SEXUAL SELECTION AMONG ANIMALS
The expression, “Sexual Selection,” was first used by Mr. Darwin.
Besides natural selection, which depends on the success of both
sexes, at all ages, in relation to the general conditions of life,
he introduced another principle, sexual selection, which depends on
the success of certain individuals over others of the same sex, in
relation to the propagation of the species. According to the former
principle, those individuals who are most successful in the struggle
for existence survive the others, and characters useful to the species
are thus inherited; according to the latter, those individuals who have
the greatest success in the struggle for mates have the most numerous
offspring, and the characters which gave them the preference pass on to
the new generation, and are afterwards intensified by the operation of
like causes. The sexual struggle is of two kinds. In both it is carried
on by individuals of the same sex; but in one these individuals,
generally the males, try to drive away or kill their rivals; in
the other, they seek to excite or charm those of the opposite sex,
generally the females, who select the most attractive males for their
partners. Therefore, the characters acquired through sexual selection,
and transmitted chiefly to offspring of the same sex, generally the
males, are, on the one hand, weapons for battle, vigour and courage; on
the other hand, certain colours, forms, ornaments, sounds, or odours,
which are felt to be pleasant. The secondary sexual characters of the
latter sort are thus due to the taste of the females. They have been
acquired because they are beautiful or otherwise agreeable, whereas the
characters resulting from natural selection have been acquired because
they are useful. How are we to explain the origin of this wonderful
æsthetic faculty? “The senses of man and of the lower animals,”
says Mr. Darwin, “seem to be so constituted that brilliant colours
and certain forms, as well as harmonious and rhythmical sounds, give
pleasure and are called beautiful; but why this should be so we know
not.”[1486] According to Mr. Darwin, natural and sexual selection are
two different sources from which animal characters have arisen. There
is some truth in the statement of one of his critics, “Mr. Darwin,
in fact, has so far abandoned his former belief in the efficacy
of ‘natural selection’ as an agent in producing the differences
which separate different species of animals, as to admit that some
supplementary cause must, in some cases at any rate, be looked for; and
this he thinks is to be found in the action, through long periods, of
‘sexual selection.’”[1487]
Far from co-operating with the process of natural selection, sexual
selection, as described by Mr. Darwin, produces effects disadvantageous
to the species. “It is evident,” he says, “that the brilliant
colours, top-knots, fine plumes, &c., of many male birds cannot
have been acquired as a protection; indeed, they sometimes lead to
danger.”[1488] When we consider what an important part is played by
colours, as means of protection, in the whole animal kingdom, it is
certainly surprising that many male animals display brilliant hues,
which cannot fail to make them conspicuous to their enemies. The strong
odours emitted by certain reptiles and mammals, during the pairing
season, and the sounds produced by various species at the same period,
have also the effect of attracting hostile animals that are searching
for food. And the danger arising for the species from these secondary
sexual characters is all the greater because they generally appear
at the time when offspring is about to be produced. Thus, besides
colours, structures, and functions, adapted in the most marvellous way
to the requirements of each species, there are others highly dangerous,
which, according to Mr. Darwin, depend upon an æsthetic sense, the
origin of which we do not know, and which is absolutely useless.
Mr. Darwin, in his many works, has shown how immense is the influence
exercised by natural selection on the organic world. A disciple,
therefore, naturally feels perplexed when he is told of a series of
facts, which, according to the explanation given by the master, are
opposed to natural selection. When the contradiction between the
theories of natural and sexual selection is distinctly realized, the
question arises:—Can we be sure that the secondary sexual characters
are so useless as Mr. Darwin suggests? May not they also be explained
by the principle of the survival of the fittest? The larger size and
greater strength of the males, and the weapons of offence or defence
many of them possess, may easily be so accounted for, as, among the
higher animals, the males generally fight with each other for the
possession of the females. The point is whether the other secondary
sexual characters can be due to the same cause.
* * * * *
It is an established fact that the colours of flowers serve a definite
end. Through them the flowers are recognized by insects in search
of honey; and the insects, during their visits, involuntarily carry
the pollen of one flower to the stigma of another, and thus effect
cross-fertilization, which is proved to be of great importance for
the vigour and fertility of the next generation of plants. Now it
is extremely interesting to note that brilliant colours are found
only in species of flowers to which they are useful as means of
attracting insects; they never occur in plants which are fertilized
by the wind.[1489] Mr. Wallace observes that plants rarely need to be
concealed, because they obtain protection by their spines, or their
hardness, or their hairy covering, or their poisonous secretions.
