"What are your theories?" demanded Drake. The feverishness had returned to his eyes.
"It became more and more obvious to me as I proceeded with my studies that the entire direction of research into the Inhibition Death was wrong. Physically, there was no solution to its mystery. The Inhibition Death is entirely a disease of the mind."
Rose interrupted, "Surely, Dr. Tholan, it isn't psychosomatic."
A thin, gray translucent film had passed over the Hawkinsite's eyes. He no longer looked at them. He said, "No, Mrs. Smollett, it is not psychosomatic. It is a true disease of the mind; a mental infection. My patients had double minds. Beyond and beneath the one that obviously belonged to them, there was evidence of another one - an alien mind. I worked with Inhibition Death patients of other races than my own, and the same could be found. In short, there are not five intelligences in the Galaxy, but six. And the sixth is parasitic."
Rose said, "This is wild - impossible! You must be mistaken, Dr. Tholan."
"I am not mistaken. Until I came to Earth, I thought I might be. But my stay at the Institute and my researches at the Missing Persons Bureau convinced me that is not so. What is so impossible about the concept of a parasitic intelligence? Intelligences like these would not leave fossil remains, nor even leave artifacts - if their only function is to derive nourishment somehow from the mental activities of other creatures. One can imagine such a parasite, through the course of millions of years, perhaps, losing all portions of its physical being but that which remains necessary, just as a tapeworm, among your Earthly physical parasites, eventually lost all its functions but the single one of reproduction. In the case of the parasitic intelligence, all physical attributes would eventually be lost. It would become nothing but pure mind, living in some mental fashion we cannot conceive of on the minds of others. Particularly on the minds of Earthmen."
Rose said, "Why particularly Earthmen?"
Drake simply stood apart, intent, asking no further questions. He was content, apparently, to let the Hawkinsite speak.
"Have you not surmised that the sixth intelligence is a native of Earth? Mankind from the beginning has lived with it, has adapted to it, is unconscious of it. It is why the higher species of terrestrial animals, including man, do not grow after maturity and, eventually, die what is called natural death. It is the result of this universal parasitic infestation. It is why you sleep and dream, since it is then that the parasitic mind must feed and then that you are a little more conscious of it, perhaps. It is why the terrestrial mind alone of the intelligences is so subject to instability. Where else in the Galaxy are found split personalities and other such manifestations? After all, even now there must be occasional human minds which are visibly harmed by the presence of the parasite.
"Somehow, these parasitic minds could traverse space. They had no physical limitations. They could drift between the stars in what would correspond to a state of hibernation. Why the first ones did it, I don't know; probably no one will ever know. But once those first discovered the presence of intelligence on other planets in the Galaxy, there was a small, steady stream of parasitic intelligences making their way through space. We of the outer worlds must have been a gourmet's dish for them or they would have never struggled so hard to get to us. I imagine many must have failed to make the trip, but it must have been worth the effort to those who succeeded.
"But you see, we of the other worlds had not lived with these parasites for millions of years, as man and his ancestors had. We had not adapted ourselves to it. Our weak strains had not been killed off gradually through hundreds of generations until only the resistants were left. So, where Earthmen could survive the infection for decades with little harm, we others die a quick death within a year."
"And is that why the incidence has increased since interstellar travel between Earth and the other planets has begun?"
"Yes." For a moment there was silence, and then the Hawkinsite said with a sudden access of energy, "Give me back my cylinder. You have your answer."
Drake said, coolly, "What about the Missing Persons Bureau?" He was swinging the cylinder again; but now the Hawkinsite did not follow its movements. The gray translucent film on his eyes had deepened and Rose wondered whether that was simply an expression of weariness or an example of the changes induced by cyanide lack.
The Hawkinsite said, "As we are not well adapted to the intelligence that infests man, neither is it well adapted to us. It can live on us - it even prefers to, apparently - but it cannot reproduce with ourselves alone as the source of its life. The Inhibition Death is therefore not directly contagious among our people."
Rose looked at him with growing horror. "What are you implying, Dr. Tholan?"
"The Earthman remains the prime host for the parasite. An Earthman may infect one of us if he remains among us. But the parasite, once it is located in an intelligence of the outer worlds, must somehow return to an Earthman, if it expects to reproduce. Before interstellar travel, this was possible only by a re-passage of space and therefore the incidence of infection remained infinitesimal. Now we are infected and reinfected as the parasites return to Earth and come back to us via the mind of Earthmen who travel through space."
Rose said faintly, "And the missing persons - "
"Are the intermediate hosts. The exact process by which it is done, I, of course, do not know. The masculine terrestrial mind seems better suited for their purposes. You'll remember that at the Institute I was told that the life expectancy of the average human male is three years less than that of the average female. Once reproduction has been taken care of, the infested male leaves, by spaceship, for the outer worlds. He disappears."