In Guns, Germs, and Steel how can Diamond generalize about all societies as he does in Chapter 14?Diamond says that societies,as they develop,pass...

Basically, Diamond can make such generalizations because
that is what all social scientists do.  Social scientists have to do things like
categorizing people or societies so that they can have some way of thinking about them. 
For example, we talk about both the United States and Great Britain as democracies even
though they have very different political systems.

Diamond
makes these generalizations because he has to be able to talk about the different stages
that societies go through.  It is clear that there are differences between societies and
it is important for Diamond's thesis to talk about what attributes each sort of society
has.  In order to make any sense of the range of possible societies, it is necessary to
make broad categories.

As Diamond says, categorizations are
"doomed to imperfection" but they are better than nothing.  That is why Diamond can make
such broad statements.