or cannot adopt the higher conceptions of a civilized
people; the fact is that they have not actually become civilized by
themselves. Thus, while evolution in mental respects has not resulted in
the loss of plasticity in the case of the brain and the nervous system as
a whole, wherefore the activities of these organs still remain capable of
individual and racial modifications that are impossible in the case of the
skeleton and in the color and shape of the eye, it remains true that races
do differ intellectually, and that their differences are marks of a mental
evolution quite as definite as their physical natural histories of change.

* * * * *

In my own view the strongest and most impressive evidence bearing upon the
great problem before us is provided by the series of transformations by
which the human intellect develops during an individual life. Mind has an
embryology no less significant than that of the skull or of any other
element of the body; and its investigation leads to the evolutionary
interpretation quite as surely as the study of the various grades of adult
psychology constituting the anatomical sequence, which we have reviewed
previously. When in the earlier part of the book we dealt with embryology
in general, we learned how the changes which take place when an organism
develops from an egg demonstrate the actuality of true organic
transformation without the necessity of concluding or inferring that this
process might occur. It is not superfluous to insist again that the
essential fact in evolution is the alteration of one organic
characteristic into another type; must we not recognize at the very outset
that mental transformation is as real as physical development?

In the first instance we might concern ourselves with the physical basis
of mind and its history. In the earliest stages of human embryology no
nervous system whatsoever is present, and it is unreasonable to suppose
that there is anything going on which corresponds to human thought. A
little later a cellular tube is established as a primitive nerve axis,
which at first is nearly uniform throughout its entire length and displays
no differentiation into brain and spinal cord. Before long an enlargement
of the anterior end expands and develops into a primitive three-parted
brain. It is not yet a real brain, however, and it is entirely incapable
of functioning in such a way as to justify the use of the word _ment