I've been reading The Deer Slayer, and I can't help but notice that some words at the beginning of sentences display their first letter within square brackets. Here are some examples:
[W]hen five or six had discharged their bullets into the trees, he could not refrain from expressing his contempt at their want of hand and eye.
[T]hen he turned and showed the astonished Hurons the noble brow, fine person, and eagle eye of a young warrior, in the paint and panoply of a Delaware.
[T]hat pale-face is my friend. My heart was heavy when I missed him . . .
(The brackets and their contents are included in the book).
I considered that maybe this was the use of dialect, but it doesn't seem likely or sound natural. What is the reason for this?