eties of many tints may have occurred, _yet those
races having colours best adapted to concealment from their enemies
would inevitably survive the longest_. We have also here an acting cause
to account for that balance so often observed in nature,--a deficiency
in one set of organs always being compensated by an increased
development of some others--powerful wings accompanying weak feet, or
great velocity making up for the absence of defensive weapons; for it
has been shown that all varieties in which an unbalanced deficiency
occurred could not long continue their existence. The action of this
principle is exactly like that of the centrifugal governor of the steam
engine, which checks and corrects any irregularities almost before they
become evident; and in like manner no unbalanced deficiency in the
animal kingdom can ever reach any conspicuous magnitude, because it
would make itself felt at the very first step, by rendering existence
difficult and extinction almost sure soon to follow. An origin such as
is here advocated will also agree with the peculiar character of the
modifications of form and structure which obtain in organized
beings--the many lines of divergence from a central type, the increasing
efficiency and power of a particular organ through a succession of
allied species, and the remarkable persistence of unimportant parts,
such as colour, texture of plumage and hair, form of horns or crests,
through a series of species differing considerably in more essential
characters. It also furnishes us with a reason for that "more
specialized structure" which Professor Owen states to be a
characteristic of recent compared with extinct forms, and which would
evidently be the result of the progressive modification of any organ
applied to a special purpose in the animal economy.

_Conclusion._

We believe we have now shown that there is a tendency in nature to the
continued progression of certain classes of _varieties_ further and
further from the original type--a progression to which there appears no
reason to assign any definite limits--and that the same principle which
produces this result in a state of nature will also explain why domestic
varieties have a tendency, when they become wild, to revert to the
original type. This progression, by minute steps, in various
directions, but always checked and balanced by the necessary conditions,
subject to which alone existence can be preserved, may, it is believed,
be