160 POPULAR SCIENCE MONTHLY. 



tail] 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 ex- 

 tended further and further over the geographical field. At first ex- 

 planation 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 classi- 

 fication. 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 geo- 

 logical surveys; from books of travel and from geographical and geo- 

 logical 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 con- 

 tinents; 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, Kloden'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 state- 

 ments, 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, Eaab, Drave, 

 Save, Morave, and Isker. On the left are the Altmiihl, Eegen, Waag, 



