THE FLORA. 



351 



Elevation and subsidence in particular would seem to have been more marked, 

 though the actual rate and the successional consequences may have been httle 

 different than at present. Climatic causes were probably little effective in 

 denudation, and biotic initial causes were certainly much less important, due 

 almost wholly to the absence of hmnan activities. 



THE CENOPHYTIC ERA. 

 THE FLORA. 



Nature and origin. — ^While angiopserms were universally characteristic of 

 the Cenophytic era, the floras of the Mesophytic and Paleophytic eras were 

 still represented, though constantly decreasing in niunber to the present time. 

 Hence, while the angiosperms were dominant and constituted the major and 

 typical portion of the eosere, fernworts and gymnosperms were still intrinsic 

 and sometimes important members of the succession. It is highly probable 

 that there was a fairly long transition epoch, during which the dominance was 

 passing from Gymnosperms to Angiosperms, and the successional sequence 

 was marked by a grand mictium, in which two floras were competing for the 

 mastery. A glance at the "Tables of Genera" (p. 445) for the Mesophytic 

 and Cenophytic reveals the well-known fact that the Angiosperms appeared 

 suddenly at the opening of the Comanchean period. It does not seem likely 

 that the evolution of the new type could have been as rapid as this would 

 indicate, particularly in view of the much slower differentiation of gymno- 

 sperms in the preceding era. It appears much more probable that angio- 

 sperms arose much earlier in the Mesophytic, and developed slowly to the 

 dominance which they assumed at the beginning of the Cretaceous. This 

 suggestion is supported by the fact that the known floras of the Triassic and 

 Jurassic are scanty, especially in North America, which was a center of angio- 

 spermous evolution. 



Fascinating as the problem of the origin of the Cenophytic flora may be, it 

 is of minor importance in comparison with its assumption of dominance. 

 This appears to have become marked as well as general during the earUest 

 epochs of the Cretaceous period, but there must still have existed great gjrmno- 

 spermous climaxes, as is true even at present. The evolution of new forms, 

 and especially of monocotyledons, must have proceeded with rapidity during 

 the Cretaceous, for we find a striking extension and differentiation in the 

 Eocene. The Tertiary was thus marked by a floral luxuriance which per- 

 sisted to the deformation cycle of the Mocene-Pleistocene. More than 250 

 genera had appeared in North America by the Eocene, and this number was 

 considerably increased in the Miocene and the PHocene, as indicated by the 

 tables already mentioned. Of these genera, the great majority are modem 

 thus proving the generic composition of the flora to have been essentially 

 uniform throughout the era. This is further indicated by the distribution of 

 the great dominants, such as Acer, Fagus, Hicoria, Juglans, Quercus, and Tilia. 

 As to species, however, there were marked differences between the various 

 periods and epochs, corresponding to a progressive differentiation of climaxes. 



Relation to mesophytic and paleophytic floras.— While pteridophytes and 

 gymnosperms are associated with angiosperms in the modern floras of many 

 regions, phylogenetically they are relicts. This is essentially true, whether 



