The interdisciplinary field of animal studies owes much to Darwin’s work, particularly in the Descent of Man, where he claims that mental and emotional capacities ranging from an ability to reason to an aesthetic sense exist in other animals besides humans. This chapter examines the significance of Darwin’s work for animal ethics, the science of animal behavior, theories of companion species, and the age of mass extinction. Activists, scientists, humanists, and environmentalists continue to find new uses for Darwin’s work as they pursue greater knowledge about the animal kingdom and more just ecological communities.