502 PROPULSION OF WHALES AND FISHES BY TAIL. 



the flat surface, the boat is driven to the depth of many fa- 

 thoms with the swiftness of an arrow. Hence this tail is a 

 most efficient instrument for the propulsion of the bulky 

 body of the Whale through the water ; and it is, in fact, its 

 principal organ of locomotion. The paddles formed by the 

 fore-feet are placed near the centre of gravity of the whole 

 mass ; and thus can readily exert their peculiar action, which 

 is that of changing the direction of the movement, and espe- 

 cially of raising and lowering the body. 



666. The propulsion of the body by the stroke of the tail 

 in Whales and Fishes, is effected precisely in the same manner 

 as the urging-forwards a boat through the water, by the 



Fig. 243. SKELETON OF PERCH. 



lateral strokes of an oar at the stern, in the mode commonly 

 termed sculling. The expansion of the Whale's tail-fin being 

 horizontal, its stroke is vertical, and may thus readily bring 

 the animal to the surface of the water for occasional respira- 

 tion, as well as propel it forwards ; but that of the Fish's 

 body and tail being vertical, its stroke is horizontal, and its 

 action will simply be to urge the body through the water. 

 The power of ascending and descending, as well as of changing 

 the direction of the motion, is principally due to the side-fins, 

 which represent the arms and legs. The direction of the 

 surface and stroke of these side-fins varies in different species. 

 In the Cod, Halibut, and others, their action appears to be 

 principally directed towards keeping the body in its right 

 position in the water; since, without such an action, the body 

 would be liable to turn over, in consequence of the position 



