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most of ... the species inhabiting one region are different from those of another region, but are closely allied or belong to the same genera, if in all such cases it can be shown that there probably has been at some former period migration from the one region to the other, 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 the 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." And I now know that he attributes this coincidence to descent with modification. ↑
The discussion on "single and multiple centres of creation" does not directly bear on another allied question,— namely whether all the individuals of the same species are descended from a single pair, or single hermaphrodite, or whether, as some authors suppose, from many individuals simultaneously created. With .. organic beings which never intercross, if such exist, each species must be descended from a succession of modified varieties, which have supplanted each other, but which have never blended with other individuals or varieties
most of its inhabitants are closely related to, or belong to the same genera with the species of a second region, has probably received at some former period immigrants from this other region, my theory will be strengthened; for we can clearly understand, on the principle of modification, why the inhabitants of a region should be related to those of another region, whence it has been stocked. 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 plainly related by inheritance to the inhabitants of that continent. Cases of this nature are common, and are, as we shall hereafter more fully see, inexplicable on the theory of independent creation. This view of the relation of species in one region to those in another, does not differ much (by substituting the word variety for species) from that lately advanced in an ingenious paper by Mr. Wallace, in which he concludes that "every species has come into existence coincident both in space and time with a pre-existing closely allied species." And I now know from correspondence, that this coincidence he attributes to generation with modification.
The previous remarks on "single and multiple centres of creation" do not directly bear on another allied question,— namely, whether all the individuals of the same species have descended from a single pair, or single hermaphrodite, or whether, as some authors suppose, from many individuals simultaneously created. With those organic beings which never intercross (if such exist), the species, on my theory, must have descended from a succession of improved varieties, which will never have blended with other individuals or varie- ties,