"But that is Cooper's way; frequently he will explain and justify little things that do not need it and then make up for this by as frequently failing to explain important ones that do need it. For instance he allowed that astute and cautious person, Deerslayer-Hawkeye, to throw his rifle heedlessly down and leave it lying on the ground where some hostile Indians would presently be sure to find it - a rifle prized by that person above all things else in the earth - and the reader gets no word of explanation of that strange act. There was a reason, but it wouldn't bear exposure. Cooper meant to get a fine dramatic effect out of the finding of the rifle by the Indians, and he accomplished this at the happy time; but all the same, Hawkeye could have hidden the rifle in a quarter of a minute where the Indians could not have found it. Cooper couldn't think of any way to explain why Hawkeye didn't do that, so he just shirked the difficulty and did not explain at all." Mark Twain, "Cooper's Prose Style," Letters from the Earth

What is Twain's tone in this passage? What is central to the tone of this passage: the attitude toward the speaker, the subject, or the reader?