Shakespeare Reference in Agatha Christie's The A.B.C. Murders

There is one Shakespeare reference in Agatha Christie's The A.B.C. Murders. Near the end, when Hercule Poirot is on the verge of revealing his conclusions regarding the case, he says, "Is it not your great Shakespeare who has said, 'You cannot see the trees for the wood'?" (p. 174). The answer, by the way, is No. And in fact the next line of Christie's book is "I did not correct Poirot's literary reminiscences" (p. 174). The line is "cannot see the wood for the trees," and it was written by Frederick Engels in the 1870s.