selection was the deus ex machina ;
to quote his own words, Alles ist angepasst.

Romanes and other writers, on the other hand, had
always maintained that in many cases the constant
peculiarities of closely allied species had no known
utility whatever, so that the problem presented
by these characters was not explained by any
theory of the origin of adaptations.

Mendelism, since 1900, has studied experimentally
the transmission of definite characters, and main-
tains that the characters of species are of the same
nature as the characters which segregate in Mendelian
experiments. Such characters are not in any way
related to external conditions, and cannot, therefore,
be adaptive except by accident. Professor Bateson
goes so far as to admit that such large variations
or mutations offer more definite material to selection
than minute variations too small to make any impor-
tant difference in survival, but as regards species the
important factor is the occurrence of mutations
which are inherited and at once form a distinct
definite difference between allied species which is
not due to selection and has nothing to do with
adaptation.

In a book entitled Problems of Genetics, 1913,

18 CLASSIFICATION AND ADAPTATION

Bateson describes several particular cases which
show how impossible it is to find any relation at all
between the diagnostic characters of certain species
or local forms and their mode of life. One of these
cases is that of the species of Colaptes, a genus of
Woodpeckers in North America, of which a detailed
study was published in the Bull. Am. Mus. Nat.
Hist., 1892. The two forms specially considered
are named C. auratus and C. cafer, and they differ in
the following seven characters : —

C. auratus. C. cafer.

1. Quills yellow. 1. Quills red.

2. Male with black cheek 2. Male with red cheek

stripe. stripe.

3. Adult female with no 3. Adult female with

cheek stripe. usually brown cheek

stripe.

4. A scarlet nuchal cres- 4. No nuchal crescent in

cent in both sexes. either sex.

5. Throat and fore-neck 5. Throat and fore-neck

brown. grey.

6. Top of head and hind- 6. Top of head and hind-

neck grey. neck brown.

7. General tone of plum- 7. General tone of plum-

age olivaceous. age rufescent.

C. auratus occurs all over Canada, and the United
States, from the north to Galveston ; westwards it
extends to Alaska and the Pacific coast to the
northern border of British Columbia. C. cafer in
comparatively pure form occupies Mexico, Arizona,
California, part of Nevada, Utah, Oregon, and is
bounded on the east by a line drawn from the Pacific
south of Washington State, south and eastward

CLASSIFICATION AND ADAPTATION 19

through Colorado to the mouth of the Rio Grande
on the Gulf of Mexico. Between the two areas thus
roughly defined is a tract of country about 300 to
400 miles wide, which contains some normal birds
of each type, but chiefly birds exhibiting irregular
mixtures of the characters of both. Bateson re-
marks that some naturalists may be disposed once
more to appeal to our ignorance, and suggest that if
we only knew more we should find that the yellow
quills, the black ' moustache,' and the red nuchal
crescent specially adapt auratus to the conditions of
the northern and eastern region, while the red quills,
red moustache, and absence of crescent fit cafer to
the conditions of the more southern and western
territory. But, as the author we are quoting
points out, when we think of the wide range of con-
ditions in the country occupied by auratus, extend-
ing from Florida to the Arctic, it is impossible to
believe that there is any common element in the
conditions which demands a scarlet nuchal patch in
auratus, while the equally varied conditions in the
cafer area do not require that character. It may be
added that the same objection is equally valid
whether we apply it to the utility of such a character
or to the supposition that the character has been
caused by external conditions; in other words,
whether we attempt to explain the facts by selec-
tion or by the Lamarckian principle.

Another case quoted by Bateson is that of the
two common British Wasps, Vespa vulgaris and
Vespa germanica. Both usually make subter-
ranean nests, but of somewhat different materials.
That of V. vulgaris is of a characteristic yellow
colour, because made of rotten wood, while that of

20 CLASSIFICATION AND ADAPTATION

V. germanica is grey, from the weathered surface
wood of palings or other exposed timber which is
used in its construction. In characters the differ-
ences of the two forms are so slight as to be dis-
tinguishable only by the expert. V. vulgaris often
has black spots on the tibiae, which are wanting in
germanica. A horizontal yellow stripe on the thorax
is enlarged downwards in the middle in germanica,
not in vulgaris. There are distinct though slight
differences in the genital appendages of the males
in the two species. Here there are differences of
habit, and slight but constant differences of struc-
ture ; but it is impossible to find any relation
between the former and the latter.

Mendelism in itself affords no evidence of the
origin of new characters, since it deals only with the
heredity of the characters which it finds usually in
the varieties of cultivated animals and plants.
But indirectly it draws the inference that new
characters arose in the form in which they are found
to be inherited, as complete units, and not by gradual,
continuous increase, that specific characters are due
to mutations, and that all evolution has been the
result of similar hereditary factors, arising by some
internal process in the divisions of reproductive
cells, and not determined by external conditions.
Some Mendelians maintain that if the mutations
are not compatible with the existing conditions of
life, the organism must either die or find new con-
ditions in which it can live.

Bateson remarks (Mendel's Principles of Heredity,
1909, p. 288) : ' Mendelism provides no fresh clue
to the problem of adaptation except in so far as it is
easier to believe that a definite integral change in

CLASSIFICATION AND ADAPTATION 21

attributes can make a perceptible difference to the
prospect of success, than that an indefinite and im-
palpable change should entail such consequences.'
Here the distinction between adaptive and non-
adaptive characters is recognised, but both are
emphatically attributed to the same origin.

The American evolutionist, T. H. Morgan, also a
specialist in Mendelism, goes further, and maintains,
not merely that mutations which happened to make
a c difference to the prospect of success ' survived,
or were selected, but that if a mutation arising from
a change in the gametes was not compatible with
the conditions of the animal's life at the time, it
either died, or found other conditions, or adopted
new habits which were adapted to the new char-
acter or structure. He takes Flat-fishes as an
example, and suggests that having by mutation
become asymmetrical, and having both eyes on one
side, etc., the fish adopted the habit of lying on the
ground on one side of its body. This is, of course, the
exact opposite of the older conception : the struc-
ture of the animal has not been changed by new
habits or conditions, but new habits and conditions
have been sought and found in order to meet the
requirements of the change of structure.

The present writer, on the other hand, believes
that not only are adaptive characters distinct from
non-adaptive specific characters, and from non-
adaptive diagnostic characters in general, but that
their origin and evolution are entirely distinct and
different. There are two separate problems, the
origin of adaptations and the origin of species, and
the investigation of these two problems leads not to
one explanation common to both, but to two entirely

22 CLASSIFICATION AND ADAPTATION

different explanations, to two different processes
going on throughout the organic world and affecting
every individual and every group in classification.

The Flat-fishes, now regarded not as merely a
family but a sub-order of Teleosteans, afford
a good example of the contrast between adaptive
and non-adaptive diagnostic characters. For the
whole group the adaptive characters are diagnostic,
distinguishing it from other sub-orders. It is
conceivable that different phyletic groups of fishes,
that is fishes of different descent, might have been
modified in the same way, as, for instance, grass-
hoppers and fleas have been adapted for leaping
without being closely related to each other. It is
generally held, however, that the Flat-fishes are
of common descent. In this group the adaptive
characters are diagnostic ; that is to say, they dis-
tinguish the group from other sub-orders, though
there are other non-adaptive characters which indi-
cate the relationship to other groups and which are
not adapted to the horizontal position of the original
median plane of symmetry. The principal adaptive
characters are : both eyes and the pigmentation
on the side which is uppermost in the natural
position, lower side without eyes and colourless ;
dorsal and ventral fins continuous and extending
nearly the whole length of the dorsal and ventral
edges ; dorsal fin extending forwards on the head,
not along the morphological median line, which is
between the eyes, but between the more dorsal eye
and the lower side of the body, in the same horizontal
plane as the posterior part of the same fin. The
4 adaptive ' quality in these characters, as in other
cases, does not necessarily consist in their utility

CLASSIFICATION AND ADAPTATION 23

to the animal, but in the definite relation between
them and the external conditions. When the re-
lation is one of function, the organ may be said to be
useful : for example, the position of the two eyes
is adaptive because they are on the upper side where
alone light can reach them, the other side resting
on the ground ; and the adaptation is one of function,
and therefore useful, because if the eyes were in
their normal position one of them would be useless,
being generally in contact with the ground or buried
in it. Similarly with the extension of the dorsal
and ventral fins, the undulations of which serve to
move the fish gently along in a plane parallel to the
ground. If the dorsal fin was not extended forward,
the head would not be so well supported. But when
we consider the pigmentation of the upper side
and the normally white lower side, although the
adaptation is equally obvious, the utility is by no
means certain. To any naturalist who has observed
these fishes in the living state the protective re-
semblance of the pigmentation of the upper side is
very evident, especially because, as in many other
fishes and amphibians, the intensity of the colour
varies in harmony with the colour of the ground on
which the fish rests. But the utility of the white
lower side is not so easy to prove. Would the fish
be any worse off if the lower side were coloured like
the upper ? Probably it would not, although it
has been maintained that the white lower side serves
to render the fish less visible when seen against the
sky by an enemy below it. Ambicolorate specimens
occur, and there is no evidence that their lives are less
secure than those of normal specimens. The essential
and universal quality of adaptation, then, is not

24 CLASSIFICATION AND ADAPTATION

utility, but relation to surroundings or to function
or to habit. In this case colour is related to in-
cidence of light, absence of colour to absence of light.
Position of eyes is also related to light; they are
situated where they can see, absent from the side
which is shut off from light. The marginal fins are
extended where their movements best support and
move the body.

It is to be noted also that these adaptations of
different organs of the body, eyes, fins, colour, are
entirely independent of each other physiologically.
It may appear on first consideration that eyes and
colour, being both on the upper side, may have
been somehow connected in the constitution of the
body, whereas the only connexion is external in
their common relation to light. This independence
is well shown in the modification of the dorsal
fin: if this were physiologically affected by the
change in the eyes, which is brought about by
the twisting of the interorbital region of the skull,
the anterior end of the fin would be between the
two eyes, since the morphological median line of
the body is in that position. In fact, on the con-
trary, the attachment of the dorsal fin is continued
forward where it is required for its mechanical
function, regardless entirely of the morphology of
the head.

This is even more clearly evident in the structure
of the jaws and teeth. These are entirely un-
affected by the torsion of the interorbital part of
the skull. In cases where the mouth is large and
teeth are required on both sides, the prey being
active fish of other species, as in Turbot, Brill, and
Halibut, the jaws and teeth are equally developed

CLASSIFICATION AND ADAPTATION 25

on the upper and lower sides, and there is almost
complete symmetry in these parts of the skull.
In Soles and Plaice, on the other hand, whose food
consists of worms, molluscs, etc., living on or in the
ground, the jaws of the lower side are well developed
and strong, these of the upper side diminished,
and teeth are confined to the lower side. Here
it is not a question of the jaws being twisted, but
simply unequally developed. There is no general
and constitutional asymmetry of head or body,
but a modification of different organs independently
of each other in relation to external conditions —
light, food, movement.

On the other hand, let us consider some of the
diagnostic characters by which species and genera
are distinguished in the Flat-fishes or Pleuro-
nectidae. The genus Pleuronectes is distinguished
by the following characters : eyes on the right side,
mouth terminal and rather small, teeth most de-
veloped on the blind (left) side. Of this genus there
are five British species, namely :—

P. platessa, the Plaice : scales small, mostly
without spinules, reduced and not imbricated,
imbedded in the skin ; bony knobs on the head
behind the eyes, red spots on the upper side.

P. flesus, the Flounder : no ordinary scales ;
rough tubercles along the bases of the marginal
fins and along the lateral line; these are modified
and enlarged scales; elsewhere scales of any kind
are absent.

In these two species the lateral line is nearly
straight, having only a slight curve above the
pectoral fin.

P. limanda, the Dab : scales uniform all over

26 CLASSIFICATION AND ADAPTATION

the body, with spinules on the projecting edges,
making the skin rough ; lateral line with a semi-
circular curve above the pectoral fin.

P. microcephalus, the Lemon-dab : scales small,
smooth, and imbedded ; skin slimy, head and mouth
very small, colour yellowish brown with large round
darker marks.

P. cynoglossus, the Witch or Pole-dab : head and
mouth smaller than in the Plaice, eyes rather larger ;
scales all alike and uniformly distributed, slightly
spinulate on upper side, smooth on the lower ;
blister-like cavities beneath the skin of the head
on the lower side.

With regard to the generic characters, it is difficult
to give any reason why the mouth should be at the
end of the head instead of behind the apex of the
snout as in the genus Solea, but, as we have seen
already, the small size of the mouth and the greater
development of teeth on the lower side are adapted
to the food and mode of feeding. It is impossible
to say why one genus of Flat-fishes should have the
right side uppermost and others, e.g. Sole and Turbot,
the left ; it would almost seem to have been a matter
of chance at the commencement of the evolution :
reversed specimens occur as variations in most of
the species.

When we consider the specific differences, we find
very definite characters in the structure and dis-
tribution of the scales, and no evidence has yet been
discovered that these differences are related to
external conditions. There are, of course, slight
differences in habits and habitat, but no constant
relation between these and the structural differences
of the scales. Plaice and Dab are taken together

CLASSIFICATION AND ADAPTATION 27

on the same ground, and nothing has been discovered
to indicate that the spinulate scales of the Dab are
adapted to one peculiarity in habits or conditions,
the spineless scales of the Plaice to another. In
comparing certain geographical races of Plaice and
Flounder the facts seem to suggest that differences of
habitat may have something to do with the develop-
ment of the scales. In the Baltic the Flounders are
as large as those on our own coasts, but the thorny
tubercles are much more developed, nearly the
whole of the upper surface being covered with them.
The Plaice, on the other hand, are smaller than those
of the North Sea, and the males have the scales
spinulate over a considerable portion of the upper
side. The chief difference between the Baltic and
the North Sea is the reduced salinity of the former, so
that it might be supposed that fresher water caused
the greater development of the dermal skeleton.
On the other hand, a species or geographical variety
of the Plaice, whose proper name is P. glacialis, is
found on the Arctic coasts of Asia and America, on
both sides of the extreme North Pacific, and on the
east coast of North America. In this form the bony
tubercles on the head in the Plaice are replaced by
a continuous rough osseous ridge, and the scales are
as much spinulated as in the Plaice of the Baltic.
On the east coast of North America the males in this
form are more spinulated than the females : on the
Alaskan coast, and apparently the Arctic coast, the
females are spinulated, and the sexual difference in
this respect is slight or absent. Lower salinity
cannot be the cause of greater spinulation in this
case, and thus it might be suggested that the con-
dition was due to lower temperature. But we do

28 CLASSIFICATION AND ADAPTATION

not find that northern or Arctic species of fish in
general have the scales more developed than southern
species.

The Dab, which occurs in the same waters as the
Plaice, has the spines more spinulated than any of the
forms of plaice above mentioned, therefore the
absence or slight development of spinules in the
typical Plaice is not explained by physical conditions
alone. Freshness of water again will not explain
the difference of the structure and distribution of
scales in Flounder and Plaice, considering the variety
of squamation in fishes confined to fresh water.
Still less can we attribute any of the peculiarities of
scales to utility. We can discover no possible
benefit of the condition in one species which would
be absent in the case of other species. We can go
much further than this, and maintain that there is
no reason to believe that scales in general in Teleo-
steans, or any of their various modifications, are of
special utility : they are not adaptive structures at
all, although of great importance as diagnostic
characters. It may be urged that in some cases,
such as the little Agonus cataphractus or the Sea-
horse among the Syngnathidae, the body is pro-
tected by a complete suit of bony armour; but
accompanying these in the littoral region are numer-
ous other species such as the Gobies, and even other
species of Syngnathidae which have soft unpro-
tected skins.

Similarly with colour characters : the power of
changing the colour so as to harmonise with the
ground is obviously beneficial and adaptive, but in
each species there is a specific pattern or marking
which remains constant throughout life and has

CLASSIFICATION AND ADAPTATION 29

nothing to do with protective resemblance, variable
or permanent. The red spots of the Plaice are
specific and diagnostic, but they confer no advan-
tage over the Dab or the Lemon-dab, in which they
are absent, nor can any relation be discovered
between these spots and mode of life or habits.

The function of the lateral line organs is still some-
what obscure. The theory that they are sensitive
to differences of hydrostatic pressure as the fish
moves from one depth to another rests on no foun-
dation, since it has yet to be shown how a change of
pressure within the limits of the incompressibility
of water can produce a sensation in an organ per-
meated throughout with water. It is more probable
that the organs are affected by vibrations in the
water, but we are unable to understand how a differ-
ence in the anterior curvature of the lateral line
would make a difference in the function in any way
related to the difference in conditions of life between
Plaice and Dab. There is, however, reason to con-
clude that the organs, especially on the head, are
more important and larger in deeper water, and
thus the enlargement of the sensory canals in the
head of the Witch, which lives in deeper water than
other species, may be an adaptive character.

Another genus of whose characters I once made a
special study is that named Zeugopterus. The
name was originally given by Gottsche to the largest
species Z. punctatus, from the fact that the pelvic
fins are united to the ventral, but this character does
not occur in other species now included in the
genus. There are three species, occurring only in
European waters, which form this genus and agree
in the following characters. The outline of the

30 CLASSIFICATION AND ADAPTATION

body is more nearly rectangular than in other Flat-
fishes from the obtuseness of the snout and caudal
end, and the somewhat uniform breadth of the body.
The surface is rough from the presence of long
slender spines on the scales. There is a large
perforation in the septum between the gill cavities,
but this occurs also in Arnoglossus megastoma,
which is placed in another genus. But the generic
character of Zeugoptems, which is most important
for the present discussion, is the prolongation of
the dorsal and ventral fins on to the lower side of
the body at the base of the tail, the attachments
of these accessory portions being transverse to the
axis of the body. These fishes have the peculiar
habit of adhering to the vertical surfaces of the sides
of aquaria, even the smooth surfaces of slate or
glass. In nature they are taken occasionally on
gravelly or sandy ground, but probably live also
among rocks and adhere to them in the same way
as to vertical surfaces in captivity. Many years
ago (Journ. Mar. Biol. Assn., vol. iii. 1893-95) I
made a careful investigation of the means by which
these fishes were able to adhere to a smooth surface,
at least in the case of the largest and commonest
species Z. punctatus. It was observed that so long
as the fish was clinging to a vertical surface the
posterior parts of the marginal fins were in rhythmical
motion, undulations passing along them in succession
from before backwards, the edge of the body to
which they were attached moving with them.
The effect of these movements was to pump out
water backwards from the space between the body
and the surface it was clinging to, and to cause
water to flow into this space at the anterior edges

CLASSIFICATION AND ADAPTATION 31

of the head. The subcaudal flaps were perfectly
motionless and tightly pressed between the base
of the tail and the surface of support, so that any
movement of them was impossible. The question
arose, however, whether the tail and these flaps
acted as a sucker which aided in the adhesion.
The flaps were therefore cut off with scissors — an
operation which caused practically no pain or injury
to the fish — and it adhered afterwards quite as well
as when the fin-flaps were intact. The subcaudal
prolongations of the fins are therefore not necessary
to the adhesion, nor to the pumping action, of
the muscles and fins, which went on as before. It
seemed probable, therefore, that the pumping action
was itself the cause of the adhesion. But the
difficulty in accepting this conclusion was that there
was a distinct though gentle respiratory movement
of the jaws and opercula ; and if the pumping of
the water from beneath the body caused a negative
pressure there, and a positive pressure on the outer
side of the body, it seemed equally certain that the
respiratory movement must force water into the
space beneath the body and so cause a positive
pressure there which would tend to force the fish
away from the surface with which it was in contact.
Examination of the currents of water around the
edges of the fish, by means of suspended carmine,
snowed that water passed in at the mouth and out
at the lower respiratory orifice, but also into the
space below the body at the upper and lower edges
of the head, without passing through the respiratory
channel. It was thus proved that the rate at which
water was pumped out at the sides of the tail was
greater than that at which it passed in by the

32 CLASSIFICATION AND ADAPTATION

respiratory movements, and consequently there was
a resultant negative pressure beneath the body.
By means of a model made of a thin flexible
sheet of rubber, at each end of which on one side
was fastened a short piece of glass tube, I was
able to imitate the physical action observed in the
fish. A long piece of rubber tube was attached to
one of the pieces of glass tube, and brought over the
edge of the glass front of an aquarium. The long
rubber tube was set in action as a siphon and the
sheet of rubber placed against the glass. As long
as water was running through the siphon the sheet
of rubber remained pressed against- the glass and
supported. As soon as the current of water was
stopped the apparatus fell to the bottom of the
tank. In this model water passed out from beneath
the rubber tlirough the glass tube attached to the
siphon and passed in by the opposite glass tube,
and at the sides of it. The latter tube represented
the respiratory channel of the fish, and the space
between tube and rubber represented the spaces
between the head of the fish and the vertical surface
to which it clung.

In the fish the marginal fins not only extend to the
base of the tail, but are broader at the posterior
end than elsewhere, whereas in other Flat-fishes
the posterior part of the marginal fins are the
narrowest parts. The shape of the fins and the
breadth of the body posteriorly, then, are adaptations
which have a definite function, that of enabling
the fish to adhere to vertical surfaces. But, on the
other hand, the extension of the marginal fins in a
transverse direction beneath the tail has no use in
the process of adhesion, nor has any other use been

CLASSIFICATION AND ADAPTATION 33

found for it. It is a generic character, so far as we
know, without utility. On the other hand, it is very
probable that this subcaudal extension of the fins
is merely a result of the posterior extension and
enlargement of these fins which has taken place in
the evolution of the adaptation. If the Lamarckian
explanation of adaptation were true, it would be
possible to understand that the constant movements
of the fins and muscles by which the adhesion was
effected caused a longitudinal growth of the fins
in excess of the length actually required, and that
this extra growth extended on to the body beneath
the tail, although the small flaps on the lower side
were not necessary to the new function which the
fins performed.

When we consider such cases as this we are led
to the conclusion that the usual conception of adap-
tation is not adequate. We require something more
than function or utility to express the difference
between the two kinds of characters to be distin-
guished. For example, the absence of pigmen-
tation from the lower sides of Flat-fishes may have
no utility whatever, but we see that it differs from
the specific markings of the upper side in the fact
that it shows a relation to or correspondence with
a difference of external conditions — namely, the
incidence of light, while in such a case as the red
spots of the Plaice we can discover no such corres-
pondence.

We know that the American artist and naturalist
Thayer has shown that the lighter colour of the
ventral side of birds and other animals aids greatly
in reducing their visibility in their natural surround-
ings, the diminution in coloration compensating for

c

34 CLASSIFICATION AND ADAPTATION

the diminution in the amount of light falling on the
lower side, so that the upper and lower sides reflect
approximately the same amount of light, and con-
trast, which would be otherwise conspicuous, is
avoided. But the white lower sides of Flat-fishes
are either not visible at all, or, if visible, are very
conspicuous, so that the utility of the character is
very doubtful.

We may distinguish then between characters
which correspond to external conditions, functions,
or habits, and those which do not. The word
' adaptation,' which we have hitherto used, does not
express satisfactorily the peculiarities of all the
characters in the former of these two divisions. If
we consider three examples — enlarged hind-legs for
jumping as in kangaroo or frog, absence of colour
from the lower sides of Flat-fishes, and, thirdly, the
finlets on the lower side of Zeugopterus — we see that
they represent three different kinds of characters,
all related to habits or external conditions. We may
say that the third kind are correlated with some
other character that has a relation to function or
external conditions, as the extension of the fins on
the under side of Zeugopterus is correlated with the
enlargement of the fins, whose function is to cause
the adhesion of the fish to a vertical surface.

With regard to the specific characters of the species
of Zeugopterus nothing is known of peculiarities in
mode of life which would give an importance in the
struggle for existence to the concrescence of the
pelvic fins with the ventral in punctatus, to the
absence of this character and the elongation of the
first dorsal ray in unimaculatus, or to the absence
of both characters in norvegicus. No use is known

CLASSIFICATION AND ADAPTATION 35

for any of the other specific characters, which tend
in each case to form a series. Thus in size norvegicus
is the smallest, unimaculatus larger, and punctatus
largest, the last reaching a length of 8J inches. The
subcaudal fin-flaps are least developed in norvegicus,
most in punctatus ; each has four rays in norvegicus
and unimaculatus., six in punctatus. The shorten-
ing and spinulation of the scales are greatest in
punctatus, least in norvegicus. In punctatus there
are teeth on the vomer, in unimaculatus none, in
norvegicus they are very small.

If we consider fishes in general, we see that there
is no evidence of any relation between many of the
most important taxonomic characters and function
or external conditions. In the sea Elasmobranchs
and Teleosteans exist in swarming numbers side by
side, but it is impossible to say that one type is more
adapted to marine life than the other. There is
good reason to believe that bony fishes were evolved
from Elasmobranchs in fresh water which was
shallow and foul, so that lungs were evolved for
breathing air, and that marine bony fishes are
descended from fishes with lungs ; but no reason
has been given for the evolution of bone in place of
cartilage or for the various kinds of scales. Professor
Houssaye, on the other hand, believes that the
number and position of fins is adapted to the shape
and velocity of movement of each kind of fish.

If we turn to other groups of animals we find
everywhere similar evidence of the distinction
between adaptive and non-adaptive characters.
Birds are adapted in their whole organisation
for flight, the structure of the wing, of the sternum,
breast muscles, legs, etc., are all co-ordinated for

36 CLASSIFICATION AND ADAPTATION

this end. But how do we know that feathers in
their origin were connected with flight ? It seems
equally probable that feathers arose as a mutation
in place of scales in a reptile, and the feathers were
then adapted for flight. Nothing shows the dis-
tinction better than convergent adaptation. Owls
resemble birds of prey in bill and claw and mode
of life, yet they are related to insect-eating swifts
and goat-suckers and not to eagles and hawks.
Swifts and swallows are similar in adaptive characters,
but not in those which show relationship. It may
be said that the characters believed to show true
affinities were originally adaptive, but we do not
know this. Similarly, in reptiles the Chelonia are
distinguished by the most extraordinary union of
skin-bones and internal skeleton enclosing the body in
rigid armour : it may be said that the function of this
is protection, that it is adaptation, and can be ex-
plained by natural selection, but the adaptation in
this case is so indefinite that it is difficult to be
convinced of it.

Systematists have always distinguished between
adaptive characters and those of taxonomic value—
those which show the true affinities — and they are
perfectly right : also they have always distrusted
and held aloof from theories of evolution which
profess to explain all characters by one universal
formula. In my opinion, those who, like Weismann,
consider all taxonomic characters adaptive, are
equally mistaken with Bateson and his followers,
who regard all characters as mutational. No system
of evolution can be satisfactory unless it recognises
that these two kinds of characters are distinct
and quite different in their nature. But it may be

CLASSIFICATION AND ADAPTATION 37

asked, What objection is there to the theory of
natural selection as an explanation of adaptations ?
The objection is that all the evidence goes to show
that the necessary variations only arose under
the given conditions, and, further, that the actions
of the conditions and the corresponding actions of
the organism give rise to stimuli which would
produce somatic modifications in the same direction
as the permanent modifications which have occurred.
My view is, then, that specific characters are usually
not adaptations, that other characters of taxonomic
value are some adaptive and some unrelated to
conditions of life, and that while non-adaptive
characters are due to spontaneous blastogenic varia-
tions or mutations, adaptive characters are due to
the direct influence of stimuli, causing somatic
modifications which become hereditary, in other
words, to the inheritance of acquired characters.
It has become a familiar statement that every
individual is the result of its heredity and its en-
vironment. The thesis that I desire to establish is
that the heredity of each individual and each type
is compounded of variations or changes of two
distinct origins, one external and one internal;
that is to say, of variations resulting from changes
originating in the germ-cells or gametes, and of
modifications produced originally in the soma by
the action of external stimuli, and subsequently
affecting the gametes.

When we study the characters of animals in
relation to sex we find that in many cases there are
conspicuous organs or characters present in one
sex, usually the male, which are absent or rudi-
mentary in the other. The conception of adapta-

38 CLASSIFICATION AND ADAPTATION

tion applies to these also, since we find that these
characters consist often of weapons such as horns,
antlers, and spurs, used in sexual combat, of
copulatory or clasping organs such as the pads on
a frog's forefeet, of ornamental plumage like the
peacock's tail serving to charm the female, or of
special pouches as in species of pipe-fish and frog
for holding the eggs or young. Darwin attempted
to explain sexual adaptation by sexual selection.
The selective process in this case was supposed to
be, not the survival of individuals best adapted to
secure food or shelter or to escape from enemies, but
the success of those males which were victorious in
combat, or which were most attractive to the
females, and therefore left the greater number of
offspring which inherited their variations. But, as
Darwin himself admitted, this theory of selection
does not in any way explain the differences between
the sexes — in other words, the limitation of the
characters or organs to one sex — since there is no
reason in the process of selection itself why the
peculiarity of a successful male should not be in-
herited by his female offspring as well as by his
male offspring. The real problem, then, is the sex-
limited heredity, and we shall consider later whether
in this kind of heredity also there are characters
of internal as well as external origin, blastogenic
as well as somatogenic.

CHAPTER II

MENDELISM AND THE HEREDITY OF SEX

WE know that new individuals are developed from
single cells which have either been formed by the
union of two cells or which develop without such
union, and that these reproductive cells are separated
from pre-existing organisms : the gametes or
gonocytes are separated from the parents and
develop into the offspring. The zygote has the
power of developing particular structures and
characters in the complicated organisation of the
adult, and we recognise that these characters are de-
termined by the properties and constitution of the
zygote ; that is to say, of one or both of the gametes
which unite to form the zygote. The distinction
between peculiarities or ' characters,' determined
in the ovum before development, and modifications
due to influences acting on the individual during
its development or life, is often obvious enough.
A child's health, size, mode of speech, and behaviour
may be greatly influenced by feeding, training, and
education, but the colour of his or her eyes and hair
were determined before birth. A human individual
has, we know, a number of congenital or innate
characters, by which we mean characters which
arise from the constitution of the individual at the
time of birth, and not from influences acting on him
or her after birth. We have to remember, however,

40 MENDELISM AND

that modifications may be caused during develop-
ment in the uterus, as, for example, birth-marks on
the skin, and these would not be due to peculiarities
in the constitution of the ovum. Professor Karl
Pearson and other devotees of the cult of Eugenics
have been lately impressing on the public by
pamphlets, lectures, and addresses the great im-
portance of nature as compared with nurture,
maintaining that the latter is powerless to counteract
either the good or bad qualities of the former, and
that the effects of nurture are not transmitted to the
next generation.

We recognise that the characters of varieties of
flowers, fruits, and domesticated animals are not
to be produced by any particular mode of treatment.
We see the various kinds of orchids or carnations
in the same greenhouse, of sweet peas and roses in
the same garden. We go to a show and see the
extraordinary variety of breeds of pigeons, rabbits,
or fowls, and we know that these cannot be produced
by treating the progeny of individuals of one kind
in special ways, but are the progeny of parents of
the same various races. If we want fowls of a
particular breed we obtain eggs of that breed and
hatch them with the certainty born of experience
that we shall obtain chickens of that breed which
will develop the colour, comb, size, and qualities
proper to it. Similarly, in nature we recognise that
the ' characters ' of species or varieties are not
due to circumstances acting on the individual during
its development, but to the properties of the ova
or seeds from which the individuals were developed.

Formerly we regarded these congenital or innate
characters as derived from the parents or inherited,

THE HEREDITY OF SEX 41

and heredity was the transmission of constitutional
characters from parent to offspring. Now that
we fix our attention on the fertilised ovum or the
gametes by which it is formed we see that the
characters are determined by some properties in
the constitution of the gametes. What, then, is
heredity ? Clearly, it is merely the development
in the offspring of the same characters which were
present in the ova from which the parents developed.
When the characters persist unchanged from genera-
tion to generation, we call the process by which they
are continued heredity. When new characters
appear, i.e. new characters determined in the ovum
not due to changes in the environment, we call them
variations. When a fertilised ovum develops into
a new individual, it divides repeatedly to form a
very large number of cells united into a single mass.
Gradually the parts of this mass are differentiated
to form the tissues and organs of the body or soma,
but some of the cells remain in their original con-
dition and become the reproductive cells which will
give rise to the next generation. The reproductive
cells also undergo division and increase in number,
and when they separate from the new individual
and unite in fertilisation they still possess all the
determinants of the fertilised ovum from which
they are descended. Heredity thus continues from
gamete to gamete, not from zygote to soma, and
then from soma to gamete.

Modern researches have shown that the nucleus,
when the cell divides, assumes the form of a spindle
of fibres, associated with which are distinct bodies
called chromosomes, that the number of these
chromosomes where it can be counted is constant

42 MENDELISM AND

for all individuals of the same species, and that
before the gametes are ready for fertilisation two
cell-divisions take place, which result in the re-
duction of the number of chromosomes to half
the original number. When two gametes unite,
the specific number is restored. Since the male
gamete is very small and seems to contribute to the
zygote almost nothing except the chromosomes,
which carry with them all the characters of the
male parent, it seems a necessary conclusion that
the chromosomes alone determine the character
of the adult. There are, however, facts which
point to an opposite conclusion.

Hegner,1 for example, found that in the egg of the
beetle Leptinotarsa, which is an elongated oval in
shape, there is at the posterior end in the superficial
cytoplasm a disc-shaped mass of darkly staining
granules, while the fertilised nucleus is in the middle
of the egg. When the protoplasm containing these
granules was killed with a hot needle, development
in some cases took place and an embryo was formed,
but the embryo contained no germ cells. Here no
injury had been done to the zygote nucleus, but
these particular granules and the portion of pro-
toplasm containing them were necessary for the
formation of germ cells. In other experiments
a large amount of protoplasm at the posterior end
of the ovum was killed before the nucleus had begun
to segment, and the result was the development
of an embryo consisting of the head and part of the
thorax, while the rest was wanting. The nucleus
segmented and migrated into that part of the

1 R. W. Hegner, 'Experiments with Chrysomelid Beetles/ ni.,
Biological Bulletin, vol. xx. 1910-11.

THE HEREDITY OF 8EX 43

superficial cytoplasm which remained alive, and
this proceeded to develop that particular part of
the embryo to which it would have given rise if
the rest of the egg had not been killed. There was
no regeneration of the part killed, no formation
of a complete embryo. It may be pointed out
that segmentation in the insect egg is peculiar.
The nuclei multiplied by segmentation migrate
into the superficial cytoplasm surrounding the yolk,
and then this cytoplasm segments, and each part
of the cytoplasm develops into a particular region
of the embryo. This, of course, does not prove
that the nuclei or their chromosomes do not de-
termine the characters of the parts of the embryo
developed, but they show that the parts of the
non-nucleated cytoplasm correspond to particular
parts of the embryo. The most important object
of investigation at the present time is to find the
origin of these properties of the chromosomes. We
may say, using the word < determinant ' as a con-
venient term for that which determines the adult
characters, that in order to explain the origin of
species or the origin of adaptations we must discover
the origin of determinants. Mendelism does not
throw any direct light on this question, but it
certainly has shown how characters may be inherited
as separate and independent units. When one
difference between two breeds is considered, e.g.
rose comb and single in fowls, and individuals are
crossed, we have the determinant for rose and
the determinant for single in the same zygote.
The result is that rose develops and single is not
apparent. In the next generation rose and single
appear, as at the beginning, in separate individuals.

44 MENDELISM AND

When two or three or more differences are studied
we find that they are usually inherited separately
without connexion with each other, although in
some cases they are connected or coupled. The
facts of Mendelism are of great interest and im-
portance, but we have to consider the general theory
based on them. This theory is that characters
are generally separate units which can exist side by
side, but do not mingle, and cannot be divided into
parts. When an apparently single character shows
itself double or treble, it is concluded that it has
not been really divided, but consists of two or three
units (Castle). Further, although Mendelism in
itself shows no evidence of the origin of the characters,
it assumes that they arose as complete units, and
one suggestion is that a dominant factor might
at some of the divisions in gametegenesis pass
entirely into one daughter cell, and therefore be
absent from the other, and thus individuals might
be developed in which a dominant character was
absent. Bateson in his well-known books, Mendel's
Principles of Heredity, 1909, and Problems of Genetics,
1913, discusses this question of the origin of the
factors which are inherited independently. The
difficulty that troubles him is the origin of a dominant
character. Naturally, if he persists in regarding
the determinant factor as a unit which does not
grow nor itself evolve in any way, it is difficult
to conceive where it came from. The dominant,
according to Bateson, must be due to the presence
of something which is absent in the recessive. He
gives as an' instance the black pigment in the Silky
fowl, which is present in the skin and connective
tissues. In his own experiments he found this was

THE HEREDITY OF SEX 45

recessive to the white-skin character of the Brown
Leghorn, and he assumes that the genetic properties
of Oallus bankiva with regard to skin pigment are
similar to those of the Brown Leghorn. Therefore
in order that this character could have arisen in
the Silky, the pigment-producing factor P must be
added and the inhibiting factor D must drop out
or be lost. He says we have no conception of the
process by which these events took place.1 Now
my experiment in crossing Silky with bankiva
shows that no inhibiting factor is present in the
latter, so that only one change, not two, was necessary
to produce the Silky. Mendelians find it so difficult
to conceive of the origin of a new dominant that
they even suggest that no such thing ever occurs :
what appears as a new character was present from
the beginning, but its development was prevented
by an inhibiting factor : when this goes into one
cell of a division and leaves the other free, the sup-
pressed character appears. This is the principle
proposed to get over the difficulty of the origin of a
new dominant. All characters are due to factors,
and all factors were present in the original ancestor
—say Amoeba. Evolution has been merely ' the
rejection of various factors from an original complex,
and a reshuffling of those that were left.' Professor
Lotsy goes so far as to say that difference in species
arose solely from crossing, that all domestic animals
are of mixed stocks, and that it is easier to believe
that a given race was derived from some ancestor
of which all trace has been lost than that all races
of fowls, for example, arose by variation from a single
species. But the evidence that our varieties of

1 Problems of Genetics, p. 85.

46 MENDELISM AND

pigeons have been derived from C. livia, and of fowls
from G. bankiva, is too strong to be disregarded
because it does not agree with theoretical con-
ceptions.

My own experiments in crossing Silky fowls with
Gallus bankiva (P.Z.S., 1919) show that the re-
cessive is not always pure, that segregation is not
in all cases complete. The colour of the bankiva
is what is called black-red, these being probably
the actual pigments present, mixed in some parts
of the plumage, in separate areas in other parts :
the Silky is white. There are seven pah's of char-
acters altogether in which the S,ilky differs from
the bankiva. Both the pigmented skin of the
Silky and the colour in the plumage of the bankiva
are dominant, so that all the offspring in F1 or the
first generation are coloured fowls with pigmented
skins. But in later generations I found that with
regard to skin pigment there were no pure recessives.
Since the heterozygote in F^ was deeply pigmented,
it is certain that a bird with only a small amount
of pigment in its skin was a recessive resulting from
incomplete segregation of the pigmented character.
The pigment occurred chiefly in the skin of the
abdomen and round the eyes, and also in the
peritoneum and in the connective tissue of the
abdominal wall. It varied in different individuals,
but in some, at any rate, was greater in later genera-
tions than in the earlier. The condition bred true,
as pure recessives do ; and when such an impure
recessive was mated with a heterozygote with
black skin, the offspring were half pigmented
and half recessive, with some pigment on the
abdomen of the latter.

PLATE I.

CaJe it 7)rt?u C/.-.-SOH. Imp.

