94 Darwin, and after Dai^n. 



and especially the higher animals— appear to depend 

 for their specific differentiations upon such barriers 

 much more than in the case with plants. This is no 

 more than we should expect ; for^ in accordance with 

 our theory, selective fertility is not so likely to work 

 alone in the case of the higher animals which mate 

 together, as in plants which are fertilized through the 

 agency of wind or insects. In the former case there 

 is no opportunity given for the first rise of cross- 

 infertility, in the form of prepotency ; and even where 

 selective fertility has gained a footing in other ways, 

 the chances against the suitable mating of " physio- 

 logical complements " must be much greater than it 

 is in the latter case. Hence, among the higher animals, 

 selective fertility ought much more frequently to be 

 found in association with other forms of homogamy 

 than it is among plants. And this is exactly what 

 we find. Thus it seems to me that this contrast 

 between the comparative absence and presence of 

 physical barriers, where allied species of plants and of 

 higher animals are respectively concerned, is entitled 

 to be taken as a further corroboration of our theory. 

 For while it displays exactly such a general corre- 

 lation as this theory would expect, the correlation is 

 one which cannot possibly be explained on any other 

 theory. It is just where physiological selection can 

 be seen to have the best opportunity of acting (viz. 

 in the vegetable kingdom) that we find the most 

 unequivocal evidence of its action ; while, on the 

 other hand, it is just where it can be seen to have 

 the least opportunity of asserting itself (viz. among 

 the higher animals) that we find it most associated 

 with, and therefore assisted by, other forms of homo- 



