Title: Hormones and Heredity
 - A Discussion of the Evolution of Adaptations and the Evolution of Species
Author: Cunningham, J. T. (Joseph Thomas)
Language: English
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Juliet Sutherland, Charles Bidwell


HORMONES 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



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 imperfectly 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, chromosomes, 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 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 - Historical Survey Of Theories Or Suggestions Of Chemical
               Influence In Heredity

CHAPTER I - Classification And Adaptation

CHAPTER II - Mendelism And The Heredity Of Sex

CHAPTER III - Influence Of Hormones On Development Of Somatic
              Sex-Characters

CHAPTER IV - Origin Of Somatic Sex-Characters In Evolution

CHAPTER V - Mammalian Sexual Characters,
            Evidence Opposed To The Hormone Theory

CHAPTER VI - Origin Of Non-Sexual Characters: The Phenomena Of Mutation

CHAPTER VII - Metamorphosis and Recapitulation

INDEX


  LIST OF PLATES

PLATE I. Recessive Pile Fowls

PLATE II. Abnormal Specimen Of Turbot

PLATE III. Flounder, Showing Pigmentation Of Lower Side
           After Exposure To Light



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 [Footnote: Yves Delage, _L'Hérédité_
(Paris, 1895), pp. 806-812.] 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 empêche l'oeuf de recevoir la modification reversible c'est
qu'étant constitué autrement que les cellules différenciées de l'organisme
il est influencé autrement qu'elles par les mêmes causes perturbatrices.
Mais est-il impossible que malgré la différence de constitution
physico-chimiques il soit influencé de la même façon?'

The author's meaning would probably have been better expressed if he had
written 'ce qui paraît empêcher.' 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 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 substances 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 in 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 modification
which reacts on the constitution of the egg.

Suppose the internal secretion of a gland (_e.g._ glucose for the liver,
glycolytic for the ferment for the pancreas) is the physiological excitant
for the gland. If the gland is removed in whole or in part the proportion
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 generation 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, for their removal causes increase in the blood
of the exciting urea and uric acid.

The effects of disuse are similar to those of mutilations and of use vice
versa. Delage, as seen above, does not consider that increase or decrease
of particular muscles can be inherited, but only the muscular system in
general. If, however, in consequence 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 individual: there would thus be a heredity produced indirectly.
With regard to general conditions of life, Delage states that there are
only two of which we know anything--namely, climate and alimentation--and
he merely suggests that temperature 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.

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 vigour.
But only those characters of which the corresponding 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
[Footnote: P. Schiefferdecker, _Ueber Symbiose_. S.B. d. Niederrhein.
Gesellsch. zu Bonn.  Sitzung der Medicinischen Sektion, 13 Juni 1904.]
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 found a
connexion between the modified parts of the body and the germ-cells, the
only connexion in existence. It is to 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 nourishment 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 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 für Entwicklungsmechanik der Organismen_,
in which it was published in 1908.

In his volume on the Embryology of the Invertebrata, 1914 (_Text-Book of
Embryology_, edited by Walter Heape, vol. i.), Professor E. W. MacBride in
his general summary (chapter xviii.) 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 period of development at which they were
originally formed, and reinforce the action of the environment. 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 unpublished suggestions of
Professor Langley, and not to the publications of authors who had
previously discussed the possible action of hormones in connexion 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 Contributions 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 determinants, 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 of
Acquired Conditions in Man.' The belief that this paper had two years'
priority over the volume of Delage entitled _L'Hérédité_ appears to have
arisen from the fact that Adami consulted the bibliographical list in
Thomson's compilation, _Heredity_ 1908, where the date of Delage's work is
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 properties, 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. 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 characters 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 intoxication, 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 somatogenic. 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 evolution 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
stimulation, 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 of the part of the paper considered does Adami
mention internal secretions. His actual words are: '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
Taxonomics, deals, in fact, with organisms as dead specimens, and it
emphasises especially the distinguishing characters of the ultimate
subdivisions of the various tribes of animals and plants--namely, species
and varieties. The investigation, on the other hand, of the different
modes of life of animals or 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 observing 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
zoological 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 functions is to capture active prey
such as prawns. They act as a kind of double lasso. Sepia, for instance,
approaches gently and cautiously till it is within striking distance of a
prawn, then the two long tentacles 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 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
respiration of the crab while it is buried, to a depth of two or three
inches. The results of the investigation 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 Taxonomics or Bionomics
according as they deal with the structure of the dead organism or the
action of the living. Anatomy and its more theoretical interpretation,
morphology, are related to Taxonomics, physiology and its branches to
Bionomics. 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, the better we can understand the sexual and parental habits
of different kinds of animals.

The two branches of biological study which we are contrasting cannot,
however, be completely separated even by those whose studies are most
specialised. In Bionomics it is necessary to distinguish 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. Taxonomics, 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 independent form. In the end
taxonomy had to be altered so as to conform with the fact of development,
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
existing 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 evolution has taken place, that the history of organisms has
been a process of descent with modification. 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
arguments 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 evolution 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 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
modifications, 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 he 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, to distinguish between two kinds
of becoming--namely, the development of the organ in the individual and
its evolution in the course of descent. The word 'adaptation' is itself
the cause of much fallacious reasoning and confusion of ideas, inasmuch 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
knocking 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, such as colour and patterns, or scales and armour,
which may he 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 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 universal, or that
it existed in the original ancestors. How then was it evolved? The second
great problem 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
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 considered 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 _Philosophie 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:--

'Il sera en effet évident que l'état où nous voyons tous les animaux, est
d'une part, le produit de la composition croissante de l'organisation, qui
tend à former une gradation régulière, et de l'autre part qu'il est celui
des influences d'une multitude de circonstances très différentes qui
tendent continuellement à détruire la régularité dans la gradation de la
composition croissante de l'organisation.

'Ici il devient nécessaire de m'expliquer sur le sens que j'attache à ces
expressions: Les circonstances influent sur la forme et l'organisation des
animaux, c'est-à-dire qu'en devenant très différentes elles changent avec
le temps et cette forme et l'organisation elle-même par des modifications
proportionnées.

'Assurément si l'on prenait ces expressions à la lettre, on m'attribuerait
une erreur; car quelles que puissent être les circonstances elles
n'opèrent directement sur la forme et sur l'organisation des animaux
aucune modification quelconque. Mais de grands changements dans les
circonstances amènent pour les animaux de grands changements dans leurs
besoins et de pareils changements dans les besoins en amènent
nécessairement dans les actions. Or, si les nouveaux besoins deviennent
constants ou très durables, les animaux prennent alors de nouvelles
habitudes qui sont aussi durables que les besoins qui les ont fait naître.
Il en sera résulté l'emploi de telle partie par préférence à celui de
telle autre, et dans certains cas le défaut 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':--

PREMIÈRE LOI

'Dans tout animal qui n'a point dépassé le terme de ses développements
l'emploi plus fréquent et soutenu d'un organe quelconque, fortifie peu à
peu cet organe, le développe, l'agrandit et lui donne une puissance
proportionée à la durée de cet emploi; tandis que le défaut constant
d'usage de tel organe Paffaiblit insensiblement, le détériore, diminue
progressivement ses facultés, et finit par le faire disparaître.

DEUXIÈME LOI

'Tout ce que la nature a fait acquérir ou perdre aux individus par
l'influence des circonstances ou leur race se trouve depuis longtemps
exposée, et par conséquent, par l'influence de l'emploi prédominant de tel
organe, ou par celle d'un défaut constant d'usage de telle partie, elle le
conserve par la génération aux nouveaux individus qui en proviennent,
pourvu que les changements acquis soient communs aux deux sexes, ou à 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 outgrowths 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.'

