urbanisierung-und-migration#definition 2 "Approximately one in five international migrants are estimated to live in just 20 cities

. For 18 of these cities, international migrants represented around 20 per cent of the total population."

Migrants in and between the cities of the world

Armelle Choplin 1 , Hasnia-Sonia Missaoui 2 and Olivier Pliez 3

1 Université de Genève, Department of Geography and Environment & Global Studies Institute, Geneva, Switzerland. 2 Université Jean-Jaurès, Laboratoire LISST-CERS, Toulouse, France. 3 Centre National de la Recherche Scientifique (CNRS), Laboratoire ART-Dev, Montpellier, France. In Clementine Cottineau, Denise Pumain, 2022, « Cities at the Heart of Inequalities », ISTE-WILEY, London, p. 173-195.

Introduction: migration, urbanization and inequalities

While issues within the field of international migration are generally associated by international bodies with those of development, the UN Commission on Population and Development recently highlighted the fact that "urbanization and international migration are both inseparable from development" (Committee for Development Policy 2018). By linking these two dynamics, the UN thus intends to emphasize the importance of the urban dimension of migration flows, and it may therefore be tempting to see them as interconnected in multiple ways. The measurement of migratory flows would even support this hypothesis, since the majority of migrants are directed towards cities, more particularly "global cities" (Sassen 1991), in other words, those considered to be the most important and influential at the global level 1 . The International Organization for Migration estimates that about one in five international migrants live in just 20 cities: Beijing, Berlin, Brussels, Buenos Aires, Chicago, Hong Kong, London, Los Angeles, Madrid, Moscow, New York, Paris, Seoul, Shanghai, Singapore, Sydney, Tokyo, Toronto, Vienna, and Washington DC (IOM 2015), and that in 18 of these cities, international migrants make up nearly 20 percent of the total population 2 . These orders of magnitude alone fully justify the significance (in number and quality) of the academic work that has focused on the place of international migration, whether inter-or intra-urban, in global cities. However, we are talking about cities that are "intrinsically linked to each other (...) made up of hyper-skilled individuals (... and) unskilled individuals, mostly immigrants from poor countries", as Romain Lecler (2019, p. 365) summarizes, rereading Sassen. However, the insight provided by urban studies should not be limited to global cities. Many other areas of research focus on a broader spectrum of cities, old, recent or in the making, that are less, or even not very global, and yet fully integrated into migratory systems. The term international migration is just as generic as that of the city, since it encompasses many types of international mobility, both in terms of categories (from diasporic movements to those of transnational communities, from refugees to emigrants/immigrants), and of flows (those who stay, those who leave and those who pass through). If the proponents of a transnational reading of migratory flows have been tempted to go beyond these divisions by grouping these movements under the term transmigrants (Schiller et al. 1992), it was first of all to shift the focus of studies that until then had favored a "methodological nationalism" that was more attentive to the settlement of immigrants than to the circulation of migrants. Yet, ceaselessly, "these social groups recreate territory through their connections, imaginaries, and social practices [..., fabricating] a renewed territoriality, imbued with old ties and new networks" (Lima et al. 2017: 3).

If we look at international migration routes and flows as a whole, from a point of departure to a point of arrival, we see that they only partially correspond to the commonly accepted idea of a distinction between emigration and immigration spaces, constantly weaving and renewing links between here and there and more and more places between these two poles. It thus appears that statistical data on international migration do not allow for a detailed description of these dynamics, which are nevertheless growing in scope. Migration stocks as envisaged by demographics -i.e., the total number of immigrants living in a country or region -generate a certain inertia over time, due to the fact that the oldest destinations are also those with the largest migrant communities. In this way, the world's most attractive large cities are located in states whose demography has been fed by migratory settlement flows since the 19th century (United States, Canada, Australia, Argentina, etc.) or in those that have enjoyed global influence and a central position linked to the industrial revolutions of the 19th century and to the capture of financial flows since the 20th century. Flows (the inflow and outflow of migrants over a given period of time) allow us to understand in greater detail major trends -but here again in an incomplete mannersuch as the feminization of flows, the migration of the poorest and not only of the elite, workers or refugees. The result is an increasing regionalization of flows, more often over short than long distances. It is therefore the way in which migration systems are structured that is now gradually being called into question by new forms of articulation between cities and migration, at all scales. This is obviously not reduced to the metropolization of the world but more generally to its urbanization (Wihtol de Wenden 2017).

Migration flows are one of the reflections of inequalities in the world, between North and South, between cities themselves and within them. It is these links that we propose to explore by trying to see how migrants shape cities, highlighting and sometimes also reinforcing inequalities on a global, inter and intra-urban scale. The very term "migrant" in the common sense is generally connoted negatively, associated with poverty and sometimes even with the figure of the pariah or the deviant individual; moreover, the terms "expatriates" or "cosmopolitan elites" are more readily used to evoke migrants belonging to the upper classes. These two groups, poor migrants and expatriates, do not frequent the same places and do not mark urban spaces in the same way. In this way, they reveal the social and spatial inequalities highlighted by numerous studies, from the founding surveys of the Chicago School at the beginning of the 20th century to recent research. While the latest research raises these questions within a framework of mobility that is now transnational -and leads to a refinement of the conceptual and empirical angles of approach -most of it attempts to understand the ways in which migrants deploy strategies that contribute to the construction of cities and urban identities, and thereby mitigate or reinforce these inequalities. Migrants are not only actors of their own mobility but also of a wide range of urban dynamics. This chapter thus sets out to show that migrants' footprints on urban spaces vary considerably depending on the type of mobility considered, whether they are immigrants, diasporas or refugees, but also according to the temporality of the movement, and this at all scales. As a result, migration generates spatial and social distance, but also, necessarily, relations aiming to compensate for its effects or even to benefit from it. We will first look at the links that exist and are constantly being renewed between city networks and migratory networks, and then focus on a few specific examples of urban forms taken in various parts of the world, fuelled by migratory flows, in particular in Asia, the Arab world and Africa.

