MIGRATION OF ANIMALS. 475 MIGRATION OF ANIMALS. in droves mi'i- a widu ranjic of foinitiy. all travel- ing; steadily in one direetion, until they {jradually vanished. The writings of Auduhon, tiodman, and other early naturalist.s contain many records of these niovenients, which did not cease until about 1S40. 'I'lio theoretic and liistoric incur- sions of human hosts from Asia into Europe, the spread of the IJantu races which overran Africa, and similar 'waves' or 'migrations' of conquering men, fall into the same category, but their superior adaptability has enabled them, or some of them, lo remain and possess the land. Insects. The insects afTord many 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 Roel^y Moun- tains eastward. They are of irregular occur- rence and the returning swarm the succeeding year is composed only of the <lesccndants of the original emigrants — a fact which contains a hint as to the possible origin of the tnie migratory habit in others. Irregular movements, without, so far as we know, any attempt to return to the original home, are illustrated by the armj'-worm, chinch-bug (qq.v. ), and cotton-worm (see COT- Tox-lxsEcTS) . 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, spreaa eastward, when suitable food was offered it l)y 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 ibutterthes) are known to migrate in the sense that fish and birds do. Reptile.s and Fishes. Such phenomena are entirely unknown among reptiles, for obvious reasons, with the possible exception of sea-going turtles, which may withdraw into dee])er water or more southerly latitudes in winter than in summer, ilany fishes ])erforni long and com- plex wanderings, but how they are guided in some cases across and up and down the ocean will be a very difficult problem to solve. Salmon and other anadromes come from the sea each spring, and ascend hundreds of miles up rivers 80 as to spawn in places suited to the needs of the young. The fish .so bred return to spawn in the water of their birth, as has been demon- strated by marking smelt that have been trans- planted and hatched in rivers previously un- occupied by them ; the marked smelt returned from the sea to spawn in the river of their adoption. Experiments upon herring along the Massachusetts coast confirm this conclusion. Sea fishes generally retire to comparatively deep water, and probably many species go southward in winter, while in summer they spread north- ward and approach the shores, river-mouths, or other spawning jilaces. These migrations are in- duced by reproductive desires and necessities, and the slight variation in the time of the com- ing of each species, which fishermen expect with Vol. X(II._31. fair regularity, seems due to variations in the temperature of the water. It is probable that even these ocean wanderers return to the same part of the coast where they were bred, and that in some cases, as of the Atlantic salmon, exag- gerated notions have prevailed as to the distance to which they go in winter. Sec Salmo.v. The Migk.tion of Birds. More conspicuous and interesting, and quite as difiieult to explain, are the migrations of the birds, which have been the theme of poetry, homily, and fable, as well as a baffling subject of inquiry, ever since men began to notice the ways of animals. Most persons have a vague idea that the habit of yearly migration among birds is uni- form and universal ; but this is not so. Most birds do not migrate at all, and among those that do great diversity exists, so great that the custom seems almost an individual rather than a racial one. The whole body of ratite birds — ostriches, rheas, cassowaries, and the like — are non-migratory. The fish-eating sea-fowl make no more of an anniial migration than is necessary to escape from the ice and darkness of their most polar haunts to where there may be open sea. These are wanderers rather than migrants. Gulls and terns, geese, ducks, and the wading marsh and beach birds are in the main migra- tory, and include some of the most remarkable examples. Of the game-birds fewer are real migrants, but here again a few notable exceptions exist, of which one of the most familiar is that of the common Eui'opean quail, which has been taken so numerous!}- for centuries on both sides of the Mediterranean, and whose migratory flocks still feed travelers wandering in the wilderness of Sinai. The pigeon tribe is sedentary as a rule, also, yet one of its species — the passenger pigeon of North America — has be- come the very tyjje and exem|)lar of a migratory bird. Many, but not all, birds of prey regularly migrate, but it is a question whether thej' do not, in most cases, accompany the movements of the smaller birds rather than travel of their own impulse. Parrots are almost wholly non- migratory. It is not, then, until we have passed twenty-one of the twenty-three classified orders of birds (with the exceptions above noted) that we come to those groups — the picarian and pas- serine birds — in which the custom of seasonal migration is a prominent characteristic. These are. to be sure,, the most numerous as well as the most highly organized orders; yet a large number even here do not migrate at all from tem- perate regions, but form a 'resident' or 'partially migrant' population in all moderate latitudes, where they remain all the year round. On the whole, the large majority of the total list of the birds of the world are non-migratory to any con- siderable degree. ^^'hen we examine the minority which does an- nually alternate between southerly winter and northerly sunnner residences, many curious facts are discernible. First, it is noticeable that all migratory birds belong to the colder latitudes of the globe: and. on the other hand, that those groups which are wholly non-migratory represent the primitive types — birds whose ancestry goes back to times when a comparatively warm cli- mate prevailed over the now unbearably cold and sterile polar regions. In general, two-thirds of the birds of the middle temperate zones, both
