"The moon," said Mandamus, "is slowly receding from Earth, due to its tidal effect on the Earth. Earth's large tides are a unique consequence of the existence of this large satellite. Earth's sun produces tides, too, but to only a third of the extent of the moon's tides - just as our sun produces small tides on Aurora.
"Since the moon recedes because of its tidal action, it was far closer to Earth during the early history of its planetary system. The closer the moon to the Earth, the higher the tides on Earth. These tides had two important effects on Earth. It flexed the Earth's crust continually as the Earth rotated and it slowed the Earth's rotation, both through that flexing and through the friction of the ocean's water tides on shallow sea bottoms - so that rotational energy was converted to heat.
"The Earth, therefore, has a thinner crust than any other habitable planet we know of and it is the only habitable planet that displays volcanic action and that has a lively system of plate tectonics."
Amadiro said, "But even all this can have nothing to do with Earth's profusion of life. I think you must either get to the point, Dr. Mandamus, or leave."
"Please bear with me, Dr. Amadiro, for just a little while longer. It is important to understand the point once we get to it. I have made a careful computer simulation of the chemical development of Earth's crust, allowing for the effect of tidal action and plate tectonics, something that no one has ever done before in as meticulous and elaborate a way as I have managed to do - if I may praise myself."
"Oh, by all means," murmured Amadiro.
"And it turns out, quite clearly - I will show you all the necessary data at any time you wish - that uranium and thorium collect in Earth's crust and upper mantle in concentrations of up to a thousand times as high as in any other habitable world. Moreover, they collect unevenly, so that scattered over the Earth are occasional pockets where uranium and thorium are even more concentrated."
"And, I take it, dangerously high in radioactivity?"
"No, Dr. Amadiro. Uranium and thorium are very weakly radioactive and even where they are relatively concentrated, they are not very concentrated in an absolute sense. - All this, I repeat, is because of the presence of a large moon."
"I assume, then, that the radioactivity, even if not intense enough to be dangerous to life, does suffice to increase the mutation rate. Is that it, Dr. Mandamus?"
"That is it. There would be more rapid extinctions now and then, but also more rapid development of new species - resulting in an enormous variety and profusion of life-forms. And, eventually, on Earth alone, this would have reached the point of developing an intelligent species and a civilization."
Amadiro nodded. The young man was not a crank. He might be wrong, but he was not a crank. And he might be right, too -
Amadiro was not a planetologist, so he would have to check books on the subject to see whether Mandamus had perhaps discovered only the already-known, as so many enthusiasts did. There was, however, a more important point he had to check at once.
He said in a soft voice, "You've spoken of the possible destruction of Earth. Is there some connection between that and Earth's unique properties?"
"One can take advantage of unique properties in a unique manner," said Mandamus just as softly.
"In this particular case - in what way?"
"Before discussing the method, Dr. Amadiro, I must explain that, in one respect, the question as to whether destruction is physically possible depends on you."
"On me?"
"Yes," said Mandamus firmly. "On you. Why, otherwise, should I come to you with this long story if not to persuade you that I know what I'm talking about, so that you would be willing to cooperate with me in a manner that will be essential to my success?"
Amadiro drew a long breath. "And if I refused, would anyone else serve your purpose?"
"It might be possible for me to turn to others if you refuse. Do you refuse?"
"Perhaps not, but I am wondering how essential I am to you."
"The answer is, not quite as essential as I am to you. You must cooperate with me."
"Must?"
"I would like you to - if you prefer it phrased in that fashion. But if you wish Aurora and the Spacers to triumph, now and forever, over Earth and the Settlers, then you must cooperate with me, whether you like the phrase or not."
Amadiro said, "Tell me what it is, exactly, that I must do."
"Begin by telling me if it is not true that the Institute has, in the past, designed and constructed humanoid robots."
"Yes, we did. Fifty of them all together. That was between fifteen and twenty decades ago."
"That long ago? And what happened to them?"
"They failed," said Amadiro indifferently.
Mandamus sat back in his chair with a horrified expression on his face. "They were destroyed?"
Amadiro's eyebrows shot upward. "Destroyed? No one destroys expensive robots. They are in storage. The power units are removed and a special long-lived microfusion battery is in each to keep the positronic paths minimally alive."
"Then they can be brought back to full action?"
"I am sure they can."
Mandamus's right hand beat out a tightly controlled rhythm against the arm of the chair. He said grimly, "Then we can win!"
12. THE PLAN AND THE DAUGHTER
52
It had been a long time since Amadiro had thought of the humanoid robots. It was a painful thought and he had, with some difficulty, trained himself to keep his mind away from that topic. And now Mandamus had unexpectedly brought it up.