PARTS OF ANIMALS, IV. xi. 



life, because they fly about ; but it would be very 

 little good to these creatures, because they all spend 

 their time in holes and corners. 



Their head has two divisions : the upper part, and 

 the lower jaw. In man and in the viviparous 

 quadrupeds the lower jaw moves from side to side as 

 well as up and down ; in fishes, however, and birds 

 and these oviparous quadrupeds it moves up and 

 down only. The reason is that this vertical motion is 

 useful for biting and cutting up food, while the 

 sideways motion is useful for grinding the food down. 

 Of course this sideways motion is useful to animals 

 which possess grinder-teeth ; but it is of no use to 

 those which lack grinders, and so not one of them 

 has it. Nature never makes or does anything that is 

 superfluous. All these animals, then, move the lower 

 jaw — with one exception, the river crocodile, which 

 moves the upper jaw, and the reason for this is that 

 his feet are no use for seizing and holding things : 

 they are too small altogether. So Nature has given 

 him a mouth which he can use for these purposes 

 instead of his feet. And when it comes to seizing 

 things and holding them, the most useful direction 

 for a blow to take is that which gives it the greatest 

 strength. Now a blow from above is always stronger 

 than one from below. And to an animal who has no 

 hands and no proper feet, who has to use his mouth 

 for seizing his food as well as for biting it, the power 

 to seize it is the more necessary ; and therefore it is 

 more useful to him to be able to move his upper jaw 

 than his lower one. For the same reason crabs move 

 the upper part of their claws and not the lower : 

 claws are their substitute for hands, so the claws have 

 to be useful for seizing things (not for cutting them 



. 397 



