SUCCRSSION 241 



changed. The former is of universal occurrence and recurrence; the latter 

 operates upon relatively few ultimate formations. In the origin of normal 

 successions, nudation may be brought about by the production of new soils 

 or habitats, or by the destruction of the formation which already occupies a 

 habitat. In a few cases, the way in which the habitat arises or becomes de- 

 nuded is not decisive as to the vegetation that is developed upon it, but as a 

 rule the cause of nudation plays as important a part in the development of 

 a succession as does the reaction exerted by the invaders. The importance 

 of this fact has been insisted upon under invasion. New soils present ex- 

 treme conditions for ecesis, possess few or no dormant disseminules, and in 

 consequence their successions take place slowly and exhibit many stages. 

 Denuded soils as a rule offer optimum conditions for ecesis as a result of 

 the action of the previous succession, dormant seeds and propagules are 

 abundant, and the revegetation of such habitats takes place rapidly and 

 shows few stages. The former may be termed primary succession, the latter 

 secondary succession. 



PRIMARY SUCCESSIONS 



290. These arise on newly formed soils, or upon surfaces exposed for the 

 first time, which have in consequence never borne vegetation^ before. In 

 general they are characteristic of mountain regions, where weathering is the 

 rule, and of lowlands and shores, where sedimentation or elevation con- 

 stantly occur. The principal physical plienomena which bring about the 

 formation of new soils are: (i) elevation, (2) volcanic action, (3) weather- 

 ing, with or without transport. 



291. Succession through elevation. Elevation w^as of very frequent oc- 

 currence during the earlier, more plastic conditions of the earth, and the suc- 

 cessions arising as a result of it must have been important features of the 

 vegetation of geological periods. To-day, elevation is of much less impor- 

 tance in changing physiography, and its operation is confined to volcanic 

 islands, coral reefs, and islets, and to rare movements or displacements in 

 seacoasts, lake beds, shore lines, etc. There has been no investigation of the 

 development of vegetation on islands that are rising, or have recently been 

 elevated, probably because of the slow growth of coral reefs and the rare 

 appearance of volcanic islands. On coral reefs, the first vegetation is in- 

 variably marine, but as the reef rises higher above the surf line and the tide, 

 the vegetation passes into a xerophytic terrestrial type adapted to an imper- 

 vious rock soil, and ultimately becomes mesophytic. In volcanic islands, 

 unless they are mere rocks over which the Avaves rush, the succession must 

 always begin with a xerophytic rock formation. The best known example 

 of a rising coast line is found in Norway and Sweden, where the southeast- 



