PALEOBOTANICAL EVIDENCE 503 



toleration must be less than 200° F. — that is to say, a permanent raising 

 of terrestrial temperatures above 200° F., or a lowering below 32° F., 

 would have inhibited life on this globe. It is of course possible that life 

 forms were more tolerant of temperature range in the earlier geologic 

 ages, but from what is now known of the nature and demands of proto- 

 plasm this seems very unlikely. 



PALEOBOTANICAL CRITERIA FOR DETERMINATION OF CLIMATIC CONDITIONS 



During the slightly more than one hundred years that practically cover 

 the period of the scientific study of paleobotany, a vast body of facts has 

 naturally been accumulated regarding the vegetation of the past. The 

 imperfection of the paleontologic record of course still leaves many lam- 

 entable gaps in our knowledge, though many of these are gradually 

 being filled in. At the same time the study of the living floras, by which 

 and through which the ancient floras must in large measure be inter- 

 preted, has also progressed by leaps and bounds. Through keenly prose- 

 cuted systematic studies the floras of the world have been made known 

 to us, while through the newly developed study of ecology we have come 

 to know much of the physical and environmental requirements of these 

 floras. To this must also be added a greatly increased knowledge of 

 histological and structural details, as well as marked advance in embryo- 

 logical, developmental, and evolutionary knowledge. Each of these fac- 

 tors is, to a great or less extent, now available in interpreting the floras 

 of the past. 



In seeking to ascertain the bearing of the floras on the climatic con- 

 ditions that obtained at the time they were living, it is desirable to set 

 forth the criteria that must form the basis of such judgment, and which 

 it is believed will furnish the most reliable evidence. As I have stated 

 on a former occasion : 



"In drawing conclusions from individual organisms in an inquiry of this 

 kind, dependence must of course be placed on our knowledge of the present- 

 day requirements of similar species, and the results must always be subject 

 to possibility of error from two sources — first, from the incorrect placing bio- 

 logically of the organism, and, second, from the fact that its requirements in 

 past geologic time may not have been the same as those which now dominate 

 the life activities of its supposed analogue. However, when all the elements 

 of a flora appear to point in the same direction the liability to serious error 

 Is minimized, if not eliminated." 



In the present connection the criteria that may seemingly be relied on 

 to furnish a reliable interpretation of climatic conditions have been so 

 succinctly enunciated by White in his paper 8n the Origin of coal that I 



XXXV— Bull. Geol. Soc. Am., Vol. 30, 1918 



