PLANTS, ANIMALS, AND HUMANS 42] 
taining larger human populations in arid zones?” there is implied 
a great potential danger, since an immoderate increase in the 
population of such zones may be encouraged, with all the risks 
that this may involve. 
Conditions exist in Mexico which make it necessary to consider 
this point very carefully. It has been estimated that between 
50% and 80% of the country’s national territory are arid and 
semi-arid zones, depending on the criterion used, and it has been 
roughly calculated that, in spite of adverse conditions, those 
zones support about fifteen million inhabitants, representing 
more than half of the Republic’s total. 
This population has traditionally been sustained by the desert 
it lives in, either availing itself of the native animal and vegetable 
elements or supplementing their possibilities by means of uncer- 
tain farm crops and an extremely limited livestock. The meager- 
ness of the products obtained gives rise to extremely poor living 
conditions for the inhabitants, above all in zones of greater water 
scarcity. And what happens in Mexico 1s repeated in many other 
regions of the world. 
With the aid of modern science and technology, it will be pos- 
sible to improve the living standards of such communities, espe- 
cially if the effort is directed toward balancing the utilization of 
native resources with what may be obtained from crop plants 
and domestic animals, improved as much as possible. 
But it would be dangerous to apply to those zones a demo- 
sraphic policy of maximum increase in local population or of 
transportation from other areas in an attempt to benefit them 
or to solve resettlement problems. 
It is evident that many arid zones could materially increase 
their yields in order to achieve a substantial improvement in the 
living standards of the groups living there at present. But if the 
population density goes beyond a certain maximum, which can- 
not be very high, not only will it have a greater impact on the 
feeble environment which supports it, but the existing living 
standards will surely fall. 
Since the time available does not permit a fuller or more de- 
tailed discussion, the basic aim of this brief explanation is to 
