Sentence:  Sherlock Holmes rose and lit his pipe. "No doubt you think that you are complimenting me in comparing me to Dupin," he observed. "Now, in my opinion, Dupin was a very inferior fellow. That trick of his of breaking in on his friends' thoughts with an <WSD>apropos</WSD> remark after a quarter of an hour's silence is really very showy and superficial. He had some analytical genius, no doubt; but he was by no means such a phenomenon as Poe appeared to imagine." 
Meaning of the word marked with <WSD> tags:  by the way, incidental.