corridor of 40,000 years ago, and that another body of immigrants came during the last of the three great interglacials, which means about 100,000 years ago.[25] Interglacial migration finds support from Albrecht Penck, the German glacialist who with Eduard Brückner established the four great glaciations of central Europe. Penck gives two reasons for believing that man came to the Americas in the last interglacial. Both theories lie outside his field of special knowledge. First, Penck doubts that man had time enough after the glaciers melted to adapt himself to the seven or eight climates in which he lived and labored in 1492. In the 10,000 years since the Wisconsin glaciation, people of an arctic habitat could not possibly have adjusted their physical nature so perfectly “first to forests, then to steppes and deserts in temperate zones, then to steaming tropical forests and to the plateaus of the tropics, the steppes and deserts of the southern hemisphere, and finally to the damp, cool south.” Penck’s second reason for supporting interglacial migration has to do with the marked changes of climate that took pace during the Great Ice Age. As the glaciers grew and moved farther and farther south, the temperature belts of Canada and the United States moved south with them. As these belts moved, vegetation altered. Tundra crept to the south, pursuing grasslands and forest. Deserts and jungle moved before them. When the glaciers began to melt, this movement went into reverse. Penck feels that primitive man migrates easily only within a single climatic zone. Tundra folk avoid forests; forest folk avoid tundra. Man moves with the climate, not against it; a New Yorker goes to Florida in the winter, not the summer. Early man would not have traveled southward through Canada and the United States while the climate was moving northward with the melting of the ice. On the contrary, man would have moved southward only as the glaciers grew and the temperature belts moved southward. When the glaciers melted he would have tended to move northward again. Thus man must have entered the New World toward the end of an interglacial, and gone southward with the weather. After that, shifts in the climate would have distributed man all over the Americas. “Under the influence of a number of alternating glacial and interglacial periods, we can understand the gradual settlement, but not solely on the assumption of one glacial migration.” With the end of the last glaciers men spread north and south once more, and even drifted back to Asia.[26] Penck’s argument becomes all the stronger if we grant that early man followed the animals he killed for food and animals followed the movements of the vegetation on which they fed.