The Evolution Theory, Vol. 2 of 2

by August Weismann

I do not believe that so many species with very divergent plumage in the males could have developed if they had all lived together on a large connected area. But here, distributed over a large number of isolated mountain forests, the decorative colouring or the distinctive shape which chances to arise through germinal selection on any of these terrestrial islands can go on increasing, undisturbed by crossing with individuals of the ancestral species, and furthered, moreover, by sexual selection.

In this way, if I mistake not, numerous new species have arisen as a result of isolation, and it is quite intelligible that several new species may have arisen from one and the same ancestral species, as we may see from the nearly related yet constantly different species of mocking-bird on the different islands of the Galapagos group.

A number of similar examples might be given from among birds. Thus Dixon calls attention to the species of the thrush genus Catharus, twelve of which live in the mountain forests of Mexico and of South America as far as Bolivia, all differing only slightly from one another and all locally separated. They came from the plains, migrated to the highlands, were isolated there, and then no longer varied together all in the same direction, but each isolated group evolved in a different direction according to the occurrence of chance germinal variations: one developed a chestnut-brown head, another a slate-grey mantle, a third a brown-red mantle, and so on. From what we have already seen in regard to the importance of sexual selection in evolving the plumage of birds, it is probable that this factor has been operative in this case also.

Another example is afforded by the weaver-birds (Ploceus) of South Africa, those ingenious singing-birds resembling blackbirds in size and form, whose pouch-shaped nests, hanging freely from a branch, usually over the water, and with their little openings on the under side, are excellently protected from almost every form of persecution. These birds have in South Africa split up into twenty or more species, but the areas of each are not sharply isolated, and the division into species cannot, therefore, be due to isolation. But it is not difficult to guess upon what it depends, when we know that the males alone are of a beautiful yellow and black colour, while the females are of a greenish protective colouring all over.

Thus, in my opinion, sexual selection plays a part more or less important in the origin of the numerous endemic species of diurnal Lepidoptera which are characteristic especially of the islands of the Malay Archipelago, and which make the Lepidopteran fauna there so rich in individuality. A large number, indeed the majority of the types of Papilionidæ, have a peculiar species, a local form, on most of the larger islands, which is sharply and definitely distinguished from those of the other islands, usually in both sexes, but most markedly in the much more brilliantly coloured males.

Thus each of these types forms a group of species, each of which is restricted to a particular locality, and has usually originated where we now find it, although of course the diffusion of one of these large strong-flying insects from one island to the other is in no way excluded. As an example we may take the Priamus group, the blackish yellow Helena group, the blue Ulyssus group, and the predominantly green Peranthus group.

If we inquire into the causes of this divergence of forms and their condensation into numerous species, we shall find that their roots lie in this case, as in that of all transformations, in germinal selection and the variational tendencies resulting therefrom, but we must regard their fixation us the result of isolation, which prevented the variational tendencies which happened to develop on any one island from being neutralized and swamped by mingling with the variations of other islands. But that sexual selection took control of these striking colour-variations and increased them still further is obvious from the rarely absent dimorphism of the sexes. Even if the females do not consciously select mates from among the males, they will more readily accept as a mate the one among several suitors which excites them most strongly. And that will be the one which exhibits the most brilliant colours or exhales the most agreeable perfume, for we know from their behaviour in regard to flowers how sensitive butterflies are to both these influences.

Although isolation has an important rôle in the formation of all these species, it seems to me an exaggeration to maintain, as many naturalists do, that the splitting up of a species is impossible without isolation. Certainly the splitting up of species is, in numerous cases, facilitated by isolation, and indeed could only have been brought about in its present precision by that means, but it is underestimating the power of natural selection not to credit it with being able to adapt a species on one and the same area to different conditions of life, and we shall return to this point later on in a different connexion. But in the meantime it must suffice to point out that the polymorphism of the social insects affords a proof that a species may break up into several forms in the same area through the operation of natural selection alone.

I am therefore of opinion, with Darwin and Wallace, that adaptation to new conditions of life has, along with isolation, had a material share in the evolution of the large number of endemic species of snail on the oceanic islands. This brings us to the co-operation of natural selection and isolation. If, thousands of years ago, by one of the rarest chances, an Achatina-like snail was carried by birds to the Sandwich Islands, it would spread slowly, at first unvaried, from the spot where it arrived over the whole of the snailless island. But during this process of diffusion it would frequently come in contact with conditions of life which would not prevent it from penetrating further, but to which it was imperfectly adapted, and in such places a process of transformation would begin, which would consist in the fostering of favourably varying individuals, and which would run its course quietly by means of personal selection, based upon the never-ceasing germinal selection, and unhindered by any occasional intrusion of still unvaried members of the species from the original settlement on the island. But these new conditions were not merely different from those of the ancestral country; the island region itself presented very diverse conditions, to which the snail immigrant had to adapt itself in the course of time, as far as its constitution allowed. Terrestrial snails are almost all limited to quite definite localities with quite definite combinations of conditions; none of our indigenous species occurs everywhere, but one species frequents the woods, another the fields; one lives on the mountains, another in the valleys; one on gneiss soil, another on limy soil, a third on rich humus, a fourth on poor river-sand; one in clefts and hollows among damp moss, and another in hot, dry banks of loess, and so on. Although we cannot see in the least from the structure of the animal why this or that spot should be the only suitable one for this or that species, we may say with certainty that each species remains permanently in a particular place because its body is most exactly adapted to the conditions of life there, and therefore it remains victorious in the competition with other species in that particular spot.