250 Merriam—Laws of Temperature Control. 
The area of overlapping of Boreal and Austral types is con- 
fined in most parts of the country to the narrow Transition zone, 
but along the Pacific coast it reaches all the way from southern 
California to Puget sound. This Pacific coast strip has always 
proved a stumbling-block to students of geographic distribution 
of life in America, but has now become the means of verifying the 
fundamental laws governing this distribution, as shown later. 
But while the boundaries of the several zones rarely coincide 
with absolute mechanical barriers, being fixed in the main by 
temperature, difference of opinion prevails as to the period during 
which the temperature exerts its restraining influence, and no 
formula for the expression of the temperature control has been 
heretofore discovered. None of the temperature data computed 
and platted on maps as isotherms are ayailable in locating the 
exact boundaries of the zones, because these isotherms invariably 
show the temperature of arbitrary periods, such as months, 
seasons and years—periods whose beginning and ending have 
reference to a particular time of year rather than a particular 
degree or quantity of heat. Thus the temperature for July, 
which is by far the most important of those commonly shown in 
isotherms, bears an inconstant relation to the hottest part of the 
year. Incertain localities the four hottest weeks may fall within 
the month of July, but in other localities they cover the period 
from the middle of June to the middle of July; in others from 
the middle of July to the middle of August, and in others still 
from the early part of August to early September. Similarly, 
the isotherms showing the mean annual temperature fail to con- 
form to the boundaries of the life zones, although in the far south 
they may be nearly coincident. The mean summer temperature 
is obviously inapplicable because of the varying length of the 
season in different localities. 
Several years ago I endeavored to show that the distribution 
of terrestrial animals and plants is governed by the temperature 
of the period of growth and reproductive activity, not by the 
temperature of the whole year; but how to measure the tem- 
peratures concerned was not then worked out. The period of 
growth and reproductive activity is of variable duration, accord- 
ing to latitude, altitude and local conditions of each particular - 
locality. In the tropics and a few other areas it extends over 
nearly the whole year, while within the Arctic circle and on the 
summits of high mountains it is of less than two months’ dura- 
