502 F. H. KXO^'LTOX EA-OLUTIOX OF GEOLOGIC CLIMATES 



tween relatively narrow limits, else life coTild not have been conidnnoiis, 

 as Tve have every reason to believe it has been. "'Tlie control of secular 

 climates/" says Professor Chamberlin/ "'is obviously a condition pre- 

 requisite to biologic continnity. The preservation of a narrow range of 

 temperature and a limited variation of atmospheric constituents throngh- 

 ont the millions of years of the biologic past was absolntely essential to 

 organic evoltition. Continued preservation for millions of years to come 

 seems equally a condition precedent to an intellectnal and spiritual evo- 

 lution commensurate with the physical and biological evolutions that have 

 preceded it." 



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

 perature falls below 32° F. (0. C), though during their resting stage 

 many plants, especially m polar lands, are accustomed to endure a tem- 

 perature of — 70° F. or even lower. The opposite extreme is shown by 

 certain simple types of alg^ that thrive in the waters of hot springs under 

 a temperature approximating 200° F., and there is very considerable 

 diversity of algal life in thermal springs with temperatiires ranging be- 

 tween 110° F. and 180° F. 



It is now known that c-ertain bacteria and protozoa when in an encysted 

 or resting stage can successfully endure a temperature of nearly — 200° 

 C. for six months and about — 250° C. for shorter periods, and the spores 

 of certatQ other bacteria can withstand for a shorter time a temperature 

 as high as 120° C. These extremes of tolerance, however, are far greater 

 than the organism could endure when in active growth. 



The optimum temperature for plant life is usually between 22° and 

 3T~ C. (71° and 98° F.). There is, however, quite a wide range for spe- 

 cies and even ditferent individuals of the same species. *Tn tropical 

 plants the minimum temperature may be as high as 4-10° C, while those 

 of higher latitudes, where the first plants of spring often grow through 

 a covering of snow, as well as those of the higher Alps and polar regions, 

 grow vigorously at a temperature but little above zero.*^ — Strasburger. 



The extremes of temperature between which animal life is possible are 

 apparently less than for plants. For instance, no known animal finds a 

 congenial habitat, either aquatic or terrestrial, that has a permanent tem- 

 perature of 200° F., though many of them can and do carry on their 

 essential life processes at a temperature considerably below 32° F.. yet 

 were such temperature to be reached they must ultimately succumb, siuce 

 in its last analysis "all food of all animals is supplied by plants." 



From the foregoing discussion it appears that the range of biologic 



3 T. C. Chamberlin : Jour. Geology, vol. 14. 1906. p. .36.3. 



