Anthropology

One concept that he discussed frequently was the idea that not all features of organisms are (or need to be) explained by natural selection for advantageous characteristics, that some features just happen by random chance and are carried forward by species that are successful for other reasons. He called it the "contingency" principle of evolution, and I think of it every time I see someone trying to explain every characteristic of an animal as somehow an evolutionary advantage — there's no logical reason to expect that.