"Let us recapitulate for a moment some of the fundamental principles of evolution," he said. "How do different animal species arise? Well, we know that variations of a given species arise from mutations caused by various agencies. It follows from elementary genetics that in a freely mixing and interbreeding population, any new characteristic will tend to be diluted, and will disappear within relatively few generations. However"—the professor's tone became deadly serious—"when sections of the population became reproductively isolated from one another—for example, by geographical separation, by segregation of behavior patterns, or by seasonal differences, say, in mating times—dilution through interbreeding will be prevented. When a new characteristic appears within an isolated group, it will be confined to and reinforced within that group; thus, generation by generation, the group will diverge from the other group or groups from which it has been isolated. Finally a new species will establish itself. This principle is fundamental to the whole idea of evolution: Given isolation, divergence will occur. The origins of all species on Earth can be traced back to the existence at some time of some mechanism or other of isolation between variations within a single species. The animal life peculiar to Australia and South America, for instance, demonstrates how rapidly divergence takes effect even when isolation has existed only for a short time.

"Now we seem to be satisfied that for the best part of twenty-five million years, two groups of terrestrial animals—one on Earth, the other on Minerva—were left to evolve in complete isolation. As a scientist who accepts fully the validity of the principle I have just outlined, I have no hesitation in saying that divergence between these two groups must have taken place. That, of course, applies equally to the primate lines that were represented on both planets."

He stopped and stood looking from one to the other of his colleagues, giving them time to think and waiting for a reaction. The reaction came from the far end of the room.

"Yes, now I see what you're saying," somebody said. "But why speculate? What's the point in saying they should have diverged, when it's clear that they didn't?"

Danchekker beamed and showed his teeth. "What makes you say they didn't?" he challenged.

The questioner raised his arms in appeal. "What my two eyes tell me—I can see they didn't."

"What do you see?"

"I see humans. I see Lunarians. They're the same. So, they didn't diverge."

"Didn't they?" Danchekker's voice cut the air like a whiplash. "Or are you making the same unconscious assumption that everyone else has made? Let me go over the facts once again, purely from an objective point of view. I'll simply list the things we observe and make no assumptions, conscious or otherwise, about how they fit in with what we think we already know.

"First: The two populations were isolated. Fact.