Before entering on this new line of investigation, it may be well to sum up briefly the evidence as to the starting-point from history and tradition. The commencement of the strictly historical period takes us back certainly for 6000 and in all probability for 7000 years in Egypt, and certainly for 5000 and probably for 6000 or 7000 years in Chaldæa. In each case we find populous cities, important temples and public works, writing and other advanced arts and industries, and all the signs of an old civilization already existing. Other nations also evidently then existed with whom these ancient empires had relations of war and of commerce, though the annals of even the oldest of them, such as China, do not carry us back further than from 4000 to 5000 years. Traditions do not add much to our information from monuments, and fade rapidly away into myths and legends. The oldest and most authentic, those of Egypt, simply confirm the inference of great antiquity for its civilization prior to Menes, but give no clue as to its origin. They neither trace it up to the stone age, which we know existed in the valley of the Nile, nor refer it to any foreign source. The Egyptian people thought themselves autochthonous, and attributed their arts, industries, and sciences to the inventions of native gods, or demi-gods, who reigned like mortal kings, in a remote and fabulous antiquity. We can gather nothing therefore from tradition that would enable us to add even 1000 years with certainty to the date of Menes; while from the high state of civilization which had been evolved prior to his accession, from the primitive conditions of the stone period whose remains are found at Cairo and Thebes, we might fairly add 10,000 or 20,000 years to his date of 5004 years _B.C._, as a matter of probable conjecture for the first dawn of historical civilization. In any case we shall be well within the mark if we take 10,000 years as our first unit, or standard of chronological measurement, with which to start in our further researches, as we do with terrestrial standards in gauging the distances of suns and stars. It may be well also to supplement this statement of the historical standard by a brief review of the previous geological periods through which evidences of man's existence can be traced. Immediately behind the historic age lies the recent period during which the existing fauna and flora, climate and configuration of seas and lands, have undergone no material change. It is characterized generally as the neolithic period, in which we find polished stone superseding the older and ruder forms of chipped stone, and passing itself into the copper, bronze, and iron ages of early history. It may also be called the recent or post-glacial period, for it coincides with the final disappearance of the last great glaciation, and the establishment of conditions of climate resembling those of the present day.