Most kinds of aggressive behavior among members of the same species are responsive to crowding in the environment. - Edward O. Wilson

We seem to be able to be fully comfortable only when the remainder of humanity can be labeled as members versus nonmembers.

The genius of human society is in fact the ease with which alliances are formed, broken, and reconstituted, always with strong emotional appeals to rules believed to be absolute.

Stable climates with muted seasons allow more kinds of organisms to specialize on narrower pieces of the environment, to outcompete the generalists around them, and so persist for longer periods of time. Species are packed more tightly. No niche, it seems goes unfilled. Specialization is likely to be pushed to bizarre, beautiful extremes.