David Quammen's 'Darwin's Doubt' delves into one of the most intriguing aspects of Charles Darwin's monumental work: his own moments of profound uncertainty and hesitation concerning the mechanisms and completeness of natural selection. This insightful book meticulously examines the specific scientific puzzles that troubled Darwin, such as the sudden appearance of complex life forms in the fossil record, particularly during the Cambrian explosion. Quammen explores how Darwin grappled with these challenges, often acknowledging their difficulty in his private notes and published works. The narrative reveals a human side to the scientific giant, showcasing his intellectual honesty and the internal debates that shaped his revolutionary ideas, offering a nuanced perspective on the birth of evolutionary theory.