Hence the reproductive system of this one variety must have been in some manner and in some degree modified.

>From these facts it can no longer be maintained that varieties when crossed are invariably quite fertile.

From the great difficulty of ascertaining the infertility of varieties in a state of nature, for a supposed variety, if proved to be infertile in any degree, would almost universally be ranked as a species; from man attending only to external characters in his domestic varieties, and from such varieties not having been exposed for very long periods to uniform conditions of life; from these several considerations we may conclude that fertility does not constitute a fundamental distinction between varieties and species when crossed.

The general sterility of crossed species may safely be looked at, not as a special acquirement or endowment, but as incidental on changes of an unknown nature in their sexual elements.

Independently of the question of fertility, the offspring of species and of varieties when crossed may be compared in several other respects.