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.