What are some ultimate causes (i.e. geographic and environmental factors) presented by Diamond in Guns, Germs, and Steel that explain why white Eurasians have been able to conquer people on other continents and spread their culture and technology?
The best way to find the ultimate causes that Diamond says are important in Guns, Germs, and Steel is to look at Figure 4.1 on p. 87. There, Diamond gives ultimate causes, proximate causes, and causes that fall somewhere in between.
Diamond says, of course, that technology and germs were the main proximate causes that allowed the Europeans to conquer people from other continents. This is why the book is entitled Guns, Germs, and Steel. He then goes on to say that the people who first started to farm would be the ones who got technology and germs first as well. If farming is the source of the proximate causes, we then need to ask why it was that people in Eurasia got farming before people in other regions. This is where the ultimate factors come into play.
Diamond cites two main ultimate factors. First, there is the number of wild species of plants and animals in a region that are suitable for domestication. Diamond says that regions with many such species would be more likely to start farming early. Second, there is the ease of species spreading. Diamond says that it is easier for farming to spread across land masses that have a long east-west axis. Species can spread on an east-west axis because places with similar latitudes tend to have similar climates and growing conditions.
The Eurasians, then, were able to conquer people from other continents because their continent had many domesticable plants and animals and a long east-west axis to help spread farming from place to place.