[Footnote 78: A true comparison of the mesa habitation and the cliff-dwelling can be made only by renewed work on the former, which is now little more than a huge pile of fallen walls. Present indications show a greater antiquity of the mesa ruin, the site of which afforded more adequate protection. On this supposition the mesa ruins would be considered older than the cliff ruins, and those of the valley the most ancient. If the ruins in Montezuma valley are the oldest, we can not suppose that the culture originated in the cliffs and spread to the valley. The circular subterranean kiva bears indication of having originated in valleys rather than in caverns. Nordenskiöld does not mention the large ruin on the bluff west of Cliff Palace.] We are forced to conclude that they [cliff-houses] were abandoned later than the villages on the mesa. Some features, for example, the superposition of walls constructed with the greatest proficiency on others built in a more primitive fashion (see plate XIII) indicate that the cliff-dwellings have been inhabited at two different periods. They were first abandoned, and had partly fallen into ruin, but were subsequently repeopled, new walls being now erected on the ruins of the old. The best explanation hereof seems to be the following: On the plateaux and in the valleys the Pueblo tribes attained their widest distribution and their highest development. The numerous villages at no great distance from each other were strong enough to defy their hostile neighbors. But afterwards, from causes difficult of elucidation, a period of decay set in, the number and population of the villages gradually decreased, and the inhabitants were again compelled to take refuge in the remote fastnesses. Here the people of the Mesa Verde finally succumbed to their enemies. The memory of their last struggle is preserved by the numerous human bones found in many places, strewn among the ruined cliff-dwellings. These human remains occur in situations where it is impossible to assume that they have been interred. Closely connected with the relative age and the identity of the Mesa Verde cliff-house and pueblo culture are the age and relationship of different cliff-houses of the same region, for example, Cliff Palace and Spruce-tree House. The relative number of kivas may shed light on this point.