Excerpt from The Origin of Species, Vol. 2 of 2: By Means of Natural Selection, or the Preservation of Favored Races in the Struggle for Life

Hybridism.

Distinction between the sterility of first crosses and of hybrids - Sterility various 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, not accumulated by natural selection - Causes of the sterility of first crosses and of hybrids - Parallelism between the effects of changed conditions of life and crossing - Dimorphism an trimorphism - Fertility of varieties when crossed and of their mongrel offspring not universal - Hybrids and mongrels compared independently of their fertility - Summary.

The view commonly entertained by naturalists is that species, when intercrossed, have been specially endowed with sterility, in order to prevent their confusion. This view certainly seems at first highly probable, for species living together could hardly have been kept distinct had they been capable of freely crossing.

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