Poetics 1449a

So when tragedy and comedy appeared side by side, each starting toward the poesis appropriate according to nature, there came to be some making comedies instead of iambs and others directing tragedies instead of epics, because the forms of these were greater and more honored than the forms of those.

Now to examine whether tragedy is already, with respect to its forms, adequate or not, judging it both by itself and with respect to the audience, is another account. But apart from this then, having come to be from an extemporaneous beginning (both itself and comedy, one from those who lead in dithyrambs and the other from those in phallic songs, which even now in many of the cities continue as customary practice), it increased little by little as the leaders moved forward in as much as came to be apparent to them, and after suffering many changes tragedy stopped when it realized its own nature. And Aeschylus was the first to bring the number of actors from one to two, lessen the chorus, and allow the speech to take the leading role; then Sophocles brought the number of actors to three and allowed scene painting. Still, because it [tragedy] came from satyric drama -- that is, the large from the small stories, and the spoken word from the ridiculous way of speaking -- it has only lately assumed its worthy status. And the meter became iambic from tetrameter. For at first they used tetrameter because the poesis was satyric and suited to dancing, but after the way of speaking came to be, nature herself found the appropriate meter -- iambic is the best of meters for speaking. And a sign of this is that we speak mostly iambs in discussion with one another, but hexameters infrequently and when departing from the settled order of speech. Further, the episodes increased.[1] And so for each of the other things said to have been organized, let them have been spoken of by us, for it would be rather difficult to go through each one.[2]

5. But, as we said, comedy is an imitation of mere trivialities, not however with respect to all vice, but the ridiculous is a proper part of the shameful[3]. For the ridiculous is a sort of mistake and disgrace that is painless and not destructive, such as the ridiculous mask[4]: something ugly and distorted without pain. So the alterations of tragedy and how it came to be have not gone unnoticed, but comedy, because from the beginning it was not serious, went unnoticed.

[1] I'm really not sure what this sentence refers to.
[2] Ignore this footnote.
[3] "shameful" = aiscrhos, which is the same word translated as "ugly" in the next sentence; there's also been some attempt at interpolation in this sentence, which I ignored.
[4] "persona" = prwsopon, which usually means face or visage, but "mask" is the translation actually given in LSJ, s.v., prwsopon, III.