Still another consideration prevents us from regarding the theory of localized brain defects as masterly. This is the fact referred to in our preliminary discussion, that every single mental function, which yields to measurement, is found to be distributed among human beings according to a probability curve. (See Figure 1, page 8.) The functions which we herein consider are not exceptions to this principle. Performance in reading, spelling, arithmetic, drawing, music, and so forth, shows children or adults, chosen at random, to be distributed in the given form. Those who have exceptional talents or defects in the function fall within the _symmetrical surface_ of this curve, at its opposite extremes. Nowhere is there a point of demarkation, denoting absolute lack of the trait in a group falling below that point, as there would be if a number of individuals suffering from lesions were introduced into the distribution. We may fairly demand of a theory which undertakes the explanation of the most extreme deviations, the explanation of the deviations of lesser magnitude, as well. The curve obtained by test approximates that form which mathematicians tell us appears when an infinite number of factors act together in an infinite number of ways, the extreme deviations occurring inevitably, by chance. A theory introducing the adventitious circumstance of lesion or atrophy is thus superfluous to the explanation of the extreme unfortunate deviations. To admit it would violate the rule of scientific method known as the law of parsimony, for we do not need it in order to explain the facts. IV. RESTITUTION OF FUNCTION WITHOUT REGENERATION OF STRUCTURE IN INJURED BRAINS Fully as important as any of the facts mentioned above, for criticism of the theory that special deficiencies are due to localized defects in brain structure, are the experiments with reëducation of those who have suffered _loss_ of an ability. Persons who have lost the power to read, or write, or speak after destruction of brain tissue, may learn to perform these functions again, without regeneration of the area impaired. If the neurones destroyed, and no others, were the special mechanisms rendering possible the functions lost, how would restitution of function be possible, without repair of the destroyed tissues? V. ATTEMPTS TO ESTABLISH A NEURAL BASIS FOR THE “TWO FACTOR THEORY” AND “THE TWO LEVEL THEORY” In prosecuting their researches from the psychological point of view, by the method of testing _performance_, Spearman, Moore, Thomson, and other investigators referred to in the preceding chapter, did not neglect the attempt to reconcile their findings with a possible neural basis.