The reader will have perceived that most of the foregoing cases are not ex amples of true migration because the element of habitual return is absent, or at best a very few survive to return; or else the movement, when seasonal and regular, is purely local, such as going to the nearest shore for spawn ing; or is merely the pursuit of traveling prey. In this class must be put most of the so-called migrations of mammals. From time to time certain small animals, as lemmings, field-mice, squirrels and the like, develop enormous num bers in some region (or formerly did so, before civilization was so worldwide) and overflow in great °armies') into neighboring parts of the country, where they gradually expire. In the plains regions of the world a lack of good pasture in one place will often cause movement of antelopes, bisons, etc., to some better dis trict in great herds; and in other situations the wild animals are accustomed to go up into the hills in summer and come down to the shelter of the valleys in winter, but these are local movements. The only examples of real migra tion afforded by the mammals are the case of certain bats that regularly journey every year between the tropics and more northern climes, and the case of the caribou and, to a less extent, the reindeer. These deer do make a real fall migration from the barren Arctic coast to the margin of the forested region southward and go back in the spring. It is to the birds, then, that we must turn for a study of migra tion in the stricter sense of the term, and even here it is only partial as regards many species and groups.
Migration of Birds.— This subject is so large that we can give no more than a super ficial sketch, following in general the lines of investigation conducted by the late Wells W. Cooke, of the United States Biological Survey, who devoted almost his whole life to a study of this phase of ornithology as exhibited espe cially in North America.
The motive or cause of Inquiry periodical migra tion of birds has excited nquiry since ancient times, and at present two different methods of explaining it are in vogue. The opinion is general that in Pleistocene times, just previous to the advance of the cold climate and finally to the great accumulations of ice and snow over the northern parts of the world, the whole of the northern hemisphere possessed a mild climate, and birds of every sort dwelt com fortably all the year round throughout virtually its whole extent. The coming of the Glacial Period so affected the north, as to limit more and more the residence in winter of birds there, although in summer they might venture somewhat toward it, when vegetation and in sect life annually revived. As the ice advanced very gradually, now and then receding, but on the whole enlarging itself, these enforced north ward and southward movements of the birds increased both in distance and duration, until migration became a fixed habit with all birds whose life was affected by the change of cli matic conditions. Finally most northern birds were restricted all the year to middle America and the Mediterranean region and southern Asia. But the habit of migration had been formed, and when the glacial ice began to retreat toward its present position, the birds annually followed its receding margin, until at last they had established their present long and diversified migration routes.
Thus far all theorists are in substantial agreement. The divergence is as to the pre vailing motive. One school argues that a long ing to continue their inherited habit of resi dence in the north, and individually to return to their birthplaces, is the incentive that com pels them to leave the tropics and make a jour ney, often of surprising length, every spring. The other school maintains that "the birds' real home is in the Southland)); that that re gion becomes overcrowded, and the birds in annually flying northward are seeking a region where there is less crowding and less com petition for food. The truth perhaps lies in
a combination of these influences, varying in intensity with different kinds of birds. It is an important circumstance, especially with ref erence to the second theory, that no similar migration occurs southward from the tropics to Bolivia and Argentina, whose plains and mountains offer a poor supply of bird-food, and not much more from the equatorial to South-African districts. "The conclusion is in evitable,)) Cooke believes, "that the advantages of the United States and Canada as a summer home, and the superb conditions of climate and food for successful rearing of a nestful of voracious young, far overbalanced the hazards and disasters of the journey thither. It must be remembered too that the migratory species have acquired various adaptations relating to their migrating habits that tend to fit them more and more to endure the exertion and danger required; also that the regular routes followed by each species are the products of thousands of generations of experience, and presumably represent the easiest way in each case.
Phenomena of In the restricted space of this article it is impossible to go into detail as to the general subject, and attention must be confined mainly to what ap pears in North America. Australasia and the South Pacific islands share to some extent in the annual movements of continental species, but have an inter-insular migratory system of their own. "In Europe,') says a recent re viewer, "and central Asia there are numerous routes, at least nine, according to Palmen. Of these one begins on the Siberian shores of the Polar Sea, Nova Zembla, and the north of Russia, and passes down the western coast of Norway to the North Sea and the British Isles; another arising in Spitzbergen follows much the same course, but is prolonged past France and Spain to the west coast of Africa. Many migrants wintering in North Africa (Algeria, etc.) have flown there from northern Russia, by way of the Baltic Sea, Holland, passing up the Rhine Valley, and crossing to the Rhone, the column splitting on reaching the Mediterra nean, one line of migration passing along western Italy and Sicily, a second crossing by way of Corsica and Sardinia, the third by southern France and eastern Spain. Egypt re ceives its winter visitors from the Russian river-valleys of the Obi and Volga, the line crossing the Black, Bosporus and iEgean seas to the Nile Valley. One important migration route is to and from India along the Danube Valley and across Persia?' An important fact to consider at first is that the migratory habit is possessed in its com pleteness by comparatively few birds, and these belong almost wholly to a single order, that of the Passeres, or insect-eating song-birds. The exceptions are mainly seafowl and certain water-birds. Even in these groups two classes are to be found, one of species that are resi dent the year round in the regions they sever ally occupy; and the other whose migrations are of slight extent. Even in the northern half of the United States many birds are pres ent in winter, some of which are those that retreat from the North only so far as driven by deep snow and excessive cold; while the Southern States have a longer list of resident birds, supplemented in winter by many kinds that moved only a little way southward to es cape the dearth of food at that season in the snowy parts of the country. This shows that in respect to distance, migration varies from a distance almost as great as the breadth of the globe (the Arctic tern passes annually from Patagonia to Alaska, and back again) to no change of residence at all, even on the Arctic coast and islands.