Why, within Eurasia, were European societies, rather than those of the Fertile Crescent or China or India, the ones that colonized America and Australia, took the lead in technology, and became politically and economically dominant in the modern world?

A historian who had lived at any time between 8500 B.C. and A.D. 1450, and who had tried then to predict future historical trajectories, would surely have labelled Europe's eventual dominance as the least likely outcome, because Europe was the most backward of those three Old World regions for most of those 10,000 years. From 8500 B.C. until the rise of Greece and then Italy after 500 B.C., almost all major innovations in western Eurasia—animal domestication, plant domestication, writing, metallurgy, wheels, states, and so on—arose in or near the Fertile Crescent. Until the proliferation of water mills after about A.D. 900, Europe west or north of the Alps contributed nothing of significance to Old World technology or civilization; it was instead a recipient of developments from the eastern Mediterranean, Fertile Crescent, and China. Even from A.D. 1000 to 1450 the flow of science and technology was predominantly into Europe from the Islamic societies stretching from India to North Africa, rather than vice versa. During those same centuries China led the world in technology, having launched itself on food production nearly as early as the Fertile Crescent did.

Why, then, did the Fertile Crescent and China eventually lose their enormous leads of thousands of years to late-starting Europe? One can, of course, point to proximate factors behind Europe's rise: its development of a merchant class, capitalism, and patent protection for inventions, its failure to develop absolute despots and crushing taxation, and its Greco-Judeo-Christian tradition of critical empirical inquiry. Still, for all such proximate causes one must raise the question of ultimate cause: why did these proximate factors themselves arise in Europe, rather than in China or the Fertile Crescent?

For the Fertile Crescent, the answer is clear. Once it had lost the head start that it had enjoyed thanks to its locally available concentration of domesticable wild plants and animals, the Fertile Crescent possessed no further compelling geographic advantages. The disappearance of that head start can be traced in detail, as the westward shift in powerful empires. After the rise of Fertile Crescent states in the fourth millennium B.C., the centre of power initially remained in the Fertile Crescent, rotating between empires such as those of Babylon, the Hittites, Assyria, and Persia. With the Greek conquest of all advanced societies from Greece east to India under Alexander the Great in the late fourth century B.C., power finally made its first shift irrevocably westward. It shifted farther west with Rome's conquest of Greece in the second century B.C., and after the fall of the Roman Empire it eventually moved again, to western and northern Europe.

The major factor behind these shifts becomes obvious as soon as one compares the modern Fertile Crescent with ancient descriptions of it. Today, the expressions "Fertile Crescent" and "world leader in food production" are absurd. Large areas of the former Fertile Crescent are now desert, semi desert, steppe, or heavily eroded or salinized terrain unsuited for agriculture. Today's ephemeral wealth of some of the region's nations, based on the single non-renewable resource of oil, conceals the region's long-standing fundamental poverty and difficulty in feeding itself.

In ancient times, however, much of the Fertile Crescent and eastern Mediterranean region, including Greece, was covered with forest. The region's transformation from fertile woodland to eroded scrub or desert has been elucidated by paleobotanists and archaeologists. Its woodlands were cleared for agriculture, or cut to obtain construction timber, or burned as firewood or for manufacturing plaster. Because of low rainfall and hence low primary productivity (proportional to rainfall), regrowth of vegetation could not keep pace with its destruction, especially in the presence of overgrazing by abundant goats. With the tree and grass cover removed, erosion proceeded and valleys silted up, while irrigation agriculture in the low-rainfall environment led to salt accumulation. These processes, which began in the Neolithic era, continued into modern times. For instance, the last forests near the ancient Nabataean capital of Petra, in modern Jordan, were felled by the Ottoman Turks during construction of the Hejaz railroad just before World War I.

Thus, Fertile Crescent and eastern Mediterranean societies had the misfortune to arise in an ecologically fragile environment. They committed ecological suicide by destroying their own resource base. Power shifted westward as each eastern Mediterranean society in turn undermined itself, beginning with the oldest societies, those in the east (the Fertile Crescent). Northern and western Europe has been spared this fate, not because its inhabitants have been wiser but because they have had the good luck to live in a more robust environment with higher rainfall, in which vegetation regrows quickly. Much of northern and western Europe is still able to support productive intensive agriculture today, 7,000 years after the arrival of food production. In effect, Europe received its crops, livestock, technology, and writing systems from the Fertile Crescent, which then gradually eliminated itself as a major center of power and innovation.

