Darwin and the Islands of Evolution

by N. J. BERRILL

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A YOUNG man of twenty-two, Charles Darwin left England as naturalist on board a warship, H.M.S. Beagle, on a journey that lasted five years. The greater part of the voyage was a protracted circumnavigation of South America which gave him opportunities for landings and journeys into the interior, both into southern Patagonia and the pampas country from the east and at various places along the Pacific coast. This was the experience that was to culminate in The Origin of Species more than twenty years later.

Darwin had no more than a fair formal education, was an indifferent student, and had failed in medical school. He went on to Cambridge to study for the ministry, while his father, a country doctor, shook his head in grave misgiving. He was offered the chance to go on the Beagle almost by default, for few men could afford to spend several years without financial reward. But despite his failures in school, there is every indication that Darwin was a deep, active thinker. From a very early age he was an ardent collector of minerals, insects, and flowers, and a man or a boy thinks most about what interests him most. And as a youth he read Humboldt’s Personal Narratives and dreamed of adding to its luster. Yet more than all of this, he was the grandson of his father’s father, Erasmus Darwin, a man great in his own right, who had already formulated a theory of evolution of animals, and who must have had some influence upon the thoughts and reflections of his grandson.

The young man who sailed on the Beagle was already tensed to the experience which lay before him, although he left England with a firm belief in the creationist point of view and the immutability of species.

The Beagle left Plymouth late in December, 1831, with its seasick naturalist on board, and took the old course to the south to the Canaries and Cape Verde Islands. Here we find Darwin improving on the dip bucket of his predecessors in an effort to capture the small drifting creatures of the sea surface, by towing a deep bag of bunting behind the vessel, an invention which has in later elaborations become the chief tool of biological investigations of the ocean. From Cape Verde to Brazil only a landing on the bird-crowded St. Paul’s Rocks, the minute tip of a volcano arising from the mid-Atlantic abyss, where no land should be, and an uncomfortable shaving by Neptune’s barber when the ship crossed the line, marked an uneventful passage.

Landfall came at Bahia, and Darwin’s first few walks in the Brazilian forest left him at a loss for words to describe it — if his eye followed a gaudy butterfly it was stopped by some strange fruit or tree; in watching an insect he forgot it in the stranger flower. He wrote that his mind was a chaos of delight.

After two weeks at Bahia the Beagle left on a southern survey of coasts and harbors; and apart from visits to Rio de Janeiro and Montevideo, Darwin was at sea for most of the time from March, 1832, until May, 1833. Darwin never became accustomed to the motion of the vessel, and he suffered continual seasickness, although when the sea was quiet enough he found the ship a marvelous place in which to study. During this period he saw something of Tierra del Fuego and its miserable savage inhabitants, and visited the bleak and treeless Falkland Islands.

At the end of July, 1833, the Beagle arrived at the Rio Negro, the largest of the rivers between La Plata and the Strait of Magellan, and from here Darwin traveled across the plains of Patagonia to Buenos Aires and Montevideo. At first the level plain, stretching as far as the eye could see, was covered with dry, gravelly soil with only tufts of brown withered grass and low scattered thorny bushes to save it from being completely desert. A solitary tree, visible from a great distance, served as a sacrificial altar for the Indians. The first night was spent under the open sky amid the deathlike stillness of the plain, with dogs keeping watch and a gypsy group of Gauchos making their beds around the fire. Only the harelike agouti was common, hopping in twos and threes in a straight line across the wild plains, though occasionally a wild llama, the guanaco, could be seen keeping its dis’tance.

Desolation slowly gave way to plains covered with green turf, tall purple clover, and little owls, and even where the country deteriorated into a wide level waste of sand, salt marsh, or mud, ostriches, deer, cavies, and armadillos were abundant. Both Darwin and his guide watched the animals closely as they rode along, Darwin as a naturalist and the guide as a man who knew the danger of riding abroad while the war of extermination was still being carried on against the Indians by the Spaniards. At last, however, they “rode on in peace and quietness to a low point called Punta Alta, whence we could see nearly the whole of the great harbour of Bahía Blanca.”

