This disparity, between tens of miles as the distance between islands within the archipelago, and hundreds of miles as the distance of the whole archipelago from the mainland, leads the evolutionist to expect that the different islands might house species that are pretty similar to each other but more different from their counterparts on the mainland. And that is exactly what we do find. Darwin himself put it well, coming tantalizingly close to evolutionary language, even before he had properly formulated his ideas. I have placed the key clause in italics, and shall repeat it throughout this chapter in different contexts.
Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends. In a like manner it might be fancied that a bird, originally a buzzard, had been induced here to undertake the office of the carrion-feeding Polybori of the American continent.
The last sentence is a reference to the Galapagos hawk, Buteo galapagoensis, another species that is found only on Galapagos, but which somewhat resembles species on the mainland, especially Buteo swainsoni, which annually migrates between North and South America and could well have been blown off course on one or two freak occasions. Nowadays, we should refer to the Galapagos hawk and the flightless cormorant as 'endemic' to the islands, meaning that this is the only place where they are found. Darwin himself, who had not yet fully embraced evolution, used the then current phrase 'aboriginal creations', which meant that God had created them here and nowhere else. He used the same phrase of the giant tortoises, which then abounded on all the islands, and also of the two species of iguana, the Galapagos land iguana and the Galapagos marine iguana. The marine iguanas are truly remarkable creatures, quite different from anything seen anywhere else in the world. They dive to the sea bottom and graze seaweed, which seems to be their only food. They are graceful swimmers, although not, in Darwin's outspoken view, beautiful to look at:
It is a hideous looking creature, of a dirty black colour, stupid,* and sluggish in its movements. The usual length of a full-grown one is about a yard, but there are some even four feet long . . . their tails are flattened sideways, and all four feet partially webbed . . . When in the water this lizard swims with perfect ease and quickness, by a serpentine movement of its body and flattened tail - the legs being motionless and closely collapsed on its sides.
Since marine iguanas are so good at swimming, it might be supposed that they, rather than the land iguanas, made the long crossing from the mainland and subsequently speciated, in the archipelago, to give rise to the land iguana. This is almost certainly not the case, however. The Galapagos land iguana is not greatly different from iguanas still living on the mainland, whereas the marine iguanas are unique to the Galapagos archipelago. No lizard with the same marine habits has ever been found elsewhere in the world. We are nowadays confident that it was the land iguana that originally arrived from the South American mainland, perhaps carted on driftwood like the modern ones from Guadeloupe that were blown to Anguilla. On Galapagos, they subsequently speciated to give rise to the marine iguana. And it was almost certainly the geographical isolation permitted by the spaced-out pattern of the islands that made possible the initial separation between the ancestral land iguanas and the newly speciating marine iguanas. Presumably some land iguanas were accidentally rafted across to a hitherto iguana-free island, and there adopted a marine habit, free from contamination by genes flowing in from the land iguanas on the original island. Much later, they spread to other islands, eventually returning to the island from which their land ancestors had originally hailed. By now they could no longer interbreed with them, and their genetically inherited marine habits were safe from contamination by land iguana genes.
In example after example, Darwin noticed the same thing. The animals and plants of each island of Galapagos are largely endemic to the archipelago ('aboriginal creations'), but they are also for the most part unique, in detail, from island to island. He was especially impressed with the plants in this respect:
Hence we have the truly wonderful fact, that in James Island [Santiago], of the thirty-eight Galapageian plants, or those found in no other part of the world, thirty are exclusively confined to this one island; and in Albemarle Island [Isabela], of the twenty-six aboriginal Galapageian plants, twenty-two are confined to this one island, that is, only four are at present known to grow in the other islands of the archipelago; and so on . . . with the plants from Chatham [San Cristobal] and Charles [Floreana] Islands.
He noticed the same thing with the distribution of mockingbirds over the islands.
My attention was first thoroughly aroused, by comparing together the numerous specimens, shot by myself and several other parties on board, of the mocking-thrushes, when, to my astonishment, I discovered that all those from Charles Island belonged to one species (Mimus trifasciatus); all from Albemarle Island to M. parvulus; and all from James and Chatham Islands (between which two other islands are situated, as connecting links) belonged to M. melanotis.
