PROGRESSION ON AND IN THE WATER. 



75 



A peculiarity is met with in the swimming of the seal, to 

 which I think it proper to direct attention. When the lower 

 portion of the body and posterior extremities of these creatures 

 are flexed and tilted, as happens during the back and least 

 effective stroke, the naturally expanded feet are more or less 

 completely closed or pressed together, in order to diminish 

 the extent of surface presented to the water, and, as a con- 

 sequence, to reduce the resistance produced. The feet are 

 opened to the utmost during extension, when the more efiec- 

 tive stroke is given, in which case they present their maximum 

 of surface. They form powerful propellers, both during 

 flexion and extension. 



The swimming apparatus of the seal is therefore more 

 highly differentiated than that of the whale, porpoise, dugong, 

 and manatee ; the natatory tail in these animals being, from 

 its peculiar structure, incapable of lateral compression.^ It 

 would appear that the swimming appliances of the seals (where 

 the feet open and close as in swimming-birds) are to those of 

 the sea-mammals generally, what the feathers of the bird's 

 wing (these also open and close in flight) are to the continuous 

 membrane forming the wing of the insect and bat. 



The anterior extremities or flippers of the seal are not 

 engaged in swimming, but only in balancing and in changing 

 position. When so employed the fore feet open and close, 

 though not to the same extent as the hind ones ; the resist- 

 ance and non-resistance necessary being secured by a partial 

 rotation and tilting of the flippers. By this twisting and 

 untwisting, the narrow edges and broader portions of the 

 flippers are applied to the water alternately. The rotating 

 and tilting 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 a few instances the caudal fin of the fish, as has been ah'eady stated, 

 is more or less pressed together during the back stroke, the comi)ression and 

 tilting or twisting of the tail taking place synchronously. 



