PARTS OF ANIMALS, IV. x. 



which provide another example of Nature's habit of 

 using an organ for secondary purposes, for she 

 employs the tail not only as a guard and covering for 

 the fundament but also in other serviceable ways. 



There are differences too in the feet of quadrupeds. Hoofs, etc 

 Some have a solid hoof, some a cloven hoof ; others 

 have a foot that is divided into several parts. Solid 

 hoofs are present in those animals which are large 

 and contain much earthy substance,*^ which instead of 

 making horns and teeth forms an abscession^ so as 

 to produce nail, and owing to the abundance of it, it 

 produces not several separate nails but a single one, 

 in other words, a hoof. Because of this, these 

 animals in general have no hucklebone ; and also 

 because the presence of a hucklebone makes it 

 rather difficult to bend the hind leg freely, since a 

 limb that has one angle can be bent to and fro more 

 quickly than one that has several. It is a sort of 

 connecting-rod, and therefore practically interpolates 

 another bit of a limb between the two, thereby in- 

 creasing the weight ; but it makes the animal's footing 

 more reliable. This explains why, when hucklebones 

 are present, they are present in the hind limbs 

 only, never in the front : the front limbs have to be 

 light and flexible because they go first, while the 

 hind limbs must be reliable and able to stretch. 

 Further, a hucklebone puts more force into a blow — a 

 useful point in self-defence — and animals which have 

 one use their hind limbs in this way : if anything 

 hurts them they kick out at it. 



Cloven-hoofed animals have a hucklebone, as their 

 hind limbs are on the light side ; and that is the very 

 reason why they are cloven-hoofed : the bony sub- 

 stance stays in the joint and therefore is deficient in 



389 



