BIOME AND BIOME-TYPE IN WORLD DISTRIBUTION 



591 



spread. Valvelike closure of nostrils, eyes, 

 and mouth in lizards and snakes burrowing 

 in loose sand are found equally in Califor- 

 nia and Arabia; both Old World and New 

 World lizards and snakes may have wid- 

 ened bodies for burrowing by lateral and 

 vertical movement instead of forward plow- 

 ing into loose soil or sand; and the ex- 



west, the grasses interspersed among sage- 

 brush and greasewood, and even in the 

 creosote bush desert, support some hardy 

 cattle. Such vegetation persists even in rock 

 desert, where the sagebrush may be no 

 higher than the surface blocks of lava, and 

 where narrow ribbons of excellent grass 

 may follow dry drainage courses. In our 



*« I 



i .■■ 



Fig. 224. Desert landscape in southwestern North America: Saguaro desert, Superior, Arizona. 

 (Photograph by Howard K. Gloyd, Chicago Academy of Sciences.) 



tremely remarkable helical "side-winding" 

 of snakes is employed by the sidewinder 

 of southwestern United States and in north 

 Africa and Arabia by the small vipers of the 

 genus Cerastes.'' 



From the standpoint of general prin- 

 ciples, by far the most interesting aspect 

 of the desert biomes is their extremely grad- 

 ual gradation into, and transition to, 

 grassland and to scrub forest. In Chile, 

 tor example, the blank desert, with extreme 

 impoverishment of flora and fauna at the 

 north, grades into the bush scrub and 

 scrub forest of the gradually better-watered 

 central Chile. In the North American South- 



* Aspis of nomenclaturists. 



North American Southwest, grazing, since 

 the turn of the century, sometimes on land 

 that would support no more than one head 

 of cattle on 20 acres, has changed what 

 must have been grassy steppe into man- 

 made desert. The high incidence of plants 

 of truly desert character in these areas be- 

 speaks an original interspersal of desert and 

 grassland according to edaphic conditions. 

 Mesquite may hold widely spaced mounds 

 of sand, and be no higher than these 

 mounds, in a mesquite desert; and may 

 grade by insensible degrees into a mesquite 

 forest Uke that of southern Texas, which is 

 low forest rather than grassland, to which 

 it is commonlv referred. 



