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in PROTECTIVE RESEMBLANCES AMONG ANIMALS 71to inquire ifanything of the same kind is to be observedamong vertebrated animals. When we consider all theconditions necessary to produce a good deceptive imitation,we shall see at once that such can very rarely occur in thehigher animals, since they possess none of those facilities forthe almost infinite modifications of external form which existin the very nature of insect organisation. The outer coveringof insects being more or less solid and horny, they are capableof almost any amount of change of form and appearance withoutany essential modification internally. In many groupsthe wings give much of the character, and these organs maybe much modified both in form and colour without interferingwith their special functions. Again, the number of species ofinsects is so great, and there is such diversity of form andproportion in every group, that the chances of an accidentalapproximation in size, form, and colour of one insect toanother 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 ofthose varieties only which tend in the right direction.In the Vertebrata, on the contrary, the skeleton beinginternal, the external form depends almost entirely on theproportions and arrangement of that skeleton, which again isstrictly adapted to the functions necessary for the well-beingof the animal. The form cannot, therefore, be rapidly modifiedby variation, and the thin and flexible integument will notadmit of the development of such strange protuberances asoccur continually in insects. The number of species of eachgroup in the same country is also comparatively small, andthus the chances of that first accidental resemblance whichis necessary for natural selection to work upon are muchdiminished. We can hardly see the possibility of a mimicryby which the elk could escape from the wolf, or the buffalofrom the tiger. There is, however, in one group of Vertebratasuch a general similarity of form, that a very slightmodification, if accompanied by identity of colour, wouldproduce the necessary amount of resemblance ;and at thesame time there exist a number of species which it would beadvantageous for others to resemble, since they are armedwith the most fatal weapons of offence. We accordingly find