ZOOLOGY 69 



places in this area and formation. For this same reason the 

 use of these formations is usually to be preferred to that of 

 the much more inclusive zones of Merriam. The Upper 

 Sonoran for instance includes such diverse formations as the 

 grassy steppe about Albuquerque and the Pinon forest of 

 the Sandias, and, as can be readily seen by referring to the 

 lists, the latter includes very many forms not found in the 

 former and vice versa. In fact the species not common to 

 the two are nearly as numerous as those that are. In other 

 words these formations represent a very definite complex of 

 environmental factors and one which is fairly uniform where- 

 ever it occurs, whereas "New Mexico" or "Sonoran" does 

 not stand for anything nearly so uniform in the matter of 

 environment. On the other hand, when an animal occurs in 

 several formations the term "Upper Sonoran" may be very 

 convenient as it obviates the necessity of repeating all of the 

 formations and I have frequently used it in the following 

 pages. 



The formations divide themselves into the same three 

 great groups as the topography but each is capable of f u rther 

 subdivision as follows: 



