^'ol-XII"! Recent Literature. 17 R 



1895 J 



accordingly attacked not only the subject of the "species constant" but 

 that of the " zone constant." " In conformity with the usage of botanists," 

 he continues, " a minimum temperature of 6° 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 minimum have been 

 added together for each station, beginning when the normal mean daily 

 temperature rises higher than 6^C. in spring and continuing until it falls 

 to the same point at the end of the season. The sums thus obtained 

 have been platted on a large scale map of the United States, and isotherms 

 have been run which are found to conform in a most gratifying manner 

 to the northern boundaries of the several life zones." The available data, 

 though not so full as desirable, appear "to justify the belief that animals 

 and plants are restricted in northward distribution by the total quantity 

 of heat during the season of growth and reproduction." 



The southern limit of distribution of species, however, must evidently 

 be determined by some other cause, as, probably, a degree of heat greater 

 than they are able to sustain. "The difficulty," as Dr. Merriam observes, 

 "is in ascertaining ihe length of the period whose mean temperature acts 

 as a barrier." This for the present he has "arbitrarily" assumed to be 

 the "six hottest consecutive weeks of summer"; and on platting the 

 mean normal of this period it is found to agree so closely "with the 

 southern boundary of the Boreal region ""as to justify the belief that 

 animals and plants are restricted in southward distribution by the mean 

 temperature of a brief period covering the hottest part of the year." 



Three maps accompany the paper, giving (i) the 'Distribution of the 

 total quantity of Heat during Season of Growth and Reproductive 

 Activity"; (2) 'Mean Temperature of Hottest Six consecutive weeks 

 of the"Year'; (3) 'Life Zones of the United States.' The boundaries 

 limiting the temperature areas and the life zones are so nearly coin- 

 "cident that thev are practically identical ! The temperature observation 

 stations are shown on the first two maps, and from their remote posi- 

 tions along some of the boundaries platted it is evident that these lines 

 are to some extent tentative and hypothetical; yet it is not probable 

 that they will be found far from their correct position when tuller data 



are secured. 



Dr. Merriam's paper is not onlv a step in the right direction but a 

 most important contribution to the subject in hand. The premises are 

 in the main sound, but it is evident that no one temperature limit will 

 tit all species as a point of departure for computing the species constant, 

 reproductive activity, in many animals at least, beginning at a tempera- 

 ture far below 6° C, the '• inception of reproductive activity" being often 

 determined by a combination of circumstances having little to do with 

 a definite temperature of 6- C, however different the case may be with 

 plants. — J. A. A. 



