in 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 



