MIGRATION. The term migration is often used very loosely in popular writings about animals, so that it seems wise to define it as limited in the present article to: (1) The annual change of residence by a species with the change of seasons from winter to sum mer or the reverse; (2) irregular mass-move meats of a species under pressure of famine, over-population of a locality, or some more obscure influence. While these classes have been enumerated in the order of their promi nence to our eyes, especially in the behavior of birds, it will be well to consider the second sort of migrations first, as these sporadic cases may throw light on the more regular phenom ena, and how they came to be habitual.
An eminent entomologist tells us that certain butterflies, as our milkweed frit illary (Arsosia plexippsts) and the cotton moth (Melia argillacea), pass northward in the United States for hundreds of miles in spring, and again in huge swarms southward in au tumn; but whether the individuals are the same is not determined. (See MILKWEED Burrgastv). Among other butterflies periodical migrations occur, as in movements of vast columns across the Isthmus of Panama out to sea, and flights miles in breadth have been observed to cross Ceylon, the individuals occupying several con tinuous days in their passage. Wallace ob served the swarming of pierid butterflies in the Indian Ocean, and Clark in Venezuela, the vast throng composed of males moving steadily east ward for several days in the face of the trade winds.
The late Dr. A. S. Packard, whose special studies of the habits of the Rocky Mountain locust about 1880 were so valuable, reported that that destructive insect is migratory in cer tain seasons favorable to the species when over production occurs; the young on hatching, after having devoured every green thing at hand, are forced, when becoming winged, to rise in enor mous swarms and sail on the wings of the wind for hundreds of miles to other regions where they lay their eggs. The next year's brood sometimes returns to the original spawn ing ground to lay their eggs. The same thing
is characteristic of similar locusts in Syria and central Africa.
The members of several fam ilies of crabs, mostly tropical, have acquired the power of living out of water, and even of wandering extensively inland, hut regularly re turn to the sea, sometimes in marching hordes, to deposit their eggs in the water, after which they go back to the highlands. See LAND CRABS.
Many kinds of fishes are regular migrants; the anadromous families, such as those of the shad, herring and salmon, annually ascend the rivers to spawn, whence in some cases they return to the sea, in others never get back, but their young, after the succeeding winter, go back to salt water. Certain fishes retire to the deeper or warmer parts of the ocean during the winter, but in early summer travel toward the shore-shallows, or to the cool north, in vast swarms; and the same is true of a large variety of other marine creatures, in cluding some of the humblest and most minute forms, in which cases the direction of the mass movements are largely determined by the ocean currents. Moreover there occurs in the ocean a regular movement of deep-sea forms toward the surface in the night, the animals sinking again as daylight approaches. Fishermen in the north Atlantic and on the coast of Norway are familiar with the vast influx in the spring of such fishes as herrings, cod, plaice and cape lan. The eminent Norwegian naturalist, G. O. Sars, concluded that some of these fish-migra tions were undertaken in order to obtain food, and others for the purpose of reproduction. the capelan gathers in millions on the coast-banks of Finmark or Labrador, when countless numbers of cod approach the banks of Lofoten, and when the herrings flock to western Norway, they migrate to spawn. The fat-herring collecting off the coast of Nordland, and the cod gathering around the shoals of capelan in the Barents Sea, are examples of f eeding-migrations.)) Sea-turtles have a similar history, going regularly toward shore in the breeding season to deposit eggs in the beach sand.