82 ORGANIC EVOLUTION CONSIDERED 



varieties are even more fertile by cross-breeding than 

 by close breeding? Is it not evident that, with every 

 generation, the slight varieties would cross-breed with 

 one another and with the parent stock, and thus all 

 varietal differences be funded into a common stock, 

 and the type would be preserved unchanged? This, 

 as already pointed out, has always been the chief dif- 

 ficulty in the way of imagining how varieties can grow 

 into species ; and the difficulty is only increased by 

 our discussion of the law of cross-breeding. Now 

 just here Dr. Romanes's most important and prolific 

 idea comes to our help, and, as it seems to us, com- 

 pletely solves the difficulty." * 



Thus, in the face of the fact that slight variations 

 render animals more fertile by cross-breeding, it is 

 assumed that new varieties are born cross-sterile with 

 the parent form and cross-fertile with their own kind. 



I know of no assumption among scientific men that 

 seems more extreme and unwarranted than this — none 

 more in opposition to the facts at present known. 



Le Conte says further: "But suppose among 

 these divergent variations there arise, from time 

 to time, some which affect the reproductive or- 

 gans in such wise that the variety, though perfectly 

 fertile with its own kind, is infertile, or imperfectly 

 fertile, with other varieties, and especially with the 



parent stock. "t 



Here we find several things that must happen in a 

 single generation. 



First, a number of individuals must be born and 

 exist at the same time, which possess the same varia- 

 tion. 



Second, this variation must, if it can be preserved, 

 be of such a kind that it will give the individuals pos- 



* Evolution and Creation, p. 226. t Ibid, p. 227. 



