62 University of California ruhlivalioits i)i Zoology [Vol.12 



points to act as a barrier to the dispersal of some animals and as a 

 highway of dispersal for others. 



The effects of the extraordinary and continuous load of sediment 

 of the Colorado River, together with the inconstancy of its channel, 

 doubtless account directly or indirectly for many of the peciiliarities 

 in the fauna. As far as known to the writer, in the Needles-to-Yuma 

 section of the river valley there are no aquatic molluscs or decapod 

 crustaceans, or tailed amphibians. Toads and frogs are present but 

 not abundant. Tlie fisli fauna in the main stream is sparse in both 

 species and individuals. 



Our party seined at three diffei'cnt points in the main stream. At 

 two of these nothing was caught ; in the third, a backwater slough on 

 the Arizona side above Mellen, four sorts of iishes were taken, catfish 

 {AmeinrHs iiebulosus), bony-tail {Gila elegans), hump-backed sucker 

 {Xyrauchen cypho). and carp {Cyprinus carpio). A huge minnow 

 {Ptycliodieihts lucius). called locally "Colorado salmon," was caught 

 with hook and line in back-water on the California side opposite 

 Cibola, and was plentiful immediately below the Laguna dam, where 

 many were being taken by the Indians living near there. In lateral 

 sloughs and overflow depressions carp and catfish were often observed 

 in numbers, and in these relatively clear waters they were far more 

 accessible to piscivorous animals than in the opaque water of the 

 river itself. It may be said, in conclusion, that the portion of the 

 Colorado River under consideration has relatively a very poor aquatic 

 fauna and flora, and that this poverty has had its effect in limiting 

 the occurrence of vertebrate animals dependent upon such sources of 

 food-supply. 



ZONAL AND FAUXAL POSITION OF THE REGION 



Two .schools of faunistic students are repr&sented among American 

 zoogeographic wTiters of the present day. One, of which C. H. Merriam 

 (1894, etc.) is the most prominent exponent, sees in temperature the 

 chief controlling cause of distribution, and deals with the ranges of 

 species in terms of "life-zones." The other school, of which C. C. 

 Adams (1905, etc.), A. G. Ruthven (1907). and Spencer Trotter (1912) 

 are active advocates, assigns to temperature but a minor role, looking 

 rather to a composite control, of many factors, resulting in ecologic 

 "associations," of which plants are essential elements, and which are 



