Networks of Exchange

What actually changed

For most of its history the overland trade crossed a dozen sovereignties, each with its own tolls, its own currency, its own courts and its own bandits, and the traffic rose and fell with how many of those states were currently stable. After the Mongol conquests of the thirteenth century, related Chinggisid regimes ruled from Korea to the Black Sea. Be careful how you state what that bought, because the empire split into competing khanates after 1260 and they fought each other. What survived the split was not one government or one law but a shared political culture: comparable merchant privileges, recognized diplomatic practice, family ties across the khanates, and rulers who each had an interest in the traffic. The tolls were fewer, and a merchant robbed in one khanate had a better chance of redress from a neighboring authority than an unrelated patchwork of states would have offered.

The Mongols did not merely tolerate this trade, they were invested in it, in the literal financial sense described in section 03. Steppe rulers had always needed what settled societies produced and had always taxed the exchange; what was new was the scale, and the fact that the men taxing the road at one end and the other were cousins. That is what the term Pax Mongolica means: not peace in any general sense, since the conquests themselves were catastrophic, but a period in which the commercial arteries of Eurasia ran through a family of related regimes with a shared interest in keeping them open.

The route in this period

The eastern terminus was the Yuan capital at Khanbaliq, modern Beijing, with goods gathered from the Chinese interior. Westward the road ran through the Gansu corridor, skirted the Taklamakan desert north or south, and reached the oasis cities of Central Asia, Kashgar, Samarkand and Bukhara among them, where the routes braided and re-braided. From there one arm ran southwest through Persia to Tabriz, the Ilkhanate's great commercial capital, and on to the Mediterranean ports of the Levant and to Trebizond on the Black Sea. Another ran northwest across the steppe to the Golden Horde's territories and the Genoese and Venetian trading colonies on the Black Sea coast, of which Kaffa in the Crimea matters enormously in section 05.

Notice what those endpoints have in common: the overland road terminated in ports. The overland and maritime systems in this unit are not rivals in the way students usually imagine; goods routinely traveled by land to a coast, by sea to another coast, and by land again. A comparison question that treats the Silk Roads and the Indian Ocean as separate worlds is missing the fact that the same Venetian firm often used both.

Kashgar and Samarkand were commercial nodes

Kashgar sat near the western edge of the Tarim Basin where routes around the Taklamakan converged before continuing toward Central Asia. Samarkand occupied another strategic position farther west. Neither city produced every good passing through it. Their value came from connecting merchants, routes, services and markets.

A busy node supports warehouses, brokers, money changing, animal markets, lodging and specialized labor. As the number of merchants rises, demand for those services rises too. That is how greater trade volume can translate into urban growth.

The system reinforced itself

The causal chain loops back on itself: luxury demand gives merchants an incentive; caravanserais, credit, banking and money economies lower cost and risk; relative political stability can lower risk further; trade then expands in volume and geographic range; cities such as Kashgar and Samarkand grow; and larger markets encourage producers to expand output.

This is the main Topic 2.1 story. Religions, technologies and disease also traveled through connected Afro-Eurasia, but those consequences receive their own focused treatment in Topics 2.5 and 2.6. Topic 2.2 next examines the Mongol Empire that provided one important period of political support for overland exchange.

Why the steppe had never done this before

Pastoral nomadic society on the Mongolian steppe was organized by kinship: clans and tribes, with alliances built on marriage, feud and grazing rights, and confederations that formed for a campaign and dissolved after it. That structure produced superb light cavalry, since every man rode and hunted with a bow from childhood, and it reliably produced instability, because a confederation held together only as long as its leader was winning and its component clans agreed.

Temujin, born into that world and left destitute as a boy when his father was killed, spent roughly twenty years defeating, absorbing and reorganizing the steppe tribes. In 1206 an assembly of the united tribes proclaimed him Chinggis Khan. The reorganization is the point, and it is what the success criteria mean by the unification of the steppe.

