The Ancestor's Tale Part 10

In the Redwood's Tale, we shall meet the ingenious dating technique known as dendrochronology. Tree rings result from the fact that trees have an annual growing season, and not all years are equally favourable, so a signature pattern of thick and thin rings develops. Such fingerprint signatures, when they occasionally arise in nature, are a natural gift to science, to be seized eagerly whenever we encounter them. It is a particularly fortunate fact that something like tree rings, although on a larger timescale, is imprinted into volcanic lava as it cools and solidifies. It works like this. While lava is still liquid, molecules within it behave like tiny compa.s.s needles, and become aligned to the magnetism of the Earth. When the lava solidifies as rock, the compa.s.s needles are petrified in their current position. Igneous rock therefore acts as a weak magnet, whose polarity is a frozen record of the Earth's magnetic field at the moment of solidification. This polarity, which is easy to measure, tells us the direction of the magnetic North Pole at the moment when the rock solidified.

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Magnetic stripes on either side of an oceanic ridge. Dark stripes represent normal polarity; white, reverse polarity. Geologists group these into magnectic intervals dominated by normal or reverse polarity. The symmetry of the stripes was first identified as evidence for seafloor spreading by Fred Vine and Drummond Matthews in a cla.s.sic 1963 paper in Dark stripes represent normal polarity; white, reverse polarity. Geologists group these into magnectic intervals dominated by normal or reverse polarity. The symmetry of the stripes was first identified as evidence for seafloor spreading by Fred Vine and Drummond Matthews in a cla.s.sic 1963 paper in Nature Nature [ [296]. The crust and rigid top layer of the mantle, together known as the lithosphere, is pushed apart by convection currents in magma in the semi-rigid layer of the mantle below (the asthenosphere). The distinctive pattern of stripes allows us to identify the age of rocks on the sea bed back to about 150 million years ago. Older seafloor has been destroyed by subduction.

Now comes the fortunate fact. The polarity of the Earth's magnetic field reverses at irregular but, by geological standards, quite frequent intervals, on a timescale of tens, or hundreds, of thousands of years. You can immediately see the exciting consequence of this. As the two conveyor belts stream out west and east from the Mid-Atlantic Ridge, their measured magnetic polarity will exhibit stripes, reflecting the flips of the Earth's field, frozen at the moment of solidification of the rock. The pattern of stripes on the west side will mirror the precise pattern of stripes on the east side, because both sets of rocks shared the same magnetic field when they spewed together, as liquid, out of the mid-ocean ridge. It is possible to match up exactly which stripe on the east side of the ridge belongs with which stripe on the west side, and the two stripes can be dated (they have the same date as each other, of course, because they were liquid at the same time when they gushed together out of the ridge). The same pattern of stripes will be found on either side of the spreading zones on all the other ocean floors, although the distances between the mirrored stripes will vary because not all conveyor belts move at the same speed. You could not ask for more compelling evidence.

There are complications. The pattern of parallel stripes does not snake down the seafloor in a simple unbroken way. It is subject to numerous fractures 'faults'. I chose the tenth parallel south of the equator deliberately for our caterpillar tractor journey, because it happens not to be complicated by any fault lines. At another lat.i.tude, our sequence of gradually changing age would have been interrupted by occasional hiccups as we crossed a fault line. But the general picture of parallel isochrons is entirely clear from the geological map of the whole Atlantic floor.

The evidence for the seafloor spreading theory of plate tectonics is, then, very solid, and the dating of the various tectonic events such as the parting of particular continents, is, by geological standards, accurate. The plate tectonics revolution has been one of the swiftest, yet at the same time most decisive, in the whole history of science.

1 Other books have done them proud, for example David Norman's Other books have done them proud, for example David Norman's Dinosaur! Dinosaur! and Robert Bakker's and Robert Bakker's The Dinosaur Heresies The Dinosaur Heresies, not forgetting Robert Mash's delightfully affectionate How to Keep Dinosaurs How to Keep Dinosaurs.

2 Stephen Gould discusses the matter in 'Darwin at sea and the virtues of port', one of the essays collected in Stephen Gould discusses the matter in 'Darwin at sea and the virtues of port', one of the essays collected in The Flamingo's Smile The Flamingo's Smile.

3 See his 1947 book See his 1947 book Darwin's Finches Darwin's Finches. In 1994, the work of the Grants was the basis of another excellent book, The Beak of the Finch The Beak of the Finch, by Jonathan Weiner. Peter Grant's own cla.s.sic monograph of 1986, Ecology and Evolution of Darwin's Finches Ecology and Evolution of Darwin's Finches, was republished in 1999.

4 Hawaii is an even more remote volcanic archipelago, and about as young as the Galapagos. Hawaii's Robinson Crusoe bird was a honey-creeper, whose descendants rapidly evolved to 'do a Galapagos' even evolving a 'woodp.e.c.k.e.r'. Similarly, about 400 original insect immigrant species sp.a.w.ned all 10,000 endemic Hawaiian species, including a unique carnivorous caterpillar and a semi-marine cricket. Apart from a bat and a seal, there are no native Hawaiian mammals. Alas, to quote E. O. Wilson's beautiful book, Hawaii is an even more remote volcanic archipelago, and about as young as the Galapagos. Hawaii's Robinson Crusoe bird was a honey-creeper, whose descendants rapidly evolved to 'do a Galapagos' even evolving a 'woodp.e.c.k.e.r'. Similarly, about 400 original insect immigrant species sp.a.w.ned all 10,000 endemic Hawaiian species, including a unique carnivorous caterpillar and a semi-marine cricket. Apart from a bat and a seal, there are no native Hawaiian mammals. Alas, to quote E. O. Wilson's beautiful book, The Diversity of Life The Diversity of Life, 'Most of the honeycreepers are gone now. They retreated and vanished under pressure from overhunting, deforestation, rats, carnivorous ants, and malaria and dropsy carried in by exotic birds introduced to "enrich" the Hawaiian landscape.'

5 Of course, like my colleague Desmond Morris, I am using 'naked' to mean hairless rather than unclothed. Of course, like my colleague Desmond Morris, I am using 'naked' to mean hairless rather than unclothed.

6 Helena Cronin's term, in her wonderful book Helena Cronin's term, in her wonderful book The Ant and the Peac.o.c.k The Ant and the Peac.o.c.k.

