b CALIFORNIA ACADEMY OF SCIENCES. 



lasts only a short time, except on the highest peaks, 

 where, in sheltered spots on northern slopes, it may be 

 seen in midsummer. At intervals of many years the 

 ground is whitened for a few hours in the valleys. 



The locally diverse conditions of temperature and 

 rainfall are of necessity accompanied by a variation in 

 the character and abundance of the vegetation, and 

 consequently of the insect fauna. Only along the 

 crests or upper slopes of the higher ranges — notably of 

 the San Bernardino Mountains — is there anything 

 approaching forest growth. At these altitudes — 5,000- 

 8,000 feet — there are considerable areas covered sparsely 

 with pine, cedar, fir, and scattered oaks. Alders line 

 the streams in the mountain caiions, while willows, 

 sycamores and cottonwoods grow along the watercourses 

 at lower levels wherever the supply of moisture is suffi- 

 ciently permanent. Oaks of several species are more or 

 less common throughout the lower foothills, and occur 

 here and there in park-like groves in the valleys. The 

 foothills are everywhere covered with a mixed growth of 

 scrub-oak, Ceanothus, greasewood, manzanita, and a 

 variety of other bushes and low trees, which, in some 

 places, form almost impenetrable thickets. The valleys 

 are green in the rainy season and in early spring are 

 bright with a profusion of wild, flowers, but are burned 

 dry and brown by the sun in summer and autumn. 

 The vegetation of the Colorado and Mojave deserts is 

 naturally limited in amount and peculiar in kind, con- 

 sisting mainly of Yucca, cactus, and mesquite, with the 

 usual "sagebrush". 



From the preceding remarks it may be seen at once 

 that Southern California is separable into two principal 

 faunal districts — one the desert region to the eastward, 

 occupying more than half the entire area and extending 



