But one of these five
varieties, when used either as 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; 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 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.
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