In the other part of the area, however, where hybridism occurs with perfect freedom, hybrids of various degrees may increase till they equal or even exceed in number the pure species--that is, the incipient species will be liable to be swamped by intercrossing.
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 two pure forms only, while in the remaining part these will be in a minority,--which is the same as saying that the new _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 _varieties_ which are _sterile_ when crossed will become established as the only ones.
10.

Now let variations in the _amount of sterility_ and in the _disinclination to crossed unions_ continue to occur--also in certain parts of the area: exactly the same result must recur, and the progeny of this new physiological variety will in time occupy the whole area.
11.

There is yet another consideration that would facilitate the process.