Why did very large numbers of people begin to depart the British Isles for the New Worlds after about 1770? They were the vanguard of mass economic migration, the carriers of new global labour forces, agents of dispossession and settlement, of family dreams, of individual aspirations, of imperial strategies. But it was new in scale, and it was a pioneering movement, a rehearsal for modern international migration. These first mass inter-continental stirrings began most of all in the British Isles. What activated these great exchanges of humanity, the precursors of so much modern population transfer and turmoil around the globe? This is a question in the middle of most genealogies and central to the making of the modern world.