With organic beings which never intercross, if such exist, each
species, must be descended from a succession of modified varieties, that
have supplanted each other, but have never blended with other individuals
or varieties of the same species, so that, at each successive stage of
modification, all the individuals of the same form will be descended from a
single parent. But in the great majority of cases, namely, with all
organisms which habitually unite for each birth, or which occasionally
intercross, the individuals of the same species inhabiting the same area
will be kept nearly uniform by intercrossing; so that many individuals will
go on simultaneously changing, and the whole amount of modification at each
stage will not be due to descent from a single parent. To illustrate what
I mean: our English race-horses differ from the horses of every other
breed; but they do not owe their difference and superiority to descent from
any single pair, but to continued care in the selecting and training of
many individuals during each generation.
Before discussing the three classes of facts, which I have selected as
presenting the greatest amount of difficulty on the theory of "single
centres of creation," I must say a few words on the means of dispersal.
MEANS OF DISPERSAL.
Sir C. Lyell and other authors have ably treated this subject. I can give
here only the briefest abstract of the more important facts.
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