The problems dealing with the make-up of the human mind and with the evidences of mental evolution bring the student to matters of more vivid human interest. Mental phenomena are so complex and intricate that it is well-nigh impossible to analyze their history without a knowledge of the principles derived from the broad study of evolution as a general doctrine, where human prejudice is not so large a factor and where his perspective is less affected by the proximity of the observer to his facts. For these and other reasons the foregoing treatment of human evolution has been confined to the purely structural characteristics of man as a species and of human races as so many varieties of this type. When the broad comparative methods of biological science are employed for the elucidation of human anatomical facts, the result in this special case, like that established through the study of the characteristics of living things in general, is the proof that evolution gives the most rational and natural explanation of the observed data. This being true, the naturalist who turns from purely structural matters to human intellect and its history, finds well-tried methods of inquiry already available, and he approaches his further studies with a conviction that evolution, having proved to be universal so far, in all probability will be found equally true in the case of psychological phenomena. This expectation is indeed realized, and the scope of the doctrine is extended over a new field, when the facts of human psychology are treated as materials for impersonal comparative study; and this result is not only useful and valuable in and by itself, but it also provides in the principles of mental evolution the transition to the field of social relations and ethical ideas and ideals which are apparently the unique possessions of men as individuals and as associated groups. The field of comparative psychology might seem at first sight to be a foreign territory to the average well-informed layman in science, but the contrary is really the case. Every one has thought at one time or another about his own mental make-up, and about the minds of others. No one can watch a child at play with his toys or at work with his schoolbooks without being struck by many evidences of marked differences between the immature and the experienced types of mind. Every one knows also that the mental "scheme of things" is by no means the same for all nations or races of mankind existing to-day, while furthermore the fact is entirely familiar that the intellectual heritage of a present race has changed in the course of previous ages. Therefore in this field as before we need only to amplify our knowledge of such representative psychological facts as these by drawing upon the full stores of the special investigator, in order to learn that human thought, like the human frame, has undergone a natural history of transformation to become what it is and what it was not.