City networks and migration networks: a coincidence rather than a given

Do city networks generate migration networks or vice versa?

A nagging question about migration flows is whether city networks generate migration networks and vice versa. There is no doubt a sense of nostalgia at times, similar to that expressed by Ulf Hannerz (1983), who notes that by focusing on the various networks that take place in the city, we lose sight of the city as "a network of networks". While he pays tribute to the perspective proposed by Paul Craven and Barry Wellman (1973) by using their formula of the "network city", he also points out what he identifies as its limitation: the fact of privileging an illumination of some parts rather than a vision of the whole. Today, we would be tempted to see, on the contrary, in these "partial" approaches the possibility of shedding new light on old dynamics and of considering differently the apprehension of a totality that the human and social sciences have long since mourned.

According to Denise Pumain (2004b), "a city system is a set of cities whose development has become interdependent because of the multiple networks of relationships they have with each other and which connect them with greater intensity than with other cities located in the system's environment. In this sense, migrants are major actors in the networking of city systems. It often appears that migratory networks have their own logics that vary according to the type of migration considered. Migrants weave links between cities that are generally large metropolises and sometimes capitals, those that are best connected to international transport networks. Within these cities, neighborhoods often bear the memory of the successive arrival of different waves of migrants who then gradually settled in the cities and states of arrival, as the work of the Chicago School has clearly shown. At the beginning of the 20th century, the work of Robert E. Park and Ernest W. Burgess focused more specifically on analyzing the links between migration, inter-ethnic relations and urban transformation in large North American cities on an intra-urban scale. They have helped to disprove certain preconceived ideas about delinquency and to demonstrate that problems and conflicts were the result of difficult social conditions before being linked to possible ethnic criteria. From the same school, sociologists William I. Thomas andFlorian Znaniecki (1918-1920) studied the way Polish peasants maintained interpersonal relations between Europe and the United States, which highlighted the way distant relations were based on initial migratory movements, repeated migrations and return migrations.

Later on, the extent to which the work carried out at the beginning of the 20th century in the North American metropolises had a strong impact on subsequent research work became clear. The sociology of migration in France, referring to the Anglophone works of the 1950s to the 1980s on the analysis of social networks, did not really succeed in mobilizing this notion in a clear way in its scientific field and remained "metaphorical rather than analytical" (Hily et al. 2004). Instead, its members follow urban approaches, whose reading has been greatly renewed thanks to translations of research conducted by the Chicago School (Grafmeyer and Joseph 1979), to develop an approach based on "immigrant social networks" located in the cities or rather the neighborhoods where they settle. Their objective is above all to qualify or challenge the culturalist approaches, dominant until the 1970s, at an intraurban level.

In the regions of departure, however, a multitude of villages and small and medium-sized towns of passage are increasingly anchored to these migratory networks. And in the territorial in-between between the emigration and immigration territories, the rapid growth, visible for two decades, of places of passage with uncertain permanence is made necessary by the lengthening, in time and space, of migratory routes. What could sometimes be perceived as sources of opportunity for local populations has subsequently become a major fact. These different types of spaces, which have since been widely studied, refer to the pioneering definition of social networks by anthropologist John Barnes (1954), which Alessandro Monsutti (2009) refers to in his work on Afghan migration networks: "The image I have is that of a series of dots, some of which are connected by lines. The dots in the image are people, or sometimes groups, and the lines indicate which person interacts with which other. We can obviously think of the whole of social life as producing a network of this kind" (p. 90). These intermediate points are, for the geographer, places which, in a geographical refine, can be considered as migratory anchors connected by lines (or roads, or corridors). While they have long thwarted the link between "'spatial ' proximity and 'relational' proximity" (Bidart et al. 2011), it is clear that this dissociation is increasing as a result of the growth of modes of transportation and communication. Between emigration and immigration, circulation has thus taken on an increasing role within migratory networks. As a result, the links between city networks and migration networks are becoming denser and more diffuse and therefore more difficult to grasp.

Obviously, these observations explain why the notion of relational chains in migration has mobilized researchers from the whole of the human and social sciences for several decades, often working from decompartmentalized disciplinary perspectives. It is always useful to trace the genealogy of this concept. The beginnings of these approaches date back to the 1940s, with the pioneering work of the American sociologist Samuel Stouffer (1940) who, with the theory of "intervening opportunities", introduced the idea that distance and spatial organization are necessary for the study of migratory movements, particularly the effects of spatial proximity. Stouffer's reflections were fertile since Torsten Hägerstrand (1957) tested his intuitions by analyzing migratory movements in Sweden. His observations led him to question the fact that certain regions send many more migrants to a particular place than one might have expected, given its population and the distance separating it from that destination. As a quantitative geographer, he developed an explanatory model to show that this was due to the flow of information within a social network that linked regions of departure and arrival of migrants and thus determined where they settle. He was thus one of the pioneers in the reconstruction and analysis of migratory chains3 .

It was John and Leatrice Mac Donald (1964) who mobilized, adapted and mediatized this notion in academic circles in the United States in the 1960s. The term "network" was then explicitly introduced into the sociology of migration in order to analyze the phenomenon of the "migration chain" as a "movement in which a person seeking to migrate is kept abreast of migration opportunities [...] through his or her primary social relations with older migrants" (Mac Donald and Mac Donald, p. 82)4 . The two sociologists made sense of the different stages experienced by migrants during their mobility, highlighting their ability to create social capital that is also urban capital.

