linear progression but was marked by phases of growth and decline (like the 'collapse' of archaeological cultures). Its roots lie in the societies of the Neolithic period, and the process is characterised by the increase of social inequality over the long term, visible in particular in the creation of monumental architecture and funerary materials by groups of the elite, which reinforced itself as a collective and managed to exercise its power in a firmer and firmer manner. Among the main causes proposed by proponents of the functionalist model of the state are a collective response to practical problems (particularly following serious crises or a deadlocks), like the need to better manage the demographic growth of a community or to provide it with resources through agricultural production or trade, alternatively others suggest that it was driven by the need to soothe or direct conflicts arising from the process of securing those resources. Other explanatory models put more stress on the personal interest of individuals in their quest for power and prestige. It is likely that several of these explanations are relevant. Urbanisation The Uruk period saw some settlements achieve a new importance and population density, as well as the development of monumental civic architecture. They reached a level where they can properly be called cities. This was accompanied by a number of social changes resulting in what can fairly be called an 'urban' society as distinct from the 'rural' society which provided food for the growing portion of the population that did not feed itself, although the relationship between the two groups and the views of the people of the time about this distinction remain difficult to discern. This phenomenon was characterised by Gordon Childe at the beginning of the 1950s as an 'urban revolution', linked to the 'Neolithic revolution' and inseparable from the appearance of the first states. This model, which is based on material evidence, has been heavily debated ever since. The causes of the appearance of cities have been discussed a great deal. Some scholars explain the development of the first cities by their role as ceremonial religious centres, others by their role as hubs for long-distance trade, but the most widespread theory is that developed largely by Robert McCormick Adams which considers the appearance of cities to be a result of the appearance of the state and its institutions, which attracted wealth and people to central settlements,