Lamarck's actual words (_Phil. Zool.,_ edit. 1873, p. 254) are: 'Dans
leurs accès de colière qui sont fréquents surtout entre les mâles, leur
sentiment intérieurs par ses efforts dirige plus fortement les fluides
vers cette partie de leur tete, et il s'y fait une secrétion de matière
cornée dans les uns (_Bovidae_) et de matière osseuse mélangée de matière
cornée dans les autres (_Cervidae_), qui donne lieu à des protubérances
solides: de là l'origine des cornes, et des bois, dont la plupart de ces
animaux ont la tête armée.'

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 principle of the persistence and fundamentally unchangeable 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. 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 distinguishing 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 maintained 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 adaptation. 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
requirements. He pointed out that there is always a 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 predetermined in the fertilised ovum. It is to
Weismann that we owe precise and definite conceptions, 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
redeveloped in the soma of the next generation. He 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 altogether, 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 maintains 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 important 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, 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 stripe.       2. Male with red cheek stripe.

3. Adult female with no                3. Adult female with usually
   cheek stripe.                          brown cheek stripe.

4. A scarlet nuchal crescent           4. No nuchal crescent in
   in both sexes.                         either sex.

5. Throat and fore-neck brown.         5. Throat and fore-neck grey.

6. Top of head and hind-neck grey.     6. Top of head and hind-neck brown.

7. General tone of plumage             7. General tone of plumage
   olivaceous.                            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 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 remarks 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 conditions in the country occupied by
_auratus_, extending 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 selection 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 subterranean
nests, but of somewhat different materials. That of _V. vulgaris_ is of a
characteristic yellow colour, because made of rotten wood, while that of
_V. germanica_ is grey, from the weathered surface wood of palings or
other exposed timber which is used in its construction. In characters the
differences of the two forms are so slight as to be distinguishable 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 structure; 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 conditions 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
attributes can make a perceptible difference to the prospect of success,
than that an indefinite and impalpable 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 '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 character 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 structure 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 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, grasshoppers 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 distinguish the group from
other sub-orders, though there are other non-adaptive characters which
indicate 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 'adaptive' quality in these characters, as in other cases, does
not necessarily consist in their utility to the animal, but in the
definite relation between them and the external conditions. When the
relation 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 resemblance 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 utility, but relation to surroundings or to
function or to habit. In this case colour is related to incidence 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 contrary, 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 unaffected 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 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, those of the upper side diminished,
and teeth are confined to the lower side. Here it is not a question of the
jaws 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
Pleuronectidae. The genus _Pleuronectes_ is distinguished by the following
characters: eyes on the right side, mouth terminal and rather small, teeth
most developed 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 tuberoles 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
slignt curve above the pectoral fin.

_P. limanda_, the Dab: scales uniform all over the body, with spinules on
the projecting edges, making the skin rough; lateral line with a
semicircular 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 distribution 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 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 development 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 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 condition was due to lower temperature. But we do
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 Teleosteans, 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 Seahorse among the Syngnathidae, the body is
protected by a complete suit of bony armour; but accompanying these in the
littoral region are numerous other species such as the Gobies, and even
other species of Syngnathidae which have soft unprotected skins.

Similarly with colour characters: the power of changing the colour so as
to harmonize 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 nothing to do with protective resemblance,
variable or permanent. The red spots of the Plaice are specific and
diagnostic, but they confer no advantage 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 somewhat obscure. The
theory that they are sensitive to differences of hydrostatic pressure as
the fish moves from one depth to another rests on no foundation, 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 permeated
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 difference
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 conclude 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 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 _Zeugopterus,_ which is most important for the
present discussion, is the prolongation of the dorsal and ventral fins on
to the lower 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 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 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 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, showed 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 respiratory movements, and consequently there 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 through 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 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 adaptation is not adequate. We require something more
than function or utility to express the difference between the two kinds
of characters to be distinguished. For example, the absence of
pigmentation 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
correspondence.

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 surroundings, the diminution
in coloration compensating for 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 contrast, 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 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 of 8-1/2 inches. The subcaudal
fin-flaps are developed in _norvegicus_, most in _punctatus_; each has
four rays in _norvegicus_ and _unimaculatus_, six in _punctatus_. The
shortening 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 seas 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 organization for flight, the structure of the wing,
of the sternum, breast muscles, legs, etc., are all co-ordinated for 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 distinction 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 explained 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
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 variations 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 environment.
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 rudimentary in the other. The
conception of adaptation applies to these also, since we find that
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 inherited 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 now 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 the characters are determined 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,
that modifications may be caused during development 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. Karl Pearson and other
devotees of the cult of Eugenics have been lately impressing on the public
by pamphlets, lectures, and addresses the great importance 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, 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 generation 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 condition 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 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 reduction 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, [Footnote: R. W. Hegner, 'Experiments with Chrysomelid Beetles,'
III., _Biological Bulletin_, vol. xx. 1910-11.] 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 protoplasm 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 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 determine 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 convenient 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. 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 importance, 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 recessive to the white-skin character of
the Brown Leghorn, and he assumes that the genetic properties of _Gallus
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. [Footnote: _Problems of
Genetics_, p. 85.] 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 suppressed 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 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 conceptions.

My own experiments in crossing Silky fowls with _Gallus bankiva_
(_P.Z.S._, 1919) show that the recessive 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 pairs of characters altogether
in which the Silky 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 _F1_
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 generations
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.

Still more striking was the incomplete segregation in the plumage colour.
The white of the Silky was recessive, all the birds of the _F1_ generation
being fully coloured. In the _F2_ 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.

 [Illustration: PLATE I. Recessive 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 experiment. From the statements of John Douglas in
_Wright's 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 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
_F1_ 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 an 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 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 race 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 represented 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 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 permanent and
indestructible, he is satisfied that they may occasionally undergo a
quantitative disintegration, 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
consequences 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.


  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
maintenance 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 chromosomes
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 subsequent to fertilisation,
but now there is good reason to believe that generally the sex of the
individual 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 individual 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 chromosomes 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 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 _f_, we have the ordinary sexual union represented by

  _Ff_x_ff_;

the gametes will then be

  _F_+_f_ and _f_+_f_

and the fertilisations

  _Ff_ and _ff_,

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 megagamete 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 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 chromosomes
coming from the original female gamete which would disappear, and the
spermatozoon would be incapable of transmitting characters derived 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 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 segregate in the reduction divisions, how is it
that the male quality in the _f_ 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
different 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 remarkable facts concerning the
relation of chromosomes 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 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 reduction 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 plentiful 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 parthenogenesis, 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, _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 2_N_. 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 2_N_-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_-l 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 parthenogenesis, only gametes of
one kind each with _N_ chromosomes, but the male forms gametes of two
sorts, one with N chromosomes, the other with _N_-l or _N_-2 chromosomes.
On fertilisation two kinds of zygotes are formed, female-producing eggs
with 2_N_ chromosomes, and male-producing eggs with 2_N_-1 or 2_N_-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 2_N_ chromosomes,
female-producing with 2_N_+.

