We have now seen, that in every quarter of the globe, and in every climate of the earth, there is formed, by means of the decay of solid rocks, and by the transportation of those moveable materials, that beautiful system of mountains and valleys, of hills and plains, covered with growing plants, and inhabited by animals. We have seen, that, with this system of animal and vegetable economy, which depends on soil and climate, there is also a system of moving water, poured upon the surface of the earth[30], in the most beneficial manner possible for the use of vegetation, and the preservation of our soil; and that this water is gathered together again by running to the lowest place, in order to avoid accumulation of water upon the surface, which would be noxious. [Footnote 30: See Dissertations upon Subjects of Natural Philosophy, Part I.] It is in this manner that we first have streams or torrents, which only run in times of rain. But the rain-water absorbed into the earth is made to issue out in springs, which run perpetually, and which, gathering together as they run, form rivulets, watering valleys, and delighting the various inhabitants of this earth. The rivulets again are united in their turn, and form those rivers which overflow our plains, and which alternately bring permanent fertility and casual devastation to our land. Those rivers, augmenting in their volume as they unite, pour at last their mighty waters into the ocean; and thus is completed that circulation of wholesome fluids, which the earth requires in order to be a habitable world. Our Theory farther shows, that in the ocean there is a system of animals which have contributed so materially to the formation of our land. These animals are necessarily maintained by the vegetable provision, which is returned in the rivers to the sea, and which the land alone or principally produces. Thus we may perceive the mutual dependence upon each other of those two habitable worlds,--the fluid ocean and the fertile earth. The land is formed in the sea, and in great part by inhabitants of that fluid world. But those animals, which form with their _exuviae_ such a portion of the land, are maintained, like those upon the surface of the earth, by the produce of that land to which they formerly had contributed. Thus the vegetable matter, which is produced upon the surface of the earth in such abundance for the use of animals, and which, in such various shapes, is carried by the rivers into the sea, there sustains that living system which is daily employed to make materials for a future land.