Intercrossing will chiefly affect those animals which unite for each birth and wander much, and which do not breed at a very quick rate.

Hence with animals of this nature, for instance birds, varieties will generally be confined to separated countries; and this I find to be the case.

With hermaphrodite organisms which cross only occasionally, and likewise with animals which unite for each birth, but which wander little and can increase at a rapid rate, a new and improved variety might be quickly formed on any one spot, and might there maintain itself in a body and afterward spread, so that the individuals of the new variety would chiefly cross together.