In all these cases the migration is certainly of recent date, and it is quite possible that, when a longer time has elapsed, some variations will take place in the new home, but in any case these instances prove that an immigrant species can spread over its new area without immediately varying.

Similarly, it must be admitted that species which have belonged to two continents ever since Tertiary times need not have diverged since that time, and we know, for instance, thirty-two species of nocturnal Lepidoptera which are common to North America and to Europe and yet exhibit no differences, while twenty-seven other nocturnal Lepidoptera are, according to Grote, represented in America by 'vicarious' species, that is, by species which have varied slightly in one or other of the two areas, perhaps in both.

To sum up: we must undoubtedly admit that isolation has a considerable influence in the evolution of species, though only in association with selection in its various grades and modes, especially germinal selection, natural selection, and sexual selection. We can say generally that each grade and mode of selection will more readily lead to the transformation if it be combined with isolation. Thus germinal selection may call forth slight divergences in colour and marking, which will be permanent if the individuals concerned are in an isolated region. In isolation these variations will increase undisturbed, and in some circumstances will be intensified by sexual selection, so that the male sex will vary alone in the first place, though the female may follow, so that ultimately the whole species will be transformed. Finally, the most marked effect of isolation is seen when individual members of a species are transferred to virgin territory which offers unoccupied areas, suitable not to one particular species alone, but to many nearly related species, so that the immigrant colony can adapt itself to all the different possibilities of life, and develop into a whole circle of species. But we saw that such an aftergrowth of new forms, whether varieties, species, or even genera, may far exceed the number of different kinds of localities, if there be relative isolation between the different groups of immigrants within the insular region, as happens in the case of slow-moving animals like the terrestrial snails, or of small singing-birds, to which each island of a little archipelago is a relatively isolated region (Galapagos).

We may thus fully recognize the importance of local isolation without regarding the absence of crossing with the members of the species in the original habitat as the sole cause of species-formation, without setting 'isolation' in the place of the processes of selection. These last, taken in the wide sense, always remain the indispensable basis of all transformations, but they certainly do not operate only in the form of personal selection, but, wherever indifferent characters are concerned, in that of germinal selection. Here, too, we see the possibility of reconciliation with those naturalists who regard transformations as primarily dependent upon internal forces of development. The fact is that all variations depend upon internal causes, and their course must be guided by forces which work in an orderly way. But the actual co-operation of all these forces and variations is not predetermined, but depends to a certain extent upon chance, for of the possible modes of evolution the one which gains the upper hand in the play of forces at the moment is alone followed, the better are everywhere preferred, from the most minute vital units of the germ-plasm, up to the struggle between individuals and between species.