178 



THAT S IT; 



mously muscular tails, 20, con- 

 tribute. The feeble young would 

 soon be left behind, in the rapid 

 flight of the parent, but for the 

 admirable provision by which the 

 female kanguroo, 17, is enabled 

 to cany her young without 

 impeding her own movements. 



EODENTIA F. 



The animals of this order 

 occupy in some respects an inter- 

 mediate place between the purely 

 carnivorous and herbivorous mam- 

 malia. The name is derived from 

 the Latin, and means gnawing. 

 Rodents, f, have two long chisel- 

 shaped incisor teeth in each jaw, 

 and a vacant space between these 

 and the molars. These gnawing 

 teeth are admirably adapted to 

 the purpose they have to fulfil ; 

 they are sharp and adapted to 

 make their way through vegetable 

 substances ; and strong and firm, 

 so as to bear the constant action 

 to which they are subjected. 

 They are, generally speaking, of 

 small size, and are very nume- 

 rous, sometimes prevailing in such 

 multitudes as at certain seasons 

 to destroy the vegetation of ex- 

 tensive districts. Some of them 

 are the most gentle of the mam- 

 malia, while others are so fero- 

 cious that if their size and 

 strength were in proportion, they 

 would be exceedingly formidable. 

 Squirrels, rats, mice, hares, rabbits, 

 Jbeavers, &c, belong to this order.* 



The beaver, 1, is found only in 

 the colder parts of temperate 

 latitudes in the northern hemi- 



* Some interesting information upon rats, mice, 

 squirrel*, hares, Ac. 'will be given in our account of 

 tlie Fields and Woods 



sphere, in wild places, by the 

 banks of pools, lakes, and streams, 

 which lie in the fastnesses of the 

 forests, and distant from the 

 abodes of man. 



518. 



The beaver is so decidedly a gnawing animal 

 in the structure of its teeth, that it may without 

 impropriety be considered as the most perfectly 

 typical one of the order to which it belongs. 

 Its cutting teeth are two in each jaw, very large 

 and strong, and standing so far clear of the lips, 

 that the animal can easily gnaw or cut a hard 

 substance without the least danger to those 

 organs. The structure of these teeth, though 

 common to most of the order, is peculiar and well 

 worthy of notice. On their front sides the teeth 

 are broad and flat ; not white like those of most 

 animals, but of a brownish yellow. They have 

 along the front surfaces a plate of very hard 

 enamel, which covers the bone or principal sub- 

 stance of the tooth, just as a piece of steel may 

 be seen covering the iron on the cutting edge of 

 a chisel, which is both sharp and not liable to 

 be broken. The body of the tooth is compact, 

 but not very hard bone ; and it is strengthened 

 by a projecting ridge on the posterior surface, in 

 the same manner as hoes and various other tools 

 are fortified, by a ridge exte nding from the socket 

 which receives the handle. The tooth is thus 

 formed upon the most skilful mechanical prin- 

 ciples, in the distribution of the materials and 

 in the form. Bone, though not so hard a sub- 

 stance as enamel, is tougher, just as iron is 

 tougher than hardened steel. The softness 

 makes it wear faster than the enamel, ana the 

 toughness makes the tooth less easily broken. It 

 the whole tooth had been of enamel, it would in 

 time have become blunted, by the cutting edge 

 wearing down faster than the rest ; but the 

 bone wears with 'less action than the enamel ; 

 and thus the enamel always stands highest, and 

 forms a cutting edge, while the bone supports it 

 behind, like the basil or stopping edge of a tool. 

 Thus the cutting tooth of the beaver is a chisel, 

 and whether the carpenter's chisel has been 

 made in imitation of it or not, the same arrange- 

 ment of iron and steel as there is of bone and 

 enamel in the tooth is the best possible for a 



