ZOOLOGY. 161 



■nished with J^iditating Membranes, by which their eyes are 

 secured from exterual injury, to which they are particularly 

 subject from the velocity of their motions ; not that we con- 

 sider their motions comparately speaking, so rapid as those 

 of many kinds of birds ; as a fish when swimming, needs but 

 little exertion to propel it forward, its specific grayity being 

 so near that of the elemeut in which it moves ; whereas, in 

 the flight of a bird, whose specific gravity greatly exceeds 

 that of air, a very considerable expenditure of exertion is 

 necessary, merely to sustain it in air, without assisting to 

 impel them forward. 



Most of this class are exceedingly voracious ; they feed on 

 the lesser kinds offish, moluscae, aquatic insects, worms, sea 

 weed, and on most kinds of dead carcasses : they are used as 

 food by Man, the larger kinds of birds of prey, the 

 Aquatic Mammaliae, and Amphibeae. Their increase is asto- 

 nishingly great, most species are oviparous, but a few produce 

 their young alive. 



The migration of Fishes has attracted the attention of 

 Mankind from the earliest ages, and like that of Birds, is 

 attributable to the want of proper places, wherein to deposit 

 their eggs orspawn, or to a dearth of food. Those that spawn 

 early in the year, generally deposit their eggs in shallow 

 waters, where the rays of the sun call them into life ; 

 these, as they advance in age, soon quit the shallows, and 

 finally abandon the shore for the season. The diflBculties 

 overcome by various species, when seeking out a place 



