--has been
repeatedly and easily refuted by all writers on the subject of varieties
and species, and it seems to have been considered that when this was
done the whole question has been finally settled; but the view here
developed renders such an hypothesis quite unnecessary, by showing that
similar results must be produced by the action of principles constantly
at work in nature. The powerful retractile talons of the falcon- and the
cat-tribes have not been produced or increased by the volition of those
animals; but among the different varieties which occurred in the earlier
and less highly organized forms of these groups, _those always survived
longest which had the greatest facilities for seizing their prey_.
Neither did the giraffe acquire its long neck by desiring to reach the
foliage of the more lofty shrubs, and constantly stretching its neck for
the purpose, but because any varieties which occurred among its
antitypes with a longer neck than usual _at once secured a fresh range
of pasture over the same ground as their shorter-necked companions, and
on the first scarcity of food were thereby enabled to outlive them_.
Even the peculiar colours of many animals, especially insects, so
closely resembling the soil or the leaves or the trunks on which they
habitually reside, are explained on the same principle; for though in
the course of ages varieties 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
mod