"Humboldt's survey of contemporary knowledge of the physical world and the cosmos -- the last such scientific survey undertaken by a single individual -- occupied him for the last three decades of his life. The first two volumes, in which Humboldt described the entire material world from the galaxies to the minutiae of the various mosses, proved enormously popular. The later three volumes, containing Humboldt's special research findings, were less successful, but the fifth volume, completed after Humboldt's death, cites over 9,000 sources to which he felt indebted, and is thus a valuable reference for the history of science" (Norman).