The deviations in performance are almost certainly biological, and not pathological. Each mental function is by original nature possible in some degree to every person, the degrees of potentiality being of enormous range, and distributed among members of the species according to a frequency curve. The form of this curve indicates that the determinants of aptitude are approximately infinite in possibility of combination. The extremes of deviation from the _typical result_ of these determinants acting together, are, as stated, very widely separated, as in any game of chance combining many factors, but they nevertheless have limits, which are knowable. The determinants exist chiefly (perhaps exclusively) in the germ-plasm, from which human organisms spring, and which carries inheritance from countless combinations of ancestry for persons now alive. It is neither necessary nor plausible to introduce a theory of brain lesion or atrophy to explain the extreme minus deviations, leaving the equally extreme plus deviations thus unexplained. The sum total of a child’s standings on these curves, in the multitude of mental functions which are possible to human beings, constitutes his psychograph or _mentality_. The physiological aspects of this inheritance may ultimately be found in brain chemistry, or in the discovery of some principle of physics at present unknown. It may be an inheritance of function, rather than of structure. We do not know. The present status of the problems indicated in this chapter may be recapitulated in the words of Ladd and Woodworth: “The analysis of mental functions into their elements, in a manner suitable for physiological use, has scarcely been begun.” REFERENCES BROCA, P. P.—_Sur le siège de la faculté du langage articulé, avec deux observations d’aphémie_; V. Masson et Fils, Paris, 1861. FRANZ, S. I.—“Cerebral-Mental Relations”; _Psychological Review_, 1921. HEAD, H.—“Aphasia and Kindred Disorders of Speech”; _Brain_, 1920. HEAD, H.—“Release of Function in the Nervous System”; _Proceedings of the Royal Society_, Series B, 1921. HEAD, H.—“Disorders of Symbolic Thinking Due to Local Lesions of the Brain”; _British Journal of Psychology_, 1921. LADD, G., and WOODWORTH, R. S.—_Physiological Psychology_; Charles Scribner’s Sons, New York, 1911. LASHLEY, K. S.—“Studies of Cerebral Functions in Learning”; _Psycho-Biology_, 1920. MONAKOW, C. VON—_Die Lokalisation im Grosshirn_; Bergmann, Wiesbaden, 1914. TILNEY, F., and RILEY, H. A.—_The Form and Functions of the Central Nervous System_; Hoeber, New York, 1921. CHAPTER IV READING I. RELATION BETWEEN IQ AND CAPACITY FOR READING