€hap. IX. of first Crosses and of Hybrids. 251 



by us ; in the other case, or that of hybrids, the external conditions 

 have remained the same, but the organisation has been disturbed 

 by two distinct structures and constitutions, including of course the 

 reproductive systems, having been blended into one. For it is 

 scarcely possible that two organisations should be compounded 

 into one, without some disturbance occurring in the development, or 

 periodical action, or mutual relations of the different parts and 

 organs one to another or to the conditions of life. When hybrids 

 are able to breed inter se, they transmit to their offspring from 

 generation to generation the same compounded organisation, and 

 hence we need not be surprised that their sterility, though in some 

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

 being generally the result, as before explained, of too close inter- 

 breeding. The above view of the sterility of hybrids being caused 

 by two constitutions being compounded 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 fertility 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 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. 

 Ap;ain, 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 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. 



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