A In June 1325 a twenty-one-year-old from Tangier named Ibn Battuta left home to make the pilgrimage to Mecca. His intention, like that of countless devout North Africans before him, was to fulfil a religious obligation and then come back. Instead, almost a quarter of a century passed before he saw Morocco again, and by the time he finally settled in Fez in the 1350s he had passed through territory that now belongs to some forty modern states, from the steppes north of the Black Sea to the Maldives and the coast of China. A figure of roughly 117,000 kilometres is frequently quoted for the total distance he covered. It can only be an approximation, yet even a generous margin of error would leave him among the most widely travelled individuals of the pre-modern era.

B What makes journeys on this scale intelligible is not so much personal daring as the world through which he moved. In the fourteenth century a broad belt of land stretching from the Atlantic to the Indian Ocean was governed by Muslim rulers, and a common body of religious law, together with Arabic as the language of scholarship, meant that an educated man carried credentials that were recognised far beyond his birthplace. Ibn Battuta had been trained as a jurist in the Maliki tradition, and this training functioned rather like a passport and a letter of credit combined. Governors and sultans received him as a man of learning, and gifts of robes, horses and money frequently followed. He was not, then, a solitary wanderer surviving on his wits. For most of his journeys he travelled in company, often with an entourage that grew to include servants, companions and, at various points, several wives.

C Practical arrangements for food and shelter reinforced these networks. Across much of the region, Sufi lodges known as zawiyas offered hospitality to travellers, and in the towns of Anatolia he was repeatedly entertained by the akhis, associations of young craftsmen who competed with one another for the privilege of hosting a visitor. Long sea crossings depended on a different kind of system altogether. The monsoon winds, which reverse direction with the seasons, dictated when ships could sail between Arabia, India and South-East Asia, and a traveller who missed the favourable wind might be obliged to wait in port for months.

D His scholarship also brought employment. In Delhi, Sultan Muhammad bin Tughluq, a ruler notorious for alternating between lavish generosity and sudden cruelty, appointed him a qadi, or judge, on a substantial salary, a position he held for several years. The post was not without danger: when a holy man with whom Ibn Battuta had associated fell from favour, he himself came under suspicion, and he withdrew for a time into religious seclusion. In the early 1340s the sultan unexpectedly selected him to lead an embassy to the Mongol emperor of China. The mission ended almost before it had begun, on the Malabar coast of south-west India, when a storm wrecked the vessels carrying the gifts while Ibn Battuta was still ashore. Reluctant to return empty-handed to so unpredictable a master, he spent much of the next period in the Maldives, where he again served as a judge and discovered that his efforts to impose strict legal penalties made him unpopular with the islands’ leading officials.

E From there he made his way by stages through Bengal, where, by his own account, he visited the celebrated Sufi Shah Jalal in the hills of Sylhet, and eventually reached the southern Chinese port of Quanzhou, known to Arab merchants as Zaytun. He was struck by the security of the roads, observing that a merchant could journey alone through the country carrying great wealth and have nothing to fear. He was less comfortable with the discovery that officials had portraits made of foreign arrivals so that anyone who committed an offence could be traced. For all its order, China did not appeal to him, and he spent most of his time among the communities of Muslim traders in its ports.

F He reached Morocco in 1349, only to learn that his mother had died of the plague a few months before. After a brief visit to Granada, he set off in 1351 across the Sahara towards the empire of Mali. At Taghaza, a desolate mining settlement, he noted that the houses and even the mosque were built from blocks of salt, roofed with camel skins. In Mali itself he complained bitterly about the meagre welcome present he received from the ruler, Mansa Sulayman, though he praised the country’s freedom from crime and the people’s devotion to prayer.

G On his return, the Sultan of Morocco, Abu Inan, commissioned a young scholar, Ibn Juzayy, to record the traveller’s recollections, and the resulting book, usually known as the Rihla, was completed around 1355. Its reliability has long been debated. Modern scholars have shown that Ibn Juzayy inserted descriptions taken, without acknowledgement, from the account of an earlier traveller, Ibn Jubayr, and some historians doubt whether Ibn Battuta made every journey he describes, a voyage up the Volga being a frequently cited example. The book remained largely unknown in Europe until the nineteenth century, when manuscripts reached European scholars and a French translation appeared in the 1850s. Despite its flaws, it is now regarded as indispensable for regions such as Mali, where few written sources from the period survive.

