Part 27

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.

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 through sexual 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 cornutus_ Gm.) 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 than sexual 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 on _a priori_ grounds 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 famous _Kallima_ has 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 atalanta_ and _Vanessa 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, and lines 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 genus _Catocala_, 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 the _type_ of 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 of _Vanessa 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.

Darwin has shown convincingly that a surprising number of characters in animals, from worms upwards, have their roots in sexual selection, and has pointed out the probability that this process has played an important part in the evolution of the human race also, though, in this case, all is not yet so clearly and certainly known as among animals.

To conclude this section, I should like once again to call attention to the deficiency which is necessarily involved in the assumption of any selection, sexual selection included, namely, that the first beginning of the character which has been intensified by selection remains obscure. Darwin attached importance to the occurrence of ordinary individual variation, but it is open to question whether the insignificant variations thus produced could give an adequate advantage in the competition for the possession of the females; and, further, whether we have not grounds for the assumption that larger variations also occur. This question may also be asked in regard to ordinary natural selection, although in that case we can imagine the beginnings to be smaller, since here the advantage of a variation lies only in the fact that it is useful, not in its being appreciated by others. As a matter of fact, this very difficulty as to the first beginnings of variations has been frequently urged against both hypotheses of selection, and rightly so, inasmuch as this must be above all else the point of attack for further investigations. But it is a mistake to deny the whole processes of selection simply because this point is not yet clear. Later on we shall attempt to gain some insight into the causes of variation, and then we shall return to this question of the beginnings of the selective processes. In the meantime let it suffice to say that Darwin was very well aware that, in addition to the ordinary individual variations, there were also larger deviations which occurred discontinuously in single forms. He believed, however, that such occurrences were very rare, and, on the whole, he was not inclined to ascribe to them any particular importance in the transformation of species. He rather referred the organic transformations which have taken place in the course of the earth's history, in the main, to the intensification of the ordinary individual variations, and I believe that he was right in so doing, since adaptations from their very nature cannot have been brought about by sudden chance leaps in organization, but can only have become exactly suited to chance conditions of life through a gradual accumulation of minute variations in the direction of utility. Whether, however, purely sexual distinctions may not have had their primary roots in discontinuous variations must be inquired into later. Theoretically, there is nothing against this assumption, when such characters are not adaptations like the lasso antennæ of the Copepods, or the turban eyes of the Ephemerids; mere distinctive markings, decorative coloration, peculiar outgrowths, and the like, may, if they arose discontinuously, very well have formed the basis for further sexual selection, as long as they were not prejudicial to the existence of the species.

LECTURE XII

INTRA-SELECTION OR SELECTION AMONG TISSUES

Does the Lamarckian principle really play a part in the transformations of species?--Darwin's position in regard to this question--Doubts expressed by Galton and others--Neo-Lamarckians and Neo-Darwinians--Results of exercise and practice: functional adaptation--Wilhelm Roux, _Kampf der Theile_.

WE have devoted a whole series of lectures to studying the Darwin-Wallace principle of Natural Selection and the range of its operation. It seemed to us to make innumerable adaptations intelligible up to a certain point. We now understand how the purposefulness, which we meet with everywhere among organisms, can have arisen without the direct interference of a Power working intentionally towards an end--simply as the outcome and result of the survival of the fittest. The two forms of the processes of selection, 'natural selection' in the narrower sense, and 'sexual selection,' dominate, so to speak, all parts and all functions of the organism, and are striving to adapt these as well as possible to the conditions of their life. And although the range of operation of Natural Selection is incomparably greater, because it actually affects every part, yet we must attribute to sexual selection also, at least among animals, a range of influence by no means unimportant, since through it, as far as we can see at present, not only do the secondary sexual characters in all their diversity arise, but by the transference of these to the other sex that too is modified, and thus the whole species may be influenced, and may indeed be started afresh on an unlimited series of further transformations.

But although the processes of selection play such an important part in the transformations of the forms of life, we have to inquire whether they are the _sole_ factors in these transformations, whether the accumulation of chance variations in the direction of utility has been the sole factor in bringing about the evolution of the animate world, or whether other factors have not also co-operated with it.