7 As we saw in the Beaver's Tale, phenotype normally means the external appearance by which a gene manifests itself, for example eye colour. Obviously I am here using it in an a.n.a.logous sense: the visible phenotype of a meme otherwise buried in the brain, as opposed to the phenotype of a gene buried in a chromosome. This is also a good a.n.a.logy for the 'self-normalising' that I mentioned in the General Prologue, under 'Renewed Relics'. See also my foreword to Blackmore's book. As we saw in the Beaver's Tale, phenotype normally means the external appearance by which a gene manifests itself, for example eye colour. Obviously I am here using it in an a.n.a.logous sense: the visible phenotype of a meme otherwise buried in the brain, as opposed to the phenotype of a gene buried in a chromosome. This is also a good a.n.a.logy for the 'self-normalising' that I mentioned in the General Prologue, under 'Renewed Relics'. See also my foreword to Blackmore's book.

Dennett makes constructive use of the theory of memes in various places, including Dennett makes constructive use of the theory of memes in various places, including Consciousness Explained Consciousness Explained (from which the quotation is taken) and (from which the quotation is taken) and Darwin's Dangerous Idea Darwin's Dangerous Idea.

8 However, my astoundingly knowledgeable research a.s.sistant Sam Turvey informs me that the white dodo almost certainly never existed: 'White dodos are figured in a few seventeenth-century paintings, and contemporary travellers made reference to large, white birds on Reunion, but the accounts are vague and possibly confused, and no raphid skeletal material is known from the island. Although the species has been given the scientific name However, my astoundingly knowledgeable research a.s.sistant Sam Turvey informs me that the white dodo almost certainly never existed: 'White dodos are figured in a few seventeenth-century paintings, and contemporary travellers made reference to large, white birds on Reunion, but the accounts are vague and possibly confused, and no raphid skeletal material is known from the island. Although the species has been given the scientific name Raphus solitarius Raphus solitarius, and the eccentric j.a.panese naturalist Masauji Hachisuka defended the occurrence of two dodo species on Reunion (which he named Victoriornis imperialis Victoriornis imperialis and and Ornithaptera solitaria Ornithaptera solitaria), it is more likely that the early accounts either refer to an extinct Reunion ibis (Threskiornis solitarius), for which skeletal material is known and which was apparently similar to the living white sacred ibis, or to immature specimens of the grey brown dodo of Mauritius. Alternatively, they may merely be the product of artistic licence.'

9 A fossilised giant flightless pigeon, A fossilised giant flightless pigeon, Natunaornis Natunaornis, approaching the dodo in size, has recently been found in Fiji.

10 Also memorably celebrated by Douglas Adams, in Also memorably celebrated by Douglas Adams, in Last Chance to See Last Chance to See.

11 Actually there were several related species, in two genera, Actually there were several related species, in two genera, Aepyornis Aepyornis and and Mullerornis Mullerornis. But A. maximus A. maximus, as its name suggests, most deserves to be called the elephant bird.

12 Not diameter it is not quite as surprising as it sounds. Not diameter it is not quite as surprising as it sounds.

13 Kiwis are smaller than turkeys but are no longer regarded as dwarf moas. As we shall see, they are closer cousins of the emus and ca.s.sowaries and arrived later from Australia. Kiwis are smaller than turkeys but are no longer regarded as dwarf moas. As we shall see, they are closer cousins of the emus and ca.s.sowaries and arrived later from Australia.

14 The Maori name for New Zealand. The Maori name for New Zealand.

15 With the probable exception of the kiwi, as we shall see. With the probable exception of the kiwi, as we shall see.

16 'Gondwa.n.a.land' is criticised as a tautology, because 'Gondwa.n.a.land' is criticised as a tautology, because vana vana in Sanskrit means land (actually forest). I shall not use it. But it has the virtue of distinguis.h.i.+ng the giant continent from the region of central Madhya Pradesh where the Gonds live, which is still called Gondwana and which gave its name to the Gondwana geological series. in Sanskrit means land (actually forest). I shall not use it. But it has the virtue of distinguis.h.i.+ng the giant continent from the region of central Madhya Pradesh where the Gonds live, which is still called Gondwana and which gave its name to the Gondwana geological series.

17 Although, strange-seeming to us, the southernmost parts would still have spent a substantial fraction of the year in darkness. Presumably this led to all sorts of behavioural adaptations for which there are no modern counterparts, for today extreme lat.i.tude goes with extreme cold. Although, strange-seeming to us, the southernmost parts would still have spent a substantial fraction of the year in darkness. Presumably this led to all sorts of behavioural adaptations for which there are no modern counterparts, for today extreme lat.i.tude goes with extreme cold.

Rendezvous 17.

AMPHIBIANS.

Three hundred and forty million years ago, in the early Carboniferous Period, only about 30 million years beyond the great milestone of Rendezvous 16 Rendezvous 16, we amniotes (the name that unites mammals with reptiles and birds) meet our amphibian cousins at Rendezvous 17 Rendezvous 17. Pangaea had not yet come together, and northern and southern landma.s.ses surrounded a pre-Tethys ocean. A south polar ice cap was beginning to form, there were tropical forests of club mosses around the equator, and the climate was probably something like that of today, although the flora and fauna were of course very different.

Concestor 17, in the vicinity of our 175-million-greats-grandparent, is the ancestor of all surviving tetrapods. Tetrapod means four feet. We who don't walk on four feet are lapsed tetrapods, recently lapsed in our case, much less recently in the case of birds, but we are all called tetrapods. More to the point, Concestor 17 is the grand ancestor of the huge throng of land vertebrates. Despite my earlier strictures on the conceit of hindsight, the emergence of fish onto the land was a major transition in our evolutionary history.

Three main bands of modern amphibian pilgrims have joined forces long 'before' they meet up with us amniotes. They are the frogs (and toads: the distinction is not a zoologically helpful one), the salamanders (and newts, which are those species that return to the water to breed), and the caecilians (moist, legless burrowers or swimmers, with a superficial resemblance to earthworms or snakes). The frogs have no tail as adults but a vigorously swimming tail as larvae. The salamanders have a long tail in the adult as well as the larval stage, and their body proportions most resemble ancestral amphibians, as judged by fossils. The caecilians have no limbs not even internal traces of the pectoral and pelvic girdles that supported the limbs of their ancestors. The great length of the caecilian body is achieved by multiplying up the vertebrae in the trunk region (up to 250, compared to 12 in frogs), and their ribs, which provide useful support and protection. The tail, oddly, is very short or even absent: if caecilians had legs, their hind legs would be right at the posterior tip of the body, which is where some extinct amphibians actually kept them.

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Amphibians join. At odds with several fossil studies, genetic studies consistently unite the 5,000 or so described amphibian species in a single group, sister to the amniotes. Molecular taxonomy has been followed here, but there are disagreements on the order of branching between the three amphibian groups. At odds with several fossil studies, genetic studies consistently unite the 5,000 or so described amphibian species in a single group, sister to the amniotes. Molecular taxonomy has been followed here, but there are disagreements on the order of branching between the three amphibian groups.

