ARBOREAL AND SWIMMING MAMMALS 53 



more and more on to the hind-quarters, and the arms, 

 thus relieved of their primitive supporting function, 

 are used for prehension ; whilst in South American 

 monkeys the tail is converted into a sensitive fifth 

 hand for grasping boughs and giving support. In 

 this way many of the Quadrumana have lost the 

 power of rapid and effective running on the ground, 

 and have acquired a wholly new arboreal mode of life 

 and a new dexterity in the performance of their won- 

 derful gymnastics. For grasping the twigs monkeys 

 employ the hand as we do, with the grasp between 

 the thumb and the palm, and the most arboreal of 

 all are either thumbless or their fingers partially 

 adhere, so that their limbs are converted into mere 

 grasping hooks. 



III. Adaptations for swimming and flight in 

 mammals. The most remarkable adaptations to 

 aquatic and aerial life are found in a few groups of 

 mammals ; amongst the seals and whales for swimming 

 and the bats for flight. 



Mammals are so emphatically land animals, only 

 swimming by the aid of their limbs to reach new 

 feeding-grounds, that the existence of two aquatic 

 groups, one of which passes the whole of its life in the 

 sea, is a striking fact. So different are whales and 

 porpoises from ordinary mammals that the idea of 

 their being fish is still widely prevalent, and, though 

 erroneous, is intelligible by the many fishlike characters 

 which these animals possess. 



The seal is amphibious, spending the greater part 



