82 CARNEGIE INSTITUTION OF WASHINGTON. 



ment begins with a bare area — rock, water, or soil — and progresses 

 slowly but inevitably to a climax. The latter is regarded as the organic 

 unit of vegetation and hence is designated by the term "formation." 

 The climax or formation is a mature or adult form, the development of 

 which is seen in the successional sequence. The nature and perma- 

 nence of the climax are determined by climatic control. As a conse- 

 quence, each formation persists until an effective change of climate 

 causes its disappearance in the old region with its concomitant inva- 

 sion into a new one, or until it gives rise to a new flora through evolution. 

 Such changes are phylogenetic in nature, and are of the first importance 

 in unraveling the successions of the geological past. Thus, as in the 

 case of the individual plant, ontogeny in vegetation comprises the 

 periodic reproductive process of the formation as an organism under the 

 same climate, while phylogeny deals with the change of one cHmax 

 into another, or its differentiation into two or more under the stress 

 of the changing climate. 



The phylogenetic study of plant formations, i. e., the course of suc- 

 cession in the geologic past, has been made possible only by the recent 

 great advances in chmatology. The existence and recurrence of 

 climatic cycles has been estabhshed beyond question, and it has proved 

 possible to recognize a complete series of such cycles from the familiar 

 annual one tlirough sun-spot and deformation cycles of varying dura- 

 tion and intensitj' to the grand deformation cycles extending over 

 millions of years. Each of these has an appreciable effect upon vegeta- 

 tion, but the major cycles alone are able to produce phylogenetic 

 changes. The phylogenetic events of the first importance in the 

 history of vegetation are those recorded in the evolution of new floras 

 and hence of chmax units, characteristic of the four great vegeta- 

 tion eras, viz, Eophytic, Paleophytic, Mesophytic, and Cenophytic. 

 Hence, it is possible to recognize four primary periods in the terrestrial 

 history of vegetation and to divide the geosere, which comprises the 

 whole course of succession on the globe, into corresponding eoseres, 

 namely, Paleosere, Meseosere, and Ceneosere. The flora of the first 

 was pteroid, and it seems probable that several climax formations were 

 already differentiated, corresponding to the respective dominance of 

 Cordaites, Lepidodendrese, and Calamites. The dominants of the 

 Meseosere were gymnospermous, and the number of climaxes seems 

 to have increased with cUmatic differentiation. The Ceneosere was 

 initiated by the change from gymnosperms to angiosperms, and the 

 course of the succession began to approximate that which is seen to-day. 

 The eosere of the Eophytic period is v/holly hypothetical, but the con- 

 clusion is unavoidable that it was marked by cHmaxes of a bryophytic 

 or pteridophytic nature. 



Next in importance to the phylogenetic changes which character- 

 ized each era were those produced by the glacial-interglacial cycles of 



