THE CLASSIFICATION OF COMMUNITIES 367 



geographic region, i.e., regionally bounded communities. In contrast 

 to these there are "extraregional" communities whose range extends 

 over several vegetational regions. Plankton communities with similar 

 composition are distributed over large portions of the earth. They 

 are generally extraregional and of little value for characterizing 

 divisions of the earth. The regionally defined circles of vegetation 

 are accompanied by at least two extraregional circles, those of the air 

 and those of the water. Probably the soil, with its microcommunities 

 (edaphon), constitutes another extraregional circle. 



3. The Taxonomy of Communities. — As a basis for taxonomy 

 (Braun-Blanquet, 1919, 1921), for determining the sequence of the 

 higher units, the principle of sociologic progression, rather than a 

 phylogenetic basis, has been proposed. Each unit is placed by means 

 of comparison with all other units and not in a preconceived series of 

 steps. This gives rise to true concepts of relationship. It depends 

 primarily upon the orderly relations of the objects to be classified. 

 The arrangement is thus brought into harmony with the prevailing 

 views of the theory of knowledge. 



Sociological Progression. — The arrangement of communities accord- 

 ing to sociological progression means according to their progressive 

 advance in organization. The "sociologically" simplest come at the 

 beginning; the most complex, at the end of the series. First are those 

 of extremely primitive ecology, the rather unstable floating com- 

 munities of air and water. In these, the social union, if it exists at all, 

 is so loose that one can scarcely speak of associations or even of alliances 

 and, indeed, can consider them only as belonging to the more compre- 

 hensive units, orders, or classes (cf. Pavillard, 1925, p. 432). Between 

 the members of these most primitive unions there is no competition for 

 space or food; and no interdependence can be demonstrated except in 

 cases of parasitism. The constituents of these floating communities 

 stand on the lowest step of evolution and are of extremely simple 

 structure, without adaptations. Each is an ecologic unit. They are, 

 in general, widely distributed. There is no genuine layering in 

 floating communities. 



At the opposite end of the series stands the tropical rain forest in its 

 majestic splendor and luxuriance, the type of the highest possible 

 phytosociologic completeness. In its complex, many-layered structure 

 it presents a marvelous picture of closely knit community life. Its 

 manifold systematic and ecologic types of plants and communities are 

 the most highly evolved, the most minutely adapted (Fig. 179). 

 Somewhat less complex are the chmax forests of temperate regions 

 (see Frontispiece and Fig. 180). 