The decimal army, which is really a political reform

The army was rebuilt into a decimal organization: units of ten, a hundred, a thousand and ten thousand, the last called a tumen. Crucially, the men in a unit were deliberately drawn from different clans and tribes, and a warrior could not leave his unit for another. His commander was appointed, and appointed increasingly on merit rather than birth. Collective responsibility ran through the unit, so a man's ten stood or fell with him.

Read that as politics rather than as military administration and it becomes the founding act of the empire. If your unit is your clan, your general is your uncle and your loyalty is to a lineage that existed before the khan and will exist after him. If your unit is a mixed hundred under an appointed captain, your standing depends entirely on the khan's system, and the old tribal identities have nothing to attach to. Chinggis dismantled the very structure that had made previous steppe confederations fall apart, and did it before he ever crossed a border.

Merit was real and is worth a specific example, because a claim about meritocracy needs one. Subutai, one of the two or three most successful commanders of the century, who campaigned from China to Hungary, was the son of a blacksmith, from a forest people rather than the aristocracy. In a steppe confederation of the old kind he could not have commanded anything.

What the conquests destroyed

Cities were annihilated: Nishapur, Merv, Herat and Baghdad among them, with populations massacred or enslaved and defenses razed. In Iran and Iraq the damage went beyond the killing to the infrastructure that made the land habitable, because the region's agriculture depended on qanats, underground irrigation channels requiring constant skilled maintenance. Kill or scatter the people who maintain a qanat and the system silts up; the land it watered goes out of cultivation and takes generations and capital to recover. Some of it never did.

The Islamic world lost the Abbasid Caliphate in Iraq in 1258, and the political center of gravity moved to Cairo and to the east. The Russian principalities spent two centuries under the Golden Horde's tribute and its intervention in their politics, and one consequence was the rise of Moscow, which collected tribute for the khans and grew powerful doing it.

Why the empire came apart

Three causes, and the first is structural. Succession was never solved. Mongol practice required a kurultai to choose a khan from among the eligible descendants, which meant every death was a crisis, campaigns were abandoned so that commanders could travel thousands of miles to attend, and by 1260 the contest between rival claimants had become open war between branches of the family. An empire whose leadership transfer is a contested election among armed cousins will fragment; the only question is when.

Assimilation was the second. Each khanate adopted the religion, language and administrative culture of the region it governed, which made it a competent local government and made the four of them foreign to one another. By the fourteenth century a Muslim khan on the Volga and a Buddhist-influenced emperor in Beijing shared an ancestor and very little else.

Then the fourteenth century arrived. The Ilkhanate dissolved after 1335 without a viable successor. The plague swept the whole zone. In China, the Yuan faced flooding of the Yellow River, hyperinflation of the paper currency, famine and rebellion, and the Ming took the capital in 1368. The Chagatai lands broke up and Timur, who ruled in the name of Chinggisid legitimacy without being of the line, rebuilt a conquering empire out of the wreckage and devastated much of the same ground again.

How to write the balanced judgment

The instruction the success criteria give is to identify destructive and connective effects together. The way to do that without producing mush is to keep them causally linked rather than merely listed. The conquests that killed the population of Nishapur are the same conquests that put one legal system across the trade routes. The relay network built to hold a conquering empire together is the network that carried Rabban Bar Sauma west and the plague in every direction. The elite who took a share in ortoq merchant capital were the elite who had sacked the cities those merchants traded through.

That is not a compromise between two views, it is a single claim: integration of that kind was achieved by conquest, and it carried whatever moved. If you can write one paragraph in which the destruction and the connection are the same fact seen from two sides, you have understood this topic better than an answer that spends half its length praising and half condemning.

The dhow

The characteristic vessel of the western Indian Ocean was the dhow, a general term for a family of ships built along the Arabian, Persian and East African coasts. Two features define it. Its hull planks were traditionally sewn together with coconut fiber cord rather than nailed, which sounds fragile and is not: a sewn hull flexes under stress instead of splitting, can be repaired at any beach where coir and a needle are available, and needs no iron in regions where iron was expensive and timber for it had to be imported anyway.

