124 SUCCESSION 



of prairie and plains successions as well as of many arctic- 

 alpine ones. 



CAUSES AND REACTIONS. 



The initial cause of a succession must be sought in a 

 physical change in the habitat : its continuance depends up- 

 on the reaction which each stage of vegetation exerts upon 

 the physical factors which constitute the habitat. A single 

 exception to this is found in anomalous successions, where 

 the change of formation often hinges upon the appearance 

 of remote or foreign disseminules. The causes which initi- 

 ate successions have already been considered : they may be 

 summarised as follows: (1) weathering, (2) erosion, (3) ele- 

 vation, (4) subsidence, (5) climatic changes, (6) artificial 

 changes. The effect of succeeding stages of vegetation up- 

 on a new or denuded habitat usually finds expression in a 

 change of the habitat with respect to a particular factor, 

 and in a definite direction. Often, there is a primary re- 

 action, and one or more secondary ones, which are corol- 

 laries of it. Rarely, there are two or more coordinate re- 

 actions. The general ways in which vegetation reacts upon 

 the habitat are the following : (1) by preventing weathering; 

 (2) by binding aeolian soils ; (3) by reducing run-off and pre- 

 venting erosion; (4) by filling with silt, and plant remains; 

 (5) by enriching the soil; (6) by exhausting the soil; (7) by 

 accumulating humus; (8) by modifying atmospheric factors. 

 The direction of the movement of a succession is the im- 

 mediate result of its reaction. Prom the fundamental 

 nature of vegetation, it must be expressed in terms of water 

 content. The reaction is often so great that the habitat 

 undergoes a profound change in the course of the suc- 

 cession, changing from hydrophytic to mesopbytic or 

 xerophytic, or the reverse. This is characteristic of newly 

 formed or exposed soils. Such successions are xerotropic 

 (fj;joos, dry, tjoowikos turning), mesotropic, or hydrotropic, ac- 

 cording to the ultimate condition of the habitat. When the 

 reaction is less marked, the type of habitat does not change 

 materially, and the successions are xerostatic, (oraTucos 

 stopping) mesostatic, or hydrostatic, depending upoH the 



