The history of these early races rests mostly upon tradition; yet a diversity of architectural ruins, and the few meagre records that remain, present certain general facts. These positive proofs leave no doubt that this valley was inhabited from a very early period by tribes or nations which made distinct advances in civilization. These tribes had developed certain of the useful arts and had evolved a social system that exhibited some refinement. The first of these races of whom we have reliable record are the Toltecs, who appeared in the Valley of Mexico in the seventh century at almost the same time that Mohammed was spreading his religion over Asia and Africa. Their sway lasted about five centuries, when they disappeared as silently and mysteriously as they came. These peaceful and agricultural people were succeeded by the Chichimecs, a more barbarous race, who came from the north. They in turn were followed by the Nahuals. Lastly came the Aztecs, who entered the valley about 1196, and reached a higher state of civilization than any of their predecessors. War was their choicest profession, for they considered that warriors slain in battle were immediately transported to scenes of ineffable bliss. They offered human sacrifices to their gods. Prescott tells us of a procession of captives two miles long, and numbering seventy thousand persons who were sacrificed at one time. This is incredible, for at that rate the population would soon have been exhausted even in this prolific land. Furthermore we know that the Aztecs were not always successful in war, and may have furnished victims from their own numbers, for sacrifice to the gods of the other nations in the same land. [Illustration: THE CALENDAR STONE]