Chap. I] THE FORMATIONS 161 



type, the characteristics of which are exactly repeated on the same 

 kinds of soil so long as the climate is unchanged, whereas the different 

 kinds of soil bear different kinds of plants. The communities of plants 

 as determined by the qualities of the soil are termed formations. 



In each formation one species of plant, or a group of species, is 

 characteristic ; plants that merely occur sporadically are unessential to 

 the formation, and commoner subsidiary constituents can only give a 

 different facies to the formation. Thus, in Europe, we are acquainted 

 with the formation of the beech-forest, where Fagus sylvatica predominates, 

 and with at least two facies of dissimilar herbaceous vegetation ! . If 

 the composition of the vegetation should alter while the nature of the 

 soil remains unchanged, this is a certain indication of transition into 

 another climate. A sudden change of formations while the quality of 

 the soil remains unaltered is only found in mountain ranges in relation 

 to the sudden change in climate. 



Whilst every formation is in its fioristic and oecological character 

 a product of climate and soil, yet the influence of the several climatic 

 and edaphic factors is not equal. The influence of the soil is always 

 subordinate to that of the climatic temperature, whereas under certain 

 conditions that are indeed merely local it neutralizes that of the 

 atmospheric precipitation. Thus woods occur in many spots where the 

 climate would give rise to grassland, or we may find the converse, and 

 vigorous forest thrives in patches under a desert climate with a very 

 scanty atmospheric precipitation. Definite properties of the soil may 

 also bring forth a character of vegetation that belongs to none of the 

 climatic types. These climatic types demand a favourable constitution 

 of the soil congenial to the vast majority of the plants. Extreme 

 properties of the soil that are unfavourable to the life of most plants 

 set vegetation free from the controlling influence of atmospheric pre- 

 cipitation. Consequently the vegetation of rocks, gravel, swamps, and 

 other special spots, bears in the highest degree the oecological impress 

 of the substratum, and this impress for the most part remains identical 

 under very dissimilar conditions of climatic humidity, which on such 

 soils plays only a subordinate part. 



From what has preceded it appears that two oecological groups of 

 formations should be distinguished — the climatic or district formations, 

 the character of whose vegetation is governed by atmospheric precipitations, 

 and the edaphic or local formations, whose vegetation is chiefly determined 

 by the nature of the soil. 



1 See p. in. 



M 



