Civilization is a recent phenomenon, arising 5,000 years ago in Mesopotamia. Indeed, only about 5 per cent of human history has been characterized by irrigated agriculture, political administration, writing, calendars, institutionalized long-distance and other civic technologies. Yet, during this brief period, world population has grown from about 20 million to 5.7 billion people, an increase of staggering proportions. This expansive process merits analysis. What are the organizing structures of civilization? How can we measure long-term, large-scale change? What does it mean for civilization to 'evolve'?

As building-blocks of civilization, cities become the most vital units of analysis for long-term study. Economically, cities represent markets and production centers in their most concentrated form; as such, they are vital to growth and innovation. Pendulum-like cycles of economic expansion and contraction - such as the 250-year A and B phases described by Barry Gills and Andre Gunder Frank (1992) -find support in rates of urban population growth (Bosworth 1995a,b). Politically, cities embody the very essence of politics, 'polls' being Greek for 'city' but also for 'politics.' Socially, cities are microcosms of regional populations and centers for education and religion. And culturally, cities reflect a relationship between humans and the earth dependent on large-scale agriculture and expanding markets. In sum, cities reflect the four interrelated dimensions of human experience: economic, political, social and cultural.

More crucially, cities presuppose connections. In 1891, a German geographer, F. Ratzel, argued that cities develop wherever one or more of three conditions exist: (1) the end of a transport route; (2) the junction of two transport routes of the same kind; or (3), die junction of two transport routes of different kinds. Indeed, no city is an island; each is part of a larger network, a 'world-city system' that provides the circuitry for civilization. This world-city system exhibits a structural order, or 'architectonics,' defined primarily by vital connections (rather than cities, which rise and fall in more rapid succession than the trade routes of which they are a part). As Janet Abu-Lughod argues: 'In a system it is the connections between parts that must be studied. When these strengthen and reticulate, the

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system may be said to 'rise'; when they fray, the system declines, although it may later undergo reorganization and revitalization' (1989:367). The world-city system's connections have clearly undergone reorganization from the dawn of civilization to today, and, broadly speaking, there have been six successive yet overlapping 'architectonic orders.'

The 'Fertile Crescent' period extends from approximately 3000 BGE to 1500 BCE, which for some represents the Bronze Age. First, there was a breakthrough to civilization in Mesopotamia, specifically in Sumeria. Second, there was a similiar breakthrough in Egypt along the Nile, where there emerged a unified kingdom in contrast to the more complex array of Mesopotamian city-states. During this time, Mesopotamia and Egyptian civilizations, inherently expansive, fused, creating what might be called 'Central Civilization.' And third, there was the emergence of civilization in Syria, Palestine, the Levant and even parts of Asia Minor - largely sparked by the original riverine civilizations.

The 'Regional Eurasian' period extends from about 1500 BCE-1 AD and approximates what many have called the Iron Age. This was a time when western Asian, Mediterranean, Chinese and Indian societies developed along relatively independent lines. Regional transportation arteries emerged: The Royal Road of Persia, the Grand Trunk Road of India, the Ambassador Road and Yellow River system of China, the Incense Road of Arabia and, adjacent to it, the Phoenician sea lanes of the Mediterranean. Regional constellations of cities sharpened in resolution; a world galaxy of cities was still embryonic. This regional Eurasian order no longer reflected Middle Eastern dominance but rather a fourfold cultural balance among the regions of high culture. By 200 BCE, China, India, the Middle East and the Mediterranean had robust populations, each with between 30-45 million people.

The 'Silk Road' period begins about 200 BCE when Chinese crossed Central Asia to obtain horses, jade, fur and gold (and to sell silk). The road soon linked the powerful Han dynasty of China with the Romans. The period ends around 1350 AD when bubonic plague gutted Silk Road cities and eroded the Mongol Empire

Table 14.1 Architectonic orders

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that had linked them together from the Mediterranean to the Pacific. Few merchants ever traversed the entire length of the Silk Roads; instead, a city-to-city relay system arose that detoured according to climate, plagues, civil wars, bandits taxes and tolls. There were two main southern branches and a northern steppe route, each with its own variations. Ideas as well as goods coursed from one end to the other. Religious pilgrims and wayfarers diffused Nestorian Christianity, Buddhism, Zoroastrianism and other faiths.

