Sentence:  We are well accustomed as we look out upon Nature at close range to see great creatures harrassed by little creatures. The lot of each big one seems to be in the keeping of some little one, which never quits it, <WSD>nags</WSD> it, stings it, wears it out, drives it desperate, makes life somewhat a burden to it and death somewhat a relief. 
Meaning of the word marked with <WSD> tags:  To bother or disturb persistently in any way.