It is from this point of view that we must understand the helpfulness of rhythm in work. That all definite stimulus, and especially sound stimulus, rhythmical or not, sets up a diffusive wave of energy, increasing blood circulation, dynamogenic phenomena, etc., is another matter, which has later to be discussed. But the essential is that this additional stimulus is rhythmical, and therefore a reinforcement of the nervous activity, and therefore a lightening and favorable condition of work itself. So it is, too, that we can understand the tremendous influence of rhythm just among primitive peoples, and those of a low degree of culture. Work is hard for savages, not because bodily effort is hard, but because the necessary concentration of attention is for them almost impossible; and the more, that in work they are unskilled, and without good tools, so that generally every movement has to be especially attended to. Now rhythm in work is especially directed to lighten that effort which they feel as hardest; it rests, renews, and frees the attention. Rhythm is helpful not primarily because it enables many to work together by making effort simultaneous, but rhythm rests and encourages the individual, and working together is most naturally carried out in rhythm. To this explanation all the other facts of life-enhancement, etc., can be attached. Rhythm is undoubtedly favorable stimulation. Can it be brought under the full aesthetic formula of favorable stimulation with repose? A rhythm once established has both retrospective and prospective reference. It looks before and after, it binds together the first and the last moments of activity, and can therefore truly be said to return upon itself, so as to give a sense of equilibrium and repose. But when we turn from the fundamental facts of simple rhythm to the phenomena of art we find straightway many other problems. It is safe to say that no single phrase of music or line of poetry is without variation; more, that a rhythm without variation would be highly disagreeable. How must we understand these facts? It is impossible within the natural limitations of this chapter to do more than glance at a few of them.