The biggest symbol in the play is the ape which takes Yank's life at the finale. It is both a double for Yank yet also the final sign that he is utterly alone.
Yank himself is constantly likened to an ape due to his physical strength, lower class, simple manner of speaking, and labor-heavy job. Monkey imagery abounds, with Yank noticing a coat made of monkey fur in a shop window and at one point trying to use a lamp post as a club to attack someone.
At the end, Yank goes to the zoo and encounters an ape in a cage. He frees the ape and tries to shake hands with it, feeling it is his brother since both are looked down upon and imprisoned (the ape literally, Yank spiritually); however, the ape kills him and throws him in the cage.
The ape is a double for Yank, representing how he is viewed by the upper classes and imprisoned within an unforgiving, unfair class system. However, the ape's rejection of Yank also sets Yank apart from it. The ape is only concerned with survival; Yank is more human because he is concerned with abstract notions such as belonging and self-worth. The ape does not care.