Our present knowledge of the nature and history of human mentality enables us to learn many lessons that have a direct practical value, although it is impossible under the present limitations to give them the full discussion they deserve.  Starting from the dictum that physical inheritance provides the mechanism of intellect, education and training of any kind prove to be effective as agents for developing hereditary qualities or for suppressing undesirable tendencies.  Just as wind-strewn grains of wheat may fall upon rock and stony soil and loam, to grow well or poorly or not at all according to their environmental situations, so children with similar intellectual possibilities would have their growth fostered or hampered or prevented by the educational systems to which they were subjected.  But the common-sense of science demonstrates that the mental qualities themselves could not be altered in nature by the circumstances controlling their development any more than the hereditary capability of the wheat grains to produce wheat would be altered by the character of the ground upon which they fell.  Education and training thus find their sphere of usefulness is developing what it is worth while to bring out, and inhibiting the growth of what is harmful.  That heredity in mental as well as in physical aspects provides the varying materials with which education must deal is a fundamental biological fact which is too often disregarded.  It would be as futile for an instructor to attempt the task of forcing the children in a single schoolroom into the same mental mold, as it would be for a gymnasium master to expect that by a similar course of exercise he could make all of his students conform to the same identical stature, the same shape of the skull, or the same color of the eye and hair.

* * * * *

Before leaving the subject of mental evolution we must return to the conception of inseparable mind and matter with which the present discussion began.  The whole problem of human mental evolution is solved when we accept the conclusion that the nervous mechanism and the total series of its functional operations have evolved together in the production of the human brain and human faculty.  The case regarding the physical organs rests solidly on the basis of the evidences outlined in a previous chapter; the special examination of purely mental phenomena has likewise been made in the foregoing sections.  Just here we must pause to give further attention to the invariable relation between the human mind and the human brain.

The personality of human consciousness consists of the current of thoughts and feelings flowing continuously as one of them rises for a time to dominance only to fade when it leads to and is replaced by another dominant element of thought.  This current is affected by the messages brought to the brain by nerves from the outer parts of the body where lie the eye and ear and other sense-organs.  In like manner the