36 INITIAL CAUSES. 



necessary it may prove to combine them later because of their effect succes- 

 sionally. In consequence, such causes may be distinguished as (1) physio- 

 graphic, (2) climatic, (3) edaphic, (4) biotic. In the analysis of each an attempt 

 is made to distinguish between processes and agents in so far as possible. 

 Special attention is given to the results of each in terms of kind of bare area 

 and the degree of departure from the holard or other mean. This is followed 

 by a discussion of the directive effect upon succession in connection with an 

 endeavor to point out the essential nature of each process from the standpoint 

 of vegetational development. While every effort has been made to appre- 

 ciate the viewpoints of the physiographer and the climatologist, it is felt that 

 these are necessarily subordinate to the main object of analyzing the develoj)- 

 ment of vegetation. 



Physiography. — It is necessary at the outset to indicate the scope assigned 

 to physiography in the present treatise, since the many definitions of the 

 term differ greatly. Physiography is here imderstood much in the sense 

 used by Salisbury (1907:4), who defines it as having "to do primarily with 

 the surface of the lithosphere, and the relations of air and water to it. Its 

 field is the zone of contact of air and water with land, and of air with water." 

 In this definition the emphasis is considered to be upon the phrase "zone of 

 contact," and climate is not regarded as covered by the definition. While 

 physiography and climate are in constant and universal interrelation, they 

 are regarded as coordinate fields. An initial cause is termed physiographic 

 when it originates a sere in consequence of a changing land form, as in dunes, 

 the cutting down of a lake outlet, or the formation of a delta. It is termed 

 climatic when succession results from denudation due to a climatic change 

 which critically affects the water or temperature relations of a community. 



Cowles (1911:168) has evidently felt something of the difl&culty inhering 

 in the various uses of the term physiography, for he contrasts topographic 

 with climatic. He apparently also furnishes an example of the double use of 

 physiographic. After speaking of biotic changes and climatic changes as 

 initial causes of succession, he says: "A third and equally diverse kind of suc- 

 cession phenomena was recorded by Reissek in his study of islands in the Dan- 

 ube, for here there was clearly recognized the influence of physiographic change 

 in vegetation." Here physiographic seems clearly coordinate with climatic 

 and biotic, while in the next two sentences it is used to include climatic: 

 "Thus, in succession we may distinguish the influence of physiographic and 

 biotic agencies. The physiographic agencies have two aspects, namely, regional 

 (chiefly climatic) and topographic." Since physiography and topography are 

 here regarded as essentially synonymous, it seems desirable for the sake of 

 clearness to speak of topographic causes and processes hereafter. 



TOPOGRAPHIC CAUSES. 



Topographic processes. — ^All the forces which mold land surfaces have one 

 of two effects. They may add to the land or take away from it. The same 

 topographic agent may do both, as when a stream erodes in its upper course 

 and deposits a delta at its mouth, or undercuts one shore and forms a mud- 

 bank or sand-bank along the other. In similar fashion, a glacier may scoop 

 out a pond or a lake in one region and deposit the material as a moraine 



