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.

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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.

Heredity of mental qualities has also been demonstrated notably by Galton, Pearson, Woods, and Thorndike, who have also shown that the strength of inheritance in the case of mental traits is approximately the same as for physical characteristics like stature and eye-color.  Just as a worker-bee inherits a specific form of nervous system which cooeperates with the other equally determined organic systems, wherefore the animal is forced to perform “instinctively” its peculiar specialized tasks, so the mental capacity of a human being is largely determined by congenital factors.  Upon these primarily depends his success or failure.  It is quite true that environment has a high degree of influence, so great indeed that some speak of a “social heredity”; they mean by this phrase that the mental equipment of an individual is determined by the things he finds about him, or learns from others without having to invent or originate them himself.  Thus a Zulu boy acquires the habits of a warrior and a huntsman when he grows up in his native village, although he would undoubtedly develop quite different aptitudes if he should be taken as an infant to a city of white men.  Nevertheless his mental machinery itself would be no less surely determined by heredity, even though the things with which it dealt would be provided by an alien environment.