Then the teacher lets me softly down with the remark that whenever the word "wegen" drops into a sentence, it always throws that subject into the Genitive case,

Then the teacher lets me softly down with the remark that whenever the word "wegen" drops into a sentence, it always throws that subject into the Genitive case, regardless of consequences -- and that therefore this bird stayed in the blacksmith shop "wegen des Regens." Interestingly, this grammatical rule has changed since Mark Twain's times - minimally in spoken language where the Dative case is commonly used. But also in more formal or more educated contexts, standard German allows the Dative case after "wegen" in certain cases. In general, the Genitive case is used less and less, it seems that German will in the long run only entertain three grammatical cases instead of the current four.