THE ECOLOGY OF MAN—SEARS 387 
volved. Here something like a sedentary system developed, while 
the technical stimulus of dealing with water by storage, ditching, flood- 
ing, and draining, had its impact not only on food production but 
on engineering, mathematics, and physics. It was in such places as 
the empires of Mesopotamia, the kingdoms of the Nile, the terraced 
highlands of Peru, and the lake-margin gardens of Mexico that the 
arts really flourished. 
By contrast there developed within forest regions an exploitive and 
transient type of land use, resting upon removal and destruction of 
the original cover by chopping and burning. Planting with simple 
tools might yield returns for a year or two, but grass and weeds 
would presently come in. If domestic animals were at hand, the area 
might then be pastured for a time while fresh clearings were made 
and the process repeated. In the absence of grazing animals such 
areas would revert to brush and then to forest, as they would ulti- 
mately through the growth of briers even if pastured. This type of 
use is not too bad so long as the cycle permits some recuperation. But 
it cannot sustain a dense population for any great period of time. 
In contrast to the control of water, which enhanced the potential of 
environment and encouraged the concentration of people, forest clear- 
ance destroyed the very constructive agency that had made the area 
productive in the first place, without substituting any proper 
equivalent. 
Therefore the effect of clearing and burning, whether in Neolithic 
Europe, Mexico and Honduras, or the Southern United States, was 
to encourage a shifting economy and produce a scattered population. 
On the whole, the occurrence of large permanent metropolitan cen- 
ters in humid forest regions is usually due to something other than 
the efficient local production of food. 
Where men followed their flocks and herds instead of tilling the 
soil it was likewise not possible to accumulate large sedentary con- 
centrations of people. Because this type of culture neither enhanced 
the environment as did irrigation, nor destroyed its cover as did 
clearing, but rather utilized that cover directly, it was immediately 
dependent upon favorable seasons and places. These nomads tended 
likewise to be alert, aggressive, mobile, and accustomed to the sight 
of blood. When continued drought or neighborly pressure threat- 
ened they sought relief by movement—with or without raiding and 
plundering. Much history has its explanation in this fact. 
These early phases of human culture can speak to us only in 
snatches, through their more durable remains and their effects on 
the landscape. But the lesson they leaves is clear enough. Only 
where the effort was made to maintain and improve the environment 
was anything like an efficient and sustained relationship achieved. 
Even here the balance could be destroyed, as when invaders ruined 
