The essay reviews historical views on the origins of species and evaluates rival mechanisms of biological change, weighing natural selection against the inheritance of functionally produced modifications. It argues that while selection accounts for many adaptations, numerous structural and physiological traits in plants and animals are not satisfactorily explained by selection alone or by direct use and disuse during an individual’s life. Drawing on botanical and zoological examples, the author maintains that changes induced by habit and function are sometimes transmitted across generations, and considers the broader consequences of this claim for mental development, moral tendencies, and the evolution of social institutions.