Also interested in what leads to social change, he was a contemporary of the European social theorists Karl Marx and Friedrich Engels, who were influenced by reading his work on social structure and material culture, the influence of technology on progress. Morgan is the only American social theorist to be cited by such diverse scholars as Marx, Charles Darwin, and Sigmund Freud. Elected as a member of the National Academy of Sciences, Morgan served as president of the American Association for the Advancement of Science in 1879.[1]