Why was agriculture delayed in some areas?

In 's
Guns, Germs and Steel, his primary thesis is that geography, not any kind
of racial superiority, is the reason that some cultures developed agriculture and some
didn't.

In places like the European continent and the Fertile Crescent in the
Middle East, geography and topography produced fertile soil in relatively stable, temperate
climates, meaning that farming happened earlier than, say, Mexico, most of Africa and Australia,
each of which have significant deserts.

Desert climates dictated more
hunter/gatherer food production because of the obvious difficulty of growing plants in sand and
the lack of water. Most of those areas also have dense, impenetrable rainforests, which would
have required the labor of hundreds of people to clear. Most humans lived in relatively small
groups, and how does one obtain food when they spend all day clearing forest?

He also argues that the climate in areas like those, even when primitive societies
could develop agriculture of any kind, was especially inhospitable to food plants. In short,
everything burned up.

Europe, by contrast, has long growing seasons, very
fertile soil and stable climate bands from east to west, where rain is plentiful and extreme
temperatures are relatively rare throughout at least 9 months a year. Those cultures developed
the technologies to preserve food, leaving more time for other forms of industry, specialization
of labor (certain clans or tribes became blacksmiths, for example) and methods of exchange where
other items could be traded for food.

Because of agriculture, European, etc.
societies began to domesticate animals they could actually feed with excess food production.
Those animals brought with them viruses and disease that jumped to human populations, just as
they do today (the bird flu, etc), giving Europeans immunities to disease much earlier than
their counterparts in other parts of the world.

All of this gave those
cultures time and resources to begin to explore the world, taking disease and steel with them,
resulting in conquest of most of the world.