Such is the summary given by Castle in 1912. [Footnote: _Heredity and
Eugenics_, by Castle and Others. University of Chicago Press, 1912.] 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, [Footnote: '_The Determination of Sex_,' _Science
Progress_, April 1910.] a very instructive _résumé_ 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,' McCluny the 'accessory chromosome,'
Montgomery the 'hetero-chromosome,' while the names 'heterotropic
chromosome' and idiochromosome have also been used. For the purpose of the
present discussion we may conveniently 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 chromosomes, 2_N_-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
2_N_-2 in one spermatocyte and 2_N_-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_-1 chromosomes from one of the
first spermatocytes and one with _N_ and one with _N_-1 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 _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 _Syromaster, 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 determined
by the male gamete, the _X_ chromosome united with that of the female
gamete producing female individuals, while the _Y_ united with _X_
produces 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, but has obtained numerous mutations, so that his work is a very
important contribution to the mutation 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 [Footnote: _A Critique of the
Theory of Evolution_. Princeton University Press and Oxford University
Press, 1916.] 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-chromosomes: 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 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_ chromosome 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_[female]
must be eliminated since the reduced egg develops always into a male. But
on fertilisation, since the fertilised egg develops into a female, a
dominant _X_[female] must come from the 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-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 gravitation 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 XY chromosomes 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 unfertilised _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 following reduction division, give rise to ova,
while an ovum containing one X sex-chromosome, or two different, XY,
chromosomes, at the next reduction 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 spermatozoon 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 compounded 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 XY 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 differentiation 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 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 cytoplasm. From this point of view the
hypothesis mentioned 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
metabolism, the ovum being more anabolic, the sperm more katabolic. A
double quantity of special chromatin may be the cause of the greater
anabolism 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 accumulation 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 femaleness 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 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 development of the sperms is the result of the
presence of the single X chromosome in the successive generations 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 femaleness, were segregated
in the same way as a pair of somatic allelomorphs, but the words maleness
and femaleness expressed no real conceptions. The view above suggested
merely attempts to bring our real knowledge of the difference between 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 in
the two sexes. On the one hand, we have extraordinary 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 invertebrate 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 enlargement 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 characters 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 differences we find that they can be classified according to their
relation to fertilisation and reproduction, 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 Elasmobranchs (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 fertilisation 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, 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 outgrowths 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 produced 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
physical agents. Mechanical irritation, especially of the interrupted
kind, repeated blows or friction causes hypertrophy of the epidermis and
of superficial 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 hypertrophied 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 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 absorption 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 antler
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 rudimentary in the other,
but that their development is connected with the functional maturity and
activity 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, generally 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 development corresponding to the periodic activity of the testes
or ovaries. Stags have a limited breeding or '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 commencement 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-breeding 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 consequence 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?

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.' [Footnote: _Archiv für Entwicklungesmechanik_, 1908.]
When castration is performed soon after birth a minute, simple spike
antler is developed, 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 their 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 stated that the capon will rear chickens, though he does not
incubate, 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 [Footnote: _Proc. Linn. Soc.,
1857.] (1857), who writes as follows:--

'The capon ceases to crow, the comb and gills do 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
characters 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 intestines, and produce
spermatozoa, which, of course hare no outlet. In such cases the secondary
male characters may fee more or less completely developed. 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 minute
nodule of testicular tissue showing normal spermatogenesis was found on
post mortem examination attached to the intestine. In this bird there was
no male development of comb or wattles, a full development of neck
hackles, a certain development of saddle hackles, a few straggling badly
curved feathers in the tail and short blunt spurs on the legs. Lode
[Footnote: _Wiener klin. Wochenschr._, 1895.] (1895) found that testes
could easily be transplanted into subcutaneous tissue and elsewhere, and
that the male characters then developed normally. Hanau [Footnote: _Arch.
f. ges. Physiologie_, 1896.] (1896) obtained the same result.

The question, however, to what degree the male characters of the cock are
suppressed after complete 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
Foges [Footnote: _Pfügers Archiv_, 1902.] (1902) state that the true capon
is characterised 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
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 reduction of the spurs, not entire
absence. It is important in discussing the effects of castration in cocks
to bear in mind the actual course of development 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 developing, 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 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, however, whether castration in the cock
is ever quite complete. In the original wild species and in the majority
of domesticated breeds the spurs are confined 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
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 he 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 attachment 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.

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 1-1/3 inches high by 2-3/8 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 15-5/8
inches, blue-black in colour.

I had no entire cock of same breed, but measured the entire White Leghorn
for comparison. Comb 1-3/4 inches high by 3-3/4 inches in length. (It is
to be remembered that the comb and wattles are especially large in
Leghorns.) Wattle 1-1/4 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 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 [Footnote: 'Castration of Drakes.' _Biol.
Bulletin_, Wood's Hole, Mass., vol. xx., 1910] 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 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 (b) 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, (b) 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 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
therefore 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 [Footnote: Relation between
Seasonal Assumption of the Eclipse Plumage in the Mallard _(Anas boscas_)
and the Functions of the Testicle.' _Proc. Zool. Soc._ 1914.] begins with
a comparison between the stages of the development 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 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 spermatogenesis 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.
Spermatogenesis 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 nuptial plumage takes place in June when
spermatogenesis has ceased and the testes are diminishing in size, but the
redevelopment of this plumage takes place in September without any renewed
activity of the testis and long before the beginning of spermatogenesis.
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 operation
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 imperfection
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
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 vermiculated 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 appearance 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 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, [Footnote: 'Ueber
Geweihbildung und Geweihentwicklung.' _Arch. Ent.-Mech._ x. and xi.] 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 [Footnote: 'True Hermaphroditism in Domestic Fowl, etc.' _Trans.
Path. Soc._, Lond., 57. 1, 1906.] 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 character 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. They
regard the condition of the ovary as insufficient to explain the
development of the male characters, and suggest that such birds are really
hermaphrodite, a male element being possibly concealed 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 developed, 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 contained 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, [Footnote: _Mendel's Principles of
Heredity_. Camb. Univ. Press, 1909.] Bateson referred to the effects of
castration as evidence that in different types sex may be differently
constituted. Castration, he urged, in the male vertebrate on the whole
leads merely to the non-appearance of male features, not to the assumption
of female characters, while injury or disease of the ovaries may lead to
the assumption of male characters by the female. This was supposed to
support the view that the male is homozygous in sex, the 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 profoundly,
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 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.

  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 [Footnote: 'Transplantation der Hoden,' _Archiv. f Anat. u.
Phys._, 1849.] of Göttingen 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 maintained 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-Séquard, in his
address before the Medical Faculty of Paris in 1869, was the first to
suggest that glands, with or without ducts, supplied special substances to
the blood which were useful or necessary to the normal health, and in 1889
at a meeting of the Société de Biologie he described the experiment he had
made upon himself by the injection of testicular extract. This was the
commencement of organotherapy. 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 constitute 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
anatomical, 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, [Footnote: _C. R. Soc. Biol., iv._]
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 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
experiments 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 spermatogonia, though Sertoli's cells (cells
on the walls of the tubules to which the true spermatic cells are
attached) were unchanged; 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
interstitial cells and not for the spermatic, and, moreover, if the
interstitial cells are really the source of the hormone on which the
somatic characters 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 cryptorchidism.

Many investigators have found that the Röntgen 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
[Footnote: _Wiener klinische Wochenschrift_, 1907.] 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 condition of the antlers. Simmonds, [Footnote:
_Fortschr. a. d. G. d. Röntgenstr._, xiv., 1909-10.] 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 investigators
have an important bearing on this question--namely, that of Miss Boring
[Footnote: _Biol. Bull._, xxiii. 1912.] that there is no interstitial
tissue in the bird's testis, and that of Miss Lane-Claypon, [Footnote:
_Proc. Roy. Soc._, 1905] 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 spermatogenesis
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 spermatocytes are sufficient to account for periodic
development of somatic sex-characters, and the same reasoning applies to
the antlers of stags.

