Chap. IX.] OF FIEST CEOSSES AND OF HYBEIDS. 27 



sterility of hybrids being caused by two constitutions 

 being compounded into one has been strongly main- 

 tained by Max Wichura. 



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 

 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 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, 

 &c., 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 inchviduals of the 

 same species, which differ to a certain extent, gives 

 vigour and fertility to the offspring; and that close 



