When Drummond chews out Hornbeck for insulting Brady after his death.
Also, just before that in the play, Rachel's big speech about the story's aesop. She says that she read Darwin's book and has decided not to agree with the evolutionary theory, but says that everyone must be allowed to think and decide for themselves, and that ideas should not be repressed.
After all is said and done Drummond tells Hornbeck that he lives an empty, miserable life, and no one will be by his grave to defend his ways after he's gone. Hornbeck seems sad for a moment, then he smiles a little.
Hornbeck: You're wrong, Henry. You'll be there. You're the type. Who else would defend my right to be lonely?