present the variation which was most useful for them. The sable is brown,
but it lives in trees, where the brown colouring protects and conceals it
more effectively. The musk-sheep (Ovibos moschatus) is also brown, and
contrasts sharply with the ice and snow, but it is protected from beasts of
prey by its gregarious habit, and therefore it is of advantage to be
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