what does henry louis gates mean when he calls the creation of the slave economy in the deep south a “second middle passage”?

Thanks to the Trans-Atlantic Slave Trade Database, edited by David Eltis and David Richardson, we know that about 388,000 Africans were transported directly to the United States over the course of the slave trade, which ended officially in 1808. This brutally cruel and disruptive phase of the trade, as all American schoolchildren should be taught, is known as “the Middle Passage.” But what is often left out of many survey courses is the second Middle Passage, and that dark chapter in American history involved far more black people than were taken from Africa to the United States. It was also uniquely cruel and brutally destructive. And it unfolded during the era when cotton was “king.”

That second forced migration was known as the domestic, or internal, slave trade: “In the seven decades between the ratification of the Constitution [in 1787] and the Civil War [1861],” the historian Walter Johnson tells us in his book Soul by Soul: Life Inside the Antebellum Slave Market, “approximately one million enslaved people were relocated from the upper South to the lower South … two thirds of these through … the domestic slave trade.” In other words, two and a half times more African Americans were directly affected by the second Middle Passage than the first one.

When we think of the image of slaves being sold “down the river” on auction blocks — mothers separated from children, husbands from wives — it was during this period that these scenes became increasingly common. The enslaved were sometimes marched hundreds of miles to their destinations, on foot and in chains. Indeed, the years between 1830 and 1860 were the worst in the history of African-American enslavement.

c. wright mills's sociological imagination suggest investigating to explain this increase?