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; from the great difficulty of ascertaining the infertility
of varieties in a state of nature, for a supposed variety if infertile in
any degree would generally be ranked as species; from man selecting only
{272} external characters in the production of the most distinct domestic
varieties, and from not wishing or being able to produce recondite and
functional differences in the reproductive system; from these several
considerations and facts, I do not think that the very general fertility of
varieties can be proved to be of universal occurrence, or to form a
fundamental distinction between varieties and species. The general
fertility of varieties does not seem to me sufficient to overthrow the view
which I have taken with respect to the very general, but not invariable,
sterility of first crosses and of hybrids, namely, that it is not a special
endowment, but is incidental on slowly acquired modifications, more
especially in the reproductive systems of the forms which are crossed.

_Hybrids and Mongrels compared, independently of their
fertility._--Independently of the question of fertility, the offspring of
species when crossed and of varieties when crossed may be compared in
several other respects. Gaertner, whose strong wish was to draw a marked
line of distinction 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. And, on the other hand, they agree most closely in very many
important respects.

I shall here discuss this subject with extreme brevity. The most important
distinction is, that in the first generation mongrels are more variable
than hybrids; but Gaertner admits that hybrids from species which have long
been cultivated are often variable in the first generation; and I have
myself seen striking instances of this fact. Gaertner further admits that
hybrids between very closely allied species are more variable {273} than
those from very distinct species; and this shows that the difference in the
degree of variability graduates away. When mongrels and the more fertile
hybrids are propagated for several generations an extreme amount of
variability in their offspring is notorious