86 THE COLOURS OF ANIMALS 



by the observation of living animals in their natural 

 surroundings. Thus Pouchet noticed that one single 

 plaice out of a large number upon a light sandy 

 surface was dark-coloured, and thus unlike its sur- 

 roundings. Examination showed that this individual 

 was blind, and therefore unable to respond to the 

 stimulus of reflected light. 1 



Another very interesting example of the same kind 

 was brought under my notice by my friend, Mr. H. 

 Nicoll. This gentleman had observed that in addition 

 to the light- coloured trout usually seen in a chalk 

 stream in Hampshire (a tributary of the Test), very 

 dark individuals are occasionally met with. He was 

 puzzled for a long time, but the fact that the dark 

 fish could never be induced to rise to a fly finally led 

 him to examine them, when he found that they were 

 invariably blind, the crystalline lens being opaque. 

 Sometimes the fish were blind in one eye, but this did 

 not affect their colour. The darkness appears to come 

 on gradually with increasing blindness, for the depth 

 of the tint varies in different individuals, and some- 

 times only part of the body (e.g. the tail) is affected. 

 The blindness probably comes on with age, for the 

 dark fish are always large, generally between one and 

 two pounds in weight. 



1 Quoted by Semper, Animal Life, International Scientific Series, 

 pp. 95-9ti. 



