THE PHYSICAL GEOGRAPHY OF THE LANDS. 169 



his followers, especially Gilbert, Dutton and McGee, have consistently 

 carried the consequences of subaerial erosion to their legitimate end in a 

 featureless lowland, and have recognized the controlling influence of the 

 baselevel during all the sequence of changes from the initial to the ulti- 

 mate form. It is not here essential whether such a featureless lowland 

 exists or ever has existed, but it is absolutely essential to follow the lead 

 of deduction until all the consequences of the theory of erosion are 

 found; and then to accept as true those theoretical deductions which 

 successfully confront the appropriate facts of observation. Only in this 

 way can the error of regarding geography as a purely observational 

 natural science be corrected. Following the acceptance of the doctrine 

 of baselevels came the method of reconstituting the original form in- 

 itiated by deformation, as a means of more fully understanding the 

 existing form; for only by beginning at the initial form can the sys- 

 tematic sequence of the changes wrought by destructive processes be 

 fully traced and the existing form appreciated. This had often been 

 done before in individual cases, but it now became a habit, an essential 

 step in geomorphological study. Naturally enough, the terms of or- 

 ganic growth, such as young, mature, old, revived, and so on, came to 

 be applied to stages in the development of inorganic forms; and thus 

 gradually the idea of the systematic physiographic development of land 

 forms has taken shape. This idea is to-day the most serviceable and 

 compact summation of all the work of the century on the physical 

 geography of the lands. It recognizes the results of deformation in 

 providing the broader initial forms on which details are to be carved. 

 It gives special attention to the work of destructive processes on these 

 forms, and especially to the orderly sequence of various stages of de- 

 velopment, recognizing that certain features are associated with youth, 

 and others with maturity and old age. It gives due consideration to 

 the renewed movements of deformation that may occur at any stage in 

 the cycle of change, whereby a new sequence of change is introduced. 

 It gives appropriate place, not only to the forms produced by the ordi- 

 nary erosive action of rain and rivers, but to the forms produced by ice 

 and by wind action as well; and it co-ordinates the changes that are pro- 

 duced by the sea on the margin of the land with the changes that are 

 produced by other agencies upon its surface. It considers not only 

 the various forms assumed by the water of the land, such as torrents, 

 rapids, falls and lakes, appropriately arranged in a river system as 

 to time and place, but also the forms assumed by the waste of the land, 

 which, like the water, is on its way to the sea. In a word, it lengthens 

 our own life, so that we may, in imagination, picture the life of a 

 geographical area as clearly as we now witness the life of a quick- 

 growing plant, and thus as readily conceive and as little confuse the 

 orderly development of the many parts of a land form, its divides, 



