388 THE INFLUENCE OF LIVING SUKEOUNDINGS. 



control a mode of colouring that has originated \>j a physio- 

 logical process, by no means proves that in every case without 

 exception the distribution of colouring in the animal Vrorld 

 must have originated in the same manner, any moi"e than the 

 established fact that chlorophyll is formed in most green-leaved 

 plants under the influence of light, alters the other fact that in 

 certain cases, as in the Coniferse, the same matter can be elabo- 

 rated in the dark. 



A very striking instance of protective colouring is exhibited 

 in the so-called Chromatic Function which has only recently 

 been made the subject of exact investigation. It has already 

 been discussed in the chapter on the influence of light (see 

 p. 92 and note ^''). It consists in the power possessed by many 

 Fishes, Crustaceans, Amphibia, and Reptiles, of adapting their 

 general colouring — often by extremely rapid alternations — ^to 

 the colouring of the surrounding objects, so that they seem to be 

 helped by it in the pursuit of their prey, or especially protected 

 against the attacks of their enemies; 



Hence it is perfectly evident that all such adaptations of 

 colouring to that of surrounding objects must be a powerful 

 instrument of selection. Those individuals which are best 

 qualified in this respect must have a conspicuous advantage 

 over their less well-fitted companions in the struggle for exist- 

 ence. Thus every cause which might give a species the capa- 

 bility of rapidly assuming certain changes of the colour of the 

 skin by a contraction of the chromatophores, would indirectly be 

 the cause of a further perfecting of this capability by natural 

 selection ; but this selection could not come into play till the 

 contractile power of the chromatophores was actually existent 

 and a protective mode of colouring was already produced by it. 

 Neither selection, nor the struggle for existence, could in this 

 case, any more than in ?i-ny other, by itself effect a modification of 

 the functions or of the morphological peculiarities of an animal. 

 And the same is evidently the case in those instances, now to 

 be considered, of protective resemblance in form and colour 

 which is commonly known by the name of mimicry. 



The mimicry or imitation of one animal by another. — Bates 

 and Wallace gave the name of mimicry to all those cases of 



