HYBRIDISM 33 



crosses; or tlie increased sterility in those hybrids which 

 occasionally and exceptionally resemble closely either 

 pure parent. Nor do I pretend that the foregoing re- 

 marks go to the root of the matter; no explanation is 

 offered why an organism, when placed under unnatural 

 conditions, is rendered sterile. All 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 life having been disturbed, in the other 

 case from the organization having been disturbed by two 

 organizations being compounded into one. 



A similar parallelism holds good with an allied yet 

 very different class of facts. It is an old and almost 

 universal belief, founded on a considerable body of evi- 

 dence 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 evi- 

 dence 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 

 1 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, 



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



