ADAPTATION OF FISHES TO DEPTHS OF WATER. 



Few departments of natural history are more interesting, both in a philosophi- 

 cal and in an economical point of view, than the natural history of fishes. They 

 live in an element which, exclusive of lakes and rivers, covers seven tenths of the 

 surface of our globe ; and they inhabit that element, not merely in the breadth of 

 its surface, as mammalia inhabit the land, but they inhabit it to the depth of a 

 considerable number of fathoms. In consequence of this great' breadth and depth 

 of their pasture, as compared with the pasture of land animals, their numbers, and 

 their powers of keeping up those numbers, are correspondingly great. The shoals 

 of some of the surface fishes, and also of some of the ground ones — as, for in- 

 stance, the common Herring and the Cod, — are numerous beyond all the powers 

 of arithmetic ; and their fertility corresponds, for a single individual of the Cod 

 produces four millions at a birth, and there are many other species scarcely less 

 productive ; while land animals, whether mammalia or birds, are reckoned exceed- 

 ingly prolific if they average a dozen, and some of the more important and highly 

 developed races have very rarely indeed more than one. 



This vast abundance of the finny tribes and the extensive means of keeping up 

 their succession, not only in the individual race, but that the one may supply food 

 for the support of the others, give them a great deal of interest in a philosophical 

 point of view, by showing us how much we are mistaken when we suppose that 

 the waters are the waste places of our globe. There is another consideration : we 

 do not need, generally speaking, to cultivate the waters as we cultivate the land ; 

 or to breed fishes as we breed land animals. It is true that fresh-water fishes, 

 and in some instances salt-water ones also, are bred for domestic purposes ; but 

 this is done more for the gratification of luxury than for economical purposes. 



We need hardly mention that, besides the cartilaginous fishes, which approxi- 

 mate some of the reptiles in some points of their physiology, there are two distinct 

 divisions of true or bony fishes, distinguished from each other by the characters of 

 their fins, or swimming organs. These are acanthopterygii, or fishes which have 

 the rays of the fins in one continuous piece, more or less flexible, but sometimes 

 an absolute spine of bone ; and rnalacopterygii, or fishes which have the rays of 

 the fins jointed, and, generally speaking, of a less bony consistency than those in 

 the others. 



Both of these grand divisions inhabit, in their different genera, different 

 depths of the sea ; but it may be said, that, taking them on the whole, the fishes 

 with spinous rays are the most discursive through the waters, and inhabit nearest 

 the surface. Those with jointed rays to the fins are more divisible according to 

 the grades of depths which they occupy ; and these grades follow pretty closely 

 the arrangement of the fins on the under part of the body. In considering the 

 mechanical action of a fish, it is distinctly to be understood that the tail is the 



