CAUSES OF THE STERILITY 317 



tinct 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 occur- 

 ring 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 dim.inish ; it is even apt to increase, 

 this being generally the result, as before explained, of too 

 close interbreeding. The above view of the sterility of hy- 

 brids 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 hy- 

 brids produced from reciprocal crosses ; or the increased ster- 

 ility 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 ex- 

 planation is offered why an organism, when placed under nat- 

 ural 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 dif- 

 ferent class of facts. It is an old and almost universal be- 

 lief founded on a considerable body erf 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 individuals of the same spe- 



