At first, of course, metrical form was stumbled upon blindly. Having two arms and two legs, men naturally moved with a symmetry that gradually impressed their minds; obliged by the facts of anatomy to group their motions in twos, they soon took the hint, and beat their drums or struck their cymbals accordingly. The primeval dance was doubtless the march. But soon they began to carry out the principle they had thus chanced upon, and despite anatomy devised the group of three. The existence of triple meter is all the proof needed that metrical form is essentially a process of intelligence, not a physical fatality; men grouped their steps or leaps or drum-taps in twos or in threes because such groups were easy to make, to imitate, and to remember. And once perceived, no matter how, such groupings tended to cling, to perpetuate themselves. For they were definite, memorable forms, and they survived all haphazard gestures and vague motions by virtue of the law that what is adapted to its environment will live longer than what is not. In this case the environment was the human mind; and the definite organisms, the metrical forms, survived and developed because the mind could remember them, while all the vague gestures out of which they grew shared the fate of what is indefinite, accidental, inorganic. Thus Dance, which was gesticulation systematized by metrical form, emerged and grew in the human mind, like an animal in a congenial habitat. For a long while the metrical forms that men could perceive and remember were most rudimentary. Probably it took them centuries to grasp the simple group of three, the basis of such accent-schemes as the waltz and the mazurka. Even to-day, psychologists agree, we are unable to grasp a group of seven, and we perceive larger groups than three only as compounded of the elementary twos and threes.[A] But gradually men learned to recombine their groups in still larger forms, of which the first groups constituted the elements. Just as in chemistry the basic elements like oxygen and hydrogen, nitrogen and carbon, can combine only in a few simple ways, but the compound molecules thus produced can recombine into the myriad substances of organic chemistry, the sugars and starches and all the rest, so the simple dual and triple measures of music can be built into an infinite variety of figures and phrases. In early dance and folk-song a more and more complex metrical plan thus slowly developed. Two or more of the simple groups of beats, called measures, were combined into a larger group, a recognizable figure or motif; then again motifs were combined into still larger phrases; and finally, as the musical medium became more definite, plastic and various, phrases were combined in many different types of design, into complete «tunes.» In all these regroupings, the wonderful variety of which is one of the most precious resources of modern music, the fundamental procedure