The Malay Archipelago, by Alfred Russel Wallace

Chapter 33

The Aru Islands — Physical Geography and Aspects of Nature.

IN this chapter I propose to give a general sketch of the physical geography of the Aru Islands, and of their relation to the surrounding countries; and shall thus be able to incorporate the information obtained from traders, and from the works of other naturalists with my own observations in these exceedingly interesting and little-known regions.

The Aru group may be said to consist of one very large central island with a number of small ones scattered round it. The great island is called by the natives and traders “Tang-busar” (great or mainland), to distinguish it as a whole from Dobbo, or any of the detached islands. It is of an irregular oblong form, about eighty miles from north to south, and forty or fifty from east to west, in which direction it is traversed by three narrow channels, dividing it into four portions. These channels are always called rivers by the traders, which puzzled me much till I passed through one of them, and saw how exceedingly applicable the name was. The northern channel, called the river of Watelai, is about a quarter of a mile wide at its entrance, but soon narrows to abort the eighth of a mile, which width it retains, with little variation, during its whole, length of nearly fifty miles, till it again widens at its eastern mouth. Its course is moderately winding, and the hanks are generally dry and somewhat elevated. In many places there are low cliffs of hard coralline limestone, more or less worn by the action of water; while sometimes level spaces extend from the banks to low ranges of hills a little inland. A few small streams enter it from right and left, at the mouths of which are some little rocky islands. The depth is very regular, being from ten to fifteen fathoms, and it has thus every feature of a true river, but for the salt water and the absence of a current. The other two rivers, whose names are Vorkai and Maykor, are said to be very similar in general character; but they are rather near together, and have a number of cross channels intersecting the flat tract between them. On the south side of Maykor the banks are very rocky, and from thence to the southern extremity of Aru is an uninterrupted extent of rather elevated and very rocky country, penetrated by numerous small streams, in the high limestone cliffs bordering which the edible birds’ nests of Aru are chiefly obtained. All my informants stated that the two southern rivers are larger than Watelai.

The whole of Aru is low, but by no means so flat as it has been represented, or as it appears from the sea. Most of it is dry rocky ground, with a somewhat undulating surface, rising here and there into abrupt hillocks, or cut into steep and narrow ravines. Except the patches of swamp which are found at the mouths of most of the small rivers, there is no absolutely level ground, although the greatest elevation is probably not more than two hundred feet. The rock which everywhere appears in the ravines and brooks is a coralline limestone, in some places soft and pliable, in others so hard and crystalline as to resemble our mountain limestone.

The small islands which surround the central mass are very numerous; but most of them are on the east side, where they form a fringe, often extending ten or fifteen miles from the main islands. On the west there are very few, Wamma and Palo Pabi being the chief, with Ougia, and Wassia at the north-west extremity. On the east side the sea is everywhere shallow, and full of coral; and it is here that the pearl-shells are found which form one of the chief staples of Aru trade. All the islands are covered with a dense and very lofty forest.

The physical features here described are of peculiar interest, and, as far as I am aware, are to some extent unique; for I have been unable to find any other record of an island of the size of Aru crossed by channels which exactly resemble true rivers. How these channels originated were a complete puzzle to me, till, after a long consideration of the whole of the natural phenomena presented by these islands, I arrived at a conclusion which I will now endeavour to explain. There are three ways in which we may conceive islands which are not volcanic to have been formed, or to have been reduced to their present condition, by elevation, by subsidence, or by separation from a continent or larger island. The existence of coral rock, or of raised beaches far inland, indicates recent elevation; lagoon coral-islands, and such as have barrier or encircling reefs, have suffered subsidence; while our own islands, whose productions are entirely those of the adjacent continent, have been separated from it. Now the Aru Islands are all coral rock, and the adjacent sea is shallow and full of coral, it is therefore evident that they have been elevated from beneath the ocean at a not very distant epoch. But if we suppose that elevation to be the first and only cause of their present condition, we shall find ourselves quite unable to explain the curious river-channels which divide them. Fissures during upheaval would not produce the regular width, the regular depth, or the winding curves which characterise them; and the action of tides and currents during their elevation might form straits of irregular width and depth, but not the river-like channels which actually exist. If, again, we suppose the last movement to have been one of subsidence, reducing the size of the islands, these channels are quite as inexplicable; for subsidence would necessarily lead to the flooding of all low tracts on the banks of the old rivers, and thus obliterate their courses; whereas these remain perfect, and of nearly uniform width from end to end.

Now if these channels have ever been rivers they must have flowed from some higher regions, and this must have been to the east, because on the north and west the sea-bottom sinks down at a short distance from the shore to an unfathomable depth; whereas on the east. a shallow sea, nowhere exceeding fifty fathoms, extends quite across to New Guinea, a distance of about a hundred and fifty miles. An elevation of only three hundred feet would convert the whole of this sea into moderately high land, and make the Aru Islands a portion of New Guinea; and the rivers which have their mouths at Utanata and Wamuka, might then have flowed on across Aru, in the channels which are now occupied by salt water. Then the intervening land sunk down, we must suppose the land that now constitutes Aru to have remained nearly stationary, a not very improbable supposition, when we consider the great extent of the shallow sea, and the very small amount of depression the land need have undergone to produce it.

