LECTURE XI

SEXUAL SELECTION

Decorative colouring of male butterflies and birds—Wallace's interpretation—Preponderance of males—Choice of the females—Sense by sight in butterflies—Attractive odours—Scent-scales—Fragrance of the females—The limits of natural and sexual selection not clearly defined—Odours of particular species—Odours of other animals at the breeding season—Song of the Cicadas, and of birds—Diversity of decoration successively acquired—Humming-birds—Substitution of other aids to wooing in place of personal decoration—Smelling organs of male insects and crabs—Contrivances for seizing and holding the female—Small size of certain males—Weapons of males used in struggle for the females—Turban eyes of Ephemerids—Hoods that can be inflated on the head of birds—Absence of secondary sexual characters in lower animals—Transference of male characters to the females—Lycæna—Parrots—Fashion operative in the phyletic modifications of colour—Pattern of markings on the upper surface of a butterfly's wing simpler than on the under side—Conclusion.

Wefound in the process of Natural Selection an explanation of numerous effective adaptations in plants and animals, as regards form, colouring, and metabolism, of the most diverse weapons and protective devices, of the existence of those forms of blossoms which we call flowers, of instincts, and so on. The origin of the most characteristic parts of whole orders of insects can only be understood as adaptations to the environment brought about by means of natural selection. Impressed by this, we have now to ask whetherallthe transformations of organisms may not be referred to adaptation to the continually changing conditions of life? We shall return to this question later, but in the meantime we are far from being able to answer it in the affirmative, for there are undoubtedly a great many characters, at least in animals, which cannot have owed their origin to natural selection in the form in which we have studied it so far.

How could the splendid plumage of the humming-birds, of the pheasants, of the parrots, the wonderful colour-patterns of so many diurnal butterflies, be referred to the process of natural selection, since all these characters can have no significance for their possessors in the struggle for existence? Or of what use in the struggle for existence could the possession of its gorgeous dress of feathers be to the bird of Paradise; or of what service is the azure blue iridescence of theMorphoof Brazil, which makes it conspicuous from a distancewhen it plays about the crowns of the palm-trees? We might indeed suppose that they are warning signs of unpalatableness, like those of the Heliconiides or of the gaily coloured caterpillars, but, in the first place, these gay creatures are by no means inedible, and are indeed much persecuted; and, secondly, the females have quite different and very much darker and simpler colours. The gleaming splendour of all these birds of Paradise and humming-birds, as well as that of many butterflies, is found in the male sex only. The females of the birds just mentioned are dark in colour and without the sparkling decorative feathers of the males; they are plain—just like the females of many butterflies. Alfred Russel Wallace has suggested that the explanation of this lies in the greater need of the females for protection, since, as is well known, they usually perform the labours of brooding, and are thus frequently exposed to the attacks of enemies. It is undoubtedly true that the dark and inconspicuous colouring of many birds and butterflies depends on this need for protection, but this does not explain the brilliant colours of the males of these species. Or can it be that these require no explanation further than that they are, so to speak, a chance secondary outcome of the structural relations of the feathers and wing-scales respectively, which brought with it some other advantage not known to us? Perhaps something in the same way as the red colour of the blood in all vertebrates, from fishes upwards, cannot be useful on the ground that it appears red to us, but because it is the expression of the chemical constitution of the hæmoglobin, a body which is indispensable to the metabolism, which here has the secondary and intrinsically quite unimportant peculiarity of reflecting the red rays of light.

No one can seriously believe this in regard to butterflies who knows that their colours are dependent on the scales which thickly cover the wings, and the significance of which, in part at least, is just to give this or that colour to the wing. They are degenerate or colourless among the transparent-winged butterflies, and their colour depends partly on pigment, partly on fluorescence and interference conditioned by the fine microscopical structure of a system of intercrossing lines on faintly coloured scales. The scales of our male 'blue' butterflies (Lycæna) only appear blue because of their structure, while the brown scales of their mates are due to a brown pigment. If the pigment be removed from the scales of the female by boiling with caustic potash, and they be then dried, they do not look blue like those of the male; the scales of the male, therefore, must possess something which those of the female do not.

Still less will any one be disposed to regard the marvellous splendour of the plumage of the male bird of Paradise, with its erectile collars—glistening like burnished metal—on the neck, breast or shoulders, with its tufts, with its specially decorative feathers standing singly out from the rest of the plumage, on head, wings, or tail, with its mane-like bunch of loose, pendulous feathers on the belly and on the sides, in short, with its extraordinary, diverse, and unique equipment of feathers, as a mere unintentional accessory effect of a feather dress designed for flight and protective warmth. Such conspicuous, diverse, and unusual specializations of plumage must have some other significance than that just indicated.

Alfred Russel Wallace regards these distinctive features of the male as an expression of the greater vigour, and the more active metabolism of the males, but it is unproved that the vigour of the male birds is greater than that of the females, and it is not easy to see why a more active metabolism should be necessary for the production of strikingly bright colours than for that of a dark or protective colour. Moreover, there are brilliantly coloured females, both among birds and butterflies, and in nearly allied species the males may be either gorgeous or quite plain like the females.

Darwin refers the origin of these secondary sexual characters to processes of selection quite analogous to those of ordinary natural selection, only that in this case it is not the maintenance of the species which is aimed at, but the attainment of reproduction by the single individual. The males are to some extent obliged to struggle for the possession of the females, and every little variation which enables a male to gain possession of a female more readily than his neighbour has for this reason a greater likelihood of being transmitted to descendants. Thus, attractive variations which once crop up will be transmitted to more and more numerous males of the species, and among these it will always be those possessing the character in question in the highest degree which will have the best chance of securing a mate, and so the character will continue to be augmented as long as variations in this direction appear.

Two kinds of preliminary conditions, however, must be assumed. As the ordinary natural selection could never have operated but for the fact that in every generation a great many individuals, indeed the majority of them, perish before they have had time to reproduce, so the process of sexual selection could never have come into operation if every male were able ultimately to secure a mate, no matter what degree of attractiveness to the latter he possessed. If the numbers of males and females were equal, so that there wasalways one female to one male, there could be no choice exercised either by male or female, for there would always remain individuals enough of both sexes, so that no male need remain unmated.

