PHYSIOGRAPHIC PROBLEMS OF TO-DAY 635 



serve the r61e of well-defined species in the study of the surface fea- 

 tures of the earth. Ideal types may be likened to composite photo- 

 graphs. They should combine critical studies of many actual forms, 

 within a chosen range, and in addition be ideally perfect representa- 

 tives of the results reached by specific agencies operating under the 

 most favorable conditions. Like the idealized personalities of history 

 and religions, the types of physiographic forms might well be more per- 

 fect than any actual example. When such idealized types shall have 

 been chosen after careful study, described with care, and illustrated 

 by means of diagrams, maps, pictures, models, etc., a comparison with 

 them of actual examples on any portion of the earth for the purpose 

 of identification and classification would be practicable. A well- 

 arranged catalogue of ideal types would be an analytical table of 

 contents to the history of the evolution of the features of the earth's 

 surface, and constitute a scheme of physiographic classification. 



In illustration of what is meant by an idealized physiographic type : 

 We find in nature a great variety of alluvial deposits, now designated 

 as alluvial cones or alluvial fans. They present a wide range and in- 

 finite gradations in size, shape, composition, structure, angle of slope, 

 degree of completeness, stage of growth or decadence, etc. Complica- 

 tions also arise because of the association and intergrowth of such 

 alluvial deposits with other topographic forms. In constructing the 

 ideal type the characteristics of many of the most perfect actual allu- 

 vial cones, aided by a study of the essential features of similar artificial 

 structures, should be combined in an ideally perfect and representa- 

 tive example which would serve as the type of its species. All actual 

 examples might be compared with such a type, their specific and gen- 

 eric relations determined, and their individual variations noted. In 

 like manner, other topographic forms, ranging from the more concrete 

 species such as constructional plains, cinder-cones, sea-cliffs, river- 

 terraces, etc., to the more comglex forms, as, for example, moun- 

 tain 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- 



