Having then determined that we can regard the oldest form of pueblo building as a religious structure, let us pass to the probable steps in the evolution from this early condition into the highest development of that strictly American type of habitation. It is evident, if the tower be looked on as the sanctuary of the clan, that the existence of two or more clans united would necessitate the same number of towers, a condition which we find repeated in the areas under consideration. Granted that the first step in the evolution of the pueblo would be the union of the secular with the sacred room, this might be accomplished either by adding the tower to the group of dwellings, if the latter were situated in a cave, or by moving the habitations out of the cave and annexing them to the base of the tower. Both of these methods seem to have been adopted, resulting on the one hand in cliff-dwellings, and on the other in communal buildings in the open or on top of a plateau. Subsequent stages in the evolution of the pueblo consist in the enlargement because of the growth of the clan of the outlines of the dwelling clustered around the base of the tower until subsequently contiguous groups joined, making one village, composed of as many clans as there are architectural units. The sacred building lost its predominance in this enlargement, and the tower passed without morphological changes into the kiva. We can trace all these modifications in the canyons and plateaus of southwestern Colorado. Sociological advance goes hand in hand with architectural complication. In the beginning the number of social units is indicated by the number of kivas; the next stage is the diminution in relative number of sacred rooms and other changes which appear in the relative size of the kivas. The several social units brought in such intimate contact naturally evolved a system of worship reflecting that union. This appears most clearly in the formation of a fraternity of priests to perform the ceremony resulting from consolidation, which leads to the abandonment of kivas rendered unnecessary, or to the fusion of several into one, and the enlargement of those remaining to accommodate the fraternity composed of men of several social units. This enlargement is shown at Far View House, a pueblo lately excavated in the Mesa Verde National Park, Colorado. The total population of this pueblo was probably as large as that of Cliff Palace, but whereas in this cliff dwelling we find twenty-three sacred rooms, in Far View House there are but four, one of which (the central) is four times as large as any in Cliff Palace. It is easy to see why the central kiva in the pueblo is more centrally placed than the others, when we remember that it was probably the oldest, and was the first settled, and in subsequent growth of the village remained the predominant one of the group.