These lines of thought were further developed up to the 1980s, in particular by Douglas Massey and his team, who drew on a body of work that enabled them to define the network as " the set of interpersonal ties that link migrants, future migrants, and non-migrants in the spaces of origin and destination, through bonds of kinship, friendship, and a shared community origin5 " (Massey et al.,p. 448). Thus, when flows become significant, interpersonal ties multiply and consolidate. A process is then set in motion that modifies as much as it is modified by the social context in which migrants evolve, allowing them to increase their mobility by connecting to other migrants and other places of migration.

One of the main contributions of this theory of cumulative causality is therefore to demonstrate how the accumulation of social capital is established over time, through successive waves of migration between Mexico and the United States -the world's most important migration corridor -and over long distances, from the regions of departure to those where migrants settle. The number of interactions increases in proportion to the number of migrants from the same community and the territorial anchors they create and rely on. This allows migration to be self-sustaining in ways that organize migratory chains linking many localities around the world. Its success is also largely due to the banality of the situations analyzed, which migration researchers find in more or less similar forms elsewhere in the world. The multiplication of specific case studies thus identified allows for the generalization of this practice as inherent to migration systems.

The notion of relational chains in migration has thus produced and enriched numerous studies, to the point that Étienne Piguet concludes that "(the) theoretical pluralism suggested by Douglas S. Massey et al. two decades ago (Massey et al., 1993;1994) is still relevant" (Piguet 1993, p. 150, 153). However, if a fertile dialogue has been established between demographers and economists, who are sensitive to the articulation of the levels of action, it remains less operative for the geography or sociology of migration, except in order to provide an overall framework for the growth of migratory flows.

Many studies on international migration prefer to give "importance to the mesolevel, which attempts to link macro-and micro-conditions" (Ma Mung et al. 1998, p.39). It is this meso-level (or intermediate level) that sociologist Thomas Faist (1997) has been interested in, in order to seek a consensual approach to the reading of migration facts between macro approaches -that of the general frameworks of migration -and micro approaches -that of fine research, on an individual scale, the scope of which is not necessarily generalizable -in order to enrich a reading of the intermediate social levels, those that emerge and are structured in a flexible manner.

The question of the life span of these migratory chains also arises, as does that of their contours, i.e. the individuals and places included or not included in these studies. Several works have thus qualified the relevance of the contributions of Massey's theory, such as those of Monica Boyd (1989) or Fred Krissman (2005). Most consider that this focus on the "migration fact" overlooks the involvement of numerous nonmigrant actors in migration chains as well as the spatial structures on which these mobilities are based (Korinek et al. 2005). Since then, the widespread use of new information and communication technologies (NICTs) has further increased the number of migrant and non-migrant actors outside the community of origin acting in the migration chain (Nedelcu 2010).

Much subsequent research has built on this work, from the early contributions of British anthropologists in the 1950s -the Manchester School -to the migration theories of the 1980s, such as those concerning migratory fields and transnational networks.

Migratory field or circulatory territory?

In the 1960s and 1970s, the idea of mobilizing and refining these theories spread in France. The demographer Daniel Courgeau began with the observation that his discipline lacked the methodological tools to analyze migratory phenomena. He identified three types of variables: "the force of repulsion of the departure zone [...]; the power of attraction of the arrival zone [...]; the interaction between the places of departure and the places of arrival " (Courgeau 1970(Courgeau , p. 1180)). While the measurement of the first two variables poses problems that demography can partially resolve, the third variable introduces what he calls the "spatial effect", understood as physical distance and social distance. On the first point, he refers to the work of Hägerstrand and on the second to that of Stouffer. In order to take into consideration so many parameters, he proposes to group them under the notion of migration field. Thus, from Stouffer's sociological intuition to Hägerstrand's geographical analysis, both empirical and modelling, and then to Courgeau's under-studied demographic theme, the approach to the strictly relational dimension of international migration has been enriched by contact with different disciplines and fields over the past forty years. Inscribed in increasingly extended spaces and temporalities, it then gradually returned to disciplinary elaborations.

Gildas Simon reminds us that it is only since the 1970s that "the convergence of a double dynamic has been taking place, on the North American side, in logics of a more systemic type [...] and on the European side, in approaches [...] more 'spatializing' around, in particular, the notions of migratory fields, migratory spaces, then circulatory territories, territories of mobility..." (Simon 2006, p. 4). (Simon 2006, p. 4). When Gildas Simon introduced the notion of migratory fields in geography, he also laid the groundwork for the notion of circulation, which was very present in the work of the 1990s and 2000s.

Since the 1980s, the notions of migratory field or circulatory territory have thus made it possible to highlight continuities rather than ruptures and to pose the question of the link between migratory flows and urban spaces differently. In this spirit, Alain Tarrius intends to break with the immigration/insertion pairing, which is largely dominant in the field of international migration, preferring the migration/territory pairing, which is better suited to understanding the way in which foreigners contribute to the social construction of the city as a crossroads of mobilities rather than as a place of sedentarities. To consider the city as a crossroads and not (only) as the receptacle of migratory flows is to define the territory as the whole of what is connected, so that migrants' journeys "mix the here, where we are today, and the there, where we come from, an in-between, which does not succeed in linking these two ends points of these trajectories" (Tarrius 1993, p. 51) and thus relativize the strength of the so-called Chicago School model, which, by considering mainly settlement, does not sufficiently take into account their inter-urban spatial mobilities. This is a question of period, scale of reading and choice of research object. However, from the 1970s onwards -i.e. half a century after the North American work on urban sociology -it appears that circulations are no longer limited to flows, since they generate "circulatory territories" which, according to Tarrius, are based on "centralities subverted" in relation to the visions of planning policies. These "territorial corridors" link in a whole the mobilities of the countryside to the medium-sized cities and the capitals of the countries of departure to the cities of installation, and for some they are resources for developing transnational activities in "invisible metropolises" that are born in the heart of those that attract the greatest number of poor migrants. Such arrangements, Marie-Antoinette Hily and Emmanuel Ma Mung later summarize, reflect "the emergence of a world that is not so much that of globalization "from above" and of global cities (this world does exist) but of local and global connections of places, individuals, collectives and territories" (Hily and Ma Mung 2003, p. 39). If cities are at the heart of it, they can hardly be considered without the routes they mark out, and this goes far beyond the richest -and most studied -countries of the planet.

