314 ANNUAL REPOET SMITHSONIAN INSTITUTION, 1919. 
Such years of famine would tend naturally to keep a primitive 
people from occupying a very humid district unless driven by pres- 
sure of population or other necessity, or unless there were a system of 
storehouses for feeding the people in famine years, as in ancient 
Peru. The chief danger in Peru was from unseasonable frosts in the 
high altitudes, but at ordinary elevations either too much raiii or too 
little would represent a limiting factor, tending to destroy or drive 
out a primitive people that had ventured beyond the margin of a 
safe existence or depleted the resources of its native district. 
If the population remains in a district after all the lands have 
been cut over, resort must be had to repeated clearing of the same 
lands as soon as the " bush " is large enough to burn, without waiting 
for trees to grow and new soil to be formed. And since the forest 
growth and the fertility of the soil are renewed more slowly after 
each burning, the adverse effects tend to be multiplied. Once the bal- 
ance is upset, so that the natural agencies for renewal of the soil do 
not have time to work, milpa agriculture becomes an actively de- 
structive system. How often the land may be cleared, or how many 
times the woody growth will renew itself, must be determined by the 
local conditions of soil and climate, but a definite limit is reached 
when the woody vegetation ceases to grow and the land becomes oc- 
cupied by grasses. The larger the population the more complete 
and extensive is the agricultural catastrophe which must ensue 
when a people who depend entirely upon the milpa system have ex- 
hausted their resources of production. 
CENTERS OF POPULATION. 
Primitive tropical peoples may live either in villages or large 
communal houses or the families may be widely scattered over the 
land. Even among the Maya peoples of Central America there were 
tribes like the Kekchis of eastern Guatemala who seem originally to 
have had no villages until assembled by the Spanish missionaries 
for religious control and instruction. The fact that all the so-called 
"Old Empire" cities of the Mayas in Central America have been found 
buried in deep forests shows what the country was like before it was 
occupied by the builders of the ruins. But wide areas of the Maya 
country must have been cleared when there were people to build 
such cities. 
The more centralized a population becomes the more definite and 
obvious are the effects of its agricultural activities. The lands im- 
mediately surrounding large Indian towns in Central America at 
the present time are not merely deforested after the manner of clear- 
ings for milpas, but are completely denuded, in order to furnish fuel 
for the towns. Firewood and charcoal for many of the towns are 
