ural 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 physical
conditions at least 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 {104} 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 believe that occasional intercrosses take place
with all animals and with all plants. Even if these take place only at long
intervals, I am convinced that the young thus produced will gain so much in
vigour and fertility over the offspring from long-continued
self-fertilisation, that they will have a better chance of surviving and
propagating their kind; and thus, in the long run, the influence of
intercrosses, even at rare intervals, will be great. If there exist organic
beings w