"You mean I've simply been working around a circular argument?" Hunt said, but even as he spoke he could see it was true.

"An elaborate one, but when you analyze the logic of it--yes," ZORAC confirmed.

Hunt nodded to himself and flicked his cigarette to the ashtray.

"Okay. What question should I be asking?"

"Forget about Minervan life and terrestrial life for a moment, and just concentrate on the terrestrial," ZORAC replied. "Now ask why Man is so different from any other species."

"I thought we knew all that," Hunt said. "Bigger brains, opposable thumbs, high-quality vision all in one species together--all the tools you need to stimulate curiosity and learning. What's new?"

"I know what the differences are," ZORAC stated. "My question was why are they?"

Hunt rubbed his chin with his knuckle for a while as he reflected on the question. "Do you think that's significant?"

"Very."

"Okay. I'll buy it. Why is Man so different from any other species?"

"I don't know."

"Great!" Hunt exhaled a long stream of smoke with a sigh. "And how exactly is that supposed to tell us more than my answers did?"

"It doesn't," ZORAC conceded. "But it's a question that needs answering. If you're looking for something unusual, that's a good place to start. There's something very unusual about Man."

"Oh, how come?"

"Because by rights Man shouldn't exist. It shouldn't have been possible for him to evolve. Man simply can't happen, but he did. That seems very unusual to me."

Hunt shook his head, puzzled. The machine was speaking in riddles.

"I don't understand. Why shouldn't Man have happened?"

"I have computed the interaction matrix functions that describe the responses of neuron trigger potentials in the nervous systems of higher terrestrial vertebrates. Some of the reaction coefficients are highly dependent on the concentrations and distributions of certain microchemical agencies. Coherent response patterns in key areas of the cerebral cortex could not stabilize with the levels that are usual in all species except Man."

Pause.

"ZORAC, what are you talking about?"

"I'm not making sense?"

"To put it mildly--no."

"Okay." ZORAC paused for a second as if getting its thoughts organized. "Are you familiar with Kaufmann and Randall's recent work at the University of Utrecht, Holland? It is fully recorded in Jupiter Five's data bank."

"Yes, I did come across some references to it," Hunt replied. "Refresh my memory on it."

"Kaufmann and Randall conducted extensive research on the way in which terrestrial vertebrates protect themselves against toxic agents and harmful microorganisms that enter their systems," ZORAC said. "The details vary somewhat from species to species, but essentially the basic mechanism is the same--presumably handed down and modified from common remote ancestral forms."

"Ah yes, I remember," Hunt said. "A kind of natural self-immunization process, wasn't it?"

He was referring to the discovery by the scientists at Utrecht that the animals of Earth manufactured a whole mixture of contaminants and toxins on a small scale, which were injected into the bloodstream in quantities just high enough to stimulate the production of specific antitoxins. The "blueprint" for manufacturing these antitoxins was thus permanently impressed into the body's chemical system in such a way that production would multiply prodigiously in the event of the body being invaded on a dangerous scale.

"Correct," ZORAC answered. "It explains why animals are far less bothered by unwholesome environments, polluted diets and so on than Man is."

"Because Man is different; he doesn't work that way--right?"

"Right."

"Which brings us back to your question."

"Right."

Hunt regarded the blank screen of the console for a while, frowning to himself in an effort to follow what the machine was getting at. Whatever it was, it failed to register.

"I still don't see where it gets us," Hunt said at last. "Man's different because he's different. It's just as much a pointless question as before."

"Not quite," ZORAC said. "The point is that it shouldn't have been possible for Man to become different. That's what's interesting."

"How come? I'm not with you."

"Permit me to show you some equations that I have solved," ZORAC suggested.

"Go ahead."

"If you key in a channel-activate command I'll put them on the large screen via the UNSA comnet."

Hunt obliged by tapping a quick sequence of characters into the keyboard in front of him. A second later the screen above kaleidoscoped into a blaze of colors which immediately stabilized into a mass of densely packaged mathematical expressions. Hunt stared at the display for a few seconds and then shook his head.

"What's it all supposed to be?" he asked.

ZORAC was happy to explain. "Those expressions describe quantitatively certain aspects of behavior of the generalized central nervous system of the terrestrial vertebrate. Specifically they define how the basic nervous system will respond to the presence of given concentrations and mixes of various chemical agents in the bloodstream. The coefficients indicated in red are modifiers that would be fixed for a given species, but the dominant factors are the general ones shown in green."

"So?"

"It reveals a fundamental drawback in the method that was adopted by terrestrial animals to protect themselves from their chemical environment. The drawback is that the substances introduced into the bloodstream by the self-immunization process will interfere with the functions of the nervous system. In particular, they will inhibit the development of higher brain functions."

Suddenly Hunt realized what ZORAC was driving at. Before he could voice his thoughts, however, the machine went on.

"In particular, intelligence shouldn't be capable of emerging at all. Larger and more complex brains demand a greater supply of blood; a greater supply of blood carries more contaminants and concentrates them in the brain cells; contaminated brain cells can't coordinate sufficiently to exhibit higher levels of activity, that is, intelligence.

"In other words, intelligence should never have been able to evolve from the terrestrial line of vertebrate evolution. All the figures there say that terrestrial life should have gotten itself truly stuck up a dead end."

Hunt gazed for a long time at the symbols frozen on the screen while he pondered the meaning of all this. The ancient architecture evolved by the remote ancestors of the vertebrates hundreds of millions of years before had met a short-term need but failed to anticipate the longer-term consequences. But Man, somewhere along his evolutionary line, had abandoned the self-immunization mechanism. In doing this he had increased his vulnerability to his surroundings, but at the same time he had opened up the way to evolving the superior intelligence that would, in time, more than make up for the initial disadvantage.

The intriguing question of course was: How and when had Man done it? The theory offered by the Utrecht researchers was: during the forced exodus of his ancestors to Minerva, during the period that lasted from twenty-five million to fifty thousand years ago. Twenty-five million years before, many species of ordinary terrestrial life had been shipped there; nearly that long later, only one had come back--one that had been very far from ordinary. Homo sapiens , in the shape of the Lunarians, had returned--the most ferocious adversary that the survival arena of either world had ever witnessed. He had dominated Minerva while contemporaneous anthropoids on Earth groped around in the dim twilight zone on the fringes of self-awareness, and then, having destroyed that world, had returned to Earth to claim his place of origin, completely and ruthlessly extinguishing his remote cousins in the process.