As far as tragedy is concerned, the art of acting may be said to have commenced in the time of Thespis. But actors did not come into existence as a separate class until many years afterwards. Before the period of Aeschylus, when only a single actor was required, his part was taken by the poet. It is expressly said that Thespis was ‘himself acting, according to ancient custom’, at that performance which excited the disapproval of Solon.[682] But when a second actor was introduced by Aeschylus, then the actor’s profession became of necessity distinct from that of the poet. For some time afterwards the poets continued to act occasionally in their own tragedies, side by side with the professional actors. But the practice went gradually out of fashion in the course of the earlier part of the fifth century. Aeschylus appears, from the statement in his Life, to have abandoned the stage even before the introduction of a second actor.[683] Sophocles was prevented from appearing as an actor by the weakness of his voice. It is true that he sometimes performed in public. In the Thamyris he played the harp, and in the Nausicaa he delighted the spectators by his skill with the ball. But it is not likely that on either of these occasions he took a regular actor’s part. He probably appeared upon the scene merely as a mute character, in order to show his skill with the harp and the ball.[684] After the time of Sophocles there are no further instances of tragic poets performing in their own plays.[685] As to the early history of comic acting very little is known. Cratinus is mentioned as one of the old poets who were called ‘dancers’, and it is therefore probable that he acted in his own comedies. Crates is said to have begun his career as an actor of Cratinus.[686] But after his time there is no certain instance of a comic poet appearing upon the stage. The professional actor was universally employed. The statement that Aristophanes acted the part of Cleon in the Knights is due to a misconception on the part of the scholiast.[687]