Or maybe we can grasp the length of time better in terms of generations. As you know, primitive peoples tend to marry and have children rather early in life. So suppose we say that twenty years will make an average generation. At this rate there would be 25,000 generations in a half-million years. But our United States is much less than ten generations old, twenty-five generations take us back before the time of Columbus, Julius Caesar was alive just 100 generations ago, David was king of Israel less than 150 generations ago, 250 generations take us back to the beginning of written history. And there were 24,750 generations of men before written history began! I should probably tell you that there is a new method of prehistoric dating which would cut the earliest dates in my reckoning almost in half. Dr. Cesare Emiliani, combining radioactive (C14) and chemical (oxygen isotope) methods in the study of deep-sea borings, has developed a system which would lower the total range of human prehistory to about 300,000 years. The system is still too new to have had general examination and testing. Hence, I have not used it in this book; it would mainly affect the dates earlier than 25,000 years ago. CHANGES IN ENVIRONMENT The earth probably hasn’t changed much in the last 5,000 years (250 generations). Men have built things on its surface and dug into it and drawn boundaries on maps of it, but the places where rivers, lakes, seas, and mountains now stand have changed very little. In earlier times the earth looked very different. Geologists call the last great geological period the _Pleistocene_. It began somewhere between a half million and a million years ago, and was a time of great changes. Sometimes we call it the Ice Age, for in the Pleistocene there were at least three or four times when large areas of earth were covered with glaciers. The reason for my uncertainty is that while there seem to have been four major mountain or alpine phases of glaciation, there may only have been three general continental phases in the Old World.[2] [2] This is a complicated affair and I do not want to bother you with its details. Both the alpine and the continental ice sheets seem to have had minor fluctuations during their _main_ phases, and the advances of the later phases destroyed many of the traces of the earlier phases. The general textbooks have tended to follow the names and numbers established for the Alps early in this century by two German geologists. I will not bother you with the names, but there were _four_ major phases. It is the second of these alpine phases which seems to fit the traces of the earliest of the great continental glaciations. In this book, I will use the four-part system, since it is the most familiar, but will add the word _alpine_ so you may remember to make the transition to the continental system if you wish to do so.