Here is a link to the original, full-length novel, which was first published in 1903 as a magazine serial over a two-month period. Many famous novels (notably several by Charles Dickens) first appeared in magazine serial format. It was an effective way of getting an audience "hooked" on the story and boost magazine sales while giving the author time to write the rest of the book. If the story was good enough, readers were sure to buy the next issue to see what would happen next. I'm not sure that this was true in Jack London's case, but it's said that Dickens often did not know how his stories would end before they became serialized. Stephen King is the most recent practioner of this method of publishing fiction. His novel "The Green Mile" was printed in six parts, one month at a time.
This photograph of Jack London was taken three years before "The Call of the Wild" was published.