'Yeah, OK, because by then we'd killed off most of the others.'

'Well, yes, but anyway. There are so many of us and increasing all the time that we're getting instances of reproductive isolation by chance alone.'

I shook my head. 'That can't be right. I mean, if various populations had remained separate, like ... I don't know ... suppose we a the Europeans, I mean a had never discovered the Americas, or Australia or something, and people had been isolated on separate continents for millions of years, heck even if none of the descendants of those who left had ever gone back to Africa after the migration and no more Africans had ever come out, maybe what people used to call races might have become species. But nothing like that ever happened, and it would have taken millions of years anyway, and people now are ... um ... interbreeding like never before, so-'

'This has nothing to do with races,' she said, almost spitting the word. 'And it wouldn't have taken millions of years, by the way, there's ten million years between us and the chimps and what is it, I think the latest count is thirty-two distinct hominid species emerged in that time, six or seven in the last million years alone, so ... Anyway, that's all beside the point because what I'm talking about isn't like that, except it does involve the same mechanism, it's just come about in a quite different way. The mechanism is reproductive isolation a groups that don't breed together, even if it's just because there's a mountain range between them or whatever, gradually become unable to breed together. What's happening now is that because there are so many of us, by chance alone you're going to find lineages of people whose last common ancestor was way, way back, time enough for mutations to build up and make them reproductively incompatible. It's completely invisible until a until two people from these lineages come together, and, and ...'

She spread her hands, then put them to her face.

I didn't move.

'This is what you think?'

'It's not just what I think,' she said, slapping her hands down to her knees. 'It's a fact, though as I said it's ... not widely talked about.'

I knew this was true. I didn't want to admit it. 'Even if it could happen to humans, which I doubt, it can't be what's happened to us. Come on, we're both Scottish, we're all Jock Tamson's bairns-'

'Oh, Christ! How many times have I heard that stupid phrase?'

'OK, OK, it's a cliche but that's because it's true,' I said. 'Most of us in these islands are descended from people who came into Europe when the ice went.' I tried a disarming smile. 'Or maybe before, in some cases.'

Gabrielle moved a hand as if swatting at a fly. 'As I said, that's true, but it's irrelevant. I'm talking about a chance thing a in effect it's long chains of genetic coincidence like those long chains of premises in pork chop logic, but with enough people it becomes statistically inevitable.'

'Well how come,' I said, 'the problem hasn't come up before?'

She looked at me as if I were stupid. 'Who says it hasn't? Infertility has been around since forever, and it seems to be increasing, or maybe it only seems to be because it's now seen as a medical problem that can be fixed rather than as a secret shame or a joke or the will of God. Cases of people who were infertile with each other but fertile with other partners ... well, it's not new or unheard of, put it that way.'

I didn't want to think about the personal implications. I certainly didn't want to talk about them.

'So why isn't this common knowledge?'

'Like I said a the can of worms thing: race and all that rubbish. And of course for quite a lot of people it doesn't matter because they don't want to have kids anyway.'

Now she'd brought it back to where I didn't want to go. Back where she'd started, in fact. Our problem is that we're different species ...

'But it matters to you.'

'Yes.' She paused and added, 'It matters to us.'

'Oh, fuck,' I said. I refilled our glasses. 'You know what our real problem is?'

'Drinking too much?' said Gabrielle, raising her glass.

'There's that,' I allowed. 'But no. That's a symptom. Our real problem, our problem, right, is that' a I drew a deep, ragged breath a 'you're too focused on having kids, when really we have so much else.'

'Like what?'

'Each other. Our jobs.' I waved around at the room. 'This.'

'Yes, this,' she said.

'What does that mean?'

'You're a lazy bugger.'

'What? I work hard, I do my share in the flat.'

'You work hard? Hah! You're coasting. Riding the up wave. Recycling press releases for any company whose new shiny you've spotted on your morning trawl. Hell, you don't even need to trawl, you have spider apps to do that for you.'

'Who cares?' I cried, stung. 'It brings in the dosh.'

'Yeah, for now. And enough for a cosy little couple flat. That's what I mean by lazy. If you worked at something where you actually used your abilities, we could afford a proper house.'

