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In view of the notable civilization developed at the older cultural sites in the Central Andes, not only at Cuzco and Tiahuanaco, but in many other places on a lesser scale, it is natural to think of human life as going back so far there that it may have been affected by the uplift of the mountains. We know the mountains to be young. They have attained their present great height since the Pliocene (p. 254). In the great period of Andean uplift in South America man is known to have inhabited North America. If he also then inhabited the southern continent he would have felt at least the latest climatic effects of the uplift.

Sites of settlement that were once at a lower altitude and therefore warm enough to be cultivated might now be so cold as to support only grass for grazing animals. <A little change might conceivably have far-reaching influence upon popula- tion. Sir Clements Markham once speculated upon such a possibility. An elevation of 500 feet would have a pronounced effect upon human distributions at the upper limit of settle- ment. In the Puna de Atacama the belt of pasture would be shifted upon the mountain slopes. The volume of springs and the discharge of streams would be changed from place to place. There would be a shifting of the edges of the belt of woodland shown in Figure 86. The upper limit of the growth of cereals and vegetables would be pushed down the mountain side to an extent roughly corresponding to the uplift. How delicately these products are now balanced on basin floors and mountain sides may be seen at Lake Titicaca. Corn is grown only at the lake border. Barley ripens on the surrounding slopes if the elevation does not exceed one thousand feet above the