This whole question has a fundamental interest for us as educators, because it affords an indirect proof that cerebral exercise develops the brain, or in other words, that education has a physical and morphological influence as well as a psychic one.
This question, coupled with that of the influence of alimentation upon the development of the head, leads to the conclusion that a two-fold nutriment is necessary for the normal development of man: material nutriment and nutriment of the spirit.
It follows that education must be considered from two different points of view: that of the progress of civilisation, and that of the perfectionment of the species.
In regard to variations of cranial volume, just as in the case of variations of stature, there are a number of different factors which may be summed up in such a way as to afford us certain determining characteristics of social caste. Delicate questions these, which we may sum up in a single question equally delicate, that lends itself to a vast amount of discussion; namely, what is the relation between the volume of the brain and the development of the intellect?
Individual Variations of Cerebral (and Cranial) Volume. Relation between the Development of the Cerebral Volume and the Development of the Intelligence.—The series of arguments in reference to the cerebral volume ought to be considered independently of the biological and biopathological factors which we have up to this point been considering; namely, race, sex, age, degeneration and disease.
That is to say, in normal individuals, other conditions being equal, volumetric differences of the brain may be met with, analogous to those other infinite individual variations, in which nature expresses her creative power, even while preserving unchanged the general morphology of the species.
It is due to this fact that the innumerable individuals of a race, while all bearing a certain resemblance to one another, are never any two of them identically alike.
Variations of this sort, which might be called biological individualisations, are in any case subject to the most diverse influences of environment, which concur in producing individual varieties.
This is in accordance with general laws which are applicable to any biological question whatever, but that in our case assume a special interest. There are certain men who have larger or smaller brains; and there are men of greater or of less intelligence. Is there a quantitative relation between these two manifestations, the morphological and the psychic?