SECONDARY SUCCESSIONS 118 



dunes have received much attention, with the result that 

 their successions are the most thoroughly known of all. 

 Prairie and steppe formations are probably to be regarded 

 as the ultimate staees of successions established on wind- 

 borne loess, and it is possible that the same is true of sand- 

 hill vegetation in the prairie province. 



7. Succession in glacial soils. The formation of 

 glacial deposits is at present confined to alpine and arctic 

 regions. Recent successions in such soils are localised in 

 these regions, and are in consequence relatively unimport- 

 ant. There can be little question, however, that the thor- 

 ough investigation 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 allu- 

 vial cones represent the various kinds of glacial deposits. 

 They agree in being heterogeneous in composition, and are 

 covered today with ultimate stages of vegetation, except in 

 the immediate vicinity of glaciers. 



SECONDARY SUCCESSIONS 



Generally speaking, all successions on denuded soils are 

 secondary. "When vegetation is completely removed by 

 excessive erosion, it is an open question whether the result- 

 ing 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, moreover, especially characteristic of 

 newly formed soils, and, in studying succession 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 success- 

 ions 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. 



8. Succession in erodedsoils. Eroded soils show 

 considerable differences, as they arise in consequence of 



