770 MAMMALIA. 



are furnished with teeth adapted to gnaw even the wood and the bark, or 

 to crack nuts and other hard fruits, from which they derive nourishment. 

 (2214.) This order of Mammals, therefore, is distinguished by the 

 possession of two incisor teeth in each jaw, so constructed as to erode 

 hard substances, and which, moreover, by a peculiar mechanism, to be 

 described in another place, are always kept sharp and trenchant : such 

 are the incisor teeth of the Beaver or of the Hare (fig. 390). 



Fig. 390. 



Incisor teeth of the Hare. 



(2215.) The skeletons of the RODENTIA are slight and feeble, adapted 

 to the bird-like activity of their habits. Their fingers and toes are well 

 developed, and the bones of the leg and fore-arm free throughout their 

 whole length, although the movements of pronation and supination are 

 as yet much limited. In many genera, more especially in such as climb 

 trees like the Squirrels, the clavicles are very perfectly formed, so that 

 the fore legs can be employed to a certain extent as hands, for convey- 

 ing food to the mouth. 



(2216.) Very generally the hind legs of the RODENTIA are consider- 

 rably longer than their anterior extremities : hence such genera run by 

 bounds or leaps, and their course is extremely rapid. In the Jerboa 

 (Dipus) (fig. 391) this disproportionate size of the hind legs is excessive, 

 insomuch that the creature moves by leaps, like a Kangaroo ; and the 

 metatarsal bones of the three middle toes being consolidated into one 

 bone, the whole limb resembles more that of a bird than of a quadruped. 



(2217.) Among all the countless races of the animal kingdom, Man 

 alone is permitted, in a state of nature, to arrive at old age ; that is to 

 say, at such an age as to allow feebleness and decrepitude to usurp the 

 place of strength and activity. Man only is capable of such a privilege, 

 because he alone possesses that foresight which enables him to prepare 

 in youth against the decline of his faculties, and is endowed with sym- 

 pathies and affections directing the strong and the vigorous to maintain 

 the aged and the infirm. 



(2218.) Among the lower animals, sickness and decay are not per- 