Punta Alta was a low cliff about twenty feet high, of alternating layers of gravel and a reddish muddy clay, with sea shells scattered through them of the same kinds found along the beach. The gravel was a perfect graveyard, and Darwin, already on the lookout for fossil bones, during the one day he had to spend, found enough ancient relics to start a museum. This is the list: a nearly perfect head of the extinct giant sloth; another member of the same more or less toothless order with great scratching claws and as large as a pony; two more Edentates related to the giant sloth, fully as large as an ox; a Toxodon, which was something like a guinea pig built on the scale of a prize-winning bull; a large piece of skin armor of an armadillo but of gigantic size; a tusk like that of a boar; and lastly a tooth that Darwin thought was identical with that of the common horse. Horses had been introduced into South America by the Spaniards, but this tooth was embedded in the gravel with the other bones and was in the same state of decay.

It is an imposing and startling list and it is no wonder Darwin stayed awake at nights thinking about it. For it was evident that a now extinct fauna of large quadrupeds lived in this region not so long ago, geologically speaking. The bones were not even true fossils, for they were not mineralized, but were just bones undergoing decay, though they belonged to animals of kinds no longer existing. That a new fauna had replaced an old one was in itself not surprising, for there was already a theory of successions of faunas in relation to a series of cataclysms. Strange extinct remains were to be expected, but what is new is Darwin’s recognition not only of the relatively giant size of the extinct forms but their general relation to existing animals, and his speculations concerning what could have brought about the change. This was the first impact of the past: others were to come.

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IN THE meantime Darwin continued his journey to Buenos Aires and saw much of the ostrich, a bird shy, wary, and solitary. “And although so fleet in its pace, it falls a prey to the Indian or Gaucho armed with the bolas. They generally prefer running against the wind; yet at the first start they expand their wings, and like a vessel make all sail.” The ride to Buenos Aires was enlivened also by armadillos of several kinds, each an unbelievable sort of animal.

Food on the way presented no difficulty, for always there were eggs — one ostrich nest yielded twenty-seven eggs, which Darwin calculated, on the basis of one to eleven, was equivalent in food value to two hundred ninety-seven hen’s eggs. There were also many plover since it was southern spring and the golden plover was already present in large flocks that had winged their way from the approaching winter of the Arctic tundra. Partridges, so called, abounded. They gave good meat and were easily caught — a boy on a quiet old horse, with a running noose made of the stem of an ostrich feather, fastened to the end of a long stick, could catch thirty or forty in a day.

Beyond Buenos Aires the journey continued for about two hundred miles up the Parana River to Santa Fe, through occasional cattle ranches separated by extensive purple tracts of acrid clover and the great impenetrable thistles, inhabited only by a long-tailed burrowing rodent and its friend the little owl. The riverbank of course meant another opportunity for searching for fossil bones, for year by year the freshets sliced away new layers of the steep banks, leaving skeleton after crumbling skeleton exposed. Bones there were, and plenty of them, though the greater number were rotten and as soft as clay, especially those of the mastodon, the kin of elephant and mammoth.

The men who took Darwin by canoe to the cliffs of the mastodon bones said they had long known of them and had often wondered how they got there — “the necessity of a theory being felt they came to the conclusion that the mastodon formerly was a burrowing animal!” He found there also more casing of the giant armadillo and another tooth, which this time was unmistakably that of a horse. At another point he found his second Toxodon and more armadillo armor, which differed, however, from what he already possessed. More than one gigantic animal of a former age was protected by a coat of mail! Darwin already knew of discoveries of fossil elephants and mastodons in Europe and Asia, and even at this early time suspected that his mastodons had walked across a Siberian-Alaskan bridge that had since disappeared.

After he had rejoined his ship at Buenos Aires in early December, “the Beagle sailed from the Rio Plata, never again to enter its muddy stream. Our course was directed to Port Desire on the coast of Patagonia.” On the way, ten miles or more offshore the seamen cried out that it was snowing butterflies, myriads of them extending in all directions as far as the eye could see, flocks as large as Columbus saw to the south of Cuba.

Butterfly migrations such as these are a little better known than they used to be, although what determines their course is as obscure as ever. As a rule the flocks consist of males alone and they fly to their death. As much as anything, it seems to be a way of removing an excess of comparatively useless individuals from overcrowded areas, for biologically speaking the sexes are not equal, and the run of males for the most part are dispensable.

