WHEJ!f GROSSED. 303 



tinned 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 ani- 

 mals, varieties have not been produced which are mutually 

 sterile; and why with plants only a few such cases, imme- 

 diately 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 de- 

 gree 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 exist- 

 ence 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 emi- 

 nently sensitive to the influence of an unnatural cross. 

 Domesticated productions, on the other hand, which, as 

 shown by the mere fact of their domestication, were not 

 originally highly sensitive to changes in their conditions of 

 life, and which can now generally resist with undimin- 

 ished fertility repeated changes of conditions, might be 

 expected to produce varieties, which would be little liable 

 to have their reproductive powei;s injuriously affected by 

 the act of crossing with other varieties which had origi- 

 nated in a like manner. 



I have as yet spoken as if the varieties of the same species 

 were invariably fertile when intercrossed. But it is impos- 

 sible to resist the evidence of the existence of a certain 

 amount of sterility in the few following oases, which I will 

 briefly abstract. The evidence is at least as good as that 

 from which we believe in the sterility of a multitude of 



