The Science of Discworld Revised Edition

this sort of thing lately!’ snapped the Dean. He waved his finger at one of the creatures, who watched it carefully in case it turned out to be a fish.
    ‘What would your ancestors say, my friend, if they saw you rushing into the water just because times are a bit tough on land?’ he said.
    ‘Er … ‘“Welcome back”?’ suggested Rincewind, trying to avoid the snapping jaws.
    ‘’Long time no sea’?’ said the Senior Wrangler, cheerfully.
    The creature begged, uncertainly.
    ‘Oh, go on, if you must,’ said the Dean. ‘Fish, fish, fish … you’ll turn into a fish one of these days!’
    ‘Y’know, going back to the sea might not be a bad idea,’ said Ridcully, as they strolled away along the beach. ‘Beaches are edges. You always get interestin’ stuff on the edge. Look at those lizards we saw on the islands. Their world was
all
edges.’
    ‘Yes, but giving up the land to just go swimming around in the water? I don’t call
that
evolution.’
    ‘But if you go on land where you have to grow a decent brain and some cunning and a bit of muscle in order to get anything done, and then you go back to see the sea where the fish have never had to think about anything very much, you could really, er, kick butt.’
    ‘
Do
fish have –?’
    ‘All right, all right. I meant, in a manner of speaking. It was just a thought, anyway.’ Uncharacteristically, the Archchancellor frowned.
    ‘Back to the sea,’ he said. ‘Well, you can’t blame them.’

FORTY
MAMMALS ON THE MAKE

    AFTER THE DINOSAURS came the mammals –
    Not exactly.
    Mammals constitute the most obvious class of animal alive on Earth today. When we say ‘animal’ in ordinary conversation, we’re mostly referring to mammals – cats, dogs, elephants, cows, mice, rabbits, whatever. There are about 4,000 species of mammals, and they are astonishingly diverse in shape, size, and behaviour. The largest mammal, the blue whale, lives in the ocean and looks like a fish but isn’t; it can weigh 150 tons (136,000 kg). The smallest mammals, various species of shrew, live in holes in the ground and weigh about half an ounce (15 g). Roughly in the middle come humans which, paradoxically, have specialized in being generalists. We are the most intelligent of the mammals – sometimes.
    The main distinguishing feature of mammals is that when they are young their mother feeds them on milk, produced by special glands. Other features that (nearly) all mammals have in common include their ears, specifically the three tiny bones in the middle ear known as the anvil, stirrup, and hammer, which send sound to the eardrum; hair (except on adult whales); and the diaphragm, which separates the heart and lungs from the rest of the internal organs. Virtually all mammals bear live young: the exceptions are the duckbilled platypus and the echidna, which lay eggs. Another curious feature is that mammalian red blood cells lack a nucleus, whereas the red cells of all other vertebrates possess a nucleus. All is this is evidence for a lengthy common evolutionary history, subject to a few unusual events of which the most significant was the early separation of Australia from the rest of Gondwanaland, the southern half of the original supercontinent Pangaea. Modern studies of mammalian DNA confirm that basically we are all one big happy family .
    When the dinosaurs died out, the mammals had a field day. Released from dinosaurian thrall, they could occupy environmental niches that, only a few million years before, would merely have presented a dinosaur with an easy meal. It seems likely that the current diversity of mammals has a lot to do with the suddenness with which they came into their kingdom – for a while, almost any lifestyle was good enough to make a living. However, it would be wrong to imagine that the mammals came into existence to fill the gaps left by the vanished dinosaurs. Mammals had coexisted with dinosaurs for at least 150 million years.
    Harry Jerison has suggested that before the dinosaurs became really dominant, many mammals were able to make their living in daylight, and they evolved good eyesight to do so. As the dinosaurs became a bigger and bigger problem, the mammals adopted a lower profile, mostly staying hidden undergound during the day. If you’re a nocturnal animal, you rely on a really good sense of hearing, so evolutionary pressures then equipped the mammals with excellent ears – including those three little bones. However, they