What does "keep up to the mark" mean in this letter from Darwin to Hooker?

Question

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"?

Answer 1

The "mark" here is an antiquated term for "target". So, here, "keep up to the mark" means to stay on task or "on target", as we would say today, towards meeting one's goal (the target). Here, of course, the goal was revising the book. From etymology online (etymonline.com/search?q=mark): "The Middle English sense of "target" (c. 1200) is the notion in marksman and slang sense "victim of a swindle" (1883)."