John H. Watson might have been many things - a doctor, a storyteller, and by most accounts a kind and decent man-but he clearly wasn't a zoologist. There's no such thing as a swamp adder. And the idea that Sherlock Holmes deduced its existence from a saucer of milk is ridiculous- snakes have zero interest in milk. They also can't hear anything but vibrations, so they wouldn't hear a whistle. But they do breathe, so a snake couldn't survive in a locked safe.