The textbook draws three arrows. The ships were messier than that. The commodities were not. England, Africa, and the American colonies locked into one trade.

The Royal African Company was a chartered monopoly, with shareholders, a governor, and a license to buy and sell people. When the monopoly cracked, the trade got bigger.

On January 1, 1808, the United States banned the import of enslaved people. The internal market in human beings expanded to fill the gap.