C. H. TYLER TOWNSEND-BIO-GEOGRAPHY OF TEXAS. 
89 
COMMENTS ON DR. MERRIAM’S LAST PAPER. 
Dr. Merriam’s last paper, “ Laws of Temperature Control of the Geo¬ 
graphic Distribution of Terrestrial Animals and Plants,” being the an¬ 
nual address before the National Geographic Society (December, 1894), 
marks a real advance in our knowledge of the biogeography of North 
America. I most heartily concur in all of the views there laid down, 
excepting only the following points: 
The law that the northward distribution of animals and plants in the 
northern hemisphere is determined by the sum of the effective temperatures 
(or total quantity of heat during the season of groivth and reproduction ) 
finds its complement in the following, which modifies it and acts as a 
constant check to it: 
Animals and plants are restricted in northward distribution in the 
northern hemisphere by low temperatures during the season of hiberna¬ 
tion (or winter season). 
For example, there is a sufficient total of heat during the season of 
growth and reproduction in most portions of Dr. Merriam’s Lower Aus¬ 
tral zone (Nat. Geog. Mag., vol. vi, 1894, pi. 14) to satisfy the proper 
growth and fruiting of' the date palm, but in many of the same portions 
there is not a sufficient total of heat during the winter season to prevent 
it from perishing of cold. I notice that Dr. Merriam refers in a foot¬ 
note to killing frosts, thus touching upon this point, but it is not a few 
“sensitive species” that are affected, especially when it comes to the 
region along the natural boundary between the Upper and Lower Sonoran , 
but a very large percentage of them. 
In running his isotherms and computing the total quantities of effective 
heat in the southwestern United States, Dr. Merriam has overlooked a 
most important factor in the climatic conditions of the arid region. It 
is well known that the same “apparent temperature” in degrees affects 
animals and plants in an arid climate very differently from what it does 
in humid climates, the actual difference in effect being equal to 25° or 
even more. Therefore any plan of the total effective temperatures which 
does not take into consideration this factor of the absence of humidity is 
wide of the mark. What we need to deal with is not the “ apparent 
temperature,” but the “ sensible temperature.’’ ’ The latter is obtained by 
the reading of the wet bulb, and represents the actual amount of heat 
felt by animals and plants, especially the former. Thus we see the pres¬ 
ence here of very important counter-conditions which bend the isotherms 
of “sensible temperature” out of the course they would otherwise fol¬ 
low. As an illustration of this I cite the statement of Capt. Glassford, 
that the very highest official record of heat (“apparent”) at Yuma, 
