In most people's minds, there seems to be a divide between mankind and the natural world. The natural world is the stuff we put in state parks and pay five dollars to park near when we want to see some trees. Mankind, well, that's everything else. The Island of Dr. Moreau tries to break down that divide by showing us that mankind and the natural world are one and the same. More to the point, mankind is part of the natural world. We can't escape it no matter how big we build our cities or how many flush toilets we add to our state parks. Like the Beast Folk, we came from nature, we live in nature, and to nature we shall return.
The divide between man and the natural world breaks down with the sinking of the Lady Vain. Yep, page one, and it's gone.
Montgomery's drinking is an attempt to keep the divide between mankind and the natural world. Unfortunately, the more he drinks, the less human he becomes.