354 PAST succession: the ceneosebe. 



Agrostideae had already been evolved. Similar results are possible among 

 the dicotyledons, in spite of less agreement as to the proper sequence of 

 orders. The appearance in the Dakota of trees with highly specialized 

 flowers, such as Juglans, Hicoria, Pojndus, and Fraxinus, leads irresistibly to 

 the conclusion that roses, pinks, and olives had already been developed in the 

 Comanchean. The occurrence of Thalidrum, an apetalous diclinic anemo- 

 phile, in the Eocene, makes practically certain the earlier existence of the more 

 primitive Ranunculaceae. 



THE LIFE-FORMS. 



The record.— The "Tables of Life-Forms and Dominants" on page 462 

 show that practically all the life-forms known to-day occurred throughout the 

 Cenophytic era. The evidence as to trees and shrubs is complete, chiefly 

 because of the greater ease of fossilization. While Popukis is the only woody 

 genus recorded for the Comanchean, the majority of tree dominants and a 

 large number of shrubs appeared promptly in the Cretaceous. Floating 

 forms are recorded from the Cretaceous in fair number, and both floating and 

 reed-swamp dominants occur abundantly in the Eocene. Grasses and herbs, 

 apart from hydrophytic forms, are but poorly represented, but the frequent 

 occurrence of amphibious species indicates that this poverty is due primarily 

 to the conditions of fossilization. The difficulty of fossilization also explains 

 the rare occurrence of fossil bryophytes, fungi, lichens, and algae. The records 

 of these become more frequent in the Cenophytic era, but the conclusion seems 

 inevitable that these four life-forms had existed since early Paleophytic times. 



The genera included in the table are those which are dominants or sub- 

 dominants in the present vegetation of North America, but especially in the 

 Great Plains and Rocky Mountain regions. The majority of them are the 

 dominants which form the characteristic consocies and consociations, though 

 the larger number of herbaceous genera are the subdominants of the important 

 socies and societies. In the case of the thallophytes, all the genera accepted 

 as warranted are recorded. 



Methods of inference. — ^The record of life-forms in the table permits us to 

 draw two kinds of inferences, namely, (1) phylogenetic and (2) associational. 

 The phylogenetic inferences are partly a matter of systematic relationship, and 

 partly of the phylogeny or origin of the life-form itself. In the case of grasses 

 and thallophytes, the taxonomic or reproduction form and the Ufe-form are 

 regularly inseparable. Thus, the presence of Phragmites in the Cretaceous, 

 Poa in the Eocene, and Stipa in the Miocene seems to make probable the 

 existence of the grass-form, and hence grass consocies and climaxes, throughout 

 the Cenophytic era. Moreover, the occurrence of the tree-form presupposes 

 the shrub-form, since many of the genera, Acer, Populus, Quercus, Salix, etc., 

 exhibit both. The record of submerged species of Potamogeton impUes an 

 earlier floating form of the genus, and the presence of floating species of Spar- 

 ganium leads to the inference that they were preceded by amphibious forms. 

 Furthermore, the existence of every higher life-form in any epoch or period 

 would seem to require the presence of all the simpler life-forms of angiosperms. 



Inference from association may lead to the recognition of earlier consocies, 

 as indicated in the last statement, or it may suggest the presence of unrecorded 

 dominants of the associes. In addition, it may point out the presence of sub- 



