gle for existence is
often very keen, and the reasons why the struggle for existence is keen
are four. First, there is the tendency to over-population in many
animals, especially those of low degree. Second, there is the fact that
the scheme of nature involves nutritive chains or successive
incarnations, one animal depending upon another for food, and all in the
long run on plants; thirdly, every vigorous animal is a bit of a
hustler, given to insurgence and sticking out his elbows. There is a
fourth great reason for the struggle for existence, namely, the frequent
changefulness of the physical environment, which forces animals to
answer back or die; but the first three reasons have most to do with the
very common assumption of some sort of disguise. Even when an animal is
in no sense a weakling, it may be very advantageous for it to be
inconspicuous when it is resting or when it is taking care of its young.
Our problem is the evolution of elusiveness, so far at least as that
depends on likeness to surroundings, on protective resemblance to other
objects, and in its highest reaches on true mimicry.

Colour Permanently Like That of Surroundings

Many animals living on sandy places have a light-brown colour, as is
seen in some lizards and snakes. The green lizard is like the grass and
the green tree-snake is inconspicuous among the branches. The spotted
leopard is suited to the interrupted light of the forest, and it is
sometimes hard to tell where the jungle ends and the striped tiger
begins. There is no better case than the hare or the partridge sitting a
few yards off on the ploughed field. Even a donkey grazing in the dusk
is much more readily heard than seen.

The experiment has been made of tethering the green variety of Praying
Mantis on green herbage, fastening them with silk threads. They escape
the notice of birds. The same is true when the brown variety is tethered
on withered herbage. But if the green ones are put on brown plants, or
the brown ones on green plants, the birds pick them off. Similarly, out
of 300 chickens in a field, 240 white or black and therefore
conspicuous, 60 spotted and inconspicuous, 24 were soon picked off by
crows, but only one of these was spotted. This was not the proportion
that there should have been if the mortality had been fortuitous. There
is no doubt that it often pays an animal to be like its habitual
surroundings, like a little piece of scenery if the animal is not
movi