i6o FORMATIONS AND GCILDS [Part I] 



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

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

 is more gradual. The eastern forest district of North America graduall) 

 passes westward into the grassland district of the prairies, and the latte; 

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

 phenomenon is exhibited in the transition from the Russian forest distric 

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

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

 to a change in climatic humidity. 



Tlie type of vegetation in the tropical and temperate zones is detenuiiiti 

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

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

 vegetation onh' by their desiccating influence. 



T/ie type of the flora in so far as it depends on existing factors i 

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

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

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

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

 tundra. 



On nearer approach the uniform character of the vegetation of 

 district appears much less distinct, for to the irregularities alread}' visibl 

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

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

 the like. Moreover, woodland, grassland, and desert display man\- fin 

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

 hygrophilous, there more xerophilous, with countless stages between th 

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

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

 changes. This fine differentiation of the vegetation and flora withi 

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

 is considerable unevenness of surface does the inequality of the insolatio 

 operate as well ; but the influence of this factor is alwaj-s subordinat 

 to the physical and chemical nature of the soil. 



The differentiation of the earth's vegetation is thus controlled by thri 

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

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

 merely picks out and blends the material supplied bj- these two climat 

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



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

 smaller and sometimes larger groups of uniform oecological and floristic 



* [The introduction of this adjective in the sense of relation to 'flora,' as ' faunisti' 

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

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



