Curiously enough, heat both must have been and then must have been lost. Like the loss of fortune or of friends sometimes in the ennobling of character, it is through its passing away that its effects are realized. For in cooling down from a once heated condition, that train of events occurs which we most commonly particularize as evolution. So far in our survey the march of advance has been through masses of matter, a molar evolution; from this point on it passes into its minute constituents and becomes a molecular one. The one is the necessary prelude to the other. Up to this great turning-point in the history of each member of a solar system we have been busied with the acquisition of heat, though we may not have been aware of it the while. All the motions we have studied tended to that end. During these three chapters, I, II, V, we have been gradually rising in our point of view until we stand at the temperature pinnacle of the whole process. In the next three we are to descend upon the other side. The slope we have come up was of necessity barren; the one we are to go down brings us to verdure and the haunts of men. Coming from the causes above, we reach at each step effects more and more related to ourselves which those causes will help us to explain. CHAPTER VI A PLANET’S HISTORY _Self-sustained Stage_ Up to this point in our retrospective survey the long course of evolution has taken one line, that of dynamical separation of the system’s parts with subsequent reunitement of them according to the laws of celestial mechanics. Of this action I have submitted the reader my brief: departing in it from common-law practice, in which the cause of action is short and the brief long. And I have, I trust, guarded against his appealing on exceptions. From this point on we have two kinds of development to follow: the one intrinsic, the chemical; the other incidental, the physical. Not that, in a way, the one is divorcible from the other. For the physical makes possible the chemical by furnishing it the conditions to act. But in another sense, and that which is most thrust upon our notice, the two are independent. Thus oceans and land, hills and valleys, clouds and blue sky, as we know them,—everything, pretty much, which we associate with a world,—are not universal, inevitable, results of planetary evolution, but resultant, individual, characteristics of our particular abode. They are as much our own as the peculiar arithmetic of waiters is theirs, or as used to be the sobriety of the country doctor’s horse—his and no other’s. Our whole geologic career is essentially earthly. Not that its fundamental laws are not of universal application, but the kaleidoscopic patterns they produce depend on the little idiosyncrasies of the constituents and the mode in which these fall together. Our everyday experiences we should find quite changed, could we alight on Venus or on Mars.