28 STERILITY OF HYBEIDS. [Chap. IX. 



interbreeding continued during several generations 

 between the nearest relations, if these be kept under 

 the same conditions of life, almost always leads to 

 decreased size, weakness, or sterility. 



Hence it seems that, on the one hand, slight changes 

 in the conditions of life benefit all organic beings, and 

 on the other hand, that slight crosses, that is crosses 

 between the males and females of the same species, 

 which have been subjected to slightly different con- 

 ditions, or which have slightly varied, give vigour and 

 fertility to the offspring. But, as we have seen, organic 

 beings long habituated to certain uniform conditions 

 under a state of nature, when subjected, as under con- 

 finement, to a considerable change in their conditions, 

 very frequently are rendered more or less sterile ; and 

 we know that a cross between two forms, that have 

 become widely or specifically different, produce hybrids 

 which are almost always in some degree sterile. I am 

 fully persuaded that this double parallelism is by no 

 means an accident or an illusion. He who is able to 

 explain why the elephant and a multitude of other 

 animals are incapable of breeding when kept under 

 only partial confinement in their native country, will 

 be able to explain the primary cause of hybrids being 

 so generally sterile. He will at the same time be able 

 to explain how it is that the races of some of our do- 

 mesticated animals, which have often been subjected 

 to new and not uniform conditions, are quite fertile 

 together, although they are descended from distinct 

 species, which would probably have been sterile if 

 aboriginally crossed. The above two parallel series of 

 facts seem to be connected together by some common 

 but unknown bond, which is essentially related to the 



