HYBRIDISM 239 



stand, excepting on vague hypotheses, several facts with 

 respect to the sterility of hybrids ; for instance, the un- 

 equal fertility of hybrids produced from reciprocal 

 crosses ; or the increased sterility in those hybrids 

 which occasionally and exceptionally resemble closely 

 either pure parent. Nor do I pretend that the fore- 

 going remarks go to the root of the matter : no ex- 

 planation 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 dis- 

 turbed, in the other case from the organisation having 

 been disturbed by two organisations having been com- 

 pounded into one. 



It may seem fanciful, but I suspect that a similar 

 parallelism extends to an allied yet very different class 

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

 founded, I think, on a considerable body of evidence, 

 that slight changes in the conditions of life are bene- 

 ficial 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, 

 we plainly see that great benefit is derived from almost 

 any change in the habits of life. Again, both with 

 plants and animals, there is abundant evidence, that a 

 cross between very distinct individuals of the same 

 species, that is between members of different strains or 

 sub-breeds, gives vigour and fertility to the offspring. 

 I believe, indeed, from the facts alluded to in our 

 fourth chapter, that a certain amount of crossing is in- 

 dispensable even with hermaphrodites ; and that close 

 interbreeding continued during several generations 

 between the nearest relations, especially if these be 

 kept under the same conditions of life, always induces 

 weakness and sterility in the progeny. 



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 



