Great cities have always been the melting-pots of races and of cultures. Out of the vivid and subtle interactions of which they have been the centers, there have come the newer breeds and the newer social types. The great cities of the United States, for example, have drawn from the isolation of their native villages great masses of the rural populations of Europe and America. Under the shock of the new contacts the latent energies of these primitive peoples have been released, and the subtler processes of interaction have brought into existence not merely vocational, but temperamental, types. _Mobilization of the individual man._—Transportation and communication have effected, among many other silent but far-reaching changes, what I have called the “mobilization of the individual man.” They have multiplied the opportunities of the individual man for contact and for association with his fellows, but they have made these contacts and associations more transitory and less stable. A very large part of the populations of great cities, including those who make their homes in tenements and apartment houses, live much as people do in some great hotel, meeting but not knowing one another. The effect of this is to substitute fortuitous and casual relationship for the more intimate and permanent associations of the smaller community. Under these circumstances the individual’s status is determined to a considerable degree by conventional signs—by fashion and “front”—and the art of life is largely reduced to skating on thin surfaces and a scrupulous study of style and manners. Not only transportation and communication, but the segregation of the urban population tends to facilitate the mobility of the individual man. The processes of segregation establish moral distances which make the city a mosaic of little worlds which touch but do not interpenetrate. This makes it possible for individuals to pass quickly and easily from one moral milieu to another, and encourages the fascinating but dangerous experiment of living at the same time in several different contiguous, but otherwise widely separated, worlds. All this tends to give to city life a superficial and adventitious character; it tends to complicate social relationships and to produce new and divergent individual types. It introduces, at the same time, an element of chance and adventure which adds to the stimulus of city life and gives it, for young and fresh nerves, a peculiar attractiveness. The lure of great cities is perhaps a consequence of stimulations which act directly upon the reflexes. As a type of human behavior it may be explained, like the attraction of the flame for the moth, as a sort of tropism.