160 FORMATIONS AND GUILDS [Part II 



Llanos, the forest of Brazil and the Argentine by the Andes from the 

 desert of Peru, Bolivia, and North Chili. In other cases the transition 

 is more gradual. The eastern forest district of North America gradually 

 passes westward into the grassland district of the prairies, and the latter 

 towards the west gradually assumes the condition of a desert ; a similar 

 phenomenon is exhibited in the transition from the Russian forest district 

 to the South Russian steppes, and from the latter to the Caspian desert. 

 Whether the change be more sudden or more gradual, it always corresponds 

 to a change in climatic humidity. 



The type of vegetation in the tropical and temperate zones is determined 

 by the amount and distribution of the rainfall, by the humidity of the 

 air, and by the movements of the atmosphere, which essentially affect 

 vegetation only by their desiccating influence. 



The type of the flora in so far as it depends on existing factors is 

 dependent primarily on heat, especially if we consider, not the groups 

 of lower order (genera and species), but those of higher order (cohorts, 

 orders, and families). Only in polar areas is the temperature important 

 as a climatic cause of a type of vegetation — in the cold desert or 

 tundra. 



On nearer approach the uniform character of the vegetation of a 

 district appears much less distinct, for to the irregularities already visible 

 from a distance a number of fresh ones are added, such as small patches 

 covered with reeds in the midst of a forest, scantily stocked gravel, and 

 the like. Moreover, woodland, grassland, and desert display many fine 

 shades of differences within their types ; here the character is more 

 hygrophilous, there more xerophilous, with countless stages between the 

 two extremes. Finally, the composition of the flora that could in most 

 cases not be discerned from a distance is subject to more or less sudden 

 changes. This fine differentiation of the vegetation and flora within 

 a climatic district is chiefly determined by the soil. Only when there 

 is considerable unevenness of surface does the inequality of the insolation 

 operate as well ; but the influence of this factor is always subordinate 

 to the physical and chemical nature of the soil. 



The differentiation of the earth's vegetation is tJins controlled by three 

 factors — heat, atmospheric precipitation {including winds), soil. Heat 

 determines the flora, climatic humidity the vegetation ; the soil as a rule 

 merely picks out and blends the material supplied by these two climatic 

 factors, and on its own account adds a few details. 



The blending activity of the soil leads to a differentiation into sometimes 

 smaller and sometimes larger groups of uniform oecological and floristic x 



1 [The introduction of this adjective in the sense of relation to 'flora,' as 'faunistic' 

 is used in relation to ' fauna,' appears to be necessary because of the botanical restriction 

 of 'floral ' to the flower and its parts.] 



