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The reason why varieties do not usually exhibit a similar amount of
infertility is not difficult to explain. The popular conclusions on this
matter have been drawn chiefly from what occurs among domestic animals,
and we have seen that the very first essential to their becoming
domesticated was that they should continue fertile under changed
conditions of life. During the slow process of the formation of new
varieties by conscious or unconscious selection, fertility has always
been an essential character, and has thus been invariably preserved or
increased; while there is some evidence to show that domestication
itself tends to increase fertility.
Among plants, wild species and varieties have been more frequently
experimented on than among animals, and we accordingly find numerous
cases in which distinct species of plants are perfectly fertile when
crossed, their hybrid offspring being also fertile _inter se_. We also
find some few examples of the converse fact--varieties of the same
species which when crossed are infertile or even sterile.


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