919 



date themselves to the soils more dysgeogenous, whilst the xerophiles 

 shun move the eugeogenous. In proportion as we advance southward 

 the hygrophiles require soils more eugeogenous, whilst the xerophiles 

 accommodate themselves to soils less dysgeogenous. 



The extreme limits of the physical characters of soils induce vege- 

 table sterility by three different conditions. Hard rocks absolutely 

 dysgeogenous are sterile in consequence of their unchangeability, 

 which is opposed to the production of detritus. The soft rocks of an 

 eugeogenous and very clayey nature are sterile occasionally in con- 

 sequence of their compactness and impermeability. The eugeogenous 

 and very sandy rocks absolutely moveable may be and really are often 

 sterile, not in consequence of their extreme separation, but in conse- 

 quence of their mobility. The first become an element of the soil 

 and contribute to establish at their surface fertility by disintegration, 

 the second by division, the third by fixedness. The first are essen- 

 tially dry ; the second essentially humid ; the third essentially divided, 

 more or less dry according as they are more or less mobile, and, when 

 fixed, necessarily more humid than the first, much less so than the 

 second. 



All things equal in respect of latitude and elevation, a district of 

 subjacent eugeogenous rocks is more cool (/rats), more humid, more 

 watery, and probably more cold [froid) than a dysgeogenous district. 

 Its vegetation is less dependent on levels, more common, more boreal, 

 more social, usually more rich in species, and particularly more rich 

 in plants of the lower families, more herbaceous, with roots deeper 

 and branched, more numerous, &c. It presents characters opposite 

 to the dysgeogenous soil. 



The more aquatic the vegetation, the more is it independent of 

 latitude and levels ; the more terrestrial it is, and the more is it under 

 the influence of those conditions. The rupestral (saxicoles) species 

 of the dysgeogenous rocks are essentially the best climatological 

 characteristic. 



In a country of moderate extent, the mean annual temperature of 

 the air, although an incomplete expression of the climate, is however 

 an element sufficiently predominant to be in constant and appreciable 

 relation with the principal facts of phyto-statistics, such as the locali- 

 zation (cantonnement) of groups of species the most southern, the 

 most northern, the most alpine. 



The same regions of elevation will not offer the same character of 

 vegetation, except so far as they belong to zones almost equally eu- 

 geogenous or dysgeogenous. These regions are not then to be com- 



