MIGRATION OF MAMMALS — MATTHEWS 279 



counterclockwise course during the winter, and then to bear away 

 northwest to the barren lands in spring. In the Mount McKinley 

 area of the Alaska range, however, the caribou that spend the summer 

 on the southern slopes actually move over in the winter to the northern 

 side where the snowfall is lighter. The migrations of the caribou are 

 connected partly with the food that is available, but they are connected 

 to an even larger extent with the appearance of the clouds of mosqui- 

 toes that make life a misery for man and beast. Shortly after the 

 deer move north the mosquitoes emerge in almost incredible hordes, but 

 as soon as the mosquito season is over the caribou return to their winter 

 quarters and thus they miss the worst of the mosquito plague farther 

 north. 



The other large North American migratory mammal was the bison. 

 It was long doubtful whether the bison was a truly migratory mammal, 

 but the analysis of a great mass of old records by the American 

 naturalist E. T. Seton has plainly shown that it was. But here again 

 the migration routes were not simply to and fro from north to south, 

 but were more or less in circular clockwise paths, some of the herds 

 joining one circuit, others another circuit. And the winter quarters 

 of at least one circuit were in the northern part of the range, although 

 other herds moved southward from 200 to 400 miles at the approach 

 of winter. 



Many other land mammals have similar seasonal migrations, though 

 on a much smaller scale. Our own red deer move down from the 

 higher hills during the winter to the less severe conditions of the 

 valleys, but the distances covered in such movements are trijfling com- 

 pared with the journeys performed by birds, some bats, and the aquatic 

 mammals. At the other end of the scale a migration to be measured 

 in yards rather than miles occurs every year when in the autumn our 

 population of feral house mice moves in from the fields to the corn 

 stacks. Such local moA^ements of limited extent are commonly found 

 in the smaller mammals. 



A kind of population movement that should not be confused with 

 true migration, which is essentially an outward journey followed by a 

 return, is the periodical emigration that is found in some species. In 

 this there is an outward journey, but no return ; the animals that quit 

 the land of their birth perish and leave no offspring to come back. 

 The most familiar example of an animal that emigrates is the lemming 

 of Scandinavia. Lemmings are small mouselike rodents that normally 

 live high up on the Norwegian mountains close to the timber line. 

 Their numbers, like those of many small rodents, are subject to periodi- 

 cal cycles of increase and decrease, which may reach an astonishing 

 size; as their numbers increase the population is crowded out of its 

 normal range and overflows down the hillsides. Finally, when it 



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