CLASSIFICATION OF PLANT COMMUNITIES 343 



Primary and secondary edaphic formations. The term second- 

 ary is here used with reference to edaphic formations in which 

 the vegetation has been modified by factors other than those 

 which are attributable to biotic (not including human) or phys- 

 iographic agencies. Formations whose vegetation has not been 

 so modified are regarded as primary. Secondary edaphic for- 

 mations commonly arise through the partial or complete de- 

 struction of the original vegetation by fire, lumbering opera- 

 tions, or cultivation. 



The edaphic formation-type. In any climatic region, owing 

 largely to the existence of numerous parallel series of physio- 

 graphic unit areas, there have been developed correspondingly 

 numerous parallel series of edaphic formations. Different in- 

 dividual formations which are correlated with the same type 

 of physiographic unit area may be referred to a common edaphic 

 formation-type ; thus : ravine formation- type, rock hill formation, 

 type, sand plain formation-type, etc. Like the association- 

 type, the edaphic formation-type is an abstract conception. 

 It may be defined as a type of edaphic formation correlated 

 with a given type of physiograph3^ As in the case of the as- 

 sociation-type, the conception of the edaphic formation-type 

 may be extended beyond the boundaries of a given climatic 

 region. Thus, the ra\ane, or the flood-plain, or the salt marsh 

 formation-type of one region may resemble that of another; 

 etc. Edaphic formation-types are referred to in terms of the 

 physiographic unit area concerned, as has been done above. 



The classification of edaphic formations. Although worked 

 out from a somewhat different point of view and therefore some- 

 what differently formulated, the developmental concept of the 

 edaphic formation was one of the fundamental features of Cowles' 

 physiographic classification of plant associations (8, 9). Cowles 

 was the first to fully appreciate the significance of physiography 

 in relation to the local distribution of plant associations. ''The 

 keynote," he writes (9, p. 8), "is that each particular topographic 

 form has its own peculiar vegetation. This is due to the fact 

 that the soil conditions upon which plants depend are deter- 

 mined by the surface geology and the topography." He further 



