Geographic Distribution of Life. 237 
Tropical species require a total quantity of heat of at least 
14,400° C. (26,000° F.); and, since the Tropical Life region is a 
broad equatorial belt, it is probable that both its northern and 
southern boundaries are marked by the isotherm showing a sum 
of normal positive temperatures of 14,400° C. (26,000° F.). 
An interesting fact respecting the relative values of the zones 
is brought/out by the isotherms showing the total quantity of 
heat necessary for each. It appears that the Transition and 
Upper Austral zones are not of equal value, but that together 
they are the exact equivalent of the Lower Austral zone. 
¥ GOVERNING TEMPERATURES. 
Northern limit. Southern limit. 
Regions. Zones. a 
Sum of normal mean | Normal mean tem- 
daily temperatures perature of six 
above 6° C. (48° hottest consecu- 
ECR ae tive weeks. 
C F, (Ge F 
VANCE CRE ay Seer eee ratee te ete iararecee oh Ace NOR 50° * 
OLGA 3) LRG SO MAM sce 5 stellen sleet s rel|t ccnsveracis ole ales Ola * 
(Gharmeyc hiewny 3.0.5 aameect lated cohcore SORES Pimper 18° 64°.4 
Transition 1. .-... 5,500° 10,000° 4 228 TALS6 
Austral...|{ Upper Austral?..| — 6,400° 11,500° 26° 78°.8 
Lower Austral*..| 10,000° TS ODOLS octets tall Gtste seneatee 
“RIQOy SNC) Dei Se OR ee a ee 14,500° ZOLOOOSS oiveres spre a dinckersracrsete 
* Estimated from insufficient data. 
1The Transition zone comprises three principal subdivisions: an east- 
ern or Alleghenian humid area, a western arid area, and a Pacific coast 
humid area. : 
*The Upper Austral zone comprises two principal subdivisions: an 
eastern or Carolinian area and a western or Upper.Sonoran area. 
The Lower Austral. zone comprises two principal subdivisions : 
eastern or Austroriparian area and a western or Lower Sonoran area. 
*The Fahrenheit equivalents of Centigrade sum temiperatures are stated 
in round numbers to avoid small figures of equivocal value. 
an 
SECONDARY CAUSES AFFECTING DISTRIBUTION. 
It is not the purpose of the present essay to discuss the secon- 
dary causes affecting distribution. At the same time it seems 
desirable to contrast for a moment the influence of humidity, 
which is by far the most potent of the secondary causes, with 
that of temperature, which has been shown to be the primary 
