282 PAST CLIMATES AND CLIMAXES. 



doubtless mosses and liverworts also, in spite of their infrequence in the record. 

 Although grasses, sedges, and hydrophytic angiosperms were absent in the 

 Paleophytic and Mesophytic eras, there is much evidence that their r61e in 

 succession was taken by cycads and pteridophytes. The ecesic causes or 

 processes, viz, aggregation, migration, ecesis, competition, invasion, and 

 reaction, must have been universal, and their action essentially the same then 

 as to-day. The basic reactions of communities upon their habitats must 

 have affected water-content in the normal way, causing extremes of wetness or 

 dryness to approach the climatic mean. 



The geological record makes it clear that each era had its dominant life- 

 forms, which were characteristic of climax vegetations. While climate was 

 usually more uniform, there is increasing evidence of the existence of great 

 changes of climate, and of cold or dry areas. These must have resulted in 

 the sorting of species into major and minor zones much like those of to-day, 

 and these formations must have exhibited climax and developmental units not 

 very different from those of the present. It seems clear that the forests of the 

 past showed layers, and that these were continued beyond the forest to form 

 scrub or thicket. Even seasonal changes, with their corresponding effect upon 

 vegetation, appeared relatively early. The great changes induced by glacia- 

 tion in Devonian and Permian times must have had effects similar to those 

 established for the Pleistocene, though the differentiation of life-forms and 

 hence of zones could hardly have been so marked. At any rate, the develop- 

 ment of the eosere must have showed seres, coseres, and cliseres in their essen- 

 tial relation. However, for the great change of flora and vegetation by which 

 the eosere of one era passed into that of the next, we have no parallel in modem 

 vegetation, or, more accurately perhaps, the parallel can not be discerned 

 because we are in the midst of its extremely slow operation, if indeed a similar 

 evolution is in process to-day. At present we have no evidence as to the 

 causes which brought about the changes in dominance from fernworts to gym- 

 nosperms and then to angiosperms, or as to the manner in which the change 

 was effected. This is due largely to the imperfection of the record, but also to 

 the absence of any convincing proof as to the manner of evolution of existing 

 species and genera. It is practically certain that the next few decades will 

 bring a great accession of knowledge in both these fields, and the developmental 

 continuity of the geosere can then be completed, in its larger expressions at 

 least. 



The appHcation of the two methods sketched above is of the simplest. 

 Every fact discovered, whether climatic, topographic, geological, botanical or 

 zoological, has its place in the causal sequence, as well as in the successional 

 development. The place in each once determined, it is a comparatively simple 

 task to ascertain its correlations with both antecedent and subsequent facts 

 or processes. The most critical part of the analysis, however, consists in 

 coordinating and harmonizing the evidence thus obtained. If the evidence 

 as to climate and topography drawn from the known facts of the paleobiotic 

 record agrees with the facts as to climate and topography revealed by the 

 geological record, the causal sequence is complete and trustworthy. The 

 converse must likewise be true, that is, the presence of highly ferruginous 

 rocks and of beds of gypsum and salt is thought to indicate a dry period, and 

 hence a dry region and a xerophytic vegetation. However plausible the 

 geological evidence, such a conlusion can be but tentative until the study of 



