PROGRESSION OF ANIMALS, viii.-ix. 



walk using either three legs or one leg ; for if it uses 

 one leg it has absolutely no support on which it is to 

 rest the weight of the body, and if it uses three it will 

 rest it on a pair of opposite legs, so that, if it attempts 

 to move thus, it necessarily falls. Polypods, however, 

 for instance the scolopendrae, can achieve progression 

 with an odd number of legs, as they can be immediately 

 seen to do if you mutilate one of their feet, because 

 the maiming of some of the feet in the opposing rows 

 is compensated by the greater number of feet still 

 remaining on either side ; the result is that the 

 maimed leg is as it were dragged along by the others, 

 and the animal does not walk properly. How^ever, 

 it is clear that these maimed animals would achieve 

 the change of position better if they had an even 

 number of feet, that is, if none were lacking and they 

 had all the feet in the corresponding rows ; for then 

 they would be able to distribute their weight evenly 

 and would not sway to one side, if they had corre- 

 sponding supports on each side and had not one space 

 in the opposite rows devoid of a leg. An animal, 

 then, when it walks progresses by means of each of 

 its limbs alternately ; for thus its state is restored so 

 as to be identical with its original form. 



It has now been established that all animals have 

 an even number of feet, and the reason for this has 

 been stated. IX. That, if nothing were at rest, there 

 could be no bending or straightening is clear from the 

 following considerations. Bending is the change from 

 what is straight to what is curved or angular; straighten- 

 ing is the change of either of these to what is straight. 

 In all the above changes the bending or straightening 



8vuaLTo scripsi : 5iVarat Z : dvi/aLUTo ceteri. 

 * Tr]v ante runf add. Z. 



509 



