I'll break the sentence up a little. First, "theologian's ecstasy" is the feeling of intense adoration and pleasure or fervor a theologian or religious person would have in the presence of God. Second, "Mahomet’s houri in the dark" is a reference to the Koran's description of paradise where a beautiful maiden awaits the devout Muslim in paradise. So when the author says, 'What is your theologian’s ecstasy but Mahomet’s houri in the dark', I think the author is perhaps saying "Your intense religious fervor (extreme enthusiasm) is the same as the belief that virgins await the devout in Paradise. I haven't read the book, so I can't tell if the tone of the words are sarcastic, mocking or just being literal (stating a comparison and nothing more). This is only my interpretation. It's a difficult read.

I don't think it is possible to give you a good answer on italki, because it would involve discussing not just one, but two prohibited subject areas. Let me just say this much: "Mahomet" is an old-fashioned spelling of the name of Muhammad, the great prophet of Islam.

Let me see if I can be more helpful than I was. You can find a key to the meaning of your passage in an earlier paragraph. If you are reading the text electronically, just search for the word "pugnacity." Otherwise, go back eleven paragraphs from your paragraph. The key is the last phrase of the sentence, which begins "Very much indeed of what we call moral education, he said, is such an artificial modification and perversion of instinct; pugnacity is trained into courageous self-sacrifice..." (That sentence, as well as the Seven's quotation, is from Chapter XIV, "Dr. Moreau Explains." You can find it online at https://www.gutenberg.org/files/159/159-h/159-h.htm#ch14 ) The last phrase of that sentence is the key. The passage you quoted is saying much the same thing, in an elaborate, colorful, sarcastic, and nasty way. In reply to Greg's remark about mockery, yes, the speaker is definitely mocking. The passage is spoken by Dr. Moreau himself. Moreau is an archetypical "mad scientist." An evil mad scientist, in fact. He has an obsession that leads him to perform cruel and painful experiments on animals in order to make them partially human. Most people would say that what he is doing is evil. Nowadays in the United States, he could be arrested and jailed for cruelty to animals. He is justifying himself to Prendick. He is mocking two different traditional belief systems at the same time. In fact, I would say he is mocking the whole idea of ethics, morality, right and wrong.