body; which begins afresh to draw the now arrested mass back from its aphelion. The only conditions under which there could be absence of rhythm—the only conditions, that is, under which there could be a continuous motion through space in the same straight line for ever, would be the existence of an infinity void of everything but the moving body. And neither of these conditions can be represented in thought. Infinity is inconceivable; and so also is a motion which never had a commencement in some pre-existing source of power. Thus, then, rhythm is a necessary characteristic of all motion. Given the coexistence everywhere of antagonist forces—a postulate which, as we have seen, is necessitated by the form of our experience—and rhythm is an inevitable corollary from the persistence of force. Hence, throughout that re-arrangement of parts which constitutes Evolution, we must nowhere expect to see the change from one position of things to another, effected by continuous movement in the same direction. Be it in that kind of Evolution which the inorganic creation presents, or in that presented by the organic creation, we shall everywhere find a periodicity of action and reaction—a backward and forward motion, of which progress is a differential result. ----- Footnote 15: After having for some years supposed myself alone in the belief that all motion is rhythmical, I discovered that my friend Professor Tyndall also held this doctrine. CHAPTER XII. THE CONDITIONS ESSENTIAL TO EVOLUTION. § 100. One more preliminary is needful before proceeding. We have still to study the conditions under which alone, Evolution can take place.