eminent naturalists believe that a long course of domestication tends
to eliminate sterility in the successive generations of hybrids, which
were at first only slightly sterile; and if this be so, we surely ought
not to expect to find sterility both appearing and disappearing under
nearly the same conditions of life. Lastly, and this seems to me by far
the most important consideration, new races of animals and plants are
produced under domestication by man s methodical and unconscious power
of selection, for his own use and pleasure: he neither wishes to
select, nor could select, slight differences in the reproductive
system, or other constitutional differences correlated with the
reproductive system. He supplies his several varieties with the same
food; treats them in nearly the same manner, and does not wish to alter
their general habits of life. Nature acts uniformly and slowly during
vast periods of time on the whole organisation, in any way which may be
for each creature s own good; and thus she may, either directly, or
more probably indirectly, through correlation, modify the reproductive
system in the several descendants from any one species. Seeing this
difference in the process of selection, as carried on by man and
nature, we need not be surprised at some difference in the result.
I have as yet spoken as if the varieties of the same species were
invariably fertile when intercrossed. But it seems to me 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 with
the 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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