As a kid, around the same time I began reading the Sherlock Holmes canon, I was also a pretty big fan of comic books. These were the days (the late 70s and early 80s) when the average price of a comic book was about 30 cents, and we used to get marked down comics at the local five-and-dime for ten cents. Back in those days, one of my favorite types of comic books were the ones that featured "crossovers": The Brave & the Bold, for example, which always featured Batman teaming up with another hero; or one of my other favorites, a DC/Marvel collaboration that featured Spiderman and Superman teaming up against the Incredible Hulk. (In fact, Batman even teamed up with a 135-year-old Sherlock Holmes in Detective Comics #572.)
But those crossovers are pretty small potatoes when compared to the Wold Newton Universe, a massive crossover concept developed by science fiction author Philip José Farmer. Farmer, who hailed from Peoria, IL (very close to my hometown), wrote many novels that featured collaborations and connections among a host of famous literary characters, including but not limited to: Tarzan, Doc Savage, the Scarlet Pimpernel, Lord Peter Wimsey, Solomon Kane, Arsène Lupin, and of course, Sherlock Holmes. The basic idea presented by Farmer in some of his novels, beginning with Tarzan Alive in 1972, was that a mysterious meteorite fell to earth near Wold Newton in Yorkshire in the year 1795, and that radiation from the impact caused genetic mutations in the occupants of a nearby coach. The descendants of these travelers were, according to Farmer, some of the famous fictional characters mentioned above.
The Wold Newton Universe, as it was dubbed by Win Scott Eckert, is not limited to Farmer's work: Eckert and other fans of Farmer's work have greatly expanded the Wold Newton mythology to a host of other fictional characters. The development of the Wold Newton Universe is not unlike the "great game" that is so popular with Sherlockians around the world. Indeed, Mr. Farmer, like William S. Baring-Gould and various other Sherlockians, treated Holmes and Lord Greystoke and others as if they were real people whose stories were deliberately fictionalized by Arthur Conan Doyle, Edgar Rice Burroughs, et al.
I wish I could report that Philip José Farmer was as skilled at writing further adventures of Sherlock Holmes and Lord Greystoke as were the authors to whose work he paid tribute. But sadly, I have read one of the novels that teamed up Tarzan with Holmes and Watson, The Adventure of the Peerless Peer (1974). I found it to be an absolutely painful read, nothing like either Doyle's or Burroughs's work. Holmes in particular is presented as pompous ass, for the most part. Lord Greystoke fares a bit better, but not very well. And Watson is an absolute bumbling oaf in the book. Still, there is something about the idea of the Wold Newton Universe that appeals to my twelve year old, comic book-reading self.
You can find out more about the mythology of the Wold Newton Universe at Win Scott Eckert's site devoted to the subject. The website is a bit scattered and difficult to navigate, but there is quite a bit of information about the subject of the Wold Newton Family, including a list of Farmer's works that incorporate the various characters of that universe, as well as a variety of works by other authors that have been woven into the tapestry.