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592 directly to the sphere of natural science. The anthropologist and sociologist alike are called upon to take cognizance of the same phenomena. The physical and social sciences are equally involved in the determination of their laws. Certain problems of city life are foremost among these questions, which lie on the border line between what were once widely separate sciences.

The most conservative societies in Europe are really to-day a seething mass of moving particles, viewed with the statistical eye. To borrow a familiar figure, a great population almost anywhere is like the atmosphere; even when apparently most quiescent, in the sunlight of investigation, revealing itself surcharged with myriad motes in ceaseless agitation. These particles, microscopic or human, as the case may be, are swept along in currents, determined both in their direction and intensity by definite causes. With men, the impelling forces are reducible mainly to economic and social factors. Most powerful of these movements of population to-day is the constant trend from the rural districts to the city. Its origin is perfectly apparent. Economically it is induced by the advantages of co-operation in labor; perhaps it would be nearer the truth to say, by the necessity of aggregation imposed by nineteenth-century industrialism. This economic incentive to migration to the towns is strengthened by the social advantages of urban life, the attractions of the crowd; often potent enough in themselves, as we know, to hold people to the tenement despite the opportunity for advancement, expansion, or superior comfort afforded elsewhere outside the city walls. The effect of these two combined motives, the economic plus the social, is to produce a steady drift of population toward the towns. This has a double significance. It promises to dissolve the bonds of geographical individuality—nay, even of nationality; for a political frontier is no bar against such immigration, provided the incentive be keen enough. At the same time it opens the way for an upheaval of the horizontal or social stratification of population; since in the city, advancement or degradation in the scale of living is alike possible, as nowhere else in the quiet life of the country.

The sudden growth of great cities is the first result of the phenomenon of immigration which we have to note. We think of this as essentially an American problem. We comfort ourselves in our failures of municipal administration with that thought. This is a grievous deception. Most of the European cities have increased in population more rapidly than in America. Shaw has emphasized