"Okay. It isn't complete—I need a lot more data—but I'll show you what I have. It's fairly strong medicine and it comes in big chunks."
"It would have to—it covers the whole macrocosmic universe, doesn't it?"
"Yes. I'll start with the striking fact that, on every out-galaxy planet we visited, the human beings were Homo sapiens to N decimal places. Fertile with each other and, according to expert testimony, with us. All planets had humanoid 'guardians,' the Arpalones and Arpales. Some, but not all, had one or more non-human, more-or-less-intelligent races, such as the Fumapties, the Lemarts, the Sencors, and so on. These other races never seemed to fight each other, but both races of Guardians fought any and all of them, on sight and to the death. What do those facts mean to you?"
"Nothing beyond face value. I've thought about them but I haven't been able to come up with anything."
"I have." He unrolled a sheet of drafting paper covered with diagrams, symbols, and equations. "But before I go into this stuff, consider the human body. How many red cells are there in your blood stream?"
"Billions, I suppose."
"And there are billions of human beings on billions of planets; each having red blood cells identical, as far as we know, with yours and mine. Also white cells. Also, sometimes, various kinds of pathogenic micro-organisms, such as staphs, streps, viruses, spiros, and so on.
"Okay. My thought is that the Lemarts, Ozobes, and the like are analogous to disease-producing organisms. We saw the full range of effects—from none at all up to death itself."
"But they—the Ozobes and so on—died, too."