PRINCIPLES OF PROTECTIVE COLOURATION 213 



With the governing purpose then of making living forms 

 indistinguishable from their normal background, the develop- 

 ment of animal colouration has, according to Thayer, been, so 

 to speak, directed by three great principles, which he calls 

 respectively those of " Countershading," those of the " Averaging 

 of the Background " and those of the " Obliteration of Outline." 



In these three principles lies the key to the whole subject. 

 The principle of Countershading is Thayer's cardinal discovery. 

 Though his book is called Concealing Coloration it is one of 

 his main contentions that " shading is really more important 

 than colour, because it is primarily an attribute of form, while 

 colour is only secondarily so." " Make the rounded body of a 

 creature look flat and, however it is coloured, it will lose much 

 of its conspicuousness." It is then for this reason that "animals 

 are painted by Nature darkest in those parts which tend to be 

 most lighted by the sky's light and vice versa. 1 ' Thayer insists 

 that the fact that the great majority of living species are shaded 

 more heavily on the back and lightly on the belly is evidence 

 of the working of this principle. The object is to create " a 

 gradation of shading counter to the gradation of shading which 

 light thrown upon the object would produce and having the 

 same note of gradation," with the result that the object appears 

 " perfectly flat, retaining its length and breadth but losing all 

 appearance of thickness." Thus a new emphasis on the distri- 

 bution of dark and light shading on the animal form is one of 

 the chief results of approaching the problem of colouration from 

 the standpoint of concealment alone. 



The principle of Averaging of the Background again gives 

 a new significance to colouration proper. If the illusion of 

 flatness is to have its full effect, the object must be so marked 

 or mottled as to be indistinguishable from its background. 

 Thayer claims that on animals which need the highest degree 

 of inconspicuousness there has been developed " a sort of com- 

 pound picture of their normal background, a picture seemingly 

 made up of the averaging of innumerable backgrounds." Ac- 

 cording to Thayer then the principle on which protective 

 colouration proper works is far more subtle than is usually 

 assumed. Thought on this subject has in fact been vitiated by 

 too much bias towards the theory of mimicry. The forces really 

 governing colouration, however, are not working towards making 

 the object resemble some other specific object in the back- 



