“Yes,” replied Thorndyke. “They render it certain that this blood was shed not earlier than nine o’clock and probably nearer midnight.” “Really!” the astonished coroner exclaimed. “Now, how is it possible to fix the time in that exact manner?” “By inference from the habits of the parasite,” Thorndyke explained. “This particular filaria is distributed by the mosquito, and its habits are adapted to the habits of the mosquito. During the day, the worms are not found in the blood; they remain hidden in the tissues of the body. But about nine o’clock at night they begin to migrate from the tissues into the blood, and remain in the blood during the hours when the mosquitoes are active. Then, about six o’clock in the morning, they leave the blood and migrate back into the tissues. “There is another very similar species--_Filaria diurna_--which has exactly opposite habits, adapted to day-flying suctorial insects. It appears in the blood about eleven in the forenoon and goes back into the tissues about six o’clock in the evening.” “Astonishing!” exclaimed the coroner. “Wonderful! By the way, the parasites that you found could not, I suppose, have been _Filaria diurna_?” “No,” Thorndyke replied. “The time excludes that possibility. The blood was certainly shed after six. They were undoubtedly _nocturna_, and the large numbers found suggest a late hour. The parasites come out of the tissues very gradually, and it is only about midnight that they appear in the blood in really large numbers.” “That is very important,” said the coroner. “But does this disease affect any particular class of persons?” “Yes,” Thorndyke replied. “As the disease is confined to tropical countries, the sufferers are naturally residents of the tropics, and nearly always natives. In West Africa, for instance, it is common among the negroes but practically unknown among the white residents.” “Should you say that there is a distinct probability that this unknown person was a negro?” “Yes. But apart from the filariæ, there is direct evidence that he was. Searching for some cause of the bleeding, I noticed a lamp-hook screwed into the ceiling and low enough to strike a tall man’s head. I examined it closely, and observed on it a dark, shiny mark, like a blood-smear, and one or two short coiled hairs which I recognized as the scalp-hairs of a negro. I have no doubt that the unknown man is a negro, and that he has a wound of the scalp.” “Does filarial disease produce any effects that can be recognized?”