Page:The New International Encyclopædia 1st ed. v. 13.djvu/526

MIGRATION. 474 MIGRATION OF ANIMALS. cases the tribes traii.-lVn-Lil themselves to the conquered countries to a considerable extent and maintained a restless life until their unstable military States were broken up by new conquerors or by their own dissensions and lack of capable leadership. .See Mongol Dynasties; Timur; TrRKEY. section Ethnology. For the effect of the Germanic migrations on the history of Kuropean civilization, see Er- HOPE. section on History. For bibliojp-aphy, see the articles on the various barbarian nations, as Franks. Goths, etc. MIGRATION, Wagner's Law of. In evolu- tion, one lit tlie main agencies leading to the isolation of animals, and consequently to the for- mation of local races, varieties, and -species. This was first pointed out by Moritz Wagner in 1868. He stated his views in three general propositions: ( 1 ) The greater the change in the conditions to which individuals are subjected on emigrating to another territory, the more intense must be the inherent individual variability of each organism ; (2) the less the even tenor of this increased in- dividual variability of organisms is interrupted by frequent crosses with emigrants of the old stock, the more frequently will nature be success- ful in forming a new variety or incipient species by the accumulation and inheritance of fresh char- acteristics: (3) the more advantageous to the variety the change in each single organ, the bet- ter it will be able to adapt itself to surrounding circumstances: and the longer the selection of an incipient variety of colonists remains undis- turbed by the old stock, the more frequently will a new species arise out of the variety. These laws arc fully illustrated by Wagner in his works, and by later observers. Wagner claimed that evolution by natural selection is impossible, unless it be assisted by geographical isolation, in order to prevent the swamping elfects of inter- crossing. The numerous facts obse^^■ed by ^ ag- ncr and others show that some of the barriers isolating incipient species are broad rapid rivers, oceans, and mountains. He speaks of the ex- traordinary phenomena of so many species of plants and animals peculiar to the volcanoes and isolated mountains of Quito. "Without the dis- tribution of organisms by migration in connection with local selection it would be inexplicable. The gigantic isolated mountains of this highland act a similar part in the forinatiim of varieties and species as the islands of the archipelago, for instance, of the Galapagos." Wagner also claimed that adaptation to changed conditions of life and transformation appear to be tantamount to a renovation. Species, he said, which did not mi- grate, and consequently did not alter in form, liecame extinct. Consult Wagner. The Dnririn- ■ian Theory nml the Liiir of the .Uirjrntion of Oryan- ■i-ims (Jlimich. 1808: trans, bv Laird, London, 187.'?). MIGRATION OF ANIMALS. The word mi- gration is used in two senses: either to refer to those periodical changes of location such as are made by many species of birds and fishes, by some mammals, and by a few insects: or to those irregular di-*persions caused by overcrowding and lack of fond or water. MA>t.tAi,s. Among mammals migrations in the first «cnse are mainly confined lo certain Cetacea. which regularly move from and to the polar seas, with the alternating seasons. Regular seasonal moveiiieuts, truly migratiiry but short, are practiced by many land animals, especially the herbivores. All deer, goats, sheej). antelopes, and the like, which dwell in mountainous regions, regularly ascend the heights in early summer to get the new grass, find safer solitudes, escape the lowland flies, and otherwise better themselves. In the fall they come down as the snow and cold increase upon the heights, and seek the valleys or the neighboring plains. The American bison formerly was wont to retreat from the moim- tains to the plains during severe winters, while those on the northerly plains tended to move south. Before the time of railroads the great body of the pronghorns of the plains used to migrate from the northern area of their range to the milder regions south of the Platte River, and returned north in the spring. Still more striking is the regular and prolonged annual migration of the caribou from the Arctic shores of America to the southerly interior, especially in the region north and east of Hudson Hay: they cannot live so far north during the winter, but go back as soon as the snow melts in spring. In all these cases there is an accompanying migra- tion of certain large predaceous animals, such as wolves, which depend upon the grazers for food. Similar facts may be cited from the plain re- gions of Asia, Australia, Patagonia, the Sahara, and South Africa, where seasonal changes, either of cold or drought or the parching of pasturage, compel annual migrations to and from other regions not far distant. It will be seen that these movements are under compulsion of the lack of food (or frequently in desert regions of water), and are eontintied only when and so far as is necessary. Mammals are too slow and hampered in their movements on land to make long, rapid journeys, such as a bird or fish is able to accomplish through the unobstructed air or water ; and most mammals either can find fond all the year round, or have acquired the [xiwer, by storage of provisions or by sinking into dormancy, of tiding over the seasons of scarcity. Overflows of Mammalian Life. Mammals, including man, take jiart in certain much more rare but more universal and permanent removals. The most conspicuous instance is afTorded by the lemmings of Central Xorw;iy and Sweden, which at uncertain intervals come down in vast hordes into the lowlands, as is fiillv described under Le.mminc. They travel by ni.ixht. feed and mul- tiplv excessively, and in from one to three years the few which escape the hordes of enemies following them reach the .tlantic or the Gulf of Bothnia. It is believed that these sudden incursions are the dispersal of an overpopula- tion in the ordinary habitat of the species, due to a combination of favoring circumstances caus- ing an increase of a nalurally fcctind race until the countrv cannot longer su])port the nmnbers. The animals are started abroad by famine, and continue the flight in aimless restlessness until an equilibrium is restored. The same thing hap- pens oceasionallv with various other small rodents. 'Plagues' of mice have broken out fre- quentl.v in the grain-growing regions of Southern Russia and in other parts of the world. In the western part of the I'niled States, imlil bunting kept down fhc stock, there used to be irregular but prodigious movements of squirrels (normally extremely numerous there), which would appear