So it is, all over the world. The fauna and flora of a particular region are just what we should expect if, to quote Darwin on the finches that now bear his name, 'one species had been taken and modified for different ends'.
The Vice-Governor of the Galapagos Islands, Mr Lawson, intrigued Darwin by informing him that the tortoises differed from the different islands, and that he himself could with certainty tell from which island any one was brought. I did not for some time pay sufficient attention to this statement, and I had already partially mingled together the collections from two of the islands. I never dreamed that islands, about fifty or sixty miles apart, and most of them in sight of each other, formed of precisely the same rocks, placed under a quite similar climate, rising to a nearly equal height, would have been differently tenanted.
All the Galapagos giant tortoises are similar to a particular mainland species of land tortoise, Geochelone chilensis, which is smaller than any of them. At some point during the few million years that the islands have existed, one or a few of these mainland tortoises inadvertently fell in the sea and floated across. How could they have survived the long and doubtless arduous crossing? Surely most of them didn't. But it would have only taken one female to do the trick. And tortoises are astonishingly well-equipped to survive the crossing.
The early whalers took thousands of giant tortoises from the Galapagos islands away in their ships for food. To keep the meat fresh, the tortoises were not killed until needed, but they were not fed or watered while waiting to be butchered. They were simply turned on their backs, sometimes stacked several deep, so they couldn't walk away. I tell the story not in order to horrify (although I have to say that such barbaric cruelty does horrify me), but to make a point. Tortoises can survive for weeks without food or fresh water, easily long enough to float in the Humboldt Current from South America to the Galapagos archipelago. And tortoises do float.
Having reached and multiplied upon their first Galapagos island, the tortoises would with comparative ease - again accidentally - have island-hopped the much shorter distances to the rest of the archipelago by the same means. And they did what many animals do when they arrive on an island: they evolved to become larger. This is the long-noticed phenomenon of island gigantism (confusingly, there is an equally well-known phenomenon of island dwarfism). *
If the tortoises had followed the pattern of Darwin's famous finches, they would have evolved a different species on each of the islands. Then, after subsequent accidental driftings from island to island, they would have been unable to interbreed (that's the definition, remember, of a separate species) and would have been free to evolve a different way of life uncontaminated by genetic swamping.
You could say that the different species' incompatible mating habits and preferences constitute a kind of genetic substitute for the geographic isolation of separate islands. Though they overlap geographically, they are now isolated on separate 'islands' of mating exclusivity. So they can diverge yet further. The Large, the Medium and the Small Ground Finch originally diverged on different islands; the three species now coexist on most of the Galapagos islands, never interbreeding and each specializing in a different kind of seed diet.
The tortoises did something similar, evolving distinctive shell shapes on the different islands. The species on the larger islands have high domes. Those on the smaller islands have saddle-shaped shells with a high-lipped window at the front for the head. The reason for this seems to be that the large islands are wet enough to grow grass, and the tortoises there are grazers. The smaller islands are mostly too dry for grass, and the tortoises resort to browsing on cactuses. The high-lipped saddle shell allows the neck to reach up to the cactuses which, for their part, grow higher in an evolutionary arms race against the browsing tortoises.
The tortoise story adds to the finch model the further complication that, for tortoises, volcanoes are islands within islands. Volcanoes provide high, cool, damp, green oases, surrounded at low altitude by dry lava fields which, for a grazing giant tortoise, constitute hostile deserts. Each of the smaller islands has a single large volcano and its own single species (or sub-species) of giant tortoise (except those few islands that have none at all). The big island of Isabela ('Albemarle' to Darwin) consists of a string of five major volcanoes, and each volcano has its own species (or sub-species) of tortoise. Truly, Isabela is an archipelago within an archipelago: a system of islands within an island. And the principle of islands in the literal geographical sense, setting the stage for the evolution of islands in the metaphorical genetic sense of species, has never been more elegantly demonstrated than here in the archipelago of Darwin's blest youth.*
Islands don't come much more isolated than St Helena, a single volcano in the South Atlantic some 1,200 miles from the coast of Africa. It has about 100 endemic plants (the young Darwin would have called them 'aboriginal creations' and the older Darwin would have said they evolved there). Among these are (or were, for some of them are now extinct) forest trees belonging to the daisy family.
