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Show 216 INSTINCT. [CnAP. VII. liar Inanner as does our British thrush: how it is that the male wrens (Troglodytes) of North America, .ht~ild "c?cknests" to roost in, like the males of our distinct Kittywren~,- a habit wholly unlike that. of any otl:er ~nown bird. Finally, it may not be a ~o~wal deduction, but to my imagination it is far 1nore satisfactory to loo~r at such instincts as the young cuckoo ejecting. its foster . brothers, -ants making slaves,-the larvre of Ichneun1onidre feeding within the live bodies of caterpillars,-not as especially endowed or created instincts, but as small consequences of one general law, leading to the advancement of all organic beings, namely, multiply, vary, let the strongest live and the weakest die. OnAP. VIII.] HYBRIDISM. 217 CHAPTER VIII. HYBRIDISM. Distinction between the sterility of first crosses and of hybrids-Sterility various in degree, not universal, affected by close interbreeding, removed by domesticationLaws governing the sterility of hybrids-Sterility not a special endowment, but incidental on other difference!-Causes of the sterility of first crosses and of hybrids-Parallelism between the effects of changed, conditions of life and crossing- Fertility of varieties when crossed and of their mongrel offspring not universal- Hybrids and mongrels compared independently of their fertility-Sum· mary. THE view generally entertained by naturalists is that species, when intercrossed, have been specially endowed with the quality of sterility, in order to prevent the confusion of all organic forms. This view certainly seems at first probable, for s~e~ies within the same country c~uld hardly have kept distinct had they been capable of crossing freely. The importance of the fact that hybrids are very generally sterile, has·, I think, been much underrated by some late writers. On the theory of natural selection the case is especially important, inasmuch as the sterility of hybrids could not possibly be of any advantage to them, and therefore could not have been acquired by the continued preservation of successive profitable degrees of sterility. I hope, however, to be able to show that sterility is not a specially acquired or endowed quality, but is incidental on other acquired differences. In treating this subject, two classes of facts, to a large extent fundamentally different, have generally been confounded together; namely, the sterility of two species when first crossed, and the sterility of the hybrids produced from thein. Pure species have of course their organs of reproduction in a perfect condition, yet when intercrossed they 10 |