"You and I have heard of it," he answered. "And that is enough. The proposition that people are not carried off by fairies is a mere working hypothesis, liable to be overthrown by any one case to the contrary. Well, we've got a case to the contrary, and that's the end of the hypothesis." "I'm arguing against myself, daddy, you know," she said. "I want to believe that we do know where he is." "No difficulty at all," said the Student, "to anyone with a properly trained mind, like yours and mine. Take it this way. No one has ever crossed the South Arabian desert or explored the snow ranges of New Guinea, have they? Well, for all anyone can say to the contrary, people may be carried off by fairies every day of the week in New Guinea or South Arabia, mayn't they? It may even be the rule there. It may be a working hypothesis among the pygmies of New Guinea that such a thing _always_ happens--at death, for instance. It would be just as good a working hypothesis as it is that it _never_ happens." "But, daddy, it would be so extraordinary, wouldn't it?" "Not a bit more extraordinary," he said, "than the inside of a bit of radium, or the inside of an egg, for that matter. It is probably simpler for the Urchin to become a fairy than for an egg to become a bird, or a caterpillar a butterfly. It would not be nearly as strange as it is that there is a water beast which can shed its gills and become a land beast, or that Uranus moons go round the wrong way. You can't knock it out by any reasoning of that kind, Fiona. It's merely a matter of fact; and if we have found a case we _have_ found a case." "Then you knew yesterday, daddy?" she said. "I had a very fair idea," he answered. "That is why I was tapping in the cave with a hammer. Can you guess why?" Fiona saw. "To find the rest of the cave," she said. "That is where he would be." "Just so," said the Student. "These caves cannot end in a wall, as that one seems to. I thought the wall must ring hollow somewhere, and the hollow is in the recess where the stone nearly fell on me. The apparent end of the cave is not in the line of the true cave at all." "It is the same place where the stones fell on Mr. Johnson," said Fiona. "That is strange," said the Student. And then Fiona told about the hand she had seen. "Of course, of course," said the Student. "That explains the whole thing. They threw the stone down on me too. They did not wish me to know that the wall was hollow just there. They must use it as a doorway. They will have carried the boy through at the moment that you turned your back, of course. I suppose he invited them in some way; they could have no power otherwise." "He said he would go _anywhere_ to find his treasure," said Fiona. "That would be quite sufficient for them to act on," said the Student. "Then the stories about the cruelty of the Little People are true," asked Fiona.