This article examines five letters from the correspondence of American zoologist Edwin Grant Conklin that highlight his theories of genetic and social inheritance, in order to suggest that Conklin's eugenic beliefs--like those of many American authorities during this time--were complex and sometimes contradictory. The letters reveal the international prestige of American science after the two world wars and illuminate key moments in the emergence of the concepts of heredity and inheritance, within both the science of genetics and the social movement of eugenics.