PART III. 



LAWS OF TEMPERATURE CONTROL OF THE GEOGRAPHIC 

 DISTRIBUTION OF ANIMALS AND PLANTS.' 



Apart from obvious mechanical barriers, such as oceans, temj)era- 

 ture is the most important single factor in fixing the limits beyond 

 which particular species of animals and plants can not go. Investi- 

 gations conducted b}'^ the ]3iological Survey have shown that the north- 

 ward distribution of terrestricd animcds and iiJants is governed hy the 

 sum. of the 'positive temjjeratures for the entire season of growth and 

 rej)roductio7i,^ and that the southward distribution is governed by the 

 mean temperature of a brief period during the hottest part of the year.'^ 



zone temperatures. 



The 1>oreal Region. 



Arctic, Hudsonian, and Canadian zones. — Tlie distinctive tempera- 

 tures of uhe three Boreal zones (Arctic, Hudsonian, and Canadian) 

 are not positivelj' kno^^^l, but the southern limit of tlie Boreal as a 

 whole is marked by tlio isotherm of 18° C. (04.4° F.) for the six hottest 

 consecutive weeks of summer. It seems probable, from the few data 

 available, that the limiting temperatures of the southern boundaries 

 of the Hudsonian and Arctic zones are, respectively, 14° C. (57.2° F.) 

 and 10° C. (50° F.) for the same period. 



' Abridpjed from a paper by the author entitled ' Law.s of temperature control of 

 the Reo.i^raphic distribution of terrestrial animals and plants,' National Geo- 

 graphic Magazine, Vol. VI, pj). 220-'2;}8, '^ colored maps, December, 1894. 



-In computing thesiunof the positive or effective temperatures a minimum tem- 

 perature of G^ C. (43*^ F.) has been assumed as marking the inception of the period 

 of physiological activity in plants and of reproductive activity in animals. The 

 effective temperatures or degrees of normal mean daily heat in excess of this mini- 

 mum havel)een added together for each station, beginning when the normal mean 

 daily temperature rises higher than G C. in spring and continuing until it falls to 

 the same point at the end of the season. The sums thus obtained were platted on 

 a large scale map of the United States and connected by isotherms, which were 

 found to conform in the most gratifying manner with the northern boundaries of 

 the several life zones. 



■'The exact length of this period has not been determined. It must be short 

 enough to fall wthin the hottest part of the summer in high northern latitudes, 

 and probably increases in length from the north southward. For experiment, the 

 mean normal temperature of the six hottest consecutive weeks was arbitrarily 

 chosen and platted on a large scale map. Isotherms were then drawn, which con- 

 formed .so closely with the southern boundaries of the Boreal, Transition, and 

 Upper Austral zones that the matter was not carried further. 

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