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estimated, London already had more than seven thousand tobacco “houses”—café-like places where the city’s growing throng of nicotine junkies could buy and drink tobacco. Unfortunately, because the sole source of fine tobacco were the colonies of hated Spain, the weed in England was hard to obtain, costly (the best tobacco sold for its weight in silver), and vaguely unpatriotic. London tobacco houses were thrilled by the sudden appearance of an English alternative: Virginia leaf. They clamored for more. Ships from London tied up to the Jamestown dock and took in barrels of rolled-up tobacco leaves. Typically four feet tall and two and a half feet across at the end, each barrel held half a ton or more. To balance the weight, sailors dumped out ballast, mostly stones, gravel, and soil—that is, for Virginia tobacco they swapped English dirt.

That dirt very possibly contained the common nightcrawler and the red marsh worm. So, almost certainly, did the rootballs of plants the colonists imported. Until the nineteenth century, worms like these were viewed as agricultural pests. Charles Darwin was among the first to realize they were something more; his last book was a three-hundred-page celebration of earthworm power. Huge numbers of these beasts, he noted, live beneath our feet; indeed, the total mass of the earthworms in a cow pasture may be many times the mass of the animals grazing above them. Literally eating their way through the soil, earthworms create networks of tunnels that let in water and air. In temperate places like Virginia, earthworms can turn over the upper foot of soil every ten or twenty years; tiny ecological engineers, they reshape entire expanses. “It may be doubted,” Darwin wrote, “whether there are many other animals which have played so important a part in the history of the world, as have these lowly organized creatures.”

The exact path of these migrants into North America is impossible to trace. What is clear is that before the arrival of Europeans, New England and the upper Midwest had no earthworms—they were wiped out in the last Ice Age. Earthworms from the south didn’t move north after the glaciers melted because the creatures don’t travel long distances unless they are transported by human agency. “If they’re born in your backyard, they’ll stay inside the fence their whole lives,” John W. Reynolds, editor of Megadrilogica, perhaps the premier U.S. earthworm journal, explained to me. They arrived with Europeans, probably in Virginia, and spread with them. Like the colonists, the worms were conquering a new place. In both cases, the arrival of foreigners was an ecological watershed.

In worm-free woodlands, leaves pile up in drifts on the forest floor. When earthworms are introduced, they can do away with the leaf litter in a few months, packing the nutrients into the soil in the form of castings (worm excrement). As a result, according to Cindy Hale, a worm researcher at the University of Minnesota, “everything changes.” Trees and shrubs in wormless places depend on litter for food. If worms tuck nutrients into the soil, the plants can’t find them. Many species die off. The forest becomes more open and dry, losing its understory, including tree seedlings. Meanwhile, earthworms compete for food with small insects, driving down their numbers. Birds, lizards, and mammals that feed in the litter decline as well. Nobody knows what happens next. “Four centuries ago, we launched this gigantic, unplanned ecological experiment,” Hale told me. “We have no idea what the long-term consequences will be.”

In some ways this is unsurprising: Jamestown itself was a case study in unintended consequences. The Virginia colony was an attempt by a group of merchants to snatch up the vast stores of gold and silver they imagined—incorrectly, alas—existed around Jamestown, in the big, shallow estuary of Chesapeake Bay. Equally important, the merchants wanted to find a route through North America, which they imagined, again incorrectly, to be only a few hundred miles wide, less than a month’s journey. And when the colonists