e
first place, be clearly shown that mere external dissimilarity between two
species does not determine their greater or lesser degree of sterility when
crossed; and we may apply the same rule to domestic varieties. 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 {270} 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. Gaertner 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 on