First published in 1974 as a companion volume to Darwin on Man by Howard E. Gruber, Paul Barrett's transcriptions of Darwin's M and N notebooks served to shed new light on the evolutionist's methods and motivation. According to Stephen Jay Gould in the New York Times Book Review, "Darwin kept these notebooks] primarily in 1838, when he was 29 years old. In them, he recorded his early conviction of evolutionary continuity between...