In regards to Guns, Germs, and Steel, how does the fact that farming is an "auto-catalytic" process account for the great disparities in societies?

The answer to this can be neatly summed up by one line
from Guns, Germs, and Steel.  On the bottom of p. 258 in my edition
of the book (about 4 pages from the end of the chapter), Diamond says "...technology
begets more technology."  This is why farming's autocatalytic nature
matters.

When a society starts to farm, it needs various
technologies.  As it invents those, they lead to other technologies.  Soon, the farming
society has all sorts of technologies that have branched off from one another. 
Meanwhile, a society that did not start farming, has no technology.  It never invented
the first set of technololgy that "begat" all the other technologies.  Because it never
invented the farm-related technology, it could not autocatalyze and create more
technologies.  This means that, over time, the non-farming society falls farther and
farther behind the farming society in terms of
technology.

Because one technology leads to more and more
technology (because farming is autocatalytic), societies can develop huge disparities in
term so their technological levels.