Images, left to right: Monterey ensatina ( Monterey ensatina (Ensatina eschscholtzii eschscholtzii); blue poison frog (Dendrobates azureus); caecilian (Ichthyophis sp.). sp.).

Even if they live on land as adults, most amphibians reproduce in water, while amniotes (except in secondarily evolved cases such as whales, dugongs and ichthyosaurs) reproduce on land. Amniotes reproduce either viviparously, giving birth to live young, or with a relatively large, tough-sh.e.l.led waterproof egg. In both cases the embryo floats in its own 'private pond'. Amphibian embryos are much more likely to float in a real pond, or something equivalent to one. The amphibian pilgrims who join us at Rendezvous 17 Rendezvous 17 may spend part of their time on land, but they are seldom far from water and, at least at some stage in their life cycle, they usually return to it. Those that reproduce on land go to some lengths to contrive watery conditions. may spend part of their time on land, but they are seldom far from water and, at least at some stage in their life cycle, they usually return to it. Those that reproduce on land go to some lengths to contrive watery conditions.

Trees provide relatively safe havens, and frogs have discovered ways to reproduce in them without losing the vital tie to water. Some exploit the small pools of rainwater that form in the rosettes of bromeliad plants. Male African grey tree frogs, Chiromantis xerampelina Chiromantis xerampelina, co-operate to whip up a thick white foam, with their back legs, from a liquid secreted by the females. This foam hardens to a crust on the outside, protecting the moist interior which serves as a nest for the group's eggs. The tadpoles develop inside the wet foam nest, up a tree. When they are ready, in the next rainy season, they wriggle free and drop into puddles of water below the tree, where they develop into frogs. Other species use the foam-nest technique, but they don't co-operate to do so. Instead one male beats up a foam from the secretion of one female.

Some frog species have made interesting transitions in the direction of true viviparity live birth. The female of the South American marsupial frog (various species of the genus Gastrotheca Gastrotheca) transfers her fertilised eggs to her back, where they become covered by a layer of skin. There the tadpoles develop and can clearly be seen wriggling under the skin of their mother's back until they eventually burst out. Again, several other species do something similar, probably independently evolved.

Another South American frog species, named Rhinoderma darwinii Rhinoderma darwinii after its ill.u.s.trious discoverer, practises a most unusual version of viviparity. The male appears to eat the eggs that he has fertilised. The eggs don't travel down his gut, however. Like many male frogs he has a commodious vocal sac, used as a resonator to amplify the voice, and it is in this moist chamber that the eggs lodge. There they develop, until they are finally vomited out as fully formed froglets, forgoing the freedom to swim as tadpoles. after its ill.u.s.trious discoverer, practises a most unusual version of viviparity. The male appears to eat the eggs that he has fertilised. The eggs don't travel down his gut, however. Like many male frogs he has a commodious vocal sac, used as a resonator to amplify the voice, and it is in this moist chamber that the eggs lodge. There they develop, until they are finally vomited out as fully formed froglets, forgoing the freedom to swim as tadpoles.

The key difference between amphibians and amniotes is that amniote skins and eggsh.e.l.ls are waterproof. Amphibian skin typically lets water evaporate through it, at the same rate you'd expect from a body of standing water of the same area. As far as the water under the skin is concerned, there might almost as well not be any skin. This is very different from reptiles, birds and mammals, where one of the main roles of the skin is to serve as a barrier to water. There are exceptions among amphibians most notably among various species of desert frogs in Australia. These exploit the fact that even deserts can have flood times, though brief and well s.p.a.ced out. During such rare and intermittent times of high rainfall, each frog makes a water-filled coc.o.o.n in which it buries itself in a state of torpor, for two years or, by some accounts, even as long as seven years. Some species of frogs can withstand temperatures well below the normal freezing point of water, by manufacturing glycerol as an antifreeze.

Almost no amphibians live in sea water, and it is therefore not surprising that, unlike lizards, they are seldom found on remote islands.1 Darwin noted this in more than one of his books, and also the fact that frogs that have been artificially introduced to just such islands thrive there. He presumed that lizard eggs are protected from sea water by their hard sh.e.l.ls, whereas sea water promptly kills frogsp.a.w.n. Frogs are, however, found on all continents except Antarctica, and have probably been there continuously since before the continents broke up. They are a very successful group. Darwin noted this in more than one of his books, and also the fact that frogs that have been artificially introduced to just such islands thrive there. He presumed that lizard eggs are protected from sea water by their hard sh.e.l.ls, whereas sea water promptly kills frogsp.a.w.n. Frogs are, however, found on all continents except Antarctica, and have probably been there continuously since before the continents broke up. They are a very successful group.

Frogs remind me of birds in one respect. Both have a body plan which is a somewhat bizarre modification of the ancestral one. That is not particularly remarkable, but birds and frogs have taken this bizarre body plan and made it the basis for a whole new range of variation. There are not quite so many frog species as birds, but the more than 4,000 species of frogs, in every part of the world, are impressive enough. Just as the bird body plan is obviously a design for flying, even in birds such as ostriches that don't fly, the best way to understand the body plan of an adult frog is as a highly specialised jumping machine. Some species can jump spectacular distances, up to 50 body lengths in the well-named rocket frog of Australia (Litoria nasuta). The largest frog in the world, the goliath frog (Conraua goliath) of West Africa, which is the size of a small dog, is said to jump three metres. Not all frogs jump, but all are descended from jumping ancestors. They are at least lapsed jumpers, just as ostriches are lapsed fliers. Some tree-dwelling species, such as Wallace's flying frog, Rhacophorus nigropalmatus Rhacophorus nigropalmatus, prolong the jump by spreading their long fingers and toes, whose webbing acts as a parachute. Indeed, they glide a bit like flying squirrels.

Salamanders and newts swim like fish when they are in water. Even on land, their legs are too small and feeble to walk or run in the sense we would recognise, and the salamander uses a sinuous swimming motion like a fish, with the legs just helping it along. Most salamanders today are quite small. The largest reach a respectable 1.5 metres, but this is still far smaller than the giant amphibians of past times, which dominated the land before the rise of the reptiles.

But what did Concestor 17 look like: the ancestor that amphibians share with reptiles and ourselves? Certainly more like an amphibian than an amniote, and more like a salamander than a frog but probably not much like either. The best fossils are in Greenland which, during the Devonian Period, was on the equator. These possibly transitional fossils have been much studied,2 among them among them Acanthostega Acanthostega, which seems to have been wholly aquatic (showing that 'legs' originally evolved for movement in water, not on land), and Ichthyostega Ichthyostega.

