PROGRESSION OF ANIMALS, xii. 



for this to happen and for the animal at the same 

 time to progress if the leading leg can bend forward, 

 but impossible if it bends backAvards. For in the 

 first case the extension of the leg will take place with 

 the forward movement of the body, in the second 

 case with its backward movement. Further, if the 

 bending Avere backwards, the planting of the foot 

 would be carried out by two movements contrary to 

 one another, one backwards and the other forwards. 

 For in bending the leg it is necessary to draw the 

 extremity of the thigh backwards, and the sliin 

 would move the foot forwards from the point of 

 bending ; but if the bending be forAvard, the 

 progression described above will take place not by 

 two contrary movements but by a single forward 

 movement. 



Man then, being a biped and carrying out the 

 change from place to place in a natural manner by 

 means of his legs, bends his legs forwards for the 

 reason already stated, but bends his arms concavely. 

 This is only in accordance with reason ; for if they 

 were bent in the opposite direction, they would be 

 useless for the purpose of the hands and for taking 

 food. But viviparous quadrupeds of necessity bend 

 their front legs in an outAvard curve, because these 

 legs lead the way when they Avalk, and are also 

 situated in the front part of their bodies ; and the 

 reason is the same as in man, for in this they resemble 

 man. Thus the quadrupeds too bend their legs 

 forAvard in the manner already described ; for indeed, 

 since they bend their legs in this way, they will be 

 able to raise their feet high in the air, whereas, if 

 they bent them in the opposite direction, they would 



525 



