In LIFE AND LETTERS OF CHARLES DARWIN Volume 2, Darwin was writing a letter to his friend about finishing the revision of his own book:
Congratulate me, for I have finished the last revise of the last sheet of my book. It has been an awful job: seven and a half months correcting the press: the book, from much small type, does not look big, but is really very big. I have had hard work to keep up to the mark, but during the last week only few revises came, so that I have rested and feel more myself.
I found the general meaning of "up to the mark", but I can't get its exact meaning in this context, does it mean "to keep the quality of the book high" or "the rate of revision high"?