'Meaning,' I said, 'one big enough to have kids?'

'Yes, dammit! Yes! I sometimes think you don't want us to have kids. You certainly don't act like you're expecting to have to cope with having kids any time soon.'

'Oh, for fuck's sake!'

'And meanwhile, I'm working my arse off for my PhD, and scraping a bit on teaching and side jobs, all so I can earn more in the future, whereas you're just tarting your knack for glib writing around every high-tech fly-by-night cowboy company that needs its Japanese-robot-written hype turned into proper English. How long can you keep doing that?'

'As long as it takes,' I said.

'No,' said Gabrielle. 'You're a hack, and a flack, and there'll always be younger and hungrier hacks after that sort of job. You're stuck in a fucking comfort zone where you think because right now you earn more than me, and that's enough to furnish a wee poky flat in fucking Leith, that's enough to be going on with. Well as far as I'm concerned it fucking isn't.'

'Is that what this is about?' I said. 'Money? Location, location, location?'

'No, it isn't. It's not just that. You're not contributing all you could.'

'All I could to what? All my money goes on this place and on us.'

'I'm not talking about contributing money. You're not contributing to the cause.'

'The cause?' I was puzzled, then I remembered. 'Oh, science.'

'Yes, science! The war of knowledge against ignorance, all that, remember?'

'Yes, I remember,' I said. 'And I'm right in it. OK, it's a war and you're a soldier on the front line.' I spread my arms. 'And yes, I admit it, I put my hands up to that one, I'm just a hack propagandist, a war correspondent at best. But come on, be fair. We can't all be scientists, just like we can't all be soldiers. Us hacks and flacks have their place, keeping up morale, entertaining the troops-'

'Yeah,' she said. 'That's you, all right. You're like those big-mouthed pro-war singers and actors who enlisted a oh, how brave of them, knowing full well they'd get sent straight to the rear.'

'That's so fucking unfair,' I said. 'I'm doing what I do best.' I shrugged. 'Sorry that's not good enough.'

'Oh, take your self-pity and stuff it up your arse! Nobody's asking you to be a scientist-'

'I thought you just had.'

'That's because you weren't listening. What I'm saying is, you're not using the abilities you do have. I know fine well they don't run to doing actual science, though to be honest I'm not sure that isn't another of your lazy excuses for not doing the work when you were at school. OK, so you ended up doing English. You weren't even willing to break out a bit and do Divinity as an open atheist. Now that would have been smart, that would have been a unique selling point in the fucking intellectual labour market. But OK, you get English first-class honours, whoop-de-doo and bully for you. You know where most people with that kind of degree are working? In business and in government, that's where, not in some fucking ivory tower and especially not in fucking dead-end freelance hackwork that any junior reporter could do if they'd paid attention in science class.'

'Well, exactly,' I said. 'They didn't and I did, and that's what gives me an edge, that's what gets me commissions.'

'Yeah, dribs and drabs, which add up to more than I get at the moment but three or four times less than what other people your age with degrees are getting, and five times less than what I'll earn when I get my PhD.'

'Well, I don't see myself doing well in management.'

'Don't say it like it's beneath you, your dad's in management and so's mine.'

'A lot of good that did them in the depression!'

'Yes, as it happens it did do them a lot of good! They kept their jobs! The point is, you won't be earning any more in five years, ten years than you're earning now, if you're lucky enough to earn that.'

'Hey, come on, I'm building up a good rep in science journalism.'

'Don't you see?' she said. 'That's only a seller's market at the start of a boom with lots of new tech coming on stream. What about when all that shit is routine?'

She mimed covering a yawn, which set me off on a real one and wishing I could just go to bed.

'Christ, by the time that happens I'll have written a fucking book. Stop worrying.'

'I can't stop worrying. In fact I wouldn't be surprised if it's the worry that's making it difficult for me to conceive.'

'Oh, so all that stuff about separate species was just bullshit, was it?'

'No, it fucking wasn't, but the worry and the tension aren't helping at all, and neither is your attitude.'

