In the case of insectivorous Vertebrata and
their prey such differences exist in an exaggerated form. Cryptic
colouring, whether used for defence or attack, may be either _general_
or _special_. In _general resemblance_ the animal, in consequence of its
colouring, produces the same effect as its environment, but the
conditions do not require any special adaptation of shape and outline.
General resemblance is especially common among the animals inhabiting
some uniformly coloured expanse of the earth's surface, such as an ocean
or a desert. In the former, animals of all shapes are frequently
protected by their transparent blue colour; on the latter, equally
diverse forms are defended by their sandy appearance. The effect of a
uniform appearance may be produced by a combination of tints in
startling contrast. Thus the black and white stripes of the zebra blend
together at a little distance, and "their proportion is such as exactly
to match the pale tint which arid ground possesses when seen by
moonlight" (F. Galton, _South Africa_, London, 1889). _Special
resemblance_ is far commoner than general, and is the form which is
usually met with on the diversified surface of the earth, on the shores,
and in shallow water, as well as on the floating masses of Algae on the
surface of the ocean, such as the Sargasso Sea. In these environments
the cryptic colouring of animals is usually aided by special
modifications of shape, and by the instinct which leads them to assume
particular attitudes. Complete stillness and the assumption of a certain
attitude play an essential part in general resemblance on land; but in
special resemblance the attitude is often highly specialized, and
perhaps more important than any other element in the complex method by
which concealment is effected. In special resemblance the combination of
colouring, shape and attitude is such as to produce a more or less exact
resemblance to some one of the objects in the environment, such as a
leaf or twig, a patch of lichen, or flake of bark. In all cases the
resemblance is to some object which is of no interest to the enemy or
prey respectively. The animal is not hidden from view by becoming
indistinguishable from its background, as in the cases of general
resemblance, but it is mistaken for some well-known object.

In seeking the interpretation of these most interesting and elaborate
adaptations, attempts have been made along two lines. First, it is
sought to expl