In chapter 3, section 6 of Nabokov's Pnin, the main character is carrying a reference work "mainly devoted to Tolstoyana" across the Waindell campus when he drops it by accident:
Pnin, on the dirty black ice of the flagged path, slipped again, threw up one arm in an abrupt convulsion, regained his balance, and, with a solitary smile, stooped to pick up Zol. Fond Lit., which lay wide open to a snapshot of a Russian pasture with Lyov Tolstoy trudging across it toward the camera and some long-maned horses behind him, their innocent heads turned toward the photographer too.
V boyu li, v stranstvii, v volnah? In fight, in travel, or in waves? Or on the Waindell campus?
--Page 73 of the Vintage International edition
From which work is the passage in the second paragraph a quotation? (I tried Google, but I suspect the transliteration of the Russian is obscuring the search.) Why would Pnin think of this specific quotation after dropping the book and seeing this photo?