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 of ten-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 the most diverse ways,
sometimes by mere variations in the form of the insect and in its
colour, sometimes by an elaborate marking, like that which occurs in
the Indian leaf-butterflies, _Kallima inachis_. In the single
butterfly-genus Anaea, in the woods of South America, there are about
a hundred species which are all gaily coloured on the upper surface,
and on the reverse side exhibit the most delicate imitation of the
colouring and pattern of a leaf, generally without any indication of
the leaf-ribs, but extremely deceptive nevertheless. Anyone who has
seen only one such butterfl