The collection and study of these fragments has been of great interest to ethnologists and philosophers and has given rise to numerous theories regarding the origin of music. Herbert Spencer gives a physiological explanation of its origin, claiming that intense emotion acts in a particular manner on the vocal and respiratory organs, thereby causing the person thus affected to emit sounds; either high or low, loud or soft, according to the kind of emotion with which he is filled. Beginning with the proposition that 'All music is originally vocal,’ he goes on to say: 'All vocal sounds are reproduced by the agency of certain muscles. These muscles, in common with those of the body at large, are excited to contraction by pleasurable and painful feelings.’ And again: 'We have here, then, a principle underlying all vocal phenomena, including those of vocal music, and by consequence those of music in general. The muscles that move the chest, larynx, and vocal cords, contracting like other muscles in proportion to the intensity of the feelings; every different contraction of these muscles involving, as it does, a different adjustment of the vocal organs; every different adjustment of the vocal organs causing a change in the sound emitted; it follows that variations of voice are the physiological results of variations of feeling.’ Charles Darwin attempts to explain the existence of primitive music by considering it as a secondary sexual manifestation. He asserts that primitive song was used as a method of charming the opposite sex; that the first songs were love songs, and that from these all others were developed. In the 'Descent of Man’ he says: 'The male alone of the tortoise utters a noise, and this only during the season of love. Male alligators roar or bellow during the same season. Every one knows how much birds use their vocal organs as a means of courtship; and some species likewise perform what may be called instrumental music.’ And later: 'Women are thought to possess sweeter voices than men, and so far as this serves as any guide, we may infer that they first acquired musical powers in order to attract the other sex.’