EXPLANATION OF PLATE I.

Fig. 1. Ventral surface of hen produced from cross between silky hen and
black-red Bankiva cock, showing " pile " coloration. The dorsal
surface is white without any of the reddish brown colour. The
hen represented was Fa II 3 ? in the record of the pedigree, i.e., a
hen of the second brood of the 5th generation. It was killed
Dec. 12, 1915, when 1 year and 8 months old.

Fig. 2. Dorsal surface of cock from the same cross, showing the "pile"
coloration in the male. The colour is very slight compared with
that of the female, and consists of a slight yellow tinge across the
loins and on the upper wing-coverts. The rest of the body is pure
white. The specimen was FG V 1 c? in the pedigree, i.e., a cock of
the fifth brood of the 6th generation. It was killed on Dec. 31
1915, when 7 months old.

THE HEREDITY OF SEX 47

Still more striking was the incomplete segregation
in the plumage colour. The white of the Silky
was recessive, all the birds of the Fl generation
being fully coloured. In the jP2 generation there
were two recessive white cocks which when mature
showed slight yellow colour across the loins. These
two were mated with coloured hens, and in later
generations all the recessives instead of being pure
white, like the Silky, had reddish-brown pigment
distributed as in pile fowls. In the hens (Plate i.,
fig. 1) it was chiefly confined to the breast and
abdomen, and was well developed, not a mere tinge
or trace, but a deep coloration, extending on to the
dorsal coverts at the lower edge of the folded wings.
The back and tail were white. In the cocks the
colour was much paler, and extended over the
dorsal surface of the wings, where it was darker
than on the back and loins (Plate I., fig. 2). These
pile-coloured fowls when mated together bred true,
with individual differences in the offspring.

The pile fowl as recognised and described by
fanciers is dominant in colour, not recessive as in
the case above described. In fact, a recessive
pile does not appear ever to have been mentioned
before the publication of the results of my experi-
ment. From the statements of John Douglas in
Wrights Book of Poultry (London, 1885), it appears
that fanciers knew long ago that the pile could be
produced from a female of the black-red Game
mated with a white Game-cock. It would seem,
therefore, that the pile is the heterozygote of black-
red and ' dominant ' white. Bateson, however
(Principles of Heredity, 1909, p. 120), writes that
the whole problem of the pile is very obscure, and

48 MENDELISM AND

treats it as a case of peculiarity in the genetics of
yellow pigments. On p. 102 of the same volume
he describes the results of crossing White Leghorn
with Indian Game or Brown Leghorn, the FI being
substantially white birds with specks of black and
brown, though cocks have sometimes enough red in
the wings to bring them into the category known
as pile. To test the matter I have crossed White
Leghorns with a pure-bred black-red Game-cock, and
in the offspring out of eight cocks six were fairly
good piles, but with not quite so much red on the
back as in typical birds : one was a pile with yellow
on the back instead of red, and one. was white with
irregular specks. Of the hens, four were of pile
coloration with breast and abdomen of uniform
reddish-brown colour, back, neck, and saddle
hackles laced with pale brown, tail white. The
other four were white with black and brown specks.
Whether these pile heterozygotes will breed true I
do not yet know.

These results tend to show that factors are not
indivisible units, and segregation is rather the
difficulty of chromatin or germ plasm from different
races uniting together. It must be remembered
that the fertilised ovum which forms one individual
gives rise also to dozens or hundreds or thousands or
millions of gametes. If a given character is re-
presented by a portion of the chromatin in the
original ovum, this has to be divided so many times,
and each time to grow to the same condition as
before. How can we suppose that the divisions
shall be exactly equal or the growth always the
same ? It is inevitable that irregularities will occur,
and if the original chromatin produced a certain

THE HEREDITY OF SEX 49

character, who shall say what more or less of that
chromatin will produce ?

In the case of my recessive pile, my interpretation
is that when the chromosomes corresponding to
two distinct characters such as colour and absence
of colour are formed they do not separate from
each other completely. Whether the mixture of
the chromosomes occurs in every resting stage of
the nucleus in the successive generations of the
gametocytes, or whether it occurs only in the
synapsis stage preceding reduction division, it is
not surprising that the colloid substance of the
chromosomes should form a more or less complete
intermixture, and that the two original chromosomes
should not be again separated in the pure condition
in which they came into contact. A part, greater
or less, of each may be left mixed with the other.
This is the probable explanation of the fact that
the recessive white plumage has some of the pigment
from the dominant form. Segregation, the repulsion
between chromosomes, or chromatin, from gametes
of different races may occur in different degrees
from complete segregation to complete mixture.
When the latter occurs there would be no segregation
and the heterozygote would breed true. The most
interesting fact is that a given factor in the cases I
have described, namely, colour of plumage and
pigmentation of skin in the Jungle fowl and the
Silky, is not a permanent and indivisible unit,
but is capable of subdivision in any proportion.
Bateson has already (in his Address to the Australian
Meeting of the British Association) expressed the
same conclusion. He states that although some
Mendelians have spoken of genetic factors as per-

D

50 MENDELISM AND

manent and indestructible, lie is satisfied that they
may occasionally undergo a quantitative disintegra-
tion, the results of which he calls subtraction or
reduction stages. For example, the Picotee Sweet
Pea with its purple edges can be nothing but a
condition produced by the factor which ordinarily
makes the fully purple flower, quantitatively
diminished. He remarks also that these fractional
degradations are, it may be inferred, the con-
sequences of irregularities in segregation.

Bateson, however, proceeds to urge that the
history of the Sweet Pea belies those ideas of a
continuous evolution with which we had formerly
to contend. The big varieties came first, the little
ones arose later by fractionation, although now the
devotees of continuity could arrange them in a
graduated series from white to deep purple. Now
this may be historically true of the Sweet Pea,
but I would point out that once the dogma of the
permanent indivisible unit or factor is abandoned,
there is nothing in Mendelism inconsistent with
the possibility of the gradual increase or decrease
of a character in evolution. I do not suggest
that the colour and markings of a species or variety
were, in all cases, due to external conditions, but if
the effect of external stimuli can be inherited, can
affect the chromosomes, then the evidence concerning
unit factors no longer contradicts the possibility
of a character gradually increasing, under the
influence of external stimuli acting on the soma,
from zero to any degree whatever.

THE HEREDITY OF SEX 51

SEX AND SECONDARY SEXUAL CHARACTERS

The mystery of sex is hidden ultimately in the
phenomenon of conjugation, that union of two
cells which in general seems necessary to the main-
tenance of life, to be a process of rejuvenation.
We know nothing of the nature of this process, or
why in general it should produce a reinvigoration of
the cell resulting from it. We know little if anything
of the relation between the two conjugating cells
or gametes, of the real nature of the attraction
that causes them to approach each other and
ultimately unite together. We have, it is true,
some evidence that one cell affects the other by
some chemical action, as for instance in the fact
that the mobile male gametes of a fern are attracted
to a tube containing malic acid, but this may be
merely an influence on the direction of movement
of the male gamete, while there are cases in which
neither cell is actively mobile. What we know in
higher animals and plants is that each gamete
contains in its nucleus half the number of chromo-
somes found in the other cells of the parent, and
that in the fertilised ovum the chromosomes of both
gametes form the new nucleus, in which therefore
the original number of chromosomes is restored.

The remarkable fact is that from this fertilised
ovum or zygote is developed usually an individual
of one sex or the other, male or female, other cases
being comparatively exceptional, although each
act of fertilisation is the union of the two sexes
together. Various attempts have been made to
prove that the sex of the organism is determined by
conditions affecting it during development sub-

52 MENDELISM AND

sequent to fertilisation, but now there is good
reason to believe that generally the sex of the in-
dividual is determined at fertilisation, though as
we shall see there is evidence that it may in certain
cases be changed at a later stage.

In Mendelian experiments, a heterozygote in-
dividual is one arising from gametes containing
opposite members of a pair of characters, in other
words, from the union of a gamete carrying a
dominant with another carrying a recessive. A
pure recessive individual is one arising from the
union of two gametes both carrying recessives. If
a heterozygote is bred with a pure recessive the
offspring are half heterozygote and half recessive.
The heterozygote individual in typical cases shows
the dominant character. In the formation of its
gametes when the reduction division of the chromo-
somes takes place, half of them receive the dominant
character, half the recessive. When the division
in the gametes of the recessive individual takes
place its gametes all contain the recessive character.
Thus, if we indicate the dominant character by D
and the recessive by d, the constitution of the two
individuals is

Dd and dd.
The gametes they produce are

D+d and d+d,

and the fertilisations are therefore
Dd, Dd, dd, dd,

or heterozygote dominants and pure recessives in
equal numbers.

It is evident that the reproduction of the sexes
is very similar to this. One of the remarkable facts
about sex is that, although the uniting gametes

THE HEREDITY OF SEX 53

are male and female yet they give rise to males and
females in equal numbers. If one sex were a dominant
this would be in accordance with Mendelian theory.
In accordance with the view that the dominant is
something present which is absent in the recessive,
the Mendelian theory of sex assumes that femaleness
is dominant, and that maleness is the absence of
femaleness, the absence of something which makes
the individual female. If we represent the character
of femaleness by F and maleness or the recessive by
/, we have the ordinary sexual union represented by

ffxff:

the gametes will then be

F+faudf+f,
and the fertilisations

F/and/jf,

or males and females in equal numbers, as they are,
at least approximately, in fact.

The close agreement of this theory with what
actually happens is certainly important and suggests
that it contains some truth. But it cannot be said
to be a satisfactory explanation. It ignores the
question of the nature of sex. According to the
theory the female character is entirely wanting in
the male. But what is sex but the difference
between ovum and spermatozoon, between mega-
gamete and microgamete ? The theory then asserts
that an individual developed from a cell formed by
the union of male and female gametes is entirely
incapable of producing female gametes again. Every
zygote after conjugation or fertilisation may be said
to be bisexual or hermaphrodite. How comes
it then that the female quality entirely disappears ?
Whether the gametocytes are distinguishable at

54 MENDELISM AND

an early stage in the segmentation of the ovum,
or only at a later stage of development, we know
that the gametes ultimately formed have descended
by a series of cell-divisions from the fertilised ovum
or zygote cell from which development commenced.
If segregation takes place at the reduction divisions
we might suppose that half the gametes formed
are sperms and half are ova, and that in the male
the latter do not survive but perish and disappear.
But in this case it would be the whole of the chromo-
somes coming from the original female gamete
which would disappear, and the spermatozoon
would be incapable of transmitting* characters de-
rived from the female parent of the individual in
which the spermatozoa were formed. An individual
could never inherit character from its paternal
grandmother. This, of course, is contrary to the
results of ordinary Mendelian experiments, for
characters are inherited equally from individuals
of either sex, except secondary sexual characters
and sex-linked characters which we shall consider
later.

Similarly, if we suppose that segregation of ovum
and sperm occurs in the female, the sperms must
disappear and the ovum would contain no factors
derived from the male parent. But the theory
supposes that the segregation of male and female
does occur in the female, that half the ova are female
and half are male. What meaning are we to attach
to the words ' male ovum ' or even ' male producing
ovum ' ? It is a fundamental principle of Mendelism
that the soma does not influence the gametocytes
or gametes; we have therefore only to consider
the sex of the gametes themselves, derived from a

THE HEREDITY OF SEX 55

zygote which is formed by the union of two sexes.
The quality of maleness consists only in the size,
form, and mobility of the sperm in the higher
animals and of the microgamete in other cases.
In what sense then can an ovum be male ? It
may perhaps be said that though it is itself female,
it has some property or factor which when united
with a sperm causes the zygote to be capable of
producing only sperms, and conversely the female
ovum has a quality which causes the zygote to
produce only ova. But since these qualities segre-
gate in the reduction divisions, how is it that the
male quality in the / ovum does not make it a
sperm ? We are asked to conceive a quality, or
the absence of a factor, in an ovum which is incapable
of causing that ovum to be a sperm, but which, when
segregated in the gametes descended from that ovum,
causes them all to be sperms. It is impossible to
conceive a single quality or factor which at differ-
ent times produces directly opposite effects. The
Mendelian theory is merely a theory in words,
which have an apparent relation to the facts, but
which when examined do not correspond to any
real conceptions.

However, we have to consider a number of re-
markable facts concerning the relation of chromo-
somes to sex. In the ants, bees, and wasps the
unfertilised ovum always develops into a male,
the fertilised into a female. The chromosomes
of the ovum undergo reduction in the usual way,
and are only half the number of those present in
the nucleus before reduction. We may call this
reduced number N and the full number 2N. The
ova developing by parthenogenesis and giving

56 MENDELISM AND

rise to males segment in the usual way, and all the
cells both of soma and gametocytes contain only
N chromosomes. In the maturation divisions re-
duction does not occur, N chromosomes passing to
one gamete, none to the other, and the latter perishes
so that the sperms all contain N chromosomes.
When fertilisation occurs the zygote therefore
contains 2N chromosomes and becomes female.
Here then we have no segregation of Fxf in the
ova. The difference of sex merely corresponds
to duplex and simplex conditions of nucleus, but it
is curious that the simplex condition in the gametes
occurs in both ova and sperms.

In Daphnia and Rotifers the facts are different.
Parthenogenesis occurs when food supply is plenti-
ful and temperature high. In this case reduction
of the chromosomes does not occur at all, the eggs
develop with 2N chromosomes and all develop into
females. Under unfavourable conditions reduction
or meiosis occurs, and two kinds of eggs larger and
smaller are formed, both with N chromosomes.
The larger only develops when fertilised and give
rise to females with 2N chromosomes. The smaller
eggs develop without fertilisation, by partheno-
genesis, and become males. Here then we have
three kinds of gametes, large eggs, small eggs, and
sperms, each with the same number of chromosomes.
It is not the mere number then which makes the
difference, but we find a segregation in the ova
into what may for convenience be called female
ova and male ova.

In Aphidae or plant lice a third condition is found.
Here again parthenogenesis continues for generation
after generation so long as conditions are favourable,

THE HEREDITY OF SEX 57

i.e. in summer, and the eggs are in the same condition
as in Daphnia, etc., that is to say, reduction does not
occur, and the number of chromosomes is 2N.
Under unfavourable conditions males are developed
as well as females by parthenogenesis, but the males
arise from eggs which undergo partial reduction
of chromosomes, only one or two being separated
instead of half the whole number. The number
then in an egg which develops into a male is 2N— 1,
while other eggs undergo complete reduction and
then have N chromosomes. The latter, however,
do not develop until they have been fertilised.
In the males, when mature, reduction takes place
in the gametes, so that two kinds of sperms are
formed, those with N chromosomes and those with
N— 1 chromosomes. The latter degenerate and
die, the former fertilise the ova, and the fertilised
ova develop only into females. The chief difference
in this case then is that the reduction in the male
to the N or simplex condition takes place in two
stages, one in the parthenogenetic ovum, one in the
gametes of the mature male. In Hymenoptera
and in Daphnia, etc., the whole reduction takes
place in the parthenogenetic ovum, and in the
mature male, though reduction divisions occur, no
separation of chromosomes takes place: at the
first division one cell is formed with N chromosomes
and one with none, and the latter perishes.

In many insects and other Arthropods which
are not parthenogenetic the male has been found to
possess fewer chromosomes than the female. The
female forms, as in the above cases of partheno-
genesis, only gametes of one kind each with N
chromosomes, but the male forms gametes of two

58 MENDELISM AND

sorts, one with N chromosomes, the other with N— I
or N— 2 chromosomes. On fertilisation two kinds
of zygotes are formed, female-producing eggs with
2N chromosomes, and male-producing eggs with
2 N— I or 2N— 2 chromosomes. There is also evidence
that in some cases, e.g. the sea-urchin, the female
is heterozygous, forming gametes, some with N
and some with N+ chromosomes, while the male
gametes are all N. Fertilisation then produces
male-producing eggs with 2N chromosomes, female-
producing with 2N+.

Such is the summary given by Castle in 1912.1
It will be seen that he treats the differences as purely
quantitative, mere differences in the number of the
chromosomes. Professor E. B. Wilson, however,
who had contributed largely by his own researches
to our knowledge of sex from the cytological point
of view, had already published, in 1910,2 a very
instructive resume of the facts observed up to that
time. The important fact which is generally true
for insects, according to Wilson, is that there is a
special chromosome or chromosomes which can be
distinguished from the others, and which is or are
related to sex differentiation. This chromosome,
to speak of it for convenience in the singular, has
been variously named by different investigators.
Wilson called it the ' X chromosome,' M'Cluny the
4 accessory chromosome,' Montgomery the ' hetero-
chromosome,' while the names c heterotropic chromo-
some ' and idiochromosome have also been used.
For the purpose of. the present discussion we may con-

1 Heredity and Eugenics, by Castle and Others. University of Chicago
Press, 1912.

8 ' The Determination of Sex.* Science Progress, April 1910,

THE HEREDITY OF SEX 59

veniently name it the sex-chromosome. It is often
distinguished by its larger size and different shape.
Wilson describes the following different cases :—

(1) The sex-chromosome in the male gametocytes
is single and fails to divide with the others, but
passes undivided to one pole. This may occur in
the first reduction division (Orthoptera, Coleoptera,
Diptera) or in the second (many Hemiptera). But
it is difficult to understand what is meant by ' fails
to divide.' In one of the reduction divisions all the
chromosomes divide as in ordinary or homotypic
nucleus division, but in the other the chromosomes
simply separate into two equal groups without
division. If there are an odd number of chromo-
somes, 2N—1, in all the gametocytes of the male, as
stated in most accounts of the subject, then if one
chromosome fails to divide in the homotypic division,
we shall have 2N— 2 in one spermatocyte and 2N— 1
in the other. Then when the heterotypic division
takes place and the number of chromosomes is
halved, we shall have two spermatocytes with N— I
chromosomes from one of the first spermatocytes
and one with N and one with N— I from the other.
Thus there will be three spermatozoa with N— 1
chromosomes and one with N chromosomes, whereas
we are supposed to find equal numbers with N and
N— 1 chromosomes. It is evident that what Dr.
Wilson means is that the sex-chromosome is unpaired,
and that although it divides like the others in the
homotypic division, in the heterotypic division it has
no mate and so passes with half the number of
chromosomes to one pole of the division spindle,
while the other group of chromosomes has no sex-
chromosome. Examples of this are the genera

60 MENDELISM AND

Pyrrhocoris and Protenor (Hemiptera) Brachystola
and many other Acrididae, Anasa, Euthoetha,
Narnia, Anax. In a second class of cases the sex-
chromosome is double, consisting of two components
which pass together to one pole. Examples of this
are Syrotnaster, Phylloxera, Agalena. In a third
class the sex-chromosome is accompanied by a fellow
which is usually smaller, and the two separate at the
differential division. The sizes of the two differ in
different degrees, from cases as in many Coleoptera
and Diptera in which the smaller chromosome is very
minute, to those (Benacus, Mineus) in which it is
almost as large as its fellow, and ,others (Nezara,
Oncopeltus) in which the two are equal in size.
Again, there are cases in which one sex-chromosome,
say X, is double, triple, or even quadruple, while the
other, say Y, is single. In all these cases there are
two X chromosomes in the oocytes (and somatic
cells) of the female, and after reduction the female
gametes or unfertilised ova are all alike, having a
single X chromosome or group. On fertilisation
half the zygotes have XX and half XY, whether Y
is absence of a sex-chromosome, or one of the other
Y forms above mentioned. The sex is thus deter-
mined by the male gamete, the X chromosome
united with that of the female gamete producing
female individuals, while the Y united with X pro-
duces male individuals.

Professor T. H. Morgan has made numerous
observations and experiments on a single culture
of the fruit-fly, Drosophila ampelophila, bred in
bottles in the laboratory for five or six years. He
has not only studied the chromosomes in the gametes
of this fly, and made Mendelian crosses with it,

THE HEREDITY OF SEX 61

but has obtained numerous mutations, so that his
work is a very important contribution to the muta-
tion doctrine. Drosophila in the hands of Professor
Morgan and his students and colleagues has thus
become as classical a type as Oenothera in those of
the botanical mutationists. Different branches of
Morgan's work are discussed elsewhere in this
volume, but here we are concerned only with its
bearing on the question of the determination of sex.
He describes 1 the chromosomes of Drosophila as
consisting in the diploid condition of four pairs,
that is to say, pairs which separate in the reduction
division so that the gamete contains four single
chromosomes, one of each pair. In two of these
pairs the chromosomes are elongated and shaped
like boomerangs, in the third they are small, round
granules, and the fourth pair are the sex-chromo-
somes : in the female these last are straight rods,
in the male one is straight as in the female, the
other is bent. The straight ones are called the
X chromosomes, the bent one the Y chromosome.
The fertilisations are thus XX which develops into
a female fly, and XY which develops into a male.
Drosophila therefore is an example of one of the
cases described by Wilson.

Dr. Wilson (loc. cit.) discusses the question of
how we are to interpret these facts, in particular,
the fact that the X chromosome in fertilisation
gives rise to females. He remarks that the X
chromosome must be a male-determining factor
since in many cases it is the only sex-chromosome
in the males, yet its introduction into the egg

1 A Critique of the Theory of Evolution. Princeton University Press
and Oxford University Press, 1916.

62 MENDELISM AND

establishes the female condition. This is the same
difficulty which I pointed out above in connection
with the Mendelian theory that the female was
heterozygous and the male homozygous for sex.
Dr. Wilson points out that in the bee, where fertilised
eggs develop into females and unfertilised into
males, we should have to assume that the -X" chromo-
some in the female gamete is a female determiner
which meets a recessive male determiner in the
X chromosomes of the sperm. When reduction
occurs, the X $ must be eliminated since the reduced
egg develops always into a male. But on fertilisa-
tion, since the fertilised egg develops into a female,
a dominant X $ must come from thfc sperm, so that
our first assumption contradicts itself.

Dr. Wilson, T. H. Morgan, and Richard Hartwig
have therefore suggested that the sex-difference
as regards gametes is not a qualitative but a
quantitative one. In certain cases there is no
evident quantitative difference of chromatin as a
whole, but there may in all cases be a difference
in the quantity of special sex-chromatin contained
in the X element. The theory put forward by
Wilson then is that a single X element means per se
the male condition, while the addition of a second
element of the same kind produces the female
condition. Such a theory might apply even to
cases where no sex-chromosomes can be distinguished
by the eye : the ova, in such cases (probably the
majority), might also have a double dose of sex-
chromatin, the males a single dose. This theory,
however, is still open to the objection that the
female gametes before fertilisation, and half the
male gametes, have the half quantity of sex-

THE HEREDITY OF SEX 63

chromatin which by hypothesis determines the
male condition, so that here again we have the
male condition as something which is distinct from
the characteristics of the spermatozoon. But if
this is the case, what is the male condition ? The
parthenogenetic ovum of the bee is male, and yet
it is an ovum capable only of producing spermatozoa.
If the single X chromosomes is the cause of the
development of spermatozoa in the male bee, why
does it not produce spermatozoa in the gametes
of the female bee, since when reduction takes place
all these gametes have a single X chromosome ?

In biology, as in every other science, we must
admit facts even when we cannot explain them.
The facts of what we call gravitation are obvious,
and any attempt to disregard them would result
in disaster, yet no satisfactory explanation of gravita-
tion has yet been discovered : many theories have
been suggested, but no theory has yet been proved
to be true. In the same way it may be necessary
to admit that two X chromosomes result in the
development of a female, and one X , or X Y chromo-
somes result in the development of a male. But
Mendelians have omitted to consider what is meant
by male and female. The soma with its male and
female somatic characters has nothing to do with
the question, since somatic sex-differences may be
altogether wanting, and moreover, the essential
male character, the formation of spermatozoa, is
by the Mendelian hypothesis due to descent of the
male gametes from the original fertilised or un-
fertilised ovum. The Mendelian theory therefore
is that when an ovum has two X sex-chromosomes
it can only after a number of cell-divisions, at the

64 MENDELISM AND

following reduction division, give rise to ova, while
an ovum containing one X sex-chromosome, or
two different, XY, chromosomes, at the next re-
duction division gives rise to spermatozoa. The X
sex-chromosome is not in itself either female or
male, since, as we have seen, either ovum or sper-
matozoon may contain a single X chromosome. The
ovum then with one X chromosome or one X and
one Y changes its sex at the next reduction division
and becomes male. In parthenogenetic ova this
happens without conjugation with a spermatozoon
at all : in other cases, since the zygote is com-
pounded of spermatozoon and ovum, we can only say
that in the XX zygote, the ovum developing only
ova, the female is dominant, in the X or X Y zygote
developing only spermatozoa the male is dominant.
Hermaphrodite animals, as has been pointed out
by Correns and Wilson, cannot be brought under
this scheme at all. In the earthworms, for instance,
we have, in every individual developed from a
zygote, ova and spermatozoa developing in different
gonads in different parts of the body. The dif-
ferentiation here, therefore, must occur in some
cell - division preceding the reduction divisions.
Every zygote must have the same composition,
and yet give rise to two sexes in the same individual.
Further light on the sex problem, as in many other
problems in biology, can only be obtained by more
knowledge of the physical and chemical processes
which take place in the chromosomes and in the
relations of these structures to the rest of the cell.
The recent advances in cytology, remarkable as they
are, consist almost entirely of observations of
microscopic structure. They may be said to reveal

THE HEREDITY OF SEX 65

the statics of the cell rather than its dynamics.
Cytology is in fact a branch of anatomy, and in the
anatomy of the cell we have made some progress,
but our knowledge of the physiology of the cell is
still infinitesimal. The nucleus, and especially the
chromosomes, are supposed in some unknown way
to influence or govern the metabolism of the cyto-
plasm. From this point of view the hypothesis men-
tioned above that the sex-difference in the gametes
is not qualitative but quantitative is probably nearer
to the truth. Geddes and Thomson and others have
maintained that the sex-difference is one of meta-
bolism, the ovum being more anabolic, the sperm
more katabolic. A double quantity of special
chromatin may be the cause of the greater ana-
bolism of the ovum. In that case the difficulty
indicated in a previous part of this chapter, that the
ovum after reduction resembles the sperm in having
only one X chromosome, may be explained by the
fact that the growth of the ovum and its accumula-
tion of yolk substances has been already accomplished
under the influence of the two chromosomes before
reduction. Other difficulties previously discussed
also appear to be diminished if we adopt this point
of view. We need not regard maleness and female-
ness as unit characters in heredity of the same kind
as Mendelian characters of the soma. Instead of
saying that the zygote composed of ovum and
spermatozoon is incapable of giving rise in the male
to ova, or in the female to sperms, we should hold
that the gametocytes ultimately give rise to ova or
to sperms according to the metabolic processes set
up and maintained in them through their successive
cell-divisions under the influence of the double or

E

66 MENDELISM AND HEREDITY OF SEX

single X chromosome. There still remains the
difficulty of explaining why the male gametocytes
after reduction develop into similar sperms, with
their heads and long flagella, although half of them
possess one X chromosome each and the other half
none. We can only suppose that the final develop-
ment of the sperms is the result of the presence of
the single X chromosome in the successive genera-
tions of male gametocytes before the reduction
divisions.

The Mendelian theory of sex-heredity assumed that
in the reduction divisions the two sex-characters,
maleness and f emaleness, were segregated in the same
way as a pair of somatic allelomorphs, but the words
maleness and femaleness expressed no real concep-
tions. The view above suggested merely attempts
to bring our real knowledge of the difference be-
tween ovum and sperm into relation with our real
knowledge of the sex-chromosomes and their
behaviour in reduction and fertilisation.

CHAPTER III

INFLUENCE OF HORMONES ON DEVELOPMENT
OF SOMATIC SEX-CHARACTERS

WE have next to consider what are commonly called
secondary sexual characters. These are characters
or organs more or less completely limited to one
sex. When we distinguish in the higher animals
the generative organs or gonads on the one hand
from the body or soma on the other, we see that
all differences between the sexes, except the gonads,
are somatic, and we may call them somatic sexual
characters. The question at once arises whether
the soma itself is sexual, that is to say, whether on the
assumption that the sex of the zygote is already
determined before it begins to develop, the somatic
cells as well as the gametocytes are individually
and collectively either male or female. In previous
discussions of the subject I have urged that the
only meaning of sex was the difference between the
megagamete or ovum, and the microgamete or sperm.
But if the zygote, although compounded of ovum
and sperm, is predestined to give rise in the gametes
descended from it, either to sperms only or to ova
only, it may be suggested that all the somatic cells
descended from the zygote are likewise either male
or female, although they do not give rise to gametes.
It is evident, however, that the somatic cells, organs,
and characters do not differ necessarily or universally

67

68 INFLUENCE OF HORMONES

in the two sexes. On the one hand, we have extra-
ordinary and very conspicuous peculiarities in the
male, entirely absent in the female, such as the
antlers of stags, and the vivid plumage of the gold
pheasant ; on the other we have the sexes externally
alike and only distinguished by their sexual organs,
as in mouse, rabbit, hare, and many other Rodents,
most Equidae, kingfisher, crows and rooks, many
parrots, many Reptiles, Amphibia, Fishes, and in-
vertebrate animals. In the majority of fishes, in
which fertilisation is external and no care is taken
of the eggs or young, there are no somatic sexual
differences. Moreover, somatic sexual characters
where they do occur have no common characteristics
either in structure or position in the body. It may
be said that any part of the soma may in different
cases present a sex-limited development. In the
stag the male peculiarity is an enormous development
of bone on the head, in the peacock it is the enlarge-
ment of the feathers of the tail. In some birds there
are spurs on the legs, in others spurs on the wings.
It is no explanation, therefore, to say that these
various organs and characters are the expression of
sex in the somatic cells.

As I pointed out in my Sexual Dimorphism (1900),
the common characteristic of somatic sexual char-
acters is their adaptive relation to some function
in the sexual habits of the species in which they
occur. There is no universal characteristic of sex
except the difference between the gametes and the
reproductive organs (gonads) in which they are
produced. All other differences, therefore, including
genital ducts and copulatory or intromittent organs,
are somatic. When we examine these somatic

ON SOMATIC SEX-CHARACTERS 69

differences we find that they can be classified
according to their relation to fertilisation and re-
production, including the care or protection of the
offspring. The precise classification is of no great
importance, but we may distinguish the following
kinds to show the chief functions to which the
characters or organs are adapted.

1. GENITAL DUCTS AND INTROMITTENT ORGANS. —
According to the theory of the coelom which we owe to
Goodrich, in all the coelomata the coelom is primarily
the generative cavity, on the walls of which the
gametocytes are situated, and the coelomic ducts
are the original genital ducts. In Vertebrates we
find two such ducts in both sexes in the embryo,
originally formed apparently by the splitting of a
single duct. In the male one of these ducts becomes
connected with the testis while the other degenerates:
the one which degenerates in the male forms the
oviduct in the female, while the one which is
functional in the male degenerates in the female.

Intromittent organs are formed in all sorts of
different ways in different animals. In Elasmo-
branchs (sharks and skates) they are enlarged
portions of the pelvic fins, and therefore paired.
In Lizards they are pouches of the skin at the
sides of the cloacal opening. In Mammals the single
penis is developed from the ventral wall of the
cloaca. In Crustacea certain appendages are used
for this function. There are a great many animals,
from jelly-fishes to fishes and frogs, in which fertilisa-
tion is external, and there are no intromittent organs
at all.

2. ORGANS FOR CAPTURING OR HOLDING THE
FEMALE : for example, the thumb -pads of the frog,

70 INFLUENCE OF HORMONES

and a modification of the foot in a water- beetle.
Certain organs on the head and pelvic fins of the
Chimaeroid fishes are believed to be used for this
purpose.

3. WEAPONS. — Organs which are employed in
combats between males for the exclusive possession
of the females. For example, antlers of stags, horns
of other Ruminants, tusks of elephants, spurs of
cocks and Phasiamidae generally, horns and out-
growths in males of Reptiles and many Beetles,
probably used for this purpose.

4. ALLUREMENTS. — Organs or characters used to
attract or excite the female. These might be called
the organs of courtship, such as the peacock's tail,
the plumes of the birds-of -paradise, and the brilliant
plumage of humming birds and many others. The
song of birds is another example, and sound is pro-
duced in many Fishes for a similar purpose.

5. ORGANS FOR THE BENEFIT OF THE OFFSPRING :
for example, the extraordinary pouches in which
the eggs are developed in certain Frogs. In the
South American species, Rhinoderma darwinii, the
enlarged vocal sacs are used for this purpose.
Pouches with the same function are developed
in many animals, for instance in Pipe-fishes and
Marsupials. Abdominal appendages are enlarged
in female Crustacea for the attachment of the eggs,
the abdomen also being larger and broader.

The argument in favour of the Lamarckian
explanation of the evolution of these adaptive
characters is the same as in the case of adaptations
common to both sexes, namely that in every case
the function of the organs and characters involves
special irritations or stimulations by external

ON SOMATIC SEX-CHARACTERS 71

physical agents. Mechanical irritation, especially
of the interrupted kind, repeated blows or friction
causes hypertrophy of the epidermis and of super-
ficial bone. I have stated this argument and the
evidence for it in some detail in my volume on
Sexual Dimorphism. It is one of the most striking
facts in support of this argument that the hyper-
trophied plumage which constitutes the somatic
sexual character of the male in so many birds is
habitually erected by muscular action for the purpose
of display in the sexual excitement of courtship.
I doubt if there is a single instance in which the male
bird takes up a position to present his ornamental
plumage to the sight of the female without a special
erection and movement of the feathers themselves.
Such a stimulation must affect the living epidermic
cells of the feather papilla. Even supposing that
the feather is not growing at the time, it is probable,
if not certain, that the stimulation will affect the
papilla at the base of the feather follicle, so as to
cause increased growth of the succeeding feather.
But we have no reason to believe that erection in
display occurs only when the growth of the feathers
is completed, still less that it did so always at the
beginning of the evolution.

The antlers of stags are the best case in favour
of the Lamarckian view of the evolution of somatic
sexual characters. The shedding of the skin
('velvet') followed by the death of the bone, and
its ultimate separation from the skull, are so closely
similar to the pathological processes occurring in
the injury of superficial bones, that it is impossible
to believe that the resemblance is only apparent
and deceptive. In an individual man or mammal, if

72 INFLUENCE OP HORMONES

the periosteum of a bone is destroyed or removed
the bone dies, and is then either absorbed, or
separated from the living bone adjoining, by ab-
sorption of the connecting part. In the stag both
skin and periosteum are removed from the antler :
probably they would die and shrivel of their own
accord by hereditary development, but as a matter of
fact the stag voluntarily removes them by rubbing
the ant]er against tree trunks, etc. When the bone is
dead the living cells at its base dissolve and absorb it,
and when the base is dissolved the antler must fall off.
The adaptive relation is not the only common
characteristic of these somatic sexual characters.
Another most important fact is not only that they
are fully developed in one sex, absent or rudi-
mentary in the other, but that their development
is connected with the functional maturity and ac-
tivity of the gonads. There is usually an early
immature period of life in which the male and
female are similar, and then at the time of puberty
the somatic sexual characters of either sex, gener-
ally most marked in the male, develop. In some
cases, where the activity of the gonads is limited to a
particular season of the year, the sexual characters
or organs are developed at this season, and then
disappear again, so that there is a periodic develop-
ment corresponding to the periodic activity of the
testes or ovaries. Stags have a limited breeding or
4 rutting ' season in autumn (in north temperate
regions), and the antlers also are shed and developed
annually. In this case we cannot assert that the
development of the antler takes place during the
active state of the testes. The antlers are fully
developed and the velvet is shed at the commence-

ON SOMATIC SEX-CHARACTERS 73

ment of the rutting season, and development of
the antlers takes place between the beginning of the
year and the month of August or September. In
ducks and other birds there is a brilliant male-breed-
ing plumage in the breeding season which disappears
when breeding is over, so that the male becomes
very similar to the female. In the North American
fresh-water crayfishes of the genus Cambarus there
are two forms of males, one of which has testes in
functional activity, while in the other these organs
are small and quiescent : the one form changes into
the other when the testes pass from the one condition
to the other.

It has long been known that the development of
male sex-characters is profoundly affected by the
operation of castration. The removal of the testes
is most easily carried out in Mammals, in conse-
quence of the external position of the organs in
these animals, and the operation has been practised
on domesticated animals as well as on man himself
from very ancient times. The effect is the more or
less complete suppression of the male insignia, in
man, for example, the beard fails to develop, the
voice does not undergo the usual change to lower
pitch which takes place at puberty, and the eunuch
therefore has much resemblance to the boy or
woman. Many careful experimental researches have
been made on the subject in recent years. The
consideration of the subject involves two questions :
(1) What are the exact effects of the removal of the
gonads in male and female? (2) By what means are
these effects brought about, what is the physiological
explanation of the influence of the gonads on the
soma ?

74 INFLUENCE OF HORMONES

I have quoted the evidence concerning the effects
of castration on stags in my Sexual Dimorphism and
in my paper on the ' Heredity of Secondary Sexual
Characters.' 1 When castration is performed soon
after birth a minute, simple spike antler is de-
veloped, only two to four inches in length : it
remains covered with skin, is never shed, and
develops no branches. When the operation is
performed on a mature stag with antlers, the latter
are shed soon after the operation, whether they have
lost their velvet or not. In the following season new
antlers develop, but these never lose then" velvet or
skin and are never shed.

CASTRATION IN FOWLS

The removal of the testes from young cocks has
been commonly practised in many countries, e.g.
France, capons, as such birds are called, being
fatter and more tender for the table than entire
birds. The actual effect, however, on the secondary
sexual characters has not in former times been
very definitely described. The usual descriptions
represent the castrated birds as having rather fuller
plumage than the entire birds ; but the comb and
wattles are much smaller than in the latter, more
similar to those of a hen. It is also stated that the
capon will rear chickens, though he does not incu-
bate, and that they are used in this way in France.

The most precise of the statements on the subject
by the earlier naturalists is that of William Yarrell 2
(1857), who writes as follows: —

' The capon ceases to crow, the comb and gills do

1 Archiv fiir Entwicklungsmechanik, 1908.

2 Proc. Linn. Soc., 1857.

ON SOMATIC SEX-CHARACTERS 75

not attain the size of those parts in the perfect male,
the spurs appear but remain short and blunt, and
the hackle feathers of the neck and saddle instead
of being long and narrow are short and broadly
webbed. The capon will take to a clutch of chickens,
attend them in their search for food, and brood them
under his wings when they are tired.'

