Prior to August Weismann’s 1889 germ-plasm theory, social reformers believed that humans could inherit the effects of a salubrious environment and, by passing environmentally-induced modifications to their offspring, achieve continuous progress. Weismann’s theory disrupted this logic and caused many to fear that they had little control over human development. As numerous historians have observed, this contributed to the birth of the eugenics movement. However, through an examination of the work of social scientists Lester Frank Ward, Richard T. Ely, Amos Griswold (...) Warner, James Mark Baldwin, Simon Nelson Patten, Alfred Kroeber, Walter Robinson Smith, and Luther Lee Bernard, I demonstrate that Weismann’s ideas also prompted scholars to create of theories of human progress in which the social environment had a central role and biological heredity had a diminished one. Furthermore, in creating a new theory of social progress based on a concept called “social heredity,” the thinkers surveyed in this article separated biological and social thought and asserted the independence of the American social sciences. I argue that this represents an important moment in the maturation of the human sciences, and I suggest that the germ-plasm theory of heredity deserves a larger place in histories of the development of the American social scientific disciplines. (shrink)