30. The law of variety is applied in such great profuseness in the works of nature, that one demands how naturalists, astronomers, or philosophers have invented so many systems in order to link comets to planetary bodies, and in order to see in them only stars more or less advanced in development or decay. The pictures which nature is ever presenting ought, however, amply to suffice for the removal from the observer’s mind of all search for parallels which do not exist, and leave to the comets the modest but useful role of wandering stars serving as advance-guards for solar empires; for the celestial bodies are found in many forms other than planetary. Comets have not, like the planets, to fulfill the mission of affording an abiding place for humanity. They travel in successive journeys from sun to sun, enriching themselves sometimes on their route by planetary fragments reduced to a vaporous state, bringing to their focuses the vivifying and renovating principle that they cast upon terrestrial bodies (Chap. IX, n° 12).