“What? Hmm, I hadn’t even thought about that,” said Arnold.

Ponter was confused. “Reuben said you were a geologist…?”

“Yes, I am,” said Arnold, “but I don’t work for Inco. I’m with Environment Canada. I flew here from Ottawa as soon as word came that contact with your world had been reestablished.”

“Ah,” said Ponter, still not understanding.

“My job is protecting the environment,” said Arnold.

“Is that not everyone’s job?” asked Ponter, being, he knew, a bit disingenuous.

But again the subtlety was lost on Arnold. “Yes, indeed,” he said. “Yes, indeed. But I wanted to find out what your people might know about environmental effects associated with magnetic-field collapses. I was hoping you might have some data from the fossil record—but to have complete studies of a recent collapse! That’s fabulous.”

“There were no appreciable environmental effects,” said Ponter. “Some migratory birds were confused, but that was about it.”

“I suppose they would be, at that,” said Arnold. “How did they adapt?”

“The affected birds have a powerfully magnetic substance in their brains…”

“Magnetite,” supplied Arnold. “Lodestone. Three iron atoms and four oxygens.”

“Yes,” said Ponter. “Other kinds of birds navigate by the stars, and some individuals of the species that use brain magnetite for determining direction turned out to be able to use the stars, too. It is ever the way in nature: variation within a population provides vigor when the environment changes, and most crucial capabilities have a backup system.”

“Fascinating,” said Arnold. “Fascinating. Tell me, though: how did you originally determine that Earth’s magnetic field does, in fact, periodically reverse? That’s a fairly new insight for us.”

“The alternation of the planet’s magnetic-field polarity is recorded at meteor-impact sites.”

“It is?” said Arnold, his one long eyebrow—how refreshing to see someone who looked normal, at least in that regard!—rising up his forehead.

“Yes,” said Ponter. “When an iron-nickel meteor slams into the Earth, the impact aligns the meteor’s magnetic field.”

Arnold frowned. “I suppose it would, at that. Just like hitting an iron bar with a hammer and turning it into a magnet.”

“Exactly,” said Ponter. “But if you did not learn of this from meteorites, how did your people come to know that Earth’s magnetic field periodically reverses?”

“Sea-floor spreading,” replied Arnold.

“What?” said Ponter

“Do you know about plate tectonics?” asked Arnold. “You know, continental drift?”

“The continents drift?” said Ponter, making his face agog. But then he held up a hand. “No, that time I was making a joke. Yes, my people know this. After all, the coastlines of Ranilass and Podlar clearly once were attached to each other.”

“You must mean South America and Africa,” said Arnold, nodding. He smiled ruefully. “Yes, you’d think it would be blindingly obvious to everyone, but it took decades for our people to accept the notion.”

“Why?”

Arnold spread his arms. “You’re a scientist; surely you understand. The old guard thought they knew how the world worked, and they weren’t about to give up their theories. As with so many paradigm shifts, it wasn’t really a case of convincing anyone to change their minds. Rather, it was waiting for the previous generation to pass on.”

Ponter tried to conceal his astonishment. What an extraordinary approach to science these Gliksins had!

“In any event,” continued Arnold, “we ultimately found proof for continental drift. At the middle of the oceans there are places where magma wells up from the mantle, forming new rock.”

“We surmised such things must exist,” said Ponter. “After all, since there are places where old rock is pushed down—”

“Subduction zones,” supplied Arnold.

“As you say,” said Ponter. “If there are places where old rocks go down, we knew there must be places where new rock comes up, although, of course, we have never seen them.”

“We’ve taken core samples from them,” said Arnold.

Ponter’s face went honestly agog this time. “In the middle of the oceans?”

“Yes, indeed,” said Arnold, clearly glad for once that his side was coming out ahead. “And if you look at rocks on both sides of the rifts from which magma is welling up, you see symmetrical patterns of magnetism—normal on either side of the rift, reversed equal distances to the left and right of the rift, normal again on either side but farther out, and so on.”

“Impressive,” said Ponter.

“We have our moments,” said Arnold. He grinned, and was clearly inviting Ponter to do the same.

“Sorry?” said Ponter.

“It’s a pun; a play on words. You know: ‘magnetic moment’—the product of the distance between a magnet’s poles and the strength of either pole.”

“Ah,” said Ponter. This Gliksin obsession with word play…he would never understand it.

Arnold looked disappointed. “Anyway,” he said, “I’m surprised that your magnetic field collapsed before ours did. I mean, I understand the Benoît model: that this universe split from your universe forty thousand years ago, at the dawn of consciousness. Fine. But I can’t see how anything your people or mine might have done in the last four hundred centuries could have possibly affected the geodynamo.”

“It is puzzling,” agreed Ponter.

Arnold clambered off his chair and rose to his feet. “Still, because of it, you’ve been able to satisfy my particular concern better than I would have thought possible.”

Ponter nodded. “I am glad. You should indeed—how would you phrase it?—you should sail effortlessly through the period of magnetic-field collapse.” He blinked. “After all, we certainly did.”

Mary tried to concentrate on her work, but her thoughts kept turning to Ponter—not surprisingly, she supposed, since Ponter’s DNA was precisely what she was working on.

Mary cringed every time she read a popular article that tried to explain why mitochondrial DNA is only inherited from the maternal line. The explanation usually given was that only the heads of sperm penetrate eggs, and only the midsections and tails of sperm contain mitochondria. But although it was true that mitochondria were indeed deployed that way in sperm, it wasn’t true that only the head made it into the ovum. Microscopy and DNA analyses both proved that mtDNA from the sperm’s midsection does end up in fertilized mammalian eggs. The truth was no one knew why the paternal mitochondrial DNA isn’t incorporated into the zygote the way maternal mitochondrial DNA is; for some reason it just disappears, and the explanation that it had never gotten in there in the first place was nice and pat, but absolutely not true.

Still, since there were thousands of mitochondria in each cell, and only one nucleus, it was much easier to recover mitochondrial rather than nuclear DNA from ancient specimens. No nuclear DNA had ever been extracted from any of the Neanderthal fossils known from Mary’s Earth, and so Mary had been concentrating on studying Ponter’s mitochondrial DNA, comparing and contrasting it with Gliksin mtDNA. But there didn’t seem to be any one sequence she could point to that was present in Ponter and the known fossil Neanderthal mitochondrial DNA, but in none of the Gliksins, or vice versa.

And so Mary at last turned her attention to Ponter’s nuclear DNA. She’d thought it would be even more difficult to find a difference there, and indeed, despite much searching, she hadn’t found any sequence of nucleotides that was reliably different between Neanderthals and Homo sapiens sapiens; all her primers matched strings on DNA from both kinds of humans.

Bored and frustrated, waiting for Ponter to be released from quarantine, waiting to renew their friendship, Mary decided to make a karyotype of Neanderthal DNA. That meant culturing some of Ponter’s cells to the point where they were about to divide (since that’s the only time that chromosomes become visible), then exposing them to colchicine to immobilize the chromosomes at that stage. Once that was done, Mary stained the cells—the word “chromosomes,” after all, meant “colored bodies,” referring to their tendency to easily pick up dye. She then sorted the chromosomes in descending order of size, which was the usual sequence for numbering them. Ponter was male, and so had both an X and a Y chromosome, and, just as in a male of Mary’s kind, the Y was only about one-third the size of the X.