PROGRESSION OF ANIMALS, iv. 



and whence every kind of creature derives it are at 

 the front, and the opposite parts to these are at the 

 back. 



Those animals which not only partake of sense- 

 perception but can also of themselves make the 

 change from place to place, in addition to the dis- 

 tinctions already mentioned," have a further distinc- 

 tion of left and right, these being each, like the above, 

 distinctions of function and not of position. For the 

 part of the body where the origin of change from 

 place to place naturally arises is the right in each 

 kind of animal, while the part which is opposed to 

 this and naturally ΐοΙΙοΛνβ its lead is the left. 



There is a greater differentiation between right 

 and left in some animals than in others. All animals 

 which make the above-mentioned change ^ by the use 

 of instrumental parts — for example, feet or wings 

 or the like — shoΛv a greater differentiation between 

 right and left in such parts ; those, on the other hand, 

 that progress not by means of such parts but by 

 moving the body itself in sections — like some of the 

 footless animals, such as snakes and the caterpillars, 

 and also earthworms — possess, it is true, this differ- 

 entiation, but it is not nearly so clearly defined. 



That the origin of movement is from the right side 

 is showTi by the fact that men always carry burdens 

 on the left shoulder ; for then it is possible for that 

 which bears the weight to be set in motion, that 

 which is to initiate the movement being free. (For 

 this reason, too, it is easier to hop on the left leg ; for 

 it is natural to the right leg to initiate movement, 

 and to the left to be set in motion.) The burden, 

 therefore, must rest not on the part Avhich is to 

 initiate movement, but on that Λvhich is to be set in 



493 



