As an author he is chiefly known by his numerous works on ornithology, mammalogy, herpetology, bibliography, lexicography, comparative anatomy, natural philosophy, and psychical research. He was one of the authors of the Century Dictionary of the English Language, in seven years contributing 40,000 words and definitions in general biology, comparative anatomy, and all branches of zoölogy. During the last few years he contributed several volumes on western history, in all twelve volumes, and by study and research was enabled to correct many errors. In 1877 he received the highest technical honor to be attained by an American scientist in his election to the Academy of National Science and was for some years the youngest academician. The same year saw his election to the chair of anatomy of the National Medical College in Washington, where he had graduated in '63. He then entered upon a professorship and lectured upon his favorite branch of the medical sciences for ten years. He appears to have been the first in Washington to teach human anatomy upon the broadest basis of morphology and upon the principle of evolution. Nearly all his life Dr. Coues has been a collaborator of the Smithsonian Institution of Washington, his name being most frequently mentioned in that connection. Many of the numberless specimens of natural history he presented to the United States government were found new to science and several have been named in compliment to their discoverer. At the height of his intellectual activity in physical science the spiritual side of Dr. Coues' nature was awakened. He became interested in the phenomena of spiritualism, as well as in the speculations of theosophy. Belonging distinctively to the materialistic school of thought and skeptical to the last degree by his whole training and turn of mind, he nevertheless began to feel the inadequacy of formal orthodox science to deal with the deeper problems of human life and destiny. Convinced of the soundness of the main principles of evolution, as held by his peers in science, he wondered whether these might not be equally applicable to psychical research, and hence took up the theory of evolution at the point where Darwin left it, proposing to use it in explanation of the obscure phenomena of hypnotism, clairvoyance, telepathy and the like. He visited Europe to see Mme. Blavatsky, founded and became president of the Gnostic Theosophical Society of Washington, and later became the perpetual president of the Esoteric Theosophical Society of America. In 1890 he published an exposé of the impostures of Blavatsky, and from that time his interest in the cult gradually ceased.