432 THE POPULAR SCIENCE MONTHLY 



Forested slopes and bare rock}' surfaces do not lose heat at the same rate 

 at night or warm up at the same rate in the daytime. The air cooled on 

 the bare slopes flows down the declivity, collecting in the valleys beneath 

 as would water. Consequently the main building of the Desert Labora- 

 tory, 400 feet above the station at the base of Tumamoc hill, is ordinarily 

 15° to 20° F. warmer than the plantation below at night. This exercises 

 marked influence on the organisms inhabiting these places. In the 

 Santa Catalina mountains in which much of our experimentation is 

 done, this inversion of temperature and collection of cold air operates 

 to give valleys climates equivalent in temperature to the great ridges 

 half a vertical mile higher, in illustration of which it is cited that the 

 cacti, characteristic of warm places, spread highest on the crests of the 

 mountain ridges. The divergence of the temperatures from the normal 

 rate to be expected from increase of temperature may be also illustrated 

 by the following facts: 



During the low extremes which characterized the climate of the 

 southwest during the first month of the present year the minima were 

 as follows : 



Actual Estimated 



Desert Laboratory (2,700 feet) 17° F 



Breeding plantation (2,300 feet) 1° F. 18° F. 



Xero-montane plantation (6,000 feet) —6° F. 7.6° F. 



Rim of Bear Canon (7,000 feet) 5° F. 3° F. 



Montane plantation (8,000 feet) —2° F. ±.5° F. 



If now these places are plotted on a scale in which the vertical ele- 

 ment was magnified, but the fall in temperature was computed on the 

 basis of one degree for about every three hundred feet in elevation, it 

 may be seen that the 2,300 feet locality diverges 17° F. from the ex- 

 pectancy by reason of cold air drainage, the 6,000 feet locality is 13.6° 

 F. colder, the 7,000 feet location 2.5° F. colder and the 8,000 feet location 

 2.5° F. colder. The difference between the mountain top at 8,000 feet 

 and the Desert Laboratory as correspondent to within the limit of pos- 

 sible error, but between these two places ranges the vegetation from the 

 subtropics to the pines under conditions of temperature largely influ- 

 enced by the relief and orography. The facts which have been brought 

 to light in this single thermometric traverse of a valley and up a moun- 

 tain slope shows the need of extended surveys for the purpose of evalu- 

 ating the temperature factor as an agency affecting the distribution of 

 organisms, and when our generalizations can be broadly based and ra- 

 tionally formulated we may also be in a position to furnish the paleon- 

 tologist with criteria for the better interpretation of the occurrence of 

 the plants and animals found in ancient deposits. 



The number of things to be considered in the study of the compara- 

 tive effects of the different climatic complexes represented at our various 

 plantations make necessary long extended observations coupled with 



