266 HYBKIDISM. Chap. VIII. 



be inappreciable 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 different 

 structures and constitutions having been blended into 

 one. For it is scarcely possible that two organisations 

 should be compounded into one, without some disturb- 

 ance occurring in the development, or periodical action, 

 or mutual relation 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, rarely 

 diminishes. 



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

 stand, excepting on vague hypotheses, 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 unna- 

 tural 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 having been compounded into one. 



It may seem fanciful, but I suspect that a similar 

 parallelism extends to an allied yet very different class 

 of facts. It is an old and almost universal belief, 

 founded, I think, on a considerable body of evidence, 

 that slight changes in the conditions of life are bene- 

 ficial to all living things. We see this acted on by 



