GENERAL PRINCIPLES. 349 



Correlations of seres. — ^At one end of the series, geosere and eosere bear a 

 definite relation to each other; at the other end, eosere and sere have a simi- 

 larly fixed relation. The geosere consists of eoseres, the eosere of seres, 

 varying in the number of primary and secondary ones. No such fixed relation 

 exists between the eosere and cUsere, or between the eosere and the eosere. 

 The clisere, on the other hand, is distinctly related to the eosere. It marks 

 within the latter the development which recurs whenever a change of climate 

 produces a shifting of climax zones. It corresponds essentially to periods of 

 glaciation, and perhaps of marked coohng as well, and hence there must have 

 been at least one clisere in each of the four vegetation periods. In any region 

 with the same climax throughout an era, the eosere would be nearly contem- 

 poraneous with the eosere. In boreal and temperate regions, where there is a 

 shifting of zones, the eosere of the original climax is complicated by the super- 

 position of new climaxes. The latter might also have shorter eoseres of their 

 own if the glacial-interglacial cycle were sufficiently long. It is readily seen 

 that the eosere exists in several forms, aU of which will be recognized in the 

 ultimate analysis. For the present, it seems sufficient to distinguish the 

 simple eosere, consisting of priseres and subseres, and the cliseral eosere, in 

 which successive climaxes occur. 



Processes and principles of past succession. — ^There appears to be ample 

 warrant for the conclusion that the processes of vegetation, like those of 

 geology and climate, were essentially the same in the past as at present. It 

 would be more exact, perhaps, to say that they are continuous, the processes 

 which began in early Paleophytic times continuing through the various periods 

 into the present. Thus, while the materials of succession varied from era to 

 era, the processes were identical for the most part. In fact, there was essen- 

 tial identity of materials as to the life-forms after the Devonian, the differences 

 being confined to the genera and species. The sole outstanding process of 

 the past which is lacking at present is the evolution of a new flora, such as 

 marks each eosere. It would seem, however, that this is probably a conse- 

 quence of perspective. It is certain that slow but universal evolution is 

 taking place at present, and it seems probable that this will result in the 

 increasing dominance of grasses and composites. In short, the clue to suc- 

 cession in the past lies in the basic assumption that the processes were identical 

 unless there is convincing evidence to the contrary. No evidence of this sort 

 is available as yet, and the detailed appUcation of suecessional principles to 

 the past strengthens the conviction as to the continuity of developmental 

 processes in vegetation. 



During the geological eras, succession must have been as universal, and 

 primary succession at least, as frequent as at present. Subseres are so largely 

 due to human activities that they are vastly more typical of the present than 

 of any other period in the history of the earth. It is not improbable that 

 Cambrian and Silurian-Devonian glaciation produced some differentiation of 

 vegetation into climaxes, and the Permian must certainly have done so. 

 Hence it seems reasonable to infer that climaxes or formations have charac- 

 terized vegetation from the Devonian to the present. Such formations are 

 also to be regarded as complex organisms, with a development and structure 

 such as we study to-day but with necessarily fewer stages and parts in 

 Eophytic times. The process of development or reproduction was succession 



