THE MIGRATIONS OF MAMMALS. 245 



snowless, and such pools of water as have been formed by the 

 extremely slight fall of rain or snow, are covered with a thick sheet 

 of ice. As soon as this sheet becomes so strong that the animals 

 inhabiting the Gobi are unable to break it, they are obliged to 

 change their quarters, and they travel not only to southern but to 

 northern lands, whose only advantage is that they are covered with 

 snow, for this affords ready refreshment to the parched tongues of 

 the wanderers, and oflFers less resistance to their weak feet than 

 the hard, unbreakable, and less easily melted ice. This is the 

 explanation of the fact that the antelope, of which great numbers 

 are found in the Gobi, forsakes a land which, save for the lack of 

 snow and therefore of available water, is exactly the same as that 

 which it chooses for its winter quarters. Not hunger, but thirst, 

 drives it from its home. At the beginning of winter, the antelopes, 

 at all times gregarious, assemble in herds of many thousands, spread- 

 ing over all the low grounds around their native plateau; they 

 often travel at the rate of fifty or sixty miles in a single night, 

 and extend their wanderings many hundreds of miles beyond the 

 boundaries of their proper habitat. The observer who follows them 

 can detect their tracks everywhere, and in such numbers that it 

 seems as though vast herds of sheep, far exceeding in number any 

 ordinary flock, had just passed by. 



Before the Chinese antelope begins its migration, restlessness 

 seizes the kulan or dziggetai, probably the ancestor of our horse, 

 and certainly the most beautiful and the proudest of all wild horses. 

 The foals of the summer are by autumn strong enough to be able to 

 endure a long journey with quick marches, and to bid defiance to 

 all the accidents and dangers of a wandering life. The young 

 stallions attain their full strength at the end of their fourth year, 

 and towards the end of September they leave the parent-herd and 

 press forward. Finally, the impulse to mate begins to animate the 

 older stallions and mares, and with it comes unrest and the desire 

 to wander. Thus the fleet, enterprising animals begin their annual 

 migration long before winter has set in, before even its approach 

 has become at all apparent; and on this account their migrations 

 at first lack steadiness and regularity, and have something of the 



