The cause of the glacial period has been much discussed. It seems to have been largely due to changes in land levels,[394] and to other causes not now understood. The people inhabiting the world before it may have been originally much alike in kind and colour with local variations, and the isolation produced by glacial conditions modified this colour and increased the variations, those finally left in hot lands becoming darker, medium temperatures producing brown, still cooler the reds and yellows, and the forests of Europe evolving a shade or shadow people, shrinking from the strong sun; the so-called white race. The glacial epoch is often spoken of as if the whole world were frozen solid, whereas in North America, from the Ohio and the Columbia to the Isthmus, the climate was doubtless about relatively the same as it is now from Davis Strait to the Potomac and from Yakutat Bay to northern California. The ice extended down about to the Ohio River in the East and on lowlands not below the Columbia in the West. The Western mountain tops must have been completely glaciated and all elevated regions were cold, the conditions prevailing resembling those now found in Southern Alaska. The Sierra Nevadas, receiving the warm, moist airs from the Pacific, must have been far more heavily glaciated than the Rockies, which received less moisture in consequence. The ice period is estimated to have endured from ten to twenty thousand years, with an interval of recession in it and subsequent advance. The people were driven southward, and those most favourably situated developed the most. The people most favourably situated were all _who were already in_, or could fight their way to, the temperate lowlands of southern Mexico and Central America, which were rendered somewhat more extensive by the recession of the sea, caused by the withdrawal of the immense quantities of water that were heaped up in ice thousands of feet in thickness.[395] This has been estimated to have lowered the waters of the ocean by from 600 to 1000 feet.[396] The lands thus laid bare were climatically inviting and probably were soon covered with vegetation. In South America the people were crowded northward, or held there by the cold coming from the south. It would be in the northern portions, particularly the lowlands, that we ought to find evidence of the highest development, especially on the side receiving warm currents, and there is where we do find it. We apparently have then a northern and a southern limit to the ancient inhabitants of this hemisphere, within which climatic conditions during the period of great cold, and for some time thereafter, were most favourable to human development. This limit in the Northern continent is latitude 23 and in the Southern also 23. Within these lines the great precolumbian development took place, and the heart of this development on the Northern continent seems to have rested between the Isthmus of Tehuantepec and the present