SEXUAL DIMORPHISM
Meaning of the term—Fatal to Wallaceism—Sexual Selection—The law of
battle—Female preference—Mutual Selection—Finn’s experiments—Objections
to the theory of Sexual Selection—Wallace’s explanation of sexual
dimorphism stated and shown to be unsatisfactory—The explanation of
Thomson and Geddes shown to be inadequate—Stolzmann’s theory stated and
criticised—Neo-Lamarckian explanation of sexual dimorphism stated and
criticised—Some features of sexual dimorphism—Dissimilarity of the
sexes probably arises as a sudden mutation—The four kinds of
mutations—Sexual dimorphism having shown itself, Natural Selection
determines whether or not the organisms which display it shall survive.
In some species the sexes are so similar in appearance that it is not
possible to tell by mere outward inspection to which sex a given
individual belongs.
In other species the sexes differ so widely in external appearance that
it is difficult to believe that the male and the female belong to the
same species. Between these two extremes are a great number of species in
which the sexes are more or less dissimilar. Those species in which the
sexes differ in appearance are said to be sexually dimorphic. The
phenomena of sexual dimorphism are fatal to that form of Neo-Darwinism
which sees in natural selection an explanation of all the peculiarities
of animal structure and colouration.
It is not easy to understand how natural selection can have caused marked
sexual dimorphism in a species where the habits of the sexes are the
same, in the Paradise Flycatcher (_Terpsiphone paradisi_), for example,
where the cock and the hen obtain their food in the same way, and share
equally the duties of nest-building, incubation, and feeding the young.
Of course, in all species where each individual carries only one of the
two kinds of sexual organs, there must of necessity be some slight
difference between the individuals that carry the male organ, which
performs one function, and those that carry the female organ, which
performs another function.
But in many species the sexes display differences which have no direct
connection with the generative organs—for example, the deer, where the
stag alone has horns.
Those characters which differ with the sex, but are not directly
connected with the organs of reproduction, are known as secondary sexual
characters.
[Illustration: QUEEN WHYDAH]
Theory of Sexual Selection
In nearly all species where the male and female differ in beauty, it is
the male who surpasses the female. Natural selection is, in many cases,
not able to explain the origin of these differences, or why, when they
occur, the male should be more beautiful than the female. This Darwin
saw. In order to account for the phenomena of sexual dimorphism, he
formulated the theory of sexual selection. This hypothesis is based on
the assumption that there is, in all species of animals, a competition
among the males to secure females as mates. It is not difficult to
understand how this competition arises in polygamous species. Assuming
that approximately equal numbers of males and females are born (an
assumption which appears to be justified as regards the majority of
species), it is clear that for every male who secures more than one wife,
at least one male will be obliged to live in a state of single
blessedness.
But how can there be competition in the case of monogamous species? The
sexes being approximately equal in number, there are sufficient females
to allow of a mate for every male.
The Law of Battle
Such is the nature of things, said Darwin, that, even under these
circumstances, there is competition among the males for females.
“Let us take any species,” he writes, on page 329 of _The Descent of Man_
(Ed. 1901), “a bird for instance, and divide the females inhabiting a
district into two equal bodies, the one consisting of the more vigorous
and better-nourished individuals, and the other of the less vigorous and
healthy. The former, there can be little doubt, would be ready to breed
in the spring before the others; and this is the opinion of Mr Jenner
Weir, who has carefully attended to the habits of birds during many
years. There can also be no doubt that the most vigorous, best nourished,
and earliest breeders would on an average succeed in rearing the largest
number of fine offspring. The males, as we have seen, are generally ready
to breed before the females; the strongest, and with some species the
best armed of the males, drive away the weaker; and the former would then
unite with the more vigorous and better-nourished females, because they
are the first to breed. Such vigorous pairs would surely rear a larger
number of offspring than the retarded females, which would be compelled
to unite with the conquered and less powerful males, supposing the sexes
to be numerically equal; and this is all that is wanted to add, in the
course of successive generations, to the size, strength, and courage of
the males, or to improve their weapons.”
From this competition among the males there arise, firstly, contests
between the males for mates; secondly, the preference of the females for
favoured males.
It is a matter of common knowledge that at the breeding season the males
of nearly all, if not all, species are very pugnacious. Two males often
engage in desperate fights for one or more females; the victor drives
away his foe and secures the harem. In such contests the stronger male
wins, and thus emerges that particular form of sexual selection which
Darwin termed “the law of battle.”
“There are,” writes Darwin, on page 324 of _The Descent of Man_, “many
other structures and instincts which must have developed through sexual
selection—such as the weapons of offence and the means of defence of the
males for fighting with and driving away their rivals—their courage and
pugnacity—their various ornaments—their contrivances for producing vocal
or instrumental music—and their glands for emitting odours.” The former
characters have, according to Darwin, been developed by the law of
battle, and the latter, since they serve only to allure or excite the
female, by the preference of the female.
“It is clear,” continues Darwin, “that these characters are the result of
sexual and not of ordinary selection, since unarmed, unornamented, or
unattractive males would succeed equally well in the battle for life and
in leaving a numerous progeny, but for the presence of better-endowed
males. We may infer that this would be the case, because the females,
which are unarmed and unornamented, are able to survive and procreate
their kind. . . . Just as man can improve the breed of his game-cocks by
the selection of those birds which are victorious in the cockpit, so it
appears that the strongest and most vigorous males, or those provided
with the best weapons, have prevailed under nature, and have led to the
improvement of the natural breed or species.”
Selection by Females
“With mammals,” says Darwin (_loc. cit._, p. 763), “the male appears to
win the female much more through the law of battle than through the
display of his charms.”
In the case of birds, however, feminine preference comes more into play.
It is well known that cocks display their charms to the hens at the
breeding season, and Darwin believed that the hen selected the most
beautiful of her rival suitors.
“Just as man,” he writes (p. 326 of _The Descent of Man_, new edition,
1901), “can give beauty, according to his standard of taste, to his male
poultry, or, more strictly, can modify the beauty originally acquired by
the parent species, can give to the Sebright bantam a new and elegant
plumage, an erect and peculiar carriage, so it appears that female birds
in a state of nature have, by a long selection of the more attractive
males, added to their beauty or other attractive qualities.”
Thus the theory of sexual selection is based on three assumptions.
Firstly, that there is in all species competition among the males for
females with which to mate. Secondly, that this results in either “the
law of battle” among the males, or selection by the female of one among
several admirers. Thirdly, that the female selects, as a rule, the most
attractive of her suitors.
