In Guns, Germs, and Steel, what is meant by ultimate and proximate causes?

When Diamond talks about proximate and ultimate causes, he
is trying to explain what has caused Europeans to become dominant in the world. 
Proximate causes are those that led immediately to this dominance.  Proximate causes
include the technology (guns and steel) that allowed the Europeans to defeat people like
the Incas.

But Diamond wants to know
why the Europeans were able to develop the technology. 
This is where the ultimate causes come in to play.  These are factors that allowed the
proximate causes to come into being.  The ultimate causes that Diamond is talking about
are things like the length of the east-west axis in a given place or the number of
species of plants and animals that could be domesticated in that particular
area.

Proximate causes, then, are the things the led
immediately to European domination.  Ultimate causes are those factors that caused the
proximate causes to come into existence.