"Yes," said Roger, "I remember that."
"Then when Maggie and I were staying at Drumnalorry we met old Dr. MacKenzie. He was out in Africa a great deal, fifty or sixty years ago. He was a great friend of my mother's. He told us at dinner one night that sleeping sickness is not a new thing at all, but a very old thing. The natives used to get it even in his day. He said that the tsetse fly theory was really all nonsense. He called it a pure invention, based on the discovery that yellow fever is spread by the white-ribbed mosquito. His own theory was that it was caused by manioc intoxication."
"That seems to me to be the prejudice of an old man. What is manioc?"
"A kind of a root, like cassava, isn't it?"
"Probably. What is cassava?"
"It's what they make bread of; cassava bread. It's poisonous until you bake it. Isn't that the stuff? Are you interested in sleeping sickness?"
"Yes. It has been running in my head all day. Look here. Here's a picture of two Africans suffering from it. Do they just sleep away like that?"
"I suppose so. They become more and more lethargic, probably, until at last they cannot be roused."
"How long are they in that condition?"
"I believe for weeks. Poor fellows; it must be ghastly to watch."