168 BOTANICAL GAZETTE [MARCH 
While most observers very properly have paid chief attention to 
the actual facts of succession rather than to their underlying causes, 
a scrutiny of past results shows very clearly that the phenomena 
considered have differed greatly in kind. Obviously the phenomena 
of bog development, as observed by WiLt1Am KIN«, had to do with 
a succession in which the activities of the plants themselves played 
the leading part; the humus accretions of the bog plants, such as 
the peat moss Sphagnum, made possible the development of another 
vegetation on a higher soil level. In a comparable manner, the 
successions observed by Burron, by BiBERG, and by DUREAU DE 
LA Matte had to do with plant activities; the forest trees of a 
given generation cast the shade necessary for the development of 
other trees which need shade rather than light for their development; 
BrwerG’s lichens accumulated a soil which made possible the 
development of higher vegetation on rock surfaces. STEENSTRUP, 
however, in his study of the fossils, introduced to the scientific 
world a new kind of succession phenomena, for in his elucidation of 
the postglacial history of Denmark there were recorded changes of 
broader significance than those hitherto observed; it was clear that 
the transition from the tundra vegetation through the birch and 
pine vegetation to the oak and beech, as developed by him and by 
his student VAUPELL, was a record of climatic change, inasmuch 
as the very same horizontal succession may be observed today in 
journeying from northern Scandinavia to Denmark. A third and 
equally diverse kind of succession phenomena was recorded by 
REISSEK in his study of the islands in the Danube, for here there 
was clearly recognized the influence of physiographic change 0? 
vegetation. Thus in succession we may distinguish the influence 
of physiographic and of biotic agencies. The physiographic 
agencies have two aspects, namely, regional (chiefly climatic) am 
topographic. 
4. Regional successions 
Regional successions are so slow in their development that they 
can be studied almost alone by the use of fossils. Hence the experi- 
mental method, which has proven so potent in unraveling many # 
biological tangle, is here of no avail. It is not strange, therefore, 
