310 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1919. 
have remarked, while few have taken account of the milpa system. 
Eepeated burning of the forest for agricultural purposes produces 
an effect entirely different from the clearing for purposes of study 
of a group of forest-covered ruins, like those of Palenque. The 
woody growth is restored less rapidly after each agricultural clear- 
ing, and a state of complete denudation and exhaustion of the soil 
may be reached if the burnings continue. A region that has been 
exploited thoroughly by the milpa system may require many decades, 
and even centuries, before the fertility of the soil is fully restored. 
In a virgin-forest clearing the wild vegetation may begin to re- 
assert itself even in advance of the maturity of the crop. Sprouts 
may come up from the stumps or from plants with underground 
rootstocks that are not killed by the fire and rank weeds appear. 
With a moist climate and a rich soil the growth of woody plants 
may be sufficient in a few months to permit the same land to be 
burned and planted again in the second or third year, but this is 
true only of clearings in old or virgin forest. A longer period of 
renewal is required after the second burning, before there is enough 
" bush " to burn again, and the interval lengthens gradually to the 
fifth, seventh, or tenth year, depending upon the soil and other local 
conditions, but also very largely upon the length of time that the 
district has been occupied since the original forest growth was de- 
stroyed. 
That the " bush " takes longer to renew itself after each successive 
cutting and burning means, of course, that the soil is becoming less 
fertile. The genuine forest growth gives place to other plants that 
are adapted to the more open and exposed conditions of the burnt- 
over lands, and eventually some of the large perennial grasses become 
established. Though grass burns readily in the dry season, the roots 
and rootstocks are not injured and continue to occupy the soil to the 
exclusion of other plants. The method of cutting and burning serves 
to clear land of woody vegetation, but becomes ineffective when the 
land is occupied with grasses that resist fire. Accumulations of dry 
grass make fires hot enough to destroy seedlings of other plants, or 
even to kill large trees when the heat is carried by wind. 
The self-limiting character of milpa agriculture does not result 
solely from burning the clearings that have been cut for planting, 
but also from the fire spreading to neighboring bush or grass lands. 
Where only 5 or 10 acres have been cut 100 or 1,000 acres may be 
burned over. The Tarahumare Indians, of northwestern Mexico, 
according to Hartmann, keep up a custom of burning off all the grass 
during the dry season from April to June in the belief that the smoke 
clouds produce rain, " wherefore it becomes almost impossible to 
travel in the mountains during that time of the year, there being no 
