hich term individual differences are always included, will
evidently be favourable. A large number of individuals, by giving a
better chance within any given period for the appearance of profitable
variations, will compensate for a lesser amount of variability in each
individual, and is, I believe, a highly important element of success.
Though nature grants long periods of time for the work of natural
selection, she does not grant an indefinite period; for as all organic
beings are striving to seize on each place in the economy of nature, if
any one species does not become modified and improved in a corresponding
degree with its competitors it will be exterminated. Unless favourable
variations be inherited by some at least of the offspring, nothing can
be effected by natural selection. The tendency to reversion may often
check or prevent the work; but as this tendency has not prevented man
from forming by selection numerous domestic races, why should it prevail
against natural selection?

In the case of methodical selection, a breeder selects for some definite
object, and if the individuals be allowed freely to intercross, his work
will completely fail. But when many men, without intending to alter
the breed, have a nearly common standard of perfection, and all try to
procure and breed from the best animals, improvement surely but slowly
follows from this unconscious process of selection, notwithstanding that
there is no separation of selected individuals. Thus it will be under
nature; for within a confined area, with some place in the natural
polity not perfectly occupied, all the individuals varying in the right
direction, though in different degrees, will tend to be preserved.
But if the area be large, its several districts will almost certainly
present different conditions of life; and then, if the same species
undergoes modification in different districts, the newly formed
varieties will intercross on the confines of each. But we shall see in
the sixth chapter that intermediate varieties, inhabiting intermediate
districts, will in the long run generally be supplanted by one of the
adjoining varieties. 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