A mountain environment often occasions a forced development in the form of agriculture among peoples who otherwise still linger in a low stage of barbarism or savagery.  The wild, head-hunting Igorots, inhabiting the Cordilleras of north central Luzon, have levelled the face of their mountains into a series of platforms, held by retaining walls from twenty to thirty feet high.  On these they cultivate upland rice at an altitude of 5000 feet.  The Igorot province of Bontoc contains valleys in which every available foot of land is terraced for rice, and which present artificial landscapes vividly recalling Japan.  Labor is the heritage of each inhabitant.  Every man, woman and child down to ten years of age shares in the work of providing food.[1287] Africa shows parallel cases.  The Angoss people, a savage negro tribe who occupy part of the Murchison Range in northern Nigeria, have mapped out all their sloping land into little terraces, sometimes only a foot or two wide.  One of their peaks, 4135 feet high, has its plateau top covered with populous villages, owing to the protection of the site, and every inch of its slope cut into terraces planted with millet and guinea corn.[1288] A more primitive form of this tillage is found in the country of the Marunga negroes, who occupy the steep western face of the rift valley filled by Lake Tanganyika.  Here Cameron found the surface not regularly terraced, but retaining walls of loose stones disposed at intervals, which served to hold the soil in place, without greatly altering the natural slope.  The scene recalled the terraced heights of Switzerland, and the people working there looked like flies on a wall.[1289] In the semi-arid country of Sudanese Darfur, where only the mountain districts are well watered and thickly populated, small terraces for grain and melons cover all the slopes.[1290]

[Sidenote:  Fertilizing]

Mountain agriculture is necessarily laborious.  The paucity of arable land precludes the possibility of letting fields lie fallow.  These, to prevent exhaustion, must be constantly and abundantly fertilized, all the more as conditions of excessive subaerial denudation found in the steep slope and usual heavy rainfall of mountains, as well as possible glacial scouring of the land in the past, have greatly attenuated the layer of soil called upon to support plant life.  The Swiss or Tyrolese farmer cherishes his manure pile as at once source and badge of his wealth.  After harvest it is carted or carried in baskets not only to the terraces, but also to the wide alluvial fan that grows his oats and rye, to his meadows and hay fields.  Both in Mexico and Peru the soil received a dressing of poudrette.  Manuring was most extensive where population was densest, as in the isolated mountain valleys opening out upon the desert coast of Peru.  Every kind of organic refuse was utilized, and fish was buried with the kernels of maize as a fertilizer.  The deposits of guano found on the headlands and off-shore islands were used from the remotest times.  Different guano beds were assigned to the several provinces, and the breeding places of the birds were protected by law.[1291] Ashes and decayed wood were employed for the same purpose, or plants were dug into the soil, while human manure was in Mexico a marketable commodity as in China.[1292]