Most modern consumers of popular culture are familiar with the concept of the crossover--that is, when characters or concepts from two or more discrete texts or series of texts meet. Today crossovers are, if not common, not so uncommon enough as to baffle readers or viewers. Most people understand the idea of Professor Moriarty appearing on a Star Trek holodeck, or Superman and Batman teaming up to stop the Joker and Lex Luthor, or Shaft appearing to help the Jack of Spades. But the concept of the crossover is much older than many people realize, and in fact goes back centuries.
The first real crossover is difficult to place. In a liberal definition we could start with the Greek myth of Jason and the Argonauts. The myth, which dates to the ninth or tenth century B.C.E., is about the hero Jason, who sails to Colchis in search of the Golden Fleece. Jason is accompanied by a band of fifty notable heroes on his ship, the Argo. The 50 heroes are the "noblest" of Greece and many, such as Castor, Polydeuces, and Hercules, are the subjects of myths of their own. While the origin of the myth of the Argonauts is unclear, there is some evidence that the myth’s author brought together the heroes of various disparate myths and performed what may be popular culture's first "team up."
This sort of crossover, where characters from folklore and legend meet in new stories, would be repeated over the centuries, with Lancelot du Lac being taken from the myths of Brittany and dragooned into Arthurian legend, and the legendary folk hero Judge Bao appearing in any number of Chinese myths from crime stories to flying swordsman epics. However, this syncretism of myth is clearly not the same as the modern popular culture crossover but is rather a precursor to it as were Virgil's Aeneid, where the ghosts of characters from Homer's Iliad speak with Aeneas, Lucian of Samosata's True History, where space travelers fly past Aristophanes' Cloudcuckooland, and Walter Savage Landor's Imaginary Conversations (1824), in which various historical and fictional characters converse.
The next significant kind of crossover began in 1834, when Honoré de Balzac finally articulated to himself the idea of making his individual novels part of a coherent whole, an individual fictional universe. Before that year his books had possessed an internal consistency, but it was in 1834 when he systematically began making use of recurring characters, with 23 of them appearing in the first edition of Le Père Goriot (1835). By the end of his La Comédie humaine cycle of novels, almost 600 recurring characters appeared in around 90 novels, creating an unmatched fictional universe.
Balzac was the first 19th century author to do this in an organized and ambitious way, but he was far from the last. Alexandre Dumas père linked together several of his novels into series as well as into an overarching universe, so that from 1844 onward his historical novels are often tied together by recurring characters. Beginning in 1859 Emile Gaboriau, who created the detective novel, crafted an entire universe of characters in his novels, often involving his series character "Monsieur Lecoq.” Paul Féval, greatest of the French pulp novelists, linked eight separate novels into the Les Habits Noir cycle, running from 1863-1875. Emile Zola did this as well, in twenty novels about the Second Empire and the Rougon and Macquart families, beginning in 1868.
The most notable example of this use of linked, reappearing characters was Jules Verne. Many non-French readers are unaware of the links between his books, thanks in large part to the many bad translations of his work and to a general ignorance of his less famous work, but Verne, like Balzac, Zola and Gaboriau before him, set many of his works, famous and less so, in the same universe:
The Adventures of Captain Hatteras (1864) refers to Journey to the Center of the Earth (1864); Journey to the Center of the Earth (1864) refers to both Captain Hatteras and to Five Weeks in a Balloon (1863); 20,000 Leagues Under The Sea (1869) refers to Hector Servadac and Journey to the Center of the Earth; The Mysterious Island (1870), the sequel to 20000 Leagues Under the Sea, refers to Captain Grant's Children (1867), _Five Weeks In A Balloon, Captain Hatteras, and Around the Moon (1870); The Far Country (1873) refers to Captain Hatteras; Hector Servadac (1877) and Black Indies (1877) refer to each other; The Clipper of the Clouds (1886) refers to The Begum's Fortune (1879) and to 20000 Leagues Under The Sea; Topsy Turvy (1889), a sequel to From The Earth To the Moon (1865), refers to The Robinsons' School (1882), Captain Hatteras, and Hector Servadac; and The Ice Sphinx (1897), Verne's sequel to Edgar Allen Poe's The Narrative of Arthur Gordon Pym, refers to 20000 Leagues Under the Sea.
