The inhabitants of each planet belong to different stages of evolution: some higher and some lower. This applies not only to the human inhabitants but also to the animal and even the plant inhabitants. They pass in great waves from one planet to another at certain stages of this evolution, going where they can find the most appropriate conditions either for expression of their present stage or for the stimulation of their next immediate stage. Consequently the stragglers and laggards who fall behind pass to a planet where the conditions are of a lower nature, for there they are more at home. On the other hand, the pioneers who have outstripped the mass and can find no conditions suitable for their further development pass to a planet in a higher stage.
Source: The Notebooks of Paul Brunton