1. Let there be a species which has varied into two forms, each adapted to existing conditions (211/1. "Existing conditions," means of course new conditions which have now come into existence. And the "two" being both better adapted than the parent form, means that they are better adapted each to a special environment in the same area--as one to damp, another to dry places; one to woods, another to open grounds, etc., etc., as Darwin had already explained. A.R.W. (1899).) better than the parent form, which they supplant.
2. If these two forms, which are supposed to co-exist in the same district, do not intercross, Natural Selection will accumulate favourable variations, till they become sufficiently well adapted to their conditions of life and form two allied species.
3. But if these two forms freely intercross with each other and produce hybrids which are also quite fertile inter se, then the formation of the two distinct races or species will be retarded or perhaps entirely prevented; for the offspring of the crossed unions will be more vigorous owing to the cross, although less adapted to their conditions of life than either of the pure breeds. (211/2. After "pure breeds," add "because less specialised." A.R.W. (1899).)
4. Now let a partial sterility of some individuals of these two forms arise when they intercross; and as this would probably be due to some special conditions of life, we may fairly suppose it to arise in some definite portion of the area occupied by the two forms.
5. The result is that in this area hybrids will not increase so rapidly as before; and as by the terms of the problem the two pure forms are better suited to the conditions of life than the hybrids, they will tend to supplant the latter altogether whenever the struggle for existence becomes severe.
6. We may fairly suppose, also, that as soon as any sterility appears under natural conditions, it will be accompanied by some disinclination to cross-unions; and this will further diminish the production of hybrids.
7. In the other part of the area, however, where hybridism occurs unchecked, hybrids of various degrees will soon far outnumber the parent or pure form.
8. The first result, then, of a partial sterility of crosses appearing in one part of the area occupied by the two forms, will be, that the GREAT MAJORITY of the individuals will there consist of the pure forms only, while in the rest of the area these will be in a minority,--which is the same as saying, that the new sterile or physiological variety of the two forms will be better suited to the conditions of existence than the remaining portion which has not varied physiologically.
9. But when the struggle for existence becomes severe, that variety which is best adapted to the conditions of existence always supplants that which is imperfectly adapted; therefore by Natural Selection the sterile varieties of the two forms will become established as the only ones.
10. Now let a fresh series of variations in the amount of sterility and in the disinclination to crossed unions occur,--also in certain parts of the area: exactly the same result must recur, and the progeny of this new physiological variety again in time occupy the whole area.
11. There is yet another consideration that supports this view. It seems probable that the variations in amount of sterility would to some extent concur with and perhaps depend upon the structural variations; so that just in proportion as the two forms diverged and became better adapted to the conditions of existence, their sterility would increase. If this were the case, then Natural Selection would act with double strength, and those varieties which were better adapted to survive both structurally and physiologically, would certainly do so. (211/3. The preceding eleven paragraphs are substantially but not verbally identical with the statement of the argument in Mr. Wallace's "Darwinism," 1889. Pages 179, 180, note 1.)
12. Let us now consider the more difficult case of two allied species A, B, in the same area, half the individuals of each (As, Bs) being absolutely sterile, the other half (Af, Bf) being partially fertile: will As, Bs ultimately exterminate Af, Bf?
13.