al 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.

Many who would be ready to accept the evolution of physical
characteristics find it impossible