The evidence upon which Darwin founds this theory may be thus
summarised:—
1. In cases where the sexes differ in appearance, or power of song, it is
almost invariably the cock who is the more beautiful or the better
singer, as the case may be.
2. All male birds that possess accessory plumes or other attractions,
make a most elaborate display of these before the females at the mating
season, hence “it is obviously probable that these appreciate the beauty
of their suitors.”
3. Darwin was able to cite specific instances in which the hens showed
preference.
In the case of polygamous species there can be no doubt that there is
considerable competition among males for their wives. It cannot be said
that the contention is so well established in the case of monogamous
species. D. Dewar suggests that circumstances may occur in which the hens
have to fight for the cock, or in which the male is in the happy position
of being able to select his mate. He states his belief that in many cases
the selection is mutual, as in the case of human beings.
“I have seen,” he writes, on page 13 of _Birds of the Plains_, “one hen
Paradise Flycatcher (_Terpsiphone paradisi_) drive away another and then
go and make up to a cock bird. Similarly, I have seen two hen orioles
behave in a very unladylike manner to one another all because they both
had designs on the same cock. He sat and looked on from a distance at the
contest.”
Darwin quotes, on page 500 of _The Descent of Man_, a case of a male
exercising selection: “It appears to be rare when the male refuses any
particular female, but Mr Wright of Geldersley House, a great breeder of
dogs, informs me that he has known some instances: he cites the case of
one of his own deerhounds who would not take any notice of a particular
female mastiff, so that another deerhound had to be employed.”
Similarly, Finn records, in _The Country-Side_ for August 29th, 1908,
that the male Globose Curassow (_Crax globicera_) in the London
Zoological Gardens, which bred with the female Heck’s Curassow (_C.
hecki_), as related on p. 104, selected the hen of this very distinctly
coloured form or species in preference to any of the typical hens of his
own kind.
Male Attractiveness
The cases on record of cocks being in a position to select their mates
are comparatively rare, while instances of selection on the part of the
hens are far more numerous.
Hence it would seem that the sex, which is in a minority, and so has the
opportunity of selecting a mate, does exert a choice and prefer one
particular individual; and that, for the reasons pointed out by Darwin,
it is in most cases the female who is in the position of being able to
pick and choose her mate. It is, as Darwin truly said, far more difficult
to decide what qualities determine the choice of the female. He believed
that it is “to a large extent the external attractions of the male,
though no doubt his vigour, courage, and other mental qualities come into
play.”
Darwin argued that it is the love of hen birds for “external attractions”
in cock birds that has brought into being all the wonderful plumes that
characterise such birds as the peacock. “Many female progenitors of the
peacock,” he writes, on page 661 of _The Descent of Man_ (ed. 1901),
“during a long line of descent, have appreciated this superiority, for
they have unconsciously, by the continued preference of the most
beautiful males, rendered the peacock the most splendid of living birds.”
This conclusion has been vigorously attacked. It is argued, with some
show of reason, that it is absurd to credit birds with æsthetic tastes
equal, if not superior, to those of the most refined and civilised of
human beings.
Is it likely, it is asked, that a bird, which will nest in an old shoe
cast off by a tramp, can appreciate beauty of plumage?
As Geddes and Thomson say (page 29 of _The Evolution of Sex_), “When we
consider the complexity of the markings of the male bird or insect, and
the slow gradations from one step of perfection to another, it seems
difficult to credit birds or butterflies with a degree of æsthetic
development exhibited by no human being without special æsthetic
acuteness and special training. Moreover, the butterfly, which is
supposed to possess this extraordinary development of psychological
subtlety, will fly naively to a piece of white paper on the ground, and
is attracted by the primary æsthetic stimulus of an old-fashioned
wall-paper, not to speak of the gaudy and monotonous brightness of some
of our garden flowers. Thus we have the further difficulty, that we must
suppose the female butterfly to have a double standard of taste, one for
the flowers which she and her mate both visit, the other for the far more
complex colourings and markings of the males. And even among birds, if we
take those unmistakable hints of real awakening of the æsthetic sense
which are exhibited by the Australian bower-bird or by the common jackdaw
in its fondness for bright objects, how very rude is his taste compared
with the critical examination of infinitesimal variations of plumage on
which Darwin relies. Is not, therefore, his essential supposition too
glaringly anthropomorphic?
“Again, the most beautiful males are often extremely combative; and on
the conventional view this is a mere coincidence, yet a most unfortunate
one for Mr Darwin’s view. Battle thus constantly decides the question of
pairing, and in cases where, by hypothesis, the female should have most
choice, she has simply to yield to the victor.”
Darwin, with characteristic fairness, quotes some instances which appear
to be opposed to the theory that the hen selects the most beautiful of
her suitors. He informs us that Messrs Hewitt, Tegetmeier, and Brent, who
have all had a long experience of domesticated birds, “do not believe
that the females prefer certain males on account of the beauty of their
plumage. . . . Mr Tegetmeier is convinced that a game-cock, though
disfigured by being dubbed and with his hackles trimmed, would be
accepted as readily as a male retaining all his natural ornaments. Mr
Brent, however, admits that the beauty of the male probably aids in
exciting the female; and her acquiescence is necessary. Mr Hewitt is
convinced that the union is by no means left to mere chance, for the
female almost invariably prefers the most vigorous, defiant, and
mettlesome male”; and, in consequence, when there is a game-cock in the
farmyard, the hens will all resort to him in preference to the cock of
their own breed. Darwin thinks that “some allowance must be made for the
artificial state in which these birds have long been kept,” and cites in
his favour the case of Mr Cupples’ female deerhound that thrice produced
puppies, and on each occasion showed a marked preference for one of the
largest and handsomest, but not the most eager, of four deerhounds living
with her, all in the prime of life.
The question what is it that determines the choice of the female is
obviously one of considerable importance, and it was to be expected that
many zoologists would have conducted experiments with a view to deciding
it. This legitimate expectation has not been realised.
The matter of sexual selection remains to-day practically where Darwin
left it. Wallace rejects the whole theory, and believes that natural
selection alone can explain all the phenomena of sexual dimorphism. To
such an extent does the enticing idea of the all-puissance of natural
selection dominate the minds of scientific men that but few of them have
paid any attention to the question of sexual selection. This neglect of
the subject affords an example of the baneful results of the too-ready
acceptance of an enticing theory, “Natural selection explains everything,
why then investigate further?” seems to be the general attitude of our
present-day naturalists.
Edmund Selous and D. Dewar have made some observations on birds, and the
Peckhams on spiders, in a state of nature. Such observations demonstrate
that selective mating occurs in nature, but, for the most part, fail to
show what it is that determines the choice.
