Fritz Müller (1821-1897), though not as well known as his colleague Charles Darwin, belongs in the cohort of great nineteenth-century naturalists. Recovering Müller's legacy, David A. West describes the close intellectual kinship between Müller and Darwin and details a lively correspondence that spanned seventeen years. The two scientists, despite living on separate continents, often discussed new research topics and exchanged groundbreaking ideas that unequivocally moved the field of evolutionary biology forward. Müller was unique among naturalists testing Darwin's theory of natural selection because he investigated an enormous diversity of plants and animals, corresponded with prominent scientists, and published important articles in Germany, England, the United States, and Brazil. Darwin frequently praised Müller's powers of observation and interpretation, counting him among those scientists whose opinions he valued most. Despite the importance and scope of his work, however, Müller is known for relatively few of his discoveries. West remedies this oversight, chronicling the life and work of this remarkable and overlooked man of science.