CHAP. XIX EFFECT OF A LIVING COVERING 77 



soil, they dry it to a less degree than does other vegetation, and they 

 protect dry easily-heated soil from desiccation. Evaporation takes place 

 more rapidly from a covering of moss than from a dead covering, but 

 the former keeps soil moist and cool upon the whole, and may easily 

 occasion swampiness in wet, shaded soil. 



8. The chemical relations of soil are influenced by a covering of plants, 

 for different kinds of vegetation differently affect the nutriment in soil and 

 its absorbent faculty, abstracting different inorganic substances and 

 enriching it with organic bodies. A rotation of crops and the apphcation 

 of manure are matters of necessity to the farmer, because with each crop 

 he constantly removes from the soil certain quantities of nutritive sub- 

 stances. The forester does this to a smaller extent, except when, as in 

 Germany, he carries away forest-Utter, and the application of manure 

 is as a rule not necessary in forest or, at any rate, has been but Httle 

 practised. Nevertheless wind blows leaves out of many forests, and 

 consequently brings about great changes in the soil and vegetation. 

 A change in the forest-vegetation is known to have occurred in Denmark 

 during past millenia,^ yet one would be only partially correct were one to 

 seek to attribute this to a kind of rotation of crops practised by Nature, 

 and due to each species of tree impoverishing the soil in such a manner 

 as to render the soil less suitable for its own maintenance but more so 

 for that of other species. Certain it is that the annual removal of wood 

 from the soil withdraws some of the most essential plant-nutriment 

 potash for example. Where no forest-litter is taken away, one can trace 

 no deterioration in the soil. The exodus of nutrient substances may be 

 balanced by the advent of suitable salts derived from the weathering 

 of deeper layers of soil. But where the removal of forest-litter is 

 practised, easily satisfied species, e. g. Scots pine, replace species that 

 are more exacting in their demands, e. g. beech and oak. 



CHAPTER XX. THE ACTIVITY OF ANIMALS AND 



PLANTS IN SOIL 



Between the plant-life and the animal-life of a place there exists 

 an intimate and complex reciprocal relation, which expresses itself in 

 various ways, and promises biological results of the deepest interest. 

 Here we shall consider only two aspects of the matter : the effect on 

 the soil of the tunnelling by animals and the effect of saprophytic plants. 



Tunnelling of the soil by animals. 



Soil is traversed by many species of animals : terrestrial soil, particu- 

 larly by earthworms, larvae of insects, millipedes, wood-lice, ants, as 

 well as by animals, such as moles, which search for these ; marine soil, 

 by small Crustacea, Sedentaria or Tubicolae, and others. 



Terrestrial soil. The uppermost layer of soil in forest and field usually 

 consists of an intimate mixture of mineral matter, animal remains, and 



' See Sect. XVII, Chapter XCVI. 



