Geologists make certain main divisions of the Cainozoic period, and it will be convenient to name them here and to indicate their climate. First comes the Eocene (dawn of recent life), an age of exceptional warmth in the world’s history, subdivided into an older and newer Eocene; then the Oligocene (but little of recent life), in which the climate was still equable. The Miocene (with living species still in a minority) was the great age of mountain building, and the general temperature was falling. In the Pliocene (more living than extinct species), climate was very much at its present phase; but with the Pleistocene (a great majority of living species) there set in a long period of extreme conditions—it was the Great Ice Age.
The Project Gutenberg EBook of The Outline of History - Being a Plain History of Life and Mankind, by H. G. Wells published 1920