Nigel Harris shows exactly how and why immigration is the lifeline of the developed world's economy, making both the rich and the poor richer. There have been poor countries and rich countries since countries first began, but only in the 20th century--the century of nationalisms and ethnic cleansings--have controls been implemented to stop movement between them. In this survey of immigration throughout the world, Harris examines the net effects of population change on income, employment, wealth, growth, and their areas.