It is the manner in which the numerous individual conformations of the vocal tract affect the overtones in the voice that makes voices different from each other; for the overtones are the chief agency in determining the timbre, quality, or physiognomy of any tone. Every tone consists of a fundamental or ground tone with its overtones. The fundamental tone determines the pitch; the overtones determine the quality, tone-color, timbre, or physiognomy of the tone. The overtones, or harmonics, as they also are called, vibrate in certain simple harmonic relations with the fundamental--from twice to five times as often per second, sounding the octave above, the fifth of that octave, the second octave, the major third of that octave, etc. So important is it to the individual musical quality of tone, to secure the coöperation of overtones, that in certain large open organ pipes, which are deficient in these, extra pipes of higher pitch and corresponding with the overtones of the fundamental note, are added and joined to the register. Overtones without the fundamental can be obtained on stringed instruments by stopping one of the strings and then touching it lightly at other points. The soft, sweet, ethereal character of the harmonics produced in this manner on a violin conveys some idea of the manner in which the many overtones of a note give it its distinctive quality. In a way the overtones may be said to echo the fundamental, but the ear receives fundamental and overtones blended as one tone of a certain timbre. What that timbre is, is determined by the shape of the resonating cavity or cavities, the shape of which in turn is determined by the shape of the instrument, and in different voices by infinitesimal differences in the shape of various parts of the vocal tract. All instruments of a kind are made more or less on the same pattern and vary but little in shape. For this reason we have the distinct violin, horn, clarinet or pianoforte timbre, and so on down the list, but I repeat here that there are not such minute and individual differences between instruments of the same kind as there are between voices of the same range, because there are no such minute and individual structural differences in instruments as in the vocal organs of individuals--differences that each individual can multiply _ad infinitum_ by the subtle and delicate play of muscles acting in response to mental suggestion, art sense, inspiration, temperament, psychic impulse, or by whatever cognate term one may choose to call it.