Hence there are very few cases of what seems to be true protective
colouring among them.[1490] In animals, on the contrary, colour is
greatly influenced by their need of protection from, or warning to,
their numerous enemies; colours of other kinds must always, to a
certain extent, be dangerous for the species. Is it probable, then,
that, whilst gay colours occur only in the flowers of those plants to
which they are of real use, conspicuous colours should occur in animals
to which they are of real danger—merely because the females find them
beautiful?
Mr. Wallace, whose well-known criticism of Mr. Darwin’s theory of
sexual selection[1491] seems, in many points, to be conclusive,
suggests that the very frequent superiority of the male bird or insect
in brightness or intensity of colour is due to the greater vigour
and activity and the higher vitality of the male. This intensity of
coloration is therefore most manifest in the male during the breeding
season, when the vitality is at a maximum. It would be further
developed by the combats of the males for the possession of the
females; and the most vigorous and energetic usually leaving the most
numerous and most healthy offspring, natural selection would indirectly
become a preserver and intensifier of colour.[1492] Mr. Wallace has
made it very probable that there is some connection between vigour and
colour, but another question is whether this connection, depending
on some unknown physiological law, is so necessary that it takes
place even when colour is positively disadvantageous to the species.
Nothing of the kind is found in the vegetable kingdom. We know, as Mr.
Wallace himself remarks, that colours which rarely or never appear in
the species in a state of nature, continually occur among cultivated
plants and domesticated animals—a fact which shows that the capacity
to develop colour is ever present.[1493] Among wild plants such colour
variations are never preserved except when they are useful. Is it not
most reasonable to suppose that the like is the case with animals?
The truth seems to be that colour subserves the same purpose in both of
the great kingdoms of the organic world. Just as flowers are coloured
that insects may recognize where honey is to be found, and thus may
be led to promote fertilization, so the sexual colours of animals
have been developed to make it easier for the sexes to find each other
during the pairing time. Protective colours are useful so far as they
conceal the animal from its enemies, but, at the same time, they
conceal it from individuals of its own species. Sexual colours are
therefore useful as well, because they make the animal more visible.
It is quite in accordance with the theory of natural selection that,
where such colours occur, the advantage from them should be greater
than the disadvantage. We can see the reason for the brilliant colours
of humming-birds, as these birds, on account of their great activity
“are practically unmolested,”[1494] and for the bright hues of the
rose chafers, who are saved from attack by a combination of protecting
characters.[1495] But generally there is danger in sexual colours,
so that nature has given them with the utmost cautiousness. Usually
they occur in males only, because of the females’ greater need of
protection.[1496] They are not developed till the age of reproduction,
and they appear, in a great many species, only during the pairing
season. The greatest advantage is won with the least possible peril.
It is a fact of great importance that sexual colours occur exactly
in those species whose habits make these colours most visible. Thus
the nocturnal moths, taken as a body, are much less gaily decorated
than butterflies, all of which are diurnal in their habits, although,
according to Mr. Wallace, the general influence of solar light and heat
is no adequate cause for the variety, intensity, and complexity of the
colours. The females of the ghost moth are yellow with darker markings,
whereas the males are white, that they may be more easily seen by the
females whilst flying about in the dusk; and it is remarkable that,
in the Shetland Islands, the male of this moth, instead of differing
widely from the female, frequently resembles her closely in colour,—as
Mr. Fraser suggests,[1497] because, at the season of the year when
the ghost moth appears in these northern latitudes, the whiteness of
the males is not needed to render them visible to the females in the
twilight night. Both Mr. Darwin[1498] and Mr. Wallace[1499] think that,
in this case, colour may be a means of recognition. Sexual colours
occur chiefly in species which, because of their manner of living, are
to be seen at a distance; they seldom occur in sedentary or slowly
moving terrestrial animals.[1500] The members of the lowly organized
order Thysanura are wingless and dull-coloured. The Hemiptera, which
usually lurk about plants, and prey upon hapless insects, are not,
as a rule, remarkable for conspicuous hues. The Orthoptera are all
terrestrial in their habits, generally feeding upon plants, and,
although some exotic locusts are beautifully ornamented, their bright
tints, according to Mr. Darwin, do not seem to fall under the head of
sexual coloration. On the other hand, the dragon-flies, which live
in the open air, possess splendid green, blue, yellow, and vermilion
metallic tints, and the sexes often differ in their coloration. Every
one has admired the extreme beauty of many butterflies, especially
of the males. Amongst the Fishes, living in a medium through which
bright colours may be observed at a distance, we often find, besides
protective colours, conspicuous hues which are especially intense
and visible during the pairing time. Among the Reptiles, the little
lizards of the genus Draco especially deserve attention; they glide
through the air on their rib-supported parachutes, and the beauty of
their colours baffles description. Mammals, on the other hand, do not
generally present the splendid tints so common among male birds; and
the brighter colours of certain arboreal mammals serve chiefly as means
of concealment.
