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.
23. And as the infinite extent which we necessarily ascribe to space, allows us to find room, without any mental difficulty, for the vast distances which astronomy reveals, and even leaves us rather embarrassed with the infinite extent which lies beyond our farthest explorations; so the infinite duration which we, in like manner, necessarily ascribe to past time, makes it easy for us, so far as our powers of intellect are concerned, to go millions of millions of years backwards, in order to trace the beginning of the earth's existence,--the first step of terrestrial creation. It is as easy for the mind of man to reason respecting a system which is billions or trillions of miles in extent, and has endured through the like number of years, or centuries, as it is to reason about a system (the earth, for instance,) which is forty million feet in extent, and has endured for a hundred thousand million of seconds, that is, a few thousand years.