Chap. XIX. 



HYBRIDISM. 187 



believe that modifications in their structure have been slowly 

 accumulated by natural selection, from an advantage having 

 been thus indirectly given to the community to which they 

 belonged over other communities of the same species; but an 

 individual animal, if rendered slightly sterile when crossed with 

 some other variety, would not thus in itself gain any advantage, 

 or indirectly give any advantage to its nearest relatives or to 

 other individuals of the same variety, leading to their preserva- 

 tion. I infer from these considerations that, as far as animals 

 are concerned, the various degrees of lessened fertility which 

 occur with species when crossed cannot have been slowly accu- 

 mulated by means of natural selection. 



With plants, it is possible that the case may be somewhat 

 different. With many kinds, insects constantly carry pollen 

 from neighbouring plants to the stigmas of each flower; and 

 with some species this is effected by the wind. Now, if the 

 pollen of a variety, when deposited on the stigma of the same 

 variety, should become by spontaneous variation in ever so 

 slight a degree prepotent over the pollen of other varieties, this 

 would certainly be an advantage to the variety; for its own 

 pollen would thus obliterate the effects of the pollen of other 

 varieties, and prevent deterioration of character. And the more 

 prepotent the variety's own pollen could be rendered through 

 natural selection, the greater the advantage would be. We 

 know from the researches of Gartner that, with species which 

 are mutually sterile, the pollen of each is always prepotent on 

 its own stigma over that of the other species ; but we do not 

 know whether this prepotency is a consequence of the mutual 

 sterility, or the sterility a consequence of the prepotency. If 

 the latter view be correct, as the prepotency became stronger 

 through natural selection, from being advantageous to a species 

 in process of formation, so the sterility consequent on prepo- 

 tency would at the same time be augmented ; and the final 

 result would be various degrees of sterility, such as occurs with 

 existing species. This view might be extended to animals, if 

 the female before each birth received several males, so that the 

 sexual element of the prepotent male of her own variety obli- 

 terated the effects of the access of previous males belonging 

 to other varieties; but we have no reason to believe, at least 



