The creatures were exposed to an infection of fear, and a frightened beast or bird or insect uses its weapons, whatever they may be. If, for example, there had been anybody with those horses when they took their panic they would have lashed out at him with their heels."

"Yes, I dare say that that is so. Well."

"Well; my belief is that the Germans have made an extraordinary discovery. I have called it the Z ray. You know that the ether is merely an hypothesis, we have to suppose that it's there to account for the passage of the Marconi current from one place to another. Now, suppose that there is a psychic ether as well as a material ether, suppose that it is possible to direct irresistible impulses across this medium, suppose that these impulses are towards murder or suicide; then I think that you have an explanation of the terrible series of events that have been happening in Meirion for the last few weeks. And it is quite clear to my mind that the horses and the other creatures have been exposed to this Z ray, and that it has produced on them the effect of terror, with ferocity as the result of terror. Now what do you say to that? Telepathy, you know, is well established: so is hypnotic suggestion. You have only to look in the Encyclopædia Britannica to see that, and suggestion is so strong in some cases to be an irresistible imperative. Now don't you feel that putting telepathy and suggestion together, as it were, you have more than the elements of what I call the Z ray? I feel myself that I have more to go on in making my hypothesis than the inventor of the steam-engine had in making his hypothesis when he saw the lid of the kettle bobbing up and down. What do you say?"

Dr. Lewis made no answer. He was watching the growth of a new, unknown tree in his garden.

The doctor made no answer to Remnant's question. For one thing, Remnant was profuse in his eloquence—he has been rigidly condensed in this history—and Lewis was tired of the sound of his voice. For another thing, he found the Z-ray theory almost too extravagant to be bearable, wild enough to tear patience to tatters. And then as the tedious argument continued Lewis became conscious that there was something strange about the night.

It was a dark summer night. The moon was old and faint, above the Dragon's Head across the bay, and the air was very still. It was so still that Lewis had noted that not a leaf stirred on the very tip of a high tree that stood out against the sky; and yet he knew that he was listening to some sound that he could not determine or define. It was not the wind in the leaves, it was not the gentle wash of the water of the sea against the rocks; that latter sound he could distinguish quite easily. But there was something else. It was scarcely a sound; it was as if the air itself trembled and fluttered, as the air trembles in a church when they open the great pedal pipes of the organ.

The doctor listened intently. It was not an illusion, the sound was not in his own head, as he had suspected for a moment; but for the life of him he could not make out whence it came or what it was. He gazed down into the night over the terraces of his garden, now sweet with the scent of the flowers of the night; tried to peer over the treetops across the sea towards the Dragon's Head. It struck him suddenly that this strange fluttering vibration of the air might be the noise of a distant aeroplane or airship; there was not the usual droning hum, but this sound might be caused by a new type of engine. A new type of engine? Possibly it was an enemy airship; their range, it had been said, was getting longer; and Lewis was just going to call Remnant's attention to the sound, to its possible cause, and to the possible danger that might be hovering over them, when he saw something that caught his breath and his heart with wild amazement and a touch of terror.

He had been staring upward into the sky, and, about to speak to Remnant, he had let his eyes drop for an instant. He looked down towards the trees in the garden, and saw with utter astonishment that one had changed its shape in the few hours that had passed since the setting of the sun. There was a thick grove of ilexes bordering the lowest terrace, and above them rose one tall pine, spreading its head of sparse, dark branches dark against the sky.

As Lewis glanced down over the terraces he saw that the tall pine tree was no longer there. In its place there rose above the ilexes what might have been a greater ilex; there was the blackness of a dense growth of foliage rising like a broad and far-spreading and rounded cloud over the lesser trees.

Here, then, was a sight wholly incredible, impossible. It is doubtful whether the process of the human mind in such a case has ever been analysed and registered; it is doubtful whether it ever can be registered. It is hardly fair to bring in the mathematician, since he deals with absolute truth (so far as mortality can conceive absolute truth); but how would a mathematician feel if he were suddenly confronted with a two-sided triangle? I suppose he would instantly become a raging madman; and Lewis, staring wide-eyed and wild-eyed at a dark and spreading tree which his own experience informed him was not there, felt for an instant that shock which should affront us all when we first realize the intolerable antinomy of Achilles and the tortoise.

Common sense tells us that Achilles will flash past the tortoise almost with the speed of the lightning; the inflexible truth of mathematics assures us that till the earth boils and the heavens cease to endure, the tortoise must still be in advance; and thereupon we should, in common decency, go mad. We do not go mad, because, by special grace, we are certified that, in the final court of appeal, all science is a lie, even the highest science of all; and so we simply grin at Achilles and the tortoise, as we grin at Darwin, deride Huxley, and laugh at Herbert Spencer.

Dr.