Thoreau in his early career did not consider nature a worthy subject for his pen. Beginning with only a superficial knowledge of nature--even while living at Walden Pond--he later began to study the subject more intensely in 1849. Over the next dozen years, he applied himself especially to botany and ornithology, seeking to integrate knowledge into the larger patterns of life. Independently deriving what today would be considered an ecological worldview, Thoreau devoted the last years of his writing career to nature studies, written in his own distinctive voice. In this revised edition of a standard study of Thoreau and nature, the author traces the origins and development of Thoreau's shift in viewpoint and his painstaking efforts thereafter.