This is one of his stand-alones. It takes place in southeast Asia, with savage tribes and monstrous orangutans in his usual style on Earth (as contrasted to under Earth as well as on Mars) -- except that a major factor in it is that a character is a Mad Scientist, out to Create Life, which shifts it into science fiction as least as much as H. G. Wells's The Island of Dr. Moreau.
It opens with Professor Maxon destroying the corpse of a malformed humanoid -- close enough to look human -- that had lived only for moments, and then taking an ocean voyage with his daughter to recover his nerves. And, it turns out, his desire for further experiments. Taking on an assistant, von Horn, and a Chinese cook, Sing, he sets up camp on a deserted island, despite an early attack by pirates, and soon creates twelve living but hideous malformed beings -- ranging from animalistic to barely able to think in intelligence. He confides to von Horn that he wishes to make a perfect one and marry it off to Virginia.
Well, with Number 13, he comes to find the experiment ended abruptly when he wasn't there, but he has a perfect handsome and intelligent specimen. von Horn tries to woo Virginia while the professor instructs his newest specimen. Number 1 escapes, abducts Virginia, and runs off to the jungle; Number 13 saves her and starts, merely from imitating it, to carry her off -- fortunately toward the camp, but seriously unnerving both Professor Maxon and von Horn.
von Horn, intending to marry Virginia by force, and prevent the professor from returning to civilization to change his will, conspires with the native crew of the ship, offering them a chest the professor treasures. Which just happens to coincide with a pirate attack that intends to carry off the chest and Virginia. And the confluence happens to let the twelve created men escape, and Virginia get carried off, and so start the adventures thoroughly. They are soon borne off to Bourno, where abductions, fights, arguments over the chests, and dealing with the jungle's dangers soon fill the pages, and Number 13 is renamed Bulan by the natives before all is resolved in the end.
Sing speaks in a horrible pidgin, but is otherwise treated as a perfectly respectable character, even being the one to treat all injuries (and he is, after all, from the southeast Asia and probably didn't have much practice at English). The other non-white characters -- sometimes serve as striking examples of the racial science of his day.
It plays about with the notion of these monster men. von Horn views them as soulless creatures that can be killed as soon as they have demonstrated the results of the experiment, and when Professor Maxon recovers his sanity (through a blow to the head no less), he agrees -- and is not portrayed as an ungrateful bastard, despite his owing his life to Number 13. But Number 13/Bulan wrestles with the notion, and Virginia argues he must have a soul, and the "soulless" creatures do do things that poke at the notion -- but it doesn't go deeply into it, and in the end, the matter is shoved aside by the course of events and various revelations. More adventures and less contemplation that Wells would have given it, I think.