ichneumonid feeding within the live bodies of caterpillars, not as
specially 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.
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 domestication. Laws governing the sterility
of hybrids. Sterility not a special endowment, but incidental on other
differences. 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. Summary.
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 species within the same
country could 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;
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