Stevenson remarked on the ‘attractive power’ of the Pacific islands in In the South Seas:
Few men who come to the islands leave them; they grow grey where they alighted; the palm shades and the trade-winds fan them till they die, perhaps cherishing to the last the fancy of a visit home…No part of the world exerts the same attractive power.
37
Two-thirds of Krakatau Island, originally six miles long and clothed in tropical rain forest, disappeared in the huge eruption of 1883, but a remnant of the southern volcano was left standing, along with two close neighbors, Sertung and Panjang. All of these were covered by a thirty-foot blanket of hot ash, so that ‘not a plant, not a blade of grass, not a fly, survived,’ in Ian Thornton’s account. Three years later, ferns were the first plants to recolonize the island. These were followed by casuarinas, birds which had migrated from Australia, and a monitor lizard.
38
Biologically, as well as geologically, continental islands (such as, for instance, New Zealand, Madagascar, or New Guinea) are entirely different from oceanic ones. For continental islands are broken-off pieces of the main and (at least initially) may have all the species of the parent continent. Once broken off, of course, they become as isolated as any other island, and their isolation (and altered conditions) may promote the most extravagant speciation, as with the unique primates of Madagascar or the flightless birds of New Zealand.
There are also diseases endemic to islands, diseases which have emerged or persisted because of their isolation, and are thus analogous to an island’s endemic flora and fauna. This too was recognized more than a century ago, by the great German epidemiologist Hirsch. The study of such diseases, he thought, would constitute a ‘geographical and historical pathology’ and such a science, he wrote, ‘in an ideally complete form, would furnish a medical history of mankind.’
39
More than forty varieties of banana are grown on Pohnpei, and some of these seem to be unique to the island. The banana has a remarkable tendency to somatic mutation, to ‘sports’ – some of these are disadvantageous, but others may lead to plants which are more disease resistant, or fruit which is more delectable in one way or another; and this has stimulated cultivation of some five hundred varieties worldwide.
The major banana sports are regarded as species (and given binomial, Linnaean names), the minor sports as varieties only (which bear only local names). But the difference, as Darwin remarks, is only one of degree: ‘Species and variations,’ he writes in the Origin, ‘blend into each other by an insensible series; and a series impresses the mind with the idea of an actual passage.’ In time, many varieties will diverge sufficiently to become distinct species.
The importation of bananas onto islands, as it happens, has also shown us something of the rate of evolution in sympatric species. Thus, as H.W. Menard notes, ‘Five new species of banana moths have evolved in Hawaii since the Polynesians introduced the banana to Hawaii only about one thousand years ago.’ For islands are forcing grounds for evolutionary change, whether of plants or animals, insects or microbes; under the special conditions of island life, the slow processes of mutation and specialization may be amplified and accelerated to a spectacular degree.
J.B.S. Haldane once proposed a way of quantifying the rate of change of any variable – a bird’s beak, an ammonite’s whorl – as it evolved, suggesting that a change of one percent per million years be called a ‘darwin.’ Evolution generally proceeded, he thought, in ‘millidarwins,’ and he imagined (as Darwin himself did) that with this infinitesimal rate evolution could never actually be seen. But we are now finding (as Jonathan Weiner recounts in The Beak of the Finch ) that evolution can occur at a very much faster rate when selection pressures are high. This has been studied by Peter and Rosemary Grant, with the very finch populations Darwin himself observed, on the small Galapagos island of Daphne Major. Following a catastrophic drought, the finch population showed clear evolutionary changes (in beak and body size) in a matter of months, an ‘evolutionary rate,’ Weiner calculates, of 25,000 darwins.
One does not need to deal only with rare and catastrophic circumstances to see evolution in action. A beautiful example has recently been observed by Martin Cody and Jacob Overton with the seeds of some daisies, which are blown by the wind to small islets off the Pacific coast of Canada. A fluffball or pappus holds the seed aloft, and its size determines, other things being equal, how far the seed is liable to be carried. Once the plants have settled on an island, their pappi become shorter, so they are less liable to be dispersed. These changes, like those of finches, have been observed within the span of a year or two.
But the most astounding example of very rapid, massive evolution relates to the more than three hundred species of cichlid fish unique to Lake Victoria. DNA studies (by Axel Meyer) have indicated that these species diverged very recently in evolutionary terms, and there is now strong geologic evidence that the lake itself is only 12,000 years old. While Darwin’s Galapagos finches evolved perhaps twenty different species over four million years, the cichlids of Lake Victoria have shown a rate of speciation more than five thousand times greater.
40
Jack London, in Uaitape, found Bora-Borans dancing ‘with strange phosphorescent flowers in their hair that pulsed and dimmed and glowed in the moonlight.’
41
Paul Theroux has called sakau (known on many islands as kava) ‘the most benign drug in the world.’ Its benignness was also stressed by Cook when he encountered it on his first visit to Tahiti (a related variety of pepper in New Zealand is now named captaincookia in his honor). Though it was described by naturalists on Cook’s first voyage, credit for its ‘discovery’ is usually given to the Forsters, the botanical father and son who accompanied Cook on his second voyage, and the plant has since been known by the name they gave it, Piper methysticum Forst.
An eloquent description of its effects was given by Lewin in his Phantastica; I had read this years before, as a student, and had been curious to try it myself. All is benign, stresses Lewin, if one does not overdo it:
When the mixture is not too strong, the subject attains a state of happy unconcern, well-being and contentment, free of physical or psychological excitement.…The drinker never becomes angry, unpleasant, quarrelsome or noisy, as happens with alcohol… The drinker remains master of his conscience and his reason. When consumption is excessive, however, the limbs become tired, the muscles seem no longer to respond to the orders and control of the mind, walking becomes slow and unsteady and the drinker looks partly inebriated. He feels the need to lie down. The eyes see the objects present, but cannot or do not want to identify them accurately. The ears also perceive sounds without being able or wanting to realize what they hear. Little by little, objects become vaguer and vaguer…[until] the drinker is overcome by somnolence and finally drifts off to sleep.
We had all been struck, when we arrived in Pohnpei, by the extraordinary slowness of drivers and pedestrians in Kolonia, but put this down to unhurriedness, a sense of leisure, ‘island time.’ But some of this slowness was clearly physiological, a sakau-induced psychomotor retardation. Sakau use and abuse is widespread here, although the effects of this are generally not dangerous. Dr. G.A. Holland mentions having seen only one sakau-related accident in his many years of practice in Micronesia; this was an elderly man who stumbled while returning home from a sakau party, fell, and broke his neck.