248 THE FUTURE OF ARID LANDS 
to a greater extent than before. Other causes of the destruction of 
soil were the introduction of herds of grazing animals and the 
change from intensive cultivation to extensive cultivation with 
the introduction of the plow. Also, the transfer of manpower from 
agriculture to other activities and the decrease in population re- 
sulting from the epidemics introduced by the Spaniards brought 
about the abandonment of pre-Spanish irrigation systems. 
These technological changes, however, were not always detri- 
mental. The introduction of the plow, which, as previously noted, 
was conducive to the destruction of soil in some areas, permitted 
the opening up of new lands to cultivation, in regions of heavy soils 
not cultivatable with the native digging stick. The introduction 
of new plants also extended cultivation beyond the limits of the 
pre-Columbian crops. 
Andean America 
The Peruvian coast and the Peru-Bolivia highlands comprise 
the central Andean area. The coast is a desert interrupted by the 
valleys of the rivers flowing from the Andes to the Pacific. In 
one of these cases, the Viru Valley, archaeologists have found evi- 
dence of occupation by sedentary farmers going back to the third 
millenium B.c., some 4,500 years ago. At first, farming probably 
depended on flood waters, but since the first millenium B.c. the 
agricultural potentialities of the valley soils were fully exploited 
by means of canal irrigation. More than 2,000 years ago the irriga- 
tion system had snondet to its maximum potentiality, permitting 
the cultivation of 40% more land than at the present time (24,200 
acres against 17,300 in 1946). There is some evidence of population 
decline in the north coast of Peru during the late pre-Spanish pe- 
riods (after A.D. 1200), but the possible causes are not yet known. 
They may have been socio-political in nature or may have in- 
volved the diminishing efficiency of the irrigation systems, or both. 
In the valleys of the south coast, the flow of water available 
for irrigation is smaller than in those of the north coast, and there 
are reasons to assume that it was the same in the past. Archaeology 
shows that the density and concentration of population, the de- 
gree of urbanization, and the complexity of the socio-political 
