tree-spirits
When a tree comes to be viewed, no longer as the body of the tree-spirit, but simply as its abode which it can quit at pleasure, an important advance has been made in religious thought. Animism is passing into polytheism. In other words, instead of regarding each tree as a living and conscious being, man now sees in it merely a lifeless, inert mass, tenanted for a longer or shorter time by a supernatural being who, as he can pass freely from tree to tree, thereby enjoys a certain right of possession or lordship over the trees, and, ceasing to be a tree-soul, becomes a forest god. As soon as the tree-spirit is thus in a measure disengaged from each particular tree, he begins to change his shape and assume the body of a man, in virtue of a general tendency of early thought to clothe all abstract spiritual beings in concrete human form.

JG Frazer, from The Golden Bough, 1922