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.
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