Chap. V] THE SOIL loi 



In the face of such phenomena, which are nuiltiplied by each new 

 investigation, so that the number of species truly characteristic of certain 

 soils becomes more and more reduced, botanists, in the middle of the 

 nineteenth century, gradually began to doubt the chemical influence of 

 the soil on the character of the flora and to trace back the difference 

 between the floras on lime and on silica, respectively, to physical factors. 

 The very able Swiss investigator Thurmann for a time carried the day 

 with his ' physical theory,' which completely denied the chemical influence 

 not only of silica but also of calcium carbonate, and attributed the differ- 

 ences in the flora exclusively to those in the humidity and consistency 

 of the soil. 



Thurmann distinguished rocks as eitgeogenoiis, which supply an abundant 

 detritus, and dysgeogeiioiis, which disintegrate very sHghtly or not at all into detritus. 

 Hygrophytes are associated with eugeogenous soil and xerophytes with dysgeogenous 

 soil. According to the physical consistence of the detritus, Thurmann further 

 distinguished pclogciioiis kinds of soil, of very fine-grained earthy consistence, and 

 \psaiiiinogenoiis, of more or less coarse-grained sandy consistence. According to their 

 degree of subdivision pelogenous soils were further classified as perpelic, licinipelic, 

 oligopi/ic, and the psammogenous, similarly, as pcrpsainiuic, Iit'iuipsaminic, oligo- 

 bsaminic. Transition states between the two groups were termed pelopsmnmic. 



According to Thurmann the so-called silicicolous plants are hygrophilous and the 

 :alcicolous plants xerophilous. It is not the presence of silica nor of lime, but the 

 Dresence of larger or smaller quantities of water, that determines their appearance, 

 .vhilst the other physical differences are said to evoke finer distinctions in the com- 

 position of the vegetation. 



That this ' theory ' so long enjoyed such universal assent and threw the ' chemical 

 heory' almost into oblivion has been- explained by Nageli, who in 1865 wrote 

 I masterly paper in favour of the 'chemical theory,' as due to the fact 'that the prin- 

 ;iples of the physical theorj' exhibit a certain want of precision, so that criticism has 

 lowhere a firm basis for refuting them and nothing is more difficult than to correct 

 I vague proposition.' Nageli, however, did not succeed in making many converts, 

 md this chiefly because the best men were almost entirely occupied with laboratory 

 vork, whilst others at that time fortunately kept away from such general questions. 

 Only since 1880 has discussion on this question recommenced, with the result that 

 he ' chemical theory ' has now been indisputably maintained, being supported 

 Jy a correct apprehension of the problem, as well as by better material from field 

 ibservations, by analyses of soils, and by cultures. 



A principal cause for the discredit into which the ' chemical theory ' fell is to be 

 bund in the then prevailing false conception of the influence of the soil. It was 

 issumed that lime-plants require as food lime but not silica, and that silica-plants, on 

 he contrary, require silica but not lime. It needs no longer to be stated that such 

 deas, which strange to say are still held by some geographical botanists, are irre- 

 :oncilable with facts. 



The untenability of the ' physical theory ' follows most clearly from the 