Its sail was the lateen, a triangular sail on a long yard set at an angle to the mast. A square sail pushes a ship along the wind; a lateen can be trimmed to sail at an angle across it, which means a dhow could make progress when the wind was not directly behind it and could work in and out of harbors that a square-rigged ship would have to wait outside. Combine that with the monsoon's reliability and you have a system in which a captain could plan a two-year round trip in advance and be right.

The junk

From the east came the Chinese junk, a fundamentally different design and in several respects a more advanced one. Its hull was divided by watertight bulkheads into separate compartments, so a hole in one section flooded that section and not the ship, an idea European shipbuilding did not adopt for centuries. It carried a stern-post rudder for controlled steering, multiple masts with battened sails that could be reefed quickly, and on the largest vessels enough space that Ibn Battuta describes cabins with private facilities for merchants and their households.

The two traditions met at the great ports of India and Southeast Asia. Ibn Battuta, at Calicut on the Malabar coast in the 1340s, describes Chinese junks lying in the harbor waiting out the season alongside vessels from Arabia and Persia. That image, ships from opposite ends of the ocean anchored in an Indian port under an Indian ruler, is the whole system in one sentence, and it is worth using.

Finding your way

Navigation combined instruments with memory. The magnetic compass, developed in China and in use at sea by the twelfth century, spread across the ocean and gave a heading in cloud or at night. Latitude was found by measuring the height of a star above the horizon: the kamal, a small wooden card on a knotted string held at arm's length with the string in the teeth, did this with an accuracy sufficient to run down a latitude to a known port, and the mariner's astrolabe did it more precisely.

The largest body of navigational knowledge, though, was written and memorized: pilot books recording courses, distances, star bearings, landmarks, currents, depths and the timing of the winds for particular routes. The Arab navigator Ahmad ibn Majid, writing in the fifteenth century, compiled navigational works of remarkable sophistication, and this literature is the reason a captain crossing open water for weeks knew where he would come out.

What happened

Between 1405 and 1433 the Ming court sent seven expeditions into the Indian Ocean under the admiral Zheng He, a Muslim court eunuch from Yunnan. The fleets were enormous by any standard of the age, hundreds of vessels and tens of thousands of men, and they reached Southeast Asia, India, the Persian Gulf, Arabia and the Swahili coast of East Africa.

They were not voyages of discovery. They sailed established routes to ports that had traded with China for centuries, using pilots and charts that already existed. Their purposes were political: to display Ming power, to enroll foreign rulers in the tribute system by which they acknowledged the emperor's superiority and received lavish gifts in return, to install or support friendly rulers at strategic ports, and to suppress piracy along the routes. The fleets did fight when they judged it necessary, intervening in Sri Lanka and against a pirate stronghold at Palembang, so this was projection of power and not merely pageantry.

The most famous single object of the voyages is a giraffe, brought from the Swahili coast by way of Bengal and presented at the Ming court, where it was received as an auspicious beast whose appearance testified to the virtue of the emperor's rule. That is the tribute system working exactly as designed: the point of the animal was what its arrival said about the throne.

Why they stopped

The voyages ended, and the reasons are concrete rather than mysterious. They were extraordinarily expensive and generated prestige rather than revenue, at a time when the dynasty faced a far more urgent and dangerous problem on its northern land frontier with the Mongols. The capital had been moved north to Beijing, an immense construction project in its own right, which shifted the court's strategic attention inland. And the voyages were a project of the eunuch establishment at court, which Confucian officials opposed both on principle and as a matter of factional advantage; when those officials gained the upper hand, the fleets were discontinued and later shipbuilding of that class was restricted.