D. Dewar, however, states (_Birds of the Plains_, p. 42) that the
coloured peahens in the Zoological Gardens at Lahore show a decided
preference for the white cocks, which are kept in the aviary along with
normally coloured cocks. He gives it as his opinion that “the hens select
the white cocks, not because they are white, but because of the strength
of the sexual instincts of these latter. The white cocks continually show
off before the hens; the sexual desire is developed more highly in them
than in the ordinary cocks, and it is this that attracts the hens.”
Pearson’s Investigations
The only zoologists who have investigated experimentally the question of
sexual selection appear to be Karl Pearson and Frank Finn. The former
tried to determine, by actual measurements, whether there is any
preferential mating among human beings as regards physical
characteristics. “Our statistics,” he writes, on page 427 of _The Grammar
of Science_, “run to only a few hundreds, and were not collected _ad
hoc_. Still, as far as they go, they show no evidence of preferential
mating in mankind on the basis of stature, or of any character _very_
closely correlated with stature. Men do not appear, for example, to
select tall women for their wives, nor do they refuse to mate with very
tall or very short women.” As regards eye-colour, Pearson seems to have
arrived at somewhat more definite results. “We conclude,” he writes (p.
428), “that in mankind there certainly exists a preferential mating in
the matter of eye-colour, or of some closely allied character in the
male; in the case of the female there also appears to be some change of
type due to preferential mating. . . . The general tendency is for
lighter-eyed to mate, the darker-eyed being relatively less frequently
mated.”
But Pearson’s experiments seem to show that as regards stature and
eye-colour there is “a quite sensible tendency of like to mate with
like.” “In fact,” writes Pearson, “husband and wife for one of these
characters are more alike than uncle and niece, and for the other more
alike than first cousins.” He adds, “Such a degree of resemblance in two
mates, which we reasonably assume to be not peculiar to man, could not
fail to be of weight if all the stages between like and unlike were
destroyed by differential selection.”
Two obvious criticisms of the results obtained by Prof. Pearson occur to
us. The first is that his conclusions do not seem to be in accordance
with the popular notion that fair-haired men prefer dark hair in a woman,
while dark-haired men prefer fair-haired women, and _vice versa_. The
second is that the human animal is not a typical one. Husbands and wives
are selected for mental and moral qualities rather than physical ones.
The same may, of course, be to some extent true of animals, but in these
there must of necessity be far less variation as regards mental
attributes. Moreover, the question of income is much bound up with human
matrimonial alliances; a rich man or woman has the same advantage in
selection as is possessed by an animal endowed with more than the average
physical strength of its species.
Finn’s Experiments
Finn adopted the plan of experiment suggested by Prof. Moseley. His
apparatus consisted of a cage divided into three compartments by wire
partitions, so that a bird living in one of them could see its neighbour
in the next compartment. In the middle compartment he placed a hen
Amadavat (_Sporæginthus amandava_), and in each of the other compartments
he put a cock bird. Under such circumstances, the hen in the middle
compartment will sit and roost beside the cock she prefers. The male
amadavat, he writes, in _The Country-Side_, vol. i. p. 142, “is in
breeding plumage red with white spots, and the hen brown. The red varies
in intensity even in full-plumaged birds, and I submitted to the hen
first of all two male birds, one of a coppery and the other of a rich
scarlet tint. In no long time she had made her choice of the latter bird;
the other, I am sorry to say, very soon died; and, as he had appeared
perfectly healthy, I fear grief was accountable for his end—a warning to
future experimenters to remove the rejected suitor as early as possible.
In the present case I took away the favoured bird, and put in the side
compartments he and his rival had occupied two other cocks, which
differed in a similar way, though not to the same extent. Again the hen
kept at the side of the rich red specimen, so, deeming I knew her views
about the correct colour for an amadavat, I took her away too, and tried
a second hen with these two males. This was an unusually big bird, and a
very independent one, for she would not make up her mind at all, and
ultimately I released all three without having gained any result.
“Subsequently I made another experiment with linnets. In this case all
three were allowed to fly in a big aviary-cage together, a method which I
do not recommend.
“In this case, however, the handsomest cock, which showed much richer red
on the breast, had a crippled foot, and proved, as I had expected, to be
in fear of the other; nevertheless, the hen mated with him. It must be
said, in justice to the duller bird, that he did not press the advantage
his soundness gave him, but with a less gentle bird than the linnet this
would have happened.”
It is obvious that there is a wide field for observation on these lines.
In the case of large birds the experiment could be made still more
conclusive by confining the three birds to be experimented on in a single
enclosure, divided into three compartments by fences. The males should be
placed each in a separate compartment, and have a wing clipped so as to
prevent them leaving their respective compartments, while the hen should
be allowed the power of flight so that she can visit at will any
compartment.
Finn has also recorded (_loc. cit._) some other observations bearing on
the question of sexual selection. He writes:—
“One cannot observe or read about the habits of birds very much without
finding out that, whatever may be the value of beauty, strength counts
for a great deal. Male birds constantly fight for their mates, and the
beaten individual, if not killed, is at any rate kept at a distance by
his successful rival, so that, if he be really more beautiful, his beauty
is not necessarily of much service to him. I was particularly impressed
by this about a couple of years ago, when I frequently watched the
semi-domesticated mallards in Regent’s Park in the pairing season. These
birds varied a good deal in colour; in some the rich claret breast was
wanting, and others had even a slate-coloured head instead of the normal
brilliant green. Yet I found these ‘off-coloured’ birds could succeed in
getting and keeping mates when correctly-dressed drakes pined in lonely
bachelorhood; one grey-breasted bird had even been able to indulge in
bigamy. That strength ruled here was obvious from the way in which the
wedded birds drove away their unmated rivals, a proceeding in which their
wives most thoroughly sympathised.
“Evidently, beauty does not count for much with the park duck, and the
same seems to be the case with the fowl. As a boy, I often used to visit
a yard wherein was a very varied assortment of fowls. Among these was one
very handsome cock, of the typical black and red colouring of the wild
bird, and very fully ‘furnished’ in the matter of hackle and sickle
feathers. Yet the hens held him in no great account, while the master of
the yard, a big black bird, with much Spanish blood, provided with a huge
pair of spurs, was so admired that he was always attended by some little
bantam hens, although they might have had diminutive husbands of their
own class.
“It must be remembered, however, that these ducks and fowls had an
unnaturally wide choice. In nature, varieties are rare, and the competing
suitors are likely to be all very much alike; this makes matters very
difficult for the observer, who may easily pass over small differences
which are plain enough to the eyes of the hen birds.”
