tinuous agricultural population, such as that of the Mississippi Valley, dependent on rain for its water supply. Two main types of civilization prevail: the condition of nomadism with its independent mode of life, due to the scattered state of the sparse population, and the condition of intensive agriculture in irrigated oases with its centralized mode of life, due to the crowding together of population in communities whose size is directly proportional to that of the streams. Because of the arid climate and the consequent physical characteristics of Central Asia, its types of civilization have been, are, and probably must continue to be fundamentally different from those of well-watered regions such as most of America and Europe.
My acquaintance with Central Asia began in 1903, when I was appointed by the Carnegie Institution of Washington to assist Professor William M. Davis of Harvard University in the physiographic work of an expedition to Russian Turkestan, under the lead of Mr. Raphael Pumpelly. I remained in Central Asia from May, 1903, to July, 1904, spending most of the time in Russian Turkestan. I crossed into Chinese Turkestan for a month, however, the first summer, and spent four months in eastern Persia during the winter. The results of the expedition are recorded in " Explorations in Turkestan," a volume published by the Carnegie Institution. The following year I had the good fortune to be invited by Mr. Robert L. Barrett to accompany him on an expedition to Chinese Turkestan. Arriving in India in February, 1905, we proceeded north to the Vale of Kashmir among the Himalayas, crossed them in May, and reached Chinese Turkestan in June. There we worked
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