 The district had originally been developed in the 19th century as an exclusive suburb for the white middle and upper middle classes; its affluent beginnings led to the development of stately houses, grand avenues, and world-class amenities such as the Polo Grounds and the Harlem Opera House.During the enormous influx of European immigrants in the late 19th century, the once exclusive district was abandoned by the white middle class, who moved farther north.Harlem became an African-American neighborhood in the early 1900s.