738 TEXTBOOK OF BOTANY 



since their origin on the earth, gradually developed from preexisting 

 ones. 



Diverse groups appeared, became abundant and widespread over the 

 earth, waned, and either became extinct or are represented in our 

 present flora by isolated remnants. Early evolution of land plants seems 

 to have resulted in the development and dominance of woody plants, 

 often of tree size. Herbs are apparently more recent, for they constitute 

 a far larger proportion of the fossil flora of the Cenozoic than of pre- 

 vious eras. 



Evolution of plants previous to the close of the Paleozoic had re- 

 sulted in the formation of most of the fundamental features found in 

 plants today, excepting those found only in the flowering plants, or 

 angiosperms. The early evolution of the angiosperms in the Mesozoic 

 seems to have been both rapid and widespread in the temperate lands 

 of the northern hemisphere. From there they spread across the equator 

 to the southern continents. 



The vegetation of the present is composed of remnants of past vegeta- 

 tions, together with new combinations of old and new species. The 

 distribution of modern species and plant communities is dependent 

 partly upon the present physical and biological environments, and partly 

 upon the historical background, such as the elevation and submergence 

 of the land, the formation and erosion of mountains, land connections 

 with other continents, changes in climate and local diversification of 

 the climatic factors, prolonged intense drought, and glaciation. Some 

 appreciation of all these biological and physical factors is essential to 

 an understanding of the vegetation of a continent, such as that of North 

 America. 



REFERENCES 



Antevs, E. The last glaciation. Amer. Geog. Soc. Research Series No. 17. Amer- 

 ican Geog. Soc. 1928. 



Berry, E. W. Tree Ancestors. Williams & Wilkins Company. 1923. 



Chaney, Ralph. Ancient forests of Oregon. Carnegie Institution of Washing- 

 ton, Pub. 501, pp. 631-648. 1938. 



Chaney, Ralph. Paleoecological interpretations of Cenozoic plants in Western 

 North America. Bot. Rev. 4:371-396. 1938. 



Dahlgren, B. E. A forest of the coal age. Geology Leaflet 14. Field Museum, 

 Chicago. 1933. 



Daly, R. A. The Changing World of the Ice Age. Yale Univ. Press. 1934. 



