In this chart, Diamond is attempting to show us what
            factors led to the Europeans' possession of "guns, germs and steel" and other "proximate
            causes" of their ability to defeat people such as the
            Incas.
At the bottom of the chart, we see the things that
            actually allowed the Europeans to be so dominant.  These are things like horses, guns,
            ships, political organization, and epidemic diseases.  We all know that these things
            helped the Europeans, but Diamond says knowing this is not enough.  Instead, he says, we
            have to understand why the Europeans had these things while the Incas and others did
            not.  This is the purpose of this chart.
The chart shows
            the factors that made these proximate causes happen.  For example, we can see that
            Europeans had epidemic diseases because they had many domesticated plant and animal
            species.  We can see that they had many domesticated species because of things like the
            east-west axis of Eurasia and the consequent ease of species
            spreading.
Overall, then, this chart shows us the ultimate
            causes of the things (horses, guns, diseases, etc) that allowed the Europeans to
            dominate the world.
