MIGRATION. 789 



ing much exposed to storms are often driven far out of tlieir path; 

 but this is not always the case, for the great albatross follows ships 

 across the whole breadth of the South Pacific, or nearly half the 

 circumference of the earth. Many birds seem to make their whole 

 journey by a single flight, for some which are common in the West 

 Indies and in Nova Scotia are almost unknown within the limits of 

 the United States, making the whole journey past our borders by 

 water and probably by a single flight. The bluethroat, which 

 breeds in the northern part of Scandinavia, is so seldom found in 

 Europe south of the Baltic that there seems to be good evidence that 

 it makes its whole journey to its winter quarters in the region of the 

 upper Nile by a single flight. 



There is no reason to suppose all migratory birds inherit this 

 habit from a common ancestor, nor that its purpose is always the 

 same, and many birds of prey seem to have acquired it by ranging far 

 in winter in search of food, and by following their prey into warmer 

 regions, to return to their birthplace in the breeding season. 



In those cases the birthplace may have been the original home, 

 before the migratory habit was acquired, and scarcity of food the 

 reason why it was acquired; and 'the influence of scarcity in causing 

 migration is well shown by the occasional or irregular migrations of 

 certain prolific animals which do not ordinarily leave their birth- 

 places, although, when these become overstocked, migrations take 

 place, just as human colonists go out from thickly settled countries 

 to find new room for growth in foreign lands. From time to time, 

 at irregular intervals, great armies of the smaller and more prolific 

 rodents, which usually spend their lives where they are born, are met 

 on the march from homes where overproduction has exhausted the 

 food; and several of the older American naturalists have described 

 the migrations of our gray squirrel, although the phenomenon has 

 been most carefully studied in the Norwegian lemming, whose re- 

 markable migrations have figured in literature for centuries. The 

 lemming is a small, restless, pugnacious, and very prolific rodent, 

 which at uncertain and irregular intervals of from five to twenty 

 years migrates from its home in the central mountain chain of Nor- 

 way, and invades the low lands so suddenly and in such numbers 

 that it is still popularly believed, as in the day of Olaus Magnus, 

 who wrote in 1490, to drop from the sky. 



The great army of lemmings travels in a straight line and over- 

 runs the cultivated country, swimming the lakes and rivers, and 

 causing so much destruction that a special formula to be employed 

 against it was at one time authorized by the Church, which attempted 

 to check its march by exorcism, just as the old Bishop of Montreal 

 tried to drive away the wild pigeons by anathemas. The lemmings 



