IMPORTANCE AND PROPERTIES OF THE SOIL 175 



remains of the plants themselves produces a little soil, poor in character 

 though it must be. It can, however, afford lodgement for plants less 

 modest in their demands than lichens, and these continue the work of 

 their predecessors, until finally, perhaps after frequent changes, an abundant 

 vegetation grows from a fruitful humus soil as a monument to the first 

 settlers upon the original barren rocks. On volcanic lavas, and upon the 

 debris from alpine avalanches, the gradual progress of the new vegetation 

 may frequently be followed to its final luxuriance from its modest com- 

 mencement by mosses and lichens l . 



Provided a supply of moisture is assured, even phanerogamic plants 

 are able to take part in the work of colonization on pure sand, or pulverized 

 stones, and to aid in the creation of a rich and fertile soil. No detailed 

 account is possible here of this perpetual creation and change, or of the 

 unceasing organic circulation upon our planet, in which the plant-world 

 plays so important a part (cf. Sect. 51). It may however be mentioned 

 that micro-organisms play a most important part in the changes and de- 

 compositions which take place in the soil, and that by the oxidizing activity 

 of certain bacteria, nitrogenous compounds may give rise to the potassium 

 nitrate or saltpetre, which most of the higher plants prefer as a source of 

 nitrogen. Attention must also be paid to the mechanical actions of roots in 

 binding together loose sand and in disintegrating rocks, while animals such 

 as earthworms may aid in various ways in the production of a fruitful soil 2 . 



1 Cf. Humboldt, Reisen in d. Aequinoctialgegenden, I, p. 143 ; Gb'ppert, Flora, 1860, p. 161 ; 

 Senft, ibid., iS6o, p. 193; Ffeffer, Jahrb. d. Schweizer Alpenclubs, 1867-68, iv. Jahrgang, p. 462, \ 

 and Bryeograph. Stud. a. d. rhatisch. Alpen, 1870, p. 135 (Sep.-abdr. a. d. Denkschr. d. Schweiz. \ ! 

 Naturf.-Ges.). [The gradual return of vegetation to the island of Krakatoa near Java, which was \ 

 completely covered by lava and tuff more than twenty-five years ago, illustrates the same process 

 admirably. Here the first plants to appear were bacteria and lowly organized algae (Cyanophyceae, 

 &c.), owing probably to climatic conditions, &c. ; now large trees are growing by the shore, and 

 many phanerogamic shrubs, &c. more inland.] 



2 Ch. Darwin, The Formation of Mould by Earthworms, 1882 ; Wollny, Forsch. a. d. Geb. d. 

 Agr.-Physik, 1890, Bd. XIII, p. 381. 



