“Retrograde movement in time is impossible,” Auruthmer said. Murther’s transmitted voice made an objection which reeked of scholarly pedantry, even across the species barrier, and so she corrected herself: “Save in one very specific circumstance. Even then, we can’t travel back in time, as in us individuals.”

Kay filed that away for further questioning. “So what the fuck is this?”

“It’s a world where this form of life never went away,” Alison said. She had a faraway look in her eyes, and Kay wasn’t sure if she was interfacing with the Posticthyan machine or if this was just general wonder.

Murther said something that sounded disparaging, and Auruthmer translated, probably diplomatically, “Unlikely. That would imply stagnation for half a billion years. In every branch that we’ve visited, a different scenario has played out, evolutionarily. This has resulted in a different biota, differing types of sentience, but still change. The world does not stand still.”

The land that time forgot, Kay thought, and then her imagination sank its teeth into the thought and worried it for a while, because there was something in it. “You’re right,” she said slowly. Murther’s sharp reply, untranslated, was probably to confirm that, yes, she knew she was right, but Kay wasn’t finished.

“Life succeeds life—that’s how evolution goes. And evolution must therefore have produced what we are seeing below—developed it from a microscopic ancestor to the macroscopic. All these weird squiggly shapes, they must have evolved to do a thing, and then they ought to keep evolving to do it better. Because the best thing-doers have more offspring, survival, fittest, all that. That’s just nature’s way, right?”

She looked from face to face, and at last to Cam’s false features—currently a stand-in for Murther, aboard one of the other trilobites.