vI COLORATION 175 
limited range of vision, even in the clearest water, would render 
coloration unsuitable for this purpose. Recognition 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- 
swunming 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-larvae 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.° 
Instances 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 Flat-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. 
1 See p. 364. 2 E. Ray Lankester, Proc. Roy. Soc. 1873, p. 70. 
3 Cunningham and MacMunn, op. cit. p. 781. 