These trees resemble in habit trees on the African mainland to which they are not closely related. The mainland plants to which they are related are herbs or small shrubs. What must have happened is that a few seeds of small herbs or shrubs chanced across the thousandmile gap from Africa, settled on St Helena and, because the niche of forest trees was unfilled, evolved larger and more woody trunks until they became proper trees. Similar tree-like daisies have evolved independently on the Galapagos archipelago. It is the same pattern on islands the world over.
Each of the great African lakes has its own unique fish fauna, dominated by the group called cichlids. The cichlid faunas of Lake Victoria, Lake Tanganyika and Lake Malawi, each several hundred species strong, are completely distinct from each other. They have evidently evolved separately in the three lakes, which makes it all the more fascinating that they have converged on the same range of 'trades' in all three. In each lake, it looks as though one or two founder species somehow made their way in, perhaps from rivers, in the first place. And in each lake these founders then speciated and speciated again, to populate the lake with the hundreds of species that we see today. How, within the confines of a lake, did the budding species achieve the initial geographical isolation that enabled them to split apart?
When introducing islands, I explained that, from a fish's point of view, a lake surrounded by land is an island. Slightly less obviously, even an island in the conventional sense of land surrounded by water can be an 'island' for a fish, especially a fish that lives only in shallow water. In the sea, think of a coral-reef fish, which never ventures into deep water. From its point of view, the shallow fringe of a coral island is an 'island', and the Great Barrier Reef is an archipelago. Something similar can happen even in a lake. Within a lake, especially a large one, a rocky outcrop can be an 'island' for a fish whose habits confine it to shallow water. This is almost certainly how at least some of the cichlids in the African great lakes achieved their initial isolation. Most individuals were confined to shallow water around islands, or in bays and inlets. This achieved partial isolation from other such pockets of shallow water, linked by occasional traversings of the deeper water between them to form the watery equivalent of a Galapagos-like 'archipelago'.
There's good evidence (for example from sediment core samples) that the level of Lake Malawi (it was called Lake Nyasa when I spent my first bucket-and-spade holidays on its sandy beaches) rises and falls dramatically over the centuries, and reached a low point in the eighteenth century, more than 100 metres lower than the present level. Many of its islands were not islands at all during that time, but hills on the land around the then smaller lake. When the lake level rose, in the nineteenth and twentieth centuries, the hills became islands, ranges of hills became archipelagoes, and the process of speciation took off among the cichlids that live in shallow water, known locally as Mbuna. 'Almost every rocky outcrop and island has a unique Mbuna fauna, with endless colour forms and species. As many of these islands and outcrops were dry land within the last 200-300 years, the establishment of the faunas has taken place within that time.'
Such rapid speciation is something the cichlid fishes are extremely good at. Lake Malawi and Lake Tanganyika are old, but Lake Victoria is extremely young. The lake basin was formed only about 400,000 years ago, and it has dried up several times since then, most recently about 17,000 years ago. This seems to mean that its endemic fauna of 450 or so species of cichlid fishes have all evolved over a timescale of centuries, not the millions of years that we usually associate with evolutionary divergence on this grand scale. The cichlids of Africa's lakes impress us mightily with what evolution can do in a short space of time. They almost qualified for inclusion in the 'before our very eyes' chapter.
