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.