Concestor 17 might have been something like Ichthyostega Ichthyostega or or Acanthostega Acanthostega, although both were larger than we normally expect grand ancestors to be. There are some other surprises for zoologists preconditioned by acquaintance with modern animals. We tend to think the possession of five digits is deeply stamped in the hands and feet of tetrapods the 'pentadactyl' limb is a cla.s.sic zoological totem. Yet recent evidence shows that Ichthyostega Ichthyostega had seven toes, had seven toes, Acanthostega Acanthostega had eight, and had eight, and Tulerpeton Tulerpeton, a third genus of Devonian tetrapod, had six. It is tempting to say the number of digits doesn't matter, is functionally neutral. I doubt that. My tentative guess is that in those early times the different species really did benefit from their respective numbers of toes. They really were more efficient than other numbers would have been, for swimming or walking. Later, the tetrapod limb design hardened at five digits, probably because some internal embryological process came to rely upon that number. In the adult, the number is frequently reduced from the embryonic number in extreme cases such as modern horses, to just one, the middle toe.

The fish group from which the amphibians sprang is the one known as the lobefins. The only surviving lobefins are the lungfish and the coelacanths,3 and we shall meet them at and we shall meet them at Rendezvous 18 Rendezvous 18 and and 19 19 respectively. In Devonian times, lobefins were much more prominent in both the marine and freshwater faunas. The tetrapods probably evolved from an otherwise extinct group of lobefins called the osteolepiforms. Among osteolepiforms are respectively. In Devonian times, lobefins were much more prominent in both the marine and freshwater faunas. The tetrapods probably evolved from an otherwise extinct group of lobefins called the osteolepiforms. Among osteolepiforms are Eusthenopteron Eusthenopteron and and Panderichthys Panderichthys, both dating from the late Devonian, about the time when the first tetrapods were starting to emerge onto the land.

Why did fish first develop the changes that permitted the move out of water onto the land? Lungs, for example? And fins that you could walk on rather than, or as well as, swim with? It wasn't that they were trying to initiate the next big chapter in evolution! For years, the favoured answer to the question was one that the eminent American palaeontologist Alfred Sherwood Romer derived from the geologist Joseph Barrell. The idea was that if these fish were trying to do anything it was to get back to water. In times of drought, fish can easily become stranded in drying pools. Individuals capable of walking and of breathing air have the enormous advantage that they can forsake a doomed, drying pond and set out for a deeper one elsewhere.

This admirable theory has become unfas.h.i.+onable but not, I think, for uniformly good reasons. Unfortunately, Romer quoted the prevailing belief of his day that the Devonian was a time of drought, a belief that has more recently been called into question. But I don't think Romer needed his Devonian desiccated. Even at times of no particular drought, there will always be some ponds shallow enough to be in danger of becoming too shallow for some particular kind of fish. If ponds three feet deep would have been at risk under severe drought conditions, mild drought conditions will render ponds one foot deep at risk. It is sufficient for the Romer hypothesis that there are some ponds that dry up, and therefore some fish that could save their lives by migrating. Even if the world of the late Devonian was positively waterlogged, one could say this simply increases the number of ponds available to dry up, thereby increasing opportunities for saving the life of walking fish and the Romer theory. Nevertheless, it is my duty to record that the theory is now unfas.h.i.+onable. A further point against the theory is that modern fish that venture onto land do so in humid, wet areas that is, when conditions on land are 'good' for water animals, not poor as in the Romer hypothesis.

And, to be sure, there are plenty of other good reasons for a fish to emerge, temporarily or permanently, onto land. Streams and ponds can become unusable for reasons other than drying up. They can become choked with weeds, in which case, again, a fish that can migrate over land to deeper water might benefit. If, as has been suggested contra contra Romer, we are talking Devonian swamps rather than Devonian droughts, swamps provide plenty of opportunities for a fish to benefit by walking, or slithering or flip-flopping or otherwise travelling through the marshy vegetation, in search of deep water or, indeed, food. This still retains the essential Romer idea that our ancestors left the water, not at first to colonise land, but to return to water. Romer, we are talking Devonian swamps rather than Devonian droughts, swamps provide plenty of opportunities for a fish to benefit by walking, or slithering or flip-flopping or otherwise travelling through the marshy vegetation, in search of deep water or, indeed, food. This still retains the essential Romer idea that our ancestors left the water, not at first to colonise land, but to return to water.

The group of lobefins from which we tetrapods are derived, are today reduced to a pitiful four genera, but they once dominated the seas almost as the teleost fish do today. We are not due to meet the teleosts until Rendezvous 20 Rendezvous 20, but they will help our discussion because some of them breathe air, at least occasionally, and a few even come out of the water and walk on land. A little further into our pilgrimage, we shall hear from one of them, the mudskipper, whose tale is a tale of independent, more recent encroachment onto the land.

THE SALAMANDER'S TALE.

Names are a menace in evolutionary history. It is no secret that palaeontology is a controversial subject in which there are even some personal enmities. At least eight books called Bones of Contention Bones of Contention are in print. And if you look at what two palaeontologists are quarrelling about, as often as not it turns out to be a name. Is this fossil are in print. And if you look at what two palaeontologists are quarrelling about, as often as not it turns out to be a name. Is this fossil h.o.m.o erectus h.o.m.o erectus, or is it an archaic h.o.m.o sapiens? h.o.m.o sapiens? Is this one an early Is this one an early h.o.m.o habilis h.o.m.o habilis or a late or a late Australopithecus? Australopithecus? People evidently feel strongly about such questions, but they often turn out to be splitting hairs. Indeed, they resemble theological questions, which I suppose gives a clue to why they arouse such pa.s.sionate disagreements. The obsession with discrete names is an example of what I call the tyranny of the discontinuous mind. The Salamander's Tale strikes a blow against the discontinuous mind People evidently feel strongly about such questions, but they often turn out to be splitting hairs. Indeed, they resemble theological questions, which I suppose gives a clue to why they arouse such pa.s.sionate disagreements. The obsession with discrete names is an example of what I call the tyranny of the discontinuous mind. The Salamander's Tale strikes a blow against the discontinuous mind (see plate 21) (see plate 21).

The Central Valley runs much of the length of California, bounded by the Coastal Range to the west and by the Sierra Nevada to the east. These long mountain ranges link up at the north and the south ends of the valley, which is therefore surrounded by high ground. Throughout this high ground lives a genus of salamanders called Ensatina Ensatina. The Central Valley itself, about 40 miles wide, is not friendly to salamanders, and they are not found there. They can move all round the valley but normally not across it, in an elongated ring of more or less continuous population. In practice any one salamander's short legs in its short lifetime don't carry it far from its birthplace. But genes, persisting through a longer timescale, are another matter. Individual salamanders can interbreed with neighbours whose parents may have interbred with neighbours further round the ring, and so on. There is therefore potentially gene flow all around the ring. Potentially. What happens in practice has been elegantly worked out by the research of my old colleagues at the University of California at Berkeley, initiated by Robert Stebbins and continued by David Wake.

