East-West Orientation of Historical Empires and Modern States

In a chapter entitled “Spacious Skies and Tilted Axes” Jared Diamond (1997) argues that crops and domestic animals are spread more easily along lines of latitude (along an East-West axis) rather than along lines of longitude (along a North-South axis). Diamond suggests that East-West spread is easier because similar climates and soil types tend to be arranged in east-west oriented bands. Th is geographic pattern is fundamental to natural vegetation types and wild animal distributions, and is best illustrated by a map of the global distribution of biomes (Figure 1). A biome is a major type of ecological community such as the grassland, desert, or temperate seasonal forest (Ricklefs 2001). Although Diamond focused primarily on the spread of crop cultivars and domesticated animals, the same principle should infl uence the military/political, demographic, and cultural dynamics of societies. An obvious example which seems to fi t this pattern is the Mongol empire under Chinggis Khan and his

immediate successors; the largest historical empire in terms of contiguous territory.Th e core of the Mongolian Empire was the Eurasian Steppe that stretches for many thousands of kilometers from the Khingan Mountains in the east to the Carpathians in the west (McNeill 1964).Th e Mongols were steppe warriors, and they were able to extend rapidly their infl uence over this whole region (Barfi eld 1994).Th e regions inhabited by settled agriculturalists adjacent to the steppe were incorporated more slowly and to a lesser degree than the steppe.For example, the Russian principalities of the forest zone were not occupied by the steppe-dwellers, and were instead subjected to tribute.As a result, the Mongol Empire, based on the steppe, was much wider in the latitudinal rather than longitudinal direction.
Th e ease of conquest was not the only factor promoting the latitudinal spread of large empires.Societies inhabiting similar ecological zones tend to be more similar to each other than societies located in very diff erent zones.Techniques developed for integrating and controlling a certain type of society should, therefore, be easier to extend latitudinally.Th ere is also a scale aspect to this "ecological factor."It will be detectable primarily at large geographic scales.Small states or empires, as long as they stay within the same biome, should fi nd it equally easy (or equally hard) to expand in any direction because the climatic diff erences in any direction will be minor.A major exception here might be those states which encompass highly varied terrain.
Finally, we note that the latitudinal eff ect should be much stronger for landbased, contiguous empires than for sea-borne empires.Th us, we would expect stronger latitudinal eff ect for historical empires than for modern empires.Also as we note below we omit modern colonial empires from the analysis.However we revisit this issue in the conclusion.