But this is not the case: the proportions of the sexes are very rarely as 1 : 1; there is usually a preponderating number of males, more rarely of females. Among birds the males are usually in the majority, still more so among fishes; and among diurnal butterflies there are often a hundred males to one female (Bates), although there seem to be a few tropical Papilionidæ among which the females have rather the preponderance. Darwin called attention to the fact that one could infer the greater rarity of the females even from the pricelists of butterflies issued by the late Dr. Staudinger in connexion with his business, for the females in most species, except the very common ones, are priced much higher than the males, often twice as high. In the whole list of many thousands of species there are only eleven species of nocturnal Lepidoptera in which the males are dearer than the females.

Among the Mayflies or Ephemerides, too, the males are in the majority; in many of them there are sixty males to one female: but there are other kinds of insects, such as the dragon-flies (Libellulidæ), in which the females are three or four times as numerous. There are also, it may be remembered, some kinds of insects, such as Aphides, which have become capable of parthenogenetic reproduction, and in which the males are becoming extinct, e.g. in the case ofCerataphisin British orchid-houses.

The first postulate implied in 'sexual selection,' namely, that there be an unequal number of individuals in the two sexes, is therefore fulfilled in Nature; we have now to inquire whether the second condition postulated—the power of choice—may also be regarded as a reality.

This point has been disputed from many sides, and even by one of the founders of the whole selection theory, Alfred Russel Wallace. This naturalist doubts whether a choice is exercised among birds by either sex in regard to pairing, and maintains that, even if there could be a choice, this could not have produced such differences in colour and character of the plumage, since that would presuppose the existence of similar taste in the females through many generations. In a similar way it has been doubted whether butterflies can be said to exercise any real power of sexual choice, whether a more beautiful male is as such preferred to a less beautiful suitor.

It must be admitted that direct observation of choosing isdifficult, and that as yet there is very little that can be said with certainty on this point. But there are, after all, some precise observations on mammals and birds which prove that the female shows active inclination to, or disinclination for, a particular male. If we hold fast to this fact, and add to it that the distinctive markings of the males are wonderfully developed during the period of courtship, and are displayed before the females, and that they only appear in mammals, birds, amphibians, and fishes at the time of sexual maturity, it seems to me that there can be no doubt that they are intended to fascinate the females, and to induce them to yield themselves to the males. The opponents of the theory of sexual selection attach too much importance to isolated cases; they imagine that each female must make a choice between several males. But the theory of sexual selection does not demand this, any more than the theory of natural selection requires the assumption that every individual of a species which is better equipped for the struggle for existence must necessarily survive and attain to reproduction, or, conversely, that the less well equipped must necessarily perish.

All that the theory requires is, that the selective and eliminative processes do,on an average, secure their ends, and in the same way the theory of sexual selection does not need the assumption that every female is in a position to exercise a scrupulous choice from among a troop of males, but only that, on an average, the males more agreeable to the females are selected, and those less agreeable rejected. If this is the case, it must result in the male characters most attractive to the females gaining preponderance, and becoming more and more firmly established in the species, increasing in intensity, and finally becoming a stable possession of all the males.

When we go more into details we shall see that theparticular qualitiesof the distinctive masculine characters are exactly such as they would be if they owed their existence to processes of selection; in other words, from this point of view the phenomena of the decorative sexual characters can be understood up to a certain point. It seems to me that we are bound to accept the process of sexual selection as really operative, and instead of throwing doubt upon it, because the choice of the females can rarely be directly established, we should rather deduce from the numerous sexual characters of the males, which have a significance only in relation to courtship, that the females of the species are sensitive to these distinguishing characters, and are really capable of exercising a choice.

In my mind at least there remains no doubt that the 'sexualselection' of Darwin is an important factor in the transformation of species, even if I only take into consideration those secondary sexual characters which are related to wooing. We shall see, however, that there are others in regard to whose origin through processes of selection doubt is still less legitimate, and from which, on this account, we can argue back to the courtship characters.

The first beginning of transformation is not, even in ordinary natural selection, to be understood as due to selection, but is to be regarded asa given variation(the causes of which we shall discuss later on); it is only the increase of such incipient variations in a definite direction that can depend on natural selection, and theymustdepend on it in so far as the transformations are purposeful. Now, all secondary sexual characters can be recognized as useful, save only the decorative distinctions, although these also undoubtedly represent intensifications of originally unimportant variations. Are we then to regard these alone as the mere outcome of the internal impulsive forces of the organism, while in the case of the analogous sexual characters for tracking, catching, and holding the female, and so forth, the augmentation and the directing must be referred to processes of selection? But if there be any utility at all in the decorative sexual characters it can only lie in their greater attractiveness to the females, and it can only be of any account if the females have, in a certain sense, the power of choice. Independently, therefore, of direct observations as to the actual occurrence of choosing, we should be compelled by our chain of reasoning to assume that there was such a power of choice—and I shall immediately discuss it more precisely.

If we consider the decorative, distinctive characters of the males more closely, we find that they are of very diverse kinds. The males of many animals are distinguished from the females chiefly by greater beauty of form, and especially of colour. This is the case in many birds, some amphibians, like the water-salamander, many fishes, many insects, and above all, in diurnal Lepidoptera. Especially among birds the dimorphism between the sexes is in obvious relation to the excess in the number of male individuals, or—what practically comes to the same thing—to polygamy. For when a male attaches to himself four or ten females the result is the same as if the number of female individuals were divided by four or by ten. Thus the fowls and pheasants, which are polygamous, are adorned by magnificent colours in the male sex, while the monogamous partridges and quails exhibit the same colouring in both sexes. Of course 'beautiful' is a relative term, and we must not simply assume that what seems beautiful tous appears so to all animals; yet when we see that all the male birds which are beautifully decorative according to our taste—whether humming-birds, pheasants, birds of Paradise, or rock-cocks (Rupicola crocea)—unfold their 'feather-wheels, 'fans,' 'collars,' and so forth, before the eyes of the females in the breeding season, and display them in all their brilliance, we must conclude that, in these instances at least, human taste accords with that of the animals. That birds have sharp vision and distinguish colours is well known; it is not for nothing that the service berries and many other berries suitable for birds are red, the mistletoe berries white, in contrast to the evergreen foliage of this plant, the juniper berries black so that they stand out amid the snows of winter; in this direction, then, there is no difficulty in the way of sexual selection.

