THE PHYSICAL GEOGRAPHY OF THE LANDS. 167 



the Ehine below Bingen, the Meuse in the Ardennes, or several of the 

 Himalayan rivers in the gorges that they have cut through the youngest 

 marginal ridges of the range. 



Eapidly following the establishment of these two important classes 

 of valleys came the recognition of the very antithesis of antecedent 

 rivers in those streams which have grown by headward erosion along 

 belts of weak structure, without relation to the initial trough lines. 

 To these the term 'subsequent' has been applied. It is frequently 

 in association with streams of this class that drainage areas are re- 

 arranged by the migration of divides, and that the upper waters of 

 one river are captured by the headward growth of another. This is 

 accomplished by a most beautiful process of inorganic natural selection, 

 which leads to a survival of the fittest and thus brings about a most 

 intimate adjustment of form to structure, whereby the more resistent 

 rock masses come to constitute the divides, and the less resistent are 

 cbosen for the excavation of valleys. Many workers have contributed 

 to the solution of problems of this class; notably Heim, in his studies 

 of the northern Alps (1876), and Lowl, who showed that, in folded 

 mountain structures of great age, the original courses of streams might 

 be greatly altered through the development of new lines of drainage 

 (1882). A valuable summary of this subject is given by Philippson in 

 his 'Studien uber Wasserscheiden' (1886). The extraordinary depre- 

 dations committed by the waxing Severn on the waning Thames 

 have recently been set forth by Buckman. The turning of side 

 branches from the slender trunk of the Meuse has been recognized in 

 France. Many remarkable instances of stream captures have been 

 found in the Appalachians, where the opportunity for the adjustment 

 of streams to structures has been exceptionally good. Hayes and 

 Campbell have, on the other hand, emphasized the importance of drain- 

 age modifications independent of the growth of subsequent streams on 

 weak structures, but governed by a slight tilting of the region, whereby 

 some streams are accelerated and their opponents are retarded. It 

 should be noted that the proof of the adjustment or rearrangement of 

 drainage marks a victory for the uniformitarian school that is even 

 more significant than that gained in the case of the antecedent rivers; 

 for in one case a growing mountain range is subdued by the concen- 

 trated discharge of a large drainage area; but in the other case, the 

 mountain slowly melts away under the attacks of the weather alone on 

 the headwater slopes of the growing valleys. 



The reason why all these studies of land carving are of importance to 

 the geographer is that they greatly enlarge the number of type forms 

 that he may use in descriptions, and that they recognize the natural 

 correlations among various forms which must otherwise be set forth 

 in successive itemized statements. The brief terminology learned in 



