elt, being healthful, dry, and fairly well supplied
with irrigation streams from the Andes, is better developed than any
other similar district in tropical America.[1256] In Bolivia, 72 per cent
of the total population live at an altitude of 6000 to 14,000 feet,
while five out of the nine most densely peopled provinces lie at
elevations over 11,000 feet.[1257] [See map page 9.]

From Mexico to central Chile, the heavy rains from the trade-winds
clothe the slopes with dense forests, except on the lee side of the high
Andean wall of Peru and Chile, and reduce much of the piedmont to
malarial swamp and jungle. The discouragement to primitive tillage
found in the unequal fight with a tropical forest, the dryer, more
bracing and healthful climate of the high intermontane basins, their
favorable conditions for agriculture by irrigation, and their naturally
defined location stimulating to early cultural development, all combined
to concentrate the population of prehistoric America upon the high
valleys and plateaus. In historic times these centers have persisted,
because the civilized or semi-civilized districts could be best
exploited by the Spanish conquerors and especially because they yielded
rich mineral wealth. Furthermore, the white population which has
subsequently invaded tropical America has to a predominant degree
reinforced the native plateau populations, while the imported negroes
and mulattoes have sought the more congenial climatic conditions found
in the hot lowlands.

[Sidenote: Increasing density with motive of protection.]

The relativity of geographical advantages in different historical
periods warns us against assuming in all times a sparsity of population
in mountains, even when the adjoining lowlands offer many attractions of
climate and soil. In ages of incessant warfare, when the motive of
safety has strongly influenced distribution of population, protected
mountain sites have attracted settlement from the exposed plains, and
thus increased the relative density of population on the steep slopes.
The corrugated plateau of Armenia and Kurdistan, located on the uneasy
political frontier of Russia, Persia and Asiatic Turkey, exposed for
centuries to nomadic invasion from the east, shows a sparser population
on its broad intermontane plains than on the surrounding ranges.
Security makes the latter the choicer places of residence. Hence they
are held by the overbearing and marauding Kurds, late-comers int