Evidences of Physiological Selection. 93 



with the observations of Jordan and Nageli) would 

 necessarily demand. 



Moreover, this view of the matter is still lurther 

 corroborated by certain other facts and considerations. 

 For example, the phenomena of prepotency (whether 

 as between varieties or between closely allied species) 

 are found to occur when the two forms occupy a 

 common area, i.e. are growing intermingled with 

 one another. Therefore, but for this physiological 

 differentiation, there could be absolutely nothing to 

 prevent free intercrossing. Yet the fact that hybrids 

 are so comparatively rare in a state of nature — a fact 

 which Sir Joseph Hooker has pointed out to me as 

 otherwise inexplicable —proves the efficacy of even 

 a low degree of such differentiation in preventing 

 the physiologically-differentiated forms from inter- 

 crossing. Even in cases where there is no difficulty 

 in producing artificial hybrids or mongrels between 

 species or varieties growing on common areas, it is 

 perfectly astonishing what an extremely small per- 

 centage of the hybrid or mongrel forms are found to 

 occur in nature. And there can be no question that 

 this is due to the very efficient manner in which 

 prepotency does its work — efficient, I mean, from 

 the point of view of the new theory ; for upon any 

 other theory prepotency is a meaningless pheno- 

 menon, which, notwithstanding its frequent occur- 

 rence, plays no part whatever in the process of organic 

 evolution. 



I attach considerable importance to the phenomena 

 of prepotency in view of the contrast which is pre- 

 sented between plants and animals in the relation of 

 their species to physical barriers. For animals — 



