372 PAST succession: the ceneosere. 



and their persistence would be expected to be greater than that of species of 

 warmer zones. A glance at the "Tables of Dominants" (p. 262) shows that 

 all of the dominant genera of trees listed for the present occurred in North 

 America during the Pleistocene. Of the shrub dominants, practically aU of- 

 the boreal genera are recorded for Europe or America, and this is true also 

 of the aquatic dominants. As a consequence, we must conclude that the 

 remarkable alternations of climate from the PUocene to the present operated 

 upon a flora essentially identical with the existing one. In short, after the 

 first shifting of climax zones in consequence of the first or Jerseyan advance 

 of the ice, the successional effects of the glacial cycles may be analyzed as 

 though they were occtming to-day. The assimiption has already been made 

 that climax zones existed in North America drniog the Miocene, and that the 

 effect of Pleistocene glaciation was hence not to originate these, but to shift 

 them repeatedly, to modify their relative width, and to differentiate them 

 internally, or at least to increase their differentiation. 



Pliocene climax zones. — ^If the geologic evidences of maximum aridity dming 

 the Permian, Triassic, and Jurassic, and of marked or maximum cooling and 

 aridity in the Cretaceous-Eocene, OHgo-Miocene, and the Pliocene are accepted, 

 they lead inevitably to the assumption of corresponding differentiations of 

 climate and climaxes. It has already been assumed that climax zones and 

 altemes of some small extent at least had persisted from the Mesophytic into 

 the Cretaceous, and that these were emphasized and extended by each of the 

 major deformations of the Tertiary period just mentioned. Accordingly, the 

 opening of the PKocene must have seen a marked zonation of vegetation in 

 boreal and polar regions and on the higher mountain ranges, as well as the 

 presence of arid altemes in the interior plains and basins. This condition 

 must have been still further emphasized by the exceptional deformations 

 during the Pliocene and at its close. In view of these facts, it does not seem 

 unwarranted to assiune that the climax zones and alternes of vegetation before 

 the oncoming of the ice were very similar to those seen in North American 

 vegetation to-day. This conclusion is strongly reinforced by the essential 

 identity of the flora of the Pliocene with that of the Pleistocene and the present. 

 In short, the assumption seems to be justified that the chmax or vegetation 

 centers suggested in Chapter IX were already in existence and possessed much 

 of their present character and extent. 



The Jerseyan-Aftonlan clisere.— The probable shifting of the zones of North 

 American vegetation in consequence of the advance of the ice has been 

 sketched in Chapter VI. There is ahnost unanimous agreement as to the 

 general features of the migration (Transeau, 1903 : 410; Chamberlin and 

 Salisbury, 1906:3:485; Harshberger, 1911 : 183, 203), and they wiU be dis- 

 cussed here only in so far as it is necessary in order to exhibit their successional 

 relations. For each glacial-interglacial cycle these may be summed up m the 

 clisere. As already suggested, the clisere must be regarded as both structural 

 and developmental, as static and dynamic at the same time. In this respect 

 it is very Hke the process of growth in the individual plant, which is at once 

 developmental and structiu-al. In its static aspect the cKsere is the series of 

 climax zones, which are set in motion by a marked change of climate and 

 cause a developmental or successional replacement of climaxes, and hence a 

 modification of the course of each sere. At first thought it would seem desir- 