It would naturally be expected, on the analogy
of the case of stags, that when a young cock was
completely castrated all the male secondary char-
acters would be suppressed, namely, the greater
size of the comb and wattles in comparison with the
hen, the long neck hackles, and saddle hackles, long
tail feathers, especially the sickle-feathers, and the
spurs. As a matter of fact, the castrated specimen
usually shows only the first of these effects to any
conspicuous degree. The comb and wattles of the
capon are similar to those of the hen, but he still has
the plumage and the spurs of the entire cock. Many
investigators have made experiments in relation to
this subject, and most of them have found that
complete castration is difficult, and that portions
of the testes left in the bird during the operation
become grafted in some other position either on the
parietal peritoneum, or on that covering the in-
testines, and produce spermatozoa, which of course
have no outlet. In such cases the secondary male
characters may be more or less completely de-
veloped. Thus Shattock and Seligmann (1904)
state that ligature of the vas deferens made no
difference to the male characters, and that after
castration detached fragments were often left in
different positions as grafts, when the secondary
characters developed. In one particular case only a

76 INFLUENCE OF HORMONES

minute nodule of testicular tissue showing normal
spermatogenesis was found on post mortem ex-
amination attached to the intestine. In this bird
there was no male development of comb or wattles,
a full development of neck hackles, a certain develop-
ment of saddle hackles, a few straggling badly
curved feathers in the tail and short blunt spurs on
the legs. Lode1 (1895) found that testes could easily
be transplanted into subcutaneous tissue and else-
where, and that the male characters then developed
normally. Hanau2 (1896) obtained the same result.
The question, however, to what degree the male
characters of the cock are suppressed after com-
plete castration is not so definitely answered in the
literature of the subject. Shattock and Seligmann
in their 1904 paper make no definite statement on
the subject. Rieger (1900), Selheim (1901), and
Foges3 (1902) state that the true capon is char-
acterised by shrivelling of the comb, wattles, and
spurs ; poor development of the neck and tail
feathers ; hoarse voice and excessive deposit of fat.
Shattock and Seligmann, on the other hand, have
placed in the College of Surgeons Museum the head
of a Plymouth Rock which was castrated in 1901.
It was hatched in the spring of that year. In
December 1901 the comb and wattles were very
small, the spurs fairly well developed, and the
tail had a somewhat masculine appearance. In
September 1902, when the bird was killed, the comb
and wattles were still poorly developed, the neck
hackles fairly well so ; saddle hackles rather well

1 Wiener klin. Wochenschr., 1895.

2 Arch. f. ges. Physiologic, 1896.

I'fluyers Archiv, 1902.

ON SOMATIC SEX-CHARACTERS 77

developed ; the tail contained rather loosely-
grouped long sickle feathers ; the spurs stout.
The description states that dissection showed no
trace of either testicle, and I am informed by Mr.
Shattock that there were no grafts. The description
ends with the conclusion that the growth of the spurs,
and to a certain extent that of the long, curved
sickle feathers, is not prevented by castration. With
regard to the spurs this result does not agree with
that of the German investigators, but it must be
remembered that the latter speak only of the re-
duction of the spurs, not entire absence. It is im-
portant in discussing the effects of castration in
cocks to bear in mind the actual course of develop-
ment of the secondary sexual characters. When
the chicks are first hatched they are in the down :
rudimentary combs are present, wattles can scarcely
be distinguished, and there is no external difference
between the sexes. The ordinary plumage begins
to develop immediately after hatching, the primaries
of the wings being the first to appear. The feathers
are completely developed in about five weeks,
and still there is no difference between the sexes.
The first sexual difference is the greater size of the
combs in the males, and this is quite distinct at the
age of six weeks. At nine to ten weeks in black-red
fowls, in which the cocks have black breasts and red
backs with yellow hackles, the black feathers on the
breast and red on the back are gradually develop-
ing, both sexes previously having been a dull
speckled brown, closely similar to the adult hens.
The spurs are the last of the male characters to
develop, these at the age of four months being still
mere nodules, scarcely, if at all, larger than the

78 INFLUENCE OP HORMONES

rudiments visible in adult hens. This is the age
at which castration is usually performed, as at an
earlier age the birds are too small to operate on
successfully. It follows, therefore, that the spurs
develop after castration, and it would seem that
their development does not depend upon the
presence of the sexual organs. It is a question, how-
ever, whether castration in the cock is ever quite
complete. In the original wild species and in the
majority of domesticated breeds the spurs are con-
fined to the male sex, and are typical secondary sex-
characters, as much so as the antlers of stags or the
beard of man, yet the above discussion shows that
there is some doubt whether their' development is
prevented as much as in other cases by the absence
of the sexual organs. Even if it should be proved
that in supposed cases of complete castration, such
as that of Shattock and Seligmann, some testicular
tissue remained at the site of the testes, it would
still be true that the development of the comb and
wattles is more affected by the removal of the
sexual organs than that of the spurs or tail feathers.
My own experiments in castrating cocks were as
follows: On August 20, 1910, I operated on a White
Leghorn cock about five months old. One testis
was removed, with a small part of the end broken
off, but the other, after it was detached, was lost
among the intestines. On the same day I operated
on another about thirteen weeks old, a speckled
mongrel. In this case both testes were extracted
but one was slightly broken at one end, although
I was not sure that any of it was left in the body.
An entire White Leghorn of the same age as the first
was kept as a control. On August 27 the two

ON SOMATIC SEX-CHARACTERS 79

castrated birds had recovered and were active.
Their combs had diminished in size and lost colour
considerably, that of the White Leghorn was scarcely
more than half as large as that of the control. Such
a rapid diminution can scarcely be due to absorption
of tissue, but shows that the size of the normal
cock's comb is largely due to distension with blood,
which ceases when the sexual organs are removed.
In the following January, the second cock, supposed
to be completely castrated, was seen to make a sexual
gesture like a cock, though not a complete action
like an entire animal: this showed that the sexual
instinct was not completely suppressed. In February
this same bird was seen to attempt to tread a hen,
while the white one, supposed to be less perfectly
emasculated, had never shown such male instinct.

The White Leghorn cock was killed and dissected
on May 13, 1911, nine months after castration. I
found an oval body of dark, dull brown colour loose
among the intestines: this was evidently the left
testis which was separated from its natural attach-
ment and lost in the abdomen at the time of the
operation. I examined the natural sites of the
testes : on the right side there was a small testis of
considerable size, about half an inch in diameter.
When a portion of this was teased up and examined
under the microscope moving spermatozoa were
seen, but they were not in swarms as in a normal
testis, but scattered among numerous cells. On the
left side was a much smaller testis, in the tissue of
which I with difficulty detected a few slowly moving
spermatozoa. The vasa deferentia were seen as
white convoluted threads on the peritoneum, but
contained no spermatozoa.

80 INFLUENCE OF HORMONES

On July 29, 1911, a little more than eleven months
after the operation, I examined and killed the second
of these castrated cocks, the speckled mongrel-bred
bird. I measured the comb and wattles while it
was alive, in case there might be reduction in the size
of these appendages when the bird was killed. The
comb was 1| inches high by 2f inches in length. The
spurs were 1 inch long, curved and pointed. Saddle
hackles short, hanging only a little below the end
of the wing. Neck hackles well developed, similar
to those of an entire cock. Longest tail feather
15f inches, blue-black in colour.

I had no entire cock of same breed, but measured
the entire White Leghorn for comparison. Comb
If inches high by 3f inches in length. (It is to be
remembered that the comb and wattles are especially
large in Leghorns.) Wattle 1J inches in vertical
length. Spur 1 inch long, stouter and less pointed
than in the capon. Longest tail feather 12 inches
long.

When killed the capon was found to be very fat :
there were masses of fat around the intestines and
under the peritoneum, which made it impossible to
make out details such as ureter and vas deferens
properly. I found a white nodule about half an inch
in diameter attached to mesentery. The liquid
pressed from this was swarming with spermatozoa
in active motion. Two other masses about the same
size or a little larger were found on the sites of the
original testes. These also were full of mobile
spermatozoa, and must have grown from portions of
the testes left behind at castration.

In ducks the sexual characters of the male differ
from those in the fowl, especially in the fact that they

ON SOMATIC SEX-CHARACTERS 81

almost completely disappear after the breeding

season and reappear in the following season. In the

interval the drake passes into a condition of plumage

in which he resembles the female ; and this condition

is known as ' eclipse.' The male plumage, therefore,

in the drake has a history somewhat similar to that of

the antlers in deer. Two investigations of the effects

of castration on ducks and drakes have been recorded.

H. D. Goodale 1 removed the generative organs from

both drakes and ducks of the Rouen breed, which

is strongly dimorphic in plumage. One drake was

castrated in the early spring of 1909 when a little less

than a year old. This bird did not assume the

summer plumage in 1909, that is, did not pass into

eclipse. It was in the nuptial plumage when

castrated. This breeding or nuptial plumage is well

known : it includes a white neck-ring, brilliant green

feathers on the head, much claret on the breast,

brilliant metallic blue on the wing, and two or more

upward curled feathers on the tail. The drake

mentioned above was accidentally killed in the

spring of 1910. Another drake was castrated on

August 8, 1909 : only the left testis was removed,

the other being ligatured. At this time the bird

would be in eclipse plumage. It appears from the

description that it assumed the nuptial plumage in

the winter of 1909, and did not pass into eclipse

again in the summer of 1910. Thus in drakes the

effect of castration is that the secondary sexual

character remains permanently instead of being

lost and renewed annually. Goodale, however, does

not describe the moults in detail. In the natural

1 ' Castration of Drakes.' Biol. Bulletin, Wood's Hole, Mass., vol. xx.,
1910.

F

82 INFLUENCE OF HORMONES

condition the drake must moult twice in the year,
once when he sheds the nuptial plumage, and again
when he drops the summer dress. Goodale insists,
from some idea about secondary sexual characters
which is not very obvious, that the eclipse or summer
plumage is not the same as that of the female. He
states that the male in summer plumage merely
mimics the female but does not become entirely like
her. In certain parts of the body there are no
modifications toward the female type. In others,
i.e. head, breast, and keel region, the feathers of the
male become quite like those of the female. ' It
can hardly be maintained that this is an example
of assumption by the male of the female's plumage,
especially as the presence of the testis is necessary for
its appearance.' The idea here seems to be that since
the eclipse plumage is only assumed when the testis
is present, therefore it must be a male character.

Out of five females on which the operation was
performed only two lived more than a few days
afterwards. One of these (a) was castrated in the
spring of 1909 when a little less than a year old, the
other (6) on August 13 when twelve weeks old. In
October 1909 they showed no marked modifications.
In July 1910 it was noticed that they had the male
curled feathers in the tail, and (a) had breast feathers
similar to those of the male in summer plumage,
(6) was rather more strongly modified : she had a
very narrow white neck-ring, and breast feathers
distinctly of male type. The next moult began in
September, and in November was well advanced.
On the whole (a) had made little advance towards
the male type, but (b) closely resembled the male
in nuptial plumage. It had brilliant green feathers

ON SOMATIC SEX-CHARACTERS 83

on the head, a white neck-ring, much claret colour
on the breast, and some feathers indistinguishable
from those of the male, and also the male sex feathers
on the tail. Goodale concludes that the female
owes her normal colour to the ovaries or something
associated with them which suppresses the male
characters and ensures the development of her own
type. He considers it is quite as conceivable that
selection should operate to pick out inconspicuously
coloured females as that selection of brilliantly
coloured males should bring about an addition to the
female type. But as pointed out above, selection
cannot explain the dimorphism in either case.

It may be mentioned here that owing to the fact
that the single (left) ovary in birds is very closely
attached to the peritoneum immediately covering
the great post-caval vein, it is generally impossible
to remove the whole of the ovary without cutting
or tearing the wall of the vein and so causing
fatal hemorrhage. The above results observed by
Goodale are thereJJpre all the more remarkable, and it
may be assumed that he removed at any rate nearly
all the ovary.

The research of Seligmann and Shattock 1 begins
with a comparison between the stages of the develop-
ment of the nuptial plumage and the stages of
spermatogenesis. In the young pheasant the male
plumage is fully developed in the autumn of its first
year, but no pairing occurs and no sexual instinct is
exhibited till the following spring. The wild duck
pairs in autumn or early winter, after the assumption

1 ' Relation between Seasonal Assumption of the Eclipse Plumage in
the Mallard (Anas boscas) and the Functions of the Testicle.' Proc. Zool.
Soc., 1914.

84 INFLUENCE OF HORMONES

of the nuptial plumage, but copulation does not occur
till spring is advanced. The investigation here
considered was made upon specimens of semi-
domesticated Anas boscas, such as are kept in London
parks and supplied from game farms. The testes
attain their maximum size during the breeding
season — end of March or beginning of April. At
this time each organ is almost as large as a pigeon's
egg, is very soft, and the liquid exuding from it when
cut is swarming with spermatozoa. The bird is of
course in full nuptial plumage. By the end of May,
although the plumage is unchanged, the testes have
diminished to the size of a haricot bean, and sper-
matogenesis has ceased. They diminish still further
during June, July, and August, and acquire a yellow
or brownish colour, while microscopically there is no
sign of activity in the spermatic cells. The change
from nuptial plumage to eclipse takes place between
the beginning of June and the middle of July. The
reappearance of the nuptial plumage takes place in
the month of September, and while this process
takes place there is no sign of change or renewed
activity in the testes. During October and November,
when the brilliant plumage is fully developed, the
testes increase slowly in size but remain yellow and
firm and exude no liquid on incision. Spermato-
genesis does not commence until the end of November
or beginning of December. The testes increase
greatly in size in January and February, and again
reach their maximum size by the end of March. It
is shown, therefore, that the loss of the nuptiaJ
plumage takes place in June when spermatogenesis
has ceased and the testes are diminishing in size,
but the redevelopment of this plumage takes place

ON SOMATIC SEX-CHARACTERS 85

in September without any renewed activity of the
testis and long before the beginning of spermato-
genesis. The case of the antlers in the stag is
probably very similar.

The important statement is made with regard to
castration (under anaesthetics, of course) that it was
found impossible to extirpate the testes completely.
When the bird was killed some months after the
operation, a greater or lesser amount of regenerated
testicular tissue was found either on the original site
of the organs or engrafted upon neighbouring organs.
This experience, it will be noted, agrees with my
own in the case of fowls. There were, however,
reasons for believing that the results observed
within the first six or eight months after the opera-
tion are not much different from those which would
follow complete castration.

Castration carried out when the drake was in
nuptial plumage produced the same effect which
was observed by Goodale, namely, delay, and im-
perfection in the assumption of the eclipse condition,
but the observations of Seligmann and Shattock are
more precise and detailed. One example described
was castrated in full winter plumage in December
1906. On July 11, when normally it would have
been in eclipse, the nuptial plumage was unmodified
except for a diffuse light-brown coloration on the
abdomen, which is stated to be due not to any growth
of new feathers but to pigmentary modification in
the old. By September 1 this bird was almost in
eclipse but not quite ; curl feathers in the tail had
disappeared, the breast was almost in full eclipse,
the white ring was slightly indicated at the sides of
the neck, the top of the head and the nape had still a

86 INFLUENCE OF HORMONES

good deal of gloss. After this the nuptial plumage
developed again, and on November 12 the bird
was in full nuptial plumage, with good curl feathers
in the tail. The only trace of the eclipse was the
presence of a few brown feathers on the flanks. This
bird was killed July 30, 1908, when the bird was in
eclipse, but not perfectly so, as there were vermi-
culated feathers mixed with eclipse feathers on the
breast, abdomen, and flanks. Dissection showed on
the right side a series of loosely attached nodular
grafts of testicular tissue, in total volume about
the size of a haricot bean: on the left side two
small nodules, together about the size of a pea,
and two other grafts at the root of the liver and on
the mesentery. Several other cases are described,
and the general result was that the eclipse was
delayed and never quite complete, while although
the nuptial plumage was almost fully developed in
the following winter, it retained some eclipse feathers,
and was also delayed and developed slowly.

Several drakes were castrated in July when in the
eclipse condition, and although the authors state, in
their general conclusions, that this does not produce
any constant appreciable effect upon the next
passage of the bird into winter plumage, they
describe one bird so treated which on November 18
retained many eclipse feathers : the general appear-
ance of the chestnut area of the breast was eclipse.

It must be remembered that not only was the
castration in these cases incomplete, but also that
it was performed on mature birds. Birds differ
from Mammals, firstly, in the difficulty of carrying
out complete castration, and secondly, in the fact
that the occurrence of puberty is not so definite, and

ON SOMATIC SEX-CHARACTERS 87

that immature birds are so small and delicate
that it is almost impossible to operate upon them
successfully.

ASSUMPTION OF MALE CHARACTERS BY THE
FEMALE

That male somatic sexual characters are latent
in the female is shown by the frequent appearance
of such characters in old age, or in individual cases.
The development of hair on the face of women in
old age, or after the child-bearing period, is a well-
known fact. Rorig,1 who carefully studied the
antlers of stags, states that old sterile females, and
those with diseased ovaries, develop antlers to some
degree. Cases of crowing hens, and female birds
assuming male plumage have long been known, but
the exact relation of the somatic changes to the
condition of the ovaries in these cases is worthy of
consideration in view of the results obtained by
Goodale after removal of the ovaries from ducks.
Shattock and Seligmann 2 record the case of a gold
pheasant hen which assumed the full male plumage
after the first moult : it had never laid eggs or
shown any sexual instincts. The only male char-
acter which was wanting was that of the spurs. The
ovary was represented by a smooth, slightly elevated
deep black eminence 1 cm. in length and 1*5 mm. in
breadth at its upper end. These authors also
mention three ducks in male plumage in which the
ovary was similarly atrophied but not pigmented.

1 * Ueber Geweihbildung und Geweihentwicklung.' Arch. Ent.-Mech.,
x. and xi.

2 * True Hermaphroditism in Domestic Fowl, etc.' Trans. Path. Soc.,
Lond., 57. 1, 1906.

88 INFLUENCE OF HORMONES

They regard the condition of the ovary as insuf-
ficient to explain the development of the male
characters, and suggest that such birds are really
hermaphrodite, a male element being possibly con-
cealed in a neighbouring organ such as the adrenal
or kidney. This hypothesis is not supported by
observation of testicular tissue in any such case,
but by the condition found in a hermaphrodite
specimen of the common fowl described in the paper.
This bird presented the fully developed comb and
wattles and the spurs of the cock, but the tail was
quite devoid of curved or sickle feathers, and
resembled that of the hen. Internally there were
two oviducts, that of the left side normally de-
veloped, that of the right diminutive and less than
half the full length. The gonad of the left side had
the tubular structure of a testis, but showed no signs
of active spermatogenesis, but in its lower part con-
tained two ova. The organ of the right side was
somewhat smaller, it had the same tubular structure,
and in one small part the tubules were larger, showed
division of nuclei (mitotic figures), and one of them
showed active spermatogenesis.

In discussing Heredity and Sex in 1909,1 Bateson
referred to the effects of castration as evidence
that in different types sex may be differently con-
stituted. Castration, he urged, in the male verte-
brate on the whole leads merely to the non-appear-
ance of male features, not to the assumption of
female characters, while injury or disease of the
ovaries may lead to the assumption of male char-
acters by the female. This was supposed to support
the view that the male is homozygous in sex, the

1 MendeVs Principles of Heredity. Camb. Univ. Press, 1909.

ON SOMATIC SEX-CHARACTERS 89

female heterozygous in Vertebrates : that is to say,
the female sex-character and the female secondary
sex-characters are entirely wanting in the male.
This argument assumes that the secondary characters
are essentially of sexual nature without inquiring
how they came to be connected with sex, and it
ignores the fact that the influence of castration on
such characters is a phenomenon entirely beyond the
scope of Mendelian principles altogether. The fact
that castration does affect, in many cases very pro-
foundly, somatic characters confined to one sex,
proves that Mendelian conceptions, however true up
to a certain point, are by no means the whole truth
about heredity and development. For it is the
essence of Mendelism as of Weismannism that not
only sex but all other congenital characters are
determined in the fertilised ovum or zygote. The
meaning of a recessive character in Mendelian
terminology is one that is hidden by a dominant
character, and both of them are due to factors in
the gametes, particularly in the chromosomes of the
gametes which come together in fertilisation. For
example, in fowls rose comb is dominant over single.
A dominant is something present which is absent
in the recessive : the rose comb is due to a factor
which is absent from the single. The two segregate
in the gametes of the hybrid or heterozygote, and if
a recessive gamete is fertilised by another recessive
gamete the single comb reappears. But castration
shows that the antlers of stags and other such
characters are not determined in the zygote when
the sex is determined, but owe their development,
partly at least, to the influence of another part of
the body, namely, the testes during the subsequent

90 INFLUENCE OF HORMONES

life of the individual. According to Mendelism the
structure and development of each part of the soma
is due to the constitution of the chromosomes of
the nuclei in that part. The effects of castration
show that the development of certain characters is
greatly influenced in some way by the presence of
the testes in a distant part of the body. The
Mendelians used to say it was impossible to believe in
the heredity of somatic modifications due to external
conditions, because it was impossible to conceive of
any means by which such modifications could affect
the constitution of the chromosomes in the gametes
within the modified body. It would, have been just
as logical to deny the proved effects of castration,
because it was impossible to conceive of any means
by which the testes could affect the development of a
distant part of the body.

But this is not all. The supposed fact that female
secondary characters in Vertebrates are absent in the
male is completely disproved for Mammals by the
presence of rudimentary mammary glands in the
male. It is true that secondary sex-characters are
usually positive in the male, while those of the
female are apparently negative, but in the case of
the mammary glands the opposite is the case. There
is no room for doubt that the mammary glands are
an essentially female somatic sex-character, not only
in their function but in the relation between the
periodicity of that function and those of the
ovaries and uterus, and it is equally certain from
their presence in rudimentary condition in the
male that they are not absent from the male
constitution.

ON SOMATIC SEX-CHARACTERS 91

INFLUENCE OF GONADS DUE TO HORMONES

The existence and the influence of hormones or
internal secretions may be said to have been first
proved in the case of the testes, for Professor A. A.
Berthold l of Gottingen in 1849 was the first to
make the experiment of removing the testicles from
cocks and grafting them in another part of the body,
and finding that the animals remained male in
regard to voice, reproductive instinct, fighting spirit,
and growth of comb and wattles. He also drew the
conclusion that the results were due to the effect
of the testicle upon the blood, and through the blood
upon the organism. Little attention was paid to
Berthold' s experiment at the time. The credit of
having been the first to formulate the doctrine of
internal secretion is generally given to Claude
Bernard. He discovered the glycogenic function
of the liver, and proved that in addition to secreting
bile, that organ stores up glycogen from the sugar
absorbed in the stomach and intestines, and gives it
out again as sugar to the blood. In 1855 he main-
tained that every organ of the body by a process of
internal secretion gives up products to the blood.
He did not, however, discover the action of such
products on other parts or functions of the body.
Brown-Sequard, in his address before the Medical
Faculty of Paris in 1869, was the first to suggest that
glands, with or without ducts, supplied special sub-
stances to the blood which were useful or necessary
to the normal health, and in 1889 at a meeting of the
Societe de Biologie he described the experiment he
had made upon himself by the injection of testicular

1 ' Transplantation der Hoden,' Archiv /. Anat. u. Phys., 1849.

92 INFLUENCE OF HORMONES

extract. This was the commencement of organo-
therapy. Since that time investigation of the more
important organs of internal secretion — namely,
the gonads, thyroid, thymus, suprarenals, pituitary,
and pineal bodies — has been carried on both by
clinical observation and experiment by a great
number of physiologists with very striking results,
and new hormones have been discovered in the walls
of the intestine and other organs.

Here, however, we are more especially concerned
with the gonads and other reproductive organs. A
great deal of evidence has now been obtained that
the influence of the testes and ovaries on secondary
sexual characters is due to a hormone formed in the
gonads and passing in the blood in the course of
the circulation to the organs and tissues which consti-
tute those characters. The fact that transplanted
portions of testes in birds (cocks and drakes) are
sufficient to maintain the secondary characters in
the same condition as in normal individuals shows
that the nexus between the primary and somatic
organs is of a liquid chemical nature and not ana-
tomical, through the nervous system for example.
Many physiologists in recent years have maintained
that the testicular hormone is not derived from the
male germ-cells or spermatocytes, but from certain
cells between the spermatic tubuli which are known
as interstitial cells, or collectively as the interstitial
gland.

The views of Ancel and Bouin,1 published in 1903,
may be described in large part as theory. They
state that the interstitial cells appear in the male
embryo before the gametocytes present distinctive

i C. R. Soc. de Biol., Iv,

ON SOMATIC SEX-CHARACTERS 93

sex-characters. They conclude that the interstitial
cells supply a nutritive material (hormone ?), which
has an effect on the sexual orientation of the primitive
generative cells. In addition to this function, the
interstitial cells by their hormone also give the
sexual character to the soma. When castration is
carried out at birth the male somatic characters do
not entirely disappear, because the hormone of the
interstitial cells has acted during intrauterine life.
The functional independence between the interstitial
cells and the seminal tubules is shown by the fact
that if the vasa deferentia are closed the seminal
gland (i.e. tubules) degenerates while the interstitial
cells do not. In the embryo the interstitial gland is
large, in the adult proportionately small.

There is complete disagreement between the
results of Ancel and Bouin on the one hand, and
those of Shattock and Seligmann on the other, with
regard to the effects of ligature of the vasa deferentia.
The latter authors, as mentioned above, found that
after ligature not only the somatic characters but
the testis itself developed normally. The experi-
ments were performed on Herdwick sheep and
domestic fowls. They state that on examination
the testes were found to be normally developed, and
spermatogenesis was in progress. The experiments
of Ancel and Bouin were carried out on rabbits
seven to eight weeks old, and consisted in removing
one testis, and ligaturing the vas deferens of the
other. About six months after the operation the
testis left in situ was smaller, the seminal tubules
contained few sperrnatogonia, though Sertoli's cells
(cells on the walls of the tubules to which the true
spermatic cells are attached) were unchanged ;

94 INFLUENCE OF HORMONES

while the interstitial cells were enormously developed,
by compensatory hypertrophy in consequence of the
removal of the other testis. At the same time the
male instincts and the other generative organs were
unchanged. In a few cases, however, Ancel and
Bouin observed atrophy of the interstitial cells as
well as the spermatic cells. They believe this is due
to the nerves supplying the testis being included in
the ligature. This is rather a surprising conclusion
in view of the fact that testicular grafts show active
spermatogenesis. It is difficult to understand why
nerve connection should be necessary for the in-
terstitial cells and not for the spermatic, and,
moreover, if the interstitial cells* are really the
source of the hormone on which the somatic char-
acters depend, they must be acting in the grafts in
which the nerve connections have been all severed.

The facts concerning cryptorchidism, that is to
say, failure of the descent of the testes in Mammals,
seem to show that the hormone of the testis is not
derived from semen or spermatogenesis, for in the
testes which have remained in the abdomen there is
no spermatogenesis, while the interstitial cells are
present, and the animals in some cases exhibit
normal or even excessive sexual instinct, and all the
male characteristics are well marked. It may be
remarked, however, in criticism of this conclusion
that the descent of the testes being itself a somatic
sexual character of the male, its failure when the
interstitial cells are normal and the spermatic cells
defective, would rather tend to prove that the
defect of the latter is itself the cause of cryptor-
chidism.

Many investigators have found that the Rontgen

ON SOMATIC SEX-CHARACTERS 95

rays destroy the spermatic cells of the testis in
Mammals, leaving the cells of Sertoli, the interstitial
tissue, nerves, and vessels uninjured. Tandler and
Gross 1 found that the antlers of roebuck were not
affected after the testes had been submitted to the
action of the rays, showing that the interstitial
cells were sufficient to maintain the normal condi-
tion of the antlers. Simmonds,2 however, found that
isolated seminal tubules remained, and regeneration
took place, and concludes that both spermatic cells
and interstitial cells take part in producing the testis
hormone. The conclusions of two other investi-
gators have an important bearing on this question-
namely, that of Miss Boring 3 that there is no inter-
stitial tissue in the bird's testis, and that of Miss
Lane-Claypon,4 that the interstitial cells of the ovary
arise from the germinal epithelium, and are perfectly
equipotential with those which form the ova and
Graafian follicles. It seems possible, although no
such suggestion has been made, that the interstitial
cells might either normally or exceptionally give
rise to ova and spermatocytes. The observations
of Seligmann and Shattock on the relation of sperma-
togenesis to the development of nuptial plumage in
drakes probably receive their explanation from the
above facts. Spermatogenesis is not the only
source of the testicular hormone : changes in the
secretory activity of the interstitial cells or sperma-
tocytes are sufficient to account for periodic de-
velopment of somatic sex-characters, and the same
reasoning applies to the antlers of stags.

1 Wiener klinische Wochenschrift, 1907.

2 Fortschr. a. d. O. d. Kdntgenstr., xiv., 1909-10.

3 Biol Bull., xxiii. 1912. « Proc. Roy. Soc., 1905.

96 INFLUENCE OF HORMONES

THE MAMMARY OR MILK GLANDS

The milk glands in Mammals constitute one of the
most remarkable of secondary sexual characters.
Except in their functional relations to the primary
organs, the ovaries, and to the uterus, there is
nothing sexual about them. They are parts of the
skin, being nothing more or less than enormous
enlargements of dermal glands, either sebaceous
or sudoriparous. Uterine and mammary functions
are generally regarded as essentially female char-
acteristics, and are included in the popular idea of
the sex of woman. Scientifically, of course, they are
not at all necessary or universal features of the
female sex, but are peculiar to the mammalian class
of Vertebrates in which they have been evolved.
Milk glands, then, are somatic sex-characters common
to a whole class, instead of being restricted to a
family like the antlers in Cervidae. There is not
the slightest trace or rudiment of them in other
classes of Vertebrates, such as Birds or Reptiles.
They are not actually sexual in their nature, since
their function is to supply food for the young, not to
play a part in the relations of the sexes. What is
sexual about them is — firstly, that they are normally
fully developed only in the female, rudimentary in
the male ; secondly, that their periodical develop-
ment and functional activity depends on the changes
which take place in the ovary and uterus. Many
investigators have endeavoured to discover the
nature of the nexus between the latter organs and
the milk glands.

That this nexus is of the nature of a hormone is
generally agreed, and may be regarded as having

ON SOMATIC SEX-CHARACTERS 97

been proved in 1874 when Goltz and Ewald l
removed the whole of the lumbo-sacral portion of
the spinal cord of a bitch and found that the
mammae in the animal developed and enlarged in
the usual way during pregnancy and secreted milk
normally after parturition. Ribbert 2 in 1898 trans-
planted a milk gland of a guinea-pig to the neighbour-
hood of the ear, and found that its development
and function during pregnancy and at parturition
were unaffected. The effective stimulus, therefore,
is not conveyed through the nervous system, but
must be a chemical stimulus passing through the
vascular system.

Physiologists, however, are not equally in agree-
ment concerning the source of the hormone which
regulates lactation. Starling and Miss Lane-Claypon
concluded from their experiments on rabbits that
the hormone originated in the fetuses themselves
within the pregnant uterus. In virgin rabbits it is
difficult to find the milk glands at all. When found
the nipple is minute and sections through it show the
gland to consist of only a few ducts a few millimetres
in length. Five days after impregnation the gland
is about 2 cm. in diameter. Nine days after im-
pregnation the glands have grown so much that the
whole inner surface of the skin of the abdomen is
covered with a thin layer of gland tissue. In six
cases by injecting subcutaneously extracts of foetus
tissue Starling and Lane-Claypon obtained a certain
amount of growth of the milk glands. The hormone
in the case of the pregnant rabbit is of course acting
continuously for the whole period of pregnancy,
while the artificial injection took place only once

1 Pfliigers Archiv, ix., 1874. 2 Fortschritte der Medicin, Bd. 7.

G

98 INFLUENCE OF HORMONES

in twenty-four hours, and the amount of hormone it
contained may have been absorbed in a very short
time. The amount of growth obtained experi-
mentally in five weeks was less than that occurring
in pregnancy in nine days. Extracts of uterus,
placenta, or ovary produced no growth, although the
ovaries used were taken from rabbits in the middle
of pregnancy. In one experiment ovaries from a
pregnant rabbit were implanted into the peritoneum
of a non-pregnant rabbit, but on post-mortem
examination of the latter eleven days later the im-
planted ovaries were found to be necrosed and no
proliferation of milk gland had taken place.

The conclusions of Starling and Lanfc-Claypon were
confirmed by Foa,1 and by Biedl and Konigstein.2
Foa states that extracts of foetuses of cows also pro-
duced swelling of the mammae in a virgin rabbit.

O'Donoghue, however, concludes from a study of
the Marsupial Dasyurus that the stimulus which acts
upon the milk glands proceeds from the corpora
lutea in the ovary. In this animal changes in the
pouch occur in pregnancy, which are doubtless also
due to hormone stimulation, but which we will not
consider here. The most important evidence in
O'Donoghue' s paper 3 is that development of the milk
glands takes place after ovulation not succeeded by
pregnancy ; that is to say, when corpora lutea are
formed but no fertilised ova or foetus are present in the
uterus. In one case, eighteen days after heat, the
milk gland was in a condition resembling that found
in the stages twenty-four and thirty-six hours after

1 Archivio d. Fisiologia, v., 1909.

2 Zeitschrift /. exp. Path, und Therap., 1910.
8 Quart. Journ. Mic. Sci., Ivii., 1911-12.

ON SOMATIC SEX-CHARACTERS 99

parturition. In another specimen, twenty-one days
after heat, the milk glands were still more advanced,
with distended alveoli and enlarged ducts. The
alveoli contained a secretion which was almost
certainly milk. O'Donoghue states that the entire
series of growth changes in these animals up to
twenty-one days after heat is identical with that
which occurs in normally pregnant animals.

O'Donoghue' s conclusion is in agreement with
that of Basch,1 who states that implantation of the
ovaries from a pregnant bitch under the skin of the
back of a one-year-old bitch that was not pregnant
was followed by proliferation of the mammary
glands of the latter. After six weeks the glands
were considerably enlarged, and after eight weeks
they were caused to secrete milk by the injection of
extract of the placenta. It has to be remembered,
however, that the milk glands undergo considerable
growth, especially in the human species, at puberty
and at every menstruation, or at oestrus in animals,
which corresponds to menstruation. In these cases
there is no question of any influence of the foetus, and
experiment has shown that if the ovaries are removed
before puberty, neither the milk glands nor the uterus
undergo the normal development, and menstruation
does not occur. According to Marshall and Jolly 2
the symptoms of oestrus in castrated bitches were
found to result from the implantation of ovaries
from other individuals in the condition of oestrus.

Before considering further the question of the
corpora lutea as organs of internal secretion, we may
briefly refer to the origin and structure of these

1 Monatsschr. f. Kinderh. V., No. ix., Dec. 1909.

2 Quart. Journ. Exp. Phys., i. and ii., 1908.

100 INFLUENCE OF HORMONES

bodies and of other parts of the mammalian ovary.
The mature follicle containing the ovum differs from
that of other Vertebrates in the fact that it is not
completely filled by the ovum and the follicular cells
surrounding it, but there is a cell-free space of large
size into which the ovum covered by follicular cells
projects. In the wall of the follicle two layers are
distinguished, the theca externa, which is more
fibrous, and the theca interna, which is more cellular.
In the connective tissue stroma of the ovary between
the follicles are scattered, or in some cases aggregated,
epithelioid cells known as the interstitial cells, and
it is stated that the cells of the theca interna are
exactly similar to the interstitial cells. According
to Limon 1 and Wallart 2 the interstitial cells are
actually derived from those of the theca interna of
the follicles. Numbers of ova die without reaching
maturity, the follicular cells degenerate, and the
follicle becomes filled with the cells of the theca
interna, which have a resemblance to those of the
true corpus luteum. These degenerate follicles have
been termed spurious corpora lutea, or atretic
vesicles. The interstitial cells are the remains of
these atretic vesicles. The true corpora lutea arise
from follicles in which the ova have become mature
and from which they have escaped through the sur-
face of the ovary. As a result of the escape of the
ovum and the contents of the cell-free space, the
follicle contracts and the follicular (so-called granu-
losa) cells secrete a yellow substance, lutein, and
enlarge. Buds from the theca interna invade the
follicle and form the connective tissue of the corpus
luteum.

1 Arch. d'Anat. micr., v., 1902. 2 Arch. f. Gynoek, vi. 271.

ON SOMATIC SEX-CHARACTERS 101

Somewhat similar processes take place in the ovaries
of Teleostean fishes, as I know from my own obser-
vations, but no corpora lutea are formed in these,
although the degenerating follicles in course of
absorption correspond to corpora lutea. The spawn-
ing of Fishes, usually annual, corresponds to ovula-
tion in Mammals, and in the ovary after spawning the
numerous collapsed follicles containing the follicular
cells may be seen in all stages of absorption.1 At
other times of the year sections of the ovary show here
and there ova which after developing to a certain
stage die and undergo absorption with their follicles.

In the higher Mammals (Eutheria) the corpora
lutea show a special relation in their development to
the occurrence of pregnancy, that is to say, they
have a different history when ovulation is followed
by pregnancy to that which they have when the ova,
from the escape of which they arise, are not fertilised.
When fertilisation occurs the corpus luteum in-
creases in size during the first part of the period of
gestation (four months, or nearly a half of the whole
period in the human species). It then remains
without much change till parturition, after which it
shrinks and is absorbed. When pregnancy does not
occur the corpus luteum is formed, but begins to
diminish within ten or twelve days in the human
species and is then gradually absorbed. According
to O'Donoghue, in the Marsupial Dasyurus there
seems to be no difference either in the development
of the milk glands or of the corpora lutea between the
pregnant and the non-pregnant animal. Sandes 2

1 Cunningham, 'Ovaries of Teleosteans.' Quart. Journ. Mic. Sci.,
vol. xl. pt. i., 1897.

2 Proc. Lin. Soc., New South- Wales, 1903.

102 INFLUENCE OP HORMONES

showed that in the same species the corpora lutea
persisted not only during the whole of pregnancy,
which Professor J. P. Hill x estimates at a little over
eight days, but during the greater part of the period
of lactation, which according to the same authority
is about four months. In the specimens of Dasyurus
described by O'Donoghue, in which the milk glands
developed after ovulation without ensuing preg-
nancy, normally developed corpora lutea were
present in the ovary. Of the five females which he
mentions, the first three, one with unfertilised ova
in the uteri, two five and six days after heat, could
not have been pregnant, but the other two killed
eighteen and twenty-one days after heat might, since
pregnancy lasts only eight days, have been pregnant,
the young having died at parturition or before. To
make certain on this point it would have been neces-
sary to examine the ovaries and milk glands of females
which had been kept separate from a male the whole
time. There is no doubt, however, about the de-
velopment of the milk glands in the first three
specimens, which were certainly not pregnant.

It is difficult to reconcile entirely the evidence
described by O'Donoghue from Dasyurus, with that
obtained from higher Mammals, although on the
whole there is reason to conclude that the corpora
lutea have an important influence on the develop-
ment of the milk glands. According to Lane-
Claypon and Starling, if the ovaries and uteri are
removed from a pregnant rabbit before the fourteenth
day the development of the mammary gland ceases,
retrogression takes place, and no milk appears in the
gland. If, on the other hand, the operation be

1 Anat. Anz.t xviii., 1900.

ON SOMATIC SEX-CHARACTERS 103

performed after the fourteenth day, milk appears
within two days after the operation. It is to be
concluded from this that the cause of secretion of
milk is the withdrawal of a stimulus proceeding from
ovary or uterus. But O'Donoghue believes that
milk is secreted in Dasyurus when no pregnancy has
occurred. Ancel and Bouin l have shown that the
growth of the mammary glands was produced in
rabbits by the artificial rupture of egg follicles
and consequent production of corpora lutea : the
growth of the glands continued up to the fourteenth
day, after which regression set in. This shows that
the development of the milk glands in rabbits is due
to the corpora lutea. On the other hand, Lane-
Claypon and Starling state that in rabbits the
corpora lutea diminish after the first half of preg-
nancy, while the growth of the milk glands is many
times greater during the second half than during
the first half of the period, and during the second
half the ovaries may be removed entirely without
interfering with the course of pregnancy or the
normal development of the milk glands. It is
evident, therefore, that in rabbits, whatever influence
the corpora lutea may have in the first half of preg-
nancy, they have none in the second half, and that
at this period the essential hormone proceeds from
the developing fcetus or foetal placenta. Again, if
it is the withdrawal of a hormone stimulus which
changes the milk gland from growth to secretion, it
cannot be the corpora lutea which are exclusively
concerned even in Dasyurus, for they persist during
lactation, while secretion begins shortly after par-
turition.

