1877.] Vegetation in Nevada and Arizona. 343 
vegetal growth upon its neighbors.” Where an area of vegeta- 
tion has not been disturbed by mechanical or agricultural causes, 
the species and genera growing thereon are to all appearances 
occupying their limit of growth and local distribution, as over 
various valleys and low elevations in Northern Nevada. “ But let 
these statical conditions be once changed, . . . . and this equi- 
librium is immediately disturbed. The chained forces are set 
free ; a general swarming begins ; some individuals are destroyed, 
others are liberated; each pushes its advantage to the utmost, 
and all move forward in the direction of least resistance, till at 
length they again mutually neutralize each other, and again come 
under new conditions and modified forms, into the former state 
of quiescence.” 1 
There has been great difficulty experienced in some of the fer- 
tile valleys of Nevada and California in attempting to prevent 
the rapid encroachment of native plants upon partially cultivated 
and irrigated patches of soil. They are stronger in vegetal 
power, and in a short time depauperate and stunt the introduced 
cereals and garden vegetables. 
These vegetal “ struggles for space and existence” are stronger 
and the results more perceptible in the tropics than elsewhere ; 
rising in altitude with the superlying belts of vegetation, they 
decrease until the region beyond the timber-line is reached, in a 
similar manner as when we proceed towards either pole in al- 
most a fixed proportion to the latitude. This is caused by a 
variety of influences, prominent amongst which are 
(1.) The presence of saline matter to such an extent as to cause 
the destruction of any but the lower types of vegetable life, — 
Chenopodiacee. 
(2.) In the sub-alpine belt or latitudes, where the Conifere 
predominate and where there is a corresponding temperature un- 
favorable to other types generally, and 
(3.) Beyond the timber-line or at extremely high latitudes, 
Where the superincumbent mass of snow in winter and the ex- 
tremely short temperate season prevent the growth of almost 
anything save lichens and mosses. 
1 Mr. L. F. Ward, Popular Science Monthly, October, 1876. 
