COLOURS. XX ill 



COLOURS. 



Intimately related to the tegumentary system and the composition of scales 

 is the subject of colour. Among the finny tribes we may perceive external 

 colours of the most varied description, some of which are permanent, many 

 transient, and others again of periodical occurrence. Some of these colours 

 are due to the influence of light, while they may vary in the same species, 

 owing to the character of the water which they inhabit; for should the latter 

 be opaque and muddy, they as a rule are darker than those obtained from 

 localities where it is clear, those in running streams are generally lighter 

 and brighter than when from stagnant pools, or from shallow pieces of water, 

 than from such as are deep, while fish captured- in dark caverns are often 

 destitute of both colour and vision. 



Age and season likewise exercise an influence in this respect, as do also 

 the state of the health and temporary local emotions. In the very young 

 one sees but few markings or colours, but these rapidly develop themselves, 

 more or less distinctly, by the time/ or even before, the first breeding season 

 has been reached, when the brilliancy of the individual has often attained to 

 its maximum. This nuptial adornment is generally acquired a short time 

 prior to the breeding season, subsequent to which it usually disappears. 

 Some see in the colours of these creatures grounds for assuming the proba- 

 bility of the descent of many forms from some common ancestral progenitor ; 

 thus vertical bands are almost invariable in the young of the various trout, 

 salmon, and char. However, they are likewise present in many other 

 families, as Scombresocidcv , in which they are usually a diagnostic sign of 

 the immaturity of the individual. Large spots or blotches, especially when 

 surrounded by a light margin, are likewise a good reason for suspecting that 

 the individual has not attained its full size. We also see in some forms 

 longitudinal bands modified, two narrow ones taking the place of a single 

 wide one, or they may be broken up into blotches. Some fish which are of a 

 silvery colour in one district are spotted in another, and others which have 

 no marks on their bodies throughout their lives frequently have the colour 

 of their fins varying with age. 



Pouchet and others have pointed out that the changeable tegumentary 

 colours of fish depend more especially upon two conditions. First, we have 

 iridescence effected by an interference with the rays of light, owing to the 

 presence in the scales of thin plates or ridges, and in these forms the tints 

 change with great rapidity in accordance with the angle at which they are 

 viewed : such lamellar colouring is common among insects, Crustacea, and 

 some fishes it is beautifully seen in the Dolphin, Conjphcena, and the scale 

 of a common herring furnishes a good example. Secondly, a distinct anato- 

 mical element, as chromatophores or colour-sacs, which are often highl 