Chinese maritime trade did not end with the voyages. Private Chinese merchants and shipping continued throughout the period, and Chinese communities remained in Southeast Asian ports. What ended was the state's presence at sea, and the practical result was that when armed European ships entered this ocean at the end of the century, they met a rich, plural, well-organized commercial world that no single power was trying to police.

What the camel actually does

The one-humped dromedary of North Africa and Arabia is not a water tank; the hump stores fat, not liquid. What it has is a set of adaptations that together make a long waterless march survivable. It tolerates losing a quarter or more of its body weight in water, a level of dehydration that would kill most mammals, then rehydrates enormously and rapidly when it does reach a well. Its body temperature rises through the day rather than being held constant by sweating, so it loses far less water to cooling. Its kidneys concentrate urine, its nostrils recapture moisture from exhaled breath, and it eats thorn scrub and dry vegetation that would not sustain a horse or an ox.

The practical result is an animal that can carry roughly one hundred and fifty to two hundred kilograms, cover something like forty kilometers a day, and go for days between waterings. That converts an impassable barrier into a crossing of weeks: the great routes from the Maghreb to the Sahel took on the order of two months, timed to leave in the cooler part of the year and organized around known wells.

The second half of the technology is the saddle. Different regional designs solved different problems, but the essential development was a frame that sat stably on the animal and distributed a heavy load or a rider's weight without injuring it. That sounds trivial and it is not: it is the difference between an animal that can be used for occasional transport and one that can be loaded to capacity, repeatedly, as a commercial vehicle. When the success criteria ask for the technology that made the Sahara viable, the answer is the camel and the saddle together, and saying both is what marks a precise answer.

The caravan as an organization

Crossings were made in large caravans, sometimes of hundreds and by some accounts thousands of camels, and the size was a safety requirement rather than a boast. A big caravan can absorb the loss of animals, defend itself against raiders, and justify the expense of the specialists it needs.

The indispensable specialist was the guide, usually a Berber of the desert peoples whose knowledge of the wells, the terrain and the seasonal conditions of a particular stretch was the caravan's actual life insurance. Desert peoples also supplied protection, and were paid for it in a relationship that ran between escort and toll. Alongside them traveled the merchants, drivers, water carriers and, on many crossings, enslaved people who were themselves part of the cargo.

Everything about the route was dictated by water. The stops were the wells and the oases, so the map of the trade is really a map of aquifers, and a settlement that sat on reliable water at a route junction could tax, provision and house a traffic it did not create. That is the same logic as the Silk Road oasis cities, and it is why the two networks produced such similar-looking towns in such different landscapes.

People as cargo

Enslaved people were among the principal exports moving north across the Sahara throughout this period, taken largely from the savanna and forest regions south of the Sahel and sold into North Africa, Egypt and the wider Islamic world, where they worked as domestic servants, soldiers, laborers and concubines. Estimates of the total over the many centuries of the trade vary widely and are contested, which is a reason to describe the traffic as sustained and large rather than to attach a false precision to it.

The desert crossing itself was lethal for people traveling on foot in a caravan organized for the survival of the animals. And at Taghaza the miners cutting the salt were themselves enslaved, working a site that produced no food, so the commodity at the heart of the exchange was extracted by forced labor at both ends of its value chain.

The point for an essay is not to append a line of condemnation but to include this in the account of what the system was. A description of trans-Saharan trade as gold, salt, books and scholarship is not wrong and it is incomplete, and the incompleteness is exactly what a strong answer notices.

Where the gold ended up, and what happened next

West African gold was not a regional curiosity. It was a major source of the metal for the coinages of North Africa and the Mediterranean, and by the fourteenth century it was underwriting European currency as well: when Italian and Iberian mints struck gold coins, a substantial part of the metal had crossed the Sahara on a camel. That is a good fact to hold, because it makes the point that Europe in this period was a customer on the edge of somebody else's system rather than the center of anything.

