634 PHYSIOGRAPHY 



ence to movement in the earth's crust; the extrusion of gases and 

 vapors from the earth's interior, and the resultant changes in progress 

 in the volume and composition of the atmosphere and hydrosphere. 

 In these and still other fundamental conceptions of the primary 

 causes of many of the changes in progress on the earth's surface the 

 planetesimal hypothesis seemingly furnishes the corner-stone of a 

 broader physiography than has as yet been framed. 



Ideal Physiographic Types 



During the descriptive stage of the study of biology the relation- 

 ships among plants and animals were the chief end in view, and as 

 a result of the conditions confronted, a systematic classification of 

 animate forms under species, genera, families, etc., was formulated, 

 which has been of infinite assistance during the more philosophical 

 investigations that followed. Biological classification was facilitated, 

 as learned later, by the fact that with the evolution of species there 

 was concurrent extinction of species. As the tree of life grew, its 

 branches became more and more widely separated. 



Throughout the many changes the surface features of the earth 

 have experienced, there has also been development, not of the same 

 grade, but analogous to that recognized in the realm of life; but the 

 process of extinction has been far less complete than in the organic 

 kingdom, and the connecting links between the various groups of 

 topographic and other physiographic forms produced have persisted, 

 and to a conspicuous extent still exist. The task of arranging the 

 infinitely varied features of the earth's surface in orderly sequence, or 

 systematic physiography, is thus far more difficult than the similar 

 task which the flora and fauna of the earth present. 



The utility of classification is fully recognized by physiographers, 

 and various attempts have been made from time to time to meet 

 the demand, but thus far without producing a generally accept- 

 able result. Remembering that a scheme of classification of topo- 

 graphic and related forms is to be considered as a means for attaining 

 a higher end, namely, the history of the evolution of the surface fea- 

 tures of the earth, and should be of the nature of a table of contents 

 to a systematically written treatise, the task of preparing such an index 

 becomes of fundamental importance to the physiographer. Since 

 extinction of species among physiographic features has probably not 

 occurred, and connected series of forms which grade one into another 

 confront us, the practical lesson taught by the success of schemes of 

 biological classification seems to be that ideal physiographic types 

 should be chosen correlative with species among plants and animals. 



By "ideal physiographic types" is meant complete .synthetic 

 examples of topographic and other physiographic forms, which will 



