f 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. And, on the
other hand, they agree most closely in 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 the offspring in both
cases is notorious; but some few instances of both hybrids and mongrels
long retaining a uniform character could be given. The variability,
however, in the successive generations of mongrels is, perhaps, greater
than in hybrids.

This greater variability in mongrels than in hybrids does not seem 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 that there has been recent variability;
which would often continue and would augment that arising from the act
of crossing. The slight variability of hybrids in the first generation,
in contrast with that in the succeeding generations, is a curious fact
and deserves attention. For it bears on the view which I have taken of
one of the causes of ordinary variability; namely, that the reproductive
system, from being eminently sensitive to changed conditions of life,
fails under these circumstances to perform its proper function of
producing offspring closely similar in all respects to the parent-form.
Now, hybrids in the first generation are descended from species
(excluding those long cultivated) which have not had their reproductive
systems in any way affected, and they are not variable; but hybrids
themselves have their reproductive systems