214 



THE AMERICAN NATURALIST 



[Vol. XLVI 



Another example of similar import is afforded by the 

 early Pennsylvanian flora, that is, the flora of the lower 

 part of the Upper Carboniferous. Wherever terriger- 

 ous beds of this age have been discovered, representa- 

 tives of this peculiar flora, which includes such common 

 genera as Lepidodendron, Sigillaria, Sphenophyllum, 

 etc., have been found, this distribution ranging from 

 South Africa to Brazil and Argentina, and thence over 

 the northern hemisphere. 



Similarly, the Mississippian flora (Lower Carbonifer- 

 ous) has been found in Spitzbergen, Greenland and arctic 

 Alaska, and thence south over Europe and America, and 

 although somewhat older than the last, is distinctly re- 

 lated to that in Argentina. 



On passing up in the geologic time scale we find that 

 during late Mesozoic and early Cenozoic time the present 

 dominant types of vegetation were firmly established. 

 With what probability of success may these floras be in- 

 terrogated as to the climatic conditions under which they 

 existed? We find from a study of the present flora that 

 certain types of vegetation, as well as certain plant asso- 

 ciations, have definite climatic requirements. Thus, A r- 

 tocarpus, or the bread fruit trees, are now confined to 

 within 20° of the tropics, showing that they require the 

 moist heat of the torrid regions. If, now, we find that 

 Artocarpus once throve in Greenland 70° or more north, 

 during Cretaceous time, we feel justified in assuming that 

 its climatic requirements were not very different from 

 those of its living representatives. And when we find 

 that it was then in association, as it is to-day, with cycads, 

 tree-ferns, cinnamons, palms and other distinctly tropical 

 forms we are confirmed in the opinion that at that time 

 Greenland must have enjoyed a tropical or at least a 

 sub-tropical climate. 



Another example is afforded by the Fort Union forma- 

 tion. In the rocks of this horizon, which now occur on 

 the wind-swept, almost treeless plains of the Dakotas, 

 Wyoming and Montana and thence northward to the val- 



