Wilson recently calculated that the only way humanity could stave off a mass extinction crisis, as devastating as the one that killed the dinosaurs 65 million years ago, would be to set aside half the planet as permanently protected areas for the ten million other species. “Half Earth,” in other words, as I began calling it—half for us, half for them. A version of this idea has been in circulation among conservationists for some time.

“It’s been in my mind for years,” Wilson told me, “that people haven’t been thinking big enough—even conservationists. Half Earth is the goal, but it’s how we get there, and whether we can come up with a system of wild landscapes we can hang onto. I see a chain of uninterrupted corridors forming, with twists and turns, some of them opening up to become wide enough to accommodate national biodiversity parks, a new kind of park that won’t let species vanish.”