As America's first national park, Yellowstone has long been the focal point for contentious public debate over federal resource management policies. Few such policies have been as hotly contested in recent years as what has come to be called “natural regulation”—a policy of letting ecological processes, such as fire, take their natural course within Yellowstone’s boundaries. Critics of natural regulation, most notably Alston Chase in his 1987 jeremiad Playing God in Yellowstone, attribute this policy to “a new philosophy of nature” invented by “California cosmologists” in the 1960s. The sixties were, indeed, an era of shifting popular and scientific ideas about the environment and consequent changes in federal approaches to managing national parks. It is, however, a serious misreading of Yellowstone’s history to suggest that ecological rationales emerged fully formed in the 1960s and then spread within National Park Service ranks like an insidious foreign plant species. Such ideas, in fact, had been the subject of study and discussion among park managers and scientists for many decades. Charles C. Adams, an early twentieth-century animal ecologist, conceived a scientific rationale for Yellowstone in the 1920s, arguing that the park preserved “natural conditions” and thus enabled scientists (and the public) to observe nature’s processes free from human intervention. An examination of Adams’s work demonstrates that the idea of Yellowstone as a place to preserve natural conditions has been a powerful and enduring theme in the park’s history.