352 PAST succession: the ceneosere. 



actual genera are relict, as in the case of Cycas, Eguisetum, and Selaginella, or 

 whether they are represented by direct generic descendants, as may be true of 

 Cordaites and Araucaria. As has already been pointed out, the fragments of 

 the Paleophytic flora now play only subordinate r61es in vegetation, and this is 

 true also of the great bulk of gymnosperms of the Mesophytic, in spite of the 

 exceptional importance of the Abietineae. As would be expected, the differ- 

 entiation of the angiospermous flora has been greatest in great land-masses, 

 on account of their more marked climatic differentiation, and least in oceanic 

 regions. As a consequence, it is in regions of this sort that the older floras 

 have persisted longest, as in Australia and New Zealand. 



The general relations of the three floras, and of their most important types, 

 are shown graphically in the phylogenetic table (fig. 44, p. 418), which has 

 been slightly modified from the original by Stopes (1910 : 177). 



Inferences from distribution.— An examination of the "Tables of Genera" 

 (p. 445) reveals the fact that gaps occur constantly in the distribution, both 

 in time and space. In the case of distribution in time particularly, it seems 

 clear that these gaps must be due to the imperfection of the record or of our 

 knowledge, and not to the disappearance of a genus for a certain time. The 

 Colorado epoch of the Cretaceous is an almost complete blank, and the Oligo- 

 cene and PUocene are but little better. But the conclusion is imavoidable 

 that the vegetation of the Colorado epoch is clearly indicated by that of the 

 Dakota which preceded and the Montana which followed, so far as families and 

 genera are concerned. Likewise, the flora of the Oligocene was essentially 

 that of the Eocene, somewhat reduced by deformation, and the plants of the 

 Pliocene are practically those of the Miocene, but with a striking reduction. 

 The poverty of the Oligocene and PUocene is partly due to the fact that the 

 relative area for water deposit was much reduced, while conversely during the 

 Eocene and Miocene, the areas of sedimentation and fossilization were rela- 

 tively larger. 



The time distribution of many genera also supports the idea of continuity 

 through epochs where they have not been found as fossils. This is shown by 

 Anona, which appears in the Dakota, but misses the Colorado, Montana, and 

 Laramie, to reappear in the Eocene and again in the Miocene. Even such 

 a dominant as Betula with 34 species skips the OUgocene and Pliocene, though 

 it is all but certain that it occmred abimdantly in both of them. Likewise the 

 record of the universal Carex jumps from the Dakota to the Eocene and then 

 to the Pleistocene. Ceanothus, which is perhaps the most widely spread genus 

 of the chaparral of western North America, occurs in the Dakota and Montana, 

 again in the Eocene and Oligocene, and again in the Pleistocene and Rficent. 

 Even with such a cosmopohtan as Cham, the record is regularly broken by 

 gaps. As would be expected from the relative ease of fossilization, the record 

 of trees is most nearly continuous, but there are gaps even in the case of Ficus, 

 Pinus, Populus, and Qitercus. As a consequence, the general assumption 

 seems warranted that an existing genus must have continued from the time 

 of its first appearance down to the present, though one must admit at least 

 the possibihty of repeated origin, i. e., of polygenesis. This assumption is 

 especially significant in helping to bridge the gap between the relatively good 

 record of the Miocene and the present, and to reconstrutet the vegetation of 

 the Pliocene and Pleistocene. Moreover, it is necessary to test the record of 



