38 THE FUTURE OF ARID LANDS 
only a few inches apart, depending upon the type of shrub above 
them. The relationship between plant and soil in the natural 
landscape is so intimate that rarely can we say that one is due to 
the other: they evolve together. 
Actually, useful plants (and useless ones) are distributed un- 
evenly over the world. One need think only of the great value of 
subterranean clover to Australia, and of the great harm done in 
Australia by the prickly pear cactus from the United States, now 
being brought under biological control by introduced pests. 
Research is now going on in order to find out where other useful 
transfers may be made. 
Plant scientists are probably making their greatest contribu- 
tions to more efficient grazing just now through improved plant 
selection and better management practices. Perennials with 
drought resistance, or at least drought tolerance under heavy use, 
are being identified. Such plants include palatable shrubs as well 
as grasses and legumes. 
Research has found ways to increase these good species through 
management and to spread some of them through seeding. For 
others, practical methods of collecting seed are not yet available. 
Although we must be on the lookout for good exotics, greatest 
progress is being made with local varieties that have persisted for 
a long time. 
Along with selection and reseeding, where possible, is the 
usually overriding matter of management, of controlled use. 
Almost phenomenal effects of temporary rest or ‘‘guarding”’ of 
the range to bring it into a high state of production have been 
reported from many parts of the world, especially where adequate 
measures for controlling water by contouring, terraces, spreading, 
and the like can be installed. Such guarding for even two years 
has raised the level of productivity of old ranges several fold; and 
the new high levels can be maintained through controlled use. 
Besides the normal cultural practices, the new methods for weed 
control are proving to be important tools for the ranchers and 
farmers in arid lands. Already brush is being eradicated from arid 
lands for as little as three dollars per acre. Many differential 
plant-killing hormones and other chemicals are in practical use, 
and testing is going forward with thousands of new kinds. We can 
