52 THE BOOK OF NATURE STUDY 



walks upon two legs and a dog upon four is one which escapes 

 comment, because the rule is universal. Yet a moment's reflec- 

 tion shows that these two very different modes of progres- 

 sion are of stupendous importance. For thereby the fore-limbs 

 in the one have become available for service on the head, while 

 in the other they are employed exclusively in the support 

 of the body. But to effect this emancipation of the fore-limbs 

 the most profound structural changes in man's bodily framework 

 have taken place. The muscles of the thighs, for example, have 

 been enormously increased, while the curvature of the backbone 



FlG. 16. Kangaroo in the act of leaping. Note the enormous size of the 

 hind-legs and tail. 



and the poise of the head have been no less materially changed 

 when compared with that of the dog. 



The development of this bipedal method of locomotion was 

 a very gradual and laborious process. A very interesting stage 

 in this development may be studied in the monkeys, more especi- 

 ally in the higher apes, such as in the chimpanzee and orang, for 

 example. Here the fore-limbs, as in man, have assumed the 

 form of " hands and arms/' but in walking the hands are still 

 indispensable, the weight of the body being thrown upon the 

 backs of the fingers, which are bent towards the palms of the 

 hands, while the body is swung between the arms, as though 

 the creature walked on crutches. But this method of pro- 

 gression is only possible where the legs are extremely short, 



