4. Each main current of migration produces a compensating countercurrent.
5. Migrants proceeding long distances generally go by preference to one of the great cities of commerce or industry.
6. The natives of towns are less migratory than those of the rural parts of the country.
7. Females are more migratory than males.
Authorities.—The statistics of migration are to be found in the official returns of different countries, especially the statistical tables relating to emigration and immigration published by the British Board of Trade, and the Reports (annual) of the Commissioner-General of Immigration of the United States. For general discussion see Philippovich, Auswanderung und Auswanderungspolitik (Leipzig, 1892). An exhaustive bibliography will be found in an article by same author, “Auswanderung,” in Handwörterbuch der Staatswissenschaften; R. Mayo-Smith, Emigration and Immigration, with bibliography (New York, 1890). For internal migration see A. F. Weber, Growth of Cities (New York, 1899). See also Ravenstein, “The Laws of Migration,” in Journal of Royal Statistical Society (1885 and 1889). Professor Flinders Petrie, in his Huxley Lecture for 1906 on Migrations (reprinted by the Anthropological Institute), deals with the mutations and movements of races from an anthropological standpoint with profound knowledge and originality. (R. M.-S.; T. A. I.)
Migration, in Zoology. In zoology considerable importance attaches to the problems of migration, by which is meant the wandering of living creatures into another, usually distant, locality in order to breed there; this implies a return, and the double phenomenon is annual. All other changes of the abode are either sporadic, epidemic, or fluctuating within lesser limits. Further, migration should not be confounded with “spreading,” which proceeds steadily, and in epicycles, with a totally different result. It need not be emphasized that hard and fast lines between these phenomena do not exist; they are often a question of degree. For instance, when the common toad, which is a strictly terrestrial creature, wanders every spring to a frequently distant pool in order to spawn there, this is a true migration. The same applies, strictly speaking, to those insects which hibernate in the ground, at the root of the tree on which they feed and breed. The grey plover breeds in the arctic circle and winters in equatorial countries. To complicate matters further, it is not necessary that the migration be undertaken periodically, more than once, by the same individual. For instance, the common eel ascends the rivers as an elver in its youth; years after it returns to the sea, there to breed and to die, whilst other fishes come and go, year after year. Further, some of the larger birds, for instance swans and cranes, are still immature in their second year, and yet they migrate like their older relations. It seems permissible to use this fact as an indication that the breeding as such is not the prime reason of their wanderings. The fundamental impelling agent must have been the want of food, and what we usually understand by migration cannot suddenly have sprung into existence to its full extent, but is more likely the cumulative effect of the doings of countless generations. The faculty of shifting the abode was of course always there, the necessity of moving further on was also present, and those which went in the wrong direction came to grief, while the others flourished and returned with their progeny. They did not at first cover enormous distances, but just enough to find unoccupied ground. The annual repetition became an established habit, at last an ineradicable instinct. There can be but little doubt that the prime impulse was want of food. The new growing grass on the prairie or on the veldt attracts every year those creatures which live upon pasture. The inter-tropical belt of the world is so crowded with creatures that there is the keenest competition, whilst in the temperate and cold regions is a long winter quiescence unfit for the support of many creatures, whereas in the summer these same regions are covered with new vegetation, with its concomitant abundance of insects and other invertebrates. The tables are decked again, and these opportunities are not wasted.
The process of migration, in its most striking cases, is now very complicated. Many a bird goes actually to the arctic regions for the shortest of summers, but spends most of the year within the tropics. On the other hand there are many species which do not go so far north, but stop to breed in the intermediate regions. We must not take the extremes when trying to unravel the development of the problem. The periodical migrations of mammals, with their more limited extent and greater leisure, are less perplexing. It has been argued with some show of reason that the real home of a bird is that country in which it was born, in other words where the species breeds, but this is not in every case a valid conclusion. It applies to most creatures, but it can well bear exceptions if we leave sentiment aside. When it comes to a question of domicile, the ten weeks’ sojourn of the swift, Cypselus, in England are more than weighted down by the nine months or more which these birds spend in southern countries, although we do not know whether they are resident there or roam about. The breeding time is the busiest period of a bird’s life; then the numbers of each species are suddenly multiplied, and so is the stress of providing food, and the particular food which is best for the young may not be available in every country. The idea that the arctic circle is the original home of the numerous kinds of birds which breed in it, whence they are now periodically driven away by stress, has been coupled with the glacial epoch, that supposed solution of so many difficulties. We have only to assume that the old, permanent home of these migrants was in the arctic region, that the progressing glaciation drove them away, of course towards the equator, and that, when times improved again, the birds returned to their old home. This sounds very plausible, but it involves huge assumptions. The birds, not the individuals, but the species, are supposed to have inherited such a loving reminiscence of their old home, that after thousands of years—with most of the small birds meaning as many generations—they returned at the first opportunity. It implies that their long continued sojourn in foreign lands, where—under this assumption—thousands of generations must have been bred and have spent all their lives, was not sufficient to naturalize them, so to speak, in other words to supplant the instinctive love of the primary ancient home. That the last glacial epoch has driven the limit of many kinds of animals and plants farther south is as certain as that many have recovered the lost ground after the reversion of the glaciation, but it must have been a very slow and steady process of spreading. It may, and probably does, account for the present annual visitations of arctic lands, as a phenomenon which has been evolved de novo, which would have come to pass even if no birds had existed in pre-glacial times.
How do birds manage to find their way, thousands and thousands of miles across land and water? This question has been extolled as a mystery of mysteries. It has been stated that the old birds show the way to the young, a speculation which does not apply to those many cases in which old and young notoriously travel at different times. It has been assumed that they travel by sight, taking advantage of certain landmarks; another untenable idea, since—experience having to be excluded in a flock of birds which made the journey for the first time—it implies that the young must have inherited the reminiscence of those landmarks! Others have likened the bird to a kind of compass, because in eastern Siberia E. von Middendorff found some migration routes to coincide with the direction of the magnetic pole. The whole question reduces itself to a sense of direction, a faculty which is possessed by nearly all animals; in some it is present to an astonishing extent; but the manifestations of this sense vary only in degree. The cat which escapes out of the bag finds its way back, directly or after many adventures. The bee, after having loaded itself with pollen, returns by the proverbial line to the hive which may be a mile away, but, move the small entrance hole in the meantime an inch to the right or left, and the bee will knock its head against the hive and blunder about; move the hive a few yards and bee after bee returning will be puzzled to find its hive again. They, maybe with the help of landmarks, have accustomed themselves to steer a course. Such instances need not be multiplied. The principle is the same whether