[Illustration: COURTSHIP OF SKYLARK]
Display of Undecorated Cocks
Finn observed that a young hen Bird of Paradise (_Paradisea apoda_) in
the London Zoological Gardens, mated with a fully adult cock in the next
compartment although a young cock in female plumage in her own
compartment did his best to show off.
It would thus seem that the very limited evidence at present available is
not sufficient to sustain the theory that the hens select the most
attractive of their suitors. It is significant that plainly-coloured
species of birds show off with as much care as their gaily-plumaged
brethren; and, if they be nearly allied, assume similar courting
attitudes. Thus the homely-attired males of the Spotted-bill (_Anas
poecilorhyncha_), Gadwall, and Black Duck (_Anas superciliosa_), show off
in precisely the same way as does the handsome mallard.
Howard describes and figures in his excellent and beautifully illustrated
monograph the elaborate display at the pairing season of some of our
plain-coloured little warblers. The skylark has also a notable display.
The common partridge assumes a nuptial attitude similar to that of the
pheasant, and, although the cock of the former species has nothing
brilliant to show off, the hen partridge pays far more attention to the
display of her suitor than does the hen pheasant.
The fact that some cock birds show off _after_ the act of pairing seems
to tell against the theory of sexual selection, or at any rate to
indicate the purely mechanical nature of the performance. Finn has
witnessed this post-nuptial display at the Zoological Gardens (London) in
the pied wagtail, the peacock, the Andaman Teal (_Nettium albigulare_),
the Avocet, the Egyptian Goose (_Chenatopex ægyptiaca_), and the Maned
Goose (_Chenonetta jubata_).
Another objection to the theory that the bright colours of cock birds are
due to feminine selection is presented by those birds which breed in
immature plumage. Darwin admits that this objection would be a valid one
“if the younger and less ornamental males were as successful in winning
females and propagating their kind as the older and more beautiful males.
But,” he continues, “we have no reason to suppose that this is the case.”
Unfortunately for the theory of sexual selection, there is evidence to
show that the cock Paradise Fly-catcher (_Terpsiphone paradisi_) in
immature plumage is quite as successful in obtaining a mate as is the
cock in his final plumage. The cock of this beautiful species has a
chestnut plumage in his second year, and a white one in the third and
subsequent years of his life. Nevertheless, a considerable proportion of
the nests found belong to chestnut cocks.
Plumage of Herons
Darwin was of opinion that any novelty in colouring in the male is
admired by the female; and in this manner he sought to overcome some
difficulties to his theory which certain birds presented.
Writing of the heron family, he says:—
“The young of the _Ardea asha_ are white, the adults being
slate-coloured; and not only the young, but the adults of the allied
_Buphus coromandus_ in their winter plumage are white, their colour
changing into a rich golden buff during the breeding season. It is
incredible that the young of these two species, as well as of some other
members of the same family, should have been specially rendered pure
white, and thus made conspicuous to their enemies; or that the adults of
one of these two species should have been specially rendered white during
the winter in a country which is never covered with snow. On the other
hand, we have reason to believe that whiteness has been gained by many
birds as a sexual ornament. We may therefore conclude that an early
progenitor of the _Ardea asha_ and the _Buphus_ acquired a white plumage
for nuptial purposes, and transmitted this colour to their young; so that
the young and the old became white like certain existing egrets, the
whiteness having afterwards been retained by the young whilst exchanged
by the adults for more strongly pronounced tints. But if we could look
still further backwards in time to the still earlier progenitors of these
two species, we should probably see the adults dark-coloured. I infer
that this would be the case, from the analogy of many other birds, which
are dark whilst young, and when adult are white; and more especially from
the adult of the _Ardea gularis_, the colours of which are the reverse of
those of _A. asha_, for the young are dark-coloured and the adults white,
the young having retained a former state of plumage. It appears,
therefore, that the progenitors in their adult condition of the _A.
asha_, the _Buphus_, and of some allies have undergone, during a long
line of descent, the following changes of colour: firstly a dark shade,
secondly pure white, and thirdly, owing to another change of fashion (if
I may so express myself), their present slaty, reddish or golden-buff
tints. These successive changes are intelligible only on the principle of
novelty having been admired by the birds for the sake of novelty.”
This reasoning may appear far-fetched and unconvincing. It seems,
however, quite likely that the hen may select as her mate the suitor who
is conspicuously different from the others, not because she admires
novelty, but because his conspicuousness attracts her attention and
enables her to make up her mind quickly to take him and thus rid herself
of the other troublesome admirers, who are all very much alike.
Sexual Dissimilarity
It is perhaps worthy of note that, after the most successful of her
suitors has succeeded in securing the hen, it may happen that a
disappointed rival makes love to her in the absence of her lord and
master and thereby nullifies the effect of her previous selection.
It is to be observed that, even if we take it as proved, as Darwin
believed, that the hens alone exercise a choice of mates, and that they
select the most beautiful of their suitors, we are still far from
arriving at an explanation of the fact that the males alone have acquired
beauty. Admitting that the hens always mate with the most beautiful
cocks, we should expect the offspring of each union to be all more or
less alike in beauty—that is to say, more beautiful than the mother and
less so than the cock. How are we to explain the one-sided inheritance of
this beauty? Why is it confined to the cocks?
In order to meet this objection Darwin had to call to his aid unknown
laws of inheritance. “The laws of inheritance,” he writes (_Descent of
Man_, p. 759), “irrespectively of selection, appear to have determined
whether the characters acquired by males for the sake of ornament, for
producing various sounds, and for fighting together, have been
transmitted to the males alone or to both sexes, either permanently or
periodically, during certain seasons of the year. Why various characters
should have been transmitted sometimes in one way and sometimes in
another is not in most cases known; but the period of variability seems
often to have been the determining cause. When the two sexes have
inherited all characters in common, they necessarily resemble each other;
but, as the successive variations may be differently transmitted, every
possible gradation may be found, even within the same genus, from the
closest similarity to the widest dissimilarity between the sexes.”
This statement, although it does not throw any light upon the problem, is
somewhat damaging to the theory of sexual selection. If it be admitted
that dissimilarity between the sexes is due to the fact that the males
have varied in one way and the females in another way, there seems no
necessity for invoking the aid of feminine preference.
