SUCCESSION 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 
tule, and of lowlands and shores, where sedimentation or elevation. con- 
stantly occur. The principal physical phenomena which bring about the 
formation of new soils are: (1) elevation, (2) volcanic action, (3) weather- 
ing, with or without transport. 
291. Succession through elevation. Elevation was 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 waves 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- 
