CHAP, xcvi SUCCESSION OF VEGETATION 361 



Fruticose lichens first appear, together with some dwarf-shrubs, so that 

 lichen-heath arises. At a later stage both the former and the latter 

 become sickly and perish ; at the same time greyish-white patches of 

 Lecanora tartarea show themselves and gradually cover everything with 

 their brittle fissured crusts, out of which project puny shoots of Empetrum, 

 Vaccinium Myrtillus, Ledum, and others. In various parts of Lapland 

 the highest parts of the undulating covering of mosses are shrouded in 

 these crusts. Development is not always concluded at this stage ; for 

 the buried plants gradually decompose and become earth-like, and the 

 incrustation of Lecanora loses its firm attachment. Fissures produced by 

 frost or drought provide points of attack for the wind, and the incrusta- 

 tions are disintegrated. The black peat then lies open to colonization by 

 any plants ; but the cohesion of its particles is too slight to permit of 

 vegetation making permanent settlement. Storms rage without ceasing 

 among the loose masses, excavate large hollows, just as in sand-dunes, 

 and drifting dust results. 1 At the bottom and sides of the hollows, which 

 often reach down to the original moraine-soil, a new type of vegetation 

 may then establish itself. 2 



Several examples have been described in the foregoing paragraphs 

 showing that a leading part is played by the depth of the water-table, or 

 the level to which the water present can rise. Too great stress cannot be 

 laid on the fact that the amount of water in soil is of the most profound 

 importance, and that extremely slight, almost imperceptible, differences 

 in that amount often exercise a decisive influence. 3 



The examples given have demonstrated transitions from hydrophytic 

 to mesophytic or xerophytic communities. The reverse course of develop- 

 ment may be seen when for any reason the amount of water in soil increases 

 for instance, by the damming of a stream or brook by dunes, by block- 

 ing an outlet, and so forth. According to Blytt's theory, 4 dry and humid 

 periods of great length alternate, and to these correspond the alternating 

 strata of tree-trunks and moss the trees growing on the moor in the dry 

 periods, and the mosses dominating and suppressing the forest during 

 the moist periods. Facts militating against Blytt's theory have already 

 been mentioned. 5 



The great moors of northern Germany are regarded as having arisen 

 after the conversion of the large tracts of forest into marsh. In Sweden 

 the conversion of forest into swamp and its devastation by Sphagnum 

 are still frequent. 6 In North America the habitations constructed by 

 beavers are allowed to occasion floods : this provides an example of the 

 intervention of animals. 



The developmental succession on rocks is well known. First, bare 

 rock clothes itself with algae and crustaceous lichens ; these prepare 

 a substratum suitable for fruticose lichens (cladinetum, and so forth) or 

 for communities of mosses ; in the more or less thick cushion of the last- 

 named some Phanerogamia occur ; and then a callunetum may develop, 

 and perhaps finally a coniferous forest. 7 



All other changes taking place in the conditions prevailing in any 

 habitat will have like results, that is to say, will induce changes in the 

 vegetation by putting certain species in a position to suppress other older 



1 See pp. 61, 208. * See Cajander, 19046, 19056. ' See p. 44. 



* Blytt, 1882. * See p. 203. * See A. Nilsson, 1897. ' Seep. 242. 



