The great glaciations presented early man with an opportunity and a difficulty; they threw a land-bridge across Bering Strait, but, for long periods of time, they also laid a barrier of ice and snow across his path to the south. Look at the map at the top of page 26 and you will see that about 65,000 years ago the barrier covered the whole depth of Canada. This would have meant a trip of some 2,000 miles across ice. For a time, as we have explained earlier, a corridor opened up for his passage. By 18,000 years ago, however, it had closed again. To be sure, there was a tongue of ice-free land that ran southward across part of Canada and shortened the journey over snow and ice to a thousand miles; but the invader would have had to be extremely lucky to hit the upper end of the open country. Far more important, none of the animals that he hunted, and that therefore led him on his southward journey, would have taken that thousand-mile trek across a frozen, foodless waste. If early man came in the time of the corridor, his trip would not have been too difficult and he would have found game along the way. At any period he could have come by boat or possibly afoot along the Pacific coast. But, no matter how he came, he would have faced almost insuperable difficulties during the first quarter and the third quarter of the last glaciation. This fact affects different students differently. Antevs, feeling that man must have come after the ice began to melt, gives our migrant not much over 15,000 years in the New World. If man arrived earlier, he feels, it must have been when the corridor opened through the ice some 40,000 years ago, and he sees no evidence for so early a migration.[21] Kirk Bryan disagrees. Accepting the theory that the last glaciation waxed and waned three times, he places the first invasion by man in the second of these wanings, or inter-stadials, just before the final burgeoning of the ice 12,000 years ago.[22] Sauer goes further. He suggests that the shores of western Siberia may have been inhabited during the second interglacial, and that during the third glaciation, preceding the Wisconsin, “a first colonization of the New World is not improbable.”[23] Erwin H. Barbour and C. Bertrand Schultz say that “evidence is constantly accumulating to show that man actually had reached North America before the last glacial advance.”[24] George F. Carter believes that artifacts in the glacial gravels of Trenton and Lake Lahontan, spear points with musk oxen in New Mexico, the Vero skull partnered in Florida with flora and fauna more appropriate to Pennsylvania, stone implements deeply buried under aged soil profiles, all argue that man was in North America during the last glaciation. He does not believe that primitive hunters—let alone the Cochise food gatherers—would have survived Arctic travel at the height of the glaciations. He thinks that Folsom man came through the ice-free