"In 1870-1 Marx taught himself Russian with the purpose of approaching directly evidence and debate published in that language. In a letter to Engels, his wife complained about the manner in which he applied himself to the new task - ‘he has begun to study Russian as if it was a matter of life and death.’  Marx proceeded with similar vigour to study Russian sources, indeed, he turned the books of the Russian radical scholars into his textbooks of language, beginning with Herzen and giving particular attention to Flerovskii and Chernyshevskii. A major library of Russian books, marked and remarked, rapidly accumulated on his shelves and their summaries increasingly entered his notes. 
In an 1870 letter to Engels, Marx praised Flerovskii’s description of the ‘labouring classes’ of Russia - a major populist analysis, as ‘the most substantial book since yours, "The Condition of the Working Class....".  He has subsequently added to the very short list of theorists he respected and publicly applauded to a degree alloted previously only to Engels, the name of Nikolai Chernyshevskii.
In 1877, Marx rebuked in a letter the ‘supra-historical theorising’, i.e. an evolutionist interpretation of his own writings as related to Russia, and rejected it again, much more specifically, in 1881 in relation to the Russian peasant commune. Marx’s quip of those very times about himself ‘not being a marxist’ was coming true with particular vengeance in so far as Russia was concerned."