One thing that murder mysteries have in common is that there is some kind of evidence that links the killer to the crime. The way that the sleuth finds that evidence varies, of course, depending on the sleuth, the kind of crime fiction, and the author's expertise. There's also variety in the way that the author shares that evidence with readers.
In early mystery novels, such as The Purloined Letter and just about all of the Sherlock Holmes novels, the evidence was obvious, physical evidence such as footprints, distinctive markings, ashes and the like. These novels were written before the advent of modern-day psychology and forensic technology, and you can see that clearly as you think about the way the sleuth gets the evidence. Holmes, for instance, is an expert on a wide variety of different sorts of tobacco ash, chemicals, and even London mud. He recognizes evidence because he has studied so much.
Some kinds of modern murder mysteries also make extensive use of physical evidence. Many are police procedurals. I had an interesting comment exchange about police procedurals with Mack from Mack Captures Crime (a blog well-worth visiting, and one of my Splashed and Lovely nominees). In our comment exchange, Mack mentioned that in police procedurals, the police get evidence bit by bit as they investigate crime. It's not usually all there at once. Good procedurals show how the police accumulate the evidence, and they acknowledge modern forensic technology such as DNA matching and ballistics. Patricia Cornwell's Kay Scarpetta series and Kevin Hughes' novels are good examples of solid police procedurals. I respect police procedurals in which it's obvious that the author has a good knowledge of gathering and analyzing evidence.
Not all evidence is physical, of course. Some evidence, for instance, comes from what witnesses have seen (or say they've seen). A great deal of Golden Age crime fiction emphasizes that kind of evidence. For example, many of the Ellery Queen mysteries include interviews with witnesses. For instance, in The King is Dead, Queen and his father rely heavily on what they've seen and heard, and what witnesses saw and heard as they unravel the death of a megalomaniac tycoon. They sift through what they're told, compare it with what they've seen and in the end, Queen uses that evidence, together with what he learns when he visits the tycoon's hometown, to solve the crime. You can see the same kind of evidence in several of Colin Dexter's Inspector Morse mysteries, as well as in Dorothy Sayers' Lord Peter Wimsey series.
As we all know, physical evidence and "witness" evidence can be deceiving, and part of what engages the reader in a good mystery is sorting out which evidence is real and which is a "red herring." Agatha Christie was a master at offering both kinds of evidence, and challenging the reader to sort things out. She even addresses that issue directly in The Murder on the Links. In that novel, Poirot investigates the death of Paul Renauld, a wealthy Canadian who has relocated to France. As he investigates the murder, he finds several pieces of physical evidence (among them footprints, a piece of pipe and some tattered clothes). Inspector Giraud, who is the official investigator, sees that evidence, too, but draws the wrong conclusions and therefore, discounts important physical evidence. In the end, Poirot shows that a long piece of pipe can be just as valuable a clue as a short piece of pipe, and that physical evidence itself isn't enough to solve a crime. Making sense of that evidence, and organizing what one learns from the evidence, is more important.
Christie returns to that theme in Murder on the Orient Express, in which there is a great deal of physical evidence. Most of it is intended to deceive; some of it is actual evidence. In the novel, in which he investigates the murder of a wealthy passenger on a luxury train, Poirot shows that reasoning, and fitting all of the evidence into a pattern, is much more important than the physical evidence one finds. For instance, one piece of evidence is a set of threatening letters. Another piece of evidence is a pipe cleaner, and another an expensive lady's handkerchief. As he interviews the suspects and forms his ideas about the crime, Poirot shows that physical evidence can be extremely misleading if one doesn't use, as he put it, "the little grey cells of the brain." Murder on the Orient Express, a real Christie classic, is one of several Christie novels reviewed in this month's Agatha Christie Reading Challenge Blog Carnival. The carnival is a wonderful place to learn about Christie works you may not have read, and to see others' insights on the books - I recommend it highly. Thanks, Kerrie, for setting up the carnival!
In my own Joel Williams mysteries, Williams has a healthy respect for careful gathering of evidence. He's a former police detective, and cooperates with the active police force. He doesn't gather evidence himself (although he would sometimes like to), but when he comes across it, he fits it into the overall pattern of the crime. One of the challenges I face as a mystery author is that I am not a forensics or police expert. So when I write about the gathering and analyzing of evidence, I try to be careful not to overstep the bounds of my expertise. I rely heavily on input from people who are experts in those fields, and, to be candid, I avoid lengthy discussions of the analysis of evidence. Not only is it not my area of expertise, but too much detail, even of evidence, can detract from the focus I like best - the focus on the mystery.
One final note on getting evidence: In some of the best mystery novels, the sleuth gets the evidence from conversation. Poirot makes this point in several Christie novels, and so does Mma Precious Ramotswe in Alexander McCall Smith's No. 1 Ladies' Detective Agency series. Both sleuths have found that people like to talk. When people talk, they cannot help revealing themselves, and the sleuth can learn as much from what people say (even when they lie) as from anything else, including physical evidence.
What do you think about getting evidence? How does your favorite sleuth find out the truth? Are you more attracted to novels where the evidence is mostly physical, and the sifting of it yields the truth? Or do you prefer novels in which the evidence comes from what the sleuth sees and hears and learns from conversation?