INTRODUCTORY 3 



its essential parts just like that of the human hand, and the 

 so-called dorsal fin is nothing more than a ridge of fatty tissue. 

 Furthermore, although a Whale is able to remain under water 

 for considerable periods of time, it is forced to come to the 

 surface at intervals to empty its lungs of air and to inhale a 

 fresh supply of oxygen — the familiar process of spouting or 

 blowing. Whales also bring forth their young alive, and after 

 birth suckle them just like any other mammal. In short, a 

 Whale is a mammal which has left its kindred and exchanged 

 a terrestrial life for one passed entirely in the water, a change 



Fig. 2. PECTORAL LIMB OF CETACEAN AND FISH COMPARED. 



A. Skeleton of paddle of the Common Dolphin {Delphinus delphis) ; b. Skeleton 

 of pectoral fin of the Comb-toothed Shark (Heptranchias perlo). Both much 



reduced. 



which has led to the fore limbs being converted into paddles 

 for swimming, while the hind limbs have completely dis- 

 appeared. The Seals give us some idea how this change has 

 come about, representing, as it were, a half-way stage between 

 a typical walking mammal and a specialised swimming one. 

 A Seal is amphibious; that is to say, it is equally at home on 

 land or in the water; but the hind limbs have lost a great deal 

 of their power of supporting the body on terra Jirma, and the 

 fore limbs are becoming more and more paddle-like, the shape 

 of the body tapering and fish-like, and the external ears have 

 more or less disappeared. The form of the tail provides a 

 rough-and-ready means of distinguishing at a glance any 



