The master said, with vehemence, "It's far above instinct; it's REASON, and many a man, privileged to be saved and go with you and me to a better world by right of its possession, has less of it *that this poor silly quadruped that's foreordained to perish;"* and then he laughed, and said: "Why, look at me—I'm a sarcasm! bless you, *with all my grand intelligence, the only thing I inferred was that the dog had gone mad and was destroying the child, *whereas but for the beast's intelligence—it's REASON, I tell you!—the child would have perished!"**
From A Dog's Tale by Mark Twain,
From what I understand many who have been privileged to possess REASON have less of it than a dog who is destined to be destroyed. am I correct? Does that mean than here? With all my great intelligence I could only infer that the dog went crazy and tried to destroy the baby. I don't understand the rest of the sentence " whereas but for..."