Yikes!
In defense of Zep, this wasn't really an uncommon practice for blues artists before the 70s. Many famous blues songs are variations of older blues songs with no mention of original songwriter. "Dazed and Confused" should have been credited to Jake Holmes (and maybe he is now because they have gone back and given writing credits on many of their old songs) because that is practically the same song but the Spirit song is one of those iffy situations. There are tons of songs with borrowed notes, chords and riffs. Neil Young's, "Mr. Soul" is basically "Satisfaction" with different lyrics. Neil has another called, "Borrowed Tune" that uses the music of "Lady Jane" and has the line, "I'm singing this borrowed tune I took from The Rolling Stones." Bob Dylan's, "Love and Theft" album has some borrowing issues and I don't think he was trying to fool anyone since he put the word Theft in the album title.
I think there is a fine line of what you change enough to make your own. Dazed and Confused crossed the line.
