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
individuals of the same species, or of the same variety, true and
uniform in character. It will obviously thus act far more efficiently
with those animals which unite for each birth; but I have already
attempted to show that we have reason to belie