d their own products. Each tribe, family,
village was a more or less self-sufficient unit. When they were
threatened or invaded people defended themselves as best they could. At
worst they abandoned their homes to the invaders and fled into the
forests, mountains or deserts.

Towns and cities, with their industries, trade, commerce, their
permanent housing and capital equipment faced a radically different
situation. Since they could not carry their wealth on their backs they
must stay put and defend themselves or face irreparable losses. Defense
required careful, extensive, expensive preparations: walls, equipment,
stored food, personnel. Unless the city was sacked and burned during
survival struggles it remained as a vantage point to be held at all
costs. If surrendered and occupied by assailants, it was equally
valuable to invaders who were prepared to settle down, take advantage of
the site, the capital equipment and exploit the available manpower.

Whether occupied by friend or enemy, towns and cities were centers of
actual or potential wealth and power. They were also consumers of goods
and services many of which could not be home-produced. Food must come
from herdsmen or farmers. Building materials must come from forests or
mines. Such raw materials, the essentials of daily life, must be brought
into urban centers when and as wanted.

Food and raw materials could be secured occasionally by plunder. A
regular supply depended on trade and commerce, or on tribute levied and
collected periodically from associated or dependent peoples. In the long
run trade and commerce proved to be more reliable and more productive
than plunder.

As urban centers grew and developed, they established regular channels
of trade and communication, by land and water. Along these channels
needed imports moved into the urban centers and exports in exchange
moved from the urban centers into the back country or the provinces. At
every stage in the process care must be taken to prevent intervention by
thieves, robbers or envious rivals. Two devices were used to meet this
situation: money to facilitate exchange and a defense organization to
deal with intruders.

Money and its uses developed money changers, money lenders and banks.
Bankers and banks exchanged currency at a profit and extended credit.

Weapons in the hands of trained personnel evolved into locally employed
police and centrally organized armed services, performing police
fun