62 STEPPE FAUNAS AND TEMPERATE 
horns are shed annually and are branched. The con- 
trast between the living, or recently exterminated, 
ungulates of the Old and New Worlds is the more 
remarkable when we find that America has many fossil 
horses, though no living ones. 
In proof of the statement just made as to the resem- 
blance between the steppe rodents in the Old and New 
Worlds, we may note that marmots occur in both 
regions, that the susliks of Asia are represented by the 
forms called gophers in America, which belong to the 
same genus, and that the jumping mouse of the American 
prairies takes the place of the jerboas and their allies 
in the Old World. As the adaptation to steppe life is 
similar, we shall refer chiefly in the following descrip- 
tion to the rodents of the Asiatic steppes. 
The susliks (Spermophilus) are related to the squirrels, 
but the common steppe form (8. citillus) differs in the 
very short tail and the minute ears, both adaptations 
to the terrestrial and burrowing habit, and markedly 
contrasted with the long tail and large tufted ears of 
the common squirrel. Like other steppe rodents the 
susliks are social and burrowing animals, hibernating in 
winter, but storing in autumn a large collection of roots, 
seeds, berries, &c., for winter use. Like the squirrels, 
they are not averse to a certain amount of animal food, 
taking small birds and their eggs when occasion offers, 
and also small rodents belonging to other genera. 
The marmots (Arctomys) form an interesting genus, 
with representatives both on mountains and in the 
steppes. We shall see later that it is not unusual to find 
relationships between steppe and mountain animals, 
a fact which has a double significance. In the first 
place, just as the tundra forms a steppe because it is 
too remote from the Equator for tree growth to occur, 
