From the Victorian era to our own, critics of Herbert Spencer have portrayed his science-based philosophical system as irrelevant to the concerns of practicing scientists. Yet, as a number of scholars have recently argued, an extraordinary range of reformist and experimental projects across the human and life sciences took their bearings from Spencer's work. This essay examines Spencerian science as practiced by the biologist, sociologist, and town planner Patrick Geddes (1854-1932). Through a close examination of his experimental natural history of the late 1870s and early 1880s, his wider writings on the evolutionary process in the 1880s, and his efforts later in that decade to improve the slums of Edinburgh, this essay reveals Geddes's deep, unifying, and energizing commitment to Spencer's program. Recapturing that commitment can not only help historians rethink Spencerian science as practice but also suggest ways of overcoming the dichotomy between elite ideas and practical activity that bedevils much contemporary historiography of science.