354 F. H. KNOWLTON CRITERIA FOR DETERMINATION OF CLIMATE 



Fossil plants have long been called the thermometers of the past; in 

 this light they have been regarded by most students as more trustworthy 

 guides than fossil animals, though when correctly interpreted there is 

 usually little discordance in the two lines of evidence. It is easy to see 

 why plants may furnish more reliable data than animals. A great ma- 

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

 power 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. AVhen, for whatever reason, 

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

 change its habitat, but the plant is largely fixed in position and when 

 stresses come must usually succumb. 



Stabilization of Temperatures for Plants and Animals 



Before presenting the paleobotanical criteria for interpreting ancient 

 climates, it is pertinent to call attention to the fact that since at least 

 Algonkian time terrestrial temperature must have been stabilized be- 

 tween relatively narrow limits, else life could not have been continuous, 

 as we have every reason to believe it has been. 



The vital processes in plants are practically suspended when the tem- 

 perature falls below 32° Fahrenheit (0° centigrade), though during their 

 resting stage many plants, especially in polar lands, are accustomed to 

 endure a temperature of — 70° Fahrenheit or even lower. The opposite 

 extreme is shown by certain simple types of algae that thrive in waters 

 of hot springs under a temperature approximating 200° Fahrenheit, and 

 there is a very considerable diversity of algal life in thermal springs with 

 temperatures ranging between 140° Fahrenheit and 180° Fahrenheit. 

 Certain bacteria, it is known, can, when in an encysted stage, withstand 

 temperatures ranging between -j-120° centigrade and — 200° centigrade, 

 or even — 250° centigrade for short periods. The optimum temperature 

 for plant life, however, is usually between 22° and 37° centigrade (71° 

 to 9S° Fahrenheit). 



The extremes of temperature between which animal life is possible are 

 apparently less than for plants. No known animal finds a congenial 

 habitat, either aquatic or terrestrial, that has a permanent temperature 

 of 200° Fahrenheit. 



From this discussion it appears that the range of biologic toleration 

 can hardly exceed 200° Fahrenheit — that is to say, a permanent raising 

 of terrestrial temperature above 200° Fahrenheit or lowering it below 32° 

 Fahrenheit would have inhibited life on this globe. 