At another time, sixty miles at sea, vast numbers of small spiders, of all ages, drifted by in the air, each hanging on to the end of a long thread that served as a kite.

The Beagle, it must be remembered, was a naval ship on an extensive surveying expedition, and the captain’s concern lay with coastlines and harbors. Darwin, as a naturalist, just went along and made the best of his opportunities, although it was at Captain Fitzroy’s request that a naturalist accompanied him. Only brief trips ashore could be made at places that were not to be revisited by the ship, and little more than glimpses were possible of the southern Patagonian river courses.

The Beagle left Patagonia and sailed east to the brown and dreary windand rain-swept Falklands. They appeared to be no place for man or beast, yet the islands were a sanctuary for murderers and flightless birds.

The birds were important, for they made it clear to Darwin how flight could become transformed into other forms of locomotion. His own descriptions are vivid: “One day, having placed myself between a penguin and the water, I was much amused by watching its habits. It was a brave bird, and till reaching the sea, it regularly fought and drove me backwards. . . . This bird is commonly called the jackass penguin, from its habit, while on shore, of throwing its head backwards and making a loud strange noise, very like the braying of that animal; but while at sea and undisturbed its note is very deep and solemn and is often heard in the night. In diving, its little plumeless wings are used as fins, but on the land as front legs. When crawling (it may be said on four legs) through the tussocks or on the side of a grassy cliff, it moves so quickly that it might be readily mistaken for a quadruped. When at sea and fishing it comes to the surface, for the purpose of breathing, with such a spring, and dives again so instantaneously that I defy anyone at first sight to be sure that it is not a fish leaping for sport.”

There were geese as well as penguins, an upland species that fed on grass and other vegetation and built its nest on the small outlying islets, and the snow-white gander and its darker mate, rock geese that found a living on the beaches. But the one that caught a young man’s fancy was the steamer duck, a goose weighing more than twenty pounds. All South Atlantic voyagers between the islands and the Horn have watched it with amazement. “Their wings are too small and weak to allow of flight, but by their aid, partly swimming and partly flapping the surface of the water, they move very quickly. The manner is something like that by which the common duck escapes when pursued by a dog, but I am nearly sure that the steamer moves its wings alternately instead of both together as in other birds. The steamer is able to dive only a very short distance. It feeds entirely on shell-fish from the kelp and tidal rocks; hence the beak and head, for the purpose of breaking them, are surprisingly heavy and strong. Thus we find in South America three birds which use their wings for other purposes besides flight: the penguin as fins, the steamer as paddles, and the ostrich as sails to a vessel.”

With the Falkland Islands left behind, the ship sailed once more to the west and beat against the wind to the Strait of Magellan, to pass through the Strait during the southern midwinter. In June, 1834, the Beagle entered the Pacific between the East and West Furies, through a sea with so many breakers it was called the Milky Way.

For more than a year the ship worked its observing way up the Pacific coast; and like Humboldt before him, Darwin found more to do as geologist than naturalist. The last call before swinging out along the equator to the Galápagos Islands was at Lima, and for once Darwin shows his actual age, for what impressed him most, as better worth looking at than all the churches and buildings of the town, was the ladies, fitted in close elastic gowns, with a black silk veil brought over the head, allowing only one brilliant and powerfully expressive eye free to appraise the young foreigner. He felt, he says, as much surprised as if he had been introduced among a number of nice round mermaids!

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SEPTEMBER 15, 1835. “The Beagle arrived at the southernmost of the Galápagos islands, of which five much exceed the others in size. They are situated under the equatorial line, and between five and six hundred miles to the westward of the coast of America. The constitution of the whole is volcanic. . . . I have no exact data — but I do not hesitate to affirm that there must be at least two thousand craters. . . . The natural history is very remarkable: it seems to be a little world within itself, the greater number of its inhabitants, both vegetable and animal, being found nowhere else.”

There you have it! Two problems in a nutshell, even though slightly petrified. How did animals and plants ever reach the islands, and why are they so distinctive?

