366 Causes and Course of Organic Evolution 



in a unique cohort Casuarinales, that has evolved directly 

 from a gnetal ancestry, and so separate from the main angio- 

 spermic line. 



The above conclusion the writer hopes to elaborate else- 

 where, but as a necessary corollary it involves the view that 

 the Angiospermia had at least a diphyletic origin.* 



The highest and remaining division of the Protoangiospermia 

 can now concern us. Up to the close of the Jurassic period 

 the predominant vegetation was gymnospermic, and to a less 

 degree of a fern character. Such was largely true also during 

 the earlier cretaceous age. But towards the close of the Juras- 

 sic age, or very early in the cretaceous, primitive angiosperms 

 must have been evolving. As already set forth above, the 

 only living group that combines in its genera most structural 

 details of angiosperms is that of the Gnetales. But neither 

 of it nor of undoubted angiosperms have we as yet traces, till 

 the early cretaceous rocks are reached. It is however a neces- 

 sity of the case to consider that during triassic times numerous 

 and diversified gnetal genera evolved, and that these in turn 

 gave rise along the ephedral line to Casuarinales, along another 

 and slowly diverging line to progenitors of the Angiospermia 

 proper, that we may term the Protoangiospermia. 



In attempting to trace the evolution of this grouj^, one is 

 confronted by the fact that, so far as fossiliferous rocks have 

 hitherto yielded evidence, the two great divisions of dicotyl- 

 edons and monocotyledons appear practically simultaneously, 

 and in rocks of lower cretaceous age. The question then at 

 once occurs as to how these two very^ natural divisions arose, 

 and whether one preceded the other. 



On the whole the writer believes he is correct in saying that, 

 till within the past decade, the great balance of opinion leaned 

 toward the view that the monocotyledons were the more an- 

 cient and in structure the more primitive group, and either 

 arose separately from, or gave origin later to the dicotyledons. 



* Since the above was written, now nearly five years ago, the valuable and 

 suggestive paper by \V. P. Thompson has appeared (Amer, Jour, of Bot. Ill 

 (191G) 13o). His views form a welcome confirmation to the above state- 

 ments. 