The system changed at the end of the period rather than collapsing. Mali declined through the fifteenth century and Songhai rose in its place, taking Timbuktu and Gao and running the same trade from a new capital. The larger change came from the coast: Portuguese ships reached the West African shore and began buying gold directly at coastal forts from the 1480s onward, which meant that a portion of the metal that had crossed the desert now went by sea instead, and the merchants who had taken a margin on the desert leg were cut out of it.

That redirection belongs to Unit 4 and is worth flagging here for one reason. It is the same explanation this unit has offered twice already: cargo goes where the cost and the risk are lowest, and a route declines because a cheaper one appears, not because anyone closed it.

The four channels

Merchants, who are the main channel and are not trying to convert anyone. A trading community that settles in a foreign port needs a place to worship, brings its own specialists, eats its own food, and demonstrates by simply existing that a different way of doing things is workable and profitable. Almost every case in sections 02 to 04 begins here.

Specialists in transmission: missionaries, Sufi teachers, monks, and scholars traveling to study. These people are trying to move something, and they follow the merchants because the merchants have already established the routes, the lodging and the local contacts.

Rulers, who adopt something deliberately and can impose or endow it: a khan funding an observatory, a mansa returning from Mecca with architects, a sultan establishing madrasas. State adoption is fast and shallow; it changes the court long before it changes the village.

Captives and the enslaved, the channel textbooks routinely omit. Craftsmen taken in war carried their techniques to wherever they were resettled, and the Mongols in particular deported artisans systematically. Skills moved because the people holding them were moved against their will, and any account of diffusion that mentions only merchants and missionaries has left out one of the most effective mechanisms of the period.

Three things "it spread" can mean

Diffusion proper: the thing itself and the knowledge of how to make it move, and the receiving society adopts it. Papermaking is the clean case, and you can trace the workshops moving west city by city.

Stimulus diffusion: the knowledge that something is possible arrives without the method, and the receiving society invents its own way to do it. Much of the movement of gunpowder weaponry looks like this: knowing that a powder can throw a projectile is enough to set metalworkers to solving the rest locally.

Independent invention: two societies solve the same problem separately, and a diffusion story is simply wrong. The positional zero in Maya mathematics owes nothing to India. European movable type in the fifteenth century is very probably a separate invention rather than a Korean import, whatever the tempting chronology suggests.

Three travelers the CED names

Marco Polo, a Venetian associated with travel across Eurasia in the late thirteenth century, produced an account that gave European readers descriptions of Asian societies and the Yuan world. Ibn Battuta, a Moroccan Muslim jurist, traveled across Africa and Asia during the fourteenth century. His Rihla records both the reach of Islamic networks and the variety within them.

Margery Kempe, an English Christian pilgrim of the early fifteenth century, traveled to religious destinations including Jerusalem and Rome and later dictated an account of her experiences. Her purpose and route differed from Polo and Ibn Battuta, which is exactly why she is useful: intensified networks supported many kinds of mobility, not only merchants and diplomats.

Use travel writing as evidence, not as unsourced fact

Travel accounts tell us what a particular observer chose to notice, understood and wanted an audience to hear. Polo’s text was shaped in literary collaboration, Ibn Battuta dictated from memory after decades of travel, and Kempe’s account is intensely devotional. Those limits determine the kinds of claims each source can support.

For Topic 2.5, the larger pattern is straightforward: intensified trade, pilgrimage, diplomacy and religious networks moved more people over long distances, and some of those travelers produced written accounts that circulated knowledge about distant societies.

The organism and its reservoir

The Black Death was caused by Yersinia pestis, a bacterium that lives in populations of burrowing rodents and circulates among them through fleas. In its bubonic form it reaches a human when an infected flea bites, producing swollen lymph nodes and a high mortality rate; in its pneumonic form it settles in the lungs and passes directly between people through the air, killing faster and needing no flea at all. Both forms were present in the fourteenth century, which is part of why it moved so quickly through crowded settlements in winter.

