SWIMMING ANIMALS 



101 



so characteristic of fast-swimming fish. They possess 

 two large flippers where their front legs or arms should be. 

 Many of them carry a vertical fin on their back and their 

 tails are developed into powerful flukes ; but unlike those 

 of fish, these tails are flattened horizontally instead of 

 vertically, probably to aid in the continual journeyings 

 to the surface to breathe. An examination of the skeleton 

 reveals the fact that the front flippers are undoubtedly the 

 same as a true leg, for buried in the flesh are typical leg 

 bones ending in numbers of small bones similar to those 

 of our hands or feet (Figs. 20 and 21). But all these bones 

 are completely buried and there is no external division of the 

 flipper into arm, hand, and fingers. Although, externally, 

 there is no sign of any hind limbs, yet in some species of 



Fig. 20. — Outline and skeleton of Sperm whale or Cachalot {Physeter macrocephalus), 



b, blow hole ; p, rudimentary pelvis. 



whales, buried in the body in just the region where the hind 

 legs should be, are to be found small remnants of bones 

 much reduced in size and serving no purpose — the last 

 vestiges of the back legs of the land mammal from which 

 through countless ages the whales have been evolved. 



Hair is one of the chief characteristics ^f mammals. It 

 is essential for helping it to keep the warm blood that flows 

 through their veins at the correct temperature. Yet a whale 

 is practically hairless save for a few fine bristles in the 

 neighbourhood of the mouth, which may or may not persist 

 throughout life. Living as it does in the cool waters of 

 the ocean it must therefore have some other means of 



