236 HEREDITY AND EVOLUTION IN PLANTS 



152. Summary of Results. From what has been said, 

 in this and in Chapter VI, we recognize that the method 

 of evolution is to be ascertained chiefly by experiment by 

 studying living plants in action; but the course of evolution 

 chiefly by the study of comparative morphology, with special 

 attention to fossil forms, and supplemented by the facts of 

 geographical distribution. Other points are necessary to 

 complete the history of the evolution of plants; the above 

 paragraphs give only the barest outline of the problem, 

 for the entire history is much too long and much too 

 difficult to be treated here. To summarize; the facts now 

 known have led some investigators to infer: 



1. The origin of Angiosperms from Cycadophyta (pro- 

 angiosperms) . 



2. The origin of Cycadophyta from Cycadonlicales. 



3. The origin of Cycadonlicales from Primofilices. 1 



4. The origin of Filicales from Primonlices. 



5. The origin of Cordaitales from Primonlices. 



6. The origin of Coniferales from Cordaitales. 



An ancestral tree embodying these views is shown in 

 Fig. no. 



What was the origin of the Primonlices? Here, as 

 often, in every science, we have to acknowledge that 

 we do not know; the group is a hypothetical one, and 

 some investigators doubt its actual existence altogether. 



153. Other Views. (a) Other and equally competent 

 students of the problem take exception to one or more 

 of the six points tabulated above. Not all of their views 

 can here be discussed, but mention may be made of 

 that first elaborated by Jeffrey, of Harvard University. 



1 The term Primofilices, not hitherto used in this text, refers to a hypo- 

 thetical, primitive fern stock. 



