How much of man's history do we know? We really know very little. Written records exist for only a fraction of what we suppose to have been man's time as a unique species. Furthermore, the accuracy of these records is often suspect, and the scope and selection of significant detail in them often needs improvement. At times there is no more than a collection of a few songs, myths and legends. Even in recent times, the not uncommon lack of truly factual historical data makes it difficult to reconstruct an accurate picture of what actually did happen in history. It is worse when we try to reconstruct man's history before the development of writing, and this is unfortunate because the history of the greatest discoveries, such as fire, the wheel and the sail, as well as the history of the early development of human society are lost to us. The most that we can do is to use traces, deduction, speculation and the knowledge we have of the habits of those animals which have some elementary social order to help us make a partial reconstruction. This is hardly a satisfactory substitute for precise information. With our knowledge of human history, which is only fragmentary at best, it is therefore nearly impossible to reconstruct the beginning, and to deduce the end, of the story of man. Thus, there have developed many schools of thought on the subject, each of which attempts to give coherence to the human past by fitting it into the framework of a theory of history.