One of the youngest of the large volcanic islands of the world is Ascension in the South Atlantic. During the Second World War the American airmen sang
If we don’t find Ascension
Our wives will get a pension
this island being the only piece of dry land between the hump of Brazil and the bulge of Africa. It is a forbidding mass of cinders, in which the vents of no less than forty extinct volcanoes can be counted. It has not always been so barren, for its slopes have yielded the fossil remains of trees. What happened to the forests no one knows; the first men to explore the island, about the year 1500, found it treeless, and today it has no natural greenness except on its highest peak, known as Green Mountain.
In modern times we have never seen the birth of an island as large as Ascension. But now and then there is a report of a small island appearing where none was before. Perhaps a month, a year, five years later, the island has disappeared into the sea again. These are the little, stillborn islands, doomed to only a brief emergence above the sea.
About 1830 such an island suddenly appeared in the Mediterranean between Sicily and the coast of Africa, rising from 100-fathom depths after there had been signs of volcanic activity in the area. It was little more than a black cinder pile, perhaps 200 feet high. Waves, wind, and rain attacked it. Its soft and porous materials were easily eroded; its substance was rapidly eaten away and it sank beneath the sea. Now it is a shoal, marked on the charts as Graham’s Reef.
Falcon Island, the tip of a volcano projecting above the Pacific nearly two thousand miles east of Australia, suddenly disappeared in 1913. Thirteen years later, after violent eruptions in the vicinity, it as suddenly rose again above the surface and remained as a physical bit of the British Empire until 1949. Then it was reported by the Colonial Under Secretary to be missing again.
Almost from the moment of its creation, a volcanic island is foredoomed to destruction. It has in itself the seeds of its own dissolution, for new explosions, or landslides of the soft soil, may violently accelerate its disintegration. Whether the destruction of an island comes quickly or only after long ages of geologic time may also depend on external forces: the rains that wear away the loftiest of land mountains, the sea, and even man himself.
South Trinidad, or in the Portuguese spelling, ‘Ilha Trinidade,’ is an example of an island that has been sculptured into bizarre forms through centuries of weathering—an island in which the signs of dissolution are clearly apparent. This group of volcanic peaks lies in the open Atlantic, about a thousand miles northeast of Rio de Janeiro. E. F. Knight wrote in 1907 that Trinidad ‘is rotten throughout, its substance has been disintegrated by volcanic fires and by the action of water, so that it is everywhere tumbling to pieces.’ During an interval of nine years between Knight’s visits, a whole mountainside had collapsed in a great landslide of broken rocks and volcanic debris.
Sometimes the disintegration takes abrupt and violent form. The greatest explosion of historic time and the literal evisceration of the island of Krakatoa. In 1680 there had been a premonitory eruption of this small island in Sunda Strait, between Java and Sumatra in the Netherlands Indies. Two hundred years later there had been a series of earthquakes. In the spring of 1883, smoke and steam began to ascend from fissures in the volcanic cone. The ground became noticeably warm, and warning rumblings and hissings came from the volcano. Then, on 27 August, Krakatoa literally exploded. In an appalling series of eruptions, that lasted two days, the whole northern half of the cone was carried away. The sudden inrush of ocean water added the fury of superheated stream to the cauldron. When the inferno of white-hot lava, molten rock, steam, and smoke had finally subsided, the island that had stood 1400 feet above the sea had become a cavity a thousand feet below sea level. Only along one edge of the former crater did a remnant of the island remain.
Krakatoa, in its destruction, became known to the entire world. The eruption gave rise to a hundred-foot wave that wiped out villages along the Strait and killed people by tens of thousands. The wave was felt on the shores of the Indian Ocean and at Cape Horn; rounding the Cape into the Atlantic, it sped northward and retained its identity even as far as the English Channel. The sound of the explosions was heard in the Philippine Islands, in Australia, and on the Island of Madagascar, nearly 3000 miles away. And clouds of volcanic dust, the pulverized rock that had been torn from the heart of Krakatoa, ascended into the stratosphere and were carried around the globe to give rise to a series of spectacular sunsets in every country of the world for nearly a year.
