SUCCESSION 249 



mesophytes of meadows, or dissophytes of sandbars. In the case of ponds 

 and pools, the process of washing-out or silting-up merely removes or de- 

 stroys the vegetation, without effectively modifying the habitat, and the sec- 

 ondary successions that follow are extremely short. 



302. Succession by subsidence. Subsidence is a factor of the most pro- 

 found importance in changing vegetation. It operates over vast areas 

 through immense periods of time. For these reasons, the changes are so 

 slow as to be almost imperceptible, and the resulting successions can be 

 studied only in the geological record. Extensive subsidence is confined to- 

 day to coastal plains, as in Greenland, the south Atlantic coast, and the region 

 of the Mississippi delta, where its effects are merged with the paludation of 

 tidal rivers, and the wave and tide erosion of the sea shore. Such succes- 

 sions are unique, inasmuch as the denuding force operates very slowly in- 

 stead of quickly, and the first pioneers of the new vegetation appear before 

 the original formation has been destroyed. In all cases, the succession is 

 from mesophytic or halophytic formations to paludose, and, finally, marine 

 vegetation. In small areas of subsidence, such as shore slips along lakes 

 and streams, sink holes, and sunken bogs, the succession is usually both short 

 and simple, mesophytes giving place to amphibious and ultimately to aquatic 

 forms. 



303. Successions in landslips. Landslips occur only in montane and hilly 

 regions, and here they are merely of local importance. In many respects, 

 they are not unlike talus ; they show essential differences, however, in that 

 they are not sorted by gravity, and in that they destroy vegetation almost 

 instantly. The succession arises as a rule, not upon the original soil, but 

 upon that of the landslip, and, as pointed out elsewhere, might well be re- 

 garded as primary. 



304. Succession in drained, or dried soils. In geological times, the subsi- 

 dence of barriers must often have produced drainage and drying-out, just as 

 elevation frequently resulted in flooding and lake formation. At the present 

 time, the drying-out of lakes and ponds is the result of artificial drainage, or 

 of climatic changes. The former will be considered under successions 

 brought about by the agency of man. Climatic changes when general oper- 

 ate so slowly that the stages of such successions are perceptible only when 

 recorded in strata. More locally, climate swings back and forth through a 

 period of years, with the result that in dry years the swamps and ponds of 

 wetter seasons are dried out, and the vegetation destroyed or changed. If 

 the process be gradual, the succession passes from hydrophytic through am- 