The woods and forests of Australia are dominated by trees of a single genus, Eucalyptus, and there are more than 700 species of them, filling a huge range of niches. Once again, Darwin's dictum about finches can be coopted: one could almost imagine that one species of eucalypt had been 'taken and modified for different ends'. And, along parallel lines, an even more famous example is the Australian mammal fauna. In Australia there are, or were until recent extinctions possibly caused by the arrival of aboriginal people, the ecological equivalents of wolves, cats, rabbits, moles, shrews, lions, flying squirrels and many others. Yet they are marsupials, quite different from the wolves, cats, rabbits, moles, shrews, lions and flying squirrels with which we are familiar in the rest of the world, the so-called placental mammals. The Australian equivalents are all descended from just a few, or even one, ancestral marsupial species, 'taken and modified for different ends'. This beautiful marsupial fauna has also produced creatures for which it is harder to find a counterpart outside Australia. The many species of kangaroo mostly fill antelope-like niches (or monkey or lemur-like niches in the case of the tree kangaroos) but get about by hopping rather than galloping. They range from the large red kangaroo (and some even larger extinct ones, including a fearsome, bounding carnivore) to the small wallabies and tree kangaroos. There were giant, rhinoceros-sized marsupials, Diprotodonts, related to modern wombats but 3 yards long, 6 feet tall at the shoulder, and weighing 2 tons. I shall return to the marsupials of Australia in the next chapter.
It is almost too ridiculous to mention it, but I'm afraid I have to because of the more than 40 per cent of the American population who, as I lamented in Chapter 1, accept the Bible literally: think what the geographical distribution of animals should look like if they'd all dispersed from Noah's Ark. Shouldn't there be some sort of law of decreasing species diversity as we move away from an epicentre - perhaps Mount Ararat? I don't need to tell you that that is not what we see.
Why would all those marsupials - ranging from tiny pouched mice through koalas and bilbys to giant kangaroos and Diprotodonts - why would all those marsupials, but no placentals at all, have migrated en masse from Mount Ararat to Australia? Which route did they take? And why did not a single member of their straggling caravan pause on the way, and settle - in India, perhaps, or China, or some haven along the Great Silk Road? Why did the entire order Edentata (all twenty species of armadillo, including the extinct giant armadillo, all six species of sloth, including extinct giant sloths, and all four species of anteater) troop off unerringly for South America, leaving not a rack behind, leaving no hide nor hair nor armour plate of settlers somewhere along the way? Why were they joined by the entire infraorder of caviomorph rodents, including guinea pigs, agoutis, pacas, maras, capybaras, chinchillas and lots of others, a large group of characteristically South American rodents, found nowhere else? Why did an entire sub-order of monkeys, the platyrrhine monkeys, end up in South America and nowhere else? Shouldn't at least a few of them have joined the rest of the monkeys, the catarrhines, in Asia or Africa? And shouldn't at least one species of catarrhine have found itself in the New World, along with the platyrrhines? Why did all the penguins undertake the long waddle south to the Antarctic, not a single one to the equally hospitable Arctic?
An ancestral lemur, again very possibly just a single species, found itself in Madagascar. Now there are thirty-seven species of lemur (plus some extinct ones). They range in size from the pygmy mouse lemur, smaller than a hamster, to a giant lemur, larger than a gorilla and resembling a bear, which went extinct quite recently. And they are all, every last one of them, in Madagascar. There are no lemurs anywhere else in the world, and there are no monkeys in Madagascar. How on Earth do the 40 per cent history-deniers think this state of affairs came about? Did all thirty-seven and more species of lemur troop in a body down Noah's gangplank and hightail it (literally in the case of the ringtail) for Madagascar, leaving not a single straggler by the wayside, anywhere throughout the length and breadth of Africa?
Once again, I am sorry to take a sledgehammer to so small and fragile a nut, but I have to do so because more than 40 per cent of the American people believe literally in the story of Noah's Ark. We should be able to ignore them and get on with our science, but we can't afford to because they control school boards, they home-school their children to deprive them of access to proper science teachers, and they include many members of the United States Congress, some state governors and even presidential and vice-presidential candidates. They have the money and the power to build institutions, universities, even a museum where children ride life-size mechanical models of dinosaurs, which, they are solemnly told, coexisted with humans. And, as recent polls have shown, Britain is not far behind (or should that read 'ahead'?), along with parts of Europe and most of the Islamic world.
