256 THE ORIGIN OF SPECIES 



that I have attempted to show is, that in two cases, in some 

 respects allied, sterility is the common result — in the one case 

 from the conditions of hfe having been disturbed, in the other 

 case from the organization having been disturbed by two organiza- 

 tions being compounded into one. 



A similar parallelism holds good with an allied yet very differ- 

 ent class of facts. It is an old and almost universal belief, founded 

 on a considerable body of evidence, which I have elsewhere given, 

 that slight changes in the conditions of life are beneficial to all 

 living things. We see this acted on by farmers and gardeners in 

 their frequent exchanges of seed, tubers, etc., from one soil or 

 climate to another, and back again. During the convalescence of 

 animals, great benefit is derived from almost any change in their 

 habits of life. Again, both with plants and animals, there is the 

 clearest evidence that a cross between individuals of the same 

 species, which differ to a certain extent, gives vigor and fertility 

 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 steriHty. 



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 differ- 

 ent 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 per- 

 suaded 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 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 to- 