The Spice Routes period, which can be broken down into four phases, largely overlaps with the Silk Road period because these maritime routes, which emerged around 50 BCE, represented alternate linkages between East and West. The Spice Routes began, in their first phase, as a relay system between three circuits (Arabian Sea, Bay of Bengal and South China Sea) overlapping in Ceylon and the Malaccan Straits, where cargoes would be exchanged. The circuits were marked by quarterly shifts of monsoon winds that imparted cyclical rhythms to maritime trade, in effect stranding Muslim and Buddhist merchants in foreign ports for months at a time and allowing for religious diffusion. During the Spice Routes's second phase, a single circuit emerged as Abbasid trading colonies were founded in China. In the third phase the relay system was restored, partly because of xenophobic massacres of Muslims in China. In its fourth phase Portugal, the Netherlands, France and Britain founded colonies in Arabian, Indian and Malaccan ports. Over the course of the entire Spice Routes period there was a great increase in the bulk of traded goods but also in its diversity: carpets, wine, sugar, salted fish, fruits and, in the sixteenth century, American tobacco and silver. Coffee was also widely distributed for the first time, diffusing from Mocha and Aden in Arabia. Despite the bulk and dynamism of Spice Route trade, this system was eclipsed by the Atlantic, when, by at least 1750, European expansion turned the Indian Ocean into a backwater.

The 'Atlantic System' begins after 1492 with the European discovery of the Americas. There arose an amalgam of Spanish, Portuguese, Dutch, French, British and North American city systems, each with its own patterned flow of traders, soldiers, missionaries, colonizers and slaves. There was a historical momentum that began with an Iberian system anchored in Lisbon and Seville and ended in the twentieth century, in a reversal of polarity, with a North American system anchored in New York. This Atlantic period of the world-city system is striking. The world's largest migration occurred as over fifty million people entered the Americas. The Industrial Revolution, with colonial trade at its base, evolved out of this Atlantic matrix and concentrated world economic and military power in northern Europe for 300 years. Technology revolutionized medicine, and the world's population exploded. Today the Atlantic system declines in importance as civilization's center-of-gravity continues its westward trajectory and shifts to the Pacific, the new Mare Nostrum.

The 'Pacific-Global' period is so named because unlike the dominance of the Atlantic system in its day, the Pacific system is merely 'prominent' within a truly global civilization that is likely half-a-century away from being orbital as well. This Pacific-Global order has its origins in the annual Spanish Manila Galleon

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trade between 1565 and 1810 that exported Mexican silver, gold and cacao from Acapulco and imported Asian silk, spices and porcelain from Manila. A more clear beginning can be traced to the 1890s and early twentieth century: US troops occupied the Philippines and were deployed in China; the US absorbed Hawaii; a diaspora of Chinese labor continued to reach California and Peru; and, in 1914, the Panama Canal linked the Atlantic and Pacific oceans. After World War II, the first war to define the Pacific as a military theatre, the growth of East Asian economies - of Japan, then of the 'Four Tigers' (South Korea, Taiwan, Hong Kong and Singapore) and now coastal China - has created a third industrial core in addition to Europe and North America. Today, trans-Pacific trade exceeds trans-Atlantic trade, and in North America, innovations in aviation, bioengineering, and computer technologies are clustering around Los Angeles, San Francisco and Seattle.

These successive architectonic orders do not appear out of thin air; restructuring, rather than substitution, is the operative principle. This restructuring often emerges out of challenges or crises that can take the form of trade blockages. (Of all connections, trade routes are the most visible, and although long-distance trade was not immense in ancient times it had enormous political and ideological consequences.) The process of restructuring is one of transcendence and inclusion; each order contains its own logic but also those of the previous orders, however decayed. Architectonic orders reflect routes of trade, invasion, migration and colonization, and although not explored here, they also reflect intellectual and spiritual world-views. This is most apparent in the maturation of the Atlantic system, one so influenced by industrial, rational and secular paradigms. For some observers, it is also evident in the shift to the Pacific-Global order, one increasingly shaped by a synthesis of East and West.

Evolutionary theory represents a broad research program in which disagreement abounds, particularly in regard to the relative importance of selection, adaptation and chance. Yet, minimally defined, we can define 'evolution' as a process of change in which forms, driven by pressures of survival and the capacity or will to change, tend toward greater structure, connectivity and differentiation. Thus, evolution can provide direction; it does not necessarily provide, in a ideological sense, destination.

The direction toward greater structure, connectivity and differentiation - or from simplicity to complexity — is significant in terms of evolution. (Movements from complexity to simplicity do happen, such as the cave crayfish that loses its eyes, but they are far more rare.) Clearly, systems of a higher order are advantageous: 'It is a fundamental characteristic of the material world,' observes Peter Corning (1994:3), 'that things in various combinations, sometimes with others of like kind and sometimes with very different kinds of things, are prodigious generators of novelty'. Generators of novelty, he adds, are 'extravagantly favored' by natural selection. This principle of evolution - that evolution engenders complex-

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ity - can in fact be applied to the world-city system, which, like any complex system, is a product of three factors: (1) the number and size of parts included in the system; (2) the connectivity and integration among the parts; and (3) the differentiation, diversity, division of labor, or degree of hierarchy among the parts. For the world-city system, it is possible to evaluate and even measure change in each of these dimensions (supporting the claim that evolutionary theory can produce testable hypotheses).

