Certain anomalies of structure in the ear give rise to tonal “gaps” and “islands.” The ear does not discriminate among pitches, in certain segments of the scale for pitch. Such a condition may occur in but one ear of a given individual, the other ear then hiding the defect. The child who is extensively deaf to tones has, of course, no means, save the testimony of others, of knowing whether he is or is not singing properly (unless he sees his singing on a tonoscope). He cannot be taught to sing in key, because the receptors which would enable him to profit by training are absent from the structure of the ear. Many a tone-deaf child has doubtless suffered much from persistent, conscientious efforts to make him sing. VIII. RANGE OF INDIVIDUAL DIFFERENCES In a previous chapter it has been pointed out that quantitative psychology is still struggling toward the invention of scales which shall measure mental traits in terms of units, every one of which shall be equal to every other, as every inch is equal to every other inch. Until this is achieved, we cannot use “times as” comparisons in speaking of the relation of one individual to another, in respect to a function. We can now say that one person is three times as heavy as another person, because we can measure them in pounds, each one of which is equal to every other. But we cannot yet say that one person is three times as intelligent as another, because we have not captured the unit which would enable us to do so. In some of the traits which go to make up musical talent, it is possible to use the “times as” comparison, because we have physical units whereby the differences may be gauged. Pitch, for instance, may be measured thus. It depends physically upon the frequency of vibrations, proceeding from a sounding stimulus, and is measurable in terms of the constant number of double vibrations per second. Seashore has found variations in power of discrimination from one-fourth of a double vibration to fifty double vibrations per second. This means that there exist individuals who are at least two hundred times as sensitive as others to pitch, in terms of the physical unit.