OVERFLOWS OF MANI MALIAN LIFE. 11 alnant1R, including man, take part in certain much more rare but more universal and permanent removals. The most conspicuous instance is afforded by the lemmings of Central Norway and Sweden. which at uncertain intervals come down in vast hordes into the lowlands, as is fully described under LEM:SUNG. They travel by night, feed and mul tiply excessively, and in from one to three years the few which escape the hordes of enemies them reach the Atlantic or the Gulf of Bothnia. It is believed that these sudden ineursions are the dispersal of an tion in the ordinary habitat of the species, due to a combination of favoring eireumstances ing an increase of a naturally fecund race until the country cannot longer support the numbers. The animals are started abroad by famine, and continue the flight in aimless restlessness until an equilibrium is restored. The same thing hap 114." neea-ioual(t' with various other small rodents, 'Plagues' of mice have broken out fre quently in the grain-growing regions of Russia and in other parts of the world. In the western part of the l'nited Slates. until hunting kept down the stoek, there used to be irregular but prodigious movements of squirrels (normally extremely numerou. there). which would appear in droves over a wide range of country, all tr•avel ing steadily in one direction, until they gradually vanished. The writings of Audubon, I7odman, and other early naturalists contain many records of these movements, which did not cease until about 1840. The theoretic and historic incur sions of hinnan hosts from Asia into Europe, the spread of the Bantu races which overcall Africa, and similar 'waves' or 'migrations' of conquering men, fall into the same category. lint their
superior• adoptability has enabled them, or some of them, to remain and possess the land.
INsEers. The insects afford litany cases of mass movements, similar to those of mammals, and also a rare approach to true migrations. The swarms of 'grasshoppers,' or locusts, which oc casionally visit parts of Africa and Asia, are among one of the most familiar phenomena of those regions; and they are accompanied by a rapacious following of birds and mammals feed ing upon the traveling, hosts of insects, which disperse, dwindle, and finally disappear. In the United States the most disastrous incursions of these insects have been from the Rocky Noun taMs eastward. They are of irregnlar occur rence and the returning swarm the succeeding year is composed only of the descendants of the original emigrants—a fact which contains a hint as to the origin of the true migratory habit in others. Irregular movements, without, so far as we know, any a ttempi to return to the original home, are illustrated by the army-worm, chinch-bug (qq.v.), and cotton-worm (see C'oT TON-]NsECTs). These migrations are due to over crowding and lack of food. There are still slower migrations among insects, which may be termed 'spreading.' Thus the Colorado potato beetle, a native of the Rocky Mountain Plateau, spread eastward, when suitable food was offered it by the cultivation of potatoes, until it now occurs all over the potato region of Eastern North America, and, like the brown rat, it permanently occupies all the new territory it enters. A few insects (butterflies) are known to migrate in the sense that fish and birds do.