way as to justify the use of the word mental 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 powers are elementary and restricted, but these same powers change and intensify so as to render the older mind quite capable of grasping the highest of human conceptions and ideas.  In my judgment the unbroken transformation of a child’s mind that exhibits only instinct and intelligence into an adult’s mind with its power of reasoning, is far more conclusive as proof of mental evolution than the inference drawn from the comparisons we have made above of the adult psychological phenomena of man, ape, cat, and fish.  It is surely natural for such mental transformations to take place, for they do take place in the vast majority of human beings; when they do not, in cases where the brain fails to mature, we speak of unnatural or diseased minds.