The general scheme under which all land forms may receive explanatory description must consider chiefly the movement and erosion of the earth’s crust. Deformation offers a part of the earth’s crust to be worked upon. Various destructive processes of erosion work upon the offered mass, and the streams, with their transported waste, follow the depressions in the carved surface. So important is the element of erosion, and so leading is the part played by rivers in erosive work, that McGee would gather all land forms under a classification determined by their drainage systems.[E] Others have preferred a classification based, first on peculiarities of structure as determined by accumulation and deformation; and, secondly, on the progress of erosion; but in either scheme, the erosive work of rivers is so important that a sketch of the progress of the physical geography of the lands towards a systematic classification of its items may well follow the order in which valleys have been explained, branching off, as occasion may require, from the leading theme of rivers that flow under a normal humid climate to special conditions of erosion under an arid or a frigid climate. The progress which has made the physical geography of the lands what it is to-day is more the work of geologists than of geographers; and the chief reason for this is the indifference of many geographers to the physical side of their subject; an indifference that was undoubtedly favored by the cultivation of historical geography in continental Europe, and by the acceptance of the traveler or explorer as a full-fledged geographer in Great Britain. In the United States, it is only in the latter part of the century that the physical geography of the lands has gained a scientific standing, and the advantages that it now enjoys are geographical grafts upon a geological stock. [E] _Nat. Geogr. Magazine_, i, 1889, 27-36.