Page:Popular Science Monthly Volume 52.djvu/812

790 travel at night, but their march is not continuous, for they make long halts in fertile spots, where they are even more prolific than they were at home, so that they become more and more numerous, although they are attended by bears, wolves, foxes, dogs, and cats, and by hawks and owls, and other beasts and birds of prey, and although even the cattle and reindeer are said to kill and eat them. The march may last for several years, but as they never go back, but continue to move forward, they at last reach the ocean, and, attempting to swim this as they have all the rivers in their course, all are drowned.

While the migration of the lemmings is undoubtedly due to scarcity, it is difficult to understand its use, for at the present day the only ones to profit by it are those who have the instinct least developed and stay at home in the mountains, although it may have been useful to the species before the low lands were occupied by man, who now destroys the stragglers and prevents them from scattering and finding permanent homes.

While the determining influence is the scarcity which comes from overcrowding, we have no reason to believe the lemmings consciously and deliberately set out to find a better feeding ground, or that they have traditions of the rich low lands which attract them as the wealth and luxury of China and Mesopotamia and of the Roman Empire attracted the Tartars and Scythians and Goths from the sterile and desolate northern lands into the fertile homes of southern civilization.

Their journeys are no doubt initiated by an unconscious impulse, which, before it brought them into conflict with man, was useful in some way to the species; and this seems to be true of the migrations of certain prolific species of locusts and grasshoppers, which, inhabiting sandy deserts, often overflow the limits of their natural home, and invade more fertile regions where they are not usually found. While there is no reason to suppose these movements are undertaken through deliberate intention to find new feeding grounds, lack of food is no doubt the chief factor in the development of the migratory instinct of rodents as well as locusts, which latter resemble birds in ability to make long journeys on the wing without rest. The African locust has been met at sea in great clouds more than twelve hundred miles from land, and this species sometimes wanders from its home in Africa to England.

While the movements of rodents and locusts show that the search for food has much to do with migration, they lack the features which make the migrations of birds so remarkable. They occur at irregular intervals, while the movements of birds are almost as regular as the almanac, for, while sea birds seem much exposed to storms,