One surviving manuscript of Aristotle's Poetics breaks off just as the author turns to comedy. This suggests deliberate censorship, perhaps performed by a puritanical monk during the Middle Ages or later. The lost treatise on comedy would have told us much, of course, but even more importantly, its existence would have granted to comedy the seal of Aristotelian authority that tragedy has always enjoyed--an authority that survives even today in the privileging of 'serious' novels over 'funny' ones. (Count the number of comic novels that have won the Pulitzer prize; you can probably do it on one hand, maybe one finger.)
It's also interesting that even in the most famous passage of the discussion of tragedy, the stuff of comedy creeps in. Aristotle's word katharsis, signifying the supreme benefit of tragedy, also carries the signification of purgation, the action of a laxative. So scatology, the lowest of comedy, invades the heights of tragedy; the generic line is crossed even as it is being constructed.