The difference between these two types of causes is that proximate causes lead immediately and directly to a given result while ultimate causes lead indirectly to that result. Ultimate causes do this by leading to the proximate causes. Let us look at this with reference to Diamond’s argument.
In Chapter 3, Diamond identifies a number of proximate causes that allowed the Spanish to defeat the Incas. He boils these proximate causes down to “guns, germs, and steel.” Another way to list them is shown in Figure 4.1 where Diamond lists horses, guns and steel swords, ocean-going ships, political organization and writing, and epidemic diseases as the proximate causes of European domination. Because the Europeans had these things, they could defeat the Incas.
But Diamond wants to know why the Europeans had these proximate causes and the Incas did not. This is where the search for ultimate causes comes in. Diamond will spend the entire rest of the book laying out what he sees as the ultimate causes. He will argue that geographic luck, such as the number of domesticable species in an area and the ease with which they could spread around that area, is the major ultimate cause of European domination.
We can say, then, that ultimate causes lead to proximate causes and proximate causes lead to results such as (in this book) European domination.