Guns, Germs & Steel

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 note 11 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 polities' 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 purposes 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 'politicaTsciehce''and the Nobel Prize in economics refers to 'economic science,' history departments rarely if ever

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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 f 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

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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 color?). 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

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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