Ill PROTECTIVE RESEMBLANCES AMONG ANIMALS 71 
to inquire 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 organisation. 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 with- 
out 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. We can hardly see the possibility of a mimicry 
by which the elk could escape from the wolf, or the buffalo 
from the tiger. There is, however, in one group of Verte- 
brata such a general similarity of form, that a very slight 
modification, if accompanied by identity of colour, would 
produce the necessary amount of resemblance ; and at the 
same time there exist a number of species which it would be 
advantageous for others to resemble, since they are armed 
with the most fatal weapons of offence. We accordingly find 
