should expect that they must, if the former were the natural progenitors of the latter. On the other hand, if they were not the natural progenitors, but all the species, both living and extinct, were the supernatural and therefore independent creations which the rival theory would suppose, then no reason can be given why the extinct species should thus resemble the living--any more than why the living species should resemble one another. For, as we have seen, there are almost always many other habitats on other parts of the globe, where any members of any given group of species might equally well have been deposited; and this, of course, applies to geological no less than to historical time. Yet throughout all time we meet with this most suggestive correlation between continuity of a geographical area and structural affinity between the forms of life which have lived, or are still living, upon that area. Similarly, we find the further, and no less suggestive, correlation between the birth of new species and the immediate pre-existence of closely allied species on the same area--or, at most, on closely contiguous areas. Where a continuous area has long been circumscribed by barriers of any kind, which prevent the animals from wandering beyond it, then we find that all the species, both extinct and living, constitute more or less a world of their own; while, on the other hand, where the animals are free to migrate from one area to another, the course of their migrations is marked by the origination of new species springing up _en route_, and serving to connect the older, or metropolitan, forms with the younger, or colonising, forms in the way of a graduated series. This principle, however, admits of being traced only in certain cases of species belonging to the same genus, of genera belonging to the same family, or, at most, of families belonging to the same order. In other words, the more general the structural affinity, the more general is the geographical extension--as we should expect to be the case on the theory of descent with branching modifications, seeing that the larger, the older, and the more diverse the group of organisms compared, the greater must be their chances of dispersal.