DESERTS OF THE UNITED STATES AND MEXICO 703 



The armature of desert plants is often 

 thoughtlessly cited as an adaptation by 

 which these forms protect themselves 

 against the ravages of animals. The 

 presence of spines undoubtedly operates 

 to prevent a plant from being eaten by 

 animals, but the action of the animals 

 has in no wise induced their formation 

 by the plant. As a matter of fact, the 

 fatality among desert plants by injury 

 from animals is greatest in the seedling 

 stage. For every prickly-pear that sur- 

 vives, tens of thousands of seedlings are 

 eaten by rodents, and these seedlings are 

 as unarmed as those of any other type. 



CHANGE OF CIvIMATE: 



A change of the climatic conditions 

 throughout the Southwest, and especially 

 in the semi-desert region of Arizona and 

 New Alexico, is marked everywhere by 

 the evidence of a much heavier rainfall 

 than we now have. River valleys in 

 many cases show only dry gravelly or 

 sandy beds which evidently were for- 

 merly occupied by continuous streams. 

 The floods that once carved their way 

 across the slopes or over the plains are 

 no longer seen, at least not in the same 

 volume as in former time. Even existing 

 streams do not reach in times of great 

 flood their former volume and carrying 

 capacity. All tell of diminished volume, 

 whether in the desert regions or in the 

 regions of abundant plant-growth. 



We may believe that the cause is ex- 

 traterrestrial and cosmic, and a part of 

 the great era of climatic changes giving 

 to the earth the glacial era, and its grad- 

 ual decay. We may believe that the era 

 of greatest precipitation in the Southwest 

 and elsewhere was coincident with the 

 widest extension of the glaciers and that 

 while the higher mountains were being 

 loaded with snow, the lower slopes were 

 deluged with rain or watered freely by 

 the melting snows and enjoyed a verdure 

 no longer possible. 



EXTINCTION OF THE GREAT MAMMALS 



The fact of the existence and wide 

 geographical range in Arizona of the 

 great mammals, the mammoth and the 



mastodon, shows a very dii¥erent condi- 

 tion of vegetation up to comparatively 

 recent geologic time. The extinction of 

 these giant herbivores may be best ex- 

 plained upon the theory of the desicca- 

 tion of the region rather than by a change 

 of temperature or increasing cold, as ap- 

 parently was the case in Siberia, and may 

 have been in the glaciated regions of Cali- 

 fornia. A great change in the rainfall 

 and the drying up of the slopes and 

 mesas of Arizona must of necessity have 

 caused a great change in the growth of 

 plants, involving their destruction over 

 great areas. It would appear that the 

 extinction of the giant mammals and the 

 disappearance of suitable vegetation for 

 their sustenance proceeded together, and 

 were due to increasing heat and dryness 

 rather than to increasing cold. 



We have ample evidence that in the 

 Cretaceous era conditions in Arizona 

 were favorable to forest growth and lux- 

 uriant vegetation. The coal-beds of Deer 

 Creek near Saddle Mountain in Pinal 

 County, described by Emerson, reveal 

 such conditions. 



Quantities of silicified tree trunks in 

 the vicinity of Yuma and the prostrate 

 forms of giant trees turned to stone in 

 the Petrified Forest Park bear eloquent 

 testimony to such forest growths and to 

 destructive climatic changes in Tertiary 

 time. 



More recent evidence is found in 

 springs surrounded by relics of vegeta- 

 tion, such, for example, as Andrade's 

 Spring east of Tucson and on the right 

 bank of Davidson's Canyon, where there 

 is a thick accumulation of sphagnum 

 with stumps of trees and, at the bottom, 

 teeth of the mastodon. 



The former existence in i\rizona of a 

 species of Bos of unusual size is shown 

 by the discovery of enormous horn-cores 

 in the gravels of the secondary or deriva- 

 tive slopes of the Santa Ritas at Greater- 

 ville. 



THE CACTUS FLOWERS AND PRICKLY 

 PEARS 



The greatest activity among the cacti 

 is displayed by the cereuses and opun- 



