Explain the advantage of an east-west continental axis in Guns, Germs, and Steel.

The reason that this is an advantage is because it allows
crops to spread from one area to another very easily.  This meant that an area like
Eurasia with a long east-west axis could have domesticated crops spread over it
quickly.

Crops can spread from east to west because the
climate along an east to west line is generally similar.  If you go north to south (or
vice versa) the climate changes a lot.  This is not true when you go east to
west.

Crop diffusion is good because it allows more
civilizations to emerge.  One area can "invent" agriculture and their invention can
spread to other places, allowing civilization to emerge.  In areas with long north-south
axes, each place has to "invent" agriculture on its own and fewer civilizations
emerge.