866 



SCIENCE. 



[N. S. Vol. XXIII. No. 597. 



east and west, plateaus, plains, basins, 

 buttes, broad valleys and narrow canyons 

 giving great diversity of the most remark- 

 able natural features to be found in the 

 world. 



The geology is that of later rocks, prin- 

 cipally the easily eroded Cretaceous and 

 other Mesozoic formations. Tremendous 

 volcanic activity in former times has 

 poured out vast floods of lava which, to- 

 gether with tufas or agglomerated ash, 

 form the most noticeable physiographic 

 features of the region. 



Latitude, elevation and natural barriers 

 here conspire to produce modifications in 

 climate. This is seen in the convolutions 

 of the isotherms of 50°, 59° and 68° cross- 

 ing the region, the scanty rainfall occur- 

 ring in the winter and summer months, the 

 excessive insolation, the extremes of day 

 and night temperature, the high winds and 

 rarefied air, which characterize the arid 

 environments. 



The fitness of the southwest to sustain 

 I)iotic forms depends mainly upon rainfall, 

 ■which itself is regulated by cosmic and geo- 

 graphic conditions. Thus the uprush of 

 heated air from the sun-baked plateaus dur- 

 ing the summer draws in the moisture-laden 

 air from the oceans, producing rains Avhich 

 are unequally distributed ; the higher moun- 

 lains, acting as condensing centers, receive 

 the most, while the plains are scantily 

 watered. The receptivity of the land must 

 also be considered, the mountains covered 

 with vegetation storing water, and the bare 

 land shedding it into the rivers, which must 

 carry at times vast floods and during long 

 periods remain perfectly dry. Everywhere 

 is evidence of the colossal agencies which 

 are at work reducing the land to sea level. 

 This workshop is littered with the bones of 

 the mountains, and the dust that is sorted 

 by water and wind moves freely to lower 

 levels or is blown higher to again resume 

 its gravitational coiirse. 



Despite the generally adverse conditions 

 set forth, there flourishes here a flora of a 

 peculiar character which forms the basis 

 of subsistence for an extensive fauna, and 

 also the newcomer, man, who has pinned 

 much of his faith to nature 's supply. The 

 region is not in any part a desert in the 

 true sense of the term, which is applied 

 to lands deserted because of an inhibition 

 of life, but it is rather a semi-arid environ- 

 ment, in which a preponderance of desic- 

 eative and other factors have restricted and 

 minimized life. These restrictions are ob- 

 served in full force among the plants fixed 

 in the earth, and therefore played upon 

 by all the natural forces, to which they 

 must adapt themselves by slower changes 

 than are required by higher biotie forms. 

 The characteristic climatic flora is thus 

 xerophytic where the leaves are small, with 

 structures for preventing too rapid evap- 

 oration, stems contain chlorophyl and act 

 when leaves fall away, etc. ; these are adap- 

 tations which give some plants the freedom 

 of the desert. Other plants are succulent 

 and spring to quick fruition when rains 

 occur; other plants have perfected water- 

 storing organs in stem, root and branches, 

 as the cacti, yucca, atriplex, sareobatus, 

 etc., and still others can live in soils con- 

 taining an excess of mineral salts. 



Most of the desert plants bear witness to 

 the struggle with sun, wind, rarefied air 

 and inhospitable soil; thus they present a 

 gnarled, wrinkled and bizarre appearance, 

 often simulating trees dwarfed by the 

 gardener's art. Unlimited opportunity is 

 here for isolation by natural boundaries, 

 which, if not a factor in the origin of spe- 

 cies, at least powerfully aids in their pres- 

 ervation.^ 



- Discussion by Jordan, Bailey and others, in re- 

 cent numbers of Science. The Desert Labora- 

 tory of the Carnegie Institution, near Tucson, 

 Ariz., is attacking these problems with en- 

 thusiasm. 



