The period opened with a long phase of considerable warmth; then the world cooled. And in the opening of this third part of the record, this Cainozoic period, a gigantic crumpling of the earth’s crust and an upheaval of mountain ranges was in progress. The Alps, the Andes, the Himalayas, are all Cainozoic mountain ranges; the background of an early Cainozoic scene, to be typical, should display an active volcano or so. It must have been an age of great earthquakes. 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. Glaciers spread from the poles towards the equator, until England to the Thames was covered in ice. Thereafter to our own time came a period of partial recovery. § 2 In the forests and following the grass over the Eocene plains there appeared for the first time a variety and abundance of mammals. Before we proceed to any description of these mammals, it may be well to note in general terms what a mammal is. [Illustration: Some Oligocene Mammals]