390 MURIDA—LEMMI 
[Group LEMMI. 
Lemmings are short-tailed microtines. Their external 
appearance as typified by Lememus 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 7umdren und Steppen (1899, i., 60), and by Stejneger 
(Amer. Nat., 1901, tot), relying on Merriam’s “law” of the distribution of animals 
and plants in definite climatic life-zones (Vational 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. [rish Acad., September 1912, No. 17, 9). All 
