ancient cities

938 Urban System
well documented. The role of cities as centers for
the integration of human capital and as incubators
of invention was rediscovered by the “new” economic
growth theory, which posits that knowledge
spillovers among individuals and firms are the necessary
underpinnings of growth. Moreover the
creation and repositioning of knowledge in cities
increases their attractive pull for educated, highly
skilled, entrepreneurial, and creative individuals
who, by locating in urban centers, generate further
knowledge spillovers
The two processes of innovation and specialization
have consequences for the dynamics of
systems of cities. The activities that can diffuse
widely through the system tend to reinforce the
relative weight of the large cities, because of their
growth advantage and attraction from the earlier
stages of innovation, whereas the activities that
focus on a few specialized towns, because of some
specific location factors, after boosting development,
with sometimes spectacular growth rates,
hamper further development by weakening their
ability to adapt.
These differential evolutions likely will continue
and do so not at the scale of national systems but
in global urban networks. The colonial era already
had introduced durable asymmetries in urban
growth and perturbed the organization of urban
systems in colonized countries. Foreign investments
and the redistribution of economic activities
that characterize the globalization of economies
will reproduce or reinforce these effects.
Coevolving Cities Through
Spatial Interaction
Over time, urban systems have changed in size and
form. Even if they ensure the same social functionality,
which is to control territories or networks,
the nature of control has evolved, from politics to
economics. Political control at first coincided with
the envelope of one city but then shifted to kingdoms
or empires and then to national territories
that associate several cities. Economic control, as
expressed first by local entrepreneurs, has broadened
to national and then multinational firms.
Individual cities that once controlled their development
by building networks to exploit distant
resources have become instruments for the control
of wider territories and networks ensured by
national or supranational actors using the networks
that cities have built through their interactions.
Systems of cities are evolutionary objects that may
include subsets of cities connected by long-
distance networks or cities belonging to unified
political territories. The general trend is toward
historical enlargement of the number of cities that
are integrated through more intense and frequent
interactions, but political events or economic crisis
can block such growth. The changing nature of
interactions and the fluctuations of their spatial
extension make it difficult to identity systems
of cities.
That is why urban systems are now conceptualized
as evolutionary systems, which are self-
adapting to the change generated by human
societies and also contribute to that change. The
urban system is an invention, the technical nature
of which is usually not apparent in collective representations.
Urban systems are admirable territorial
adaptors to social change. Like other social systems,
urban systems are the product of historical
self-organizing processes that mix deliberate actions
and involuntary outcomes of social interactions.
Their dynamics are driven by a general expansive
trend rooted in social practices aiming to increase
symbolic power, available resources, and the space
for action; in cities, this trend is converted into
invention intended to reduce the local uncertainties
that constrain the development of a site and to
search further afield for complementary resources,
either in the surrounding territory or in more distant
networks. It follows that interurban interactions
contribute, by emulation, to hastening the
process of globalization through urban networks
and to enhancing the complexity of human activities
through the division of labor and specialization.
This trend has already generated a major
bifurcation, known as the urban transition, which
has transformed the way we inhabit the planet by
converting a set of settlements whose original function
was the agricultural use of a territory into a
much more concentrated, hierarchical, and qualitatively
differentiated system of towns and cities. The
resulting regular urban hierarchies that emerge
from the interactions between cities are not produced
by conscious design. Their structure is, however,
constrained by the competitive process
of growth, which explains their form, similar to
the size distribution of elements in other complex