428 USE OF NATUKAL INSTRUMENTS. 



had, or have, advanced a stage or two beyond the prehistoric 

 troglodytes or cave-dwellers. 



Many animals show their sense of the value of, or neces- 

 sity for, their natural tools or weapons by keeping them in 

 good working order sharpening them in the majority of 

 cases. The Felince generally whet or sharpen their claws ; 

 the bear sharpens both teeth and claws ; the boar whets its 

 tusks; for this purpose, probably, the cat defaces leather- 

 bound books in libraries, and the tiger is fond of scratching 

 the bark of trees, especially the Indian fig. Hence, too, the 

 propensity of the stag to rub his antlers against trees in 

 summer, to clear them of their skin and to polish them. 



