CHAPTER XXXI. 
The Rodents,— Order Rodentia. 
Squirrels, Marmots, and Beavers. 
Under the common title of Gnawing or Rodent Mammals are grouped that 
extensive assemblage of small or medium-sized species which, like rats, porcupines, 
beavers, squirrels, and hares, are characterised by their habit of gnawing. At the 
present day these creatures are more numerously represented than in any other 
Mammalian order, both as regards individuals and species; the number of the 
latter being probably fully one thousand, while the swarms of individuals by which 
some species are represented are too well known to need more than passing mention. 
Nearly all the Rodents are inhabitants of the dry land, and a large proportion of 
them seek protection from enemies by living in subterranean burrows which they 
construct for themselves, and wherein they generally associate in larger or smaller 
colonies. Some, however, like the water-vole and the beaver, are aquatic; while 
others, like the squirrels and tree-porcupines, lead an arboreal life. A few, again, 
have acquired the power of spurious flight through the development of folds of 
skin along the sides of the body and limbs, by the aid of which they are enabled 
to take long flying leaps. 
The Rodents are some of the most easily defined of all mammals, and are best 
characterised by the number and nature of their teeth—especially those in the 
front of the jaws. They are distinguished by the presence in each jaw of a pair 
of large chisel-like front or incisor teeth, which grow continuously throughout the 
life of their owners. As a rule, no other incisor teeth, save these two pairs, are 
developed, but in the hares and rabbits and their allies a second smaller pair occur 
behind those of the upper jaw. There are no tusks or canine teeth in either jaw, 
and in the cheek-series the number of premolars is always reduced below the normal 
four, very generally only one of these teeth being present, while in some cases even 
this may be wanting. In consequence of the reduced number of incisor teeth, 
coupled with the absence of canines and the reduction in the premolars, the skull 
of a Rodent is always distinguished by the presence of a long gap between the 
front and the cheek-teeth. Indeed, the presence in all Rodents of only a single 
pair of chisel - shaped and permanently-growing lower incisors, opposed to a 
corresponding pair in the upper jaw, the total absence of canines, the long gap 
between the incisors and the cheek-teeth, and the reduction in the number of 
the premolars, are of themselves sufficient to distinguish the Rodent order from 
all other mammals, with the exception of the aye-aye among the lemurs. 
Among other distinctive characteristics of the group, the following may be 
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