Then, for six or seven hundred years, with short-period variations of from thirty to fifty years, the world inhabited by civilized man and large areas in the temperate zone not yet civilized, grew drier. The Caspian Sea fell to a lower level than it now maintains, for the ends of great walls, constructed to keep out barbarians, and other evidences of the handiwork of man, are now many feet below the surface of the water. This is the driest time known to history. Ellsworth Huntington of Yale, acting under the auspices of the Carnegie Foundation at Washington, made an examination of many of the stumps of the big trees of California, ranging in age from one to four thousand years. The thickness of each ring of annual growth is a legible record of the wetness or the dryness of the year. One would hardly think of these towering giants of the floral kingdom as being both thermometers and rain gauges, accurately measuring and recording the dry-hot and the wet-cold periods for thousands of years, and now at the end of their majestic careers revealing the hidden secrets of past ages. Huntington and Cushing, in “Principles of Human Geography”, say: “The rings dating from the time of Christ are thick and indicate that at that time, when Palmyra had an abundant supply of water, when Owens Lake overflowed and there was high water in the Caspian Sea, the big trees also had plenty of water and grew rapidly. Six or seven hundred years later, when Palmyra was abandoned and when the Caspian Sea stood fifteen or twenty feet lower than at present, the trees formed only narrow rings, because the climate was dry. The way in which the growth of the trees has varied is shown in Figure 30. The high part of the curve indicates abundant rainfall. The black shading at the bottom indicates periods of comparative aridity.” [Illustration: FIG. 30.—Changes in Climate in California during the Christian Era. Black shading indicates Drought.]