  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
characteristics, 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 development 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 been proved in 1874 when Goltz and Ewald [Footnote:
_Pflügers Archiv,_ ix., 1874.] 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 [Footnote: _Fortschritte
der Medicin,_ Bd. 7.] in 1898 transplanted a milk gland of a guinea-pig to
the neighbourhood 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 agreement 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 foetuses 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 impregnation 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
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
experimentally 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 implanted ovaries were
found to be necrosed and no proliferation of milk gland had taken place.

The conclusions of Starling and Lane-Claypon were confirmed by Foa,
[Footnote: _Archivo d. Fisiologia_, v., 1909.] and by Biedl and
Königstein, [Footnote: _Zeitschrift f. exp. Path. und Therap_., 1910.] Foa
states that extracts of foetuses of cows produced swelling of the mammae
in a virgin rabbit.

O'Donoghue, however, concludes from a study of the Marsupial _Dasyurus_
that the stimulus which 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 [Footnote: _Quart. Journ. Mic. Sci_., lvii., 1911-12.] 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 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 in identical with that which occurs in normally pregnant
animals.

O'Donoghue's conclusion is in agreement with that of Basch,[Footnote:
_Monatesschr. f. Kinderh. V._, No. ix., Dec. 1909.] 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 correspond 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, the milk glands nor
the uterus undergo the normal development and menstruation does not occur.
According to Marshall to Jolly [Footnote: _Quart. Journ. Exp. Phys._, i.
and ii., 1906.] 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 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 [Footnote: _Arch. d'Anat. micr._,
v., 1902.] and Wallart [Footnote: _Arch. f. Gynock_, vi. 271.] 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 surface 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 granulosa) 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.

Somewhat similar processes take place in the ovaries of Teleostean fishes,
as I know from my own observations, but no corpora lutea are formed in
these, although the degenerating follicles in course of absorption
correspond to corpora lutea. The spawning of Fishes, usually annual,
corresponds to ovulation in Mammals, and in the ovary after spawning the
numerous collapsed follicles containing the follicular cells may be seen
in all stages of absorption. [Footnote: Cunningham, 'Ovaries of
Teleosteans.' _Quart. Journ. Mic. Sci._, vol. xl. pt. 1., 1897.] 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 increases 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 [Footnote: _Proc. Lin. Soc._, New South Wales, 1903.]
showed that in the same species the corpora lutea persisted not only
during the whole of pregnancy, which Professor J. P. Hill [Footnote:
_Anat. Anz._, xviii., 1900.] 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 pregnancy, 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 necessary 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 development 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 development 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 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
[Footnote: _C. R. Soc. de Biol._, t. lxvii., 1909.] 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 pregnancy, 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 pregnancy, they have none
in the second half, and that at this period the essential hormone proceeds
from the developing foetus 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 parturition.

Gustav Born suggested, and Fränkel 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. Fränkel 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 removal 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. 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 hare 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 intensification 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 perfection of femaleness is to be shown in a later
publication. By acting with Röntgen 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 seminal tubules
took place, with increase of the interstitial or Leydig'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 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 confirmation, 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, entitles _The
Organism as a Whole_. [Footnote: Putnam's Sons, 1916.] 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 are these characters are not gametogenic, but were
originally due to external stimulation of particular parts of the soma.



CHAPTER IV

  Origin Of Somatic Sex-Characters In Evolution


In his _Mendel'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 or female respectively. As
mentioned above, he argues from the fact that injury or disease of the
ovaries may lead to the development 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 _F1_
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[male]_ was horned
and _Hh[female]_ was hornless, or, as Bateson expresses it, the horned
character was dominant in males, recessive in females. Bateson 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 _F1_, 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 dominant 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,
remarks that we should expect that disease or removal 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 when bred with
_grossulariata_ [male] produces [male]'s and [female]'s all
_grossulariata_, these of course being heterozygous. When the _F1
grossulariata_ [male] was bred with the wild _lacticolor_ [female] it
produced both forms in both sexes, and thus _lacticolor_ [male] was
obtained for the first time. When this _lacticolor_ [male] was bred with
_F1 grossulariata_[female] it produced all the [male]'s _grossulariata_
and all the [female]'s _lacticolor_. Bateson's explanation is that the
female, according to the Mendelian theory of sex, is heterozygous 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,

        lact. male, LL male male x F, gross. female,  GL female male
  Gametes    L male  +  L male   x  G male  +  L female
            _____________________|______________________
            |                                           |
       GL male male                              LL male female
       gross. male                                lact. female

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_ [male]. It is so coupled only in
the heterozygous _grossulariata_ [female], and at the same time the
_grossulariata_ character is repelled.

According to Doncaster [Footnote: _Determination of Sex_, Camb. Univ.
Press, 1914.] 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 1/2 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 gives [Footnote: _Mendel's Principles of
Heredity_, 1909.] 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 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:--

                          Nn (male)  x  NN (female)
  Gametes     n (male) + N (female)  x  N    +    N

              n (male) +  N             N (female) +  N
                       |                           |
                    Nn (male)                   NN (female)

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 (male)  x  NN (female)
  Gametes     n (male) + n (female)  x  N    +    N

              n (male) +  N             N (female) +  N
                       |                           |
                    nN (male)                   nN (female)

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

                          nN (male)  x  nN (female)
  Gametes     n (male) + N (female)  x  n    +    N
               ______________________|______________________
               |             |             |               |
            nn (male)    nN (male)     nN (female)    NN (female)

that is, half the sons are normal and half colour-blind, while half the
females are homozygous and normal, and the other half heterozygous and
normal.

T. H. Morgan [Footnote: _A Critique of the Theory of Evolution._] 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 wards 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 crisscross
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 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 _F1_ 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 below, 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 grandsons 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 below.

 Parents    Red-eyed male                 White-eyed female
                XR  XR            x             XW  YW

 F1         Red-eyed male                  Red-eyed female
                XR  XW                           XR  YW

 F2  Red-eyed male  Red-eyed male    Red-eyed female  White-eyed female
          XR  XR         XW  XR            XR  YW           XW  YW
        Homozygous.   Heterozygous.     Heterozygous.     Homozygous.


            White-eyed male                 Red-eyed female
                XW  XW            x             XR  YW

 F1        Red-eyed male                  White-eyed female
                XW  XR                           XW  YW

 F2  White-eyed male  Red-eyed male   White-eyed female  Red-eyed female
          XW  XW         XR  XW             XW  YW           XR  YW
        Homozygous.   Heterozygous.       Homozygous.     Heterozygous.

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; 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 Winiwarter [Footnote: 'Spermatogénèse humaine,' _Arch. de Biol._,
xxvii., 1912.] on spermatogenesis in man, the latter is actually the case
in the human species. This investigator found that there were 48
chromosomes 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 spontaneous 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 in animals, referring the reader for a fuller
account to the works cited. One example in the barred character of the
feathers in the breed of fowls called Plymouth Rock. In this 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 and females. The female thus
transmits the character only to her sons. If we represent the barred
character by _B_, and its absence by _b_, we can represent the heredity as
follows:--

            BARRED FEMALE WITH UNBARRED MALE

    B female  b male        X          b male  b male

        Bb male                           bb female

      Barred male.                     Unbarred female.
      Heterozygous.                      Homozygous.


