If early man first came to the New World in the Great Ice Age or in the time when the glaciers were beginning to melt, he could have crossed dry-shod on a land-bridge. Geologists have calculated that the water withdrawn from the ocean to form the glaciers—which were half a mile to two miles thick over much of Canada and the northern portion of the United States—would have lowered the water level in the Bering Strait region by as much as 200 to 300 feet toward the end of the Great Ice Age.[7] In addition, the ocean floor of the strait—relieved of so much weight of water—would doubtless have risen to some extent. Since, at present, portions of the strait reaching from shore to shore are not more than 120 feet deep, a land-bridge is a perfectly plausible hypothesis. Of course the bridge would have disappeared with the end of the glaciers, which means that, if man had to come over dry-shod in the summer, he must have invaded America while the glaciers were still fairly extensive. [Illustration: THE LAND-BRIDGE TO THE NEW WORLD _A conservative map of shorelines during the last glaciation, based on a drop in sea level of 180 feet. Geologists believe that the ice impounded in the great glaciers and ice fields of the world lowered the ocean 200 to 300 feet. The southern shore of Alaska during the last glaciation may have been much nearer its present position. (After Johnston, 1933.)_] [Illustration: A GREAT-CIRCLE ROUTE TO NORTH AMERICA _On the flat, distorted map of Mercator, on page 4, the path of early man across Asia to the New World seems a roundabout curve. On a globe, it is very nearly a great circle. This is indicated on a map such as this, projected from a point above the North Pole. From above Bering Strait, the route would appear still straighter._] Aleš Hrdlička has said that not many men would have frequented northeastern Siberia because of its inhospitable climate, and so only a few would have “trickled over.”[8] The opposite seems to have been true in the time of the glaciers. Then northeastern Asia was an excellent jumping-off place for the Old World migrant. In the first place, there was not much glaciation in this area, certainly nothing to interfere with the passage of people along the coast. The last glaciation was “far less extensive than its predecessor,” say R. F. Flint and H. G. Dorsey, “and was confined to the higher parts of the higher mountain ranges.”[9] Secondly, the land-bridge, which made crossing easy, also altered the climate of Siberia south of the bridge.[10] It cut off the arctic currents and therefore to some extent the arctic damp which now makes the Asiatic coast inhospitable. Hrdlička—one of the first and most violent opponents of early man in America—said that the land-bridge was not essential. Even if it had existed, “man would not have used it, but would have followed the much easier route over the water.”[11]