The 1580s and 1590s, an intense point in the Little Ice Age, were two decades of hard cold rains that flooded Fujianese valleys, washing away rice paddles and drowning the crop. Famine shadowed the rains. Poor families were reduced to eating bark, grass, insects, and even the seeds found in wild-goose excrement. Chen Zhenlong and his friends seem initially to have thought of the fanshu—foreign tubers—as an amusing novelty; they gave them away as presents, a slice or two at a time, neatly wrapped in a box. … As hunger tightened its grip, Chen's son, Chen Jinglun, showed the fanshu to the provincial governor, to whom he was an adviser. The younger Chen was asked to conduct a trial planting near his home. Successful results persuaded the governor to distribute cuttings to farmers and instruct farmers how to grow and store them. "It was a great fall harvest; both near and far food was abundant and disaster was no longer a threat," exulted the great-great-great-grandson. Near Yuegang, as much as 80 percent of the locals were living on sweet potatoes.

Governmental promotion of foreign crops was nothing new in Fujian. Sometime before 1000 A.D., Fujianese merchants brought in a novel type of rice—early-ripening Champa rice—from Southeast Asia. Because the new rice matured quickly, it could be planted in areas with shorter growing seasons. After intensive breeding, farmers created varieties that grew quickly enough to let them plant two crops a year in the same field—one of rice, then a second of wheat or millet. Harvesting twice as much from the same amount of land, Chinese farms became more productive than farms elsewhere in the world. The then-ruling Song dynasty actively promoted the new rice, distributing free seeds, publishing illustrated how- to brochures, sending out agents to explain cultivation techniques, and even providing some low-interest loans to help smallholders adapt. This aggressive adaptation and promotion of a new technology was a key reason for the nation's subsequent prosperity, and its preeminence. 

Still, Fujian was lucky that sweet potatoes arrived when they did. The crop spread though the province just in time for the fall of the Ming dynasty, which ushered in decades of violent chaos. Incoming Manchu forces seized Beijing in 1644, beginning a new dynasty: the Qing. The last Ming emperor hanged himself, and pretenders emerged to lead a rump state. Initially it was based in Fujian. In a disordered interlude, pieces of the Ming military splintered away and became, in effect, wokou [pirates]. Meanwhile actual wokou took advantage of the confusion. To deny supplies to the Ming/wokou, the Qing army forced the coastal population from Guangdong to Shandong—the entire eastern "bulge" of China, a 2,500-mile stretch of coastline—to move en masse into the interior. 

Beginning in 1652, soldiers marched into seaside villages and burned houses, knocked down walls, and smashed ancestral shrines; families, often given only a few days' warning, evacuated with nothing but their clothes. All privately owned ships were set afire or sunk. Anyone who stayed behind was slain. "We became vagrants, fleeing and scattering," one Fujianese family history recalled. People "simply went in one direction until they halted," another said. "Those who did not die scattered over distant and nearby localities." For three decades the shoreline was emptied to a distance inland of as much as fifty miles. It was a scorched-earth policy, except that the Qing scorched the enemy's earth, not theirs

For Fujian, the coastal evacuation amounted to a spectacularly harsh version of the Ming dynasty's ban on overseas trade. In the 1630s, before the political convulsions and the trade bans, twenty or more big junks went to Manila every year, each carrying hundreds of traders. During the evacuation, the number fell to as low as two or three, all illicit. Like the Ming trade bans, the Qing coastal clearance effectively turned over the silver trade to wokou. …

… The Qing had ordered the coastal evacuation, but it had disastrous consequences for them, too. As the treasury official Mu Tianyan complained, closing down the silver trade effectively froze the money supply. Because silver was always being wasted, lost, and buried, the pool of Chinese money was actually shrinking. "Every day there is less and less to meet the demand, with no way to restore it," Mu wrote to the emperor. When the money supply falls, each unit becomes more valuable; prices fall in a classic deflationary spiral. To stop the importation of silver "yet desire the wealth of fortune and convenience of use," Mu asked, "how is this different from blocking a source of water while expecting to benefit from its flow?" Reluctantly agreeing, the Qing lifted the ban in 1681. 

Meanwhile, though, coastal people had flooded into the mountains of Fujian, Guangdong, and Zhejiang. Inconveniently, these areas were already inhabited. Most of the inhabitants belonged to a different ethnic group, the Hakka, famed for their tulou—fortress-like complexes, usually but not always circular, whose earthen outer walls contain scores of apartments, all facing onto a central courtyard. (Today these amazing structures are a tourist attraction.) Decades before the expulsion, the Fujianese scholar Xie Zhaozhe had observed that the Hakka in the hills were packed into every scrap of available real estate: There is not an inch of open ground.... Truly as someone once said, "Not a drop of water goes unused, and as much as possible even the most rugged parts of mountains are cultivated." One could say that there isn't a bit of land left. 

Nobody knew how many shack people were in the hills. Hoping, perhaps, that hiding the problem would avoid the need to solve it, Qing bureaucrats left them out of census reports. But all evidence suggests that the number was not small. In Jiangxi, Fujian's western neighbor, the rigid, nit-picking provincial treasurer insisted in 1773 that the shack people, many of had lived in Jiangxi for decades, counted as actual inhabitants of the province and therefore should be included in the reports sent to Beijing. He dispatched field workers to enumerate every Hakka head and every Hakka shack. In lugged Ganxian County, they tallied 58,340 settled inhabitants, most in the main town of Ganzhou—-and 274,280 shack people in the surrounding slopes. In county after county the story was repeated, sometimes with a few thousand wanderers, other times a hunch-ed thousand or more. Hidden from the government, more than a million shack people had been slashing and burning their way across Jiangxi. And that, as the Qing court must have realized, was only one medium-sized province. 

Coupled with the outflow of shack people was a second, parallel, even bigger wave of migration into the parched, mountainous, thinly settled west. In their quest for social stability, the Ming had prohibited people from leaving their home regions. Reversing course, the Qing actively promoted a westward movement. Much as the United States encouraged its citizens to move west in the nineteenth century and Brazil provided incentives to occupy the Amazon in the twentieth, China 's new Qing masters believed that filling up empty spaces was essential to the national destiny. ("Empty," that is, from the Qing point-of -view; dozens of non-Chinese peoples—Tibetans, Yao, Uighurs, Miao—lived in them. By sending in people from the center, the Qing were forcibly incorporating these previously autonomous cultures into the nation.) Lured by tax subsidies and cheap land, migrants from the east swanned into the western hills. Most of the newcomers were, like the shack people, poor, politically luckless, and scorned by urban elites. They looked at the weathered, craggy landscape, so unwelcoming to lice—and they, too, planted American crops. 

China's fifth-largest province is Sichuan, adjacent to Tibet and nearly as alpine. Back in 1795, according to Lan Yong, a historian at Sichuan's Southwest University, it was a big, roomy place: more land than California, a population as low as 9 million. Just 2,300 square miles of its surface, an area half the size of Los Angeles County, were considered arable. During the next twenty years, Lan has mitten, American crops moved into the ridges and highlands, increasing the pool of farmland to almost 3,700 square miles. As Sichuan's agricultural capacity increased, its population increased in tandem, to 25 million. Something similar occurred in Shaanxi Province, Sichuan's even emptier neighbor to the northeast. Migrants poured into the steep, arid hills along the border between them, knocking down the trees that clung to the slopes to make room for sweet potatoes, maize, and, later, potatoes. The amount of cropland soared, followed by the amount of food grown on that cropland, and then the population. In some places the number of inhabitants increased a hundredfold in little more than a century.