No. 400.] ZONES IN ARIZONA AND NEW MEXICO. 289 
ceed commercially at Phoenix. The Larrea in the Mesilla 
Valley occupies only the higher levels, its lower limit, a little 
distance from the valley bottom, being as clean-cut as if it had 
been planted. But in the Salt River Valley the Larrea covers 
the whole area, and grows to a great size in the bottom lands, 
which are never so cold as to injureit. It is the desert land, 
once occupied by the Larrea, which has under irrigation been 
converted into the most flourishing orchards ; and could the 
Larrea land of the Mesilla Valley be irrigated the results would 
doubtless be most gratifying. 
These examples show us the value, on the one hand, of an 
exact knowledge of temperature conditions; and the fallacy, 
on the other, of mapping the temperature for the entire country 
from the observations made in a few widely scattered localities. 
Thus the temperature tables for Phoenix would not apply to 
Ingleside, nor those for Mesilla Park to the bench a couple 
of miles away. The writer has discussed the subject from his 
own standpoint, that of the fauna and flora ; but he would not 
be understood to undervalue in any degree the knowledge of 
temperature in mapping life zones; all he would urge is, that 
for the proper mapping of zones on the temperature basis, we 
need a mass of information we do not possess, and are not 
likely to possess in the near future. 
REPRESENTATIVE Facts REGARDING THE NATIVE FAUNA 
AND FLORA. 
(1) Species Common to the Mesilla Valley and Salt River Valley. 
PLANTS. 
Perezia nana Gray. Larrea tridentata DC. 
Pluchea (Tessaria) borealis Gray. _ Cladothrix lanuginosa Nutt. 
Pectis papposa Harv, and Gray. Hoffmanseggia stricta Benth. 
Aster spinosus Benth. Populus Jremonti inm 
Helianthus annuus L. Salix fluviatilis 
Baccharis glutinosa Pers. Atriplex canescens rn Tun) James. 
Verbesina encelioides (Cav.) Gray. Datura meteloides 
No attempt was made to catalogue the flora. The plants 
above cited are merely some of those which are so abundant 
and conspicuous as to give a character to the landscape. 