     B male  B male         X          b female  b male

     B male  b female                  b male  b male

      Barred female.                     Barred male.
      Heterozygous.                      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
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-chromosome, and use
the black rod for the dominant character:--

  BARRED female  x  unbarred male
         BX  uY     uX  uX
          |      \/      |
          |      /\      |
         BX  uX     uY  uX
    BARRED male     unbarred female
    Heterozygous    Homozygous


    BARRED male  x  unbarred female
         BX  BX     uX  uY
          |      \/      |
          |      /\      |
         BX  uX     BX  uY
    BARRED male     BARRED female
    Heterozygous    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. 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_, _YY_, or _BY_. Thus the crosses are:--

          YELLOW male  x  BLACK female
              YO male     BB female
              |      \/   |
              |      /\   |
              YB female   BO male
  Tortoise-shell female   BLACK male


          BLACK male  x  YELLOW female
             BO male     YY female
             |      \/   |
             |      /\   |
             BY female   YO male
  Tortoise-shell female  YELLOW male

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.


  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 exclusively in one sex, but the direct contrary--
transmitted by one sex to the opposite sex--and in the second place there
is no suggestion that the development 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 question
there are only two alternatives: either the characters are blastogenic--
that is, they arise from some change in the gametocytes occurring
somewhere 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 sexual dimorphism arose
as a mutation in his cultures of _Drosphilia_. 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
originated 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 determinant in the chromosomes. To say that certain mutations consist
of new factors which only 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_ [Footnote: Camb.
Univ. 1914, p. 99.] in which he remarks: '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 regarded 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 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
influenced 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 produced by the
reproductive organs exists, the characters 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 transmitting 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 mutations not caused by any influence from the
exterior, what is the reason why these particular characters 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 hormones. There is
no more striking example of the inadequacy of the current conceptions of
Mendelism and mutation to cover the 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 coexistence 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 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 products, and that these are not merely pure CO2
and H2O, 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
factors or determinants might be stimulated by products circulating in the
blood and derived from the parts of the soma corresponding 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.

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 different 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 developed in a new
individual, product 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 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 effect would produce a result equal to
_x+x/n_. The latter fraction being already hereditary, a new fraction
_x/n_ would be added to the heredity in each generation, so that after _m_
generations the amount of hereditary development would be _x+mx/n_. 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 fraction _x/n_ would increase, for the heredity might
very well consist not only in a growth independent of stimulation, but in
an increasing response to stimulation, so that _x_ itself might be
increasing, and the fraction _x/n_ would become larger in each generation.
The death and loss of the skin over the antler, originally duo 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 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 produced.

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 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 from
which the antlers arise are formed, each of these cells must be supposed
to contain the same chromosomes 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 themselves 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 in 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, adaptation 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 complicated theory of sexual selection. The possibility
of a Lamarckian explanation he does not even mention. He would doubtless
assume that the antlers of stags arose as a mutation, without explaining
how they came to be affected by the testicular 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
Ruminants 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
characters; in fact all hereditary characters are subject to mutation.
This in 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.

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 observations 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 hereditary, 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,
[Footnote: 'Observations and Experiments on Japanese Long-tailed Fowls,'
_Proc. Zool. Soc._, 1903.] 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
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
outwards. 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 ware just over three months old, the
adult feathers of the tail were all growing. The growing condition can be
distinguished 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, 2 feet 4-1/2 inches long. One
of the feathers--namely, one of the anterior tail coverts--was
accidentally pulled out on 11th February 1902, when it was 15-1/4 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 the original feathers, one, the left central
posterior covert, continued to grow till 13th July 1902, when it was 2
feet 9-1/2 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 9-7/8 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 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 5-1/2 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.

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 in duration of growth the feathers of the pulled side were
longer than those of the right, and this was the result of treatment
continued 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; (_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 teleological: 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 _Gallus_ are
sexually dimorphic, being in the original species larger in the cock than
in the hen. There is no convincing evidence that these appendages 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 bleeding 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 universal in the whole class of Mammalia, the mammary
glands in the female, the scrotum in the male. We have considered the
evidence concerning the relation 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 epithelium. In the earliest
Mammals we may suppose that the young were born in a well-developed
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
Bresslau [Footnote: Stuttgart, 1901.] and by O'Donoghue. [Footnote:
_Q.J.M.S._, lvii., 1911-12.] The rudiment of the teat is a depression or
invagination 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 sebaceous 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 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 chromosomes 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 _XY_ 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 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 parturition
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. Moreover, lactation may be induced by the
repeated act of sucking without any gestation. This has happened in mares,
virgin bitches, mules, virgin women, and in one woman lactation continued
uninterruptedly for forty-seven years, to her eighty-first year, long
after the ovary had ceased to be functional. Lactation has also been
induced in male animals, _e.g._ in a bull, a male goat, male sheep, and in
men. [Footnote: Knott, 'Abnormal Lactation,' _American Medicine_, vol. ii
(new series), 1907.] 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 withdrawal 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 physiologically
inhibits the dissimilative process and augments 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 become 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 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
gestation, 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 accidental 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 undergoes 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 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 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 this 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 of 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.

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 [Footnote: _Ztschr. f. Physiol. Chem._, 83
(1912).] found that the lutein of the corpus luteum had the formula
C{40}H{56} and was apparently identical with the carotin of the carrot,
while the lutein of egg-yolk was C{40}H{56}O{2} 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 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_, [Footnote: London, 1910, p.
151.] 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, [Footnote: 'The Corpus Luteum
of Dasyurus,' _Proc. Lin. Soc._, New South Wales, 1903.] in the latter
there is a pronounced 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 hypertrophied 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 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 be interesting to know how long the
rudimentary corpus luteum persists in _Ornithorhynchus_: 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 Elasmobranchs _Myliobatis_ and _Spinax_; in
Teleosteans, in _Zoarces_; in Reptiles, in _Anguis_ and _Seps_. Bühler 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 retained 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 viviparous Vertebrates arose as a
mutation the result of which was to cause internal development of the
ovum. Habits might easily bring about retention of the fertilised ova for
gradually increasing periods, [Footnote: 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.] 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, congested, 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_, therefore, 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 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 menstruation. This was the theory
of Born and Fränkel. [Footnote: See Biedl, _Internal Secretory Organs_
(Eng. trans.), 1912, p. 404.] 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 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 development 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 determines 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 development 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 parturition 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 oestrus cycle
is about the as that of gestation--namely, six months.

The so-called descent of the testicles occurs exclusively 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
epididymis, closely attached to it, projects into the coelom, 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 formation of the scrotum, but
no one has ever been able to suggest a use for it. It has always been
quite impossible 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 phenomenon. A mutation is a change of structure
affecting more or less the whole soma, but showing itself especially in
some particular organ or structure. But I know of no mutation occurring
under observation 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
continuous 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 [Footnote: _Proc. Zool. Soc._, 1903,
Part 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 produced 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 the ground after being hurled upwards
and forwards, the abdominal organs have acquired a rapid movement
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.

It is evident that the violent shocks of the galloping 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
correspondence between the 'impulsive' mode of progression 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
unnecessary to inquire whether this is the original position, or whether
they are descended from ancestors which had a scrotum: in either case the
position of the testes corresponds to the absence of what Dr. Woodland
calls impulsiveness in progression. 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 their structure
and massive shape, locomotion in 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
abdominal 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 terrestrial
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 with the hormone theory, for it is to be noted that
although the development of 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 scrotal sacs, at other times of the year
being apparently more or less abdominal. In rabbits and hares, which have
a much more impulsive 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 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 disadvantageous, 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 blastogenic
origin.


