The Origin of the Desert Climax and Climate loi 



both respects is even greater, if possible, in Fraxinus ameri- 

 cana-texensis-veltttina-coriacea-oregona; the first two overlap in 

 Texas; velutina reaches from Texas to Inyo County, California; 

 coriacea occurs in the desert region from southern Utah to south- 

 eastern California, and oregona from the mountains of southern 

 California to Washington. Platanus occidentalis-tvrighti-race- 

 mosa extends from New England to southern and Lower Cali- 

 fornia, touching the borders of the desert on both sides. Populus 

 deltoidea and fremonti meet in southern Colorado and western 

 Texas to span the continent, and the closely related wislizeni 

 overlaps them both in Texas and New Mexico. 



Though best regarded as one polymorphic species consisting 

 of monophylla, edtdis, cembroides, and quadrifolia, with com- 

 plete overlapping in the needle number, the pinyon is usually 

 treated as four. Pinas edulis ranges from Utah and Colorado 

 to western Texas, Arizona, and northern Mexico; cembroides 

 through the southern half of Arizona and adjacent Mexico to 

 Lower CaHfornia; quadrifolia from Lower California to the 

 arid Santa Rosa Mountains along the Colorado Desert, and 

 monophylla from northeastern Utah to Nevada, northern Ari- 

 zona, and the southern Sierras into Lower California. Jumperus 

 utahensis and californica meet today in southeastern California, 

 while the general areas of monosperjna and occidentalis confront 

 each other over a long line. 



Arbutus menziesi makes its home from British Columbia to 

 the mountains of San Diego County, California; it is separated 

 by the desert from A. arizonica in southern Arizona and Chi- 

 huahua, where A. arizonica is in proximity to A. xalapensis of 

 Mexico and Texas. Aesculus californica reaches the margin of 

 the Mohave in Antelope Valley; its cognates are glabra and 

 octandra, which range from Pennsylvania to Oklahoma and to 