Global city, secondary city and metropolitan bias

During the 1990s and 2000s, the macro-structural approach, notably put forward by the work of Saskia Sassen (1991), presented immigration as "an essentially urban phenomenon and concerns in particular the major urban centers of the developed world" (Piché, 2013, p. 159). This observation has resulted in an emphasis on migration flows from the South to the North, and more specifically to their main urban areas.

Saskia Sassen sees the global city as a dual city insofar as two groups make up the core of its functioning: on the one hand, mobile, highly qualified executives and highprofit service companies that make it the place of innovation and creativity, and on the other hand, low-skilled employees, often poor migrants, whose social protection is weak. As a result, according to Sassen, interdependent relationships link these two groups. The better-off are able to pursue their lifestyles because they have local access to a range of cheap services provided by migrants at all hours of the day (cleaning, childcare, concierge, catering, delivery, dry cleaning, etc.). These contrasts are at the heart of the urban dailies of global cities, which are ultimately the crucible of a reinforcement of inequalities, marked by a muted violence, both between spaces, but also between inhabitants. In other words, Sassen shows that the connection to global circuits leads to a high level of development but also generates strong inequalities. However, this dichotomy needs to be weighed against many factors (Deboulet 2012), starting with the emergence of the middle classes, which on a global scale calls into question such an image of the dual city, but also the fact that the mobility of the "creative classes" is not as strong as one might suppose (Martin-Brelot et al. 2010). Since Sassen's work, the diversification of migratory routes observed by specialists leads one to think that the global city is only one receptacle among others for international migratory flows. These flows are not "mechanically" linked to city systems. At the global level, immigrants represent only 4% of the world's population, or 258 million people in 2017 according to the UN. It appears that they migrate rather from the South to the South (97 million) before migrating from the South to the North (89 million) and from the North to the North (57 million). Conversely, only "14 million people born in the North live in the South" according to Gilles Pison (2019). This focus on the global city in the North, combined with the politicization and (over)media coverage of migrants attempting to penetrate American and European fortresses en masse, has resulted in other migratory flows, sometimes more significant in number, being ignored, particularly South-South flows and those from the countryside to secondary cities. Work has focused first on the global cities of the North and then, in the wake of the postcolonial approach and the Southern Turn in urban studies (Robinson 2006), on the "worlding cities" (Ong and Roy 2011) and other megacities of the Global South. This infatuation and over-study of certain metropolises of the South, such as Dubai, Mumbai, Shanghai or Johannesburg, has resulted in other urban and migratory dynamics becoming invisible. However, the "metropolitan convergence" observed does not mean the end of small cities as some have predicted (Dick and Rimmer 1998).

For nearly 40 years, researchers have been highlighting the predominant role of secondary cities in migration routes. While for some migrants they are only a stopover, many make them their main place of residence. These cities, which have between 100,000 and 500,000 inhabitants, according to UN-Habitat classifications, play a major role in reducing poverty, as has been demonstrated for Southern Africa (Christiaensen and Kanbur 2016). Mathieu Hilgers (2012) reminds us that West African secondary cities are home to "the invisible majority of urban dwellers." Recently, taking the case of India, the work of Éric Denis and Marie-Hélène Zérah (2017) has also insisted on this "subaltern urbanization", in other words, this massive urbanization that takes place outside of the large Indian millionaire metropolises. They propose to take the opposite view of these trends by seeking to understand urbanization through the small towns and rural villages that abound in India.

The various works on the global city and the larger ones make us forget that migratory networks are ultimately more complex, and that the "city" object, whether small, secondary or large, as a place of passage or arrival, is above all a point marking out a complex journey.

Migration flows that create urban systems

Since the 1990s, overland migration routes have lengthened and, with them, the migratory stages have multiplied and diversified to the point of constituting systems of cities of different sizes, not very visible but very well connected to each other, between Africa and Europe (Bredeloup and Pliez 2005), Latin and North America (Baby-Collin et al. 2009) or from China (Roulleau-Berger and Shi 2004) to all continents. The urban forms that emerge from these migratory flows tend to amplify inequalities and make them visible, since they mainly concern the most modest migrants; in fact, the organization and functions of these "pieces" of cities evolve with the migration, sometimes even the city as a whole. One thinks of such emblematic forms as ghettos, historical markers of diasporas and multiple forms of urban segregation (Bruneau 2004), as well as ethnic enclaves (Chabrol and Dubucs 2014), sometimes perceived as modern forms of ghettoization in large metropolises, but which, with globalization, change their function, and also to the camps (Agier 2014), long associated with refugee or internally displaced populations, but whose number is multiplying with the detention centers that line the new migratory routes. This distancing, whether desired or suffered, does not only concern the most modest but also the circulating elites who settle in closed neighborhoods (gated communities and other condominiums) in large metropolises around the world (Varrel 2020), the diasporas returning to African cities, more or less copying the network of global cities and those closely associated with them.

We propose here to present some of the urban spaces shaped by transnational migration, which highlight the dynamism of these migratory networks and the active role of migrants in urban economies, whether in globalized forms or in a more banal way, the multiple territorial forms of departure or passage. To do this, we will take as our starting point two well-known migratory situations in order to understand their evolution over the past half-century: the Maghreb migratory field, which has gradually become an urban field, and migration in sub-Saharan Africa, a powerful engine of plural urbanizations. We will then focus on two situations, one generated by globalization from below and the other accompanying the spread of viruses around the world, which highlight global-scale dynamics that result in a subtle multi-scalar game, where variations in levels are constantly entangled, from the planetary urban to the urban interstices.