  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
characteristics on one side of the body, and female on the other, is in
consistent 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 characters assumes that these characters are potentially present by
heredity in both sexes. The occurrence of male somatic characters on one
side or in some 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 instances 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 experiments 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 white-ringed Formosan
variety, _P. torquatus_, of the Chinese pheasant. [Footnote: C. J. Bond,
'Unilateral Development of Secondary Male Characters in a Pheasant,'
_Journ. of Genetics_, vol. iii., 1914.] 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 rudiment 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, 3/4 inch long by 1/2
inch broad, and microscopic sections showed in one part actively growing
areas of tubular gland structure in some of which bodies like spermatozoa
could be detected, while in another were fibrous tissue with degenerating
cysts. The latter appear to have been degenerating egg follicles. The
author concludes that the organ was originally a functional ovary, and
that the ovarian portion had atrophied while a male portion had become
functionally active.

Another case in birds was described by Poll [Footnote: _B.B. Ges. Naturf.
Freunde_, Berlin, 1909.] and is mentioned by Doncaster. [Footnote:
_Determination of Sex_, Cambridge, 1914.] 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. [Footnote:
_Trans. Pathol. Soc._ (London), vol. 57, Part i., 1906.] 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 consisting entirely of tubuli lined by an epithelium consisting 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. 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 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 the evidence concerning castration tends to
prove that hormones from the gonads play no part at all in the development
of somatic sexual characters. Kellog, an American zoologist, in 1905
[Footnote: _Journ. Exper. Zool._ (Baltimore), vol. i., 1905.] described
experiments in which he destroyed by means of a hot needle the gonads in
silkworm caterpillars (_Bombyx 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 [Footnote: _Experimentelle Studien zur Soma- und
Geschlechtedifferenzierung_. Jena, 1909.] 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
male characters, and in the females the only difference was that some of
them were slightly darker than the normal. Meisenheimer and Kopee 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.[Footnote:
See Doncaster, _Determination of Sex_ (Camb. Univ. Press, 1914), chap.
ix.] 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.
Presumably not only the antennae and markings, but also the genital
appendages and the gonads themselves, are male and female on the two
sides. On the view that both sexes and the somatic sex-characters 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
development of somatic sexual characters here. Various attempts have been
made to explain gynandromorphism in insects in accordance with the
chromosome theory of sex-determination. These are discussed by Doncaster
in the volume 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, contain an X chromosome, and half a Y,
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 XY or female. It
may be supposed that as there was only the cytoplasm 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 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  x  MALE
        XY     XX
        |  \/   |
        |  /\   |
        XX     YX
      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 suppose 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. 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 reduction 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 modification, 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,  MmXY 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 _MMXY_. On this theory, therefore, there would always be
some females heterozygous for the male character.

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
[Footnote: 'Ergebuisse der Anat. und Entwicklungsgesch.,' Bd. xv.;
_Pflügers Archiv_, Bd. cxxvi, 1909.] had recorded experiments on _Rana
fusca_ (which is identical with the British species commonly called _R.
temporaria_) 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 castrated frog the thumb-pads
and muscles developed as in a normal frog. Geoffrey Smith and Edgar
Schuster [Footnote: _Quart Journ. Mic. Sci_., lvii, 1911-12.] 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 development, after the
discharge of spermatozoa in the breeding season the spermatogonia divide
and proliferate, forming groups of cells known as spermatocysts. 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
epidermic 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 argument 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 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 implantation 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 sufficient
evidence that the effect is due to a hormone derived from the testis. It
is equally probable, 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 substances. Smith's theory therefore is that the
action of the testes in metabolism is rather to take something from the
blood than to add something to it, and that it is this subtractive effect
which influences the development of somatic sexual organs.

Geoffrey Smith in fact, in the paper above considered, attempts to apply
to the frog the views he put forward [Footnote: _Fauna und Flora des
Golfes van Neapel_, 29 Monographie Rhizocephala.] 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
host may be infected by the parasite, but the presence 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 nevertheless 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 appendages covered
with hairs, the normal function of which is to carry the eggs. The effect
of the parasite 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 development of female secondary characters,
but it is obvious from the condition in Macrura or tailed Decapods, like
the lobster or crayfish, that the abdomen or tail of the male originally
carried appendages 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 involves 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 concerning
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 explanation 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 characters. He describes this change as
the production 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 prevents 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 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 fertilisation. In this case the influence of a
parasite predominantly 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 substance, and of the
influence of the gonads by subtraction 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, modifications produced by external
stimuli in the soma will also be inherited in some slight degree in each
generation when they have no relation to sex or reproduction. 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
chromosomes 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 their experiments, but they have found a difficulty, as already
mentioned in Chapter II, in forming any idea of the origin of a 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
heterozygous, 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 discussed the question in his Silliman Lectures.
[Footnote: _Problems of Genetics_, Oxford Univ. Press, 1913.] 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 spontaneous
origin of various abnormalities which behave as dominants, such as
brachydactyly, and Bateson considers the authenticity of some of 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 F(2) generation are coloured.[Footnote:
Bateson, _Principles of Heredity_, p. 104.] 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 remarkable than the subject
matter. The more true-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 hereditary 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 removal 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 characters, 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 important examples of such
research, namely, that by Professor T. H. Morgan, which deals with the
American fruit-fly _Drosophla_, and the other which concerns the mutations
of the genus of plants OEnothera, exemplified by our well-known Evening
Primrose.

Professor T. H. Morgan informs us [Footnote: _A Critique of the Theory of
Evolution_ (Oxford Univ. Press, 1916), p. 60] 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 completely
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 segment 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 duplication
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. Wingless 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 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 somewhat 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 reduction
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 chromosome, 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 mutations 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 hypothesis 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.
Exceptions 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.

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 _Oenothera_ 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 _Oenothera_ was first noticed by De Vries, the Dutch
botanist, in the neighbourhood of Amsterdam in 1886. He found a large
number of specimens of _Oenothera Lamarckiana_ growing in an abandoned
potato-field at Hilversum, and these plants showed an unusual amount of
variation. He transplanted nine young plants to the Botanic Garden of
Amsterdam, and cultivated them and their descendants for seven generations
in one experiment. Similar experiments have been made by himself and
others. The large majority of the plants produced from the _Oe.
Lamarckiana_ 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 observations 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--was
one called _lata._ It must be explained that the young plant of
_Oenothera_ has practically no stem, but a number of leaves radiating in
all directions 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 DeVries 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 _Oenothera_ 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, 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 _Oenothera_ 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
_Oenothera Lamarckiana_ was the result of a cross, or repeated crosses,
between plants differing in many factors, that the numerous mutations were
similar to the variety of different types which are produced by breeding
together the grey mice arising from a cross between an albino and a
Japanese waltzing mouse in Darbishire's experiment. Since that time,
however, the natural distribution and the cultural history of _Oenothera_
has been very thoroughly worked out. _Oenothera 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 Muséum d'Histoire Naturelle at Paris, collected by
Michaux in North America about 1796, which agrees exactly with the
_Oenothera Lamarckiana_ naturalised or cultivated in Europe. The plant was
first described by Lamarck from plants grown in the gardens of the Muséum
d'Histoire Naturelle, under the name _OE. grandiflora_, 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
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 _grandiflora_. 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 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 _Lamarckiana_, 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 _laevifolia_ 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 heterozygote, 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.

