Symbolism, Imagery, Allegory
During the preparations for the arrival of The Great Prosecutor, Mr. Brady, the town takes on a carnival atmosphere… complete with hot dogs and, yes, a monkey. No really, a monkey. An organ-grinder shows up with a monkey on a string, who is trained to take money from the crowd.
Hornbeck, the city slicker journalist, immediately seizes this opportunity to use the monkey to criticize human nature. He's a sly one, you see:
(MELINDA hands the monkey a penny.)
MELINDA. Look. He took my penny.
HORNBECK. How could you ask of better proof than that? There's the father of the human race! (I, I, 316-19)
By mockingly calling this cash-taking monkey "the father of the human race," Hornbeck is both avowing his evolutionist beliefs and calling people fundamentally greedy. The fact that the monkey takes a coin is proof that he's a great-great-great-…-grandfather of modern-day humans, because, according to Hornbeck, people are slimy; they'll do anything for a buck.
The playwrights, Lawrence and Lee, deliberately use a trained monkey here—not a puppy or a squid or whatever—because one of the biggest problems that people in the play have with evolution is that it posits that humans descended from monkeys.
And who can blame them, really? Who wants to admit that their ancestors had blue bottoms or ate each other's lice? Still, science is science. Proof is proof.