THE BROWN RAT. 167 



mere growth of its teeth ; for unless their chisel -like edges were con- 

 tinually kept sharp, they would be of little use for cutting their way 

 through the hard substances which the Rodents are in the habit of 

 gnawing. This result is attained as follows: 



The enamel which covers the front face of the incisor teeth is much 

 harder than that which is laid upon the remaining surfaces, while the 

 dentine which makes up the solid mass of each tooth is also harder in 

 front than behind. It is evident that when these teeth are employed 

 in their usual task, the softer enamel and dentine are worn away very 

 much more rapidly than the remainder of the teeth, so that the pecu- 

 liar chisel-edge of the teeth is continually preserved. Following — 

 perhaps unconsciously — the structure of these teeth, our cutlers have 

 long been accustomed to make their chisels on the same principle, a 

 thin plate of steel being strengthened with a thick backing of iron. 



The Rodent animals are widely spread over the entire globe, and 

 are very numerous, comprising nearly one-third of the mammalia. 



Few animals are so well known or so thoroughly detested as the 

 common Brown Rat, or Norway Rat, as it is sometimes erroneously 

 called. 



It is an exceedingly voracious animal, eating all kinds of strange 

 food, and not sparing its own species in times of scarcity. The havoc 

 which an army of Ruts will make among the 

 corn-ricks is almost incredible, while they carry 

 on their depredation with so much secrecy that 

 an unpractised eye would think the stacks to be 

 sound and unharmed. Fortunately, they can 

 easily be dislodged from any rick by taking it 

 down and replacing it on proper "staddles," 

 taking great care that no stray weeds or 

 branches afford a foothold to these persevering The Rat (Mus Decima- 

 marauders. While the rick is being rebuilt, '"'^^' 



no particular care need be taken to shake the Rats out of the sheaves, 

 for, as they are thirsty animals, they will be forced to leap from the 

 stack in search of water, and then will not be able to return. 



Mice can subsist in a stack by means of the rain and dew which 

 moisten the thatch, and may often be seen licking the straws in order 

 to quench their thirst. But the Rats are less tolerant of thirst, and are 

 forced to evacuate their premises. When mice and Rats are found 

 inhabiting the same stack, the former animals reside in the upper parts, 

 and the Rats in the lower. 



Rats are not without their use, especially in large towns, which but 

 for their never-failing appetites would often be in very sad case. 

 Taking, for example, London itself, we find that the sewers which 

 underlie its whole extent are inhabited by vast hordes of Rats, which 



