thus: He adheres to the general theory of Descent, i.e., he
believes the simplest explanation which has yet been offered of the
structural _similarities_ between species within the same group,
is the hypothesis of a common descent from a parent species. But he
emphatically rejects the notion--and this is the quintessence of
Darwinism--that the _dissimilarities_ between species have been
brought about by the purely mechanical agency of natural selection.

To find out what, precisely, Darwin meant by the term "natural
selection" let us turn for a moment, to his great work, _The Origin
of Species by Means of Natural Selection_. In the second chapter of
that work, Darwin observes that small "fortuitous" variations in
individual organisms, though of small interest to the systematist, are
of the "highest importance" for his theory, since these minute
variations often confer on the possessor of them, some advantage over
his fellows in the quest for the necessaries of life. Thus these chance
individual variations become the "first steps" towards slight
varieties, which, in turn, lead to sub-species, and, finally, to
species. Varieties, in fact, are "incipient species." Hence, small
"fortuitous" fluctuating, individual variations--i.e., those which
chance to occur without predetermined direction--are the "first-steps"
in the origin of species. This is the first element in the Darwinian
theory.

In the third chapter of the same work we read: "It has been seen in the
last chapter that amongst organic beings in a state of nature there is
some individual variability. * * * But the mere existence of individual
variability and of some few well-marked varieties, though necessary as
a _foundation_ of the work, helps us but little in understanding
how species arise in nature. How have all those exquisite adaptations
of one part of the organization to another part, and to the conditions
of life, and of one organic being to another being, been perfected?
* * *" Again it may be asked, how is it that varieties, which I have
called incipient species, become ultimately converted into good and
distinct species, which in most cases obviously differ from each other
far more than do the varieties of the same species? How do those groups
of species which constitute what are called distinct genera arise?
All of these results follow from the _struggle for life_. Owing to
this struggle, variations, however slight and from whatever cause
proceeding,