Why was there so much difficulty with the spreading of crops/animals in the Americas in Guns, Germs, and Steel?Chapter 10

If you want to answer this question, simply look at Figure
10.1 -- the map showing the major axes of all the continents.  You will notice that the
major axis of the Americas runs north and south.   This is in contrast to the major axis
of Eurasia, which runs east and west.  Diamond argues that this is the major factor that
made it difficult for crops and animals to spread in the
Americas.

If a continent's major axis is east-west, much
more of the land mass lies along the same latitude.  This means that it will have more
or less the same weather and more or less the same plants and animals can thrive all
along that axis.  That way, plants that are domesticated in one place can spread along
that long axis.

But with the axis of the Americas, this is
not possible.  That axis covers lots of different latitudes from the equator up to much
higher latitudes.  A crop that was discovered in Mesoamerica would not be able to spread
up and be raised, for example, in what is now Kansas.

The
Americas also had other problems (deserts, thick jungles), but the north-south axis was
the main problem.  This is why Diamond even mentions the term in the title of the
chapter.