C. H. TYLER TOWNSEND — BIOGEOGRAPHY OE MEXICO, ETC. 
65 
side of the Organ Mountains, and several hundred feet (200 to 400 feet) 
higher up, than it is in the Rio Grande Valley at Las Cruces. This is 
due to the same causes pointed out by Dr. Rodgers. 
The greater summer heat of these river bottoms, over what exists on 
the surrounding plains and mesas, naturally paves the way for the north- 
ward extension of the fauna and flora from the somewhat hotter regions 
of the Lower Sonoran . The bottoms and valleys form an outlet, so to 
speak, for the overflow of the fauna immediately to the south, which is 
thus enticed to spread northward as far as possible. Here it finds some- 
what different conditions; under these influences, those forms which are 
able to adapt themselves remain, and in time become more or less modi- 
fied. Their northward limit is marked by the point beyond which they 
can no longer adapt themselves to the low winter temperatures. We 
can thus see that an annual faunal overflow, continued through long 
periods of time, would finally result in certain forms becoming perma- 
nent in these valleys, for a greater or less distance up. In this, way, cer- 
tain Lower Sonoran forms have extended up the Rio Grande Valley from 
the mouth of the Pecos to Albuquerque. Others have found their limit 
farther south, while a large number extend up half or two-thirds of the 
way to El Paso. The Lower Sonoran forms need a high summer tem- 
perature, which is here supplied. At the same time they become gradu- 
ally adapted to the low winter temperatures, so far as their capability in 
that direction goes. Such river bottoms offer more or less shelter and 
protection in winter, which can be taken advantage of by certain ani- 
mals, insects, etc. The same forms, however, could not thrive on the 
surrounding Upper Sonoran plains and mesas,- from the lack of a suffi- 
cient total of summer heat for their needs; notwithstanding the fact 
that the winter temperatures would be higher than those of the bottoms, 
while as a complement, shelter would be much less. The element of in- 
creased humidity in river bottoms threading the Upper Sonoran is, there- 
fore, the factor which has caused the Lower Sonoran extensions up these 
valleys. 
Further on Sensible Temperatures. — I wish also to quote here 
the following on actual and sensible temperatures, from an article by 
Prof. Edw. M. Boggs, on the climate of Arizona (Bull. Ho. 20 of the Ari- 
zona Agric. Exper. Station), as substantiating the stand taken by me 
on sensible temperatures in my first paper: 
“The simple thermometer, no matter how accurate it may be, does 
not measure temperature as felt by animal life. Its records must be con- 
sidered in connection with certain other data in order to afford a mode 
of comparison with the climates of other portions of the earth. We may 
term the reading of an accurate thermometer the actual , and the sensa- 
