SUCCESSION 261 
often throw back also the sediment that has been deposited in the sea, the 
‘marsh vegetation acting as a filter in both cases. Successions of the kind 
indicated above are regularly mesotropic. Where the soil is sandy, and the 
filling-up process sufficiently great, or where salts or humus occur in excess, 
xerophytic formations result. In certain cases, these successions appear to 
be permanently hydrostatic, changing merely from floating or submerged to 
amphibious conditions, but this is probably due to the slowness of the reac- 
tion. As a rule, the accumulation of plant remains is relatively slight, and 
Fig. 67. Pine forest formation (Pinus-xerohylium), stage VI of the 
talus succession. 
plays an unimportant part in the reaction. In peat bogs and other extensive 
swamps, the amount of organic matter is excessive, and plays an important 
role in the building up of the swamp bed. 
320. Succession by enriching the soil. This reaction occurs to some de- 
gree in the great majority of all successions. , The relatively insignificant 
lichens and mosses produce this result upon the most barren rocks, while 
the higher forms of later stages, grasses, herbs, shrubs, and trees, exhibit it 
in marked progressicn. The reaction consists chiefly in the incorporation 
of the decomposed remains of each generation and each stage in the soil. A 
very important part is played by the mechanical and chemical action of the 
