Yet these varieties differ in no respect, except in the colour of their flowers; and one variety can sometimes be raised from the seed of another.
Kolreuter, whose accuracy has been confirmed by every subsequent observer, has proved the remarkable fact that one particular variety of the common tobacco was more fertile than the other varieties, when crossed with a widely distinct species.

He experimented on five forms which are commonly reputed to be varieties, and which he tested by the severest trial, namely, by reciprocal crosses, and he found their mongrel offspring perfectly fertile.

But one of these five varieties, when used either as the father or mother, and crossed with the Nicotiana glutinosa, always yielded hybrids not so sterile as those which were produced from the four other varieties when crossed with N.glutinosa.
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.
HYBRIDS AND MONGRELS COMPARED, INDEPENDENTLY OF THEIR FERTILITY.
Independently of the question of fertility, the offspring of species and of varieties when crossed may be compared in several other respects.
Gartner, whose strong wish it was to draw a distinct line between species and varieties, could find very few, and, as it seems to me, quite unimportant differences between the so-called hybrid offspring of species, and the so-called mongrel offspring of varieties.