Science Marches On: Wells states that the changes to the animals are the result of various surgical techniques. Later adaptations of the same story state that genetic engineering is responsible for altering the animals.
Technology Marches On: While most adaptions of the story uses genetic engineering for Moreau's creations, the original novel actually uses vivisection, since it predates the discovery of genetics and DNA. Hence the House of Pain location in the novel, as the surgeries Moreau performs on his unwilling experiments are both unnatural and incredibly painful for the subjects.
Executive Meddling / Troubled Production: Hoo boy, did this one go through hell getting to the screen, and the final result shows how bad it was. It was the subject of a 2014 documentary, Lost Soul: The Doomed Journey of Richard Stanley’s Island of Dr. Moreau, which only scratched the surface as to how insane things got.
To start with, director Richard Stanley feared that he might be kicked off the production and replaced with Roman Polanski before a single frame was even shot, as New Line Cinema had little faith in his ability to helm a big-budget blockbuster, so he enlisted a British warlock to carry out a blood magic ritual to ensure his job security and get star Marlon Brando (who played Dr. Moreau and enthusiastically endorsed Stanley's vision for the film) to vouch for him at meetings. One could say that it worked, as he kept his job, but things started going wrong almost from the moment production started up near Cairns, Queensland. The boat bringing the exotic animals to the set got caught in a hurricane, and Stanley stayed on the ship to ensure the animals' safety — which meant that he got peed on by a restless puma.
Bruce Willis was originally cast as Edward Douglas, but had to drop out due to the proceedings for his divorce from Demi Moore preventing him from leaving the country. Willis was replaced by Val Kilmer — who immediately started behaving like a prima donna, demanding a 40% cut in the days he was required on set and the construction of a treehouse to "get into character", having Marco Hofschneider’s role heavily cut down to avoid being outshined, and frequently butting heads with Stanley to the point that all of his footage from the first few days of filming was deemed unusable. As such, he was recast in the smaller part of Dr. Montgomery so as to limit the amount of damage he could do; the part of Douglas was recast with Rob Morrow, but he only lasted two days before the sheer hostility on set led him to drop out, causing him to be replaced in turn with David Thewlis. (Kilmer attributes his obnoxious behavior to learning, upon the start of filming, that his own wife was suing him for divorce.)
Brando, meanwhile, didn't show up to the set at all initially. His daughter Cheyenne had just killed herself, sending him into a deep depression that prevented him from even leaving his private island, let alone flying out to Australia. Not only did this force Stanley to shoot Kilmer's scenes first, but not having Brando to vouch for him left him more vulnerable to pressure from New Line. When he finally did get to the set, Brando proved to be almost as bad as Kilmer. He stopped trying to memorize lines and would hear from a radio receiver instead; according to Thewlis, the receiver also picked up other transmissions like police scanners, meaning Brando would randomly announce things like "there's been a robbery at Woolworth's" in the middle of a scene. Brando also had the script revised to give more screen time to Nelson de la Rosa, the "world's smallest man" who he'd befriended during filming, he and Kilmer got along spectacularly poorly, and in one famous instance, he wore a bucket on his head and refused to take it off; this wound up in the finished film.
Stanley was eventually fired on the third day of shooting, and he did not take it well, destroying his notes, storyboards, and production art and then disappearing to a remote farm in the jungle, where he lived for two months. Co-star Fairuza Balk, upon learning of Stanley's firing, walked off the set in outrage and tried to escape the shoot with the help of one of the crew, only relenting upon being informed that, if she dropped out, her career would likely be ruined. Stanley would later be discovered by a number of crew members still loyal to him, and he was smuggled back to the set in disguise as an extra wearing a rubber dog mask (security had been tightened in case he tried to sabotage the film). Nobody was the wiser.
John Frankenheimer took over after Stanley's firing, using New Line's desperation as leverage to secure a massive paycheck and a three-picture deal. He faced Kilmer and Brando on the same coin: apparently, he once replied to Kilmer with "I don't give a fuck. Get off my set!" (He had nothing but bad things to say about his experience directing Kilmer, and vowed to never work with him again.) Stanley's script was also discarded, and the new one was being rewritten on a daily basis. Frankenheimer's arrival was by all accounts a case of Tyrant Takes the Helm — he was a very "old-fashioned" director whose dictatorial control of the production led to constant clashes with the cast, the crew, and the studio.
The constant delays meant that the extras playing Moreau's "children" were frequently bored and had nothing to do... so they descended into sex, drugs, and all-around debauchery. Desperate for extras to replace them, Frankenheimer eventually hired some random hippies.
The film finally entered theaters after a harrowing six-month shoot, whereupon it was met with a scathing reception and bombed at the box office. This will probably go down in history as a rare example of a film adaptation that turned out to be less interesting than both the book that inspired it and the production that went into it!
Old Shame: David Thewlis had such a horrible time doing this he skipped the premiere and vowed to never watch the movie.
The Other Marty: Rob Morrow was originally cast as Edward, but quit when Richard Stanley was fired. He was replaced by David Thewlis, who, ironically, was one of the actors Stanley wanted but couldn't get.
One scene has Moreau wearing an ice bucket on his head, because Brando showed up with it and no one asked him to take it off.
Majai, Moreau's Mini Mook, was Nelson de la Rosa. The production hired him because of his 2'4" height to play a fetal creature. What the producers didn't know was that de la Rosa was famous in South America. Brando met him on the set, and immediately took a liking to him, replacing Daniel Rigney's The Dragon role with de la Rosa, expanding his original role as background.