That is how the Fertile Crescent lost its huge early lead over Europe. Why did China also lose its lead? Its falling behind is initially surprising, because China enjoyed undoubted advantages: a rise of food production nearly as early as in the Fertile Crescent; ecological diversity from North to South China and from the coast to the high mountains of the Tibetan plateau, giving rise to a diverse set of crops, animals, and technology; a large and productive expanse, nourishing the largest regional human pop¬ulation in the world; and an environment less dry or ecologically fragile than the Fertile Crescent's, allowing China still to support productive intensive agriculture after nearly 10,000 years, though its environmental problems are increasing today and are more serious than western Europe's.

These advantages and head start enabled medieval China to lead the world in technology. The long list of its major technological firsts includes cast iron, the compass, gunpowder, paper, printing, and many others mentioned earlier. It also led the world in political power, navigation, and control of the seas. In the early 15th century it sent treasure fleets, each consisting of hundreds of ships up to 400 feet long and with total crews of up to 28,000, across the Indian Ocean as far as the east coast of Africa, decades before Columbus's three puny ships crossed the narrow Atlantic Ocean to the Americas' east coast. Why didn't Chinese ships proceed around Africa's southern cape westward and colonize Europe, before Vasco da Gama's own three puny ships rounded the Cape of Good Hope eastward and launched Europe's colonization of East Asia? Why didn't Chinese ships cross the Pacific to colonize the Americas' west coast? Why, in brief, did China lose its technological lead to the formerly so backward Europe?

The end of China's treasure fleets gives us a clue. Seven of those fleets sailed from China between A.D. 1405 and 1433. They were then suspended as a result of a typical aberration of local politics that could happen anywhere in the world: a power struggle between two factions at the Chinese court (the eunuchs and their opponents). The former faction had been identified with sending and captaining the fleets. Hence when the latter faction gained the upper hand in a power struggle, it stopped sending fleets, eventually dismantled the shipyards, and forbade oceangoing shipping. The episode is reminiscent of the legislation that strangled development of public electric lighting in London in the 1880s, the isolationism of the United States between the First and Second World Wars, and any number of backward steps in any number of countries, all motivated by local political issues. But in China there was a difference, because the entire region was politically unified. One decision stopped fleets over the whole of China. That one temporary decision became irreversible, because no shipyards remained to turn out ships that would prove the folly of that temporary decision, and to serve as a focus for rebuilding other shipyards.

Now contrast those events in China with what happened when fleets of exploration began to sail from politically fragmented Europe. Christopher Columbus, an Italian by birth, switched his allegiance to the duke of Anjou in France, then to the king of Portugal. When the latter refused his request for ships in which to explore westward, Columbus turned to the duke of Medina-Sedonia, who also refused, then to the count of Medina-Celi, who did likewise, and finally to the king and queen of Spain, who denied Columbus's first request but eventually granted his renewed appeal. Had Europe been united under any one of the first three rulers, its colonization of the Americas might have been stillborn.

In fact, precisely because Europe was fragmented, Columbus succeeded on his fifth try in persuading one of Europe's hundreds of princes to sponsor him. Once Spain had thus launched the European colonization of America, other European states saw the wealth flowing into Spain, and six more joined in colonizing America. The story was the same with Europe's cannon, electric lighting, printing, small firearms, and innumerable other innovations: each was at first neglected or opposed in some parts of Europe for idiosyncratic reasons, but once adopted in one area, it eventu¬ally spread to the rest of Europe.

These consequences of Europe's disunity stand in sharp contrast to those of China's unity. From time to time the Chinese court decided to halt other activities besides overseas navigation: it abandoned development of an elaborate water-driven spinning machine, stepped back from the verge of an industrial revolution in the 14th century, demolished or virtually abolished mechanical clocks after leading the world in clock construction, and retreated from mechanical devices and technology in general after the late 15th century. Those potentially harmful effects of unity have flared up again in modern China, notably during the madness of the Cultural Revolution in the 1960s and 1970s, when a decision by one or a few leaders closed the whole country's school systems for five years.

