140 ADAPTATIONS. OECOLOGICAL CLASSIFICATION SECT, in 



term 'formation'. Hult, in his excellent papers on the phytogeography 

 of Finland l establishes about fifty ' formations ' as existent in North 

 Finland an Empetrum-formation, Phyllodoce-formation, Azalea-forma- 

 tion, Betula nana-formation, Juncus trifidus-formation, Carex rupestris- 

 formation, Nardus-formation, Scirpus caespitosus-formation, and others. 

 Similarly Kjellman 2 divides the algal vegetation into numerous 'forma- 

 tions ' named after predominant species ; and the same use of the term 

 is made by Stebler and Schroter in connexion with the types of Swiss 

 meadows that they recognized. 



In like fashion we must distinguish as different formations beech- 

 forest, oak-forest, birch-forest, and other dicotylous forest ; ling-heath, 

 Empetrum-heath, Erica-heath ; or in fresh water, Scirpus lacustris- 

 formation, Phragmites-formation, Equisetum limosum-formation, and 

 so forth. 



This means a subdivision of the vegetation based upon the local 

 domination of certain species, and may consequently result in matters 

 of wide and general import escaping notice, and communities naturally 

 belonging together, as evidenced by their pursuance of identical economy, 

 may not be recognized as such. The Empetrum-formation, Azalea- 

 formation, and Phyllodoce-formation are oecologically essentially alike, 

 and may be regarded as members of a more comprehensive natural 

 community, the dwarf-shrub heath ; the Scirpus-formation and Phrag- 

 mites-formation are likewise members of one community, and it often 

 evidently depends upon mere accident whether the one or the other 

 of these ' formations ' prevails at a given spot. 



Obviously, such special communities play a part, and must be dis- 

 tinguished in any detailed description of the vegetation of a definite area ; 

 but it is better to speak of them as associations, and to follow Grisebach 

 in the use of the term 'formation ', since he was the first to introduce and 

 define it, although he laid less stress on its oecological meaning. 3 



A formation may then be defined as a community of species, all 

 belonging to definite growth-forms, which have become associated together 

 by definite external (edaphic or climatic) characters of the habitat to 

 which they are adapted. Consequently, so long as the external condi- 

 tions remain the same, or nearly so, a formation appears with a certain 

 determined 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 or families. Therefore 



A formation is an expression of certain defined conditions of life, and 

 is not concerned with floristic differences. 



The majority of growth-forms can by themselves compose formations 

 or can occur as dominant members in a formation. Hence, in sub- 

 dividing the groups of hydrophilous, xerophilous, and mesophilous plants, 

 it will be natural to employ the chief types of growth-forms as the prime 

 basis of classification, or, in other words, to depend on the distinctions 



1 Hult, 1 88 1, 1887. 2 Kjellman, 1878. 



3 The term 'formation ' is so employed also by Ascherson, 1883, p. 728 ; Kerner, 

 1891, p. 830; C. Schroter und Kirchner, 1902; Kearney, 1900; Ganong, 1902; 

 Clements, 1902 a. Adamovicz, 1898, and Cowles, 1901, adopt at least approximately 

 the same course. On the other hand, Flahault and W. G. Smith employ the word 

 ' association ' in this sense. 



