In the 19th century, the US rapidly evolved from a predominantly rural to an overwhelmingly urban society. At play were the same catalysts which previously transformed industrial Europe, but the pace and magnitude eclipsed all precedents. High birth rates and decreasing mortality rates, combined with mass immigration, produced soaring population growth. Agricultural industrialization revolutionized food production, but reduced rural employment opportunities. These factors combined to cause an explosive expansion in the number and size of American cities. New York had 800,000 inhabitants in 1860, 3.4 million by 1900. Over the same period, Chicago grew by a factor of 15. Innovation and resourcefulness accentuated these trends. The invention of safety elevators and steel frame building design enabled the skyscraper, and with it the emblematic, condensed American downtown. By the turn of the century, peak urbanization was spreading centrifugally south and west, with Los Angeles growing from nothing to metropolis by the 1920s.
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