Even greater is the difficulty presented by those species in which the
males alone are provided with horns or antlers. “When,” writes Darwin
(_Descent of Man_, p. 767), “the males are provided with weapons which in
the females are absent, there can hardly be a doubt that these serve for
fighting with other males; and that they were acquired through sexual
selection, and were transmitted to the male sex alone. It is not
probable, at least in most cases, that the females have been prevented
from acquiring such weapons on account of their being useless,
superfluous, or in some way injurious. On the contrary, as they are often
used by the males for various purposes, more especially as a defence
against their enemies, it is a surprising fact that they are so poorly
developed, or quite absent, in the females of so many animals.”
We have, we believe, demonstrated that Darwin’s theory of sexual
selection is unable to account satisfactorily for all the phenomena of
sexual dimorphism. But, as we have seen, it is quite possible that sexual
selection is a real factor of evolution.
We trust that what we have said will stimulate some leisured naturalist
to study the question of male and female preference.
We now pass on to consider briefly some of the other attempts that have
been made to explain the phenomena of sexual dimorphism.
Wallace’s Explanation of Sexual Dissimilarity
Wallace does not accept the theory of sexual selection. He admits that
the form of male rivalry, which Darwin calls “the law of battle,” is “a
real power in nature,” and believes that “to it we must impute the
development of the exceptional strength, size, and activity of the male,
together with the possession of special offensive and defensive weapons,
and of all other characters which arise from the development of these, or
are correlated with them” (_Darwinism_, p. 283). But the view that the
female selects the most beautiful of her suitors has always seemed to
Wallace “to be unsupported by evidence, while it is also quite inadequate
to account for the facts.” For example, the accessory plumes of birds
“usually appear in a few definite parts of the body. We require some
cause to initiate the development in one part rather than in another.”
Wallace considers that natural selection is able to explain all the
phenomena of sexual dimorphism. He points out that, when the sexes are
dissimilar among birds, it is almost invariably the female which is
duller coloured. The reason for this is, he believes, that the hen birds,
while sitting, “are exposed to observation and attack by the numerous
devourers of eggs and birds, and it is of vital importance that they
should be protectively coloured in all those parts of the body which are
exposed during incubation. To secure this, all the bright colours and
showy ornaments which decorate the male have not been acquired by the
female, who often remains clothed in the sober hues which were probably
once common to the whole order to which she belongs. The different
amounts of colour acquired by the females have no doubt depended on
peculiarities of habits and environment, and on the powers of defence and
concealment possessed by the species.”
In support of his contention, Wallace asserts that all species of birds,
of which the hens are as conspicuously coloured as the cocks, nest in
holes or build domed nests. The plumes and other ornaments, which the
cocks of certain species display, Wallace would attribute to a surplus of
strength, vitality, and growth power, which is able to expend itself in
this way without injury.
“If,” he writes, “we have found a _vera causa_ for the origin of
ornamental appendages of birds and other animals in a surplus of vital
energy, leading to abnormal growths in those parts of the integument
where muscular and nervous action are greatest, the continuous
development of these appendages will result from the ordinary action of
natural selection in preserving the most healthy and vigorous
individuals, and the still further selective agency of sexual struggle in
giving to the very strongest and most energetic the parentage of the next
generation.” (_Darwinism_, p. 293.) “Why,” he says, “in allied species
the development of accessory plumes has taken different forms we are
unable to say, except that it may be due to that individual variability
which has served as the starting point for so much of what seems to us
strange in form, or fantastic in colour, both in the animal and vegetable
world.”
Wallace’s Theory Criticised
Wallace’s view that the dull plumage of the hen bird is due to her
greater need of protection is based on the assumption that the hen bird
alone takes part in incubation.
Is this assumption a correct one?
It certainly is not in all cases. As D. Dewar has stated in _Birds of the
Plains_, the showy white cock Paradise Fly-catcher (_Terpsiphone
paradisi_) sits in broad daylight on the open nest quite as much as the
hen does. And this may prove to be true of many other species of birds.
Again, the cocks of the various species of Indian sunbirds are brightly
coloured while the hens are dull brown. In these species the hen alone
sits on the eggs, but, as the nest is well covered-in, the hen might
display all the colours of the rainbow without being visible to passing
birds. Moreover, as D. Dewar pointed out in a paper read before the Royal
Society of Arts (_Journal_, vol. lvii., p. 104), although, in most
species of Indian dove, the sexes show little or no dissimilarity, there
is one species (_Œnopopelia tranquebarica_) which exhibits considerable
sexual dimorphism. But the nesting habits of this peculiar species are in
all respects similar to those of the other species of dove. Why then the
marked dissimilarity of the sexes?
Another objection to the theory of Wallace is that urged by J. T.
Cunningham (_Archiv für_ _Entwicklungsmechanik der Organismen_, vol.
xxvi., p. 378), namely, that the secondary sexual characters in those
species which possess them show an entire absence of uniformity in nature
and position. “Why,” asks Cunningham, “should the male constitution of
the stag show itself in bony excrescences of the skull, in the peacock in
excessive growth of the other end of the body? Why should the larynx be
modified in one mammal, the teeth in another, the nose in another? Why is
the male newt distinguished by a dorsal fin, the male frog by a swelling
on the fore foot?”
Another objection to the explanation of sexual dimorphism suggested by
Wallace, is that in many species of bird, as, for example, the house
sparrow and the green paroquets of India, the external differences
between the sexes are so slight that it is unreasonable to believe that
they are the result of natural selection. It seems impossible to hold
that the Rose-ringed Paroquet (_Palæornis torquatus_)—a species which
nests in holes—would have become extinct if the hens had developed the
narrow rose-coloured collar that characterises the cocks.
Darwin pointed out that while Wallace’s hypothesis might appear plausible
if applied to colour, it can scarcely be said to explain the origin of
such structures as the musical apparatus of certain male insects, or the
larger size of the larynx in some birds and mammals. We thus see that
suggestions offered by Wallace, although they contain a modicum of truth,
fail to explain the phenomena of sexual dimorphism.
The fairest possible criticism of these views is that of Darwin:—
“It will have been seen that I cannot follow Mr Wallace in the belief
that dull colours, when confined to the females, have been in most cases
specially gained for the sake of protection. There can, however, be no
doubt, as formerly remarked, that both sexes of many birds have had their
colours modified, so as to escape the notice of their enemies; or in some
instances, so as to approach their prey unobserved, just as owls have had
their plumage rendered soft, that their flight may not be overheard”
(_The Descent of Man_, p. 745).
The Theory of Thomson and Geddes
Thomson and Geddes have attempted to explain sexual dimorphism on the
hypothesis that males are essentially dissipators of energy, while
females tend to conserve energy. They point out that the spermatozoon is
a small intensely active body, which dissipates its energy in motion,
while the ovum is a large inert body—the result of the female tendency to
conserve energy and to build up material. The various ornaments and
excrescences which appear in male organisms are the result of this male
tendency to dissipate energy. In the spermatozoon the dissipated energy
appears in the form of active movement; in the adult organism it takes
the shape of plumes and other ornaments, of song and contests for the
females.
