In the
second place, some 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.
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