"Quammen brilliantly and powerfully re-creates the 19th century naturalist's intellectual and spiritual journey."--Los Angeles Times Book ReviewTwenty-one years passed between Charles Darwin's epiphany that "natural selection" formed the basis of evolution and the scientist's publication of On the Origin of Species. Why did Darwin delay, and what happened during the course of those two decades? The human drama and scientific basis of these years constitute a fascinating, tangled tale that elucidates the character of a cautious naturalist who initiated an intellectual revolution.