In a study area called Camp Wolahi, in the mountains to the south of the valley, there are two clearly distinct species of Ensatina Ensatina which do not interbreed. One is conspicuously marked with yellow and black blotches. The other is a uniform light brown with no blotches. Camp Wolahi is in a zone of overlap, but wider sampling shows that the blotched species is typical of the eastern side of the Central Valley which, here in Southern California, is known as the San Joaquin Valley. The light brown species, on the contrary, is typically found on the western side of the San Joaquin. which do not interbreed. One is conspicuously marked with yellow and black blotches. The other is a uniform light brown with no blotches. Camp Wolahi is in a zone of overlap, but wider sampling shows that the blotched species is typical of the eastern side of the Central Valley which, here in Southern California, is known as the San Joaquin Valley. The light brown species, on the contrary, is typically found on the western side of the San Joaquin.

Non-interbreeding is the recognised criterion for whether two populations deserve distinct species names. It therefore should be straightforward to use the name Ensatina eschscholtzii Ensatina eschscholtzii for the plain western species, and for the plain western species, and Ensatina klauberi Ensatina klauberi for the blotched eastern species straightforward but for one remarkable circ.u.mstance, which is the nub of the tale. for the blotched eastern species straightforward but for one remarkable circ.u.mstance, which is the nub of the tale.

If you go up to the mountains that bound the north end of the Central Valley, which up there is called the Sacramento Valley, you'll find only one species of Ensatina Ensatina. Its appearance is intermediate between the blotched and the plain species: mostly brown, with rather indistinct blotches. It is not a hybrid between the two: that is the wrong way to look at it. To discover the right way, make two expeditions south, sampling the salamander populations as they fork to west and east on either side of the Central Valley. On the east side, they become progressively more blotched until they reach the extreme of klauberi klauberi in the far south. On the west side, the salamanders become progressively more like the plain in the far south. On the west side, the salamanders become progressively more like the plain eschscholtzii eschscholtzii that we met in the zone of overlap at Camp Wolahi. that we met in the zone of overlap at Camp Wolahi.

This is why it is hard to treat Ensatina eschscholtzii Ensatina eschscholtzii and and Ensatina klauberi Ensatina klauberi with confidence as separate species. They const.i.tute a 'ring species'. You'll recognise them as separate species if you only sample in the south. Move north, however, and they gradually turn into each other. Zoologists normally follow Stebbins's lead and place them all in the same species, with confidence as separate species. They const.i.tute a 'ring species'. You'll recognise them as separate species if you only sample in the south. Move north, however, and they gradually turn into each other. Zoologists normally follow Stebbins's lead and place them all in the same species, Ensatina eschscholtzii Ensatina eschscholtzii, but give them a range of subspecies names. Starting in the far south with Ensatina eschscholtzii eschscholtzii Ensatina eschscholtzii eschscholtzii, the plain brown form, we move up the west side of the valley through Ensatina eschscholtzii xanthoptica Ensatina eschscholtzii xanthoptica and and Ensatina eschscholtzii oregonensis Ensatina eschscholtzii oregonensis which, as its name suggests, is also found further north in Oregon and Was.h.i.+ngton. At the north end of California's Central Valley is which, as its name suggests, is also found further north in Oregon and Was.h.i.+ngton. At the north end of California's Central Valley is Ensatina eschscholtzii picta Ensatina eschscholtzii picta, the semi-blotched form mentioned before. Moving on round the ring and down the east side of the valley, we pa.s.s through Ensatina eschscholtzii platensis Ensatina eschscholtzii platensis which is a bit more blotched than which is a bit more blotched than picta picta, then Ensatina eschscholtzii croceater Ensatina eschscholtzii croceater until we reach until we reach Ensatina eschscholtzii klauberi Ensatina eschscholtzii klauberi (which is the very blotched one that we previously called (which is the very blotched one that we previously called Ensatina klauberi Ensatina klauberi when we were considering it to be a separate species). when we were considering it to be a separate species).

Stebbins believes that the ancestors of Ensatina Ensatina arrived at the north end of the Central Valley and evolved gradually down the two sides of the valley, diverging as they went. An alternative possibility is that they started in the south as, say, arrived at the north end of the Central Valley and evolved gradually down the two sides of the valley, diverging as they went. An alternative possibility is that they started in the south as, say, Ensatina eschscholtzii eschscholtzii Ensatina eschscholtzii eschscholtzii, then evolved their way up the west side of the valley, round the top and down the other side, ending up as Ensatina eschscholtzii klauberi Ensatina eschscholtzii klauberi at the other end of the ring. Whatever the history, what happens today is that there is hybridization all round the ring, except where the two ends of the line meet, in the far south of California. at the other end of the ring. Whatever the history, what happens today is that there is hybridization all round the ring, except where the two ends of the line meet, in the far south of California.

As a complication, it seems that the Central Valley is not a total barrier to gene flow. Occasionally, salamanders seem to have made it across, for there are populations of, for example, xanthoptica xanthoptica, one of the western subspecies, on the eastern side of the valley, where they hybridise with the eastern subspecies, platensis platensis. Yet another complication is that there is a small break near the south end of the ring, where there seem to be no salamanders at all. Presumably they used to be there, but have died out. Or maybe they are still there but have not been found: I am told that the mountains in this area are rugged and hard to search. The ring is complicated, but a ring of continuous gene flow is, nevertheless, the predominant pattern in this genus, as it is with the better-known case of herring gulls and lesser black-backed gulls around the Arctic Circle.

In Britain the herring gull and the lesser black-backed gull are clearly distinct species. Anybody can tell the difference, most easily by the colour of the wing backs. Herring gulls have silver-grey wing backs, lesser black-backs, dark grey, almost black. More to the point, the birds themselves can tell the difference too, for they don't hybridise although they often meet and sometimes even breed alongside one another in mixed colonies. Zoologists therefore feel fully justified in giving them different names, Larus argentatus Larus argentatus and and Larus fuscus Larus fuscus.

