Rockhill, 1888 and 1891; Hamilton Bower, 1891–92; Dutreuil de Rhins and F. Grenard, 1893–94; St. George Littledale, 1895; Captains W. S. Wellby, 1896, and H. H. P. Deasy, 1896, whose work was afterwards extended by Captain C. G. Rawling and by Sir M. A. Stein; and Sven Hedin, 1896–98, 1899–1902, 1906–08, whose last journey revealed the existence, long suspected, of the great mountain system north of the upper Tsan-po. This list is by no means exhaustive, nor can be that of the Russians who worked from the opposite direction, from their own territory; their leader was Nicolai Prjevalsky, who in 1871–73 and in 1876 worked in the Tsaidam region and made the first contribution to the mapping of the important hydrographical area above referred to, and in 1879 studied the vast physical changes which have taken place in Central Asia within historic times, and form one of the most remarkable geographical problems in the world. He continued his work in 1883–85, and was followed by Pevtsov and Roborovsky (1889 and 1894), P. K. Kozlov, Potanin, and many others. The names of Russian scientists, such as Baron A. Kaulbars and L. Griesbach, are also associated with the problems of the Aral and Caspian depressions. The former extensions of human settlement over areas now covered by the Central Asian deserts has been brought to light in great measure through the researches of Sven Hedin, and especially of Sir M. A. Stein. The general result of all these investigations has been to modify profoundly, even during the present generation, preexisting ideas of the physical geography of the central region. Nor should we overlook the work of recent travellers in China proper, a broad canvas on which outlines had been sketched earlier; but details remained, and still in great part remain, to be filled in, though Ferdinand Baron von Richthofen, in the course of his seven journeys in 1868–72, left few districts entirely unvisited. The problem of the former existence of flourishing communities in areas now desert, and of the causes of the change, has a partial counterpart in southern Arabia. The modern period of Arabian exploration began earlier than that of Central Asia. The journeys of J. Halévy (1869), E. Glaser (1889), and J. T. Bent (1893) in the south were primarily archæological in purpose. In other parts of the peninsula the work of J. L. Burckhardt (1815), Sir R. F. Burton, Captain G. F. Sadlier, W. G. Palgrave, Charles Doughty, Wilfrid Blunt, C. Huber, Musil, Leachman, and others, has made it possible to lay down at least the position of the chief towns and settlements, and the main physical outlines, with close accuracy, save in the Dahna or great desert of the southern interior, which remains untrodden.