Science of Discworld III

regulatory system, ‘text messages’ that let the placenta use ERV-3 without running the risk of setting the corresponding virus loose.
    At any rate, whatever the introns are for, warm-bloodedness is not the only trick that mammalian evolution managed to find and exploit.It also indulged in wholesale theft of a virus’s genome, to stop mother’s immune system booting out baby because it ‘smelled’ of father. And we also get another lesson that DNA isn’t selfish . ERV-3 is present in the human genome, but not because it’s a bit of junk that gets copied along with everything else and has remained because it does no harm. It’s there because, in a very real sense, humans could not survive – could not even reproduce – without it.
    1 They made exceptions for manifestly ‘unimportant’ but very diverse sets of alleles like blood groups, but in those cases it didn’t seem to matter much which kind you had.
    2 That’s very important for a few species. Zebra-fish eggs in the wild must hatch in just under 72 hours, because they’re laid just before dawn and must hide before the third dawn when predators could see them.
    3 Sorry about the proliferation of barnyard metaphors.

TWENTY-ONE
NOUGAT SURPRISE
    T HE ACTIVITY IN THE G REAT H ALL was slowing down now. All along the rainbow lines of time, the nodes were closed. Or shut, or denoded, Rincewind thought. Whatever you did with nodes, anyway. There was a little cheer as the last glowing wizard symbol faded away, and a roar from outside as three wizards and a lot of tentacles landed in the fountain. Rincewind had been surprised about that, and then dismayed. Since it did not have his name on it, this meant that Ridcully had something worse in mind.
    ‘Looks like I’m not needed now,’ he said hopefully, just in case.
    ‘Haha, professor, what a card you are and no mistake,’ said the lobster next to him. ‘The Archchancellor was very definite that we was to keep you here no matter what you said.’
    ‘But I wasn’t running away!’
    ‘No, you was only inspecting the wall with the loose bricks in it,’ said the lobster. 1 We quite understand. Lucky for you we caught you before you dropped over into the alley, eh? Could’ve done yerself a mischief.’
    Rincewind sighed. The lobsters were always hard to outrun. They hunted in packs, appeared to share a common brain, and many yearsof harrying erring students had given them a malignant street cunning that verged on the supernatural.
    Some of the senior wizards swept in … There was an argument going on between Ridcully and the Dean.
    ‘I don’t see why I shouldn’t.’
    ‘Because you get too excited in the presence of combat, Dean. You run around making silly “hut, hut” noises,’ said Ridcully. ‘Remember why we had to stop those paintball afternoons? You didn’t seem to be able to get the hang of the term “people on my side”.’
    ‘Yes, but this is—’
    ‘It took us a week to get the Senior Wrangler looking halfway suitable for polite company – Ah, Rincewind. Still with us, I see. Good man. Very well, Mr Stibbons: report!’
    Ponder coughed.
    ‘Hex confirms that, er, that our recent activities may have left camiloops between our word and Roundworld, sir. That is to say, residual connections that may be used deliberately or inadvertently from either side. Magic doors, in fact, drifting without anchor. These will evaporate within a matter of days. Um …’
    ‘I don’t want to hear “um”, Mr Stibbons. “Um” is not a word we entertain here.’
    ‘Well, the fact is that since the camiloops are spread over centuries, the Auditors may very well have been in Roundworld for some time. We have no way of knowing for how long. Hex, um, sorry, does report some circumstantial evidence that humans are dimly aware of their malign meddling, albeit at a very mundane level as evidenced by the findings of a researcher called Murphy. Roundworld would be difficult for them. They would be bewildered. Things would not work the way they expect. They are not flexible thinkers.’
    ‘They were able to mess around with Mr Darwin’s voyage!’
    ‘By doing lots of small and rather stupid things at great effort, sir. They don’t react well to adversity. They get petulant. Fromwhat Professor Rincewind tells us, many hundreds of them have to combine to perform even a simple physical action.’
    He stood back and indicated some items laid out on a dining table.
    ‘There is some evidence that