This dissertation examines the origins of British weird horror fiction, an understudied literary genre that had an extraordinary impact on later writers whose works appeared in popular magazines such as The Argosy (1882-1978) and Weird Tales (1923-1954). By far the most popular writer associated with the latter publication is H.P. Lovecraft, an American practitioner of cosmic weird horror whose astounding fictions have become emblematic of the genre in the mainstream imagination. This dissertation locates Lovecraft's early modernist predecessors in British authors Vernon Lee (Violet Paget), Arthur Machen, Algernon Blackwood, and William Hope Hodgson. By tracking the evolution of the genre through these authors' works, this study addresses the following question: "How does weird horror fiction distinguish itself from prior supernatural traditions, in particular the Gothic romance and the Victorian ghost story?" The chapters answer that inquiry by demonstrating that British weird horror fiction destabilizes scientific and philosophical accounts of physical matter, as well as the materialistic theories of biological life and the cosmos that issue from such accounts. Accordingly, weird horror writers devise characteristic strategies to "darken" matter, injecting it with incomprehensible, vitalistic energies; hidden, metaphysical realities; and higher alien dimensions. And yet, the genre is hardly mere anti-science; British weird horror draws from mathematics, chemistry, and biology to launch its virtuoso speculations.

The first chapter examines Vernon Lee's 1880 essay on the supernatural, "Faustus and Helena," and her short story, "Amour Dure" (1887). While Lee is not typically associated with weird horror, this chapter demonstrates how her work marks the emergence of this genre. The second chapter looks at Arthur Machen's novella, "The Great God Pan" (1894), and his novel, The Hill of Dreams (1907), which contest T.H. Huxley's and Ernst Haeckel's theories of biological life. The third chapter analyzes the outdoor horror tales of Algernon Blackwood, in particular "The Willows" (1907), which exemplifies the genre's engagement with mathematics and physics. The last chapter takes an in-depth look at some maritime short fiction by William Hope Hodgson. These works pit the creative powers of ecology against Darwinism, and therefore invite new critical approaches to weird horror that depart from familiar narratives of evolution and degeneration.