In the late 19th century and for most of the 20th century, New York was host to millions of immigrants each year. Many of the immigrants crowded into tenements and other apartment buildings in the inner city. This resulted in overpopulation of the buildings, and congestion of city streets. Manhattan's population had risen from 516,000 people in 1850 to 2.33 million people in 1910. The population of the entire city had grown from 1.17 million people in 1860 to 3.44 million in 1900 and 4.77 million in 1910. Living in Manhattan was becoming a hazard due to the higher probability of crime and overcrowding, and for the most part, the first subway line only served areas that were already developed. The first subway lines to the outer boroughs were planned during the early 20th century. Dispersion resulted in the expansion and development of the boroughs.
