GENERAL DISCUSSION. 



175 



The annual average from data covering 36 years is seen to be 2.74 inches and the char- 

 acter of the precipitation phenomena suggests a high degree of aridity. The maximum 

 amount received in one year was 7.10 inches (1906) and the lowest, a "trace" (less than 

 0.01 inch) in 1904, giving a variation as 1 to 1000, a proportion occurring in deserts of a 

 pronounced degree of aridity only. In addition to this expression, the ratio of possible 

 evaporation from a free-water surface to the annual amount of precipitation has been 

 useful in characterizing deserts. About 116 inches of water would evaporate from the 

 surface of a small vessel on the ground in the open in the Cahuilla during a year; this is 

 15 times the amount which has fallen in any one year, 43 times the average, and many 

 thousands of times the minimum. (These estimates are to supersede the figures given in 

 a statement made before the Royal Geographical Society of London in December 1911, 

 and published in the Geographical Journal, vol. 40, p. 105, 1912.) The possible evapora- 

 tion in northern Africa at Algiers is about 60 inches annually, but in the interior of the 

 Sahara it is much greater. The figures in table 36, compiled by Dr. W. A. Cannon, give 

 data from four points, of which Laghouat is far inland and Ghardaia the most arid. 

 The measurements were made in millimeters with a Piche evaporimeter and should be 

 multiplied by 0.74 to obtain the evaporation from a free-water surface. 



The possible evaporation for a year at Ghardaia is about 160 inches, and as the 

 average precipitation is 0.5 inch the ratio would be as 1 to 330 as compared with 1 to 

 43 in the Cahuilla. While this basin is by no means so arid as the Sahara, the desicca- 

 tion results in the reduction of the soil moisture to a minimum during the warmer season, 

 and organisms would not only be subjected to intense aridity, but also to the action of 

 soil solutions of a high concentration. This effect is highly accentuated in places where 

 the evaporation results in the deposition of soil salts, given a charged substratum in which 

 only halophytes can survive. Seepages and flowing wells afford a supply of ground water 

 in places which furnish conditions for vegetation not essentially different from those in 

 any warm climate, although the species which inhabit them are subjected to a high rate 

 of water-loss. The occasional rains call into action a few plants of short cycle, which like- 

 wise may be of more or less mesophytic habit. The number of such "annuals," however, 

 is very small, as it is in the Libyan desert in Northern Africa. 1 



Outside of these types the specialized conditions of soil moisture, soil salts, evaporating 

 capacity of the air, and aspect of the precipitation offered by the more arid portions of 

 the basin and of deserts in general might be expected to be accompanied by the presence 

 of specialized types of vegetation, especially since it is reasonably certain that the desert 

 of the Colorado, in which the Cahuilla lies, has been arid since Tertiary times. It seems 

 highly probable that the entire number of characteristic desert plants have undergone 

 their evolution within the period during which this region has been arid. No fossil material 

 has been recovered which suggests reduced spinose shoots, such as those of Zizyphus, Con- 

 dalia, or Parkinsonia, on the one hand, or the succulent stems of the Cactacese, Euphorbi- 

 acese, Asclepiaceae, or Crassulaceae on the other, although some of the ancient Bennettiales 

 and Cycadales suggest that they were suitable for endurance in regions in which the water- 



1 See MacDougal, The Deserts of Western Egypt. Plant World, vol. 16, p. 291. 1913. 



