236 Merriam—Laws of Temperature Control. 
The fundamental laws here developed, phrased for the northern 
hemisphere, may be briefly formulated as follows : 
(1) The northward distribution of animals and plants is de- 
termined by the total quantity of heat—the sum of the effective 
temperatures. 
(2) The southward distribution of Boreal, Transition zone, and 
Upper Austral species is determined by the mean temperature of the 
hottest part of the year. 
ZONE TEMPERATURES. 
Boreal Zones.—The distinctive temperatures of the three Boreal 
zones (Arctic, Hudsonianand Canadian) are not positively known, 
but the southern limit of the Boreal as a whole is marked by the 
isotherm of 18° C. (64°.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. 
Transition Zone species require a total quantity of heat of at 
least 5,500° C. (10,000° F.), but cannot endure a summer tem- 
perature the mean of which for the six hottest consecutive weeks 
exceeds 22° C.(71°.6 F.). The northern boundary of the Transi- 
tion zone, therefore, is marked by the isotherm showing a sum 
of normal positive temperatures of 5,500° C. (10,000° F.), while 
its southern boundary is coincident with the isotherm of 22° C. 
(71°.6 F.) for the six hottest consecutive weeks. 
Upper Austral species require a total quantity of heat of at 
least 6,400° C. (11,500° F.), but apparently cannot endure a sum- 
mer temperature the mean of which for the six hottest consecu- 
tive weeks exceeds 26° C. (78°.8 F.). The northern boundary of 
the Upper Austral zone, therefore, is marked by the isotherm 
showing a sum of normal positive temperatures of 6,400° C. 
(11,500° F.), while its southern boundary agrees very closely 
with the isotherm of 26° C. (78°.8 F.) for the six hottest consecu- 
tive weeks. | 
Lower Austral species require a total quantity of heat of at 
least 10,000° C. (18,000° F.). The northern boundary of the 
Lower Austral zone, therefore, is marked by the isotherm showing 
a sum of normal positive temperatures of 10,000° C. (18,000° F-.). 
A formula expressing the temperature-control of its southern 
boundary has not yet been found. 
