"Pa, what is all this talk about Evolution?"
"Son, I'll have to consult my attorney before I can answer that question. I might be sent to jail for it."
Okay. It's a reference to the Scopes trial, which was in 1925. (A case in which a schoolteacher was tried for teaching about evolution.) So is it just a simple matter of the father referring to the Scopes prosecution, or is it some strange devolution of the boy into a monkey that the father is invoking Scopes to avoid dealing with? I'm going to guess the former. Finding this cartoon inscrutable doesn't necessarily prove that humor changes a lot over the decades, because there are always some current New Yorker cartoons that you can't quite figure out. (There was a "Seinfeld" episode about that.)
Anyway, looking for some info on the Scopes trial, I came up with some contemporaneous and non-New Yorker cartoons: here and here.