64 r inri rsity of California Publications in Zoology [Vol. 12 



the plotting of the ranges of many animals as well as of plants, 

 coincidence in this regard is found in so many cases as to warrant the 

 recognition of a number of ' ' f aunal areas, ' ' on the causative basis of 

 relative uniformity in humidity. It is probable that every species is 

 affected by both orders of geographic control. 



As to which is the more important, assembled data seem to show 

 that more genera and higher groups are delimited by zonal boundaries 

 than by f aunal boundaries (see Merriam, 1892, p. 49, etc.) The 

 arresting power of temperature barriers would therefore seem to be 

 relatively the greater. 



In the third category of distributional control there is a con- 

 spicuous association of the majority of so-called adaptive structures of 

 animals (often of high taxonomic value) with certain mechanical 

 or physical features of their environment. An animal may thus 

 intimately depend upon certain peculiarities, inorganic or organic, or 

 both, of a given area, and be unable to maintain existence beyond the 

 limits of occurrence of those features of the environment. For instance, 

 Bipodomys deserti is delimited by soil of certain texture and depth. 

 Tracts of relatively uniform environmental condition, including their 

 inanimate as well as living elements, are here called associations. 



The geographical distribution of any animal is correctly diagnosed 

 in terms of each of the above three groupings. In other words, an 

 animal belongs simultaneously to one or more zones, to one or more 

 faunas, and to one or more associations. No one of these groupings 

 can be stated in terms of the other, any more than a person can com- 

 pute liquids by the peck, or weight in miles. The constituent species 

 within each of these groupings always belong to the other two. To 

 illustrate: the southern white-headed woodpecker inhabits the con- 

 iferous forest association of the San Bernardino fauna of the Transi- 

 tion zone; the Abert towhee belongs to the mesquite and quail-brush 

 associations of the Colorado Desert fauna, of the Lower Sonoran zone; 

 the Pacific shrew belongs to the upland riparian association of the 

 northern coast redwood fauna of the Transition and Boreal zones. 



Referring now to the region contiguous to the lower Colorado River, 

 we have good reason, both biotic and meteorologic, for assigning it 

 all to one zone, namely, the Lower Sonoran, and to one fauna, (lie 

 Colorado Desert ; but many associations are represented. In other 

 words, the variation in altitude and latitude included is not great 

 enough to bring sufficient modification of the characteristically high 

 temperature to affect profoundly the distribution of the plant and 



