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.

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