DISTRIBUTION OF FISHES.- ORDER SELACIIII. 29 



food even to the depth of several hundred feet ; and, as there is 

 no obstacle in the water to bar the movements of Fish, we see 

 that their pasture-grounds are almost unlimited in extent. 

 Different tribes are formed to inhabit different situations ; thus 

 we have some Fish adapted to live in fresh-water only, others 

 which can only live in salt-water, and others, again, which can 

 pass from the one to the other without inconvenience. Of fresh- 

 water fishes, some are the inhabitants of rivers, others of lakes, 

 whilst others prefer small streams. Of the marine fishes, some 

 keep near the shore, whilst others pass most of their lives in the 

 open sea ; some, again, float near the surface ; whilst others 

 never rise much above the bottom. It is probable that we 

 might regard the deeper parts of the ocean as divided (so to 

 speak) into strata., each tenanted by certain species of fish ; for 

 it is quite certain, that there is a particular range of depth, in 

 which each species is usually found, and beyond which it seldom 

 strays, either towards the surface or the bottom of the ocean. 

 It is among the Fishes of shallow waters, and those which 

 habitually tenant the higher stratum of the deep seas, that we 

 find the most beautiful display of colours ; those of deep waters 

 are for the most part comparatively dull in tint. This differ- 

 ence is probably due to the absence of light, which seems neces- 

 sary to the development of the most brilliant colours ; for the 

 diminution in the intensity of the sun's rays, as they traverse 

 water, is very rapid. What may be the absolute depth of water, 

 at which all light ceases, and death and darkness reign, we have 

 no direct means of ascertaining. It varies, of course, with the 

 latitude ; since, the more obliquely the sun's rays fall upon the 

 water, the less will be the depth to which they will penetrate. 



ORDER I. SELACHII. 



575. Although Cuvier, taking the degree of ossification of 

 the skeleton as his guide, placed the Selachii with the rest of the 



