214 
COLOUR IN NATURE 
CHAP. 
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 
