248 CAUSES OF THE STERILITY Chap. VIII. 



(lifTcront. With many kinds, insects constantly ciirry pollen 

 from neigliboriniT plants to the stif^mas of each flower ; and 

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

 pollen of a variet}', when deposited on the stij]^ia of the same va- 

 riety, should become by spontaneous variation in ever so slig'ht 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 efl\3cts of the pollen of other varieties, 

 and prevent deterioration of character. And the more pre- 

 ])(^tcnt the variety's own pollen could be rendered through nat- 

 ural selection, the greater the advantage would be. We know 

 from the researches of Giirtncr that, with s^xjcies 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 spe- 

 cies in process of formation, so the sterility consequent on pre- 

 potency 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 ob- 

 literated the effects of the access of previous males belonging 

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

 with terrestrial animals, that this is the case ; as most males 

 and females pair for each birth, and some few for life. 



On the whole we may conclude, that with animals the ster- 

 ility of crossed species has not been slowly augmented, through 

 natural selection ; and as this sterility follows the same general 

 laws in the vegetable as in the animal kingdom, it is improba- 

 ble, though apparently possible, that with plants crossed spe- 

 cies should have been rendered sterile by a different process. 

 From this consideration, and remembering that species Avhich 

 have never coexisted in the same countiy, and Avhich therefore 

 could not have received any advantage from having been ren- 

 dei'ed mutually infertile, yet are generally sterile when cix)ssed ; 

 and bearing in mind that in reciprocal crosses between the 

 same two species there is sometimes the widest difference in 

 their sterility, we must give up the belief that natural selection 

 has come into play. We are thus driven to our former prop- 

 osition, namely, that the sterility of first crosses, and indi- 



