We shall have to learn much more of what makes the cultures of men “receptive” and experimental. Did change in the environment alone force it? Was it simply a case of Professor Toynbee’s “challenge and response?” I cannot believe the answer is quite that simple. Were it so simple, we should want to know why the change hadn’t come earlier, along with earlier environmental changes. We shall not know the answer, however, until we have excavated the traces of many more cultures of the time in question. We shall doubtless also have to learn more about, and think imaginatively about, the simpler cultures still left today. The “mechanics” of culture in general will be bound to interest us. It will also be necessary to learn much more of the environments of 10,000 to 12,000 years ago. In which regions of the world were the natural conditions most promising? Did this promise include plants and animals which could be domesticated, or did it only offer new ways of food-collecting? There is much work to do on this problem, but we are beginning to get some general hints. Before I begin to detail the hints we now have from western Asia, I want to do two things. First, I shall tell you of an old theory as to how food-production might have appeared. Second, I will bother you with some definitions which should help us in our thinking as the story goes on. AN OLD THEORY AS TO THE CAUSE OF THE REVOLUTION The idea that change would result, if the balance between nature and culture became upset, is of course not a new one. For at least twenty-five years, there has been a general theory as to _how_ the food-producing revolution happened. This theory depends directly on the idea of natural change in the environment. The five thousand years following about 10,000 B.C. must have been very difficult ones, the theory begins. These were the years when the most marked melting of the last glaciers was going on. While the glaciers were in place, the climate to the south of them must have been different from the climate in those areas today. You have no doubt read that people once lived in regions now covered by the Sahara Desert. This is true; just when is not entirely clear. The theory is that during the time of the glaciers, there was a broad belt of rain winds south of the glaciers. These rain winds would have kept north Africa, the Nile Valley, and the Middle East green and fertile. But when the glaciers melted back to the north, the belt of rain winds is supposed to have moved north too. Then the people living south and east of the Mediterranean would have found that their water supply was drying up, that the animals they hunted were dying or moving away, and that the plant foods they collected were dried up and scarce.