Sherlock:

“Then, of course, this blood belongs to a second individual presumably the murderer, if a murder has been committed. It reminds me of the circumstances attendant on the death of Van Jansen, in Utrecht, in the year ’34. Do you remember the case, Gregson?”

Gregson:

“No, Sir.”

Sherlock:

“Read it up－you really should. There is nothing new under the sun. It has all been done before.”