A When the ice sheets retreated from Britain at the end of the last glaciation, oak trees spread northwards from their refuges in southern Europe with remarkable speed. In 1899 the botanist Clement Reid pointed out a difficulty that this created. Acorns are heavy, and most of them fall directly beneath the parent tree. Reid calculated that, if acorns travelled no further than gravity and an occasional roll downhill could take them, the oak could not conceivably have reached the north of Scotland in the time that had elapsed since the ice melted. Something, he concluded, must occasionally carry seeds far beyond the reach of the parent’s canopy. A century later the ecologist James Clark and his colleagues gave the problem a name, ‘Reid’s paradox’.

B Part of the answer lies in the mathematics of what ecologists call the dispersal kernel, a curve describing the probability that a seed will land at any given distance from its source. For most plants the curve falls steeply: the great majority of seeds come to rest within a few tens of metres. What matters for the rate at which a population expands, however, is the shape of the curve’s far end, its ‘tail’. Clark’s models showed that if even a minute fraction of seeds travels exceptionally far, the speed at which a species advances can increase many times over. The irony is that the events of greatest consequence are precisely those that are hardest to observe: a field researcher can count thousands of seeds under a tree without ever recording the handful that decided the species’ future.

C Plants have evolved an extraordinary variety of means of moving their offspring, and some of these have appeared independently many times. Thousands of plant species, including violets and many shrubs of the dry heathlands of Australia and South Africa, attach to their seeds a small fatty structure called an elaiosome. Ants carry the seed back to their nest, feed the appendage to their larvae and then discard the seed intact in sites that tend to be rich in nutrients and relatively safe from seed-eating rodents. A 2010 review led by Szabolcs Lengyel estimated that this arrangement, known as myrmecochory, occurs in at least 11,000 species and has arisen on more than a hundred separate occasions in the history of flowering plants. Ants rarely move seeds more than a few metres; it is the destination, rather than the distance, that appears to benefit the plant.

D Wind dispersal, too, still holds surprises. In 2018 a team at the University of Edinburgh used a vertical wind tunnel to examine the dandelion’s parachute-like tuft, the pappus, a crown of around a hundred fine filaments. They found that air passing through and around the filaments generates a stable ring of circulating air, a ‘separated vortex ring’, which hovers just above the seed without being attached to it and sharply increases drag. Because the pappus is mostly open space, the researchers concluded, the seed can remain aloft while using far less material than a solid parachute of similar performance would require.

E Following seeds that have been eaten by animals has historically been even harder. Since the early 2000s, however, molecular techniques have transformed the field. The hard inner wall of many seeds is formed from the tissue of the mother plant, so its DNA can be matched to one specific parent tree. Applying this method to mahaleb cherry trees in southern Spain, a team led by Pedro Jordano found that small songbirds dropped most seeds close to their source, whereas larger birds and mammals such as foxes and stone martens were responsible for a disproportionate share of long-distance movements.

F Some plants appear to be waiting for helpers that no longer exist. In 1982 the ecologist Daniel Janzen and the palaeontologist Paul Martin argued that many large, hard-shelled fruits in Central America had evolved to be eaten by giant mammals such as gomphotheres, which disappeared roughly 10,000 years ago. Without them, such fruits often rot beneath the parent tree, and the species may persist partly because horses and cattle, introduced by Europeans, have stepped into the vacant role. These ‘evolutionary anachronisms’ are difficult to prove, however, and the history of one celebrated example counsels caution. In 1977 Stanley Temple claimed that a Mauritian tree, the tambalacoque, had almost ceased to reproduce because its seeds needed to pass through the gut of the dodo before they could germinate. Other botanists, however, later pointed out that younger trees existed and that the species’ poor regeneration could be explained in other ways.

G Today the concern is less about animals lost in prehistory than about those disappearing now. As the climate warms, many plant species will need to shift their ranges to remain within suitable conditions; yet hunting and habitat fragmentation are removing precisely the large animals capable of moving seeds over such distances. The consequences can be measured in the seeds themselves. In Brazil’s Atlantic Forest, Mauro Galetti and his colleagues reported in 2013 that where toucans and other large-billed birds had vanished, populations of the juçara palm were producing noticeably smaller seeds, apparently because only small seeds could still be swallowed by the birds that remained. The change seems to have occurred within roughly a century, an evolutionary eye-blink, and smaller seeds are thought to be more vulnerable to drying out. For organisms that cannot walk, the loss of a carrier can amount to the loss of a future.

