These ruins belong to the great Mayan civilization. Mayan history has been briefly summarized by Huntington as follows: “First we have a long period of active development, during which the calendar was evolved and the arts of architecture and sculpture were gradually developed.... This time of marked growth must have preceded the Christian era. Then comes ... the building of the great cities of Copan, Quirigua, Tikal and others. These first great cities were in the southern part of the Maya area, on the borders of Honduras or in eastern Guatemala. They lasted perhaps three or four centuries; then quickly declined. So far as we have any evidence, civilization never revived in this southern area, for the structures of the great period have not been rebuilt by later inhabitants. Towards the end of the period of greatness the centre of Mayan culture moved northward.... The great period, according to Bowditch, lasted from 100 B.C. to A.D. 350 ... then came a time of very low civilization, lasting for centuries.... A revival ensued about A.D. 900 or A.D. 1000, and architecture once more reached a high pitch, but ... only in northern Yucatan; all the rest of the country seems to have remained in darkness. Moreover, this mediæval revival was relatively shortlived. Since that time the condition of the Mayas has fluctuated more or less, but on the whole there has been a decline.” Now at the present day the densest and most progressive population in Yucatan is found in the driest part of the country, where the forest gives place to jungle. If the line of separation between jungle and forest were moved southward 300 miles, the former would include all the districts where ruins are now found. We see from the above summary that the prosperous periods of Mayan history were just those periods which in California were moist; in Yucatan they must have been dry. Huntington’s explanation is the theory of the “shifting of climatic belts”; during the rainy period in California the temperate storm-tracks were shifted further southward. At the same time the sub-tropical high-pressure belt, which at present lies over the West Indies, was also shifted southwards, and this brought a dry cool winter to Yucatan, with an increased contrast of seasons, and consequently a more invigorating climate. In Asia, Huntington and other explorers have found similar traces of past variations of climate, a fascinating account of which is given in “The Pulse of Asia.” Space will not permit of a summary in detail, but the following general conclusions may be quoted:[6] “If we omit the Volga and the European portions of the Caspian drainage area, the limits (of the six basins considered) lie over sixteen hundred miles apart from north to south and over three thousand from east to west. All this great area seems to have been subject to the same great waves of climatic change.