INTRODUCTION 501 



familiar and as seemingly permanent as this climatic zoning is at the 

 present time, it is believed that it can be conclusively demonstrated that 

 it is more or less abnormal or exceptional — that is, when earth history is 

 viewed as a whole it is increasingly evident that the time during which 

 climate has been distributed in zones represents but a small fraction of 

 geologic time. 



The following may be taken as the thesis of the first part of this paper. 



Eelative uniformity, mildness, and comparative equability of 

 climate, accompanied by high humidity, have prevailed over the 

 greater part of the earth, extending to, or into, polar circles, dur- 

 ing the greater part of geologic time — since, at least, the Middle 

 Paleozoic. This is the regular, the ordinary, the normal condi- 

 tion.^ 



The study of climatic conditions that obtained during the past — the 

 study of fossil climates, so to speak — is beset with some difficulties. Nat- 

 urally, these conditions can only be interpreted through the impress re- 

 tained by the physical and biological surroundings, and these are not 

 always of equal distinctness or equal value. It is not the intention in the 

 present paper to consider fully the purely geologic criteria ; nor is it the 

 intention, or indeed the desire, to draw any invidious comparisons be- 

 tween the two great classes of biologic criteria, but it is, I believe, gen- 

 erally admitted that plants inherently possess the qualities which permit 

 them to exhibit the more reliable criteria as to the climatic conditions 

 under which they passed through their life processes. A great majority 

 of animals are endowed at some stage in their life cycle with the poAvei 

 of locomotion, which enables them to move about more or less freely in 

 response to various external forces, perhaps the most important of which 

 is that of the climatic environment. When, for whatever reason, the 

 conditions become unfavorable, the animal is more or less free to change 

 its habitat: but with the plant it is usually quite different. 



Part I. Paleobotanical Evidence in the Interpretation oe 

 GEOLOGIC Climates 



TEMPERATURE TOLERATION IN PLANTS AND ANIMALS 



Before presenting the paleobotanical evidence regarding ancient cli- 

 mates, it is pertinent to call attention to the fact that, whatever the 

 decision may be regarding geologic climates, it is certain that since at 

 least Algonkian time terrestrial temperatures have been stabilized be- 



2 David White and F. H. Knowlton : Science, new ser., vol, 31, 1910, p. 760. 



