28 DISTRIBUTION OF FISHES. FAMILY PERCIDJ3. 



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 difference is 

 probably due to the absence of light, which seems necessary to 

 the development of the most brilliant colours ; for the dimi- 

 nution 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. ACANTHOPTERYGII. 



555. Of this Order, one of the principal families is that of 

 PERCIDJS, or Perches; of which the common Perch of this 

 country may be taken as an example. In this group, the body 

 is oblong, and covered with hard or rough scales ; and the oper- 

 cular bone is edged with spines or small teeth, which do not, 

 however, extend over the cheek. The Perch is one of our most 

 common fresh- water fishes, abounding in rivers, lakes, and 

 ponds, especially such as are clear, and lurking under the banks, 

 or swimming near the surface. It is spread throughout the 



