In any kind of musical narration it is usually necessary to represent the principal characters or ideas in particular fashion, and the leit-motif is the natural means to this end. And, though theoretically not indispensable, the leit-motif has become a distinguishing feature of the symphonic poem and inseparable from it. Sometimes the themes are many (Strauss has scores of them in his _Heldenleben_), but Liszt took a particular pleasure in economy of means. Sometimes a single theme served him for the development of the whole work. He took the delight of a short-story writer in making his work as compact and unified as possible. In fact, the formal theory of the symphonic poem would read much like Poe’s well known theory of the short story. Let there be some predominant character or idea--‘a single unique effect,’ in Poe’s language--and let this be developed through the various incidents of the narration, changing according to the changing conditions, but always retaining an obvious relation to the central idea. Or, in musical terms, select a single theme (or at most two or three) representing the central character or idea, and repeat and develop this in various forms and moods. This principle brought to a high efficiency a device which Berlioz used only tentatively--that of _transformation_. To Liszt a theme should always be fluid, rarely repeating itself exactly, for a story never repeats itself. And his musicianship and invention show themselves at their best (and sometimes at their worst) in his constant variation of his themes through many styles and forms.