"[T]he phenomena which constitute the subject-matter of Comparative Psychology... have at least as great a claim to accurate classification as those phenomena of structure which constitute the subject-matter of Comparative Anatomy. ...[W]ithin the last twenty years the facts of animal intelligence have suddenly acquired a new and profound importance, from the proved probability of their genetic continuity with those of human intelligence ...[M]y first object ...amounts merely to passing the animal kingdom in review in order to give a trustworthy account of the grade of psychological development which is presented by each group. My second, and much more important object, is that of considering the facts of animal intelligence in their relation to the theory of Descent. With the exception of Mr. Darwin's admirable chapters on the mental powers and moral sense, and Mr. Spencer's great work on the Principles of Psychology, there has hitherto been no earnest attempt at tracing the principles which have been probably concerned in the genesis of Mind. ...[T]he present volume... has for its more ultimate purpose the laying of a firm foundation for my future treatise on Mental Evolution."