In January 1940, Doc Savage Magazine published a story written by Lester Dent titled The Other World.  It could have just as easily carried the title “The Other Lost World.”  Dent's tale owes a strong literary ancestry to a story written by A. Conan Doyle titled The Lost World, which was first published in 1912.

Before discussing Dent's work it would be proper to lay the groundwork and provide a synopsis for those not familiar with Doyle's tale.  In this story of the unexplored South American jungle, Professor Challenger encounters a dying white man, which results in the professor hunting a lost prehistoric land.  Searching, he locates a large secluded plateau of significant elevation.  Unable to fully ascend the heights, the professor achieves a certain vantage point whereby he confirms the existence of prehistoric animals on this isolated island towering over the region.

Returning to England, Challenger is subjected to generous ridicule with the end result being a scientific expedition to the lost land to ascertain the validity of the good professor's claims.  After a journey up wild rivers, the team arrives at the site and ascends the escarpment.    There they discover the world as it once was, complete with dinosaurs and ape-like cavemen.  A minor survey of this zoological wonderland is made until the explorers become embroiled in an evolutionary war between the primitive cavemen and a modern race of humans who also dwell on this island in the sky.

After a successful prosecution of the war, Challenger and his friends at last manage to escape their towering prison.  They return to England and present a scientific exposition on their expedition.  Their show is capped off by the exhibition of a live pterodactyl that they have secretly brought back.

Let us now move ahead nearly four decades to the Doc Savage universe and visit a place hidden within the earth in the frozen northern wastes.  It is The Other World, and like its namesake is populated with a plethora of amazing dinosaurs and extinct creatures. Before reading The Lost World my inclination was to credit this story's pedigree to Edgar Rice Burroughs and his Pellucidar stories.  Others pointed toward Doyle's influence.  After reading Doyle's work, my opinions are distinctly in favor of crediting The Lost World with no small measure of influence on this Doc Savage story.
Consider the similarities between Dent's story and Doyle's original: Each of the lost worlds is isolated from the earth's surface.  High cliffs segregate Doyle's lost land while Dent's is hidden away within the earth.  Doc Savage and his crew enter the other world via a huge crack in the earth.  Similarly we learn that Maple White ascended the escarpment to reach his lost world by way of a tunnel.

In both stories there are two distinct human species; one of modern appearance while the second is a prehistoric ape-like race.  The adventurers side with the modern species to the detriment of the primitives. Professor Challenger and Monk both bear a marked resemblance to the prehistoric cavemen.  Finally in both stories the location of the lost land is kept secret.

The basic plot of The Other World is recycled in a later Doc Savage story, The Time Terror from January 1943.  Probably the most notable feature about this story is the fact that a pterodactyl is brought out of this prehistoric world to be used as living evidence that the place is genuine.  This seems to be Dent's way of tipping his hat to Doyle.

However, there is another Doc Savage story from early in the series that also bears some striking similarities to The Lost World.  The story in question is The Phantom City from December 1933.

The lost civilization is hidden within the vast regions of the Arabian Desert.  Yet, exactly like Maple White, Doc and his men arrive at their destination by way of a tunnel.  Just as in Doyle's lost land, this inhospitable spot is populated by two distinct human species – one that is modern in appearance while the other is distinctly ape-like.  These two races are at war with one another.

The primitive ape-men of The Lost World toss their captives off the edge of their world. Correspondingly, the primitive creatures in the Doc Savage story hurl their captives off a cliff into the underground river.

Monk Mayfair and Professor Challenger share some traits.  Both enjoy fighting and go into a berserker rage when doing so.  Furthermore, like Professor Challenger who was a twin in appearance of the ape king, Monk Mayfair's striking resemblance to the beast men in his story is noted.  So much so in fact, that Ja initially believes him to be a dark-furred variant of the white-furred beast-men who terrorize her people.

Just as in The Lost World, the explorers aid the modern humans and establishing their dominance over their evolutionary inferiors.  Readers should also note that Monk and Ham carry on the tradition of the Challenger-Summerlee arguments.

The native human habitations are remarkably similar in both novels:

Doyle: Along the base of these red cliffs, some distance above the ground, I could see a number of dark holes through the glass, which I conjectured to be the mouths of caves. 

Dent: It was a city carved from solid rock - a mountain of stone, hewn and hollowed into walls, streets, and dwellings. The rock was a pale red in color.

The passages are unremarkable on their own but given the other parallels Dent's stories shares with Doyle's they become yet another solid piece of evidence.  An additional intriguing point dealing with Dent's stories is that in every tale involving dinosaurs, the very first such creature encountered is a pterodactyl. All in all, it appears a safe bet to say that Dent read The Lost World and incorporated some of its ideas into the Doc Savage series.