262 NATURAL THEOLOGY. 



proportion as it is wasted in the process of attri- 

 tion and sharpening. This is a mode of growth 

 which takes place in no other animal's incisor 

 tooth. 



Let us now observe how the grinding surface 

 of the tooth of an herbivorous animal is composed. 

 It must be rough or irregular, so as to catch the 

 grain. A smooth mill-stone, for example, would 

 not bruise the grain into meal. The burr-stone, 

 accordingly, is sought for the nether mill-stone. 

 This stone contains small portions of feldspar, 

 imbedded in a softer material : and thus, however 

 the surface may be ground down, the harder ma- 

 terial, by yielding less easily to friction than the 

 softer, projects above the general level, and pre- 

 serves a roughness of the surface even whilst it is 

 yielding. It is exactly so with the tooth of a 

 graminivorous animal. It is composed of alter- 

 nate layers of the hardest bone, or rather of ivory, 

 and a denser material still, the enamel. The 

 consequence of this inequality in the composition 

 is, that, notwithstanding the surface of the tooth 

 is worn down, the roughness is preserved. 



There is something curious too in this irregu- 

 larity, showing that it is as far as possible from 

 accident. The lines of enamel which stand up 

 differ in their arrangement according to the mo- 

 tion to be given to the jaw. In the horse and 

 cow, these ridges run parallel with the jaw, and 

 consequently lie across the direction of the mo- 

 tion of chewing, w hich is from side to side. In 



