ing much exposed to storms are often driven far out of their 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 birthplaces, 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 remarkable 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 Norway, 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 overruns 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