Even if we leave Mount Ararat to one side; even if we refrain from lampooning those who take the Noah's Ark myth literally, similar problems apply to any theory of the separate creation of species. Why would an all-powerful creator decide to plant his carefully crafted species on islands and continents in exactly the appropriate pattern to suggest, irresistibly, that they had evolved and dispersed from the site of their evolution? Why would he put lemurs in Madagascar and nowhere else? Why put platyrrhine monkeys in South America only, and catarrhine monkeys in Africa and Asia only? Why no mammals in New Zealand, except bats who could fly there? Why do the animals in island chains most closely resemble those on neighbouring islands, and why do they nearly always resemble - less strongly but still unmistakably - those on the nearest continent or large island? Why would the creator put only marsupial mammals in Australia, again except bats who could fly there, and those who could arrive in man-made canoes? The fact is that, if we survey every continent and every island, every lake and every river, every mountaintop and every Alpine valley, every forest and every desert, the only way to make sense of the distribution of animals and plants is, yet again, to follow Darwin's insight about the Galapagos finches: 'One might really fancy that from an original paucity . . . one species had been taken and modified for different ends.'
Darwin was fascinated by islands, and he tramped the length and breadth of a good few during the voyage of the Beagle. He even worked out the surprising truth about how islands of one major class, those built by the animals called corals, are formed. Darwin later came to recognize the crucial importance of islands and archipelagoes for his theory, and he did several experiments to settle questions about the theory of geographical isolation as a prelude to speciation (he didn't use the word). For example, in a number of experiments he kept seeds in sea water for long periods, and demonstrated that some retained the power to germinate even after immersion for long enough to have drifted from continents to neighbouring islands. Frogspawn, on the other hand, he found to be immediately killed by sea water, and he made good use of this to explain a signal fact about the geographical distribution of frogs:
With respect to the absence of whole orders on oceanic islands, Bory St. Vincent long ago remarked that Batrachians (frogs, toads, newts) have never been found on any of the many islands with which the great oceans are studded. I have taken pains to verify this assertion, and I have found it strictly true. I have, however, been assured that a frog exists on the mountains of the great island of New Zealand; but I suspect that this exception (if the information be correct) may be explained through glacial agency. This general absence of frogs, toads, and newts on so many oceanic islands cannot be accounted for by their physical conditions; indeed it seems that islands are peculiarly well fitted for these animals; for frogs have been introduced into Madeira, the Azores, and Mauritius, and have multiplied so as to become a nuisance. But as these animals and their spawn are known to be immediately killed by sea-water, on my view we can see that there would be great difficulty in their transportal across the sea, and therefore why they do not exist on any oceanic island. But why, on the theory of creation, they should not have been created there, it would be very difficult to explain.
Darwin was well aware of the significance of the geographical distribution of species for his theory of evolution. He noted that most of the facts could be accounted for if we assume that animals and plants have evolved. From this, we should expect - and we find - that modern animals tend to live on the same continent as fossils that could plausibly be their ancestors, or close to their ancestors. We should expect, and we find, that animals share the same continent with species that resemble them. Here is Darwin on the subject, paying special attention to the animals of South America that he knew so well:
the naturalist in travelling, for instance, from north to south never fails to be struck by the manner in which successive groups of beings, specifically distinct, yet clearly related, replace each other. He hears from closely allied, yet distinct kinds of birds, notes nearly similar, and sees their nests similarly constructed, but not quite alike, with eggs coloured in nearly the same manner. The plains near the Straits of Magellan are inhabited by one species of Rhea (American ostrich), and northward the plains of La Plata by another species of the same genus; and not by a true ostrich or emeu, like those found in Africa and Australia under the same latitude. On these same plains of La Plata, we see the agouti and bizcacha, animals having nearly the same habits as our hares and rabbits, . . . but they plainly display an American type of structure. We ascend the lofty peaks of the Cordillera and we find an alpine species of bizcacha; we look to the waters, and we do not find the beaver or musk-rat, but the coypu and capybara, rodents of the American type.
This is mostly common sense, and Darwin was able to account for an enormous range of observations by means of it. But there are certain facts about the geographical distribution of animals and plants, and the distribution of rocks, that need a different kind of explanation: one that is anything but common sense, and which would have staggered and enthralled Darwin, if only he had known about it.
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