Ingenuity, knowledge, and organization alter but cannot cancel humanity's vulnerability to invasion by parasitic forms of life. Infectious disease wh… - William Hardy McNeill

When variations could be so extravagantly successful, displacement of one humanoid population by another even more effective group of hunters must have occurred frequently. Survival was more likely for the more formidable in battle as well as for the more efficient in the hunt.

Categories of understanding along with everything else alter as societies change.

The diffusion of alchemy westward, like the movement of astrology eastward, became significant only in the centuries after 200 A.D. The conservatism of learning was such that, even when commercial intercourse had made intellectual contacts possible, little serious interchange took place until severe social stress had disturbed the even tenor of the times in China, India, and Europe. Technology was a little, but only a little, less conservative.