PROGRESSION OF ANIMALS, xi. 



constituted would it be able to carry itself easily. 

 Therefore man, the only erect animal, has legs larger 

 and stronger in proportion to the upper part of his 

 body than any of the other animals which have legs. 

 What happens with children illustrates this : they 

 cannot walk erect because they are ahvays dwarfish 

 and have the upper parts of their body too big and 

 too strong in proportion to the loAver parts. As they 

 grow older, the lower parts increase more quickly, 

 until they attain their proper size ; and it is only then 

 that they can walk with their bodies erect. Birds 

 are lightly built but can stand on two feet because 

 their weight is at the back, just like bronze horses 

 which are made by sculptors with their fore-legs 

 raised in the air. The chief reason why birds can 

 stand although they are bipeds is that their hip-joint 

 resembles a thigh and is of such a size that they seem 

 to have two thighs, one on the leg above the joint and 

 the other between this and the fundament ; but it is 

 not really a thigh but a hip. If it were not so large, 

 a bird could not be a biped ; for then, just as in man 

 and the quadrupeds, the thigh and the rest of the 

 leg would be directly attached to a short hip, and so 

 the whole body would tend to fall forward too much. 

 But, as it is, the hip, being long, extends up to the 

 middle of the belly, and so the legs form supports at 

 that point and carry the whole body. It is clear too 

 from this that it is impossible for a bird to stand erect 

 in the way that a man stands ; for the way that birds' 

 wings grow is useful to them in the position in which 

 they now hold themselves, but if they stood erect, 



521 