Even among much lower animals, like the butterflies, there seems to me no reason for the assumption that they do not see the gorgeous colours and often very complicated markings, the bars and eye-spots, on the wings of their fellows of the same species. Of course if each facet of the insect eye contributed only a single visual impression, as Johannes Müller supposed, then even an eye with 12,000 facets would give but a rough and ill-defined picture of objects more than a few feet away, and I confess that for a long time I regarded this as an obstacle in the way of referring the sexual dimorphism of butterflies to processes of selection. But we now know, through Exner, that this is not the case; we know that each facet gives a little picture, and not an 'inverted' but an 'upright' one, and experiment with the excised insect eye has directly shown that it throws on a photographic plate a tolerably clear image of even distant objects, such as the frame of a window, a large letter painted on the window, or even a church tower visible through it.

Furthermore, the structure of the eye allows of incomparably clearer vision of near objects, for in that case the eyes act like lenses, and reveal much more minute details than we ourselves are able to make out. Here again, therefore, there is no obstacle to the Darwinian hypothesis of a choice on the part of the females, for although it cannot be demonstrated from the structure of the eye itself that insects see colour, and that colours have a specially exciting influence on them, yet we can deduce this with certainty from the phenomena of their life. The butterflies fly to gaily coloured flowers, and as they find in them their food, the nectar of the flowers, we may take for granted that the sight of the colour of their food-providing plants is associated with an agreeable sensation, and thisis an indication that similar colours in their fellows may awaken similar agreeable sensations.

Fig. 53.Scent-scales of diurnal butterflies.a, of Pieris.b, of Argynnis paphia.c, of a Satyrid.d, of Lycæna. All highly magnified.

This conclusion is furthermore confirmed by the fact that, in the male sex, numerous species of butterfly possess another means of exciting the females, namely, by pleasant odours. Volatile ethereal oils are secreted by certain cells of the skin, and exhale into the air through specially constructed scales. Usually the apparatus for dispersing fragrance occurs on the wing in the form of the so-called scent-scales (Duftschuppen), peculiar modifications of the ordinary colour-scales of the wing, but sometimes they take the form of brush-like hair-tufts on the abdomen, and they are in all cases so arranged that the volatile perfume from the cells of the skin penetrates into them, and then evaporates through very thin spots on the surface of the scale, or through brush-like, expanded fringes on their tips. Many of these have long been known to entomologists, because their divergence in form from the ordinary scales attracted attention; and it was also observed that they never occurred on the females, but only on the males. Their significance, however, remained obscure until, by a happy chance, Fritz Müller, in his Brazilian garden, discovered the fact that there are butterflies which give off fragrance like a flower, and then close investigation revealed to him the connexion between this delicate odour and the so-called 'male scales.' One can convince oneself of the correctness of the observation even in some of our own butterflies by brushing the finger over the wing of a newly caught male Garden White (Pieris napi). The finger will be found covered with a white dust, the rubbed-off wing-scales,and it will have a delicate perfume of lemon or balsam, thus proving that the fragrance adheres to the scales.

Fig. 54.A portion of the upper surfaceof the wing of a male 'blue' (Lycænamenalcas); after Dr. F. Köhler.bl, ordinaryblue scales.d, scent-scales. Highlymagnified.

Fig. 55.Zeuxidia wallacei,male, showing four tuftsof long, bristle-like, brightyellow scent-scales (d) onthe upper surface of theposterior wing.

In the last case, that is, among the Whites (Pieridæ) (Fig. 53,a), the scent-scales are distributed fairly regularly over the upper surface of the wing, and the same is true of our blue butterflies, the Lycænnidæ whose minute lute-shaped scales are shown singly in Fig. 53,d, but in their natural position among the ordinary scales in Fig. 54. In many other diurnal, and also in nocturnal Lepidoptera, the fragrant scales are united into tufts and localized in definite areas. They then often form fairly large spots, stripes, or brushes, which are easily visible to the naked eye. Thus the males of our various species of grass-butterflies (Satyridæ) have velvet-like black spots on the anterior wings, while the fritillary,Argynnis paphia, has coal-black stripes on four longitudinal ribs of the anterior wing which are absent in the females, and which are composed of hundreds of odoriferous scales. Certain large forest butterflies of South America, resembling ourApatura, bear in the middle of the gorgeous green shimmering posterior wing a thick expansible brush of long, bright yellow scent-scales, and a similar arrangement obtains in the beautiful violet butterfly of the Malay Islands, theZeuxidia wallaceidepicted in Fig. 55. In many of the Danaides, which we have already considered in relation to mimicry, the scent apparatus is even more perfect, for it is sunk in a fairly deep pocket on the posterior wings, and in this the scent-producing, hair-like scales lie concealed until the butterfly wishes to allow the fragrance to stream forth. In many South American and Indian species ofPapiliothe fragrant hairs are disposed in a sort of mane on a fold of the edge of the posterior wing, and so on. The diversity of these arrangements is extreme, and they are widely distributed among both diurnal and nocturnal Lepidoptera, in the latter sometimes in the form of a thick, glistening,white felt which fills a folded-over portion of the edge of the posterior wing. In many cases the perfume can be retained, and then, by a sudden turning out of the wing-fold, be allowed to stream forth. But there are a great many species of butterfly which do not possess odoriferous scales, and they are often wanting in near relatives of fragrant species; they are obviously of very late origin, and arose only after the majority of our modern species were already differentiated. It often seems as if they bore a compensatory relation to beauty of colour, somewhat in the same way as many modestly coloured flowers develop a strong perfume, while, conversely, many magnificently coloured flowers have no scent at all. Although among butterflies, as among flowers, there are species which possess both beauty and fragrance, yet our most beautiful diurnal butterflies, the Vanessas, the Apaturas, and Limenitis, possess no scent-scales; and many inconspicuous, that is, protectively coloured nocturnal Lepidoptera, are strongly fragrant, like most night-flowers: I need only mention the convolvulus hawk-moth (Sphinx convolvuli), whose musk-like odour was known to entomologists long before the discovery of scent-scales.

