if infertile in any degree would generally be ranked as species; from
man selecting only 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. 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.
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