This theory, however, does not explain what we might call the haphazard
nature of sexual dimorphism. If sexual dissimilarity is due to the
tendency of the male to dissipate energy, why do we see very marked
dimorphism in one species, and no dimorphism in a very nearly allied
species? Why are the males larger than the females in some species, and
smaller in other species? Again, how is it that in certain species of
birds—the quails of the genus _Turnix_, the Painted Snipe (_Rhynchæa_),
and the Phalaropes—it is the female who possesses the more showy plumage?
Moreover, this theory, equally with that of Wallace, does not explain why
the excrescences which characterise the male appear in various parts of
the body in different species.
Stolzmann’s Theory
Stolzmann has made an ingenious attempt to explain why in birds the cock
is so frequently more conspicuously coloured than the hen. He asserts
that among birds the males are more numerous than the females, and that
this preponderance is not advantageous to the species. Those males which
have not managed to secure a mate are apt to persecute the females while
sitting on the eggs, to the detriment of these latter. Natural selection,
says Stolzmann, is concerned with the well-being of the species rather
than of the individual. Hence anything that would tend to lessen the
number of males would be a good thing for the species, so that a
peculiarity, such as bright plumage, which renders the males conspicuous,
or ornamental plumes, which cause their flight to be slow, and so leads
to their destruction, will be seized upon and perpetuated by natural
selection. He points out that the cock of one species of
hummingbird—_Loddigesia mirabilis_—has not only longer tail feathers, but
a shorter wing than the female, and must, in consequence, find it
comparatively difficult to obtain food, and be more liable to fall a
victim to birds of prey than the hen. Stolzmann further suggests that the
excessive pugnacity of male birds at the breeding season may lead to the
destruction of some individuals, and so prove of advantage to the
species.
Several objections seem to present themselves to this most ingenious
theory.
In the first place, there does not appear to be any satisfactory evidence
to show that more cocks than hens are born.
We may grant that a superfluity of cocks is injurious to any species,
since the unmated ones are likely to persecute the hens; we may also
grant that many cocks are handicapped in the struggle for existence by
the excessive growth of certain of their feathers, but we fail to see how
this excessive development has been caused by natural selection in the
manner suggested by Stolzmann. Although it may be advantageous to the
species for the cocks to be showy, natural selection can perpetuate this
only by weeding out the least conspicuous of the cocks. But it is the
more gaudy ones, those, according to Stolzmann, whose presence is
beneficial to the species, which will be eliminated by natural selection.
So that, in this case, that force will act in a manner contrary to the
interests of the species, if Stolzmann’s idea is a correct one.
The theory in question would therefore seem to be untenable. Nevertheless
there is doubtless some truth in the notion that too many males spoil the
species. Thus, excessive showiness and high mortality among the males may
be beneficial to the species. But we must not forget that the more
beneficial it is, the stronger must be the tendency of natural selection
to eliminate the males that possess the desired peculiarity.
Neo-Lamarckian Explanation
Cunningham’s Theory
J. T. Cunningham makes an attempt to explain the phenomena of sexual
dimorphism on Neo-Lamarckian principles. His theory is set forth in a
paper entitled _The Heredity of Secondary Sexual Characters in relation
to Hormones_, which was read before the Zoological Society of London, and
published in full in the _Archiv für Entwicklungsmechanik der
Organismen_. “The significant correlation of male sexual characters,” he
writes, “is not with any general or essential property of the male sex,
such as katabolism (or the tendency to dissipate energy, as we have
called it), but with certain habits and functions confined to one sex,
but differing in different animals. . . . In those animals which possess
such (_i.e._ secondary sexual) characters, the parts of the soma (_i.e._
the body) affected differ as much as they can differ; any part of the
soma may show a sexual difference: teeth in one mammal, skull in another;
feathers of the tail in one bird, those of the neck in another, and so
on. But in all cases such unisexual characters correspond to their
functions or use in habits and instincts which are associated, but only
indirectly, with sexual production. These habits are as diverse and as
irregular in their distribution as the characters. The cocks of common
fowls and of the Phasianidæ generally are polygamous, fight with each
other for the possession of the females, and take no part in incubation
or care of the young, and they differ from the hens in their enlarged
brilliant plumage, spurs on the legs, and combs, wattles, or other
excrescences on the head. In the Columbidæ _per contra_ the males are not
polygamous, but pair for life, the males do not fight, and share equally
with the females in parental duties.
“Corresponding with this contrast of sexual habits is the contrast of
sexual dimorphism, which is virtually absent in the Columbidæ.
“I think, then, the only scientific explanation is that the difference of
habits is the cause of the sexual dimorphism, and that the special sexual
habits which occur in some species but not in others are the causes of
the sexual characters. . . . The habits in question always involve
certain definite stimulations applied to those parts of the body whose
modification constitutes the somatic sexual characters. The stimulations
are confined, as the characters are confined, to one sex, to one period
of life, to one season of the year, to those animals which have the
characters, to those parts of the body which are modified.” Mr Cunningham
believes that these stimulations cause hypertrophy or excessive growth of
the part affected, and that this peculiarity is transmitted to the
offspring. And thus he supposes all the ornaments and excrescences of the
males of various species to have arisen.
As evidence in favour of his view, he points out that these excrescences
are, in many species, not only functionless but absolutely injurious, as
in the case of the comb and wattles of the jungle cock and his domestic
descendants, which merely serve as a handle for enemies to seize.
Cunningham asserts that the only objection to his theory is the dogma
that acquired characters cannot be inherited. This assertion is, however,
not correct. It is, indeed, a very serious objection that all the
evidence available seems to show that acquired characters are not
inherited, but this is by no means the only difficulty.
Before mentioning these further objections, let us say a word on the
subject of the inheritance of acquired characters. Mr Cunningham himself
compares the formation of a splint or spavin in a horse as the result of
special strain, to the acquisition of secondary sexual characters.
Unfortunately for Cunningham’s theory, but fortunately for mankind in
general, spavined horses and mares do not beget spavined offspring. If,
then, spavin is not inherited, is it not unreasonable to assert that the
thickening of the bone that develops on the head of a butting animal is
inherited?
Another objection to Cunningham’s theory is that many birds which show
off their plumage most vigorously possess no ornamental plumes. As Howard
has recorded, many of our dull-coloured British warblers show off in the
same manner as bright-coloured birds do. If the exercise has caused the
development and inheritance of plumes in some species, why not in the
others?
