CHAP. IX.] OF FIKST CROSSES AND OF HYBRIDS. 235 



It must, however, be owned that we cunnot understand, on the 

 above or any other view, several facts with respect to the sterility 

 of hybrids ; for instance, the unequal fertility of hybrids pro- 

 duced 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 foregoing remarks 

 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 organisation having been disturbed by two 

 organisations 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 evidence, which I have else- 

 where 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, <fcc., 

 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 indi- 

 viduals of the same species, which differ to a certain extent, gives 

 vigour 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 sterility. 



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 

 different conditions, or which have slightly varied, give vigour 

 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 con- 

 siderable 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 



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