Chapter X 



-179- 



Historical Causes 



Numerous attempts were made to solve these enigmatic peculiarities 

 in the distribution and biology of fossil floras. It was assumed that 

 plants of former periods possessed considerably greater ability, as com- 

 pared with present-day plants, to adapt themselves to different climates 

 and were considerably less sensitive to heat and cold. Or again, in 

 order to explain the uniformity of climate over a considerable extent 

 of the earth's surface, it was presumed that this uniformity of climate 

 was due to the intense heat in the center of the earth and the insignifi- 

 cant amount of losses of this heat from irradiation owing to the fact 

 that the earth was enveloped in a thick blanket of clouds. Lastly, it 

 was considered possible to assume that the absence of annual rings 

 was a peculiarity of plants of those times and that, consequently, this 

 could not serve as a basis for conclusions regarding climatic condi- 

 tions. 



Fig. 26. — Same as in Fig. 25, but in the Cretaceous period, (.\fter Koppen and 

 Wegener). 



These suppositions are refuted by the finding of traces of glaciers in 

 the most ancient deposits of the earth and also by the fact that even 

 in the Carboniferous period the climate was not every^vhere uniform, 

 since Carboniferous remains of trees having annual rings are known 

 from the Falkland Islands and from Australia. In the floras of the 

 succeeding geological stages periodicity in plant growth becomes of 

 ever more widespread occurrence. All this indicates that climatic zones 

 existed in past geological periods but that the location of these zones 

 was undoubtedly entirely different from now. 



It is likewise impossible to assume that there were any radical 

 differences in the physiology and biology of plants of former geological 

 periods. Paleobotanical data show that they were approximately the 

 same as they are today. According to these data, fossil plants must 

 have grown in climatic zones corresponding to the physiological peculi- 

 arities of these plants, whose requirements as regards light and heat 

 must have been the same as those of their present-day descendants. 



