Sentence:  But they had already discovered that he could be bullied, and they had it their own way; and presently Selwyn lay prone upon the nursery floor, impersonating a ladrone while pleasant shivers chased themselves over Drina, whom he was <WSD>stalking</WSD>. 
Meaning of the word marked with <WSD> tags:  To approach slowly and quietly in order not to be discovered when getting closer.