The Problems of Plant Distribution 255 



there were probably others which at present are 

 restricted to limited areas, but which were more 

 widely distributed during the Tertiary. 



By the end of the Tertiary there was an evident 

 lowering of the temperature, and the zonal climates 

 were already well marked, but less pronounced than 

 at present. Toward the end of the Tertiary the 

 general distribution of the land areas was much 

 as at present and the land connection between Eu- 

 rope and America was permanently severed. The 

 end of the Tertiary was followed by the gradual 

 formation of the great ice-sheet, inaugurating the 

 Glacial epoch. The great climatic disturbances 

 due to the development of the great polar ice-sheet, 

 resulted in very marked changes in the distribution 

 of the uniform northern Tertiary flora, and the ad- 

 vance of the ice-sheet was the principal factor in 

 determining the distribution -of the present flora 

 of the Northern Hemisphere. With the increasing 

 cold, and the southward extension of the great 

 glaciers, vegetation of all kinds must have been 

 forced southward. The result of this was very dif- 

 ferent in different parts of the world. In Europe, 

 which lies mostly within the region of severe glacia- 

 tion and whose great mountain ranges formed bar- 

 riers against the southward retreat of the more 

 tender plants, many plants were destroyed which 

 have survived under the more favorable conditions 

 presented in Eastern Asia and America. 



It is evident from a study of European Cretaceous 



