The most used weedkiller on American corn turns male frogs into egg-laying females at a tenth of the dose the law allows in drinking water, Europe banned it in 2004, and the company that makes it put the scientist who found out under surveillance. Atrazine goes on half the corn in America, seventy million pounds a year, and every spring the rain washes it off the fields into the reservoirs of the corn belt in a pulse the water companies can set their calendar by. In 2002 Tyrone Hayes at Berkeley, who had been hired by the maker to test it, found that at a fraction of the legal limit it castrated male leopard frogs and turned some of them into functioning females with eggs. The company tried to keep the results. He published. The documents that came out in court a decade later were the company's plan for him, by name: discredit him, investigate him, have him followed, and buy the search results for his name so his work came up last. The frogs were the warning. The water is the point. For most of the growing season the taps of Illinois, Iowa, Indiana, Ohio, Kansas and Nebraska run above the European limit, and the water utilities sued the maker to pay for the filters and settled for a hundred million dollars. The European Union looked at the data and banned it because it could not be kept out of the water. The United States looked at the same data and renewed it, twice. The corn it protects is fed to pigs, burned in cars, and refined into the syrup in the drink. A pasture has never needed atrazine, and the cows on it drink from the pond. The corn belt puts a chemical through its own taps every June that changes the sex of the thing in the pond, and the official position is that the frogs are not people.

The same campaign that killed China's sparrows built a hundred dams across one province of Henan in ten years, the hydrologist who said the dams were too many and too weak was removed from the project, and in August 1975 sixty-two of them failed in a single night and drowned more people than any dam failure before or since. Banqiao was the showpiece, finished in 1952 to harness the Huai River, and when the Great Leap began in 1958 the slogan over it was water accumulation first. Reservoirs went up everywhere, built by peasants with baskets and shovels on the same timetable as the furnaces and the sparrow drives, to the same reporting standard. Chen Xing, the engineer who knew the basin best, said the province was building so many that it was raising the water table into the fields and the salt into the soil, and that Banqiao needed twelve sluice gates to pass a flood. It was given five. He was a rightist. He was sent away. The dam was called the Iron Dam. Typhoon Nina came up from Taiwan in the first week of August 1975 and stalled against a cold front over the Huai basin, and a year's rain fell in a day. Banqiao's five gates could not pass it. The water came over the top an hour after midnight, the dam dissolved, and a wall of water six miles wide and seven metres high went down the valley at the speed of a car, taking the next dam with it, and the next, until more than sixty had gone. Whole towns were under water before anyone had lit a lamp. The official figure for the flood is twenty-six thousand dead; the figure that includes the famine and the typhoid in the weeks after, when a province sat in standing water with the dead floating in it, runs to a hundred and seventy thousand, and some put it over two hundred. The Party kept it secret for thirty years. Chen Xing was brought back after the famine to say what he had said before, and brought back again after the flood. A sparrow weighs an ounce and a dam weighs a mountain, and the same men measured both by the number they could report, with the same result.

In December 1958 the Polish embassy in Beijing refused to let the sparrow killers onto its grounds, so the Chinese surrounded it with drummers and kept the drums going for two days until every bird sheltering in the garden had dropped dead from the air, and the diplomats cleared them up with shovels. That is how thorough it was. The campaign had been running since the spring. Every village, school and factory was turned out with gongs, pans and firecrackers to keep the sparrows flying until their hearts gave out. Nests were torn from eaves, eggs smashed, chicks killed, and children went through the fields with slingshots and were paid by the bird. The dead came in by the basket, strung on wires, piled on carts, and the totals went up the chain as a harvest. Something close to two billion sparrows in two years. The embassy garden was the last place in the capital with birdsong in it, and the drums got that too. A zoologist called Zhu Xi had told them before it started that a king had tried this and lost. Frederick the Great of Prussia had put a bounty on sparrows in the 1740s to save his grain and his cherries, and the sparrows went, and the caterpillars they had been eating stripped his orchards to the stalk, and the king cancelled the bounty and paid to have sparrows brought back from neighbouring states and let loose. The whole episode was in the library. The ministry had a figure for what a sparrow ate and no figure for what it did, and a man with the second figure was told to sit down. The locusts arrived on schedule. Then the grain borers. A sparrow takes a few grains from a field in autumn and feeds its chicks nothing but insects all spring, and with the sparrows gone the insects had the rice and the wheat to themselves. The counties with the most sparrows to lose saw their grain fall the furthest, and the rice and wheat that had been holding the country up went down together. That was 1959, and the state took its grain quota anyway, on the harvest that had been reported. What was left in the villages was not enough, and the famine that followed over the next three years is the largest in human history: tens of millions dead, the estimates running from fifteen million to forty-five, whole counties in Anhui and Sichuan where the living could not bury the dead. The sparrows were one cause among several and the economists who have gone back to the county records put the bird's share at something over two million people, with another four hundred thousand never born. Two million, from a campaign against a thing that weighs an ounce. By 1960 the ornithologist Tso-hsin Cheng, who had been cutting open dead sparrows and counting the insects in their stomachs, got the bird struck off the list and bed bugs put in its place. There were almost no sparrows left in China to pardon. So the government did what Prussia had done two centuries before: it bought a quarter of a million sparrows from the Soviet Union and shipped them east in crates, to be released into a country that had drummed every one of their relatives out of the sky. The animal that does the invisible job is always the one on the ledger as a cost. Take the sheep off a chalk down and the orchids are under scrub in twenty years. Fence the cattle off a Welsh hill and the bracken takes it. Empty a savannah of its herds and the grass goes to fire. All of it written down, all of it in the library, all of it available to the next committee with a figure for what a cow eats. Beijing paid Moscow for the birds in hard currency. The men who had said so in 1958 were still not allowed to say so in 1960.