.

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, 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 of crossing--Dimorphism and 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.

The subject is in many ways important for us, more especially as the sterility of species when first crossed, and that of their hybrid offspring, cannot have been acquired, as I shall show, by the preservation of successive profitable degrees of sterility.

It is an incidental result of differences in the reproductive systems of the parent-species.
In treating this subject, two classes of facts, to a large extent fundamentally different, have generally been confounded; namely, the sterility of species when first crossed, and the sterility of the hybrids produced from them.
Pure species have of course their organs of reproduction in a perfect condition, yet when intercrossed they produce either few or no offspring.