714 



SCIENCE. 



[N. S. Vol. XII. No. 306. 



the consequence of carrying a great richness 

 of forms into the South American region. 



Thei-e has also apparently, been an en- 

 croachment of elements developed in South 

 America northward, as shown in the Loas- 

 acecB (Mentzelias) and species of Prosopis, 

 whose great development occurs in the 

 Chilean and Argentine regions respectively. 



Greater details of distribution may be 

 discussed as follows : (1) The mountain 

 forms; (2) Forms of the arid basins and val- 

 leys of the Lower Sonoran Zone; (3) Sub- 

 tropical xerophilous forms of Gulf Zone dis- 

 tribution. 



(1) The Mountain Xerophilous Sonoran 

 Elements. 



In North America this element occupies 

 the arid mountain slopes and high plateaus 

 of the Transition and Upper Sonoran zones, 

 extending also into the deserts of the Lower 

 Sonoran. Its southward distribution has 

 been favored by the existence of an arid 

 zone comprising the moistureless west 

 slopes and enclosed plateaus of the Mexican 

 and Tropical Andes, lying mostly below the 

 altitudes of Alpine conditions. Both the 

 aridity and continuity of this zone have 

 varied with the changes in elevation, and 

 in all probability a north-south distribu- 

 tion of xerophilous mountain elements was 

 much easier at some earlier period than at 

 present. The facts of endemism are much 

 the same for the North American, Mexican 

 and Andean regions as in the case of Alpine 

 forms. 



Illustrations of this element include Xer- 

 ophilous ferns of the genera Gymnogramme, 

 Pellcea, § Eupelloea and § Cincinalis, Notho- 

 IcBtia and Cheilanthes, many of which range 

 from West Texas, New Mexico, Arizona, 

 etc., to Mexico, Guatemala and in the South 

 American Andes to Chile ; of the Leguminosa: 

 Astragalus, Dalea, I/apinus, Trifolium, Vicia 

 and Lathyrus ; Rosacece-Quillajece, Onagracece: 

 CEnothera, Oayophytum, Chammi^sonia, La- 



vauxia, Godetia, and Boisduvallia; Artemisia, 

 Peresia and AsterecB-Soladiginece of the Com- 

 positce; many Cadacece ; BorragwiaeecB-Eritri- 

 chece ; Gilia and many others. 



(2) Lower Sonoran Elements. 



These forms are of special interest because 

 they include the most extreme xerophytes 

 and halophytes occupying the most arid 

 deserts of both North and South America 

 in the extra-tropical regions, and mostly unrep- 

 resented in the long stretch of moist, trop- 

 ical and high mountain areas between. 

 Such are the mimosece, Prosopis, § Strombocarpa 

 with 3 species in Argentine, and 3 Lower 

 Sonoran species of west Texas, north Mex- 

 ico and westward ; § Algarobia with 19 spe- 

 cies mostly Argentine; Polygonacece-EriogonecB 

 with eleven Lower Sonoran genera (ex- 

 cept some Eriogonums) and the peculiar 

 subgenus Chorizanthopsis of the Chorisanthes, 

 endemic in Chile, and three species common 

 to both zones ; namely, Oxytheca dendroidea, 

 Ghorizanthe commissuralis and Lastarricea ehi- 

 lensis, all originally from the Californian re- 

 gion ; FrankeniacecE, with the very distinct 

 Franhenia jamesii of the west Texas region, 

 F. Palmeri of the southern California region, 

 F. triandra of the Puna region six nearly 

 allied Chilean species, one of which is in 

 California and Arizona and Nederleinia 

 juniperoides of the Argentine Salt Steppes, 

 more nearly related to the Lower Sonoran 

 than to the Chilean species. These, appar- 

 ently, constitute remnants of a previously 

 widespread development. 



The Zygophyllaoe also present an excellent 

 illustration of the phenomena of distribu- 

 tion here considered. Perhaps no plant is 

 more prominent as an indicator of the 

 Lower Sonoran Zone than Larrea mexieana 

 which is exceedingly abundant and wide- 

 spread over this zone. No representatives 

 of this genus occur between the southern 

 limits of the Lower Sonoran in Mexico, and 

 the Andes and Salt Steppes of Cardoba and 



