208 DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 



and untwisting, the narrow edges and natter portions of the nippers are applied to the 

 water alternately. The rotating and-tiltmg of the anterior and posterior extremities, and 

 the opening and closing of the hands and feet in the balancing and swimming of the Seal, 

 form a series of strictly progressive and very graceful movements. They are, however, 

 performed so rapidly, and glide into each other so perfectly, as to render an analysis of 



them exceedingly difficult. 



In the Sea-Bear ( Otaria Hooheri) (Plate XII. fig. 6), the anterior extremities attain 

 snilicient magnitude and power to enable the animal to progress by their aid alone— the 

 feet and the lower portions of the body being moved only sufficiently to maintain, correct, 



or alter the course pursued. 

 This remarkable creature, which I have repeatedly watched at the Zoological Gardens 



* 



uses its hands as paddles, the universal joint by which the arm is attached to the shoulder 

 enabling it, by a partial rotation and twisting of the extremity (Plate XV. fig. 51c), to 

 present the palm or flat of the hand to the water at one instant and the edge or narrow 

 part the next. In swimming, the anterior or thick margin of the flipper is directed 

 downwards as in the anterior extremities of the Walrus, Great Auk, and Turtle 

 (Plate XII. fig. 7)t. 



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 con- 

 siderable resemblance to the motions witnessed in the pectoral fins of fishes. It may be re- 

 marked that the twisting or spiral movements 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 disentangle itself with astonishing facility, and with 

 the minimum 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 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 %. The Walrus, a 

 living specimen of which I had an opportunity of frequently examining, is nearly 

 allied to the Seal and Sea-Bear, but differs 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 effected 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 physiologically those of 



* The unusual opportunities afforded by this unrivalled collection have enabled me to determine with something 

 like accuracy the movements of the various land-animals, as well as the motions of the wings and feet of birds, both 

 in and out of the water. I have also studied under the most favourable circumstance the movements of the Otter, 

 Sea-Bear, Seal, Walrus, Porpoise, Turtle, Triton, Crocodile, Frog, Lepidosiren, Proteus, Axolotl, and the several 



orders ot fishes. 



t This is the reverse of what takes place in flying, the anterior or thick margin of the wing being invariably 

 directed upwards. 



+ f 



fhe air-bladder is wanting in the Dermopteri, Plagiostomi, and Pleuroneetid*. (Owen, op. cit. p. 255.) 



