Complex sentence: Fans of Edgar Rice Burroughs acknowledge Robertson's contribution to the works of H. DeVere Stacpoole, particularly "The Blue Lagoon", and that both Robertson and Stacpoole's writings influenced Burroughs in his creation of Tarzan of the Apes.
Split into simpler sentences: Fans of Edgar Rice Burroughs acknowledge Robertson's contribution to the works of Henry DeVere Stacpoole, particularly "The Blue Lagoon". They believe that both Robertson's and Stacpoole's writings influenced Burroughs in his creation of Tarzan of the Apes.