[Enclosure]⁸

Sterility of Hybrids produced by Natural Selection

1. Let there be a species which has varied into two forms, each adapted to existing conditions 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, Nat. Select. 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.

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 forms.

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 ⁹ 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.

12. Let us now consider the more difficult case of two allied species A. B. in the same area, half the individuals of each (A^S. B^S.) being absolutely sterile, the other half (A^F. B^F.) being partially fertile. Will A^S. B^S. ultimately exterminate A^F. B^F.?

13. To avoid complication it must be granted, that between A^S. and B^S. no cross unions take place, while between A^F. and B^F. cross unions are as frequent as direct unions, though much less fertile. We must also leave out of consideration crosses between A^S. and A^F., B^S. and B^F. with their various approaches to sterility, as I believe they will not affect the final result although they will greatly complicate the problem.

14. In the first generation there will result; 1st. The pure progeny of A^S. and of B^S. 2nd. The pure progeny of A^F. and of B^F. and 3rd. The hybrid progeny of A^F. B^F.

15. Supposing that, in ordinary years, the increased constitutional vigour of the hybrids exactly counterbalances their imperfect adaptation to conditions, there will be in the 2nd. generation, besides these three classes, hybrids of the 2^nd. degree between the first hybrids and A^F. and B^F. respectively. In succeeding generations there will be hybrids of all degrees, varying between the first hybrids and the almost pure types of A^F. and B^F.

16. Now if at first the number of individuals of A^S. B^S. A^F. and B^F. were equal, and year after year the total number continues stationary, I think it can be proved, that while one half will be the pure progeny of A^S. and B^S., the other half will become more and more hybridised, until the whole will be hybrids of various degress.

17. Now this hybrid and somewhat intermediate race, cannot be so well adapted to the conditions of life as the two pure species, which have been formed by the minute adaptation to conditions through Nat. Select.;— therefore, in a severe struggle for existence, the hybrids must succomb, especially as, by hypothesis, their fertility would not be so great as that of the two pure species.

18. If we were to take into consideration the unions of A^S. with A^F. and B^S. with B^F., the results would become very complicated, but it must still lead to there being a number of pure forms entirely derived from A^S. and B^S. and of hybrid forms mainly derived from A^F. and B^F.; and the result of the struggle of these two sets of individuals can not be doubtful.

19. If these arguments are sound, it follows—that sterility may be accumulated and increased, and finally made complete by Natural Selection whether the sterile varieties originate together in a definite portion of the area occupied by the two species, or occur scattered over the whole area.

Alfred R. Wallace—

P.S. In answer to the objection as to the unequal Sterility of reciprocal crosses (“Variation &c.” Vol. 2. p. 186) I reply, that, as far as it went, the sterility of one cross would be advantageous even if the other cross was fertile: and just as characters now coordinated may have been separately accumulated by Nat. Select., so the reciprocal crosses may have become sterile one at a time.