The world-city system's movement toward greater structure is virtually self-evident. Around 3000 BCE this system rested on die riverine civilizations of the Fertile Crescent and contained no more than five million people. Today the system is global in scope, and about half of the world's 6 billion people are urban. The largest twenty-five cities alone contain over 350 million people, more than 5 per cent of world population. Geographically, the world-city system has enveloped ever-larger regions (the Middle East, Eurasia, and die Americas) and ever-larger bodies of water (the Aegean Sea, the Mediterranean Sea, and the Indian, Atlantic and Pacific oceans, in that order). This structural expansion was not gradual or continuous. Centuries of stagnation were followed by decades of accelerated, quantum change - punctuated evolution.

The world-city system has also moved toward further connectivity and integration. Economically, there has been a historic and well-documented shift in trade from low-weight, high-priced goods to heavy-weight, low-priced ones. There Has also been a general expansion and diversification of all trade goods to the point where today 'information' is added to the world's commodity pool. This integration is even visible at the household level. A century ago household items were of local and regional origin, with a few treasured valuables like porcelain (from China) or lace (from Britain) representing imports. Today, a much higher portion of household items are imported, and they come from all over the world.

Political integration, furthered in ancient times by me expanding average size of empires, was more recently furthered by colonial empires and their transformation into a global nation-state system. Within this expanding community of nations, democratization integrates populations into an alliance structure and encourages the emergence of common norms and values. The process of democratization, which depends on diffusion, can be viewed as a self-reinforcing learning process whose rewards include peace: history demonstrates that democratic societies do not make war against one another. With about half of the world's population now living in democratic polities (a percentage that has steadily increased in modern times exept for the 1930s and 1940s), it is entirely possible that, with more complete democratization, war can be made extinct.

Social integration is also notable, especially the diffusion of English as the world's lingua franca. More deplorable to some is the rise of a consumerist monoculture sustained by multinational corporations. More tragic signs of integration are global epidemics. Just as increased movement across Eurasia made possible the fourteenth-century bubonic plague, and just as the expansion of Europe in the sixteenth century brought smallpox and influenza to Mexico and Peru, contemporary migration from developing to developed regions, travel, tourism

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and the rise of a global sex industry have helped AIDS reach intercontinental proportions. The world has indeed become smaller, often with tremendous costs.

The third major criterion of evolution considered here is differentiation. A system under evolutionary pressure tends to become internally complex, specialized and hierarchic. There arises a greater division of labor among the parts. This is also true for the world-city system whose functional hierarchy increased with the Industrial Revolution and the emergence of manufacturing cities of unprecedented size. The Information Revolution of the last few decades has further complexified the world-city system. The largest cities of the developed world are no longer industrial; they are 'postindustrial' centers of banking, education and service.

Another dimension of differentiation is cyclical: 'world urban hierarchy.' In this study, this hierarchy is defined by the ratio between the population of the world's largest city and its twenty-fifth largest (whose population is usually similar to all cities in the fifteenth to fiftieth range). High ratios of this indicator, produced when the largest city in the world surges in population, can be associated with economic growth and political concentration - at least until the late twentieth century. Figure 14.1 measures the changing ratio of the world's largest city to the twenty-fifth largest from about 500 BCE to AD 2000. Adapted from urban population estimates, this figure is compatible with the historical record. Each of these cities is widely recognized as having been the world's largest for a considerable period of time.

Figure 14.1 World urban hierarchy between 500 BCE and AD 2000.

Source: Based on data from Chandler (1987).

Note: This figure displays the ratio of size difference between the world's largest and twenty-fifth largest cities. Increases of this ratio are produced by the pulling with periods of general economic and political expansion (and decreases with contraction).

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This figure, which exhibits a harmonious wave-like pattern (supporting, incidentally, the existence of fluctuation at the world level), shows the first major wave produced by Rome, which integrated the Mediterranean basin and exerted a pull of attraction on the world economy; the second by Baghdad, the center of an Abbassid Muslim commercial and intellectual movement; the third by Peking, which, although more isolated than Hangchow, was Asia's largest city when the world's center-of-gravity still rested in that continent; and the fourth by London, capital for a network of cities that unleashed the explosive Industrial Revolution.

