What does Jared Diamond mean by "tilted axis?" a
Diamond discusses this concept in Chapter 10 of Guns, Germs, and Steel. It refers to the fact that the axis of Eurasia is longest from east to west, while the axes of South America and Africa, and for that matter North America, are longest from north to south. This means that Eurasia has more territory lying along similar latitudes. The effect of this is described in the following sentence:
Localities distributed east and west of each other at the same latitude share exactly the same day length and its seasonal variations. To a lesser degree, they also tend to share similar diseases, regimes of temperature and rainfall, and habitats or biomes.
So crops domesticated in the Fertile Crescent could spread with relative ease from that region to others in Eurasia. Agriculture, which gave rise to the settled societies that developed the technologies, diseases and cultural and political institutions that we define as civilization, could thus also spread throughout this vast region. On the other hand, it was very difficult to grow the same crops in Canada as in Mesoamerica, so civilization spread less rapidly there. Or crops domesticated in Ethiopia could not spread into the many different climate zones of Africa, which also lies on a north-south axis.
Source: Diamond, Guns, Germs and Steel, 183.