CHAPTER IX. HYBRIDISM.
6. FERTILITY OF VARIETIES WHEN CROSSED, AND OF THEIR MONGREL OFFSPRING, NOT UNIVERSAL. (continued)
The real difficulty in our present subject is not, as it appears to me, why
domestic varieties have not become mutually infertile when crossed, but why
this has so generally occurred with natural varieties, as soon as they have
been permanently modified in a sufficient degree to take rank as species.
We are far from precisely knowing the cause; nor is this surprising, seeing
how profoundly ignorant we are in regard to the normal and abnormal action
of the reproductive system. But we can see that species, owing to their
struggle for existence with numerous competitors, will have been exposed
during long periods of time to more uniform conditions, than have domestic
varieties; and this may well make a wide difference in the result. For we
know how commonly wild animals and plants, when taken from their natural
conditions and subjected to captivity, are rendered sterile; and the
reproductive functions of organic beings which have always lived under
natural conditions would probably in like manner be eminently sensitive to
the influence of an unnatural cross. Domesticated productions, on the
other hand, which, as shown by the mere fact of their domestication, were
not originally highly sensitive to changes in their conditions of life, and
which can now generally resist with undiminished fertility repeated changes
of conditions, might be expected to produce varieties, which would be
little liable to have their reproductive powers injuriously affected by the
act of crossing with other varieties which had originated in a like manner.
I have as yet spoken as if the varieties of the same species were
invariably fertile when intercrossed. But it is impossible to resist the
evidence of the existence of a certain amount of sterility in the few
following cases, which I will briefly abstract. The evidence is at least
as good as that from which we believe in the sterility of a multitude of
species. The evidence is also derived from hostile witnesses, who in all
other cases consider fertility and sterility as safe criterions of specific
distinction. Gartner kept, during several years, a dwarf kind of maize
with yellow seeds, and a tall variety with red seeds growing near each
other in his garden; and although these plants have separated sexes, they
never naturally crossed. He then fertilised thirteen flowers of the one
kind with pollen of the other; but only a single head produced any seed,
and this one head produced only five grains. Manipulation in this case
could not have been injurious, as the plants have separated sexes. No one,
I believe, has suspected that these varieties of maize are distinct
species; and it is important to notice that the hybrid plants thus raised
were themselves PERFECTLY fertile; so that even Gartner did not venture to
consider the two varieties as specifically distinct.
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