The treatment of physical geography will be much more even, to the great advantage of its students, when explanatory description is applied to all its parts. The alluvial fans at the base of arid mountains should be accounted for as well as the dunes of deserts. The fault cliffs of broken plateau blocks and the weathered cliffs of retreating escarpments deserve to be considered as carefully as the wave-cut cliffs of coasts; the essential differences of these forms are reached most easily through their explanation. The varied sculpturing of a mountain slope may, in time, come to be as well understood as is now the erosion of a simple valley in a low plain. One of the most notable elements of the century’s progress is the increasing breadth of view gained as explanatory descriptions are extended further and further over the geographical field. At first explanation was given to various individual features, item by item; now it is recognized that an appropriate place must be provided for all kinds of land forms in a comprehensive scheme of physiographic classification. Many instances of the earlier stage might be given, beginning with examples from the works of Humboldt, the acknowledged leader of scientific explorers in the opening decades of the century. His attempts, more or less completely successful, to explain the facts that he observed, as well as to correlate life with environment, may be traced all through his writings; but his ‘Cosmos’ (1845) did not reach a careful discussion of land forms, although it entered so far into an explanatory treatment as to consider the formation of mountain ranges.