Physiological Selection. 53 



been the case wherever there was an absence of any 

 other form of isolation), it is nothing less than a 

 necessary consequence that many allied species should 

 now present the physiological peculiarity in question. 

 Thus the association between the physiological pecu- 

 liarity and the morphological divergence is explained 

 by the simple hypothesis, that the former has acted 

 as a necessary condition to the occurrence of th,e 

 latter. In the absence of other forms of isolation, 

 the morphological divergence could not have taken 

 place at all, had not the physiological peculiarity 

 arisen ; and hence it is that we now meet with so 

 many cases where such divergence is associated with 

 this peculiarity. 



So far we have been considering the physiological 

 change as historically the prior one. Here, at first 

 sight, it may seem that the segregative power of 

 physiological selection must end ; for it may well 

 seem impossible that the physiological chan^^e can 

 ever be necessary for the divergence of morphological 

 varieties into true species in cases where it has not 

 been the prior change, but has only set in after mor- 

 phological changes have proceeded far enough to have 

 already constituted definite varieties. A little thought, 

 however, will show that physiological selection is quite 

 as potent a condition to the differentiation of species 

 when it occurs after varietal divergence has begun, as 

 it is when it occurs before the divergence — and hence 

 that it really makes no difference to the theory of 

 physiological selection whether, in particular cases, the 

 cross- infertihty arises before or after any structural or 

 other modifications with which it is associated. 



