vii ON THE INFERTILITY OF CROSSES 179 



such infertility there is no proof. This is admitted ; but it is 

 urged that facts have been adduced which render such 

 infertility probable, at least in some cases, and this is all 

 that is required. It is by no means necessary that all varieties 

 should exhibit incipient infertility, but only some varieties ; 

 for we know that, of the innumerable varieties that occur 

 but few become developed into distinct species, and it may be 

 that the absence of infertility, to obviate the effects of inter- 

 crossing, is one of the usual causes of their failure. All I 

 have attempted to show is, that when incipient infertility does 

 occur in correlation with other varietal differences, that in- 

 fertility can be, and in fact must be, increased by natural 

 selection ; and this, it appears to me, is a decided step in 

 advance in the solution of the problem. 1 



1 As this argument is a rather difficult one to follow, while its theoretical 

 importance is very great, I add here the following briefer exposition of it, in a 

 series of propositions ; being, with a few verbal alterations, a copy of what I 

 wrote on the subject about twenty years back. Some readers may find this 

 easier to follow than the fuller discussion in the text : — 



Can Sterility of Hybrids have been Produced by Natural Selection ? 



1. Let there be a species which has varied into two forms each adapted to 

 certain existing conditions better than the parent form, which they soon 

 supplant. 



2. If these two forms, which are supposed to coexist in the same 

 district, do not intercross, natural selection will accumulate all favourable 

 variations till they become well suited to their conditions of life, and form 

 two slightly differing 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 the hybrids of some considerable propor- 

 tion of these two forms arise ; 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 will be that, in that area, the hybrids (although continually 

 produced by first crosses almost as freely as before) will not themselves 

 increase so rapidly as the two pure forms ; and as the two pure forms are, by 

 the terms of the problem, better suited to their several conditions of life than 

 the hybrids, they will inevitably increase more rapidly, and will continually 

 tend to supplant the hybrids altogether at every recurrent severe struggle for 

 existence. 



6. We may fairly suppose, also, that as soon as any sterility appears some 

 disinclination to cross unions will appear, and this will further tend to the 

 diminution of the production of hybrids. 