China's frequent unity and Europe's perpetual disunity both have a long history. The most productive areas of modern China were politically joined for the first time in 221 B.C. and have remained so for most of the time since then. China has had only a single writing system from the beginnings of literacy, a single dominant language for a long time, and substantial cultural unity for two thousand years. In contrast, Europe has never come remotely close to political unification: it was still splintered into 1,000 independent statelets in the 14th century, into 500 statelets in A.D. 1500, got down to a minimum of 25 states in the 1980s, and is now up again to nearly 40 at the moment that I write this sentence. Europe still has 45 languages, each with its own modified alphabet, and even greater cultural diversity. The disagreements that continue today to frustrate even modest attempts at European unification through the European Economic Community (EEC) are symptomatic of Europe's ingrained commitment to disunity.

Hence the real problem in understanding China's loss of political and technological pre-eminence to Europe is to understand China's chronic unity and Europe's chronic disunity. The answer is again suggested by maps.

Europe has a highly indented coastline, with five large peninsulas that approach islands in their isolation, and all of which evolved independent languages, ethnic groups, and governments: Greece, Italy, Iberia, Denmark, and Norway / Sweden. China's coastline is much smoother, and only the nearby Korean Peninsula attained separate importance. Europe has two islands (Britain and Ireland) sufficiently big to assert their political independence and to maintain their own languages and ethnicities, and one of them (Britain) big and close enough to become a major independent European power. But even China's two largest islands, Taiwan and Hainan, have each less than half the area of Ireland; neither was a major independent power until Taiwan's emergence in recent decades; and Japan's geographic isolation kept it until recently much more isolated politically from the Asian mainland than Britain has been from mainland Europe. Europe is carved up into independent linguistic, ethnic, and political units by high mountains (the Alps, Pyrenees, Carpathians, and Norwegian border mountains), while China's mountains east of the Tibetan plateau are much less formidable barriers. China's heartland is bound together from east to west by two long navigable river systems in rich alluvial valleys (the Yangtze and Yellow Rivers), and it is joined from north to south by relatively easy connections between these two river systems (eventually linked by canals). As a result, China very early became dominated by two huge geographic core areas of high productivity, themselves only weakly separated from each other and eventually fused into a single core. Europe's two biggest rivers, the Rhine and Danube, are smaller and connect much less of Europe. Unlike China, Europe has many scattered small core areas, none big enough to dominate the others for long, and each the center of chronically independent states.

Once China was finally unified, in 221 B.C., no other independent state ever had a chance of arising and persisting for long in China. Although periods of disunity returned several times after 221 B.C., they always ended in reunification. But the unification of Europe has resisted the efforts of such determined conquerors as Charlemagne, Napoleon, and Hitler; even the Roman Empire at its peak never controlled more than half of Europe's area.

Thus, geographic connectedness and only modest internal barriers gave China an initial advantage. North China, South China, the coast, and the interior contributed different crops, livestock, technologies, and cultural features to the eventually unified China. For example, millet cultivation, bronze technology, and writing arose in North China, while rice cultivation and cast-iron technology emerged in South China. For much of this book I have emphasized the diffusion of technology that takes place in the absence of formidable barriers. But China's connectedness eventually became a disadvantage, because a decision by one despot could and repeatedly did halt innovation. In contrast, Europe's geographic balkanization resulted in dozens or hundreds of independent, competing statelets and centres of innovation. If one state did not pursue some particular innovation, another did, forcing neighbouring states to do likewise or else be con¬quered or left economically behind. Europe's barriers were sufficient to prevent political unification, but insufficient to halt the spread of technology and ideas. There has never been one despot who could turn off the tap for all of Europe, as of China.

These comparisons suggest that geographic connectedness has exerted both positive and negative effects on the evolution of technology. As a result, in the very long run, technology may have developed most rapidly in regions with moderate connectedness, neither too high nor too low. Technology's course over the last 1,000 years in China, Europe, and possibly the Indian subcontinent exemplifies those net effects of high, moderate, and low connectedness, respectively.

Naturally, additional factors contributed to history's diverse courses in different parts of Eurasia. For instance, the Fertile Crescent, China, and Europe differed in their exposure to the perennial threat of barbarian invasions by horse-mounted pastoral nomads of Central Asia. One of those nomad groups (the Mongols) eventually destroyed the ancient irrigation systems of Iran and Iraq, but none of the Asian nomads ever succeeded in establishing themselves in the forests of western Europe beyond the Hungarian plains. Environmental factors also include the Fertile Crescent's geographically intermediate location, controlling the trade routes linking China and India to Europe, and China's more remote location from Eurasia's other advanced civilizations, making China a gigantic virtual island within a continent. China's relative isolation is especially relevant to its adoption and then rejection of technologies, so reminiscent of the rejections on Tasmania and other islands (Chapters 13 and 15). But this brief discussion may at least indicate the relevance of environmental factors to smaller-scale and shorter-term patterns of history, as well as to history's broadest pattern.

