In Guns, Germs, and Steel, what does Diamond think is the most glaring difference between the Americas and Eurasia?
There are many differences between Eurasia and the Americas that Diamond discusses in this book. However, there is only one place in the book where Diamond uses the word “glaring” with reference to these two land masses. He does this in Chapter 18. Specifically, he does so on pages 354 through 356 in the paperback edition of the book.
In this book, Diamond argues that food production (agriculture) is what makes some societies stronger than others. Societies that start having food production first will tend to become stronger. This is because they have a longer period of time in which to build their strength (this strength comes about because they have agriculture). Diamond says that it is very important that Eurasia got food production long before the Americas did.
So why did Eurasia get agriculture before the Americas? Diamond says this very clearly on page 354. There, he says
The most glaring difference between American and Eurasian food production involved big domestic mammal species.
Diamond is arguing that American food production was slow to develop and was not very productive when it did develop. This was because the Americas did not have large domestic mammals. He says that Eurasia had 13 species that could be domesticated. By contrast, the Americas only had one, and that mammal, the llama/alpaca, only lived in a very small area of the land mass.
This was very important for Diamond. Animals provide protein through their meat and milk. Americans did not have this source of protein. Animals also provide fertilizer through their manure. They can pull plows. Plows and manure help to improve agriculture. The Americans had none of these things.
Thus, this lack of big domestic mammals is the most glaring difference between these areas.