Thoreau was a great great man, but... you mean great grandfather, right? Or even great-great grandfather? Cause like... assuming a generation procreates every 30 years or so, and let's just say you're 30 for kicks.. A great grandfather would had to have been born in the 1920s, and Thoreau died in the 1860s. I mean, even if you get more liberal than 30 years, that's still a stretch. Oldest man I've known was born in 1896, and.. well.. he was friggin old.
...Unless you meant to list two different people, but given that it was a sentence fragment separated by a comma and not a semicolon, i assume they're the same person (and "grandfather" is just an easier term to say than great great great grandfather or soemthing).