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 reduction 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 mutation, and the results of these abnormalities in the
number of chromosomes. The original number of chromosomes in _OEnothera_
is 14. In the mutation _lata_ this has become 15, and also in another
mutation called _semilata_. The chromosomes before the reduction 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 produced 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_. 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 chromosome 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 chromosomes 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 _OEnothera_ 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 _OEnothera 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 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 discoveries 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 _Oenothera_ 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, [Footnote:
_Journ. Exper. Zool._, vol. xviii., 1915.] for example, crossed 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 in the _F1_. An _F1_ Pintail-Mallard
[female] was mated with a wild Pintail [male]. 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
individual 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 _F2_'s
obtained from crossing _Anas tristis_ [male] with _Anas boscas_ [female]
Phillips obtained 23 females and 16 males. The females were all alike and
similar to _F1_ 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 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
domestication, abundant food, security and reduced 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 conditions 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 considering 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, [Footnote: _A Critique of the Theory of Evolution_, p. 67
(Princeton, U.S.A., and London, 1916).] 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 perfectly 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 _Ambtyopsis_ (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
[Footnote: _The Organism as a Whole_, p. 319 (New York and London, 1916).]
also endeavours to prove that the blindness of cave animals is no 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 considered 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.' According 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
_Gillichthys_, which have eyes; but these fishes live outside the holes,
and only retreat 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
complete 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 associated with absence of pigment, since
we know that the latter in Fishes and Amphibia is due to the 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 pigmentation 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 'tropism'
should occur which caused the animal 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 physicochemical 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 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
portion of rabbit's thyroid, to the other a piece of rabbit's liver. They
fed eagerly on both. Afterwards 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 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 metamorphosis 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 "
                   tail, 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 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 experiment 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, earthworm, 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 metamorphosis 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
gametogenic mutations and adaptive modifications. In several species
specimens occur occasionally in which the asymmetry is not fully
developed. [Footnote:  See 'Coloration of Skins of Fishes, especially of
Pleuronectidae,' _Phil. Trans. Royal Soc_., 1894.] 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 tipper. Minor 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, [Footnote: _Evolution and Adaptation_.] 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. According to
this view habits have been adapted to structure, not structure to habits.
We are thus to 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 individual 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 side 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
transformation 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 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. Morgan
[Footnote: _A Critique of the Theory of Evolution_ (1916), p. 18.] states
that a variation arising in the germ-plasm, no matter what its cause, may
affect any stage in the development 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 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 discontinuous 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 hypothetically 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 condition 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 symmetrical 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 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 representing 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 development
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 existing 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 gradually became earlier in the
life history: it is the new character which is pushed back, not the adult
character of the ancestor.

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 _Drosophila_ 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 _OEnothera_. 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
consideration 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,
[Footnote: _Proc. Zool Soc._, 1907.] and in that paper have discussed
other possible explanations of these mutations. In the 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 II, fig. 2), while the upper side was unpigmented
excepting a scattering of minute black specks and a little pigment on the
head (Plate II., fig. 1).

  [Illustration: PLATE II, Fig. 1 and Fig. 2,
   Abnormal Specimen Of Turbot]

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 ambicolorate
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 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 attachment 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 parts. In a sinistral Flat-fish, whether it is
normally sinistral like the Turbot or abnormally like a 'reversed'
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
characters 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 determinants
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
fertilisation. On this last hypothesis the mutation here considered 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 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 MgCl[2] [Footnote: Stockard, _Arch. Eut. Mech._, xxiii.
(1907).] 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 symmetrical
fish, and the subsequent _continuous gradual_ 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 metamorphosis.

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 development 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 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 inference 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 individual, for in some cases the young fish were placed in the
apparatus before the pigment had entirely disappeared from the lower side,
and the metamorphosis 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 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
III.). Microscopically 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 developed 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 life. 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 difficult 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 adaptations are due to the
heredity of the effects of stimulation assumes that the same stimulus has
been acting for many generations.

 [Illustration: PLATE III - Flounder, Showing Pigmentation Of Lower Side
After Exposure To Light]

It is necessary, however, to consider how far the conclusions drawn from
these experiments are contradicted 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 pigmentation 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 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
experiments. 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 4-1/2 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 entirely white. The theory of
sympathy or correlation 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
pigmented 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 chromosomes of the gametes.

Experimental evidence concerning the movement 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 conditions 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 prefrontal
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 interorbitl septum to
pressure of the lower eye-ball against it, pressure which is probably due
partly if not chiefly to the action of the 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 themselves chiefly by the tail; in shuffling 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 hypertrophy, 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. 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 objection
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 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 modification, _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 combination 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 modification 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 tubercles of the Flounder, and the scattered thornlike 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 contradiction 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 developing in
the males only at maturity. The character was described by E. W. L. Holt
in specimens of the Baltic variety of the Plaice, _Pleuronectes platessa_,
[Footnote: _Journ. Mar. Biol. Assn._, vol iii. (Plymouth, 1893-95.)] 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 myself to occur constantly
in Plaice from the English Channel and North Sea. [Footnote: _Ibid._, vol.
iv. p. 323.] 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 whatever of such function or
such stimulation has been discovered. It is possible that the case differs
from other secondary sexual characters as the antlers of stags in one
respect, namely, that the Dab (_P. limanda_), the Sole, and other species
of _Solea._ and several other Pleuronectidae have what are called etenoid
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 and 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 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, [Footnote: Darwin,
_Descent of Man_ (2nd edit., 1885), p. 331.] apparently by Yarrell, that
these spines are developed 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
_Raia 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 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 correspond 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 characters 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 Amphibia 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
condition. 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 condition. 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 terrestrial animal. This may
be regarded as evidence that the conditions of life which prevent the
metamorphosis 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 congenital
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 metamorphosis 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 mutation was the original cause
of the neoteny, _i.e._ the peisistence 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 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 metamorphosis which had already in numerous experiments
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
development 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. 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 metamorphosis 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. 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
compared 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
modifications 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 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 expected, 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 winch, 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, 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 modification 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
conclude that feathers are not an adaptation, and not due to somatogenic
modification, but must be 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
growth of feathers was modified greatly by the degree of stimulation
applied to the papillae at roots by the movement and bending strain of the
feathers. The modification of the hones and 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 hypertrophy
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 pterodactyls 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
climbing 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 wing. 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 metacarpals 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 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 immensely greater than that which has
elapsed since the blind fish, _Amblyoysis_, was modified by cave-life, so
that we can understand why the eye is developed 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 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 relation 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 unnecessary. 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 he 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.

T. H. Morgan [Footnote: _A Critique of the Theory of Evolution_, p.18.]
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 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
experiments a caterpillar without the three pairs of thoracic legs and yet
developing into a moth that had normal three pairs. Morgan, with all his
mutations of the adult _Drosophila_, says nothing of mutants possessing
legs. The only rational conclusion is that legless larvae have lost the
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 influence of somatic hormones might be 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 [Footnote:
_Journ. Exper. Zool._ (1908), v.] 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 produced 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 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 operation, 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
[Footnote: _Arch. Ital. de Bid._ (1901), Tome xxxv.] states that he made
bilateral grafts of ovaries from newborn 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 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 [Footnote: W. E, Castle and J. C. Phillips, _On Germinal
Transplantation in Vertebrates_, Pub. Carnegie Institution in Washington
(1911), No. 144.] 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 dominance. 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 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 experiments 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.
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 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 gametogenic 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
harmonize with the nature of the majority of specific and varietal
characters, and with 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 recapitulation.