analysis of the shapes of historical empires
Territorial expansion by states is, of course, a complex process, infl uenced by many factors other than the environment.Th e question of interest here is whether this ecological factor has a detectable eff ect on the projection of military/political power, or if its infl uence is lost in the "noise" of complex interactions.To answer this question we compiled a list of all large historical empires with peak territories exceeding 1 Mm² (= 1,000,000 km²), and measured the distances from their eastern to western extremes, as well as from the northern to southern extremes.¹Many of the historical empires in our analysis rose and fell starting from the same territory, for instance the diff erent Chinese dynasties.However, this does not invalidate the analysis because each dynastic empire had the opportunity to expand either north-south or east-west (omitting repeat empires from the analysis produced substantially the same result).¹.Our list of large historical states was based on the compilation by Taagepera (a, b, , ), which has been systematized and posted on the web by Chase-Dunn and coworkers http://irows.ucr.edu/.We checked the Taagepera list with all major historical atlases in the library of the University of Connecticut and found eight additional empires that fi t our criteria (Axum, Hsi-Hsia, Kara-Khitai, Srivijaya, Maurian, Kushan, Gupta, and Maratha).For historical empires, we used states that peaked before .We excluded the maritime empires of the European Great Powers, because these empires were not contiguous (widely distributed collections of territories).One diffi culty in constructing the list was presented by the repeated rise of empires in the same location, such as in China.We adopted the middle road of counting each major dynasty (Han, Tang, Ming, etc.) as a separate empire, but did not distinguish between cycles within any one dynasty (e.g., Early versus Late Han).Analysis of a reduced dataset, which included only the largest empire for each geographic location, yielded qualitatively the same result.Th is lends support to our argument that successive dynasties had signifi cantly independent opportunities to expand in any direction.See Table  for the list of empires.².Log-transformation of the ratio of distances was necessary to make the distribution of the index normal, because the ratio cannot be less than zero.Logically, the metric chosen to quantify the East-West versus North-South spread should give the same magnitude to ratios of : and : (but with the opposite sign), and logtransformation accomplishes this.Positive values of the log-transformed ratio, thus, indicate east-west orientation, and negative values north-south orientation.Th e frequency distribution of the latitudinal index in our sample of 62 historic empires is strongly skewed to the right (Figure 2), and the mean index is signifi cantly greater than zero (t = 4.83, P < 0.001).Th e great majority of empires, nearly 80, have a positive latitudinal index-that is, they are wider in the east-west compared to the north-south direction.Th ere are only three empires that have a strong north-south orientation, and these are the proverbial exceptions that prove the rule.Th e New Kingdom of Egypt had at its core the valley of a major river running south-north, the Nile.Th e Inca empire is located on the west coast of South America where ecological zones are longitudinal (see Figure 1) along the Andean mountain chain.We do note, however, that Andean empires, especially the Inca, did transcend ecological zones from the altiplano to the coast, but these are over very short distances.Indeed, many empires, if examined more locally, would also exhibit short range biome diversity.Finally, the Khmer empire was located entirely within the wet tropical forest biome.Th us, even though these three cases do not conform to the rule of latitudinal spread, they obey a more general rule of expansion within an ecological zone.
All of the largest empires (with territory over 10 Mm²) were oriented in the east-west direction.We have already discussed the case of the Mongol empire.Th e Islamic Caliphate is a variation on the same pattern, except that the "native biome" of the Arabs was the subtropical desert, rather than the temperate grassland/desert of the Mongols.Th e next largest state in history after the Mongols, the Russian empire (peak area of 22.8 Mm² in 1895), originated in the transitional zone between the steppe and the forest (ecologists call such transitional zones ecotones).When the Muscovite state began to expand in the sixteenth century, it spread fastest precisely within the same ecotone-eastward along the boundary between the Eurasian steppe and northern taiga.Eastward expansion was extremely rapid, so that the Pacifi c was reached by the mid-seventeenth century.In contrast, the southern advance into the steppes and deserts of Central Asia took a much longer time, and they were conquered only by the late nineteenth century.In addition to ecological considerations the presence of strong pastoral confederacies abetted this slowing (Khodarkovsky 2002).But this is also indirectly ecological since these confederacies, many remnants of the Mongol Empire, depended on a steppe environment to make their living.
Another example of the same dynamic is the early expansion of Rome.Th e territory of the Roman Empire in the fi rst century b.c.e.coincides almost precisely with the woodland/shrubland biome (also known as the Mediterranean zone).Subsequent expansion took the Romans into the forests of northern Europe.However, severe reverses, such as the battle of Teutoburg Forest in 9 c.e., in which 20,000 legionnaires were obliterated by the tribal Germans (Wells 2003), persuaded the Romans to abandon plans of further conquest.Th e general rule, thus, seems to be that expansion is easiest and most lasting when occurring within the same ecological zone.
Expansion into other biomes is possible, but more diffi cult, slow, and requires greater state resources.China is probably the best illustration of this principle.Th e native biome for China is the temperate seasonal forest,³ and this was precisely the area that was fi rst unifi ed by each of a long succession of Chinese empires.Th e strength of the Chinese state, however, allowed it to expand into alien biomes.At their peaks the Chinese empires intruded into the steppe (Inner Mongolia, Chinese Turkestan), the alpine biome (Tibet), and the tropical rain forest (Vietnam).³.It may seem strange to call the Chinese home biome a "forest," because in present-day China, of course, very few forests are left.Remember, however, that the biome names refl ect the types of ecological communities that would be present before substantial human impact.Th e names are simply a short-hand reference to particular combinations of the climate and soil types.Th e same principle applies to the "subtropical desert."Some examples of this biome (e.g.Sahara) extend well beyond the subtropics in the strictly geographic sense of the word.

