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 acantliopterygii , 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 malacopterygii, or fishes which have the rays of 
the fins jointed, and, generally speaking, of a less bony consistency than those in 
the others. 
Roth 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 
