the common rabbit will crouch as steadily as the
normal brownish-gray type, notwithstanding that, owing to its abnormal
colour, a "nigger-rabbit" thus renders itself the most conspicuous
object in the landscape. In all such cases, of course, there has been a
deviation from the normal type in respect of colour, with the result
that the inherited instinct is no longer in tune with the other
endowments of the animal. Such a variation of colour, therefore, will
tend to be suppressed by natural selection; while any variations which
may bring the animal still more closely to resemble its habitual
surroundings will be preserved. Thus we can understand the truly
wonderful extent to which this principle of protective colouring has
been carried in many cases where the need of it has been most urgent.

Not only colour, but structure, may be profoundly modified for the
purposes of protective concealment. Thus, caterpillars which resemble
twigs do so not only in respect of colour, but also of shape; and this
even down to the most minute details in cases where the adaptation is
most complete: certain butterflies and leaf-insects so precisely
resemble the leaves upon which, or among which, they live, that it is
almost impossible to detect them in the foliage--not only the colour,
the shape, and the venation being all exactly imitated, but in some
cases even the defects to which the leaves are liable, in the way of
fungoid growths, &c. There are other insects which with similar
exactness resemble moss, lichens, and so forth. A species of fish
secures a complete resemblance to bunches of sea-weed by a frond-like
modification of all its appendages, and so on through many other
instances. Now, in all such cases where there is so precise an
imitation, both in colour and structure, it seems impossible to suggest
any other explanation of the facts than the one which is supplied by
Mr. Darwin's theory--namely, that the more perfect the resemblance is
caused to become through the continuous influence of natural selection
always picking out the best imitations, the more highly discriminative
becomes the perception of those enemies against the depredations of
which this peculiar kind of protection is developed; so that, in virtue
of this action and re-action, eventually we have a degree of imitation
which renders it almost impossible for a naturalist to detect the animal
when living in its natural environment.

[Illustration: FIG. 110.--Im