"The land is often barren, but once it and the ocean undergo biological terraforming - that is, once they are seeded with Earth life - such life flourishes and the planet can then be settled. Hundreds of such planets have been recorded and studied and about half of them are already occupied by Settlers.
"And yet not one habitable planet of all those which have been discovered to date has the enormous variety and excess of life that Earth has. Not one has anything larger or more complex than a small array of wormlike or insect like invertebrates or, in the plant world, anything more advanced than some fernlike shrubbery. No question of intelligence, of anything even approaching intelligence."
Amadiro listened to the stiff sentences and thought: He's speaking by rote. He's memorized all this. - He stirred and said, "I am not a planetologist, Dr. Mandamus, but I ask you to believe that you are telling me nothing I don't already know."
"As I said, Dr. Amadiro, I am starting from the beginning. - Astronomers are increasingly of the belief that we have a fair sample of the habitable planets of the Galaxy and that all - or almost all - are markedly different from Earth. For some reason, Earth is a surprisingly unusual Planet and evolution has proceeded on it at a radically rapid pace and m a radically abnormal manner."
Amadiro said, "The usual argument is that if there were another intelligent species in the Galaxy that was as advanced as we are, it would have become aware of our expansion by now and have made themselves known to us one way or another."
Mandamus said, "Yes, sir. In fact, if there were another intelligent species in the Galaxy that was more advanced than we are, we would not have had a chance to expand in the first place. That we are the only species in the Galaxy capable of traveling in hyperspace would seem certain, then. That we are the only species in the Galaxy that is intelligent is perhaps not quite certain, but there is a very good chance that we are."
Amadiro was now listening with a weary half-smile. The young man was being didactic, like a man stamping out the rhythm of his monomania in a dull beat. It was one of the marks of the crank and the mild hope Amadiro had had that Mandamus might actually have something that would turn the tide of history was beginning to fade.
He said, "You continue to tell me the known, Dr. Mandamus. Everyone knows Earth seems unique and that we are probably the only intelligent species in the Galaxy."
"But no one seems to ask the simple question: 'Why?' The Earthpeople and the Settlers don't ask it. They accept it. They have a mystic attitude toward Earth and consider it a holy world, so that its unusual nature is taken as a matter of course. As for the Spacers, we don't ask it. We ignore it. We do our best not to think of Earth at all, since if we do, we are liable to go further and think of ourselves as having descended from Earthpeople."
Amadiro said, "I see no virtue in the question. We need not seek for complex answers to the 'Why?' Random processes play an important role in evolution and, to some extent, in all things. If there are millions of habitable worlds, evolution may proceed on each of them at a different rate. On most, the rate will have some intermediate value; on some the rate will be distinctly slow, on others distinctly fast; on perhaps one it would proceed exceedingly slow and on another exceedingly fast. Earth happens to be the one on which it proceeded exceedingly fast and we are here because of that. Now if we ask 'Why?' the natural - and sufficient - answer is 'Chance.'"
Amadiro waited for the other to betray the crank by exploding in rage at a preeminently logical statement presented in an amused way, that served to shatter his thesis completely. Mandamus, however, merely stared at him for a few moments out of his deep-set eyes and then said quietly, "NO."
Mandamus let that stand for perhaps two beats and then said, "It takes more than a lucky chance or two to speed evolution a thousand fold. On every planet but Earth, the speed of evolution is closely related to the flux of cosmic radiation in which that planet is bathed. That speed is not the result of chance at all but the result of cosmic radiation producing mutations at a slow rate. On Earth, something produces many more mutations than are produced on other habitable planets and that has nothing to do with cosmic rays, for they do not strike Earth in any remarkable profusion. Perhaps you see a little more clearly, now, why the 'Why?' could be important."
"Well, then, Dr. Mandamus, since I am still listening, with rather more patience than I would have expected myself to possess, answer the question you so insistently raise. Or do you merely have the question and no answer?"
"I have an answer," said Mandamus, "and it depends upon the fact that Earth is unique in a second way."
Amadiro said, "Let me anticipate. You are referring to its large satellite. Surely, Dr. Mandamus, you are not advancing this as a discovery of yours."
"Not at all," said Mandamus stiffly, "but consider that large satellites seem to be common. Our planetary system has five, Earth's has seven and so on. All the known large satellites but one, however, circle gas giants. Only Earth's satellite, the moon, circles a planet not much larger than itself."
"Dare I use the word 'chance' again, Dr. Mandamus?"
"In this case, it may be chance, but the moon remains unique."
"Even so. What possible connection can the satellite have with Earth's profusion of life?"
"That may not be obvious and a connection may be unlikely - but it is far more unlikely that two such unusual examples of uniqueness in a single planet can have no connection at all. I have found such a connection."
"Indeed?" said Amadiro alertly. Now ought to come unmistakable evidence of crackpotism. He looked casually at the time strip on the wall. There really wasn't much more time he could possibly spend on this, for all that his curiosity continued to be aroused.