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
moving. It is safe to say that in process of time wide departures from
the safest coloration will be wiped out in the course of Nature's
ceaseless sifting.
But we must not be credulous, and there are three cautions to be borne
in mind. (1) An animal may be very like its surroundings without there
being any protection implied. The arrow-worms in the sea are as clear as
glass, and so are many open-sea animals. But this is because their
tissues are so watery, with a specific gravity near that of the salt
water. And the invisibility does not save them, always or often, from
being swallowed by larger animals that gather the harvest of the sea.
(2) Among the cleverer animals it looks as if the creature sometimes
sought out a spot where it was most inconspicuous. A spider may place
itself in the middle of a little patch of lichen, where its
self-effacement is complete. Perhaps it is more comfortable as well as
safer to rest in surroundings the general colour of which is like that
of the animal's body. (3) The fishes that live among the coral-reefs are
startling in their brilliant coloration, and there are many different
patterns. To explain this it has been suggested that these fishes are so
safe among the mazy passages and endless nooks of the reefs, that they
can well afford to wear any colour that suits their constitution. In
some cases this may be true, but naturalists who have put on a diving
suit and walked about among the coral have told us that each kind of
fish is particularly suited to some particular place, and that some are
suited for midday work and others for evening work. Sometimes there is a
sort of Box and Cox arrangement by which two different fishes utilise
the same corner at different times.
[Illustration: THE PRAYING MANTIS (_Mantis Religiosa_)
A very voracious insect with a quiet, unobtrusive appearance. It holds
its formidable forelegs as if in the attitude of prayer; its movements
are very slow and stealthy; and there is a suggestion of a leaf in the
forewing. But there is no reason to credit the creature with conscious
guile!]
[Illustration: PROTECTIVE COLORATION: A WINTER SCENE IN NORTH
SCANDINAVIA
Showing Variable Hare, Willow Grouse, and Arctic Fox, all white in
winter and inconspicuous against the snow. But the white dress is also
the dress that is physiologically best, for it loses least of the animal
heat.]
[Illustration: THE VARIABLE MONITOR (_Varanus_)
The monitors are the largest of existing lizards, the Australian species
represented in the photograph attaining a length of four feet. It has a
brown colour with yellow spots, and in spite of its size it is not
conspicuous against certain backgrounds, such as the bark of a tree.]
§ 2