4 
1877.] Vegetation in Nevada and Arizona. 341 
ainous districts and elevated plains of Southern Mexico and the 
chain of the Andes at the equator.” 
_ This rule is also followed by representative species of birds, 
taking, for example, the ruby-throated humming-bird ( Zrochilus 
colubris Linn.), which is found from latitude 61° N. to Terra 
del Fuego, the southern remnant of the American continent, 
while it has been observed in the tropics at an altitude of 14,600 
feet. I have observed it in Nevada, latitude 38° N., at an eleva- 
tion of 9700 feet (timber-line), and it appears thus to be found 
nearly everywhere within that line, at which phenogamous veg- 
etation ceases to exist. — 
Species found in Mexico are also found at an altitude in the 
tropics at which the same temperature and the same belt of veg- 
etation occurs, which would place an outside limit of altitude at 
about 10,000 feet. M. Becquerel 1 says, “In the equatorial 
zone no change is observed in the vegetation from the level of 
the sea to the height of 600 metres (1969 feet), and beyond 
this even to an altitude of 1200 metres (3937 feet) we still rec- 
ognize the flora of the tropical zone.” 
It is apparent, then, that in the distribution of a flora from 
north to south, or in equal directions (or nearly so) from the 
equator, there is a downward tendency as the latitude increases. 
This zone of vegetation being divided into successive layers, ver- 
tically at the torrid zone, which, as they rise in altitude, spread 
their termini over a section of country where they descend, give 
rise to a succession of changes from a torrid to an arctic flora. 
This zone forms an arch, when viewed in a barometric profile, 
from the northern to the southern hemisphere, having the great- 
est depression over the equatorial region. 
hat the regularity in location of these various belts is gov- 
erned by climatic or meteorological laws, modified to some ex- 
tent by geological causes, is apparent and undoubtedly true, but 
regarding the local distribution throughout any one of these belts 
there are slow changes, as on some of the deserts in Arizona or 
in the salt marshes of Nevada. 
On the Gila Desert, as elsewhere, we observed the remains of 
an undergrowth of acacias (Algarobia glandulosa) which were 
destroyed by the encroaching Cereus giganteus, and here the law 
Of mutual repulsion is forcibly illustrated. Dr. J. M. Bigelow ? 
Noticed the same fact in the valley of Bill Williams Fork, and 
1 In Smithsonian Report, 1869, page 401. (Translation.) 
2 Pacific R. R. Report, vol. iv., page 21. (No. 2 Botany.) 
