itation of keeled scales on the crown produced by the recumbent feet,
as the caterpillar threw itself backward!

The attitudes of many of the tropical spiders are most extraordinary and
deceptive, but little attention has been paid to them. They often mimic
other insects, and some, Mr. Bates assures us, are exactly like flower
buds, and take their station in the axils of leaves, where they remain
motionless waiting for their prey.

_Cases of Mimicry among the Vertebrata._

Having thus shown how varied and extraordinary are the modes in which
mimicry occurs among insects, we have now to enquire if anything of the
same kind is to be observed among vertebrated animals. When we consider
all the conditions necessary to produce a good deceptive imitation, we
shall see at once that such can very rarely occur in the higher animals,
since they possess none of those facilities for the almost infinite
modifications of external form which exist in the very nature of insect
organization. The outer covering of insects being more or less solid
and horny, they are capable of almost any amount of change of form and
appearance without any essential modification internally. In many groups
the wings give much of the character, and these organs may be much
modified both in form and colour without interfering with their special
functions. Again, the number of species of insects is so great, and
there is such diversity of form and proportion in every group, that the
chances of an accidental approximation in size, form, and colour, of one
insect to another of a different group, are very considerable; and it is
these chance approximations that furnish the basis of mimicry, to be
continually advanced and perfected by the survival of those varieties
only which tend in the right direction.

In the Vertebrata, on the contrary, the skeleton being internal the
external form depends almost entirely on the proportions and arrangement
of that skeleton, which again is strictly adapted to the functions
necessary for the well-being of the animal. The form cannot therefore be
rapidly modified by variation, and the thin and flexible integument will
not admit of the development of such strange protuberances as occur
continually in insects. The number of species of each group in the same
country is also comparatively small, and thus the chances of that first
accidental resemblance which is necessary for natural selection to work
upon are much diminished.