THE LAND ETHIC 
lengthen the circuit. Evolutionary changes, however, are 
usually slow and local. Man's invention of tools has enabled 
him to make changes of unprecedented violence, rapidity, 
and scope. 
One change is in the composition of floras and faunas. 
The larger predators are lopped off the apex of the pyramid; 
food chains, for the first time in history, become shorter 
rather than longer. Domesticated species from other lands 
are substituted for wild ones, and wild ones are moved to 
new habitats. In this world-wide pooling of faunas and 
floras, some species get out of bounds as pests and diseases, 
others are extinguished. Such effects are seldom intended or 
foreseen; they represent unpredicted and often untraceable 
readjustments in the structure. Agricultural science is largely 
a race between the emergence of new pests and the emer- 
gence of new techniques for their control. 
Another change touches the flow of energy through plants 
and animals and its returnto the soil. Fertility is the ability 
of soil to receive, store, and release energy. Agriculture, by 
overdrafts on the soil, or by too radical a substitution of 
domestic for native species in the superstructure, may de- 
range the channels of flow or deplete storage. Soils depleted 
of their storage, or of the organic matter which anchors it, 
wash away faster than they form. This is erosion. 
Waters, like soil, are part of the energy circuit. Industry, 
(by polluting waters or obstructing them with dams, may 
exclude the plants and animals necessary to keep energy 
5  m circulation. 
r ansportation brings about another basic change: the 
plants or animals grown in one region are now consumed 
and returned to the soil in another. Transportation taps the 
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