246 THE FUTURE OF ARID LANDS 
west to northwestern Argentina. This zone included the areas of 
native civilizations of Mexico-Central America (called Meso- 
america by the anthropologists) and Peru-Bolivia (called 4ndean) 
as cultural climaxes. Because of limitations of space, and also 
because of the reason noted above, I shall concentrate my atten- 
tion on these two areas. 
Mesoamerica 
Mesoamerica includes central and southern Mexico, Guatemala, 
British Honduras, E] Salvador, and western Honduras. Owing to 
a diversity of factors, the most important of which are great differ- 
ences in altitude and broken relief, it is far from being a geographi- 
cal unit. In the lowlands, which include the coasts and some of the 
interior depressions, tropical rain forests and monsoon forests, 
tropical savannas, and tracts of hot steppe and cactus desert are 
found. In the mountains and plateaus there are temperate forests, 
mesothermal savannas and cold steppes. 
In the semi-arid zones, constituting most of the area, the degree 
of aridity depends on the duration of the winter dry season and the 
intensity, reliability, and efficiency of the summer rains. Also, the 
regularity of the beginning of the rains—as a determinant of the 
planting period—is an important factor in the highlands, where 
early frost might destroy the unripe crops when planted late. 
In the rain-deficient sections and in many of those having a 
markedly seasonal distribution of rains, canal irrigation or other 
specialized forms of horticulture were important, although not 
exclusive of rain farming, in pre-Columbian times. The distribu- 
tion of irrigated lands included the highlands, the intermountain 
depressions, the Pacific coast and the semi-arid zone of central 
Veracruz. Hydraulic engineering was most developed in the valley 
of Mexico. Elsewhere the irrigation works were built on a small 
scale, possibly with the resources of single local communities, but 
requiring in many basins centralized control for the allocation of 
water rights. 
The other specialized forms of horticulture to which I referred 
were: 
(a) Gardens irrigated with water manually elevated from wells, 
dug within these plots. 
