534 ISRAEL C. RUSSELL 



from the more concrete species — such as constructional plains, cinder 

 cones, sea cliffs, river terraces, etc. — to the more complex forms — as, 

 for example, mountain ranges, mountain systems, and yet larger earth- 

 features— could be represented by ideally perfect examples free from 

 accidental and secondary complexities and accessories. 



While individual examples of idealized topographic and other 

 features of the earth's surface would serve as species, their arrange- 

 ment under genera, families, etc., offers another problem, in which 

 relationship or genesis should be the controlling idea. 



The selection of idealized physiographic types, as just suggested, 

 has for its chief purpose the reduction of endless complexities and 

 intergradations to practicable limits. It is a method of artificial 

 selection so governed that, while no link in the chain of evolution 

 need be lost to view, certain links are chosen to represent their nearest 

 of kin and serve as types. A danger to be marked by a conspicuous 

 signal, in case this plan for aiding physiographic study is put in prac- 

 tice, is that it may tend toward empty ritualism. To give the idealized 

 types chosen for convenience of classification an appropriate atmos- 

 phere, the fact that changes are constantly in progress — that moun- 

 tains come and go even as the clouds of the air form and reform — 

 should be ever present in the mind. 



The process of evolution without concurrent extinction which 

 characterizes the development of physiographic features finds expres- 

 sion also in related departments of nature, as, for example, in petrog- 

 raphy, where, as is well known, it has greatly delayed the framing of 

 a serviceable and logical system of classification. Indeed, the prin- 

 ciple referred to may be said to be one of the chief distinctions 

 between the organic and the inorganic kingdoms of nature. 



THE PRIMARY FEATURES OF THE EARTH's SURFACE. 



The primary features of the earth's surface may consistently be 

 defined as those resulting from the growth and internal changes of 

 the lithosphere, while the modifications of relief resulting from the 

 action of agencies which derive their energy from without the earth 

 may be termed secondary features. The primary or major character- 

 istics of the earth's surface, so far as now known, may be ranked in 

 three groups, in accord with the agency by which they were principally 



