342 GEORGE E. NICHOLS 



sociation-complex which is related to a specific physiographic 

 unit area. 



The developmental concept of the edaphic formation. The so- 

 called developmental concept of the formation was first suggested 

 by Moss who, in his account of the geographical distribution 

 of the vegetation of Somerset (16, p. 12; fide Clements, 4, p. 

 118), states that: ''The series of plant associations which be- 

 gins its history as an open or unstable association, passes 

 through intermediate stages, and eventually becomes a closed 

 or stable formation, is in this paper termed a plant formation. "^ 

 In a later paper (17, p. 36) he defines the [edaphic] formation 

 as comprising "the progressive associations which culminate 

 in one or more stable or chief associations [ = edaphic climax 

 associations], and the retrogressive associations which result from 

 the decay of the chief associations, so long as these changes 

 occur on the same habitat." This concept of the formation 

 has been adopted by the Committee for the Survey and Study 

 of British Vegetation (see 21, p. IX), and the edaphic formation 

 is so interpreted by the writer, except that he would substitute 

 "physiographic unit area" for "habitat" in the definition just 

 quoted. 



By some ecologists the edaphic climax association alone is 

 treated as a formation. In the opinion of the writer, however, 

 not the edaphic climax association, but rather the entire as- 

 sociation-complex should be regarded as constituting the physio- 

 graphic unit of vegetation : the edaphic formation. The edaphic 

 climax association represents merely the culminating phase of a 

 specific successional series: the most mesophytic type of vegeta- 

 tion capable of attainment in a specific physiographic unit area. 

 It is the indicator, so to speak, of the degree of mesophytism 

 which is attainable within the edaphic formation. As Moss ex- 

 presses it (16, 17), it is the "chief" association of a successional 

 series. It should be stated once more that the edaphic climax 

 association of an edaphic formation may or may not coincide with 

 the regional climax association-type. 



1 The term formation is used by Moss in the sense that the term edaphic 

 formation is used by the writer. 



