THE MIGRATIONS OF MAMMALS. 241 



be the sole cause of migration; there is at the same time a further 

 incentive supplied by a plague much feared in the far north. For 

 the short summer on these expanses calls to life an insect-world 

 poor in species, but endlessly rich in individuals, particularly an 

 indescribable number of mosquitoes and bot-flies, which make life 

 a burden to the reindeer, as well as to man. To escape these the 

 reindeer forsakes the marshy tundra, over which dense clouds of 

 mosquitoes hover during summer, and hies to where the scourge is 

 less severely felt to the Alpine heights, which, in the summer season 

 afford their most fragrant pasturage. From inherited habit, the rein- 

 deer migrate not only at the same time, but along the same paths, 

 thus forming tracks which may be distinctly traced, traversing the 

 tundra for many miles, and crossing streams and rivers at definite 

 places. At the beginning of the journey, the cows with their calves 

 arrange themselves in herds of from ten to a hundred, and precede 

 the young stags and hinds, which are followed again by the old 

 stags. One troop follows directly behind another, and the observer 

 can count thousands as they pass. All hurry incessantly on, turning 

 aside neither for the mountains nor the broad streams which cross 

 their path, and resting only when they have reached their winter- 

 quarters. Packs of wolves, bears, and gluttons follow close on their 

 heels and often pursue them no small part of the way. In spring, 

 on the return journey, the animals keep to the same order, but the 

 herds are much smaller, and they travel in a much more leisurely 

 fashion, and keep less strictly to the paths by which they went. 



Journeys still longer than those of the reindeer are taken by 

 the American bison, the "buffalo" of the prairies. 58 What distance 

 individual animals travel cannot be stated with certainty, but herds 

 in course of migrating have been met from Canada to Mexico, from 

 the Missouri to the Rocky Mountains, and it may be assumed that 

 a single herd traverses a considerable part of the country lying 

 between these limits. The bisons have been seen in summer scat- 

 tered over the boundless prairie, and in winter in the same places, but 

 assembled in many thousands; their migrations have been observed, 

 for they have been followed for hundreds of miles along the tracks 

 the so-called "buffalo-paths", trodden out straight across plains 



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