Although Krakatoa’s dramatic passing was the most violent eruption that modern man has witnessed, Krakatoa itself seems to have been the product of an even greater one. There is evidence that an immense volcano once stood where the waters of Sunda Strait now lie. In some remote period a titanic explosion blew it away, leaving only its base represented by a broken ring of islands. The largest of these was Krakatoa, which, in its own demise, carried away what was left of the original crater ring. But in 1929 a new volcanic island arose in this place—Anak Krakatoa, Child of Krakatoa.
Subterranean fires and deep unrest disturb the whole area occupied by the Aleutians. The islands themselves are the peaks of a thousand-mile chain of undersea mountains, of which volcanic action was the chief architect. The geologic structure of the ridge is little known, but it rises abruptly from oceanic depths of about a mile on one side and two miles on the other. Apparently this long narrow ridge indicates a deep fracture of the earth’s crust. On many of the islands volcanoes are now active, or only temporarily quiescent. In the short history of modern navigation in this region, it has often happened that a new island has been reported but perhaps only the following year could not be found.
The small island of Bogoslof, since it was first observed in 1796, has altered its shape and position several times and has even disappeared completely, only to emerge again. The original island was a mass of black rock, sculptured into fantastic, tower-like shapes. Explorers and sealers coming upon it in the fog were reminded of a castle and named it Castle Rock. At the present time there remain only one or two pinnacles of the castle, a long spit of black rocks where sea lions haul out, and a cluster of higher rocks resounding with the cries of thousands of sea birds. Each time the parent volcano erupts, as it has done at least half a dozen times since men have been observing it, new masses of steaming rocks emerge from the heated waters, some to reach heights of several hundred feet before they are destroyed in fresh explosions. Each new cone that appears is, as described by the volcanologist Jaggar, ‘the live crest, equivalent to a crater, of a great submarine heap of lava six thousand feet high, piled above the floor of Bering Sea where the Aleutian mountains fall off to the deep sea.’
One of the few exceptions to the almost universal rule that oceanic islands have a volcanic origin seems to be the remarkable and fascinating group of islets known as the Rocks of St. Paul. Lying in the open Atlantic between Brazil and Africa, St. Paul’s Rocks are an obstruction thrust up from the floor of the ocean into the midst of the racing Equatorial Current, a mass against which the seas, which have rolled a thousand miles unhindered, break in sudden violence. The entire cluster of rocks covers not more than a quarter of a mile, running in a curved line like a horseshoe. The highest rock is no more than sixty feet above the sea; spray wets it to the summit. Abruptly the rocks dip under water and slope steeply down into great depths. Geologists since the time of Darwin have puzzled over the origin of these black, wave-washed islets. Most of them agree that they are composed of material like that of the sea floor itself. In some remote period, inconceivable stresses in the earth’s crust must have pushed a solid rock mass upward more than two miles.
So bare and desolate that not even a lichen grows on them, St. Paul’s Rocks would seem one of the most unpromising places in the world to look for a spider, spinning its web in arachnidan hope of snaring passing insects. Yet Darwin found spiders when he visited the Rocks in 1833, and forty years later
the naturalists of H.M.S. Challenger also reported them, busy at their web-spinning. A few insects are there, too, some as parasites on the sea birds, three species of which nest on the Rocks. One of the insects is a small brown moth that lives on feathers. This very nearly completes the inventory of the inhabitants of St. Paul’s Rocks, except for the grotesque crabs that swarm over the islets, living chiefly on the flying fish brought by the birds to their young.
St. Paul’s Rocks are not alone in having an extraordinary assortment of inhabitants, for the faunas and floras of oceanic islands are amazingly different from those of the continents. The pattern of island life is peculiar and significant. Aside from forms recently introduced by man, islands remote from the continents are never inhabited by any land mammals, except sometimes the one mammal that has learned to fly—the bat. There are never any frogs, salamanders, or other amphibians. Of reptiles, there may be a few snakes, lizards, and turtles, but the more remote the island from a major land mass, the fewer reptiles there are, and the really isolated islands have none. There are usually a few species of land birds, some insects, and some spiders. So remote an island as Tristan da Cunha in the South Atlantic, 1500 miles from the nearest continent, has no land animals but these: three species of land birds, a few insects, and several small snails.
