tain 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.
Innumerable examples of isolated facts and special explanations, unrelated to a comprehensive scheme of physiographic classification, might be taken from the reports of exploring expeditions and of geological surveys; from books of travel and from geographical and geological journals with which the nineteenth century has filled so many library shelves; but lack of space will prevent mention of all sources, save a few treatises in which the accumulated knowledge of their time is summarized. Such a work as Mrs. Somerville's 'Physical Geography' (1848) gives in the early pages a brief general consideration of land forms, and then enters at once upon the areal description of the continents; later pages present a short outline of the features of rivers, and then the rivers of the world are taken up. This is as if a text-book of botany should pass rapidly over the structure and classification of plants, and devote most of its pages to the flora of different regions. Again, Klöden's compendious geography includes a volume on 'Physical Geography,' in which much material is gathered (3d ed., 1873); but the treatment is very uneven, as is natural in the absence of a good scheme of classification. Glaciers receive much attention, but valleys are rather curtly dismissed; deltas are elaborately described, but little space is given to other forms assumed by the waste of the land on the way to the sea. Ansted's 'Physical Geography' (5th ed., 1871) contains abundant fact, but much of it is a kind that is better presented on a map than in verbal form. Many pages are devoted to statistical statements, from which no student can gain inspiration for further study, for example: "The Danube receives a large number of tributaries, of which the most important are, on the right, the Isar, Inn, Raab, Drave, Save, Morave, and Isker. On the left are the Altmühl, Regen, Waag,