158 DAVID WHITE 



distinct terrestrial climatic zones, possibly completely into the polar_ 

 regions. 



Some of the criteria above mentioned are susceptible of different 

 interpretations; but taken collectively they appear to admit of but 

 one conclusion. Whether or not we admit that climatic changes may 

 be caused by reasonable or practicable changes in the amount of 

 carbonic-acid gas in the air it is certain that in geological times the 

 vegetation of the earth must have been more or less influenced by the 

 constitution of the atmosphere from which the plant derives so 

 important a part of its real food. 



Gradual loss of unijormity of climate, with brief glacial interruption 

 in Permian. — As has already been indicated the Westphalian probably 

 witnessed the greatest extension of uniformity and equability of 

 climate over the earth. In the Stephanian the flora is hardly so 

 homogeneous, though the world-climate appears still to have been so 

 equable as to allow free migration of the larger part of the flora from 

 a moderate latitude on one side of the equator to the opposite without 

 encountering seriously obstructive seasonal changes. In the Permian 

 the regional distinctions between the floras are much clearer; and 

 presently climatic zones, and consequently botanical provinces, are 

 recognized. Yet, about the North Atlantic the climate of the Lower 

 Permian was still relatively uniform so that moderately free migration 

 of the floras without the development, so far as we know, of pro- 

 nounced annual rings, took place in the Autunian of France, the 

 Permian of Prince Edward Island, the Dunkard of southwestern 

 Pennsylvania, the Chase of Kansas, and the Wichita of Texas. 



Red beds and climate. — It will be remembered that the period now 

 considered is characterized in western Europe, England, Eastern 

 Canada, the Appalachian trough, and the Western Interior basin, by 

 the deposition of red beds which in some areas carry deposits of 

 gypsum, etc., and which are generally regarded as laid down under 

 an arid climate. Viewing the matter from the paleobotanical stand- 

 point, we may ask whether equability and uniformity of climate, such 

 as is shown by the fossil floras, is compatible with aridity in latitudes 

 so high as northern England and the Gulf of St. Lawrence. If all 

 the regions of Permian red-bed deposition were arid it would seem 

 that humidity could not have been essential to equability of climate. 



