28 STKRILITY OF HYBEIDS. [Chap. IX. 



Hence it seems that, on the one hand, slight 

 changes in the conditions of life benefit all organic be- 

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

 crosses between the males and females of the same spe- 

 cies, 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, or- 

 ganic beings long habituated to certain uniform condi- 

 tions under a state of nature, when subjected, as under 

 confinement, to a considerable change in their condi- 

 tions, 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 gen- 

 erally sterile. He will at the same time be able to ex- 

 plain 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, al- 

 though 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 con- 

 nected 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 



