92 Darwin , and after Darwin. 



when co-operating with physiological selection. But 

 I also stated my belief that in many cases selective 

 fertility is presumably of itself capable of splitting 

 a specific type ; and the reason why I still believe 

 this is, that I do not otherwise understand these pheno- 

 mena of prepotency. I cannot believe that in all the 

 innumerable cases where they arise, they have been 

 super-induced by some prior morphological changes 

 going on in some other part of the organism, or by 

 "prolonged exposure to uniform conditions of life," 

 on the part of two wellnigh identical forms which 

 have arisen intimately commingled in exactly the 

 same environment, and under the operation of a pre- 

 viously universal intercrossing. Even if such a thing 

 could be imagined as happening occasionally, I feel 

 it difficult to imagine that it can happen habitually, 

 and yet this view must be held by those who would 

 attribute prepotency to natural selection. 



It must never be forgotten that the relatively 

 enormous changes as to size, structure, habit, &c., 

 which are presented by our domesticated plants as 

 results of artificial selection, do not entail the physio- 

 logical character of cross-sterility in any degree, 

 save possibly in some small number of cases. Although 

 in wild species any correspondingly small percentage 

 of cases (where natural selection happens to hit upon 

 parts of the organism modifications of which produce 

 the physiological change by way of correlation) would 

 doubtless be the ones to survive on common areas, 

 still it is surely incredible that such an accidental 

 association between natural selection and cross- 

 infertility is so habitually the means of specific 

 differentiation as the facts of prepotency (together 



