XIII. PAST SUCCESSION: THE CENEOSERE. 

 GENERAL PRINCIPLES, 



Phylogeny and ontogeny. — ^Each climax formation has its individual or onto- 

 genetic development, as shown in its priseres and subseres, and sometimes 

 recorded in stases. In addition, it shows a phylogenetic development from 

 a preceding climax or community. In fact, the cUmaxes of the past are con- 

 nected in a phyletic series or line of descent in the same general fashion as the 

 component genera and species. The descent is more involved and more 

 difficult to retrace, since the climax is a complex organism of wide extent, and 

 the fossil record of it is very imperfect, except in the case of coal and peat 

 stases. Moreover, while a single well-preserved plant establishes the species 

 and genus clearly, an adequate outline of a geological climax requires a large 

 if not the major number of genera in both serai and climax units. However, 

 we do know the three great floras of the vegetation eras with steadily increas- 

 ing fulness. Our knowledge of the differentiation of these into regional 

 climaxes must likewise increase with the study of fossil horizons. Once 

 formed, a climax must have persisted as long as the flora to which it belonged, 

 though it may have moved back and forth over a wide area, as in the case of 

 the cUseres of the Permian and Pleistocene glacial periods. As the flora 

 became subordinate to a new one, as Bryophyta to Pteridophyta, the latter to 

 Gymnosperms, and these to Angiosperms, corresponding new climaxes 

 appeared. Whether each climax passed into a corresponding climax in the 

 new series is uncertain, but it is probable that the climatic differences which 

 produced cUmaxes out of an original homogeneous climax would have a similar 

 effect upon the new flora. The phylogenetic behavior of the community can 

 not well have been very different from that of the species. In the former, as 

 in the latter, we must expect relatively rapid differentiation into several 

 forms, i. e., climaxes, as well as purely Unear descent involving the disappear- 

 ance of the parent. Less marked differentiation must have produced frequent 

 divergence, and there must likewise have been occasional convergence of two 

 climaxes into one. 



Thus the conclusion seems unavoidable that the various coniferous climaxes 

 of the North American continent are all descendants of an original coniferous 

 climax of the later Mesophytic era. The boreal belt of conifers may perhaps 

 be regarded as the linear representative of this climax, though greatly impover- 

 ished as a result of repeated climatic changes and the consequent migrations. 

 By divergence arising out of the invasion of new climatic areas, the original 

 climax gave rise to the two closely related climaxes of the northeast and the 

 northwest. Each of these in turn seems to have a linear descendant, the one 

 in the southeastern pine forest, the other in the interrupted and often fragmen- 

 tary coniferous forests of Arizona, Mexico, and southern California. Finally, a 

 further differentiation has taken place in the Cascade, Sierra Nevada, and 

 Rocky Mountains in response to the climatic differentiation due to altitude. 

 This is perhaps best seen in the Colorado mountains, in which the coniferous 

 mass falls into three cUmatic climaxes, namely, (1) Pinus edulis-Juniperus, 

 (2) Pinus ponderosa-Pseudotsuga d<yuglasii-Abies concolor, (3) Picea engel- 

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