It is, however, always only in the males that this odoriferous apparatus is present. It must not be believed on this account that this fragrance has the significance of a means of attraction comparable to the perfume of the flowers which induces butterflies to visit them; indeed, we cannot assume that the odour carries to a distance, for, as far as we can make out, it is perceptible only within a very short radius, and this is indicated also by the manifold arrangements of the odoriferous organs, which are all calculated to retain the fragrance, and then—in the immediate neighbourhood of the female—to let it suddenly stream forth. Obviously, this arrangement can have no other significance than that of a sexual excitant; its use is to incline the female to the male, to fascinate her, just as do the beautiful colours, in regard to which we must draw the same inference. It is in this direction that the already mentioned relation of compensation between beautiful colours and pleasant odours is particularly interesting, for it confirms our interpretation of the decorative colours as a means of sexual excitement. The most delicately fragrant or the most beautifully coloured males were those which most excited the females, and thus most easily attained to reproduction. The expression used by Darwin, that the females 'choose,' must be taken metaphorically; they do not exercise a conscious choice, but they follow the male which excites them most strongly. Thus there arises a process of selection among these distinctively male characteristics.

If the odoriferous organs we have been discussing had merely been a means of attraction, serving to announce the proximity of a member of the species, then they should have occurred, not in the males but in the females, for these are sought out by the males, not conversely. The males are able to track their desired mates from great distances, and many remarkable examples of this are known, some of them indeed sounding almost fabulous. The females must therefore also exhale a fragrance, and perhaps continually, but it is much more delicate, carries extraordinarily far, and is quite imperceptible to our weak sense of smell. It is possible that it streams out from all the scales covering the wings and body, for, as I long ago pointed out, all the scales retain a connexion with the living cells of the skin, however minute these may be, and it is therefore quite possible that the cells produce scent imperceptible by us, and let it exhale through the ordinary scales, since the male scent-scales owe their ethereal oil to the large gland-like cells of the hypodermis on which they are placed.

Here we see very clearly the difference between ordinary natural selection and sexual selection. The male odoriferous organs depend on the latter, for they do not serve for the maintenance of the species, but are of advantage in the courting competitions among the males for the possession of the females, while the assumed fragrant cells of the females must depend on natural selection, since they are of general importance for the mutual discovery of the sexes, which would otherwise be in most cases impossible. This hypothetical 'species scent,' as we may call it, is first of all useful in securing the existence of the species, and must therefore be referred to natural selection. The other, the 'male scent,' might be, and actually is, wanting in many species, although it may be necessary to reproduction in cases where it has become a male specific character, and could not be absent from any male without dooming him to sterility.

That the 'species scent' really exists admits of no doubt, although we may be unable to perceive it. Entomologists have long been in the habit of catching the males of the rarer Lepidoptera, especially of the nocturnal forms, by freely exposing a captive female. Some years ago I kept for some time in my study, with a view to certain experiments, females of the eyed hawk-moth (Smerinthus ocellatus), and placed them at first, without any special intention, in a gauze-covered vessel near the open window. The very next morning several males had gathered and were sitting on the window-sill, or on the wall of the room close to the vessel, and by continuing the experiment I caught, in the course of nine nights, no fewer than forty-two malesof this species, which I had never believed to be so numerous in the gardens of the town. The males of the nocturnal Lepidoptera obviously possess an incredibly delicate organ of smell, and its bearers, the antennæ, are usually larger and more complex in structure in the male sex than in the female.

Butterflies are by no means the only creatures that produce a peculiar odour at the breeding season; many other animals do the same, though in their case it does not seem so pleasant to our sense of smell. It is true that the scent of the musk-deer and that of the beaver (Castoreum), when much diluted, are agreeable to man, but others, like the odours exhaled by stags or by beasts of prey, are very disagreeable to us, though they have for the species that produce them the same significance as the others, and are therefore to be referred to sexual selection.

Darwin referred all the differentmechanisms for the production of sounds, up to the song of birds, to sexual selection, but it is probable that natural selection has also to do with this in many ways. It is certainly only the males which produce the well-known song of the Cicadas, crickets, grasshoppers and birds, and I do not see any reason to doubt that this 'music' affects the females by arousing sexual excitement. To some extent, then, the rivalry among the males for the possession of the females—that is to say, sexual selection—must have produced these mechanisms of song; and how long-continued and gradual the accumulations must have been which produced the song of the thrush or of the nightingale from the chirping of the sparrow we may learn from the innumerable species which, as regards beauty of song, may be ranged between these two extremes.

My assumption that natural selection has also been operative in the case of the song of insects and birds is based on the fact that many of our songsters live widely scattered, and that the characteristic note must be a means by which the two sexes find each other. That they should find each other is an indispensable condition for the maintenance of the species. Thus it is well known that each species has a characteristic 'note' or love-call, which the male utters during the breeding season, and which is answered by the female. From this simple love-call the modern song of many species must have developed by means of sexual selection.

It is remarkable that here again the various distinguishing characters of the male seem to be often mutually restrictive or mutually exclusive. The best singers among our birds are inconspicuously coloured, grey or brown-grey, and this can hardly be regarded as due to chance, but as the outcome of a greater sensitivenesson the part of the females either to the song or to the beauty of their mates. And since, according to the theory, only those characters of the males could be increased which decided the choice, it therefore seems to me that this mutual exclusiveness of the two kinds of distinguishing characters is another indication of the reality of sexual selection. It proves—so at least I am inclined to believe—that the excitement of the female has been essentially affected byonly oneof the characters of the male, that in the bird of Paradise it was mainly the brilliance of the plumage which roused excitement, while in the nightingale it was mainly the song.

It might be objected to this that there are brilliant butterflies which also possess scent-scales. This is really the case; thus a magnificent blue iridescentApaturafrom Brazil has on the posterior wings a large yellow brush of scent-hairs, and even the beautiful blue males of our Lycænids have scent-scales in addition to their beautiful colour. But this can hardly be considered as a contradiction, but is rather an exception, which is the easier to explain since the odoriferous apparatus is a relatively simple arrangement, which did not require such a long series of generations for its evolution as the complicated song-box and brain-mechanism of the singing-birds.

