rendering it difficult to distinguish from its immediate
environment. In most cases the effect is PROTECTIVE; but in snakes,
spiders, mantids, and other preying animals it is termed
AGGRESSIVE, since it enables these animals to stalk their prey
undetected. It is probable that this power, when possessed by a
vertebrate animal, nearly always bears the double meaning, as in
the green tree frog, where the colouration is protective so far as
it provides concealment from snakes, which are particularly fond of
these frogs, and aggressive in that it allows flies and other
insects to approach without suspicion.

There may be either General Resemblance to surrounding objects or
Special Resemblance to definite objects. The plain sandy colour of
desert animals, the snow white of the inhabitants of the arctic
regions, the inconspicuous hues of nocturnal animals, the stripes
of the tiger and the zebra, the spots of the leopard and the
giraffe have all a cryptic effect which at a very short distance
renders the creatures invisible amid their natural surroundings.
Nor is it necessary in order to attain this invisibility that the
colouring should be really dull and plain. It all depends upon the
habitat. Mr. Wallace has described "a South American goatsucker
which rests in the bright sunshine on little bare rocky islets in
the upper Rio Negro where its unusually light colours so closely
resemble those of the rock and sand that it can scarcely be
detected till trodden upon." A little observation will supply large
numbers of instances of such protective colouration.

It is, however, in the insect world that this principle of
adaptation of animals to their environment is most fully and
strikingly developed. "There are thousands of species of insects,"