Seite - 317 - in The Origin of Species

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-