e these begin to arise, we must
remember that first of all they start of necessity from very slight and
indefinite resemblances, which succeed as it were by accident in
occasionally eluding the vigilance of enemies. Thus, there are stick
insects which only look like long round cylinders, not obviously
stick-shaped, but rudely resembling a bit of wood in outline only. These
imperfectly mimetic insects may often obtain a casual immunity from
attack by being mistaken for a twig by birds or lizards. There are
others, again, in which natural selection has gone a step further, so as
to produce upon their bodies bark-like colouring and rough patches which
imitate knots, wrinkles, and leaf-buds. In these cases the protection
given is far more marked, and the chances of detection are
proportionately lessened. But sharp-eyed birds, with senses quickened by
hunger, the true mother of invention, must learn at last to pierce such
flimsy disguises, and suspect a stick insect in the most
innocent-looking and apparently rigid twigs. The final step, therefore,
consists in the production of that extraordinary actor, the _Xeroxylus
laceratus_, whose formidable name means no more than 'ragged dry-stick,'
and which really mimics down to the minutest particular a broken twig,
overgrown with mosses, liverworts, and lichens.

Take, on the other hand, the well-known case of that predaceous mantis
which exactly imitates the white ants, and, mixing with them like one of
their own horde, quietly devours a stray fat termite or so, from time to
time, as occasion offers. Here we must suppose that the ancestral mantis
happened to be somewhat paler and smaller than most of its
fellow-tribesmen, and so at times managed unobserved to mingle with the
white ants, especially in the shade or under a dusky sky, much to the
advantage of its own appetite. But the termites would soon begin to
observe the visits of their suspicious friend, and to note their
coincidence with the frequent mysterious disappearance of a
fellow-townswoman, evaporated into space, like the missing young women
in neat cloth jackets who periodically vanish from the London suburbs.
In proportion as their reasonable suspicions increased, the termites
would carefully avoid all doubtful looking mantises; but, at the same
time, they would only succeed in making the mantises which survived
their inquisition grow more and more closely to resemble the termite
pattern in all particulars. For any m