But the fact of the Aru Islands having once been connected with New Guinea does not rest on this evidence alone. There is such a striking resemblance between the productions of the two countries as only — exists between portions of a common territory. I collected one hundred species of land-birds in the Aru Islands, and about eighty of them, have been found on the mainland of New Guinea. Among these are the great wingless cassowary, two species of heavy brush turkeys, and two of short winged thrushes; which could certainly not have passed over the 150 miles of open sea to the coast of New Guinea. This barrier is equally effectual in the case of many other birds which live only in the depths of the forest, as the kinghunters (Dacelo gaudichaudi), the fly-catching wrens (Todopsis), the great crown pigeon (Goura coronata), and the small wood doves (Ptilonopus perlatus, P. aurantiifrons, and P. coronulatus). Now, to show the real effect of such barrier, let us take the island of Ceram, which is exactly the same distance from New Guinea, but separated from it by a deep sea. Cut of about seventy land-birds inhabiting Ceram, only fifteen are found in New Guinea, and none of these are terrestrial or forest-haunting species. The cassowary is distinct; the kingfishers, parrots, pigeons, flycatchers, honeysuckers, thrushes, and cuckoos, are almost always quite distinct species. More than this, at least twenty genera, which are common to New Guinea and Aru, do not extend into Ceram, indicating with a force which every naturalist will appreciate, that the two latter countries have received their faunas in a radically different manner. Again, a true kangaroo is found in Aru, and the same species occurs in Mysol, which is equally Papuan in its productions, while either the same, or one closely allied to it, inhabits New Guinea; but no such animal is found in Ceram, which is only sixty miles from Mysol. Another small marsupial animal (Perameles doreyanus) is common to Aru and New Guinea. The insects show exactly the same results. The butterflies of Aru are all either New Guinea species, or very slightly modified forms; whereas those of Ceram are more distinct than are the birds of the two countries.

It is now generally admitted that we may safely reason on such facts as those, which supply a link in the defective geological record. The upward and downward movements which any country has undergone, and the succession of such movements, can be determined with much accuracy; but geology alone can tell us nothing of lands which have entirely disappeared beneath the ocean. Here physical geography and the distribution of animals and plants are of the greatest service. By ascertaining the depth of the seas separating one country from another, we can form some judgment of the changes which are taking place. If there are other evidences of subsidence, a shallow sea implies a former connexion of the adjacent lands; but i£ this evidence is wanting, or if there is reason to suspect a rising of the land, then the shallow sea may be the result of that rising, and may indicate that the two countries will be joined at some future time, but not that they have previously been so. The nature of the animals and plants inhabiting these countries will, however, almost always enable us to determine this question. Mr. Darwin has shown us how we may determine in almost every case, whether an island has ever been connected with a continent or larger land, by the presence or absence of terrestrial Mammalia and reptiles. What he terms “oceanic islands “possess neither of these groups of animals, though they may have a luxuriant vegetation, and a fair number of birds, insects, and landshells; and we therefore conclude that they have originated in mid-ocean, and have never been connected with the nearest masses of land. St. Helena, Madeira, and New Zealand are examples of oceanic islands. They possess all other classes of life, because these have means of dispersion over wide spaces of sea, which terrestrial mammals and birds have not, as is fully explained in Sir Charles Lyell’s “Principles of Geology,” and Mr. Darwin’s “Origin of Species.” On the other hand, an island may never have been actually connected with the adjacent continents or islands, and yet may possess representatives of all classes of animals, because many terrestrial mammals and some reptiles have the means of passing over short distances of sea. But in these cases the number of species that have thus migrated will be very small, and there will be great deficiencies even in birds and flying insects, which we should imagine could easily cross over. The island of Timor (as I have already shown in Chapter XIII) bears this relation to Australia; for while it contains several birds and insects of Australian forms, no Australian mammal or reptile is found in it, and a great number of the most abundant and characteristic forms of Australian birds and insects are entirely absent. Contrast this with the British Islands, in, which a large proportion of the plants, insects, reptiles, and Mammalia of the adjacent parts of the continent are fully represented, while there are no remarkable deficiencies of extensive groups, such as always occur when there is reason to believe there has been no such connexion. The case of Sumatra, Borneo, and Java, and the Asiatic continent is equally clear; many large Mammalia, terrestrial birds, and reptiles being common to all, while a large number more are of closely allied forms. Now, geology has taught us that this representation by allied forms in the same locality implies lapse of time, and we therefore infer that in Great Britain, where almost every species is absolutely identical with those on the Continent, the separation has been very recent; while in Sumatra and Java, where a considerable number of the continental species are represented by allied forms, the separation was more remote.

