http://www.leeds.ac.uk/classics/heath/Aristotle%20on%20comedy.pdf
Here is an excerpt from Aristotle's Poetics:
Comedy is, as we have said, an imitation of characters of a lower type - not, however, in the full sense of the word bad, the ludicrous being merely a subdivision of the ugly. It consists in some defect or ugliness which is not painful or destructive. To take an obvious example, the comic mask is ugly and distorted, but does not imply pain.
The successive changes through which tragedy passed, and the authors of these changes, are well known, whereas comedy has had no history, because it was not at first treated seriously. It was late before the archon granted a comic chorus to a poet; the performers were till then voluntary. Comedy had already taken definite shape when comic poets, distinctively so called, are heard of. Who furnished it with masks, or prologues, or increased the number of actors - these and other similar details remain unknown. As for the plot, it came originally from Sicily; but of Athenian writers Crates was the first who abandoning the 'iambic' or lampooning form, universalised his themes and plots.