With so selective a list, it is hard to see how, as some biologists believe, the islands could have been colonized by migration across land bridges, even if there were good evidence for the existence of the bridges. The very animals missing from the islands are the ones that would have had to come dry-shod, over the hypothetical bridges. The plants and animals that we find on oceanic islands, on the other hand, are the ones that could have come by wind or water. As an alternative, then, we must suppose that the stocking of the islands has been accomplished by the strangest migration in earth’s history—a migration that began long before man appeared on earth and is still continuing, a migration that seems more like a series of cosmic accidents than an orderly process of nature.
We can only guess how long after its emergence from the sea an oceanic island may lie uninhabited. Certainly in its original state it is a land bare, harsh, and repelling beyond human experience. No living thing moves over the slopes of its volcanic hills; no plants cover its naked lava fields. But little by little, riding on the winds, drifting on the currents, or rafting in on logs, floating brush, or trees, the plants and animals that are to colonize it arrive from the distant continents.
So deliberate, so unhurried, so inexorable are the ways of nature that the stocking of an island may require thousands or millions of years. It may be that no more than half a dozen times in all these eons does a particular form, such as a tortoise, make a successful landing upon its shores. To wonder impatiently why man is not a constant witness of such arrivals is to fail to understand the majestic pace of the process.
Yet we have occasional glimpses of the method. Natural rafts of uprooted trees and matted vegetation have frequently been seen adrift at sea, more than a thousand miles off the mouths of such great tropical rivers as the Congo, the Ganges, the Amazon, and the Orinoco. Such rafts could easily carry an assortment of insect, reptile, or mollusk passengers. Some of the involuntary passengers might be able to withstand long weeks at sea; others would die during the first stages of the journey. Probably the one best adapted for travel by raft are the wood-boring insects, which, of all the insect tribe, are most commonly found on oceanic islands. The poorest raft travelers must be the mammals. But even a mammal might cover short interisland distances. A few days after the explosion of Krakatoa, a small monkey was rescued from some drifting timber in Sunda Strait. She had been terribly burned, but survived the experience.
No less than the water, the winds and the air currents play their part in bringing inhabitants to the islands. The upper atmosphere, even during the ages before man entered it in his machines, was a place of congested traffic. Thousands of feet above the earth, the air is crowded with living creatures, drifting, flying, gliding, ballooning, or involuntarily swirling along on the high winds. Discovery of this rich aerial plankton had to wait until man himself had found means to make physical invasion of these regions. With special nets and traps, scientists have now collected from the upper atmosphere many of the forms that inhabit oceanic islands. Spiders, whose almost invariable presence on these islands is a fascinating problem, have been captured nearly three miles above the earth’s surface. Airmen have passed through great numbers of the white, silken filaments of spiders’ ‘parachutes’ at heights of two to three miles. At altitudes of 6000 to 16,000 feet, and with wind velocities reaching 45 miles an hour, many living insects have been taken. At such heights and on such strong winds, they might well have been carried hundreds of miles. Seeds have been collected at altitudes up to 5000 feet. Among those commonly taken are members of the Composite family, especially the so-called ‘thistle-down’ typical of oceanic islands.
An interesting point about transport of living plants and animals by wind is the fact that in the upper layers of the earth’s atmosphere the winds do not necessarily blow in the same direction as at the earth’s surface. The trade winds are notably shallow, so that a man standing on the cliffs of St. Helena, a thousand feet above the sea, is above the wind, which blows with great force below him. Once drawn into the upper air, insects, seeds, and the like can easily be carried in a direction contrary to that of the winds prevailing at island level.
The wide-ranging birds that visit islands of the ocean in migration may also have a good deal to do with the distribution of plants, and perhaps even of some insects and minute land shells. From a ball of mud taken from a bird’s plumage, Charles Darwin raised eighty-two separate plants, belonging to five distinct species! Many plant seeds have hooks or prickles, ideal for attachment to feathers. Such birds as the Pacific golden plover, which annually flies from the mainland of Alaska to the Hawaiian Islands and even beyond, probably figure in many riddles of plant distribution.
