rmanent, as steps leading to more {52} 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 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 essentially differ from
the term variety, which is given to less distinct and more fluctuating
forms. The term variety, again, in comparison with mere individual
differences, is also applied arbitrarily, and for mere convenience' sake.
{53}

Guided by theoretical considerations, I thought that some interesting
results might be obtained in regard to the nature and relations of the
species which vary most, by tabulating all the varieties in several
well-worked floras. At first this seemed a simple task; but Mr. H. C.
Watson, to whom I am much indebted for valuable advice and assistance on
this subject, soon convinced me that there were many difficulties, as did
subsequently Dr. Hooker, even in stronger terms. I shall rese