'My attitude? Fuck's sake, if I'd known-'

'Known what?'

I didn't heed the warning in her voice. 'Known that you're so obsessed with money.'

'I'm not obsessed with money,' Gabrielle said, 'and I know what you were about to say. Well, if I'd known you were so lazy and inconsiderate, I might not have either.'

'It's not just money,' I said, plunging on. 'It's this thing about kids. I mean, can't we just accept that maybe it's not going to happen for us, at least not without-'

'I am so not going through IVF.'

'Oh, me neither, that was my point, but look, what I mean is can't we just accept it then? That there's more to life than having kids? What I said, before we got off on this about money a and you're right, I could look for something more stable and secure, sure, there's loads of jobs. I could find a better job. I'm happy to do that now I know it's worrying you. I'm sorry I was so dismissive about it just now, yeah, I mean I take your point. But the other thing I said, I meant it. We have each other. We love each other. Isn't that what matters?'

As I said that I leaned towards her, and reached out a hand. She swiped it aside. The gesture seemed to me far more contemptuous and dismissive than Gabrielle perhaps intended. Looking back at it, replaying it in slow motion again and again, all I see is a moment of irritation. It also stung my hand quite hard. In another moment I've revisited many times more than is good for me, I drew that hand sharply back, above my shoulder, poised to swing and hit.

Gabrielle's hand shot forward like a striking snake, grabbing my wrist as she jumped to her feet. Off balance, I rocked back on the sofa. My foot, in helpless reflex, jerked forward, fortunately missing her but kicking the coffee table. Glasses fell and rolled. There was a reek of spilled whisky.

Gabrielle, still gripping my wrist, glared down at me, her face filled with shock and fury.

'You were going to hit me!' she cried, as if she could hardly believe it. 'You tried to kick me!'

'No, no,' I pleaded. 'I was just a I pulled my hand back.'

'You raised your hand to me,' she said, in a duller voice, as if coming to terms with something terrible. 'And then you lashed out with your foot.'

'That was just-'

'Oh, I know what happened.' She let go of my wrist, and stepped back, and away. Her eyes welled up.

'I used to love you so much,' she said.

She wrenched the ring off and dropped it on the table, where it came to rest in a puddle of whisky.

In the morning we had a tearful reconciliation. It was like an inverted image of the morning after our handfasting in the tent on Orkney. We couldn't believe that it all hadn't been a nightmare; that we had said such wounding things to each other. She surprised me by slipping her ring back on. I had such an epic hangover that I felt my relief and pleasure and renewal of hope only as a diminution of misery, still below zero on the felicific calculus.

That night wasn't what ended it. What ended it was all the other nights like that, before and after. One day in October I came home mid-evening from a trek around the syn bio labs of Aberdeen to find her gone. She had taken all her stuff except some opened scent bottles, seldom-worn jewellery, and used cosmetics from the top of the dressing-table; and below it half a drawer's worth of short nighties which she'd seemed delighted with when I'd bought them for her.

PART FIVE.

20.

'Whole thing sounds like a total bummer,' said Calum, after I'd given him my side of the story a couple of weekends later, over Sunday lunchtime drinks in a quiet corner of a Greenock pub. 'Jeez, man. She seemed so nice.'

'She is nice,' I said. 'More than nice. She's lovely. I'm still in love with her.'

'Yeah, man, I understand. Fuck's sake, man. So what are you doing about it?'

'Apart from trying to contact her so often she's fucking blocked me? And then knocking on her folks' door and getting a severe warning from her old man about the legal penalties for harassment and assault?'

'Assault?'

'Yes,' I said. 'That incident I told you about ...'

'Yeah, but you didn't actually-'

'I was very firmly told that lifting your hand to someone counts as assault. Following through with it and connecting is battery.'

'So she told her da about ...?'

'Aye. No doubt her account is a bit more graphic than mine, and no doubt that'll grow in the telling. Plus, her dad's ... well, he's never been exactly hostile but I can bet he's willing to believe anything bad about me. But honest, Calum, whatever you hear on the family grapevine, I never hit her or even really or intentionally threatened her.'