{ Permian period { Carboniferous period Palæozoic era { Devonian period { Silurian period { Ordovician period { Cambrian period Pre-Cambrian eras { Algonkian period { Archæan period It must not be supposed that all the divisions of similar rank, such as the eras, for example, were of equal length, as measured by the thickness of the rocks assigned to those divisions. On the contrary, they must have been of very unequal length and are of very different divisibility. The Pre-Cambrian eras, with only two periods, were probably far longer than all subsequent time, and all that the major divisions imply is that they represent changes in the system of life of approximately equivalent importance. It is impossible to give any trustworthy estimate of the actual lengths of these divisions in years, though many attempts to do so have been made. All that can be confidently affirmed is that geological time, like astronomical distances, is of inconceivable vastness and its years can be counted only in hundreds of millions. To discuss in any intelligible manner the history of mammals, it will be necessary to go much farther than the above table in the subdivision of that part of geological time in which mammalian evolution ran its course. As mammals represent the highest stage of development yet attained in the animal world, it is only the latter part of the earth’s history which is concerned with them; the earlier and incomparably longer portion of that history may be passed over. Mammals are first recorded in the later Triassic, the first of the three periods which make up the Mesozoic era. They have also been found, though very scantily, in the other Mesozoic periods, the Jurassic and Cretaceous, but it was the Cenozoic era that witnessed most of the amazing course of mammalian development and diversification, and hence the relatively minute subdivisions necessary for the understanding of this history deal only with the Cenozoic, the latest of the great eras. In the subjoined table the periods and epochs are those which are in general use throughout the world, the ages and stages are those which apply to the western interior of North America, each region, even of the same continent, requiring a different classification. The South American formations are given in a separate table, as it is desirable to avoid the appearance of an exactitude in correlation which cannot yet be attained. CENOZOIC ERA Quaternary period { Recent epoch { Pleistocene epoch = Glacial and Interglacial stages. { Pliocene epoch { Miocene epoch Tertiary period { Oligocene epoch { Eocene epoch { Paleocene epoch Continuing the subdivision of the Tertiary period still farther, we have the following arrangement: