CHARACTERISTICS AND HABITAT. 19 



separated from the molars by an empty space. These 

 incisors are the distinctive characteristic upon which 

 the order is founded. With jaws thus mounted, the 

 rodents are physically incapable of seizing a living 

 prey, and consequently are formed to draw their nu- 

 triment from the vegetable kingdom. The general 

 characteristics of this order are given by Cuvier as 

 follows : 



"Two large incisors in each jaw, separated from 

 the molars by a wide interval, cannot well seize a 

 living prey or devour flesh. They are unable even 

 to cut the aliment; but they serve to file, and by con- 

 tinued labor to reduce it into small particles; in a 

 word, to gnaw it; hence the word rodentia applied to 

 animals of this order; it is thus that they successfully 

 attack the hardest substances, frequently feeding on 

 wood and the bark of trees. The better to accom- 

 plish this object, these incisors have enamel only in 

 front, so that their posterior edges wearing away faster 

 than the anterior, they are always' naturally sloped 

 [or chisel like]. Their prismatic form causes them to 

 grow from the root as fast as they wear away from 

 the tip [their formative pulp being persistent], and 

 this tendency to increase in length is so powerful that 

 if either of them be lost or broken, its antagonist in 

 the other jaw, having nothing to oppose or commi- 

 nute, becomes developed to a monstrous extent. The 

 inferior jaw is articulated by a longitudinal condyle 

 in such a way as to allow of no horizontal motion, 

 except from back to front, and vice versa^ as is requis- 

 ite for the action of gnawing. The molars also have 

 flat crowns, the enameled eminences of which are 

 always transversely, so as to be in opposition to the 



