Historians have resorted to a language of mystery and metaphor when they come to grapple with the great structural changes which underpin the array of contributory causes of migration. The British Isles was the prototype case of agrarian transformation associated with industrial growth and mass migration. Frank Thistlethwaite in the early 1960s re-shaped the subject by insisting on linking the two sides of the Atlantic into a connected explanation of the migratory turmoil. There were links along the chain of causation towards the migration of millions of the British people in their confusing permutations. Migration history comes in three main schematic forms: first the individual account, second the general narrative of migratory behaviour, and third the grand theories of migration. International emigration has depended on the basic facilities of migration. The British case was the prototype of modern rural-urban migration and has been replicated, with important variations, across the world.