There is no better way to explain author intent than through the use of the author's own words. Therefore, the following reasoning behind London's purpose for writing Call of the Wild was best described in another text of his.
As a side note, the controversy between London and Roosevelt did not concern only his text Call of the Wild. Roosevelt also had a problem with London's novel White Fang. This was written about in an article published by the New York Times on November 1, 1908 ("A Question of Bulldogs and Fakers").
In "The Other Animals" in Revolution and Other Essays, London describes his reasoning behind the novel. His answer came as a reaction to the critical remarks made by both President Roosevelt and John Burroughs when they accused him of being a "nature-faker". Here is what London stated regarding his writing both Call of the Wild and White Fang:
I have been guilty of writing two animal stories—two books about dogs. The writing of these two stories, on my part, was in truth a protest against the “humanizing” of animals, of which it seemed to me several “animal writers” had been profoundly guilty. Time and again, and many times, in my narratives, I wrote, speaking of my dog heroes: “He did not think these things; he merely did them,” etc. And I did this repeatedly to the clogging of my narrative and in violation of my artistic canons; and I did it in order to hammer into the average human understanding that these dog-heroes of mine were not directed by abstract reasoning. Also, I endeavored to make my stories in line with the facts of evolution; I hewed them to the mark set by scientific research, and awoke, one day, to find myself bundled neck and crop into the camp of the nature-faker.
Not only was London able to explain the purpose behind his texts, he was able to also address the name-calling of Roosevelt.