The globalization of the Maghreb migratory field

Rereading the articles by Gildas Simon and Daniel Noin (1972;1996) on North African migration to Europe, the first written together and the second written by Simon in homage to Noin, 25 years apart, provides an overview of the themes dealt with from one generation to the next and sheds light on the reasons for the emergence of the field of research on North African migration. The article published in 1972 starts from the observation of the rapid growth of North African immigration to France, particularly that of Algerians, who constituted the largest foreign community in France in 1971. The causes of this immigration are well known -the decades of post-war economic growth, the links forged by decades of colonization, European integration... -as well as the countries where migrants settle -France, Germany, the Netherlands and Belgium -particularly in industrial basins, irrigated areas and metropolises, and those from which they leave, mostly "rural regions with a long tradition of migration: Western Anti-Atlas, Souss, Western High Atlas, Eastern Rif, Trara and Dahra in Morocco, Greater and Lesser Kabylie in Algeria, Sahel of Bizerte, Sousse and Sfax, Djerba and southern coastal edge in Tunisia as well as the three national metropolises of Casablanca, Algiers and Tunis. This Maghrebian migratory space became global from the 1980s onwards with the increasing diversification of destinations in Europe but also in the Arab-Persian Gulf, Libya and North America. Gildas Simon, once again taking stock of the changes that have occurred since the early 1970s, notes that "classic labor migration, essentially male, with a high turnover rate, is giving way to a real relational system, structured by working-class and commercial diasporas" (Simon 1996, p. 29).

It is around the notion of the migratory field that geographical rather than sociological approaches are being developed, following the reflections of Daniel Courgeau. However, the first theoretical elaborations can be found in the work of Roger Montagne (1954), who spoke of a migratory noria in the 1950s in order to describe the organization of migratory flows of North African workers to France, then a colonial metropolis. Abdelmalek Sayad (1977) later criticizes the risks of such an approach, which is not only a situated point of view -that of the colonial power, which imposed "a view that is both partial and ethnocentric" -but which also fixed a figure, that of "the immigrant being fixed once and for all in the image of the rural (or peasant) emigrant alone (i.e. without his family), for a necessarily limited period of time" (Sayad 1977, p. 60).

But the notion of migratory field was then mainly used in the context of research on internal rather than international migration. As early as 1971, Roland Schwab, using the example of migration from/to Alsace, reserved "the term 'migratory fields' for all flows of migrants travelling through a geographical space, whatever their origin or destination" (Schwab 1971, p. 369). Roger Béteille extended these analyses to the Aveyron. He later clarified their scope by stating that migration "creates a relational space in which impulses and values specific to the migrants' region of origin are mixed with a socio-economic dynamic of an urban nature" (Béteille 1981, p. 197).

Gildas Simon (1979) confirms the relevance of these observations on an international scale, between Tunisia and France. The perspectives opened up on the Franco-Tunisian migratory field are particularly fertile because they enhance the importance of the spatiality of migratory relational spaces between the country of origin and the host country. The study of international migration is then quickly established in all its plurality since it does not only consider the logics of settlement and circulation but that upstream undeniable territorial strategies at the initiative of migrants echo the process of urbanization of countries of emigration. In Tunisia, this is visible at all levels of the urban framework, from rural villages (Baduel 1980) to the economic activities of medium-sized cities (Ma Mung 1986) and cross-border areas (Chandoul, Boubakri 1991). ). Within the vast field opened up by the transition from analysis of a migratory field to the way it becomes transnational, globalization from below is already fully installed as one of the fertile paths in order to grasp the emergence of social and spatial networks still active today that link cities of different sizes no longer in a particular field but according to intercontinental logics.

Globalization from below, transnational "ethnic niches"?

On all continents, marketplaces created by transnational communities (North African, Turkish, Lebanese, etc.) and diasporas (Chinese, Indian, Syrian-Lebanese) are now highly connected places of globalization. Gernot Grabher ( 2006) was surprised that geographers are so slow to perceive in urban ethnic enclaves a transnational economy driven by migrant communities that goes far beyond individual and local strategies.

At first, in the 1980s, these enclaves developed in cities where migrant communities settled and arrived, mainly in the neighborhoods of northern metropolises, often with a long history as a gateway for migrants, based on the logic analyzed by the Chicago School. The examples of Miami in the United States (Portes 1997) or Marseille in France (Tarrius 1995) are emblematic of the way in which communities from Central America, the Caribbean or the Maghreb have developed economic activities between a "here", place of settlement, and a "there", place of origin. These commercial enclaves were thus born within migratory fields where migratory movements were particularly active. They quickly became intertwined with similar arrangements in other fields, at the crossroads of multiple routes.

The scale of economic activity quickly transformed some of these enclaves into "trading posts" whose model spread from the Mediterranean and the Caribbean to Brazil, China, Eurasia and the African continent to serve a growing number of consumers. Thus, this economy linked to migration is profoundly urban if we want to consider this term in a broad sense. On a micro-local scale, these markets respond to localization logics that facilitate complex logistical chains, because they must respond to numerous constraints: that of the targeted consumer market, made up of consumers with low income levels, which marketing strategists describe as the "bottom of the pyramid" (BOP); that of entrepreneurs who have the capital and know-how that allow them to circulate in an extremely competitive transnational space; that of the rigidity or flexibility of border crossings... Chinatowns, present in major metropolises around the world, are one of the most emblematic markers of these chains of exchange. However, while the oldest of them are generally the object of a patrimonialization with a tourist vocation, we note at the same time that contemporary Chinese diasporas invest in wholesale markets located on the outskirts of metropolises, often in disused industrial zones (Madrid, Aubervilliers, Budapest, Naples... in Europe) or on sites created ex-nihilo. Other spatial forms have developed, at the initiative of different communities that settle or intersect in metropolises as was the case of the Libyan souk of Omdourman (now destroyed) created by Sudanese returning from Libya in the Sudanese capital of Khartoum (Chevillon-Guibert 2017), port cities such as Odessa in Ukraine (Marsden and Skvirskaja 2018), or medium-sized cities occupying a central position within a national territory, such as El Eulma in Algeria (Belguidoum and Pliez 2015), but also in border positions along urban pioneer fronts, such as Ciudad del Este in Paraguay (Rabossi 2012), or as a result of incentive-based public policies in medium-sized cities, the most emblematic of which remains Yiwu in China (Belguidoum and Pliez 2015).

