You are, doubtless, all aware that the crust of the earth, the superficial part of the earth, is not of an homogeneous character, but that it is made up of a number of beds or strata, the titles or the principal groups of which are placed upon the accompanying diagram—beds of sand, beds of stone, beds of clay, of slate, and of various other materials.
On further examination, it is found that these beds of solid material are of exactly the same nature as these which are at present being formed under known conditions at the surface of the earth; that the chalk, for example, which forms a great part of the Cretaceous formation in some parts of the world, is identical in its physical and chemical characters, or practically so, with a substance which is now being formed at the bottom of the Atlantic Ocean, and covers an enormous area; that other bodies of rock are comparable with the sands which are being formed upon sea-shores, packed together, and so on. Thus it comes to be certain that, omitting rocks of igneous origin, all these beds of stone, of which a total of not less than seventy thousand feet is known, have been deposited and formed by natural agencies, either out of the waste and washing of the dry land, or else as the product of plants and animals. Now, these rocks or strata are full of the remains of animals and plants. Countless thousands of species of animals and plants, as perfectly recognizable as those which you meet with in museums at the present day, or as the shells and remains which you pick up upon the beach—countless thousands of species of these creatures have been imbedded in the sand or mud, or limestone, just as they are being imbedded now. They furnish us with a record, the general nature of which cannot be subject to any misinterpretation, as to the kind of things that have lived upon the surface of the earth during the time that is registered by this great thickness of stratified rock. The most superficial study of these remains shows us that the animals and plants which live at the present time have had only a temporary duration; that you will find them and such as they are now, for the most part, only in those uppermost of the strata called Tertiary. As you go back in time their places are taken by other forms as numerous and diversified, but different, and you will find yet others different from the Cretaceous or Tertiary, and from those of the present day, and so on, as you go further and further back. Thus, the circumstantial evidence absolutely negatives the conception of the eternity of the present condition of things. We can say with certainty that such has not been the course of Nature. We can say with certainty that the present condition of things has existed for a comparatively short period; and that, so far as animal and vegetable nature are concerned, it has been preceded by a different condition of things. We can pursue this evidence until we reach the lowest of stratified rocks, in which we lose the indications of life altogether. The hypothesis of the eternity of the present condition of things may, therefore, be put out of court.