We analyse the original cosmology of P. Jordan through his 1939 key paper entitled "Bemerkungen zur Kosmologie"or "Comments on cosmology". In this almost forgotten work, the author introduced a model of dynamical cosmology with spontaneous creation of matter, based on the Large Numbers study, initiated by Eddington and further developed by Dirac. Jordan's will to explore heuristically all possible cosmological models in order to be prepared in case of surprising future astronomical data is very compelling in this article. Since we think it is wise to learn from our predecessors and from the unsuccessful theories that were later left behind, the present article also offers an overview of Jordan's work during the 1930s through the analysis of a series of some of his other original pieces. An English translation of Jordan's key paper can be found in the appendix.