[Illustration: Miocene Mammals] The Record of the Rocks tells of the increasing chill. The Pliocene was a temperate time, and many of the warmth-loving plants and animals had gone. Then, rather less deliberately, some feet or some inches every year, the ice came on. An arctic fauna, musk ox, woolly mammoth, woolly rhinoceros, lemming, ushers in the Pleistocene. Over North America, and Europe and Asia alike, the ice advanced. For thousands of years it advanced, and then for thousands of years it receded, to advance again. Europe down to the Baltic shores, Britain down to the Thames, North America down to New England, and more centrally as far south as Ohio, lay for ages under the glaciers. Enormous volumes of water were withdrawn from the ocean and locked up in those stupendous ice caps so as to cause a world-wide change in the relative levels of land and sea. Vast areas were exposed that are now again sea bottom. The world to-day is still coming slowly out of the last of four great waves of cold. It is not growing warmer steadily. There have been fluctuations. Remains of bog oaks, for example, which grew two or three thousand years ago, are found in Scotland at latitudes in which not even a stunted oak will grow at the present time. And it is amidst this crescendo and diminuendo of frost and snow that we first recognize forms that are like the forms of men. The Age of Mammals culminated in ice and hardship and man. § 5 Guesses about the duration of the great age of cold are still vague, but in the Time diagram on page 60 we follow H. F. Osborn in accepting as our guides the estimates of Albrecht Penck[18] and C. A. Reeds.[19] [Illustration: TIME DIAGRAM OF THE GLACIAL AGES. The reader should compare this diagram carefully with our first time diagram, Chapter II, § 2, p. 14. That diagram, if it were on the same scale as this one, would be between 41 and 410 feet long. The position of the Eoanthropus is very uncertain: it may be as early as the Pliocene] BOOK II THE MAKING OF MEN VIII THE ANCESTRY OF MAN[20] § 1. _Man Descended from a Walking Ape._ § 2. _First Traces of Man-like Creatures._ § 3. _The Heidelberg Sub-man._ § 4. _The Piltdown Sub-man._ § 5. _The Riddle of the Piltdown Remains._ § 1