These markets, where imported goods are traded at wholesale and semi-wholesale levels and then transported to innumerable retail sites, are connected to each other around the world, forming a heterogeneous, inconspicuous, but durable urban framework. Each of these markets also occupies a function at the level of mesoregions where transnational entrepreneurs, former migrants or children of migrants, specialize in a niche of importing products that they then resell in different parts of the territory (Zhipeng, 2020). However, if these activities form an urban network, they should not be confused with the great intercontinental migratory movements but only with the small entrepreneurial communities that have emerged within them. Julien Thorez, studying the highly political Silk Roads megaproject in Central Asia, considers that it is a "myth that obscures international migration" (2016, p. 297). Yet the power of these new myths often obscures the vitality of initiatives born from below, which have structured perennial exchange routes at the origin of institutional initiatives that then tend to obscure them.

"Why sell the dream of 'New Silk Road' when a multiplicity of silk -or perhaps more aptly nylon -roads already exist? Why impose a utopian vision of a connected world on people who are already familiar in sophisticated and intimate ways with the dynamics of a complex region?" asks the anthropologist Magnus (2017, p. 23) .

Migrants and urbanization in Africa

Migration and urbanization are intimately linked in Africa and have been the subject of numerous studies, beginning with those of the Manchester School and the Rhode Livingstone Institute (RLI), which opened in the 1950s in the Zambian Copper Belt, the mining area straddling Zambia and the Democratic Republic of Congo. Although less well known in research on the North, outside the network analysis community, the Manchester School shares with the Chicago School an interest in the study of new migrants moving to the city. Scholars such as Max Gluckman and Clyde Mitchell examined how Africans left their villages to work in the extractive industries, emphasizing the figure of the modern African, with Gluckman explaining that "an African townsman is a townsman, an African miner is a miner" (1960:57). But the Manchester School's observations on urban integration differ from those made by the Chicago School, as geographer Amandine Spire reminds us in her work on the figure of the foreigner in the cities of Lomé and Accra (Spire 2011). While the Chicago School emphasizes the loss of rurality and processes of acculturation, with the migrant becoming invisible, the Manchester School reveals that migrants from southern Africa maintain various social, cultural or ethnic networks to negotiate their place in the city. Recently, Jennifer Robinson (2006) has recalled the importance and interest of the Manchester School's work in understanding urbanization in Africa, in addition to that of the Chicago School. In a postcolonial approach, she emphasized that the Chicago School's models are valuable, but above all they were thought up by and for Western urban culture and from Western cities. In this sense, they are not intended to be universal and valid for all ordinary cities in the world.

Urban growth, in connection with the massive rural exodus, became a key issue in the 1970s. Policies sought to limit the flow of people to cities, based on the urban bias theories developed by Lipton (1977), who warned against overurbanization. According to him, cities tended to become parasites that sucked up all the strengths and resources of the countryside, and that to avoid this process it was necessary to encourage the development of the countryside and limit development aid to the cities in order to reduce their attractiveness. But in the 1980s, the theories of the "urban bias" were challenged and the World Bank proposed to strengthen its development projects in cities, a sign of consideration of the close relationship between rural and urban areas. Since the 1970s, it has been emphasized that the departure from the countryside to the cities is rarely definitive, and the term rural exodus, like that of definitive settlement, is ill-suited to describe these multiple circulations. Florence Boyer and Harouna Mounkaila (2010) have thus clearly shown these pendular migrations between villages and Sahelian cities in Niger and cities in the Gulf of Guinea (Abidjan, Accra, Lomé, etc.) where men go to work for several months before returning to their village of origin for the harvest season. She speaks of "fixed routes but nomadic spatialities. Different research programs have also shown these complex links, such as the MAFE (Migrations entre l'Afrique et l'Europe) program coordinated by demographer Cris Beauchemin (2015), which highlighted different forms of settlement and return, more complex than simple immigration. In the MIDDAS program, the economist Flore Gubert and her team followed Senegalese families between West African countries and cities, France and Italy, and highlighted complex circulations, made up of round trips, circulatory loops (Chort et al. 2016).

Other studies have highlighted the reversibility of migration flows to cities. Deborah Potts (2016) has highlighted the importance of counter-urbanization, in other words, of reverse migration flows, this time from cities to the countryside. This reverse rural exodus has been observed in southern Africa, particularly in the Copper Belt and the declining mining towns, which neo-urbanites have ended up leaving because they could not find decent living conditions. Cris Beauchemin (2013) arrived at similar findings for Côte d'Ivoire and Burkina Faso. These counterurbanization trends suggest that traditional rural-urban migration is not, or no longer, the only factor driving urban growth.