Fluctuations play a role in evolution. As Ilya Prigogine (1984) argues, when fluctuations are of greatest amplitude a complex system is at a 'bifurcation point' and able to 'choose' among varying regimes of order. If true for the world-city system, fluctuations of urban hierarchy represent opportunities for structural metamorphosis. Indeed, each fluctuation of urban hierarchy coincides with the transition to a new architectonic order; in other words, each peak of urban hierarchy matches one turning point in the evolution of the world-city system. The first wave coincides with the development of the Silk Roads as the most prominent (if not dominant) mode of cross-cultural transmission. The wave of AD 900 coincides with the mid-point in the 'struggle' between Silk Roads and Spice Routes leading to the eventual primacy of the latter. The wave of AD 1600 coincides with the rise of an Atlantic network and its rapid eclipse of its Indian Ocean counterpart. Finally, the last wave of AD 1900 coincides with the modern development of a Pacific-Global system.

These synchronicities - between ratios of world urban hierarchy and architectonic change - raise interesting questions: are transitions from one architectonic order to another dependent on fluctuations in the system? Do these fluctuations represent 'structural instability' and therefore 'bifurcation points'? Do great concentrations of economic and political power precipitate shifts of world connections? It is a tantalizing possibility. A final question emerges: why does each fluctuation contract in time span from about 1,000 to 800 to 500 to 300 years? Are world-level processes accelerating as communication networks tighten?

Another realm of concern is adaptation, an important hallmark of evolution. For the world-city system, the capacity to respond to a changing environment is of course the product of local adaptive behavior trickling-up to effect systemic change. At this local level, adaptive behavior emerges in the face of 'blockages' brought about by military and political choke-holds on world trade: taxes, tolls and other obstacles. (Some blockages, particularly for the Silk Roads, were induced by epidemics and desertification.) Thus, blockages represent a form of 'selection' and circumventions a form of 'adaptation,' with the cities forging new connections becoming, if not more powerful and prosperous, at least more secure. Indeed, the two most important transitions of the world-city system — from an

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overland Silk Road to a maritime Spice Route system, and then from a pre-industrial Spice Route to an industrial Atlantic system - were each induced by a series of blockage-circumvention sequences. Table 14.2 lists important blockage-circumvention sequences and the rise of associated cities.

The first blockage was caused by the loss of Chinese control over the western Tarim Basin. According to Franck and Brownstone (1986), the Parthian empire capitalized on these developments:

In response, Augustus Caesar (who ruled from 27 BCE to AD 14) tried to find another route to the East. The route north of the Caspian Sea was afflicted by similar problems, so he turned to the Red Sea. The Romans were aware that Arabs and Greeks had been sailing from Egypt to India for about a century, and Caesar hoped to tap this connection. His first military expedition to the region, as large as eighty warships and 10,000 men, was a fiasco, with thousands succumbing to heat and exhaustion. The second expedition was successful. The Romans sacked the major port of the Incense region, Arabia Eudaemon (Aden), forcing traffic up to Roman ports such as Aela. Nearby Alexandria became the western anchor for the Spice Routes and a distribution center for the entire Mediterranean. Of the Roman effort John Firth (1902:281) writes: 'Certainly the Red Sea became a Roman water and the trade of the Far East was largely diverted from the old caravan ports through Arabia into the Egyptian ports and the Nile Route, to the great profit of Egyptian revenue.' By the beginning of the first century AD trade along the Silk Roads had been reduced to a 'trickle,'

Table 14.2 World-city system blockages and circumventions

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having given way to the 'larger flow' of the Spice Routes (Franck and Brownstone 1986).

The second blockage also happened along the Silk Roads. Because Silk Roads remained the most direct link between the Mediterranean and China, and because China's population was still largely northwestern, Constantinople restored the overland route but soon found it hindered by the Persian empire. Thus, Emperor Justinian (who ruled from AD 527 to 565) tried to forge an alliance with the Ethiopian kingdom of Axum (connected to Ceylonese trade) by appealing to their common religion, Christianity The Axumites, however, putting profit before principle, declined Justinian's overtures. Constantinople then looked to the northern Eurasian steppe. From the northeast shores of the Black Sea, the city's merchants allied themselves widi the Jews of Khazaria and the Turkish tribes to link up, once again, with China. This opened up a new Silk Road, a more northerly track around the top of the Caspian Sea that would be inherited centuries later by the Mongols.

