Wold Newton Family

The "Wold Newton Family" is an early example of crossover, and shared universe fiction. 

It was created by author Philip José Father. The idea is this: In 1975, in real life, a meteorite crashed to the earth. It hit near a town, in England, called Wold Newtom.

In Farmer's stories, some people who came into close contact with the meteorite ended with genetic mutations which led to special abilities (strength, intelligence, etc...).

In Farmer's world, pretty much all extraordinary and\or super characters are somehow related to this incident either directly or as descendents of someone else who was involved. 

Characters in this shared universe include Tarzan, Did Savage, Sherlock Holmes, Professor Moriarty, Arsene Lupin, James Bond...