THE RODENTIA, OR GNAWING ANIMALS 387 



constructs in the rivers it frequents, that it is unnecessary to 

 restate these well-known facts here. 



The largest and most typical family of the Rodents is the 

 Muridcz, which includes the Rats and Mice, and their numerous 

 allies. Wallace estimates the number of known species at 330, 

 which is probably well within the mark. As might be expected 

 in so large an assemblage of species, the variety of forms is very 

 great, but, broadly, the Common Rats and Mice, which are only too 

 familiar to most people, are characteristic types of the whole 

 series. The family, however, includes jumping forms, swimming 

 forms, arboreal and burrowing forms, and is almost absolutely 

 cosmopolitan in its distribution. It includes, as well as the Common 

 Brown Rat and the Common Mouse, the Black Rat, the Field and 

 Harvest Mice, the Hamsters and Tree Mice, the Voles, Musquash, 

 and Lemmings. 



The last-named animals are among the most remarkable of 

 the Muridcs, on account of the great migrations which they perform. 

 These migrations do not take place with any rhythmic regularity, 

 but from time to time the Lemmings start in vast swarms from 

 their mountain fastnesses, and make their way in a straight line 

 in some definite direction. Nothing seems to turn them from 

 their course, and they go straight on over hill and dale, and across 

 any lake or stream that may come in their way. Preyed upon by 

 carnivorous beasts and birds, their numbers diminish as the march 

 proceeds, and when they come to the cultivated regions, where they 

 do fearful damage to the vegetation, man joins in waging war 

 upon them. 



Crotch, who has published two or three papers on the Lemming 

 and its migrations, says that in Norway these animals always 

 proceed from the central backbone of the country in an easterly 

 or westerly direction, and that in either case the survivors of the 

 march drown themselves those that go westward in the Atlantic, 

 those that go eastward in the Gulf of Bothnia. 



The Common Dormouse is an elegant little somewhat squirrel- 

 like creature belonging to the family Myo%id<% ; its fur is of a 

 light reddish-tawny colour above, becoming pale and yellowish 

 on the under surface. It is widely distributed in Europe, ranging 

 from Britain and Sweden in the north to Tuscany and Northern 

 Turkey in the south. Nocturnal in habit, the Dormouse sleeps 



