Evolution, during the early nineteenth century, was an idea in the air. Other thinkers had suggested it, but no one had proposed a cogent explanation for how evolution occurs. Then, in September 1838, a young Englishman named Charles Darwin hit upon the idea that natural selection among competing individuals would lead to wondrous adaptations and species diversity. Twenty-one years passed between that epiphany and publication of On the Origin of Species. The human drama and scientific basis of Darwins twenty-one-year delay constitute a fascinating, tangled tale that elucidates the character of a cautious naturalist who initiated an intellectual revolution.