366 PLANT SOCIOLOGY 



It thus brings to its most comprehensive expression the individuahty of 

 vegetation, based partly upon climatic isolation and partly on floristic 

 history. 



Since we base our system of vegetation upon units (associations 

 and species) which are strictly circumscribed in space, the spatial factor 

 has a profound systematic importance. A division which thus rests 

 upon the species must necessarily culminate in the regional unit of 

 geographic and developmental history — the circle of vegetation. 

 Consequently, in this highest unit the phytosociologic and phyto- 

 geographic divisions of the earth are the same. The areal basis of both 

 is the "vegetational region" (c/. p. 355). 



The vegetational region corresponds, in general, to the great life 

 zones of our planet, the biogeographic regions. Thus, in these highest 

 categories of the division of space, the unity of life and its organic 

 interrelations find their most complete expression. 



Similar combinations of life forms (formations, vegetation types) 

 may occur in regions widely separated geographically, where they have 

 never been in contact and therefore have no floristic relation to one 

 another. These are to be classed as parallelisms, due to similarity of 

 cHmate. Under similar chmatic conditions similar life forms "evolved." 

 This is the explanation of the similarity of the regions of broad sclero- 

 phyll forest in Mediterranean climates found in the Mediterranean 

 region, Cahfornia, Cape Colony, Chile, and southwestern Austraha. 

 Gams (1918) has proposed the name "isocies" for such communities; 

 the term "homologies," used by Chodat, seems preferable. 



2. Regional and Extraregional Units. — The more complicated the 

 structure, the more highly organized the floristically defined social 

 units the more strictly are they confined to certain parts of the earth. 

 Forest associations, as at present understood, can hardly extend from 

 one vegetational region into another. If they appear to do so, they 

 may be regarded as "outliers." The beech forests of some Mediter- 

 ranean mountains are outliers of the Euro-Sibiric-North American 

 region; the areas of Quercus ilex scrub of the central Rhone valley, 

 north of Valence, are outliers of the Mediterranean region. 



Water and swamp communities, on the other hand, show a much 

 greater capacity for distribution. For example, the order of Pota- 

 metaha proposed by W. Koch (1926) is found in the Mediterranean as 

 well as the Euro-Sibiric-North American circle of vegetation. Prim- 

 itive floating communities (phytoplankton) have, of course, a still 

 broader and more general range 



In characterizing a circle of vegetation, only those communities 

 can be used which are more or less confined to a strictly bounded 



