A “Nature Cruise of the Century” to the Galápagos collapses before it sails: a global money-panic makes every currency worthless overnight, small wars break out, and a new disease quietly renders almost every human on Earth infertile. By sheer dumb luck, the future of the species ends up marooned on a barren volcanic island — a widowed high-school biology teacher, a vain and failing sea captain, six orphaned Kanka-bono girls, a blind heiress, and not much else.

A million years later, natural selection has done its patient work: we are sleek, furry, flippered, fish-catching sea-creatures with small, contented brains, and we have never been happier. The whole catastrophe, the narrator insists, was the fault of a single organ — the oversized human brain, that three-pound show-off that dreams up money and missiles and misery and simply cannot stop itself.

Our narrator would know. He is the ghost of Leon Trout, who caught syphilis in Vietnam, lost his head in a Swedish shipyard, and has haunted the ship ever since. Mandarax — the gadget that meets real human suffering with an apt quotation — is that big brain distilled into something you can hold in one hand. The captain, “as the new Adam,” hurled his into the deep blue sea.