io6 F' E' Clements 



of the smaller number of woody species, these often give a dis- 

 tinctive character to the Colorado Desert. The most notable are 

 Washingtonia filifera, Cereus giganteus, Fouquiera splendens, 

 Par\insonia torreyana, Olneya tesota, Dalea spinosa, Beloperone 

 californica, and Simmondsia californica. It is understandable 

 that these are all of southern derivation, while the Mohave has 

 obtained most of its important shrubs from the west, north, 

 and especially the east. Even the Joshua tree (Yucca brevi folia) y 

 though characteristic, is not a strict endemic. 



Fossil Plants 



As is well known, grassland and desert do not aflEord ready op- 

 portunity for the production of plant fossils, partly because 

 leaves and stems are not easily detached, but chiefly for the 

 reason that water bodies are relatively few in those regions. The 

 paucity of fossils is naturally more pronounced in desert than in 

 grassland and, so far as the climax desert is concerned, amounts 

 to almost complete absence from the Pleistocene up to the pres- 

 ent. Conditions were more favorable on both counts during 

 the Pliocene and still more so in the Miocene, with the result 

 that the fossil floras are increasingly better known for these 

 periods of greater rainfall. Hence, in scrutinizing the evidence 

 of the course of development, it is desirable as usual to proceed 

 from the present and thus to consider the Pleistocene horizons 

 first. The value of these depends in large measure upon proxim- 

 ity, apart from number of species; those in southern California 

 come first, none being found in the true desert so far; those 

 farther north demand more interpretation in terms of differ- 

 ence of climate and vegetation. 



Although the general practice of paleo-botany is to provide a 

 separate name for a fossil species, even though it is regarded as 