Darwin’s first walk, on a glowing hot day among the little craters, brought him face to face with two large tortoises, each a two-hundred-pounder, busy eating cactus. They looked at him and hissed, and slowly walked away. Among the leafless shrubs, the tall cacti, and (he black forbidding lava they seemed not out of place but out of time.

Along the shores the rocks abounded with great black lizards between three and four feet long, and on the hills another kind was just as common. There were smaller lizards too, and several kinds of snakes, but no frogs or toads. And there was a large mouse with large thin ears, like a kind he knew from the sterile deserts west of the Andes. Birds, especially turtledoves, were tame, but not so tame as they used to be — “Dampier says that a man in a morning’s walk might kill six or seven dozen of these birds. . . . At present they do not suffer themselves to be killed in such numbers.”

The three great reptiles hold the stage, and Darwin’s accounts of them are more detailed and accurate than his buccaneering predecessors’, and fresher than those left by later visitors. The tortoises, which Dampier reckoned could support an army of men for several months, lived on juicy cactus on those islands, where the land was low and arid; but in higher, damper regions where trees grew, they ate leaves and berries and hanging lichens. On the larger islands, where there were springs high up towards the central parts, the thirsty, mud-loving perambulating tanks made well-beaten paths in every direction from the wells even down the seacoast, and Spaniards first discovered the watering places by following the tortoises. It was curious to see one set of the monsters trudging eagerly uphill with outstretched thirsting necks, and another set stumbling down and almost spilling over. Darwin found they could speed along at a brisk six yards a minute.

The two big lizards drove home the lesson of the birds and their wings — “a variation in habit accompanying change in structure.” When Darwin found it, the aquatic lizard was extremely common on all the islands of the archipelago, living exclusively on the rocky sea beaches and never even ten yards inshore. Black imps of darkness, they swam with perfect ease by a sinuous movement of the body and flattened tail, feeding on submerged Seaweed. Even a forced submersion of over an hour did not drown one.

The land lizards were confined to the central islands—“it would appear as if this species had been created in the centre of the archipelago, and thence had been dispersed only to a certain distance.” This is creationist language, and the fact that Darwin does not seem concerned with how the lizards came to be on the islands suggests that he is using it literally, but biological thought generally was in transition between creation and some sort of evolution, and the meanings of words were slowly shifting.

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I KNOW of no better substitute for counting sheep on a sleepless night than thinking of the questions raised by the Galápagos and trying to dream up the answers. Where have the animals and plants come from, how did they get there, how have they managed to live there after they arrived, and what has happened to them since their arrival? Almost every problem of evolution and world history finds its place in this cluster of small equatorial islands.

As reptiles go, the tortoise is thoroughly antique. There were tortoises of a kind, with something like their present peculiar armor, stumping around in the warm and deserty part of the earth before there were dinosaurs, before the great age of reptiles was ever under way. Within certain latitudes the tortoise is world-wide, and true tortoises have been ready to make the best of a landing on the Galápagos for nearly one hundred million years. Whether the Galápagos were there to land upon is another matter.

One thing is certain: no tortoise ever swam the distance from Peru to the Galápagos, or even drifted with the current, without some sort of help. Many persons have worried over how the tortoise made the trip, and some insist that it walked on dry land and, like the plowman, plodded its weary way every step of the journey. The traveling tortoise is explained by making a land bridge from Central or South America to the Galápagos. A submarine plateau is already there, but to make it emerge above the surface means raising it by another ten thousand feet, and it is a case of out of the frying pan into the fire, for the absent bridge is more disturbing than the present tortoise.

A land bridge implies that the islands have been a part of the continent, and as such they should show the continental structure in the rocks. They do not, and few islands anywhere flaunt their volcanic origin and character as blatantly as these craterridden tops of mountains rising from the floor of the deep sea. Even if we agree to the land bridge, forgetting the geological difficulty, the bridge merely changes the nature of the problem. It makes a road for a tortoise but it also makes a road for a toad, and neither frogs nor toads nor salamanders, nor the host of creeping and crawling things of the tropical mainland that would have had ample time to travel a few hundred miles, ever reached the islands. There was no bridge, and the torioise is back where it started.

