232 Merriam—Laws of Temperature Control. 
year. This formula was based on the belief that plants of the 
same species living in different places arrive at the same phase 
of development by utilizing the same proportion of the total heat 
which they receive in the course of a season. 
Students of geographic distribution may dismiss this phase of 
the inquiry as not pertinent to the problem in hand, for we are 
concerned with the physiological constant of the species itself, not 
of any stage or period in its life history. But what is the physio- 
logical constant of a species, and how can it be measured? If 
it is true that the same stage of vegetation is attained in different 
years when the sum of the mean daily temperatures reaches the 
same value, it is obvious that the physiological constant of a species 
must be the total quantity of heat or sum of positive temperatures re- 
quired by that species to convplete its cycle of development and repro- 
duction. The difficulty in computing such sums is in fixing the 
end of the period during which temperature exerts its influence 
upon the organism. In the case of plants this can be done by 
direct observation of a particular individual or crop, in con- 
nection with careful thermometric readings covering the whole 
period of vegetative activity, and data of this sort have been 
actually recorded by certain European phenologists, but I am 
not aware that an attempt has been made to correlate the facts 
thus obtained with the boundaries of the life zones. Since, 
however, all forms of life are affected by temperature and it is 
manifestly impracticable to ascertain by direct observation the 
total quantity of heat necessary to enable the various species of 
mammals, birds and reptiles to complete the annual cycle of 
reproduction, and since the areas inhabited by definite assem- 
blages of animals and plants have been found to be essentially 
coincident, it is evident that a more generalized formula is neces- 
sary. If the computation can be transferred from the species to 
the zone it inhabits—if a zone constant can be substituted for a 
species constant—the problem will be well nigh solved. This I 
have attempted to do. In conformity with the usage of bota- 
nists, a Minimum temperature of 6° C. (48° F.) has been as- 
sumed as marking the inception of the period of physiolggical 
activity in plants and of reproductive activity in animals. The 
effective temperatures or degrees of normal mean daily heat in 
excess of this minimum have been added together for each 
station, beginning when the normal mean daily temperature 
rises higher than 6° C. in spring and continuing until it falls to 
