By 1900, America had become an urban nation in its dominant tone, in its economy and work, and in its culture and aspiration as well as its demography. Gradually-and then seemingly suddenly-cities emerged as the focal points of the nation's economic growth, social mobility, and a changing culture of leisure and entertainment. They comprised the market of a consumer economy, and the petri dish of a social experiment that was breathtaking in its ambition to see if the disparate peoples of the earth could live together in one place.