But now here's the interesting observation, and the point of resemblance to the salamanders. If you follow the population of herring gulls westward to North America, then on around the world across Siberia and back to Europe again, you notice a curious fact. The 'herring gulls', as you move round the pole, gradually become less and less like herring gulls and more and more like lesser black-backed gulls until it turns out that our Western European lesser black-backed gulls actually are the other end of a ring-shaped continuum which started with herring gulls. At every stage around the ring, the birds are sufficiently similar to their immediate neighbours in the ring to interbreed with them. Until, that is, the ends of the continuum are reached, and the ring bites itself in the tail. The herring gull and the lesser black-backed gull in Europe never interbreed, although they are linked by a continuous series of interbreeding colleagues all the way round the other side of the world.

Ring species like the salamanders and the gulls are only showing us in the spatial dimension something that must always happen in the time dimension. Suppose we humans, and the chimpanzees, were a ring species. It could have happened: a ring perhaps moving up one side of the Rift Valley, and down the other side, with two completely separate species co-existing at the southern end of the ring, but an unbroken continuum of interbreeding all the way up and back round the other side. If this were true, what would it do to our att.i.tudes to other species? To apparent discontinuities generally?

Many of our legal and ethical principles depend on the separation between h.o.m.o sapiens h.o.m.o sapiens and all other species. Of the people who regard abortion as a sin, including the minority who go to the lengths of a.s.sa.s.sinating doctors and blowing up abortion clinics, many are unthinking meat-eaters, and have no worries about chimpanzees being imprisoned in zoos and sacrificed in laboratories. Would they think again, if we could lay out a living continuum of intermediates between ourselves and chimpanzees, linked in an unbroken chain of interbreeders like the Californian salamanders? Surely they would. Yet it is the merest accident that the intermediates all happen to be dead. It is only because of this accident that we can comfortably and easily imagine a huge gulf between our two species or between any two species, for that matter. and all other species. Of the people who regard abortion as a sin, including the minority who go to the lengths of a.s.sa.s.sinating doctors and blowing up abortion clinics, many are unthinking meat-eaters, and have no worries about chimpanzees being imprisoned in zoos and sacrificed in laboratories. Would they think again, if we could lay out a living continuum of intermediates between ourselves and chimpanzees, linked in an unbroken chain of interbreeders like the Californian salamanders? Surely they would. Yet it is the merest accident that the intermediates all happen to be dead. It is only because of this accident that we can comfortably and easily imagine a huge gulf between our two species or between any two species, for that matter.

I have previously recounted the case of the puzzled lawyer who questioned me after a public lecture. He brought the full weight of his legal ac.u.men to bear on the following nice point. If species A evolves into species B, he reasoned closely, there must come a point when a child belongs to the new species B but his parents still belong to the old species A. Members of different species cannot, by definition, interbreed with one another, yet surely a child would not be so different from its parents as to be incapable of interbreeding with their kind. Doesn't this, he wound up, wagging his metaphorical finger in the special way that lawyers, at least in courtroom dramas, have perfected as their own, undermine the whole idea of evolution?

That is like saying, 'When you heat a kettle of cold water, there is no particular moment when the water ceases to be cold and becomes hot, therefore it is impossible to make a cup of tea.' Since I always try to turn questions in a constructive direction, I told my lawyer about the herring gulls, and I think he was interested. He had insisted on placing individuals firmly in this species or that. He didn't allow for the possibility that an individual might lie half way between two species, or a tenth of the way from species A to species B. Exactly the same limitation of thought hamstrings the endless debates about exactly when in the development of an embryo it becomes human (and when, by implication, abortion should be regarded as tantamount to murder). It is no use saying to these people that, depending upon the human characteristic that interests you, a foetus can be 'half human' or 'a hundredth human'. 'Human', to the qualitative, absolutist mind, is like 'diamond'. There are no halfway houses. Absolutist minds can be a menace. They cause real misery, human misery. This is what I call the tyranny of the discontinuous mind, and it leads me to develop the moral of the Salamander's Tale.

For certain purposes names, and discontinuous categories, are exactly what we need. Indeed, lawyers need them all the time. Children are not allowed to drive; adults are. The law needs to impose a threshold, for example the seventeenth birthday. Revealingly, insurance companies take a very different view of the proper threshold age.

Some discontinuities are real, by any standards. You are a person and I am another person and our names are discontinuous labels that correctly signal our separateness. Carbon monoxide really is distinct from carbon dioxide. There is no overlap. A molecule consists of a carbon and one oxygen, or a carbon and two oxygens. None has a carbon and 1.5 oxygens. One gas is deadly poisonous, the other is needed by plants to make the organic substances that we all depend upon. Gold really is distinct from silver. Diamond crystals really are different from graphite crystals. Both are made of carbon, but the carbon atoms naturally arrange themselves in two quite distinct ways. There are no intermediates.

But discontinuities are often far from so clear. My newspaper carried the following item during a recent flu epidemic. Or was it an epidemic? That question was the burden of the article.

Official statistics show there are 144 people in every 100,000 suffering from flu, said a spokeswoman for the Department of Health. As the usual gauge of an epidemic is 400 in every 100,000, it is not being officially treated as an epidemic by the Government. But the spokeswoman added: 'Professor Donaldson is happy to stick by his version that this is an epidemic. He believes it is many more than 144 per 100,000. It is very confusing and it depends on which definition you choose. Professor Donaldson has looked at his graph and said it is a serious epidemic.'What we know is that some particular number of people are suffering from flu. Doesn't that, in itself, tell us what we want to know? Yet for the spokeswoman, the important question is whether this counts as an 'epidemic'. Has the proportion of sufferers crossed the rubicon of 400 per 100,000? This is the great decision which Professor Donaldson had to make, as he pored over his graph. You'd think he might have been better employed trying to do something about it, whether or not it counted officially as an epidemic.

As it happens, in the case of epidemics, for once there really is a natural rubicon: a critical ma.s.s of infections above which the virus, or bacterium, suddenly 'takes off' and dramatically increases its rate of spreading. This is why public health officials try so hard to vaccinate more than a threshold proportion of the population against, say, whooping cough. The purpose is not just to protect the individuals vaccinated. It is also to deprive the pathogens of the opportunity to reach their own critical ma.s.s for 'take-off'. In the case of our flu epidemic, what should really worry the spokeswoman for the Ministry of Health is whether the flu virus has yet crossed its rubicon for take-off, and leapt abruptly into high gear in its spread through the population. This should be decided by some means other than reference to magic numbers like 400 per 100,000. Concern with magic numbers is a mark of the discontinuous mind, or qualitative mind. The funny thing is that, in this case, the discontinuous mind overlooks a genuine discontinuity, the take-off point for an epidemic. Usually there isn't a genuine discontinuity to overlook.

