In the beginning of Chapter 10: Spacious Skies and Tilted Axes Diamond points out that “the Americas span a much greater distance north-south (9,000 miles) than east-west: only 3,000 miles at the widest…That is, the major axis of the Americas is north-south. The same is also true…for Africa. In contrast, the major axis of Eurasia is east-west.” (176) Diamond also uses a world map in order to help us visualize this. He then poses the chief question of the chapter, “What effect, if any, did those differences in the orientation of the continents’ axes have on human history?” (176) He goes on to answer this foremost question through numerous strategies, including the use of scientific data. For example, Diamond contends that food production spreads much faster along east-west axes as opposed to north-south axes by offering scientific data. He asserts that the rates of spread for crops latitudinally from Southwest Asia to Europe, Egypt, and the Indus Valley (0.7 miles/year) and from the Philippines east to Polynesia (3.2 miles/year) was much faster than the rates of north-south spread. For example, food spread from Mexico to the U.S. Southwest (less than 0.5 miles/year), Mexico to the eastern U.S. (less than 0.3 miles/year), and Peru to Ecuador (0.2 miles per year) was considerably slower than the latitudinal spread in Europasia. Therefore, through his use of scientific data, he establishes a clear correlation between the rapid spread of food latitudinally as opposed to the slower speed longitudinally.
Having already proved that there is a correlation between the speed of food spread and the orientation of continents, Diamond goes on to reason why this is true. He initially asks “Why was the spread of crops from the Fertile Crescent so rapid?” (183) To answer this question, Diamond uses a simple train of logic with a few examples concerning the climates, day lengths, and environments of locations lying on the same latitudinal plane. He starts with the widely known fact that locations that lie along the same latitudinal lines share the exact same day length as well as seasonal variations. However, he also asserts that “To a lesser degree, they also tend to share similar diseases, regimes of temperature and rainfall, and habitats or biomes (types of vegetation).” (183) He then presents the examples of Portugal, northern Iran, and Japan which are all located at approximately the same latitude but lie “successively 4,000 miles east or west of each other, [and they] are more similar to each other in climate than each is to a location lying even a mere 1,000 miles due south.” (183) Therefore, due to this similarity in climate, the germination, growth, and disease resistance of plants are developed accordingly. Each plant population becomes genetically programmed, through natural selection, to respond to the climate under which it has evolved. Thus, as Diamond simply puts it, “That’s part of the reason why Fertile Crescent domesticates spread west and east so rapidly: they were already well adapted to the climates of the regions to which they were spreading.” (185) In other words, since the crop or animal was already adapted to a certain climate, it would not have trouble surviving elsewhere as long as the climate remained constant. On the other hand, on numerous occasions crops were unable to spread from north to south. For example, many of the Fertile Crescent crops were able to spread south to Egypt and Ethiopia where the climates are similar. However, once the crops reached Ethiopia, it was unable to spread further although South Africa’s climate would be ideal for them. This is because “the 2,000 miles of tropical conditions between Ethiopia and South Africa posed an insuperable barrier.” (186) Also, the trypanosome disease carried by tsetse flies to which the animals had no immunity halted the spread of animals through Africa. Nonetheless, this dichotomy in environment along latitudinal versus longitudinal lines was able to either positively or adversely affect food spread.
Although Diamond seems to have proven that food spread was directly proportional to latitude, towards the end of the chapter he offers some concessions to this theory. He states that, “latitude is of course not the only such determinant, and it is not always true that adjacent places at the same latitude have the same climate. Topographic and ecological barriers…were locally important obstacles to diffusion.” (189) Hence, Diamond concedes that climate does not always remain constant along latitudinal lines. For example, crop diffusion was difficult from the U.S. Southeast to the Southwest because of the dry, infertile land between the two areas. Another example Diamond offers is the difficulty in the spread of crops east of the Indus Valley because of the presence of the Himalayas. Therefore, he clearly shows that latitude is not the only determinant in food spread, but rather the combination of climate, terrain, and disease.
In Guns, Germs, and Steel, Jared Diamond aptly reveals his theory that the “ultimate cause” for civilization in certain places, or lack thereof, is not because of superiority or inferiority of one race over another, but rather because of the environment and climate of the people’s land. In Chapter 10, he specifically discusses why the food spread in certain regions of the world occurred faster, such as Eurasia, than in other places, such as the Americas and Africa. He proves that a major reason (not the only) for the rapid spread of crops was due to a constant climate. Hence, since seasonal variation and day length remains virtually the same across latitudinal lines, it is more viable for crops to spread from east to west, or vice versa. Therefore, the intention in Spacious Skies and Tilted Axes is to expose why the spread of food and, in turn, the rise of civilization occurred at the rate that it did. His logical reasoning, scientific data, and concessions expose and explain the relation between food diffusion and continent orientation because this rational reasoning uncovers the reason for the rate of spread of food.
Spacious Skies and Tilted Axes serves as an integral piece to Diamond’s overall thesis in Guns, Germs, and Steel: history followed different courses in different places because of the differences in their environments, not because of genetic differences amongst the people. As he logically describes in Chapter 4, the east-west axis is an “ultimate cause” which led to the “proximate cause” of guns, technology, political organization, and disease. He illustrates via a simple chart that continent orientation led to the ease of the spread and domestication of species of plants and animals. The domestication of animals led to epidemic diseases because the diseases evolved from germs of the domestic animals themselves. Furthermore, this domestication also led to food surpluses which are integral to large, stable, and developed societies. Only with these sedentary, stratified societies can technology be produced and political organizations created. Technology (guns included), political organization, and disease are the “proximate causes” for the outcome of our world. Therefore, this “ultimate cause” refutes the notion that biological differences are the reason for European dominance. Diamond disproves the view of Richard J. Herrnstein and Charles Murray, in their book The Bell Curve, that there are substantial individual and group differences in intelligence; these differences profoundly influence the social structure and organization of modern industrial societies. Hence, Diamond’s proof that history is based upon differences among people’s environments clarifies his intention that race is not the determining factor of the differences in society, but rather ones geographic setting.
