195 



III. Geologic Evolution, Theoretic Considerations and 

 Statistics on the Distribution of North American Plants. 

 If the historical factors, climatic, geological, and ethnological, 

 have been the most important in the fixing of the permanent 

 complexion of our vegetation, then this part of the book will 

 doubtless be considered as of chief interest, for it deals with the 

 most fascinating part of the origin and development of the North 

 American flora. To the botanist, or even to the intelligent gen- 

 eral reader, Dr. Harshberger has presented, almost dramatically, 

 a picture of the beginnings of things floral on this continent, 

 that will perhaps evoke criticism, but must meet with general 

 admiration. The alternate rising and falling of the earth's crust, 

 the encroachment of inland oceans over what is now dry land, 

 the upheavals of our great mountain chains, the advance and 

 recession of the continental glaciers, and many other minor geo- 

 logical phenomena, have had profound and fundamental in- 

 fluences on the migration of whole floras, the creation of interest- 

 ing endemisms, and the struggle between heat- and cold-resisting 

 floras. 



The Cretaceous and Tertiary floras are first discussed (pp. 

 120-182), and a general review of the fossil-bearing strata, to- 

 gether with a list of the better known preglacial plants, is given. 

 This list, to the botanist, will convey a very fair idea of the state 

 of North American vegetation just before the beginning of the 

 southward extension of the great continental glacier; and it serves 

 also to fix in one's mind the vast climatic significance of the 

 encroaching ice-sheet. That such genera as Anona, Araucaria, 

 Artocarpus, Bombax, Casuarina, Dalhergia, Eugenia, Inga, Grewia, 

 Sabal, and Sterculia should ever have flourished in what is now 

 temperate America is evidence of the far-reaching change wrought 

 by the ice. 



The second chapter (pp. 182-203) deals with the development 

 of the flora during the glacial periods, and calls attention to the 

 facts of the alternate encroachment and recession of the glaciers 

 and of the consequent see-sawing of heat- and cold-resistant 

 types of plant life. The treatment of the endemisms created 

 by the final recession of the glacier and of the formation of 



