PROGRESSION OF ANIMALS, ix. 



must necessarily be relative to a single point. Further, 

 if there were no bending, there would be no walking 

 or swimming or flying. For since animals with feet 

 stand and rest their weight alternately on each of 

 their two opposite legs, as one leg advances the other 

 must necessarily be bent. For the corresponding 

 legs on either side are naturally equal in length, and 

 the leg which supports the weight must be straight, 

 at right angles, as it were, to the ground. But when 

 a leg advances, it is assuming the position of the side 

 subtending a right angle, ° the square upon which 

 equals the squares ^ on the side which is at rest and 

 the hne between the two legs ; but since the legs 

 are equal, the leg which is at rest must bend either 

 at the knee or, in any kneeless animal that walks, at 

 the joint. That this is so is shown by the fact that 

 if a man were to walk on the ground alongside a wall 

 [with a reed dipped in ink attached to his head],^ 

 the Hne traced [by the reed] would not be straight 

 but zigzag, because it goes lower when he bends and 

 higher when he stands upright and raises himself. 



It is possible, however, to move even if the leg has 

 no bend in it, as happens when children crawl. (The 

 old account attributed such motion to elephants, but 

 it is untrue.) Movement of this kind takes place 

 through a bending in the shoulders or hips. But no 

 creature could walk erect in this way continuously 

 and safely, but could only move like those who drag 

 themselves forward through the dust in the wrestling- 

 school on their knees. For the upper portion of the 



can be formed by constructing squares on the side of, e.g. a 

 triangle. 



* The text here is corrupt and something has fallen out in 

 all our Mss. : the words here bracketed are supplied from the 

 explanation given by Mich. 



511 