The histories of the Fertile Crescent and China also hold a salutary lesson for the modern world: circumstances change, and past primacy is no guarantee of future primacy. One might even wonder whether the geographical reasoning employed throughout this book has at last become wholly irrelevant in the modern world, now that ideas diffuse everywhere instantly on the Internet and cargo is routinely airfreighted overnight between continents. It might seem that entirely new rules apply to competition between the world's peoples, and that as a result new powers are emerging—such as Taiwan, Korea, Malaysia, and especially Japan.

On reflection, though, we see that the supposedly new rules are just variations on the old ones. Yes, the transistor, invented at Bell Labs in the eastern United States in 1947, leapt 8,000 miles to launch an electronics industry in Japan—but it did not make the shorter leap to found new industries in Zaire or Paraguay. The nations rising to new power are still ones that were incorporated thousands of years ago into the old centres of dominance based on food production, or that have been repopulated by peoples from those centres. Unlike Zaire or Paraguay, Japan and the other new powers were able to exploit the transistor quickly because their populations already had a long history of literacy, metal machinery, and centralized government. The world's two earliest centres of food production, the Fertile Crescent and China, still dominate the modern world, either through their immediate successor states (modern China), or through states situated in neighbouring regions influenced early by those two centres (Japan, Korea, Malaysia, and Europe), or through states repopulated or ruled by their overseas emigrants (the United States, Australia, Brazil). Prospects for world dominance of sub-Saharan Africans, Aboriginal Australians, and Native Americans remain dim. The hand of history's course at 8000 B.C. lies heavily on us.

AMONG OTHER FACTORS relevant to answering Yali's question, cultural factors and influences of individual people loom large. To take the former first, human cultural traits vary greatly around the world. Some of that cultural variation is no doubt a product of environmental variation, and I have discussed many examples in this book. But an important question concerns the possible significance of local cultural factors unrelated to the environment. A minor cultural feature may arise for trivial, temporary local reasons, become fixed, and then predispose a society toward more important cultural choices, as is suggested by applications of chaos theory to other fields of science. Such cultural processes are among history's wild cards that would tend to make history unpredictable.

As one example, I mentioned in Chapter 13 the QWERTY keyboard for typewriters. It was adopted initially, out of many competing keyboard designs, for trivial specific reasons involving early typewriter construction in America in the 1860s, typewriter salesmanship, a decision in 1882 by a certain Ms. Longley who founded the Shorthand and Typewriter Institute in Cincinnati, and the success of Ms. Longley's star typing pupil Frank McGurrin, who thrashed Ms. Longley's non-QWERTY competitor Louis Taub in a widely publicized typing contest in 1888. The decision could have gone to another keyboard at any of numerous stages between the 1860s and the 1880s; nothing about the American environment favored the QWERTY keyboard over its rivals. Once the decision had been made, though, the QWERTY keyboard became so entrenched that it was also adopted for computer keyboard design a century later. Equally trivial specific reasons, now lost in the remote past, may have lain behind the Sumerian adoption of a counting system based on 12 instead of 10 (leading to our modern 60-minute hour, 24-hour day, 12-month year, and 360-degree circle), in contrast to the widespread Mesoamerican counting system based on 20 (leading to its calendar using two concurrent cycles of 260 named days and a 365-day year).

Those details of typewriter, clock, and calendar design have not affected the competitive success of the societies adopting them. But it is easy to imagine how they could have. For example, if the QWERTY keyboard of the United States had not been adopted elsewhere in the world as well— say, if Japan or Europe had adopted the much more efficient Dvorak key-board—that trivial decision in the 19th century might have had big consequences for the competitive position of 20th-century American technology.

Similarly, a study of Chinese children suggested that they learn to write more quickly when taught an alphabetic transcription of Chinese sounds (termed pinyin) than when taught traditional Chinese writing, with its thousands of signs. It has been suggested that the latter arose because of their convenience for distinguishing the large numbers of Chinese words possessing differing meanings but the same sounds (homophones). If so, the abundance of homophones in the Chinese language may have had a large impact on the role of literacy in Chinese society, yet it seems unlikely that there was anything in the Chinese environment selecting for a language rich in homophones. Did a linguistic or cultural factor account for the otherwise puzzling failure of complex Andean civilizations to develop writing? Was there anything about India's environment predisposing toward rigid socioeconomic castes, with grave consequences for the development of technology in India? Was there anything about the Chinese environment predisposing toward Confucian philosophy and cultural conservatism, which may also have profoundly affected history? Why was proselytizing religion (Christianity and Islam) a driving force for colonization and conquest among Europeans and West Asians but not among Chinese?

