Geographic Distribution of Life. 235 
as it may seem, San Francisco has a lower normal mean tem- 
perature during the hottest part of the year than Eastport, Maine, 
the mean at Eastport being 16° C. (61° F.).. On the other hand, 
the sum of positive temperatures (the normal mean daily tem- 
peratures above 6° C.) at San Francisco is more than 10,000° 
Fahrenheit higher than at Eastport, being 11,290° C. (20,360° F.) 
at the former and only 5,470° C. (9,880° F.) at the latter locality. 
At no point in the Pacific coast strip is the sum of the positive 
temperatures known to fall below 7,330° C. (13,600° F.), and it 
reaches 8,200° C. (14,800° F.) at Tatoosh island, off cape Flat- 
tery, the extreme northwestern point of the United States. Even 
at cape Flattery, therefore, the total of heat for the season is 
260° C. (600° F.) greater than at Eastport, Maine, though the 
latter is the more southern locality and has the higher mean 
summer temperature. 
The data at hand for the Pacific coast strip are amply sufli- 
cient to demonstrate two important facts: (1) that the tempera- 
ture of the summer season, the hottest part of the year, is phe- 
nomenally low for the latitude and altitude—so low, indeed, as 
to enable Boreal types to push south to latitude 35°; (2) that 
the total quantity of heat (the sum of the positive temperatures) 
for the entire season is phenomenally high for the latitude—so 
high, indeed, as to enable Austral types to push north to Puget 
sound. The total of heat is even greater at Puget sound than at 
Philadelphia, Pittsburg, Cleveland, Indianapolis, Keokuk, or 
Omaha, though five hundred miles north of the latitude of these 
places. In other words, the mean temperature of the hottest 
part of the year is sufficiently low for Boreal species, while the 
total quantity of heat is sufficiently great for Austral species. 
It is evident, therefore, that the principal climatic factors that 
permit Boreal and Austral types to live together along the Pa- 
cific coast are a low summer temperature combined with a high sum 
total of heat. The temperature is remarkably equable throughout 
the year; it never rises high for any length of time, and killing 
frosts are rare. 
The study of the accompanying maps was the means of lead- _ 
ing me, first, to the explanation of the anomalous distribution of 
species on the Pacific coast, where for a distance of more than a 
thousand miles a curious intermingling of northern and southern 
forms occurs ; and, second, to what I now conceive to be the true 
theory of the temperature control of the geographic distribution 
of species. 
32—Nar. Grog. Maa., vor. VI, 1894. 
