Hi John,
the spelling of german surnames wasn't defined until 1900 the "Bürgerliches Gesetzbuch" (BGB) was introduced.
So it isn't unusual to find a great variance in surnames.
E.g. variations of my surname are Hungerige, Hungrige, Hungerge, Hungringe, etc.
Best regards
Heiko
Hello John,
the pronunciation in the northern parts of Germany is often slurred. Especially the „r” is not pronunced in many words. The name Marx (other spelling Marks) sounds as Maaax („a” like the o in other), and this might be the reason for the different spelling.
Hello John and Heiko,
registry offices were introduced in Germany since 01.01.1876 (partwise already in 1870). Since that times the spellings of names (first names and surnames) were official.
Kind regards
Detlef
Yes, indeed:
"Mit der Einführung des Personenstandsgesetzes vom 6. Februar 1875 trat dann im Deutschen Reich die Versteinerung der Familiennamen ein."
(German Wikipedia)
Best regards
Heiko
Hi John,
In addition to possible reasons that were already mentioned it could be that one of your ancestors changed the name after emigrating to the US (I assume you are from the US), because Marx is related to communism; maybe during the McCarthy era?
I was thinking the same thing, that my ancestor changed his name when he emigrated to the US. The site admin has told me, though, that she found some of his siblings christening records have the surname of Max. I'll have to search the records myself to see when it was changed. I'm really glad they changed it, though, because I actually like the name Max better than Marx. 
Thanks for everyone's input!
John