These examples illustrate the broad range of questions concerning cultural idiosyncrasies, unrelated to environment and initially of little significance, that might evolve into influential and long-lasting cultural features. Their significance constitutes an important unanswered question. It can best be approached by concentrating attention on historical patterns that remain puzzling after the effects of major environmental factors have been taken into account.

WHAT ABOUT THE effects of idiosyncratic individual people? A famil¬iar modern example is the narrow failure, on July 20, 1944, of the assassination attempt against Hitler and of a simultaneous uprising in Berlin. Both had been planned by Germans who were convinced that the war could not be won and who wanted to seek peace then, at a time when the eastern front between the German and Russian armies still lay mostly within Russia's borders. Hitler was wounded by a time bomb in a briefcase placed under a conference table; he might have been killed if the case had been placed slightly closer to the chair where he was sitting. It is likely that the modern map of Eastern Europe and the Cold War's course would have been significantly different if Hitler had indeed been killed and if World War II had ended then.

Less well known but even more fateful was a traffic accident in the summer of 1930, over two years before Hitler's seizure of power in Germany, when a car in which he was riding in the "death seat" (right front passenger seat) collided with a heavy trailer truck. The truck braked just in time to avoid running over Hitler's car and crushing him. Because of the degree to which Hitler's psychopathology determined Nazi policy and success, the form of an' eventual World War II would probably have been quite different if the truck driver had braked one second later.

One can think of other individuals whose idiosyncrasies apparently influenced history as did Hitler's: Alexander the Great, Augustus, Buddha, Christ, Lenin, Martin Luther, the Inca emperor Pachacuti, Mohammed, William the Conqueror, and the Zulu king Shaka, to name a few. To what extent did each really change events, as opposed to "just" happening to be the right person in the right place at the right time? At the one extreme is the view of the historian Thomas Carlyle: "Universal history, the history of what man [sic] has accomplished in this world, is at bottom the History of the Great Men who have worked here." At the opposite extreme is the view of the Prussian statesman Otto von Bismarck, who unlike Carlyle had long firsthand experience of politics' inner workings: "The statesman's task is to hear God's footsteps marching through history, and to try to catch on to His coattails as He marches past."

Like cultural idiosyncrasies, individual idiosyncrasies throw wild cards into the course of history. They may make history inexplicable in terms of environmental forces, or indeed of any generalizable causes. For the pur¬poses of this book, however, they are scarcely relevant, because even the most ardent proponent of the Great Man theory would find it difficult to interpret history's broadest pattern in terms of a few Great Men. Perhaps Alexander the Great did nudge the course of western Eurasia's already literate, food-producing, iron-equipped states, but he had nothing to do with the fact that western Eurasia already supported literate, food-producing, iron-equipped states at a time when Australia still supported only non-literate hunter-gatherer tribes lacking metal tools. Nevertheless, it remains an open question how wide and lasting the effects of idiosyncratic individuals on history really are.

THE DISCIPLINE OF history is generally not considered to be a science, but something closer to the humanities. At best, history is classified among the social sciences, of which it rates as the least scientific. While the field of government is often termed "political science" and the Nobel Prize in economics refers to "economic science," history departments rarely if ever label themselves "Department of Historical Science." Most historians do not think of themselves as scientists and receive little training in acknowledged sciences and their methodologies. The sense that history is nothing more than a mass of details is captured in numerous aphorisms: "History is just one damn fact after another," "History is more or less bunk," "There is no law of history any more than of a kaleidoscope," and so on.

One cannot deny that it is more difficult to extract general principles from studying history than from studying planetary orbits. However, the difficulties seem to me not fatal. Similar ones apply to other historical subjects whose place among the natural sciences is nevertheless secure, including astronomy, climatology, ecology, evolutionary biology, geology, and paleontology. People's image of science is unfortunately often based on physics and a few other fields with similar methodologies. Scientists in those fields tend to be ignorantly disdainful of fields to which those methodologies are inappropriate and which must therefore seek other methodologies—such as my own research areas of ecology and evolutionary biology. But recall that the word "science" means "knowledge" (from the Latin scire, "to know," and scientia, "knowledge"), to be obtained by whatever methods are most appropriate to the particular field. Hence I have much empathy with students of human history for the difficulties they face.

