The population grew dramatically at the end of the 19th and beginning of the 20th centuries, through immigration from Ireland, Southern Europe, Eastern Europe, and Asia, as well as the Great Migration of blacks from the rural South and Puerto Ricans from the Caribbean, all attracted to the city's expanding industrial jobs. The Pennsylvania Railroad was expanding and hired 10,000 workers from the South. Manufacturing plants and the US Navy Yard employed tens of thousands of industrial workers along the rivers, and the city was also a center of finance and publishing, with major universities. By the 1950s, much Philadelphia housing was aged and substandard. In the post-World War II era of suburbanization and construction of area highways, many middle-class families met their demand for newer housing by leaving the city for the suburbs. Population decline accompanied the industrial restructuring and the loss of tens of thousands of jobs in the mid 20th century. With increasing poverty and social dislocation in the city, gang and mafia warfare plagued the city in from the mid-20th century to the early 21st century.