RODENTIA. 515 



perpetually worn away by their action upon each 

 other, and upon the hard food which they are 

 formed for nibbling, so that the wearing away of 

 the extremities and the growth from the bases 



FlG. 442 SKULL OF FOKCL'PLNK. 



balance each other with exact precision. When, 

 however, by accident, an opposing incisor is lost, 

 or when by the distorted union of a broken jaw, 

 the lower incisors no longer meet the upper ones, 

 as sometimes happens to a wounded Hare, the in- 

 cisors grow until they project like the tusk of an 

 Elephant. The Rodents are all timid and feeble, 

 trusting for self-protection to flight or concealment. 

 The prey of ferocious beasts, and birds, and reptiles, 

 their fertility, by a wise provision, counterbalances 

 their annual diminution. Spread over the earth from 

 the equator to the coldest latitudes, they tenant 

 rocks and mountains, plains and woods, and often 

 devastate the cultivated domains of man, About 

 half of the Mammalia known belong to this Order, 

 which contains above six hundred species. Of these, 

 three hundred and six are of the Family Muridse, 

 the Rats and Mice, while the Squirrels (Sciuridee) 

 contain exactly half that number. Fifteen species 

 are enumerated by Professor Bell as belonging to 

 the British Isles. The Rodentia are classed as 

 follows : 



