This series of changes perhaps occupied centuries, for the artistic power of primitive men advances very slowly. The singers had not yet thought of putting this song in rhythm; and it was yet many centuries before they would feel anything like a musical scale. Rhythm had probably developed among people as a thing in itself, not directly connected with musical tones. Rhythmic movement is common to all men--in walking, in running, and in the beating of the heart. Dancing must have arisen spontaneously from an excess of emotion. Such dancing was perhaps accompanied by the clapping of hands, the beating of sticks, or the pounding of some primitive drum. Then with the invention of some primitive musical instrument the tribal musician began to play a simple dance tune in rhythm, a tune of two notes, at first an aimless shifting from one note to another, and later an organized melody with tonal figures and balances and contrasts. It is probable that these two elements--rhythm and melody--were developed separately and were at first regarded as separate arts. At least it seems certain that singing was at first without rhythm. It must have been a wonderful day when somebody thought of combining the two--of singing the old song with metre and accents. But this, too, must have developed very slowly, being at first no more than an accenting of each alternative note. At the same time the song had probably been divided into line divisions. The singer then probably developed his rhythm from both ends, so to speak, building a more complex series of accents from the single group of two notes, and at the same time subdividing the line into sections. After several more centuries, perhaps, the tribe possessed a song that was fairly metrical and regular. All this time the notion of a scale--a set succession of notes from which the notes of the tune were to be chosen--was entirely lacking. It came comparatively late in musical development, though it is the first incident in musical _history_. At first each melody was its own scale. Then, as people began to observe similarities in the tone relations between several melodies, they began to have a conception of the regular succession of tones which was common to all of them. Such a succession, a mode or primitive scale, was of but three or four notes. By arranging the semi-tones several new scales could be formed. And by here and there adding a note above or below the scales could be enlarged. With the enlargement of the scales melodies became enlarged in range. More opportunities arose for the balancing and contrasting of melodic devices. The tune became more complex, more regular, and more beautiful.