Agriculture spread from East to West, then from North to South in Guns, Germs, and Steel. Why?

To understand this, think about climate.  Climate stays
more or less the same as you go from east to west.  Of course, there are differences
near oceans and there are differences if the elevation changes.  However, the climate
stays more or less the same at the same latitude.  By contrast, climate changes
radically as you move north or south.  The climate in Seattle is very different from
that in Los Angeles.

Because of this, it was much easier
for agriculture to spread on an East-West axis.  A crop that was domesticated in one
place could be spread to other cultures at the same latitude.  This crop might not be
able to spread north or south because of differences in
climate.

Diamond believes that this is a major reason why
Eurasia (long East-West axis) produced so many civilizations while the Americas and
Africa (long North-South axis) did not.