analysis of the shapes of modern political states
Is the infl uence of ecology detectable in the shapes of modern states?At fi rst glance, no.Th e average latitude index for the 29 modern states whose territory exceeds 1 Mm² is positive, but not signifi cantly diff erent from 0. However, if we exclude South American countries, where biomes extend in the longitudinal direction, the statistical test indicates that the pattern is detectable even today (t = 2.66, P = 0.014).Th e tendency to east-west orientation in modern countries, nevertheless, is much weaker than for historical empires.Th ere are multiple reasons for this.First, most modern colonial states were sea-borne and not land based.Second, modern transportation technology made and continues to make long-distance travel much cheaper (Ciccantell and Bunker 1998;Bunker andCiccantell 2005a, 2005b).Th ird there has been a propensity among colonial states to claim territory that is either not inhabited by the home populations, or little used, or being held in reserve for future use.A striking example of this tendency is Canada, whose population is squeezed into a narrow band running east-west along its southern border with the us, but which nevertheless claims extensive territories in the Arctic.Because of the addition of these lands, which are very sparsely populated, the latitudinal index of Canada is slightly negative.Algeria and Lybia provide other examples of the same tendency-their populations are largely confi ned to the east-west band along the Mediterranean littoral, but their latitudinal indices are essentially zero, because they claim huge territories to the south, in the Saharan desert.
Fourth, and probably the most important in world-system terms, with advent of industrial technology and the rise of modern capitalism, states intentionally sought new resources (Bunker andCiccantell 2005a, 2005b).Where those resources were ecologically based, increased biological and ecological diversity became a disiderata if not an explicit goal.Th is marked a signifi cant change in world-system logic as argued by Chase-Dunn and Hall (1997).

conclusions
Our results indicate that the physical and biological environment has a detectable eff ect on the shapes of historic states and to a lesser extent on modern states.It appears that projection of military/political power is easier within the same ecological zone (biome).Th is, however, does not support "ecological determinism."Although ecology is important, its infl uence on state expansion patterns is transmitted by social mechanisms which can either abate, or sometimes overturn these ecological eff ects.Despite the complexities of the human world, certain techniques and ideas from ecological sciences have proven to be fruitful in suggesting novel approaches to the study of social systems (Turchin and Hall 2003;Hall and Turchin 2007).Diamond's original insight, which motivated our study, is one example.Another is the recent demonstration that cultural variability exhibits a latitudinal gradient (Pagel and Mace 2004).Our results also have interesting implications for the study of historical dynamics (Turchin 2003).
Researchers working within the world-system paradigm have noted that the rise and fall of populations, cities, and empires is characterized by a broad-scale synchronicity (Chase-Dunn andHall 1997, Chase-Dunn et al. 2000;Chase-Dunn, Hall, and Turchin 2007).For example, there is a substantial correlation between the dynamics of Western Europe and China.On the other hand, South Asian dynamics are completely uncorrelated with the rest of Eurasia.Our fi nding that the propagation of "signals" within military-political networks is facilitated in the latitudinal, but not longitudinal, directions suggests another possible explanation for this pattern.
Finally, these results support the arguments originally advanced by Wallerstein (1974Wallerstein ( , 2004) ) and elaborated by Chase-Dunn andHall (1997). Contra Frank andGills (1993), there was signifi cant break in world-system logic with rise of modern capitalism.It is here evidenced in an attenuation of the latitudinal, or ecological, eff ects on shapes of states and empires with the rise of industrial capitalism.Th e logic of capitalist accumulation of capital⁴ emphasizes ecological diversity over contiguity, shored up by radical decreases in transportation costs and regimes.It remains fascinating, yet problematic, that even in modern states there is still some residual ecological eff ect.Th is is a topic that warrants further, and more nuanced, research than the broad-brush analysis presented here.⁴. Th e phrase "capitalist accumulation of capital," which comes from Chase-Dunn and Hall (), serves to distinguish capitalist modes of accumulation from tributary or kin modes of accumulation.Our point is that the logic of capitalist accumulation emphasizes ecological diversity much more than any other mode of accumulation, thus overcoming much of the latitude eff ect.

Figure 1 -
Figure 1 -Distribution of World Biomes (Ricklefs 2001) Figure 2 -Frequency Distribution of the Latitude Index in the Sample of Large Empires Our measure of the tendency to expand in the latitudinal direction is the log-transformed ratio of the east-west distance to north-south distance.²