234 Merriam—Laws of Temperature Control. 
counter, sooner or later, a degree of heat they are unable to 
endure. The difficulty is in ascertaining the length of the period 
whose mean temperature acts asa 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 
from the north southward. For experimental purposes, and 
without attempting tinnecessary refinement, the mean normal 
temperature of the six hottest consecutive weeks of summer was 
arbitrarily chosen and platted on a large contour map of the 
United States, as in the case of the total quantity of heat. On 
comparing a reduced copy of this map (plate 13) with the zone 
map (plate 14) it appears that the isotherms conform to. the 
southern boundaries of the Boreal, Transition and Upper Aus- 
tral life zones, and that the isotherm of 18° C. (64°.4 F.) agrees 
almost precisely with the southern boundary of the Boreal re- 
gion. The coincidence is indeed so close 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. 
If the isotherm of 18° C. (64°.4 F.) for the six hottest consecu- 
tive weeks (see plate 13) iscompared with that of 5,500° C-(10,000° 
F.), showing the sum of positive temperatures (see plate 12), it will 
be observed that the two are coincident in the main-except in a 
few localities. The principal discrepancy is along the Pacific 
coast from Puget sound to southern California. In this strip 
maps 12 and 13 not only fail to agree, but are fundamentally dif- 
ferent, showing that no constant relation exists between the mean 
temperature of the six hottest consecutive weeks and the total 
of heat for the season. The mean temperature of the hottest 
part of the year from about latitude 35° northward along the 
coast is truly boreal, being as low as the mean of the correspond- 
ing period in northern Maine and other points well within the 
Boreal zone. The mean of the six consecutive hottest weeks at 
several points on the coast of California is as follows: At Eureka, 
on Humboldt bay, 18°.5 C. (56° F.);* at San Francisco, 15°.5 C. 
(60° F.); at Monterey and Ventura, 17°.5 C. (63°.5 F.).} Strange 
* In the following mean temperatures, fractions smaller than one-half a 
degree are ignored. 
+ Santa Barbara, between Monterey and Ventura, has a slightly higher 
mean (67° F.), which is explained by its situation on alow, narrow coastal 
plain facing the south, with a range of mountains immediately on the 
north. 
