You have seen the big Ferris Wheel in Chicago. The wheel revolves, and the little rooms in the wheel are regularly coming one after another; one set of persons gets into these, and after they have gone round the circle, they get out, and a fresh batch of people gets in. Each one of these batches is like one of these manifestations, from the lowest animals to the highest man. Nature is like the chain of the Ferris Wheel, endless and infinite, and these little carriages are the bodies or forms in which fresh batches of souls are riding, going up higher and higher until they become perfect and come out of the wheel. (CW, 2:230)
Source: The Complete Works of Swami Vivekananda/Volume 2/Jnana-Yoga/Immortality