1 C. R. Soc. de Biol, t. Ixvii., 1909.

104 INFLUENCE OF HORMONES

Gustav Born suggested, and Frankel tested the
suggestion experimentally, that the corpus luteum
of pregnancy is a gland of internal secretion whose
function is to cause the attachment of the ovum in
the uterus and the normal development of uterus
and placenta. Frankel found that removal of both
ovaries in rabbits between the first and sixth days
after fertilisation prevented pregnancy, and that the
same result followed if the corpora lutea were
merely destroyed in situ by galvano-cautery. Either
process carried out between the eighth and twentieth
days of pregnancy causes abortion.

Lane-Claypon and Starling also found that re-
moval of both ovaries in the rabbit before the
fifteenth day was apt to cause abortion, but at a
later stage the same operation could be performed
without interfering with the course of pregnancy.
According to these authors numberless instances
prove that in women double ovariotomy does not
necessarily interfere with the course of pregnancy or
the development of the milk glands. Parturition
may take place and be followed by normal lactation.
This shows that a hormone from the corpora lutea is
not necessary either to the uterus or the milk glands,
at any rate in the last third of pregnancy, though
of course this does not prove that such a hormone is
not necessary for the earlier stages both of pregnancy
and growth of the milk glands.

The results of Steinach, if confirmed, would prove
conclusively that the ovaries and testes produce
hormones which determine the development of all
the sexual characters, not merely physical but
psychical. He adopts the view that the interstitial
cells or gland are the source of the active hormone.

ON SOMATIC SEX-CHARACTERS 105

He claims by transplantation of the gonads in young
rats and guinea-pigs to have feminised males and
masculised females. The females are smaller, and
have finer, softer hair than the males. The testes
were removed and ovaries implanted in young males.
The animals so treated grew less than the merely
castrated specimens, and therefore when full-grown
resembled females in size. In the young state both
sexes have fine, soft hair, the feminised males had the
same character, like the normal females. They also
developed teats and milk glands like the females,
and were sought and treated as females by the normal
males. When the implanted ovaries are able to
resist the influence of their new surroundings, the
female interstitial gland, which Steinach calls the
puberty gland, develops so much that an intensifica-
tion of the female character takes place : the animals
are smaller than normal females, the milk glands
develop and secrete milk, which can be easily
pressed out, and if young are given to them they
suckle them and show all the maternal instincts.

Why the ovary in normal circumstances only
when in the gravid condition calls forth this per-
fection of f emaleness is to be shown in a later publica-
tion. By acting with Rontgen rays on the region
where the ovaries lie, Steinach and his colleague
Holzknecht brought about all the symptoms of
pregnancy, development of teats and milk glands,
secretion of milk, and great growth of the uterus in
all its layers.

Masculising of females was much more difficult
than feminising of males because the testicular tissue
was less resistent, and could not be grafted so easily.
When it succeeded, however, degeneration of the

106 INFLUENCE OP HORMONES

seminal tubules took place, with increase of the
interstitial or Ley dig's cells. The vaginal opening in
rats disappeared, partly or completely. The sexual
instincts became male, the animals recognised a
female in heat from one that was not, and attempted
to copulate.

Steinach considers that he has proved from these
results that sex is not fixed or predetermined but
dependent on the puberty gland. By sex here he
obviously means the instincts and somatic characters,
for sex in the first instance, as we have already
pointed out, means the difference between ovary and
testis, between ova and spermatozoa. It is difficult
to accept all Steinach' s results without confirma-
tion, especially those which show that the feminised
male is more female than the normal female. Such
a conclusion inevitably suggests that the investigator
is proving too much.

The subject of the influence of hormones from the
gonads is mentioned, but not fully discussed, in a
volume by Dr. Jacques Loeb, entitled The Organism
as a Whole.1 Loeb entirely omits the problem of the
origin of somatic sex-characters, and fails to perceive
that the fact that such characters are dependent to a
marked degree on hormones derived from the gonads,
together with their relation to definite habits and
functions connected with the behaviour of the sexes
to each other, is proof that these characters are not
gametogenic, but were originally due to external
stimulation of particular parts of the soma.

1 Putnam's Sons, 1916.

CHAPTER IV
ORIGIN OF SOMATIC SEX-CHARACTERS IN EVOLUTION

IN his Mender s Principles of Heredity, 1909, Bateson
does not discuss the nature of somatic sex-characters
in general, but appears to regard them as essential
sex - features, as male OT female respectively. As
mentioned above, he argues from the fact that injury
or disease of the ovaries may lead to the develop-
ment of male characters in the female, that the
female is heterozygous for sex, and from the supposed
fact that castration of the male leads merely to the
non-appearance of male somatic characters, that the
female sex-factor is wanting in the male. He does
not distinguish somatic sex-characters from primary
sex-factors, and discusses certain cases of heredity
limited by sex as though they were examples of the
same kind of phenomenon as somatic sex-characters
in general. One of these cases is the crossing by
Professor T. B. Wood of a breed of sheep horned in
both sexes with another hornless in both sexes. In
the F! generation the males were horned, the females
hornless. Here, with regard to the horned character,
both sexes were of the same genetic composition,
i.e. heterozygous, or if we represent the possession of
horns by H, and their absence by h, both sexes were
Hh. Thus Hh$ was horned and Hh% was hornless,
or, as Bateson expresses it, the horned character was
dominant in males, recessive in females. Bateson

107

108 ORIGIN OF SOMATIC

offers no explanation of this, but it obviously
suggests that some trace of the original dimorphism
of the sheep in this character was retained in both
horned and hornless breeds. We may suppose that
the factor for horns had disappeared entirely from
the hornless sheep by a mutation, but in the horned
breed another mutation had been a weakening of the
influence of the sexual hormones on the development
of the character, which, as in all such cases, is really
inherited in both sexes. In the F19 when the horned
character in the female is only inherited from one
side, the hereditary tendency is not enough to
overcome the influence of the absence of the testis
hormone and presence of the ovarian -hormone, and
so the horns do not develop. The Mendelian merely
sees a relation of the character to sex, but overlooks
entirely the question of the dimorphism in the
original species from which the domesticated breeds
are descended. Similarly, with regard to cattle
where it has been found that hornlessness is domi-
nant or nearly so in both sexes, no reference is
made to the opposite fact that wild cattle have
horns in both sexes and are not dimorphic in this
character.

Bateson proceeds to consider colour-blindness as
though its heredity were of similar kind. He refers
to it as a male character latent in the female, and
remarks that we should expect that disease or re-
moval of the ovaries might lead to the occasional
appearance of colour-blindness in females. He also
discusses the case of Abraxas grossulariata and its
variety lacticolor, and other cases of sex-linked
heredity, apparently with the idea that all such
cases are similar to those of sexual dimorphism. A.
lacticolor occurs in nature only in the female sex, and

SEX-CHARACTERS IN EVOLUTION 109

when bred with grossulariata <J produces £'s and $'s
all grossulariata, these of course being heterozygous.
When the Fl grossulariata $ was bred with the wild
lacticolor $ it produced both forms in both sexes,
and thus lacticolor $ was obtained for the first time.
When this lacticolor $ was bred with Fl grossulariata
$ it produced all the <J's grossulariata and all the $'s
lacticolor. Bateson's explanation is that the female,
according to the Mendelian theory of sex, is hetero-
zygous in sex, the male homozygous and recessive, and
that lacticolor is linked with the female sex-character,
grossulariata being repelled by that character. Thus
we have, the lacticolor character being recessive,

tact, cf , LL<3<3 X F, gross.Q GL^6
Gametes L$ + L<5 X G6 + L<^

| B

r n

GL tfcf LL 90

gross. Cf tact. 9

It will be seen that although in the progeny of this
mating all the grossulariata were males and all the
lacticolor females, yet this case is by no means
similar to that of sexual dimorphism in which the
characters are normally always confined to the same
sex. For the lacticolor character in the parent was
in the male, while in the offspring it was in the female.
We cannot say here that in the theoretical factors
which are supposed to represent what happens, the
lacticolor character is coupled with the female sex-
factor, for we find it with the male sex-character in
the lacticolor $. It is so coupled only in the hetero-
zygous grossulariata $, and at the same time the
grossulariata character is repelled.

110 ORIGIN OP SOMATIC

According to Doncaster 1 sex-limited, or as it is now
proposed to call it sex-linked, transmission in this
case means that the female grossulariata transmits
the character to all her male offspring and to none of
the female, while a heterozygous male grossulariata
mated with lacticolor female transmits the character
equally to both sexes : that is to say, the heredity is
completely sex-limited in the female but not at all in
the male. This is evidence that the female produces
two kinds of eggs, one male producing and the other
female producing.

With regard to the ordinary form of colour-
blindness, Bateson's first explanation was that it was
like the horns in the cross-bred sheep, dominant in
males, recessive in females. About 4 per cent, of
males in European countries are colour-blind, but
less than \ per cent, of females. Affected males
may transmit the defect to their sons but not to
their daughters : but daughters of affected persons
transmit the defect frequently to their sons. Bateson
gives2 a scheme of the transmission, but corrects
this in a note stating that colour-blindness does not
descend from father to son, unless the defect was
introduced by the normal sighted mother also, i.e.
was carried by her as a recessive. The fact that
unaffected males do not transmit the defect shows,
according to Bateson, that it is due to the addition
of a factor to the normal, not to omission of a factor.

According to later researches as quoted by
Doncaster, colour-blindness is due to the loss of some
factor which is present in the normal individual.
The normal male is heterozygous for this normal
factor. If we denote the presence of the normal

1 Determination of Sex, Camb. Univ. Press, 1914.

2 Menders Principles of Heredity, 1909.

SEX-CHARACTERS IN EVOLUTION 111

factor by N and its absence or recessive by n, then the
male is Nn, while the female is homozygous or NN.
But in addition to this it is the male in this case
which is heterozygous for sex, and n goes to the
male-producing sperms, N to the female-producing.
Thus in the mating of normal man with normal
woman the transmission is as follows : —

Gametes n d

Nn (5 X NN 9
9 x

d /v/v 9

That is all offspring normal, but the males again
heterozygous.

An affected male has the constitution nn, and if he
marries a normal woman the descent is as follows : —

nn<$ x /WV9
Gametes n. d + /z o x /V + /V

When a normal male is mated with a heterozygous
nN female we get

X /2/V9

Gametes

n<3 + /V

'£ X n + N
1

r~

1

T "I

/2/V9 /w

that is, hah° the sons are normal and half colour-blind,

112 ORIGIN OF SOMATIC

while half the females are homozygous and normal,
and the other half heterozygous and normal.

T. H. Morgan1 has observed a number of cases
of sex-linked inheritance in the mutations which
occurred in his cultures of Drosophila. The eye of
the wild original fly is red, one of the mutants has a
white eye, i.e. the red colour and its factor are absent.
When a white-eyed male is mated to a red-eyed
female all the offspring have red eyes. If these are
bred inter se, there are, as in ordinary Mendelian
cases, three red-eyed to one white-eyed in the F2
generation, but white eyes occur only in the males,
in other words half the males are white-eyed. On
the other hand, when a white-eyed female is mated to
a red-eyed male all the daughters have red eyes, and
all the sons white eyes. This has been termed criss-
cross inheritance. If these are bred together the result
in F2 is equal numbers of red-eyed and white-eyed
females, and equal numbers of red-eyed and white-
eyed males. The ration of dominant to recessive is 2
to 2 instead of the usual Mendelian ration of 3 to 1.

According to Morgan the interpretation is as
follows : In the nucleus of the female gametocytes
there are two X chromosomes related to sex, in those
of the male there is one X chromosome and one Y
chromosome of slightly different shape. The factor
for red eye occurs in the sex-chromosomes, that is to
say, according to this theory, the sex-chromosome
does not merely determine sex but carries other
factors as well, and this fact is the explanation of
sex-linked inheritance. The factor for red eye then
is present in both X chromosomes of the wild
female, absent from both X and Y chromosomes

1 A Critique of the Theory of Evolution.

SEX-CHARACTERS IN EVOLUTION 113

of the white-eyed male. The gametes of the female
each carry one X red chromosome, of those of the
male half carry an X white chromosome, and half
the Y white chromosome. The fertilised female ova
therefore carry an X red chromosome + an X white
chromosome, the male producing ova one X red
chromosome and one Y white chromosome. They
are all therefore red-eyed, but heterozygous — that
is, the red eye is due to one red-eye factor, not two.
When the Ft are bred together, half the female
gametes carry one X red chromosome, the other half
one X white chromosome ; half the male gametes
carry one X red chromosome, the other half one Y
white chromosome. The fertilisations are therefore
one X red X red, one X red X white, one X red Y
white, and one X white Y white. These last are the
white-eyed males. The two different crosses are
represented diagrammatically on p. 114, the dark
rod representing the X red chromosome, the clear
rod the X white chromosome, and the bent clear
rod the Y white chromosome.

According to Morgan, the heredity of colour-
blindness in man is to be explained exactly in the
same way as that of white eye in Drosophila. A
colour-blind man married to a normal (homozygous)
woman transmits the peculiarity to half his grand-
sons and to none of his grand-daughters. Colour-
blind women are rare, but in the few cases known
where such women have married normal husbands
the defect has appeared only in the sons, as in the
second of the diagrams on p. 1 1 4. It must be explained
that according to this theory the normal male is
always heterozygous, because the Y chromosome
never carries any other factor except that for sex ;

H

114

ORIGIN OF SOMATIC

RED EYED

PARENTS

F.I

F.2

II 01 Iff OC?

fiEDEYEDQ REOEYEOQ ffEO EYE0*<S WH/TE EYEO Q

Homozygous. Heterozygous. Heterozygous. Homozygous,

WH/TE fYEO O R£0 EYED (5

00. x Iff

ftED EYED

WHfTEEYED (5

Off

F.I

00 10 Off, Iff,

WH/TEEYEDQ RED EYED Q WH/TE EVEO <J #ED EYED<J

Homozygous. Heterozygous. Homozygous. Heterozygous,

SEX-CHARACTERS IN EVOLUTION 115

it is thus of no more importance than the absence
of an X chromosome which occurs in those cases
where the male has one sex - chromosome and
the female two. According to the researches of
von Winiwarter1 on spermatogenesis in man, the
latter is actually the case in the human species.
This investigator found that there were 48 chromo-
somes in the female cell, 47 in the male ; after
the reduction divisions the unfertilised ova had
24 chromosomes, half the spermatids 24 and half
23, so that sex is determined in man by the
spermatozoon.

Morgan believes that the heredity of haemophilia
(the constitutional defect which prevents the spon-
taneous cessation of bleeding) follows the same
scheme, and also at least some forms of stationary
night-blindness — that is, the inability to see in
twilight.

We may mention a few other cases in animals,
referring the reader for a fuller account to the works
cited. One example is the barred character of the
feathers in the breed of fowls called Plymouth Rock.
In this case the female is heterozygous for sex as in
Abraxas grossulariata, and the barred character is
sex-linked. When a barred hen is crossed with an
unbarred cock all the male offspring are barred, all
the females plain. On the other hand, if a barred
cock is crossed with an unbarred hen, the barred
character appears in all the offspring, both males and
females. The female thus transmits the character
only to her sons. If we represent the barred char-
acter by By and its absence by 6, we can represent
the heredity as follows : —

1 ' Spermatogen^se humaine,' Arch, de Biol, xxvii., 1912.

116

ORIGIN OF SOMATIC

BARRED FEMALE WITH UNBARRED MALE
X

Barred male.
Heterozygous.

B6 B6

Unbarred female
Homozygous.

Barred female.
Heterozygous.

Barred male.
Heterozygous.

This case is thus exactly similar to that of Abraxas
grossulariata and A. lacticolor. The barred character,
like grossulariata^ is dominant, the unbarred recessive,
and to explain the results it is necessary to assume
that the female is not only heterozygous for the
barred character, but also for sex, with the female
sex-factor dominant. The recessive character in
this case is linked to the female sex-chromosome,
or, as Bateson described it, the dominant character is
repelled by the female sex-factor. We may make a
diagram of the kind given by Morgan if we use a
rod of different shape for the female-producing sex-

SEX-CHARACTERS IN EVOLUTION 117

chromosome, and use the black rod for the dominant
character : —

Heterozygous.

UNBAftRED C?

00

Heterozygous,

Another case is that of tortoise-shell, i.e. black
and yellow cats. The tortoise-shell with very rare
exceptions is female, the corresponding male being
yellow, without any black colour. Doncaster found
that a yellow male mated to a black female produced
black male offspring and tortoise-shell females.

ORIGIN OF SOMATIC

When a black male is mated to a yellow female, the
female kittens are tortoise-shell as before, but the
males yellow. The Mendelian hypothesis which
explains these results is that the male is always
heterozygous, or has only one colour factor whether
yellow or black, and transmits these colours only
to his daughters, while the female has two
colour factors, either BB, 77, or BY. Thus the
crosses are: —

TORTOISE SH£LL

The sex must be determined therefore by the
spermatozoa, as in the case of colour-blindness, etc.,
in man, and the colour factor must always be in the
female-producing sperm.

SEX-CHARACTERS IN EVOLUTION 119

SEXUAL DIMORPHISM

It is obvious from the above facts that however
interesting and important sex-linked heredity may be,
it is not the same thing as the heredity of secondary
sexual characters, and does not in the least explain
sexual dimorphism. In the first place, the term
sex-linked does not mean occurring always ex-
clusively in one sex, but the direct contrary — trans-
mitted by one sex to the opposite sex — and in the
second place there is no suggestion that the develop-
ment of the character is dependent in any way on
the presence or function of the gonad. The problem
I am proposing to consider is what light the facts
throw on the origin of the secondary sexual characters
in evolution. In endeavouring to answer this ques-
tion there are only two alternatives: either the
characters are blastogenic — that is, they arise from
some change in the gametocytes occurring some-
where in the succession of cell-divisions of these
cells — or they arise in the soma and are impressed
on the gametocytes by the influence of the soma
within which these gametocytes are contained —
that is to say, they are somatogenic. That characters
do originate by the first of these processes may be
considered to be proved by recent researches, and
such characters are called mutations. There can be
little doubt that the so-called sex-linked characters,
of which examples have been given above, have
originated in this way, and that their relation to sex
is part of the mutation. According to T. H. Morgan,
it is simply due to the fact that the determinants for
such characters are situated in the sex-chromosome.
Morgan, however, also states that a case of true

120 ORIGIN OF SOMATIC

sexual dimorphism arose as a mutation in his
cultures of Drosophila. The character was eosin
colour in the eye instead of the red colour of the eye
in the original fly. In the female this was dark
eosin colour, in the male yellowish eosin. But this
case differs from the characters particularly under
consideration here in two points: (1) there is no
suggestion that it was adaptive, (2) or that it was
influenced by hormones from the gonads.

No character whose development is dependent in
greater or less degree on the stimulation of some
substance derived from the gonads can have origin-
ated as a mutation, because the term mutation
means a new character which develops in the soma
as a result of the loss or gain of some factor or de-
terminant in the chromosomes. To say that certain
mutations consist of new factors which only cause
the development of characters in the soma when
the part of the soma concerned is stimulated by a
hormone, is a mere assertion unsupported at present
by any evidence. As an example of the way in
which Mendelians misunderstand the problem to be
considered, I may refer to Doncaster's book, The
Determination of Sex,1- in which he remarks : 4 It
follows that the secondary sexual characters cannot
arise simply from the action of hormones ; they
must be due to differences in the tissues of the body,
and the activity of the ovary or testis must be re-
garded rather as a stimulus to their development
than as their source of origin.' This seems to imply
a serious misunderstanding of the idea of the action
of the hormones from the gonads and of hormones
in general. No one would suggest that the hormones

1 Carub. Univ. Press, 1914, p. 99.

SEX-CHARACTERS IN EVOLUTION 121

from the testis should be regarded as in any sense
the origin of the antlers of a stag. If so, why should
not antlers equally develop in the stallion or in the
buck rabbit, or indeed in man ? How far Doncaster
is right in holding that the soma is different in the
two sexes is a question already mentioned, but it is
obvious that in each individual the somatic sexual
characters proper to its species are present potentially
in its constitution by heredity — in other words, as
factors or determinants in the chromosomes of the
zygote from which it was developed ; but the normal
development of such characters in the individual
soma is either entirely dependent on the stimulus
of the hormone of the gonad or is profoundly in-
fluenced by the presence or absence of that stimulus.
The evidence, as we have seen, proves that, at any
rate in the large number of cases where this relation
between somatic sex-characters and hormones pro-
duced by the reproductive organs exists, the char-
acters are inherited by both sexes. In one sex they
are fully developed, in the other rudimentary or
wanting. But the sex, usually the female, in which
they are rudimentary or wanting is capable of trans-
mitting them to offspring, and also is capable of
developing them more or less completely when the
ovaries are removed, atrophied or diseased. If
we state these facts in the terms of our present
conceptions of chromosomes and determinants or
factors, we must say that the factors for these
characters are present in the chromosomes of both
male and female gametes. The question then is,
how did these factors arise ? If they were muta-
tions not caused by any influence from the exterior,
what is the reason why these particular characters

122 ORIGIN OF SOMATIC

which alone have an adaptive relation to the sexual
or reproductive habits of the animal are also
the only characters which are influenced by the
hormones of the reproductive organs ? The idea
of mutations implies neither an external relation
nor an internal relation in the organ or character;
but these characters have both, the external relation
in the function they perform in the sexual life of
the individual, the internal relation in the fact that
their development is affected by the sexual hor-
mones. There is no more striking example of the
inadequacy of the current conceptions of Mendelism
and mutation to cover the facts of bionomics and
evolution.

The truth is that facts and experiments within
a somewhat narrow field have assumed too much
importance in recent biological research. No
increase in the number of facts or experimental
results of a particular class will compensate for the
want of sound reasoning and a comprehensive
grasp of the phenomena to be explained. The co-
existence of the external and the internal relation in
the characters we are considering suggests that one
is the cause of the other, and as it is obvious that the
relation for instance of a stag's antlers to a testicular
hormone could not very well be the cause of the use
of the antlers in fighting, the reasonable suggestion
is that the latter is the cause of the former. We have
already seen that the development and shedding
of the antler are processes of essentially the same
kind physiologically, or pathologically, as these
which can be and are occasionally produced in the
individual soma by mechanical stimulus and injury
to the periosteum. The fact that a hormone from

SEX-CHARACTERS IN EVOLUTION 123

the testis affects the development of the antler, as
well as our knowledge of hormones in general,
suggests a special theory of the heredity of somatic
modifications due to external stimuli. Physiologists
are apt to look for a particular gland to produce
every internal secretion. But the fact that the wall
of the intestine produces secretion, which carried by
the blood causes the pancreas to secrete, shows that
a particular gland is not necessary. There is nothing
improbable in supposing that a tissue stimulated to
excessive growth by external irritation would give
off special substances to the blood. We know that
living tissues give off waste products, and that these
are not merely pure C02 and H20, but complicated
compounds. The theory proposed by me in 1908
was that we have within the gonads numerous
gametocytes whose chromosomes contain factors
corresponding to the different parts of the soma,
and that these factors or determinants might be
stimulated by waste products circulating in the
blood and derived from the parts of the soma corre-
sponding to them. There is no reason to suppose
that an exostosis formed on the frontal bone as a
result of repeated mechanical stimulation due to the
butting of stags would give off a special hormone
which was never formed in the body before, but it
would probably in its increased growth give off an
increased quantity of intermediate waste products
of the same kind as the tissues from which it arose
gave off before. These products would act as a
hormone on the gametocytes, stimulating the factors
which in the next generation would control the
development of the frontal bone and adjacent
tissues.

124 ORIGIN OF SOMATIC

The difficulty of this theory is one which has
occurred to biologists who have previously made
suggestions of a connexion between hormones and
heredity — namely, how hormones or waste products
from one part of the body could differ from these
from the same tissue in another part of the body.
If there were no special relation, hypertrophy of
bone on one part of the body such as the head would
merely stimulate the factor for the whole skeleton in
the gametocytes, and the result would merely be an
increased development of the whole skeleton. On
the other hand, we have the evident fact that a
number of chromosomes formed apparently of the
same substance, by a series of equal chromosome
divisions determine all the various special parts of
the complicated body. This is not more difficult to
understand than that every part of the body should
give off special substances which would have a
special effect on the corresponding parts of the
chromosomes. We know that skin glands in dif-
ferent parts of the body produce special odours,
although all formed of the same tissue and all
derived from the epidermis. It seems not impossible
that bones of different parts of the body give off
different hormones. If the factors in the gametes
were thus stimulated they would, when they de-
veloped in a new individual, produce a slightly
increased development of the part which was
hypertrophied in the parent soma. No matter
how slight the degree of hereditary effect, if the
stimulation was repeated in every generation, as in
the case of such characters as we are considering
it undoubtedly was, the hereditary effect would
constantly increase until it was far greater than the

SEX-CHARACTERS IJJ EVOLUTION 125

direct effect of the stimulation. We may express
the process mathematically in this way. Suppose
the amount of hypertrophy in such a case as the
antlers to be x, and that some fraction of this is
inherited. Then in the second generation the same
amount of stimulation together with the inherited

/y*

effect would produce a result equal to x+-. The

n

latter fraction being already hereditary, a new

fraction - would be added to the heredity in each

n

generation, so that after m generations the amount
of hereditary development would be #+— . If n

were 1000, then after 1000 generations the inherited
effect would be equal to x. This, it is true, would not
be a very rapid increase. But it is possible that the

/y»

fraction - • would itself increase, for the heredity
7i

might very well consist not only in a growth in-
dependent of stimulation, but in an increasing
response to stimulation, so that x itself might be

/y«

increasing, and the fraction - would become larger

in each generation. The death and loss of the skin
over the antler, originally due to the laceration of
the skin in fighting, has also become hereditary, and
it is certainly difficult to conceive the action of
hormones in this part of the process. All we can
suggest is that the hormone from the rapidly growing
antler, including the covering skin, is acting on the
corresponding factor in the gametocytes for a
certain part of every year, and then, when the skin
is stripped off, the hormone disappears. The factor
then may be said to be stimulated for a time and then

126 ORIGIN OF SOMATIC

the stimulus suddenly ceases. The bone also begins
to die when the skin and periosteum is stripped off,
and the hormone from this also ceases to be pro-
duced.

The annual shedding and recrescence of the
antler, however, is only to be understood in connexion
with the effect of the testicular hormone. According
to my theory there are two hormone actions, the
centripetal from the hypertrophied tissue to the
corresponding factor in the gametocytes, and the
centrifugal from the testis to the tissue of the antler
or other organ concerned. The reason why the
somatic sexual character does not develop until the
time of puberty, and develops again each breeding
season in such cases as antlers, is that the original
hypertrophy due to external stimulation occurred
only when the testicular hormone was circulating in
the blood. The factor in the gametocytes then was
in each generation acted upon by both hormones,
and we must suppose that in some way the result
was produced that the hereditary development of the
antler in the soma only took place when the testicular
hormone was present. It is to be remembered that
we are unable at present to form a clear conception
of the process of development, to understand how
the simple fertilised ovum is able by cell-division
and differentiation to develop into a complicated
organism with organs and characters predetermined
in the single cell which constitutes the ovum. If we
accept the idea that characters are represented by
particular parts of the chromosomes, according to
Morgan's scheme, our theory of development is the
modern form of the theory of preformation. When
in the course of development the cells of the head

SEX-CHARACTERS IN EVOLUTION 127

from which the antlers arise are formed, each of these
cells must be supposed to contain the same chromo-
somes as the original ovum from which the cells have
descended by repeated cell-division. The factors in
these chromosomes corresponding to the forehead
have been stimulated while in the parent animal
by hormones from the outgrowth of tissue produced
by external mechanical stimulation, while at the
same time they were permeated by the testicular
hormone produced either by the gametocytes them-
selves or by interstitial cells of the testis. When the
head begins to form in the process of individual
development, the factors, according to my theory,
have a tendency to form the special growth of tissue
of which the incipient antler consists, but part of
the stimulus is wanting, and is not completed until
the testicular hormone is produced and diffused into
the circulation — that is to say, when the testes are
becoming mature and functional.

I do not claim that this theory is complete — it is
impossible to understand the process completely in
the present state of knowledge — but I maintain that
it is the only theory which affords any explanation
of the remarkable facts concerning the influence of
the hormones from the reproductive organs on the
development of secondary sexual characters, while
at the same time explaining the adaptive relation of
these characters or organs to the sexual habits of the
various species. On the mutation hypothesis, adapta-
tion is purely accidental. T. H. Morgan considers
that the appearance of two slightly different shades of
eye colour in male and female in a culture of a fruit-
fly in a bottle is sufficient to settle the whole problem
of sexual dimorphism, and to supersede Darwin's

128 ORIGIN OF SOMATIC

complicated theory of sexual selection. The possi-
bility of a Lamarckian explanation he does not even
mention. He would doubtless assume that the
antlers of stage arose as a mutation, without ex-
plaining how they came to be affected by the testi-
cular hormone, and that when they arose the stags
found them convenient as fighting weapons. But
the complicated adaptive relations are not to be
disposed of by the simple word mutation. The
males have sexual instincts, themselves dependent
on the testicular hormone, which develop sexual
jealousy and rivalry, and the Euminants fight by
butting with their heads because they have no
incisor teeth in the upper jaw, or tusks, which
are used in fighting in other species. Doubtless,
mutations have occurred in antlers as in other char-
acters ; in fact all hereditary characters are subject
to mutation. This is the most probable explanation,
not only of the occasional occurrence of hornless
individual stags, but of the differences between the
antlers of different species, for there is no reason to
believe that the special character of the antler in
each species is adapted to a special mode of fighting
in each species.

The different structure of the horns of the Bovine
and Ovine Ruminants is, in my view, the result of a
different mode of fighting. If we suppose that the
fighting was slower and less fierce in the Bovidae,
so that the skin over the exostosis was subject to
friction but not lacerated, the result would be a
thickening of the horny layer of the epidermis as we
find it, and the fact that the skin and periosteum are
not destroyed explains why the horns are not shed
but permanent.

SEX-CHARACTERS IN EVOLUTION 129

There is a tendency among Mendelians and
mutationists to overestimate the importance of
experiments in comparison with reasoning, either
inductive or deductive. Bateson, however, has
admitted that Mendelian experiments and observa-
tions on mutation have not solved the problem of
adaptation. It seems to be demanded, nevertheless,
that characters must be produced experimentally
and then inherited before the hereditary influence
of external stimuli can be accepted. Kammerer's
experiments in this direction have been sceptically
criticised, and it must be granted that the evidence
he has published is not sufficient to produce complete
conviction. But experiments of this kind are from
the nature of the case difficult if not impossible.
There is, however, another method — namely, to take
a character which is certainly to some extent heredi-
tary, and then to ascertain by experiment if it is
' acquired.' If it be proved that a hereditary
character was originally somatogenic, it follows that
somatogenic characters in time become hereditary.
This is the reasoning I have used in reference to my
experiments on the production of pigment on the
lower sides of Flat-fishes, and I obtained similar
evidence with regard to the excessive growth of the
tail feathers in the Japanese Tosa-fowls,1 which is a
modification of a secondary sexual character. In
these fowls the feathers of the tail in the hens are only
slightly lengthened.

I learned from Mr. John Sparks, who himself
brought specimens of the breed from Japan, that the
Japanese not only keep the birds separately on high

1 'Observations and Experiments on Japanese Long-tailed Fowls,'
Proc. Zool. Soc., 1903.

I

130 ORIGIN OF SOMATIC

perches in special cages, but pull the tail feathers
gently every morning in order to cause them to grow
longer. One question which I had to investigate on
my specimens, hatched from eggs obtained from
Mr. Sparks, was the relation of the growth of the
feathers to the moult which occurs in ordinary birds.
My experiment consisted in keeping two cocks, A
and B, the first of which was left to itself, while in
the second the feathers were gently pulled by stroking
between the finger and thumb from the base out-
wards. The feathers in the tail were seven pairs of
rectrices, two rows of tail coverts, anterior and
posterior, four or five pairs in each row, a number
of transition feathers : all these were steel-blue,
almost black ; in front of them on the saddle were a
number of reddish yellow, very slender saddle hackles.

In September 1901, when the birds were just over
three months old, the adult feathers of the tail were
all growing. The growing condition can be distin-
guished by the presence of a horny tubular sheath
extending up the base of the feather for about one
inch. When growth ceases this sheath is shed. In
cock A growth continued till the end of the following
March, when the longest feathers, the central rectrices,
were 2 feet 4J inches long. One of the feathers —
namely, one of the anterior tail coverts — was acci-
dentally pulled out on llth February 1902, when it
was 15 J inches long and had nearly ceased to grow
and formed its quill, and it immediately began to
grow again and continued to grow till the following
September, when it was accidentally broken off at
the base : it was then 18 inches (44-5 cm.) long.

The effect of stroking in cock B was to pull out
from time to time one of the growing feathers. Of

SEX-CHARACTERS IN EVOLUTION 131

the original feathers, one, the left central posterior
covert, continued to grow till 13th July 1902, when it
was 2 feet 9J inches long without the part contained
in the follicle. All the feathers pulled out immediately
commenced to grow again, except the last two pulled
out 27th May and 13th July, which did not grow again
till the following moulting season, in September.

The first right central rectrix in cock B was
accidentally pulled out on 13th April 1902, when it
was 2 feet 9J inches long. Its successor began to
grow immediately, and in course of time pieces of it
were broken off accidentally without injury to the
base in the socket, which continued to grow until
16th June 1905, when it was torn out of its socket.
The total length of the feather with the pieces
previously broken off, which were measured and
preserved, was 11 feet 5J inches. It therefore
continued to grow without interruption for three
years and two months at an average rate of 3*6
inches per month.

In cock A only four of the short outer rectrices
were moulted in the beginning of September 1902 :
the longer feathers — namely, central rectrices and
tail coverts — which ceased to grow naturally in the
spring of 1902, were not moulted till the beginning of
October. This shows the great importance of pulling
out the feathers as soon as they show signs of ceasing
to grow, in order to obtain the abnormally long
feathers. The central rectrices continued to grow
till the beginning of September 1903, when that of the
left side was 3 feet 6 inches long, that of the right about
an inch shorter. The coverts had ceased to grow of
their own accord some time before this, and the central
ones of the posterior row were about 3 feet long.

132 ORIGIN OF SOMATIC

As it seemed possible that there was some natural
congenital difference in growth of feathers between
cocks A and B, I commenced early in March 1903 to
pull and stroke the feathers of the left side only in
cock A, leaving those of the right side untouched.
On 30th July on the left side the central rectrix and
the first and second posterior coverts were still
growing, on the right side the central rectrix was also
growing, but the first and second posterior coverts
had ceased growth and formed their quills. The first
posterior covert on the left or pulled side was 3 inches
longer than that of the right. The second posterior
covert on the left side was still longer. The first and
second posterior coverts of left side did not cease
growth till 26th August. On 2nd September the
left central rectrix was almost at the end of its
growth, the right had ceased to grow a little before.
The left was about an inch longer than the right.
Thus both in length and in duration of growth the
feathers of the pulled side were longer than those of
the right, and this was the result of treatment con-
tinued only six months, and commenced some
months after the feathers had begun to grow. I
have no doubt, however, that the pulling out of the
feather as soon as it shows signs of forming quill, so
that its successor at once grows again, is even more
important in producing the great length of feather
than the stroking of the feather itself.

In this case, then, there is no doubt (a) that
the long-tailed birds are artificially treated with the
utmost care and ingenuity by the Japanese, who
produced them; (b) that the mechanical stimulus
in my experiments did cause the feathers to grow
for a longer period and attain greater length;

SEX-CHARACTERS IN EVOLUTION 133

(c) that the tendency to longer growth is, even when
no treatment is applied, distinctly inherited. It is a
legitimate and logical conclusion that the inherited
tendency is the result of the artificial treatment.
No other breed of fowls shows such excessive growth
of tail feathers. It may be admitted that individuals
differ considerably in their congenital tendency
to greater growth, i.e. greater length of the tail
feathers, but according to my views this is not
contradictory to the main conclusion, for every
hereditary character shows individual variation.

It may be pointed out here that on the Lamarckian
theory the conception of adaptations is not teleo-
logical : they do not exist for a certain purpose, but
are the result of external stimulations arising from
the actions and habits of the organism. The latter
conception is the more general, for cases of somatic
sexual characters exist which cannot be said to have
a use or function. For example, the comb and
wattles of Oallus are sexually dimorphic, being in the
original species larger in the cock than in the hen.
There is no convincing evidence that these ap-
pendages are either for use or ornament. They are,
in fact, a disadvantage to the bird, being used by
his adversary to take hold of when he strikes. The
first thing that happens when cocks fight is the bleed-
ing and laceration of the comb, as they peck at each
other's heads. This laceration of the skin is, in my
view, the primary cause of the evolution of these
structures, leading to hypertrophy. But in this,
as in other cases, the hereditary result is regular,
constant, and symmetrical, while the immediate
effect on the individual is doubtless irregular.

CHAPTER V

MAMMALIAN SEXUAL CHARACTERS. EVIDENCE
OPPOSED TO THE HORMONE THEORY

PERHAPS the most remarkable of all somatic
sexual characters are those which are almost uni-
versal in the whole class of Mammalia, the mammary
glands in the female, the scrotum in the male. We
have considered the evidence concerning the re-
lation of the development and functional action of
the milk glands to hormones arising in the ovary
or uterus, now we have to consider the origin of the
glands and of their peculiar physiology in evolution.
The obvious explanation from the Lamarckian point
of view, and in my opinion the true one, is that they
owed their origin at the beginning to the same
stimulation which is applied to them now in every
female mammal that bears young. There is, as we
have seen, a difficulty in explaining how the occurrence
of parturition causes the secretion of milk to begin,
but it is certain that the secretion soon stops if the
milk is not drawn from the glands by the sucking
action of the offspring, or the artificial imitation of
that action. A cow that is not milked or milked
incompletely ceases to give milk. When the
stimulus ceases, lactation ceases. The pressure
of the secretion in the alveoli causes the cells to
cease to secrete, much in the same way that pressure
in the ureters injures the secretory action of the renal

131

MAMMALIAN SEXUAL CHARACTERS 135

epithelium. In the earliest Mammals we may
suppose that the young were born in a well-de-
veloped condition, for at first the supply of milk
would not have been enough to sustain them for a
long time as their only food. We must also suppose
that the mother began to cherish the young, keeping
them in contact with her abdomen. Then being
hungry they began to suck at her hair or fur. The
actual development of the milk glands in Marsupials
has been described by Bresslau1 and by O'Donoghue.2
The rudiment of the teat is a depression or invagina-
tion of the epidermis from the bottom of which six
stout hairs arise. The follicles of these hairs extend
down into the derma, and from the upper end of the
follicle, i.e. near the aperture of the invagination, a
long cellular outgrowth extends down into the derma,
branches at its end, and becomes hollow. These
branches are the tubules of the future milk gland.
Another outgrowth from the follicle forms a sebaceous
gland. Later on the hairs and the sebaceous glands
entirely disappear, and the milk gland alone is left
with its tubules and ducts opening into the cavity
of the teat. This is clear evidence that the milk
gland was evolved in connexion with hairs, and was
an enlargement of glands opening into the hair
follicle, but it is difficult to understand why a seba-
ceous gland is developed and afterwards disappears.
This would seem to indicate that the milk gland was
not a hypertrophied sebaceous gland, but a distinct
outgrowth, which however had nothing to do with
sweat glands.

