146 ADAPTATIONS. OECOLOGICAL CLASSIFICATION SECT, in 



dependent upon minor differences in an association. 1 A beech-forest, 

 f agetum Fagi sylvaticae, is not everywhere and on every terrain absolutely 

 uniform and constant in composition, but owes its modification, inter 

 alia, to differences in soil ; in one beech-forest the ground- vegetation 

 may be dominated by Asperula odorata, which denotes alkaline, loose, 

 well-aerated humus, while in others Aira flexuosa or Vaccinium Myrtillus 

 may play a dominant part and thus indicate acid humus. There are 

 also beech-forests in which still other species characterize the flora on 

 the ground. Accordingly, beech-forests may be described as fageta 

 asperulosa, fageta myrtillosa, and so on. Similarly there are betuleta 

 hylocomiosa, betuleta cladinosa, and others, according as the ground is 

 clothed with mosses of the genus Hylocomium, or lichens of the genus 

 Cladina. The ground-flora in forests of Pinus sylvestris differs widely 

 according to various conditions, including the character of the soil ; in 

 Baltic forest there occur together many species that in the Hercynian 

 hills and lower highlands are distributed in different formations. 2 



Grabner 3 distinguishes in the heath-formation various associations : 

 namely, ling-heath (callunetum), with several varieties, according as 

 Pulsatilla, Genistae, or Solidago, or others predominate as perennial 

 herbs ; a tetralix-heath (ericetum), which also has several varieties ; an 

 Empetrum-heath, and others. 



Certain species are so accommodating that they can grow on very 

 different soils, and can therefore give rise to different associations. 

 Woodhead 4 has pointed out that this is the case with Pteris aquilina ; 

 and he therefore distinguishes between two varieties of associations, 

 which he terms meso-pteridetum (an association of Pteris with Holcus 

 lanatus and Scilla festalis), and xero-pteridetum (an association of Pteris 

 with Calluna, Vaccinium Myrtillus, Aira flexuosa, and others). 



Edaphic varieties. Varieties in an association may be distributed 

 in patches, or zones, according to the conditions prevailing in the soil ; 

 in a meadow we can see both these forms of arrangement, corresponding 

 to the shapes and extent of the depressions in the ground. Here we 

 are dealing with edaphic varieties. 



Geographical varieties. But varieties may have arisen through histo- 

 ric or climatic causes; for example, according to Beck von Mannagetta 5 

 Pinus Laricio is distributed as a high-forest tree over so wide an area 

 that its ground-vegetation belongs to three different floral districts : 

 the Pontic, Baltic, and Mediterranean. Here, then, we have three 

 geographical varieties. Hock's investigations show that the ' companion- 

 plants ' of the respective forest-trees in different parts of Europe may 

 be widely different, even if the limiting boundaries of the trees and of 

 their companion-plants coincide to some extent. 



In the succeeding Sections an attempt will be made to give a survey 



1 Cowles, 1899, p. in. These varieties are sometimes spoken of under the name 

 ' facies '. The term ' facies ' seems to have been employed by Lorenz, 1863, first 

 to denote small local differences in a formation, but subsequently to denote an 

 association, and in a somewhat similar or slightly different sense it is employed 

 by others. [The term ' facies ' has already the recognized meaning in English 

 biological science of ' general aspect or appearance ' (N.E.D.), and its use in a 

 restricted sense as that of a variety of an association is barred. P.O.] 



' Drude, 1902. . * Grabner, 1901. 



4 Woodhead, 1906, pp. 114, 363. * Beck von Mannagetta, 1902. 



