Answer:
"It's rather a mystery--" provides evidence that Whitney is a more sensitive person than Rainsford.
Step-by-step explanation:
When Whitney expresses his view on the dank tropical island as βItβs rather a mystery--β explains that he is more sensitive than Rainford. Because, Whitney expressed his views upon the island fearfully where Rainford was trying to fear Whitney by saying dark things about Island and how sailors perceive that place to Whitney which makes him even more dreadful. But from the given context explains that Whitney is kind.