That the intra-uterine gestation, or its cessation,
were not originally necessary to determine the

1 Stuttgart, 1901. • Q.J.M.S., Ivii., 1911-12.

136 MAMMALIAN SEXUAL CHARACTERS

functional periodicity of the milk glands is proved
by their presence in the Monotremes, which are
oviparous. It is evident from the conditions in
these mammals that both hair and milk glands
were evolved before the placenta.

It may also be pointed out here that, according
to the evidence of Steinach, in the milk glands at
least among somatic sexual characters there is no
difference between the male and female in the
heredity of the organs. The zygote therefore,
whether the sex of it is determined as male or female,
has the same factor for the development of milk
glands. On the chromosome theory as formulated
by Morgan this factor must be in the somatic chromo-
somes and not in the sex-chromosomes, and must be
present in every zygote. All the cells of the body,
assuming that somatic segregation does not occur,
must possess the same chromosomes as the zygote
from which it developed, and whether the sex
chromosomes are XX or XT or X, there must be
at any rate one chromosome bearing the factor for
milk glands. The functional development of these
depends normally, according to the evidence hitherto
discovered, on the presence or absence of hormones
from the ovary or from the uterus.

If we attribute, as in my opinion we must, the
primary origin of the milk glands in evolution to
the mechanical stimulus of sucking, we may attempt
to reconstruct the stages of the evolution of the
present relation of the glands to the other organs
and processes of reproduction. In the earliest stage
represented by the Monotremata or Prototheria,
there was no intra-uterine development. We must
suppose that in the beginning the sucking stimulus

MAMMALIAN SEXUAL CHARACTERS 137

caused both growth and secretion, for at first there
was nothing but sebaceous or sweat glands, and
although a mutation might be supposed to have
produced larger glands, no mutation could explain
the influence of hormones on the growth and function
of such glands. Then heredity of the effect of stimulus
took place to some slight degree, and this would occur,
according to my theory, only in the presence of the
hormone from the ovary in the same condition as that
in which the modification was first caused. This would
be of course after ovulation, and after hatching of
the eggs. In the next stage, if we adopt the modern
view that Marsupials are descended from Placental
Mammals, the eggs would be retained for increasing
periods in the uteri, and would be born in a well-
developed condition, since lactation would demand
active sucking effort on the part of the young. The
early Placentalia would inherit from the Monotreme-
like ancestors the development of the milk glands
after ovulation, although no sucking was taking
place while the young were inside the uterus. It
seems probable that the relation between parturi-
tion and actual milk secretion originated with the
sucking stimulus of the young after birth.

There is good evidence that the secretion of milk
may continue almost indefinitely under the stimulus
of sucking or milking. Neither menstruation nor
gestation put an end to it. Cows may continue to
give milk until the next parturition, and if castrated
during lactation will continue to yield milk for years.
Women also may continue to produce milk as long
as the child is allowed to suck, and this has been in
some cases two or three years or even more. More-
over, lactation may be induced by the repeated act

138 MAMMALIAN SEXUAL CHARACTERS

of sucking without any gestation. This has happened
in mares, virgin bitches, mules, virgin women, and in
one woman lactation was continued uninterruptedly
for forty-seven years, to her eighty-first year, long
after the ovary had ceased to be functional. Lacta-
tion has also been induced in male animals, e.g. in a
bull, a male goat, male sheep, and in men.1 We may
conclude, therefore, that the secretion of milk
normally begins by heredity after parturition, and
this, in accordance with what we have learned
about hormones in connexion with the reproductive
system, is probably the consequence of the with-
drawal of the hormone absorbed from the foetus. I
do not think it is necessary to suppose, as do Lane-
Claypon and Starling, that the hormone physio-
logically inhibits the dissimilative process and aug-
ments the assimilative, and that the withdrawal of
the hormone at parturition therefore causes the
dissimilative process, i.e. secretion of milk. My
conclusion is that the process of secretion set up by
the mechanical stimulus of sucking is inherited as it
was acquired, so that it only begins to take place in
the individual in the absence of the hormone from
the foetus, which was absent when the process was
acquired. The growth of the gland during gestation
would then be due to the postponement of the process
of secretion in consequence of the presence of the
foetal hormone, and in this way this hormone has be-
come in the course of evolution at once the stimulus
to growth and the cause of the inhibition of secretion.
This interpretation does not, however, agree with
the case of Dasyurus. If the foetal hormone is

1 Knott, 'Abnormal Lactation,' American Medicine, vol. ii. (new
series), 1907.

MAMMALIAN SEXUAL CHARACTERS 139

absorbed from the pouch, as I have suggested in order
to explain the persistence of the corpora lutea during
lactation, then the secretion of milk after parturition
ought not to take place. But in this case the sucking
stimulus has been applied to the glands after a very
short gestation, while the hormone from the foetus
is being absorbed in the pouch, and therefore the
hereditary correlation between secretion and absence
of foetal hormone may be assumed to have been lost
in the course of evolution.

We have next to consider the question of the
evolution of the corpora lutea. If these bodies are
formed only in Mammals which have uterine gesta-
tion, and not in Prototheria, they cannot be the only
essential source of the hormone which stimulates the
development of the milk glands, since the latter
develop in Prototheria. Again it is difficult, it
might be said impossible, to believe that an acci-
dental mutation gave rise to corpora lutea the
secretion of which caused uterine gestation and
ultimately the formation of the placenta. It seems
more probable that the retention of the originally
yolked ova within the oviduct, however this retention
arose, was the essential cause of the formation of the
placenta and all the changes which the uterus under-
goes in gestation. The absorption of nutriment
from the walls of the uterus, and the chemical and
mechanical stimulation of those walls, might well be
the cause of the diversion of nutrition from the
ovary, leading gradually to the decline of the process
of secretion of yolk in the ova.

The conceptions and the mode of reasoning of the
physiologist are very different from those of the
evolutionist. The former concludes from certain

140 MAMMALIAN SEXUAL CHARACTERS

experiments that a given organ of internal secretion
has a certain function. The corpora lutea, for
example, according to one theory are ductless glands,
the function of whose secretion is to establish the
ova in the uterus and promote their development.
Another function suggested for the secretion of the
corpora lutea is to prevent further ovulation during
pregnancy. The evolutionist, on the other hand,
asks what was the origin of the corpora lutea, why
should the ruptured ovarian follicles after the
escape of the ova in Mammals undergo a progressive
development and persist during the greater part or
the whole of pregnancy ? It seems obvious that the
corpora lutea in evolution were a consequence of
intra-uterine gestation, for they occur only in
association with this condition, and it is impossible
to suppose that a mutation could arise accidentally
by which the ruptured follicles should produce a
secretion which would cause the fertilised ova to
develop within the oviducts. The developing ovum
within the uterus may, however, reasonably be
supposed to give off something which is absorbed
into the maternal blood, and this something would
be of the same nature as that which was given off
by the ovum while still within the ovarian follicle.
The presence of this hormone might cause the
follicular cells to behave as though the ovum was
still present in the follicle, so that they would
persist and not die and be absorbed. But this
leaves the question, what is lutein and why is it
secreted ? Lutein is a colouring matter sometimes
found in blood-clots, and probably derived from
haemoglobin. In the corpus luteum the lutein is
contained in the cells, not in a blood-clot.

MAMMALIAN SEXUAL CHARACTERS 141

Chemical investigation shows that the lutein of
the corpus luteum is almost if not quite identical with
the colouring matter of the yolk in birds and reptiles.
Escher 1 found that the lutein of the corpus luteum
had the formula C40H56 and was apparently identical
with the carotin of the carrot, while the lutein of
egg-yolk was C40H5602 and more soluble in alcohol,
less soluble in petroleum ether, than that of the
corpus luteum. The difference, if it exists, is very
slight, and it is evident that one compound could
easily be converted into the other. Moreover, the
hypertrophied follicular cells which constitute the
corpus luteum secrete fat which is seen in them in
globules. The similarity of their contents therefore
to yolk is very remarkable, and it may be suggested
that the hormones absorbed from the ovum or
embryo in the uterus acts upon the follicular cells in
such a way as to cause them to secrete substances
which in the ancestor were passed on to the ovum
and formed the yolk. It may be urged that this
idea is contradictory to the previous suggestion that
the absorption of nourishment by the intra-uterine
embryo was the cause of the gradual decline of the
process of yolk-secretion by the ova in the ovary,
but it is not really so. Originally in the reptilian
ancestor, or in the Monotreme, the ovum in the
follicle secreted yellow-coloured yolk. The materials
for this, at any rate, passed through the follicle cells,
and it is probable that these cells were not entirely
passive, but actively secretory in the process.
Substances diffusing from the ovum would be
present in the follicle cells during this process, and
probably act as a stimulus. The same substances

1 Ztschr. f. Physiol Chem., 83 (1912).

142 MAMMALIAN SEXUAL CHARACTERS

diffusing from the ovum during its development in
the uterus would continue to stimulate the follicle
cells, and thus explain not merely their persistence,
but their secretory activity. The ovum being no
longer present in the ovary, the secretions would
remain in the follicular cells, and the corpus luteum
would be explained.

If this theory is sound, it would follow that corpora
lutea are not formed in cases where the ova are not
retained in the oviduct during their development.
The essential process in the development of these
structures is the hypertrophy and, in some cases at
least, multiplication of the follicular cells in the
ruptured follicle. I have already mentioned that
this process does not occur in Teleosteans whose
ovaries were studied by me. These were species
of Teleosteans in which fertilisation is external.
Marshall, in his Physiology of Reproduction,1 quotes
a number of authors who have published observations
on the changes occurring in the ruptured follicle in
the lower Vertebrata, and also in the Monotremes.
According to Sandes,2 in the latter there is a pro-
nounced hypertrophy of the follicular epithelium
after ovulation, but no ingrowth of connective
tissue or blood-vessels from the follicular wall.
Marshall himself examined sections of the corpus
luteum of Ornithorhynchus and saw much hyper-
trophied and apparently fully developed luteal
cells, but no trace of any ingrowth from the wall of
the follicle. This fact would appear to be quite
inconsistent with the theory above proposed, but

1 London, 1910, p. 151.

2 'The Corpus Luteum of Dasyurus/ Proc. Lin. Soc.t New South
Wales, 1903.

MAMMALIAN SEXUAL CHARACTERS 143

it must be remembered that the ovum of Monotremes
is known to remain for a short period in the oviduct,
or in other words to pass through it very slowly, and
to absorb fluid from its walls, as shown by the
considerable increase in size which the ovarian
ovum undergoes before it is laid. It would bfc*in-
teresting to know how long the rudimentary corpus
luteum persists in Ornifhorhynchus : the period,
according to my views, should be very short. It
is remarkable that in the results quoted by Marshall
a well-developed corpus luteum was found and
exclusively found in the lower Vertebrates which are
viviparous. For example, among fishes in the Elas-
mobranchs Myliobatis and Spinax ; in Teleosteans,
in Zoarces ; in Reptiles, in Anguis and Seps. Biihler,
on the other hand, confirmed my own negative
result with regard to oviparous Teleosteans, and also
found no hypertrophy of the follicle in Cyclostomes
which are also oviparous. In the viviparous forms
mentioned there is yolk in the ovum which is re-
tained in oviduct or ovary, but additional nutriment
is also absorbed from the uterine or ovarian walls.
In these cases there is no placenta and generally no
adhesion of ovum or embryo to walls of oviduct or
ovary. These facts alone would be sufficient to
disprove the theory that the corpora lutea are organs
producing a secretion whose function is to cause the
attachment of the embryo to the uterine mucosa.
It is also, in my opinion, unreasonable to suppose
that the rudimentary corpora lutea of lower vivi-
parous Vertebrates arose as a mutation the result
of which was to cause internal development of
the ovum. Habits might easily bring about reten-
tion of the fertilised ova for gradually increasing

144 MAMMALIAN SEXUAL CHARACTERS

periods,1 and the correlation between the retained
developing ova and the hypertrophy of the ruptured
follicles is comprehensible on my theory of the
influence of substances absorbed by the walls of
oviduct or ovary from the developing ovum.

The case of Dasyurus, however, seems inconsistent
with this argument, for, as previously mentioned,
Sandes found that in this Marsupial the corpora
lutea persisted during the greater part of the period
of lactation, which continues for four months after
parturition. During the whole of this time there
are no embryos in the uteri, and therefore it might
be urged absorption of hormones from the embryos
cannot be the cause of the persistence of corpora
lutea in pregnancy. But it seems* to me that a
complete answer to this objection is supplied by the
peculiar relations of the embryos to the pouch in
Dasyurus and other Marsupials. The skin of the
pouch while the embryos are in it is very soft, con-
gested, and glandular ; at the same time the embryos
when transferred to the pouch at parturition are
very small, immature, and have a soft delicate skin.
The relation of embryos to pouch in Dasyurus, there-
fore, is closely similar to that of embryos to uterus
after the first few days of pregnancy in the Eutheria.
It is true there is no placenta, but the mouths of the
embryos are in very close contact with the teats, and
both the skin of the embryos and that of the pouch
are soft and moist. If any special substances are
given off by the embryos in the uterus in ordinary
gestation, the same substances would continue to be

1 According to Geddes and Thomson (Evolution of Sex, 1889), the
common grass-snake has been induced under artificial conditions to
bring forth its young alive.

MAMMALIAN SEXUAL CHARACTERS 145

given off by the embryos in the marsupial pouch,
and these must be absorbed by the skin of the
pouch. In this way it seems to me we have a logical
explanation of the fact that the corpora lutea in the
Marsupial are not absorbed at parturition as in
Eutheria. As Sandes says the ' greater part of the
period of lactation,' it would appear that absorption
of the corpora lutea takes place when the young
Dasyurus have grown to some size, become covered
with hair, and are able to leave the teats or even the
pouch at will. Under these conditions it is obvious
that diffusion of chemical substances from the
young through the walls of the pouch would come to
an end. It would be interesting in this connexion to
know more of the relation of egg and embryo to the
pouch and to the corpora lutea in Echidna. In
Ornithorhynchus the eggs are hatched in a nest
and there is no pouch.

On this view that the corpora lutea are the result,
not the cause, of intra-uterine gestation, it would no
longer be possible to maintain the theory that the
corpus luteum in the human species is the cause by
its internal secretion of the phenomenon of menstrua-
tion. This was the theory of Born and Frankel.1
Biedl's conclusion is that the periodic development
and disintegration of the uterine mucous membrane
in the menstrual cycle is due to the hormone of the
interstitial cells of the ovary. Leopold and Ravana
found that ovulation as a rule coincides with
menstruation, but may take place at any time.
Here, again, the problem must be considered from
the point of view of evolution. It can scarcely be
doubted that the thickening and growth of the

1 See Biedl, Internal Secretory Organs (Eng. trans.), 1912, p. 404.

K

146 MAMMALIAN SEXUAL CHARACTERS

mucous membrane in the menstrual cycle is of the
same nature as that which takes place in pregnancy.
When the ovum or ova are not fertilised the develop-
ment comes to an end after a certain time, differing
in different species of Mammals, and the membrane
sloughs, returns to its original state, and then begins
the same process of development again.

Menstruation, then, must be interpreted as an
abortive parturition, both in woman and lower
Mammals, though in the latter it is not usually
accompanied by hemorrhage, and is called pro-oestrus.
The question then to be considered is, what deter-
mines parturition and menstruation ? The presence
of the fertilised ovum must have been the original
cause of the hypertrophy of the 'uterine mucous
membrane, and in its congenital or hereditary de-
velopment the chemical substances diffusing from
the ova in the uterus or even in the Fallopian tube
may well be the stimulus starting the hypertrophy.
But what determines the end of the pregnancy ?
Is it merely the increasing distension of the uterus
by the developing foetus ? This could scarcely be
the case in the Marsupials in which the foetus when
born is quite minute. Nor can we attribute parturi-
tion to renewed ovulation, for this occurs in Dasyurus
only once a year. All we can suggest at present is
that a certain periodic development takes place by
heredity in presence of the hormones exuded by the
fertilised ovum and the embryo developed from it.
When the ovum or ova, not being fertilised, die, the
period of development is (usually) shortened and
pro-oestrus or menstruation occurs. In the dog,
however, the period of the oestrous cycle is about the
same as that of gestation — namely, six months.

MAMMALIAN SEXUAL CHARACTERS 147

The so-called descent of the testicles occurs ex-
clusively in Mammals, in which with a few important
exceptions it is universal. This is a very remarkable
case of the change of position of an organ in the
course of development. The original position of the
testis on either side is quite similar to that of the
same organ in birds or reptiles. The genital ridge
runs along the inner edge of the mesonephros, with
which the testicular tubules become connected.
The testis, with the mesonephros, forming the epi-
didymis, closely attached to it, projects into the
ccelom, and without losing its connexion with the
peritoneum changes its position gradually during
development, passing backwards and downwards
until it comes to lie over the wall of the abdomen just
in front of the pubic symphysis of the pelvic girdle.
There the abdominal wall on either side of the middle
line becomes thin and distended to form a pouch,
the scrotal sac, into which the testis passes, still
remaining attached to the peritoneum which lines
the pouch, while the distal end of the vas deferens
retains its original connexion with the urethra. The
movement of the testis can thus be accurately
described as a transposition or dislocation.

Various causes have been suggested for the forma-
tion of the scrotum, but no one has ever been able to
suggest a use for it. It has always been quite im-
possible to bring it within the scope of the theory of
natural selection. The evolution of it can only be
explained either on the theory of mutation or some
Lamarckian hypothesis. The process of dislocation
of the testis does not conform to the conception of
mutation, nor agree with other cases of that pheno-
menon. A mutation is a change of structure affecting

148 MAMMALIAN SEXUAL CHARACTERS

more or less the whole soma, but showing itself
especially in some particular organ or structure.
But I know of no mutation occurring under observa-
tion which consisted, not in a change of structure or
function, but merely in a change of position of an
organ from one part of the body to another, and
moreover a change which takes place by a con-
tinuous process in the course of development. If
the testes were developed from the beginning in a
different part of the abdomen, there might be some
reason in calling the change a mutation. Moreover,
if it is a mutation, why has it never occurred in any
other class of Vertebrates except Mammals ?

In 1903 Dr. W. Woodland published 1 a
Lamarckian theory of this mammalian feature, the
probability of which it seems to me has been increased
rather than decreased by the progress of research
concerning heredity and evolution since that date.
Dr. Woodland correlated the dislocation of the testes
with the special mechanical features of the mode of
locomotion in Mammalia. His words are : ' The
theory here advocated is to the effect that the
descent of the testes in the Mammalia has been pro-
duced by the action of mechanical strains causing
rupture of the mesorchial attachments, such strains
being due to the inertia of the organs reacting to the
impulsiveness involved in the activity of the animals
composing the group.' The ' impulsiveness ' is the
galloping or leaping movement which is characteristic
of most Mammals when moving at their utmost
speed, as seen, for example, in horses, deer, antelopes,
dogs, wolves, and other Ungulata and Carnivora.
It is obvious that when the body is descending to

1 Proc. Zool Soc., 1903, Part i.

MAMMALIAN SEXUAL CHARACTERS 149

the ground after being hurled upwards and forwards,
the abdominal organs have acquired a rapid move-
ment downwards and forwards; when the body
reaches the ground its movement is stopped
suddenly, while the abdominal organs continue to
move. The testes therefore are violently jerked
downwards away from their attachments and at the
same time forward. The check to the forward
movement, however, is momentary, while the body is
immediately thrown again upwards and forwards,
which by the law of inertia means that the testes are
thrown still more downwards and backwards. There
is no reason to suppose, as Dr. Woodland suggests,
that any rupture of the mesorchium was the usual
result of these strains, but a constant pull or tension
was caused in the direction in which the testes
actually move during development. On this theory
we have to consider (1) how such strains could cause
a shifting of the peritoneal attachment, (2) why the
testes should be supposed to be particularly affected
more than other abdominal organs. The answer
to the first question is that the strains would cause
a growth of the connecting membrane (mesorchium)
at the posterior end, accompanied by an absorption of
it at the anterior end. The answer to the second
question is that the testes are at once the most
compact and heaviest organs in the abdomen, and
at the same time the most loosely attached. The
latter statement does not apply to the mesonephros
or epididymis which has moved with the testis, but
the latter cannot function without the former, and
it may be supposed that the close attachment of the
epididymis to the testis had come about in the early
Mammalia before the change of position was evolved.

150 MAMMALIAN SEXUAL CHARACTERS

It is evident that the violent shocks of the gallop-
ing or leaping movement do not occur in Birds,
Reptiles, or Amphibia. Ostriches run very fast and
do not fly, but their progression is a stride with each
foot alternately, not a gallop. The Anura among
the Amphibia are saltatory, but their leaps are
usually single, or repeated only a few times, not
sustained gallops. The exceptions among the
Mammalia still more tend to prove the close corre-
spondence between the ' impulsive ' mode of pro-
gression and the dislocation of the male gonads.
In the Monotremata there is no scrotum, the testes
are in a position similar to that which obtains in
Reptiles, and they are the only Mammals in which
these organs are anterior to the kidneys. In
locomotion they are sluggish, there is no running or
galloping among them. Ornithorhynchus is aquatic
in its habits, and Echidna is nocturnal and moves
very slowly. In Marsupials the scrotum is in front
of the penis, but really in the same position as in
other Mammals — that is, in front of the ventral part
of the pelvic girdle. It is the penis which is different,
as the skin around the organ has not united in a
ventral suture below it, while the organ itself has not
grown forward adnate to the abdominal skin as in
most other Mammals. The scrotum is always
anterior to the origin of the penis, although in the
Eutheria apparently behind that organ. The larger
Marsupials like the kangaroos are eminently
saltatory, and the others are active in locomotion.
The aquatic Mammals Sirenia and Cetacea have no
scrotum, the testes being abdominal. It is un-
necessary to inquire whether this is the original
position, or whether they are descended from

MAMMALIAN SEXUAL CHARACTERS 151

ancestors which had a scrotum : in either case the
position of the testes corresponds to the absence of
what Dr. Woodland calls impulsiveness in progres-
sion. The Fissipedia offer an instructive example,
for while the Otariidae have the hind feet turned
forward and can move on land somewhat like
ordinary Mammals, the Phocidae cannot move their
hind legs independently or turn them forward, and
can only drag themselves about on land for short
distances. In the former the testes are situated in a
well-defined scrotum, in the latter these organs are
abdominal. The Phocidae are probably descended
from Mammals of the terrestrial type with a scrotum,
which has disappeared in the course of evolution.
Perhaps the most curious exception is that of the
elephants, in which the testes are abdominal. Here,
in consequence of then" structure and massive shape,
locomotion is usually a walk, and though they
run occasionally the gait is a trot, not a sustained
gallop, and leaping is out of the question. Sloths
which hang from branches upside down have ab-
dominal testes, but even here they are in a posterior
position, between the rectum and the bladder, so
there has apparently been a degree of dislocation,
probably inherited from ancestors with more ter-
restrial habits.

The fact that the ovaries do not occupy normally
a position similar to that of the testes is in accordance
with the theory, for they are very much smaller than
the testes; and yet they have undergone some
change of position, for they are posterior to the
kidneys.

The facts agree also with the hormone theory, for
it is to be noted that although the development of

152 MAMMALIAN SEXUAL CHARACTERS

the scrotum is confined to the males, the ' descent '
or dislocation takes place in the foetus, and not at
the period of puberty. This is in accordance with
the fact that the mechanical conditions to which the
change is attributed are not related to sexual habits,
but to the general habits of life which begin soon
after birth. The development, therefore, may be
considered to be related to the presence of a hormone
derived from the normal testis, but not to a special
quantity or quality of hormone associated with
maturity or the functional activity of the organ. In
Rodents, however, there is a difference in the organs,
not only at maturity, but in every rutting season,
at any rate in Muridae such as rats and others. In
the rutting season the testes become much larger and
descend into the scrota! sacs, at other times of the
year being apparently more or less abdominal. In
rabbits and hares, which have a much more im-
pulsive progression, the organs seem to be always in
the scrotal sacs.

It might be thought that in this case, although the
hormone theory of heredity might be applied, there
was no reason to suppose that a hormone derived
from the testis in the individual development was
necessary in order that the hereditary change
should take place. If the individual was male and
therefore had a testis, this organ would by heredity
go through the process of dislocation. But there is
the curious fact that when the descent is not normal
and complete, in what is called cryptorchidism, the
organs are always sterile. The retention of the
testes within the abdomen may be regarded as a
case of arrested development, like many other
abnormalities, but this does not explain why the

MAMMALIAN SEXUAL CHARACTERS 153

retained testes should always be sterile, without
spermatogenesis. If the inherited or congenital
process of dislocation requires the presence of
hormones produced by a normal testis, then we can
understand why a defective testis does not descend
completely, because it does not produce the hormone
which is necessary to stimulate the hereditary
mechanism to complete dislocation. It is often
stated that in cryptorchidic individuals the sexual
instincts and somatic sexual characters are well
developed, which would appear contradictory to the
above explanation, but according to Ancel and
Bouin such individuals in the case of the pig show
considerable differences in the secondary signs of sex
and in the external genital organs, presenting
variations which lie between the normal and the
castrated animal.

We have here, then, in the position of the testes in
Mammalia a condition which is not in the slightest
degree 'adaptive' in the ordinary sense — that is,
fulfilling any special function or utility. The
condition must be regarded as distinctly disad-
vantageous, since the organs are more exposed to
injury, and the abdominal wall is weakened, as we
know from the risk of scrotal hernia in man. But
from the Lamarckian point of view the facts support
the conclusion that the condition is the effect of
certain mechanical strains, and is of somatic origin,
while the correlations here reviewed are entirely
unexplained by any theory of mutation or blasto-
genic origin.

154 EVIDENCE OPPOSED TO

OPPOSING EVIDENCE

We have now to review certain cases which seem
to support conclusions contrary to those which we
have maintained in the preceding pages, and to
consider the evidence which has been published in
support of other theories. It must be admitted that
the occurrence of male secondary characters on one
side of the body, and female on the other, is incon-
sistent with the view that the development of such
characters is due to the stimulus of a hormone, since
the idea of a hormone means something which
diffuses by way of the blood-vessels, lymph- vessels,
and interstices of the tissues, throughout the body,
and the hormone theory of secondary sexual char-
acters assumes that these characters are potentially
present by heredity in both sexes. The occurrence
of male somatic characters on one side or in one part
of the body and female on the other, usually
associated with the corresponding gonads, has been
termed gynandromorphism, and has long been
known in insects. Cases of this condition have been
observed, though much more rarely, in Vertebrates.
I am not aware of any authentic instance in Mammals,
and the supposition that in stags reduction or
abnormality of one antler may be the result of
removal or injury to the testis of one side, or the
opposite, have been completely disproved by ex-
periments in which unilateral castration has been
carried out without any effect on the antlers at all.
In birds, however, a few cases have been recorded by
competent observers with a definiteness of detail
which leaves no possibility of doubt. One of the
more recent of these is that of a pheasant of the

THE HORMONE THEORY 155

white-ringed Formosan variety, P. torquaius, of the
Chinese pheasant.1 On the left side this bird shows
the plumage, colour, and the spur of the male; on
the right leg there is no spur except the small rudi-
ment normally occurring in the hen. The difference
in plumage between the two sides, however, is not
complete. The white collar is strictly limited to the
left side, but the iridescent blue green of head and
neck is present on both sides, though more marked
on the left. Only a few male feathers appear in the
wing coverts of the left side. The breast feathers
are rufous, especially on the left side. The tail
coverts show marked male characters, more especially
on the left side. In the tail, however, the barred
character of the male is not present on one side,
absent on the other, but in most of the feathers is
confined to one, the outer side of each feather. With
regard to the gonads, in this bird a single organ was
found on the left side, i.e. in the position of the
ovary in normal females, and there was no trace of
a gonad on the right side. The organ present was
small, | inch long by | inch broad, and micro-
scopic sections showed in one part actively
growing areas of tubular gland structure in some
of which bodies like spermatozoa could be de-
tected, while in another were fibrous tissue with
degenerating cysts. The latter appear to have
been degenerating egg follicles. The author con-
cludes that the organ was originally a functional
ovary, and that the ovarian portion had atrophied
while a male portion had become functionally
active.

1 C. J. Bond, ' Unilateral Development of Secondary Male Characters
in a Pheasant,' Journ. oj Genetics, vol. iii., 1914,

156 EVIDENCE OPPOSED TO

Another case in birds was described by Poll 1 and
is mentioned by Doncaster.2 It is that of a Bullfinch
which had the male and female plumage sharply
separated on the two sides of the body. The right
side of the ventral surface was red like a normal
male, the left side grey like a normal female. In this
case there was a testis on the right side, on the left
an ovary as in normal females.

A third case in birds, somewhat different from
the two first mentioned, is that of a domestic fowl
described by Shattock and Seligmann.3 It was a
bird of the Leghorn breed, two years old, and had
the fully developed comb and wattles of the cock.
Each leg bore a thick blunt spur, nearly an inch in
length, but in the Leghorn breed spurs are by no
means uncommon in hens of mature age, before they
have ceased to lay eggs. In plumage the characters
were mainly female. The colour being white could
not show sexual differences, the neck hackles were
but moderately developed, saddle hackles practically
absent, the tail resembled that of the hen. There
was a fully developed oviduct on the left side, on the
right another less than half the full length. There
was also a vas deferens on each side. There was a
gonad on each side, that of the right about one
fourth the size of that on the left. In microscopic
structure the right gonad resembled a testis con-
sisting entirely of tubuli lined by an epithelium con-
sisting of a single layer of cells. In one part of this
organ the tubules were larger than elsewhere, and
one of them exhibited spermatogenesis in progress.

1 S.B. Qes. Naturf. Freunde, Berlin, 1909.

2 Determination of Sex, Cambridge, 1914.

8 Trans. Pathol Soc. (London), vol. 57, Part i., 1906.

THE HORMONE THEORY 157

The left and larger gonad had a quite similar
structure, but at its lower end were found two ova
enclosed within a follicular epithelium.

With regard to the last case it is to be remarked
that though the gonad on the right side was entirely
male, there was no unilateral development of male
characters. With regard to the other two cases it
must be pointed out (1) that the difference between
the two somatic sex-characters on the two sides is
chiefly a difference of colour, except the difference in
the spurs in Bond's pheasant ; (2) that the evidence
already cited shows that in fowls castration does not
prevent the development of the colour and form
of the male plumage, nor of the spurs : that in
drakes, although castration does not seem to have
been carried out on young specimens before the male
plumage was developed, when performed on the
mature bird it prevents the eclipse, and does not
cause the male to resemble the hen. Castration,
then, tends to prove that in Birds the development
of the male characters is not so closely dependent on
the stimulation of testicular hormone as in Mammals.
The characters must therefore be developed by
heredity in the soma, which implies that the soma
must itself be differentiated in the two sexes. The
development must therefore be more in the nature
of gametic coupling. It does not follow that the
primary sex-character or the somatic characters are
exclusive in either sex. We may suppose that the
zygote contains both sexes, one or other of which is
dominant, and that dominance of one primary sex
involves dominance of the corresponding sexual
characters. This does not, however, agree with the
result of removal of the ovaries in ducks, for this

158 EVIDENCE OPPOSED TO

causes the characters of the male to appear, so that
the dominance of the female is not a permanent
condition of the soma but is dependent on the
ovarian hormone.

In the hermaphrodite individuals mentioned above
the difference of dominance is on two sides of the
body instead of two different individuals. It may
also be remarked here that while it is very difficult
to believe that spurs were not due in evolution to the
mechanical stimulation of striking with the legs in
combat, and while specially enlarged feathers are
erected in display, we cannot at present attribute the
varied and brilliant colour of male birds to the direct
influence of external stimuli.

In Lepidoptera among insects tKe evidence con-
cerning castration tends to prove that hormones
from the gonads play no part at all in the develop-
ment of somatic sexual characters. Kellog, an
American zoologist, in 1905 l described experiments
in which he destroyed by means of a hot needle the
gonads in silkworm caterpillars (Bonibyx mori), and
found no difference in the sexual characters of the
moths reared from such caterpillars. Oudemans
had previously obtained the same result in the Gipsy
Moth, Limantria dispar. Meisenheimer 2 made more
extensive experiments on castration of caterpillars
in the last-mentioned species, in which the male is
dark in colour and has much-feathered antennae,
while the female is very pale and has antennae only
slightly feathered. In the moths developed from
the castrated larvae there was no alteration in the

1 Journ. Exper. Zool. (Baltimore), vol. i., 1905.

2 Experimentelle Studien zur Soma- und Geschlechtsdifferenzierung.
Jena, 1909.

THE HORMONE THEORY 159

male characters, and in the females the only difference
was that some of them were slightly darker than the
normal. Meisenheimer and KopeS after him claim
to have grafted ovaries into males and testes into
females, with the result that the transplanted organs
remained alive and grew, and in some cases at least
became connected with the genital ducts. Even in
these cases the moth when developed showed the
original characters of the sex to which belonged
the caterpillar from which it came, although it was
carrying a gonad of the opposite sex. It will be seen
that these results are the direct opposite of those
obtained by Steinach on Mammals. We have no
evidence that the darker colour of the normal male
in this case is adaptive, or due to external stimuli,
but the feathering of the antennae is generally
believed to constitute a greater development of the
olfactory sense organs, and is therefore adaptive,
enabling the male to find the female. This is
therefore the kind of organ which would be expected
to be affected by hormones from the generative
organs. It is stated that the sexual instincts were
also unaltered, a male containing ovaries instead of
testes readily copulating with a normal female.

These results, almost incredible as they appear, are
in harmony with the relatively frequent occurrence
of gynandromorphism in insects.1 One of the most
remarkable cases of this is that of an ant (Myrmica
scabrinodis) the left half of which is male, the right
half not merely female, but worker — that is, sterile
female, without wing. Cases in Lepidoptera, e.g.
Amphidasys betularia, have frequently been recorded.

1 See Doncaster, Determination of Sex (Camb. Univ. Press, 1914),
chap. ix.

160 EVIDENCE OPPOSED TO

Presumably not only the antennae and markings, but
also the genital appendages and the gonads them-
selves, are male and female on the two sides. On
the view that both sexes and the somatic sex-char-
acters of both sexes are present in each zygote, and
that the actual sex is due to dominance, we must
conclude that the male primary and secondary
characters are dominant on one side, and the female
on the other, and it is evident that hormones diffusing
throughout the body cannot determine the develop-
ment of somatic sexual characters here. Various
attempts have been made to explain gynandro-
morphism in insects in accordance with the chromo-
some theory of sex-determination. These are dis-
cussed by Doncaster in the vohime already cited,
but from the point of view of the present work the
important question is that concerning the somatic
sex-characters. According to Doncaster it has been
found that in some Lepidoptera the different sex-
chromosomes occur in the female, not in the male
as in other insects. Half the eggs, therefore, con-
tain an X chromosome, and half a 7, while all the
sperms contain an X chromosome. Doncaster has
seen in Abraxas grossulariata ova with two nuclei
both undergoing maturation. If one of these in
reduction expelled a Y chromosome, the other an
X, then one would retain an X and the other a Y.
Each was fertilised by a sperm, one becoming
therefore XX or male and the other X Y or female. It
may be supposed that as there was only the cyto-
plasm of one ovum, each nucleus would determine the
characters of half the individual developed. The
question remains, therefore, where are the factors of
the somatic sex-characters ? One suggestion which

THE HORMONE THEORY

161

might be made is that the female characters are
present in the Y, in this case female producing
chromosome, or, if the female characters are merely
negative, that the male characters are in the X
chromosome, but only show themselves in the
homozygous condition, thus :—

FEMALE MALE

MALE

FEMALE

The male characters in the male, XX, would
appear because present in two chromosomes, but
would be recessive in the female because present
only in one chromosome. The validity of this
scheme, however, is disproved by the fact that males
can transmit the female characters of their race, as
in the case mentioned by Doncaster where a male
Nyssia zonaria when crossed transmits the wingless
character of its own female.

Another, perhaps better, suggestion is that the
somatic characters of both sexes are present in each.
Then as each somatic cell is descended without
segregation from the fertilised ovum, we may sup-
pose that the presence of the sex-chromosomes in
the somatic cells themselves in some way determines
whether male or female characters shall develop,
without the aid of any hormones from the gonads.

162 EVIDENCE OPPOSED TO

This theory would be quite compatible with the
belief that adaptive somatic sex-characters may be
due to external stimulation, for supposing that the
hypertrophy or modification is conveyed to the
determinants in the gametocytes, and was confined
to one sex, e.g. the male, then these determinants
would be modified in association with the sex-
chromosomes of that sex, and thus though after re-
duction and fertilisation they would be present in
the female zygote also, they would not develop in that
sex. Thus supposing M to represent a modification
acquired in the male and m the absence of the modi-
fication, such as the feathered antenna of a moth, and
the sex-chromosomes to be X and Y> then we should
have in the gametocytes

Male Female

MM mm

XX XY

Gametes. . MX, MX: mXmY
Zygotes . . MmXX male, MmX Y female,

and the character M would only appear in the male
because it only develops in association with XX in
the somatic cells descended from the male zygote.
This would be the result in the first generation in
which a somatic modification affected the factors in
the chromosomes. In the next generation m in the
male would be affected, and the male for the sake
of simplicity might be supposed to become MMXX.
When the female gametes segregated, some would
always be mY, and some zygotes therefore MXmY.
Others might be MMX Y. On this theory, therefore,
there would always be some females heterozygous
for the male character.

THE HORMONE THEORY 163

Geoffrey Smith, one of the many promising young
scientific investigators whose careers were cut short
in the War, maintained views concerning somatic
sex-characters different from that which explains
their development as due to a hormone from the
testis or ovary. Nussbaum in 1905 1 had recorded
experiments on Eana fusca (which is identical with
the British species commonly called R. temporaries)
which appeared to prove that in the male frog after
castration the annual development of the thumb-pad
and the muscles of the fore-leg does not take place,
and if these organs have begun to enlarge before
castration they atrophy again. When pieces of testis
were introduced into the dorsal lymph-sac of a cas-
trated frog the thumb-pads and muscles developed
as in a normal frog. Geoffrey Smith and Edgar
Schuster 2 investigated the subject again with results
contrary to those of Nussbaum.

Smith and Schuster begin by describing the normal
cycle of changes in the testes on the one hand and
the thumb-pad on the other. After the discharge
of the spermatozoa in March or April the testes are
at their smallest size. From this time onwards till
August they steadily increase in size, attaining their
maximum at the beginning of September. From
then till the breeding season no increase in size or
alteration of cellular structure occurs, the testes
apparently remaining in a state of complete inactivity
during this period. With regard to internal de-
velopment, after the discharge of spermatozoa in
the breeding season the spermatogonia divide and

1 ' Ergebnisse der Anat. und Entwicklungsgesch./ Bd. xv. ; Pflugers
Archiv, Bd. cxxvi., 1909.

2 Quart. Journ. Mic. Sci., Ivii., 1911-12.

164 EVIDENCE OPPOSED TO

proliferate, forming groups of cells known as sper-
matocysts. In June and July spermatogenesis is
active, and from August to October the formation of
ripe spermatozoa is completed.

