Zeuner has an explanation of why the periodic decrease in the heat from the sun produced glaciation during the last million years and not for 200,000,000 years before. He introduces the geological factor called Eustatism,[15] meaning by it simply a progressive drop in sea level. According to the hypothesis, this began before the Great Ice Age, and was caused by the sinking of very deep portions of the sea floor. As the sea level sank, the temperature of the mountains and plains dropped also, for the higher we rise above the surface of the ocean the cooler the air grows. The snowline fell, the mountain glaciers grew larger, and the snow and ice on the northern plains could not be completely melted by the reduced summer heat, and gradually grew deeper and more extensive. Thus the lowering of general temperature made it possible for the periodic decrease in solar radiation to cause the glaciations of the Great Ice Age. The theory of a general and steady lowering of the sea level is based on a series of four raised beaches occurring uniformly in many parts of the world. Other students believe these terraces were products of a regional rise of land. Considering that the Great Ice Age ranges back at least 600,000 years—and probably 1,000,000, if we credit evidence of three earlier Danubian glaciations—it is small wonder that scientists are not entirely agreed on many factors in its story. “The difficulties are such,” says the French archaeologist A. Vayson de Pradenne, “that after fifty years of study to which the greatest geologists have devoted all their energies, there is no certainty yet as to the exact number of glaciations and the way in which the faunal changes are related to them.”[16] Much more important, of course, than the cause of glaciation is its effect on early man. Ice covered 27 per cent of the earth’s surface during the Würm-Wisconsin period, according to Flint. This created the land-bridge over Bering Strait. It connected Santa Rosa Island with the coast of California. It broadened the Isthmus of Panama, so that man did not have to pass through a semi-mountainous jungle, which suggests that he came south during the Wisconsin glaciation. It seems to have been the ice that urged man to the south in the Americas and provided freeways. 5 EARLY MAN IN THE OLD WORLD _Bone of our bone, and flesh of our flesh, are these half-brutish prehistoric brothers._ —WILLIAM JAMES Archaeology, a New Science