Since both earth and heaven must co-operate if the sowing is to be propitious and the harvest bountiful, vegetation festivals are intermediate between demon cults and celestial cults. In respect to origin, they belong to the former; in the degree in which more adequate conceptions of nature are attained, they give rise to the latter. In many cases, moreover, elements of ancestor cult still exercise an influence towards bringing about this transition. The cloud that bestows rain and blessing is regarded as dependent upon a controlling will. Back of the clouds, therefore, according to the ideas of the Zuni and other Pueblo tribes, dwell the ancestors. The prayer of the priests to the clouds is also a prayer to the ancestors for protection and aid. The procession of the rain-priesthood through the village is a representation of the ancestors who are hidden behind the mask of clouds, and is supposed to exercise a magical influence. These cult festivals also include invocations to the sun, whose assistance is likewise necessary to the prosperity of the crop. Thus, in the ceremonial customs of the Navajos, who occupy the same territory, the yellow sand that covers the festival place represents the coloured expanse of the rainbow, the sun, and the moon. All the heavenly forces are to co-operate in bringing about the ripening of the harvest. In this wise it is possible to trace an advance, stage by stage, from the cults of terrestrial demons, who dwell within the growing grain itself, to celestial cults. The fact that the aid of the heavens is indispensable draws the attention upwards. If, now, there are other causes such as give rise to the idea of a celestial migration of the souls of departed ancestors, the cloud demons become merged with ancestor spirits, and there are combined with them the supra-terrestrial powers that are conceived as inherent in the other celestial phenomena.