In Guns Germs and Steel, what major factor of unification allows for a smooth flow of technology and crops from all parts of East Asia?history

There are at least two important factors of unification
that allowed for this to happen.

First, there were
geographic factors.  Diamond argues that the topography of East Asia made diffusion
relatively easy.  He notes that there were long east-west rivers in China along which
technology and crops could flow.  In addition, there were no major features (like
mountain ranges) blocking diffusion along that
axis.

Second, there was a political factor.  China was so
dominant in the area that it created an environment in which technology and crops could
spread.  Since China created a huge states, there were no political barriers to
diffusion of technology and crops.