The Tertiary Period marks the true beginning of modern geological history, when the great outlines of geography were laid down, and the first representatives of modern plants and animals made their appearance. EOCENE SYSTEM. The Eocene rocks are mainly limestones, with sandstone and hardened clays. We owe them to the sea and its organisms. Modern evergreen trees now first appeared. The mammals come to the front, with the tapir-like palæotherium and the first recognisable ancestor of the horse. MIOCENE SYSTEM. The Miocene Age was a mountain-building period, when the great chain which runs from the Alps into Central Asia received its final uplift. Deciduous trees, like the beech and elm, now made their appearance. The giant mastodon and the formidable sabre-toothed tiger roamed the Miocene forest, and true apes--man’s first forerunners--mopped and mowed in the boughs. PLIOCENE SYSTEM. The last of the Tertiary ages set the final stamp on the geological moulding of the earth’s crust. Its plants were transitional to the flora of modern Europe. Great herds of herbivora now appeared. The Fourth Period of the Earth The Quaternary Period is that in which we are still living. Its outstanding feature is the appearance of man. PLEISTOCENE OR GLACIAL SYSTEM. Its essential feature was the appearance of glacial conditions over most of the northern hemisphere, when great ice sheets rubbed our land into shape. The vegetation was Arctic, and only animals like the reindeer and the hairy mammoth could endure the cold. HUMAN OR RECENT SYSTEM. The precise antiquity of man is still uncertain, but it was only after the close of the Glacial Period that he made his home in Europe, where he shared a precarious existence with mammoth, cave-bear, and rhinoceros. Man developed through the _Palæolithic_ and _Neolithic_ ages of stone implements to the _Bronze_ and _Iron_ ages, when metal was first worked. In the last of these we live. GEOLOGICAL CLOCK OF THE WORLD’S LIFE This page is an effort, based on Professor Lester Ward’s calculations in “Pure Sociology,” to show the comparative length of each geological period, and the thin white line between Tertiary and Archæan indicates the period of human history. Thin as this line is--and we could not show it thinner--it is too thick, and out of proportion to the rest of the clock. If we assume that from the beginning of the world--from its first forming into a solid sphere--to the present, time may be represented by a day of twenty-four hours, the time occupied by human history does not exceed twelve seconds. This is reckoning human history as ten thousand years. There is, of course, no possibility of obtaining more than relative figures for such a scheme as this, which should be regarded in connection with the previous page and the chart of the Beginnings of Life, facing page 96 [Illustration: The thin white line between the Tertiary and the Archæan periods represents the duration of human history]