THE FORMATION OF VEGETATION. 127 



much clearer understanding of the climatic and edaphic factors and the essen- 

 tial balance between them. 



The developmental limitation of formations demands long investigation. 

 Hence it is necessary to appeal first to physiognomy and floristic for tentative 

 units, except in regions where successional studies are already well advanced. 

 Such tentative units must be tested and confirmed by development before 

 they can be accepted. Such a test will necessarily involve the use of habitat 

 criteria to an increasing degree. Thus, over the whole of the Great Plains 

 region, life-forms and population indicate a vast grassland formation. The 

 existence of such a climax is confirmed by numerous developmental studies 

 which have already been made upon it. In the matter of temperature the 

 region is far from uniform, but in the critical water relations investigation 

 shows it to be essentially a unit. Over this wide stretch from Texas and 

 northern Mexico far into Alberta the dominant genera are the same, and this 

 is true of many of the species. This is also true of the genera and some of the 

 species of the scrub or chaparral formation which extends from Minnesota 

 westward to British Columbia, southward to California and Mexico and east- 

 ward to Texas, Colorado, and Nebraska. 



According to the developmental idea, the formation is necessarily an organic 

 entity, covering a definite area marked by a climatic climax. It consists of 

 associations, but these are actual parts of the area with distinct spatial rela- 

 tions. The climax formation is not an abstraction, bearing the same relation 

 to its component associations that a genus does to its species. It is not a 

 pigeon-hole in which are filed physiognomic associations gathered from all 

 quarters of the earth. Hence it differs radically from the formation of Warm- 

 ing and other writers who have adopted his concept. According to the latter 

 (1909 : 140; cf. Moss, 1910 : 43), "a formation appears with a certain deter- 

 mined uniformity and physiognomy, even in different parts of the world, and 

 even when the constituent species are very different and possibly belong to 

 different genera and families. Therefore a formation is an expression of cer- 

 tain defined conditions of life, and is not concerned with floristic differences." 

 The formation as developmentally limited would include the closely related 

 chief associations of Drude (1896 : 286) and Moss (1910 : 38). The formations 

 of many writers are associations as here understood, and those of Hult and 

 his followers are merely consocies and societies. The current conceptions of 

 formation and association in the larger sense were regarded as fairly final by 

 the writer, until 15 months of continuous field-work in 1913 and 1914 made 

 this position appear to be no longer tenable. This change of view was not 

 only a direct consequence of the application of developmental principles to a 

 wide range of communities, but it was also rendered unavoidable by the oppor- 

 tunity of comparing all the formations and associations in the region from the 

 prairies to the Pacific Coast, and between Mexico and middle Canada dm-ing 

 the summer of 1914. 



Names of formations. — The need of being able to designate formations more 

 acciu-ately than by the use of vernacular names led to the proposal-by Clem- 

 ents (1902 : 5) that they be designated by Greek names of habitats or com- 

 munities, to which the suffix, -eiov, place, was added. This suggestion has 

 been adopted by Ganong (1902 : 53; 1903 : 303), Diels (1908 : 70; 1910 : 18), 

 and Moss (1910 : 142; 1913 : 167). More recently, Brockmann and Rtibei 



