THEOUOn CTJMULATIVE SEGEEaATlOIT. 



243 



fifth edition * of ' The Origin of Species/ in tlie section on " The 

 Origin and Causes of Sterilit}^," Darwin,, while maintaining tliat 

 the mutual sterility of species is not due to Natural Selection, 

 refers to prepotency of the kind we are now considering as a 

 quality which, occurring in ever so slight a degree, would prevent 

 deterioration of character, and which would therefore be an ad- 

 vantage to a species in the process of formation, and accordingly 

 subject to accumulation through Natural Selection. In order to 

 construct a possible theory for the introduction of sterility 

 between allied species by means of Natural Selection, he finds it 

 necessary simply to add the supposition that sterility is directly 

 caused by this prepotency. He, however, for several reasons 

 concludes that there is no such dependence of mutual sterility on 

 the process of Natural Selection. Concerning the prepotency 

 he makes no reservation, and I accordingly judge that he con- 

 tinued to regard it as strengthened and developed through the 

 action of Natural Selection. 



It is concerning this last point that I wish to give reasons for 

 a different opinion. I believe that qualities simply producing 

 Segregation can never be accumulated by Natural Selection ; 

 for : — 



(1) When separate generation comes in between two sections 

 of a species they cease to be one aggregate, subject to modifi- 

 cation through the elimination of certain parts. Both will be 

 subject to similar forms of natural selection only so long as the 

 circumstances of both and the variations pf both are nearly the 

 same, but they will no longer be the members of one body 

 between which the selecting process is carried out. On the con- 

 trary, if they occupy the same district each group will stand in 

 the relation of environment to the other, modifying it, and being 

 modified by it, without mutually sharing in the same modifi- 

 cation. 



(2) Though one may exterminate the other, the change that 

 comes to the successful group through the contest is not due to 

 its superiority over the other, but to the superiority of some of 

 its own members over others. 



(3) When any Segregate form begins to arise we cannot attri- 

 bute its success to the advantage of segeneratiou, for the inter- 



* Since my comments on this passage wore written, I have discovered that 

 Darwin has omitted it from the sixth etlition. 