Long-standing rodent reservoirs exist in Central Asia, and it is there that the current evidence places the beginning. That combination, a durable animal reservoir plus a route to carry infected rodents, fleas and people out of it, is what turns a regional disease into a pandemic.

The route west

The disease moved along the commercial arteries: westward through the territories of the Golden Horde, reaching the Black Sea ports where Italian merchants traded. In 1346 it was in the army besieging Kaffa, the Genoese colony in the Crimea described in the Topic 2.1 chapter. From the Black Sea, Genoese shipping carried it to Constantinople and to the Mediterranean, and by 1347 it was in Sicily, Alexandria and the Levantine ports. Through 1348 and 1349 it moved across Italy, France, Iberia, England, the German lands, Egypt and Syria, and by 1351 to 1353 it had reached Scandinavia and Russia.

Notice the shape of the spread. It jumps along shipping lanes rather than crawling across the land, appearing first in port cities and then radiating inland, which is what a disease moving with shipping looks like rather than one walking overland with refugees. The map of the plague is a map of the trade. How it actually traveled inside those ships is a live argument: infected rodents and their fleas are the traditional answer, and researchers have since modeled human fleas and body lice as more important in some European outbreaks, alongside pneumonic spread person to person and repeated fresh introductions by sea. The routes are settled; the vector is not, and saying so is better history than picking one.

Four archives and what each one holds

Tree rings. A tree adds a ring each year, wide in a good year and narrow in a poor one, and the pattern is shared across a region. Overlapping patterns from living trees, dead wood and construction timbers produces a continuous year-by-year record of growing conditions reaching back centuries, and dates any beam to the year its tree was felled. It is how the droughts behind the Ancestral Puebloan migrations in Topic 1.4 and the failures of Angkor's water system in Topic 1.3 are dated.

Ice cores. Snow falling on a glacier traps air, dust, ash and industrial pollution, and the layers can be counted and dated. Volcanic ash in the ice dates eruptions, and their sulfur explains cold years far away.

Pollen cores. Pollen grains preserved in lake and bog sediments are identifiable by species, so a dated column of sediment shows what was growing nearby, century by century. Forest species giving way to cereals and weeds is a picture of land clearance; the reverse is a picture of land being abandoned. Pollen is how the reforestation of abandoned farmland after the plague is measured.

Ancient DNA. Genetic material recovered from human, animal and microbial remains identifies pathogens directly and reconstructs their family trees, which is how the plague's identity, ancestry and probable geography were established, as section 01 describes.

How to use physical evidence well

Two rules. First, physical archives are excellent for what and when and poor for why. Tree rings can tell you that rainfall failed for twenty-three years; they cannot tell you whether a society responded by migrating, fighting or reorganizing, and only documents and archaeology can. The strongest arguments in this unit pair the two: the drought record plus the abandoned settlement, the lead record plus the tax rolls.

Second, be careful with correlation. A cold decade and a rebellion in the same years is a hypothesis, not a finding. What upgrades it is a mechanism, harvest failure, then grain prices, then unrest, with evidence at more than one step. That is exactly what the good work on the fourteenth century does, and it is a standard you can apply to any environmental claim you meet for the rest of the course.

The Silk Roads

Environment: deserts, steppe and mountain passes across Central Asia. Unit of transport: the Bactrian camel, about two hundred kilograms. Constraint: security, since the danger lies along every mile between towns. Cargo: high value per kilogram, silk, musk, gems, medicines, plus horses and enslaved people. Who ran it: relay merchants working single segments, with Sogdian, Uyghur, Persian, Armenian and Italian networks at different points. Institutions: caravanserais funded by charitable endowment, partnership contracts, bills of exchange, ortoq associations financed by Mongol nobles. State role: decisive; the network peaked when one authority governed its length and contracted when that dissolved after 1335. Endpoint: ports, because overland cargo terminated at the Black Sea and the Levant and continued by sea.

