SUCCESSION 247 



that their successions are the most thoroughly known of all. Prairie and 

 steppe formations are probably to be regarded as the ultimate stages of suc- 

 cessions established on wind-borne loess, and it is possible that the same is 

 true of sand-hill vegetation in the prairie province. 



298. Succession in glacial soils. The formation of glacial deposits is at 

 present confined to alpine and arctic regions. Recent successions in such 

 soils are localized in these regions, and are in consequence relatively unim- 

 portant. There can be little question, however, that the thorough investiga- 

 tion of succession in and near the moraines of existing glaciers will throw 

 much light upon the successions of the glacial period. Moraines, drumlins, 

 eskars, and alluvial cones represent the various kinds of glacial deposits. 

 They agree in being heterogeneous in composition, and are covered to-day 

 with ultimate stages of vegetation, except in the immediate vicinity of 

 glaciers. 



SECONDARY SUCCESSIONS 



299. Generally speaking, all successions on denuded soils are secondary. 

 When vegetation is completely removed by excessive erosion, it is an open 

 question whether the resulting habitat is to be regarded as new or denuded. 

 Erosion is rarely so extreme and so rapid, however, as to produce such a 

 condition, even when it results from cultivation or deforestation. It is, more- 

 over, especially characteristic of newly formed soils, and in studying succes- 

 sion in eroded habitats, it is fundamentally important to determine whether 

 erosion has produced denudation, or has operated upon a new soil. The 

 great majority of secondary successions owe their origin to floods, animals, 

 or the activities of man, and they agree in occurring upon decomposed soils 

 of medium water-content, which contain considerable organic matter, and a 

 large number of dormant migrants. These successions consist of relatively 

 few stages, and are rarely of extreme character. 



300. Succession in eroded soils. Eroded soils show considerable differ- 

 ences, as they arise in consequence of erosion by water or by wind, though 

 the initial stages of revegetation derive their character more from the aggre- 

 gation of the soil than from the nature of the erosive agent. Eroded soils 

 are as a rule xerophytic. In the case of erosion by water, dysgeogenous soils 

 are readily worn away in consequence of their lack of cohesion, as in sand- 

 draws, etc., while eugeogenous soils are easily eroded only on slopes, as in 

 the case of ravines, hillsides, etc. In the former, the extreme porosity and 

 slight capillarity of the sand and gravel result in a low water-content. In 

 the finer soils, the water-content is also low, on account of the excessive run- 