The catastrophe of Krakatoa gave naturalists a perfect opportunity to observe the colonization of an island. With most of the island itself destroyed, and the remnant covered with a deep layer of lava and ash that remained hot for weeks, Krakatoa after the explosive eruptions of 1883 was, from a biological standpoint, a new volcanic island. As soon as it was possible to visit, scientists searched for signs of life, although it was hard to imagine how any living thing could have survived. Not a single plant or animal could be found. It was not until nine months after the eruption that the naturalist Cotteau was able to report: ‘I only discovered one microscopic spider—only one. This strange pioneer of the renovation was busy spinning its web.’ Since there were no insects on the island, the web-spinning of the bold little spider was presumably in vain, and except for a few blades of grass, practically nothing lived on Krakatoa for a quarter of a century. Then the colonists began to arrive—a few mammals in 1908; a number of birds, lizards, and snakes; various mollusks, insects, and earthworms. Ninety percent of Krakatoa’s new inhabitants, Dutch scientists found, were forms that could have arrived by air.
Isolated from the great mass of life on the continents, with no opportunity for the crossbreeding that tends to preserve the average and to eliminate the new and unusual, island life has developed in a remarkable manner. On these remote bits of earth, nature has excelled in the creation of strange and wonderful forms. As though to prove her incredible versatility, almost every island has developed species that are endemic—that is, they are peculiar to it alone and are duplicated nowhere else on earth.
It was from the pages of earth’s history written on the lava fields of the Galapagos that young Charles Darwin got his first inkling of the great truths of the origin of species. Observing the strange plants and animals—giant tortoises, black, amazing lizards that hunted their food in the surf, sea lions, birds in extraordinary variety—Darwin was struck by their vague similarity to mainland species of South and Central America, yet was haunted by the differences
, differences that distinguish them not only from the mainland species but from those on other islands of the archipelago. Years later he was to write in reminiscence: ‘Both in space and time, we seem to be brought somewhat near to that great fact—that mystery of mysteries—the first appearance of new beings on earth.’
Of the ‘new beings’ evolved on islands, some of the most striking examples have been birds. In some remote age before there were men, a small, pigeonlike bird found its way to the island of Mauritius, in the Indian Ocean. By processes of change at which we can only guess, this bird lost the power of flight, developed short, stout legs, and grew larger until it reached the size of a modern turkey. Such was the origin of the fabulous dodo, which did not long survive the advent of man on Mauritius. New Zealand was the sole home of the moas. One species of these ostrich-like birds stood twelve feet high. Moas had roamed New Zealand from the early part of the Tertiary; those that remained when the Maoris arrived soon died out.
Other island forms besides the dodo and the moas have tended to become large. Perhaps the Galapagos tortoise became a giant after its arrival on the islands, although fossil remains on the continents cast doubt on this. The loss of wing use and even of the wings themselves (the moas had none) are common results of insular life. Insects on small, wind-swept islands tend to lose the power of flight—those that retain it are in danger of being blown out to sea. The Galapagos Islands have a flightless cormorant. There have been at least fourteen species of flightless rails on the islands of the Pacific alone.
One of the most interesting and engaging characteristics of island species is their extraordinary tameness—a lack of sophistication in dealings with the human race, which even the bitter teachings of experience do not quickly alter. When Robert Cushman Murphy visited the island of South Trinidad in 1913 with a party from the brig Daisy, terns alighted on the heads of the men in the whaleboat and peered inquiringly into their faces. Albatrosses on Laysan, whose habits include wonderful ceremonial dances, allowed naturalists to walk among their colonies and responded with a grave bow to similar polite greetings from the visitors. When the British ornithologist David Lack visited the Galapagos Islands, a century after Darwin, he found that the hawks allowed themselves to be touched, and the flycatchers tried to remove hair from the heads of the men for nesting material. ‘It is a curious pleasure,’ he wrote, ‘to have the birds of the wilderness settling upon one’s shoulders, and the pleasure could be much less rare were man less destructive.’