PROGKESSION ON AND IN THE WATER. 



77 



them, to present the palms or flat of the hands to the water 

 the one instant, and the edge or narrow parts the next. In 

 swimming, the anterior or thick margins of the flippers are 

 directed downwards, and similar remarks are to be made of the 

 anterior extremities of the walrus, great auk, and turtle.-^ 



The flippers are advanced alternately; and the twisting, 

 screw-like movement which they exhibit in action, and which 

 I have carefully noted on several occasions, bears considerable 

 resemblance to the motions witnessed in the pectoral fins of 

 fishes. It may be remarked that the twisting or spiral move- 

 ments of the anterior extremities are calculated to utilize the 

 water to the utmost — the gradual but rapid operation of the 

 helix enabling the animal to lay hold of the water and dis- 

 entangle itself with astonishing facility, and with the mini- 

 mum expenditure of power. In fact, the insinuating motion 

 of the screw is the only one which can contend successfully 

 with the liquid element ; and it appears to me that this 

 remark holds even more true of the air. It also applies 

 within certain limits, as has been explained, to the land. 

 The otaria or sea-bear swims, or rather flies, under the water 

 with remarkable address and with apparently equal ease in 

 an upward, downward, and horizontal direction, by muscular 

 efforts alone — an observation which may likewise be made 

 regarding a great number of fishes, since the swimming- 

 bladder or float is in many entirely absent.^ Compare with 

 figs. 33, 34, 35, and 36, pp. 73 and 74. The walrus, a living 

 specimen of which I had an opportunity of frequently examin- 

 ing, is nearly allied to the seal and sea-bear, but diff'ers from 

 both as regards its manner of swimming. The natation of this 

 rare and singularly interesting animal, as I have taken great 

 pains to satisfy myself, is efl'ected by a mixed movement — the 

 anterior and posterior extremities participating in nearly an 

 equal degree. The anterior extremities or flippers of the 

 walrus, morphologically resemble those of the seal, but physio- 

 logically thosa of the sea-bear ; while the posterior extremities 



' This is the reverse of what takes place in flying, the anterior or thick 

 margins of the wings being invariably directed ujnvards, 



2 The air-bladder is wanting in the dermopteri, plagiostonii, and plenronec- 

 tidie. — Owen, o]). cit. p. 255. 



