Thank you for your help on Inspiration particle and Third elephant. I have read and re-skimmed the books, it is just that I seem to be very bad at finding anything at the re-skim stage. I seem to recall in Science of Discworld, one of the science-authors mentioning a theory on races of men in Africa, but not outside, if one were to follow the same line of thought that lead to the forest elephants being classified as different species. That is, they may be able to mate, but they will not do so. The idea was that there may indeed by a biological idea of race among men, it is just that it is not the one we are used to. I may have imagined it, cause I can not seem to re-find it.