20G CA USES OF THE STERILITY 



some disturbance occurring in the development, or periodi- 

 cal 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 com- 

 pounded organization, 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 gener- 

 ally the result, as before explained, of too close interbreed- 

 ing. 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 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 dis- 

 turbed 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 inter- 

 breeding 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. 