These phenomena seem to show that sexual colours have been evolved for
the purpose of _being seen_. They can scarcely be due merely to the
fact that coloration is connected with the degree of vitality, since
the Mammals, for instance, are certainly not less vigorous than any
of the other Vertebrate orders. It may perhaps be suggested that,
as flying animals more easily escape their enemies than terrestrial,
they may with less danger be decorated with conspicuous hues. But here
we have to observe the most important fact, that animals which do
not possess sexual colours generally have some other means of making
themselves discoverable.
Flowers which need the help of insects for fertilization attract them,
in some cases, not by bright colours, but by peculiar odours. And as
we do not find conspicuous colours in plants fertilized by the wind,
so flowers have no perfume except where it is of real use. The most
brilliant flowers, as a rule, are those which possess least odour,
whilst many of them have no scent at all. White or very pale flowers
are generally the most odoriferous. M. Mongredien gives a list of about
160 species of hardy trees and shrubs with showy flowers, and another
list of sixty species with fragrant flowers; but only twenty of the
latter are included among the showy species, and these are almost all
white-flowered.[1501] Most of the white flowers are scented only at
night, or their perfumes are most powerfully emitted at that time; the
reason being that white flowers are fertilized chiefly by night-flying
insects. We arrive thus at two conclusions: first, that powerful odours
and conspicuous colours as guides to insect fertilizers are, as a rule,
complementary to each other; secondly, that they occur alternately in
the way most useful to the species.
In the animal kingdom various odours and sounds are closely connected
with the reproduction of the species. During the season of love a
musky odour is emitted by the submaxillary glands of the crocodile,
and pervades its haunts. At the same period the anal scent-glands of
snakes are in active function, and so are the corresponding glands of
the lizards. Many mammals are odoriferous. In some cases the odour
appears to serve as a defence or a protection, but in other species the
glands are confined to the males, and almost always become more active
during the rutting season. Again, a great many insects have the power
of producing stridulous sounds. In two families of the Homoptera and in
three of the Orthoptera, the males alone possess organs of sound in
an efficient state, and these are used incessantly during the pairing
season. Some male fishes have sound-producing instruments, and the
fishermen of Rochelle assert that the males alone make the noise during
the spawning-time. Of frogs and toads the males emit various sounds
at the pairing time, as in the case of the croaking of our common
frog. During the rutting season, and at no other time, the male of
the huge tortoise of the Galapagos Islands utters a hoarse bellowing
noise, which can be heard at a distance of more than a hundred yards.
Professor Aughey states that on two occasions, being himself unseen,
he watched from a little distance a rattle-snake coiled up with head
erect, which continued to rattle at short intervals for half an hour;
at last he saw another snake approach, and when they met they paired.
Among Birds the power of song, or of giving forth strange cries, or
even instrumental music, is exceedingly common, particularly in the
males during the pairing season; and almost all male mammals use their
voices much more during that period than at any other time. Some, as
the giraffe and porcupine, are stated to be completely mute except
during the rutting season.
The colours, odours, and sounds of animals, like the colours and odours
of plants—so far as they may be assumed to be in some way connected
with the reproductive functions—are, as a rule, complementary to each
other. Stridulating insects are generally not conspicuously coloured.
