EARLY INVESTIGATIONS. 17 



scattered willows, most frequently Salix purpurea. The willows became 

 bushy and caught the water-borne sand, building hummocks which gradually 

 united to form a sandy level 6 to 8 feet above the gravel. The willows them- 

 selves came to be half-buried in the sand. All other invaders established 

 themselves in the sand among the willow crowns. They entered in a definite 

 succession, so that a sequence of stages results, each with its characteristic 

 woody plants. Salix purpurea, S. riparia, and Myrica germanica belonged 

 solely to the first stage. The second stage consisted of Alnus incana, Populus 

 alba, and Cornus sanguinea, and the last stage of Fraxinus excelsior, Ulmus 

 campestris, Acer campestre, Qvsrcus pedunculata, Pirus malus, P. communis, 

 etc. High water and drift-ice often destroyed young islands entirely or 

 partly, exposing the gravel-bank on which the sere might be repeated. Par- 

 tial destruction of the sandy plain permitted the development to begin again 

 in new areas alongside of those in later stages. The pioneer willows died off 

 as soon as the trees of the second stage developed much shade, a fate which 

 also overtook the groups of Phragmites which occurred among the willows. 



Vaupell, 1857. — In discussing the invasion of the beech into Denmark, 

 Vaupell (1857:55) reviewed the evidence obtained from submerged forests, 

 deposits of calcareous tufa, and peat-bogs. The ancient forests of Denmark, 

 and especially of Jutland, were a mixture of coniferous and deciduous species. 

 Betula was the most commaon, with Quercus and Pinus silvestris next in impor- 

 tance; the aspen, willow, hazel, elm, and maple played but a secondary part. 

 In existing forests Fagus is the universal dominant. Since remains of the 

 beech are lacking in peat, tufa, and in the submerged forests, Vaupell con- 

 cluded that it had entered Denmark at a subsequent time. In seeking an 

 explanation of the change of dominance, he cited the opinions of Dureau de la 

 MaUe, Laurent, and Cotta in favor of the natiu-al "alternation of essences," 

 but reached the conclusion that it must be produced by other causes than the 

 exhaustion of the soil. Where the beech invades forests of birch, it gains the 

 upper hand by overshadowing the birch trees, suppressing them and causing 

 their death. The birch fails also to reproduce because its seedHngs do not 

 thrive in the dense shade of the beech. The plantations of pine are likewise 

 invaded by the beech with similar results, unless protected by man. In the 

 cases where beech has yielded to pine, the explanation is always to be found 

 in intervention by man. The author concluded that the beech had migrated 

 from its center in France and Germany during the present geological period, 

 establishing itself wherever the soil became drier or richer, and dispossessing 

 the birches and pines everywhere but in marshy or sterile soil. 



von Post, 1861. — von Post (1861) appears to have been the first to give a 

 complete and detailed account of the reactions by which plants and animals 

 produce soils. Ramann (1888) has summarized his work upon the copro- 

 genous formation of the various biogenous soils. Muck (Schlamm, gyttja) 

 consists of plant fragments, including diatom shells. It forms very elastic 

 masses which are deposited on the bottom in waters, springs, brooks, lakes, 

 etc. Muck is formed by the deposition of insect excreta, together with the 

 remains of dead infusoria, Crustacea, and insects, diatom shells, and algae. 

 Such muck deposits are often found beneath peat moors; "Lebertorf" is a 

 kind of fossil muck. Moor soil is deposited more rapidly than muck in waters 

 colored brown by humus material. Moor soil consists of a dark-brown, soft 



