We have seen that through the ages we have been studying the climate was mostly warmer than at the present day. The climate of the Eocene was tropical. The Miocene was sub-tropical and becoming cooler. Palms become rarer in the Upper strata. Evergreens, which form three-fourths of the flora in the Lower Miocene, divide the flora with deciduous trees in the Upper. And through the Pliocene the climate, though still warmer than now, was steadily becoming cooler; till in the beginning of the next period, the Pleistocene, it had become considerably colder than that of the present day. And then followed a time which is known as the great Ice Age, or the Glacial Period,--a time which has left its traces all over this country, and, indeed all over Northern Europe and America, and even into southern lands. The cold increased, heavy snowfalls piled up snow on the mountains of Wales, the Lake District, and Scotland; and the snow remained, and did not melt, and more fell and pressed the lower snow into ice, which flowed down the valleys in glaciers, as in Switzerland to-day. Gradually all the vegetation of temperate lands disappeared, till only the dwarf Arctic birch and Arctic willows were to be seen. The sea shells of temperate climates were replaced by northern species. Animals of warm and temperate climates wandered south, and the Arctic fox, and the Norwegian lemming, and the musk ox which now lives in the far north of America took their place; and the mammoth, an extinct elephant fitted by a thick coat of hair and wool for living in cold countries, and a woolly-haired rhinoceros, and other animals of arctic regions occupied the land. When the cold was greatest, the glaciers met and formed an ice-sheet; and Scotland, northern England and the Midlands, Wales, and Ireland were buried in one vast sheet of ice as Greenland is to-day.