In late-nineteenth and early-twentieth-century Britain, rival visions of sociology's relationship to biology dominated debate about sociology's aims and scope. Each of these visions offered a different future for sociologists and each had its defenders and detractors. Why was it, then, that L.T. Hobhouse-a thinker who battled to separate the biological and social sciences-became Britain's first chair of sociology in 1907? This thesis answers this question by charting the emergence of sociology in Britain from the mid-1870s through to the early twentieth century. In telling this story, the thesis has four main aims in view. First, it shows how the collapse of an earlier social science, political economy, conditioned the evolutionary programmes of the three most important figures in British sociology of the early 1900s: the eugenicist Francis Galton; the Scottish biologist Patrick Geddes; and Hobhouse. Second, by using previously unknown or underutilised archival sources relating to the early years of the Sociological Society (the prime mover in establishing sociology within the British academy), the dissertation provides the first detailed account of the events that led to Hobhouse's appointment at the London School of Economics. Third, the more comprehensive reconstruction of those events and their contexts thus developed gives fresh insight into what was at stake for sociologists when they decided to follow Hobhouse rather than his biologically engaged rivals. Finally, in its attention to the scientific practices of Galton, Geddes, and Hobhouse, and their relations to the debate about the role of biology in social explanation, the dissertation contributes to the modem historiography of science by showing why taking scientific practice seriously requires us to take scientific ideas seriously and vice versa.