The Problems of Plant Distribution 241 



of the Mesozoic, especially the Cretaceous, the cli- 

 matic conditions were apparently less uniform than 

 at the beginning, and zonal climates were already 

 indicated, although much less pronounced than at 

 present. 



Quite suddenly toward the end of the Mesozoic, 

 in the Sub-Cretaceous, the angiosperms, the 

 highest of all plants, first appear. Their origin 

 is very obscure, but once developed, they show an 

 extraordinary power of adaptation, and soon out- 

 number all the other plants, increasing their su- 

 premacy until now they are by far the most impor- 

 tant of living plants. The earliest seed-bearing 

 plants, the Cordaitales and Pteridosperms, became 

 extinct towards the end of the Paleozoic, and were 

 replaced by other types which have persisted down 

 to the present time. The Cycads, Ginkgoales, and 

 the lowest Conifers were probably all in existence 

 before the end of the Paleozoic. 



During the whole of the Mesozoic, North Amer- 

 ica was connected with the Eurasian Continent, and 

 although this connection was probably broken down 

 during the Tertiary, it was reestablished from time 

 to time, so that there was free intermingling of the 

 floras throughout the whole extent of the Northern 

 Hemisphere, and this flora maintained its similarity 

 up to the end of the Tertiary. 



Cretaceous Plants. A good many existing gen- 

 era occur in the Cretaceous, and it would seem from 

 their distribution that the climate of the Northern 



