" In 1940, 77 per cent of black Americans still lived in, the South—49 per cent in the rural South. The invention of the cotton picker was crucial to the great migration by blacks from the Southern countryside to the cities of the South, the West, and the North. Between 1910 and 1970, six and a half million black Americans moved from the South to the North; five million of them moved after 1940, during the time of the mechanization of cotton farming. "

― Nicholas Lemann , The Promised Land: The Great Black Migration and How It Changed America