The difficulty of historical research in reconstructing the records of these aboriginal peoples is due partly to poor methods of transmission and also to the fact that so many of the original documents were lost at the time of the Spanish conquest and before. Chronological reckonings were kept for the purpose of marking the days on which tributes and sacrifices were due. To this were added the figures of chieftains, the notices of tribal conquests, and such events as floods, famines, and eclipses. All of this miscellaneous popular lore was embodied in paintings, executed by a large class of artists, some of whom were women, on paper or fiber rolls or on prepared skins. For this picture-writing skins, oblong in shape and of great length, were employed. Along with these “pinturas” there was handed down an oral method of interpretation. Our knowledge of Mexican history has to be derived from the surviving examples of these picture rolls and from the traditional explanations which were taken down in writing at the time of the entrance of the Spaniards into the country. The tradition existing in Mexico at this period told how the primitive stock inhabiting the land were giants, many of whom had perished by flood, fire, and earthquake. Then came a body of men who wished to reach the sun, and for this purpose they reared a tower. The sun, angered at the presumption of the earth-dwellers in aspiring to share with the gods the dwellings in the heavens, summoned all of the supernatural powers; the building was destroyed; and the guilty mortals were scattered over the earth. A mythical legislator then appears in Central America who teaches the people, the offspring of the giants, the arts of civilized life. The basis of this folklore may not unreasonably be ascribed to the finding of the bones of large extinct animals and, on the site of the Central American ruined cities, of mammoth statues of human beings. The residuum of truth seems to be that the Mexicans of the Conquest were correct in their common tradition that their ancestors had come from the north, and that the country had been gradually occupied by successive swarms of invaders who came south while they were still dependent on hunting game for their food and were finally reduced to settled forms of life by the cultivation of maize. The various tribes who took part in this migration are called by the Mexican word “Nahuatlaca,” used to denote those communities who were dependent on agriculture and followed a nahua or rule of life dictated by a custom administered by hereditary chiefs. At the beginning of the sixteenth century the Nahuatlaca had reached the present limits of Costa Rica. That there were aboriginal inhabitants is inferred from the mention of the Otomi, the Huaxtecs, the Totonacs, and the Ulmecs, who at the time of the Conquest occupied districts not overrun by the Nahuatlacan immigration.