21. Nor is there anything which need startle us, in the necessity of assuming such vast intervals of time, when we have once brought ourselves to deal with the question of the antiquity of the earth upon scientific evidence alone. For if geology thus carries us far backwards through thousands, it may be, millions of years, astronomy does not offer the smallest argument to check this regressive supposition. On the contrary, all the most subtle and profound investigations of astronomers have led them to the conviction, that the motions of the earth may have gone on, as they now go on, for an indefinite period of past time. There is no tendency to derangement in the mechanism of the solar system, so for as science has explored it. Minute inequalities in the movements exist, too small to produce any perceptible effect on the condition of the earth's surface; and even these inequalities, after growing up through long cycles of ages, to an amount barely capable of being detected by astronomical scrutiny, reach a maximum; and, diminishing by the same slow degrees by which they increased, correct themselves, and disappear. The solar system, and the earth as part of it, constitute, so for as we can discover, a Perpetual Motion. 22. There is therefore nothing, in what we know of the Cosmical conditions of our globe, to contradict the Terrestrial evidence for its vast antiquity, as the seat of organic life. If for the sake of giving definiteness to our notions, we were to assume that the numbers which express the antiquity of these four Periods;--the Present organic condition of the earth; the Tertiary Period of geologists, which preceded that; the Secondary Period, which was anterior to that; and the Primary Period which preceded the Secondary; were on the same scale as the numbers which express these four magnitudes:--the magnitude of the Earth; that of the Solar System compared with the Earth; the distance of the nearest Fixed Stars compared with the solar system; and the distance of the most remote Nebulæ compared with the nearest fixed stars; there is, in the evidence which geological science offers, nothing to contradict such an assumption.