84 THE ORIGIN OF SPECIES 



between the males and females of the same species, which 

 have been subjected to slightly different conditions, or 

 which have slightly varied, give vigor 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 confinement, 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 breed- 

 ing 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 domesticated 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 

 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 toward an equilib- 

 rium; and when this tendency is slightly disturbed by an] 

 change, the vital forces gain in power. 



