Books and Current Literature 49 



of plants, each referable to its special rock-formation, are clearly 

 recognized and described. 



The section on invasion, competition and succession includes 

 records with maps showing the exact present position of certain 

 invading species, thus recording in permanent form the data 

 for future comparison. There is abundant evidence that within 

 the area under consideration severe competition prevails between 

 many of the plants, notwithstanding the generally received 

 view that with desert plants the struggle for existence is almost 

 exclusively with their physical environment rather than with 

 each other; a view, it is to be said, which approaches the truth 

 the more distinctively arid are the prevailing conditions. Thus 

 in the zone of the creosote bush, where really desert conditions 

 exist, competition between species is reduced to a minimum, 

 and the struggle with environmental factors really determines the 

 outcome. Well-marked successions of permanent character, 

 corresponding with changes of topography, are readily ob- 

 served. In the cases described an essential change of topograph- 

 ical boundaries has been promptly followed by the advance 

 of certain associations into an extension of their appropriate 

 habitats. 



In the third part of the work, devoted specifically to environ- 

 mental and historical factors, there are discussed in order the 

 geology (by Professor C. F. Tolman), soils (Dr. B. E- Livingston), 

 climatic conditions, and animals of the Desert Laboratory do- 

 main. The data are too numerous for reproduction. It is 

 sufficient for the present purpose to state that in various particu- 

 lars, especially as regards soils, and atmospheiic conditions, 

 some of the most striking facts of local distribution are correlated, 

 with a certainty equivalent to demonstration, with factors 

 which stand to them in a causal relation. 



The section on vegetation groups of the Desert Laboratory 

 domain, contributed by Professor J. J. Thornber, shows that of 

 the 449 species listed no less than 264 are limited in their distri- 

 bution to the southwestern United States and Mexico, in other 

 words, at least 59% of this flora as it exists today is to be thought 

 of as belonging' distinctively to the desert region of Southwestern 

 North America. With regard, therefore, to the geographical 



