C. H. TYLER TOWNSEND — BIOGEOGRAPHY OE MEXICO, ETC. 63 
Sonoran broadly up this valley through New Mexico, as far north as 
Albuquerque. Such extensions up river bottoms would naturally result 
to a certain extent from a difference in elevation, but this is so slight in 
most cases that it can not explain the long range of some species up river 
valleys out of their natural life areas. We must therefore look for other 
causes tending to produce this result. Dr. Mark A. Rodgers, in a very 
valuable and interesting paper, “The Climate of Arizona” (read before 
the American Climatological Society at Lakewood, N. J., May 13, 1896; 
and published in the Medical and Surgical Reporter of May 16, 1896), 
touches upon the differences in climatic conditions between river bottoms 
and plains in the arid region. As he presents this subject as plainly as possi- 
ble, I shall quote from his paper at some length. It will be seen that he 
points out climatic changes that have already occurred, and may take 
place in future, through putting large areas under irrigation. At the 
same time his remarks also apply to some extent to river bottoms under 
ordinary natural conditions, especially where an annual rise takes place 
and overflows the bottoms, constituting a natural method of irrigation; 
and where the water permeates the soil up to within a comparatively 
short distance of the surface, forming what are called subirrigated lands. 
Dr. Rodgers says: 
“It will be noticed by Table III that of all the stations in southern 
Arizona, Phoenix shows the greatest variation of temperature, being the 
hottest in summer and the coldest in winter. Phoenix is at an altitude 
of 1068 feet, just midway between Yuma and Tucson. Under ordinary 
conditions we would expect to see the temperature there also midway 
in extremes between these two points. Although we have not at our 
command records of the relative humidity in the Salt River Yalley for 
more than a period of six months, we feel quite safe in assuming that 
the difference is due to this cause. Phoenix is situated low down in the 
valley near the Salt River, and according to the well known principles of 
gravity as applied to atmospheric drainage, the cold moi^t air settles in 
the most dependent localities. This is quite apparent by the sense on 
appreciation in the differences between the sensible and actual tempera- 
Table III. — Showing the Relation between Temperature and Altitude. 
Stations. 
Altitude. 
Mean 
Minimum. 
Mean 
Maximum. 
Yuma 
141 
42 
106.3 
Phoenix 
10G8 
32 
107.3 
Tucson 
2400 
35 
100.8 
*Prescott 
5389 
20 
84.9 
*Prescott, in Northern Arizona. But the conditions are the same as for regions in Southern 
Arizona at corresponding altitudes. 
