252 STERILITY OF HYBRIDS. Chap. VIII. 



or any other view, scvenil facts with respect to the sterility of 

 hybrids ; for instance, the unequal fertility of hybrids produced 

 from reciprocal crosses; or the increased sterility in those hy- 

 brids Avhich occasionally and exceptionally reseniljle closely 

 cither pure parent. Nor do I pretend that the foroi^oing re- 

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

 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 

 disturljed, in the other case from the organization or constitu- 

 tion having been disturbed by two organizations being com- 

 pounded into one. 



A similar parallelism apparently extends to an allied yet 

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

 belief, founded on a considerable body of evidence, that slight 

 changes in the conditions of life are beneficial to all living 

 things. We sec this acted on by fiirmcrs 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 cliange in 

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

 is abundant evidence that a cross between indiWduals of the 

 same species, which difltr to a certain extent, gives vigor and 

 fertility to the oflspring; and that close interbreeding con- 

 tinued during several generations between the nearest rela- 

 tions, especially if these be kept under the same conditions of 

 life, almost always induces weakness and sterility. 



Hence it seems that, on the one hand, slight clianges 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 varied and become 

 slightly different, give vigor and fertility to the offspring. But 

 we have seen that greater changes, or changes of a jiarticular 

 nature, often render organic beings in some degree sterile ; 

 and that greater crosses, tliat is, crosses between males and 

 females which have become widely or specifically different, 

 produce hybrids which are generally sterile in some degree. 

 I cannot persuade myself that this parallelism is an accident 

 or an illusion. Both series of facts seem to be connected to- 

 gether by some common but unknown bond, which is essen- 

 tially related to the principle of life ; this principle apparently 

 being that life, as Mr. Herbert Spencer has remarked, depends 



