MILPA AGRICULTURE COOK. 325 
milpa system is taken into account. The tree crops usually clo not 
become productive for several seasons and require an investment of 
labor not likely to be made among tribes who do not have private 
property in land, or other assurances of permanent tenure that are 
the basis of agriculture in northern countries. The only tropical 
tree crop that seems to have been cultivated systematically and on a 
large scale in ancient America was cacao. The native plantations 
of cacao in Guatemala were compared by an early Spanish writer 
to the vines and olive trees of Spain. Cacao was grown as a sub- 
culture under leguminous trees called " mother of cacao," and the 
same method is still applied to cacao and coffee in many tropical 
countries (pi. 14, fig. 1) . Instead of forming artificial grasslands and 
deserts, an ideal for tropical agriculture is to develop artificial for- 
ests that not only will yield food or other useful products but at the 
same time maintain or increase the fertility of the soil. 
SUMMARY. 
LIMITATIONS OF NATIVE AGRICULTURE IN CENTRAL AMERICA. 
Milpa agriculture, the system that is used generally by primitive 
peoples in the Tropics, is based on the cutting and burning of new 
areas of forest each year, in order to clear the land for maize or other 
crops. The milpa system is adapted to sparsely inhabited regions, 
with long intervals between burnings, but has definite limitations in 
populous districts. If the land is burned over too frequently, not 
only the forest trees but all other types of woody vegetation eventu- 
ally are exterminated. Perennial grasses become established, which 
render the land useless for agricultural purposes. 
Replacement of all the forests of a tropical region by grasslands 
sets a natural limit to a period of agricultural occupation under the 
milpa system, because the method of clearing the land by cutting and 
burnin,g is not effective against grasses. Every extension of the 
grass-covered areas means that more land has become unproductive, 
and eventually the food supply becomes restricted. A district that 
may have been able for a time to support a large population may be 
entirely denuded and completely abandoned as a fire-swept waste of 
grasslands. This condition continues until a period of reforestation 
has intervened to exterminate the grasses and renew the soil so that 
the land can be cleared again by burning and reoccupied by agri- 
cultural people. Many tropical forests are found to represent not 
truly virgin growth, but various stages of reforestation, which re- 
quire a long period of time. 
The limitations of the milpa system and the resulting periodicity 
of agricultural populations tends to be more definite in districts oc- 
12573^—21 22 
