The first run of Universal Horror films established Universal’s iconic monsters, and included some genuine classics.
The Phantom of the Opera
Frankenstein
The Bride of Frankenstein
Dracula
The Mummy
The Invisible Man
The Werewolf of London
You might also include in there The Old Dark House and the silent Cat and the Canary, although those had no iconic monsters, and got a lot less exposure later on, when most of the others were leased to independent TV stations.
Those films were made over a ten year period, from 1925 to 1935. There were arguably others (see Universal Classic Monsters - Wikipedia), but these films contained the nucleus of the Universal legacy. When one film was the sequel to another, as with Frankenstein and Bride of Franklenstein, the sequel picked up literally from the last scene of the original. aside from the opening scene, with Mary soon-to-be-Shelley recounting the events of the first Frankenstein story, the sequels picks up at the remains of the burning windmill where we apparently saw the monster destroyed. Similarly, Dracula’s Daughter starts in the crypt where Dracula has been staked, with the police interviewing Edward van Sloan as Abraham van Helsing.
In 1936, Universal turned away from horror films, dropping them altogether from their production schedule and moving on to other subjects. But a double-bill showing of Dracula and Frankenstein was an unexpected and significant success, and convinced the studio to go back into the horror business.
But there was a substantial difference. Not only was there a Changing of the Guard – Carl Laemmle Jr. no longer produced, John Balderston no longer wrote screenplays, Boris Karloff and Bela Lugosi, although they still appeared, would each only portray their signature character only once more. Completely new incarnations of The Werewolf (now The Wolfman, played by Lon Chaney Jr. instead of Henry Hull) and The Mummy (now Kharis, played mostly by Chaney, rather than Karloff’s Im-Ho-Tep; and Kharis remained mute and bandage-wrapped, unlike the eloquent Im-Ho-Tep) and The Phantom of the Opera (for the first time he became an acid-scarred and burned composer seeking revenge, rather than the deformed since birth loner of Gaston Leroux’s dark fantasy). Kurt Siodmak did a lot of scripting, Hans J. Salter composed a lot of the music. The setting for the Frankenstein movies was changed completely.
A lot of people (especially those involved) felt that the new films were a lot less classy than the original ones, more mechanical, pandering to the audience, more commercial. But there was also another change I just realized – the internal time between sequels had lengthened appreciably.
The studio continued to crank out the movies continuously, several each year, with sequels appearing every year. But the subjective time between these sequels, within the universe of the stories, now stretched out to downright absurd lengths.
Son of Frankenstein was the first of the iconic monster sequels in the new regime. Instead of being set in Ingolstadt, with the laboratory in an old watchtower far from everything else, it’s now set in the town of Frankenstein, with a laboratory right behind the family house. Wolf von Frankenstein (Basil Rathbone) has had time to grow up into a young doctor, and Inspector Krogh (Lionel Atwill), who had his arm “torn out by the roots” by the Monster, has grown up, as well so this sequel, filmed only four years after Bride of Frankenstein, actually takes place some 25 years later.
Ghost of Frankenstein, made three years later, is filled with inconsistencies (wasn’t Ygor killed in the previous film?), but could have taken place three years later, but clearly some time has passed since the events of the prior film.
Frankenstein Meets the Wolf Man, made the next year, takes place many years after the events of both The Wolfman and Ghost Larry Talbot (the Wolfman), clearly WAS killed in the earlier film, but that is explained, kinda, at the beginning of this film. The Frankenstein monster is found frozen in ice.
House of Frankenstein takes place after the events of the previous film, with this time both The Wolfman and the Monster frozen in ice, with no indication of how long they’ve been there. At the end, the monster carries the Mad Scientist (played by Karloff) into a swamp.
In House of Dracula, made the next year, they find the skeleton of the Mad Scientist and the body of the Frankenstein monster in a cave far below where the swamp was, so clearly a lot of time has passed for the bodies to work their way through and for the Scientist’s body to have completely decayed.
So from the first to the last in the series (ignoring the Abbott and Costello comedy), we have something in excess of forty years for films that cover fewer than 15.
The Mummy movies are even worse in this respect. The first film starts in 1921 and finishes a decade later. The second film, The Mummy’s Hand, was made in 1940, and could have been set then.
The Mummy’s Tomb, made two years later, is set Thirty Years afterwards. at the end, though, the hero gets a draft notice that is clearly intended for WWII, so it’s very confusing.
The Mummy’s Ghost, made two years late, again could have been set then. But its sequel, The Mummy’s Curse, is set about 25 years later. So the second series of Mummy films , made over a span of only four years, covers about sixty in “internal time”. If you include the original film, add about twenty years to that. There are whopping inconsistencies of Mummy behavior and of setting in the second series of Mummy films, too.
(The Invisible Man films are a hopeless mess of tangled relationships and time – better not to go there.)