Some of the undertakings described in these pages have not been completed. The foot of man has not yet stood on the last snows of Everest, or on the summit of Carstensz. One notable discovery I have not dealt with--the great Turfan Depression in the heart of Central Asia, far below the sea level, the existence of which was first established by the Russian, Roborowski, before the close of last century, and the details of which have been described by Sir Aurel Stein in his _Ruins of Desert Cathay_ and _Serindia_. But Sir Aurel's interest was chiefly in the antiquities of the place, and the more strictly geographical results have not yet been given to the world. Today, if we survey the continents, we find nothing of which the main features have not been already expounded. The Amazon basin might be regarded as an exception, and only a little while ago men dreamed of discovering among the wilds of the Bolivian frontier the remains, perhaps even the survival, of an ancient civilization. It would appear that these dreams are baseless. The late President Roosevelt did, indeed, succeed in putting upon the map a new river, the Rio Roosevelt, 1,500 kilometres long, of which the upper course was entirely unknown, and the lower course explored only by a few rubber collectors--a river which is the chief affluent of the Madeira, which is itself the chief affluent of the Amazon. But now all the tributaries have been traced, and though there is much unexplored ground in the Amazon valley, it consists of forest tracts lying between the rivers, all more or less alike in their general character, and with nothing to repay the explorer except their flora and fauna. Africa is now an open book, even though many parts have been little travelled. The map of Asia alone holds one blank patch which may well be the last of the great secrets--the Desert of Southern Arabia, which lies between Yemen and Oman, 800 by 500 miles of waterless sands. Long ago there were routes athwart it, and hidden in its recesses some great news may await the traveller. But its crossing will be a hazardous affair for whoever undertakes it, since he will have to lean upon the frail reed of milk camels for food and transport. For the rest, the problems are now of survey and scientific enquiry rather than of exploration in the grand manner.