THE PHYSICAL GEOGBAPHY OF THE LANDS. 157 



THE PHYSICAL GEOGEAPHY OF THE LANDS. 



By Professor W. M. DAVIS. 



HARVARD UNIVERSITY. 



THE most important principles established in physical geography 

 during the ninetenth century are that the description of the 

 earth's surface features must be accompanied by explanation, and that 

 the surface features must be correlated with their inhabitants. During 

 the establishment of these evolutionary principles, exploration at home 

 and abroad has greatly increased the store of recorded facts; the more 

 civilized countries have been in large part measured and mapped; the 

 coasts of the world have been charted; the less civilized continents 

 have been penetrated to their centers. This harvest of fact has been an 

 .indispensable stimulus to the study of physical geography; yet it can 

 not be doubted that the spirit which has given life to the letter of the 

 subject is the principle of evolution — inorganic and organic. This is 

 especially true of the geography of the lands. 



The century has seen the measurement of higher peaks in the Hima- 

 layas than had been previously measured in the Andes. The Nile has 

 been traced to its source in the lakes of equatorial Africa, verifying the 

 traditions of the ancients; and the Kongo has been found to cross 

 the equator twice on its way to the sea. Facts without number have 

 been added to the previous sum of knowledge. But at the same time, it 

 has been discovered that the valleys of mountain ranges are the work of 

 erosion; that the product of valley erosion is often seen in extensive 

 piedmont fluviatile plains; that waterfalls are retrogressively worn away 

 until they are reduced to the smooth grade of a maturely established 

 river; and that interior basins are slowly filling with the waste that is 

 washed in from their rims upon their floors. Here are explanatory 

 generalizations, involving, yet going far beyond matter of direct ob- 

 servation. Such generalizations in geography correspond to the recog- 

 nition in astronomy that planetary movements exemplify the law of 

 gravitation; they are the Newton as against the Kepler of the subject. 



The sufficient justification of the demand that has now arisen for ex- 

 planation and correlation in the study of land forms is found in the 

 repeated experience that until an explanatory description of a region 

 can be given, one may be sure that some of its significant elements 

 pass unnoticed; and until the controls that it exerts on living forms are 

 studied, one may be confident that its geographical value is but half 



