DENTITION. 161 



the nature of their food, less liable to be worn, 

 than those of animals living on grain, or on the 

 harder kinds of vegetable substances ; so that 

 the simple plating of enamel is sufficient to pre- 

 serve them, even during a long life. But in 

 many herbivorous quadrupeds we find that in 

 proportion as the front teeth are worn away in 

 mastication, other teeth are formed, and advance 

 from the back of the jaw to replace them. This 

 happens in a most remarkable manner in the 

 elephant, and is the cause of the curved form 

 which the roots assume ; for in proportion as the 

 front teeth are worn away, those immediately 

 behind them are pushed forwards by the growth 

 of a new tooth at the back of the jaw ; and this 

 process goes on continually, giving rise to a suc- 

 cession of teeth, each of which is larger than 

 that which has preceded it, during the w^hole 

 period that the animal lives. A similar suc- 

 cession of teeth takes place in the icilcl boar, and 

 also, though to a less extent, in the Sits (Ethio- 

 picjis* This mode of dentition appears to be 

 peculiar to animals of great longevity, and 

 which subsist on vegetable substances con- 

 taining a large proj^ortion of tough fibres, or 

 other materials of great hardness ; and requiring 

 for their mastication teeth so large as not to 

 admit of both the old and new tooth being: 



o 



* Home, Phil. Trans, for 1799, p. 237 ; and 1801, p. 319. 

 VOL. II. M 



