144 STRUCTURE AND UNITS OF VEGETATION. 



is well understood, but it must always remain largely a matter of speculation. 

 It is only in the case of fairly complete records, such as those of peat-bogs, 

 that the actual origin of a climax formation can be traced. From their very 

 nature such formations are dependent upon climate. This fact furnishes the 

 best basis for a natural classification at present. In this coimection it is 

 instructive and convenient to group the climaxes of similar climates together, 

 as, for example, the plains of America and the steppes of Eurasia. Such a clas- 

 sification emphasizes the essential relation of climax and climate, but is not 

 necessarily genetic. Such a genetic or developmental classification can be 

 based at present only upon the regional relation of climaxes, as indicated in 

 Chapter IX. A system of this sort is suggested by the regional classification 

 of Clements (1905:304), in which (1) lowland, (2) midland, (3) upland, 

 (4) foot-hill, (5) subalpine, (6) alpine, and (7) niveal formations correspond 

 closely to a similar series of climaxes, namely, (1) deciduous forest, (2) prairie, 

 (3) plains, (4) scrub, (5) montane forest, (6) alpine grassland, (7) lichen and 

 moss tundra. A similar relation exists in the case of continental zones of tem- 

 perature (I. c, 283), the (1) polar-niveal, (2) arctic-alpine, (3) boreal-subal- 

 pine, (4) temperate, (5) subtropical, and (6) tropical zones, corresponding 

 essentially to as many climatic climaxes, more or less interrupted by the super- 

 imposed series indicated above. 



The sequence of climates and climaxes in either of the above series indicates 

 the course of development in the event of any normal climatic change. If the 

 climate of the Mississippi basin becomes drier, prairie will encroach upon and 

 replace deciduous forest, and the plains will conquer prairie to the east and 

 scrub to the west, etc. If the rainfall increases the deciduous forest will extend 

 more and more into the prairie, the latter will move westward over the plains, 

 and the plaias will be further narrowed by the creeping out of scrub and forest 

 from foot-hUls and mountains. The appearance of another period of glacia- 

 tion would produce a similar shifting of climaxes. The polar-niveal climax 

 would move into the arctic-alpine climax, the latter into the boreal-subalpine 

 climax, etc., the amount of movement and replacement depending upon the 

 extent and duration of the ice. The reverse migration of climaxes would 

 occur upon the melting of the ice-sheet, as it must have occurred at the end of 

 the glacial epoch, and to a certain extent in interglacial intervals. A climatic 

 series of climaxes or formations is an epitome of past and potential develop- 

 ment, i. e., of phylogeny. It is both genetic and natural, and furnishes the 

 basis for a natural classification of climax formations. Such a series is the 

 connecting link between the coseres of one climatic period and another, that is, 

 between two different vegetation periods geologically speaking, or eoseres. 

 From its step-like nature and its relation to climate and climax, such a regional- 

 historical series may be tgrmed a clisere. This term is formed by combining 

 sere with the unmodified root cli, found in Gr. kKIvu, make to bend or slope, 

 (cXi/to, slope, region, cHmate, and Lat. clivus, slope, hill. In accordance with 

 what has been said above, it is here proposed to group formations in climatic 

 series or cliseres. The illustrations already given would constitute two 

 cliseres, one dependent upon water primarily, the other upon temperature. 

 Cliseres in turn would be related to definite eoseres, as shown in Chapters 

 XII to XIV. 



