make up his own mind; but he will succeed in this at the expense of
admitting much variation, and the truth of this admission will often be
disputed by other naturalists. When he comes to study allied forms brought
from countries not now continuous, in which case he cannot hope to find
intermediate links, he will be compelled to trust almost entirely to
analogy, and his difficulties will rise to a climax.

Certainly no clear line of demarcation has as yet been 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 by 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 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