PALEIC INDICATORS. 101 



made by the developmental study of vegetation during the last twenty years. 

 When successional studies become the rule in zoo-ecology as well, there will 

 seem to be no Umit to the increasing perfection of detail in picturing the rise 

 and fall of past populations and conmiunities. In the case of vegetation, this 

 method has already gone so far as to bring conviction that all the essential 

 features of successional processes and climax communities as seen to-day 

 already existed in the past. 



"As indispensable corollaries of the methods of phylogeny and succession 

 are inferences from distribution in space and in time, and from association. 

 The former enables us to close many a gap in the fossil record and to fill in the 

 areas outlined by the known distribution of dominants. Inference from asso- 

 ciation, for example, aided by phylogeny, makes it all but certain that swamps 

 of reed-grass, bulrushes, and cattails existed as far back as the Cretaceous, 

 though Phragmites is the only one of the three dominants recorded for that 

 period. The most recent is the method of cycles, which gives promise of becom- 

 ing one of the most important. It is perhaps too soon to insist that cyclic 

 processes are universal in time and in space; but the great mass of evidence 

 from geology and climatology is matched by an increasing body of facts from 

 biological succession." 



Kinds. ^The indicator values of a fossil plant or animal clearly depend 

 upon the accuracy of its identification and stratigraphic position. With 

 reference to the former, its generic position, together with the vegetation- 

 form and habitat-form, is of paramount importance, partly because speciflc 

 determinations are often very uncertain among plants at least, and partly 

 because the majority of genera are uniform as to the ecological type of their 

 constituent species. While definite stratigraphic allocation is necessary for 

 finer analysis, the assignment of a plant to a particular era or period has much 

 value, owing to the fact that many dominant genera persist throughout most 

 or all of an era. The indicator value also depends greatly upon whether the 

 plants were fossilized in position and hence in their conmiunity relations, or 

 whether they have been scattered and carried to points more or less remote 

 from their home. The distinction between the corresponding deposits, termed 

 sta-ses and strates, is discussed at some length in "Plant Succession" (291). 

 Here it wtII suffice to point out that the water stase as exempUfied in peat- 

 bogs has nearly the complete indicator values of an existing sere, while those 

 of the much more universal strate are usually incomplete and subject to 

 interpretation. 



It is evident that fossil plants, and animals also to a lesser degree, may 

 serve as indicators of factors, processes, or practices, in essentially the same 

 way that existing species do. Practice indicators are naturally connected 

 with the presence of man, and hence are restricted to the Pleistocene and 

 Recent periods. Grazing must have been the earliest of these, perhaps reach- 

 ing back into the late Pleistocene, but agriculture was relatively well-advanced 

 by the time of the Lake-dwellers, and construction, as well as a crude sort of 

 forest utilization, was at least begun. Moreover, it must be recognized that, 

 while grazing took on some new features as herds came under the control of 

 man, it must have existed as a natural process throughout the Tertiary at 

 least. In the case of fire, this agency must have begun its modifying influence 

 upon vegetation as early as the Paleozoic, but its effect must have greatly 

 increased with the differentiation of deciduous forest and grassland in early 



