ADAPTATION OF EXTREMITIES FOR SWIMMING. 501 



far backwards, so as almost to occupy the position of the tail. 

 In the Whale and its allies, on the other hand, the posterior 

 extremities are almost entirely wanting, and the tail is- greatly 

 prolonged and expanded at its extremity (fig. 241), This 



Fig. 241. SKELETON OF DUGONG. 



expansion, however (which is in the horizontal direction, fig. 

 242), is not supported by bones, except in its centre ; but it 

 consists internally of cartilages and tendons, which last are 

 prolonged from a set of very powerful muscles that are at- 

 tached to the spine, and give to this organ an enormous force 



Fig. 242. TAIL-FIN OF WHALE. 



and great variety of motion. The texture of the portion of it 

 by which the blow is usually given, is such that it can hardly 

 be injured ; it is so tough that it cannot be torn, and so free 

 from feeling, that a stroke of it against a hard substance gives 

 no pain to the animal. If it strike a boat across the middle 

 with its edge, the boat is cut asunder as clean and suddenly 

 as if by one stroke of a giant axe ; whereas, if it strike with 