From these examples we may see how important a supplement to geological evidence is the study of the geographical distribution of animals and plants, in determining the former condition of the earth’s surface; and how impossible it is to understand the former without taking the latter into account. The productions of the Aru Islands offer the strangest evidence, that at no very distant epoch they formed a part of New Guinea; and the peculiar physical features which I have described, indicate that they must have stood at very nearly the same level then as they do now, having been separated by the subsidence of the great plain which formerly connected them with it.

Persons who have formed the usual ideas of the vegetation of the tropics who picture to themselves the abundance and brilliancy of the flowers, and the magnificent appearance of hundreds of forest trees covered with masses of coloured blossoms, will be surprised to hear, that though vegetation in Aru is highly luxuriant and varied, and would afford abundance of fine and curious plants to adorn our hothouses, yet bright and showy flowers are, as a general rule, altogether absent, or so very scarce as to produce no effect whatever on the general scenery. To give particulars: I have visited five distinct localities in the islands, I have wandered daily in the forests, and have passed along upwards of a hundred miles of coast and river during a period of six months, much of it very fine weather, and till just as I was about to leave, I never saw a single plant of striking brilliancy or beauty, hardly a shrub equal to a hawthorn, or a climber equal to a honeysuckle! It cannot be said that the flowering season had not arrived, for I saw many herbs, shrubs, and forest trees in flower, but all had blossoms of a green or greenish-white tint, not superior to our lime-trees. Here and there on the river banks and coasts are a few Convolvulaceae, not equal to our garden Ipomaeas, and in the deepest shades of the forest some fine scarlet and purple Zingiberaceae, but so few and scattered as to be nothing amid the mass of green and flowerless vegetation. Yet the noble Cycadaceae and screw-pines, thirty or forty feet high, the elegant tree ferns, the lofty palms, and the variety of beautiful and curious plants which everywhere meet the eye, attest the warmth and moisture of the tropics, and the fertility of the soil.

It is true that Aru seemed to me exceptionally poor in flowers, but this is only an exaggeration of a general tropical feature; for my whole experience in the equatorial regions of the west and the east has convinced me, that in the most luxuriant parts of the tropics, flowers are less abundant, on the average less showy, and are far less effective in adding colour to the landscape than in temperate climates. I have never seen in the tropics such brilliant masses of colour as even England can show in her furze-clad commons, her heathery mountain-sides, her glades of wild hyacinths, her fields of poppies, her meadows of buttercups and orchises — carpets of yellow, purple, azure-blue, and fiery crimson, which the tropics can rarely exhibit. We, have smaller masses of colour in our hawthorn and crab trees, our holly and mountain-ash, our boom; foxgloves, primroses, and purple vetches, which clothe with gay colours the whole length and breadth of our land, These beauties are all common. They are characteristic of the country and the climate; they have not to be sought for, but they gladden the eye at every step. In the regions of the equator, on the other hand, whether it be forest or savannah, a sombre green clothes universal nature. You may journey for hours, and even for days, and meet with nothing to break the monotony. Flowers are everywhere rare, and anything at all striking is only to be met with at very distant intervals.

The idea that nature exhibits gay colours in the tropics, and that the general aspect of nature is there more bright and varied in hue than with us, has even been made the foundation of theories of art, and we have been forbidden to use bright colours in our garments, and in the decorations of our dwellings, because it was supposed that we should be thereby acting in opposition to the teachings of nature. The argument itself is a very poor one, since it might with equal justice be maintained, that as we possess faculties for the appreciation of colours, we should make up for the deficiencies of nature and use the gayest tints in those regions where the landscape is most monotonous. But the assumption on which the argument is founded is totally false, so that even if the reasoning were valid, we need not be afraid of outraging nature, by decorating our houses and our persons with all those gay hues which are so lavishly spread over our fields and mountains, our hedges, woods, and meadows.

It is very easy to see what has led to this erroneous view of the nature of tropical vegetation. In our hothouses and at our flower-shows we gather together the finest flowering plants from the most distant regions of the earth, and exhibit them in a proximity to each other which never occurs in nature. A hundred distinct plants, all with bright, or strange, or gorgeous flowers, make a wonderful show when brought together; but perhaps no two of these plants could ever be seen together in a state of nature, each inhabiting a distant region or a different station. Again, all moderately warm extra-European countries are mixed up with the tropics in general estimation, and a vague idea is formed that whatever is preeminently beautiful must come from the hottest parts of the earth. But the fact is quite the contrary. Rhododendrons and azaleas are plants of temperate regions, the grandest lilies are from temperate Japan, and a large proportion of our most showy flowering plants are natives of the Himalayas, of the Cape, of the United States, of Chili, or of China and Japan, all temperate regions. True, there are a great number of grand and gorgeous flowers in the tropics, but the proportion they bear to the mass of the vegetation is exceedingly small; so that what appears an anomaly is nevertheless a fact, and the effect of flowers on the general aspect of nature is far less in the equatorial than in the temperate regions of the earth.

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