According to Guns, Germs,and Steel, why did the east-west axis allow for "diffusion"?

The reason
for this has to do with climate.  Think about what happens to the climate as you move in a line
from north to south.  Typically, the climate gets warmer as you go (if you are in the Northern
Hemisphere).  Now think about what happens to the climate as you go from east to west.  In most
cases, the climate does not change all that much.

Now think
about...