Chap. V] THE SOIL 



]0I 



In the face of such phenomena, which are multiplied 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 eugeogenous, which supply an abundant 

 detritus, and dysgeogenous, which disintegrate very slightly 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 pelogenons kinds of soil, of very fine-grained earthy consistence, and 

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

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

 oligopclic, and the psammogenous, similarly, as perpsammic, hcmipsammic, oligo- 

 psammic. Transition states between the two groups were termed pelopsammic. 



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

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

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

 whilst 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 

 theory 1 almost into oblivion has been explained by Nageli, who in 1865 wrote 

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

 ciples of the physical theory exhibit a certain want of precision, so that criticism has 

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

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

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

 work, 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 

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

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

 observations, by analyses of soils, and by cultures. 



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

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

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

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

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

 concilable with facts. 



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



