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 \vings 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 func- 

 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. 