Many Western countries at present are suffering what is described as an epidemic of obesity. I seem to see evidence of this all around me, but I am not impressed by the preferred way of turning it into numbers. A percentage of the population is described as 'clinically obese'. Once again, the discontinuous mind insists on separating people out into the obese on one side of a line, the non-obese on the other. That is not the way real life works. Obesity is continuously distributed. You can measure how obese each individual is, and you can compute group statistics from such measurements. Counts of numbers of people who lie above some arbitrarily defined threshold of obesity are not illuminating, if only because they immediately prompt a demand for the threshold to be specified and maybe redefined.

The same discontinuous mind also lurks behind all those official figures detailing the numbers of people 'below the poverty line'. You can meaningfully express a family's poverty by telling us their income, preferably expressed in real terms of what they can buy. Or you can say 'X is as poor as a church mouse' or 'Y is as rich as Croesus' and everybody will know what you mean. But spuriously precise counts or percentages of people said to fall above or below some arbitrarily defined poverty line line are pernicious. They are pernicious because the precision implied by the percentage is instantly belied by the meaningless artificiality of the 'line'. Lines are impositions of the discontinuous mind. Even more politically sensitive is the label 'black', as opposed to 'white', in the context of modern society especially American society. This is the central issue in the Gra.s.shopper's Tale, and I'll leave it for now, except to say that I believe race is yet another of the many cases where we don't need discontinuous categories, and where we should do without them unless an extremely strong case in their favour is made. are pernicious. They are pernicious because the precision implied by the percentage is instantly belied by the meaningless artificiality of the 'line'. Lines are impositions of the discontinuous mind. Even more politically sensitive is the label 'black', as opposed to 'white', in the context of modern society especially American society. This is the central issue in the Gra.s.shopper's Tale, and I'll leave it for now, except to say that I believe race is yet another of the many cases where we don't need discontinuous categories, and where we should do without them unless an extremely strong case in their favour is made.

Here's another example. Universities in Britain award degrees that are cla.s.sified into three distinct cla.s.ses, First, Second and Third Cla.s.s. Universities in other countries do something equivalent, if under different names, like A, B, C etc. Now, my point is this. Students do not really separate neatly into good, middling and poor. There are not discrete and distinct cla.s.ses of ability or diligence. Examiners go to some trouble to a.s.sess students on a finely continuous numerical scale, awarding marks or points that are designed to be added to other such marks, or otherwise manipulated in mathematically continuous ways. The score on such a continuous numerical scale conveys far more information than cla.s.sification into one of three categories. Nevertheless, only the discontinuous categories are published.

In a very large sample of students, the distribution of ability and prowess would normally be a bell curve with few doing very well, few doing very badly and many in between. It might not actually be a symmetrical bell like the picture on page 316, but it would certainly be smoothly continuous, and it would become smoother as more and more students are added in.

A few examiners (especially, I hope I'll be forgiven for adding, in non-scientific subjects) seem actually to believe that there really is a discrete ent.i.ty called the First-Cla.s.s Mind, or the 'alpha' mind, and a student either definitely has it or definitely hasn't. The task of the examiner is to sort out the Firsts from the Seconds and the Seconds from the Thirds, just as one might sort sheep from goats. The likelihood that in reality there is a smooth continuum, sliding from pure sheepiness through all intermediates to pure goatiness, is a difficult one for some kinds of mind to grasp.

If, against all my expectations, it should turn out that the more students you add in, the more the distribution of exam marks approximates to a discontinuous distribution with three peaks (see lower picture), it would be a fascinating result. The awarding of First, Second and Third Cla.s.s degrees might then actually be justifiable.

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But there is certainly no evidence for this, and it would be very surprising given everything we know about human variation. As things are, it is clearly unfair: there is far more difference between the top of one cla.s.s and the bottom of the same cla.s.s, than there is between the bottom of one cla.s.s and the top of the next cla.s.s. It would be fairer to publish the actual marks obtained, or a rank order based upon those marks. But the discontinuous or qualitative mind insists on forcing people into one or other discrete category.

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Returning to our topic of evolution, what about sheep and goats themselves? Are there sharp discontinuities between species, or do they merge into each other like first-cla.s.s and second-cla.s.s exam performances? If we look only at surviving animals, the answer is normally yes, there are sharp discontinuities. Exceptions like the gulls and the Californian salamanders are rare, but revealing because they translate into the spatial domain the continuity which is normally found only in the temporal domain. People and chimpanzees are certainly linked via a continuous chain of intermediates and a shared ancestor, but the intermediates are extinct: what remains is a discontinuous distribution. The same is true of people and monkeys, and of people and kangaroos, except that the extinct intermediates lived longer ago. Because the intermediates are nearly always extinct, we can usually get away with a.s.suming that there is a sharp discontinuity between every species and every other. But in this book we are concerned with evolutionary history, with the dead as well as the living. When we are talking about all the animals that have ever lived, not just those that are living now, evolution tells us there are lines of gradual continuity linking literally every species to every other. When we are talking history, even apparently discontinuous modern species like sheep and dogs are linked, via their common ancestor, in unbroken lines of smooth continuity.

Ernst Mayr, distinguished elder statesman of twentieth-century evolution, has blamed the delusion of discontinuity under its philosophical name of Essentialism as the main reason why evolutionary understanding came so late in human history. Plato, whose philosophy can be seen as the inspiration for Essentialism, believed that actual things are imperfect versions of an ideal archetype of their kind. Hanging somewhere in ideal s.p.a.ce is an essential, perfect rabbit, which bears the same relation to a real rabbit as a mathematician's perfect circle bears to a circle drawn in the dust. To this day many people are deeply imbued with the idea that sheep are sheep and goats are goats, and no species can ever give rise to another because to do so they'd have to change their 'essence'.

There is no such thing as essence.

No evolutionist thinks that modern species change into other modern species. Cats don't turn into dogs or vice versa. Rather, cats and dogs have evolved from a common ancestor, who lived tens of millions of years ago. If only all the intermediates were still alive, attempting to separate cats from dogs would be a doomed enterprise, as it is with the salamanders and the gulls. Far from being a question of ideal essences, separating cats from dogs turns out to be possible only because of the lucky (from the point of view of the essentialist) fact that the intermediates happen to be dead. Plato might find it ironic to learn that it is actually an imperfection the sporadic ill-fortune of death that makes the separation of any one species from another possible. This of course applies to the separation of human beings from our nearest relatives and, indeed, from our more distant relatives too. In a world of perfect and complete information, fossil information as well as recent, discrete names for animals would become impossible. Instead of discrete names we would need sliding scales, just as the words hot, warm, cool and cold are better replaced by a sliding scale such as Celsius or Fahrenheit.

