rather, they are the living "usages of other [writers]," "the shared practice of those who come together." Thinking of generic conventions as a practice shared by many users, allows later writers to exercise the same degree of control over convention as those who predated them. Far from constraining writers, convention provides flexibility to preserve certain aspects of a genre and transform others. Convention in this sense enables "individual expression, because the [writer] is seen as responsive to, even when challenging, his predecessors and fellows." Genre theorist David Fishelov also deals with generic conventions—he calls them "generic rules"—in elaborating his explanatory metaphor of "literary genres as social institutions" in the book Metaphors of Genre: The Role of Analogies in Genre Theory. Fishelov, like Alpers, sees generic conventions as an inescapably "vital part of the literary communicative situation," linking present and past writers to each other, as well as to readers. Established conventions are "a challenge, or a horizon, against which the writer and his reader have to define themselves." The writer may respond to this challenge by "stretch[ing] the generic rules." Fishelov draws his metaphor of genre as social institution from a passage in René Welleck and Austin Warren's Theory of Literature: The literary kind [genre] is an 'institution'—as Church, University, or State is an institution. It exists not as an animal exists or even as a building, chapel, library, or capital, but as an institution exists. One can work through, express oneself through, existing institutions, create new ones, or get on, so far as possible, without sharing in politics or rituals; one can also join, but then reshape institutions. This formulation ascribes agency to actors within social institutions. In the same way institutions like churches, universities, and states organize social actors to accomplish collective social purposes, literary genres organize relationships between writers and readers to accomplish communicative purposes, which change over time. Genres are not static, but rather, like social institutions, persist through the constant renovation of their conventions by individuals. Fishelov is particularly helpful in theorizing the role of the reader in alternately constraining and motivating generic change: [T]he reader demands compliance with the established generic conventions so that he can integrate the new text, but at the same time he expects the writer to manipulate these established conventions so that the new text is more than a tedious repetition of the generic tradition. Reader expectations operate as both