If, when most of the species inhabiting one region are different from those of another region, though closely allied to them, it can be shown that migration from the one region to the other has probably occurred at some former period, our general view will be much strengthened; for the explanation is obvious on the principle of descent with modification.

A volcanic island, for instance, upheaved and formed at the distance of a few hundreds of miles from a continent, would probably receive from it in the course of time a few colonists, and their descendants, though modified, would still be related by inheritance to the inhabitants of that continent.

Cases of this nature are common, and are, as we shall hereafter see, inexplicable on the theory of independent creation.

This view of the relation of the species of one region to those of another, does not differ much from that advanced by Mr.

Wallace, who concludes that "every species has come into existence coincident both in space and time with a pre-existing closely allied species.