Historical sciences in the broad sense (including astronomy and the like) share many features that set them apart from nonhistorical sciences such as physics, chemistry, and molecular biology. I would single out four: methodology, causation, prediction, and complexity.

In physics the chief method for gaining knowledge is the laboratory experiment, by which one manipulates the parameter whose effect is in question, executes parallel control experiments with that parameter held constant, holds other parameters constant throughout, replicates both the experimental manipulation and the control experiment, and obtains quantitative data. This strategy, which also works well in chemistry and molecular biology, is so identified with science in the minds of many people that experimentation is often held to be the essence of the scientific method. But laboratory experimentation can obviously play little or no role in many of the historical sciences. One cannot interrupt galaxy formation, start and stop hurricanes and ice ages, experimentally exterminate grizzly bears in a few national parks, or rerun the course of dinosaur evolution. Instead, one must gain knowledge in these historical sciences by other means, such as observation, comparison, and so-called natural experiments (to which I shall return in a moment).

Historical sciences are concerned with chains of proximate and ultimate causes. In most of physics and chemistry the concepts of "ultimate cause," "purpose," and "function" are meaningless, yet they are essential to understanding living systems in general and human activities in particular. For instance, an evolutionary biologist studying Arctic hares whose fur color turns from brown in summer to white in winter is not satisfied with identifying the mundane proximate causes of fur color in terms of the fur pigments' molecular structures and biosynthetic pathways. The more important questions involve function (camouflage against predators?) and ultimate cause (natural selection starting with an ancestral hare population with seasonally unchanging fur colour?). Similarly, a European historian is not satisfied with describing the condition of Europe in both 1815 and 1918 as having just achieved peace after a costly pan-European. war. Understanding the contrasting chains of events leading up to the two peace treaties is essential to understanding why an even more costly pan-European war broke out again within a few decades of 1918 but not of 1815. But chemists do not assign a purpose or function to a collision of two gas molecules, nor do they seek an ultimate cause for the collision.

Still another difference between historical and nonhistorical sciences involves prediction. In chemistry and physics the acid test of one's understanding of a system is whether one can successfully predict its future behavior. Again, physicists tend to look down on evolutionary biology and history, because those fields appear to fail this test. In historical sciences, one can provide a posteriori explanations (e.g., why an asteroid impact on Earth 66 million years ago may have driven dinosaurs but not many other species to extinction), but a priori predictions are more difficult (we would be uncertain which species would be driven to extinction if we did not have the actual past event to guide us). However, historians and historical scientists do make and test predictions about what future discoveries of data will show us about past events.

The properties of historical systems that complicate attempts at prediction can be described in several alternative ways. One can point out that human societies and dinosaurs are extremely complex, being characterized by an enormous number of independent variables that feed back on each other. As a result, small changes at a lower level of organization can lead to emergent changes at a higher level. A typical example is the effect of that one truck driver's braking response, in Hitler's nearly fatal traffic accident of 1930, on the lives of a hundred million people who were killed or wounded in World War II. Although most biologists agree that biological systems are in the end wholly determined by their physical properties and obey the laws of quantum mechanics, the systems' complexity means, for practical purposes, that that deterministic causation does not translate into predictability. Knowledge of quantum mechanics does not help one understand why introduced placental predators have exterminated so many Australian marsupial species, or why the Allied Powers rather than the Central Powers won World War I.

Each glacier, nebula, hurricane, human society, and biological species, and even each individual and cell of a sexually reproducing species, is unique, because it is influenced by so many variables and made up of so many variable parts. In contrast, for any of the physicist's elementary particles and isotopes and of the chemist's molecules, all individuals of the entity are identical to each other. Hence physicists and chemists can formulate universal deterministic laws at the macroscopic level, but biologists and historians can formulate only statistical trends. With a very high probability of being correct, I can predict that, of the next 1,000 babies born at the University of California Medical Center, where I work, not fewer than 480 or more than 520 will be boys. But I had no means of knowing in advance that my own two children would be boys. Similarly, historians note that tribal societies may have been more likely to develop into chiefdoms if the local population was sufficiently large and dense and if there was potential for surplus food production than if that was not the case. But each such local population has its own unique features, with the result that chiefdoms did emerge in the highlands of Mexico, Guatemala, Peru, and Madagascar, but not in those of New Guinea or Guadalcanal.

