214 COLOUR IN NATURE 



Coloration of Flat-fishes 



It is impossible to speak of the coloration of 

 fishes without touching, however briefly, on the 

 classical problems connected with the coloration of 

 flat-fishes. As is well known, these fishes are much- 

 modified forms, being flattened from side to side, 

 and having the upper surface, that is one of 

 the sides, coloured usually in relatively dull tints, 

 while the actual lower surface, strictly the other 

 side, is of a silvery white colour without pigment 

 According to many naturalists, the colours of the 

 upper surface are protective, the lower surface is 

 without pigment, because it is usually concealed in 

 these ground fishes, and therefore colour, if present, 

 would be useless. Others, notably Mr. Cunningham, 

 regard this explanation as totally inadequate, and 

 hold that the absence of colour on the lower surface 

 is due directly to the absence of light, or rather was 

 so due in the first instance. To use Mr. Cunning- 

 ham's own words (1893, a) : "The disappearance of 

 the pigment from the lower side in the normal flat- 

 fish is an hereditary character, not due to the with- 

 drawal of the action of light in the individual. . 

 On the other hand, the fact that the pigment, after 

 prolonged action of the light, actually reappears, is 

 strong evidence that originally, in the beginning of 

 the evolution, the pigment disappeared, in conse- 

 quence of the withdrawal of the lower sides from 

 the action of the light. If so, an acquired character 

 has become hereditary." We cannot here discuss 

 the main question of the inheritance of acquired 



