Cal. Civ. Proc. Code § 1172

On the trial of any proceeding for any forcible entry or forcible detainer, the plaintiff shall only be required to show, in addition to the forcible entry or forcible detainer complained of, that he was peaceably in the actual possession at the time of the forcible entry, or was entitled to the possession at the time of the forcible detainer. The defendant may show in his defense that he or his ancestors, or those whose interest in such premises he claims, have been in the quiet possession thereof for the space of one whole year together next before the commencement of the proceedings, and that his interest therein is not then ended or determined; and such showing is a bar to the proceedings.