342 ě ` Vegetation in Nevada and Arizona. [June, 
says “that it [Acacia] forms a shelter for the propagation of the 
Cereus giganteus of that region. Every young Cereus is pro- 
tected and fostered by this tree until the cactus attains the size 
and hardihood that enables it to withstand the war of elements 
waged against it, when it ungraciously spurns its protector, ulti- 
mately destroying it, as we saw in numerous instances on our 
journey. 
That geological and climatie influences and effects modify 
and in time alter the flora of a district is perceptible in the salt 
marshes. Here the lower forms of vegetation flourish in luxuri- 
ance, especially the Chenopodiacee, in which the higher type» 
seem unable to exist on account of the strongly alkaline soil, and 
even in waters strongly impregnated with salt, forms exist 
which retain these places in spite of the force brought to bear 
upon them from the invading species which perish not from a 
‘* mutual repulsion and subjection,’ but from the alkalinity of 
the soil itself. 
Mr. Lester F. Ward proposes “ what might be called the law 
of mutual repulsion, by which every individual, to the extent of 
its influence, repels the approach of every other and seeks the 
sole possession and enjoyment of the inorganic conditions sur- 
rounding it; this mutual repulsion results at length in a stati- 
eal condition which is always brought about through the action 
of the vital forces themselves, and which, as soon as reached, de- 
termines absolutely the exact place and degree of development 
of each species and each individual.” 
This is at present not the case in the salt marshes, but, when 
an accumulation of organic and silicious matter is the result, 
through the decomposition of the plants and the dust from sur- 
rounding sources, these lower types lose their predominance, 
and higher types replace them. This will ultimately be the re- 
sult, as there is no aqueous precipitation, and the constant evap- 
oration from the marshes will leave them nothing but alkaline 
deserts. 
Upon the foot-hills in the upper portion of Nevada different 
species of plants occupy distinct patches, but it is apparent that 
there are changes going on, and that in time some will be de- 
stroyed, giving place for hardier varieties. Mr. Ward further 
says, “ Each species is the perpetual and inexorable antagonist of 
every other. The ‘struggle’ is not alone ‘for existence,’ it 1s 
also for space. ... . But the first principle, as in the rest of 
nature, is force. Each one encroaches with all the power of 
