Using case studies from the Caribbean borderlands, Catherine Mas explores the history of the human sciences and the ethnicization of bodies, both human and nonhuman, in a context of shifting U.S. immigration discourses. From comparative psychology in early-twentieth-century Havana to medical anthropology in 1970s Miami, the vantage point from the Caribbean brings into view the uncertain boundaries—of nation as well as nature—for which the body has served as a key site in navigating.