The Physiology of the Multitude addresses a paradox that played out over the eighteenth and nineteenth centuries around the British novel. During precisely the years when rational-choice economics and democratic liberalism both began to insist that individual free will would decide the political future, scientific vocabularies equally forcefully attributed action and decision to involuntary biological and social-systemic processes, rather than conscious choice. Over the course of the eighteenth and nineteenth centuries, natural sciences of the individuated body and social sciences of collectivity redefined the standards for what counted as realistic representations of human experience. As a result, fiction emerged as the field in which an increasingly mechanistic material body and deterministic forms of mass behavior could contest political, racial, and gendered fictions of why individuals behave as they do. Focusing on fiction's unique formal ability to move between dramatically different scales, I show how novels in the years 1748-1907 combined the insights of scientific disciplines that were not otherwise in conversation with one another. Across a broad range of fiction, action and emotion get usurped from the conscious mind and are re-attributed either to the involuntary impulses of the nervous system, or to crowds and masses that seem to move, think, and feel as one. These novels introduce a model of non-volitional action that draws a connection between the body's unconscious biological processes and the mindless behavior of large-scale masses. The seemingly alien and often threatening features of the masses come to be lodged beneath the skin of the individual. At different points during these years, novelists seized on new ways of aligning evolving conceptions of the physical body with historically-specific kinds of masses. Each of my chapters explains the conceptual tools novelists used to superimpose the category of involuntary behavior at the scale of the mass onto the category of involuntary behavior in the body. In the first chapter, Montesquieu's 1748 climate theory and Mary Shelley's 1818 Frankenstein explore the way a body whose biology contains the features of a population unsettled conventions of political theory and literary form. The second chapter reads Emily Brontë's 1847 Wuthering Heights alongside the emergent concept of the species to ask what new kinds of relations form among individuals when their bodies are understood to be composed of the same kind of flesh. The third chapter uses the physiological psychology popularized by George Eliot's life-partner, G.H. Lewes, to put Eliot's strange short work The Lifted Veil (1859) into conversation with Daniel Deronda (1876), arguing that Eliot points to the preconscious modes of thought that arise from the body as a way of navigating the chaos of crowd behavior and transforming anonymous encounters into meaningful relationships. The final chapter addresses globalism in late-century adventure and spy fiction; the "deep time" of Victorian geology in H. Rider Haggard's 1885 King Solomon's Mines and standardized Greenwich time in Joseph Conrad's 1907 The Secret Agent both work to embed individual bodies within planetary configurations of human life.