THE LEWIS BKOOKS MUSEUM. 43 



known, a poplar, which is hence the oldest of our arbores- 

 cent genera. In this formation the Mesozoic type still pre- 

 dominated. Its flora consisted of ferns, cycads, conifers 

 and a few equisetas. 



In the middle Cretaceous we find the predominant veg- 

 etation to be angiospermous, not differing very greatly from 

 modern forms. The Mesozoic vegetation, which like the 

 Protozoic, had prevailed all over the earth, even as far to- 

 wards the poles as man has penetrated, no longer is found 

 in the extreme north. It begins to retreat towards the equa- 

 torial regions. Heer has found in Greenland a lower Cre- 

 taceous formation, with an abundant Mesozoic flora, and 

 over it upper Cretaceous beds filled with modern forms of 

 plants. Here the middle Cretaceous is wanting. In the 

 Dakota group, which is extensively displayed in our Terri- 

 tories, on about our latitude, we find an abundant flora 

 closely resembling our present forest trees. This group 

 corresponds to the middle Cretaceous of Europe. 



We thus find in the middle Cretaceous clear indications 

 of the commencement of the formation of zones of tempera- 

 ture, and along with this of the introduction of the modern 

 forms of plants. The question arises what was the origin 

 of the first angiosperms, a type of plants so different from 

 all which preceded. 



Clearly their appearance was connected with the in- 

 creasing refrigeration of some parts of the earth. The first 

 forms we perhaps have not seen, for the oldest known to us 

 do not differ greatly from those now living. 



The close of the lower Cretaceous was then the most 

 important era in the history of plant life. Ic marks a 

 great change from ancient to modern condition?, a change 

 which is nowhere else equaled in extent. There has since 

 been no essential modification of the vegetation then intro- 

 duced. 



The changes since have been principally in the direc- 

 tion of increasing the variety of forms and a stricter limjta- 



