416 MESEOSERE AND PALEOSERE. 



oak-like leaves, and analogous names for plants whose leaves resembled those 

 of the elm, waWt, maple, eucalyptus, and others). To these were added, in 

 the Amboy (N. J.) clays at the very close of the period, figs, magnohas, tulip 

 trees, laurels, cinnamon, and other forms referred to modern genera, but not 

 to modern species. The cycadeans had dropped to an insignificant place, and 

 the conifers and ferns, while not equally reduced, were markedly subordinate 

 to the angiosperms." 



An examination of the "Tables of Genera" shows that practically no angio- 

 spermous dominants, either serai or climax, are recorded for the Comanchean, 

 but that these appeared in great number in the Cretaceous. The assumption 

 of dominance then marked a transition period from the Comanchean to the 

 Cretaceous, and closed the Comanchean period of evolution and migration. 

 If we ignore the possibility of a polytopic origin for angiosperms, we are free 

 to consider the successional significance of their westward migration from the 

 Atlantic coast, though there would have been Httle or no difference in the suc- 

 cession had they originated in two or more regions. From the analogy with 

 invasion at present, it seems that a more or less definite mass invasion moved 

 in waves from the rapidly increasing center or zone along the seaboard. This 

 would result in the establishment of dominance first in the original area of 

 evolution, and dominance would follow migration in its somewhat pulsatory 

 movement to the west and north. The consequence would be a shifting 

 essentially similar to that of the cUsere. In fact, it may be designated by this 

 term, though the movement was in but one direction, and the efficient factor 

 in it was competition alone rather than the influence of a changing cUmate. 

 As a consequence, we may assume the existence of three zones over the mass of 

 the continent after angiosperms had become dominant along the Atlantic. 

 These were the cUmax zone of angiosperms, the climax zone of gymnosperms, 

 and the transition zone or ecotone in which the two climaxes were variously 

 mixed. The constant westward movement of the transition and its concomi- 

 tant conversion into an angiospermous climax show clearly that the ecotone is 

 developmentally the mictium of two serai stages. 



The forward movement of the clisere would have been similar to that already 

 noted for the Pleistocene, except that it seems to have been independent of a 

 change of climate. As the more mobile and vigorous dominants invaded the 

 gymnospermous climax, they established a new ecotone, while upon the old 

 arose the new climax of angiosperms. This process continued across the whole 

 continent until the angiosperms became the masters of all the regions where 

 their advantage in competition was not offset by climatic conditions. In other 

 areas the gymnospermous climax must have been favored, and hence it must 

 have persisted in the higher latitudes and altitudes, with the emphasis upon 

 the rapid evolution of the Abietineae, to give rise to the coniferous cUmaxes of 

 to-day. However, while the angiosperms were unable to become the cUmax 

 dominants in such regions, they did succeed in estabhshing themselves as serai 

 dominants, all but completely replacing the ferns and horse-tails. When the 

 invading angiosperms reached the drier interior, the Great Plains and Great 

 Basin of to-day, they replaced the assumed cycadean chmax completely, but 

 with the probable evolution of grasses and shrubs, such as are characteristic 

 of the chmaxes in these regions to-day. 



The development of the sere during the Comanchean was necessarily com- 

 plex, since it dealt for the first and last time with a complete change of domi- 