The third blockage considered here was a series of barbarian invasions plaguing China from the ninth through the eleventh centuries and disrupting continental trade. The Khitai, an early tribe of Mongols, initiated this disruption and paved the way for other tribes as Franck and Brownstone argue:

Ironically, many of these tribes intended to facilitate Silk Road trade and even established bureaucracies to this end. The disruption was largely due to the unpredictable coming and going of short-lived empires.

Other trends conspired to weaken the Silk Roads. In 960 a new Chinese dynasty; the Sung, took the throne in Kaifeng, a city integrated into the canal and coastal trade. In 1126 war with the northern Chin forced the Sung to relocate their capital further south to Hangchow, the bustling port of the Spice Routes Marco Polo would visit. China's center of population, reflecting these developments, became coastal.

The last case of blockage considered here precipitated oceanic expansion and led to a global system. Clearly, Muslim powers in the Middle East hindered European access to the Black Sea and the Red Sea, gateways to the East - a hindrance that stemmed less from the seldom-enforced ban on trading with infidels and more from regulations designed to favor Muslim traders. This was one reason for Portuguese exploration of the West African coast in the fifteenth century, a drive that culminated with Vasco de Gama's 1497 rounding of Africa's

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Cape of Good Hope. On repeat voyages, the Portuguese blocked the Red Sea to Muslim shipping. Cairo declined as a direct result and its Mamluke Slave Sultanate was conquered by Ottoman Turks in 1516. Lisbon rose in size and importance along with its trading partners: Antwerp and, later, Amsterdam.

Unlike many schoolbook stories, it is in fact correct that Muslim blockages of trade contributed to the European discovery of the Americas. After all, Christopher Columbus expressed his determination to find a shorter and unfettered route to 'Cathay' (China) and the islands of 'Cipangu' (Japan), and he mistook Caribbean islands for the Indies. (It would be only after Columbus died that Amerigo Vespucci concluded otherwise.) Columbus's 'circumvention' led to commercial expansion and cultural conquest.

These cycles of Silk Road and Spice Route alternation lead to a compelling proposition: the world-city system, as civilization's highest level of organization, reflected a tension between continental and maritime systems, each a strategy for building systemic structure, connectivity and differentiation - for moving the system toward complexity. By implication, the rise and fall of cities and empires is deeply embedded in die survival contest between Silk Roads and Spice Routes.

During this survival contest, a higher proportion of the world's largest twenty-five cities became, as the centuries wore on, oceanic ports. This 'maritime shift' of the world-city system, compatible with a historical record that has long recognized a shift from land-based to sea-based empires, is further evidence for the existence of evolutionary processes. Interestingly, Figure 14.2, which displays this maritime shift, can be compared to a 'learning curve' in mat initial progress levels off before resuming.

Figure 14.2 Maritime shift of the world-city system between 1000 BCE and AD 2000, as a percentage.

Source: Based on data from Chandler (1987) and McEvedy and Jones (1978).

Note: This figure demonstrates that a significant percentage of the world's largest twenty-five cities have become oceanic ports.

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Ports have a clear advantage: less dependent on the viability of one or few connections, they are less vulnerable to blockages. Ports, especially oceanic ports are better able to forge linkages with other parts of me system, and over time this creates a self-reinforcing trend: the maritime shift. Cooperation, as me maritime shift suggests, is as important in evolution as competition. More precisely, the world-city system reveals cooperation to be the dominant principle within trade routes and competition to be the dominant principle among trade routes. Pulses of local cooperation and competition translate into systemic adaptation.

This study demonstrates that me world-city system's structure, connectivity and degree of differentiation have all increased. The system has cycled its way to a maritime orientation. It has evolved. Hopefully, this study also suggests the merits of evolutionary theory for formulating and testing hypotheses of long-term, large-scale change. This approach can bridge the gap between social and biological analysis, with the former emphasizing qualitative change in structures and me latter quantitative change in populations. Indeed, through this prism civilization appears to be a technology (economic, political, social and cultural) of species survival and reproduction - me ultimate imperative for all life.

The evolutionary approach is capable of moving beyond the nineteenth century Social Darwinist focus on competition (an oversimplification of Darwin). Cooperation is an ingredient as important as competition in the evolutionary cauldron. Ironically, it is me 'realist' school, with its Machiavellian emphasis on state struggle amid perpetual anarchy, that unwittingly adopts, wholesale, me calculus of Social Darwinism.

Finally, there are advantages in retiring nations as primary units of analysis in favor of species-wide structures like the world-city system. One advantage is mat such structures are better windows into die diversity of human experience. Cities are simultaneously local and global agents in economic, political, social and cultural processes. Another advantage is that our research program is brought into line with me borderless challenges facing our civilization.