100 
STRUCTURE OF FISHES. 
a Fish, or as a Keptile, so complete is the mixture of charac¬ 
ters which it presents. 
88. The class of Fishes is distinguished from all other 
Vertebrata, by the adaptation of the animals composing it to 
breathe by means of water in their adult state, so as to be 
capable of hying in that element only. Like Keptiles, they 
are oviparous and cold-blooded; and in these characters they 
differ completely from the Whales and other Mammals, 
which are, like them, inhabitants of the great deep, but which 
are warm-blooded, viviparous, and air-breathing animals. 
There is a simple external character, by which the members 
of the two classes may be at once distinguished. The animals 
of the Whale tribe are, like fishes, chiefly propelled through 
the water by means of a flattened tail; but in the former the 
tail is flattened horizontally, so that its downward stroke may 
serve to bring the animal to the surface to breathe; whilst in 
Fishes it is flattened vertically, that its strokes from side to 
side may simply propel the fish through the water. A 
flattening or compression of the body is seen more or less 
in almost all fishes, and is intimately connected with the 
nature of their motion through the element they inhabit; as 
it serves the double purpose of diminishing the resistance 
which is offered to their progress, and of increasing the extent 
of the oar-like surface, by the lateral stroke of which the 
body is propelled forwards (Chap. xn.). This stroke is given 
by- a series of muscles of great power, which pass from the 
prolonged extensions of one vertebra to those of another, and 
altogether make up the principal part of the bulk of the 
animal. The fins which represent the limbs are not so much 
used in propelling the Fish, as in changing its direction 
either laterally or vertically. Thus in the lowest group of 
the Yertebrated series, the act of motion is chiefly performed 
by the vertebral column itself, instead of being committed to 
the limbs, as in Mammals, Birds, and most Keptiles. The 
larger number of Fishes swim with great activity; and their 
lives may be said to be passed in seeking their subsistence 
and in flying from their enemies. 
89. Fishes are for the most part very voracious, and their 
food consists in great part of the members of their own class. 
In seeking it, they appear to be chiefly guided by the sight; 
for their eyes are usually large and highly developed, while 
