Where the early settlers found squirrels or coyotes or deer or turkeys, they knew that they were venturing into wilderness. These wild creatures they hunted for food, and for sport. Onto the clear-cut land they brought new creatures—pigs and dogs and cows. They even brought tame turkeys, domesticated from a Mexican subspecies, taken to Europe, and then reimported across the Atlantic.

As Europeans advanced across the continent, they drove out its native species. And as these creatures dwindled and disappeared, the settlers began to regret what they had done. Some, like Teddy Roosevelt, saw the fauna of the New World as a natural resource, a treasure to be protected and cherished so that it could be harvested by future generations. Others, like John Muir, viewed the remaining patches of wilderness as sacred trusts, little patches of Eden to be preserved and enjoyed.

But both groups of conservationists drew a clear line between the pure preserves of nature, and the soiled, sullied domain of man. The conservation movement aimed to cordon off parts of the country from development, where woodland creatures could thrive and survive. Without such spaces, they warned, they would vanish altogether.

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The squirrels were the first to return. In the mid-19th century, Boston, New Haven, and Philadelphia released small numbers of squirrels on to their commons, as curiosities. Hundreds came to marvel at the incongruity; imagine, a squirrel in the city! They dined at the public trough, of course; such wild creatures could hardly be expected to find their own food in an urban environment.

Those first released did not survive for long. But within a few decades, squirrels became fashionable appurtenances for upscale urban parks. Their presence, one contemporary gushed, “excites feelings of admiration akin to those awakened by the birds and the fairer forms of nature.” They remained dependent on the kindness of people, a necessarily fragile existence. But city dwellers cherished them, little avatars of the natural world dwelling peacefully in their midst.

Meanwhile, the forests of New England hit their nadir. They had once covered Massachusetts; by the mid-19th century, they accounted for less than a third of the state, a patchwork of land too hilly or marshy or rocky to farm. As agriculture exhausted the soil, new land to the west promised greater returns, and burgeoning industries offered steadier wages. Farmers abandoned the land their grandfathers had cleared, and trees sprouted up around the stone walls they left behind.

Today, more than 60 percent of Massachusetts is forestland. (Only a few thousand acres of that total are old-growth forest.) For conservationists, the return of the trees seemed a dream come true. As the population emptied off the land, returning to cities and their suburbs, it seemed possible to reestablish the frontier, to restore the woods to their pristine splendor, to again draw a line between nature and civilization.