Name a country where internal migration has occurred explain the reasons for this migration

The United States saw a massive internal migration from the eastern states toward the west coast during the mid-19th century, a similar large-scale migration of African Americans from the agricultural south to the industrialized northeast in the early to mid-twentieth century and a large-scale reverse migration of African Americans from other parts of the country to the urban south beginning in the late 20th century and continuing to the present.

What exactly do you mean 

Except common history