THE DISTRIBUTION OF COMMUNITIES 355 



A preliminary attempt to define vegetational regions on this 

 phytosociological basis appeared in 1923 in "Origine et Developpement 

 des Flores dans le Massif Central de France," The outline and 

 definitions of the regional units used therein (first propounded in 1919) 

 have been made available to the general public by being quoted almost 

 verbatim in De Martonne's classic "Traite de Geographie" (vol. III. 

 pp. 1284-1286). 1 



Following the floristic divisions of Engler, Flahault, Diels, etc., 

 we recognize six regional units of different ranks : region, province, sec- 

 tor, subsector, district, and subdistrict. The content of these terms is 

 as follows : 



Region. — The region is the most comprehensive unit, characterized by numerous 

 well-defined climax communities and many peculiar transition communities. It 

 shows long-standing endemism in groups of high taxonomic rank (families, sub- 

 families, sections, etc.). The unity of the region is shown by the occurrence 

 throughout, or nearly throughout, of identical or closely allied species of high 

 sociologic importance (examples: Euro-Sibiric-North American region, Medi- 

 terranean region, Capetown region, oceanic (thalassic) region). 



Province. — The province is a subdivision of the region characterized by at least 

 one climax community and by various edaphic communities. It exhibits endemism 

 of species and genera, and has genera which are only feebly represented in neighbor- 

 ing provinces (examples: Central European, Illyrian, and circumboreal provinces. 

 The subalpine-alpine levels of the Alps and Carpathians would be a subprovince of 

 the Central European province). 



Sector. — An area without any peculiar climax community of high rank (alli- 

 ance). The special climatic or edaphic communities and geographic variants are 

 usually without generic endemism but often have notable endemic species (exam- 

 ples: Alpine, Carpathian, north European, Baltic, north Atlantic sectors). 



Subsector. — Less clearly marked by communities and endemism (micro- 

 endemism); possessing mostly some peculiar geographic races of more widely 

 distributed communities, as well as species and communities which are lacking in 

 the neighboring areas (examples: mountains of south-central Germany; Jurassic, 

 north Alpine, central Alpine, south Alpine subsectors). 



District. — Characterized by the presence of communities and species which are 

 rare or lacking in adjacent areas, e.g., district of the low plains of the upper Rhine, 

 of the Mainz basin; Ilex district and Erica cinerea district of the European north 

 Atlantic sector (Holmboe, 1913; Wille, 1915). 



Subdistrict. — This lowest synchorologic unit is characterized by the mere domi- 

 nance or suppression of edaphic or biotic communities and by the presence or 

 absence of distinctive species (precinct of the Schaffhausen basin of Hegau, of the 

 upper Danube). 



The above definitions are given rather as an outline than as a 

 finished plan. In striving to make the vegetational regions comparable 

 and the units of equal rank, some kind of outUne must be followed, 



1 A still more inclusive biogeographic classification could well be built up on 

 that foundation. 