Evolution is now universally accepted as a fact by thinking people, so one might have hoped that essentialist intuitions in biology would have been finally overcome. Alas, this hasn't happened. Essentialism refuses to lie down. In practice, it is usually not a problem. Everyone agrees that h.o.m.o sapiens h.o.m.o sapiens is a different species (and most would say a different genus) from is a different species (and most would say a different genus) from Pan troglodytes Pan troglodytes, the chimpanzee. But everyone also agrees that if you follow human ancestry backward to the shared ancestor and then forward to chimpanzees, the intermediates all along the way will form a gradual continuum in which every generation would have been capable of mating with its parent or child of the opposite s.e.x.

By the interbreeding criterion every individual is a member of the same species as its parents. This is an unsurprising, not to say plat.i.tudinously obvious conclusion, until you realise that it raises an intolerable paradox in the essentialist mind. Most of our ancestors throughout evolutionary history have belonged to different species from us by any criterion, and we certainly couldn't have interbred with them. In the Devonian Period our direct ancestors were fish. Yet, although we couldn't interbreed with them, we are linked by an unbroken chain of ancestral generations, every one of which could have interbred with their immediate predecessors and immediate successors in the chain.

In the light of this, see how empty are most of those pa.s.sionate arguments about the naming of particular hominid fossils. h.o.m.o ergaster h.o.m.o ergaster is widely recognised as the predecessor species that gave rise to is widely recognised as the predecessor species that gave rise to h.o.m.o sapiens h.o.m.o sapiens, so I'll play along with that for what follows. To call h.o.m.o ergaster h.o.m.o ergaster a separate species from a separate species from h.o.m.o sapiens h.o.m.o sapiens could have a precise meaning in principle, even if it is impossible to test in practice. It means that if we could go back in our time machine and meet our could have a precise meaning in principle, even if it is impossible to test in practice. It means that if we could go back in our time machine and meet our h.o.m.o ergaster h.o.m.o ergaster ancestors, we could not interbreed with them. ancestors, we could not interbreed with them.4 But suppose that, instead of zooming directly to the time of But suppose that, instead of zooming directly to the time of h.o.m.o ergaster h.o.m.o ergaster, or indeed any other extinct species in our ancestral lineage, we stopped our time machine every thousand years along the way and picked up a young and fertile pa.s.senger. We transport this pa.s.senger back to the next thousand-year stop and release her (or him: let's take a female and a male at alternate stops). Provided our one-stop time traveller could accommodate to local social and linguistic customs (quite a tall order) there would be no biological barrier to her interbreeding with a member of the opposite s.e.x from 1,000 years earlier. Now we pick up a new pa.s.senger, say a male this time, and transport him back another 1,000 years. Once again, he too would be biologically capable of fertilising a female from 1,000 years before his native time. The daisy chain would continue on back to when our ancestors were swimming in the sea. It could go back without a break, to the fishes, and it would still be true that each and every pa.s.senger transported 1,000 years before its own time would be able to interbreed with its predecessors. Yet at some point, which might be a million years back but might be longer or shorter, there would come a time when we moderns could not interbreed with an ancestor, even though our latest one-stop pa.s.senger could. At this point we could say that we have travelled back to a different species.

The barrier would not come suddenly. There would never be a generation in which it made sense to say of an individual that he is h.o.m.o sapiens h.o.m.o sapiens but his parents are but his parents are h.o.m.o ergaster h.o.m.o ergaster. You can think of it as a paradox if you like, but there is no reason to think that any child was ever a member of a different species from its parents, even though the daisy chain of parents and children stretches back from humans to fish and beyond. Actually it isn't paradoxical to anybody but a dyed-in-the-wool essentialist. It is no more paradoxical than the statement that there is never a moment when a growing child ceases to be short and becomes tall. Or a kettle ceases to be cold and becomes hot. The legal mind may find it necessary to impose a barrier between childhood and majority the stroke of midnight on the eighteenth birthday, or whenever it is. But anyone can see that it is a (necessary for some purposes) fiction. If only more people could see that the same applies to when, say, a developing embryo becomes 'human'.

Creationists love 'gaps' in the fossil record. Little do they know, biologists have good reason to love them too. Without gaps in the fossil record, our whole system for naming species would break down. Fossils could not be given names, they'd have to be given numbers, or positions on a graph. Or instead of arguing heatedly over whether a fossil is 'really', say, an early h.o.m.o ergaster h.o.m.o ergaster or a late or a late h.o.m.o habilis h.o.m.o habilis, we might call it habigaster habigaster. There's a lot to be said for this. Nevertheless, perhaps because our brains evolved in a world where most things do fall into discrete categories, and in particular where most of the intermediates between living species are dead, we often feel more comfortable if we can use separate names for things when we talk about them. I am no exception and neither are you, so I shall not bend over backwards to avoid using discontinuous names for species in this book. But the Salamander's Tale explains why this is a human imposition rather than something deeply built into the natural world. Let us use names as if they really reflected a discontinuous reality, but let's privately remember that, at least in the world of evolution, it is no more than a convenient fiction, a pandering to our own limitations.

THE NARROWMOUTH'S TALE.

Microhyla (sometimes confused with (sometimes confused with Gastrophryne Gastrophryne) is a genus of small frogs, the narrowmouthed frogs. There are several species, including two in North America: the eastern narrowmouth Microhyla carolinensis Microhyla carolinensis, and the Great Plains narrowmouth Microhyla olivacea Microhyla olivacea. These two are so closely related that they occasionally hybridise in nature. The eastern narrowmouth's range extends down the east coast from the Carolinas to Florida, and west until half way across Texas and Oklahoma. The Great Plains narrowmouth extends from Baja California in the west, as far as eastern Texas and eastern Oklahoma, and as far north as northern Missouri. Its range is therefore a western mirror of the eastern narrowmouth's, and it might as well be called the western narrowmouth. The important point is that their ranges meet in the middle: there is an overlap zone running up the eastern half of Texas and into Oklahoma. As I said, hybrids are occasionally found in this overlap zone, but mostly the frogs distinguish just as well as herpetologists do. This is what justifies our calling them two different species.

As with any two species, there must have been a time when they were one. Something separated them: to use the technical term, the single ancestral species 'speciated' and became two. It is a model for what happens at every branch point in evolution. Every speciation begins with some sort of initial separation between two populations of the same species. It isn't always a geographical separation, but, as we shall see in the Cichlid's Tale, an initial separation of some kind makes it possible for the statistical distribution of genes in the two populations to move apart. This usually results in an evolutionary divergence with respect to something visible: shape or colour or behaviour. In the case of these two populations of American frogs, the western species became adapted to life in drier climates than the eastern, but the most conspicuous difference lies in their mating calls. Both are squeaky buzzes, but each b