from this unconscious process of selection, notwithstanding a large
amount of crossing with inferior animals. Thus it will be in nature;
for within a confined area, with some place in its polity not so
perfectly occupied as might be, natural selection will always tend to
preserve all the individuals varying in the right direction, though in
different degrees, so as better to fill up the unoccupied place. But if
the area be large, its several districts will almost certainly present
different conditions of life; and then if natural selection be
modifying and improving a species in the several districts, there will
be intercrossing with the other individuals of the same species on the
confines of each. And in this case the effects of intercrossing can
hardly be counterbalanced
by natural selection always tending to modify all the individuals in
each district in exactly the same manner to the conditions of each; for
in a continuous area, the conditions will generally graduate away
insensibly from one district to another. The intercrossing will most
affect those animals which unite for each birth, which wander much, and
which do not breed at a very quick rate. Hence in animals of this
nature, for instance in birds, varieties will generally be confined to
separated countries; and this I believe to be the case. In
hermaphrodite organisms which cross only occasionally, and likewise in
animals which unite for each birth, but which wander little and which
can increase at a very rapid rate, a new and improved variety might be
quickly formed on any one spot, and might there maintain itself in a
body, so that whatever intercrossing took place would be chiefly
between the individuals of the same new variety. A local variety when
once thus formed might subsequently slowly spread to other districts.
On the above principle, nurserymen always prefer getting seed from a
large body of plants of the same variety, as the chance of
intercrossing with other varieties is thus lessened.
Even in the case of slow-breeding animals, which unite for each birth,
we must not overrate the effects of intercrosses in retarding natural
selection; for I can bring a considerable catalogue of facts, showing
that within the same area, varieties of the same animal can long remain
distinct, from haunting different stations, from breeding at slightly
different seasons, or from varieties of the same kind preferring to
pair together.
Intercrossing plays a very important part in nature in keeping the
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