54 



SCIENCE. 



[N. S. A'OL. I. No. 2. 



attained in different j^ears when the sum of 

 the mean daily tempei-atures reaches the 

 same value, it is obvious that the physio- 

 logical constant of a species must be the 

 total quantity of heat or sum of positive tempera- 

 tures required by that spiecies to complete its cycle 

 of development and reproduction.'''' Now, "if 

 the computation can be transferred fi-om the 

 species to the zone it inhabits — if a zone 

 constant can be substituted for a species con- 

 stant — ^the problem will be well nigh solved." 

 This Dr. Merriam has attempted to do. 

 " In conformity with the usage of botanists, 

 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 ani- 

 mals. The effective temperatures or degi-ees 

 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 were plotted on a 

 large scale map of the United States, and 

 isotherms were run which were found to 

 conform to the northern boundaries of the 

 several zones. This is shown by colored 

 maps. The data seem to justify the state- 

 ment that " animals and plants are restricted 

 in northward distribution by the total quantity 

 of heat during the season of (jrouih and repro- 

 ductive activity. ' ' 



In the case of the southern boundaries of 

 the zones, it was assumed that animals 

 and plants in ranging southward would en- 

 counter, sooner or later, a degree of mean 

 summer heat they are unable to endure. 

 " The difficulty is in ascertaining the length 

 of the period whose mean temperature acts 

 as a barrier. It must be short enough to 

 be included within the hottest part of the 

 summer in high northern latitudes, and 

 would naturally increase in length fi-om the 

 north southward. For experimental pur- 



poses, and without attempting unnecessary 

 refinement, the mean normal temj)erature 

 of the six hottest consecutive weeks of sum- 

 mer was arbitrarily chosen and plotted on 

 a large contour map of the United States, 

 as in the case of the total quantity of 

 heat." 



On comparing this map with the zone 

 map, the isotherms of 18°, 22° and 26°C 

 were found to conform respectively to the 

 southern boundaries of the Boreal, Transi- 

 tion and Upper Austral zones, leadmg to 

 the belief that " animals and plants are re- 

 stricted in southivard distribution by the mean 

 temperature of a brief period covering the hottest 

 part of the year." 



Except in a few localities the northern 

 boundary of Austral species coincides with 

 the southern boundary of Boreal species, 

 but for a distance of more than a thousand 

 miles along the Pacific coast a curious over- 

 lapping and intermingling of northern and 

 southern tj'pes occurs. On looking at the 

 temperature maps this is at once explained, 

 for the mean temperature of the six hottest 

 consecutive weeks from about lat. 35° north- 

 ward to Puget Sound is truly Boreal, being 

 as low as the mean of the corresponding- 

 period in northern Maine and other points 

 well within the Boreal zone. On the other 

 hand, the total quantity of heat is found to 

 be the same as that required by Austral 

 species. " It is evident, therefore, that the 

 principal climatic factors that permit Boreal 

 and Austral types to live together along the 

 Pacific coast are a low summer teinpera 

 ture combined with a high sum total of 

 heat." 



A table is given showing the actual gov- 

 ei-ning temperatures, so far as kno\Aai, of the 

 northern and southern boundaries of the 

 several zones. 



In conclusion. Dr. Merriam calls attention 

 to the subordinate value of humidity as 

 compared with temperature. " Humidity 

 and other secondary causes determine the 



