No. 400.] ZONES IN ARIZONA AND NEW MEXICO. 287 
entirely different. The contrast, instead of being weak, is of 
the strongest possible kind; and if the valleys compared are 
not in different zones, then life zones have no meaning for the 
horticulturist. We thus arrive at this conclusion : 
In the arid west the influences of temperature upon the 
cultivated, irrigated plants, derived from moister regions, are 
very much greater than upon the native plants or animals, 
which have become so adapted that they endure without harm 
enormous variations of heat and cold. 
Hence it results that from the horticulturist’s standpoint, 
the Middle Sonoran zone is a very real division, and belongs 
rather with the Upper than the Lower Sonoran. We may 
almost define the Middle Sonoran as a zone having the culti- 
vated products of the Upper Sonoran and the native products 
of the Lower. 
It will of course be understood that the data here given are 
only illustrative, and the time is not ripe for statistical tables. 
It will be a matter for future research to determine the exact 
details of the differences between the two zones. It is reason- 
ably to be expected, however, that we may by patient study 
find native species of plants and insects which afford reasonably 
close indications of the horticultural zones, and thus enable us 
to decide the zonal position in advance of cultivation. Such 
decisions will be greatly assisted when we are in possession of 
more precise meteorological data, but unfortunately we know too 
little at present about the daily variations of temperature in the 
several localities where weather observations have been made,and 
the differences between the temperatures of adjacent localities. 
Although Tucson is considerably higher than Phoenix, it is 
undoubtedly Lower, as distinguished from Middle, Sonoran. 
Oleanders, olives, Washingtonia and date palms, pepper trees, 
and a cultivated Parkinsonia were seen there; and Professor 
Toumey tells me there is a “hot pocket" some fourteen miles 
to the east, where orange trees are growing. - 
One circumstance which artificially emphasizes the horti- 
cultural distinction between the Mesilla and Salt River Valleys 
is, that in the latter it is possible to irrigate the higher levels 
at the side of the valley, while in the former only the bottom 
