visible from
as great a distance as possible. That so many species have been able to
give rise to white varieties does not depend on a special sensitiveness
of the skin to the influence of cold, but to the fact that Mammals and
Birds have a general tendency to vary towards white. Even with us, many
birds--starlings, blackbirds, swallows, etc.--occasionally produce white
individuals, but the white variety does not persist, because it readily
falls a victim to the carnivores. This is true of white fawns, foxes,
deer, etc. The whiteness, therefore, arises from internal causes, and
only persists when it is useful. A great many animals living in a
GREEN ENVIRONMENT have become clothed in green, especially insects,
caterpillars, and Mantidae, both persecuted and persecutors.

That it is not the direct effect of the environment which calls forth
the green colour is shown by the many kinds of caterpillar which rest on
leaves and feed on them, but are nevertheless brown. These feed by night
and betake themselves through the day to the trunk of the tree, and hide
in the furrows of the bark. We cannot, however, conclude from this that
they were UNABLE to vary towards green, for there are Arctic animals
which are white only in winter and brown in summer (Alpine hare, and
the ptarmigan of the Alps), and there are also green leaf-insects which
remain green only while they are young and difficult to see on the leaf,
but which become brown again in the last stage of larval life, when they
have outgrown the leaf. They then conceal themselves by day, sometimes
only among withered leaves on the ground, sometimes in the earth itself.
It is interesting that in one genus, Chaerocampa, one species is brown
in the last stage of larval life, another becomes brown earlier, and in
many species the last stage is not wholly brown, a part remaining green.
Whether this is a case of a double adaptation, or whether the green is
being gradually crowded out by the brown, the fact remains that the same
species, even the same individual, can exhibit both variations. The case
is the same with many of the leaf-like Orthoptera, as, for instance, the
praying mantis (Mantis religiosa) which we have already mentioned.

But the best proofs are furnished by those often-cited cases in which
the insect bears a deceptive resemblance to another object. We now know
many such cases, such as the numerous imitations of green or withered
leaves, which are brought about in