Again, Cunningham is not correct in saying that sexual dimorphism is
“virtually absent” in the Columbidæ. Few birds display so striking a
sexual dimorphism as the Orange Dove (_Chrysœna victor_) of Fiji, in
which the male is bright orange and the hen green. We have already cited
the case of the curious sexually dimorphic red turtle-dove. Now, the
courting attitudes and actions of this species are precisely the same as
those of other allied turtle-doves; why, then, have these exercises
caused only one species to become sexually dimorphic?
Existing Theories not Satisfactory
Our survey of the more important attempts which have been made to explain
the phenomena of sexual dimorphism leads to the conclusion that these
still require elucidation. We have weighed each theory in the balance and
found it wanting.
The outstanding feature of sexual dissimilarity is the apparently
haphazard manner of its occurrence.
We have already alluded to the case of the doves in India. In that
country four species are widely distributed—namely, the Spotted Dove
(_Turtur suratensis_), the Ring or Collared Dove (_Turtur risorius_), the
Little Brown Dove (_Turtur cambayensis_), and the Red Turtle-dove
(_Œnopopelia_ _tranqebarica_). The habits of all these four species
appear to be identical, nevertheless in the first three the sexes show
little or no dissimilarity in outward appearance, while in the last the
sexual dimorphism is so great that the cock and hen were formerly thought
to belong to different species.
Another very curious case is that of the South American geese of the
genus _Chloëphaga_, in which some species, as the familiar Upland or
Magellan Goose of our parks (_C. magellanica_), have the sexes utterly
unlike, while in others, as the Ruddy-headed Goose (_C. rubidiceps_),
they are quite similar to each other.
The ducks furnish us with another very good example of the apparently
haphazard nature of sexual dimorphism. In the Common Mallard or Wild Duck
(_Anas boscas_) the cock is far more showily coloured than the hen, but
in all the species most nearly allied to it the males are as
inconspicuous as the females, _e.g._ in the Indian Spotted-bill (_Anas
pœcilorhyncha_), the Australian Grey Duck (_A. superciliosa_), the
African Yellow Bill (_Anas undulata_), and the American Dusky Duck (_A.
obscura_). As the dusky duck inhabits North America, where the mallard is
also found, the case is particularly striking.
Among mammals the lion and the tiger and the sable and roan antelopes
(_Hippotragus niger_ and _H. equinus_) furnish familiar examples of
nearly-related species, in one of which the sexes are alike and in the
other dissimilar in appearance.
Hormones
Another important point to be borne in mind is the intimate correlation
that exists between the reproductive organs and the general appearance of
the organism, more especially of the secondary sexual characters. These
last, in most cases, do not show themselves until the maturity of the
sexual organs. The well-known effects of castration illustrate this
connection. Again, females in which the reproductive organs have ceased
to be functional often assume male characters.
It has lately been proved by experiment that, in many cases at any rate,
the development of the ornaments, etc., characteristic of the sexes is
due to the secretion by the sexual cells of what are known as
hormones—that is to say, secretions which excite development of the
secondary sexual characters. The tendency to produce the external
characteristics of the sex to which an organism belongs is inherited, but
the actual development thereof is in many cases dependent on the
secretion of these hormones. Accordingly, if a male individual be
completely castrated it ceases to develop the external characters of its
sex. The evidence upon which the doctrine of hormones is based is
admirably summarised in the above-quoted paper by Cunningham. Into this
evidence we cannot go. It must suffice that the doctrine is quite in
accordance with all the observed results of castration.
It is worthy of notice that the various features which characterise the
sexes in sexually dimorphic animals are not associated with any
particular organ or parts of the body, nor do they necessarily affect the
same part in allied species. “We cannot say,” writes J. T. Cunningham,
“that any part of the soma (_i.e._ the body tissue) is specially sexual
more than another part, except that such differences between the sexes
are usually external. They usually affect the skin, and especially
epidermic appendages, and the superficial parts of the skeleton, or whole
limbs and appendages; or the difference may be one of size of the whole
soma. In mammals and birds the male is often the larger, sometimes very
much so, but there are cases in which the female is larger. There is no
general rule.”
Another important point is, that females, although they themselves show
no trace of the male character, are capable of transmitting it to their
progeny. This can be proved by crossing a hen pheasant with a cock
barn-door-fowl; the male offspring of the union display the plumes so
characteristic of the cock pheasant. These cannot have been derived from
the barn-door-fowl father; they must have come from the dull-coloured hen
pheasant.
In this connection we may mention the curious fact recorded by Bonhote,
on page 245 of the _Proceedings of the Fourth International
Ornithological Congress_, that in the case of ducks descended from
crosses between the pintail, the mallard, and the spotbill, the drakes in
full breeding plumage showed a mixture of pintail and mallard
characteristics, while, in their non-breeding plumage, the colouring of
the spotbill is predominant.
Eye-colour, Comb, and Spurs
An important point, and one which does not seem to have been pointed out
by any zoologist, is that eye-colour, comb, and spurs in birds and horns
in mammals do not stand in the same relation to the sexual organs as do
the other external characteristics. For example, the castrated Nilgai
(_Boselaphus tragocamelus_) acquires horns, but not the characteristic
male colour. In the common Indian Francolin Partridge (_Francolinus
pondicerianius_), the cock differs from the hen only in the possession of
spurs. The same applies to the various species of Snow Cock
(_Tetraogallus_). There is a breed of game-cocks which display plumage
like that of the hen, but such birds have the comb and spurs developed as
in normally feathered cocks.
The white eye of the white-eyed Pochard Drake (_Nyroca africana_), and
the yellow eye of the cock Golden Pheasant (_Chrysolophus pictus_), which
are purely male characters, show themselves earlier than the male
plumage. Occasionally a hen golden pheasant assumes the plumage of the
cock, but she never acquires the yellow eye.
Many birds when kept in captivity lose some of the beauty of their
plumage, and this is usually attributed to the sexual organs becoming
impaired and reacting on the somatic tissue. But this explanation cannot
in all cases be the correct one, because the linnet, although losing the
male plumage in captivity, lives long and well in a cage and breeds
readily with hen canaries.
Another curious fact is that the male plumage sometimes appears
pathologically in hen birds, more especially in those which have become
sterile from age or disease. This phenomenon occurs comparatively
frequently in the gold pheasant, and more rarely in the common pheasant,
the fowl, and the duck.
