PROTECTIVE PIGMENTS AND APPEARANCES 359 



words, the resemblance to the model does not protect. Moreover, 

 in a part of the island where monkeys are supposed to be the chief 

 enemies of the butterflies, the most abundant forms are those that 

 are supposed to be defenseless forms, whereas the mimics are scarce. 



FIG. 176. Desert plants 



Cholla cactus on the western deserts. The thickened leaves and short stems, or the 



entire absence of leaves, may be considered as a more or less direct adaptation to the 



high temperature and the dry soil, which together make up the danger of excessive loss 



of water. (From photograph by the United States Reclamation Service) 



405. Reduction of surface. Some organisms may derive a 

 kind of protection from a reduction of surface. This is espe- 

 cially common among plants that are exposed to the danger 

 of drought. In desert plants we observe a comparatively small 

 surface in proportion to their bulk (Fig. 176). 



