GENERAL RELATIONS. 109 



wider range, but this is probably to be explained by the assumption that it is 

 really a subclimax consocies as described below, and persists as an apparent 

 society well into the climax. In general, however, the climatic and floristic 

 differences between associations are sufficiently marked to restrict each 

 society to a particular association. 



When a species exhibits a local or restricted subdominance covering a few 

 square yards or a few acres, it constitutes a clan. It is clear that the differ- 

 ence between society and clan is merely one of degree. Theoretically, there is a 

 point at which they are indistinguishable, but practically very few difficult 

 cases have been encountered. The best examples of clans are species of gre- 

 garious habit, especially stoloniferous ones, and of low growth. Such clans 

 are capable of holding ground very tenaciously, and of slowly extending it, 

 but they are able to make only limited headway against the double control 

 of dominants and subdominants. Clans are best exemplified by Delphinium 

 penardi and Erigeron flagellaris in grassland, and by Pirola, Goodyera, Heu- 

 chera, etc., in forest. 



Names of climax communities. — An endeavor has already been made to 

 devise a system of names for plant communities, in which the names would 

 be short, significant, and usable, as well as international in character (Clem- 

 ents 1916 : 127, 129, 133, 137, 138). Some such system will be indispensable 

 as ecology becomes more and more definite in nature as well as international 

 in scope. In the present treatise, which is purposely limited to the western 

 half of the United States, the technical terms are unnecessary and are used 

 only as an occasional convenience. Hence, the practice will be to secure the 

 maximum of definiteness consistent with brevity and clearness by uniformly 

 distinguishing between associations, consociations, societies, and clans by 

 means of the one or two most characteristic genera or species. At the same 

 time, an endeavor is made to furnish a somewhat more usable equivalent in 

 vernacular terms, in the expectation that these will come into practical use. 

 Thus the Stipa-Bouteloua poion will be referred to as the Stipa-Bouteloua 

 climax or formation, or as the grassland climax or formation, and the Stipa- 

 Koeleria association as the Stipa-Koeleria prairie or true prairie. The alter- 

 native terms for the various formations and associations are given in the 

 summary on page 114. 



Serai communities. — The limits of space make it impossible to give an ade- 

 quate account of the basic process of succession as exhibited in the develop- 

 ment of climax formations, and for this the reader must be referred to "Plant 

 Succession," especially Chapters I, V, VI, and VII. Here it must suffice to 

 point out that succession is a universal phenomenon by which bare areas 

 become colonized by plants and slowly develop through successive stages into 

 the climax formation which surrounds them. Bare areas are initially bare, 

 as in the case of bodies of water, rock ridges, and fields and sand-dunes, or 

 they are denuded of vegetation by various forces, especially fire, lumbering, 

 grazing, and cultivation. The course of succession is much longer and slower 

 in the former case than in the latter, but the essential features of development 

 are the same. Each population or community reacts upon the area or habitat 

 in such a way as to make conditions less extreme and correspondingly more 

 favorable to species of greater requirements. These enter gradually and 

 compete successfully with the occupants, finally driving them out or com- 



