But lice aren’t utterly doomed. When people started wearing clothing, some lice evolved to take advantage of the new situation. They developed claws that are adapted for clinging to fabric instead of hair. So these days, there are two species of human lice: hair-loving head lice and clothes-loving body lice.

Evolution marches on. Maybe one day we’ll have space suit lice.

So what does this have to do with the invention of clothing?

Not long ago, scientists compared the DNA of three kinds of lice: head lice, body lice, and the old original chimp lice. As time passes, DNA changes at a fixed rate, so scientists can tell roughly how long ago any two species split up from each other. Comparing lice DNA soon settled the question of what came first—the clothes or the nakedness.

Here’s how it happened:

Human lice and monkey lice split off from each other about 1.8 million years ago. That’s when ancient humans lost their body hair and the lice we inherited from the chimps had to adapt, evolving to stick to our heads.

But head lice and body lice didn’t split until seventy-two thousand years ago, an eternity later (especially in lice years). That’s when human beings invented clothes, and body lice evolved to reclaim some of their lost real estate. They got brand-new claws and spread down into our brand-new clothing.

So that’s the answer: Clothes got invented after we lost our body hair. And not right away; our primate ancestors ran around naked and hairless for well over a million years.

That part of human evolution is written in lousy history, in the genes of the things that suck our blood.

Just as I finished Freddie’s apartment (finding no glimmer of bodily fluids), my phone buzzed. One of the Shrink’s minders was on the other end, saying that she wanted to see me again. My stack of forms had returned from Records, chock-full of enough intrigue to bounce all the way up to the Shrink. That was always a sign of progress.

Still, I sometimes wished she would just talk to me on the phone and not insist on quite so much face time. But she’s so old-school that telephones just aren’t her thing. In fact, electricity isn’t her thing.

I wonder if I’ll ever get that ancient.

I took the subway down to Wall Street, then walked across. The Shrink’s house is on a crooked alley paved with cobblestones, barely one car wide. It’s one of those New Amsterdam originals that the Dutch laid down four centuries ago, running on the diagonal, flouting the grid in the same grumpy way that the Shrink ignores telephones. Those early streets possess their own logic; they were built atop the age-old hunting trails of the Manhattan Indians. Of course, the Indians were only following even more ancient paths created by deer.

And who were the deer copying? I wondered. Maybe my route had first been cut through the primeval forest by a line of hungry ants.

One thing about carrying the parasite—it makes you feel connected to the past. As a peep, I’m a blood brother to every other parasite-positive throughout the ages. There’s an unbroken chain of biting, scratching, unprotected sex, rat reservoirs, and various other forms of fluid-sharing between me and that original slavering maniac, the poor human who was first infected with the disease.

So where did he or she get it from? you may ask. From elsewhere in the animal kingdom. Most parasites leap to humanity from other species. Of course, it was a long time ago, so the original parasite-positive wasn’t exactly what we’d call human. More likely the first peep was some early Cro-Magnon who was bitten by a dire wolf or giant sloth or saber-toothed weasel.

I kicked a bag of garbage next to the Shrink’s stoop and heard the skittering of tiny claws inside it. A few little faces peeked out to glare at me; then one rat jumped free and scampered a few yards down the alley, disappearing down a hole among the cobblestones.

There are more of those holes than you’d think.

When I first came to the city, I saw only street level, or sometimes caught glimpses of the netherworld through exhaust grates or down empty subway tracks. But in the Night Watch we see the city in layers. We feel the sewers and the hollow sidewalks carrying electrical cables and steam pipes, and below that the older spaces: the basements of fallen buildings, the giant buried caskets of abandoned breweries, the ancient septic tanks, the forgotten graveyards. And, struggling to get free underneath, the old streambeds and natural springs—all those pockets where rats, and much bigger things, can thrive.

Dr. Rat says that the only creatures that ever come out onto the surface are the weak ones, the punks who aren’t competitive enough to feed themselves down where it’s safe. The really big things, the rat kings and the other alpha beasties, live and die without ever troubling the daylight world. Think about that for a second: There are creatures down there who’ve never seen a human being.

The laden sky rumbled overhead, and I smelled rain.

History. Nature. Weather. My head was pounding, full of those big, abstract words that have their own cable channels.

But it was the sound of those tiny scratching feet inside the garbage bag that followed me into the Shrink’s house and down the corridor to her session room, pushed along by an invisible wind.

“Most impressive, Cal.” She leafed through papers on her desk. “All it took was a few drinks to get you back to Morgan’s house.”

“Yeah. But it was an apartment, Dr. Prolix. Not many houses in Manhattan these days, you might have noticed.”

The guy from Records in the other visitor’s chair raised his eyebrows at my tone, but the Shrink only folded her hands and smiled. “Still glum? But you’re making such progress.”

I chewed my lip. The Shrink didn’t need to know what I was bummed about. Not that it mattered anymore, the whole stupid way I’d had Lace help me. Even if she’d gone on believing me, hanging out with her would have just gotten more and more torturous.

Worse than that, it had been dangerous. Lace hadn’t showed a bit of interest—not that kind of interest anyway—and I’d still come close to kissing her.

Never again. Lesson learned. Move on. I was back in lone-hunter mode now.

“Yeah, gallons of progress,” I said. “You saw what I found on the wall?”

“I read your 1158-S from this morning, yes.”

“Well, I went back there today but didn’t find anything more on the creepy graffiti front. Or much else. Morgan moved out at least seven months, ago. Not exactly an oven-fresh trail.”

“Cal, eight months is the blink of an eye for Records. To find out where Morgan has gone, perhaps we should look at where she came from.”

“What do you mean?”

“The history of that property has proven interesting.” She turned to the Records guy and waved her pale hand.

“When the landlords in question filed their initial rent-control forms,” he began, “there were four residents on the seventh floor.” His voice quivered slightly, and once or twice as he read, his eyes darted up to the creepy dolls, confirming that he wasn’t comfortable in the Shrink’s office. Not a hunter, just an average working stiff with a city job. His chair was backed as far away from the red line as it could go. No typhoid germs for him. “We ran the names of these individuals through the city databases and hit a missing persons report from March this year.”

“Only one?” I asked. “I figured they’d all be missing.”

He shook his head. “More than one missing person from the same address, and we would have already filed an MP-2068 with you guys. But there was only one hit. NYPD has no leads, and at this point it’s pretty much a dead investigation.”