Anyone who agrees with me in deriving variations from germinal selection will regard that process as an essential aid towards explaining the selection of distinctive courtship-characters, such as coloured spots, decorative feathers, horny outgrowths in birds and reptiles, combs, feather-tufts, and the like, since the beginnings of these would be presented with relative frequency in the struggle between the determinants within the germ-plasm. The process of transmission of decorative feathers to the female results, as Darwin pointed out and illustrated by interesting examples, in the COLOUR-TRANSFORMATION OF A WHOLE SPECIES, and this process, as the phyletically older colouring of young birds shows, must, in the course of thousands of years, have repeated itself several times in a line of descent.
If we survey the wealth of phenomena presented to us by secondary sexual characters, we can hardly fail to be convinced of the truth of the principle of sexual selection. And certainly no one who has accepted natural selection should reject sexual selection, for, not only do the two processes rest upon the same basis, but they merge into one another, so that it is often impossible to say how much of a particular character depends on one and how much on the other form of selection.
An actual proof of the theory of sexual selection is out of the question, if only because we cannot tell when a variation attains to selection-value. It is certain that a delicate sense of smell is of value to the male moth in his search for the female, but whether the possession of one additional olfactory hair, or of ten, or of twenty additional hairs leads to the success of its possessor we are unable to tell. And we are groping even more in the dark when we discuss the excitement caused in the female by agreeable perfumes, or by striking and beautiful colours. That these do make an impression is beyond doubt; but we can only assume that slight intensifications of them give any advantage, and we MUST assume this SINCE OTHERWISE SECONDARY SEXUAL CHARACTERS REMAIN INEXPLICABLE.
The same thing is true in regard to natural selection. It is not possible to bring forward any actual proof of the selection-value of the initial stages, and the stages in the increase of variations, as has been already shown. But the selection-value of a finished adaptation can in many cases be statistically determined. Cesnola and Poulton have made valuable experiments in this direction. The former attached forty-five individuals of the green, and sixty-five of the brown variety of the praying mantis (Mantis religiosa), by a silk thread to plants, and watched them for seventeen days. The insects which were on a surface of a colour similar to their own remained uneaten, while twenty-five green insects on brown parts of plants had all disappeared in eleven days.
The experiments of Poulton and Sanders ("Report of the British Association" (Bristol, 1898), London, 1899, pages 906-909.) were made with 600 pupae of Vanessa urticae, the "tortoise-shell butterfly." The pupae were artificially attached to nettles, tree-trunks, fences, walls, and to the ground, some at Oxford, some at St Helens in the Isle of Wight. In the course of a month 93 per cent of the pupae at Oxford were killed, chiefly by small birds, while at St Helens 68 per cent perished. The experiments showed very clearly that the colour and character of the surface on which the pupa rests--and thus its own conspicuousness--are of the greatest importance. At Oxford only the four pupae which were fastened to nettles emerged; all the rest--on bark, stones and the like--perished. At St Helens the elimination was as follows: on fences where the pupae were conspicuous, 92 per cent; on bark, 66 per cent; on walls, 54 per cent; and among nettles, 57 per cent. These interesting experiments confirm our views as to protective coloration, and show further, THAT THE RATIO OF ELIMINATION IN THE SPECIES IS A VERY HIGH ONE, AND THAT THEREFORE SELECTION MUST BE VERY KEEN.
We may say that the process of selection follows as a logical necessity from the fulfilment of the three preliminary postulates of the theory: variability, heredity, and the struggle for existence, with its enormous ratio of elimination in all species. To this we must add a fourth factor, the INTENSIFICATION of variations which Darwin established as a fact, and which we are now able to account for theoretically on the basis of germinal selection. It may be objected that there is considerable uncertainty about this LOGICAL proof, because of our inability to demonstrate the selection- value of the initial stages and the individual stages of increase. We have therefore to fall back on PRESUMPTIVE EVIDENCE. This is to be found in THE INTERPRETATIVE VALUE OF THE THEORY. Let us consider this point in greater detail.
In the first place, it is necessary to emphasise what is often overlooked, namely, that the theory not only explains the TRANSFORMATIONS of species, it also explains THEIR REMAINING THE SAME; in addition to the principle of varying, it contains within itself that of PERSISTING. It is part of the essence of selection, that it not only causes a part to VARY till it has reached its highest pitch of adaptation, but that it MAINTAINS IT AT THIS PITCH. THIS CONSERVING INFLUENCE OF NATURAL SELECTION is of great importance, and was early recognised by Darwin; it follows naturally from the principle of the survival of the fittest.
We understand from this how it is that a species which has become fully adapted to certain conditions of life ceases to vary, but remains "constant," as long as the conditions of life FOR IT remain unchanged, whether this be for thousands of years, or for whole geological epochs. But the most convincing proof of the power of the principle of selection lies in the innumerable multitude of phenomena which cannot be explained in any other way. To this category belong all structures which are only PASSIVELY of advantage to the organism, because none of these can have arisen by the alleged LAMARCKIAN PRINCIPLE. These have been so often discussed that we need do no more than indicate them here. Until quite recently the sympathetic coloration of animals--for instance, the whiteness of Arctic animals--was referred, at least in part, to the DIRECT influence of external factors, but the facts can best be explained by referring them to the processes of selection, for then it is unnecessary to make the gratuitous assumption that many species are sensitive to the stimulus of cold and that others are not. The great majority of Arctic land-animals, mammals and birds, are white, and this proves that they were all able to present the variation which was most useful for them. The sable is brown, but it lives in trees, where the brown colouring protects and conceals it more effectively. The musk-sheep (Ovibos moschatus) is also brown, and contrasts sharply with the ice and snow, but it is protected from beasts of prey by its gregarious habit, and therefore it is of advantage to be visible from as great a distance as possible. That so many species have been able to give rise to white varieties does not depend on a special sensitiveness of the skin to the influence of cold, but to the fact that Mammals and Birds have a general tendency to vary towards white. Even with us, many birds--starlings, blackbirds, swallows, etc.--occasionally produce white individuals, but the white variety does not persist, because it readily falls a victim to the carnivores. This is true of white fawns, foxes, deer, etc. The whiteness, therefore, arises from internal causes, and only persists when it is useful. A great many animals living in a GREEN ENVIRONMENT have become clothed in green, especially insects, caterpillars, and Mantidae, both persecuted and persecutors.