Crossovers of this variety, in which one author had two of his series characters meet, grew more common during the heyday of dime novels, with Albert Aiken writing about Joe Phenix's pursuit of Dick Talbot, in Beadle’s New York Dime Library #419, in 1886, and Luis Senarens having Frank Reade, Jr. race Jack Wright for $10,000 in The Boys’ Star Library #375, in 1896. This continued during the years of the pulps, most notably with Edwy Searles Brooks, whose Norman Conquest, Dixon Hawke, Nelson Lee, Marko the Miracle Man, and Waldo the Wonder Man met (and often clashed) in a dizzying variety of crossovers.
The first truly modern crossover--that is, one in which characters from different creators are brought together in a story or novel by another creator--took place in 1872, when "Theopholis M'Crib" published Kennaquhair. A Narrative of Utopian Travel. Kennaquhair (a Scots word meaning an imaginary place) is a city in which various characters from fiction live, dying only when they are forgotten by the outside world. The narrator is escorted through Kennaquhair by Yorick and meets several fictional characters, including a number of Dickensians.
The next example, and one of the most stereotypical of its kind, appeared in 1897, when John Kendrick Bangs wrote the The Pursuit of the Houseboat, the sequel to his 1895 book A Houseboat On The River Styx. Houseboat was the book that spawned the phrase "Bangsian fantasy," or a fantasy of the afterlife in which the ghosts of various famous men and women come together and have various, usually genial, adventures. However, Houseboat featured the ghosts of real people, from Dr. Johnson to Shakespeare to Homer to Napoleon. Pursuit took this idea a step further and showed the ghosts of fictional characters, including Sherlock Holmes, Shylock, Lecoq, Hawkshaw, and Old Sleuth, interacting, trying to solve the mystery of the missing houseboat, and competing with each other.
In 1902 Walter de la Mare published Henry Brocken. In it the titular hero travels through various fictional locations, meeting Annabel Lee, Electra, Jane Eyre and Rochester, and Rosinante and Don Quixote. However, while Henry Brocken is a crossover of a kind, it involves Brocken meeting the characters individually, rather than the characters coming together in a group, as Bangs wrote and later crossovers would feature.
1906 saw what may be the first crossover between Sherlock Holmes and another series character. In 1897 Guy Boothby, in his Simon Carne story, "The Duchess of Wiltshire's Diamonds," referred to "the late Sherlock Holmes," but Holmes did not appear in the story itself. In 1906 Maurice LeBlanc pitted Arsène Lupin, the "Prince of Thieves," against Holmes in "Sherlock Holmes Arrive Trop Tard" (Sherlock Holmes Arrives Too Late), in Je Sais Tout #17 (June 15, 1906). (After pressure from A. Conan Doyle's lawyers the title of LeBlanc's story was changed to "Herlock Sholmes Arrive Trop Tard," thus placing it in the traditions of Holmes pastiches which had begun with R.C. Lehmann's "Picklock Holes" in 1893.) This was probably the first crossover between Holmes and another series character, but was far from the last; Holmes was often illegally used in crossovers like this, occasionally even being killed (!) so that another series character, like the anonymously-created French character "Miss Boston," could avenge Holmes' death and begin her own career. (This fate also befell Nick Carter in another European dime novel.)