Among the Homoptera, there do not seem to be any well-marked cases
of ornamental differences between the sexes. Among crickets, the
Locustidæ, and grasshoppers, some species are beautifully coloured;
but Mr. Darwin says, “It is not probable that they owe their bright
tints to sexual selection. Conspicuous colours may be of use to these
insects by giving notice that they are unpalatable.” Other species have
directly protective colours. The bright hues of stridulating beetles
seem to be of use chiefly for protective and warning purposes; whereas
species belonging to the orders Neuroptera and Lepidoptera, often
extremely conspicuously coloured, are not remarkable for any stridulous
sounds. Frogs and toads, which have an interesting sexual character
in the musical powers possessed by the males, are evidently coloured
according to the principle of protection, or sometimes tinted with
conspicuous hues in order to be more easily recognized by their enemies
as a nauseous food. Of Reptiles, the Lacertilia excel mainly in bright
tints; the Chelonia, Crocodilia, and Ophidia, in sounds and odours.
Among Birds, in one instance at least, the male is remarkable for his
scent. “During the pairing and breeding season,” says Mr. Gould, with
reference to the Australian musk-duck, “ ... this bird emits a strong
musky odour;” it is not ornamented with any conspicuous hues.[1502]
Sexual colours and the power of song are generally complementary to
each other among Birds. “As a general rule,” Mr. Wood remarks, “it
is found that the most brilliant songsters among the birds are attired
in the plainest garb; and it may safely be predicted of any peculiarly
gorgeous bird, that power, quality and sweetness of voice are in
inverse ratio to its beauty of plumage.”[1503] Thus, of the British
birds, with the exception of the bullfinch and goldfinch, the best
songsters are plain-coloured, and the brilliant birds of the tropics
are hardly ever songsters. The wild camel in the desert of Kum-tagh has
a reddish, sandy hue, and the males, “even during the rutting season,
utter no sound, but find their consorts by scent.”[1504] The musk-deer,
well known for the intolerable perfume which the males emit at the
pairing time, is also entirely silent.[1505]
Moreover, as appears from what has just been said, the sexual colours,
the perceptible scents and sounds of animals are complementary to
each other in the way that is best suited to make the animals easily
discoverable. As bright colours would be of no advantage to flowers
fertilized by night-flying insects, so they would be of comparatively
little advantage to animals living among grass and plants, in woods
and bushes; whereas sounds and scents make the animal recognizable at
a considerable distance. We have also seen that it is among flying and
aquatic animals that sexual colours chiefly occur, whereas terrestrial
animals excel in sound and scents. Thus most of the stridulating
insects are terrestrial. Whilst brightly-coloured lizards, living on
trees or running from stone to stone, must attract attention by the
brilliance of their covering, crocodiles inhabiting rivers and jungles,
and frogs crawling among the grass, allure their mates, the former by
emitting musky odours, the latter by producing loud sounds. The odour
of the Australian musk-duck, which depends for its food and for its
preservation from danger upon its powers of diving rather than upon
those of flying, is, as Mr. Gould observes, often perceptible long
before the animal can be seen.[1506]
Mr. Darwin remarks, as regards birds, “Bright colours and the power of
song seem to replace each other. We can perceive that, if the plumage
did not vary in brightness, or if bright colours were dangerous to
the species, other means would be employed to charm the females; and
melody of voice offers one such means.”[1507] But if we accept Mr.
Darwin’s theory of sexual selection, we are compelled to suppose that
that inexplicable æsthetic sense of the females has been developed in
the way most dangerous to the species. Conspicuous colours are admired
by the females of those animals which, by means of such colours, are
most easily discovered by their enemies, and sounds and odours are
appreciated exactly in those species to which they are most perilous.
If, on the contrary, we accept the explanation that, although sexual
colours, odours, and sounds are in some ways hurtful to the species,
they are upon the whole advantageous, inasmuch as they make it easier
for the sexes to find each other, we have a theory in accordance
with all known facts, as well as with the great principle of natural
selection. It may be objected that it is not the females but the
males that are the seekers, whilst the secondary sexual characters
generally occur in the males only. But we have no reason to think
that the females are entirely passive during the pairing season; and
several of the statements collected by Mr. Darwin directly indicate
that females are attracted by the sounds of their future partners. If
Burdach is correct in saying that the male sex generally possesses
more acute senses than the female,[1508] it is obvious that secondary
sexual characters would be of less use to females than to males, as it
certainly would be of greater danger.
