CHAP, xxxv VARIETIES OF ASSOCIATION 147 



of the most important formations existing on the Earth. But to associa- 

 tions and their varieties less attention can be devoted, partly because 

 the subject has not been sufficiently investigated, and partly because 

 it would involve too much detail ; for such detail reference should be 

 made to special papers. It must be said that there still prevails great 

 confusion in the use of the terms * formation ', ' subformation ', and 

 the like, and this places difficulties in the way of a clear comprehension 

 of the subject. 



In the following scheme an endeavour is made to arrange the for- 

 mations within the respective classes according to definite principles in 

 a definite order of succession. The scheme commences, so far as is practi- 

 cally possible, with the simplest and shortest types, which form a low stratum 

 of vegetation corresponding to the growth-forms described in Chapter II 

 and on page 141 ; they can not only form separate communities, but also 

 occur as subordinate constituents in formations composed of taller forms. 

 For example, the formations on acid humus soil are, as a whole, arranged 

 as follows : First, the low formations composed of thallophytes and mosses ; 

 secondly, those composed of two storeys, dwarf-shrubs, and those forms 

 belonging to the preceding type ; and, thirdly, still taller formations 

 composed of several storeys, shrubs and trees, among which representa- 

 tives of the first two types of formations are usually found and may, 

 indeed, have developed as the real foundation of the whole formation. 

 An attempt has been made to show between the different formations 

 a logical and biological connexion, which in many cases is also develop- 

 mental in significance. Formations commence often as open, not fixed, 

 communities that are composed of short and lowly organized species ; 

 with the passage of time their development is continued by the immigra- 

 tion of species that are taller and more successful in the struggle for 

 existence, until a final stage is reached. 1 



Cowles 2 strongly insists ' that the plant societies must be grouped 

 according to origins and relationships, and the idea of constant change 

 must be strongly emphasized '. A. Nilsson 3 followed the same train 

 of thought in his arrangement of Swedish formations into four series 

 the heath-series, meadow-series, marsh-series, and moor-series as he 

 particularly took cognizance of the amount of water and nutriment in 

 the soil. Many of the formations brought into his series are closely allied 

 and can readily develop into one another, but there are others that are 

 certainly not allied, for instance, littoral meadow, alpine meadow, wood- 

 meadow, 4 meadow spruce-forest, and meadow oak-forest. 5 



It is true that changes in the physical relationships of soil are every- 

 where and always taking place, and that in close correlation with this 

 plant-communities also undergo modification. 6 This will be discussed 

 in the final Section of this book. In the future it will be an interesting 

 and important problem to trace out in each country or district the 

 development of the vegetation, as regards not only flora but also plant- 



1 Warming, 1890, 1906; Moss, 1906, 1907; A. Ernst, 1907. 

 1 Cowles, 1901 b. 'A. Nilsson, 1902. 



4 [Very open, thinly wooded forest, \vith an abundance of grass and other herbs 

 Representing meadow.] 



6 In regard to the genesis of plant -communities consult Engler, 1899, p. 179. 

 6 Warming, 1899. 



L 2 



