PROGRESSION OF ANIMALS, i.-ii. 



himself bends his legs and his arms in opposite 

 directions, the arms concavely and the knees con- 

 vexly. And viviparous quadrupeds bend their limbs 

 in the opposite way to a man's and in opposite 

 ways to one another ; for they bend their front legs 

 convexly and their back legs concavely. Further, 

 quadrupeds which are not viviparous but oviparous 

 have the peculiarity of bending their legs sideways. 

 A further question is why do quadrupeds move their 

 legs diagonally. ** 



We must examine the reasons of all these and 

 similar facts ; that they are facts is clear from our 

 Natural History,^ and we have now to examine their 

 causes. 



II. We must begin our inquu-y by assuming the 

 principles which we are frequently accustomed to 

 employ in natural investigation, namely, by accept- 

 ing as true what occurs in accordance with these 

 principles *^ in all the works of nature. One of these 

 principles is that nature never creates anything 

 without a purpose, but always what is best in view 

 of the possibilities allowed by the essence of each 

 kind of animal ; therefore, if it is better to do a thing 

 in a particular manner, it is also in accordance with 

 nature. Further, we must accept the dimensions of 

 magnitude in the size and quahty in which they are 

 present in various objects. For there are six dimen- 

 sions grouped in three pairs, the first being the 

 superior and the inferior, the second the front and 

 the back, and the third the right and the left. We 

 must further postulate that the origins of movement 

 from place to place are thrusting and pulling. These 

 are movements per se ; that which is carried by 



487 



