The inspector puffed out his cheeks and blew softly. “This is getting out of my depth,” he said. “Still, sir, you can’t get over the bloodhounds. They tell us distinctly that the knife is Ellis’s knife and I don’t see any answer to that.”
“There is no answer because there has been no statement. The bloodhounds have told you nothing. You have drawn certain inferences from their actions, but those inferences may be totally wrong and they are certainly not evidence.”
“You don’t seem to have much opinion of bloodhounds,” the inspector remarked.
“As agents for the detection of crime,” replied Thorndyke, “I regard them as useless. You cannot put a bloodhound in the witness-box. You can get no intelligible statement from it. If it possesses any knowledge, it has no means of communicating it. The fact is,” he continued, “that the entire system of using bloodhounds for criminal detection is based on a fallacy. In the American plantations these animals were used with great success for tracking runaway slaves. But the slave was a known individual. All that was required was to ascertain his whereabouts. That is not the problem that is presented in the detection of a crime. The detective is not concerned in establishing the whereabouts of a known individual, but in discovering the identity of an unknown individual. And for this purpose bloodhounds are useless. They may discover such identity, but they cannot communicate their knowledge. If the criminal is unknown they cannot identify him: if he is known, the police have no need of the bloodhound.
“To return to our present case,” Thorndyke resumed, after a pause; “we have employed certain agents—the hounds—with whom we are not en rapport, as the spiritualists would say; and we have no ‘medium.’ The hound possesses a special sense—the olfactory—which in man is quite rudimentary. He thinks, so to speak, in terms of smell, and his thoughts are untranslatable to beings in whom the sense of smell is undeveloped. We have presented to the hound a knife, and he discovers in it certain odorous properties; he discovers similar or related odorous properties in a tract of land and a human individual—Ellis. We cannot verify his discoveries or ascertain their nature. What remains? All that we can say is that there appears to exist some odorous relation between the knife and the man Ellis. But until we can ascertain the nature of that relation, we cannot estimate its evidential value or bearing. All the other ‘evidence’ is the product of your imagination and that of the general. There is, at present, no case against Ellis.”
“He must have been pretty close to the place when the murder happened,” said the inspector.
“So, probably, were many other people,” answered Thorndyke; “but had he time to wash and change? Because he would have needed it.”
“I suppose he would,” the inspector agreed dubiously.
“Undoubtedly. There were seven wounds which would have taken some time to inflict. Now we can’t suppose that Pratt stood passively while the other man stabbed him—indeed, as I have said, the position of the wounds shows that he did not. There was a struggle. The two men were locked together. One of the murderer’s hands was against Pratt’s back; probably both hands were, one clasping and the other stabbing. There must have been blood on one hand and probably on both. But you say there was no blood on Ellis, and there doesn’t seem to have been time or opportunity for him to wash.”
“Well, it’s a mysterious affair,” said the inspector; “but I don’t see how you are going to get over the bloodhounds.”