Geographic Distribution of Life. 231 
tion.* It is evident, therefore, that while in the tropics there 
may bea close agreement between the mean annual temperature 
and the life zones, in the north the widest discrepancy exists 
between them. 
At one time I believed that the mean temperature of the actual 
period of reproductive activity in each locality was the factor 
needed, but such means are almost impossible to obtain, and 
subsequent study has convinced me that the real temperature 
control may be better expressed by other data. 
For more than a century physiological botanists have main- 
tained that the various events in the life of plants, as leafing, 
flowering and maturing of fruit, take place when the plant has 
been exposed to a definite quantity of heat, which quantity is 
the sum total of the daily temperatures above a minimum as- 
sumed to; be necessary for functional activity. The minimum 
used. by Boussingault and early botanists generally was the freez- 
ing point (0° C. or 32° F.), but Marie-Davy and other recent 
writers believe that 6° C. or 43° F.{ more correctly indicates the 
temperature of the awakening of plant life in spring. In either 
case the substance of the theory is that the same stage of vegetation 
is attained in any year when the sum of the mean daily temperatures 
reaches the same value, which value or total is essentially the same 
for the same plant in all localities. This implies that the period 
necessary for the accomplishment of a definite physiological act, 
blossoming for instance, may be short or long, according to local 
climatic pecuharities, but.the total quantity of heat must be the 
same. The total amount of heat necessary to advance a plant 
to a given stage came to be known as the physiological constant of 
that stage. Linsser beheved this law to be fallacious and main- 
tained that the physiological constant of any particular stage of 
vegetation was not the sum total of heat acquired that time, but 
the ratio or proportion of this sum to the sum total for the entire 
season. Thus Linsser’s physiological constant is the ratio of the 
sum of the mean daily temperatures at the time when any par- 
ticular stage of vegetation is attained to the sum total for the 
*S6e N. Am. Fauna, No. 3, September, 1890, pp. 26, 27, 29-32; also 
Presidential Address, Biological Soc. Wash., vol. vii, April, 1892, pp. 
45, 46. 
+I began work on this line about fifteen years ago and continued at 
intervals for ten years before convinced of its impracticability. 
t The exact equivalent of 6° C. is 42°.8 F. 
