Genres are constituted, implicitly and explicitly, through their construction of the past. Genres continually reconstitute themselves, as authors, producers and, most importantly, readers situate texts in relation to one another; each text implies a reader who will locate the text on a spectrum of previously developed generic characteristics. Though science fiction appears to be a genre concerned with the future, I argue that the persistent presence of lost race stories – where the contemporary world and groups of people thought to exist only in the past intersect – in science fiction demonstrates that the past is crucial in the operation of the genre. By tracing the origins and evolution of the lost race story from late nineteenth-century novels through the early twentieth-century American pulp science fiction magazines to novel-length narratives, and narrative series, at the end of the twentieth century, this thesis shows how the consistent presence, and varied uses, of lost race stories in science fiction complicates previous critical narratives of the history and definitions of science fiction.