1895. ] Current Literature. 85 
is remarked that the question is not one of spreading over all avail- 
able areas, but how this tendency to spread has been checked. The 
circumpolar belts of distribution in the northern hemisphere are pri- 
marily three: boreal, austral, tropical. These have subdivisions of 
various rank, the austral, for example, having the transition, upper 
austral, lower austral subdivisions. The most remarkable case of over- 
lapping is that found on the Pacific coast, where the 1,000 miles from 
southern California to Puget sound belongs to the transition zone, 
elsewhere narrow, in which boreal and austral forms freely mingle, 
and which must account for that wonderfully varied flora. The lim- 
itation of these life zones Dr. Merriam finds to be due to temperature 
as the great primary cause. The two fundamental laws worked out 
are formulated as follows: (1) the northward distribution of animals 
and plants is determined by the total quantity of heat, the sum of the 
effective temperatures (that is, those above the assumed minimum); 
(2) the southward distribution of boreal, transition, and upper austral 
Species is determined by the mean temperature of the hottest part of 
the year. The application of these laws to the Pacific coast strip will 
Serve both as an illustration and as an explanation of that remarkable 
region. The data at hand demonstrate: (1) that the temperature of 
the summer season is phenomenally low for the latitude and altitude, 
so low as to enable boreal types to push south to latitude 35°; (2) that 
the total quantity of heat for the entire season is phenomenally high 
for the latitude, so high as to enable austral types to push north to 
Puget sound. It should be said that the minimum temperature of 
6°C. (43°F.), has been assumed as marking the inception of the period 
of physiological activity in plants and of reproductive activity among 
animals, and that the total quantity of heat is obtained by adding 
temperatures above this minimum. Some of the temperature limits 
(64.°4F.), for the six hottest consecutive weeks; /vansifion, northern 
boundary, isotherm with a sum of normal positive Sot siete 
5:500°C. (10,000°F.), southern boundary, isotherm of 22°C. (71.°6 
for the six hottest consecutive weeks; upper austral, northern baeut 
ary, isotherm with a sum of 6,400°C. (11,500°F.), southern boundary, 
isotherm of 26°C. (78.°8F.) for the six hottest consecutive weeks; ower 
austra/, northern boundary, isotherm with a sum of 10,000°C. (18,000° F.); 
‘ropical, northern and southern boundaries marked by isotherm show- 
ing a sum of 14,400°C. (26,000°F,). The most prominent secondary 
cause affecting distribution is said to be humidity. Three colored 
maps of the United States show distribution of total quantity of heat, 
mean temperature of six hottest. consecutive weeks, and life zones. 
The close resemblance of the maps is remarkable. 
