Chap. IX.] OF FIRST CROSSES AND OF HYBRIDS. 27 



It must, however, be owned that we cannot under- 

 stand, on the above or any other view, several facts 

 with respect to the sterility of hybrids; for instance, 

 the unequal fertility of hybrids produced from recipro- 

 cal 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 explana- 

 tion is offered why an organism, when placed under un- 

 natural 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 dis- 

 turbed bytwo 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 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, &c., from one soil 

 or climate to another, and back again. During the con- 

 valescence of animals, great benefit is derived from al- 

 most 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 vigour and fer- 

 tility to the offspring; and that close interbreeding 

 continued during several generations between the near- 

 est relations, if these be kept under the same conditions 

 of life, almost always leads to decreased size, weakness, 

 or sterility. 