A To an economist, the tip is something of an embarrassment. Standard models of behaviour assume that people pay for goods and services because they are obliged to, and that they will avoid any expense they can escape. Yet every day millions of diners hand over money they are under no legal obligation to pay to staff they may never see again. A traveller passing through an unfamiliar town, who will never return to the restaurant and has nothing to gain from being remembered kindly, typically tips anyway. Whatever sustains the practice, it is plainly not narrow self-interest. Nor, the evidence suggests, does tipping achieve what its defenders claim for it.

B The custom is often assumed to be ancient and universal. In fact, its modern form owes much to the aristocratic households of Europe, from which it spread to the United States in the decades after the Civil War, carried partly by wealthy Americans eager to display their familiarity with European manners. Its arrival provoked fierce resistance. In The Itching Palm (1916), the writer William Rufus Scott denounced tipping as a relic of servitude that had no place in a democracy. Between 1909 and 1915, moreover, several states, beginning with Washington, passed laws prohibiting the practice. Such legislation proved almost impossible to enforce, and by 1926 every one of these laws had been repealed. The Pullman Company, which operated sleeping cars on American railways, paid its porters, almost all of them Black men, wages so low that they depended heavily on whatever passengers chose to give them.

C Defenders of tipping argue that the customer, who experiences the service at first hand, is better placed than any manager to judge it, and that the tip motivates staff to perform well. Unfortunately for this argument, decades of research suggest that the link between the quality of service and the size of a tip is remarkably weak. Michael Lynn of Cornell University found in a statistical review of earlier studies, conducted with Michael McCall, that diners’ ratings of service accounted for only a tiny fraction of the variation in what they left. Far more influential were factors that have little to do with competence: the size of the bill, whether the server gave their name or crouched beside the table, and even whether the sun was shining. More troubling are studies reporting that Black servers in the United States tend to receive smaller tips than their white colleagues. Such findings should give pause to anyone who regards a tip as a purely private exchange between a grateful customer and a deserving worker.

D United States federal law permits employers to pay tipped staff a cash wage of just $2.13 an hour, a figure unchanged since 1991, provided that tips bring their earnings up to the ordinary minimum wage. In effect, part of the cost of employing staff is transferred from the business to the customer, while menu prices appear lower than they really are. For workers, the arrangement creates incomes that fluctuate from shift to shift and season to season, making household budgeting difficult. It also widens inequality within restaurants, since cooks and dishwashers, traditionally excluded from sharing in tips, may take home considerably less than the waiting staff.

E In 2015 the New York restaurateur Danny Meyer announced that his company’s restaurants would no longer accept tips, raising menu prices instead and sharing the extra revenue more evenly among all employees. Some experienced servers, accustomed to high earnings on busy nights, left for competitors, and in 2020, as the pandemic upended the restaurant trade, the company reinstated tipping. The chain Joe’s Crab Shack abandoned a similar experiment within a year, reporting that many customers had chosen to eat elsewhere. The lesson is less that service-inclusive pricing cannot work than that it is extremely hard for one business to adopt alone. A restaurant that builds the full cost of labour into its prices appears more expensive than the competitor next door whose menu leaves it out, even if the final bill is identical; diners compare listed prices, not totals.

F In Japan, tipping is not customary, and an attempt to leave money on the table may even cause confusion; yet nobody who has eaten in Tokyo would conclude that the absence of tips has made waiters careless. Why, then, does the American custom persist, and even expand, as payment screens at coffee counters and takeaway shops now invite customers to add 15, 20 or 25 per cent? The economist Ofer Azar has argued that tipping is sustained above all by social norms: people tip less to reward good service than to avoid the guilt and embarrassment of being seen to break an expected rule. Once a norm of this kind takes hold, no individual has much reason to defy it.

G None of this means that the answer is to outlaw tipping; the experience of the early twentieth century suggests that prohibition merely drives the practice out of sight. A more promising route is to remove the conditions that make tips necessary. Seven states, including California and Washington, already require employers to pay the full minimum wage before tips, and their restaurant trades have not collapsed. If every business were bound by the same rule, the competitive penalty that undid the pioneers of service-inclusive pricing would largely disappear. Customers who wished to express gratitude for exceptional service would remain free to do so. What they should not be asked to do is pay someone’s wages and call it generosity.