Jared Diamond North-South axis thesis

Personally I think Diamond is just Toynbee rehashed. This way of doing history through sweeping generalizations just doesn't work for me - it's for the paperback shelf at the supermarket, imo. It's far too heavy on interpretation and very light on facts.

As for the North-South variation, it doesn't really make alot of sense to me. Agricultural crops like corn and so on had excellent dispersal throughout the Americas, from Argentina to Canada. Sufficient varieties were introduced for a good diet, as early settlers in English and French colonies in the north discovered - their own crops were not sufficient to support the settlers, and the colonies that thrived were the ones who began growing native foodstuffs. Look at the traditional Thanksgiving dinner - turkey, corn, potatoes, cranberries, American Chestnuts, bell peppers, etc etc, almost all of which are native to the Americas and were being cultivated in the northeast, thousands of miles from Mesoamerica.

I think the easiest and simplest way to explain the technological gap between the two hemispheres is simply that one was settled much, much later and remained in a hunter-gatherer phase for much longer. Much of the best forage areas were picked clean and many big game animals were wiped out in Eurasia early on, forcing them to turn to alternatives sooner. In the Americas, game and forage remained plentiful in many areas for a long time, so there was no need to practice agriculture. Why scrabble hard at the soil for bland staple foods when you could have abundant, very very fresh meat and vegetables with so little effort? The population has to get to a certain concentration - become established to a level that can't be supported by game and forage anymore - before agriculture is desirable. Where it had reached that concentration, agriculture was already being practiced. Where it had not, it was unnecessary and inefficient. 

Diamond's thesis is not that it is impossible for plants and animals to spread from north to south or vice versa.

Well sure. But roses aren't really a necessary foodstuff for agrarian peoples. And you're not going to make a breadbasket out of the desert regardless of which axis your diffusion is happening in. That land is just not suited to it.

All the most important agricultural products of the Americas just happen to grow excellently in many many different climates. Sweet potatoes, corn, squash, peppers, beans, etc.

Diamond tends to be a bit short on specifics. If he had really studied Mesoamerican and Peruvian agriculture, he'd know that the crops developed there had to be adapted to a wide variety of climates. In both Peru and Mesoamerica, there isn't just one climate. It developed in highland ranges situated next to the coast in both regions, where altitude plays a large role in determining climate. There are cool, temperate zones on the altiplano in Peru and in the fertile uplands of the Sierra Madre in Mesoamerica. There are arid areas (some of which were highly agriculturally productive, such as the Valley of Mexico or the area around Lake Titicaca), wet areas, cool areas, hot areas. Different microclimates were farmed extensively in early times, and the crops developed in those regions were bred to be suitable for growth in the variety of climates found in those regions - from cool, temperate climes to very warm, subtropical and tropical climes. It's also one of the reasons why these particular areas were able to develop such an impressive portfolio of domesticated species, because they had access to so many different microclimates in such a small area. 

As wikipedia puts it - "The region possesses a complex combination of ecological systems. Archaeologist and anthropologist Michael D. Coe groups these different niches into two broad categories: lowlands (those areas between sea level and 1000 meters) and altiplanos or highlands (those situated between 1000 and 2000 meters above sea level). In the low-lying regions, sub-tropical and tropical climates are most common, as is true for most of the coastline along the Gulf of Mexico and the Caribbean Sea. The highlands show much more climatic diversity, ranging from dry tropical to cold mountainous climates, the dominant climate is temperate with warm temperatures and moderate rainfall."

http://en.wikipedia.org/wiki/Geography_of_Mesoamerica