156 COMPARATIVE ZOOLOGY. 
progression through water, are the most perfect swimmers, 
Thus, the Salmon can go twenty miles an hour, and even 
’ g by ’ 
Fig. 122.—The Fins of a Fish (Pike-perch). 
ascend cataracts. They have fins of two kinds: those set 
obliquely to the body, and in pairs; and those which are 
vertical, and single. 
The former, called pectoral and ven- 
tral fins, represent the fore and hind limbs of quadrupeds. 
The vertical fins, which are only expansions of the skin, 
vary in number; but in most Fishes there are at least 
three: the caudal, or tail-fin; the dorsal, or back-fin; 
Fra. 123.—Diagram illustrat- 
ing the locomotion of a 
Fish. The tail describes 
the arc of an ellipse; the 
resultant of the two im- 
pulses is the straight line 
in front. 
and the anal, situated on the abdo- 
men, near the tail. The chief loco- 
motive agent is the tail, which sculls 
like a stern-oar; the other fins are 
mainly used to balance and raise the 
body. When the two lobes of the 
tail are equal, and the vertebral col- 
umn stops short at its base, as in the 
Trout, it is said to be homocercal. 
If the vertebrze extend into the up- 
per lobe, making it longer than the 
lower one, as in the Shark, the tail is 
called heterocercal. The latter is the 
more effective for varying the course; 
the Shark, e. g., will accompany and 
gambol around a ship in full sail 
across the Atlantic. The Whale swims by striking the 
