On the other hand, where we find an art or institution possessed in common by dozens or hundreds of tribes situated without any gaps between them on the map, it would be far-fetched to assume that each of them independently evolved this identical phenomenon. Why presuppose a hundred parallel causes, each operating quite separately, when one will suffice, in view of the fact that human beings imitate each other’s manners and borrow knowledge. We know that Christianity, gun-powder, the printing press, were originated but once. Even with history wiped out, we could infer as much, from their universality among the nations of Europe.