Interlingue is a constructed language. It was called Occidental between 1922 and 1947. Edgar von Wahl, one of the first Esperantists created it. Von Wahl was from the Russian Empire. He spoke German, Russian, Estonian and French since he was a child. He had natural ability in languages. He is often called de Wahl.
De Wahl was not happy with Esperanto. He decided to create a language called "Occidental". He published it in 1922. In 1949 the name of the language was changed to Interlingue.
There were already many types of constructed languages during de Wahl's time. Some of them, like Esperanto and Ido, were called schematic because they followed their own design. Others, like Latino sine flexione and Idiom Neutral, were called naturalistic because they tried to look like natural languages. The difficulty was that creators had to choose between being regular or looking natural. Louis Couturat, the creator of Ido, wrote the same thing in 1903, and called it a paradox: "The international words are not regular, and the regular words are not international." The language that de Wahl made was meant to be a language that was both regular and international at the same time.
Haas, Fritz. 1956. Grammatica de Interlingue in Interlingue. Interlingue-Servicie Winterthur (Svissia).  (in Interlingue)
Jacob, Henry. 1947 Occidental (1922) by Edgar de Wahl en 0' A Planned Auxiliary Language Publicate: London, Dobson, London. 1947 Archived 2009-10-27 at the Wayback Machine (in English)
Rodriguez, José María. 1999. Breve gramática de Interlinguie/Occidental en Gazeto Andaluzia (órgano oficial de la Asociación Andaluza de Esperanto) N° 57 marzo 1999. Archived 2009-04-08 at the Wayback Machine (in Esperanto)
Stenström, Ingvar. 1997. Occidental-Interlingue: Factos e fato de un lingua international. Societate Svedese pro Interlingua. ISBN91-971940-2-6. (in Interlingua)