In 1870, there were only two American cities with a population of more than 500,000; by 1900, there were six, and three of these — New York, Chicago, and Philadelphia — boasted over one million inhabitants. Roughly 40 percent of Americans lived in major cities and the number was climbing. Although much of the urbanization in the U.S. occurred in the industrial regions of the Northeast and Midwest, it was a natural phenomenon that often corresponded to the presence of railroads. Because the birth rate in the United States declined in the late nineteenth century, urban growth reflected an internal migration of Americans from farms and small towns to the larger cities and the overseas migration that brought millions of people to U.S. shores.