[link to original on tumblr]
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"Call of the Night" was recorded by the original Zombies (albeit only as a demo), but "Girl Help Me" was recorded after they broke up and - according to the liner notes in Zombie Heaven - may have featured Argent personnel.
Also, the lyrics are substantially different (I actually prefer "Call of the Night" because at times I feel like love songs as a genre have become a cliché, so it's interesting to hear something that's about something else - in this case, transience), and the chord progressions are slightly different. "Girl Help Me" is also in a lower key - probably because Rod Argent sang it instead of Colin Blunstone.