Analysis of information sources in references of the Wikipedia article "Undine" in English language version.
Silver, Carole G. (2000). Strange and Secret Peoples: Fairies and Victorian Consciousness. Oxford University Press. ISBN 978-0-19-512199-5.The subdivisions and elaborations [of nature spirits] ... by Paracelsus, the Rosicrucians, and the modern theosophists are no doubt amplifications of that popular belief in the existence of a race, neither divine nor human, but very like to human beings, who existed on a "plane" different from that of humans, though occupying the same space which ... resembles the theory of these mystics in its main outlines, and was probably what suggested it to them.