50 THE BOOK OF NATURE STUDY 



is called, leaving the solid matter behind to be swallowed. The 

 loss of hair, and the consequent loss of warmth, has been com- 

 pensated for by the development of a layer of fat of enormous 

 thickness just beneath the skin; while in the sperm-whale the 

 top of the head is loaded with oil to an enormous thickness, 

 and this, which is of great commercial value, has caused these 

 animals to be so ruthlessly persecuted that they are now in 

 danger of extinction. 



The whale, it should be remarked, has now become so per- 

 fectly fitted to live in the water that it can no longer live on land, 

 and when stranded in shallow water, as sometimes happens, 

 it speedily dies. 



The seal (Fig. 15) presents a most interesting half-way stage 

 between the whale on the one hand and land animals on the 



other. Essentially a sea- 

 dweller, the whole of its 

 food is obtained from 

 the water, and this, since 

 it consists exclusively 

 of living fish, at least 

 in the seals common on 

 British coasts, must be 



FlG. 15. The common seal, showing the gradual trans- , , , . , 



formation of the limbs into paddles. chased under water witl 



considerable skill and 



agility. But when not feeding the greater part of its time is 

 passed ashore, or at least resting on rocks overhanging the 

 sea. 



And just as the seal is less exclusively aquatic than the whale, 

 so it has undergone less striking modifications of its bodily 

 structure. The clothing of hair, for example, has been retained, 

 but the external ears have been lost. The channel leading to 

 the inner ear, the organ of hearing proper, is of considerable 

 size, and in consequence muscles have been developed whereby 

 this entrance may be closed when the animal is under 

 water. 



The most striking of the changes which this creature has 

 undergone are to be found in the fore- and hind-limbs, which now 

 have the form of " flippers " or swimming organs, which are but 



