The period of time that has elapsed since the so-called disappearance of the ice was formerly believed to be very great, but latterly views on this point have been much modified. Gilbert has declared, after a study of the Niagara gorge, that the time since the ice left that region is not more than seven thousand years, perhaps less. More recent investigations have tended to confirm his suggestion of fewer years. Immediately after the recession of glacial ice, as may be seen in Alaska to-day, erosion is extremely rapid. I have not space to discuss this point at length, but it is apparent that the rate of erosion is variable, and I doubt if more than five thousand years have passed since the ice left the vicinity of the Niagara gorge. As it still lingers in the North, far down on the Pacific side, it _is probably not more than a thousand years since its influence was powerful in affecting the climate of all the region southward_. The North is undoubtedly growing warmer. Some five hundred years ago Alaska was still covered with glacial ice. Five hundred years from now there will scarcely be a glacier to be found there, except in the highest mountains. “The next generation will find few of them with their fronts still in the sea,” says Henry Gannett.[399] [Illustration: A PUEBLOAN WARRIOR OF NAMBÉ, NEW MEXICO, IN BATTLE ARRAY] The most widely spread stocks are made up of those that were forced to occupy a middle position during the cold, like the Algonquins and Athapascans, who were invigorated by it. Other stocks, for reasons not understood, dwindled to mere handfuls of people, like the Karankawan, now extinct, the Adaizan, the Natchezan, the Uchean, the Zuñian, Keresan, and others. The oldest people of the Valley of Mexico mentioned are the Xicalancas, Olmecas, and the Toltecs. Brinton believed the latter never existed, but other authors, fully as distinguished, accept them as a _bona-fide_ tribe. They may have been kindred to the Nahuatls, coming from the crowded lowlands, as the waters rose and the heat increased, and occupying the cooler plateau. Their wilder relatives later became influenced by them and adopting their learning began the famous development in the Valley of Mexico. The period of evolution in the crowded region was very long. Tribes rose to power and declined.[400] Other tribes, profiting by their experience, took up some of their ways and progressed. Many of these tribes we have no reminiscence of.