In the lower animals, which are lacking not only in intelligence but also in the higher and more complex differentiation of the sensory system, the development of such secondary sex characters is rare or altogether absent. Animals which have no sense of hearing can develop no song, and animals which do not see cannot acquire gorgeous colours as a means of exciting one sex through the other. But distinctive sex coloration may arise even in lowly animals, though there can be no question of æsthetic pleasure associated therewith; if the animals are able to see the colours at all, sexual excitement may be associated with these.
We need not wonder, therefore, that in the somewhat stupid fishes, in the butterflies, and in the lower crustaceans, like the Daphnids, we still find brilliant colours, which we can hardly interpret otherwise than as the results of sexual selection. On the other hand, the absence of such characters in animals of a still lower order, with still simpler sense-organs, like the Polyps, Medusæ, Echinoderms, most Worms, and the Sponges, affords an indirect confirmation of the correctness of our view as to the reality of a sexual selection in the more highly organized animals.
We see, then, that numerous peculiarities which distinguish the males of a species from the females depend on the process of sexual selection. This may be said of ornamental outgrowths, colours, remarkable feathers and feather-groups, peculiar odoriferous organs, vocal organs, artistic instincts, and also weapons, like antlers, tusks, and spurs, notable size and strength of body, and protective devices like manes; and again, the various organs for catching and holding the females, or for finding them out by sight or smell, must also be referred, at least in part, to sexual selection. The diversity of the male sexual characters is so great that I cannot give more than a faint idea of them without entering on a long catalogue; whoever wishes a complete survey has only to consult Darwin's Descent of Man.
But the significance of sexual selection is by no means exhausted with the production of the male sexual characters, for these characters are often more or less completely transferred to the females, and thus give rise to a transformation of the whole species, and not only of the male section of it. This is obviously a very important consequence of sexual selection, one which, as we shall see, materially deepens our insight into the mode of origin of new species.
First let us try to determine the facts. Many male characters are not represented in the female in any degree, and therefore have never been transmitted to them at all. Such are the mane of the lion, the grasping antennæ of Moina, the turban eyes of the Ephemerides, the intensification of the sense of smell in Leptodora, the lasso-like antennæ of the Copepods, the scent-scales of the butterflies, and the musk glands of the alligators and stags. But in other cases there has been transmission, though only to a slight extent. Thus many female humming-birds have a faint indication of the magnificent metallic colouring of the males; many female blue butterflies have a tinge of the beautiful blue of their mates; the females of the stag-beetle (Lucanus cervus) possess a diminutive suggestion of the antler-like jaws of the male, and the female crickets, although they do not chirp, have a slight indication of the 'musical' mechanism of the male on the wing-coverts, and some of them even produce feeble notes at certain times.
It can be proved, however, that such transmissions may, in the course of many successive generations, become intensified until the characters are exhibited by the females in the same degree as in the males. I know no better example of this than that afforded by the beautiful butterflies of the genus Lycæna. In this genus, which is rich in species and widely distributed over the whole earth, and must therefore be an old one, the upper surface of the wing is blue in by far the greater number of species, at least in the male sex. But there are three or four species which are dark-brown, and quite or nearly alike in the two sexes; such are the species Lycæna agestis, L. eumedon, L. admetus, and others. Everything indicates that this is the primitive colour of the genus. Moreover, there are some species with brown females, in which the males are not completely blue, but which have a slight bluish tinge, like L. alsus, the smallest of our indigenous Blues. Then follows a host of beautiful species, like L. alexis, L. adonis, L. damon, L. corydon, and many others, with brown females, and among these there occasionally occur females more or less tinged with blue. These lead on to L. meleager, which has two forms of female, a common brown and a rarer blue; and thus we reach L. tiresias, L. optilete, and L. argiolus, in which all the females are blue, although less intensely and completely so than their mates. The climax of this evolutionary series is reached by some species like L. beatica, belonging to tropical or at least warm countries, in which both sexes are of an equally intense blue. As we know that, in species with an excess of males, sexual characters always begin in the males, there can be no doubt as to the direction of evolution—from brown to blue—in this series. Furthermore, the entire absence of scent-scales in most of the species with brown males indicates the great age of these species, for, as far as I have been able to investigate, all the males of the blue species possess them.
Darwin regarded this transferring of the male characters to the females as due to inheritance, and it really seems as if it were simply a case of transmission by inheritance to one sex of what has been acquired by the other. Yet we have to ask whether we can continue to regard the facts in this light. In any case this 'transmission' is not an inevitable physiological process, necessarily resulting from the intrinsic conditions of inheritance, for we see that it often does not occur, even in many cases in which we can see no external reasons why it should not do so, though in other cases the failure may be presumably correlated with the external conditions of life. Thus, for instance, the persistent retention of the brown colour in the majority of our female Lycænidæ has probably its reason in the greater need of protection on the part of the much rarer females, and this must be so also in the case of many birds in which the brilliant colours of the males have not been transferred to the females. Wallace first pointed out that all birds whose females brood in exposed nests are inconspicuously coloured in the female sex, even if the males are brightly coloured, while those whose nests are concealed in holes of trees or the like, or which build domes over them, not rarely exhibit brilliant colouring in both sexes. This is the case in woodpeckers and parrots, while the gallinaceous birds, which brood in the open, have usually inconspicuously coloured females, for the most part very well adapted to their surroundings.
If we grasp the fact that a transference of the characters which have arisen through sexual selection can take place, we have a valuable aid in the interpretation of many phenomena which would otherwise remain quite inexplicable. What is the meaning of the gay colours of the parrots, which occur in such incredibly diverse combinations in this large and widely distributed family? Or of the marvellously complex markings and colour-patterns of the butterflies? In some cases they may be protective, as is the green of many parrots; in others, warning signs of unpalatability, like the bright colours and contrasted markings of many Heliconiidæ and Eusemiidæ and other butterflies with a nauseous taste; but there remain a great many cases to which neither of these explanations applies, which could only be regarded as pure freaks of nature if we did not know that male sexual characters can be transferred to the females, and that thus all the individuals of a species can be totally altered in their colouring.
Thus the occurrence not only of conspicuous, but of complicated, coloration is explained.
Darwin has shown that, in the equipment developed by the males in their competition for the possession of the females, it is by no means only those characters which may be considered 'beautiful' in themselves that have to be considered; it is rather the striking characteristics which mark their possessor and distinguish it from others that are primarily important. In fact, it is the principle of 'mode' or 'fashion' which is operative; something new is demanded, and as far as possible something quite different from that which was previously considered beautiful. Thus the starting-point for such processes of selection may have been afforded by white spots on a black ground, or, indeed, by any light spots on a dark ground, which may have been the primitive colour in most cases. If in the course of a long series of generations these spots became the common property of all the males, a possibility of further change was opened up as soon as a new contrast cropped up as a chance variation, which would then, under favourable conditions, be the starting-point of a new process of selection. Darwin has cited some cases in which, from a comparison of the dress of the young bird with that of the adult, we may conclude that a transformation of the colouring of the whole plumage must have taken place in the course of the phylogenetic history.