Forested slopes and bare rocky 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 Laboratory, 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 element 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 expectancy 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 possible error, but between these two places ranges the vegetation from the subtropics to the pines under conditions of temperature largely influenced by the relief and orography. The facts which have been brought to light in this single thermometric traverse of a valley and up a mountain slope shows the need of extended surveys for the purpose of evaluating the temperature factor as an agency affecting the distribution of organisms, and when our generalizations can be broadly based and rationally formulated we may also be in a position to furnish the paleontologist 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 comparative effects of the different climatic complexes represented at our various plantations make necessary long extended observations coupled with