The corresponding changes in the thumb-pads
are as follows. Immediately after the breeding
season the horny epidermis of the pad with its
deeply pigmented papillae is cast off, and the
thumb remains comparatively smooth from April
or May until August or September. When the
large papillae are shed, smaller papillae remain
beneath, and are gradually obliterated by the
epidermis growing up between them. The epidermis
is therefore growing while the spermatogenesis is
taking place. In August and September the epi-
dermic papillae begin to be obvious, and from this
time till February a continuous increase in the
papillae and their pigmentation occur. Geoffrey
Smith argues that the development of this somatic
character occurs while the testes are inactive and
unchanged. Considering that the testes throughout
the winter months are crammed with spermatozoa,
which must require some nourishment, and which
may be giving off a hormone all the time, the argu-
ment has very little weight. Smith and Schuster
found that ovariotomy, with or without subsequent
implantation of testes or injection of testis extract,
had no effect in causing the thumb of the female to
assume any male characters.

Castration during the breeding season causes the
external pigmented layer with its papillae to be
cast off very soon — that is to say, it has the same
effect as the normal discharge of the spermatozoa.
Smith and Schuster found that castration at other

THE HORMONE THEORY 165

seasons caused the pad to remain in the condition in
which it was at the time, that there was no reduction
or absorption as Nussbaum and Meisenheimer found,
and that allo-transplantation of testes — that is, the
introduction of testes from other frogs either into the
dorsal lymph-sacs or into the abdominal cavity — or
the injection of testis extract, had no effect in
causing growth or development of the thumb -pad.

There seems to be one defect in the papers of both
Nussbaum and Smith and Schuster — namely, that
neither of them mentions or apparently appreciates
the fact that the thumb-pads, apart from the dermal
glands, consist of horny epidermis developed from
the living epidermis beneath. The horny layer is
not shown clearly in the figures of Smith and Schuster.
It seems impossible that the horny layer or its
papillae could atrophy in consequence of castration,
or be absorbed. The horny part of the frog's
thumb-pad is comparable with the horny sheath of
the horns in the mammalian Prong-buck (Antilocapra)
which are shed after the breeding season and annually
redeveloped. Meisenheimer claims that he produced
development of papillae on the thumb-pad, not only
by implantation of pieces of testis, but also by im-
plantation of pieces of ovary. This seems so very
improbable that it suggests a doubt whether the
same investigator was not mistaken with regard to
the results of his experiments in transplanting
gonads in Moths.

Smith and Schuster conclude that the normal
development of the thumb-pad depends on the
presence of normal testes, but that there is no suffi-
cient evidence that the effect is due to a hormone
derived from the testis. It is equally probable,

166 EVIDENCE OPPOSED TO

according to Smith, that the testicular cells take up
some substance or substances from the blood, thus
altering the composition of the latter and perhaps
stimulating the production of these substances in
some other organ of the body. These substances
may be provisionally called sexual formative sub-
stances. Smith's theory therefore is that the action
of the testes in metabolism is rather to take some-
thing from the blood than to add something to it,
and that it is this subtractive effect which influ-
ences the development of somatic sexual organs.

Geoffrey Smith in fact, in the paper above con-
sidered, attempts to apply to the frog the views he
put forward l in relation to the effect ,of the parasite
Sacculina on the sexual organs of crabs. The species
in which he made the most complete investigation of
the influence of the parasite was Inachus scorpio (or
dorsettensis). Figures showing the changes in the
abdomen produced by the presence of Sacculina are
given in Doncaster's Determination of Sex, PL xv.
Sacculina is one of the Cirripedia, and therefore allied
to the Barnacles. It penetrates into the crab in its
larval stage, and passes entirely into the crab's
body, where it develops a system of branching
root-like processes. When mature the body of the
Sacculina containing its generative organs forms a
projection at the base of the abdomen of the crab
on its ventral surface, and after this is formed the
crab does not moult. Crabs so affected do not
show the usual somatic sexual characters, and at
one time it was supposed that only females were
attacked. It is now known that both sexes of the

1 Fauna und Flora des Ool/es von Neapel, 29 Monographic
Rhizocephala.

THE HORMONE THEORY 167

host may be infected by the parasite, but the pres-
ence of the latter causes suppression of the somatic
sex-differences. The entry of the parasite is effected
when the crab is young and small, before the somatic
sex-characters are fully developed. The gonads are
not actually penetrated, at least in some cases, by
the fibrous processes of the parasite, but neverthe-
less they are atrophied and almost disappear. In
Inachus the abdomen of the normal male is very
narrow and has no appendages except two pairs of
copulatory styles. The abdomen of the female is
very broad, and has four pairs of biramous appen-
dages covered with hairs, the normal function of
which is to carry the eggs. The effect of the para-
site in the male is that the abdomen is broader, the
copulatory styles reduced, and biramous hairy
appendages are developed similar to those of the
female, but smaller. In the female the abdomen
remains broad, but the appendages are much
smaller than in the normal female, about equal in
size to those of the ' sacculinised ' male. Smith
interpreted the alteration in the male as a develop-
ment of female secondary characters, but it is
obvious from the condition in Macrura or tailed
Decapods, like the lobster or crayfish, that the ab-
domen or tail of the male originally carried appen-
dages similar to those of the female, and that the
male character is a loss of these appendages. The
absence of the male character therefore necessarily in-
volves a development of these appendages, and there
is not much more reason for saying that the male
under the influence of the parasite develops female
characters, than for saying that the male character
is absent. There is no evidence in the facts con-

168 EVIDENCE OPPOSED TO

cerning parasitic castration for Geoffrey Smith's
conclusion that the female characters are latent in
the male, but the male characters not latent in the
female : both return to a condition in which they
resemble each other, and the primitive form from
which they were differentiated.

By his studies of parasitic castration Geoffrey
Smith was led to formulate a theory for the explana-
tion of somatic sex-characters different from that
of hormones. He found that in the normal female
crab the blood contained fatty substances which
were absorbed by the ovaries for the production of
the yolk of the ova. When Sacculina is present
these substances are absorbed by the parasite ; the
ovary is deprived of them, and therefore atrophies.
In the male the parasite requires similar substances,
and its demand on the blood of the host stimulates
the secretion of such substances, so that the whole
metabolism is altered and assimilated to that of
the female. It is this physiological change which
causes the development of female secondary char-
acters. He describes this change as the produc-
tion of a hermaphrodite sexual formative substance,
on the ground that in at least one case eggs were
found in the testis of a male Inachus which had been
the host of a Sacculina, but had recovered. It
must however be noted that the Sacculina itself is
hermaphrodite, with ovaries much larger than the
testes. It is possible that while the parasite pre-
vents the development of testis or ovary in the host,
it gives up to the body of the host a hormone from
its own ovaries which tends to develop the female
secondary characters : for the parasite is itself a
Crustacean, and therefore the hormone from its

THE HORMONE THEORY 169

ovaries would not be of too different a nature to
act upon the tissues of the host.

The observation of Geoffrey Smith that eggs may
occur in the testis of a crab after recovery from the
parasite appears of more importance than his
peculiar theoretical suggestions, for it tends to show
that sex is not always unalterably fixed at fertilisa-
tion. In this case the influence of a parasite pre-
dominantly female would seem to be the real cause
of the development of eggs in the testis of the host.
Geoffrey Smith does not discuss the origin of the
somatic sexual characters in evolution, or attempt
to show how his theories of sexual formative sub-
stance, and of the influence of the gonads by sub-
traction rather than addition, would bear upon the
problem.

CHAPTER VI

ORIGIN OF NON-SEXUAL CHARACTERS : THE
PHENOMENA OF MUTATION

ACCORDING to the theory here advocated, modifica-
tions produced by external stimuli in the soma will
also be inherited in some slight degree in each genera-
tion when they have no relation to sex or reproduc-
tion. In this case the habits and the stimuli which
they involve will be common to both sexes, and the
hormones given off by the hypertrophied tissues will
act upon the corresponding determinants in the
gametocytes. The modifications thus produced will
therefore be related to habits, and the theory
will include all adaptations of structure to function,
but other characters may also be included which are
the result of stimuli and yet have no function or
utility.

The majority of evolutionists in recent years have
taught that influences exerted through the soma
have no effect on the determinants in the chromo-
somes of the gametes, that all hereditary variations
are gametogenic and none somatogenic. Mendelians
believe that evolution has been due to the appearance
of characters or factors of the same kind as those
which distinguish varieties in cultivated organisms,
and which are the subject of then: experiments, but
they have found a difficulty, as already mentioned
in Chapter II, in forming any idea of the origin of a

170

THE PHENOMENA OF MUTATION 171

new dominant character. A recessive character is
the absence of some positive character, and if in the
cell-divisions of gametogenesis the factor for the
positive character passes wholly into one cell, the
other will be without it, will not ' carry ' that factor.
If such a gamete is fertilised by a normal gamete the
organism developed from the zygote will be hetero-
zygous, and segregation will take place in its gametes
between the chromosome carrying the factor and the
other without it, so that there will now be many
gametes destitute of the factor in question. When
two such gametes unite in fertilisation the resulting
organism will be a homozygous recessive, and the
corresponding character will be absent. In this way
we can conceive the origin of albino individuals from
a coloured race, supposing the colour was due to a
single factor.

In Bateson's opinion the origin of a new dominant
is a much more difficult problem. In 1913 he dis-
cussed the question in his Silliman Lectures.1 He
considers the difficulty is equally hopeless whether
we imagine the dominants to be due to some change
internal to the organism or to the assumption of
something from without. Accounts of the origin of
new dominants under observation in plants usually
prove to be open to the suspicion that the plant was
introduced by some accident, or that it arose from
a previous cross, or that it was due to the meeting
of complementary factors. In medical literature,
however, there are numerous records of the spon-
taneous origin of various abnormalities which
behave as dominants, such as brachydactyly, and
Bateson considers the authenticity of some of

1 Problems of Genetics, Oxford Univ. Press, 1913.

172 ORIGIN OF NON-SEXUAL CHARACTERS

these to be beyond doubt. He concludes that it is
impossible in the present state of knowledge to offer
any explanation of the origin of dominant characters.
In a note, however, he suggests the possibility that
there are no such things as new dominants. Factors
have been discovered which simply inhibit or
prevent the development of other characters. For
example, the white of the plumage in the White
Leghorn fowl is due to an inhibiting factor which
prevents the development of the colour factor which
is also present. Withdraw the dominant inhibiting
factor, and the colour shows itself. This is shown
by crossing the dominant white with a recessive
white, when some birds of the F2 -generation are
coloured.1 Similarly, brachydactyly in man may
be due to the loss of an inhibiting factor which
prevents it appearing in normal persons. It is
evident, however, that it is difficult to apply this
suggestion to all cases. For example, the White
Leghorn fowl must have descended from a coloured
form, probably from the wild species Gallus bankiva.
If Bateson's suggestion were valid we should have to
suppose that the loss of the factor for colour caused
the dominant white to appear, and then when this
is withdrawn colour appears again, so that the colour
factors and the inhibiting factors must lie over one
another in a kind of stratified alternation. And
then how should we account for the recessive white ?
In his Presidential Address to the meeting of the
British Association in Australia, 1914, Bateson
explains his suggestion somewhat more fully with a
command of language which is scarcely less re-
markable than the subject matter. The more true-

1 Bateson, Principles of Heredity, p. 104.

THE PHENOMENA OF MUTATION 173

breeding forms are studied the more difficult it is to
understand how they can vary, how a variation can
arise. When two forms of Antirrhinum are crossed
there is in the second generation such a profusion of
different combinations of the factors in the two
grandparents, that Lotsy has suggested that all
variations may be due to crossing. Bateson does
not agree with this. He believes that genetic factors
are not permanent and indestructible, but may
undergo quantitative disintegration or fractionation,
producing subtraction or reduction stages, as in the
Picotee Sweet Pea, or the Dutch Rabbit. Also
variation may take place by loss of factors as in the
origin of the white Sweet Pea from the coloured.
But regarding a factor as something which, although
it may be divided, neither grows nor dwindles,
neither develops nor decays, the Mendelian cannot
conceive its beginning any more than we can
conceive the creation of something out of nothing.
Bateson asks us to consider therefore whether all the
divers types of life may not have been produced by
the gradual unpacking of an original complexity in
the primordial, probably unicellular forms, from
which existing species and varieties have descended.
Such a suggestion in the present writer's opinion is in
one sense a truism and in another an absurdity.
That the potentiality of all the characters of all the
forms that have existed, pterodactyls, dinosaurs,
butterflies, birds, etc. etc., including the characters
of all the varieties of the human race and of human
individuals, must have been present in the primordial
ancestral protoplasm, is a truism, for if the possibility
of such evolution did not exist, evolution would not
have taken place. But that every distinct heredi-

174 ORIGIN OF NON-SEXUAL CHARACTERS

tary character of man was actually present as a
Mendelian factor in the ancestral Amoeba, and that
man is merely a group of the whole complex of
characters allowed to produce real effects by the re-
moval of a host of inhibiting factors, is incredible.
The truth is that biological processes are not within
our powers of conception as those of physics and
chemistry are, and Bateson's hypothesis is nothing
but the old theory of preformation in ontogeny.
Just as the old embryologists conceived the adult
individual to be contained with all its organs to the
most minute details within the protoplasm of the
fertilised ovum or one of the gametes, so the modern
Mendelian, because he is unable to conceive or to
obtain the evidence of the gradual development of a
hereditary factor, conceives all the hereditary factors
of the whole animal kingdom packed in infinite
complexity within the protoplasm of the primordial
living cells. That man is complex and Amoeba
simple is merely a delusion ; the truth according to
Mendelism is that man is merely a fragment of the
complexity of the original Amoeba.

Mendelism studies especially the heredity of char-
acters, and only incidentally deals with recorded
instances of the appearance of new forms, such as
the origin of a salmon-coloured variety of Primula
from a crimson variety. The occurrence of new
characters, or mutations as they are called, has
been specially studied by other investigators, and
I propose briefly to consider the two most im-
portant examples of such research, namely, that
by Professor T. H. Morgan, which deals with the
American fruit - fly Drosophila, and the other
which concerns the mutations of the genus of plants

THE PHENOMENA OF MUTATION 175

CEnothera, exemplified by our well-known Evening
Primrose.

Professor T. H. Morgan informs us l that within
five or six years in laboratory cultures of the fruit-
fly, Drosophila ampelophila, arose over a hundred
and twenty-five new types whose origin was com-
pletely known. The first of these which he mentions
is that of eye colour, differing in the two sexes, in
the female dark eosin, in the male yellowish eosin.
Another mutation was a change of the third seg-
ment of the thorax into a segment similar to the
second. Normally the third segment bears minute
appendages which are the vestiges of the second
pair of wings ; in the mutant the wings of the third
segment are true wings though imperfectly developed.
A factor has also occurred which causes duplica-
tion of the legs. Another mutation is loss of the
eyes, but in different individuals pieces of the eye
may be present, and the variation is so wide that it
ranges from eyes which until carefully examined
appear normal, to the total absence of eyes. Wing-
less flies also arose by a single mutation. These
were found on mating with normal specimens
to be all recessive characters, thus agreeing with
Bateson' s views. The next one described is dominant.
A single male appeared with a narrow vertical red
bar instead of the broad red normal eye. When
this male was bred with normal females all the eyes
of the offspring were narrower than the normal
eye, though not so narrow as in the abnormal male
parent. It may be pointed out that this is scarcely
a sufficient proof of dominance. If the mutation

1 A Critique of the Theory of Evolution (Oxford Univ. Press, 1916),
p. 60.

176 ORIGIN OF NON-SEXUAL CHARACTERS

were due to the loss of one factor affecting the eye,
the heterozygote carrying the normal factor from
the mother only might very well develop a some-
what imperfect eye.

Morgan arranges the numerous mutations observed
in Drosophila in four groups, corresponding in his
opinion to the four pairs of chromosomes occurring
in the cells of the insect. After the meiotic or reduc-
tion divisions each gamete of course contains in
its nucleus four single chromosomes. One of the
four pairs consists of the sex-chromosomes. All the
factors of one group are contained in one chromo-
some, and it is found in experiments that the members
of each group tend to be inherited together — that is
to say, if two or more enter a cross together, in other
words, if a specimen possessing two or more muta-
tions is crossed with another in which they are
absent, they tend to segregate as though they were
a single factor. This fact agrees with the hypo-
thesis that the factors in such a case are contained
in a single chromosome which segregates from the
fellow of its pair in the reduction divisions. Excep-
tions may occur, however, and these are explained
by what is called ' crossing over.' When one
chromosome of a pair, instead of being parallel to
the other in the gametocyte, crosses it at a point of
contact, then when the chromosomes separate, part of
one chromosome remains connected with the part of
the other on the same side and the two parts separate
as a new chromosome, so that two factors originally
in the same chromosome may thus come to lie in
different chromosomes. In consequence of this, two or
more factors which are usually ' coupled ' or inherited
together may come to appear in different individuals.

THE PHENOMENA OF MUTATION 177

Morgan emphasises the statement that a factor
does not affect only one particular organ or part of
the body. It may have a chief effect in one kind of
organ, e.g. the wings or eyes, but usually affects
several parts of the body. Thus the factor that causes
rudimentary wings also produces sterility in females,
general loss of vigour, and short hind legs.

The facts to which I shall refer concerning (Eno-
thera are for the most part quoted on the authority
of Dr. Ruggles Gates, and taken from his book The
Mutation Factor in Evolution (London, 1915). The
occurrence of mutations in (Enothera was first
noticed by De Vries, the Dutch botanist, in the
neighbourhood of Amsterdam in 1886. He found a
large number of specimens of (Enothera Lamarck-
iana growing in an abandoned potato-field at Hil-
versum, and these plants showed an unusual amount
of variation. He transplanted nine young plants
to the Botanic Garden of Amsterdam, and culti-
vated them and their descendants for seven genera-
tions in one experiment. Similar experiments have
been made by himself and others. The large
majority of the plants produced from the (E. Lamarck-
iana by self-fertilisation were of the same form
with the same characters, but a certain percentage
presented ' mutations ' — that is, characters different
from the parent form, and in some cases identical
with those of plants occurring occasionally among
those growing wild in the field where the observa-
tions began. Nine of these mutants have been
recognised and defined, and distinguished by different
names. The characters are precisely described and
in many cases figured by Gates in the volume cited
above. The first mutant to be recognised — in 1887—

M

178 ORIGIN OF NON-SEXUAL CHARACTERS

was one called lata. It must be explained that
the young plant of (Enothera has practically no
stem, but a number of leaves radiating in all direc-
tions from the growing point which is near the
surface of the soil. The plant is normally biennial,
and in the first season the internodes are not
developed. This first stage is called the ' rosette.'
From the reduced stem are afterwards developed
one or more long stems with elongated internodes,
bearing leaves and flowers. In the mutation lata
the rosette leaves are shorter and more crinkled
than those of Lamarckiana, and the tips of the leaves
are very broad and rounded. The stems of the
mature plant are short and usually more or less
decumbent with irregular branches. The flower-
buds are peculiarly stout and barrel-shaped, with
a protrusion on one side. The seed-capsules are
short and thick, containing relatively few seeds, and
the pollen is wholly or almost wholly sterile.

It is to be noted here, a fact emphasised by De
Vries in his earliest publications on the subject, that
in nearly all, if not all cases, a mutation does not
consist in a peculiarity of a single organ, but in an
alteration of the whole plant in every part. In this
respect mutations as observed in (Enothera seem to
be in striking contrast to the majority of Mendelian
characters. Mutation in fact seems to be a case of
what the earlier Darwinians called correlation, while
Mendelian characters may apparently be separated
and rejoined in any combination. For example, in
breeds of fowls any colour or any type of plumage
may be obtained with single comb or with rose comb.
In my own experiments on fowls the loose kind of
plumage first known in the Silky fowl, which is white,

THE PHENOMENA OF MUTATION 179

could be combined with the coloured plumage of the
type known as black-red. At the same time it must
be borne in mind that since the factor, whether a
portion of a chromosome or not, is transmitted in
heredity as a part of a single cell, the gamete, and
since every cell of the developed individual is
derived by division from the single zygote cell
formed by the union of the two gametes, the factor or
determinant must be contained in every cell of the
soma, except in cases where differential division, or
what is called somatic segregation, takes place.
Thus the factor which causes the comb to be a rose
comb in a fowl must be present in the cells that
produce the plumage or the toes or any other part of
the body. Morgan, as mentioned above, finds in
Drosophila that factors do affect several parts of the
body. It is, however, curious to consider that the
factor which produces intense pigmentation of the
skin and all the connective tissue in the Silky fowl
has no effect on the colour of the plumage in that
breed, which is a recessive white. The plumage is
an epidermic structure, and therefore distinct from
the connective tissue, but it is difficult to understand
why a pigment factor though present in every cell
has no effect on epidermic cells.

The Mendelians, when the mutations of (Enothem
were first described, endeavoured to show that they
were merely examples of the segregation of factors
from a heterozygous combination. They suggested
in fact that (Enothem Lamarckiana was the result
of a cross, or repeated crosses, between plants
differing in many factors, that the numerous muta-
tions were similar to the variety of different types
which are produced by breeding together the grey

180 ORIGIN OF NON-SEXUAL CHARACTERS

mice arising from a cross between an albino and a
Japanese waltzing mouse in Darbishire's experi-
ment. Since that time, however, the natural dis-
tribution and the cultural history of (Enothera has
been very thoroughly worked out. (Enothera
Lamarckiana is the common Evening Primrose of
English gardens. The species of the sub-genus
Onagra to which Lamarckiana belongs were originally
confined to America (Canada, United States, and
Mexico), but Lamarckiana itself has never been
found there in a wild state. Attempts, however, to
produce it by crossing of other forms have not
succeeded, and a specimen has been discovered at the
Museum d'Histoire Naturelle at Paris, collected by
Michaux in North America about 1796, which agrees
exactly with the (Enothera Lamarckiana naturalised
or cultivated in Europe. The plant was first
described by Lamarck from plants grown in the
gardens of the Museum d'Histoire Naturelle, under
the name CE. yrandi flora, which had been introduced
by Solander from Alabama, but Seringe subsequently
decided that Lamarck's species was distinct from
grandiflora, and named it Lamarckiana. Gates
states that Michaux was in the habit of collecting
seeds with his specimens, and that it is therefore
highly probable that Lamarck's specimens were
grown directly from seeds collected in America by
Michaux. Gates considers that the suggestion of
the hybrid origin of Lamarckiana in culture is thus
finally disposed of. By the year 1805, Lamarckiana
was apparently naturalised and flourishing on the
coast of Lancashire, and in 1860 it was brought into
commerce, probably from these Lancashire plants,
by Messrs. Carter. The cultures of De Vries are

THE PHENOMENA OF MUTATION 181

descended from these commercial seeds, but the
Swedish race of Lamarckiana, as well as those of
English gardens, differ in several features and must
have come from another source or been modified
by crossing with grandiflom. This last remark is
quoted from Gates, but it seems improbable that
the Dutch plants should be derived from those of
Lancashire, and those of English gardens from a
different source. The fact seems to be, according to
other parts of Gates' s volume, that there are various
races of Lamarckiana in English gardens and in the
Isle of Wight, as well as in Sweden, etc., and that
these races differ from one another less than the
mutants of De Vries and his followers.

An important point about these mutations is that
their production is a constant feature of Lamarckiana.
Whenever large numbers of the seeds of this plant are
grown, a certain proportion of the plants developed
present these same mutations ; not always all of
them — some may be absent in one culture, present
in another, but four of them are fairly common and
of constant occurrence. The total proportion of
mutant plants compared with the normal was 1*55
per cent, in one family, 5*8 per cent, in another. It
would appear therefore, supposing that mutations
arose subsequently in the same determinate way
from previous mutations, that evolution, though in
a number of divergent directions from one ancestral
form, would proceed along definite lines, and that
there would be nothing accidental about it. We
should thus arrive at a demonstration of what
Eimer called orthogenesis, or evolution in definite
directions.

The mutation lata cannot be said to breed true, as

182 ORIGIN OF NON-SEXUAL CHARACTERS

the pollen is almost entirely sterile. It has therefore
been propagated by crossing with Lamarckiana
pollen, with the result that both forms are obtained
with lata varying in proportion from 4 per cent, to
45 per cent.

Rubrinervis is a mutation from Lamarclciana,
chiefly distinguished by red midribs in the leaves
and red stripes on the sepals. When propagated
from self-fertilised seed it produced about 95 per
cent, of offspring with the same characters, and the
remaining 5 per cent, mutants, one of which was
laemfolia which had been found by De Vries among
plants growing wild at Hilversum. Gates obtained
a single plant among offspring of rubrinervis in which
the sepals were red throughout, and to this he gave
the name rubricalyx. When selfed this plant gave
rise to both rubricalyx and rubrinervis, and in the
second generation when the rubricalyx was selfed
again the numbers of the two were approximately
3 to 1. Rubricalyx is therefore a dominant hetero-
zygote, and this fact was further confirmed in the
third generation when a selfed plant gave 200
offspring all rubricalyx, the mother plant having
evidently been homozygous for the red character.
In this case, therefore, we have what Bateson was
seeking, the origin of a new dominant character
under observation, the original mutation having
arisen in a single gamete of the zygote which gave rise
to the plant. It is claimed by mutationists that
mutations are not new combinations or separations
of Mendelian unit characters already present, but
are themselves new characters, though not always
necessarily, as in the case of rubricalyx, new unit
characters in the Mendelian sense.

THE PHENOMENA OF MUTATION 183

Perhaps the most interesting of the researches on
the phenomena of mutation are those concerning the
relation of the characters to the chromosomes of the
cell, in which Gates has been a pioneer and one of
the most industrious and successful investigators.
The behaviour of the chromosomes in meiosis or re-
duction division both in the pollen mother-cells and
in the megaspore mother- cells which give rise to the
so-called embryo-sac are fully described by Gates.
Here it is only necessary to refer to the abnormalities
in the reduction division which are related to muta-
tion, and the results of these abnormalities in the
number of chromosomes. The original number of
chromosomes in (Enothera is 14. In the mutation
lata this has become 15, and also in another mutation
called semilata. The chromosomes before the re-
duction division are arranged in pairs, each pair
consisting, it is believed, of one paternal and one
maternal chromosome. One of each pair goes into
one daughter-cell and the other into the other, but
not all maternal into one and all paternal into the
other. Thus each daughter-cell after the first or
heterotypic division in normal cases contains 7
chromosomes. A second homotypic division takes
place in which each chromosome splits into two as
in somatic divisions, and thus we have 4 gametes
with 7 chromosomes each. Now when lata is pro-
duced it is believed that in the heterotypic division
one pair passes into one daughter-cell instead of one
chromosome of the pair into each daughter-cell, the
other pairs segregating in the usual way. We thus
have one daughter-cell with 8 chromosomes and the
other with 6. This 6+8 distribution has actually
been observed in the pollen mother-cell in rubrinervis.

184 ORIGIN OF NON-SEXUAL CHARACTERS

When a gamete with 8 chromosomes unites in
fertilisation with a normal gamete with 7 the zygote
has 15. The lata mutants having an odd chromo-
some are almost completely male-sterile, and their
seed production is also much reduced : but this
partial sterility cannot be attributed entirely to the
odd chromosome because semilata, which has also
15 chromosomes, does not show the same degree of
sterility.

Other cases occur in which the number of chromo-
somes in the somatic cells is double the ordinary
number — namely, 28 — and others in which the
number is 21. The normal number in the gamete,
7, is considered the simple or haploid number, and
therefore the number 28 is called tetraploid. This
doubling of the somatic number of chromosomes is
now known in a number of plants and animals. It
occurs in the (Enothera mutant gigas. The origin of
it has not been clearly made out, but it must result
either from the splitting of each chromosome or
from the omission of the chromosome reduction.
In many cases the more numerous chromosomes are
individually as large as those in normal plants, and
consequently the nucleus is larger, the cell is larger,
and the whole plant is larger in every part. But
giantism may occur without tetraploidy, and vice
versa. In the (Enothera gigas the rosette leaves are
broadly lanceolate with obtuse or rounded tips,
more crinkled than in Lamarckiana, petioles shorter.
The stem-leaves are also larger, broader, thicker,
more obtuse, and more crinkled than in Lamarckiana.
The stem is much stouter, almost double as thick,
but not taller because the upper internodes are
shorter and less numerous. It is difficult to avoid

THE PHENOMENA OF MUTATION 185

the conclusion that the stouter character of the
organs in this plant is causally connected with the
increased number of chromosomes. Where the
number of cells formed is approximately similar, as
in two allied forms of plant in this case, the greater
size of the cells would naturally give a stouter habit,
but it is clear that large cells do not necessarily
mean greater size. The cells of Salamander and
Proteus are the largest found among Vertebrates,
but those Amphibia are not the largest Vertebrates.
It is curious to note how different are these dis-
coveries concerning differences in the number of
chromosomes from the conception of Morgan that
a mutation depends on a factor situated in a part
of one chromosome.

More copious details concerning mutations will
be found in the publications cited. The question
to be considered here is how far the claim is justified
that the facts of this kind hitherto discovered afford
an explanation of the process of evolution. It
seems probable that mutations are of different kinds,
as exemplified in (Enoihera by gigas and rubricalyx
respectively, the former producing only sterile
hybrids, the latter behaving exactly like a Mendelian
unit. There can be little doubt that, as Bateson
states, numerous forms recognised as species or
varieties in nature differ in the same way as the
races or breeds of cultivated organisms which differ
by factors independently inherited. There are facts,
however, which prove that all species are not sterile
inter se, and that their characters when they are
hybridised do not always segregate in Mendelian
fashion. John C. Phillips,1 for example, crossed

1 Journ. Exper. ZooL, vol. xviii., 1915,

186 ORIGIN OF NON-SEXUAL CHARACTERS

three wild species of duck, Anas boscas (the Mallard)
with Dafila acuta (the Pintail) and with Anas tristis.
In the former cross he states that except for one or
two characters there seemed to be no more tendency
to variation in the F2 generation than hi the Fr An
Fl Pintail-Mallard ? was mated with a wild Pintail $.•
According to Mendelian expectation the offspring of
this mating should have been half Pintail and half
Pintail-Mallard hybrids, but Phillips states that
on casual inspection the plumage of all the males
appeared pure Pintail although the shape was
distinctly Mallard-like. The statement is, however,
open to criticism. The question is, what were the
unit characters in the parent species ? If the unit
characters were very small and numerous, an in-
dividual in which all the characters of the Pintail
existed together among the offspring of the hybrid
mated with pure Pintail would be rare in proportion
to the individuals presenting other combinations.
Of the F% s obtained from crossing Anas tristis $ with
Anas boscas $, Phillips obtained 23 females and
16 males. The females were all alike and similar to
F! females. Of the males one was a variate specially
marked, about half-way between the F1 type and
the Mallard parent. This, according to Phillips, was
a segregate. The rest showed a range of variation
but no distinct segregation.

It is somewhat surprising that Mendelian experts,
who seem to believe that species are distinguished
by Mendelian characters, have not made systematic
experiments on the crossing of species in order to
prove or disprove their belief.

For my own part I cannot help thinking that the
origin of varieties in species in a domesticated or

THE PHENOMENA OF MUTATION 187

cultivated state is in a sense pathological. Such
variation doubtless occurs in nature, but not with
such luxuriance. The breeds of domestic fowls
differ so greatly that Bateson and others refuse to
believe that they have all arisen from the single
species Gallus bankiva. It seems to me from the
evidence that there cannot be any doubt that they
have so arisen. One fact that impresses my mind is
that if we consider colour variations in domesticated
animals, we find that a similar set of colours has
arisen in the most diverse kinds of animals with
sometimes certain markings or colours peculiar to
one group, e.g. dappling in horses, wing bars in
pigeons. Thus in various kinds of Mammals and
Birds we have white and black, red or yellow,
chocolate with various degrees of dilution, and
piebald combinations. Why should forms originally
so different, as the cat with its striped markings and
the rabbit with no markings at all, give rise to the
same colour varieties ? It seems probable that the
reason is that the original form had the small number
of pigments which occur mixed together in very
small particles, and that in the descendants the single
pigments have separated out, with increase or
decrease in different cases. It is true that historical
evidence tends to show that the greatest variations,
such as albinism in one direction or excess of
pigment in the other in the Sweet Pea, were the first
to arise (see Bateson, Presidential Address to British
Association, Australia, 1914, Part i.), and the splitting
appears often to be intentionally produced by crossing
these extreme variations with the original form, but
the possibility remains that the conditions of domes-
tication, abundant food, security and reduced

188 ORIGIN OF NON-SEXUAL CHARACTERS

activity, lead to irregularity in the process of
heredity. In any case the mere separation among
different individuals of factors originally inherited
together in one complex does not account for the
origin of the complex or of the factors. This is
somewhat the same idea as that of Bateson when he
states that it is easy to understand the origin of a
recessive character but difficult to conceive the
origin of a dominant.

The point, however, which I desire most to
emphasise is that the investigations we have been
discussing are concerned with variations which have
no relation whatever to adaptation, and afford no
explanation of the evolution of adaptations. These
variations perform no function in the life of the
individual, have no relation to external conditions,
either in the sense of being caused by special con-
ditions or fitting the individual to live in special
conditions. A still more important fact is that they
do not explain the origin of metamorphosis. They
do not arise by a metamorphosis : in the case of the
rose comb of fowls the chick is not hatched with a
single comb which gradually changes into a rose
comb, but the rose comb develops directly from the
beginning. Mutationists and Mendelians do not
seem in the least to appreciate the importance of
metamorphosis or of development generally in con-
sidering the relation of the mutations or factors
which they study to evolution in general, because
they have not grasped the fact that there are two
kinds of characters to be explained, adaptational and
non-adaptational. T. H. Morgan, for example,1

1 A Critique of the Theory of Evolution, p. 67 (Princeton, U.S.A., and
London, 1916).

THE PHENOMENA OF MUTATION 189

describes a mutation in Drosophila consisting in
the loss of the eyes, and triumphantly remarks :
' Formerly we were taught that eyeless animals
arose in caves. This case shows that they may also
arise suddenly in glass milk-bottles by a change in
a single factor.' As it stands the statement is per-
fectly true, but it is obvious that the writer does not
believe that the darkness of caves ever had anything
to do with the loss of eyes. It is almost as though
a man should discover that blindness in a certain
case was due to a congenital, i.e. gametic, defect,
and should then scoff at the idea that any person
could become blind by disease. Some of those
who specialise in the investigation of genetics seem
to give inadequate consideration to other branches
of biology. It is a well-established fact that in the
mole, in Proteus, and in Amblyopsis (the blind fish
of the Kentucky caves), the eyes develop in the
embryo up to a certain stage in a perfectly normal
way and degenerate afterwards, and that they are
much better developed in the very young animal
than in the adult. Does this metamorphosis take
place in the blind Drosophila of the milk-bottle ?
The larva of the fly is, I believe, eyeless like the
larvae of other Diptera, but Morgan says nothing of
the eye being developed in the imago or pupa and
then degenerating. There is therefore no relation or
connexion between the mutation he describes and
the evolution of blindness in cave animals. It is a
truth, too often insufficiently appreciated by biologists,
that sound reasoning is quite as important in science
as fact or experiment. Loeb *• also endeavours to
prove that the blindness of cave animals is no

1 The Organism as a Whole, p. 319 (New York and London, 1916).

190 ORIGIN OF NON-SEXUAL CHARACTERS

evidence of the influence of darkness in causing
degeneration of the eyes. He refers to experiments
by Uhlenhuth, who transplanted eyes of young
Salamanders into different parts of their bodies
where they were no longer connected with the optic
nerves. These eyes underwent a degeneration which
was followed by a complete regeneration. He
showed that this regeneration took place in complete
darkness, and that the transplanted eyes remained
normal when the Salamanders were kept in the dark
for fifteen months. Hence the development of the
eyes does not depend on the influence of light or on
the functional action of the organs. But it must be
obvious to any biologist who has thoroughly con-
sidered the problem, that this experiment has little
to do with the question of the cause of blindness in
cave animals. No one ever supposed that cave
fishes became blind in fifteen months, or in fifteen
years. The experiment cited merely proves that in
the individual the embryonic or young eye will
continue developing by heredity even after it is
transplanted and in the absence of light. But the
eye of the Mammal normally develops in the uterus
in the absence of light.

In his remarks concerning Typhlogobius, a blind
fish on the coast of southern California, Loeb seems
to be mistaken with regard to the facts. He states
that this fish lives ' in the open, in shallow water
under rocks, in holes occupied by shrimps.' Accord-
ing to Professor Eigenmann the same species of
shrimp is found all over the Bay of San Diego, and
is accompanied by other genera of goby, such as
Clevelandia and Gillichihys, which have eyes : but
these fishes live outside the holes, and only retreat

THE PHENOMENA OF MUTATION 191

into them when frightened, while the blind species is
found only at Point Loma, and never leaves the
burrows of the shrimp. It would appear, therefore,
that Typhlogobius lives in almost if not quite com-
plete darkness, instead of being, as Loeb states,
' blind in spite of exposure to light,' while the
closely allied forms which are exposed to light are
not blind.

Loeb states, on the authority of Eigenmann, that
all those forms which live in caves were adapted to
life in the dark before they entered the cave, because
they are all negatively heliotropic and positively
stereotropic, and with these tropisms would be
forced to enter a cave whenever they were put at the
entrance. Even those among the Amblyopsidae
which live in the open have the tropisms of the cave
dweller. But these latter are not blind, and the
argument only tends to show that the blind fish
Amblyopsis entered the caves before it was blind.
Nocturnal animals generally must be said to be
negatively heliotropic, but these usually have larger
and more sensitive eyes than the diurnal.

It is said, however, that Chologaster agassizii,
which is not blind, lives in the underground streams
of Kentucky and Tennessee, but I think it is open
to doubt whether it is a species entirely confined to
darkness.

Another point which Loeb omits to mention is
the absence of pigment in cave animals, especially
Vertebrates such as Amblyopsis and Proteus. If
absence of light is not the cause of blindness in these
cases, how is it that the blindness is always as-
sociated with absence of pigment, since we know that
the latter in Fishes and Amphibia is due to the

192 ORIGIN OF NON-SEXUAL CHARACTERS

absence of light ? It has been shown that Proteus
when kept in the light develops some amount of
pigment, although it does not become pigmented to
the same degree as ordinary Amphibia. We have
here, I think, an example of the essential difference
between mutations and somatic modifications.
Absence of the gametic factor or factors for pig-
mentation results in albinism, and no amount of
exposure to light produces pigmentation in albinos,
e.g. albino Axolotls which are well known in
captivity. Absence of light, on the other hand,
prevents the development of pigment. The question
therefore is whether the somatic modification is
inherited. The fact that Proteus does not rapidly
become as deeply coloured when exposed to light
as ordinary Amphibia shows that the gametic
factors for pigmentation have been modified as well
as the somatic tissues.

Loeb attributes the blindness of cave fishes to a
disturbance in the circulation and mutation of the
eyes originally occurring as a mutation. But how
could an explanation of this kind be applied to the
case of Anableps tetrophthalmus, in which each eye is
divided by a partition of the cornea and lens into an
upper half adapted for vision in air and a lower half
for vision in water ? This fish lives in the smooth
water of estuaries in Central America, and swims
habitually with the horizontal partition of the lens
level with the surface of the water. It is impossible
to understand in this case, firstly, how a mutation
could cause the eyes to be divided and doubly
adapted to two different optic conditions, and,
secondly, how at the same time a convenient
4 tropism ' should occur which caused the animal

THE PHENOMENA OF MUTATION 193

to swim with its eyes half in and half out of water.
Are we to suppose that the upper half of the body or
eye had a positive heliotropism and the lower half a
negative heliotropism ? The fact is that the fish
swims at the surface in order to watch for and feed
on floating particles. The tropism concerned is the
food tropism, but what is gained by calling the
search for food common to all active animals a
tropism, and how is the search for food before the
food is perceptible to the senses, before it can act as
a stimulus on a food-sensitive substance in the body,
to be compared to a tropism at all ?

