What brought me to "The Island of Doctor Moreau" was the movie with Marlon Brando. Yup, you read that right: It was the catastrophic (by IMDB comments) movie that made me read the book.
For all that I can remember, the movie goes to explore the fact that some people want to be Gods of others. It explores much of our egocentrism, how we find outselves better than anyone else and such (but my memory could be fading after all this time).
The book, on the other hand, goes in a way more simpler and way more interesting concept: our intellect vs our instincts.
Moreau turns animals into anthropomorphic beings, including changes in the brain to allow them talk and understand most basic stuff. But something keeps bringing their instincts back, to the point that they lose their "humanity" and revert to... animals. From that point, from that basic premise, Wells explores what it is to be a human and what it is to be a beast.
The best way to surmise the whole thing is this little gem in the very end of the book (hence why I marked everything a spoiler):
I, Moreau (by his passion for research), Montgomery (by his passion for drink), the Beast People with their instincts and mental restrictions, were torn and crushed, ruthlessly, inevitably, amid the infinite complexity of its incessant wheels.