36 FERTILITY OF VAEIETIES [Cjap. IX 



whilst Still retaining the capacity of fertilising, and 

 being fertilised by, other species. If the Pallasian 

 doctrine of the elimination of sterility through long- 

 continued domestication be admitted, and it can hardly 

 be rejected, it becomes in the highest degree improbable 

 that similar conditions long-continued should likewise 

 induce this tendency; though in certain cases, with 

 species ha\ing a peculiar constitution, sterility might 

 occasionally be thus caused. Thus, as I believe, we 

 can understand why with domesticated animals varieties 

 have not been produced which are mutually sterile; 

 and why with plants only a few such cases, immediately 

 to be given, have been observed. 



The real difficulty in our present subject is not, as it 

 appears to me, why domestic varieties have not become 

 mutually infertile when crossed, but why this has so 

 generally occurred with natural varieties, as soon as 

 they have been permanently modified in a sufficient 

 degree to take rank as species. We are far from 

 precisely knowing the cause; nor is this surprising, 

 seeing how profoundly ignorant we are in regard to the 

 normal and abnormal action of the reproductive system. 

 But we can see that species, owing to their struggle for 

 existence with numerous competitors, will have been 

 exposed during long periods of time to more uniform 

 conditions, than have domestic varieties ; and this may 

 well make a wide difference in the result. For we 

 know how commonly wild animals and plants, when 

 taken from their natural conditions and subjected to 

 captivity, are rendered sterile; and the reproductive 

 functions of organic beings which have always lived 

 under natural conditions would probably in like 

 manner be eminently sensitive to the influence of an 



