EARLY INVESTIGATIONS. 19 



entrance of Sphagnum, or of Nardus, Sdrpus caespitosus, Azalea procumbens, 

 Empetrum nigrum, etc., which form peat, often a meter deep. The last plants. 

 Azalea and Empetrum, finally disappear and the naked peat alone remains, to 

 be again colonized when soil is drifted upon it by the wind. 



Mvller, 1878-1887.— Mviler (1878, 1884, 1887) made a critical investiga- 

 tion of the hmnus soils of beech and oak woods and of heath, which is of the 

 first importance for the study of the factors which affect invasion and replace- 

 ment in forests. The soil of the beech forest is distinguished as of two types. 

 In the first, the surface is covered with a layer of leaves and twigs which build 

 an incoherent mass. This covers the upper soil, which consists of loose earth, 

 and is 3 to 6 feet or more deep. Sometimes the entire upper soil is dark gray- 

 brown, but frequently only the uppermost layer has this color. The latter 

 is then called mull. It has a characteristic ground-cover of Asperula, Mercuri- 

 alis. Milium, Melica, Stellaria, Anemone, etc. It is defined by Muller as 

 follows: "Beech mull is a loose incoherent layer of converted leaves, twigs, 

 etc., of the beech forest, rich in animal life and with the organic material 

 intimately mixed with the mineral earth." In the second type, the leaf litter 

 is lacking. The soil is firm, filmy, and absorbs rain like a sponge. The upper 

 part is composed of a tenacious brown-black layer of humus. The vegetation 

 consists characteristically of Aira, Trientalis, Maianthemum, Potentilla, etc., 

 and many mosses. The beech thrives poorly in contrast with its growth in 

 beech mull, and the old trees are mostly in a pathological condition. Beech 

 turf is regarded as consisting of a leaf-mold of the beech woods which is poor 

 in animal life; it is miited into a firm peat by roots and by a very persistent 

 mycelium. It is significant that the reproduction of beech upon muU is easy, 

 while upon peat young trees can not come to maturity. This indicates that 

 the peat was formerly clothed with mull. If a beech forest upon mull is com- 

 pletely cut off so that no beech peat is naturally formed, there appears another 

 vegetation which in its turn prepares the soil and opens the way for other 

 forms. The mull may retain its essential character or may be converted into 

 heath peat. After the destruction of beech forest upon beech peat, no new 

 forest can appear, as a rule, but the soil is densely clothed with Aira flexuosa, 

 and the peat layer is finally destroyed by the grass. In similar thorough 

 fashion, the author considered the soil of oak woods and of heath in reference 

 to the changes in them which affect the succession. 



Other investigations. — From 1802 to 1885, when Hult's classic work upon the 

 developmental study of vegetation was published, there appeared a large num- 

 ber of works in which succession was treated more or less incidentally. These 

 dealt mostly with peat-bogs, or with succession after fire or distiu-bance by 

 man. Among the former were the important monographs or handbooks of 

 Eiselen (1802), Dau (1823), Wiegmann (1837), Lesquereux (1844), Grisebach 

 (1845), Vaupell (1851), Lorenz (1853, 1858), Pokorny (1858, 1860), and Senft 

 (1861, 1862). The others may be mentioned briefly here. De CandoUe 

 (1820:27) mentioned the cultures on the dunes of the "Landes," in which the 

 rapidly growing Genista, after having served as cover for seedlings of Pinus, 

 was finally driven out by the latter. Lund (1835) and Reinhardt (1856) 

 studied the origin of the BraziUan campos or savannahs, the former concluding 

 that they had been derived from forest as a consequence of fire, while the 

 latter regarded the effect of fire as secondary. Berg (1844) studied the sue- 



