PROGRESSION OF ANIiMALS, viii. 



a purpose but always with a view to what is best for 

 each thing within the bounds of possibiHty, preserv- 

 ing the particular essence and purpose of each ; and, 

 secondly, as we have already said, because no red- 

 blooded animal can move by means of more than four 

 points. It is clear from this that all red-blooded 

 animals whose length is out of proportion to the rest 

 of their bodily constitution, like the snakes, can none 

 of them have feet ; for they cannot have more than 

 four feet (for if they had, they would be bloodless), 

 whereas, if they had two or four feet, they would be 

 practically incapable of any movement at all, so slow 

 and useless would their movement necessarily be. 



Every animal which has feet must necessarily have 

 an even number of feet ; for those which move from 

 place to place by jumping only do not require feet 

 (at least not for this movement), while those which 

 jump but do not find this mode of locomotion suffi- 

 cient by itself and need to walk also, must clearly 

 either progress better with an even number of legs 

 or else cannot otherwise progress at all.^ For since 

 this kind of change from place to place is carried out 

 by a part and not, like jumping, with the whole of 

 the body at once, some of the feet during the change 

 of position must remain at rest while others are in 

 motion, and the animal must rest and move ^^'ith 

 opposite legs, transferring the weight from the legs 

 in motion to those at rest. Hence no animal can 



708b 5 ff.), would walk better with an even number; quad- 

 rupeds and bipeds, on the other hand, cannot walk at all with 

 an uneven number of legs. Farquharson's insertion of dXXws 

 seems therefore a certain emendation : the omission of ctWws, 

 however, in our mss. would be better accounted for if it is 

 inserted before 6'Xws rather than before iropeveadai. 



R 507 



