After the epic party, Doc wakes up, has a beer and a peanut butter sandwich (part of a balanced breakfast), and gets started on all the dishes. Right at the end of the book he recites a little more of that poem he was reading during the party. Then,
he wiped his eyes with the back of his hand. And the white rats scampered and scrambled in their cages. And behind the glass the rattlesnakes lay still and stared into space with their dusty frowning eyes. (32.11)
Let's get this straight. Doc had all his friends over and lots of (kind of) cool gifts, and now he's getting all misty-eyed? Maybe he should stop reading such depressing poetry. The poem he's reciting is about a breakup, and its author spends about 50 stanzas talking about how he remembers what used to be. (Check out "Shout Outs" for more on that.)
So, we get the idea that something is making Doc sad and nostalgic. Is it that everything on Cannery Row is just too perfect and can't possibly stay that way? Since World War II and the end of the canning business are right around the corner, this is definitely true. But how could Doc know that? Well, he couldn't. But Steinbeck sure could.
And what about our final images of rats running around and snakes sitting still? After all, the rats and the snakes would still be there doing what they do (being dead and captive) no matter what Doc was up to. So why end this light-hearted romp through Cannery Row with such a depressing image?