PROGRESSION OF ANIMALS, xi.-xii. 



as winged ciipids are represented in pictures, the 

 wings would serve no purpose. 



At the same time it is clear from what has been 

 said that man, or any other creature of like form, 

 cannot be winged, not only because, being red- 

 blooded, he w^ould then move at more points than 

 four, but also because the possession of wings would 

 be useless to him when moving in a natural manner. 

 Now nature creates nothing unnatural. 



XII. It has already been stated that, if there were 

 no bending in the legs or shoulders and hips, none 

 of the animals which are red-blooded and have feet 

 could progress ; and that bending would be impos- 

 sible if something were not at rest ; and that men 

 and birds, being both bipeds, bend their legs in 

 opposite directions ; and, furthermore, that quadru- 

 peds bend their pairs of legs in opposite directions 

 to one another and in an opposite manner to men. 

 For men bend their arms concavely and their legs 

 convexly, but quadrupeds bend their front legs con- 

 vexly and their back legs concavely ; birds too do 

 the latter. The reason is that nature never does 

 anything without a purpose, as has been said before, 

 but creates all things with a view to the best that 

 circumstances allow. And so since in all creatures 

 which possess by nature the power of locomotion by 

 means of their two legs, when each leg is stationary 

 the weight must be upon it, but when they move 

 forward, the leading leg must have no weight upon 

 it, and as progression continues it is necessary to 

 transfer the weight on to this leg ; it is clearly essential 

 that the leg after being bent should become straight 

 again, the point at which the leg is thrust forward 

 and the shin remaining at rest. And it is possible 



523 