In his work on ‘Darwinism,’ Mr. Wallace expresses the opinion that
the various sounds and odours which are peculiar to the male serve
as a call to the female, or as an indication of his presence; and,
as he says, “the production, intensification, and differentiation
of these sounds and odours are clearly within the power of natural
selection.”[1509] Mr. Wallace has also shown the immense importance of
colour as a means of recognition. The theory here set forth thus, in
fact, very nearly approaches his views. The only difference is that
the sexual colours have been classified under the head of “colour
for recognition,” though the positive cause by which they have been
produced may be a surplus of vital energy.
We have still to consider certain secondary sexual characters which,
according to Mr. Darwin, must be regarded as ornaments. With these he
classes the great horns which rise from the head, thorax, and clypeus
of many male beetles; the appendages with which some male fishes and
reptiles are provided; the combs, plumes, crests, and protuberances
of many male birds; and various crests, tufts, and mantles of hair
which are found in certain mammals. But some of these characters may
be of use to the males in their fights for females, or serve as means
of recognition. Mr. Wallace suggests that crests and other erectile
feathers may have been useful in making the bird more formidable in
appearance, and in thus frightening away enemies; while long tail or
wing feathers might serve to distract the aim of a bird of prey.[1510]
Moreover, characters of which we cannot yet perceive the use may in
the future be brought under the law of utility, as has been the case
in so many other instances. According to Mr. Wallace, the ornamental
appendages of birds and other animals are due to a surplus of vital
energy, leading to abnormal growths in those parts of the integument
where muscular and nervous action are greatest.[1511] And where these
“ornaments” are of no positive disadvantage to the species, certainly
no other explanation is needed.
For other arguments which may be advanced against Mr. Darwin’s
theory of sexual selection, reference may be made to Mr. Wallace’s
criticisms in ‘Tropical Nature’ and ‘Darwinism.’ We have sufficient
evidence that females are pleased or excited by the males’ display of
their sexual colours,[1512] and are charmed by their songs. But Mr.
Darwin’s theory presupposes, amongst many other things, that almost
all the females of a species, over a wide area and for many successive
generations, prefer exactly the same modification of the colour, or
ornament or sounds.[1513] Moreover, if the secondary sexual characters
are due to female choice, how shall we explain the strange fact that
the taste of the females varies so much that there are scarcely two
species in which the standard of perfection is exactly the same? This
difficulty did not escape Mr. Darwin. “It is a curious fact,” he says,
“that in the same class of animals sounds so different as the drumming
of the snipe’s tail, the tapping of the woodpecker’s beak, the harsh
trumpet-like cry of certain waterfowl, the cooing of the turtle-dove,
and the song of the nightingale, should all be pleasing to the females
of the several species.” And further, “What shall we say about the
harsh screams of, for instance, some kinds of macaws; have these birds
as bad taste for musical sounds as they apparently have for colour,
judging by the inharmonious contrast of their bright yellow and blue
plumage?”[1514]
The theory now suggested accounts fully for this difference in taste.
The immense variability of the secondary sexual characters is
precisely what might be expected, if their object is to make it easier
for the sexes to find and recognize each other. And it is natural that
the females should be pleased by colours, odours, or sounds which, by
the association of ideas, are to them the symbols of the most exciting
period of their lives. On the other hand, we know that differently
coloured races of the same species may be disinclined to pair
together.[1515] And here, I think, we may draw an important conclusion.
The great stability of the secondary sexual characters which we find in
wild species, but certainly not in animals under domestication, seems
to be due chiefly to the fact that those males which most typically
represent the peculiarities of their species have the best chance of
finding mates.
* * * * *
The reader may have felt some surprise at this strange jump from the
_patria potestas_ to a discussion of merely zoological facts, which
have nothing to do, directly, with the history of human marriage. But
we have now to deal with the sexual selection of man, and, for the
right understanding of this, it was necessary to show that the sexual
selection of the lower animals is entirely subordinate to the great law
of natural selection. Mr. Darwin discussed the origin of the secondary
sexual characters as a preliminary to the statement of his theory
regarding the origin of man, and of the different races of men. At the
end of the next chapter we shall consider whether this theory appears
to be in accordance with facts or not.