Moreover, it may also be that the scent-scales have arisen later than the decorative colouring, and they would do so the more easily since the brilliant blue, when once it was perfectly developed, and was common to all the males of the species in an equal degree, was no longer distinctive, and would have no specially exciting effect, while a novel preferential character in the male would have a much stronger effect. In the same way, the different parts of the body would be furnished in succession with decorative and, therefore, exciting distinctive characters. To understand this effect on the opposite sex we need only think of analogous phenomena in human kind, and of the strongly exciting effect that the sight of the secondary sexual characters of the woman has upon the man.

By the successive additions of new decorative characters after the older ones became general and reached a climax, the origin of the extraordinary diversity of the decorative plumage in one and the same species of bird, can be readily understood, and the same is true of the complicated decorative coloration of the butterflies in so far as it depends on sexual selection, and not on other factors. The details did not arise all at once, but one after the other, and every character went on increasing till it had reached its limit of increase, but whenever it was common in its highest development to all the males itwas no longer an object of preference or the cause of specially violent excitement, so that a new process of selection would begin in reference to some other part of the body. We thus understand how, among male birds of Paradise and humming-birds, such a marvellous diversity of colours and of decorative feathers is found combined in one and the same species.

Whoever has seen the Gould Collection of humming-birds in London must have observed with amazement that among the 130 or so species of these beautiful little birds nearly every group of feathers in the body has been affected by the decorative colouring. In one species the little feathers on the region of the throat are emerald green, metallic blue, or rose; in another the feathers of the neck have been transformed into an erectile collar of rose-coloured feathers with a metallic sheen; or, again, it is the little feathers round the ear that stand erect and are brilliantly coloured. Sometimes we find that the feathers of the tail are lengthened, it may be only two of them, or the various lengths may be graduated like steps; sometimes the tail has assumed the form of a wedge, or is fan-like, or is shaped like the tail of a swallow, and all this in combination with the most diverse colours and patterns, black and white, ultramarine blue, and so forth. Or it may be the outermost tail-feathers which are the longest, the inner ones the shortest, or the four outer feathers are broad, pointed, directed outwards, and only half as long as the other two, which are very long and straight. Some species exhibit a sort of fine swan's down on the legs, others have a gorgeous metallic red cap on the head—in short, the variety is beyond description, just as we should expect it to be if now this and now that chance variation attracted the favourable regard of the selecting sex, and thus attained to its highest pitch of development.

The decorative colouring of male birds may be replaced, not only by the power of song, but in other ways also. Not all the male birds of Paradise possess the familiar feather ornaments. The Italian traveller Beccari has called attention to a species, the males of which are simply coloured brown, like the females of other species. ThisAmblyornis inornataentices its mate to itself in the pairing time in a very peculiar manner, for it arranges in the midst of the primitive forests of New Guinea a little 'love garden' or bower, a spot several feet in extent, strewn with white sand, on which it places shining stones and shells, and brightly coloured berries. In this case a special instinct has developed, which has replaced the personal charm of the bird in the eyes of the female. For this very reason the case seems to me to have some theoretical importance, for it serves indirectly toshow that the personal excellences do actually function as a means of exciting and attracting, if any one should still doubt it.

All the distinguishing characters of the male which we have hitherto considered have had reference to gaining the favour of the female, but there are many other secondary sexual characters which are employed in quite a different manner to secure possession of the female. I have already mentioned that in many butterflies the males possess a much larger organ of smell. The antennæ of the males of numerous beetles, such as the cockchafer and its relatives, are also much larger, and furnished with much broader accessory branches, than those of the female, and the same is the case in many of the lower crustaceans, like the large transparent Daphnid of our lakes,Leptodora hyalina. Here the anterior antenna bears (Fig. 56,AandB,at´) olfactory filaments; in the female this appendage is small and stump-like, while in the male (A) it grows to a long, somewhat curved rod, which is extended obliquely into the water, and in addition to the nine olfactory filaments of the female (ri) bears from sixty to ninety more (ri´).

Fig. 56.Leptodora hyalina.A, head of the male.B, head of the female.Au, eye.g. opt, optic ganglion.gh, brain.at´, first antenna with olfactory filamentsriandri´.sr, œsophageal nerve-ring.n, nerve.m, muscles.

In this and many other such cases it is not the struggle of the species for existence which has so markedly augmented this distinctive characteristic of the male; it is undoubtedly the struggle of the males among themselves, their competition for the possession of the females. In regard to decorative distinctions, the reality of a rivalry in wooing and the ultimate victory of the most decorative may perhaps be still doubted; but it is quite certain that, on an average, the male which can smell and track best will also gain possession of the females more easily than one less well equipped. Exactly the same is also true of those cases in which the male distinguishing character does not refer merely to finding the female, but to holding her fast, or, as we may say, to capturing her.

Thus the males of the Copepods possess on their anterior antennæ an arrangement which enables them to throw a long whiplike structure like a lasso round the head of the female as she rapidly swims away. The antennæ of the male Daphnids, too, are in one genus (Moina) developed into a grasping apparatus, instead of into smelling organs as inLeptodora. Fig. 57 shows the male, Fig. 58 the female ofMoina paradoxa; the first antennæ of the male are not only much longer and stronger than those of the female (at1), but they are also armed with claws at the end, so that the males can catch their mates as with a fork, and hold them fast. And even that was not enough, for, in addition, the males of most Daphnids possess a large sickle-shaped but blunt claw on the first pair of legs (Fig. 57,fkr), which enables them to cling to the smooth shell of the female, and to clamber up on it to get into the proper position for copulation.

Fig. 57.Moina paradoxa, male.at1, first antennæ, with claws at the tip for capturing the female.at2, second antennæ.fkr, claws on the first pair of legs for clambering.gh, brain.lbr, upper lip.md, mandible.md, mid-gut, with the liver lobes (lh).h, heart.sp, testis.aft, anus.sb, caudal setæ.skr, caudal claws.sch, shell.schr, cavity of the shell.kie, gill-plates. Magnified 100 times.

