In several voluminous works, Jennings developed the theory that the origin of all religion is to be sought in phallic worship of the Sun and Fire, which he properly called "phallism." In addition to the works to which he affixed his own name, Jennings is thought by some researchers to have written a number of anonymous volumes in the privately printed "Nature Worship and Mystical Series" As Jennings made clear in several of his books, he used the word "phallic" in its non-gendered sense, meaning "having to do with the sexual organs"; thus he included worship of the female genitalia under the heading of "phallic." In later editions he reluctantly acceded to popular (although incorrect) usage and called his subject "phallicism." Phallism: A Description of the Worship of Lingam-Yoni in Various Parts of the World, and in Different Ages, with an Account of Ancient and Modern Crosses, Particularly the Crux Ansata (or Handled Cross) and Other Symbols Connected with the Mysteries of Sex Worship. Partial Contents: Definitions; History of the Phallic symbol-structures; Classes of the Phalli; Celestial or Theosophical doctrine of the unsexual transcendental Phallicism; Mysteries of Phallus; its idealized Gnostic, Rosicrucian or Christian renderings; Rites and ceremonies; Hebrew Phallicism; Rosicrucian and Gnostic meanings of the obelisks; Priapic illustrations; Transcendental ideas of the Rosicrucian's; their Cabalistic philosophy as to the occult interchange of Nature and Magic; Mystic anatomy;