een drawn between
species and sub-species--that is, the forms which in the opinion of some
naturalists come very near to, but do not quite arrive at the rank of
species; or, again, between sub-species and well-marked varieties, or
between lesser varieties and individual differences. These differences
blend into each other in an insensible series; and a series impresses
the mind with the idea of an actual passage.

Hence I look at individual differences, though of small interest to
the systematist, as of high importance for us, as being the first step
towards such slight varieties as are barely thought worth recording
in works on natural history. And I look at varieties which are in any
degree more distinct and permanent, as steps leading to more strongly
marked and more permanent varieties; and at these latter, as leading to
sub-species, and to species. The passage from one stage of difference
to another and higher stage may be, in some cases, due merely to the
long-continued action of different physical conditions in two different
regions; but I have not much faith in this view; and I attribute the
passage of a variety, from a state in which it differs very slightly
from its parent to one in which it differs more, to the action of
natural selection in accumulating (as will hereafter be more fully
explained) differences of structure in certain definite directions.
Hence I believe a well-marked variety may be justly called an incipient
species; but whether this belief be justifiable must be judged of by
the general weight of the several facts and views given throughout this
work.

It need not be supposed that all varieties or incipient species
necessarily attain the rank of species. They may whilst in this
incipient state become extinct, or they may endure as varieties for very
long periods, as has been shown to be the case by Mr. Wollaston with the
varieties of certain fossil land-shells in Madeira. If a variety were
to flourish so as to exceed in numbers the parent species, it would then
rank as the species, and the species as the variety; or it might come to
supplant and exterminate the parent species; or both might co-exist, and
both rank as independent species. But we shall hereafter have to return
to this subject.

From these remarks it will be seen that I look at the term species,
as one arbitrarily given for the sake of convenience to a set of
individuals closely resembling each other, and that it does not
es