This book argues that pragmatism—the most famous movement in American philosophy in the early 1900s—was the outgrowth of a rich conversation between late nineteenth-century philosophers, biologists, and social scientists. For the pragmatists, biological ideas such as evolution, adaptation, and environment were central to debates about scientific inquiry and moral progress. The initial chapters focus on how Charles Sanders Peirce, William James, and other early pragmatists engaged with the evolutionary ideas of Charles Darwin and Herbert Spencer in the 1860s and 1870s. The book then turns to the education of a second cohort of pragmatists, who entered an intellectual world that took biological evolution for granted: by the 1880s and 1890s, the debate had moved away from the fact of evolution and towards its causes or factors. John Dewey, Jane Addams, and W. E. B. Du Bois were excited by biological ideas as students, going on to apply these ideas in their own writing and teaching. They also saw important connections between evolution and idealism. The book’s final chapters demonstrate that in the years around 1900, the pragmatists developed an experimental-evolutionary model of science and social reform. That is, they argued that both epistemic and moral progress involve evolutionary change guided by experimental inquiry.