The physical continuity of the race is insured, as we have just seen, by a mechanism, which, though it may be subjected to rational consideration, is instinctive in its operation. The human beings that people the earth to-day are offspring of human ancestors reaching back to the appearance of the human animal in the long process of the evolution of life on earth. So far as we can see, posterity will be for countless generations physically similar to ourselves, as they certainly will, unless all records or evidences of the fact are obscured, trace their ancestry continuously back to us. Not only is there continuity of physical descent, however, but continuity of cultural achievement. The past, in any literal temporal sense, is over and done with. The Romans are physically dead, as are the generations of barbarians of the Dark Ages, and all the inhabitants of mediaeval and modern Europe, save our own contemporaries. Yesterdays are irrevocably over. The past, in any real sense, exists only in the form of achievements that have been handed down to us from previous generations. The only parts of the past that survive physically are the actual material products and achievements of bygone generations, the temples and the cathedrals, the sculptures and the manuscripts, the roads and the relics of earlier civilizations. Even these exist in the present; they are evidences, memorials, mementos of the past. These heritages from past civilizations may be interesting, intrinsically, as in the case of paintings and statues, or useful, as in the case of roads, reservoirs, or harbors. But we inherit the past in a more vital sense. We inherit ways of thought and action, social systems, scientific and industrial methods, manners and morals, educational bequests and ideals, all that we have and are. Without these, each generation would have to start anew. If the whole of existing society were destroyed, and a newborn generation could be miraculously preserved to maturity, its members would have to start on the same level, with the same ignorances, uncertainties, and impotences as primitive savages.