1914] Grinnell: Mammals and Birds of the Colorado Valley 53 



The present paper concerns itself with the mammals and birds 

 of the region, and is based upon the collections and field-notes obtained 

 in 1910, as above enumerated. No attempt has been made as yet to 

 work np the reptiles and amphibians, though these promise interesting 

 results. The writer's efforts have been concentrated upon the mam- 

 mals and birds, with a view to establishing first of all the systematic 

 status of the various included forms; in other words, to determine 

 their relationships with similar species or races of the surrounding 

 regions. Such determination has been performed in a manner wholly 

 satisfactory to the writer in only a part of the critical cases. As must 

 always obtain in work of this sort, lack of material in certain direc- 

 tions often puts abrupt limitations upon investigation. 



The obvious principle has been followed, that no generalized treat- 

 ment of a concrete subject like geographic distribution is justifiable 

 upon any but the soundest basis of systematic analysis. 



Because of the close dependence of most mammals and birds upon 

 plants, the latter have an important place in any treatment of animal 

 distribution. The set of plants secured by the Colorado Kiver Expedi- 

 tion has been named by Professor H. M. Hall, of the University of 

 California. Names so provided are used throughout the present paper, 

 particularly in the discussion of associational areas. 



My sincere acknowledgements are hereby extended to Professor 

 Charles II. Gilbert, Department of Zoology, Leland Stanford Junior 

 University, and to Professor Charles A. Kofoid, Department of 

 Zoology. University of California, for critical suggestions concern- 

 ing general considerations; to Professor H. M. Hall, of the University 

 of California, for critical reading of the chapter on associations; 

 and to Messrs. Harry S. Swarth and Walter P. Taylor, fellow staff- 

 members in the California Museum of Vertebrate Zoology, for help 

 from time to time on many points of detail. 



ITINERARY 



Our party assembled at the town of Needles, California, on the 

 evening of February 14, 1910, and on the following clay our first 

 collecting station was established in the river bottom close by. 



A scow was constructed for the transportation of our rather bulky 

 outfit, while the skiff purchased, being a much readier means of loco- 

 motion, enabled us to traverse the river expeditiously where need be. 

 It was found practicable to divide the party during a portion of the 



