DEFORMATION AND GRADATION. 307 



necessary perspective. At first thought, the striking dominance of arigio- 

 sperms, with the seed habit perfected through the development of the pistil, 

 and with predominant secondary growth, would seem to indicate that the last 

 glaciation had failed of its evolutionary sequence because of the lack of rudi- 

 mentary materials. Such is almost certainly not the case if one turns to the 

 groups in which the angiospermous type shows its greatest and most recent 

 modifications. These are the grasses and sedges, composites, the capitate 

 mints and globularias, and the caryophyllals (Centrospermae) in particular. 

 The first three are characterized by the development of a highly specialized 

 flower community, the spikelet and head, while all agree in having a highly 

 perfected pistil and fruit.. It is unnecessary to point out that grasses and 

 sedges, composites and centrosperms are the dominant groups in grassland 

 and arid scrub the world over. If they were to add secondary growth as a 

 general feature to their reproductive efficiency, a new dominant vegetation 

 would result. In the usual sense, such an evolution has probably been ren- 

 dered largely or wholly impossible by man. In another sense, this very 

 change of dominance is being brought about by him as a result of the fact that 

 the grasses have been the plants best adapted to a habitat controlled directly 

 or indirectly by man. In other words, artificial grassland, e. g., grain fields, 

 is already the dominant vegetation of a large part of the earth, and its extent 

 wiU stUl be greatly increased. 



Effects upon vegetation. — As already indicated, deformation and its grada- 

 tional and climatic consequences will have one of two effects upon vegeta- 

 tion. The latter will be destroyed or changed. The change may be one of 

 differentiation or distribution, or of evolution. The destruction of vegetation 

 must have been the regular consequence of gradation and submergence. 

 Denudation by gradation must have produced complementary but unlike 

 areas of erosion and deposit, as it does to-day. Submergence in general must 

 merely have destroyed vegetation, and the production of a new area must 

 have waited upon later emergence of the land. In the shallow portions of 

 the submerged areas, and especially about the border, water seres must have 

 developed wherever the depth was not too great. In such periods as the 

 Pennsylvanian and the Cretaceous, oscillating submergence and emergence 

 seem to have produced and maintained shallow swamps over vast areas. 



The climatic changes due to deformation directly or indirectly destroyed 

 vegetation only when they eventuated in glaciation, or in fatal cooling or 

 drouth. Their normal effect was to differentiate and shift vegetations, and 

 to bring about the evolution of a new dominant vegetation in the greatest 

 cycles. These consequences are discussed in the next section, as well as in 

 detail in the treatment of the vegetational eras. The denudation effects of 

 gradation and submergence are characteristic of the smaller deformation cycles 

 or cycles of erosion, and may well be considered here. As at the present 

 denudation was concerned with the formation of new successional areas' 

 and the initiation of seres and coseres. Climatic changes, on the contrary' 

 find their outcome in the major successional developments, cliseres and eoseres' 



The cycle of erosion.— As akeady indicated, a cycle of erosion is conceived 

 to be the gradation phase between two local or regional deformations. The 

 cycle of erosion is a more or less continuous process, but it consists also of two 

 major phases: (1) one of elongation, in which the main streams and the tribu- 



