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. Gartner, 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 Gartner 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.
Gartner further admits that hybrids between very closely allied species
are more variable 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; but some few cases both of hybrids and mongrels long
retaining uniformity of character could be given. The variability,
however, in the successive generations of mongrels is, perhaps, greater
than in hybrids.

This greater variability of mongrels than of hybrids does not seem to me
at all surprising. For the parents of mongrels are varieties, and mostly
domestic varieties (very few experiments having been tried on natural
varieties), and this implies in most cases that there has been recent
variability; and therefore we might expect that such variability would
often continue and be super-added to that arising from the mere act of
crossing. The slight degree of variability in hybrids from the first
cross or in the first generation, in contrast with their extreme
variability in the succeeding generations, is a curious fact and
deserves attention. For it bears on and corroborates the view which I
have taken on the cause of ordinary variability; namely, that it is due
to the reproductive system being eminently sensitive to any change in
the conditions of life, being thus often rendered either impotent or at
least i