ISOLATION 293 



Vernon's theory rests upon an arbitrary supposi- 

 tion: the greater fertility of similar mates. Since 

 Darwin's time, however, the superiority of cross- 

 breeds upon the interbreeding of identical individuals 

 has been a fact universally recognised and one which 

 the few examples cited by Vernon in support of his 

 view are insufficient to invalidate. His theory also 

 includes a mathematical demonstration the conclusions 

 of which are similar to those embodied in Delboeuf's 

 law and are open to the same criticism. 



Romanes' theory of physiological selection has not 

 won many supporters among naturalists. It presup- 

 poses too many coincidences. For instance, variations 

 in genitalia must necessarily occur in both sexes at 

 the same time; also variations of this order would be 

 insufficient to create a new race of beings, for it is 

 not clear why the progeny of individuals, varied in 

 that respect only, should differ from the rest in many 

 other respects. Finally, sexual variations could not 

 accumulate without the assistance of natural selec- 

 tion, whose intervention would remove precisely that 

 cause of sterility within a species. 



It is most likely that if sexual variation affects a 

 restricted number of individuals, if, in other words, it 

 remains, as Darwin thought, an individual variation, 

 those individuals are more apt to die off without leav- 

 ing any progeny than to constitute the nucleus of a 

 new species. If the variation is more general, it may 



