90 
TRANSACTIONS OF THE TEXAS ACADEMY OF SCIENCE. 
Arizona, is 116° F., but the “sensible heat” at the same time was only 
84° F. This is a difference of 32° between the “apparent” and “sen¬ 
sible” temperatures. In the Eastern States, 84° of sensible heat are not 
uncommon when the thermometer stands only as high as the nineties. 
The same “ apparent temperature” is in effect cooler in the summer and 
warmer in the winter in an arid climate, but the “sensible temperature” 
is identical in both. It thus follows that animals and plants are subjected 
to a more limited range of sensible temperature between the same par¬ 
allels of latitude in the arid region than in the humid, conditions of ele¬ 
vation being equal. Therefore, aided greatly by the peculiar configura¬ 
tion of the country, which at first stretches southward in a gradually 
rising mountain chain and then in a very long, broad, and gradually 
rising plateau, the transcontinental life zones widen in the arid region to 
a remarkable degree. Enough has been said above to demonstrate that 
only isotherms representing the “sensible temperature” can be used in 
biogeographic work. 
Dr. Merriam’s two fundamental laws of temperature control of the 
geographic distribution of terrestrial life, modified so as to agree with 
the above, and including a wider application of the second, would thus 
read as follows: 
I. The northward or higher distribution of life in the northern hemis¬ 
phere is determined by the total quantity of sensible heat during the season 
of growth and reproduction , checked by low sensible temperatures during the 
season of hibernation. 
II. The southward or lower distribution of extra-tropical life in the 
northern hemisphere is determined by the mean sensible temperature of the 
hottest part of the year. 
The frost line should be used for the southern boundary of the Lower 
Austral zone. The line marking the southernmost limit of frost, during 
a period of any twenty or even ten consecutive years, would define the 
actual southern limit of the Temperate region more nearly than any other. 
Temperature observations are needed at various points on the frost line 
to determine the approximate total of sensible heat expressing the tem¬ 
perature control. 
Dr. Merriam gives the least total quantity of apparent heat required 
by Ti'opical species as 14,400° C. or 26,000° F. I believe that 27,000° 
F. of apparent heat expresses it more nearly, as being the least total with 
which the majority of Tropical species can exist. The mean daily tem¬ 
perature (apparent) of Kingston, Jamaica, for the ten consecutive years 
from June, 1880, to May, 1890, was 78.1° F. On no occasion did the 
temperature sink to 6° C. Therefore 365 times 78.1° F. gives the sum 
of the effective apparent temperatures, or over 28,500° F. 
I believe that Dr. Merriam’s totals of temperature control are too low 
