The urbanization of the United States, depicted in statistics in Chapter 1, may be identified with an assortment of social and historical changes. Certainly the most obvious of these changes is an increase, during the late nineteenth and early twentieth centuries, in the populations of existing urban places. United States Census data reported in Chapter 1 indicated that the populations of many American cities more than doubled in the short period bounding the turn of the century. More importantly, the same factors that produced such substantial increments in urban population also brought about qualitative changes within the growing populations of individual cities. These changes left cities not simply with more inhabitants, but with a class of residents whose presence signaled a newly diversified urban environment.

Many cities in the Northeast and Middle West were points of destination for widening streams of foreign immigration. Others, located in the South and in Border States, proved inhospitable to the foreign-born, but harbored large communities of black Americans. In either case, turn-of-the-century cities constituted a type of settlement different, in terms of both the scale and the composition of their populations, from what had prevailed in America at any previous point in its history.

Louis Wirth realized as much in 1938, when he commented that