If we inquire into the manner of the origin of secondary sexual characters of this kind, we shall find that both may have been increased by sexual selection, for a male with a better sickle will succeed more quickly in getting into the proper position for copulation than one with a less perfect mechanism. This assumption does not reston mere theory, for I was once able, by a happy chance, to observe for a considerable time, under the microscope, a female to whose shell two males were clinging, each trying to push the other off. Nevertheless it seems to me very questionable whether the origin of this sickle-claw can be referred to sexual selection, for without this clamping-organ copulation in most Daphnids would not be possible. It was thus not as an advantage which one male had over another that the clamping-sickle evolved, but rather as a necessary acquisition of the whole family, which must have developed in all the species at the same time as the other peculiarities, and notably those of the shell. The competition of the males among themselves is thus in this case simply an expression of the struggle for existence on the part of the species as such, and it is not a question merely of a character which makes it easier for the males to gain possession of the females, but of one which had necessarily to arise lest the species should become extinct. In other words, in this case natural selection and sexual selection coincide.

Fig. 58.Moina paradoxa, female. The letters of Fig. 57 applymutatis mutandis.brr, brood-pouch.ov, ovary.sr, margin of shell.

The case of the antennæ ofMoina, which have been modified into grasping organs, is quite different; these owe their origin not to natural selection, but to sexual selection, for antennæ of that kind are not indispensable to the existence of the species, as we can see from the closely related genera,DaphniaandSimocephalus, where the males have quite short stump-like antennæ, furnished with olfactory filaments not much more numerous than the females possess. Just as these supernumerary olfactory filaments were produced bysexual selection, and not by the ordinary natural selection, because those males with the more acute sense of smell had an advantage over those in which it was blunter, so the males of the genusMoinawhich could grasp most securely had an advantage over those that gripped less firmly, and thus arose these two different kinds of male characteristics. Neither of them is of advantage to the species as such, but only to the males in their competition for the possession of the females.

But, where the production of a novel character in the male is concerned, natural selection cannot proceed in a different manner from sexual selection; the process of selection is exactly the same: the better equipped males survive, the less well-equipped die without begetting offspring; the difference lies only in the fact that in the one case the improvement is in the species as such, in the other case only in one sex without the existence of the species being thereby made more secure. Such cases are instructive, because they make a denial of the process of sexual selection quite impossible if that of species-selection is admitted. If processes of selection are operative at all as factors in transformation, they must act even where the advantage is not to the species but only 'intra-sexual,' and the one process must often run into the other, so that it is often quite impossible to draw an exact line of demarcation between them.

Numerous secondary sexual differences probably depend purely on species selection, that is to say, they include an improvement of the species in relation to the struggle for existence. We may find a case in point in the dwarf-like smallness of the males in many parasitic crustaceans, in some worms, in many Rotifers, and in the Cirripedes. It can hardly have been of advantage for the individual male to be smaller than his fellows, but it was of advantage for the species to produce as many males as possible in order to ensure a meeting with the females, and, as the enormous production of males made it advantageous for the species that as little material as possible should be used in their individual production, we can readily understand the minuteness of the males, and in some cases, as in the Rotifers andBonellia, their poor equipment, lack of nutritive organs, and ephemeral existence. The marine worm,Bonellia viridis, whose female may be a foot in length, is not the only case in which a microscopically small male lives like a parasite inside the female. Among the round-worms, too, there is a species calledTrichosomum crassicauda, discovered by Leuckart in the rat, the dwarf males of which live in the reproductive organs of the female. All these are arrangements for securing the propagation of the species, whichmight have been endangered if the males had had to seek out the females, which, in the case ofBonellia, live in holes in the rocks on the sea-floor, and, in the case ofTrichosomum, are concealed in the urinary bladder of the rat. Obviously, this is the reason which, in addition to the one already mentioned, has conditioned and produced, or helped to produce, the remarkable minuteness of certain males.

From another category of sexual differences we see in how many ways species-selection and sexual selection play into each other's hands. In many species of animals the males are eager for combat, and they are equipped with special weapons, or excel the females in general strength of body. As these males struggle, in the literal sense of the word, for the possession of the females, Darwin referred to sexual selection those distinguishing characters which gave the stronger male the victory over the weaker, and thus raised the victorious characters to the rank of general characters of the species. And it certainly cannot be doubted that, for instance, the strength and the antlers of the stag must have been increased through the combats which recurred every year at the breeding season, for the stronger always win in these battles. The case is the same with the strength and the weapons of many other male animals. The lion is effectively protected by his mane from the bite of a rival, and the same protective arrangement occurs in quite a different family of mammals—in an eared seal, which is called the 'sea-lion' for this very reason. Among the seals the secondary sexual characters are often very strongly developed, at least in all the polygamous species, for in these the struggle for the females is very keen. In the 'sea-lions' and 'sea-elephants' there are often fifty females to one male, and the latter are 'enormously larger' than the females, while in monogamous species of seal the two sexes are alike in size.

Darwin has shown that actual combat for the females takes place among most mammals, not only among stags, lions, and seals, but even among the moles and the timid hares. Even among birds such combats occur, and this is sometimes particularly noteworthy in those species in which the males possess the most decorative colouring, like the humming-birds. In some cases among birds there has also been a development of weapons. Witness the spur of the cock, whose merciless combats with his rivals Man has, as is well known, made positively atrocious for his own amusement, by preventing the flight of the vanquished.

In Darwin's great work on sexual selection a considerable number of cases are cited from among lower vertebrates, such as crocodiles and fishes, and even from insects, in which the males fightfor the possession of the females, and exhibit distinctive masculine characters adapted to such combats. But I do not propose to enter upon a discussion of such cases, since my aim is rather to elucidate the relation between sexual selection and species-selection than to discuss all the phenomena of the former in detail. But the combats of males illustrate with particular clearness the relation of sexual selection and species-selection, since many of the weapons or protective arrangements which may have arisen through sexual selection imply at the same time an improvement to the species in relation to the struggle for existence. Thus greater strength or sharper and larger teeth in the males mean a gain to the species, and it is indifferent to the species whether the weaker males succumb to a strange enemy (species-selection) or to their stronger rivals (sexual selection), provided only that the better equipped survive and leave descendants similarly endowed.

