17. Although spirits are everywhere, the globes of the universe are centers in which they assemble by preference, according to the similarity existing between themselves and those by whom they are inhabited. Globes of great advancement are surrounded by the shining hosts of the higher spirits; around globes of low degree, low and backward spirits swarm in crowds. The Earth is still one of the latter. Each globe has, so to say, its own population of incarnate and discarnate spirits, supplied, for the most part, by the incarnation and discarnation of the same spirits. The population of the various globes is more stable in proportion to their backwardness, because, the lower the globe, the more closely are its spirits attached to matter; whereas in the globes of higher degree the inhabitants are more freely floating. But the higher spirits voluntarily quit the splendid worlds which are foci of light and joy, and go to worlds of lower degree, in order to sow therein the germs of progress, to bring consolation and hope to the spirits incarnated in them, to raise the courage of those who are sinking under the trials and struggles of corporeal life; — and they sometimes incarnate themselves in the world whose improvement they wish to help forward, in order to accomplish their undertaking with greater efficiency and success.