Evolution of Plants 373 



in the extreme. This would even more apply to plants of 

 cretaceous age, which since fossilization have been subjected 

 to upheaval, depression, denudation changes, and other ter- 

 rene alterations, along with the rocks that enclose them. 



Reviewing the evidence for dicotyledons now to hand, the 

 writer would consider that at least three, possibly four or 

 five, diverging groups early arose, that may be designated 

 by the existing cohort names of the Fagales, the Laurales, 

 the Chenopodiales, and the Ranales. Fairly recognizable 

 and probably correctly identified remains of the three first 

 named are included amongst the oldest dicotyledonous fossils. 

 Of the fourth it seems fairly assured that magnoliaceous re- 

 mains coexist Tvith the other three. 



Given the existence then of a few primitive representatives, 

 both herbaceous and arborescent, of each of the above great 

 morphological upbuildings, it seems by no means difficult 

 to understand how the existing dicotyledonous flora may haA^e 

 been evolved therefrom. In this connection the probable 

 appearance of the Hymenoptera and Lepidoptera during the 

 cretaceous period, and their undoubted specialization during 

 oligocene times into divisions like those now existing, doubtless 

 contributed not a little to rapid variation and specific selection 

 amongst both dicotyledons and monocotyledons, from early 

 tertiary times onward. 



The origin and widespread distribution of a northern and 

 simultaneously of a southern monocotyledonous and dicoty- 

 ledonous flora, each largely distinct from the other, seems to 

 have occurred during the cenomanian and senonian periods 

 of cretaceous age, and on into eocene times, when a gradual 

 separation into the present continental masses seems to have 

 been effected. Throughout these times the genera and species 

 that now exist gradually spread. But only after an exhaustive 

 investigation has been made of palaeontological remains from 

 the early cretaceous up to our day, and when botanical com- 

 parison is then instituted between these and the geographic 

 localities of living genera, as well as the relative morphological 

 dignity of these genera, will we be able to trace successfully 

 the history of angiospermic development. 



