This study traces the origins of conservation thinking in America to the naturalists who explored the middle-western frontier between 1740 and 1840. Their inquiries yielded a comprehensive natural history of America and inspired much of the conservation and ecological thinking we associate with later environmental and ecological philosophy. These explorers witnessed one of the great environmental transformations in American history, as the vast forests lying between the Appalachian Mountains and the Mississippi gave way to a landscape of fields, meadows, and pastures. In debating these changes, naturalists translated classical ideas like the balance of nature and the spiritual unity of all species into an American idiom. This book highlights the contributions made by the generation of natural historians who pioneered the utilitarian, ecological, and aesthetic arguments for protecting or preserving nature in America.