I have intentionally begun the consideration of sexual selection with the cases most difficult to interpret on this theory, with those which have called forth the greatest divergence of opinion—the decorative colours and forms, the song of birds and of insects, the alluring odours—in short, all the courtship-adaptations of the males; these are the most difficult to deal with, because it is not easy to demonstrate directly that the femalesdochoose. But if we revise them briefly in reverse order, I believe that all doubt as to the reality of choice on the part of the females will disappear. Thus the last-mentioned sexual characters of greater strength and greater perfection of weapons and defence in the males have been evolved by sexual selection in close co-operation with species-selection. We should have to deny species-selection altogether if we were to dispute this form of sexual selection, which is closely connected with pure species-selection, such, for instance, as is revealed in the production of dwarf males, where there does not seem to be any aid from sexual selection at all.

Then came the cases in which the tracking and grasping organs of the males were strengthened or were increased in number, and here too species-selection may have had its share, for instance, in evolving the sickle-claws of the Daphnids, which were inevitably advanced and perfected through sexual selection, which must in this case have operated independently of any choice on the part of the female. In other cases the result may be referred to pure sexual selection, as in the grasping antennæ of the maleMoina, or in the highly developed olfactory antennæ of the maleLeptodora. That new organs, too, can arise in this way is shown by the 'turban eyes'—to which little attention has hitherto been paid—of some Ephemerids of the generaCloëandPotamanthus, which were long ago described by Pictet, the monographer of this family. These are large turban-shaped compound eyes, occurring beside the ordinary eyes in the males alone, which in these genera are in a majority of sixty to one. Whole swarms of these males fly about over the water on the search for females, and their highly developed organ of vision seems to decide matters for them just as the organ of smell does forLeptodora. Neither of these sense-organs can have any other advantage than that of making their possessors aware of the female, for the whole activity of the short-lived adult Ephemerides is limited to reproduction; they take no food, and have nothing whatever to do except to reproduce.

Finally, when in an enormous number of cases we find in addition to one or the other of the already mentioned male distinguishing characters some which do not directly lead to gaining possession of the female, but do so only by sexually exciting her, can we doubt that the same principle has been operative, that here too processes of selection are fundamental, depending on the fact that in the wooing of the female the successful male is the one who most strongly excites her? There is no question of æsthetic pleasure in this, as the opponents of the theory of sexual selection have often urged, but only of sexual excitement, which may be aroused by very different means, by colours and shapes, but also by love-calls, songs, or odours. There are a few tropical birds (Chasmorhynchus) which have as the only distinguishing character of the male sex a hollow and soft appendage several inches long borne on the head. Usually it hangs down limply at the side of the head, but during the breeding season it is inflated from the mouth-cavity, and then stands erect like a spur. One species of this genus has as many as three of these horns, one of which is upright, while the other two stand out laterally from the head. Can it be supposed that these remarkable horns satisfy the female's 'sense of beauty'? To human beings they appear rather ugly than beautiful, both when limp and when inflated, but at any rate they are striking, and will be regarded by the female bird as something out of the common, and, since they are only fully displayed during the breeding season, that is, when the male is sexually excited, they will have an exciting effect on the female too. These inflated horns are symptoms of excitement, and they arouse it in the female. In exactly the same way the decorative feathers, the ruby-red and emerald-green feather collars of the humming-birds and birds of Paradise, are only erected and displayed when the males are wooing, and they, too, act as signs of excitement. This is not to saythat the gorgeousness of colour, the eye-spots on the train of the peacock and the Argus pheasant, and the hundreds of different kinds of beautiful feathers, do not also exercise a fascinating influence; on the contrary, we cannot avoid assuming this, since otherwise we could find no sufficient reason for their origin. But the primary effect in wooing is not due to the mere pleasure in the sight, or in the odour, or in the song, but to the contagious excitement which these express. The females do not behave as dispassionate judges, but as excitable persons which fall to the lot of the male who is able to excite them most strongly. It may be, however, that a sense of æsthetic satisfaction in perceiving such symptoms of excitement may also have been evolved as an accessory effect, at least in the higher and more intelligent animals.

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 thana faint idea of them without entering on a long catalogue; whoever wishes a complete survey has only to consult Darwin'sDescent 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 ofMoina, the turban eyes of the Ephemerides, the intensification of the sense of smell inLeptodora, 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 genusLycæna. 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 speciesLycæna 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, likeL. alsus, the smallest of our indigenous Blues. Then follows a host of beautiful species, likeL. alexis,L. adonis,L. damon,L. corydon, and many others, withbrown females, and among these there occasionally occur females more or less tinged with blue. These lead on toL. meleager, which has two forms of female, a common brown and a rarer blue; and thus we reachL. tiresias,L. optilete, andL. 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 likeL. 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.

In other cases the course of the process of selection has been such that, though the general colouring has not been changed, variations have appeared in particular regions of the body—spots or stripes which accumulated through the ages and co-operated to form an increasingly diverse and complex colour-scheme, such a 'marking' of the animal as we may observe to-day, especially in butterflies, but also in birds.

It is a fine corroboration of the origin of bright colours throughsexual selection that, even in those groups of the animal kingdom which are in general sexually monomorphic, there always occur some species in which male and female are quite different, and a host of species in which both sexes are alike in the main, yet with differences in certain minor points. Among the parrots similarity of colouring prevails as a general rule, but in New Guinea there lives a parrot the female of which is a gorgeous blood-red and the male a beautiful light-green; minor differences occur in many species, for instance, the female of the horned parrot (Cyanorhamphus cornutusGm.) lacks the two long black and red feathers on the head, that of the grass-parakeet (Melopsittacus undulatus) is a slightly paler green and has not the beautiful blue spots on the cheeks which the male possesses. Innumerable similar instances might be cited, serving to show that all these distinguishing characters of the males have been acquired step by step and piece by piece, and are slowly and independently transferred to the females—if, indeed, at all.

In yet another way the correctness of the Darwinian theory of sexual selection may be deduced from the markings and coloration of birds and butterflies.

