Thesis advisor: James M. O'Toole / This dissertation argues that the movement to restrict European immigration to the United States in the early 1900s was critically supported by a set of ideas that the dissertation refers to as "classic racialism." Derived from several intellectual traditions - such as anthropology, biology, criminology, eugenics and zoology - classic racialism posited that differences in human population groups were biologically determined and hereditary, and because of this fact, American nativists held that the "new" immigration to the United States had to be curtailed in order to save the American Anglo-Saxon racial stock. The dissertation uses Italian immigration to the United States as a case study for understanding the fluidity of racial and biological thought. While classic racialism played a key role in supporting nativists' calls for immigration restriction, advances in methods of scientific research were revolutionizing the fields of biology, genetics and anthropology. Research in these fields cast doubts on the veracity of intellectual claims made by classic racialists, which were increasingly untenable in the light of advancing scientific knowledge. The tensions between these competing intellectual paradigms of classic racialism and modern experimentalism in the late nineteenth- and early twentieth-centuries reveal the esoteric nature of scientific revolutions, in that the uncertainty and complexity of the developing biological and genetic sciences kept knowledge of scientific advances in these fields restricted to a narrow audience of professional scientists and academics. While modern experimental biology raised significant scientific doubts about the principles of classic racialism, it was the latter that influenced American immigration policy in the 1920s because of classic racialism's simplicity and the broad public recognition that "like produces like." / Thesis (PhD) — Boston College, 2009. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: History.