MIGRA'TIONS OF ANIMALS, which must not be confounded with their diffusion over a rnore or less extended area, are apparently always guided by an instinct operating on all, or nearly all, the individuals of a species, and leading them to move in a definite direction in search of food or (in the case of fishes) of a fit position for spawning.
Among mammals, such migrations are comparatively rare. The most remarkable instance is that of the lemmings, which at no definite epochs, but generally once or twice in a quarter of a century, traverse Nordland and Finmark in vast hosts, ending their career in the western ocean, into which they eiater, and come to a suicidal end; or, taking a direction through Swedish Lapland, are drowned in the gulf of Bothnia. M. Martins, who WAS a member of the great scientific Scandinavian expedition, seems to doubt the generally entertained view of these animals casting themselves into the West ern ocean, and believes that most of them perish from the cold in crossing the rivers, while many are killed by dogs, foxes, and a species of horned owl (strix ln.ackyolos), which in large numbers always accompanies these emigrations.
According to Gmelin, the Arctic fox (vulpcs lagoints) always accompanies the lem mings in such numbers that, on this ground, it is entitled to be considered a migratory animal; but independently of these special migrations, it is stated by sir James Ross that "the young generally migrate to the southward late in the autumn, and collect in vast multitudes on the shores of Hudson's bay; they return early the following spring to the northward, and seldom again leave the spot they select as a breeding-place." The spring-bok (antidoreas euchore) is accustomed to make pilgrimages from one spot to another in the vast plains of southern Africa. Herds of many thousands are led by their chiefs in these migrations, and the wonderful density of the moving mass may be imagined from the fact that a flock of sheep has been inextricably entangled and carried along without the possibility of escape. Want of water is said to be the cause of these migrations, but Dr. Livingstone thinks that there must be other causes.
The occasional incursions of wolves, in very severe winters, into districts in which they are not commonly found, and the long excursions of large groups of monkeys (entellus and rhesus), hardly fall within the scope of this article.
.Many of the cetacea are probably migratory. " The migrations V the porpoise (phoecena communis) appear—says Marcel de Serres in his prize-essay, Des Causes des .Migrations des divers Animavo, p. 63--to be as periodic as those of certain species of birds. During the winter, they constantly proceed from n. to s.; and when they feel the
warmth of summer, they turn northwards. Thus they are common in summer in Green land, while they are rare on our own coasts, where they abound in winter." The number of species of birds that periodically migrate is so great that it is impossi ble to find space for a list of them. Marcel de Serrcs, in the work already quoted, gives a " Tableau de l'Epoque des Passages des Oiseaux," which extends over nearly 100 pages. See BIRDS OF PASSAGE. The desire for a suitable temperature and the search for their proper food are the apparent causes stimulating birds to these migrations; and in most instances especially in the case of insectivorous birds, the food is intimately associated with the temperature.
The migrations of many species of fishes are as remarkable for their regular perio dicity as those of birds. In some cases, fishes that are produced in fresh-water streams migrate to the ocean, and after spending some time in salt water, return (generally, with singular instinct, to their own birthplace) to fresh water to propagate their species. Soine of these fishes—as, for example, the lamprey (petrinnyzon marinus)—spend most of their lives at sea, and others, a.s the salmon, in fresh water. The remarkable migrations formerly, but ertoneously supposed to be made by herrings, are noticed in the article on that fish. Many fishes of the same family as the herring, the for example, the sprat and pilchard—leave the deep sea for shallow water during the spawning period, when they approach our coasts in vast shoals. All such migrations as these seem mainly due to a reproductive impulse. See FISHES, LAND-CRAB.
Amongst insects, the locust (loeusta migratoria)is most remarkable for its migrations. These insects are probably produced much more abundantly some years than others, and as in such years their birthplace cannot afford them sufficient vegetation, they are led to migrate in search of food. Some idea of the occasional extent of their wanderings may be formed from the fact that, in the early part of 1810, myriads of locusts appeared in Bengal, from whence they proceeded westward completely across the great Indian penin sula to Guzerat and the neighboring provinces, from whence they pursued their course southwards towards Bombay, the whole period of their migration extending over between two and three years; while, in relation to their numbers, capt. Beaufort calculated a swarm that appeared at Sardis, in Asia 3finor, in 1811, at upwards of 168,000,000,000,000.