2§4 FERTILITY OF VARIETIES 



tivated plants have been subjected, have had so little ten. 

 dency toward modifying the reproductive system in a manner 

 leading to mutual sterility, that we have good grounds for 

 admitting the directly opposite doctrine of Pallas, namely, 

 that such conditions generally eliminate this tendency ; su 

 that the domesticated descendants of species, which in their 

 natural state probably would have been in some degree 

 sterile when crossed, become perfectly fertile together. With 

 plants, so far is cultivation from giving a tendency toward 

 sterility between distinct species, that in several well-authen- 

 ticated cases already alluded to, certain plants have been 

 affected in an opposite manner, for they have become self- 

 impotent, while still retaining the capacity of fertilizing, 

 and being fertilized by, other species. If the Pallasian 

 doctrine of the elimination of sterility through long-con- 

 tinued 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 having 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 gener- 

 ally 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 igno- 

 rant 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 compet- 

 itors, 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 cap- 

 tivity, 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 unnatural cross. Domesti- 

 cated productions, on the other hand, which, as shown by 



