CHAPTER XXIX 



THE RODENTIA, OR GNAWING ANIMALS 



" MICE, rats, and such small deer," to use Shakespeare's phrase, 

 make up a great proportion of the order Rodentia. The biggest 

 of them is only about the size of a small pig ; while the common 

 house rat, or the squirrel, may be taken as examples of the average 

 size of a Rodent. But although small of size, they are very numerous 

 in species and individuals, being extraordinarily prolific creatures, 

 and, for the most part, remarkably destructive, while in the East, 

 as one of the recognised carriers of plague, the rat is a serious 

 pest to mankind. The destruction of grain by rats and mice 

 amounts to many thousands of pounds in value annually. 



The Rodents form a very definitely circumscribed order of 

 the Mammalia, so that a cursory inspection of the jaws is always 

 sufficient to decide whether a quadruped belongs to the Rodents 

 or not, so characteristic is the dentition of the group. The teeth 

 are only of two kinds, incisors and grinders, and the number of 

 efficient teeth of the former kind is never more than two in each 

 jaw. Almost throughout the order, indeed, there are actually, 

 even from the first, only two incisors present ; but in the hares 

 and rabbits, and some allied forms, there are in the upper jaw, 

 in addition to the working teeth, a pair of rudimentary incisors 

 placed immediately behind the large ones, but quite incapable 

 of taking any part in this business of gnawing, for which the latter 

 are so admirably fitted. Their presence is, however, of interest 

 as indicating the direction in which an alliance with other forms 

 of Mammalia more abundantly supplied with teeth is to be sought. 

 The great incisors possess no roots, but spring from a permanent 

 pulp, so that they continue to grow during the whole life of the 

 animal ; and their form, and that of the cavity which constitutes 

 their socket, is always that of a segment of a circle, in consequence 

 of which they always protrude from the front of the jaws in the 



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