al_
for the results of its operations. We know that it is only in the cerebral
hemisphere of the adult brain that the processes of true human
consciousness go on. But it is not until long after the three-parted stage
that the cerebral hemispheres make their appearance therefore we cannot
speak of mind as present when the cell and tissue basis of mind is not
present. When, now, the cerebral hemispheres do appear, they are small
bean-shaped structures no larger relatively than those of a fish. Later
they enlarge so as to attain the relative size of the cerebral hemispheres
of an amphibian, and still later they are like those of a reptilian brain.
Continuing to enlarge, they begin to fold so that the total surface is
increased without very much addition to their bulk. At this time the
cerebral hemispheres of the brain of the human embryo are like those of an
adult cat or dog. The process of general enlargement and of progressive
convolution are continued, and stages are reached and passed which
correspond with the monkey and ape conditions.

Nothing in human development is more impressive than the origin of the
cerebrum and its development by passing through successive stages which
are counterparts in the main of the adult brains of other and lower
animals. The alteration of a tissue-mechanism constructed in one way into
a tissue-mechanism of a more complex nature, provides the most conclusive
evidence of the reality of brain evolution, because the process of
transformation actually takes place.

But in the present connection we are more interested in the dynamic or
functional aspects of mental evolution, which it must be remembered are
inseparably bound up with the physical structures and their modifications.
After a human infant is born its activities are reflex and mechanical like
those of the adult members of lower groups. As it grows it performs
instinctive acts because its inherited nervous system operates in the
purely mechanical manner of a lower mammal's nervous system. For these
reasons an eminent psychologist has said that the mental ability of an
infant six months old is about that of a well-bred fox terrier. The same
infant at nine months displays an intelligence of a higher order equal to
that of a well-trained chimpanzee; it has become what it was not, and in
so far it has truly evolved in mental respects. At two years of age the
child is incapable of solving problems of the calculus, for its reasoning
powe