Geographic Distribution of Life. 233 
the same point at the end of the season. The sums thus ob- 
tained have been platted on a large scale map of the United 
States,* and isotherms have been run which are found to con- 
form in a most gratifying manner to the northern boundaries of 
the several life zones, as may be seen on comparing a reduced copy 
of this map (see plate 12) with a map of the life zones (see plate 
14). The latter, it may be observed, is identical, save a few correc- 
tions in minor details, with the third edition of my Bio-geographic 
map of North America (prepared a year ago and published in the 
Annual Report of the Secretary of Agriculture for 1893).+ While 
the available data are not so numerous as might be desired, the 
stations in many instances being too far apart, still enough are 
at hand to justify the belief that animals and plants are restricted 
in northward distribution by the total quantity of heat during the sea- 
son of growth and reproduction.t 
The isotherm indicating a sum total of 5,500° C. (10,000° F.) 
coincides with the northern limit of distribution of Transition 
zone species, agreeing in the main with the dividing line be- 
tween the two primary life regions of the northern hemi- 
sphere—Austral and Boreal. But in areas where extensive over- 
lapping of Austral and Boreal types occurs, as along the Pacific 
coast from southern California northward to Puget sound, it will . 
be observed that the isotherm in question points, as elsewhere, 
to the northern limit of Austral types and bears no relation what- 
ever to the southward limit of Boreal types. It is evident, 
therefore, that the southward range of Boreal species, and per- 
haps of others also, is regulated by some cause other than the 
total quantity of heat. This cause was believed to be the mean 
temperature of the hottest part of the year,§ for it is reasonable 
to suppose that Boreal species in ranging southward will en- 
*Gannett’s ‘‘ Nine-sheet contour map,’’ published by the U. 8. Geo- 
logical Survey. 
{+ The only changes worth mentioning are the introduction of the Trop- 
ical along the lower Colorado valley, the extension of the Tropical across 
the peninsula of Florida, and the extension of the Transition along the 
Pacific coast strip. 
t In the case of certain sensitive species another factor enters into the 
problem, namely, killing frosts, for a few species are excluded by the occur- 
rence of frosts from areas having a sufficient total quantity of heat for 
their needs. 
2 This was indicated by mean summer temperatures platted from time 
to time during the past fifteen years, but the length of the period was 
never satisfactorily ascertained. 
