Life Zones 
xxiii 
cus iruei, P. rowleyi, and P. auripectus, Neotoma fallax, and 
N. arizoncB are all found in this zone, as well as in the zone 
below. 
In Colorado the Upper Sonoran Zone is that portion of the 
State lower than say 5,800 feet; it covers most of the plains 
region, and also a certain amount of rather rough country 
in the southern and western parts of the State. In the dry 
regions the greasewood (Sarcobatus) is found, also various 
cacti not found higher up, while it shares the sage-brush 
(Artemisia), junipers, and pinons with the Transition. 
Among our mammals Geomys lutes cens, Cratogeomys cas- 
tanops, Perognathus paradoxus, Citellus obsoletus, C. s, 
major, Peromyscus tornillo, Reithrodontomys nebrascensis, 
Neotoma baileyi, N. micropus, N. alb. warreni, N. deserto- 
rum, Antrozous pallidus, Pipistrellus hesperus, Nyctinomus 
depressus, and N. mexicanus seem at present to be confined 
to it, while various other species are shared with the Transi- 
tion, to say nothing of those which range into the zones 
above that. 
It should be stated that it is almost impossible to define 
exactly the limits of any of these zones, except perhaps the 
Arctic-Alpine. The most reliable data is afforded by the 
flora, as plants do not have the power of movement that 
animals have, and are also in many cases much more suscep- 
tible to climatic influences. On the other hand many of our 
birds and mammals are found in several zones. We have 
in Colorado three species of mammals which are found in 
every zone from Upper Sonoran to and including the Alpine. 
Many others are found in three and four of the zones, so 
that it will be seen that, on the whole, these forms do not 
specially characterize any one zone as a rule. Of course all 
have their limits, above or below which they seldom or never 
go, yet occasional individuals do wander surprisingly at 
times. 
