That was Bernal all over, Matthew thought. He had always been a lateral thinker, ceaselessly trying to find increasingly odd angles from which to approach intractable problems. He was—had been—exactly the kind of man to think it desirable to make an odyssey into alien territory in a vehicle that was “in keeping with local traditions.” Bernal had not recorded any of this in his notepad—but it was exactly the kind of mental exercise that was difficult to commit to text, even as a series of doodles. Bernal must have spent the last few months of his life trying to figure out what analogues of “upstream” and “downstream” the local ecosystems possessed, whose subtle effects favored versatility in so many of the local organisms.

“Instead of seasons,” Matthew murmured.

“What?” Lynn queried.

“Just a stray thought,” Matthew said, slowly. He had to take a deep breath before carrying on, but talking was a lot less energy-expensive than climbing and he certainly didn’t want to move on too quickly. “On Earth,” he said, pensively, “the versatility of organisms is mostly a series of responses to seasonal variations. In winter, deciduous trees shed their leaves and some vertebrates hibernate. Most flowering plants and most invertebrate imagos die, leaving their seeds and eggs to withstand the cold spell. Large numbers of species opt for an annual life cycle, because the year-on-year advantages gained thereby far outweigh the problems raised by occasional disruptive ecocatastrophes. There are seasons even in the tropics—dry and rainy—generated by ocean currents.”

“Not here,” Lynn told him, although he’d already noted the fact. “Tyre’s axial tilt is less pronounced, and the ocean is as stable as the atmosphere. That constancy seems to be reflected in the relative lack of biodiversity—and, of course, in the dearth of species with dramatic life cycles, like metamorphic insects. Bernal said it wasn’t quite that simple, though, because of the complicity of ecosystems and their inorganic environment.”

“That’s right,” Matthew agreed. “Ecosystems aren’t helpless prisoners of their inorganic frames. Life manages its own atmosphere; to some extent, it manages its own weather too. The rain that falls on rain forests evaporates from the rain forests in a disciplined fashion—take away the forest and the rain goes too. Here, where the world’s axial tilt is less, seasonal variations would be less extreme anyway, but the ecosphere may well play an active role massaging them into near-uniformity, thus nullifying the kinds of advantages insects and other ephemerae derive from their chimerical life cycles. It’s easy enough to grasp the fact that there’s a whole new ballgame here, with a very different set of constraints and strategic opportunities—but it’s not easy to imagine what they might be. Take winter and summer out of the equation, and what might substitute for them as forces of variation? Is there another kind of cycle, or something much more arbitrary? If there is a cycle, it might take a lot longer than three years to work through—and if there isn’t … how often, and how swiftly, do major changes happen? Confusing as it is, thiscan’t be the whole picture.”

As he voiced the last sentence Matthew drew a wide arc with his right arm, taking in the limited panorama spread out before them and a much greater one whose horizons they were not yet in a position to see.

“Yeah,” said Lynn, quietly. “That’s exactlywhat Bernal sounded like, when he got going. Did you really know him that well, or is it a case of great minds thinking alike?”

“We were two peas in a pod,” Matthew told her, his gaze lingering for a moment longer on the visible fragment of the distant boat. Then he turned away, saying: “Okay, I’m rested. Onward and upward.”

Having visited several of the ancient walled cities of Earth, Matthew had a reasonably good idea of the way in which the scale of cities had shifted with the centuries. His memory retained a particularly graphic image of the Old City of Jerusalem surrounded by its vast sprawl of twentieth-century concrete suburbs. He was not unduly surprised, therefore, to find that what had apparently been the living space of the aliens’ city was mostly compressed into an area not much more than a couple of kilometers square—although the shape of the hills meant that it was anything but square, and only vaguely round.

Like ancient Rome, the city seemed to have been built on seven hills, although the hills were very various in size and reach. Lynn was guiding him toward the summit of the highest of them all. His limbs felt like lead, and he was glad that Rand Blackstone was not present to witness his weakness.

Had they not been walking through the relics of ancient streets the surrounding territory would have yielded much more to Matthew’s enquiring eyes, but they always seemed to be closely surrounded by huge hedgerows that stopped him seeing anything at all except multitudes of purple flaps, fans, spikes, and florets. Eventually, though, they began to climb something that looked like—and presumably was—a flight of ancient steps. It took them to the top of a lumpen mount that must once have been a building of some kind.

Matthew was exceedingly glad to reach the top. He mopped his brow with the back of his right hand, awkwardly conscious of the fact that both the hand and the moist forehead were intangibly encased in false skin.

The sun was high in the sky now, and its glare was uninterrupted by clouds. Although he knew that the faint purpling of the blue sky had nothing to do with ultraviolet light, Matthew could not help feeling that the alien light might somehow be dangerous, and Blackstone’s wide-brimmed hat suddenly seemed far less ridiculous than it had the previous day. But Lynn had no hat, and no hair either, so he was probably being oversensitive.

From the summit of the mound, the extensive vistas surrounding him seemed quite different from the limited ones accessible from the lower vantage point.

Unpracticed as they were, Matthew’s eyes were suddenly able to pick out the lines inscribed upon the landscape long ago by artificers’ hands, and not yet completely obscured by the patient work of nature. From here, he could see enough of the undulations imposed on the vegetation by ancient walls to comprehend the unobliterated pattern.

The most astonishing thing of all, now that he could judge it properly, was the sheer extent of the walls appended to the city. They covered an area at least twelve times as vast. The residential part of the city—“downtown,” as Matthew could not help calling it in the privacy of his thoughts—was by no means at the center of the complex, most of which was downslope of it. If the whole resembled a falling teardrop, gradually spreading over a landscape of tiny freckles and follicles, “downtown” would have been fairly near to the trailing edge.

“It’s not obvious from anywhere else,” Lynn told him, “but from up here you can see how the pattern must have developed. They moved gradually outward from the primary rampart, preferring downhill to uphill, gradually clearing more ground and surrounding the fields they’d created with new walls. The city itself continued to grow, mostly in upslope directions, so that some of the fields were built over, and there were subsidiary islands of residential building way out there to the east and south, but most of the development as the population swelled seems to have been a matter of building higher and filling in. As they cleared more land for crops, though, they built more walls: rank after rank after rank. The innermost walls are the lowest, perhaps because they routinely cannibalized them to help in the building of the outer ones, even though their quarrying techniques had come on by leaps and bounds. You can see a couple of their biggest quarries way over there in the northwest.”

“How did they move the blocks?” Matthew asked, still feeling distinctly breathless.