278 CAUSES OF THE STERILITY 



degree variable, does not diminish ; it is even apt to 

 increase, this being generally the result, as before explained, 

 of too close interbreeding. The above view of the sterility 

 of hybrids being caused by two constitutions being com- 

 pounded into one has been strongly maintained by Max 

 Wichura. 



It must, however, be owned that we cannot understand, 

 on the above or any other view, several facts with respect 

 to the sterility of hybrids ; for instance, the unequal fer- 

 tility 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 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 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 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 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 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 



