lower slope is found the vineyard belt, a region of highly intensive tillage, large returns upon labor, and hence of closely distributed settlement.  Above that is the zone of field agriculture, less productive and less thickly peopled.  Higher still is the wide zone of hay farming and stock-raising, supporting a sparse, semi-nomadic population and characterized by villages which diminish with the altitude and cease beyond 2000 meters.  On Aetna, located in the tropical Mediterranean, three girdles of altitude have long been recognized,—­the girdle of agriculture, the forest belt, and the desert summit.  But the tourist who ascends Aetna, passes from the coast through a zone of orange and lemon groves, which are protected by temporary matting roofs against occasional frosts; then through vineyards and olive orchards which rise to 800 meters; then through a belt of summer crops rising to 1550 meters, and varied between 1400 and 1850 meters elevation by stretches of chestnut groves, whose green expanse is broken here and there by the huts of the forest guards, the highest tenants of the mountain.  From these lonely dwellings down to the sea, density of population increases regularly to a maximum of over 385 to the square mile (150 to the square kilometer) near the coast.

[Sidenote:  Altitude and density belts in tropical highlands.]

In the tropical highlands of Mexico, Central and South America, on the other hand, concentration of population and its concomitant cultural development begin to appear above the 2000 meter line.  Here are the chief seats of population.  Mexico has three recognized altitude zones, the cold, the temperate and the hot, corresponding to plateau, high slopes and coastal piedmont up to 1000 meters or 3300 feet; but the first two contain nine-tenths of the people.  While the plateau has in some sections a population dense as that of France, the lowlands are sparsely peopled by wild Indians and lumbermen.  Ecuador has three-fourths of its population crowded into the plateau basins (mean elevation 8000 feet or 2500 meters), enclosed by the ranges of the Andes.  Peru presents a similar distribution, with a comparatively dense population on a plateau reaching to 11,000 feet (3500 meters) or more, though its coastal belt, 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.]