Loeb undertakes to prove that the organism as
a whole acts automatically according to physico-
chemical laws. But he misses the question of
evolution altogether. For example, he quotes
Gudernatsch as having proved that legs can be
induced to grow in tadpoles at any time, even in
very young specimens, by feeding them with thyroid
gland. Loeb writes : ' The earlier writers explained
the growth of the legs in the tadpole as a case of
an adaptation to life on land. We know through
Gudernatsch that the growth of the legs can be
produced at any time by feeding the animal with the
thyroid gland.' Obviously he thinks that these
two propositions are contradictory to each other,
whereas there is no contradiction between them at
all. Loeb actually supposes that the thyroid is the
cause of the development of the legs. Logically, if
this were the case it would follow that if we fed an
eel or a snake with thyroid it would develop legs like
those of a frog, and if a man were injected with
extract of the testes of a stag he would develop
antlers on his forehead. It will be obvious to most

N

194 ORIGIN OF NON-SEXUAL CHARACTERS

biologists that the thyroid, whether that of the
tadpole itself or that which is supplied as food, only
causes the development of legs because the hereditary
power to develop legs is already present. The
question is how this hereditary power was evolved.
Legs are an adaptation to life on land. What we
have to consider and to investigate is whether the
legs arose as a gametic mutation or as a direct result
of locomotion on land.

The general result of clinical and experimental
evidence is to show that the hormone of the thyroid
is necessary to normal development. The arrest of
development in cretinous children is due to some
deficiency of thyroid secretion, and' is counteracted
by the administration of thyroid extract. Excess of
the secretion produces a state of restlessness and
excitement associated with an abnormally rapid
rate of metabolism and protrusion of the eye-balls
(Graves' disease). The physiological text-books,
however, say nothing of precocity of development
in children as a result of hyperthyroidism. This,
however, is undoubtedly what occurs in the case of
tadpoles. The legs would naturally develop at some
time or other, after a prolonged period of larval life.
Feeding with thyroid causes them to develop at
once. I have repeated Gudernatsch's experiment
with the following results : —

This year I had a considerable number of tadpoles
of the common English frog, which were hatched
between March 26 and March 29. On April 12,
when they had all passed the stage of external
gills and developed internal gills and opercula, I
divided them into two lots, one in a shallow pie-dish,
the other in a glass cylinder. To one lot I gave a

THE PHENOMENA OF MUTATION 195

portion of rabbit's thyroid, to the other a piece of
rabbit's liver. They fed eagerly on both. After-
wards I obtained at intervals of a week or so the
thyroid of a sheep. I have seen no precise details of
Gudernatsch's method of feeding tadpoles, but my
own method was simply to put a piece of thyroid
into the water containing the tadpoles and leave it
there for several days, then to take it out and put in
another piece, changing the water when it seemed to
be getting foul.

April 22. Noticed that the non-thyroid tadpoles
were larger than those fed on thyroid. Changed the
former into the pie-dish and the latter into the glass
jar, to make sure that the difference in size was not
due to larger space.

May 3. Only eighteen of the non-thyroid tadpoles
surviving, owing to the water having become foul,
but these are three times as large as those fed on
thyroid. In the latter no trace of hind-legs was
visible, but the abdominal region was much emaciated
and contracted, while the head region was broader.

May 4. Noticed minute white buds of hind-legs in
the thyroid-fed tadpoles.

May 6. A number of the thyroid-fed were dying,
and the skin and opercular membranes were swollen
out away from the tissues beneath.

Largest normal tadpole, . . 2*7 cm. long.

body, 1-0

tail, 1-7 „

Largest thyroid-fed tadpole, 1*1 cm. long.

body, 0-5 „
tail, 0-6

May 10. A great number of the thyroid-fed dead

196 ORIGIN OP NON-SEXUAL CHARACTERS

and the rest dying, lying at the bottom motionless.
They now had the tail much shorter, and the fore-legs
showing as well as the hind, but the latter not very
long, and without joints or toes.

Period from first feeding with thyroid, thirty days.
I now decided to feed the controls with thyroid,
expecting that as they were large and vigorous they
would have strength enough to complete the meta-
morphosis and become frogs.

May 15. Fed the controls with thyroid for first
time.

The smallest of them was in total length 1-7 cm.

body, 0-7 „
tail, 1-0 „

The largest measured was in total length 2-2 „

body, 0-8 „
tail, 1*4 „

May 25. All but two of the tadpoles dead. The
tails were only half the original length, all had well-
developed hind-legs, some with toes, but the fore-legs
were beneath the opercula, not projecting from the
surface.

Smallest total length, 1-2 cm.
body, 0-5 „
taU, 0-7 „

Largest total length, 1-8 „

body, 0-7 „

tail, 1-1 „

These last measurements were made after the
tadpoles had been preserved in spirit, and were
therefore doubtless somewhat less than in the fresh
condition. Making allowance for this it is evident
that the tails had undergone reduction as part of the

THE PHENOMENA OF MUTATION 197

metamorphosis, but the body was also shorter.
There is some reason therefore for concluding that
actual reduction in size of body occurs as the result
of metamorphosis induced by thyroid feeding. As
in the other case the skin and opercular membranes
were distended by liquid beneath them.

The total period of the change in this second ex-
periment was ten days.

I conclude that the amount of thyroid eaten was so
excessive as to cause pathological conditions as well
as precocious metamorphosis, so that the animals died
without completing the process.

On June 10 I still had four tadpoles which had
never had thyroid, but only pieces of meat, earth-
worm, or fish. These were very much larger than any
of the others, were active and vigorous, and the
largest one showed small rudiments of hind-legs, the
others none at all.

CHAPTER VII
METAMORPHOSIS AND RECAPITULATION

As one of the most remarkable examples of meta-
morphosis and recapitulation in connexion with
adaptation we will consider once more the case
of the Flat-fishes which I have already mentioned
in an earlier chapter. These fishes offer perhaps
the best example of the difference between gameto-
genic mutations and adaptive modifications. In
several species specimens occur occasionally in
which the asymmetry is not fully developed.1
These abnormalities are most frequent in the
Turbot, Brill, Flounder, and Plaice. The chief
abnormal features are pigmentation of the lower
side as well as of the upper, the eye of the lower
side, left or right according to the species, on the
edge of the head instead of the upper side, and the
dorsal fin with its attachment ceasing behind this
eye, the end of the fin projecting freely forwards over
the eye in the form of a hook. Such specimens have
been called ambicolorate, but it is an important fact
that they are also ambiarmate — that is to say, the
scales or tubercles which in the normal Flat-fish are
considerably reduced or absent on the lower side, in
these abnormal specimens are developed on the
lower side almost as much as on the upper. Minor

1 See 'Coloration of Skins of Fishes, especially of Pleuronectidao,'
Phil. Trans. Royal Soc., 1894.

198

RECAPITULATION 199

degrees of the abnormality occur : in Turbot with
the hook-like projection of the dorsal fin the lower
side of the head is often without pigment, while the
rest of the lower side is pigmented. Less degrees
of pigmentation of the lower side occur without
structural abnormality of the eye and dorsal fin.

There is no evidence that these abnormalities are
due to abnormal conditions of life. One specimen
of Plaice of this type was kept alive in the aquarium,
and it lay on its side, buried itself in the sand, and
when disturbed swam horizontally, like a normal
specimen. The abnormalities are undoubtedly
mutations of gametic origin. The development of
one of these abnormal specimens from the egg has
not to my knowledge been traced, but there is no
reason to suppose that the fish develops first into the
normal asymmetrical condition and then changes
gradually to the abnormal condition described. On
the contrary, everything points to the conclusion
that the abnormality is an arrest or incomplete
occurrence of the normal process of development, i.e.
of the normal metamorphosis. T. H. Morgan, in a
volume published some years ago,1 put forward the
extraordinary view that the Pleuronectidae arose
from symmetrical fishes by a mutation which was
entirely gametogenetic and entirely independent of
habits or external conditions, and then finding itself
with two eyes on one side of its head, and no air-
bladder, adopted the new mode of life, the new habit
of lying on the ground on one side in order to make
better use of its asymmetrically placed eyes. Accord-
ing to this view habits have been adapted to
structure, not structure to habits. We are thus to

1 Evolution and Adaptation.

200 METAMORPHOSIS AND

believe that Amphibia came out of the water and
breathed air because by an accidental mutation they
possessed lungs and a pulmonary circulation capable
of atmospheric respiration. Such is the result of
applying conclusions derived from phenomena of
one kind to phenomena of a totally different kind.
One of the chief differences between structural
features and correlations which are adaptive from
those which are not is the process of metamorphosis,
where we see the structure changing in the in-
dividual life history as the mode of life changes.
The egg of the Flat-fish develops into a symmetrical
pelagic larva similar to that of many other marine
fishes. The larva has an eye on each^ide of its head
and swims with its plane of symmetry in a vertical
position : it has also colour on both sides equally.
When the skeleton begins to develop the transforma-
tion takes place : the eye of one side, left in some
species, right in others, moves gradually to the edge
of the head and then on to the other side. The
dorsal fin extends forward, preserving its original
direction, and so passes between the eye that has
changed its position and the lower side of the fish,
on which that eye was originally situated. In some
cases this extension of the fin takes place earlier and
the eye passes beneath the base of the fin to reach the
other side. Any one who takes the trouble to make
himself acquainted with the facts will see that the
three chief features of the Pleuronectid — namely,
the position of the eyes, the extension of the dorsal
and ventral fins, and the absence of pigment from
the lower side — are not structurally correlated with
one another at all as changes in different parts of the
organism in a mutation are said to be, but are all

RECAPITULATION 201

closely related to their functions in the new position
of the body. A mutation consisting in general
asymmetry would be comprehensible, but the head
of the Pleuronectid is not asymmetrical in a general
sense, but only so far as to allow of the changed
position of the eyes. The posterior end of the skull
is as symmetrical as in any other fish, and in some
cases the mouth and jaws are also symmetrical,
entirely unaffected by the change in the position of
the eyes. In other cases the jaws are asymmetrical
in a direction opposite to that of the eyes, there is no
change of position but a much greater development
of the lower half of the jaws, reduction, with absence
of teeth, of the upper half. In the latter case the
fish feeds on worms and molluscs living on the
ground and seized with the lower half of the jaws,
in the former the food consists of small fish swimming
above the Flat-fish and seized with the whole of the
jaws (Turbot, Halibut, etc.).

I contend, then, that the mode in which the normal
Flat-fish develops is quite different from that in
which mutations arise. T. H. Morgan1 states that
a variation arising in the germ-plasm, no matter
what its cause, may affect any stage in the develop-
ment of the next individuals that arise from it. In
certain cases this is true, that is to say, when there
are very distinct stages already. For example, a
green caterpillar becomes a white butterfly with
black spots. A mutation might affect the black
spots, an individual might be produced which had
two spots on each wing instead of one, and no sign of
this mutation would be evident in the caterpillar.
But my contention is that when this mutation

1 A Critique of the Theory of Evolution (1916), p. 18.

202 METAMORPHOSIS AND

occurred, the original condition of one spot would
not be first developed and then gradually split into
two. Morgan proceeds to state clearly what I wish
to insist upon concerning mutations. He writes
that in recent times the idea that variations are dis-
continuous has become current. Actual experience,
he tells us, shows that new characters do not add
themselves to the line of existing characters, but if
they affect the adult characters, they change them
without as it were passing through and beyond them.

Now in the case of the ancestors of the Flat-fish
the adult and the larva must have had the same
symmetry with regard to eyes and colour and the
dorsal fin terminated behind the level of the eyes.
Thus the variations which gave rise to the Flat-fish
were not discontinuous but continuous. In each
individual development now, not merely hypo-
thetically in the ancestor, the condition of the adult
arises by an absolutely continuous change of the eyes,
fins, and colour. Such a continuous change cannot
be explained by a discontinuous variation, i.e. a
mutation. The abnormalities above mentioned on
the other hand, although they doubtless arise from
the same kind of symmetrical larva as the normal
Flat-fish, and develop by a gradual and continuous
process, do not presumably pass through the con-
dition of the normal adult Flat-fish and then change
gradually into the condition we find in them. As
compared with the normal Flat-fish they arise by a
discontinuous variation, they are mutations, whereas
the normal Flat-fish as compared with its sym-
metrical ancestor arises by a continuous change.

In order to make my meaning clear I must point
out that I have been using the word continuous in a

RECAPITULATION 203

different sense from that in which it is used by other
biologists, Bateson for example. The word has
been applied previously to variations which form a
continuous series in a large number of individuals,
each of which differs only slightly from those most
similar to it. No two individuals are exactly alike,
and thus such continuous variations are universal.
According to the theory of natural selection the
course of evolutionary change in any organ or
character would form a similar continuous series,
the mean of each generation differing only by a small
difference from that of the preceding. According
to the modern mutationists such small differences
are to be called fluctuations, and have no effect on
evolution at all, are not even hereditary, are not due
to genetic factors in the gametes. Discontinuous
variations, on the other hand, are as a rule differences
in an individual from the normal type and from its
parents of considerable degree, and are conspicuous :
these are what are called mutations.

The mutationists and Mendelians have not shown
how the essential characteristics of mutations are to
be reconciled with the facts of metamorphosis, or
with recapitulation in development which is so often
associated with metamorphosis. T. H. Morgan is
the only mutationist, so far as my reading has gone,
who has attempted to do this, and he seems to me to
have failed to understand the difficulties or even the
nature of the problem. He points out that the
embryos of Birds and Mammals have gill slits re-
presenting the same structures as those of the adult
Fish, but the young stage of the Fish also possessed
gill slits, therefore it is ' more probable that the
Mammal and Bird possess this stage in their de-

204 METAMORPHOSIS AND

velopment simply because it has never been lost.'
He concludes therefore that the gill slits of the
embryo Bird represent the gill slits of the embryo
Fish, and not the adult gill slits of the Fish, which
have been in some mysterious way pushed back into
the embryo of the Bird.

Morgan evidently does not realise that the Birds
and Reptiles must have been derived from Amphibia,
and that the embryo Reptile or Bird with gill slits and
gill arches is merely a tadpole enclosed in an egg shell.
The Frog in its adult state differs much from a Fish,
while the larva in its gill arches and gill slits resembles
a Fish. Morgan contends that the new characters
do not add themselves to the end of the line of
already existing characters. But in the case of the
Frog this is exactly what they have done. The exist-
ing characters were in this case the gill arches and
slits. Those who believe in recapitulation do not
suppose that the animal had to live a second life
added on to the life of its ancestors and that the
new characters appeared in the second life. They
believe that in the ancestor a certain character or
general structure of body when developed persisted
without change throughout life like the gill arches
and slits in a Fish. At some stage of life before
maturity this character underwent a change, and in
the descendants the development of the original
character and the change were repeated by heredity.
There is no ' mysterious pushing back of adult
characters into the embryo,' although it is possible or
even probable that in some cases the change gradu-
ally became earlier in the life history : it is the new
character which is pushed back, not the adult
character of the ancestor.

RECAPITULATION 205

It is perfectly true, as Morgan says, that new
characters which arise as discontinuous variations
— in other words, those kinds of variation which are
called mutations — do not add themselves to the line
of already existing characters, but * change the
adult characters without as it were passing through
and beyond them.' The mutations which Morgan
describes in his own experiments on Drosopkila
illustrate this in every case. In no case is the
original organ or character, e.g. wings, of the normal
Fly first developed and then changed by a gradual
continuous process into the new character. It
might perhaps be said that this took place in the
pupa, but that seems impossible, for the complete
wing is not fully developed in the pupa. The same
truth is equally apparent in the mutations described
in (Enothera. It follows, therefore, that none of
the evolutionary changes which have produced what
are called recapitulations can have been due to
changes of that kind which is known as mutation.

The abnormalities in Pleuronectidae to which I
have referred are of the kind usually regarded as
due to arrested development. But closer considera-
tion gives rise to doubt concerning the validity of
this explanation. It might be supposed that the
attached base of the dorsal fin is unable to extend
forward because the eye on the edge of the head is in
the way, but if the metamorphosis is arrested, why
should the fin grow forward in a free projection ? I
have described a very abnormal specimen of Turbot
in a paper communicated to the Zoological Society of
London,1 and in that paper have discussed other
possible explanations of these mutations. In the

1 Proc. Zool Soc., 1907.

206 METAMORPHOSIS AND

specimen to which I refer the pigmentation instead of
being present on both sides was reversed : the lower
side was pigmented from the posterior end to the edge
of the operculum (Plate n., fig. 2), while the upper side
was unpigmented excepting a scattering of minute
black specks and a little pigment on the head (Plate
n., fig. 1). I have suggested that the explanation
here is that in the zygote the primordia of a normal
body and a reversed head have been united together.
We may suppose that different parts of the body are
represented in the gametes by different determinants
or factors, and therefore it is possible that these
factors may be separated. In the specimen we are
considering the body is normal or nearly so, with
the pigmentation on the left side, which is normal
for the Turbot, while the head has both eyes with
some pigment on the right side and the left side
unpigmented. Reversed specimens occasionally
occur in many species of Pleuronectidae, and if the
determinants for a reversed head and a normal
body were united in one zygote, the curious
abnormality observed might be the result. It is
just a possibility that if this fish which was only
4 '4 cm. long had lived to adult size, the upper side
would have become pigmented under the influence
of light, while the strong hereditary influence would
have prevented the disappearance of the pigment
from the lower side. In that case the adult condition
would have been similar to that of ordinary ambi-
colorate specimens, but reversed, with eyes on the
right side instead of the left. Other explanations
of the more frequent ambicolorate mutation are
possible: the body may consist of two left sides
instead of a left and right, joined on to a normal

PLATE II.

FIG. I UPPER SIDE, FIG. 2 LOWER .SIDE OF
AN ABNORMAL SPECIMEN OF THE "fURBOT
(RHOMBUS MAXIMUS). THE SPECIMEN WAS
4.4 CM. IN LENGTH, AND WAS CAPTURED
ALIVE AT PADSTOW ON THE COAST OF
CORNWALL. REPRODUCED BY PERMISSION
OF THE COUNCIL FROM THE PROCEEDINGS
OF THE ZOOLOGICAL SOCIETY.

f.o.I

FIG. II

RECAPITULATION 207

head. But the first suggestion seems the more
probable, as two rights or two lefts would not be
symmetrical. Supposing the head and body not
properly to belong to each other, one being reversed
and one normal, we can in a way understand why
the dorsal fin does not form the usual connexion with
the edge of the head, because the determinants would
not be in the normal intimate relation to each other.
In thus writing of reversed and normal it must be
understood that the former word does not mean
merely turned over, for in that case right side of the
body would be joined to the left side of the head, and
the dorsal fin would be next to the ventral side of the
head, which is not the case. What is meant is that a
left side of the body which is normally pigmented is
joined to a left side of the head which instead of
having both eyes has neither, the two eyes being on
the right side of the head which is joined to the right
side of the body, and this is normal and unpigmented.
The dorsal fin belonging to the normal sinistral body
would therefore have a congenital tendency in the
metamorphosis to unite with the head on the outer
side of the original lower or right eye after it has
moved to the left side. Actually, however, in this
abnormal specimen it finds itself on the outer side of
the left eye which has passed to the right side, and it
has no tendency to unite with this part of the head.
At the same time it has no tendency to bend over at
an angle to reach the outer side of the right eye, and
therefore it grows directly forward without attach-
ment to the head at all.

It will be seen, therefore, that what is changed in
relative position in these mutations is not the actual
parts of the body, but merely the characters of those

208 METAMORPHOSIS AND

parts. In a sinistral Flat-fish, whether it is normally
sinistral like the Turbot or abnormally like a ' re-
versed ' Flounder, the viscera are in the same position
as in a dextral specimen : the liver is on the left side,
the coils of the intestine on the right. Thus in a
reversed or sinistral Flounder, which is normally
dextral, the left side which is uppermost is still the
left side, but it has colour and two eyes, whereas in
the normal specimen the right side has these char-
acters and not the left. Thus we are forced to
conceive of the determinants in the chromosomes of
the fertilised ovum which correspond to the two sides
of the body, as entirely distinct from the deter-
minants which cause the condition or ' characters '
of the two sides, unless indeed we suppose that
determinants of right side with eyes and colour occur
in some gametes and of right side without eyes and
colour in others, and vice versa, and that homozygous
and heterozygous combinations occur in f ertilisation.
On this last hypothesis the mutation here con-
sidered might be a heterozygous specimen, with the
dextral condition dominant in the head and the
sinistral in the body. Or it might be somehow due
to what Morgan and his colleagues have called
crossing over in the segregation of heterozygous
chromosomes, so that a part corresponding to a
sinistral body is united with a part corresponding to
a dextral head.

My conclusion from the evidence is that any
process of congenital development may in particular
zygotes exhibit a mutation, a departure from the
normal. We need not use the term heredity at all,
or if we do, must remember that in the present
argument it does not refer to any transmission from

RECAPITULATION 209

the parent. The factors in the gametes of the
normal Flat-fish egg cause the normal metamorphosis
to take place after the larval symmetry has lasted a
certain time. In occasional individuals the factors
whatever they are, portions of the chromosomes or
arrangement of the chromosomes or anything else,
are different from those of the normal egg, and in
consequence the abnormalities above described are
developed. But the chief fact which I cannot too
strongly emphasise is that the development of the
abnormality from the symmetrical larva is direct,
whether it is merely an arrest of development or an
abnormal combination of reversed and normal parts.
The abnormal development is not due to a change
occurring after the normal asymmetry has been
developed. These abnormalities are true mutations.
The evolution of the normal Flat-fish, on the other
hand, was obviously due to a change of a different
kind. Here we are dealing with the change from
a symmetrical fish to the asymmetrical. Judging
from what takes place in other mutations, it was
quite possible for asymmetry to have developed
directly from the egg, in consequence of some
difference in the chromosomes of the nucleus. It
has been shown that placing a fish egg for a short
time in MgCl2 * causes a cyclopean monstrosity to be
developed in which the two eyes are united into one :
but the two eyes do not develop separately first and
then gradually approach each other and unite, the
development of the optic cups is different from the
first. In the normal Flat-fish the evolution that has
occurred is the original development of the sym-
metrical fish, and the subsequent continuous gradual

1 Stockard, Arch. Eut. Mech., xxiii. (1907).
O

210 METAMORPHOSIS AND

change in eyes, fin, and colour to the adult Flat-fish
as we see it. All the evidence accumulated by the
experiments and observations of mutationists and
Mendelians goes to prove that this change is of an
entirely different kind from those variations which
are described as mutations, or as loss or addition of
genetic factors.

This being the case, we have to inquire what is the
explanation of the evolution of the normal meta-
morphosis.

The important fact is that the original symmetrical
structure of the larva and the asymmetrical structure
of the adult Flat-fish correspond to the different
positions of the body of the fish in relation to the
vertical, the horizontal ground at the bottom of the
water, and incidence of light. The larva swims
with its plane of symmetry vertical like most other
fishes ; its locomotion requires symmetrical de-
velopment of muscles and fins ; the two sides being
equally exposed to light, it requires an eye on each
side, and the pigment on each side is also related to
the equal exposure to light. The adult lying with
one side on the ground has its original plane of
symmetry horizontal and parallel to the ground, and
only the other side exposed to light, and on this side
only eyes and colour, i.e. pigment. The change of
structure corresponds with the change of habit. It
consists in the change of position of the lower eye,
the extension of the dorsal fin forwards, and the
disappearance of pigment from the lower side. In
the actual metamorphosis these changes take place
as the skeleton develops, before the hard bones are
fully formed, while the fish is still small, but the
young Turbot reaches a much larger size before

RECAPITULATION 211

metamorphosis is complete, namely, about one inch
in length, than the young Plaice or Flounder. It is
of little importance to consider whether at the
beginning of the evolution the change of position
occurred late or early in life. It may have become
earlier in the course of the evolution. The important
matter is to consider the evidence in support of the
conclusion that the relation to external conditions
has been the cause of the evolutionary change. We
have already seen that the nature of the change
and the relation of the change of structure to the
change of conditions necessarily tend to the infer-
ence that the latter is the cause of the former.
But we have to consider the particular changes in
detail.

To take first the loss of pigmentation from the
lower side. I have shown experimentally that
exposure of the lower sides of Flounders to light
reflected upwards from below causes development of
pigment on the lower side. At the same time the
experiments proved that the loss of pigment in the
fish in the natural state and the development of
it under exposure to light were not merely direct
results of the presence or absence of light in the in-
dividual, for in some cases the young fish were placed
in the apparatus before the pigment had entirely
disappeared from the lower side, and the meta-
morphosis went on, the lower side becoming quite
white, and the pigment only developed gradually
after long exposure to the light. In the principal
experiment four specimens were placed in the
apparatus on September 17, 1890, when about six
months old and 7 to 9 cm. in length. One of
these died on July 1, 1891, and had no pigment on

212 METAMORPHOSIS AND

the lower side. The other three all developed
pigment on that side. In one it was first noticed
in April 1891, and in the following November the
fish was 22 cm. long and had pigmentation over
the greater part of the lower side (Plate HI.). Micro-
scopically examined, the pigmentation was found to
consist of black and orange chromatophores exactly
similar to those of the upper side. Some hundreds
of young Flounders were reared at the same time
under ordinary conditions and none of them de-
veloped pigment.

It is clear, therefore, that exposure of the lower side
to light and reduction of the amount of light falling
on the upper side (for the tops of the aquaria used
were covered with opaque material) does not cause
the two sides to behave in the same way in respect of
pigment, as they would if the normal condition of the
fish was merely due to the difference in the exposure
to light of the two sides in the individual lif e. There
is a very strong congenital or hereditary tendency to
the disappearance of pigment from the lower side,
and this is only overcome after long exposure to the
light. On the other hand, if the disappearance of the
pigment were due to a mutation, were gametogenic
and entirely independent of external conditions,
there would be no development of pigment after
the longest exposure. To prove that an inherited
character is an acquired character is quite as good
evidence as to show that an acquired character is
inherited. The latter kind of evidence is very dif-
ficult to get, for the effect of conditions in a single
lifetime is but slight, and is not likely to show a
perceptible inherited effect. The theory that adap-
tations are due to the heredity of the effects of

PLATE III.

I-IGMKNTATION OF THE LOWER SIDE OF A

FLOUNDER (PLEURONECTES FLESUS) WHICH
WAS EXPOSED TO DAYLIGHT REFLECTED
FROM A MIRROR, FROM SEPT. 1890 JO
NOV. 1891. THE SPECIMEN WAS ABOUT SIX
MONTHS OLD AT THE BEGINNING OF THE
EXPERIMENT, AND THEN HAD NO PIGMENT
'ON THE LOWER SIDE. REPRODUCED BY
PERMISSION OF THE ROYAL SOCIETY FROM
PHIL. TRANS. 1893. B.

RECAPITULATION 213

stimulation assumes that the same stimulus has been
acting for many generations.

It is necessary, however, to consider how far the
conclusions drawn from these experiments are con-
tradicted by the mutations occurring in nature, some
of which have already been mentioned. We will
consider first ambicolorate specimens. If the
absence of pigment from the lower side in normal
Flat-fishes is due to the absence of light, how is it
that the pigmentation persists on the lower side of
ambicolorate specimens, which is no more exposed to
light than in normal specimens ? The answer is that
in the mutants the determinants for pigmentation
are united with the determinants for the lower side
of the fish. My view is that the differentiation of
these determinants for the two sides was due in the
course of evolution to the different exposure to light,
was of somatic origin, but once the congenital
factors or determinants were in existence they were
liable to mutation, and thus in the ambicolorate
specimens there is a congenital tendency to pig-
mentation on the lower side, which would only be
overcome by exclusion of light for another series of
generations.

Mutations also occur in which part or whole of
the upper side is white and unpigmented. Several
such specimens are mentioned in the memoir by
myself and Dr. MacMunn in the Phil. Trans, already
cited, one being a Sole which was entirely white on
the lower side, and also on the upper, which was
pigmented only over the head region from the free
edge of the operculum forwards. Since the upper
sides in these specimens are fully exposed to light in
the natural state and yet remain unpigmented, it

214 METAMORPHOSIS AND

would appear impossible to believe that the action of
light was the cause of the development of pigment on
the lower sides of normal specimens in my experi-
ments. To some it may be so, but in my own
opinion the one fact is as certain as the other. I
believe the two facts can be reconciled. I had one
specimen of Plaice in the living condition which had
the middle third of its upper surface white, and
the whole of the lower side white as usual. This
specimen was kept for 4J months with its lower
surface exposed to light and the upper side shaded.
At the end of that period there were numerous small
patches of pigment scattered over the lower side
principally in the regions of the interspinous bones,
above and below the lateral line. In the area of the
upper side, which was originally unpigmented, there
were also numerous small pigment spots. I believe,
therefore, that in this case there were determinants
for absence of pigment not only on the lower side but
on part of the upper side also, and that so long as
light was excluded from the lower side the patch on
the upper side remained unpigmented in sympathy.
When the congenital tendency of the determinants
on the lower side was overcome by the action of light,
the white patch on the upper side also began to
develop pigment.

Lastly, I may refer again to the specially abnormal
Turbot mentioned above. In this case the lower side
was over the greater part pigmented and the upper
side white, and this would appear to contradict the
conclusion just drawn concerning the piebald Plaice.
But this Turbot was only 4-4 cm. long, and is the
only case known to me where so much of the lower
side was pigmented with the upper side almost

RECAPITULATION 215

entirely white. The theory of sympathy or correla-
tion might apply here since the lower side of the head
was unpigmented, but from the small size of the
specimen and the amount of pigment on the lower
side, it seems to me most probable that if the
specimen had lived to be adult the upper side
would have developed pigment under the action
of light and the specimen would have become
ambicolorate.

When we compare the results reached by the
mutationists with those obtained by the Mendelians
we find that they tend to two different conceptions
of the relation between the gametes and the organism
developed from them. The effect of a change in
the determinants of the gametes according to the
mutationists is evident in every part of the plant.
A factor in Mendelian experiments usually affects
only one organ or one part of the organism. The
factor for double hallux in fowls, for instance, may
coexist with single comb or rose comb. The general
impression produced on the mind by study of
Mendelian phenomena is that the organism is a
mosaic of which every element corresponds to a
separate element in the chromosomes. Thus we
know that what we call a single factor may cause the
whole plumage of a fowl to have the detached barbs,
which constitutes the Silky character, but we also
know that an animal may be piebald, strongly pig-
mented in one part and white or unpigmented in
another. So we find in these Flat-fish mutations
mosaic-like forms which evidently result from
mosaic-like factors in the gametes, or in the chromo-
somes of the gametes.

Experimental evidence concerning the movement

216 METAMORPHOSIS AND

of the lower eye to the upper side and of the forward
extension of the dorsal fin has not been obtained,
though years ago I made some attempts, at the
suggestion of Mr. G. J. Romanes, to obtain such
evidence with regard to the eye by keeping young
Flounders, already partially metamorphosed, in a
reversed position. I did not succeed in devising
apparatus which would keep the young fish alive in
the reversed position for a sufficiently long time.
We can only consider, therefore, whether those other
changes can reasonably be attributed to the con-
ditions of life. Anatomical investigation shows that
the bony interorbital septum composed principally
of the frontal bones, which in symmetrical fish passes
between the eyes, is still between the eyes in the Flat-
fish, but has been bent round through an angle of
90 degrees on the upper side, while in the lower side
a new bony connexion has been formed on the outer
side of the eye which has moved from the lower side.
This connexion is due to a growth from the pre-
frontal backwards to join a process of the frontal, and
is entirely absent in symmetrical fishes. It is along
this bony bridge that the dorsal fin extends. The
origin of the eye muscles and of the optic nerves is
morphologically the same as in symmetrical fishes.
On the theory of modification by external stimuli we
must naturally attribute the dislocation of the eye of
the lower side to the muscular effort of the fish to
direct this eye to the dorsal edge, but something
may also be due to the pressure of the flat ground on
the eye-ball. There is little difficulty in attributing
the bending of the interorbital septum to pressure
of the lower eye-ball against it, pressure which is
probably due partly if not chiefly to the action of the

RECAPITULATION 217

eye muscles. The formation of the bony bridge
outside the dislocated eye is more difficult to explain,
as I have never had the opportunity to study the
relation of this bridge to the muscles. It is worth
mentioning that in the actual development of
Turbot and Brill the metamorphosis takes place to a
considerable degree while the young fish is pelagic,
before the habit of lying on the ground is assumed,
but of course this is no evidence that the change
was not originally caused by the habit of lying on
the ground.

With regard to the extension of the dorsal fin there
is no difficulty in discovering a stimulus which would
account for it. Symmetrical fishes propel them-
selves chiefly by the tail ; in shiffling over the
ground or swimming a little above it, Flat-fishes
move by means of undulations of the dorsal and
ventral fins. Increased movement produces hyper-
trophy, and according to the theory here maintained,
not merely enlargement of parts existing, but
phylogenetic increase in the number of such parts,
here fin rays and their muscles. In Flat-fishes the
dorsal and ventral fins extend along the whole
length of the dorsal and ventral edges : the dorsal
from the head, in some cases from a point anterior
to the eyes, to the base of the tail, the ventral from
the anus, which is pushed very far forward, to the
base of the tail, and in some species of Solidae these
fins are confluent with the caudal fin.

Formerly it was dogmatically maintained that
the effect of an external stimulus on somatic organs
or tissues could have no influence on the determinants
in the chromosomes of the gametes to which the
hereditary characters of the organism were due.

218 METAMORPHOSIS AND

As we have tried to show, this dogma is no longer
credible in face of the discoveries concerning
hormones. The hormone theory supposes that the
somatic modifications due to external stimuli — in the
case of the Flat-fish the disappearance of pigment
from the lower side, the torsion of the orbital region
of the skull, and the extension of the dorsal fin-
modify the hormones given off by these parts,
increasing some and decreasing others, and that these
changes in the hormones affect the determinants,
whatever they are, in the gametocytes within the
body.

Here arises an interesting question — namely, how
does the hormone theory explain the phenomenon of
metamorphosis any better than the mutation theory ?
It might be agreed that if the determinants are
stimulated or deprived of stimulation, the effect of
the change should logically show itself from the
beginning of development, and that therefore the
process of metamorphosis or indirect development
does not support the hormone theory any more than
the theory of gametogenic mutations. This objec-
tion may be answered in the following way. The
reason why the determinants give rise to the original
structure first and then change it into the new
structure is probably the same as that which causes
secondary sexual characters to develop only at the
stage of puberty. By the hypothesis the new habits
and new stimuli begin to act at some stage after the
complete development of the original structure of the
body. The differences in the original hormones of
the modified parts are therefore acting simultaneously
with the hormones, that is, the chemical substances
derived from all other parts of the body in its fully

RECAPITULATION 219

developed condition. It is very probable that in
the early stages of development the metabolism of
the body would be considerably different from that
of the adult stage, and the same combination of
hormones would not be present. We may suppose,
therefore, that the determinants of the zygote have
acquired a tendency to produce the increases and
decreases of tissue which constitute a certain modi-
fication, e.g. the change in the position of the eyes
in a Flat-fish, but the stimulus which caused this
tendency has always acted when the adult com-
bination of hormones was present. In consequence
of this the developed tissues do not undergo the
inherited modification until the adult combination is
again present. In this way we can form a definite
conception of the reason why an adaptive modifica-
tion is inherited at the same stage in which it was
produced, just as the antlers of a stag are only
developed when the hormone of the mature testis is
present. At the same time it is probable that the
age at which the inherited development takes place
tends to become earlier in later generations, to occur
in fact as soon as the necessary hormone medium is
present.

The diagnostic characters of some of the species of
Pleuronectidae have been mentioned in an earlier
part of this volume, in order to point out that they
have no relation to differences of habit or external
conditions. Here it is to be pointed out that there
is no evidence that they arise by metamorphosis.
The scales, for example, afford distinct and constant
diagnostic characters both of species and genera,
but their peculiarities have not been found to arise
by modification of a primitive form. The rough

220 METAMORPHOSIS AND

tubercles of the Flounder, and the scattered thorn-
like tubercles of the Turbot, develop directly, not by
the continuous modification of imbricated scales.
There is, however, one scale-character among the
Pleuronectidae which appears to stand in direct con-
tradiction to the conclusions drawn by me concerning
scales in general. It not only develops by a gradual
change, but it is a secondary sexual character de-
veloping in the males only at maturity. The char-
acter was described by E. W. L. Holt in specimens
of the Baltic variety of the Plaice, Pleuronectes
platessa,1 and consists in the spinulation of the
posterior edges of the scales, especially on the upper
side, in mature males. The same condition, but to a
much slighter degree, was afterwards shown by my-
self to occur constantly in Plaice from the English
Channel and North Sea.2 It occurs also in P.
glacialis, the representative of the Plaice in more
northern seas. I have shown that the spinules
develop in the mature males not as a modification
of the scale, but as separate calcareous deposits the
bases of which afterwards become united to the
scale. It would seem that the development of this
character is dependent on the hormone from the
mature testis, and in order to conform with the
arguments used by me in other cases, the spinulation
should have some definite function in relation to the
habits of the sexes, and this function should involve
some kind of external stimulation restricted to the
mature male. So far, however, no evidence what-
ever of such function or such stimulation has been
discovered. It is possible that the case differs from

1 Jvurn. Mar. Biol Assn , vol. iii. (Plymouth, 1893-95).
* Ibid., vol. iv. p. 323.

RECAPITULATION 221

other secondary sexual characters such as the antlers
of stags in one respect, namely, that since the Dab
(P. limanda), the Sole, and other species of Solea, and
several other Pleuronectidae have what are called
ctenoid scales — that is, scales furnished with spines
on the posterior edge — and since the ordinary scales
of the Plaice are reduced, the spinulation of scales in
the mature male Plaice is not a new character but
the retention of a primitive character. Then the
question would remain why the scales in the mature
female and immature male have degenerated, or
rather why the primitive character develops only in
the mature stage of the male.

There is one point in which this sexual dimorphism
in the Plaice appears to differ from typical cases, and
which suggests that the greater spinulation of scales
in the males has no function at all in the relations
of the sexes, and is therefore not subject to any
external stimulation. This point is the remarkable
way in which the degree of development of spiny
armature differs in different regions and in local
races, and seems to correspond to different climatic
conditions. Both Plaice and Flounders in the Baltic
are much more spiny than in the North Sea, although
in the Flounder no sexual difference in this respect
has been noted. On the east coast of North America
occurs P. glacialis, in which the scales of the male are
strongly spinulate and those of the female smooth.
On the coast of Alaska females of this species seem
to be more spinulate than elsewhere. The Flounder
does not occur in the Arctic, but on the west coast of
North America occurs a local form called P. stellatus,
scarcely distinct as a species, which has a strong
development of spiny tubercles all over the upper

222 METAMORPHOSIS AND

side. The Flounders of the Mediterranean are much
less spinous than those of the North Sea or Channel.
The Dab (P. limanda) occurs on the American
coast in a local form called Limanda ferruginea,
and in the North Pacific there is a rougher form
called L. aspera. In these three species therefore,
apart from mutations, the northern forms all show a
greater development of spines on the scales. Whether
this is an effect of colder temperature it is difficult
to say. It is possible that the difference is due to
external conditions, of which lower temperature of
the water is the most obvious, and it may be that
these conditions have a greater effect on the male
than on the female in the Plaice.