Still another way of describing the complexity and unpredictability of historical systems, despite their ultimate determinacy, is to note that long chains of causation may separate final effects from ultimate causes lying outside the domain of that field of science. For example, the dinosaurs may have been exterminated by the impact of an asteroid whose orbit was completely determined by the laws of classical mechanics. But if there had been any paleontologists living 67 million years ago, they could not have predicted the dinosaurs' imminent demise, because asteroids belong to a field of science otherwise remote from dinosaur biology. Similarly, the Little Ice Age of A.D. 1300-1500 contributed to the extinction of the Greenland Norse, but no historian, and probably not even a modern climatologist, could have predicted the Little Ice Age.

THUS THE DIFFICULTIES historians face in establishing cause-and-effect relations in the history of human societies are broadly similar to the difficulties facing astronomers, climatologists, ecologists, evolutionary biologists, geologists, and paleontologists. To varying degrees, each of these fields is plagued by the impossibility of performing replicated, controlled experimental interventions, the complexity arising from enormous numbers of variables, the resulting uniqueness of each system, the consequent impossibility of formulating universal laws, and the difficulties of predicting emergent properties and future behaviour. Prediction in history, as in other historical sciences, is most feasible on large spatial scales and over long times, when the unique features of millions of small-scale brief events become averaged out. Just as I could predict the sex ratio of the next 1,000 newborns but not the sexes of my own two children, the historian can recognize factors that made inevitable the broad outcome of the collision between American and Eurasian societies after 13,000 years of separate developments, but not the outcome of the 1960 U.S. presidential election. The details of which candidate said what during a single televised debate in October 1960 could have given the electoral victory to Nixon instead of to Kennedy, but no details of who said what could have blocked the European conquest of Native Americans.

How can students of human history profit from the experience of scientists in other historical sciences? A methodology that has proved useful involves the comparative method and so-called natural experiments. While neither astronomers studying galaxy formation nor human historians can manipulate their systems in controlled laboratory experiments, they both can take advantage of natural experiments, by comparing systems differing in the presence or absence (or in the strong or weak effect) of some putative causative factor. For example, epidemiologists, forbidden to feed large amounts of salt to people experimentally, have still been able to identify effects of high salt intake by comparing groups of humans who already differ greatly in their salt intake; and cultural anthropologists, unable to provide human groups experimentally with varying resource abundances for many centuries, still study long-term effects of resource abundance on human societies by comparing recent Polynesian populations living on islands differing naturally in resource abundance. The student of human history can draw on many more natural experiments than just comparisons among the five inhabited continents. Comparisons can also utilize large islands that have developed complex societies in a considerable degree of isolation (such as Japan, Madagascar, Native American Hispaniola, New Guinea, Hawaii, and many others), as well as societies on hundreds of smaller islands and regional societies within each of the continents.

Natural experiments in any field, whether in ecology or human history, are inherently open to potential methodological criticisms. Those include confounding effects of natural variation in additional variables besides the one of interest, as well as problems in inferring chains of causation from observed correlations between variables. Such methodological problems have been discussed in great detail for some of the historical sciences. In particular, epidemiology, the science of drawing inferences about human diseases by comparing groups of people (often by retrospective historical studies), has for a long time successfully employed formalized procedures for dealing with problems similar to those facing historians of human societies. Ecologists have also devoted much attention to the problems of natural experiments, a methodology to which they must resort in many cases where direct experimental interventions to manipulate relevant ecological variables would be immoral, illegal, or impossible. Evolutionary biologists have recently been developing ever more sophisticated methods for drawing conclusions from comparisons of different plants and animals of known evolutionary histories.

In short, I acknowledge that it is much more difficult to understand human history than to understand problems in fields of science where history is unimportant and where fewer individual variables operate. Nevertheless, successful methodologies for analyzing historical problems have been worked out in several fields. As a result, the histories of dinosaurs, nebulas, and glaciers are generally acknowledged to belong to fields of science rather than to the humanities. But introspection gives us far more insight into the ways of other humans than into those of dinosaurs. I am thus optimistic that historical studies of human societies can be pursued as scientifically as studies of dinosaurs—and with profit to our own society today, by teaching us what shaped the modern world, and what might shape our future.