believe that there have been alternations of cold or glacial periods and of warm periods, of periods of subsidence and re-elevation, and of periods of greater and less activity of certain of the leading agents of geological change. But as to the extent of these differences and their bearing on the geological history, there is still much uncertainty and difference of opinion.[141] In the sediment _now_ accumulating in the bottom of the waters are being buried remains of the existing animals and plants. A geological formation is being produced, and it contains the skeletons and other solid parts of a vast variety of creatures belonging to all climates, and which have lived on land as well as in fresh and salt water. Let us now suppose that by a series of changes, sudden or gradual, all the present organized beings were swept away, and that, when the earth was renewed by the power of the Creator, a new race of intelligent beings could explore those parts of the former sea basins that had been elevated into land. They would find the remains of multitudes of creatures not existing in their time; and by the presence of these they could distinguish the deposits of the former period from those that belonged to their own. They could also compare these remains with the corresponding parts of creatures which were their own contemporaries, and could thus infer the circumstances in which they had lived, the modes of subsistence for which they had been adapted, and the changes in the distribution of land and water and other physical conditions which had occurred. This, then, is precisely the place which fossil organic remains occupy in modern geology, except that our present system of nature rests on the ruins, not of one previous system, but of several.