Sexual differences in scales, which have a function
in the relations of the sexes, occur in a few other
fishes, and these can be attributed with good reason
to mechanical stimulation. For example, in the
Rajidae among Elasmobranchs the males possess
on each ' wing ' or pectoral two series of large,
recurved, hooked spines. It has been stated,1
apparently by Yarrell, that these spines are de-
veloped only in the breeding season. It is doubtful
if there is any marked breeding season in these fishes,
but it is probable that the spines are absent in the
immature male, as it is known that in Raid clavata
the adult male has sharp pointed teeth, while the
young male and the female at all ages have broad
flat teeth. It is supposed that the spines and perhaps
the sharp teeth are used for holding the female, but
it seems equally probable that these structures are
really used by the males in fighting with each other.
The habits of these marine fish have not been much

1 Darwin, Descent of Nan (2nd edit., 1885), p. 331.

RECAPITULATION 223

observed, but there is little reason to doubt that
these differences in scales and teeth correspond with
differences of mechanical stimulation. This does not
at all imply that the scales and teeth themselves
have been produced by mechanical stimulation, or
that the difference between the dermal denticles of
Elasmobranchs and the scales of Teleosteans corre-
spond to differences of stimulation. But the degree
of development of a structure whose presence is due
to gametic factors may very probably be modified
by external stimulation, and the modification may
become hereditary. If the views here advocated are
true, the two processes mutation and modification
must be always acting together and affecting the
development not only of the individual but of any
organ or structure. Thus the peculiarities of antlers
in stags, it seems to me, prove that the mechanical
stimulation due to fighting was the cause of the
evolution of antlers, that without the habit of fighting
in the males antlers would not exist. At the same
time each species of the Cervidae has its special char-
acters in the antlers, in shape and branching, and it
would be impossible to attribute these to differences
in mode of fighting : they are due to mutation.

In connexion with the metamorphosis of Am-
phibia the case of the Axolotl has always been of
very great interest. In the few small lakes near
the city of Mexico where it occurs it has never
been known to undergo metamorphosis but is
aquatic throughout its life and breeds in that con-
dition. Yet in captivity by reducing the quantity
of water in which it is placed the young Axolotl can
be forced to breathe air, and then it undergoes
complete metamorphosis to the abranchiate con-

224 METAMORPHOSIS AND

dition. The same species in other parts of North
America normally goes through the metamorphosis,
like other species of the Urodela. It is evident,
therefore, that the Mexican Axolotls, although they
have been perennibranchiate for a great number of
generations, have not lost the hereditary tendency
to the metamorphosis which changes the larvae of
Amblystoma elsewhere into an air-breathing ter-
restrial animal. This may be regarded as evidence
that the conditions of life which prevent the meta-
morphosis in the Mexican Axolotl have produced no
hereditary effect. The fact, however, that Axolotls
require special treatment to induce metamorphosis
seems to show that they have distinctly less con-
genital tendency to metamorphosis than larvae of
the same species, Amblystoma tigrinum, in other
parts of North America, and this difference must be
attributed to the inherited effect of the conditions.
The most important of these conditions seems to
be abundance of oxygen in solution in the water, and
the next in importance abundance of food in the
water. Recently it has been shown that the meta-
morphosis may be induced by feeding Axolotls on
thyroid gland. But there is no reason to suppose
that a congenital defect of thyroid arising as a muta-
tion was the original cause of the neoteny, i.e. the
persistence of the larval or aquatic, branchiate
condition. Such a supposition would imply that
the association between Axolotls and the peculiar
Mexican lakes, supplied with oxygenated water
by springs at the bottom, was purely accidental.
Moreover, there is no evidence that there is any
deficiency of thyroid in the Axolotl. The secretion
of the thyroid gland is necessary for the normal

RECAPITULATION 225

growth and development of all Vertebrates, and we
are only beginning to understand the effects of
defect or excess of this secretion. There is nothing
very surprising in the fact that excess in the case
of the Axolotl causes the occurrence of the meta-
morphosis which had already in numerous experi-
ments been produced by forcing the animals to
breathe air.

Metamorphosis, as in the development of gill
arches and gill slits in the embryos of Birds, Reptiles,
and Mammals, exhibits a recapitulation of the stages
of evolution of certain organs. But in the case
of other organs the absence of recapitulation is
remarkable by contrast. If, as I believe, the de-
velopment of lungs and disappearance of gills was
directly due to the necessity of breathing air, it is
difficult to avoid the conclusion that the terrestrial
legs were originally evolved from some type of fishes'
fins by the use of the fins for terrestrial locomotion.
Yet neither the amphibian larva nor the embryo
of higher Vertebrates develops anything closely
similar to a fin. There is no gradual change of a fin-
like limb into a leg, but the leg develops directly
from a simple bud of tissue. The larva of the
Urodela is probably more primitive than the tadpole
of the Frogs and Toads, and in the former the legs
develop while the external gills are still large, long
before the animal leaves the water.

It is possible that the limbs were transformed to
the terrestrial type before the animal itself became
terrestrial, the habit of swimming having been
partly abandoned for that of crawling or walking
at the bottom of the water, and the tail being used
merely for swimming to the surface to obtain air.

p

226 METAMORPHOSIS AND

But the condition of the Dipnoi, which possess lungs
but do not walk on land, does not support this
supposition, for they possess fins which are either
filamentous or fin-like, having a central axis with
rays on each side. There can be little doubt that
the digits of the terrestrial limb are homologous
with endoskeletal fin-rays, but the evolution of the
axis of the limb is not to be ascertained either from
development or palaeontology. The absence of meta-
morphosis here may perhaps be due to the fact that
the lateral fins ceased to function in the earlier aquatic
stages, only the caudal fin being used for swimming.
If this were the case the absence of metamorphosis in
the legs is itself an adaptation, the disuse of the
paired limbs in the larva having caused the earlier
fin-like stages of these limbs to disappear, while the
terrestrial leg was developed later by heredity, just
as the legs have disappeared in the larvae of many
insects, though fully developed in the adult.

Metamorphosis of structure in Amphibia and in
Flat-fishes corresponds to the change of conditions
of life in the free-living animal. In the case of the
eyes of the Cave-fishes the -conditions in respect of
absence of light are constant throughout life, and
we find only an embryonic development of the
eye taking place by heredity. The question arises
whether, when there is no embryonic recapitulation,
it must be concluded that apparent adaptations are
due to mutation and not to function or external
conditions. One case of this kind is that of the
limbs of Snakes, where, if we except the vestiges of
hind limbs in the Pythons, there is no trace of limbs
either in the embryo or after hatching. There are
several similar cases among Reptiles and Amphibia.

RECAPITULATION 227

The Slow-worm (Anguis fragilis) is limbless, and
so are the members\of the sub-class Apoda among
the Amphibia. In these also rudiments of limbs
are entirely absent in the embryos or larval stages.
Considering the recent evolution of Snakes as com-
pared with the origin of lungs and loss of gills
and gill slits in terrestrial Vertebrates in general,
we have here a remarkable contrast which shows in
the first place the difference resulting when the
change in habits and conditions in the one case takes
place from one stage of life to another, and in the
other case the new habits are constant throughout
life from the moment of hatching. It seems to me
that in the present state of our knowledge we cannot
form a decisive opinion on the question whether the
absence of limbs in such cases is the result of
mutation or of disuse — that is, absence of functional
stimulation.

The power of flight is an excellent example of
adaptation. It has been evolved independently in
Pterodactyls, Bats, and Birds. In the two first
groups, and to a slight degree in the third, the expanse
of the wing is formed by an extension of the skin into
a thin membrane, supported by the fore-limbs. It is
not necessary to argue in detail that the evolution of
this membrane and of the modifications of bones and
muscles by which it is supported and moved, can be
satisfactorily explained on the theory that modifica-
tions due to mechanical and functional stimulation
are ultimately inherited. In birds, however, the
surface of the wing is supplied chiefly by feathers,
and consideration of the matter affords no reason for
supposing that the evolution of feathers was due to
any external or functional stimulation. It is often

228 METAMORPHOSIS AND

stated that the feathers of birds are a modification
of the epidermic scales of reptiles, but investigation
does not fully confirm this statement. The reptilian
scales are retained on the tarso-metatarsal region of
the leg in the majority of birds, and it would be ex-
pected, if the view just quoted were correct, that a
transition from scales to feathers would be visible
at the ankle- joint. This, however, is not the case.
In fowls some breeds have scaly shanks and others
feathered. In those with scaly legs I have found
cases in which, in the chicks, there were two or three
very minute feathers, and I have examined these
microscopically by means of sections of the skin.
The result was to show that the minute feathers were
not a prolongation of the tips or edges of the scales,
but arose from follicles between the scales. The
scale is flat and is a fold of the epidermis not arising
from an invaginated follicle. The feather, on the
other hand, is a tubular structure arising from a
papilla at the base of a deep follicle extending
inwards from the surface of the skin. As the feather
grows the papilla grows with it. This papilla
consists of vascular dermal, i.e. mesodermic tissue,
and if the feather is pulled out during growth
bleeding occurs. The epidermic horny tube splits
posteriorly towards the apex of the feather, and is
divided into rachis and barbs, and thus the dermal
tissue within, by this time dead and dry, is exposed
and is shed. Every feather is in fact an open
wound, and is perhaps the only other case, in
addition to that of the antlers of stags, in which
vascular mesodermic tissue is normally shed in such
considerable quantities. When the development of
the feather is complete, growth gradually ceases,

RECAPITULATION 229

the proximal part of the feather remains tubular and
does not split, and the vascular tissue within dies,
shrivels, and dries up, forming the pith of the quill.
When the papilla recommences to grow the old
feather is pushed out, and this process causes the
moult. It would appear, therefore, that the feather
must have been evolved, not by a continuous modi-
fication from the scale but by a development of a
new kind between the scales. I have been unable
to discover hitherto any evidence suggesting an
external stimulus which could cause this remarkable
process of development in feathers, or indicating
that the function of flight would involve such a
stimulus. For the present, therefore, we must con-
clude that feathers are not an adaptation, and not
due to somatogenic modification, but must be the
result of a gametogenic mutation.

Feathers, having been evolved, served in the
wings and tail as important organs of flight. There
is reason to believe that, once present, the size and
growth of feathers was modified greatly by the
degree of stimulation applied to the papillae at their
roots by the movement and bending strain of the
feathers. The modification of the bones and muscles
of the wing, shoulders, and sternum by the functional
stimuli involved in flying are obviously adaptations,
and in my opinion are only to be explained as the
hereditary effects of functional stimulation, like all
skeleto-muscular adaptations. The strains produced
in bones by muscular contraction produce hyper-
trophy of the part of the bone to which the muscles
are attached, and thus we can understand the origin
of the carina of the sternum in flying birds, and its
absence in flightless forms. In bats and in ptero-

230 METAMORPHOSIS AND

dactyls also the sternum is produced into a carina
along the median line. The reduction of the digits
of the wing in birds to three, with the bones firmly
united together, would follow from their use in flight
and their disuse as digits, and it would seem, from
the fact that the flight-feathers must have been
always on the posterior edge of the wing, and that
the ulna is larger than the radius, that the three
digits which have persisted are the 3rd, 4th, and 5th,
and not the 1st, 2nd, and 3rd as usually taught. A
comparison of the hind-limbs of birds with those
of bats and pterodactyls suggests strongly that the
patagium flyers have arisen from arboreal or climb-
ing animals, while the birds arose from terrestrial
forms which acquired the bipedal habit, as certain
reptiles have. An arboreal animal would necessarily
use all four limbs, as climbing animals actually
do. The wings of birds, on the other hand,
would have arisen from the endeavour to increase
speed by movements of the fore-limbs. The
perching birds would therefore have arisen by
later adaptations after the power of flight had been
evolved.

Complete recapitulation does not occur in the
development of the digits of the whig. Only a
rudiment of a fourth digit has been found in the
embryonic wing, not, as might be expected, rudiments
of five digits of which two disappear. The meta-
carpals are free, not united as in the adult, and there
are separate distal carpals, which in the adult are
united with the metacarpals. In other respects the
modifications of wings and sternum are so obviously
adaptive that it is difficult to believe that the
reduction of digits was not due to disuse. This is

RECAPITULATION 231

another of those cases in which the function to which
structure is adapted is constant from the beginning
of independent life to the end, and there is some
ground for believing that in course of time in such
cases embryonic recapitulation may be much
diminished or disappear. The period of time since
birds were first evolved is in all probability im-
mensely greater than that which has elapsed since
the blind fish, Amblyopsis, was modified by cave-
life, so that we can understand why the eye is de-
veloped to a certain stage in the embryo of the blind
fish, although it lives in darkness all its life, while
embryonic recapitulation in the wing of the bird is
very incomplete.

In another class of adaptations the embryonic or
larval stage is adapted to new conditions, while the
adult condition is either less changed or not changed
at all. One of the most obvious examples of this
is the allantois in the Amniota. The embryos of
Reptiles, Birds, and Mammals all develop two
embryonic or foetal membranes, the amnion and
the allantois. Of the function or origin of the
amnion little is known : to state that it is protective
affords little explanation. It seems possible that
it is merely the mechanical result of the weight of the
embryo and the development of the allantois. The
latter is a precocious hypertrophy of the cloacal
bladder found in Amphibia, with the function of
embryonic respiration. In the water the amphibian
larva respires by means of gills and gill slits. In
adaptation to terrestrial life it is necessary, if the
free aquatic larval stage is to be eliminated, that the
embryo should be able to breathe air before hatching.
Various Amphibia show how this requirement was

232 METAMORPHOSIS AND

met in various ways. In the South American tree-
frogs of the genus Nototrema the eggs are developed
in a dorsal pouch of the skin of the female, and
within this pouch the respiration of the embryo is
carried on by a membranous expansion of the second
and third external gills on each side. In the Reptilia
the bladder is expanded for the same function, and
absorbs oxygen and gives off carbon dioxide through
the pores of the shell. It is impossible to reconcile
the conception of mutation with the adaptive re-
lation between this allantois and the expulsion of the
egg enclosed in a shell on land. The transition
probably came about gradually from the deposition
of the eggs in moist places but not in water. In the
midwife toad (Alytes obstetricans) the male carries
the eggs about attached to his legs, respiration is
effected by enlarged external gills, and the larvae
are hatched in water. In the ancestral reptiles
external gills may have helped at first, until by the
enlargement of the bladder they were rendered un-
necessary. In all such cases the absorption of oxygen
must be regarded as the stimulus which caused the
enlargement of the respiratory membrane. As the
allantois could not be absorbed or retracted again
into the abdomen, the umbilicus was evolved — that is
to say, the scar formed by the union of the folded
edge between the body wall and amnion surrounding
the stalk of the allantois. It would be difficult for a
mutationist to explain how a mutation should affect
the development of the cloacal bladder to such an
enormous degree, just when it was required for
embryonic respiration, and cause the sides of the body
to unite ventrally at the time of hatching, cutting off
the allantois and the amnion.

RECAPITULATION 233

T. H. Morgan l states that a mutation of gametic
origin may affect any stage in the development of
the individual. This may be true when there are
already distinct stages in the life history. The
more important question is whether distinct stages
can be caused by mutation. It is true that in
heterozygous individuals characters may develop
more fully in the adult stage than in the young. But
when we find different stages evidently adapted to
different modes of life, it is impossible to explain them
by mutations affecting different stages of life. In
such cases as the larval stages of Insects we find that
the larvae have become adapted to new habits while
the adults have remained unchanged, or have evolved
quite independent adaptations. For example, the
adults in the chief orders of Insects have the typical
three pairs of legs, while the maggots or grubs of
the Diptera or Hymenoptera have no legs at all,
the caterpillars of Lepidoptera have evolved pseudo-
legs on the abdomen, and the larvae of Coleoptera
have the ordinary legs and no more. This is the
reverse of recapitulation : in the case of legless
maggots, and caterpillars with pro-legs, the adult is
more similar to the ancestor than the larva. But
the same principle holds, that where functions and
habits are different, there organs are different. No
mutationist has yet produced by breeding experi-
ments a caterpillar without the three pairs of thoracic
legs and yet developing into a moth that had the
normal three pairs. Morgan, with all his mutations
of the adult Drosophila, says nothing of larval
mutants possessing legs. The only rational con-
clusion is that legless larvae have lost the legs through

1 A Critique of the Theory of Evolution, p. 18.

234 METAMORPHOSIS AND

disuse, since those larvae which are destitute of legs
do not go in search of food but either live in the
midst of it or are fed by others, and that the pro-legs
of the caterpillar have been developed by the
muscular action of the insect in clinging to leaves.
Here again the hormone theory, although we cannot
pretend to understand the matter completely, helps
us to form a conception of the process of heredity and
evolution. The disuse of legs in the larva affects the
determinants, so that they remain inactive in the
presence of the hormones produced in the body
generally in this stage. In the adult stage activity
of the legs produces hormones which influence the
same determinants in the gametes, to develop legs,
but again in the presence of the different hormones
which are present in the body generally in the adult
stage. As the habits of larva and adult became
more specialised and contrasted, the change became
less and less gradual, and the intermediate stage, not
being adapted to any transitional mode of life,
became an inactive pupa in which the adult organs
develop.

In conclusion I will briefly consider the attempts
which have been made to prove the influence of
somatic modifications or characters on the gametes
by direct experiment. The method of Kammerer of
inducing changes of habit or structure by conditions,
and then showing that the change is in some degree
inherited, has already been mentioned. One
obvious criticism of this evidence is that it seems to
prove too much, for it is difficult to believe that a
change produced in individuals would show so much
hereditary effect in their immediate offspring.
Two other methods are conceivable by which the

RECAPITULATION 235

influence of somatic hormones might be made
evident. One of these is to graft ovaries or testes
from one animal into another which possesses a
certain somatic character, and then to see if the
offspring produced from these gonads shows any
trace of the character of the foreign soma in which it
was nourished. C. C. Guthrie x claimed to have done
this in his experiments on hens. He grafted the
ovaries of two Black Leghorn pullets into two White
pullets of the same breed, and vice versa. The
black and the white birds bred true when mated
to cocks of their own colour. The black hen with
white ovary mated with black cock produced four
black chicks and two black chicks with white legs,
the white hen with black ovary mated with white
cock produced some white chicks, some black and
some white with black spots. This is held to prove
that the transplanted ovaries were functional,
because they produced evidence of the character
originally belonging to them. On the other hand,
the black hen with white ovary mated with white
cock produced nine white chicks, and eleven chicks
which were white spotted with black, and the white
hen with black ovary mated with black cock pro-
duced not black chicks but white chicks spotted
with black. This was held to prove that the
somatic characters of the ' foster mothers ' were
transmitted.

Davenport repeated Guthrie' s experiments on
different fowls, grafting the ovary from a cinnamon-
coloured hen into a white hen, and mating her with
a cinnamon-coloured cock. The chicks were exactly
similar to those obtained from crossing such a cock

1 Journ. Exper. Zool (1908), v.

236 METAMORPHOSIS AND

with a normal white hen, and Davenport concludes
that the engrafted ovary was not functional but had
degenerated. It is known to be almost if not quite
impossible to remove the ovary completely from a
hen, owing to its close attachment over the great
post-caval vein. At the same time it is difficult to
see how Guthrie could have obtained black and
spotted chicks from a white hen mated with a white
cock if the grafted ovary from a black hen had not
been functional. One point which Guthrie does not
mention, and of which apparently he was not aware,
is that the white of the White Leghorn is dominant
to colour, the heterozygotes not being pure white but
white with spots. Thus when he mated a black cock
with a white hen with grafted ovary and obtained
spotted chicks, this would have been the result if
the original white ovary was functional. None of his
results prove conclusively the influence of the soma
of the hen into which ovaries were grafted, but would
all be explained if some eggs were derived from the
part of the original ovary not removed in the opera-
tion, and others from the grafted ovary.

The grafting of ovaries in Mammals has often been
tried, but very rarely with success. The introduced
ovary usually dies and is absorbed. C. Foa *• states
that he made bilateral grafts of ovaries from new-
born rabbits into adult rabbits, and two months
after the operation one of the operated females was
fecundated and produced five normal young. In
other cases he placed ovaries from new-born young
in positions far from the normal position, such as the
space between the uterus and bladder, and in one
case the female so treated became pregnant, and

1 Arch. Ital. de Bid. (1901), Tome xxxv.

RECAPITULATION 237

when killed had a single embryo in one uterus and no
trace of the original ovaries in the normal position.
But Foa was not investigating the influence of
somatic characters on ova in the grafted ovaries, and
does not even mention the characters or breed of the
rabbits he used or of the young which were produced
from the grafted ovaries. Castle x carried out
seventy-four transplantations of ovaries principally
in guinea-pigs. Out of all these only one grafted
female produced young. In this case the ovaries
of two different black guinea-pigs about one month
old were grafted into an albino female about five
months old. After recovery the grafted female
was kept with an albino male. She produced six
young in three pregnancies, first two, then one, and
lastly died with three foetus in the uteri. All these
were black, with some red hairs among the black.
One of the first two young had a white forefoot. In
this case black is dominant, and therefore there is
nothing extraordinary in the offspring from a black
grafted ovary being black. The presence of red
hairs and a white foot is no evidence of the influence
of the foster soma, but is due to imperfect domin-
ance. When the same male was mated with a normal
black female the offspring were black with red
hairs interspersed.

All these experiments are open to the following
criticism. It has been the main argument of this
volume that there are two distinct kinds of characters
in all organisms — namely, those of somatogenic
origin and those of gametogenic origin. Theory
supposes that somatic modifications by means of

1 W. E. Castle and J. C. Phillips, On Germinal Transplantation in
Vertebrates, Pub. Carnegie Institution in Washington (1911), No. 144.

238 METAMORPHOSIS AND

hormones affect the determinants in the gametes.
But it is obvious that the black and white of Leghorn
fowls and of guinea-pigs are gametogenic characters,
and are strongly established in the gametes of their
respective varieties. It is not even certain that the
black or white hair or feathers are giving off special
hormones which would or could influence the
gametes. The hormone theory only postulates such
influence from hormones issuing from tissues modified
by external stimuli. It is quite certain that the
black colour in Leghorns or guinea-pigs is not due
to any external stimulus or influence. The experi-
ments therefore are entirely irrelevant to what has
been called the inheritance of acquired characters.
All that they can be said to prove is that an albino
soma does not convert ingrafted ova of black race
into ova carrying the albino character.

It is probably impossible to prove experimentally
the influence of a modified soma in one generation.
I have endeavoured to find a case which would not
be open to the above criticism — that is, to find a
character which could be considered somatogenic and
which was absent in a closely allied variety. Most
of the characters in domesticated varieties are
obviously gametogenic mutations, but the lop-ear in
rabbits may be, partly at least, somatogenic. Since
many breeds have upright ears, we cannot say that
disuse of the external ear has produced lop-ears in
domesticated rabbits generally, but in lop-eared
breeds the ears are much enlarged ; and though this
may be gametogenic, the increased weight may
have been the cause of the loss of the power to erect
the ears. I therefore tried grafting ovaries from
straight-eared females into lop-eared individuals.

RECAPITULATION 239

The operation was perfectly successful in seven
specimens — that is to say, they recovered completely
and lived for many months, up to a year or more,
afterwards, but none of them became pregnant.
When killed no trace of ovary was found in any of
them ; in every case it had been completely absorbed,
and the uteri and vagina were diminished in size, and
anaemic. For grafting I used ovaries from young
rabbits of various ages, from seven days to six
weeks or more, but all were equally unsuccessful.
Satisfactory evidence by direct experiment of the
inheritance of somatogenic modifications due to
external stimuli cannot be said to have been yet
produced, and, as I have shown, such evidence from
the nature of the case must be very difficult to
obtain. The indirect evidence, however, which has
been considered in this volume is too strong to be
ignored — namely, the case of Japanese long-tailed
fowls, that of colour on the lower sides of Flat-fishes,
and the similarity of the congenital development
of the antlers in stags, to the generally admitted
effects of mechanical stimulation and injury on the
skin and superficial bones of Mammals.

The general conclusions which are logically to be
drawn from our present knowledge with regard to
the problems of heredity and evolution in animals
are in my opinion as follows : —

1. All attempts to explain adaptation by gameto-
genic mutations, or changes in gametic factors or
' genes,' have completely failed, as Bateson himself
has admitted.

2. The facts discovered concerning mutations and
Mendelian heredity harmonise with the nature of the
majority of specific and varietal characters, and with

240 METAMORPHOSIS AND

the diagnostic characters of many larger divisions in
classification.

3. Some of the most striking cases of adaptation,
such as the organs of respiration and circulation
in terrestrial Vertebrates, and the asymmetry of
Flat-fishes, are developed in the individual by a
metamorphosis which is generally regarded as a
recapitulation of the ancestral evolution. No cases
of mutation or gametogenic variation hitherto
described exhibit a similar metamorphosis or re-
capitulation.

4. Secondary sexual characters, usually in the
male sex, correspond in their development with the
development of maturity and functional activity in
the gonads, and it has been proved that the latter
influence the former by means of c hormones ' or
internal secretions. The evidence concerning sex
and sex-linked characters and the localisation of
their factors in the chromosomes of the gametes has
no bearing on the action of hormones.

5. The facts concerning the action of hormones
are beyond the scope of current conceptions of the
action of factors or genes localised in the gametes
and particularly in the chromosomes. According
to these conceptions, characters are determined
entirely by the genes in the chromosomes, whereas in
certain cases the development of organs or characters
depends on a chemical substance secreted in some
distant part of the body.

6. It was formerly stated that no process was
known or could be conceived by which modifications
produced in the soma by external stimuli could
affect the determinants in the gametes in such a way
that the modifications would be inherited. The

RECAPITULATION 241

knowledge now obtained concerning the nature and
action of hormones shows that such a process
actually exists, and in modern theory real substances
of the nature of special chemical compounds take
the place of the imaginary gemmules of Darwin's
theory of pangenesis or the ' constitutional units '
of Spencer.

7. The theory of the heredity of somatogenic modi-
fications by means of hormones harmonises with
and goes far to explain the facts of metamorphosis
and recapitulation in adaptive characters, and also
the origin of secondary sexual characters, their
correlation with the periodical changes in the gonads
and the effects of castration. At the same time
there are some somatic sex-characters, e.g. hi insects
and birds, which do not appear to be correlated with
changes in the gonads, and which are probably
gametogenic, not somatogenic in origin.

8. The theory of the heredity of somatogenic
modifications is not in opposition to the mutation
theory. The author's view is that there are two
kinds of variation in evolution, one somatogenic and
due to external stimuli, acting either directly on
passive tissues or indirectly through function, and
the other gametogenic and due to changes in the
chromosomes of the gametes which are spontaneous
and not in any way due to modifications of the soma.
Adaptations are due to somatogenic modifications,
non-adaptive diagnostic characters to gametogenic
mutations. It is a mistake to attempt to explain
all the results of evolution by a single principle.
There are two kinds of congenital, constitutional or
hereditary characters in all organisms, namely, the
adaptive and the non-adaptive, and every distinct

Q

242 RECAPITULATION

type in classification exhibits a combination of the
two. To assert that all characters are adaptive is as
erroneous as to state that all characters are blasto-
genic mutations, and therefore in their origin non-
adaptive.

9. Finally it may be urged, although the question
has not been directly discussed in this volume, that
no biologist is justified in the present state of know-
ledge in dogmatically teaching the lay public that
gametogenic characters are alone worthy of attention
in questions of eugenics and sociology. Hereditary
or constitutional factors are of course of the highest
importance, but there exists very good evidence that
modifications due to external stimulus do not perish
with the individual, but are in some degree handed
on to succeeding generations, and that good qualities
and improvement of the race are not exclusively
due to mutations which are entirely independent of
external stimuli and functional activity. It is
important to produce good stock, but it is also
necessary to exercise and develop the moral, mental,
and physical qualities of that stock, not merely for
the benefit of the individual, but for the benefit of
succeeding generations and to prevent degeneration.

INDEX

Abraxas grossulariata and lacticolor,
108.

Adaptations, origin of, 7, 21 ; evolu-
tion of, 188.

Agonus cataphractus, 28.

Albinism, 187.

Allantois, 231.

Allurements, 70.

Alytes obatetricans, 232.

Amblyopsis, eyes of, 189.

Amblystoma tigrinum, 224.

Amnion, 231.

Anableps tetr ophthalmia, 192.

Anas boscas, 84 ; crosses of, 186.

tristis, crosses, 186.

Ancel and Bouin, 92.

Anguis fragilis, 227.

Antilocapra, 165.

Antirrhinum, crossing of, 173.

Antlers of stags, 71, 122.

Ants, heredity of sex in, 55.

Aphidae, heredity of sex in, 56.

Apoda, 227.

Axolotl, albino, 192 ; metamorphosis,
223 ; influence of thyroid feeding,
224.

BARRED plumage in fowls, 115.

Basch, 99.

Bateson, 17, 44, 171.

Bees, heredity of sex in, 55.

Bernard, Claude, 91.

Berthold, A. A., 91.

Biedl, 145 ; and Konigstein, 98.

Bionomics, 3.

Blindness in cave animals, 189.

Bombyx mori, 158.

Boring, Miss, 95.

Born and Frankel, 103, 145.

Brachydactyly, 171.

Bresslau, 135.

Brown-Sequard, 91.

Biihler, 143.

Cambarus, males of, 73.
Capons, 74.

Castle, experiments in grafting, 237 ;
on sex, 58.

Castration, 73 ; in ducks, 81 ; of

frog, 163 ; of Lepidoptera, 158.
Cats, heredity of colour in, 117.
Cave animals, absence of pigment,

191.

Cephalopoda, 2.

Cetacea, absence of scrotum, 150.
Chelonia, 36.
Chologaster agassizii, 191.
Chromosomes, 41 ; in mutations,

183.

Clevelandia, 190.
Colaptes, 18.
Colour-blindness, 113 ; heredity of,

108.
Colours, origin of, in domesticated

breeds, 187.
Comb of fowls, uselessness of,

Corpora lutea, evolution of, 139; in
viviparous lower vertebrates, 143 ;
origin of, 98, 100.

Corystes cassivelaunus, 2.

Courtship, organs of, 70.

Criss-cross inheritance, 112.

Crossing over, 176.

Cryptorchidism, 94, 152.

Cuttle-fishes, 2.

Cyclostomes, absence of corpora
lutea in, 143.

Cytology, 65.

Cytoplasm, in heredity, 43.

Dafila acuta, crosses, 186.
Daphnia, heredity of sex in, 56.
Darwin, 6, 14.
Dasyurus, 146 ; corpora lutea, 144 ;

lactation, 98, 101, 138.
Davenport, 235.
Determinants, 43.
Determination of sex, 120.
Dipnoi, fins, 226.
Dog-fishes, oviparous and viviparous,

4.
Dominant characters, origin of, 44,

171.
I Doncaster, 110; on heredity in cats,

117.

248

244

HORMONES AND HEREDITY

Drosophila, blind mutation, 189 ;
heredity of sex, 61, 112; muta-
tions, 175.

Ducks, crosses of, 186.

Dutch rabbit, 173.

EARTHWORMS, sex in, 64.

Eclipse plumage, 82.

Eigenmann, 190, 191.

Eimer, 181.

Elasmobranchs, 35 ; corpus luteum

in, 143.

Elephants, testes, 151.
Eugenics, 242.
Eunuch, 73.
Evolution, evidence of, 6.

FACTORS, origin of, 44.
Feathers, 36 ; evolution of, 227.
Flat-fishes, 21, 22, 33 ; mutations of,

198-203.

Flight, evolution of, 227.
Flounder, 25.
Foa, on lactation, 98 ; on grafting

ovaries, 236.
— Eoges, 76.
Fowls, castration of, 74 ; origin of

breeds, 187.
Fractionation of Mendelian factors,

50.

Frankel, 103.
Frog, thumb-pad, 163.

Qallus banJciva, 45, 46, 187.
Gates, Dr. R. Ruggles, 177.
Geddes and Thomson, 65.
Gemmules, 241.
Genital ducts, 69.
Oigas, (Enothera, 184.
Gillichthy*, 190.
Gipsy moth, 158.
Goltz and Ewald, 97.
Gonads, hormones of, 91, 92.
Goodale, H. D., 81.
Grafting, of ovaries or testes, 235.
Graves' disease, 194.
Gudernatsch, 193.
Guthrie, C. C., 235.
Gynandromorphism, 154, 159.

HAEMOPHILIA, 115.

Hanau, 76.

Hegner, 42.

Herdwick sheep, castration in, 93.

Heredity, 41 ; and sex, 88.

Hermaphroditism, 64.

Hill, J. P., 102.

Horns, 128.

Houssaye, 35.

Inachus scorpio, 166.
Insects, heredity of sex in, 57.
Interstitial cells, 94.
Intromittent organs, 69.

JAPANESE long-tailed fowls, 129 ;
artificial treatment of, 132.

KAMMERER, 234.
Kellog, 158.
Kopec, 159.

LACTATION, dependence on stimula-
tion, 137 ; in males, 138 ; regula-
tion of, 97.

Laevifolia, (Enothera, 182.

Lamarck, 11.

Lamarckian theory, 133.

Lane-Claypon, Miss, 95 ; and Star-
ling, on ovaries of rabbit, 102.

Larvae of insects, 233.

Lata, (Enothera, 184.

Leghorn, White, 172.

Lemon-dab, 2,6.

Leopold and Ravana, 145.

Lepidoptera, castration in, 158.

Leptinotarsa, 42.

Limantria dispar, 158.

Limon, 100.

Linnaeus, 14.

Lode, 76.

Loeb, on blind fish, 190, 191 ; on
blindness in cave animals, 189 ; on
tadpoles and thyroid, 193.

Lop-eared rabbits, grafting experi-
ments, 238.

Lotsy, Professor, 45 : on crossing,
173.

Lutein, of corpora lutea, 140.

MALE characters in female, 87.

Mallard crosses, 186.

Mammary glands, 8 ; origin of, 134 ;
rudimentary in male, 90.

Marshall, 142 ; and Jolly, 99.

Marsupials, relation of foetus to
pouch, 144 ; scrotum of, 150.

Masked crab, 2.

Meisenheimer, 158 ; thumb-pad of
frog, 165.

Mendel's Principles of Heredity, 20,
107.

Mendelism, 17, 20, 43 ; and castra-
tion, 89.

Menstruation, 145.

Metamorphosis, 188 ; in Flat-fishes,
200 ; causes of, 212-217 ; and hor-
mones, 218 ; and diagnostic charac-
ters, 219.

INDEX

245

Michaux, 180.

Midwife toad, 232.

Milk glands, 96.

Mole, eyes of, 189.

Monotremata, origin of milk glands,
136.

Morgan, T. H., 21, 233; on blind-
ness in cave animals, 189 ; on
mutations, 174, 175 ; on sex, 60 ;
on sex-linked heredity, 112; on
sexual dimorphism in Drosophila,
120 ; on variation, 201.

Mutations, 174 ; in antlers, 128.

NATURAL selection, 6, 15, 37.
Nuptial plumage, 83.
Nussbaum, 163.
Nyssia zonaria, 161.

Q'DoNOGHUE, 98, 101 ; development

of milk glands, 135.
{Enothera, mutations, 180 ; grandi-

flora, 180 ; lata, 178, 181 ; La-

marckiana, 178-181.
Onagra, species of, 180.
Origin of Species, Darwin's, 6.
Ornithorhynchus, corpus luteum, 142.
Orthogenesis, 181.
Otariidae, scrotum, 151.
Ovaries, position of, 151.
Ovary, in birds,) 83.
Ovulation, 145.

PANGENESIS, 241.

Parthenogenesis, 56.

Parturition, 146.

Pearson, Karl, 40.

Pheasant, male, 83 ; gynandromor-

phism in, 155.
Phillips, John C., 185.
Philosophic Zoologique, 11.
Phocidae, testes, 151.
Physiology of Reproduction, 142.
Picotee Sweet Pea, 50, 173.
Pigeons, 46.
Pigment, absence in cave animals,

191.

Pile fowls, 47.
Pintail duck, crosses, 186.
Plaice, 25.
Pleuronectes Jlesus, 25 ; glacialis, 27 ;

platessa, 25.

Plymouth Rock fowl, 115.
Pole-dab, 26.
Poll, 156.
Preformation, 174.
Problems of Genetics, 17.
Prong-buck, 165.

Pro-03strus, 146.

Proteut, 185 ; eyes of, 189,

Prototheria, milk glands in, 136.

RABBITS, lactation in, 97.
Recapitulation, absence of, 225 ; and

mutations, 203.

Reptiles, corpora lutea in, 143.
Reversal, in Flat-fishes, 206.
Rhinoderma darwinii, 70.
Ribbert, 97.
Rieger, 76.
Rodents, testes, 152.
Romanes, 17.

Rontgen rays, effect on testes, 95.
Rose comb, in fowls, 43.
Rotifers, heredity of sex in, 56.
Rubricalyx, (Enothera, 182.
Rubrinervis, (Enothera, 182.

Sacculina, 166.

Salamanders, transplantation of eye,

190.

Sandes, 144.
Schuster, Edgar, 163.
Scrotum, origin of, 147.
Sea-horse, 28.

Secondary sexual characters, 67.
Selheim, 76.

Semilata, (Enothera, 183.
Sertoli's cells, 93.
Sex, 37; chromosomes, 59 ; Mendelian

theory of, 53.
Sex-linked heredity, 108.
Sexual Dimorphism, 68.
Sexual dimorphism, 119 ; in Rajidae,

222 ; in Plaice, 220, 221.
Shattock and Seligmann, 75, 156.
Silkworm, 158.

Silky fowl, 44, 46 ; plumage of, 178.
Sirenia, absence of scrotum, 150.
Slow-worm, 227.
Smith, Geoffrey, 163, 166-168.
Snakes, absence of limbs, 226.
Sociology, 242.

Somatic sexual characters, 67, 68.
Species, conception of, 9 ; origin of,

11, 14, 21 ; characters of, 37 ;

sterility and hybridism, 185.
Spermatogenesis, in man, 115.
Starling and Lane-Claypon, on lacta-
tion, 97.
Steinach, 104 ; heredity of milk

glands, 136.

Sternum, carina of, 229.
Swallows, 36.
Sweet Pea, 187.
Swifts, 36.

246 HORMONES AND HEREDITY

TADPOLES, effect of thyroid in, 193-

197.

Tandler and Gross, 95.
Taxonomies, 1.
Teleosteans, 35 ; corpora lutea in,

143 ; ovarian follicles, 101.
Testes, descent of, 147.
Tetraploidy, 184.
Thayer, 33.

Thumb-pad of frog, 69.
Thyroid-gland feeding, 193-197.
Tortoise-shell colour in cats, 117.
Tosa fowls, Japanese, 129.
Transplantation of gonads, 104.
Typhlogobius, 190, 191.

UHLENHUTH, 190.
Urodela, larva, 225.

VARIATIONS, 41.

Vespa vulgaris, 19 ; germanica, 19.

Vries, De, 177.

WALLART, 100.

Wasps, 18 ; heredity of sex in, 55.

Weapons, organs used as, 70.

Weismann, 16.

Whale, paddle of, 8.

White Leghorn, crosses, 48.

Wilson, E. B., 58, 61.

Wing, development of,|230.

Winiwarter, von, 115.

Witch, 26.

Wood, T. B., on crossing of sheep,

107.

Woodland, W., 148.
Woodpecker, 8, 18.

X chromosome, 58.

Zeugopterus, 29, 34.
I Zoaea, 4.

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