In this configuration of intense transnational mobility, African cities themselves must be seen not simply as places of arrival or departure. They are part of the mobility experience, as stopping points, transit points, meeting points, pilgrimage points, and religious resources (Bava 2011), or trading places along complex routes, often spanning thousands of kilometers, but also weeks or months and several countries. This is at least what we have been able to observe by analyzing mobilities between Mauritania, Senegal, and Mali: between these three countries, male and female merchants travel in buses and at the borders, carrying with them all kinds of goods (fabrics, car parts, hi-fi equipment, and telephones), people visiting family members, who are scattered among different countries, and a few single men ready to "try the adventure" towards Europe (Choplin and Lombard 2008).

These migrants transform the cities through which they pass or end up settling more permanently as a result of restrictive migration policies. They generally rely on these resource neighborhoods to which migrants converge, such as the Zongos, which are clearly visible in the cities of the Gulf of Guinea. In Kumasi in Ghana, Lomé in Togo and Cotonou in Benin, the zongo (from the Hausa word for "stopover") refers to the neighborhood where Muslim traders from the north of the country, often Hausa, Fulani or Songhai, gather (Agier 1983). In these zongos, otherness is clearly expressed: these neighborhoods are marked by the imprint of Islam (mosques, daily prayers, Friday gatherings) (Spire 2011). Conversely, foreigners tend to be invisible, or even to become invisible, as soon as they leave the neighborhood to face the city. The same observation was made in Sahelian and Saharan transit cities at the time of the migrations towards the Mediterranean coasts in the 2000s (Bredeloup and Pliez 2005): migrants intensely transform pieces of cities that are connected to each other, even though they are very distant, while remaining strangers to the city on which they are grafted (Agadez, Gao, Tamanrasset, Nouadhibou...). These studies have sought to break with the image of these migrant neighborhoods, which are too generally stigmatized and perceived as problem areas or as areas of deviance. On the contrary, they have shown that these neighborhoods are resourceful spaces, producing wealth and allowing connections with the outside world. Alice Hertzog (2020) made a similar observation when analyzing the links between urbanization and migration along the Abidjan-Dakar corridor. She explains that far from reinforcing urban poverty, migrants move into, through, between, and out of cities, which has an impact on urbanization and produces specific urban forms. Migrants in the cities of Accra, Lomé, Cotonou, or Lagos should be viewed positively, as they are part of these cities, their urban identities, as well as sending remittances and building and investing in infrastructure in their places of origin (figure 6.2). It is interesting to compare these migrant neighborhoods, which are considered poor even though they are often key and positive elements of urbanization and local African economies, with the rich neighborhoods built by the diasporas, which are perceived positively for the money they transfer to their regions of origin. In West Africa, members of the diaspora and recent returners, known as "returnees" in English-speaking Africa, have become key actors in the urban fabric, fueling the boom in private real estate development (Choplin 2020; Goodfellow 2020). It responds to a growing demand for quality individual housing that meets international standards. This is what the discerning middle and upper classes are looking for, as well as Africans in the diaspora, many of whom invest in real estate to hoard their money from afar (Mercer et al. 2008). In Africa, as elsewhere, these migration flows highlight the growing inequalities between cities, but also within cities, between the neighborhoods of poor migrants seen as outcasts and those of rich migrants, the expatriates, whose urban lifestyles are envied.

Epidemic perspective on global urbanization: the case of Dengue

Since 2019, the health crisis has led us to focus our collective attention on a largescale pandemic, that of COVID-19, which we are beginning to perceive as bringing powerful structural changes in the medium and long term. However, beyond COVID-19, there is a growing awareness that other pandemics will occur, and this raises questions about how societies, which are increasingly urban and mobile, will deal with them.

Viral threats are global but the responses to them are first and foremost local (Frutos et al. 2020) and, in fact, increasingly urban. Scientific communities agree that "the mobility of people and merchandise can trigger large epidemics and has favored the emergence of coronaviruses and arboviruses in regions of the world where these viruses were absent" (ibid, 2020) in what Gubler (2011) calls the "Unholy Trinity" by establishing close links between the spread of Dengue and more generally of viruses with globalization and urbanization of the world. This dialogue needs to be extended beyond the scientific circles most directly concerned, those of health specialists, in order to shed light on the territorial facets of the manufacture and spread of viruses, of which dengue fever is particularly exemplary. It is indeed a vector-borne viral infection that is inexorably spreading around the world at the rate of 390 million infected people per year, affecting 9 countries before 1970 and 124 today (Pham Thi et al. 2017). While it is mainly found in the intertropics, it is now also spreading outside of its initial foci, including the southern United States, the Mediterranean basin, and southern China (Brady and Hay 2020) while the flight distance of the affected mosquitoes, of the genus Aedes aegypti and Aedes albopictus is limited to an average of 400 meters.

These territories increasingly coincide with what Neil Brenner and Christian Schmid (2014) call planetary urbanization, based on the observation that "Today, the urban is constituted by a complex of places, whose links spread across multiple geographic scales, ranging from the district or even the housing unit, to the planet" (Charmes and Rousseau 2020). Such a definition leads us to go beyond the morphological criteria on the basis of which the urban is generally qualified, and to favor a relational approach -from individual to individual, from site to site, from community to community -. However, we note that the increasing dispersion of human communities at the national and international levels is generating more and more mobility. Thus, with the acceleration of these two processes, that of the spread of the virus and that of human mobility, our vision of their interactions is blurred. However, "urban lives" are taking shape at the intersection of these two globalization processes. We can try to identify the issues at stake at several levels: At the local level, the link between mobility, urbanization and dengue is now established. While the factors of its spread are multiple and complex, it appears in Delhi that "this geography is also shaped by individuals with increasing mobility" (Telle, 2015). Work conducted on Santa Cruz de la Sierra, a Bolivian city of 2.5 million people, leads to similar results, namely that while mosquito breeding grounds are fairly well identified (abandoned tin cans, old tires, or flower pot saucers), "the mobilities of the population are likely to generalize its spread to the entire city." As a result, economic and social inequalities have obvious spatial implications, as they are reflected both in the ecological conditions of different parts of the city but also "in terms of screening and care of the sick" (Donnat et al. 2011). These findings can be found wherever dengue fever takes hold, such as in China, where it was absent until the late 1970s. Yet, a single construction site in rapidly expanding cities can contribute to infected migrant workers introducing dengue, which then spreads from there to the neighborhood populations (Liu et al. 2021).

