390 MURID^— LEMMI 



[Group LEMMI. 



Lemmings are short-tailed microtines. Their external 

 appearance as typified by Lemmus is described below. The 

 skull is broad and massive ; the lower incisors are short, their 

 roots ending on the inner sides of the cheek-teeth, the crowns 

 of which in the upper jaw are of about the same width from 

 front to back. 



The lemmings are a highly specialised modern group of a 

 relatively low and decadent type, formerly of wider distribu- 

 tion, but now in course of replacement by more dominant 

 genera. They reached their zenith in the late Pleistocene, the 

 mammalian fauna of which indicates a climate supporting 

 a plentiful vegetation. The bulk of this fauna is now 

 extinct or the members widely dispersed, some to arctic 

 regions, others to mountains, others to islands of mild climate 

 — a combination showing that retreat from competition with 

 the present entirely distinct fauna is the key to their present 

 distribution. Far from being confined by choice to a polar 

 environment, it is probable that the latest glacial period was 

 injurious to the lemmings to the extent of partial extermina- 

 tion, and they would not now be restricted to polar regions did 

 their foes permit them to exist elsewhere. They certainly, in 

 their well-known "migrations," make many attempts to colonise 

 new districts, and these are believed to be invariably failures, 

 owing to the ease with which, in the absence of snow, 

 they are captured by carnivorous enemies. They survive in 

 apparently undesirable regions, because, like the varying hares, 

 they have the power of thriving on coarse, innutritious food, 

 and because, thanks to their small size, they are enabled to 

 shelter themselves under the snow, and thus not only to escape 

 being eaten, but to remain active throughout the winter. 



Note. — The arguments for lemmings being essentially "arctic" animals have 

 been given by Nehring in his Tundren und Steppen (1890, i., 60), and by Stejneger 

 {Amer. Nat., 1901, loi), relying on Merriam's "law "of the distribution of animals 

 and plants in definite climatic life-zones {National Geog. Mag., 29th December 1894, 

 229-238). The dependence of mammals, except in very special cases, primarily on 

 food rather than on climate has been emphasised by me in the " Mammalia of the 

 Clare Island Survey" {Proc. R. Irish Acad., September 1912, No. 17, 9). AH 



