Max Reger's Variations on a Theme of Mozart (1914) are notable for adhering closely to the original melody and its proportions, in contrast his many variation sets that make extensive motivic transformations and insert newly composed passages. Paired with this strict measure-to-measure correspondence is a process of introducing increasingly elaborate harmonic and functional transformations. Drawing from theories of tonality by Daniel Harrison, David Huron, Christopher Doll, and Steven Rings, I demonstrate how the variations split apart different ways that listeners perceive a tonic -- as a referent pitch, as a harmonic function, and as a property of a theme's melody. Walter Frisch suggests that some of Reger's variation sets use a "historicist modernism" in that balance past forms with insertions of expressionist, modernist passages. My presentation argues that even the more conservative variation sets offer a modernist tonality by juxtaposing contradictory passages and creating tonics that are splintered and provisional.