Other elements in musical sensitivity cannot be so readily measured in stimulus units, so that the “times as” comparison cannot be made. The great diversity of sensitivity to pitch may, perhaps, be regarded as a token of the range of individual differences in musical sensitivity, especially since pitch is a fundamental capacity. It is probably no exaggeration to say that, in an ordinary class in the elementary school, children are being taught together, some of whom are at least a hundred times as musical as others. If children of the same age differed as much from each other in height as they do in sense of pitch, it would be impossible to teach them in unassorted groups, for some would be two hundred times as tall as others. The diversity in mental traits, so much greater than in physical traits, leaves us complacent, for the eye cannot behold the incongruities, as it can in physical matters. The eye cannot see the waste of time, effort, and joy which follows from the attempt to train, equally and together, children of such widely differing capacities for learning. IX. CAN MUSICAL CAPACITY BE INCREASED BY EDUCATION? Musical sensitivity is inborn, and probably cannot be increased in any respect by training. If the various elements are not present in amount and combination suitable for a given degree of achievement in music, no course of training will supply the lack. This is not to say that ultimate achievement, for those who do possess capacity, does not depend on training. Achievement depends upon both training and capacity, but the latter cannot be supplied except by hereditary endowment. The question of improvement through education becomes especially important in a case where the psychograph is excellent, but for one element. Much depends, of course, upon what the inferior element is, and the degree of the inferiority, as to whether the person will be able to succeed in a given musical career. Inferiorities that appear in capacity for musical _action_ are possibly much more susceptible to improvement by training than are inferiorities of _sensitivity_. For example, there are persons whose psychographs show excellent musical talent, except that they falter from the pitch in singing. The voice may be excellent in range, quality, and volume, yet with a falter in control which leads to “flatting” or “sharping.” This is a defect in musical action, an inaccuracy of movement. It has long been known that the control of movement is brought about not only through the kinæsthetic sensations, but through aid from the other special senses as well. Vision is a first rate aid to the acquisition of motor control. It is a more efficient aid than hearing, because much finer differences can be detected by vision. The problem, then, in an endeavor to improve by training those who “flat” or “sharp,” is to devise some method whereby visual aid may be administered to control.