The approximate date of the immigration of the Patki people to Tusayan is as yet a matter of conjecture. It may have been in prehistoric times, or more likely at a comparatively late period in the history of the people. It seems well substantiated, however, that when this Water-house people joined the other Hopi, the latter inhabited pueblos and were to all intents a pueblo people. If this hypothesis be a correct one, the Snake, Horn, and Bear peoples, whom the southern colonists found in Tusayan, had a culture of their own similar to that of the people from the south. Whence that culture came must be determined by studies of the component clans of the Hopi before the arrival of the Patki people.[30] The origin of the round shape of the estufa, according to Nordenskiöld (p. 168), is most easily explained on the hypothesis that it is a reminiscence of the cliff-dwellers' nomadic period. "There must be some very cogent reason for the employment of this shape," he says, "for the construction of a cylindrical chamber within a block of rectangular rooms involves no small amount of labor. We know how obstinately primitive nations cling to everything connected with their religious ideas. Then what is more natural than the retention, for the room where religious ceremonies were performed, of the round shape characteristic of the original dwelling place, the nomadic hut? This assumption is further corroborated by the situation of the hearth and the structure of the roof of the estufa, when we find points of analogy to the method employed by certain nomadic Indians in the erection of their huts." This theory of the origin of the round form of dwelling and its retention in the architecture of the kiva, advanced by Nordenskiöld in 1893, has much in its favor, but the rectangular form, which, so far as known, is the only shape of these sacred rooms in the Tusayan region, is still unexplained. From Castañeda's narrative of the Coronado expedition it appears that in the middle of the sixteenth century the eastern pueblos had both square and round estufas or kivas, and that these kivas belonged to the men while the rooms of the pueblo were in the possession of the women. The apparent reason why we find no round rooms or kivas in the southern cliff houses and in Tusayan may be due to several causes. Local conditions, including the character of the building sites on the Hopi mesa, made square rooms more practical, or the nomadic stage was so far removed that the form of the inclosure in which the ancients held their rites had not been preserved. Moreover, some of the most ancient and secret observances at Walpi, as the Flute ceremony, are not performed in special kivas, but take place in ordinary living rooms.