GENERAL PRINCIPLES. 347 



western North America will be traced through the various eoseres. The dis- 

 cussion of the Ceneosere follows in the present chapter, and that of the Meseo- 

 sere and Paleosere is found in the following chapter. The plan of beginning 

 with the Ceneosere rather than the Proteosere seems much the most desirable 

 for several reasons. Chief among these are the vastly greater amount of 

 knowledge, especially of Pleistocene glaciation and its stases, and the great 

 advantage of tracing the successional clues through a vegetation essentially 

 the same as that of to-day. 



Clisere. — The effect of climatic differentiation upon a uniform vegetation 

 must have been the production of as many climaxes as there were climates. 

 Since climates arrange themselves more or less regularly in a series from excess 

 to deficiency, it is clear that the corresponding climaxes will have a similar 

 arrangement. Such series are found to-day in the more or less obvious zones 

 of continents and mountain ranges. Once differentiated in Paleozoic times, 

 they would seem to have been a constant feature of vegetation since, although 

 greatly compressed polewards in periods of equatorial climates. The evolu- 

 tion of a new flora at the beginning of each era must have had to reckon with 

 the persistence of climatic zones of some sort, even though origin occurred in 

 but few places, and migration was widespread. Hence, it is assumed that 

 vegetation zones or centers have existed at practically all times since the 

 Silurian. A change of climate subordinate to that which distinguished the 

 era or eosere would operate upon the series of vegetation zones or climaxes 

 exactly as it seems to have done during the Pleistocene. Shifting of the zones 

 in consequence of migration and ecesis in the favorable direction would take 

 place much more readily and rapidly than adaptation of the flora. Indeed, 

 the latter seems to have been present in only a minor degree during Pleistocene 

 glaciation. Moreover, shifting of the climax zones appears to have been the 

 only adequate process, in view of the relatively rapid advance and retreat of 

 the ice. 



The clisere is the series of climax formations or zones which follow each 

 other in a particular climatic region in consequence of a distinct change of 

 climate. The clisere of the zone just south of the ice-mass must have con- 

 sisted of tundra, scrub, conifer forest, and deciduous forest at each retreat 

 of the ice. A similar cUsere must have existed at the base of every heavily 

 glaciated mountain or range. Moreover, cliseres must have occurred through- 

 out geological times, whenever marked cooling or glaciation took place. Of 

 e \^en greater importance, however, is the fact that the stages of a cUsere per- 

 sist in the form of climax zones from one period of shifting to another. In 

 short, as has been indicated previously, a series of zones or cHmaxes constitutes 

 a potential chsere, which reveals its fundamentally successional nature when- 

 ever marked cooling or other climatic change produces shifting. Hence, as 

 pointed out in Chapter VI, climaxes always stand in cliseral relation to each 

 other, and the series of zones is an index of the successional sequence of the 

 actual clisere. Whether the climatic change is plus or minus, the resulting 

 clisere will consist of stages whose order wiU be identical with the spatial order 

 of the climax zones. If the climatic ehangei s favorable to succession the 

 stages of the clisere will pass from lower to higher, e. g., from tundra to decidu- 

 ous forest, as m the case of a retreat of the ice. If the change is unfavorable 

 as in front of an ice advance, the deciduous forest is replaced stage by stage by 



