Tucked away in Charles Darwin's surviving papers is a manuscript of almost 300,000 words that he never completed. It was his sequel to The Origin of Species. It was the book he had promised would finally supply solid empirical evidence for the creative power of natural selection, evidence he admitted was absent from the Origin, which he repeatedly described as a "mere abstract." Darwin soon abandoned his sequel, though he never revealed that decision to those who awaited its appearance. The mystery of why Darwin didn't finish his sequel has never been satisfactorily resolved. In this fascinating piece of historical detective work, Robert Shedinger draws on Darwin's letters, private notebooks, and the unfinished manuscript itself to piece together the puzzle and reveal an embarrassing truth: Darwin never finished his sequel because in the end he could not deliver the promised goods. His book, begun in earnest, devolved into a bluff.