As we look back over the facts that have been cited, and as we contemplate the large departments of knowledge about human psychology, mental development, and racial culture which these few details illustrate, we come to realize how securely founded is the doctrine that even the human mind with all its varied powers has grown to be what it is. Indeed, it is solely due to his mental prowess that man has attained a position above that of any lower animal. And yet every human organ and its function can be traced to something in the lower world; it is a difference only in degree and not in category that science discovers. The line connecting civilized man with the savage leads inevitably through the ape to the lower mammalia possessing intelligence, and on down to the reflex organic mechanisms which end with the _Amoeba_. It is a long distance from the mechanical activities of the protozooen to the processes of human thought; yet the physical basis of the latter is a cellular mechanism and nothing more, developed during a single human life in company with all other organs from a one-celled starting-point--the human egg. * * * * * The method by which mental evolution has been accomplished is likewise demonstrable, because the factors are identical with those which bring about specific transformation in physical respects. This is to be expected, for the contention that the structures and the functions of the several organs constituting any system are inseparable has never been gainsaid. Mental variation is real. It needs no scientist to tell us that human beings differ in intellectual qualifications and attainments, and that no two people are exactly similar even though they may be brothers or sisters. The struggle for existence or competition on the basis of mental ability is equally real, and every day we see the prize awarded to the more fit, while those who lose are crowded ever closer to the wall. As in all other fields of endeavor, the goal of success can be attained only by adaptation, which involves an adjustment to all of the conditions of existence--to social and ethical as well as to the more expressly material biological circumstances.