The other circumstance, on which the stability of our system depends, is the small eccentricity of the planetary orbits, or their near approach to circles. Were their orbits very eccentric, an opening would be given to progressive change, that might so far increase, as to prove the destruction of the whole. But neither the movement of all the planets in the same direction, nor the small eccentricity of their orbits, can be ascribed to accident, since that either of these should happen by chance, in as many instances as there are planets, both primary and secondary, is almost infinitely improbable. Again, that any necessity in the nature of things should have either determined the _direction_ of the planetary motions, or proportioned the _quantity_ of them to the intensity of the central force, cannot be admitted, as these are things unavoidably conceived to be quite independent of one another. It remains, therefore, that we consider the laws, which make the disturbances in our system correct themselves, and by that means give firmness and permanence to it, as a proof of the consummate wisdom with which the whole is constructed. 387. The geological system of Dr Hutton, resembles, in many respects, that which appears to preside over the heavenly motions. In both, we perceive continual vicissitude and change, but confined within certain limits, and never from a certain, mean condition, which is such, that, in the lapse of time, the deviations from it on the one side, must become just equal to the deviations from it on the other. In both, a provision is made for duration of unlimited extent, and the lapse of time has no effect to wear out or destroy a machine, constructed with so much wisdom. Where the movements are all so perfect, their beginning and end must be alike invisible. NOTE XXI. § 122. _Changes in the apparent Level of the Sea._ 388. In speaking of the natural epochs marked out by the phenomena of the mineral kingdom, we have supposed a greater simplicity, and separation of effects from one another, than probably takes place in nature. We have, for instance, abstracted, in speaking of the waste and degradation of the land, from that elevation which may have been carried on at the same time. This appeared necessary to be done, in order to simplify as much as possible the view that was to be given of the whole; but there can be no doubt, that, while the land has been gradually worn down by the operations on its surface, it has been raised up by the expansive forces acting from below. There is even reason to think, that the elevation has not been uniform, but has been subject to a kind of oscillation, insomuch, that the continents have both ascended and descended, or have had their level alternately raised and depressed, independently of all action at the surface, and this within a period comparatively of no great extent.