RECENT INVESTIGATIONS. 21 



Middendorff (1864:641) considered the succession of dominants to be the 

 exception rather than the rule in the case of burn forests in Siberia, and 

 explained the cases in which other species replaced the original forest domi- 

 nants as due to the influence of man. Kabsch (1865 : 75) pointed out the pri- 

 mary rdle of lichens in succession on rock surfaces: 



"Lichens are the real pioneers in vegetation; they corrode the hardest 

 basalt as they do the softest limestone, decompose the rock, and mix its 

 particles with their own remains, in such a way as to give opportunity for the 

 growth of a higher vegetation." 



Engler's pioneer work (1879) upon the developmental history of vegetation 

 deals primarily with the geological development and the relationship of floras, 

 but has little bearing upon succession. Nathorst (1870, 1873) was the first 

 to demonstrate the presence of arctic plants, Salix herbacea, S. polaris, S. 

 reticulata, and Dryas octopetala, in beds of postglacial clay in southern Sweden. 

 These and other arctic species were also found at the bottom of moors in See- 

 land. Nathorst discovered Betula nana, Salix retusa, S. reticulata. Polygonum 

 viviparum, and Loiseleuria procumbens in layers resting directly upon glacial 

 deposits in Switzerland. Salix polaris was also found under the glacial 

 boulder clay at Cromer in England, and various other willows between the 

 clay and the "forest beds." His later papers are abstracted in Chapter XIII. 



RECENT INVESTIGATIONS. 



Blytt, 1876.— Blytt (1876, 1881) advanced the theory that since the glacial 

 period the climate of Norway has undergone secular changes in such fashion 

 that dry periods of continental climate have alternated with moist periods 

 of insular or oceanic climate, and that this has happened not once but repeat- 

 edly. As long as land connections permitted a mass invasion, continental 

 species entered diu"ing one period and insular species during the other. This 

 theory is supported by investigations of the peat-beds of Norway, the oldest 

 of which have an average depth of 16 feet. They consist of four layers of 

 peat with three intervening layers of remains of rootstocks and forests. The 

 surface of the drier moors is more or less completely covered with heather, 

 lichens, and forest. With increasing moisture, forest and heath disappeared, 

 and were replaced by moor, while at the same time species of Sphagnum domi- 

 nated the wetter places almost wholly. The root layers, on the other hand, 

 represent periods when the moor was drier than formerly, and during which 

 peat formation probably ceased for thousands of years, to begin again later. 

 In the oldest moors there are traces of three such dry periods, and such moors 

 are often covered to-day with forests for the fourth time. 



The explanation of such changes has been sought in local causes, but Blytt 

 is convinced that it lies in the alternation of dry and wet periods. When the 

 rainfall and humidity changed, the surface of the moor must have become 

 drier or wetter in consequence, and have produced the vegetation found in the 

 alternating layers of peat and forest remains. The absence of forest beds in 

 the wet moors, and their presence only in the dry ones, seem to indicate that 

 this has not been produced by local causes, The moors of Norway are at 

 present drier than formerly, and are mostly covered with forest or heath, while 

 the Sphagnum layer just below the surface indicates that the period just pre- 



