1897] REPRODUCTIVE DIVERGENCE 189 



victuals of greater vigour and fitness than the parents. As far as I 

 am aware, there is no evidence to show that this greater vigour is 

 the result of the differences of morphological form, but rather that 

 it is due to the individuals being descended from different stocks, 

 whereby the evil effects of in-and-in breeding are avoided, or to 

 being exposed to differences of environmental conditions, whereby 

 they may perhaps be rendered physiologically unlike individuals 

 to a slight degree, rather than morphologically unlike. That mere 

 exposure to differences of environmental conditions may be sufficient 

 to give rise to a vigorous race even when this is propagated by the 

 closest in-and-in breeding, is shown by the case of the rabbits on 

 the Island of Porto Santo, all of which are descended from a single 

 pregnant individual. 



Another objection which might be raised is, that in the case of 

 both plants and animals it has frequently been found that varieties 

 showing considerable differences of external form are perfectly 

 fertile inter sc. Even if this is the case, it is no argument against 

 the theory of reproductive divergence, for it was specially men- 

 tioned that this is not supposed to be invariably in operation when 

 a species is in course of splitting up into varieties. At the same 

 time, it may reasonably be doubted whether this statement as to 

 the perfect fertility of varieties is a fact, because a very slight 

 degree of sterility would easily escape notice unless extensive series 

 of breeding experiments were made, and careful records kept. 



H. M. Vernon. 