It has frequently struck me, during the long period in which I have been studying brightly coloured birds and butterflies, that those colour-patterns which are referable to sexual selection are much simpler than those which must be referred to species-selection, especially in the case of what we call 'sympathetic coloration.' How crude is the decorative pattern of most parrots, notwithstanding all the brilliance of their colour. Large tracts of the body are red, others green, yellow, blue, and occasionally one finds a red and blue striped feather collar, a head which is red above and yellow underneath, but it is seldom that the colours vary enough in a small space to give rise to a delicate decorative pattern. The gayest of parrots are the Brush Tongues (Trichoglossus), and even among them subtlety of coloration does not go further than the combination of three colours on one of the long tail-feathers, or the production of a double band round the neck, and so forth. If we compare with this the complex markings of the inconspicuously coloured females of the pheasants, of the partridges, or that of the upper surface of the many birds in mingled grey, blackish-brown and white, which resemble the ground or the dried leaves when they crouch, we find that the colour-pattern in these cases is infinitely finer and more complex.

This seems to me quite intelligible when we remember, on the one hand, that species-selection must operate far more intensively thansexual selection, and that in the production of a protective colouring it is a question of deceiving the eye of a sharp-sighted enemy, while the aim of sexual selection is to secure the approval of others of the same species. As long as the enemy on the search for prey perceives the difference between the markings of its victim and those of the surroundings, so long will the gradual and steady improvement of the protective coloration continue, so long will new shades and new lines be added. We can thus understand how there would be gradually reached a complexity of marking to which sexual selection can never attain, or at least only in regard to a few specially favourable points. The eye-spots on the train feathers of the Argus pheasant and the peacock are such points, and these occur among polygamous birds in which sexual selection must be very intense; they are placed, too, on a part of the body, the wheel-shaped train, which is peculiarly suited for communicating the excitement of the male to the female, and must therefore be especially influenced by the latter. In general, however, we may say ona priorigrounds that the intensity of species-selection is greater than that of sexual selection, because the former ceaselessly and pitilessly eliminates the less perfect, while the claims of the latter are in any case less imperative, and are also often mollified by a variety of chance circumstances.

But in the case of insects, in particular, we have to add that the protective colours and the decorative colours have been, so to speak, painted by different artists—the former by birds, lizards, and other persecutors endowed with well-developed eyes, the latter by the insects themselves, whose eyes can hardly possess, for objects not quite near, that acuteness of vision which the bird's eye has. Thus we find that the protective coloration of butterflies has often a very complex marking, while the same butterfly may exhibit only a rather crude though brilliant pattern on its upper surface, where the coloration is due to sexual selection. Thus the famousKallimahas on its under surface the likeness of a dry or decayed leaf composed of a number of colour-tones—quite a complex painting. But if we look at the upper surface we see a deep brown with a shimmer of steel blue as the ground-colour of the wings, and on it a broad yellow band and a white spot: that is the whole pattern. We find a similar state of things among many of the forest butterflies of Brazil, and also among our indigenous butterflies. The pattern of our gayest diurnal butterflies, the red Admiral and the tortoiseshell butterfly (Vanessa atalantaandVanessa cardui), is somewhat crude on the upper surface, and very simple compared with the protective colouring of the under surface, which is made up of hundreds of points, spots, strokes, andlines of every shape and colour. On the other hand, the upper surface of the anterior wings in the hawk-moths and the Noctuidæ exhibits protective coloration, and is made up of curious zigzag complex lines, strokes, and spots, so that it resembles the bark of a tree or a bit of an old wooden fence—a painting, like the modern impressionist work, which, with an apparently meaningless confusion of colour splashes, conveys a perfect impression even of the details of a landscape. In the owl-moths (Noctuidæ) the wing surfaces, which are brightly coloured, are simple, almost crude, in pattern, as in the moths of the genusCatocala, with their red, blue, or yellow posterior wings, traversed by a large black band; while in the Geometer-moths, whose wings are spread out flat when at rest, the protective upper surface of all four wings is covered with a complex pattern of lines, spots, and streaks in different shades of grey, yellow, white, and black, so that it bears a deceptive resemblance to the bark of a tree or the side of a wall. For a long time I could not understand how such a definite and constant pattern could arise through natural selection if it was a case of mimicking the impression of bark or of any other irregularly covered surface, the colours of which are not mingled in exactly the same way everywhere. But now I think I understand it; for in the apparently meaningless colour-splashes of an 'impressionist' landscape the different splashes must be exactly where they are, otherwise on stepping back from the picture one would see, not a Haarlem hyacinth-field, or an avenue of poplars with their golden autumn leaves, but a mere unintelligible daub. It is thetypeof the colour-pattern that must be attained, and in nature this is attained very slowly, step by step, spot after spot, and therefore, obviously, no correct stroke once attained will be given up again, since, in combination with the rest, it secures the proper type of colour-pattern. Only thus, it seems to me, can we understand how apparently meaningless lines, like the figures 1840 on the under surface ofVanessa atalanta, could have become a constant characteristic of the species.

To sum up briefly, we may say that sexual selection is a much more powerful factor in transformation than we should at first be inclined to believe. It cannot, of course, have been operative in the case of plants, nor can it be taken into consideration in regard to the lower animals, for these, like the plants, do not pair, or, at any rate, do so without any possibility of choice. Animals which live on the sea-floor, or which are attached there, must simply liberate their reproductive cells into the water, and cannot secure that they unite with those of this or that individual. This is the case among sponges,corals, and Hydroid polyps. In some other classes the sense organs are too poorly developed, and the eyes in particular too imperfect to be excited in different degrees by any peculiarities in the appearance or behaviour of the males. This is what Darwin meant when he ascribed to these animals 'too imperfect senses and much too low intelligence' 'to estimate the beauty or other attractive points of the opposite sex, or to feel anything like rivalry.' Accordingly, in the Protozoa, Echinoderms, Medusæ, and Ctenophores, secondary sexual characters are entirely absent, as pairing also is.

In those worms that pair we first meet with secondary sexual characters, and from this level upwards they are never quite absent from any large group, and gradually play an increasingly important rôle.

But the significance of sexual selection lies, as we have seen, not only in the fact that one sex of a species, usually the male, is modified, but in the possibility of the transference of this modification to the females, and further, in the fact that the process of variation may start afresh at any time, and thus one variation may be developed upon or alongside of another. In this way we can explain certain complex and often fantastic forms and colourings which we could not otherwise understand; thus the extraordinary number of nearly related species in some animal groups, such as butterflies and birds, in which the differences mainly concern the colour-patterns.

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