Natural History 459 



has also been utilised by reptiles, either living or extinct. This 

 suggests that evolution has proceeded on an ever-ascending 

 spiral. 



Birds and mammals have evolved, as we have already said, 

 on entirely different tacks, but it is not unprofitable to notice 

 that they have often made the same kinds of experiments. The 

 Ostrich is a running bird, the Antelope a running mammal; 

 the Owl is a nocturnal bird, the Hedgehog is a nocturnal mammal ; 

 the Storm-Petrel is an open-sea bird, the Dolphin an open-sea 

 mammal; the Sand-Martin is a burrowing bird, the Mole a bur- 

 rowing mammal, and so on. For a long time there were no 

 flying mammals to vie with the flying birds; but eventually 

 there was the evolution of bats, doubtless from an arboreal in- 

 sectivorous stock. 



Aquatic Mammals 



It is instructive to consider some of the thousand and one 

 ways in which mammals are specially adapted to the various 

 haunts and conditions in which they live. But only a few illustra- 

 tions can be given, beginning with aquatic mammals. In whales 

 the tail has been transformed into a propeller, which sculls the 

 water first to one side and then to the other, and great speed is 

 attained in swimming and diving. With these swimming powers 

 is associated the almost worldwide distribution of many cetaceans, 

 like the Sperm Whale and the Southern Right Whale. In seals 

 the hind-limbs are bound up with the tail, a conjoint propeller 

 which churns the water from side to side being the result. In 

 the walrus the hind-limbs are helped by the great paddle-like 

 fore-limbs, which are also used for clambering on the slippery 

 ice. The Common Seal has a remarkable way of moving on 

 land. It arches up its body, bringing the hind-limbs and tail 

 towards the head, and then suddenly straightens itself away, thus 

 jerking the body forwards. In swimming the Beaver uses its 

 trowel-like flattened tail; the Duckmole has webbed fingers; the 