On a global scale, the pioneering investigations into the global trade in used tires (Reiter and Sprenger 1987), for example, demonstrate the value of analyzing specific channels in order to understand the introduction of exotic species in the wake of the rise of containerization; Lucky bamboo played the same role as a vector. However, it appears that the issue goes beyond the mere movement of goods, as it is closely associated with human migrations (Steverding 2020). Phylogeography allows us to identify, for example, how "the probable sources of dengue virus detected in Guangzhou" reflect the mapping of transnational trade exchanges (Shaowei et al. 2015) or how air traffic (Tian et al. 2017) is a powerful vector.

Today, the mega-project of the physical and/or digital "New Silk Roads" as well as the multiple routes of the "commercial democratization of products and materials, sometimes high tech but most often low tech and low cost" (Ferchaud et al. 2020) represent both a real challenge in terms of commercial and urban logistic planning as well as in terms of opening new air, sea and land routes to the spread of viruses worldwide. The roads and the markets, ports, airports and intra-urban commercial areas they link are indeed urban and transnational interfaces where dengue fever is transmitted and spread from. A Sanitary Silk Road is thus planned according to the protocol signed between the WTO and China in 2017. It provides for collaborative actions (Chen et al. 2019), particularly with regard to Dengue fever along the Maritime Silk Road.

Finally, while the two scalar levels of the local and the global are most frequently invoked in spatial readings of virus circulation, the mobilities generated by containment in India demonstrate that the flows generated by metropolitan areas there have implications for the viral diffusion of COVID-19 (Figure 6.3), dengue and other pathogens not only at the intra-regional but also at the inter-regional level (Denis et al. 2020).

Conclusion. Migrants and cities: creators and gas pedals of inequalities?

Cities, like international migration, are in many ways sufficiently broad objects for each of them to allow us to question socio-spatial inequalities at different scales. Starting from this perspective at the intersection of these two themes, we can nevertheless refine our approaches to inequalities on several key points, in particular those that can be grouped under the question of people's rights of access.

Although Lefebvre's work (1968) on the "right to the city" is constantly relevant to urban research and, for this reason, is regularly questioned, it appears that it refers to many related issues that point to the strong inequalities suffered by the most vulnerable migrants. Beyond the rights that freedom of movement poses and, in particular, the fact that "the question raised by movement is ultimately that of the right of access to places: the right to live in the neighboring country, but also, once the border has been crossed, to return home" (Baby-Collin et al. 2009, p. 108), other rights of access arise, such as the right to health care, education, housing, and legal resources, to which migrants have difficulty gaining access and which are often intermittent... In this way, cities are spaces that reveal or amplify the inequalities suffered by migrants. This is particularly the case with dengue fever, as it appears that in the metropolis of Delhi, India, "the virus takes advantage of precarious environmental conditions (high population density, limited access to infrastructure) and inadequate disease governance to sustain itself in the urban system" (Telle 2015). This is also the case with the inflation of relegation sites that from transit corridors to settlement neighborhoods line globalized migration routes.

However, migrants are also both individual and collective actors in the urban fabric. In this respect, while the inequalities inherent in urban systems and migratory systems no longer need to be demonstrated, it seems just as necessary to identify their "city-dweller skills" (Berry-Chikhaoui and Deboulet 2000) in the places from which they leave, where they pass through and where they settle. In this plurality of places, the initiatives that migrants deploy in order to deal with, mitigate or escape from the inegalitarian systems that are imposed on them are omnipresent, whether it be the vulnerabilities of a migration that is becoming more feminized but also what it offers in terms of the possibility of women's emancipation (Schmoll, 2020) or migrants who, in the harsh context of the industrial crisis of 1973, invent new ways of playing with the borders and the profits generated by transnational companies... The little visible but strong links that connect cities and regions across national borders are part of these strong competences. Thus, if "the network has a bad reputation" (Dewitte 2004, p.1) because it is often confused with "communitarianism, clandestine immigration, mafia networks, trafficking, underground activities, religious fundamentalism...", research will be more inclined to identify the modalities of mutual aid between villagers or members of an extended family. This oscillation between scholarly use and commonplace use contrasts with the richness of the bridges that have gradually emerged at the intersection of the two fields of research, that of networks and that of migrations, which often refer only to a mode of social organization of populations which, even when they are settled, are led to move more often than those who are described as sedentary. It is, in short, a mode of social and economic organization that allows migrants to achieve social fulfillment by benefiting from the solidarity of the group and, ultimately, to seek better integration into the host society. Today, the framework is both urban and global.

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Figure 6

 6 Figure 6.1: The Mediterranean migration and trade space

Figure 6 . 2 :

 62 Figure 6.2: The Accra-Lagos Urban Corridor: Intense Traffic Space

Figure 6 . 3 :

 63 Figure 6.3: Mobilities generated by COVID-19 lockdown

On this point, we refer to the tribute paid to him by the American geographer Kevin R. Cox: https://kevinrcox.wordpress.com/2017/06/14/torsten-hagerstrand-and-migration-sixty-yearson-and-still-relevant/

4 "that movement in which prospective migrants learn of opportunities (...) arranged by means of primary social relationships with previous migrants"(Ibid, p. 82

). 5 Translated by Swanie Potot in Potot Swanie, 2006, "Le réseau migrant : une organisation entre solidarité communautaire et 'zone de libre échange'", Migrations Société, vol. 18,