Hunting metaphors and jungle imagery are motifs, recurring in nearly every story in The Return of Sherlock Holmes. Watson frequently compares crime-fighting to hunting, casting himself and Holmes as hunters or predators stalking their prey. Generally, Watson mentions hunting briefly, but he sometimes extends the metaphor for entire paragraphs. Here's an example of one of the longest hunting metaphors:
It was a long and melancholy vigil, and yet it brought with it something of the thrill which the hunter feels when he lies beside the water-pool, and waits for the coming of the thirsty beast of prey. What savage creature was it which might steal upon us out of the darkness? Was it a fierce tiger of crime [...] or would it prove to be some skulking jackal [....] (Black Peter.20)
"Fierce tiger of crime" (Black Peter.20)? Oh that Watson. He does get over-the-top at times. Since hunting references crop up so frequently, it begs the question as to why Watson links crime-solving with hunting. Well, both require patience. Watson and Holmes often go on stake-outs, just like hunters do. Hunting, for Watson, also seems to be an activity involving skill, strategy and adventure. This is also how he views fighting crime.
It's also worth noting that all these hunting references help to link solving crimes, especially in the "jungle" of London (Empty House.36), to the British Empire. In the 1890s, hunting was considered a "manly" sport that occurred out in the "wilderness" of the British Empire, in places like African savannas or Indian jungles. Holmes and Watson often go after foreign criminals in England, London especially. So crime-solving for Holmes and Watson is a sort of exciting sport where they track down and capture some sort of foreign "prey."
Watson's "tiger of crime" spiels might seem ridiculous, but his hunting references give us hints as to how detective work, criminals, and foreigners especially were thought of in the late nineteenth century.