What does all this difference mean? It means that, at one moment in time, within each different area, men tended to look somewhat alike. From area to area, men tended to look somewhat different, just as they do today. This is all quite natural. People _tended_ to mate near home; in the anthropological jargon, they made up geographically localized breeding populations. The simple continental division of “stocks”--black = Africa, yellow = Asia, white = Europe--is too simple a picture to fit the facts. People became accustomed to life in some particular area within a continent (we might call it a “natural area”). As they went on living there, they evolved towards some particular physical variety. It would, of course, have been difficult to draw a clear boundary between two adjacent areas. There must always have been some mating across the boundaries in every case. One thing human beings don’t do, and never have done, is to mate for “purity.” It is self-righteous nonsense when we try to kid ourselves into thinking that they do. I am not going to struggle with the whole business of modern stocks and races. This is a book about prehistoric men, not recent historic or modern men. My physical anthropologist friends have been very patient in helping me to write and rewrite this chapter--I am not going to break their patience completely. Races are their business, not mine, and they must do the writing about races. I shall, however, give two modern definitions of race, and then make one comment. Dr. William G. Boyd, professor of Immunochemistry, School of Medicine, Boston University: “We may define a human race as a population which differs significantly from other human populations in regard to the frequency of one or more of the genes it possesses.” Professor Sherwood L. Washburn, professor of Physical Anthropology, Department of Anthropology, the University of California: “A ‘race’ is a group of genetically similar populations, and races intergrade because there are always intermediate populations.” My comment is that the ideas involved here are all biological: they concern groups, _not_ individuals. Boyd and Washburn may differ a bit on what they want to consider a “population,” but a population is a group nevertheless, and genetics is biology to the hilt. Now a lot of people still think of race in terms of how people dress or fix their food or of other habits or customs they have. The next step is to talk about racial “purity.” None of this has anything whatever to do with race proper, which is a matter of the biology of groups. Incidentally, I’m told that if man very carefully _controls_ the breeding of certain animals over generations--dogs, cattle, chickens--he might achieve a “pure” race of animals. But he doesn’t do it. Some unfortunate genetic trait soon turns up, so this has just as carefully to be bred out again, and so on. SUMMARY OF PRESENT KNOWLEDGE OF FOSSIL MEN