Out of Time's Abyss

Out of Time's Abyss is a direct sequel to The Land That Time Forgot and The People That Time Forgot, continuing the lost world saga begun in the earlier stories. It connects the previous two instalments, bringing in characters introduced in each. Burroughs completes the revelation of his lost world's unique biological system, only hinted at in the previous instalment, in which the slow progress of evolution in the world outside is recapitulated as a matter of individual metamorphosis. This system forms a thematic element serving to unite the three otherwise rather loosely linked Caspak stories. (Wikipedia)