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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； whi