If not by land then by sea, for a tortoise has yet to learn to fly. What are the chances of drifting undrowned to the Galapagos? Depending on where you start, they may be not at all bad, especially if you happen to be as tough as a tortoise — few animals put up with so much and need so little. Two currents reach for the Galápagos from the American coast: the cold Peruvian or Humboldt Current from the southeast during the three quarters of the year that the southeast trades blow, and a warm current from the general direction of Central America during the time of the northeast trade winds of the northern winter.

Between them the two currents account for much that is peculiar to the archipelago. For the anomaly of the islands is that the land supports a tropical continental American fauna and flora, while in the surrounding equatorial sea the life has an Antarctic likeness. There is no coral in the waters round these islands.

One of the most striking passages in William Beebe’s Arcturus Adventure describes a rip marking the line separating two currents running side by side in these seas, where the water was thick with floating land debris of all kinds, from seeds and grasses to the trunks of fair-sized trees. Torrents pouring down from the tropical forests of the Isthmus carry flotsam and sediment into the sea and out with the warm current towards the Galápagos, and the ocean floor is covered with decaying vegetable matter far out along the line of drift. And the eastern shore of Chatham Island, the first island to be encountered from the east, has driftwood of a larger size than any of the trees growing locally, not to mention bamboos, sugar cane, and coconut, none of which grow in the archipelago. Here is the road that the reptiles traveled, clinging to the trees and rafts of jungle growth that drifted to the islands. It would lake but two or three weeks, and in the course of many million years the wonder is so few have found the way. A single tortoise, lizard, or snake with eggs about to be laid would do the trick, and so would a pregnant mouse or rat. Frogs may have started the journeys as often as lizards, but a touch of the sea and a frog is soon dead: its naked skin needs moisture pure and unsalted.

The great lizards as well as the little up-to-date fellows must have taken the same journey, although neither they nor the tortoise may have been as big as they now are, when they first emigrated. There has been plenty of time for change since they made their landing. The question is, did one kind of these lizards, or two, make the initial voyage? In any case, either before departure or after landing the sea lizard evolved from the land lizard — it doesn’t work the other way round.

The closest relatives of the big Galápagos land lizards are the iguanas of tropical America, although they are much more alert and lively creat ures that lift their bodies clear of the ground when they start to go places. They have to, or be eaten by warm-blooded and quicker mammals. The Galápagos kind never gets its weight fully onto its feet and cannot put on anything like the same burst of speed. In this they are old-fashioned, as reptiles go, but there are two ways of looking at it. The mainland crowd that stayed behind were also heavy and weak-legged at one time but have since had to get moving to stay alive; or the island group, free in an isolated sanctuary without need to move except to eat or breed, has grown weak in the legs and heavy. The position of the legs, however, well out to the sides of the body, favors the first and labels the islanders as archaic.

However it was, the black sea lizard and the brown land lizard are more closely kin than either of them is to any other, and it is likely that either the land form took to the sea or the sea beast took to the land and gave rise to the other. Supposing some of the land lizards took to the sea to add seaweed to what may well have been a scanty diet on shore, those with the most bladelike tail would swim better, avoid more sharks, and live longer. Their eggs would be the ones to produce the next generation. And from generation to generation flatter tails and better swimmers would survive the longest and lay the most eggs. Gradually the sea lizard would come into being, with a flat swimming tail in place of the thick clumsy one its ancestors had dragged around. This way it works, from land to sea.

The land lizard is a landlubber whose one ocean crossing was enough of the sea for all time. Yet on the Galápagos even the land lizard has had to change in some degree: and while selection has left the tail alone, those that finally flourished have jaws, and teeth so strong they can literally chew shoe leather. This tells something of the general toughness of the plants that also drifted and settled on the black volcanic islands.

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ISLANDS have had a small part to play in the grand sweep of evolution as a whole, but they have played a large part in the progress of our understanding of it. First and foremost, islands are sanctuaries. The lizards and tortoises of the Galapagos arrived there to find no enemies of any kind. They were free to live in happy isolation, provided they didn’t eat themselves out of house and home. They went about their own business undisturbed until buccaneers and whalers raided them for food, and various transient settlers brought in cattle, horses and goats, and dogs and cats, all of which in one or two generations have gone back to their original wild and undomesticated stale. There are no islands any more; and as men in ships make moving bridges between continents and islands, all that made them sanctuaries disappears and the archaic living fossils go down before the more aggressive newcomers.

