The Descent of Man

Von Baer has defined advancement or progress
in the organic scale better than any one else, as
resting on the amount of differentiation and
specialisation of the several parts of a being,—
when arrived at maturity, as I should be inclined
to add. Now as organisms have become
slowly adapted to diversified lines of life by
means of natural selection, their parts will have
become more and more differentiated and specialised
for various functions from the advantage
gained by the division of physiological
labour. The same part appears often to have
been modified first for one purpose, and then
long afterwards for some other and quite distinct
purpose; and thus all the parts are rendered
more and more complex. But each organism
still retains the general type of structure of
the progenitor from which it was aboriginally
derived. In accordance with this view it seems,
if we turn to geological evidence, that organisation
on the whole has advanced throughout the
world by slow and interrupted steps. In the