The Yukon valley rivers could have taken early man to the Mackenzie and to the northern edge of the plateau; but during a large part of the Great Ice Age he still faced the gigantic fields of snow and ice that covered half of North America. Although ice journeys may not be so very difficult, and early man had learned to live in the chill of northern Siberia (the Eskimo of today proves that human existence is possible in a land of little sun and much cold), we cannot believe that he attempted to cross the icy wastes. A journey on foot across a thousand or two thousand miles of ice becomes a sheer impossibility if there is no provender to be found along the way. There was certainly no food for musk ox or mammoth, and therefore no food for man. Without game to hunt, he would not have felt the impulse to invade the ice fields. [Illustration: MIGRATION ROUTES _The pathways available to early man, as mapped by Carl Sauer—to which have been added a problematical route by sea along the southern coast of Alaska and another down the corridor between the eastern Rockies in Canada and the coastal range. (After Sauer, 1944.)_] Too many authorities have written as if early man made a free choice of routes through Alaska and Canada. Actually, the animals he hunted chose his route for him—doubtless many routes. Unless he had learned to spear and net fish, the first invader probably pursued a herd of mammoth or musk ox across the land-bridge or over the frozen ice of later winters. You in your armchair are likely to suggest that, once man was in Alaska, he turned south of his own free will and intelligence in order to avoid the cold. The first trouble with that line of thought is that man had gone north in Asia. The second is that primitive man had no conception of where south lay or of the possibility of greater comfort there. At this point, you will probably say that, though he had no ideas about the nature of the south, he had brains enough to follow the sun. Unfortunately this would have spun him round like a slow-motion teetotum; for he entered North America in the neighborhood of the Arctic Circle where the summer sun moves in a great low circle, and sinks out of sight when winter sets in.