ystematist, as of the highest 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 toward more strongly marked
and permanent varieties; and at the latter, as leading to sub-species,
and then to species. The passage from one stage of difference to another
may, in many cases, be the simple result of the nature of the organism
and of the different physical conditions to which it has long been
exposed; but with respect to the more important and adaptive characters,
the passage from one stage of difference to another may be safely
attributed to the cumulative action of natural selection, hereafter
to be explained, and to the effects of the increased use or disuse
of parts. A well-marked variety may therefore be called an incipient
species; but whether this belief is justifiable must be judged by the
weight of the various facts and considerations to be given throughout
this work.

It need not be supposed that all varieties or incipient species attain
the rank of species. They may 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,
and with plants by Gaston de Saporta. 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 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, for convenience sake.

WIDE-RANGING, MUCH DIFFUSED, AND COMMON SPECIES VARY MOST.

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.