100 MIMICRY, AND OTHER PROTECTIVE — 
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 en- 
tirely on the proportions and arrangement of that 
skeleton, which again is strictly adapted to the fune- 
tions 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. 
