VEGETATION ERAS. 283 



the fossils derived from such an area shows that the plant life was xerophytic. 

 Such conditions seem to have obtained during the Jurassic of the Great Plains, 

 and the geologic evidence of dryness is rendered almost if not quite convincing 

 by the apparent dominance over much of the region of a scrub of xerophytic 

 cycadoideans. 



In using the method of succession, the procedure is similar. The presence 

 of a forest of Lepidodendreae and Cordaites in the Paleophytic era argues for 

 the existence of forest layers and of adjacent zones of scrub and grassland 

 equivalents. These in turn indicate the general course of the sere with cer- 

 tainty. If we turn now to the paleobotanic evidence, we find that fernworts, 

 climbing ferns, and tree-ferns show the gross and minute structure to be 

 expected of layer societies, and that the horse-tails and tree-ferns furnish the 

 constituents for a plausible scrub and grassland. In some cases, moreover, 

 the paleontic evidence may be supported by that drawn from the persistence 

 of archaic communities. It is at least suggestive that a dwarf tree-fern, 

 Dicksonia lanata, and a climbing fern, Lygodium articidatum, to-day form a 

 large part of the undergrowth in New Zealand forests of Agathis australis, one 

 of the Araucarieae (Kirk, 1889 : 144). 



VEGETATION ERAS. 



Criteria. — It is weU understood that the divisions of geological history are 

 based primarily upon critical events in the development of the earth itself, 

 and secondarily upon the behavior of the animal populations. In view of 

 what has been said as to the basic effect-and-cause position of vegetation 

 between climate and topography on the one hand, and animal life upon the 

 other, it is a question whether plant life should not have played a more 

 important part in the analysis. However this may be for the history of the 

 earth as a whole, it seems clear that plants must take the leading r61e in 

 characterizing the major divisions since the emergence of land. While there 

 can be Httle question that land life is phylogenetically connected with that 

 of the sea, its general development has been essentially distinct since the time 

 of its origin. Moreover, the conclusion is irresistible that a land vegetation 

 preceded any considerable evolution of land animals, and that the vegetation 

 played a controlling and probably the controlling role in this evolution. 

 While no special warrant seems necessary for basing eras of vegetation upon 

 vegetation itself, the essential dependence of animal life upon plants furnishes 

 a cogent reason. When we recall the effect of great continental and climatic 

 changes upon vegetation, and the record made by the latter of them, there 

 seems to be ample justification for using primary divisions based upon vege- 

 tation. This course gains support also from the fact that the close of the 

 first era of vegetation dominance coincides in general with the end of the 

 Palaeozoic era. No such correspondence occurs in the Mesozoic period as 

 limited at present, since the dominance of the gymnosperms yielded to that 

 of the angiosperms in the Cretaceous. 



It is not to be expected, however, that the present limits of geological eras 

 shall be either definite or final. While the Paleozoic era is generally regarded 

 as brought to a close by the Permian period, the following statements of 

 Chamberlin and Sahsbury (1906:2:639, 642; 3:38) seem to indicate that 

 it really terminated before the Permian: 



