The First Epoch.—The Archilithic, or Primordial Epoch, constitutes the Age of Skull-less Animals and Sea-weed Forests, and is made up of the Laurentian, Cambrian, and Silurian Period.
The Second Epoch.—The Palæolithic, or Primary Epoch, constitutes the Age of Fishes and Fern Forests, and is made up of the Devonian, Coal, and Permian Period.
The Third Epoch.—The Mesolithic, or Secondary Epoch, constitutes the Age of Reptiles and Pine Forests, Coniferæ, and is made up of the Triassic, Jurassic, and Chalk Period.
The Fourth Epoch.—The Cænolithic, or Tertiary Epoch, constitutes the Age of Mammals and Leaf Forests, and is made up of the Eocene, Miocene, and Phocene Period.
The Fifth Epoch.—The Anthropolithic, or Quaternary Epoch, constitutes the Age of Man and Cultivated Forests, and is made up of the Glacial and Postglacial Period, and the Period of Culture.
During the archilithic epoch the inhabitants of our planet, as has been already stated, consisted of skull-less animals, or aquatic forms. No remains of terrestrial animals or plants, dated from this period, have as yet been found.
The archilithic period was longer than the whole long period between the close of the archilithic and the present time; for if the total thickness of all sedimentary strata be estimated as about one hundred and thirty thousand feet, then seventy thousand feet belong to this epoch. It was during this epoch that the little mass of protoplasm, which has been so often spoken of, came into existence.
It has been stated above that palæontology is quite deficient. This is not only true of the record, but of the lack as yet of sufficient investigations. The greatest fields of investigation in this department have never been explored. The whole of the petrifactions accurately known do not probably amount to a hundredth part of those which, by more elaborate explorations, are yet to be discovered. The most ancient of all distinctly preserved petrifactions is the Eozoon Canadense, which was found in the lowest Laurentian strata in the Ottawa formation.
Probably no discovery in palæontology ranks higher than the discovery of the descendants of the horse. The horse, for example, as far as his limbs and teeth go, differs far more from extant graminivora than man differs from the ape. Had not fossil ungulates been found, which demonstrate the common origin of the horse with didactyles and multidactyles, some would have deemed the horse a special miraculous creation. But now the links are complete, and the descent of the horse is found to follow exactly what the doctrine of evolution could have predicted.