i864— 1869] STERILITY OF HYBRIDS 291 



occurs unchecked, hybrids of various degrees will soon far Letter 211 

 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 pro- 

 portion 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. 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 

 (A 8 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 be- 

 tween A F and B F cross-unions are as frequent as direct 

 unions, though much less fertile. We must also leave out of 



1 The preceding eleven paragraphs are substantially but not verbally 

 identical with the statement of the argument in Mr. Wallace's Darwinism) 

 1889, pp. 179, 180, note 1. 