On the islands we have found the relics of more ancient times, but the problem now is to save them. The oldest of all lizards, the tuatara, once lived throughout New Zealand, but the rats and dogs brought in by Maoris and Europeans have killed them off, except in some tiny offshore islands.

To save them, all visitors are barred, even the conservationists themselves. Yet it is worth the fuss, for the tuatara still retains the third eye which all backboned animals once possessed, an eye that looks straight up through a hole in the top of the skull. The rest of us have closed the hole, no longer having anything to see, and keep no more than a peculiar blob of pineal tissue deep between the hemispheres of the brain, which, having nothing else to do, was said to be the seat of the soul!

Yet life that reached the islands anywhere has gone on evolving or changing, and it is the small scale on which it has taken place which makes it easier to see. What islands have done, as the Galápagos finches have shown, is to demonstrate the effect of isolation, apart from the simple question of safety. Sports may arise within a certain population of animals and give rise to more of their kind. If there is free breeding throughout the population, the character of the community as a whole may change slightly, but that is all.

But if part of a community reaches an island and sports arise there, the distance or the water interferes with travel, there is no mixed breeding with the rest of the community, and the islanders before long become so different that they could not breed with the mainlanders even if the opportunity arose. They become a distinct species. At the Galápagos the ground finches a long time ago scattered themselves among the various islands. Being inherently lazy, like all living things, each little island group fed and bred upon its own little spot of ground and lost contact even with those on the other islands ten to fifty miles away. And the sports that arose from time to time, on this island or that, never spread beyond the finch community of the particular island, until at last each group became so inbreeding and different that interbreeding with finches of other islands became impossible— and several species of Galápagos finches take the place of the original one and only.

In his collections of birds, Darwin found twentysix different species of land birds, all but one unique to the archipelago. The most significant were a group of thirteen species of finches, still known as Darwin’s finches, and three species of mockingbirds. What makes them so important is the manner of their distribution among the various Galápagos islands. One species of mockingbird lived only on Charles Island, another on Albemarle, and the third only on James and Chatham Islands. The ground finches varied all the way from thick-beaked kinds to some with beaks as fine as a warbler’s, with other differences as well, and they too were distributed one kind to one island. With these things in mind it came time for him to go, for “it is the fate of every voyager, when he has just discovered what object . . . in any place is more particularly worthy of his attention, to be hurried from it.” And the Beagle departed, with Darwin aboard once more, for Tahiti and the long voyage west around the world.

There is no speculation in Darwin’s journal concerning the meaning of the variations of the Galápagos birds and reptiles, no bringing of the extinct South American giants and the rising land into the picture with the rest. He went on to see the rest of the world and to wonder about the nature and origin of coral reefs, to arrive at last in England in October, 1836.

Yet questions bothered him: Why should species have been created slightly but distinctly different on each island? Could they have all come from a single stock and become different in the course of generations? Less than a year after his return, in July of 1837, he started his “first note-book on Transmutation of Species,” and in a little diary of the same date noted that he had been greatly struck with the character of the South American fossils and the species on the Galápagos Archipelago, and that these facts were the origin of all his views. Many years later the notebooks reached their maturity as The Origin of Species.

Other men had played with the idea of evolution but no one had proved that it really happened. Darwin returned from the voyage of the Beagle with his mind buzzing with new thoughts; he returned to the life of a country gentleman in a farming community where the selective breeding of sheep and cattle was already a fine art. And he was able to see that if selection among the offspring of domestic animals could produce new strains, the same sort of process might do the same in nature. Finally it all came together, and after twenty years of hard labor he showed to enthusiast and skeptic alike that animals vary and that some variation is heritable; that animals breed excessively but their numbers are forever being cut down by adverse circumstances; and that the inevitable result of all this is that the varieties best suited to meet their circumstances will survive at the expense of others; and that this leads to the gradual evolution of new kinds in response to changing surroundings. It is the beginning of our own journey into wonder, believing in the continuity of all life in spite of its diversity.