VI COLORATION 1 7 5 



limited range of vision, even in the clearest water, would render 

 coloration unsuitable for this purpose. Eecognition sounds are 

 likely to be far more effective, and there is evidence of their 

 production by a special vocal mechanism in many Fishes.^ 



The examples given above show how natural selection may 

 lead to the evolution of distinctive forms of coloration which are 

 advantageous to the Fish either for concealment, aggression, or 

 protection, and in conclusion it may be pointed out that by the 

 same cause colour may be eliminated or its development checked 

 if in any way harmful to the animal ; and further, that if a par- 

 ticular coloration becomes useless to the Fish by reason of a 

 change in its habits or environment, natural selection ceasing to 

 act where its intervention is no longer necessary to maintain the 

 coloration, the latter will sooner or later tend to disappear. 



The absence of pigment is sometimes protective. The surface- 

 swimming larvae of many Teleosts have no chromatophores, and 

 therefore no obvious pigmentary colours. Their bodies are so 

 translucent that they can be seen through, and hence are visible 

 only with difficulty. The transparency of the body may even be 

 increased by the absence of the red haemoglobin of the blood, as is 

 the case with the pelagic Leptocephalus-lenYae of the Eel.^ The 

 iridocytes of the reflecting tissue may also disappear under the 

 influence of changed surroundings. The larvae of various species 

 of Onus (Gadidae) are silvery in hue during their pelagic 

 career, owing to the presence of iridocytes in the skin, but on 

 becoming mature they change to a dull dark colour, and live under 

 stones or in holes and crevices in the rocks. During the change 

 of habit the reflecting tissue (argenteum) is lost, and the needful 

 chromatophores are acquired.^ 



Iftstances of the loss of pigmentary colours, owing to the 

 cessation of the controlling influence of natural selection, are to be 

 found in the absence of chromatophores on the white under surface 

 of the Fiat-Fishes, where such colours are useless but not 

 necessarily harmful, and in the colourless, cave-inhabiting Fishes, 

 of which the Blind-Fish {Amblyopsis) of North America may be 

 taken as an example. 



^ See p. 364. ^ E. Ray Lankester, Proc. Roy. Soc. 1873, p. 70. 



' Cunningham and MaoMunn, op. cit. p. 781. 