In 1915 Carolyn Wells took Bangs one step further and wrote "The Adventure of the Clothes-Line" (The Century, May 1915 issue), in which the "Society of Infallible Detectives" meets to solve crimes. They are presided over by Sherlock Holmes, and their membership includes Jacques Futrelle's S.S. Van Dusen, aka "The Thinking Machine," E.W. Hornung's Raffles, Maurice LeBlanc's Arsene Lupin, Edgar Allen Poe's C. Auguste Dupin, Gaboriau's M. Vidocq, Philip Trent's Luther Trant, William MacHarg and Edwin Balmer's Luther Trant, Arthur Reeve's Craig Kennedy, and Gaston Leroux' Rouletabille. "The Adventure of the Clothes-Line" was the first real team-up of characters created by various authors and the clear prototype of the comic book superhero teams (see below).
In 1921 Cheng Xiaoqing, a Chinese writer, began a series of crossover stories between his character Huo Sang and Sun Liaohong's Lu Ping. The joke was that Huo Sang had begun life as a Sherlock Holmes pastiche and was consciously written as the "Oriental Sherlock Holmes," while Sun Liaohong had created Lu Ping in imitation of LeBlanc's Lupin (the similarity in names is deliberate). The duel between Huo Sang and Lu Ping was an homage to LeBlanc's Holmes-Lupin crossover, but like Huo Sang and Lu Ping themselves, the homage was repeated several times by Cheng Xiaoqing and took on a life of its own.
In 1928 Ralph Smith wrote "Frank Merriwell vs. Fred Fearnot" (The Frank Library, September-December 1928), set in Fredonia, New York, at the wedding of the long-running (14 years' worth of stories) dime novel hero, Fred Fearnot, to his sweetheart, Evelyn Olcott. The guests include Frank and Dick Merriwell, Dick "the Millionaire Detective" Dobbs, Tex Rickard, Old and Young King Brady, Ted Strong and his Rough Riders, Rex Bright, Tom True, and Don Dare (aka "The Three Chums"), Old Joe Crowfoot, Cap'n Wiley, Bowery Billy, Dick Daresome, and Chickering Carter. Nick Carter and Frank Reade, Jr. are mentioned has having been invited but unable to attend due to active cases. And Young Klondike, Klondike Kit, Old Broadbrim and Old Cap Collier are referred to as having died in action before the wedding. All of these characters were perennials, with some, like the Merriwells, appearing in stories for over thirty years.
The 1930s saw a few crossovers between individual pulp characters, as when Emile Tepperman's Red Falcon teamed up with Robert J. Hogan's G-8 in "Dynamite Cargo," in the January 1935 issues of Dare-Devil Aces, but the next really significant crossover came in the comics, with All-Star Comics #3, published in the winter of 1940. Several months before comic books had seen their first crossover, when Bill Everett and Carl Burgos had their characters the Sub-Mariner and the Human Torch meet and fight in Marvel Mystery Comics #7-9, but All-Star Comics #3 saw the first mass crossover and formation of a superhero team, with the creation of the Justice Society of America. Gardner Fox and Sheldon Mayer brought together characters from various DC comics--Sandman and Hourman from Adventure Comics, Flash and Hawkman from Flash Comics, Green Lantern and the Atom from All-American Comics, and the Spectre and Dr. Fate from More Fun Comics--and had them form a team, the Justice Society of America. The Justice Society, or JSA, remained in All-Star Comics until issue #57, in 1951.
I believe that All-Star Comics #3 is particularly significant in terms of crossovers because it was arguably the single greatest vector for the concept of the crossover. During World War Two comics had very high circulation rates, with some, like Superman, Batman, and Captain America, selling over a million copies an issue, hundreds of thousands more than Time. Comics were bought by children, teenagers, and adults, and thanks to their distribution via the United States Armed Services during the war millions more adults read comics without buying them. Given the high rate of exposure during these years of adults and children to comics, I would argue that a significant portion of the population had been exposed to the concept of the crossover, via the Justice Society of America and other, similar superhero teams and crossovers. In this way the idea of the crossover became a familiar one and continues to be a common part of modern popular culture.