VOL. I. ] The Economy of Nature. i Se 
which the uniformity of the coniferous plantation was more fre- 
quently relieved by interspersed birch, oak, beech, or mixed plan- 
tations, produced a favorable change. 
The insect pests of course did not become extinct—nature takes as 
much care as possible even of her naughty children—but they ceased 
to be pests; 
Variety of plantation, therefore, prevents to a certain degree in- 
sect pests, or at least diminishes their power. It is very probable 
that the prevalence of insect pests in this country to some extent is 
owing to the uniformity of cultivation, as compared to the variety 
in other countries where agriculture and stock-raising are much 
more combined on the same farm, and the fields themselves are 
cultivated to and grown over by a greater variety of plants. 
This is the place te discuss a class of insect pests not created by 
the monotonous cultivation of the soil, but owing their origin to an 
analogous condition of things without human interference. I refer 
to those pests that owe their origin to the monotony of desert 
lands. : 
There are certain regions where a dead level of the surface, the - 
chemical compound of the soil, absence of water, or imperfect 
drainage, oppose vegetation in general. Still there are certain or- 
ganisms adapted to conditions detrimental to the existence of the 
majority of types. The vegetation of such localities in some in- 
stances can exist only under the very conditions of the locality; in 
other instances it could exist in cultivation, but would be super- 
seded by the more powerful organization of other types, so that it 
is driven to the desert regions by its inability to stand competition. 
The number of types that are original inhabitants or adapted im- 
migrants in such localities is naturally a limited one, but the small 
number of species is frequently made up by an enormous number 
of individuals. The sage-brush vegetation of our alkali plains, the 
heath ( Zrica) that covers the plains of northern Europe, the lichen 
tundra of the Arctic, the pampas of the Argentine Republic, the 
Russian and Siberian steppes, and many other regions, are well 
characterized:instances of this state of things. 
The small number of species and great number of individuals 
produces monotony, and the monotony is the cause of a series of 
_ phenomena similar to those produced by exclusive occupation of 
cultivated grounds by one or a few species. The differences in the 
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