ATLANTIC CROSSINGS WERE ONLY THE FIRST half of the Columbian Exchange. Voyages across the Pacific completed the interconnection of the world. Cities such as Potosí, in South America; Manila, in the Philippines; and Yuegang, in southeast China, became bustling, essential links in an economic exchange that knit the world together. Along the way, the exchange brought sweet potatoes and corn to China—with accidental, devastating effects on Chinese ecosystems. Those effects, in turn, shaped economic and political developments. Carried across the sea as part of the Columbian Exchange, sweet potatoes and corn played a major part in the rise and fall of China’s last imperial dynasty, and in the history of the Communist state that came afterward and that still rules the country.