But as the river of ice proceeded south, plants and animals retreated before it, some of them changing their nature to endure the excessive cold. Then came a climatic change which melted the ice and gradually drove the margin of the glacier farther north. Immediately under the influence of the warm winds the vegetation and animals followed slowly at a distance the movement of the glacier. Then followed a long inter-glacial period before the southerly movement of the returning ice cap. This in turn retreated to the north, and thus four separate times this great movement, one of the greatest geological phenomena of the earth, occurred, leaving an opportunity to study four different glacial periods with three warmer interglacial and one warm post-glacial. This movement gave great opportunity for the study of {63} geology, paleontology, and the archeology of man. That is, the story of the relationship of the earth to plant, animal, and man was revealed. The regularity of these movements and the amount of material evidence found furnish a great opportunity for measuring geological time movements and hence the life of plants and animals, including man. The table on page 64 will contribute to the clearness of this brief statement about the glacial periods. ====================================================================== {64} THE ICE AGE IN EUROPE[5] Geological time-unit 25,000 years RELA- TIVE TOTAL TIME TIME HUMAN ANIMAL AND GLACIERS UNIT YRS. YRS. LIFE PLANT LIFE ----------------------------------------------------------------------------- Post-Glacial 1 25,000 25,000 Cro-Magnon Horse, Stag, Rein- Daum Azilian deer, Musk-Ox, Geschintz Magdalenian Arctic Fox, Pine, Buehl Solutrian Birch, Oak Aurignacian ----------------------------------------------------------------------------- 4th Glacial 1 25,000 50,000 Mousterian Reindeer, period of Wurm Ice Neanderthal Tundra, Alpine, Steppe, Meadow ----------------------------------------------------------------------------- Q 3d Inter- 4 100,000 150,000 Pre-Neander- Last warm Asiatic U glacial thal and African ani- A Piltdown mals R --------------------------------------------------------------------------- T 3d Glacial 1 25,000 175,000 Woolly Mammoth, E Riss Rhinoceros,