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 '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 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 modifications 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._ in 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 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 principle. There are two
kinds of congenital, constitutional or hereditary characters in all
organisms, namely, the adaptive and the non-adaptive, and every distinct
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 blastogenic 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 knowledge 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 groussularioun_ and _lacticolor_
Adaptations, origin of; evolution of
_Agonus entaphractus_
Albinism
Allantois
Allurements
_Alytes obstetricans_
_Amblyopsis_, eyes of
_Amblystoma tigrinum_
Amnion
_Anableps tetrophthalmus_
_Anas boscas_, crosses of
_Anas tristis_, crosses
Ancel and Bouin
_Anguis fragilis_
_Antilocapra_
_Antirrhinum_, crossing of
Antlers of stags
Ants, heredity of sex in
Aphidae, heredity of sex in
Apoda
Axolotl, albino; metamorphosis; influence of thyroid feeding

Barred plumage in fowls
Basoh
Bateson
Bees, heredity of sex in
Bernard, Claude
Berthold, A. A.
Biedl and Konigstein
Bionomies
Blindness in cave animals
_Bombyx mori_
Boring, Miss
Born and Fränkel
Brachydactyly
Bresslau
Brown-Séquard
Bühler

_Cambarus_, males of
Capons
Castle, experiments in grafting; on sex
Castration; in ducks; of frog; of Lepidoptera
Cats, heredity of colour in
Cave animals, absence of pigment
Cephalopoda
Cetacea, absence of scrotum
Chelonia
_Chologaster agassixii_
Chromosomes; in mutations
_Clevelandia_
_Colaptes_
Colour-blindness; heredity of
Colours, origin of, in domesticated breeds
Comb of fowls, uselessness of
Corpora lutea, evolution of; in viviparous lower vertebrates; origin of
_Corystes cassivelaunus_
Courtship, organs of
Criss-cross inheritance
Crossing over
Cryptorchidism
Cuttle-fishes
Cyclostomes, absence of corpora lutea in
Cytology
Cytoplasm, in heredity

_Dafila acuta_ crosses
_Daphnia_, heredity of sex in
Darwin
_Dasyurus_; corpora lutea; lactation
Davenport
Determinants
Determination of sex
Dipnoi, fins
Dog-fishes, oviparous and viviparous
Dominant characters, origin of
Doncaster; on heredity in cats
_Drosophila_, blind mutation, heredity of sex, mutations
Ducks, crosses of
Dutch rabbit

Earthworms, sex in
Eclipse plumage
Eigenmann
Eimer
Elasmobranchs; corpus luteum in
Elephants, testes
Eugenics
Eunuch
Evolution, evidence of

Factors, origin of
Feathers, evolution of
Flat-fishes, mutations of
Flight, evolution of
Flounder
Foa, on lactation; on grafting
ovaries
Foges
Fowls, castration of; origin of breeds
Fractionation of Mendelian factors
Fränkel
Frog, thumb-pad

_Gallus bankiva_
Gates, Dr. R. Ruggles
Geddes and Thomson
Gemmules
Genital ducts
_Gigas, Oenothera_
_Gillichthys
Gipsy moth
Goltz and Ewald
Gonads, hormones of
Goodale, H. D.
Grafting, of ovaries or testes
Graves' disease
Gudernatsch
Guthrie, C. C.
Gynandromorphism

Haemophilia
Hanau
Hegner
Herdwick sheep, castration in
Heredity; and sex
Hermaphroditism
Hill, J. P.
Horns
Houssaye

_Inachus scorpio_
Insects, heredity of sex in
Interstitial cells
Intromittent organs

Japanese long-tailed fowls; artificial treatment of

Kammerer
Kellog
Kopec

Lactation, dependence on stimulation, in males; regulation of
_Laevifolia, Oenothera_
Lamarck
Lamarckian theory
Lane-Claypon, Miss; and Starling, on ovaries of rabbit
Larvae of insects
_Lata, Cenothera_
Leghorn, White
Lemon-dab
Leopold and Ravana
Lepidoptera, castration in
_Leptinotarsa_
_Limantria dispar_
Limon
Linnæus
Lode
Loeb, on "blind fish; on blindness in cave animals;
  on tadpoles and thyroid
Lop-eared rabbits, grafting experiments
Lotsy, Professor; on crossing
Lutein, of corpora lutes

Male characters in female
Mallard crosses
Mammary glands; origin of rudimentary in male
Marshall; and Jolly
Marsupials, relation of foetus to pouch; scrotum of
Masked crab
Meisenheimer; thumb-pad of frog
_Mendel's Principles of Heredity_
Mendelism; and castration
Menstruation
Metamorphosis; in Flat-fishes; causes of; and hormones;
  and diagnostic characters
Michaux,
Midwife toad,
Milk glands,
Mole, eyes of,
Monotremata, origin of milk glands,
Morgan, T. H., on blindness in cave animals, on mutations, on sex:,
  on sex-linked heredity, on sexual dimorphism in _Drosophila_,
  on variation,
Mutations, in antlers,

Natural selection,
Nuptial plumage,
Nussbaum,
_Nyssia zonaria_

O'Donoghue, development of milk glands,
_OEnothera_, mutations, _grandiflora_, lata_, _Lamarckiana_,
Onagra, species of,
_Origin of Species_, Darwin's,
_Ornithorhyncus_, corpus luteum
Orthogenesis,
Otariidae, scrotum,
Ovaries, position of,
Ovary, in birds,
Ovulation,

Pangenesis,
Parthenogenesis,
Parturition,
Pearson, Karl,
Pheasant, male, gynandramorphism in
Phillips, John C.,
_Philosophie Zoologique_
Phoeidae, testes,
_Physiology of Reproduction_,
Picotee Sweet Pea,
Pigeons,
Pigment, absence in cave animals,
Pile fowls,
Pintail duck, crosses,
Plaice,
_Pleuronectes flesus_,
  _glacialis_,
  _platesca_,
Plymouth Rock fowl,
Pole-dab,
Poll,
Preformation,
_Problems of Genetics_,
Prong buck,
Pro-oestrus,
_Proteus_, eyes of,
Prototheria, milk glands in,

Rabbits, lactation in,
Recapitulation, absence of, and mutations,
Reptiles, corpora lutea in,
Reversal, in Flat-fishes,
_Rhinoderma darwinii_,
Ribbert,
Rieger,
Rodents, testes,
Romanes, GJ
Röntgen rays, effect on testes,
Rose comb, in fowls,
Rotifers, heredity of sex in,
_Rubricalyx, Oenothera_,
_Rubrinervis, Oenothera_,

_Sacculina_,
Salamanders, transplantation of eye,
Sandes,
Schuster, Edgar,
Scrotum, origin, of,
Sea-horse,
Secondary sexual characters,
Selheim,
_Semilata, Oenothera_,
Sertoli's cells,
Sex, chromosomes; Mendelian theory of,
Sex-Linked heredity,
_Sexual Dimorphism_,
Sexual dimorphism, in Rajidae, in Plaice,
Shattock and Seligmann,
Silkworm,
Silky fowl, plumage of,
Sirenia, absence of scrotum,
Slow-worm,
Smith, Geoffrey,
Snakes, absence of limbs,
Sociology,
Somatic sexual characters,
Species, conception of, origin of, characters of, sterility and hybridism,
Spermatogenesis, in man,
Starling and Lane-Claypon, on lactation,
Steinach, heredity of milk glands,
Sternum, carina of,
Swallows,
Sweet Pea,
Swifts,

Tadpoles, effect of thyroid in
Tandler and Gross
Taxonomies
Teleosteans; corpora lutea in; ovarian follicles
Testes, descent of
Tetraploidy
Thayer
Thumb-pad of frog
Thyroid-gland feeding
Tortoise-shell colour in cats
Tosa fowls, Japanese
Transplantation of gonads
_Typhiogobius_

Uhlenhuth
Urodela, larva

Variations
_Vespa vulgaris_; _germanica_
Vries, De

Wallart
Wasps; heredity of sex in
Weapons, organs used as
Weismann
Whale, paddle of
White Leghorn, crosses
Wilson, E. B.
Wing, development of
Winiwarter, von
Witch
Wood, T. B., on crossing of sheep
Woodland, W.
Woodpecker

X chromosome

_Zeugopterus_
_Zoaea_





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