The Wings of Victory by Fred M. White

The assault was so unexpected that Sylvia could only wonder if Bevill had been suddenly afflicted with a phase of madness into which she seemed to have been plunged during the last few hours. She could see nothing except a little patch of moonlight on the far side of the avenue, could hear nothing but the hard breathing of her companion, and presently catch his whispered words to the effect that she must be quite silent and all would be well. That there was some great danger close at hand she realised. Perhaps some wild animal had escaped and was even at that moment stalking them through the bushes.

Then suddenly that strange chattering sound broke out, a chatter that was more than half-snarling rage, and a queer figure, like some misshapen human being, darted out from the shadow of one of the big trees and went headlong to the ground. The strange monstrosity rose again, and then Sylvia's amazed eyes made out the outline of a huge ape that seemed, so far as she could see, to be fastened by a long chain to the base of one of the big trees in the avenue.