318 ORIGIN OF SPECIES 



cies, which differ to a certain extent, gives vigour and fer- 

 tility to the offspring; and that close 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 conditions, or which have slightly varied, 

 give vigour and fertility to the offspring. But, as we have 

 seen, organic beings long habituated to certain uniform condi- 

 tions 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 al- 

 ways 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 gener- 

 ally sterile. He will at the same time be able to explain 

 how it is that the races of some of our domesticated animals, 

 which have often been subjected to new and not uniform con- 

 ditions, 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 principle of 

 life ; this principle, according to Mr. Herbert Spencer, being 

 that life depends on, or consists in, the incessant action and 

 reaction of various forces, which, as throughout nature, are 

 always tending towards an equilibrium; and when this ten- 

 dency is slightly disturbed by any change, the vital forces 

 gain in power. 