Phenomena such as these seem to suggest that in some cases the bright
colours of the male may be pathological, that the hormones which the male
sexual cells secrete may exercise an injurious effect on the somatic or
body tissues. Decay is known to be accompanied by the production of
brightly coloured pigment in the case of leaves. Finn suggests that the
white plumage which the cock paradise fly-catcher assumes in the fourth
year of his existence may be a livery of decay, a sign of senility.
The Four Kinds of Mutations
It is our belief that sexual dimorphism arises frequently, if not
invariably, as a mutation. Mutations may be of four different kinds.
Those which appear only, or especially, in conjunction with the male
organs, for example, whiteness in domesticated geese allowed to breed
indiscriminately.
Those which appear only, or especially, in conjunction with the female
organs; mutations of this description appear to be very rare, but it may
be noted that in fowls allowed to breed indiscriminately, as in India,
completely black hens are common, but completely black cocks are rarely,
if ever, seen. This indicates an association between blackness and
femininity.
Those which appear in the same manner in both sexes. The great majority
of mutations appear to be of this kind.
Lastly, those that appear in both sexes but take a different form in the
case of the two sexes; thus in cats a mutation has given rise to sandy
males and tortoise-shell females. The mutation which has produced the
black-winged peacock shows itself in the form of a black wing in the
cock, while it causes the plumage of the hen to be grizzly white.
We shall deal with the phenomenon of correlation at some length in the
next chapter. It is a subject to which sufficient attention has not been
paid. Even as certain characters are correlated in certain species, so in
some cases are certain characters correlated with sex.
Why this should be so we are not in a position to say; this, however,
does not affect the indisputable fact that such correlation does exist.
Physicians in the course of their practice sometimes come across very
curious cases of correlation in human beings.
Unilateral Transmission
“It is,” writes Thomson (_Heredity_, p. 290), “an interesting fact that
an abnormal element in the inheritance may find expression in the males
only or in the females only. If we could understand this we should be
nearer understanding what sex really means.
“Hæmophilia, or a tendency to bleeding, is a heritable abnormality,
partly associated with weakness in the blood-vessels, which do not
contract as they should and are apt to break, and partly connected with a
lack of coagulating power in the blood. It is usually confined to males.
But as it passes from a father through a daughter to a grandson, and so
on, it must be a latent part of the germinal inheritance of the females,
though for some obscure physiological reason it fails to find expression
in them, or has its expression quite disguised. Colour-blindness or
Daltonism has been recorded (Horner) through the males only of seven
generations. Dejerine cites another case (_fide_ Appenzeller) in which
all the males in a family history had cataract through four generations.
There are other instances of what is sometimes awkwardly called the
unilateral transmission of abnormal qualities. Edward Lambert, born in
1717, is said to have been covered with ‘spines.’ His children showed the
same peculiarity, which began to be manifest from the sixth to the ninth
month after birth. One of his children grew up and handed on the
peculiarity to another generation. Indeed, it is said to have persisted
for five generations, and in the males only—unilateral transmission.”
In our view, these abnormalities are of such a kind that they are only
possible in connection with the male organ; in other words, they are
mutations of the first of the four kinds cited above—those which appear
only in connection with the male organ.
It is a curious fact that the general rule in nature seems to be that the
male is ahead of the female in the course of evolution. The sexes may be
alike at a given period in the life-history of the species. Presently a
mutation appears which is confined to the male alone; thus arises the
phenomenon of sexual dimorphism. The next step in the evolution of the
species is frequently a mutation on the part of the female which brings
her once again into line with the male, and so the sexual dimorphism
disappears, for a time at any rate. A good example of this is furnished
by the sparrows; in the common sparrow of a large part of Africa (_Passer
swainsoni_) both sexes are very plain, like the hen of the house-sparrow;
in this species (_P. domesticus_) as every one knows, the cock, though by
no means brilliant, is noticeably handsomer than his mate; while in the
Tree-sparrow (_P. montanus_) both sexes have a plumage of masculine type,
much like that of the cock house-sparrow.
If we consider in conjunction with one another the various facts we have
cited above, we begin to grasp the nature of the phenomena of sexual
dimorphism.
Let us consider an imaginary case of a defenceless little bird which
builds an open nest. Let us suppose that it is inconspicuously plumaged.
Now suppose that a mutation of the first kind shows itself, a mutation
which affects the cock only and makes him more conspicuous. Let us
further suppose that the cock does not share in the duties of incubation.
It is quite possible that, in spite of this apparently unfavourable
mutation, the species may survive, for, as we have seen, it does not
affect the hen, and she, since she alone incubates, stands the most in
need of protective colouring. Moreover, as Stolzmann has suggested, the
species can possibly afford to lose a few males. But suppose that both
cock and hen share in the duties of incubation, it is then quite likely
that the mutation will cause the species to become extinct, by the
elimination of all the males. Or, let us suppose that the mutation in the
direction of showy plumage affects both sexes, then in such a case the
species will almost certainly become extinct. If, however, the
hypothetical species nested in holes in trees, it is quite possible that
it might survive notwithstanding its showy plumage.
Greater Value of Females
Whether, as Wallace suggests, the hen does most of the incubating, and is
exposed to special danger when sitting on her eggs in an open nest, or,
as Stolzmann urges, it is of advantage to the species that there should
not be too many males, the result is the same, that the species can
afford to allow the cock to be more gaily attired than the hen. In either
case the colouration of the cock becomes a matter of comparatively little
importance to the species, and this, coupled with the fact that the male
tends to mutate more readily than the female, will explain why, in most
species which exhibit sexual dimorphism, it is the cocks that are the
more conspicuous. In certain species the cocks alone incubate, and these
then become more important than the females to the race, so that they
have not been permitted to become showy, while the hens have been allowed
more freedom in this respect. The extreme variability of the Ruff
(_Pavoncella pugnax_) in breeding plumage points to the fact that his
colour is a matter of comparative indifference to the species; in
consequence plenty of latitude is allowed to his tendency to vary.
Our view, then, is that evolution proceeds by mutations, which may be
large or small.
The mutation is the result of a rearrangement in part or parts of the
fertilised egg, and this rearrangement shows itself in the adult organism
as a change in one or more of its characteristics. The mutation may be
correlated with only one of the sexual organs, and when this is the case,
it gives rise to the phenomenon of sexual dimorphism. The appearance in
the adult of certain, if not of all, characteristics is affected by
causes other than the nature of the biological molecules from which they
are derived. The tendency to develop in a certain direction is there, but
something else, such as the secretion of hormones from the sexual cells,
is frequently necessary to enable a given tendency to fully develop
itself. Thus it is that castration often affects the bodily appearance of
those animals operated on. When a mutation appears, natural selection
decides whether or not it shall persist.