510 



CENOZOIC ERA— AGE OF MAMMALS. 



tries. The Australian flora is now only where the European flora was 

 in Eocene times, and the American flora now where the European was 

 in the Pliocene. The probable reason is that, in Europe, in these later 

 geological times,* changes of physical geography and climate, and con- 

 sequent migrations of species, were more frequent, and the struggle for 

 life more severe. Australia especially, probably on account of its isola- 

 tion, has advanced more slowly than most other countries. Many rem- 

 nants of extinct faunas and floras exist there still. 



Still another conclusion is, that the floras of Europe, America, and 

 Australia, were far less differentiated from one another then than now. 



Diatoms. — If the highest of plants — Dicotyls and Monocotyls — were 

 abundant, probably more abundant than now, so also were the lowest 

 order of uni-celled plants — the Diatoms. Immense deposits, consisting 

 wholly of the siliceous shells of these microscopic plants, are found in 

 the Tertiary. In Europe the Bohemian deposit is celebrated. It is 



....... -"^ \a r r -~ ^■•_^ _ 



Fig. 860.— Microscopic View of Richmond Infusorial Earth (by Ehrenberg). 



fourteen feet thick, and every cubic inch of the material, according to 

 Ehrenberg, contains 40,000,000,000 shells. The Eichmond (Virginia) 

 deposit is equally well known. It is thirty feet thick, and many miles 



* In Cretaceous times the flora of America seems to have been more advanced than 

 that of Europe. 



