DR, TETTIGREW ON THE MECHANISM OF FLIGHT. 207 





seen to advantage in the Shark and Sturgeon, the former of which requires to turn on it- 

 side before it can seize its prey, — and likewise in the Pipefish, whose motions are un- 

 wontedly sluggish. The twisting of the tail is occasion.! lly well marked in the swiinmin 

 of the Salamander. In the Whale*, Porpoise (Plate XII. fig. 10), Du ong, and Manatee 

 (Plate XII. fig. 9) the movements are strictly analogous to those of the fish, the only differ- 

 ence being that the tail acts from above downwards, or verticall;- instead of from side to 

 side or laterally. The anterior extremities, which in those animals are comparatively per- 

 fect, are rotated on their long axis, and applied obliquely and non-obliquch to the water, 

 as in the Seal, Sea-Bear, and "Walrus, and assist in balancing and turning the animal. 



Natation, however, is performed almost exclusively by the tail and lower half of the 

 trunk — the tail of the Whale exerting prodigious power. It is otherwise with the Hay--, 

 where the hands are principally concerned in progression. In the Beaver the tail is ilnt- 

 tened from above downwards as in the foregoing mammals, but in sa\ immin- it is mad' 

 to act upon the water laterally as in the fish. The tail of the bird, which is also com- 

 pressed from above downwards, can be twisted obliquely, and whan in this position may 

 be made to perform the office of a rudder. 



Swimming of the Seal, Sea-Bear, and Walrus.— In the Seal (Plate XII. fig. 1) 



the anterior and posterior extremities are more perfect than in the Whale, Porpoise, 

 Dugong, and Manatee; the general form, however, and mode of progression (if the fact 



of its occasionally swimming on its back be taken into account) is essentially fish- 

 like. 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 or non-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 consequence, to reduce 

 the resistance produced. The feet are opened to the utmost during extension, when 

 the effective stroke is given, in which case they present their maximum of surface. 



The swimming-apparatus of the Seal is therefore more highly differentiated than that 

 of the Whale, Porpoise, Dugong, or Manatee; the natatory tail in th< animals being, 

 from its peculiar structure, incapable of lateral compression f. It would consequently 

 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 employed in swimming, but 

 only in balancing and in changing position. The fore feet also open and close in natation, 

 thou-h not to the same extent as the hind ones; and the resistance and non-resistance 



necessary are secured by a partial rotation and tilting of the flippers. By this twisting 



digits of both pectorals and ventrals (the homologues of the posterior extremities) can be divaricated and approximated, 

 and the intervening webs spread out or folded up."— 0/). cit. vol. i. p. 252. 



* Fide remarks on the swimming of Cetaceans, by Dr. Marie, Proc. Zool. Soc. 1865, pp. 209-210. 



t In a few instances the caudal fin of the fish, as has been already toted, is more or less pressed together daring 



2f2 



the back stroke, the compression and tilting or twisting of the tail taking place synchronously. 