The Indian Ocean

Environment: a seasonal wind system of extraordinary reliability. Unit of transport: the dhow and the junk, cargo measured in hundreds of tons. Constraint: timing, since the wind decides when a voyage can begin and how long you must stay. Cargo: bulk as well as luxury, Indian cotton cloth above all, plus rice, timber, mangrove poles, porcelain, spices, horses and enslaved people. Who ran it: resident merchant diasporas, Arab, Persian, Gujarati, Tamil, Chinese and Swahili, settled in each other's ports. Institutions: the harbor master for each community, warehouses, brokers, partnership contracts, the Karimi wholesale financiers. State role: minimal and local; no power controlled the ocean before the Portuguese, and ports competed for traffic by being hospitable. Endpoint: everywhere, from Guangzhou to Sofala.

The trans-Saharan routes

Environment: the largest hot desert on earth, with no water for weeks at a stretch. Unit of transport: the dromedary, about one hundred and fifty to two hundred kilograms. Constraint: water, which fixes the routes to wells and the departures to seasons. Cargo: gold north, salt south, plus cloth, copper, books, and enslaved people north. Who ran it: North African and Berber merchants on the desert leg, Wangara and Dyula Muslim merchant networks in the savanna. Institutions: the guide, the caravan, Islamic commercial law and Arabic literacy, market towns at the desert's edge. State role: significant at the southern end, where Mali and later Songhai taxed and policed the traffic. Endpoint: Mediterranean ports at one end and the Niger river system at the other, where the desert traffic met river transport.

The five failures

1. The parallel description. A paragraph on the Silk Roads, then a paragraph on the Indian Ocean, with no shared category and no sentence connecting them. Fix: name one question and ask it of both in the same sentence.

2. The single highway. Writing "the Silk Road" as one road with one route and one owner. Fix: a shifting braid of routes worked in segments by merchants who each knew one stretch, whose volume rose and fell with the stability of the states along it.

3. The spice-route error. Describing Indian Ocean trade as a luxury spice trade. Fix: its largest manufactured cargo was Indian cotton cloth, and it also carried rice, timber and mangrove poles, because sea transport is cheap enough for bulk. This single correction improves more essays in this unit than any other.

4. The mechanism-free consequence. "All three networks spread religion and disease." True and worth nothing without the how. Fix: religion moved with resident merchants and the teachers who followed them, which is why ports converted and interiors did not; disease moved in ships' cargo with rodents and fleas, which is why it appeared in ports first.

5. The periodization slip. Comparing the overland routes under Mongol integration with the Indian Ocean after the plague, without dates. Fix: give the years, and if the difference you are describing is really change over time, say so and make that your point instead.

The paragraph, assembled

Four sentences. Name the category and state the claim. Give evidence from case one, specific and dated. Give evidence from case two at the same level of detail. Explain the difference by naming a prior condition that differed and how it produced the outcome.

Worked example, entirely from earlier chapters. Category and claim: the two networks depended on political protection to completely different degrees. Evidence one: overland trade reached its greatest volume under Mongol rule, when related Chinggisid regimes reduced tolls, honored each other's merchant privileges and made a journey's risk calculable, and it contracted after the Ilkhanate dissolved in 1335 and the Yuan fell in 1368. Evidence two: Indian Ocean trade had no dominant power before the Portuguese arrived at the end of the fifteenth century, and Calicut, Hormuz, Kilwa and Melaka were separately governed and competed for shipping by offering predictable duties, warehousing and harbor masters for each merchant community. Reasoning: the difference follows from where the danger lies, since a caravan is exposed to bandits and tolls at every mile between towns and therefore needs the ground policed, while a ship is exposed only in port and therefore needs a welcoming harbor and nothing at all in between.

That is one category, two mechanisms, one causal explanation, and it took no memorization that is not in the Topic 2.1 and 2.3 chapters. The skill is not writing; it is knowing what each network actually required.