THE WHALES. 33 



be moved from side to side, or up and down, accord- 

 ing to the will of the animal. The end of the body is 

 flattened out into a broad and powerful tail, popularly 

 called the "flukes/' This tail, though shaped like 

 that of a fish, is differently set. When a fish is in its 

 natural position, the tail is perpendicular, and a for- 

 ward movement is obtained by moving it from side to 

 side ; but, in the whales, the tail is set horizontally, 

 so that when the animal is at rest, it lies flatly upon 

 the surface of the water. The forward movement is 

 therefore obtained, not by sweeping the tail from side 

 to side, but by moving it up and down. It is believed 

 that this modification of structure is due to the habits 

 of the whale, which must possess the power of diving 

 rapidly. In so doing, it strikes the water with its tail, 

 so as to throw itself partly into the air, and then dives 

 almost perpendicularly, the flukes waving in the air as 

 it descends. So powerful is this instrument of pro- 

 pulsion, that in spite of its enormous weight, a whale 

 measuring a hundred feet in length can throw itself 

 completely out of the water, and can urge itself along 

 the surface at a rate of some fifteen miles an hour. 



The organs of hearing are curiously modified when 

 compared with those of the terrestrial mammalia. 

 Water is a much better conductor of sound than air ; 

 and, if the ear of the whale were constructed like that 

 of a land animal, a single blow struck on the water 

 by the flukes of one whale, would stun all the rest in 

 the immediate neighbourhood. 



This transmissive power of water is utilized by 

 beaver-hunters. In the winter-time a hunter some- 

 times sees a beaver attempting to escape by swimming 



3 



